recent aging research in caenorhabditis elegans
TRANSCRIPT
Mini review
Recent aging research in Caenorhabditis elegans
Elke Schaffitzel a,b, Maren Hertweck a,*
a Bio 3, Bioinformatics and Molecular Genetics, University of Freiburg, Schaenzlestr. 1, D-79104 Freiburg (Brsg.), Germanyb Renal Division, University Hospital Freiburg, Freiburg, Germany
Received 5 January 2006; received in revised form 16 February 2006; accepted 21 February 2006
Available online 3 April 2006
Abstract
Evidence gathered over the past 15 years shows that the nematode Caenorhabditis elegans is excellently suited as a model to study aging
processes in the entire organism. Genetic approaches have been used to identify and elucidate multiple mechanisms and their corresponding genes
that limit the life span of C. elegans. These highly conserved pathways include the well-studied insulin/IGF-1 receptor-like signaling pathway,
which is thought to be a central determinant of life span, since several other mechanisms depend or converge on the insulin/IGF-1 pathway
transcription factor DAF-16/FoxO. In this review we focus on new insights into the molecular mechanisms of aging in C. elegans, including new
genes acting in the insulin/IGF-1 pathway and germline signaling. In addition, stress response pathways and mitochondrial mechanisms, dietary
restriction, SIR2 deacetylase activity, TOR and TUBBY signaling, as well as telomere length contribution are discussed in relation to recent
developments in C. elegans aging research.
q 2006 Elsevier Inc. All rights reserved.
Keywords: Aging; Caenorhabditis elegans; DAF-16/FoxO; SKN-1/Nrf; JNK; Germline; SIR2; TUBBY
1. The insulin-/IGF-1-like signaling pathway is a major
determinant of life span
So far one of the best characterized genetic regulatory
networks that influences aging is the insulin/insulin growth
factor 1 (IGF-1)-like endocrine system (Fig. 1). Inhibition of
the insulin/IGF-1 signaling results in constitutive dauer
formation (an alternative L3 larval stage) during development,
and leads to an extension of life span in animals that reach the
adulthood. In addition, insulin/IGF-1 signaling mutants show
increased fat storage, defective egg-laying and high tolerance
to a variety of stressors (Kenyon, 2005; Kenyon et al., 1993;
Kimura et al., 1997; Paradis and Ruvkun, 1998).
The binding of insulin-like molecules to the insulin-like
receptor DAF-2 initiates a cascade of protein kinases (for
review, see Kenyon, 2005). The AGE-1 PI3-kinase catalytic
subunit generates 3-phosphoinositides such as PIP2 and PIP3
(phosphatidylinositol-3,4-bisphosphate and phosphatidyl-
inositol-3,4,5-trisphosphate), required to activate PDK-1
(PI3K-dependent kinase). The adaptor subunit of PI3-kinase
AAP-1 potentiates signaling by the DAF-2/AGE-1 pathway
0531-5565/$ - see front matter q 2006 Elsevier Inc. All rights reserved.
doi:10.1016/j.exger.2006.02.008
* Corresponding author. Tel.: C49 761 2038364; fax: C49 761 2038352.
E-mail address: [email protected] (M. Hertweck).
(Wolkow et al., 2002). DAF-18, a homolog of the human tumor
suppressor PTEN, has lipid phosphatase activity and antagon-
izes AGE-1 (Ogg and Ruvkun, 1998). PDK-1 activates a
multimeric protein complex formed by AKT-1, AKT-2 and
SGK-1. All three kinases of this complex are able to control the
activity of the FoxO family transcription factor DAF-16, but
have different functions in the insulin-like signaling. SGK-1
acts in parallel to the AKT/PKB kinases, and is the crucial
factor for the regulation of post-embryonic development, stress
response and life span, whereas AKT-1 and AKT-2 are mainly
involved in the control of the dauer larval stage (Hertweck et
al., 2004; Ogg et al., 1997; Paradis and Ruvkun, 1998)
(Table 1).
Many mutations within genes of the insulin/IGF-1 signaling
pathway have been shown to extend life span. This life span
extension, the Age phenotype, depends on the function of
DAF-16, indicating that DAF-16 is the major downstream
effector of this pathway that is negatively regulated by its
upstream components (Ogg et al., 1997). The inactivation of
DAF-16 involves AKT/SGK-mediated phosphorylation and
subsequent cytoplasmic retention of DAF-16 (Lee et al., 2001;
Lin et al., 2001). In conditions of reduced DAF-2 signaling,
DAF-16 translocates into the nucleus and modulates transcrip-
tion. Multiple DAF-16 transcriptional targets probably mediate
the distinct functions of the insulin/IGF-1 signaling pathway.
Candidate gene approaches, comparative genomics, DNA
microarray analysis, genome-wide RNAi screens, and
Experimental Gerontology 41 (2006) 557–563
www.elsevier.com/locate/expgero
Fig. 1. Several evolutionary conserved mechanisms contribute to the complex
regulation of life span in Caenorhabditis elegans. Decreased activity (indicated
in blue) and increased activity (indicated in orange) of the following signaling
pathways and processes lead to longevity in the animal. In some of these
mechanisms, including insulin/IGF-1-, germline-, TOR-, TUBBY-, JNK-
signaling, and SIR2 activity, life span extension occurs in a DAF-16/FoxO-
dependent manner. See text for details.
