Wnt/beta-catenin signaling: new (and old) players and new insights

WNT protein-independent constitutive nuclear localization of beta-catenin protein and its low degradation rate in thalamic neurons

Nuclear localization of β-catenin is a hallmark of canonical Wnt signaling, a pathway that plays a crucial role in brain development and the neurogenesis of the adult brain. We recently showed that β-catenin accumulates specifically in mature thalamic neurons, where it regulates the expression of the Ca(v)3.1 voltage-gated calcium channel gene. Here, we investigated the mechanisms underlying β-catenin accumulation in thalamic neurons. We report that a lack of soluble factors produced either by glia or cortical neurons does not impair nuclear β-catenin accumulation in thalamic neurons. We next found that the number of thalamic neurons with β-catenin nuclear localization did not change when the Wnt/Dishevelled signaling pathway was inhibited by Dickkopf1 or a dominant negative mutant of Dishevelled3. These results suggest a WNT-independent cell-autonomous mechanism. We found that the protein levels of APC, AXIN1, and GSK3β, components of the β-catenin degradation complex, were lower in the thalamus than in the cortex of the adult rat brain. Reduced levels of these proteins were also observed in cultured thalamic neurons compared with cortical cultures. Finally, pulse-chase experiments confirmed that cytoplasmic β-catenin turnover was slower in thalamic neurons than in cortical neurons. Altogether, our data indicate that the nuclear localization of β-catenin in thalamic neurons is their cell-intrinsic feature, which was WNT-independent but associated with low levels of proteins involved in β-catenin labeling for ubiquitination and subsequent degradation.

E-cadherin mediates contact inhibition of proliferation through Hippo signaling-pathway components

Contact inhibition of cell growth is essential for embryonic development and maintenance of tissue architecture in adult organisms, and the growth of tumors is characterized by a loss of contact inhibition of proliferation. The recently identified Hippo signaling pathway has been implicated in contact inhibition of proliferation as well as organ size control. The modulation of the phosphorylation and nuclear localization of Yes-associated protein (YAP) by the highly conserved kinase cascade of the Hippo signaling pathway has been intensively studied. However, cell-surface receptors regulating the Hippo signaling pathway in mammals are not well understood. In this study, we show that Hippo signaling pathway components are required for E-cadherin-dependent contact inhibition of proliferation. Knockdown of the Hippo signaling components or overexpression of YAP inhibits the decrease in cell proliferation caused by E-cadherin homophilic binding at the cell surface, independent of other cell-cell interactions. We also demonstrate that the E-cadherin/catenin complex functions as an upstream regulator of the Hippo signaling pathway in mammalian cells. Expression of E-cadherin in MDA-MB-231 cells restores the density-dependent regulation of YAP nuclear exclusion. Knockdown of β-catenin in densely cultured MCF10A cells, which mainly depletes E-cadherin-bound β-catenin, induces a decrease in the phosphorylation of S127 residue of YAP and its nuclear accumulation. Moreover, E-cadherin homophilic binding independent of other cell interactions is sufficient to control the subcellular localization of YAP. Therefore, Our results indicate that, in addition to its role in cell-cell adhesion, E-cadherin-mediated cell-cell contact directly regulates the Hippo signaling pathway to control cell proliferation.

β-catenin as a potential key target for tumor suppression

β-catenin is a multifunctional protein identified to be pivotal in embryonic patterning, organogenesis and adult homeostasis. It plays a critical structural role in mediating cadherin junctions and is also an essential transcriptional co-activator in the canonical Wnt pathway. Evidence has been documented that both the canonical Wnt pathway and cadherin junctions are deregulated or impaired in a plethora of human malignancies. In the light of this, there has been a recent surge in elucidating the mechanisms underlying the etiology of cancer development from the perspective of β-catenin. Here, we focus on the emerging roles of β-catenin in the process of tumorigenesis by discussing novel functions of old players and new proteins, mechanisms identified to mediate or interact with β-catenin and the most recently unraveled clinical implications of β-catenin regulatory pathways toward tumor suppression.

En1 and Wnt signaling in midbrain dopaminergic neuronal development

Dopaminergic neurons of the ventral mesodiencephalon are affected in significant health disorders such as Parkinson's disease, schizophrenia, and addiction. The ultimate goal of current research endeavors is to improve the clinical treatment of such disorders, such as providing a protocol for cell replacement therapy in Parkinson's disease that will successfully promote the specific differentiation of a stem cell into a dopaminergic neuronal phenotype. Decades of research on the developmental mechanisms of the mesodiencephalic dopaminergic (mdDA) system have led to the identification of many signaling pathways and transcription factors critical in its development. The unraveling of these pathways will help fill in the pieces of the puzzle that today dominates neurodevelopment research: how to make and maintain a mdDA neuron. In the present review, we provide an overview of the mdDA system, the processes and signaling molecules involved in its genesis, with a focus on the transcription factor En1 and the canonical Wnt pathway, highlighting recent findings on their relevance--and interplay--in the development and maintenance of the mdDA system.

Molecular and cellular pathways associated with chromosome 1p deletions during colon carcinogenesis

Chromosomal instability is a major pathway of sporadic colon carcinogenesis. Chromosome arm 1p appears to be one of the "hot spots" in the non-neoplastic mucosa that, when deleted, is associated with the initiation of carcinogenesis. Chromosome arm 1p contains genes associated with DNA repair, spindle checkpoint function, apoptosis, multiple microRNAs, the Wnt signaling pathway, tumor suppression, antioxidant activities, and defense against environmental toxins. Loss of 1p is dangerous since it would likely contribute to genomic instability leading to tumorigenesis. The 1p deletion-associated colon carcinogenesis pathways are reviewed at the molecular and cellular levels. Sporadic colon cancer is strongly linked to a high-fat/low-vegetable/low-micronutrient, Western-style diet. We also consider how selected dietary-related compounds (eg, excess hydrophobic bile acids, and low levels of folic acid, niacin, plant-derived antioxidants, and other modulatory compounds) might affect processes leading to chromosomal deletions, and to the molecular and cellular pathways specifically altered by chromosome 1p loss.

