Induction of cardiomyocytes by GATA4 in Xenopus ectodermal explants

Towards Understanding the Gene-Specific Roles of GATA Factors in Heart Development: Does GATA4 Lead the Way?

Transcription factors play crucial roles in the regulation of heart induction, formation, growth and morphogenesis. Zinc finger GATA transcription factors are among the critical regulators of these processes. GATA4, 5 and 6 genes are expressed in a partially overlapping manner in developing hearts, and GATA4 and 6 continue their expression in adult cardiac myocytes. Using different experimental models, GATA4, 5 and 6 were shown to work together not only to ensure specification of cardiac cells but also during subsequent heart development. The complex involvement of these related gene family members in those processes is demonstrated through the redundancy among them and crossregulation of each other. Our recent identification at the genome-wide level of genes specifically regulated by each of the three family members and our earlier discovery that gata4 and gata6 function upstream, while gata5 functions downstream of noncanonical Wnt signalling during cardiac differentiation, clearly demonstrate the functional differences among the cardiogenic GATA factors. Such suspected functional differences are worth exploring more widely. It appears that in the past few years, significant advances have indeed been made in providing a deeper understanding of the mechanisms by which each of these molecules function during heart development. In this review, I will therefore discuss current evidence of the role of individual cardiogenic GATA factors in the process of heart development and emphasize the emerging central role of GATA4.

Exploring the Expression of Cardiac Regulators in a Vertebrate Extremophile: The Cichlid Fish Oreochromis (Alcolapia) alcalica

Although it is widely accepted that the cellular and molecular mechanisms of vertebrate cardiac development are evolutionarily conserved, this is on the basis of data from only a few model organisms suited to laboratory studies. Here, we investigate gene expression during cardiac development in the extremophile, non-model fish species, Oreochromis (Alcolapia) alcalica. We first characterise the early development of O. alcalica and observe extensive vascularisation across the yolk prior to hatching. We further investigate heart development by identifying and cloning O. alcalica orthologues of conserved cardiac transcription factors gata4, tbx5, and mef2c for analysis by in situ hybridisation. Expression of these three key cardiac developmental regulators also reveals other aspects of O. alcalica development, as these genes are expressed in developing blood, limb, eyes, and muscle, as well as the heart. Our data support the notion that O. alcalica is a direct-developing vertebrate that shares the highly conserved molecular regulation of the vertebrate body plan. However, the expression of gata4 in O. alcalica reveals interesting differences in the development of the circulatory system distinct from that of the well-studied zebrafish. Understanding the development of O. alcalica embryos is an important step towards providing a model for future research into the adaptation to extreme conditions; this is particularly relevant given that anthropogenic-driven climate change will likely result in more freshwater organisms being exposed to less favourable conditions.

TAK1/Map3k7 enhances differentiation of cardiogenic endoderm from mouse embryonic stem cells

Specification of the primary heart field in mouse embryos requires signaling from the anterior visceral endoderm (AVE). The nature of these signals is not known. We hypothesized that the TGFβ-activated kinase (TAK1/Map3k7) may act as a cardiogenic factor, based on its expression in heart-inducing endoderm and its requirement for cardiac differentiation of p19 cells. To test this, mouse embryonic stem (ES) cells overexpressing Map3k7 were isolated and differentiated as embryoid bodies (EBs). Map3k7-overexpressing EBs showed increased expression of AVE markers but interestingly, showed little effect on mesoderm formation and had no impact on overall cardiomyocyte formation. To test whether the pronounced expansion of endoderm masks an expansion of cardiac lineages, chimeric EBs were made consisting of Map3k7-overexpressing ES and wild type ES cells harboring a cardiac reporter transgene, MHCα::GFP, allowing cardiac differentiation to be assessed specifically in wild type ES cells. Wild type ES cells co-cultured with Map3k7-overexpressing cells had a 4-fold increase in expression of the cardiac reporter, supporting the hypothesis that Map3k7 increases the formation of cardiogenic endoderm. To further examine the role of Map3k7 in early lineage specification, other endodermal markers were examined. Interestingly, markers that are expressed in both the VE and later in gut development were expanded, whereas transcripts that specifically mark the early definitive (streak-derived) endoderm (DE) were not. To determine if Map3k7 is necessary for endoderm differentiation, EBs were grown in the presence of the Map3k7 specific inhibitor 5Z-7-oxozeaenol. Endoderm differentiation was dramatically decreased in these cells. Western blot analysis showed that known downstream targets of Map3k7 (Jnk, Nemo-like kinase (NLK) and p38 MAPK) were all inhibited. By contrast, transcripts for another TGFβ target, Sonic Hedgehog (Shh) were markedly upregulated, as were transcripts for Gli2 (but not Gli1 and Gli3). Together these data support the hypothesis that Map3k7 governs the formation, or proliferation of cardiogenic endoderm.

Liver Specification in the Absence of Cardiac Differentiation Revealed by Differential Sensitivity to Wnt/β Catenin Pathway Activation

Embryonic precursors of liver and heart, whilst not sharing cellular origin, develop in close proximity through a dynamic series of inductive signaling events. During gastrulation anterior endoderm (AE) provides cardiogenic signals that act on adjacent mesoderm, resulting in induction of cardiac precursors. Subsequently cardiogenic mesoderm generates a FGF signal that acts on adjacent AE to induce foregut organ specification. Additional signals such as BMP and Wnt provide further information required for liver specification. Most findings on liver specification were derived from mouse explant studies as well as experiments with Xenopus and zebrafish embryos. To address some of the limitations of these models, here we used two complementary ex vivo models based on Xenopus embryos: pluripotent animal cap explants expressing Gata4 transcription factor and conjugates of gastrula-stage AE with animal caps (AC). We show that in these models liver specification is not sensitive to Wnt signaling manipulation, in contrast to the requirement for Wnt antagonism shown in vivo. FGF pathway is not necessary for Gata4-induced liver specification in animal cap explants but is required for prolonged period in sandwiches of AE and AC. In contrast, BMP signaling is shown to be essential for Gata4-induced liver specification. Our findings may have implications for research on liver differentiation from embryonic stem cells.

