Zfpm1 regulates endothelial-mesenchymal transition and endocardial cell differentiation

The endocardium is the endothelial component of the heart that forms valves. Endocardial (EC) cells undergo endothelial-mesenchymal transition (EndoMT) which distinguishes them from endothelial (ET) cells, despite sharing markers and a mesodermal origin. Nfatc1, uniquely labels EC cells but is not sufficient for EC cell specification, since knockouts form valves, albeit underdeveloped, therefore the regulators that specify EC fate remain incompletely defined. Using an in vivo Nfatc1 reporter system, we show that EC and ET cells are transcriptionally distinct and identify the transcription factor Zfpm1 (Fog1), as a potential regulator of EC cell specification. In an in vitro embryonic stem cell differentiation model, Zfpm1 knockdown results in an impaired EC cell differentiation, with reduced endothelial-mesenchymal transition and altered contractility and beating phenotype, consistent with reduced differentiation of EC cells affecting the crosstalk with myocardial cells. These findings are consistent with data linking mutations in the human ZFPM1 gene with non-syndromic tetralogy of Fallot. This and the broad expression pattern across all regions of the cardiac outflow tract support a role for Zfpm1 in mammalian EC cell differentiation and endothelial-mesenchymal transition. This study extends the in vivo evidence of its relevance to cardiac development and provides new insights into the transcriptional control of EC lineages.