Activin/Nodal and FGF pathways cooperate to maintain pluripotency of human embryonic stem cells

Developments in cell culture systems for human pluripotent stem cells

Human pluripotent stem cells (hPSCs) are important resources for cell-based therapies and pharmaceutical applications. In order to realize the potential of hPSCs, it is critical to develop suitable technologies required for specific applications. Most hPSC technologies depend on cell culture, and are critically influenced by culture medium composition, extracellular matrices, handling methods, and culture platforms. This review summarizes the major technological advances in hPSC culture, and highlights the opportunities and challenges in future therapeutic applications.

TGFβ Family Signaling Pathways in Pluripotent and Teratocarcinoma Stem Cells' Fate Decisions: Balancing Between Self-Renewal, Differentiation, and Cancer

The transforming growth factor-β (TGFβ) family factors induce pleiotropic effects and are involved in the regulation of most normal and pathological cellular processes. The activity of different branches of the TGFβ family signaling pathways and their interplay with other signaling pathways govern the fine regulation of the self-renewal, differentiation onset and specialization of pluripotent stem cells in various cell derivatives. TGFβ family signaling pathways play a pivotal role in balancing basic cellular processes in pluripotent stem cells and their derivatives, although disturbances in their genome integrity induce the rearrangements of signaling pathways and lead to functional impairments and malignant transformation into cancer stem cells. Therefore, the identification of critical nodes and targets in the regulatory cascades of TGFβ family factors and other signaling pathways, and analysis of the rearrangements of the signal regulatory network during stem cell state transitions and interconversions, are key issues for understanding the fundamental mechanisms of both stem cell biology and cancer initiation and progression, as well as for clinical applications. This review summarizes recent advances in our understanding of TGFβ family functions in naїve and primed pluripotent stem cells and discusses how these pathways are involved in perturbations in the signaling network of malignant teratocarcinoma stem cells with impaired differentiation potential.

Reprogrammed Cells Display Distinct Proteomic Signatures Associated with Colony Morphology Variability

Human induced pluripotent stem cells (hiPSCs) are of high interest because they can be differentiated into a vast range of different cell types. Ideally, reprogrammed cells should sustain long-term culturing in an undifferentiated state. However, some reprogrammed cell lines represent an unstable state by spontaneously differentiating and changing their cellular phenotype and colony morphology. This phenomenon is not fully understood, and no method is available to predict it reliably. In this study, we analyzed and compared the proteome landscape of 20 reprogrammed cell lines classified as stable and unstable based on long-term colony morphology. We identified distinct proteomic signatures associated with stable colony morphology and with unstable colony morphology, although the typical pluripotency markers (POU5F1, SOX2) were present with both morphologies. Notably, epithelial to mesenchymal transition (EMT) protein markers were associated with unstable colony morphology, and the transforming growth factor beta (TGFB) signalling pathway was predicted as one of the main regulator pathways involved in this process. Furthermore, we identified specific proteins that separated the stable from the unstable state. Finally, we assessed both spontaneous embryonic body (EB) formation and directed differentiation and showed that reprogrammed lines with an unstable colony morphology had reduced differentiation capacity. To conclude, we found that different defined patterns of colony morphology in reprogrammed cells were associated with distinct proteomic profiles and different outcomes in differentiation capacity.

Regulation of Cell Fate Decisions in Early Mammalian Embryos

Early embryogenesis is characterized by the segregation of cell lineages that fulfill critical roles in the establishment of pregnancy and development of the fetus. The formation of the blastocyst marks the emergence of extraembryonic precursors, needed for implantation, and of pluripotent cells, which differentiate toward the major lineages of the adult organism. The coordinated emergence of these cell types shows that these processes are broadly conserved in mammals. However, developmental heterochrony and changes in gene regulatory networks highlight unique evolutionary adaptations that may explain the diversity in placentation and in the mechanisms controlling pluripotency in mammals. The incorporation of new technologies, including single-cell omics, imaging, and gene editing, is instrumental for comparative embryology. Broadening the knowledge of mammalian embryology will provide new insights into the mechanisms driving evolution and development. This knowledge can be readily translated into biomedical and biotechnological applications in humans and livestock, respectively.

CDK11 safeguards the identity of human embryonic stem cells via fine-tuning signaling pathways

Signaling pathways transmit extracellular cues into cells and regulate transcriptome and epigenome to maintain or change the cell identity. Protein kinases and phosphatases are critical for signaling transduction and regulation. Here, we report that CDK11, a member of the CDK family, is required for the maintenance of human embryonic stem cell (hESC) self-renewal. Our results show that, among the three main isoforms of CDK11, CDK11^p46 is the main isoform safeguarding the hESC identity. Mechanistically, CDK11 constrains two important mitogen-activated protein kinase (MAPK) signaling pathways (JNK and p38 signaling) through modulating the activity of protein phosphatase 1. Furthermore, CDK11 knockdown activates transforming growth factor β (TGF-β)/SMAD2/3 signaling and upregulates certain nonneural differentiation-associated genes. Taken together, this study uncovers a kinase required for hESC self-renewal through fine-tuning MAPK and TGF-β signaling at appropriate levels. The kinase-phosphatase axis reported here may shed new light on the molecular mechanism sustaining the identity of hESCs.

Interplay between Metabolites and the Epigenome in Regulating Embryonic and Adult Stem Cell Potency and Maintenance

The environment surrounding stem cells has the ability to elicit profound, heritable epigenetic changes orchestrated by multiple epigenetic mechanisms, which can be modulated by the level of specific metabolites. In this review, we highlight the significance of metabolism in regulating stem cell homeostasis, cell state, and differentiation capacity, using metabolic regulation of embryonic and adult muscle stem cells as examples, and cast light on the interaction between cellular metabolism and epigenetics. These new regulatory networks, based on the dynamic interplay between metabolism and epigenetics in stem cell biology, are important, not only for understanding tissue homeostasis, but to determine in vitro culture conditions which accurately support normal cell physiology.

A novel chemically defined serum- and feeder-free medium for undifferentiated growth of porcine pluripotent stem cells

Development and improvement of in vitro culture system supporting self-renewal and unlimited proliferation of porcine pluripotent stem cells (pPSCs) is an indispensable process for the naïve pPSCs establishment. In this study, we modified the previous culture system and attempted to develop a novel chemically defined medium (KOFL) for the establishment of pPSCs. It has been cultured >45 passages with flat colony morphology and normal karyotypes in in vitro environment. These cells exhibited alkaline phosphatase activity and expressed pluripotency markers such as OCT4, SOX2, and NANOG, and also possessed differentiation abilities both in vitro and in vivo, proving by the formation of embryonic bodies and teratomas into three germ layers. Then the cells transfected with a green fluorescent protein (GFP) and the GFP positive cells contribute to the porcine preimplantation embryo development. In addition, these cells maintained long duration under feeder-free condition. In conclusion, our results demonstrated that the pPSCs could be derived from preimplantation porcine embryos in serum-free medium and cultured under the feeder-free condition, providing an effective reference for further optimization of the pPSCs culture system.