Structural and functional characterization of the Wnt inhibitor APC membrane recruitment 1 (Amer1)

Amer1/WTX binds to the tumor suppressor adenomatous polyposis coli and acts as an inhibitor of Wnt signaling by inducing β-catenin degradation. We show here that Amer1 directly interacts with the armadillo repeats of β-catenin via a domain consisting of repeated arginine-glutamic acid-alanine (REA) motifs, and that Amer1 assembles the β-catenin destruction complex at the plasma membrane by recruiting β-catenin, adenomatous polyposis coli, and Axin/Conductin. Deletion or specific mutations of the membrane binding domain of Amer1 abolish its membrane localization and abrogate negative control of Wnt signaling, which can be restored by artificial targeting of Amer1 to the plasma membrane. In line, a natural splice variant of Amer1 lacking the plasma membrane localization domain is deficient for Wnt inhibition. Knockdown of Amer1 leads to the activation of Wnt target genes, preferentially in dense compared with sparse cell cultures, suggesting that Amer1 function is regulated by cell contacts. Amer1 stabilizes Axin and counteracts Wnt-induced degradation of Axin, which requires membrane localization of Amer1. The data suggest that Amer1 exerts its negative regulatory role in Wnt signaling by acting as a scaffold protein for the β-catenin destruction complex and promoting stabilization of Axin at the plasma membrane.

Intervertebral disc development is regulated by Wnt/β-catenin signaling

STUDY DESIGN

Histologic analysis of intervertebral disc (IVD) in three types of transgenic mice.

OBJECTIVE

To investigate the role of Wnt/β-catenin signaling in regulation of IVD development and organization.

SUMMARY OF BACKGROUND DATA

β-catenin dependent Wnt signaling is one of the central regulators in cartilage development during limb skeletal formation. Little is known, however, about the physiologic relevance of this signaling pathway to IVD development and organization.

METHODS

Temporal-spatial distribution of Wnt/β-catenin signaling activity was examined in IVD using Wnt/β-catenin reporter (TOPGAL) mice. The structural changes in the mouse IVD components such as the nucleus pulposus (NP), endplate (EP), annulus fibrosus (AF), and the growth plate (GP) of the vertebral body were analyzed after transient activation of Wnt/β-catenin signaling or deletion of β-catenin in the mice.

RESULTS

Activity of Wnt/β-catenin signaling was high in EP, AF, and GP in the embryonic stages and decreased at the postnatal stage; it was undetectable in the embryonic NP but upregulated after birth. The transient activation of Wnt/β-catenin signaling caused severe deterioration of the GP and the AF, whereas deficiency of β-catenin accelerated bone formation in between EP and GP.

CONCLUSION

The findings in this study suggest that proper regulation of Wnt/β-catenin signaling is required for development and organization of IVD.

Wnt5a is a transcriptional target of Dlx homeogenes and promotes differentiation of interneuron progenitors in vitro and in vivo

During brain development, neurogenesis, migration, and differentiation of neural progenitor cells are regulated by an interplay between intrinsic genetic programs and extrinsic cues. The Dlx homeogene transcription factors have been proposed to directly control the genesis and maturation of GABAergic interneurons of the olfactory bulb (OB), subpallium, and cortex. Here we provide evidence that Dlx genes promote differentiation of olfactory interneurons via the signaling molecule Wnt5a. Dlx2 and Dlx5 interact with homeodomain binding sequences within the Wnt5a locus and activate its transcription. Exogenously provided Wnt5a promotes GABAergic differentiation in dissociated OB neurons and in organ-type brain cultures. Finally, we show that the Dlx-mutant environment is unfavorable for GABA differentiation, in vivo and in vitro. We conclude that Dlx genes favor interneuron differentiation also in a non-cell-autonomous fashion, via expression of Wnt5a.

A paraptosis-like cell death induced by δ-tocotrienol in human colon carcinoma SW620 cells is associated with the suppression of the Wnt signaling pathway

Tocotrienol is considered a beneficial effect agent on inhibition of tumor development. In this study, we focused on the effects of δ-tocotrienol and its possible mechanism on induction of death in human colon cancer SW620 cells. δ-Tocotrienol inhibited proliferation of SW620 cell in a dose-dependent manner. Our findings showed that δ-tocotrienol effectively induced paraptosis-like death in SW620 cells, correlated with the vacuolation that may be from welling and fusion of mitochondria and/or the endoplasmic reticulum (ER) as well as caspase-3 nonactivated. However, there were no changes in apoptosis based on flow cytometry analysis. Of being noted, δ-tocotrienol reduced the expression of β-catenin and wnt-1 proteins by about 50% at the highest dose (20μmol/L). δ-Tocotrienol also decreased cyclin D1, c-jun and MMP-7 protein levels in SW620 cells. Altogether, these data indicate that δ-tocotrienol induces paraptosis-like cell death, which is associated with the suppression of the Wnt signaling pathway. Thus, our findings may provide a novel application in treatment of human colon carcinoma.

Jerky/Earthbound facilitates cell-specific Wnt/Wingless signalling by modulating β-catenin-TCF activity

Wnt/Wingless signal transduction directs fundamental developmental processes, and upon hyperactivation triggers colorectal adenoma/carcinoma formation. Responses to Wnt stimulation are cell specific and diverse; yet, how cell context modulates Wnt signalling outcome remains obscure. In a Drosophila genetic screen for components that promote Wingless signalling, we identified Earthbound 1 (Ebd1), a novel member in a protein family containing Centromere Binding Protein B (CENPB)-type DNA binding domains. Ebd1 is expressed in only a subset of Wingless responsive cell types, and is required for only a limited number of Wingless-dependent processes. In addition, Ebd1 shares sequence similarity and can be functionally replaced with the human CENPB domain protein Jerky, previously implicated in juvenile myoclonic epilepsy development. Both Jerky and Ebd1 interact directly with the Wnt/Wingless pathway transcriptional co-activators β-catenin/Armadillo and T-cell factor (TCF). In colon carcinoma cells, Jerky facilitates Wnt signalling by promoting association of β-catenin with TCF and recruitment of β-catenin to chromatin. These findings indicate that tissue-restricted transcriptional co-activators facilitate cell-specific Wnt/Wingless signalling responses by modulating β-catenin-TCF activity.

Two structural and functional domains of MESD required for proper folding and trafficking of LRP5/6

How the endoplasmic reticulum (ER) folding machinery coordinates general and specialized chaperones during protein translation and folding remains an important unanswered question. Here, we show two structural domains in MESD, a specialized chaperone for LRP5/6, carry out dual functions. The chaperone domain forms a complex with the immature receptor, maintaining the β-propeller (BP) domain in an interaction competent state for epidermal growth factor-repeat binding. This promotes proper folding of the BP domain, causing a binding switch from the chaperone domain to the escort domain. The escort complex ensures LRP5/6 safe-trafficking from the ER to the Golgi by preventing premature ligand-binding. Inside the Golgi, the BP domain may contain a histidine switch, regulating MESD dissociation and retrieval. Together, we generate a plausible cell biology picture of the MESD/LRP5/6 pathway, suggesting that it is the specialized chaperones, MESD, that serves as the folding template to drive proper folding and safe trafficking of large multidomain proteins LRP5/6.

