The people behind the papers – Anish Dattani and Ge Guo

Anish Dattani (L) and Ge Guo (R)

Ge, can you give us your scientific biography and the questions your lab is trying to answer?

GG: I received my first degree in Genetics from Nankai University. After that, I was admitted to the PhD programme at Baylor College of Medicine, USA, where I learned about ESCs for the first time from Professor Allan Bradley. I was immediately stunned by such a powerful genetic tool. I followed Allan to the University of Cambridge in 2001 to continue my PhD, where I explored approaches for recessive genetic screens in mouse ESCs. In 2006, I joined Professor Austin Smith's laboratory to investigate genes regulating mouse pluripotency. Soon, I became intrigued by the difference between mouse and human pluripotent stem cell states and redirected my research towards human pluripotency. How to establish a type of human pluripotent stem cells, similar to mouse ESCs, has been the only focus of my research over the past 10 years. I had some very difficult times, but, fortunately, together with colleagues from the Smith lab, we established a human nPSC culture and validated that these cells are a close approximate to their embryonic origin, the epiblast of the blastocyst. We also showed that, unlike mouse ESCs, human nPSCs can readily differentiate to the extra-embryonic trophectoderm. In 2020, I established my own research group at the Living Systems Institute, University of Exeter. My lab is working on understanding the regulatory mechanism of cell fate transition and tissue organisation during early human embryo development using human nPSCs and the recently established human blastoid model.

Anish, how did you come to work in Ge's lab and what drives your research today?

AD: For my PhD, I studied the pluripotent stem cells of planarian flatworms, which bestow these animals with a remarkable regenerative ability. Although planarian regeneration is fascinating, my real interest was always the fundamental nature of pluripotency: what signals maintain self-renewal of pluripotent cells, and how the production of specialised cell types is controlled. Through this interest, I came across a paper from Austin Smith's lab describing the capture of human nPSCs in vitro. It was clear these cells had a close identity to the actual epiblast cells of human blastocyst, but also showed some interesting differences to mouse naïve ESC counterparts. I knew that for my postdoc I wanted to work with these cells and was excited by the new biology they could reveal about the early human development.

I was fortunate enough to join Ge's new lab at the Living Systems Institute soon after her work with Austin had introduced something of a paradigm shift in how we viewed early lineage segregation in the human embryo. She had showed that human naïve cells, unlike mouse ESCs, have the intrinsic ability to make extra-embryonic trophectoderm. I want to understand the molecular mechanisms underpinning how human naïve cells differentiate to extra-embryonic lineages in vitro, and this first paper shows how XAV939, a factor in the media cocktail ‘PXGL’, buffers against this intrinsic differentiation capacity.

Can you give us the key results of the paper in a paragraph?

GG & AD: We have previously shown that XAX939, a tankyrase inhibitor, can stabilise human nPSCs and is implicated in trophectoderm (TE) suppression. In this paper, we dissected the mechanism of this effect. XAV939 is a well-known canonical Wnt/β-catenin signalling inhibitor. We showed, however, that nPSCs depleted of β-catenin remain dependent on XAV939. We found instead that XAV939 prevents TE induction by reducing activation of YAP, a co-factor of TE-inducing TEAD transcription factors. Tankyrase inhibition stabilises angiomotin, which limits nuclear accumulation of YAP. Upon deletion of the angiomotin-family members AMOT and AMOTL2, nuclear YAP increases and XAV939 fails to prevent TE induction. Expression of constitutively active YAP similarly precipitates TE differentiation. Conversely, nPSCs lacking YAP1 or its paralogue TAZ resist TE differentiation and self-renewal efficiently without XAV939. These findings explain the distinct requirement for tankyrase inhibition in human, but not mouse, nPSCs and highlight the pivotal role of YAP activity in human naïve pluripotency and TE differentiation.

Given the contradicting literature and its importance in mouse ESCs, were you surprised to find that β-catenin is not important for human nPSC self-renewal?

GG & AD: Actually, it is not surprising to see that β-catenin is not important for human nPSC self-renewal. We have long been aware that the effect of Chiron (CHIR99021), a Wnt agonist, in promoting mouse ESC culture is mediated by two key transcription factors, Tcf3 and Esrrb. However, neither is expressed in the naïve epiblast in human embryos and in the human nPSCs. We also never observed similar effects of Chiron on human nPSC culture as on mouse ESCs. We were, of course, very pleased to confirm that depletion of β-catenin indeed doesn't have a major influence on human naïve pluripotency. However, we cannot rule out the possibility that Chiron may have β-catenin-independent effects on the propagation of human nPSCs.

Do you have a favourite hypothesis as to why the double YAP1/TAZ mutant nPSCs fail to expand?

GG & AD: YAP/TAZ has a multiplicity of functions in regulating cellular proliferation, metabolism and apoptosis. It is therefore not surprising to see that minimal function of YAP/TAZ is required for long-term cell propagation.

When doing the research, did you have any particular result or eureka moment that has stuck with you?

AD: I would not say there was a single ‘eureka’ moment during this project. However, it was satisfying to look at the experimental evidence showing that removal of XAV939 clearly reduced the levels of AMOTL2 protein expression, and that this was associated with the upregulation of YAP-TEAD markers expressed during naïve-to-TE conversion.

And what about the flipside: any moments of frustration or despair?

AD: Most of the experiments in this paper were conducted soon after coming out of lockdown when there were still restrictions in working hours and interactions with colleagues. This slowed things down a little, but, overall, there were no real moments of despair. As with any scientific project, I did have a few head-scratching moments, but I could always discuss them with Ge to work things out!

Anish, what is next for you after this paper?

AD: Recently, we showed that human naïve cells derived in our lab can be used to make a blastocyst model, a ‘blastoid’, with both high efficiency and high fidelity to the real human embryo. My next ambition is to use a genetic approach to understand the mechanisms underpinning trophectoderm and hypoblast lineage segregation during blastoid formation and look at how these are similar to the real embryo.

Ge, where will this story take your lab next?

GG: This story consolidates our previous discovery and emphasises that naïve pluripotency to TE differentiation is indeed an intrinsic feature of human naïve pluripotency, and a mechanism must be put in place to counteract this effect to enable long-term propagation of pluripotent stem cells. We are moving towards investigating how these lineage potentials are established and molecularly regulated both in cells and in human embryos.

Finally, let's move outside the lab – what do you like to do in your spare time?

GG: As a mother of two, family always comes first in my spare time. I do wish we could travel more to explore the beautiful world!

AD: We are very lucky in Exeter to be so close to some stunning coastline, so in my spare time I like taking long walks along the beach with my partner.