Comparative Biochemistry and Physiology, Part A 143 (2006) 12 – 23
www.elsevier.com/locate/cbpa
Review
Energy, quiescence and the cellular basis of animal life spans
Jeffrey A. Stuart ⁎, Melanie F. Brown
Department of Biological Sciences, Brock University, St. Catharines, Ontario, Canada L2S 3A1
Received 30 June 2005; received in revised form 2 November 2005; accepted 3 November 2005
Available online 27 December 2005
Abstract
Animals are routinely faced with harsh environmental conditions in which insufficient energy is available to grow and reproduce. Many
animals adapt to this challenge by entering a dormant, or quiescent state. In some animals, such as the nematode Caenorhabditis elegans,
quiescence is coincident with increased stress resistance and longevity. Here we review evidence that the rules of life span extension established in
C. elegans may be generally true of most animals. That is, that the rate of animal aging correlates inversely with cellular resistance to physiological
stress, particularly oxidative stress, and that stress resistance is co-regulated with the quiescence adaptation (where the latter occurs). We discuss
evidence for highly conserved intracellular signalling pathways involved in energy sensing that are sensitive to aspects of mitochondrial energy
transduction and can be modulated in response to energetic flux. We provide a broad overview of the current knowledge of the relationships
between energy, metabolism and life span.
© 2005 Elsevier Inc. All rights reserved.
Keywords: Energy; Quiescence; Life span; Mitchondria; Stress resistance; Reactive oxygen species
Contents
1.
2.
3.
Introduction . . . . . . . . . . . . . . . . . . . .
Energy, quiescence and the rate of aging . . . . .
Stress resistance in quiescence and aging . . . . .
3.1. Oxidative stress . . . . . . . . . . . . . .
3.2. Stress response proteins . . . . . . . . . .
3.3. The caloric restriction model . . . . . . . .
4. Inter- and intracellular signalling of energy status.
5. Mitochondria, ROS and cell life span . . . . . . .
6. Mitochondrial ROS production and proton leak. .
7. ROS as a signal of mitochondrial respiratory flux
8. DNA damage, repair and life span . . . . . . . .
9. Animal cell life span in culture . . . . . . . . . .
10. Conclusions . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
12
13
14
14
15
16
16
17
18
18
18
19
19
20
1. Introduction
⁎ Corresponding author. Tel.: +1 905 688 5550x4814; fax: +1 905 688 1855.
E-mail address: [email protected] (J.A. Stuart).
1095-6433/$ - see front matter © 2005 Elsevier Inc. All rights reserved.
doi:10.1016/j.cbpa.2005.11.002
Much progress has been made in understanding biological
determinants of animal life spans using the model organisms
Caenorhabditis elegans, Drosophila melanogaster and Mus
musculus. Various molecular determinants of cellular aging
J.A. Stuart, M.F. Brown / Comparative Biochemistry and Physiology, Part A 143 (2006) 12–23
and the hormones and intracellular signalling pathways involved in their regulation have now been identified and characterized. A recurrent theme in this literature is the associations
between energy status, metabolic rate, stress resistance and life
span. In many animals, including C. elegans, low energy status
elicits an altered pattern of gene expression that results in
reduced rates of aging and metabolism, coincident with entry
into a quiescent state. High energy status, on the other hand,
promotes growth and reproduction, but is associated with
shorter life spans. A theoretical framework for interpreting
such observations has been provided by the disposable soma
theory (reviewed in Kirkwood, 2005), which rationalizes tradeoffs between energetic investment in somatic maintenance and
in growth, development and reproduction. Recent evidence
from C. elegans indicates that hormonal signalling pathways
regulating cellular aging overlap with those that regulate metabolic rate and entry into quiescence.
In this review we compare the relationships between energy
status, metabolic rate, quiescence and life span that have been
established in model organisms with those of other animals,
particularly those that have served as models of metabolic rate
suppression. These aspects of organismal energetics have a
strong basis in cell physiology. Organismal aging, for example,
is the manifestation of cellular aging and degeneration. We
therefore focus on aspects of cell physiology common to all
animal (and yeast) cells that may participate in the regulation of
energetics and life span.
2. Energy, quiescence and the rate of aging
Hepatocyte oxygen consumption
(nmol dioxygen/min/mg dry mass
of cells)
The rate of living theory predicts an inverse relationship
between cellular metabolic rate and life span. Indeed, an inverse
correlation exists between mass-specific metabolic rate and life
span in mammal species. Liver cells isolated from short-lived
mammals consume up to 18-fold more oxygen per unit time
than do cells from long-lived mammals (Porter and Brand,
1995) (Fig. 1). The meaning of this correlation alone is uncertain because numerous characteristics covary with body mass
100
y = 30.61x-0.6976
Mouse
R 2 = 0.7526
Rat
10
Ferret
Dog
Sheep
Pig
Rabbit
Horse
1
1
10
100
MLSP (yrs)
Fig. 1. The metabolic rate of mammalian hepatocytes is inversely correlated to
maximum species life span. Data were replotted from Porter and Brand (1995)
using life span data from Kirkwood (1992), Rohme (1981) and Altman and
Dittmer (1964).
13
including life span (Speakman, 2005). However, metabolic rate
and life span are also inversely related in other experimental
contexts. Lowering metabolic rate in ectotherms by decreasing
environmental temperature extends life span (Lamb, 1968; Van
Voorhies and Ward, 1999). Similarly, inhibiting mitochondrial
respiration by RNAi extends life span in C. elegans (Dillin et
al., 2002; Lee et al., 2002a,b). Long-lived mutants of C. elegans
also have decreased metabolic rates (Van Voorhies and Ward,
1999). While these data do not prove that metabolic rate regulates life span, they do indicate that the inverse relationship
between life span and metabolic rate occurs in disparate contexts. The relationship can be understood by studying animals
in their quiescent states, which are characterized by reduced
metabolic rates.
All animals must acquire and assimilate sufficient energy to
successfully reproduce and this requires a substantial, but limited, investment in cells of the soma to successfully propagate
the germline (see Kirkwood, 2005 for review). Under ideal
environmental conditions, including typical conditions of laboratory rearing, animals are able to meet the high energetic
requirements of reproduction. Harsh environmental conditions,
on the other hand, limit either resources or access to resources.
Animals in such environments must adapt and await a return of
conditions more favourable for growth, development and reproduction. Many animals enhance their chances of survival
under these circumstances by entering a dormant, or quiescent
state. Examples of animal quiescence include hibernation, torpor, estivation, and diapause. Animals using this adaptive strategy include both endothermic and ectothermic vertebrates and
also numerous invertebrate species. Recently, genes linking
reduced rates of aging with quiescence have been identified
and their protein products characterized in C. elegans (see
Kenyon, 2005 for review).
Quiescence is characterized by starvation and profound
reductions of metabolic rate. For example, estivating snails
reduce their metabolic rates by about 85% (Pedler et al.,
1996). Hibernating and/or torpid squirrels and other mammals suppress their metabolism to less than 10% of active
animals (Thomas et al., 1990; Szewczak and Jackson, 1992;
Heldmaier et al., 2004). Quiescent cysts of the brine shrimp
Artemia franciscana can reduce their metabolism to essentially zero (Clegg, 1997). The dauer larval stage of C. elegans is also a hypometabolic quiescent state, with metabolic
rates of 50% or less that of active individuals (Anderson,
1978). Metabolism during quiescence is generally aerobic
and supported by lipid fuels (see Carey et al., 2003a; Storey,
2002 for reviews).
The metabolic rate suppression observed in quiescent animals appears to result from intrinsic reductions of metabolic
rate in individual cells, as it persists in cells isolated from
quiescent animals. For example, mantle tissue excised from
the estivating terrestrial snail Helix aspersa maintains a lower
metabolic rate than mantle from active snails (Pedler et al.,
1996). Cells isolated from the hepatopancreas of estivating H.
aspersa consume oxygen at only about 30% the rate of those
isolated from active snails (Guppy et al., 2000). Similar observations have been made in vertebrates: liver slices from the
14
J.A. Stuart, M.F. Brown / Comparative Biochemistry and Physiology, Part A 143 (2006) 12–23
estivating frog Neobatrachus centralis have depressed metabolic rates relative to those from active frogs (Fuery et al.,
1998). The metabolic rate suppression of cells from quiescent
animals can be traced to changes both in mitochondrial respiratory capacity and also in the rates of ATP-consuming processes (Bishop and Brand, 2000; St. Pierre and Boutilier, 2001;
Barger et al., 2003). Thus, there is an intrinsic cellular basis to
metabolic rate suppression in animals.
Metabolic rate suppression is a defining characteristic of
animal quiescence. Though the rate of aging in quiescent
states has not been widely considered from a comparative
perspective, there is evidence from model organisms that it
is associated with reduced rates of cellular degeneration and
aging, manifesting as slower aging of the organism. This has
been perhaps best characterized in C. elegans. The hypometabolic dauer larva has a significantly reduced rate of aging: C.
elegans can live up to 6 months as a dauer larva, whereas
normal adult life span is about 3 weeks (Riddle and Albert,
1997). Reduced rates of aging have also been observed in
insect diapause (Tatar and Yin, 2001). Insects diapause while
overwintering, and in response to other harsh environmental
conditions. Like other forms of quiescence, diapause is characterized by hypometabolism and stress resistance (Denlinger,
2002). It is also associated with a reduced rate of aging in D.
melanogaster, butterflies and grasshoppers (Tatar and Yin,
2001). The brine shrimp A. franciscana is perhaps the animal
champion of metabolic rate suppression, capable of entering
an encysted quiescent state in which metabolic rate is reduced
to essentially zero (Clegg, 1997). Whereas active Artemia live
less than 6 months, encysted individuals can last for as long as
332 years (Hairston et al., 1995), and then emerge and resume
activity. Thus, experimental evidence from invertebrates suggests that the rate of aging is generally reduced during
quiescence.
Studies examining the effect of quiescence on rates of aging
in mammals are far more limited. If bat species are examined on
the basis of those that undergo regular daily torpor and/or
seasonal hibernation and those that do not, members of the
former group are characterized by life spans more than 50%
longer (Wilkinson and South, 2002). Caloric restriction (CR) is
a well studied experimental model of life span extension
(Browner et al., 2004). In some strains of mice, reducing caloric
intake to 60% of ad libitum fed mice elicits periodic torpor and
extends life span by up to 50% (Koizumi et al., 1992). These
limited results suggest a relationship between quiescence and
reduced rates of aging in mammals as well, though more data
are clearly required.
