New insights into the developmental mechanisms of coronary vessels and epicardium

Human Cardiac Development

Many aspects of heart development are topographically complex and require three-dimensional (3D) reconstruction to understand the pertinent morphology. We have recently completed a comprehensive primer of human cardiac development that is based on firsthand segmentation of structures of interest in histological sections. We visualized the hearts of 12 human embryos between their first appearance at 3.5 weeks and the end of the embryonic period at 8 weeks. The models were presented as calibrated, interactive, 3D portable document format (PDF) files. We used them to describe the appearance and the subsequent remodeling of around 70 different structures incrementally for each of the reconstructed stages. In this chapter, we begin our account by describing the formation of the single heart tube, which occurs at the end of the fourth week subsequent to conception. We describe its looping in the fifth week, the formation of the cardiac compartments in the sixth week, and, finally, the septation of these compartments into the physically separated left- and right-sided circulations in the seventh and eighth weeks. The phases are successive, albeit partially overlapping. Thus, the basic cardiac layout is established between 26 and 32 days after fertilization and is described as Carnegie stages (CSs) 9 through 14, with development in the outlet component trailing that in the inlet parts. Septation at the venous pole is completed at CS17, equivalent to almost 6 weeks of development. During Carnegie stages 17 and 18, in the seventh week, the outflow tract and arterial pole undergo major remodeling, including incorporation of the proximal portion of the outflow tract into the ventricles and transfer of the spiraling course of the subaortic and subpulmonary channels to the intrapericardial arterial trunks. Remodeling of the interventricular foramen, with its eventual closure, is complete at CS20, which occurs at the end of the seventh week. We provide quantitative correlations between the age of human and mouse embryos as well as the Carnegie stages of development. We have also set our descriptions in the context of variations in the timing of developmental features.

Single-cell and spatial heterogeneity landscapes of mature epicardial cells

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

Tenascin-C in Heart Diseases-The Role of Inflammation

Tenascin-C (TNC) is a large extracellular matrix (ECM) glycoprotein and an original member of the matricellular protein family. TNC is transiently expressed in the heart during embryonic development, but is rarely detected in normal adults; however, its expression is strongly up-regulated with inflammation. Although neither TNC-knockout nor -overexpressing mice show a distinct phenotype, disease models using genetically engineered mice combined with in vitro experiments have revealed multiple significant roles for TNC in responses to injury and myocardial repair, particularly in the regulation of inflammation. In most cases, TNC appears to deteriorate adverse ventricular remodeling by aggravating inflammation/fibrosis. Furthermore, accumulating clinical evidence has shown that high TNC levels predict adverse ventricular remodeling and a poor prognosis in patients with various heart diseases. Since the importance of inflammation has attracted attention in the pathophysiology of heart diseases, this review will focus on the roles of TNC in various types of inflammatory reactions, such as myocardial infarction, hypertensive fibrosis, myocarditis caused by viral infection or autoimmunity, and dilated cardiomyopathy. The utility of TNC as a biomarker for the stratification of myocardial disease conditions and the selection of appropriate therapies will also be discussed from a clinical viewpoint.

Tenascin-C in cardiac disease: a sophisticated controller of inflammation, repair, and fibrosis

Tenascin-C (TNC) is a large extracellular matrix glycoprotein classified as a matricellular protein that is generally upregulated at high levels during physiological and pathological tissue remodeling and is involved in important biological signaling pathways. In the heart, TNC is transiently expressed at several important steps during embryonic development and is sparsely detected in normal adult heart but is re-expressed in a spatiotemporally restricted manner under pathological conditions associated with inflammation, such as myocardial infarction, hypertensive cardiac fibrosis, myocarditis, dilated cardiomyopathy, and Kawasaki disease. Despite its characteristic and spatiotemporally restricted expression, TNC knockout mice develop a grossly normal phenotype. However, various disease models using TNC null mice combined with in vitro experiments have revealed many important functions for TNC and multiple molecular cascades that control cellular responses in inflammation, tissue repair, and even myocardial regeneration. TNC has context-dependent diverse functions and, thus, may exert both harmful and beneficial effects in damaged hearts. However, TNC appears to deteriorate adverse ventricular remodeling by proinflammatory and profibrotic effects in most cases. Its specific expression also makes TNC a feasible diagnostic biomarker and target for molecular imaging to assess inflammation in the heart. Several preclinical studies have shown the utility of TNC as a biomarker for assessing the prognosis of patients and selecting appropriate therapy, particularly for inflammatory heart diseases.

