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

ETV2 transcriptionally activates Rig1 gene expression and promotes reprogramming of the endothelial lineage

ETV2 is an essential transcription factor as Etv2 null murine embryos lack all vasculature, blood and are lethal early during embryogenesis. Previous studies have established that ETV2 functions as a pioneer factor and directly reprograms fibroblasts to endothelial cells. However, the underlying molecular mechanisms regulating this reprogramming process remain incompletely defined. In the present study, we examined the ETV2-RIG1 cascade as regulators that govern ETV2-mediated reprogramming. Mouse embryonic fibroblasts (MEFs) harboring an inducible ETV2 expression system were used to overexpress ETV2 and reprogram these somatic cells to the endothelial lineage. Single-cell RNA-seq from reprogrammed fibroblasts defined the induction of the transcriptional network involved in Rig1-like receptor signaling pathways. Studies using ChIP-seq, electrophoretic mobility shift assays, and transcriptional assays demonstrated that ETV2 was a direct upstream activator of Rig1 gene expression. We further demonstrated that the knockdown of Rig1 and separately, Nfκb1 using shRNA significantly reduced the efficiency of endothelial cell reprogramming. These results highlight that ETV2 reprograms fibroblasts to endothelial cells by directly activating RIG1. These findings extend our current understanding of the molecular mechanisms underlying ETV2-mediated reprogramming and will be important in the design of revascularization strategies for the treatment of ischemic tissues such as ischemic heart disease.

Transcription factors regulating vasculogenesis and angiogenesis

Transcription factors (TFs) play a crucial role in regulating the dynamic and precise patterns of gene expression required for the initial specification of endothelial cells (ECs), and during endothelial growth and differentiation. While sharing many core features, ECs can be highly heterogeneous. Differential gene expression between ECs is essential to pattern the hierarchical vascular network into arteries, veins and capillaries, to drive angiogenic growth of new vessels, and to direct specialization in response to local signals. Unlike many other cell types, ECs have no single master regulator, instead relying on differing combinations of a necessarily limited repertoire of TFs to achieve tight spatial and temporal activation and repression of gene expression. Here, we will discuss the cohort of TFs known to be involved in directing gene expression during different stages of mammalian vasculogenesis and angiogenesis, with a primary focus on development.

Blastocyst complementation and interspecies chimeras in gene edited pigs

The only curative therapy for many endstage diseases is allograft organ transplantation. Due to the limited supply of donor organs, relatively few patients are recipients of a transplanted organ. Therefore, new strategies are warranted to address this unmet need. Using gene editing technologies, somatic cell nuclear transfer and human induced pluripotent stem cell technologies, interspecies chimeric organs have been pursued with promising results. In this review, we highlight the overall technical strategy, the successful early results and the hurdles that need to be addressed in order for these approaches to produce a successful organ that could be transplanted in patients with endstage diseases.

Mechanisms and strategies to promote cardiac xenotransplantation

End stage heart failure is a terminal disease, and the only curative therapy is orthotopic heart transplantation. Due to limited organ availability, alternative strategies have received intense interest for treatment of patients with advanced heart failure. Recent studies using gene-edited porcine organs suggest that cardiac xenotransplantation may provide a future source of organs. In this review, we highlight the historical milestones for cardiac xenotransplantation and the gene editing strategies designed to overcome immunological barriers, which have culminated in a recent cardiac pig-to-human xenotransplant. We also discuss recent results of studies on the engineering of human-porcine chimeric organs that may provide an alternative and complementary strategy to overcome some of the major immunological barriers to producing a new source of transplantable organs.

The regulatory role of pioneer factors during cardiovascular lineage specification – A mini review

Cardiovascular disease (CVD) remains the number one cause of death worldwide. Ischemic heart disease contributes to heart failure and has considerable morbidity and mortality. Therefore, alternative therapeutic strategies are urgently needed. One class of epigenetic regulators known as pioneer factors has emerged as an important tool for the development of regenerative therapies for the treatment of CVD. Pioneer factors bind closed chromatin and remodel it to drive lineage specification. Here, we review pioneer factors within the cardiovascular lineage, particularly during development and reprogramming and highlight the implications this field of research has for the future development of cardiac specific regenerative therapies.