How might the rate of aging be reduced during quiescence?
The basis of the life span extension appears to be the enhancement of resistance to ‘physiological stress.’ Selective breeding
of D. melanogaster on the basis of resistance to various forms
of stress results in populations with increased life span (Rose et
al., 1992). Similarly, selection for heat stress resistance in C.
elegans also results in a population composed of long-lived
mutants (Munoz, 2003; Munoz and Riddle, 2003). Thus, stress
resistance appears to be a key determinant of the rate of cellular
degeneration and aging.
3. Stress resistance in quiescence and aging
Stress resistance may be permissive of life span extension
in animals utilizing a quiescence adaptation to environmental
extremes. Though not always provided, the implied definition
of stress resistance is typically ‘resistance to normally detrimental or even fatal exogenous and/or endogenous perturbations’. Quiescent animals are generally resistant to a wide
range of environmental stressors, including radiation, temperature extremes, chemical carcinogens/mutagens, and strong
oxidants like hydrogen peroxide. However, resistance to oxidative stress has been perhaps the most commonly measured
parameter.
3.1. Oxidative stress
Oxidative stress is a strong correlate of animal aging (Finkel
and Holbrook, 2000). Oxidized protein, lipid and DNA products accumulate with age in animal cells (Pamplona et al.,
2000; Hamilton et al., 2001; Levine, 2002). This accumulation
of damaged macromolecules has been widely proposed to play
a role in limiting animal cell life spans. Indeed, high levels of
oxidized cellular macromolecules correlate with shorter life
spans in mammals (Barja and Herrero, 2000).
Oxidative damage is limited by ROS detoxifying enzymes
like superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase, and removed by DNA repair enzymes, and by
protein and phospholipid turnover. Results from many experiments in which the activities of antioxidant enzymes have been
manipulated support the hypothesis that ROS are important
determinants of aging rate. For example, simultaneously increasing the cytosolic superoxide dismutase 1 (CuZnSOD)
and catalase (CAT) activities via transgenic overexpression
increases life span by up to 33% in D. melanogaster (Orr and
Sohal, 1994). Life span extension has also been reported in
mice overexpressing human CAT in mitochondria (Schriner et
al., 2005). Simultaneous overexpression of CuZnSOD and
MnSOD has been shown to increase life span of D. melanogaster in a dose-dependent manner (Sun et al., 2004). Increasing
CuZnSOD activity alone in adults or exclusively in neurons of
Drosophila extends life span by 40–50% (Parkes et al., 1998;
Sun and Tower, 1999). Similarly, transgenic overexpression of
mitochondrial MnSOD activity increases Drosophila life span
by 75% (Sun et al., 2002). All of these results point to a role for
ROS detoxifying enzymes in extending the life span of animal
cells (but, see also Parker et al., 2004; Keaney et al., 2004; Van
Remmen et al., 2003).
Is resistance to oxidative and/or other forms of stress a
common characteristic of quiescent states? This has been
most extensively studied in C. elegans. Dauer stage C. elegans
are highly resistant to stress, including heat shock, UV irradiation, heavy metals and oxidative stress from hydrogen peroxide
or paraquat (Lithgow, 2000). The activities of antioxidant
enzymes are elevated in dauer larvae, compared to adult
worms (Anderson, 1982). Similarly, they are elevated in the
long-lived C. elegans mutant strains harbouring loss of function
mutations in age-1 and daf-2 genes (Larsen, 1993; Vanfleteren,
J.A. Stuart, M.F. Brown / Comparative Biochemistry and Physiology, Part A 143 (2006) 12–23
1993). Thus, increased oxidative stress resistance is a feature of
the C. elegans quiescent state.
Activities of SOD, CAT and selenium-dependent glutathione
peroxidase are also increased during estivation in the terrestrial
snail Otala lactea (Hermes-Lima and Storey, 1995) (but see
also Ramos-Vasconcelos and Hermes-Lima, 2003). There is
evidence for the up-regulation of ROS detoxifying enzymes
also in hibernating animals. Exposure of hibernating ground
squirrels (Citellus citellus) to cold stress induces increased
activities of SOD and CAT that do not occur in cold-stressed
non-hibernators (Rattus norvegicus) (Buzadzic et al., 1997).
Similarly, levels of the antioxidant enzyme thioredoxin peroxidase measured by Western blot are several-fold higher in the
hearts of hibernating versus euthermic little brown bats, Myotis
lucifugus (Eddy et al., 2005). Hibernation also induces increases in levels of blood-borne antioxidants; plasma ascorbate
levels increase 4-fold during hibernation in the arctic ground
squirrel Spermophilus parryii, but return rapidly to normal
levels at arousal (Toein et al., 2001). Similarly, intracellular
levels of glutathione are elevated in estivating snails (RamosVasconcelos and Hermes-Lima, 2003). Taken together, these
data suggest that the oxidative stress resistance characterized in
quiescent C. elegans dauers and long-lived mutant strains may
occur in other quiescent animals. Thus, there is compelling
evidence that up-regulation of ROS detoxifying capacity may
be a global characteristic of animal quiescence.
One hypothesis to explain the up-regulation of antioxidant
enzymes and substrates in quiescent animals is that antioxidant
capacity is enhanced in anticipation of oxidative stress associated with arousal (Hermes-Lima and Zenteno-Savin, 2002).
This hypothesis suggests a switch from mild hypoxia to hyperoxia during arousal, with an associated burst of ROS production. Where measured, quiescence is known to be characterized
by episodic ventilation (Barnhart, 1986; Thomas et al., 1990)
and significant regional alterations in perfusion, which could
lead to an ischemia–reperfusion type of insult upon a return to
activity. Thus, even though pO2 drops only marginally from
63.5 Torr (awake) to 44 Torr (estivating) in quiescent H.
aspersa (Pedler et al., 1996), regional hypoperfusion could
create hypoxic conditions, and resultant ROS production, in
some tissues (Chandel et al., 1998). Modifications of mitochondrial inner membrane phospholipid composition following 6
weeks of estivation in the terrestrial snail Cepaea nemoralis
(Stuart et al., 1998) are consistent with accumulation of peroxidative damage in fatty acyl chains. Similarly, there is evidence
of oxidative stress in intestinal cells of hibernating 13-lined
ground squirrels (Spermophilus tridecemlineatus) (Carey et
al., 2003b). There is currently no direct evidence that hyperoxia
occurs during arousal in snails, though levels of oxidized glutathione in hepatopancreas and foot muscle of snails increase
transiently by about 15% during arousal (Hermes-Lima and
Zeneno-Savin, 2002). Thus, up-regulation of antioxidant defenses in terrestrial snails could be a defense against oxidative
stress associated with estivation and/or emergence from
estivation.
An alternative hypothesis, however, is that the enhanced
resistance to oxidative stress is merely part of the general stress
15
resistance program employed during quiescence. Perhaps the
signalling pathway promoting both life span extension and
entry into the quiescent state anticipates harsh environmental
conditions that will require maximal defenses from various
unpredictable environmental stressors, including oxidants, extreme temperatures, ultraviolet light, and chemical toxins. A
robust stress-resistance response would be under particularly
high selective pressure under such conditions as only individuals mounting a successful response would survive to later
reproduce (Shanley and Kirkwood, 2000). Consistent with this
hypothesis, oxidative stress resistance is observed in quasiquiescent states that involve no regional tissue hypoxia or
hyperoxia. For example, calorie restricted rats live longer but
do not undergo torpor or substantial hypometabolism (Berg and
Simms, 1961; Yu et al., 1982). Nonetheless, they also upregulate some antioxidant activities, at least in selected tissues
(Semsei et al., 1989; Rao et al., 1990; Kim et al., 1996).
Similarly, yeast in which mitochondrial respiration is inhibited
experience no change in oxygen availability but up-regulate
oxidative stress resistance pathways and live longer (Jazwinski,
2005).
3.2. Stress response proteins
Heat shock proteins (Hsps) are both stress response proteins
and constitutively expressed intracellular chaperones. The former category of Hsps appears to play an important role in
determining animal life spans (Soti and Csermely, 2000; Hunt
et al., 2004). Levels of heat-shock proteins are elevated in the
long-lived C. elegans mutants daf-2 and age-1 (Lithgow et al.,
1995; see Lithgow and Walker, 2002 for review). RNAi interference with heat shock factor (HSF-1), which regulates transcription of a suite of stress-response proteins, reduces the life
spans of both wild type and daf mutant C. elegans (Morley and
Morimoto, 2004). In contrast, overexpression of specifically
Hsp-16 in C. elegans extends life span (Walker and Lithgow,
2003). Similar genetic manipulations of Hsp expression in D.
melanogaster also suggest a role for these proteins in determining life span. Overexpression of the mitochondrial Hsp22
extends life span and increases resistance to oxidative stress
in D. melanogaster (Morrow et al., 2004a), while disruption of
the same gene shortens life span (Morrow et al., 2004b). Overexpression of other Hsps can also confer life span extension in
Drosophila (Wang et al., 2004). Thus, there is abundant evidence that intracellular stress resistance conferred by Hsps is an
important determinate of life span.
Are Hsps similarly important in the stress resistance of animal quiescent states? The data are somewhat equivocal, and
support only the tentative conclusion that, in many species,
specific Hsps are up-regulated in some tissues as part of the
quiescence adaptation. C. elegans up-regulates the levels of
several Hsp mRNAs, including Hsp90 (Dalley and Golomb,
1992), upon entry into the dauer larval stage (Cherkasova et
al., 2000). Hsp90 is also up-regulated upon entry into the
encysted diapausing stage in A. franciscana (Clegg et al.,
1996). A. franciscana also up-regulates expression of a small
Hsp (p26) (Clegg et al., 1996), which appears to confer cellular
16
J.A. Stuart, M.F. Brown / Comparative Biochemistry and Physiology, Part A 143 (2006) 12–23
resistance to oxidative stress (Collins and Clegg, 2004). There is
evidence that some Hsps may also play important roles in
mediating environmental stress resistance during diapause in
insects (Denlinger, 2002). A profound up-regulation of Hsp70
transcripts occurs upon entry into diapause in the flesh eating fly
Sarcophaga crassipalpis, and is sustained throughout the duration of diapause (Rinehart et al., 2000). Hsp70 expression is also
increased in diapausing Colorado potato beetles (Leptinotarsa
decemlineata). Data from estivators is absent in the literature
and data from hibernating species is limited (see Carey et al.,
2003 for review). Hsp70 protein levels are increased two- to
seven-fold during hibernation in some tissues of the 13-lined
ground squirrel (S. tridecemlineatus) (Carey et al., 1999), including intestine (Carey et al., 2003b). A general role for Hsps in
conferring stress resistance and cellular life span extension in
quiescence cannot be concluded from the limited data available,
but should be further examined from this perspective.
