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First published online December 21, 2007
doi: 10.1242/10.1242/dev.001883
Review |
1 Massachusetts General Hospital - Cardiovascular Research Center, Charles River
Plaza/CPZN 3208, 185 Cambridge Street, Boston, MA 02114, USA.
2 Klinikum rechts der Isar und Deutsches Herzzentrum - Technische
Universität München, I. Medizinische Klinik - Molekulare
Kardiologie, Ismaninger Strasse 22, 81675 München, Germany.
3 Department of Stem Cell and Regenerative Biology, Harvard University and the
Harvard Stem Cell Institute, Cambridge, MA 02138, USA.
* Authors for correspondence (e-mails: klaugwitz{at}med1.med.tum.de, amoretti{at}med1.med.tum.de, kchien{at}partners.org)
SUMMARY
The creation of regenerative stem cell therapies for heart disease requires that we understand the molecular mechanisms that govern the fates and differentiation of the diverse muscle and non-muscle cell lineages of the heart. Recently, different cardiac cell types have been reported to arise from a common, multipotent Islet1 (Isl1)-positive progenitor, suggesting that a clonal model of heart lineage diversification might occur that is analogous to hematopoiesis. The ability to isolate, renew and differentiate Isl1+ precursors from postnatal and embryonic hearts and from embryonic stem cells provides a powerful cell-based system for characterizing the signaling pathways that control cardiovascular progenitor formation, renewal, lineage specification and conversion to specific differentiated progeny.
"Is myogenesis controlled by a single master gene? Possibly, combinatorial schemes and cell lineage are both used to generate positional identity in the embryo for each individual cell."
Hal Weintraub and colleagues (Davis et
al., 1987
)
Introduction
Heart is muscle. Or so it would seem to the outside observer. From this
vantage point, the discovery of MyoD almost 20 years ago by Weintraub, Lassar
and colleagues (Davis et al.,
1987) represents one of the seminal advances in our understanding
of myogenesis, not only in skeletal muscle, but also in cardiac muscle.
Subsequent studies by many investigators (for a review, see
Srivastava and Olson, 2000;
Olson, 2006) have underscored
this point and have documented the conservation of the pathways and principles
of myogenesis across muscle types, spanning the evolutionary spectrum from
invertebrates to humans. This beautiful body of work has had major
implications for our understanding of both development and disease, and has
uncovered a host of genetic regulatory circuits that contribute to the control
of cardiogenesis. As such, the molecular paradigm for cardiogenesis has
largely been based on studies of skeletal myogenesis, an approach that has
yielded spectacular advances in the field of cardiovascular science and
medicine.
But is heart more than muscle? And if so, might there be another major
paradigm that accounts for the generation of these diverse cell types, which
include endothelial, smooth muscle and conduction system cells? Early cell
lineage-tracing studies in avian systems indicated that a common muscle cell
precursor exists for both working myocardium (atrial and ventricular
myocardium) and the conduction system
(Mikawa, 1999). At the same
time, other studies identified a common progenitor, the hemangioblast, for
endothelial and blood cell lineages, a finding which indicated that a common
precursor might also exist for endothelial cells in the heart
(Choi et al., 1998;
Fehling et al., 2003;
Kouskoff et al., 2005). The
recent discovery of multipotent Islet1-positive (Isl1+) progenitors
in several species, including mouse, rat and human, and in different regions
of the embryonic and adult heart, implies that there might be a stem cell
paradigm for the generation of diverse cell lineages in the heart
(Moretti et al., 2006).
Unexpectedly, a growing body of evidence from multiple independent
laboratories now suggests that, with respect to lineage diversification, the
heart could be like blood, an organ in which a single stem/progenitor cell is
able to generate all of the major cell types of the system
(Kattman et al., 2006;
Wu et al., 2006). The concept
of progressive lineage restriction is well accepted for hematopoiesis but has
not been established in such detail in the development of solid organs such as
the heart. In this review, we explore this idea further by focusing on the
Isl1 cardiovascular progenitor story, placing it into the context of the
generation of diverse cardiovascular lineages, and discussing its implications
for cardiovascular development and disease. The reader is also directed to
reviews on the potential general role of progenitors in cardiogenesis and
disease (see Buckingham et al.,
2005; Chien and Karsenty,
2005; Srivastava,
2006; Black,
2007).
Cardiovascular cell lineages arise from discrete embryonic precursors
The heart is composed of diverse muscle and non-muscle cell lineages:
atrial/ventricular cardiac myocytes, conduction system cells, smooth
muscle/endothelial cells of the coronary arteries and veins, endocardial
cells, valvular components and connective tissue
(Fig. 1). During cardiogenesis,
the differentiation of these multiple heart lineages is under tight spatial
and temporal control, resulting in the coordinated formation of the distinct
tissue components of the heart, including the four specialized chambers,
diverse structures of the conduction system, the endocardium, the heart
valves, the coronary arterial tree and the outflow tract
(Harvey, 2002;
Brand, 2003). Understanding
how this diversity of heart cell lineages arises is a fundamental question
that has major implications for understanding and treating both congenital and
adult heart diseases, a subset of which have recently been shown to be due to
defects in the pathways involved in heart lineage specification
(Schott et al., 1998;
Benson et al., 1999;
Garg et al., 2003;
Pashmforoush et al., 2004)
(Table 1).
|
;
Epstein and Buck, 2000). The
mesenchyme portion of the developing heart and the majority of epicardial
cells are derived from the proepicardium
(Dettman et al., 1998;
Moore et al., 1999;
Manner et al., 2001). The
epicardial mantle begins to envelop the myocardium at the same time that
coronary precursor cells first appear in the heart tube
(Mikawa and Gourdie, 1996) and
retroviral genetic labeling experiments have shown that single vasculogenic
cells of the proepicardium differentiate into solitary vessel-associated
clusters that consist of three cell types: endothelial, smooth muscle and
perivascular connective tissue cells
(Mikawa and Gourdie, 1996).
This evidence suggests that the coronary vasculature might be partially
derived from mesodermal cells in the proepicardium, although the contribution
of the proepicardium to the endothelial lineage is still controversial
(Poelmann et al., 2002). Taken
together, these studies on the contribution of the primordial heart field,
cardiac neural crest, and of the proepicardium to specialized structures
during cardiogenesis indicate that lineage diversification in the heart can
arise, in part, via distinct pools of cardiovascular progenitors that are
spatially and temporally segregated in the developing embryo (see
Fig. 1B).
|
One of the earliest steps in cardiogenesis is the formation of the cardiac
crescent (Fig. 2 and see
Box 1), which is derived from
cells of the mesoderm that become instructed to adopt a cardiac fate in
response to signals from adjacent tissues
(Harvey, 2002;
Srivastava and Olson, 2000).
