Foxa2 identifies a cardiac progenitor population with ventricular differentiation potential

Early coordination of cell migration and cardiac fate determination during mammalian gastrulation

During gastrulation, mesodermal cells derived from distinct regions are destined to acquire specific cardiac fates after undergoing complex migratory movements. Here, we used light-sheet imaging of live mouse embryos between gastrulation and heart tube formation to track mesodermal cells and to reconstruct lineage trees and 3D migration paths for up to five cell divisions. We found independent progenitors emerging at specific times, contributing exclusively to left ventricle/atrioventricular canal (LV/AVC) or atrial myocytes. LV/AVC progenitors differentiated early to form the cardiac crescent, while atrial progenitors later generated the heart tube's Nr2f2+ inflow tract during morphogenesis. We also identified short-lived multipotent progenitors with broad potential, illustrating early developmental plasticity. Descendants of multipotent progenitors displayed greater dispersion and more diverse migratory trajectories within the anterior mesoderm than the progeny of uni-fated progenitors. Progenitors contributing to extraembryonic mesoderm (ExEm) exhibited the fastest and most dispersed migrations. In contrast, those giving rise to endocardial, LV/AVC, and pericardial cells showed a more gradual divergence, with late-stage behavioural shifts: endocardial cells increased in speed, while pericardial cells slowed down in comparison to LV/AVC cells. Together, these data reveal patterns of individual cell directionality and cardiac fate allocation within the seemingly unorganised migratory pattern of mesoderm cells.

HAND1 level controls the specification of multipotent cardiac and extraembryonic progenitors from human pluripotent stem cells

Diverse sets of progenitors contribute to the development of the embryonic heart, but the mechanisms of their specification have remained elusive. Here, using a human pluripotent stem cell (hPSC) model, we deciphered cardiac and non-cardiac lineage trajectories in differentiation and identified transcription factors underpinning cell specification, identity and function. We discovered a concentration-dependent, fate determining function for the basic helix-loop-helix transcription factor HAND1 in mesodermal progenitors and uncovered its gene regulatory network. At low expression level, HAND1 directs differentiation towards multipotent juxta-cardiac field progenitors able to make cardiomyocytes and epicardial cells, whereas at high level it promotes the development of extraembryonic mesoderm. Importantly, HAND1-low progenitors can be propagated in their multipotent state. This detailed mechanistic insight into human development has the potential to accelerate the delivery of effective disease modelling, including for congenital heart disease, and cell therapy-based regenerative medicine.

Synthetic organizer cells guide development via spatial and biochemical instructions

In vitro development relies primarily on treating progenitor cells with media-borne morphogens and thus lacks native-like spatial information. Here, we engineer morphogen-secreting organizer cells programmed to self-assemble, via cell adhesion, around mouse embryonic stem (ES) cells in defined architectures. By inducing the morphogen WNT3A and its antagonist DKK1 from organizer cells, we generated diverse morphogen gradients, varying in range and steepness. These gradients were strongly correlated with morphogenetic outcomes: the range of minimum-maximum WNT activity determined the resulting range of anterior-to-posterior (A-P) axis cell lineages. Strikingly, shallow WNT activity gradients, despite showing truncated A-P lineages, yielded higher-resolution tissue morphologies, such as a beating, chambered cardiac-like structure associated with an endothelial network. Thus, synthetic organizer cells, which integrate spatial, temporal, and biochemical information, provide a powerful way to systematically and flexibly direct the development of ES or other progenitor cells in different directions within the morphogenetic landscape.

Histone Demethylase PHF8 Confers Protection against Oxidative Stress and Cardiomyocyte Apoptosis in Heart Failure by Upregulating FOXA2

Oxidative stress and cardiomyocyte apoptosis are hallmarks of heart failure (HF) development. Plant homeodomain finger protein 8 (PHF8) is a histone demethylase downregulated in failing human hearts. Nevertheless, the potential role of PHF8 in HF remains unclear. Therefore, this study aimed to explore the biological action and molecular mechanism of PHF8 in HF.A rat model of left anterior descending coronary artery (LAD) ligation-induced HF and a cardiomyocyte model of oxygen-glucose deprivation/reperfusion (OGD/R) were developed after gain- or loss-of-function experiments in rats and cardiomyocytes, respectively. Heart function indexes, such as left ventricular end-diastolic diameter, left ventricular end-systolic diameter, left ventricular ejection fraction, and left ventricular fractional shortening, were detected. Changes in myocardial tissues were examined by pathological staining. Cardiomyocyte apoptosis and oxidative stress markers, such as malondialdehyde, reactive oxygen species, superoxide dismutase, and catalase, were examined. The relationship between PHF8 and forkhead box A2 (FOXA2) was analyzed by luciferase and chromatin immunoprecipitation-quantitative polymerase chain reaction assays.PHF8 was downregulated in LAD-ligated rats and OGD/R-exposed cardiomyocytes. Following PHF8 upregulation, pathological changes in myocardial tissues and heart dysfunction were improved in LAD-ligated rats. Importantly, cardiomyocyte apoptosis and oxidative stress were diminished in vivo and in vitro upon PHF8 upregulation. Mechanistically, PHF8 increased FOXA2 expression in a histone demethylase-dependent manner. FOXA2 silencing abrogated the protective effect of PHF8 upregulation on cardiomyocytes against OGD/R-induced apoptosis and oxidative stress.PHF8 exerts protective functions against cardiomyocyte apoptosis, oxidative stress, and heart dysfunction in HF, in correlation with FOXA2 upregulation. These results suggest that the PHF8/FOXA2 axis may be a promising therapeutic target to prevent HF.

A Robust Protocol for Pluripotent Stem Cell Modeling of 3D Chamber-Like Cardiac Organoids

The formation of cardiac organoids brings a new 3D model of cardiac development. The organoid is also very helpful in screening drug efficiency or toxicity. Inspired by cardiac biology of development, a step-by-step protocol is described to generate cardiac chamber-like organoids from pluripotent stem cells. A few examples of functional tests and the use of these organoids for drug toxicity are reported.

In vitro modelling of anterior primitive streak patterning with human pluripotent stem cells identifies the path to notochord progenitors

Notochord progenitors (NotoPs) represent a scarce yet crucial embryonic cell population, playing important roles in embryo patterning and eventually giving rise to the cells that form and maintain intervertebral discs. The mechanisms regulating NotoPs emergence are unclear. This knowledge gap persists due to the inherent complexity of cell fate patterning during gastrulation, particularly within the anterior primitive streak (APS), where NotoPs first arise alongside neuro-mesoderm and endoderm. To gain insights into this process, we use micropatterning together with FGF and the WNT pathway activator CHIR9901 to guide the development of human embryonic stem cells into reproducible patterns of APS cell fates. We show that CHIR9901 dosage dictates the downstream dynamics of endogenous TGFβ signalling, which in turn controls cell fate decisions. While sustained NODAL signalling defines endoderm and NODAL inhibition is imperative for neuro-mesoderm emergence, timely inhibition of NODAL signalling with spatial confinement potentiates WNT activity and enables us to generate NotoPs efficiently. Our work elucidates the signalling regimes underpinning NotoP emergence and provides insights into the regulatory mechanisms controlling the balance of APS cell fates during gastrulation.

Early autonomous patterning of the anteroposterior axis in gastruloids

Minimal in vitro systems composed of embryonic stem cells (ESCs) have been shown to recapitulate the establishment of the anteroposterior (AP) axis. In contrast to the native embryo, ESC aggregates - such as gastruloids - can break symmetry, which is demarcated by polarization of the mesodermal marker T, autonomously without any localized external cues. However, associated earliest patterning events, such as the spatial restriction of cell fates and concomitant transcriptional changes, remain poorly understood. Here, we dissect the dynamics of AP axis establishment in mouse gastruloids, particularly before external Wnt stimulation. Through single-cell RNA sequencing, we identify key cell state transitions and the molecular signatures of T+ and T- populations underpinning AP polarization. We also show that this process is robust to modifications of aggregate size. Finally, transcriptomic comparison with the mouse embryo indicates that gastruloids develop similar mesendodermal cell types, despite initial differences in their primed pluripotent populations, which adopt a more mesenchymal state in lieu of an epiblast-like transcriptome. Hence, our findings suggest the possibility of alternate ESC states in vivo and in vitro that can converge onto similar cell fates.

