Differential requirements for β-catenin during mouse development

FOXF1 promotes tumor vessel normalization and prevents lung cancer progression through FZD4

Cancer cells re-program normal lung endothelial cells (EC) into tumor-associated endothelial cells (TEC) that form leaky vessels supporting carcinogenesis. Transcriptional regulators that control the reprogramming of EC into TEC are poorly understood. We identified Forkhead box F1 (FOXF1) as a critical regulator of EC-to-TEC transition. FOXF1 was highly expressed in normal lung vasculature but was decreased in TEC within non-small cell lung cancers (NSCLC). Low FOXF1 correlated with poor overall survival of NSCLC patients. In mice, endothelial-specific deletion of FOXF1 decreased pericyte coverage, increased vessel permeability and hypoxia, and promoted lung tumor growth and metastasis. Endothelial-specific overexpression of FOXF1 normalized tumor vessels and inhibited the progression of lung cancer. FOXF1 deficiency decreased Wnt/β-catenin signaling in TECs through direct transcriptional activation of Fzd4. Restoring FZD4 expression in FOXF1-deficient TECs through endothelial-specific nanoparticle delivery of Fzd4 cDNA rescued Wnt/β-catenin signaling in TECs, normalized tumor vessels and inhibited the progression of lung cancer. Altogether, FOXF1 increases tumor vessel stability, and inhibits lung cancer progression by stimulating FZD4/Wnt/β-catenin signaling in TECs. Nanoparticle delivery of FZD4 cDNA has promise for future therapies in NSCLC.

A Mouse Model for the Rapid and Binomial Assessment of Putative WNT/β-Catenin Signalling Inhibitors

Specific signalling thresholds of the WNT/β-catenin pathway affect embryogenesis and tissue homeostasis in the adult, with mutations in this pathway frequently occurring in cancer. Excessive WNT/β-catenin activity inhibits murine anterior development associated with embryonic lethality and accounts for the driver event in 80% of human colorectal cancers. Uncontrolled WNT/β-catenin signalling arises primarily from impairment mutation in the tumour suppressor gene APC that otherwise prevents prolonged stabilisation of β-catenin. Surprisingly, no inhibitor compounds for WNT/β-catenin signalling have reached clinical use in part owing to the lack of specific in vivo assays that discriminate between on-target activities and dose-limiting toxicities. Here, we present a simple in vivo assay with a binary outcome whereby the administration of candidate compounds to pregnant and phenotypically normal Apcflox/flox mice can rescue in utero death of Apcmin/flox mutant conceptus without subsequent post-mortem assessment of WNT/β-catenin signalling. Indeed, the phenotypic plasticity of born Apcmin/flox conceptus enables future refinement of our assay to potentially enable dosage finding and cross-compound comparisons. Thus, we show for the first time the suitability of endogenous WNT/β-catenin signalling during embryonic development to provide an unambiguous and sensitive mammalian in vivo model to assess the efficacy and bioavailability of potential WNT/β-catenin antagonists.

Embryonic β-Catenin Is Required for Priming of the Uterus to Implantation

Repeated implantation failure is a major cause of infertility among healthy women. Uterine β-catenin (CTNNB1) plays a critical role in implantation. However, the role of embryonic CTNNB1 during implantation remains unclear. We addressed this topic by analyzing mice carrying Ctnnb1-deficient (Ctnnb1^Δ/Δ) embryos. Ctnnb1^Δ/Δ embryos were produced by intercrossing mice bearing Ctnnb1-deficient eggs and sperms. We found that Ctnnb1^Δ/Δ embryos developed to the blastocyst stage; thereafter, they were resorbed, leaving empty decidual capsules. Moreover, leukemia inhibitory factor, a uterine factor essential for implantation, was undetectable in Ctnnb1^Δ/Δ blastocysts. Furthermore, CDX2, a transcription factor that determines the fate of trophectoderm cells, was not observed in Ctnnb1^Δ/Δ blastocysts. Intrauterine injection with uterine fluids (from control mice) and recombinant mouse leukemia inhibitory factor proteins rescued the uterine response to Ctnnb1^Δ/Δ blastocysts. These results suggest that embryonic CTNNB1 is required for the secretion of blastocyst-derived factor(s) that open the implantation window, indicating that the uterine response to implantation can be induced using supplemental materials. Therefore, our results may contribute to the discovery of a similar mechanism in humans, leading to a better understanding of the pathogenesis of repeated implantation failure.

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

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

Molecular Markers in Maternal Blood Exosomes Allow Early Detection of Fetal Alcohol Spectrum Disorders

Prenatal alcohol exposure can cause developmental abnormalities (fetal alcohol spectrum disorders; FASD), including small eyes, face and brain, and neurobehavioral deficits. These cannot be detected early in pregnancy with available imaging techniques. Early diagnosis could facilitate development of therapeutic interventions. Banked human fetal brains and eyes at 9−22 weeks’ gestation were paired with maternal blood samples, analyzed for morphometry, protein, and RNA expression, and apoptotic signaling. Alcohol (EtOH)-exposed (maternal self-report) fetuses were compared with unexposed controls matched for fetal age, sex, and maternal race. Fetal brain-derived exosomes (FB-E) were isolated from maternal blood and analyzed for protein, RNA, and apoptotic markers. EtOH use by mothers, assessed by self-report, was associated with reduced fetal eye diameter, brain size, and markers of synaptogenesis. Brain caspase-3 activity was increased. The reduction in eye and brain sizes were highly correlated with amount of EtOH intake and caspase-3 activity. Levels of several biomarkers in FB-E, most strikingly myelin basic protein (MBP; r > 0.9), correlated highly with morphological abnormalities. Reduction in FB-E MBP levels was highly correlated with EtOH exposure (p < 1.0 × 10−10). Although the morphological features of FAS appear long before they can be detected by live imaging, FB-E in the mother’s blood may contain markers, particularly MBP, that predict FASD.

The origins of skin diversity: lessons from dermal fibroblasts

Skin is largely composed of an epidermis that overlies a supporting dermis. Recent advancements in our understanding of how diverse groups of dermal fibroblasts regulate epidermal and hair follicle growth and differentiation have been fueled by tools capable of resolving molecular heterogeneity at a single-cell level. Fibroblast heterogeneity can be traced back to their developmental origin before their segregation into spatially distinct fibroblast subtypes. The mechanisms that drive this lineage diversification during development are being unraveled, with studies showing that both large- and small-scale positional signals play important roles during dermal development. Here, we first delineate what is known about the origins of the dermis and the central role of Wnt/β-catenin signaling in its specification across anatomical locations. We then discuss how one of the first morphologically recognizable fibroblast subtypes, the hair follicle dermal condensate lineage, emerges. Leveraging the natural variation of skin and its appendages between species and between different anatomical locations, these collective studies have identified shared and divergent factors that contribute to the extraordinary diversity of skin.

