Modular regulation of cGATA-5 gene expression in the developing heart and gut

Understanding Epicardial Cell Heterogeneity during Cardiogenesis and Heart Regeneration

The outermost layer of the heart, the epicardium, is an essential cell population that contributes, through epithelial-to-mesenchymal transition (EMT), to the formation of different cell types and provides paracrine signals to the developing heart. Despite its quiescent state during adulthood, the adult epicardium reactivates and recapitulates many aspects of embryonic cardiogenesis in response to cardiac injury, thereby supporting cardiac tissue remodeling. Thus, the epicardium has been considered a crucial source of cell progenitors that offers an important contribution to cardiac development and injured hearts. Although several studies have provided evidence regarding cell fate determination in the epicardium, to date, it is unclear whether epicardium-derived cells (EPDCs) come from specific, and predetermined, epicardial cell subpopulations or if they are derived from a common progenitor. In recent years, different approaches have been used to study cell heterogeneity within the epicardial layer using different experimental models. However, the data generated are still insufficient with respect to revealing the complexity of this epithelial layer. In this review, we summarize the previous works documenting the cellular composition, molecular signatures, and diversity within the developing and adult epicardium.

Single-cell and spatial heterogeneity landscapes of mature epicardial cells

Tbx18, Wt1, and Tcf21 have been identified as epicardial markers during the early embryonic stage. However, the gene markers of mature epicardial cells remain unclear. Single-cell transcriptomic analysis was performed with the Seurat, Monocle, and CellphoneDB packages in R software with standard procedures. Spatial transcriptomics was performed on chilled Visium Tissue Optimization Slides (10x Genomics) and Visium Spatial Gene Expression Slides (10x Genomics). Spatial transcriptomics analysis was performed with Space Ranger software and R software. Immunofluorescence, whole-mount RNA in situ hybridization and X-gal staining were performed to validate the analysis results. Spatial transcriptomics analysis revealed distinct transcriptional profiles and functions between epicardial tissue and non-epicardial tissue. Several gene markers specific to postnatal epicardial tissue were identified, including Msln, C3, Efemp1, and Upk3b. Single-cell transcriptomic analysis revealed that cardiac cells from wildtype mouse hearts (from embryonic day 9.5 to postnatal day 9) could be categorized into six major cell types, which included epicardial cells. Throughout epicardial development, Wt1, Tbx18, and Upk3b were consistently expressed, whereas genes including Msln, C3, and Efemp1 exhibited increased expression during the mature stages of development. Pseudotime analysis further revealed two epicardial cell fates during maturation. Moreover, Upk3b, Msln, Efemp1, and C3 positive epicardial cells were enriched in extracellular matrix signaling. Our results suggested Upk3b, Efemp1, Msln, C3, and other genes were mature epicardium markers. Extracellular matrix signaling was found to play a critical role in the mature epicardium, thus suggesting potential therapeutic targets for heart regeneration in future clinical practice.

A Mesp1-dependent developmental breakpoint in transcriptional and epigenomic specification of early cardiac precursors

Transcriptional networks governing cardiac precursor cell (CPC) specification are incompletely understood owing, in part, to limitations in distinguishing CPCs from non-cardiac mesoderm in early gastrulation. We leveraged detection of early cardiac lineage transgenes within a granular single-cell transcriptomic time course of mouse embryos to identify emerging CPCs and describe their transcriptional profiles. Mesp1, a transiently expressed mesodermal transcription factor, is canonically described as an early regulator of cardiac specification. However, we observed perdurance of CPC transgene-expressing cells in Mesp1 mutants, albeit mislocalized, prompting us to investigate the scope of the role of Mesp1 in CPC emergence and differentiation. Mesp1 mutant CPCs failed to robustly activate markers of cardiomyocyte maturity and crucial cardiac transcription factors, yet they exhibited transcriptional profiles resembling cardiac mesoderm progressing towards cardiomyocyte fates. Single-cell chromatin accessibility analysis defined a Mesp1-dependent developmental breakpoint in cardiac lineage progression at a shift from mesendoderm transcriptional networks to those necessary for cardiac patterning and morphogenesis. These results reveal Mesp1-independent aspects of early CPC specification and underscore a Mesp1-dependent regulatory landscape required for progression through cardiogenesis.

