Vascular endothelial and endocardial progenitors differentiate as cardiomyocytes in the absence of Etsrp/Etv2 function

Transcriptional inhibition of etv2 expression is essential for embryonic cardiac development

E-twenty six variant 2 (Etv2) transcription factor participates in cardiac, vascular-endothelial and blood cell lineage specification decisions during embryonic development. Previous studies have identified genomic elements in the etv2 locus responsible for vascular endothelial cell specification. Using transgenic analysis in zebrafish, we report here an etv2 proximal promoter fragment that prevents transgene misexpression in myocardial progenitor cells. This inhibition of etv2 expression in the cardiac progenitor population is partly mediated by Scl and Nkx2.5, likely through direct binding to the etv2 promoter, and cis-regulatory elements located in the first and second introns. The results identify an etv2 cis-regulatory mechanism controlling cardiovascular fate choice implying that etv2 participates in a transcriptional network mediating developmental plasticity of endothelial progenitor cells during embryonic development.

Early lineage restriction in temporally distinct populations of Mesp1 progenitors during mammalian heart development

Cardiac development arises from two sources of mesoderm progenitors, the first heart field (FHF) and the second (SHF). Mesp1 has been proposed to mark the most primitive multipotent cardiac progenitors common for both heart fields. Here, using clonal analysis of the earliest prospective cardiovascular progenitors in a temporally controlled manner during early gastrulation, we found that Mesp1 progenitors consist of two temporally distinct pools of progenitors restricted to either the FHF or the SHF. FHF progenitors were unipotent, whereas SHF progenitors were either unipotent or bipotent. Microarray and single-cell PCR with reverse transcription analysis of Mesp1 progenitors revealed the existence of molecularly distinct populations of Mesp1 progenitors, consistent with their lineage and regional contribution. Together, these results provide evidence that heart development arises from distinct populations of unipotent and bipotent cardiac progenitors that independently express Mesp1 at different time points during their specification, revealing that the regional segregation and lineage restriction of cardiac progenitors occur very early during gastrulation.

Hand2 elevates cardiomyocyte production during zebrafish heart development and regeneration

Embryonic heart formation requires the production of an appropriate number of cardiomyocytes; likewise, cardiac regeneration following injury relies upon the recovery of lost cardiomyocytes. The basic helix-loop-helix (bHLH) transcription factor Hand2 has been implicated in promoting cardiomyocyte formation. It is unclear, however, whether Hand2 plays an instructive or permissive role during this process. Here, we find that overexpression of hand2 in the early zebrafish embryo is able to enhance cardiomyocyte production, resulting in an enlarged heart with a striking increase in the size of the outflow tract. Our evidence indicates that these increases are dependent on the interactions of Hand2 in multimeric complexes and are independent of direct DNA binding by Hand2. Proliferation assays reveal that hand2 can impact cardiomyocyte production by promoting division of late-differentiating cardiac progenitors within the second heart field. Additionally, our data suggest that hand2 can influence cardiomyocyte production by altering the patterning of the anterior lateral plate mesoderm, potentially favoring formation of the first heart field at the expense of hematopoietic and vascular lineages. The potency of hand2 during embryonic cardiogenesis suggested that hand2 could also impact cardiac regeneration in adult zebrafish; indeed, we find that overexpression of hand2 can augment the regenerative proliferation of cardiomyocytes in response to injury. Together, our studies demonstrate that hand2 can drive cardiomyocyte production in multiple contexts and through multiple mechanisms. These results contribute to our understanding of the potential origins of congenital heart disease and inform future strategies in regenerative medicine.