Recent studies have revealed that the cardiogenic mesoderm in fact consists of
two populations or fields (see Box
1) of cardiac precursor cells that contribute to different parts
of the heart. The earliest population of cardiac progenitors, referred to as
the first heart field, originates in the anterior splanchnic mesoderm, gives
rise first to the cardiac crescent, later to the linear heart tube, and
ultimately contributes to parts of the atrial chambers and the left
ventricular region. The second cardiogenic region, known as the second heart
field, lies anterior and dorsal to the linear heart tube and is derived from
the pharyngeal mesoderm medial to the cardiac crescent (see
Fig. 2A and
Box 1). Cells from this second
heart lineage are added to the developing heart tube and give rise to the
outflow tract, the right ventricular region and the main parts of the atrial
tissue (reviewed by Kelly and Buckingham,
2002; Buckingham et al.,
2005). The discovery of the second heart fieldreads like a
scientific detective story, in which independent pieces of evidence were
brought together to ultimately identify a prime suspect.
|
| Box 1. Glossary of specialized terms Branchial arches: A series of paired segmental structures composed of ectoderm, mesoderm and neural crest cells that are positioned on either side of the developing pharynx. In mammals, the branchial arches contribute to pharyngeal organs and to the connective, skeletal, neural and vascular tissues of the head. Cardiac crescent: Crescent-shaped epithelium that represents the first recognizable cardiac progenitors at the cranial and cranial-lateral parts of the embryo. The cardiac crescent fuses at the midline to form the early heart tube. Cardiogenic mesoderm: Mesoderm is one of the three layers of cells in the early embryo that, together with the endoderm and ectoderm, provides the source of all subsequent cell types that appear during embryogenesis. The cardiogenic mesoderm gives rise to the specialized cells of the heart and includes both first and second heart field precursors. Ductus arteriosus: A shunt connection between the pulmonary artery and the aortic arch that allows most of the blood from the right ventricle to bypass the embryonic lungs. Heart field: A defined special area of cells in the mesoderm that is fated to form the cardiac organ. Linear heart tube: Transient structure of the early developing heart composed of an inner endothelial tube shrouded by a myocardial layer. Myocardial cell lineage: The term lineage emphasizes the history of a cell and its descendents, and the characterization of their clonal contribution to the heart. The first and second myocardial lineages represent the progenitors of the first and second heart fields and their differentiated progeny. Neural crest: Progenitor cells present in vertebrates that arise from the dorsal neural tube and migrate to other sites in the embryo and differentiate into various cell types depending on location. Pharyngeal mesoderm: The mesoderm that is located below the head in the pharynx, which is the part of the embryo where the developing respiratory and digestive systems are situated. Primitive streak: Transitory embryonic structure, present as a strip of cells that prefigures the anterior-posterior axis of the embryo. Proepicardial organ: A cluster of mesothelial cells located on the right side of the external surface of the sinus venosus. These cells contact the dorsal wall of the tubular heart in the region of the atrioventricular junction and form a monolayer that gradually covers the heart. Retrospective clonal analysis: A genetic approach for lineage analysis that is based on random labeling of precursor cells. The example cited in this review employed a lacZ carrying a duplication (laacZ) that renders it non-functional. Only upon a rare, spontaneous intragenic recombination which removes the duplication do cells express the lacZ reporter and can be clonally analyzed.
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The initial evidence that the outflow tract was not present in the linear
heart-tube stage came from a series of in vivo lineage-tracing experiments
performed in chick embryos by de la Cruz and co-workers in the 1970s, which
indicated that the outflow tract myocardium originates from a progenitor
population that is different from the cardiac crescent and is situated
anterior to the heart tube (reviewed by de
la Cruz and Sanchez-Gomez, 2000). Over the past few years, the
source of the outflow tract and the right ventricle has been addressed by
studies from three different laboratories, two performed in chick embryos and
one in mouse embryos (Waldo et al.,
2001; Kelly et al.,
2001; Mjaatvedt et al.,
2001). The lineage studies in chick demonstrated that the
splanchnic mesoderm adjacent to the pharyngeal endoderm migrates in through
the aortic sac to contribute cells to the outflow tract and the right
ventricle of the heart, and that the flow of these progenitors along this path
gives rise to distinct outflow tract regions at different times during cardiac
development (Mjaatvedt et al.,
2001; Waldo et al.,
2001) (see Fig.
2B). In mouse embryos, a lacZ reporter gene integrated
into the genomic fibroblast growth factor 10 (Fgf10) locus marked the
outflow tract and the right ventricle of the developing embryonic heart
(Kelly et al., 2001). This
transgene was expressed at the cardiac-crescent stage of heart development at
embryonic day (E) 7.5, and β-galactosidase (β-gal) activity was
observed in splanchnic mesoderm that lies medial to the classical first heart
field. As development progressed, β-gal+ cells were found in
anterior splanchnic mesoderm adjacent to pharyngeal endoderm and were
subsequently observed in branchial arch mesoderm proximal to the heart, in the
outflow tract and in the right ventricle. Taken together, these studies
demonstrated that cells comprising the earliest fusing myocardium do not
contain all of the progenitors of the outflow tract and the right ventricle
and that these heart structures derive, wholly or in part, from precursor
cells of a second heart field, which are added to and supplement the
myocardium that originates from the first heart field progenitors
(Kelly and Buckingham, 2002).
Furthermore, an elegant retrospective clonal analysis (see
Box 1) in the mouse embryo
suggested that the first and second lineages (see
Box 1) of cardiac progenitors
originate from a common precursor population that segregates prior to the
cardiac-crescent stage (Meilhac et al.,
2004). However, a clear delineation of the existence of the second
myocardial lineage as a separate subset of cardiovascular precursors, and the
ultimate identification of the heart components that it generates, would await
the identification of a suitable genetic marker.
In this regard, recent studies have revealed that the expression of the
LIM-homeodomain transcription factor Isl1 is a marker of the second myocardial
lineage during mammalian cardiogenesis
(Cai et al., 2003), a finding
that ultimately allowed the isolation of the Isl1+ cardiovascular
progenitors themselves (Laugwitz et al.,
2005). LIM/HD proteins are a subset of homeodomain (HD)-containing
transcription factors that are defined on the basis of a common LIM domain,
which consists of a conserved cysteine- and histidine-rich structure of two
tandemly repeated zinc fingers. The acronym of LIM is derived from the first
identified members of this family, namely LIN-11 from Caenorhabditis
elegans (Frey et al.,
1990), Isl1 from rat (Karlsson
et al., 1990), and MEC-3 from C. elegans
(Way and Chalfie, 1988).
Subsequent studies of LIM/HD proteins in embryonic motoneurons have led to the
identification of a combinatorial code of several LIM/HD proteins, including
the Isl1 gene, that controls various aspects of motoneuron identity
(Tsuchida et al., 1994;
Thor et al., 1999;
Thaler et al., 2004). Aside
from its expression in embryonic motoneurons, Isl1 is expressed in a variety
of cell lineages of pancreatic endocrine origin during embryogenesis, as well
as in normal adult islet cells (Karlsson
et al., 1990).