Lineage tracing using Wnt2b-2A-CreERT2 knock-in mice reveals the contributions of Wnt2b-expressing cells to novel subpopulations of mesothelial/epicardial cell lineages during mouse development

Mesothelial and epicardial cells give rise to various types of mesenchymal cells via epithelial (mesothelial)-to-mesenchymal transition during development. However, the genes controlling the differentiation and diversification of mesothelial/epicardial cells remain unclear. Here, we examined Wnt2b expression in the embryonic mesothelium and epicardium and performed lineage tracing of Wnt2b-expressing cells by using novel Wnt2b-2A-CreERT2 knock-in and LacZ-reporter mice. Wnt2b was expressed in mesothelial cells covering visceral organs, but the expression was restricted in their subpopulations. Wnt2b-expressing cells labeled at embryonic day (E) 10.5 were distributed to the mesothelium and mesenchyme in the lungs, abdominal wall, stomach, and spleen in Wnt2b2A-CreERT2/+;R26RLacZ/+ mice at E13.0. Wnt2b was initially expressed in the proepicardial organ (PEO) at E9.5 and then in the epicardium after E10.0. Wnt2b-expressing PEO cells labeled at E9.5 differentiated into a small fraction of cardiac fibroblasts and preferentially localized at the left side of the postnatal heart. LacZ+ epicardium-derived cells labeled at E10.5 differentiated into a small fraction of fibroblasts and smooth muscle cells in the postnatal heart. Taken together, our results reveal novel subpopulations of PEO and mesothelial/epicardial cells that are distinguishable by Wnt2b expression and elucidate the unique contribution of Wnt2b-expressing PEO and epicardial cells to the postnatal heart.

Embryonic Stem Cell Differentiation to Definitive Endoderm As a Model of Heterogeneity Onset During Germ Layer Specification

Embryonic stem cells (ESCs) hold great promise for regenerative medicine thanks to their ability to self-renew and differentiate into somatic cells and the germline. ESCs correspond to pluripotent epiblast - the tissue from which the following three germ layers originate during embryonic gastrulation: the ectoderm, mesoderm, and endoderm. Importantly, ESCs can be induced to differentiate toward various cell types by varying culture conditions, which can be exploited for in vitro modeling of developmental processes such as gastrulation. The classical model of gastrulation postulates that mesoderm and endoderm specification is made possible through the FGF-, BMP-, Wnt-, and Nodal-signaling gradients. Hence, it can be expected that one of these signals should direct ESC differentiation towards specific germ layers. However, ESC specification appears to be more complicated, and the same signal can be interpreted differently depending on the readout. In this research, using chemically defined culture conditions, homogeneous naïve ESCs as a starting cell population, and the Foxa2 gene-driven EGFP reporter tool, we established a robust model of definitive endoderm (DE) specification. This in vitro model features formative pluripotency as an intermediate state acquired by the epiblast in vivo shortly after implantation. Despite the initially homogeneous state of the cells in the model and high Activin concentration during endodermal specification, there remains a cell subpopulation that does not reach the endodermal state. This simple model developed by us can be used to study the origins of cellular heterogeneity during germ layer specification.

Transient stabilization of human cardiovascular progenitor cells from human pluripotent stem cells in vitro reflects stage-specific heart development in vivo

AIMS

Understanding the molecular identity of human pluripotent stem cell (hPSC)-derived cardiac progenitors and mechanisms controlling their proliferation and differentiation is valuable for developmental biology and regenerative medicine.

METHODS AND RESULTS

Here, we show that chemical modulation of histone acetyl transferases (by IQ-1) and WNT (by CHIR99021) synergistically enables the transient and reversible block of directed cardiac differentiation progression on hPSCs. The resulting stabilized cardiovascular progenitors (SCPs) are characterized by ISL1pos/KI-67pos/NKX2-5neg expression. In the presence of the chemical inhibitors, SCPs maintain a proliferation quiescent state. Upon small molecules, removal SCPs resume proliferation and concomitant NKX2-5 up-regulation triggers cell-autonomous differentiation into cardiomyocytes. Directed differentiation of SCPs into the endothelial and smooth muscle lineages confirms their full developmental potential typical of bona fide cardiovascular progenitors. Single-cell RNA-sequencing-based transcriptional profiling of our in vitro generated human SCPs notably reflects the dynamic cellular composition of E8.25-E9.25 posterior second heart field of mouse hearts, hallmarked by nuclear receptor sub-family 2 group F member 2 expression. Investigating molecular mechanisms of SCP stabilization, we found that the cell-autonomously regulated retinoic acid and BMP signalling is governing SCP transition from quiescence towards proliferation and cell-autonomous differentiation, reminiscent of a niche-like behaviour.

CONCLUSION

The chemically defined and reversible nature of our stabilization approach provides an unprecedented opportunity to dissect mechanisms of cardiovascular progenitors' specification and reveal their cellular and molecular properties.

Cardiac construction-Recent advances in morphological and transcriptional modeling of early heart development

During human embryonic development the early establishment of a functional heart is vital to support the growing fetus. However, forming the embryonic heart is an extremely complex process, requiring spatiotemporally controlled cell specification and differentiation, tissue organization, and coordination of cardiac function. These complexities, in concert with the early and rapid development of the embryonic heart, mean that understanding the intricate interplay between these processes that help shape the early heart remains highly challenging. In this review I focus on recent insights from animal models that have shed new light on the earliest stages of heart development. This includes specification and organization of cardiac progenitors, cell and tissue movements that make and shape the early heart tube, and the initiation of the first beat in the developing heart. In addition I highlight relevant in vitro models that could support translation of findings from animal models to human heart development. Finally I discuss challenges that are being addressed in the field, along with future considerations that together may help move us towards a deeper understanding of how our hearts are made.

In vitro and in vivo models define a molecular signature reference for human embryonic notochordal cells

Understanding the emergence of human notochordal cells (NC) is essential for the development of regenerative approaches. We present a comprehensive investigation into the specification and generation of bona fide NC using a straightforward pluripotent stem cell (PSC)-based system benchmarked with human fetal notochord. By integrating in vitro and in vivo transcriptomic data at single-cell resolution, we establish an extended molecular signature and overcome the limitations associated with studying human notochordal lineage at early developmental stages. We show that TGF-β inhibition enhances the yield and homogeneity of notochordal lineage commitment in vitro. Furthermore, this study characterizes regulators of cell-fate decision and matrisome enriched in the notochordal niche. Importantly, we identify specific cell-surface markers opening avenues for differentiation refinement, NC purification, and functional studies. Altogether, this study provides a human notochord transcriptomic reference that will serve as a resource for notochord identification in human systems, diseased-tissues modeling, and facilitating future biomedical research.

The MEK-ERK signaling pathway promotes maintenance of cardiac chamber identity

Ventricular and atrial cardiac chambers have unique structural and contractile characteristics that underlie their distinct functions. The maintenance of chamber-specific features requires active reinforcement, even in differentiated cardiomyocytes. Previous studies in zebrafish have shown that sustained FGF signaling acts upstream of Nkx factors to maintain ventricular identity, but the rest of this maintenance pathway remains unclear. Here, we show that MEK1/2-ERK1/2 signaling acts downstream of FGF and upstream of Nkx factors to promote ventricular maintenance. Inhibition of MEK signaling, like inhibition of FGF signaling, results in ectopic atrial gene expression and reduced ventricular gene expression in ventricular cardiomyocytes. FGF and MEK signaling both influence ventricular maintenance over a similar timeframe, when phosphorylated ERK (pERK) is present in the myocardium. However, the role of FGF-MEK activity appears to be context-dependent: some ventricular regions are more sensitive than others to inhibition of FGF-MEK signaling. Additionally, in the atrium, although endogenous pERK does not induce ventricular traits, heightened MEK signaling can provoke ectopic ventricular gene expression. Together, our data reveal chamber-specific roles of MEK-ERK signaling in the maintenance of ventricular and atrial identities.