From Nucleus to Organs: Insights of Aryl Hydrocarbon Receptor Molecular Mechanisms

The aryl hydrocarbon receptor (AHR) is a markedly established regulator of a plethora of cellular and molecular processes. Its initial role in the detoxification of xenobiotic compounds has been partially overshadowed by its involvement in homeostatic and organ physiology processes. In fact, the discovery of its ability to bind specific target regulatory sequences has allowed for the understanding of how AHR modulates such processes. Thereby, AHR presents functions in transcriptional regulation, chromatin architecture modifications and participation in different key signaling pathways. Interestingly, such fields of influence end up affecting organ and tissue homeostasis, including regenerative response both to endogenous and exogenous stimuli. Therefore, from classical spheres such as canonical transcriptional regulation in embryonic development, cell migration, differentiation or tumor progression to modern approaches in epigenetics, senescence, immune system or microbiome, this review covers all aspects derived from the balance between regulation/deregulation of AHR and its physio-pathological consequences.

Dermal EZH2 orchestrates dermal differentiation and epidermal proliferation during murine skin development

Skin development and patterning is dependent on factors that regulate the stepwise differentiation of dermal fibroblasts concomitant with dermal-epidermal reciprocal signaling, two processes that are poorly understood. Here we show that dermal EZH2, the methyltransferase enzyme of the epigenetic Polycomb Repressive Complex 2 (PRC2), is a new coordinator of both these processes. Dermal EZH2 activity is present during dermal fibroblast differentiation and is required for spatially restricting Wnt/β-catenin signaling to reinforce dermal fibroblast cell fate. Later in development, dermal EZH2 regulates the expression of reticular dermal markers and initiation of secondary hair follicles. Embryos lacking dermal Ezh2 have elevated epidermal proliferation and differentiation that can be rescued by small molecule inhibition of retinoic acid (RA) signaling. Together, our study reveals that dermal EZH2 is acting like a rheostat to control the levels of Wnt/β-catenin and RA signaling to impact fibroblast differentiation cell autonomously and epidermal keratinocyte development non-cell autonomously, respectively.

Wnt-Dependent Activation of ERK Mediates Repression of Chondrocyte Fate during Calvarial Development

Wnt signaling regulates cell fate decisions in diverse contexts during development, and loss of Wnt signaling in the cranial mesenchyme results in a robust and binary cell fate switch from cranial bone to ectopic cartilage. The Extracellular signal-regulated protein kinase 1 and 2 (ERK1/2) and Wnt signaling pathways are activated during calvarial osteoblast cell fate selection. Here, we test the hypothesis that ERK signaling is a mediator of Wnt-dependent cell fate decisions in the cranial mesenchyme. First, we show that loss of Erk1/2 in the cranial mesenchyme results in a diminished domain of osteoblast marker expression and increased expression of cartilage fate markers and ectopic cartilage formation in the frontal bone primordia. Second, we show that mesenchyme Wnt/β-catenin signaling and Wntless are required for ERK activation in calvarial osteoblasts. Third, we demonstrate that Wnt and ERK signaling pathways function together to repress SOX9 expression in mouse cranial mesenchyme. Our results demonstrate an interaction between the Wnt and ERK signaling pathways in regulating lineage selection in a subset of calvarial cells and provide new insights into Wnt-dependent cell fate decisions.

Fetuin-A is a HIF target that safeguards tissue integrity during hypoxic stress

Intrauterine growth restriction (IUGR) is associated with reduced kidney size at birth, accelerated renal function decline, and increased risk for chronic kidney and cardiovascular diseases in adults. Precise mechanisms underlying fetal programming of adult diseases remain largely elusive and warrant extensive investigation. Setting up a mouse model of hypoxia-induced IUGR, fetal adaptations at mRNA, protein and cellular levels, and their long-term functional consequences are characterized, using the kidney as a readout. Here, we identify fetuin-A as an evolutionary conserved HIF target gene, and further investigate its role using fetuin-A KO animals and an adult model of ischemia-reperfusion injury. Beyond its role as systemic calcification inhibitor, fetuin-A emerges as a multifaceted protective factor that locally counteracts calcification, modulates macrophage polarization, and attenuates inflammation and fibrosis, thus preserving kidney function. Our study paves the way to therapeutic approaches mitigating mineral stress-induced inflammation and damage, principally applicable to all soft tissues.

Adipsin promotes bone marrow adiposity by priming mesenchymal stem cells

Background

Marrow adipose tissue (MAT) has been shown to be vital for regulating metabolism and maintaining skeletal homeostasis in the bone marrow (BM) niche. As a reflection of BM remodeling, MAT is highly responsive to nutrient fluctuations, hormonal changes, and metabolic disturbances such as obesity and diabetes mellitus. Expansion of MAT has also been strongly associated with bone loss in mice and humans. However, the regulation of BM plasticity remains poorly understood, as does the mechanism that links changes in marrow adiposity with bone remodeling.

Methods

We studied deletion of Adipsin, and its downstream effector, C3, in C57BL/6 mice as well as the bone-protected PPARγ constitutive deacetylation 2KR mice to assess BM plasticity. The mice were challenged with thiazolidinedione treatment, calorie restriction, or aging to induce bone loss and MAT expansion. Analysis of bone mineral density and marrow adiposity was performed using a μCT scanner and by RNA analysis to assess adipocyte and osteoblast markers. For in vitro studies, primary bone marrow stromal cells were isolated and subjected to osteoblastogenic or adipogenic differentiation or chemical treatment followed by morphological and molecular analyses. Clinical data was obtained from samples of a previous clinical trial of fasting and high-calorie diet in healthy human volunteers.

Results

We show that Adipsin is the most upregulated adipokine during MAT expansion in mice and humans in a PPARγ acetylation-dependent manner. Genetic ablation of Adipsin in mice specifically inhibited MAT expansion but not peripheral adipose depots, and improved bone mass during calorie restriction, thiazolidinedione treatment, and aging. These effects were mediated through its downstream effector, complement component C3, to prime common progenitor cells toward adipogenesis rather than osteoblastogenesis through inhibiting Wnt/β-catenin signaling.

Conclusions

Adipsin promotes new adipocyte formation and affects skeletal remodeling in the BM niche. Our study reveals a novel mechanism whereby the BM sustains its own plasticity through paracrine and endocrine actions of a unique adipokine.

Funding

This work was supported by the National Institutes of Health T32DK007328 (NA), F31DK124926 (NA), R01DK121140 (JCL), R01AR068970 (BZ), R01AR071463 (BZ), R01DK112943 (LQ), R24DK092759 (CJR), and P01HL087123 (LQ).

β-Catenin and Associated Proteins Regulate Lineage Differentiation in Ground State Mouse Embryonic Stem Cells

Mouse embryonic stem cells (ESCs) cultured in defined medium resemble the pre-implantation epiblast in the ground state, with full developmental capacity including the germline. β-Catenin is required to maintain ground state pluripotency in mouse ESCs, but its exact role is controversial. Here, we reveal a Tcf3-independent role of β-catenin in restraining germline and somatic lineage differentiation genes. We show that β-catenin binds target genes with E2F6 and forms a complex with E2F6 and HMGA2 or E2F6 and HP1γ. Our data indicate that these complexes help β-catenin restrain and fine-tune germ cell and neural developmental potential. Overall, our data reveal a previously unappreciated role of β-catenin in preserving lineage differentiation integrity in ground state ESCs.