MiR-301a promotes embryonic stem cell differentiation to cardiomyocytes

BACKGROUND

Cardiovascular disease is the leading cause of death worldwide. Tissue repair after pathological injury in the heart remains a major challenge due to the limited regenerative ability of cardiomyocytes in adults. Stem cell-derived cardiomyocytes provide a promising source for the cell transplantation-based treatment of injured hearts.

AIM

To explore the function and mechanisms of miR-301a in regulating cardiomyocyte differentiation of mouse embryonic stem (mES) cells, and provide experimental evidence for applying miR-301a to the cardiomyocyte differentiation induction from stem cells.

METHODS

mES cells with or without overexpression of miR-301a were applied for all functional assays. The hanging drop technique was applied to form embryoid bodies from mES cells. Cardiac markers including GATA-4, TBX5, MEF2C, and α-actinin were used to determine cardiomyocyte differentiation from mES cells.

RESULTS

High expression of miR-301a was detected in the heart from late embryonic to neonatal mice. Overexpression of miR-301a in mES cells significantly induced the expression of cardiac transcription factors, thereby promoting cardiomyocyte differentiation and beating cardiomyocyte clone formation. PTEN is a target gene of miR-301a in cardiomyocytes. PTEN-regulated PI3K-AKT-mTOR-Stat3 signaling showed involvement in regulating miR-301a-promoted cardiomyocyte differentiation from mES cells.

CONCLUSION

MiR-301a is capable of promoting embryonic stem cell differentiation to cardiomyocytes.

Retinoic acid signaling in heart development

Retinoic acid (RA) is a vitamin A metabolite that acts as a morphogen and teratogen. Excess or defective RA signaling causes developmental defects including in the heart. The heart develops from the anterior lateral plate mesoderm. Cardiogenesis involves successive steps, including formation of the primitive heart tube, cardiac looping, septation, chamber development, coronary vascularization, and completion of the four-chambered heart. RA is dispensable for primitive heart tube formation. Before looping, RA is required to define the anterior/posterior boundaries of the heart-forming mesoderm as well as to form the atrium and sinus venosus. In outflow tract elongation and septation, RA signaling is required to maintain/differentiate cardiogenic progenitors in the second heart field at the posterior pharyngeal arches level. Epicardium-secreted insulin-like growth factor, the expression of which is regulated by hepatic mesoderm-derived erythropoietin under the control of RA, promotes myocardial proliferation of the ventricular wall. Epicardium-derived RA induces the expression of angiogenic factors in the myocardium to form the coronary vasculature. In cardiogenic events at different stages, properly controlled RA signaling is required to establish the functional heart.

Coronary artery disease genes SMAD3 and TCF21 promote opposing interactive genetic programs that regulate smooth muscle cell differentiation and disease risk

Although numerous genetic loci have been associated with coronary artery disease (CAD) with genome wide association studies, efforts are needed to identify the causal genes in these loci and link them into fundamental signaling pathways. Recent studies have investigated the disease mechanism of CAD associated gene SMAD3, a central transcription factor (TF) in the TGFβ pathway, investigating its role in smooth muscle biology. In vitro studies in human coronary artery smooth muscle cells (HCASMC) revealed that SMAD3 modulates cellular phenotype, promoting expression of differentiation marker genes while inhibiting proliferation. RNA sequencing and chromatin immunoprecipitation sequencing studies in HCASMC identified downstream genes that reside in pathways which mediate vascular development and atherosclerosis processes in this cell type. HCASMC phenotype, and gene expression patterns promoted by SMAD3 were noted to have opposing direction of effect compared to another CAD associated TF, TCF21. At sites of SMAD3 and TCF21 colocalization on DNA, SMAD3 binding was inversely correlated with TCF21 binding, due in part to TCF21 locally blocking chromatin accessibility at the SMAD3 binding site. Further, TCF21 was able to directly inhibit SMAD3 activation of gene expression in transfection reporter gene studies. In contrast to TCF21 which is protective toward CAD, SMAD3 expression in HCASMC was shown to be directly correlated with disease risk. We propose that the pro-differentiation action of SMAD3 inhibits dedifferentiation that is required for HCASMC to expand and stabilize disease plaque as they respond to vascular stresses, counteracting the protective dedifferentiating activity of TCF21 and promoting disease risk.