Generation, characterization, and use of EKLF(Klf1)/CRE knock-in mice for cell-restricted analyses

Introduction

EKLF/Klf1 is a tissue-restricted transcription factor that plays a critical role in all aspects of erythropoiesis. Of particular note is its tissue-restricted pattern of expression, a property that could prove useful for expression control of a linked marker or enzymatic gene.

Methods and results

With this in mind, we fused the CRE recombinase to the genomic EKLF coding region and established mouse lines. We find by FACS analyses that CRE expression driven by the EKLF transcription unit recapitulates erythroid-restricted expression with high penetrance in developing embryos. We then used this line to test its properties in the adult, where we found EKLF/CRE is an active and is a robust mimic of normal EKLF expression in the adult bone marrow. EKLF/CRE is also expressed in erythroblastic island macrophage in the fetal liver, and we demonstrate for the first time that, as seen during embryonic development, EKLF is also expressed in adult BM-derived erythroblastic island macrophage. Our data also support lineage studies showing EKLF expression at early stages of hematopoiesis.

Discussion

The EKLF/CRE mouse lines are novel reagents whose availability will be of great utility for future experiments by investigators in the red cell field.

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.

Age-associated changes in lineage composition of the enteric nervous system regulate gut health and disease

The enteric nervous system (ENS), a collection of neural cells contained in the wall of the gut, is of fundamental importance to gastrointestinal and systemic health. According to the prevailing paradigm, the ENS arises from progenitor cells migrating from the neural crest and remains largely unchanged thereafter. Here, we show that the lineage composition of maturing ENS changes with time, with a decline in the canonical lineage of neural-crest derived neurons and their replacement by a newly identified lineage of mesoderm-derived neurons. Single cell transcriptomics and immunochemical approaches establish a distinct expression profile of mesoderm-derived neurons. The dynamic balance between the proportions of neurons from these two different lineages in the post-natal gut is dependent on the availability of their respective trophic signals, GDNF-RET and HGF-MET. With increasing age, the mesoderm-derived neurons become the dominant form of neurons in the ENS, a change associated with significant functional effects on intestinal motility which can be reversed by GDNF supplementation. Transcriptomic analyses of human gut tissues show reduced GDNF-RET signaling in patients with intestinal dysmotility which is associated with reduction in neural crest-derived neuronal markers and concomitant increase in transcriptional patterns specific to mesoderm-derived neurons. Normal intestinal function in the adult gastrointestinal tract therefore appears to require an optimal balance between these two distinct lineages within the ENS.

Gene editing and cardiac disease modelling for the interpretation of genetic variants of uncertain significance in congenital heart disease

BACKGROUND

Genomic sequencing in congenital heart disease (CHD) patients often discovers novel genetic variants, which are classified as variants of uncertain significance (VUS). Functional analysis of each VUS is required in specialised laboratories, to determine whether the VUS is disease causative or not, leading to lengthy diagnostic delays. We investigated stem cell cardiac disease modelling and transcriptomics for the purpose of genetic variant classification using a GATA4 (p.Arg283Cys) VUS in a patient with CHD.

METHODS

We performed high efficiency CRISPR gene editing with homology directed repair in induced pluripotent stem cells (iPSCs), followed by rapid clonal selection with amplicon sequencing. Genetic variant and healthy matched control cells were compared using cardiomyocyte disease modelling and transcriptomics.

RESULTS

Genetic variant and healthy cardiomyocytes similarly expressed Troponin T (cTNNT), and GATA4. Transcriptomics analysis of cardiomyocyte differentiation identified changes consistent with the patient's clinical human phenotype ontology terms. Further, transcriptomics revealed changes in calcium signalling, and cardiomyocyte adrenergic signalling in the variant cells. Functional testing demonstrated, altered action potentials in GATA4 genetic variant cardiomyocytes were consistent with patient cardiac abnormalities.

CONCLUSIONS

This work provides in vivo functional studies supportive of a damaging effect on the gene or gene product. Furthermore, we demonstrate the utility of iPSCs, CRISPR gene editing and cardiac disease modelling for genetic variant interpretation. The method can readily be applied to other genetic variants in GATA4 or other genes in cardiac disease, providing a centralised assessment pathway for patient genetic variant interpretation.

FOXA2 Suppression by TRIM36 Exerts Anti-Tumor Role in Colorectal Cancer Via Inducing NRF2/GPX4-Regulated Ferroptosis

The forkhead box transcription factor A2 (FOXA2) is a transcription factor and plays a key role in embryonic development, metabolism homeostasis and tumor cell proliferation; however, its regulatory potential in CRC is not fully understood. Here, it is found that FOXA2 expression is markedly up-regulated in tumor samples of CRC patients as compared with the normal tissues, which is closely associated with the worse survival in patients with CRC. Notably, a positive correlation between FOXA2 and nuclear factor erythroid 2-related factor 2 (Nrf2)/glutathione peroxidase 4 (GPX4) gene expression is observed in CRC patients. Mechanistically, FOXA2 depletion weakens the activation of Nrf2 pathway and decreases GPX4 level in CRC cells, thereby leading to ferroptosis, which is further supported by bioinformatic analysis. More intriguingly, the E3 ubiquitin ligase tripartite motif containing 36 (TRIM36) is identified as a key suppressor of FOXA2, and it is observed that TRIM36 can directly interact with FOXA2 and induce its K48-linked polyubiquitination, resulting in FOXA2 protein degradation in vitro. Taken together, all the studies demonstrate that FOXA2 mediated by TRIM36 promotes CRC progression by inhibiting the Nrf2/GPX4 ferroptosis signaling pathway, thus providing a new therapeutic target for CRC treatment.

Conditional blastocyst complementation of a defective Foxa2 lineage efficiently promotes the generation of the whole lung

Millions suffer from incurable lung diseases, and the donor lung shortage hampers organ transplants. Generating the whole organ in conjunction with the thymus is a significant milestone for organ transplantation because the thymus is the central organ to educate immune cells. Using lineage-tracing mice and human pluripotent stem cell (PSC)-derived lung-directed differentiation, we revealed that gastrulating Foxa2 lineage contributed to both lung mesenchyme and epithelium formation. Interestingly, Foxa2 lineage-derived cells in the lung mesenchyme progressively increased and occupied more than half of the mesenchyme niche, including endothelial cells, during lung development. Foxa2 promoter-driven, conditional Fgfr2 gene depletion caused the lung and thymus agenesis phenotype in mice. Wild-type donor mouse PSCs injected into their blastocysts rescued this phenotype by complementing the Fgfr2-defective niche in the lung epithelium and mesenchyme and thymic epithelium. Donor cell is shown to replace the entire lung epithelial and robust mesenchymal niche during lung development, efficiently complementing the nearly entire lung niche. Importantly, those mice survived until adulthood with normal lung function. These results suggest that our Foxa2 lineage-based model is unique for the progressive mobilization of donor cells into both epithelial and mesenchymal lung niches and thymus generation, which can provide critical insights into studying lung transplantation post-transplantation shortly.