GATA4 as a novel regulator involved in the development of the neural crest and craniofacial skeleton via Barx1

The role of GATA-binding protein 4 (GATA4) in neural crest cells (NCCs) is poorly defined. Here we showed that mouse NCCs lacking GATA4 exhibited developmental defects in craniofacial bone, teeth, and heart. The defects likely occurred due to decreased cell proliferation at the developmental stage. The in vitro results were consistent with the mouse model. The isobaric tags for relative and absolute quantitation assay revealed that BARX1 is one of the differentially expressed proteins after GATA4 knockdown in NCCs. On the basis of the results of dual-luciferase, electro-mobility shift, and chromatin immunoprecipitation assays, Barx1 expression is directly regulated by GATA4 in NCCs. In zebrafish, gata4 knockdown affects the development of NCCs derivatives. However, the phenotype in zebrafish could be partly rescued by co-injection of gata4 morpholino oligomers and barx1 mRNA. This study identified new downstream targets of GATA4 in NCCs and uncovered additional evidence of the complex regulatory functions of GATA4 in NCC development.

Genome-wide transcriptomics analysis identifies sox7 and sox18 as specifically regulated by gata4 in cardiomyogenesis

The transcription factors GATA4, GATA5 and GATA6 are important regulators of heart muscle differentiation (cardiomyogenesis), which function in a partially redundant manner. We identified genes specifically regulated by individual cardiogenic GATA factors in a genome-wide transcriptomics analysis. The genes regulated by gata4 are particularly interesting because GATA4 is able to induce differentiation of beating cardiomyocytes in Xenopus and in mammalian systems. Among the specifically gata4-regulated transcripts we identified two SoxF family members, sox7 and sox18. Experimental reinstatement of gata4 restores sox7 and sox18 expression, and loss of cardiomyocyte differentiation due to gata4 knockdown is partially restored by reinstating sox7 or sox18 expression, while (as previously reported) knockdown of sox7 or sox18 interferes with heart muscle formation. In order to test for conservation in mammalian cardiomyogenesis, we confirmed in mouse embryonic stem cells (ESCs) undergoing cardiomyogenesis that knockdown of Gata4 leads to reduced Sox7 (and Sox18) expression and that Gata4 is also uniquely capable of promptly inducing Sox7 expression. Taken together, we identify an important and conserved gene regulatory axis from gata4 to the SoxF paralogs sox7 and sox18 and further to heart muscle cell differentiation.

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Gata 4, 5 and 6 have redundant and non-redundant functions in heart development.

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RNA-seq analysis of Gata4, 5 and 6 knockdown experiments was carried out.

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Genes specifically regulated by Gata4, 5 and 6 were identified.

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The SoxF genes sox7 and sox18 were identified as specifically regulated by Gata4.

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Epistasis demonstrates a regulatory axis from Gata4 to Sox7/18 to cardiomyogenesis.

In vivo reprogramming for tissue regeneration and organismal rejuvenation

Transcription factor-mediated reprogramming has enabled us to induce the fate conversion of somatic cells into other cell types. Although the study of reprogramming mostly occurs at the cellular level in vitro, previous studies have demonstrated that somatic cells are reprogrammable in multicellular organisms too. Recent studies using in vivo reprogramming have provided important insights on regenerative medicine for diseased organs. Moreover, similar studies have revealed unappreciated mechanisms in various biological phenomena, including tissue regeneration, aging, rejuvenation and cancer development in multicellular organisms. Here, we review recent progress and future perspectives of in vivo reprogramming.

YY1 Expression Is Sufficient for the Maintenance of Cardiac Progenitor Cell State

During cardiac development, DNA binding transcription factors and epigenetic modifiers regulate gene expression in cardiac progenitor cells (CPCs). We have previously shown that Yin Yang 1 (YY1) is essential for the commitment of mesodermal precursors into CPCs. However, the role of YY1 in the maintenance of CPC phenotype and their differentiation into cardiomyocytes is unknown. In this study, we found, by genome-wide transcriptional profiling and phenotypic assays, that YY1 overexpression prevents cardiomyogenic differentiation and maintains the proliferative capacity of CPCs. We show further that the ability of YY1 to regulate CPC phenotype is associated with its ability to modulate histone modifications specifically at a developmentally critical enhancer of Nkx2-5 and other key cardiac transcription factor such as Tbx5. Specifically, YY1 overexpression helps to maintain markers of gene activation such as the acetylation of histone H3 at lysine 9 (H3K9Ac) and lysine 27 (H3K27Ac) as well as trimethylation at lysine 4 (H3K4Me3) at the Nkx2-5 cardiac enhancer. Furthermore, transcription factors associated proteins such as PoIII, p300, and Brg1 are also enriched at the Nkx2-5 enhancer with YY1 overexpression. The biological activities of YY1 in CPCs appear to be cell autonomous, based coculture assays in differentiating embryonic stem cells. Altogether, these results demonstrate that YY1 overexpression is sufficient to maintain a CPC phenotype through its ability to sustain the presence of activating epigenetic/chromatin marks at key cardiac enhancers. Stem Cells 2017;35:1913-1923.

A Novel Missense Mutation of GATA4 in a Chinese Family with Congenital Heart Disease

Background

Congenital heart disease (CHD) is the most prevalent type of birth defect in human, with high morbidity in infant. Several genes essential for heart development have been identified. GATA4 is a pivotal transcription factor that can regulate the cardiac development. Many GATA4 mutations have been identified in patients with different types of CHD.

Aims

In this study, the NKX2-5, HAND1 and GATA4 coding regions were sequenced in a family spanning three generations in which seven patients had CHD. Disease-causing potential variation in this family was evaluated by bioinformatics programs and the transcriptional activity of mutant protein was analyzed by the dual luciferase reporter assay.

Results

A novel GATA4 mutation, c.C931T (p.R311W), was identified and co-segregated with the affected patients in this family. The bioinformatics programs predicted this heterozygous mutation to be deleterious and the cross-species alignment of GATA4 sequences showed that the mutation occurred within a highly conserved amino acid. Even though it resided in the nuclear localization signal domain, the mutant protein didn’t alter its intracellular distribution. Nevertheless, further luciferase reporter assay demonstrated that the p.R311W mutation reduced the ability of GATA4 to activate its downstream target gene.

