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1.  Ectopic γ-catenin Expression Partially Mimics the Effects of Stabilized β-catenin on Embryonic Stem Cell Differentiation 
PLoS ONE  2013;8(5):e65320.
β-catenin, an adherens junction component and key Wnt pathway effector, regulates numerous developmental processes and supports embryonic stem cell (ESC) pluripotency in specific contexts. The β-catenin homologue γ-catenin (also known as Plakoglobin) is a constituent of desmosomes and adherens junctions and may participate in Wnt signaling in certain situations. Here, we use β-catenin(+/+) and β-catenin(−/−) mouse embryonic stem cells (mESCs) to investigate the role of γ-catenin in Wnt signaling and mESC differentiation. Although γ-catenin protein is markedly stabilized upon inhibition or ablation of GSK-3 in wild-type (WT) mESCs, efficient silencing of its expression in these cells does not affect β-catenin/TCF target gene activation after Wnt pathway stimulation. Nonetheless, knocking down γ-catenin expression in WT mESCs appears to promote their exit from pluripotency in short-term differentiation assays. In β-catenin(−/−) mESCs, GSK-3 inhibition does not detectably alter cytosolic γ-catenin levels and does not activate TCF target genes. Intriguingly, β-catenin/TCF target genes are induced in β-catenin(−/−) mESCs overexpressing stabilized γ-catenin and the ability of these genes to be activated upon GSK-3 inhibition is partially restored when wild-type γ-catenin is overexpressed in these cells. This suggests that a critical threshold level of total catenin expression must be attained before there is sufficient signaling-competent γ-catenin available to respond to GSK-3 inhibition and to regulate target genes as a consequence. WT mESCs stably overexpressing γ-catenin exhibit robust Wnt pathway activation and display a block in tri-lineage differentiation that largely mimics that observed upon overexpression of β-catenin. However, β-catenin overexpression appears to be more effective than γ-catenin overexpression in sustaining the retention of markers of naïve pluripotency in cells that have been subjected to differentiation-inducing conditions. Collectively, our study reveals a function for γ-catenin in the regulation of mESC differentiation and has implications for human cancers in which γ-catenin is mutated and/or aberrantly expressed.
PMCID: PMC3664634  PMID: 23724138
2.  Beta-Catenin Signaling Plays a Disparate Role in Different Phases of Fracture Repair: Implications for Therapy to Improve Bone Healing 
PLoS Medicine  2007;4(7):e249.
Delayed fracture healing causes substantial disability and usually requires additional surgical treatments. Pharmacologic management to improve fracture repair would substantially improve patient outcome. The signaling pathways regulating bone healing are beginning to be unraveled, and they provide clues into pharmacologic management. The β-catenin signaling pathway, which activates T cell factor (TCF)-dependent transcription, has emerged as a key regulator in embryonic skeletogenesis, positively regulating osteoblasts. However, its role in bone repair is unknown. The goal of this study was to explore the role of β-catenin signaling in bone repair.
Methods and Findings
Western blot analysis showed significant up-regulation of β-catenin during the bone healing process. Using a β-Gal activity assay to observe activation during healing of tibia fractures in a transgenic mouse model expressing a TCF reporter, we found that β-catenin-mediated, TCF-dependent transcription was activated in both bone and cartilage formation during fracture repair. Using reverse transcription-PCR, we observed that several WNT ligands were expressed during fracture repair. Treatment with DKK1 (an antagonist of WNT/β-catenin pathway) inhibited β-catenin signaling and the healing process, suggesting that WNT ligands regulate β-catenin. Healing was significantly repressed in mice conditionally expressing either null or stabilized β-catenin alleles induced by an adenovirus expressing Cre recombinase. Fracture repair was also inhibited in mice expressing osteoblast-specific β-catenin null alleles. In stark contrast, there was dramatically enhanced bone healing in mice expressing an activated form of β-catenin, whose expression was restricted to osteoblasts. Treating mice with lithium activated β-catenin in the healing fracture, but healing was enhanced only when treatment was started subsequent to the fracture.
These results demonstrate that β-catenin functions differently at different stages of fracture repair. In early stages, precise regulation of β-catenin is required for pluripotent mesenchymal cells to differentiate to either osteoblasts or chondrocytes. Once these undifferentiated cells have become committed to the osteoblast lineage, β-catenin positively regulates osteoblasts. This is a different function for β-catenin than has previously been reported during development. Activation of β-catenin by lithium treatment has potential to improve fracture healing, but only when utilized in later phases of repair, after mesenchymal cells have become committed to the osteoblast lineage.
In a study in mice Benjamin Alman and colleagues show that β-catenin functions differently in different stages of fracture repair; moreover, activation of β-catenin by lithium improves fracture healing when used in later phases of repair.
Editors' Summary
Most people break at least one bone during their life. If the damaged bone is immobilized with a plaster cast or with metal plates and pins, most fractures heal naturally and quickly. Soon after a bone is damaged, cells called pluripotent mesenchymal cells collect at the injury site. Here, they multiply and change (differentiate) into osteoblasts (cells that make bone) and chondrocytes (cells that make cartilage, the dense connective tissue that covers joints). Osteoblasts and chondrocytes mend the fracture by making new bone, a process called ossification. Bone healing involves two types of ossification. In intramembranous ossification, mesenchymal cells and osteoblast progenitor cells make bone directly, forming a hard “callus” within the fracture. In endochondral ossification, mesenchymal cells differentiate into chondrocytes and make cartilage at the fracture site, which osteoblasts turn into bone. Finally, the bone made by both types of ossification is remodeled so that it closely resembles the damaged bone's original shape and strength.
Why Was This Study Done?