Anatomy of the coronary artery and cardiac vein in the quail ventricle: patterns are distinct from those in mouse and human hearts

Coronary vessel development has been investigated in avian and mouse embryonic hearts. Quail embryos are a useful tool to examine vascular development, particularly because the QH1 antibody and transgenic quail line, Tg (tie1:H2B-eYFP), are useful to trace endothelial cells. However, there are only a few descriptions of the quail coronary vessels. Using ink injection coronary angiography, we examined the course of coronary vessels in the fetal quail heart. The major coronary arteries were the right and left septal arteries, which, respectively, branched off from the right and left coronary stems. The right septal artery ran posteriorly (dorsally) and penetrated the ventricular free wall to distribute to the posterior surface of the ventricles. The left septal artery ran anteriorly (ventrally) and penetrated the ventricular free wall to distribute to the anterior surface of the ventricles. The right and left circumflex arteries were directed posteriorly along the atrioventricular sulci. The cardiac veins consisted of three major tributaries: the middle, great, and anterior cardiac veins. The middle cardiac vein ascended along the posterior interventricular sulcus and emptied into the right atrium. The great cardiac vein ran along the anterior interventricular sulcus, entered the space between the left atrium and conus arteriosus and emptied into the right atrium behind the aortic bulb. The anterior cardiac vein drained the anterior surface of the right ventricle and connected to the anterior base of the right atrium. The course of coronary vessels in the quail heart was basically the same as that observed in chick but was different from those of mouse and human.

Avian coronary endothelium is a mosaic of sinus venosus- and ventricle-derived endothelial cells in a region-specific manner

The origin of coronary endothelial cells (ECs) has been investigated in avian species, and the results showed that the coronary ECs originate from the proepicardial organ (PEO) and developing epicardium. Genetic approaches in mouse models showed that the major source of coronary ECs is the sinus venosus endothelium or ventricular endocardium. To clarify and reconcile the differences between avian and mouse species, we examined the source of coronary ECs in avian embryonic hearts. Using an enhanced green fluorescent protein-Tol2 system and fluorescent dye labeling, four types of quail-chick chimeras were made and quail-specific endothelial marker (QH1) immunohistochemistry was performed. The developing PEO consisted of at least two cellular populations in origin, one was sinus venosus endothelium-derived inner cells and the other was surface mesothelium-derived cells. The majority of ECs in the coronary stems, ventricular free wall, and dorsal ventricular septum originated from the sinus venosus endothelium. The ventricular endocardium contributed mainly to the septal artery and a few cells to the coronary stems. Surface mesothelial cells of the PEO differentiated mainly into a smooth muscle phenotype, but a few differentiated into ECs. In avian species, the coronary endothelium had a heterogeneous origin in a region-specific manner, and the sources of ECs were basically the same as those observed in mice.

Non-muscle myosin IIB (Myh10) is required for epicardial function and coronary vessel formation during mammalian development

The coronary vasculature is an essential vessel network providing the blood supply to the heart. Disruptions in coronary blood flow contribute to cardiac disease, a major cause of premature death worldwide. The generation of treatments for cardiovascular disease will be aided by a deeper understanding of the developmental processes that underpin coronary vessel formation. From an ENU mutagenesis screen, we have isolated a mouse mutant displaying embryonic hydrocephalus and cardiac defects (EHC). Positional cloning and candidate gene analysis revealed that the EHC phenotype results from a point mutation in a splice donor site of the Myh10 gene, which encodes NMHC IIB. Complementation testing confirmed that the Myh10 mutation causes the EHC phenotype. Characterisation of the EHC cardiac defects revealed abnormalities in myocardial development, consistent with observations from previously generated NMHC IIB null mouse lines. Analysis of the EHC mutant hearts also identified defects in the formation of the coronary vasculature. We attribute the coronary vessel abnormalities to defective epicardial cell function, as the EHC epicardium displays an abnormal cell morphology, reduced capacity to undergo epithelial-mesenchymal transition (EMT), and impaired migration of epicardial-derived cells (EPDCs) into the myocardium. Our studies on the EHC mutant demonstrate a requirement for NMHC IIB in epicardial function and coronary vessel formation, highlighting the importance of this protein in cardiac development and ultimately, embryonic survival.