Conclusions

Our study identified a novel mutation in GATA4 that likely contributed to the CHD in this family. This finding expanded the spectrum of GATA4 mutations and underscored the pathogenic correlation between GATA4 mutations and CHD.

Ventricular cell fate can be specified until the onset of myocardial differentiation

The mechanisms that govern specification of various cell types that constitute vertebrate heart are not fully understood. Whilst most studies of heart development have utilised the mouse embryo, we have used an alternative model, embryos of the frog Xenopus laevis, which permits direct experimental manipulation of a non-essential heart. We show that in this model pluripotent animal cap explants injected with cardiogenic factor GATA4 mRNA express pan-myocardial as well as ventricular and proepicardial markers. We found that cardiac cell fate diversification, as assessed by ventricular and proepicardial markers, critically depends on tissue integrity, as it is disrupted by dissociation but can be fully restored by inhibition of the BMP pathway and partially by Dkk-1. Ventricular and proepicardial cell fates can also be restored in reaggregated GATA4-expressing cells upon transplantation into a host embryo. The competence of the host embryo to induce ventricular and proepicardial markers gradually decreases with the age of the transplant and is lost by the onset of myocardial differentiation at the late tailbud stage (st. 28). The influence of the host on the transplant was not limited to diversification of cardiac cell fates, but also included induction of growth and rhythmic beating, resulting in generation of a secondary heart-like structure. Our results additionally show that efficient generation of secondary heart requires normal axial patterning of the host embryo. Furthermore, secondary hearts can be induced in a wide range of locations within the host, arguing that the host embryo provides a permissive environment for development of cardiac patterning, growth and physiological maturation. Our results have implications for a major goal of cardiac regenerative medicine, differentiation of ventricular myocardium.

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Ventricular and proepicardial fate can be induced by cardiogenic factor GATA4.

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This process requires tissue integrity.

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Ventricular and proepicardial cell fate can be restored by BMP inhibition.

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A secondary heart-like structure can be induced from GATA4-expressing cells.

Direct cellular reprogramming for cardiac repair and regeneration

Heart failure is a major cause of morbidity and mortality, characterized by depletion of functioning cardiomyocytes, myocardial remodelling, and impaired contractile function. As the heart has a limited capacity for repair, and current treatments do not reverse myocardial attrition, novel regenerative strategies are imperative. Although cell delivery-based approaches remain promising, in situ reprogramming of endogenous cardiac fibroblasts (which are pathophysiologically implicated in cardiac remodelling) into functional cardiomyocytes may represent an advantageous approach. Several groups report successful in vitro and in vivo reprogramming of murine fibroblasts, using critical transcription factors, microRNA mimics, and small molecules, to cells demonstrating cardiomyocyte-like morphology, gene expression, and spontaneous contraction, which improve cardiac function in post-infarct models. Although proof-of-concept studies demonstrate reprogramming in human fibroblasts, significant barriers to therapeutic reprogramming remain. In this review, we evaluate the current status of reprogramming strategies for cardiac repair, and explore future perspectives within the context of clinical translation.

Macro advances in microRNAs and myocardial regeneration

PURPOSE OF REVIEW

Myocardial injury and disease often result in heart failure, a major cause of death worldwide. To achieve myocardial regeneration and foster development of efficient therapeutics for cardiac injury, it is essential to uncover molecular mechanisms that will promote myocardial regeneration. In this review, we examine the latest progress made in elucidation of the roles of small non-coding RNAs called microRNAs (miRs) in myocardial regeneration.

RECENT FINDINGS

Promising progress has been made in studying cardiac regeneration. Several miRs, which include miR-590, miR-199a, miR-17-92 cluster, miR-199a-214 cluster, miR-34a, and miR-15 family, have been recently shown to play an essential role in myocardial regeneration by regulating different processes during cardiac repair, including cell death, proliferation, and metabolism. For example, miR-590 promotes cardiac regeneration through activating cardiomyocyte proliferation, whereas miR-34a inhibits cardiac repair through inducing apoptosis.

SUMMARY

These recent findings shed new light on our understanding of myocardial regeneration and suggest potential novel therapeutic targets to treat cardiac disease.

Nkx2.5 is involved in myeloid cell differentiation at anterior ventral blood islands in the Xenopus embryo

We have shown previously that two populations of myeloid cells emerge in the anterior and posterior ventral blood islands (aVBI and pVBI) at the different stages in Xenopus laevis embryo. In order to elucidate the regulatory mechanism of myeloid cell differentiation in the aVBI, we examined the role of Nkx2.5, an essential transcription factor for heart differentiation, in regulation of the myeloid cell differentiation in this region. Knockdown of endogenous Nkx2.5 by introducing MO into the dorsal marginal zone (DMZ) suppressed the expression of MHCα as well as that of mpo and spib in the resultant embryos and in DMZ explants made from the injected embryos. Expression of c/ebpα was less affected in the embryos injected with Nkx2.5 MO. The effect of Nkx2.5 MO in myeloid cell differentiation was recovered by coinjection of nkx2.5 or c/ebpα mRNA, indicating that Nkx2.5 functions at the same or the upper level of C/EBPα for the specification of myeloid cells. An attempt to identify transcription factors for myeloid cell differentiation in ventral marginal zone (VMZ) explants demonstrated that coinjection of two transcription factors out of three factors, namely C/EBPα, Nkx2.5 and GATA4, was sufficient to induce a certain amount of mpo expression. We suggest that C/EBPα is an unequivocal factor for myeloid cell differentiation in the aVBI and that Nkx2.5 and GATA4 cooperate with C/EBPα for promotion of myeloid cell differentiation.

Carboxy terminus of GATA4 transcription factor is required for its cardiogenic activity and interaction with CDK4

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Carboxy terminal region of GATA4 is required for cardiogenesis in Xenopus pluripotent explants and in embryos.

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Carboxy terminus of GATA4 interacts with CDK4.

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CDK4 enhances transcriptional and cardiogenic activity of GATA4.