Endothelial Cell Differentiation and Hemogenic Specification

Formation of the vasculature is a critical step within the developing embryo and its disruption causes early embryonic lethality. This complex process is driven by a cascade of signaling events that controls differentiation of mesodermal progenitors into primordial endothelial cells and their further specification into distinct subtypes (arterial, venous, hemogenic) that are needed to generate a blood circulatory network. Hemogenic endothelial cells give rise to hematopoietic stem and progenitor cells that generate all blood cells in the body during embryogenesis and postnatally. We focus our discussion on the regulation of endothelial cell differentiation, and subsequent hemogenic specification, and highlight many of the signaling pathways involved in these processes, which are conserved across vertebrates. Gaining a better understanding of the regulation of these processes will yield insights needed to optimize the treatment of vascular and hematopoietic disease and generate human stem cell-derived vascular and hematopoietic cells for tissue engineering and regenerative medicine.

Etv2 regulates enhancer chromatin status to initiate Shh expression in the limb bud

Sonic hedgehog (Shh) is essential for limb development, and the mechanisms that govern the propagation and maintenance of its expression has been well studied; however, the mechanisms that govern the initiation of Shh expression are incomplete. Here we report that ETV2 initiates Shh expression by changing the chromatin status of the developmental limb enhancer, ZRS. Etv2 expression precedes Shh in limb buds, and Etv2 inactivation prevents the opening of limb chromatin, including the ZRS, resulting in an absence of Shh expression. Etv2 overexpression in limb buds causes nucleosomal displacement at the ZRS, ectopic Shh expression, and polydactyly. Areas of nucleosome displacement coincide with ETS binding site clusters. ETV2 also functions as a transcriptional activator of ZRS and is antagonized by ETV4/5 repressors. Known human polydactyl mutations introduce novel ETV2 binding sites in the ZRS, suggesting that ETV2 dosage regulates ZRS activation. These studies identify ETV2 as a pioneer transcription factor (TF) regulating the onset of Shh expression, having both a chromatin regulatory role and a transcriptional activation role.

Endocardial-Myocardial Interactions During Early Cardiac Differentiation and Trabeculation

Throughout the continuum of heart formation, myocardial growth and differentiation occurs in concert with the development of a specialized population of endothelial cells lining the cardiac lumen, the endocardium. Once the endocardial cells are specified, they are in close juxtaposition to the cardiomyocytes, which facilitates communication between the two cell types that has been proven to be critical for both early cardiac development and later myocardial function. Endocardial cues orchestrate cardiomyocyte proliferation, survival, and organization. Additionally, the endocardium enables oxygenated blood to reach the cardiomyocytes. Cardiomyocytes, in turn, secrete factors that promote endocardial growth and function. As misregulation of this delicate and complex endocardial-myocardial interplay can result in congenital heart defects, further delineation of underlying genetic and molecular factors involved in cardiac paracrine signaling will be vital in the development of therapies to promote cardiac homeostasis and regeneration. Herein, we highlight the latest research that has advanced the elucidation of endocardial-myocardial interactions in early cardiac morphogenesis, including endocardial and myocardial crosstalk necessary for cellular differentiation and tissue remodeling during trabeculation, as well as signaling critical for endocardial growth during trabeculation.

Endocardial identity is established during early somitogenesis by Bmp signalling acting upstream of npas4l and etv2

The endocardium plays important roles in the development and function of the vertebrate heart; however, few molecular markers of this tissue have been identified and little is known about what regulates its differentiation. Here, we describe the Gt(SAGFF27C); Tg(4xUAS:egfp) line as a marker of endocardial development in zebrafish. Transcriptomic comparison between endocardium and pan-endothelium confirms molecular distinction between these populations and time-course analysis suggests differentiation as early as eight somites. To investigate what regulates endocardial identity, we employed npas4l, etv2 and scl loss-of-function models. Endocardial expression is lost in npas4l mutants, significantly reduced in etv2 mutants and only modestly affected upon scl loss-of-function. Bmp signalling was also examined: overactivation of Bmp signalling increased endocardial expression, whereas Bmp inhibition decreased expression. Finally, epistasis experiments showed that overactivation of Bmp signalling was incapable of restoring endocardial expression in etv2 mutants. By contrast, overexpression of either npas4l or etv2 was sufficient to rescue endocardial expression upon Bmp inhibition. Together, these results describe the differentiation of the endocardium, distinct from vasculature, and place npas4l and etv2 downstream of Bmp signalling in regulating its differentiation.

Summary: A zebrafish transgenic reporter of the endocardium is identified, permitting transcriptomic analysis and identification of new endocardial markers. Epistasis experiments demonstrate npas4l and etv2 act downstream of Bmp signalling to regulate endocardial differentiation.