Hey2 enhancer activity defines unipotent progenitors for left ventricular cardiomyocytes in juxta-cardiac field of early mouse embryo

The cardiac crescent is the first structure of the heart and contains progenitor cells of the first heart field, which primarily differentiate into left ventricular cardiomyocytes. The interface between the forming cardiac crescent and extraembryonic tissue is known as the juxta-cardiac field (JCF), and progenitor cells in this heart field contribute to the myocardium of the left ventricle and atrioventricular canal as well as the epicardium. However, it is unclear whether there are progenitor cells that differentiate specifically into left ventricular cardiomyocytes. We have previously demonstrated that an enhancer of the gene encoding the Hey2 bHLH transcriptional repressor is activated in the ventricular myocardium during mouse embryonic development. In this study, we aimed to investigate the characteristics of cardiomyocyte progenitor cells and their cell lineages by analyzing Hey2 enhancer activity at the earliest stages of heart formation. We found that the Hey2 enhancer initiated its activity prior to cardiomyocyte differentiation within the JCF. Hey2 enhancer-active cells were present rostrally to the Tbx5-expressing region at the early phase of cardiac crescent formation and differentiated exclusively into left ventricular cardiomyocytes in a lineage distinct from the Tbx5-positive lineage. By the late phase of cardiac crescent formation, Hey2 enhancer activity became significantly overlapped with Tbx5 expression in cells that contribute to the left ventricular myocardium. Our study reveals that a population of unipotent progenitor cells for left ventricular cardiomyocytes emerge in the JCF, providing further insight into the mode of cell type diversification during early cardiac development.

Charting the Path: Navigating Embryonic Development to Potentially Safeguard against Congenital Heart Defects

Congenital heart diseases (CHDs) are structural or functional defects present at birth due to improper heart development. Current therapeutic approaches to treating severe CHDs are primarily palliative surgical interventions during the peri- or prenatal stages, when the heart has fully developed from faulty embryogenesis. However, earlier interventions during embryonic development have the potential for better outcomes, as demonstrated by fetal cardiac interventions performed in utero, which have shown improved neonatal and prenatal survival rates, as well as reduced lifelong morbidity. Extensive research on heart development has identified key steps, cellular players, and the intricate network of signaling pathways and transcription factors governing cardiogenesis. Additionally, some reports have indicated that certain adverse genetic and environmental conditions leading to heart malformations and embryonic death may be amendable through the activation of alternative mechanisms. This review first highlights key molecular and cellular processes involved in heart development. Subsequently, it explores the potential for future therapeutic strategies, targeting early embryonic stages, to prevent CHDs, through the delivery of biomolecules or exosomes to compensate for faulty cardiogenic mechanisms. Implementing such non-surgical interventions during early gestation may offer a prophylactic approach toward reducing the occurrence and severity of CHDs.

Harnessing developmental cues for cardiomyocyte production

Developmental research has attempted to untangle the exact signals that control heart growth and size, with knockout studies in mice identifying pivotal roles for Wnt and Hippo signaling during embryonic and fetal heart growth. Despite this improved understanding, no clinically relevant therapies are yet available to compensate for the loss of functional adult myocardium and the absence of mature cardiomyocyte renewal that underlies cardiomyopathies of multiple origins. It remains of great interest to understand which mechanisms are responsible for the decline in proliferation in adult hearts and to elucidate new strategies for the stimulation of cardiac regeneration. Multiple signaling pathways have been identified that regulate the proliferation of cardiomyocytes in the embryonic heart and appear to be upregulated in postnatal injured hearts. In this Review, we highlight the interaction of signaling pathways in heart development and discuss how this knowledge has been translated into current technologies for cardiomyocyte production.

Regulating the cell shift of endothelial cell-like myofibroblasts in pulmonary fibrosis

Pulmonary fibrosis is a common and severe fibrotic lung disease with high morbidity and mortality. Recent studies have reported a large number of unwanted myofibroblasts appearing in pulmonary fibrosis, and shown that the sustained activation of myofibroblasts is essential for unremitting interstitial fibrogenesis. However, the origin of these myofibroblasts remains poorly understood. Here, we create new mouse models of pulmonary fibrosis and identify a previously unknown population of endothelial cell (EC)-like myofibroblasts in normal lung tissue. We show that these EC-like myofibroblasts significantly contribute myofibroblasts to pulmonary fibrosis, which is confirmed by single-cell RNA sequencing of human pulmonary fibrosis. Using the transcriptional profiles, we identified a small molecule that redirects the differentiation of EC-like myofibroblasts and reduces pulmonary fibrosis in our mouse models. Our study reveals the mechanistic underpinnings of the differentiation of EC-like myofibroblasts in pulmonary fibrosis and may provide new strategies for therapeutic interventions.

Generation of left ventricle-like cardiomyocytes with improved structural, functional, and metabolic maturity from human pluripotent stem cells

Decreased left ventricle (LV) function caused by genetic mutations or injury often leads to debilitating and fatal cardiovascular disease. LV cardiomyocytes are, therefore, a potentially valuable therapeutical target. Human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) are neither homogeneous nor functionally mature, which reduces their utility. Here, we exploit cardiac development knowledge to instruct differentiation of hPSCs specifically toward LV cardiomyocytes. Correct mesoderm patterning and retinoic acid pathway blocking are essential to generate near-homogenous LV-specific hPSC-CMs (hPSC-LV-CMs). These cells transit via first heart field progenitors and display typical ventricular action potentials. Importantly, hPSC-LV-CMs exhibit increased metabolism, reduced proliferation, and improved cytoarchitecture and functional maturity compared with age-matched cardiomyocytes generated using the standard WNT-ON/WNT-OFF protocol. Similarly, engineered heart tissues made from hPSC-LV-CMs are better organized, produce higher force, and beat more slowly but can be paced to physiological levels. Together, we show that functionally matured hPSC-LV-CMs can be obtained rapidly without exposure to current maturation regimes.

An atlas of lamina-associated chromatin across twelve human cell types reveals an intermediate chromatin subtype

BACKGROUND

Association of chromatin with lamin proteins at the nuclear periphery has emerged as a potential mechanism to coordinate cell type-specific gene expression and maintain cellular identity via gene silencing. Unlike many histone modifications and chromatin-associated proteins, lamina-associated domains (LADs) are mapped genome-wide in relatively few genetically normal human cell types, which limits our understanding of the role peripheral chromatin plays in development and disease.

RESULTS

To address this gap, we map LAMIN B1 occupancy across twelve human cell types encompassing pluripotent stem cells, intermediate progenitors, and differentiated cells from all three germ layers. Integrative analyses of this atlas with gene expression and repressive histone modification maps reveal that lamina-associated chromatin in all twelve cell types is organized into at least two subtypes defined by differences in LAMIN B1 occupancy, gene expression, chromatin accessibility, transposable elements, replication timing, and radial positioning. Imaging of fluorescently labeled DNA in single cells validates these subtypes and shows radial positioning of LADs with higher LAMIN B1 occupancy and heterochromatic histone modifications primarily embedded within the lamina. In contrast, the second subtype of lamina-associated chromatin is relatively gene dense, accessible, dynamic across development, and positioned adjacent to the lamina. Most genes gain or lose LAMIN B1 occupancy consistent with cell types along developmental trajectories; however, we also identify examples where the enhancer, but not the gene body and promoter, changes LAD state.

CONCLUSIONS

Altogether, this atlas represents the largest resource to date for peripheral chromatin organization studies and reveals an intermediate chromatin subtype.

Platr4 is an early embryonic lncRNA that exerts its function downstream on cardiogenic mesodermal lineage commitment

The mammalian genome encodes thousands of long non-coding RNAs (lncRNAs), many of which are developmentally regulated and differentially expressed across tissues, suggesting their potential roles in cellular differentiation. Despite this expression pattern, little is known about how lncRNAs influence lineage commitment at the molecular level. Here, we demonstrate that perturbation of an embryonic stem cell/early embryonic lncRNA, pluripotency-associated transcript 4 (Platr4), directly influences the specification of cardiac-mesoderm-lineage differentiation. We show that Platr4 acts as a molecular scaffold or chaperone interacting with the Hippo-signaling pathway molecules Yap and Tead4 to regulate the expression of a downstream target gene, Ctgf, which is crucial to the cardiac-lineage program. Importantly, Platr4 knockout mice exhibit myocardial atrophy and valve mucinous degeneration, which are both associated with reduced cardiac output and sudden heart failure. Together, our findings provide evidence that Platr4 is required in cardiac-lineage specification and adult heart function in mice.