E. Schaffitzel, M. Hertweck / Experimental Gerontology 41 (2006) 557–563558
chromatin immunoprecipitation (ChIP) analysis revealed
several genes that are regulated by DAF-16, including
metabolic and developmental genes, antioxidant genes,
chaperones, particularly small heat-shock genes, and anti-
bacterial genes (Halaschek-Wiener et al., 2005; Hamilton et al.,
2005; Hansen et al., 2005; Lee et al., 2003a; McElwee et al.,
Table 1
Genes involved in the Caenorhabditis elegans aging process
Gene Description
Insulin/IGF-1-like signaling
daf-2 Insulin/IGF-1 receptor
aap-1 Phosphoinositide 3-kinase p50/p55 adaptor subunit
age-1 Phosphatidylinositol-3-kinase p110 catalytic subunit
daf-18 PTEN lipid phosphatase
pdk-1 3-Phosphoinositide-dependent kinase 1
sgk-1 Serum- and glucocorticoid- inducible kinase
akt-1/2 Akt/protein kinase B (PKB)
daf-16 Forkhead/winged helix transcription factor
hsf-1 Heat shock transcription factor
JNK signalling
jkk-1 JNK kinase
jnk-1 c-Jun N-terminal kinase
Oxidative stress
gsk-3 Glycogen synthase kinase
skn-1 Similar to bZIP transcription factors
Germline signaling
glp-1 Notch family receptor
mes-1 Receptor tyrosine kinase
daf-12 Nuclear hormone receptor
Mitochondrial
mechanisms
isp-1 ‘Rieske’ iron sulphur protein of mitochondrial
complex III of ETC
clk-1 Enzyme necessary for ubiquinone biosynthesis
TOR signalling
let-363 TOR (target of rapamycin)
daf-15 Raptor (regulatory associated protein of TOR)
pep-2 Intestinal proton-coupled oligopeptide transporter
Other mechanisms
aak-2 Catalytic subunit of AMP-activated protein kinase
sir-2.1 NADC-dependent histone deacetylase
eat-2 Subunit of nicotinic acetylcholine receptor
tub-1 TUBBY protein family
2003; Murphy et al., 2003; Wook Oh et al., 2005). Over-
expression of at least some of the DAF-16 target genes that are
upregulated in long-lived mutants, such as the superoxide
dismutase sod-3 and the catalases ctl-1 and ctl-2, can prolong
life span and also protect the animal from harsh conditions,
such as heat and oxidative stress. In summary, DAF-16
responds to the cues of a changing environment to provide
resources at all stages of life.
2. HSF-1 and DAF-16 have overlapping and distinct
functions in stress response and life span
In addition to DAF-16, the heat-shock transcription factor
HSF-1 is also required for the extension of life span in daf-2
mutants (Hajdu-Cronin et al., 2004; Hsu et al., 2003; Morley
and Morimoto, 2004). As for daf-16, inhibiting hsf-1 activity
shortens life span and accelerates aging. daf-16; hsf-1 double
mutants show no further decrease of life span, suggesting that
daf-16 and hsf-1 function, at least in part, through similar
mechanisms. This is likely because HSF-1 and DAF-16
together activate the expression of a specific subset of genes,
including genes that encode small heat-shock proteins, which
promote longevity (Hsu et al., 2003). Small heat-shock proteins
may delay aging by preventing the accumulation of protein
aggregates. DAF-16 and HSF-1 have overlapping functions in
the insulin/IGF-1 signaling pathway, yet have distinct
functions in the regulation of gene expression. HSF-1 can
also function independently of DAF-16 and, thus, mediates the
expression of a subset of stress-inducible genes after heat-
shock, encoding molecular chaperones (e.g. HSP-70, AIP-1).
Likewise, hsf-1 loss-of-function does not prevent DAF-16 from
accumulating in the nucleus and activating downstream genes.
Thus, neither DAF-16 nor HSF-1 is completely required for the
other’s activity. These findings provide a molecular link
between genes that regulate longevity, and those that are
involved in stress protection and the restoration of protein-
folding homeostasis.
3. Sensing energy levels is linked to insulin/IGF-1 signaling
The life span extension caused by the daf-2 insulin-like
receptor mutant also requires the function of aak-2, which
encodes the catalytic subunit of the AMP-activated protein
kinase (AMPK) (Table 1). AAK-2 senses low energy levels,
and becomes activated when the AMP:ATP ratio is high.
Animals with a loss-of-function mutation in aak-2 have a
significantly shorter life span than wild-type animals.
Accordingly, overexpression of aak-2 leads to an elongated
life span, indicating that AAK-2 has an important role in life
span regulation and functions in a dose-dependent manner.
However, combination of daf-16 and aak-2 loss-of-function
mutation further decreases life span, suggesting that the effect
of AAK-2 on life span is not dependent on the activity of the
transcription factor DAF-16. Thus, AAK-2 and DAF-16 act in
parallel to mediate longevity, but so far the targets of AAK-2
phosphorylation and the mechanism by which AAK-2
promotes longevity are not known (Apfeld et al., 2004).
E. Schaffitzel, M. Hertweck / Experimental Gerontology 41 (2006) 557–563 559
Hence, AAK-2 is a sensor of energy levels and regulates life
span in response to insulin/IGF-1 signaling.
