Wnt/β-catenin signaling maintains the mesenchymal precursor pool for murine sinus horn formation

Biomaterial-induced conversion of quiescent cardiomyocytes into pacemaker cells in rats

Pacemaker cells can be differentiated from stem cells or transdifferentiated from quiescent mature cardiac cells via genetic manipulation. Here we show that the exposure of rat quiescent ventricular cardiomyocytes to a silk-fibroin hydrogel activates the direct conversion of the quiescent cardiomyocytes to pacemaker cardiomyocytes by inducing the ectopic expression of the vascular endothelial cell-adhesion glycoprotein cadherin. The silk-fibroin-induced pacemaker cells exhibited functional and morphological features of genuine sinoatrial-node cardiomyocytes in vitro, and pacemaker cells generated via the injection of silk fibroin in the left ventricles of rats functioned as a surrogate in situ sinoatrial node. Biomaterials with suitable surface structure, mechanics and biochemistry could facilitate the scalable production of biological pacemakers for human use.

Implementing Biological Pacemakers: Design Criteria for Successful

Each heartbeat that pumps blood throughout the body is initiated by an electrical impulse generated in the sinoatrial node (SAN). However, a number of disease conditions can hamper the ability of the SAN's pacemaker cells to generate consistent action potentials and maintain an orderly conduction path, leading to arrhythmias. For symptomatic patients, current treatments rely on implantation of an electronic pacing device. However, complications inherent to the indwelling hardware give pause to categorical use of device therapy for a subset of populations, including pediatric patients or those with temporary pacing needs. Cellular-based biological pacemakers, derived in vitro or in situ, could function as a therapeutic alternative to current electronic pacemakers. Understanding how biological pacemakers measure up to the SAN would facilitate defining and demonstrating its advantages over current treatments. In this review, we discuss recent approaches to creating biological pacemakers and delineate design criteria to guide future progress based on insights from basic biology of the SAN. We emphasize the need for long-term efficacy in vivo via maintenance of relevant proteins, source-sink balance, a niche reflective of the native SAN microenvironment, and chronotropic competence. With a focus on such criteria, combined with delivery methods tailored for disease indications, clinical implementation will be attainable.

The role of Sfrp and DKK proteins in cardiomyocyte development

In this review, we summarize the role of Wnt proteins in cardiomyogenesis. More specifically, we focus on how the development of cardiomyocytes from precursor cells involves a complex interplay between Wnt canonical β-catenin signaling pathways and Wnt noncanonical signaling pathways involving PCP and JNK. We also describe recent literature which suggests that endogenous Wnt inhibitors such as the Sfrp and DKK proteins play important roles in regulating the cardiomyocyte differentiation.

Control of sinus venous valve and sinoatrial node development by endocardial NOTCH1

AIMS

Sinus venous valve (SVV) and sinoatrial node (SAN) develop together at the sinoatrial junction during embryogenesis. SVV ensures unidirectional cardiac input and SAN generates sinus rhythmic contraction, respectively; both functions are essential for embryonic survival. We aim to reveal the potential role of endocardial NOTCH signalling in SVV and SAN formation.

METHODS AND RESULTS

We specifically deleted Notch1 in the endocardium using an Nfatc1Cre line. This deletion resulted in underdeveloped SVV and SAN, associated with reduced expression of T-box transcription factors, Tbx5 andTbx18, which are essential for the formation of SVV and SAN. The deletion also led to decreased expression of Wnt2 in myocardium of SVV and SAN. WNT2 treatment was able to rescue the growth defect of SVV and SAN resulted from the Notch1 deletion in whole embryo cultures. Furthermore, the Notch1 deletion reduced the expression of Nrg1 in the SVV myocardium and supplement of NRG1 restored the growth of SVV in cultured Notch1 knockout embryos.

CONCLUSION

Our findings support that endocardial NOTCH1 controls the development of SVV and SAN by coordinating myocardial WNT and NRG1 signalling functions.

