Hindgut defects and transformation of the gastro-intestinal tract in Tcf4(-/-)/Tcf1(-/-) embryos

Spatiotemporal map of key signaling factors during early penis development

The formation of the external genitalia is a highly complex developmental process, considering it involves a wide range of cell types and results in sexually dimorphic outcomes. Development is controlled by several secreted signalling factors produced in complex spatiotemporal patterns, including the hedgehog (HH), bone morphogenic protein (BMP), fibroblast growth factor (FGF) and WNT signalling families. Many of these factors act on or are influenced by the actions of the androgen receptor (AR) that is critical to masculinisation. This complexity of expression makes it difficult to conceptualise patterns of potential importance. Mapping expression during key stages of development is needed to develop a comprehensive model of how different cell types interact in formation of external genitalia, and the global regulatory networks at play. This is particularly true in light of the sensitivity of this process to environmental disruption during key stages of development. The goal of this review is to integrate all recent studies on gene expression in early penis development to create a comprehensive spatiotemporal map. This serves as a resource to aid in visualising potentially significant interactions involved in external genital development.

Diagrams of published RNA and protein localisation data for key secreted signalling factors during early penis development.

Unconventional expression patterns are identified that suggest novel signalling axes during development.

Key research gaps and limitations are identified and discussed.

Frizzled7 Activates β-Catenin-Dependent and β-Catenin-Independent Wnt Signalling Pathways During Developmental Morphogenesis: Implications for Therapeutic Targeting in Colorectal Cancer

Frizzled₇ activates β-catenin-dependent and β-catenin-independent Wnt signalling pathways, is highly conserved through evolution from the ancient phylum hydra to man, plays essential roles in stem cells, tissue homeostasis and regeneration in the adult, and is upregulated in diverse cancers. Much of what is known about the core components of the Wnt signalling pathways was derived from studying the function of Frizzled₇ orthologues in the development of lower organism. As we interrogate Frizzled₇ signalling and function for therapeutic targeting in cancer, it is timely to revisit lower organisms to gain insight into the context dependent and dynamic nature of Wnt signalling for effective drug design.

Negative regulation of IL-8 in human astrocytes depends on β-catenin while positive regulation is mediated by TCFs/LEF/ATF2 interaction

Expression of cytokines/chemokines is tightly regulated at the transcription level. This is crucial in the central nervous system to maintain neuroimmune homeostasis. IL-8 a chemoattractant, which recruits neutrophils, T cells, and basophils into the brain in response to inflammation and/or injury is secreted predominantly by neurons, microglia, and astrocytes. Here, we investigated the mechanism by which astrocytes regulate IL-8 expression. We demonstrate that while β-catenin negatively regulated IL-8 transcription, its canonical transcriptional partners, members of the TCF/LEF transcription factors (TCF1, TCF3, TCF4 and LEF1) and Activating transcription factor 2 (ATF2) positively regulated IL-8 transcription. We further identified a putative TCF/LEF binding site at -175nt close to the minimal transcription region on the IL-8 promoter, mutation of which caused a significant reduction in IL-8 promoter activity. Chromatin immunoprecipitation demonstrated binding of TCF1, TCF4, LEF1 and ATF2 on the IL-8 promoter suggesting that TCFs/LEF partner with ATF2 to induce IL-8 transcription. These findings demonstrate a novel role for β-catenin in suppression of IL-8 expression and for TCFs/LEF/ATF2 in inducing IL-8. These findings reveal a unique mechanism by which astrocytes tightly regulate IL-8 expression.

Large intestine embryogenesis: Molecular pathways and related disorders (Review)

The large intestine, part of the gastrointestinal tract (GI), is composed of all three germ layers, namely the endoderm, the mesoderm and the ectoderm, forming the epithelium, the smooth muscle layers and the enteric nervous system, respectively. Since gastrulation, these layers develop simultaneously during embryogenesis, signaling to each other continuously until adult age. Two invaginations, the anterior intestinal portal (AIP) and the caudal/posterior intestinal portal (CIP), elongate and fuse, creating the primitive gut tube, which is then patterned along the antero‑posterior (AP) axis and the radial (RAD) axis in the context of left‑right (LR) asymmetry. These events lead to the formation of three distinct regions, the foregut, midgut and hindgut. All the above‑mentioned phenomena are under strict control from various molecular pathways, which are critical for the normal intestinal development and function. Specifically, the intestinal epithelium constitutes a constantly developing tissue, deriving from the progenitor stem cells at the bottom of the intestinal crypt. Epithelial differentiation strongly depends on the crosstalk with the adjacent mesoderm. Major molecular pathways that are implicated in the embryogenesis of the large intestine include the canonical and non‑canonical wingless‑related integration site (Wnt), bone morphogenetic protein (BMP), Notch and hedgehog systems. The aberrant regulation of these pathways inevitably leads to several intestinal malformation syndromes, such as atresia, stenosis, or agangliosis. Novel theories, involving the regulation and homeostasis of intestinal stem cells, suggest an embryological basis for the pathogenesis of colorectal cancer (CRC). Thus, the present review article summarizes the diverse roles of these molecular factors in intestinal embryogenesis and related disorders.

β-Catenin and TCFs/LEF signaling discordantly regulate IL-6 expression in astrocytes

Background

The Wnt/β-catenin signaling pathway is a prolific regulator of cell-to-cell communication and gene expression. Canonical Wnt/β-catenin signaling involves partnering of β-catenin with members of the TCF/LEF family of transcription factors (TCF1, TCF3, TCF4, LEF1) to regulate gene expression. IL-6 is a key cytokine involved in inflammation and is particularly a hallmark of inflammation in the brain. Astrocytes, specialized glial cells in the brain, secrete IL-6. How astrocytes regulate IL-6 expression is not entirely clear, although in other cells NFκB and C/EBP pathways play a role. We evaluated here the interface between β-catenin, TCFs/LEF and C/EBP and NF-κB in relation to IL-6 gene regulation in astrocytes.

Methods

We performed molecular loss and/or gain of function studies of β-catenin, TCF/LEF, NFκB, and C/EBP to assess IL-6 regulation in human astrocytes. Specifically, siRNA mediated target gene knockdown, cDNA over expression of target gene, and pharmacological agents for regulation of target proteins were used. IL-6 levels was evaluated by real time quantitative PCR and ELISA. We also cloned the IL-6 promoter under a firefly luciferase reporter and used bioinformatics, site directed mutagenesis, and chromatin immunoprecipitation to probe the interaction between β-catenin/TCFs/LEFs and IL-6 promoter activity.

Results

β-catenin binds to TCF/LEF to inhibits IL-6 while TCFs/LEF induce IL-6 transcription through interaction with ATF-2/SMADs. β-catenin independent of TCFs/LEF positively regulates C/EBP and NF-κB, which in turn activate IL-6 expression. The IL-6 promoter has two putative regions for TCFs/LEF binding, a proximal site located at -91 nt and a distal site at -948 nt from the transcription start site, both required for TCF/LEF induction of IL-6 independent of β-catenin.

