Inhibition of α4-integrin stimulates epicardial-mesenchymal transformation and alters migration and cell fate of epicardially derived mesenchyme

Epithelial-mesenchymal transformation of the embryonic epicardium produces the subepicardial mesenchyme that is essential for normal coronary vascular development. Gene targeting experiments in mice have demonstrated an essential role for α4-integrin in normal epicardial development, but the precise cellular consequences of α4-integrin loss remain uncertain. To better understand the function of α4-integrin in epicardial development, we constructed a replication-incompetent adenovirus (AdlacZα4AS) that expresses antisense chicken α4-integrin as the 3′ untranslated region of a lacZ reporter gene. This construct effectively labeled cells while greatly reducing levels of α4-integrin mRNA and protein. In quail chick chimeras, transplanted epicardial cells infected with AdlacZα4AS adhered to the heart and were incorporated into the epicardium, but 4 days after grafting, were largely absent from the epicardial epithelium, recapitulating the defect in α4-null mice. This did not result from epicardial cell apoptosis or anomalous migration of epicardial cells to extracardiac sites. Rather, AdlacZα4AS-infected epicardial cells were particularly invasive, being three to four times more likely to migrate to the interstitium of the myocardium than AdlacZ-infected epicardial cells. Accelerated epicardial-mesenchymal transformation and migration of α4-negative epicardium was observed in an organ culture system that does not require prior culture of epicardial cells. Remarkably, AdlacZα4AS infection also prevented targeting of epicardially derived mesenchyme to the media of developing coronary vasculature in the myocardial interstitium. This study provides evidence that epicardial α4-integrin normally restrains epicardial-mesenchymal transformation, invasion, and migration and is essential for correct targeting of epicardially derived mesenchyme to the developing coronary vasculature.