4. JNK-1 confers longevity and stress resistance andconverges on DAF-16/FoxO
The c-Jun N-terminal kinase (JNK) family, a subgroup of
the mitogen-activated protein kinase superfamily, is part of a
signal transduction cascade that is activated by exposure to
environmental stress. Studies in Drosophila have established
that JNK signaling represents an important genetic factor of life
span regulation (Wang et al., 2003; 2005). Recently, it was also
shown in C. elegans that activating the JNK pathway by
overexpression of jnk-1 confers tolerance to oxidative and heat
stress, and increases life span up to 40% (Oh et al., 2005)
(Table 1). The life span extension by jnk-1 overexpression is
completely suppressed by daf-16 RNAi, indicating that the
transcription factor DAF-16 plays a critical role in JNK-1
mediated longevity. Genetic analysis suggests that the JNK
pathway acts in parallel with the insulin/IGF-1 signaling
pathway to regulate life span, and both pathways converge on
DAF-16 (Fig. 1). Interestingly, JNK-1 and insulin/IGF-1
signaling regulate DAF-16 in opposite ways. In contrast to
the AKT/SGK complex, JNK-1 is a positive regulator of DAF-
16. JNK activates DAF-16 by phosphorylation, and modulates
the translocation of DAF-16 into the nucleus in response to
stress. Thus, JNK-1 induces the expression of DAF-16 target
genes important for stress response and life span.
5. A post-embryonic function for SKN-1/Nrf in oxidative
stress response and life span
Typically, mutants with decreased life span show decreased
thermo tolerance, and are sensitive to UV radiation and
reactive oxygen species (ROS, including superoxide, peroxide
or hydroxyl radicals). These physiological alterations are
consistent with the free radical theory of aging, which argues
that senescence results from cumulative cellular and systemic
oxidative damage. Eukaryotic cells respond to oxidative stress
by inducing the expression of phase II detoxification genes.
Phase II enzymes synthesize the critical reducing agent
glutathione, scavenge free radicals, and detoxify reactive
intermediates of the phase I cytochrome p450 system.
In C. elegans, mesendodermal development is initiated by
the transcription factor SKN-1. Recently, it was shown that this
transcription factor, which is related to the mammalian NF-E2
stress response proteins Nrf1 and Nrf2, also mediates oxidative
stress response (An and Blackwell, 2003) (Table 1). SKN-1
induces the transcription of phase II detoxification genes
through constitutive and stress-inducible mechanisms. Inhibit-
ing skn-1 activity leads to increased stress sensitivity and
shortens life span. Accordingly, overexpressing skn-1 enhances
resistance to oxidative stress (An et al., 2005). This suggests
that skn-1 loss-of-function mutants do not die prematurely due
to an unrelated general pathology, but rather that skn-1
functions in the oxidative stress response in the adult worm.
Under normal conditions SKN-1 is present in ASI
chemosensory neurons and functions constitutively. In con-
trast, in the intestine glycogen synthase kinase GSK-3
phosphorylates and inhibits SKN-1, and thus prevents SKN-1
to function constitutively in the absence of stress (An et al.,
2005). In response to oxidative stress SKN-1 accumulates in
the intestinal cell nuclei and activates the transcription of phase
II genes. The activation and nuclear localization of SKN-1
under stress conditions depends on p38 mitogen-activated
protein kinase (MAPK) signaling (Inoue et al., 2005). Thus,
under oxidative stress conditions p38 MAPK signaling
counteracts the inhibition of SKN-1 by GSK-3 and is
independently required for SKN-1 to induce phase II gene
expression.
6. Germline signals influence life span in a DAF-16/FoxO-
and DAF-12/NHR-dependent manner
Signals from the reproductive system modulate the rate of
aging in C. elegans (Fig. 1) (Hsin and Kenyon, 1999). Insulin/
IGF-1 signaling and steroid hormone signaling are critical for
the regulation of life span by the reproductive system. There
are two types of gonad-dependent signals that influence life
span. A signal from the germ cells decreases life span by
downregulating the activity of the forkhead transcription factor
DAF-16/FoxO and the nuclear hormone receptor DAF-12/
NHR. A counterbalancing signal from the somatic gonad
increases life span by downregulating DAF-2/insulin receptor
activity. In fact, removal of the germline precursors results in
increased DAF-16 nuclear localisation in adults, most
prominently in intestinal cells, accompanied by a significant
life span extension of w60%. It remains unclear how germ
cells signal to the intestine to regulate aging. Consistent with
the negative effect of germ cells on life span, mes-1 loss-of-
function mutants, which lack germ cells, as well as glp-1 loss-
of-function mutants (glp-1 encodes the receptor for a germline
proliferation signal), show life span extension that is
suppressed by an additional loss-of-function mutation in daf-
16 (Table 1). However, when the entire gonad is absent, the
worm lives its normal life span, indicating that sterility alone is
insufficient to lengthen life and that the somatic gonad
produces counterbalancing signals to alter the rate of aging
in C. elegans (Arantes-Oliveira et al., 2002; Lin et al., 2001).
7. Different mitochondrial mechanisms affect the aging
process
Mitochondria, the crucial organelles for the overall function
of the cell, are a major source of endogenous reactive oxygen
species (ROS) and participate in life span determination in C.
elegans. ROS are generated by misplacement of electrons from
the electron transport chain (ETC) by ubisemiquinones and are
effectively detoxified by superoxide dismutases and catalases.
