Mesp1 at the Heart of Mesoderm Lineage Specification

Article (PDF Available)inCell stem cell 3(1):1-2 · August 2008with6 Reads
DOI: 10.1016/j.stem.2008.06.017 · Source: PubMed
Abstract
Stem cell-based cardiac regeneration requires a detailed understanding of the factors that induce cardiac lineage commitment. In this issue of Cell Stem Cell, Lindsley et al. (2008) and Bondue et al. (2008) use embryonic stem cells to identify a key role for Mesp1 in this process.

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Cell Stem Cell
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Mesp1 at the Heart of Mesoderm Lineage Specification
Sean M. Wu
1,2,
*
1
Cardiovascular Research Center, Massachusetts General Hospital, Boston, MA 02114, USA
2
Harvard Stem Cell Institute, Cambridge, MA 02138, USA
*Correspondence: smwu@partners.org
DOI 10.1016/j.stem.2008.06.017
Stem cell-based cardiac regeneration requires a detailed understanding of the factors that induce cardiac
lineage commitment. In this issue of Cell Stem Cell, Lindsley et al. (2008) and Bondue et al. (2008) use embry-
onic stem cells to identify a key role for Mesp1 in this process .
Recent advances in stem cell biology are
poised to transform our therapeutic ap-
proaches to a variety of degenerative
cardiovascular diseases. For embryonic
stem cells (ESCs) or induced pluripotent
stem cells (iPSCs) to play a role, a compre-
hensive understanding of the regulatory
pathways that expand and functionally
differentiate cardiac cells from their multi-
potent mesoderm precursors is required.
By dissecting the transcriptional hierarchy
governing cardiovascular lineage commit-
ment, Lindsley et al. and Bondue et al.
have now demonstrated a key role for
mesoderm posterior 1 (Mesp1) in this
process (Lindsley et al., 2008; Bondue
et al., 2008).
During embryonic development, the
formation of the nascent mesoderm re-
quires the spatially and temporally regu-
lated expression of Wnt, BMP, and Nodal
pathway molecules (Rossant and Tam,
2004). These factors regulate the ingress
and migration of epithelial cells in the gas-
trulating epiblast (via epithelial-to-mesen-
chymal transformation), resulting in the
generation of mesodermal cells marked
by the expression of Brachyury T. Subse-
quent fate restriction of mesodermal pre-
cursors toward cardiovascular and hema-
topoietic progenitors can be identified
by the expression of Mesp1 and Flk1 (re-
viewed in Wu et al., 2008). Although the
morphological events that occur during
this period have been well described,
the molecular regulators that drive lineage
commitment remain poorly understood.
The introduction of ESCs as an in vitro
model of mammalian embryogenesis has
opened new avenues for study. Despite
lacking spatial organization for the estab-
lishment of morphogen gradients essen-
tial in embryogenesis in vivo, ESC in vitro
differentiation recapitulates many signal-
ing events required for germ layer induc-
tion and lineage-specific differentiation
(Keller, 2005). Importantly, this system
shows remarkable conservation of the
temporal sequence of gene expression
needed to trigger lineage commitment.
To dissect the developmental require-
ment of Wnt signaling in mesoderm in-
duction, Lindsley et al. have previously
shown, using ESC in vitro differentiation
assays, that Wnt pathway activation is
essential for the expression of primitive
streak-associated genes such as Bra-
chyury T, Mixl1, and Evx1, while the loss
of Wnt signal results in failed induction of
mesendoderm lineages as demonstrated
by the absence of Sox17, Goosecoid,
PDGFRa, Flk1, and Mesp1-expressing
cells (Lindsley et al., 2006).
To further explore the role of Wnt sig-
naling in mesoderm lineage specification,
Lindsley et al. (2008) have now found that
inducible overexpression of Mesp1 during
in vitro ESC differentiation can expand
myocardial cells independently of Wnt.
Interestingly, this effect was enhanced
when the cells were cotreated with
Dkk-1, an extracellular inhibitor of Wnt
signaling. These findings are consistent
a recent study demonstrating that ubi-
quitous Mesp1 overexpression results in
the formation of ectopic beating cardiac
cells in zebrafish embryos and enhances
the formation of beating embryoid
bodies in differentiating ESCs (David
et al., 2008).
Although both Lindsley et al. and David
et al. have reached the same conclusion
using similar approaches, the two groups
reported different mechanisms responsi-
ble for the observed effect. In David
et al., the effect of Mesp1 on myocardial
differentiation was ascribed to the ability
of Mesp1 to directly transactivate Dkk-1
expression resulting in an inhibition of
Wnt signaling. In Lindsley et al., the inhibi-
tion of Wnt signaling by Dkk-1 in the
absence of Mesp1 overexpression could
not achieve the level of cardiac lineage
induction seen with Mesp1. These differ-
ences are clarified in Bondue et al., where
Dkk-1 was found not to be a direct tran-
scriptional target of Mesp1 by chromatin
immunoprecipitation assays in differenti-
ating ESCs, nor elevated in expression
in ESCs that overexpressed Mesp1.