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β-Catenin depletion irreversibly compromised lineage development of ground state ESCs

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TCF3-independent role of β-catenin in determining lineage differentiation potential

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E2F6, HP1γ, and HMGA2 are β-catenin interaction partners and co-bound to target genes

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β-Catenin and protein partners fine-tune germline and neural development potential

β-catenin-mediated Wnt signal transduction proceeds through an endocytosis-independent mechanism

The Wnt pathway is a key intercellular signaling cascade that regulates development, tissue homeostasis, and regeneration. However, gaps remain in our understanding of the molecular events that take place between ligand-receptor binding and target gene transcription. We used a novel tool for quantitative, real-time assessment of endogenous pathway activation, measured in single cells, to answer an unresolved question in the field-whether receptor endocytosis is required for Wnt signal transduction. We combined knockdown or knockout of essential components of clathrin-mediated endocytosis with quantitative assessment of Wnt signal transduction in mouse embryonic stem cells (mESCs). Disruption of clathrin-mediated endocytosis did not affect accumulation and nuclear translocation of β-catenin, as measured by single-cell live imaging of endogenous β-catenin, and subsequent target gene transcription. Disruption of another receptor endocytosis pathway, caveolin-mediated endocytosis, did not affect Wnt pathway activation in mESCs. Additional results in multiple cell lines support that endocytosis is not a requirement for Wnt signal transduction. We show that off-target effects of a drug used to inhibit endocytosis may be one source of the discrepancy among reports on the role of endocytosis in Wnt signaling.

Developmentally Programmed Tankyrase Activity Upregulates β-Catenin and Licenses Progression of Embryonic Genome Activation

Embryonic genome activation (EGA) is orchestrated by an intrinsic developmental program initiated during oocyte maturation with translation of stored maternal mRNAs. Here, we show that tankyrase, a poly(ADP-ribosyl) polymerase that regulates β-catenin levels, undergoes programmed translation during oocyte maturation and serves an essential role in mouse EGA. Newly translated TNKS triggers proteasomal degradation of axin, reducing targeted destruction of β-catenin and promoting β-catenin-mediated transcription of target genes, including Myc. MYC mediates ribosomal RNA transcription in 2-cell embryos, supporting global protein synthesis. Suppression of tankyrase activity using knockdown or chemical inhibition causes loss of nuclear β-catenin and global reductions in transcription and histone H3 acetylation. Chromatin and transcriptional profiling indicate that development arrests prior to the mid-2-cell stage, mediated in part by reductions in β-catenin and MYC. These findings indicate that post-transcriptional regulation of tankyrase serves as a ligand-independent developmental mechanism for post-translational β-catenin activation and is required to complete EGA.

Characterization of Stem-like Circulating Tumor Cells in Pancreatic Cancer

Pancreatic ductal adenocarcinoma (PDAC) is the fourth most frequent cause of death from cancer. Circulating tumor cells (CTCs) with stem-like characteristics lead to distant metastases and thus contribute to the dismal prognosis of PDAC. Our purpose is to investigate the role of stemness in CTCs derived from a genetically engineered mouse model of PDAC and to further explore the potential molecular mechanisms. The publically available RNA sequencing dataset GSE51372 was analyzed, and CTCs with (CTC-S) or without (CTC-N) stem-like features were discriminated based on a principal component analysis (PCA). Differentially expressed genes, weighted gene co-expression network analysis (WGCNA), and further functional enrichment analyses were performed. The prognostic role of the candidate gene (CTNNB1) was assessed in a clinical PDAC patient cohort. Overexpression of the pluripotency marker Klf4 (Krüppel-like factor 4) in CTC-S cells positively correlates with Ctnnb1 (β-Catenin) expression, and their interaction presumably happens via protein-protein binding in the nucleus. As a result, the adherens junction pathway is significantly enriched in CTC-S. Furthermore, the overexpression of Ctnnb1 is a negative prognostic factor for progression-free survival (PFS) and relapse-free survival (RFS) in human PDAC cohort. Overexpression of Ctnnb1 may thus promote the metastatic capabilities of CTCs with stem-like properties via adherens junctions in murine PDAC.

NOTO Transcription Factor Directs Human Induced Pluripotent Stem Cell-Derived Mesendoderm Progenitors to a Notochordal Fate

The founder cells of the Nucleus pulposus, the centre of the intervertebral disc, originate in the embryonic notochord. After birth, mature notochordal cells (NC) are identified as key regulators of disc homeostasis. Better understanding of their biology has great potential in delaying the onset of disc degeneration or as a regenerative-cell source for disc repair. Using human pluripotent stem cells, we developed a two-step method to generate a stable NC-like population with a distinct molecular signature. Time-course analysis of lineage-specific markers shows that WNT pathway activation and transfection of the notochord-related transcription factor NOTO are sufficient to induce high levels of mesendoderm progenitors and favour their commitment toward the notochordal lineage instead of paraxial and lateral mesodermal or endodermal lineages. This study results in the identification of NOTO-regulated genes including some that are found expressed in human healthy disc tissue and highlights NOTO function in coordinating the gene network to human notochord differentiation.

Ubiquitin Ligases Involved in the Regulation of Wnt, TGF-β, and Notch Signaling Pathways and Their Roles in Mouse Development and Homeostasis

The Wnt, TGF-β, and Notch signaling pathways are essential for the regulation of cellular polarity, differentiation, proliferation, and migration. Differential activation and mutual crosstalk of these pathways during animal development are crucial instructive forces in the initiation of the body axis and the development of organs and tissues. Due to the ability to initiate cell proliferation, these pathways are vulnerable to somatic mutations selectively producing cells, which ultimately slip through cellular and organismal checkpoints and develop into cancer. The architecture of the Wnt, TGF-β, and Notch signaling pathways is simple. The transmembrane receptor, activated by the extracellular stimulus, induces nuclear translocation of the transcription factor, which subsequently changes the expression of target genes. Nevertheless, these pathways are regulated by a myriad of factors involved in various feedback mechanisms or crosstalk. The most prominent group of regulators is the ubiquitin-proteasome system (UPS). To open the door to UPS-based therapeutic manipulations, a thorough understanding of these regulations at a molecular level and rigorous confirmation in vivo are required. In this quest, mouse models are exceptional and, thanks to the progress in genetic engineering, also an accessible tool. Here, we reviewed the current understanding of how the UPS regulates the Wnt, TGF-β, and Notch pathways and we summarized the knowledge gained from related mouse models.

