Animal Models of Aging Research: Implications for Human Aging and Age-Related Diseases*

Abstract

Aging is characterized by an increasing morbidity and functional decline that eventually results in the death of an organism. Aging is the largest risk factor for numerous human diseases, and understanding the aging process may thereby facilitate the development of new treatments for age-associated diseases. The use of humans in aging research is complicated by many factors, including ethical issues; environmental and social factors; and perhaps most importantly, their long natural life span. Although cellular models of human disease provide valuable mechanistic information, they are limited in that they may not replicate the in vivo biology. Almost all organisms age, and thus animal models can be useful for studying aging. Herein, we review some of the major models currently used in aging research and discuss their benefits and pitfalls, including interventions known to extend life span and health span. Finally, we conclude by discussing the future of animal models in aging research.

Full text

Animal Models of Aging
Research: Implications
for Human Aging and
Age-Related Diseases

Sarah J. Mitchell,
1,†
Morten Scheibye-Knudsen,
2,†
Dan L. Longo,
3
and Rafael de Cabo
1
1
Translational Gerontology Branch,
2
Laboratory of Molecular Gerontology, and
3
Laboratory of Genetics, National Institute on Aging, National Institutes of Health,
Baltimore, Maryland 21224; email: sarah.mitchell@nih.gov; scheibyem@mail.nih.gov;
longod@grc.nia.nih.gov; decabora@grc.nia.nih.gov
Annu. Rev. Anim. Biosci. 2015. 3:283–303
The Annual Review of Animal Biosciences is online
at animal.annualreviews.org
This article’s doi:
10.1146/annurev-animal-022114-110829

This is a work of the U.S. Government and is not
subject to copyright protection in the United States.
†
Authors contributed equally to this work.
Keywords
aging, animal models, rodents, nonhuman primates
Abstract
Aging is characterized by an increasing morbidity and functional de-
cline that eventually results in the death of an organism. Aging is the
largest risk factor for numerous human diseases, and understanding
the aging process may thereby facilitate the development of new treat-
ments for age-associated diseases. The use of humans in aging re-
search is complicated by many factors, including ethical issues;
environmental and social factors; and perhaps most importantly, their
long natural life span. Although cellular models of human disease pro-
vide valuable mechanistic information, they are limited in that they
may not replicate the in vivo biology. Almost all organisms age, and
thus animal models can be useful for studying aging. Herein, we re-
view some of the major models currently used in aging research and
discuss their benefits and pitfalls, including interventions known to
extend life span and health span. Finally, we conclude by discussing
the future of animal models in aging research.
283
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ANNUAL
REVIEWS

INTRODUCTION
The aging process is associated with a time-dependent progressive increase in disease suscepti-
bility. Almost all known organisms age, and although the maximum life span differs between
organisms, the shape of the curve, often considered representative of the health of the organism, is
remarkably consistent across species (Figure 1). In the human context, aging is becoming an
increasing socioeconomic problem for countries around the world. By the end of the twenty-first
century, the percentage of the population aged above 65 is projected to increase from approx-
imately 7% to more than 20% worldwide (http://esa.un.org/wpp/). Further adding to this aging
epidemic, the older population, and indeed the population in general, is becoming increasingly
unhealthy independent of a slight increase in life span over the past decades (1). Further, at least
80% of health care costs are accrued after a person turns 45 years of age (2). It is thus clear that
society is facing an enormous economic challenge in the decades to come, and investigating
interventions that ensure healthy aging is becoming increasingly important.
In the past decades, research into the underlying causes of aging has led to remarkable
breakthroughs, not only in the understanding of mechanisms of aging but also in interventions
that may increase life span and, more importantly, health span. Model organisms have been at the
forefront of this research and have yielded a wealth of information, allowing us to find conserved
pathways that may also regulate human aging.
One of the most successful examples was the initial discovery that inhibition of the target of
rapamycin (mTOR) pathway increases life span in yeast, nematodes, and flies, with later work
demonstrating these life-extending properties appear to be conserved in vertebrates (3–7). This led
to the discovery that rapamycin (named for its discovery on Easter Island, Rapa Nui), an inhibitor
of mTOR, may be able to ameliorate aspects of the accelerated aging diseases Hutchinson-Gilford
progeria and Cockayne syndrome (8, 9), as well as extending life span in mice (3, 10). Another
0
Age (days)
0
0
20
40
60
80
100
0
20
40
60
80
100
510152025
Age (days)
Survival (%)
Survival (%)
Age (days)
0
Age (days)
Caenorhabditis
elegans
Mus musculus
Macaca mullata
7,060 10,5603,520
30 0 200 400 600 800 1,000 1,200
31,68021,12010,560
Homo sapiens
Figure 1
The universality of aging. Theoretical life-span curves depicting the similarity in the aging process across model
organisms relative to humans. Despite the differences in life span, the shape of the curve, often considered
a measure of the health (or health span) of the organism, is similar.
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famous, but controversial, discovery underscoring the use of model organisms was the finding that
overexpressionof the sirtuinSir2 in yeast, nematodes, and flies leads to life-span extension (11–13).
The implication of Sir2 in aging across several species led to the identification of the small molecule
resveratrol, which was able to activate Sir2 as well as the mammalian homolog SIRT1 (14). Later,
resveratrol was found to extend the life span of mice fed a high-fat diet, as well as having beneficial
effects in nonhuman primates (NHPs) fed a high-sugar, high-fat diet (15–18). Compounds with
higher specificity and potency as SIRT1 activators were later synthesized, and two of these,
SRT1720 and SRT2104, have been shown to extend the life span of mice fed a standard diet (19,
20). These animal studies have led to the initiation of several clinical trials using SIRT1 activators in
humans (http://www.clinicaltrials.gov/ct2/results?term5resveratrol&Search5Search).