Isl1 knockout mice have been generated and studied for defects
both in motoneuron specification and pancreatic development
(Pfaff et al., 1996;
Ahlgren et al., 1997).
Homozygous Isl1 mutants exhibit growth retardation around E9.5-10,
and die at approximately E10.5-11. Histological analysis of mutant hearts
between E9.0 and 9.5 revealed that homozygous Isl1 mutants have a
severe cardiac phenotype. Isl1-null hearts fail to undergo looping
morphogenesis and appear to have a common atrium and a uni-ventricular
chamber, whereas the right ventricle and the outflow tract are absent
(Cai et al., 2003). Genetic
marker analysis for expression of Tbx5, Hand1 and Fgf10
demonstrated that the remaining ventricular tissue had a left ventricular
identity and confirmed the lack of the right ventricle and the outflow
tract.
These observations suggested that Isl1 is expressed in cells of the second heart field, which contribute to both the venous and arterial poles of the heart. Lineage tracing of Isl1-expressing cells using the Cre-loxP strategy showed that these cells colonize the outflow tract, the right ventricle, part of the atria and a minor portion of the inner curvature of the left ventricle, confirming that this transcription factor marks cardiac progenitors of the second myocardial lineage (see Box 1). Furthermore, Isl1 seems to be required for the survival, proliferation and migration of these cells into the cardiac tube, and its transcription is turned off as the precursor cells differentiate. Since Isl1 expression delineates undifferentiated and differentiated progenitor states, it represents an excellent lineage tracer for cardiac mesodermal cells during embryogenesis.
Is Isl1 expression restricted to the second heart field of cardiac
progenitor cells or is it also transiently expressed in the first? Two lines
of evidence support the latter notion. First, recent lineage-tracing
experiments that used a highly efficient Isl1-Cre knock-in mouse line
showed that the majority of cells in the left ventricle express the genetic
marker β-gal (Park et al.,
2006). However, it is possible that the inefficiency of the
excision of the original Isl1-IRES-Cre allowed cells in which
Isl1 was expressed for a longer period of time to be preferentially
detected (Srinivas et al.,
2001; Cai et al.,
2003). The second piece of evidence comes from recent studies in
mouse and Xenopus. Harvey and co-workers report that, in contrast to
Isl1 mRNA, Isl1 protein is expressed at E7.5 throughout the anterior
intra-embryonic coelomic walls and proximal head mesenchyme, regions that
encompass both first and second heart fields in mouse
(Prall et al., 2007).
Similarly, during neurula stages in Xenopus, Isl1 is co-expressed
with Nkx2-5 throughout the cardiac crescent, which is the first heart
field in amphibians (Brade et al.,
2007). These data suggest that Isl1 might be a pan-cardiac
progenitor marker, but additional work is needed to clarify this issue. In
this regard, the identification of specific markers for the first heart field
would be extremely valuable. However, analyses of the cardiovascular phenotype
of Isl1 knockout mice suggests that Isl1 does not play such
an essential role in the first myocardial precursor lineage as compared with
the second. So far, the earliest molecular pan-cardiac markers for both
myocardial cell lineages are the transcription factors Mesp1/2 (mesoderm
posterior 1/2) and Fgf8, which are expressed transiently in cells of the newly
formed mesoderm at the primitive-streak stage (see
Box 1), the descendents of
which colonize the whole myocardium (Saga
et al., 2000; Kitajima et al., 2003;
Ilagan et al., 2006).
Isl1: an early nodal point in cardiogenesis
A comparison of the cardiovascular defects that arise in Isl1-deficient mouse embryos with those of mice with mutations in other cardiac transcription factors has begun to uncover a genetic regulatory network that controls the fate of Isl1+ cardiovascular precursors in the second heart field (Table 2; Fig. 3).
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) and NK2 transcription factor related, locus 5
(Nkx2-5) (Harvey, 1996).
Recent studies have identified Mef2c (myocyte enhancer factor 2C;
also known as RSRF, related to serum response factor) as a direct
transcriptional target of Isl1 and GATA factors in the second heart field
(Dodou et al., 2004). The
forkhead box H1 transcription factor Foxh1 is also expressed in the second
lineage of cardiac precursors, and Foxh1 mutant mouse embryos, like
Isl1- and Mef2c-deficient embryos, display defects in the
right ventricle and in outflow tract formation
(von Both et al., 2004). Isl1
and Foxh1 directly activate transcription of Mef2c by regulating two
independent enhancer regions in collaboration with GATA factors and Nkx2-5
(Arceci et al., 1993;
Dodou et al., 2004). Thus,
these genes appear to act at the top of a cascade of cardiac transcription
factors in the second myocardial lineage. The SET-domain protein Smyd1 (Bop)
is a direct target of Mef2c and regulates Hand2 (heart and neural
crest derivatives expressed transcript 2) expression during second heart field
development, implying that Smyd1 is an indirect downstream target of Isl1/GATA
factors and Foxh1/Nkx2-5 (Gottlieb et al.,
2002; Phan et al.,
2005). Recently, it has been shown that Forkhead factors are
central components of additional regulatory circuits that intersect and
reinforce the Isl1-GATA-Mef2c main pathway in the second heart lineage
transcriptional network. Foxa2, Foxc1 and Foxc2 can bind and activate in vitro
a Tbx1 enhancer that is sufficient to direct expression to the second
heart field (Maeda et al.,
2006). Tbx1, in turn, activates Fgf8
(Hu et al., 2004), whose
loss-of-function in the second heart field results in a reduction of
Isl1 expression in the pharyngeal mesoderm and outflow tract
(Park et al., 2006).
Taken together, the early segregation of the two lineages, the different
time-course of myocytic differentiation and the distinct regional
contributions to the embryonic heart support the idea that the two progenitor
populations may have discrete properties and distinct transcriptional
hierarchies for cardiac development, with Nkx2-5 as the critical transcription
factor in the first and second lineages, and Isl1, along with Foxh1 and GATA
factors, as the key transcriptional regulators in the second heart progenitor
field (Biben and Harvey, 1997)
(see Fig. 3).
Alternatively, one might question the existence of two unbridgeable
progenitor lineages regulated by distinct transcriptional networks whose
descendents form distinct heart compartments, and instead propose the complex
patterning of one primordial precursor field
(Abu-Issa et al., 2004). Such a
view carries the implication that cardiac precursors within this solitary
field have the capacity to form different heart structures, depending on
positional cues. Temporal microenvironmental stimuli and differing
concentrations of diffusing morphogens could create cellular diversity that
leads to a progressive restriction of developmental potency within a field
that was initially homogenous (Moorman et
al., 2007).
