Expression of all Wnt genes and their secreted antagonists during mouse blastocyst and postimplantation development

3D reconstruction of a gastrulating human embryo

Progress in understanding early human development has been impeded by the scarcity of reference datasets from natural embryos, particularly those with spatial information during crucial stages like gastrulation. We conducted high-resolution spatial transcriptomics profiling on 38,562 spots from 62 transverse sections of an intact Carnegie stage (CS) 8 human embryo. From this spatial transcriptomic dataset, we constructed a 3D model of the CS8 embryo, in which a range of cell subtypes are identified, based on gene expression patterns and positional register, along the anterior-posterior, medial-lateral, and dorsal-ventral axis in the embryo. We further characterized the lineage trajectories of embryonic and extra-embryonic tissues and associated regulons and the regionalization of signaling centers and signaling activities that underpin lineage progression and tissue patterning during gastrulation. Collectively, the findings of this study provide insights into gastrulation and post-gastrulation development of the human embryo.

The Wnt-dependent master regulator NKX1-2 controls mouse pre-implantation development

Embryo size, specification, and homeostasis are regulated by a complex gene regulatory and signaling network. Here we used gene expression signatures of Wnt-activated mouse embryonic stem cell (mESC) clones to reverse engineer an mESC regulatory network. We identify NKX1-2 as a novel master regulator of preimplantation embryo development. We find that Nkx1-2 inhibition reduces nascent RNA synthesis, downregulates genes controlling ribosome biogenesis, RNA translation, and transport, and induces severe alteration of nucleolus structure, resulting in the exclusion of RNA polymerase I from nucleoli. In turn, NKX1-2 loss of function leads to chromosome missegregation in the 2- to 4-cell embryo stages, severe decrease in blastomere numbers, alterations of tight junctions (TJs), and impairment of microlumen coarsening. Overall, these changes impair the blastocoel expansion-collapse cycle and embryo cavitation, leading to altered lineage specification and developmental arrest.

Source cell-type epigenetic memory persists in induced pluripotent cells but is lost in subsequently derived germline cells

Introduction: Retention of source cell-type epigenetic memory may mitigate the potential for induced pluripotent stem cells (iPSCs) to fully achieve transitions in cell fate in vitro. While this may not preclude the use of iPSC-derived somatic cell types for therapeutic applications, it becomes a major concern impacting the potential use of iPSC-derived germline cell types for reproductive applications. The transition from a source somatic cell type to iPSCs and then on to germ-cell like cells (GCLCs) recapitulates two major epigenetic reprogramming events that normally occur during development in vivo-embryonic reprogramming in the epiblast and germline reprogramming in primordial germ cells (PGCs). We examined the extent of epigenetic and transcriptomic memory persisting first during the transition from differentiated source cell types to iPSCs, and then during the transition from iPSCs to PGC-like cells (PGCLCs). Methods: We derived iPSCs from four differentiated mouse cell types including two somatic and two germ cell types and tested the extent to which each resulting iPSC line resembled a) a validated ES cell reference line, and b) their respective source cell types, on the basis of genome-wide gene expression and DNA methylation patterns. We then induced each iPSC line to form PGCLCs, and assessed epigenomic and transcriptomic memory in each compared to endogenous PGCs/M-prospermatogonia. Results: In each iPSC line, we found residual gene expression and epigenetic programming patterns characteristic of the corresponding source differentiated cell type from which each was derived. However, upon deriving PGCLCs, we found very little evidence of lingering epigenetic or transcriptomic memory of the original source cell type. Discussion: This result indicates that derivation of iPSCs and then GCLCs from differentiated source cell types in vitro recapitulates the two-phase epigenetic reprogramming that normally occurs in vivo, and that, to a significant extent, germline cell types derived in vitro from pluripotent cells accurately recapitulate epigenetic programming and gene expression patterns corresponding to equivalent endogenous germ cell types, suggesting that they have the potential to form the basis of in vitro gametogenesis as a useful therapeutic strategy for treatment of infertility.

In Vitro Embryogenesis and Gastrulation Using Stem Cells in Mice and Humans

During early mammalian embryonic development, fertilized one-cell embryos develop into pre-implantation blastocysts and subsequently establish three germ layers through gastrulation during post-implantation development. In recent years, stem cells have emerged as a powerful tool to study embryogenesis and gastrulation without the need for eggs, allowing for the generation of embryo-like structures known as synthetic embryos or embryoids. These in vitro models closely resemble early embryos in terms of morphology and gene expression and provide a faithful recapitulation of early pre- and post-implantation embryonic development. Synthetic embryos can be generated through a combinatorial culture of three blastocyst-derived stem cell types, such as embryonic stem cells, trophoblast stem cells, and extraembryonic endoderm cells, or totipotent-like stem cells alone. This review provides an overview of the progress and various approaches in studying in vitro embryogenesis and gastrulation in mice and humans using stem cells. Furthermore, recent findings and breakthroughs in synthetic embryos and gastruloids are outlined. Despite ethical considerations, synthetic embryo models hold promise for understanding mammalian (including humans) embryonic development and have potential implications for regenerative medicine and developmental research.

Inhibition of Wnt activity improves peri-implantation development of somatic cell nuclear transfer embryos

Somatic cell nuclear transfer (SCNT) can reprogram differentiated somatic cells into totipotency. Although pre-implantation development of SCNT embryos has greatly improved, most SCNT blastocysts are still arrested at the peri-implantation stage, and the underlying mechanism remains elusive. Here, we develop a 3D in vitro culture system for SCNT peri-implantation embryos and discover that persistent Wnt signals block the naïve-to-primed pluripotency transition of epiblasts with aberrant H3K27me3 occupancy, which in turn leads to defects in epiblast transformation events and subsequent implantation failure. Strikingly, manipulating Wnt signals can attenuate the pluripotency transition and H3K27me3 deposition defects in epiblasts and achieve up to a 9-fold increase in cloning efficiency. Finally, single-cell RNA-seq analysis reveals that Wnt inhibition markedly enhances the lineage developmental trajectories of SCNT blastocysts during peri-implantation development. Overall, these findings reveal diminished potentials of SCNT blastocysts for lineage specification and validate a critical peri-implantation barrier for SCNT embryos.

