Klf5 establishes bi-potential cell fate by dual regulation of ICM and TE specification genes

Early blastomeres of mouse preimplantation embryos exhibit bi-potential cell fate, capable of generating both embryonic and extra-embryonic lineages in blastocysts. Here, we identified three major 2 cell (2C) specific endogenous retroviruses (ERVs) as the molecular hallmark of the bi-potential plasticity. Using the LTRs of all three 2C-ERVs, we identified Klf5 as their major upstream regulator. Klf5 is essential for bi-potential cell fate: a single Klf5-overexpressing ESC generated terminally differentiated embryonic and extra-embryonic lineages in chimeric embryos, and Klf5 directly induces both ICM and TE specification genes. Intriguingly, Klf5 and Klf4 act redundantly during ICM specification, whereas Klf5 deficiency alone impairs TE specification. Klf5 is regulated by multiple 2C-specific transcription factors, particularly Dux, and the Dux/Klf5 axis is evolutionarily conserved. Altogether, the 2C-specific transcription program converges on Klf5 to establish bi-potential cell fate, enabling a cell state with dual activation of ICM and TE genes.