Mouse↔rat aggregation chimaeras can develop to adulthood

Exclusive generation of rat spermatozoa in sterile mice utilizing blastocyst complementation with pluripotent stem cells

Blastocyst complementation denotes a technique that aims to generate organs, tissues, or cell types in animal chimeras via injection of pluripotent stem cells (PSCs) into genetically compromised blastocyst-stage embryos. Here, we report on successful complementation of the male germline in adult chimeras following injection of mouse or rat PSCs into mouse blastocysts carrying a mutation in Tsc22d3, an essential gene for spermatozoa production. Injection of mouse PSCs into Tsc22d3-Knockout (KO) blastocysts gave rise to intraspecies chimeras exclusively embodying PSC-derived functional spermatozoa. In addition, injection of rat embryonic stem cells (rESCs) into Tsc22d3-KO embryos produced interspecies mouse-rat chimeras solely harboring rat spermatids and spermatozoa capable of fertilizing oocytes. Furthermore, using single-cell RNA sequencing, we deconstructed rat spermatogenesis occurring in a mouse-rat chimera testis. Collectively, this study details a method for exclusive xenogeneic germ cell production in vivo, with implications that may extend to rat transgenesis, or endangered animal species conservation efforts.

Attempts to obtain fully xenogeneic fetuses in rat ↔ mouse model†,‡

The full-term development of the xenogeneic embryo in the uterus of the mother of different species is very restricted and can occur only in certain groups of closely related mammals. In the case of mouse ↔ rat chimeras, the interspecific uterine barrier is less hostile to interspecific chimeric fetuses. In current work, we tested the development of mouse and rat fetuses in uteri of females of the opposite species. We created chimeric mouse ↔ rat blastocysts by injection of mouse embryonic stem cells (ESCs) into eight-cell rat embryos and rat ESCs into eight-cell mouse embryos. Chimeras were transferred to the foster mothers of the opposite species. Despite a huge number of transferred embryos (>1000 in total for both variants), only one live fetus derived solely from the mouse ESCs was isolated at E13.5 from the rat uterus. All other fetuses and newborns were chimeric or were built only from the cells of the recipient embryo. We examined the possible reason for such an outcome and found that the xenogeneic fetuses are eliminated at the perigastrulation stage of development. Thus, we conclude that in the rat ↔ mouse combination even when extraembryonic tissues of the chimeric embryo are composed solely of the cells of the same species as the female to which embryos are transferred, the full-term development of the pure xenogeneic fetus is very unlikely.

Generation of interspecies mouse-rat chimeric embryos by embryonic stem (ES) cell microinjection

Interspecies chimerism is a useful tool to study interactions between cells of different genetic makeup in order to elucidate the mechanisms underlying non-cell-autonomous processes, including evolutionary events. However, generating interspecies chimeras with high efficiency and chimerism level remains challenging. Here, we describe a protocol for generating chimeras between mouse and rat. Donor embryonic stem cells of one species are microinjected into early embryos of the other species (recipient), which are implanted into host foster mothers of the recipient species.

For complete details on the use and execution of this protocol, please refer to Stepien et al. (2020).

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Isolation of ES cells from mouse blastocysts and culture of rodent ES cells

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Microinjection of mouse ES cells into early rat embryos and vice versa

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Transfer of chimeric embryos into pseudo-pregnant foster mothers

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Generation of mouse-rat chimeric brains

Perspectives of pluripotent stem cells in livestock

The recent progress in derivation of pluripotent stem cells (PSCs) from farm animals opens new approaches not only for reproduction, genetic engineering, treatment and conservation of these species, but also for screening novel drugs for their efficacy and toxicity, and modelling of human diseases. Initial attempts to derive PSCs from the inner cell mass of blastocyst stages in farm animals were largely unsuccessful as either the cells survived for only a few passages, or lost their cellular potency; indicating that the protocols which allowed the derivation of murine or human embryonic stem (ES) cells were not sufficient to support the maintenance of ES cells from farm animals. This scenario changed by the innovation of induced pluripotency and by the development of the 3 inhibitor culture conditions to support naïve pluripotency in ES cells from livestock species. However, the long-term culture of livestock PSCs while maintaining the full pluripotency is still challenging, and requires further refinements. Here, we review the current achievements in the derivation of PSCs from farm animals, and discuss the potential application areas.

Common principles of early mammalian embryo self-organisation

Pre-implantation mammalian development unites extreme plasticity with a robust outcome: the formation of a blastocyst, an organised multi-layered structure ready for implantation. The process of blastocyst formation is one of the best-known examples of self-organisation. The first three cell lineages in mammalian development specify and arrange themselves during the morphogenic process based on cell-cell interactions. Despite decades of research, the unifying principles driving early mammalian development are still not fully defined. Here, we discuss the role of physical forces, and molecular and cellular mechanisms, in driving self-organisation and lineage formation that are shared between eutherian mammals.

Human-animal chimeras for autologous organ transplantation: technological advances and future perspectives

Organ transplantation is the most promising curation for end-stage organ disease. However, the donor organ shortage has become a global problem that has limited the development of organ transplantation. Human-animal chimeras provide the ability to produce human organs in other species using autologous stem cells [e.g., induced pluripotent stem cells (iPSCs) or adult stem cells], which would be patient-specific and immune-matched for transplantation. Due to the potential application prospect of interspecies chimeras in basic and translational research, this technology has attracted much interest. This review focuses primarily on technological advances, including options of donor stem cell types and gene editing in donor cells and host animals, in addition to perspectives on human-animal chimeras in clinical and basic research.

Contribution of rat embryonic stem cells to xenogeneic chimeras in blastocyst or 8-cell embryo injection and aggregation

The ultimate goal of regenerative medicine is the transplantation of a target organ generated by the patient's own cells. Recently, a method of organ generation using pluripotent stem cells (PSCs) and blastocyst complementation was reported. This approach is based on chimeric animal generation using an early embryo and PSCs, and the contribution of PSCs to the target organ is key to the method's success. However, the contribution rate of PSCs in target organs generated by different chimeric animal generation methods remains unknown. In this study, we used 8-cell embryo aggregation, 8-cell embryo injection, and blastocyst injection to generate interspecies chimeric mice using rat embryonic stem (ES) cells and then investigated the differences in the contribution rate of the rat ES cells. The rate of chimeric mouse generation was the highest using blastocyst injection, followed in order by 8-cell embryo injection and 8-cell embryo aggregation. However, the contribution rate of rat ES cells was the highest in chimeric neonates generated by 8-cell embryo injection, and the difference was statistically significant in the liver. Live functionality was confirmed by analyzing the expression of rat hepatocyte-derived drug-metabolizing enzyme. Collectively, these findings indicate that the 8-cell embryo injection method is the most suitable for generation of PSC-derived organs via chimeric animal generation, particularly for the liver.