Epicardial Contribution to the Developing and Injured Heart: Exploring the Cellular Composition of the Epicardium

The epicardium is an essential cell population during cardiac development. It contributes different cell types to the developing heart through epithelial-to-mesenchymal transition (EMT) and it secretes paracrine factors that support cardiac tissue formation. In the adult heart the epicardium is a quiescent layer of cells which can be reactivated upon ischemic injury, initiating an embryonic-like response in the epicardium that contributes to post-injury repair processes. Therefore, the epicardial layer is considered an interesting target population to stimulate endogenous repair mechanisms. To date it is still not clear whether there are distinct cell populations in the epicardium that contribute to specific lineages or aid in cardiac repair, or that the epicardium functions as a whole. To address this putative heterogeneity, novel techniques such as single cell RNA sequencing (scRNA seq) are being applied. In this review, we summarize the role of the epicardium during development and after injury and provide an overview of the most recent insights into the cellular composition and diversity of the epicardium.

Heart Enhancers: Development and Disease Control at a Distance

Bound by lineage-determining transcription factors and signaling effectors, enhancers play essential roles in controlling spatiotemporal gene expression profiles during development, homeostasis and disease. Recent synergistic advances in functional genomic technologies, combined with the developmental biology toolbox, have resulted in unprecedented genome-wide annotation of heart enhancers and their target genes. Starting with early studies of vertebrate heart enhancers and ending with state-of-the-art genome-wide enhancer discovery and testing, we will review how studying heart enhancers in metazoan species has helped inform our understanding of cardiac development and disease.

Disparate expression specificities coded by a shared Hox-C enhancer

Can a single regulatory sequence be shared by two genes undergoing functional divergence? Here we describe a single promiscuous enhancer within the Drosophila Antennapedia Complex, EO053, that directs aspects of the expression of two adjacent genes, pb (a Hox2 ortholog) and zen2 (a divergent Hox3 paralog), with disparate spatial and temporal expression patterns. We were unable to separate the pb-like and zen2-like specificities within EO053, and we identify sequences affecting both expression patterns. Importantly, genomic deletion experiments demonstrate that EO053 cooperates with additional pb- and zen2-specific enhancers to regulate the mRNA expression of both genes. We examine sequence conservation of EO053 within the Schizophora, and show that patterns of synteny between the Hox2 and Hox3 orthologs in Arthropods are consistent with a shared regulatory relationship extending prior to the Hox3/zen divergence. Thus, EO053 represents an example of two genes having evolved disparate outputs while utilizing this shared regulatory region.

Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Epicardial TGFβ and BMP Signaling in Cardiac Regeneration: What Lesson Can We Learn from the Developing Heart?

The epicardium, the outer layer of the heart, has been of interest in cardiac research due to its vital role in the developing and diseased heart. During development, epicardial cells are active and supply cells and paracrine cues to the myocardium. In the injured adult heart, the epicardium is re-activated and recapitulates embryonic behavior that is essential for a proper repair response. Two indispensable processes for epicardial contribution to heart tissue formation are epithelial to mesenchymal transition (EMT), and tissue invasion. One of the key groups of cytokines regulating both EMT and invasion is the transforming growth factor β (TGFβ) family, including TGFβ and Bone Morphogenetic Protein (BMP). Abundant research has been performed to understand the role of TGFβ family signaling in the developing epicardium. However, less is known about signaling in the adult epicardium. This review provides an overview of the current knowledge on the role of TGFβ in epicardial behavior both in the development and in the repair of the heart. We aim to describe the presence of involved ligands and receptors to establish if and when signaling can occur. Finally, we discuss potential targets to improve the epicardial contribution to cardiac repair as a starting point for future investigation.