Isl1+ progenitor cells in the embryonic and postnatal heart
The purification, renewal and differentiation of native Isl1+
cardiac progenitors provides a means to unravel the steps for both cardiac
lineage formation and regeneration, and to identify how these steps are linked
to certain forms of congenital and adult cardiac diseases. Taking advantage of
the fact that Isl1 expression is downregulated in most cardiac precursor cells
as they differentiate, recent studies have utilized inducible
Isl1-Cre and knock-in Isl1-nlacZ mice to analyze the timing
of Isl1+ progenitor migration into the looping heart and the
distinct subdomains of the heart that they colonize during embryonic
development (Laugwitz et al.,
2005; Sun et al.,
2007) (Fig. 4).
Interestingly, a subset of Isl1+ undifferentiated progenitors
remains embedded in the embryonic heart after its formation and a few cells
are still detectable after birth in the compartments that arise from
Isl1+ second lineage precursors during cardiac development.
Tamoxifen-inducible Cre-lox technology has enabled this novel postnatal
Isl1+ cell population and its progeny to be selectively marked at a
defined time and purified to relative homogeneity
(Laugwitz et al., 2005). The
ability of these cells to self-renew in vitro on a cardiac mesenchymal feeder
layer and to be stimulated to differentiate into fully mature functional
cardiomyocytes indicates that these cells represent native cardiac
progenitors, remnants of the embryonic Isl1+ precursors
(Laugwitz et al., 2005).
Postnatal Isl1-expressing cells can be detected in mouse, rat and human
myocardium and appear to be distinct from the previously reported cardiac
c-Kit+ and Sca1+ cells (Sca1 is also known as Ly6a),
which were reported to activate cardiomyocyte-specific genes in vitro and to
differentiate into cardiac muscle cells in vivo
(Beltrami et al., 2003;
Oh et al., 2003).
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Isl1+ cardiovascular progenitors and heart lineage diversification
In vivo cell lineage tracing using the Cre-loxP strategy in the mouse has
been an invaluable tool for precisely delineating the specific cell types that
derive from Isl1+ precursors, as well as for defining their
specific locations in the components of the heart, vascular and conduction
systems. To date, several separate Cre mouse lines have proven to be useful
for the in vivo lineage tracing of cardiovascular progenitors from the first
and second heart field. The Isl1-IRES-Cre line
(Srinivas et al., 2001) has
the advantage of driving Cre expression only in undifferentiated cardiac
progenitors, but the disadvantage of being widely expressed later on in
embryonic development. The inducible Isl1-mER-Cre-mER line offers
temporal control over Cre activity
(Laugwitz et al., 2005). In
this line, the presence of the two mutated oestrogen-receptors (mERs) results
in Cre being sequestered in the cytoplasm. In the presence of Tamoxifen, the
mER-Cre-mER protein undergoes rapid nuclear translocation, which allows
Cre-mediated recombination to then occur in those cells that express
Isl1. A transgenic Cre mouse line that allows gene inactivation to be
restricted to the second heart field lineage has been recently described in
which Cre is expressed under a specific Mef2c promoter/enhancer
region (Verzi et al., 2005).
Nkx2-5-Cre and Mesp1-Cre deleter lines have also been well
characterized, and can be utilized to trace lineages from both the first and
second heart fields (Moses et al.,
2001; Kitajima et al.,
2000). Fate-mapping experiments utilizing these different Cre
lines have demonstrated that Mef2c, Nkx2-5 and Mesp1 can mark cell populations
that contribute to myocardial cells and to subsets of endocardium
(Verzi et al., 2005;
Stanley et al., 2002;
Kitajima et al., 2000).
Furthermore, the Cre-mediated lineage tracing of cells that express Flk1 (also
known as Kdr), one of the earliest mesodermal progenitor markers for vascular
endothelial and hematopoietic lineages has, interestingly, shown the potential
of Flk1+ precursors to give rise to both cardiac and smooth muscle
during development (Motoike et al.,
2003; Coultas et al.,
2005).
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|
; Moretti et al.,
2006). In these mice, the Cre-mediated removal of a stop sequence
in the cells that express Isl1 results in the expression/translation
of lacZ, which is itself under the control of the endogenous Rosa26
(R26) promoter. Thus, this approach marks all the cells that once expressed
Isl1 during development, and its use has revealed that a high proportion of
the right ventricular myocardium and parts of both atria consist of cells that
express lacZ and the myocytic protein sarcomeric 
-actinin
(Laugwitz et al., 2005). The
histochemical analysis of β-gal and acetylcholinesterase expression has
revealed that Isl1+ progenitors make a remarkable contribution to
the cells of the conduction system, primarily to the sino-atrial node, the
pacemaker of the heart. Additionally, β-gal staining was observed
throughout the proximal aorta, the trunk of the pulmonary artery and the stems
of the main left and right coronary arteries (see
Fig. 5). The co-expression of
lacZ with endothelial and smooth muscle-specific markers, such as
CD31 (Pecam1), VE-cadherin (cadherin 5) and smooth muscle myosin heavy chain
(SM-MHC; also known as myosin heavy chain 11), revealed that Isl1+
precursors can give rise to vascular lineages. Additional studies in the mouse
embryo have confirmed that Isl1+ cardiovascular progenitors
contribute to over two-thirds of all the cells in the embryonic heart, and to
the major cell types in all of the cardiovascular compartments and conduction
system, with the exception of the free left ventricular wall
(Cai et al., 2003;
Moretti et al., 2006;
Sun et al., 2007).
Taken together, these results demonstrate that Isl1 marks cardiac
precursors that give rise to working cardiac muscle, to the conduction system
and to endothelial/smooth muscle cells in multiple heart tissue compartments
during cardiogenesis (see Fig.
5). Furthermore, this evidence raises the question of what role
Isl1 plays in the specification of each of these mesodermal lineages. Is Isl1
marking an early anterior mesodermal progenitor not yet committed to a cardiac
fate? Or is Isl1 defining a multipotent primordial cardiovascular progenitor,
which contributes to distinct cell lineages within heart components known to
originate from the second heart field? Or do different lineage-restricted
precursors exist, all of which independently express Isl1? The early onset of
Isl1 expression is consistent with the first two possibilities, although
previous retroviral marking experiments have shown, for the proepicardial
progenitors of the coronary vasculature, that clones of precursors can give
rise only to a single phenotype (for example, only to epicardial cells,
endothelial cells, smooth muscle cells or fibroblasts)
(Reese et al., 2002). A clear
delineation of the developmental potency of Isl1+ cells during
cardiogenesis awaits in vivo clonal differentiation analysis.
Multipotent Isl1+ cardiovascular progenitors
Understanding how embryonic precursors generate and control the formation
of distinct endothelial, pacemaker, atrial, ventricular and vascular smooth
muscle lineages, as well as how these cells become positioned to form the
specific chambers, aorta, coronary arteries and conduction system of the
heart, is of fundamental importance for understanding the developmental logic
and molecular cues that underlie both cardiovascular development and disease.