Developmental hematopoiesis: ontogeny, genetic programming and conservation

Hematopoietic stem cells (HSCs) sustain blood production throughout life and are of pivotal importance in regenerative medicine. Although HSC generation from pluripotent stem cells would resolve their shortage for clinical applications, this has not yet been achieved mainly because of the poor mechanistic understanding of their programming. Bone marrow HSCs are first created during embryogenesis in the dorsal aorta (DA) of the midgestation conceptus, from where they migrate to the fetal liver and, eventually, the bone marrow. It is currently accepted that HSCs emerge from specialized endothelium, the hemogenic endothelium, localized in the ventral wall of the DA through an evolutionarily conserved process called the endothelial-to-hematopoietic transition. However, the endothelial-to-hematopoietic transition represents one of the last steps in HSC creation, and an understanding of earlier events in the specification of their progenitors is required if we are to create them from naïve pluripotent cells. Because of their ready availability and external development, zebrafish and Xenopus embryos have enormously facilitated our understanding of the early developmental processes leading to the programming of HSCs from nascent lateral plate mesoderm to hemogenic endothelium in the DA. The amenity of the Xenopus model to lineage tracing experiments has also contributed to the establishment of the distinct origins of embryonic (yolk sac) and adult (HSC) hematopoiesis, whereas the transparency of the zebrafish has allowed in vivo imaging of developing blood cells, particularly during and after the emergence of HSCs in the DA. Here, we discuss the key contributions of these model organisms to our understanding of developmental hematopoiesis.

Recent advances in heart regeneration

Although cardiac stem cells (CSCs) and tissue engineering are very promising for cardiac regenerative medicine, studies with model organisms for heart regeneration will provide alternative therapeutic targets and opportunities. Here, we present a review on heart regeneration, with a particular focus on the most recent work in mouse and zebrafish. We attempt to summarize the recent progresses and bottlenecks of CSCs and tissue engineering for heart regeneration; and emphasize what we have learned from mouse and zebrafish regenerative models on discovering crucial genetic and epigenetic factors for stimulating heart regeneration; and speculate the potential application of these regenerative factors for heart failure. A brief perspective highlights several important and promising research directions in this exciting field.

Fli1 acts downstream of Etv2 to govern cell survival and vascular homeostasis via positive autoregulation

RATIONALE

Cardiovascular health depends on proper development and integrity of blood vessels. Ets variant 2 (Etv2), a member of the E26 transforming-specific family of transcription factors, is essential to initiate a transcriptional program leading to vascular morphogenesis in early mouse embryos. However, endothelial expression of the Etv2 gene ceases at midgestation; therefore, vascular development past this stage must continue independent of Etv2.

OBJECTIVE

To identify molecular mechanisms underlying transcriptional regulation of vascular morphogenesis and homeostasis in the absence of Etv2.

METHODS AND RESULTS

Using loss- and gain-of-function strategies and a series of molecular techniques, we identify Friend leukemia integration 1 (Fli1), another E26 transforming-specific family transcription factor, as a downstream target of Etv2. We demonstrate that Etv2 binds to conserved Ets-binding sites within the promoter region of the Fli1 gene and governs Fli1 expression. Importantly, in the absence of Etv2 at midgestation, binding of Etv2 at Ets-binding sites in the Fli1 promoter is replaced by Fli1 protein itself, sustaining expression of Fli1 as well as selective Etv2-regulated endothelial genes to promote endothelial cell survival and vascular integrity. Consistent with this, we report that Fli1 binds to the conserved Ets-binding sites within promoter and enhancer regions of other Etv2-regulated endothelial genes, including Tie2, to control their expression at and beyond midgestation.

CONCLUSIONS

We have identified a novel positive feed-forward regulatory loop in which Etv2 activates expression of genes involved in vasculogenesis, including Fli1. Once the program is activated in early embryos, Fli1 then takes over to sustain the process in the absence of Etv2.

Flk1+ and VE-cadherin+ endothelial cells derived from iPSCs recapitulates vascular development during differentiation and display similar angiogenic potential as ESC-derived cells

RATIONALE

Induced pluripotent stem (iPS) cells have emerged as a source of potentially unlimited supply of autologous endothelial cells (ECs) for vascularization. However, the regenerative function of these cells relative to adult ECs and ECs derived from embryonic stem (ES) cells is unknown. The objective was to define the differentiation characteristics and vascularization potential of Fetal liver kinase (Flk)1(+) and Vascular Endothelial (VE)-cadherin(+) ECs derived identically from mouse (m)ES and miPS cells.

METHODS AND RESULTS

Naive mES and miPS cells cultured in type IV collagen (IV Col) in defined media for 5 days induced the formation of adherent cell populations, which demonstrated similar expression of Flk1 and VE-cadherin and the emergence of EC progenies. FACS purification resulted in 100% Flk1(+) VE-cadherin(+) cells from both mES and miPS cells. Emergence of Flk1(+)VE-cadherin(+) cells entailed expression of the vascular developmental transcription factor Er71, which bound identically to Flk1, VE-cadherin, and CD31 promoters in both populations. Immunostaining with anti-VE-cadherin and anti-CD31 antibodies and microscopy demonstrated the endothelial nature of these cells. Each cell population (unlike mature ECs) organized into well-developed vascular structures in vitro and incorporated into CD31(+) neovessels in matrigel plugs implanted in nude mice in vivo.