3.3. The caloric restriction model
Given the coincidence of quiescence with reduced rates of
aging, it is reasonable to ask whether quiescence is necessary
for life span extension. This question can be addressed by
examining the caloric restriction (CR) model of life span extension, and the answer is no. Reduction of caloric intake
appears to universally extend life span in animals (Browner et
al., 2004). Many species, including C. elegans, respond to CR
by entering a quiescent state. In some strains of mice, CR elicits
torpor (Koizumi et al., 1992), with core temperature reductions
to ∼30° C, decreased heart rate and metabolic rate suppression
(Heldmeier et al., 2004). This is associated with an approximately 50% increase in mean life span, when the CR regime is
60% of ad libitum caloric intake. Importantly however, similar
results are found in animals that undergo CR without entering a
quiescent state. Life span extension in rats, which do not
undergo torpor, is similar to that observed in mice. Similarly,
CR extends life span without quiescence in fish (Gerhard and
Cheng, 2002), dogs (Kealy et al., 2002) and probably some
primate species (Mattison et al., 2003). Slight reductions in
metabolic rate, heart rate and core temperature are common
characteristics of CR mammals, including rats and primates;
however these are not sufficiently hypometabolic to represent a
true quiescent state. Thus, while a reduced rate of aging may be
characteristic of quiescence, quiescence is not required for life
span extension. The explanation for this difference appears to
reside in the architecture of intracellular signalling pathways
that regulate both phenomena and share common segments
(Hertweck et al., 2004) (see below).
4. Inter- and intracellular signalling of energy status
Energy sensing, or perception of available energy, plays a
key role in regulating quiescence and life span in C. elegans
(Apfeld and Kenyon, 1999). The importance of hormonal signalling of energy status, via insulin/insulin-like growth factor-1
(IGF-1), has been elucidated primarily by work with C. elegans
(see Kenyon, 2005 for review) and D. melanogaster (Partridge
and Gems, 2002). In C. elegans, low insulin/IGF-1 levels, or an
inability to perceive insulin/IGF-1 levels due to loss of function
mutations in components of the intracellular signalling pathway
instigates entry into the quiescent dauer larval stage, or adoption of certain dauer-like characteristics. Thus, for example,
mutations to daf-2, the IGF-1 receptor can cause dauer-like
characteristics including hypometabolism, stress resistance
and life span extension to be expressed.
A similar role of the insulin/IGF-1 signalling pathway has
also been described in D. melanogaster (Broughton et al.,
2005; see Hughes and Reynolds, 2005 for review) and other
animals. Insulin/IGF-1 signalling regulates life span and stress
resistance in mice: disruption of the IGF-1 receptor is lethal, but
heterozygotes live up to 26% longer and are resistant to oxidative stress (Holzenberger et al., 2003). There is evidence that
insulin/IGF-1 signalling plays a role in regulating quiescence in
other animals. In hibernating golden-mantled ground squirrels
(Spermophilus lateralis), plasma IGF-1 concentrations are reduced concomitantly with a 50% reduction in plasma glucose
concentrations (Schmidt and Kelley, 2001). Hibernation is also
associated with a profound reduction in plasma insulin in the
bat Scotophilus heathi (Krishna et al., 1998). Increased insulin/
IGF-1 signalling stimulates emergence from a quiescent diapause in both C. elegans and Ancylostoma caninum, suggesting
a common function in nematodes (Tissenbaum et al., 2000).
The life span extending effects of CR are also regulated in part
by reduced plasma insulin and IGF-1 levels (see Hursting et al.,
2003 for review). Thus, insulin/IGF-1 signalling appears to be
highly conserved as a mechanism for signalling energy status in
animals.
The intracellular signalling pathway communicating IGF-1/
receptor binding events within the cell has been partially characterized in C. elegans and also in other animal models (see
Richardson et al., 2004 for review). The pathway involves a PI
3-kinase (phosphatidylinositol 3-kinase), Pdk (phosphoinositide-dependent protein kinase) and Akt (protein kinase B) and
leads to the translocation of a forkhead transcription factor into
the nucleus which subsequently regulates gene expression
favouring growth, development and reproduction (high IGF1) or stress-resistance and longevity (low IGF-1) (see Barthel et
al., 2005 for review). Loss of function mutations in the gene
products of daf-2, age-1 (PI 3-kinase), pdk-1 or akt all increase
life span in C. elegans, apparently by initiating a transcriptional
program associated with stress resistance (Richardson et al.,
2004).
The basic structure of this intracellular signalling pathway is
conserved from yeast to mammals (Barbieri et al., 2003), and
there is evidence that its function in regulating quiescence is
also conserved. During hibernation in both squirrels and bats
elements within the insulin/IGF-1 signalling pathway are modulated, primarily by reversible phosphorylation. Phosphorylation and activity of the signalling intermediate Akt are both
reduced during hibernation in multiple tissues of the 13-lined
ground squirrel S. tridecemlineatus (Cai et al., 2004b). Similarly, Akt phosphorylation is reduced in brain, kidney and liver of
the little brown bat M. lucifugus during hibernation (Eddy and
Storey, 2003). Hibernating Vespertilionid bats (S. heathi) show
J.A. Stuart, M.F. Brown / Comparative Biochemistry and Physiology, Part A 143 (2006) 12–23
signs of reduced responsiveness to insulin/IGF-1, consistent
with inhibition of signalling pathway (Krishna and Abhilasha,
2000). Recently, a forkhead transcription factor homologue
FoxO1a, with about 90% sequence identity to its mammalian
homologues has been identified in S. tridecemlineatus (Cai et
al., 2004a). Whether it functions similarly to its homologues is
not yet known.
Interestingly, the insulin/IGF-1 signalling pathway in C.
elegans is forked (Davenport, 2004) so that insulin/IGF-1
may stimulate two separate subpathways, which independently
regulate quiescence and life span in response to the same signal
(Hertweck et al., 2004). In C. elegans, Akt appears to be
important for regulating entry into the dauer, but it is not
essential for life span extension, which may be mediated instead by a protein resembling mammalian SGK. This could
explain why life span extension can be dissociated from quiescence in some animals in CR. Thus, hormonal signalling of low
cellular energy availability appears to diverge and initiate separate programs of lowered metabolic rate and stress resistance
that can both be understood from the perspective of promoting
survival and eventual reproductive success in the face of poor
environmental conditions.
In C. elegans, deficient insulin/IGF-1 signalling, for example by genetic mutations, is associated with decreased metabolic rate (VanVoorhies and Ward, 1999). It is interesting that
modulation of gene expression and/or behaviour can also be
elicited by intracellular inhibition of energy metabolism. Interference with glucose oxidation via the administration of 2deoxyglucose, which enters cells and inhibits glycolysis, stimulates entry into torpor in rodents and marsupials (Dark et al.,
1994; Bac et al., 2003; Westman and Geiser, 2004). Similarly,
inhibition of mitochondrial oxidative metabolism stimulates
quiescence and/or life span extension. For example, RNAi
inhibition of mitochondrial respiration in developing C. elegans
extends life span while slowing the apparent rate of activity
(Dillin et al., 2002; Lee et al., 2002a,b). The C. elegans clk-1
mutant, which lacks the ability to synthesize coenzyme Q, and
therefore has deficient electron transport, is also long-lived
(Lakowski and Hekimi, 1996). Dietary provision of CoQ
restores normal life span (Jonassen et al., 2001). Inhibition of
cytochrome c oxidase with hydrogen sulfide (H2S) has recently
been shown to elicit a reversible torpor in mice (Blackstone et
al., 2005).
An important molecular target linking insulin-IGF-1 signalling to life span may be the sirtuin family of nicotinamide
adenine dinucleotide (NAD)-dependent deacetylases (Cohen
et al., 2004). In Saccharomyces cerevisiae (Kaeberlein et al.,
1999), C. elegans (Tissenbaum and Guarente, 2001), and mammals (Cohen et al., 2004) sirtuin overexpression is associated
with increased cellular stress resistance. In S. cerevisiae and C.
elegans, this extends life span, though this has not been shown
in mammals, and sirtuin overexpression does not affect the
replicative life span of mammalian cells in culture (Michishita
et al., 2005). Nonetheless, it will be interesting to examine
sirtuins from the perspective of animal quiescence.
The studies outlined above suggest the participation of
mechanisms for sensing and responding to flux through intra-
17
cellular energy metabolism. Such ‘retrograde signalling pathways’ have been characterized in animal cells (see Butow and
Avadhani, 2004 for review). Similar pathways that connect
mitochondrial respiration, stress resistance and life span have
been established in yeast (Jazwinski, 2005). Low mitochondrial
activity in S. cerevisiae stimulates the expression of stressresistance genes, a switch from carbohydrate to fatty acid
catabolism and increased life span. Interestingly, RNAi inhibition of mitochondrial respiration also induces a stress-resistant
phenotype in C. elegans, up-regulating the expression of Hsps,
including the mitochondrial form of Hsp70 (Kuzmin et al.,
2004). Thus, mitochondria may generate signals of energy
status within animal cells, which in turn allows mitochondrial
activity to regulate the rate of aging. This is quite different from
the commonly proposed role of mitochondria in aging—the
production of toxic ROS.
5. Mitochondria, ROS and cell life span
Organisms appear to age because the cells that constitute
them age, leading to deficits in tissue and organ activities and
eventually organ failure. Most tissues composed primarily of
post-mitotic cells show a decline in cell number with age,
presumably due to the death of dysfunctional cells. A strong
correlate of cellular aging is the accumulation within cells of
oxidized macromolecules. This increase in the levels of oxidized macromolecules appears to accelerate in the latter third of
maximal animal life span (Levine, 2002; Hamilton et al., 2001).