Unfortunately, fractures do not always heal efficiently. If healing is delayed, additional surgery may be needed to repair the break. But surgery can be risky, so drug-based ways of encouraging bone repair would be very useful. To develop such treatments, researchers need to understand what controls the differentiation and activity of osteoblasts and chondrocytes during normal healing. In this study, the researchers have investigated the role of the β-catenin signaling pathway in bone repair. This pathway regulates bone formation during embryonic development, a process that closely resembles bone healing. β-catenin is usually degraded rapidly in cells. However, if a member of a particular family of proteins known as the WNT family binds to a WNT receptor on the surface of a cell, β-catenin moves into the cell's nucleus where it interacts with a protein called T cell factor (TCF). This interaction activates the transcription (the copying of DNA into messenger RNA, which is used to make proteins) of numerous genes and alters the behavior of the cell.
What Did the Researchers Do and Find?
The researchers first measured β-catenin levels in mouse and human bones. In both species, much more β-catenin was made in bones undergoing repair than in intact bones. Then they studied TCF reporter mice—animals in which TCF controls the expression of a marker gene. β-catenin-mediated TCF-dependent transcription, they report, was activated during both bone and cartilage formation after a fracture in these mice. Next, the researchers made mice that could be induced to express an inactive form of β-catenin or a stabilized (permanently active) form of β-catenin in all the cells in a bone fracture. Expression of inactive β-catenin slowed the rate of healing but, unexpectedly, so did expression of stabilized β-catenin. Osteoblast-specific expression of inactive β-catenin also delayed bone healing, whereas osteoblast-specific expression of stabilized β-catenin enhanced the process. Finally, treatment of wild-type mice with lithium (which prevents the degradation of β-catenin) enhanced bone healing if given after a fracture, but interfered with it if given before.
What Do These Findings Mean?
These findings indicate that β-catenin signaling (which, the researchers show, is mainly activated by WNT signaling) has different effects at different stages of bone repair. Early in the process, it controls the ratio of osteoblasts and chondrocytes made from the pluripotent mesenchymal cells. Consequently, too much or too little β-catenin interferes with bone healing at this stage. Later on, β-catenin promotes the differentiation of osteoblasts and enhances their ability to make bone, and so too little β-catenin at this stage prevents healing, whereas increased β-catenin levels stimulate healing. These findings need to be confirmed in people before testing agents that affect β-catenin signaling for their effects on human bone healing. Nevertheless, the researchers' final discovery that lithium improves bone healing if given at the right time is particularly encouraging; lithium is widely used to treat one form of depression so could be readily tested in clinical trials.
Additional Information.
Please access these Web sites via the online version of this summary at
MedlinePlus encyclopedia contains pages on broken bones and on bone fracture repair (in English and Spanish)
Wikipedia has pages on bone fracture and on bone healing (note: Wikipedia is a free online encyclopedia that anyone can edit; available in several languages)
The UK National Health Service Direct encyclopedia provides pages on broken bones
Animations of intramembranous and endochondral ossification are available from the Ministry of Advanced Education, Training and Technology, Province of British Columbia, Canada
The American Academy of Orthopedic Surgeons has an informative discussion of fractures
The Hospital for Sick Children in Toronto (where the authors of this study are affiliated) has a Web site called SickKids, which contains a page on child physiology, including diagrams of bone development
PMCID: PMC1950214  PMID: 17676991
3.  Pleiotropy of Glycogen Synthase Kinase-3 Inhibition by CHIR99021 Promotes Self-Renewal of Embryonic Stem Cells from Refractory Mouse Strains 
PLoS ONE  2012;7(4):e35892.
Inhibition of glycogen synthase kinase-3 (GSK-3) improves the efficiency of embryonic stem (ES) cell derivation from various strains of mice and rats, as well as dramatically promotes ES cell self-renewal potential. β-catenin has been reported to be involved in the maintenance of self-renewal of ES cells through TCF dependent and independent pathway. But the intrinsic difference between ES cell lines from different species and strains has not been characterized. Here, we dissect the mechanism of GSK-3 inhibition by CHIR99021 in mouse ES cells from refractory mouse strains.
Methodology/Principal Findings
We found that CHIR99021, a GSK-3 specific inhibitor, promotes self-renewal of ES cells from recalcitrant C57BL/6 (B6) and BALB/c mouse strains through stabilization of β-catenin and c-Myc protein levels. Stabilized β-catenin promoted ES self-renewal through two mechanisms. First, β-catenin translocated into the nucleus to maintain stem cell pluripotency in a lymphoid-enhancing factor/T-cell factor–independent manner. Second, β-catenin binds plasma membrane-localized E-cadherin, which ensures a compact, spherical morphology, a hallmark of ES cells. Further, elevated c-Myc protein levels did not contribute significantly to CH-mediated ES cell self-renewal. Instead, the role of c-Myc is dependent on its transformation activity and can be replaced by N-Myc but not L-Myc. β-catenin and c-Myc have similar effects on ES cells derived from both B6 and BALB/c mice.
Our data demonstrated that GSK-3 inhibition by CH promotes self-renewal of mouse ES cells with non-permissive genetic backgrounds by regulation of multiple signaling pathways. These findings would be useful to improve the availability of normally non-permissive mouse strains as research tools.
PMCID: PMC3335080  PMID: 22540008
4.  Esrrb Is a Pivotal Target of the Gsk3/Tcf3 Axis Regulating Embryonic Stem Cell Self-Renewal 
Cell Stem Cell  2012;11(4):491-504.
