Wnt3 signaling in the epiblast is required for proper orientation of the anteroposterior axis

New findings on the orientation of the mouse anterior-posterior (A-P) axis before and during the initiation of gastrulation using a more refined embryo staging

A clinically significant event of early mammalian embryogenesis is the generation and early development of the anterior-posterior (A-P) axis, the imaginary line along which the structures from head to tail will form. This axis not only appears before gastrulation but is also oriented in a specific way in relation to the long and short diameters of the bilaterally symmetric epiblast. In mice, the most widely used mammalian in vivo model of early embryogenesis, the A-P axis is normally aligned with the long epiblast diameter by the early streak (ES) stage, a time during early gastrulation around embryonic day 6.5 (E6.5). Incorrect orientation of the A-P axis by the ES stage, that is, being aligned with the short epiblast diameter, leads to failure in completing gastrulation and results in embryo death soon after. Knowing the orientation of this axis from when it forms before gastrulation (around E5.5) until just before the ES stage is crucial for: (a) understanding the ill-defined factors involved in its formation and early development since they must be spatially related to it, and (b) providing explanations for the underlying mechanism when it is incorrectly orientated. However, the orientation of the A-P axis in pre-ES embryos of the E5.5-E6.5 period remains unclear. Specifically, although it is thought that this axis initially aligns with the short epiblast diameter and subsequently changes its orientation to become aligned with the long diameter by an unidentified pre-gastrulation stage before the ES stage, this proposition remains unresolved. This is largely due to the lack of clearly defined morphological criteria for staging certain periods of pre-ES mouse embryos (especially when the A-P axis initiates and when gastrulation begins prior to the ES stage), which are a prerequisite for identifying A-P axis orientation at specific pre-ES stages. Furthermore, although the orientation of an extraembryonic trophoblast asymmetry, specifically the tilt of the ectoplacental cone (EPC), coincides with that of the A-P axis by the ES stage, it is unknown whether such an association also exists at pre-gastrulation stages during A-P axis formation. Knowing this would exclude or implicate this trophoblast asymmetry as an upstream factor in orientating the A-P axis when it forms. To address these issues, we established a more refined embryo staging for the E5.5-E6.5 period using a novel combination of live morphological criteria and used it to examine the orientation of the A-P axis and that of the EPC tilt at specific stages. First, contrary to current thinking, we show that when the A-P axis first appears at our newly described anterior visceral endoderm-1 (AVE-1) and AVE-2 stages, it aligns with the long epiblast diameter in all embryos. This orientation is maintained in most embryos at all subsequent pre-gastrulation stages, specifically at our AVE-3 and pre-streak stages (the remaining embryos of these stages had this axis aligned with the short epiblast diameter). Second, we identified for the first time the pre-ES stage when gastrulation initiates, which we named the nascent streak (NS) stage, and further subdivided it into NS-1 and NS-2. At variance with current belief, we provide evidence that the earliest stage just before the ES stage when all embryos align their A-P axis with the long epiblast diameter is not a pre-gastrulation stage, but the NS-2 stage (at NS-1, most but not all embryos had this A-P axis orientation). Third, we implicate the EPC tilt as a possible extraembryonic factor in promoting correct A-P axis orientation, as this tilt exists before the AVE-1 stage and its orientation coincided with that of the A-P axis in all embryos at AVE-1, AVE-2 and ES stages and almost all embryos at AVE-3, pre-streak and NS stages. Overall, our work: (a) identified the previously unresolved orientation of the mouse A-P axis within the epiblast before the ES stage during the E5.5-E6.5 period; (b) provides an alternative explanation for when this axis is incorrectly oriented by the ES stage, namely, its defective alignment with the short epiblast diameter by this stage could be due to its failure to align with the long epiblast diameter from the time of its formation; and (c) implicates the pre-existing orientation of the EPC tilt as a possible factor in orientating the newly formed A-P axis.

Degradation meets development: Implications in β-cell development and diabetes

Pancreatic development is orchestrated by timely synthesis and degradation of stage-specific transcription factors (TFs). The transition from one stage to another stage is dependent on the precise expression of the developmentally relevant TFs. Persistent expression of particular TF would impede the exit from the progenitor stage to the matured cell type. Intracellular protein degradation-mediated protein turnover contributes to a major extent to the turnover of these TFs and thereby dictates the development of different tissues. Since even subtle changes in the crucial cellular pathways would dramatically impact pancreatic β-cell performance, it is generally acknowledged that the biological activity of these pathways is tightly regulated by protein synthesis and degradation process. Intracellular protein degradation is executed majorly by the ubiquitin proteasome system (UPS) and Lysosomal degradation pathway. As more than 90% of the TFs are targeted to proteasomal degradation, this review aims to examine the crucial role of UPS in normal pancreatic β-cell development and how dysfunction of these pathways manifests in metabolic syndromes such as diabetes. Such understanding would facilitate designing a faithful approach to obtain a therapeutic quality of β-cells from stem cells.

The role of Wnt signaling in the development of the epiblast and axial progenitors

Understanding how the body plan is established during embryogenesis remains a fundamental biological question. The Wnt/β-catenin signaling pathway plays a crucial and highly conserved role in body plan formation, functioning to polarize the primary anterior-posterior (AP) or head-to-tail body axis in most metazoans. In this chapter, we focus on the roles that the mammalian Wnt/β-catenin pathway plays to prepare the pluripotent epiblast for gastrulation, and to elicit the emergence of multipotent axial progenitors from the caudal epiblast. Interactions between Wnt and retinoic acid (RA), another powerful family of developmental signaling molecules, in axial progenitors will also be discussed. Gastrulation movements and somitogenesis result in the anterior displacement of the RA source (the rostral somites and lateral plate mesoderm (LPM)), from the posterior Wnt source (the primitive streak (PS)), leading to the establishment of antiparallel gradients of RA and Wnt that control the self-renewal and successive differentiation of neck, trunk and tail progenitors.

