Developmental cues from epicardial cells simultaneously promote cardiomyocyte proliferation and electrochemical maturation.

Accumulating evidence indicates that maturation limits cardiomyocyte proliferation. We expand on that theory by co-culturing human induced pluripotent stem cell (hiPSC)-cardiomyocytes (CM) with epicardial cells (EPCs) and epicardial-derived cells in both 2D co-cultures and 3D engineered heart tissues (EHTs). In 2D co-cultures, the percentage of proliferating CM increased in parallel with stark electrophysiologic improvements. Single-cell transcriptomics revealed a significant shift in the bulk CM population of the epicardial-CM co-cultures as characterized by more fetal-like myofilament isoforms but with enhanced pathways associated with electrochemical maturation. The 3D-EHTs containing EPCs showed more limited proliferation but a similar improvement in CM electrophysiologic function. Next, epicardial-derived fibroblasts (EPD-FBs) were added to the EHTs containing EPCs, and we observed significant myofilament maturation and increased force generation. Our results suggest that some aspects of CM maturation (i.e., electrochemical) can occur when proliferation rates are relatively high, and that sarcomere-associated mechanical maturation occurs at later developmental stages when proliferation has largely ceased.