Conclusion

IL-6 regulation in human astrocytes engages a discordant interaction between β-catenin and TCF/LEF. These findings are intriguing given that no role for β-catenin nor TCFs/LEF to date is associated with IL-6 regulation and suggest that β-catenin expression in astrocytes is a critical regulator of anti-inflammatory responses and its disruption can potentially mediate persistent neuroinflammation.

A dorsal-ventral gradient of Wnt3a/β-catenin signals controls mouse hindgut extension and colon formation

Despite the importance of Wnt signaling for adult intestinal stem cell homeostasis and colorectal cancer, relatively little is known about its role in colon formation during embryogenesis. The development of the colon starts with the formation and extension of the hindgut. We show that Wnt3a is expressed in the caudal embryo in a dorsal-ventral (DV) gradient across all three germ layers, including the hindgut. Using genetic and lineage-tracing approaches, we describe novel dorsal and ventral hindgut domains, and show that ventrolateral hindgut cells populate the majority of the colonic epithelium. A Wnt3a-β-catenin-Sp5/8 pathway, which is active in the dorsal hindgut endoderm, is required for hindgut extension and colon formation. Interestingly, the absence of Wnt activity in the ventral hindgut is crucial for proper hindgut morphogenesis, as ectopic stabilization of β-catenin in the ventral hindgut via gain- or loss-of-function mutations in Ctnnb1 or Apc, respectively, leads to severe colonic hyperplasia. Thus, the DV Wnt gradient is required to coordinate growth between dorsal and ventral hindgut domains to regulate the extension of the hindgut that leads to colon formation.

dTcf/Pangolin suppresses growth and tumor formation in Drosophila

Wnt/Wingless (Wg) signaling controls many aspects of animal development and is deregulated in different human cancers. The transcription factor dTcf/Pangolin (Pan) is the final effector of the Wg pathway in Drosophila and has a dual role in regulating the expression of Wg target genes. In the presence of Wg, dTcf/Pan interacts with β-catenin/Armadillo (Arm) and induces the transcription of Wg targets. In absence of Wg, dTcf/Pan partners with the transcriptional corepressor TLE/Groucho (Gro) and inhibits gene expression. Here, we use the wing imaginal disk of Drosophila as a model to examine the functions that dTcf/Pan plays in a proliferating epithelium. We report a function of dTcf/Pan in growth control and tumorigenesis. Our results show that dTcf/Pan can limit tissue growth in normal development and suppresses tumorigenesis in the context of oncogene up-regulation. We identify the conserved transcription factors Sox box protein 15 (Sox15) and Ftz transcription factor 1 (Ftz-f1) as genes controlled by dTcf/Pan involved in tumor development. In conclusion, this study reports a role for dTcf/Pan as a repressor of normal and oncogenic growth and identifies the genes inducing tumorigenesis downstream of dTcf/Pan.

Msx1 loss suppresses formation of the ectopic crypts developed in the Apc-deficient small intestinal epithelium

The first step in the development of human colorectal cancer is aberrant activation of the Wnt signaling pathway. Wnt signaling hyperactivation is predominantly caused by loss-of-function mutations in the adenomatous polyposis coli (APC) gene that encodes the pathway negative regulator. In order to identify genes affected by the Apc loss, we performed expression profiling of intestinal epithelium isolated from mice harboring a conditional Apc allele. The gene encoding transcriptional factor msh homeobox 1 (Msx1) displayed robust upregulation upon Apc inactivation. Histological analysis of the Apc-deficient epithelium revealed that in the small intestine, the Msx1 protein was localized exclusively in ectopic crypts, i.e., in pockets of proliferating cells abnormally positioned on the villi. Ablation of the Msx1 gene leads to the disappearance of ectopic crypts and loss of differentiated cells. Moreover, tumors arising from Msx1-deficient cells display altered morphology reminiscent of villous adenomas. In human tumor specimens, MSX1 displayed significantly increased expression in colonic neoplasia with a descending tendency during the lesion progression towards colorectal carcinoma. In summary, the results indicate that Msx1 represents a novel marker of intestinal tumorigenesis. In addition, we described the previously unknown relationship between the Msx1-dependent formation of ectopic crypts and cell differentiation.

Modeling Wnt signaling by CRISPR-Cas9 genome editing recapitulates neoplasia in human Barrett epithelial organoids

Primary organoid cultures generated from patient biopsies comprise a novel improved platform for disease modeling, being genetically stable and closely recapitulating in vivo scenarios. Barrett esophagus (BE) is the major risk factor for esophageal adenocarcinoma. There has been a dearth of long-term in vitro expansion models of BE neoplastic transformation. We generated a long-term virus-free organoid expansion model of BE neoplasia from patient biopsies. Both wild-type and paired APC-knockout (APC^KO) BE organoids genome-edited by CRISPR-Cas9 showed characteristic goblet cell differentiation. Autonomous Wnt activation was confirmed in APC^KO organoids by overexpression of Wnt target genes and nuclear-translocated β-catenin expression after withdrawal of Wnt-3A and R-spondin-1. Wnt-activated organoids demonstrated histologic atypia, higher proliferative and replicative activity, reduced apoptosis, and prolonged culturability. Wnt-activated organoids also showed sustained protrusive migration ability accompanied by disrupted basement membrane reorganization and integrity. This CRISPR-Cas9 editing human-derived organoid model recapitulates the critical role of aberrant Wnt/β-catenin signaling activation in BE neoplastic transformation. This system can be used to study other 'driver' pathway alterations in BE-associated neoplasia, avoiding signaling noise present in immortalized or cancer-derived cell lines.

Modeling Wnt signaling by CRISPR-Cas9 genome editing recapitulates neoplasia in human Barrett epithelial organoids

Primary organoid cultures generated from patient biopsies comprise a novel improved platform for disease modeling, being genetically stable and closely recapitulating in vivo scenarios. Barrett esophagus (BE) is the major risk factor for esophageal adenocarcinoma. There has been a dearth of long-term in vitro expansion models of BE neoplastic transformation. We generated a long-term virus-free organoid expansion model of BE neoplasia from patient biopsies. Both wild-type and paired APC-knockout (APC^KO) BE organoids genome-edited by CRISPR-Cas9 showed characteristic goblet cell differentiation. Autonomous Wnt activation was confirmed in APC^KO organoids by overexpression of Wnt target genes and nuclear-translocated β-catenin expression after withdrawal of Wnt-3A and R-spondin-1. Wnt-activated organoids demonstrated histologic atypia, higher proliferative and replicative activity, reduced apoptosis, and prolonged culturability. Wnt-activated organoids also showed sustained protrusive migration ability accompanied by disrupted basement membrane reorganization and integrity. This CRISPR-Cas9 editing human-derived organoid model recapitulates the critical role of aberrant Wnt/β-catenin signaling activation in BE neoplastic transformation. This system can be used to study other 'driver' pathway alterations in BE-associated neoplasia, avoiding signaling noise present in immortalized or cancer-derived cell lines.

