Download Genetic regulation of skeletal muscle development

E XP E RI ME N T AL C E L L R E S EA RC H 31 6 ( 20 1 0) 3 0 8 1– 3 08 6
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Review
Genetic regulation of skeletal muscle development
Keren Bismuth, Frédéric Relaix⁎
INSERM UMR S 787-Myology Group, Avenir Team Mouse Molecular Genetics, UPMC- Faculté de Médecine Pitié-Salpêtrière, Institut de Myologie,
105 Bd de l'Hôpital 75013 Paris, France
A R T I C L E I N F O R M A T I O N
A B S T R A C T
Article Chronology:
During development, skeletal muscles are established in a highly organized manner, which
Received 1 March 2010
persists throughout life. Molecular and genetic experiments over the last decades have identified
Revised version received 30 August 2010
many developmental control genes critical for skeletal muscle formation. Developmental studies
Accepted 31 August 2010
have shown that skeletal muscles of the body, limb and head have distinct embryonic and cellular
Available online 7 September 2010
origin, and the genetic regulation at work in these domains and during adult myogenesis are
starting to be identified. In this review we will summarize the current knowledge on the
Keywords:
regulatory circuits that lead to the establishment of skeletal muscle in these different anatomical
Muscle development
regions.
Transcription factor
© 2010 Elsevier Inc. All rights reserved.
Myogenic Regulatory Factors
Contents
Introduction . . . . . . . . . . . . . . . . . .
Genetic control of trunk musculature . . . . .
Genetic control of limb muscle development .
Genetic control of facial muscles development
From the embryo to the adult . . . . . . . . .
Concluding remarks . . . . . . . . . . . . . .
Acknowledgments . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . .
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Introduction
Skeletal muscle cells, or myotubes, are contractile, multinucleated
cells, which constitute the minimal functional unit of all muscles in
the body. Undifferentiated muscle cells are called myoblasts; they
are mononucleated and are characterized by the expression of
members of the myogenic regulatory factors (MRFs). Classic
experiments in embryology have clearly established that all body
⁎ Corresponding author. Fax: +33 1 53 60 08 02.
E-mail address: [email protected] (F. Relaix).
0014-4827/$ – see front matter © 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.yexcr.2010.08.018
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muscles, but not head, are derived from the condensation of
paraxial mesoderm into epithelial structures called somites.
Somites are formed along the rostro-caudal axis of the embryo
and are organized in dorso-ventral compartments. The most
ventral part is the sclerotome (that will give rise to the axial
skeleton); most dorsally, the dermomyotome is comprised of cells
that will give rise to dermis and muscle progenitors cells (MPC).
The borders of the dermomyotome will undergo an epithelial to
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E XP E RI ME N T AL C E L L R E SE A RC H 31 6 ( 20 1 0) 3 0 81 – 3 08 6
mesenchyme transition and form the third somitic compartment:
the myotome, which contains the first differentiated myofibers.
The epaxial (dorso-medial) part of the dermomyotome and
myotome will give rise to the back muscles while the hypaxial
(ventro-lateral) somite will generate the rest of the trunk and limb
muscles [1]. Genetic in vivo invalidation of genes in the mouse, as
well as studies in the avian model and in vitro experiments made it
possible to identify genetic regulatory networks that are involved
in specifying muscle progenitors and precursors cells (see Fig. 1).
chick somite explants lead to an up-regulation of Pax3 [5]. Six1 and
Six4 control the hypaxial expression of Pax3 [6], and in their absence,
limb muscles, which originate from the migrating hypaxial cells, are
absent. Six genes also control early MRFs expression in the myotome
[7] (Fig. 1) and misexpression of both Six1 and Eya2 is sufficient to
trigger MRFs expression in avian somite [5]. SIX proteins bind MEF3
sites, which have been found in Myogenin promoter and many others
myogenic differentiation genes [8–10].
On the next level down in the myogenic genetic hierarchy, in
the myotome, myogenesis is driven by the four MRFs (Myf5, Mrf4,
MyoD and Myogenin). These are basic helix–loop–helix (bHLH)
transcription factors, which play somewhat redundant role to
ensure proper muscle differentiation, in combination with less
specific factors such as members of the MEF2 family. The first MRF
expressed in the myogenic differentiation network is Myf5, it is
first transiently expressed in the paraxial mesoderm before the
onset of myogenesis [11], and is then expressed in the lips of the
dermomyotome, inducing migration within the myotome and
initiation of the myogenic differentiation pathway. The regulation
of Myf5 expression has been thoroughly investigated leading to
the identification of a large number of discrete enhancers [12].
