Proliferation pause as an early blockade of human cellular reprogramming toward pluripotency

Human induced Pluripotent Stem cells (iPSCs) can be generated by enforced-expression of OCT3/4, SOX2, KLF4 and c-MYC (OSKM) via intermediate state that is labeled with a surface antigen TRA-1-60. However the event during conversion of somatic cells to TRA-1-60 positive (+) fate by OSKM is still unclear. Here we show that TRA-1-60 (+) cells emerged from ESRG (+) cells with proliferation independent manner. Overcoming the proliferation pausing phase is crucial for further progression of reprogramming. Inactivation of RB pathway is associated with the expansion of early TRA-1-60 (+) cells. Indeed, extended RB activation drastically decreased the proportion of TRA-1-60 (+) cells with marked premature senescence. An RNA-binding protein LIN41 promotes the RB inactivation and facilitates reprogramming efficiency in early stage via post-transcriptional suppression of cycline dependent kinase inhibitor p21. This LIN41/p21/RB axis was invalidated in immortalized cells which showed a resistance to oncogene-induced senescence. Thus, LIN41- mediated RB inactivation promotes to bail out of transient quiescence as a major blockade in initial phase of reprogramming. Reprogrammed cells from five different somatic cell lines, N=90 Human dermal fibroblasts expressing various combination of reprogramming factors, N=48 Human induced Pluripotent Stem cells (iPSCs), N=3 Somatic cell lines N=17