Inhibition of glycogen synthase kinase-3 (Gsk3) supports mouse embryonic stem cells (ESCs) by modulating Tcf3, but the critical targets downstream of Tcf3 are unclear. We analyzed the intersection between genome localization and transcriptome data sets to identify genes repressed by Tcf3. Among these, manipulations of Esrrb gave distinctive phenotypes in functional assays. Knockdown and knockout eliminated response to Gsk3 inhibition, causing extinction of pluripotency markers and loss of colony forming capability. Conversely, forced expression phenocopied Gsk3 inhibition or Tcf3 deletion by suppressing differentiation and sustaining self-renewal. Thus the nuclear receptor Esrrb is necessary and sufficient to mediate self-renewal downstream of Gsk3 inhibition. Leukaemia inhibitory factor (LIF) regulates ESCs through Stat3, independently of Gsk3 inhibition. Consistent with parallel operation, ESCs in LIF accommodated Esrrb deletion and remained pluripotent. These findings highlight a key role for Esrrb in regulating the naive pluripotent state and illustrate compensation among the core pluripotency factors.
► Esrrb is the principal target of Tcf3 repression in the pluripotency network ► Esrrb is essential for self-renewal downstream of Gsk3 inhibition ► Esrrb potently suppresses differentiation and sustains ESC self-renewal ► Esrrb is a core pluripotency factor but can be compensated by LIF/Stat3
Inhibition of glycogen synthase kinase 3 keeps embryonic stem cells in a naïve ground state. Surprisingly this effect is mediated mainly by upregulating expression of a single transcription factor Esrrb. Esrrb function can be compensated by independent activation of Stat3, however, demonstrating the plasticity of the core network underpinning pluripotency.
PMCID: PMC3465555  PMID: 23040478
5.  Robust Self-Renewal of Rat Embryonic Stem Cells Requires Fine-Tuning of Glycogen Synthase Kinase-3 Inhibition 
Stem Cell Reports  2013;1(3):209-217.
Germline-competent embryonic stem cells (ESCs) have been derived from mice and rats using culture conditions that include an inhibitor of glycogen synthase kinase 3 (GSK3). However, rat ESCs remain susceptible to sporadic differentiation. Here, we show that unsolicited differentiation is attributable to overinhibition of GSK3. The self-renewal effect of inhibiting GSK3 is mediated via β-catenin, which abrogates the repressive action of TCF3 on core pluripotency genes. In rat ESCs, however, GSK3 inhibition also leads to activation of differentiation-associated genes, notably lineage specification factors Cdx2 and T. Lowered GSK3 inhibition reduces differentiation and enhances clonogenicity and self-renewal. The differential sensitivity of rat ESCs to GSK3 inhibition is linked to elevated expression of the canonical Wnt pathway effector LEF1. These findings reveal that optimal GSK3 inhibition for ESC propagation is influenced by the balance of TCF/LEF factors and can vary between species.
Graphical Abstract
•Rat ESCs are susceptible to sporadic differentiation•High GSK3 inhibition induces lineage specification genes•Reduced GSK3 inhibition derepresses pluripotency factors via TCF3•Threshold sensitivity is due to high LEF1, which activates differentiation genes
PMCID: PMC3849254  PMID: 24319657
6.  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.
PMCID: PMC3817399  PMID: 24104477
β-catenin; mathematical modelling; Oct4; pluripotency; protein network
7.  Inhibition of glycogen synthase kinase-3 alleviates Tcf3 repression of the pluripotency network and increases embryonic stem cell resistance to differentiation 
Nature cell biology  2011;13(7):838-845.
Self-renewal of rodent embryonic stem (ES) cells is enhanced by partial inhibition of glycogen synthase kinase-3 (Gsk3)1 2. This effect has variously been attributed to stimulation of Wnt signalling via β-catenin1, stabilisation of cMyc3, and global de-inhibition of anabolic processes4. Here we demonstrate that β-catenin is not necessary for ES cell identity or expansion, but its absence eliminates the self-renewal response to Gsk3 inhibition. Responsiveness is fully restored by truncated β-catenin lacking the C-terminal transactivation domain5. However, requirement for Gsk3 inhibition is dictated by expression of Tcf3 and mediated by direct interaction with β-catenin. Tcf3 localises to many pluripotency genes6 in ES cells. Our findings confirm that Tcf3 acts as a transcriptional repressor and reveal that β-catenin directly abrogates Tcf3 function. We conclude that Gsk3 inhibition stabilises the ES cell state primarily by reducing repressive influence on the core pluripotency network.
PMCID: PMC3160487  PMID: 21685889
8.  Gene regulatory networks mediating canonical Wnt signal directed control of pluripotency and differentiation in embryo stem cells 
Stem cells (Dayton, Ohio)  2013;31(12):2667-2679.
Canonical Wnt signaling supports the pluripotency of embryonic stem cells (ESCs) but also promotes differentiation of early mammalian cell lineages. To explain these paradoxical observations, we explored the gene regulatory networks at play. Canonical Wnt signaling is intertwined with the pluripotency network comprising Nanog, Oct4, and Sox2 in mouse ESCs. In defined media supporting the derivation and propagation of ESCs, Tcf3 and β-catenin interact with Oct4; Tcf3 binds to Sox motif within Oct-Sox composite motifs that are also bound by Oct4-Sox2 complexes. Further, canonical Wnt signaling up-regulates the activity of the Pou5f1 distal enhancer via the Sox motif in ESCs. When viewed in the context of published studies on Tcf3 and β-catenin mutants, our findings suggest Tcf3 counters pluripotency by competition with Sox2 at these sites, and Tcf3 inhibition is blocked by β-catenin entry into this complex. Wnt pathway stimulation also triggers β-catenin association at regulatory elements with classic Lef/Tcf motifs associated with differentiation programs. The failure to activate these targets in the presence of a MEK/ERK inhibitor essential for ESC culture suggests MEK/ERK signaling and canonical Wnt signaling combine to promote ESC differentiation.
PMCID: PMC3830733  PMID: 23505158
Mouse embryonic stem cells; Wnt; β-catenin; 2i; pluripotency; differentiation
9.  Opposing Effects of Tcf3 and Tcf1 Control Wnt-Stimulation of Embryonic Stem Cell Self Renewal 
Nature cell biology  2011;13(7):762-770.
