Interspecies chimeras for human stem cell research

Debates on humanization of human-animal brain chimeras - are we putting the cart before the horses?

Research on human-animal chimeras have elicited alarms and prompted debates. Those involving the generation of chimeric brains, in which human brain cells become anatomically and functionally intertwined with their animal counterparts in varying ratios, either via xenografts or embryonic co-development, have been considered the most problematic. The moral issues stem from a potential for "humanization" of the animal brain, as well as speculative changes to the host animals' consciousness or sentience, with consequential alteration in the animal hosts' moral status. However, critical background knowledge appears to be missing to resolve these debates. Firstly, there is no consensus on animal sentience vis-à-vis that of humans, and no established methodology that would allow a wholesome and objective assessment of changes in animal sentience resulting from the introduction of human brain cells. Knowledge in interspecies comparative neuropsychology that could allow effective demarcation of a state of "humanization" is also lacking. Secondly, moral status as a philosophical construct has no scientific and objective points of reference. Either changes in sentience or humanization effects would remain unclear unless there are some neuroscientific research grounding. For a bioethical stance based on moral status of human-animal brain chimera to make meaningful contributions to regulatory policies, it might first need to be adequately informed by, and with its arguments constructed, in a manner that are factually in line with the science. In may be prudent for approved research projects involving the generation of human-animal brain chimera to have a mandatory component of assessing plausible changes in sentience.

Establishment of porcine embryonic stem cells in simplified serum free media and feeder free expansion

BACKGROUND

The establishment of stable porcine embryonic stem cells (pESCs) can contribute to basic and biomedical research, including comparative developmental biology, as well as assessing the safety of stem cell-based therapies. Despite these advantages, most pESCs obtained from in vitro blastocysts require complex media and feeder layers, making routine use, genetic modification, and differentiation into specific cell types difficult. We aimed to establish pESCs with a single cell-passage ability, high proliferative potency, and stable in long-term culture from in vitro-derived blastocysts using a simplified serum-free medium.

METHODS

We evaluated the establishment efficiency of pESCs from in vitro blastocysts using various basal media (DMEM/F10 (1:1), DMEM/F12, and a-MEM) and factors (FGF2, IWR-1, CHIR99021, and WH-4-023). The pluripotency and self-renewal capacity of the established pESCs were analyzed under feeder or feeder-free conditions. Ultimately, we developed a simplified culture medium (FIW) composed of FGF2, IWR-1, and WH-4-023 under serum-free conditions.

RESULTS

The pESC-FIW lines were capable of single-cell passaging with short cell doubling times and expressed the pluripotency markers POU5F1, SOX2, and NANOG, as well as cell surface markers SSEA1, SSEA4, and TRA-1-60. pESC-FIW showed a stable proliferation rate and normal karyotype, even after 50 passages. Transcriptome analysis revealed that pESC-FIW were similar to reported pESC maintained in complex media and showed gastrulating epiblast cell characteristics. pESC-FIW were maintained for multiple passages under feeder-free conditions on fibronectin-coated plates using mTeSR™, a commercial medium used for feeder-free culture, exhibiting characteristics similar to those observed under feeder conditions.

CONCLUSIONS

These results indicated that inhibition of WNT and SRC was sufficient to establish pESCs capable of single-cell passaging and feeder-free expansion under serum-free conditions. The easy maintenance of pESCs facilitates their application in gene editing technology for agriculture and biomedicine, as well as lineage commitment studies.

