Embryonic stem cells (ESCs) undergo coordinated epigenetic and metabolic changes to differentiate properly. However, the precise mechanisms by which these alterations are fine-tuned in the early stages of differentiation have not been identified. In this study, we demonstrate that phosphoserine aminotransferase 1 (Psat1), an Oct4/Sox2/Nanog (OSN) target protein, regulates changes in α-ketoglutarate (α-KG), determining the fate of mouse ESCs (mESCs). Maintaining Psat1 levels was essential for mESC self-renewal and pluripotency. Moderate knockdown (KD) of Psat1 in mESCs lowered DNA 5'-hydroxymethylcytosine (5'-hmC) and increased histone methylation levels by downregulating permissive amounts of α-KG, ultimately accelerating differentiation. We found that intracellular α-KG declined transiently during differentiation and that its dysregulation by treatment with dimethyl-α-KG impeded differentiation. Further, by in vivo teratoma formation assay, pluripotency of Psat1 KD mESCs was impaired, especially into the ectodermal lineage. Thus, we have established how Psat1 is regulated in maintaining intracellular α-KG levels and determining the fate of mESCs.
Cyclin-dependent kinase 1 (Cdk1) is indispensable for embryonic stem cell (ESC) maintenance and embryo development. Even though some reports have described a connection between Cdk1 and Oct4, there is no evidence that Cdk1 activity is directly linked to the ESC pluripotency transcription program. We recently reported that Aurkb/PP1-mediated Oct4 resetting is important to cell cycle maintenance and pluripotency in mouse ESCs (mESCs). In this study, we show that Cdk1 is an upstream regulator of the Oct4 phosphorylation state during cell cycle progression, and it coordinates the chromatin associated state of Oct4 for pluripotency-related gene expression within the cell cycle. Upon entry into mitosis, Aurkb in the chromosome passenger complex becomes fully activated and PP1 activity is inhibited downstream of Cdk1 activation, leading to sustaining Oct4(S229) phosphorylation and dissociation of Oct4 from chromatin during the mitotic phase. Cdk1 inhibition at the mitotic phase abnormally results in Oct4 dephosphorylation, chromosome decondensation and chromatin association of Oct4, even in replicated chromosome. Our study results suggest a molecular mechanism by which Cdk1 directly links the cell cycle to the pluripotency transcription program in mESCs.
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