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Describing the Barriers to the Totipotent State of ESCs

Review of "CTCF is a barrier for 2C-like reprogramming" from Nature Communications by Stuart P. Atkinson

The finding that cultures of embryonic stem cells (ESCs) contain a rare transient population of cells with totipotent-like potential similar to the cells of the zygote and 2-cell (2C) embryo [1-3] has prompted research into the mechanisms mediating the in-vitro interconversion of these two distinct cell types. While previous studies found that the expression of the transcription factor DUX (Double homeobox, 4) could convert ESCs into a 2C-like state in vitro, studies evaluating the development of DUX knockout mice [4-6] suggested that other mechanisms regulate the appearance of the totipotent state in vivo [7, 8].

Now, researchers led by Sergio Ruiz (National Cancer Institute, NIH, Bethesda, MD, USA) demonstrate that the zinc-finger binding protein CTCF (CCCTC-binding factor), which regulates the organization of higher-order chromatin structure, represents a significant barrier to the conversion of mouse ESCs into a 2C-like totipotent state [9]. Could this exciting new study reveal more of the mechanisms controlling the totipotent state?

Olbrich et al. first discovered that the forced conversion of ESCs into a 2C-like state in response to DUX expression, which occurred alongside the re-expression of a wide range of previously silenced genes, led to replication stress-mediated DNA damage at some CTCF binding sites and increased cell death. In line with CTCF acting as a barrier to conversion, the lost expression of this critical chromatin regulator in ESCs eased their spontaneous and asynchronous conversion into the 2C-like state and facilitated the expression of the 2C transcriptional program. Of note, the reinstatement of CTCF expression reversed these improvements.

The authors also revealed that successful 2C-like reprogramming required the transcriptional activation of the ZSCAN4 (Zinc finger and SCAN domain containing 4) cluster during the early stages after 2C-like reprogramming upon CTCF-depletion. Of note, previous related studies had established that the levels of ZSCAN4 progressively increase during 2C conversion [10, 11]; however, the precise roles of ZSCAN4 in 2C-like reprogramming remain relatively unclear.

While many questions remain unanswered, these exciting new findings firmly link CTCF and 2C-like reprogramming and provide new insight into the totipotent state. Furthermore, the authors anticipate that their findings may also have relevance to the reprogramming of somatic cells into induced pluripotent cells, perhaps by allowing increased efficiency by removing barriers to the reprogramming process.

For more about totipotency and barriers to reprogramming, stay tuned to the Stem Cells Portal!

References

  1. Riveiro AR and Brickman JM, From pluripotency to totipotency: an experimentalist's guide to cellular potency. Development 2020;147.
  2. Macfarlan TS, Gifford WD, Driscoll S, et al., Embryonic stem cell potency fluctuates with endogenous retrovirus activity. Nature 2012;487:57-63.
  3. Genet M and Torres-Padilla M-E, The molecular and cellular features of 2-cell-like cells: a reference guide. Development 2020;147.
  4. Hendrickson PG, Doráis JA, Grow EJ, et al., Conserved roles of mouse DUX and human DUX4 in activating cleavage-stage genes and MERVL/HERVL retrotransposons. Nature Genetics 2017;49:925-934.
  5. De Iaco A, Planet E, Coluccio A, et al., DUX-family transcription factors regulate zygotic genome activation in placental mammals. Nature Genetics 2017;49:941-945.
  6. Whiddon JL, Langford AT, Wong C-J, et al., Conservation and innovation in the DUX4-family gene network. Nature Genetics 2017;49:935-940.
  7. Chen Z and Zhang Y, Loss of DUX causes minor defects in zygotic genome activation and is compatible with mouse development. Nature Genetics 2019;51:947-951.
  8. De Iaco A, Verp S, Offner S, et al., DUX is a non-essential synchronizer of zygotic genome activation. Development 2020;147.
  9. Olbrich T, Vega-Sendino M, Tillo D, et al., CTCF is a barrier for 2C-like reprogramming. Nature Communications 2021;12:4856.
  10. Rodriguez-Terrones D, Gaume X, Ishiuchi T, et al., A molecular roadmap for the emergence of early-embryonic-like cells in culture. Nature Genetics 2018;50:106-119.
  11. Fu X, Djekidel Mohamed N, and Zhang Y, A transcriptional roadmap for 2C-like–to–pluripotent state transition. Science Advances;6:eaay5181.