Transcription Regulation of Tceal7 by the Triple Complex of Mef2c, Creb1 and Myod

Simple Summary

We have previously reported a striated muscle-specific gene during embryogenesis, Tceal7. Our studies have characterized the 0.7 kb promoter of the Tceal7 gene, which harbors important E-box motifs driving the LacZ reporter in the myogenic lineage. However, the underlying mechanism regulating the dynamic expression of Tceal7 during skeletal muscle regeneration is still elusive. In the present work, we have defined a cluster of Mef2#3–CRE#3–E#4 motifs through bioinformatic analysis and transcription assays. Our studies suggested that the triple complex of Mef2c, Creb1 and Myod binds to the Mef2#3–CRE#3–E#4 cluster region, therefore driving the dynamic expression of Tceal7 during skeletal muscle regeneration. The novel mechanism may throw new light on understanding transcription regulation in skeletal muscle myogenesis.

Abstract

Tceal7 has been identified as a direct, downstream target gene of MRF in the skeletal muscle. The overexpression of Tceal7 represses myogenic proliferation and promotes cell differentiation. Previous studies have defined the 0.7 kb upstream fragment of the Tceal7 gene. In the present study, we have further determined two clusters of transcription factor-binding motifs in the 0.7 kb promoter: CRE#2–E#1–CRE#1 in the proximal region and Mef2#3–CRE#3–E#4 in the distal region. Utilizing transcription assays, we have also shown that the reporter containing the Mef2#3–CRE#3–E#4 motifs is synergistically transactivated by Mef2c and Creb1. Further studies have mapped out the protein–protein interaction between Mef2c and Creb1. In summary, our present studies support the notion that the triple complex of Mef2c, Creb1 and Myod interacts with the Mef2#3–CRE#3–E#4 motifs in the distal region of the Tceal7 promoter, thereby driving Tceal7 expression during skeletal muscle development and regeneration.

ETV2/ER71 Transcription Factor as a Therapeutic Vehicle for Cardiovascular Disease

Cardiovascular diseases have long been the leading cause of mortality and morbidity in the United States as well as worldwide. Despite numerous efforts over the past few decades, the number of the patients with cardiovascular disease still remains high, thereby necessitating the development of novel therapeutic strategies equipped with a better understanding of the biology of the cardiovascular system. Recently, the ETS transcription factor, ETV2 (also known as ER71), has been recognized as a master regulator of the development of the cardiovascular system and plays an important role in pathophysiological angiogenesis and the endothelial cell reprogramming. Here, we discuss the detailed mechanisms underlying ETV2/ER71-regulated cardiovascular lineage development. In addition, recent reports on the novel functions of ETV2/ER71 in neovascularization and direct cell reprogramming are discussed with a focus on its therapeutic potential for cardiovascular diseases.

Etv2 as an essential regulator of mesodermal lineage development

The 'master regulatory factors' that position at the top of the genetic hierarchy of lineage determination have been a focus of intense interest, and have been investigated in various systems. Etv2/Etsrp71/ER71 is such a factor that is both necessary and sufficient for the development of haematopoietic and endothelial lineages. As such, genetic ablation of Etv2 leads to complete loss of blood and vessels, and overexpression can convert non-endothelial cells to the endothelial lineage. Understanding such master regulatory role of a lineage is not only a fundamental quest in developmental biology, but also holds immense possibilities in regenerative medicine. To harness its activity and utility for therapeutic interventions, it is essential to understand the regulatory mechanisms, molecular function, and networks that surround Etv2. In this review, we provide a comprehensive overview of Etv2 biology focused on mouse and human systems.

IP3R-mediated Ca2+ signals govern hematopoietic and cardiac divergence of Flk1+ cells via the calcineurin-NFATc3-Etv2 pathway

Ca2+ signals participate in various cellular processes with spatial and temporal dynamics, among which, inositol 1,4,5-trisphosphate receptors (IP3Rs)-mediated Ca2+ signals are essential for early development. However, the underlying mechanisms of IP3R-regulated cell fate decision remain largely unknown. Here we report that IP3Rs are required for the hematopoietic and cardiac fate divergence of mouse embryonic stem cells (mESCs). Deletion of IP3Rs (IP3R-tKO) reduced Flk1+/PDGFRα- hematopoietic mesoderm, c-Kit+/CD41+ hematopoietic progenitor cell population, and the colony-forming unit activity, but increased cardiac progenitor markers as well as cardiomyocytes. Concomitantly, the expression of a key regulator of hematopoiesis, Etv2, was reduced in IP3R-tKO cells, which could be rescued by the activation of Ca2+ signals and calcineurin or overexpression of constitutively active form of NFATc3. Furthermore, IP3R-tKO impaired specific targeting of Etv2 by NFATc3 via its evolutionarily conserved cis-element in differentiating ESCs. Importantly, the activation of Ca2+-calcineurin-NFAT pathway reversed the phenotype of IP3R-tKO cells. These findings reveal an unrecognized governing role of IP3Rs in hematopoietic and cardiac fate commitment via IP3Rs-Ca2+-calcineurin-NFATc3-Etv2 pathway.

