Interaction between Wnt and TGF-beta signalling pathways during formation of Spemann's organizer

Modulation of proliferation-specific and differentiation-specific markers in human keratinocytes by SMAD7

We examined the potential role of SMAD7 in human epidermal keratinocyte differentiation. Overexpression of SMAD7 inhibited the activity of the proliferation-specific promoters for the keratin 14 and cdc2 genes and reduced the expression of the mRNA for the proliferation-specific genes cdc2 and E2F1. The ability of SMAD7 to suppress cdc2 promoter activity was lost in transformed keratinocyte cell lines and was mediated by a domain(s) located between aa 195-395 of SMAD7. This domain lies outside the domain required to inhibit TGFbeta1 signaling, suggesting that this activity is mediated by a novel functional domain(s). Examination of AP1, NFkappaB, serum response element, Gli, wnt, and E2F responsive reporters indicated that SMAD7 significantly suppressed the E2F responsive reporter and modestly increased AP1 activity in proliferating keratinocytes. These data suggest that SMAD7 may have a role in TGFbeta-independent signaling events in proliferating/undifferentiated keratinocytes. The effects of SMAD7 in differentiated keratinocytes indicated a more traditional role for SMAD7 as an inhibitor of TGFbeta action. SMAD7 was unable to initiate the expression of differentiation markers but was able to superinduce/derepress differentiation-specific markers and genes in differentiated keratinocytes. This latter role is consistent with the ability of SMAD7 to inhibit TGFbeta-mediated suppression of keratinocyte differentiation and suggest that the opposing actions of SMAD7 and TGFbeta may serve to modulate squamous differentiation.

Convergence of Wnt, beta-catenin, and cadherin pathways

The specification and proper arrangements of new cell types during tissue differentiation require the coordinated regulation of gene expression and precise interactions between neighboring cells. Of the many growth factors involved in these events, Wnts are particularly interesting regulators, because a key component of their signaling pathway, beta-catenin, also functions as a component of the cadherin complex, which controls cell-cell adhesion and influences cell migration. Here, we assemble evidence of possible interrelations between Wnt and other growth factor signaling, beta-catenin functions, and cadherin-mediated adhesion.

Cooperation between TGF-beta and Wnt pathways during chondrocyte and adipocyte differentiation of human marrow stromal cells

Human marrow stromal cells have the potential to differentiate to chondrocytes or adipocytes. We show interactions between TGF-beta and Wnt signaling pathways during stimulation of chondrogenesis and inhibition of adipogenesis. Combining these signals may be useful in marrow stromal cell therapies.

INTRODUCTION

Human bone marrow stromal cells (hMSCs) have the potential to differentiate to lineages of mesenchymal tissues, including cartilage, fat, bone, tendon, and muscle. Agents like transforming growth factor (TGF)-beta promote chondrocyte differentiation at the expense of adipocyte differentiation. In other processes, TGF-beta and Wnt/wingless signaling pathways play major roles in controling certain developmental events and activation of specific target genes. We tested whether these pathways interact during differentiation of chondrocytes and adipocytes in human marrow stromal cells.

MATERIALS AND METHODS

Both a line of human marrow stromal cells (KM101) and freshly isolated hMSCs were studied. Reverse transcriptase-polymerase chain reaction (RT-PCR), Western blot, and macroarrays were used for analysis of the modulation of TGF-beta1 on Wnt signaling-associated genes, chondrocyte differentiation genes, and TGFbeta/bone morphogenetic protein (BMP) signaling-associated genes in KM101 cells. Early passage hMSCs obtained from 42- and 58-year-old women were used for the effects of TGF-beta and/or Wnt (mimicked by LiCl) signals on chondrocyte and adipocyte differentiation in two-dimensional (2-D) cultures, 3-D pellet cultures, and collagen sponges.

RESULTS

As indicated by macroarray, RT-PCR, and Western blot, TGF-beta activated genes in the TGF-beta/Smad pathway, upregulated Wnt2, Wnt4, Wnt5a, Wnt7a, Wnt10a, and Wnt co-receptor LRP5, and increased nuclear accumulation and stability of beta-catenin in KM101 cells. TGF-beta upregulated chondrocyte gene expression in KM101 cells and also stimulated chondrocyte differentiation and inhibited adipocyte differentiation in hMSCs, synergistically with Wnt signal. Finally, hMSCs cultured in 3-D collagen sponges were stimulated by TGF-beta1 to express aggrecan and collagen type II mRNA, whereas expression of lipoprotein lipase was inhibited.

CONCLUSIONS

In summary, TGF-beta stimulated chondrocyte differentiation and inhibited adipocyte differentiation of hMSCs in vitro. The activation of both TGF-beta and Wnt signal pathways by TGF-beta, and synergy between TGF-beta and Wnt signals, supports the view that Wnt-mediated signaling is one of the mechanisms of TGF-beta's effects on chondrocyte and adipocyte differentiation of hMSCs.

TGFbeta3 signaling activates transcription of the LEF1 gene to induce epithelial mesenchymal transformation during mouse palate development

Epithelial mesenchymal transformation (EMT) of the medial edge epithelial (MEE) seam creates palatal confluence. This work aims to elucidate the molecular mechanisms by which TGFβ₃ brings about palatal seam EMT. We collected mRNA for PCR analysis from individual transforming MEE cells by laser microdissection techniques and demonstrated that TGFβ₃ stimulates lymphoid-enhancing factor 1 (LEF1) mRNA synthesis in MEE cells. We show with antisense β-catenin oligonucleotides that up-regulated LEF1 is not activated by β-catenin in palate EMT. We ruled out other TGFβ₃ targets, such as RhoA and MEK1/2 pathways, and we present evidence using dominant-negative Smad4 and dominant-negative LEF1 showing that TGFβ₃ uses Smads both to up-regulate synthesis of LEF1 and to activate LEF1 transcription during induction of palatal EMT. When phospho-Smad2 and Smad4 are present in the nucleus, LEF1 is activated without β-catenin. Our paper is the first to show that the Smad2,4/LEF1 complex replaces β-catenin/LEF1 during activation of EMT in vivo by TGFβ₃.

Dynamic expression of Lef/Tcf family members and β-catenin during chick gastrulation, neurulation, and early limb development

Members of the Lef/Tcf family of HMG-box transcription factors mediate the response to Wnt as part of the canonical Wnt signaling cascade. Positive and negative cofactors, including beta-catenin, CtBP, and Smad3, regulate the activity of Lef/Tcf transcription complexes. Interaction of Lef/Tcfs with beta-catenin results in target gene activation or repression, depending on the context. Here, we report the cloning of a novel chick Tcf-1 splice variant and of a partial cDNA for chick Tcf-3. We describe their expression patterns during early development and have compared them with the expression profiles of Lef-1 and beta-catenin. We found restricted patterns during gastrulation, neurulation, somitogenesis, and early limb development. beta-catenin and Lef/Tcf expression did not always coincide, indicating developmental contexts in which Lef/Tcf proteins may interact with other cofactors and conversely, the areas in which beta-catenin may interact with other coregulators, or be involved in regulating adhesive properties of cells.

Beta-catenin regulates Cripto- and Wnt3-dependent gene expression programs in mouse axis and mesoderm formation

Gene expression profiling of beta-catenin, Cripto and Wnt3 mutant mouse embryos has been used to characterise the genetic networks that regulate early embryonic development. We have defined genes whose expression is regulated by beta-catenin during formation of the anteroposterior axis and the mesoderm, and have identified Cripto, which encodes a Nodal co-receptor, as a primary target of beta-catenin signals both in embryogenesis as well as in colon carcinoma cell lines and tissues. We have also defined groups of genes regulated by Wnt3/beta-catenin signalling during primitive streak and mesoderm formation. Our data assign a key role to beta-catenin upstream of two distinct gene expression programs during anteroposterior axis and mesoderm formation.