Intensified Stiffness and Photodynamic Provocation in a Collagen-Based Composite Hydrogel Drive Chondrogenesis

Directed differentiation of bone-marrow-derived stem cells (BMSCs) toward chondrogenesis has served as a predominant method for cartilage repair but suffers from poor oriented differentiation tendency and low differentiation efficiency. To overcome these two obstacles, an injectable composite hydrogel that consists of collagen hydrogels serving as the scaffold support to accommodate BMSCs and cadmium selenide (CdSe) quantum dots (QDs) is constructed. The introduction of CdSe QDs considerably strengthens the stiffness of the collagen hydrogels via mutual crosslinking using a natural crosslinker (i.e., genipin), which simultaneously triggers photodynamic provocation (PDP) to produce reactive oxygen species (ROS). Experimental results demonstrate that the intensified stiffness and augmented ROS production can synergistically promote the proliferation of BMSCs, induce cartilage-specific gene expression and increase secretion of glycosaminoglycan. As a result, this approach can facilitate the directed differentiation of BMSCs toward chondrogenesis and accelerate cartilage regeneration in cartilage defect repair, which routes through activation of the TGF-β/SMAD and mTOR signaling pathways, respectively. Thus, this synergistic strategy based on increased stiffness and PDP-mediated ROS production provides a general and instructive approach for developing alternative materials applicable for cartilage repair.

Evolutionary view of pluripotency seen from early development of non-mammalian amniotes

Early embryonic cells are capable of acquiring numerous developmental fates until they become irreversibly committed to specific lineages depending on intrinsic determinants and/or regional interactions. From fertilization to gastrulation, such pluripotent cells first increase in number and then turn to undergoing differentiation. Mechanisms regulating pluripotency in each species attract great interest in developmental biology. Also, outlining the evolutionary background of pluripotency can enhance our understanding of mammalian pluripotency and provide a broader view of early development of vertebrates. Here, we introduce integrative models of pluripotent states in amniotes (mammals, birds and reptiles) to offer a comprehensive overview of widely accepted knowledge about mammalian pluripotency and our recent findings in non-mammalian amniotes, such as chicken and gecko. In particular, we describe 1) the IL6/Stat3 signaling pathway as a positive regulator of naive pluripotency, 2) Fgf/Erk signaling as a process that prepares cells for differentiation, 3) the role of the interactions between these two signaling pathways during the transition from pluripotency to differentiation, and 4) functional diversification of two transcription factors, Class V POUs and Nanog. In the last section, we also briefly discuss possible relationships of unique cell cycle properties of early embryonic cells with signaling pathways and developmental potentials in the pluripotent cell states.

Multifactorial Modeling Reveals a Dominant Role of Wnt Signaling in Lineage Commitment of Human Pluripotent Stem Cells

The human primed pluripotent state is maintained by a complex balance of several signaling pathways governing pluripotency maintenance and commitment. Here, we explore a multiparameter approach using a full factorial design and a simple well-defined culture system to assess individual and synergistic contributions of Wnt, FGF and TGFβ signaling to pluripotency and lineage specification of human induced pluripotent stem cells (hiPSC). Hierarchical clustering and quadratic models highlighted a dominant effect of Wnt signaling over FGF and TGFβ signaling, drawing hiPSCs towards mesendoderm lineages. In addition, a synergistic effect between Wnt signaling and FGF was observed to have a negative contribution to pluripotency maintenance and a positive contribution to ectoderm and mesoderm commitment. Furthermore, FGF and TGFβ signaling only contributed significantly for negative ectoderm scores, suggesting that the effect of both factors for pluripotency maintenance resides in a balance of inhibitory signals instead of proactive stimulation of hiPSC pluripotency. Overall, our dry-signaling multiparameter modeling approach can contribute to elucidate individual and synergistic inputs, providing an additional degree of comprehension of the complex regulatory mechanisms of human pluripotency and commitment.

Roles of MicroRNAs in Establishing and Modulating Stem Cell Potential

Early embryonic development in mammals, from fertilization to implantation, can be viewed as a process in which stem cells alternate between self-renewal and differentiation. During this process, the fates of stem cells in embryos are gradually specified, from the totipotent state, through the segregation of embryonic and extraembryonic lineages, to the molecular and cellular defined progenitors. Most of those stem cells with different potencies in vivo can be propagated in vitro and recapitulate their differentiation abilities. Complex and coordinated regulations, such as epigenetic reprogramming, maternal RNA clearance, transcriptional and translational landscape changes, as well as the signal transduction, are required for the proper development of early embryos. Accumulated studies suggest that Dicer-dependent noncoding RNAs, including microRNAs (miRNAs) and endogenous small-interfering RNAs (endo-siRNAs), are involved in those regulations and therefore modulate biological properties of stem cells in vitro and in vivo. Elucidating roles of these noncoding RNAs will give us a more comprehensive picture of mammalian embryonic development and enable us to modulate stem cell potencies. In this review, we will discuss roles of miRNAs in regulating the maintenance and cell fate potential of stem cells in/from mouse and human early embryos.

Targeting Nodal and Cripto-1: Perspectives Inside Dual Potential Theranostic Cancer Biomarkers

BACKGROUND

Elucidating the mechanisms of recurrence of embryonic signaling pathways in tumorigenesis has led to the discovery of onco-fetal players which have physiological roles during normal development but result aberrantly re-activated in tumors. In this context, Nodal and Cripto-1 are recognized as onco-developmental factors, which are absent in normal tissues but are overexpressed in several solid tumors where they can serve as theranostic agents.

OBJECTIVE

To collect, review and discuss the most relevant papers related to the involvement of Nodal and Cripto-1 in the development, progression, recurrence and metastasis of several tumors where they are over-expressed, with a particular attention to their occurrence on the surface of the corresponding sub-populations of cancer stem cells (CSC).

RESULTS

We have gathered, rationalized and discussed the most interesting findings extracted from some 370 papers related to the involvement of Cripto-1 and Nodal in all tumor types where they have been detected. Data demonstrate the clear connection between Nodal and Cripto-1 presence and their multiple oncogenic activities across different tumors. We have also reviewed and highlighted the potential of targeting Nodal, Cripto-1 and the complexes that they form on the surface of tumor cells, especially of CSC, as an innovative approach to detect and suppress tumors with molecules that block one or more mechanisms that they regulate.

CONCLUSION

Overall, Nodal and Cripto-1 represent two innovative and effective biomarkers for developing potential theranostic anti-tumor agents that target normal as well as CSC subpopulations and overcome both pharmacological resistance and tumor relapse.

