Extraembryonic Endoderm (XEN) Cells Capable of Contributing to Embryonic Chimeras Established from Pig Embryos

Establishment of bovine extraembryonic endoderm stem cells enables efficient blastoid formation

Extraembryonic endoderm stem cells (XENs) resemble the primitive endoderm of blastocyst and provide valuable alternatives for understanding hypoblast development and building stem cell-based embryo models. Here, we report that a chemical cocktail (FGF4, BMP4, IL-6, XAV939, and A83-01) enables the de novo derivation and long-term culture of bovine XENs (bXENs) from blastocysts. Transcriptomic and epigenomic analyses confirm the identity of bXENs and reveal that bXENs resemble the hypoblast lineages of early bovine peri-implantation embryos. Furthermore, we show that bXENs maintain the stemness of expanded embryonic stem cells (EPSCs) and prevent them from undergoing differentiation. The chemical cocktail sustaining bXENs also facilitates the growth of epiblasts in developing pre-implantation embryos. Finally, 3D assembly of bXENs with bovine EPSCs and trophoblast stem cells enables efficient generation of blastoids that resemble blastocysts. bXENs and blastoids established here represent accessible in vitro models for understanding embryogenesis and improving reproductive efficiency in domestic livestock.

Wnt Inhibition Safeguards Porcine Embryonic Stem Cells From the Acquisition of Extraembryonic Endoderm Cell Fates

Porcine embryonic stem cells (ESCs) are excellent models for exploring embryogenesis, producing genetically enhanced farm animals, and improving breeding. Various chemicals have been applied to generate porcine ESCs from embryos, which differ from mouse and human ESC derivation. Wnt inhibitors XAV939 or IWR1 are required to isolate and maintain porcine ESCs. How Wnt inhibitors specify porcine ESC fate decisions remains poorly understood. Additionally, whether porcine ESCs can be converted to extraembryonic endoderm (XEN) cells without genetic interventions has not been reported. Here, it is reported that Wnt inhibitors (i.e., XAV939 and IWR1) safeguard porcine ESCs from acquiring the XEN lineage. Porcine ESCs rely on Wnt inhibitors to maintain pluripotency. Without them, porcine ESCs exit from pluripotency and convert to XEN cells. An efficient strategy and culture conditions are further developed to directly derive porcine XEN cells from ESCs without gene editing. The resulting XEN cells from ESCs exhibit similar transcriptome and chromatin accessibility features to XEN cells from embryos and contribute to mouse extraembryonic tissues. This study will deepen the understanding of porcine pluripotency, lay the foundation for deriving high-quality porcine ESCs with germline chimerism and transmission, and provide valuable materials to study extraembryonic development and lineage segregation in livestock.

Establishment of bovine extraembryonic endoderm cells

Understanding the mechanisms of hypoblast development and its role in the implantation is critical for improving farm animal reproduction, but it is hampered by the lack of research models. Here we report that a chemical cocktail (FGF4, BMP4, IL-6, XAV939, and A83-01) enables de novo derivation and long-term culture of bovine extraembryonic endoderm cells (bXENs). Transcriptomic and epigenomic analyses confirmed the identity of bXENs and revealed that they are resemble hypoblast lineages of early bovine peri-implantation embryos. We showed that bXENs help maintain the stemness of bovine ESCs and prevent them from differentiation. In the presence of a signaling cocktail sustaining bXENs, the growth and progression of epiblasts are also facilitated in the developing pre-implantation embryo. Furthermore, through 3D assembly of bXENs with bovine ESCs and TSCs, we developed an improved bovine blastocyst like structure (bovine blastoid) that resembles blastocyst. The bovine XENs and blastoids established in this study represent accessible in vitro models for understanding hypoblast development and improving reproductive efficiency in livestock species.

Current Status of Synthetic Mammalian Embryo Models

Advances in three-dimensional culture technologies have facilitated the development of synthetic embryo models, such as blastoids, through the co-culturing of diverse stem cell types. These in vitro models enable precise investigation of developmental processes, including gastrulation, neurulation, and lineage specification, thereby advancing our understanding of early embryogenesis. By providing controllable, ethically viable platforms, they help circumvent the limitations of in vivo mammalian embryo studies and contribute to developing regenerative medicine strategies. Nonetheless, ethical challenges, particularly regarding human applications, persist. Comparative studies across various species-such as mice, humans, non-human primates, and ungulates, like pigs and cattle-offer crucial insights into both species-specific and conserved developmental mechanisms. In this review, we outline the species-specific differences in embryonic development and discuss recent advancements in stem cell and synthetic embryo models. Specifically, we focus on the latest stem cell research involving ungulates, such as pigs and cattle, and provide a comprehensive overview of the improvements in synthetic embryo technology. These insights contribute to our understanding of species-specific developmental biology, help improve model efficiency, and guide the development of new models.