Model organisms continue to form the basis of aging research, as ethical issues, long natural life
span, environmental influences, genetic heterogeneity, and various other limiting factors com-
plicate use of human subjects in aging research (http://www.afar.org). But how do we assess the
ability of an intervention to improve both the health and longevity of an organism? Great strides
have been made since the pivotal reports of McCay describing the life-span extension of rats on
caloric restriction (CR) in the early twentieth century (21). A host of more sophisticated
assessments of health span and life span are now available (Figure 2). Nevertheless, we must still
consider the limitations of these models to accurately reflect human aging. In this review, we
attempt to describe vertebrate animal models that have been used to study aging and age-related
diseases, as well as suggest future directions for this research.
Interventions
Genetic Nutritional
Food intake
Body
temperature
Body
composition
Serum/
urine
analysis
PK data,
metabolomics
Glucose
Homeostasis
Glucose and
insulin levels
Glucose, insulin,
and pyruvate
tolerance tests
Behavior
Learning,
memory, and
cognition
Open eld,
rotarod,
MWM, fear
conditioning
Physical
performance
Treadmill,
strength test,
wirehang
Tissue
analysis
Microarray,
metabolomics,
proteomics
Immune
stress test
LPS, tumor
injection,
cold stress
Metabolic
assessment
Home cage
activity,
clams
Necropsy and
histology
Environmental
Figure 2
Testing interventions for longitudinal studies. Longitudinal assessments every three to six months on at least n ¼ 10 animals per
intervention are performed on animals across their life span to assess health span. Abbreviations: LPS, lipopolysaccharide; MWM, Morris
water maze; PK, pharmacokinetic.
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RODENT MODELS OF AGING
The laboratory mouse and rat are common models for the study of aging and age-related diseases.
The wealth of background knowledge, convenience of use, capacity to regulate environmental
factors, genetic manipulability, and expense have led to an explosion of aging-related research
focused on these models. Furthermore, their short life span relative to humans makes them easier
to study than long-lived animals. Indeed, rodents paved the way for both dietary and genetic
interventions in aging, as best illustrated by the discovery that CR extends rodent life span, as well
as the finding that mutations in certain genes are associated with longevity. In the following
sections, we discuss these common aging models, including their possible limitations.
Mouse Models
Inbred mice. Inbred mice have been the most extensively used strains for the study of aging and
age-related diseases to date. This method of breeding between relatives (usually a brother and
sister) increases the genetic similarity between the offspring; thus, differences between animals of
the same genetic strain can be attributed to environmental or treatment effects. The idea is to
minimize other factors that may affect an outcome or complicate interpretation of a study. Al-
though inbred strains have had considerable use in the study of aging, the major concern sur-
rounding their use is that some commonly used strains show only a limited range of pathology. For
example, C57BL/6 mice, upon which 70% of published animal studies have relied, show high
prevalence of lymphoma and increased susceptibility to metabolic dysregulation (22). But whether
one strain is more appropriate than another remains contentious. In particular, the assessment of
health span in inbred mice can be confounded owing to premature vision or hearing loss compared
with other inbred strains (23). Furthermore, differences in reported mean life span can vary up to
20% depending on the strain and sex of the mouse, despite the same genetic background and
environment (24, 25). The power of inbreeding is remarkable given its capacity to minimize genetic
variability; however, conclusions must be interpreted with caution, as data from a single inbred
strain may not be representative of the species as a whole. Further, the resulting genetic uniformity
of inbred strains is not representative of the human population.
With this in mind, the body of information surrounding the development, reproduction,
physiology, behavior, and genetics ofthese mice is vast. The Mouse Phenome Project conducted by
the Jackson Laboratories is an in-depth study of the physiology and life span of 31 genetically
diverse inbred mouse strains (http://phenome.jax.org/). Launched in 2001, the Mouse Phenome
Database (MPD) is the data coordination center for the international Mouse Phenome Project. The
MPD integrates quantitative phenotype, gene expression, and genotype data into a common
annotated framework to facilitate query and analysis (26). With more than 3,500 phenotype
measurements or traits relevant to human health, including cancer, aging, cardiovascular dis-
orders, obesity, infectious disease susceptibility, blood disorders, neurosensory disorders, drug
addiction, and toxicity, the MPD represents an important resource for the study of aging biology
and its relevance to human disease.
Outbred and F1 mice. Outbred and F1 mice are generally used for the same reason: hybrid vigor,
with long life spans, high disease resistance, early fertility, large and frequent litters, rapid growth,
and large size. However, unlike F1 hybrids, outbred mice are genetically undefined. This brings an
advantage, as they can be considered to be more representative of the human population; however,
it represents an obstacle when assessing the benefits of an intervention (Figure 2). These outbred
stocks should not be used in situations where smaller numbers of mice from a range of inbred
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strains would give optimal results, such as determining sensitivities to substances or examining
physiological parameters (27). Caloric and methionine restriction are two of the most frequently
used interventions to extend life span in mice. In (BALB/cJ x C57BL/6 J)F1 mice, methionine
restriction has been shown to increase maximal life span as well as lower levels of serum insulin-like
growth factor 1 (IGF-1), insulin, glucose, and thyroid hormone (28). Forty percent CR extends
maximal life span in male B6D2F1 mice by 20% relative to ad libitum–fed controls (29); however,
whether this effect also extends to females and to other F1 and outbred strains remains to be seen.