Mammary gland development requires syndecan-1 to create a beta-catenin/TCF-responsive mammary epithelial subpopulation

Mice with a null mutation in the cell surface heparan sulfate (HS) proteoglycan, syndecan-1 (Sdc1), develop almost normally, but resist mammary tumor development in response to Wnt-1. Here, we test the hypothesis that Sdc1 promotes Wnt-1-induced tumor development by interacting with the Wnt cell surface signaling complex. Thus, the response of Sdc1-/- mammary epithelial cells (mecs) to the intracellular, activated Wnt signal transducer, DeltaNbeta-catenin, was assayed both in vitro and in vivo, to test whether beta-catenin/TCF transactivation was Sdc1-independent. Surprisingly, we found that the expression of a canonical Wnt pathway reporter, TOP-FLASH, was reduced by 50% in both unstimulated Sdc1-/- mecs and in stimulated cells responding to Wnt1 or DeltaNbeta-catenin. Tumor development in response to DeltaNbeta-catenin was also significantly delayed on a Sdc1-/- background. Furthermore, the average beta-catenin/TCF transactivation per cell was normal in Sdc1-/- mec cultures, but the number of responsive cells was reduced by 50%. Sdc1-/- mecs show compensatory changes that maintain the number of HS chains, hence these experiments cannot test the coreceptor activity of HS for Wnt signaling. We propose that TCF-dependent transactivational activity is suppressed in 50% of cells in Sdc1-/- glands, and conclude that the major effect of Sdc1 does not map to the activity of the Wnt signaling complex, but to another pathway to create or stabilize the beta-catenin/TCF-responsive tumor precursor cells in mouse mammary gland.

XenopusMeis3 protein forms a hindbrain-inducing center by activating FGF/MAP kinase and PCP pathways

Knockdown studies in Xenopus demonstrated that the XMeis3gene is required for proper hindbrain formation. An explant assay was developed to distinguish between autonomous and inductive activities of XMeis3 protein. Animal cap explants caudalized by XMeis3 were recombined with explants neuralized by the BMP dominant-negative receptor protein. XMeis3-expressing cells induced convergent extension cell elongations in juxtaposed neuralized explants. Elongated explants expressed hindbrain and primary neuron markers, and anterior neural marker expression was extinguished. Cell elongation was dependent on FGF/MAP-kinase and Wnt-PCP activities. XMeis3 activates FGF/MAP-kinase signaling, which then modulates the PCP pathway. In this manner, XMeis3 protein establishes a hindbrain-inducing center that determines anteroposterior patterning in the brain.

In vivo functions of catenins

The adhesion of cells to neighbor cells determines cellular and tissue morphogenesis and regulates major cellular processes including motility, growth, survival, and differentiation. Regions of cell-cell adhesion are adherens junctions, desmosomes, and tight junctions. Cadherins are transmembrane molecules whose extracellular domains transmit the direct interaction of two cells. The intracellular cadherin domains bind directly or indirectly to the submembranous catenins, which are linked to the cytoskeleton. Four types of catenins, alpha-catenin, beta-catenin, gamma-catenin, and p120 catenin are known. Three of them, beta-, gamma-, and p120 catenin, are structurally related and possess similar protein interaction domains, the so-called armadillo repeats. These catenins are also parts of signal transduction pathways and play a role in phenotypical changes of cells, e.g., during switches from adherent to migratory cells. The function of catenins in such basic cellular processes also determines a role of catenins in embryogenesis, adult tissue homeostasis, and disease. In particular, beta-catenin is known to be an important oncoprotein in human cancer development.

Notch 1 impairs osteoblastic cell differentiation

Notch receptors are single pass transmembrane receptors activated by membrane-bound ligands with a role in cell proliferation and differentiation. As Notch 1 and 2 mRNAs are expressed by osteoblasts and induced by cortisol, we postulated that Notch could regulate osteoblastogenesis. We investigated the effects of retroviral vectors directing the constitutive expression of the Notch 1 intracellular domain (NotchIC) in murine ST-2 stromal and in MC3T3 cells. NotchIC overexpression was documented by increased Notch 1 transcripts and activity of the Notch-dependent Hairy Enhancer of Split promoter. In the presence of bone morphogenetic protein-2 (BMP-2), ST-2 cells differentiated toward osteoblasts forming mineralized nodules, and Notch 1 opposed this effect and decreased the expression of osteocalcin, type I collagen, and alkaline phosphatase transcripts and Delta2Delta FosB protein. Further, NotchIC decreased Wnt/beta-catenin signaling. As cells differentiated in the presence of BMP-2, they underwent apoptosis, and Notch opposed this event. In the presence of cortisol, NotchIC induced the formation of mature adipocytes and enhanced the effect of cortisol on adipsin, peroxisome proliferator-activated receptor-gamma2 and CCAAT enhancer binding protein alpha and delta mRNA levels. NotchIC also opposed MC3T3 cell differentiation and the expression of a mature osteoblastic phenotype. In conclusion, NotchIC impairs osteoblast differentiation and enhances adipogenesis in stromal cell cultures.

Role of β-catenin in the developing cortical and hippocampal neuroepithelium

beta-Catenin plays a pivotal role in Wnt signaling during embryogenesis and is a component of adherens junctions. Since targeted disruption of the beta-catenin gene is lethal at gastrulation we have used a D6-Cre mouse line for conditional inactivation of beta-catenin in the mouse cerebral cortex and hippocampus after embryonic day (E) 10.5. In D6-Cre floxed beta-catenin mice, hippocampal CA1-CA2 fields are disrupted in similar manner as in Wnt-3a and LEF-1 mutants. The cortex of D6-Cre floxed beta-catenin mutants is strongly affected which contrasts with the normal cortex observed in Wnt-3a and LEF-1 mutants. Severe abnormalities in the organization of the neuroepithelium are observed that include disrupted interkinetic nuclear migration, loss of adherens junctions, impaired radial migration of neurons toward superficial layers and decreased cell proliferation after E15.5. At newborn stage, a premature disassembly of the radial glial scaffold and increased numbers of astrocytes are found in the cortex.

Smad4 and beta-catenin co-activators functionally interact with lymphoid-enhancing factor to regulate graded expression of Msx2

Recent in vivo evidence suggests that Wnt signaling plays a central role in determining the fate of stem cells in the ectoderm and in the neural crest by modulating bone morphogenetic protein (BMP) levels, which, in turn, influence Msx gene expression. However, the molecular mechanism regulating the expression of the Msx genes as key regulators of cell fate has not been elucidated. Here we show in murine embryonic stem cells that BMP-dependent activation of Msx2 is mediated via the cooperative binding of Smad4 at two Smad binding elements and of lymphoid enhancing factor (Lef1) at two Lef1/TCF binding sites. Lef1 can synergize with Smad4 and Smad1 to activate Msx2 promoter, and this transcriptional complex is assembled on the endogenous promoter in response to BMP2. The Wnt/beta-catenin signaling pathway can activate Msx2 via the binding of Lef1 to its promoter and synergizes with BMP2 to activate Msx2 expression, possibly via enhanced recruitment of the p300/cAMP-response element-binding protein-binding protein co-factor. Interestingly, the Wnt/beta-catenin-dependent activation of Msx2 was defective in Smad4-deficient embryonic stem cells or when Smad binding elements were mutated but persisted in the presence of various BMP antagonists, indicating that Smad4 was involved in transducing the Wnt/beta-catenin signals in the absence of a BMP autocrine loop. A chromatin immunoprecipitation analysis revealed that endogenous Smad4, but not Smad1, was part of the Lef1 transcriptional complex in response to beta-catenin activation, dismissing any implication of BMP signaling in this response. We propose that Wnt signaling pathway could dictate cell fate not only by modulating BMP levels but also by directly regulating cooperatively BMP-target genes.

