Mef2c is a direct transcriptional target of ISL1 and GATA factors in the anterior heart field during mouse embryonic development

A retinoic acid:YAP1 signaling axis controls atrial lineage commitment

In cardiac progenitor cells (CPCs), retinoic acid (RA) signaling induces atrial lineage gene expression and acquisition of an atrial cell fate. To achieve this, RA coordinates a complex regulatory network of downstream effectors that is not fully identified. To address this gap, we applied a functional genomics approach (i.e., scRNA-seq and snATAC-seq) to untreated and RA-treated human embryonic stem cell (hESC)-derived CPCs. Unbiased analysis revealed that the Hippo effectors YAP1 and TEAD4 are integrated with the atrial transcription factor enhancer network and that YAP1 activates RA enhancers in CPCs. Furthermore, Yap1 deletion in mouse embryos compromises the expression of RA-induced genes, such as Nr2f2, in the CPCs of the second heart field. Accordingly, in hESC-derived patterned heart organoids, YAP1 regulates the formation of an atrial chamber but is dispensable for the formation of a ventricle. Overall, our findings revealed that YAP1 cooperates with RA signaling to induce atrial lineages during cardiogenesis.

Targeting GATA6 with pedunculoside inhibits fetal gene expression to attenuate pathological cardiac hypertrophy

BACKGROUND

Pathological cardiac hypertrophy is a characteristic feature of numerous cardiovascular diseases and significantly impacts human health. However, effective treatment options for cardiac hypertrophy are still significantly unmet. Pedunculoside, a pentacyclic triterpenoid saponin from the traditional Chinese herb Ilex rotunda Thunb., exhibits various pharmacological properties such as anti-inflammatory and cardiovascular therapeutic effects, but its anti-hypertrophy efficacy and mechanisms have not yet been reported.

PURPOSE

This study aimed to confirm the ameliorating effect of pedunculoside on cardiac hypertrophy and elucidate its underlying mechanism.

METHODS

To investigate the effect of pedunculoside on cardiac hypertrophy, we used transverse aortic constriction (TAC) and isoproterenol hydrochloride (ISO) infusion to induce cardiac hypertrophy model in mice. Angiotensin II (Ang II) was used to mimic hypertrophy model in myocardial cells. Then, we utilized a biotin-tagged carabrone chemical probe and validation experiments to pinpoint pedunculoside's key targets. Further, molecular docking study and sites mutation were used to predict and identify the binding modes of pedunculoside to target. Finally, structural optimization was carried out to find new pedunculoside derivatives with stronger anti-hypertrophy activity and binding affinity to the target.

RESULTS

Our findings revealed for the first time that pedunculoside treatment significantly attenuated hypertrophic phenotypes in response to TAC and ISO. It also effectively reduced hypertrophy and fibrosis in myocardial cells exposed to Ang II stimulation. Mechanically, we identified transcription factor GATA-6 (GATA6) as a key target of pedunculoside for treating cardiac hypertrophy. Further studies demonstrated that pedunculoside blocks cardiac hypertrophy progression by inhibiting the transcriptional activation of GATA6 on promoting fetal gene expression. More importantly, a new pedunculoside derivative PE-3 with stronger anti-hypertrophy activity and affinity for GATA6 was discovered.

CONCLUSION

Our findings suggest that pedunculoside and PE-3 could be developed as promising drug candidates for cardiac hypertrophy treatment.

BubR1 Controls Heart Development by Promoting Expression of Cardiogenesis Regulators

BACKGROUND

Congenital heart defects are structural anomalies present at birth that can affect the function of the heart. Aneuploidy is a significant risk factor for congenital heart defects. Mosaic variegated aneuploidy syndrome, caused by mutations in Bub1b (encoding BubR1, a mitotic checkpoint protein), leads to congenital heart defects such as septal defects. However, the molecular rationale for how Bub1b mutations promote congenital heart defects associated with mosaic variegated aneuploidy syndrome remains unresolved.

METHODS

To study morphological, structural, and cellular consequences of BubR1 deletion in the heart, we crossed mice carrying conditional alleles of Bub1b with Nkx2.5-cre mice. Single-cell RNA sequencing was carried out to determine differentially expressed genes and biological processes in various cell types present in the developing heart. Trajectory analysis was carried out to determine the differentiation trajectory of BubR1 knockout embryonic hearts. Finally, CellChat analysis provided details on the major signaling interactions that were either absent or hyperactive in the BubR1 knockout heart.

RESULTS

Here, we show that cardiac-specific BubR1 deletion causes embryonic lethality due to developmental stalling after cardiac looping with defects in cardiac maturation including chamber wall thickness, septation, and trabeculation. Single-cell transcriptomic profiling further revealed that the differentiation trajectory of cardiomyocytes is severely impacted with suppression of critical cardiogenesis genes. Hyperactivation of Wnt signaling in BubR1 knockout hearts indicated a disturbed homeostasis in cellular pathways essential for proper tissue morphogenesis of the heart.

CONCLUSIONS

Taken together, these findings reveal that BubR1 is a crucial regulator of cardiac development in vivo, which ensures the proper timing of heart morphogenesis.

The molecular mechanisms of cardiac development and related diseases

Cardiac development is a complex and intricate process involving numerous molecular signals and pathways. Researchers have explored cardiac development through a long journey, starting with early studies observing morphological changes and progressing to the exploration of molecular mechanisms using various molecular biology methods. Currently, advancements in stem cell technology and sequencing technology, such as the generation of human pluripotent stem cells and cardiac organoids, multi-omics sequencing, and artificial intelligence (AI) technology, have enabled researchers to understand the molecular mechanisms of cardiac development better. Many molecular signals regulate cardiac development, including various growth and transcription factors and signaling pathways, such as WNT signaling, retinoic acid signaling, and Notch signaling pathways. In addition, cilia, the extracellular matrix, epigenetic modifications, and hypoxia conditions also play important roles in cardiac development. These factors play crucial roles at one or even multiple stages of cardiac development. Recent studies have also identified roles for autophagy, metabolic transition, and macrophages in cardiac development. Deficiencies or abnormal expression of these factors can lead to various types of cardiac development abnormalities. Nowadays, congenital heart disease (CHD) management requires lifelong care, primarily involving surgical and pharmacological treatments. Advances in surgical techniques and the development of clinical genetic testing have enabled earlier diagnosis and treatment of CHD. However, these technologies still have significant limitations. The development of new technologies, such as sequencing and AI technologies, will help us better understand the molecular mechanisms of cardiac development and promote earlier prevention and treatment of CHD in the future.