Cardiac Development in the Presence of Cadmium: An in Vitro Study Using Human Embryonic Stem Cells and Cardiac Organoids

BACKGROUND

Exposure to cadmium (Cd) is associated with cardiovascular diseases. Maternal Cd exposure is a significant risk factor for congenital heart disease. However, mechanisms of Cd on developmental cardiotoxicity are not well defined.

OBJECTIVES

We evaluated the effects of Cd on the different stages (mesoderm, cardiac induction, cardiac function) of cardiac development using an early embryo development in vitro model and two- or three-dimensional (2- or 3D) cardiomyocyte and cardiac organoid formation models mimicking early cardiac development.

METHODS

Embryonic stem cells (ESCs) form 3D aggregates, called embryoid bodies, that recapitulate events involved with early embryogenesis (e.g., germ layer formation). This model was used for early germ layer formation and signaling pathway identification. The 2D cardiomyocyte differentiation from the NKX2-5eGFP/w human ESCs model was used to explore the effects of Cd exposure on cardiomyocyte formation and to model mesoderm differentiation and cardiac induction, allowing us to explore different developmental windows of Cd toxicity. The 3D cardiac organoid model was used in evaluating the effects of Cd exposure on contractility and cardiac development.

RESULTS

Cd (0.6μM; 110 ppb) lowered the differentiation of embryoid bodies to mesoderm via suppression of Wnt/β-catenin-signaling pathways. During early mesoderm induction, the mesoderm-associated transcription factors MESP1 and EOMES showed a transient up-regulation, which decreased later in the cardiac induction stage. Cd (0.15μM) lowered mesoderm formation and cardiac induction through suppression of the transcription factors and mesoderm marker genes HAND1, SNAI2, HOPX, and the cardiac-specific genes NKX2-5, GATA4, troponin T, and alpha-actinin. In addition, Cd-induced histone modifications for both gene activation (H3K4me3) and repression (H3K27me3), which play vital roles in regulating mesoderm commitment markers. The effects of Cd inhibition on cardiomyocyte differentiation were confirmed in 3D cardiac organoids.

DISCUSSION

In conclusion, using a human ESC-derived 2D/3D in vitro differentiation model system and cardiac organoids, we demonstrated that low-dose Cd suppressed mesoderm formation through mesoderm gene histone modification, thus inhibiting cardiomyocyte differentiation and cardiac induction. The studies provide valuable insights into cellular events and molecular mechanisms associated with Cd-induced congenital heart disease. https://doi.org/10.1289/EHP11208.

Human-gained heart enhancers are associated with species-specific cardiac attributes

The heart, a vital organ which is first to develop, has adapted its size, structure and function in order to accommodate the circulatory demands for a broad range of animals. Although heart development is controlled by a relatively conserved network of transcriptional/chromatin regulators, how the human heart has evolved species-specific features to maintain adequate cardiac output and function remains to be defined. Here, we show through comparative epigenomic analysis the identification of enhancers and promoters that have gained activity in humans during cardiogenesis. These cis-regulatory elements (CREs) are associated with genes involved in heart development and function, and may account for species-specific differences between human and mouse hearts. Supporting these findings, genetic variants that are associated with human cardiac phenotypic/disease traits, particularly those differing between human and mouse, are enriched in human-gained CREs. During early stages of human cardiogenesis, these CREs are also gained within genomic loci of transcriptional regulators, potentially expanding their role in human heart development. In particular, we discovered that gained enhancers in the locus of the early human developmental regulator ZIC3 are selectively accessible within a subpopulation of mesoderm cells which exhibits cardiogenic potential, thus possibly extending the function of ZIC3 beyond its conserved left-right asymmetry role. Genetic deletion of these enhancers identified a human gained enhancer that was required for not only ZIC3 and early cardiac gene expression at the mesoderm stage but also cardiomyocyte differentiation. Overall, our results illuminate how human gained CREs may contribute to human-specific cardiac attributes, and provide insight into how transcriptional regulators may gain cardiac developmental roles through the evolutionary acquisition of enhancers.

Dissecting mechanisms of chamber-specific cardiac differentiation and its perturbation following retinoic acid exposure

The specification of distinct cardiac lineages occurs before chamber formation and acquisition of bona fide atrial or ventricular identity. However, the mechanisms underlying these early specification events remain poorly understood. Here, we performed single cell analysis at the murine cardiac crescent, primitive heart tube and heart tube stages to uncover the transcriptional mechanisms underlying formation of atrial and ventricular cells. We find that progression towards differentiated cardiomyocytes occurs primarily based on heart field progenitor identity, and that progenitors contribute to ventricular or atrial identity through distinct differentiation mechanisms. We identify new candidate markers that define such differentiation processes and examine their expression dynamics using computational lineage trajectory methods. We further show that exposure to exogenous retinoic acid causes defects in ventricular chamber size, dysregulation in FGF signaling and a shunt in differentiation towards orthogonal lineages. Retinoic acid also causes defects in cell-cycle exit resulting in formation of hypomorphic ventricles. Collectively, our data identify, at a single cell level, distinct lineage trajectories during cardiac specification and differentiation, and the precise effects of manipulating cardiac progenitor patterning via retinoic acid signaling.

The circRNA CNEACR regulates necroptosis of cardiomyocytes through Foxa2 suppression

Circular RNAs (circRNAs) are differentially expressed in various cardiovascular disease including myocardial ischemia-reperfusion (I/R) injury. However, their functional impact on cardiomyocyte cell death, in particular, in necrotic forms of death remains elusive. In this study, we found that the level of mmu_circ_000338, a cardiac- necroptosis-associated circRNA (CNEACR), was reduced in hypoxia-reoxygenation (H/R) exposed cardiomyocytes and I/R-injured mice hearts. The enforced expression of CNEACR attenuated the necrotic form of cardiomyocyte death caused by H/R and suppressed of myocardial necrosis in I/R injured mouse heart, which was accompanied by a marked reduction of myocardial infarction size and improved cardiac function. Mechanistically, CNEACR directly binds to histone deacetylase (HDAC7) in the cytoplasm and interferes its nuclear entry. This leads to attenuation of HDAC7-dependent suppression of forkhead box protein A2 (Foxa2) transcription, which can repress receptor-interacting protein kinase 3 (Ripk3) gene by binding to its promoter region. In addition, CNEACR-mediated upregulation of FOXA2 inhibited RIPK3-dependent necrotic/necroptotic death of cardiomyocytes. Our study reveals that circRNAs such as CNEACR can regulate the cardiomyocyte necroptosis associated activity of HDACs, promotes cell survival and improves cardiac function in I/R-injured heart. Hence, the CNEACR/HDAC7/Foxa2/ RIPK3 axis could be an efficient target for alleviating myocardial damage caused by necroptotic death in ischemia heart diseases.