ROS that are able to escape detoxification can react with
macromolecules, including nucleic acids, proteins, and lipids.
This reaction leads to aberrant and dysfunctional molecules
impaired in the regulation of cell maintenance and survival.
E. Schaffitzel, M. Hertweck / Experimental Gerontology 41 (2006) 557–563560
Research in C. elegans uncovered that mitochondrial
oxidative phosphorylations are regulators of the aging process.
One mitochondrial mechanism that affects aging is to protect
from mitochondrial ROS production, as given in the
insulin/IGF-1-like receptor mutant of daf-2 and the mutant of
isp-1 (encoding the ‘Rieske’ iron sulphur protein of complex
III of the ETC) (Table 1). The hypomorphic mutant daf-
2(e1370) has a powerful effect on life span and is highly
resistant to a variety of stressors, which are mediated at least in
part by the production of ROS. The neomorphic mutant isp-
1(qm150) leads to a large decrease in oxygen consumption and
a substantial increase in life span. This suggests that the
longevity phenotypes of these two mutants are due to high ROS
detoxification accompanied by low ROS production (reviewed
in Hekimi and Guarente, 2003).
Another possibility to affect ROS and aging is represented
in clk-1 mutants. clk-1 is directly linked to mitochondria and
their metabolism since it encodes an enzyme that is necessary
for ubiquinone biosynthesis (Table 1). Ubiquinone is an
antioxidant in all cellular membranes, and a redox factor in
the mitochondrial ETC and elsewhere. The clk-1 loss-of-
function mutant accumulates the ubiquinone intermediate
demethoxyubiquinone (DMQ). DMQ reduces levels of extra-
mitochondrially produced ROS, resulting in low levels of
oxidative damaged lipoproteins, decreased activation of
oncogenic ras signaling, and, as a consequence, increased
life span (Shibata et al., 2003). Recently, it was shown that
abolishing the activity of clk in worms and mice results in a
similar reduction of ROS levels, oxidative stress and oxidative
damage, and prolongs life span of both species (Liu et al.,
2005). Such a correlation between life span and the level of
oxidative stress has been frequently observed and led to the
free radical theory of aging.
A third mitochondrial mechanism that indirectly affects
aging is thought to act during larval development. Dillin et al.
(2002); Lee et al. (2003b) demonstrated that abolishing the
activity of genes encoding major subunits of the mitochondrial
ETC complexes by RNAi during larval development, not
during adulthood, produces a reduction of oxygen consumption
and ATP levels. These changes are accompanied by severe
developmental changes that slow adult aging. This observation
suggests that a signal established during development is
maintained during adulthood to result in longer life span.
Thus, low levels of ATP and oxygen consumption during
development induce a morphological and physiological state
that leads to longevity in these ETC mutants (Aguilaniu et al.,
2005).
8. Dietary restriction extends life span through different
mechanisms
The life span of a number of organisms, including yeast,
worms, flies, and rodents can be increased up to 50% simply by
reducing their calorie intake. Dietary restriction (DR) also
lowers the incidence of age-related disorders, e.g. cancer,
cardiovascular diseases and diabetes in mammals. The
mechanisms through, which DR leads to life span extension
are not fully understood, partially controversial, and under
intensive investigation. The two hypotheses postulated in C.
elegans are, DR reduces the metabolic rate, and DR reduces the
insulin/IGF-1 signaling (Bordone and Guarente, 2005; Walker
et al., 2005).
An early hypothesis from mammals postulated that the
influence of calories on life span was excited via reducing the
metabolic rate and production of ROS. In C. elegans, the
effects of DR on aging are not exclusively related to a reduced
metabolic rate since the metabolic rate in DR-restricted loss-of-
function mutants of clk-1 and eat-2 (encoding a subunit of the
nicotinic acetylcholine receptor) is not reduced (Table 1). This
was shown by direct measurements of the metabolic rate
(including oxygen consumption or heat production) in worms
subjected to DR (Houthoofd et al., 2002a,b).
The second hypothesis implies that the insulin/IGF-1
pathway mediates the effect of DR. This appears to be true in
Drosophila since the life span of animals subjected to DR is not
further increased by mutations in the insulin/IGF-1 pathway
(Clancy et al., 2002). However, DR in C. elegans insulin/IGF-1
mutants further extends life span. In addition, the FoxO
transcription factor DAF-16 is not required for the effects of
DR on life span (Lakowski and Hekimi, 1998) (Fig. 1). The
additive interaction between the DAF-2 pathway and DR
argues against the possibility that insulin/IGF-1 signaling
mediates the response to DR. Therefore, it is more likely that
DR and insulin/IGF-1 act in parallel on a common longevity
assurance process. The different concepts by which DR may
regulate aging in C. elegans, including a number of other
organisms, are extensively discussed in the reviews of Bordone
and Guarente (2005) and Walker et al. (2005).
9. SIR-2.1 functions in a DAF-16/FoxO- and DR-dependent
manner, and regulates ER stress genes to determine life
span
Another evolutionary conserved regulator of longevity is
SIR2 (silent information regulator 2). SIR2 and its orthologs
belong to a family of NADC-dependent protein deacetylases
that respond to metabolic changes in the cellular environment,
including nutrient/energy availability and cellular stress.