Direct cardiac reprogramming via combined CRISPRa-mediated endogenous Gata4 activation and exogenous Mef2c and Tbx5 expression

Direct cardiac reprogramming of fibroblasts into induced cardiomyocytes (iCMs) can be achieved by ectopic expression of cardiac transcription factors (TFs) via viral vectors. However, risks like genomic mutations, viral toxicity, and immune response limited its clinical application. Transactivation of endogenous TFs emerges as an alternative approach that may partially mitigate some of the risks. In this study, we utilized a modified CRISPRa/dCas9 strategy to transactivate endogenous reprogramming factors MEF2C, GATA4, and TBX5 (MGT) to induce iCMs from both mouse and human fibroblasts. We identified single-guide RNAs (sgRNAs) targeting promoters and enhancers of the TFs capable of activating various degrees of endogenous gene expression. CRISPRa-mediated Gata4 activation, combined with exogenous expression of Mef2c and Tbx5, successfully converted fibroblasts into iCMs. Despite extensive sgRNA screening, transactivation of Mef2c and Tbx5 via CRISPRa remained less effective, potentially due to de novo epigenetic barriers. While future work and refined technologies are needed to determine whether cardiac reprogramming could be achieved solely through CRISPRa activation of endogenous factors, our findings provide proof of concept that reliance on exogenous TFs for reprogramming can be reduced through CRISPRa-mediated activation of endogenous factors, particularly Gata4, offering a novel strategy to convert scar-forming fibroblasts into iCMs for regenerative purposes.

Single-cell multi-modal integrative analyses highlight functional dynamic gene regulatory networks directing human cardiac development

Illuminating the precise stepwise genetic programs directing cardiac development provides insights into the mechanisms of congenital heart disease and strategies for cardiac regenerative therapies. Here, we integrate in vitro and in vivo human single-cell multi-omic studies with high-throughput functional genomic screening to reveal dynamic, cardiac-specific gene regulatory networks (GRNs) and transcriptional regulators during human cardiomyocyte development. Interrogating developmental trajectories reconstructed from single-cell data unexpectedly reveal divergent cardiomyocyte lineages with distinct gene programs based on developmental signaling pathways. High-throughput functional genomic screens identify key transcription factors from inferred GRNs that are functionally relevant for cardiomyocyte lineages derived from each pathway. Notably, we discover a critical heat shock transcription factor 1 (HSF1)-mediated cardiometabolic GRN controlling cardiac mitochondrial/metabolic function and cell survival, also observed in fetal human cardiomyocytes. Overall, these multi-modal genomic studies enable the systematic discovery and validation of coordinated GRNs and transcriptional regulators controlling the development of distinct human cardiomyocyte populations.

Gata6 functions in zebrafish endoderm to regulate late differentiating arterial pole cardiogenesis

Mutations in GATA6 are associated with congenital heart disease, most notably conotruncal structural defects. However, how GATA6 regulates cardiac morphology during embryogenesis is undefined. We used knockout and conditional mutant zebrafish alleles to investigate the spatiotemporal role of gata6 during cardiogenesis. Loss of gata6 specifically impacts atrioventricular valve formation and recruitment of epicardium, with a prominent loss of arterial pole cardiac cells, including those of the ventricle and outflow tract. However, there are no obvious defects in cardiac progenitor cell specification, proliferation or death. Conditional loss of gata6 starting at 24 h is sufficient to disrupt the addition of late differentiating cardiomyocytes at the arterial pole, with decreased expression levels of anterior secondary heart field (SHF) markers spry4 and mef2cb. Conditional loss of gata6 in the endoderm is sufficient to phenocopy the straight knockout, resulting in a significant loss of ventricular and outflow tract tissue. Exposure to a Dusp6 inhibitor largely rescues the loss of ventricular cells in gata6-/- larvae. Thus, gata6 functions in endoderm are mediated by FGF signaling to regulate the addition of anterior SHF progenitor derivatives during heart formation.

Multisite Assembly of Gateway Induced Clones (MAGIC): a flexible cloning toolbox with diverse applications in vertebrate model systems

Here we present the Multisite Assembly of Gateway Induced Clones (MAGIC) system, which harnesses site-specific recombination-based cloning via Gateway technology for rapid, modular assembly of between 1 and 3 "Entry" vector components, all into a fourth, standard high copy "Destination" plasmid backbone. The MAGIC toolkit spans a range of in vitro and in vivo uses, from directing tunable gene expression, to driving simultaneous expression of microRNAs and fluorescent reporters, to enabling site-specific recombinase-dependent gene expression. All MAGIC system components are directly compatible with existing multisite gateway Tol2 systems currently used in zebrafish, as well as existing eukaryotic cell culture expression Destination plasmids, and available mammalian lentiviral and adenoviral Destination vectors, allowing rapid cross-species experimentation. Moreover, herein we describe novel vectors with flanking piggyBac transposon elements for stable genomic integration in vitro or in vivo when used with piggyBac transposase. Collectively, the MAGIC system facilitates transgenesis in cultured mammalian cells, electroporated mouse and chick embryos, as well as in injected zebrafish embryos, enabling the rapid generation of innovative DNA constructs for biological research due to a shared, common plasmid platform.

Epigenetic Regulation of Mammalian Cardiomyocyte Development

The heart is the first organ formed during mammalian development and functions to distribute nutrients and oxygen to other parts of the developing embryo. Cardiomyocytes are the major cell types of the heart and provide both structural support and contractile function to the heart. The successful differentiation of cardiomyocytes during early development is under tight regulation by physical and molecular factors. We have reviewed current studies on epigenetic factors critical for cardiomyocyte differentiation, including DNA methylation, histone modifications, chromatin remodelers, and noncoding RNAs. This review also provides comprehensive details on structural and morphological changes associated with the differentiation of fetal and postnatal cardiomyocytes and highlights their differences. A holistic understanding of all aspects of cardiomyocyte development is critical for the successful in vitro differentiation of cardiomyocytes for therapeutic purposes.

Gastrulation-stage gene expression in Nipbl+/- mouse embryos foreshadows the development of syndromic birth defects

In animal models, Nipbl deficiency phenocopies gene expression changes and birth defects seen in Cornelia de Lange syndrome, the most common cause of which is Nipbl haploinsufficiency. Previous studies in Nipbl+/- mice suggested that heart development is abnormal as soon as cardiogenic tissue is formed. To investigate this, we performed single-cell RNA sequencing on wild-type and Nipbl+/- mouse embryos at gastrulation and early cardiac crescent stages. Nipbl+/- embryos had fewer mesoderm cells than wild-type and altered proportions of mesodermal cell subpopulations. These findings were associated with underexpression of genes implicated in driving specific mesodermal lineages. In addition, Nanog was found to be overexpressed in all germ layers, and many gene expression changes observed in Nipbl+/- embryos could be attributed to Nanog overexpression. These findings establish a link between Nipbl deficiency, Nanog overexpression, and gene expression dysregulation/lineage misallocation, which ultimately manifest as birth defects in Nipbl+/- animals and Cornelia de Lange syndrome.

Cardiac Transcription Factors and Regulatory Networks

Cardiac development is a fine-tuned process governed by complex transcriptional networks, in which transcription factors (TFs) interact with other regulatory layers. In this chapter, we introduce the core cardiac TFs including Gata, Hand, Nkx2, Mef2, Srf, and Tbx. These factors regulate each other's expression and can also act in a combinatorial manner on their downstream targets. Their disruption leads to various cardiac phenotypes in mice, and mutations in humans have been associated with congenital heart defects. In the second part of the chapter, we discuss different levels of regulation including cis-regulatory elements, chromatin structure, and microRNAs, which can interact with transcription factors, modulate their function, or are downstream targets. Finally, examples of disturbances of the cardiac regulatory network leading to congenital heart diseases in human are provided.

