In vivo induction of cardiac Purkinje fiber differentiation by coexpression of preproendothelin-1 and endothelin converting enzyme-1

The rhythmic heart beat is coordinated by electrical impulses transmitted f rom Purkinje fibers of the cardiac conduction system. During embryogenesis, the impulse-conducting cells differentiate from cardiac myocytes in direct association with the developing endocardium and coronary arteries, but not with the venous system. This conversion of myocytes into Purkinje fibers r equires a paracrine interaction with blood vessels in vivo, and can be indu ced in vitro by exposing embryonic myocytes to endothelin-l (ET-1), an endo thelial cell-associated paracrine factor, These results suggest that an end othelial cell-derived signal is capable of inducing juxtaposed myocytes to differentiate into Purkinje fibers, It remains unexplained how Purkinje fib er recruitment is restricted to subendocardial and periarterial sites but n ot those juxtaposed to veins. Here we show that while the ET-receptor is ex pressed throughout the embryonic myocardium, introduction of the ET-1 precu rsor (preproET-1) in the embryonic myocardium is not sufficient to induce m yocytes to differentiate into conducting cells, ET converting enzyme-1 (ECE -1), however, is expressed preferentially in endothelial cells of the endoc ardium and coronary arteries where Purkinje fiber recruitment takes place. Retroviral-mediated coexpression of both preproET-1 and ECE-1 in the embryo nic myocardium induces myocytes to express Purkinje fiber markers ectopical ly and precociously. These results suggest that expression of ECE-1 plays a key role in defining an active site of ET signaling in the heart, thereby determining the tinting and location of Purkinje fiber differentiation with in the embryonic myocardium.