Transport of receptors, receptor signaling complexes and ion channels via neuropeptide-secretory vesicles

Stimulus-induced exocytosis of large dense-core vesicles (LDCVs) leads to discharge of neuropeptides and fusion of LDCV membranes with the plasma membrane. However, the contribution of LDCVs to the properties of the neuronal membrane remains largely unclear. The present study found that LDCVs were associated with multiple receptors, channels and signaling molecules, suggesting that neuronal sensitivity is modulated by an LDCV-mediated mechanism. Liquid chromatography-mass spectrometry combined with immunoblotting of subcellular fractions identified 298 proteins in LDCV membranes purified from the dorsal spinal cord, including G-protein-coupled receptors, G-proteins and other signaling molecules, ion channels and trafficking-related proteins. Morphological assays showed that δ-opioid receptor 1 (DOR1), β2 adrenergic receptor (AR), G(αi2), voltage-gated calcium channel α2δ1 subunit and P2X purinoceptor 2 were localized in substance P (SP)-positive LDCVs in small-diameter dorsal root ganglion neurons, whereas β1 AR, Wnt receptor frizzled 8 and dishevelled 1 were present in SP-negative LDCVs. Furthermore, DOR1/G(αi2)/G(β1γ5)/phospholipase C β2 complexes were associated with LDCVs. Blockade of the DOR1/G(αi2) interaction largely abolished the LDCV localization of G(αi2) and impaired stimulation-induced surface expression of G(αi2). Thus, LDCVs serve as carriers of receptors, ion channels and preassembled receptor signaling complexes, enabling a rapid, activity-dependent modulation of neuronal sensitivity.

Amer1/WTX couples Wnt-induced formation of PtdIns(4,5)P2 to LRP6 phosphorylation

Phosphorylation of the Wnt receptor low-density lipoprotein receptor-related protein 6 (LRP6) by glycogen synthase kinase 3β (GSK3β) and casein kinase 1γ (CK1γ) is a key step in Wnt/β-catenin signalling, which requires Wnt-induced formation of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)). Here, we show that adenomatous polyposis coli membrane recruitment 1 (Amer1) (also called WTX), a membrane associated PtdIns(4,5)P(2)-binding protein, is essential for the activation of Wnt signalling at the LRP6 receptor level. Knockdown of Amer1 reduces Wnt-induced LRP6 phosphorylation, Axin translocation to the plasma membrane and formation of LRP6 signalosomes. Overexpression of Amer1 promotes LRP6 phosphorylation, which requires interaction of Amer1 with PtdIns(4,5)P(2). Amer1 translocates to the plasma membrane in a PtdIns(4,5)P(2)-dependent manner after Wnt treatment and is required for LRP6 phosphorylation stimulated by application of PtdIns(4,5)P(2). Amer1 binds CK1γ, recruits Axin and GSK3β to the plasma membrane and promotes complex formation between Axin and LRP6. Fusion of Amer1 to the cytoplasmic domain of LRP6 induces LRP6 phosphorylation and stimulates robust Wnt/β-catenin signalling. We propose a mechanism for Wnt receptor activation by which generation of PtdIns(4,5)P(2) leads to recruitment of Amer1 to the plasma membrane, which acts as a scaffold protein to stimulate phosphorylation of LRP6.

The Wnt/beta-catenin pathway in adrenocortical development and cancer

Signaling by the Wnt family of secreted glycolipoproteins plays key roles in embryonic development of organisms ranging from nematodes to mammals and is also implicated in several types of human cancers. Canonical Wnt signaling functions by regulating the translocation of β-catenin to the nucleus, where it controls key gene expression programs through interaction with Tcf/Lef and other families of transcription factors. Wnts can also act through non-canonical pathways that do not involve β-catenin activation, but implicate small GTPases/JNK kinase and intracellular calcium. Here we review recent studies that have revealed the expression of several components of Wnt/β-catenin signaling in the adrenal cortex and discovered a key role for this pathway in the regulation of proliferation/differentiation of progenitor cells and in tumorigenesis of that endocrine organ.

GATA transcription factors in the developing reproductive system

Previous work has firmly established the role for both GATA4 and FOG2 in the initial global commitment to sexual fate, but their (joint or individual) function in subsequent steps remained unknown. Hence, gonad-specific deletions of these genes in mice were required to reveal their roles in sexual development and gene regulation. The development of tissue-specific Cre lines allowed for substantial advances in the understanding of the function of GATA proteins in sex determination, gonadal differentiation and reproductive development in mice. Here we summarize the recent work that examined the requirement of GATA4 and FOG2 proteins at several critical stages in testis and ovarian differentiation. We also discuss the molecular mechanisms involved in this regulation through the control of Dmrt1 gene expression in the testis and the canonical Wnt/ß-catenin pathway in the ovary.

Independent interactions of phosphorylated β-catenin with E-cadherin at cell-cell contacts and APC at cell protrusions

Background

The APC tumour suppressor functions in several cellular processes including the regulation of β-catenin in Wnt signalling and in cell adhesion and migration.

Findings

In this study, we establish that in epithelial cells N-terminally phosphorylated β-catenin specifically localises to several subcellular sites including cell-cell contacts and the ends of cell protrusions. N-terminally phosphorylated β-catenin associates with E-cadherin at adherens junctions and with APC in cell protrusions. We isolated APC-rich protrusions from stimulated cells and detected β-catenin, GSK3β and CK1α, but not axin. The APC/phospho-β-catenin complex in cell protrusions appears to be distinct from the APC/axin/β-catenin destruction complex. GSK3β phosphorylates the APC-associated population of β-catenin, but not the cell junction population. β-catenin associated with APC is rapidly phosphorylated and dephosphorylated. HGF and wound-induced cell migration promote the localised accumulation of APC and phosphorylated β-catenin at the leading edge of migrating cells. APC siRNA and analysis of colon cancer cell lines show that functional APC is required for localised phospho-β-catenin accumulation in cell protrusions.

Conclusions

We conclude that N-terminal phosphorylation of β-catenin does not necessarily lead to its degradation but instead marks distinct functions, such as cell migration and/or adhesion processes. Localised regulation of APC-phospho-β-catenin complexes may contribute to the tumour suppressor activity of APC.

