The Pluripotency Continuum and Interspecies Chimeras

Reevaluation by the CRISPR/Cas9 knockout approach revealed that multiple pluripotency-associated lncRNAs are dispensable for pluripotency maintenance while Snora73a/b is essential for pluripotency exit

Many long noncoding RNAs (lncRNAs) have been identified through siRNA-based screening as essential regulators of embryonic stem cell (ESC) pluripotency. However, the biological and molecular functions of most lncRNAs remain unclear. Here, we employed CRISPR/Cas9-mediated knockout technology to explore the functions of 8 lncRNAs previously reported to promote pluripotency in mouse ESCs. Unexpectedly, all of these lncRNAs were dispensable for pluripotency maintenance and proliferation in mouse ESCs when disrupted individually or in combination. Single-cell transcriptomic analysis also showed that the knockout of these lncRNAs has a minimal impact on pluripotency gene expression and cell identity. We further showed that several small hairpin RNAs (shRNAs) previously used to knock down lncRNAs caused the downregulation of pluripotency genes in the corresponding lncRNA-knockout ESCs, indicating that off-target effects likely responsible for the pluripotency defects caused by these shRNAs. Interestingly, linc1343-knockout and linc1343-knockdown ESCs failed to form cystic structures and exhibited high expression of pluripotency genes during embryoid body (EB) differentiation. By reintroducing RNA products generated from the linc1343 locus, we found that two snoRNAs, Snora73a and Snora73b, but not lncRNAs, could rescue pluripotency silencing defects during EB differentiation of linc1343 knockout ESCs. Our results suggest that the 8 previously annotated pluripotency-regulating lncRNAs have no overt functions in conventional ESC culture; however, we identified snoRNA products derived from an annotated lncRNA locus as essential regulators for silencing pluripotency genes.

[Interspecies systemic chimeras]

Inter-species chimeras are both fantastic and monstrous creatures from Greek or Egyptian mythology, and a long-established research tool. Recent advances in the field of pluripotent stem cells have made it possible to extend the repertoire of inter-species chimeras to "systemic" chimeras, in which the mixing of cells from both species involves all organs including the germline. These chimeric embryos and fetuses open up new research avenues and potential medical applications. We will review the latest advances in the field. We will discuss the concepts of developmental complementation and developmental equivalence. We will discuss the methodological hurdles to be unlocked, as well as the biological and ethical limits of these new technologies.

Building Pluripotency Identity in the Early Embryo and Derived Stem Cells

The fusion of two highly differentiated cells, an oocyte with a spermatozoon, gives rise to the zygote, a single totipotent cell, which has the capability to develop into a complete, fully functional organism. Then, as development proceeds, a series of programmed cell divisions occur whereby the arising cells progressively acquire their own cellular and molecular identity, and totipotency narrows until when pluripotency is achieved. The path towards pluripotency involves transcriptome modulation, remodeling of the chromatin epigenetic landscape to which external modulators contribute. Both human and mouse embryos are a source of different types of pluripotent stem cells whose characteristics can be captured and maintained in vitro. The main aim of this review is to address the cellular properties and the molecular signature of the emerging cells during mouse and human early development, highlighting similarities and differences between the two species and between the embryos and their cognate stem cells.

Chimeric contribution of human extended pluripotent stem cells to monkey embryos ex vivo

Interspecies chimera formation with human pluripotent stem cells (hPSCs) represents a necessary alternative to evaluate hPSC pluripotency in vivo and might constitute a promising strategy for various regenerative medicine applications, including the generation of organs and tissues for transplantation. Studies using mouse and pig embryos suggest that hPSCs do not robustly contribute to chimera formation in species evolutionarily distant to humans. We studied the chimeric competency of human extended pluripotent stem cells (hEPSCs) in cynomolgus monkey (Macaca fascicularis) embryos cultured ex vivo. We demonstrate that hEPSCs survived, proliferated, and generated several peri- and early post-implantation cell lineages inside monkey embryos. We also uncovered signaling events underlying interspecific crosstalk that may help shape the unique developmental trajectories of human and monkey cells within chimeric embryos. These results may help to better understand early human development and primate evolution and develop strategies to improve human chimerism in evolutionarily distant species.

Parsing the pluripotency continuum in humans and non-human primates for interspecies chimera generation

Pluripotency refers to the potential of single cells to form all cells and tissues of an organism. The observation that pluripotent stem cells can chimerize the embryos of evolutionarily distant species, albeit at very low efficiencies, could with further modifications, facilitate the production of human-animal interspecies chimeras. The generation of human-animal interspecies chimeras, if achieved, will enable practitioners to recapitulate pathologic human tissue formation in vivo and produce patient-specific organs inside livestock species. However, little is known about the nature of chimera-competent cellular states in primates. Here, I discuss recent advances in our understanding of the pluripotency continuum in humans and non-human primates (NHPs). Although undefined differences between humans and NHPs still justify the utility of studying human cells, the complementary use of NHP PS cells could also allow one to conduct pilot studies testing interspecies chimera generation strategies with reduced ethical concerns associated with human interspecies neurological chimerism. However, the availability of standardized, high-quality and validated NHP PS cell lines covering the spectrum of primate pluripotent states is lacking. Therefore, a clearer understanding of the primate pluripotency continuum will facilitate the complementary use of both human and NHP PS cells for testing interspecies organogenesis strategies, with the hope of one day enabling human organ generation inside livestock species.