CD4 + Tregs Regulate Heart Growth and Regeneration Through MRG15/TIP60-Mediated Epigenomic Remodeling in Proliferating Cardiomyocytes

BACKGROUND: Cardiovascular disease remains a leading cause of mortality globally, with the adult mammalian heart exhibiting limited regenerative capacity. The chromatin regulatory network plays a crucial role in the dynamic changes in gene expression that orchestrate the regenerative response in the neonatal heart. This study aims to identify key chromatin regulators in neonatal cardiomyocytes and to elucidate their roles in heart regeneration. METHODS: We generated genetic knockout mouse models by crossing Mrg15 fl/fl and Tip60 fl/fl mice with various Cre-driver lines such as Isl1-Cre and Myh6-MerCreMer to evaluate the function of MRG15 (MORF-related gene 15)/TIP60 in cardiac progenitor cells and cardiomyocytes during heart development and regeneration. The epigenetic regulation of cardiomyocyte proliferation by MRG15/TIP60 was investigated through a variety of methods, including histological, cellular, genomic, transcriptomic, computational, and pharmacological approaches. In addition, we assessed the regulation of transient MRG15 induction in the regenerating neonatal heart by CD4 + regulatory T cells through both adoptive transfer and monoclonal antibody–based depletion in mice. RESULTS: MRG15, but not its cofactor TIP60, was transiently expressed in neonatal cardiomyocytes, with its expression downregulated as the regenerative potential of the heart declined. We generated knockout mice targeting Mrg15 in cardiac progenitor cells and cardiomyocytes, revealing that MRG15 is critical for neonatal heart regeneration and plays a key role in regulating cardiomyocyte proliferation. Mechanistically, MRG15 forms an activator complex with TIP60, p300, and RNA polymerase II, facilitating histone acetylation at the Ccnd1 enhancer region. Furthermore, regulatory T cells were shown to induce MRG15 expression, promoting cardiomyocyte proliferation through paracrine signaling. Notably, adeno-associated virus 9–mediated overexpression of Mrg15 rescued the impaired heart regeneration after Treg depletion in vivo. Furthermore, recapitulating MRG15 expression into the juvenile heart promoted regeneration by enhancing cardiomyocyte proliferation. CONCLUSIONS: This study elucidates the critical role of regulatory T cells in regulating the transient expression of MRG15 in regenerating cardiomyocytes and its subsequent control of cardiomyocyte proliferation and heart regeneration through TIP60-mediated chromatin modification. Our findings highlight the important interplay between the cardiac and the immune system during neonatal development, particularly through the MRG15/TIP60 axis regulated by regulatory T cells. This cross-talk suggests promising therapeutic strategies for enhancing cardiac repair by targeting these pathways.