Mitochondrial Dynamics Is Critical for the Full Pluripotency and Embryonic Developmental Potential of Pluripotent Stem Cells

While the pluripotency of stem cells is known to determine the fate of embryonic development, the mechanisms underlying the acquisition and maintenance of full pluripotency largely remain elusive. Here, we show that the balance between mitochondrial fission and fusion is critical for the full pluripotency of stem cells. By analyzing induced pluripotent stem cells with differential developmental potential, we found that excess mitochondrial fission is associated with an impaired embryonic developmental potential. We further uncover that the disruption of mitochondrial dynamics impairs the differentiation and embryonic development of pluripotent stem cells; most notably, pluripotent stem cells that display excess mitochondrial fission fail to produce live-born offspring by tetraploid complementation. Mechanistically, excess mitochondrial fission increases cytosolic Ca²⁺ entry and CaMKII activity, leading to ubiquitin-mediated proteasomal degradation of β-Catenin protein. Our results reveal a previously unappreciated fundamental role for mitochondrial dynamics in determining the full pluripotency and embryonic developmental potential of pluripotent stem cells.

Role of β-Catenin Activation Levels and Fluctuations in Controlling Cell Fate

Cells have developed numerous adaptation mechanisms to external cues by controlling signaling-pathway activity, both qualitatively and quantitatively. The Wnt/β-catenin pathway is a highly conserved signaling pathway involved in many biological processes, including cell proliferation, differentiation, somatic cell reprogramming, development, and cancer. The activity of the Wnt/β-catenin pathway and the temporal dynamics of its effector β-catenin are tightly controlled by complex regulations. The latter encompass feedback loops within the pathway (e.g., a negative feedback loop involving Axin2, a β-catenin transcriptional target) and crosstalk interactions with other signaling pathways. Here, we provide a review shedding light on the coupling between Wnt/β-catenin activation levels and fluctuations across processes and cellular systems; in particular, we focus on development, in vitro pluripotency maintenance, and cancer. Possible mechanisms originating Wnt/β-catenin dynamic behaviors and consequently driving different cellular responses are also reviewed, and new avenues for future research are suggested.

Wnt/β-catenin signaling in the mouse embryonic cranial mesenchyme is required to sustain the emerging differentiated meningeal layers

Cranial neural crest cells (CNCCs) give rise to cranial mesenchyme (CM) that differentiates into the forebrain meningeal progenitors in the basolateral and apical regions of the head. This occurs in close proximity to the other CNCC-CM-derivatives, such as calvarial bone and dermal progenitors. We found active Wnt signaling transduction in the forebrain meningeal progenitors in basolateral and apical populations and in the non-meningeal CM preceding meningeal differentiation. Here, we dissect the source of Wnt ligand secretion and requirement of Wnt/β-catenin signaling for the lineage selection and early differentiation of the forebrain meninges. We find persistent canonical Wnt/β-catenin signal transduction in the meningeal progenitors in the absence of Wnt ligand secretion in the CM or surface ectoderm, suggesting additional sources of Wnts. Conditional mutants for Wntless and β-catenin in the CM showed that Wnt ligand secretion and Wnt/β-catenin signaling were dispensable for specification and proliferation of early meningeal progenitors. In the absence of β-catenin in the CM, we found diminished laminin matrix and meningeal hypoplasia, indicating a structural and trophic role of mesenchymal β-catenin signaling. This study shows that β-catenin signaling is required in the CM for maintenance and organization of the differentiated meningeal layers in the basolateral and apical populations of embryonic meninges.

Expression and localization of the polarity protein CRB2 in adult mouse brain: a comparison with the CRB1rd8 mutant mouse model

Acquisition of cell polarization is essential for the performance of crucial functions, like a successful secretion and appropriate cell signaling in many tissues, and it depends on the correct functioning of polarity proteins, including the Crumbs complex. The CRB proteins, CRB1, CRB2 and CRB3, identified in mammals, are expressed in epithelial-derived tissues like brain, kidney and retina. CRB2 has a ubiquitous expression and has been detected in embryonic brain tissue; but currently there is no data regarding its localization in the adult brain. In our study, we characterized the presence of CRB2 in adult mice brain, where it is particularly enriched in cortex, hippocampus, hypothalamus and cerebellum. Double immunofluorescence analysis confirmed that CRB2 is a neuron-specific protein, present in both soma and projections where colocalizes with certain populations of exocytic and endocytic vesicles and with other members of the Crumbs complex. Finally, in the cortex of CRB1^rd8 mutant mice that contain a mutation in the Crb1 gene generating a truncated CRB1 protein, there is an abnormal increase in the expression levels of the CRB2 protein which suggests a possible compensatory mechanism for the malfunction of CRB1 in this mutant background.

Identification of circular RNAs and their alterations involved in developing male Xenopus laevis chronically exposed to atrazine

Atrazine (AZ) is an environmental endocrine disrupting chemical which can affect the development of amphibians. In our past studies, we demonstrated that chronical exposure to 100 μg/L AZ can cause abnormalities in development and related genes expression of gonads in developing male Xenopus laevis (X. laevis) tadpoles. Recent studies by others have demonstrated that circular RNAs (circRNAs) are implicated in multiple developmental anomalies. However, whether circRNAs involve in the effects in AZ-exposed X. laevis remains unknown. In this study, over 68575 circRNAs were detected by circRNA sequencing of testis tissues from control groups (n = 3) and AZ-treated X. laevis (n = 3). Treatment of AZ led to 405 circRNAs differentially expressed including 44 upregulated and 361 downregulated compared with froglets in the control groups. Two upregulated and 6 downregulated circRNAs were further validated by real-time PCR assay which displayed consistent regulation patterns as shown by the transcriptome sequencing results. Two hundreds and eighty two differentially expressed circRNAs played miRNA sponges roles. Kyoto Encyclopedia of Genes and Genomes pathway analysis of miRNA targets showed that AZ-affected circRNAs are mainly involved in 19 pathways. The Wnt signaling pathway and progesterone-mediated oocyte maturation pathway may be two involved signal pathways. This study for the first time provides evidence that AZ can alter circRNAs which play a role in AZ-induced testicular degeneration of developing male X. laevis through regulation of expressions of functional genes in the testes of X. laevis.

Neural Differentiation Is Inhibited through HIF1α/β-Catenin Signaling in Embryoid Bodies

Extensive research in the field of stem cells and developmental biology has revealed evidence of the role of hypoxia as an important factor regulating self-renewal and differentiation. However, comprehensive information about the exact hypoxia-mediated regulatory mechanism of stem cell fate during early embryonic development is still missing. Using a model of embryoid bodies (EBs) derived from murine embryonic stem cells (ESC), we here tried to encrypt the role of hypoxia-inducible factor 1α (HIF1α) in neural fate during spontaneous differentiation. EBs derived from ESC with the ablated gene for HIF1α had abnormally increased neuronal characteristics during differentiation. An increased neural phenotype in Hif1α^-/- EBs was accompanied by the disruption of β-catenin signaling together with the increased cytoplasmic degradation of β-catenin. The knock-in of Hif1α, as well as β-catenin ectopic overexpression in Hif1α^-/- EBs, induced a reduction in neural markers to the levels observed in wild-type EBs. Interestingly, direct interaction between HIF1α and β-catenin was demonstrated by immunoprecipitation analysis of the nuclear fraction of wild-type EBs. Together, these results emphasize the regulatory role of HIF1α in β-catenin stabilization during spontaneous differentiation, which seems to be a crucial mechanism for the natural inhibition of premature neural differentiation.