As noted above, the formation of cardiac, smooth muscle and endothelial cell
lineages in the heart has largely been ascribed to a set of non-overlapping
embryonic precursors that have distinct origins. The discovery of several
heart lineage-restricted genes has lead to the suggestion that the generation
of different cardiac cell types could be driven by a unique combinatorial
subset of transcriptional networks that operate within distinct cardiovascular
progenitors (for a review, see Srivastava
and Olson, 2000). An alternative possibility exists that diverse
muscle and non-muscle lineages arise from multipotent, primordial
cardiovascular stem cells, which give rise to a hierarchy of downstream
cellular intermediates that represent the tissue-restricted precursors of
fully differentiated heart cells (Fig.
6). This clonal model of heart lineage diversification is similar
to that of hematopoiesis, which is initiated by a few multipotent
hematopoietic stem cells that generate large numbers of differentiated progeny
by a process of amplification and progressive lineage restriction
(Morrison and Weissman, 1994;
Weissman, 2000).
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). MICPs have been cloned from both mouse ES cells and mouse
embryos, and can make the decision to enter the muscle or endothelial
differentiation pathways at the single-cell level, hinting at a
hematopoietic-like system for how the diverse cardiovascular lineages can be
generated (Moretti et al.,
2006). In support of this concept, a hierarchy of distinct
Isl1+ cardiovascular progenitors has been uncovered
(Laugwitz et al., 2005;
Moretti et al., 2006) (see
Fig. 6), including a rare
subset of Isl1+ cardioblasts that persists until birth and can
develop into fully mature cardiac and smooth muscle cells
(Laugwitz et al., 2005;
Sun et al., 2007). At the same
time, independent in vivo and in vitro studies from other laboratories have
also identified other multipotent and bi-potent cardiovascular precursors
(Kattman et al., 2006;
Moretti et al., 2006;
Wu et al., 2006), which might
also arise from the early heart fields. However, the multi-lineage potential
of MICPs still needs to be confirmed by fate-mapping techniques in vivo.
Uncovering the molecular pathways that control the formation, renewal and
differentiation of cardiovascular progenitors into specific mature cellular
progeny will be crucial for unlocking the potential of stem cell therapy for
use in a myriad of cardiovascular degenerative diseases, such as heart
failure, conduction system disorders and congenital heart disease (see
Table 1). Wnt/β-catenin signaling controls renewal of Isl1+ cardiovascular progenitors
The microenvironment provided by a stem cell niche plays a vital role in
stem cell/progenitor maintenance (for a review, see
Scadden, 2006). Previous
studies have shown that cardiac mesenchymal cells serve in vitro as an
effective microenvironment that allows embryonic-, ES cell- and
postnatal-derived Isl1+ progenitors to renew in culture
(Laugwitz et al., 2005;
Moretti et al., 2006).
Utilizing a chemical screen, recent work from our laboratory has identified
the Wnt/β-catenin pathway as being a major component of the cardiac
mesenchymal microenvironment that controls the pre-specification, renewal, and
subsequent differentiation of a hierarchy of Isl1+ cardiovascular
precursors from mouse ES cells, embryos and postnatal hearts
(Qyang et al., 2007). This
study reported that the inhibition of canonical Wnt signaling in the
mesenchymal cells of the feeder layer promotes the pre-specification of
mesodermal precursors into Isl1+ cardiac progenitors in
vitro, while reducing the expansion of already specified
Isl1+ precursors. Furthermore, this study and several
others demonstrate that the in vivo activation of β-catenin within the
Isl1+ progenitors in the second heart field leads to their
massive accumulation in the pharyngeal mesoderm, the inhibition of their
differentiation, and the onset of outflow tract morphogenetic defects
(Cohen et al., 2007;
Kwon et al., 2007). Similarly,
loss-of-function studies have confirmed the requirement for β-catenin for
the expansion and survival of Isl1+ cardiac precursors in the
embryo in vivo (Lin et al.,
2007). Interestingly, chemical agents that inhibit the activity of
glycogen synthase kinase 3 (GSK3), the kinase that phosphorylates
β-catenin and promotes its degradation, can markedly promote the in vitro
proliferation of ISL1+ cells from human neonatal hearts,
representing a key advance towards the eventual cloning of human
ISL1+ cardiac progenitors
(Qyang et al., 2007).
Isl1+ progenitors and congenital heart disease
Congenital heart disease (CHD), which is one of the most important and
prevalent forms of human birth defect, is present in nearly 1 in 100 live
births and is responsible for the vast majority of prenatal losses
(Hoffman, 1995). Some
congenital cardiovascular malformations occur as syndromes in which multiple
organs are affected. Over the past decade, a number of single-gene mutations
have been correlated with syndromic CHDs, such as the association of mutations
in Tbx1 with DiGeorge syndrome and of Tbx5 with Holt-Oram
syndrome (for reviews, see Gruber and
Epstein, 2004; Ransom and
Srivastava, 2007). Although these syndromes have helped
researchers to elucidate some of the mechanisms of CHD, most CHD occurs in the
absence of any other organ malformation. In humans, 50% of these non-syndromic
CHDs manifest as failed atrial/ventricular septation or outflow tract defects,
and necessitate open-heart surgery to restore normal circulation. A subset of
non-syndromic CHD is familial, and causative genes have been identified, most
of which encode transcription factors that are part of a conserved regulatory
network that controls cardiogenesis (see
Table 1). Mutations in
NKX2-5 have been identified in individuals with atrial/ventricular
septal defects and with conduction system abnormalities, whereas mutations in
GATA4 and TBX20 are found in patients with septation
defects, valve abnormalities and cardiomyopathy (Gruber and Epstein, 2006;
Schott et al., 1998;
Garg et al., 2003;
Kirk et al., 2007). However,
most non-syndromic CHD cases are sporadic and multifactorial, with no single
gene being wholly responsible. Therefore, a major challenge is to dissect the
transcriptional and signaling pathways that regulate cardiogenesis and to
better understand at a genetic and mechanistic level how a cardiac progenitor
cell can develop into each of the specific cell lineages that form the
heart.