Long-term expansion of human functional epidermal precursor cells: promotion of extensive amplification by low TGF-beta1 concentrations

We have previously introduced the concept of high proliferative potential-quiescent (HPP-Q) cells to refer to primitive human hematopoietic progenitors, on which transforming growth factor-beta1 (TGF-beta1) exerts a pleiotropic effect. TGF-beta1 confers to these slow-dividing cells a mitogenic receptor(low) phenotype and maintains immature properties by preventing differentiation and apoptosis. However, the effect of TGF-beta1 on long-term expansion has not yet been clearly demonstrated. Here, we describe the characterization of a human skin keratinocyte subpopulation, highly enriched for primitive epidermal precursors, on the basis of high adhesion capacity (Adh+++) and low expression of the epidermal growth factor receptor (Adh+++EGF-Rlow). In our standard culture condition without feeder cells, the mean estimated output for cells from an unfractionated population of primary foreskin keratinocytes was 10(7)-10(8), increasing to 10(12)-10(13) in cultures initiated with selected Adh+++EGF-Rlow precursors. Characterization of these cells revealed a hitherto unknown property of TGF-beta1: its addition at a very low concentration (10 pg/ml) in long-term cultures induces a very significant additional increase of expansion. In this optimized system, outputs obtained in cultures initiated with Adh+++EGF-Rlow cells repeatedly reached 10(16)-10(17) ( approximately 60 population doublings, approximately 4 x 10(18) keratinocytes produced per clonogenic cell present in the initial population). At the molecular level, this effect is associated with an increase in Smad1, Smad2 and Smad3 phosphorylation and an increase in alpha6 and beta1 integrin expression. No such effect could be observed on mature keratinocytes with low adhesion capacity (Adh-/+). We finally demonstrated that the progeny of Adh+++EGF-Rlow precursors after long-term expansion is still capable of generating a pluristratified epidermis in a model for skin reconstruction. In conclusion, after further characterizing the phenotype of primitive epidermal precursors, we demonstrated a new function of TGF-beta1, which is to promote undifferentiated keratinocyte amplification.

The molecular phenotype of polycystic ovary syndrome (PCOS) theca cells and new candidate PCOS genes defined by microarray analysis

Polycystic ovary syndrome (PCOS) affects 5% of reproductive aged women and is the leading cause of anovulatory infertility. A hallmark of PCOS is excessive theca cell androgen secretion, which is directly linked to the symptoms of PCOS. Our previous studies demonstrated that theca cells from PCOS ovaries maintained in long term culture persistently secrete significantly greater amounts of androgens than normal theca cells, suggesting an intrinsic abnormality. Furthermore, previous studies suggested that ovarian hyperandrogenemia is inherited as an autosomal dominant trait. However, the genes responsible for ovarian hyperandrogenemia of PCOS have not been identified. In this present study, we carried out microarray analysis to define the gene networks involved in excess androgen synthesis by the PCOS theca cells in order to identify candidate PCOS genes. Our analysis revealed that PCOS theca cells have a gene expression profile that is distinct from normal theca cells. Included in the cohort of genes with increased mRNA abundance in PCOS theca cells were aldehyde dehydrogenase 6 and retinol dehydrogenase 2, which play a role in all-trans-retinoic acid biosynthesis and the transcription factor GATA6. We demonstrated that retinoic acid and GATA6 increased the expression of 17alpha-hydroxylase, providing a functional link between altered gene expression and intrinsic abnormalities in PCOS theca cells. Thus, our analyses have 1) defined a stable molecular phenotype of PCOS theca cells, 2) suggested new mechanisms for excess androgen synthesis by PCOS theca cells, and 3) identified new candidate genes that may be involved in the genetic etiology of PCOS.

New intracellular components of bone morphogenetic protein/Smad signaling cascades

Bone morphogenetic proteins (BMPs) regulate many processes in the embryo, including cell type specification, patterning, apoptosis, and epithelial-mesenchymal interaction. They also act in soft and hard tissues in adult life. Their signals are transduced from the plasma membrane to the nucleus through a limited number of Smad proteins. The list of Smad-interacting proteins is however growing and it is clear that these partners determine the outcome of the signal. We summarize the present status in BMP/Smad signaling, with emphasis on recently identified Smad partners and how these proteins may cooperate in the regulation of the expression of BMP target genes.

TGF-beta1-mediated alterations of renal proximal tubular epithelial cell phenotype

The aim of this study was to characterize the mechanism of transforming growth factor (TGF)-beta1-mediated alteration of renal proximal tubular cell phenotype. TGF-beta1 altered cell phenotype, with cells appearing elongated and spindle shaped. This was associated with loss of cell-cell contact and rearrangement of the actin cytoskeleton, increased formation of stress fibers, and focal adhesions. Addition of the tyrosine phosphatase inhibitor sodium orthovanadate also led to rapid but transient loss of cell-cell contact, but it did not lead to a change of phenotype comparable to that seen following addition of TGF-beta1. There was, however, no change in the formation of focal adhesions and no associated reorganization of the Factin cytoskeleton. Disruption of the actin cytoskeleton with cytochalasin D prevented phenotypic alterations following addition of TGF-beta1. Transient transfection with Smad2/4 or Smad3/4 expression vectors did not alter cell phenotype. Previously, we demonstrated beta-catenin translocation to proximal tubule cell nuclei and its association with Smad proteins following addition of TGF-beta1, suggesting the possibility that TGF-beta1 may modulate Wnt signaling. The Wnt-responsive Xtwn-reporter construct was, however, silent in response to TGF-beta1. Similarly, a second Wnt/LEF-1-regulated element, Toplflash, which does not contain Smad binding sites, was insensitive to TGF-beta1 signaling. In contrast, phenotypic changes in response to TGF-beta1 were abrogated by inhibitors of the RhoA downstream target ROCK, which also prevented loss of cell-cell contact and adherens junction disassembly.

Role of tissue stroma in cancer cell invasion

Maintenance of epithelial tissues needs the stroma. When the epithelium changes, the stroma inevitably follows. In cancer, changes in the stroma drive invasion and metastasis, the hallmarks of malignancy. Stromal changes at the invasion front include the appearance of myofibroblasts, cells sharing characteristics with fibroblasts and smooth muscle cells. The main precursors of myofibroblasts are fibroblasts. The transdifferentiation of fibroblasts into myofibroblasts is modulated by cancer cell-derived cytokines, such as transforming growth factor-beta (TGF-beta). TGF-beta causes cancer progression through paracrine and autocrine effects. Paracrine effects of TGF-beta implicate stimulation of angiogenesis, escape from immunosurveillance and recruitment of myofibroblasts. Autocrine effects of TGF-beta in cancer cells with a functional TGF-beta receptor complex may be caused by a convergence between TGF-beta signalling and beta-catenin or activating Ras mutations. Experimental and clinical observations indicate that myofibroblasts produce pro-invasive signals. Such signals may also be implicated in cancer pain. N-Cadherin and its soluble form act as invasion-promoters. N-Cadherin is expressed in invasive cancer cells and in host cells such as myofibroblasts, neurons, smooth muscle cells, and endothelial cells. N-Cadherin-dependent heterotypic contacts may promote matrix invasion, perineural invasion, muscular invasion, and transendothelial migration; the extracellular, the juxtamembrane and the beta-catenin binding domain of N-cadherin are implicated in positive invasion signalling pathways. A better understanding of stromal contributions to cancer progression will likely increase our awareness of the importance of the combinatorial signals that support and promote growth, dedifferentiation, invasion, and ectopic survival and eventually result in the identification of new therapeutics targeting the stroma.