The observation that Tcf3 (MGI name: Tcf7l1) bound the same genes as core stem cell transcription factors, Oct4 (MGI name:Pou5f1), Sox2 and Nanog, revealed a potentially important aspect of the poorly understood mechanism whereby Wnts stimulate self renewal of pluripotent mouse embryonic stem (ES) cells. Although the conventional view of Tcf proteins as the β-catenin-binding effectors of Wnt signaling suggested Tcf3-β-catenin mediated activation of target genes would stimulate ES cell self renewal, here we show that an antagonistic relationship between Wnt3a and Tcf3 on gene expression is important for regulating ES cell self renewal. Genetic ablation of Tcf3 replaced the requirement for exogenous Wnt3a or GSK3-inhibition for self renewal of ES cells, demonstrating that inhibition of Tcf3-repressor is the necessary downstream effect of Wnt signaling. Interestingly, the molecular mechanism underlying Wnt’s effects required both Tcf3-β-catenin and Tcf1-β-catenin interactions, as they each contributed to Wnt stimulation of self renewal and gene expression. Finally, the combination of Tcf3 and Tcf1 was necessary to recruit Wnt-stabilized β-catenin to Oct4 binding sites in ES cell chromatin. These results elucidate the molecular link between the effects of Wnt and the regulation of the Oct4/Sox2/Nanog network.
PMCID: PMC3129424  PMID: 21685894
10.  Glycogen Synthase Kinase-3 Inhibition Enhances Translation of Pluripotency-Associated Transcription Factors to Contribute to Maintenance of Mouse Embryonic Stem Cell Self-Renewal 
PLoS ONE  2013;8(4):e60148.
Maintenance of embryonic stem cell (ESC) self-renewal and pluripotency are controlled by extrinsic factors, molecular signaling pathways and transcriptional regulators. While many of the key players have been studied in depth, how the molecular signals interact with transcription factors of the pluripotency network to regulate their action remains less well understood. Inhibition of glycogen synthase kinase 3 (Gsk-3) has been implicated in the maintenance of mouse ESC pluripotency, although there is contradictory data on its role, with enhancement of cell survival and metabolism, stabilisation of c-Myc and activation of Wnt signalling proposed as potential mechanisms. We have discovered that suppression of Gsk-3 activity leads to enhanced protein levels of key transcriptional regulators of the pluripotency network, notably Nanog, Tbx3 and c-Myc. Protein stability was unchanged following Gsk-3 inhibition, although interestingly, Nanog and Tbx3 proteins were found to have half-lives of 1–3 h, while that of Oct4 protein was longer, at 6 h. We demonstrate that the effects on protein levels seen following inhibition of Gsk-3 are due to both enhanced de novo synthesis of Nanog protein and increases in the proportion of Nanog and Tbx3 RNAs bound to polysomes, findings consistent with Gsk-3 regulating translation of these factors. These effects were not due to changes in regulators of general translation initiation machinery nor mediated via the 5′ or 3′ UTR sequences of Nanog alone. The data we present provide both new conceptual insight into the mechanisms regulated by Gsk-3 that may contribute to ESC self-renewal and, importantly, establish control of protein translation as an additional mechanism involved in modulation of ESC pluripotency.
PMCID: PMC3618116  PMID: 23577087
11.  β-Catenin-Independent Activation of TCF1/LEF1 in Human Hematopoietic Tumor Cells through Interaction with ATF2 Transcription Factors 
PLoS Genetics  2013;9(8):e1003603.
The role of Wnt signaling in embryonic development and stem cell maintenance is well established and aberrations leading to the constitutive up-regulation of this pathway are frequent in several types of human cancers. Upon ligand-mediated activation, Wnt receptors promote the stabilization of β-catenin, which translocates to the nucleus and binds to the T-cell factor/lymphoid enhancer factor (TCF/LEF) family of transcription factors to regulate the expression of Wnt target genes. When not bound to β-catenin, the TCF/LEF proteins are believed to act as transcriptional repressors. Using a specific lentiviral reporter, we identified hematopoietic tumor cells displaying constitutive TCF/LEF transcriptional activation in the absence of β-catenin stabilization. Suppression of TCF/LEF activity in these cells mediated by an inducible dominant-negative TCF4 (DN-TCF4) inhibited both cell growth and the expression of Wnt target genes. Further, expression of TCF1 and LEF1, but not TCF4, stimulated TCF/LEF reporter activity in certain human cell lines independently of β-catenin. By a complementary approach in vivo, TCF1 mutants, which lacked the ability to bind to β-catenin, induced Xenopus embryo axis duplication, a hallmark of Wnt activation, and the expression of the Wnt target gene Xnr3. Through generation of different TCF1-TCF4 fusion proteins, we identified three distinct TCF1 domains that participate in the β-catenin-independent activity of this transcription factor. TCF1 and LEF1 physically interacted and functionally synergized with members of the activating transcription factor 2 (ATF2) family of transcription factors. Moreover, knockdown of ATF2 expression in lymphoma cells phenocopied the inhibitory effects of DN-TCF4 on the expression of target genes associated with the Wnt pathway and on cell growth. Together, our findings indicate that, through interaction with ATF2 factors, TCF1/LEF1 promote the growth of hematopoietic malignancies in the absence of β-catenin stabilization, thus establishing a new mechanism for TCF1/LEF1 transcriptional activity distinct from that associated with canonical Wnt signaling.
Author Summary
The Wnt signaling pathway plays a crucial role during embryonic development and in the maintenance of stem cell populations in various organs and tissues. Aberrant activation of this pathway through different mechanisms participates in the onset and progression of several types of human cancers. In the presence of Wnt ligands, stabilized β-catenin acts as a transcriptional activator to induce the expression of target genes through binding to the TCF/LEF family of transcription factors. Using in vitro and in vivo models, we show that TCF/LEF proteins can be activated independently of β-catenin through cooperation with members of the ATF2 subfamily of transcription factors. This novel alternative mechanism of TCF/LEF activation is constitutively up-regulated in certain hematopoietic tumor cells, where it regulates the expression of TCF/LEF target genes and promotes cell growth.
