Epicardial function of canonical Wnt-, Hedgehog-, Fgfr1/2-, and Pdgfra-signalling.

Aims: The embryonic epicardium is a source of smooth muscle cells and fibroblasts of the coronary vasculature and of the myocardium, but the signalling pathways that control mobilization and differentiation of epicardial cells are only partly known. We aimed to (re-)evaluate the relevance of canonical Wnt-, Hedgehog (Hh)-, Fibroblast growth factor receptor (Fgfr)1/2-, and platelet-derived growth factor receptor alpha (Pdgfra)-signalling in murine epicardial development.

Methods And Results: We used a T-box 18 (Tbx18)(cre)-mediated conditional approach to delete and to stabilize, respectively, the downstream mediator of canonical Wnt-signalling, beta-catenin (Ctnnb1), to delete and activate the mediator of Hh-signalling, smoothened (Smo), and to delete Fgfr1/Fgfr2 and Pdgfra in murine epicardial development.

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.

Partial absence of pleuropericardial membranes in Tbx18- and Wt1-deficient mice.

The pleuropericardial membranes are fibro-serous walls that separate the pericardial and pleural cavities and anchor the heart inside the mediastinum. Partial or complete absence of pleuropericardial membranes is a rare human disease, the etiology of which is poorly understood. As an attempt to better understand these defects, we wished to analyze the cellular and molecular mechanisms directing the separation of pericardial and pleural cavities by pleuropericardial membranes in the mouse.

Wnt/Ctnnb1 signaling and the mesenchymal precursor pools of the heart.

Lineage tracing has shown that the different regions of the four-chambered heart of mammalian embryos derive from molecularly distinct precursor pools in a spatially and temporally tightly controlled manner. Cells of the first heart field differentiate early and form the linear heart tube of headfold-stage embryos, the future left ventricle. The right ventricle, atria, and outflow tract derive from the second heart field by recruitment and delayed local myocardial differentiation.

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

Rationale: Canonical (β-catenin [Ctnnb1]-dependent) wingless-related MMTV integration site (Wnt) signaling plays an important role in the development of second heart field-derived structures of the heart by regulating precursor cell proliferation. The signaling pathways that regulate the most posterior elongation of the heart, that is, the addition of the systemic venous return from a Tbx18(+) precursor population, have remained elusive.

Objective: To define the role of Ctnnb1-dependent Wnt signaling in the development of the cardiac venous pole.

Notch signaling regulates smooth muscle differentiation of epicardium-derived cells.

Rationale: The embryonic epicardium plays a crucial role in the formation of the coronary vasculature and in myocardial development, yet the exact contribution of epicardium-derived cells (EPDCs) to the vascular and connective tissue of the heart, and the factors that regulate epicardial differentiation, are insufficiently understood.

Objective: To define the role of Notch signaling in murine epicardial development.

Methods And Results: Using in situ hybridization and RT-PCR analyses, we detected expression of a number of Notch receptor and ligand genes in early epicardial development, as well as during formation of coronary arteries.

Developmental origin, growth, and three-dimensional architecture of the atrioventricular conduction axis of the mouse heart.

Rationale: The clinically important atrioventricular conduction axis is structurally complex and heterogeneous, and its molecular composition and developmental origin are uncertain.

Objective: To assess the molecular composition and 3D architecture of the atrioventricular conduction axis in the postnatal mouse heart and to define the developmental origin of its component parts.

Methods And Results: We generated an interactive 3D model of the atrioventricular junctions in the mouse heart using the patterns of expression of Tbx3, Hcn4, Cx40, Cx43, Cx45, and Nav1.

Wt1 and retinoic acid signaling in the subcoelomic mesenchyme control the development of the pleuropericardial membranes and the sinus horns.

Rationale: The cardiac venous pole is a common focus of congenital malformations and atrial arrhythmias, yet little is known about the cellular and molecular mechanisms that regulate its development. The systemic venous return myocardium (sinus node and sinus horns) forms only late in cardiogenesis from a pool of pericardial mesenchymal precursor cells.

Objective: To analyze the cellular and molecular mechanisms directing the formation of the fetal sinus horns.

The sinus venosus progenitors separate and diversify from the first and second heart fields early in development.

Aims: During development, the heart tube grows by differentiation of Isl1(+)/Nkx2-5(+) progenitors to the arterial and venous pole and dorsal mesocardium. However, after the establishment of the heart tube, Tbx18(+) progenitors were proposed to form the Tbx18(+)/Nkx2-5(-) sinus venosus and proepicardium. To elucidate the relationship between these contributions, we investigated the origin of the Tbx18(+) sinus venosus progenitor population in the cardiogenic mesoderm and its spatial and temporal relation to the second heart field during murine heart development.

Tbx20 interacts with smads to confine tbx2 expression to the atrioventricular canal.

Rationale: T-box transcription factors play critical roles in the coordinated formation of the working chambers and the atrioventricular canal (AVC). Tbx2 patterns embryonic myocardial cells to form the AVC and suppresses their differentiation into chamber myocardium. Tbx20-deficient embryos, which fail to form chambers, ectopically express Tbx2 throughout the entire heart tube, providing a potential mechanism for the function of Tbx20 in chamber differentiation.

The Tbx2+ primary myocardium of the atrioventricular canal forms the atrioventricular node and the base of the left ventricle.

The primary myocardium of the embryonic heart, including the atrioventricular canal and outflow tract, is essential for septation and valve formation. In the chamber-forming heart, the expression of the T-box transcription factor Tbx2 is restricted to the primary myocardium. To gain insight into the cellular contributions of the Tbx2+ primary myocardium to the components of the definitive heart, genetic lineage tracing was performed using a novel Tbx2Cre allele.

Tbx18 and the fate of epicardial progenitors.

Uncovering the origins of myocardial cells is important for understanding and treating heart diseases. Cai et al. suggest that Tbx18-expressing epicardium provides a substantial contribution to myocytes in the ventricular septum and the atrial and ventricular walls.