Neurobiology of Aging 20 (1999) 479 – 486

Mutations in signal transduction proteins increase stress resistance and
longevity in yeast, nematodes, fruit flies, and mammalian neuronal cells
Valter D. Longo*,1
Division of Neurogerontology Andrus Gerontology Center and Department of Biological Sciences, University of Southern California, 3715 McClintock
Avenue, Los Angeles, CA 90089-0191, USA
Received 11 May 1999; received in revised form 25 August 1999; accepted 3 September 1999

Abstract
Mutations in Ras and other signal transduction proteins increase survival and resistance to oxidative stress and starvation in stationary
phase yeast, nematodes, fruit flies, and in neuronal PC12 cells. The chronological life span of yeast, based on the survival of nondividing
cells in stationary phase, has allowed the identification and characterization of long-lived strains with mutations in the G-protein Ras2. This
paradigm was also used to identify the in vivo sources and targets of reactive oxygen species and to examine the role of antioxidant enzymes
in the longevity of yeast. I will review this model system and discuss the striking phenotypic similarities between long-lived mutants ranging
from yeast to mammalian neuronal cells. Taken together, the published studies suggest that survival may be regulated by similar
fundamental mechanisms in many eukaryotes and that simple model systems will contribute to our understanding of the aging process in
mammals. © 1999 Elsevier Science Inc. All rights reserved.
Keywords: Aging; Yeast; Neuron; Ras; SOD; Oxidative damage; Stationary phase; G-protein

1. Introduction
Oxidative damage to macromolecules has been implicated in aging and certain aging-related diseases [1,40,56]
and is believed to result from stochastic microenvironmental fluctuations in the balance between oxidants, such as
O2Ϫ, H2O2, and ⅐OH, and antioxidants, including superoxide dismutases, peroxidases, and glutathione. However, the
demonstrated ability of a single protein, such as Ras, to
regulate the generation of reactive oxygen species, antioxidant defenses, and cell death in mammalian cells [11,44,61]
raises the possibility that oxidative damage and aging may
be regulated by a limited number of genes.
Caenorhabditis elegans (nematode), Drosophila (fruit
fly), and mice are the three main model systems that are
being genetically manipulated to experimentally address
this topic [7]. Saccharomyces cerevisiae (yeast), thanks to
straightforward genetic techniques and to the wealth of
information available at the biochemical, molecular, and
* Corresponding author. Tel.: ϩ1-213-740-4915; fax: ϩ1-213-7400853.
E-mail address: vlongo@usc.edu (V.D. Longo)
1
VDL is a John Douglas French Alzheimer’s Foundation fellow.

cellular level, is emerging as a novel and powerful model
system to study the genetics of aging [14,22,35].

2. Budding life span and stationary phase
Yeast is a simple, unicellular, eukaryote for which extensive genetic and molecular biology are known. The entire
genome has been completely sequenced and contains 5885
potential genes [9,71]. The similarities of a large number
of signal transduction and other housekeeping proteins between yeast and humans have enhanced our understanding
of human systems, thanks in part to the ability of mammalian proteins to functionally substitute for their yeast analogs. Examples include the antioxidant superoxide dismutase [35], Ras [16], and heat shock proteins [45]. In
contrast to mammalian systems, the simplicity of genetic
manipulations in yeast allows the removal or over expression of one or multiple genes to study the function of a
particular protein [15]. In addition, this small eukaryote can
be grown to large, stationary-phase populations of billions
of organisms that can be used to screen for longevity mutations or to identify novel genes involved in the long-term
resistance to insults, such as oxidative and thermal stress.

0197-4580/99/$ – see front matter © 1999 Elsevier Science Inc. All rights reserved.
PII: S 0 1 9 7 - 4 5 8 0 ( 9 9 ) 0 0 0 8 9 - 5

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V.D. Longo / Neurobiology of Aging 20 (1999) 479 – 486

Fig. 1. The chronological life span of yeast. Typical survival curves for yeast stationary phase populations maintained in water or in expired medium. Cells
are normally inoculated at a density of 1 ϫ 106 cells/mL and grown in complete minimal medium at 30°C in a shaking incubator. After reaching a density
of 2– 4 ϫ 107 cells/mL, external nutrients become scarce, growth slows down, and the population begins to store glycogen and other nutrients. By Day 4
(in strain EG103) cells enter the nondividing stationary phase characterized by decreased metabolic rates and increased protection against heat and oxidative
stress. Mean survival, depending on the strain and the incubation medium (water vs. expired medium), ranges from 8 to 50 days. Viability in the culture is
measured every 24 or 48 h by serially diluting aliquots of the population and plating onto rich medium plates. The ability of each live cell to form a colony
is used to estimate viability in the culture.