Nodal pathway activation due to Akt1 suppression is a molecular switch for prostate cancer cell epithelial-to-mesenchymal transition and metastasis

Several studies have unraveled the negative role of Akt1 in advanced cancers, including metastatic prostate cancer (mPCa). Hence, understanding the consequences of targeting Akt1 in the mPCa and identifying its downstream novel targets is essential. We studied how Akt1 deletion in PC3 and DU145 cells activates the Nodal pathway and promotes PCa epithelial-to-mesenchymal transition (EMT) and metastasis. Here we show that Akt1 loss increases Nodal expression in PCa cells accompanied by activation of FoxO1/3a, and EMT markers Snail and N-cadherin as well as loss of epithelial marker E-cadherin. Treatment with FoxO inhibitor AS1842856 abrogated the Nodal expression in Akt1 deleted PCa cells. Akt1 deficient PCa cells exhibited enhanced cell migration and invasion in vitro and lung metastasis in vivo, which were attenuated by treatment with Nodal pathway inhibitor SB505124. Interestingly, Nodal mRNA analysis from two genomic studies in cBioportal showed a positive correlation between Nodal expression and Gleason score indicating the positive role of Nodal in human mPCa. Collectively, our data demonstrate Akt1-FoxO3a-Nodal pathway as an important mediator of PCa metastasis and present Nodal as a potential target to treat mPCa patients.

EGFL7 Mediates BMP9-Induced Sprouting Angiogenesis of Endothelial Cells Derived from Human Embryonic Stem Cells

Human embryonic stem cells (hESCs) are instrumental in characterizing the molecular mechanisms of human vascular development and disease. Bone morphogenetic proteins (BMPs) play a pivotal role in cardiovascular development in mice, but their importance for vascular cells derived from hESCs has not yet been fully explored. Here, we demonstrate that BMP9 promotes, via its receptor ALK1 and SMAD1/5 activation, sprouting angiogenesis of hESC-derived endothelial cells. We show that the secreted angiogenic factor epidermal growth factor-like domain 7 (EGFL7) is a downstream target of BMP9-SMAD1/5-mediated signaling, and that EGFL7 promotes expansion of endothelium via interference with NOTCH signaling, activation of ERK, and remodeling of the extracellular matrix. CRISPR/Cas9-mediated deletion of EGFL7 highlights the critical role of EGFL7 in BMP9-induced endothelial sprouting and the promotion of angiogenesis. Our study illustrates the complex role of the BMP family in orchestrating hESC vascular development and endothelial sprouting.

•

BMP9/ALK1 signaling induces sprouting of hESC-derived endothelial cells

•

EGFL7 mediates BMP9-induced sprouting angiogenesis of hESC-derived endothelial cells

•

EGFL7 inhibits the NOTCH pathway and activates the ERK pathway in HUVECs

•

EGFL7 affects the extracellular matrix in HUVECs

The Role of Ubiquitination in Regulating Embryonic Stem Cell Maintenance and Cancer Development

Ubiquitination regulates nearly every aspect of cellular events in eukaryotes. It modifies intracellular proteins with 76-amino acid polypeptide ubiquitin (Ub) and destines them for proteolysis or activity alteration. Ubiquitination is generally achieved by a tri-enzyme machinery involving ubiquitin activating enzymes (E1), ubiquitin conjugating enzymes (E2) and ubiquitin ligases (E3). E1 activates Ub and transfers it to the active cysteine site of E2 via a transesterification reaction. E3 coordinates with E2 to mediate isopeptide bond formation between Ub and substrate protein. The E1-E2-E3 cascade can create diverse types of Ub modifications, hence effecting distinct outcomes on the substrate proteins. Dysregulation of ubiquitination results in severe consequences and human diseases. There include cancers, developmental defects and immune disorders. In this review, we provide an overview of the ubiquitination machinery and discuss the recent progresses in the ubiquitination-mediated regulation of embryonic stem cell maintenance and cancer biology.

Genome-scale screens identify JNK-JUN signaling as a barrier for pluripotency exit and endoderm differentiation

Human embryonic and induced pluripotent stem cells (hESCs/hiPSCs) hold great promise for cell-based therapies and drug discovery. However, homogeneous differentiation remains a major challenge, highlighting the need for understanding developmental mechanisms. We performed genome-scale CRISPR screens to uncover regulators of definitive endoderm (DE) differentiation, which unexpectedly uncovered five JNK/JUN family genes as key barriers of DE differentiation. The JNK/JUN pathway does not act through directly inhibiting the DE enhancers. Instead JUN co-occupies ESC enhancers with OCT4, NANOG and SMAD2/3, and specifically inhibits the exit from the pluripotent state by impeding the decommissioning of ESC enhancers and inhibiting the reconfiguration of SMAD2/3 chromatin binding from ESC to DE enhancers. Therefore, the JNK/JUN pathway safeguards pluripotency from precocious DE differentiation. Direct pharmacological inhibition of JNK significantly improves the efficiencies of generating DE and DE-derived pancreatic and lung progenitor cells, highlighting the potential of harnessing the knowledge from developmental studies for regenerative medicine.

Physiological profile of undifferentiated bovine blastocyst-derived trophoblasts

Trophectoderm of blastocysts mediate early events in fetal-maternal communication, enabling implantation and establishment of a functional placenta. Inadequate or impaired developmental events linked to trophoblasts directly impact early embryo survival and successful implantation during a crucial period that corresponds with high incidence of pregnancy losses in dairy cows. As yet, the molecular basis of bovine trophectoderm development and signaling towards initiation of implantation remains poorly understood. In this study, we developed methods for culturing undifferentiated bovine blastocyst-derived trophoblasts and used both transcriptomics and proteomics in early colonies to categorize and elucidate their functional characteristics. A total of 9270 transcripts and 1418 proteins were identified and analyzed based on absolute abundance. We profiled an extensive list of growth factors, cytokines and other relevant factors that can effectively influence paracrine communication in the uterine microenvironment. Functional categorization and analysis revealed novel information on structural organization, extracellular matrix composition, cell junction and adhesion components, transcription networks, and metabolic preferences. Our data showcase the fundamental physiology of bovine trophectoderm and indicate hallmarks of the self-renewing undifferentiated state akin to trophoblast stem cells described in other species. Functional features uncovered are essential for understanding early events in bovine pregnancy towards initiation of implantation.

In vitro culture and characterization of duck primordial germ cells

This study aimed to isolate, culture, and characterize duck primordial germ cells (PGCs) and to compare these cells with chicken PGCs. We first cultured Muscovy duck (Cairina moschata) circulating PGCs and gonadal PGCs (gPGCs) in the modified serum-containing medium used to amplify chicken PGCs. gPGCs were found to proliferate better in serum-free chemically defined medium than in serum-containing medium. Thereafter, gPGCs were similarly isolated from 2 other duck breeds, the Pekin duck (Anas platyrhynchos) and the hybrid mule duck (C. moschata × A. platyrhynchos), and amplified for a limited period of time in the chemically defined culture condition, but sufficiently to be characterized and transplanted. Cultured gPGCs of all 3 duck breeds were characterized by Periodic acid-Schiff staining, immunocytochemical staining, and expression analysis of germline-specific and pluripotency genes. Cultured duck gPGCs colonized the gonads after being genetically labeled and injected into recipient embryos. Taken together, these results demonstrate that duck PGCs retain their germline characteristics after being isolated, expanded in vitro, and genetically modified. Further studies are required to establish the optimal conditions for long-term culture of duck PGCs, which may involve supplementing the culture medium with other growth factors or compounds.

