The role of secondary heart field in cardiac development

Origin and fates of the proepicardium

The embryonic heart initially consists of only two cell layers, the endocardium and the myocardium. The epicardium, which forms an epithelial layer on the surface of the heart, is derived from a cluster of mesothelial cells developing at the base of the venous inflow tract of the early embryonic heart. This cell cluster is termed the proepicardium and gives rise not only to the epicardium but also to epicardium-derived cells. These cells populate the myocardial wall and differentiate into smooth muscle cells and fibroblasts, while the contribution to the vascular endothelial lineage is uncertain. In this review we will discuss the signaling molecules involved in recruiting mesodermal cells to undergo proepicardium formation and guide these cells to the myocardial surface. Marker genes which are suitable to follow these cells during proepicardium formation and cell migration will be introduced. We will address whether the proepicardium consists of a homogenous cell population or whether different cell lineages are present. Finally the role of the epicardium as a source for cardiac stem cells and its importance in cardiac regeneration, in particular in the zebrafish and mouse model systems is discussed.

Expression of the Tgfβ2 gene during chick embryogenesis

We performed a comprehensive analysis of the expression of transforming growth factor (TGF) β2 during chick embryogenesis from stage 6 to 30 (Hamburger and Hamilton, J Morphol 1951;88:49-92) using in situ hybridization. During cardiogenesis, Tgfβ2 was expressed in the endothelial/mesenchymal cells of the valvulo-septal endocardial cushion tissue and in the epicardium until the end of embryogenesis. During the formation of major arteries, Tgfβ2 was localized in smooth muscle progenitors but not in the vascular endothelium. During limb development, Tgfβ2 was expressed in the mesenchymal cells in the presumptive limb regions at stage 16, and thereafter it was localized in the skeletal muscle progenitors. In addition, strong Tgfβ2 expression was seen in the mesenchymal cells in the pharyngeal arches. Tgfβ2 mRNA was also detected in other mesoderm-derived tissues, such as the developing bone and pleura. During ectoderm development, Tgfβ2 was expressed in the floor plate of the neural tube, lens, optic nerve, and otic vesicle. In addition, Tgfβ2 was expressed in the developing gut epithelium. Our results suggest that TGFβ2 plays an important role not only in epithelial-mesenchymal interactions but also in cell differentiation and migration and cell death during chick embryogenesis. We also found that chick and mouse Tgfβ2 RNA show very similar patterns of expression during embryogenesis. Chick embryos can serve as a useful model to increase our understanding in the roles of TGFβ2 in cell-cell interactions, cell differentiation, and proliferation during organogenesis.

Distinct phases of Wnt/β-catenin signaling direct cardiomyocyte formation in zebrafish

Normal heart formation requires reiterative phases of canonical Wnt/β-catenin (Wnt) signaling. Understanding the mechanisms by which Wnt signaling directs cardiomyocyte (CM) formation in vivo is critical to being able to precisely direct differentiated CMs from stem cells in vitro. Here, we investigate the roles of Wnt signaling in zebrafish CM formation using heat-shock inducible transgenes that increase and decrease Wnt signaling. We find that there are three phases during which CM formation is sensitive to modulation of Wnt signaling through the first 24 h of development. In addition to the previously recognized roles for Wnt signaling during mesoderm specification and in the pre-cardiac mesoderm, we find a previously unrecognized role during CM differentiation where Wnt signaling is necessary and sufficient to promote the differentiation of additional atrial cells. We also extend the previous studies of the roles of Wnt signaling during mesoderm specification and in pre-cardiac mesoderm. Importantly, in pre-cardiac mesoderm we define a new mechanism where Wnt signaling is sufficient to prevent CM differentiation, in contrast to a proposed role in inhibiting cardiac progenitor (CP) specification. The inability of the CPs to differentiate appears to lead to cell death through a p53/Caspase-3 independent mechanism. Together with a report for an even later role for Wnt signaling in restricting proliferation of differentiated ventricular CMs, our results indicate that during the first 3days of development in zebrafish there are four distinct phases during which CMs are sensitive to Wnt signaling.

