Regulation of heart morphogenesis by the myocardial oxygen sensing mechanism

Hypoxia stress causes defective heart morphogenesis. Under hypoxia, hydroxylation of HIF-α proteins by prolyl hydroxylase domain containing proteins (PHDs) is inefficient, allowing them to accumulate to high levels by escaping von Hippel Lindau protein (VHL) dependent polyubiquitination and proteasomal degradation. This thesis examines how PHDs and VHL, which are important for oxygen sensing and oxygen dependent degradation of HIF-α, regulate coronary vascular patterning and myocardial development. When Phd2 was specifically deleted in the myocardium by Nkx2.5Cre, overt heart defects did not occur but modest coronary vascular defects were found. In contrast, Nkx2.5Cre-mediated triple knockout of Phd1, Phd2 and Phd3 caused heart defects similar to those reported for hypoxia-stressed embryos, such as thin myocardial compact layer and ventricular septation defects. Nkx2.5Cre-mediated knockout of Vhl resulted in similar heart defects. In either VHL or triple PHD deficient hearts, coronary vascular patterning was defective. Coronary vascular networks covered smaller areas, and failed to evolve into mature vascular trees with distinct large and small vascular branches. In avascular areas, there were numerous CD31+/VEGFR-2+ foci, suggesting failure of endothelial cells to organize into vascular structures. Dye injection into embryonic circulation confirmed that cardiac tissues in VHL or triple PHD deficient hearts were poorly irrigated. These data indicate the essentiality of the myocardial oxygen sensing mechanism to regulate HIF-1α levels. Hyperactive HIF signaling in the myocardium interferes with normal coronary vascular patterning, which in turn hinders myocardial development. It is likely that this mechanism is also operational under hypoxia stress.