PMCID: PMC3744423  PMID: 23966864
12.  A membrane-associated β-catenin/Oct4 complex correlates with ground-state pluripotency in mouse embryonic stem cells 
Development (Cambridge, England)  2013;140(6):1171-1183.
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.
PMCID: PMC3585656  PMID: 23444350
Oct4; Wnt signaling; Mouse embryonic stem cells; Pluripotency; β-Catenin
13.  Glycogen synthase kinase 3 beta inhibits microRNA-183-96-182 cluster via the β-Catenin/TCF/LEF-1 pathway in gastric cancer cells 
Nucleic Acids Research  2013;42(5):2988-2998.
Glycogen synthase kinase 3 beta (GSK3β) is a critical protein kinase that phosphorylates numerous proteins in cells and thereby impacts multiple pathways including the β-Catenin/TCF/LEF-1 pathway. MicroRNAs (miRs) are a class of noncoding small RNAs of ∼22 nucleotides in length. Both GSK3β and miR play myriad roles in cell functions including stem cell development, apoptosis, embryogenesis and tumorigenesis. Here we show that GSK3β inhibits the expression of miR-96, miR-182 and miR-183 through the β-Catenin/TCF/LEF-1 pathway. Knockout of GSK3β in mouse embryonic fibroblast cells increases expression of miR-96, miR-182 and miR-183, coinciding with increases in the protein level and nuclear translocation of β-Catenin. In addition, overexpression of β-Catenin enhances the expression of miR-96, miR-182 and miR-183 in human gastric cancer AGS cells. GSK3β protein levels are decreased in human gastric cancer tissue compared with surrounding normal gastric tissue, coinciding with increases of β-Catenin protein, miR-96, miR-182, miR-183 and primary miR-183-96-182 cluster (pri-miR-183). Furthermore, suppression of miR-183-96-182 cluster with miRCURY LNA miR inhibitors decreases the proliferation and migration of AGS cells. Knockdown of GSK3β with siRNA increases the proliferation of AGS cells. Mechanistically, we show that β-Catenin/TCF/LEF-1 binds to the promoter of miR-183-96-182 cluster gene and thereby activates the transcription of the cluster. In summary, our findings identify a novel role for GSK3β in the regulation of miR-183-96-182 biogenesis through β-Catenin/TCF/LEF-1 pathway in gastric cancer cells.
PMCID: PMC3950676  PMID: 24335145
14.  Intrinsic properties of Tcf1 and Tcf4 splice variants determine cell-type-specific Wnt/β-catenin target gene expression 
Nucleic Acids Research  2012;40(19):9455-9469.
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.
PMCID: PMC3479169  PMID: 22859735
15.  14-3-3σ Regulates β-Catenin-Mediated Mouse Embryonic Stem Cell Proliferation by Sequestering GSK-3β 
PLoS ONE  2012;7(6):e40193.
Pluripotent embryonic stem cells are considered to be an unlimited cell source for tissue regeneration and cell-based therapy. Investigating the molecular mechanism underlying the regulation of embryonic stem cell expansion is thus important. 14-3-3 proteins are implicated in controlling cell division, signaling transduction and survival by interacting with various regulatory proteins. However, the function of 14-3-3 in embryonic stem cell proliferation remains unclear.
Methodology and Principal Findings
In this study, we show that all seven 14-3-3 isoforms were detected in mouse embryonic stem cells. Retinoid acid suppressed selectively the expression of 14-3-3σ isoform. Knockdown of 14-3-3σ with siRNA reduced embryonic stem cell proliferation, while only 14-3-3σ transfection increased cell growth and partially rescued retinoid acid-induced growth arrest. Since the growth-enhancing action of 14-3-3σ was abrogated by β-catenin knockdown, we investigated the influence of 14-3-3σ overexpression on β-catenin/GSK-3β. 14-3-3σ bound GSK-3β and increased GSK-3β phosphorylation in a PI-3K/Akt-dependent manner. It disrupted β-catenin binding by the multiprotein destruction complex. 14-3-3σ overexpression attenuated β-catenin phosphorylation and rescued the decline of β-catenin induced by retinoid acid. Furthermore, 14-3-3σ enhanced Wnt3a-induced β-catenin level and GSK-3β phosphorylation. DKK, an inhibitor of Wnt signaling, abolished Wnt3a-induced effect but did not interfere GSK-3β/14-3-3σ binding.
Our findings show for the first time that 14-3-3σ plays an important role in regulating mouse embryonic stem cell proliferation by binding and sequestering phosphorylated GSK-3β and enhancing Wnt-signaled GSK-3β inactivation. 14-3-3σ is a novel target for embryonic stem cell expansion.
PMCID: PMC3387134  PMID: 22768254
16.  Gene Expression Profiling in Mouse Embryonic Stem Cells Reveals Glycogen Synthase Kinase-3-Dependent Targets of Phosphatidylinositol 3-Kinase and Wnt/β-Catenin Signaling Pathways 
Glycogen synthase kinase-3 (Gsk-3) activity is an important regulator of numerous signal transduction pathways. Gsk-3 activity is the sum of two largely redundant proteins, Gsk-3α and Gsk-3β, and in general, Gsk-3 is a negative regulator of cellular signaling. Genetic deletion of both Gsk-3α and Gsk-3β in mouse embryonic stem cells (ESCs) has previously been shown to lead to the constitutive activation of the Wnt/β-catenin signaling pathway. However, in addition to Wnt signaling, all Gsk-3-regulated pathways, such as insulin signaling, are also affected simultaneously in Gsk-3α−/−; Gsk-3β−/−ESCs. In an effort to better understand how specific signaling pathways contribute to the global pattern of gene expression in Gsk-3α−/−; Gsk-3β−/−ESCs, we compared the gene expression profiles in Gsk-3α−/−; Gsk-3β−/− ESCs to mouse ESCs in which either Wnt/β-catenin signaling or phosphatidylinositol 3-kinase (PI3K)-dependent insulin signaling are constitutively active. Our results show that Wnt signaling has a greater effect on up-regulated genes in the Gsk-3α−/−; Gsk-3β−/−ESCs, whereas PI3K-dependent insulin signaling is more responsible for the down-regulation of genes in the same cells. These data show the importance of Gsk-3 activity on gene expression in mouse ESCs, and that these effects are due to the combined effects of multiple signaling pathways.