Aging in yeast is often measured by counting the finite
number of buds that can be generated by a single mother
cell maintained in the growth phase (budding life span) [48].
Published studies that used this paradigm suggest that replicative aging is caused by the accumulation of rDNA circles resulting in nucleolar fragmentation [59]. During the
budding life span, each daughter cell must be separated
from the mother cell by micromanipulation to prevent overcrowding and entry of the population in stationary phase
[69]. In this nondividing phase, the cells can survive for
long periods without dividing. Although the relationship
between the budding life span and stationary phase survival
is not clear [58], a recent study demonstrated that yeast cells
maintained in stationary phase show a decrease in replicative life span after reentering the cell cycle [2]. The mean
replicative life span was 27 after 1 day, 20 after 13 days, and
16 after 33 days in stationary phase, suggesting that nondividing yeast undergo senescence. Furthermore, one of the
mutants with increased budding life span, sir4 – 42, was
isolated by screening for cells able to survive longer than
the wild-type in stationary phase [26]. However, other studies demonstrate that the over expression of the G-protein
Ras2, which causes rapid death in stationary phase, increases the budding life span [62], whereas the deletion of
Ras2, which doubles stationary phase survival (Longo et al.,
unpublished results), decreases budding life span [62]. The
effect of Ras2 on budding life span was proposed to involve
retrograde regulation; a regulation of signaling from the
mitochondrion to the nucleus [29]. In summary, although
some of the genes and mechanisms that regulate budding
potential can also affect survival in stationary phase, further
studies are necessary to clarify the relationship between
these two paradigms.
Stationary phase is relatively well-understood from de-

cades of study [70]. As the level of external nutrients decreases, yeast cells store glycogen and other nutrients intracellularly, decrease metabolic rates and protein synthesis,
and develop increased thermotolerance and antioxidant defenses [70]. Cells can survive in stationary phase for days to
months depending on the strain and on the incubation medium (Fig. 1) and can, under very low nutrient conditions,
undergo meiosis and form spores able to survive for months
to years, with metabolic rates lower than those of stationary
phase cells [4].
Although the majority of multicellular eukaryotes, including the ones that can enter a hypometabolic state, such
as nematodes, will spend most of their adult life with normal
metabolic rates, yeast, however, either sporulate or unavoidably survive in the low metabolism stationary phase. In fact,
“much of the microorganismal mass in the world is estimated to exist under nutrient-depleted conditions” [69].
Thus, it may be more correct to view the short growth phase
of yeast as a hypermetabolic state aimed at quickly generating a large population and the long stationary phase as the
normal metabolic state for long-term survival during which
internal nutrient reserves are used slowly. The very low
metabolism spore state, entered only by a minority of cells
under extreme nutrient conditions, may be viewed as a true
hypometabolic state, analogous to the nematode dauer larva
(as recently pointed out by Kenyon and colleagues [5]).

3. Chronological life span of yeast
Most studies of yeast are performed by using logarithmically growing cells. However, the growth phase is not
suited to study the accumulation of oxidative and other
forms of macromolecular damage because individual cells

