Inhibition of Apoptosis Overcomes Stage-Related Compatibility Barriers to Chimera Formation in Mouse Embryos

Foxa1 disruption enhances human cell integration in human-mouse interspecies chimeras

Blastocyst complementation can potentially generate a rodent model with humanized nasopharyngeal epithelium (NE) that supports sustained Epstein-Barr virus (EBV) infection, enabling comprehensive studies of EBV biology in nasopharyngeal carcinoma. However, during this process, the specific gene knockouts required to establish a developmental niche for NE remain unclear. We performed bioinformatics analyses and generated Foxa1 mutant mice to confirm that Foxa1 disruption could potentially create a developmental niche for NE. Subsequently, MYD88-inactivated human pluripotent stem cells (hPSCs) were constructed and complemented with Foxa1-deficient mouse blastocysts, with Nosip-deficient mouse blastocysts as a control. The chimerism of human cells in mouse embryos was evaluated from E8.5 to E12.5 using genomic DNA PCR and immunohistochemistry. Our bioinformatics analysis indicated that the expression patterns of Foxa1 in E8.5 to E16.5 mouse embryos underscore its critical role in NE development. The generated mice with Foxa1 disordered region mutations displayed morphological abnormality in NE, suggesting Foxa1-knockouts could potentially establish a developmental niche for NE. In chimeric assays, human cells integrated into 80.00% of Foxa1-deficient embryos, compared with the 4.17% in controls. Immunohistochemistry results revealed robust proliferation of human cells in Foxa1-deficient mouse embryos. However, chimeras from Foxa1-deficient mouse embryos did not survive beyond E10.5, hindering the evaluation of human cell integration in mouse NE. Foxa1 disruption in mouse embryos significantly enhances the integration of human cells in human-mouse interspecies chimeras, thereby facilitating the generation of endoderm-derived organs through blastocyst complementation. Overcoming chimeras' embryonic lethality is crucial for successfully generating humanized NE in Foxa1-deficient mouse embryos.

Research Guideline Recommendations for Research on Stem Cells, Human Embryos, and Gene Editing

Recent advances in biomedical technologies have extended the boundaries of previously established regulatory guidelines pertaining to stem cell research. These guidelines constrained the study of human pluripotent stem cells (hPSCs) and their derivatives from use under various conditions, including the introduction of hPSCs into the brains of host animals because of concerns of humanizing the brains of animal species. Other guidelines constrained the use of hPSCs in creating human-animal chimeras because of the potential contribution of human stem cells not only to the brain but also to the germline. Some regulatory guidelines forbid the growing of human embryos ex vivo beyond the stage of primitive streak development because of concerns regarding the creation of human forms of life ex vivo. At the subcellular level, there are guidelines regulating the transfer of mitochondria within human embryos. At the molecular level, there are guidelines regulating genome editing to prevent permanent genetic alterations in germline cells. These and other issues related to stem cells have been reviewed, and new research guidelines established by the International Society for Stem Cell Research (ISSCR) for its membership. Because many of the recommended changes by the ISSCR impact research being conducted by members of the American Society for Neural Therapy and Repair (ASNTR), the ASNTR established a task force to review relevant recommendations by the ISSCR to determine which new guidelines to adopt for research conducted by the ASNTR society membership. The final ASNTR recommendations are presented in this document.

Incompatibility in cell adhesion constitutes a barrier to interspecies chimerism

Interspecies blastocyst complementation holds great potential to address the global shortage of transplantable organs by growing human organs in animals. However, a major challenge in this approach is the limited chimerism of human cells in evolutionarily distant animal hosts due to various xenogeneic barriers. Here, we reveal that human pluripotent stem cells (PSCs) struggle to adhere to animal PSCs. To overcome this barrier, we developed a synthetic biology strategy that leverages nanobody-antigen interactions to enhance interspecies cell adhesion. We engineered cells to express nanobodies and their corresponding antigens on their outer membranes, significantly improving adhesion between different species' PSCs during in vitro assays and increasing the chimerism of human PSCs in mouse embryos. Studying and manipulating interspecies pluripotent cell adhesion will provide valuable insights into cell interaction dynamics during chimera formation and early embryogenesis.

Producing Human Livers From Human Stem Cells Via Blastocyst Complementation

The need for organ transplants exceeds donor organ availability. In the quest to solve this shortage, the most remarkable area of advancement is organ production through the use of chimeric embryos, commonly known as blastocyst complementation. This technique involves the combination of different species to generate chimeras, where the extent of donor cell contribution to the desired tissue or organ can be regulated. However, ethical concerns arise with the use of brain tissue in such chimeras. Furthermore, the ratio of contributed cells to host animal cells in the chimeric system is low in the production of chimeras associated with cell apoptosis. This review discusses the latest innovations in blastocyst complementation and highlights the progress made in creating organs for transplant.

Parathyroid Gland Generation from Pluripotent Stem Cells

Patients with permanent hypoparathyroidism require lifelong treatment. Current replacement therapies sometimes have adverse effects (e.g., hypercalciuria and chronic kidney disease). Generating parathyroid glands (PTGs) from the patient's own induced pluripotent stem cells (PSCs), with transplantation of these PTGs, would be an effective treatment option. Multiple methods for generating PTGs from PSCs have been reported. One major trend is in vitro differentiation of PSCs into PTGs. Another is in vivo generation of PSC-derived PTGs by injecting PSCs into PTG-deficient embryos. This review discusses current achievements and challenges in present and future PTG regenerative medicine.

