SRSF3 is a key regulator of epicardial formation

The epicardium is a fundamental regulator of cardiac development and regeneration, functioning to secrete essential growth factors and to produce epicardium-derived cells (EPDCs) that contribute most coronary mural cells and cardiac fibroblasts. The molecular mechanisms controlling epicardial formation have not been fully elucidated. In this study, we found that the RNA-binding protein SRSF3 is highly expressed in the embryonic proepicardium and epicardial layer. Deletion of Srsf3 from the murine proepicardium led to proliferative arrest, which prevented proper epicardial formation. Induction of Srsf3 deletion after the proepicardial stage resulted in impaired epicardial proliferation and EPDC formation by E13.5. Single-cell RNA-sequencing showed SRSF3-depleted epicardial cells were eliminated, however, the surviving non-recombined cells became hyperproliferative and, remarkably, compensated for the early deficit, via a mechanism that involved Srsf3 up-regulation This unexpected finding attests the importance of SRSF3 in controlling epicardial proliferation, and highlights the significant confounding effect of mosaic recombination on embryonic phenotyping. Mapping the SRSF3-RNA interaction network by endogenous irCLIP identified binding to major cell cycle regulators, such as Ccnd1 and Map4k4, with both splicing and non-splicing roles. This research defines SRSF3 as a key regulator of epicardial cell proliferation.