V.D. Longo / Neurobiology of Aging 20 (1999) 479 – 486

can only be exposed to a short period of stress, and damage
is rapidly diluted by the synthesis of new macromolecules
required for rapid growth. In fact, yeast cells grow well even
in the presence of elevated concentrations of O2Ϫ and H2O2
[35,36]. By contrast, the long-term survival of cells maintained in stationary phase can serve as a valuable system to
monitor long-term macromolecular damage and mortality
[34 –36,68]. This paradigm was termed the chronological
life span to distinguish it from the budding life span. In most
of the aging studies performed that use the chronological
life span, cells are either grown and maintained throughout
the study in minimal glucose medium, or they are transferred to water on Day 3 and washed every 48 h (Fig. 1)
[13,35]. The cultures, grown in shaking flasks maintained at
30°C, normally reach a density of ϳ100 million cells/mL
and, depending on the strain and the incubation medium,
have a mean survival of 8 –50 days (Fig. 1) [34,35].
These paradigms have allowed the simulation of two
conditions commonly encountered by stationary phase yeast
in the wild: 1) an environment with limited nutrient resources (expired medium) that causes a rapid increase in
mortality after Day 5 (strain EG103) and that allows some
growth to occur after the majority of cells are dead (usually
Ͼ99.9%), and 2) an environment with low or no nutrients
(water), that does not allow growth to occur, in which cells
survive for extended time. Notably, the mechanisms that
regulate survival under these environments appear to be
similar because strains with increased survival in expired
medium also survive longer in water.

4. Oxidative damage and longevity in yeast
Bacteria and yeast have been used for many years as
simple model systems to study the function of antioxidant
enzymes and to identify the sources of reactive oxygen
species. Similarly to human cells, S. cerevisiae expresses a
cytosolic CuZn superoxide dismutase (SOD1) and catalase
(CTT1) as well as a mitochondrial Mn SOD (SOD2). The
first report on the yeast chronological life span model system showed that cytoplasmic and mitochondrial superoxide
dismutases, but not catalase or metallothionein, are required
for the long-term survival of yeast. Sod2 was found to be
required under both low and normal oxygen conditions,
whereas cytoplasmic Sod1 was mainly required under normal aeration [35]. The expression of human SOD1 in yeast
sod1 null mutants completely reversed the survival defects.
These results are consistent with studies performed with the
use of mice lacking sod1. Although sod1 knockout mice
showed few abnormalities, cultured fibroblast obtained
from these mice were 80 times more sensitive to the superoxide generator paraquat than were wild-type cells. Furthermore these fibroblasts grew poorly in air [19], indicating
that, as reported for yeast, Sod1 is only required under high
concentration of oxygen or superoxide. The exposure of
cultured cells to air, and therefore to a concentration of

481

Table 1
Activity of mitochondrial enzymes in yeast and mice lacking Sod2
Yeasta (Days 1–3)
Survival
Aconitase
Succinate dehydrogenase
Cyt c ox.
ATPase

Miceb (Days 4–6)

5–10 days
2 67–96%
2 52–84%
2 22–40%c
2 13–26%c

Ͻ10 days
2 67–89%
2 65–76%
No change
ND

a

See reference 36.
See reference 43.
c
No change when adjusted for viability in addition to protein concentration.
b

oxygen at least fourfold higher than that in the brain of live
animals, may explain why sod1 animals show few abnormalities in vivo, whereas cultured fibroblasts obtained from
these animals have severe defects.
Unlike most experimental organisms, yeast have the ability to grow either by respiration by using nonfermentable
carbon sources or exclusively by fermentation by using
glucose (low respiration). This simple feature, termed index
of respiratory competence (IRC), is very useful when designing experiments to separate mitochondrial from extramitochondrial damage. Using the IRC, Longo et al. [36]
were able to define the sequence of events that lead to the
death of sod2 mutants and were able to explore the role of
mitochondrial damage in the mortality of wild-type yeast in
stationary phase. The characterization of yeast sod2 null
mutants resulted in the identification of mitochondrial aconitase and succinate dehydrogenase, both 4Fe-4S cluster
binding proteins, as the primary targets of mitochondrial
superoxide [36]. These results are consistent with studies in
sod2 knockout mice in which the activity of mitochondrial
aconitase was reduced by Ͼ67% in the heart and brain, and
succinate dehydrogenase was reduced by Ͼ65% in the heart
and skeletal muscle (Table 1). As expected, in sod2 mice the
cells most severely affected by the increased concentration
of mitochondrial superoxide were postmitotic. The similarities between stationary phase yeast and mice lacking either
SOD1 or SOD2 suggest that the chronological life span of
yeast is a valuable model system for the study of mechanisms of oxidative damage in mammalian cells, particularly
postmitotic cells.
The over expression of CuZn SOD has been convincingly demonstrated to increase the life span of fruit flies. Orr
et al. [51] showed that flies with extra copies of both CuZn
SOD and cytosolic catalase, but neither gene alone, survived 30% longer than controls. However, more recent
studies showed that the over expression of CuZn SOD alone
is sufficient to extend the life span of fruit flies. The over
expression of CuZn SOD in only the motor neurons extends
life span by up to 40% [52]. A similar extension in longevity
was obtained when the over expression of CuZn SOD
alone was induced in adult flies by using the yeast recombinase system [63]. To test whether the over expression
of antioxidant enzymes could also extend the life span

