Control of early anterior-posterior patterning in the mouse embryo by TGF-beta signalling

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.

Deciphering epiblast lumenogenesis reveals proamniotic cavity control of embryo growth and patterning

During the peri-implantation stages, the mouse embryo radically changes its appearance, transforming from a hollow-shaped blastocyst to an egg cylinder. At the same time, the epiblast gets reorganized from a simple ball of cells to a cup-shaped epithelial monolayer enclosing the proamniotic cavity. However, the cavity's function and mechanism of formation have so far been obscure. Through investigating the cavity formation, we found that in the epiblast, the process of lumenogenesis is driven by reorganization of intercellular adhesion, vectoral fluid transport, and mitotic paracellular water influx from the blastocoel into the emerging proamniotic cavity. By experimentally blocking lumenogenesis, we found that the proamniotic cavity functions as a hub for communication between the early lineages, enabling proper growth and patterning of the postimplantation embryo.

Deletion of the Dishevelled family of genes disrupts anterior-posterior axis specification and selectively prevents mesoderm differentiation

The Dishevelled proteins transduce both canonical Wnt/β-catenin and non-canonical Wnt/planar cell polarity (PCP) signaling pathways to regulate many key developmental processes during embryogenesis. Here, we disrupt both canonical and non-canonical Wnt pathways by targeting the entire Dishevelled family of genes (Dvl1, Dvl2, and Dvl3) to investigate their functional roles in the early embryo. We identified several defects in anterior-posterior axis specification and mesoderm patterning in Dvl1+/-; Dvl2-/-; Dvl3-/- embryos. Homozygous deletions in all three Dvl genes (Dvl TKO) resulted in defects in distal visceral endoderm migration and a complete failure to induce mesoderm formation. To identify potential mechanisms that lead to the defects in the developmental processes preceding gastrulation, we generated Dvl TKO mouse embryonic stem cells (mESCs) and compared the transcriptional profile of these cells with wild-type (WT) mESCs during germ lineage differentiation into 3D embryoid bodies (EBs). While the Dvl TKO mESCs displayed similar morphology, self-renewal properties, and minor transcriptional variation from WT mESCs, we identified major transcriptional dysregulation in the Dvl TKO EBs during differentiation in a number of genes involved in anterior-posterior pattern specification, gastrulation induction, mesenchyme morphogenesis, and mesoderm-derived tissue development. The absence of the Dvls leads to specific down-regulation of BMP signaling genes. Furthermore, exogenous activation of canonical Wnt, BMP, and Nodal signaling all fail to rescue the mesodermal defects in the Dvl TKO EBs. Moreover, endoderm differentiation was promoted in the absence of mesoderm in the Dvl TKO EBs, while the suppression of ectoderm differentiation was delayed. Overall, we demonstrate that the Dvls are dispensable for maintaining self-renewal in mESCs but are critical during differentiation to regulate key developmental signaling pathways to promote proper axis specification and mesoderm formation.

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.

Dynamics of Wnt activity on the acquisition of ectoderm potency in epiblast stem cells

During embryogenesis, the stringent regulation of Wnt activity is crucial for the morphogenesis of the head and brain. The loss of function of the Wnt inhibitor Dkk1 results in elevated Wnt activity, loss of ectoderm lineage attributes from the anterior epiblast, and the posteriorisation of anterior germ layer tissue towards the mesendoderm. The modulation of Wnt signalling may therefore be crucial for the allocation of epiblast cells to ectoderm progenitors during gastrulation. To test this hypothesis, we examined the lineage characteristics of epiblast stem cells (EpiSCs) that were derived and maintained under different signalling conditions. We showed that suppression of Wnt activity enhanced the ectoderm propensity of the EpiSCs. Neuroectoderm differentiation of these EpiSCs was further empowered by the robust re-activation of Wnt activity. Therefore, during gastrulation, the tuning of the signalling activities that mediate mesendoderm differentiation is instrumental for the acquisition of ectoderm potency in the epiblast.

Implantation initiation of self-assembled embryo-like structures generated using three types of mouse blastocyst-derived stem cells

Spatially ordered embryo-like structures self-assembled from blastocyst-derived stem cells can be generated to mimic embryogenesis in vitro. However, the assembly system and developmental potential of such structures needs to be further studied. Here, we devise a nonadherent-suspension-shaking system to generate self-assembled embryo-like structures (ETX-embryoids) using mouse embryonic, trophoblast and extra-embryonic endoderm stem cells. When cultured together, the three cell types aggregate and sort into lineage-specific compartments. Signaling among these compartments results in molecular and morphogenic events that closely mimic those observed in wild-type embryos. These ETX-embryoids exhibit lumenogenesis, asymmetric patterns of gene expression for markers of mesoderm and primordial germ cell precursors, and formation of anterior visceral endoderm-like tissues. After transplantation into the pseudopregnant mouse uterus, ETX-embryoids efficiently initiate implantation and trigger the formation of decidual tissues. The ability of the three cell types to self-assemble into an embryo-like structure in vitro provides a powerful model system for studying embryogenesis.

Effects of Activin in Embryoid Bodies Expressing Fibroblast Growth Factor 5

Nodal/activin signaling is indispensable for embryonic development. We examined what activin does to the embryoid bodies (EBs) produced from mouse embryonic stem cells (mESCs) expressing an epiblast marker. The EBs were produced by culturing mESCs by the hanging drop method for 24 hours. The resulting EBs were transferred onto gelatin-coated dishes and allowed to further differentiate. The 24-hour EBs showed a stronger expression of fibroblast growth factor (FGF)5 and Brachyury (specific to the epiblast) in comparison with mESCs. Treating the transferred EBs with activin A maintained transcript levels of FGF5 and Oct4, while inhibiting definitive endoderm differentiation. The activin A treatment reversed the endoderm differentiation induced by retinoic acid (RA), while the inhibition of nodal/activin signaling promoted RA-induced endoderm differentiation. Inhibition of nodal/activin signaling in EBs, including epiblast-like cells, promotes differentiation into the endoderm, facilitating the transition from the pluripotent state to specification of the endoderm.

Single-cell analysis reveals lineage segregation in early post-implantation mouse embryos

The mammalian post-implantation embryo has been extensively investigated at the tissue level. However, to unravel the molecular basis for the cell-fate plasticity and determination, it is essential to study the characteristics of individual cells. In particular, the individual definitive endoderm (DE) cells have not been characterized in vivo Here, we report gene expression patterns in single cells freshly isolated from mouse embryos on days 5.5 and 6.5. Initial transcriptome data from 124 single cells yielded signature genes for the epiblast, visceral endoderm, and extra-embryonic ectoderm and revealed a unique distribution pattern of fibroblast growth factor (FGF) ligands and receptors. Further analysis indicated that early-stage epiblast cells do not segregate into lineages of the major germ layers. Instead, some cells began to diverge from epiblast cells, displaying molecular features of the premesendoderm by expressing higher levels of mesendoderm markers and lower levels of Sox3 transcripts. Analysis of single-cell high-throughput quantitative RT-PCR data from 441 cells identified a late stage of the day 6.5 embryo in which mesoderm and DE cells emerge, with many of them coexpressing Oct4 and Gata6 Analysis of single-cell RNA-sequence data from 112 cells of the late-stage day 6.5 embryos revealed differentially expressed signaling genes and networks of transcription factors that might underlie the segregation of the mesoderm and DE lineages. Moreover, we discovered a subpopulation of mesoderm cells that possess molecular features of the extraembryonic mesoderm. This study provides fundamental insight into the molecular basis for lineage segregation in post-implantation mouse embryos.