Notochord and floor plate produce Sonic Hedgehog, regulating
epaxial expression of Myf5. Mice mutant for Shh have a reduced
Myf5 expression and lack epaxial musculature [13]. This regulation
signals through GLI1, which directly binds the somite enhancer of
Myf5 [14]. Furthermore compound Gli mutant display somite
defects [15]. In mouse, PAX3 acts upstream of Myf5, in the hypaxial
somite, by directly binding to a separate 145 bp regulatory
element [2]. Strikingly, mice lacking Pax3 in addition to Myf5 and
Mrf4 will lack all body muscles, demonstrating that these factors
also act genetically upstream of MyoD [16] (Fig. 1).
In the absence of Mrf4, MyoD or Myogenin, Myf5 alone is not
able to drive muscle differentiation [17]. In the myotome, Mrf4
expression closely follows that of Myf5 [18]. However, Mrf4 is
not necessary for muscle differentiation and its absence does not
lead to muscle defect [19]. Nevertheless, the importance of MRF4
function as a determination and differentiation factor is revealed
Genetic control of trunk musculature
The dermomyotome contains the reservoir of proliferative skeletal
muscle progenitors cells [1]. Key markers of these cells are the
Paired-Homeobox transcription factors Pax3 and Pax7. Pax3 expression is found during organogenesis in various tissues among
which the dorsal neural tube, neural crest cells, and body muscle
cells. In addition to neural crest cell loss and dorsal neural tube
closure defects, the absence of PAX3 causes impaired muscle
development. Mice homozygous for the Pax3 Splotch allele, which
have a spontaneous mutation in Pax3, have a complete loss of the
hypaxial domain of the somite and as a result a loss of limb and
some of the trunk muscles, while epaxial-derived muscles are less
affected, revealing that hypaxial and epaxial muscles do not have
the same requirement for PAX3 (Fig. 1), which also acts as a
survival factor in these domains [2]. Pax7, however, is expressed
later in PAX3-expressing MPCs and is not mandatory for muscle
development; the consequence of Pax7 absence is only revealed
later on, at the post-natal stage [3]. The combined lack of Pax3 and
Pax7 however, is deleterious for trunk muscles formation since
Pax3/Pax7-deficient MPCs are unable to enter the myogenic
program, underlining the importance and overlapping functions
of these two factors in building the muscle lineage [4].
What, then, induces these factors? The genetic network Six-EyaDach is a major regulator of Pax3 expression in the dermomyotome.
Notably, the overexpression of both Dach2/Eya2 or Six1/Eya2 in
GENETIC HIERARCHY IN MYOGENESIS
Hypaxial somite
Paraxis
Six1/4
Epaxial somite
Paraxis
Six1/4
Mrf4
Myogenin
MyoD
My5
>E10
My5
Mrf4
Branchiomeric muscles
Meox2 Six1/4
Pax3
Pax3
>E10
Limb
Pitx2
Pax3
Myf5
Tbx1
Mrf4
Six1/4
MyoR, Capsulin
Myf5
Adult
Pax3/7
Myf5
Mrf4
Myogenin
MyoD
MyoD
MyoD
Myogenin
Myogenin
MyoD
Mrf4
Myogenin
Fig. 1 – The molecular genetic pathways engaged in hypaxial and epaxial somite, limb, branchiomeric and adult myogenesis, are
schematically represented. We distinguished two modes of interaction: known direct interaction at the transcriptional level (full
line), from genetic interaction (dashed line). In both hypaxial and epaxial somites, MyoD starts to play its role in the genetic
hierarchy at E10 (red line).
E XP E RI ME N T AL C E L L R E S EA RC H 31 6 ( 20 1 0) 3 0 8 1– 3 08 6
in a MyoD−/−; Myf5loxp/loxp context, indeed, when MyoD and Myf5
are lacking, MRF4 alone is able to sustain muscle development
[20]. Interestingly, if both Mrf4 and Myf5 are missing, there is a
delayed formation of the myotome accompanied by a delayed
expression of MyoD [16].