Cardiac Progenitor Cells of the First and Second Heart Fields

The heart forms from the first and second heart fields, which contribute to distinct regions of the myocardium. This is supported by clonal analyses, which identify corresponding first and second cardiac cell lineages in the heart. Progenitor cells of the second heart field and its sub-domains are controlled by a gene regulatory network and signaling pathways, which determine their behavior. Multipotent cells in this field can also contribute cardiac endothelial and smooth muscle cells. Furthermore, the skeletal muscles of the head and neck are clonally related to myocardial cells that form the arterial and venous poles of the heart. These lineage relationships, together with the genes that regulate the heart fields, have major implications for congenital heart disease.

The benign nature and rare occurrence of cardiac myxoma as a possible consequence of the limited cardiac proliferative/ regenerative potential: a systematic review

BACKGROUND

Cardiac Myxoma is a primary tumor of heart. Its origins, rarity of the occurrence of primary cardiac tumors and how it may be related to limited cardiac regenerative potential, are not yet entirely known. This study investigates the key cardiac genes/ transcription factors (TFs) and signaling pathways to understand these important questions.

METHODS

Databases including PubMed, MEDLINE, and Google Scholar were searched for published articles without any date restrictions, involving cardiac myxoma, cardiac genes/TFs/signaling pathways and their roles in cardiogenesis, proliferation, differentiation, key interactions and tumorigenesis, with focus on cardiomyocytes.

RESULTS

The cardiac genetic landscape is governed by a very tight control between proliferation and differentiation-related genes/TFs/pathways. Cardiac myxoma originates possibly as a consequence of dysregulations in the gene expression of differentiation regulators including Tbx5, GATA4, HAND1/2, MYOCD, HOPX, BMPs. Such dysregulations switch the expression of cardiomyocytes into progenitor-like state in cardiac myxoma development by dysregulating Isl1, Baf60 complex, Wnt, FGF, Notch, Mef2c and others. The Nkx2-5 and MSX2 contribute predominantly to both proliferation and differentiation of Cardiac Progenitor Cells (CPCs), may possibly serve roles based on the microenvironment and the direction of cell circuitry in cardiac tumorigenesis. The Nkx2-5 in cardiac myxoma may serve to limit progression of tumorigenesis as it has massive control over the proliferation of CPCs. The cardiac cell type-specific genetic programming plays governing role in controlling the tumorigenesis and regenerative potential.

CONCLUSION

The cardiomyocytes have very limited proliferative and regenerative potential. They survive for long periods of time and tightly maintain the gene expression of differentiation genes such as Tbx5, GATA4 that interact with tumor suppressors (TS) and exert TS like effect. The total effect such gene expression exerts is responsible for the rare occurrence and benign nature of primary cardiac tumors. This prevents the progression of tumorigenesis. But this also limits the regenerative and proliferative potential of cardiomyocytes. Cardiac Myxoma develops as a consequence of dysregulations in these key genes which revert the cells towards progenitor-like state, hallmark of CM. The CM development in carney complex also signifies the role of TS in cardiac cells.

Drosophila as a Model to Understand Second Heart Field Development

The genetic model system Drosophila has contributed fundamentally to our understanding of mammalian heart specification, development, and congenital heart disease. The relatively simple Drosophila heart is a linear muscular tube that is specified and develops in the embryo and persists throughout the life of the animal. It functions at all stages to circulate hemolymph within the open circulatory system of the body. During Drosophila metamorphosis, the cardiac tube is remodeled, and a new layer of muscle fibers spreads over the ventral surface of the heart to form the ventral longitudinal muscles. The formation of these fibers depends critically upon genes known to be necessary for mammalian second heart field (SHF) formation. Here, we review the prior contributions of the Drosophila system to the understanding of heart development and disease, discuss the importance of the SHF to mammalian heart development and disease, and then discuss how the ventral longitudinal adult cardiac muscles can serve as a novel model for understanding SHF development and disease.

The MEF2A transcription factor interactome in cardiomyocytes

Transcriptional regulators encoded by the Myocyte Enhancer Factor 2 (MEF2) gene family play a fundamental role in cardiac development, homeostasis and pathology. Previous studies indicate that MEF2A protein-protein interactions serve as a network hub in several cardiomyocyte cellular processes. Based on the idea that interactions with regulatory protein partners underly the diverse roles of MEF2A in cardiomyocyte gene expression, we undertook a systematic unbiased screen of the MEF2A protein interactome in primary cardiomyocytes using an affinity purification-based quantitative mass spectrometry approach. Bioinformatic processing of the MEF2A interactome revealed protein networks involved in the regulation of programmed cell death, inflammatory responses, actin dynamics and stress signaling in primary cardiomyocytes. Further biochemical and functional confirmation of specific protein-protein interactions documented a dynamic interaction between MEF2A and STAT3 proteins. Integration of transcriptome level data from MEF2A and STAT3-depleted cardiomyocytes reveals that the balance between MEF2A and STAT3 activity exerts a level of executive control over the inflammatory response and cardiomyocyte cell survival and experimentally ameliorates Phenylephrine induced cardiomyocyte hypertrophy. Lastly, we identified several MEF2A/STAT3 co-regulated genes, including the MMP9 gene. Herein, we document the cardiomyocyte MEF2A interactome, which furthers our understanding of protein networks involved in the hierarchical control of normal and pathophysiological cardiomyocyte gene expression in the mammalian heart.

Graded mesoderm assembly governs cell fate and morphogenesis of the early mammalian heart

Using four-dimensional whole-embryo light sheet imaging with improved and accessible computational tools, we longitudinally reconstruct early murine cardiac development at single-cell resolution. Nascent mesoderm progenitors form opposing density and motility gradients, converting the temporal birth sequence of gastrulation into a spatial anterolateral-to-posteromedial arrangement. Migrating precardiac mesoderm does not strictly preserve cellular neighbor relationships, and spatial patterns only become solidified as the cardiac crescent emerges. Progenitors undergo a mesenchymal-to-epithelial transition, with a first heart field (FHF) ridge apposing a motile juxta-cardiac field (JCF). Anchored along the ridge, the FHF epithelium rotates the JCF forward to form the initial heart tube, along with push-pull morphodynamics of the second heart field. In Mesp1 mutants that fail to make a cardiac crescent, mesoderm remains highly motile but directionally incoherent, resulting in density gradient inversion. Our practicable live embryo imaging approach defines spatial origins and behaviors of cardiac progenitors and identifies their unanticipated morphological transitions.

The second heart field: the first 20 years

In 2001, three independent groups reported the identification of a novel cluster of progenitor cells that contribute to heart development in mouse and chicken embryos. This population of progenitor cells was designated as the second heart field (SHF), and a new research direction in heart development was launched. Twenty years have since passed and a comprehensive understanding of the SHF has been achieved. This review provides retrospective insights in to the contribution, the signaling regulatory networks and the epithelial properties of the SHF. It also includes the spatiotemporal characteristics of SHF development and interactions between the SHF and other types of cells during heart development. Although considerable efforts will be required to investigate the cellular heterogeneity of the SHF, together with its intricate regulatory networks and undefined mechanisms, it is expected that the burgeoning new technology of single-cell sequencing and precise lineage tracing will advance the comprehension of SHF function and its molecular signals. The advances in SHF research will translate to clinical applications and to the treatment of congenital heart diseases, especially conotruncal defects, as well as to regenerative medicine.