Wild-derived mice. But what about the wild-derived mice? It has been suggested that laboratory
mice eat roughly 20% more than wild mice under ad libitum laboratory conditions on a weight-
adjusted basis, indicating that they are metabolically obese (30). Thus, the life-span extension in
these fat CR mice may simply be due to the reduction of food intake to what they should normally
be eating if they were in the wild (31). Nevertheless, CR does extend life span in wild mice, but
whether there is a beneficial effect on health span remains to be determined (31). These results do
agree with some aspects of the CR literature in that the incidence of tumors was remarkably
reduced in wild mice on CR (31). Few studies since Harper et al. (31) in 2006 have used wild-
derived mice, most likely owing to the tedious nature of catching wild mice. However, one should
consider genetically heterogeneous models of mice and their utility in aging research, in particular
the four- or eight-way cross (see next section).
Genetically heterogeneous mouse models. Genetically heterogeneous mouse models provide
many advantages for research on aging but have been used infrequently. These mice are the off-
spring of four or eight different grandparent lines. In this cross, each mouse is genetically unique,
but replicate populations of essentially similar genetic structure can be generated quickly, at low
cost, and of arbitrary size from commercially available, genetically stable hybrid parents (32). A
recent study of genetically heterogeneous mice created from four inbred strains (BALB/c, C57BL/6,
C3H, and DBA2), referred to as HET3 mice, found that more than 90% died of cancer (33).
Although this homogeneity in the cause of death could be considered beneficial under some
circumstances, it highlights the importance of natural variation in causes of death for mouse
models to parallel the human situation. We do know that CR extends life span in HET3 mice (34).
Accelerated Aging
An important step in our understanding of aging was the description of several inherited human
diseases that show accelerated aging (35). Notably, all of these diseases appear to be caused by
mutations in genes that are involved in maintaining genome integrity, supporting the idea that the
accumulation of DNA damage may be involved in aging. Each of the diseases displays only
a few features of normal aging phenotype, and the disorders are therefore also called segmental
progerias. The diseases include Werner syndrome (36), Hutchinson-Gilford progeria (37),
Rothmund-Thomson syndrome (38), Bloom syndrome (39), Nestor-Guillermo progeria (40),
dyskeratosis congenita (41), ataxia telangiectasia (42), xeroderma pigmentosum (43), and
Cockayne syndrome (44). The genetic mutations underlying all of these disorders have been
characterized, and knockout mouse models have been created for most of them (45). Interestingly,
many of these knockout mice show much milder phenotypes, and premature aging often occurs
only when multiple genes are knocked out. Thus, it appears that backup systems exist for some of
these key pathways in mice that likely involve genes not yet defined. We describe some of the mouse
models in greater detail below.
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Human Werner syndrome is the accelerated aging disorder that most closely reflects human
aging and is caused by mutations in the RecQ-like DNA helicase Wrn. Patients develop normally
until they reach early adulthood. At this time, premature aging, such as cerebral atrophy, hair loss,
cataracts, osteoporosis, diabetes, and cardiovascular disease, becomes apparent, and the patients
generally die of heart disease at approximately 40–50 years of age (46, 47). In the late nineties,
a mouse model was generated that showed little phenotype (48). However, crossing the Wrn
/
mice with mice lacking the tumor suppressor P53 led to a modest decrease in life span, whereas
knockout of the telomerase complex in a Wrn
/
background replicated many features of human
aging, such as osteoporosis and diabetes, possibly owing to defects in cellular replicative potential
(49, 50).
Hutchinson-Gilford progeria is caused by mutations in the LMNA gene, which encodes the
nuclear filament proteins lamin A and C. The LMNA mutation creates an alternative splice site
leading to the formation of a shortened mRNA transcript encoding a protein dubbed progerin that
accumulates in cells from Hutchinson-Gilford progeria patients. Accumulation of progerin dis-
rupts normal nuclear architecture, leading to DNA damage and replication problems (51). Patients
suffering from Hutchinson-Gilford progeria display aging-associated pathology, such as car-
diovascular disease, osteoporosis, hair loss, and loss of adipose tissue, and have a mean life span of
approximately 12 years (52). Hutchinson-Gilford patients in general do not display neurological
aging to any significant extent. Several mouse models have been created that recapitulate aspects of
the disorder, such as cardiovascular disease and osteoporosis (53–55). More recently, an inducible
transgenic mouse that overexpresses progerin was created that showed premature skin aging and
hair loss (56). Notably, the skin aging was reversed when progerin expression was inhibited.
Possible cardiovascular changes in this mouse model were, however, not reported.
Dyskeratosis congenita is caused by mutations in genes believed to be involved in maintenance
of telomeres, the specialized structures that form the ends of the chromosomes. These include the
RNA component of telomerase, TERC, and protein components of telomerase, such as TERT and
DKC1 (57). Dyskeratosis congenita is characterized by the triad of nail dystrophy, reticular
hypopigmentation, and leukoplakia. In addition, bone marrow failure, idiopathic lung fibrosis,
graying of hair, and hair loss occur with varying penetrance. The disease thus shows only relatively
minor features of normal aging. Telomeres have, however, been implicated in aging, particularly at
the cellular level. This correlation originally stemmed from the observation that primary cells in
culture divide only a limited number of times. This phenomenon, termed the Hayflick limit, is
believed to be caused by telomere shortening that occurs with each division owing to problems in
replicating the very ends of the telomeres (58). Telomerase helps to maintain the telomere length,
and deficiencies in this enzyme lead to defects in proliferating cells, such as stem cells in the skin.
Interestingly, mice have very long telomeres, and knockout of TERC or TERT does not lead to any
immediate phenotype. However, inbreeding of telomerase-deficient mice leads to progressive loss
of telomere length with each generation. Thus, third-generation telomerase null mice show ac-
celerated aging (58–60). As in humans, proliferative tissues are particularly prone to telomere
shortening, and TERC or TERT knockout mice may therefore represent good models for
interventions that aim at maintaining stem cell pools. It remains largely unexplained why telo-
merase deficiency affects the largely nonreplicating lung and has no phenotype in the vigorously
replicating intestinal mucosa. Furthermore, it is not well explained why mice that have long
telomeres have short life spans, while humans have short telomeres and live substantially longer. In
addition, although telomere shortening has been shown in circulating leukocytes with age in
several studies, it is unknown whether this correlates with increased mortality (61–63). It is most
certain that telomeres play a role in cellular senescence in vitro; however, the effect on aging in vivo
is still being questioned.