Disentangling cell-intrinsic and extrinsic factors underlying evolution

A key goal of developmental biology is to determine the extent to which cells and organs develop autonomously, as opposed to requiring interactions with other cells or environmental factors. Chimeras have played a foundational role in this by enabling qualitative classification of cell-intrinsically vs. extrinsically driven processes. Here, we extend this framework to precisely decompose evolutionary divergence in any quantitative trait into cell-intrinsic, extrinsic, and intrinsic-extrinsic interaction components. Applying this framework to thousands of gene expression levels in reciprocal rat-mouse chimeras, we found that the majority of their divergence is attributable to cell-intrinsic factors, though extrinsic factors also play an integral role. For example, a rat-like extracellular environment extrinsically up-regulates the expression of a key transcriptional regulator of the endoplasmic reticulum (ER) stress response in some but not all cell types, which in turn strongly predicts extrinsic up-regulation of its target genes and of the ER stress response pathway as a whole. This effect is also seen at the protein level, suggesting propagation through multiple regulatory levels. Applying our framework to a cellular trait, neuronal differentiation, revealed a complex interaction of intrinsic and extrinsic factors. Finally, we show that imprinted genes are dramatically mis-expressed in species-mismatched environments, suggesting that mismatch between rapidly evolving intrinsic and extrinsic mechanisms controlling gene imprinting may contribute to barriers to interspecies chimerism. Overall, our conceptual framework opens new avenues to investigate the mechanistic basis of developmental processes and evolutionary divergence across myriad quantitative traits in any multicellular organism.

Interspecies chimerism with human embryonic stem cells generates functional human dopamine neurons at low efficiency

Interspecies chimeras offer great potential for regenerative medicine and the creation of human disease models. Whether human pluripotent stem cell-derived neurons in an interspecies chimera can differentiate into functional neurons and integrate into host neural circuity is not known. Here, we show, using Engrailed 1 (En1) as a development niche, that human naive-like embryonic stem cells (ESCs) can incorporate into embryonic and adult mouse brains. Human-derived neurons including tyrosine hydroxylase (TH)+ neurons integrate into the mouse brain at low efficiency. These TH+ neurons have electrophysiologic properties consistent with their human origin. In addition, these human-derived neurons in the mouse brain accumulate pathologic phosphorylated α-synuclein in response to α-synuclein preformed fibrils. Optimization of human/mouse chimeras could be used to study human neuronal differentiation and human brain disorders.

RNA Sensing and Innate Immunity Constitutes a Barrier for Interspecies Chimerism

Interspecies chimera formation with human pluripotent stem cells (PSCs) holds great promise to generate humanized animal models and provide donor organs for transplant. However, the approach is currently limited by low levels of human cells ultimately represented in chimeric embryos. Different strategies have been developed to improve chimerism by genetically editing donor human PSCs. To date, however, it remains unexplored if human chimerism can be enhanced in animals through modifying the host embryos. Leveraging the interspecies PSC competition model, here we discovered retinoic acid-inducible gene I (RIG-I)-like receptor (RLR) signaling, an RNA sensor, in "winner" cells plays an important role in the competitive interactions between co-cultured mouse and human PSCs. We found that genetic inactivation of Ddx58/Ifih1-Mavs-Irf7 axis compromised the "winner" status of mouse PSCs and their ability to outcompete PSCs from evolutionarily distant species during co-culture. Furthermore, by using Mavs-deficient mouse embryos we substantially improved unmodified donor human cell survival. Comparative transcriptome analyses based on species-specific sequences suggest contact-dependent human-to-mouse transfer of RNAs likely plays a part in mediating the cross-species interactions. Taken together, these findings establish a previously unrecognized role of RNA sensing and innate immunity in "winner" cells during cell competition and provides a proof-of-concept for modifying host embryos, rather than donor PSCs, to enhance interspecies chimerism.

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.

Generation of chimeric kidneys using progenitor cell replacement: Oshima Award Address 2021

It is believed that the development of new renal replacement therapy (RRT) will increase treatment options for end-stage kidney disease and help reduce the mismatch between supply and demand. Technological advancement in the development of kidney organoids derived from pluripotent stem cells and xenotransplantation using porcine kidneys has been accelerated by a convergence of technological innovations, including the discovery of induced pluripotent stem cells and genome editing, and improvement of analysis techniques such as single-cell ribonucleic acid sequencing. Given the difficulty associated with kidney regeneration, hybrid kidneys are studied as an innovative approach that involves the use of stem cells to generate kidneys, with animal fetal kidneys used as a scaffold. Hybrid kidney technology entails the application of local chimerism for the generation of chimeric kidneys from exogenous renal progenitor cells by borrowing complex nephrogenesis programs from the developmental environment of heterologous animals. Hybrid kidneys can also utilize the urinary tract and bladder tissue of animal fetuses for urine excretion. Generating nephrons from syngeneic stem cells to increase self-cell ratio in xeno-tissues can reduce the risk of xeno-rejection. We showed that nephrons can be generated by ablation of host nephron progenitor cells (NPCs) in the nephron development region of animals and replacing them with exogenous NPCs. This progenitor cell replacement is the basis of hybrid kidney regeneration from progenitor cells using chimera technology. The goal of xeno-regenerative medicine using hybrid kidneys is to overcome serious organ shortage.

