Dazl is required for TET-mediated reprogramming to a naïve pluripotent state (I)

Embryonic stem cell (ESC) cultures display a heterogeneous gene expression profile, ranging from a pristine naïve pluripotent state to a primed epiblast state. While it is known that the addition of inhibitors of GSK3β and MEK (so-called 2i conditions) push ESC cultures towards a more homogeneous naïve pluripotent state, the molecular underpinnings of this naïve transition are not completely understood. Here we demonstrate that Dazl, a RNA-binding protein previously thought to be expressed specifically in developing primordial germ cells (PGCs), marks a subpopulation of ESCs in vitro that is actively transitioning toward naïve pluripotency. In the absence of Dazl expression, ESCs fail to induce proper expression of Tet enzymes required for 5-hydroxymethylation in 2i-culture conditions. As a result, 5-hydroxymethylation of methylated cystosine residues is impaired. Indeed, we demonstrate that Tet1 and Tet2 are mRNA targets of Dazl, indicating that Dazl might play a role in protection or stabilizing these mRNA molecules. Our results provide insight in the regulation of the acquisition of naïve pluripotency and demonstrate that Dazl is required for TET-mediated cytosine hydroxymethylation in cells that are actively reprogramming to a pluripotent ground state. Two independent mouse ES cell lines, Dazl-GFP and Stella-GFP, were cultured on γ-irradiated feeder MEFs in DMEM containing 15% FBS or serum-free B27N2 medium both supplemented with leukemia inhibitory factor (LIF) (Ying 2008). For the 2i experiments, 1µM MEK inhibitor PD0325901 (Axon Medchem), and 5 µM GSK3β inhibitor Kenpaullone (Tocris), were used. Cells were harvest at 0, 3 and 10 days in each condition for expression microarray analysis.