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Xeroderma pigmentosum, particularly complementation group A, as well as ataxia telan-
giectasia and Cockayne syndrome are the only accelerated aging disorders in which severe
neurodegeneration is highly prevalent (64–6 6). Xeroderma p igmentosum is caused by muta-
tions in several genes (XPA, XPB, XPC, XPD, XPE, XPF, XPG, and XPV) involved in a DNA
repair pathway called nucleotide excision repair (43). Cockayne syndrome is most commonly
caused by mutations in CSA or CSB, two proteins involved in transcription-coupled nucleotide
excision repair. Ataxia telangiectasia is caused by mutations in the ATM kinase, an e nzyme
primarily involved in the signaling cascade after double-stranded DNA b reaks (64). Although
they work in different DNA repair pathways, the neurodegenerative phenotypes are r elatively
similar, with cerebellar degeneration, ataxia, and neuropathy. All of these diseases manifest in
early childhood, and patients have an average life span of 12 years (Cockayne syndrome) to 30–
40 years (ataxia telangiectasia and xeroderma pigmentosum group A). Several mouse models
have been created to describe these diseases. None of them capture the severity of the disease in
humans, and only very minor degeneration of the cerebellum has been reported in one model of
ataxia telangiectasia (38).
The Cockayne syndrome mice mirror some aspects of human diseases, such as wasting and loss
of cells in the inner ear, and show a 10% reduction in brain size (9). XPA mice, however, appear
completely normal, although they show higher propensity for UV-induced skin cancer (65).
Crossing the XPA mice with the Cockayne syndrome mice produces a profound neurodegen-
erative phenotype with greatly shortened life span and global neurological deterioration (67, 68).
ATM null mice display only minor neurodegenerative phenotypes, although they do recapitulate
aspects of ataxia telangiectasia, such as immunodeficiency (69, 70). ATM
/
mice may thereby
represent an interesting model for the study of immune senescence. More recently, mice harboring
catalytically dead ATM show early embryonic lethality, perhaps indicating that nonfunctional
ATM may interfere with a general DNA damage response and that other kinases may compensate
if the ATM protein is completely absent (71).
Because of the idea that deficient DNA repair may contribute to aging, several mouse models
have been created with disruption of various enzymes in this pathway. One interesting model is the
ERCC1 and XPF knockout mice. ERCC1, in complex with the endonuclease XPF, participates in
nucleotide excision repair as well as interstrand crosslink DNA repair. Interestingly, ERCC1 and
XPF knockout mice show a strong multisystemic degeneration and die of liver failure upon
weaning (72, 73). The hepatic phenotype and early death of ERCC1 mice can be rescued by
overexpression of liver-specific ERCC1, which leads to survival after weaning and death from
kidney failure at two to three months of age (74). Notably, transcriptional profiling in the liver of
ERCC1-deficient mice at postnatal day 15 shows attenuation of the IGF-1 axis (75). As we touch
on below, loss of IGF-1 signaling is known to extend life span in mice and nematodes, indicating
that loss of this pathway in the ERCC1-deficient mice may be a compensatory response to DNA
damage accumulation. Indeed, the same transcriptional changes are observed in Xpa
/
/Csa
/
double-knockout mice (76).
Accumulation of mitochondrial damage has been proposed to be the underlying cause of
aging (77). Considerable research has supported a role for mitochondria in the aging process,
and a large number of animal models have been generated that support the mitochondrial theory
of aging. The most famous example of this may be the mutator mouse. This mouse model
harbors a mutation in the proofreading domain of the murine mitochondrial DNA po lymerase
gamma (POLG) (78). This leads to the accumulation of mutations in mitochondrial DNA but
interestingly does not lead to increased reactive oxygen species (ROS) production. The phe-
notype of the mice is characterized by weight loss, alopecia, osteoporosis, cardiomyopathy, and
hypogonadism and thereby shows significant overlap with many features of human aging.
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Notably, the mice show stem cell renewal defects but no overt neurodegenerative phenotype
(79). This is in contrast to humans with mitochondrial diseases, in whom neurodegeneration is
prominent but osteoporosis, hair loss, and anemia are rare (80). Even though no increase in
ROS production is observed in the mutatormouse, other mouse models have supported the role
of free radicals in aging. Particularly strong support came from the observation that over-
expression of catalase, a ROS scavenging enzyme, targeted to mitochondria leads to life-span
extension in mice ( 81). However, other models with decreased capacity to scavenge ROS have
not demonstrated shortened l ongevity (82).
Delayed Aging
Caloric (83) and methionine (84) restriction remain the only non-genetic, non-pharmacological
interventions to increase life span in mice. In fact, it has been nearly a century since the potential
life-extending effects of CR were first reported, and we are still searching for the elusive mech-
anism. CR extends life span in most species tested (as reviewed in 85). But recent evidence suggests
that the subtleties of CR may be more complex than initially thought. Indeed, the effect of 40% CR
on 41 recombinant inbred strains (ILSXISS) of mice, both males and females, found a huge
variation in the response to CR, with CR being detrimental to some strains (86). There are clear
examples of the differential response to CR in the literature. For example, reports of CR on the
DBA2 strain show anywhere from a detrimental effect on life span of approximately 6% to
a beneficial effect on life span of 20–50% depending on the sex of the animals (87–89). Fur-
thermore, diet composition plays a major role. Most recently, it has been shown that longevity can
be manipulated through altering macronutrient content, with mice fed a low-protein, high-
carbohydrate diet having maximal life span (90). And this is before we even consider the ef-
fect (if any) of CR. However, the translational potential for humans is low given the proven
difficulty of altering diet to manage diseases in people and the aversion to consuming 40% less
calories for years. Thus, alternative strategies are in demand. Perhaps if one understood how CR
works, an alternative approach could be developed.