In vivo generation of bone marrow from embryonic stem cells in interspecies chimeras

Generation of bone marrow (BM) from embryonic stem cells (ESCs) promises to accelerate the development of future cell therapies for life-threatening disorders. However, such approach is limited by technical challenges to produce a mixture of functional BM progenitor cells able to replace all hematopoietic cell lineages. Herein, we used blastocyst complementation to simultaneously produce BM cell lineages from mouse ESCs in a rat. Based on fluorescence-activated cell sorting analysis and single-cell RNA sequencing, mouse ESCs differentiated into multiple hematopoietic and stromal cell types that were indistinguishable from normal mouse BM cells based on gene expression signatures and cell surface markers. Receptor-ligand interactions identified Cxcl12-Cxcr4, Lama2-Itga6, App-Itga6, Comp-Cd47, Col1a1-Cd44, and App-Il18rap as major signaling pathways between hematopoietic progenitors and stromal cells. Multiple hematopoietic progenitors, including hematopoietic stem cells (HSCs) in mouse-rat chimeras derived more efficiently from mouse ESCs, whereas chondrocytes predominantly derived from rat cells. In the dorsal aorta and fetal liver of mouse-rat chimeras, mouse HSCs emerged and expanded faster compared to endogenous rat cells. Sequential BM transplantation of ESC-derived cells from mouse-rat chimeras rescued lethally irradiated syngeneic mice and demonstrated long-term reconstitution potential of donor HSCs. Altogether, a fully functional BM was generated from mouse ESCs using rat embryos as 'bioreactors'.

Pluripotent stem cells related to embryonic disc exhibit common self-renewal requirements in diverse livestock species

Despite four decades of effort, robust propagation of pluripotent stem cells from livestock animals remains challenging. The requirements for self-renewal are unclear and the relationship of cultured stem cells to pluripotent cells resident in the embryo uncertain. Here, we avoided using feeder cells or serum factors to provide a defined culture microenvironment. We show that the combination of activin A, fibroblast growth factor and the Wnt inhibitor XAV939 (AFX) supports establishment and continuous expansion of pluripotent stem cell lines from porcine, ovine and bovine embryos. Germ layer differentiation was evident in teratomas and readily induced in vitro. Global transcriptome analyses highlighted commonality in transcription factor expression across the three species, while global comparison with porcine embryo stages showed proximity to bilaminar disc epiblast. Clonal genetic manipulation and gene targeting were exemplified in porcine stem cells. We further demonstrated that genetically modified AFX stem cells gave rise to cloned porcine foetuses by nuclear transfer. In summary, for major livestock mammals, pluripotent stem cells related to the formative embryonic disc are reliably established using a common and defined signalling environment.

This article has an associated ‘The people behind the papers’ interview.

Summary: We report the derivation of similar, stable and continuously expandable pluripotent stem cells related to embryonic disc epiblast from embryos of pig, sheep and cow.

[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.

All models are wrong, but some are useful: Establishing standards for stem cell-based embryo models

Detailed studies of the embryo allow an increasingly mechanistic understanding of development, which has proved of profound relevance to human disease. The last decade has seen in vitro cultured stem cell-based models of embryo development flourish, which provide an alternative to the embryo for accessible experimentation. However, the usefulness of any stem cell-based embryo model will be determined by how accurately it reflects in vivo embryonic development, and/or the extent to which it facilitates new discoveries. Stringent benchmarking of embryo models is thus an important consideration for this growing field. Here we provide an overview of means to evaluate both the properties of stem cells, the building blocks of most embryo models, as well as the usefulness of current and future in vitro embryo models.