Increased dosage of SIR2 in yeast, flies and worms leads to
life span extensions up to 50% by acting on biological
processes that promote survival in times of scarcity and stress
(Kaeberlein et al., 1999; Rogina and Helfand, 2004;
Tissenbaum and Guarente, 2001). C. elegans SIR-2.1 regulates
aging through the insulin/IGF-1 pathway transcription factor
DAF-16 (Fig. 1). SIR-2.1 acts either in the insulin/IGF-1
signaling pathway upstream of the insulin receptor DAF-2, or
in a parallel pathway, which converges on DAF-16. Addition-
ally, SIR-2.1 has been shown to be required for life span
extension by DR. Since, the sir-2.1(ok434) null mutant
suppresses the enhanced longevity of the caloric restricted
loss-of-function mutants such as eat-2(ad465) and unc-
13(e450) (encoding a neurotransmitter release regulator), sir-
2.1 is thought to function downstream of the DR pathway
(Wang and Tissenbaum, 2006). This reveals overlapping and
E. Schaffitzel, M. Hertweck / Experimental Gerontology 41 (2006) 557–563 561
distinct roles of DAF-16 and SIR-2.1 in the regulation of life
span.
Recently, it was shown that resveratrol, a plant-derived
polyphenolic compound, extends life span in dependence of
sir-2.1, but independent of daf-16 (Viswanathan et al., 2005).
Genome-wide microarray analysis revealed that resveratrol
upregulates a family of endoplasmic reticulum (ER) stress
genes, including the prion-like glutamine/asparagine-rich gene
abu-11. This gene is necessary and sufficient for the longevity
induced by resveratrol, and its expression (as the expression of
other members of this gene family) is repressed by sir-2.1.
Resveratrol might activate abu-11 and other genes of this
family to extend C. elegans life span by inhibiting the
repressive activity of sir-2.1. Since sir-2.1 also functions in a
DAF-16- and DR-dependent manner, sir-2.1 influences life
span by modulating multiple pathways.
10. LET-363/TOR mediates life span control
by insulin/IGF-1 signaling and nutrient sensing
The target of rapamycin (TOR) kinase is part of a putative
amino acid sensing pathway that regulates various cellular
processes, including initiation of translation, ribosome syn-
thesis, protein degradation, and autophagy in response to
nutrients, and hormone-dependent mitogenic signals in many
species (Schmelzle and Hall, 2000). Under high amino acid
levels, TOR up-regulates translation through the ribosomal S6
kinase (S6K). When amino acid levels are low, reduced TOR
signaling results in increased protein degradation and
autophagy.
Recently, it was shown in worms and flies that TOR
signaling modulates aging (Kapahi et al., 2004; Vellai et al.,
2003) (Fig. 1). C. elegans LET-363/TOR together with DAF-
15/raptor (regulatory associated protein of TOR) (Table 1) act
together to regulate dauer morphogenesis, fat storage and life
span. It was shown that loss of daf-16 function suppresses the
effect of let-363 RNAi and heterozygous daf-15 loss-of-
function mutants on longevity, but does not suppress fat
deposition phenotypes. This suggests that LET-363/TOR and
DAF-15/raptor might act in parallel to the transcription factor
DAF-16 regarding fat storage. So far the mechanism by which
the daf-16 mutant suppresses let-363 RNAi and daf-15/Cmutant longevity phenotypes are not fully understood (Jia et
al., 2004; Vellai et al., 2003).
The oligopeptide transporter PEP-2 functions in the
intestinal absorption of di- and tri-peptides and is therefore
important for the overall protein nutrition of the organism
(Table 1). pep-2 loss-of-function mutants enhance the
phenotype of the insulin receptor loss-of-function mutant
daf-2(e1370) and intensify the let-363/TOR RNAi phenotype
(construct for weak RNAi inactivation). Since pep-2 mutant
animals do not affect the let-363/TOR (strong RNAi)
phenotype, pep-2 is thought to be genetically located upstream
of let-363 and in parallel to daf-2. This indicates that the
obvious restriction of amino acid availability in pep-2 mutants
affects both TOR and insulin/IGF-1 signaling (Meissner et al.,
2004). Hence, pep-2 might have a predominant role in the
delivery of peptides for growth and development, which
consequently affect signaling pathways that regulate metab-
olism and aging. Furthermore, insulin/IGF-1 signaling regu-
lates the expression of daf-15 and pep-2 via the transcription
factor DAF-16, which implies an integrated control of nutrient
sensing and uptake by TOR and insulin/IGF-1 signaling
(Walker et al., 2005).
11. TUB-1/TUBBY regulates life span through
the insulin/IGF-1 pathway
One of few single-gene mutations that cause increased fat
deposition in mammals is found in the tubby gene. Mutations in
tubby result in adult onset obesity, insulin resistance, infertility,
and progressive sensorineural degeneration of retinal and
cochlear hair cells (Carroll et al., 2004). Several studies have
suggested possible functions of TUBBY, including a function
as a transcription factor, an adaptor molecule that integrates
several pathways, or a protein that regulates vesicle transport.
In C. elegans, a tub-1 null mutant accumulates triglycerides,
the major form of stored fat (Ashrafi et al., 2003).
Recently, a novel role for tub-1 in the regulation of life span
through the insulin/IGF-1 signaling pathway was observed
(Mukhopadhyay et al., 2005) (Fig. 1). However, the increased
fat storage in tub-1 mutants is independent of this pathway.