The cardiogenic niche as a fundamental building block of engineered myocardium

Cardiac muscle engineering is evolving rapidly, aiming at the provision of innovative models for drug development and therapeutic myocardium. The progress in this field will depend crucially on the proper exploitation of stem cell technologies. Understanding the processes governing stem cell differentiation towards a desired phenotype and subsequent maturation in an organotypic manner will be key to ultimately providing realistic tissue models or therapeutics. Cardiogenesis is controlled by milieu factors that collectively constitute a so-called cardiogenic niche. The components of the cardiogenic niche are not yet fully defined but include paracrine factors and instructive extracellular matrix. Both are provided by supportive stromal cells under strict spatial and temporal control. Detailed knowledge on the exact composition and functionality of the dynamic cardiogenic niche during development will likely be instrumental to further advance cardiac muscle engineering. This review will discuss the concept of myocardial tissue engineering from the stem cell/developmental biology perspective and put forward the hypothesis of the cardiogenic niche as a fundamental building block of tissue-engineered myocardium.

Decreased fetal cardiac performance in the first trimester correlates with hyperglycemia in pregestational maternal diabetes

OBJECTIVE

In-vitro animal studies suggest that high glucose levels impair fetal cardiac function early in gestation. We aimed to study whether evidence of first-trimester myocardial dysfunction can be detected in fetuses of women with pregestational diabetes mellitus.

METHODS

Women with diabetes mellitus underwent fetal echocardiography at 11-14 weeks' gestational age. In fetuses with normal anatomy, the cardiac preload, diastolic function, global myocardial performance and placental afterload were studied by Doppler of the ductus venosus (DV), mitral and tricuspid early/atrial (E/A) ratios, left and right ventricular myocardial performance index (MPI) and umbilical artery (UA) Doppler, respectively. Cases were matched for gestational age and UA and DV Doppler with controls that had no diabetes mellitus.

RESULTS

Sixty-three singleton diabetic pregnancies were matched with 63 controls. Mean gestational age at enrollment was 12.6 (range, 11.1-13.6) weeks. Diabetic mothers had moderate to poor glycemic control (median (range) glycosylated hemoglobin A1 (HbA1c), 7.5 (5.1-12.7)%, and the HbA1c level was ≥ 7% in 37 (59%)). Fetuses of diabetic mothers exhibited worse measures of diastolic dysfunction: the isovolumetric relaxation time (IRT) was significantly prolonged (left ventricle: 36.9 ± 7.4 ms vs. 45.8 ± 6.8 ms; right ventricle: 35.6 ± 8 ms vs. 46.4 ± 7.3 ms, P < 0.0001 for both). The mitral E/A ratio was lower in diabetics (0.55 ± 0.06 vs. 0.51 ± 0.08, P = 0.03), and the global myocardial performance was lower in both ventricles (left ventricle MPI: 0.5 ± 0.08; right ventricle MPI: 0.52 ± 0.08, P = 0.03 and P < 0.0001, respectively). This lower global myocardial performance was caused by a prolonged myocardial relaxation time, which was most marked in diabetics with an HbA1c of ≥ 7% (P < 0.001 vs. controls for both ventricles). There were no significant correlations between cardiac Doppler parameters and DV, UA indices and fetal heart rate (P > 0.05 for all).

CONCLUSIONS

Fetuses of poorly controlled diabetic mothers demonstrate significant differences in first-trimester diastolic myocardial function compared with non-diabetic controls. The decrease in myocardial performance is more marked with increasing HbA1c and appears to be independent of preload and afterload. The ability to document these cardiac functional changes this early in pregnancy opens potential new avenues to understand the consequences of maternal glycemic status.

Redundant and dosage sensitive requirements for Fgf3 and Fgf10 in cardiovascular development

Heart development requires contributions from, and coordinated signaling interactions between, several cell populations, including splanchnic and pharyngeal mesoderm, postotic neural crest and the proepicardium. Here we report that Fgf3 and Fgf10, which are expressed dynamically in and near these cardiovascular progenitors, have redundant and dosage sensitive requirements in multiple aspects of early murine cardiovascular development. Embryos with Fgf3(-/+);Fgf10(-/-), Fgf3(-/-);Fgf10(-/+) and Fgf3(-/-);Fgf10(-/-) genotypes formed an allelic series of increasing severity with respect to embryonic survival, with double mutants dead by E11.5. Morphologic analysis of embryos with three mutant alleles at E11.5-E13.5 and double mutants at E9.5-E11.0 revealed multiple cardiovascular defects affecting the outflow tract, ventricular septum, atrioventricular cushions, ventricular myocardium, dorsal mesenchymal protrusion, pulmonary arteries, epicardium and fourth pharyngeal arch artery. Assessment of molecular markers in E8.0-E10.5 double mutants revealed abnormalities in each progenitor population, and suggests that Fgf3 and Fgf10 are not required for specification of cardiovascular progenitors, but rather for their normal developmental coordination. These results imply that coding or regulatory mutations in FGF3 or FGF10 could contribute to human congenital heart defects.

