The mouse secreted frizzled-related protein 5 gene is expressed in the anterior visceral endoderm and foregut endoderm during early post-implantation development

Spatial transcriptomic characterization of a Carnegie stage 7 human embryo

Gastrulation marks a pivotal stage in mammalian embryonic development, establishing the three germ layers and body axis through lineage diversification and morphogenetic movements. However, studying human gastrulating embryos is challenging due to limited access to early tissues. Here we show the use of spatial transcriptomics to analyse a fully intact Carnegie stage 7 human embryo at single-cell resolution, along with immunofluorescence validations in a second embryo. Employing 82 serial cryosections and Stereo-seq technology, we reconstructed a three-dimensional model of the embryo. Our findings reveal early specification of distinct mesoderm subtypes and the presence of the anterior visceral endoderm. Notably, primordial germ cells were located in the connecting stalk, and haematopoietic stem cell-independent haematopoiesis was observed in the yolk sac. This study advances our understanding of human gastrulation and provides a valuable dataset for future research in early human development.

Study on gene expression in the liver at various developmental stages of human embryos

Background

The normal development of the liver during human embryonic stages is critical for the functionality of the adult liver. Despite this, the essential genes, biological processes, and signal pathways that drive liver development in human embryos remain poorly understood.

Methods

In this study, liver samples were collected from human embryos at progressive developmental stages, ranging from 2-month-old to 7-month-old. Highly expressed genes and their associated enrichment processes at various developmental stages of the liver were identified through transcriptomic sequencing.

Results

The findings indicated that genes associated with humoral immune responses and B-cell-mediated immunity were highly expressed during the early developmental stages. Concurrently, numerous genes related to vitamin response, brown adipocyte differentiation, T cell differentiation, hormone secretion, hemostasis, peptide hormone response, steroid metabolism, and hematopoietic regulation exhibited increased expression aligned with liver development. Our results suggest that the liver may possess multiple functions during embryonic stages, beyond serving hematopoietic roles. Moreover, this study elucidated the complex regulatory interactions among genes involved in lymphocyte differentiation, the regulation of hemopoiesis, and liver development. Consequently, the development of human embryonic liver necessitates the synergistic regulation of numerous genes. Notably, alongside conventionally recognized genes, numerous previously uncharacterized genes involved in liver development and function were also identified.

Conclusion

These findings establish a critical foundation for future research on liver development and diseases arising from fetal liver abnormalities.

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

Network inference with Granger causality ensembles on single-cell transcriptomics

Cellular gene expression changes throughout a dynamic biological process, such as differentiation. Pseudotimes estimate cells' progress along a dynamic process based on their individual gene expression states. Ordering the expression data by pseudotime provides information about the underlying regulator-gene interactions. Because the pseudotime distribution is not uniform, many standard mathematical methods are inapplicable for analyzing the ordered gene expression states. Here we present single-cell inference of networks using Granger ensembles (SINGE), an algorithm for gene regulatory network inference from ordered single-cell gene expression data. SINGE uses kernel-based Granger causality regression to smooth irregular pseudotimes and missing expression values. It aggregates predictions from an ensemble of regression analyses to compile a ranked list of candidate interactions between transcriptional regulators and target genes. In two mouse embryonic stem cell differentiation datasets, SINGE outperforms other contemporary algorithms. However, a more detailed examination reveals caveats about poor performance for individual regulators and uninformative pseudotimes.

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.

Wnt-β-catenin signalling in liver development, health and disease

The canonical Wnt-β-catenin pathway is a complex, evolutionarily conserved signalling mechanism that regulates fundamental physiological and pathological processes. Wnt-β-catenin signalling tightly controls embryogenesis, including hepatobiliary development, maturation and zonation. In the mature healthy liver, the Wnt-β-catenin pathway is mostly inactive but can become re-activated during cell renewal and/or regenerative processes, as well as in certain pathological conditions, diseases, pre-malignant conditions and cancer. In hepatocellular carcinoma (HCC) and cholangiocarcinoma (CCA), the two most prevalent primary liver tumours in adults, Wnt-β-catenin signalling is frequently hyperactivated and promotes tumour growth and dissemination. A substantial proportion of liver tumours (mainly HCC and, to a lesser extent, CCA) have mutations in genes encoding key components of the Wnt-β-catenin signalling pathway. Likewise, hepatoblastoma, the most common paediatric liver cancer, is characterized by Wnt-β-catenin activation, mostly as a result of β-catenin mutations. In this Review, we discuss the most relevant molecular mechanisms of action and regulation of Wnt-β-catenin signalling in liver development and pathophysiology. Moreover, we highlight important preclinical and clinical studies and future directions in basic and clinical research.

A single-cell molecular map of mouse gastrulation and early organogenesis

Across the animal kingdom, gastrulation represents a key developmental event during which embryonic pluripotent cells diversify into lineage-specific precursors that will generate the adult organism. Here we report the transcriptional profiles of 116,312 single cells from mouse embryos collected at nine sequential time points ranging from 6.5 to 8.5 days post-fertilization. We construct a molecular map of cellular differentiation from pluripotency towards all major embryonic lineages, and explore the complex events involved in the convergence of visceral and primitive streak-derived endoderm. Furthermore, we use single-cell profiling to show that Tal1-/- chimeric embryos display defects in early mesoderm diversification, and we thus demonstrate how combining temporal and transcriptional information can illuminate gene function. Together, this comprehensive delineation of mammalian cell differentiation trajectories in vivo represents a baseline for understanding the effects of gene mutations during development, as well as a roadmap for the optimization of in vitro differentiation protocols for regenerative medicine.

TGF-β Family Signaling in Early Vertebrate Development

TGF-β family ligands function in inducing and patterning many tissues of the early vertebrate embryonic body plan. Nodal signaling is essential for the specification of mesendodermal tissues and the concurrent cellular movements of gastrulation. Bone morphogenetic protein (BMP) signaling patterns tissues along the dorsal-ventral axis and simultaneously directs the cell movements of convergence and extension. After gastrulation, a second wave of Nodal signaling breaks the symmetry between the left and right sides of the embryo. During these processes, elaborate regulatory feedback between TGF-β ligands and their antagonists direct the proper specification and patterning of embryonic tissues. In this review, we summarize the current knowledge of the function and regulation of TGF-β family signaling in these processes. Although we cover principles that are involved in the development of all vertebrate embryos, we focus specifically on three popular model organisms: the mouse Mus musculus, the African clawed frog of the genus Xenopus, and the zebrafish Danio rerio, highlighting the similarities and differences between these species.