CONCLUSION

Thus, iPS cell-derived Flk1(+)VE-cadherin(+) cells expressing the Er71 are as angiogenic as mES cell-derived cells and incorporate into CD31(+) neovessels. Their vessel forming capacity highlights the potential of autologous iPS cells-derived EC progeny for therapeutic angiogenesis.

tal1 Regulates the formation of intercellular junctions and the maintenance of identity in the endocardium

The endocardium forms the inner lining of the heart tube, where it enables blood flow and also interacts with the myocardium during the formation of valves and trabeculae. Although a number of studies have identified regulators in the morphogenesis of the myocardium, relatively little is known about the molecules that control endocardial morphogenesis. Prior work has implicated the bHLH transcription factor Tal1 in endocardial tube formation: in zebrafish embryos lacking Tal1, endocardial cells form a disorganized mass within the ventricle and do not populate the atrium. Through blastomere transplantation, we find that tal1 plays a cell-autonomous role in regulating endocardial extension, suggesting that Tal1 activity influences the behavior of individual endocardial cells. The defects in endocardial behavior in tal1-deficient embryos originate during the earliest steps of endocardial morphogenesis: tal1-deficient endocardial cells fail to generate a cohesive monolayer at the midline and instead pack tightly together into a multi-layered aggregate. Moreover, the tight junction protein ZO-1 is mislocalized in the tal1-deficient endocardium, indicating a defect in intercellular junction formation. In addition, we find that the tal1-deficient endocardium fails to maintain its identity; over time, a progressively increasing number of tal1-deficient endocardial cells initiate myocardial gene expression. However, the onset of defects in intercellular junction formation precedes the onset of ectopic myocardial gene expression in the tal1-deficient endocardium. We therefore propose a model in which Tal1 has distinct roles in regulating the formation of endocardial intercellular junctions and maintaining endocardial identity.

Enhanced hemangioblast generation and improved vascular repair and regeneration from embryonic stem cells by defined transcription factors

The fetal liver kinase 1 (FLK-1)(+) hemangioblast can generate hematopoietic, endothelial, and smooth muscle cells (SMCs). ER71/ETV2, GATA2, and SCL form a core transcriptional network in hemangioblast development. Transient coexpression of these three factors during mesoderm formation stage in mouse embryonic stem cells (ESCs) robustly enhanced hemangioblast generation by activating bone morphogenetic protein (BMP) and FLK-1 signaling while inhibiting phosphatidylinositol 3-kinase, WNT signaling, and cardiac output. Moreover, etsrp, gata2, and scl inhibition converted hematopoietic field of the zebrafish anterior lateral plate mesoderm to cardiac. FLK-1(+) hemangioblasts generated by transient coexpression of the three factors (ER71-GATA2-SCL [EGS]-induced FLK-1(+)) effectively produced hematopoietic, endothelial, and SMCs in culture and in vivo. Importantly, EGS-induced FLK-1(+) hemangioblasts, when codelivered with mesenchymal stem cells as spheroids, were protected from apoptosis and generated functional endothelial cells and SMCs in ischemic mouse hindlimbs, resulting in improved blood perfusion and limb salvage. ESC-derived, EGS-induced FLK-1(+) hemangioblasts could provide an attractive cell source for future hematopoietic and vascular repair and regeneration.

Transcriptional regulation of endothelial cell and vascular development

The establishment and maintenance of the vascular system is critical for embryonic development and postnatal life. Defects in endothelial cell development and vessel formation and function lead to embryonic lethality and are important in the pathogenesis of vascular diseases. Here, we review the underlying molecular mechanisms of endothelial cell differentiation, plasticity, and the development of the vasculature. This review focuses on the interplay among transcription factors and signaling molecules that specify the differentiation of vascular endothelial cells. We also discuss recent progress on reprogramming of somatic cells toward distinct endothelial cell lineages and its promise in regenerative vascular medicine.

