Isthmin-1 is a key regulator of induced pluripotent stem cell–derived cardiomyocytes maturation through activation of p53 signaling

Human induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) remain structurally, metabolically, and functionally immature, limiting their value for disease modeling and regenerative therapy. Here, we identify Isthmin-1 (ISM1) as an intrinsic regulator of cardiomyocyte maturation and define its mechanism of action. Cross-species transcriptomic comparisons of adult versus fetal hearts and hypertrophic cardiomyopathy versus healthy myocardium highlighted ISM1 as a conserved maturation-associated gene. In human iPSC-CMs, ISM1 overexpression enhanced sarcomere organization, increased contractile protein expression, elevated ATP production, promoted oxidative phosphorylation (Seahorse OCR), and improved calcium-handling kinetics, whereas ISM1 knockdown produced the opposite phenotype. RNA sequencing showed that ISM1 drives a global shift toward an adult-like transcriptional state, characterized by suppression of cell-cycle programs and activation of oxidative metabolic pathways. Pathway enrichment analyses consistently identified p53 signaling as the most significantly activated pathway in ISM1-overexpressing iPSC-CMs. Mechanistically, co-immunoprecipitation and imaging demonstrated a direct ISM1-p53 interaction; ISM1 increased p53 protein stability, promoted nuclear accumulation, and enhanced transcription of canonical p53 targets involved in metabolic remodeling and cell-cycle exit. Pharmacological inhibition of p53 abolished ISM1-induced structural and metabolic maturation, establishing p53 as a required mediator. Collectively, these data define ISM1 as a previously unrecognized driver of cardiomyocyte maturation that promotes structural, metabolic, and functional maturation of iPSC-CMs through p53-dependent transcriptional reprogramming.