This indicates that increased life span is not the result of
increased fat storage. For life span control, TUB-1 couples with
the insulin/IGF-1 signaling pathway and requires the forkhead
transcription factor DAF-16. In contrast, fat metabolism is
controlled through direct regulation of RBG-3, a novel TBC
domain-containing RabGTPase-activating protein (RabGAP).
RBG-3 was found in a yeast two-hybrid screen as a TUB-1
interactor and is thought to regulate neuronal transport of yet-
unknown molecules (Mukhopadhyay et al., 2005).
12. Telomere length has no effect on longevity in C. elegans
Telomeres, the specialized nucleoprotein structures that
protect chromosome ends from degradation and fusions, have
long been implicated in the replicative aging process of
dividing cells (Bodnar et al., 1998). Cellular aging in mitotic
cultures is defined by both initial telomere length and the rate
of telomere shortening per division, in absence of the
telomerase. Nonetheless, it has been reported that the short
telomeres in wild-derived mice strains have no inverse effect
on longevity, suggesting that the organismal life span is
independent of telomere length. C. elegans spends its adult life
as an organism consisting exclusively of post-mitotic cells
(except for the germ cells, which are mitotic throughout
adulthood). Therefore, it represents a unique model system to
study the importance of telomere function in a post-mitotic
setting and its implications on organismal aging. In a recent
study, telomere length did not determine the potential life span
of C. elegans (Raices et al., 2005), and worms with short
telomeres lived as long as worms with long telomeres.
Furthermore, animals with an altered life span and stress
response due to mutations in the insulin/IGF-1 signaling
E. Schaffitzel, M. Hertweck / Experimental Gerontology 41 (2006) 557–563562
pathway did not show any changes in telomere length.
Telomere length was also found to be constant in isolated,
aging worm populations, implicating that organismal post-
mitotic aging can be uncoupled from mitotic aging (Raices et
al., 2005). On the other hand, telomerase-deficient trt-1(ok410)
worms show a progressive decrease in brood size and display a
high frequency of telomeres with fewer repeats than wild-type.
This indicates that telomerase is required to prevent or repair
sporadic telomere truncations that are unrelated to the typical
end-replication problems (Cheung et al., 2006).
13. Conclusions
Genetic analysis, primarily in model organisms such as
yeast, worms, and flies, identified multiple distinct genetic
networks that control the aging process. Either up- or
downregulation of these evolutionary conserved signaling
pathways and processes result in organismal longevity, as
summarized for C. elegans in Fig. 1. This review highlights
new discoveries that help to understand the complex frame-
work of mechanisms in C. elegans aging which will, in turn,
provide important insights into human aging. Surprisingly,
telomere shortening is an unlikely general cause of aging since
(post-mitotic) worms as well as mice with short and long
telomeres age at the same pace. In contrast, recent findings
support the view that telomeres may affect aging in humans.
Hence, distinguishing between species-specific aging-affecting
mechanisms and universal processes remains a major challenge
in this field. Nevertheless, a detailed knowledge of the
pathways that mediate or modulate aging in worms may,
1 day, lead to novel therapies for a wide range of human age-
related diseases, including diabetes, cancer, atherosclerosis,
Alzheimer’s, Parkinson’s and Huntington’s disease.
Acknowledgements
We thank G. Walz and B. Schulze for suggestions and
critical comments on our manuscript. E.S. and M.H. are
supported by the Bundesministerium fur Bildung und
Forschung (BMBF), and a training grant of the Medical
School Freiburg (E.S.).
References
Aguilaniu, H., Durieux, J., Dillin, A., 2005. Metabolism, ubiquinone synthesis,
and longevity. Genes Dev. 19, 2399–2406.
An, J.H., Blackwell, T.K., 2003. SKN-1 links C. elegans mesendodermal
specification to a conserved oxidative stress response. Genes Dev. 17,
1882–1893.
An, J.H., Vranas, K., Lucke, M., Inoue, H., Hisamoto, N., Matsumoto, K.,
Blackwell, T.K., 2005. Regulation of the Caenorhabditis elegans oxidative
stress defense protein SKN-1 by glycogen synthase kinase-3. Proc. Natl
Acad. Sci. USA 102, 16275–16280.
Apfeld, J., O’Connor, G., McDonagh, T., DiStefano, P.S., Curtis, R., 2004. The
AMP-activated protein kinase AAK-2 links energy levels and insulin-like
signals to lifespan in C. elegans. Genes Dev. 18, 3004–3009.
Arantes-Oliveira, N., Apfeld, J., Dillin, A., Kenyon, C., 2002. Regulation of
life-span by germ-line stem cells in Caenorhabditis elegans. Science 295,
502–505.
Ashrafi, K., Chang, F.Y., Watts, J.L., Fraser, A.G., Kamath, R.S., Ahringer, J.,
Ruvkun, G., 2003. Genome-wide RNAi analysis of Caenorhabditis elegans
fat regulatory genes. Nature 421, 268–272.
Bodnar, A.G., Ouellette, M., Frolkis, M., Holt, S.E., Chiu, C.P., Morin, G.B.,
Harley, C.B., Shay, J.W., Lichtsteiner, S., Wright, W.E., 1998. Extension of
life-span by introduction of telomerase into normal human cells. Science
279, 349–352.
Bordone, L., Guarente, L., 2005. Calorie restriction, SIRT1 and metabolism:
understanding longevity. Nat. Rev. Mol. Cell Biol. 6, 298–305.