There is extensive evidence that genomic instability, especially
an inability to appropriately regulate the synthesis and turnover
of normal cellular proteins due to accumulating damage within
genes and their promoters (Lu et al., 2004), underlies the
degeneration of animal cells in aging.
Mitochondria are thought to be the most important endogenous source of ROS in most animal cells. Mitochondria
produce ROS at very low rates during normal oxidative metabolism (see Turrens, 2003 for review; also St. Pierre et al., 2002),
but can overproduce ROS under certain conditions, including
pathological ones. The mitochondrial production of ROS has
been integrated with the rate of living theory to predict that
lower mitochondrial activity and/or ROS production should be
associated with prolonged life (Barja, 2002). Indeed, there is an
inverse correlation between longevity and the rate of mitochondrial ROS generation in mammals (reviewed in Barja, 2002;
but see Speakman, 2005). Similarly, mitochondria isolated
from relatively long-lived birds produce less ROS than those
isolated from rats (Barja, 2002). Also, mitochondria from longlived (maximum life span = 34 years) little brown bats (M.
lucifugus) produce ROS at lower rates than do mitochondria
from the relatively short-lived (maximum life span = 2 years)
short-tailed shrew (Blarina brevicauda) (Brunet-Rossinni,
2004). Finally, CR lowers hydrogen peroxide production and
markers of oxidative damage in rat liver (Hagopian et al., 2005)
and skeletal muscle (Bevilacqua et al., 2005) mitochondria.
Taken together, these studies suggest that the relationship between mitochondrial activity and life span could be via ROS
production.
18
J.A. Stuart, M.F. Brown / Comparative Biochemistry and Physiology, Part A 143 (2006) 12–23
Differences in mitochondrial ROS production may explain
an apparent contradiction in experimental results obtained for
C. elegans and mice. Lowering mitochondrial activity extend
life span in C. elegans, as outlined above. This has been
achieved, for example, by decreasing environmental temperature (Van Voorhies and Ward, 1999) or by using RNAi to
inhibit the synthesis of mitochondrial respiratory complexes
(Dillin et al., 2002; Lee et al., 2002a,b). However, while
inhibition of mitochondrial respiration in C. elegans leads to
significant life span extension, the opposite effect has been
shown in mice. Mice with deficient mtDNA polymerase proofreading activity accumulate mtDNA mutations at an accelerated rate, develop mitochondrial respiratory dysfunction and die
prematurely (Trifunovic et al., 2004). Why does this impairment of mitochondrial activity have apparently the opposite
effect in mice (Anson and Hansford, 2004)? One possibility is
that preventing the assembly of respiratory complexes by
RNAi limits the number of functional respiratory complexes,
but does not alter their function, whereas allowing mutant
subunits to be assembled into respiratory complexes alters
the function of the complexes in such a way that ROS production is increased. ROS overproduction is associated with specific mutations in respiratory complex proteins in C. elegans
mutants that shorten life span (Ishii et al., 1998; Grad and
Lemire, 2004).
6. Mitochondrial ROS production and proton leak
Mitochondrial ROS production is increased at high membrane potentials (Korshunov et al., 1997). Uncoupling of oxidative phosphorylation by creating an alternative pathway for
protons to re-enter the matrix lowers both the mitochondrial
inner membrane potential and the rate of ROS production
(Korshunov et al., 1997). Presumably, such an effect would
decrease the incidence of oxidative damage to DNA, particularly mtDNA, and thereby promote increased life span (Brand,
2000). Indeed, individual mice with greater mitochondrial inner
membrane proton conductance, and therefore presumably
lower membrane potential and lower rates of ROS production,
have longer life spans (Speakman et al., 2004). These interindividual differences in mitochondrial proton conductance may
be related to levels of uncoupling protein 3 and the adenine
nucleotide translocase. The former protein functions in the
inner membrane to open a proton conductance pathway when
stimulated by mitochondrial ROS (Echtay et al., 2002). Thus, it
may represent a mechanism for preventing overproduction of
ROS. Uncoupling proteins appear to be present in all cephalochordates (Stuart et al., 1999, 2001) though their functions
remain incompletely characterized, with the exception of
uncoupling protein 1 of brown adipose tissue, which functions
in thermogenesis (Enerback et al., 1997; reviewed in Cannon
and Nedergaard, 2004). Overexpression of uncoupling protein
2 can prevent cell death following oxidative stress (Mattiasson
et al., 2003; Teshima et al., 2003), and underexpression of
UCP3 increases intracellular oxidative damage in vivo (VidalPuig et al., 2000; Brand et al., 2002). Uncoupling proteins thus
appear to be important in cellular ROS metabolism. However,
there is no report of shortened life span in any of the UCP-null
mice. Also, longer lived mammalian species actually have
reduced mitochondrial proton conductance compared to shorter
lived species, indicating that the intraspecies relationship observed in mice does not apply to interspecies comparisons
(Porter and Brand, 1993). Similarly, hyperthyroidism increases
mitochondrial proton conductance (Harper and Brand, 1993),
but decreases life span. Thus, there is as yet no persuasive
evidence that increased mitochondrial uncoupling plays a causal role in extending animal cell life span under unstressed
conditions, though this remains an attractive hypothesis.
7. ROS as a signal of mitochondrial respiratory flux
The connection between ROS, mitochondria and cell life
span could be oxidative damage to macromolecules. Alternatively, ROS may be important as signals of mitochondrial
respiratory flux, an indirect indicator of energy availability.
ROS are important intracellular messengers (Droge, 2002).
They are strong mitogenic stimuli (Esposito et al., 2004),
which stimulate mtDNA replication and mitochondrial biogenesis (Ogihara et al., 2002; Lee et al., 2002a,b; Suliman et al.,
2004). Presumably the small but significant levels of ROS
produced during metabolism in normal animal mitochondria
could serve as a signal of normal mitochondrial function. Decreased ROS production might result from either increased
uncoupling or lower overall rates of mitochondrial activity (as
in hypometabolic or ametabolic states). This could signal insufficient energy available for growth and reproduction and
therefore a need to slow the rate of aging and wait for higher
energy levels. Many experimental results are consistent with
this hypothesis. However, in almost all instances the coincident
oxidation of macromolecules is interpreted as the causal mechanism for life span shortening. Pathologically high rates of
mitochondrial ROS production can also be interpreted from
within this framework. They may simultaneously stimulate
mitochondrial and cellular division, and macromolecular damage, which would have important implications for the formation and progression of tumours.
8. DNA damage, repair and life span
Mitochondrial ROS is typically considered from the perspective of its induction of macromolecular damage, which
promotes aging. Consistent with this idea, there is an inverse
correlation between steady-state levels of the oxidative DNA
adduct 8-oxodeoxyguanine (8-oxodG) in mtDNA and life span
of mammals (Barja and Herrero, 2000; but see Speakman,
2005). Levels of 8-oxodG increase with age in both nuclear
and mitochondrial DNA of animal cells (Hamilton et al., 2001),
and this has been interpreted as a cause of cellular dysfunction
in aging. However, elevated levels of 8-oxodG in mitochondrial
and nuclear DNA of Ogg1 knockout mice have no effect on life
span or mitochondrial respiratory function (Stuart et al., 2005).
Nonetheless, there is extensive evidence for a general relationship between mitochondrial ROS production, genomic damage
and rate of animal aging (Lombard et al., 2005).
J.A. Stuart, M.F. Brown / Comparative Biochemistry and Physiology, Part A 143 (2006) 12–23
DNA sustains various forms of chemical and oxidative
insults daily, resulting in modifications of the structures of
individual bases (such as 8-oxodG) (Cooke et al., 2003), formation of abasic sites, and strand breaks. Over 100 enzymes
participate in identifying and repairing DNA damage in animal
cells (see Barnes and Lindahl, 2004 for review). There is
considerable evidence that the balance between the rate of
DNA damage occurrence and its repair is central to the progression of aging (Hasty et al., 2003). Most of the human
progeria (premature aging) syndromes, including Werner,
Cockayne and Rothmund-Thomson, result from genomic instability (Bohr, 2002). They are caused by specific mutations
affecting the functions of proteins that are either directly involved in repairing damaged DNA, and/or participate in signalling pathways that regulate DNA repair. Hutchinson-Gilford
progeria is caused by a defective nuclear Lamin A (Mounkes et
al., 2003) which, in an unknown way, promotes genomic instability in affected individuals. The human progeria syndromes
shorten life span to between 10 and 50 years. Animal models
that are deficient in DNA repair also have greatly shortened life
spans (Treuting et al., 2002).
The most important pathway for enzymatic removal of oxidative DNA damage is base excision repair (BER). Disruption
of genes whose products are central to this pathway, such as
apurinic/apyrimidinic endonuclease (APEX, or APE1) or polymerase β, are lethal during embryonic development. On the
other hand, DNA BER polymerase β activity is up-regulated in
longer lived CR rodents (Cabelof et al., 2003; Stuart et al.,
2004). As a result, steady-state levels of DNA oxidative damage are lowered (Gredilla et al., 2001; Hamilton et al., 2001)
and the mutation load of aged animals appears also to be lower
in some tissues (Dempsey et al., 1993; Kang et al., 1998),
though this is somewhat equivocal (Stuart et al., 2000; Newell
and Heddle, 2002). However, the accumulation of mutations
may not be the primary means by which DNA damage affects
cell function. Evidence suggests that interference of DNA damage with gene expression may also play a critical role in cell
dysfunction and aging (Lu et al., 2004). How is genomic
stability maintained in animal cells during quiescent states?
Perhaps this is achieved in part by lowering the rate of ROS
production (Barja, 2004), as occurs in skeletal muscle of CR
rats (Bevilacqua et al., 2005). Whether modulation of DNA
repair also plays an important role during animal quiescence is
not yet known.
9. Animal cell life span in culture
An intrinsic cellular basis of animal cell life span, independent of modulating signals, is suggested by experiments in
cultured cells taken from different species. Fibroblast cell cultures established from a range of species with different maximal
life spans have correspondingly different replicative life spans
(Rohme, 1981). Thus, human fibroblasts can typically undergo
between 50 and 70 population doublings in culture, whereas
mouse fibroblasts begin to show signs of senescence after as
little as 10 population doublings. Interestingly, oxidative stress
may in part determine this difference. Normal cell culture
19
conditions use oxygen concentrations approaching atmospheric
(21%), and this superphysiological oxygen concentration appears to impose a significant oxidative stress on the growing
cells. Human skin fibroblasts cultured in atmospheric oxygen
grow slowly relative to those cultured at lower oxygen tensions
(Balin and Pratt, 2002). Mouse embryonic fibroblasts grown at
atmospheric oxygen show multiple signs of oxidative stress,
including DNA oxidative damage (Parrinello et al., 2003).