Keeping a lid on nodal: transcriptional and translational repression of nodal signalling

Nodal is an evolutionarily conserved member of the transforming growth factor-β (TGF-β) superfamily of secreted signalling factors. Nodal factors are known to play key roles in embryonic development and asymmetry in a variety of organisms ranging from hydra and sea urchins to fish, mice and humans. In addition to embryonic patterning, Nodal signalling is required for maintenance of human embryonic stem cell pluripotency and mis-regulated Nodal signalling has been found associated with tumour metastases. Therefore, precise and timely regulation of this pathway is essential. Here, we discuss recent evidence from sea urchins, frogs, fish, mice and humans that show a role for transcriptional and translational repression of Nodal signalling during early development.

Genetic and Epigenetic Regulation of Human Cardiac Reprogramming and Differentiation in Regenerative Medicine

Regeneration or replacement of lost cardiomyocytes within the heart has the potential to revolutionize cardiovascular medicine. Numerous methodologies have been used to achieve this aim, including the engraftment of bone marrow- and heart-derived cells as well as the identification of modulators of adult cardiomyocyte proliferation. Recently, the conversion of human somatic cells into induced pluripotent stem cells and induced cardiomyocyte-like cells has transformed potential approaches toward this goal, and the engraftment of cardiac progenitors derived from human embryonic stem cells into patients is now feasible. Here we review recent advances in our understanding of the genetic and epigenetic control of human cardiogenesis, cardiac differentiation, and the induced reprogramming of somatic cells to cardiomyocytes. We also cover genetic programs for inducing the proliferation of endogenous cardiomyocytes and discuss the genetic state of cells used in cardiac regenerative medicine.

Defining the three cell lineages of the human blastocyst by single-cell RNA-seq

Here, we provide fundamental insights into early human development by single-cell RNA-sequencing of human and mouse preimplantation embryos. We elucidate conserved transcriptional programs along with those that are human specific. Importantly, we validate our RNA-sequencing findings at the protein level, which further reveals differences in human and mouse embryo gene expression. For example, we identify several genes exclusively expressed in the human pluripotent epiblast, including the transcription factor KLF17. Key components of the TGF-β signalling pathway, including NODAL, GDF3, TGFBR1/ALK5, LEFTY1, SMAD2, SMAD4 and TDGF1, are also enriched in the human epiblast. Intriguingly, inhibition of TGF-β signalling abrogates NANOG expression in human epiblast cells, consistent with a requirement for this pathway in pluripotency. Although the key trophectoderm factors Id2, Elf5 and Eomes are exclusively localized to this lineage in the mouse, the human orthologues are either absent or expressed in alternative lineages. Importantly, we also identify genes with conserved expression dynamics, including Foxa2/FOXA2, which we show is restricted to the primitive endoderm in both human and mouse embryos. Comparison of the human epiblast to existing embryonic stem cells (hESCs) reveals conservation of pluripotency but also additional pathways more enriched in hESCs. Our analysis highlights significant differences in human preimplantation development compared with mouse and provides a molecular blueprint to understand human embryogenesis and its relationship to stem cells.

Summary: Single-cell RNA-sequencing of human and mouse embryos reveals conserved and human-specific transcriptional programmes as well as a functional requirement for TGFβ signalling in human embryos.

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

Mouse epiblast stem cells (EpiSCs) display temporal differences in the upregulation of Mixl1 expression during the initial steps of in vitro differentiation, which can be correlated with their propensity for endoderm differentiation. EpiSCs that upregulated Mixl1 rapidly during differentiation responded robustly to both Activin A and Nodal in generating foregut endoderm and precursors of pancreatic and hepatic tissues. By contrast, EpiSCs that delayed Mixl1 upregulation responded less effectively to Nodal and showed an overall suboptimal outcome of directed differentiation. The enhancement in endoderm potency in Mixl1-early cells may be accounted for by a rapid exit from the progenitor state and the efficient response to the induction of differentiation by Nodal. EpiSCs that readily differentiate into the endoderm cells are marked by a distinctive expression fingerprint of transforming growth factor (TGF)-β signalling pathway genes and genes related to the endoderm lineage. Nodal appears to elicit responses that are associated with transition to a mesenchymal phenotype, whereas Activin A promotes gene expression associated with maintenance of an epithelial phenotype. We postulate that the formation of definitive endoderm (DE) in embryoid bodies follows a similar process to germ layer formation from the epiblast, requiring an initial de-epithelialization event and subsequent re-epithelialization. Our results show that priming EpiSCs with the appropriate form of TGF-β signalling at the formative phase of endoderm differentiation impacts on the further progression into mature DE-derived lineages, and that this is influenced by the initial characteristics of the cell population. Our study also highlights that Activin A, which is commonly used as an in vitro surrogate for Nodal in differentiation protocols, does not elicit the same downstream effects as Nodal, and therefore may not effectively mimic events that take place in the mouse embryo.

Extra-embryonic Wnt3 regulates the establishment of the primitive streak in mice

The establishment of the head to tail axis at early stages of development is a fundamental aspect of vertebrate embryogenesis. In mice, experimental embryology, genetics and expression studies have suggested that the visceral endoderm, an extra-embryonic tissue, plays an important role in anteroposterior axial development. Here we show that absence of Wnt3 in the posterior visceral endoderm leads to delayed formation of the primitive streak and that interplay between anterior and posterior visceral endoderm restricts the position of the primitive streak. Embryos lacking Wnt3 in the visceral endoderm, however, appear normal by E9.5. Our results suggest a model for axial development in which multiple signals are required for anteroposterior axial development in mammals.

Tgf-Beta family signaling in embryonic stem cells

Ligands of transforming growth factor beta (TGF-β) family members have been implicated in the development and patho-physiological process of various organs. Embryonic stem cells (ESCs) are characterized by their ability to proliferate indefinitely and differentiated into all three germ layer cells, which are termed as pluripotency and self-renewal,respectively. For successful therapeutic application of ESCs, it is essential to understand the mechanisms underlying self-renewal and pluripotency, which involve complex networks among key factors including transcription factors, epigenetic control, microRNAs and signaling pathways. In this review, we discuss recent progress on the function of TGF beta family ligands and their canonical SMAD signaling in the maintenance of ESC' s identity.

Generation of β cells from human pluripotent stem cells: are we there yet?

In 1998, the landmark paper describing the isolation and culture of human embryonic stem cells (ESCs) was published. Since that time, the main goal of many diabetes researchers has been to derive β cells from ESCs as a renewable cell-based therapy for the treatment of patients with diabetes. In working toward this goal, numerous protocols that attempt to recapitulate normal pancreatic development have been published that result in the formation of pancreatic cell types from human pluripotent cells. This review examines stem cell differentiation methods and places them within the context of pancreatic development. We additionally compare strategies that are currently being used to generate pancreatic cell types and contrast them with approaches that have been used to generate functional cell types in different lineages. In doing this, we aim to identify how new approaches might be used to improve yield and functionality of in vitro-derived pancreatic β cells as an eventual cell-based therapy for type 1 diabetes.

