Capturing Identity and Fate Ex Vivo: Stem Cells from the Mouse Blastocyst

Single-cell analysis of bidirectional reprogramming between early embryonic states reveals mechanisms of differential lineage plasticities

Two distinct fates, pluripotent epiblast (EPI) and primitive (extra-embryonic) endoderm (PrE), arise from common progenitor cells, the inner cell mass (ICM), in mammalian embryos. To study how these sister identities are forged, we leveraged embryonic (ES) and eXtraembryonic ENdoderm (XEN) stem cells - in vitro counterparts of the EPI and PrE. Bidirectional reprogramming between ES and XEN coupled with single-cell RNA and ATAC-seq analyses uncovered distinct rates, efficiencies and trajectories of state conversions, identifying drivers and roadblocks of reciprocal conversions. While GATA4-mediated ES-to-iXEN conversion was rapid and nearly deterministic, OCT4, KLF4 and SOX2-induced XEN-to-iPS reprogramming progressed with diminished efficiency and kinetics. The dominant PrE transcriptional program, safeguarded by Gata4, and globally elevated chromatin accessibility of EPI underscored the differential plasticities of the two states. Mapping in vitro trajectories to embryos revealed reprogramming in either direction tracked along, and toggled between, EPI and PrE in vivo states without transitioning through the ICM.

Coordination between patterning and morphogenesis ensures robustness during mouse development

The mammalian preimplantation embryo is a highly tractable, self-organizing developmental system in which three cell types are consistently specified without the need for maternal factors or external signals. Studies in the mouse over the past decades have greatly improved our understanding of the cues that trigger symmetry breaking in the embryo, the transcription factors that control lineage specification and commitment, and the mechanical forces that drive morphogenesis and inform cell fate decisions. These studies have also uncovered how these multiple inputs are integrated to allocate the right number of cells to each lineage despite inherent biological noise, and as a response to perturbations. In this review, we summarize our current understanding of how these processes are coordinated to ensure a robust and precise developmental outcome during early mouse development. This article is part of a discussion meeting issue 'Contemporary morphogenesis'.

In vitro modeling of early mammalian embryogenesis

Synthetic embryology endeavors to use stem cells to recapitulate the first steps of mammalian development that define the body axes and first stages of fate assignment. Well-engineered synthetic systems provide an unparalleled assay to disentangle and quantify the contributions of individual tissues as well as the molecular components driving embryogenesis. Experiments using a mixture of mouse embryonic and extra-embryonic stem cell lines show a surprising degree of self-organization akin to certain milestones in the development of intact mouse embryos. To further advance the field and extend the mouse results to human, it is crucial to develop a better control of the assembly process as well as to establish a deeper understanding of the developmental state and potency of cells used in experiments at each step of the process. We review recent advances in the derivation of embryonic and extraembryonic stem cells, and we highlight recent efforts in reconstructing the structural and signaling aspects of embryogenesis in three-dimensional tissue cultures.

Expected advances in human fertility treatments and their likely translational consequences

BACKGROUND

Due to rapid research progress in reproductive biology and reproductive clinical endocrinology, many human infertility treatments are close to potential breakthroughs and translational applications. We here review current barriers, where such breakthroughs will likely come from, what they will entail, and their potential clinical applications.

MAIN TEXT

The radical nature of change will primarily benefit older women, reduce fertility treatment costs and thereby expand access to treatment. A still widely overlooked prerequisite for implantation and normal pregnancy maintenance is timely development of maternal immunological tolerance toward an implanting paternal semi-allograft, if malfunctioning associated with implantation failure and pregnancy loss, while premature termination of tolerance appears associated with premature labor, pre-eclampsia/eclampsia and gestoses of pregnancy. Common denominators between pregnancy and invasive malignancies have again been attracting attention, suggesting that, like in malignant tumors, degrees of embryo aneuploidy may affect invasiveness and ability to "disarm" the immune system's innate response against implanting embryos. Linking tolerance to implantation, we offer evidence that the so-called "implantation window" is likely immunological rather than hormonally defined.

CONCLUSIONS

Because many here outlined treatment changes will disproportionally benefit older women, they will exert a pronounced effect on society, as increasing numbers of women at grandparental ages will become mothers.

Links between DNA Replication, Stem Cells and Cancer

Cancers can be categorized into two groups: those whose frequency increases with age, and those resulting from errors during mammalian development. The first group is linked to DNA replication through the accumulation of genetic mutations that occur during proliferation of developmentally acquired stem cells that give rise to and maintain tissues and organs. These mutations, which result from DNA replication errors as well as environmental insults, fall into two categories; cancer driver mutations that initiate carcinogenesis and genome destabilizing mutations that promote aneuploidy through excess genome duplication and chromatid missegregation. Increased genome instability results in accelerated clonal evolution leading to the appearance of more aggressive clones with increased drug resistance. The second group of cancers, termed germ cell neoplasia, results from the mislocation of pluripotent stem cells during early development. During normal development, pluripotent stem cells that originate in early embryos give rise to all of the cell lineages in the embryo and adult, but when they mislocate to ectopic sites, they produce tumors. Remarkably, pluripotent stem cells, like many cancer cells, depend on the Geminin protein to prevent excess DNA replication from triggering DNA damage-dependent apoptosis. This link between the control of DNA replication during early development and germ cell neoplasia reveals Geminin as a potential chemotherapeutic target in the eradication of cancer progenitor cells.

Geminin Is Essential for Pluripotent Cell Viability During Teratoma Formation, but Not for Differentiated Cell Viability During Teratoma Expansion

Pluripotent embryonic stem cells (ESCs) are unusual in that geminin has been reported to be essential either to prevent differentiation by maintaining expression of pluripotency genes or to prevent DNA rereplication-dependent apoptosis. To distinguish between these two incompatible hypotheses, immune-compromised mice were inoculated subcutaneously with ESCs harboring conditional Gmnn alleles alone or together with a tamoxifen-dependent Cre recombinase gene. Mice were then injected with tamoxifen at various times during which the ESCs proliferated and differentiated into a teratoma. For comparison, the same ESCs were cultured in vitro in the presence of monohydroxytamoxifen. The results revealed that geminin is a haplosufficient gene that is essential for ESC viability before they differentiate into a teratoma, but once a teratoma is established, the differentiated cells can continue to proliferate in the absence of Gmnn alleles, geminin protein, and pluripotent stem cells. Thus, differentiated cells did not require geminin for efficient proliferation within the context of a solid tissue, although they did when teratoma cells were cultured in vitro. These results provide proof-of-principle that preventing geminin function could prevent malignancy in tumors derived from pluripotent cells by selectively eliminating the progenitor cells with little harm to normal cells.