482

V.D. Longo / Neurobiology of Aging 20 (1999) 479 – 486

Table 2
Signal transduction mutations, stress resistance and longevity in eukaryotes
Gene(s)
S. cerevisiae
(chronological life span)
ras2⌬a
1 SOD1-SOD2a
C. elegans
age-1a
daf-2a
Drosophila
Mtha
1 SOD1a
Neuronal cells (PC12)
p21RASa
a

Protein function

G-protein
Antioxidants

Life span
(% increase)

Increased
resistance to

100%
10–33%

Oxidants, heat, starvation
Oxidants

65%
100%

Oxidants, heat, starvation
Oxidants, heat, starvation

G-protein coupled rec.
Antioxidant

35%
30–40%

Oxidants, heat, starvation
Oxidants

G-protein

Ͼ100%

Oxidants, serum withdrawal

PI3K
Insulin receptor-like

See text.

of yeast, Longo et al. constructed yeast strains expressing
several-fold higher concentrations of these antioxidant enzymes. In yeast, the over expression of SOD1 and SOD2
together increased viability in stationary phase (Longo et
al., unpublished results). The over expression of each Sod
alone had a significant, but modest, effect on survival.
Yeast has been used for many years as a model system
to understand the function and mechanism of action of
human proteins. The anti-apoptotic human Bcl-2 protein
was over expressed in yeast sod mutants and in wild-type
cells to investigate its mechanism of action and to understand whether yeast may have components of a programmed
cell-death pathway [34]. The results suggested that human
Bcl-2 partially reversed several defects of yeast that lacked
sod1 or both sod1 and sod2, which is consistent with the
demonstrated antioxidant function of Bcl-2 in mammalian
cells [24]. Bcl-2 over expression increased the long-term
viability and growth in 100% oxygen of sod1 and sod1sod2
mutants [34]. Bcl-2 also delayed the death of wild-type cells
by ϳ2 days in stationary phase, raising the possibility that
portions of an apoptotic pathway are present in yeast. This
hypothesis was recently supported by studies that show that
the expression of the human pro-apoptotic protein Bax
causes cell death in yeast, which can be reversed by coexpression of Bcl-2 [42,57]. Furthermore, the depletion of
glutathione or the treatment of yeast cells with hydrogen
peroxide was shown to induce a form of apoptosis that was
blocked by inhibiting protein synthesis and by incubating
the cells under low oxygen [37].

5. Control of survival by signal transduction proteins
in nondividing cells
Central signal transduction proteins, by virtue of controlling a wide range of cellular functions, are ideal candidates
as regulators of survival in simple eukaryotes. Mutations in
signal transduction proteins were found to increase survival

in C. elegans [23,27,28,47] and in Drosophila [32]. A
mutation that increases the chronological life span, as well
as the thermotolerance and antioxidant defenses, was recently identified in yeast (Longo et al., unpublished results).
This work suggests that the G-protein Ras2, which is highly
conserved in many organisms and functionally interchangeable between yeast and human cells, decreases stress protection and survival in yeast (Table 2). In two strains lacking
the ras2 gene, mean survival increased by Ͼ100% compared to wild-type. In both strains, ras2 null mutations
caused increased resistance to superoxide toxicity. Although over expressing various combinations of antioxidant
enzymes increased survival, the effect was much smaller
compared to that observed in ras2, indicating that changes
in multiple systems are necessary to achieve a major extension in longevity.
The Ras2/protein kinase A (PKA) pathway (Fig. 2) negatively regulates a number of genes that contain the stress
response element (STRE) in their promoters [38,55]
through its action on transcription factors Msn2 and Msn4
[12,60]. Among the genes reported to be regulated in this
manner are those for several heat shock proteins, catalase
(CTT1), polyubiquitin (UBI4) and the DNA damage-inducible gene DDR2 [41]. The CuZn SOD promoter also contains a potential STRE sequence. In the yeast strain SP1, the
deletion of ras2 caused a twofold increase in SOD activity
and increased resistance to the superoxide-generating agent
paraquat (Longo et al., unpublished results). ras2 mutations
were also shown to double the expression of SOD2 in the
yeast strain JC482 [8]. Over expression of Msn2 and Msn4
was shown to increase survival, in stationary phase, and
thermotolerance [41]. These results are consistent with the
data obtained by using ras2 mutants (Longo et al., unpublished results), and indicate roles for Msn2, Msn4, and
STRE response elements in life span regulation in the pathway downstream of Ras2. Taken together, these studies
suggest that, in the presence of all essential nutrients, yeast
Ras2 promotes growth, while downregulating the expres-

