Reprogramming roadblocks are system-dependent

Since the first generation of induced Pluripotent Stem cells (iPSCs), several reprogramming systems have been used to study its molecular mechanisms. However, the system of choice largely affects the reprogramming efficiency, influencing our view on the mechanisms. Here we demonstrate that reprogramming triggered by less efficient polycistronic reprogramming cassettes not only highlights Mesenchymal-Epithelial Transition (MET) as a roadblock, but also faces more severe difficulties to attain a pluripotent state even post-MET. Also, in contrast to previous findings, more efficient cassettes can reprogram both wild type and Nanog-/- fibroblasts with comparable efficiencies, routes and kinetics, rebutting previous studies that Nanog is critical for iPSC generation. We revealed that the 9 amino acids in the N-terminus of Klf4 in polycistronic reprogramming cassettes are the dominant factor causing these critical differences. Our data establishes that some reprogramming roadblocks are system-dependent, highlighting the need to pursue mechanistic studies with close attention to the systems to better understand reprogramming. Overall design: The aim of the experiment is to compare the reprogramming pathways driven by two different polycistronic cassettes (MKOS and OKMS). We have isolated cells at intermediate stages of both MKOS and OKMS reprogramming and analysed their gene expression profiles. 2N- are CD44- ICAM1-, Nanog-GFP-, 3N- are CD44- ICAM1+, Nanog-GFP-, 3N+ are CD44- ICAM1+, Nanog-GFP+, all from day 10 of reprogramming. MKOS/OKMS iPSCs are established iPSC clones, TNG an Embryonic Stem Cell line carrying a Nanog-GFP reporter published in Chambers et al. Cell, 113, 643-655, from this line TNG MKOS and OKMS Embryonic Stem Cells were generated after targeting the Sp3 locus with the MKOS or the OKMS cassette respectively,E14 a reference Embryonic Stem Cell line and MEF are Mouse Embryonic Fibroblasts either wild type or generaterd from TNG MKOS or OKMS ESCs. D6 is the D6s4B5 iPSC line published in O''Malley et al. Nature, 499, 88-91.

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Chantzoura E, Skylaki S, Menendez S, Kim SI, Johnsson A, Linnarsson S, Woltjen K, Chambers I, Kaji K