Tcf7L2 is essential for neurogenesis in the developing mouse neocortex

Generation of neurons in the embryonic neocortex is a balanced process of proliferation and differentiation of neuronal progenitor cells. Canonical Wnt signalling is crucial for expansion of radial glial cells in the ventricular zone and for differentiation of intermediate progenitors in the subventricular zone. We detected abundant expression of two transcrtiption factors mediating canonical Wnt signalling, Tcf7L1 and Tcf7L2, in the ventricular zone of the embryonic neocortex. Conditional knock-out analysis showed that Tcf7L2, but not Tcf7L1, is the principal Wnt mediator important for maintenance of progenitor cell identity in the ventricular zone. In the absence of Tcf7L2, the Wnt activity is reduced, ventricular zone markers Pax6 and Sox2 are downregulated and the neuroepithelial structure is severed due to the loss of apical adherens junctions. This results in decreased proliferation of radial glial cells, the reduced number of intermediate progenitors in the subventricular zone and hypoplastic forebrain. Our data show that canonical Wnt signalling, which is essential for determining the neuroepithelial character of the neocortical ventricular zone, is mediated by Tcf7L2.

Patterning the gastrointestinal epithelium to confer regional-specific functions

The gastrointestinal (GI) tract, in simplest terms, can be described as an epithelial-lined muscular tube extending along the cephalocaudal axis from the oral cavity to the anus. Although the general architecture of the GI tract organs is conserved from end to end, the presence of different epithelial tissue structures and unique epithelial cell types within each organ enables each to perform the distinct digestive functions required for efficient nutrient assimilation. Spatiotemporal regulation of signaling pathways and downstream transcription factors controls GI epithelial morphogenesis during development to confer essential regional-specific epithelial structures and functions. Here, we discuss the fundamental functions of each GI tract organ and summarize the diversity of epithelial structures present along the cephalocaudal axis of the GI tract. Next, we discuss findings, primarily from genetic mouse models, that have defined the roles of key transcription factors during epithelial morphogenesis, including p63, SOX2, SOX15, GATA4, GATA6, HNF4A, and HNF4G. Additionally, we examine how the Hedgehog, WNT, and BMP signaling pathways contribute to defining unique epithelial features along the cephalocaudal axis of the GI tract. Lastly, we examine the molecular mechanisms controlling regionalized cytodifferentiation of organ-specific epithelial cell types within the GI tract, concentrating on the stomach and small intestine. The delineation of GI epithelial patterning mechanisms in mice has provided fundamental knowledge to guide the development and refinement of three-dimensional GI organotypic culture models such as those derived from directed differentiation of human pluripotent stem cells and those derived directly from human tissue samples. Continued examination of these pathways will undoubtedly provide vital insights into the mechanisms of GI development and disease and may afford new avenues for innovative tissue engineering and personalized medicine approaches to treating GI diseases.

Tcf7l2 plays pleiotropic roles in the control of glucose homeostasis, pancreas morphology, vascularization and regeneration

Type 2 diabetes (T2D) is a disease characterized by impaired insulin secretion. The Wnt signaling transcription factor Tcf7l2 is to date the T2D-associated gene with the largest effect on disease susceptibility. However, the mechanisms by which TCF7L2 variants affect insulin release from β-cells are not yet fully understood. By taking advantage of a tcf7l2 zebrafish mutant line, we first show that these animals are characterized by hyperglycemia and impaired islet development. Moreover, we demonstrate that the zebrafish tcf7l2 gene is highly expressed in the exocrine pancreas, suggesting potential bystander effects on β-cell growth, differentiation and regeneration. Finally, we describe a peculiar vascular phenotype in tcf7l2 mutant larvae, characterized by significant reduction in the average number and diameter of pancreatic islet capillaries. Overall, the zebrafish Tcf7l2 mutant, characterized by hyperglycemia, pancreatic and vascular defects, and reduced regeneration proves to be a suitable model to study the mechanism of action and the pleiotropic effects of Tcf7l2, the most relevant T2D GWAS hit in human populations.

Morphogenesis and maturation of the embryonic and postnatal intestine

The intestine is a vital organ responsible for nutrient absorption, bile and waste excretion, and a major site of host immunity. In order to keep up with daily demands, the intestine has evolved a mechanism to expand the absorptive surface area by undergoing a morphogenetic process to generate finger-like units called villi. These villi house specialized cell types critical for both absorbing nutrients from food, and for protecting the host from commensal and pathogenic microbes present in the adult gut. In this review, we will discuss mechanisms that coordinate intestinal development, growth, and maturation of the small intestine, starting from the formation of the early gut tube, through villus morphogenesis and into early postnatal life when the intestine must adapt to the acquisition of nutrients through food intake, and to interactions with microbes.

In vitro patterning of pluripotent stem cell-derived intestine recapitulates in vivo human development

The intestine plays a central role in digestion, nutrient absorption and metabolism, with individual regions of the intestine having distinct functional roles. Many examples of region-specific gene expression in the adult intestine are known, but how intestinal regional identity is established during development is a largely unresolved issue. Here, we have identified several genes that are expressed in a region-specific manner in the developing human intestine. Using human embryonic stem cell-derived intestinal organoids, we demonstrate that the duration of exposure to active FGF and WNT signaling controls regional identity. Short-term exposure to FGF4 and CHIR99021 (a GSK3β inhibitor that stabilizes β-catenin) resulted in organoids with gene expression patterns similar to developing human duodenum, whereas longer exposure resulted in organoids similar to ileum. When region-specific organoids were transplanted into immunocompromised mice, duodenum-like organoids and ileum-like organoids retained their regional identity, demonstrating that regional identity of organoids is stable after initial patterning occurs. This work provides insights into the mechanisms that control regional specification of the developing human intestine and provides new tools for basic and translational research.

Summary: Human embryonic stem cell-derived intestinal organoids can be patterned into duodenum-like or ileum-like tissue, recapitulating in vivo human development.

Mitochondrial function controls intestinal epithelial stemness and proliferation

Control of intestinal epithelial stemness is crucial for tissue homeostasis. Disturbances in epithelial function are implicated in inflammatory and neoplastic diseases of the gastrointestinal tract. Here we report that mitochondrial function plays a critical role in maintaining intestinal stemness and homeostasis. Using intestinal epithelial cell (IEC)-specific mouse models, we show that loss of HSP60, a mitochondrial chaperone, activates the mitochondrial unfolded protein response (MT-UPR) and results in mitochondrial dysfunction. HSP60-deficient crypts display loss of stemness and cell proliferation, accompanied by epithelial release of WNT10A and RSPO1. Sporadic failure of Cre-mediated Hsp60 deletion gives rise to hyperproliferative crypt foci originating from OLFM4⁺ stem cells. These effects are independent of the MT-UPR-associated transcription factor CHOP. In conclusion, compensatory hyperproliferation of HSP60⁺ escaper stem cells suggests paracrine release of WNT-related factors from HSP60-deficient, functionally impaired IEC to be pivotal in the control of the proliferative capacity of the stem cell niche.

