issn 1389-5729, volume 11, number 4 2010... · genetic, epigenetic and posttranslational mechanisms...
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ISSN 1389-5729, Volume 11, Number 4
REVIEW ARTICLE
Genetic, epigenetic and posttranslational mechanismsof aging
L. Robert • J. Labat-Robert • A. M. Robert
Received: 14 September 2009 / Accepted: 15 January 2010 / Published online: 16 February 2010
� Springer Science+Business Media B.V. 2010
Abstract Gerontological experimentation is and
was always strongly influenced by ‘‘theories’’. The
early decades of molecular genetics inspired deter-
ministic thinking, based on the ‘‘Central Dogma’’
(DNA ? RNA ? Proteins). With the progress of
detailed knowledge of gene-function a much more
complicated picture emerged. Regulation of gene-
expression turned out to be a highly complicated
process. Experimental gerontology produced over the
last decades several ‘‘paradigms’’ incompatible with
simple genetic determinism. The increasing number
of such detailed experimental ‘‘facts’’ revealed the
importance of epigenetic factors and of posttransla-
tional modifications in the age-dependent decline of
physiological functions. We shall present in this
review a short but critical analysis of genetic and
epigenetic processes applied to the interpretation of
the more and more precisely elucidated experimental
paradigms of aging followed by some of the most
relevant aging-mechanisms at the post-translational
level, the posttranslational modifications of proteins
such as the Maillard reaction, the proteolytic pro-
duction of harmful peptides and the molecular
mechanisms of the aging of elastin with the role of
the age-dependent uncoupling of the elastin receptor,
as well as the loss of several other receptors. We
insist also on the well documented influence of
posttranslational modifications on gene expression
and on the role of non-coding RNA-s. Altogether,
these data replace the previous simplistic concepts on
gene action as related to aging by a much more
complicated picture, where epigenetic and posttrans-
lational processes together with environmentally
influenced genetic pathways play key-roles in aging
and strongly influence gene expression.
Keywords Aging � Genetics � Epigenetics �Posttranslational changes � Maillard reaction �Receptor-aging � Proteolysis � Fibronectin �mi-RNA-s � Elastin � Atherosclerosis
Abbreviations
JH Juvenile hormone
ECM Extracellular matrix
CR Calorie restriction
nc-RNA Non coding RNA-s
RNAi RNA-interference
mTOR Mammalian target of rapamycin
S6K1 A ribosomal S6-protein kinase
CNV Copy number variation
SNP Single nucleotide polymorphism
Corresponding to a lecture delivered at the Congress of the
International Association of Gerontology and Geriatrics
(IAGG) in Paris in July 2009.
L. Robert (&) � J. Labat-Robert � A. M. Robert
Laboratoire de Recherche Ophtalmologique, Hopital
Hotel Dieu, Universite Paris 5, 1 Place du Parvis Notre
Dame, 75181 Paris cedex 04, France
e-mail: [email protected]
123
Biogerontology (2010) 11:387–399
DOI 10.1007/s10522-010-9262-y
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Introduction
The elucidation of the structure and function of DNA,
the ‘‘double helix’’ resulted during the first decades
after 1953 in a rigid, deterministic thinking which
penetrated also experimental gerontology. See for
instance the chapters on developmentally ‘‘pro-
grammed’’ aging in Warner et al. (1987), and the
critical remarks of Hayflick (1987). The elucidation
of the genetic code, closely followed by the ‘‘Central
Dogma’’ (DNA ? RNA ? Protein) further rigidi-
fied by Beadle and Tatum’s ‘‘one gene—one
enzyme’’ thesis outlawed for several decades envi-
ronmental effects or any other mechanisms outside
direct gene-action. This limited knowledge of gene
action gave rise to the only acceptable theories of
aging, mutations, which at that time were recognised
as the major mechanism capable of changing nucle-
otide sequences and were proposed to be causally
involved in aging (Medawar 1952; Burnet 1974 and
Carnes et al. 2003 for a recent review). During the
later decades of the twentieth century several impor-
tant discoveries complicated progressively this pic-
ture: retrotranscription, alternative splicing and
finally RNA-interference definitely outlawed the
simplistic approach to gene-action, as first proposed
(for details on gene-action and inheritance see for
instance Levin 2008; Pierce 2008). Some decades
before the turn of the twentyfirst century epigenetics
invaded the arena. First proposed for the interpreta-
tion of evolutionary processes in the embryo by
Waddington (1968), it rapidly arose interest in
experimental biology outside the field of embryonic
development. The recent organisation of international
symposia and publication of treatises (see for instance
Stillmann and Stewart 2004; Allis et al. 2007) further
confirmed the recognition of the importance of
epigenetic mechanisms in a wide range of biological
and pathological processes, as for instance the
development of malignant tumors (see for instance
Verma et al. 2003). The most recent and rapidly
spreading field is RNA-interference which showed
that nearly all steps from gene-action to protein
production can be controlled by RNA-interference
(see for instance Grosshans and Slack 2002; Morris
2008a). Some of the experimental findings of geron-
tology further displaced the site of action, far from
the genes, to what has to be designated as
posttranslational processes. The first to be described
and the last to be interpreted in these terms is the
Maillard reaction (Ikan 1996; Baynes et al. 2005;
Robert 2009 for reviews). But most data published on
this topic concern ‘‘molecular aging’’, a designation
which concerns posttranslationally modified proteins
(Adelman and Roth 1983). This trend was initiated by
the discovery made by the Gershons demonstrating
inactive enzymes, modified proteins in aging cells
(Gershon and Rott 1988 for a review). Attributed first
to age-dependent increase in errors of aminoacid
incorporation in proteins (the Error-Catastrophe
Theory of Orgel), it was shown later convincingly
that the above modifications and inactivations are all
of a posttranslational nature. The accumulation of
such modified, inactive proteins could be attributed to
a slow-down of their turnover leaving time for
modifications not seen in more rapidly renewed
proteins. Several other age-related processes, as for
instance the proteolytic production of harmful
peptides (Labat-Robert 2002, 2003, 2004) and the
age-dependent loss of elastin’s elasticity with its
consequences on cardiovascular aging (Robert et al.
2008) are of more recent recognition and increase the
number of well documented posttranslational mech-
anisms of aging.
This succinct enumeration of some examples
which came to complicate the interpretation of
gene-action and aging shows clearly that this review
can not be exhaustive. We consider it as illustrative
of the evolution of genetic thinking as applied to
experimental gerontology over the last decades
around the turn of the century.