Ectodermal Wnt/β-catenin signaling shapes the mouse face

The canonical Wnt/β-catenin pathway is an essential component of multiple developmental processes. To investigate the role of this pathway in the ectoderm during facial morphogenesis, we generated conditional β-catenin mouse mutants using a novel ectoderm-specific Cre recombinase transgenic line. Our results demonstrate that ablating or stabilizing β-catenin in the embryonic ectoderm causes dramatic changes in facial morphology. There are accompanying alterations in the expression of Fgf8 and Shh, key molecules that establish a signaling center critical for facial patterning, the frontonasal ectodermal zone (FEZ). These data indicate that Wnt/β-catenin signaling within the ectoderm is critical for facial development and further suggest that this pathway is an important mechanism for generating the diverse facial shapes of vertebrates during evolution.

The Wnt /β-catenin signaling pathway in the adult neurogenesis

The Wnt/β-catenin signaling pathway plays an important role in neural development, β-catenin is a central component of the Wnt/β-catenin signaling pathway, which not only performs the function of transmitting information in the cytoplasm, but also translocates to the nucleus-activating target gene transcription. The target genes in neural tissues have not been fully revealed, but the effects of the Wnt/β-catenin signaling pathway in adult neurogenesis have been demonstrated by ongoing research, which are significative to the basic research and treatment of neuronal degeneration diseases. Here, we review key findings to show the characteristics of β-catenin and its pivotal nature in the Wnt/β-catenin signaling pathway in a number of molecular studies. We also review current literature on the role of β-catenin in adult neurogenesis, which consists of an active process encompassing the proliferation, migration, differentiation and final synaptogenesis.

Drug discovery approaches to target Wnt signaling in cancer stem cells

Cancer stem cells (CSCs) represent a unique subset of cells within a tumor that possess self-renewal capacity and pluripotency, and can drive tumor initiation and maintenance. First identified in hematological malignancies, CSCs are now thought to play an important role in a wide variety of solid tumors such as NSCLC, breast and colorectal cancer. The role of CSCs in driving tumor formation illustrates the dysregulation of differentiation in tumorigenesis. The Wnt, Notch and Hedgehog (HH) pathways are developmental pathways that are commonly activated in many types of cancer. While substantial progress has been made in developing therapeutics targeting Notch and HH, the Wnt pathway has remained an elusive therapeutic target. This review will focus on the clinical relevance of the Wnt pathway in CSCs and tumor cell biology, as well as points of therapeutic intervention and recent advances in targeting Wnt/β-catenin signaling.

Colon cancer stem cells

Colorectal cancer (CRC) is the second leading cause of death from cancer in the United States. Aggressive research in the last decade has led to a wealth of information about this disease; for example, we now know that more than 80% of sporadic colon tumors contain mutations in the Wnt and TGFβ signaling pathways. The latest avenue of research is revealing the existence of and role for the cancer stem cell (CSC) model, which promotes the idea that malignancies originate from a small fraction of cancer cells that show self-renewal and multi- or pluripotency. The model also endorses that CSCs are capable of initiating and sustaining tumor growth. The body of evidence in favor of the CSC model is rapidly growing and includes analyses from flow cytometry of numerous CSC biomarkers, abnormal signaling pathways, symmetric division, dietary augmentation, and analysis of the behavior of these cells in spheroid culture formation. Although the incidence of death from CRC remains high, fervent research, both basic and translational, is beginning to improve patient outcomes. This paper focuses on stem cell biology in the context of CRC to help understand the mechanisms leading to tumor development and therapy resistance, with possible therapeutic indications.

Proliferation and pluripotency of human embryonic stem cells maintained on type I collagen

Human embryonic stem cells (hESC) require a balance of growth factors and signaling molecules to proliferate and retain pluripotency. Conditioned medium (CM) from a human embryonic germ-cell-derived cell culture, SDEC, was observed to support the growth of hESC on type I collagen (COL I) and on Matrigel (MAT) biomatricies. After 1 month, the population doubling of hESC grown in SDEC CM on COL I was equivalent to that of hESC grown in mouse embryonic fibroblast (MEF) CM on MAT. hESC grown in SDEC CM on COL I expressed OCT4, NANOG, SSEA-4, alkaline phosphatase (AP), and TRA-1-60; retained a normal karyotype; and were capable of forming teratomas. DNA microarray analysis was used to compare the transcriptional profiles of SDEC and the less supportive WI38 and Detroit 551 human cell lines. The mRNA level of secreted frizzled-related protein (sFRP-1), a known antagonist of the WNT/β-catenin signaling pathway, was significantly reduced in SDEC as compared with the other 2 cell lines, whereas the mRNA levels of prostaglandin-endoperoxide synthase 2 (PTGS2 or COX-2) and prostaglandin I₂ synthase (PGIS), two prostaglandin biosynthesis genes, were significantly increased in SDEC. The level of sFRP-1 protein was significantly reduced, and levels of 2 prostaglandins that are downstream products of PTGS2 and PGIS, prostaglandin E₂ and 6-keto-prostaglandin F(1α), were significantly elevated in SDEC CM compared with WI38, Detroit 551, and MEF CM. Further, addition of purified sFRP-1 to SDEC CM reduced the proliferation of hESC grown on COL I as well as MAT in a dose-dependent manner.

Junctional music that the nucleus hears: cell-cell contact signaling and the modulation of gene activity

Cell-cell junctions continue to capture the interest of cell and developmental biologists, with an emerging area being the molecular means by which junctional signals relate to gene activity in the nucleus. Although complexities often arise in determining the direct versus indirect nature of such signal transduction, it is clear that such pathways are essential for the function of tissues and that alterations may contribute to many pathological outcomes. This review assesses a variety of cell-cell junction-to-nuclear signaling pathways, and outlines interesting areas for further study.

Planar cell polarity: keeping hairs straight is not so simple

The fur on a cat's back, the scales on a fish, or the bristles on a fly are all beautifully organized, with a high degree of polarization in their surface organization. Great progress has been made in understanding how individual cell polarity is established, but our understanding of how cells coordinate their polarity in forming coherent tissues is still fragmentary. The organization of cells in the plane of the epithelium is known as planar cell polarity (PCP), and studies in the past decade have delineated a genetic pathway for the control of PCP. This review will first briefly review data from the Drosophila field, where PCP was first identified and genetically characterized, and then explore how vertebrate tissues become polarized during development.