It is unclear what role mitochondrial function plays in maintaining intestinal epithelial cell (IEC) homeostasis. Here, the authors deplete a mitochondrial chaperone, heat shock protein 60 (HSP60) in IEC and observe a loss of stemness and cell proliferation, and suggest this is accompanied by a compensatory release of WNT-related factors.

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.

The Role of the C-Clamp in Wnt-Related Colorectal Cancers

T-cell Factor/Lymphoid Enhancer Factor (TCF/LEF) transcription factors are major regulators of Wnt targets, and the products of the TCF7 and TCF7L2 genes have both been implicated in the progression of colorectal cancer in animal models and humans. TCFs recognize specific DNA sequences through their high mobility group (HMG) domains, but invertebrate TCFs and some isoforms of vertebrate TCF7 and TCF7L2 contain a second DNA binding domain known as the C-clamp. This review will cover the basic properties of C-clamps and their importance in Wnt signaling, using data from Drosophila, C. elegans, and mammalian cell culture. The connection between C-clamp containing TCFs and colorectal cancer will also be discussed.

TCF/LEF Transcription Factors: An Update from the Internet Resources

T-cell factor/lymphoid enhancer-binding factor (TCF/LEF) proteins (TCFs) from the High Mobility Group (HMG) box family act as the main downstream effectors of the Wnt signaling pathway. The mammalian TCF/LEF family comprises four nuclear factors designated TCF7, LEF1, TCF7L1, and TCF7L2 (also known as TCF1, LEF1, TCF3, and TCF4, respectively). The proteins display common structural features and are often expressed in overlapping patterns implying their redundancy. Such redundancy was indeed observed in gene targeting studies; however, individual family members also exhibit unique features that are not recapitulated by the related proteins. In the present viewpoint, we summarized our current knowledge about the specific features of individual TCFs, namely structural-functional studies, posttranslational modifications, interacting partners, and phenotypes obtained upon gene targeting in the mouse. In addition, we employed several publicly available databases and web tools to evaluate the expression patterns and production of gene-specific isoforms of the TCF/LEF family members in human cells and tissues.

Stomach development, stem cells and disease

The stomach, an organ derived from foregut endoderm, secretes acid and enzymes and plays a key role in digestion. During development, mesenchymal-epithelial interactions drive stomach specification, patterning, differentiation and growth through selected signaling pathways and transcription factors. After birth, the gastric epithelium is maintained by the activity of stem cells. Developmental signals are aberrantly activated and stem cell functions are disrupted in gastric cancer and other disorders. Therefore, a better understanding of stomach development and stem cells can inform approaches to treating these conditions. This Review highlights the molecular mechanisms of stomach development and discusses recent findings regarding stomach stem cells and organoid cultures, and their roles in investigating disease mechanisms.

Tcf7l1 protects the anterior neural fold from adopting the neural crest fate

The neural crest (NC) is crucial for the evolutionary diversification of vertebrates. NC cells are induced at the neural plate border by the coordinated action of several signaling pathways, including Wnt/β-catenin. NC cells are normally generated in the posterior neural plate border, whereas the anterior neural fold is devoid of NC cells. Using the mouse model, we show here that active repression of Wnt/β-catenin signaling is required for maintenance of neuroepithelial identity in the anterior neural fold and for inhibition of NC induction. Conditional inactivation of Tcf7l1, a transcriptional repressor of Wnt target genes, leads to aberrant activation of Wnt/β-catenin signaling in the anterior neuroectoderm and its conversion into NC. This reduces the developing prosencephalon without affecting the anterior-posterior neural character. Thus, Tcf7l1 defines the border between the NC and the prospective forebrain via restriction of the Wnt/β-catenin signaling gradient.

Wnt/β-Catenin Signaling Activation beyond Robust Nuclear β-Catenin Accumulation in Nondysplastic Barrett's Esophagus: Regulation via Dickkopf-1

INTRODUCTION: Wnt/β-catenin signaling activation has been reported only during the late steps of Barrett’s esophagus (BE) neoplastic progression, but not in BE metaplasia, based on the absence of nuclear β-catenin. However, β-catenin transcriptional activity has been recorded in absence of robust nuclear accumulation. Thus, we aimed to investigate the Wnt/β-catenin signaling in nondysplastic BE. METHODS: Esophageal tissues from healthy and BE patients without dysplasia were analyzed for Wnt target gene expression by quantitative reverse transcription polymerase chain reaction (qRT-PCR) and immunohistochemistry. Esophageal squamous (EPC1-& EPC2-hTERT), BE metaplastic (CP-A), and adenocarcinoma (OE33) cell lines were characterized for Wnt activation by qRT-PCR, Western blot, and luciferase assay. Wnt activity regulation was examined by using recombinant Wnt3a and Dickkopf-1 (Dkk1) as well as Dkk1 short interfering RNA. RESULTS: Wnt target genes (AXIN2, c-MYC, Cyclin D1, Dkk1) and Wnt3a were significantly upregulated in nondysplastic BE compared with squamous mucosa. Elevated levels of dephosphorylated β-catenin were detected in nondysplastic BE. Nuclear active β-catenin and TOPflash activity were increased in CP-A and OE33 cells compared with squamous cells. Wnt3a-mediated β-catenin signaling activation was abolished by Dkk1 in CP-A cells. TOPFlash activity was elevated following Dkk1 silencing in CP-A but not in OE33 cells. Dysplastic and esophageal adenocarcinoma tissues demonstrated further Dkk1 and AXIN2 overexpression. CONCLUSIONS: Despite the absence of robust nuclear accumulation, β-catenin is transcriptionally active in nondysplastic BE. Dkk1 overexpression regulates β-catenin signaling in BE metaplastic but not in adenocarcinoma cells, suggesting that early perturbation of Dkk1-mediated signaling suppression may contribute to BE malignant transformation.

TCF/Lef regulates the Gsx ParaHox gene in central nervous system development in chordates

BACKGROUND

The ParaHox genes play an integral role in the anterior-posterior (A-P) patterning of the nervous system and gut of most animals. The ParaHox cluster is an ideal system in which to study the evolution and regulation of developmental genes and gene clusters, as it displays similar regulatory phenomena to its sister cluster, the Hox cluster, but offers a much simpler system with only three genes.

RESULTS

Using Ciona intestinalis transgenics, we isolated a regulatory element upstream of Branchiostoma floridae Gsx that drives expression within the central nervous system of Ciona embryos. The minimal amphioxus enhancer region required to drive CNS expression has been identified, along with surrounding sequence that increases the efficiency of reporter expression throughout the Ciona CNS. TCF/Lef binding sites were identified and mutagenized and found to be required to drive the CNS expression. Also, individual contributions of TCF/Lef sites varied across the regulatory region, revealing a partial division of function across the Bf-Gsx-Up regulatory element. Finally, when all TCF/Lef binding sites are mutated CNS expression is not only abolished, but a latent repressive function is also unmasked.