Paraxis, a bHLH transcription factor is also involved in this early
control of myogenic specification. In Paraxis null embryo, the paraxial
mesoderm does not form epithelial somites, but disrupted skeletal
muscles develop [21], despite the loss of Pax3 expression. Analysis of
Paraxis: Myf5 mutant mice demonstrated that Paraxis, acting upstream
of Pax3 in the epaxial somite also controls MyoD expression [22].
MYOD is considered as a major transcription factor for muscle
lineage formation, on the basis of its ability (in fact shared with other
MRFs) to induce myogenic fate in fibroblasts and other cell types [1].
Moreover, double and triple compound mutant mice have shown that
MyoD, in combination with other MRFs, is involved both in myogenic
determination and differentiation of skeletal muscle cells, underlining
MYOD pivotal role in myogenesis (Fig. 1). During development, MyoD
is first detectable at E10 in hypaxial somite, its expression is under the
control of the transcription factors PAX3, MRF4, MYF5 and SIX1/SIX4.
Indeed, overexpression of Pax3 in chick somite explants is sufficient to
trigger MyoD expression [23]. Extra-cellular signals coming from the
dorsal ectoderm (such as WNTs) and the lateral plate mesoderm
(including BMPs and Notch signaling) regulate positively and
negatively, respectively, the expression of MyoD [1]. The absence of
MyoD is not deleterious for embryonic development, still its
importance is revealed during adult muscle regeneration. MyoD−/−
embryos show no obvious phenotype in the trunk or in head muscles,
likely due to functional compensation by other MRFs. In the limb,
however, myogenesis is paused between E11.5 and E13.5. MYOD
triggers myoblasts differentiation by activating the expression of the
fourth and last MRF implicated in myogenesis: Myogenin.
In somites Myogenin expression is first detectable at E9.25 in
the myotome, at that stage Myogenin is genetically downstream of
Myf5 and Mrf4 (Fig. 1), and indeed the Myogenin promoter regions
contain two E-Boxes directly bound by MYF5, MRF4 or MYOD, in
addition to MEF2 and MEF3 sites. Myogenin−/− pups die at birth
due to respiratory failure and the histology analysis revealed that
limb muscles area are populated with mononucleated cells and
rare myofibers, revealing that in vivo, Myogenin is necessary for
mononucleated myoblasts fusion to form myotubes [1]. Interestingly, Myogenin-derived myoblasts fuse normally in vitro, suggesting that in vivo, in the absence of Myogenin, the environment
negatively regulates myoblasts fusion, or that other MRFs are
deregulated in this context and can compensate for the requirement of Myogenin. In vivo, however, other MRFs, whose expression
is apparently not altered in Myogenin−/−, could not compensate
for the loss of Myogenin. Strikingly, MyoD:Mrf4 double mutant
mice also display similar differentiation defects, yet, Myogenin is
still expressed in these double mutant mice, suggesting that MYF5
may be sufficient to activate Myogenin, but the overall level of MRF
(MYOGENIN and MYF5) in this context does not reach a threshold
required for myogenic differentiation.
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process starts around E9.25 in the forelimb and ends around E11.0
in the hindlimb. During their migration, progenitor cells proliferate and do not express any MRF. Once they reach their final
destination in the limb, they will quickly start to differentiate (first
expressing Myf5, then after a few hours, MyoD and Myogenin) and
form the ventral and dorsal muscle masses of the fore- and
hindlimbs.
Pax3 but not Pax7 is expressed in migrating progenitor cells.
Pax3 mutant embryos are devoid of limb muscles, due to apoptosis
in the hypaxial somites, and display a loss of the tyrosine kinase
receptor c-MET expression. c-MET and its ligand, Hepatocyte
Growth Factor (HGF) or Scatter factor (SF), are essential for the
delamination and migration of progenitors cells. While c-MET is
expressed in muscle progenitors cells, its ligand is secreted by
surrounding mesenchymal cells [24]. The absence of c-MET
or HGF/SF prevents the progenitor cells from delaminating and
migrating from the dermomyotome and no muscles are subsequently found in the limbs [25].
Lbx1 is another homeobox gene co-expressed with Pax3 in
migrating MPCs. Dorsal muscles from forelimb and all muscles
from hindlimb are missing in Lbx1 mutants. While cells do migrate
out from the dermomyotome they cannot find their way to the
limb bud [25]. This observation also revealed that the cell autonomous effect of Lbx1 affects differently forelimb from hindlimb
and ventral from dorsal muscle masses.