Extracellular Vesicles Secreted by Pre-Hatching Bovine Embryos Produced In Vitro and In Vivo Alter the Expression of IFNtau-Stimulated Genes in Bovine Endometrial Cells

The embryo-maternal interaction occurs during the early stages of embryo development and is essential for the implantation and full-term development of the embryo. In bovines, the secretion of interferon Tau (IFNT) during elongation is the main signal for pregnancy recognition, but its expression starts around the blastocyst stage. Embryos release extracellular vesicles (EVs) as an alternative mechanism of embryo-maternal communication. The aim of the study was to determine whether EVs secreted by bovine embryos during blastulation (D5-D7) could induce transcriptomic modifications, activating IFNT signaling in endometrial cells. Additionally, it aims to assess whether the EVs secreted by embryos produced in vivo (EVs-IVV) or in vitro (EVs-IVP) have different effects on the transcriptomic profiles of the endometrial cells. In vitro- and in vivo-produced bovine morulae were selected and individually cultured for 48 h to collect embryonic EVs (E-EVs) secreted during blastulation. E-EVs stained with PKH67 were added to in vitro-cultured bovine endometrial cells to assess EV internalization. The effect of EVs on the transcriptomic profile of endometrial cells was determined by RNA sequencing. EVs from both types of embryos induced several classical and non-classical IFNT-stimulated genes (ISGs) and other pathways related to endometrial function in epithelial endometrial cells. Higher numbers of differentially expressed genes (3552) were induced by EVs released by IVP embryos compared to EVs from IVV (1838). Gene ontology analysis showed that EVs-IVP/IVV induced the upregulation of the extracellular exosome pathway, the cellular response to stimulus, and the protein modification processes. This work provides evidence regarding the effect of embryo origin (in vivo or in vitro) on the early embryo-maternal interaction mediated by extracellular vesicles.

The Wnt/TCF7L1 transcriptional repressor axis drives primitive endoderm formation by antagonizing naive and formative pluripotency

Early during preimplantation development and in heterogeneous mouse embryonic stem cells (mESC) culture, pluripotent cells are specified towards either the primed epiblast or the primitive endoderm (PE) lineage. Canonical Wnt signaling is crucial for safeguarding naive pluripotency and embryo implantation, yet the role and relevance of canonical Wnt inhibition during early mammalian development remains unknown. Here, we demonstrate that transcriptional repression exerted by Wnt/TCF7L1 promotes PE differentiation of mESCs and in preimplantation inner cell mass. Time-series RNA sequencing and promoter occupancy data reveal that TCF7L1 binds and represses genes encoding essential naive pluripotency factors and indispensable regulators of the formative pluripotency program, including Otx2 and Lef1. Consequently, TCF7L1 promotes pluripotency exit and suppresses epiblast lineage formation, thereby driving cells into PE specification. Conversely, TCF7L1 is required for PE specification as deletion of Tcf7l1 abrogates PE differentiation without restraining epiblast priming. Taken together, our study underscores the importance of transcriptional Wnt inhibition in regulating lineage specification in ESCs and preimplantation embryo development as well as identifies TCF7L1 as key regulator of this process.

Allelic to genome wide perspectives of swine genetic variation to litter size and its component traits

Litter size is a complex and sex limited trait that depends on various biological, managemental and environmental factors. Owing to its low heritability it is inefficaciously selected by traditional methods. However, due to higher heritability of ovulation rate and embryo survival, selection based on component traits of litter size is advocated. QTL analysis and candidate gene approach are among the various supplementary/alternate strategies for selection of litter size. QTL analysis is aimed at identifying genomic regions affecting trait of interest significantly. Candidate gene approach necessitates identification of genes potentially affecting the trait. There are various genes that significantly affect litter size and its component traits viz. ESR, LEP, BF, IGFBP, RBP4, PRLR, CTNNAL1, WNT10B, TCF12, DAZ, and RNF4. These genes affect litter size in a complex interacting manner. Lately, genome wide association study (GWAS) have been utilized to unveil the genetic and biological background of litter traits, and elucidate the genes governing litter size. Favorable SNPs in these genes have been identified and offers a scope for inclusion in selection programs thereby increasing breeding efficiency and profit in pigs. The review provides a comprehensive coverage of investigations carried out globally to unravel the genetic variation in litter size and its component traits in pigs, both at allelic and genome wide level. It offers a current perspective on different strategies including the profiling of candidate genes, QTLs, and genome wide association studies as an aid to efficient selection for litter size and its component traits.

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.

Overview of junctional complexes during mammalian early embryonic development

Cell-cell junctions form strong intercellular connections and mediate communication between blastomeres during preimplantation embryonic development and thus are crucial for cell integrity, polarity, cell fate specification and morphogenesis. Together with cell adhesion molecules and cytoskeletal elements, intercellular junctions orchestrate mechanotransduction, morphokinetics and signaling networks during the development of early embryos. This review focuses on the structure, organization, function and expressional pattern of the cell-cell junction complexes during early embryonic development. Understanding the importance of dynamic junction formation and maturation processes will shed light on the molecular mechanism behind developmental abnormalities of early embryos during the preimplantation period.

wnt16 regulates spine and muscle morphogenesis through parallel signals from notochord and dermomyotome

Bone and muscle are coupled through developmental, mechanical, paracrine, and autocrine signals. Genetic variants at the CPED1-WNT16 locus are dually associated with bone- and muscle-related traits. While Wnt16 is necessary for bone mass and strength, this fails to explain pleiotropy at this locus. Here, we show wnt16 is required for spine and muscle morphogenesis in zebrafish. In embryos, wnt16 is expressed in dermomyotome and developing notochord, and contributes to larval myotome morphology and notochord elongation. Later, wnt16 is expressed at the ventral midline of the notochord sheath, and contributes to spine mineralization and osteoblast recruitment. Morphological changes in wnt16 mutant larvae are mirrored in adults, indicating that wnt16 impacts bone and muscle morphology throughout the lifespan. Finally, we show that wnt16 is a gene of major effect on lean mass at the CPED1-WNT16 locus. Our findings indicate that Wnt16 is secreted in structures adjacent to developing bone (notochord) and muscle (dermomyotome) where it affects the morphogenesis of each tissue, thereby rendering wnt16 expression into dual effects on bone and muscle morphology. This work expands our understanding of wnt16 in musculoskeletal development and supports the potential for variants to act through WNT16 to influence bone and muscle via parallel morphogenetic processes.