Deregulation of Frizzled Receptors in Hepatocellular Carcinoma

G protein-coupled receptors (GPCRs) have a substantial role in tumorigenesis and are described as a “cancer driver”. Aberrant expression or activation of GPCRs leads to the deregulation of downstream signaling pathways, thereby promoting cancer progression. In hepatocellular carcinoma (HCC), the Wnt signaling pathway is frequently activated and it is associated with an aggressive HCC phenotype. Frizzled (FZD) receptors, a family member of GPCRs, are known to mediate Wnt signaling. Accumulating findings have revealed the deregulation of FZD receptors in HCC and their functional roles have been implicated in HCC progression. Given the important role of FZD receptors in HCC, we summarize here the expression pattern of FZD receptors in HCC and their corresponding functional roles during HCC progression. We also further review and highlight the potential targeting of FZD receptors as an alternative therapeutic strategy in HCC.

From Endoderm to Liver Bud: Paradigms of Cell Type Specification and Tissue Morphogenesis

The early specification, rapid growth and morphogenesis, and conserved functions of the embryonic liver across diverse model organisms have made the system an experimentally facile paradigm for understanding basic regulatory mechanisms that govern cell differentiation and organogenesis. This essay highlights concepts that have emerged from studies of the discrete steps of foregut endoderm development into the liver bud, as well as from modeling the steps via embryonic stem cell differentiation. Such concepts include understanding the chromatin basis for the competence of progenitor cells to develop into specific lineages; the importance of combinatorial signaling from different sources to induce cell fates; the impact of inductive signaling on preexisting chromatin states; the ability of separately specified domains of cells to merge into a common tissue; and the marked cell biological dynamics, including interactions with the developing vasculature, which establish the initial morphogenesis and patterning of a tissue. The principles gleaned from these studies, focusing on the 2 days it takes for the endoderm to develop into a liver bud, should be instructive for many other organogenic systems and for manipulating tissues in regenerative contexts for biomedical purposes.

Transcriptomic analysis of murine embryos lacking endogenous retinoic acid signaling

Retinoic acid (RA), an active derivative of the liposoluble vitamin A (retinol), acts as an important signaling molecule during embryonic development, regulating phenomenons as diverse as anterior-posterior axial patterning, forebrain and optic vesicle development, specification of hindbrain rhombomeres, pharyngeal arches and second heart field, somitogenesis, and differentiation of spinal cord neurons. This small molecule directly triggers gene activation by binding to nuclear receptors (RARs), switching them from potential repressors to transcriptional activators. The repertoire of RA-regulated genes in embryonic tissues is poorly characterized. We performed a comparative analysis of the transcriptomes of murine wild-type and Retinaldehyde Dehydrogenase 2 null-mutant (Raldh2^−/−) embryos — unable to synthesize RA from maternally-derived retinol — using Affymetrix DNA microarrays. Transcriptomic changes were analyzed in two embryonic regions: anterior tissues including forebrain and optic vesicle, and posterior (trunk) tissues, at early stages preceding the appearance of overt phenotypic abnormalities. Several genes expected to be downregulated under RA deficiency appeared in the transcriptome data (e.g. Emx2, Foxg1 anteriorly, Cdx1, Hoxa1, Rarb posteriorly), whereas reverse-transcriptase-PCR and in situ hybridization performed for additional selected genes validated the changes identified through microarray analysis. Altogether, the affected genes belonged to numerous molecular pathways and cellular/organismal functions, demonstrating the pleiotropic nature of RA-dependent events. In both tissue samples, genes upregulated were more numerous than those downregulated, probably due to feedback regulatory loops. Bioinformatic analyses highlighted groups (clusters) of genes displaying similar behaviors in mutant tissues, and biological functions most significantly affected (e.g. mTOR, VEGF, ILK signaling in forebrain tissues; pyrimidine and purine metabolism, calcium signaling, one carbon metabolism in posterior tissues). Overall, these data give an overview of the gene expression changes resulting from embryonic RA deficiency, and provide new candidate genes and pathways that may help understanding retinoid-dependent molecular events.

Wnt/beta-catenin signaling in hepatic organogenesis

Wnt/beta-catenin signaling has come to the forefront of liver biology in recent years. This pathway regulates key pathophysiological events inherent to the liver including development, regeneration and cancer, by dictating several biological processes such as proliferation, apoptosis, differentiation, adhesion, zonation and metabolism in various cells of the liver. This review will examine the studies that have uncovered the relevant roles of Wnt/beta-catenin signaling during the process of liver development. We will discuss the potential roles of Wnt/beta-catenin signaling during the phases of development, including competence, hepatic induction, expansion and morphogenesis. In addition, we will discuss the role of negative and positive regulation of this pathway and how the temporal expression of Wnt/beta-catenin can direct key processes during hepatic development. We will also identify some of the major deficits in the current understanding of the role of Wnt/beta-catenin signaling in liver development in order to provide a perspective for future studies. Thus, this review will provide a contextual overview of the role of Wnt/beta-catenin signaling during hepatic organogenesis.

Transcriptional networks in liver and intestinal development

The development of the gastrointestinal tract is a complex process that integrates signaling processes with downstream transcriptional responses. Here, we discuss the regionalization of the primitive gut and formation of the intestine and liver. Anterior-posterior position in the primitive gut is important for establishing regions that will become functional organs. Coordination of signaling between the epithelium and mesenchyme and downstream transcriptional responses is required for intestinal development and homeostasis. Liver development uses a complex transcriptional network that controls the establishment of organ domains, cell differentiation, and adult function. Discussion of these transcriptional mechanisms gives us insight into how the primitive gut, composed of simple endodermal cells, develops into multiple diverse cell types that are organized into complex mature organs.