The identification of two distinct populations of cardiac precursors, one
that exclusively forms the left ventricle and the other that mainly forms the
outflow tract, the right ventricle and most of the atria, hints at a new
approach to understanding CHDs, not as a defect in a specific gene or
transcription factor, but rather as a defect in the lineage decisions of a
defined subset of cardiac precursors. In this manner, CHDs might be associated
with alterations in the formation, expansion and differentiation of embryonic
cardiac progenitor cells, which in turn form essential components of the
heart, such as the atria, ventricles, coronary arteries and conduction system
(Goldmuntz et al., 2001;
Pashmforoush et al., 2004;
Prall et al., 2007). The
discovery of a unique Isl1+ cardiovascular progenitor that
contributes to all of these structures in the heart has important implications
for understanding the mechanistic origins of CHD. In this regard, a series of
studies of the localization of Isl1+ progenitors in patients with
diverse forms of CHD (atresia of the aortic and mitral valve, transposition of
the great arteries, ventricular septal defects and an interrupted aortic arch)
have added important new insights into the potential role of Isl1+
progenitors in the closure of the atrial septum, in remodeling events that
occur in the latest stages of cardiac morphogenesis or in the immediate
postnatal window when the right ventricle starts to pump blood into the
pulmonary circulation (Laugwitz et al.,
2005). Moreover, the recent discovery of the role of
β-catenin pathways in regulating the renewal and differentiation of
Isl1+ cardiovascular precursors in vivo
(Cohen et al., 2007;
Kwon et al., 2007;
Lin et al., 2007;
Qyang et al., 2007) is likely
to have a significant impact on our understanding of several forms of CHDs
that involve outflow tract defects, especially those implicating mal-rotations
or outflow tract dysplasia.
Isl1+ progenitors and cardiovascular regenerative medicine
Whereas the pivotal role of Isl1+ cardiovascular progenitor
cells in the formation of the major heart lineages is clear, the importance of
these cells in endogenous programs of cardiovascular regeneration is still
unknown. Cardiomyocytes rarely seem to enter the cell cycle after birth and,
consequently, the heart has a very limited regenerative capacity following
injury. Given that the number of Isl1+ cells within the heart is
vanishingly small after the postnatal window, it is unlikely that they play a
role in the regeneration of the adult working myocardium. However, there is a
persistence of the postnatal Isl1+ cells within the cardiac
autonomic nervous system in regions that intersect with the cardiac conduction
system, raising the question as to their role in the maintenance of normal
cardiac conduction (Moretti et al.,
2006; Sun et al.,
2007). The location of Isl1+ cells in the
embryonic/fetal structures that are associated with CHDs (outflow tract,
inter-atrial septum, etc) suggests that these cells might participate in
regenerative pathways in these tissues in response to genetic or
environmentally induced injury in the hearts of newborns. Uncovering the
stimuli that might lead to the in vivo mobilization of Isl1+
cardiovascular progenitor cells will be of importance for understanding their
contribution to endogenous regenerative pathways.
The ability to isolate and clonally expand multipotent Isl1+
cardiovascular progenitors from mouse ES cells encourages the hope that the
same could be achieved in human ES cells. If so, it could form the basis for
the generation of models of human CHD and of adult forms of heart disease. The
technology to genetically manipulate human ES cells is moving forward rapidly,
as are technologies that will allow the generation of ES cells via somatic
cell nuclear transfer (SCNT) without requiring the use of eggs
(Hochedlinger and Jaenisch,
2002). Moreover, recent work from several independent groups has
demonstrated the possibility of reprogramming in vitro, differentiated mouse
adult fibroblasts into a pluripotent ES cell-like state through the ectopic
expression of only a few defined factors: Oct3/4 (also known as Pou5f1), Sox2,
c-Myc and Klf4 (Maherali et al.,
2007; Okita et al.,
2007; Takahashi and Yamanaka,
2006; Wernig et al.,
2007). This finding represents an important achievement in
controlling pluripotency and might allow patient-specific pluripotent ES-like
cells to be created directly from somatic cells, which could be a powerful
tool for studying human disease in a `culture dish'. The human ES cell-based
ISL1+ cardiovascular progenitor model system could prove to be
extremely valuable in designing new human-based assay systems for screening
the toxicity of new drugs during the early stages of cardiac development, as
well as for identifying and validating new therapeutic targets in specific
human cardiovascular cell types, such as in coronary vascular smooth muscle,
pulmonary arterial smooth muscle, coronary endothelial cells and conduction
system cells.
Conclusion
In the search for master cardiovascular genes, it has become increasingly
apparent that the generation of the diverse cell lineages of the heart is
likely to be due to the employment of combinatorial codes for specific cell
types, akin to the theory proposed by Weintraub and colleagues almost 20 years
ago (Davis et al., 1987). At
the same time, recent studies of Isl1+ cardiovascular progenitors
suggest that a key element of lineage diversification in the heart relates to
the presence of a renewable, rare subset of multipotent cells, perhaps a
master heart progenitor, that ultimately gives rise to over two-thirds of the
heart and to the heart's three major cell types: cardiac muscle, smooth muscle
and endothelium. The generation of specific cardiovascular cell types might
ultimately relate more to a series of decisions that result in a sequential
increase in the restriction of multipotent cardiovascular progenitors to
specific cellular intermediates and their differentiated derivatives, than to
the functioning of dominantly acting genes that enforce lineage specification.
As such, understanding cardiogenesis at the level of specific decisions that
are made by discrete heart cell lineages requires that we identify the
pathways that govern critical steps in the formation, renewal, specification
and differentiation of the hierarchy of Isl1+ progenitors and their
derivatives. The identification of specific markers that enable FACS
purification of these intermediates, and genetic approaches to reveal key
steps in their cell fate decisions, will be essential. In short, if heart is
like blood, then approaching cardiogenesis as `cardiopoiesis' might represent
the road ahead.
ACKNOWLEDGMENTS
The authors especially thank Sylvia Evans for her continuous support. We also thank members of the laboratories of Karl-Ludwig Laugwitz and Kenneth R. Chien for their helpful discussions and comments. We apologize to colleagues whose work is not mentioned here owing to space limitations. The authors are supported by the Massachusetts General Hospital and the Cardiovascular Disease Program of the Harvard Stem Cell Institute, a Marie Curie Excellence Team Grant from the European Research Council (MEXT-23208), the German Research Foundation (La 1238 3-1/4-1), the National Heart, Lung and Blood Institute, and the Jean Le Ducq Foundation.
REFERENCES
Abu-Issa, R., Smyth, G., Smoak, I., Yamamura, K. and Meyers, E.
N. (2002). Fgf8 is required for pharyngeal arch and
cardiovascular development in the mouse. Development
129,4613
-4625.
Abu-Issa, R., Waldo, K. and Kirby, M. L. (2004). Heart fields: one, two or more? Dev. Biol. 272,281 -285.[CrossRef][Medline]
Ahlgren, U., Pfaff, S. L., Jessell, T. M., Edlund, T. and Edlund, H. (1997). Independent requirement for ISL1 in formation of pancreatic mesenchyme and islet cells. Nature 385,257 -260.[CrossRef][Medline]
Arceci, R. J., King, A. A., Simon, M. C., Orkin, S. H. and
Wilson, D. B. (1993). Mouse GATA-4: a retinoic acid-inducible
GATA-binding transcription factor expressed in endodermally derived tissues
and heart. Mol. Cell. Biol.
13,2235
-2246.
