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For this decade, heart development has been extensively elucidated by the introduction of the concepts of “heart fields” and “cardiac progenitor cells (CPCs)”. It is believed that multipotent CPCs are specified among the most anterior part of lateral plate mesoderm as belonging to the two anatomical fields; the first heart field (FHF), which is the future left ventricle and atria, and the second heart field (SHF), which is the future right ventricle and outflow tract of the heart. However, the paradigm of two heart fields dependent on conventional marker genes is still disputed, so the existence of independent CPCs specific to each HF remains an open question. In addition, the molecular mechanism underlying the specification of CPCs remains largely unknown. A single-cell transcriptomic approach, which is realized by the recent advances in molecular biology, can be one of the solutions to bring some breakthrough in this subject.

The neonatal mammalian heart is capable of substantial regeneration following injury through cardiomyocyte proliferation (Porrello et al, Science 331:1078–1080, 2011; Proc Natl Acad Sci U S A 110:187–92, 2013). However, this regenerative capacity is lost by postnatal day 7 and the mechanisms of cardiomyocyte cell cycle arrest remain unclear. The homeodomain transcription factor Meis1 is required for normal cardiac development but its role in cardiomyocytes is unknown (Paige et al, Cell 151:221–232, 2012; Wamstad et al, Cell 151: 206–220, 2012). Here we identify Meis1 as a critical regulator of the cardiomyocyte cell cycle. Meis1 deletion in mouse cardiomyocytes was sufficient for extension of the postnatal proliferative window of cardiomyocytes and for reactivation of cardiomyocyte mitosis in the adult heart with no deleterious effect on cardiac function. In contrast, overexpression of Meis1 in cardiomyocytes decreased neonatal myocyte proliferation and inhibited neonatal heart regeneration. Finally, we show that Meis1 is required for transcriptional activation of the synergistic CDK inhibitors p15, p16, and p21. These results identify Meis1 as a critical transcriptional regulator of cardiomyocyte proliferation and a potential therapeutic target for heart regeneration.

The heart is the first organ to develop in the embryo, and its formation is an exquisitely regulated process. Inherited mutations in genes required for cardiac development may cause congenital heart disease (CHD), manifested in the newborn or in the adult. Notch is an ancient, highly conserved signaling pathway that communicates adjacent cells to regulate cell fate specification, differentiation, and tissue patterning. Mutations in Notch signaling elements result in cardiac abnormalities in mice and humans, demonstrating an essential role for Notch in heart development. Recent work has shown that endocardial Notch activity orchestrates the early events as well as the patterning and morphogenesis of the ventricular chambers in the mouse and that inactivating mutations in the NOTCH pathway regulator MIND BOMB-1 (MIB1) cause left ventricular non-compaction (LVNC), a cardiomyopathy of poorly understood etiology. Here, we review these data that shed some light on the etiology of LVNC that at least in the case of that caused by mutations has a developmental basis.

The epicardium has several essential functions in development of cardiac architecture and differentiation of the myocardium in vertebrates. We uncovered a novel function of the epicardium in species with partial or complete ventricular septation including reptiles, birds and mammals. Most reptiles have a complex ventricle with three cava, partially separated by the horizontal and vertical septa. Crocodilians, birds and mammals, however, have completely separated left and right ventricles, a clear example of convergent evolution. We have investigated the mechanisms underlying epicardial involvement in septum formation in embryos. We find that the primitive ventricle of early embryos becomes septated by folding and fusion of the anterior ventricular wall, trapping epicardium in its core. This ‘folding septum’, as we propose to call it, develops in lizards, snakes and turtles into the horizontal septum and, in the other taxa studied, into the folding part of the interventricular septum. The vertical septum, indistinct in most reptiles, arises in crocodilians and pythonids at the posterior ventricular wall. It is homologous to the inlet septum in mammals and birds. Eventually, the various septal components merge to form the completely septated heart. In our attempt to discover homologies between the various septum components, we draw perspectives to the development of ventricular septal defects in humans.

During embryogenesis, zebra fish cardiac precursor cells (CPCs) originating from anterior lateral plate mesoderm migrate toward the midline between the endoderm and the yolk syncytial layer (YSL) to form cardiac tube. The endoderm functions as a foothold for CPCs as evidenced by the endodermal mutants (/, , , /, and ) showing two hearts (cardia bifida) [1]. Furthermore, mutant zebra fish () lacking sphingosine-1-phosphate (S1P) transporter which is expressed in the YSL show two hearts [2], indicating the essential role for S1P-mediated signal in cardiac development. This is also supported by a S1p2 receptor mutant () which exhibits two hearts [3]. However, it is still unclear how S1P released from YSL regulates CPC migration.

