MesP1 and MesP2 are essential for the development of cardiac mesoderm

Mesp1 controls the speed, polarity, and directionality of cardiovascular progenitor migration

During embryonic development, Mesp1 marks the earliest cardiovascular progenitors (CPs) and promotes their specification, epithelial-mesenchymal transition (EMT), and cardiovascular differentiation. However, Mesp1 deletion in mice does not impair initial CP specification and early cardiac differentiation but induces cardiac malformations thought to arise from a defect of CP migration. Using inducible gain-of-function experiments during embryonic stem cell differentiation, we found that Mesp2, its closest homolog, was as efficient as Mesp1 at promoting CP specification, EMT, and cardiovascular differentiation. However, only Mesp1 stimulated polarity and directional cell migration through a cell-autonomous mechanism. Transcriptional analysis and chromatin immunoprecipitation experiments revealed that Mesp1 and Mesp2 activate common target genes that promote CP specification and differentiation. We identified two direct Mesp1 target genes, Prickle1 and RasGRP3, that are strongly induced by Mesp1 and not by Mesp2 and that control the polarity and the speed of cell migration. Altogether, our results identify the molecular interface controlled by Mesp1 that links CP specification and cell migration.

The Apelin receptor enhances Nodal/TGFβ signaling to ensure proper cardiac development

The Apelin receptor (Aplnr) is essential for heart development, controlling the early migration of cardiac progenitors. Here we demonstrate that in zebrafish Aplnr modulates Nodal/TGFβ signaling, a key pathway essential for mesendoderm induction and migration. Loss of Aplnr function leads to a reduction in Nodal target gene expression whereas activation of Aplnr by a non-peptide agonist increases the expression of these same targets. Furthermore, loss of Aplnr results in a delay in the expression of the cardiogenic transcription factors mespaa/ab. Elevating Nodal levels in aplnra/b morphant and double mutant embryos is sufficient to rescue cardiac differentiation defects. We demonstrate that loss of Aplnr attenuates the activity of a point source of Nodal ligands Squint and Cyclops in a non-cell autonomous manner. Our results favour a model in which Aplnr is required to fine-tune Nodal output, acting as a specific rheostat for the Nodal/TGFβ pathway during the earliest stages of cardiogenesis.

DOI:http://dx.doi.org/10.7554/eLife.13758.001

In one of the first events that happens as an embryo develops, cells become the different stem cell populations that form the body’s organs. So what makes a cell become one stem cell type rather than another? In the case of the heart, the first important event is the activity of a signaling pathway called the Nodal/TGFβ pathway. Nodal signaling can drive cells to become many different stem cell types depending on its level of activity. Many different levels of regulation fine-tune Nodal signaling to produce these activity thresholds.

Zebrafish that have a mutation in the gene that encodes a protein called the Apelin receptor have no heart. The loss of this receptor interferes with how heart stem cells (called cardiac progenitors) are made and how they move to where heart development occurs.

Deshwar et al. have now studied mutant zebrafish in order to investigate how the Apelin receptor influences early heart development. This revealed that Nodal signaling levels are slightly lower in the mutant zebrafish embryos than in normal fish at the time when Nodal activity induces cardiac progenitors to form. When Nodal activity is experimentally boosted in zebrafish that lack the Apelin receptor, they become able to develop hearts.

Deshwar et al. also found that the Apelin receptor does not work in cells that produce or receive Nodal signals. This suggests that the Apelin receptor modulates Nodal signaling levels by acting in cells that lie between the cells that release Nodal signals and the cardiac progenitors. An important question for future work to address is how this modulation works. As Nodal is a key determinant of many cell types in developing embryos, learning how Apelin receptors regulate its activity could help researchers to derive specific cell types from cultured stem cells for use in regenerative medicine.

DOI:http://dx.doi.org/10.7554/eLife.13758.002

The Early Stages of Heart Development: Insights from Chicken Embryos

The heart is the first functioning organ in the developing embryo and a detailed understanding of the molecular and cellular mechanisms involved in its formation provides insights into congenital malformations affecting its function and therefore the survival of the organism. Because many developmental mechanisms are highly conserved, it is possible to extrapolate from observations made in invertebrate and vertebrate model organisms to humans. This review will highlight the contributions made through studying heart development in avian embryos, particularly the chicken. The major advantage of chick embryos is their accessibility for surgical manipulation and functional interference approaches, both gain- and loss-of-function. In addition to experiments performed in ovo, the dissection of tissues for ex vivo culture, genomic, or biochemical approaches is straightforward. Furthermore, embryos can be cultured for time-lapse imaging, which enables tracking of fluorescently labeled cells and detailed analysis of tissue morphogenesis. Owing to these features, investigations in chick embryos have led to important discoveries, often complementing genetic studies in mice and zebrafish. As well as including some historical aspects, we cover here some of the crucial advances made in understanding early heart development using the chicken model.

Novel Mode of Defective Neural Tube Closure in the Non-Obese Diabetic (NOD) Mouse Strain

Failure to close the neural tube results in birth defects, with severity ranging from spina bifida to lethal anencephaly. Few genetic risk factors for neural tube defects are known in humans, highlighting the critical role of environmental risk factors, such as maternal diabetes. Yet, it is not well understood how altered maternal metabolism interferes with embryonic development, and with neurulation in particular. We present evidence from two independent mouse models of diabetic pregnancy that identifies impaired migration of nascent mesodermal cells in the primitive streak as the morphogenetic basis underlying the pathogenesis of neural tube defects. We conclude that perturbed gastrulation not only explains the neurulation defects, but also provides a unifying etiology for the broad spectrum of congenital malformations in diabetic pregnancies.