V.D. Longo / Neurobiology of Aging 20 (1999) 479 – 486

483

Fig. 2. The Ras pathway, protection, survival, and death in S. cerevisiae. In the presence of the nutrients necessary for growth, in wild-type yeast (RAS2)
the activation of both Ras1 and Ras2 in turn increases cyclic AMP generation [66] and activates protein kinase A [65] as well as a cyclic AMP-independent
pathway [49,53]. Activation of Ras promotes growth when all the required nutrients are available, but causes loss of viability when essential nutrients are
absent [66]. ras2 null mutations (ras2⌬) increase glycogen accumulation [64], SOD activity (Longo et al., unpublished results; [8]), catalase activity [55],
and thermotolerance [38], and increase chronological survival by twofold (see text). Metabolic rates are similar to that of wild-type before and after the entry
into the hypometabolic state. However, in ras2⌬ mutants the entry into the hypometabolic state occurs 1–2 days earlier than in wild-type (Longo et al.,
unpublished results). In the presence of all essential nutrients, the activity of Ras1 is sufficient for growth in ras2⌬ mutants [25]. The Ras2/PKA-dependent
regulation of stress response and glycogen accumulation is mediated by transcription factors Msn2 and Msn4 [12,60] through the induction of genes that
contain a response element (STRE) in their promoter, such as SOD, catalase, and several heat shock genes (see text). The life-extension effect of the double
over expression of Sod1 and Sod2 suggest that physiological concentrations of cytoplasmic and mitochondrial O2Ϫ act synergistically to decrease survival
in wild-type strains (Longo et al., unpublished results). The increased resistance to superoxide toxicity and the induction of multiple protection systems
through STREs are likely to mediate the extension of survival in ras2 mutants (see text). It is not known whether in addition to downregulating antioxidant
enzymes, Ras2 activity causes an increased generation of superoxide, as shown in mammalian neuronal PC12 and other cells (see text). Dashed lines indicate
possible functions; solid lines indicate known functions (see text for details and further references).

sion of antioxidants and heat shock proteins (Fig. 2). When
essential nutrient are missing, Ras2 promotes the loss of
viability that appears to be mediated in part by superoxide
(Longo et al., unpublished results; [66]). It is not clear
whether superoxide causes death in yeast only by damaging
macromolecules directly, and through the generation of the
highly reactive hydroxyl radical, or by acting as a signal
transduction messenger that promotes death. Recent results
suggest that activation of Ras increases superoxide generation, which in turn functions as a mitogenic signaling molecule in fibroblasts and PC12 cells [20,21,44]. Although a
signaling role for superoxide has not been demonstrated in
yeast, we cannot exclude that superoxide may act both as a
toxic oxygen species that causes direct macromolecular
damage and as a signal transduction messenger that promotes either apoptosis or necrosis.
There are many phenotypic similarities between long-

lived yeast ras2 mutants and long-lived mutants of certain
higher eukaryotes (Table 2). C. elegans age-1 and daf-2
mutations, which have been shown to increase the life span
in adult organisms by 65–100%, occur in signal transduction genes that are involved in regulating the formation of
the hypometabolic, long-lived dauer larva [23,27,28]. Analogously to ras2 mutations in yeast, these longevity mutations in nematodes also cause increased accumulation of
nutrients, thermotolerance, and antioxidant defense [28,30,
33,67]. Recently, the over expression of SOD1 was shown
to increase survival in Drosophila by up to 40% [51,52,63],
and a Drosophila line with a mutation in the G-protein
coupled receptor homolog MTH gene displayed a 35% increase in life span [32]. Drosophila mth mutants are resistant to starvation and superoxide toxicity. The striking similarities between long-lived mutants of organisms as
phylogenetically distant as yeast, nematodes, and fruit flies

484

V.D. Longo / Neurobiology of Aging 20 (1999) 479 – 486

suggest that longevity may be regulated by similar mechanisms in many eukaryotes (Table 2).