Elevated SMAD1/beta-catenin molecular complexes and renal medullary cystic dysplasia in ALK3 transgenic mice

Renal dysplasia, the most frequent cause of childhood renal failure in humans, arises from perturbations in a complex series of morphogenetic events during embryonic renal development. The molecular pathogenesis of renal dysplasia is largely undefined. While investigating the role of a BMP-dependent pathway that inhibits branching morphogenesis in vitro, we generated a novel model of renal dysplasia in a transgenic (Tg) model of ALK3 receptor signaling. We report the renal phenotype, and our discovery of molecular interactions between effectors in the BMP and WNT signaling pathways in dysplastic kidney tissue. Expression of the constitutively active ALK3 receptor ALK3(QD), in two independent transgenic lines caused renal aplasia/severe dysgenesis in 1.5% and 8.4% of hemizygous and homozygous Tg mice, respectively, and renal medullary cystic dysplasia in 49% and 74% of hemizygous and homozygous Tg mice, respectively. The dysplastic phenotype, which included a decreased number of medullary collecting ducts, increased medullary mesenchyme, collecting duct cysts and decreased cortical thickness, was apparent by E18.5. We investigated the pathogenesis of dysplasia in these mice, and demonstrated a 30% decrease in branching morphogenesis at E13.5 before the appearance of histopathogical features of dysplasia, and the formation of beta-catenin/SMAD1/SMAD4 molecular complexes in dysplastic renal tissue. Increased transcriptional activity of a beta-catenin reporter gene in ALK3(QD);Tcf-gal mice demonstrated functional cooperativity between the ALK3 and beta-catenin-dependent signaling pathways in kidney tissue. Together with our results in the dysplastic mouse kidney, our findings that phospho-SMAD1 and beta-catenin are overexpressed in human fetal dysplastic renal tissue suggest that dysregulation of these signaling effectors is pathogenic in human renal dysplasia. Our work provides novel insights into the role that crucial developmental signaling pathways may play during the genesis of malformed renal tissue elements.

Axin and the Axin/Arrow-binding protein DCAP mediate glucose-glycogen metabolism

Axin was found as a negative regulator of the canonical Wnt pathway. Human LRP5 was originally found as a candidate gene of insulin dependent diabetes mellitus (IDDM), but its Drosophila homolog, Arrow, works as a co-receptor of the canonical Wnt signal. In our previous paper, we found a new Drosophila Axin (Daxin)-binding SH3 protein, DCAP, a homolog of mammalian CAV family protein. Among the subtypes, DCAPL3 shows significant homology with CAP, an essential component of glucose transport in insulin signal. Further binding assay revealed that DCAP binds to not only Axin but also Arrow, and Axin binds to not only GSK3beta but also Arrow. However, overexpression and RNAi experiments of DCAP do not affect the canonical Wnt pathway. As DCAP is expressed predominantly in insulin-target organs, and as RNAi of DCAP disrupts the pattern of endogenous glycogen accumulation in late stage embryos, we suggest that DCAP is also involved in glucose transport. Moreover, early stage embryos lacking maternal Axin show significant delay of initial glycogen decomposition, and RNAi of Axin in S2 cells revealed quite increase of endogenous glycogen level as well as GSK3beta. These results suggest that Axin and DCAP mediate glucose-glycogen metabolism in embryo. In addition, the interaction among Axin, Arrow, and DCAP implies a possible cross-talk between Wnt signal and insulin signal.

Two tcf3 genes cooperate to pattern the zebrafish brain

Caudalizing factors operate in the context of Wnt/beta-catenin signaling to induce gene expression in discrete compartments along the rostral-caudal axis of the developing vertebrate nervous system. In zebrafish, basal repression of caudal genes is achieved through the function of Headless (Hdl), a Tcf3 homolog. In this study, we show that a second Tcf3 homolog, Tcf3b, limits caudalization caused by loss of Hdl function and although this Lef/Tcf family member can rescue hdl mutants, Lef1 cannot. Wnts can antagonize repression mediated by Tcf3 and this derepression is dependent on a Tcf3 beta-catenin binding domain. Systematic changes in gene expression caused by reduced Tcf3 function help predict the shape of a caudalizing activity gradient that defines compartments along the rostral-caudal axis. In addition, Tcf3b has a second and unique role in the morphogenesis of rhombomere boundaries, indicating that it controls multiple aspects of brain development.

TGF-beta and the Smad signal transduction pathway

Transforming growth factor beta (TGF-beta) superfamily members are important regulators of many diverse developmental and homeostatic processes and disruption of their activity has been implicated in a variety of human diseases ranging from cancer to chondrodysplasias and pulmonary hypertension. TGF-beta family members signal through transmembrane Ser-Thr kinase receptors that directly regulate the intracellular Smad pathway. Smads are a unique family of signal transduction molecules that can transmit signals directly from the cell surface receptors to the nucleus, where they regulate transcription by interacting with DNA binding partners as well as transcriptional coactivators and corepressors. In addition, more recent evidence indicates that Smads can also function both as substrates and adaptors for ubiquitin protein ligases, which mediate the targeted destruction of intracellular proteins. Smads have thus emerged as multifunctional transmitters of TGF-beta family signals that play critical roles in the development and homeostasis of metazoans.

Negative regulation of beta-catenin signaling by tyrosine phosphatase SHP-1 in intestinal epithelial cells

Protein-tyrosine phosphatase SHP-1 is expressed at high levels in hematopoietic cells and at moderate levels in many other cell types including epithelial cells. Although SHP-1 has been shown to be a negative regulator of multiple signaling pathways in hematopoietic cells, very little is known about the biological role of SHP-1 in epithelial cells. In order to elucidate the mechanism(s) responsible for the loss of proliferative potential once committed intestinal epithelial cells begin to differentiate, the role and regulation of SHP-1 were analyzed in both intact epithelium as well as in well established intestinal cell models recapitulating the crypt-villus axis in vitro. Results show that SHP-1 was expressed in the nuclei of all intestinal epithelial cell models as well as in epithelial cells of intact human fetal jejunum and colon. Expression and phosphatase activity levels of SHP-1 were much more elevated in confluent growth-arrested intestinal epithelial cells and in differentiated enterocytes as well. Overexpression of SHP-1 in intestinal epithelial crypt cells significantly inhibited dhfr, c-myc, and cyclin D1 gene expression but did not interfere with c-fos gene expression. In contrast, a mutated inactive form of SHP-1 had no effect on these genes. SHP-1 expression significantly decreased beta-catenin/TCF-dependent transcription in intestinal epithelial crypt cells. Immunoprecipitation experiments revealed that beta-catenin is one of the main binding partners and a substrate for SHP-1. Taken together, our results indicate that SHP-1 may be involved in the regulation of beta-catenin transcriptional function and in the negative control of intestinal epithelial cell proliferation.

Bone morphogenetic proteins, their antagonists, and the skeleton

Skeletal homeostasis is determined by systemic hormones and local factors. Bone morphogenetic proteins (BMP) are unique because they induce the differentiation of mesenchymal cells toward cells of the osteoblastic lineage and also enhance the differentiated function of the osteoblast. However, the activity of BMPs needs to be tempered by intracellular and extracellular antagonists. BMPs bind to specific receptors and signal by phosphorylating the cytoplasmic proteins mothers against decapentaplegic (Smad) 1 and 5, which form heterodimers with Smad 4, and after nuclear translocation regulate transcription. BMP antagonists can be categorized as pseudoreceptors that compete with signaling receptors, inhibitory Smads that block signaling, intracellular binding proteins that bind Smad 1 and 5, and factors that induce ubiquitination and proteolysis of signaling Smads. In addition, a large number of extracellular proteins that bind BMPs and prevent their binding to signaling receptors have emerged. They are the components of the Spemann organizer, noggin, chordin, and follistatin, members of the Dan/Cerberus family, and twisted gastrulation. The antagonists tend to be specific for BMPs and are regulated by BMPs, indicating the existence and need of local feedback mechanisms to temper BMP cellular activities.