In humans, genetic variants (DNA sequences that vary amongst individuals) have been identified that appear to influence two tissues, bone and skeletal muscle. However, how single genes and genetic variants exert dual influence on both tissues is not well understood. In this study, we found that the wnt16 gene is necessary for specifying the size and shape of both muscle and bone during development in zebrafish. We also disentangled how wnt16 affects both tissues: distinct cellular populations adjacent to muscle and bone secrete Wnt16, where it acts as a signal guiding the size and shape of each tissue. This is important because in humans, genetic variants near the WNT16 gene have effects on both bone- and muscle-related traits. This study expands our understanding of the role of WNT16 in bone and muscle development, and helps to explain how genetic variants near WNT16 affect traits for both tissues. Moreover, WNT16 is actively being explored as a target for osteoporosis therapies, thus our study could have implications with regard to the potential of targeting WNT16 to treat bone and muscle simultaneously.

Porcn is essential for growth and invagination of the mammalian optic cup

Microphthalmia, anophthalmia, and coloboma (MAC) are congenital ocular malformations causing 25% of childhood blindness. The X-linked disorder Focal Dermal Hypoplasia (FDH) is frequently associated with MAC and results from mutations in Porcn, a membrane bound O-acyl transferase required for palmitoylation of Wnts to activate multiple Wnt-dependent pathways. Wnt/β-catenin signaling is suppressed in the anterior neural plate for initiation of eye formation and is subsequently required during differentiation of the retinal pigment epithelium (RPE). Non-canonical Wnts are critical for early eye formation in frog and zebrafish. However, it is unclear whether this also applies to mammals. We performed ubiquitous conditional inactivation of Porcn in mouse around the eye field stage. In Porcn CKO , optic vesicles (OV) arrest in growth and fail to form an optic cup. Ventral proliferation is significantly decreased in the mutant OV, with a concomitant increase in apoptotic cell death. While pan-ocular transcription factors such as PAX6, SIX3, LHX2, and PAX2 are present, indicative of maintenance of OV identity, regional expression of VSX2, MITF, OTX2, and NR2F2 is downregulated. Failure of RPE differentiation in Porcn CKO is consistent with downregulation of the Wnt/β-catenin effector LEF1, starting around 2.5 days after inactivation. This suggests that Porcn inactivation affects signaling later than a potential requirement for Wnts to promote eye field formation. Altogether, our data shows a novel requirement for Porcn in regulating growth and morphogenesis of the OV, likely by controlling proliferation and survival. In FDH patients with ocular manifestations, growth deficiency during early ocular morphogenesis may be the underlying cause for microphthalmia.

Germline stem cells in human

The germline cells are essential for the propagation of human beings, thus essential for the survival of mankind. The germline stem cells, as a unique cell type, generate various states of germ stem cells and then differentiate into specialized cells, spermatozoa and ova, for producing offspring, while self-renew to generate more stem cells. Abnormal development of germline stem cells often causes severe diseases in humans, including infertility and cancer. Primordial germ cells (PGCs) first emerge during early embryonic development, migrate into the gentile ridge, and then join in the formation of gonads. In males, they differentiate into spermatogonial stem cells, which give rise to spermatozoa via meiosis from the onset of puberty, while in females, the female germline stem cells (FGSCs) retain stemness in the ovary and initiate meiosis to generate oocytes. Primordial germ cell-like cells (PGCLCs) can be induced in vitro from embryonic stem cells or induced pluripotent stem cells. In this review, we focus on current advances in these embryonic and adult germline stem cells, and the induced PGCLCs in humans, provide an overview of molecular mechanisms underlying the development and differentiation of the germline stem cells and outline their physiological functions, pathological implications, and clinical applications.

Actions of DKK1 on the preimplantation bovine embryo to affect pregnancy establishment, placental function and postnatal phenotype†

One mechanism by which the maternal environment regulates the early embryo is by secretion of cell-signaling molecules. One of these is dickkopf WNT signaling pathway inhibitor 1 (DKK1). Objectives were to A) resolve discrepancies in the literature regarding effects of DKK1 in the bovine embryo on development of trophectoderm (TE) and competence to establish pregnancy after embryo transfer and B) determine whether there are long-term consequences of DKK1 on placental function and postnatal phenotype. Embryos produced in vitro were cultured with vehicle or 100 ng/mL recombinant human DKK1 from day 5 to 7.5 of development (i.e. the morula and blastocyst stages of development). DKK1 increased the number of cells positive for the TE marker CDX2 at day 7.5 of development while having no effect on numbers of cells positive for the inner cell mass marker SOX2. There was no effect of DKK1 on pregnancy or calving rate after transfer of blastocysts produced with Y-sorted semen to either lactating dairy cows or suckling beef cows. Treatment with DKK1 at the morula-to-blastocyst stages programmed placental function, as measured by an effect of DKK1 on plasma concentrations of pregnancy associated glycoproteins and placental lactogen at day 160 of gestation (although not on other days examined). DKK1 treatment also resulted in calves that were heavier at birth as compared to calves derived from control embryos. After birth, DKK1 calves grew slower than controls. Results confirm that DKK1 alters the developmental program of the bovine embryo to affect both prenatal and postnatal phenotype.

Maternal age affects equine day 8 embryo gene expression both in trophoblast and inner cell mass

BACKGROUND

Breeding a mare until she is not fertile or even until her death is common in equine industry but the fertility decreases as the mare age increases. Embryo loss due to reduced embryo quality is partly accountable for this observation. Here, the effect of mare's age on blastocysts' gene expression was explored. Day 8 post-ovulation embryos were collected from multiparous young (YM, 6-year-old, N = 5) and older (OM, > 10-year-old, N = 6) non-nursing Saddlebred mares, inseminated with the semen of one stallion. Pure or inner cell mass (ICM) enriched trophoblast, obtained by embryo bisection, were RNA sequenced. Deconvolution algorithm was used to discriminate gene expression in the ICM from that in the trophoblast. Differential expression was analyzed with embryo sex and diameter as cofactors. Functional annotation and classification of differentially expressed genes and gene set enrichment analysis were also performed.

RESULTS

Maternal aging did not affect embryo recovery rate, embryo diameter nor total RNA quantity. In both compartments, the expression of genes involved in mitochondria and protein metabolism were disturbed by maternal age, although more genes were affected in the ICM. Mitosis, signaling and adhesion pathways and embryo development were decreased in the ICM of embryos from old mares. In trophoblast, ion movement pathways were affected.

CONCLUSIONS

This is the first study showing that maternal age affects gene expression in the equine blastocyst, demonstrating significant effects as early as 10 years of age. These perturbations may affect further embryo development and contribute to decreased fertility due to aging.