Spatial multi-omic map of human myocardial infarction

Myocardial infarction is a leading cause of death worldwide1. Although advances have been made in acute treatment, an incomplete understanding of remodelling processes has limited the effectiveness of therapies to reduce late-stage mortality2. Here we generate an integrative high-resolution map of human cardiac remodelling after myocardial infarction using single-cell gene expression, chromatin accessibility and spatial transcriptomic profiling of multiple physiological zones at distinct time points in myocardium from patients with myocardial infarction and controls. Multi-modal data integration enabled us to evaluate cardiac cell-type compositions at increased resolution, yielding insights into changes of the cardiac transcriptome and epigenome through the identification of distinct tissue structures of injury, repair and remodelling. We identified and validated disease-specific cardiac cell states of major cell types and analysed them in their spatial context, evaluating their dependency on other cell types. Our data elucidate the molecular principles of human myocardial tissue organization, recapitulating a gradual cardiomyocyte and myeloid continuum following ischaemic injury. In sum, our study provides an integrative molecular map of human myocardial infarction, represents an essential reference for the field and paves the way for advanced mechanistic and therapeutic studies of cardiac disease.

The arginine methyltransferase Carm1 is necessary for heart development

To discover genes implicated in human congenital disorders, we performed ENU mutagenesis in the mouse and screened for mutations affecting embryonic development. In this work, we report defects of heart development in mice homozygous for a mutation of coactivator-associated arginine methyltransferase 1 (Carm1). While Carm1 has been extensively studied, it has never been previously associated with a role in heart development. Phenotype analysis combining histology and microcomputed tomography imaging shows a range of cardiac defects. Most notably, many affected midgestation embryos appear to have cardiac rupture and hemorrhaging in the thorax. Mice that survive to late gestation show a variety of cardiac defects, including ventricular septal defects, double outlet right ventricle, and persistent truncus arteriosus. Transcriptome analyses of the mutant embryos by mRNA-seq reveal the perturbation of several genes involved in cardiac morphogenesis and muscle development and function. In addition, we observe the mislocalization of cardiac neural crest cells at E12.5 in the outflow tract. The cardiac phenotype of Carm1 mutant embryos is similar to that of Pax3 null mutants, and PAX3 is a putative target of CARM1. However, our analysis does not support the hypothesis that developmental defects in Carm1 mutant embryos are primarily due to a functional defect of PAX3.

Stage-specific regulation of signalling pathways to differentiate pluripotent stem cells to cardiomyocytes with ventricular lineage

BACKGROUND

Pluripotent stem cells (PSCs) can be an ideal source of differentiation of cardiomyocytes in vitro and during transplantation to induce cardiac regeneration. However, differentiation of PSCs into a heterogeneous population is associated with an increased incidence of arrhythmia following transplantation. We aimed to design a protocol to drive PSCs to a ventricular lineage by regulating Wnt and retinoic acid (RA) signalling pathways.

METHODS

Mouse embryonic stem cells were cultured either in monolayers or three-dimensional hanging drop method to form embryonic bodies (EBs) and exposed to different treatments acting on Wnt and retinoic acid signalling. Samples were collected at different time points to analyse cardiomyocyte-specific markers by RT-PCR, flow cytometry and immunofluorescence.

RESULTS

Treatment of monolayer and EBs with Wnt and RA signalling pathways and ascorbic acid, as a cardiac programming enhancer, resulted in the formation of an immature non-contractile cardiac population that expressed many of the putative markers of cardiac differentiation. The population exhibited upregulation of ventricular specific markers while suppressing the expression of pro-atrial and pro-sinoatrial markers. Differentiation of EBs resulted in early foetal like non-contractile ventricular cardiomyocytes with an inherent propensity to contract when stimulated.

CONCLUSION

Our results provide the first evidence of in vitro differentiation that mimics the embryonic morphogenesis towards ventricular specific cardiomyocytes through regulation of Wnt and RA signalling pathways.

Genomic enhancers in cardiac development and disease

The Human Genome Project marked a major milestone in the scientific community as it unravelled the ~3 billion bases that are central to crucial aspects of human life. Despite this achievement, it only scratched the surface of understanding how each nucleotide matters, both individually and as part of a larger unit. Beyond the coding genome, which comprises only ~2% of the whole genome, scientists have realized that large portions of the genome, not known to code for any protein, were crucial for regulating the coding genes. These large portions of the genome comprise the 'non-coding genome'. The history of gene regulation mediated by proteins that bind to the regulatory non-coding genome dates back many decades to the 1960s. However, the original definition of 'enhancers' was first used in the early 1980s. In this Review, we summarize benchmark studies that have mapped the role of cardiac enhancers in disease and development. We highlight instances in which enhancer-localized genetic variants explain the missing link to cardiac pathogenesis. Finally, we inspire readers to consider the next phase of exploring enhancer-based gene therapy for cardiovascular disease.

Spotlight on Isl1: A Key Player in Cardiovascular Development and Diseases

Cardiac transcription factors orchestrate a regulatory network controlling cardiovascular development. Isl1, a LIM-homeodomain transcription factor, acts as a key player in multiple organs during embryonic development. Its crucial roles in cardiovascular development have been elucidated by extensive studies, especially as a marker gene for the second heart field progenitors. Here, we summarize the roles of Isl1 in cardiovascular development and function, and outline its cellular and molecular modes of action, thus providing insights for the molecular basis of cardiovascular diseases.

Vascular Smooth Muscle Cell Subpopulations and Neointimal Formation in Mouse Models of Elastin Insufficiency

OBJECTIVE

Using a mouse model of Eln (elastin) insufficiency that spontaneously develops neointima in the ascending aorta, we sought to understand the origin and phenotypic heterogeneity of smooth muscle cells (SMCs) contributing to intimal hyperplasia. We were also interested in exploring how vascular cells adapt to the absence of Eln. Approach and Results: We used single-cell sequencing together with lineage-specific cell labeling to identify neointimal cell populations in a noninjury, genetic model of neointimal formation. Inactivating Eln production in vascular SMCs results in rapid intimal hyperplasia around breaks in the ascending aorta's internal elastic lamina. Using lineage-specific Cre drivers to both lineage mark and inactivate Eln expression in the secondary heart field and neural crest aortic SMCs, we found that cells with a secondary heart field lineage are significant contributors to neointima formation. We also identified a small population of secondary heart field-derived SMCs underneath and adjacent to the internal elastic lamina. Within the neointima of SMC-Eln knockout mice, 2 unique SMC populations were identified that are transcriptionally different from other SMCs. While these cells had a distinct gene signature, they expressed several genes identified in other studies of neointimal lesions, suggesting that some mechanisms underlying neointima formation in Eln insufficiency are shared with adult vessel injury models.

CONCLUSIONS

These results highlight the unique developmental origin and transcriptional signature of cells contributing to neointima in the ascending aorta. Our findings also show that the absence of Eln, or changes in elastic fiber integrity, influences the SMC biological niche in ways that lead to altered cell phenotypes.