Rapid changes in morphogen concentration control self-organized patterning in human embryonic stem cells

During embryonic development, diffusible signaling molecules called morphogens are thought to determine cell fates in a concentration-dependent way. Yet, in mammalian embryos, concentrations change rapidly compared to the time for making cell fate decisions. Here, we use human embryonic stem cells (hESCs) to address how changing morphogen levels influence differentiation, focusing on how BMP4 and Nodal signaling govern the cell-fate decisions associated with gastrulation. We show that BMP4 response is concentration dependent, but that expression of many Nodal targets depends on rate of concentration change. Moreover, in a self-organized stem cell model for human gastrulation, expression of these genes follows rapid changes in endogenous Nodal signaling. Our study shows a striking contrast between the specific ways ligand dynamics are interpreted by two closely related signaling pathways, highlighting both the subtlety and importance of morphogen dynamics for understanding mammalian embryogenesis and designing optimized protocols for directed stem cell differentiation.

Editorial note

This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Microphysiological systems meet hiPSC technology - New tools for disease modeling of liver infections in basic research and drug development

Complex cell culture models such as microphysiological models (MPS) mimicking human liver functionality in vitro are in the spotlight as alternative to conventional cell culture and animal models. Promising techniques like microfluidic cell culture or micropatterning by 3D bioprinting are gaining increasing importance for the development of MPS to address the needs for more predictivity and cost efficiency. In this context, human induced pluripotent stem cells (hiPSCs) offer new perspectives for the development of advanced liver-on-chip systems by recreating an in vivo like microenvironment that supports the reliable differentiation of hiPSCs to hepatocyte-like cells (HLC). In this review we will summarize current protocols of HLC generation and highlight recently established MPS suitable to resemble physiological hepatocyte function in vitro. In addition, we are discussing potential applications of liver MPS for disease modeling related to systemic or direct liver infections and the use of MPS in testing of new drug candidates.

Pluripotency and X chromosome dynamics revealed in pig pre-gastrulating embryos by single cell analysis

High-resolution molecular programmes delineating the cellular foundations of mammalian embryogenesis have emerged recently. Similar analysis of human embryos is limited to pre-implantation stages, since early post-implantation embryos are largely inaccessible. Notwithstanding, we previously suggested conserved principles of pig and human early development. For further insight on pluripotent states and lineage delineation, we analysed pig embryos at single cell resolution. Here we show progressive segregation of inner cell mass and trophectoderm in early blastocysts, and of epiblast and hypoblast in late blastocysts. We show that following an emergent short naive pluripotent signature in early embryos, there is a protracted appearance of a primed signature in advanced embryonic stages. Dosage compensation with respect to the X-chromosome in females is attained via X-inactivation in late epiblasts. Detailed human-pig comparison is a basis towards comprehending early human development and a foundation for further studies of human pluripotent stem cell differentiation in pig interspecies chimeras.

Lineage segregation from conception to gastrulation has been mapped at the single cell level in mouse, human and monkey. Here, the authors provide a comprehensive analysis of porcine preimplantation development using single cell RNA-seq; mapping metabolic changes, X chromosome inactivation and signalling pathways.

Two decades of embryonic stem cells: a historical overview

STUDY QUESTION

How did the field of stem cell research develop in the years following the derivation of the first human embryonic stem cell (hESC) line?

SUMMARY ANSWER

Supported by the increasing number of clinical trials to date, significant technological advances in the past two decades have brought us ever closer to clinical therapies derived from pluripotent cells.

WHAT IS KNOWN ALREADY

Since their discovery 20 years ago, the use of human pluripotent stem cells has progressed tremendously from bench to bedside. Here, we provide a concise review of the main keystones of this journey and focus on ongoing clinical trials, while indicating the most relevant future research directions.

STUDY DESIGN, SIZE, DURATION

This is a historical narrative, including relevant publications in the field of pluripotent stem cells (PSC) derivation and differentiation, recounted both through scholarly research of published evidence and interviews of six pioneers who participated in some of the most relevant discoveries in the field.

PARTICIPANTS/MATERIALS, SETTING, METHODS

The authors all contributed by researching the literature and agreed upon body of works. Portions of the interviews of the field pioneers have been integrated into the review and have also been included in full for advanced reader interest.

MAIN RESULTS AND THE ROLE OF CHANCE

The stem cell field is ever expanding. We find that in the 20 years since the derivation of the first hESC lines, several relevant developments have shaped the pluripotent cell field, from the discovery of different states of pluripotency, the derivation of induced PSC, the refinement of differentiation protocols with several clinical trials underway, as well as the recent development of organoids. The challenge for the years to come will be to validate and refine PSCs for clinical use, from the production of highly defined cell populations in clinical grade conditions to the possibility of creating replacement organoids for functional, if not anatomical, function restoration.

LIMITATIONS, REASONS FOR CAUTION

This is a non-systematic review of current literature. Some references may have escaped the experts’ analysis due to the exceedingly diverse nature of the field. As the field of regenerative medicine is rapidly advancing, some of the most recent developments may have not been captured entirely.

WIDER IMPLICATIONS OF THE FINDINGS

The multi-disciplinary nature and tremendous potential of the stem cell field has important implications for basic as well as translational research. Recounting these activities will serve to provide an in-depth overview of the field, fostering a further understanding of human stem cell and developmental biology. The comprehensive overview of clinical trials and expert opinions included in this narrative may serve as a valuable scientific resource, supporting future efforts in translational approaches.

STUDY FUNDING/COMPETING INTEREST(S)

ESHRE provided funding for the authors’ on-site meeting and discussion during the preparation of this manuscript. S.M.C.S.L. is funded by the European Research Council Consolidator (ERC-CoG-725722-OVOGROWTH). M.P. is supported by the Special Research Fund, Bijzonder Onderzoeksfonds (BOF01D08114). M.G. is supported by the Methusalem grant of Vrije Universiteit Brussel, in the name of Prof. Karen Sermon and by Innovation by Science and Technology in Flanders (IWT, Project Number: 150042). A.V. and B.A. are supported by the Plataforma de Proteomica, Genotipado y Líneas Celulares (PT1770019/0015) (PRB3), Instituto de Salud Carlos III. Research grant to B.H. by the Research Foundation—Flanders (FWO) (FWO.KAN.2016.0005.01 and FWO.Project G051516N). There are no conflicts of interest to declare.

TRIAL REGISTRATION NUMBER

Not applicable.

ESHRE Pages are not externally peer reviewed. This article has been approved by the Executive Committee of ESHRE.

Targeted gene silencing in human embryonic stem cells using cell-penetrating peptide PepFect 14

Background

Human embryonic stem (hES) cells serve as an invaluable tool for research and future medicine, but their transfection often leads to unwanted side effects as the method itself may induce differentiation. On the other hand, RNA interference (RNAi)-based targeted gene silencing is a quick, cost-effective, and easy-to-perform method to address questions regarding the function of genes, especially when hypomorphic knockdowns are needed. Therefore, effective transfection method with minimal side effects is essential for applying RNAi to hES cells. Here, we report a highly promising approach for targeted gene silencing in hES cells with siRNA complexed with cell-penetrating peptide PepFect 14 (PF14). This strategy provides researchers with efficient tool for unraveling the functions of genes or addressing the differentiation of pluripotent stem cells.