Dickkopf Wnt signaling pathway inhibitor 1 regulates the differentiation of mouse embryonic stem cells in vitro and in vivo

Embryonic stem cells (ESCs) are pluripotent stem cells derived from early stage embryos. It remains unclear whether inhibiting the Wnt/β‑catenin signaling pathway using dickkopf Wnt signaling pathway inhibitor 1 (DKK1) impacts on the differentiation potential of mouse ESCs in vitro and in vivo. In the present study, immunohistochemical staining was used to measure the expression of markers of the three germ layers in ESCs and teratomas derived from ESCs. The expression of markers for the Wnt/β‑catenin signaling pathway were detected by reverse transcription‑polymerase chain reaction (RT‑qPCR). Immunohistochemistry and western blotting indicated that the expression levels of octamer‑binding transcription factor 4 in the DKK1‑treated ESC group were significantly greater compared with the control ESCs. Reduced expression levels of NeuroD and bone morphogenetic protein 4 were observed in the DKK1‑treated ESCs and teratomas derived from DKK1‑treated ESCs compared with the control group. Increased expression levels of SOX17 were observed in the DKK1‑treated ESCs compared with the control group. RT‑qPCR indicated that β‑catenin expression was significantly reduced in DKK1‑treated ESCs and teratomas derived from DKK1‑treated ESCs compared with the control groups. Western blotting indicated no alterations in the expression of GSK‑3β, however, the levels of phosphorylated‑GSK‑3β were significantly greater in the DKK1 treatment groups, while cyclin D1 and c‑Myc expression levels were significantly reduced in the DKK1 treatment groups compared with the control groups. These results suggest that inhibiting Wnt signaling in ESCs using DKK1 may promote mouse ESCs to differentiate into endoderm in vitro and in vivo, and suppress the tumorigenicity of ESCs.

Genome-Wide Identification of MESP1 Targets Demonstrates Primary Regulation Over Mesendoderm Gene Activity

MESP1 is considered the first sign of the nascent cardiac mesoderm and plays a critical role in the appearance of cardiac progenitors, while exhibiting a transient expression in the developing embryo. We profiled the transcriptome of a pure population of differentiating MESP1-marked cells and found that they chiefly contribute to the mesendoderm lineage. High-throughput sequencing of endogenous MESP1-bound DNA revealed that MESP1 preferentially binds to two variants of E-box sequences and activates critical mesendoderm modulators, including Eomes, Gata4, Wnt5a, Wnt5b, Mixl1, T, Gsc, and Wnt3. These mesendoderm markers were enriched in the MESP1 marked population before the appearance of cardiac progenitors and myocytes. Further, MESP1-binding is globally associated with H(3)K(27) acetylation, supporting a novel pivotal role of it in regulating target gene epigenetics. Therefore, MESP1, the pioneer cardiac factor, primarily directs the appearance of mesendoderm, the intermediary of the earliest progenitors of mesoderm and endoderm organogenesis.

A time frame permissive for Protein Kinase D2 activity to direct angiogenesis in mouse embryonic stem cells

The protein kinase D isoenzymes PKD1/2/3 are prominent downstream targets of PKCs (Protein Kinase Cs) and phospholipase D in various biological systems. Recently, we identified PKD isoforms as novel mediators of tumour cell-endothelial cell communication, tumour cell motility and metastasis. Although PKD isoforms have been implicated in physiological/tumour angiogenesis, a role of PKDs during embryonic development, vasculogenesis and angiogenesis still remains elusive. We investigated the role of PKDs in germ layer segregation and subsequent vasculogenesis and angiogenesis using mouse embryonic stem cells (ESCs). We show that mouse ESCs predominantly express PKD2 followed by PKD3 while PKD1 displays negligible levels. Furthermore, we demonstrate that PKD2 is specifically phosphorylated/activated at the time of germ layer segregation. Time-restricted PKD2-activation limits mesendoderm formation and subsequent cardiovasculogenesis during early differentiation while leading to branching angiogenesis during late differentiation. In line, PKD2 loss-of-function analyses showed induction of mesendodermal differentiation in expense of the neuroectodermal germ layer. Our in vivo findings demonstrate that embryoid bodies transplanted on chicken chorioallantoic membrane induced an angiogenic response indicating that timed overexpression of PKD2 from day 4 onwards leads to augmented angiogenesis in differentiating ESCs. Taken together, our results describe novel and time-dependent facets of PKD2 during early cell fate determination.

The roles of Mesp family proteins: functional diversity and redundancy in differentiation of pluripotent stem cells and mammalian mesodermal development

Mesp family proteins comprise two members named mesodermal posterior 1 (Mesp1) and mesodermal posterior 2 (Mesp2). Both Mesp1 and Mesp2 are transcription factors and they share an almost identical basic helix-loop-helix motif. They have been shown to play critical regulating roles in mammalian heart and somite development. Mesp1 sits in the core of the complicated regulatory network for generation of cardiovascular progenitors while Mesp2 is central for somitogenesis. Here we summarize the similarities and differences in their molecular functions during mammalian early mesodermal development and discuss possible future research directions for further study of the functions of Mesp1 and Mesp2. A comprehensive knowledge of molecular functions of Mesp family proteins will eventually help us better understand mammalian heart development and somitogenesis as well as improve the production of specific cell types from pluripotent stem cells for future regenerative therapies.