Single cell multi-omic analysis identifies a Tbx1-dependent multilineage primed population in murine cardiopharyngeal mesoderm

The poles of the heart and branchiomeric muscles of the face and neck are formed from the cardiopharyngeal mesoderm within the pharyngeal apparatus. They are disrupted in patients with 22q11.2 deletion syndrome, due to haploinsufficiency of TBX1, encoding a T-box transcription factor. Here, using single cell RNA-sequencing, we now identify a multilineage primed population within the cardiopharyngeal mesoderm, marked by Tbx1, which has bipotent properties to form cardiac and branchiomeric muscle cells. The multilineage primed cells are localized within the nascent mesoderm of the caudal lateral pharyngeal apparatus and provide a continuous source of cardiopharyngeal mesoderm progenitors. Tbx1 regulates the maturation of multilineage primed progenitor cells to cardiopharyngeal mesoderm derivatives while restricting ectopic non-mesodermal gene expression. We further show that TBX1 confers this balance of gene expression by direct and indirect regulation of enriched genes in multilineage primed progenitors and downstream pathways, partly through altering chromatin accessibility, the perturbation of which can lead to congenital defects in individuals with 22q11.2 deletion syndrome.

Single-Nucleotide Polymorphisms in Exonic and Promoter Regions of Transcription Factors of Second Heart Field Associated with Sporadic Congenital Cardiac Anomalies

Multiple second heart field (SHF) transcription factors are involved in cardiac development. In this article we evaluate the relationship between SHF transcription factor polymorphisms and congenital heart disease (CHD). Ten polymorphisms were used for genotyping, and three of these were used for the luciferase assay. The risk of CHD was increased 4.31 times and 1.54 times in the C allele of GATA5: rs6061243 G>C and G allele of TBX20: rs336283 A>G, respectively. The minor alleles of SMYD1: rs1542088 T>G, MEF2C: rs80043958 A>G and GATA5: rs6587239 T>C increased the risk of the simple types of CHD. The minor alleles of GATA5: rs41305803 G>A and MEF2C: rs304154 A>G increased the risk of tetralogy of Fallot (TOF). The minor alleles of TBX20: rs336284 A>G and SMYD1: rs88387557 T>G only increased the risk of a single ventricle (SV). Luciferase assays revealed that the minor alleles of rs304154 and rs336284 decreased the transcriptional levels of MEF2C and TBX20, respectively (p<0.01). When combined with HLTF, the G promoter showed a higher expression level than the A promoter in rs80043958 (p<0.01). Our findings suggest that minor alleles of SNPs in the exonic and promoter regions of transcription factors in the SHF can increase the risks of sporadic CHD.

Cardiac Progenitor Cells

Cardiovascular diseases top the list of fatal illnesses worldwide. Cardiac tissues is known to be one of te least proliferative in the human body, with very limited regenraive capacity. Stem cell therapy has shown great potential for treatment of cardiovascular diseases in the experimental setting, but success in human trials has been limited. Applications of stem cell therapy for cardiovascular regeneration necessitate understamding of the complex and unique structure of the heart unit, and the embryologic development of the heart muscles and vessels. This chapter aims to provide an insight into cardiac progenitor cells and their potential applications in regenerative medicine. It also provides an overview of the embryological development of cardiac tissue, and the major findings on the development of cardiac stem cells, their characterization, and differentiation, and their regenerative potential. It concludes with clinical applications in treating cardiac disease using different approaches, and concludes with areas for future research.

Direct Reprogramming of Cardiac Fibroblasts to Repair the Injured Heart

Coronary heart disease is a leading cause of mortality and morbidity. Those that survive acute myocardial infarction are at significant risk of subsequent heart failure due to fibrotic remodelling of the infarcted myocardium. By applying knowledge from the study of embryonic cardiovascular development, modern medicine offers hope for treatment of this condition through regeneration of the myocardium by direct reprogramming of fibrotic scar tissue. Here, we will review mechanisms of cell fate specification leading to the generation of cardiovascular cell types in the embryo and use this as a framework in which to understand direct reprogramming. Driving expression of a network of transcription factors, micro RNA or small molecule epigenetic modifiers can reverse epigenetic silencing, reverting differentiated cells to a state of induced pluripotency. The pluripotent state can be bypassed by direct reprogramming in which one differentiated cell type can be transdifferentiated into another. Transdifferentiating cardiac fibroblasts to cardiomyocytes requires a network of transcription factors similar to that observed in embryonic multipotent cardiac progenitors. There is some flexibility in the composition of this network. These studies raise the possibility that the failing heart could one day be regenerated by directly reprogramming cardiac fibroblasts within post-infarct scar tissue.

DAND5 Inactivation Enhances Cardiac Differentiation in Mouse Embryonic Stem Cells

Deciphering the clues of a regenerative mechanism for the mammalian adult heart would save millions of lives in the near future. Heart failure due to cardiomyocyte loss is still one of the significant health burdens worldwide. Here, we show the potential of a single molecule, DAND5, in mouse pluripotent stem cell-derived cardiomyocytes specification and proliferation. Dand5 loss-of-function generated the double of cardiac beating foci compared to the wild-type cells. The early formation of cardiac progenitor cells and the increased proliferative capacity of Dand5 KO mESC-derived cardiomyocytes contribute to the observed higher number of derived cardiac cells. Transcriptional profiling sequencing and quantitative RT-PCR assays showed an upregulation of early cardiac gene networks governing cardiomyocyte differentiation, cell cycling, and cardiac regenerative pathways but reduced levels of genes involved in cardiomyocyte maturation. These findings prompt DAND5 as a key driver for the generation and expansion of pluripotent stem cell-derived cardiomyocytes systems with further clinical application purposes.

Persistent Ventricle Partitioning in the Adult Zebrafish Heart

The vertebrate heart integrates cells from the early-differentiating first heart field (FHF) and the later-differentiating second heart field (SHF), both emerging from the lateral plate mesoderm. In mammals, this process forms the basis for the development of the left and right ventricle chambers and subsequent chamber septation. The single ventricle-forming zebrafish heart also integrates FHF and SHF lineages during embryogenesis, yet the contributions of these two myocardial lineages to the adult zebrafish heart remain incompletely understood. Here, we characterize the myocardial labeling of FHF descendants in both the developing and adult zebrafish ventricle. Expanding previous findings, late gastrulation-stage labeling using drl-driven CreERT2 recombinase with a myocardium-specific, myl7-controlled, loxP reporter results in the predominant labeling of FHF-derived outer curvature and the right side of the embryonic ventricle. Raised to adulthood, such lineage-labeled hearts retain broad areas of FHF cardiomyocytes in a region of the ventricle that is positioned at the opposite side to the atrium and encompasses the apex. Our data add to the increasing evidence for a persisting cell-based compartmentalization of the adult zebrafish ventricle even in the absence of any physical boundary.

