Differential response of epiblast stem cells to Nodal and Activin signalling: a paradigm of early endoderm development in the embryo

Cross-activation of FGF, NODAL, and WNT pathways constrains BMP-signaling-mediated induction of the totipotent state in mouse embryonic stem cells

Embryonic stem cells (ESCs) are an attractive model to study the relationship between signaling and cell fates. Cultured mouse ESCs can exist in multiple states resembling distinct stages of early embryogenesis, such as totipotent, pluripotent, primed, and primitive endoderm. The signaling mechanisms regulating the totipotent state and coexistence of these states are poorly understood. Here we identify bone morphogenetic protein (BMP) signaling as an inducer of the totipotent state. However, we discover that BMP's role is constrained by the cross-activation of FGF, NODAL, and WNT pathways. We exploit this finding to enhance the proportion of totipotent cells by rationally inhibiting the cross-activated pathways. Single-cell mRNA sequencing reveals that induction of the totipotent state is accompanied by suppression of primed and primitive endoderm states. Furthermore, reprogrammed totipotent cells we generate in culture resemble totipotent cells of preimplantation embryo. Our findings reveal a BMP signaling mechanism regulating both the totipotent state and heterogeneity of ESCs.

A chemically defined system supports two distinct types of stem cell from a single blastocyst and their self-assembly to generate blastoid

The pluripotent stem cells exist in a narrow window during early development and its derivation depends on intrinsic and extrinsic growth signalling in vitro. It has remained challenging to derive two or three distinct cell lines that are representative of blastocyst-stage lineages from one preimplantation embryo simultaneously in a chemical defined condition. Therefore, it is desirable to establish a system by manipulating extrinsic signalling in culture to derive multiple types of stem cells from a single blastocyst. This study used a defined medium containing Activin A, WNT activator and LIF (ACL medium), enabling establishment of ACL-ESCs and ACL-XEN cells from one blastocyst. ACL-blastoids were generated by suspending ACL-ESCs and ACL-XEN cells with ACL-blastoid medium in three-dimensional culture system. Lineage markers expression of ACL-blastoids were performed by immunofluorescence. Our results indicate that ACL-ESCs and ACL-XEN cells derived from one blastocyst represent ICM and PrE lineages. Importantly, we obtained ACL-blastoid from ACL-ESCs and ACL-XEN cells self-aggregation, partially recapitulating early development and initiation of early implantation events. This study would not only provide ACL culture system for derivation and maintenance of two types of cell lines corresponding to ICM as well as PrE, but also reconstruct blastoids with them to deepen our understanding of early embryogenesis and widen insights into translational application of stem cells.

Differential impact of TGFβ/SMAD signaling activity elicited by Activin A and Nodal on endoderm differentiation of epiblast stem cells

Allocation of cells to an endodermal fate in the gastrulating embryo is driven by Nodal signaling and consequent activation of TGFβ pathway. In vitro methodologies striving to recapitulate the process of endoderm differentiation, however, use TGFβ family member Activin in place of Nodal. This is despite Activin not known to have an in vivo role in endoderm differentiation. In this study, five epiblast stem cell lines were subjected to directed differentiation using both Activin A and Nodal to induce endodermal fate. A reporter line harboring endoderm markers FoxA2 and Sox17 was further analyzed for TGFβ pathway activation and WNT response. We demonstrated that Activin A-treated cells remain more primitive streak-like when compared to Nodal-treated cells that have a molecular profile suggestive of more advanced differentiation. Activin A elicited a robust TGFβ/SMAD activity, enhanced WNT signaling activity and promoted the generation of DE precursors. Nodal treatment resulted in lower TGFβ/SMAD activity, and a weaker, sustained WNT response, and ultimately failed to upregulate endoderm markers. This is despite signaling response resembling more closely the activity seen in vivo. These findings emphasize the importance of understanding the downstream activities of Activin A and Nodal signaling in directing in vitro endoderm differentiation of primed-state epiblast stem cells.

Establishment of Mouse Epiblast Stem Cells

Epiblast stem cells are made from the epiblast of mouse post-implantation embryo. They have been critical in the understanding of mammalian pluripotent stem cells as they share similar properties, such as their incapability to contribute to the formation of an embryo after injection into blastocyst. The epiblast stem cells (EpiSC) have delineated a novel status of pluripotency called "primed." How to establish EpiSC from mouse embryo is described in detail in this chapter.

A case for revisiting Nodal signaling in human pluripotent stem cells

Nodal is a transforming growth factor-β (TGF-β) superfamily member that plays a number of critical roles in mammalian embryonic development. Nodal is essential for the support of the peri-implantation epiblast in the mouse embryo and subsequently acts to specify mesendodermal fate at the time of gastrulation and, later, left-right asymmetry. Maintenance of human pluripotent stem cells (hPSCs) in vitro is dependent on Nodal signaling. Because it has proven difficult to prepare a biologically active form of recombinant Nodal protein, Activin or TGFB1 are widely used as surrogates for NODAL in hPSC culture. Nonetheless, the expression of the components of an endogenous Nodal signaling pathway in hPSC provides a potential autocrine pathway for the regulation of self-renewal in this system. Here we review recent studies that have clarified the role of Nodal signaling in pluripotent stem cell populations, highlighted spatial restrictions on Nodal signaling, and shown that Nodal functions in vivo as a heterodimer with GDF3, another TGF-β superfamily member expressed by hPSC. We discuss the role of this pathway in the maintenance of the epiblast and hPSC in light of these new advances.