α-Catenin inhibits glioma cell migration, invasion, and proliferation by suppression of β-catenin transactivation

Patients with glioblastomas, the most common primary tumors of the central nervous system, have poor prognoses because of uncontrolled tumor cell invasion and proliferation. β-Catenin plays an important role in tumor development. However, whether α-catenin expression contributes to β-catenin transactivation in glioma cells is largely unknown. We report here that α-catenin expression abrogates epidermal growth factor receptor (EGFR)-activation-induced β-catenin nuclear translocation in human glioblastoma cells, thereby attenuating β-catenin transactivation and the expression of its downstream genes CCND1 and c-myc. In addition, ectopic expression of α-catenin or depletion of β-catenin suppresses EGF-promoted glioblastoma cell migration, invasion, and proliferation. In contrast, α-catenin depletion promotes β-catenin nuclear translocation and transactivation, and tumor cell motility and growth. These findings reveal the importance of β-catenin regulation by α-catenin in cellular activities of glioblastoma cells.

Zebrafish Nkd1 promotes Dvl degradation and is required for left-right patterning

The establishment of the left-right (LR) axis in zebrafish embryos relies on signals from the dorsal forerunner cells (DFC) and the Kupffer's vesicle (KV). While the Wnt signaling network influences many aspects of embryonic development, its precise role in LR patterning is still unclear. One branch of the Wnt network leads to stabilization of β-catenin and activation of downstream target genes. Other Wnt ligands appear to act independently of β-catenin to modulate calcium release and influence cell polarity. Central to regulation of β-catenin and coordination of convergent extension (CE) movements is Dishevelled (Dvl). Naked Cuticle (Nkd) binds Dvl and modulates β-catenin-dependent and independent Wnt signaling. Here, we analyze the expression patterns of three zebrafish Nkd homologs and find enriched expression of nkd1 in DFCs and KV. Dvl is degraded upon Nkd1 overexpression in zebrafish. Knockdown of Nkd1 specifically in the DFC results in β-catenin nuclear localization and transcriptional activation as well as alterations to DFC migration, KV formation, ciliogenesis and LR patterning. Furthermore, we identify asymmetric expression of the Nodal antagonist charon around the KV and show that Nkd1 knockdown impacts asymmetric charon expression. Our findings show that Nkd1 acts as a β-catenin antagonist in the DFCs necessary for LR patterning.

Wnt/β-catenin signaling regulates postnatal development and regeneration of the salivary gland

Regenerative therapy of the salivary gland (SG) is a promising therapeutic approach for irreversible hyposalivation in patients with head and neck cancer treated by radiotherapy. However, little is known about the molecular regulators of stem/progenitor cell activity and regenerative processes in the SG. Wnt/β-catenin signaling regulates the function of many adult stem cell populations, but its role in SG development and regeneration is unknown. Using BAT-gal Wnt reporter transgenic mice, we demonstrate that in the submandibular glands (SMGs) of newborn mice Wnt/β-catenin signaling is active in a few cells at the basal layer of intercalated ducts, the putative location of salivary gland stem/progenitor cells (SGPCs). Wnt activity decreases as mice age, but is markedly enhanced in SG ducts during regeneration of adult SMG after ligation of the main secretory duct. The Hedgehog (Hh) pathway is also activated after duct ligation. Inhibition of epithelial β-catenin signaling in young Keratin5-rtTA/tetO-Dkk1 mice impairs the postnatal development of SMG, particularly affecting maturation of granular convoluted tubules. Conversely, forced activation of epithelial β-catenin signaling in adult Keratin5-rtTA/tetO-Cre/Ctnnb1((Ex3)fl) mice promotes proliferation of ductal cells, expansion of the SGPC compartment, and ectopic activation of Hh signaling. Taken together, these results indicate that Wnt/β-catenin signaling regulates the activity of SGPCs during postnatal development and regeneration upstream of the Hh pathway, and suggest the potential of modulating Wnt/β-catenin and/or Hh pathways for functional restoration of SGs after irradiation.

Poly(ADP-ribose) polymerase inhibitors as promising cancer therapeutics

The year of 2005 was a watershed in the history of poly(ADP-ribose) polymerase (PARP) inhibitors due to the important findings of selective killing in BRCA-deficient cancers by PARP inhibition. The findings made PARP inhibition one of the most promising new therapeutic approaches to cancers, especially to those with specific defects. With AZD2281 and BSI-201 entering phase III clinical trials, the final application of PARP inhibitors in clinic would come true soon. This current paper will review the major advances in targeting PARP for cancer therapy and discuss the existing questions, the answers to which may influence the future of PARP inhibitors as cancer therapeutics.

Dynamic expression of a LEF-EGFP Wnt reporter in mouse development and cancer

We have characterized a transgenic mouse line in which enhanced green fluorescent protein (EGFP) is expressed under the control of multimerized LEF-1 responsive elements. In embryos, EGFP was detected in known sites of Wnt activation, including the primitive streak, mesoderm, neural tube, somites, heart, limb buds, mammary placodes, and whisker follicles. In vitro cultured transgenic embryonic fibroblasts upregulated EGFP expression in response to activation of Wnt signaling by GSK3beta inhibition. Mammary tumor cell lines derived from female LEF-EGFP transgenic mice treated with the carcinogen 7, 12-dimethylbenz[a]anthracene (DMBA) also express EGFP. Thus, this transgenic line is useful for ex vivo and in vitro studies of Wnt signaling in development and cancer.

Resveratrol modulates angiogenesis through the GSK3β/β-catenin/TCF-dependent pathway in human endothelial cells

Vascular endothelial growth factor (VEGF) plays a critical role in angiogenesis due to its potent and specific ability to promote the proliferation and migration of endothelial cells. Resveratrol has been shown to have many health-benefiting effects, including the protection of cardiovascular system. In this study we examined the effect of resveratrol on angiogenesis in human umbilical vein endothelial cells (HUVECs). We observed that resveratrol was able to modulate the expression of VEGF and the formation of vascular network in a biphasic pattern. While resveratrol at low concentrations, from 1 to 10μM, up-regulated the expression of VEGF and promoted angiogenesis, it had opposite effect at high concentrations (20μM and higher). The biphasic effect of resveratrol on angiogenesis was confirmed by chick chorioallantoic membrane assay. Up-regulation of VEGF expression depended on the nuclear accumulation and transcriptional activity of β-catenin. Correspondingly, GSK3β, a negative regulator of β-catenin, turned into a less active state (phosphorylated at Ser9) in cells exposed to 5μM of resveratrol, but became more active at 20μM. We demonstrated that both Akt and ERK signaling pathways, which are known to be critical for angiogenesis, became activated in response to 5μM of resveratrol and functioned to inactivate GSK3β. Our findings may have implications in the management of cardiovascular diseases and other conditions such as cancer by the use of resveratrol.