Stepwise pluripotency transitions in mouse stem cells

Pluripotent cells in mouse embryos, which first emerge in the inner cell mass of the blastocyst, undergo gradual transition marked by changes in gene expression, developmental potential, polarity, and morphology as they develop from the pre-implantation until post-implantation gastrula stage. Recent studies of cultured mouse pluripotent stem cells (PSCs) have clarified the presence of intermediate pluripotent stages between the naïve pluripotent state represented by embryonic stem cells (ESCs-equivalent to the pre-implantation epiblast) and the primed pluripotent state represented by epiblast stem cells (EpiSCs-equivalent to the late post-implantation gastrula epiblast). In this review, we discuss these recent findings in light of our knowledge on peri-implantation mouse development and consider the implications of these new PSCs to understand their temporal sequence and the feasibility of using them as model system for pluripotency.

Specification and role of extraembryonic endoderm lineages in the periimplantation mouse embryo

During mammalian embryo development, the correct formation of the first extraembryonic endoderm lineages is fundamental for successful development. In the periimplantation blastocyst, the primitive endoderm (PrE) is formed, which gives rise to the parietal endoderm (PE) and visceral endoderm (VE) during further developmental stages. These PrE-derived lineages show significant differences in both their formation and roles. Whereas differentiation of the PE as a migratory lineage has been suggested to represent the first epithelial-to-mesenchymal transition (EMT) in development, organisation of the epithelial VE is of utmost importance for the correct axis definition and patterning of the embryo. Despite sharing a common origin, the striking differences between the VE and PE are indicative of their distinct roles in early development. However, there is a significant disparity in the current knowledge of each lineage, which reflects the need for a deeper understanding of their respective specification processes. In this review, we will discuss the origin and maturation of the PrE, PE, and VE during the periimplantation period using the mouse model as an example. Additionally, we consider the latest findings regarding the role of the PrE-derived lineages and early embryo morphogenesis, as obtained from the most recent in vitro models.

Formal proof of the requirement of MESP1 and MESP2 in mesoderm specification and their transcriptional control via specific enhancers in mice

MESP1 and MESP2 are transcriptional factors involved in mesoderm specification, somite boundary formation and somite polarity regulation. However, Mesp quadruple mutant zebrafish displayed only abnormal somite polarity without mesoderm specification defects. In order to re-evaluate Mesp1/Mesp2 mutants in mice, Mesp1 and Mesp2 single knockouts (KOs), and a Mesp1/Mesp2 double KO were established using genome-editing techniques without introducing selection markers commonly used before. The Mesp1/Mesp2 double KO embryos exhibited markedly severe mesoderm formation defects that were similar to the previously reported Mesp1/Mesp2 double KO embryos, indicating species differences in the function of MESP family proteins. However, the Mesp1 KO did not display any phenotype, including heart formation defects, which have been reported previously. We noted upregulation of Mesp2 in the Mesp1 KO embryos, suggesting that MESP2 rescues the loss of MESP1 in mesoderm specification. We also found that Mesp1 and Mesp2 expression in the early mesoderm is regulated by the cooperation of two independent enhancers containing T-box- and TCF/Lef-binding sites. Deletion of both enhancers caused the downregulation of both genes, resulting in heart formation defects. This study suggests dose-dependent roles of MESP1 and MESP2 in early mesoderm formation.

Rab7-Mediated Endocytosis Establishes Patterning of Wnt Activity through Inactivation of Dkk Antagonism

Endocytosis has been proposed to modulate cell signaling activities. However, the role of endocytosis in embryogenesis, which requires coordination of multiple signaling inputs, has remained less understood. We previously showed that mouse embryos lacking a small guanosine triphosphate (GTP)-binding protein Rab7 implicated in endocytic flow are defective in gastrulation. Here, we investigate how subcellular defects associated with Rab7 deficiency are related to the observed developmental defects. Rab7-deficient embryos fail to organize mesodermal tissues due to defects in Wnt-β-catenin signaling. Visceral endoderm (VE)-specific ablation of Rab7 results in patterning defects similar to systemic Rab7 deletion. Rab7 mutants accumulate the Wnt antagonist Dkk1 in the extracellular space and in intracellular compartments throughout the VE epithelium. These data indicate that Rab7-dependent endocytosis regulates the concentration and availability of extracellular Dkk1, thereby relieving the epiblast of antagonism. This intercellular mechanism therefore organizes distinct spatiotemporal patterns of canonical Wnt activity during the peri-gastrulation stages of embryonic development.

Quantitative analysis of signaling responses during mouse primordial germ cell specification

During early mammalian development, the pluripotent cells of the embryo are exposed to a combination of signals that drive exit from pluripotency and germ layer differentiation. At the same time, a small population of pluripotent cells give rise to the primordial germ cells (PGCs), the precursors of the sperm and egg, which pass on heritable genetic information to the next generation. Despite the importance of PGCs, it remains unclear how they are first segregated from the soma, and if this involves distinct responses to their signaling environment. To investigate this question, we mapped BMP, MAPK and WNT signaling responses over time in PGCs and their surrounding niche in vitro and in vivo at single-cell resolution. We showed that, in the mouse embryo, early PGCs exhibit lower BMP and MAPK responses compared to neighboring extraembryonic mesoderm cells, suggesting the emergence of distinct signaling regulatory mechanisms in the germline versus soma. In contrast, PGCs and somatic cells responded comparably to WNT, indicating that this signal alone is not sufficient to promote somatic differentiation. Finally, we investigated the requirement of a BMP response for these cell fate decisions. We found that cell lines with a mutation in the BMP receptor (Bmpr1a^−/−), which exhibit an impaired BMP signaling response, can efficiently generate PGC-like cells revealing that canonical BMP signaling is not cell autonomously required to direct PGC-like differentiation.