Pigs lacking TMPRSS2 displayed fewer lung lesions and reduced inflammatory response when infected with influenza A virus

Influenza A virus (IAV) infection is initiated by hemagglutinin (HA), a glycoprotein exposed on the virion's lipid envelope that undergoes cleavage by host cell proteases to ensure membrane fusion, entry into the host cells, and completion of the viral cycle. Transmembrane protease serine S1 member 2 (TMPRSS2) is a host transmembrane protease expressed throughout the porcine airway epithelium and is purported to play a major role in the HA cleavage process, thereby influencing viral pathogenicity and tissue tropism. Pigs are natural hosts of IAV and IAV disease causes substantial economic impact on the pork industry worldwide. Previous studies in mice demonstrated that knocking out expression of TMPRSS2 gene was safe and inhibited the spread of IAV after experimental challenge. Therefore, we hypothesized that knockout of TMPRSS2 will prevent IAV infectivity in the swine model. We investigated this hypothesis by comparing pathogenesis of an H1N1pdm09 virus challenge in wildtype (WT) control and in TMPRSS2 knockout (TMPRSS2 -/-) pigs. We demonstrated that TMPRSS2 was expressed in the respiratory tract in WT pigs with and without IAV infection. No differences in nasal viral shedding and lung lavage viral titers were observed between WT and TMPRSS2 -/- pigs. However, the TMPRSS2 -/- pig group had significantly less lung lesions and significant reductions in antiviral and proinflammatory cytokines in the lung. The virus titer results in our direct challenge model contradict prior studies in the murine animal model, but the reduced lung lesions and cytokine profile suggest a possible role for TMPRSS2 in the proinflammatory antiviral response. Further research is warranted to investigate the role of TMPRSS2 in swine IAV infection and disease.

- Invited Review - Reproductive technologies needed for the generation of precise gene-edited pigs in the pathways from laboratory to farm

Gene editing (GE) offers a new breeding technique (NBT) of sustainable value to animal agriculture. There are 3 GE working sites covering 5 feasible pathways to generate GE pigs along with the crucial intervals of GE/genotyping, microinjection/electroporation, induced pluripotent stem cells, somatic cell nuclear transfer, cryopreservation, and nonsurgical embryo transfer. The extension of NBT in the new era of pig breeding depends on the synergistic effect of GE and reproductive biotechnologies; the outcome relies not only on scientific due diligence and operational excellence but also on the feasibility of application on farms to improve sustainability.

Establishment, characterization, and validation of novel porcine embryonic fibroblasts as a potential source for genetic modification

Fibroblasts are the common cell type in the connective tissue-the most abundant tissue type in the body. Fibroblasts are widely used for cell culture, for the generation of induced pluripotent stem cells (iPSCs), and as nuclear donors for somatic cell nuclear transfer (SCNT). We report for the first time, the derivation of embryonic fibroblasts (EFs) from porcine embryonic outgrowths, which share similarities in morphology, culture characteristics, molecular markers, and transcriptional profile to fetal fibroblasts (FFs). We demonstrated the efficient use of EFs as nuclear donors in SCNT, for enhanced post-blastocyst development, implantation, and pregnancy outcomes. We further validated EFs as a source for CRISPR/Cas genome editing with overall editing frequencies comparable to that of FFs. Taken together, we established an alternative and efficient pipeline for genome editing and for the generation of genetically engineered animals.

New Animal Models for Understanding FMRP Functions and FXS Pathology

Fragile X encompasses a range of genetic conditions, all of which result as a function of changes within the FMR1 gene and abnormal production and/or expression of the FMR1 gene products. Individuals with Fragile X syndrome (FXS), the most common heritable form of intellectual disability, have a full-mutation sequence (>200 CGG repeats) which brings about transcriptional silencing of FMR1 and loss of FMR protein (FMRP). Despite considerable progress in our understanding of FXS, safe, effective, and reliable treatments that either prevent or reduce the severity of the FXS phenotype have not been approved. While current FXS animal models contribute their own unique understanding to the molecular, cellular, physiological, and behavioral deficits associated with FXS, no single animal model is able to fully recreate the FXS phenotype. This review will describe the status and rationale in the development, validation, and utility of three emerging animal model systems for FXS, namely the nonhuman primate (NHP), Mongolian gerbil, and chicken. These developing animal models will provide a sophisticated resource in which the deficits in complex functions of perception, action, and cognition in the human disorder are accurately reflected and aid in the successful translation of novel therapeutics and interventions to the clinic setting.

Derivation of Porcine Extra-Embryonic Endoderm Cell Lines Reveals Distinct Signaling Pathway and Multipotency States

The inner cell mass of the pre-implantation blastocyst consists of the epiblast and hypoblast from which embryonic stem cells (ESCs) and extra-embryonic endoderm (XEN) stem cells, respectively, can be derived. Importantly, each stem cell type retains the defining properties and lineage restriction of its in vivo tissue origin. We have developed a novel approach for deriving porcine XEN (pXEN) cells via culturing the blastocysts with a chemical cocktail culture system. The pXEN cells were positive for XEN markers, including Gata4, Gata6, Sox17, and Sall4, but not for pluripotent markers Oct4, Sox2, and Nanog. The pXEN cells also retained the ability to undergo visceral endoderm (VE) and parietal endoderm (PE) differentiation in vitro. The maintenance of pXEN required FGF/MEK+TGFβ signaling pathways. The pXEN cells showed a stable phenotype through more than 50 passages in culture and could be established repeatedly from blastocysts or converted from the naïve-like ESCs established in our lab. These cells provide a new tool for exploring the pathways of porcine embryo development and differentiation and providing further reference to the establishment of porcine ESCs with potency of germline chimerism and gamete development.

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.

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.