Carroll, K., Gomez, C., Shapiro, L., 2004. Tubby proteins: the plot thickens.
Nat. Rev. Mol. Cell Biol. 5, 55–63.
Cheung, I., Schertzer, M., Rose, A., Lansdorp, P.M., 2006. High incidence of
rapid telomere loss in telomerase-deficient Caenorhabditis elegans.
Nucleic Acids Res. 34, 96–103.
Clancy, D.J., Gems, D., Hafen, E., Leevers, S.J., Partridge, L., 2002. Dietary
restriction in long-lived dwarf flies. Science 296, 319.
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
behavior and aging specified by mitochondrial function during develop-
ment. Science 298, 2398–2401.
Hajdu-Cronin, Y.M., Chen, W.J., Sternberg, P.W., 2004. The L-type cyclin
CYL-1 and the heat-shock-factor HSF-1 are required for heat-shock-
induced protein expression in Caenorhabditis elegans. Genetics 168, 1937–
1949.
Halaschek-Wiener, J., Khattra, J.S., McKay, S., Pouzyrev, A., Stott, J.M.,
Yang, G.S., Holt, R.A., Jones, S.J., Marra, M.A., Brooks-Wilson, A.R.,
Riddle, D.L., 2005. Analysis of long-lived C. elegans daf-2 mutants using
serial analysis of gene expression. Genome Res. 15, 603–615.
Hamilton, B., Dong, Y., Shindo, M., Liu, W., Odell, I., Ruvkun, G., Lee, S.S.,
2005. A systematic RNAi screen for longevity genes in C. elegans. Genes
Dev. 19, 1544–1555.
Hansen, M., Hsu, A.L., Dillin, A., Kenyon, C., 2005. New genes tied to
endocrine, metabolic, and dietary regulation of lifespan from a
Caenorhabditis elegans genomic RNAi screen. Plos. Genet. 1, 119–128.
Hekimi, S., Guarente, L., 2003. Genetics and the specificity of the aging
process. Science 299, 1351–1354.
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.
Houthoofd, K., Braeckman, B.P., Lenaerts, I., Brys, K., De Vreese, A., Van
Eygen, S., Vanfleteren, J.R., 2002a. Axenic growth up-regulates mass-
specific metabolic rate, stress resistance, and extends life span in
Caenorhabditis elegans. Exp. Gerontol. 37, 1371–1378.
Houthoofd, K., Braeckman, B.P., Lenaerts, I., Brys, K., De Vreese, A., Van
Eygen, S., Vanfleteren, J.R., 2002b. No reduction of metabolic rate in food
restricted Caenorhabditis elegans. Exp. Gerontol. 37, 1359–1369.
Hsin, H., Kenyon, C., 1999. Signals from the reproductive system regulate the
lifespan of C. elegans. Nature 399, 362–366.
Hsu, A.L., Murphy, C.T., Kenyon, C., 2003. Regulation of aging and age-
related disease by DAF-16 and heat-shock factor. Science 300, 1142–1145.
Inoue, H., Hisamoto, N., An, J.H., Oliveira, R.P., Nishida, E., Blackwell, T.K.,
Matsumoto, K., 2005. The C. elegans p38 MAPK pathway regulates
nuclear localization of the transcription factor SKN-1 in oxidative stress
response. Genes Dev. 19, 2278–2283.
Jia, K., Chen, D., Riddle, D.L., 2004. The TOR pathway interacts with the
insulin signaling pathway to regulate C. elegans larval development,
metabolism and life span. Development 131, 3897–3906.
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.
Kapahi, P., Zid, B.M., Harper, T., Koslover, D., Sapin, V., Benzer, S., 2004.
Regulation of lifespan in Drosophila by modulation of genes in the TOR
signaling pathway. Curr. Biol. 14, 885–890.
Kenyon, C., 2005. The plasticity of aging: insights from long-lived mutants.
Cell 120, 449–460.
Kenyon, C., Chang, J., Gensch, E., Rudner, A., Tabtiang, R., 1993. A C.
elegans mutant that lives twice as long as wild type. Nature 366, 461–464.
E. Schaffitzel, M. Hertweck / Experimental Gerontology 41 (2006) 557–563 563
Kimura, K.D., Tissenbaum, H.A., Liu, Y., Ruvkun, G., 1997. daf-2, an insulin
receptor-like gene that regulates longevity and diapause in Caenorhabditis
elegans. Science 277, 942–946.
Lakowski, B., Hekimi, S., 1998. The genetics of caloric restriction in
Caenorhabditis elegans. Proc. Natl. Acad. Sci. USA 95, 13091–13096.
Lee, R.Y., Hench, J., Ruvkun, G., 2001. Regulation of C. elegans DAF-16 and
its human ortholog FKHRL1 by the daf-2 insulin-like signaling pathway.
Curr. Biol. 11, 1950–1957.
Lee, S.S., Kennedy, S., Tolonen, A.C., Ruvkun, G., 2003a. DAF-16 target
genes that control C. elegans life-span and metabolism. Science 300, 644–
647.
Lee, S.S., Lee, R.Y., Fraser, A.G., Kamath, R.S., Ahringer, J., Ruvkun, G.,
2003b. A systematic RNAi screen identifies a critical role for mitochondria
in C. elegans longevity. Nat. Genet. 33, 40–48.