Summary: A subpopulation of pluripotent cells of the embryo give rise to the primordial germ cells (PGCs), the precursors of the sperm and egg, which pass on heritable genetic information to the next generation. To determine how PGCs are first segregated from the soma, we investigated BMP, MAPK and WNT signaling over time in PGCs and their surrounding niche in vitro and in vivo at single-cell resolution.

MiR-184 directly targets Wnt3 in cardiac mesoderm differentiation of embryonic stem cells

Embryonic stem (ES) cells have the property of self-renewal and multi-directional differentiation, and provide an ideal model for studying early embryo development in vitro. Wnt3, as Wnt family member 3, plays a vital role during ES cell differentiation. However, the exact regulatory mechanism of Wnt3 remains to be elucidated. MicroRNAs can directly regulate gene expression at the post-transcriptional level and play critical function in cell fate determination. Here, we found the expression level of miR-184 decreased when ES cells differentiated into cardiac mesoderm then increased during the process as differentiated into cardiomyocytes, which negatively correlated with the expression of Wnt3. Overexpression of miR-184 during the process of ES cell differentiation into cardiac mesoderm repressed cardiac mesoderm differentiation and cardiomyocyte formation. Bioinformatics prediction and mechanism studies showed that miR-184 directly bound to the 3'UTR region of Wnt3 and inhibited the expression level of Wnt3. Consistently, knockdown of Wnt3 mimicked the effects of miR-184-overexpression on ES cell differentiation into cardiac mesoderm, whereas overexpression of Wnt3 rescued the inhibition effects of miR-184 overexpression on ES cell differentiation. These findings demonstrated that miR-184 is a direct regulator of Wnt3 during the differentiation process of ES cells, further enriched the epigenetic regulatory network of ES cell differentiation into cardiac mesoderm and cardiomyocytes.

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.

Canonical WNT/β-Catenin Signaling Activated by WNT9b and RSPO2 Cooperation Regulates Facial Morphogenesis in Mice

The R-spondin (RSPO) family of proteins potentiate canonical WNT/β-catenin signaling and may provide a mechanism to fine-tune the strength of canonical WNT signaling. Although several in vitro studies have clearly demonstrated the potentiation of canonical WNT signaling by RSPOs, whether this potentiation actually occurs in normal development and tissue function in vivo still remains poorly understood. Here, we provide clear evidence of the potentiation of canonical WNT signaling by RSPO during mouse facial development by analyzing compound Wnt9b and Rspo2 gene knockout mice and utilizing ex vivo facial explants. Wnt9b;Rspo2 double mutant mice display facial defects and dysregulated gene expression pattern that are significantly more severe than and different from those of Wnt9b or Rspo2 null mutant mice. Furthermore, we found suggestive evidence that the LGR4/5/6 family of the RSPO receptors may play less critical roles in WNT9b:RSPO2 cooperation. Our results suggest that RSPO-induced cooperation is a key mechanism for fine-tuning canonical WNT/β-catenin signaling in mouse facial development.

Vertebrate cranial mesoderm: developmental trajectory and evolutionary origin

Vertebrate cranial mesoderm is a discrete developmental unit compared to the mesoderm below the developing neck. An extraordinary feature of the cranial mesoderm is that it includes a common progenitor pool contributing to the chambered heart and the craniofacial skeletal muscles. This striking developmental potential and the excitement it generated led to advances in our understanding of cranial mesoderm developmental mechanism. Remarkably, recent findings have begun to unravel the origin of its distinct developmental characteristics. Here, we take a detailed view of the ontogenetic trajectory of cranial mesoderm and its regulatory network. Based on the emerging evidence, we propose that cranial and posterior mesoderm diverge at the earliest step of the process that patterns the mesoderm germ layer along the anterior-posterior body axis. Further, we discuss the latest evidence and their impact on our current understanding of the evolutionary origin of cranial mesoderm. Overall, the review highlights the findings from contemporary research, which lays the foundation to probe the molecular basis of unique developmental potential and evolutionary origin of cranial mesoderm.

Signaling regulation during gastrulation: Insights from mouse embryos and in vitro systems

Gastrulation is the process whereby cells exit pluripotency and concomitantly acquire and pattern distinct cell fates. This is driven by the convergence of WNT, BMP, Nodal and FGF signals, which are tightly spatially and temporally controlled, resulting in regional and stage-specific signaling environments. The combination, level and duration of signals that a cell is exposed to, according its position within the embryo and the developmental time window, dictates the fate it will adopt. The key pathways driving gastrulation exhibit complex interactions, which are difficult to disentangle in vivo due to the complexity of manipulating multiple signals in parallel with high spatiotemporal resolution. Thus, our current understanding of the signaling dynamics regulating gastrulation is limited. In vitro stem cell models have been established, which undergo organized cellular differentiation and patterning. These provide amenable, simplified, deconstructed and scalable models of gastrulation. While the foundation of our understanding of gastrulation stems from experiments in embryos, in vitro systems are now beginning to reveal the intricate details of signaling regulation. Here we discuss the current state of knowledge of the role, regulation and dynamic interaction of signaling pathways that drive mouse gastrulation.

Canonical Wnt5b Signaling Directs Outlying Nkx2.5+ Mesoderm into Pacemaker Cardiomyocytes

Pacemaker cardiomyocytes that create the sinoatrial node are essential for the initiation and maintenance of proper heart rhythm. However, illuminating developmental cues that direct their differentiation has remained particularly challenging due to the unclear cellular origins of these specialized cardiomyocytes. By discovering the origins of pacemaker cardiomyocytes, we reveal an evolutionarily conserved Wnt signaling mechanism that coordinates gene regulatory changes directing mesoderm cell fate decisions, which lead to the differentiation of pacemaker cardiomyocytes. We show that in zebrafish, pacemaker cardiomyocytes derive from a subset of Nkx2.5+ mesoderm that responds to canonical Wnt5b signaling to initiate the cardiac pacemaker program, including activation of pacemaker cell differentiation transcription factors Isl1 and Tbx18 and silencing of Nkx2.5. Moreover, applying these developmental findings to human pluripotent stem cells (hPSCs) notably results in the creation of hPSC-pacemaker cardiomyocytes, which successfully pace three-dimensional bioprinted hPSC-cardiomyocytes, thus providing potential strategies for biological cardiac pacemaker therapy.