β-catenin-mediated adhesion is required for successful preimplantation mouse embryo development

β-catenin (CTNNB1) is integral to cell adhesion and to the canonical Wnt signaling pathway. The effects of maternal and zygotic CTNNB1 on embryogenesis have each been separately assessed, whereas the effect of its total absence has not. As the 'traditional' conditional Ctnnb1 knockout alleles give rise to truncated CTNNB1 fragments, we designed a new knockout allele incapable of CTNNB1 production. Mouse embryos lacking intact maternal/zygotic CTNNB1 from two knockout strains were examined in detail. Preimplantation embryos are formed, yet abnormalities in their size and shape were found throughout pre- and early postimplantation development. In the absence of the zona pellucida, embryos lacking CTNNB1 undergo fission and these separated blastomeres can become small trophoblastic vesicles, which in turn induce decidual reactions. Comparing the severity of this defective adhesion phenotype in embryos bearing the null allele with those carrying the 'traditional' knockout allele suggests a hypomorphic effect of the truncated CTNNB1 protein fragment, an important observation with possible impact on previous and future studies.

Βeta-catenin N-terminal domain: An enigmatic region prone to cancer causing mutations

The Wnt/β-catenin is a highly conserved signaling pathway involved in cell fate decisions during various stages of development. Dysregulation of canonical Wnt/β-catenin signaling has been associated with various diseases including cancer. β-Catenin, the central component of canonical Wnt signaling pathway, is a multi-functional protein playing both structural and signaling roles. β-Catenin is composed of three distinct domains: N-terminal domain, C-terminal domain and a central armadillo repeat domain. N-terminal domain of β-catenin harbours almost all of the cancer causing mutations, thus deciphering its critical structural and functional roles offers great potential in cancer detection and therapy. Here, in this review, we have collected information from pharmacological analysis, bio-physical and structural studies, molecular modeling, in-vivo and in-vitro assays, and transgenic animal experiments employing various N-terminal domain variants of β-catenin to discuss the interaction of β-catenin with its binding partners that specifically interact with this domain and the implications of these interactions on signaling, cell fate determination, and in tumorigenesis. A thorough understanding of interactions between β-catenin and its binding partners will enable us to more effectively understand how β-catenin switches between its multiple roles, and will lead to the development of specific assays for the identification of small molecules as chemotherapeutic agents to treat diseases, including cancer and neurological disorders, where Wnt/β-catenin signaling is dysregulated.

Reduced β-catenin expression affects patterning of bone primordia, but not bone maturation

Wnt/β-catenin signaling is involved in patterning of bone primordia, but also plays an important role in the differentiation of chondrocytes and osteoblasts. During these processes the level of β-catenin must be tightly regulated. Excess β-catenin leads to conditions with increased bone mass, whereas loss of β-catenin is associated with osteoporosis or, in extreme cases, the absence of limbs. In this study, we examined skeletogenesis in mice, which retain only 25% of β-catenin. These embryos showed severe morphological abnormalities of which the lack of hindlimbs and misshaped front paws were the most striking. Surprisingly however, calcification of bone primordia occurred normally. Moreover, the Wnt-dependent regulatory network of transcription factors driving the differentiation of cartilage and bone, as well as the expression of extracellular matrix components, were preserved. These findings show that 25% β-catenin is insufficient for the correct patterning of bone primordia, but sufficient for their mineralization. Our approach helps to identify bone morphogenetic processes that can proceed normally even at low β-catenin levels, in contrast to those that require high β-catenin dosages. This information could be exploited to improve the treatment of bone diseases by fine-tuning the individual β-catenin dosage requirements.

Summary: The correct allocation and patterning of skeletal elements during development requires a higher dosage of β-catenin than the maturation and mineralization of these primordia.

Sevoflurane anesthesia in pregnant rats negatively affects nerve function in offspring potentially via inhibition of the Wnt/β-catenin pathway

Due to the rapid development of medical technology used to perform intrauterine procedures during pregnancy, the number of patients receiving fetal surgery under general anesthesia is increasing. The aim of the present study was to determine the effects of anesthetics on the offspring of rats, and to identify the potential mechanisms underlying these effects. On day 14 of pregnancy, Sprague‑Dawley rats were equally divided into the following 3 groups (n=9): Control group (n=3), 3% sevoflurane group (n=3) and 4% sevoflurane group (n=3). Following birth of the offspring, the juvenile rats were assessed using an open‑field test, Morris water maze and a continuous passive avoidance test on different days to determine their learning abilities and memory. Western blot and reverse transcription‑quantitative polymerase chain reaction (RT‑qPCR) analyses were used to examine the expression of multiple critical factors associated with the proliferation and apoptosis of nerve cells, including Ki67, nestin, B cell leukemia/lymphoma 2 (Bcl-2), BCL2 associated X (Bax) and caspase‑3. Additionally, the level of adenosine triphosphate production among the 3 groups were compared. Furthermore, expression alterations in of glycogen synthase kinase‑3β (GSK‑3β) and β‑catenin were examined. The Morris water maze experiment revealed that an increased concentration of sevoflurane exposure significantly reduced the learning and memory abilities of the juvenile rats when compared with controls. In addition, western blotting and RT-qPCR analyses determined that the protein and mRNA expression levels of Bax, caspase‑3 and GSK‑3β were significantly increased relative to the controls. By contrast, the expression levels of nestin, Ki‑67, Bcl‑2 and β‑catenin were significantly reduced. The results of the present study suggest that exposure of pregnant mice to sevoflurane anesthesia demonstrates a negative effect on the learning and memory abilities of their offspring, and the Wnt/β-catenin signaling pathway may be involved in this process.

Epiblast-specific loss of HCF-1 leads to failure in anterior-posterior axis specification

Mammalian Host-Cell Factor 1 (HCF-1), a transcriptional co-regulator, plays important roles during the cell-division cycle in cell culture, embryogenesis as well as adult tissue. In mice, HCF-1 is encoded by the X-chromosome-linked Hcfc1 gene. Induced Hcfc1(cKO/+) heterozygosity with a conditional knockout (cKO) allele in the epiblast of female embryos leads to a mixture of HCF-1-positive and -deficient cells owing to random X-chromosome inactivation. These embryos survive owing to the replacement of all HCF-1-deficient cells by HCF-1-positive cells during E5.5 to E8.5 of development. In contrast, complete epiblast-specific loss of HCF-1 in male embryos, Hcfc1(epiKO/Y), leads to embryonic lethality. Here, we characterize this lethality. We show that male epiblast-specific loss of Hcfc1 leads to a developmental arrest at E6.5 with a rapid progressive cell-cycle exit and an associated failure of anterior visceral endoderm migration and primitive streak formation. Subsequently, gastrulation does not take place. We note that the pattern of Hcfc1(epiKO/Y) lethality displays many similarities to loss of β-catenin function. These results reveal essential new roles for HCF-1 in early embryonic cell proliferation and development.