The road to regenerative liver therapies: the triumphs, trials and tribulations

The liver is one of the few organs that possess a high capacity to regenerate after liver failure or liver damage. The parenchymal cells of the liver, hepatocytes, contribute to the majority of the regeneration process. Thus, hepatocyte transplantation presents an alternative method to treating liver damage. However, shortage of hepatocytes and difficulties in maintaining primary hepatocytes still remain key obstacles that researchers must overcome before hepatocyte transplantation can be used in clinical practice. The unique properties of pluripotent stem cells (PSCs) and induced pluripotent stem cells (iPSCs) have provided an alternative approach to generating enough functional hepatocytes for cellular therapy. In this review, we will present a brief overview on the current state of hepatocyte differentiation from PSCs and iPSCs. Studies of liver regenerative processes using different cell sources (adult liver stem cells, hepatoblasts, hepatic progenitor cells, etc.) will be described in detail as well as how this knowledge can be applied towards optimizing culture conditions for the maintenance and differentiation of these cells towards hepatocytes. As the outlook of stem cell-derived therapy begins to look more plausible, researchers will need to address the challenges we must overcome in order to translate stem cell research to clinical applications.

An essential role for maternal control of Nodal signaling

Growth factor signaling is essential for pattern formation, growth, differentiation, and maintenance of stem cell pluripotency. Nodal-related signaling factors are required for axis formation and germ layer specification from sea urchins to mammals. Maternal transcripts of the zebrafish Nodal factor, Squint (Sqt), are localized to future embryonic dorsal. The mechanisms by which maternal sqt/nodal RNA is localized and regulated have been unclear. Here, we show that maternal control of Nodal signaling via the conserved Y box-binding protein 1 (Ybx1) is essential. We identified Ybx1 via a proteomic screen. Ybx1 recognizes the 3’ untranslated region (UTR) of sqt RNA and prevents premature translation and Sqt/Nodal signaling. Maternal-effect mutations in zebrafish ybx1 lead to deregulated Nodal signaling, gastrulation failure, and embryonic lethality. Implanted Nodal-coated beads phenocopy ybx1 mutant defects. Thus, Ybx1 prevents ectopic Nodal activity, revealing a new paradigm in the regulation of Nodal signaling, which is likely to be conserved.

DOI:http://dx.doi.org/10.7554/eLife.00683.001

In many organisms, embryonic development is controlled in part by RNAs that are deposited into the egg as it forms inside the mother. These ‘maternal RNAs’ may localize to particular regions of the egg or embryo, where they are then exclusively translated into protein and carry out their specific function. This helps to establish asymmetry in the developing organism—that is, to produce tissues that will eventually become the top or bottom, front or back, and left or right of the organism.

One such maternal RNA encodes Nodal, a key signaling molecule that is conserved across vertebrate and some invertebrate organisms. In zebrafish, the equivalent RNA is called squint, and plays an important role in embryonic development. The squint RNA deposited by the mother localizes to the dorsal region—the embryo’s back—and signals that region to make dorsal tissues, but how squint is regulated is not well understood. Now, Kumari et al. identify a protein that controls the positioning of squint RNA, and find that it can also prevent this RNA from being translated into protein.

The squint RNA contains a ‘dorsal localization element’ that recruits it to the dorsal cells of the embryo by the 4-cell stage (i.e., within two cell divisions after the egg is fertilized). Kumari et al. identified a protein called Ybx1 that could bind to this element: this protein may help to correctly position RNAs in many other organisms, including fruit flies and mammals. Strikingly, embryos formed abnormally when their maternally derived Ybx1 protein was mutant, and these mutations also prevented the squint RNA from localizing properly. This suggests that maternally derived Ybx1 protein directly regulates the squint RNA.

As well as positioning the squint RNA correctly, the embryo must translate this RNA into protein at the right time. In embryos with mutant maternal Ybx1 protein, the Squint protein could be detected at the 16-cell stage, whereas in wild-type embryos this protein is not translated until the 256-cell stage; this indicates that Ybx1 protein might normally repress the translation of the squint RNA. Indeed, Kumari et al. found that Ybx1 binds to another protein—eIF4E—that recruits mRNAs to the ribosome (the cell’s translational machinery). Ybx1 might therefore prevent eIF4E from associating with other components of the ribosomal complex, and initiating the translation of the squint RNA, until additional signals have been received. It will be interesting to determine how widespread this regulatory mechanism is in other organisms.

DOI:http://dx.doi.org/10.7554/eLife.00683.002

Early patterning in a chondrichthyan model, the small spotted dogfish: towards the gnathostome ancestral state

In the past few years, the small spotted dogfish has become the primary model for analyses of early development in chondrichthyans. Its phylogenetic position makes it an ideal outgroup to reconstruct the ancestral gnathostome state by comparisons with established vertebrate model organisms. It is also a suitable model to address the molecular bases of lineage-specific diversifications such as the rise of extraembryonic tissues, as it is endowed with a distinct extraembryonic yolk sac and yolk duct ensuring exchanges between the embryo and a large undivided vitelline mass. Experimental or functional approaches such as cell marking or in ovo pharmacological treatments are emerging in this species, but recent analyses of early development in this species have primarily concentrated on molecular descriptions. These data show the dogfish embryo exhibits early polarities reflecting the dorso-ventral axis of amphibians and teleosts at early blastula stages and an atypical anamniote molecular pattern during gastrulation, independently of the presence of extraembryonic tissues. They also highlight unexpected relationships with amniotes, with a strikingly similar Nodal-dependent regional pattern in the extraembryonic endoderm. In this species, extraembryonic cell fates seem to be determined by differential cell behaviors, which lead to cell allocation in extraembryonic and embryonic tissues, rather than by cell regional identity. We suggest that this may exemplify an early evolutionary step in the rise of extraembryonic tissues, possibly related to quantitative differences in the signaling activities, which shape the early embryo. These results highlight the conservation across gnathostomes of a highly constrained core genetic program controlling early patterning. This conservation may be obscured in some lineages by taxa-specific diversifications such as specializations of extraembryonic nutritive tissues.

Yin-Yang1 is required for epithelial-to-mesenchymal transition and regulation of Nodal signaling during mammalian gastrulation

The ubiquitously expressed Polycomb Group protein Yin-Yang1 (YY1) is believed to regulate gene expression through direct binding to DNA elements found in promoters or enhancers of target loci. Additionally, YY1 contains diverse domains that enable a plethora of protein-protein interactions, including association with the Oct4/Sox2 pluripotency complex and Polycomb Group silencing complexes. To elucidate the in vivo role of YY1 during gastrulation, we generated embryos with an epiblast specific deletion of Yy1. Yy1 conditional knockout (cKO) embryos initiate gastrulation, but both primitive streak formation and ingression through the streak is severely impaired. These streak descendants fail to repress E-Cadherin and are unable to undergo an appropriate epithelial to mesenchymal transition (EMT). Intriguingly, overexpression of Nodal and concomitant reduction of Lefty2 are observed in Yy1 cKO embryos, suggesting that YY1 is normally required for proper Nodal regulation during gastrulation. Furthermore, definitive endoderm is specified but fails to properly integrate into the outer layer. Although anterior neuroectoderm is specified, mesoderm production is severely restricted. We show that YY1 directly binds to the Lefty2 locus in E7.5 embryos and that pharmacological inhibition of Nodal signaling partially restores mesoderm production in Yy1 cKO mutant embryos. Our results reveal critical requirements for YY1 during several important developmental processes, including EMT and regulation of Nodal signaling. These results are the first to elucidate the diverse role of YY1 during gastrulation in vivo.