Outflow Tract Formation-Embryonic Origins of Conotruncal Congenital Heart Disease

Anomalies in the cardiac outflow tract (OFT) are among the most frequent congenital heart defects (CHDs). During embryogenesis, the cardiac OFT is a dynamic structure at the arterial pole of the heart. Heart tube elongation occurs by addition of cells from pharyngeal, splanchnic mesoderm to both ends. These progenitor cells, termed the second heart field (SHF), were first identified twenty years ago as essential to the growth of the forming heart tube and major contributors to the OFT. Perturbation of SHF development results in common forms of CHDs, including anomalies of the great arteries. OFT development also depends on paracrine interactions between multiple cell types, including myocardial, endocardial and neural crest lineages. In this publication, dedicated to Professor Andriana Gittenberger-De Groot and her contributions to the field of cardiac development and CHDs, we review some of her pioneering studies of OFT development with particular interest in the diverse origins of the many cell types that contribute to the OFT. We also discuss the clinical implications of selected key findings for our understanding of the etiology of CHDs and particularly OFT malformations.

Transcription Factor Reprogramming in the Inner Ear: Turning on Cell Fate Switches to Regenerate Sensory Hair Cells

Non-mammalian vertebrates can restore their auditory and vestibular hair cells naturally by triggering the regeneration of adjacent supporting cells. The transcription factor ATOH1 is a key regulator of hair cell development and regeneration in the inner ear. Following the death of hair cells, supporting cells upregulate ATOH1 and give rise to new hair cells. However, in the mature mammalian cochlea, such natural regeneration of hair cells is largely absent. Transcription factor reprogramming has been used in many tissues to convert one cell type into another, with the long-term hope of achieving tissue regeneration. Reprogramming transcription factors work by altering the transcriptomic and epigenetic landscapes in a target cell, resulting in a fate change to the desired cell type. Several studies have shown that ATOH1 is capable of reprogramming cochlear non-sensory tissue into cells resembling hair cells in young animals. However, the reprogramming ability of ATOH1 is lost with age, implying that the potency of individual hair cell-specific transcription factors may be reduced or lost over time by mechanisms that are still not clear. To circumvent this, combinations of key hair cell transcription factors have been used to promote hair cell regeneration in older animals. In this review, we summarize recent findings that have identified and studied these reprogramming factor combinations for hair cell regeneration. Finally, we discuss the important questions that emerge from these findings, particularly the feasibility of therapeutic strategies using reprogramming factors to restore human hearing in the future.

Understanding Heart Field Progenitor Cells for Modeling Congenital Heart Diseases

PURPOSE OF REVIEW

Heart development is a meticulously coordinated process that involves the specification of two distinct populations of cardiac progenitor cells, namely the first and the second heart field. Disruption of heart field progenitors can result in congenital heart defects. In this review, we aim to describe the signaling pathways and transcription factors that link heart field development and congenital heart disease.

RECENT FINDINGS

Single-cell transcriptomics, lineage-tracing mouse models, and stem cell-based in vitro modeling of cardiogenesis have significantly improved the spatiotemporal characterization of cardiac progenitors. Additionally, novel functional genomic studies have now linked more genetic variants with congenital heart disease. Dysregulation of cardiac progenitor cells causes malformations that can be lethal. Ongoing research will continue to shed light on cardiac morphogenesis and help us better understand and treat patients with congenital heart disease.

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.

From Stripes to a Beating Heart: Early Cardiac Development in Zebrafish

The heart is the first functional organ to form during vertebrate development. Congenital heart defects are the most common type of human birth defect, many originating as anomalies in early heart development. The zebrafish model provides an accessible vertebrate system to study early heart morphogenesis and to gain new insights into the mechanisms of congenital disease. Although composed of only two chambers compared with the four-chambered mammalian heart, the zebrafish heart integrates the core processes and cellular lineages central to cardiac development across vertebrates. The rapid, translucent development of zebrafish is amenable to in vivo imaging and genetic lineage tracing techniques, providing versatile tools to study heart field migration and myocardial progenitor addition and differentiation. Combining transgenic reporters with rapid genome engineering via CRISPR-Cas9 allows for functional testing of candidate genes associated with congenital heart defects and the discovery of molecular causes leading to observed phenotypes. Here, we summarize key insights gained through zebrafish studies into the early patterning of uncommitted lateral plate mesoderm into cardiac progenitors and their regulation. We review the central genetic mechanisms, available tools, and approaches for modeling congenital heart anomalies in the zebrafish as a representative vertebrate model.

Cell-Extracellular Matrix Interactions Play Multiple Essential Roles in Aortic Arch Development

RATIONALE

Defects in the morphogenesis of the fourth pharyngeal arch arteries (PAAs) give rise to lethal birth defects. Understanding genes and mechanisms regulating PAA formation will provide important insights into the etiology and treatments for congenital heart disease.

OBJECTIVE

Cell-ECM (extracellular matrix) interactions play essential roles in the morphogenesis of PAAs and their derivatives, the aortic arch artery and its major branches; however, their specific functions are not well-understood. Previously, we demonstrated that integrin α5β1 and Fn1 (fibronectin) expressed in the Isl1 lineages regulate PAA formation. The objective of the current studies was to investigate cellular mechanisms by which integrin α5β1 and Fn1 regulate aortic arch artery morphogenesis.

METHODS AND RESULTS

Using temporal lineage tracing, whole-mount confocal imaging, and quantitative analysis of the second heart field (SHF) and endothelial cell (EC) dynamics, we show that the majority of PAA EC progenitors arise by E7.5 in the SHF and contribute to pharyngeal arch endothelium between E7.5 and E9.5. Consequently, SHF-derived ECs in the pharyngeal arches form a plexus of small blood vessels, which remodels into the PAAs by 35 somites. The remodeling of the vascular plexus is orchestrated by signals dependent on the pharyngeal ECM microenvironment, extrinsic to the endothelium. Conditional ablation of integrin α5β1 or Fn1 in the Isl1 lineages showed that signaling by the ECM regulates aortic arch artery morphogenesis at multiple steps: (1) accumulation of SHF-derived ECs in the pharyngeal arches, (2) remodeling of the EC plexus in the fourth arches into the PAAs, and (3) differentiation of neural crest-derived cells adjacent to the PAA endothelium into vascular smooth muscle cells.

CONCLUSIONS

PAA formation is a multistep process entailing dynamic contribution of SHF-derived ECs to pharyngeal arches, the remodeling of endothelial plexus into the PAAs, and the remodeling of the PAAs into the aortic arch artery and its major branches. Cell-ECM interactions regulated by integrin α5β1 and Fn1 play essential roles at each of these developmental stages.

Cardiac Development: A Glimpse on Its Translational Contributions

Cardiac development is a complex developmental process that is initiated soon after gastrulation, as two sets of precardiac mesodermal precursors are symmetrically located and subsequently fused at the embryonic midline forming the cardiac straight tube. Thereafter, the cardiac straight tube invariably bends to the right, configuring the first sign of morphological left–right asymmetry and soon thereafter the atrial and ventricular chambers are formed, expanded and progressively septated. As a consequence of all these morphogenetic processes, the fetal heart acquired a four-chambered structure having distinct inlet and outlet connections and a specialized conduction system capable of directing the electrical impulse within the fully formed heart. Over the last decades, our understanding of the morphogenetic, cellular, and molecular pathways involved in cardiac development has exponentially grown. Multiples aspects of the initial discoveries during heart formation has served as guiding tools to understand the etiology of cardiac congenital anomalies and adult cardiac pathology, as well as to enlighten novels approaches to heal the damaged heart. In this review we provide an overview of the complex cellular and molecular pathways driving heart morphogenesis and how those discoveries have provided new roads into the genetic, clinical and therapeutic management of the diseased hearts.