Genetics of cardiovascular development

Structural malformations of the heart are the commonest abnormalities found at the time of birth and the incidence is higher in fetuses that are lost during the first trimester. Although the form of the heart has been studied for centuries, it is in the past decades that the genetic pathways that control heart development have been unraveled. Recently, the concept of the second heart field, a population of multipotent cardiac cells that augment the initial simple heart tube, has clarified the development of the heart. Understanding how the second heart field is used in morphogenesis and how genes interact in a subtle and more complex way is moving us closer to understanding how the normal heart forms and why abnormalities occur. In this chapter, we present a description of the morphological processes that create the formed postnatal human heart and emphasize key genetic pathways and genes that control these aspects. Where possible, these are also linked to the common patterns of human cardiac malformation. Undoubtedly, the details will refine or change with further research but emphasis has been placed on areas of greatest certainty and the presentation designed to promote a general understanding.

The transcription factor Mesp1 interacts with cAMP-responsive element binding protein 1 (Creb1) and coactivates Ets variant 2 (Etv2) gene expression

Mesoderm posterior 1 (Mesp1) is well recognized for its role in cardiac development, although it is expressed broadly in mesodermal lineages. We have previously demonstrated important roles for Mesp1 and Ets variant 2 (Etv2) during lineage specification, but their relationship has not been defined. This study reveals that Mesp1 binds to the proximal promoter and transactivates Etv2 gene expression via the CRE motif. We also demonstrate the protein-protein interaction between Mesp1 and cAMP-responsive element binding protein 1 (Creb1) in vitro and in vivo. Utilizing transgenesis, lineage tracing, flow cytometry, and immunostaining technologies, we define the lineage relationship between Mesp1- and Etv2-expressing cell populations. We observe that the majority of Etv2-EYFP(+) cells are derived from Mesp1-Cre(+) cells in both the embryo and yolk sac. Furthermore, we observe that the conditional deletion of Etv2, using a Mesp1-Cre transgenic strategy, results in vascular and hematopoietic defects similar to those observed in the global deletion of Etv2 and that it has embryonic lethality by embryonic day 9.5. In summary, our study supports the hypothesis that Mesp1 is a direct upstream transactivator of Etv2 during embryogenesis and that Creb1 is an important cofactor of Mesp1 in the transcriptional regulation of Etv2 gene expression.

Myeloid zinc finger 1 (Mzf1) differentially modulates murine cardiogenesis by interacting with an Nkx2.5 cardiac enhancer

Vertebrate heart development is strictly regulated by temporal and spatial expression of growth and transcription factors (TFs). We analyzed nine TFs, selected by in silico analysis of an Nkx2.5 enhancer, for their ability to transactivate the respective enhancer element that drives, specifically, expression of genes in cardiac progenitor cells (CPCs). Mzf1 showed significant activity in reporter assays and bound directly to the Nkx2.5 cardiac enhancer (Nkx2.5 CE) during murine ES cell differentiation. While Mzf1 is established as a hematopoietic TF, its ability to regulate cardiogenesis is completely unknown. Mzf1 expression was significantly enriched in CPCs from in vitro differentiated ES cells and in mouse embryonic hearts. To examine the effect of Mzf1 overexpression on CPC formation, we generated a double transgenic, inducible, tetOMzf1-Nkx2.5 CE eGFP ES line. During in vitro differentiation an early and continuous Mzf1 overexpression inhibited CPC formation and cardiac gene expression. A late Mzf1 overexpression, coincident with a second physiological peak of Mzf1 expression, resulted in enhanced cardiogenesis. These findings implicate a novel, temporal-specific role of Mzf1 in embryonic heart development. Thereby we add another piece of puzzle in understanding the complex mechanisms of vertebrate cardiac development and progenitor cell differentiation. Consequently, this knowledge will be of critical importance to guide efficient cardiac regenerative strategies and to gain further insights into the molecular basis of congenital heart malformations.

How to make a cardiomyocyte

During development, cardiogenesis is orchestrated by a family of heart progenitors that build distinct regions of the heart. Each region contains diverse cell types that assemble to form the complex structures of the individual cardiac compartments. Cardiomyocytes are the main cell type found in the heart and ensure contraction of the chambers and efficient blood flow throughout the body. Injury to the cardiac muscle often leads to heart failure due to the loss of a large number of cardiomyocytes and its limited intrinsic capacity to regenerate the damaged tissue, making it one of the leading causes of morbidity and mortality worldwide. In this Primer we discuss how insights into the molecular and cellular framework underlying cardiac development can be used to guide the in vitro specification of cardiomyocytes, whether by directed differentiation of pluripotent stem cells or via direct lineage conversion. Additional strategies to generate cardiomyocytes in situ, such as reactivation of endogenous cardiac progenitors and induction of cardiomyocyte proliferation, will also be discussed.

Irx4 identifies a chamber-specific cell population that contributes to ventricular myocardium development

BACKGROUND

The ventricular myocardium is the most prominent layer of the heart, and the most important for mediating cardiac physiology. Although the ventricular myocardium is critical for heart function, the cellular hierarchy responsible for ventricle-specific myocardium development remains unresolved.

RESULTS

To determine the pattern and time course of ventricular myocardium development, we investigated IRX4 protein expression, which has not been previously reported. We identified IRX4+ cells in the cardiac crescent, and these cells were positive for markers of the first or second heart fields. From the onset of chamber formation, IRX4+ cells were restricted to the ventricular myocardium. This expression pattern persisted into adulthood. Of interest, we observed that IRX4 exhibits developmentally regulated dynamic intracellular localization. Throughout prenatal cardiogenesis, and up to postnatal day 4, IRX4 was detected in the cytoplasm of ventricular myocytes. However, between postnatal days 5–6, IRX4 translocated to the nucleus of ventricular myocytes.