Paraspeckles / CARM1 mediates the regulation of OEVs on cell differentiation during in vitro embryonic development of yak

Mammalian embryos produced in vitro have poor embryo quality and low developmental ability compared with in vivo embryos. The main manifestations are the low number of blastocysts, the low ratio of the number of inner cell mass cells to the number of trophoblastic cells, and the high apoptosis rate of blastocysts, resulting in low embryo implantation rate. Therefore, optimizing in vitro culture conditions has become a key technology to im-prove the quality of preimplantation embryos. Oviduct Epithelial cells exosomes (OEVs) can be absorbed and internalized by embryos to improve the blastocyst rate and blastocyst quality of embryos in vitro. As a special nuclear structure, Paraspeckles are involved in the fate determination of mammalian early embryonic mammalian cells. However, the regulation of embryonic cell differentiation by OEVs remains unknown. We aimed to investigate the effects of OEVs on paraspeckle formation and cell fate determination in yak in vitro fertilization (IVF) of em-bryos. To simulate the in vivo oviduct environment after ovulation, we used follicular fluid exosomes (FEVs) to stimulate yak oviduct epithelial cells and collect OEVs. OEVs were added to the yak IVF embryo culture system. Paraspeckle formation, cell differentiation, and blastocyst quality in yak embryos were determined. Our results show that, development of yak embryos is unique compared to other bovine species, and OEVs can be used as a supplement to the in vitro culture system of yak embryos to improve embryonic development and blas-tocyst quality. And also Paraspeckles/CARM1 mediated the regulation of OEVs on cell differentiation during in vitro yak embryo production. These results provide new insights into the study of yak embryonic development and the role of OEVs in embryonic development.

Mesendodermal cells fail to contribute to heart formation following blastocyst injection

Blastocyst complementation offers an opportunity for generating transplantable whole organs from donor sources. Pluripotent stem cells (PSCs) have traditionally served as the primary donor cells due to their ability to differentiate into any type of body cell. However, the use of PSCs raises ethical concerns, particularly regarding their uncontrollable differentiation potential to undesired cell lineages such as brain and germline cells. To address this issue, various strategies have been explored, including the use of genetically modified PSCs with restricted lineage potential or lineage-specified progenitor cells as donors. In this study, we tested whether nascent mesendodermal cells (MECs), which appear during early gastrulation, can be used as donor cells. To do this, we induced Bry-GFP+ MECs from mouse embryonic stem cells (ESCs) and introduced them into the blastocyst. While donor ESCs gave rise to various regions of embryos, including the heart, Bry-GFP+ MECs failed to contribute to the host embryos. This finding suggests that MECs, despite being specified from PSCs within a few days, lack the capacity to assimilate into the developing embryo.

Determining the potency of primordial germ cells by injection into early mouse embryos

Primordial germ cells (PGCs) are the earliest precursors of the gametes. During normal development, PGCs only give rise to oocytes or spermatozoa. However, PGCs can acquire pluripotency in vitro by forming embryonic germ (EG) cells and in vivo during teratocarcinogenesis. Classic embryological experiments directly assessed the potency of PGCs by injection into the pre-implantation embryo. As no contribution to embryos or adult mice was observed, PGCs have been described as unipotent. Here, we demonstrate that PGCs injected into 8-cell embryos can initially survive, divide, and contribute to the developing inner cell mass. Apoptosis-deficient PGCs exhibit improved survival in isolated epiblasts and can form naive pluripotent embryonic stem cell lines. However, contribution to the post-implantation embryo is limited, with no functional incorporation observed. In contrast, PGC-like cells show an extensive contribution to mid-gestation chimeras. We thus propose that PGC formation in vivo establishes a latent form of pluripotency that restricts chimera contribution.

Chimeric Livers: Interspecies Blastocyst Complementation and Xenotransplantation for End-Stage Liver Disease

Orthotopic liver transplantation (OLT) currently serves as the sole definitive treatment for thousands of patients suffering from end-stage liver disease; and the existing supply of donor livers for OLT is drastically outpaced by the increasing demand. To alleviate this significant gap in treatment, several experimental approaches have been devised with the aim of either offering interim support to patients waiting on the transplant list or bioengineering complete livers for OLT by infusing them with fresh hepatic cells. Recently, interspecies blastocyst complementation has emerged as a promising method for generating complete organs in utero over a short timeframe. When coupled with gene editing technology, it has brought about a potentially revolutionary transformation in regenerative medicine. Blastocyst complementation harbors notable potential for generating complete human livers in large animals, which could be used for xenotransplantation in humans, addressing the scarcity of livers for OLT. Nevertheless, substantial experimental and ethical challenges still need to be overcome to produce human livers in larger domestic animals like pigs. This review compiles the current understanding of interspecies blastocyst complementation and outlines future possibilities for liver xenotransplantation in humans.

Advancements in medical research using fetal sheep: Implications for human health and treatment methods

Sheep are typically considered as industrial animals that provide wool and meals. However, they play a significant role in medical research in addition to their conventional use. Notably, sheep fetuses are resistant to surgical invasions and can endure numerous manipulations, such as needle puncture and cell transplantation, and surgical operations requiring exposure beyond the uterus. Based on these distinguishing characteristics, we established a chimeric sheep model capable of producing human/monkey pluripotent cell-derived blood cells via the fetal liver. Furthermore, sheep have become crucial as human fetal models, acting as platforms for developing and improving techniques for intrauterine surgery to address congenital disorders and clarifying the complex pharmacokinetic interactions between mothers and their fetuses. This study emphasizes the significant contributions of fetal sheep to advancing human disease understanding and treatment strategies, highlighting their unique characteristics that are not present in other animals.

Developmental potency of human ES cell-derived mesenchymal stem cells revealed in mouse embryos following blastocyst injection

Mesenchymal stem cells (MSCs) are present in almost all the tissues in the body, critical for their homeostasis and regeneration. However, the stemness of MSCs is mainly an in vitro observation, and lacking exclusive markers for endogenous MSCs makes it difficult to study the multipotency of MSCs in vivo, especially for human MSCs. To address this hurdle, we injected GFP-tagged human embryonic stem cell (hESC)-derived MSCs (EMSCs) into mouse blastocysts. EMSCs survived well and penetrated both the inner cell mass and trophectoderm, correlating to the higher anti-apoptotic capability of EMSCs than hESCs. Injected EMSCs contributed to skeletal, dermal, and extraembryonic tissues in the resultant chimera and partially rescued skeletal defects in Sox9+/- mouse fetuses. Thus, this study provides evidence for the stemness and developmental capability of human MSCs through chimerization with the mouse blastocyst, serving as a model for studying human mesenchymal and skeletal development.

