Tbx18 function in epicardial development

The embryonic epicardium is a source of smooth muscle cells and fibroblasts of the coronary vasculature and of the myocardium, but the molecular circuits that direct the temporal and spatial generation of these cell types from epicardium-derived cells are only partly known. We aimed to elucidate the functional significance of the conserved epicardial expression of the T-box transcription factor gene Tbx18 using transgenic technology in the mouse. We show by cellular and molecular analyses that in Tbx18-deficient mice the epicardium is formed normally and that epicardial cells undergo an epithelialmesenchymal transition, differentiate into smooth muscle cells and fibroblasts, and form a normal coronary vasculature and fibrous skeleton. Prolonged expression of Tbx18 in epicardium-derived cells by a transgenic approach in vivo does not affect the differentiation and migratory behaviour of these cells. In contrast, epicardial misexpression of a transcriptional activator version of Tbx18, Tbx18VP16, results in premature smooth muscle differentiation of epicardial cells. Inhibition of Notch and transforming growth factor beta receptor signalling in Tbx18VP16 expressing epicardial cells in explant cultures reverts this phenotype. Tbx18 is dispensable for epicardial development, yet a repressive T-box function may be required to prevent premature smooth muscle cell differentiation by repressing transforming growth factor beta receptor and Notch signalling in the embryonic epicardium.