Inhibition of β-catenin-TCF1 interaction delays differentiation of mouse embryonic stem cells

Blocking β-catenin/TCF1–mediated transcriptional activation with a specific small molecule or by TCF1 knockdown delays the mouse embryonic stem cell differentiation program and enhances pluripotency.

The ability of mouse embryonic stem cells (mESCs) to self-renew or differentiate into various cell lineages is regulated by signaling pathways and a core pluripotency transcriptional network (PTN) comprising Nanog, Oct4, and Sox2. The Wnt/β-catenin pathway promotes pluripotency by alleviating T cell factor TCF3-mediated repression of the PTN. However, it has remained unclear how β-catenin’s function as a transcriptional activator with TCF1 influences mESC fate. Here, we show that TCF1-mediated transcription is up-regulated in differentiating mESCs and that chemical inhibition of β-catenin/TCF1 interaction improves long-term self-renewal and enhances functional pluripotency. Genetic loss of TCF1 inhibited differentiation by delaying exit from pluripotency and conferred a transcriptional profile strikingly reminiscent of self-renewing mESCs with high Nanog expression. Together, our data suggest that β-catenin’s function in regulating mESCs is highly context specific and that its interaction with TCF1 promotes differentiation, further highlighting the need for understanding how its individual protein–protein interactions drive stem cell fate.

BDNF promotes human neural stem cell growth via GSK-3β-mediated crosstalk with the wnt/β-catenin signaling pathway

Brain-derived neurotrophic factor (BDNF) plays important roles in neural stem cell (NSC) growth. In this study, we investigated whether BDNF exerts its neurotrophic effects through the Wnt/β-catenin signaling pathway in human embryonic spinal cord NSCs (hESC-NSCs) in vitro. We found an increase in hESC-NSC growth by BDNF overexpression. Furthermore, expression of Wnt1, Frizzled1 and Dsh was upregulated, whereas GSK-3β expression was downregulated. In contrast, hESC-NSC growth was decreased by BDNF RNA interference. BDNF, Wnt1 and β-catenin components were all downregulated, whereas GSK-3β was upregulated. Next, we treated hESC-NSCs with 6-bromoindirubin-3'-oxime (BIO), a small molecule inhibitor of GSK-3β. BIO reduced the effects of BDNF upregulation/downregulation on the cell number, soma size and differentiation, and suppressed the effect of BDNF modulation on the Wnt signaling pathway. Our findings suggest that BDNF promotes hESC-NSC growth in vitro through crosstalk with the Wnt/β-catenin signaling pathway, and that this interaction may be mediated by GSK-3β.

BDNF promotes the growth of human neurons through crosstalk with the Wnt/β-catenin signaling pathway via GSK-3β

Brain-derived neurotrophic factor (BDNF) plays an important role in neuronal growth; however, the downstream regulatory mechanisms remain unclear. In this study, we investigated whether BDNF exerts its neurotrophic effects through the Wnt/β-catenin signaling pathway in human embryonic spinal cord neurons in vitro. We found that neuronal growth (soma size and average neurite length) was increased by transfection with a BDNF overexpression plasmid. Western blotting and real-time quantitative PCR showed that expression of the BDNF pathway components TrkB, PI3K, Akt and PLC-γ was increased by BDNF overexpression. Furthermore, the Wnt signaling factors Wnt, Frizzled and Dsh and the downstream target β-catenin were upregulated, whereas GSK-3β was downregulated. In contrast, when BDNF signaling was downregulated with BDNF siRNA, the growth of neurons was decreased. Furthermore, BDNF signaling factors, Wnt pathway components and β-catenin were all downregulated, whereas GSK-3β was upregulated. This suggests that BDNF affects the growth of neurons in vitro through crosstalk with Wnt signaling, and that GSK-3β may be a critical factor linking these two pathways. To evaluate this possibility, we treated neurons with 6-bromoindirubin-3'-oxime (BIO), a small molecule GSK-3β inhibitor. BIO reduced the effects of BDNF upregulation/downregulation on soma size and average neurite length, and suppressed the impact of BDNF modulation on the Wnt signaling pathway. Taken together, our findings suggest that BDNF promotes the growth of neurons in vitro through crosstalk with the Wnt/β-catenin signaling pathway, and that this interaction may be mediated by GSK-3β.

Wnt/ß-catenin signalling and the dynamics of fate decisions in early mouse embryos and embryonic stem (ES) cells

Wnt/ß-catenin signalling is a widespread cell signalling pathway with multiple roles during vertebrate development. In mouse embryonic stem (mES) cells, there is a dual role for ß-catenin: it promotes differentiation when activated as part of the Wnt/ß-catenin signalling pathway, and promotes stable pluripotency independently of signalling. Although mES cells resemble the preimplantation epiblast progenitors, the first requirement for Wnt/ß-catenin signalling during mouse development has been reported at implantation [1,2]. The relationship between ß-catenin and pluripotency and that of mES cells with epiblast progenitors suggests that ß-catenin might have a functional role during preimplantation development. Here we summarize the expression and function of Wnt/ß-catenin signalling elements during the early stages of mouse development and consider the reasons why the requirement in ES cells do not reflect the embryo.

Wnt/catenin signaling in adult stem cell physiology and disease

Wnt signaling plays an important role in development and disease. In this review we focus on the role of the canonical Wnt signaling pathway in somatic stem cell biology and its critical role in tissue homeostasis. We present current knowledge how Wnt/β-catenin signaling affects tissue stem cell behavior in various organ systems, including the gut, mammary gland, the hematopoietic and nervous system. We discuss evidence that canonical Wnt signaling can both maintain potency and an undifferentiated state as well as cause differentiation in somatic stem cells, depending on the cellular and environmental context. Based on studies by our lab and others, we will attempt to explain the dichotomous behavior of this signaling pathway in determining cell fate decisions and put special emphasis on the interaction of β-catenin with two highly homologous co-activator proteins, CBP and p300, to shed light on the their differential role in the outcome of Wnt/β-catenin signaling. Furthermore, we review current knowledge regarding the aberrant regulation of Wnt/β-catenin signaling in cancer biology, particularly its pivotal role in the context of cancer stem cells. Finally, we discuss data demonstrating that small molecule modulators of the β-catenin/co-activator interaction can be used to shift the balance between undifferentiated proliferation and differentiation, which potentially presents a promising therapeutic approach to stem cell based disease mechanisms.