BCL2 is a major regulator of haploidy maintenance in murine embryonic stem cells

Mammalian haploid cells are important resources for forward genetic screening and are important in genetic medicine and drug development. However, the self-diploidization of murine haploid embryonic stem cells (haESCs) during daily culture or differentiation jeopardizes their use in genetic approaches. Here, we show that overexpression (OE) of an antiapoptosis gene, BCL2, in haESCs robustly ensures their haploidy maintenance in various situations, even under strict differentiation in vivo (embryonic 10.5 chimeric fetus or 21-day teratoma). Haploid cell lines of many lineages, including epiblasts, trophectodermal lineages, and neuroectodermal lineages, can be easily derived by the differentiation of BCL2-OE haESCs in vitro. Transcriptome analysis revealed that BCL2-OE activates another regulatory gene, Has2, which is also sufficient for haploidy maintenance. Together, our findings provide an effective and secure strategy to reduce diploidization during differentiation, which will contribute to the generation of haploid cell lines of the desired lineage and related genetic screening.

Generation of a humanized mesonephros in pigs from induced pluripotent stem cells via embryo complementation

Heterologous organ transplantation is an effective way of replacing organ function but is limited by severe organ shortage. Although generating human organs in other large mammals through embryo complementation would be a groundbreaking solution, it faces many challenges, especially the poor integration of human cells into the recipient tissues. To produce human cells with superior intra-niche competitiveness, we combined optimized pluripotent stem cell culture conditions with the inducible overexpression of two pro-survival genes (MYCN and BCL2). The resulting cells had substantially enhanced viability in the xeno-environment of interspecies chimeric blastocyst and successfully formed organized human-pig chimeric middle-stage kidney (mesonephros) structures up to embryonic day 28 inside nephric-defective pig embryos lacking SIX1 and SALL1. Our findings demonstrate proof of principle of the possibility of generating a humanized primordial organ in organogenesis-disabled pigs, opening an exciting avenue for regenerative medicine and an artificial window for studying human kidney development.

Human and Pig Pluripotent Stem Cells: From Cellular Products to Organogenesis and Beyond

Pluripotent stem cells (PSCs) are important for studying development and hold great promise in regenerative medicine due to their ability to differentiate into various cell types. In this review, we comprehensively discuss the potential applications of both human and pig PSCs and provide an overview of the current progress and challenges in this field. In addition to exploring the therapeutic uses of PSC-derived cellular products, we also shed light on their significance in the study of interspecies chimeras, which has led to the creation of transplantable human or humanized pig organs. Moreover, we emphasize the importance of pig PSCs as an ideal cell source for genetic engineering, facilitating the development of genetically modified pigs for pig-to-human xenotransplantation. Despite the achievements that have been made, further investigations and refinement of PSC technologies are necessary to unlock their full potential in regenerative medicine and effectively address critical healthcare challenges.

Functional calcium-responsive parathyroid glands generated using single-step blastocyst complementation

Patients with permanent hypoparathyroidism require lifelong replacement therapy to avoid life-threatening complications, The benefits of conventional treatment are limited, however. Transplanting a functional parathyroid gland (PTG) would yield better results. Parathyroid gland cells generated from pluripotent stem cells in vitro to date cannot mimic the physiological responses to extracellular calcium that are essential for calcium homeostasis. We thus hypothesized that blastocyst complementation (BC) could be a better strategy for generating functional PTG cells and compensating loss of parathyroid function. We here describe generation of fully functional PTGs from mouse embryonic stem cells (mESCs) with single-step BC. Using CRISPR-Cas9 knockout of Glial cells missing2 (Gcm2), we efficiently produced aparathyroid embryos for BC. In these embryos, mESCs differentiated into endocrinologically mature PTGs that rescued Gcm2-/- mice from neonatal death. The mESC-derived PTGs responded to extracellular calcium, restoring calcium homeostasis on transplantation into mice surgically rendered hypoparathyroid. We also successfully generated functional interspecies PTGs in Gcm2-/- rat neonates, an accomplishment with potential for future human PTG therapy using xenogeneic animal BC. Our results demonstrate that BC can produce functional endocrine organs and constitute a concept in treatment of hypoparathyroidism.

Review: Progress in producing chimeric ungulate livestock for agricultural applications

The progress made in recent years in the derivation and culture of pluripotent stem cells from farm animals opens up the possibility of creating livestock chimeras. Chimeras producing gametes exclusively derived from elite donor stem cells could pass superior genetics on to the next generation and thereby reduce the genetic lag that typically exists between the elite breeding sector and the commercial production sector, especially for industries like beef and sheep where genetics is commonly disseminated through natural service mating. Chimeras carrying germ cells generated from genome-edited or genetically engineered pluripotent stem cells could further disseminate useful genomic alterations such as climate adaptation, animal welfare improvements, the repair of deleterious genetic conditions, and/or the elimination of undesired traits such as disease susceptibility to the next generation. Despite the successful production of chimeras with germ cells generated from pluripotent donor stem cells injected into preimplantation-stage blastocysts in model species, there are no documented cases of this occurring in livestock. Here, we review the literature on the derivation of pluripotent stem cells from ungulates, and progress in the production of chimeric ungulate livestock for agricultural applications, drawing on insights from studies done in model species, and discuss future possibilities of this fast-moving and developing field. Aside from the technical aspects, the consistency of the regulatory approach taken by different jurisdictions towards chimeric ungulate livestock with germ cells generated from pluripotent stem cells and their progeny will be an important determinant of breeding industry uptake and adoption in animal agriculture.