CONCLUSIONS

We have identified a B. floridae Gsx upstream regulatory element that drives CNS expression within transgenic Ciona intestinalis, and have shown that this CNS expression is dependent upon TCF/Lef binding sites. We examine the evolutionary and developmental implications of these results, and discuss the possibility of TCF/Lef not only as a regulator of chordate Gsx, but as a deeply conserved regulatory factor controlling all three ParaHox genes across the Metazoa.

From Definitive Endoderm to Gut-a Process of Growth and Maturation

The intestine and colon carries out vital functions, and their lifelong maintenance is of the upmost importance. Research over the past decades has carefully addressed bowel function, how it is maintained and begun to unravel how disorders such as cancer and inflammatory bowel disease form. In contrast, very little is known about the molecular mechanisms that trigger tissue maturation during development. With this review, our aim is to carefully provide a critical appraisal of the literature to give a state-of-the-art view of intestinal development. Starting from definitive endoderm at gastrulation to the emergence of a structure with mature properties, the tissue undergoes complex morphogenetic processes that rely on both biophysical changes and secreted signaling molecules. We will also discuss how new and exciting developments using in vitro models are likely to provide new insights into this process and potential therapeutic strategies for gastrointestinal disorders.

Generating human intestinal tissues from pluripotent stem cells to study development and disease

As one of the largest and most functionally complex organs of the human body, the intestines are primarily responsible for the breakdown and uptake of macromolecules from the lumen and the subsequent excretion of waste from the body. However, the intestine is also an endocrine organ, regulating digestion, metabolism, and feeding behavior. Intricate neuronal, lymphatic, immune, and vascular systems are integrated into the intestine and are required for its digestive and endocrine functions. In addition, the gut houses an extensive population of microbes that play roles in digestion, global metabolism, barrier function, and host-parasite interactions. With such an extensive array of cell types working and performing in one essential organ, derivation of functional intestinal tissues from human pluripotent stem cells (PSCs) represents a significant challenge. Here we will discuss the intricate developmental processes and cell types that are required for assembly of this highly complex organ and how embryonic processes, particularly morphogenesis, have been harnessed to direct differentiation of PSCs into 3-dimensional human intestinal organoids (HIOs) in vitro. We will further describe current uses of HIOs in development and disease research and how additional tissue complexity might be engineered into HIOs for better functionality and disease modeling.

A regulatory network controls nephrocan expression and midgut patterning

Although many regulatory networks involved in defining definitive endoderm have been identified, the mechanisms through which these networks interact to pattern the endoderm are less well understood. To explore the mechanisms involved in midgut patterning, we dissected the transcriptional regulatory elements of nephrocan (Nepn), the earliest known midgut specific gene in mice. We observed that Nepn expression is dramatically reduced in Sox17(-/-) and Raldh2(-/-) embryos compared with wild-type embryos. We further show that Nepn is directly regulated by Sox17 and the retinoic acid (RA) receptor via two enhancer elements located upstream of the gene. Moreover, Nepn expression is modulated by Activin signaling, with high levels inhibiting and low levels enhancing RA-dependent expression. In Foxh1(-/-) embryos in which Nodal signaling is reduced, the Nepn expression domain is expanded into the anterior gut region, confirming that Nodal signaling can modulate its expression in vivo. Together, Sox17 is required for Nepn expression in the definitive endoderm, while RA signaling restricts expression to the midgut region. A balance of Nodal/Activin signaling regulates the anterior boundary of the midgut expression domain.

Pulsatile exposure to simulated reflux leads to changes in gene expression in a 3D model of oesophageal mucosa

Oesophageal exposure to duodenogastroesophageal refluxate is implicated in the development of Barrett's metaplasia (BM), with increased risk of progression to oesophageal adenocarcinoma. The literature proposes that reflux exposure activates NF-κB, driving the aberrant expression of intestine-specific caudal-related homeobox (CDX) genes. However, early events in the pathogenesis of BM from normal epithelium are poorly understood. To investigate this, our study subjected a 3D model of the normal human oesophageal mucosa to repeated, pulsatile exposure to specific bile components and examined changes in gene expression. Initial 2D experiments with a range of bile salts observed that taurochenodeoxycholate (TCDC) impacted upon NF-κB activation without causing cell death. Informed by this, the 3D oesophageal model was repeatedly exposed to TCDC in the presence and absence of acid, and the epithelial cells underwent gene expression profiling. We identified ~300 differentially expressed genes following each treatment, with a large and significant overlap between treatments. Enrichment analysis (Broad GSEA, DAVID and Metacore™; GeneGo Inc) identified multiple gene sets related to cell signalling, inflammation, proliferation, differentiation and cell adhesion. Specifically NF-κB activation, Wnt signalling, cell adhesion and targets for the transcription factors PTF1A and HNF4α were highlighted. Our data suggest that HNF4α isoform switching may be an early event in Barrett's pathogenesis. CDX1/2 targets were, however, not enriched, suggesting that although CDX1/2 activation reportedly plays a role in BM development, it may not be an initial event. Our findings highlight new areas for investigation in the earliest stages of BM pathogenesis of oesophageal diseases and new potential therapeutic targets.

Pattern and process during sea urchin gut morphogenesis: the regulatory landscape

The development of the endoderm is a multistage process. From the initial specification of the endodermal domain in the embryo to the final regionalization of the gut, there are multiple stages that require the involvement of complex gene regulatory networks. In one concrete case, the sea urchin embryo, some of these stages and their genetic control are (relatively) well understood. Several studies have underscored the relevance of individual transcription factor activities in the process, but very few have focused the attention on gene interactions within specific gene regulatory networks (GRNs). Sea urchins offer an ideal system to study the different factors involved in the morphogenesis of the gut. Here we review the knowledge gained over the last 10 years on the process and its regulation, from the early specification of endodermal lineages to the late events linked to the patterning of functional domains in the gut. A lesson of remarkable importance has been learnt from comparison of the mechanisms involved in gut formation in different bilaterian animals; some of these genetic mechanisms are particularly well conserved. Patterning the gut seems to involve common molecular players and shared interactions, whether we look at mammals or echinoderms. This astounding degree of conservation reveals some key aspects of deep homology that are most probably shared by all bilaterian guts.

