Transcriptional, Electrophysiological, and Metabolic Characterizations of hESC-Derived First and Second Heart Fields Demonstrate a Potential Role of TBX5 in Cardiomyocyte Maturation

Background: Human embryonic stem cell-derived cardiomyocytes (hESC-CMs) can be used as a source for cell delivery to remuscularize the heart after myocardial infarction. Despite their therapeutic potential, the emergence of ventricular arrhythmias has limited their application. We previously developed a double reporter hESC line to isolate first heart field (FHF: TBX5+NKX2-5+) and second heart field (SHF: TBX5-NKX2-5+) CMs. Herein, we explore the role of TBX5 and its effects on underlying gene regulatory networks driving phenotypical and functional differences between these two populations.Methods: We used a combination of tools and techniques for rapid and unsupervised profiling of FHF and SHF populations at the transcriptional, translational, and functional level including single cell RNA (scRNA) and bulk RNA sequencing, atomic force and quantitative phase microscopy, respirometry, and electrophysiology.Results: Gene ontology analysis revealed three biological processes attributed to TBX5 expression: sarcomeric structure, oxidative phosphorylation, and calcium ion handling. Interestingly, migratory pathways were enriched in SHF population. SHF-like CMs display less sarcomeric organization compared to FHF-like CMs, despite prolonged in vitro culture. Atomic force and quantitative phase microscopy showed increased cellular stiffness and decreased mass distribution over time in FHF compared to SHF populations, respectively. Electrophysiological studies showed longer plateau in action potentials recorded from FHF-like CMs, consistent with their increased expression of calcium handling genes. Interestingly, both populations showed nearly identical respiratory profiles with the only significant functional difference being higher ATP generation-linked oxygen consumption rate in FHF-like CMs. Our findings suggest that FHF-like CMs display more mature features given their enhanced sarcomeric alignment, calcium handling, and decreased migratory characteristics. Finally, pseudotime analyses revealed a closer association of the FHF population to human fetal CMs along the developmental trajectory.Conclusion: Our studies reveal that distinguishing FHF and SHF populations based on TBX5 expression leads to a significant impact on their downstream functional properties. FHF CMs display more mature characteristics such as enhanced sarcomeric organization and improved calcium handling, with closer positioning along the differentiation trajectory to human fetal hearts. These data suggest that the FHF CMs may be a more suitable candidate for cardiac regeneration.

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.

Tbx5 drives aldh1a2 expression to regulate a RA-Hedgehog-Wnt gene regulatory network coordinating cardiopulmonary development

The gene regulatory networks that coordinate the development of the cardiac and pulmonary systems are essential for terrestrial life but poorly understood. The T-box transcription factor Tbx5 is critical for both pulmonary specification and heart development, but how these activities are mechanistically integrated remains unclear. Here using Xenopus and mouse embryos, we establish molecular links between Tbx5 and retinoic acid (RA)-signaling in the mesoderm and between RA signaling and sonic hedgehog expression in the endoderm to unveil a conserved RA-Hedgehog-Wnt signaling cascade coordinating cardiopulmonary development. We demonstrate that Tbx5 directly maintains expression of aldh1a2, the RA-synthesizing enzyme, in the foregut lateral plate mesoderm via an evolutionarily conserved intronic enhancer. Tbx5 promotes posterior second heart field identity in a positive feedback loop with RA, antagonizing a Fgf8-Cyp regulatory module to restrict FGF activity to the anterior. We find that Tbx5/Aldh1a2-dependent RA signaling directly activates shh transcription in the adjacent foregut endoderm through a conserved MACS1 enhancer. Hedgehog signaling coordinates with Tbx5 in the mesoderm to activate expression of wnt2/2b, which induces pulmonary fate in the foregut endoderm. These results provide mechanistic insight into the interrelationship between heart and lung development informing cardiopulmonary evolution and birth defects.

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.

Lamb1a regulates atrial growth by limiting second heart field addition during zebrafish heart development

During early vertebrate heart development the heart transitions from a linear tube to a complex asymmetric structure, a morphogenetic process which occurs simultaneously with growth of the heart. Cardiac growth during early heart morphogenesis is driven by deployment of cells from the Second Heart Field (SHF) into both poles of the heart. Laminin is a core component of the extracellular matrix (ECM), and although mutations in laminin subunits are linked with cardiac abnormalities, no role for laminin has been identified in early vertebrate heart morphogenesis. We identified tissue-specific expression of laminin genes in the developing zebrafish heart, supporting a role for laminins in heart morphogenesis. Analysis of heart development in lamb1a zebrafish mutant embryos reveals mild morphogenetic defects and progressive cardiomegaly, and that Lamb1a functions to limit heart size during cardiac development by restricting SHF addition. lamb1a mutants exhibit hallmarks of altered haemodynamics, and blocking cardiac contractility in lamb1a mutants rescues heart size and atrial SHF addition. Together this suggests that laminin mediates interactions between SHF deployment and cardiac biomechanics during heart development and growth in the developing embryo.