Geometrical confinement controls the asymmetric patterning of brachyury in cultures of pluripotent cells

Diffusible signals are known to orchestrate patterning during embryogenesis, yet diffusion is sensitive to noise. The fact that embryogenesis is remarkably robust suggests that additional layers of regulation reinforce patterning. Here, we demonstrate that geometrical confinement orchestrates the spatial organisation of initially randomly positioned subpopulations of spontaneously differentiating mouse embryonic stem cells. We use micropatterning in combination with pharmacological manipulations and quantitative imaging to dissociate the multiple effects of geometry. We show that the positioning of a pre-streak-like population marked by brachyury (T) is decoupled from the size of its population, and that breaking radial symmetry of patterns imposes polarised patterning. We provide evidence for a model in which the overall level of diffusible signals together with the history of the cell culture define the number of T+ cells, whereas geometrical constraints guide patterning in a multi-step process involving a differential response of the cells to multicellular spatial organisation. Our work provides a framework for investigating robustness of patterning and provides insights into how to guide symmetry-breaking events in aggregates of pluripotent cells.

Mechanisms of Wnt signaling and control

The Wnt signaling pathway is a highly conserved system that regulates complex biological processes across all metazoan species. At the cellular level, secreted Wnt proteins serve to break symmetry and provide cells with positional information that is critical to the patterning of the entire body plan. At the organismal level, Wnt signals are employed to orchestrate fundamental developmental processes, including the specification of the anterior-posterior body axis, induction of the primitive streak and ensuing gastrulation movements, and the generation of cell and tissue diversity. Wnt functions extend into adulthood where they regulate stem cell behavior, tissue homeostasis, and damage repair. Disruption of Wnt signaling activity during embryonic development or in adults results in a spectrum of abnormalities and diseases, including cancer. The molecular mechanisms that underlie the myriad of Wnt-regulated biological effects have been the subject of intense research for over three decades. This review is intended to summarize our current understanding of how Wnt signals are generated and interpreted. This article is categorized under: Biological Mechanisms > Cell Signaling Developmental Biology > Stem Cell Biology and Regeneration.

Cilia and Ciliopathies in Congenital Heart Disease

A central role for cilia in congenital heart disease (CHD) was recently identified in a large-scale mouse mutagenesis screen. Although the screen was phenotype-driven, the majority of genes recovered were cilia-related, suggesting that cilia play a central role in CHD pathogenesis. This partly reflects the role of cilia as a hub for cell signaling pathways regulating cardiovascular development. Consistent with this, many cilia-transduced cell signaling genes were also recovered, and genes regulating vesicular trafficking, a pathway essential for ciliogenesis and cell signaling. Interestingly, among CHD-cilia genes recovered, some regulate left-right patterning, indicating cardiac left-right asymmetry disturbance may play significant roles in CHD pathogenesis. Clinically, CHD patients show a high prevalence of ciliary dysfunction and show enrichment for de novo mutations in cilia-related pathways. Combined with the mouse findings, this would suggest CHD may be a new class of ciliopathy.

Probing early heart development to instruct stem cell differentiation strategies

Scientists have studied organs and their development for centuries and, along that path, described models and mechanisms explaining the developmental principles of organogenesis. In particular, with respect to the heart, new fundamental discoveries are reported continuously that keep changing the way we think about early cardiac development. These discoveries are driven by the need to answer long-standing questions regarding the origin of the earliest cells specified to the cardiac lineage, the differentiation potential of distinct cardiac progenitor cells, and, very importantly, the molecular mechanisms underlying these specification events. As evidenced by numerous examples, the wealth of developmental knowledge collected over the years has had an invaluable impact on establishing efficient strategies to generate cardiovascular cell types ex vivo, from either pluripotent stem cells or via direct reprogramming approaches. The ability to generate functional cardiovascular cells in an efficient and reliable manner will contribute to therapeutic strategies aimed at alleviating the increasing burden of cardiovascular disease and morbidity. Here we will discuss the recent discoveries in the field of cardiac progenitor biology and their translation to the pluripotent stem cell model to illustrate how developmental concepts have instructed regenerative model systems in the past and promise to do so in the future. Developmental Dynamics 245:1130-1144, 2016. © 2016 Wiley Periodicals, Inc.

From blastocyst to gastrula: gene regulatory networks of embryonic stem cells and early mouse embryogenesis

To date, many regulatory genes and signalling events coordinating mammalian development from blastocyst to gastrulation stages have been identified by mutational analyses and reverse-genetic approaches, typically on a gene-by-gene basis. More recent studies have applied bioinformatic approaches to generate regulatory network models of gene interactions on a genome-wide scale. Such models have provided insights into the gene networks regulating pluripotency in embryonic and epiblast stem cells, as well as cell-lineage determination in vivo. Here, we review how regulatory networks constructed for different stem cell types relate to corresponding networks in vivo and provide insights into understanding the molecular regulation of the blastocyst-gastrula transition.