Genome-wide association study analysis to resolve the key regulatory mechanism of biomineralization in Pinctada fucata martensii

Biomineralization-controlled exo-/endoskeleton growth contributes to body growth and body size diversity. Molluscan shells undergo ectopic biomineralization to form the exoskeleton and biocalcified "pearl" involved in invading defence. Notably, exo-/endoskeletons have a common ancestral origin, but their regulation and body growth are largely unknown. This study employed the pearl oyster, Pinctada fucata marntensii, a widely used experimental model for biomineralization in invertebrates, to perform whole-genome resequencing of 878 individuals from wild and breeding populations. This study characterized the genetic architecture of biomineralization-controlled growth and ectopic biomineralization. The insulin-like growth factor (IGF) endocrine signal interacted with ancient single-copy transcription factors to form the regulatory network. Moreover, the "cross-phylum" regulation of key long noncoding RNA (lncRNA) in bivalves and mammals indicated the conserved genetic and epigenetic regulation in exo-/endoskeleton growth. Thyroid hormone signal and apoptosis regulation in pearl oysters affected ectopic biomineralization in pearl oyster. These findings provide insights into the mechanism underlying the evolution and regulation of biomineralization in exo-/endoskeleton animals and ectopic biomineralization.

Genetically engineered pigs for xenotransplantation: Hopes and challenges

The shortage of donor resources has greatly limited the application of clinical xenotransplantation. As such, genetically engineered pigs are expected to be an ideal organ source for xenotransplantation. Most current studies mainly focus on genetically modifying organs or tissues from donor pigs to reduce or prevent attack by the human immune system. Another potential organ source is interspecies chimeras. In this paper, we reviewed the progress of the genetically engineered pigs from the view of immunologic barriers and strategies, and discussed the possibility and challenges of the interspecies chimeras.

Super-enhancers conserved within placental mammals maintain stem cell pluripotency

Despite pluripotent stem cells sharing key transcription factors, their maintenance involves distinct genetic inputs. Emerging evidence suggests that super-enhancers (SEs) can function as master regulatory hubs to control cell identity and pluripotency in humans and mice. However, whether pluripotency-associated SEs share an evolutionary origin in mammals remains elusive. Here, we performed comprehensive comparative epigenomic and transcription factor binding analyses among pigs, humans, and mice to identify pluripotency-associated SEs. Like typical enhancers, SEs displayed rapid evolution in mammals. We showed that BRD4 is an essential and conserved activator for mammalian pluripotency-associated SEs. Comparative motif enrichment analysis revealed 30 shared transcription factor binding motifs among the three species. The majority of transcriptional factors that bind to identified motifs are known regulators associated with pluripotency. Further, we discovered three pluripotency-associated SEs (SE-SOX2, SE-PIM1, and SE-FGFR1) that displayed remarkable conservation in placental mammals and were sufficient to drive reporter gene expression in a pluripotency-dependent manner. Disruption of these conserved SEs through the CRISPR-Cas9 approach severely impaired stem cell pluripotency. Our study provides insights into the understanding of conserved regulatory mechanisms underlying the maintenance of pluripotency as well as species-specific modulation of the pluripotency-associated regulatory networks in mammals.

Stepwise pluripotency transitions in mouse stem cells

Pluripotent cells in mouse embryos, which first emerge in the inner cell mass of the blastocyst, undergo gradual transition marked by changes in gene expression, developmental potential, polarity, and morphology as they develop from the pre-implantation until post-implantation gastrula stage. Recent studies of cultured mouse pluripotent stem cells (PSCs) have clarified the presence of intermediate pluripotent stages between the naïve pluripotent state represented by embryonic stem cells (ESCs-equivalent to the pre-implantation epiblast) and the primed pluripotent state represented by epiblast stem cells (EpiSCs-equivalent to the late post-implantation gastrula epiblast). In this review, we discuss these recent findings in light of our knowledge on peri-implantation mouse development and consider the implications of these new PSCs to understand their temporal sequence and the feasibility of using them as model system for pluripotency.

Chimpanzee and pig-tailed macaque iPSCs: Improved culture and generation of primate cross-species embryos

As our closest living relatives, non-human primates uniquely enable explorations of human health, disease, development, and evolution. Considerable effort has thus been devoted to generating induced pluripotent stem cells (iPSCs) from multiple non-human primate species. Here, we establish improved culture methods for chimpanzee (Pan troglodytes) and pig-tailed macaque (Macaca nemestrina) iPSCs. Such iPSCs spontaneously differentiate in conventional culture conditions, but can be readily propagated by inhibiting endogenous WNT signaling. As a unique functional test of these iPSCs, we injected them into the pre-implantation embryos of another non-human species, rhesus macaques (Macaca mulatta). Ectopic expression of gene BCL2 enhances the survival and proliferation of chimpanzee and pig-tailed macaque iPSCs within the pre-implantation embryo, although the identity and long-term contribution of the transplanted cells warrants further investigation. In summary, we disclose transcriptomic and proteomic data, cell lines, and cell culture resources that may be broadly enabling for non-human primate iPSCs research.

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.

Approaches to kidney replacement therapies—opportunities and challenges

One out of seven people develop chronic kidney disease (CKD). When kidney function continues to decline, CKD patients may develop end-stage renal disease (ESRD, or kidney failure). More than 2 out of 1,000 adults develop ESRD and these patients must live on dialysis or get a kidney transplant to survive. Each year, more than $51 billion is spent to treat patients with ESRD in the United States. In addition, ESRD greatly reduces longevity and quality of life for patients. Compared to dialysis, kidney transplant offers the best chance of survival, but few donor organs are available. Thus, there is an urgent need for innovative solutions that address the shortage of kidneys available for transplantation. Here we summarize the status of current approaches that are being developed to solve the shortage of donor kidneys. These include the bioartificial kidney approach which aims to make a portable dialysis device, the recellularization approach which utilizes native kidney scaffold to make an engineered kidney, the stem cell-based approach which aims to generate a kidney de novo by recapitulating normal kidney organogenesis, the xenotransplantation approach which has the goal to make immunocompatible pig kidneys for transplantation, and the interspecies chimera approach which has potential to generate a human kidney in a host animal. We also discuss the interconnections among the different approaches, and the remaining challenges of translating these approaches into novel therapies.