Mef2c factors are required for early but not late addition of cardiomyocytes to the ventricle

During heart formation, the heart grows and undergoes dramatic morphogenesis to achieve efficient embryonic function. Both in fish and amniotes, much of the growth occurring after initial heart tube formation arises from second heart field (SHF)-derived progenitor cell addition to the arterial pole, allowing chamber formation. In zebrafish, this process has been extensively studied during embryonic life, but it is unclear how larval cardiac growth occurs beyond 3 days post-fertilisation (dpf). By quantifying zebrafish myocardial growth using live imaging of GFP-labelled myocardium we show that the heart grows extensively between 3 and 5 dpf. Using methods to assess cell division, cellular development timing assay and Kaede photoconversion, we demonstrate that proliferation, CM addition, and hypertrophy contribute to ventricle growth. Mechanistically, we show that reduction in Mef2c activity (mef2ca^+/-;mef2cb^-/-), downstream or in parallel with Nkx2.5 and upstream of Ltbp3, prevents some CM addition and differentiation, resulting in a significantly smaller ventricle by 3 dpf. After 3 dpf, however, CM addition in mef2ca^+/-;mef2cb^-/- mutants recovers to a normal pace, and the heart size gap between mutants and their siblings diminishes into adulthood. Thus, as in mice, there is an early time window when SHF contribution to the myocardium is particularly sensitive to loss of Mef2c activity.

The Future of Direct Cardiac Reprogramming: Any GMT Cocktail Variety?

Direct cardiac reprogramming has emerged as a novel therapeutic approach to treat and regenerate injured hearts through the direct conversion of fibroblasts into cardiac cells. Most studies have focused on the reprogramming of fibroblasts into induced cardiomyocytes (iCMs). The first study in which this technology was described, showed that at least a combination of three transcription factors, GATA4, MEF2C and TBX5 (GMT cocktail), was required for the reprogramming into iCMs in vitro using mouse cells. However, this was later demonstrated to be insufficient for the reprogramming of human cells and additional factors were required. Thereafter, most studies have focused on implementing reprogramming efficiency and obtaining fully reprogrammed and functional iCMs, by the incorporation of other transcription factors, microRNAs or small molecules to the original GMT cocktail. In this respect, great advances have been made in recent years. However, there is still no consensus on which of these GMT-based varieties is best, and robust and highly reproducible protocols are still urgently required, especially in the case of human cells. On the other hand, apart from CMs, other cells such as endothelial and smooth muscle cells to form new blood vessels will be fundamental for the correct reconstruction of damaged cardiac tissue. With this aim, several studies have centered on the direct reprogramming of fibroblasts into induced cardiac progenitor cells (iCPCs) able to give rise to all myocardial cell lineages. Especially interesting are reports in which multipotent and highly expandable mouse iCPCs have been obtained, suggesting that clinically relevant amounts of these cells could be created. However, as of yet, this has not been achieved with human iCPCs, and exactly what stage of maturity is appropriate for a cell therapy product remains an open question. Nonetheless, the major concern in regenerative medicine is the poor retention, survival, and engraftment of transplanted cells in the cardiac tissue. To circumvent this issue, several cell pre-conditioning approaches are currently being explored. As an alternative to cell injection, in vivo reprogramming may face fewer barriers for its translation to the clinic. This approach has achieved better results in terms of efficiency and iCMs maturity in mouse models, indicating that the heart environment can favor this process. In this context, in recent years some studies have focused on the development of safer delivery systems such as Sendai virus, Adenovirus, chemical cocktails or nanoparticles. This article provides an in-depth review of the in vitro and in vivo cardiac reprograming technology used in mouse and human cells to obtain iCMs and iCPCs, and discusses what challenges still lie ahead and what hurdles are to be overcome before results from this field can be transferred to the clinical settings.

Hox-dependent coordination of mouse cardiac progenitor cell patterning and differentiation

Perturbation of addition of second heart field (SHF) cardiac progenitor cells to the poles of the heart tube results in congenital heart defects (CHD). The transcriptional programs and upstream regulatory events operating in different subpopulations of the SHF remain unclear. Here, we profile the transcriptome and chromatin accessibility of anterior and posterior SHF sub-populations at genome-wide levels and demonstrate that Hoxb1 negatively regulates differentiation in the posterior SHF. Spatial mis-expression of Hoxb1 in the anterior SHF results in hypoplastic right ventricle. Activation of Hoxb1 in embryonic stem cells arrests cardiac differentiation, whereas Hoxb1-deficient mouse embryos display premature cardiac differentiation. Moreover, ectopic differentiation in the posterior SHF of embryos lacking both Hoxb1 and its paralog Hoxa1 results in atrioventricular septal defects. Our results show that Hoxb1 plays a key role in patterning cardiac progenitor cells that contribute to both cardiac poles and provide new insights into the pathogenesis of CHD.

Effects of PPAR-γ in the Myocardium on the Development of Ventricular Septation

Ventricular septum defects (VSDs) are common types of congenital heart diseases caused by developmental defect; they contribute to 25%-30% of all adult congenital heart diseases. The peroxisome proliferator-activated receptor gamma (PPAR-γ) is widely expressed in mammalian tissues and in the immune system, regulating cell differentiation and immune and inflammatory responses. The PPAR-γ gene has recently been found crucial for heart development, but the mechanism of action is not clear. This study aims to investigate the effects of the PPAR-γ gene in the myocardium on the development of ventricular septation. In this study, we applied Cre-loxP recombination enzyme (CRE) technology to downregulate the expression of the PPAR-γ gene in different cardiac tissues, RT-PCR to examine the expression of the c-fos and TGF-β1 genes, and histology staining to check the defect of embryonic heart at embryonic day 14.5 (E14.5). We found that the downregulation of the PPAR-γ gene resulted in a ventricular membranous septation defect of the embryonic heart at E14.5. Furthermore, only conversion of a Tnt:Cre, but not Mef2c:Cre, Tie2:Cre, or Wnt:Cre PPAR-γ floxed allele to a null allele resulted in VSD. PPAR-γ^Tnt-Cre/+ embryos showed increases in atrioventricular (AV)-cushion cells and the expression of c-fos gene but no change in the expression of TGF-β1 at E10.5. Our study demonstrates PPAR-γ in the myocardium is required for ventricular septation through regulation of AV-cushion cell proliferation by a Tnt/c-fos signal.

Comprehensive Overview of Non-coding RNAs in Cardiac Development

Cardiac development in the human embryo is characterized by the interactions of several transcription and growth factors leading the heart from a primordial linear tube into a synchronous contractile four-chamber organ. Studies on cardiogenesis showed that cell proliferation, differentiation, fate specification and morphogenesis are spatiotemporally coordinated by cell-cell interactions and intracellular signalling cross-talks. In recent years, research has focused on a class of inter- and intra-cellular modulators called non-coding RNAs (ncRNAs), transcribed from the noncoding portion of the DNA and involved in the proper formation of the heart. In this chapter, we will summarize the current state of the art on the roles of three major forms of ncRNAs [microRNAs (miRNAs), long ncRNAs (lncRNAs) and circular RNAs (circRNAs)] in orchestrating the four sequential phases of cardiac organogenesis.