Controlling neural territory patterning from pluripotency using a systems developmental biology approach

Successful manufacture of specialized human cells requires process understanding of directed differentiation. Here, we apply high-dimensional Design of Experiments (HD-DoE) methodology to identify critical process parameters (CPPs) that govern neural territory patterning from pluripotency—the first stage toward specification of central nervous system (CNS) cell fates. Using computerized experimental design, 7 developmental signaling pathways were simultaneously perturbed in human pluripotent stem cell culture. Regionally specific genes spanning the anterior-posterior and dorsal-ventral axes of the developing embryo were measured after 3 days and mathematical models describing pathway control were developed using regression analysis. High-dimensional models revealed particular combinations of signaling inputs that induce expression profiles consistent with emerging CNS territories and defined CPPs for anterior and posterior neuroectoderm patterning. The results demonstrate the importance of combinatorial control during neural induction and challenge the use of generic neural induction strategies such as dual-SMAD inhibition, when seeking to specify particular lineages from pluripotency.

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Mathematical models describe pathway control of neuroectoderm marker expression

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Stage 1 media conditions optimized for regionally specific neuroectoderm in 3 days

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Optimized conditions are more consistent than dual-SMADi across hiPSC lines

Roles of Wnt Signaling Pathway and ROR2 Receptor in Embryonic Development: An Update Review Article

The Wnt family is a large class of highly conserved cysteine-rich secretory glycoproteins that play a vital role in various cellular and physiological courses through different signaling pathways during embryogenesis and tissue homeostasis 3. Wnt5a is a secreted glycoprotein that belongs to the noncanonical Wnt family and is involved in a wide range of developmental and tissue homeostasis. A growing body of evidence suggests that Wnt5a affects embryonic development, signaling through various receptors, starting with the activation of β-catenin by Wnt5a. In addition to affecting planar cell polarity and Ca2+ pathways, β-catenin also includes multiple signaling cascades that regulate various cell functions. Secondly, Wnt5a can bind to Ror receptors to mediate noncanonical Wnt signaling and a significant ligand for Ror2 in vertebrates. Consistent with the multiple functions of Wnt5A/Ror2 signaling, Wnt5A knockout mice exhibited various phenotypic defects, including an inability to extend the anterior and posterior axes of the embryo. Numerous essential roles of Wnt5a/Ror2 in development have been demonstrated. Therefore, Ror signaling pathway become a necessary target for diagnosing and treating human diseases. The Wnt5a- Ror2 signaling pathway as a critical factor has attracted extensive attention.

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.

Reconstructing aspects of human embryogenesis with pluripotent stem cells

Understanding human development is of fundamental biological and clinical importance. Despite its significance, mechanisms behind human embryogenesis remain largely unknown. Here, we attempt to model human early embryo development with expanded pluripotent stem cells (EPSCs) in 3-dimensions. We define a protocol that allows us to generate self-organizing cystic structures from human EPSCs that display some hallmarks of human early embryogenesis. These structures mimic polarization and cavitation characteristic of pre-implantation development leading to blastocyst morphology formation and the transition to post-implantation-like organization upon extended culture. Single-cell RNA sequencing of these structures reveals subsets of cells bearing some resemblance to epiblast, hypoblast and trophectoderm lineages. Nevertheless, significant divergences from natural blastocysts persist in some key markers, and signalling pathways point towards ways in which morphology and transcriptional-level cell identities may diverge in stem cell models of the embryo. Thus, this stem cell platform provides insights into the design of stem cell models of embryogenesis.

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.

Formative pluripotent stem cells show features of epiblast cells poised for gastrulation

The pluripotency of mammalian early and late epiblast could be recapitulated by naïve embryonic stem cells (ESCs) and primed epiblast stem cells (EpiSCs), respectively. However, these two states of pluripotency may not be sufficient to reflect the full complexity and developmental potency of the epiblast during mammalian early development. Here we report the establishment of self-renewing formative pluripotent stem cells (fPSCs) which manifest features of epiblast cells poised for gastrulation. fPSCs can be established from different mouse ESCs, pre-/early-gastrula epiblasts and induced PSCs. Similar to pre-/early-gastrula epiblasts, fPSCs show the transcriptomic features of formative pluripotency, which are distinct from naïve ESCs and primed EpiSCs. fPSCs show the unique epigenetic states of E6.5 epiblast, including the super-bivalency of a large set of developmental genes. Just like epiblast cells immediately before gastrulation, fPSCs can efficiently differentiate into three germ layers and primordial germ cells (PGCs) in vitro. Thus, fPSCs highlight the feasibility of using PSCs to explore the development of mammalian epiblast.

Mechanical Tension Promotes Formation of Gastrulation-like Nodes and Patterns Mesoderm Specification in Human Embryonic Stem Cells

Embryogenesis is directed by morphogens that induce differentiation within a defined tissue geometry. Tissue organization is mediated by cell-cell and cell-extracellular matrix (ECM) adhesions and is modulated by cell tension and tissue-level forces. Whether cell tension regulates development by modifying morphogen signaling is less clear. Human embryonic stem cells (hESCs) exhibit an intrinsic capacity for self-organization, which motivates their use as a tractable model of early human embryogenesis. We engineered patterned substrates that recapitulate the biophysical properties of the early embryo and mediate the self-organization of "gastrulation-like" nodes in cultured hESCs. Tissue geometries that generated local nodes of high cell-adhesion tension directed the spatial patterning of the BMP4-dependent "gastrulation-like" phenotype by enhancing phosphorylation and junctional release of β-catenin to promote Wnt signaling and mesoderm specification. Furthermore, direct force application via mechanical stretching promoted BMP-dependent mesoderm specification, confirming that tissue-level forces can directly regulate cell fate specification in early human development.

Mouse gastrulation: Coordination of tissue patterning, specification and diversification of cell fate

During mouse embryonic development a mass of pluripotent epiblast tissue is transformed during gastrulation to generate the three definitive germ layers: endoderm, mesoderm, and ectoderm. During gastrulation, a spatiotemporally controlled sequence of events results in the generation of organ progenitors and positions them in a stereotypical fashion throughout the embryo. Key to the correct specification and differentiation of these cell fates is the establishment of an axial coordinate system along with the integration of multiple signals by individual epiblast cells to produce distinct outcomes. These signaling domains evolve as the anterior-posterior axis is established and the embryo grows in size. Gastrulation is initiated at the posteriorly positioned primitive streak, from which nascent mesoderm and endoderm progenitors ingress and begin to diversify. Advances in technology have facilitated the elaboration of landmark findings that originally described the epiblast fate map and signaling pathways required to execute those fates. Here we will discuss the current state of the field and reflect on how our understanding has shifted in recent years.

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.