Calpain induces N-terminal truncation of β-catenin in normal murine liver development: diagnostic implications in hepatoblastomas

Hepatic competence, specification, and liver bud expansion during development depend on precise temporal modulation of the Wnt/β-catenin signaling. Also, loss- and gain-of-function studies have revealed pleiotropic roles of β-catenin in proliferation and hepatocyte and biliary epithelial cell differentiation, but precise mechanisms remain unknown. Here we utilize livers from different stages of murine development to determine β-catenin signaling and downstream targets. Although during early liver development full-length β-catenin is the predominant form, at late stages, where full-length β-catenin localizes to developing biliary epithelial cells only, a 75-kDa truncated β-catenin species is the principal form localizing at the membrane and in the nucleus of differentiating hepatocytes. The truncated species lacks 95 N-terminal amino acids and is transcriptionally active. Our evidence points to proteolytic cleavage of β-catenin by calpain as the mechanism of truncation in cell-free and cell-based assays. Intraperitoneal injection of a short term calpain inhibitor to timed pregnant female mice abrogated β-catenin truncation in the embryonic livers. RNA-seq revealed a unique set of targets transcribed in cells expressing truncated versus full-length β-catenin, consistent with different functionalities. A further investigation using N- and C-terminal-specific β-catenin antibodies on human hepatoblastomas revealed a correlation between full-length versus truncated β-catenin and differentiation status, with embryonal hepatoblastomas expressing full-length β-catenin and fetal hepatoblastomas expressing β-catenin lacking its N terminus. Thus we conclude that calpain-mediated cleavage of β-catenin plays a role in regulating hepatoblast differentiation in mouse and human liver, and the presence of the β-catenin N terminus correlates with differentiation status in hepatoblastomas.

Pathophysiologic role of hepatocyte nuclear factor 6

Hepatocyte nuclear factor 6 (HNF6) is one of liver-enriched transcription factors. HNF6 utilizes the bipartite onecut-homeodomain sequence to localize the HNF6 protein to the nuclear compartment and binds to specific DNA sequences of numerous target gene promoters. HNF6 regulates an intricate network and mediates complex biological processes that are best known in the liver and pancreas. The function of HNF6 is correlated to cell proliferation, cell cycle regulation, cell differentiation and organogenesis, cell migration and cell-matrix adhesion, glucose metabolism, bile homeostasis, inflammation and so on. HNF6 controls the transcription of its target genes in different ways. The details of the regulatory pathways and their mechanisms are still under investigation. Future study will explore HNF6 novel functions associated with apoptosis, oncogenesis, and modulation of the inflammatory response. This review highlights recent progression pertaining to the pathophysiologic role of HNF6 and summarizes the potential mechanisms in preclinical animal models. HNF6-mediated pathways represent attractive therapeutic targets for the treatment of the relative diseases such as cholestasis.

A targeted NKX2.1 human embryonic stem cell reporter line enables identification of human basal forebrain derivatives

We have used homologous recombination in human embryonic stem cells (hESCs) to insert sequences encoding green fluorescent protein (GFP) into the NKX2.1 locus, a gene required for normal development of the basal forebrain. Generation of NKX2.1-GFP(+) cells was dependent on the concentration, timing, and duration of retinoic acid treatment during differentiation. NKX2.1-GFP(+) progenitors expressed genes characteristic of the basal forebrain, including SHH, DLX1, LHX6, and OLIG2. Time course analysis revealed that NKX2.1-GFP(+) cells could upregulate FOXG1 expression, implying the existence of a novel pathway for the generation of telencephalic neural derivatives. Further maturation of NKX2.1-GFP(+) cells gave rise to γ-aminobutyric acid-, tyrosine hydroxylase-, and somatostatin-expressing neurons as well as to platelet-derived growth factor receptor α-positive oligodendrocyte precursors. These studies highlight the diversity of cell types that can be generated from human NKX2.1(+) progenitors and demonstrate the utility of NKX2.1(GFP/w) hESCs for investigating human forebrain development and neuronal differentiation.

The molecular and cellular basis of variable craniofacial phenotypes and their genetic rescue in Twisted gastrulation mutant mice

The severity of numerous developmental abnormalities can vary widely despite shared genetic causes. Mice deficient in Twisted gastrulation (Twsg1(-/-)) display such phenotypic variation, developing a wide range of craniofacial malformations on an isogenic C57BL/6 strain background. To examine the molecular basis for this reduced penetrance and variable expressivity, we used exon microarrays to analyze gene expression in mandibular arches from several distinct, morphologically defined classes of Twsg1(-/-) and wild type (WT) embryos. Hierarchical clustering analysis of transcript levels identified numerous differentially expressed genes, clearly distinguishing severely affected and unaffected Twsg1(-/-) mutants from WT embryos. Several genes that play well-known roles in craniofacial development were upregulated in unaffected Twsg1(-/-) mutant embryos, suggesting that they may compensate for the loss of TWSG1. Imprinted genes were overrepresented among genes that were differentially expressed particularly between affected and unaffected mutants. The most severely affected embryos demonstrated increased p53 signaling and increased expression of its target, Trp53inp1. The frequency of craniofacial defects significantly decreased with a reduction of p53 gene dosage from 44% in Twsg1(-/-)p53(+/+) pups (N=675) to 30% in Twsg1(-/-)p53(+/-) (N=47, p=0.04) and 15% in Twsg1(-/-)p53(-/-) littermates (N=39, p=0.001). In summary, these results demonstrate that phenotypic variability in Twsg1(-/-) mice is associated with differential expression of certain developmentally regulated genes, and that craniofacial defects can be partially rescued by reduced p53 levels. We postulate that variable responses to stress may contribute to variable craniofacial phenotypes by triggering differential expression of genes and variable cellular apoptosis.

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.

Embryonic and adult stem cell systems in mammals: ontology and regulation

Stem cells are defined as having the ability to self-renew and to generate differentiated cells. During embryogenesis, cells are initially proliferative and pluripotent and then they gradually become restricted to different cell fates. In the adult, tissue stem cells are normally quiescent, but become proliferative upon injury. Knowledge from developmental biology and insights into the properties of stem cells are keys to further understanding and successful manipulation. Here, we first focus on ES cells, then on embryonic development, and then on tissue stem cells of endodermally derived tissues, particularly the liver and pancreas.

Liver development, regeneration, and carcinogenesis

The identification of putative liver stem cells has brought closer the previously separate fields of liver development, regeneration, and carcinogenesis. Significant overlaps in the regulation of these processes are now being described. For example, studies in embryonic liver development have already provided the basis for directed differentiation of human embryonic stem cells and induced pluripotent stem cells into hepatocyte-like cells. As a result, the understanding of the cell biology of proliferation and differentiation in the liver has been improved. This knowledge can be used to improve the function of hepatocyte-like cells for drug testing, bioartificial livers, and transplantation. In parallel, the mechanisms regulating cancer cell biology are now clearer, providing fertile soil for novel therapeutic approaches. Recognition of the relationships between development, regeneration, and carcinogenesis, and the increasing evidence for the role of stem cells in all of these areas, has sparked fresh enthusiasm in understanding the underlying molecular mechanisms and has led to new targeted therapies for liver cirrhosis and primary liver cancers.