The role of the miR-200 family in epithelial-mesenchymal transition

MicroRNAs (miRNAs) are single-stranded, non-coding RNA molecules that regulate gene expression at the post-transcriptional level. Genes encoding miRNAs are located in regions of the genome that are commonly amplified, deleted or rearranged. They are commonly dysregulated in human cancers and known to act as oncogenes or tumor suppressors. Members of the miR-200 miRNA family are downregulated in human cancer cells and tumors due to aberrant epigenetic gene silencing and play a critical role in the suppression of epithelial-to-mesenchymal transition (EMT), tumor cell adhesion, migration, invasion and metastasis, by targeting and repressing the expression of key mRNAs that are involved in EMT (ZEB1 and ZEB2), β-catenin/Wnt signaling (β-catenin), EGFR inhibitor resistance (ERRFI-1) and chemoresistance to therapeutic agents (TUBB3). Since the miR-200 family functions as putative tumor suppressors and represent biomarkers for poorly differentiated and aggressive cancers, restoration of miR-200 expression may have therapeutic implications for the treatment of metastatic and drug-resistant tumors.

The role of the miR-200 family in epithelial-mesenchymal transition

MicroRNAs (miRNAs) are single-stranded, non-coding RNA molecules that regulate gene expression at the post-transcriptional level. Genes encoding miRNAs are located in regions of the genome that are commonly amplified, deleted or rearranged. They are commonly dysregulated in human cancers and known to act as oncogenes or tumor suppressors. Members of the miR-200 miRNA family are downregulated in human cancer cells and tumors due to aberrant epigenetic gene silencing and play a critical role in the suppression of epithelial-to-mesenchymal transition (EMT), tumor cell adhesion, migration, invasion and metastasis, by targeting and repressing the expression of key mRNAs that are involved in EMT (ZEB1 and ZEB2), β-catenin/Wnt signaling (β-catenin), EGFR inhibitor resistance (ERRFI-1) and chemoresistance to therapeutic agents (TUBB3). Since the miR-200 family functions as putative tumor suppressors and represent biomarkers for poorly differentiated and aggressive cancers, restoration of miR-200 expression may have therapeutic implications for the treatment of metastatic and drug-resistant tumors.

Silibinin exerts sustained growth suppressive effect against human colon carcinoma SW480 xenograft by targeting multiple signaling molecules

PURPOSE

Earlier, we reported the strong preventive efficacy of silibinin against colorectal cancer (CRC), but its usefulness against established CRC or effect of its withdrawal on CRC growth remained unknown. Present study focused on these important issues by employing two different treatment protocols in advanced human CRC SW480 xenograft in nude mice.

METHODS

In the first treatment protocol, silibinin was fed for 28 days (200 mg/kg body weight, 5 days/week) to mice with growing SW480 xenograft; thereafter, tumor growth was monitored for additional 3 weeks without silibinin treatment. In the second protocol, silibinin treatment was started after 25 days of SW480 cells injection (established tumors), and tumor growth was studied 4 days, 8 days and 16 days after silibinin treatment.

RESULTS

In both treatment protocols, silibinin had strong and sustained inhibitory effect on xenograft growth. Detailed xenograft analyses showed that silibinin, in both treatment protocols, exerts anti-proliferative, pro-apoptotic and anti-angiogenic effects. Further, silibinin reduced the expression of β-catenin and phospho-GSK3β in xenograft tissues. Silibinin also targeted signaling molecules involved in CRC proliferation and survival (cyclin D1, c-Myc and survivin) as well as angiogenesis regulators (VEGF and iNOS).

CONCLUSIONS

Collectively, these findings substantiate silibinin's therapeutic efficacy against CRC, advocating its translational potential.

Epigenomic alterations and gene expression profiles in respiratory epithelia exposed to cigarette smoke condensate

Limited information is available regarding epigenomic events mediating initiation and progression of tobacco-induced lung cancers. In this study, we established an in vitro system to examine epigenomic effects of cigarette smoke in respiratory epithelia. Normal human small airway epithelial cells and cdk-4/hTERT-immortalized human bronchial epithelial cells (HBEC) were cultured in normal media with or without cigarette smoke condensate (CSC) for up to 9 months under potentially relevant exposure conditions. Western blot analysis showed that CSC mediated dose- and time-dependent diminution of H4K16Ac and H4K20Me3, while increasing relative levels of H3K27Me3; these histone alterations coincided with decreased DNA methyltransferase 1 (DNMT1) and increased DNMT3b expression. Pyrosequencing and quantitative RT-PCR experiments revealed time-dependent hypomethylation of D4Z4, NBL2, and LINE-1 repetitive DNA sequences; up-regulation of H19, IGF2, MAGE-A1, and MAGE-A3; activation of Wnt signaling; and hypermethylation of tumor suppressor genes such as RASSF1A and RAR-beta, which are frequently silenced in human lung cancers. Array-based DNA methylation profiling identified additional novel DNA methylation targets in soft-agar clones derived from CSC-exposed HBEC; a CSC gene expression signature was also identified in these cells. Progressive genomic hypomethylation and locoregional DNA hypermethylation induced by CSC coincided with a dramatic increase in soft-agar clonogenicity. Collectively, these data indicate that cigarette smoke induces 'cancer-associated' epigenomic alterations in cultured respiratory epithelia. This in vitro model may prove useful for delineating early epigenetic mechanisms regulating gene expression during pulmonary carcinogenesis.

Cooperative signaling between Wnt1 and integrin-linked kinase induces accelerated breast tumor development

Introduction

Breast cancer is genetically and clinically a heterogeneous disease. However, the exact contribution of different cell types and oncogenic mutations to this heterogeneity are not well understood. Recently, we discovered an interaction between Wnt and integrin-linked kinase (ILK) within the signaling cascade that regulates cell growth and survival. Interestingly, mammary-specific expression of either one of these proteins has been shown to promote mammary tumorigenesis. In light of our recent findings and to investigate the potential interaction between Wnt and ILK proteins during mammary tumor formation and progression, we established a transgenic mouse model that expresses both Wnt and ILK in mammary epithelial cells.