Targeting the hedgehog signaling pathway for cardiac repair and regeneration

The hedgehog (Hh) signaling pathway is involved in the angiogenesis and development of the coronary vasculature in the embryonic heart. Recently, the Hh signal pathway has emerged as an important regulator that can increase cardiomyocyte proliferation, inhibit cardiomyocyte death and apoptosis, recruit endothelial progenitor cell (EPCs) into sites of myocardial ischemia, and direct stem cells to differentiate into cardiac muscle lineage. Experimental studies have tried to target the Hh signaling pathway for cardiac repair and regeneration. The purpose of this review is to discuss the role of the Hh signal pathway in cardiac repair and regeneration as well as the current strategies targeting the Hh signaling pathway and its potential in heart diseases.

Probing early heart development to instruct stem cell differentiation strategies

Scientists have studied organs and their development for centuries and, along that path, described models and mechanisms explaining the developmental principles of organogenesis. In particular, with respect to the heart, new fundamental discoveries are reported continuously that keep changing the way we think about early cardiac development. These discoveries are driven by the need to answer long-standing questions regarding the origin of the earliest cells specified to the cardiac lineage, the differentiation potential of distinct cardiac progenitor cells, and, very importantly, the molecular mechanisms underlying these specification events. As evidenced by numerous examples, the wealth of developmental knowledge collected over the years has had an invaluable impact on establishing efficient strategies to generate cardiovascular cell types ex vivo, from either pluripotent stem cells or via direct reprogramming approaches. The ability to generate functional cardiovascular cells in an efficient and reliable manner will contribute to therapeutic strategies aimed at alleviating the increasing burden of cardiovascular disease and morbidity. Here we will discuss the recent discoveries in the field of cardiac progenitor biology and their translation to the pluripotent stem cell model to illustrate how developmental concepts have instructed regenerative model systems in the past and promise to do so in the future. Developmental Dynamics 245:1130-1144, 2016. © 2016 Wiley Periodicals, Inc.

Tenascin-C in development and disease of blood vessels

Tenascin-C (TNC) is an extracellular glycoprotein categorized as a matricellular protein. It is highly expressed during embryonic development, wound healing, inflammation, and cancer invasion, and has a wide range of effects on cell response in tissue morphogenesis and remodeling including the cardiovascular system. In the heart, TNC is sparsely detected in normal adults but transiently expressed at restricted sites during embryonic development and in response to injury, playing an important role in myocardial remodeling. Although TNC in the vascular system appears more complex than in the heart, the expression of TNC in normal adult blood vessels is generally low. During embryonic development, vascular smooth muscle cells highly express TNC on maturation of the vascular wall, which is controlled in a way that depends on the embryonic site of cell origin. Strong expression of TNC is also linked with several pathological conditions such as cerebral vasospasm, intimal hyperplasia, pulmonary artery hypertension, and aortic aneurysm/ dissection. TNC synthesized by smooth muscle cells in response to developmental and environmental cues regulates cell responses such as proliferation, migration, differentiation, and survival in an autocrine/paracrine fashion and in a context-dependent manner. Thus, TNC can be a key molecule in controlling cellular activity in adaptation during normal vascular development as well as tissue remodeling in pathological conditions.