Lin, K., Hsin, H., Libina, N., Kenyon, C., 2001. Regulation of the
Caenorhabditis elegans longevity protein DAF-16 by insulin/IGF-1 and
germline signaling. Nat. Genet. 28, 139–145.
Liu, X., Jiang, N., Hughes, B., Bigras, E., Shoubridge, E., Hekimi, S., 2005.
Evolutionary conservation of the clk-1-dependent mechanism of longevity:
loss of mclk1 increases cellular fitness and lifespan in mice. Genes Dev. 19,
2424–2434.
McElwee, J., Bubb, K., Thomas, J.H., 2003. Transcriptional outputs of
the Caenorhabditis elegans forkhead protein DAF-16. Aging Cell 2,
111–121.
Meissner, B., Boll, M., Daniel, H., Baumeister, R., 2004. Deletion of the
intestinal peptide transporter affects insulin and TOR signaling in
Caenorhabditis elegans. J. Biol. Chem. 279, 36739–36745.
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.
Mukhopadhyay, A., Deplancke, B., Walhout, A.J., Tissenbaum, H.A., 2005. C.
elegans tubby regulates life span and fat storage by two independent
mechanisms. Cell Metab. 2, 35–42.
Murphy, C.T., McCarroll, S.A., Bargmann, C.I., Fraser, A., Kamath, R.S.,
Ahringer, J., Li, H., Kenyon, C., 2003. Genes that act downstream of DAF-
16 to influence the lifespan of Caenorhabditis elegans. Nature 424, 277–
283.
Ogg, S., Ruvkun, G., 1998. The C. elegans PTEN homolog, DAF-18, acts in the
insulin receptor-like metabolic signaling pathway. Mol. Cell 2, 887–893.
Ogg, S., Paradis, S., Gottlieb, S., Patterson, G.I., Lee, L.,
Tissenbaum, H.A., Ruvkun, G., 1997. The Fork head transcription
factor DAF-16 transduces insulin-like metabolic and longevity signals
in C. elegans. Nature 389, 994–999.
Oh, S.W., Mukhopadhyay, A., Svrzikapa, N., Jiang, F., Davis, R.J.,
Tissenbaum, H.A., 2005. JNK regulates lifespan in Caenorhabditis elegans
by modulating nuclear translocation of forkhead transcription factor/DAF-
16. Proc. Natl Acad. Sci. USA 102, 4494–4499.
Paradis, S., Ruvkun, G., 1998. Caenorhabditis elegans Akt/PKB transduces
insulin receptor-like signals from AGE-1 PI3 kinase to the DAF-16
transcription factor. Genes Dev. 12, 2488–2498.
Raices, M., Maruyama, H., Dillin, A., Karlseder, J., 2005. Uncoupling of
longevity and telomere length in C. elegans. Plos. Genet. 1, e30.
Rogina, B., Helfand, S.L., 2004. Sir2 mediates longevity in the fly through a
pathway related to calorie restriction. Proc. Natl Acad. Sci. USA 101,
15998–16003.
Schmelzle, T., Hall, M.N., 2000. TOR, a central controller of cell growth. Cell
103, 253–262.
Shibata, Y., Branicky, R., Landaverde, I.O., Hekimi, S., 2003. Redox
regulation of germline and vulval development in Caenorhabditis elegans.
Science 302, 1779–1782.
Tissenbaum, H.A., Guarente, L., 2001. Increased dosage of a sir-2 gene extends
lifespan in Caenorhabditis elegans. Nature 410, 227–230.
Vellai, T., Takacs-Vellai, K., Zhang, Y., Kovacs, A.L., Orosz, L., Muller, F.,
2003. Genetics: influence of TOR kinase on lifespan in C. elegans. Nature
426, 620.
Viswanathan, M., Kim, S.K., Berdichevsky, A., Guarente, L., 2005. A role for
SIR-2.1 regulation of ER stress response genes in determining C. elegans
life span. Dev. Cell 9, 605–615.
Wang, Y., Tissenbaum, H.A., 2006. Overlapping and distinct functions for a
Caenorhabditis elegans SIR2 and DAF-16/FOXO. Mech. Ageing Dev.
127, 48–56.
Walker, G., Houthoofd, K., Vanfleteren, J.R., Gems, D., 2005. Dietary
restriction in C. elegans: from rate-of-living effects to nutrient sensing
pathways. Mech. Ageing Dev. 126, 929–937.
Wang,M.C., Bohmann, D., Jasper, H., 2003. JNK signaling confers tolerance to
oxidative stress and extends lifespan in Drosophila. Dev. Cell 5, 811–816.
Wang, M.C., Bohmann, D., Jasper, H., 2005. JNK extends life span and limits
growth by antagonizing cellular and organism-wide responses to insulin
signaling. Cell 121, 115–125.
Wolkow, C.A., Munoz, M.J., Riddle, D.L., Ruvkun, G., 2002. Insulin receptor
substrate and p55 orthologous adaptor proteins function in the
Caenorhabditis elegans daf-2/insulin-like signaling pathway. J. Biol.
Chem. 277, 49591–49597.
Wook Oh, S., Mukhopadhyay, A., Dixit, B.L., Raha, T., Green, M.R.,
Tissenbaum, H.A., 2005. Identification of direct DAF-16 targets controlling
longevity, metabolism and diapause by chromatin immunoprecipitation.
Nat. Genet. 38, 251–257.