Genetic counseling in the adult with congenital heart disease: what is the role?

New discoveries using high-resolution methods for detecting genetic aberrations indicate that the genetic contribution to congenital heart disease has been significantly underestimated in the past. DNA diagnostics have become more accessible and genetic test results are increasingly being used to guide clinical management. Adult congenital heart disease specialists seeking to counsel adults with congenital heart disease about the genetic aspects of their condition face the challenge of keeping abreast of new genetic techniques and discoveries. The emphasis of this review is on the genetic basis of structural cardiovascular defects. A framework for identifying adult congenital heart disease patients most likely to benefit from genetic testing is suggested, along with a summary of current techniques for genetic testing. The clinical and ethical challenges associated with genetic counseling are highlighted. Finally, emerging technologies and future directions in genetics and adult congenital heart disease are discussed.

Zebrafish cardiac development requires a conserved secondary heart field

The secondary heart field is a conserved developmental domain in avian and mammalian embryos that contributes myocardium and smooth muscle to the definitive cardiac arterial pole. This field is part of the overall heart field and its myocardial component has been fate mapped from the epiblast to the heart in both mammals and birds. In this study we show that the population that gives rise to the arterial pole of the zebrafish can be traced from the epiblast, is a discrete part of the mesodermal heart field, and contributes myocardium after initial heart tube formation, giving rise to both smooth muscle and myocardium. We also show that Isl1, a transcription factor associated with undifferentiated cells in the secondary heart field in other species, is active in this field. Furthermore, Bmp signaling promotes myocardial differentiation from the arterial pole progenitor population, whereas inhibiting Smad1/5/8 phosphorylation leads to reduced myocardial differentiation with subsequent increased smooth muscle differentiation. Molecular pathways required for secondary heart field development are conserved in teleosts, as we demonstrate that the transcription factor Tbx1 and the Sonic hedgehog pathway are necessary for normal development of the zebrafish arterial pole.

Dkk1 and Dkk2 regulate epicardial specification during mouse heart development

BACKGROUND

Dkk1 and Dkk2 interact with LRP5 and LRP6 to modulate canonical Wnt signaling during development, and are known to be expressed in the developing heart. However, a loss-of-function mutation in either gene by itself produces no discernable heart phenotype.

METHODS

Using standard husbandry techniques, Dkk1 null and Dkk2 null mouse lines were crossed to create double null embryos, which we examined using histological and immunohistochemical methods.

RESULTS

Double null embryos die perinatally, with a gross head phenotype reminiscent of Dkk1 null embryos. Upon examination of late stage hearts, we observe myocardial defects including ventricular septal defects. At earlier stages, double mutant hearts show myocardial and epicardial hyperplasia. Myocardial hypertrophy is associated with a moderate increase in cell proliferation, but epicardial hypercellularity is not. Rather, the field of proepicardial precursor cells near the liver shows a broadening of expression for the cardiac-specific gap junction protein Connexin 43.

CONCLUSIONS

Dkk1 and Dkk2 both inhibit Wnt signaling to regulate early myocardial proliferation and each can compensate for the loss of the other in that role. Wnt signaling regulates myocardial proliferation in both heart fields at early stages. Additionally, Wnt signaling is sufficient to increase proepicardial specification as measured by Connexin 43 expression, resulting in a hypercellular epicardium and perhaps contributing to later defects.