Role of Wnt Signaling During In-Vitro Bovine Blastocyst Development and Maturation in Synergism with PPARδ Signaling

Wnt/β-catenin signaling plays vital role in the regulation of cellular proliferation, migration, stem cells cell renewal and genetic stability. This pathway is crucial during the early developmental process; however, the distinct role of Wnt/β-catenin signaling during pre-implantation period of bovine embryonic development is obscure. Here, we evaluated the critical role of Wnt/β-catenin pathway in the regulation of bovine blastocyst (BL) development and hatching. 6 bromoindurbin-3’oxime (6-Bio) was used to stimulate the Wnt signaling. Treatment with 6-Bio induced the expression of peroxisome proliferator-activated receptor-delta (PPARδ). Interestingly, the PPARδ co-localized with β-catenin and form a complex with TCF/LEF transcription factor. This complex potentiated the expression of several Wnt directed genes, which regulate early embryonic development. Inhibition of PPARδ with selective inhibitor 4-chloro-N-(2-{[5-trifluoromethyl]-2-pyridyl]sulfonyl}ethyl)benzamide (Gsk3787) severely perturbed the BL formation and hatching. The addition of Wnt agonist successfully rescued the BL formation and hatching ability. Importantly, the activation of PPARδ expression by Wnt stimulation enhanced cell proliferation and fatty acid oxidation (FAO) metabolism to improve BL development and hatching. In conclusion, our study provides the evidence that Wnt induced PPARδ expression co-localizes with β-catenin and is a likely candidate of canonical Wnt pathway for the regulation of bovine embryonic development.

Comparative analysis of human and mouse development: From zygote to pre-gastrulation

Development of the mammalian embryo begins with formation of the totipotent zygote during fertilization. This initial cell is able to give rise to every embryonic tissue of the developing organism as well as all extra-embryonic lineages, such as the placenta and the yolk sac, which are essential for the initial patterning and support growth of the fetus until birth. As the embryo transits from pre- to post-implantation, major structural and transcriptional changes occur within the embryonic lineage to set up the basis for the subsequent phase of gastrulation. Fine-tuned coordination of cell division, morphogenesis and differentiation is essential to ultimately promote assembly of the future fetus. Here, we review the current knowledge of mammalian development of both mouse and human focusing on morphogenetic processes leading to the onset of gastrulation, when the embryonic anterior-posterior axis becomes established and the three germ layers start to be specified.

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.

The role of WNT10B in normal prostate gland development and prostate cancer

BACKGROUND

WNT signaling is implicated in embryonic development, and in adult tissue homeostasis, while its deregulation is evident in disease. This study investigates the unique roles of canonical WNT10B in both normal prostate development and prostate cancer (PCa) progression.

METHODS

Organ culture and rat ventral prostates (VPs) were used to study Wnt10b ontogeny and growth effect of WNT10B protein. PB-SV40 LTag rat VPs were utilized for Wnt expression polymerase chain reaction (PCR) array and immunohistochemistry. Human localized PCa tissue microarrays (TMAs) were investigated for differential WNT10B expression. Human RNA-seq data sets were queried for differential expression of WNT10B in metastatic and localized PCa. Knockdown of WNT10B in PC3 cells was utilized to study its effects on proliferation, stemness, epithelial to mesenchymal transition (EMT), and xenograft propagation.

RESULTS

Wnt10b expression was highest at birth and rapidly declined in the postnatal rat VP. Exogenous WNT10B addition to culture developing VPs decreased growth suggesting an antiproliferative role. VPs from PB-SV40 LTag rats with localized PCa showed a 25-fold reduction in Wnt10b messenger RNA (mRNA) expession, confirmed at the protein level. Human PCa TMAs revealed elevated WNT10B protein in prostate intraepithelial neoplasia compared with normal prostates but reduced levels in localized PCa specimens. In contrast, RNA-seq data set of annotated human PCa metastasis found a significant increase in WNT10B mRNA expression compared with localized tumors suggesting stage-specific functions of WNT10B. Similarly, WNT10B mRNA levels were increased in metastatic cell lines PC3, PC3M, as well as in HuSLC, a PCa stem-like cell line, as compared with disease-free primary prostate epithelial cells. WNT10B knockdown in PC3 cells reduced expression of EMT genes, MMP9 and stemness genes NANOG and SOX2 and markedly reduced the stem cell-like side population. Furthermore, loss of WNT10B abrogated the ability of PC3 cells to propagate tumors via serial transplantation.

CONCLUSIONS

Taken together, these results suggest a dual role for WNT10B in normal development and in PCa progression with opposing functions depending on disease stage. We propose that decreased WNT10B levels in localized cancer allow for a hyperproliferative state, whereas increased levels in advanced disease confer a stemness and malignant propensity which is mitigated by knocking down WNT10B levels. This raises the potential for WNT10B as a novel target for therapeutic intervention in metastatic PCa.

Analysis of the wnt1 regulatory chromosomal landscape

One of the earliest patterning events in the vertebrate neural plate is the specification of mes/r1, the territory comprising the prospective mesencephalon and the first hindbrain rhombomere. Within mes/r1, an interface of gene expression defines the midbrain-hindbrain boundary (MHB), a lineage restriction that separates the mesencephalon and rhombencephalon. wnt1 is critical to mes/r1 development and functions within the MHB as a component of the MHB gene regulatory network (GRN). Despite its importance to these critical and early steps of vertebrate neurogenesis, little is known about the factors responsible for wnt1 transcriptional regulation. In the zebrafish, wnt1 and its neighboring paralog, wnt10b, are expressed in largely overlapping patterns, suggesting co-regulation. To understand wnt1 and wnt10b transcriptional control, we used a comparative genomics approach to identify relevant enhancers. We show that the wnt1-wnt10b locus contains multiple cis-regulatory elements that likely interact to generate the wnt1 and wnt10b expression patterns. Two of 11 conserved enhancers tested show activity restricted to the midbrain and MHB, an activity that is conserved in the distantly related spotted gar orthologous elements. Three non-conserved elements also play a likely role in wnt1 regulation. The identified enhancers display dynamic modes of chromatin accessibility, suggesting controlled deployment during embryogenesis. Our results suggest that the control of wnt1 and wnt10b expression is under complex regulation involving the interaction of multiple enhancers.

