Nature 506, 316321 (20 February 2014)
Go J. Yoshida M.D.,Ph.D. (E-mail:[email protected])Dept. of Stem Cell Biology, Medical Research Institute
Tokyo Medical and Dental University, Tokyo, Japan
Senescence regulation in stem cellsSenescence is a state in which a cell no longer has
the ability to proliferate.
Senescence accompanies changes in nuclear
morphology and formation of a distinct chromatin
structure, called senescence-associated
heterochromatic foci (SAHF).
Cell. 2003;113:703-716.
p16Ink4a promotes SAHF formation.
Epigenetic regulation of the p16INK4a locus in
senescent cells
In senescent cells, polycomb proteins are lost or displaced from the INK4A locus by a
number of established mechanisms, including (d) EZH2 downregulation resulting in the
loss of H3K27 methylation, (e) the histone demethylase JMJD3, which actively removes
the H3K27me3 mark and (f) the SWI/SNF chromatin remodelling complex, which is
proposed to displace polycombs from the INK4A locus. We also propose a novel
mechanism (g) by which polycomb proteins may be displaced by transcriptional
activators.
Oncogene. 2011;30:2901-11.
The pRb pathway and SAHF formation in senescent cells
There is a inversed relation between p16INK4a and p-Rb.
pRb protein functions to recruit repressive enzymatic activities to E2F-regulated target gene promoters.
The remarkably stable state of cellular senescence is thought to be maintained by compacted heterochromatic or closed chromatin, called SAHF, which accumulates around genes encoding cell cycle and proliferation proteins.
Oncogene. 2011;30:2901-11.
Aging Cell. 2002;1:132-9.
The arrows indicate satellite cells stained
positively for N-CAM (peroxidase, brown)
whereas the arrowheads give examples of
myonuclei (blue).
Cold Spring Harb.2011;76:101111.
Age-associated decline in muscle regeneration
Aged muscle (right, 24 months of
age) shows a significant deficit in
the number of regenerating fibers
and an increase in necrotic fibers
compared with young muscle.
Prevention of age-related increase in fibrosis during
muscle regeneration by heterochronic parabiosis
established for 1month
muscle tissue damage (freeze injuries)
3 days later, BrdU was administered.
2 days later, muscle tissue was fixed. Science. 2007;317:807-10.
6month 24month
The effects of the aged environment on myogenic
progenitor cell fate and muscle regeneration are
mediated by the Wnt signaling pathway.
Science. 2007;317:807-10.
Satellite CellsSkeletal Muscle Stem Cells
to exist close proximity to skeletal muscle fibers
to exhibit regenerative activity of skeletal muscle
to be required for early post-natal muscle growth
and for repair of catastrophic muscle injury
SC markers: Pax7, Myf5, MyoD, Cav1, N-CAM etc.
Physiol Rev. 2013;93:23-67.
Stem Cells Int. 2013;2013:420164.
Regulatory switch between
quiescent and activated states
of satellite cells
Notch and Wnt signaling antagonize
each other to define the state of
satellite cells.
Stem Cells Int. 2013;2013:420164.Notch-predominant Wnt-predominant
Functional dysregulation of stem cells during
aging: a focus on skeletal muscle stem cells
FEBS J. 2013;280:40514062.
Dev Biol. 2006;294:50-66.
: sarcopeniaLoss of muscle mass: sarcopenia
Loss of self-renewal: senescence
geroconversion
Bmi1 plays a crucial role in the maintenance of the stem cell pool in postnatal skeletal muscle and is
essential for efficient muscle regeneration after injury, possibly through its actions on p16ink4a and p19arf
Regenerating fibers express fetal myosin heavy chain (MyHC) initially and then switch to adult isoforms while they mature.
PLoS One. 2011;6(11):e27116.
Bmi-1-/- MEF underwent premature aging.
Nature. 1999;397:164-8.