In 2000, the Interventions Testing Program was developed to systematically study the effects of
diets, drugs, or other interventions on life span in mice. Unfortunately, this program is reserved
specifically for mice, as the number of rats required to obtain statistical significance for a particular
intervention far outweighs the space and financial availability to conduct these studies. One of the
first compounds tested, rapamycin, was found to extend median and maximal life span of both
male and female genetically heterogeneous mice when fed beginning at 600 days of age. Based on
age at 90% mortality, rapamycin led to an increase of 14% for females and 9% for males (3).
Rapamycin administered in the food from 9 months of age to genetically heterogeneous mice
resulted in significant increases in life span, including maximum life span, with an associated
increase in median survival of approximately 10% in males and 18% in females (33). Other
pharmacological interventions, such as resveratrol, metformin, and sirtuin activators, have been
demonstrated to increase life span in mice (15, 19, 91), through modulation of the nutrient sensing
pathways controlled by AMP-activated protein kinase and sirtuin 1 (92, 93). However, the efficacy
of these interventions might be sex and strain specific, and this warrants further investigation. It is
important to consider both males and females when determining the success of an intervention,
genetic, pharmacological, or otherwise (Figure 1). Indeed, male, but not female, transgenic mice
overexpressing Sirt6 (94) exhibit increased life span. Similarly, nordihydroguaiaretic acid and
aspirin significantly increased life span in heterogeneous male, but not female, mice (95). And more
recently, it has been shown that life-span extension of HET3 mice on rapamycin is independent of
insulin sensitivity (96).
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Genetic Models of Delayed Aging
In looking for the fountain of youth, several models have been identified through which genes are
shown to play a major role in the extension of life span. The Ames dwarf, Snell dwarf, and growth
hormone (GH) receptor knockout (GHRKO) mice are the classical mouse models of delayed
aging. These strains display exceptional longevity through alteration in the GH pathway resulting
in low-circulating IGF-1 (97, 98).
The Ames and Snell dwarf mice have loss-of-function mutations in their Prop-1 and Pit-1 genes,
respectively, resulting in deficiencies in circulating levels of thyrotropin, prolactin, and GH, which
lead to life-span extension (99). Interestingly, there is a sex-specific difference in maximal life span
of Ames dwarf mice, with an observed increase of 20% in males and 50% in females. Snell mice,
however, live up to 50% longer than their wild-type littermates (97, 99). These mice show some of
the characteristics of CR, including lower core body temperature (100, 101), improved insulin
sensitivity (98), enhanced antioxidant defenses (102), and delayed onset of neoplasia (103, 104),
which may play roles in their increased longevity. A defect in the Klotho gene leads to a premature
aging phenotype characterized by arteriosclerosis, osteoporosis, age-related skin changes, and
ectopic calcifications, together with short life span and infertility (105). Conversely, the transgenic
mice that overexpress Klotho exhibit significant resistance to oxidative stress associated with
moderate resistance to insulin/IGF-1, which may partly explain why these mice live longer than
wild-type mice (106). The GHRKO mouse was generated through the targeted disruption of the
GH receptor and GH-binding protein (97). These mice are long-lived and have a reduction in
glucose, insulin, thyroid hormones, and core body temperature that is in agreement with
observations reported for the Ames dwarf mouse (100). The GHRKO mice show a similar increase
in life span between males and females of 23% and 25%, respectively (97). Reductions in these
parameters may be important to the underlying mechanisms of delayed aging in these animals.
Interestingly, the GHRKO mice are obese but insulin sensitive (97), which is paradoxically op-
posite to what is observed in CR. A recent study that examined the role of the visceral fat in
adiposity and insulin sensitivity found that removal of visceral fat resulted in an improvement in
insulin sensitivity in wild-type mice but made the GHRKO mice more insulin resistant (107). When
GHRKO mice are put on CR, there is no life-span extension (108, 109), perhaps because CR
reduces adiposity, which may not be beneficial to these animals (107). Consistent with this idea of
altered fat signaling, removal of visceral fat at five months of age (110) leads to increased medial
and maximal life span in rats. The GHRKO mice achieve life-span extension by a mechanism that
appears to overlap the effects of CR given that CR cannot augment the effect. Thus, the available
tools to examine the mechanisms behind aging and potential interventions are vast.
Rats
Rats have been extensively used in the laboratory for research into many areas, including car-
diovascular disease, neurological disorders, neurobehavioral studies, cancer susceptibility, and
renal disease, as well as for behavioral studies of cognition. Such research has relied on the
widespread use of inbred Fischer 344 (F344) rats as well as other genetically defined (F1 hybrids)
and outbred rat populations. The National Institute on Aging (NIA) aging animal colony has
provided F344 rats since inception, possibly accounting for the relatively widespread use of this
model in aging research even today. Three options for aging rats, all genetically defined, are now
available under the NIA program: the F344, Brown-Norway (BN), and F1 hybrid of F344 3 BN
strains. Interestingly, F344 3 BN rats are used as models for progressive aortic vasculopathy, as
changes in the thoracic aorta have been shown to display age-related pathology similar to what
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occurs in humans (111). In cognitive studies, it is important to understand the phenotype of the
model that you are using to identify any pathologies or disabilities, which may affect the outcome.
For example, age-associated blindness can negatively impact and confound cognitive assessments.