Gata4 directs development of cardiac-inducing endoderm from ES cells

The transcription factor Gata4 is essential for normal heart morphogenesis and regulates the survival, growth, and proliferation of cardiomyocytes. We tested if Gata4 can specify cardiomyocyte fate from an uncommitted stem or progenitor cell population, by developing a system for conditional expression of Gata4 in embryonic stem cells. We find that in embryoid body cultures containing even a low ratio of these cells, expression of Gata4 is sufficient to enhance significantly the generation of cardiomyocytes, via a non-cell-autonomous mechanism. The Gata4-expressing cells do not generate cardiac or other mesoderm derivatives. Rather, Gata4 expression directs the development of two types of Sox17+ endoderm. This includes an epCam+Dpp4+ subtype of visceral endoderm. In addition, Gata4 generates similar amounts of epCam+Dpp4- definitive endoderm enriched for Cxcr4, FoxA2, FoxA3, Dlx5 and other characteristic transcripts. Both types of endoderm express cardiac-inducing factors, including WNT antagonists Dkk1 and Sfrp5, although the visceral endoderm subtype has much higher cardiac-inducing activity correlating with relatively enhanced levels of transcripts encoding BMPs. The Gata4-expressing cells eventually express differentiation markers showing commitment to liver development, even under conditions that normally support mesoderm development. The results suggest that Gata4 is capable of specifying endoderm fates that facilitate, with temporal and spatial specificity, the generation of cardiomyocyte progenitors from associated mesoderm.

Mechanisms of liver development: concepts for understanding liver disorders and design of novel therapies

The study of liver development has significantly contributed to developmental concepts about morphogenesis and differentiation of other organs. Knowledge of the mechanisms that regulate hepatic epithelial cell differentiation has been essential in creating efficient cell culture protocols for programmed differentiation of stem cells to hepatocytes as well as developing cell transplantation therapies. Such knowledge also provides a basis for the understanding of human congenital diseases. Importantly, much of our understanding of organ development has arisen from analyses of patients with liver deficiencies. We review how the liver develops in the embryo and discuss the concepts that operate during this process. We focus on the mechanisms that control the differentiation and organization of the hepatocytes and cholangiocytes and refer to other reviews for the development of nonepithelial tissue in the liver. Much progress in the characterization of liver development has been the result of genetic studies of human diseases; gaining a better understanding of these mechanisms could lead to new therapeutic approaches for patients with liver disorders.

Analysis of gene expressions of embryonic stem-derived Pdx1-expressing cells: implications of genes involved in pancreas differentiation

We have recently reported the method by which embryonic stem (ES) cells were induced into Pdx1-expressing cells. To gain insights into the ES cell-derived Pdx1-expressing cells, we examined gene expression profiles of the cells by microarray experiments. Microarray analyses followed by a comparison with the data of the cells in developing pancreatic and adult islet suggested that the ES cell-derived Pdx1-positive cells were immature pancreatic progenitor cells with endodermal characteristics. The analyses of the genes upregulated in the ES cell-derived Pdx1-positive cells would give us knowledge on early pancreatic development. Here, we first listed the genes and found that these contained not only those known to be expressed in the endoderm or pancreatic progenitor cells, but also those known to be involved in left-right axis formation. Second, we examined the gene expression patterns and found that several genes were expressed in the ventral foregut lip at the anterior intestinal portal in E8.5 embryo. Given that the Pdx1/GFP-expressing cells are first observed in the same region at the anterior intestinal portal, these results suggest that the pancreatic progenitor cells first give rise at the ventral endoderm prior to the formation of dorsal and ventral pancreatic buds.

Sfrp controls apicobasal polarity and oriented cell division in developing gut epithelium

Epithelial tubular morphogenesis leading to alteration of organ shape has important physiological consequences. However, little is known regarding the mechanisms that govern epithelial tube morphogenesis. Here, we show that inactivation of Sfrp1 and Sfrp2 leads to reduction in fore-stomach length in mouse embryos, which is enhanced in the presence of the Sfrp5 mutation. In the mono-cell layer of fore-stomach epithelium, cell division is normally oriented along the cephalocaudal axis; in contrast, orientation diverges in the Sfrps-deficient fore-stomach. Cell growth and apoptosis are not affected in the Sfrps-deficient fore-stomach epithelium. Similarly, cell division orientation in fore-stomach epithelium diverges as a result of inactivation of either Stbm/Vangl2, an Fz/PCP component, or Wnt5a. These observations indicate that the oriented cell division, which is controlled by the Fz/PCP pathway, is one of essential components in fore-stomach morphogenesis. Additionally, the small intestine epithelium of Sfrps compound mutants fails to maintain proper apicobasal polarity; the defect was also observed in Wnt5a-inactivated small intestine. In relation to these findings, Sfrp1 physically interacts with Wnt5a and inhibits Wnt5a signaling. We propose that Sfrp regulation of Wnt5a signaling controls oriented cell division and apicobasal polarity in the epithelium of developing gut.

The gastrointestinal tract is generated from the primitive gut tube during embryogenesis. The primitive gut differentiates regionally along the cephalocaudal axis. Individual regions simultaneously acquire specific morphologies through morphogenetic mechanisms. The regional specification of the gut tube is controlled by cross-talk between the mesenchyme and epithelium. However, the morphogenetic mechanisms governing gut formation remain poorly understood. Secreted Frizzled-related protein (Sfrp) is an inhibitor of the Wnt pathway, members of which are expressed in the developing gut. A deficiency of Sfrp genes (Sfrp1, Sfrp2, and Sfrp5) results in reduction of fore-stomach length in mice. During normal fore-stomach formation, cell division is oriented along the cephalocaudal axis; in contrast, reduced fore-stomach length in Sfrps-deficient mice is associated with the divergence of oriented cell division in tubular epithelial cells. Thus, oriented cell division is one of the essential components in fore-stomach morphogenesis. In addition, Sfrps-deficient small intestine epithelium fails to maintain proper apicobasal polarity. We also found that Wnt5a-inactivation leads to a phenotype similar to that induced by Sfrps-deficiency in the developing gut, and that Sfrp1 inhibits Wnt5a signaling. We propose that Sfrp regulation of Wnt5a signaling is required for oriented cell division and that it modulates apicobasal polarity in gut epithelium during organ elongation.