Genetic inheritance and aging
The Mendelian discovery on the inheritance of
relatively simple traits, its rapid complication by
recessive genes, transposons etc. could not be easily
applied to the field of aging. The first major obstacle
is a precise definition of aging. The still most used
definition is the endpoint, age at death. This actuarian
approach is still important but insufficient. It resulted,
however, in the first quantitative description of life
statistics by Gompertz, further refined by the first
generation of geriatricians (see for a review of early
applications Comfort 1979). This definition is, how-
ever, not satisfactory for studies on the genetic level,
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unless a convincing demonstration of relatively
simple transmission of life expectancy or age at
death can be achieved. This is not the case as best
exemplified by the identical twin studies which
reduced the role of direct genetic transmission of
age-limit to about 3%, from the previous estimation
of about 25% (Browner et al. 2004 for a review).
A further complication, still not resolved in a
satisfactory manner, is the relationship between aging
and disease. Dying of old age is still an ill-defined
concept (see for instance Carnes et al. 2008). The
strongest arguments against genetic transmission of
life expectancy is derived from evolutionary theory.
Evolutionary constraints are supposed to act essen-
tially on early reproduction and survival to reproduc-
tive age. Although the ‘‘grandmother effect’’ was
proposed to alleviate the rigor of this evolutionary
argument, at least for the human species, its selective
strength is hard to prove (Hawks 2003). The potential
role of individual differences, such as single nucleo-
tide polymorphisms (SNP-s), copy number variations
(CNV) and others as life course determinants is still in
its early beginnings. Remains as the only survival of
the evolutionary arguments proposed previously the
potential role of antagonistic pleiotropy (Williams
1957). But here again the few processes elucidated at
the cellular-molecular level which play an important
role in the decline of physiological functions is not in
favour of such mechanisms (Robert and Labat-Robert
2000; Robert and Miquel 2004; Robert et al. 2008).
This argument will be developed in the following
articles of this review.
All these critical remarks do not have to be pushed
too far, however, if one wants to give an objective
(although provisory) picture on genetics and aging.
Thanks to the rapid progress in molecular genetics
more and more genes were shown to be involved in the
regulation of physiological processes which decline
with age. The web-site on Human Aging Genomic
Resources (genomics.senescence.info) lists C261
genes identified as important in such processes,
derived from more than 2,461 references. The review
by Browner et al. (2004) on the Genetics of Human
Longevity proposes a list of seven classes of genes
involved in the most important regulatory processes
related to life support mechanisms (Table 1). The most
convincing arguments for genes involved in the
regulation of longevity are derived from genotypes
favouring life threatening diseases, in particular
cardiovascular diseases. This is the case for instance
for the e4/e4 genotype coding apolipoprotein E4, shown
to be accompanied by early onset athero-arterioscle-
rosis and also of neurodegenerative diseases, Alzhei-
mer’s disease and Creutzfeld–Jakob disease (Assmann
et al. 1984; Amouyel et al. 1994; Jacotot 1993).
Several other examples could be cited as for instance
the genetic regulation of a1-antiprotease expression as
related to early onset and severe emphysema (Robert
et al. 1980). These examples can, however, not be
considered as arguments in favour of the genetic
regulation of life expectancy. They belong to the still
open question of the relationship between aging and
age-related pathologies as mentioned before. In this
respect the proposition of Martin et al. (1996) is of
interest. He proposed to distinguish ‘‘public’’ and
‘‘private’’ genetic pathways regulating the aging
process of a variety of species. ‘‘Public’’ genetic
pathways concerne defence mechanisms against life-
threatening processes common to many if not all
species. This is the case for enzymes and molecular
processes protecting cells and tissues against damage
by reactive oxygen species (ROS). Table 2 taken from
the above cited review of Martin et al. (1996) shows
a list of such ‘‘public’’ genetic mechanisms shared by
a large number of species. On the other hand,
age-associated processes depending on mutation accu-
mulations might well be considered as ‘‘private’’
mechanisms, not necessarily shared by a large number
of organisms. We agree with these authors to consider
aging as a ‘‘…mere epiphenomena or passive by-
product of evolution’’ (Martin et al. 1996).
Epigenetics and aging
This term, introduced by Waddington (1968) for the
interpretation of embryonic development concerns
processes which do not modify directly gene-struc-
ture (nucleotide sequences) but processes which
regulate the timing of gene-accessibility for expres-
sion. Such mechanisms were described as self-
perpetuating structural modifications of chromatin
modulating the availability of genes for transcription.
Its mechanisms comprise DNA-methylation, histone
acetylation, imprinting, RNA-interference, gene-
silencing and paramutations (for definitions see
references by Stillmann and Stewart 2004; Allis
et al. 2007). Such epigenetic marks exhibit some
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remarkable properties, as for instance their environ-
mental dependence (rehabilitating Lamarck) and their
temporal variations. These last two properties illus-
trate their relevance for aging. There are several
examples of age-related epigenetic regulations as for
instance studies on sirtuins. Some of these genes
(as SIR-2 in yeast) were shown, when overexpressed,
to increase life-expectancy in some model-animals
(Bishop and Guarente 2007 for a review). It is
certainly interesting to remember that these studies
were undertaken in order to understand the mecha-
nism of calorie-restriction (CR) on longevity, as first
shown by McCay et al. on the rat (1939). These
experiments extended to several other species, clearly
demonstrated the importance of environmental fac-
tors in the regulation of longevity, discovered well
before the elucidation of the structure of DNA. A
more recent example of external influence on the
aging process is the life-prolonging effect of rapa-
mycin-feeding in rats (Harrison et al. 2009 and
Kaeberlein and Kapahi 2009 for comments). The
feeding of this drug was shown to extend significantly
the average and maximal life expectancy of mice,
even when feeding of rapamycin started on day 600
of their life (corresponding approximately to 60 years
old men). Similarly Selman et al. (2009) showed that
besides the mTOR pathway triggered by rapamycin,
the ribosomal protein S6-kinase triggered signalling
regulates significantly mammalian life-span. In their
comments on this work, Kaeberlein and Kennedy
(2009) proposed the following longevity pathways:
CR or rapamycin! mTOR! S6K1 ! longevity
with a possible interference at this last step by AMP-
dependent protein kinase (AMPK) in mice lacking
S6K1. This kinase integrates energy balance with
metabolism and stress resistance and was shown to
function in this respect as a longevity factor in
C. elegans also (see Kaeberlein and Kapahi 2009 for
further details).