A neuronal enhancer network upstream of MEF2C is compromised in patients with Rett-like characteristics

Mutations in myocyte enhancer factor 2C (MEF2C), an important transcription factor in neurodevelopment, are associated with a Rett-like syndrome. Structural variants (SVs) upstream of MEF2C, which do not disrupt the gene itself, have also been found in patients with a similar phenotype, suggesting that disruption of MEF2C regulatory elements can also cause a Rett-like phenotype. To characterize those elements that regulate MEF2C during neural development and that are affected by these SVs, we used genomic tools coupled with both in vitro and in vivo functional assays. Through circularized chromosome conformation capture sequencing (4C-seq) and the assay for transposase-accessible chromatin using sequencing (ATAC-seq), we revealed a complex interaction network in which the MEF2C promoter physically contacts several distal enhancers that are deleted or translocated by disease-associated SVs. A total of 16 selected candidate regulatory sequences were tested for enhancer activity in vitro, with 14 found to be functional enhancers. Further analyses of their in vivo activity in zebrafish showed that each of these enhancers has a distinct activity pattern during development, with eight enhancers displaying neuronal activity. In summary, our results disentangle a complex regulatory network governing neuronal MEF2C expression that involves multiple distal enhancers. In addition, the characterized neuronal enhancers pose as novel candidates to screen for mutations in neurodevelopmental disorders, such as Rett-like syndrome.

Effects of cigarette smoke components on myocardial differentiation of mouse embryonic stem cells

The heart is the first organ formed in the developing fetus, and abnormal development of the heart is a major cause of fetal death. The adverse effects of cigarette smoke on the heart have been well established, but it is not well understood how cigarette smoke components regulate signaling molecules and cardiac specific functions during the early differentiation stage of the embryonic heart. In this study, we identified changes in the size of mouse embryoid bodies (mEBs) in response to treatment with cigarette smoke extract (CSE) via regulation of HDAC2, p53, p21, and cyclin D1 protein expression, which are cardiac differentiation and cell-cycle markers, respectively. In addition, exposure of mouse embryonic stem cells (mESCs) to cigarette smoke components inhibited myocardial differentiation and development through the expression of HDAC1, HDAC2, GATA4, NKX2-5, TBX5, HAND1, and Troponin I. Long-term exposure studies showed that CSE and nicotine may delay the development of mouse cardiomyocytes from mESCs and inhibit the contractibility, which is a fundamental function of the heart. Taken together, these findings suggest that cigarette smoke components, including nicotine, may affect abnormal myocardial differentiation and development.

MiR-21, MiR-29a, GATA4, and MEF2c Expression Changes in Endothelin-1 and Angiotensin II Cardiac Hypertrophy Stimulated Isl-1+Sca-1+c-kit+ Porcine Cardiac Progenitor Cells In Vitro

Cost- and time-intensive porcine translational disease models offer great opportunities to test drugs and therapies for pathological cardiac hypertrophy and can be supported by porcine cell culture models that provide further insights into basic disease mechanisms. Cardiac progenitor cells (CPCs) residing in the adult heart have been shown to differentiate in vitro into cardiomyocytes and could contribute to cardiac regeneration. Therefore, it is important to evaluate their changes on the cellular level caused by disease. We successfully isolated Isl1⁺Sca1⁺cKit⁺ porcine CPCs (pCPCs) from pig hearts and stimulated them with endothelin-1 (ET-1) and angiotensin II (Ang II) in vitro. We also performed a cardiac reprogramming transfection and tested the same conditions. Our results show that undifferentiated Isl1⁺Sca1⁺cKit⁺ pCPCs were significantly upregulated in GATA4, MEF2c, and miR-29a gene expressions and in BNP and MCP-1 protein expressions with Ang II stimulation, but they showed no significant changes in miR-29a and MCP-1 when stimulated with ET-1. Differentiated Isl1⁺Sca1⁺cKit⁺ pCPCs exhibited significantly higher levels of MEF2c, GATA4, miR-29a, and miR-21 as well as Cx43 and BNP with Ang II stimulation. pMx-MGT-transfected Isl1⁺Sca1⁺cKit⁺ pCPCs showed significant elevations in MEF2c, GATA4, and BNP expressions when stimulated with ET-1. Our model demonstrates that in vitro stimulation leads to successful Isl1⁺Sca1⁺cKit⁺ pCPC hypertrophy with upregulation of cardiac remodeling associated genes and profibrotic miRNAs and offers great possibilities for further investigations of disease mechanisms and treatment.

The deployment of cell lineages that form the mammalian heart

The function of the mammalian heart depends on the interplay between different cardiac cell types. The deployment of these cells, with precise spatiotemporal regulation, is also important during development to establish the heart structure. In this Review, we discuss the diverse origins of cardiac cell types and the lineage relationships between cells of a given type that contribute to different parts of the heart. The emerging lineage tree shows the progression of cell fate diversification, with patterning cues preceding cell type segregation, as well as points of convergence, with overlapping lineages contributing to a given tissue. Several cell lineage markers have been identified. However, caution is required with genetic-tracing experiments in comparison with clonal analyses. Genetic studies on cell populations provided insights into the mechanisms for lineage decisions. In the past 3 years, results of single-cell transcriptomics are beginning to reveal cell heterogeneity and early developmental trajectories. Equating this information with the in vivo location of cells and their lineage history is a current challenge. Characterization of the progenitor cells that form the heart and of the gene regulatory networks that control their deployment is of major importance for understanding the origin of congenital heart malformations and for producing cardiac tissue for use in regenerative medicine.

Minor alleles of genetic variants in second heart field increase the risk of hypoplastic right heart syndrome

Hypoplastic right heart syndrome(HRHS) is characterized by hypoplastic right ventricle (RV); Numerous transcriptional cascades in the second heart field (SHF) regulate RVdevelopment. The relationship of SHF gene variants with human HRHS remains unknown. The whole lengths of 17 SHF genes were sequenced in 16 HRHS, and the selected single-nucleotide variants (SNVs) were then genotyped in HRHS, other congenital heart disease (CHD) and healthy control. Luciferase assay was performed to verify the effect of FOXC2: rs34221221A>GandTBX20: rs59854940C>Gat the transcription level. There were 151 (12.86%) novel SNVs after sequence analysis, of which three were in exons (one was synonymous SNV and two were nonsynonymous SNVs), two in promoter, and most SNVs (89.95%) were in intronic regions. Genotype analyses revealed that the minor alleles of FOXC2: rs34221221 A>G and TBX20: rs59854940 C>G could increase HRHS risk (P<0.05), but not in other CHD or healthy control. Luciferase assay showed that the minor G allele in rs34221221 significantly increased FOXC2 transcription while in rs59854940 it decreased TBX20 transcription significantly. Novel variants of SHF gene associated with HRHS were identified. Minor alleles in two variants from FOXC2 and TBX20 could increase the risk of HRHS.