Methods

A novel transgenic mouse model with mammary-specific expression of both Wnt1 and ILK was generated by crossing the two previously characterized mouse models, MMTV-Wnt1 and MMTV-ILK. The resulting MMTV-Wnt/ILK mice were closely monitored for tumor development and growth, as well as for the tumor onset. The molecular phenotypes of both tumors and premalignant mammary glands were investigated by using biochemical and global gene-expression analysis approaches.

Results

A significant acceleration in mammary tumor incidence and growth was observed in the MMTV-Wnt/ILK mice. Pre-neoplastic mammary glands also display lobuloalveolar hyperplasia and an increase in ductal epithelium proliferation. Apart from elevated expression of Wnt/ILK targets, such as β-catenin and cyclin D1, gene-expression profiling identified the surprising activation of the FOXA1 transcription factor. Upregulation of FOXA1, which is also known as the molecular marker of differentiated mammary luminal cells, was consistent with the expansion of the enriched luminal progenitor population or CD29^loCD24^hiCD61⁺ cells in MMTV-Wnt/ILK tumors.

Conclusions

These results show cooperation between Wnt1 and ILK transgenes during mammary carcinogenesis, leading to changes in a transcriptional network, which could dictate a specific breast cancer phenotype with enhanced growth dynamics. The MMTV-Wnt/ILK can be used as a model to identify further the genes downstream of the estrogen receptor-β/FOXA1 and to investigate the mechanisms targeting the expansion of the luminal progenitor cells leading to hyperplasia and tumorigenesis.

beta-Catenin regulates intercellular signalling networks and cell-type specific transcription in the developing mouse midbrain-rhombomere 1 region

β-catenin is a multifunctional protein involved in both signalling by secreted factors of Wnt family and regulation of the cellular architecture. We show that β-catenin stabilization in mouse midbrain-rhombomere1 region leads to robust up-regulation of several Wnt signalling target genes, including Fgf8. Suggestive of direct transcriptional regulation of the Fgf8 gene, β-catenin stabilization resulted in Fgf8 up-regulation also in other tissues, specifically in the ventral limb ectoderm. Interestingly, stabilization of β-catenin rapidly caused down-regulation of the expression of Wnt1 itself, suggesting a negative feedback loop. The changes in signal molecule expression were concomitant with deregulation of anterior-posterior and dorso-ventral patterning. The transcriptional regulatory functions of β-catenin were confirmed by β-catenin loss-of-function experiments. Temporally controlled inactivation of β-catenin revealed a cell-autonomous role for β-catenin in the maintenance of cell-type specific gene expression in the progenitors of midbrain dopaminergic neurons. These results highlight the role of β-catenin in establishment of neuroectodermal signalling centers, promoting region-specific gene expression and regulation of cell fate determination.

Plasminogen activator inhibitor-1 is a transcriptional target of the canonical pathway of Wnt/beta-catenin signaling

Plasminogen activator inhibitor-1 (PAI-1) is a multifunctional glycoprotein that plays a critical role in the pathogenesis of chronic kidney and cardiovascular diseases. Although transforming growth factor (TGF)-beta1 is a known inducer of PAI-1, how it controls PAI-1 expression remains enigmatic. Here we investigated the mechanism underlying TGF-beta1 regulation of PAI-1 in kidney tubular epithelial cells (HKC-8). Surprisingly, overexpression of Smad2 or Smad3 in HKC-8 cells blocked PAI-1 induction by TGF-beta1, whereas knockdown of them sensitized the cells to TGF-beta1 stimulation, suggesting that Smad signaling is not responsible for PAI-1 induction. Blockade of several TGF-beta1 downstream pathways such as p38 MAPK or JNK, but not phosphatidylinositol 3-kinase/Akt and ERK1/2, only partially inhibited PAI-1 expression. TGF-beta1 stimulated beta-catenin activation in tubular epithelial cells, and ectopic expression of beta-catenin induced PAI-1 expression, whereas inhibition of beta-catenin abolished its induction. A functional T cell factor/lymphoid enhancer-binding factor-binding site was identified in the promoter region of the PAI-1 gene, which interacted with T cell factor upon beta-catenin activation. Deletion or site-directed mutation of this site abolished PAI-1 response to beta-catenin or TGF-beta1 stimulation. Similarly, ectopic expression of Wnt1 also activated PAI-1 expression and promoter activity. In vivo, PAI-1 was induced in kidney tubular epithelia in obstructive nephropathy. Delivery of Wnt1 gene activated beta-catenin and promoted PAI-1 expression after obstructive injury, whereas blockade of Wnt/beta-catenin signaling by Dickkopf-1 gene inhibited PAI-1 induction. Collectively, these studies identify PAI-1 as a direct downstream target of Wnt/beta-catenin signaling and demonstrate that PAI-1 induction could play a role in mediating the fibrogenic action of this signaling.

Increased sclerostin serum levels associated with bone formation and resorption markers in patients with immobilization-induced bone loss

CONTEXT

Sclerostin, a Wnt signaling antagonist on the osteoblasts produced by osteocytes, is regulated by mechanical strain and is implicated in the pathogenesis of disuse bone loss. There are no data on sclerostin in humans.

OBJECTIVE

The aim of the study was to evaluate sclerostin in patients immobilized after stroke, compared with control subjects, and to analyze its relationship with markers of bone formation and resorption.

DESIGN

This was a cross-sectional study.

SETTING AND PATIENTS

We studied 40 postmenopausal women immobilized after a single episode of stroke 6 months or longer after onset, and 40 postmenopausal women from the general community. Bone status was assessed by quantitative ultrasound measurements at the calcaneus. Bone alkaline phosphatase (b-AP), carboxy-terminal telopeptide of type I collagen (CrossLaps), and sclerostin were evaluated by ELISA. We also used ELISA to measure serum levels of Dickkopf-1, another soluble inhibitor of Wnt/beta-catenin signaling, highly expressed by osteocytes.

RESULTS

Immobilized patients had higher sclerostin serum levels (median 0.975 ng/ml; 25th to 75th percentiles 0.662-1.490) than controls (median 0.300 ng/ml; 25th to 75th percentiles 0.165-0.400: P < 0.0001) and an increased bone turnover with a more significant rise in bone resorption (CrossLaps) than formation (b-AP) markers. Sclerostin correlated negatively with b-AP (r = -0.911; P < 0.0001) and positively with CrossLaps (r = 0.391; P = 0.012). Dickkopf-1 did not significantly differ between the groups. Patients also had quantitative ultrasound measurements index lower than controls (P < 0.001).

CONCLUSIONS

This study shows for the first time that long-term immobilized patients present hypersclerostinemia associated with reduced bone formation, and suggests that sclerostin could be a link between mechanical unloading and disuse osteoporosis in humans.