Table 1 Genetic mechanisms involved in the regulation of (human) longevity
Genetic mechanisms Potential candidate genes in humana
DNA-repair, nuclear structure and function WRN, LMNA
Telomere—telomerase hTR, DKC1
Stress-resistance, oxidative damage Genes for SOD, insulin—IGF-1R, PI3 K
Mitochondrial DNA mt—haplotypes
Caloric restriction Sirtuinsb
Insulin signalling Genes for insulin—IGF1R, insulinR—substrate and others
Inflammation Genes for toll-like receptors, MIF-s, IL-6, CRP and others
Modified after Table 1 of Browner et al. (2004). Genetic mechanisms favouring diseases which might shorten lifespan are excludeda The role of these genes in aging may be attributed to mutations or also to epigenetic regulations probably for other animal species
alsob More recent results point to mTOR and S6 Kinase 1 signaling
Table 2 The seven classes of genetic loci involved in the protection of organisms against oxidative damage (ROS-defence
mechanisms; modified from Box 2, p 27 of Martin et al. 1996)
Class I. Structural and regulatory genes modulating ROS-production
Class II. Structural and regulatory genes for scavenger enzymes (Ex: SOD-s, catalase etc.)
Class III. Genes regulating flux of non-enzymatic free ROS-scavengers (Ex: c-glutamyl cysteine, and uric-acid synthesis)
Class IV. Genes regulating target copy number (Ex: regulation of mt DNA replicationa)
Class V. Genes specifying target structure (Ex: structural genes for chromatin proteins and membrane lipoproteins)
Class VI. Structural and regulatory genes for target repair processes (Ex: reversal, repair or tolerance of DNA-damage)
Class VII. Genes specifying the orderly replacement of effete cells (Ex: genes modulating DNA replication, cell cycle
progression, apoptosis GF-s and GFR-s)
a And more recently CNV-s
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Holliday insisted from the nineteen nineties on the
importance of epigenetic mechanisms in aging. He
proposed transmission of epigenetic mechanisms by
epimutations (Holliday 1991, 1993, 1998). Our own
experiments on the aging of elastic tissues and
especially of blood vessels also pointed to epigenetic
mechanisms (Robert and Labat-Robert 2000; Robert
and Miquel 2004).
Another interesting and well studied example is the
epigenetic regulation of the longevity of honeybee
workers and queens (Apis mellifera; Amdam and
Seehus 2006; Seehus et al. 2006; Munch et al. 2008;
Rascon et al. 2009). Honeybee workers, according to
their social role such as foraging or brood rearing,
exhibit an exceptional plasticity of their life cycle.
Aging becomes for these insects a function of behav-
ioral control. In their hypopharingeal head glands
workers synthesise ‘‘royal gelly’’, an important ingre-
dient of queen-food. Although eating much more than
the workers, queens live longer. Apparently the
calorie-restriction (CR) paradigm does not hold up in
this society. Hundreds of genes were identified as
differentially expressed in queen- and worker-destined
larvae (Rascon et al. 2009 for a review). Among the
overexpressed genes in queen-larvae are the Insulin-
Insulin-like signalling and Target-Rapamycin (TOR)
pathway regulator genes. These pathways mediate
caste-identity via Juvenile Hormone (JH) as down-
stream metabolic regulator. DNA-methylation pat-
terns appear to play a key role in these regulatory
processes which, at the end determine life-span. DNA-
methyltransferase-3 (Dnmt3), part of the CpG-epige-
netic regulatory machinery, is lower in queen-larvae
than in worker larvae. RNAi-mediated silencing of
Dnmt3 can induce queen-like traits in worker-destined
larvae, showing the importance of DNA-methylation
for caste-regulation. Epigenetic regulation of the
quality and quantity of food intake plays a key-role
in the determination of the life-span of these insects.
Such epigenetic regulation enabled this highly organ-
ised insect society to integrate environmental signals
in their genome, rehabilitating definitively Lamarck.
The downregulation of vitellogenin production, an
important vital factor, by the JH results in the
‘‘pyknotik’’ death of hemocytes, compromising their
immune-system by reducing the availability of Zn, an
important cofactor for Vitellogenin. The longevity
determination of the honeybee worker can therefore
be represented by the sequence:
JH! Vitel log enin! Zn! hemocyte pycnosis
! loss of immundefence
! loss of somatic maintenance:
The plasticity of this life-cycle underlies the
influence of environmental factors on this causal
chain of longevity, as shown clearly by the important
variations of the life expectancy of the honeybee
worker class and their diutinus stage, strongly
influenced also by weather conditions besides nutri-
ent availability. The authors conclude, that ‘‘aging in
honeybees is not merely a collection of nonadaptative
deleterious events that happen in the shadow of
natural selection, but that natural selection has shaped
the aging pattern to come under strict regulatory
control that answers impaired allocation of resources
at the colony level’’. This epigenetic and posttrans-
lational sequence can indeed regulate life span from a
few weeks to 2 years. According to Amdam et al.
similar causal chains can be found for several other
species as Drosophila and also Caenorhabditis ele-
gans with its dauer larval stage. With the recent
sequencing of the honeybee genome more details can
soon be expected on these interesting age-regulatory
process, integrating highly specialised individuals in
a complex society (Rueppell et al. 2004).
Another recent example of the importance of
epigenetic regulations in age-dependent decline of
function for vertebrates was reported by Wolffe and
Matzke (1999), which concerns among others the
regulation of the biosynthesis of extracellular matrix
(ECM) components such as elastin, as well as of its
age-dependent variation. The availability of lysyloxi-
dase, LOXL-1, important for the crosslinking of
ECM-components, collagen and elastin appears also
to be regulated by epigenetic mechanisms (Debret
et al. 2009).
These few examples clearly demonstrate the
relevance of epigenetic, posttranslational mecha-
nisms of aging in far related species of evolution.
Non-coding RNA-s
A number of important mechanisms were attributed to
non-coding RNA-s in the regulation of gene-expres-
sion and the definition of phenotypes. Such RNA-s are
coded in the genome, their synthesis is regulated by as
yet incompletely defined mechanisms. They act at
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several crucial steps of gene-expression, from gene
silencing to destruction of messenger RNA-s (Grosshans
and Slack 2002; Morris 2008a for reviews). These
mechanisms can be considered as part of epigenetic
modulations of gene-expression, because they do not
involve modifications of nucleotide sequences in geno-
mic DNA. Mechanisms mediated by non-coding RNA-s
are, however, quite different from those attributed to
DNA-methylation or histone acetylation, the classical
mechanisms of epigenetic regulations. It was, however,
demonstrated that nuclear RNAi controls among others
heterochromatin assembly and transcriptional gene
silencing, mechanisms close to classical epigenetic
regulations of gene expression (Vavasseur et al. 2008).