Mutations in the homeobox Meox2 gene also lead to specific
loss of a subset of limb muscles [26]. Meox2 is expressed in the
paraxial mesoderm and in the migrating MPCs into the limb bud.
Interestingly, in the limb, Pax3 and Myf5 expression, but not that
of MyoD, is downregulated in Meox2 mutant embryos, indicating
that in the limb, unlike in the trunk, MyoD regulation is not under
the control PAX3 and MYF5. In any case, in the context of a
MEOX2 depleted environment, MYOD is not sufficient to sustain
myogenesis [26].
The role and regulations of the MRFs are distinct in the trunk
and limb (Fig. 1) : while SIX1/4 does not control Myf5 expression
in the dermomyotome, it does so in the limb, via the very same
145 bp enhancer that PAX3 uses to control Myf5 expression in the
hypaxial somite [27]. Moreover, myogenesis is delayed in somites
from Myf5nLacZ/nLacZ [Myf5:Mrf4 double mutant], it proceeds normally in the limb, revealing that neither MYF5 nor MRF4 are
necessary for limb muscle development. While MyoD is dispensable for trunk myogenesis, where Mrf4 is expressed and can
functionally replace MyoD, in the limb, myogenesis is stalled for
2 embryonic days between E11.5 and E13.5 in MyoD mutant
embryos, time at which Mrf4 expression is induced in the limb,
rescuing the phenotype [28].
Since Myf5 and MyoD have two distinct roles, the question
whether all trunk and limb muscles are derived from a single
lineage, has also been addressed. The specific ablation of Myf5
expressing cells, using Myf5Cre; R26RDTA/+, revealed the presence
of a distinct myogenic MyoD + lineage [29,30].
Genetic control of facial muscles development
Genetic control of limb muscle development
Limb musculature develops from a few thousands of muscle
progenitors cells that have delaminated from the hypaxial part of
the dermomyotome and migrated into the opposite limb bud. This
Head muscles find their singularity in their developmental origin
and in the unique genetic network of transcriptions factors
involved in the establishment of this musculature. While all
trunk and limb muscles originate from paraxial mesoderm, head
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E XP E RI ME N T AL C E L L R E SE A RC H 31 6 ( 20 1 0) 3 0 81 – 3 08 6
muscles are derived from pre-chordal and pharyngeal head
mesoderm. Branchiomeric muscle progenitor cells migrate and
differentiate within the mesodermal core of the different head
muscles [31], and are surrounded by endodermal and migrating
neural crest cells. Although the same MRFs are involved in the
myoblasts formation in the head, trunk and limb, the upstream
factors and regulation of the MRFs differ. The first evidence of this
difference came from the analysis of Pax3Sp/Sp; Myf5nLacZ/nLacZ
embryos [Pax3:Myf5:Mrf4 mutants], in which all trunk and limb
muscles are missing but head muscles are unaffected [16]. In the
head, Pax3 is expressed in cranial neural crest-derived cells, but
not in mesodermal derivatives. Pax7, however, is expressed in
branchiomeric muscles and other head muscles, but its role as a
master regulator of head musculature has been excluded since
Pax7−/− mice do not display head muscle phenotype [32]. Instead,
four transcription factors have been identified to control the
induction of MRFs expression: MyoR, Capsulin, Tbx1 and Pitx2.