Role of Etv2-positive cells in the remodeling morphogenesis during vascular development

Etv2 is a critical determinant for the commitment of endothelial (EC) and hematopoietic (HPC) cells from mesoderm. Etv2 is assumed to be transiently required for EC commitment but dispensable after most ECs differentiate around E9.5. To confirm the time window of Etv2 requirement, Etv2 was ablated at different time points using ROSA26CreER mice. Unexpectedly, Etv2 ablation at E9.5 caused vascular remodeling defects in cranial and yolk sac vasculature. Immunostaining showed that Etv2+/VE-cadherin (VECAD)- cells were present around forming vasculature, mostly co-expressing Flk-1 with a small number of Etv2+/VECAD+ cells, indicating that Etv2+/Flk-1+/VECAD- cells are the major Etv2+ population promoting vascular remodeling around E9.5. Gene expression analysis showed up-regulation of Fgf proteins, Il-6, Glypican-3 and matrix metalloproteases in Etv2+/VEDAC- cells over Etv2-/VECAD+ mature ECs. Blockade of those factors caused reduced EC sprouting in ex vivo explant culture from E9.5 embryos, suggesting the functional significance of environmental factors derived from Etv2+ cells. Altogether, we propose that Etv2+/VEDAC- cells around E9.5-E10.5 provide extracellular factors to complete vascular morphogenesis in addition to becoming differentiated ECs incorporated into vessels. This insight for the new role of Ets protein in perivascular Flk-1+/VECAD-/(Etv2+) cells to induce expression of angiogenic factors may provide another strategy to control angiogenesis.

Transdifferentiation of fast skeletal muscle into functional endothelium in vivo by transcription factor Etv2

Etv2, a master regulator of endothelial cell fate, can induce fast skeletal muscle cells to transdifferentiate into endothelial cells in the zebrafish embryo.

Etsrp/Etv2 (Etv2) is an evolutionarily conserved master regulator of vascular development in vertebrates. Etv2 deficiency prevents the proper specification of the endothelial cell lineage, while its overexpression causes expansion of the endothelial cell lineage in the early embryo or in embryonic stem cells. We hypothesized that Etv2 alone is capable of transdifferentiating later somatic cells into endothelial cells. Using heat shock inducible Etv2 transgenic zebrafish, we demonstrate that Etv2 expression alone is sufficient to transdifferentiate fast skeletal muscle cells into functional blood vessels. Following heat treatment, fast skeletal muscle cells turn on vascular genes and repress muscle genes. Time-lapse imaging clearly shows that muscle cells turn on vascular gene expression, undergo dramatic morphological changes, and integrate into the existing vascular network. Lineage tracing and immunostaining confirm that fast skeletal muscle cells are the source of these newly generated vessels. Microangiography and observed blood flow demonstrated that this new vasculature is capable of supporting circulation. Using pharmacological, transgenic, and morpholino approaches, we further establish that the canonical Wnt pathway is important for induction of the transdifferentiation process, whereas the VEGF pathway provides a maturation signal for the endothelial fate. Additionally, overexpression of Etv2 in mammalian myoblast cells, but not in other cell types examined, induced expression of vascular genes. We have demonstrated in zebrafish that expression of Etv2 alone is sufficient to transdifferentiate fast skeletal muscle into functional endothelial cells in vivo. Given the evolutionarily conserved function of this transcription factor and the responsiveness of mammalian myoblasts to Etv2, it is likely that mammalian muscle cells will respond similarly.

The endothelial cell is a specialized cell type that lines blood vessels. These cells are involved in normal cardiovascular function and become damaged in cardiovascular disease states such as atherosclerosis and stroke. We have discovered that developing muscle cells in the zebrafish embryo can be converted into endothelial cells by the expression of a transcription factor called Etv2. Etv2 normally functions during embryonic development to specify blood and blood vessels. When expressed in muscle cells, Etv2 induces the expression of genes that are normally expressed in endothelial cells; it also represses muscle gene expression. On expressing Etv2, muscle cells change shape and go on to form lumenized blood vessels that connect to the existing circulatory system and support blood flow. The Wnt and VEGF signaling pathways are required for this fate transformation. Our results suggest that muscle cells may be a viable source for the de novo generation of endothelial cells for use in transplantation therapies and they highlight signalling pathways that might be manipulated to improve the efficiency of this process in mammalian cells.