Retinoic acid-activated Ndrg1a represses Wnt/β-catenin signaling to allow Xenopus pancreas, oesophagus, stomach, and duodenum specification

How cells integrate multiple patterning signals to achieve early endoderm regionalization remains largely unknown. Between gastrulation and neurulation, retinoic acid (RA) signaling is required, while Wnt/β-catenin signaling has to be repressed for the specification of the pancreas, oesophagus, stomach, and duodenum primordia in Xenopus embryos. In attempt to screen for RA regulated genes in Xenopus endoderm, we identified a direct RA target gene, N-myc downstream regulated gene 1a (ndrg1a) that showed expression early in the archenteron roof endoderm and late in the developing pancreas, oesophagus, stomach, and duodenum. Both antisense morpholino oligonucleotide mediated knockdown of ndrg1a in Xenopus laevis and the transcription activator-like effector nucleases (TALEN) mediated disruption of ndrg1 in Xenopus tropicalis demonstrate that like RA signaling, Ndrg1a is specifically required for the specification of Xenopus pancreas, oesophagus, stomach, and duodenum primordia. Immunofluorescence data suggest that RA-activated Ndrg1a suppresses Wnt/β-catenin signaling in Xenopus archenteron roof endoderm cells. Blocking Wnt/β-catenin signaling rescued Ndrg1a knockdown phenotype. Furthermore, overexpression of the putative Wnt/β-catenin target gene Atf3 phenocopied knockdown of Ndrg1a or inhibition of RA signaling, while Atf3 knockdown can rescue Ndrg1a knockdown phenotype. Lastly, the pancreas/stomach/duodenum transcription factor Pdx1 was able to rescue Atf3 overexpression or Ndrg1a knockdown phenotype. Together, we conclude that RA activated Ndrg1a represses Wnt/β-catenin signaling to allow the specification of pancreas, oesophagus, stomach, and duodenum progenitor cells in Xenopus embryos.

Acetylation of human TCF4 (TCF7L2) proteins attenuates inhibition by the HBP1 repressor and induces a conformational change in the TCF4::DNA complex

The members of the TCF/LEF family of DNA-binding proteins are components of diverse gene regulatory networks. As nuclear effectors of Wnt/β-catenin signaling they act as assembly platforms for multimeric transcription complexes that either repress or activate gene expression. Previously, it was shown that several aspects of TCF/LEF protein function are regulated by post-translational modification. The association of TCF/LEF family members with acetyltransferases and deacetylases prompted us to investigate whether vertebrate TCF/LEF proteins are subject to acetylation. Through co-expression with p300 and CBP and subsequent analyses using mass spectrometry and immunodetection with anti-acetyl-lysine antibodies we show that TCF4 can be acetylated at lysine K₁₅₀ by CBP. K₁₅₀ acetylation is restricted to TCF4E splice variants and requires the simultaneous presence of β-catenin and the unique TCF4E C-terminus. To examine the functional consequences of K₁₅₀ acetylation we substituted K₁₅₀ with amino acids representing the non-acetylated and acetylated states. Reporter gene assays based on Wnt/β-catenin-responsive promoter regions did not indicate a general role of K₁₅₀ acetylation in transactivation by TCF4E. However, in the presence of CBP, non-acetylatable TCF4E with a K₁₅₀R substitution was more susceptible to inhibition by the HBP-1 repressor protein compared to wild-type TCF4E. Acetylation of K₁₅₀ using a bacterial expression system or amino acid substitutions at K₁₅₀ alter the electrophoretic properties of TCF4E::DNA complexes. This result suggests that K₁₅₀ acetylation leads to a conformational change that may also represent the mechanism whereby acetylated TCF4E acquires resistance against HBP1. In summary, TCF4 not only recruits acetyltransferases but is also a substrate for these enzymes. The fact that acetylation affects only a subset of TCF4 splice variants and is mediated preferentially by CBP suggests that the conditional acetylation of TCF4E is a novel regulatory mechanism that diversifies the transcriptional output of Wnt/β-catenin signaling in response to changing intracellular signaling milieus.

The Wnt signaling pathway effector TCF7L2 controls gut and brain proglucagon gene expression and glucose homeostasis

The type 2 diabetes risk gene TCF7L2 is the effector of the Wnt signaling pathway. We found previously that in gut endocrine L-cell lines, TCF7L2 controls transcription of the proglucagon gene (gcg), which encodes the incretin hormone glucagon-like peptide-1 (GLP-1). Whereas peripheral GLP-1 stimulates insulin secretion, brain GLP-1 controls energy homeostasis through yet-to-be defined mechanisms. We aim to determine the metabolic effect of a functional knockdown of TCF7L2 by generating transgenic mice that express dominant-negative TCF7L2 (TCF7L2DN) specifically in gcg-expressing cells. The gcg-TCF7L2DN transgenic mice showed reduced gcg expression in their gut and brain, but not in pancreas. Defects in glucose homeostasis were observed in these mice, associated with attenuated plasma insulin levels in response to glucose challenge. The defect in glucose disposal was exacerbated with high-fat diet. Brain Wnt activity and feeding-mediated hypothalamic AMP-activated protein kinase (AMPK) repression in these mice were impaired. Peripheral injection of the cAMP-promoting agent forskolin increased brain β-cat Ser675 phosphorylation and brain gcg expression and restored feeding-mediated hypothalamic AMPK repression. We conclude that TCF7L2 and Wnt signaling control gut and brain gcg expression and glucose homeostasis and speculate that positive cross-talk between Wnt and GLP-1/cAMP signaling is an underlying mechanism for brain GLP-1 in exerting its metabolic functions.

Diabetes risk gene and Wnt effector Tcf7l2/TCF4 controls hepatic response to perinatal and adult metabolic demand

Most studies on TCF7L2 SNP variants in the pathogenesis of type 2 diabetes (T2D) focus on a role of the encoded transcription factor TCF4 in β cells. Here, a mouse genetics approach shows that removal of TCF4 from β cells does not affect their function, whereas manipulating TCF4 levels in the liver has major effects on metabolism. In Tcf7l2(-/-) mice, the immediate postnatal surge in liver metabolism does not occur. Consequently, pups die due to hypoglycemia. By combining chromatin immunoprecipitation with gene expression profiling, we identify a TCF4-controlled metabolic gene program that is acutely activated in the postnatal liver. In concordance, adult liver-specific Tcf7l2 knockout mice show reduced hepatic glucose production during fasting and display improved glucose homeostasis when maintained on high-fat diet. Furthermore, liver-specific TCF4 overexpression increases hepatic glucose production. These observations imply that TCF4 directly activates metabolic genes and that inhibition of Wnt signaling may be beneficial in metabolic disease.

Cdx1 and Cdx2 exhibit transcriptional specificity in the intestine

The caudal-related homeodomain transcription factors Cdx1 and Cdx2 are expressed in the developing endoderm with expression persisting into adulthood. Cdx1^−/− mutants are viable and fertile and display no overt intestinal phenotype. Cdx2 null mutants are peri-implantation lethal; however, conditional mutation approaches have revealed that Cdx2 is required for patterning the intestinal epithelium and specification of the colon. Cdx2 is also necessary for homeostasis of the intestinal tract in the adult, where Cdx1 and Cdx2 appear to functionally overlap in the distal colon, but not during intestinal development. Cdx1 and Cdx2 exhibit complete overlap of expression in the intestine, although they differ in their relative levels, with Cdx1 maximal in the distal colon and Cdx2 peaking in the proximal cecum. Moreover, Cdx1 protein is graded along the crypt-villus axis, being abundant in the crypts and diminishing towards the villi. Cdx2 is expressed uniformly along this axis, but is differentially phosphorylated; the functional relevance of these expression domains and phosphorylation is currently unknown. Cdx1 and Cdx2 have been suggested to exhibit functional specificity in the intestinal tract. In the present study, using cell-based models, we found that relative to Cdx1, Cdx2 was significantly less potent at effecting a transcriptional response from the Cdx1 promoter, a known Cdx target gene. We subsequently assessed this relationship in vivo using a “gene swap” approach and found that Cdx2 cannot substitute for Cdx1 in this autoregulatory loop. This is in marked contrast with the ability of Cdx2 to support Cdx1 expression and function in paraxial mesoderm and vertebral patterning, thus providing novel in vivo evidence of context-dependent transcriptional specificity between these transcription factors.