Single nucleotide polymorphisms in Wnt signaling and cell death pathway genes and susceptibility to colorectal cancer

It is well known that approximately 90% of colorectal cancer (CRC) cases originate from the constitutive activation of the canonical Wnt signaling pathway. There is increasing evidence that genetic variation both in Wnt and apoptotic pathway genes affects CRC susceptibility and progression. This population-based case-control study, including 1795 CRC cases and 1805 controls, investigates the association between common, putative functional polymorphisms in DNFA5, HIF1A, NDRG1, PYGO1, SFRP2, SFRP4, WISP1 and WISP3 genes and CRC risk. We found no evidence for an association between the selected allelic variants and risk of CRC. Subsite analyses, however, revealed a significant association of HIF1A c.*191T>C with rectal cancer risk [odds ratio (OR) = 1.25, 95% confidence interval (CI), 1.03-1.51, P = 0.03] comparing minor allele carriers with major allele homozygotes. In addition, homozygosity for the minor allele of SFRP4 P320T was significantly associated with rectal cancer risk (OR = 1.37, 95% CI, 1.06-1.79, P = 0.02) and early-stage CRC (OR = 1.33, 95% CI, 1.05-1.69, P = 0.02). This study does not support the hypothesis that Wnt signaling- and apoptosis-related polymorphisms contribute to CRC risk. However, our results provide evidence that CRC subsets may be affected. If confirmed, this knowledge may be used to assess individual susceptibility and to target potential measures of cancer prevention.

Delayed dopaminergic neuron differentiation in Lrp6 mutant mice

Wnts are known to bind and activate multiple membrane receptors/coreceptors and to regulate dopaminergic (DA) neuron development and ventral midbrain (VM) morphogenesis. The low density lipoprotein receptor-related protein (Lrp6) is a Wnt co-receptor, yet it remains unclear whether Lrp6 is required for DA neuron development or VM morphogenesis. Lrp6 is expressed ubiquitously in the developing VM. In this study, we show that Lrp6(-/-) mice exhibit normal patterning, proliferation and cell death in the VM, but display a delay in the onset of DA precursor differentiation. A transient 50% reduction in tyrosine hydroxylase-positive DA neurons and in the expression of DA markers such as Nurr1 and Pitx3, as well as a defect in midbrain morphogenesis was detected in the mutant embryos at embryonic day 11.5. Our results, therefore, suggest a role for Lrp6 in the onset of DA neuron development in the VM as well as a role in midbrain morphogenesis.

Crosstalk between Wnt and bone morphogenic protein signaling: a turbulent relationship

The Wnt and the bone morphogenic protein (BMP) pathways are evolutionarily conserved and essentially independent signaling mechanisms, which, however, often regulate similar biological processes. Wnt and BMP signaling are functionally integrated in many biological processes, such as embryonic patterning in Drosophila and vertebrates, formation of kidney, limb, teeth and bones, maintenance of stem cells, and cancer progression. Detailed inspection of regulation in these and other tissues reveals that Wnt and BMP signaling are functionally integrated in four fundamentally different ways. The molecular mechanism evolved to mediate this integration can also be summarized in four different ways. However, a fundamental aspect of functional and mechanistic interaction between these pathways relies on tissue-specific mechanisms, which are often not conserved and cannot be extrapolated to other tissues. Integration of the two pathways contributes toward the sophisticated means necessary for creating the complexity of our bodies and the reliable and healthy function of its tissues and organs.

Fzd3 and Fzd6 deficiency results in a severe midbrain morphogenesis defect

Wnt/beta-catenin signaling controls the proper development of the mid-/hindbrain region (MHR) and of midbrain dopaminergic (mDA) neurons, but the Frizzled (Fzd) receptors transducing these signals are still unknown. Fzd3 is expressed throughout the mouse anterior neural tube, whereas Fzd6 is restricted to the MHR. We show that the MHR is properly established and mDA neurons develop normally in Fzd6(-/-) mutants, but the number of mDA neurons is initially reduced and recovers at later stages in Fzd3(-/-) embryos. Fzd3(-/-); Fzd6(-/-) double mutants exhibit a severe midbrain morphogenesis defect consisting of collapsed brain ventricles, apparent thickening of the neuroepithelium, focal disruption of the ventricular basal lamina and protrusion of individual cells, and increased proliferation at later stages, despite a normal closure of the anterior neural tube and the rescue of the mDA defect in these embryos. Fzd3 and Fzd6 thus control proper midbrain morphogenesis by a yet unknown mechanism in the mouse.

A co-dependent requirement of xBcl9 and Pygopus for embryonic body axis development in Xenopus

The Wnt/beta-catenin transcriptional activation complex requires the adapter protein Pygopus (Pygo), which links the basal transcription machinery to beta-catenin, by its association with legless (Lgs)/ B-cell lymphoma-9 (Bcl9). Pygo was shown to be required for development in vertebrates, but the role of Lgs/Bcl9 is unknown. We identified an amphibian orthologue of Lgs/Bcl9, XBcl9, which interacted biochemically with Xbeta-catenin and XPygo2. The body axis promoting ability of Xbeta-catenin was diminished when residues required for its interaction with XBcl9 were mutated. In blastula embryos, XBcl9 was transiently preferentially expressed in nuclei of dorsoanterior cells and ectopically expressed XBcl9 required XPygo2 to localize to nuclei. Furthermore, while neither XBcl9 nor XPygo2 alone affected development when ectopically expressed, both were required to induce supernumerary axis and dorsal gene activation. Like XPygo2, depletion of maternal XBcl9 alone caused dorsal defects. These results indicated an essential role of the Pygo-Bcl9 duet in vertebrate body axis formation.

Wnt to build a tube: contributions of Wnt signaling to epithelial tubulogenesis

Epithelial tubes are crucial to the function of organ systems including the cardiovascular system, pulmonary system, gastrointestinal tract, reproductive organ systems, excretory system, and auditory system. Using a variety of animal model systems, recent studies have substantiated the role of Wnt signaling via the canonical/beta-catenin-mediated trajectory, the non-canonical Wnt trajectories, or both, in forming epithelial tubular tissues. This review focuses on the involvement of the Wnt pathways in the induction, specification, proliferation, and morphogenesis involved in tubulogenesis within tissues including the lungs, kidneys, ears, mammary glands, gut, and heart. The ultimate goal is to describe the developmental processes forming the various tubulogenic organ systems to determine the relationships between these processes.