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GATA4-Tbx5 and GATA4-FOG2 interactions are not required for cardiogenesis.

GATA4-6 transcription factors regulate numerous aspects of development and homeostasis in multiple tissues of mesodermal and endodermal origin. In the heart, the best studied of these factors, GATA4, has multiple distinct roles in cardiac specification, differentiation, morphogenesis, hypertrophy and survival. To improve understanding of how GATA4 achieves its numerous roles in the heart, here we have focused on the carboxy-terminal domain and the residues required for interaction with cofactors FOG2 and Tbx5. We present evidence that the carboxy terminal region composed of amino acids 362–400 is essential for mediating cardiogenesis in Xenopus pluripotent explants and embryos. In contrast, the same region is not required for endoderm-inducing activity of GATA4. Further evidence is presented that the carboxy terminal cardiogenic region of GATA4 does not operate as a generic transcriptional activator. Potential mechanism of action of the carboxy terminal end of GATA4 is provided by the results showing physical and functional interaction with CDK4, including the enhancement of cardiogenic activity of GATA4 by CDK4. These results establish CDK4 as a GATA4 partner in cardiogenesis. The interactions of GATA4 with its other well described cofactors Tbx5 and FOG2 are known to be involved in heart morphogenesis, but their requirement for cardiac differentiation is unknown. We report that the mutations that disrupt interactions of GATA4 with Tbx5 and FOG2, G295S and V217G, respectively, do not impair cardiogenic activity of GATA4. These findings add support to the view that distinct roles of GATA4 in the heart are mediated by different determinants of the protein. Finally, we show that the rat GATA4 likely induces cardiogenesis cell autonomously or directly as it does not require activity of endodermal transcription factor Sox17, a GATA4 target gene that induces cardiogenesis non-cell autonomously.

Concise review: reprogramming strategies for cardiovascular regenerative medicine: from induced pluripotent stem cells to direct reprogramming

Myocardial cell-replacement therapies are emerging as novel therapeutic paradigms for myocardial repair but are hampered by the lack of sources of autologous human cardiomyocytes. The recent advances in stem cell biology and in transcription factor-based reprogramming strategies may provide exciting solutions to this problem. In the current review, we describe the different reprogramming strategies that can give rise to cardiomyocytes for regenerative medicine purposes. Initially, we describe induced pluripotent stem cell technology, a method by which adult somatic cells can be reprogrammed to yield pluripotent stem cells that could later be coaxed ex vivo to differentiate into cardiomyocytes. The generated induced pluripotent stem cell-derived cardiomyocytes could then be used for myocardial cell transplantation and tissue engineering strategies. We also describe the more recent direct reprogramming approaches that aim to directly convert the phenotype of one mature cell type (fibroblast) to another (cardiomyocyte) without going through a pluripotent intermediate cell type. The advantages and shortcomings of each strategy for cardiac regeneration are discussed, along with the hurdles that need to be overcome on the road to clinical translation.

Cyclin D2 is a GATA4 cofactor in cardiogenesis

The G1 cyclins play a pivotal role in regulation of cell differentiation and proliferation. The mechanisms underlying their cell-specific roles are incompletely understood. Here, we show that a G1 cyclin, cyclin D2 (CycD2), enhances the activity of transcription factor GATA4, a key regulator of cardiomyocyte growth and differentiation. GATA4 recruits CycD2 to its target promoters, and their interaction results in synergistic activation of GATA-dependent transcription. This effect is specific to CycD2 because CycD1 is unable to potentiate activity of GATA4 and is CDK-independent. GATA4 physically interacts with CycD2 through a discreet N-terminal activation domain that is essential for the cardiogenic activity of GATA4. Human mutations in this domain that are linked to congenital heart disease interfere with CycD2-GATA4 synergy. Cardiogenesis assays in Xenopus embryos indicate that CycD2 enhances the cardiogenic function of GATA4. Together, our data uncover a role for CycD2 as a cardiogenic coactivator of GATA4 and suggest a paradigm for cell-specific effects of cyclin Ds.

Induced regeneration--the progress and promise of direct reprogramming for heart repair

Regeneration of cardiac tissue has the potential to transform cardiovascular medicine. Recent advances in stem cell biology and direct reprogramming, or transdifferentiation, have produced powerful new tools to advance this goal. In this Review we examine key developments in the generation of new cardiomyocytes in vitro as well as the exciting progress that has been made toward in vivo reprogramming of cardiac tissue. We also address controversies and hurdles that challenge the field.

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.

Dissociation of cardiogenic and postnatal myocardial activities of GATA4

Transcription factor GATA4 is a critical regulator of the embryonic and postnatal heart, but the mechanisms and cofactors required for its diverse functions are not fully understood. Here, we show that whereas the N-terminal domain of GATA4 is required for inducing cardiogenesis and for promoting postnatal cardiomyocyte survival, distinct residues and domains therein are necessary to mediate these effects. Cardiogenic activity of GATA4 requires a 24-amino-acid (aa) region (aa 129 to 152) which is needed for transcriptional synergy and physical interaction with BAF60c. The same region is not essential for induction of endoderm or blood cell markers by GATA4, suggesting that it acts as a cell-type-specific transcriptional activation domain. On the other hand, a serine residue at position 105, which is a known target for mitogen-activated protein kinase (MAPK) phosphorylation, is necessary for GATA4-dependent cardiac myocyte survival and hypertrophy but is entirely dispensable for GATA4-induced cardiogenesis. We find that S105 is differentially required for transcriptional synergy between GATA4 and serum response factor (SRF) but not other cardiac cofactors such as TBX5 and NKX2.5. The findings provide new insight into GATA4 mechanisms of action and suggest that distinct regulatory pathways regulate activities of GATA4 in embryonic development and postnatal hearts.