Generating human artery and vein cells from pluripotent stem cells highlights the arterial tropism of Nipah and Hendra viruses

Stem cell research endeavors to generate specific subtypes of classically defined "cell types." Here, we generate >90% pure human artery or vein endothelial cells from pluripotent stem cells within 3-4 days. We specified artery cells by inhibiting vein-specifying signals and vice versa. These cells modeled viral infection of human vasculature by Nipah and Hendra viruses, which are extraordinarily deadly (∼57%-59% fatality rate) and require biosafety-level-4 containment. Generating pure populations of artery and vein cells highlighted that Nipah and Hendra viruses preferentially infected arteries; arteries expressed higher levels of their viral-entry receptor. Virally infected artery cells fused into syncytia containing up to 23 nuclei, which rapidly died. Despite infecting arteries and occupying ∼6%-17% of their transcriptome, Nipah and Hendra largely eluded innate immune detection, minimally eliciting interferon signaling. We thus efficiently generate artery and vein cells, introduce stem-cell-based toolkits for biosafety-level-4 virology, and explore the arterial tropism and cellular effects of Nipah and Hendra viruses.

Cell competition and the regulative nature of early mammalian development

The mammalian embryo exhibits a remarkable plasticity that allows it to correct for the presence of aberrant cells, adjust its growth so that its size is in accordance with its developmental stage, or integrate cells of another species to form fully functional organs. Here, we will discuss the contribution that cell competition, a quality control that eliminates viable cells that are less fit than their neighbors, makes to this plasticity. We will do this by reviewing the roles that cell competition plays in the early mammalian embryo and how they contribute to ensure normal development of the embryo.

Research progress and application prospects of stable porcine pluripotent stem cells

Pluripotent stem cells (PSCs) harbor the capacity of unlimited self-renewal and multi-lineage differentiation potential which are crucial for basic research and biomedical science. Establishment of PSCs with defined features were previously reported from mice and humans, while generation of stable large animal PSCs has experienced a relatively long trial stage and only recently has made breakthroughs. Pigs are regarded as ideal animal models for their similarities in physiology and anatomy to humans. Generation of porcine PSCs would provide cell resources for basic research, genetic engineering, animal breeding and cultured meat. In this review, we summarize the progress on the derivation of porcine PSCs and reprogrammed cells and elucidate the mechanisms of pluripotency changes during pig embryo development. This will be beneficial for understanding the divergence and conservation between different species involved in embryo development and the pluripotent regulated signaling pathways. Finally, we also discuss the promising future applications of stable porcine PSCs.

Naïve-like pluripotency to pave the way for saving the northern white rhinoceros from extinction

The northern white rhinoceros (NWR) is probably the earth's most endangered mammal. To rescue the functionally extinct species, we aim to employ induced pluripotent stem cells (iPSCs) to generate gametes and subsequently embryos in vitro. To elucidate the regulation of pluripotency and differentiation of NWR PSCs, we generated iPSCs from a deceased NWR female using episomal reprogramming, and observed surprising similarities to human PSCs. NWR iPSCs exhibit a broad differentiation potency into the three germ layers and trophoblast, and acquire a naïve-like state of pluripotency, which is pivotal to differentiate PSCs into primordial germ cells (PGCs). Naïve culturing conditions induced a similar expression profile of pluripotency related genes in NWR iPSCs and human ESCs. Furthermore, naïve-like NWR iPSCs displayed increased expression of naïve and PGC marker genes, and a higher integration propensity into developing mouse embryos. As the conversion process was aided by ectopic BCL2 expression, and we observed integration of reprogramming factors, the NWR iPSCs presented here are unsuitable for gamete production. However, the gained insights into the developmental potential of both primed and naïve-like NWR iPSCs are fundamental for in future PGC-specification in order to rescue the species from extinction using cryopreserved somatic cells.

Lima1 mediates the pluripotency control of membrane dynamics and cellular metabolism

Lima1 is an extensively studied prognostic marker of malignancy and is also considered to be a tumour suppressor, but its role in a developmental context of non-transformed cells is poorly understood. Here, we characterise the expression pattern and examined the function of Lima1 in mouse embryos and pluripotent stem cell lines. We identify that Lima1 expression is controlled by the naïve pluripotency circuit and is required for the suppression of membrane blebbing, as well as for proper mitochondrial energetics in embryonic stem cells. Moreover, forcing Lima1 expression enables primed mouse and human pluripotent stem cells to be incorporated into murine pre-implantation embryos. Thus, Lima1 is a key effector molecule that mediates the pluripotency control of membrane dynamics and cellular metabolism.

Interspecies Chimeric Barriers for Generating Exogenic Organs and Cells for Transplantation

A growing need for organs and novel cell-based therapies has provided a niche for approaches like interspecies chimeras. To generate organs from one donor species in another host species requires techniques such as blastocyst complementation and gene editing to successfully create an embryo that has cells from both the donor and the host. However, the task of developing highly efficacious and competent interspecies chimeras is met by many challenges. These interspecies chimeric barriers impede the formation of chimeras, often leading to lower levels of chimeric competency. The barriers that need to be addressed include the evolutionary distance between species, stage-matching, temporal and spatial synchronization of developmental timing, interspecies cell competition and the survival of pluripotent stem cells and embryos, compatibility of ligand–receptor signaling between species, and the ethical concerns of forming such models. By overcoming the interspecies chimera barriers and creating highly competent chimeras, the technology of organ and cellular generation can be honed and refined to develop fully functioning exogenic organs, tissues, and cells for transplantation.

Xenotransplantation and interspecies organogenesis: current status and issues

Pancreas (and islet) transplantation is the only curative treatment for type 1 diabetes patients whose β-cell functions have been abolished. However, the lack of donor organs has been the major hurdle to save a large number of patients. Therefore, transplantation of animal organs is expected to be an alternative method to solve the serious shortage of donor organs. More recently, a method to generate organs from pluripotent stem cells inside the body of other species has been developed. This interspecies organ generation using blastocyst complementation (BC) is expected to be the next-generation regenerative medicine. Here, we describe the recent advances and future prospects for these two approaches.