Signaling Control of Differentiation of Embryonic Stem Cells toward Mesendoderm

Mesendoderm (ME) refers to the primitive streak in mammalian embryos, which has the ability to further differentiate into mesoderm and endoderm. A better understanding on the regulatory networks of ME differentiation of embryonic stem (ES) cells would provide important insights on early embryo patterning and a possible guidance for ES applications in regenerative medicine. Studies on developmental biology and embryology have offered a great deal of knowledge about key signaling pathways involved in primitive streak formation. Recently, various chemically defined recipes have been formulated to induce differentiation of ES cells toward ME in vitro, which greatly facilitate the elucidation of the regulatory mechanisms of different signals involved in ME specification. Among the extrinsic signals, transforming growth factor-β/Activin signaling and Wnt signaling have been shown to be the most critical ones. On another side, intrinsic epigenetic regulation has been indicated to be important in ME determination. In this review, we summarize the current understanding on the extrinsic and intrinsic regulations of ES cells-to-ME differentiation and the crosstalk among them, aiming to get a general overview on ME specification and primitive streak formation.

Casz1 is required for cardiomyocyte G1-to-S phase progression during mammalian cardiac development

Organ growth occurs through the integration of external growth signals during the G1 phase of the cell cycle to initiate DNA replication. Although numerous growth factor signals have been shown to be required for the proliferation of cardiomyocytes, genetic studies have only identified a very limited number of transcription factors that act to regulate the entry of cardiomyocytes into S phase. Here, we report that the cardiac para-zinc-finger protein CASZ1 is expressed in murine cardiomyocytes. Genetic fate mapping with an inducible Casz1 allele demonstrates that CASZ1-expressing cells give rise to cardiomyocytes in the first and second heart fields. We show through the generation of a cardiac conditional null mutation that Casz1 is essential for the proliferation of cardiomyocytes in both heart fields and that loss of Casz1 leads to a decrease in cardiomyocyte cell number. We further report that the loss of Casz1 leads to a prolonged or arrested S phase, a decrease in DNA synthesis, an increase in phospho-RB and a concomitant decrease in the cardiac mitotic index. Taken together, these studies establish a role for CASZ1 in mammalian cardiomyocyte cell cycle progression in both the first and second heart fields.

β-Catenin Regulates Primitive Streak Induction through Collaborative Interactions with SMAD2/SMAD3 and OCT4

Canonical Wnt and Nodal signaling are both required for induction of the primitive streak (PS), which guides organization of the early embryo. The Wnt effector β-catenin is thought to function in these early lineage specification decisions via transcriptional activation of Nodal signaling. Here, we demonstrate a broader role for β-catenin in PS formation by analyzing its genome-wide binding in a human embryonic stem cell model of PS induction. β-catenin occupies regulatory regions in numerous PS and neural crest genes, and direct interactions between β-catenin and the Nodal effectors SMAD2/SMAD3 are required at these regions for PS gene activation. Furthermore, OCT4 binding in proximity to these sites is likewise required for PS induction, suggesting a collaborative interaction between β-catenin and OCT4. Induction of neural crest genes by β-catenin is repressed by SMAD2/SMAD3, ensuring proper lineage specification. This study provides mechanistic insight into how Wnt signaling controls early cell lineage decisions.

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.

The generation of definitive endoderm from human embryonic stem cells is initially independent from activin A but requires canonical Wnt-signaling

The activation of the TGF-beta pathway by activin A directs ES cells into the definitive endoderm germ layer. However, there is evidence that activin A/TGF-beta is not solely responsible for differentiation into definitive endoderm. GSK3beta inhibition has recently been shown to generate definitive endoderm-like cells from human ES cells via activation of the canonical Wnt-pathway. The GSK3beta inhibitor CHIR-99021 has been reported to generate mesoderm from human iPS cells. Thus, the specific role of the GSK3beta inhibitor CHIR-99021 was analyzed during the differentiation of human ES cells and compared against a classic endoderm differentiation protocol. At high concentrations of CHIR-99021, the cells were directed towards mesodermal cell fates, while low concentrations permitted mesodermal and endodermal differentiation. Finally, the analyses revealed that GSK3beta inhibition rapidly directed human ES cells into a primitive streak-like cell type independently from the TGF-beta pathway with mesoderm and endoderm differentiation potential. Addition of low activin A concentrations effectively differentiated these primitive streak-like cells into definitive endoderm. Thus, the in vitro differentiation of human ES cells into definitive endoderm is initially independent from the activin A/TGF-beta pathway but requires high canonical Wnt-signaling activity.

Programming and reprogramming a human heart cell

The latest discoveries and advanced knowledge in the fields of stem cell biology and developmental cardiology hold great promise for cardiac regenerative medicine, enabling researchers to design novel therapeutic tools and approaches to regenerate cardiac muscle for diseased hearts. However, progress in this arena has been hampered by a lack of reproducible and convincing evidence, which at best has yielded modest outcomes and is still far from clinical practice. To address current controversies and move cardiac regenerative therapeutics forward, it is crucial to gain a deeper understanding of the key cellular and molecular programs involved in human cardiogenesis and cardiac regeneration. In this review, we consider the fundamental principles that govern the "programming" and "reprogramming" of a human heart cell and discuss updated therapeutic strategies to regenerate a damaged heart.

Gene regulatory networks mediating canonical Wnt signal-directed control of pluripotency and differentiation in embryo stem cells

Canonical Wnt signaling supports the pluripotency of embryonic stem cells (ESCs) but also promotes differentiation of early mammalian cell lineages. To explain these paradoxical observations, we explored the gene regulatory networks at play. Canonical Wnt signaling is intertwined with the pluripotency network comprising Nanog, Oct4, and Sox2 in mouse ESCs. In defined media supporting the derivation and propagation of ESCs, Tcf3 and β-catenin interact with Oct4; Tcf3 binds to Sox motif within Oct-Sox composite motifs that are also bound by Oct4-Sox2 complexes. Furthermore, canonical Wnt signaling upregulates the activity of the Pou5f1 distal enhancer via the Sox motif in ESCs. When viewed in the context of published studies on Tcf3 and β-catenin mutants, our findings suggest Tcf3 counters pluripotency by competition with Sox2 at these sites, and Tcf3 inhibition is blocked by β-catenin entry into this complex. Wnt pathway stimulation also triggers β-catenin association at regulatory elements with classic Lef/Tcf motifs associated with differentiation programs. The failure to activate these targets in the presence of a mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) inhibitor essential for ESC culture suggests MEK/ERK signaling and canonical Wnt signaling combine to promote ESC differentiation.