MyoR and Capsulin belong to the bHLH family of transcription
factors, they play a redundant function in the specification of
the future masticatory muscles. Single MyoR−/− or Capsulin−/−
mutants do not display head muscle defects, however the
compound homozygote MyoR−/−; Capsulin−/− lack 1st branchial
arch (BA)-derived muscles, whereas other head muscles were
not affected. This defect was the result of an absence of MRFs
expression in the first BA and an increased apoptosis in the cells
that failed to enter the myogenic program [33]. This finding reveals
that distinct genetic programs regulate different groups of muscles
within the head [31]. Tbx1 belongs to the family of the T-boxcontaining genes, and is expressed in the branchial arches before
the activation of the MRFs. Tbx1 mutant mice die around birth and
have defects in craniofacial and cardiovascular structures [31], in
addition to impaired branchiomeric muscles formation. Tbx1−/−
muscle precursor cells correctly migrate into the 1st BA as
indicated by the presence of MyoR and Capsulin mRNA, however
they fail to robustly activate Myf5 or MyoD, leaving only a small
population of cells to enter the myogenic program. At later stages,
branchiomeric muscles are lost except for sparse unilateral muscle
masses retaining MRFs expression. Tbx1 ensures the strong
bilateral triggering of branchiomeric myogenesis [34]. Pitx2 is
another factor part of the core regulatory network of transcription
factor engaged in head musculature development. Pitx2 is a bicoidrelated homeobox transcription factor; its expression precedes
that of the myogenic factors and remained in all MRFs positive
cells during myogenesis. In the head, Pitx2 mutation results in the
loss of 1st BA-derived muscles including absence of peri-ocular
and jaw muscles [35,36]. Lineage tracing studies established that at
E10.5 Pitx2Cre-derived cells are found in the mesodermal core of
the 1st BA albeit in small number [36]. These cells failed to activate
Myf5 and a strong reduction of MyoD expression was also observed. Myogenic cells specification failure was accompanied by an
increased level of apoptosis and by E13.5, the 1st BA was lost. The
expression of Tbx1, Capsulin and MyoR was also compromised in
the myogenic field of the 1st BA, suggesting that PITX2 acts
genetically upstream of these myogenic progenitor cells regulators
in the mandibular arch.
Analyses of a combination of MRFs mutants shed light on head
muscle-specific requirement for Myf5 and Mrf4. Myf5nLacZ/nLacZ
embryos lack extra ocular muscles (EOM) while mandibular
muscles developed normally. In Myf5nLacZ/nLacZ EOM, MyoD
expression is not initiated despite the presence of Pitx2, revealing
that in normal condition it is MYF5 or MRF4 that trigger MyoD
expression and EOM accurate differentiation. In the BA, the absence of Myf5 and Mrf4 is not deleterious, and like in the somite,
MyoD is able to drive myogenesis. While in the somite, PAX3 is
responsible for the rescue of MyoD activation; analysis of the
compound Tbx1−/−; Myf5loxp/loxp revealed the sequence of events
during BA development. Strikingly, MyoD expression is lost in
these embryos. This has several implication, first, unlike in the
EOM, MRF4 cannot rescue MyoD expression, nor can PITX2, second
it suggests that Tbx1 is genetically equivalent to Pax3 during BA
development [37].
Hence, it appears that the core network of genes engaged in
head myogenesis not only differ from that of the body (Fig. 1), but
also differ among the different groups of head muscles.
From the embryo to the adult
Muscle masses are composed of a pool of skeletal muscle progenitors cells, which continue to proliferate and at same time
provide differentiated cells, building the embryonic and fetal
muscle masses. The differential origin of the embryonic and fetal
muscle cells has been addressed in a recent study. Using a Pax3Cre/+;
R26RDTA/+, in which cells that express Pax3 are specifically deleted,
it was shown that PAX3+ progenitor cells give rise to all PAX7+
cells and are necessary for embryonic (limb in this case) myogenesis.
Limb fetal myogenesis, however, is established from Pax7+
progenitors cells [38], that will later give rise to the adult muscle
stem cells, called satellite cells (SC). SC have first been recognized by
Alexander Mauro in 1961, and have been defined by their anatomic
location beneath the basal lamina of the myofiber [39]. Since then,
other cells present in the muscle, non-somitically derived, have been
proposed to serve too, as resident muscle stem cells [40] or, to the
least, to participate in muscle regeneration after injury [41]. Lineage
tracing experiments by way of GFP labeling as well as classic quail/
chick grafting rendered possible to track the origin of satellite cells
to the Pax3/Pax7 muscle progenitor cells in the dermomyotome,
embryonic and fetal muscle masses [42,4]. However, studies
conducted both in murine and avian models have established that
head muscle stem cells, unlike trunk and limbs, do not derive from
Pax3 expressing cells, but from cells which have expressed Isl1, a
marker of the pharyngeal mesoderm [43], therefore the origin of
SC is closely dependent on the origin of the muscle itself. Despite
their different developmental origin, SC from branchiomeric, or
extra-ocular, muscles are able to regenerate muscle when
transplanted into a damaged limb [43,37]. Interestingly, at birth
there is a switch of myogenesis mode, as first suggested by work
on single fibers that showed that quiescent muscle satellite cells
can be derived from committed MyoD+ cells [39,44]. This was
recently confirmed genetically by analysis of MyoDiCre/+ mice,
demonstrating that all satellite cells have expressed MyoD at some
point in their history [45].