Wnt signaling in mammalian development: lessons from mouse genetics

Wnts are evolutionarily conserved signaling ligands critical for animal development. Genetic engineering in the mouse has enabled investigators to acquire a detailed activation profile of the β-catenin-dependent canonical Wnt pathway during mouse development, and to manipulate Wnt pathway activities with great spatial and temporal precision. Together, these studies have not only revealed important functions of Wnt signaling at multiple stages of early mouse development, but also elucidated how the Wnt pathway interacts with other pathways to form signaling networks that confer the unique features of mammalian embryogenesis. Additionally, the planar cell polarity pathway has emerged as an essential β-catenin independent noncanonical Wnt pathway that coordinates cell polarity and regulates tissue morphogenesis in various mammalian developmental processes. Importantly, studies of Wnt signaling in mouse development have also revealed important pathogenic mechanisms of several congenital disorders in humans.

Dynamics of anterior-posterior axis formation in the developing mouse embryo

The development of an anterior-posterior (AP) polarity is a crucial process that in the mouse has been very difficult to analyse, because it takes place as the embryo implants within the mother. To overcome this obstacle, we have established an in-vitro culture system that allows us to follow the step-wise development of anterior visceral endoderm (AVE), critical for establishing AP polarity. Here we use this system to show that the AVE originates in the implanting blastocyst, but that additional cells subsequently acquire AVE characteristics. These 'older' and 'younger' AVE domains coalesce as the egg cylinder emerges from the blastocyst structure. Importantly, we show that AVE migration is led by cells expressing the highest levels of AVE marker, highlighting that asymmetry within the AVE domain dictates the direction of its migration. Ablation of such leading cells prevents AVE migration, suggesting that these cells are important for correct establishment of the AP axis.

[Wnt signaling pathway and the Evo-Devo of deuterostome axis]

A series of signal transduction pathways have been found to regulate the polarity establishment and formation of animal primary body axis. Among them, Wnt signaling pathway is extremely conserved and several key components in the pathway have been identified in the demosponge lineage. This implies that it is one of the earliest pathways involved in the ancestral metazoan axis development and might play an important role in specification and development of posterior and ventral fate of animal axis. Recently, with the establishment of functional experiments in vitro, the body plan formation has been found to be affected, in varying degrees, by many genes in the Wnt signaling pathway, such as members of wnt gene family, maternal gene beta-catenin and some transcription factor encoding genes. In this review, we analyzed the evolutionary origin of the wnt gene family involved in development of metazoan body plans, and then made a brief review on the roles of canonical Wnt/beta-catenin signaling in the polarity establishment and formation of primary body axis in diverse deuterostomes including sea urchin, amphioxus, zebrafish, frog, and mouse.

Differentiation of an embryonic stem cell to hemogenic endothelium by defined factors: essential role of bone morphogenetic protein 4

Current approaches to differentiate embryonic stem (ES) cells to hematopoietic precursors in vitro use either feeder cell, serum, conditioned culture medium or embryoid body, methods that cannot avoid undefined culture conditions, precluding analysis of the fate of individual cells. Here, we have developed a defined, serum-free and low cell-density differentiation program to generate endothelial and hematopoietic cells within 6 days from murine ES cells. Our novel approach identifies a set of factors that are necessary and sufficient to differentiate ES cells into definitive hematopoietic precursors, as documented by the time-lapse video microscopy of the stepwise differentiation processes from single progenitors. Moreover, this defined milieu revealed the essential role of bone morphogenetic protein 4 (BMP4) in determining the hematopoietic/endothelial fate and demonstrated that the hemogenic fate in mesoderm is determined as early as day 4 of our differentiation protocol. Our ability to directly convert ES cells to endothelial and hematopoietic precursors should have important utilities for studies of hematopoietic development and personalized medicine in the future.

Vertebrate intestinal endoderm development

The endoderm gives rise to the lining of the esophagus, stomach and intestines, as well as associated organs. To generate a functional intestine, a series of highly orchestrated developmental processes must occur. In this review, we attempt to cover major events during intestinal development from gastrulation to birth, including endoderm formation, gut tube growth and patterning, intestinal morphogenesis, epithelial reorganization, villus emergence, as well as proliferation and cytodifferentiation. Our discussion includes morphological and anatomical changes during intestinal development as well as molecular mechanisms regulating these processes.

Activin/nodal signaling and pluripotency

Maintenance of a pluripotent cell population during mammalian embryogenesis is crucial for the proper generation of extraembryonic and embryonic tissues to ensure intrauterine survival and fetal development. Pluripotent stem cells derived from early stage mammalian embryos are known as "embryonic stem cells." Such embryo-derived stem cells can proliferate indefinitely in vitro and give rise to derivatives of all three primary germ layers. Their potential for clinical and commercial applications has sparked great excitement within scientific and lay communities. Identification of the signaling pathways controlling stem cell pluripotency and differentiation provides knowledge-based approaches to manipulate stem cells for regenerative medicine. One of the signaling cascades that has been identified in the control of stem cell pluripotency and differentiation is the Activin/Nodal pathway. Here, we describe the differences among pluripotent cell types and discuss the latest findings on the molecular mechanisms involving Activin/Nodal signaling in controlling their pluripotency and differentiation.

Human embryonic and rat adult stem cells with primitive endoderm-like phenotype can be fated to definitive endoderm, and finally hepatocyte-like cells

Stem cell-derived hepatocytes may be an alternative cell source to treat liver diseases or to be used for pharmacological purposes. We developed a protocol that mimics mammalian liver development, to differentiate cells with pluripotent characteristics to hepatocyte-like cells. The protocol supports the stepwise differentiation of human embryonic stem cells (ESC) to cells with characteristics of primitive streak (PS)/mesendoderm (ME)/definitive endoderm (DE), hepatoblasts, and finally cells with phenotypic and functional characteristics of hepatocytes. Remarkably, the same protocol can also differentiate rat multipotent adult progenitor cells (rMAPCs) to hepatocyte-like cells, even though rMAPC are isolated clonally from cultured rat bone marrow (BM) and have characteristics of primitive endoderm cells. A fraction of rMAPCs can be fated to cells expressing genes consistent with a PS/ME/DE phenotype, preceding the acquisition of phenotypic and functional characteristics of hepatocytes. Although the hepatocyte-like progeny derived from both cell types is mixed, between 10-20% of cells are developmentally consistent with late fetal hepatocytes that have attained synthetic, storage and detoxifying functions near those of adult hepatocytes. This differentiation protocol will be useful for generating hepatocyte-like cells from rodent and human stem cells, and to gain insight into the early stages of liver development.

