Epiblast formation by Tead-Yap-dependent expression of pluripotency factors and competitive elimination of unspecified cells

Background of the preprint

During animal development, one single cell divides to generate every cell type in the body. This process is incredibly complex, highly robust, but perhaps most remarkable it is very flexible. For example, embryos containing mutant cells from early developmental stages are able to develop normally (Tam and Rossant, 2003 and references therein). Understanding how developing systems succeed in generating millions of distinct cell types while withstanding perturbation is an intriguing and poorly investigated question in biology.

Cell competition has emerged in recent years as one potential mechanism through which developing tissues tolerate disturbances and achieve quality control. During cell competition, less fit but viable cell types are selectively eliminated when in the presence of more fit cells. The process was initially described to remove mutant cells in developing Drosophila tissues and has since been described in mouse development. Specifically, during mouse gastrulation (around embryonic day (E) 6.0), cells that are karyotypically abnormal or mutant (Sancho et al, 2013; Bowling et al, 2018), those that have differentiated precociously (Diaz-Diaz et al, 2017), or those that have elevated p53 levels (Zhang et al, 2017) are eliminated, resulting in a selected epiblast cell pool (Claveria et al, 2013).

In this preprint, the authors investigate the mechanisms which safeguard cell fitness at an earlier stage of mouse development: the blastocyst (E3.5). During this stage, around 10 cells exist in the inner cell mass and specify to either epiblast fate (becoming all cells of the future embryo), or to primitive endoderm fate (contributing to extra-embryonic tissues). The authors propose that since such a small number of cells seed the entire body, it is likely that quality control mechanisms act to boost cellular fitness. Furthermore, the authors investigate roles of Hippo signalling, a pathway that has important roles in early fate decision events, in mediating pluripotency in the blastocyst. In doing so, the preprint reveals both a novel role for Hippo signaling in regulating pluripotency and the existence of cell competition mechanisms in the blastocyst.

 

Key findings of the preprint

• Hippo mutant cells are eliminated through cell competition in the blastocyst

Initially the authors demonstrate that cell competition is triggered by differences in Hippo pathway activity: deletion of Hippo signalling components Tead or Yap in a small number of cells leads to their apoptotic elimination during mid-blastocyst development (embryonic day (E) ~3.75). Importantly, the elimination is cell non-autonomous as Tead^-/- and Yap^-/- embryos survive until embryonic day (E) 11.5 and E8.5, respectively.

Elimination of Tead^-/- cells from wild-type embryos during blastocyst development

• Hippo signaling regulates cell pluripotency

Next the authors investigate the role Hippo signaling could play in the blastocyst. First, the authors identify a strong correlation between Yap and pluripotency factor expression (particularly Sox2) in the developing blastocyst, hinting at a potential link between Hippo signaling and pluripotency. Supporting this, the authors demonstrate that Tead^-/- cells have lower Sox2 expression than surrounding wild-type cells in chimeric blastocysts. Finally, culturing chimeras in 2i media, which maintains naïve pluripotency, rescues the competitive elimination of Tead^-/- cells. Together, these data indicate a role for Hippo in regulating pluripotency in the inner cell mass, but also suggest that cell competition could act to remove unspecified cells during blastocyst development.

• Cell competition removes unspecified cells in the embryo

Finally, the authors further probe the possibility that cell competition functions to remove unspecified cells from the blastocyst in normal conditions. During blastocyst development, cells either specify to epiblast fate and express Sox2, or specify to primitive endoderm fate and express Sox17. When the authors inhibit cell death in wild-type blastocysts and analyse the expression of these two markers, a new population of cells emerge that lack either Sox2 or Sox17 expression. The authors suggest that these ‘unspecified’ cells are normally eliminated through cell competition.

 

What we like about this preprint

• Use of mosaics to confront populations during pre-implantation (and generated by electroporation instead of microinjection!)
• Evidence of endogenous cell competition: a particularly high frequency of apoptosis was observed in early mid-blastocyst stage (64–95-cell) embryos in cells with weak nuclear levels of YAP.
• Novel mechanism for Hippo signalling pathway that couples growth control and fate choice, since embryos treated with inhibitors of apoptosis show a higher degree of unspecified cells and increased cell numbers in the inner cell mass.

 

Questions to the authors:

• What proportion of cells undergoing apoptosis during blastocyst development do you think are being subject to endogenous cell competition rather than cell-autonomous death?
• Do you expect to see the same effects on cell competition in primed mESCs versus naïve?
• What could be the cause some cells failing to specify during epiblast formation? Would this be expected to be intrinsic or extrinsic factors?

 

References

Tam and Rossant (2003) Mouse embryonic chimeras: tools for studying mammalian development. Development 2003 130: 6155-6163

Sancho, M et al (2013). Competitive Interactions Eliminate Unfit Embryonic Stem Cells at the Onset of Differentiation. Developmental Cell 26, 19-30.

Bowling, S et al (2018) P53 and mTOR signalling determine fitness selection through cell competition during early mouse embryonic development. Nature Communications 9(1):1763

Díaz-Díaz, C. et al. Pluripotency Surveillance by Myc-Driven Competitive Elimination of Differentiating Cells. Dev. Cell (2017)

Zhang, G. et al (2017). p53 pathway is involved in cell competition during mouse embryogenesis. Proceedings of the National Academy of Sciences 114, 498-503.

Claveria, C (2013). Myc-driven endogenous cell competition in the early mammalian embryo. Nature 500, 39-44.

 

 

 

doi: https://doi.org/10.1242/prelights.6078

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