Etv2 rescues Flk1 mutant embryoid bodies

Independent mouse knockouts of Etv2 and Flk1 are embryonic lethal and lack hematopoietic and endothelial lineages. We previously reported that Flk1 activates Etv2 in the initiation of hematopoiesis and vasculogenesis. However, Flk1 and its ligand VEGF are expressed throughout development, from E7.0 to adulthood, whereas Etv2 is expressed only transiently during embryogenesis. These observations suggest a complex regulatory interaction between Flk1 and Etv2. To further examine the Flk1 and Etv2 regulatory interaction, we transduced Etv2 and Flk1 mutant ES cells with viral integrants that inducibly overexpress Flk1 or Etv2. We demonstrated that forced expression of Etv2 rescued the hematopoietic and endothelial potential of differentiating Flk1 and Etv2 mutant cells. We further discovered that forced expression of Flk1 can rescue that of the Flk1, but not Etv2 mutant cells. Therefore, we conclude that the requirement for Flk1 can be bypassed by expressing Etv2, supporting the notion that disruption of Etv2 expression is responsible for the early phenotypes of the Etv2 and Flk1 mutant embryos.

Tcf7l1 proteins cell autonomously restrict cardiomyocyte and promote endothelial specification in zebrafish

Tcf7l1 (formerly Tcf3) proteins are conserved transcription factors whose function as transcriptional repressors is relieved through interactions with β-catenin. Although the functions of Tcf7l1 proteins have been studied in many developmental contexts, whether this conserved mediator of Wnt signaling is required for appropriate cardiomyocyte (CM) development has not been investigated. We find that Tcf7l1 proteins are necessary during two developmental periods to limit CM number in zebrafish embryos: prior to gastrulation and after the initial wave of CM differentiation. In contrast to partially redundant roles in anterior neural patterning, we find that Tcf7l1a and Tcf7l1b have non-redundant functions with respect to restricting CM specification during anterior mesodermal patterning, suggesting that between the two zebrafish Tcf7l1 paralogs there is a limit to the transcriptional repression provided during early CM specification. Using cell transplantation experiments, we determine that the Tcf7l1 paralogs are required cell autonomously to restrict CM specification and promote endothelial cell (EC) specification, which is overtly similar to the ability of Wnt signaling to direct a transformation between these progenitors in embryonic stem cells. Therefore, these results argue that during anterior-posterior patterning of the mesoderm Tcf7l1 proteins are cell autonomously required to limit Wnt signaling, which balances CM and EC progenitor specification within the anterior lateral plate mesoderm. This study expands our understanding of the in vivo developmental requirements of Tcf7l1 proteins and the mechanisms directing CM development in vertebrates.

Genetic interaction between pku300 and fbn2b controls endocardial cell proliferation and valve development in zebrafish

Abnormal cardiac valve morphogenesis is a common cause of human congenital heart disease. The molecular mechanisms regulating endocardial cell proliferation and differentiation into cardiac valves remain largely unknown, although great progress has been made on the endocardial contribution to the atrioventricular cushion and valve formation. We found that scotch tape(te382) (sco(te382)) encodes a novel transmembrane protein that is crucial for endocardial cell proliferation and heart valve development. The zebrafish sco(te382) mutant showed diminished endocardial cell proliferation, lack of heart valve leaflets and abnormal common cardinal and caudal veins. Positional cloning revealed a C946T nonsense mutation of a novel gene pku300 in the sco(te382) locus, which encoded a 540-amino-acid protein on cell membranes with one putative transmembrane domain and three IgG domains. A known G3935T missense mutation of fbn2b was also found ∼570 kb away from pku300 in sco(te382) mutants. The genetic mutant sco(pku300), derived from sco(te382), only had the C946T mutation of pku300 and showed reduced numbers of atrial endocardial cells and an abnormal common cardinal vein. Morpholino knockdown of fbn2b led to fewer atrial endocardial cells and an abnormal caudal vein. Knockdown of both pku300 and fbn2b phenocopied these phenotypes in sco(te382) genetic mutants. pku300 transgenic expression in endocardial and endothelial cells, but not myocardial cells, partially rescued the atrial endocardial defects in sco(te382) mutants. Mechanistically, pku300 and fbn2b were required for endocardial cell proliferation, endocardial Notch signaling and the proper formation of endocardial cell adhesion and tight junctions, all of which are crucial for cardiac valve development. We conclude that pku300 and fbn2b represent the few genes capable of regulating endocardial cell proliferation and signaling in zebrafish cardiac valve development.