TCF/LEFs and Wnt signaling in the nucleus

T-cell factor/lymphoid enhancer factor (TCF/LEF) transcription factors are the major end point mediators of Wnt/Wingless signaling throughout metazoans. TCF/LEFs are multifunctional proteins that use their sequence-specific DNA-binding and context-dependent interactions to specify which genes will be regulated by Wnts. Much of the work to define their actions has focused on their ability to repress target gene expression when Wnt signals are absent and to recruit β-catenin to target genes for activation when Wnts are present. Recent advances have highlighted how these on/off actions are regulated by Wnt signals and stabilized β-catenin. In contrast to invertebrates, which typically contain one TCF/LEF protein that can both activate and repress Wnt targets, gene duplication and isoform complexity of the family in vertebrates have led to specialization, in which individual TCF/LEF isoforms have distinct activities.

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.

Activation of Wnt signalling promotes development of dysplasia in Barrett's oesophagus

Barrett's oesophagus is a precursor of oesophageal adenocarcinoma, via intestinal metaplasia and dysplasia. Risk of cancer increases substantially with dysplasia, particularly high-grade dysplasia. Thus, there is a clinical need to identify and treat patients with early-stage disease (metaplasia and low-grade dysplasia) that are at high risk of cancer. Activated Wnt signalling is critical for normal intestinal development and homeostasis, but less so for oesophageal development. Therefore, we asked whether abnormally increased Wnt signalling contributes to the development of Barrett's oesophagus (intestinal metaplasia) and/or dysplasia. Forty patients with Barrett's metaplasia, dysplasia or adenocarcinoma underwent endoscopy and biopsy. Mice with tamoxifen- and β-naphthoflavone-induced expression of activated β-catenin were used to up-regulate Wnt signalling in mouse oesophagus. Immunohistochemistry of β-catenin, Ki67, a panel of Wnt target genes, and markers of intestinal metaplasia was performed on human and mouse tissues. In human tissues, expression of nuclear activated β-catenin was found in dysplasia, particularly high grade. Barrett's metaplasia did not show high levels of activated β-catenin. Up-regulation of Ki67 and Wnt target genes was also mostly associated with high-grade dysplasia. Aberrant activation of Wnt signalling in mouse oesophagus caused marked tissue disorganization with features of dysplasia, but only selected molecular indicators of metaplasia. Based on these results in human tissues and a mouse model, we conclude that abnormal activation of Wnt signalling likely plays only a minor role in initiation of Barrett's metaplasia but a more critical role in progression to dysplasia.

Intrinsic properties of Tcf1 and Tcf4 splice variants determine cell-type-specific Wnt/β-catenin target gene expression

T-cell factor (Tcf)/lymphoid-enhancer factor (Lef) proteins are a structurally diverse family of deoxyribonucleic acid-binding proteins that have essential nuclear functions in Wnt/β-catenin signalling. Expression of Wnt/β-catenin target genes is highly dependent on context, but the precise role of Tcf/Lef family members in the generation and maintenance of cell-type-specific Wnt/β-catenin responses is unknown. Herein, we show that induction of a subset of Wnt/β-catenin targets in embryonic stem cells depends on Tcf1 and Tcf4, whereas other co-expressed Tcf/Lef family members cannot induce these targets. The Tcf1/Tcf4-dependent gene responses to Wnt are primarily if not exclusively mediated by C-clamp-containing Tcf1E and Tcf4E splice variants. A combined knockdown of Tcf1/Tcf4 abrogates Wnt-inducible transcription but does not affect the active chromatin conformation of their targets. Thus, the transcriptionally poised state of Wnt/β-catenin targets is maintained independent of Tcf/Lef proteins. Conversely, ectopically overexpressed Tcf1E cannot invade silent chromatin and fails to initiate expression of inactive Wnt/β-catenin targets even if repressive chromatin modifications are abolished. The observed non-redundant functions of Tcf1/Tcf4 isoforms in acute transcriptional activation demonstrated that the cell-type-specific complement of Tcf/Lef proteins is a critical determinant of context-dependent Wnt/β-catenin responses. Moreover, the apparent inability to cope with chromatin uncovers an intrinsic property of Tcf/Lef proteins that prevents false ectopic induction and ensures spatiotemporal stability of Wnt/β-catenin target gene expression.

Patterning and shaping the endoderm in vivo and in culture

The definitive endoderm (DE) was first defined as the innermost germ layer found in all metazoan embryos. During development, it gives rise to a vast array of specialized epithelial cell types lining the respiratory and digestive systems, and contributes to associated organs such as thyroid, thymus, lungs, liver, and pancreas. In the adult, the DE provides a protective barrier against the environment and assumes many essential functions including digestion, nutrient absorption, and glucose homeostasis. Since general endoderm formation and patterning have been reviewed recently in a comprehensive manner [1], we will only provide a brief summary of how extracellular signals and downstream transcription factors control endoderm patterning. We will then focus on emerging work addressing the chromatin remodeling events occurring during endoderm organ specification and discuss how these molecular tools can be used to engineer endodermal organs in vitro.

Wnt/β-catenin signaling is disrupted in the extra-toes (Gli3(Xt/Xt) ) mutant from early stages of forebrain development, concomitant with anterior neural plate patterning defects

The zinc finger transcription factor Gli3 is essential for normal development of the forebrain. Mutant mice with no functional Gli3 (extra-toes, Gli3(Xt/Xt) mutants) display a massive reduction in the size of the telencephalic lobes and absence of dorsomedial telencephalic structures, including the cortical hem, which normally expresses a number of Wnt molecules essential for patterning the hippocampus. Dorsomedial telencephalic Wnt activity, transduced through the Wnt/β-catenin signaling pathway, is also required for hippocampal specification and dorsoventral telencephalic patterning. Wnts whose normal expression is restricted to the cortical hem are completely absent in Gli3(Xt/Xt) embryos, but some expression of those Wnts with a broader expression domain persists, raising the possibility that Wnt/β-catenin signaling may still be active in this mutant. We examined whether the Wnt expression that persists in the Gli3(Xt/Xt) mutant neocortex activates Wnt/β-catenin signaling, using the BAT-gal transgenic reporter. We found Wnt/β-catenin signaling consistently decreased in the forebrains of Gli3(Xt/Xt) mutants, even prior to the formation of the cortical hem. This is accompanied by a severe reduction in expression of Wnt7b and Wnt8b at the lateral edges of the anterior neural plate that will give rise to the pallium. In addition, we found a significant increase in the expression of rostroventral markers of the anterior neural plate that will give rise to the basal forebrain. Our data reveal that Gli3 is required at the neural plate stage to regulate Wnt expression and Wnt/β-catenin signaling in the presumptive forebrain and confirm its previously proposed role in patterning the anterior neural plate.