Canonical Wnt signaling promotes pacemaker cell specification of cardiac mesodermal cells derived from mouse and human embryonic stem cells

Cardiac differentiation of embryonic stem cells (ESCs) can give rise to de novo chamber cardiomyocytes and nodal pacemaker cells. Compared with our understanding of direct differentiation toward atrial and ventricular myocytes, the mechanisms for nodal pacemaker cell commitment are not well understood. Taking a cue from the prominence of canonical Wnt signaling during cardiac pacemaker tissue development in chick embryos, we asked if modulations of Wnt signaling influence cardiac progenitors to bifurcate to either chamber cardiomyocytes or pacemaker cells. Omitting an exogenous Wnt inhibitor, which is routinely added to maximize cardiac myocyte yield during differentiation of mouse and human ESCs, led to increased yield of spontaneously beating cardiomyocytes with action potential properties similar to those of native sinoatrial node pacemaker cells. The pacemaker phenotype was accompanied by enhanced expression of genes and gene products that mark nodal pacemaker cells such as Hcn4, Tbx18, Tbx3, and Shox2. Addition of exogenous Wnt3a ligand, which activates canonical Wnt/β-catenin signaling, increased the yield of pacemaker-like myocytes while reducing cTNT-positive pan-cardiac differentiation. Conversely, addition of inhibitors of Wnt/β-catenin signaling led to increased chamber myocyte lineage development at the expense of pacemaker cell specification. The positive impact of canonical Wnt signaling on nodal pacemaker cell differentiation was evidenced in direct differentiation of two human ESC lines and human induced pluripotent stem cells. Our data identify the Wnt/β-catenin pathway as a critical determinant of cardiac myocyte subtype commitment during ESC differentiation: endogenous Wnt signaling favors the pacemaker lineage, whereas its suppression promotes the chamber cardiomyocyte lineage.

Transcriptional regulation of the cardiac conduction system

The rate and rhythm of heart muscle contractions are coordinated by the cardiac conduction system (CCS), a generic term for a collection of different specialized muscular tissues within the heart. The CCS components initiate the electrical impulse at the sinoatrial node, propagate it from atria to ventricles via the atrioventricular node and bundle branches, and distribute it to the ventricular muscle mass via the Purkinje fibre network. The CCS thereby controls the rate and rhythm of alternating contractions of the atria and ventricles. CCS function is well conserved across vertebrates from fish to mammals, although particular specialized aspects of CCS function are found only in endotherms (mammals and birds). The development and homeostasis of the CCS involves transcriptional and regulatory networks that act in an embryonic-stage-dependent, tissue-dependent, and dose-dependent manner. This Review describes emerging data from animal studies, stem cell models, and genome-wide association studies that have provided novel insights into the transcriptional networks underlying CCS formation and function. How these insights can be applied to develop disease models and therapies is also discussed.

Transcriptome analysis of mouse and human sinoatrial node cells reveals a conserved genetic program

The rate of contraction of the heart relies on proper development and function of the sinoatrial node, which consists of a small heterogeneous cell population, including Tbx3⁺ pacemaker cells. Here, we have isolated and characterized the Tbx3⁺ cells from Tbx3 ^+/Venus knock-in mice. We studied electrophysiological parameters during development and found that Venus-labeled cells are genuine Tbx3⁺ pacemaker cells. We analyzed the transcriptomes of late fetal FACS-purified Tbx3⁺ sinoatrial nodal cells and Nppb-Katushka⁺ atrial and ventricular chamber cardiomyocytes, and identified a sinoatrial node-enriched gene program, including key nodal transcription factors, BMP signaling and Smoc2, the disruption of which in mice did not affect heart rhythm. We also obtained the transcriptomes of the sinoatrial node region, including pacemaker and other cell types, and right atrium of human fetuses, and found a gene program including TBX3, SHOX2, ISL1 and HOX family members, and BMP and NOTCH signaling components conserved between human and mouse. We conclude that a conserved gene program characterizes the sinoatrial node region and that the Tbx3 ^+/Venus allele provides a reliable tool for visualizing the sinoatrial node, and studying its development and function.