Beltrami, A. P., Barlucchi, L., Torella, D., Baker, M., Limana, F., Chimenti, S., Kasahara, H., Rota, M., Musso, E., Urbanek, K. et al. (2003). Adult cardiac stem cells are multipotent and support myocardial regeneration. Cell 114,763 -776.[CrossRef][Medline]
Benson, D. W., Silberbach, G. M., Kavanaugh-McHugh, A., Cottrill, C., Zhang, Y., Riggs, S., Smalls, O., Johnson, M. C., Watson, M. S., Seidman, J. G. et al. (1999). Mutations in the cardiac transcription factor Nkx2.5 affect diverse cardiac developmental pathways. J. Clin. Invest. 104,1567 -1573.[Medline]
Biben, C. and Harvey, R. P. (1997). Homeodomain
factor Nkx2.5 controls left/right asymmetric expression of bHLH gene eHand
during murine heart development. Genes Dev.
11,1357
-1369.
Black, B. L. (2007). Transcriptional pathways in second heart field development. Cell Dev. Biol. 18, 67-76.[CrossRef]
Brade, T., Gessert, S., Kühl, M. and Pandur, P. (2007). The amphibian second heart field: Xenopus islet-1 is required for cardiovascular development. Dev. Biol. doi: 10.1016/j.ydbio.2007.08.004.
Brand, T. (2003). Heart development: molecular insights into cardiac specification and early morphogenesis. Dev. Biol. 258,1 -19.[CrossRef][Medline]
Brown, C. B., Wenning, J. M., Lu, M. M., Epstein, D. J., Meyers, E. N. and Epstein, J. A. (2004). Cre-mediated excision of Fgf8 in the Tbx1 expression domain reveals a critical role for Fgf8 in cardiovascular development in the mouse. Dev. Biol. 267,190 -202.[CrossRef][Medline]
Buckingham, M., Meilhac, S. and Zaffran, S. (2005). Building the mammalian heart from two sources of myocardial cells. Nat. Rev. Genet. 6, 826-835.[CrossRef][Medline]
Cai, C. L., Liang, X., Shi, Y., Chu, P. H., Pfaff, S. L., Chen, J. and Evans, S. (2003). Isl1 identifies a cardiac progenitor population that proliferates prior to differentiation and contributes a majority of cells to the heart. Dev. Cell 5, 877-889.[CrossRef][Medline]
Cai, C. L., Zhou, W., Yang, L., Bu, L., Qyang, Y., Zhang, X.,
Li, X., Rosenfeld, M. G., Chen, J. and Evans, S. (2005).
T-box genes coordinate regional rates of proliferation and regional
specification during cardiogenesis. Development
132,2475
-2487.
Charron, F. and Nemer, M. (1999). GATA transcription factors and cardiac development. Semin. Cell Dev. Biol. 10,85 -91.[CrossRef][Medline]
Chien, K. R. and Karsenty, G. (2005). Longevity and lineages: toward the integrative biology of degenerative disease. Cell 120,533 -544.[CrossRef][Medline]
Ching, Y. H., Ghosh, T. K., Cross, S. J., Packham, E. A., Honeymon, L., Loughana, S., Robinson, T. E., Dearlove, A. M., Ribos, G, Bonser, A. J. et al. (2005). Mutation in myosin heavy chain 6 causes atrial septal defect. Nat. Genet. 37,423 -428.[CrossRef][Medline]
Choi, K., Kennedy, M., Kazarov, A., Papadimitriou, J. C. and Keller, G. (1998). A common precursor for hematopoietic and endothelial cells. Development 125,725 -732.[Abstract]
Cohen, E. D., Wang, Z., Lepore, J. J., Lu, M. M., Taketo, M. M., Epstein, D. J. and Morrisey, E. E. (2007). Wnt/β-catenin signalling promotes expansion of Isl1-positive cardiac progenitor cells through regulation of FGF signalling. J. Clin. Invest. 117,1794 -1804.[CrossRef][Medline]
Coultas, L., Chawengsaksophak, K. and Rossant, J. (2005). Endothelial cells and VEGF in vascular development. Nature 438,937 -945.[CrossRef][Medline]
de la Cruz, M. V. and Sanchez-Gomez, C. (2000). Straight heart tube. Primitive cardiac cavities vs. primitive cardiac segments. In Living Morphogenesis of the Heart (ed. M. V. de la Cruz and R. R. Markwald), pp. 85-99. Boston: Birkhauser.
Davis, R. L., Weintraub, H. and Lassar, A. B. (1987). Expression of a single transfected cDNA converts fibroblasts to myoblasts. Cell 51,987 -1000.[CrossRef][Medline]
Dettman, R. W., Denetclaw, W., Ordahl, C. P. and Bristow, J. (1998). Common epicardial origin of coronary vascular smooth muscle, perivascular fibroblasts, and intermyocardial fibroblasts in avian heart. Dev. Biol. 193,169 -181.[CrossRef][Medline]
Dodou, E., Verzi, M. P., Anderson, J. P., Xu, S.-M. and Black,
B. L. (2004). Mef2c is a direct transcriptional target of
isl1 and GATA factors in the anterior heart field during mouse embryonic
development. Development
131,3931
-3942.
Elliott, D. A., Kirk, E. P., Yeoh, T., Chandor, S., McKenzie,
F., Taylor, P., Grossfeld, P., Fatkin, D., Jones, O., Hayes, P. et al.
(2003). Cardiac homeobox gene NKX2.5 mutations and congenital
heart disease: associations with atrial septal defect and hypoplastic left
heart syndrome. J. Am. Coll. Cardiol.
41,2072
-2076.
Epstein, J. A. and Buck, C. A. (2000). Transcriptional regulation of cardiac development: implications for congenital heart disease and DiGeorge syndrome. Pediatr. Res. 48,717 -724.[Medline]
Fehling, H. J., Lacaud, G., Kubo, A., Kennedy, M., Robertson,
S., Keller, G. and Kouskoff, V. (2003). Tracking mesoderm
induction and its specification to the hemangioblast during embryonic stem
cell differentiation. Development
130,4217
-4227.
Frey, G., Kim, S. K. and Horvitz, H. R. (1990). Novel cysteine-rich motif and homeodomain in the product of Caenorhabditis elegans cell lineage gene Lin-11. Nature 344,876 -879.[CrossRef][Medline]
Garg, V., Kathiriya, I. S., Barnes, R., Schluterman, M. K., King, I. N., Butler, C. A., Rothrock, C. R., Eapen, R. S., Hirayama-Yamada, K., Joo, K. et al. (2003). GATA4 mutations cause human congenital heart defects and reveal an interaction with TBX5. Nature 424,443 -447.[CrossRef][Medline]
Garg, V., Muth, A. N., Ransom, J. F., Schluterman, M. K., Barnes, R., King, I. N., Grossfeld, P. D. and Srivastava, D. (2005). Mutations in NOTCH1 cause aortic valve disease. Nature 437,270 -274.[CrossRef][Medline]
Goldmuntz, E., Geiger, E. and Benson, D. W.
(2001). Nkx2.5 mutations in patients with tetralogy of Fallot.