Remarkably, growing the same mouse cells at 6-fold lower
oxygen tensions eliminates these effects, allowing 50 or more
population doublings. Thus, as in living animals, oxidative
stress appears to limit the life span of cells in culture. Consistent with this idea, RNAi inhibition of CuZnSOD expression in
cultured human fibroblasts induces senescence (Blander et al.,
2003).
Cells cultured from longer lived species are more resistant to
oxidative and other forms of stress (Kapahi et al., 1999). The
lethal dose (LD90) of various oxidants including paraquat and
hydrogen peroxide is appreciably lower for fibroblasts from
short-lived, versus long-lived, species. While oxidative stress
no doubt affects all cellular processes, it again appears that
damage to DNA may be particularly important. Fibroblast
cultures from humans with Werner syndrome, or other premature aging syndromes characterized by DNA repair deficiencies, senesce prematurely in culture (Weirich-Schwaiger et al.,
1994). Fibroblast cultures from long-lived species repair UV
damage to DNA more readily than fibroblasts from short-lived
species (Hart and Setlow, 1974). We have observed a correlation between specific BER enzyme activities and the maximal
life spans of species in cultured skin fibroblasts (M. Brown,
unpublished results). Thus, the senescence of animal cells in
culture is regulated by some of the same parameters that affect
cellular aging in vivo. Generally, more stress resistant cells can
be cultured longer before senescing. Interestingly, as has been
observed in vivo, induction of Hsps delays senescence of
cultured cells (Rattan, 1998; Fonager et al., 2002).
10. Conclusions
Possibly all animals respond to low energy status by initiating a genetic program promoting stress resistance, which is in
turn associated with longevity. In many animals this is accompanied by a profound reduction of metabolic rate and entry into
a quiescent state. Both metabolic rate suppression and stress
resistance enable animals to survive unfavourable environmental conditions associated with limited available energy. Central
regulators of the adaptive responses include the insulin/IGF-1
hormonal signalling pathway and possibly the ubiquitous sirtuins which participate in the regulation of life span and stress
resistance.
The intracellular mechanisms involved in these responses
are highly conserved amongst animals, suggesting that hibernation, estivation and diapause of vertebrates and invertebrates
might be regulated by similar energy sensing pathways and
stress resistance responses. Mitochondrial physiology is implicated in this process, possibly through production of ROS.
Substantial evidence also suggests that maintenance of genomic
20
J.A. Stuart, M.F. Brown / Comparative Biochemistry and Physiology, Part A 143 (2006) 12–23
stability by repair of DNA damage may be one of the most
important mechanisms for slowing the rate of aging and increasing life span.
References
Altman, P., Dittmer, D.S., 1964. Biology Data Book. Federation of American
Societies for Experimental Biology, Washington.
Anderson, G.L., 1978. Responses of dauer larvae of Caenorhabditis elegans
(Nematoda: Rhabditidae) to thermal stress and oxygen deprivation. Can. J.
Zool. 56, 1786–1791.
Anderson, G.L., 1982. Superoxide dismutase activity in dauer larvae of
Caenorhabditis elegans (Nematoda: Rhabditidae). Can. J. Zool. 60,
288–291.
Anson, R.M., Hansford, R.G., 2004. Mitochondrial influence on aging rate in
Caenorhabditis elegans. Aging Cell 3, 29–34.
Apfeld, J., Kenyon, C., 1999. Regulation of lifespan by sensory perception in
Caenorhabditis elegans. Nature 402, 804–809.
Bac, H.H., Larkin, J.E., Zucker, I., 2003. Juvenile Siberian hamsters display
torpor and modified locomotor activity and body temperature rhythms in
response to reduced food availability. Physiol. Biochem. Zool. 76, 858–867.
Balin, A., Pratt, L., 2002. Oxygen modulates the growth of skin fibroblasts. In
Vitro Cell. Dev. Biol., Anim. 38, 305–310.
Barbieri, M., Bonafe, M., Franceschi, C., Paolisso, G., 2003. Insulin/IGF-1
signaling pathway: an evolutionarily conserved mechanisms of longevity
from yeast to humans. Am. J. Physiol. 285, E1064–E1071.
Barger, J.L., Brand, M.D., Barnes, B.M., Boyer, B.B., 2003. Tissue-specific
depression of mitochondrial proton leak and substrate oxidation in
hibernating arctic ground squirrels. Am. J. Physiol. 284, R1306–R1313.
Barja, G., 2002. Rate of generation of oxidative stress-related damage and
animal longevity. Free Radic. Biol. Med. 33, 1167–1172.
Barja, G., 2004. Free radicals and aging. Trends Neurosci. 27, 595–600.
Barja, G., Herrero, A., 2000. Oxidative damage to mitochondrial DNA is
inversely related to maximum life span in the heart and brain of mammals.
FASEB J. 14, 312–318.
Barnes, D.E., Lindahl, T., 2004. Repair and genetic consequences of
endogenous DNA base damage in mammalian cells. Annu. Rev. Genet.
38, 445–476.
Barnhart, M.C., 1986. Respiratory gas tensions and gas-exchange in active and
dormant land snails, Otala lactea. Phsyiol. Zool. 59, 733–745.
Barthel, A., Schmoll, D., Unterman, T.G., 2005. FoxO proteins in insulin action
and metabolism. Trends Endocrinol. Metab. 16, 183–189.
Bevilacqua, L., Ramsey, J.J., Hagopian, K., Weindruch, R., Harper, M.-E., 2005.
Long-term caloric restriction increases UCP3 content but decreases proton
leak and reactive oxygen species production in rat skeletal muscle
mitochondria. Am. J. Physiol: Endocrinol. Metab. 289, E429–E438.
Berg, B.N., Simms, H.S., 1961. Nutrition and longevity in the rat: III. Food
restriction beyond 800 days. J. Nutr. 74, 23–32.
Bishop, T., Brand, M.D., 2000. Processes contributing to metabolic depressin in
hepatopancreas cells from the snail Helix aspersa. J. Exp. Biol. 203,
3603–3612.
Blackstone, E., Morrison, M., Roth, M.B., 2005. H2S induces a suspended
animation-like state in mice. Science 308, 518.
Blander, G., de Oliveira, R.M., Conboy, C.M., Haigis, M., Guarente, L., 2003.
Superoxide dismutase 1 knock-down induces senescence in human
fibroblasts. J. Biol. Chem. 278, 38966–38969.
Bohr, V.A., 2002. DNA-related pathways defective in human premature aging.
Sci. World J. 2, 1216–1226.
Brand, M.D., 2000. Uncoupling to survive? The role of mitochondrial
inefficiency in ageing. Exp. Gerontol. 35, 811–820.
Brand, M.D., Pamplona, R., Portero-Otin, M., Requena, J.R., Roebuck, S.J.,
Buckingham, J.A., Clapham, J.C., Cadenas, S., 2002. Oxidative damage and
phospholipid fatty acyl composition in skeletal muscle mitochondria from
mice underexpressing or overexpressing uncoupling protein 3. Biochem. J.
368, 597–603.
Broughton, S.J., Piper, M.D.W., Ikeya, T., Bass, T.M., Jacobson, J., Driege,
Y., Martinez, P., Hafen, E., Withers, D.J., Leevers, S.J., Partridge, L.,
2005. Longer lifespan, altered metabolism, and stress resistance in
Drosophila from ablation of cells making insulin-like ligands. Proc. Natl.
Acad. Sci. U. S. A. 102, 3105–3110.
Browner, W.S., Kahn, A.J., Ziv, E., Reinre, A.P., Oshima, J., Cawthon, R.M.,
Hsuch, W.C., Cummings, S.R., 2004. The genetics of human longevity. Am.
J. Med. 117, 851–860.
Brunet-Rossinni, A.K., 2004. Reduced free-radical production and extreme
longevity in the little brown bat (Myotis lucifugus) versus two non-flying
mammals. Mech. Ageing Dev. 125, 11–20.
Butow, R.A., Avadhani, N.G., 2004. Mitochondrial signalling: the retrograde
response. Mol. Cell 14, 1–15.
Buzadzic, B., Blagojevic, D., Korac, B., Saicic, Z.S., Spasic, M.B., Petrovic, V.
M., 1997. Seasonal variation in the antioxidant defense system of the brain
of the ground squirrel (Citellus citellus) and response to low temperature
compared with rat. Comp. Biochem. Physiol. C 117, 141–149.
Cabelof, D.C., Yanamadala, S., Raffoul, J.J., Guo, Z., Soofi, A., Heydari, A.R.,
2003. Caloric restriction promotes genomic stability by induction of base
excision repair and reversal of its age-related decline. DNA Repair 2,
295–307.
Cai, D., McCarron, R.M., Hallenbeck, J., 2004a. Cloning and characterization of
a forkhead transcription factor gene, FoxO1a, from thirteen-lined ground
squirrel. Gene 343, 203–209.
Cai, D., McCarron, R.M., Yu, E.Z., Li, Y., Hallenbeck, J., 2004b. Akt
phosphorylation and kinase activity are down-regulated during hibernation
in the 13-lined ground squirrel. Brain Res. 1014, 14–21.
Cannon, B., Nedergaard, J., 2004. Brown adipose tissue: function and
physiological significance. Physiol. Rev. 84, 277–359.
Carey, H.V., Sills, N.S., Gorham, D.A., 1999. Stress proteins in mammalian
hibernation. Am. Zool. 39, 825–835.
Carey, H.V., Andrews, M.T., Martin, S.L., 2003a. Mammalian hibernation:
cellular and molecular responses to depressed metabolism and low
temperature. Physiol. Rev. 83, 1153–1181.
Carey, H.V., Rhoads, C.A., Aw, T.Y., 2003b. Hibernation induces glutathione
redox imbalance in ground squirrel intestine. J. Comp. Physiol. B 173,
269–276.