6. Do signal transduction proteins regulate survival in
mammals?
The phenotypic similarities between long-lived eukaryotic mutants (Table 2) raises the possibility that analogous
mutations may also affect the survival of mammals. Interestingly, two of the yeast genes found to have the most
profound effect on survival, SOD1 and RAS2, have DNA
sequences that are Ͼ60% identical to their mammalian
homologs and can be functionally substituted by them [35,
46]. Although, the possibility that mutations in G-proteins
affect mammalian longevity has not been addressed at the
organismal level, several studies performed with the use of
mammalian cells suggest that Ras activity increases the
generation of superoxide and decreases survival in neurons.
Neuronal apoptosis increases in mice that lack a negative
regulator of Ras [18], whereas inhibition of p21Ras rescues
naive and neuronally differentiated PC12 cells from apoptotic death (Table 2) [6]. Analogously to yeast lacking
Ras2, the inhibition of p21Ras in PC12 cells increases
resistance to oxidative stress [61] and survival after serum
withdrawal [6]. Inhibition of p21Ras in PC12 cells also
prevents superoxide generation induced by treatment with
epidermal growth factor (EGF) [44]. Ras also mediates
apoptosis in T cells [10] and in human epithelial cells [17]
and induces replicative senescence in human diploid fibroblasts by increasing intracellular levels of reactive oxygen
species [31], suggesting that its ability to cause cell death is
not limited to neuronal cells.
However, Ras activation was also shown to prevent cell
death in rat sympathetic neurons [50] and in endothelial
cells [54]. These apparently contradictory roles of Ras in the
induction and prevention of cell death are consistent with
multiple functions of mammalian Ras. Activation of Ras
may cause or prevent cell death depending on the activity of
other signal transduction pathways and on the relative activity of proteins acting downstream of Ras, such as MAP
kinase (MAPK), Rac1, and protein kinase B, which have
been implicated in both pro-apoptotic and anti-apoptotic
signaling [3,11,39,72].

7. Conclusion
The involvement of signal transduction proteins that affect longevity in the regulation of thermotolerance, resistance to oxidative stress, and accumulation of reserve nutrients in yeast, nematodes, and flies suggest that longevity
is regulated, or at least strongly influenced, by similar mechanism in many eukaryotes. It would be very surprising if the
increased resistance to starvation heat and oxidative stress
shared by all the long-lived mutants (Table 2) was just a

coincidence. In yeast, the Ras pathway plays a central role
in cell growth, but decreases survival in nondividing organisms. In neurons, Ras functions in a pathway that mediates
cell growth and differentiation, but can induce cell death.
Although Ras function and apoptosis have not been demonstrated to decrease longevity in mammals, the disruption
of the delicate balance between multiple-signal transduction
pathways may be responsible for the age-dependent increase in oxidative damage, loss of certain cellular functions, and death of postmitotic cells. Thus the central role of
Ras and other signal transduction proteins in cellular functions ranging from growth, to differentiation, to death
makes these signaling proteins ideal candidates as longevity
assurance genes. Further biochemical and genetic studies in
simple and rodent model systems should soon reveal the
extent of the conservation of the role of signal transductiondependent regulation of multiple protection systems in the
longevity of eukaryotes.
Acknowledgments
I thank Dr. Edith B. Gralla and Dr. Joan S. Valentine in
whose laboratory I performed the majority of the yeast
studies presented in this review. I thank Dr. Caleb E. Finch
and Dr. Edith B. Gralla for their suggestions and careful
review of the manuscript. I thank Mr. John H. Martin and
Mr. George W. Fenimore for their generous donations.
Supported by grant A608761 (VDL) and by an AFAR
research grant.
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