Wt1+/Itga4+ progenitors drive epicardial-derived cell expansion via non-canonical Wnt signalling

The correct formation of the epicardium – the outermost tissue layer of the heart – is crucial to proper cardiac embryonic morphogenesis. However, our knowledge of the molecular and cellular mechanisms regulating epicardial progenitor cell conversion into primitive epicardial cells is limited. By combining in vivo mouse models for cell lineage tracing, high throughput transcriptomic technologies, and in vitro assays, we have characterised the proepicardium (PE), the transient extracardiac structure that contains epicardial progenitors. Our study reveals that only Wt1 ⁺ / Itga4 ⁺ PE cells truly represent the progenitors of primitive epicardial cells and epicardium-derived cells (EPDCs). We show that early EPDCs display a high proliferative activity that is regulated by non-canonical Wnt signals (Wnt5a/c-Jun axis). This study redefines the concept of PE/epicardial progenitors, uncovers relevant aspects of the mechanisms controlling early EPDC formation, and lays the foundation for future research on EPDC differentiation in clinically relevant cardiac interstitial cells.