NANOG alone induces germ cells in primed epiblast in vitro by activation of enhancers

In mouse embryonic stem cells converted to an epiblast fate in vitro—a state in which the cells can also gain germ cell fate if exposed to the signalling molecule BMP4—the sole expression of the transcription factor NANOG is shown to be sufficient to induce germ cell fate, in the absence of BMP4. The transcription factor gene Nanog is expressed both in pluripotent cells and in primordial germ cells in mice, but its function in germ cells has not been defined. Azim Surani and colleagues find that in embryonic stem cells converted to an epiblast fate in vitro, a state in which they can also gain germ cell fate if exposed to the signalling molecule BMP4, sole expression of Nanog is sufficient to induce germ cell fate in the absence of BMP4. They demonstrate that the resetting of the chromatin that occurred in the primed epiblast allows NANOG to bind and activate the expression of key germ cell factors. The pluripotent factor Sox2 prevents the action of Nanog on these germ cell factors in the epiblast state, while SOX2 and NANOG work together to maintain pluripotency in naive embryonic stem cells. Nanog, a core pluripotency factor in the inner cell mass of blastocysts, is also expressed in unipotent primordial germ cells (PGCs) in mice1, where its precise role is yet unclear2,3,4. We investigated this in an in vitro model, in which naive pluripotent embryonic stem (ES) cells cultured in basic fibroblast growth factor (bFGF) and activin A develop as epiblast-like cells (EpiLCs) and gain competence for a PGC-like fate5. Consequently, bone morphogenetic protein 4 (BMP4), or ectopic expression of key germline transcription factors Prdm1, Prdm14 and Tfap2c, directly induce PGC-like cells (PGCLCs) in EpiLCs, but not in ES cells6,7,8. Here we report an unexpected discovery that Nanog alone can induce PGCLCs in EpiLCs, independently of BMP4. We propose that after the dissolution of the naive ES-cell pluripotency network during establishment of EpiLCs9,10, the epigenome is reset for cell fate determination. Indeed, we found genome-wide changes in NANOG-binding patterns between ES cells and EpiLCs, indicating epigenetic resetting of regulatory elements. Accordingly, we show that NANOG can bind and activate enhancers of Prdm1 and Prdm14 in EpiLCs in vitro; BLIMP1 (encoded by Prdm1) then directly induces Tfap2c. Furthermore, while SOX2 and NANOG promote the pluripotent state in ES cells, they show contrasting roles in EpiLCs, as Sox2 specifically represses PGCLC induction by Nanog. This study demonstrates a broadly applicable mechanistic principle for how cells acquire competence for cell fate determination, resulting in the context-dependent roles of key transcription factors during development.