Methods

We present a method for delivery of siRNA into hES cells with cell-penetrating peptide PF14. Accordingly, hES cells were transfected in ROCK inhibitor containing medium for 24 h right after EDTA passaging as small cell clumps. Fluorescently labeled siRNA and siRNAs targeting OCT4 or beta-2-microglobulin (B2M) mRNA sequences were used to evaluate the efficiency of transfection and silencing. Analyses were performed at various time points by flow cytometry, RT-qPCR, and immunofluorescence microscopy.

Results

Effective downregulation of OCT4 in 70% of treated hES cells at protein level was achieved, along with 90% reduction at mRNA level in bulk population of cells. The applicability of this low-cost and easy-to-perform method was confirmed by inducing silencing of another target not associated with hES cell pluripotency (B2M). Furthermore, we discovered that downregulation of OCT4 induces neuroectodermal differentiation accompanied by reduced expression of B2M during early stage of this lineage.

Conclusions

The results demonstrate PF14 as a promising tool for studying gene function and regulatory networks in hES cells by using RNAi.

Electronic supplementary material

The online version of this article (10.1186/s13287-019-1144-x) contains supplementary material, which is available to authorized users.

DeepNEU: cellular reprogramming comes of age - a machine learning platform with application to rare diseases research

BACKGROUND

Conversion of human somatic cells into induced pluripotent stem cells (iPSCs) is often an inefficient, time consuming and expensive process. Also, the tendency of iPSCs to revert to their original somatic cell type over time continues to be problematic. A computational model of iPSCs identifying genes/molecules necessary for iPSC generation and maintenance could represent a crucial step forward for improved stem cell research. The combination of substantial genetic relationship data, advanced computing hardware and powerful nonlinear modeling software could make the possibility of artificially-induced pluripotent stem cells (aiPSC) a reality. We have developed an unsupervised deep machine learning technology, called DeepNEU that is based on a fully-connected recurrent neural network architecture with one network processing layer for each input. DeepNEU was used to simulate aiPSC systems using a defined set of reprogramming transcription factors. Genes/proteins that were reported to be essential in human pluripotent stem cells (hPSC) were used for system modelling.

RESULTS

The Mean Squared Error (MSE) function was used to assess system learning. System convergence was defined at MSE < 0.001. The markers of human iPSC pluripotency (N = 15) were all upregulated in the aiPSC final model. These upregulated/expressed genes in the aiPSC system were entirely consistent with results obtained for iPSCs.

CONCLUSION

This research introduces and validates the potential use of aiPSCs as computer models of human pluripotent stem cell systems. Disease-specific aiPSCs have the potential to improve disease modeling, prototyping of wet lab experiments, and prediction of genes relevant and necessary for aiPSC production and maintenance for both common and rare diseases in a cost-effective manner.

Highly Efficient Derivation of Pluripotent Stem Cells from Mouse Preimplantation and Postimplantation Embryos in Serum-Free Conditions

Pluripotency refers to the potential of cells to generate all cell types of the embryo proper. Pluripotency spans a spectrum of cellular states. At one polar extreme is naïve pluripotency, which is identified based on the potential to form germline chimeras. At the other polar extreme is primed pluripotency, in which pluripotent cells are primed to differentiate. Mouse naïve PS cells can be derived from preimplantation embryos. Primed epiblast stem (EpiS) cells are typically isolated from epiblasts of early postimplantation mouse embryos. In this chapter, we describe protocols for highly efficient derivation and propagation of murine naïve and primed PS cell lines in serum-free conditions from preimplantation and postimplantation embryos. We describe generation of mouse naïve PS cells using LIF and inhibitors of MEK and GSK3 kinases and of mouse primed PS cells using FGF2 and IWR1 compound which induces the stabilization of Axin proteins.

[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.

Human Pluripotent Stem Cell Culture: Current Status, Challenges, and Advancement

Over the past two decades, human embryonic stem cells (hESCs) have gained attention due to their pluripotent and proliferative ability which enables production of almost all cell types in the human body in vitro and makes them an excellent tool to study human embryogenesis and disease, as well as for drug discovery and cell transplantation therapies. Discovery of human-induced pluripotent stem cells (hiPSCs) further expanded therapeutic applications of human pluripotent stem cells (PSCs). hPSCs provide a stable and unlimited original cell source for producing suitable cells and tissues for downstream applications. Therefore, engineering the environment in which these cells are grown, for stable and quality-controlled hPSC maintenance and production, is one of the key factors governing the success of these applications. hPSCs are maintained in a particular niche using specific cell culture components. Ideally, the culture should be free of xenobiotic components to render hPSCs suitable for therapeutic applications. Substantial efforts have been put to identify effective components, and develop culture conditions and protocols, for their large-scale expansion without compromising on quality. In this review, we discuss different media, their components and functions, including specific requirements to maintain the pluripotent and proliferative ability of hPSCs. Understanding the role of culture components would enable the development of appropriate conditions to promote large-scale, quality-controlled expansion of hPSCs thereby increasing their potential applications.

WNT signaling memory is required for ACTIVIN to function as a morphogen in human gastruloids

Self-organization of discrete fates in human gastruloids is mediated by a hierarchy of signaling pathways. How these pathways are integrated in time, and whether cells maintain a memory of their signaling history remains obscure. Here, we dissect the temporal integration of two key pathways, WNT and ACTIVIN, which along with BMP control gastrulation. CRISPR/Cas9-engineered live reporters of SMAD1, 2 and 4 demonstrate that in contrast to the stable signaling by SMAD1, signaling and transcriptional response by SMAD2 is transient, and while necessary for pluripotency, it is insufficient for differentiation. Pre-exposure to WNT, however, endows cells with the competence to respond to graded levels of ACTIVIN, which induces differentiation without changing SMAD2 dynamics. This cellular memory of WNT signaling is necessary for ACTIVIN morphogen activity. A re-evaluation of the evidence gathered over decades in model systems, re-enforces our conclusions and points to an evolutionarily conserved mechanism.

Development of trophoblast cystic structures from human induced pluripotent stem cells in limited-area cell culture

We developed a novel engineering technique to induce differentiation of human induced pluripotent stem cells (hiPSCs) into organoids mimicking the trophectoderm (TE). Here, hiPSCs were cultured on a limited area of 2-4 mm in diameter. After 15-20 days, spherical cysts appeared on the surface of the limited area. Secretion of human chorionic gonadotrophin (hCG) began to increase after ∼ 20 days and remained dramatically elevated over the next 20 days. Limited-area-cultured cysts exhibited expression of hCG, which was a result of epithelial differentiation. Low expression levels of pluripotent genes and high expression levels of trophoblast lineage-specific genes were detected in the cells of spherical cysts. Multinucleated syncytia trophoblast was observed in the reseeded cystic cells. We observed hiPSC-derived cysts that morphologically resembled trophectoderm in vivo. The limited-area cell culture induced a three-dimensional (3D) trophectoderm organoid, which has potential for use in the study of human trophoblast differentiation and placental morphogenesis.