CONCLUSIONS

Given the ventricle-specific expression of Irx4 in later stages of heart development, we hypothesize that IRX4+ cells in the cardiac crescent represent the earliest cell population in the cellular hierarchy underlying ventricular myocardium development.

Incorporation of gold-coated microspheres into embryoid body of human embryonic stem cells for cardiomyogenic differentiation

Human embryonic stem cells (hESCs) are a useful cell source for cardiac regeneration by stem cell therapy. In this study, we show that incorporation of gold-coated microspheres into hESC-derived embryoid bodies (EBs) enhances the cardiomyogenic differentiation process of pluripotent embryonic stem cells. A polycaprolactone (PCL) microsphere surface was coated with gold. Either gold-coated PCL microspheres (AuMS) or PCL microspheres (MS) were incorporated into hESC-derived EBs. AuMS and MS were not cytotoxic. AuMS promoted the expression of genes for mesodermal and cardiac mesodermal lineage cells, both of which are intermediates in the process of cardiac differentiation of hESCs on day 4 and the expression of cardiomyogenic differentiation markers on day 14 compared to MS. AuMS also enhanced gene expression of cardiac-specific extracellular matrices. Incorporation of gold-coated MS into hESC-derived EBs may provide a new platform for inducing cardiomyogenic differentiation of pluripotent embryonic stem cells.

Traffic jam in the primitive streak: the role of defective mesoderm migration in birth defects

Gastrulation is the process in which the three germ layers are formed that contribute to the formation of all major tissues in the developing embryo. We here review mouse genetic models in which defective gastrulation leads to mesoderm insufficiencies in the embryo. Depending on severity of the abnormalities, the outcomes range from incompatible with embryonic survival to structural birth defects, such as heart defects, spina bifida, or caudal dysgenesis. The combined evidence from the mutant models supports the notion that these congenital anomalies can originate from perturbations of mesoderm specification, epithelial-mesenchymal transition, and mesodermal cell migration. Knowledge about the molecular pathways involved may help to improve strategies for the prevention of major structural birth defects.

The functional diversity of essential genes required for mammalian cardiac development

Genes required for an organism to develop to maturity (for which no other gene can compensate) are considered essential. The continuing functional annotation of the mouse genome has enabled the identification of many essential genes required for specific developmental processes including cardiac development. Patterns are now emerging regarding the functional nature of genes required at specific points throughout gestation. Essential genes required for development beyond cardiac progenitor cell migration and induction include a small and functionally homogenous group encoding transcription factors, ligands and receptors. Actions of core cardiogenic transcription factors from the Gata, Nkx, Mef, Hand, and Tbx families trigger a marked expansion in the functional diversity of essential genes from midgestation onwards. As the embryo grows in size and complexity, genes required to maintain a functional heartbeat and to provide muscular strength and regulate blood flow are well represented. These essential genes regulate further specialization and polarization of cell types along with proliferative, migratory, adhesive, contractile, and structural processes. The identification of patterns regarding the functional nature of essential genes across numerous developmental systems may aid prediction of further essential genes and those important to development and/or progression of disease. genesis 52:713–737, 2014.

Dose-dependent Nodal/Smad signals pattern the early mouse embryo

Nodal signals in the early post-implantation stage embryo are essential to establish initial proximal-distal (P-D) polarity and generate the final anterior-posterior (A-P) body axis. Nodal signaling in the epiblast results in the phosphorylation of Smad2 in the overlying visceral endoderm necessary to induce the AVE, in part via Smad2-dependent activation of the T-box gene Eomesodermin. Slightly later following mesoderm induction a continuum of dose-dependent Nodal signaling during the process of gastrulation underlies specification of mesodermal and definitive endoderm progenitors. Dynamic Nodal expression during the critical 72 h time window immediately following implantation, accomplished by a series of feed-back and feed-forward mechanisms serves to provide key positional cues required for establishment of the body plan and controls cell fate decisions in the early mammalian embryo.

Mesp1 patterns mesoderm into cardiac, hematopoietic, or skeletal myogenic progenitors in a context-dependent manner

Mesp1 is regarded as the master regulator of cardiovascular development, initiating the cardiac transcription factor cascade to direct the generation of cardiac mesoderm. To define the early embryonic cell population that responds to Mesp1, we performed pulse inductions of gene expression over tight temporal windows following embryonic stem cell differentiation. Remarkably, instead of promoting cardiac differentiation in the initial wave of mesoderm, Mesp1 binds to the Tal1 (Scl) +40 kb enhancer and generates Flk-1+ precursors expressing Etv2 (ER71) and Tal1 that undergo hematopoietic differentiation. The second wave of mesoderm responds to Mesp1 by differentiating into PDGFRα+ precursors that undergo cardiac differentiation. Furthermore, in the absence of serum-derived factors, Mesp1 promotes skeletal myogenic differentiation. Lineage tracing revealed that the majority of yolk sac and many adult hematopoietic cells derive from Mesp1+ precursors. Thus, Mesp1 is a context-dependent determination factor, integrating the stage of differentiation and the signaling environment to specify different lineage outcomes.