Capturing Pluripotency and Beyond

During the development of a multicellular organism, the specification of different cell lineages originates in a small group of pluripotent cells, the epiblasts, formed in the preimplantation embryo. The pluripotent epiblast is protected from premature differentiation until exposure to inductive cues in strictly controlled spatially and temporally organized patterns guiding fetus formation. Epiblasts cultured in vitro are embryonic stem cells (ESCs), which recapitulate the self-renewal and lineage specification properties of their endogenous counterparts. The characteristics of totipotency, although less understood than pluripotency, are becoming clearer. Recent studies have shown that a minor ESC subpopulation exhibits expanded developmental potential beyond pluripotency, displaying a characteristic reminiscent of two-cell embryo blastomeres (2CLCs). In addition, reprogramming both mouse and human ESCs in defined media can produce expanded/extended pluripotent stem cells (EPSCs) similar to but different from 2CLCs. Further, the molecular roadmaps driving the transition of various potency states have been clarified. These recent key findings will allow us to understand eutherian mammalian development by comparing the underlying differences between potency network components during development. Using the mouse as a paradigm and recent progress in human PSCs, we review the epiblast's identity acquisition during embryogenesis and their ESC counterparts regarding their pluripotent fates and beyond.

Fumarylacetoacetate hydrolase gene as a knockout target for hepatic chimerism and donor liver production

A reliable source of human hepatocytes and transplantable livers is needed. Interspecies embryo complementation, which involves implanting donor human stem cells into early morula/blastocyst stage animal embryos, is an emerging solution to the shortage of transplantable livers. We review proposed mutations in the recipient embryo to disable hepatogenesis, and discuss the advantages of using fumarylacetoacetate hydrolase knockouts and other genetic modifications to disable hepatogenesis. Interspecies blastocyst complementation using porcine recipients for primate donors has been achieved, although percentages of chimerism remain persistently low. Recent investigation into the dynamic transcriptomes of pigs and primates have created new opportunities to intimately match the stage of developing animal embryos with one of the many varieties of human induced pluripotent stem cell. We discuss techniques for decreasing donor cell apoptosis, targeting donor tissue to endodermal structures to avoid neural or germline chimerism, and decreasing the immunogenicity of chimeric organs by generating donor endothelium.

The exploration of pluripotency space: Charting cell state transitions in peri-implantation development

Pluripotent cells in the mammalian embryo undergo state transitions marked by changes in patterns of gene expression and developmental potential as they progress from pre-implantation through post-implantation stages of development. Recent studies of cultured mouse and human pluripotent stem cells (hPSCs) have identified cells representative of an intermediate stage (referred to as the formative state) between naive pluripotency (equivalent to pre-implantation epiblast) and primed pluripotency (equivalent to late post-implantation epiblast). We examine these recent findings in light of our knowledge of peri-implantation mouse and human development, and we consider the implications of this work for deriving human embryo models from pluripotent cells.

In Vivo Generation of Organs by Blastocyst Complementation: Advances and Challenges

The ultimate goal of regenerative medicine is to replace damaged cells, tissues or whole organs, in order to restore their proper function. Stem cell related technologies promise to generate transplants from the patients' own cells. Novel approaches such as blastocyst complementation combined with genome editing open up new perspectives for organ replacement therapies. This review summarizes recent advances in the field and highlights the challenges that still remain to be addressed.

[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.

Cell state transitions: definitions and challenges

A fundamental challenge when studying biological systems is the description of cell state dynamics. During transitions between cell states, a multitude of parameters may change - from the promoters that are active, to the RNAs and proteins that are expressed and modified. Cells can also adopt different shapes, alter their motility and change their reliance on cell-cell junctions or adhesion. These parameters are integral to how a cell behaves and collectively define the state a cell is in. Yet, technical challenges prevent us from measuring all of these parameters simultaneously and dynamically. How, then, can we comprehend cell state transitions using finite descriptions? The recent virtual workshop organised by The Company of Biologists entitled 'Cell State Transitions: Approaches, Experimental Systems and Models' attempted to address this question. Here, we summarise some of the main points that emerged during the workshop's themed discussions. We also present examples of cell state transitions and describe models and systems that are pushing forward our understanding of how cells rewire their state.

Human-animal interspecies chimerism via blastocyst complementation: advances, challenges and perspectives: a narrative review

OBJECTIVE

Interspecific human-animal chimerism via blastocyst complementation provides a promising strategy to generate function human cells, tissues or organs from human pluripotent stem cells (hPSCs), although it is still quite challenging. In this review, we will mainly focus on the recent advances, such as the options of donor hPSCs and the understanding of interspecific chimera barriers, challenges, and perspectives on the efficient generation of human-animal interspecies chimeras.

BACKGROUND

hPSCs, including the human embryonic stem cells (hESCs) and the human induced pluripotent stem cells (hiPSCs) hold great promise for regenerative medicine to treat various degenerative diseases. However, although hPSCs can differentiate to all lineage cells in dish, the functionality of these cells is limited, hinting that the in vitro differentiation system failed to fully recapture the in vivo development. A promising alternative strategy is in vivo generation of functional human cells in animals through interspecies chimerism, based on the principle that mammalian development is highly conserved across species. This strategy was inspired by the successful generation of functional rat pancreas in mice through blastocyst injection of rat pluripotent stem cells (PSCs). Over the past ten years, since this milestone work was reported, advances have been made in the human-animal interspecies chimerism. However, it is still challenging to efficiently generate human cells, tissues, or organs in the interspecies chimeras. This phenomenon suggests that there are still obstacles to illustrate and overcome implicated in human-animal interspecies chimeras.

METHODS

Narrative overview of the literatures reported the recent advances, challenges and perspectives regarding the interspecies chimerism via blastocyst complementation.

CONCLUSIONS

Human-animal interspecies chimerism via blastocyst complementation is a valuable method to generate functional human cells, tissues or organs, while there are at least three barriers need to be overcome. Firstly, conventional hPSCs should be converted to possess the chimera competency; secondly, efficient human-animal chimerism are required to robustly generate human derivatives in chimera; thirdly, the discrepancy regarding the developmental regulation network between human and host animals must be eliminated to generate certain human cells, tissues or organs in the interspecies chimeras.

Limited Expansion of Human Hepatocytes in FAH/RAG2-Deficient Swine

BACKGROUND

The mammalian liver's regenerative ability has led researchers to engineer animals as incubators for expansion of human hepatocytes. The expansion properties of human hepatocytes in immunodeficient mice are well known. However, little has been reported about larger animals that are more scalable and practical for clinical purposes. Therefore, we engineered immunodeficient swine to support expansion of human hepatocytes and identify barriers to their clinical application.