Trans-spliced long non-coding RNA: an emerging regulator of pluripotency

With dual capacities for unlimited self-renewal and pluripotent differentiation, pluripotent stem cells (PSCs) give rise to many cell types in our body and PSC culture systems provide an unparalleled opportunity to study early human development and disease. Accumulating evidence indicates that the molecular mechanisms underlying pluripotency maintenance in PSCs involve many factors. Among these regulators, recent studies have shown that long non-coding RNAs (lncRNAs) can affect the pluripotency circuitry by cooperating with master pluripotency-associated factors. Additionally, trans-spliced RNAs, which are generated by combining two or more pre-mRNA transcripts to produce a chimeric RNA, have been identified as regulators of various biological processes, including human pluripotency. In this review, we summarize and discuss current knowledge about the roles of lncRNAs, including trans-spliced lncRNAs, in controlling pluripotency.

Single-Cell Profiling Identifies Key Pathways Expressed by iPSCs Cultured in Different Commercial Media

We assessed the pluripotency of human induced pluripotent stem cells (iPSCs) maintained on an automated platform using StemFlex and TeSR-E8 media. Analysis of transcriptome of single cells revealed similar expression of core pluripotency genes, as well as genes associated with naive and primed states of pluripotency. Analysis of individual cells from four samples consisting of two different iPSC lines each grown in the two culture media revealed a shared subpopulation structure with three main subpopulations different in pluripotency states. By implementing a machine learning approach, we estimated that most cells within each subpopulation are very similar between all four samples. The single-cell RNA sequencing analysis of iPSC lines grown in both media reports the molecular signature in StemFlex medium and how it compares to that observed in the TeSR-E8 medium.

Soluble Cripto-1 Induces Accumulation of Supernumerary Centrosomes and Formation of Aberrant Mitoses in Human Embryonic Stem Cells

Chromosomal instability evoked by abnormalities in centrosome numbers has been traditionally considered as a hallmark of aberrant, typically cancerous or senescent cells. We have reported previously that pristine human embryonic stem cells (hESC) suffer from high frequency of supernumerary centrosomes and hence may be prone to undergo abnormal mitotic divisions. We have also unraveled that this phenomenon of multicentrosomal mitoses vanishes with prolonged time in culture and with initiation of differentiation, and it is strongly affected by the culture substratum. In this study, we report for the first time that Cripto-1 protein (teratocarcinoma-derived growth factor 1, epidermal growth factor-Cripto/FRL-1/Cryptic) produced by hESC represents a factor capable of inducing formation of supernumerary centrosomes in cultured hESC. Elimination of Cripto-1 signaling on the other hand restores the normal number of centrosomes in hESC. Linking the secretory phenotype of hESC to the centrosomal metabolism may help to develop better strategies for propagation of stable and safe bioindustrial and clinical grade cultures of hESC. From a broader point of view, it may lead to unravelling Cripto-1 as a micro-environmental factor contributing to adverse cell behaviors in vivo.

Synergistic effect of co-immobilized FGF-2 and vitronectin-derived peptide on feeder-free expansion of induced pluripotent stem cells

Expansion of human induced pluripotent stem cells (h-iPSCs) on mouse derived feeder layers or murine cells secretions such as Matrigel hamper their clinical applications. Alternative methods have introduced novel substrates as stem cell niches or/and optimized combinations of humanized soluble factors as fully defined mediums. Accordingly vitronectin as a main part of ECM have been commercialized significantly as a stem cell niche-forming substrate. In this work, we used a functional peptide derived from vitronectin (VTN) and co-immobilized it with FGF-2 (as an indisputable ingredient of defined culture mediums) on chitosan film surface. After chemical and physical characterization of the pristine chitosan surface as well as ones modified by VTN or/and FGF-2, h-iPS cells were cultured on them at the xeno/feeder-free conditions. Our results demonstrated that co-immobilization of these two biomolecules has a synergistic effect on adhesion and clonal growth of h-iPS cells with maintained expression of pluripotency markers in a FGF-2 density-dependent manner. This is the first report of co-immobilization of an ECM derived molecule and a growth factor for stem cell culture.

The establishment of a chemically defined serum-free culture system for human dental pulp stem cells

Background

The concept of establishing a dental stem cell (DSC) bank for oral and maxillofacial regeneration has become of great interest but it remains at a primitive stage. The routine application of serum-containing conditions for human DSC (hDSC) culture is in great controversy considering that the animal-originated serum can cause serious ethical concerns and lead to increasingly irrelevant variables, errors, and poor repeatability of experiment results. Thus, this study aimed to establish a safe, stable and efficient hDSC serum-free culturing system for future DSC bank usage.

Methods

Dental pulp stem cells (DPSCs) from human permanent tooth pulp were isolated, expanded, passaged, and divided into two groups according to their culture conditions: group 1 was the serum-containing medium (SCM) group; and group 2 was the serum-free Essential 8 medium (E8) group. DPSCs were characterized first, followed by cell proliferation, pluripotency, and migration study in SCM and E8 medium.

Results

Human DPSCs (hDPSCs) in E8 medium demonstrated greater proliferation, pluripotency, migration ability and less apoptosis. hDPSCs could be successfully induced to the adipogenic, osteogenic, neurogenic, and chondrogenic lineages in E8 group. Real-time polymerase chain reaction indicated that the expression of PPAR-γ, RUNX2, OCN and MAP-2 was higher in E8 group. 

Conclusions

Compared with serum-containing medium, E8 medium exhitibed higher ability in maintaining the cell proliferation, pluripotency, migration, and stability. This new serum-free culture environment might be applicable for hDSC culture in the future.

The Suppression of Medium Acidosis Improves the Maintenance and Differentiation of Human Pluripotent Stem Cells at High Density in Defined Cell Culture Medium

Cell density has profound impacts on the cell culture practices of human pluripotent stem cells. The regulation of cell growth, cell death, pluripotency and differentiation converge at high density, but it is largely unknown how different regulatory mechanisms act at this stage. We use a chemically defined medium to systemically examine cellular activities and the impact of medium components in high-density culture. We show that medium acidosis is the main factor that alters cell cycle, gene expression and cellular metabolism at high cell density. The low medium pH leads to inhibition of glucose consumption, cell cycle arrest, and subsequent cell death. At high cell density, the suppression of medium acidosis with sodium bicarbonate (NaHCO₃) significantly increases culture capacity for stem cell survival, derivation, maintenance and differentiation. Our study provides a simple and effective tool to improve stem cell maintenance and applications.

Epigenetic differences between naïve and primed pluripotent stem cells

It has been 8 years since the concept of naïve and primed pluripotent stem cell states was first proposed. Both are states of pluripotency, but exhibit slightly different properties. The naïve state represents the cellular state of the preimplantation mouse blastocyst inner cell mass, while the primed state is representative of the post-implantation epiblast cells. These two cell types exhibit clearly distinct developmental potential, as evidenced by the fact that naïve cells are able to contribute to blastocyst chimeras, while primed cells cannot. However, the epigenetic differences that underlie the distinct developmental potential of these cell types remain unclear, which is rather surprising given the large amount of active investigation over the years. Elucidating such epigenetic differences should lead to a better understanding of the fundamental properties of these states of pluripotency and the means by which the naïve-to-primed transition occurs, which may provide insights into the essence of stem cell commitment.

