In vivo bioreactor matures iPSC-CMs for MLP disease modeling.

Muscle LIM protein (MLP) is a critical regulator of cardiomyocytes (CMs) cytoarchitecture, and its deficiency results in late-onset dilated cardiomyopathy (DCM) in both mice and human. However, recapitulating this phenotype in vitro using induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) has been challenging largely due to their immature state. Here, we generated MLP knockout (MLP-KO) mouse iPSCs and differentiated them into CMs. We then employed both conventional in vitro and in vivo transplantation approach using immunocompromised rat hearts to promote cardiomyocytes maturation. Our results showed that while in vitro-matured MLP-KO iPSC-CMs failed to exhibit disease phenotypes, in vivo-matured MLP-KO iPSC-CMs successfully recapitulated the hallmarks of DCM, including disrupted sarcomeric architecture and upregulation of atrial natriuretic peptide (ANP), closely mirroring disease progression observed in MLP-deficient mice. These findings demonstrate that the in vivo maturation environment is essential for the maturation of iPSC-derived cardiomyocytes to better model genetic cardiac diseases like DCM and provide valuable insights for future therapeutic strategies.