Insulin inhibits cardiac mesoderm, not mesendoderm, formation during cardiac differentiation of human pluripotent stem cells and modulation of canonical Wnt signaling can rescue this inhibition

The study of the regulatory signaling hierarchies of human heart development is limited by a lack of model systems that can reproduce the precise developmental events that occur during human embryogenesis. The advent of human pluripotent stem cell (hPSC) technology and robust cardiac differentiation methods affords a unique opportunity to monitor the full course of cardiac induction in vitro. Here, we show that stage-specific activation of insulin signaling strongly inhibited cardiac differentiation during a monolayer-based differentiation protocol that used transforming growth factor β superfamily ligands to generate cardiomyocytes. However, insulin did not repress cardiomyocyte differentiation in a defined protocol that used small molecule regulators of canonical Wnt signaling. By examining the context of insulin inhibition of cardiomyocyte differentiation, we determined that the inhibitory effects by insulin required Wnt/β-catenin signaling and that the cardiomyocyte differentiation defect resulting from insulin exposure was rescued by inhibition of Wnt/β-catenin during the cardiac mesoderm (Nkx2.5+) stage. Thus, insulin and Wnt/β-catenin signaling pathways, as a network, coordinate to influence hPSC differentiation to cardiomyocytes, with the Wnt/β-catenin pathway dominant to the insulin pathway. Our study contributes to the understanding of the regulatory hierarchies of human cardiomyocyte differentiation and has implications for modeling human heart development.

Mutational analysis of the human MESP1 gene in patients with congenital heart disease reveals a highly variable sequence in exon 1

MESP1 represents an essential transcription factor to guarantee coordinated cardiac development. The expression of MESP1 is thought to be the first sign that a cell has been committed to the cardiac lineage. We analyzed the coding sequence of MESP1 in 215 patients with congenital heart disease. Our results show that the sequence of exon 1 is highly variable with up to seven alterations in individual samples. Five base pair positions (c.157_G>C A53P, rs6496598; c.174_A>C P58P, rs28377352; c.182_T>G L61R, rs28368490; c.669_C>G F223L, rs2305440; c.687_T>G P229P, rs2305441) are particularly variable. In almost half of the samples a 12 base pair insertion after position 55 (c.165_166insGTGCCGAGCCCC P55insVPSP, rs71934166) coding for VPSP was detected which was strongly correlated with the appearance of further amino acid changes (c.157_G>C A53P, c.182_T>G L61R, c.669_C>G F223L). Two missense mutations (c.33_G>C E11D, rs190259690; c.528_A>T T176S) were detected in two patients but were absent in the controls. The assessment of the biological activity of altered MESP1 proteins in a luciferase reporter assay showed an enhanced activity of the c.33_G>C E11D mutation and a reduction of the insertion without an accompanying change of c.182_T>G L61R. The modified biological properties of mutated MESP1 proteins might be associated with the appearance of certain pathological phenotypes of congenital heart disease.

Heart genetics in a small package, exploiting the condensed genome of Ciona intestinalis

Defects in the initial establishment of cardiogenic cell fate are likely to contribute to pervasive human congenital cardiac abnormalities. However, the molecular underpinnings of nascent cardiac fate induction have proven difficult to decipher. In this review we explore the participation of extracellular, cellular and nuclear factors in comprehensive specification networks. At each level, we elaborate on insights gained through the study of cardiogenesis in the invertebrate chordate Ciona intestinalis and propose productive lines of future research. In-depth discussion of pre-cardiac induction is intended to serve as a paradigm, illustrating the potential use of Ciona to elucidate comprehensive networks underlying additional aspects of chordate cardiogenesis, including directed migration and subspecification of cardiac and pharyngeal lineages.

Fibronectin mediates mesendodermal cell fate decisions

Non-cell-autonomous signals often play crucial roles in cell fate decisions during animal development. Reciprocal signaling between endoderm and mesoderm is vital for embryonic development, yet the key signals and mechanisms remain unclear. Here, we show that endodermal cells efficiently promote the emergence of mesodermal cells in the neighboring population through signals containing an essential short-range component. The endoderm-mesoderm interaction promoted precardiac mesoderm formation in mouse embryonic stem cells and involved endodermal production of fibronectin. In vivo, fibronectin deficiency resulted in a dramatic reduction of mesoderm accompanied by endodermal expansion in zebrafish embryos. This event was mediated by regulation of Wnt signaling in mesodermal cells through activation of integrin-β1. Our findings highlight the importance of the extracellular matrix in mediating short-range signals and reveal a novel function of endoderm, involving fibronectin and its downstream signaling cascades, in promoting the emergence of mesoderm.

Transient Mesp1 expression: a driver of cardiac cell fate determination

Mesp1 sits on the tip of the cardiac regulatory hierarchy, recent evidences support that it is regulated by stem cell factor Oct4, early gastrulation signal canonical Wnts and a couple of T-box factors, T and Eomes. With other transcription factors, Mesp1 programs/reprograms human cells toward cardiomyocytes.

A molecular and genetic outline of cardiac morphogenesis

Perturbations in cardiac development result in congenital heart disease, the leading cause of birth defect-related infant morbidity and mortality. Advances in cardiac developmental biology have significantly augmented our understanding of signalling pathways and transcriptional networks underlying heart formation. Cardiogenesis is initiated with the formation of mesodermal multipotent cardiac progenitor cells and is governed by cross-talk between developmental cues emanating from endodermal, mesodermal and ectodermal cells. The molecular and transcriptional machineries that direct the specification and differentiation of these cardiac precursors are part of an evolutionarily conserved programme that includes the Nkx-, Gata-, Hand-, T-box- and Mef2 family of transcription factors. Unravelling the hierarchical networks governing the fate and differentiation of cardiac precursors is crucial for our understanding of congenital heart disease and future stem cell-based and gene therapies. Recent molecular and genetic lineage analyses have revealed that subpopulations of cardiac progenitor cells follow distinctive specification and differentiation paths, which determine their final contribution to the heart. In the last decade, progenitor cells that contribute to the arterial pole and right ventricle have received much attention, as abnormal development of these cells frequently results in congenital defects of the aortic and pulmonary outlets, representing the most commonly occurring congenital cardiac defects. In this review, we provide an overview of the building plan of the vertebrate four-chambered heart, with a special focus on cardiac progenitor cell specification, differentiation and deployment during arterial pole development.

