O-linked β-d-N-acetylglucosamine (O-GlcNAc) modification has recently emerged as a key regulator of cell physiology, controlling aspects such as nutrient sensing and cell cycle progression. Youn and colleagues reveal that this is not restricted to differentiated cells; O-GlcNAcylation also regulates embryonic stem (ES) cell pluripotency.
Previous studies suggested that ES cell maintenance may be regulated by O-GlcNAcylation, in particular by reporting that the levels of glucose in medium affect whether ES cells differentiate or self-renew. Consistent with these observations, the authors found that loss of O-GlcNAc transferase (OGT; which mediates O-GlcNAcylation) led to reduced proliferation and self-renewal of ES cells as well as decreased reprogramming efficiency of induced pluripotent stem (iPS) cells. By contrast, overexpression of OGT or depletion of O-GlcNAcase (OGA; which removes O-GlcNAc) increased reprogramming efficiency and decreased differentiation, respectively. These findings indicate that O-GlcNAcylation promotes ES cell self-renewal and inhibits differentiation.
ES cell self-renewal is regulated by a pluripotency network comprising OCT4 (also known as POU5F1 and OCT3), SOX2, KLF4 (Krüppel-like factor 4) and NANOG, so the authors hypothesized that O-GlcNAc may be modifying some of these factors. Indeed, OCT4 and SOX2 were O-GlcNAcylated, and this modification decreased following ES cell differentiation. Moreover, OCT4 O-GlcNAcylation depended on the OCT4 residue Thr228, as mutation of this residue decreased O-GlcNAc levels in ES cells. Importantly, mutation of OCT4 at Thr228 resulted in reduced reprogramming efficiency of mouse embryonic fibroblasts and defective self-renewal of ES cells, which suggests that O-GlcNAcylation of OCT4 is required to maintain pluripotency.
But how is this achieved? Because OCT4 O-GlcNAcylation did not affect protein stability or cell localization, the authors postulated that it may regulate OCT4 activity as a transcription factor. Using a luciferase reporter assay, the authors confirmed that O-GlcNAcylation increases the transcriptional activity of OCT4. Further analysis indicated that O-GlcNAcylated OCT4 directly regulates the transcription of 29 genes, including pluripotency-related genes such as Nanog and Klf4.
Together, these findings reveal that OCT4 O-GlcNAcylation acts as a key regulator of pluripotency, adjusting the transcription of pluripotency-related genes according to external signals.
References
ORIGINAL RESEARCH PAPER
Jang, H. et al. O-GlcNAc regulates pluripotency and reprogramming by directly acting on core components of the pluripotency network. Cell Stem Cell 17 May 2012 (doi:10.1016/j.stem.2012.03.001)
FURTHER READING
Hanover, J. A., Krause, M. W. & Love, D. C. Bittersweet memories: linking metabolism to epigenetics through O-GlcNAcylation. Nature Rev. Mol. Cell Biol. 13, 312–321 (2012)
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David, R. ES cells have a sweet tooth. Nat Rev Mol Cell Biol 13, 410 (2012). https://doi.org/10.1038/nrm3375
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DOI: https://doi.org/10.1038/nrm3375