Etv2-miR-130a-Jarid2 cascade regulates vascular patterning during embryogenesis

Remodeling of the primitive vasculature is necessary for the formation of a complex branched vascular architecture. However, the factors that modulate these processes are incompletely defined. Previously, we defined the role of microRNAs (miRNAs) in endothelial specification. In the present study, we further examined the Etv2-Cre mediated ablation of Dicer^L/L and characterized the perturbed vascular patterning in the embryo proper and yolk-sac. We mechanistically defined an important role for miR-130a, an Etv2 downstream target, in the mediation of vascular patterning and angiogenesis in vitro and in vivo. Inducible overexpression of miR-130a resulted in robust induction of vascular sprouts and angiogenesis with increased uptake of acetylated-LDL. Mechanistically, miR-130a directly regulated Jarid2 expression by binding to its 3’-UTR region. Over-expression of Jarid2 in HUVEC cells led to defective tube formation indicating its inhibitory role in angiogenesis. The knockout of miR-130a showed increased levels of Jarid2 in the ES/EB system. In addition, the levels of Jarid2 transcripts were increased in the Etv2-null embryos at E8.5. In the in vivo settings, injection of miR-130a specific morpholinos in zebrafish embryos resulted in perturbed vascular patterning with reduced levels of endothelial transcripts in the miR-130a morphants. Further, co-injection of miR-130a mimics in the miR-130a morphants rescued the vascular defects during embryogenesis. qPCR and in situ hybridization techniques demonstrated increased expression of jarid2a in the miR-130a morphants in vivo. These findings demonstrate a critical role for Etv2-miR-130a-Jarid2 in vascular patterning both in vitro and in vivo.

A CRISPR screen identifies genes controlling Etv2 threshold expression in murine hemangiogenic fate commitment

The ETS transcription factor Etv2 is necessary and sufficient for the generation of hematopoietic and endothelial cells. However, upstream regulators of Etv2 in hemangiogenesis, generation of hematopoietic and endothelial cells, have not been clearly addressed. Here we track the developmental route of hemangiogenic progenitors from mouse embryonic stem cells, perform genome-wide CRISPR screening, and transcriptome analysis of en route cell populations by utilizing Brachyury, Etv2, or Scl reporter embryonic stem cell lines to further understand the mechanisms that control hemangiogenesis. We identify the forkhead transcription factor Foxh1, in part through Eomes, to be critical for the formation of FLK1⁺ mesoderm, from which the hemangiogenic fate is specified. Importantly, hemangiogenic fate is specified not simply by the onset of Etv2 expression, but by a threshold-dependent mechanism, in which VEGF-FLK1 signaling plays an instructive role by promoting Etv2 threshold expression. These studies reveal comprehensive cellular and molecular pathways governing the hemangiogenic cell lineage development.

How haematopoietic and endothelial cell lineages are specified is unclear. Here, the authors identify the forkhead transcription factor Foxh1 as regulating FLK1+ mesoderm formation in mouse embryonic stem cells, which in turn specifies hemangiogenic fate via Etv2.

Earlier and broader roles of Mesp1 in cardiovascular development

Mesoderm posterior 1 is one of earliest markers of the nascent mesoderm. Its best-known function is driving the onset of the cardiovascular system. In the past decade, new evidence supports that Mesp1 acts earlier with greater breadth in cell fate decisions, and through cell-autonomous and cell non-autonomous mechanisms. This review summarizes these new aspects, with an emphasis on the upstream and downstream regulation around Mesp1 and how they may guide cell fate reprogramming.

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.

ETS transcription factors in embryonic vascular development

At least thirteen ETS-domain transcription factors are expressed during embryonic hematopoietic or vascular development and potentially function in the formation and maintenance of the embryonic vasculature or blood lineages. This review summarizes our current understanding of the specific roles played by ETS factors in vasculogenesis and angiogenesis and the implications of functional redundancies between them.

