Somatic cells can be transdifferentiated to other cell types without passing through a pluripotent state by ectopic expression of appropriate transcription factors. Recent reports have proposed an alternative transdifferentiation method in which fibroblasts are directly converted to various mature somatic cell types by brief expression of the induced pluripotent stem cell (iPSC) reprogramming factors Oct4, Sox2, Klf4 and c-Myc (OSKM) followed by cell expansion in media that promote lineage differentiation. Here we test this method using genetic lineage tracing for expression of endogenous Nanog and Oct4 and for X chromosome reactivation, as these events mark acquisition of pluripotency. We show that the vast majority of reprogrammed cardiomyocytes or neural stem cells obtained from mouse fibroblasts by OSKM-induced 'transdifferentiation' pass through a transient pluripotent state, and that their derivation is molecularly coupled to iPSC formation mechanisms. Our findings underscore the importance of defining trajectories during cell reprogramming by various methods.
Bibliographical noteFunding Information:
J.H.H. is supported by a generous gift from I. and P. Mantoux; the New York Stem Cell Foundation (NYSCF), FAMRI, the Kimmel Innovator Research Award, the ERC (StG-2011-281906), the Leona M. and Harry B. Helmsley Charitable Trust, Moross Cancer Institute, the Israel Science Foundation Regular research program, the ICRF Foundation, Helen and Martin Kimmel Institute for Stem Cell research (HMKISCR), the Benoziyo Endowment fund. J.H.H. and W.G. are supported by an HFSPO research grant. J.H.H. is a New York Stem Cell Foundation - Robertson Investigator. We thank K. Hochedlinger for mutual exchange of results and discussions before publication. We thank Weizmann Institute management for providing critical financial and infrastructural support.