Lineage interactions during heart development

Variations in coronary artery anatomy are associated with a number of congenital heart diseases. Knowledge of how the coronary circulation is established is essential for understanding how these congenital defects occur. The coronary ostium are the origins of the two primary coronary arteries and their abnormal development leads to vascular defects yet little is known how they form in mammals. We examined how ostia form in mice and found that a small population of endothelial cells migrate to the aorta where they penetrate to first form the left ostia followed by the right ostia. The ostia increase their diameter by recruiting endothelial cells and extramural cells. After remodeling, the coronary ostia allows blood to flow through the coronary arteries and the accompanying shear stress of the circulation promotes growth and bifurcation of the arteries.;During our investigation of the developmental roles of the bHLH transcription factor Hand1, we found that the loss of Hand1 in neural crest resulted in mis-patterning of the coronary arteries. Loss of Hand1 does not affect cardiac neural crest's ability to populate the heart nor cause significant defects in the outflow tract. We show that Hand1 loss does not affect vasculogenesis but causes defects during angiogenesis in the heart that can be attributed to defective coronary ostia development. Comparing embryos with a deletion of Hand1 in neural crest to normal development, we show that loss of Hand1 leads to ectopic endothelial cells around the ostia and formation of accessory ostia. Mutant hearts display either complete disorganization of the coronary arteries or arteries with an increased luminal diameter. Mutant embryonic hearts also show a decrease in density of vascular network branching. Similarly, latex and corrosion cast injections in adult hearts show a decrease in microvasculature in mutant hearts. To begin addressing the molecular basis of the ostia defects in the Hand1 mutant embryos, we examined the expression of genes known to cause ostia defects. We examined the expression levels of Vegf-A, Vegf-B, Pdgf-BB, and Fgf-2 transcript and found that Vegf-A is up-regulated in the mutant hearts. Our analysis shows for the first time that in addition to the previously known functions of neural crest in heart development, they also play a role in coronary vascular development by regulating ostia formation.;Coordinated contraction of the ventricular myocardium is essential for heart function. The cardiac conduction system (CCS) is responsible for coordinating contraction with developmental defects resulting in embryonic lethality or a predisposition to arrhythmias post-natally. The CCS is derived from working cardiomyocytes that reside adjacent to the endocardium suggesting that the endocardium promotes the trans-differentiation of myocardial cells into Purkinje fiber cells of the CCS. A possible mechanism by which the endocardium signals to the myocardium is by endothelin (ET) signaling.;ET peptide binds to one of the two G-protein coupled receptors (GPCRs) known as endothelin receptor A (EDNRA) and endothelin receptor B (EDNRB). An examination of receptor distribution showed they are expressed in different cell compartments. We simultaneously inactivated the genes encoding the endothelin receptor A (EDNRA) and endothelin receptor B (EDNRB) in mice to determine if their loss resulted in novel cardiovascular defects. We found that double knockout (DKOs) embryos die at birth from mechanical asphyxia due to severe malformation of neural crest-derived facial and throat structures. Comparing hearts with deletions of both endothelin receptors to single Ednra and Ednrb nulls showed the DKO and single Ednrb null hearts have more severe thinning of their myocardial walls as compared to single Ednra nulls. The hearts have a thin myocardial wall but no obvious morphological CCS defects. Examining genes known to cause myocardial and conduction system defects we found an up-regulation of Irx3, Irx5, and Bmp10. Our analysis shows for the first time that endothelin receptors are not required for proper cardiac conduction system formation and are expressed in different cell compartments in the heart.