The Lhx9-integrin pathway is essential for positioning of the proepicardial organ.

The development of the vertebrate embryonic heart occurs by hyperplastic growth as well as the incorporation of cells from tissues outside of the initial heart field. Amongst these tissues is the epicardium, a cell structure that develops from the precursor proepicardial organ on the right side of the septum transversum caudal to the developing heart. During embryogenesis, cells of the proepicardial organ migrate, adhere and envelop the maturing heart, forming the epicardium. The cells of the epicardium then delaminate and incorporate into the heart giving rise to cardiac derivatives, including smooth muscle cells and cardiac fibroblasts. Here, we demonstrate that the LIM homeodomain protein Lhx9 is transiently expressed in Xenopus proepicardial cells and is essential for the position of the proepicardial organ on the septum transversum. Utilizing a small-molecule screen, we found that Lhx9 acts upstream of integrin-paxillin signaling and consistently demonstrate that either loss of Lhx9 or disruption of the integrin-paxillin pathway results in mis-positioning of the proepicardial organ and aberrant deposition of extracellular matrix proteins. This leads to a failure of proepicardial cell migration and adhesion to the heart, and eventual death of the embryo. Collectively, these studies establish a requirement for the Lhx9-integrin-paxillin pathway in proepicardial organ positioning and epicardial formation.