XsFRP5 modulates endodermal organogenesis in Xenopus laevis

Canonical Wnt signalling is known to be involved in the regulation of differentiation and proliferation in the context of endodermal organogenesis. Wnt mediated beta-catenin activation is understood to be modulated by secreted Frizzled-related proteins, such as XsFRP5, which is dynamically expressed in the prospective liver/ventral pancreatic precursor cells during late neurula stages, becoming liver specific at tailbud stages and shifting to the posterior stomach/anterior duodenum territory during tadpole stages of Xenopus embryogenesis. These expression characteristics prompted us to analyse the function of XsFRP5 in the context of endodermal organogenesis. We demonstrate that XsFRP5 can form a complex with and inhibit a multitude of different Wnt ligands, including both canonical and non-canonical ones. Knockdown of XsFRP5 results in transient pancreatic hypoplasia as well as in an enlargement of the stomach. In VegT-injected animal cap explants, XsFRP5 can induce expression of exocrine but not endocrine pancreatic marker genes. Both, its expression characteristics as well as its interactions with XsFRP5, define Wnt2b as a putative target for XsFRP5 in vivo. Knockdown of Wnt2b results in a hypoplastic stomach as well as in hypoplasia of the pancreas. On the basis of these findings we propose that XsFRP5 exerts an early regulatory function in the specification of the ventral pancreas, as well as a late function in controlling stomach size via inhibition of Wnt signalling.

Sfrp5 coordinates foregut specification and morphogenesis by antagonizing both canonical and noncanonical Wnt11 signaling

Cell identity and tissue morphogenesis are tightly orchestrated during organogenesis, but the mechanisms regulating this are poorly understood. We show that interactions between Wnt11 and the secreted Wnt antagonist secreted frizzled-related protein 5 (Sfrp5) coordinate cell fate and morphogenesis during Xenopus foregut development. sfrp5 is expressed in the surface cells of the foregut epithelium, whereas wnt11 is expressed in the underlying deep endoderm. Depletion of Sfrp5 results in reduced foregut gene expression and hypoplastic liver and ventral pancreatic buds. In addition, the ventral foregut cells lose adhesion and fail to form a polarized epithelium. We show that the cell fate and epithelial defects are due to inappropriate Wnt/beta-catenin and Wnt/PCP signaling, respectively, both mediated by Wnt11. We provide evidence that Sfrp5 locally inhibits Wnt11 to maintain early foregut identity and to allow an epithelium to form over a mass of tissue undergoing Wnt-mediated cell movements. This novel mechanism coordinating canonical and noncanonical Wnt signaling may have broad implications for organogenesis and cancer.

Dkk1andWnt3interact to control head morphogenesis in the mouse

Loss of Dkk1 results in ectopic WNT/β-catenin signalling activity in the anterior germ layer tissues and impairs cell movement in the endoderm of the mouse gastrula. The juxtaposition of the expression domains of Dkk1 and Wnt3 is suggestive of an antagonist-agonist interaction. The downregulation of Dkk1 when Wnt3 activity is reduced reveals a feedback mechanism for regulating WNT signalling. Compound Dkk1;Wnt3 heterozygous mutant embryos display head truncation and trunk malformation, which are not found in either Dkk1+/- or Wnt3+/- embryos. Reducing the dose of Wnt3 gene in Dkk1-/- embryos partially rescues the truncated head phenotype. These findings highlight that head development is sensitive to the level of WNT3 signalling and that DKK1 is the key antagonist that modulates WNT3 activity during anterior morphogenesis.

Gene expression profiling elucidates a specific role for RARgamma in the retinoic acid-induced differentiation of F9 teratocarcinoma stem cells

The biological effects of all-trans-retinoic acid (RA), a major active metabolite of retinol, are mainly mediated through its interactions with retinoic acid receptor (RARs alpha, beta, gamma) and retinoid X receptor (RXRs alpha, beta, gamma) heterodimers. RAR/RXR heterodimers activate transcription by binding to RA-response elements (RAREs or RXREs) in the promoters of primary target genes. Murine F9 teratocarcinoma stem cells have been widely used as a model for cellular differentiation and RA signaling during embryonic development. We identified and characterized genes that are differentially expressed in F9 wild type (Wt) and F9 RARgamma-/- cells, with and without RA treatment, through the use of oligonucleotide-based microarrays. Our data indicate that RARgamma, in the absence of exogenous RA, modulates gene expression. Genes such as Sfrp2, Tie1, Fbp2, Emp1, and Emp3 exhibited higher transcript levels in RA-treated Wt, RARalpha-/- and RARbeta2-/- lines than in RA-treated RARgamma-/- cells, and represent specific RARgamma targets. Other genes, such as Runx1, were expressed at lower levels in both F9 RARbeta2-/- and RARgamma-/- cell lines than in F9 Wt and RARalpha-/-. Genes specifically induced by RA at 6h with the protein synthesis inhibitor cycloheximide in F9 Wt, but not in RARgamma-/- cells, included Hoxa3, Hoxa5, Gas1, Cyp26a1, Sfrp2, Fbp2, and Emp1. These genes represent specific primary RARgamma targets in F9 cells. Several genes in the Wnt signaling pathway were regulated by RARgamma. Delineation of the receptor-specific actions of RA with respect to cell proliferation and differentiation should result in more effective therapies with this drug.

Sfrp1, Sfrp2, and Sfrp5 regulate the Wnt/beta-catenin and the planar cell polarity pathways during early trunk formation in mouse

Sfrp is a secreted Wnt antagonist that directly interacts with Wnt ligand. We show here that inactivation of Sfrp1, Sfrp2, and Sfrp5 leads to fused somites formation in early-somite mouse embryos, simultaneously resulting in defective convergent extension (CE), which causes severe shortening of the anteroposterior axis. These observations indicate the redundant roles of Sfrp1, Sfrp2, and Sfrp5 in early trunk formation. The roles of the Sfrps were genetically distinguished in terms of the regulation of Wnt pathways. Genetic analysis combining Sfrps mutants and Loop-tail mice revealed the involvement of Sfrps in CE through the regulation of the planar cell polarity pathway. Furthermore, Dkk1-deficient embryos carrying Sfrp1 homozygous and Sfrp2 heterozygous mutations display irregular somites and indistinct intersomitic boundaries, which indicates that Sfrps-mediated inhibition of the Wnt/beta-catenin pathway is necessary for somitogenesis. Our results suggest that Sfrps regulation of the canonical and noncanonical pathways is essential for proper trunk formation.