Bone morphogenetic protein 4 promotes the differentiation of Tbx18-positive epicardial progenitor cells to pacemaker-like cells

Clarifying the mechanisms via which pacemaker- like cells are generated is critical for identifying novel targets for arrhythmia-associated disorders and constructing pacemakers with the ability to adapt to physiological requirements. T-box 18 (Tbx18)⁺ epicardial progenitor cells (EPCs) have the potential to differentiate into pacemaker cells. Although bone morphogenetic protein 4 (Bmp4) is likely to contribute, its role and regulatory mechanisms in the differentiation of Tbx18⁺ EPCs into pacemaker-like cells have remained to be fully elucidated. In the present study, the association between Bmp4, GATA binding protein 4 (Gata4) and hyperpolarization- activated cyclic nucleotide gated potassium channel 4 (Hcn4) to regulate NK2 homeobox 5 (Nkx2.5), which is known to be required for the differentiation of Tbx18⁺ EPCs into pacemaker-like cells, was assessed. Tbx18⁺ EPCs were isolated from Tbx18:Cre/Rosa26R^{enhanced yellow fluorescence protein (EYFP)} murine embryos at embryonic day 11.5 and divided into the following four treatment groups: Control, Bmp4, Bmp4+LDN193189 (a Bmp inhibitor) and LDN193189. In vitro Bmp4 promoted the expression of Hcn4 in Tbx18⁺ EPCs via lineage tracing of Tbx18:Cre/Rosa26R^EYFP mice, which was likely due to upregulation of Gata4 expression. Gata4 knockdown experiments were then performed using the following five treatment groups: Control, control small interfering RNA (siRNA), Bmp4, Bmp4+siRNA targeting Gata4 (siGata4) and siGata4 group. Knockdown of Gata4 caused a downregulation of Hcn4 and an upregulation of Nkx2.5, but had no effect on Bmp4 expression. In conclusion, it was indicated that in Tbx18⁺ EPCs, the expression of Nkx2.5 was regulated by Bmp4 via Gata4. Taken together, these results provide important information on regulatory networks of pacemaker cell differentiation and may serve as a basis for further studies.

Co-exposure to an Aryl Hydrocarbon Receptor Endogenous Ligand, 6-Formylindolo[3,2-b]carbazole (FICZ), and Cadmium Induces Cardiovascular Developmental Abnormalities in Mice

6-Formylindolo[3,2-b]carbazole (FICZ) is a signal substance and an endogenous activator of aryl hydrocarbon receptor (AHR). Cadmium (Cd) is an environmental pollutant that can activate both AHR and Wnt/β-catenin signaling pathways. We aimed to determine how dysregulated signaling through AHR-Wnt/β-catenin cross-talk can influence mice heart development. Mice fetuses were exposed to Cd alone or in combination with FICZ in gestation day (GD) 0. In GD18, fetuses were harvested and randomly divided into two parts for stereological and molecular studies. Stereological and tessellation results revealed that when fetuses were co-exposed with FICZ and Cd, abnormalities were synergistically raised. In the presence of FICZ, mRNA expression levels of Wnt/β-catenin target genes significantly enhanced, especially when animals co-treated with FICZ and Cd. Based on these findings, we propose that chemical pollutants can interfere with the normal function of AHR that has a physiological role in regulating Wnt/β-catenin during cardiogenesis.

