Papers

Our study on tracking and mitigating imprint erasure during induction of naive human pluripotency is now online at Stem Cell Reports. In this paper, we tackled a long-standing problem in stem cell biology: the erosion of parent-specific epigenetic marks (so-called “imprints”) during the generation of naive stem cells, which more closely resemble the pre-implantation blastocyst. These imprints are protected during the genome-wide wave of demethylation in the pre-implantation embryo and ensure monoallelic expression of hundreds of genes during human development. However, imprints are demethylated under current conditions for inducing naive human pluripotency, which creates an epigenetic imbalance in naive stem cells and their derivatives. To track the dynamics of imprint erasure during naive resetting in real time, we established a dual-colored fluorescent reporter at both alleles of the imprinted SNRPN locus. During primed-to-naive resetting, SNRPN expression becomes biallelic in most naive cells, and biallelic SNRPN expression is irreversible upon re-priming. We utilized this live-cell reporter to evaluate chemical and genetic strategies to minimize imprint erasure. Decreasing the level of MEK/ERK inhibition or overexpressing the KRAB zinc-finger protein ZFP57 protected a subset of imprints during naive resetting. Combining these two strategies protected imprint levels to a further extent than either strategy alone. This study offers an experimental tool to track and enhance imprint stability during transitions between human pluripotent states in vitro.

This study was led by our former graduate student and postdoc, Laura Fischer, with support from our collaborators in the research groups of Sabine Dietmann and Ting Wang at WashU and Harald Jueppner at MGH.

A fascinating new study by Jose Silva (Thor’s former PhD advisor in Cambridge) and his team at Guangzhou Laboratory reports on the remarkable ability of naive human pluripotent stem cells to spontaneously form blastocyst-like structures (“blastoids”) in 3D suspension culture. These spontaneous blastoids mimic early-stage human blastocysts in terms of structure, size, and transcriptome characteristics and are capable of progressing to post-implantation stages on appropriate matrices, taking advantage of the recent methodology developed by Rowan Karvas and her colleagues in the Theunissen lab. This spontaneous blastoid potential of naive stem cells property is conferred by the glycogen synthase kinase-3 signalling inhibitor IM-12 present in 5iLAF self-renewing naive medium. IM-12 upregulates oxidative phosphorylation-associated genes that underly the capacity of naive stem cells to generate blastoids spontaneously. Starting from day one of self-organization, naive stem cells at the boundary of all 3D aggregates dedifferentiate into E5 embryo-like intermediates. Intermediates co-express SOX2/OCT4 and GATA6 and by day 3 specify trophoblast fate, which coincides with cavitation and blastoid formation. In summary, spontaneous blastoid formation results from 3D culture triggering dedifferentiation of naive stem cells into earlier embryo-like intermediates which are then competent to segregate blastocyst fates. Check out the paper in Nature Communications!

The featured image shows an example of a TBXT-positive primitive streak-like structure in spontaneous blastoids maintained on an optimized 3D matrix until D14 (courtesy of the Silva lab). SOX2 is indicated in red, TBXT in green, and GATA3 in grey.