Dax-1 (dosage-sensitive sex reversal-adrenal hypoplasia congenita critical region on the X chromosome, gene 1) gene transcription is regulated by wnt4 in the female developing gonad

Dax-1 [dosage-sensitive sex reversal-adrenal hypoplasia congenita critical region on the X chromosome, gene 1 (NR0B1)] is an orphan nuclear receptor acting as a suppressor of Ad4 binding protein/steroidogenic factor 1 [Ad4BP/SF-1 (NR5A1)] and as an anti-Sry factor in the process of gonadal sex differentiation. The roles of these nuclear receptors in the differentiation of the gonads and the adrenal cortex have been established through studies of the mutant phenotype in both mice and humans. However, the mechanisms underlying transcriptional regulation of these genes remain largely unknown. Here, we examined the relationship between Dax-1 gene transcription and the Wnt4 pathway. Reporter gene analysis revealed that Dax-1 gene transcription was activated by beta-catenin, a key signal-transducing protein in the Wnt pathway, acting in synergy with Ad4BP/SF-1. Interaction between beta-catenin and Ad4BP/SF-1 was observed using yeast two-hybrid and in vitro pull-down assays. The region of Ad4BP/SF-1 essential for this interaction consists of an acidic amino acid cluster, which resides in the first helix of the ligand-binding domain. Mutation of the amino acid cluster impaired transcriptional activation of Dax-1 as well as interaction of Ad4BP/SF-1 with beta-catenin. These results were supported by in vivo observations using Wnt4 gene-disrupted mice, in which Dax-1 gene expression was decreased significantly in sexually differentiating female gonads. We thus conclude that Wnt4 signaling mediates the increased expression of Dax-1 as the ovary becomes sexually differentiated.

Mapping Wnt/beta-catenin signaling during mouse development and in colorectal tumors

Wntbeta-catenin signaling plays key roles in several developmental and pathological processes. Domains of Wnt expression have been extensively investigated in the mouse, but the tissues receiving the signal remain largely unidentified. To define which cells respond to activated beta-catenin during mammalian development, we generated the beta-catenin-activated transgene driving expression of nuclear beta-galactosidase reporter (BAT-gal) transgenic mice, expressing the lacZ gene under the control of beta-cateninT cell factor responsive elements. Reporter gene activity is found in known organizing centers, such as the midhindbrain border and the limb apical ectodermal ridge. Moreover, BAT-gal expression identifies novel sites of Wnt signaling, like notochord, endothelia, and areas of the adult brain, revealing an unsuspected dynamic pattern of beta-catenin transcriptional activity. Expression of the transgene was analyzed in mutant backgrounds. In lipoprotein receptor-related protein 6-null homozygous mice, which lack a Wnt coreceptor, BAT-gal staining is absent in mutant tissues, indicating that BAT-gal mice are bona fide in vivo indicators of Wntbeta-catenin signaling. Analyses of BAT-gal expression in the adenomatous polyposis coli (multiple intestinal neoplasia+) background revealed betacatenin transcriptional activity in intestinal adenomas but surprisingly not in normal crypt cells. In summary, BAT-gal mice unveil the entire complexity of Wntbeta-catenin signaling in mammals and have broad application potentials for the identification of Wnt-responsive cell populations in development and disease.

A beta-catenin survival signal is required for normal lobular development in the mammary gland

The Wnt (wingless) family of secreted glycoproteins initiates a signalling pathway implicated in the regulation of both normal mouse mammary gland development and tumorigenesis. Multiple Wnt signals ultimately converge on the multifunctional protein beta-catenin to activate the transcription of target genes. Although beta-catenin plays a crucial role in canonical Wnt signalling, it also functions in epithelial cell-cell adhesion at the adherens junctions. This study was designed to isolate beta-catenin's signalling function from its role in adherence during mouse mammary gland development. A transgenic dominant-negative beta-catenin chimera (beta-eng), which retains normal protein-binding properties of wild-type beta-catenin but lacks its C-terminal signalling domain, was expressed preferentially in the mammary gland. Thus, beta-eng inhibits the signalling capacity of endogenous beta-catenin, while preserving normal cell-cell adhesion properties. Analysis of the mammary gland in transgenic mice revealed a severe inhibition of lobuloalveolar development and a failure of the mice to nurse their young. Expression of beta-eng resulted in an induction of apoptosis both in transgenic mice and in retrovirally transduced HC11 cells. Thus, endogenous beta-catenin expression appears to be required to provide a survival signal in mammary epithelial cells, which can be suppressed by transgenic expression of beta-eng. Comparison of the timing of transgene expression with the transgenic phenotype suggested a model in which beta-catenin's survival signal is required in lobular progenitors that later differentiate into lobuloalveolar clusters. This study illustrates the importance of beta-catenin signalling in mammary lobuloalveolar development.

Identification of a promoter-specific transcriptional activation domain at the C terminus of the Wnt effector protein T-cell factor 4

Wnt growth factors control numerous cell fate decisions in development by altering specific gene expression patterns through the activity of heterodimeric transcriptional activators. These consist of beta-catenin and one of the four members of the T-cell factor (TCF) family of DNA-binding proteins. How can the Wnt/beta-catenin pathway control various sets of target genes in distinct cellular settings with such a limited number of nuclear effectors? Here we asked whether different TCF proteins could perform specific, nonredundant functions at natural beta-catenin/TCF-regulated promoters. We found that TCF4E but not LEF1 supported beta-catenin-dependent activation of the Cdx1 promoter, whereas LEF1 specifically activated the Siamois promoter. Deletion of a C-terminal domain of TCF4E prevented Cdx1 promoter induction. A chimeric protein consisting of LEF1 and the C terminus of TCF4E was fully functional. Therefore, the TCF4E C terminus harbors a promoter-specific transactivation domain. This domain influences the DNA binding properties of TCF4 and additionally mediates an interaction with the transcriptional coactivator p300. Apparently, the C terminus of TCF4E cooperates with beta-catenin and p300 to form a specialized transcription factor complex that specifically supports the activation of the Cdx1 promoter.

Global expression profiling of fibroblast responses to transforming growth factor-beta1 reveals the induction of inhibitor of differentiation-1 and provides evidence of smooth muscle cell phenotypic switching

Transforming growth factor-beta1 (TGF-beta1) plays a central role in promoting extracellular matrix protein deposition by promoting the transformation of fibroblasts to myofibroblasts. To gain new insights into the transcriptional programs involved, we profiled human fetal lung fibroblast global gene expression in response to TGF-beta1 up to 24 hours using oligonucleotide microarrays. In this report, we present data for 146 genes that were up-regulated at least twofold at two time points. These genes group into several major functional categories, including genes involved in cytoskeletal reorganization (n = 30), matrix formation (n = 25), metabolism and protein biosynthesis (n = 27), cell signaling (n = 21), proliferation and survival (n = 13), gene transcription (n = 9), and of uncertain function (n = 21). For 80 of these genes, this is the first report that they are TGF-beta1-responsive. The early induction of two members of the inhibitor of differentiation (ID) family of transcriptional regulators, ID1 and ID3, was followed by the up-regulation of a number of genes that are usually expressed by highly differentiated smooth muscle cells, including smooth muscle myosin heavy chain, basic calponin, and smoothelin. These findings were confirmed at the protein level for primary adult lung fibroblasts. ID1 further behaved like a typical immediate-early gene and, unlike ID3, was expressed and induced at the protein level. Immunohistochemical analysis showed that ID1 was highly expressed by (myo)fibroblasts within fibrotic foci in experimentally induced pulmonary fibrosis. ID1 acts as a dominant-negative antagonist of basic helix-loop-helix transcription factors that drive cell lineage commitment and differentiation. These findings have important implications for our understanding of fibroblast transcriptional programming in response to TGF-beta1 during development, oncogenesis, tissue repair, and fibrosis.

