Fibroblasts in an endocardial fibroelastosis disease model mainly originate from mesenchymal derivatives of epicardium

Endocardial fibroelastosis （EFE） refers to the thickening of the ventricular endocardium as a result of de novo deposition of subendocardial fibrous tissue layers during neonatal heart development. The origin of EFE fibroblasts is proposed to be postnatal endocardial cells that undergo an aberrant endothelial-to-mesenchymal transition （End- MT）. Genetic lineage tracing of endocardial cells with the inducible endocardial Cre line Npr3-CreER and the endo- thelial cell tracing line Cdh5-CreER on an EFE-like model did not reveal any contribution of neonatal endocardial cells to fibroblasts in the EFE-like tissues. Instead, lineage tracing of embryonic epicardium by Wtl-CreER suggested that epicardium-derived mesenchymal cells （MCs） served as the major source of EFE fibroblasts. By labeling MCs using Sox9-CreER, we confirmed that MCs of the embryonic heart expand and contribute to the majority of neonatal EFE fibroblasts. During this pathological process, TGFp signaling, the key mediator of fibroblasts activation, was highly upregulated in the EFE-like tissues. Targeting TGFp signaling by administration of its antagonist bone mor- phogenetic protein 7 effectively reduced fibroblast accumulation and tissue fibrosis in the EFE-like model. Our study provides genetic evidence that excessive fibroblasts in the EFE-like tissues mainly originate from the epicardium-de- rived MCs through epicardial to mesenchymal transition （EpiMT）. These EpiMT-derived fibroblasts within the EFE- like tissues could serve as a potential therapeutic target.