Over the last decade, stem/progenitor cell therapy has emerged as an innovative approach to promote cardiac repair and regeneration. However, the therapeutic prospects of are currently limited by inadequate means to regulate cell proliferation, homing, engraftment, and differentiation. Autophagy, a lysosome-mediated degradation pathway for recycling organelles and protein aggregates, is recognized as important for facilitating cell differentiation. Studies have shown that induced pluripotent stem cells (iPCs), which exhibit a predominantly glycolytic metabolism, shift toward oxidative mitochondrial metabolism as a prerequisite for the formation of sarcomeres and differentiation into cardiomyocytes. C2C12 myoblasts are a mouse-derived myogenic progenitor cell line which can be induced to differentiate into myotubes. We hypothesize that autophagy is essential in coordinating transcription factor activity and metabolic reprogramming of mitochondria to support myocyte differentiation.

Congenital hypothyroidism (CH) is one of the most common diseases of the endocrine system among newborns. Infants with CH have been reported to have a high frequency of congenital cardiovascular malformations (CM), such as ventricular and atrial septal defects [1]. Some studies have demonstrated that these cases were due to gene mutations and neural crest abnormality. Infants with CH and CM have been shown to have significantly lower T levels than those with isolated CH. However, the role of thyroid hormone in the developing heart has not been reported. In this study, we show the thyroid anlage in chick embryos by immunohistochemistry and follow the expression of thyroid hormone receptor during heart development.

Malformation of the atrioventricular (AV) cushion is a common congenital heart defect. Ebstein’s anomaly, characterized by a heart defect related to the AV cushion, involves not only a valve defect but also a myocardial abnormality such as Uhl’s anomaly. The morphogenetic features of the heart in the case of these diseases can be used as a reference for investigating valvuloseptal and myocardial formations in the human heart.

The AV endocardium transforms into the cushion mesenchyme through epithelial–mesenchymal transition (EMT). After the EMT, distal outgrowth and maturation of endocardial cushions are important morphogenetic steps for AV valvuloseptal morphogenesis. While bone morphogenetic protein (BMP)-2 is known to be critical for AV EMT, little is known about the functional relationship between BMP and ECM and their roles in cushion mesenchymal cell (CMC) migration after EMT. In our previous study, we showed that BMP-2 and BMP signaling induced AV CMC migration. We have been exploring the role of BMP-2 in the regulation of valvulogenic extracellular matrix (ECM) components, periostin, versican, and hyaluronic acid (HA), and cell migration during post-EMT AV cushion expansion and maturation.

We further examined whether BMP-2-promoted cell migration is associated with expression of periostin, versican, and HA. BMP-2-promoted cell migration is significantly impaired by treatment with versican siRNA and HA oligomer. We also found that transcription of , implicated in cell migration in embryogenesis and activation of the periostin promoter, was induced by BMP-2 but repressed by noggin in CMC cultures.

Taken together, we provide evidence that BMP-2 induces expression and deposition of three major ECM proteins, periostin, versican, and HA, and that these ECM components contribute to BMP-2-induced CMC migration during post-EMT AV cushion expansion and maturation.

Based on these findings, we discuss the morphogenetic process of AV valve abnormalities and crosstalk between valve and cardiomyocytes morphogenesis.

The mature heart valves consist of stratified extracellular matrix (ECM) layers, and heart valve disease is characterized by ECM dysregulation and mineralization. There is increasing evidence that regulatory pathways that control heart valve development also are active in disease. In human diseased valves and mouse models, the expression of valve progenitor markers, including Twist1, Msx1/2 and Snail1/2, is induced. Additional markers of osteogenesis, including Runx2, osteocalcin and bone sialoprotein, also are expressed in calcific aortic valve disease (CAVD) in humans and mice. New mouse models have been developed for studies of valve disease mechanisms. Klotho-null mice are a model for premature aging and exhibit calcified nodules in aortic valves with osteogenic gene induction. Osteogenesis Imperfecta mice, bearing a collagen1a2 mutation, develop features of myxomatous valve disease, including thickening, increased proteoglycan deposition and chondrogenic gene induction. Together, these findings demonstrate specific molecular indicators of valve disease progression, including the identification of early disease markers, which represent potential targets for therapeutic intervention.

The mechanisms regulating late-gestational and early postnatal semilunar valve remodeling and maturation are poorly understood. Tie1 is a receptor tyrosine kinase with broad expression in embryonic endothelium. During semilunar valve development, Tie1 expression becomes restricted to the turbulent, arterial surfaces of the valves in the perinatal period. Previous studies in our laboratory have demonstrated that Tie1 can regulate cellular responses to blood flow and shear stress. We hypothesized that Tie1 signaling would regulate the flow-dependent remodeling of the semilunar valves associated with the conversion from maternal/placental to independent neonatal circulation. To circumvent the embryonic lethality of the Tie1 null mutation, we developed a floxed Tie1 allele and crossed it with an line that mediates gene excision exclusively in the endocardial cushion endothelium. Excision of Tie1 resulted in aortic valve leaflets displaying hypertrophy with perturbed matrix deposition. The valves demonstrated insufficiency and stenosis by ultrasound, and atomic force microscopy documented decreased stiffness in the mutant aortic valve consistent with an increased glycosaminoglycan to collagen ratio. These data suggest that active endocardial to mesenchymal signaling, at least partially mediated by Tie1, is uniquely required for normal remodeling of the aortic but not pulmonary valve in the late gestation and postnatal animal.