Heterocellular spheroids of the neurovascular blood-brain barrier as a platform for personalized nanoneuromedicine

Nanoneuromedicine investigates nanotechnology to target the brain and treat neurological diseases. In this work, we biofabricated heterocellular spheroids comprising human brain microvascular endothelial cells, brain vascular pericytes and astrocytes combined with primary cortical neurons and microglia isolated from neonate rats. The structure and function are characterized by confocal laser scanning and light sheet fluorescence microscopy, electron microscopy, western blotting, and RNA sequencing. The spheroid bulk is formed by neural cells and microglia and the surface by endothelial cells and they upregulate key structural and functional proteins of the blood-brain barrier. These cellular constructs are utilized to preliminary screen the permeability of polymeric, metallic, and ceramic nanoparticles (NPs). Findings reveal that penetration and distribution patterns depend on the NP type and that microglia would play a key role in this pathway, highlighting the promise of this platform to investigate the interaction of different nanomaterials with the central nervous system in nanomedicine, nanosafety and nanotoxicology.

In Vivo Generation of Lung and Thyroid Tissues from Embryonic Stem Cells Using Blastocyst Complementation

Rationale: The regeneration and replacement of lung cells or tissues from induced pluripotent stem cell- or embryonic stem cell-derived cells represent future therapies for life-threatening pulmonary disorders but are limited by technical challenges to produce highly differentiated cells able to maintain lung function. Functional lung tissue-containing airways, alveoli, vasculature, and stroma have never been produced via directed differentiation of embryonic stem cells (ESCs) or induced pluripotent stem cells. We sought to produce all tissue components of the lung from bronchi to alveoli by embryo complementation.Objectives: To determine whether ESCs are capable of generating lung tissue in Nkx2-1^-/- mouse embryos with lung agenesis.Methods: Blastocyst complementation was used to produce chimeras from normal mouse ESCs and Nkx2-1^-/- embryos, which lack pulmonary tissues. Nkx2-1^-/- chimeras were examined using immunostaining, transmission electronic microscopy, fluorescence-activated cell sorter analysis, and single-cell RNA sequencing.Measurements and Main Results: Although peripheral pulmonary and thyroid tissues are entirely lacking in Nkx2-1 gene-deleted embryos, pulmonary and thyroid structures in Nkx2-1^-/- chimeras were restored after ESC complementation. Respiratory epithelial cell lineages in restored lungs of Nkx2-1^-/- chimeras were derived almost entirely from ESCs, whereas endothelial, immune, and stromal cells were mosaic. ESC-derived cells from multiple respiratory cell lineages were highly differentiated and indistinguishable from endogenous cells based on morphology, ultrastructure, gene expression signatures, and cell surface proteins used to identify cell types by fluorescence-activated cell sorter.Conclusions: Lung and thyroid tissues were generated in vivo from ESCs by blastocyst complementation. Nkx2-1^-/- chimeras can be used as "bioreactors" for in vivo differentiation and functional studies of ESC-derived progenitor cells.

Humanization of Immunodeficient Animals for the Modeling of Transplantation, Graft Versus Host Disease, and Regenerative Medicine

The humanization of animals is a powerful tool for the exploration of human disease pathogenesis in biomedical research, as well as for the development of therapeutic interventions with enhanced translational potential. Humanized models enable us to overcome biologic differences that exist between humans and other species, while giving us a platform to study human processes in vivo. To become humanized, an immune-deficient recipient is engrafted with cells, tissues, or organoids. The mouse is the most well studied of these hosts, with a variety of immunodeficient strains available for various specific uses. More recently, efforts have turned to the humanization of other animal species such as the rat, which offers some technical and immunologic advantages over mice. These advances, together with ongoing developments in the incorporation of human transgenes and additional mutations in humanized mouse models, have expanded our opportunities to replicate aspects of human allotransplantation and to assist in the development of immunotherapies. In this review, the immune and tissue humanization of various species is presented with an emphasis on their potential for use as models for allotransplantation, graft versus host disease, and regenerative medicine.