Stabilization of beta-catenin affects mouse embryonic liver growth and hepatoblast fate

During hepatogenesis, after the liver has budded out of the endoderm, the hepatoblasts quickly expand and differentiate into either hepatocytes or biliary cells, the latter of which arise only within the ductal plate surrounding the portal vein. Because the Wnt/beta-catenin pathway is involved in liver homeostasis and regeneration and in liver carcinogenesis, we investigated here a role for Wnt/beta-catenin signaling in the embryonic liver. A cyclization recombination (Cre)/locus of X-over P1 (loxP) strategy was chosen to perform adenomatous polyposis coli (Apc) invalidation in order to activate ectopic beta-catenin signaling in hepatoblasts; an appropriate transgenic model expressing the Cre recombinase was used. Phenotypic and immunolocalization studies, together with messenger RNA analyses, by microarray and real-time quantitative polymerase chain reaction approaches were performed on this model during normal hepatogenesis. The loss of Apc allowed beta-catenin activation in the hepatoblasts after the formation of the liver bud and led to embryonic lethality. In this model, the liver became hypoplastic, and hepatocyte differentiation failed, whereas beta-catenin-activated ducts developed and gave rise to fully differentiated bile ducts when transplanted into adult recipient livers. Microarray analyses suggested that beta-catenin plays a role in repressing the hepatocyte genetic program and remodeling the ductal plate. According to these data, in normal embryonic livers, beta-catenin was transiently activated in the nascent bile ducts.

CONCLUSION

We demonstrated a key role for the Wnt/beta-catenin pathway in liver embryonic growth and in controlling the fate of hepatoblasts, preventing them from differentiating toward the hepatocyte lineage, and guiding them to biliary ductal morphogenesis.

A role of Wnt/beta-catenin signals in hepatic fate specification of human umbilical cord blood-derived mesenchymal stem cells

Human umbilical cord blood-derived mesenchymal stem cells (UCBMSCs) are expected to be an excellent source of cells for transplantation. In addition, the stem cell plasticity of human UCBMSCs, which can transdifferentiate into hepatocytes, has been reported. However, the mechanisms involved remain to be clarified. To identify the genes and/or signals that are important in specifying the hepatic fate of human UCBMSCs, we analyzed gene expression profiles during the hepatic differentiation of UCBMSCs with human telomerase reverse transcriptase, UCBMSCs immortalized by infection with a retrovirus carrying telomerase reverse transcriptase, but whose differentiation potential remains unchanged. Efficient differentiation was induced by 5-azacytidine (5-aza)/hepatocyte growth factor (HGF)/oncostatin M (OSM)/fibroblast growth factor 2 (FGF2) treatment in terms of function as well as protein expression: 2.5-fold increase in albumin, 4-fold increase in CCAAT enhancer-binding protein alpha, 1.5-fold increase in cytochrome p450 1A1/2, and 8-fold increase in periodic acid-Schiff staining. Consequently, we found that the expression of Wnt/beta-catenin-related genes downregulated, and the translocation of beta-catenin was observed along the cell membrane and in the cytoplasm, although some beta-catenin was still in the nucleus. Downregulation of Wnt/beta-catenin signals in the cells by Fz8-small interference RNA treatment, which was analyzed with a Tcf4 promoter-luciferase assay, resulted in similar hepatic differentiation to that observed with 5-azacytidine/HGF/OSM/FGF2. In addition, the subcellular distribution of beta-catenin was similar to that of cells treated with 5-azacytidine/HGF/OSM/FGF2. In conclusion, the suppression of Wnt/beta-catenin signaling induced the hepatic differentiation of UCBMSCs, suggesting that Wnt/beta-catenin signals play an important role in the hepatic fate specification of human UCBMSCs.

The Onecut transcription factors HNF-6/OC-1 and OC-2 regulate early liver expansion by controlling hepatoblast migration

Liver development in mammals is initiated by the formation of a hepatic bud from the ventral foregut endoderm. The hepatic cells then proliferate and invade the septum transversum mesenchyme, and further differentiate to give rise to hepatocytes and biliary cells. By analyzing mice that are knockout for the transcription factors Hepatocyte Nuclear Factor-6 (HNF-6)/Onecut-1 (OC-1) and OC-2, we show here that these factors redundantly stimulate the degradation of the basal lamina surrounding the liver bud and promote hepatoblast migration in the septum transversum. Gene expression analysis indicates that HNF-6 and OC-2 belong to a gene network comprising E-cadherin, thrombospondin-4 and osteopontin, which regulates liver bud expansion by controlling hepatoblast migration and adhesion. This network operating at the onset of liver development contains candidate genes for investigation of liver carcinogenesis.

Repression of Wnt/beta-catenin signaling in the anterior endoderm is essential for liver and pancreas development

The liver and pancreas are specified from the foregut endoderm through an interaction with the adjacent mesoderm. However, the earlier molecular mechanisms that establish the foregut precursors are largely unknown. In this study, we have identified a molecular pathway linking gastrula-stage endoderm patterning to organ specification. We show that in gastrula and early-somite stage Xenopus embryos, Wnt/beta-catenin activity must be repressed in the anterior endoderm to maintain foregut identity and to allow liver and pancreas development. By contrast, high beta-catenin activity in the posterior endoderm inhibits foregut fate while promoting intestinal development. Experimentally repressing beta-catenin activity in the posterior endoderm was sufficient to induce ectopic organ buds that express early liver and pancreas markers. beta-catenin acts in part by inhibiting expression of the homeobox gene hhex, which is one of the earliest foregut markers and is essential for liver and pancreas development. Promoter analysis indicates that beta-catenin represses hhex transcription indirectly via the homeodomain repressor Vent2. Later in development, beta-catenin activity has the opposite effect and enhances liver development. These results illustrate that turning Wnt signaling off and on in the correct temporal sequence is essential for organ formation, a finding that might directly impact efforts to differentiate liver and pancreas tissue from stem cells.