RNA mediated transcriptional gene silencing was also
proposed as a mechanism of ‘‘writing the histone code’’
(Morris 2008b).
More direct implication of non-coding RNA-s in
the regulation of age-related processes was also
produced recently, among others by Frank Slack and
colleagues (Pincus and Slack 2008; Budovskaya et al.
2008). RNAi against GATA—family transcription
factors, elt-5 or elt-6 can increase longevity of
C. elegans in an elt-3 dependent manner. elt-3
suppression by RNAi eliminates the long-lifespan
phenotype of mutations in both daf-2 and eat-2
animals, models of calorie-restriction. The elt-3/elt-5/
elt-6 circuit appears therefore to modulate the insulin-
IGF1 pathway, shown to control lifespan in C.
elegans. ncRNA-s were shown also to regulate
stress-response, an important modulator of longevity
(The New York Ac. Sci. e-briefing on short RNA-s in
stress and longevity by the Non Coding RNA Biology
Discussion Group, ref. Don Monroe, Oct. 13, 2009).
We also mentioned in a former section of this review
the RNAi-mediated silencing of Dnmt-3, a DNA
methyl transferase, involved in the epigenetic regu-
lation of honey-bee life course. This rapidly expand-
ing field of RNA-i will undoubtedly contribute a
great deal to our understanding of age-regulatory
mechanisms.
Aging in spare parts
Since the birth of experimental gerontology a number
of laboratories reported reliable determinations of the
age-dependent decline of physiological functions.
These data were collected by Weale (1993). A
simplified linear extrapolation of such correlations
will eventually reach zero value (total loss of the
considered function) at widely different ages (Robert
1995). This graphical representation confirms We-
ale’s suggestion of the selective and relatively
independent rate of decline of a number of physio-
logical functions (Fig. 1).
These data represent a strong argument against
aging as a general process acting simultaneously on
the whole organism indistinctively of individual
functions. Aging in ‘‘spare parts’’ is best exemplified
by the preservation of some ‘‘long lived’’ functions as
for instance musical or intellectual gifts, although
most body functions such as those of the musculo-
skeletal system and others are strongly affected by
age. On Fig. 1 only one line represents embryonic
and early postnatal development. This symbolises the
fact that embryonic development is a ‘‘robust’’
process, as defined by Fox-Keller (2000), a fertilised
human ovule gives only humans and no other species.
With, however, relatively large individual variations
in morphology and function. Such variations are
further amplified during the age-dependent decline of
the organism as represented by the lines on this
figure, declining towards zero function with different
slopes. The elastic functions decline fast, the speed of
nerve-conduction slowly.
Posttranslational mechanisms of aging
This chapter of experimental gerontology started with
the discovery of the Gershon-s, as mentioned in the
Introduction, describing the accumulation of modified
proteins, inactive enzymes in aging cells (Gershon
and Rott 1988 for a review). The accumulation of such
inactive, modified macromolecules was attributed to
delayed turnover leaving time for posttranslational
modifications. A number of such modifications were
described, such as oxidations, S-nitrosylation, dephos-
phorylation and others, reviewed by several authors
(Adelman and Roth 1983). Such mechanisms deprive
aging cells from important cell-and tissue constitu-
ents. Their accumulation is an indirect proof of
important functional modifications in aging cells,
leading to slowdown of biosynthesis and of degrada-
tion of macromolecules.
The situation is different with the Maillard reac-
tion. Although discovered early during the twentieth
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century (Maillard 1912), the recognition of its
importance for aging took several decades. This
was the case for calorie-restriction also, described
during the first half of the twentieth century and
‘‘explained’’ recently by epigenetic mechanisms,
although the final proof of the explanation of calorie
restriction is not yet at hand.
Processes described during the second half of the
previous century, the proteolytic degradation of
macromolecules, essentially of the extracellular
matrix, with the production of harmful peptides and
the creation of vicious circles with age-dependent
amplification belong also to posttranslational mech-
anisms and will be shortly reviewed (Labat-Robert
2002, 2003, 2004).
The third and final example which will be
described in this article concerns the postsynthetic
aging of elastic fibers, studied in our laboratory over
several decades. It represents an example of postsyn-
thetic aging, susceptible to be analysed in terms of
molecular mechanisms (Robert et al. 2008). It also
concerns another important mechanism of posttrans-
lational modifications, the age-dependent loss of
receptors (Roth 1995; Robert 1998).
The Maillard reaction
The first experiments, demonstrating its importance
for connective tissue aging, were performed by
Verzar during the 1950s (Robert 2006 for a review),
but its correct interpretation came several decades
later. As several recent symposia and books were
devoted to this reaction (Ikan 1996; Baynes et al.
2005; Robert 2009), we shall only shortly describe its
mechanisms, insisting on its role in aging. The
reaction itself is quite well understood in its details,
starting by the formation of glycosylamines (for
instance from glucose and free amino groups on
proteins, nucleotide bases), their Amadori-type of
rearrangement and stabilisation, followed by a series
of reactions, only some of them being well under-
stood, and leading to the formation of a number of
organic molecules, described jointly as advanced
glycation end-products (or AGE-s; Robert 2009 for
review). Some of the steps leading to AGE-s and
several reactions engaging AGE-s are mediated by
ROS and called glycoxidation. Such ROS-mediated
reactions explain probably the cytotoxic properties of
AGE-s. The structure and chemical composition of a
Fig. 1 Illustration of ‘‘aging in spare-parts’’. The abscissa
represents a relative time-scale. The ordinates give the percent
remaining activity of a variety of quantifiable physiological
functions. The ascending line on the left illustrates the
evolution of functions during embryonic development and
early childhood, followed by a horizontal portion at early
adulthood. The last part of the graph represents the selective
and differential decline of functions during aging. The first twoportions of the graph are oversimplifications. The development
of functions might well be different for different individuals.
Those differences are, however, negligible compared to those
of the age-dependent decline of functions. Some might be
nearly completely lost, while others are still quite well retained
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number of well identified AGE-s can be found in the
cited references. Here is a short description of the
Verzar-experiments which largely proved the role of
the Maillard reaction in aging of connective tissues.