Identification of the WNT1 residues required for palmitoylation by Porcupine

The post-translational palmitoylation of WNT morphogens is critical for proper signaling during embryogenesis and adult homeostasis. The addition of palmitoyl groups to WNT proteins is catalyzed by Porcupine (PORCN). However, the Wnt amino acid residues required for recognition and palmitoylation by PORCN have not been fully characterized. We show that WNT1 residues 214-234 are sufficient for PORCN-dependent palmitoylation of Ser224. Substitution of Ser224 with Thr, but not Cys, is tolerated in palmitoylation and biological assays. Our data highlight the importance of palmitoylation for WNT1 activity and establish PORCN as an O-acyl transferase for WNT1.

How to make a cardiomyocyte

During development, cardiogenesis is orchestrated by a family of heart progenitors that build distinct regions of the heart. Each region contains diverse cell types that assemble to form the complex structures of the individual cardiac compartments. Cardiomyocytes are the main cell type found in the heart and ensure contraction of the chambers and efficient blood flow throughout the body. Injury to the cardiac muscle often leads to heart failure due to the loss of a large number of cardiomyocytes and its limited intrinsic capacity to regenerate the damaged tissue, making it one of the leading causes of morbidity and mortality worldwide. In this Primer we discuss how insights into the molecular and cellular framework underlying cardiac development can be used to guide the in vitro specification of cardiomyocytes, whether by directed differentiation of pluripotent stem cells or via direct lineage conversion. Additional strategies to generate cardiomyocytes in situ, such as reactivation of endogenous cardiac progenitors and induction of cardiomyocyte proliferation, will also be discussed.

Traffic jam in the primitive streak: the role of defective mesoderm migration in birth defects

Gastrulation is the process in which the three germ layers are formed that contribute to the formation of all major tissues in the developing embryo. We here review mouse genetic models in which defective gastrulation leads to mesoderm insufficiencies in the embryo. Depending on severity of the abnormalities, the outcomes range from incompatible with embryonic survival to structural birth defects, such as heart defects, spina bifida, or caudal dysgenesis. The combined evidence from the mutant models supports the notion that these congenital anomalies can originate from perturbations of mesoderm specification, epithelial-mesenchymal transition, and mesodermal cell migration. Knowledge about the molecular pathways involved may help to improve strategies for the prevention of major structural birth defects.

The Dynamics of Morphogenesis in the Early Mouse Embryo

SUMMARYOver the past two decades, our understanding of mouse development from implantation to gastrulation has grown exponentially with an upsurge of genetic, molecular, cellular, and morphogenetic information. New discoveries have exalted the role of extraembryonic tissues in orchestrating embryonic patterning and axial specification. At the same time, the identification of unexpected morphogenetic processes occurring during mouse gastrulation has challenged established dogmas and brought new insights into the mechanisms driving germ layer formation. In this article, we summarize the key findings that have reinvigorated the contemporary view of early postimplantation mammalian development.

A mesodermal factor, T, specifies mouse germ cell fate by directly activating germline determinants

Germ cells ensure reproduction and heredity. In mice, primordial germ cells (PGCs), the precursors for spermatozoa and oocytes, are induced in pluripotent epiblast by BMP4 and WNT3, yet the underlying mechanism remains unclear. Here, using an in vitro PGC specification system, we show that WNT3 induces many transcription factors associated with mesoderm in epiblast-like cells through β-CATENIN. Among these, T (BRACHYURY), a classical and conserved mesodermal factor, was essential for robust activation of Blimp1 and Prdm14, two of the germline determinants. T, but not SMAD1 or TCF1, binds distinct regulatory elements of both Blimp1 and Prdm14 and directly upregulates these genes, delineating the downstream PGC program. Without BMP4, a program induced by WNT3 prevents T from activating Blimp1 and Prdm14, demonstrating a permissive role of BMP4 in PGC specification. These findings establish the key signaling mechanism for, and a fundamental role of a mesodermal factor in, mammalian PGC specification.

Wnt/β-catenin signaling directs the regional expansion of first and second heart field-derived ventricular cardiomyocytes

In mammals, cardiac development proceeds from the formation of the linear heart tube, through complex looping and septation, all the while increasing in mass to provide the oxygen delivery demands of embryonic growth. The developing heart must orchestrate regional differences in cardiomyocyte proliferation to control cardiac morphogenesis. During ventricular wall formation, the compact myocardium proliferates more vigorously than the trabecular myocardium, but the mechanisms controlling such regional differences among cardiomyocyte populations are not understood. Control of definitive cardiomyocyte proliferation is of great importance for application to regenerative cell-based therapies. We have used murine and human pluripotent stem cell systems to demonstrate that, during in vitro cellular differentiation, early ventricular cardiac myocytes display a robust proliferative response to β-catenin-mediated signaling and conversely accelerate differentiation in response to inhibition of this pathway. Using gain- and loss-of-function murine genetic models, we show that β-catenin controls ventricular myocyte proliferation during development and the perinatal period. We further demonstrate that the differential activation of the Wnt/β-catenin signaling pathway accounts for the observed differences in the proliferation rates of the compact versus the trabecular myocardium during normal cardiac development. Collectively, these results provide a mechanistic explanation for the differences in localized proliferation rates of cardiac myocytes and point to a practical method for the generation of the large numbers of stem cell-derived cardiac myocytes necessary for clinical applications.