IKKalpha regulates mitogenic signaling through transcriptional induction of cyclin D1 via Tcf

The Wnt/beta-catenin/Tcf and IkappaB/NF-kappaB cascades are independent pathways involved in cell cycle control, cellular differentiation, and inflammation. Constitutive Wnt/beta-catenin signaling occurs in certain cancers from mutation of components of the pathway and from activating growth factor receptors, including RON and MET. The resulting accumulation of cytoplasmic and nuclear beta-catenin interacts with the Tcf/LEF transcription factors to induce target genes. The IkappaB kinase complex (IKK) that phosphorylates IkappaB contains IKKalpha, IKKbeta, and IKKgamma. Here we show that the cyclin D1 gene functions as a point of convergence between the Wnt/beta-catenin and IkappaB pathways in mitogenic signaling. Mitogenic induction of G(1)-S phase progression and cyclin D1 expression was PI3K dependent, and cyclin D1(-/-) cells showed reduced PI3K-dependent S-phase entry. PI3K-dependent induction of cyclin D1 was blocked by inhibitors of PI3K/Akt/IkappaB/IKKalpha or beta-catenin signaling. A single Tcf site in the cyclin D1 promoter was required for induction by PI3K or IKKalpha. In IKKalpha(-/-) cells, mitogen-induced DNA synthesis, and expression of Tcf-responsive genes was reduced. Reintroduction of IKKalpha restored normal mitogen induction of cyclin D1 through a Tcf site. In IKKalpha(-/-) cells, beta-catenin phosphorylation was decreased and purified IKKalpha was sufficient for phosphorylation of beta-catenin through its N-terminus in vitro. Because IKKalpha but not IKKbeta induced cyclin D1 expression through Tcf activity, these studies indicate that the relative levels of IKKalpha and IKKbeta may alter their substrate and signaling specificities to regulate mitogen-induced DNA synthesis through distinct mechanisms.

Activated beta-catenin induces osteoblast differentiation of C3H10T1/2 cells and participates in BMP2 mediated signal transduction

Wnt glycoproteins are important regulators of cellular differentiation and embryonic development. Some Wnt proteins induce stabilization of beta-catenin which cooperatively regulates gene expression with LEF/Tcf transcription factors. Here we demonstrate a direct role for beta-catenin signaling in osteoblast differentiation and in BMP2-mediated signal transduction. Similar to treatment with BMP-2 protein, ectopic expression of stabilized beta-catenin in C3H10T1/2 cells or activation of endogenous beta-catenin signaling with LiCl induces expression of alkaline phosphatase mRNA and protein, a defined marker of early osteoblast differentiation. Unlike BMP2 protein, stabilized beta-catenin does not induce osteocalcin gene expression, a marker of late osteoblast differentiation. BMP2-induced differentiation also leads to activation of endogenous beta-catenin signaling thus implicating beta-catenin in early steps of BMP2-mediated osteoblast differentiation. Effects of beta-catenin and BMP2 on C3H10T1/2 differentiation are not completely overlapping, implying that some aspects of BMP2-induced differentiation may be mediated by beta-catenin signaling and that beta-catenin can also participate in non-BMP2-dependent differentiation processes.

Synaptogenesis: Wnt and TGF-beta take centre stage

The formation of functional synapses requires precise coordination between neurons and their synaptic targets. Recent studies have identified two signaling molecules, Wnt and TGF-beta, which are required for formation and growth of the neuromuscular synapse.

Nodal signaling in Xenopus gastrulae is cell-autonomous and patterned by beta-catenin

The classical three-signal model of amphibian mesoderm induction and more recent modifications together propose that an activin-like signaling activity is uniformly distributed across the vegetal half of the Xenopus blastula and that this activity contributes to mesoderm induction. In support of this, we have previously shown that the activin-response element (DE) of the goosecoid promoter is uniformly activated across the vegetal half of midgastrula-stage embryos. Here, we further examine the nature of this activity by measuring DE activation by endogenous signals over time. We find that the spatiotemporal pattern of DE activation is much more dynamic than was previously appreciated and also conclude that DE(6X)Luc activity reflects endogenous nodal signaling in the embryo. Using both the DE(6X)Luc construct and endogenous Xbra and Xgsc expression as read-outs for nodal activity, and the cleavage-mutant version of Xnr2 (CmXnr2) to regionally suppress endogenous nodal activity, we demonstrate that nodal signals act cell-autonomously in Xenopus gastrulae. Nodal-expressing cells are unable to rescue either reporter gene activation or target gene expression in distant nodal-deficient cells, suggesting that nodals function at short range in this context. Finally, we show that DE activation by endogenous signals occurs in the absence of dorsal beta-catenin-mediated signaling, but that the timing of dorsal initiation is altered. We conclude that nodal signals in Xenopus gastrulae function cell autonomously at short ranges and that the spatiotemporal pattern of this signaling along the dorsoventral axis is regulated by maternal Wnt-like signaling.

Defects of blastogenesis

Ever more frequent and closer involvement by clinical geneticists and counselors in the prenatal assessment of development mandates a better understanding of all stages of human ontogeny, but especially those of earliest development during which most of the lethal and all of the gross, multiple and complex defects of morphogenesis arise. Because of the phenomenon of universality, i.e., identical molecular inductive mechanisms involved in the process of embryonic pattern formation in all vertebrates, experimental animals indeed are a most valuable approach to an understanding of the causal and formal aspects of development and are beginning to forge essential, strong bonds between molecular biologists and clinicians in a mutually supportive discipline of developmental biology. However, to grieving parents of a stillborn fetus with, say, Pentalogy of Cantrell, sirenomelia or otocephaly, mouse data offer little comfort or reassurance about recurrence; thus, it is imperative to make ever more effective a science of human teratology (sensu lato) with participating reproductive geneticists, obstetricians, neonatologists, ultrasonographers, pediatric/fetal pathologists, cytogeneticists and pediatric geneticists to generate the diagnostic, pathogenetic and causal data necessary to counsel and to comfort the parents. Few molecular data exist on causes of blastogenetic defects in humans; however, the phenomenon of parsimony, whereby the same "morphogenetic" molecule, say, sonic hedgehog (SHH), is "deployed" simultaneously or sequentially during the morphogenesis (and even the histogenesis) of several/many embryonic primordia, makes it likely that a genetic/epigenetic disturbance of such an inductive system will have multiple effects on blastogenetic, organogenetic and perhaps also histogenetic events in the embryo. If causally defined, such a pattern of anomalies constitutes pleiotropy, and the embryo/fetus can be said to have a syndrome. If cause is unknown, the presumption of pleiotropy is less certain, and the fetus/infant may be said to have an "association" with low empiric recurrence risk.

Two major Smad pathways in TGF-beta superfamily signalling

Members of the transforming growth factor-beta (TGF-beta) superfamily bind to two different serine/threonine kinase receptors, i.e. type I and type II receptors. Upon ligand binding, type I receptors specifically activate intracellular Smad proteins. R-Smads are direct substrates of type I receptors; Smads 2 and 3 are specifically activated by activin/nodal and TGF-beta type I receptors, whereas Smads 1, 5 and 8 are activated by BMP type I receptors. Nearly 30 proteins have been identified as members of the TGF-beta superfamily in mammals, and can be classified based on whether they activate activin/TGF-beta-specific R-Smads (AR-Smads) or BMP-specific R-Smads (BR-Smads). R-Smads form complexes with Co-Smads and translocate into the nucleus, where they regulate the transcription of target genes. AR-Smads bind to various proteins, including transcription factors and transcriptional co-activators or co-repressors, whereas BR-Smads interact with other proteins less efficiently than AR-Smads. Id proteins are induced by BR-Smads, and play important roles in exhibiting some biological effects of BMPs. Understanding the mechanisms of TGF-beta superfamily signalling is thus important for the development of new ways to treat various clinical diseases in which TGF-beta superfamily signalling is involved.

