Single-cell RNA sequencing reveals regulatory mechanism for trophoblast cell-fate divergence in human peri-implantation conceptuses

Widespread transcript shortening through alternative polyadenylation in secretory cell differentiation

Most eukaryotic genes produce alternative polyadenylation (APA) isoforms. Here we report that, unlike previously characterized cell lineages, differentiation of syncytiotrophoblast (SCT), a cell type critical for hormone production and secretion during pregnancy, elicits widespread transcript shortening through APA in 3'UTRs and in introns. This global APA change is observed in multiple in vitro trophoblast differentiation models, and in single cells from placentas at different stages of pregnancy. Strikingly, the transcript shortening is unrelated to cell proliferation, a feature previously associated with APA control, but instead accompanies increased secretory functions. We show that 3'UTR shortening leads to transcripts with higher mRNA stability, which augments transcriptional activation, especially for genes involved in secretion. Moreover, this mechanism, named secretion-coupled APA (SCAP), is also executed in B cell differentiation to plasma cells. Together, our data indicate that SCAP tailors the transcriptome during formation of secretory cells, boosting their protein production and secretion capacity.

Derivation of trophoblast stem cells from naïve human pluripotent stem cells

Naïve human pluripotent stem cells (hPSCs) provide a unique experimental platform of cell fate decisions during pre-implantation development, but their lineage potential remains incompletely characterized. As naïve hPSCs share transcriptional and epigenomic signatures with trophoblast cells, it has been proposed that the naïve state may have enhanced predisposition for differentiation along this extraembryonic lineage. Here we examined the trophoblast potential of isogenic naïve and primed hPSCs. We found that naïve hPSCs can directly give rise to human trophoblast stem cells (hTSCs) and undergo further differentiation into both extravillous and syncytiotrophoblast. In contrast, primed hPSCs do not support hTSC derivation, but give rise to non-self-renewing cytotrophoblasts in response to BMP4. Global transcriptome and chromatin accessibility analyses indicate that hTSCs derived from naïve hPSCs are similar to blastocyst-derived hTSCs and acquire features of post-implantation trophectoderm. The derivation of hTSCs from naïve hPSCs will enable elucidation of early mechanisms that govern normal human trophoblast development and associated pathologies.

Toward Guidelines for Research on Human Embryo Models Formed from Stem Cells

Over the past few years, a number of research groups have reported striking progress on the generation of in vitro models from mouse and human stem cells that replicate aspects of early embryonic development. Not only do these models reproduce some key cell fate decisions but, especially in the mouse system, they also mimic the spatiotemporal arrangements of embryonic and extraembryonic tissues that are required for developmental patterning and implantation in the uterus. If such models could be developed for the early human embryo, they would have great potential benefits for understanding early human development, for biomedical science, and for reducing the use of animals and human embryos in research. However, guidelines for the ethical conduct of this line of work are at present not well defined. In this Forum article, we discuss some key aspects of this emerging area of research and provide some recommendations for its ethical oversight.