Circulation 104,2565
-2568.
Gottlieb, P. D., Pierce, S. A., Sims, R. J., Yamagishi, H., Weihe, E. K., Harriss, J. V., Maika, S. D., Kuziel, W. A., King, H. L., Olson, E. N. et al. (2002). Bop encodes a muscle-restricted protein containing MYND and SET domains and is essential for cardiac differentiation and morphogenesis. Nat. Genet. 31, 25-32.[CrossRef][Medline]
Gruber, P. J. and Epstein, J. A. (2004).
Development gone awry. Circ. Res.
94,273
-283.
Harvey, R. P. (1996). NK-2 homeobox genes and heart development. Dev. Biol. 178,203 -216.[CrossRef][Medline]
Harvey, R. P. (2002). Patterning the vertebrate heart. Nat. Rev. Genet. 3, 544-556.[CrossRef][Medline]
Hochedlinger, K. and Jaenisch, R. (2002). Monoclonal mice generated by nuclear transfer from mature B and T donor cells. Nature 415,1035 -1038.[CrossRef][Medline]
Hoffman, J. I. E. (1995). Incidence of congenital heart disease: I. Postnatal incidence. Pediatr. Cardiol. 16,103 -113.[CrossRef][Medline]
Hu, T., Yamagishi, H., Maeda, J., McAnally, J., Yamagishi, C.
and Srivastava, D. (2004). Tbx1 regulates fibroblast growth
factors in the anterior heart field through a reinforcing autoregulatory loop
involving forkhead transcription factors. Development
131,5491
-5502.
Ilagan, R., Abu-Issa, R., Brown, D., Yang, Y. P., Jiao, K.,
Schwartz, R. J., Klingensmith, J. and Meyers, E. N. (2006).
Fgf8 is required for anterior heart field development.
Development 133,2435
-2445.
Karlsson, O., Thor, S., Norberg, T., Ohlsson, H. and Edlund, T. (1990). Insulin gene enhancer binding protein isl-1 is a member of a novel class of proteins containing both homeo- and Cys-His domain. Nature 344,879 -882.[CrossRef][Medline]
Kattman, S. J., Huber, T. L. and Keller, G. M. (2006). Multipotent flk-1+ cardiovascular progenitor cells give rise to the cardiomyocyte, endothelial, and vascular smooth muscle lineages. Dev. Cell 11,723 -732.[CrossRef][Medline]
Kelly, R. G. and Buckingham, M. E. (2002). The anterior-heart forming field:voyage to the arterial pole of the heart. Trends Genet. 18,210 -216.[CrossRef][Medline]
Kelly, R. G., Brown, N. A. and Buckingham, M. E. (2001). The arterial pole of the mouse heart froms from Fgf10-expressing cells in pharyngeal mesoderm. Dev. Cell 1,435 -440.[CrossRef][Medline]
Kirby, M. L., Gale, T. F. and Stewart, D. E.
(1983). Neural crest cells contribute to normal aorticopulmonary
septation. Science 220,1059
-1061.
Kirk, E. P., Sunde, M., Costa, M. W., Rankin, S. A., Wolstein, O., Castro, M. L., Butler, T. L., Hyun, C., Guo, G., Otway, R. et al. (2007). Mutations in cardiac T-box factor gene TBX20 are associated with diverse cardiac pathologies, including defects of septation and valvulogenesis and cardiomyopathy. Am. J. Hum. Genet. 81,280 -291.[CrossRef][Medline]
Kitajima, S., Takagi, A., Inoue, T. and Saga, Y. (2000). MesP1 and MesP2 are essential for the development of cardiac mesoderm. Development 127,3215 -3226.[Abstract]
Kouskoff, V., Lacaud, G., Schwantz, S., Fehling, H. J. and
Keller, G. (2005). Sequential development of hematopoietic
and cardiac mesoderm during embryonic stem cell differentiation.
Proc. Natl. Acad. Sci. USA
102,13170
-13175.
Krantz, I. D., Smith, R., Colliton, R. P., Tinkel, H., Zackei, E. H., Piccoli, D. A., Goldmuntz, E. and Spinner, N. B. (1999). Jagged1 mutations in patients ascertained with isolated congenital heart defects. Am. J. Med. Genet. 84, 56-60.[CrossRef][Medline]
Kume, T., Jiang, H., Topczewska, J. M. and Hogan, B. L.
(2001). The murine winged helix transcription factors, Foxc1 and
Foxc2, are both required for cardiovascular development and somitogenesis.
Genes Dev. 15,2470
-2482.
Kwon, C., Arnold, J., Hsiao, E. C., Taketo, M. M., Conklin, B.
R. and Srivastava, D. (2007). Canonical Wnt signaling is a
positive regulator of mammalian cardiac progenitors. Proc. Natl.
Acad. Sci. USA 104,10894
-10899.
Laugwitz, K.-L., Moretti, A., Lam, J., Gruber, P., Chen, Y., Woodard, S., Lin, L.-Z., Cai, C.-L., Lu, M. M., Reth, M. et al. (2005). Postnatal isl1+ cardioblasts enter fully differentiated cardiomyocyte lineages. Nature 433,647 -653.[CrossRef][Medline]
Lin, L., Cui, K., Zhou, W., Dufort, D., Zhang, X., Cai, C.-L.,
Bu, L., Yang, L., Martin, J., Kemler, R. et al. (2007).
β-catenin directly regulates Islet1 expression in cardiovascular
progenitors and is required for multiple aspects of cardiogenesis.
Proc. Natl. Acad. Sci. USA
104,9313
-9318.
Lin, Q., Schwarz, J., Bucana, C. and Olson, E. N.
(1997). Control of mouse cardiac morphogenesis and myogenesis by
transcription factor MEF2C. Science
276,1404
-1407.
Lyons, I., Parsons, L. M., Hartley, L., Li, R., Andrews, L. E.,
Robb, L. and Harvey, R. P. (1995). Myogenic and morphogenetic
defects in the heart tubes of murine embryos lacking the homeobox gene Nkx2.5.
Genes Dev. 9,1654
-1666.
Maeda, J.,
E14), the chambers separate due
to septation and are connected to the pulmonary trunk (PT) and aorta (Ao).
Cranial (Cr)-caudal (Ca), right (R)-left (L), and dorsal (D)-ventral (V) axes
are indicated. (B) Cardiac cell types that arise through the lineage
diversification of the three embryonic precursor pools in the mouse heart.
Whereas the contribution of the proepicardium to the smooth muscle cells of
the coronary system and to the mesenchymal cells of the heart is well
accepted, the origin of the endothelial lineage in the coronary vasculature is
still controversial. AA, aortic arch; IVS, interventricular septum; LA, left
atrium; LV, left ventricle; PhA, pharyngeal arches; PLA, primitive left
atrium; PRA, primitive right atrium; RA, right atrium; RV, right
ventricle.