Molecular characterization and expression profiles provide new insights into GATA5 functions in tongue sole (Cynoglossus semilaevis)

GATA5 is a member of the GATA transcription factor family, which serves essential roles in varieties of cellular functions and biological processes. In this study, we have accomplished the molecular cloning, bioinformatic analysis and preliminary function study of C. semilaevis GATA5. The full-length cDNA nucleotide sequence is 1955 bp, with a coding sequence of 1167 bp, which encodes a polypeptide of 388 amino acids. Homology, phylogenetic, gene structure and synteny analysis showed that C. semilaevis GATA5 was highly conserved among vertebrates. Tissue distribution pattern exhibited that C. semilaevis GATA5 was significantly expressed in heart, intestine, liver, kidney and gonad, with a sexual dimorphic feature observed in testis and ovary. Embryonic development expression profiles showed that C. semilaevis GATA5 transcripts increased at the blastula stage, and peaked at the heat-beating period. Strong signals were detected at spermatids of male testis and stage III oocytes of female ovary by ISH. The expression of C. semilaevis GATA5 was regulated by 17α-MT and E2 after hormone stimulation to the ovary. Together, all the results pointed out that GATA5 might play a vital role during gonadal maturation and the reproductive cycle of C. semilaevis. This study lays the foundation for further researches on the sex control breeding in tongue sole.

Heterogeneity of Adult Cardiac Stem Cells

Cardiac biology and heart regeneration have been intensively investigated and debated in the last 15 years. Nowadays, the well-established and old dogma that the adult heart lacks of any myocyte-regenerative capacity has been firmly overturned by the evidence of cardiomyocyte renewal throughout the mammalian life as part of normal organ cell homeostasis, which is increased in response to injury. Concurrently, reproducible evidences from independent laboratories have convincingly shown that the adult heart possesses a pool of multipotent cardiac stem/progenitor cells (CSCs or CPCs) capable of sustaining cardiomyocyte and vascular tissue refreshment after injury. CSC transplantation in animal models displays an effective regenerative potential and may be helpful to treat chronic heart failure (CHF), obviating at the poor/modest results using non-cardiac cells in clinical trials. Nevertheless, the degree/significance of cardiomyocyte turnover in the adult heart, which is insufficient to regenerate extensive damage from ischemic and non-ischemic origin, remains strongly disputed. Concurrently, different methodologies used to detect CSCs in situ have created the paradox of the adult heart harboring more than seven different cardiac progenitor populations. The latter was likely secondary to the intrinsic heterogeneity of any regenerative cell agent in an adult tissue but also to the confusion created by the heterogeneity of the cell population identified by a single cell marker used to detect the CSCs in situ. On the other hand, some recent studies using genetic fate mapping strategies claimed that CSCs are an irrelevant endogenous source of new cardiomyocytes in the adult. On the basis of these contradictory findings, here we critically reviewed the available data on adult CSC biology and their role in myocardial cell homeostasis and repair.

The epicardium as a source of multipotent adult cardiac progenitor cells: Their origin, role and fate

Since the regenerative capacity of the adult mammalian heart is limited, cardiac injury leads to the formation of scar tissue and thereby increases the risk of developing compensatory heart failure. Stem cell therapy is a promising therapeutic approach but is facing problems with engraftment and clinical feasibility. Targeting an endogenous stem cell population could circumvent these limitations. The epicardium, a membranous layer covering the outside of the myocardium, is an accessible cell population which plays a key role in the developing heart. Epicardial cells undergo epithelial to mesenchymal transition (EMT), thus providing epicardial derived cells (EPDCs) that migrate into the myocardium and cooperate in myocardial vascularisation and compaction. In the adult heart, injury activates the epicardium, and an embryonic-like response is observed which includes EMT and differentiation of the EPDCs into cardiac cell types. Furthermore, paracrine communication between the epicardium and myocardium improves the regenerative response. The significant role of the epicardium has been shown in both the developing and the regenerating heart. Interestingly, the epicardial contribution to cardiac repair can be improved in several ways. In this review, an overview of the epicardial origin and fate will be given and potential therapeutic approaches will be discussed.