Mapping a Circular RNA-microRNA-mRNA-Signaling Regulatory Axis That Modulates Stemness Properties of Cancer Stem Cell Populations in Colorectal Cancer Spheroid Cells

Spheroidal cancer cell cultures have been used to enrich cancer stem cells (CSC), which are thought to contribute to important clinical features of tumors. This study aimed to map the regulatory networks driven by circular RNAs (circRNAs) in CSC-enriched colorectal cancer (CRC) spheroid cells. The spheroid cells established from two CRC cell lines acquired stemness properties in pluripotency gene expression and multi-lineage differentiation capacity. Genome-wide sequencing identified 1503 and 636 circRNAs specific to the CRC parental and spheroid cells, respectively. In the CRC spheroids, algorithmic analyses unveiled a core network of mRNAs involved in modulating stemness-associated signaling pathways, driven by a circRNA–microRNA (miRNA)–mRNA axis. The two major circRNAs, hsa_circ_0066631 and hsa_circ_0082096, in this network were significantly up-regulated in expression levels in the spheroid cells. The two circRNAs were predicted to target and were experimentally shown to down-regulate miR-140-3p, miR-224, miR-382, miR-548c-3p and miR-579, confirming circRNA sponging of the targeted miRNAs. Furthermore, the affected miRNAs were demonstrated to inhibit degradation of six mRNA targets, viz. ACVR1C/ALK7, FZD3, IL6ST/GP130, SKIL/SNON, SMAD2 and WNT5, in the CRC spheroid cells. These mRNAs encode proteins that are reported to variously regulate the GP130/Stat, Activin/Nodal, TGF-β/SMAD or Wnt/β-catenin signaling pathways in controlling various aspects of CSC stemness. Using the CRC spheroid cell model, the novel circRNA–miRNA–mRNA axis mapped in this work forms the foundation for the elucidation of the molecular mechanisms of the complex cellular and biochemical processes that determine CSC stemness properties of cancer cells, and possibly for designing therapeutic strategies for CRC treatment by targeting CSC.

Heterogeneity in Epiblast Stem Cells

Epiblast stem cells (EpiSCs) are pluripotent cells that are derived from mouse embryos at gastrulation stages. They represent the primed state of pluripotency, in which cells are on the verge of differentiation and already express markers of the three primary lineages (mesoderm, endoderm, neurectoderm). EpiSCs display some heterogeneity intra- and inter-cell lines in the expression of some of these lineage markers. We relate this heterogeneity to signalling pathways that are active in EpiSCs, either due to addition of growth factors (FGF2 and activin) in the culture medium, or endogenously active (FGF, Nodal, and Wnt). By modulating Wnt or activin/nodal pathways, cell lines close to EpiSCs but with different properties can be obtained. These signalling pathways are all at work in vivo to pattern the pluripotent epiblast and specify cellular fates.

TGF-β signaling pathway in early mouse development and embryonic stem cells

TGF-β superfamily signaling pathways essentially contribute to the broad spectrum of early developmental events including embryonic patterning, cell fate determination and dynamic movements. In this review, we first introduced some key developmental processes that require TGF-β signaling to show the fundamental importance of these pathways. Then we discuss how their activities are regulated, and new findings about how the TGF-β superfamily ligands bind to the chromatin to regulate transcription during embryo development.

Climbing the mountain: experimental design for the efficient optimization of stem cell bioprocessing

"To consult the statistician after an experiment is finished is often merely to ask him to conduct a post mortem examination. He can perhaps say what the experiment died of." - R.A. Fisher While this idea is relevant across research scales, its importance becomes critical when dealing with the inherently large, complex and expensive process of preparing material for cell-based therapies (CBTs). Effective and economically viable CBTs will depend on the establishment of optimized protocols for the production of the necessary cell types. Our ability to do this will depend in turn on the capacity to efficiently search through a multi-dimensional problem space of possible protocols in a timely and cost-effective manner. In this review we discuss approaches to, and illustrate examples of the application of statistical design of experiments to stem cell bioprocess optimization.

The many faces of Pluripotency: in vitro adaptations of a continuum of in vivo states

Pluripotency defines the propensity of a cell to differentiate into, and generate, all somatic, as well as germ cells. The epiblast of the early mammalian embryo is the founder population of all germ layer derivatives and thus represents the bona fide in vivo pluripotent cell population. The so-called pluripotent state spans several days of development and is lost during gastrulation as epiblast cells make fate decisions towards a mesoderm, endoderm or ectoderm identity. It is now widely recognized that the features of the pluripotent population evolve as development proceeds from the pre- to post-implantation period, marked by distinct transcriptional and epigenetic signatures. During this period of time epiblast cells mature through a continuum of pluripotent states with unique properties. Aspects of this pluripotent continuum can be captured in vitro in the form of stable pluripotent stem cell types. In this review we discuss the continuum of pluripotency existing within the mammalian embryo, using the mouse as a model, and the cognate stem cell types that can be derived and propagated in vitro. Furthermore, we speculate on embryonic stage-specific characteristics that could be utilized to identify novel, developmentally relevant, pluripotent states.

Spatial Transcriptome for the Molecular Annotation of Lineage Fates and Cell Identity in Mid-gastrula Mouse Embryo

Gastrulation of the mouse embryo entails progressive restriction of lineage potency and the organization of the lineage progenitors into a body plan. Here we performed a high-resolution RNA sequencing analysis on single mid-gastrulation mouse embryos to collate a spatial transcriptome that correlated with the regionalization of cell fates in the embryo. 3D rendition of the quantitative data enabled the visualization of the spatial pattern of all expressing genes in the epiblast in a digital whole-mount in situ format. The dataset also identified genes that (1) are co-expressed in a specific cell population, (2) display similar global pattern of expression, (3) have lineage markers, (4) mark domains of transcriptional and signaling activity associated with cell fates, and (5) can be used as zip codes for mapping the position of single cells isolated from the mid-gastrula stage embryo and the embryo-derived stem cells to the equivalent epiblast cells for delineating their prospective cell fates.