METHODS

Immunodeficient swine were engineered by knockout of recombinase activating gene 2 (RAG2) and fumarylacetoacetate hydrolase (FAH). Immature human hepatocytes (ihHCs) were injected into fetal swine by intrauterine cell transplantation (IUCT) at day 40 of gestation. Human albumin was measured as a marker of engraftment. Cytotoxicity against ihHCs was measured in transplanted piglets and control swine.

RESULTS

Higher levels of human albumin were detected in cord blood of newborn FAH/RAG2-deficient (FR) pigs compared to immunocompetent controls (196.26 ng/dL vs 39.29 ng/dL, p = 0.008), indicating successful engraftment of ihHC after IUCT and adaptive immunity in the fetus. Although rare hepatocytes staining positively for human albumin were observed, levels of human albumin did not rise after birth but declined suggesting rejection of xenografted ihHCs. Cytotoxicity against ihHCs increased after birth 3.8% (95% CI: [2.1%, 5.4%], p < 0.001) and correlated inversely to declining levels of human albumin (p = 2.1 x 10-5, R2 = 0.17). Circulating numbers of T-cells and B-cells were negligible in FR pigs. However, circulating natural killer (NK) cells exerted cytotoxicity against ihHCs. NK cell activity was lower in immunodeficient piglets after IUCT than naive controls (30.4% vs 40.1% (p = 0.011, 95% CI for difference [2.7%, 16.7%]).

CONCLUSION

Immature human hepatocytes successfully engrafted in FR swine after IUCT. NK cells were a significant barrier to expansion of hepatocytes. New approaches are needed to overcome this hurdle and allow large scale expansion of human hepatocytes in immunodeficient swine.

The road to generating transplantable organs: from blastocyst complementation to interspecies chimeras

Growing human organs in animals sounds like something from the realm of science fiction, but it may one day become a reality through a technique known as interspecies blastocyst complementation. This technique, which was originally developed to study gene function in development, involves injecting donor pluripotent stem cells into an organogenesis-disabled host embryo, allowing the donor cells to compensate for missing organs or tissues. Although interspecies blastocyst complementation has been achieved between closely related species, such as mice and rats, the situation becomes much more difficult for species that are far apart on the evolutionary tree. This is presumably because of layers of xenogeneic barriers that are a result of divergent evolution. In this Review, we discuss the current status of blastocyst complementation approaches and, in light of recent progress, elaborate on the keys to success for interspecies blastocyst complementation and organ generation.

25th ANNIVERSARY OF CLONING BY SOMATIC-CELL NUCLEAR TRANSFER: Nuclear transfer and the development of genetically modified/gene edited livestock

The birth and adult development of 'Dolly' the sheep, the first mammal produced by the transfer of a terminally differentiated cell nucleus into an egg, provided unequivocal evidence of nuclear equivalence among somatic cells. This ground-breaking experiment challenged a long-standing dogma of irreversible cellular differentiation that prevailed for over a century and enabled the development of methodologies for reversal of differentiation of somatic cells, also known as nuclear reprogramming. Thanks to this new paradigm, novel alternatives for regenerative medicine in humans, improved animal breeding in domestic animals and approaches to species conservation through reproductive methodologies have emerged. Combined with the incorporation of new tools for genetic modification, these novel techniques promise to (i) transform and accelerate our understanding of genetic diseases and the development of targeted therapies through creation of tailored animal models, (ii) provide safe animal cells, tissues and organs for xenotransplantation, (iii) contribute to the preservation of endangered species, and (iv) improve global food security whilst reducing the environmental impact of animal production. This review discusses recent advances that build on the conceptual legacy of nuclear transfer and – when combined with gene editing – will have transformative potential for medicine, biodiversity and sustainable agriculture. We conclude that the potential of these technologies depends on further fundamental and translational research directed at improving the efficiency and safety of these methods.

Conserved features of non-primate bilaminar disc embryos and the germline

Post-implantation embryo development commences with a bilaminar disc in most mammals, including humans. Whereas access to early human embryos is limited and subject to greater ethical scrutiny, studies on non-primate embryos developing as bilaminar discs offer exceptional opportunities for advances in gastrulation, the germline, and the basis for evolutionary divergence applicable to human development. Here, we discuss the advantages of investigations in the pig embryo as an exemplar of development of a bilaminar disc embryo with relevance to early human development. Besides, the pig has the potential for the creation of humanized organs for xenotransplantation. Precise genetic engineering approaches, imaging, and single-cell analysis are cost effective and efficient, enabling research into some outstanding questions on human development and for developing authentic models of early human development with stem cells.

Breakthrough of interspecific chimerism: blocking apoptosis

On January 28 in Nature online, Zheng et al. (Nature, 2021 (Online ahead of print)) reported that they developed an ingenious method of interspecies PSC co-culture system in vitro which unfolded interspecific cell competition. This study paves the way for discovering the mechanism of interspecific chimera and for further interspecific organogenesis between evolutionarily distant species.

Formative pluripotent stem cells show features of epiblast cells poised for gastrulation

The pluripotency of mammalian early and late epiblast could be recapitulated by naïve embryonic stem cells (ESCs) and primed epiblast stem cells (EpiSCs), respectively. However, these two states of pluripotency may not be sufficient to reflect the full complexity and developmental potency of the epiblast during mammalian early development. Here we report the establishment of self-renewing formative pluripotent stem cells (fPSCs) which manifest features of epiblast cells poised for gastrulation. fPSCs can be established from different mouse ESCs, pre-/early-gastrula epiblasts and induced PSCs. Similar to pre-/early-gastrula epiblasts, fPSCs show the transcriptomic features of formative pluripotency, which are distinct from naïve ESCs and primed EpiSCs. fPSCs show the unique epigenetic states of E6.5 epiblast, including the super-bivalency of a large set of developmental genes. Just like epiblast cells immediately before gastrulation, fPSCs can efficiently differentiate into three germ layers and primordial germ cells (PGCs) in vitro. Thus, fPSCs highlight the feasibility of using PSCs to explore the development of mammalian epiblast.