Cardiac specification during gastrulation - The Yellow Brick Road leading to Tinman

The question of how the heart develops, and the genetic networks governing this process have become intense areas of research over the past several decades. This research is propelled by classical developmental studies and potential clinical applications to understand and treat congenital conditions in which cardiac development is disrupted. Discovery of the tinman gene in Drosophila, and examination of its vertebrate homolog Nkx2.5, along with other core cardiac transcription factors has revealed how cardiac progenitor differentiation and maturation drives heart development. Careful observation of cardiac morphogenesis along with lineage tracing approaches indicated that cardiac progenitors can be divided into two broad classes of cells, namely the first and second heart fields, that contribute to the heart in two distinct waves of differentiation. Ample evidence suggests that the fate of individual cardiac progenitors is restricted to distinct cardiac structures quite early in development, well before the expression of canonical cardiac progenitor markers like Nkx2.5. Here we review the initial specification of cardiac progenitors, discuss evidence for the early patterning of cardiac progenitors during gastrulation, and consider how early gene expression programs and epigenetic patterns can direct their development. A complete understanding of when and how the developmental potential of cardiac progenitors is determined, and their potential plasticity, is of great interest developmentally and also has important implications for both the study of congenital heart disease and therapeutic approaches based on cardiac stem cell programming.

Change of gene expression profiles in human cardiomyocytes and macrophages infected with SARSCoV2 and its significance

OBJECTIVES

Coronavirus disease 2019 (COVID-19) is an acute respiratory infectious disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). SARS-CoV-2 can damage the myocardium directly, or activate the immune system, trigger a cytokine storm, and cause inflammatory cells to infiltrate the myocardial tissue and damage the myocardium. This study is based on the sequencing data to analyze the changes in gene expression of cardiomyocytes and macrophages after SARS-CoV-2 infection, and explore the potential effects of SARS-CoV-2 on the heart and immune system.

METHODS

The public data set GSE151879 was retrieved. The online software Network Analyst was used to preprocess the data, and the differentially expressed genes (DEGs) [log2(fold change)>2, adjusted P-value<0.05] screening between the infection group and the control group in cardiomyocytes, human embryonic stem cell-derived cardiomyocytes, and macrophages were screened. Consistent common differentially expressed genes (CCDEGs) with the same expression pattern in cardiomyocytes and macrophages were obtained, and the online analysis software String was used to conduct enrichment analysis of their biological functions and signal pathways. Protein-protein interaction network, transcription factor-gene interaction network, miRNA-gene interaction network and environmental chemical-gene interaction network were established, and Cytoscape 3.72 was used to perform visualization.

RESULTS

After data standardization, the data quality was excellent and it can ensure reliable results. Myocardial cell infection with SARS-CoV-2 and gene expression spectrum were changed significantly, including a total of 484 DEGs in adult cardiomyoblasts, a total of 667 DEGs in macrophages, and a total of 1 483 DEGs in human embryo source of cardiomyopathy. The Stum, mechanosensory transduction mediator homolog (STUM), dehydrogenase/reductase 9 (DHRS9), calcium/calmodulin dependent protein kinase II beta (CAMK2B), claudin 1(CLDN1), C-C motif chemokine ligand 2 (CCL2), TNFAIP3 interacting protein 3 (TNIP3), G protein-coupled receptor 84 (GPR84), and C-X-C motif chemokine ligand 1 (CXCL1) were identical in expression patterns in 3 types of cells. The protein-protein interaction suggested that CAMK2B proteins may play a key role in the antiviral process in 3 types of cells; and silicon dioxide (SiO2), benzodiazepine (BaP), nickel (Ni), and estradiol (E2) affect anti-SARS-CoV-2 processes of the 3 types of cells.

CONCLUSIONS

CAMK2B, CLDN1, CCL2, and DHRS9 genes play important roles in the immune response of cardiomyocytes against SARS-CoV-2. SiO2, BaP, Ni, E2 may affect the cell's antiviral process by increasing the toxicity of cardiomyocytes, thereby aggravating SARS-CoV-2 harm to the heart.

Emergence of heart and branchiomeric muscles in cardiopharyngeal mesoderm

Branchiomeric muscles of the head and neck originate in a population of cranial mesoderm termed cardiopharyngeal mesoderm that also contains progenitor cells contributing to growth of the embryonic heart. Retrospective lineage analysis has shown that branchiomeric muscles share a clonal origin with parts of the heart, indicating the presence of common heart and head muscle progenitor cells in the early embryo. Genetic lineage tracing and functional studies in the mouse, as well as in Ciona and zebrafish, together with recent experiments using single cell transcriptomics and multipotent stem cells, have provided further support for the existence of bipotent head and heart muscle progenitor cells. Current challenges concern defining where and when such common progenitor cells exist in mammalian embryos and how alternative myogenic derivatives emerge in cardiopharyngeal mesoderm. Addressing these questions will provide insights into mechanisms of cell fate acquisition and the evolution of vertebrate musculature, as well as clinical insights into the origins of muscle restricted myopathies and congenital defects affecting craniofacial and cardiac development.

Retinoic acid signaling in heart development: Application in the differentiation of cardiovascular lineages from human pluripotent stem cells

Retinoic acid (RA) signaling plays an important role during heart development in establishing anteroposterior polarity, formation of inflow and outflow tract progenitors, and growth of the ventricular compact wall. RA is also utilized as a key ingredient in protocols designed for generating cardiac cell types from pluripotent stem cells (PSCs). This review discusses the role of RA in cardiogenesis, currently available protocols that employ RA for differentiation of various cardiovascular lineages, and plausible transcriptional mechanisms underlying this fate specification. These insights will inform further development of desired cardiac cell types from human PSCs and their application in preclinical and clinical research.

Control of cardiomyocyte differentiation timing by intercellular signaling pathways

Congenital Heart Disease (CHD), malformations of the heart present at birth, is the most common class of life-threatening birth defect (Hoffman (1995) [1], Gelb (2004) [2], Gelb (2014) [3]). A major research challenge is to elucidate the genetic determinants of CHD and mechanistically link CHD ontogeny to a molecular understanding of heart development. Although the embryonic origins of CHD are unclear in most cases, dysregulation of cardiovascular lineage specification, patterning, proliferation, migration or differentiation have been described (Olson (2004) [4], Olson (2006) [5], Srivastava (2006) [6], Dunwoodie (2007) [7], Bruneau (2008) [8]). Cardiac differentiation is the process whereby cells become progressively more dedicated in a trajectory through the cardiac lineage towards mature cardiomyocytes. Defects in cardiac differentiation have been linked to CHD, although how the complex control of cardiac differentiation prevents CHD is just beginning to be understood. The stages of cardiac differentiation are highly stereotyped and have been well-characterized (Kattman et al. (2011) [9], Wamstad et al. (2012) [10], Luna-Zurita et al. (2016) [11], Loh et al. (2016) [12], DeLaughter et al. (2016) [13]); however, the developmental and molecular mechanisms that promote or delay the transition of a cell through these stages have not been as deeply investigated. Tight temporal control of progenitor differentiation is critically important for normal organ size, spatial organization, and cellular physiology and homeostasis of all organ systems (Raff et al. (1985) [14], Amthor et al. (1998) [15], Kopan et al. (2014) [16]). This review will focus on the action of signaling pathways in the control of cardiomyocyte differentiation timing. Numerous signaling pathways, including the Wnt, Fibroblast Growth Factor, Hedgehog, Bone Morphogenetic Protein, Insulin-like Growth Factor, Thyroid Hormone and Hippo pathways, have all been implicated in promoting or inhibiting transitions along the cardiac differentiation trajectory. Gaining a deeper understanding of the mechanisms controlling cardiac differentiation timing promises to yield insights into the etiology of CHD and to inform approaches to restore function to damaged hearts.