Generation of the epicardial lineage from human pluripotent stem cells

The epicardium supports cardiomyocyte proliferation early in development and provides fibroblasts and vascular smooth muscle cells to the developing heart. The epicardium has been shown to play an important role during tissue remodeling after cardiac injury, making access to this cell lineage necessary for the study of regenerative medicine. Here we describe the generation of epicardial lineage cells from human pluripotent stem cells by stage-specific activation of the BMP and WNT signaling pathways. These cells display morphological characteristics and express markers of the epicardial lineage, including the transcription factors WT1 and TBX18 and the retinoic acid–producing enzyme ALDH1A2. When induced to undergo epicardial-tomesenchymal transition, the cells give rise to populations that display characteristics of the fibroblast and vascular smooth muscle lineages. These findings identify BMP and WNT as key regulators of the epicardial lineage in vitro and provide a model for investigating epicardial function in human development and disease.

Circuitry and dynamics of human transcription factor regulatory networks

The combinatorial cross-regulation of hundreds of sequence-specific transcription factors (TFs) defines a regulatory network that underlies cellular identity and function. Here we use genome-wide maps of in vivo DNaseI footprints to assemble an extensive core human regulatory network comprising connections among 475 sequence-specific TFs and to analyze the dynamics of these connections across 41 diverse cell and tissue types. We find that human TF networks are highly cell selective and are driven by cohorts of factors that include regulators with previously unrecognized roles in control of cellular identity. Moreover, we identify many widely expressed factors that impact transcriptional regulatory networks in a cell-selective manner. Strikingly, in spite of their inherent diversity, all cell-type regulatory networks independently converge on a common architecture that closely resembles the topology of living neuronal networks. Together, our results provide an extensive description of the circuitry, dynamics, and organizing principles of the human TF regulatory network.

An in vivo map of bone morphogenetic protein 2 post-transcriptional repression in the heart

The Bmp2 3'untranslated region (UTR) sequence bears a sequence conserved between mammals and fishes that can post-transcriptionally activate or repress protein synthesis. We developed a map of embryonic cells in the mouse where this potent Bmp2 regulatory sequence functions by using a lacZ reporter transgene with a 3'UTR bearing two loxP sites flanking the ultra-conserved sequence. Cre-recombinase-mediated deletion of the ultra-conserved sequence caused strong ectopic expression in proepicardium, epicardium and epicardium-derived cells (EPDC) and in tissues with known epicardial contributions (coronary vessels and valves). Transient transfections of reporters in the epicardial/mesothelial cell (EMC) line confirmed this repression. Ectopic expression of the recombined transgene also occurred in the aorta, outlet septum, posterior cardiac plexus, cardiac and extracardiac nerves and neural ganglia. Bmp2 is dynamically regulated in the developing heart. 3'UTR-mediated mechanisms that restrain BMP2 synthesis may be relevant to congenital heart and vasculature malformations and to adult diseases involving aberrant BMP2 synthesis.

Comparative gene expression analysis and fate mapping studies suggest an early segregation of cardiogenic lineages in Xenopus laevis

Retrospective clonal analysis in mice suggested that the vertebrate heart develops from two sources of cells called first and second lineages, respectively. Cells of the first lineage enter the linear heart tube and initiate terminal differentiation earlier than cells of the second lineage. It is thought that both heart lineages arise from a common progenitor cell population prior to the cardiac crescent stage (E7.5 of mouse development). The timing of segregation of different lineages as well as the molecular mechanisms underlying this process is not yet known. Furthermore, gene expression data for those lineages are very limited. Here we provide the first comparative study of cardiac marker gene expression during Xenopus laevis embryogenesis complemented by single cell RT-PCR analysis. In addition we provide fate mapping data of cardiac progenitor cells at different stages of development. Our analysis indicates an early segregation of cardiac lineages and a fairly complex heterogeneity of gene expression in the cardiac progenitor cells. Furthermore, this study sets a reference for all further studies analyzing cardiac development in X. laevis.