Abstract MP245: Tet Proteins Regulate Second Heart Field Multipotent Progenitors Differentiating To Myocytes

DNA methylation and demethylation play an important role in shaping the epigenetic landscape and chromatin accessibility to control gene expression during development in mammals. Ten-eleven Translocation (Tet1, Tet2 and Tet3) is a family of dioxygenases that catalyze DNA methylation oxidation with ultimate DNA demethylation. Our previous study showed that cardiac-specific deletion of Tet2 and Tet3 could disrupt YY1-mediated long range chromatin interactions during heart development and lead to ventricular non-compaction cardiomyopathy. However, it is still unclear whether and how Tet protein mediated epigenetic modifications contribute to cardiac lineage specification during embryonic development. In this study, we generated cardiac specific Tet1-3 triple deficient (Tet-TKO) mouse lines using various cardiac specific Cres to evaluate the function of Tet protein in regulating cardiac lineage specification. We observed developmental defects at outflow tract (OFT) in Tet-TKO embryos, suggesting that Tet deficiency affects the second heart field (SHF) development. Single cell RNA-seq analysis further revealed the accumulation of multipotent SHF progenitors and subsequent halt of myocyte differentiation upon Tet depletion. At the molecular level, we found that Tet ablation perturbs the transcriptional network of Islet1, a transcription factor that is crucial for cardiac development in embryos. Overall, our study demonstrates a critical role of Tet-mediated epigenetic regulation for embryonic cardiac development.

Strong as a Hippo’s Heart: Biomechanical Hippo Signaling During Zebrafish Cardiac Development

The heart is comprised of multiple tissues that contribute to its physiological functions. During development, the growth of myocardium and endocardium is coupled and morphogenetic processes within these separate tissue layers are integrated. Here, we discuss the roles of mechanosensitive Hippo signaling in growth and morphogenesis of the zebrafish heart. Hippo signaling is involved in defining numbers of cardiac progenitor cells derived from the secondary heart field, in restricting the growth of the epicardium, and in guiding trabeculation and outflow tract formation. Recent work also shows that myocardial chamber dimensions serve as a blueprint for Hippo signaling-dependent growth of the endocardium. Evidently, Hippo pathway components act at the crossroads of various signaling pathways involved in embryonic zebrafish heart development. Elucidating how biomechanical Hippo signaling guides heart morphogenesis has direct implications for our understanding of cardiac physiology and pathophysiology.

Understanding Mechanisms of Chamber-Specific Differentiation Through Combination of Lineage Tracing and Single Cell Transcriptomics

The specification and differentiation of atrial and ventricular myocardial cell types during development is incompletely understood. We have previously shown that Foxa2 expression during gastrulation identifies a population of ventricular fated progenitors, allowing for labeling of these cells prior to the morphogenetic events that lead to chamber formation and acquisition of bona fide atrial or ventricular identity. In this study, we performed single cell RNA sequencing of Foxa2Cre;mTmG embryos at the cardiac crescent (E8.25), primitive heart tube (E8.75) and heart tube (E9.25) stage in order to understand the transcriptional mechanisms underlying formation of atrial and ventricular cell types at the earliest stages of cardiac development. We find that progression towards differentiated myocardial cell types occurs primarily based on heart field progenitor identity, and that different progenitor populations contribute to ventricular or atrial identity through separate differentiation mechanisms. We identified a number of candidate markers that define such differentiation processes, as well as differential regulation of metabolic processes that distinguish atrial and ventricular fated cells at the earliest stages of development. We further show that exogenous injection with retinoic acid during formation of the cardiac primordia causes defects in ventricular chamber size and is associated with dysregulation in FGF signaling in anterior second heart field cells and a shunt in differentiation towards orthogonal lineages. Retinoic acid also causes defects in cell-cycle exit in myocardial committed progenitors that result in formation of hypomorphic ventricles with decreased expression of important metabolic processes and sarcomere assembly. Collectively, our data identify, at a single cell level, distinct lineage trajectories during cardiac progenitor cell specification and differentiation, and the precise effects of manipulating cardiac progenitor field patterning via exogenous retinoic acid signaling.