Beta-catenin/Tcf-regulated transcription prior to the midblastula transition

Following fertilization, the zygotic genome in many organisms is quiescent until the midblastula transition (MBT), when large-scale transcription begins. In Xenopus embryos, for example, transcription is believed to be repressed until the twelfth cell division. Thus, although dorsal-ventral patterning begins during the first cell cycle, little attention has been given to transcriptional regulation in pre-MBT development. We present evidence that regulated transcription begins during early cleavage stages and that the beta-catenin-Tcf complex is required for the transcription of the Xenopus nodal genes Xnr5 and Xnr6 as early as the 256-cell stage. Moreover, inhibition of beta-catenin/Tcf function can block dorsal development, but only if the inhibition begins early and is maintained throughout pre-MBT stages. Dorsal development can be rescued in ventralized embryos if Tcf-dependent transcription is activated prior to MBT, but activation of Tcf after MBT cannot rescue ventralized embryos, suggesting that beta-catenin/Tcf-dependent transcription is required prior to MBT for dorsal-ventral patterning in Xenopus.

The nodal target gene Xmenf is a component of an FGF-independent pathway of ventral mesoderm induction in Xenopus

The interplay of fibroblast growth factor (FGF) and nodal signaling in the Xenopus gastrula marginal zone specifies distinct populations of presumptive mesodermal cells. Cells in the vegetal marginal zone, making up the presumptive leading edge mesoderm, are exposed to nodal signaling, as evidenced by SMAD2 activation, but do not appear to be exposed to FGF signaling, as evidenced by the lack of MAP kinase (MAPK) activation. However, in the animal marginal zone, activation of both SMAD2 and MAPK occurs. The differential activation of these two signaling pathways in the marginal zone results in the vegetal and animal marginal zones expressing different genes at gastrulation, and subsequently having different fates, with the vegetal marginal zone contributing to ventral mesoderm (e.g. ventral blood island) and the animal marginal zone giving rise to dorsal fates (e.g. notochord and somite). We report here the cloning of a cDNA encoding a novel nuclear protein, Xmenf, that is expressed in the vegetal marginal zone. The expression of Xmenf is induced by nodal signaling and negatively regulated by FGF signaling. Results from animal cap studies indicate that Xmenf plays a role in the pathway of ventral mesoderm induction in the vegetal marginal zone.

The roles of three signaling pathways in the formation and function of the Spemann Organizer

Since the three main pathways (the Wnt, VegT and BMP pathways) involved in organizer and axis formation in the Xenopus embryo are now characterized, the challenge is to understand their interactions. Here three comparisons were made. Firstly, we made a systematic comparison of the expression of zygotic genes in sibling wild-type, VegT-depleted (VegT–), β-catenin-depleted (β-catenin–) and double depleted (VegT–/β-catenin–) embryos and placed early zygotic genes into specific groups. In the first group some organizer genes, including chordin, noggin and cerberus, required the activity of both the Wnt pathway and the VegT pathway to be expressed. A second group including Xnr1, 2, 4 and Xlim1 were initiated by the VegT pathway but their dorsoventral pattern and amount of their expression was regulated by the Wnt pathway. Secondly, we compared the roles of the Wnt and VegT pathways in producing dorsal signals. Explant co-culture experiments showed that the Wnt pathway did not cause the release of a dorsal signal from the vegetal mass independent from the VegT pathway. Finally we compared the extent to which inhibiting Smad 1 phosphorylation in one area of VegT–, or β-catenin– embryos would rescue organizer and axis formation. We found that BMP inhibition with cm-BMP7 mRNA had no rescuing effects on VegT– embryos, while cm-BMP7 and noggin mRNA caused a complete rescue of the trunk, but not of the anterior pattern in β-catenin– embryos.

Repression of organizer genes in dorsal and ventral Xenopus cells mediated by maternal XTcf3

In the early Xenopus embryo, the dorsal axis is specified by a Wnt signal transduction pathway, involving the movement of β-catenin into dorsal cell nuclei and its functional association with the LEF-type transcription factor XTcf3. The subsequent function of XTcf3 is uncertain. Overexpression data has suggested that it can be both an activator and repressor of downstream genes. XTcf3 mRNA is synthesized during oogenesis in Xenopus and is stored in the egg. To identify its role in dorsal axis specification, we depleted this maternal store in full-grown oocytes using antisense deoxyoligonucleotides, and fertilized them. The developmental effects of XTcf3 depletion, both on morphogenesis and the expression of marker genes, show that primarily, XTcf3 is an inhibitor, preventing both dorsal and ventral cells of the late blastula from expressing dorsal genes. We also show that simple relief from the repression is not the only factor required for dorsal gene expression. To demonstrate this, we fertilized eggs that had been depleted of both XTcf3 and the maternal transcription factor VegT. Dorsal genes normally repressed by XTcf3 are not activated in these embryos. These data show that normal dorsal gene expression in the embryo requires the transcriptional activator VegT, whilst XTcf3 prevents their inappropriate expression on the ventral side of the embryo.

Wnt-3A enhances bone morphogenetic protein-2-mediated chondrogenesis of murine C3H10T1/2 mesenchymal cells

We have recently reported the chondrogenic effect of bone morphogenetic protein-2 (BMP-2) in high density cultures of the mouse multipotent mesenchymal C3H10T1/2 cell line and have shown the functional requirement of the cell-cell adhesion molecule N-cadherin in BMP-2-induced chondrogenesis in vitro (Denker, A. E., Nicoll, S. B., and Tuan, R. S. (1995) Differentiation 59, 25-34; Haas, A. R., and Tuan, R. S. (1999) Differentiation 64, 77-89). Furthermore, BMP-2 treatment also results in an increased protein level of beta-catenin, a known N-cadherin-associated Wnt signal transducer (Fischer, L., Haas, A., and Tuan, R. S. (2001) Signal Transduction 2, 66-78), suggesting functional cross-talk between the BMP-2 and Wnt signaling pathways. We have observed previously that BMP-2 treatment up-regulates expression of Wnt-3A in high density cultures of C3H10T1/2 cells. To assess the contribution of Wnt-3A to BMP-2-mediated chondrogenesis, we have generated C3H10T1/2 cell lines overexpressing Wnt-3A and various forms of glycogen synthase kinase-3beta (GSK-3beta), an immediate cytosolic component of the Wnt signaling pathway, and examined their response to BMP-2. We show that overexpression of either Wnt-3A or kinase-dead GSK-3beta enhances BMP-2-mediated chondrogenesis. Furthermore, Wnt-3A overexpression results in decreases in both N-cadherin and GSK-3beta protein levels, whereas Wnt-3A as well as kinase-dead GSK-3beta overexpression increase total and nuclear levels of both beta-catenin and LEF-1. Direct cross-talk between Wnts and BMP-2 was also indicated by the up-regulated interaction between beta-catenin and SMAD-4 in response to BMP-2. These results suggest that Wnt-3A acts in a manner opposite to that of other Wnts, such as Wnt-7A, which were previously identified as inhibitory to chondrogenesis, and is the first BMP-2-regulated, chondrogenesis-enhancing member of the Wnt family.

Decreased Smad4 expression in the transforming growth factor-beta signaling pathway during progression of esophageal squamous cell carcinoma

BACKGROUND

Transforming growth factor-beta (TGF-beta) has antiproliferative effects in various cells, and inactivation of the TGF-beta signaling pathway contributes to tumor progression or development. Smad4, a tumor suppressor gene, is a central mediator in the signaling pathways of the TGF-beta superfamily. This study was undertaken to clarify the correlation between Smad4 expression and the clinicopathologic characteristics of patients with esophageal squamous cell carcinoma (SCC). The authors also investigated the expression of components of the TGF-beta signaling pathway in seven established cell lines derived from esophageal SCC.

METHODS

Immunohistochemistry for Smad4 using monoclonal anti-Smad4 antibody was performed on surgical specimens obtained from 80 patients with esophageal SCC. In seven cell lines, the authors examined the expression of components of the TGF-beta signaling pathway using Western and Northern blot analyses.