Transforming Growth Factor type beta and Smad family signaling in stem cell function

Ligands of the Transforming Growth Factor type beta (TGFbeta) family exert multiple and sometimes opposite effects on most cell types in vivo depending on cellular context, which mainly includes the stage of the target cell, the local environment of this cell or niche, and the identity and the dosage of the ligand. Significant progress has been made in the molecular dissection of the regulation of the activity of the ligands and their intracellular signal transduction pathways, including via the canonical Smad pathway where Smads interact with many transcription factors. This knowledge together with results from functional studies within the embryology and stem cell research fields is giving us insight in the role of individual ligands and other components of this signaling system and where and how it regulates many properties of embryonic and adult stem/progenitor cells, which is anticipated to contribute to successful cell-based therapy in the future. We review and discuss recent progress on the effects of Nodal/Activin and Bone Morphogenetic Proteins (BMPs) and their canonical signaling in cells with stem cell properties. We focus on embryonic stem cells and their maintenance and pluripotency, and conversion into selected cell types of neuroectoderm, mesoderm and endoderm, on induced pluripotent cells and on neurogenic cells in the adult brain.

Roles of TGF-beta family signaling in stem cell renewal and differentiation

Transforming growth factor (TGF)-betas and their family members, including bone morphogenetic proteins (BMPs), Nodal and activins, have been implicated in the development and maintenance of various organs, in which stem cells play important roles. Stem cells are characterized by their ability to self-renew and to generate differentiated cells of a particular tissue, and are classified into embryonic and somatic stem cells. Embryonic stem (ES) cells self-renew indefinitely and contribute to derivatives of all three primary germ layers. In contrast, somatic stem cells, which can be identified in various adult organs, exhibit limited abilities for self-renewal and differentiation in most cases. The multi-lineage differentiation capacity of ES cells and somatic stem cells has opened possibilities for cell replacement therapies for genetic, malignant and degenerative diseases. In order to utilize stem cells for therapeutic applications, it is essential to understand the extrinsic and intrinsic factors regulating self-renewal and differentiation of stem cells. More recently, induced pluripotent stem (iPS) cells have been generated from mouse and human fibroblasts that resemble ES cells via ectopic expression of four transcription factors. iPS cells may have an advantage in regenerative medicine, since they overcome the immunogenicity and ethical controversy of ES cells. Moreover, recent studies have highlighted the involvement of cancer stem cells during the formation and progression of various types of cancers, including leukemia, glioma, and breast cancer. Here, we illustrate the roles of TGF-beta family members in the maintenance and differentiation of ES cells, somatic stem cells, and cancer stem cells.

Why the embryo still matters: CSF and the neuroepithelium as interdependent regulators of embryonic brain growth, morphogenesis and histiogenesis

The key focus of this review is that both the neuroepithelium and embryonic cerebrospinal fluid (CSF) work in an integrated way to promote embryonic brain growth, morphogenesis and histiogenesis. The CSF generates pressure and also contains many biologically powerful trophic factors; both play key roles in early brain development. Accumulation of fluid via an osmotic gradient creates pressure that promotes rapid expansion of the early brain in a developmental regulated way, since the rates of growth differ between the vesicles and for different species. The neuroepithelium and ventricles both contribute to this growth but by different and coordinated mechanisms. The neuroepithelium grows primarily by cell proliferation and at the same time the ventricle expands via hydrostatic pressure generated by active transport of Na(+) and transport or secretion of proteins and proteoglycans that create an osmotic gradient which contribute to the accumulation of fluid inside the sealed brain cavity. Recent evidence shows that the CSF regulates relevant aspects of neuroepithelial behavior such as cell survival, replication and neurogenesis by means of growth factors and morphogens. Here we try to highlight that early brain development requires the coordinated interplay of the CSF contained in the brain cavity with the surrounding neuroepithelium. The information presented is essential in order to understand the earliest phases of brain development and also how neuronal precursor behavior is regulated.

Sall4 isoforms act during proximal-distal and anterior-posterior axis formation in the mouse embryo

Reciprocal signals from embryonic and extra-embryonic tissues pattern the embryo in proximal-distal (PD) and anterior-posterior (AP) fashion. Here we have analyzed three gene trap mutations of Sall4, of which one (Sall4-1a) led to a hypomorphic and recessive phenotype, demonstrating that Sall4-1a has yet undescribed extra-embryonic and embryonic functions in regulating PD and AP axis formation. In Sall4-1a mutants the self-maintaining autoregulatory interaction between Bmp4, Nodal and Wnt, which determines the PD axis was disrupted because of defects in the extra-embryonic visceral endoderm. More severely, two distinct Sall4 gene-trap mutants (Sall4-1a,b), resembling null mutants, failed to initiate Bmp4 expression in the extra-embryonic ectoderm and Nodal in the epiblast and were therefore unable to initiate PD axis formation. Tetraploid rescue underlined the extra-embryonic nature of the Sall4-1a phenotype and revealed a further embryonic function in Wnt/beta-catenin signaling to elongate the AP axis during gastrulation. This observation was supported through genetic interaction with beta-catenin mutants, since compound heterozygous mutants recapitulated the defects of Wnt3a mutants in posterior development.

The anterior visceral endoderm of the mouse embryo is established from both preimplantation precursor cells and by de novo gene expression after implantation

Initiation of the development of the anterior-posterior axis in the mouse embryo has been thought to take place only when the anterior visceral endoderm (AVE) emerges and starts its asymmetric migration. However, expression of Lefty1, a marker of the AVE, was recently found to initiate before embryo implantation. This finding has raised two important questions: are the cells that show such early, preimplantation expression of this AVE marker the real precursors of the AVE and, if so, how does this contribute to the establishment of the AVE? Here, we address both of these questions. First, we show that the expression of another AVE marker, Cer1, also commences before implantation and its expression becomes consolidated in the subset of ICM cells that comprise the primitive endoderm. Second, to determine whether the cells showing this early Cer1 expression are true precursors of the AVE, we set up conditions to trace these cells in time-lapse studies from early periimplantation stages until the AVE emerges and becomes asymmetrically displaced. We found that Cer1-expressing cells are asymmetrically located after implantation and, as the embryo grows, they become dispersed into two or three clusters. The expression of Cer1 in the proximal domain is progressively diminished, whilst it is reinforced in the distal-lateral domain. Our time-lapse studies demonstrate that this distal-lateral domain is incorporated into the AVE together with cells in which Cer1 expression begins only after implantation. Thus, the AVE is formed from both part of an ancestral population of Cerl-expressing cells and cells that acquire Cer1 expression later. Finally, we demonstrate that when the AVE shifts asymmetrically to establish the anterior pole, this occurs towards the region where the earlier postimplantation expression of Cer1 was strongest. Together, these results suggest that the orientation of the anterior-posterior axis is already anticipated before AVE migration.