Furthermore, by coculturing Mesp1 over-
expressing ESCs with control ESCs,
Bondue et al. showed that Mesp1 pro-
motes cardiovascular lineage commit-
ment in a cell-autonomous manner. Thus,
it appears unlikely that extracellular medi-
ators such as Dkk1 could be solely re-
sponsible for the observed Mesp1 effects.
Given the complex requirement for Wnt
in the development of cardiac lineages,
how can the findings of Lindsley et al.
and Bondue et al. be integrated into the
existing literature? Furthermore, what
implications do these results raise in the
context of the recently identified multipo-
tent cardiovascular progenitor cells (Garry
and Olson, 2006)? The role of Wnt signal-
ing in the development of the embryonic
right ventricle and outflow tract (i.e., sec-
ond heart field derivatives) was recently
reported by six independent groups (for
an overview, see Tzahor, 2007). Collec-
tively, these studies describe a biphasic
role for Wnt signaling in the induction of
the primitive streak and mesoderm forma-
tion and to inhibit myocardial differentia-
tion. The requirement for Wnt in the
mesodermal transition into cardiovascu-
lar progenitor cells is much less clear. In-
hibition of Wnt signaling in the second
heart field progenitor cells revealed a
requirement for Wnt in cardiac progenitor
cell self-renewal and expansion. On the
other hand, constitutive activation of
Wnt signaling in the same progenitor cell
Cell Stem Cell 3, July 2008 ª2008 Elsevier Inc. 1
population illustrated the
need for Wnt inhibition to pro-
mote myocardial cell differen-
tiation. Hence, the critical
switch of the roles played by
Wnt signals during cardiac
development occurs at or
shortly after cardiac progeni-
tor cell commitment (Fig-
ure 1). Given this transition,
do the results from Lindsley
et al. and Bondue et al. sup-
port or contradict this view?
The answer lies in the role
of Mesp1. Mesp1 and its
closely related family mem-
ber Mesp2 have been shown
to be cell autonomously re-
quired for the development of myocardial
cells in the heart (Kitajima et al., 2000).
Mesp1 drives cardiac lineage commit-
ment from undifferentiated mesoderm by
promoting the expression of cardiovascu-
lar lineage-defining transcriptions factors
such as Nkx2.5, Isl1, and myocardin inde-
pendent of Wnt (Bondue et al., 2008;
Lindsley et al., 2008 ). Wnt signaling, how-
ever, also supports the expansion of
both cardiovascular and hematopoietic
progenitor cells. It appears that the ex-
pression of Mesp1 protects cardiac
progenitors from fate switching into he-
matopoietic cells in the presence of Wnt
pathway activation, thereby allowing their
ongoing expansion during early develop-
ment. Once these myocardial precursors
have fully committed, the presence of
Wnt inhibitor is then able to facilitate
their full maturation into differentiated
cardiomyocytes.
The findings in Lindsley et al. and Bon-
due et al. are of particular importance in
cardiac progenitor cell biology and rele-
vant to the development of translational
studies employing ESC- or iPSC-derived
cells. The generation of a large quantity
of cardiomyocytes from in vitro differenti-
ated ESCs has been extremely challeng-
ing and variable from cell line to cell line.
The isolation of a near homogenous pop-
ulation of cardiac cells, however, should
help to minimize teratoma formation fol-
lowing cell transplantation. The current
studies have illuminated the factors re-
quired to generate committed cardiac
progenitor cells from their mesodermal
precursors and demonstrated the exqui-
site timing required to properly trigger
lineage-specific differentiation. The iden-
tification of reliable surface markers to iso-
late a highly purified population of cardiac
progenitor cells and the inductive signals
that promote their full matura-
tion into differentiated, cham-
ber-specific cardiomyocytes
will be the next challenge to
overcome in order for cell-
based cardiovascular regen-
erative therapy to become
one step closer to reality.
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Figure 1. The Biphasic Effect of Wnt Signaling in Cardiac Mesoderm
Induction and Myocardial Differentiation
The role of Wnt signaling in cardiovascular lineage specification and differen-
tiation is depicted in the context of the expression of mesoderm and cardiac
transcription factors during embryogenesis. cTnT, cardiac troponin T.
Cell Stem Cell
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2 Cell Stem Cell 3, July 2008 ª2008 Elsevier Inc.