Expression ofFrizzled5, Frizzled7, andFrizzled10 during early mouse development and interactions with canonical Wnt signaling

Wnt signaling has been shown to be important in the patterning of the gastrulating mouse embryo, especially in axis formation. To this date, there is no clear indication that the Wnt receptors, Frizzleds (Fzds), are involved in such early specification. Moreover, at the gastrulation stage, the only Fzd with a known characterized expression pattern is Fzd8, which is expressed in the anterior visceral endoderm (aVE) (Lu et al. [2004] Gene Expr Patterns 4:569-572). Following a real time RT-PCR study to evaluate Fzd expression in the gastrulating embryo, we used whole-mount in situ hybridization to reveal new expression domains for Fzd5, Fzd7, and Fzd10. Fzd5 is expressed in the aVE and Fzd7 expression is restricted to the epiblast of the gastrulating embryo. The expression pattern of Fzd10 in the primitive streak of the gastrula suggests it has a role in mesoderm induction. We also show that the purified, secreted forms of the extracellular cysteine-rich domains (CRDs) of FZD5, Fzd7, and Fzd8 can antagonize Wnt3a-induced beta-Catenin accumulation in L-cells, whereas in mouse embryonic stem cells, these CRDs can inhibit spontaneous mesoderm formation and promote neural differentiation. Our data demonstrate that Fzd5, Fzd7, and Fzd10 are expressed in distinct domains of the gastrulating embryo, and that the CRDs of FZD5, Fzd7, and Fzd8 can regulate Wnts, indicating that Fzds interpret Wnt signals during embryonic mesoderm and neural induction.

Liver specification: a new role for Wnts in liver development

Secreted Wnt proteins control a diverse array of developmental decisions. A recent analysis of the zebrafish mutant prometheus points to a previously unknown role for Wnts during liver specification.

Sexually dimorphic expression of secreted frizzled-related (SFRP) genes in the developing mouse Müllerian duct

In developing male embryos, the female reproductive tract primordia (Müllerian ducts) regress due to the production of testicular anti-Müllerian hormone (AMH). Because of the association between secreted frizzled-related proteins (SFRPs) and apoptosis, their reported developmental expression patterns and the role of WNT signaling in female reproductive tract development, we examined expression of Sfrp2 and Sfrp5 during development of the Müllerian duct in male (XY) and female (XX) mouse embryos. We show that expression of both Sfrp2 and Sfrp5 is dynamic and sexually dimorphic. In addition, the male-specific expression observed for both genes prior to the onset of regression is absent in mutant male embryos that fail to undergo Müllerian duct regression. We identified ENU-induced point mutations in Sfrp5 and Sfrp2 that are predicted to severely disrupt the function of these genes. Male embryos and adults homozygous for these mutations, both individually and in combination, are viable and apparently fertile with no overt abnormalities of reproductive tract development.

Lhx5 promotes forebrain development and activates transcription of secreted Wnt antagonists

In vertebrate embryos, induction and patterning of the forebrain require the local inhibition of caudalizing signals, such as Wnts, emanating from the mesendoderm and caudal brain. Here, we report that Lhx5, expressed in the rostral neuroectoderm, regulates the local inhibition of Wnts. Activation of Lhx5 expands forebrain structures, whereas inhibition of Lhx5 function compromises forebrain development in zebrafish embryos. Lhx5 can rescue forebrain deficiencies caused by excess Wnt activity, and inhibition of Lhx5 function results in ectopic activation of Wnt signaling. Lhx5 regulates the expression of two secreted Frizzled-related Wnt antagonists, Sfrp1a and Sfrp5. These Sfrps can reduce the ectopic activation of Wnt signaling and rescue the forebrain deficiencies caused by inhibition of Lhx5 function. Our results demonstrate that Lhx5 is a required factor that promotes forebrain development and inhibits Wnt signaling by activating the transcription of secreted Wnt antagonists.

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

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

Mesodermal Wnt2b signalling positively regulates liver specification

Endodermal organs such as the lung, liver and pancreas emerge at precise locations along the primitive gut tube. Although several signalling pathways have been implicated in liver formation, so far no single gene has been identified that exclusively regulates liver specification. In zebrafish, the onset of liver specification is marked by the localized endodermal expression of hhex and prox1 at 22 hours post fertilization. Here we used a screen for mutations affecting endodermal organ morphogenesis to identify a unique phenotype: prometheus (prt) mutants exhibit profound, though transient, defects in liver specification. Positional cloning reveals that prt encodes a previously unidentified Wnt2b homologue. prt/wnt2bb is expressed in restricted bilateral domains in the lateral plate mesoderm directly adjacent to the liver-forming endoderm. Mosaic analyses show the requirement for Prt/Wnt2bb in the lateral plate mesoderm, in agreement with the inductive properties of Wnt signalling. Taken together, these data reveal an unexpected positive role for Wnt signalling in liver specification, and indicate a possible common theme for the localized formation of endodermal organs along the gut tube.

Duplicate sfrp1 genes in zebrafish: sfrp1a is dynamically expressed in the developing central nervous system, gut and lateral line

The secreted frizzled-related proteins (Sfrp) are a family of soluble proteins with diverse biological functions having the capacity to bind Wnt ligands, to modulate Wnt signalling, and to signal directly via the Wnt receptor, Frizzled. In an enhancer trap screen for embryonic expression in zebrafish we identified an sfrp1 gene. Previous studies suggest an important role for sfrp1 in eye development, however, no data have been reported using the zebrafish model. In this paper, we describe duplicate sfrp1 genes in zebrafish and present a detailed analysis of the expression profile of both genes. Whole mount in situ hybridisation analyses of sfrp1a during embryonic and larval development revealed a dynamic expression profile, including: the central nervous system, where sfrp1a was regionally expressed throughout the brain and developing eye; the posterior gut, from the time of endodermal cell condensation; the lateral line, where sfrp1a was expressed in the migrating primordia and interneuromast cells that give rise to the sensory organs. Other sites included the blastoderm, segmenting mesoderm, olfactory placode, developing ear, pronephros and fin-bud. We have also analysed sfrp1b expression during embryonic development. Surprisingly this gene exhibited a divergent expression profile being limited to the yolk syncytium under the elongating tail-bud, which later covered the distal yolk extension, and transiently in the tail-bud mesenchyme. Overall, our studies provide a basis for future analyses of these developmentally important factors using the zebrafish model.

Sfrp1 and Sfrp2 regulate anteroposterior axis elongation and somite segmentation during mouse embryogenesis

Regulation of Wnt signaling is essential for embryonic patterning. Sfrps are secreted Wnt antagonists that directly interact with the Wnt ligand to inhibit signaling. Here, we show that Sfrp1 and Sfrp2 are required for anteroposterior (AP) axis elongation and somitogenesis in the thoracic region during mouse embryogenesis. Double homozygous mutations in Sfrp1 and Sfrp2 lead to severe shortening of the thoracic region. By contrast, a homozygous mutation in one or the other exerts no effect on embryogenesis, indicating that Sfrp1 and Sfrp2 are functionally redundant. The defect of a shortened thoracic region appears to be the consequence of AP axis reduction and incomplete somite segmentation. The reduction in the AP axis is partially due to abnormalities in cell migration of pre-somitic mesoderm from the end of gastrulation. Aberrant somite segmentation is associated with altered oscillations of Notch signaling, as evidenced by abnormal Lfng and Hes7 expression during somitogenesis in the thoracic region. This study suggests that Wnt regulation by Sfrp1 and Sfrp2 is required for embryonic patterning.