Verzar’s experiment consisted in measuring the
resistance of collagen fibers to thermal denaturation
as a function of age. He found an age-dependent
exponential increase of this resistance attributed
rightly to an increase of crosslinking of collagen
fibers (Robert 2006 for review). The nature of these
crosslinks was, however, elusive in his time and took
several decades to be elucidated. It appeared that the
AGE-products are the culprits of this regular, age-
dependent increase of collagen crosslinking. As
AGE-products are present in all tissues of the
organism, an age-dependent increase of AGE-
induced post-synthetic modifications of proteins
concern the whole organism and most of its macro-
molecular components. Besides proteins, nucleotide
bases are also affected, as suggested by the genotoxic
effect of AGE-products. When in vitro prepared
AGE-products are added to fibroblast cultures, an
immediate increase of cytotoxicity could be demon-
strated, as shown by the number of dead cells floating
above the adherent cell layer (Peterszegi et al. 2006).
This effect was shown to be transmitted to (at least)
the next cell-generation. The elimination of the AGE-
containing culture medium, its replacement by fresh
medium with no further AGE-s added, still resulted in
a strongly increased cytotoxicity. The rate of cell
proliferation was also affected. What makes the
Maillard reaction an important factor for age-depen-
dent loss of cell and tissue structure and function is
the fact, that besides the in vivo generation of AGE-s
there is also a strong contribution from processed
food, as shown by the quantification of AGE-s in a
variety of food-products (Goldberg et al. 2004).
Acting on the quality of food is one way to alleviate
the nocivity of the Maillard reaction. Routinely used
anti-diabetic drugs as metformin is another. Another
important feature of this reaction is the fact that it
starts early in life, reducing sugars and derivatives are
taken up and produced constantly in the living
organism. It is their accumulation with time what
makes them important for the age-dependent modi-
fications of tissue structure and function. It also
shows that vital molecules as glucose can avoid
classical metabolic pathways and engage in organic
chemical reactions, harmful for the organism with no
efficient scavenging mechanisms which could have
evolved during evolution.
Proteolytic generation of harmful peptides
The role of proteolytic generation of harmful degra-
dation products was convincingly demonstrated dur-
ing the last decades of the twentieth century. As this
subject was also reviewed recently (Labat-Robert
2002, 2003, 2004), we shall shortly remind its most
relevant facets for the subject of this review.
The proteolytic degradation of fibronectin was
shown to result in the formation of several large
peptides endowed with unexpected biological activ-
ity. One large fragment was shown to behave as a
protease, although intact fibronectin is devoid of such
activity (Keil-Dlouha and Planchenault 1986).
Another fragment was shown by Barlati et al.
(1981) to potentiate malignant transformation. The
team of Homandberg demonstrated an active role of
fibronectin fragments in the generation of inflamma-
tion, especially in articular cartilage (Xie et al. 1994).
Another proteolytic fragment of fibronectin induced
an increased synthesis of fibronectin and of TNFarelease (Lopez-Armada et al. 1997). We could show
in our laboratory that fibronectin biosynthesis by
fibroblasts (the tissue form of fibronectin), as well as
by hepatocytes (circulating form in the blood plasma)
increase with age (Labat-Robert et al. 1981). Prote-
olytic activity in several tissues was also shown to
increase with age as well as with passage number in
cell cultures (Robert and Labat-Robert 1988).
All these data add up to a vicious circle with age-
dependent amplification of harmful effects (Fig. 2).
Fibronectin is, however, only one of the proteins of
the extracellular matrix to yield such peptides on
proteolytic degradation, endowed with harmful
effects and an age-dependent amplification. Similar
mechanisms were demonstrated for other macromol-
ecules of the extracellular matrix also (Labat-Robert
2004). Some authors proposed conformational
modifications uncovering cryptic sites for proteo-
lytic attack (matricryptic sites). It is, however,
highly probable that post-proteolytic conformational
changes are involved in the generation of peptides
capable of producing such harmful effects with age-
dependent amplification. In vivo evidence of the role
of such peptides in age-dependent decline of
394 Biogerontology (2010) 11:387–399
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functions is still lacking. The presence of fibronectin-
derived peptides was, however, demonstrated in the
blood plasma of elderly inpatients of a geriatric ward,
all suffering of age-related diseases (Labat-Robert
et al. 2000). The plasma of several centenarians in
relatively acceptable health-conditions was also stud-
ied during these same experiments These samples did
not exhibit the presence of comparable fibronectin
degradation products.
Postsynthetic aging of elastic fibers
This process, as that described by Maillard, is among
the age-related processes with loss of structure and
function, elucidated in most of its details (Robert
et al. 2008 for a recent review). It shows also the
important role of the conformation and structure of
biological macromolecules directly involved in their
age-dependent loss of function. Elastin is a strongly
hydrophobic protein. This property is largely respon-
sible for its mainly entropy-driven elasticity (Robert
and Robert 1980). This property is also directly
responsible for its strong affinity for lipids. Most
classes of lipids were shown to accumulate in elastin
fibers, essentially cholesterol and free fatty acid
(Claire et al. 1976). As a result of the specific
sequence of some peptides, part of the elastin forms
ring-like structures with a high affinity for calcium
which was also shown to accumulate with age in the
elastic fibers (Lansing 1959). These two processes,
the progressive accumulation of lipids and calcium
result in a progressive loss of elasticity and in an
increase of its susceptibility to proteolytic degrada-
tion (Hornebeck et al. 1976). Since the early decades
of microscopic pathology it was shown that aorta-
elastic fibers appear fragmented in older autopsy
samples, contrasting with mainly continuous fibers in
young specimens. Elastin derived peptides could be
demonstrated in the circulating blood (Bizbiz et al.
1997). Such peptides were shown to act as high
affinity agonists with the elastin receptor and induce
an increased release of elastolytic proteases as well as
of ROS-s as superoxide (Fulop et al. 1998; Robert
1998). The intracellular transmission pathway of this
receptor coupled to a Gi-protein is altered in cells
taken from old individuals. Superoxide release is
increased in presence of elastin peptides, but no more
inhibited in ‘‘old’’ cells by pertussis toxin, a known
antagonist of Gi-proteins, as was shown to be the case
in cells from young individuals (Fulop et al. 1992;
Robert 1999). The coupling of the elastin receptor
with iNOS in endothelial cells is also lost with age,
accompanied by a progressive loss of vasodilatation
produced by elastin peptides added to rat aorta rings
(Faury et al. 1997). Other age-dependent loss of
functions of the elastin receptor were also demon-
strated as the loss of dose-dependent inhibition by
elastin peptides of cholesterol synthesis in monocytes
from older individuals (Varga et al. 1997). This age-
dependent uncoupling of the elastin receptor goes
together with the loss of its protective functions such
as vasodilatation and limitation of cholesterol bio-
synthesis. On the contrary, it is accompanied by an
increase of its harmful effects, such as release of
elastolytic proteases and free radicals. This sequence
of events leads also to a vicious circle with age-
dependent auto-amplification (Fig. 3).