Essential and unexpected role of Yin Yang 1 to promote mesodermal cardiac differentiation

RATIONALE

Cardiogenesis is regulated by a complex interplay between transcription factors. However, little is known about how these interactions regulate the transition from mesodermal precursors to cardiac progenitor cells (CPCs).

OBJECTIVE

To identify novel regulators of mesodermal cardiac lineage commitment.

METHODS AND RESULTS

We performed a bioinformatic-based transcription factor binding site analysis on upstream promoter regions of genes that are enriched in embryonic stem cell-derived CPCs. From 32 candidate transcription factors screened, we found that Yin Yang 1 (YY1), a repressor of sarcomeric gene expression, is present in CPCs in vivo. Interestingly, we uncovered the ability of YY1 to transcriptionally activate Nkx2.5, a key marker of early cardiogenic commitment. YY1 regulates Nkx2.5 expression via a 2.1-kb cardiac-specific enhancer as demonstrated by in vitro luciferase-based assays, in vivo chromatin immunoprecipitation, and genome-wide sequencing analysis. Furthermore, the ability of YY1 to activate Nkx2.5 expression depends on its cooperative interaction with Gata4 at a nearby chromatin. Cardiac mesoderm-specific loss-of-function of YY1 resulted in early embryonic lethality. This was corroborated in vitro by embryonic stem cell-based assays in which we showed that the overexpression of YY1 enhanced the cardiogenic differentiation of embryonic stem cells into CPCs.

CONCLUSIONS

These results demonstrate an essential and unexpected role for YY1 to promote cardiogenesis as a transcriptional activator of Nkx2.5 and other CPC-enriched genes.

Cardiac differentiation in Xenopus is initiated by mespa

AIMS

Future cardiac repair strategies will require a profound understanding of the principles underlying cardiovascular differentiation. Owing to its extracorporal and rapid development, Xenopus laevis provides an ideal experimental system to address these issues in vivo. Whereas mammalian MesP1 is currently regarded as the earliest marker for the cardiovascular system, several MesP1-related factors from Xenopus-mespa, mespb, and mespo-have been assigned only to somitogenesis so far. We, therefore, analysed these genes comparatively for potential contributions to cardiogenesis.

METHODS AND RESULTS

RNA in situ hybridizations revealed a novel anterior expression domain exclusively occupied by mespa during gastrulation, which precedes the prospective heart field. Correspondingly, when overexpressed mespa most strongly induced cardiac markers in vivo as well as ex vivo. Transference to murine embryonic stem (ES) cells and subsequent FACS analyses for Flk-1 and Troponin I confirmed the high potential of mespa as a cardiac inducer. In vivo, Morpholino-based knockdown of mespa protein led to a dramatic loss of pro-cardiac and sarcomeric markers, which could be rescued either by mespa itself or human MesP1, but neither by mespb nor mespo. Epistatic analysis positioned mespa upstream of mespo and mespb, and revealed positive autoregulation for mespa at the time of its induction.

CONCLUSIONS

Our findings contribute to the understanding of conserved events initiating vertebrate cardiogenesis. We identify mespa as functional amphibian homologue of mammalian MesP1. These results will enable the dissection of cardiac specification from the very beginning in the highly versatile Xenopus system.

Early cardiac development: a view from stem cells to embryos

From the 1920s, early cardiac development has been studied in chick and, later, in mouse embryos in order to understand the first cell fate decisions that drive specification and determination of the endocardium, myocardium, and epicardium. More recently, mouse and human embryonic stem cells (ESCs) have demonstrated faithful recapitulation of early cardiogenesis and have contributed significantly to this research over the past few decades. Derived almost 15 years ago, human ESCs have provided a unique developmental model for understanding the genetic and epigenetic regulation of early human cardiogenesis. Here, we review the biological concepts underlying cell fate decisions during early cardiogenesis in model organisms and ESCs. We draw upon both pioneering and recent studies and highlight the continued role for in vitro stem cells in cardiac developmental biology.

Epithelial-mesenchymal transitions: insights from development

Epithelial-mesenchymal transition (EMT) is a crucial, evolutionarily conserved process that occurs during development and is essential for shaping embryos. Also implicated in cancer, this morphological transition is executed through multiple mechanisms in different contexts, and studies suggest that the molecular programs governing EMT, albeit still enigmatic, are embedded within developmental programs that regulate specification and differentiation. As we review here, knowledge garnered from studies of EMT during gastrulation, neural crest delamination and heart formation have furthered our understanding of tumor progression and metastasis.

T-box factors: insights into the evolutionary emergence of the complex heart

The heart as a functional organ first appeared in bilaterians as a single peristaltic pump and evolved through arthropods, fish, amphibians, and finally mammals into a four-chambered engine controlling blood-flow within the body. The acquisition of cardiac complexity in the evolving heart was a product of gene duplication events and the co-option of novel signaling pathways to an ancestral cardiac-specific gene network. T-box factors belong to an evolutionary conserved family of transcriptional regulators with diverse roles in development. Their regulatory functions are integral in the initiation and potentiation of heart development, and mutations in these genes are associated with congenital heart defects. In this review we will discuss the evolutionary conserved cardiac regulatory functions of this family as well as their implication in disease in an aim to facilitate future gene-targeted and regenerative therapeutic remedies.

