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LETTER TO THE EDITOR
Hypertrophy, replicative ageing and the ageing process
YEAST RESEARCH
DOI: 10.1111/j.1567-1364.2012.00843.x
Final version published online 14 September 2012.
Two Letters to the Editor (Ganley et al., 2012; Kaeberlein,
2012) have been published as a reaction to our commentary entitled ‘Hypertrophy hypothesis as an alternative
explanation of the phenomenon of replicative ageing of
yeast’ (Bilinski et al., 2012). Our response to those letters,
because of its limited length will concentrate only on the
most important issues raised.
The main experimental argument against the hypertrophy hypothesis was that ‘yeast mothers stop replicating
with a range of sizes: enlarged, but not uniformly so’.
This statement confirms that hypertrophy (enlargement)
is observed also in other strains. Uniformity is rather rare
in biology. The heterogeneity of the volume at which
replication stops is a simple consequence of the molecular
noise, important especially at the level of regulatory
molecules whose copy number in a cell is very low (Di
Talia et al., 2007; Frigola et al., 2012). The critical events
in cell division are regulated by such molecules.
The main point of controversy is not hypertrophy
itself, but its origin. In one of the replies, it was stated
‘there is an implicit (and as yet untested) assumption:
hypertrophy results from increasing cell cycle arrest’
(Ganley et al., 2012). However, even if cells are not
arrested, they also stop reproduction after attaining the
same maximal volume, like those arrested. Unfortunately,
these authors did not explain why they consider increasing cell cycle arrest as an element of the ageing process.
In the other reply, we find ‘one obvious hypothesis is that
one or more of these senescence factors cause hypertrophy (Kaeberlein, 2012), but no mechanism leading to
hypertrophy was suggested’. However, we have postulated
that the existence of reproduction limit in the budding
yeast cells is a consequence of hypertrophy. The reason
for hypertrophy is obvious and results from the choice of
budding as a mechanism of reproduction. Our reasoning
is as follows: Budding yeast cells increase their size during
each cell cycle (Mortimer & Johnston, 1959; Zadrag et al.,
2005; Zadrag-Tecza et al., 2009). The G1 phase, during
which the growth of the mother cell mainly takes place
(Hartwell & Unger, 1977), cannot be omitted because of
its multiple regulatory roles. Equally important is that
there is no reduction in cell volume during reproduction
by budding. In contrast, cells which reproduce by binary
fission reduce their volume during each cell division.
FEMS Yeast Res 12 (2012) 739–740
Therefore, hypertrophy is a direct and unavoidable
consequence of budding and as such, is a primary
phenomenon, not secondary to ageing. Thus, in our
opinion, there is no need for search of the molecular
mechanisms of hypertrophy, while it is necessary to elucidate molecular mechanisms by which hypertrophy arrests
cell cycle.
It was argued (Kaeberlein, 2012) that ‘if cell size is sole
limiting factor for yeast replicative life span (RLS), then
the mutations that increase RLS should always be accompanied by either a reduced mother cell size or altered
kinetics of enlargement during replicative ageing’. Our
hypothesis assumes that RLS depends on three, instead of
two independent factors mentioned: threshold size to
enter the S phase (which is most probably independent of
the initial mother cell size), maximal size and the rate of
volume increase per generation. RLS; therefore, is a resultant of three factors, each of which can be changed by
mutations.
Hypertrophy hypothesis explains differences in reproductive capacity of all tested strains, irrespective of their
origin and character. For example, cells of the mutant rad
52 stop reproducing after much smaller number of
buddings than cells of the parent strain (Kaeberlein,
2012), but both strains stop reproduction after attaining
the same maximal volume (R. Zadrag-Tecza, A. Skoneczna, M. Skoneczny, in preparation). The ad hoc
hypothesis on two independent mechanisms ‘of ageing
for short-lived and wild-type individuals’ (Delaney et al.,
2011; Kaeberlein, 2012) is thus fully redundant.
It was stated that our model is purely correlative. In no
case, the content of the presumable senescence factor was
measured in cells that have already stopped reproduction.
In contrast, we are recording changes in volume during
each cell’s entire life, and measure it in arrested cells, to
establish their final volume.
The next argument against our hypothesis is that our
‘model lacks mechanistic insight into the molecular
processes that cause replicative ageing’ (Kaeberlein, 2012),
or ‘it is not biochemically tractable’ (Ganley et al., 2012).