It can be noticed that the loss of elasticity by
progressive accumulation of lipids and calcium is the
direct consequence of the specific structure and
conformation of the elastin protein (Urry 1980). No
external factors are needed for the age-dependent loss
of elastin-elasticity. These modifications increase the
susceptibility of elastic fibers for proteolytic degra-
dation. The released elastin peptides reach a plasma-
tissue concentration several log-units above the
affinity constant (KD) of the elastin receptor, shown
to be in the nanomol range. The overload of the
elastin receptor by its agonists, elastin peptides, and
Proteolytic degradation
These processes are up-regulated with age
FIBRONECTIN
(age-dependent increase)
PEPTIDES
WITH POTENTIAL HARMFUL EFFECTS
PROTEOLYTIC ACTIVITY
INFLAMMATION
MALIGNANT TRANSFORMATION
Up-REGULATION OF FN BIOSYNTHESIS
Fig. 2 Vicious circle generated by the age-dependent increase
of fibronectin (FN) biosynthesis and its proteolytic degrada-
tion. Several of the proteolytically generated peptides exhibit
harmful properties, such as novel proteolytic activity, poten-
tiation of malignant transformation and mediation of inflam-
mation. Some peptides further increase fibronectin
biosynthesis. For more details see Labat-Robert (2004)
Biogerontology (2010) 11:387–399 395
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its uncoupling from its normal intracellular transmis-
sion pathway (with no change in KD) results in a
progressive upregulation of further elastin degrada-
tion accompanied by the amplification of its harmful
effects (Fig. 3).
Conclusions and perspectives
We attempted in this short review to explore argu-
ments for or against genetic determinism in the aging
process. Although the classic era of molecular
genetics following the elucidation of the structure
and function of DNA suggested rigid genetic deter-
minism, no convincing arguments could be found in
favour of such claims that ‘‘aging is coded in the
genome’’. This negative finding does not distract,
however, from the experimental evidence that a large
number of genes were shown to be involved in
reactions associated with age-dependent decline of
tissue structure and function. Studies on identical
twins furnished also arguments against a strong
inheritance of life expectancy. The best confirmed
life-increasing manipulations as calorie-restriction
pointed to the importance of epigenetic factors in
the regulation of life expectancy. Further experimen-
tal exploration of such mechanisms will forseeably
confirm the importance of epigenetic and posttrans-
lational mechanisms in aging. The environmental and
temporal influence of such mechanisms is in favour
of their potential role in life-course determination.
We summarised in some detail some of the postsyn-
thetic processes involved in aging, RNA interference,
the Maillard reaction, proteolytic degradation prod-
ucts of matrix macromolecules with production of
peptides producing harmful effects and the aging of
elastin with its molecular details comprising the role
of the elastin receptor in the age-dependent amplifi-
cation of the harmful effects. All these examples are
in favour of the evolutionary argument against direct
genetic determinism of life expectancy.
The age-related mechanisms with the most
detailed knowledge of their molecular details are
not in favour of the role of antagonistic pleiotropy in
aging as proposed by Williams (Robert and Labat-
Robert 2000; Robert and Miquel 2004). The above
detailed mechanisms are much more in favour of the
proposition made by Jacob (1997) that evolutionary
processes are imperfect, correspond more to tinkering
than to the production of masterpieces. The elastin
gene does not have to change with age, neither in its
structure or function to explain the progressive
accumulation of lipids and calcium in the peptide
folds of the protein it codes for. The same could be
sad about the structure of fibronectin. The harmful
role of its degradation products do not reflect an age-
dependent modification of the gene coding for this
protein. Its proteolytic degradation with the produc-
tion of harmful peptides is nowhere ‘‘coded in the
genome’’, neither do we have to claim a tissue
specific pleiotropic effect. The Maillard reaction can
also be considered as a result of ‘‘tinkering’’ by the
evolutionary development of the intermediary metab-
olism of reducing sugars. A non negligible fraction of
free glucose (together with other reducing metabo-
lites) reacts freely with macromolecular amino
groups, resulting in AGE-products exhibiting harmful
effects. Their age-dependent accumulation was
clearly shown by the Verzar phenomenon and by a
number of more recent experiments (Peterszegi et al.
2006; Robert 2009). It is important to emphasize that
some nutritional ingredients as glucose can bypass
standard metabolic pathways (catalysed by enzymes
‘‘coded’’ in the genome) and react, just as they would
in a test-tube with biological macromolecules, with
no efficient inhibitory mechanisms which could have
evolved during the course of evolution of living
species. So in our opinion no tissue- and age-
dependent shifts of gene-action (antagonistic pleiot-
ropy) should be claimed to play a role in aging.
Nature did a lousy job during evolution as proposed
by the ‘‘tinkering’’ hypothesis of Francois Jacob.
ELASTIN
Ca Lipids
Further degradation of elastin Loss of elasticity
Elastases
Elastin peptides Action on the Upregulation of Elastin receptor elastase production
and ROS release
Fig. 3 Vicious circle started by the saturation of elastin with
Ca and lipids, loss of elasticity, degradation of elastic fibers by
elastases, liberation of elastin peptides, their action on the
elastin receptor, followed by the upregulation of elastase (and
free radical) production. For more details see Robert et al.
(2008)
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Although aging is not ‘‘coded in the genome’’, a
large number of genes are somehow involved in the
age-dependent decline of tissue structure and func-
tion. The widely different rates of decline of func-
tions with time is in itself a strong argument against a
strict genetic determinism. This is probably the
reason of the flexibility of human life-expectancy,
which increased significantly over the last decades,
essentially thanks to improved nutrition (and not to
calorie restriction) and other environmental factors.
Acknowledgments The original experiments reported in this
review were carried out in our CNRS Laboratory at University
Paris XII and at the Hotel Dieu Hospital, Univ. Paris 5, Paris,
supported by Institut DERM. The hospitality of Prof. Gilles
Renard, Head of Ophthalmology at Hotel Dieu is thankfully
acknowledged.