A classic question in stem cell biology regards the mechanisms
and factors that ensure stem cells maintenance: while it remains
unclear whether embryonic muscle progenitor cells self-renew
efficiently, SC are able to accurately replenish their pool. The set of
transcription factors, which activate myogenesis during embryonic development, is redeployed in the adult (Fig. 1) to control
muscle stem cells activation during post-natal growth, tissue
injury and the replenishment of the stem cell niche. SC express
E XP E RI ME N T AL C E L L R E S EA RC H 31 6 ( 20 1 0) 3 0 8 1– 3 08 6
Pax7 and, in a subset of muscles, Pax3 [40]. Pax7-deficient mice
present a rapid loss of SC after birth, suggesting that PAX3 cannot
compensate for the loss of Pax7 and a role of Pax7 in SC post-natal
survival and self-renewal [3,46,47]. The absolute requirement for
Pax7 and Pax3 has recently been challenged. Generation of conditional Pax7, Pax3 as well as Pax7cre-ERT2 mouse line that allow the
suppression of either genes expression in a time and tissuespecific manner, revealed that the requirement of PAX3 and PAX7
exists up to three weeks after birth but is strikingly lost thereafter
[48]. This observation suggests that Pax3 and Pax7 are needed
for early post-natal growth but not after, although they remain
express in adult SC. The need for Pax3 and Pax7 is associated with
embryonic, fetal muscle progenitors and young SC, whether this
independency towards Pax3 and Pax7 marks the decline of SC
stemness, or a switch of myogenic specification mechanisms remain to be elucidated.
The role of Myf5 during adult muscle regeneration can be
brought to light in a specific Myf5 null allele [49]. After muscle
injury, these mice show an increased number of hypertrophic
fibers, delayed differentiation and less efficient muscle
regeneration.
After injury, the quiescent SC will quit the G0 cell cycle state, reenter the myogenic program and quickly express MyoD, they are
then called activated SC. Numerous studies have taken advantage
of the isolated single fibers model to dissect adult myogenesis [44].
After activation, PAX7+ MYOD+ SC actively proliferate, most of
the cells will then activate the myogenin differentiating gene
and fuse with existing myofibers, while a small proportion of the
cells will go back to an undifferentiated PAX7+ only stage and repopulate the SC niche. The role of MYOD in adult myogenesis is
revealed by the phenotype of MyoD−/− mice, these one have a
reduced body size and have an impair muscle regeneration due
to the failure of SC to differentiate into mature myoblasts, as a
consequence more SC accumulate in MyoD−/− muscles [50].
Interestingly, while Myogenin is very important during embryonic
myogenesis, adult mouse deficient for Myogenin did not show any
muscle defects [51], suggesting that MYOGENIN have different
function during development in adult myogenesis.
SC have to maintain their stemness but also their lineage
memory. Hence, it is the equilibrium between genes that will act to
prevent differentiation and promote self-renewal with genes that
will allow the dormant SC to efficiently re-enter and progress
through the myogenic lineage, that will make SC to function as
bona fide healing cells for the damaged muscle.
Concluding remarks
In the embryos and adult, it appears that a core set of transcription
factors, namely, the MRFs, are engaged in the development of
all muscle cells in the body; the expression of their upstream
regulators, however, is tissue and time specific. While our understanding of the genetic control of muscle formation has increased,
many questions are left unanswered: clearly, this battery of muscle
genes may not only be regulated by transcription factors. For
instance, to what extent microRNA [12] and epigenetic modifications control myogenesis? How do all of these pathways interact
with one another to ensure proper muscle development? How
do they respond to the neighboring environment in normal and
pathological situation? Can we mimick in non-muscle cells the
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genetic signature of muscle stem cells for regenerative medicine?
It makes no doubt that in the next decade, more and more layers
will be added to the complexity of the genetic control of muscle
lineage.
Acknowledgments
Our Research is funded by grants from INSERM Avenir program,
Association Francaise contre les Myopathies, Association Institut
de Myologie, La Ligue contre le Cancer, European consortium FP7
HEALTH-2009 Endostem, Decrypthon program grant, INCa network grant.
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