SUMMARY The coronary vessels and epicardium arise from an extracardiac rudiment called the proepicardium. Failed fusion of the proepicardium to the heart results in severe coronary and heart defects. However, it is unknown how the proepicardium protrudes toward and attaches to the looping heart tube. Here we show that ectopic expression of BMP ligands in the embryonic myocardium can cause proepicardial cells to target aberrant regions of the heart. Additionally, misexpression of a BMP antagonist, Noggin, suppresses proepicardium protrusion and contact with the heart. Finally, proepicardium explant preferentially expands toward a co-cultured heart segment. This preference can be mimicked by BMP2/4 and suppressed by Noggin. These results support a model in which myocardium-derived BMP signals regulate the entry of coronary progenitors to the specific site of the heart by directing their morphogenetic movement.

SUMMARYParacrine signals, both positive and negative, regulate the positioning and remodeling of embryonic blood vessels. In the embryos of mammals and birds, the first major remodeling event is the fusion of bilateral dorsal aortae at the midline to form the dorsal aorta. Although the original bilaterality of the dorsal aortae occurs as the result of inhibitory factors (antagonists of BMP signaling) secreted from the midline by the notochord, it is unknown how fusion is later signaled. Here, we report that dorsal aortae fusion is tightly regulated by a change in signaling by the notochord along the anteroposterior axis. During aortae fusion, the notochord ceases to exert its negative influence on vessel formation. This is achieved by a transcriptional downregulation of negative regulators while positive regulators are maintained at pre-fusion levels. In particular, Chordin, the most abundant BMP antagonist expressed in the notochord prior to fusion, undergoes a dramatic downregulation in an anterior to posterior wave. With inhibitory signals diminished and sustained expression of the positive factors SHH and VEGF at the midline, fusion of the dorsal aortae is signaled. These results demonstrate a novel mechanism by which major modifications of the vascular pattern can occur through modulation of vascular inhibitors without changes in the levels of positive vascular regulators.

Efficient blood flow depends on two developmental processes that occur within the atrioventricular junction (AVJ) of the heart: conduction delay, which entrains sequential chamber contraction; and valve formation, which prevents retrograde fluid movement. Defects in either result in severe congenital heart disease; however, little is known about the interplay between these two crucial developmental processes. Here, we show that AVJ conduction delay is locally assigned by the morphogenetic events that initiate valve formation. Our data demonstrate that physical separation from endocardial-derived factors prevents AVJ myocardium from becoming fast conducting. Mechanistically, this physical separation is induced by myocardial-derived factors that support cardiac jelly deposition at the onset of valve formation. These data offer a novel paradigm for conduction patterning, whereby reciprocal myocardialendocardial interactions coordinate the processes of valve formation with establishment of conduction delay. This, in turn, synchronizes the electrophysiological and structural events necessary for the optimization of blood flow through the developing heart.

The coronary vessel precursors and epicardium derive from a separate tissue rudiment outside of the heart called the proepicardium (PE). The PE protrudes to the heart and makes contact with the atrioventricular junction/inner curvature (AV/IC) region despite being closer to the sinoatrium. The attachment of the PE to the heart is an essential step in coronary vasculogenesis and formation of the epicardium. However, it is unknown how the specificity of PE attachment to the AV/IC is achieved. We demonstrate that BMP‐signaling from the myocardium specifies the attachment site of the PE to the heart. We show that Bmp2, which is expressed specifically at the AV/IC region of the heart, can signal alternative sites of PE attachment when misexpressed outside the AV/IC region. In contrast, misexpression of a BMP‐antagonist, Noggin, blocks PE attachment to the heart. Finally, PE explants preferentially protrude toward the AV/IC and this preference can be mimicked by BMP2/4. These results support a model in which BMP signals from the myocardium regulate PE protrusion and contact with a specific site on the heart.Grant Funding Source: NIH