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Oct4 Plays Detrimental Role in Mouse Induced Pluripotent Stem Cell Generation

Review of “Excluding Oct4 from Yamanaka Cocktail Unleashes the Developmental Potential of iPSCs” from Cell Stem Cell by Stuart P. Atkinson

A previous study from the laboratories of Guangming Wu and Hans R. Schöler (Max Planck Institute for Molecular Biomedicine/University of Münster, Münster, Germany) discovered that establishing totipotency during mouse development did not require the Oct4 transcription factor and that Oct4-null oocytes efficiently reprogrammed somatic nuclei to pluripotency following nuclear transfer [1]. Within the realms of the reprogramming of somatic cells into induced pluripotent stem cells (iPSCs), Oct4 is widely considered the most important among the four Yamanaka reprogramming factors (Oct4, Sox2, Klf4, and Myc, or OSKM) and its expression alone can drive the reprogramming process in certain cells [2-5], suggestive of the critical nature of this transcription factor. However, a new article from Wu and Schöler now reports that the SKM combination alone allows for the reprogramming of mouse somatic cells to iPSCs, thereby casting doubt on the Oct4 as a controller of the base pluripotency network [6].

Velychko et al. discovered the ability of an SKM polycistronic lentiviral reprogramming cassette to reprogram mouse embryonic fibroblasts (MEFs) into fully pluripotent iPSCs (at an efficiency of 30% of that for OSKM with delayed kinetics) while generating a negative control to compare the reprogramming ability of several factors. While Oct4-less reprogramming required high transgene expression and a high rate of cellular proliferation, the SKM-induced iPSCs displayed exceptionally high developmental potential, as evidenced by the generation of all-iPSC mice in the tetraploid complementation assay and more faithful epigenetic reprogramming when compared to OSKM-induced iPSCs. Altogether, this data suggested to the authors that exogenous Oct4 expression may act detrimentally with regards to iPSC quality. 

Indeed, subsequent gene expression analysis demonstrated that Oct4 expression generated profound off-target gene activation that prompted altered reprogramming trajectories when comparing SKM and OSKM and also prompted the appearance of epigenetic defects such as loss of imprinting and the misregulation of polycomb targets that can result in compromised differentiation potential. Of note, SKM induction induced the expression of the Nanog and Sall4 pluripotency-associated factors in the first wave of transcriptional alterations that activate the pluripotency network, with Oct4 expression appearing in a secondary activatory wave.

As an explanation as to why other studies failed to observe Oct4-less reprogramming, the study demonstrated that the co-expression of Sox2 and cMyc prompted the retroviral silencing during the early stages (around two days) of fibroblast reprogramming, highlighting the requirement for a non-retrovirally-driven reprogramming approach to produce iPSCs without Oct4. 

Overall, the authors providing convincing evidence that exogenous Oct4 plays a detrimental role in the reprogramming of mouse somatic cells to pluripotency, a finding that may bear significant implications for further development and application of iPSC technology.

Is the same true for the reprogramming of human somatic cells? Stay tuned to the Stem Cells Portal to find out!

References

  1. Wu G, Han D, Gong Y, et al., Establishment of totipotency does not depend on Oct4A. Nature Cell Biology 2013;15:1089-1097.
  2. Kim JB, Sebastiano V, Wu G, et al., Oct4-Induced Pluripotency in Adult Neural Stem Cells. Cell 2009;136:411-419.
  3. Kim JB, Greber B, Araúzo-Bravo MJ, et al., Direct reprogramming of human neural stem cells by OCT4. Nature 2009;461:649-653.
  4. Tsai S-Y, Bouwman BA, Ang Y-S, et al., Single Transcription Factor Reprogramming of Hair Follicle Dermal Papilla Cells to Induced Pluripotent Stem Cells. STEM CELLS 2011;29:964-971.
  5. Wu T, Wang H, He J, et al., Reprogramming of Trophoblast Stem Cells into Pluripotent Stem Cells by Oct4. STEM CELLS 2011;29:755-763.
  6. Velychko S, Adachi K, Kim K-P, et al., Excluding Oct4 from Yamanaka Cocktail Unleashes the Developmental Potential of iPSCs. Cell Stem Cell 2019;25:737-753.e4.