Chandel, N.S., Maltepe, E., Goldwasser, E., Mathieu, C.E., Simon, M.C.,
Schumacker, P.T., 1998. Mitochondrial reactive oxygen species trigger
hypoxia-induced transcription. Proc. Natl. Acad. Sci. U. S. A. 95,
11715–11720.
Cherkasova, V., Ayyadevara, S., Egilmez, N., Shmookler Reis, R., 2000.
Diverse Caenorhabditis elegans genes that are upregulated in dauer larvae
also show elevated transcript levels in long-lived, aged or starved adults. J.
Mol. Biol. 300, 433–448.
Clegg, J.S., 1997. Embryos of Artemia franciscana survive four years of
continuous anoxia: the case for complete metabolic rate depression. J. Exp.
Biol. 200, 467–475.
Clegg, J.S., Drinkwater, L.E., Sorgeloos, P., 1996. The metabolic status of
diapause embryos of Artemia franciscana (SFB). Physiol. Zool. 69,
49–66.
Cohen, H.Y., Miller, C., Bitterman, K.J., Wall, N.R., Hekking, B., Kessler, B.,
Howitz, K.T., Gorospe, M., de Cabo, R., Sinclair, D.A., 2004. Calorie
restriction promotes mammalian cell survival by inducing the SIRT1
deacetylase. Science 305, 390–392.
Collins, C.H., Clegg, J.S., 2004. A Small Heat-Shock Protein, p26, from the
Crustacean Artemia Protects Mammalian Cells (Cos-1) Against Oxidative
Damage.
Cooke, M.S., Evans, M.D., Dizdaroglu, M., Lunec, J., 2003. Oxidative DNA
damage: mechanisms, mutation and disease. FASEB J. 17, 1195–1214.
Dalley, B.K., Golomb, M., 1992. Gene expression in the Caenorhabditis
elegans dauer larva; developmental regulation of Hsp90 and other genes.
Dev. Biol. 151, 80–90.
Dark, J., Miller, D.R., Zucker, I., 1994. Reduced glucose availability induces
torpor in Siberian hamsters. Am. J. Physiol. 267, R496–R501.
Davenport, R.J., 2004. Fork in the road. Sci. Aging Knowl. Environ. 15 (nf40).
Dempsey, J.L., Pfeiffer, M., Morley, A.A., 1993. Effect of dietary restriction on
in vivo somatic mutation in mice. Mutat. Res. 291, 141–145.
Denlinger, D.L., 2002. Regulation of diapause. Annu. Rev. Entomol. 47,
93–122.
J.A. Stuart, M.F. Brown / Comparative Biochemistry and Physiology, Part A 143 (2006) 12–23
Dillin, A., Hsu, A.-L., Arantes-Oliveira, N., Lehrer-Graiwer, J., Hsin, H., Fraser,
A.G., Kamath, R.S., Ahringer, J., Kenyon, C., 2002. Rates of behaviour and
aging specified by mitochondrial function during development. Science 298,
2398–2401.
Droge, W., 2002. Free radicals in the physiological control of cell function.
Physiol. Rev. 82, 47–95.
Echtay, K.S., Roussel, D., St-Pierre, J., Jekabsons, M.B., Cadenas, S., Stuart, J.
A., Harper, J.A., Roebuck, S.J., Morrison, A., Pickering, S., Clapham, J.C.,
Brand, M.D., 2002. Superoxide activates mitochondrial uncoupling
proteins. Nature 415, 96–99.
Eddy, S.F., Storey, K.B., 2003. Differential expression of Akt, PPARγ, and
PGC-1 during hibernation in bats. Biochem. Cell. Biol. 81, 269–274.
Eddy, S.F., McNally, J.D., Storey, K.B., 2005. Up-regulation of a thioredoxin
peroxidase-like protein, proliferation-associated gene, in hibernating bats.
Arch. Biochem. Biophys. 435, 103–111.
Enerback, S., Jacobsson, A., Simpson, E.M., Guerra, C., Yamashita, H., Harper,
M.E., Kozak, L.P., 1997. Mice lacking mitochondrial uncoupling protein are
cold-sensitive but not obese. Nature 387, 90–94.
Esposito, F., Ammendola, R., Faraonio, R., Russo, T., Cimino, F., 2004. Redox
control of signal transduction, gene expression and cellular senescence.
Neurochem. Res. 29, 617–628.
Finkel, T., Holbrook, N.J., 2000. Oxidants, oxidative stress and the biology of
ageing. Nature 408, 239–247.
Fonager, J., Beedholm, R., Clark, B.F., Rattan, S.L., 2002. Mild stress-induced
stimulation of heat-shock protein synthesis and improved functional ability
of human fibroblasts undergoing aging in vitro. Exp. Gerontol. 37,
1223–1228.
Fuery, C.J., Withers, P.C., Hobbs, A.A., Guppy, M., 1998. The role of protein
synthesis during metabolic depression in the Australian desert frog
Neobatrachus centralis. Comp. Biochem. Physiol. A 119, 469–476.
Gerhard, G.S., Cheng, K.C., 2002. A call to fins! Zebrafish as a gerontological
model. Aging Cell 1, 104–111.
Grad, L.I., Lemire, B.D., 2004. Mitochondrial complex I mutations in
Caenorhabditis elegans produce cytochrome c oxidase deficiency, oxidative
stress and vitamin-responsive lactic acidosis. Hum. Mol. Genet. 13,
303–314.
Gredilla, R., Sanz, A., Lopez-Torres, M., Barja, G., 2001. Caloric restriction
decreases mitochondrial free radical generation at complex I and lowers
oxidative damage to mitochondrial DNA in the rat heart. FASEB J. 15,
1589–1591.
Guppy, M., Reeves, D.C., Bishop, T., Withers, P., Buckingham, J.A., Brand, M.
D., 2000. Intrinsic metabolic depression in cells isolated from the
hepatopancreas of estivating snails. FASEB J. 14, 999–1004.
Hagopian, K., Harper, M.-E., Ram, J.J., Humble, S.J., Weindruch, R., Ramsey, J.
J., 2005. Long-term calorie restriction reduces proton leak and hydrogen
peroxide production in liver mitochondria. Am. J. Physiol. 288, E674–E684.
Hairston Jr., N.G., Van Brunt, R.A., Kearns, C.M., 1995. Age and survivorship
of diapausing eggs in a sediment bank. Ecology 76, 1706–1711.
Hamilton, M.L., Van Remmen, H., Drake, J.A., Yang, H., Guo, Z.M., Kewitt,
K., Walter, C.A., Richardson, A., 2001. Does oxidative damage to DNA
increase with age? Proc. Natl. Acad. Sci. U. S. A. 98, 10269–10474.
Harper, M.E., Brand, M.D., 1993. The quantitative contributions of mitochondrial proton leak and ATP turnover reactions to the changed respiration rates
of hepatocytes from rats of different thyroid status. J. Biol. Chem. 268,
14850–14860.
Hart, R.W., Setlow, R.B., 1974. Correlation between deoxyribonucleic acid
excision repair and life-span in a number of mammalian species. Proc. Natl.
Acad. Sci. U. S. A. 71, 2169–2173.
Hasty, P., Campisi, J., Hoeijmakers, J., van Steeg, H., Vijg, J., 2003. Aging and
genome maintenance: lessons from the mouse? Science 299, 1355–1359.
Heldmaier, G., Ortmann, S., Elvert, R., 2004. Natural hypometabolism during
hibernation and daily torpor in mammals. Resp. Physiol. Neurobiol. 141,
317–329.
Hermes-Lima, M., Storey, K.B., 1995. Antioxidant defenses and metabolic
depression in a pulmonate land snail. Am. J. Physiol. 268, R1386–R1393.
Hermes-Lima, M., Zeneno-Savin, T., 2002. Animal response to drastic changes
in oxygen availability and physiological oxidative stress. Comp. Biochem.
Physiol. C 133, 537–556.
21
Hertweck, M., Gobel, C., Baumeister, R., 2004. C. elegans SGK-1 is the critical
component in the Akt/PKB kinase complex to control stress response and
life span. Dev. Cell 6, 577–588.
Holzenberger, M., Dupont, J., Ducos, B., Leneuve, P., Geloen, A., Even, P.C.,
Cervera, P., Le Bouc, Y., 2003. IGF-1 receptor regulates lifespan and
resistance to oxidative stress in mice. Nature 421, 182–187.
Hughes, K.A., Reynolds, R.M., 2005. Evolutionary and mechanistic theories of
aging. Annu. Rev. Entomol. 50, 421–445.
Hunt, C.R., Dix, D.J., Sharma, G.G., Pandita, R.K., Gupta, A., Funk, M.,
Pandita, T.K., 2004. Genomic instability and enhanced radiosensitivity in
Hsp70.1 and Hsp70.3-deficient mice. Mol. Cell. Biol. 24, 899–911.
Hursting, S.D., Lavigne, J.A., Berrigan, D., Perkins, S.N., Barrett, J.C., 2003.
Calorie restriction, aging, and cancer prevention: mechanisms of action and
applicability to humans. Annu. Rev. Med. 54, 131–152.
Ishii, N., Fujii, M., Hartman, P.S., Tsuda, M., Yasuda, K., Senoo-Matsuda, N.,
Yanase, S., Ayusawa, D., Suzuki, K., 1998. A mutation in succinate
dehydrogenase cytochrome b causes oxidative stress and aging in
nematodes. Nature 394, 694–697.
Jazwinski, S.M., 2005. Yeast longevity and aging—the mitochondrial
connection. Mech. Ageing Dev. 126, 243–248.
Jonassen, T., Larsen, P.L., Clarke, C.F., 2001. A dietary source of coenzyme Q is
essential for growth of long-lived Caenorhabditis elegans clk-1 mutants.
Proc. Natl. Acad. Sci. U. S. A. 98, 421–426.
Kaeberlein, M., McVey, M., Guarente, L., 1999. The SIR2/3/4 complex and
SIR2 alone promote longevity in Saccharomyces cerevisiae by two different
mechanisms. Genes Dev. 13, 2570–2580.
Kang, C.-M., Kristal, B.S., Yu, B.P., 1998. Age-related mitochondrial DNA
deletions: effect of dietary restriction. Free Radic. Biol. Med. 24, 148–154.