The DVE changes distal epiblast fate from definitive endoderm to neurectoderm by antagonizing nodal signaling

To assess the function of the distal visceral endoderm (DVE) of embryonic day 5.5 (E5.5) embryos, we established a system to directly ablate the DVE and observe the consequences after culture. When the DVE was successfully ablated, such embryos (DVE-ablated embryos) showed deregulated expression of Nodal and Wnt3 and ectopically formed the primitive streak at the proximal portion of the embryo. The DVE and anterior visceral endoderm (AVE) are implicated in the development of neurectoderm. We found that the distal epiblast of E5.5 embryo rotates anteriorly by the beginning of gastrulation. These cells remained to be anteriorly located during gastrulation and contributed to the ectoderm in the anterior side of the embryo. This indicates that the distal epiblast of E5.5 embryo becomes neurectoderm in normal development. In DVE-ablated embryos, the distal epiblast did not show any movement during culture and was abnormally fated to early definitive endoderm lineage. The data suggest that down-regulation of Nodal signaling in the distal epiblast of E5.5 embryo may be an initial step of neural development.

Regulated Nodal signaling promotes differentiation of the definitive endoderm and mesoderm from ES cells

Nodal signaling induces the formation of the endoderm and mesoderm during gastrulation. Nodal expression persists until the definitive endoderm progenitor has completely formed, and disappears thereafter. A tightly regulated Nodal expression system is essential for the differentiation of embryonic stem (ES) cells into distinct tissue lineages. On this basis, we established an ES cell differentiation system with the tetracycline-regulated expression of Nodal. The upregulated Nodal signaling pathway and its downstream transcriptional targets induced the specification of ES cells into definitive endoderm and mesoderm derivatives, and the subsequent downregulation of Nodal signaling promoted further maturation of the gut tube both in vitro and in vivo. Sustained expression of the Nodal gene inhibited the maturation of the definitive endoderm owing to persistent Oct3 and/or Oct4 expression and teratoma formation. Furthermore, quantitative single cell analysis by flow cytometry using CXCR4, VEGFR2 and PDGFR-α indicated that this protocol for definitive endoderm and mesoderm differentiation is superior to any other available protocol. Our findings also indicated that the Nodal or Nodal-related molecules secreted from Nodal-expressing ES cells could cause genetically unmanipulated ES cells to induce the expression of the Nodal signaling pathway and its downstream targets, which consequently leads to the differentiation of the ES cells into definitive endoderm and mesoderm. Our differentiation system, using tightly regulated Nodal expression, enabled us to investigate the mechanism of ES cell differentiation into definitive endoderm or mesoderm derivatives. Our findings also demonstrate that Nodal-expressing ES cells might be a source of highly active proteins that could be used for developing endoderm or mesoderm tissues in regenerative medicine.

Activin-Nodal signaling is involved in propagation of mouse embryonic stem cells

Embryonic stem (ES) cells are self-renewing cells that maintain pluripotency to differentiate into all types of cells. Because of their potential to provide a variety of tissues for use in regenerative medicine, there is great interest in the identification of growth factors that govern these unique properties of ES cells. However, the signaling pathways controlling ES cell proliferation remain largely unknown. Since transforming growth factor beta (TGFbeta) superfamily members have been implicated in the processes of early embryogenesis, we investigated their roles in ES cell self-renewal. Inhibition of activin-Nodal-TGFbeta signaling by Smad7 or SB-431542 dramatically decreased ES cell proliferation without decreasing ES pluripotency. By contrast, inhibition of bone morphogenetic protein (BMP) signaling by Smad6 did not exhibit such effects, suggesting that activin-Nodal-TGFbeta signaling, but not BMP signaling, is indispensable for ES cell propagation. In serum-free culture, supplementation of recombinant activin or Nodal, but not TGFbeta or BMP, significantly enhanced ES cell propagation without affecting pluripotency. We also found that activin-Nodal signaling was constitutively activated in an autocrine fashion in serum-free cultured ES cells, and that inhibition of such endogenous signaling by SB-431542 decreased ES cell propagation in serum-free conditions. These findings suggest that endogenously activated autocrine loops of activin-Nodal signaling promote ES cell self-renewal.

Gene expression pattern and progression of embryogenesis in the immediate post-implantation period of mouse development

During development of the mouse conceptus from implantation to the early gastrula stage, a multitude of genes encoding structural proteins, transcription factors and components of signalling pathways are expressed in the extraembryonic and embryonic tissues derived from the trophectoderm and the inner cell mass. Some genes are expressed widely in the extraembryonic ectoderm, the visceral endoderm or the epiblast, while others display more restricted expression domains in these tissues or are expressed upon the specification of the germ layers at gastrulation. Overall, the developmental changes in gene expression mirror key events of embryogenesis, and reveal the regionalization of signalling activity and the emergence of tissue patterning.

Human embryonic stem cells: an in vitro model to study mechanisms controlling pluripotency in early mammalian development

The property of pluripotency confers the capacity for differentiation into a large number of cell types including extra-embryonic, somatic and germinal cells. During normal development, pluripotency is acquired by the cells of the early embryo, which shortly thereafter undergo differentiation, whereas embryonic stem cells (ESCs) uniquely maintain pluripotency while undergoing extensive in vitro proliferation. Studies using ESCs have begun to unravel the network of cytokines and transcription factors responsible for their maintenance of pluripotency. Surprisingly, mouse and human ESCs display significant differences in such mechanisms despite their similar embryonic origins. In this review, we compare the properties of pluripotent embryonic cells with those of ESCs to establish a general model for the mechanisms maintaining pluripotency. We first consider whether mouse and human ESCs represent comparable stages of early embryonic development. We then describe how human embryoid body (EB) differentiation could be used as a model of embryonic development. Finally, to concretely illustrate the discussion, we discuss our recent results concerning Nodal function in controlling cell fate at early stages of human EB development. With the new perspective of these findings, we suggest a previously unrecognized role of TGF-beta pathway signaling in maintaining pluripotency at early stages of mammalian embryonic development.

Nodal activity around Kupffer's vesicle depends on the T-box transcription factors notail and spadetail and on notch signaling

The node, or its zebrafish equivalent, Kupffers Vesicle (KV), is thought to generate laterality cues through cilia-dependent signaling. An interaction between Nodal ligands and Nodal antagonists around the node/KV is also required. Here we investigate whether loss of Brachyury/Notail or Tbx16/Spadetail disrupts the balance of Nodal ligands (Southpaw) and antagonists (Charon) around Kupffers Vesicle. Reduction of Spadetail or Notail disrupts expression of southpaw in the perinodal domains flanking Kupffers Vesicle. Similar to what was published for Notail, we find Spadetail is also required for expression of charon. We present evidence for the model that Notail has a direct role in regulating the charon promoter. In particular, a flanking genomic region with putative Notail binding sites can drive KV expression of a reporter in a Notail-dependent fashion. This region also contains motifs for CSL/RBP-J/Su(H). Consistent with this, we find charon expression is strongly Notch-dependent whereas perinodal southpaw expression is not.

