Abstract Body: Introduction
Adult Cardiomyocytes (CMs) are post-mitotic cells, hence cardiac insults usually progress to heart failure. In contrary, spontaneously cycling CMs are a distinct population in the neonatal heart that has been reported but the molecular mechanisms regulating their cell cycle activity remain poorly understood. In this study, we aimed to identify the signaling pathways that define these CMs in the neonatal stage.
Methods and Results
Using single-cell RNAseq of neonatal mouse cardiomyocytes at postnatal day 1 (NMCMs-P1), we identified Cd36 as a marker of spontaneously proliferating CMs and those responsive to cell cycle induction. Global CD36 knockout (CD36^KO) and cardiomyocyte-specific knockout (CD36^CKO) mice displayed smaller hearts, reduced CM numbers, impaired CM proliferation, and diminished regenerative capacity following apical resection at P1. Bulk RNAseq of CD36^KO and CD36^CKO hearts revealed downregulation of cell cycle genes alongside key components of lipid signaling, including Fabp5, Pparδ, and Rxrs. Spatial transcriptomics in wild-type (WT) P1 hearts localized a subset of proliferative CMs co-expressing Cd36, Fabp5, and Pparδ.
To investigate the upstream regulator of this pathway, spatial metabolomics revealed reduced cardiac retinyl ester levels in CD36^KO and CD36^CKO mice, which was confirmed by quantitative assays. At nanomolar concentrations, retinoic acid (RA) enhanced cell cycle activity in WT-P1 NMCMs but not in CD36^KO CMs. FABP5 knockdown abolished the RA effect, while FABP5 overexpression amplified it. Crossbreeding CD36^KO mice with PPARδ cardiac-specific overexpressing mice restored CM proliferation and regenerative capacity after apical resection to WT levels. Furthermore, PPARδ overexpression in neonatal mice enhanced CM proliferation and improved heart regeneration following injury.
Conclusions
We identified a distinct population of spontaneously cycling neonatal CMs marked by Cd36 expression, where the CD36/FABP5 axis facilitates RA-mediated activation of PPARδ, driving CM proliferation. This study provides new insights into the regulatory mechanisms controlling neonatal CM proliferation and highlights potential therapeutic targets for promoting heart regeneration.