Pathologic calcification of adult vascular smooth muscle cells differs on their crest or mesodermal embryonic origin

Vascular calcifications can occur in the elderly and in patients suffering from various diseases. Interestingly, depending on the pathology, different regions of the arterial system can be affected. Embryonic observations have clearly indicated that vascular smooth muscle cell (VSMC) origin is notably heterogeneous. For instance, in the aorta, VSMCs colonizing the aortic arch region derive from cardiac neural crest cells, whereas those populating the descending aorta derive from the mesoderm. We examined here whether the embryonic origin of aortic VSMCs would correlate with their ability to mineralize. Under hyperphosphatemic conditions that induce vascular calcifications, we performed ex vivo aortic explant cultures as well as in vitro VSMC cultures from wild-type mice. Our data showed that VSMC embryonic origin affects their ability to mineralize. Indeed, the aortic arch media made up of VSMCs of neural crest origin calcifies significantly earlier than the descending aorta composed of VSMCs, which are mesoderm-derived. Similar results were obtained with cultured VSMCs harvested from both aortic regions. We also demonstrated that in a mouse model deficient in matrix Gla protein, a potent calcification inhibitor, developing extensive and spontaneous medial calcifications of the aorta, lesions initiate in the aortic arch. Subsequently, calcifications progress outside the aortic arch region and ultimately spread all over the entire arterial tree, including the descending aorta. Altogether, our results support an unsuspected correlation between VSMCs of embryonic origin and the timing of appearance of calcifications.

An eye on the head: the development and evolution of craniofacial muscles

Skeletal muscles exert diverse functions, enabling both crushing with great force and movement with exquisite precision. A remarkably distinct repertoire of genes and ontological features characterise this tissue, and recent evidence has shown that skeletal muscles of the head, the craniofacial muscles, are evolutionarily, morphologically and molecularly distinct from those of the trunk. Here, we review the molecular basis of craniofacial muscle development and discuss how this process is different to trunk and limb muscle development. Through evolutionary comparisons of primitive chordates (such as amphioxus) and jawless vertebrates (such as lampreys) with jawed vertebrates, we also provide some clues as to how this dichotomy arose.

ACE2 improves right ventricular function in a pressure overload model

BACKGROUND

Right ventricular (RV) dysfunction is a complication of pulmonary hypertension and portends a poor prognosis. Pharmacological therapies targeting RV function in pulmonary hypertension may reduce symptoms, improve hemodynamics, and potentially increase survival. We hypothesize that recombinant human angiotensin-converting enzyme 2 (rhACE2) will improve RV function in a pressure overload model.

RESULTS

rhACE2 administered at 1.8 mg/kg/day improved RV systolic and diastolic function in pulmonary artery banded mice as measured by in vivo hemodynamics. Specifically, rhACE2 increased RV ejection fraction and decreased RV end diastolic pressure and diastolic time constant (p<0.05). In addition, rhACE2 decreased RV hypertrophy as measured by RV/LV+S ratio (p<0.05). There were no significant negative effects of rhACE2 administration on LV function. rhACE2 had no significant effect on fibrosis as measured by trichrome staining and collagen1α1 expression. In pulmonary artery banded mice, rhACE2 increased Mas receptor expression and normalized connexin 37 expression.

CONCLUSION

In a mouse RV load-stress model of early heart failure, rhACE2 diminished RV hypertrophy and improved RV systolic and diastolic function in association with a marker of intercellular communication. rhACE2 may be a novel treatment for RV failure.

Signaling pathways in early cardiac development

Cardiomyocyte differentiation is a complex multistep process requiring the proper temporal and spatial integration of multiple signaling pathways. Previous embryological and genetic studies have identified a number of signaling pathways that are critical to mediate the initial formation of the mesoderm and its allocation to the cardiomyocyte lineage. It has become clear that some of these signaling networks work autonomously, in differentiating myocardial cells whereas others work non-autonomously, in neighboring tissues, to regulate cardiac differentiation indirectly. Here, we provide an overview of three signaling networks that mediate cardiomyocyte specification and review recent insights into their specific roles in heart development. In addition, we demonstrate how systems level, 'omic approaches' and other high-throughput techniques such as small molecules screens are beginning to impact our understanding of cardiomyocyte specification and, to identify novel signaling pathways involved in this process. In particular, it now seems clear that at least one chemokine receptor CXCR4 is an important marker for cardiomyocyte progenitors and may play a functional role in their differentiation. Finally, we discuss some gaps in our current understanding of early lineage selection that could be addressed by various types of omic analysis.