Wnt2 is a direct downstream target of GATA6 during early cardiogenesis

The GATA4, 5 and 6 subfamily of transcription factors are potent transactivators of transcription expressed within the precardiac mesoderm. However, little is known of the immediate downstream targets of GATA-factor regulation during the earliest stages of cardiogenesis. Using the P19-CL6 embryonal carcinoma (EC) cell line as an in vitro model of cardiogenesis, we show that GATA6 is the most abundantly expressed of the GATA factors in presumptive cardiac cells. Consequently, we performed a microarray screen comparing mRNA from control EC cells, early in the cardiac differentiation pathway, with those in which GATA6 had been overexpressed. These studies identified 103 genes whose expression changed significantly and this was verified in a representative array of these genes by real-time RT-PCR. We show that early cardiac expression of one of these genes, Wnt2, mirrors that of GATA6 in vitro and in vivo. In addition, its upregulation by GATA6 in differentiating EC cells is mediated by the direct binding of GATA-factor(s) to the cognate Wnt2 promoter, suggesting Wnt2 is an immediate downstream target of GATA-factor regulation during early cardiogenesis.

Epicardial retinoid X receptor alpha is required for myocardial growth and coronary artery formation

Vitamin A signals play critical roles during embryonic development. In particular, heart morphogenesis depends on vitamin A signals mediated by the retinoid X receptor alpha (RXRalpha), as the systemic mutation of this receptor results in thinning of the myocardium and embryonic lethality. However, the molecular and cellular mechanisms controlled by RXRalpha signaling in this process are unclear, because a myocardium-restricted RXRalpha mutation does not perturb heart morphogenesis. Here, we analyze a series of tissue-restricted mutations of the RXRalpha gene in the cardiac neural crest, endothelial, and epicardial lineages, and we show that RXRalpha signaling in the epicardium is required for proper cardiac morphogenesis. Moreover, we detect an additional phenotype of defective coronary arteriogenesis associated with RXRalpha deficiency and identify a retinoid-dependent Wnt signaling pathway that cooperates in epicardial epithelial-to-mesenchymal transformation.

Regulation of GATA gene expression during vertebrate development

GATA factors regulate critical events in hematopoietic lineages (GATA-1/2/3), the heart and gut (GATA-4/5/6) and various other tissues. Transgenic approaches have revealed that GATA genes are regulated in a modular fashion by sets of enhancers that govern distinct temporal and/or spatial facets of the overall expression patterns. Efforts are underway to resolve how these GATA gene enhancers are themselves regulated in order to elucidate the genetic and molecular hierarchies that govern GATA expression in particular developmental contexts. These enhancers also afford a raft of tools that can be used to selectively perturb and probe various developmental events in transgenic animals.

Cell history determines the maintenance of transcriptional differences between left and right ventricular cardiomyocytes in the developing mouse heart

The molecular mechanisms that establish and maintain transcriptional differences between cardiomyocytes in the left and right ventricular chambers are unkown. We have previously analysed a myosin light chain 3f transgene containing an nlacZ reporter gene, which is transcribed in left but not right ventricular cardiomyocytes. In this report we examine the mechanisms involved in maintaining regionalised transgene expression. Primary cardiomyocytes prepared from left and right ventricular walls of transgenic mice were found to maintain transgene expression status in culture. However, similar cultures prepared from nontransgenic mice or rats show uniform expression after transient transfection of Mlc3f constructs, suggesting that the mechanism responsible for differential expression of the transgene between left and right ventricular cells does not operate on transiently introduced molecules. These data suggest that developmental cell history determines transgene expression status. Maintenance of transgene expression status is regulated by a cell-autonomous mechanism that is independent of DNA methylation, trichostatin A-sensitive histone deacetylation and miss-expression of transcription factors that are expressed in the left or right ventricles of the embryonic heart. Parallels between Mlc3f transgene repression in right ventricular cardiomyocytes and polycomb-mediated silencing in Drosophila suggest that Mlc3f regulatory sequences included on the transgene may contain a cellular memory module that is switched into an on or off state during early cardiogenesis. Epigenetic mechanisms may therefore be involved in maintaining patterning of the mammalian myocardium.

