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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!


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