BMP signaling in congenital heart disease: new developments and future directions

Congenital heart malformations are the most common of all congenital human birth anomalies. During the past decade, research with zebrafish, chick, and mouse models have elucidated many fundamental genetic pathways that govern early cardiac patterning and differentiation. This review highlights the roles of the bone morphogenetic protein (BMP) signaling pathway in cardiogenesis and how defective BMP signals can disrupt the intricate steps of cardiac formation and cause congenital heart defects.

The representation of heart development in the gene ontology

An understanding of heart development is critical in any systems biology approach to cardiovascular disease. The interpretation of data generated from high-throughput technologies (such as microarray and proteomics) is also essential to this approach. However, characterizing the role of genes in the processes underlying heart development and cardiovascular disease involves the non-trivial task of data analysis and integration of previous knowledge. The Gene Ontology (GO) Consortium provides structured controlled biological vocabularies that are used to summarize previous functional knowledge for gene products across all species. One aspect of GO describes biological processes, such as development and signaling. In order to support high-throughput cardiovascular research, we have initiated an effort to fully describe heart development in GO; expanding the number of GO terms describing heart development from 12 to over 280. This new ontology describes heart morphogenesis, the differentiation of specific cardiac cell types, and the involvement of signaling pathways in heart development. This work also aligns GO with the current views of the heart development research community and its representation in the literature. This extension of GO allows gene product annotators to comprehensively capture the genetic program leading to the developmental progression of the heart. This will enable users to integrate heart development data across species, resulting in the comprehensive retrieval of information about this subject. The revised GO structure, combined with gene product annotations, should improve the interpretation of data from high-throughput methods in a variety of cardiovascular research areas, including heart development, congenital cardiac disease, and cardiac stem cell research. Additionally, we invite the heart development community to contribute to the expansion of this important dataset for the benefit of future research in this area.

Extracellular matrix and heart development

The extracellular matrix (ECM) of the developing heart contains numerous molecules that form a dynamic environment that plays an active and crucial role in the regulation of cellular events. ECM molecules found in the heart include hyaluronan, fibronectin, fibrillin, proteoglycans, and collagens. Tight regulation of the spatiotemporal expression, and the proteolytic processing of ECM components by proteases including members of the ADAMTS family, is essential for normal cardiac development. Perturbation of the expression of genes involved in matrix composition and remodeling can interfere with a myriad of events involved in the formation of the four-chambered heart and result in prenatal lethality or cardiac malformations as seen in humans with congenital heart disease. In this review, we summarize what is known about the specific importance of some of the components of the ECM in relation to the cardiovascular development.

Latent TGF-β binding protein 3 identifies a second heart field in zebrafish

The four-chambered mammalian heart develops from two fields of cardiac progenitor cells distinguished by their spatiotemporal patterns of differentiation and contributions to the definitive heart. The first heart field differentiates earlier in lateral plate mesoderm, generates the linear heart tube and ultimately gives rise to the left ventricle. The second heart field (SHF) differentiates later in pharyngeal mesoderm, elongates the heart tube, and gives rise to the outflow tract and much of the right ventricle. Because hearts in lower vertebrates contain a rudimentary outflow tract but not a right ventricle, the existence and function of SHF-like cells in these species has remained a topic of speculation. Here we provide direct evidence from Cre/Lox-mediated lineage tracing and loss-of-function studies in zebrafish, a lower vertebrate with a single ventricle, that latent TGF-β binding protein 3 (ltbp3) transcripts mark a field of cardiac progenitor cells with defining characteristics of the anterior SHF in mammals. Specifically, ltbp3(+) cells differentiate in pharyngeal mesoderm after formation of the heart tube, elongate the heart tube at the outflow pole, and give rise to three cardiovascular lineages in the outflow tract and myocardium in the distal ventricle. In addition to expressing Ltbp3, a protein that regulates the bioavailability of TGF-β ligands, zebrafish SHF cells co-express nkx2.5, an evolutionarily conserved marker of cardiac progenitor cells in both fields. Embryos devoid of ltbp3 lack the same cardiac structures derived from ltbp3(+) cells due to compromised progenitor proliferation. Furthermore, small-molecule inhibition of TGF-β signalling phenocopies the ltbp3-morphant phenotype whereas expression of a constitutively active TGF-β type I receptor rescues it. Taken together, our findings uncover a requirement for ltbp3-TGF-β signalling during zebrafish SHF development, a process that serves to enlarge the single ventricular chamber in this species.