References
Adelman RC, Roth GS (1983) Altered proteins and aging. CRC
Press, Boca Raton
Allis DC, Jenuwein T, Reinberg D, Caparros M-L (eds) (2007)
Epigenetics. CSHL Press, New York
Amdam GV, Seehuus SC (2006) Order, disorder, death: les-
sons from a superorganism. Adv Cancer Res 95:31–60
Amouyel P, Vidal O, Launay J-M, Laplanche J-L (1994) The
apolipoprotein E alleles as major susceptibility factors for
Creutzfeld–Jacob disease. Lancet 344:1315–1318
Assmann G, Schmitz G, Menzel HJ, Schulte H (1984) Apoli-
poprotein E polymorphism and hyperlipidemia. Clin
Chemistry 30:641–643
Barlati S, de Petro G, Vartio T, Vaheri A (1981) Transfor-
mation-enhancing activity of proteolytic fragments of
fibronectin. Proc Natl Acad Sci USA 78:4965–4969
Baynes JW, Monnier VM, Ames JM, Thorpe SR (eds) (2005)
The Maillard reaction. Chemistry at the interface of
nutrition, aging and disease, vol 1043. Annual New York
Academic Science, New York
Bishop NA, Guarente L (2007) Genetic links between diet and
lifespan: shared mechanisms from yeast to humans. Nat
Rev Genet 8:835–844
Bizbiz L, Alperovitch A, Robert L, the EVA Group (1997)
Aging of the vascular wall: serum concentration of elastin
peptides and elastase inhibitors in relation with cardio-
vascular risk factor. The EVA study. Atherosclerosis
131:73–78
Browner WS, Kahn AJ, Ziv E, Reiner AP, Oshima J, Cawthon
RM, Hsueh W-C, Cummings SR (2004) The genetics of
human longevity. Review. Am J Med 117:851–860
Budovskaya YV, Wu K, Southworth LK, Jiang M, Tedesco P,
Johnson TE (2008) Kim SK An elt-3/elt-5/elt-6 GATA
transcription circuit guides aging in Caenorhabditis ele-gans. Cell 134:291–303
Burnet MF (1974) Intrinsic mutagenesis: a genetic approach to
aging. Wiley, Chichester
Carnes BA, Olshansky SJ, Grahn D (2003) Biological evidence
for limits to the duration of life. Biogerontology 4:31–45
Carnes B, Staats DO, Sonntag WE (2008) Does senescence
give rise to disease? Mech Aging Develop 2008(129):
693–699
Claire M, Jacotot B, Robert L (1976) Characterisation of lipids
associated with macromolecules of the intercellular
matrix of human aorta. Connect Tissue Res 4:61–71
Comfort A (1979) The biology of senescence. Churchill Liv-
ingstone, Edinburgh
Debret R, Claus S, Cenizo V, Aimond G, Andre V, Megarbane
A, Devillers M, Damour O, Sommer P (2009). Silencing
of elastic fibers related genes in Cutis Laxa as model of
human accelerated skin aging. Summary SA8 185-5 (p
101) IAGG 2009 Paris
Faury G, Chabaud A, Ristori MT, Robert L, Verdetti J (1997)
Effect of age on the vasodilatory action of elastin pep-
tides. Mech Aging Develop 95:31–42
Fox-Keller E (2000) The century of the gene. Harvard Uni-
versity Press, Cambridge
Fulop T Jr, Barabas G, Varga Z, Csongor J, Hauck M, Szucs S,
Seres I, Mohacsi A, Kekessy D, Despont JP, Robert L,
Penyige A (1992) Transmembrane signalling changes
with aging. Ann New York Acad Sci 673:165–171
Fulop T, Jacob MP, Khalil A, Wallach J, Robert L (1998)
Biological effects of elastin peptides. Pathol Biol 46:497–
506
Gershon D, Rott R (1988) Studies on the nature of faulty
protein molecules and their diminished degradation in
cells of aging organisms: functional implications. In:
Bergener M, Ermini M, Stahelin HB (eds) The 1988
Sandoz lectures in gerontology. Academic Press, New
York, pp 25–33
Goldberg T, Cai W, Peppa M, Dardaine V, Baliga BS, Uribarri
J, Vlassara H (2004) Advanced glycoxidation end prod-
ucts in commonly consumed food. J Am Diet Assoc
104:1287–1291
Grosshans H, Slack FJ (2002) Micro-RNA-s: small is plentiful.
J Cell Biol 156:17–21
Harrison DE, Strong R, Sharp ZD et al (2009) Rapamycin fed
late in life extends lifespan in genetically heterogeneous
mice. Nature 460:392–395
Hawks K (2003) Grandmothers and the evolution of human
longevity. Am J Human Biol 15:380–400
Hayflick L (1987) Origins of longevity. In: Warner HR, Butler
RN, Sprott RL, Schneider EL (eds) Modern biological
theories of aging. Raven Press, New York, pp 21–34
Holliday R (1991) Mutations and epimutations in mammalian
cells. Mutat Res 250:351–363
Holliday R (1993) Epigenetic inheritance based on DNA-
methylation. EXS 64:452–468
Holliday R (1998) Endogenous DNA-methylation and epimu-
tagenesis. Mutat Res 422:97–100
Hornebeck W, Derouette J-C, Roland J, Chatelet F, Bouissou
H, Robert L (1976). Correlation entre l’age, l’art-
eriosclerose et l’activite elastolytique de la paroi aortique
humaine. C R Ac Sci 292: 2003–2006
Ikan R (ed) (1996) The Maillard reaction. Consequences for
the chemical and life sciences. Wiley, Chichester
Jacob F (1997) Evolution and tinkering. Science 196:1161–
1166
Biogerontology (2010) 11:387–399 397
123
Author's personal copy
Jacotot B (ed) (1993) Atherosclerose. Sandoz, Paris
Kaeberlein M, Kapahi P (2009) Aging Is RSKy business.
Science 326:55–56
Kaeberlein M, Kennedy BK (2009) A midlife longevity drug.
Nature 460:331–332
Keil-Dlouha V, Planchenault T (1986) Potential proteolytic
activity of human plasma fibronectin. Proc Natl Acad Sci
USA 83:5377–5381
Labat-Robert J (2002) Fibronectin in malignancy. Effect of
aging. Sem Cancer Biol 12:187–195
Labat-Robert J (2003) Age-dependent remodeling of connec-
tive tissue: role of fibronectin and laminin. Pathol Biol
51:563–568
Labat-Robert J (2004) Cell-matrix interactions in aging: role of
receptors and matricryptins. Aging Res Rev 3:233–247
Labat-Robert J, Potazman JP, Derouette JC, Robert L (1981)
Age-dependent increase of human plasma fibronectin.