Porcn-dependent Wnt signaling is not required prior to mouse gastrulation

In mice and humans the X-chromosomal porcupine homolog (Porcn) gene is required for the acylation and secretion of all 19 Wnt ligands and thus represents a bottleneck for all Wnt signaling. We have generated a mouse line carrying a floxed allele for Porcn and used zygotic, oocyte-specific and visceral endoderm-specific deletions to investigate embryonic and extra-embryonic requirements for Wnt ligand secretion. We show that there is no requirement for Porcn-dependent secretion of Wnt ligands during preimplantation development of the mouse embryo. Porcn-dependent Wnts are first required for the initiation of gastrulation, where Porcn function is required in the epiblast but not the visceral endoderm. Heterozygous female embryos, which are mutant in both trophoblast and visceral endoderm due to imprinted X chromosome inactivation, complete gastrulation but display chorio-allantoic fusion defects similar to Wnt7b mutants. Our studies highlight the importance of Wnt3 and Wnt7b for embryonic and placental development but suggest that endogenous Porcn-dependent Wnt secretion does not play an essential role in either implantation or blastocyst lineage specification.

Wnt signalling in mouse gastrulation and anterior development: new players in the pathway and signal output

Embryonic development and adult homeostasis are dependent upon the coordinated action of signal transduction pathways such as the Wnt signalling pathway which is used iteratively during these processes. In the early post-implantation mouse embryo, Wnt/β-catenin signalling activity plays a critical role in the formation of the primitive streak, progression of gastrulation and tissue patterning in the anterior-posterior axis. The net output of the signalling pathway is influenced by the delivery and post-translational modification of the ligands, the counteracting activities of the activating components and the negative modulators, and the molecular interaction of β-catenin, TCF and other factors regulating the transcription of downstream target genes.

ENU mutagenesis reveals that Notchless homolog 1 (Drosophila) affects Cdkn1a and several members of the Wnt pathway during murine pre-implantation development

Background

Our interests lie in determining the genes and genetic pathways that are important for establishing and maintaining maternal-fetal interactions during pregnancy. Mutation analysis targeted to a 34 Mb domain flanked by Trp53 and Wnt3 demonstrates that this region of mouse chromosome 11 contains a large number of essential genes. Two mutant alleles (l11Jus1 and l11Jus4), which fall into the same complementation group, survive through implantation but fail prior to gastrulation.

Results

Through a positional cloning strategy, we discovered that these homozygous mutant alleles contain non-conservative missense mutations in the Notchless homolog 1 (Drosophila) (Nle1) gene. NLE1 is a member of the large WD40-repeat protein family, and is thought to signal via the canonical NOTCH pathway in vertebrates. However, the phenotype of the Nle1 mutant mice is much more severe than single Notch receptor mutations or even in animals in which NOTCH signaling is blocked. To test the hypothesis that NLE1 functions in multiple signaling pathways during pre-implantation development, we examined expression of multiple Notch downstream target genes, as well as select members of the Wnt pathway in wild-type and mutant embryos. We did not detect altered expression of any primary members of the Notch pathway or in Notch downstream target genes. However, our data reveal that Cdkn1a, a NOTCH target, was upregulated in Nle1 mutants, while several members of the Wnt pathway are downregulated. In addition, we found that Nle1 mutant embryos undergo caspase-mediated apoptosis as hatched blastocysts, but not as morulae or blastocysts.

Conclusions

Taken together, these results uncover potential novel functions for NLE1 in the WNT and CDKN1A pathways during embryonic development in mammals.

Wnt3 function in the epiblast is required for the maintenance but not the initiation of gastrulation in mice

The formation of the anteroposterior axis in mice requires a Wnt3-dependent symmetry-breaking event that leads to the formation of the primitive streak and gastrulation. Wnt3 is expressed sequentially in two distinct areas of the mouse embryo before the appearance of the primitive streak; first in the posterior visceral endoderm and soon after in the adjacent posterior epiblast. Hence, although an axial requirement for Wnt3 is well established, its temporal and tissue specific requirements remain an open question. Here, we report the conditional inactivation of Wnt3 in the epiblast of developing mouse embryos. Contrary to previous studies, our data shows that embryos lacking Wnt3 specifically in the epiblast are able to initiate gastrulation and advance to late primitive streak stages but fail to thrive and are resorbed by E9.5. At the molecular level, we provide evidence that Wnt3 regulates its own expression and that of other primitive streak markers via activation of the canonical Wnt signaling pathway.

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.

Distinct gene expression signatures in human embryonic stem cells differentiated towards definitive endoderm at single-cell level

Characterization of directed differentiation of pluripotent stem cells towards therapeutically relevant cell types, including pancreatic beta-cells and hepatocytes, depends on molecular markers and assays that resolve the signature of individual cells. Pancreas and liver both have a common origin of anterior definitive endoderm (DE). Here, we differentiated human embryonic stem cells towards DE using three different activin A based treatments. Differentiation efficiencies were evaluated by gene expression profiling over time at cell population level. A panel of key markers was used to study DE formation. Final DE differentiation was also analyzed with immunocytochemistry and single-cell gene expression profiling. We found that cells treated with activin A in combination with sodium butyrate and B27 serum-free supplement medium generated the most mature DE cells. Cell population studies were useful to monitor the temporal expression of genes involved in primitive streak formation and endoderm formation, while single-cell analysis allowed us to study cell culture heterogeneity and fingerprint individual cells. In addition, single-cell analysis revealed distinct gene expression patterns for the three activin A based protocols applied. Our data provide novel insights in DE gene expression at the cellular level of in vitro differentiated human embryonic stem cells, and illustrate the power of using single-cell gene expression profiling to study differentiation heterogeneity and to characterize cell types and subpopulations.