Pathways involved in Drosophila and human cancer development: the Notch, Hedgehog, Wingless, Runt, and Trithorax pathway

Animal models are established tools to study basic questions of biology in a systematic way. They have greatly facilitated our understanding of the mechanisms by which nature forms and maintains organisms. Much of the knowledge on molecular changes underlying the development of organisms originates from research in the fruit fly model Drosophila melanogaster. Vertebrate models including the mouse and zebrafish model, but also other animal models coming from different corners of the animal kingdom have shown that much of the basic machinery of development is essentially identical, not just in all vertebrates but in all major phyla of invertebrates too. Moreover, key elements of this machinery have been demonstrated to be involved in recurrent molecular abnormalities detected in tumor-tissue from patients, indicating their implication in the genesis of human cancer. Thus, research in this field has become a common topic for both biologists and hemato-oncologists. In this review, we summarize current knowledge on some of these key elements and molecular pathways such as Notch, Hedgehog, Wingless, Runt, and Trithorax that have been originally described and studied in animal models and which seem to play a major role in the pathophysiology and targeted management of human cancer.

A critical role for the Wnt effector Tcf4 in adult intestinal homeostatic self-renewal

Throughout life, intestinal Lgr5+ stem cells give rise to proliferating transient amplifying cells in crypts, which subsequently differentiate into one of the five main cell types and migrate along the crypt-villus axis. These dynamic processes are coordinated by a relatively small number of evolutionarily conserved signaling pathways, which includes the Wnt signaling pathway. The DNA-binding proteins of the T-cell factor family, Tcf1/Tcf7, Lef, Tcf3/Tcf7l1, and Tcf4/Tcf7l2, constitute the downstream effectors of the Wnt signaling pathway. While Tcf4 is the major member active during embryogenesis, the role of these Wnt effectors in the homeostasis of the adult mouse intestinal epithelium is unresolved. Using Tcf1-/-, Tcf3(flox), and novel Tcf4(flox) mice, we demonstrate an essential role for Tcf4 during homeostasis of the adult mouse intestine.

Maintaining embryonic stem cell pluripotency with Wnt signaling

Wnt signaling pathways control lineage specification in vertebrate embryos and regulate pluripotency in embryonic stem (ES) cells, but how the balance between progenitor self-renewal and differentiation is achieved during axis specification and tissue patterning remains highly controversial. The context- and stage-specific effects of the different Wnt pathways produce complex and sometimes opposite outcomes that help to generate embryonic cell diversity. Although the results of recent studies of the Wnt/β-catenin pathway in ES cells appear to be surprising and controversial, they converge on the same conserved mechanism that leads to the inactivation of TCF3-mediated repression.

Concerted involvement of Cdx/Hox genes and Wnt signaling in morphogenesis of the caudal neural tube and cloacal derivatives from the posterior growth zone

Decrease in Cdx dosage in an allelic series of mouse Cdx mutants leads to progressively more severe posterior vertebral defects. These defects are corrected by posterior gain of function of the Wnt effector Lef1. Precocious expression of Hox paralogous 13 genes also induces vertebral axis truncation by antagonizing Cdx function. We report here that the phenotypic similarity also applies to patterning of the caudal neural tube and uro-rectal tracts in Cdx and Wnt3a mutants, and in embryos precociously expressing Hox13 genes. Cdx2 inactivation after placentation leads to posterior defects, including incomplete uro-rectal septation. Compound mutants carrying one active Cdx2 allele in the Cdx4-null background (Cdx2/4), transgenic embryos precociously expressing Hox13 genes and a novel Wnt3a hypomorph mutant all manifest a comparable phenotype with similar uro-rectal defects. Phenotype and transcriptome analysis in early Cdx mutants, genetic rescue experiments and gene expression studies lead us to propose that Cdx transcription factors act via Wnt signaling during the laying down of uro-rectal mesoderm, and that they are operative in an early phase of these events, at the site of tissue progenitors in the posterior growth zone of the embryo. Cdx and Wnt mutations and premature Hox13 expression also cause similar neural dysmorphology, including ectopic neural structures that sometimes lead to neural tube splitting at caudal axial levels. These findings involve the Cdx genes, canonical Wnt signaling and the temporal control of posterior Hox gene expression in posterior morphogenesis in the different embryonic germ layers. They shed a new light on the etiology of the caudal dysplasia or caudal regression range of human congenital defects.

Wnt signaling specifies and patterns intestinal endoderm

Wnt signaling has been implicated in many developmental processes, but its role in early endoderm development is not well understood. Wnt signaling is active in posterior endoderm as early as E7.5. Genetic and chemical activation show that the Wnt pathway acts directly on endoderm to induce the intestinal master regulator Cdx2, shifting global gene away from anterior endoderm and toward a posterior, intestinal program. In a mouse embryonic stem cell differentiation platform that yields pure populations of definitive endoderm, Wnt signaling induces intestinal gene expression in all cells. We have identified a set of genes specific to the anterior small intestine, posterior small intestine, and large intestine during early development, and show that Wnt, through Cdx2, activates large intestinal gene expression at high doses and small intestinal gene expression at lower doses. These findings shed light on the mechanism of embryonic intestinal induction and provide a method to manipulate intestinal development from embryonic stem cells.

Opposing effects of Tcf3 and Tcf1 control Wnt stimulation of embryonic stem cell self-renewal

The co-occupancy of Tcf3 with Oct4, Sox2 and Nanog on embryonic stem cell (ESC) chromatin indicated that Tcf3 has been suggested to play an integral role in a poorly understood mechanism underlying Wnt-dependent stimulation of mouse ESC self-renewal of mouse ESCs. Although the conventional view of Tcf proteins as the β-catenin-binding effectors of Wnt signalling suggested Tcf3-β-catenin activation of target genes would stimulate self-renewal, here we show that an antagonistic relationship between Wnt3a and Tcf3 on gene expression regulates ESC self-renewal. Genetic ablation of Tcf3 replaced the requirement for exogenous Wnt3a or GSK3 inhibition for ESC self-renewal, demonstrating that inhibition of Tcf3 repressor is the necessary downstream effect of Wnt signalling. Interestingly, both Tcf3-β-catenin and Tcf1-β-catenin interactions contributed to Wnt stimulation of self-renewal and gene expression, and the combination of Tcf3 and Tcf1 recruited Wnt-stabilized β-catenin to Oct4 binding sites on ESC chromatin. This work elucidates the molecular link between the effects of Wnt and the regulation of the Oct4/Sox2/Nanog network.