T-box binding sites are required for activity of a cardiac GATA-4 enhancer

GATA-4 is a key regulator of a poorly understood cardiac morphogenetic program. We used genomic regions of the GATA-4 gene to target GFP expression to the developing heart of living fish. In these fish, GFP-expressing cells in the lateral plate mesoderm form two tubes that migrate ventrally to fuse into a linear heart tube. In addition, we find that a 14.8-kb fragment upstream of the transcription initiation site targets expression to both chambers and the valves of the heart. Truncation of 7 kb of the distal sequences eliminates expression in the atrium and the atrioventricular valve while expression is retained in the ventricle and the bulboventricular valve. Within this 7-kb distal regulatory region, we delineated a 1300-bp region with a cluster of consensus binding sites for T-box transcription factors. Mutation of these sequences significantly reduces reporter gene expression in the heart. This provides the first evidence that T-box factors function by directly regulating GATA-4 expression. Thus, GATA-4 regulatory elements control gene expression differentially along the rostro-caudal axis, and T-box binding elements in the GATA-4 promoter contribute to heart-specific expression.

Cooperative interaction between GATA5 and NF-ATc regulates endothelial-endocardial differentiation of cardiogenic cells

In vertebrates, heart development is a complex process requiring proper differentiation and interaction between myocardial and endocardial cells. Significant progress has been made in elucidating the molecular events underlying myocardial cell differentiation. In contrast, little is known about the development of the endocardial lineage that gives rise to cardiac valves and septa. We have used a novel in vitro model to identify the molecular hierarchy of endocardial differentiation and the role of transcription factor GATA5 in endocardial development. The results indicate that GATA5 is induced at an early stage of endothelial-endocardial differentiation prior to expression of such early endocardial markers as Tie2 and ErbB3. Inhibition of either GATA5 expression or NF-ATc activation, blocks terminal differentiation at a pre-endocardial stage and GATA5 and NF-ATc synergistically activate endocardial transcription. The data reveal that transcription factor GATA5 is required for differentiation of cardiogenic precursors into endothelial endocardial cells. This, in turn, suggests that the GATA5 pathway may be relevant to early stages of valvuloseptal development, defects of which account for the majority of human birth malformations.

A GATA-6 gene heart-region-specific enhancer provides a novel means to mark and probe a discrete component of the mouse cardiac conduction system

The transcriptional programs that specify the distinct components of the cardiac conduction system are poorly understood, in part due to a paucity of definitive molecular markers. In the present study we show that a cGATA-6 gene enhancer can be used to selectively express transgenes in the atrioventricular (AV) conduction system as it becomes manifest in the developing multichambered mouse heart. Furthermore, our analysis of staged cGATA-6/lacZ embryos revealed that the activity of this heart-region-specific enhancer can be traced back essentially to the outset of the cardiogenic program. We provide evidence that this enhancer reads medial/lateral and anterior/posterior positional information before the heart tube forms and we show that the activity of this enhancer becomes restricted at the heart looping stage to AV myocardial cells that induce endocardial cushion formation. We infer that a deeply-rooted heart-region-specific transcriptional program serves to coordinate AV valve placement and AV conduction system formation. Lastly, we show that cGATA-6/Cre mice can be used to delete floxed genes in the respective subsets of specialized heart cells.

Abnormalities of the genitourinary tract in female mice lacking GATA5

Members of the GATA family of transcription factors play important roles in cell fate specification, differentiation, and morphogenesis during mammalian development. GATA5, the only one of the six vertebrate GATA factor genes not yet inactivated in mice, is expressed in a pattern that overlaps with but is distinct from that of other GATA factor genes. During mouse embryogenesis, GATA5 is expressed first in the developing heart and subsequently in the lung, vasculature, and genitourinary system. To investigate the function of GATA5 in vivo, we created mice homozygous for a GATA5 null allele. Homozygous mutants were viable and fertile, but females exhibited pronounced genitourinary abnormalities that included vaginal and uterine defects and hypospadias. In contrast, the genitourinary system was unaffected in male GATA5 mutants. These results reveal a specific role of GATA5 in development of the female genitourinary system and suggest that other GATA factors may have functions overlapping those of GATA5 in other tissues.