Stem-cell-like embryonic explants to study cardiac development

Cells from the animal pole of Xenopus blastula embryo possess pluripotency that can be used to generate various tissues and even functional organs ex vivo. This finding has sparkled development of a variety of experimental protocols to study mechanisms that underlie formation of various organs and explore strategies for organ engineering for clinical applications. In this chapter, key methods are described for using Xenopus stem-cell-like embryonic explants as an assay system for studying organ development, with a focus on cardiogenesis. This assay allows investigation of cardiogenesis in isolation from neighboring tissues, minimizes interference with other developmental processes, and presents the further advantage of a heterologous system to study cardiogenesis in isolation of endogenous development of the heart. The cardiogenic assays can be exploited to investigate molecular mechanisms and cellular processes that underlie function of different molecules involved in cardiogenesis.

A decade of advances in the molecular embryology and genetics underlying congenital heart defects

Congenital heart defects (CHD) are the most common type of human birth defect and result in significant mortality worldwide. Despite numerous epidemiologic studies in the past decades, few genetic causes have been identified until recently. CHD result from abnormal morphogenesis of the systematic cardiovascular construction during development. Recent advances in molecular embryology, including the discovery of a new source of cardiac progenitor cells termed the second heart field (SHF), have revealed that the heart arises from multiple distinct embryonic origins. Cells derived from the SHF contribute to the development of the cardiac outflow tract, together with the other progenitor cell lineage called cardiac neural crest cells. Numerous cardiac transcription factors regulate these progenitor cells during heart development. Elucidation of the transcriptional network for these cardiac progenitor cells is essential for further understanding cardiac development and providing new insights into the morphogenesis of CHD. This review outlines the recent discoveries of the molecular embryology of the normal heart and the genetic basis of CHD.

Epigenetic factors and cardiac development

Congenital heart malformations remain the leading cause of death related to birth defects. Recent advances in developmental and regenerative cardiology have shed light on a mechanistic understanding of heart development that is controlled by a transcriptional network of genetic and epigenetic factors. This article reviews the roles of chromatin remodelling factors important for cardiac development with the current knowledge of cardiac morphogenesis, regeneration, and direct cardiac differentiation. In the last 5 years, critical roles of epigenetic factors have been revealed in the cardiac research field.

Geometric control of cardiomyogenic induction in human pluripotent stem cells

Although it has been observed that aggregate size affects cardiac development, an incomplete understanding of the cellular mechanisms underlying human pluripotent stem cell-derived cardiomyogenesis has limited the development of robust defined-condition cardiac cell generation protocols. Our objective was thus to elucidate cellular and molecular mechanisms underlying the endogenous control of human embryonic stem cell (hESC) cardiac tissue development, and to test the hypothesis that hESC aggregate size influences extraembryonic endoderm (ExE) commitment and cardiac inductive properties. hESC aggregates were generated with 100, 1000, or 4000 cells per aggregate using microwells. The frequency of endoderm marker (FoxA2 and GATA6)-expressing cells decreased with increasing aggregate size during early differentiation. Cardiogenesis was maximized in aggregates initiated from 1000 cells, with frequencies of 0.49±0.06 cells exhibiting a cardiac progenitor phenotype (KDR(low)/C-KIT(neg)) on day 5 and 0.24±0.06 expressing cardiac Troponin T on day 16. A direct relationship between ExE and cardiac differentiation efficiency was established by forming aggregates with varying ratios of SOX7 (a transcription factor required for ExE development) overexpressing or knockdown hESCs to unmanipulated hESCs. We demonstrate, in a defined, serum-free cardiac induction system, that robust and efficient cardiac differentiation is a function of endogenous ExE cell concentration, a parameter that can be directly modulated by controlling hESC aggregate size.

Myocardial lineage development

The myocardium of the heart is composed of multiple highly specialized myocardial lineages, including those of the ventricular and atrial myocardium, and the specialized conduction system. Specification and maturation of each of these lineages during heart development is a highly ordered, ongoing process involving multiple signaling pathways and their intersection with transcriptional regulatory networks. Here, we attempt to summarize and compare much of what we know about specification and maturation of myocardial lineages from studies in several different vertebrate model systems. To date, most research has focused on early specification, and although there is still more to learn about early specification, less is known about factors that promote subsequent maturation of myocardial lineages required to build the functioning adult heart.

Programming pluripotent precursor cells derived from Xenopus embryos to generate specific tissues and organs

Xenopus embryos provide a rich source of pluripotent cells that can be differentiated into functional organs. Since the molecular principles of vertebrate organogenesis appear to be conserved between Xenopus and mammals, this system can provide useful guidelines for the directional manipulation of human embryonic stem cells. Pluripotent Xenopus cells can be easily isolated from the animal pole of blastula stage Xenopus embryos. These so called "animal cap" cells represent prospective ectodermal cells, but give rise to endodermal, mesodermal and neuro-ectodermal derivatives if treated with the appropriate factors. These factors include evolutionary conserved modulators of the key developmental signal transduction pathways that can be supplied either by mRNA microinjection or direct application of recombinant proteins. This relatively simple system has added to our understanding of pancreas, liver, kidney, eye and heart development. In particular, recent studies have used animal cap cells to generate ectopic eyes and hearts, setting the stage for future work aimed at programming pluripotent cells for regenerative medicine.

In vitro organogenesis using multipotent cells

Abstract The establishment of efficient methods for promoting stem cell differentiation into target cells is important not only in regenerative medicine, but also in drug discovery. In addition to embryonic stem (ES) cells and various somatic stem cells, such as mesenchymal stem cells derived from bone marrow, adipose tissue, and umbilical cord blood, a novel dedifferentiation technology that allows the generation of induced pluripotent stem (iPS) cells has been recently developed. Although an increasing number of stem cell populations are being described, there remains a lack of protocols for driving the differentiation of these cells. Regeneration of organs from stem cells in vitro requires precise blueprints for each differentiation step. To date, studies using various model organisms, such as zebrafish, Xenopus laevis, and gene-targeted mice, have uncovered several factors that are critical for the development of organs. We have been using X. laevis, the African clawed frog, which has developmental patterns similar to those seen in humans. Moreover, Xenopus embryos are excellent research tools for the development of differentiation protocols, since they are available in high numbers and are sufficiently large and robust for culturing after simple microsurgery. In addition, Xenopus eggs are fertilized externally, and all stages of the embryo are easily accessible, making it relatively easy to study the functions of individual gene products during organogenesis using microinjection into embryonic cells. In the present review, we provide examples of methods for in vitro organ formation that use undifferentiated Xenopus cells. We also describe the application of amphibian differentiation protocols to mammalian stem cells, so as to facilitate the development of efficient methodologies for in vitro differentiation.

