P53 Activation Promotes Maturational Characteristics of Pluripotent Stem Cell-Derived Cardiomyocytes in 3-Dimensional Suspension Culture Via FOXO-FOXM1 Regulation

Current protocols generate highly pure human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) in vitro that recapitulate characteristics of mature in vivo cardiomyocytes. Yet, a risk of arrhythmias exists when hiPSC-CMs are injected into large animal models. Thus, understanding hiPSC-CM maturational mechanisms is crucial for clinical translation. Forkhead box (FOX) transcription factors regulate postnatal cardiomyocyte maturation through a balance between FOXO and FOXM1. We also previously demonstrated that p53 activation enhances hiPSC-CM maturation. Here, we investigate whether p53 activation modulates the FOXO/FOXM1 balance to promote hiPSC-CM maturation in 3-dimensional suspension culture. Three-dimensional cultures of hiPSC-CMs were treated with Nutlin-3a (p53 activator, 10 μM), LOM612 (FOXO relocator, 5 μM), AS1842856 (FOXO inhibitor, 1 μM), or RCM-1 (FOXM1 inhibitor, 1 μM), starting 2 days after onset of beating, with dimethyl sulfoxide (0.2% vehicle) as control. P53 activation promoted hiPSC-CM metabolic and electrophysiological maturation alongside FOXO upregulation and FOXM1 downregulation, in n=3 to 6 per group for all assays. FOXO inhibition significantly decreased expression of cardiac-specific markers such as TNNT2. In contrast, FOXO activation or FOXM1 inhibition promoted maturational characteristics such as increased contractility, oxygen consumption, and voltage peak maximum upstroke velocity, in n=3 to 6 per group for all assays. Further, by single-cell RNA sequencing of n=2 LOM612-treated cells compared with dimethyl sulfoxide, LOM612-mediated FOXO activation promoted expression of cardiac maturational pathways. We show that p53 activation promotes FOXO and suppresses FOXM1 during 3-dimensional hiPSC-CM maturation. These results expand our understanding of hiPSC-CM maturational mechanisms in a clinically-relevant 3-dimensional culture system.