Nkx1-2 is a transcriptional repressor and is essential for the activation of Brachyury in P19 mouse embryonal carcinoma cell

Activation of Wnt/β-catenin signaling is crucial for the differentiation of pluripotent stem cells, namely the epiblast, embryonic stem, and embryonal carcinoma cells, into mesendoderm. However, downstream events of Wnt/β-catenin signaling that control the formation of mesendoderm are still unclear. In the present study, we used mouse P19 embryonal carcinoma cells as a model, and identified a homeodomain protein Nkx1-2 as a key regulator of mesendoderm formation. In the mouse embryo, Nkx1-2 was expressed in the primitive streak, in which the nascent mesendoderm emerges. In P19 cells, the expression of Nkx1-2 was activated by Wnt/β-catenin signaling independently of Brachyury, an evolutionary conserved early mesendoderm gene. In contrast, the expression of Nkx1-2 was both necessary and sufficient for the activation of Brachyury. Nkx1-2 acted as a transcriptional repressor to mediate the action of Wnt/β-catenin signaling to activate the Brachyury expression. We found Tcf3 as a potential target of gene repression by Nkx1-2, and the down-regulation of Tcf3 was partly required for effective activation of Brachyury by Wnt/β-catenin signaling. These results suggest that Nkx1-2 is a critical component of the gene regulatory network that operates downstream of Wnt/β-catenin signaling to regulate the formation of mesendoderm.

A synthetic substrate to support early mesodermal differentiation of human embryonic stem cells

Our ability to guide differentiation of human pluripotent stem cells (hPSCs) toward desired lineages efficiently and reproducibly in xeno-free conditions is the key to advancing hPSC technology from the laboratory to clinical use. Here we report an engineered biomimetic substrate functionalized with both peptide ligands for α5β1 and α6β1 integrins to support efficient early mesodermal differentiation of human embryonic stem cells (hESCs) when cultured in a differentiation medium containing BMP4. In contrast, mesodermal differentiation is not induced on substrates functionalized with either ligand alone even though the culture medium is identical. Mesodermal differentiation was characterized by immunofluorescent staining, flow cytometric analysis, and RT-PCR analysis of early mesodermal markers Brachyury, Mixl1, and Wnt3. The early mesodermal progenitors derived on the substrate functionalized with both integrin ligands have the normal developmental potential to further differentiate along the hemato-endothelial and cardiac lineages. Immobilized ligands for α5β1 and α6β1 integrins both are permissive, necessary, and sufficient insoluble ligands in this engineered system to support early mesodermal differentiation of hESCs. This synthetic substrate, in conjunction with defined soluble factors, constructs a well-controlled and xeno-free early mesodermal differentiation niche that offers advantages over the previously reported niche constructed with the Matrigel-coated substrate.

Deletion of mouse Porcn blocks Wnt ligand secretion and reveals an ectodermal etiology of human focal dermal hypoplasia/Goltz syndrome

The Drosophila porcupine gene is required for secretion of wingless and other Wnt proteins, and sporadic mutations in its unique human ortholog, PORCN, cause a pleiotropic X-linked dominant disorder, focal dermal hypoplasia (FDH, also known as Goltz syndrome). We generated a conditional allele of the X-linked mouse Porcn gene and analyzed its requirement in Wnt signaling and embryonic development. We find that Porcn-deficient cells exhibit a cell-autonomous defect in Wnt ligand secretion but remain responsive to exogenous Wnts. Consistent with the female-specific inheritance pattern of FDH, Porcn hemizygous male embryos arrest during early embryogenesis and fail to generate mesoderm, a phenotype previously associated with loss of Wnt activity. Heterozygous Porcn mutant females exhibit a spectrum of limb, skin, and body patterning abnormalities resembling those observed in human patients with FDH. Many of these defects are recapitulated by ectoderm-specific deletion of Porcn, substantiating a long-standing hypothesis regarding the etiology of human FDH and extending previous studies that have focused on downstream elements of Wnt signaling, such as β-catenin. Conditional deletion of Porcn thus provides an experimental model of FDH, as well as a valuable tool to probe Wnt ligand function in vivo.

Beta-catenin signaling in hepatic development and progenitors: which way does the WNT blow?

The Wnt/β-catenin pathway is an evolutionarily conserved signaling cascade that plays key roles in development and adult tissue homeostasis and is aberrantly activated in many tumors. Over a decade of work in mouse, chick, xenopus, and zebrafish models has uncovered multiple functions of this pathway in hepatic pathophysiology. Specifically, beta-catenin, the central component of the canonical Wnt pathway, is implicated in the regulation of liver regeneration, development, and carcinogenesis. Wnt-independent activation of beta-catenin by receptor tyrosine kinases has also been observed in the liver. In liver development across various species, through regulation of cell proliferation, differentiation, and maturation, beta-catenin directs foregut endoderm specification, hepatic specification of the foregut, and hepatic morphogenesis. Its role has also been defined in adult hepatic progenitors or oval cells especially in their expansion and differentiation. Thus, beta-catenin undergoes tight temporal regulation to exhibit pleiotropic effects during hepatic development and in hepatic progenitor biology.

Wnt-3 and Wnt-3a play different region-specific roles in neural crest development in avians

Wnt signalling regulates cell proliferation and cell fate determination during embryogenesis. However, little is known about the developmental role of one Wnt family member, Wnt-3, during avian development. To investigate the possible functions of Wnt-3, its expression pattern was determined using whole-mount in situ hybridization. Wnt-3 is expressed in important signalling centres, including the dorsal neural tube, Hensen's node and the AER (apical ectodermal ridge). Most interestingly, Wnt-3 is expressed in the dorsal neural tube as a gradient, with the strongest expression anterior in the trunk. Furthermore, this study showed that Wnt-3 and Wnt-3a play a different role in neural crest lineages derived from different axial level of neural tube. Wnt-3 might be involved in proliferation of neural crest lineages, whereas Wnt-3a plays an important role in melanogenesis in vagal. However, both Wnt-3 and Wnt-3a cause a significant increase in melanogenesis in the trunk neural crest lineage.