Tbx1, subpulmonary myocardium and conotruncal congenital heart defects

Conotruncal congenital heart defects, including defects in septation and alignment of the ventricular outlets, account for approximately a third of all congenital heart defects. Failure of the left ventricle to obtain an independent outlet results in incomplete separation of systemic and pulmonary circulation at birth. The embryonic outflow tract, a transient cylinder of myocardium connecting the embryonic ventricles to the aortic sac, plays a critical role in this process during normal development. The outflow tract (OFT) is derived from a population of cardiac progenitor cells called the second heart field that contributes to the arterial pole of the heart tube during cardiac looping. During septation, the OFT is remodeled to form the base of the ascending aorta and pulmonary trunk. Tbx1, the major candidate gene for DiGeorge syndrome, is a critical transcriptional regulator of second heart field development. DiGeorge syndrome patients are haploinsufficient for Tbx1 and present a spectrum of conotruncal anomalies including tetralogy of Fallot, pulmonary atresia, and common arterial trunk. In this review, we focus on the role of Tbx1 in the regulation of second heart field deployment and, in particular, in the development of a specific population of myocardial cells at the base of the pulmonary trunk. Recent data characterizing additional properties and regulators of development of this region of the heart, including the retinoic acid, hedgehog, and semaphorin signaling pathways, are discussed. These findings identify future subpulmonary myocardium as the clinically relevant component of the second heart field and provide new mechanistic insight into a spectrum of common conotruncal congenital heart defects.

Cardiac neural crest is dispensable for outflow tract septation in Xenopus

In vertebrate embryos, cardiac precursor cells of the primary heart field are specified in the lateral mesoderm. These cells converge at the ventral midline to form the linear heart tube, and give rise to the atria and the left ventricle. The right ventricle and the outflow tract are derived from an adjacent population of precursors known as the second heart field. In addition, the cardiac neural crest contributes cells to the septum of the outflow tract to separate the systemic and the pulmonary circulations. The amphibian heart has a single ventricle and an outflow tract with an incomplete spiral septum; however, it is unknown whether the cardiac neural crest is also involved in outflow tract septation, as in amniotes. Using a combination of tissue transplantations and molecular analyses in Xenopus we show that the amphibian outflow tract is derived from a second heart field equivalent to that described in birds and mammals. However, in contrast to what we see in amniotes, it is the second heart field and not the cardiac neural crest that forms the septum of the amphibian outflow tract. In Xenopus, cardiac neural crest cells remain confined to the aortic sac and arch arteries and never populate the outflow tract cushions. This significant difference suggests that cardiac neural crest cell migration into the cardiac cushions is an amniote-specific characteristic, presumably acquired to increase the mass of the outflow tract septum with the evolutionary need for a fully divided circulation.

Pharyngeal mesoderm development during embryogenesis: implications for both heart and head myogenesis

The pharyngeal mesoderm (PM), located in the head region of the developing embryo, recently triggered renewed interest as the major source of cells contributing to broad regions of the heart as well as to the head musculature. What exactly is PM? In this review, we describe the anatomical and molecular characteristics of this mesodermal population and its relationship to the first and second heart fields in chick and mouse embryos. The regulatory network of transcription factors and signalling molecules that regulate PM development is also discussed. In addition, we summarize recent studies into the evolutionary origins of this tissue and its multipotential contributions to both cardiac and pharyngeal muscle progenitors.

Mef2cb regulates late myocardial cell addition from a second heart field-like population of progenitors in zebrafish