SAMP8 pre-mature aging model
sarcopenia in senescence-accelerated mice (SAMP8)
Exp Gerontol. 2005;40:562-72.
type I fiber type IIA fiber
SAMP8 strains exhibit
accelerated muscle aging
as compared with SAMR1.
senescence-prone inbred
strains (SAMP)
senescence-resistant inbred
strains (SAMR)
Type-I fiber size was not
different between SAM
strains.
Major Hypothesis
The geriatric satellite stem cells show the irreversible
switch of quiescence-to-senescence (geroconversion) and
lose self-renewal potential via PRC1/p16INK4a/Rb/E2F axis.
young: 23 months of age
adult: 5-6 months of age
old: 2024 months of age
geriatric: 28-32 months of age
young: 23 months of age
adult: 5-6 months of age
old: 2024 months of age
geriatric: 28-32 months of age
A reduced number of satellite cells in aged mice is not
responsible for the significant regenerative decline.
Injection of cardiotoxin (CTX
1X10-5M) to tibialis anterior
muscle
After 21days
eMHC (embryonic myosin heavy chain) is a differentiation marker of skeletal muscle.
Pax7-positive satellite
cells in resting phase
Satellite-cell-intrinsic potential of regeneration 1week
after transplantation into young recipient mice
Equal numbers of sorted satellite cells labelled with
GFP were transplanted into muscles of young mice.
Pax7+ quiescent satellite cells from geriatric mice
showed a reduced activation rate early after injury
compared to adult/old cells.
Pax7/MyoD double-positive satellite cells
are defined as activated satellite cells.
Quiescent geriatric satellite cells exhibited a
defective activation capacity 24hr after
transplantation into a young host
The only significantly up-regulated senescence
gene in cluster G1 was the master regulator of
senescence p16INK4a (Cdkn2a)
group of genes with increased
expression in geriatric satellite cells
H2Aub mark in the INK4a locus was significantly
reduced in quiescent satellite cells from geriatric mice
Polycomb repressive complex 1
(PRC1): E3 ubiquitin ligase complex
that is specific for histone H2A.
PRC1 comple possess H2A-K119
ubiquitin E3 ligase activities
Canonical PRC1
complex
Nature 2004;431:873-878.
Mol Cell. 2005;20:845-54.
H2Aub: histone H2A mono-ubiquitination at
lysine 119 (PRC1 repressive marker)
RD domain is the main binding site for Bmi1 of
p16INK4a locus.
p16INK4a expression changed the satellite cell activation
ectopic p16INK4a overexpression in
quiescent young satellite cells
prevented their activation
p16INK4a silencing rescued the permanent
cell-cycle arrest and allowed geriatric
satellite cell activation
p16INK4a silencing in geriatric satellite cells
before engraftment into young mice
p16INK4a silencing in geriatric satellite cells
recovered the self-renewal potential
Self-renewing satellite cells; Pax7(+)/ Ki-67(-)
p16INK4a overexpression reduced the
capacity of young reserve satellite cells
to reactivate.
Activation capacity of satellite cells were
reduced after successive myogenesis rounds
(repeated injuries).
Geriatric satellite cells underwent an accelerated
entry into senescence under proliferative pressure
Transplanted geriatric satellite cells also displayed
signs of deep senescence at times of maximal
proliferative expansion after injury
Geriatric satellite cells exposed to proliferative
conditions presented high p16INK4a expression levels
correlating with reduced phosphorylated Rb protein
p16INK4a interference reduced geroconversion in
geriatric satellite cells
Ex vivo culture of muscle satellite cells
J Stem Cell Res Ther 2012;S11:003.
adult: 30 years of age
old: 65 years of age
geriatric: 96 years of age
Ex vivo analysis
Active p16INK4a/Rb axis in human geriatric stem cells
Conclusion
Geriatric age induces intrinsic alterations in
muscle stem-cell regenerative functions, which
cannot be recovered by a young host environment.
p16INK4a expression is specifically induced in
geriatric satellite cells and drives geroconversion
at the expense of proliferation.
p16INK4a seems to be positively associated with
reduced myogenic potential and increased cellular
senescence in human satellite cells from geriatric
individuals with sarcopenia.
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