Intersection of AHR and Wnt signaling in development, health, and disease

The AHR (aryl hydrocarbon receptor) and Wnt (wingless-related MMTV integration site) signaling pathways have been conserved throughout evolution. Appropriately regulated signaling through each pathway is necessary for normal development and health, while dysregulation can lead to developmental defects and disease. Though both pathways have been vigorously studied, there is relatively little research exploring the possibility of crosstalk between these pathways. In this review, we provide a brief background on (1) the roles of both AHR and Wnt signaling in development and disease, and (2) the molecular mechanisms that characterize activation of each pathway. We also discuss the need for careful and complete experimental evaluation of each pathway and describe existing research that explores the intersection of AHR and Wnt signaling. Lastly, to illustrate in detail the intersection of AHR and Wnt signaling, we summarize our recent findings which show that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced disruption of Wnt signaling impairs fetal prostate development.

β-Catenin is required for hair-cell differentiation in the cochlea

The development of hair cells in the auditory system can be separated into steps; first, the establishment of progenitors for the sensory epithelium, and second, the differentiation of hair cells. Although the differentiation of hair cells is known to require the expression of basic helix-loop-helix transcription factor, Atoh1, the control of cell proliferation in the region of the developing cochlea that will ultimately become the sensory epithelium and the cues that initiate Atoh1 expression remain obscure. We assessed the role of Wnt/β-catenin in both steps in gain- and loss-of-function models in mice. The canonical Wnt pathway mediator, β-catenin, controls the expression of Atoh1. Knock-out of β-catenin inhibited hair-cell, as well as pillar-cell, differentiation from sensory progenitors but was not required to maintain a hair-cell fate once specified. Constitutive activation of β-catenin expanded sensory progenitors by inducing additional cell division and resulted in the differentiation of extra hair cells. Our data demonstrate that β-catenin plays a role in cell division and differentiation in the cochlear sensory epithelium.

Distinct requirements for cranial ectoderm and mesenchyme-derived wnts in specification and differentiation of osteoblast and dermal progenitors

The cranial bones and dermis differentiate from mesenchyme beneath the surface ectoderm. Fate selection in cranial mesenchyme requires the canonical Wnt effector molecule β-catenin, but the relative contribution of Wnt ligand sources in this process remains unknown. Here we show Wnt ligands are expressed in cranial surface ectoderm and underlying supraorbital mesenchyme during dermal and osteoblast fate selection. Using conditional genetics, we eliminate secretion of all Wnt ligands from cranial surface ectoderm or undifferentiated mesenchyme, to uncover distinct roles for ectoderm- and mesenchyme-derived Wnts. Ectoderm Wnt ligands induce osteoblast and dermal fibroblast progenitor specification while initiating expression of a subset of mesenchymal Wnts. Mesenchyme Wnt ligands are subsequently essential during differentiation of dermal and osteoblast progenitors. Finally, ectoderm-derived Wnt ligands provide an inductive cue to the cranial mesenchyme for the fate selection of dermal fibroblast and osteoblast lineages. Thus two sources of Wnt ligands perform distinct functions during osteoblast and dermal fibroblast formation.

Craniofacial abnormalities are relatively common congenital birth defects, and the Wnt signaling pathway and its effectors have key roles in craniofacial development. Wntless/Gpr177 is required for the efficient secretion of all Wnt ligands and maps to a region that contains SNPs strongly associated with reduced bone mass, and heterozygous deletion is associated with facial dysmorphology. Here we test the role of specific sources of secreted Wnt proteins during early stages of craniofacial development and obtained dramatic craniofacial anomalies. We found that the overlying cranial surface ectoderm Wnts generate an instructive cue of Wnt signaling for skull bone and skin cell fate selection and transcription of additional Wnts in the underlying mesenchyme. Once initiated, mesenchymal Wnts may maintain Wnt signal transduction and function in an autocrine manner during differentiation of skull bones and skin. These results highlight how Wnt ligands from two specific tissue sources are integrated for normal craniofacial patterning and can contribute to complex craniofacial abnormalities.

Beta-catenin is vital for the integrity of mouse embryonic stem cells

β-Catenin mediated Wnt-signaling is assumed to play a major function in embryonic stem cells in maintaining their stem cell character and the exit from this unique trait. The complexity of β-catenin action and conflicting results on the role of β-catenin in maintaining the pluripotent state have made it difficult to understand its precise cellular and molecular functions. To attempt this issue we have generated new genetically modified mouse embryonic stem cell lines allowing for the deletion of β-catenin in a controlled manner by taking advantage of the Cre-ER-T2 system and analyzed the effects in a narrow time window shortly after ablation. By using this approach, rather then taking long term cultured β-catenin null cell lines we demonstrate that β-catenin is dispensable for the maintenance of pluripotency associated genes. In addition we observed that the removal of β-catenin leads to a strong increase of cell death, the appearance of multiple clustered functional centrosomes most likely due to a mis-regulation of the polo-like-kinase 2 and furthermore, alterations in chromosome segregation. Our study demonstrates the importance of β-catenin in maintaining correct cellular functions and helps to understand its role in embryonic stem cells.

A competitive protein interaction network buffers Oct4-mediated differentiation to promote pluripotency in embryonic stem cells

The dynamic competition for complex formation between the pluripotency network components Oct4, Nanog, Tcf3, and β-catenin prevents embryonic stem cell differentiation by controlling the levels of free Oct4.

Pluripotency is defined by the ratios between the levels of pluripotency factors rather than by their absolute levels.

Competition between different protein complexes involving Nanog, Oct4, Tcf3, and β-catenin can account for the ratios associated with pluripotency.

The unstable pluripotency of Nanog mutant cells was shown to depend on the interactions between Oct4 and β-catenin.

The function of the protein competition network is to control the levels of free Oct4, which are balanced by Nanog and β-catenin in embryonic stem cells.

Pluripotency in embryonic stem cells is maintained through the activity of a small set of transcription factors centred around Oct4 and Nanog, which control the expression of ‘self-renewal' and ‘differentiation' genes. Here, we combine single-cell quantitative immunofluorescence microscopy and gene expression analysis, together with theoretical modelling, to investigate how the activity of those factors is regulated. We uncover a key role for post-translational regulation in the maintenance of pluripotency, which complements the well-established transcriptional regulatory layer. Specifically, we find that the activity of a network of protein complexes involving Nanog, Oct4, Tcf3, and β-catenin suffices to account for the behavior of ES cells under different conditions. Our results suggest that the function of the network is to buffer the transcriptional activity of Oct4, which appears to be the main determinant to exit pluripotency. The protein network explains the mechanisms underlying the gain and loss of function in different mutants, and brings us closer to a full understanding of the molecular basis of pluripotency.

Toggling a conformational switch in Wnt/β-catenin signaling: regulation of Axin phosphorylation. The phosphorylation state of Axin controls its scaffold function in two Wnt pathway protein complexes

The precise orchestration of two opposing protein complexes - one in the cytoplasm (β-catenin destruction complex) and the other at the plasma membrane (LRP6 signaling complex) - is critical for controlling levels of the transcriptional co-factor β-catenin, and subsequent activation of the Wnt/β-catenin signal transduction pathway. The Wnt pathway component Axin acts as an essential scaffold for the assembly of both complexes. How the β-catenin destruction and LRP6 signaling complexes are modulated following Wnt stimulation remains controversial. A recent study in Science by He and coworkers reveals an underlying logic for Wnt pathway control in which Axin phosphorylation toggles a switch between the active and inactive states. This mini-review focuses on this and two other recent studies that provide insight into the initial signaling events triggered by Wnt exposure. We emphasize regulation of the β-catenin destruction and LRP6 signaling complexes and propose a framework for future work in this area.