Comparative developmental biology of the cardiac inflow tract

The vertebrate heart receives the blood through the cardiac inflow tract. This area has experienced profound changes along the evolution of vertebrates; changes that have a reflection in the cardiac ontogeny. The development of the inflow tract involves dynamic changes due to the progressive addition of tissue derived from the secondary heart field. The inflow tract is the site where oxygenated blood coming from lungs is received separately from the systemic return, where the cardiac pacemaker is established and where the proepicardium develops. Differential cell migration towards the inflow tract breaks the symmetry of the primary heart tube and determines the direction of the cardiac looping. In air-breathing vertebrates, an inflow tract reorganization is essential to keep separate blood flows from systemic and pulmonary returns. Finally, the sinus venosus endocardium has recently been recognized as playing a role in the constitution of the coronary vasculature. Due to this developmental complexity, congenital anomalies of the inflow tract can cause severe cardiac diseases. We aimed to review the recent literature on the cellular and molecular mechanisms that regulate the morphogenesis of the cardiac inflow tract, together with comparative and evolutionary details, thus providing a basis for a better understanding of these mechanisms.

Sfrp5 identifies murine cardiac progenitors for all myocardial structures except for the right ventricle

Upon acquirement of pulmonary circulation, the ancestral heart may have been remodelled coincidently with, or accompanied by, the production and rearrangement of progenitor cells. However, the progenitor populations that give rise to the left ventricle (LV) and sinus venosus (SV) are still ambiguous. Here we show that the expression of Secreted frizzled-related protein Sfrp5 in the mouse identifies common progenitors for the outflow tract (OFT), LV, atrium and SV but not the right ventricle (RV). Sfrp5 expression begins at the lateral sides of the cardiac crescent, excluding early differentiating regions, and continues in the venous pole, which gives rise to the SV. Lineage-tracing analysis revealed that descendants of Sfrp5-expressing cells at E7.5 contribute not only to the SV but also to the LV, atria and OFT and are found also in the dorsal splanchnic mesoderm accompanied by the expression of the secondary heart field marker, Islet1. These findings provide insight into the arrangement of cardiac progenitors for systemic circulation.

It is unclear which progenitors define different regions of the heart. Here, the authors find Secreted frizzled-related protein 5 is expressed in murine progenitor cells for the outflow tract, first heart field, and sinus venosus, but not the right ventricle, and Wnt inhibition prevents progenitor proliferation.

Wnt signaling in the heart fields: Variations on a common theme

Wnt signaling plays an essential role in development and differentiation. Heart development is initiated with the induction of precardiac mesoderm requiring the tightly and spatially controlled regulation of canonical and noncanonical Wnt signaling pathways. The role of Wnt signaling in subsequent development of the heart fields is to a large extent unclear. We will discuss the role of Wnt signaling in the development of the arterial and venous pole of the heart, highlighting the dual roles of Wnt signaling with respect to its time- and dosage-dependent effects and the balance between the canonical and noncanonical signaling. Canonical signaling appears to be involved in retaining the cardiac precursors in a proliferative and precursor state, whereas noncanonical signaling promotes their differentiation. Thereafter, both canonical and noncanonical signaling regulate specific steps in differentiation of the cardiac compartments. Because heart development is a contiguous, rather than a sequential, process, analyses tend only to show a single timeframe of development. The repetitive alternating and reciprocal effect of canonical and noncanonical signaling is lost when studied in homogenates. Without the simultaneous in vivo visualization of the different Wnt signaling pathways, the mechanism of Wnt signaling in heart development remains elusive.

