Nanog regulates proliferation during early fish development.

Nanog is involved in controlling pluripotency and differentiation of stem cells in vitro. However, its function in vivo has been studied only in mouse embryos and various reports suggest that Nanog may not be required for the regulation of differentiation. To better understand endogenous Nanog function, more animal models should be introduced to complement the murine model. Here, we have identified the homolog of the mammalian Nanog gene in teleost fish and describe the endogenous expression of Ol-Nanog mRNA and protein during medaka (Oryzias latipes) embryonic development and in the adult gonads. Using medaka fish as a vertebrate model to study Nanog function, we demonstrate that Ol-Nanog is necessary for S-phase transition and proliferation in the developing embryo. Moreover, inhibition or overexpression of Ol-Nanog does not affect gene expression of various pluripotency and differentiation markers, suggesting that this transcription factor may not play a direct role in embryonic germ layer differentiation. STEM CELLS 2009;27:2081-2091.