Nanog, a core pluripotency factor, is required for stabilizing pluripotency of 
inner cell mass (ICM) and embryonic stem cells (ESCs), and survival of 
primordial germ cells in mice. Here, we have addressed function and regulation 
of Nanog in epiblasts of postimplantation mouse embryos by conditional knockdown 
(KD), chromatin immunoprecipitation (ChIP) using in vivo epiblasts, and protein 
interaction with the Nanog promoter in vitro. Differentiation of Nanog-KD 
epiblasts demonstrated requirement for Nanog in stabilization of pluripotency. 
Nanog expression in epiblast is directly regulated by Nodal/Smad2 pathway in a 
visceral endoderm-dependent manner. Notably, Nanog promoters switch from 
Oct4/Esrrb in ICM/ESCs to Oct4/Smad2 in epiblasts. Smad2 directly associates 
with Oct4 to form Nanog promoting protein complex. Collectively, these data 
demonstrate that Nanog plays a key role in stabilizing Epiblast pluripotency 
mediated by Nodal/Smad2 signaling, which is involved in Nanog promoter switching 
in early developing embryos.