Another issue is the occurrence of a single severe disease in inbred animals that can confound the
interpretation of an aging study; for instance, nephropathy in F344 rats is the major cause of
mortality (112).
Transgenic models. Althoughthe use of transgenic mice in research has steadily increased over the
past years, this has not been the case for transgenic rats. There have been hurdles to the de-
velopment of transgenic rats, such as sensitivity of rats’ fertilized eggs under in vitro conditions.
Nevertheless, recent advances in the development of transgenic rats have meant they are gaining
importance in cognitive research. In Alzheimer’s disease, it has been suggested that rats are a more
appropriate model for the human disease given that rats are closer to humans and have a pre-
dictable and multifaceted behavioral display (113). However, rat Alzheimer’s disease models do
not display the human-like neurofibrillary tangles that some mouse models do (113). Transgenic
rat models have been used for the study of retinal degeneration, including the P23H transgenic
albino rat for the study of the retinitis pigmentosa mutation (114) and the Royal College of
Surgeons transgenic rat used for the study of human retinitis pigmentosa (115).
Interventions for life span extension. McCay et al. (21) presented the very first report of extended
life span in his white rats upon dietary restriction. Since this pivotal report, many labs have
confirmed this finding in rats (116–118). Notably, removal of the pituitary gland in male Wistar
rats at 70 days of age produced similar life span–extension effects as CR begun at the same time
point (119). Further supporting the role of GH-IGF-1 in longevity, heterogeneous GH knockout
rats had life-span extension of approximately 10% relative to control rats, although the ho-
mozygous GH knockout rats are actually shorter lived (120). However, not all interventions are
successful; take, for instance, metformin, which is successful in mice (91) but not in F344 rats (121),
and 2-deoxyglucose, which does not extend life span in F344 rats (122) but does in Caenorhabditis
elegans (123). Moving forward, integrated approaches of both mouse and rat models will together
advance our understanding of aging and age-related diseases.
NAKED MOLE RATS
The naked mo le rat (NMR; Heterocephalus glaber), also known as the sand puppy or desert
mole rat, is the longest-living rodent known to man, with a maximum life span of approximately
30 years (124). These mouse-sized rodents live up t o five times longer than expected based on
their small body size, but they are highly socialized rodents that a re commonly used in be-
havioral, neurological, and physiological research (124, 125). NMRs are common to the
subterranean burrows in the arid and semiarid regions of the h orn of Africa. They are the first
mammals discovered to exhibit eusociality, with the presence of a female queen and one to three
reproducing males, with the rest of the members of the colony functioning as workers for
gathering food and protection (124). But it is their biology that makes them so attractive to
gerontologists. Indeed, NMRs aged >24 years do exhibit signs of aging consistent with humans,
such as retinal degeneration and osteoarthritis (125), but display negligible senescence, no age-
related increase in mortality, and high fecundity until death. The possibilities for translation to
human health are undoubtedly significant if we discover the mechanism behind their well-
preserved health.
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Potential Mechanisms for Longevity in NMRs
Initially, enhanced antioxidant defense was thought to be one of the major mechanisms through
which NMRs had enhanced longevity (126) and extreme resistance to experimentally induced
tumorigenesis (127). The activity levels of the antioxidants, such as superoxide dismutases
(SOD1–2), do not change with age in NMRs, although they do decline with age in mice (128).
Thus, maintenance of the activity of SOD1 and -2 rather than an enhanced activity may contribute
to the extended life span of NMRs. Indeed, CR maintains the levels of these enzymes into old age in
mice (129).
The insulin/IGF signaling pathway is another important modulator of life span. In CR,
maintenance of this pathway is proposed to be one of the major factors influencing longevity
(130). Interestingly, NMRs display an abnormal response to a bolus of glucose as measured using
the glucose tolerance test with prolonged hyperglycemia (131). Their pancreata show an unusual
distribution of endocrine cells relative to most other rodents, which may explain their unusual
hyperglycemic condition. These animals show lower insulin levels (126), which further highlights
the complexity of the IGF pathway in longevity. These lower insulin levels and reduced levels of
IGF-1 are consistent with changes reported in CR (130, 132).
NMR cells produce fewer aberrant proteins, supporting the hypothesis that the more stable
proteome of the NMR contributes to its longevity (133, 134). Recently, it was shown that NMRs
have high levels of basal autophagy (135). Increased translational fidelity may play a role in the
NMR’s longevity, and differences in translational fidelity may be important in determining life
span (133). A whole genome sequencing analysis of the NMR genome found that genes related to
the degradation of macromolecules, mitochondrial encoded genes, were not altered with age in
NMRs (136). Furthermore, telomerase reverse transcriptase showed stable expression regardless of
age (136). Taken together, these results highlight differentially expressed patterns of expression
ofNMRgenes,whichmayunderlielongevitymechanismsinthisanimal.Furthermore,itwouldbeof
significant interest to compare the gene expression profile of mice or rats on CR and on ad libitum
feeding to that of NMRs. Given the tenfold difference in life expectancy of mice and NMRs and the
likely high degree of genetic homology between the species, any differences detected are likely to be
importantinexplainingthedifferencesinlongevity. WouldCRfurtherextendthelifespanofNMRs,
or would it be detrimental?
PRIMATES
NHPs are perhaps the most appropriate model for the study of aging and age-related diseases.
Traditionally,rhesus macques (Macaca mulatta) have been the prime focus of aging research. Two
programs, one at the NIA of the National Institutes of Health and the other by the University of
Wisconsin–Madison, have studied this species in ongoing longevity studies for more than 30 years.
Rhesus monkeys are commonly used in biomedical research owing to their similarity to humans
across a wide range of variables, including genetics, endocrinology, physiology, neuroanatomy,
and cognitive function. However, there are drawbacks to the use of these monkeys in research.