Cell Biol Int Rep 5:969–973
Labat-Robert J, Marques MA, N’Doye S, Alperovitch A,
Moulias R, Allard M, Robert L (2000) Plasma fibronectin
in French centenarians. Arch Gerontol Geriat 31:95–105
Lansing AI (ed) (1959) The arterial wall. The Williams &
Wilkins Company, Baltimore
Levin B (2008) Genes IX. Jones and Bartlett Publishers,
Sudbury
Lopez-Armada LP, Gonzales E, Gomez-Guerrero C, Egido J
(1997) The 80-kDa fibronectin fragment increases the
production of fibronectin and tumor necrosis factor alpha
(TNF-a) in cultured mesangial cells. Clin Exp Immunol
107:398–403
Maillard L-C (1912). Action des acides amines sur les sucres:
formation des melanoıdines par voie methodique. C R Ac
Sci 154:66–68
Martin GM, Austad SN, Johnson TE (1996) Genetic analysis of
aging: role of oxidative damage and environmental
stresses. Nat Genet 13:25–34
McCay CM, Maynard LA, Sperling G, Barnes LL (1939)
Retarded growth, lifespan, ultimate body size and age
changes in the albino rat after feeding diets restricted in
calories. J Nutr 18:1–13
Medawar PB (1952) An unsolved problem in biology. Lewis,
London
Morris KV (ed) (2008a). RNA and the regulation of gene
expression. A hidden layer of complexity. Caister Aca-
demic Press, Norfolk
Morris KV (2008b). RNA-mediated transcriptional gene
silencing: mechanism and implications in writing the
histone code. In: Morris KV (ed) RNA and the Regulation
of Gene Expression. Caister Academic Press, Norfolk
Munch D, Adam GW, Wolschin F (2008) Aging in a eusocial
insect: molecular and physiological characteristics of
lifespan plasticity in the honeybee. Funct Ecol 22:407–
421
Peterszegi G, Molinari J, Ravelojaona V, Robert L (2006)
Effect of advanced glycation end-products on cell prolif-
eration and cell death. Pathol Biol 54:396–404
Pierce BA (2008) Genetics, a conceptual approach.
W.H.Freeman and Co, New York
Pincus Z, Slack FJ (2008) Transcriptional (dys)regulation and
aging in Caenorhabditis elegans. Genome Biol 9:233–236
Rascon B, Navdeep MS, Tolfsen C, Amdam GV (2009).
Honey bee life-history plasticity—development, behav-
iour, ageing. In: Flatt T, Heymand A (eds) Mechanisms of
life history evolution. Oxford University Press (in print)
Robert L (1995) Le vieillissement. Faits et Theories. Flam-
marion, Paris
Robert L (1998) Mechanisms of aging of the extracellular
matrix: role of the elastin-laminin receptor. Gerontology
44:307–317
Robert L (1999) Interaction between cells and elastin, the
elastin-receptor. For the 80th birthday of Ines Mandl.
Connective Tissue Res 40:75–82
Robert L (2006) Fritz Verzar was born 120 years ago: his
contribution to experimental gerontology through the
collagen research as assessed after half a century. Arch
Gerontol Geriat 43:13–43
Robert L (2009) The Maillard reaction. Path Biol. doi:
10.1016/j.patbio.2009.09.004
Robert L, Labat-Robert J (1988) Aging of extracellular matrix,
its role in the development of age-associated diseases. In:
Bergener M, Ermini M, Stahelin HB (eds) Crossroads of
aging. The 1988 Sandoz lectures in gerontology. Aca-
demic Press, London, pp 105–126
Robert L, Labat-Robert J (2000) Aging of connective tissues,
from genetic to epigenetic mechanisms. Biogerontology
1:123–131
Robert L, Miquel P-A (2004) Bio-Logiques du Vieillissement.
Editions Kime, Paris
Robert L, Robert AM (1980) Elastin, elastase and arterioscle-
rosis. Front matrix biology, vol 8. Karger, Basel, pp 130–
173
Robert L, Bellon G, Hornebeck W (1980) Characterisation of
different elastases. Their possible role in the genesis of
emphysema. Bull Eur Physiopathol Resp 16:199–206
Robert L, Robert AM, Fulop T (2008) Rapid Increase in human
life expectancy: will it soon be limited by the aging of
elastin? Biogerontology 9:119–133
Roth GS (1995) Changes in tissue responsiveness to hormones
and neurotransitters during aging. Exp Gerontol 30:361–
368
Rueppell O, Amdam GV, Page RE Jr, Carey JR (2004). From
genes to societies. Sci Aging Knowledge Environ 5:pe5
Seehuus SC, Krekling T, Amdam GV (2006) Cellular senes-
cence in honey bee brain is largely independent of chro-
nological age. Exp Gerontol 41:1117–1125
Selman C, Tullet JMA, Wieser D et al (2009) Ribosomal
protein S6 kinase 1 signaling regulates mammalian life
span. Science 326:140–144
Stillmann B, Stewart D (eds) (2004) Epigenetics. Cold Spring
Harb Symp Quant Biol, LWIX. CSHL Press, New York
Urry DW (1980). Sequential Polypeptides of elastin: structural
properties and molecular pathologies. Front Matrix Biol
8:78–103
Varga Zs, Jacob MP, Robert L, Csongor J, Fulop T (1997)
Age-dependent changes of j-elastin stimulated effector
functions of human phagocytic cells: relevance for ath-
erosclerosis. Exper Gerontol 32:653–662
Vavasseur A, Touat-Todeschini L, Verdel A (2008). Hetero-
chromatin assembly and transcriptional gene silencing
under the control of nuclear RNAi: lessons from fission
398 Biogerontology (2010) 11:387–399
123
Author's personal copy
yeast. In: Morris KV (ed) RNA and the regulation of gene
expression. Caister Academic Press, pp 45–57
Verma M, Dunn BK, Umar A (eds) (2003). Epigenetics in
cancer prevention. Early detection and risk assessment.
Ann New York Acad Sci 983
Waddington CH (ed) (1968) Towards a theoretical biology. 1.
Prolegomena. An IUBS symposium. Edinburgh Univer-
sity Press, UK
Warner HR, Butler RN, Sprott RL, Schneider EL (eds) (1987)
Modern biological theories of aging. Aging vol 31. Raven
Press, New York
Weale RA (1993) Have human biological functions evolved in
support of life-span? Mech Age Develop 69:65–77
Williams GC (1957) Pleiotropy, natural selection and the
evolution of senescence. Evolution 11:398–411
Wolffe AP, Matzke MA (1999) Epigenetics: regulation through
repression. Science 286:481–486
Xie DL, Hui F, Meyers R, Homandberg GA (1994) Cartilage
chondrolysis by fibronectin fragments is associated with
release of several proteinases: stromelysin plays a major
role in chondrolysis. Arch Biochem Biophys 311:205–212
Biogerontology (2010) 11:387–399 399
123
Author's personal copy