Fgfr2 is required for the expansion of the early adrenocortical primordium

The adrenal cortex is a critical steroidogenic endocrine tissue, generated at least in part from intermediate mesoderm of the anterior urogenital ridge. Previous work has pinpointed a minor role of the FGFR2IIIb isoform in expansion and differentiation of the fetal adrenal cortex in mice but did not address the complete role of FGFR2 and FGFR1 signaling in adrenocortical development. Here, we show that a Tbx18(cre) line mediates specific recombination in the coelomic epithelium of the anterior urogenital ridge which gives rise by a delamination process to the adrenocortical primordium. Mice with conditional (Tbx18(cre)-mediated) deletion of all isoforms of Fgfr2 exhibited severely hypoplastic adrenal glands around birth. Cortical cells were dramatically reduced in number but showed steroidogenic differentiation and zonation. Neuroendocrine chromaffin cells were also reduced and formed a cell cluster adjacent to but not encapsulated by steroidogenic cells. Analysis of earlier time points revealed that the adrenocortical primordium was established in the intermediate mesoderm at E10.5 but that it failed to expand at subsequent stages. Our further experiments show that FGFR2 signaling acts as early as E11.5 to prevent apoptosis and enhance proliferation in adrenocortical progenitor cells. FGFR1 signaling does not contribute to early adrenocortical development. Our work suggests that FGFR2IIIb and IIIc isoforms largely act redundantly to promote expansion of the adrenocortical primordium.

SHOX2 overexpression favors differentiation of embryonic stem cells into cardiac pacemaker cells, improving biological pacing ability

When pluripotency factors are removed, embryonic stem cells (ESCs) undergo spontaneous differentiation, which, among other lineages, also gives rise to cardiac sublineages, including chamber cardiomyocytes and pacemaker cells. Such heterogeneity complicates the use of ESC-derived heart cells in therapeutic and diagnostic applications. We sought to direct ESCs to differentiate specifically into cardiac pacemaker cells by overexpressing a transcription factor critical for embryonic patterning of the native cardiac pacemaker (the sinoatrial node). Overexpression of SHOX2 during ESC differentiation upregulated the pacemaker gene program, resulting in enhanced automaticity in vitro and induced biological pacing upon transplantation in vivo. The accentuated automaticity is accompanied by temporally evolving changes in the effectors and regulators of Wnt signaling. Our findings provide a strategy for enriching the cardiac pacemaker cell population from ESCs.

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SHOX2 accentuates the molecular profile of pacemaker cells in differentiating ESCs

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SHOX2 increases the frequency and rate of spontaneously active cardiac derivatives

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SHOX2-overexpressing EBs function as biopacemakers when transplanted in vivo

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Wnt signaling underlies SHOX2-mediated pacemaker cell specification

Slit-roundabout signaling regulates the development of the cardiac systemic venous return and pericardium

RATIONALE

The Slit-Roundabout (Robo) signaling pathway has pleiotropic functions during Drosophila heart development. However, its role in mammalian heart development is largely unknown.

OBJECTIVE

To analyze the role of Slit-Robo signaling in the formation of the pericardium and the systemic venous return in the murine heart.

METHODS AND RESULTS

Expression of genes encoding Robo1 and Robo2 receptors and their ligands Slit2 and Slit3 was found in or around the systemic venous return and pericardium during development. Analysis of embryos lacking Robo1 revealed partial absence of the pericardium, whereas Robo1/2 double mutants additionally showed severely reduced sinus horn myocardium, hypoplastic caval veins, and a persistent left inferior caval vein. Mice lacking Slit3 recapitulated the defects in the myocardialization, alignment, and morphology of the caval veins. Ligand binding assays confirmed Slit3 as the preferred ligand for the Robo1 receptor, whereas Slit2 showed preference for Robo2. Sinus node development was mostly unaffected in all mutants. In addition, we show absence of cross-regulation with previously identified regulators Tbx18 and Wt1. We provide evidence that pericardial defects are created by abnormal localization of the caval veins combined with ectopic pericardial cavity formation. Local increase in neural crest cell death and impaired neural crest adhesive and migratory properties underlie the ectopic pericardium formation.

CONCLUSIONS

A novel Slit-Robo signaling pathway is involved in the development of the pericardium, the sinus horn myocardium, and the alignment of the caval veins. Reduced Slit3 binding in the absence of Robo1, causing impaired cardiac neural crest survival, adhesion, and migration, underlies the pericardial defects.