PMCID: PMC4131280  PMID: 25165462
embryonic stem cells; glycogen synthase kinase-3; Wnt; phosphatidylinositol 3-kinase; microarray; quantitative PCR; signal transduction; gene expression
17.  PI3K-Dependent GSK3ß(Ser9)-Phosphorylation Is Implicated in the Intestinal Epithelial Cell Wound-Healing Response 
PLoS ONE  2011;6(10):e26340.
The ability of the intestinal epithelial barrier to respond to various injurious insults is an essential component of intestinal homeostasis. However, the molecular mechanisms responsible for wound-healing and repair in the intestine are poorly understood. The glycogen synthase kinase 3ß (GSK3ß) has been implicated in various biological processes such as cellular motility, cell spreading and recently inflammation.
To investigate the role of GSK3ß in intestinal epithelial cell restitution.
Rat intestinal epithelial IEC18 cells were serum-starved for 16 to 24h and wounded by multiple scraping. Akt(Ser473)-, GSK3ß(Ser9)- and RelA(Ser536)-phosphorylation were determined by Western blot using specific phospho-antibodies. The inhibitors AG1478 (1 µM) and Ly294002 (25 µM) were used to block EGF-R autophosphorylation and PI3K-activation, respectively. ß-catenin/LEF/TCF dependent transcription was determined by reporter gene assay (TOP/FOP system). C-myc gene expression was evaluated by real-time RT-PCR. GSK3ß−/− mouse embryonic fibroblasts were used to characterize the role of GSK3ß in wounding-induced cell migration.
Wounding induced GSK3ß(Ser9) phosphorylation in IEC-18 cells, which led to ß-catenin accumulation as well as nuclear translocation of ß-catenin. ß-catenin stabilization/nuclear translocation led to enhanced LEF-TCF transcriptional activity and subsequent c-myc mRNA accumulation in wounded cell monolayers. Blocking PI3K/Akt signaling with Ly294002 prevented wound-induced GSK3ß(Ser9) phosphorylation as well as ß-catenin nuclear translocation and significantly attenuated restitution. Additionally, wounding induced rapid NF-kB(Ser536) phosphorylation, which was inhibited by AG1478, but not by Ly294002. GSK3ß−/− cells demonstrated significantly attenuated wound-induced restitution compared to wild-type cells.
We conclude that PI3K-mediated GSK3ß phosphorylation is involved in the intestinal epithelial wound-healing response. Phosphorylation of GSK3ß may be important for intestinal restitution by promoting cell motility in response to wounding.
PMCID: PMC3198390  PMID: 22039465
18.  Sox17 and Sox4 Differentially Regulate β-Catenin/T-Cell Factor Activity and Proliferation of Colon Carcinoma Cells▿  
Molecular and Cellular Biology  2007;27(22):7802-7815.
The canonical Wnt pathway is necessary for gut epithelial cell proliferation, and aberrant activation of this pathway causes intestinal neoplasia. We report a novel mechanism by which the Sox family of transcription factors regulate the canonical Wnt signaling pathway. We found that some Sox proteins antagonize while others enhance β-catenin/T-cell factor (TCF) activity. Sox17, which is expressed in the normal gut epithelium but exhibits reduced expression in intestinal neoplasia, is antagonistic to Wnt signaling. When overexpressed in SW480 colon carcinoma cells, Sox17 represses β-catenin/TCF activity in a dose-dependent manner and inhibits proliferation. Sox17 and Sox4 are expressed in mutually exclusive domains in normal and neoplastic gut tissues, and gain- and loss-of-function studies demonstrate that Sox4 enhances β-catenin/TCF activity and the proliferation of SW480 cells. In addition to binding β-catenin, both Sox17 and Sox4 physically interact with TCF/lymphoid enhancer factor (LEF) family members via their respective high-mobility-group box domains. Results from gain- and loss-of-function experiments suggest that the interaction of Sox proteins with β-catenin and TCF/LEF proteins regulates the stability of β-catenin and TCF/LEF. In particular, Sox17 promotes the degradation of both β-catenin and TCF proteins via a noncanonical, glycogen synthase kinase 3β-independent mechanism that can be blocked by proteasome inhibitors. In contrast, Sox4 may function to stabilize β-catenin protein. These findings indicate that Sox proteins can act as both antagonists and agonists of β-catenin/TCF activity, and this mechanism may regulate Wnt signaling responses in many developmental and disease contexts.
PMCID: PMC2169141  PMID: 17875931
19.  The responses of neural stem cells to the level of GSK-3 depend on the tissue of origin 
Biology Open  2013;2(8):812-821.
