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Summary Generation of induced oligodendrocyte progenitor cells (iOPCs) from somatic fibroblasts is a strategy for cell-based therapy of myelin diseases. However, iOPC generation is inefficient, and the resulting iOPCs exhibit limited expansion and differentiation competence. Here we overcome these limitations by transducing an optimized transcription factor combination into a permissive donor phenotype, the pericyte. Pericyte-derived iOPCs (PC-iOPCs) are stably expandable and functionally myelinogenic with high differentiation competence. Unexpectedly, however, we found that PC-iOPCs are metastable so that they can produce myelination-competent oligodendrocytes or revert to their original identity in a context-dependent fashion. Phenotypic reversion of PC-iOPCs is tightly linked to memory of their original transcriptome and epigenome. Phenotypic reversion can be disconnected from this donor cell memory effect, and in vivo myelination can eventually be achieved by transplantation of O4+ pre-oligodendrocytes. Our data show that donor cell source and memory can contribute to the fate and stability of directly converted cells.

Adult stem cells are responsible for the cellular turnover of many organs, and an impairment in their function leads to aging and disease. In efforts to reverse the process of tissue stem cell aging, we speculate on the promise and challenges of in vivo direct reprogramming strategies.

SummaryMouse epiblast stem cells (EpiSCs) are cultured with FGF2 and Activin A, like human embryonic stem cells (hESCs), but the action of the associated pathways in EpiSCs has not been well characterized. Here, we show that activation of the Activin pathway promotes self-renewal of EpiSCs via direct activation of Nanog, whereas inhibition of this pathway induces neuroectodermal differentiation, like in hESCs. In contrast, the different roles of FGF signaling appear to be only partially conserved in the mouse. Our data suggest that FGF2 fails to cooperate with SMAD2/3 signaling in actively promoting EpiSC self-renewal through Nanog, in contrast to its role in hESCs. Rather, FGF appears to stabilize the epiblast state by dual inhibition of differentiation to neuroectoderm and of media-induced reversion to a mouse embryonic stem cell-like state. Our data extend the current model of cell fate decisions concerning EpiSCs by clarifying the distinct roles played by FGF signaling.

SummaryInduced pluripotent stem cells (iPSCs) may represent an ideal cell source for future regenerative therapies. A critical issue concerning the clinical use of patient-specific iPSCs is the accumulation of mutations in somatic (stem) cells over an organism's lifetime. Acquired somatic mutations are passed onto iPSCs during reprogramming and may be associated with loss of cellular functions and cancer formation. Here we report the generation of human iPSCs from cord blood (CB) as a juvenescent cell source. CBiPSCs show characteristics typical of embryonic stem cells and can be differentiated into derivatives of all three germ layers, including functional cardiomyocytes. For future therapeutic production of autologous and allogeneic iPSC derivatives, CB could be routinely harvested for public and commercial CB banks without any donor risk. CB could readily become available for pediatric patients and, in particular, for newborns with genetic diseases or congenital malformations.

SummaryMouse and human stem cells with features similar to those of embryonic stem cells have been derived from testicular cells. Although pluripotent stem cells have been obtained from defined germline stem cells (GSCs) of mouse neonatal testis, only multipotent stem cells have been obtained so far from defined cells of mouse adult testis. In this study we describe a robust and reproducible protocol for obtaining germline-derived pluripotent stem (gPS) cells from adult unipotent GSCs. Pluripotency of gPS cells was confirmed by in vitro and in vivo differentiation, including germ cell contribution and transmission. As determined by clonal analyses, gPS cells indeed originate from unipotent GSCs. We propose that the conversion process requires a GSC culture microenvironment that depends on the initial number of plated GSCs and the length of culture time.