Xenopus explants as an experimental model system for studying heart development

Many developmental processes are highly conserved in all vertebrate organisms. This conservation has allowed developmental biologists to use numerous animal models to further our understanding of the molecular mechanisms that govern heart development and congenital heart disease. Amphibian embryos represent a useful model for such studies because their relatively large embryos are available in large numbers and survive simple microsurgery. In addition, until swimming tadpole stages, an amphibian embryo develops using nutrients stored in each of its many cells. This feature has the advantage that explants isolated from embryonic tissue will continue to survive in isolation and differentiate in culture. Furthermore, cells from the ectodermal layer of the blastula or gastrula embryos are stem cell like in that they are pluripotent and can be induced to form various tissues in vitro. Here, we will review work from recent studies in which explants from the amphibian embryos were used to further our understanding of vertebrate heart development. We will bring together the key facts needed for using Xenopus explants as experimental approaches for studying molecular pathways and gene regulatory networks in vertebrate cardiogenesis. The knowledge generated with these approaches supports the usefulness of amphibian explants, and the relevance of the findings strongly validates the conservation of molecular pathways that underlie heart development in all vertebrates.

Early activation of FGF and nodal pathways mediates cardiac specification independently of Wnt/beta-catenin signaling

BACKGROUND

Cardiac induction, the first step in heart development in vertebrate embryos, is thought to be initiated by anterior endoderm during gastrulation, but what the signals are and how they act is unknown. Several signaling pathways, including FGF, Nodal, BMP and Wnt have been implicated in cardiac specification, in both gain- and loss-of-function experiments. However, as these pathways regulate germ layer formation and patterning, their specific roles in cardiac induction have been difficult to define.

METHODOLOGY/PRINCIPAL FINDINGS

To investigate the mechanisms of cardiac induction directly we devised an assay based on conjugates of anterior endoderm from early gastrula stage Xenopus embryos as the inducing tissue and pluripotent ectodermal explants as the responding tissue. We show that the anterior endoderm produces a specific signal, as skeletal muscle is not induced. Cardiac inducing signal needs up to two hours of interaction with the responding tissue to produce an effect. While we found that the BMP pathway was not necessary, our results demonstrate that the FGF and Nodal pathways are essential for cardiogenesis. They were required only during the first hour of cardiogenesis, while sustained activation of ERK was required for at least four hours. Our results also show that transient early activation of the Wnt/beta-catenin pathway has no effect on cardiogenesis, while later activation of the pathway antagonizes cardiac differentiation.

CONCLUSIONS/SIGNIFICANCE

We have described an assay for investigating the mechanisms of cardiac induction by anterior endoderm. The assay was used to provide evidence for a direct, early and transient requirement of FGF and Nodal pathways. In addition, we demonstrate that Wnt/beta-catenin pathway plays no direct role in vertebrate cardiac specification, but needs to be suppressed just prior to differentiation.

Gata4 directs development of cardiac-inducing endoderm from ES cells

The transcription factor Gata4 is essential for normal heart morphogenesis and regulates the survival, growth, and proliferation of cardiomyocytes. We tested if Gata4 can specify cardiomyocyte fate from an uncommitted stem or progenitor cell population, by developing a system for conditional expression of Gata4 in embryonic stem cells. We find that in embryoid body cultures containing even a low ratio of these cells, expression of Gata4 is sufficient to enhance significantly the generation of cardiomyocytes, via a non-cell-autonomous mechanism. The Gata4-expressing cells do not generate cardiac or other mesoderm derivatives. Rather, Gata4 expression directs the development of two types of Sox17+ endoderm. This includes an epCam+Dpp4+ subtype of visceral endoderm. In addition, Gata4 generates similar amounts of epCam+Dpp4- definitive endoderm enriched for Cxcr4, FoxA2, FoxA3, Dlx5 and other characteristic transcripts. Both types of endoderm express cardiac-inducing factors, including WNT antagonists Dkk1 and Sfrp5, although the visceral endoderm subtype has much higher cardiac-inducing activity correlating with relatively enhanced levels of transcripts encoding BMPs. The Gata4-expressing cells eventually express differentiation markers showing commitment to liver development, even under conditions that normally support mesoderm development. The results suggest that Gata4 is capable of specifying endoderm fates that facilitate, with temporal and spatial specificity, the generation of cardiomyocyte progenitors from associated mesoderm.

Absence of heartbeat in the Xenopus tropicalis mutation muzak is caused by a nonsense mutation in cardiac myosin myh6

Mechanisms coupling heart function and cardiac morphogenesis can be accessed in lower vertebrate embryos that can survive to swimming tadpole stages on diffused oxygen. Forward genetic screens in Xenopus tropicalis have identified more than 80 mutations affecting diverse developmental processes, including cardiac morphogenesis and function. In the first positional cloning of a mutation in X. tropicalis, we show that non-contractile hearts in muzak (muz) embryos are caused by a premature stop codon in the cardiac myosin heavy chain gene myh6. The mutation deletes the coiled-coil domain responsible for polymerization into thick filaments, severely disrupting the cardiomyocyte cytoskeleton. Despite the lack of contractile activity and absence of a major structural protein, early stages of cardiac morphogenesis including looping and chamber formation are grossly normal. Muz hearts subsequently develop dilated chambers with compressed endocardium and fail to form identifiable cardiac valves and trabeculae.

Identification of embryonic pancreatic genes using Xenopus DNA microarrays

The pancreas is both an exocrine and endocrine endodermal organ involved in digestion and glucose homeostasis. During embryogenesis, the anlagen of the pancreas arise from dorsal and ventral evaginations of the foregut that later fuse to form a single organ. To better understand the molecular genetics of early pancreas development, we sought to isolate markers that are uniquely expressed in this tissue. Microarray analysis was performed comparing dissected pancreatic buds, liver buds, and the stomach region of tadpole stage Xenopus embryos. A total of 912 genes were found to be differentially expressed between these organs during early stages of organogenesis. K-means clustering analysis predicted 120 of these genes to be specifically enriched in the pancreas. Of these, we report on the novel expression patterns of 24 genes. Our analyses implicate the involvement of previously unsuspected signaling pathways during early pancreas development. Developmental Dynamics 238:1455-1466, 2009. (c) 2009 Wiley-Liss, Inc.