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.

Scl represses cardiomyogenesis in prospective hemogenic endothelium and endocardium

Endothelium in embryonic hematopoietic tissues generates hematopoietic stem/progenitor cells; however, it is unknown how its unique potential is specified. We show that transcription factor Scl/Tal1 is essential for both establishing the hematopoietic transcriptional program in hemogenic endothelium and preventing its misspecification to a cardiomyogenic fate. Scl(-/-) embryos activated a cardiac transcriptional program in yolk sac endothelium, leading to the emergence of CD31+Pdgfrα+ cardiogenic precursors that generated spontaneously beating cardiomyocytes. Ectopic cardiogenesis was also observed in Scl(-/-) hearts, where the disorganized endocardium precociously differentiated into cardiomyocytes. Induction of mosaic deletion of Scl in Scl(fl/fl)Rosa26Cre-ER(T2) embryos revealed a cell-intrinsic, temporal requirement for Scl to prevent cardiomyogenesis from endothelium. Scl(-/-) endothelium also upregulated the expression of Wnt antagonists, which promoted rapid cardiomyocyte differentiation of ectopic cardiogenic cells. These results reveal unexpected plasticity in embryonic endothelium such that loss of a single master regulator can induce ectopic cardiomyogenesis from endothelial cells.

Uncovering the molecular and cellular mechanisms of heart development using the zebrafish

Over the past 20 years, the zebrafish has emerged as a powerful model organism for studying cardiac development. Its ability to survive without an active circulation and amenability to forward genetics has led to the identification of numerous mutants whose study has helped elucidate new mechanisms in cardiac development. Furthermore, its transparent, externally developing embryos have allowed detailed cellular analyses of heart development. In this review, we discuss the molecular and cellular processes involved in zebrafish heart development from progenitor specification to development of the valve and the conduction system. We focus on imaging studies that have uncovered the cellular bases of heart development and on zebrafish mutants with cardiac abnormalities whose study has revealed novel molecular pathways in cardiac cell specification and tissue morphogenesis.

Overlapping cardiac programs in heart development and regeneration

Gaining cellular and molecular insights into heart development and regeneration will likely provide new therapeutic targets and opportunities for cardiac regenerative medicine, one of the most urgent clinical needs for heart failure. Here we present a review on zebrafish heart development and regeneration, with a particular focus on early cardiac progenitor development and their contribution to building embryonic heart, as well as cellular and molecular programs in adult zebrafish heart regeneration. We attempt to emphasize that the signaling pathways shaping cardiac progenitors in heart development may also be redeployed during the progress of adult heart regeneration. A brief perspective highlights several important and promising research areas in this exciting field.

Transcription factors ETS2 and MESP1 transdifferentiate human dermal fibroblasts into cardiac progenitors

Unique insights for the reprograming of cell lineages have come from embryonic development in the ascidian Ciona, which is dependent upon the transcription factors Ci-ets1/2 and Ci-mesp to generate cardiac progenitors. We tested the idea that mammalian v-ets erythroblastosis virus E26 oncogene homolog 2 (ETS2) and mesoderm posterior (MESP) homolog may be used to convert human dermal fibroblasts into cardiac progenitors. Here we show that murine ETS2 has a critical role in directing cardiac progenitors during cardiopoiesis in embryonic stem cells. We then use lentivirus-mediated forced expression of human ETS2 to convert normal human dermal fibroblasts into replicative cells expressing the cardiac mesoderm marker KDR(+). However, although neither ETS2 nor the purported cardiac master regulator MESP1 can by themselves generate cardiac progenitors de novo from fibroblasts, forced coexpression of ETS2 and MESP1 or cell treatment with purified proteins reprograms fibroblasts into cardiac progenitors, as shown by the de novo appearance of core cardiac transcription factors, Ca(2+) transients, and sarcomeres. Our data indicate that ETS2 and MESP1 play important roles in a genetic network that governs cardiopoiesis.

