recent aging research in caenorhabditis elegans

7
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.), Germany b 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 (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 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).

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Page 1: Recent aging research in Caenorhabditis elegans

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

Page 2: Recent aging research in Caenorhabditis elegans

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).

Page 3: Recent aging research in Caenorhabditis elegans

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.

Page 4: Recent aging research in Caenorhabditis elegans

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

Page 5: Recent aging research in Caenorhabditis elegans

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

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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.

Page 7: Recent aging research in Caenorhabditis elegans

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.