Sfrp5 is not essential for axis formation in the mouse

Secreted frizzled related protein (Sfrp) genes encode extracellular factors that can modulate Wnt signaling. During early post-implantation mouse development Sfrp5 is expressed in the anterior visceral endoderm (AVE) and the ventral foregut endoderm. The AVE is important in anterior-posterior axis formation and the ventral foregut endoderm contributes to multiple gut tissues. Here to determine the essential role of Sfrp5 in early mouse development we generated Sfrp5-deficient mice by gene targeting. We report that Sfrp5-deficient mice are viable and fertile. To determine whether the absence of an axis phenotype might be due to genetic redundancy with Dkk1 in the AVE we generated Sfrp5;Dkk1 double mutant mice. AVE development and primitive streak formation appeared normal in Sfrp5(-/-);Dkk1(-/-) embryos. These results indicate that Sfrp5 is not essential for axis formation or foregut morphogenesis in the mouse and also imply that Sfrp5 and Dkk1 together are not essential for AVE development.

Primitive streak formation in mice is preceded by localized activation of Brachyury and Wnt3

The prevalent model for the generation of axial polarity in mouse embryos proposes that a radial to a linear transition in the expression of primitive streak markers precedes the formation of the primitive streak on one side of the epiblast. This model contrasts with the models of mesoderm formation in other vertebrates as it suggests that the primitive streak is initially established in a radial pattern rather than a localized region of the epiblast. Here, we examine the proposed correlation between the expression of Brachyury and Wnt3, two genes reported as expressed radially in the proximal epiblast, with the movements of proximal anterior epiblast cells at stages leading to the formation of the primitive streak. Our results reveal that neither Brachyury nor Wnt3 forms a ring of expression in the proximal epiblast as previously thought. In embryos dissected between 5.5 and 6.5 dpc, Brachyury is first expressed in the distal extra-embryonic ectoderm and subsequently on one side of the epiblast. Wnt3 expression is evident first in the posterior visceral endoderm of 5.5 dpc embryos and later in the posterior epiblast. Lineage analysis shows that the movements of the proximal epiblast do not restrict Brachyury expression to the posterior epiblast. Our data suggest a model whereby the localized expression of these genes in the posterior epiblast, and hence the formation of the primitive streak, is the result of local cell-cell interactions in the future posterior portion of the egg cylinder rather than regionalization of a radial pattern of expression in proximal epiblast cells.

Beta-catenin signaling marks the prospective site of primitive streak formation in the mouse embryo

Beta-catenin signaling has been shown to be involved in triggering axis formation in several organisms, including Xenopus and zebrafish. Genetic analysis has demonstrated that the Wnt/beta-catenin signaling pathway is also involved in axis formation in the mouse, since a targeted deletion of beta-catenin results in embryos that have a block in anterior-posterior axis formation, fail to initiate gastrulation, and do not form mesoderm. However, because beta-catenin is ubiquitously expressed, the precise time and cell types in which this signaling pathway is active during early embryonic development remain unknown. Thus, to better understand the role of the Wnt/beta-catenin signaling pathway in axis formation and mesoderm specification, we have examined both the distribution and signaling activity of beta-catenin during early embryonic development in the mouse. We show that the N-terminally nonphosphorylated form of beta-catenin as well as beta-catenin signaling is first detectable in the extraembryonic visceral endoderm in day 5.5 embryos. Before the initiation of gastrulation at day 6.0, beta-catenin signaling is asymmetrically distributed within the epiblast and is localized to a small group of cells adjacent to the embryonic--extraembryonic junction. At day 6.5 and onward, beta-catenin signaling was detected in the primitive streak and mature node. Thus, beta-catenin signaling precedes primitive streak formation and is present in epiblast cells that will go on to form the primitive streak. These results support a critical role for the Wnt/beta-catenin pathway in specifying cells to form the primitive streak and node in the mammalian embryo as well as identify a novel domain of Wnt/beta-catenin signaling activity during early embryogenesis.

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

In this extensive study, real-time reverse transcriptase-polymerase chain reaction was used to analyze the expression levels of all 19 Wnt genes and their 11 potential antagonists in mouse blastocysts, pregastrula, gastrula, and neurula stages. By complementing these results with in situ hybridization, we revealed new expression domains for Wnt2b and Sfrp1, respectively, in the future primitive streak at the posterior side and in the anterior visceral endoderm before the initiation of gastrulation. Moreover, the anterior visceral endoderm expresses three secreted Wnt antagonists (Sfrp1, Sfrp5, and Dkk1) in partially overlapping domains. We also identified expression patterns for the Wnt1, Wnt3a, Wnt6, Wnt7b, Wnt9a, Wnt10b, and Sfrp1 genes at the blastocyst stage. In particular, the expression of Wnt1 and Sfrp1 predominantly in the inner cell mass and of Wnt9a in the mural trophoblast and inner cell mass cells surrounding the blastocoele suggests new roles for the Wnt pathway in preimplantation development. This article is the first report on the regional expression of Wnt genes in the mouse blastocyst.

Olfactory and lens placode formation is controlled by the hedgehog-interacting protein (Xhip) in Xenopus

The integration of multiple signaling pathways is a key issue in several aspects of embryonic development. In this context, extracellular inhibitors of secreted growth factors play an important role, which is to antagonize specifically the activity of the corresponding signaling molecule. We provide evidence that the Hedgehog-interacting protein (Hip) from Xenopus, previously described as a Hedgehog-specific antagonist in the mouse, interferes with Wnt-8 and eFgf/Fgf-8 signaling pathways as well. To address the function of Hip during early embryonic development, we performed gain- and loss-of-function studies in the frog. Overexpression of Xhip or mHip1 resulted in a dramatic increase of retinal structures and larger olfactory placodes primarily at the expense of other brain tissues. Furthermore, loss of Xhip function resulted in a suppression of olfactory and lens placode formation. Therefore, the localized expression of Xhip may counteract certain overlapping signaling activities, which inhibit the induction of distinct sensory placodes.