The Wnt/β-catenin pathway regulates the expression of the miR-302 cluster in mouse ESCs and P19 cells

MicroRNAs of the miR-302 cluster are involved in early embryonic development and somatic cell reprogramming. Expression of the miR-302 gene is regulated by the binding of the pluripotency factors Oct4, Sox2 and Nanog to the miR-302 promoter. The specific expression pattern of the miR-302 gene suggested that additional transcription factors might be involved in its regulation. Here, we show that the miR-302 promoter is a direct target of the Wnt/β-catenin signaling pathway. We found that the miR-302 promoter contains three different functional Tcf/Lef binding sites. Two of the three sites were located within the cluster of Oct4/Sox2/Nanog binding sites and were essential for Wnt/β-catenin-mediated regulation of the miR-302 gene. Tcf3, the only Tcf/Lef factor that bound to the miR-302 promoter, acted as a repressor of miR-302 transcription. Interestingly, mutations in the two Tcf/Lef binding sites and the Oct4/Nanog binding sites abolished miR-302 promoter responsiveness to Wnt signaling, suggesting that the Tcf/Lef and the Oct4/Nanog sites interact genetically.

The canonical WNT2 pathway and FSH interact to regulate gap junction assembly in mouse granulosa cells

WNTs are extracellular signaling molecules that exert their actions through receptors of the frizzled (FZD) family. Previous work indicated that WNT2 regulates cell proliferation in mouse granulosa cells acting through CTNNB1 (beta-catenin), a key component in canonical WNT signaling. In other cells, WNT signaling has been shown to regulate expression of connexin43 (CX43), a gap junction protein, as well as gap junction assembly. Since previous work demonstrated that CX43 is also essential in ovarian follicle development, the objective of this study was to determine if WNT2 regulates CX43 expression and/or gap-junctional intercellular communication (GJIC) in granulosa cells. WNT2 knockdown via siRNA markedly reduced CX43 expression and GJIC. CX43 expression, the extent of CX43-containing gap junction membrane, and GJIC were also reduced by CTNNB1 transient knockdown. CTNNB1 is mainly localized to the membranes between granulosa cells but disappeared from this location after WNT2 knockdown. Furthermore, CTNNB1 knockdown interfered with the ability of follicle-stimulating hormone (FSH) to promote the mobilization of CX43 into gap junctions. We propose that the WNT2/CTNNB1 pathway regulates CX43 expression and GJIC in granulosa cells by modulating CTNNB1 stability and localization in adherens junctions, and that this is essential for FSH stimulation of GJIC.

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

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

A membrane-associated β-catenin/Oct4 complex correlates with ground-state pluripotency in mouse embryonic stem cells

The maintenance of pluripotency in mouse embryonic stem cells (mESCs) relies on the activity of a transcriptional network that is fuelled by the activity of three transcription factors (Nanog, Oct4 and Sox2) and balanced by the repressive activity of Tcf3. Extracellular signals modulate the activity of the network and regulate the differentiation capacity of the cells. Wnt/β-catenin signaling has emerged as a significant potentiator of pluripotency: increases in the levels of β-catenin regulate the activity of Oct4 and Nanog, and enhance pluripotency. A recent report shows that β-catenin achieves some of these effects by modulating the activity of Tcf3, and that this effect does not require its transcriptional activation domain. Here, we show that during self-renewal there is negligible transcriptional activity of β-catenin and that this is due to its tight association with membranes, where we find it in a complex with Oct4 and E-cadherin. Differentiation triggers a burst of Wnt/β-catenin transcriptional activity that coincides with the disassembly of the complex. Our results establish that β-catenin, but not its transcriptional activity, is central to pluripotency acting through a β-catenin/Oct4 complex.

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

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

Wnt3a-dependent and -independent protein interaction networks of chromatin-bound β-catenin in mouse embryonic stem cells

Canonical Wnt signaling is repeatedly used during development to control cell fate, and it is often implicated in human cancer. β-catenin, the effector of Wnt signaling, has a dual function in the cell and is involved in both cell adhesion and transcription. Nuclear β-catenin controls transcription through association with transcription factors of the TCF family and the recruitment of epigenetic modifiers. In this study, we used a strategy combining the genetic manipulation of mouse embryonic stem cells with affinity purification and quantitative mass spectroscopy utilizing stable isotope labeling with amino acids in cell culture to study the interactome of chromatin-bound β-catenin with and without Wnt3a stimulation. We uncovered previously unknown interactions of β-catenin with transcription factors and chromatin-modifying complexes. Our proof-of-principle experiments show that β-catenin can recruit the H3K4me2/1 demethylase LSD1 to regulate the expression of the tumor suppressor Lefty1 in mouse embryonic stem cells. The mRNA levels of LSD1 and β-catenin are inversely correlated with the levels of Lefty1 in pancreas and breast tumors, implying that this mechanism is common to mouse embryonic stem cells and cancer cells.

E-cadherin is required for the proper activation of the Lifr/Gp130 signaling pathway in mouse embryonic stem cells

The leukemia inhibitory factor (Lif) signaling pathway is a crucial determinant for mouse embryonic stem (mES) cell self-renewal and pluripotency. One of the hallmarks of mES cells, their compact growth morphology, results from tight cell adhesion mediated through E-cadherin, β-catenin (Ctnnb1) and α-catenin with the actin cytoskeleton. β-catenin is also involved in canonical Wnt signaling, which has also been suggested to control mES cell stemness. Here, we analyze Ctnnb1(-/-) mES cells in which cell adhesion is preserved by an E-cadherin-α-catenin (Eα) fusion protein (Ctnnb1(-/-)Eα mES cells), and show that mimicking only the adhesive function of β-catenin is necessary and sufficient to maintain the mES cell state, making β-catenin/Wnt signaling obsolete in this process. Furthermore, we propose a role for E-cadherin in promoting the Lif signaling cascade, showing an association of E-cadherin with the Lifr-Gp130 receptor complex, which is most likely facilitated by the extracellular domain of E-cadherin. Without Eα, and thus without maintained cell adhesion, Ctnnb1(-/-) mES cells downregulate components of the Lif signaling pathway, such as Lifr, Gp130 and activated Stat3, as well as pluripotency-associated markers. From these observations, we hypothesize that the changes in gene expression accompanying the loss of pluripotency are a direct consequence of dysfunctional cell adhesion. Supporting this view, we find that the requirement for intact adhesion can be circumvented by the forced expression of constitutively active Stat3. In summary, we put forward a model in which mES cells can be propagated in culture in the absence of Ctnnb1, as long as E-cadherin-mediated cell adhesion is preserved.