Their weight and strength pose difficulties in husbandry, and sophisticated equipment is needed to
navigate daily life in these facilities. Furthermore, the strict social hierarchies and potential for
aggressive behavior mean that these incredibly intelligent animals need special consideration and
substantial environmental enrichment to keep them appropriately cared for. Monkeys can carry
and transmit many dangerous pathogens, making it expensive to study them in the context of
aging. Furthermore, the costs and ethical concerns of supply alone limit the contribution of NHPs
to research.
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Interventions for Longevity and Health
In recent years, two studies have highlighted the importance of the study environment for calorie
restriction and its application to humans. Most notably, the University of Wisconsin–Madison, and
the NIA NHP CR studies have highlighted the subtle differences in response to CR (137, 138).
Although we can all agree that CR delays the onset of age-associated diseases, the data on whether
thisis also associated with life-span extension are conflicting. Indeed, further studies and analysis are
needed to definitively address this question. Interestingly, we have recently shown that two years of
resveratroltreatmentimprovedthemetabolicsyndromeassociatedwitha high-fat, high-sugardiet in
rhesusmonkeys (16–18).Clearly, the translationpotentialforcompoundslikeresveratrol is great,as
resveratrol now is in clinical trials for use in humans, with at least 80 different trials ongoing or
completed as of the publication of this article (http://www.clinicaltrials.gov).
ALTERNATE AGING MODELS
Although primates and rodents have supplied a wealth of information regarding the aging process,
alternative models are useful to test ongoing hypotheses of aging. This is particularly pertinent
because species-specific changes may influence results and data interpretation. The rate-of-living
hypothesis of aging is an example of a theory that initially explained many observations in aging
but was later questioned based on data from other species. This theory was based on observations
more than a century ago by the physiologist Max Rubner, who found that longer-lived species
generally have a lower resting metabolism per gram body weight than shorter-lived species do
(139). Although this relationship has been found across several species, there are several
exceptions. Birds, for example, appear to defy this relationship by living considerably longer than
expected for their metabolic rate (140, 141). To understand aging, it is therefore clear that in-
formation from multiple species across the phylogeny of life is of value. We now discuss a few of the
alternative vertebrate aging models that have been reported in the literature. Although these
models may appear rather extraordinary, each has its own strengths and weaknesses.
Fish
Fish have been surprisingly robustly present in the aging field throughout the years. This may
partially stem from some rather controversial claims in the early twentieth century that fish do not
age (142), a statement that was later repudiated (143). Nevertheless, fish have emerged as an
interesting model system in general biology and aging research. The zebrafish (Danio rerio)
remains the most common fish in the lab setting. It has a life span of approximately two to three
years and may therefore not be particularly advantageous for life-span studies as compared with
rodents. However, zebrafish have remarkable regenerative capabilities that could be of interest for
tissue repair and thus for longevity (144).
Another species that shows promise as a model for longevity is the turquoise killifish
(Nothobranchius furzeri). N. furzeri have several advantages compared with other vertebrate
aging models. First, the fish has one of the shortest life spans ( ∼13 weeks) of any vertebrate species
(145). Second, the fish can be kept at relatively high population densities, allowing for larger and
cheaper population studies than usual for rodent life-span studies. Third, the eggs are resistant to
desiccation and can be kept at room temperature for months. Storage of strains of fish is therefore
much easier than for rodents. Fourth, each female produces several hundred eggs, allowing for
rapid expansion of a colony (146). In addition, these fish respond with an increase in life span in
response to CR and show life-span extension after resveratrol treatment under standard diet
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conditions (147). Based on these observations, N. furzeri represents an interesting and inexpensive
model system for interventions in aging and could thus represent an ideal model system for higher
throughput screening of putative life span –extending compounds. Indeed, several labs are cur-
rently pursuing research with this model (see http://www.nothobranchius.info/).
Dogs and Cats
Domesticated species, such as dogs and cats, represent interesting model systems for aging. Even
though the average canine life span of 10–12 years discourages longevity studies, dogs sponta-
neously develop many age-related phenotypes, such as muscular and neurological decline, as well
as cardiovascular disease (148–151). Rodents, however, do not develop significant neuro-
degeneration with age unless severely genetically manipulated (152). Dogs may therefore be
particularly interesting in the study of cognitive deterioration and age-associated neurodegen-
erative disorders (153). In addition, the physiology and pathology of dogs have been extremely
well characterized. Similarly, cats represent another physiologically well-characterized domes-
ticated animal that has been used in aging studies (149, 154–156). As in dogs, several pathological
age-associated processes occur in felines, including kidney disease, arthritis, sarcopenia, and
neurological decline (149, 154–156). Cats live an average of 12–14 years, and life-span studies in
this species are therefore also problematic (157); however, their aging phenotype may make them
attractive models.
Birds
When looking across the life span of multiple species, longevity tends to scale according to the size
of the animal, in agreement with the rate-of-living hypothesis of aging. Birds, however, live a re-
markably long time when considering their relatively small body size (158). Interestingly, birds
maintain blood glucose levels one- to threefold higher than most mammals but with low insulin
and high glucagon levels (159). This could indicate that the insulin/IGF-1 pathway might be
involved in the longevity of birds. Indeed, although birds retain very high GH and IGF-1 levels
during development, the levels of these hormones decrease in adulthood (158). Other possible
explanations for the apparent longevity of birds have been related to decreased susceptibility to
oxidative stress and increased telomere length (158). In addition, fertility appears to be well
preserved with age in birds (160). Several bird species can be kept in a lab setting, and their high
reproductive capacity makes them easy models to work with. Life-span studies are, however,
difficult owing to their inherent longevity. The Japanese quail is a common lab bird that lives for
a maximum of six years and interestingly responds similarly to CR as mammals (161). Birds
represent an interesting animal for comparative cross-species studies of the interplay of metab-
olism and aging.