Temporal Dynamics of Gene Expression During Endothelial Cell Differentiation From Human iPS Cells: A Comparison Study of Signalling Factors and Small Molecules

Endothelial cell (EC) therapy may promote vascular growth or reendothelization in a variety of disease conditions. However, the production of a cell therapy preparation containing differentiated, dividing cells presenting typical EC phenotype, functional properties and chemokine profile is challenging. We focused on comparative analysis of seven small molecule-mediated differentiation protocols of ECs from human induced pluripotent stem cells. Differentiated cells showed a typical surface antigen pattern of ECs as characterized with flow cytometry analysis, functional properties, such as tube formation and ability to uptake acetylated LDL. Gene expression analysis by RNA sequencing revealed an efficient silencing of pluripotency genes and upregulation of genes related to cellular adhesion during differentiation. In addition, distinct patterns of transcription factor expression were identified during cellular reprogramming providing targets for more effective differentiation protocols in the future. Altogether, our results suggest that the most optimal EC differentiation protocol includes early inhibition of Rho-associated coiled-coil kinase and activation of cyclic AMP signaling, and inhibition of transforming growth factor beta signaling after mesodermal stage. These findings provide the first systematic characterization of the most potent signalling factors and small molecules used to generate ECs from human induced pluripotent stem cells and, consequently, this work improves the existing EC differentiation protocols and opens up new avenues for controlling cell fate for regenerative EC therapy.

The SMAD2/3 interactome reveals that TGFβ controls m6A mRNA methylation in pluripotency

The TGFβ pathway has essential roles in embryonic development, organ homeostasis, tissue repair and disease. These diverse effects are mediated through the intracellular effectors SMAD2 and SMAD3 (hereafter SMAD2/3), whose canonical function is to control the activity of target genes by interacting with transcriptional regulators. Therefore, a complete description of the factors that interact with SMAD2/3 in a given cell type would have broad implications for many areas of cell biology. Here we describe the interactome of SMAD2/3 in human pluripotent stem cells. This analysis reveals that SMAD2/3 is involved in multiple molecular processes in addition to its role in transcription. In particular, we identify a functional interaction with the METTL3-METTL14-WTAP complex, which mediates the conversion of adenosine to N⁶-methyladenosine (m⁶A) on RNA. We show that SMAD2/3 promotes binding of the m⁶A methyltransferase complex to a subset of transcripts involved in early cell fate decisions. This mechanism destabilizes specific SMAD2/3 transcriptional targets, including the pluripotency factor gene NANOG, priming them for rapid downregulation upon differentiation to enable timely exit from pluripotency. Collectively, these findings reveal the mechanism by which extracellular signalling can induce rapid cellular responses through regulation of the epitranscriptome. These aspects of TGFβ signalling could have far-reaching implications in many other cell types and in diseases such as cancer.

The Pleiotropic Effects of the Canonical Wnt Pathway in Early Development and Pluripotency

The technology to derive embryonic and induced pluripotent stem cells from early embryonic stages and adult somatic cells, respectively, emerged as a powerful resource to enable the establishment of new in vitro models, which recapitulate early developmental processes and disease. Additionally, pluripotent stem cells (PSCs) represent an invaluable source of relevant differentiated cell types with immense potential for regenerative medicine and cell replacement therapies. Pluripotent stem cells support self-renewal, potency and proliferation for extensive periods of culture in vitro. However, the core pathways that rule each of these cellular features specific to PSCs only recently began to be clarified. The Wnt signaling pathway is pivotal during early embryogenesis and is central for the induction and maintenance of the pluripotency of PSCs. Signaling by the Wnt family of ligands is conveyed intracellularly by the stabilization of β-catenin in the cytoplasm and in the nucleus, where it elicits the transcriptional activity of T-cell factor (TCF)/lymphoid enhancer factor (LEF) family of transcription factors. Interestingly, in PSCs, the Wnt/β-catenin–TCF/LEF axis has several unrelated and sometimes opposite cellular functions such as self-renewal, stemness, lineage commitment and cell cycle regulation. In addition, tight control of the Wnt signaling pathway enhances reprogramming of somatic cells to induced pluripotency. Several recent research efforts emphasize the pleiotropic functions of the Wnt signaling pathway in the pluripotent state. Nonetheless, conflicting results and unanswered questions still linger. In this review, we will focus on the diverse functions of the canonical Wnt signaling pathway on the developmental processes preceding embryo implantation, as well as on its roles in pluripotent stem cell biology such as self-renewal and cell cycle regulation and somatic cell reprogramming.

Integrated analysis of single-cell embryo data yields a unified transcriptome signature for the human pre-implantation epiblast

Single-cell profiling techniques create opportunities to delineate cell fate progression in mammalian development. Recent studies have provided transcriptome data from human pre-implantation embryos, in total comprising nearly 2000 individual cells. Interpretation of these data is confounded by biological factors, such as variable embryo staging and cell-type ambiguity, as well as technical challenges in the collective analysis of datasets produced with different sample preparation and sequencing protocols. Here, we address these issues to assemble a complete gene expression time course spanning human pre-implantation embryogenesis. We identify key transcriptional features over developmental time and elucidate lineage-specific regulatory networks. We resolve post-hoc cell-type assignment in the blastocyst, and define robust transcriptional prototypes that capture epiblast and primitive endoderm lineages. Examination of human pluripotent stem cell transcriptomes in this framework identifies culture conditions that sustain a naïve state pertaining to the inner cell mass. Our approach thus clarifies understanding both of lineage segregation in the early human embryo and of in vitro stem cell identity, and provides an analytical resource for comparative molecular embryology.

The Art of Capturing Pluripotency: Creating the Right Culture

Embryonic stem cells (ESCs) are a unique tool for genetic perturbation of mammalian cellular and organismal processes additionally in humans offer unprecedented opportunities for disease modeling and cell therapy. Furthermore, ESCs are a powerful system for exploring the fundamental biology of pluripotency. Indeed understanding the control of self-renewal and differentiation is key to realizing the potential of ESCs. Building on previous observations, we found that mouse ESCs can be derived and maintained with high efficiency through insulation from differentiation cues combined with consolidation of an innate cell proliferation program. This finding of a pluripotent ground state has led to conceptual and practical advances, including the establishment of germline-competent ESCs from recalcitrant mouse strains and for the first time from the rat. Here, we summarize historical and recent progress in defining the signaling environment that supports self-renewal. We compare the contrasting requirements of two types of pluripotent stem cell, naive ESCs and primed post-implantation epiblast stem cells (EpiSCs), and consider the outstanding challenge of generating naive pluripotent stem cells from different mammals.

