Glycogen synthase kinase-3 (Gsk-3) activity regulates multiple signal transduction pathways, and is also a key component of the network responsible for maintaining stem cell pluripotency. Genetic deletion of Gsk-3α and Gsk-3β or inhibition of Gsk-3 activity via small molecules promotes stem cell pluripotency, yet the mechanism underlying the role for Gsk-3 in this process remains ambiguous. Another cellular process that has been shown to affect stem cell pluripotency is mRNA methylation (m 6 A). Here, we describe an intersection between these components -the regulation of m 6 A by Gsk-3. We find that protein levels for the RNA demethylase, FTO (fat mass and obesity-associated protein), are elevated in Gsk-3α;Gsk-3β-deficient mouse embryonic stem cells (ESCs). FTO is normally phosphorylated by Gsk-3, and mass spectrometry identified the sites on FTO that are phosphorylated in a Gsk-3-dependent fashion. Gsk-3 phosphorylation of FTO leads to polyubiquitination, but in Gsk-3 knockout ESCs, that process is impaired, resulting in elevated levels of FTO protein. As a consequence of altered FTO protein levels, mRNAs in Gsk-3 knockout ESCs have 50% less m 6 A than wild-type ESCs, and m 6 A-seq analysis reveals the specific mRNAs that have reduced m 6 A modifications. Taken together, we provide the first evidence for how m 6 A demethylation is regulated in mammalian cells, and sheds light onto a possible novel mechanism by which Gsk-3 activity regulates stem cell pluripotency.Glycogen synthase kinase-3 (Gsk-3) activity is an important regulator of numerous signal transduction pathways (1). Gsk-3 activity is the sum of two largely redundant proteins, Gsk-3α and Gsk-3β, and in general, Gsk-3 is a negative regulator of cellular signaling (2). Rare among kinases, Gsk-3 is active at a basal state, while pathway activation from upstream signaling cascades results in the inhibition of Gsk-3 activity (2). Gsk-3α and Gsk-3β together regulate signal transduction pathways such as Wnt, protein kinase A (PKA), Hedgehog, transforming growth factor-β (TGF-β), nuclear factor of activated T-cells (NF-AT) and phosphatidylinositol 3-kinase (PI3K)-dependent insulin signaling in a variety of biological settings (3)(4)(5).Gsk-3 activity can be inhibited through the use of small molecule inhibitors, such as SB- (6)(7)(8), and the clinically relevant mood-stabilizer lithium (9,10); however, a drawback to the use of small molecules to study Gsk-3 function is the potential for off-target effects (11). Cells in which Gsk-3α and Gsk-3β have been genetically deleted allows for a more confident assessment of Gsk-3-specific functions. Therefore, we utilize mouse embryonic stem cells (ESCs) deficient in both Gsk-3α and Gsk-3β (Gsk-3α -/-; Gsk-3β -/-), i.e., Gsk-3 double knockout (DKO), to assess Gsk-3-specific functions (12,13). One prominent phenotype of Gsk-3 DKO ESCs is their persistent pluripotency, assessed by their inability to differentiate into all three germs layers in a teratoma assay, as well as by analysis of global gene expression ...

A genome-wide analysis is given of DNA methylation in mouse embryonic stem cells in which both Gsk-3α and Gsk-3β have been genetically deleted. DNA methylation patterns are compared to those of wild-type cells. More than 75% of known imprinted loci have reduced DNA methylation in the Gsk-3–knockout cells.

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