Regulation of endothelial and hematopoietic development by the ETS transcription factor Etv2

PURPOSE OF REVIEW

Vasculogenesis and hematopoiesis are essential for development. Recently, the ETS domain transcription factor Etv2 has been identified as an essential regulator of vasculogenesis and hematopoiesis. Here, we review the recent studies that have established the critical role of Etv2 in the specification of mesoderm to blood and endothelial cells.

RECENT FINDINGS

Loss and gain-of-function studies have demonstrated the conserved role of Etv2 in endothelial and hematopoietic development. Recent studies have placed Etv2 at or near the top of the hierarchy in specification of these lineages and have begun to dissect the upstream regulators and downstream effectors of Etv2 function using multiple model organisms and experimental systems.

SUMMARY

Etv2 is essential for the specification of endothelial and hematopoietic lineages. Understanding the mechanisms through which Etv2 specifies endothelial and blood cells by defining upstream transcriptional regulators and cofactors will lead to greater insight into vasculogenesis and hematopoiesis, and may help to identify therapeutic targets to treat vascular disorders or to promote or inhibit vessel growth.

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.

Histone deacetylase is required for the activation of Wnt/β-catenin signaling crucial for heart valve formation in zebrafish embryos

During vertebrate heart valve formation, Wnt/β-catenin signaling induces BMP signals in atrioventricular canal (AVC) myocardial cells and underlying AVC endocardial cells then undergo endothelial-mesenchymal transdifferentiation (EMT) by receiving this BMP signals. Histone deacetylases (HDACs) have been implicated in numerous developmental processes by regulating gene expression. However, their specific roles in controlling heart valve development are largely unexplored. To investigate the role of HDACs in vertebrate heart valve formation, we treated zebrafish embryos with trichostatin A (TSA), an inhibitor of class I and II HDACs, from 36 to 48 h post-fertilization (hpf) during which heart looping and valve formation occur. Following TSA treatment, abnormal linear heart tube development was observed. In these embryos, expression of AVC myocardial bmp4 and AVC endocardial notch1b genes was markedly reduced with subsequent failure of EMT in the AVC endocardial cells. However, LiCl-mediated activation of Wnt/β-catenin signaling was able to rescue defective heart tube formation, bmp4 and notch1b expression, and EMT in the AVC region. Taken together, our results demonstrated that HDAC activity plays a pivotal role in vertebrate heart tube formation by activating Wnt/β-catenin signaling which induces bmp4 expression in AVC myocardial cells.

ER71 specifies Flk-1+ hemangiogenic mesoderm by inhibiting cardiac mesoderm and Wnt signaling

Two distinct types of Flk-1(+) mesoderm, hemangiogenic and cardiogenic, are thought to contribute to blood, vessel, and cardiac cell lineages. However, our understanding of how Flk-1(+) mesoderm is specified is currently limited. In the present study, we investigated whether ER71, an Ets transcription factor essential for hematopoietic and endothelial cell lineage development, could modulate the hemangiogenic or cardiogenic outcome of the Flk-1(+) mesoderm. We show that Flk-1(+) mesoderm can be divided into Flk-1(+)PDGFRα(-) hemangiogenic and Flk-1(+)PDGFRα(+) cardiogenic mesoderm. ER71-deficient embryonic stem cells produced only the Flk-1(+)PDGFRα(+) cardiogenic mesoderm, which generated SMCs and cardiomyocytes. Enforced ER71 expression in the wild-type embryonic stem cells skewed toward the Flk-1(+)PDGFRα(-) mesoderm formation, which generated hematopoietic and endothelial cells. Whereas hematopoietic and endothelial cell genes were positively regulated by ER71, cardiac and Wnt signaling pathway genes were negatively regulated by ER71. We show that ER71 could inhibit Wnt signaling in VE-cadherin-independent as well as VE-cadherin-dependent VE-cadherin/β-catenin/Flk-1 complex formation. Enforced β-catenin could rescue cardiogenic mesoderm in the context of ER71 overexpression. In contrast, ER71-deficient Flk-1(+) mesoderm displayed enhanced Wnt signaling, which was reduced by ER71 re-introduction. We provide the molecular basis for the antagonistic relationship between hemangiogenic and cardiogenic mesoderm specification by ER71 and Wnt signaling.