Kapahi, P.J., Boulton, M.E., Kirkwood, T.B.L., 1999. Positive correlation
between mammalian life span and cellular resistance to oxidative stress. Free
Radic. Biol. Med. 26, 495–500.
Kealy, R.D., Lawler, D.E., Ballam, J.M., Mantz, S.L., Biery, D.N., Greeley, E.
H., Lust, G., Segre, M., Smith, G.K., Stowe, H.D., 2002. Effects of diet
restriction on life span and age-related changes in dogs. J. Am. Vet. Med.
Assoc. 220, 1315–1320.
Keaney, M., Matthijssens, F., Sharpe, M., Vanfletern, J., Gems, D., 2004.
Superoxide dismutase mimetics elevate superoxide dismutase activity in
vivo but do not retard aging in the nematode Caenorhabditis elegans. Free
Radic. Biol. Med. 37, 2390250.
Kenyon, C., 2005. The plasticity of aging: insights from long-lived mutants.
Cell 120, 449–460.
Kim, J.D., Yu, B.P., McCarter, R.J., Lee, S.Y., Herlihy, J.T., 1996. Exercise and
diet modulate cardiac lipid peroxidation and antioxidant defenses. Free
Radic. Biol. Med. 20, 83–88.
Kirkwood, T.B.L., 1992. Comparative life spans of species: why do species have
the life spans they do? Am. J. Clin. Nutr. 55, 1191S–1195S.
Kirkwood, T.B., 2005. Understanding the odd science of aging. Cell 120,
437–447.
Koizumi, A., Tsukada, M., Wada, Y., Masuda, H., Weindruch, R., 1992. Mitotic
activity in mice is suppressed by energy-restriction induced torpor. J. Nutr.
122, 1446–1453.
Korshunov, S.S., Skulachev, V.P., Starkov, A.A., 1997. High protonic potential
actuates a mechanism of production of reactive oxygen species in
mitochondria. FEBS Lett. 416, 15–18.
Krishna, A., Abhilasha, 2000. Mechanism of delayed ovulation in a
vespertilonid bat, Scotophilus heathi: role of gonadotropin, insulin and
insulin-like growth factor-1. Physiol. Biochem. Zool. 73, 523–529.
Krishna, A., Singh, K., Doval, J., Chanda, D., 1998. Changes in circulating
insulin and corticosterone concentrations during different reproductive
phases and their relationships to body weight and androstenedione
concentration of male Scotophilus heathi. J. Exp. Zool. 281, 201–206.
Kuzmin, E.V., Karpova, O.V., Elthon, T.E., Newton, K.J., 2004. Mitochondrial
respiratory deficiencies signal up-regulation of genes for heat shock
proteins. J. Biol. Chem. 279, 20672–20677.
Lakowski, B., Hekimi, S., 1996. Determination of life-span in Caenorhabditis
elegans by four clock genes. Science 272, 1010–1013.
Lamb, M.J., 1968. Temperature and lifespan in Drosophila. Nature 220,
808–809.
22
J.A. Stuart, M.F. Brown / Comparative Biochemistry and Physiology, Part A 143 (2006) 12–23
Larsen, P.L., 1993. Ageing and resistance to oxidative damage in Caenorhabditis elegans. Proc. Natl. Acad. Sci. U. S. A. 90, 8905–8909.
Lee, H.-C., Yin, P.-H., Chi, C.-W., Wei, Y.-H., 2002a. Increase in mitochondrial
mass in human fibroblasts under oxidative stress and during replicative cell
senescence. J. Biomed. Sci. 9, 517–526.
Lee, S.S., Lee, R.Y.N., Fraser, A.G., Kamath, R.S., Ahringer, J., Ruvkun, G.,
2002b. A systematic RNAi screen identifies a critical role for mitochondria
in C. elegans longevity. Nat. Genet. 33, 40–48.
Levine, R.L., 2002. Carbonyl modified proteins in cellular regulation, aging and
disease. Free Radic. Biol. Med. 32, 790–796.
Lithgow, G.J., 2000. Stress response and aging in Caenorhabditis elegans. Cell
Differ. 29, 131–148.
Lithgow, G.J., Walker, G.A., 2002. Stress resistance as a determinant of C.
elegans lifespan. Mech. Ageing Dev. 123, 765–771.
Lithgow, G.J., White, T.M., Melov, S., Johnson, T.E., 1995. Thermotolerance
and extended life span conferred by single-gene mutations and induced by
thermal stress. Proc. Natl. Acad. Sci. U. S. A. 92, 7540–7544.
Lombard, D.B., Chua, K.F., Mostoslavsky, R., Franco, S., Gostissa, M., Alt, F.
W., 2005. DNA repair, genome stability and aging. Cell 120, 497–512.
Lu, T., Pan, Y., Kao, S.Y., Li, C., Kohane, I., Chan, J., Yankner, B.A., 2004.
Gene regulation and DNA damage in the ageing human brain. Nature 429,
883–891.
Mattiasson, G., Shamloo, M., Gido, G., Mathi, K., Toasevic, G., Yi, S., Warden,
C.H., Castilho, R.F., Melcher, T., Gonzalez-Zulueta, M., Nikolich, K.,
Wieloch, T., 2003. Uncoupling protein-2 prevent neuronal death and
diminishes brain dysfunction after stroke and brain trauma. Nat. Med. 9,
1062–1068.
Mattison, J.A., Lane, M.A., Roth, G.S., Ingram, D.K., 2003. Calorie restriction
in rhesus monkeys. Exp. Gerontol. 38, 35–46.
Michishita, E., Park, J.Y., Burneskis, J.M., Barrett, J.C., Horikawa, I., 2005.
Evolutionarily conserved and nonconserved cellular localizations and
functions of human SIRT1 proteins. Mol. Biol. Cell 16, 4623–4635.
Morley, J.F., Morimoto, R.I., 2004. Regulation of longevity in Caenorhabditis
elegans by heat shock factor and molecular chaperones. Mol. Biol. Cell 15,
657–664.
Morrow, G., Samson, M., Michaud, S., Tanguay, R.M., 2004a. Overexpression
of the small mitochondrial Hsp22 extends Drosophila life span and increases
resistance to oxidative stress. FASEB J. 18, 598–599.
Morrow, G., Battistini, S., Zhang, P., Tanguay, R.M., 2004b. Decreased lifespan
in the absence of expression of mitochondrial small heat shock protein
Hsp22 in Drosophila. J. Biol. Chem. 279, 43382–43385.
Mounkes, L.C., Kozlov, S., Hernandez, L., Sullivan, T., Stewart, C.L., 2003. A
progeroid syndrome in mice is caused by defects in A-type lamins. Nature
423, 298–301.
Munoz, M.J., 2003. Longevity and heat stress regulation in Caenorhabditis
elegans. Mech. Ageing Dev. 124, 43–48.
Munoz, M.J., Riddle, D.L., 2003. Positive selection of Caenorhabditis elegans
mutants with increased stress resistance and longevity. Genetics 163,
171–180.
Newell, L., Heddle, J.A., 2002. The effect of dietary restriction during
development in utero on the frequency of spontaneous somatic mutations.
Mutagenesis 17, 289–292.
Ogihara, M., Tanno, M., Izumiyama, N., Nakamura, H., Taguchi, T., 2002.
Increase in DNA polymerase gamma in the hearts of adriamycinadministered rats. Exp. Mol. Pathol. 73, 234–241.
Orr, W.C., Sohal, R.S., 1994. Extension of life-span by overexpression of
superoxide dismutase and catalase in Drosophila melanogaster. Science
263, 1128–1130.
Pamplona, R., Portero-Otin, M., Ruiz, C., Gredilla, R., Herrero, A., Barja, G.,
2000. Double bond content of phospholipids and lipid peroxidation
negatively correlate with maximum longevity in the heart of mammals.
Mech. Ageing Dev. 112, 169–183.
Parker, J.D., Parker, K.M., Sohal, B.H., Sohal, R.S., Keller, L., 2004. Decreased
expression of Cu–Zn superoxide dismutase 1 in ants with extreme lifespan.
Proc. Natl. Acad. Sci. U. S. A. 101, 3486–3489.
Parkes, T.L., Elia, A.J., Dickinson, D., Hilliker, A.J., Phillips, J.P., Boulianne, G.
L., 1998. Extension of Drosophila lifespan by overexpression of human
SOD1 in motor neurons. Nat. Genet. 19, 171–174.
Parrinello, S., Samper, E., Krtolica, A., Goldstein, J., Melov, S., Campisi, J.,
2003. Oxygen sensitivity severely limits the replicative lifespan of murine
fibroblasts. Nat. Cell Biol. 5, 741–747.
Partridge, L., Gems, D., 2002. Mechanisms of ageing: public or private? Nat.
Rev. Genet. 3, 165–175.
Pedler, S., Fuery, C.J., Withers, P.C., Flanigan, J., Guppy, M., 1996. Effectors of
metabolic depression in an estivating pulmonate snail (Helix aspersa):
whole animal and in vitro tissue studies. J. Comp. Physiol. B 166, 375–381.
Porter, R.K., Brand, M.D., 1993. Body mass dependence of H+ leak in
mitochondria and its relevance to metabolic rate. Nature 362, 628–630.
Porter, R.K., Brand, M.D., 1995. Cellular oxygen consumption depends on body
mass. Am. J. Physiol. 269, R226–R228.
Ramos-Vasconcelos, G.R., Hermes-Lima, M., 2003. Hypometabolism, antioxidant defenses and free radical metabolism in the pulmonate land snail Helix
aspersa. J. Exp. Biol. 206, 675–685.
Rao, G., Xia, E., Nadakavukaren, M.J., Richardson, A., 1990. Effect of dietary
restriction on the ag-dependent changes in the expression of antioxidant
enzymes in rat liver. J. Nutr. 120, 602–609.
Rattan, S.I.S., 1998. Repeated mild heat shock delays ageing in cultured human
skin fibroblasts. Biochem. Mol. Biol. Int. 45, 753–759.
Richardson, A., Liu, F., Adamo, M.L., Van Remmen, H., Nelson, J.F., 2004. The
role of insulin and insulin-like growth factor-1 in mammalian ageing. Best
Pract Res. Clin. Endocrinol. Metab. 18, 393–406.
Riddle, D.L., Albert, P.S., 1997. In: Riddle, D.L., Blumenthal, T., Meyer, B.