Two populations of cells, termed the first and second heart field, drive heart growth during chick and mouse development. The zebrafish has become a powerful model for vertebrate heart development, partly due to the evolutionary conservation of developmental pathways in this process. Here we provide evidence that the zebrafish possesses a conserved homolog to the murine second heart field. We developed a photoconversion assay to observe and quantify the dynamic late addition of myocardial cells to the zebrafish arterial pole. We define an extra-cardiac region immediately posterior to the arterial pole, which we term the late ventricular region. The late ventricular region has cardiogenic properties, expressing myocardial markers such as vmhc and nkx2.5, but does not express a full complement of differentiated cardiomyocyte markers, lacking myl7 expression. We show that mef2cb, a zebrafish homolog of the mouse second heart field marker Mef2c, is expressed in the late ventricular region, and is necessary for late myocardial addition to the arterial pole. FGF signaling after heart cone formation is necessary for mef2cb expression, the establishment of the late ventricular region, and late myocardial addition to the arterial pole. Our study demonstrates that zebrafish heart growth shows more similarities to murine heart growth than previously thought. Further, as congenital heart disease is often associated with defects in second heart field development, the embryological and genetic advantages of the zebrafish model can be applied to study the vertebrate second heart field.

Cardiac muscle regeneration: lessons from development

The adult human heart is an ideal target for regenerative intervention since it does not functionally restore itself after injury yet has a modest regenerative capacity that could be enhanced by innovative therapies. Adult cardiac cells with regenerative potential share gene expression signatures with early fetal progenitors that give rise to multiple cardiac cell types, suggesting that the evolutionarily conserved regulatory networks that drive embryonic heart development might also control aspects of regeneration. Here we discuss commonalities of development and regeneration, and the application of the rich developmental biology heritage to achieve therapeutic regeneration of the human heart.

Cardiac origin of smooth muscle cells in the inflow tract

Multipotent Isl1(+) heart progenitors give rise to three major cardiovascular cell types: cardiac, smooth muscle, and endothelial cells, and play a pivotal role in lineage diversification during cardiogenesis. A critical question is pinpointing when this cardiac-vascular lineage decision is made, and how this plasticity serves to coordinate cardiac chamber and vessel growth. The posterior domain of the Isl1-positive second heart field contributes to the SLN-positive atrial myocardium and myocardial sleeves in the cardiac inflow tract, where myocardial and vascular smooth muscle layers form anatomical and functional continuity. Herein, using a new atrial specific SLN-Cre knockin mouse line, we report that bipotent Isl1(+)/SLN(+) transient cell population contributes to cardiac as well as smooth muscle cells at the heart-vessel junction in cardiac inflow tract. The Isl1(+)/SLN(+) cells are capable of giving rise to cardiac and smooth muscle cells until late gestational stages. These data suggest that the cardiac and smooth muscle cells in the cardiac inflow tract share a common developmental origin. This article is part of a special issue entitled, "Cardiovascular Stem Cells Revisited".

Bmp signaling regulates myocardial differentiation from cardiac progenitors through a MicroRNA-mediated mechanism

MicroRNAs (miRNAs) are small, noncoding RNAs that regulate gene expression posttranscriptionally. We investigated the hypothesis that bone morphogenetic protein (Bmp) signaling regulates miRNAs in cardiac progenitor cells. Bmp2 and Bmp4 regulate OFT myocardial differentiation via regulation of the miRNA-17-92 cluster. In Bmp mutant embryos, myocardial differentiation was delayed, and multiple miRNAs encoded by miRNA-17-92 were reduced. We uncovered functional miRNA-17-92 seed sequences within the 3' UTR of cardiac progenitor genes such as Isl1 and Tbx1. In both Bmp and miRNA-17-92 mutant embryos, Isl1 and Tbx1 expression failed to be correctly downregulated. Transfection experiments indicated that miRNA-17 and miRNA-20a directly repressed Isl1 and Tbx1. Genetic interaction studies uncovered a synergistic interaction between miRNA-17-92 cluster and Bmp4, providing direct in vivo evidence for the Bmp-miRNA-17-92 regulatory pathway. Our findings indicate that Bmp signaling directly regulates a miRNA-mediated effector mechanism that downregulates cardiac progenitor genes and enhances myocardial differentiation.

Inactivation of Bmp4 from the Tbx1 expression domain causes abnormal pharyngeal arch artery and cardiac outflow tract remodeling

Maldevelopment of outflow tract and aortic arch arteries is among the most common forms of human congenital heart diseases. Both Bmp4 and Tbx1 are known to play critical roles during cardiovascular development. Expression of these two genes partially overlaps in pharyngeal arch areas in mouse embryos. In this study, we applied a conditional gene inactivation approach to test the hypothesis that Bmp4 expressed from the Tbx1 expression domain plays a critical role for normal development of outflow tract and pharyngeal arch arteries. We showed that inactivation of Bmp4 from Tbx1-expressing cells leads to the spectrum of deformities resembling the cardiovascular defects observed in human DiGeorge syndrome patients. Inactivation of Bmp4 from the Tbx1 expression domain did not cause patterning defects, but affected remodeling of outflow tract and pharyngeal arch arteries. Our further examination revealed that Bmp4 is required for normal recruitment/differentiation of smooth muscle cells surrounding the PAA4 and survival of outflow tract cushion mesenchymal cells.