From pluripotency to distinct cardiomyocyte subtypes

Differentiated adult cardiomyocytes (CMs) lack significant regenerative potential, which is one reason why degenerative heart diseases are the leading cause of death in the western world. For future cardiac repair, stem cell-based therapeutic strategies may become alternatives to donor heart transplantation. The principle of reprogramming adult terminally differentiated cells (iPSC) had a major impact on stem cell biology. One can now generate autologous pluripotent cells that highly resemble embryonic stem cells (ESC) and that are ethically inoffensive as opposed to human ESC. Yet, due to genetic and epigenetic aberrations arising during the full reprogramming process, it is questionable whether iPSC will enter the clinic in the near future. Therefore, the recent achievement of directly reprogramming fibroblasts into cardiomyocytes via a milder approach, thereby avoiding an initial pluripotent state, may become of great importance. In addition, various clinical scenarios will depend on the availability of specific cardiac cellular subtypes, for which a first step was achieved via our own programming approach to achieve cardiovascular cell subtypes. In this review, we discuss recent progress in the cardiovascular stem cell field addressing the above mentioned aspects.

The origin and fate of muscle satellite cells

Satellite cells represent the primary population of stem cells resident in skeletal muscle. These adult muscle stem cells facilitate the postnatal growth, remodeling, and regeneration of skeletal muscle. Given the remarkable regenerative potential of satellite cells, there is great promise for treatment of muscle pathologies such as the muscular dystrophies with this cell population. Various protocols have been developed which allow for isolation, enrichment, and expansion of satellite cell derived muscle stem cells. However, isolated satellite cells have yet to translate into effective modalities for therapeutic intervention. Broadening our understanding of satellite cells and their niche requirements should improve our in vivo and ex vivo manipulation of these cells to expedite their use for regeneration of diseased muscle. This review explores the fates of satellite cells as determined by their molecular signatures, ontogeny, and niche dependent programming.

Eomesodermin induces Mesp1 expression and cardiac differentiation from embryonic stem cells in the absence of Activin

The transcription factor Eomesodermin (Eomes) is involved in early embryonic patterning, but the range of cell fates that it controls as well as its mechanisms of action remain unclear. Here we show that transient expression of Eomes promotes cardiovascular fate during embryonic stem cell differentiation. Eomes also rapidly induces the expression of Mesp1, a key regulator of cardiovascular differentiation, and directly binds to regulatory sequences of Mesp1. Eomes effects are strikingly modulated by Activin signalling: high levels of Activin inhibit the promotion of cardiac mesoderm by Eomes, while they enhance Eomes-dependent endodermal specification. These results place Eomes upstream of the Mesp1-dependent programme of cardiogenesis, and at the intersection of mesodermal and endodermal specification, depending on the levels of Activin/Nodal signalling.

Wnt5a and Wnt11 are essential for second heart field progenitor development

Wnt/β-catenin has a biphasic effect on cardiogenesis, promoting the induction of cardiac progenitors but later inhibiting their differentiation. Second heart field progenitors and expression of the second heart field transcription factor Islet1 are inhibited by the loss of β-catenin, indicating that Wnt/β-catenin signaling is necessary for second heart field development. However, expressing a constitutively active β-catenin with Islet1-Cre also inhibits endogenous Islet1 expression, reflecting the inhibitory effect of prolonged Wnt/β-catenin signaling on second heart field development. We show that two non-canonical Wnt ligands, Wnt5a and Wnt11, are co-required to regulate second heart field development in mice. Loss of Wnt5a and Wnt11 leads to a dramatic loss of second heart field progenitors in the developing heart. Importantly, this loss of Wnt5a and Wnt11 is accompanied by an increase in Wnt/β-catenin signaling, and ectopic Wnt5a/Wnt11 inhibits β-catenin signaling and promotes cardiac progenitor development in differentiating embryonic stem cells. These data show that Wnt5a and Wnt11 are essential regulators of the response of second heart field progenitors to Wnt/β-catenin signaling and that they act by restraining Wnt/β-catenin signaling during cardiac development.

Dual actions of Meis1 inhibit erythroid progenitor development and sustain general hematopoietic cell proliferation

Myeloid ecotropic viral integration site 1 (Meis1) forms a heterodimer with Pbx1 that augments Hox-dependent gene expression and is associated with leukemogenesis and HSC self-renewal. Here we identified 2 independent actions of Meis1 in hematopoietic development: one regulating cellular proliferation and the other involved in megakaryocyte lineage development. First, we found that endogenous Mesp1 indirectly induces Meis1 and Meis2 in endothelial cells derived from embryonic stem cells. Overexpression of Meis1 and Meis2 greatly enhanced the formation of hematopoietic colonies from embryonic stem cells, with the exception of erythroid colonies, by maintaining hematopoietic progenitor cells in a state of proliferation. Second, overexpression of Meis1 repressed the development of early erythroid progenitors, acting in vivo at the megakaryocyte-erythroid progenitor stage to skew development away from erythroid generation and toward megakaryocyte development. This previously unrecognized action of Meis1 may explain the embryonic lethality observed in Meis1(-/-) mice that arises from failure of lymphatic-venous separation and can result as a consequence of defective platelet generation. These results show that Meis1 exerts 2 independent functions, with its role in proliferation of hematopoietic progenitors acting earlier in development from its influence on the fate choice at the megakaryocyte-erythroid progenitor between megakaryocytic and erythroid development.

