Apoptosis Is Required for the Proper Formation of the Ventriculo-Arterial Connections

The role of apoptosis in cardiac morphogenesis has not been directly tested. Cardiomyocyte apoptosis is prevalent during the remodeling of the embryonic chicken cardiac outflow tract (OFT) in the transition from a single to a dual circulation. We tested the hypothesis that OFT cardiomyocyte apoptosis drives the shortening and rotation of the embryonic cardiac OFT and is required to achieve the mature ventriculo-arterial configuration. Chick embryos were treated with the peptide Caspase inhibitors zVAD-fmk or DEVD-cho at HH stages 15–20 (looped heart). Morphology of control and experimental embryos was assessed at HH stage 35, at which time the control hearts have developed a dual circulation. Infection of the hearts with a recombinant adenovirus expressing green fluorescent protein was used to follow the fate of the OFT cardiomyocytes. Affected embryos displayed abnormal persistence of a long infundibulum (OFT myocardial remnant) beneath the great vessels, indicating failure of OFT shortening. In some instances, the infundibulum connected both great vessels to the right ventricle in a side-by-side arrangement with transposition of the aorta, indicating a failure of rotation of the OFT, and modeling human congenital double outlet right ventricle. Defects were also observed at other sites in the heart where apoptosis is prevalent, such as in the formation of the cardiac valves and trabeculae. To more specifically target the apoptosis of the OFT cardiomyocytes, recombinant adenovirus was used to express the X-linked inhibitor of apoptosis protein in these cells. This resulted in an effect on outflow tract shortening and rotation similar to that of the peptide inhibitors, while the effects on the other cardiac structures were not observed. These results demonstrate that elimination of OFT cardiomyocytes by apoptosis is necessary for the proper formation of the ventriculo-arterial connections, and suggest apoptosis as a potential target of teratogens and genetic defects that are associated with congenital human conal heart defects.