Synthetic embryology: Early mammalian embryo modeling systems from cell cultures

Recently, the fields of embryology, developmental biology, stem cell biology, and cell reprogramming, have intersected with synthetic embryo systems (SESs) from cultured cells. Among such SESs, several approaches have engaged early-embryo-like cells, cells with atypical potency, or assembled traditional in vitro stem cell populations with synergy, to advance life discovery systems that may yield emergent knowledge and biotechnical advance. Such models center on the competent generation of blastocyst-like and post-implantation embryo-like forms. Our group, and several others have recently pioneered unique SES strategies covering a broad spectrum of key early embryo-like developmental stages and features to seed an emerging SES field. Herein, we provide a comprehensive perspective of synthetic embryology and the powerful promise that excites us.

Capture of Mouse and Human Stem Cells with Features of Formative Pluripotency

Pluripotent cells emerge as a naive founder population in the blastocyst, acquire capacity for germline and soma formation, and then undergo lineage priming. Mouse embryonic stem cells (ESCs) and epiblast-derived stem cells (EpiSCs) represent the initial naive and final primed phases of pluripotency, respectively. Here, we investigate the intermediate formative stage. Using minimal exposure to specification cues, we derive stem cells from formative mouse epiblast. Unlike ESCs or EpiSCs, formative stem (FS) cells respond directly to germ cell induction. They colonize somatic tissues and germline in chimeras. Whole-transcriptome analyses show similarity to pre-gastrulation formative epiblast. Signal responsiveness and chromatin accessibility features reflect lineage capacitation. Furthermore, FS cells show distinct transcription factor dependencies, relying critically on Otx2. Finally, FS cell culture conditions applied to human naive cells or embryos support expansion of similar stem cells, consistent with a conserved staging post on the trajectory of mammalian pluripotency.

Embryonic Environmental Niche Reprograms Somatic Cells to Express Pluripotency Markers and Participate in Adult Chimaeras

The phenomenon of the reprogramming of terminally differentiated cells can be achieved by various means, like somatic cell nuclear transfer, cell fusion with a pluripotent cell, or the introduction of pluripotency genes. Here, we present the evidence that somatic cells can attain the expression of pluripotency markers after their introduction into early embryos. Mouse embryonic fibroblasts introduced between blastomeres of cleaving embryos, within two days of in vitro culture, express transcription factors specific to blastocyst lineages, including pluripotency factors. Analysis of donor tissue marker DNA has revealed that the progeny of introduced cells are found in somatic tissues of foetuses and adult chimaeras, providing evidence for cell reprogramming. Analysis of ploidy has shown that in the chimaeras, the progeny of introduced cells are either diploid or tetraploid, the latter indicating cell fusion. The presence of donor DNA in diploid cells from chimaeric embryos proved that the non-fused progeny of introduced fibroblasts persisted in chimaeras, which is evidence of reprogramming by embryonic niche. When adult somatic (cumulus) cells were introduced into early cleavage embryos, the extent of integration was limited and only cell fusion-mediated reprogramming was observed. These results show that both cell fusion and cell interactions with the embryonic niche reprogrammed somatic cells towards pluripotency.

Generation of inner ear sensory neurons using blastocyst complementation in a Neurog1+/--deficient mouse

This research is the first to produce induced pluripotent stem cell-derived inner ear sensory neurons in the Neurog1^+/− heterozygote mouse using blastocyst complementation. Additionally, this approach corrected non-sensory deficits associated with Neurog1 heterozygosity, indicating that complementation is specific to endogenous Neurog1 function. This work validates the use of blastocyst complementation as a tool to create novel insight into the function of developmental genes and highlights blastocyst complementation as a potential platform for generating chimeric inner ear cell types that can be transplanted into damaged inner ears to improve hearing.

Chicken Interspecies Chimerism Unveils Human Pluripotency

Human pluripotent stem cells (hPSCs) are commonly kept in a primed state but also able to acquire a more immature naive state under specific conditions in vitro. Acquisition of naive state changes several properties of hPSCs and might affect their contribution to embryonic development in vivo. However, the lack of an appropriate animal test system has made it difficult to assess potential differences for chimera formation between naive and primed hPSCs. Here, we report that the developing chicken embryo is a permissive host for hPSCs, allowing analysis of the pluripotency potential of hPSCs. Transplantation of naive-like and primed hPSCs at matched developmental stages resulted in robust chimerism. Importantly, the ability of naive-like but not of primed hPSCs to form chimera was substantially reduced when injected at non-matched developmental stages. We propose that contribution to chick embryogenesis is an informative and versatile test to identify different pluripotent states of hPSCs.

Apoptosis, G1 Phase Stall, and Premature Differentiation Account for Low Chimeric Competence of Human and Rhesus Monkey Naive Pluripotent Stem Cells

After reprogramming to naive pluripotency, human pluripotent stem cells (PSCs) still exhibit very low ability to make interspecies chimeras. Whether this is because they are inherently devoid of the attributes of chimeric competency or because naive PSCs cannot colonize embryos from distant species remains to be elucidated. Here, we have used different types of mouse, human, and rhesus monkey naive PSCs and analyzed their ability to colonize rabbit and cynomolgus monkey embryos. Mouse embryonic stem cells (ESCs) remained mitotically active and efficiently colonized host embryos. In contrast, primate naive PSCs colonized host embryos with much lower efficiency. Unlike mouse ESCs, they slowed DNA replication after dissociation and, after injection into host embryos, they stalled in the G1 phase and differentiated prematurely, regardless of host species. We conclude that human and non-human primate naive PSCs do not efficiently make chimeras because they are inherently unfit to remain mitotically active during colonization.

•

Mouse ESCs are highly effective in colonizing rabbit and non-human primate embryos

•

Rhesus monkey and human naive PSCs ineffectively colonize rabbit and monkey embryos

•

Most rhesus/human naive PSCs differentiate prematurely upon injection into embryos

•

Rhesus monkey PSCs stall in the G1 phase after transfer into rabbit embryos