RESULTS

There was a significant inverse correlation between Smad4 expression and both depth of invasion (P = 0.0008) and pathologic stage (P = 0.0079). The expression of Smad4 proteins could be detected in five of seven cell lines. The expression of TGF-beta type II receptor protein was decreased in two of seven cell lines, and the expression of both Smad2 and Smad3 proteins was decreased in only one cell line. The level of expression of Smad4 mRNA did not differ dramatically between cell lines and was not correlated with the quantity of Smad4 protein.

CONCLUSIONS

In this study, the expression of Smad4 protein appeared to be correlated with the depth of invasion of esophageal SCC. The loss of Smad4 expression was not regulated at the level of transcription.

Combinatorial signaling by an unconventional Wg pathway and the Dpp pathway requires Nejire (CBP/p300) to regulate dpp expression in posterior tracheal branches

The decapentaplegic (dpp) gene influences many developmental events in Drosophila melanogaster. We have been analyzing dpp expression in two groups of dorsal ectoderm cells at the posterior end of the embryo, in abdominal segment 8 and the telson. These dpp-expressing cells become tracheal cells in the posterior-most branches of the tracheal system (Dorsal Branch10, Spiracular Branch10, and the Posterior Spiracle). These branches are not identified by reagents typically used in analyses of tracheal development, suggesting that dpp expression confers a distinct identity upon posterior tracheal cells. We have determined that dpp posterior ectoderm expression begins during germ band extension and continues throughout development. We have isolated the sequences responsible for these aspects of dpp expression in a reporter gene. We have determined that an unconventional form of Wingless (Wg) signaling, Dpp signaling, and the transcriptional coactivator Nejire (CBP/p300) are required for the initiation and maintenance of dpp expression in the posterior-most branches of the tracheal system. Our data suggest a model for the integration of Wg and Dpp signals that may be applicable to branching morphogenesis in other developmental systems.

Acetylation of beta-catenin by CREB-binding protein (CBP)

Acetylation controls the activity of numerous proteins involved in regulating gene transcription as well as many other cellular processes. In this report we show that the CREB-binding protein (CBP) acetyltransferase acetylates beta-catenin protein in vivo. beta-Catenin is a central component of the Wnt signaling pathway, which is of key importance in development as well as being heavily implicated in a variety of human cancers. We show that the CBP-mediated acetylation of beta-catenin occurs at a single site, lysine 49. Importantly, this lysine is frequently found mutated in cancer and is in a region of importance to the regulation of beta-catenin. We show that mutation of this site leads specifically to an increase in the ability of beta-catenin to activate the c-myc gene but not other beta-catenin-regulated genes. This suggests that acetylation of beta-catenin is involved in regulating Wnt signaling in a promoter-specific fashion.

Carcinogen-induced pancreatic lesions in the mouse: effect of Smad4 and Apc genotypes

Mutations in the tumour suppressor genes SMAD4 (DPC4, deleted in pancreatic cancer locus 4) and adenomatous polyposis coli (APC) have been implicated in the development of pancreatic cancer in humans. Treatment of wild-type, Smad4(+/-), Apc(Min/+) or Apc(Min/+)Smad4(+/-) mice with N-Nitroso-N-Methyl Urea (NMU) results in abnormal foci in pancreatic acinar cells characterized by increased levels of beta-catenin. Previously such foci have been shown to be the precursors of pancreatic neoplasia. Interestingly, only NMU-treated Apc(Min/+)Smad4(+/-) mice exhibit a significant increase in abnormal pancreas, which was found to be due to increased number of abnormal foci rather than increased focus size. A range of foci sizes were analysed, but only smaller abnormal foci were characterized by morphological nuclear atypia. These studies suggest functional co-operation between TGF-beta and Wnt signalling pathways in the suppression of pancreatic tumorigenesis in the mouse.

Wnt and Bmp signalling cooperatively regulate graded Emx2 expression in the dorsal telencephalon

Pattern formation of the dorsal telencephalon is governed by a regionalisation process that leads to the formation of distinct domains, including the future hippocampus and neocortex. Recent studies have implicated signalling proteins of the Wnt and Bmp gene families as well as several transcription factors, including Gli3 and the Emx homeobox genes, in the molecular control of this process. The regulatory relationships between these genes, however, remain largely unknown. We have used transgenic analysis to investigate the upstream mechanisms for regulation of Emx2 in the dorsal telencephalon. We have identified an enhancer from the mouse Emx2 gene that drives specific expression of a lacZ reporter gene in the dorsal telencephalon. This element contains binding sites for Tcf and Smad proteins, transcriptional mediators of the Wnt and Bmp signalling pathway, respectively. Mutations of these binding sites abolish telencephalic enhancer activity, while ectopic expression of these signalling pathways leads to ectopic activation of the enhancer. These results establish Emx2 as a direct transcriptional target of Wnt and Bmp signalling and provide insights into a genetic hierarchy involving Gli3, Emx2 and Bmp and Wnt genes in the control of dorsal telencephalic development.

Mechanisms balancing skeletal matrix synthesis and degradation

Bone is regulated by evolutionarily conserved signals that balance continuous differentiation of bone matrix-producing cells against apoptosis and matrix removal. This is continued from embryogenesis, where the skeleton differentiates as a solid mass and is shaped into separate bones by cell death and proteolysis. The two major tissues of the skeleton are avascular cartilage, with an extracellular matrix based on type II collagen and hydrophilic proteoglycans, and bone, a stronger and lighter material based on oriented type I collagen and hydroxyapatite. Both differentiate from the same mesenchymal stem cells. This differentiation is regulated by a family of related signals centred on bone morphogenic proteins. Fibroblast growth factors, Indian hedgehog and parathyroid hormone-related protein are important in determining the type of matrix and the relation of skeletal and non-skeletal structures. Removal of mineralized matrix involves apoptosis of matrix cells and differentiation of acid-secreting cells (osteoclasts) from macrophage precursors. Key regulators of matrix removal are signals in the tumour-necrosis-factor family. Osteoclasts dissolve bone by isolating a region of the matrix and secreting HCl and proteinases at that site. Successive cycles of removal and replacement allow growth, repair and remodelling. The signals for bone turnover are predominantly cell-membrane-associated, allowing very specific spatial regulation. In addition to its support function, bone is a reservoir of Ca2+, PO3-(4) and OH-. Secondary modulation of mineral secretion and bone degradation are mediated by humoral signals, including parathyroid hormone and vitamin D, as well as the cytokines that also regulate the underlying cell differentiation.

An analysis using the hobo genetic system reveals that combinatorial signaling by the Dpp and Wg pathways regulates dpp expression in leading edge cells of the dorsal ectoderm in Drosophila melanogaster

Our laboratory has contributed to the development of a genetic system based upon the hobo transposable element in Drosophila melanogaster. We recently reported that hobo, like the better-known P element, is capable of local transposition. In that study, we mobilized a hobo enhancer trap vector and generated two unique alleles of decapentaplegic (dpp), a transforming growth factor-beta family member with numerous roles during development. Here we report a detailed study of one of those alleles (dpp(F11)). To our knowledge, this is the first application of the hobo genetic system to understanding developmental processes. First, we demonstrate that lacZ expression from the dpp(F11) enhancer trap accurately reflects dpp mRNA accumulation in leading edge cells of the dorsal ectoderm. Then we show that combinatorial signaling by the Wingless (Wg) pathway, the Dpp pathway, and the transcriptional coactivator Nejire (CBP/p300) regulates dpp(F11) expression in these cells. Our analysis of dpp(F11) suggests a model for the integration of Wg and Dpp signals that may be applicable to other developmental systems. Our analysis also illustrates several new features of the hobo genetic system and highlights the value of hobo, as an alternative to P, in addressing developmental questions.