We have already mentioned that the mechanism by which
hypertrophy influences reproductive capacity of cells is so
far unknown. Its nature could be either physical or
molecular. Our articles (Bilinski & Bartosz, 2006; Zadragª 2012 Federation of European Microbiological Societies
Published by Blackwell Publishing Ltd. All rights reserved
740
Tecza et al., 2009) inspired another group to postulate
that ageing is a consequence of ‘increased intracellular
water volume and density’ (Bonatto et al., 2011). It suggests new biophysical mechanism of the phenomenon.
We believe, however, that up to 10-fold increase of the
cellular volume raises demand for cell maintenance
processes, including protein turnover. In our opinion, it
is possible that hypertrophy lowers the effective concentration of some crucial regulatory protein molecules,
whose number and stability are very low. Simultaneously,
their diffusion to and from the nucleus could be less
effective in oversized cells. Therefore, we postulate that
the limit of reproductive capacity is caused by a deficiency of some factor(s) in hypertrophic cells. Our postulate is thus biochemically tractable. We do not question
any accumulation of some metabolic byproducts within
the mother cell, but only the effects of this accumulation.
Ganley et al. (2012) declare ‘We believe that yeast is a
valuable model system for ageing and will continue to
contribute to our understanding of ageing generally’.
We think that this (as any) belief should be treated
with caution, according to the basic principles of the
methodology of science. If the ageing of Saccharomyces
cerevisiae is because of a private rather than public mechanism of ageing (as we postulate), its studies may
broaden our understanding of the biology of ageing but
may have limited relevance to ageing of higher eukaryotes. Our hypothesis provides a new explanation why
yeast cells have limited reproductive potential. Further
studies are indeed necessary to clarify whether this potential is causally connected with the ageing process.
Letter to the Editor
Bilinski T, Zadrag-Tecza R & Bartosz G (2012) Hypertrophy
hypothesis as an alternative explanation of the phenomenon
of replicative aging of yeast. FEMS Yeast Res 12: 97–101.
Bonatto D, Feltes BC & Poloni JdF (2011) Aging as a
consequence of intracellular water volume and density. Med
Hypotheses 77: 982–984.
Delaney JR, Sutphin GL, Dulken B et al. (2011) Sir2 deletion
prevents lifespan extension in 32 long-lived mutants. Aging
Cell 10: 1089–1091.
Di Talia S, Skotheim JM, Bean JM, Siggia ED & Cross FR
(2007) The effects of molecular noise and size control on
variability in the budding yeast cell cycle. Nature 448:
947–951.
Frigola D, Casanellas L, Sancho JM & Ibanes M (2012)
Asymmetric stochastic switching driven by intrinsic
molecular noise. PLoS ONE 7(2): e31407.
Ganley ARD, Breitenbach M, Kennedy BK & Kobayashi T
(2012) Yeast hypertrophy: cause or consequence of aging?
Reply to Bilinski et al. FEMS Yeast Res 12: 267–268.
Hartwell LH & Unger MW (1977) Unequal division in
saccharomyces-cerevisiae and its implications for control of
cell-division. J Cell Biol 75: 422–435.
Kaeberlein M (2012) Hypertrophy and senescence factors in
yeast aging. A reply to Bilinski et al.. FEMS Yeast Res 12:
269–270.
Mortimer RK & Johnston JR (1959) Life span of individual
yeast cells. Nature 183: 1751–1752.
Zadrag R, Bartosz G & Bilinski T (2005) Replicative aging of
the yeast does not require DNA replication. Biochem Biophys
Res Commun 333: 138–141.
Zadrag-Tecza R, Kwolek-Mirek M, Bartosz G & Bilinski T
(2009) Cell volume as a factor limiting the replicative
lifespan of the yeast Saccharomyces cerevisiae. Biogerontology
10: 481–488.
Acknowledgement
The experiments, the conclusions of which were presented, were supported by a Grant No. N N303 376436
from the Polish National Science Center.
Tomasz Bilinski
Department of Biochemistry and Cell Biology
University of Rzeszow
Rzeszow, Poland
E-mail: [email protected]
References
Bilinski T & Bartosz G (2006) Hypothesis: cell volume limits
cell divisions. Acta Biochim Pol 53: 833–835.
ª 2012 Federation of European Microbiological Societies
Published by Blackwell Publishing Ltd. All rights reserved
FEMS Yeast Res 12 (2012) 739–740