FUTURE DIRECTIONS
Animal models form the basis for preclinical biomedical research and will undoubtedly continue
to do so, as their life span, although shorter, essentially mimics that of humans, highlighting the
universality of the aging process (Figure 1). Transgenic mice have contributed greatly to our
knowledge of a multitude of different biological processes; however, this animal model also has its
drawbacks. In particular, inbred mouse strains are prone to numerous diseases, perhaps masking
true physiological responses to various interventions. This is widely acknowledged, and many
large-scale investigations, such as the Aging Interventions Testing Program, now use the four-way
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cross. Nevertheless, even outbred strains of mice are still significantly limited in the aging phe-
notype. For example, normally aged mice do not develop neurodegeneration and have very low
prevalence of cardiovascular disease (162, 163). It is perhaps not surprising that some organisms,
such as mice, age differently than humans; however, this is important to remember when
attempting to extrapolate from murine data to human physiology. It is thus possible that with
expanding physiological knowledge of species not conventionally used in aging research, many
nonmurine animal models may contribute to our understanding of aging. In particular, transgenic
primate models have now been generated, and useful primate models for studying genetic
pathways involved in aging could therefore be created (164). However, rodent models still
represent one of the best tools in our toolbox, and much translational knowledge can still be
gathered from these models. In conclusion, a multifaceted approach using different model
organisms is the key to further understanding human aging and age-related diseases.
DISCLOSURE STATEMENT
The authors are not aware of any affiliations, memberships, funding, or financial holdings that
might be perceived as affecting the objectivity of this review.
ACKNOWLEDGMENTS
The preparation of this manuscript was supported entirely by the Intramural Research Program of
the National Institute of Aging, National Institutes of Health.
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Annual Review of
Animal Biosciences
Volume 3, 2015
Contents
If a Bull Were a Cow, How Much Milk Would He Give?
Morris Soller ...............................................1
One Hundred Years of Statistical Developments in Animal Breeding
Daniel Gianola and Guilherme J.M. Rosa .........................19
The Genome 10K Project: A Way Forward
Klaus-Peter Koepfli, Benedict Paten, the Genome 10K Communit y
of Scientists, and Stephen J. O’Brien .............................57
Conservation Genetics and Genomics of Amphibians and Reptiles
H. Bradley Shaffer, Müge Gidis
¸
, Evan McCartney-Melstad, Kevin M. Neal,
Hilton M. Oyamaguchi, Marisa Tellez, and Erin M. Toffelmier .......113
Elephant Natural History: A Genomic Perspective
Alfred L. Roca, Yasuko Ishida, Adam L. Brandt, Neal R. Benjamin,
Kai Zhao, and Nicholas J. Georgiadis ...........................139
Development, Regenerati on, and Evolution of Feathers
Chih-Feng Chen, John Foley, Pin-Chi Tang, Ang Li, Ting Xin Jiang,
Ping Wu, Randall B. Widelitz, and Cheng Ming Chuong .............169
The Genetics of Skeletal Muscle Disorders in Hors es
James R. Mickelson and Stephanie J. Valberg .....................197
Unraveling the Swine Genome: Implications for Human Health
Lawrence B. Schook, Tiago V. Collares, Kwame A. Darfour-Oduro,
Arun Kumar De, Laurie A. Rund, Kyle M. Schachtschneider,
and Fabiana K. Seixas ......................................219
The Domestic Piglet: An Important Model for Investigating the
Neurodevelopmental Consequences of Early Life Insults
Matthew S. Conrad and Rodney W. Johnson .....................245
vii
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A New Medical Research Model: Ethically and Responsibly Advancing
Health for Humans and Animal s
Patricia N. Olson and Robi n R. Ganzert .........................265
Animal Models of Aging Research: Implications for Human Aging and
Age-Related Diseases
Sarah J. Mitchell, Morten Scheibye-Knudsen, Dan L. Longo,
and Rafael de Cabo ........................................283
Chronic Wasting Disease of Cervids: Current Knowledge and
Future Perspectives
Nicholas J. Haley and Edward A. Hoover ........................305
Comparative Immunology of Allergic Responses
Laurel J. Gershwin .........................................327
Environmental Role in Influenza Virus Outbreaks
Harini Sooryanarain and Subbiah Elankumaran ...................347
Strategies for Design and Application of Enteric Viral Vaccines
Kuldeep S. Chattha, James A. Roth, and Linda J. Saif ...............375
Understanding the Basis of Parasite Strain-Restricted Immunity
to Theileria parva
W. Ivan Morrison, Timothy Connelley, Johanneke D. Hemmink,
and Niall D. MacHugh ......................................397
The Impact of the Milk Glycobiome on the Neonate Gut Microbiota
Alline R. Pacheco, Daniela Baril e, Mark A. Underwood,
and David A. Mills .........................................419
The Early Impact of Genomics and Metagenomics on Ruminal Microbiology
Stuart E. Denman and Christopher S. McSweeney ..................447
Lessons from Reproductive Technology Research
George E. Seidel, Jr. ........................................467
Uterine Responses to the Preattachment Embryo in Domestic Ungulates:
Recognition of Pregnancy and Preparation for Implantation
Stefan Bauersachs and Eckhard Wolf ...........................489
Thermal Biology of Domestic Animals
Robert J. Collier and Kifle G. Gebremedhin ......................513
Comparative Dendritic Cell Biology of Veterinary Mammals
Artur Summerfield, Gael Auray, and Meret Ricklin .................533
Genetically Engineered Livestock: Ethical Use for Food and Medical Models
Lydia C. Garas, James D. Murray, and Elizabeth A. Maga ...........559
viii Contents
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