Differential repression of Otx2 underlies the capacity of NANOG and ESRRB to induce germline entry

Primordial germ cells (PGCs) are induced in the embryo by signals, including BMP emanating from extra-embryonic ectoderm, that act on cells in the post-implantation epiblast. PGC development can be recapitulated in vitro through the exposure of epiblast-like cells (EpiLCs) to appropriate cytokines, resulting in differentiation into PGC-like cells (PGCLCs). Interestingly, the requirement for cytokines to induce PGCLCs can be bypassed by enforced expression of the transcription factor (TF) NANOG. However, the underlying mechanisms are not fully elucidated. Here, we show that Otx2 downregulation is essential to enable NANOG to induce PGCLC formation. Moreover, while previous work has shown that the direct NANOG target gene Esrrb can substitute for several functions of NANOG, enforced expression of ESRRB cannot promote PGCLC specification from EpiLCs. This appears to be due to differential downregulation of Otx2 by NANOG and ESRRB, since induction of ESRRB in Otx2+/- EpiLCs activates expression of the core PGC TFs, Blimp1, Prdm14 and Ap2{gamma} and emergence of PGCLCs. This study illuminates the interplay of TFs occurring at the earliest stages of PGC specification from a state of competence.