The RNA-binding protein SRSF3 controls epicardial formation by regulating splicing and proliferation.

The epicardium is a fundamental regulator of cardiac development and regeneration, functioning to secrete essential growth factors and to produce epicardium-derived cells that contribute coronary mural cells and cardiac fibroblasts. The molecular mechanisms controlling epicardial formation have not been fully elucidated. In this study, we report 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, preventing proper epicardial formation. Induction of Srsf3 deletion after the proepicardial stage resulted in impaired epicardial proliferation and epicardium-derived cell formation. Single-cell RNA sequencing showed that SRSF3-depleted epicardial cells were eliminated; however, the surviving non-recombined cells upregulated Srsf3, became hyperproliferative and, remarkably, compensated for the early deficit. This unexpected finding attests to 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, including Ccnd1 and Map4k4, mediating both splicing and non-splicing roles. This research defines SRSF3 as an important regulator of epicardial formation and function.