Neural stem cells (NSCs) can be obtained from a variety of sources, but not all NSCs exhibit the same characteristics. We have examined how the level of glycogen synthase kinase-3 activity regulates NSCs obtained from different sources: the mouse embryonic striatum, embryonic hippocampus, and mouse ES cells. Growth of striatal NSCs is enhanced by mild inhibition of GSK-3 but not by strong inhibition that is accompanied by Wnt/TCF transcriptional activation. In contrast, the growth of hippocampal NSCs is enhanced by both mild inhibition of GSK-3 as well as stronger inhibition. Active Wnt/TCF signaling, which occurs normally in the embryonic hippocampus, is required for growth of neural stem and progenitor cells. In the embryonic striatal germinal zone, however, TCF signaling is normally absent and its activation inhibits growth of NSCs from this region. Using a genetic model for progressive loss of GSK-3, we find that primitive ES cell-derived NSCs resemble striatal NSCs. That is, partial loss of GSK-3 alleles leads to an increase in NSCs while complete ablation of GSK-3, and activation of TCF-signaling, leads to their decline. Furthermore, expression of dominant negative TCF-4 in the GSK-3-null background was effective in blocking expression of Wnt-response genes and was also able to rescue neuronal gene expression. These results reveal that GSK-3 regulates NSCs by divergent pathways depending on the tissue of origin. The responses of these neural precursor cells may be contingent on baseline Wnt/TCF signaling occurring in a particular tissue.
PMCID: PMC3744073  PMID: 23951407
Neural stem cells; Wnt signaling; ES cells; Neurosphere assay
20.  Activation of β-Catenin Signaling in Prostate Cancer by Peptidyl-Prolyl Isomerase Pin1-Mediated Abrogation of the Androgen Receptor-β-Catenin Interaction 
Molecular and Cellular Biology  2006;26(3):929-939.
Androgen receptor (AR) interacts with β-catenin and can suppress its coactivation of T cell factor 4 (Tcf4) in prostate cancer (PCa) cells. Pin1 is a peptidyl-prolyl cis/trans isomerase that stabilizes β-catenin by inhibiting its binding to the adenomatous polyposis coli gene product and subsequent glycogen synthase kinase 3β (GSK-3β)-dependent degradation. Higher Pin1 expression in primary PCa is correlated with disease recurrence, and this study found that Pin1 expression was markedly increased in metastatic PCa. Consistent with this result, increased expression of Pin1 in transfected LNCaP PCa cells strongly accelerated tumor growth in vivo in immunodeficient mice. Pin1 expression in LNCaP cells enhanced β-catenin/Tcf4 transcriptional activity, as assessed using Tcf4-regulated reporter genes, and increased expression of endogenous Tcf4 and c-myc. However, in contrast to results in cells with intact PTEN and active GSK-3β, Pin1 expression in LNCaP PCa cells, which are PTEN deficient, did not increase β-catenin. Instead, Pin1 expression markedly inhibited the β-catenin interaction with AR, and Pin1 abrogated the ability of AR to antagonize β-catenin/Tcf4 binding and transcriptional activity. These findings demonstrate that AR can suppress β-catenin signaling, that the AR-β-catenin interaction can be regulated by Pin1, and that abrogation of this interaction can enhance β-catenin/Tcf4 signaling and contribute to aggressive biological behavior in PCa.
PMCID: PMC1347015  PMID: 16428447
21.  Tcf15 Primes Pluripotent Cells for Differentiation 
Cell Reports  2013;3(2):472-484.
The events that prime pluripotent cells for differentiation are not well understood. Inhibitor of DNA binding/differentiation (Id) proteins, which are inhibitors of basic helix-loop-helix (bHLH) transcription factor activity, contribute to pluripotency by blocking sequential transitions toward differentiation. Using yeast-two-hybrid screens, we have identified Id-regulated transcription factors that are expressed in embryonic stem cells (ESCs). One of these, Tcf15, is also expressed in the embryonic day 4.5 embryo and is specifically associated with a novel subpopulation of primed ESCs. An Id-resistant form of Tcf15 rapidly downregulates Nanog and accelerates somatic lineage commitment. We propose that because Tcf15 can be held in an inactive state through Id activity, it may prime pluripotent cells for entry to somatic lineages upon downregulation of Id. We also find that Tcf15 expression is dependent on fibroblast growth factor (FGF) signaling, providing an explanation for how FGF can prime for differentiation without driving cells out of the pluripotent state.
Graphical Abstract
► Tcf15 marks a subpopulation of pluripotent cells primed for somatic lineages ► Tcf15 expression is regulated by FGF signaling ► Tcf15 activity is repressed by Id proteins ► Tcf15 represses Nanog and drives differentiation once released from Id inhibition
Little is known about the transcription factors that oppose the pluripotency network to drive cells toward differentiation. Lowell and colleagues show that the basic helix-loop-helix (bHLH) transcription factor Tcf15 is expressed in the late preimplantation embryo and in a differentiation-primed subpopulation of embryonic stem cells and that an activated form of Tcf15 drives embryonic stem cells into a differentiation-primed state. These findings suggest a role for Tcf15 in manipulating and monitoring the earliest transition from pluripotency toward lineage commitment.
PMCID: PMC3607254  PMID: 23395635
22.  Identification of a Novel Gene Signature of ES Cells Self-Renewal Fluctuation through System-Wide Analysis 
PLoS ONE  2014;9(1):e83235.
Embryonic Stem cells (ESCs) can be differentiated into ectoderm, endoderm, and mesoderm derivatives, producing the majority of cell types. In regular culture conditions, ESCs' self-renewal is maintained through molecules that inhibit spontaneous differentiation enabling long-term cellular expansion. This undifferentiating condition is characterized by multiple metastable states that fluctuate between self-renewal and differentiation balance. Here, we aim to characterize the high-pluripotent ESC metastate marked by the expression of Zscan4 through a supervised machine learning framework based on an ensemble of support vector machine (SVM) classifiers. Our study revealed a leukaemia inhibitor factor (Lif) dependent not-canonical pluripotency signature (AF067063, BC061212, Dub1, Eif1a, Gm12794, Gm13871, Gm4340, Gm4850, Tcstv1/3, and Zfp352), that specifically marks Zscan4 ESCs' fluctuation. This novel ESC metastate is enhanced by high-pluripotency culture conditions obtained through Extracellular signal Regulated-Kinase (ERK) and Glycogen synthase kinase-3 (Gsk-3) signaling inhibition (2i). Significantly, we reported that the conditional ablation of the novel ESC metastate marked by the expression of Gm12794 is required for ESCs self-renewal maintenance. In conclusion, we extend the comprehension of ESCs biology through the identification of a novel molecular signature associated to pluripotency programming.