Benzo(a)pyrene inhibits endometrial cell apoptosis in early pregnant mice via the WNT5A pathway

Benzo(a)pyrene (BaP) is an endocrine-disrupting pollutant present in various aspects of daily life, and studies have demonstrated that BaP exerts reproductive toxicity. We previously showed that BaP damages endometrial morphology and decreases the number of implantation sites in early pregnant mice, but the mechanisms underlying these effects remain unclear. The endometrial function is crucial for implantation, which is associated with endometrial cell apoptosis. In this study, we focused on the effect of BaP on endometrial cell apoptosis and the role of WNT signaling during this process. Pregnant mice were gavaged with corn oil (control group) or 0.2 mg·kg^-1 ·day ^-1 BaP (treatment group) from Days 1 to 6 of pregnancy. BaP impaired endometrial function by decreasing the expression of HOXA10 and BMP2, two markers of receptivity and decidualization. WNT5A and β-catenin were activated in the BaP group. BaP affected the expression of apoptosis-related proteins and inhibited the apoptosis of endometrial stromal cells. In vitro, human endometrial stromal cells (HESCs) were treated with different concentrations of BaP (dimethyl sulfoxide (DMSO); 5, 10 µM). WNT5A and β-catenin were also upregulated in the BaP treatment group. HESC apoptosis was restrained by BaP. Inhibiting WNT5A by SFRP5 partially restored the effect of BaP on apoptosis. In summary, these results suggested that BaP exposure during early pregnancy activates WNT5A/β-catenin signaling pathway, which inhibits the endometrial cell apoptosis and potentially destroys endometrial function.

Wnt signalling: conquering complexity

The history of the Wnt pathway is an adventure that takes us from mice and flies to frogs, zebrafish and beyond, sketching the outlines of a molecular signalling cascade along the way. Here, we specifically highlight the instrumental role that developmental biology has played throughout. We take the reader on a journey, starting with developmental genetics studies that identified some of the main molecular players, through developmental model organisms that helped unravel their biochemical function and cell biological activities. Culminating in complex analyses of stem cell fate and dynamic tissue growth, these efforts beautifully illustrate how different disciplines provided missing pieces of a puzzle. Together, they have shaped our mechanistic understanding of the Wnt pathway as a conserved signalling process in development and disease. Today, researchers are still uncovering additional roles for Wnts and other members of this multifaceted signal transduction pathway, opening up promising new avenues for clinical applications.

Blastocyst-like structures generated solely from stem cells

The blastocyst (the early mammalian embryo) forms all embryonic and extra-embryonic tissues, including the placenta. It consists of a spherical thin-walled layer, known as the trophectoderm, that surrounds a fluid-filled cavity sheltering the embryonic cells ¹ . From mouse blastocysts, it is possible to derive both trophoblast ² and embryonic stem-cell lines ³ , which are in vitro analogues of the trophectoderm and embryonic compartments, respectively. Here we report that trophoblast and embryonic stem cells cooperate in vitro to form structures that morphologically and transcriptionally resemble embryonic day 3.5 blastocysts, termed blastoids. Like blastocysts, blastoids form from inductive signals that originate from the inner embryonic cells and drive the development of the outer trophectoderm. The nature and function of these signals have been largely unexplored. Genetically and physically uncoupling the embryonic and trophectoderm compartments, along with single-cell transcriptomics, reveals the extensive inventory of embryonic inductions. We specifically show that the embryonic cells maintain trophoblast proliferation and self-renewal, while fine-tuning trophoblast epithelial morphogenesis in part via a BMP4/Nodal-KLF6 axis. Although blastoids do not support the development of bona fide embryos, we demonstrate that embryonic inductions are crucial to form a trophectoderm state that robustly implants and triggers decidualization in utero. Thus, at this stage, the nascent embryo fuels trophectoderm development and implantation.

Consequences of endogenous and exogenous WNT signaling for development of the preimplantation bovine embryo

The specific role of WNT signaling during preimplantation development remains unclear. Here, we evaluated consequences of activation and inhibition of β-catenin (CTNNB1)-dependent and -independent WNT signaling in the bovine preimplantation embryo. Activation of CTNNB1-mediated WNT signaling by the agonist 2-amino-4-(3,4-(methylenedioxy)benzylamino)-6-(3-methoxyphenyl)pyrimidine (AMBMP) and a glycogen synthase kinase 3 inhibitor reduced development to the blastocyst stage. Moreover, the antagonist of WNT signaling, dickkopf-related protein 1 (DKK1), alleviated the negative effect of AMBMP on development via reduction of CTNNB1. Based on labeling for phospho c-Jun N-terminal kinase, there was no evidence that DKK1 activated the planar cell polarity (PCP) pathway. Inhibition of secretion of endogenous WNTs did not affect development but increased number of cells in the inner cell mass (ICM). In contrast, DKK1 did not affect number of ICM or trophectoderm (TE) cells, suggesting that embryo-derived WNTs regulate ICM proliferation through a mechanism independent of CTNNB1. In addition, DKK1 did not affect the number of cells positive for the transcription factor yes-associated protein 1 (YAP1) involved in TE formation. In fact, DKK1 decreased YAP1. In contrast, exposure of embryos to WNT family member 7A (WNT7A) improved blastocyst development, inhibited the PCP pathway, and did not affect amounts of CTNNB1. Results indicate that embryo-derived WNTs are dispensable for blastocyst formation but participate in regulation of ICM proliferation, likely through a mechanism independent of CTNNB1. The response to AMBMP and WNT7A leads to the hypothesis that maternally derived WNTs can play a positive or negative role in regulation of preimplantation development.

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

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

What Can Stem Cell Models Tell Us About Human Germ Cell Biology?

Fusion of sperm and egg generates a totipotent zygote that develops into a whole organism. Accordingly, the "immortal" germline transmits genetic and epigenetic information to subsequent generations with consequences for human health and disease. In mammals, primordial germ cells (PGCs) originate from peri-gastrulation embryos. While early human embryos are inaccessible for research, in vitro model systems using pluripotent stem cells have provided critical insights into human PGC specification, which differs from that in mice. This might stem from significant differences in early embryogenesis at the morphological and molecular levels, including pluripotency networks. Here, we discuss recent advances and experimental systems used to study mammalian germ cell development. We also highlight key aspects of germ cell disorders, as well as mitochondrial and potentially epigenetic inheritance in humans.