CCN1 gene polymorphisms associated with congenital heart disease susceptibility in Northwest Chinese population from different high-altitude areas

High-altitude hypoxic environment exposure is considered one of the risk factors for congenital heart disease (CHD), but the genetic factors involved are still unclear. CCN1, one of the synergistic molecules in the hypoxic response, is also an indispensable molecule in cardiac development. Considering that CCN1 may play an important role in the occurrence of CHD in high-altitude areas, we investigated the association between CCN1 polymorphisms and CHD susceptibility in Northwest Chinese population from different high-altitude areas. We conducted a case-control study with a total of 395 CHD cases and 486 controls to evaluate the associations of CCN1 polymorphisms with CHD risk. Our results showed that the protective alleles rs3753793-C (OR = 0.59, 95% CI = 0.42-0.81, P = 0.001), rs2297141-A (OR = 0.66, 95% CI = 0.49-0.90, P = 0.001), and C-A haplotype of rs3753793-rs2297141 (OR = 0.58, 95% CI = 0.42-0.82, P = 0.002) were significantly associated with a decreased atrial septal defect (ASD) risk. Further subgroup analysis in different geography populations revealed robust association of SNP rs2297141 with ASD risk in a Han population residing in high altitude of 2500-4287 m. We also found that the frequency of protective alleles was higher in high-altitude population, and the alleles were responsible for the difference of oxygen physiology-related erythrocyte parameters in different high-altitude populations. rs3753793-C and rs2297141-A are likely related to high altitude and hypoxia adaptation, which may also be the reason for the association between CCN1 polymorphism and ASD risk.

Of form and function: Early cardiac morphogenesis across classical and emerging model systems

The heart is the earliest organ to develop during embryogenesis and is remarkable in its ability to function efficiently as it is being sculpted. Cardiac heart defects account for a high burden of childhood developmental disorders with many remaining poorly understood mechanistically. Decades of work across a multitude of model organisms has informed our understanding of early cardiac differentiation and morphogenesis and has simultaneously opened new and unanswered questions. Here we have synthesized current knowledge in the field and reviewed recent developments in the realm of imaging, bioengineering and genetic technology and ex vivo cardiac modeling that may be deployed to generate more holistic models of early cardiac morphogenesis, and by extension, new platforms to study congenital heart defects.

Dysgenesis and Dysfunction of the Pancreas and Pituitary Due to FOXA2 Gene Defects

CONTEXT

Developmental disorders of the pituitary gland leading to congenital hypopituitarism can either be isolated or associated with extrapituitary abnormalities (syndromic hypopituitarism). A large number of syndromic hypopituitarism cases are linked to mutations in transcription factors. The forkhead box A2 (FOXA2) is a transcription factor that plays a key role in the central nervous system, foregut, and pancreatic development.

OBJECTIVE

This work aims to characterize 2 patients with syndromic hypopituitarism due to FOXA2 gene defects.

RESULTS

We report a novel heterozygous nonsense c.616C > T(p.Q206X) variant that leads to a truncated protein that lacks part of the DNA-binding domain of FOXA2, resulting in impaired transcriptional activation of the glucose transporter type 2 (GLUT2)-luciferase reporter. The patient is the sixth patient described in the literature with a FOXA2 mutation, and the first patient exhibiting pancreatic hypoplasia. We also report a second patient with a novel de novo 8.53 Mb deletion of 20p11.2 that encompasses FOXA2, who developed diabetes mellitus that responded to sulfonylurea treatment.

CONCLUSION

Our 2 cases broaden the molecular and clinical spectrum of FOXA2-related disease, reporting the first nonsense mutation and the first case of pancreatic dysgenesis.

Endothelial ontogeny and the establishment of vascular heterogeneity

The establishment of distinct cellular identities was pivotal during the evolution of Metazoa, enabling the emergence of an array of specialized tissues with different functions. In most animals including vertebrates, cell specialization occurs in response to a combination of intrinsic (e.g., cellular ontogeny) and extrinsic (e.g., local environment) factors that drive the acquisition of unique characteristics at the single-cell level. The first functional organ system to form in vertebrates is the cardiovascular system, which is lined by a network of endothelial cells whose organ-specific characteristics have long been recognized. Recent genetic analyses at the single-cell level have revealed that heterogeneity exists not only at the organ level but also between neighboring endothelial cells. Thus, how endothelial heterogeneity is established has become a key question in vascular biology. Drawing upon evidence from multiple organ systems, here we will discuss the role that lineage history may play in establishing endothelial heterogeneity.

Ventricular, atrial, and outflow tract heart progenitors arise from spatially and molecularly distinct regions of the primitive streak

The heart develops from 2 sources of mesoderm progenitors, the first and second heart field (FHF and SHF). Using a single-cell transcriptomic assay combined with genetic lineage tracing and live imaging, we find the FHF and SHF are subdivided into distinct pools of progenitors in gastrulating mouse embryos at earlier stages than previously thought. Each subpopulation has a distinct origin in the primitive streak. The first progenitors to leave the primitive streak contribute to the left ventricle, shortly after right ventricle progenitor emigrate, followed by the outflow tract and atrial progenitors. Moreover, a subset of atrial progenitors are gradually incorporated in posterior locations of the FHF. Although cells allocated to the outflow tract and atrium leave the primitive streak at a similar stage, they arise from different regions. Outflow tract cells originate from distal locations in the primitive streak while atrial progenitors are positioned more proximally. Moreover, single-cell RNA sequencing demonstrates that the primitive streak cells contributing to the ventricles have a distinct molecular signature from those forming the outflow tract and atrium. We conclude that cardiac progenitors are prepatterned within the primitive streak and this prefigures their allocation to distinct anatomical structures of the heart. Together, our data provide a new molecular and spatial map of mammalian cardiac progenitors that will support future studies of heart development, function, and disease.

Pathways Regulating Establishment and Maintenance of Cardiac Chamber Identity in Zebrafish

The vertebrate heart is comprised of two types of chambers-ventricles and atria-that have unique morphological and physiological properties. Effective cardiac function depends upon the distinct characteristics of ventricular and atrial cardiomyocytes, raising interest in the genetic pathways that regulate chamber-specific traits. Chamber identity seems to be specified in the early embryo by signals that establish ventricular and atrial progenitor populations and trigger distinct differentiation pathways. Intriguingly, chamber-specific features appear to require active reinforcement, even after myocardial differentiation is underway, suggesting plasticity of chamber identity within the developing heart. Here, we review the utility of the zebrafish as a model organism for studying the mechanisms that establish and maintain cardiac chamber identity. By combining genetic and embryological approaches, work in zebrafish has revealed multiple players with potent influences on chamber fate specification and commitment. Going forward, analysis of cardiomyocyte identity at the single-cell level is likely to yield a high-resolution understanding of the pathways that link the relevant players together, and these insights will have the potential to inform future strategies in cardiac tissue engineering.

Spatiotemporal sequence of mesoderm and endoderm lineage segregation during mouse gastrulation

Anterior mesoderm (AM) and definitive endoderm (DE) progenitors represent the earliest embryonic cell types that are specified during germ layer formation at the primitive streak (PS) of the mouse embryo. Genetic experiments indicate that both lineages segregate from Eomes-expressing progenitors in response to different Nodal signaling levels. However, the precise spatiotemporal pattern of the emergence of these cell types and molecular details of lineage segregation remain unexplored. We combined genetic fate labeling and imaging approaches with single-cell RNA sequencing (scRNA-seq) to follow the transcriptional identities and define lineage trajectories of Eomes-dependent cell types. Accordingly, all cells moving through the PS during the first day of gastrulation express Eomes AM and DE specification occurs before cells leave the PS from Eomes-positive progenitors in a distinct spatiotemporal pattern. ScRNA-seq analysis further suggested the immediate and complete separation of AM and DE lineages from Eomes-expressing cells as last common bipotential progenitor.

Human Pluripotent Stem Cell-Derived Cardiovascular Cells: From Developmental Biology to Therapeutic Applications

Advances in our understanding of cardiovascular development have provided a roadmap for the directed differentiation of human pluripotent stem cells (hPSCs) to the major cell types found in the heart. In this Perspective, we review the state of the field in generating and maturing cardiovascular cells from hPSCs based on our fundamental understanding of heart development. We then highlight their applications for studying human heart development, modeling disease-performing drug screening, and cell replacement therapy. With the advancements highlighted here, the promise that hPSCs will deliver new treatments for degenerative and debilitating diseases may soon be fulfilled.