PMCID: PMC3879232  PMID: 24392082
23.  Tcf3 Functions as a Steady State Limiter of Transcriptional Programs of Mouse Embryonic Stem Cell Self Renewal 
Stem cells (Dayton, Ohio)  2008;26(8):1951-1960.
Elucidating the underlying transcriptional control of pluripotent cells is necessary for the development of new methods of inducing and maintaining pluripotent cells in vitro. Three transcription factors, Nanog, Oct4, and Sox2, have been reported to form a feedforward circuit promoting pluripotent cell self renewal in embryonic stem cells (ESC). Previously, we found that a transcriptional repressor activity of Tcf3, a DNA-binding effector of Wnt signaling, reduced Nanog promoter activity and Nanog levels in mouse ESC (mESC). The objective of this study was to determine the scope of Tcf3 effects on gene expression and self renewal beyond the regulation of Nanog levels. We show that Tcf3 acts broadly on a genome-wide scale to reduce the levels of several promoters of self renewal (Nanog, Tcl1, Tbx3, Esrrb) while not affecting other ESC genes (Oct4, Sox2, Fgf4). Comparing effects of Tcf3 ablation with Oct4 or Nanog knockdown revealed that Tcf3 counteracted effects of both Nanog and Oct4. Interestingly, effects of Tcf3 were more strongly correlated with Oct4 than with Nanog, despite the normal levels of Oct4 in TCF3−/− mESC. The deranged gene expression allowed TCF3−/− mESC self renewal even in the absence of leukemia inhibitory factor (Lif) and delayed differentiation in embryoid bodies. These findings identify Tcf3 as a cell-intrinsic inhibitor of pluripotent cell self renewal that functions by limiting steady-state levels of self renewal factors.
PMCID: PMC2743928  PMID: 18483421
24.  Tumor Suppressor Pten Inhibits Nuclear Accumulation of β-Catenin and T Cell/Lymphoid Enhancer Factor 1–Mediated Transcriptional Activation 
The Journal of Cell Biology  2001;153(6):1161-1174.
β-Catenin is a protein that plays a role in intercellular adhesion as well as in the regulation of gene expression. The latter role of β-catenin is associated with its oncogenic properties due to the loss of expression or inactivation of the tumor suppressor adenomatous polyposis coli (APC) or mutations in β-catenin itself. We now demonstrate that another tumor suppressor, PTEN, is also involved in the regulation of nuclear β-catenin accumulation and T cell factor (TCF) transcriptional activation in an APC-independent manner. We show that nuclear β-catenin expression is constitutively elevated in PTEN null cells and this elevated expression is reduced upon reexpression of PTEN. TCF promoter/luciferase reporter assays and gel mobility shift analysis demonstrate that PTEN also suppresses TCF transcriptional activity. Furthermore, the constitutively elevated expression of cyclin D1, a β-catenin/TCF–regulated gene, is also suppressed upon reexpression of PTEN. Mechanistically, PTEN increases the phosphorylation of β-catenin and enhances its rate of degradation. We define a pathway that involves mainly integrin-linked kinase and glycogen synthase kinase 3 in the PTEN-dependent regulation of β-catenin stability, nuclear β-catenin expression, and transcriptional activity. Our data indicate that β-catenin/TCF–mediated gene transcription is regulated by PTEN, and this may represent a key mechanism by which PTEN suppresses tumor progression.
PMCID: PMC2192018  PMID: 11402061
integrin-linked kinase; glycogen synthase kinase 3; cyclin D1; prostate cancer; protein kinase B
25.  Temporal Perturbation of the Wnt Signaling Pathway in the Control of Cell Reprogramming Is Modulated by TCF1 
Stem Cell Reports  2014;2(5):707-720.
Cyclic activation of the Wnt/β-catenin signaling pathway controls cell fusion-mediated somatic cell reprogramming. TCFs belong to a family of transcription factors that, in complex with β-catenin, bind and transcriptionally regulate Wnt target genes. Here, we show that Wnt/β-catenin signaling needs to be off during the early reprogramming phases of mouse embryonic fibroblasts (MEFs) into iPSCs. In MEFs undergoing reprogramming, senescence genes are repressed and mesenchymal-to-epithelial transition is favored. This is correlated with a repressive activity of TCF1, which contributes to the silencing of Wnt/β-catenin signaling at the onset of reprogramming. In contrast, the Wnt pathway needs to be active in the late reprogramming phases to achieve successful reprogramming. In conclusion, continued activation or inhibition of the Wnt/β-catenin signaling pathway is detrimental to the reprogramming of MEFs; instead, temporal perturbation of the pathway is essential for efficient reprogramming, and the “Wnt-off” state can be considered an early reprogramming marker.
Graphical Abstract
•Time-dependent perturbation of Wnt pathway enhances reprogramming•TCF1 acts as repressor in cells undergoing reprogramming•TCF1 represses senescence genes and promotes MET•“Wnt-off” state is an early reprogramming marker
TCF1 is an effector of the Wnt/β-catenin pathway that can act as an activator or as a repressor. In this study, Lluis, Cosma, and colleagues show that the repressive activity of TCF1 or the inhibition of the Wnt/β-catenin at early stages of the reprogramming process is essential to generate induced pluripotent stem cells (iPSCs) from mouse embryonic fibroblasts (MEFs).
PMCID: PMC4050487  PMID: 24936456

Results 1-25 (956790)