The nodal precursor acting via activin receptors induces mesoderm by maintaining a source of its convertases and BMP4

During early mouse development, the subtilisin-like proprotein convertases (SPC) Furin and PACE4 pattern the primitive ectoderm and visceral endoderm, presumably by activating the TGFss-related Nodal precursor. Here, mutation of the SPC motif provides direct evidence that Nodal processing is essential to specify anterior visceral endoderm and mesendoderm. Surprisingly, however, the Nodal precursor binds and activates activin receptors to maintain expression of Furin, PACE4, and Bmp4 in extraembryonic ectoderm at a distance from the Nodal source. In return, Bmp4 induces Wnt3, which amplifies Nodal expression in the epiblast and mediates induction of mesoderm. We conclude that uncleaved Nodal sustains the extraembryonic source of proprotein convertases and Bmp4 to amplify Nodal signaling in two nonredundant feedback loops with dual timescales and to localize primitive streak formation at the posterior pole. Based on mathematical modeling, we discuss how these sequential loops control cell fate.

Evolution of the mechanisms that establish the embryonic axes

A long-standing debate in developmental biology concerns the extent to which embryos are largely 'mosaic' (cell fates are allocated by localization of maternal determinants that are inherited differentially) or 'regulative' (cell interactions determine cell fates). Generally, it has been thought that amniotes, especially birds and mammals, are at the extreme regulative end of the spectrum, whereas most invertebrates, lower chordates and anamnia are more mosaic. Various studies have identified additional differences, including egg size, the timing of zygotic transcription and the speed of development. However, new research is starting to reveal among the vertebrate classes an astonishing degree of conservation in the intercellular signalling mechanisms that regulate cell fate and embryonic polarity before gastrulation.

Absence of Nodal signaling promotes precocious neural differentiation in the mouse embryo

After implantation, mouse embryos deficient for the activity of the transforming growth factor-beta member Nodal fail to form both the mesoderm and the definitive endoderm. They also fail to specify the anterior visceral endoderm, a specialized signaling center which has been shown to be required for the establishment of anterior identity in the epiblast. Our study reveals that Nodal-/- epiblast cells nevertheless express prematurely and ectopically molecular markers specific of anterior fate. Our analysis shows that neural specification occurs and regional identities characteristic of the forebrain are established precociously in the Nodal-/- mutant with a sequential progression equivalent to that of wild-type embryo. When explanted and cultured in vitro, Nodal-/- epiblast cells readily differentiate into neurons. Genes normally transcribed in organizer-derived tissues, such as Gsc and Foxa2, are also expressed in Nodal-/- epiblast. The analysis of Nodal-/-;Gsc-/- compound mutant embryos shows that Gsc activity plays no critical role in the acquisition of forebrain characters by Nodal-deficient cells. This study suggests that the initial steps of neural specification and forebrain development may take place well before gastrulation in the mouse and highlights a possible role for Nodal, at pregastrula stages, in the inhibition of anterior and neural fate determination.

Mechanisms of self-renewal in human embryonic stem cells

Embryonic stem cells (ESCs) are the pluripotent cell population derived from the inner cell mass of pre-implantation embryos and are characterised by prolonged self-renewal and the potential to differentiate into cells representing all three germ layers both in vitro and in vivo. Preservation of the undifferentiated status of the ESC population requires the maintenance of self-renewal whilst inhibiting differentiation and regulating senescence and apoptosis. In this review, we discuss the intrinsic and extrinsic factors associated with self-renewal process, together with possible signalling pathway interactions and mechanisms of regulation.

Regionalised signalling within the extraembryonic ectoderm regulates anterior visceral endoderm positioning in the mouse embryo

The development of the anterior-posterior (AP) axis in the mammalian embryo is controlled by interactions between embryonic and extraembryonic tissues. It is well established that one of these extraembryonic tissues, the anterior visceral endoderm (AVE), can repress posterior cell fate and that signalling from the other, the extraembryonic ectoderm (ExE), is required for posterior patterning. Here, we show that signals from the prospective posterior ExE repress AVE gene expression and affect the distribution of the AVE cells. Surgical ablation of the prospective posterior, but not the anterior, extraembryonic region at 5.5 days of development (E5.5) perturbs the characteristic distal-to-anterior distribution of AVE cells and leads to a dramatic expansion of the AVE domain. Time-lapse imaging studies show that this increase is due to the ectopic expression of an AVE marker, which results in a symmetrical positioning of the AVE. Surgical ablation of this same ExE region after the distal-to-anterior migration has already commenced, at E5.75, does not affect the localisation of the AVE, indicating that this effect takes place within a short time window. Conversely, transplanting the prospective posterior, but not the anterior, extraembryonic region onto isolated E5.5 embryonic explants drastically reduces the AVE domain. Further, transplantation experiments demonstrate that the signalling regulating AVE gene expression originates from the posterior ExE, rather than its surrounding VE. Together, our results show that signals emanating from the future posterior ExE within a temporal window both restrict the AVE domain and promote its specific positioning. This indicates for the first time that the ExE is already regionalised a day before the onset of gastrulation in order to correctly set the orientation of the AP axis of the mouse embryo. We propose a reciprocal function of the posterior ExE and the AVE in establishing a balance between the antagonistic activities of these two tissues, essential for AP patterning.

MaternalXenopus Zic2negatively regulatesNodal-relatedgene expression during anteroposterior patterning

During the development of Xenopus laevis, maternal mRNAs and proteins stored in the egg direct early patterning events such as the specification of the dorsoventral axis and primary germ layers. In an expression screen to identify maternal factors important for early development, we isolated a truncated cDNA for maternal Zic2(tZic2), encoding a zinc-finger transcription factor. The predicted tZic2 protein lacked the N-terminal region, but retained the zinc-finger domain. When expressed in embryos, tZic2 inhibited head and axial development,and blocked the ability of full-length Zic2 to induce neural crest genes. Depletion of maternal Zic2 from oocytes, using antisense oligonucleotides, caused exogastrulation, anterior truncations and axial defects. We show that loss of maternal Zic2 results in persistent and increased expression of Xenopus nodal-related (Xnr) genes, except for Xnr4, and overall increased Nodal signaling. Injection of a Nodal antagonist, Cerberus-short, reduced the severity of head and axial defects in Zic2-depleted embryos. Depletion of Zic2 could not restore Xnr expression to embryos additionally depleted of VegT, a T-domain transcription factor and an activator of Xnr gene transcription. Taken together, our results suggest a role for maternal Zic2 in the suppression of Xnr genes in early development. ZIC2 is mutated in human holoprosencephaly (HPE), a severe defect in brain hemisphere separation,and these results strengthen the suggestion that increased Nodal-related activity is a cause of HPE.