Endothelialization and altered hematopoiesis by persistent Etv2 expression in mice

Etv2 is a master gene for the commitment of hematopoietic/endothelial cells and is a potent inducer of endothelial/hematopoietic cells from embryonic stem cells. Etv2 is highly expressed in endothelial/hematopoietic precursors but is downregulated when they are differentiated, indicating that Etv2 should have transient but not constitutive function. However, relatively little attention has been paid to the importance of transient Etv2 expression. To determine whether transient Etv2 expression is essential to normal development and cell differentiation, we generated mice that constitutively express Etv2 from a Cre-activatable ROSA26 locus in endothelial/hematopoietic, somite, or neuronal lineages. Constitutive Etv2 expression caused profound phenotypes in hematopoietic/endothelial cells, with little effect on somite or neuronal lineages. In hematopoietic/endothelial lineages, constitutive Etv2 expression induced by Tie-2 Cre transgene caused abnormal yolk sac vasculature. Prolonged vascular endothelial cadherin expression and decreased B lymphopoiesis were observed in Etv2 expressing vascular endothelial cadherin(+)/CD45(+) cells, indicating that Etv2 forces endothelial program on hematopoietic cells. Etv2 expression in adult hematopoietic cells by Vav-iCre transgene also conferred an endothelial phenotype on hematopoietic stem cells and suppressed hematopoiesis, with erythropoiesis severely affected. We conclude that constitutive Etv2 expression perturbs vascular development and hematopoiesis. While promoting hematopoiesis/vasculogenesis, Etv2 expression should be tightly regulated to achieve normal vascular development and hematopoiesis.

A fluorescence spotlight on the clockwork development and metabolism of bone

Biological phenomena that exhibit periodic activity are often referred as biorhythms or biological clocks. Among these, circadian rhythms, cyclic patterns reflecting a 24-h cycle, are the most obvious in many physiological activities including bone growth and metabolism. In the late 1990s, several clock genes were isolated and their primary structures and functions were identified. The feedback loop model of transcriptional factors was proposed to work as a circadian core oscillator not only in the suprachiasmatic nuclei of the anterior hypothalamus, which is recognized as the mammalian central clock, but also in various peripheral tissues including cartilage and bone. Looking back to embryonic development, the fundamental architecture of skeletal patterning is regulated by ultradian clocks that are defined as biorhythms that cycle more than once every 24 h. As post-genomic approaches, transcriptome analysis by micro-array and bioimaging assays to detect luminescent and fluorescent signals have been exploited to uncover a more comprehensive set of genes and spatio-temporal regulation of the clockwork machinery in animal models. In this review paper, we provide an overview of topics related to these molecular clocks in skeletal biology and medicine, and discuss how fluorescence imaging approaches can contribute to widening our views of this realm of biomedical science.

[Pluripotent stem cells: a cell model for early cardiac development]

Mouse embryonic stem cell lines were derived three decades ago and allow the process of transgenesis and in turn the generation of transgenic mice. In the past and still nowadays, these mice as well as more primitive organisms have provided models to study the first cell decisions in the embryo. Derivation of human embryonic stem cells more than a decade ago has provided a similar cell model for human early embryonic development, an issue that could not be addressed for obvious ethical reasons which limit research on human embryos. These cells allow investigating the genetic and epigenetic mechanisms underlying the first cell decisions in the human embryo. Herein, we use cardiogenesis as an example to reveal the potential of these cells to better understand the first steps of cardiac development.

The Aplnr GPCR regulates myocardial progenitor development via a novel cell-non-autonomous, Gα(i/o) protein-independent pathway

Myocardial progenitor development involves the migration of cells to the anterior lateral plate mesoderm (ALPM) where they are exposed to the necessary signals for heart development to proceed. Whether the arrival of cells to this location is sufficient, or whether earlier signaling events are required, for progenitor development is poorly understood. Here we demonstrate that in the absence of Aplnr signaling, cells fail to migrate to the heart-forming region of the ALPM. Our work uncovers a previously uncharacterized cell-non-autonomous function for Aplnr signaling in cardiac development. Furthermore, we show that both the single known Aplnr ligand, Apelin, and the canonical Gα_i/o proteins that signal downstream of Aplnr are dispensable for Aplnr function in the context of myocardial progenitor development. This novel Aplnr signal can be substituted for by activation of Gata5/Smarcd3 in myocardial progenitors, suggesting a novel mechanism for Aplnr signaling in the establishment of a niche required for the proper migration/development of myocardial progenitor cells.

MicroRNAs in heart development

MicroRNAs (miRNAs) are a class of small noncoding RNAs of ~22nt in length which are involved in the regulation of gene expression at the posttranscriptional level by degrading their target mRNAs and/or inhibiting their translation. Expressed ubiquitously or in a tissue-specific manner, miRNAs are involved in the regulation of many biological processes such as cell proliferation, differentiation, apoptosis, and the maintenance of normal cellular physiology. Many miRNAs are expressed in embryonic, postnatal, and adult hearts. Aberrant expression or genetic deletion of miRNAs is associated with abnormal cardiac cell differentiation, disruption of heart development, and cardiac dysfunction. This chapter will summarize the history, biogenesis, and processing of miRNAs as well as their function in heart development, remodeling, and disease.

Mechanisms of cardiogenesis in cardiovascular progenitor cells

Self-renewing cells of the vertebrate heart have become a major subject of interest in the past decade. However, many researchers had a hard time to argue against the orthodox textbook view that defines the heart as a postmitotic organ. Once the scientific community agreed on the existence of self-renewing cells in the vertebrate heart, their origin was again put on trial when transdifferentiation, dedifferentiation, and reprogramming could no longer be excluded as potential sources of self-renewal in the adult organ. Additionally, the presence of self-renewing pluripotent cells in the peripheral blood challenges the concept of tissue-specific stem and progenitor cells. Leaving these unsolved problems aside, it seems very desirable to learn about the basic biology of this unique cell type. Thus, we shall here paint a picture of cardiovascular progenitor cells including the current knowledge about their origin, basic nature, and the molecular mechanisms guiding proliferation and differentiation into somatic cells of the heart.