Function of FEZF1 during early neural differentiation of human embryonic stem cells

The understanding of the mechanism underlying human neural development has been hampered due to lack of a cellular system and complicated ethical issues. Human embryonic stem cells (hESCs) provide an invaluable model for dissecting human development because of unlimited self-renewal and the capacity to differentiate into nearly all cell types in the human body. In this study, using a chemical defined neural induction protocol and molecular profiling, we identified Fez family zinc finger 1 (FEZF1) as a potential regulator of early human neural development. FEZF1 is rapidly up-regulated during neural differentiation in hESCs and expressed before PAX6, a well-established marker of early human neural induction. We generated FEZF1-knockout H1 hESC lines using CRISPR-CAS9 technology and found that depletion of FEZF1 abrogates neural differentiation of hESCs. Moreover, loss of FEZF1 impairs the pluripotency exit of hESCs during neural specification, which partially explains the neural induction defect caused by FEZF1 deletion. However, enforced expression of FEZF1 itself fails to drive neural differentiation in hESCs, suggesting that FEZF1 is necessary but not sufficient for neural differentiation from hESCs. Taken together, our findings identify one of the earliest regulators expressed upon neural induction and provide insight into early neural development in human.

Mechanisms of gene regulation in human embryos and pluripotent stem cells

Pluripotent stem cells have broad utility in biomedical research and their molecular regulation has thus garnered substantial interest. While the principles that establish and regulate pluripotency have been well defined in the mouse, it has been difficult to extrapolate these insights to the human system due to species-specific differences and the distinct developmental identities of mouse versus human embryonic stem cells. In this Review, we examine genome-wide approaches to elucidate the regulatory principles of pluripotency in human embryos and stem cells, and highlight where differences exist in the regulation of pluripotency in mice and humans. We review recent insights into the nature of human pluripotent cells in vivo, obtained by the deep sequencing of pre-implantation embryos. We also present an integrated overview of the principal layers of global gene regulation in human pluripotent stem cells. Finally, we discuss the transcriptional and epigenomic remodeling events associated with cell fate transitions into and out of human pluripotency.

States and Origins of Mammalian Embryonic Pluripotency In Vivo and in a Dish

Mouse embryonic stem cells (ESC), derived from preimplantation embryos in 1981, defined mammalian pluripotency for many decades. However, after the derivation of human ESC in 1998, comparative studies showed that different types of pluripotency exist in early embryos and that these can be captured in vitro under various culture conditions. Over the past decade much has been learned about the key signaling pathways, growth factor requirements, and transcription factor profiles of pluripotent cells in embryos, allowing improvement of derivation and culture conditions for novel pluripotent stem cell types. More recently, studies using single-cell transcriptomics of embryos from different species provided an unprecedented level of resolution of cellular interactions and cell fate decisions that are informing new ways to understand the emergence of pluripotency in different organisms. These new approaches enhance knowledge of species differences during early embryogenesis and will be instrumental for improving methodologies for generating intra- and interspecies chimeric animals using pluripotent stem cells. Here, we discuss the recent developments in our understanding of early embryogenesis in different mammalian species.

Stimulatory effects of fibroblast growth factor 2 on proliferation and migration of uterine luminal epithelial cells during early pregnancy

Fibroblast growth factor 2 (FGF2) is a mitogen that induces proliferation, differentiation, and migration of cells, as well as angiogenesis and carcinogenesis via autocrine or paracrine actions. Fibroblast growth factor 2 expression is abundant in porcine conceptuses and endometrium during the estrous cycle and peri-implantation period of pregnancy. However, its intracellular actions in uterine epithelial cells have not been reported. The results of this study indicated abundant expression of FGFR1 and FGFR2 predominantly in uterine luminal and glandular epithelia during early pregnancy and that their expression decreased with increasing parity of the sows. Treatment of porcine uterine luminal epithelial (pLE) cells with FGF2 increased proliferation and DNA replication based on increases in proliferating cell nuclear antigen (PCNA) and initiation of G1/S phase progression. In addition, FGF2 increases phosphorylation of AKT, P70S6K, S6, ERK1/2, JNK, P38, and P90RSK in a time-dependent manner, and increases in their expression was suppressed by Wortmannin (a phosphatidylinositol 3-kinase [PI3K] inhibitor), U0126 (an ERK1/2 inhibitor), SP600125 (a JNK inhibitor), and SB203580 (a P38 inhibitor) based on western blot analyses. Also, the abundance of cytoplasmic p-AKT protein was decreased by Wortmannin and U0126, and p-ERK1/2 protein was reduced only by U0126. Furthermore, inhibition of each signal transduction protein reduced the ability of FGF2 to stimulate proliferation and migration of pLE cells. Collectively, these results indicate that activation of FGFR1 and FGFR2 by uterine- and endometrial-derived FGF2 stimulates PI3K/AKT and mitogen-activated protein kinase pathways for development of the porcine uterus and improvement of litter size.

OCT4 supports extended LIF-independent self-renewal and maintenance of transcriptional and epigenetic networks in embryonic stem cells

Embryonic stem (ES) cell pluripotency is governed by OCT4-centric transcriptional networks. Conventional ES cells can be derived and maintained in vitro with media containing the cytokine leukemia inhibitory factor (LIF), which propagates the pluripotent state by activating STAT3 signaling, and simultaneous inhibition of glycogen synthase kinase-3 (GSK3) and MAP kinase/ERK kinase signaling. However, it is unclear whether overexpression of OCT4 is sufficient to overcome LIF-dependence. Here, we show that inducible expression of OCT4 (iOCT4) supports long-term LIF-independent self-renewal of ES cells cultured in media containing fetal bovine serum (FBS) and a glycogen synthase kinase-3 (GSK3) inhibitor, and in serum-free media. Global expression analysis revealed that LIF-independent iOCT4 ES cells and control ES cells exhibit similar transcriptional programs relative to epiblast stem cells (EpiSCs) and differentiated cells. Epigenomic profiling also demonstrated similar patterns of histone modifications between LIF-independent iOCT4 and control ES cells. Moreover, LIF-independent iOCT4 ES cells retain the capacity to differentiate in vitro and in vivo upon downregulation of OCT4 expression. These findings indicate that OCT4 expression is sufficient to sustain intrinsic signaling in a LIF-independent manner to promote ES cell pluripotency and self-renewal.

Naive-like ESRRB+ iPSCs with the Capacity for Rapid Neural Differentiation

Several groups have reported the existence of a form of pluripotency that resembles that of mouse embryonic stem cells (mESCs), i.e., a naive state, in human pluripotent stem cells; however, the characteristics vary between reports. The nuclear receptor ESRRB is expressed in mESCs and plays a significant role in their self-renewal, but its expression has not been observed in most naive-like human induced pluripotent stem cells (hiPSCs). In this study, we modified several methods for converting hiPSCs into a naive state through the transgenic expression of several reprogramming factors. The resulting cells express the components of the core transcriptional network of mESCs, including ESRRB, at high levels, which suggests the existence of naive-state hiPSCs that are similar to mESCs. We also demonstrate that these cells differentiate more readily into neural cells than do conventional hiPSCs. These features may be beneficial for their use in disease modeling and regenerative medicine.