Design Approaches for Generating Organ Constructs

Organ constructs are organ-like structures grown in vitro or in vivo that harbor the components, architecture, and function of in vivo organs, in part or in toto. The convergence of stem cell biology, bioengineering, and gene editing tools have substantially broadened our ability to generate various types of organ constructs for regenerative medicine as well as to address pressing biomedical questions. In this Review, we highlight prevailing approaches for generating organ constructs, from organoids to chimeric organ engineering. We also discuss design principles of different approaches, their utility and limitations, and propose strategies to resolve existing hurdles.

Ethical arguments concerning human-animal chimera research: a systematic review

Background

The burgeoning field of biomedical research involving the mixture of human and animal materials has attracted significant ethical controversy. Due to the many dimensions of potential ethical conflict involved in this type of research, and the wide variety of research projects under discussion, it is difficult to obtain an overview of the ethical debate. This paper attempts to remedy this by providing a systematic review of ethical reasons in academic publications on human-animal chimera research.

Methods

We conducted a systematic review of the ethical literature concerning human-animal chimeras based on the research question: “What ethical reasons have been given for or against conducting human-animal chimera research, and how have these reasons been treated in the ongoing debate?” Our search extends until the end of the year 2017, including MEDLINE, Embase, PhilPapers and EthxWeb databases, restricted to peer-reviewed journal publications in English. Papers containing ethical reasons were analyzed, and the reasons were coded according to whether they were endorsed, mentioned or rejected.

Results

Four hundred thirty-one articles were retrieved by our search, and 88 were ultimately included and analyzed. Within these articles, we found 464 passages containing reasons for and against conducting human-animal chimera research. We classified these reasons into five categories and, within these, identified 12 broad and 31 narrow reason types.

15% of the retrieved passages contained reasons in favor of conducting chimera research (Category P), while 85% of the passages contained reasons against it. The reasons against conducting chimera research fell into four further categories: reasons concerning the creation of a chimera (Category A), its treatment (Category B), reasons referring to metaphysical or social issues resulting from its existence (Category C) and to potential downstream effects of chimera research (Category D). A significant proportion of identified passages (46%) fell under Category C.

Conclusions

We hope that our results, in revealing the conceptual and argumentative structure of the debate and highlighting some its most notable tendencies and prominent positions, will facilitate continued discussion and provide a basis for the development of relevant policy and legislation.

[Fifty shades of pluripotency]

Since the derivation of the first pluripotent embryonic stem cell lines in mice in the early 1980s, a plethora of lines has been obtained from various mammalian species including rodents, lagomorphs and primates. These lines are distinguished by their molecular and functional characteristics and correspond to the different pluripotency states observed in the developing embryo between the "blastocyst" and "gastrula" stages. These cell lines are positioned along a gradient, or continuum of pluripotency, the ends of which are epitomized by the naïve and primed states, respectively. Conventional human pluripotent stem cells self-renew in the primed state of pluripotency (ie, at the bottom of the gradient), a position that is undoubtedly the cause of their natural instability. Recent studies aim to generate naive human pluripotent stem cells (at the top of the gradient). The importance of this research in the perspective of medical applications will be discussed.

Matched Developmental Timing of Donor Cells with the Host Is Crucial for Chimera Formation

Chimeric mice have been generated by injecting pluripotent stem cells into morula-to-blastocyst stage mouse embryo or by introducing more mature cells into later stage embryos that correspond to the differentiation stage of the donor cells. It has not been rigorously tested, however, whether successful chimera formation requires the developmental stage of host embryo and donor cell to be matched. Here, we compared the success of chimera formation following injection of primary neural crest cells (NCCs) into blastocysts or of embryonic stem cells (ESCs) into E8.5 embryos (heterochronic injection) with that of injecting ESCs cells into the blastocyst or NCCs into the E8.5 embryos (isochronic injection). Chimera formation was efficient when donor and host were matched, but no functional chimeric contribution was found in heterochronic injections. This suggests that matching the developmental stage of donor cells with the host embryo is crucial for functional engraftment of donor cells into the developing embryo.