Stem cell models of cardiac development and disease

The past few years have witnessed remarkable advances in stem cell biology and human genetics, and we have arrived at an era in which patient-specific cell and tissue models are now practical. The recent identification of cardiovascular progenitor cells, as well as the identification of genetic variants underlying congenital heart disorders and adult disease, opens the door to the development of human models of human cardiovascular disease. We review the current understanding of the contribution of progenitor cells to cardiogenesis and outline how pluripotent stem cells can be applied to the modeling of cardiovascular disorders of genetic origin. A key challenge will be to implement these models in an efficient manner to develop a molecular understanding of how genes lead to disease and to screen for genes and drugs that modify the disease process.

Arterial pole progenitors interpret opposing FGF/BMP signals to proliferate or differentiate

During heart development, a subpopulation of cells in the heart field maintains cardiac potential over several days of development and forms the myocardium and smooth muscle of the arterial pole. Using clonal and explant culture experiments, we show that these cells are a stem cell population that can differentiate into myocardium, smooth muscle and endothelial cells. The multipotent stem cells proliferate or differentiate into different cardiovascular cell fates through activation or inhibition of FGF and BMP signaling pathways. BMP promoted myocardial differentiation but not proliferation. FGF signaling promoted proliferation and induced smooth muscle differentiation, but inhibited myocardial differentiation. Blocking the Ras/Erk intracellular pathway promoted myocardial differentiation, while the PLCgamma and PI3K pathways regulated proliferation. In vivo, inhibition of both pathways resulted in predictable arterial pole defects. These studies suggest that myocardial differentiation of arterial pole progenitors requires BMP signaling combined with downregulation of the FGF/Ras/Erk pathway. The FGF pathway maintains the pool of proliferating stem cells and later promotes smooth muscle differentiation.

[Lineage tracing of epicardial cells during development and regeneration]

Tracing the history of individual cells during embryonic morphogenesis in a structure as complex as the cardiovascular system is one of the major challenges of developmental biology. It involves determining the relationships between the various lineages of cells forming an organ at different stages, describing the topological rearrangements tissues undergo during morphogenesis, and characterizing the interactions between cells in different structures. However, despite the great expectations raised in the field of regenerative medicine, only limited progress has been made in using regenerative therapy to repair the cardiovascular system. Recent research has highlighted the role of the epicardium during cardiac regeneration, but it is still unclear whether it is important for molecular signaling or acts as a source of progenitor cells during this process. Consequently, increasing knowledge about the origin, diversification and potential of epicardial cells during development and homeostasis and under pathological conditions is of fundamental importance both for basic research and for the development of effective cellular therapies. The aims of this article were to provide a general overview of the classical techniques used for tracing cell lineages, including their potential and limitations, and to describe novel techniques for studying the origin and differentiation of the epicardium and its role in cardiac regeneration.

Retinoic acid regulates differentiation of the secondary heart field and TGFbeta-mediated outflow tract septation

In many experimental models and clinical examples, defects in the differentiation of the second heart field (SHF) and heart outflow tract septation defects are combined, although the mechanistic basis for this relationship has been unclear. We found that as the initial SHF population incorporates into the outflow tract, it is replenished from the surrounding progenitor territory. In retinoic acid (RA) receptor mutant mice, this latter process fails, and the outflow tract is shortened and misaligned as a result. As an additional consequence, the outflow tract is misspecified along its proximal-distal axis, which results in ectopic expression of TGFbeta2 and ectopic mesenchymal transformation of the endocardium. Reduction of TGFbeta2 gene dosage in the RA receptor-deficient background restores septation but does not rescue alignment defects, indicating that excess TGFbeta causes septation defects. This may be a common pathogenic pathway when second heart field and septation defects are coupled.