ETS Transcription Factor ETV2/ER71/Etsrp in Hematopoietic and Vascular Development

Effective establishment of the hematopoietic and vascular systems is prerequisite for successful embryogenesis. The ETS transcription factor Etv2 has proven to be essential for hematopoietic and vascular development. Etv2 expression marks the onset of the hematopoietic and vascular development and its deficiency leads to an absolute block in hematopoietic and vascular development. Etv2 is transiently expressed during development and is mainly expressed in testis in adults. Consistent with its expression pattern, Etv2 is transiently required for the generation of the optimal levels of the hemangiogenic cell population. Deletion of this gene after the hemangiogenic progenitor formation leads to normal hematopoietic and vascular development. Mechanistically, ETV2 induces the hemangiogenic program by activating blood and endothelial cell lineage specifying genes and enhancing VEGF signaling. Moreover, ETV2 establishes an ETS hierarchy by directly activating other Ets genes, which in the face of transient Etv2 expression, presumably maintain blood and endothelial cell program initiated by ETV2 through an ETS switching mechanism. Current studies suggest that the hemangiogenic progenitor population is exclusively sensitive to ETV2-dependent FLK1 signaling. Any perturbation in the ETV2, VEGF, and FLK1 balance causing insufficient hemangiogenic progenitor cell generation would lead to defects in hematopoietic and endothelial cell development.

MESP1 Mutations in Patients with Congenital Heart Defects

Identifying the genetic etiology of congenital heart disease (CHD) has been challenging despite being one of the most common congenital malformations in humans. We previously identified a microdeletion in a patient with a ventricular septal defect containing over 40 genes including MESP1 (mesoderm posterior basic helix-loop-helix transcription factor 1). Because of the importance of MESP1 as an early regulator of cardiac development in both in vivo and in vitro studies, we tested for MESP1 mutations in 647 patients with congenital conotruncal and related heart defects. We identified six rare, nonsynonymous variants not seen in ethnically matched controls and one likely race-specific nonsynonymous variant. Functional analyses revealed that three of these variants altered activation of transcription by MESP1. Two of the deleterious variants are located within the conserved HLH domain and thus impair the protein-protein interaction of MESP1 and E47. The third deleterious variant was a loss-of-function frameshift mutation. Our results suggest that pathologic variants in MESP1 may contribute to the development of CHD and that additional protein partners and downstream targets could likewise contribute to the wide range of causes for CHD.

Etv2 IS A MASTER REGULATOR OF HEMATOENDOTHELIAL LINEAGES

Ets transcription factors are important developmental regulators and have been shown to function as modulators of cell fate. We have previously discovered Ets variant 2 (Etv2) as an essential regulator of the hematoendothelial lineage. We have demonstrated that Etv2 mutant embryos are non-viable and lack hematoendothelial lineages. We have utilized gene editing technologies to define upstream regulators of the Etv2 gene and we and others have defined downstream target genes. Recent studies have demonstrated that Etv2, in combination with co-factors, promote a hematoendothelial fate in differentiating human-induced pluripotent stem cells. Collectively, these studies support the notion that Etv2 is a master regulator of the hematoendothelial lineages. Definition of these master regulators and the use of emerging technologies will provide a platform for engineering large animal models that will be useful for clinical research and regenerative medicine and will potentially impact the treatment of chronic vascular diseases.

The ETS Factor, ETV2: a Master Regulator for Vascular Endothelial Cell Development

Appropriate vessel development and its coordinated function is essential for proper embryogenesis and homeostasis in the adult. Defects in vessels cause birth defects and are an important etiology of diseases such as cardiovascular disease, tumor and diabetes retinopathy. The accumulative data indicate that ETV2, an ETS transcription factor, performs a potent and indispensable function in mediating vessel development. This review discusses the recent progress of the study of ETV2 with special focus on its regulatory mechanisms and cell fate determining role in developing mouse embryos as well as somatic cells.

Feedback Mechanisms Regulate Ets Variant 2 (Etv2) Gene Expression and Hematoendothelial Lineages

Etv2 is an essential transcriptional regulator of hematoendothelial lineages during embryogenesis. Although Etv2 downstream targets have been identified, little is known regarding the upstream transcriptional regulation of Etv2 gene expression. In this study, we established a novel methodology that utilizes the differentiating ES cell and embryoid body system to define the modules and enhancers embedded within the Etv2 promoter. Using this system, we defined an autoactivating role for Etv2 that is mediated by two adjacent Ets motifs in the proximal promoter. In addition, we defined the role of VEGF/Flk1-Calcineurin-NFAT signaling cascade in the transcriptional regulation of Etv2. Furthermore, we defined an Etv2-Flt1-Flk1 cascade that serves as a negative feedback mechanism to regulate Etv2 gene expression. To complement and extend these studies, we demonstrated that the Flt1 null embryonic phenotype was partially rescued in the Etv2 conditional knockout background. In summary, these studies define upstream and downstream networks that serve as a transcriptional rheostat to regulate Etv2 gene expression.