Directed transdifferentiation of mouse mesoderm to heart tissue by defined factors

Heart disease is the leading cause of mortality and morbidity in the western world. The heart has little regenerative capacity after damage, leading to much interest in understanding the factors required to produce new cardiac myocytes. Despite a robust understanding of the molecular networks regulating cardiac differentiation, no single transcription factor or combination of factors has been shown to activate the cardiac gene program de novo in mammalian cells or tissues. Here we define the minimal requirements for transdifferentiation of mouse mesoderm to cardiac myocytes. We show that two cardiac transcription factors, Gata4 and Tbx5, and a cardiac-specific subunit of BAF chromatin-remodelling complexes, Baf60c (also called Smarcd3), can direct ectopic differentiation of mouse mesoderm into beating cardiomyocytes, including the normally non-cardiogenic posterior mesoderm and the extraembryonic mesoderm of the amnion. Gata4 with Baf60c initiated ectopic cardiac gene expression. Addition of Tbx5 allowed differentiation into contracting cardiomyocytes and repression of non-cardiac mesodermal genes. Baf60c was essential for the ectopic cardiogenic activity of Gata4 and Tbx5, partly by permitting binding of Gata4 to cardiac genes, indicating a novel instructive role for BAF complexes in tissue-specific regulation. The combined function of these factors establishes a robust mechanism for controlling cellular differentiation, and may allow reprogramming of new cardiomyocytes for regenerative purposes.

Induction and modulation of smooth muscle differentiation in Xenopus embryonic cells

By comparison with skeletal or cardiac developmental programs, little is known regarding the specific factors that promote specification and differentiation of smooth muscle cells from pluripotent cells. We have analyzed the developmental expression of a subset of smooth muscle genes during Xenopus early development and showed that similar to mammals and avians, Xenopus smooth muscle myosin heavy chain (SM-MHC) is a highly specific marker of smooth muscle differentiation. Embryonic cells from animal pole explants of Xenopus blastula can be induced by basic fibroblast growth factor, Wnt, and bone morphogenetic protein signals to adopt the smooth muscle pathway. Explants from early embryos that contain neural crest cells can also differentiate into cells expressing smooth muscle genes. We examined the interplay of several transcription factors, that is SRF, myocardin, and GATA6, that induce the expression of SM-MHC in animal cap cells and found that myocardin-dependent expression of smooth muscle genes in animal cap cells is synergized by SRF but is strongly antagonized by GATA6.

GATA transcription factors integrate Wnt signalling during heart development

Cardiogenesis is inhibited by canonical Wnt/beta-catenin signalling and stimulated by non-canonical Wnt11/JNK signalling, but how these two signalling pathways crosstalk is currently unknown. Here, we show that Wnt/beta-catenin signalling restricts cardiogenesis via inhibition of GATA gene expression, as experimentally reinstating GATA function overrides beta-catenin-mediated inhibition and restores cardiogenesis. Furthermore, we show that GATA transcription factors in turn directly regulate Wnt11 gene expression, and that Wnt11 is required to a significant degree for mediating the cardiogenesis-promoting function of GATA transcription factors. These results demonstrate that GATA factors occupy a central position between canonical and non-canonical Wnt signalling in regulating heart muscle formation.

GATA4 and GATA5 are essential for heart and liver development in Xenopus embryos

BACKGROUND

GATA factors 4/5/6 have been implicated in the development of the heart and endodermal derivatives in vertebrates. Work in zebrafish has indicated that GATA5 is required for normal development earlier than GATA4/6. However, the GATA5 knockout mouse has no apparent embryonic phenotype, thereby questioning the importance of the gene for vertebrate development.

RESULTS

In this study we show that in Xenopus embryos GATA5 is essential for early development of heart and liver precursors. In addition, we have found that in Xenopus embryos GATA4 is important for development of heart and liver primordia following their specification, and that in this role it might interact with GATA6.

CONCLUSION

Our results suggest that GATA5 acts earlier than GATA4 to regulate development of heart and liver precursors, and indicate that one early direct target of GATA5 is homeobox gene Hex.

Gata5 and Gata6 are functionally redundant in zebrafish for specification of cardiomyocytes

An outstanding problem in vertebrate development has been to define the genetic program that specifies the cardiomyocyte lineage. It has been a challenge to define the transcription factors that control specification, since candidate gene knockouts typically cause rather complex morphogenetic defects. In contrast, Drosophila genetics identified single transcription factors that are essential for specification of cardiomyocytes from uncommitted mesoderm. For those vertebrate orthologs, it has been considered that paralogous family members might compensate for the loss-of-function of individual genes. However, this hypothesis had not been formally tested. In zebrafish, defects in gata5 can lead to a loss of myocardial tissue, but most embryos depleted for any single vertebrate Gata4/5/6 transcription factor develop a cardiac morphogenetic defect, and cardiomyocytes are specified and differentiate. Here we show that in zebrafish the gata5 and gata6 genes are redundant for specification of cardiomyocytes. Embryos depleted of these two gene products are heartless. Restoring either gene product is sufficient to rescue cardiomyocyte specification. In contrast, embryos depleted of Gata4 and Gata6, or Gata4 and Gata5, develop defective heart tubes. Our study identifies a specific pair of vertebrate transcription factor paralogs that is essential for cardiomyocyte specification.

