Theunissen TW, van Oosten AL, Castelo-Branco G, Hall J, Smith A, Silva JCR
Induced pluripotency requires the expression of defined factors and culture conditions that support the self-renewal of embryonic stem (ES) cells . Small molecule inhibition of MAP kinase (MEK) and glycogen synthase kinase 3 (GSK3) with LIF (2i/LIF) provides an optimal culture environment for mouse ES cells  and promotes transition to naive pluripotency in partially reprogrammed (pre-iPS) cells . Here we show that 2i/LIF treatment in clonal lines of pre-iPS cells results in the activation of endogenous Nanog and rapid downregulation of retroviral Oct4 expression. Nanog enables somatic cell reprogramming in serum-free medium supplemented with LIF, a culture condition which does not support induced pluripotency or the self-renewal of ES cells, and is sufficient to reprogram epiblast-derived stem cells to naive pluripotency in serum-free medium alone. Nanog also enhances reprogramming in cooperation with kinase inhibition or 5- aza-cytidine, a small molecule inhibitor of DNA methylation. These results highlight the capacity of Nanog to overcome multiple barriers to reprogramming and reveal a synergy between Nanog and chemical inhibitors that promote reprogramming. We conclude that Nanog induces pluripotency in minimal conditions. This provides a strategy for imposing naive pluripotency in mammalian cells independently of species-specific culture requirements.
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