Complexity of the Wnt/β‑catenin pathway: Searching for an activation model

Wnt signaling refers to a conserved signaling pathway, widely studied due to its roles in cellular communication, cell fate decisions, development and cancer. However, the exact mechanism underlying inhibition of the GSK phosphorylation towards β-catenin and activation of the pathway after biding of Wnt ligand to its cognate receptors at the plasma membrane remains unclear. Wnt target genes are widely spread over several animal phyla. They participate in a plethora of functions during the development of an organism, from axial specification, gastrulation and organogenesis all the way to regeneration and repair in adults. Temporal and spatial oncogenetic re-activation of Wnt signaling almost certainly leads to cancer. Wnt signaling components have been extensively studied as possible targets in anti-cancer therapies. In this review we will discuss one of the most intriguing questions in this field, that is how β-catenin, a major component in this pathway, escapes the destruction complex, gets stabilized in the cytosol and it is translocated to the nucleus where it acts as a co-transcription factor. Four major models have evolved during the past 20years. We dissected each of them along with current views and future perspectives on this pathway. This review will focus on the molecular mechanisms by which Wnt proteins modulate β-catenin cytoplasmic levels and the relevance of this pathway for the development and cancer.

Wnt/β-catenin signaling during early vertebrate neural development

The vertebrate central nervous system (CNS) is comprised of vast number of distinct cell types arranged in a highly organized manner. This high degree of complexity is achieved by cellular communication, including direct cell-cell contact, cell-matrix interactions, and cell-growth factor signaling. Among the several developmental signals controlling the development of the CNS, Wnt proteins have emerged as particularly critical and, hence, have captivated the attention of many researchers. With Wnts' evolutionarily conserved function as primordial symmetry breaking signals, these proteins and their downstream effects are responsible for simultaneously establishing cellular diversity and tissue organization. With their expansive repertoire of secreted agonists and antagonists, cell surface receptors, signaling cascades and downstream biological effects, Wnts are ideally suited to control the complex processes underlying vertebrate neural development. In this review, we will describe the mechanisms by which Wnts exert their potent effects on cells and tissues and highlight the many roles of Wnt signaling during neural development, starting from the initial induction of the neural plate, the subsequent patterning along the embryonic axes, to the intricately organized structure of the CNS. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 1239-1259, 2017.

The Dkk3 gene encodes a vital intracellular regulator of cell proliferation

Members of the Dickkopf (Dkk) family of Wnt antagonists interrupt Wnt-induced receptor assembly and participate in axial patterning and cell fate determination. One family member, DKK3, does not block Wnt receptor activation. Loss of Dkk3 expression in cancer is associated with hyperproliferation and dysregulated ß-catenin signaling, and ectopic expression of Dkk3 halts cancer growth. The molecular events mediating the DKK3-dependent arrest of ß-catenin-driven cell proliferation in cancer cells are unknown. Here we report the identification of a new intracellular gene product originating from the Dkk3 locus. This Dkk3b transcript originates from a second transcriptional start site located in intron 2 of the Dkk3 gene. It is essential for early mouse development and is a newly recognized regulator of ß-catenin signaling and cell proliferation. Dkk3b interrupts nuclear translocation ß-catenin by capturing cytoplasmic, unphosphorylated ß-catenin in an extra-nuclear complex with ß-TrCP. These data reveal a new regulator of one of the most studied signal transduction pathways in metazoans and provides a novel, completely untapped therapeutic target for silencing the aberrant ß-catenin signaling that drives hyperproliferation in many cancers.

Role of Wnt signalling in early pregnancy

The integration of a complex network of signalling molecules promotes implantation of the blastocyst and development of the placenta. These processes are crucial for a successful pregnancy and fetal growth and development. The signalling network involves both cell-cell and cell-extracellular matrix communication. The family of secreted glycoprotein ligands, the Wnts, plays a major role in regulating a wide range of biological processes, including embryonic development, cell fate, proliferation, migration, stem cell maintenance, tumour suppression, oncogenesis and tissue homeostasis. Recent studies have provided evidence that Wnt signalling pathways play an important role in reproductive tissues and in early pregnancy events. The focus of this review is to summarise our present knowledge of expression, regulation and function of the Wnt signalling pathways in early pregnancy events of human and other model systems, and its association with pathological conditions. Despite our recent progress, much remains to be learned about Wnt signalling in human reproduction. The advancement of knowledge in this area has applications in the reduction of infertility and the incidence and morbidity of gestational diseases.

Vertebrate Axial Patterning: From Egg to Asymmetry

The emergence of the bilateral embryonic body axis from a symmetrical egg has been a long-standing question in developmental biology. Historical and modern experiments point to an initial symmetry-breaking event leading to localized Wnt and Nodal growth factor signaling and subsequent induction and formation of a self-regulating dorsal "organizer." This organizer forms at the site of notochord cell internalization and expresses primarily Bone Morphogenetic Protein (BMP) growth factor antagonists that establish a spatiotemporal gradient of BMP signaling across the embryo, directing initial cell differentiation and morphogenesis. Although the basics of this model have been known for some time, many of the molecular and cellular details have only recently been elucidated and the extent that these events remain conserved throughout vertebrate evolution remains unclear. This chapter summarizes historical perspectives as well as recent molecular and genetic advances regarding: (1) the mechanisms that regulate symmetry-breaking in the vertebrate egg and early embryo, (2) the pathways that are activated by these events, in particular the Wnt pathway, and the role of these pathways in the formation and function of the organizer, and (3) how these pathways also mediate anteroposterior patterning and axial morphogenesis. Emphasis is placed on comparative aspects of the egg-to-embryo transition across vertebrates and their evolution. The future prospects for work regarding self-organization and gene regulatory networks in the context of early axis formation are also discussed.

A Comparative Perspective on Wnt/β-Catenin Signalling in Cell Fate Determination

The Wnt/β-catenin pathway is an ancient and highly conserved signalling pathway that plays fundamental roles in the regulation of embryonic development and adult homeostasis. This pathway has been implicated in numerous cellular processes, including cell proliferation, differentiation, migration, morphological changes and apoptosis. In this chapter, we aim to illustrate with specific examples the involvement of Wnt/β-catenin signalling in cell fate determination. We discuss the roles of the Wnt/β-catenin pathway in specifying cell fate throughout evolution, how its function in patterning during development is often reactivated during regeneration and how perturbation of this pathway has negative consequences for the control of cell fate.The origin of all life was a single cell that had the capacity to respond to cues from the environment. With evolution, multicellular organisms emerged, and as a result, subsets of cells arose to form tissues able to respond to specific instructive signals and perform specialised functions. This complexity and specialisation required two types of messages to direct cell fate: intra- and intercellular. A fundamental question in developmental biology is to understand the underlying mechanisms of cell fate choice. Amongst the numerous external cues involved in the generation of cellular diversity, a prominent pathway is the Wnt signalling pathway in all its forms.