    • "MESP1, a bHLH protein, is involved in the activation of several cellular processes via activation of transcription, including a cascade of cardiac-specific transcription factors [Saga et al., 2000; Bondue et al., 2008; David et al., 2008; Lindsley et al., 2008; Wu, 2008; Bondue and Blanpain, 2010; Chan et al., 2013]. Consequently, we studied whether the MESP1 variants affected activation of downstream target genes. "
    [Show abstract] [Hide abstract] ABSTRACT: Identifying the genetic etiology of congenital heart disease (CHD) has been challenging despite being one of the most common congenital malformations in humans. We previously identified a microdeletion in a patient with a ventricular septal defect containing over 40 genes including MESP1 (mesoderm posterior bHLH transcription factor 1). Because of the importance of MESP1 as an early regulator of cardiac development in both in vivo and in vitro studies, we tested for MESP1 mutations in 647 patients with congenital conotruncal and related heart defects. We identified six rare, non-synonymous variants not seen in ethnically matched controls and one likely race-specific non-synonymous variant. Functional analyses revealed that three of these variants altered activation of transcription by MESP1. Two of the deleterious variants are located within the conserved HLH domain and thus impair the protein-protein interaction of MESP1 and E47. The third deleterious variant was a loss of function frameshift mutation. Our results suggest that pathologic variants in MESP1 may contribute to the development of CHD and that additional protein partners and downstream targets could likewise contribute to the wide range of causes for CHD. This article is protected by copyright. All rights reserved.
    Full-text · Article · Dec 2015
    • "Wnt/β-catenin signal activation contributes to the promoting effect of 3D PCL nanofibrous scaffolds on the CM differentiation of miPSCs Then, we sought to investigate the underlying mechanism responsible for the promoting effect of the 3D PCL nanofibrous scaffolds on the CM differentiation of miPSCs. MESP1 and TNNT2 represent the beginning of the cardiac mesoderm stage of development [17, 18] and the definite appearance of CM, respectively; thus, the early differentiation period was divided into two stages: D0-3 for early cardiac mesoderm genesis and D4-7 for cardiac myocyte commitment. The following period (D8-15) was considered to consist primarily of miPSC-derived CM maturation (Fig. 5a). "
    [Show abstract] [Hide abstract] ABSTRACT: Environmental factors are important for stem cell lineage specification, and increasing evidence indicates that the nanoscale geometry/topography of the extracellular matrix (ECM) directs stem cell fate. Recently, many three-dimensional (3D) biomimetic nanofibrous scaffolds resembling many characteristics of the native ECM have been used in stem cell-based myocardial tissue engineering. However, the biophysical role and underlying mechanism of 3D nanofibrous scaffolds in cardiomyocyte differentiation of induced pluripotent stem cells (iPSCs) remain unclear. Here, we fabricated a 3D poly-(ε-caprolactone) (PCL) nanofibrous scaffold using the electrospinning method and verified its nanotopography and porous structure by scanning electron microscopy. We seeded murine iPSCs (miPSCs) directly on the 3D PCL nanofibrous scaffold and initiated non-directed, spontaneous differentiation using the monolayer method. After the 3D PCL nanofibrous scaffold was gelatin coated, it was suitable for monolayer miPSC cultivation and cardiomyocyte differentiation. At day 15 of differentiation, miPSCs differentiated into functional cardiomyocytes on the 3D PCL nanofibrous scaffold as evidenced by positive immunostaining of cardiac-specific proteins including cardiac troponin T (cTnT) and myosin light chain 2a (MLC2a). In addition, flow cytometric analysis of cTnT-positive cells and cardiac-specific gene and protein expression of cTnT and sarcomeric alpha actinin (α-actinin) demonstrated that the cardiomyocyte differentiation of miPSCs was more efficient on the 3D PCL nanofibrous scaffold than on normal tissue culture plates (TCPs). Furthermore, early inhibition of Wnt/β-catenin signaling by the selective antagonist Dickkopf-1 significantly reduced the activity of Wnt/β-catenin signaling and decreased the cardiomyocyte differentiation of miPSCs cultured on the 3D PCL nanofibrous scaffold, while the early activation of Wnt/β-catenin signaling by CHIR99021 further increased the cardiomyocyte differentiation of miPSCs. These results indicated that the electrospun 3D PCL nanofibrous scaffolds directly promoted the cardiomyocyte differentiation of miPSCs, which was mediated by the activation of the Wnt/β-catenin signaling during the early period of differentiation. These findings highlighted the biophysical role of 3D nanofibrous scaffolds during the cardiomyocyte differentiation of miPSCs and revealed its underlying mechanism involving Wnt/β-catenin signaling, which will be helpful in guiding future stem cell- and scaffold-based myocardium bioengineering.
    Full-text · Article · Sep 2015
    • "T expression is known to induce the expression of MESP1 [19] . MESP1 is a master regulator of cardiac progenitor specification and is needed to enter the cardiac mesodermal stage202122. The graphene group showed higher expression of FOXA2 on day 14 and SOX17 on day 14 and 21, both of which are endodermal genes, compared with the Matrigel and the glass groups (Fig. 3C). "
    Data · Sep 2014 · BMC Cell Biology
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