J., Priess, J.R. (Eds.), C. elegans: II. Cold Spring Harbor Press, New
York, pp. 739–768.
Rinehart, J.P., Yocum, G.D., Denlinger, D.L., 2000. Developmental upregulation of inducible hsp70 transcripts, but not the cognate form, during pupal
diapause in the flesh fly Sarcophaga crassipalpis. Insect Biochem. Mol.
Biol. 30, 515–521.
Rohme, D., 1981. Evidence for a relationship between longevity of mammalian
species and life spans of normal fibroblasts in vitro and erythrocytes in vivo.
Proc. Natl. Acad. Sci. U. S. A. 78, 5009–5013.
Rose, M.R., Vu, L.N., Park, S.U., Graves Jr., J.L., 1992. Selection on stress
resistance increases longevity in Drosophila melanogaster. Exp. Gerontol.
27, 241–250.
Schmidt, K.E., Kelley, K.M., 2001. Down-regulation in the insulin-like growth
factor (IGF) axis during hibernation in the golden-mantled ground squirrel,
Spermophilus lateralis: IGF-1 and the IGF-binding proteins (IGBPs). J. Exp.
Zool. 289, 66–73.
Schriner, S.E., Linford, N.J., Martin, G.M., Treuting, P., Ogburn, C.E., Emond,
M., Coskun, P.E., Ladiges, W., Wolf, N., Van Remmen, H., Wallace, D.C.,
Rabinovitch, P.S., 2005. Extension of murine life span by overexpression of
catalase targeted to mitochondria. Science 308, 1909–1911.
Semsei, I., Rao, G., Richardson, A., 1989. Changes in the expression of
superoxide dismutase and catalase as a function of age and dietary
restriction. Biochem. Biophys. Res. Commun. 164, 620–625.
Shanley, D.P., Kirkwood, T.B.L., 2000. Calorie restriction and aging: a lifehistory analysis. Evolution 54, 740–750.
Soti, C., Csermely, P., 2000. Molecular chaperones and the aging process.
Biogerontology 1, 225–233.
Speakman, J.R., 2005. Correlations between physiology and lifespan—two
widely ignored problems with comparative studies. Aging Cell 4,
167–175.
Speakman, J.R., Talbot, D.A., Selman, C., Snart, S., McLaren, J.S., Redman, P.,
Krol, E., Jackson, D.M., Johnson, M.S., Brand, M.D., 2004. Uncoupled and
surviving: individual mice with high metabolism have greater mitochondrial
uncoupling and live longer. Aging Cell 3, 87–95.
St. Pierre, J., Boutilier, R.G., 2001. Aerobic capacity of frog skeletal muscle
during hibernation. Physiol. Biochem. Zool. 74, 390–397.
St. Pierre, J., Buckingham, Roebuck, S.J., Brand, M.D., 2002. Topology of
superoxide production from different sites in the mitochondrial electron
transport chain. J. Biol. Chem. 277, 44784–44790.
Storey, K.B., 2002. Life in the slow lane: molecular mechanisms of estivation.
Comp. Biochem. Physiol. A 133, 733–754.
Stuart, J.A., Gillis, T.E., Ballantyne, J.S., 1998. Remodeling of phospholipids
fatty acids in mitochondrial membranes of estivating snails. Lipids 33,
787–793.
J.A. Stuart, M.F. Brown / Comparative Biochemistry and Physiology, Part A 143 (2006) 12–23
Stuart, J.A., Harper, J.A., Brindle, K.M., Brand, M.D., 1999. Uncoupling
protein 2 from carp and zebrafish, ectothermic vertebrates. Biochim.
Biophys. Acta 1413, 50–54.
Stuart, G.R., Oda, Y., de Boer, J.G., Glickman, B.W., 2000. No change in
spontaneous mutation frequency or specificity in dietary restricted mice.
Carcinogenesis 21, 317–319.
Stuart, J.A., Cadenas, S., Jekabsons, M.B., Roussel, D., Brand, M.D., 2001.
Mitochondrial proton leak and the uncoupling protein 1 homologues.
Biochim. Biophys. Acta 1504, 144–158.
Stuart, J.A., Karahalil, B., Hogue, B.A., de Souza-Pinto, N.C., Bohr, V.A., 2004.
Mitochondrial and nuclear DNA base excision repair are affected differently
by caloric restriction. FASEB J. 18, 595–597.
Stuart, J.A., Bourque, B.M., de Souza-Pinto, N.C., Bohr, V.A., 2005. No
evidence of mitochondrial respiratory dysfunction in OGG1-null mice
deficient in removal of 8-oxodeoxyguanine from mitochondrial DNA. Free
Radic. Biol. Med. 38, 737–745.
Suliman, H.B., Welty-Wolf, K.E., Carraway, M.S., Tatro, L., Piantadosi, C.A.,
2004. Lipopolysaccharide induces oxidative cardiac mitochondrial damage
and biogenesis. Cardiovasc. Res. 64, 279–288.
Sun, J.T., Tower, J., 1999. FLP recombinase-mediated induction of Cu/Znsuperoxide dismutase transgene expression can extend the life span of adult
Drosophila melanogaster flies. Mol. Cell. Biol. 19, 216–228.
Sun, J.t., Folk, D., Bradley, T.J., Tower, J., 2002. Induced overexpression of
mitochondrial Mn-superoxide dismutase extends the life span of adult
Drosophila melanogaster. Genetics 161, 661–672.
Sun, J., Molitor, J., Tower, J., 2004. Effects of simultaneous over-expression of
Cu/ZnSOD and MnSOD on Drosophila melanogaster life span. Mech.
Ageing Dev. 125, 341–349.
Szewczak, J.M., Jackson, D.C., 1992. Apneic oxygen uptake in the torpid bat
Eptesicus fuscus. J. Exp. Biol. 173, 217–227.
Tatar, M., Yin, C.-M., 2001. Slow aging during insect reproductive diapause:
why butterflies, grasshoppers and flies are like worms. Exp. Gerontol. 36,
723–738.
Teshima, Y., Akao, M., Jones, S.P., Marban, E., 2003. Uncoupling protein-2
overexpression inhibits mitochondrial death pathway in cardiomyocytes.
Circ. Res. 93, 192–200.
Tissenbaum, H.A., Guarente, L., 2001. Increased dosage of a sir-2 gene extends
lifespan in Caenorhabditis elegans. Nature 410, 227–230.
Tissenbaum, H.A., Hawdon, J., Perregaux, M., Hotez, P., Guarente, L., Ruvkun,
G., 2000. A common muscarinic pathway for diapause recovery in the
distantly related nematode species Caenorhabditis elegans and Ancylostoma
caninum. Proc. Natl. Acad. Sci. U. S. A. 97, 460–465.
Thomas, D.W., Cloutier, D., Gagne, D.C., 1990. Arrhythmic breathing, apnoea,
and non-steady-state oxygen uptake in little brown bats (Myotis lucifugus). J.
Exp. Biol. 149, 395–406.
23
Toein, O., Drew, K.L., Chao, M.L., Rice, M.E., 2001. Ascorbate dynamics and
oxygen consumption during arousal from hibernation in Arctic ground
squirrels. Am. J. Physiol. 281, R572–R583.
Treuting, P.M., Hopkins, H.C., Ware, C.A., Rabinovitch, P.R., Ladiges, W.C.,
2002. Generation of genetically altered mouse models for aging studies.
Exp. Mol. Pathol. 72, 49–55.
Trifunovic, A., Wredenberg, A., Falkenberg, M., Spelbrink, J.N., Rovio, A.T.,
Bruder, C.E., Bohlooy, Y.M., Gidlof, S., Oldfors, A., Wibom, R., Tornell, J.,
Jacobs, H.T., Larsson, N.G., 2004. Premature ageing in mice expressing
defective mitochondrial DNA polymerase. Nature 429, 417–423.
Turrens, J.F., 2003. Mitochondrial formation of reactive oxygen species. J.
Physiol. 552, 335–344.
Vanfleteren, J.R., 1993. Oxidative stress and aging in Caenorhabditis elegans.
Biochem. J. 292, 605–608.
Van Remmen, H., Ikeno, Y., Hamilton, M., Pahlavani, M., Wolf, N., Thorpe, S.
R., Alderson, N.L., Bayens, J.W., Epstein, C.J., Huang, T.T., Nelson, J.,
Strong, R., Richardson, A., 2003. Life-long reduction in MnSOD activity
results in increased DNA damage and higher incidence of cancer but does
not accelerate aging. Physiol. Genomics 16, 29–37.
Van Voorhies, W.A., Ward, S., 1999. Genetic and environmental conditions that
increase longevity in Caenorhabditis elegans decrease metabolic rate. Proc.
Natl. Acad. Sci. U. S. A. 96, 11399–11403.
Vidal-Puig, A.J., Grujic, D., Zhang, C.Y., Hagen, T., Boss, O., Ido, Y.,
Szczepanik, A., Wade, J., Mootha, V., Cortright, R., Muoio, D.M., Lowell,
B.B., 2000. Energy metabolism in uncoupling protein 3 gene knockout
mice. J. Biol. Chem. 275, 16258–016266.
Walker, G.A., Lithgow, G.J., 2003. Lifespan extension in C. elegans by a
molecular chaperone dependent upon insulin-like signals. Aging Cell 2,
131–139.
Wang, H.D., Kazemi-Esfarjani, P., Benzer, S., 2004. Multiple-stress analysis for
isolation of Drosophila longevity genes. Proc. Natl. Acad. Sci. U. S. A. 101,
12610–12615.
Weirich-Schwaiger, H., Weirich, H.G., Gruber, B., Schweiger, M., HirschKauffmann, M., 1994. Correlation between senescence and DNA repair in
cells from young and old individuals and in premature aging syndromes.
Mutat. Res. 316, 37–48.
Westman, W., Geiser, F., 2004. The effect of metabolic fuel availability on
thermoregulation and torpor in a marsupial hibernator. J. Comp. Physiol. B
174, 49–57.
Wilkinson, G.S., South, J.M., 2002. Life history, ecology and longevity in bats.
Aging Cell 1, 124–131.
Yu, B.P., Masoro, E.J., Murata, I., Bertrand, H.A., Lynd, F.T., 1982. Life
span study of SPF Fischer 344 male rats fed ad libitum or restricted
diets: longevity, growth, lean body mass and disease. J. Gerontol. 37,
130–141.