Abstract
Mammalian target of rapamycin (mTOR) is a serine/threonine kinase and functions through two distinct complexes, mTOR complex 1 (mTORC1) and complex 2 (mTORC2), with their key components Raptor and Rictor, to play crucial roles in cellular survival and growth. However, the roles of mTORC1 and mTORC2 in regulating cardiomyocyte differentiation from mouse embryonic stem (mES) cells are not clear. In this study, we performed Raptor or Rictor knockdown experiments to investigate the roles of mTORC1 and mTORC2 in cardiomyocyte differentiation. Ablation of Raptor markedly increased the number of cardiomyocytes derived from mES cells with well-organized myofilaments. Expression levels of brachyury (mesoderm protein), Nkx2.5 (cardiac progenitor cell protein), and α-Actinin (cardiomyocyte marker) were increased in Raptor knockdown cells. In contrast, loss of Rictor prevented cardiomyocyte differentiation. The dual ablation of Raptor and Rictor also decreased the number of cardiomyocytes. The two complexes exerted a regulatory mechanism in such a manner that knockdown of Raptor/mTORC1 resulted in a decreased phosphorylation of Rictor (Thr1135), which subsequently activated Rictor/mTORC2 in the differentiation of mES cells into cardiomyocytes. In conclusion, mTORC1 and mTORC2 played different roles in cardiomyocyte differentiation from mES cells in vitro. The activation of Rictor/mTORC2 was critical for facilitating cardiomyocyte differentiation from mES cells. Thus, this complex may be a promising target for regulating myocardial differentiation from embryonic stem cells or induced pluripotent stem cells.