Canonical Wnt signaling regulates smooth muscle precursor development in the mouse ureter

Smooth muscle cells (SMCs) are a key component of many visceral organs, including the ureter, yet the molecular pathways that regulate their development from mesenchymal precursors are insufficiently understood. Here, we identified epithelial Wnt7b and Wnt9b as possible ligands of Fzd1-mediated β-catenin (Ctnnb1)-dependent (canonical) Wnt signaling in the adjacent undifferentiated ureteric mesenchyme. Mice with a conditional deletion of Ctnnb1 in the ureteric mesenchyme exhibited hydroureter and hydronephrosis at newborn stages due to functional obstruction of the ureter. Histological analysis revealed that the layer of undifferentiated mesenchymal cells directly adjacent to the ureteric epithelium did not undergo characteristic cell shape changes, exhibited reduced proliferation and failed to differentiate into SMCs. Molecular markers for prospective SMCs were lost, whereas markers of the outer layer of the ureteric mesenchyme fated to become adventitial fibroblasts were expanded to the inner layer. Conditional misexpression of a stabilized form of Ctnnb1 in the prospective ureteric mesenchyme resulted in the formation of a large domain of cells that exhibited histological and molecular features of prospective SMCs and differentiated along this lineage. Our analysis suggests that Wnt signals from the ureteric epithelium pattern the ureteric mesenchyme in a radial fashion by suppressing adventitial fibroblast differentiation and initiating smooth muscle precursor development in the innermost layer of mesenchymal cells.

Signaling during epicardium and coronary vessel development

The epicardium, the tissue layer covering the cardiac muscle (myocardium), develops from the proepicardium, a mass of coelomic progenitors located at the venous pole of the embryonic heart. Proepicardium cells attach to and spread over the myocardium to form the primitive epicardial epithelium. The epicardium subsequently undergoes an epithelial-to-mesenchymal transition to give rise to a population of epicardium-derived cells, which in turn invade the heart and progressively differentiate into various cell types, including cells of coronary blood vessels and cardiac interstitial cells. Epicardial cells and epicardium-derived cells signal to the adjacent cardiac muscle in a paracrine fashion, promoting its proliferation and expansion. Recently, high expectations have been raised about the epicardium as a candidate source of cells for the repair of the damaged heart. Because of its developmental importance and therapeutic potential, current research on this topic focuses on the complex signals that control epicardial biology. This review describes the signaling pathways involved in the different stages of epicardial development and discusses the potential of epicardial signals as targets for the development of therapies to repair the diseased heart.

The second heart field

Ten years ago, a population of cardiac progenitor cells was identified in pharyngeal mesoderm that gives rise to a major part of the amniote heart. These multipotent progenitor cells, termed the second heart field (SHF), contribute progressively to the poles of the elongating heart tube during looping morphogenesis, giving rise to myocardium, smooth muscle, and endothelial cells. Research into the mechanisms of SHF development has contributed significantly to our understanding of the properties of cardiac progenitor cells and the origins of congenital heart defects. Here recent data concerning the regulation, clinically relevant subpopulations, evolution and lineage relationships of the SHF are reviewed. Proliferation and differentiation of SHF cells are controlled by multiple intercellular signaling pathways and a transcriptional regulatory network that is beginning to be elucidated. Perturbation of SHF development results in common forms of congenital heart defects and particular progenitor cell subpopulations are highly relevant clinically, including cells giving rise to myocardium at the base of the pulmonary trunk and the interatrial septum. A SHF has recently been identified in amphibian, fish, and agnathan embryos, highlighting the important contribution of these cells to the evolution of the vertebrate heart. Finally, SHF-derived parts of the heart share a lineage relationship with craniofacial skeletal muscles revealing that these progenitor cells belong to a broad cardiocraniofacial field of pharyngeal mesoderm. Investigation of the mechanisms underlying the dynamic process of SHF deployment is likely to yield further insights into cardiac development and pathology.