Redundancy and evolution of GATA factor requirements in development of the myocardium

The transcription factors, GATA4, 5 and 6, recognize the same DNA sequence and are all expressed in the developing myocardium. However, knockout studies in the mouse have indicated that none of them are absolutely required for the specification of the myocardium. Here we present evidence for redundancy in this family for the first time. Using morpholinos in both Xenopus and zebrafish embryos, we show that GATA4 knockdown, for example, only affects cardiac marker expression in the absence of either GATA5 or GATA6. A similar situation pertains for GATA5 in Xenopus whereas, in zebrafish, GATA5 (faust) plays a major role in driving the myocardial programme. This requirement for GATA5 in zebrafish is for induction of the myocardium, in contrast to the GATA6 requirement in both species, which is for differentiation. This early role for GATA5 in zebrafish correlates with its earlier expression and with an earlier requirement for BMP signalling, suggesting that a mutual maintenance loop for GATA, BMP and Nkx expression is the evolutionarily conserved entity.

Genetic insights into normal and abnormal heart development

Congenital heart defects (CHDs) affect 1-2% of newborn children and are the leading cause of death in infants under 1 year of age. CHDs represent the single largest class of birth defects and account for 25% of all human congenital abnormalities. Numerous epidemiologic studies have established the heritable nature of CHDs. However, despite the remarkable progress of the past decade, very few CHD-causing genes have been identified so far. Molecular and genetic analysis of heart development--which requires the execution of specific genetic programs--has led to the identification of essential cardiac regulators and mutations that are linked to human CHD. Elucidation of the mechanisms of action of these transcription factors has also provided a molecular framework that will continue to help furthering our understanding of the molecular basis of normal and abnormal heart growth. This review will summarize present knowledge of cardiac development and illustrate how analysis of heart development has helped understand the genetic basis of some CHDs and how these advances could translate into better prevention, diagnosis, and care of congenital heart disease.

Embryonic heart induction

We have characterized two signaling pathways that induce heart tissue during embryonic development. The first is initiated by the Wnt antagonist Dickkopf1 (Dkk1) and involves the homeodomain transcription factor Hex. Other Wnt antagonists are less effective and the potency of Dkk1 might be due to synergy between Wnt antagonizing and another, novel activity emanating from its amino terminal cysteine-rich domain. The second signal is initiated by Nodal and its co-receptor Cripto. Importantly, both the Dkk1/Wnt antagonism and Nodal pathways act on the endoderm that underlies the future heart to control secretion of diffusible factors that induce cardiogenesis in adjacent mesoderm. In this article, we summarize data that Dkk1 induces cardiogenic differentiation cell non-autonomously through the action of the homeodomain transcription factor Hex. We also discuss recent data showing that Nodal also acts indirectly through stimulation of the secreted protein Cerberus, which is a member of the differential-screening selected aberrant in neuroblastoma (DAN) family of secreted proteins. Finally, we present the model that signaling from Dkk1 regulates novel activities, in addition to Wnt antagonism, which are essential for progression beyond initiation of cardiogenesis to control later stages of cardiomyocyte differentiation and myocardial tissue organization.

Xenopus as a model system for vertebrate heart development

The African clawed frog, Xenopus laevis, is a valuable model system for studies of vertebrate heart development. In the following review, we describe a range of embryological and molecular methodologies that are used in Xenopus research and discuss key discoveries relating to heart development that have been made using this model system. We also discuss how the sequence of the Xenopus tropicalis genome provides a valuable tool for identification of orthologous genes and for identification of evolutionarily conserved promoter elements. Finally, both forward and reverse genetic approaches are currently being applied to Xenopus for the study of vertebrate heart development.

The Kruppel-like transcription factor KLF13 is a novel regulator of heart development

In humans, congenital heart defects occur in 1-2% of live birth, but the molecular mechanisms and causative genes remain unidentified in the majority of cases. We have uncovered a novel transcription pathway important for heart morphogenesis. We report that KLF13, a member of the Krüppel-like family of zinc-finger proteins, is expressed predominantly in the heart, binds evolutionarily conserved regulatory elements on cardiac promoters and activates cardiac transcription. KLF13 is conserved across species and knockdown of KLF13 in Xenopus embryos leads to atrial septal defects and hypotrabeculation similar to those observed in humans or mice with hypomorphic GATA-4 alleles. Physical and functional interaction with GATA-4, a dosage-sensitive cardiac regulator, provides a mechanistic explanation for KLF13 action in the heart. The data demonstrate that KLF13 is an important component of the transcription network required for heart development and suggest that KLF13 is a GATA-4 modifier; by analogy to other GATA-4 collaborators, mutations in KLF13 may be causative for congenital human heart disease.

A role for GATA factors in Xenopus gastrulation movements

Gastrulation movements in Xenopus laevis are becoming increasingly well characterised, however the molecular mechanisms involved are less clear. Active migration of the leading edge mesendoderm across the fibronectin-coated blastocoel roof is necessary for further development of tissues such as head mesoderm, heart, blood and liver. The zinc finger transcription factors GATA4 and GATA6 are expressed in this migratory tissue during gastrulation, but their role here is unknown. This study further characterises the expression of GATA4 and 6 during gastrulation, and investigates their function in migratory behaviour. Gain-of-function experiments with these GATA factors induce cell spreading, polarisation and migration in non-motile presumptive ectoderm cells. Expression of a dominant-interfering form of GATA6, which inhibits transactivation of GATA targets, severely impairs the ability of dorsal leading edge mesendoderm to spread and translocate on fibronectin. Mosaic inhibition of GATA activity indicates that GATA factors function cell autonomously to induce cell spreading and movement in dorsal mesendoderm. Knockdown of specific GATA factors using anti-sense morpholinos indicates that GATA4 and GATA6 both contribute to dorsal mesendoderm migration in vitro. GATA4 and GATA6 are known to be involved in cell-specification of mesoderm and endoderm-derived tissues, but this is the first description of an additional role for these factors in cell migration.

Gene regulatory networks in the evolution and development of the heart

The heart, an ancient organ and the first to form and function during embryogenesis, evolved by the addition of new structures and functions to a primitive pump. Heart development is controlled by an evolutionarily conserved network of transcription factors that connect signaling pathways with genes for muscle growth, patterning, and contractility. During evolution, this ancestral gene network was expanded through gene duplication and co-option of additional networks. Mutations in components of the cardiac gene network cause congenital heart disease, the most common human birth defect. The consequences of such mutations reveal the logic of organogenesis and the evolutionary origins of morphological complexity.