CFTR-β-catenin interaction regulates mouse embryonic stem cell differentiation and embryonic development

Cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-regulated anion channel capable of conducting both Cl^- and HCO₃^-, mutations of which cause cystic fibrosis (CF), a common autosomal recessive disease. Although CF patients are known to have varied degree of developmental problems, the biological role of CFTR in embryonic development remains elusive. Here, we show that CFTR is functionally expressed in mouse ESCs. CFTR^-/- mESCs exhibit dramatic defect in mesendoderm differentiation. In addition, CFTR physically interacts with β-catenin, defect of which leads to premature degradation of β-catenin and suppressed activation of β-catenin signaling. Furthermore, knockdown of CFTR retards the early development of Xenopus laevis with impaired mesoderm/endoderm differentiation and β-catenin signaling. Our study reveals a previously undefined role of CFTR in controlling ESC differentiation and early embryonic development via its interaction with β-catenin, and provides novel insights into the understanding of embryonic development.

Wnt signaling promotes hindgut fate commitment through regulating multi-lineage genes during hESC differentiation

Wnt signaling plays essential roles in both embryonic pattern formation and postembryonic tissue homoestasis. High levels of Wnt activity repress foregut identity and facilitate hindgut fate through forming a gradient of Wnt signaling activity along the anterior-posterior axis. Here, we examined the mechanisms of Wnt signaling in hindgut development by differentiating human embryonic stem cells (hESCs) into the hindgut progenitors. We observed severe morphological changes when Wnt signaling was blocked by using Wnt antagonist Dkk1. We performed deep-transcriptome sequencing (RNA-seq) and identified 240 Wnt-activated genes and 2023 Wnt-repressed genes, respectively. Clusters of Wnt targets showed enrichment in specific biological functions, such as "gastrointestinal or skeletal development" in the Wnt-activated targets and "neural or immune system development" in the Wnt-repressed targets. Moreover, we adopted a high-throughput chromatin immunoprecipitation and deep sequencing (ChIP-seq) approach to identify the genomic regions through which Wnt-activated transcription factor TCF7L2 regulated transcription. We identified 83 Wnt direct target candidates, including the hindgut marker CDX2 and the genes relevant to morphogenesis (MSX1, MSX2, LEF1, T, PDGFRB etc.) through combinatorial analysis of the RNA-seq and ChIP-seq data. Together, our study identified a series of direct and indirect Wnt targets in hindgut differentiation, and uncovered the diverse mechanisms of Wnt signaling in regulating multi-lineage differentiation.

Butyrate and bioactive proteolytic form of Wnt-5a regulate colonic epithelial proliferation and spatial development

Proliferation and spatial development of colonic epithelial cells are highly regulated along the crypt vertical axis, which, when perturbed, can result in aberrant growth and carcinogenesis. In this study, two key factors were identified that have important and counterbalancing roles regulating these processes: pericrypt myofibroblast-derived Wnt-5a and the microbial metabolite butyrate. Cultured YAMC cell proliferation and heat shock protein induction were analzyed after butryate, conditioned medium with Wnt5a activity, and FrzB containing conditioned medium. In vivo studies to modulate Hsp25 employed intra-colonic wall Hsp25 encoding lentivirus. To silence Wnt-5a in vivo, intra-colonic wall Wnt-5a silencing RNA was used. Wnt-5a, secreted by stromal myofibroblasts of the lower crypt, promotes proliferation through canonical β-catenin activation. Essential to this are two key requirements: (1) proteolytic conversion of the highly insoluble ~40 kD Wnt-5a protein to a soluble 36 mer amino acid peptide that activates epithelial β-catenin and cellular proliferation, and (2) the simultaneous inhibition of butyrate-induced Hsp25 by Wnt-5a which is necessary to arrest the proliferative process in the upper colonic crypt. The interplay and spatial gradients of these factors insures that crypt epithelial cell proliferation and development proceed in an orderly fashion, but with sufficient plasticity to adapt to physiological perturbations including inflammation.

Kremen1 regulates mechanosensory hair cell development in the mammalian cochlea and the zebrafish lateral line

Here we present spatio-temporal localization of Kremen1, a transmembrane receptor, in the mammalian cochlea, and investigate its role in the formation of sensory organs in mammal and fish model organisms. We show that Kremen1 is expressed in prosensory cells during cochlear development and in supporting cells of the adult mouse cochlea. Based on this expression pattern, we investigated whether Kremen1 functions to modulate cell fate decisions in the prosensory domain of the developing cochlea. We used gain and loss-of-function experiments to show that Kremen1 is sufficient to bias cells towards supporting cell fate, and is implicated in suppression of hair cell formation. In addition to our findings in the mouse cochlea, we examined the effects of over expression and loss of Kremen1 in the zebrafish lateral line. In agreement with our mouse data, we show that over expression of Kremen1 has a negative effect on the number of mechanosensory cells that form in the zebrafish neuromasts, and that fish lacking Kremen1 protein develop more hair cells per neuromast compared to wild type fish. Collectively, these data support an inhibitory role for Kremen1 in hair cell fate specification.

A new gain-of-function mouse line to study the role of Wnt3a in development and disease

Wnt/β-catenin signals are important regulators of embryonic and adult stem cell self-renewal and differentiation and play causative roles in tumorigenesis. Purified recombinant Wnt3a protein, or Wnt3a-conditioned culture medium, has been widely used to study canonical Wnt signaling in vitro or ex vivo. To study the role of Wnt3a in embryogenesis and cancer models, we developed a Cre recombinase activatable Rosa26(Wnt3a) allele, in which a Wnt3a cDNA was inserted into the Rosa26 locus to allow for conditional, spatiotemporally defined expression of Wnt3a ligand for gain-of-function (GOF) studies in mice. To validate this reagent, we ectopically overexpressed Wnt3a in early embryonic progenitors using the T-Cre transgene. This resulted in up-regulated expression of a β-catenin/Tcf-Lef reporter and of the universal Wnt/β-catenin pathway target genes, Axin2 and Sp5. Importantly, T-Cre; Rosa26(Wnt3a) mutants have expanded presomitic mesoderm (PSM) and compromised somitogenesis and closely resemble previously studied T-Cre; Ctnnb1(ex3) (β-catenin(GOF) ) mutants. These data indicate that the exogenously expressed Wnt3a stimulates the Wnt/β-catenin signaling pathway, as expected. The Rosa26(Wnt3a) mouse line should prove to be an invaluable tool to study the function of Wnt3a in vivo.