Molecular regulation of cardiomyocyte differentiation

The heart is the first organ to form during embryonic development. Given the complex nature of cardiac differentiation and morphogenesis, it is not surprising that some form of congenital heart disease is present in ≈1 percent of newborns. The molecular determinants of heart development have received much attention over the past several decades. This has been driven in large part by an interest in understanding the causes of congenital heart disease coupled with the potential of using knowledge from developmental biology to generate functional cells and tissues that could be used for regenerative medicine purposes. In this review, we highlight the critical signaling pathways and transcription factor networks that regulate cardiomyocyte lineage specification in both in vivo and in vitro models. Special focus will be given to epigenetic regulators that drive the commitment of cardiomyogenic cells from nascent mesoderm and their differentiation into chamber-specific myocytes, as well as regulation of myocardial trabeculation.

Epicardium is required for sarcomeric maturation and cardiomyocyte growth in the ventricular compact layer mediated by transforming growth factor β and fibroblast growth factor before the onset of coronary circulation

The epicardium, which is derived from the proepicardial organ (PE) as the third epithelial layer of the developing heart, is crucial for ventricular morphogenesis. An epicardial deficiency leads to a thin compact layer for the developing ventricle; however, the mechanisms leading to the impaired development of the compact layer are not well understood. Using chick embryonic hearts, we produced epicardium-deficient hearts by surgical ablation or blockade of the migration of PE and examined the mechanisms underlying a thin compact myocardium. Sarcomeric maturation (distance between Z-lines) and cardiomyocyte growth (size) were affected in the thin compact myocardium of epicardium-deficient ventricles, in which the amounts of phospho-smad2 and phospho-ERK as well as expression of transforming growth factor (TGF)β2 and fibroblast growth factor (FGF)2 were reduced. TGFβ and FGF were required for the maturation of sarcomeres and growth of cardiomyocytes in cultured ventricles. In ovo co-transfection of dominant negative (dN)-Alk5 (dN-TGFβ receptor I) and dN-FGF receptor 1 to ventricles caused a thin compact myocardium. Our results suggest that immature sarcomeres and small cardiomyocytes are the causative architectures of an epicardium-deficient thin compact layer and also that epicardium-dependent signaling mediated by TGFβ and FGF plays a role in the development of the ventricular compact layer before the onset of coronary circulation.

Tenascin-C and mechanotransduction in the development and diseases of cardiovascular system

Living tissue is composed of cells and extracellular matrix (ECM). In the heart and blood vessels, which are constantly subjected to mechanical stress, ECM molecules form well-developed fibrous frameworks to maintain tissue structure. ECM is also important for biological signaling, which influences various cellular functions in embryonic development, and physiological/pathological responses to extrinsic stimuli. Among ECM molecules, increased attention has been focused on matricellular proteins. Matricellular proteins are a growing group of non-structural ECM proteins highly up-regulated at active tissue remodeling, serving as biological mediators. Tenascin-C (TNC) is a typical matricellular protein, which is highly expressed during embryonic development, wound healing, inflammation, and cancer invasion. The expression is tightly regulated, dependent on the microenvironment, including various growth factors, cytokines, and mechanical stress. In the heart, TNC appears in a spatiotemporal-restricted manner during early stages of development, sparsely detected in normal adults, but transiently re-expressed at restricted sites associated with tissue injury and inflammation. Similarly, in the vascular system, TNC is strongly up-regulated during embryonic development and under pathological conditions with an increase in hemodynamic stress. Despite its intriguing expression pattern, cardiovascular system develops normally in TNC knockout mice. However, deletion of TNC causes acute aortic dissection (AAD) under strong mechanical and humoral stress. Accumulating reports suggest that TNC may modulate the inflammatory response and contribute to elasticity of the tissue, so that it may protect cardiovascular tissue from destructive stress responses. TNC may be a key molecule to control cellular activity during development, adaptation, or pathological tissue remodeling.

FoxA2 hunting research identifies the early trail of mesenchymal differentiation

Epigenetic regulation offers a flexible means to instruct cell functions and fate. In human embryonic stem cells (hESCs), thousands of genes are targets for histone modifications leading to activation or suppression of transcription. Novel research now indicates that, in hESCs, the transcription start site of FOXA2, encoding a member of the forkhead family of transcription factors, is bivalently marked with histone modifications for both gene activation and repression. Moreover, FOXA2 is remarkably upregulated at an early stage of endothelial differentiation. These discoveries provide better understanding of the natural program of differentiation and also open up new opportunities for large scale production of endothelial progenitors.