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

Maintenance of pluripotency is crucial to the mammalian embryo's ability to generate the extra-embryonic and embryonic tissues that are needed for intrauterine survival and foetal development. The recent establishment of embryonic stem cells from human blastocysts (hESCs) provides an opportunity to identify the factors supporting pluripotency at early stages of human development. Using this in vitro model, we have recently shown that Nodal can block neuronal differentiation, suggesting that TGFbeta family members are involved in cell fate decisions of hESCs, including preservation of their pluripotency. Here, we report that Activin/Nodal signalling through Smad2/3 activation is necessary to maintain the pluripotent status of hESCs. Inhibition of Activin/Nodal signalling by follistatin and by overexpression of Lefty or Cerberus-Short, or by the Activin receptor inhibitor SB431542, precipitates hESC differentiation. Nevertheless, neither Nodal nor Activin is sufficient to sustain long-term hESC growth in a chemically defined medium without serum. Recent studies have shown that FGF2 can also maintain long-term expression of pluripotency markers, and we find that inhibition of the FGF signalling pathway by the tyrosine kinase inhibitor SU5402 causes hESC differentiation. However, this effect of FGF on hESC pluripotency depends on Activin/Nodal signalling, because it is blocked by SB431542. Finally, long-term maintenance of in-vitro pluripotency can be achieved with a combination of Activin or Nodal plus FGF2 in the absence of feeder-cell layers, conditioned medium or Serum Replacer. These findings suggest that the Activin/Nodal pathway maintains pluripotency through mechanism(s) in which FGF acts as a competence factor and therefore provide further evidence of distinct mechanisms for preservation of pluripotency in mouse and human ESCs.

Transcriptional regulation of the homeobox gene Mixl1 by TGF-beta and FoxH1

Mixl1 is a paired-type homeodomain protein that plays a crucial role in morphogenesis and endoderm differentiation in the murine embryo. To understand how Mixl1 directs embryogenesis, we studied the regulation of Mixl1 expression at a transcriptional level. In HepG2 cells, a genomic fragment encompassing the Mixl1 promoter conferred strong TGF-beta-induced transcription that was dependent on the presence of the DNA-binding protein FoxH1. Further analysis of the Mixl1 promoter identified a proximal response element (PRE) containing SMAD- and FoxH1-binding sites required for TGF-beta responsiveness. The PRE was also responsive to signalling by Nodal, a TGF-beta ligand required for normal embryonic patterning. These results demonstrate for the first time a functional role for TGF-beta ligands in regulation of mammalian Mixl1, identify FoxH1 as an essential transcriptional co-activator, and implicate Nodal as the embryonic regulator of Mixl1 in mesendoderm morphogenesis.

Induction and migration of the anterior visceral endoderm is regulated by the extra-embryonic ectoderm

The anterior visceral endoderm (AVE) is an extra-embryonic tissue required for specifying anterior pattern in the mouse embryo. The AVE is induced at the distal tip of the 5.5 dpc embryo and then migrates to the prospective anterior, where it imparts anterior identity upon the underlying epiblast (the tissue that gives rise to the embryo proper). Little is known about how the AVE is induced and what directs its migration. In this paper, we describe an essential role for another extra-embryonic tissue, the extra-embryonic ectoderm (ExE), in patterning the AVE and epiblast. Removal of the ExE in pre-gastrulation embryos leads to ectopic AVE formation, to a failure of AVE cell migration and to the assumption by the entire epiblast of an anterior identity. Ectopic transplantation of ExE cells inhibits AVE formation and leads to an expansion of the posterior epiblast marker T. These results demonstrate that the ExE restricts the induction of the AVE to the distal tip of the mouse embryo and is required to initiate the migration of these cells to the prospective anterior. Together, these data reveal a novel role for the ExE in the specification of the anteroposterior axis of the mouse embryo.

Nodal inhibits differentiation of human embryonic stem cells along the neuroectodermal default pathway

Genetic studies in fish, amphibia, and mice have shown that deficiency of Nodal signaling blocks differentiation into mesoderm and endoderm. Thus, Nodal is considered as a major inducer of mesendoderm during gastrulation. On this basis, Nodal is a candidate for controlling differentiation of pluripotent human embryonic stem cells (hESCs) into tissue lineages with potential clinical value. We have investigated the effect of Nodal, both as a recombinant protein and as a constitutively expressed transgene, on differentiation of hESCs. When control hESCs were grown in chemically defined medium, their expression of markers of pluripotency progressively decreased, while expression of neuroectoderm markers was strongly upregulated, thus revealing a neuroectodermal default mechanism for differentiation in this system. hESCs cultured in recombinant Nodal, by contrast, showed prolonged expression of pluripotency marker genes and reduced induction of neuroectoderm markers. These Nodal effects were accentuated in hESCs expressing a Nodal transgene, with striking morphogenetic consequences. Nodal-expressing hESCs developing as embryoid bodies contained an outer layer of visceral endoderm-like cells surrounding an inner layer of epiblast-like cells, each layer having distinct gene expression patterns. Markers of neuroectoderm were not upregulated during development of Nodal-expressing embryoid bodies, nor was there induction of markers for definitive mesoderm or endoderm differentiation. Moreover, the inner layer expressed markers of pluripotency, characteristic of undifferentiated hESCs and of epiblast in mouse embryos. These results could be accounted for by an inhibitory effect of Nodal-induced visceral endoderm on pluripotent cell differentiation into mesoderm and endoderm, with a concomitant inhibition of neuroectoderm differentiation by Nodal itself. There could also be a direct effect of Nodal in the maintenance of pluripotency. In summary, analysis of the Nodal-expressing phenotype suggests a function for the transforming growth factor-beta (TGF-beta) growth factor superfamily in pluripotency and in early cell fate decisions leading to primary tissue layers during in vitro development of pluripotent human stem cells. The effects of Nodal on early differentiation illustrate how hESCs can augment mouse embryos as a model for analyzing mechanisms of early mammalian development.

Emerging Asymmetry and Embryonic Patterning in Early Mouse Development

Recent studies have revealed asymmetries in the mouse zygote and preimplantation embryo, well before the establishment of anterior-posterior polarity after implantation. Whether these asymmetries are causally related to embryonic patterning or are coincidental outcomes of the topology of normal development remains uncertain.

Epigenesis versus preformation during mammalian development. Introduction

The elaboration of a physiologically integrated organism from a fertilized egg depends on processes of cellular growth and diversification that require very precise coordination in both space and time. The extent to which the final outcome is presaged in the egg is an issue that has engaged those seeking to explain embryonic development since antiquity. According to the concept of preformation espoused by Charles Bonnet, the new organism was already present in its final form in the egg, so that development simply entailed enlargement without any accompanying increase in complexity (Oppenheimer 1967). This extreme view was shown to be untenable by Caspar Friedrich Wolff, whose relevant studies included careful documentation of the emergence of increasingly complex organization during the course of embryogenesis in the chick. As a result, the opposing concept of epigenesis, namely that order and form emerge de novo during the course of development, rapidly gained dominance (Oppenheimer 1967). However, by the latter half of the nineteenth century, preformation was revived in a subtler guise to explain the development of various marine invertebrates in which both patterns of cleavage and differentiation of the resulting blastomeres were essentially invariant. Here, cellular diversification was attributed to the localization within the cytoplasm of the egg of factors, or ‘determinants’, which dictated the fate of the cells that inherited them. The stereotypical pattern of cleavage exhibited by such organisms was referred to as ‘mosaic’, in contrast to the variable or ‘regulative’ pattern shown by many others. Mosaic soon came to be equated with a determinant–based or neo–preformationist, and regulative with an epigenetic, view of development. We now know that this distinction is not valid because cellular interactions play a vital part in the development even of organisms with invariant lineage like Caenorhabditis elegans, whereas ‘determinants’ such as those for the germline occur in species whose lineage is variable.