Mouse gastrulation: Coordination of tissue patterning, specification and diversification of cell fate

During mouse embryonic development a mass of pluripotent epiblast tissue is transformed during gastrulation to generate the three definitive germ layers: endoderm, mesoderm, and ectoderm. During gastrulation, a spatiotemporally controlled sequence of events results in the generation of organ progenitors and positions them in a stereotypical fashion throughout the embryo. Key to the correct specification and differentiation of these cell fates is the establishment of an axial coordinate system along with the integration of multiple signals by individual epiblast cells to produce distinct outcomes. These signaling domains evolve as the anterior-posterior axis is established and the embryo grows in size. Gastrulation is initiated at the posteriorly positioned primitive streak, from which nascent mesoderm and endoderm progenitors ingress and begin to diversify. Advances in technology have facilitated the elaboration of landmark findings that originally described the epiblast fate map and signaling pathways required to execute those fates. Here we will discuss the current state of the field and reflect on how our understanding has shifted in recent years.

Cardiopharyngeal Progenitor Specification: Multiple Roads to the Heart and Head Muscles

During embryonic development, the heart arises from various sources of undifferentiated mesodermal progenitors, with an additional contribution from ectodermal neural crest cells. Mesodermal cardiac progenitors are plastic and multipotent, but are nevertheless specified to a precise heart region and cell type very early during development. Recent findings have defined both this lineage plasticity and early commitment of cardiac progenitors, using a combination of single-cell and population analyses. In this review, we discuss several aspects of cardiac progenitor specification. We discuss their markers, fate potential in vitro and in vivo, early segregation and commitment, and also intrinsic and extrinsic cues regulating lineage restriction from multipotency to a specific cell type of the heart. Finally, we also discuss the subdivisions of the cardiopharyngeal field, and the shared origins of the heart with other mesodermal derivatives, including head and neck muscles.

A single-cell transcriptomic landscape of primate arterial aging

Our understanding of how aging affects the cellular and molecular components of the vasculature and contributes to cardiovascular diseases is still limited. Here we report a single-cell transcriptomic survey of aortas and coronary arteries in young and old cynomolgus monkeys. Our data define the molecular signatures of specialized arteries and identify eight markers discriminating aortic and coronary vasculatures. Gene network analyses characterize transcriptional landmarks that regulate vascular senility and position FOXO3A, a longevity-associated transcription factor, as a master regulator gene that is downregulated in six subtypes of monkey vascular cells during aging. Targeted inactivation of FOXO3A in human vascular endothelial cells recapitulates the major phenotypic defects observed in aged monkey arteries, verifying FOXO3A loss as a key driver for arterial endothelial aging. Our study provides a critical resource for understanding the principles underlying primate arterial aging and contributes important clues to future treatment of age-associated vascular disorders.

Fetal Hypoxia Impacts on Proliferation and Differentiation of Sca-1+ Cardiac Progenitor Cells and Maturation of Cardiomyocytes: A Role of MicroRNA-210

Hypoxia is one of the most frequent and severe stresses to an organism’s homeostatic mechanisms, and hypoxia during gestation has profound adverse effects on the heart development increasing the occurrence of congenital heart defects (CHDs). Cardiac progenitor cells (CPCs) are responsible for early heart development and the later occurrence of heart disease. However, the mechanism of how hypoxic stress affects CPC fate decisions and contributes to CHDs remains a topic of debate. Here we examined the effect of hypoxic stress on the regulations of CPC fate decisions and the potential mechanism. We found that experimental induction of hypoxic responses compromised CPC function by regulating CPC proliferation and differentiation and restraining cardiomyocyte maturation. In addition, echocardiography indicated that fetal hypoxia reduced interventricular septum thickness at diastole and the ejection time, but increased the heart rate, in mouse young adult offspring with a gender-related difference. Further study revealed that hypoxia upregulated microRNA-210 expression in Sca-1⁺ CPCs and impeded the cell differentiation. Blockage of microRNA-210 with LNA-anti-microRNA-210 significantly promoted differentiation of Sca-1⁺ CPCs into cardiomyocytes. Thus, the present findings provide clear evidence that hypoxia alters CPC fate decisions and reveal a novel mechanism of microRNA-210 in the hypoxic effect, raising the possibility of microRNA-210 as a potential therapeutic target for heart disease.

Loss of Asb2 Impairs Cardiomyocyte Differentiation and Leads to Congenital Double Outlet Right Ventricle

Defining the pathways that control cardiac development facilitates understanding the pathogenesis of congenital heart disease. Herein, we identify enrichment of a Cullin5 Ub ligase key subunit, Asb2, in myocardial progenitors and differentiated cardiomyocytes. Using two conditional murine knockouts, Nkx^+/Cre.Asb2^fl/fl and AHF-Cre.Asb2^fl/fl, and tissue clarifying technique, we reveal Asb2 requirement for embryonic survival and complete heart looping. Deletion of Asb2 results in upregulation of its target Filamin A (Flna), and concurrent Flna deletion partially rescues embryonic lethality. Conditional AHF-Cre.Asb2 knockouts harboring one Flna allele have double outlet right ventricle (DORV), which is rescued by biallelic Flna excision. Transcriptomic and immunofluorescence analyses identify Tgfβ/Smad as downstream targets of Asb2/Flna. Finally, using CRISPR/Cas9 genome editing, we demonstrate Asb2 requirement for human cardiomyocyte differentiation suggesting a conserved mechanism between mice and humans. Collectively, our study provides deeper mechanistic understanding of the role of the ubiquitin proteasome system in cardiac development and suggests a previously unidentified murine model for DORV.

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Flna removal partially rescues embryonic lethality of Asb2-heart-specific knockout

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AHF-Asb2 knockouts harboring one Flna allele have double outlet right ventricle

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Asb2-Flna regulate TGFβ-Smad2 signaling in the heart

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Conserved role of Asb2 in heart morphogenesis between mice and humans

Role of carotenoids and retinoids during heart development

The nutritional requirements of the developing embryo are complex. In the case of dietary vitamin A (retinol, retinyl esters and provitamin A carotenoids), maternal derived nutrients serve as precursors to signaling molecules such as retinoic acid, which is required for embryonic patterning and organogenesis. Despite variations in the composition and levels of maternal vitamin A, embryonic tissues need to generate a precise amount of retinoic acid to avoid congenital malformations. Here, we summarize recent findings regarding the role and metabolism of vitamin A during heart development and we survey the association of genes known to affect retinoid metabolism or signaling with various inherited disorders. A better understanding of the roles of vitamin A in the heart and of the factors that affect retinoid metabolism and signaling can help design strategies to meet nutritional needs and to prevent birth defects and disorders associated with altered retinoid metabolism. This article is part of a Special Issue entitled Carotenoids recent advances in cell and molecular biology edited by Johannes von Lintig and Loredana Quadro.

Pre-marked chromatin and transcription factor co-binding shape the pioneering activity of Foxa2

Pioneer transcription factors (PTF) can recognize their binding sites on nucleosomal DNA and trigger chromatin opening for recruitment of other non-pioneer transcription factors. However, critical properties of PTFs are still poorly understood, such as how these transcription factors selectively recognize cell type-specific binding sites and under which conditions they can initiate chromatin remodelling. Here we show that early endoderm binding sites of the paradigm PTF Foxa2 are epigenetically primed by low levels of active chromatin modifications in embryonic stem cells (ESC). Priming of these binding sites is supported by preferential recruitment of Foxa2 to endoderm binding sites compared to lineage-inappropriate binding sites, when ectopically expressed in ESCs. We further show that binding of Foxa2 is required for chromatin opening during endoderm differentiation. However, increased chromatin accessibility was only detected on binding sites which are synergistically bound with other endoderm transcription factors. Thus, our data suggest that binding site selection of PTFs is directed by the chromatin environment and that chromatin opening requires collaboration of PTFs with additional transcription factors.