Propagation and localization of Wnt signaling

The functional activity of E-cadherin controls tumor cell metastasis at multiple steps

E-cadherin is a tumor suppressor protein, and the loss of its expression in association with the epithelial mesenchymal transition (EMT) occurs frequently during tumor metastasis. However, many metastases continue to express E-cadherin, and a full EMT is not always necessary for metastasis; also, positive roles for E-cadherin expression in metastasis have been reported. We hypothesize instead that changes in the functional activity of E-cadherin expressed on tumor cells in response to environmental factors is an important determinant of the ability of the tumor cells to metastasize. We find that E-cadherin expression persists in metastatic lung nodules and circulating tumor cells (CTCs) in two mouse models of mammary cancer: genetically modified MMTV-PyMT mice and orthotopically grafted 4T1 tumor cells. Importantly, monoclonal antibodies that bind to and activate E-cadherin at the cell surface reduce lung metastasis from endogenous genetically driven tumors and from tumor cell grafts. E-cadherin activation inhibits metastasis at multiple stages, including the accumulation of CTCs from the primary tumor and the extravasation of tumor cells from the vasculature. These activating mAbs increase cell adhesion and reduce cell invasion and migration in both cell culture and three-dimensional spheroids grown from primary tumors. Moreover, activating mAbs increased the frequency of apoptotic cells without affecting proliferation. Although the growth of the primary tumors was unaffected by activating mAbs, CTCs and tumor cells in metastatic nodules exhibited increased apoptosis. Thus, the functional state of E-cadherin is an important determinant of metastatic potential beyond whether the gene is expressed.

Sclerostin inhibition promotes TNF-dependent inflammatory joint destruction

Sclerostin, an inhibitor of the Wnt/β-catenin pathway, has anti-anabolic effects on bone formation by negatively regulating osteoblast differentiation. Mutations in the human sclerostin gene (SOST) lead to sclerosteosis with progressive skeletal overgrowth, whereas sclerostin-deficient (Sost(-/-)) mice exhibit increased bone mass and strength. Therefore, antibody-mediated inhibition of sclerostin is currently being clinically evaluated for the treatment of postmenopausal osteoporosis in humans. We report that in chronic TNFα (tumor necrosis factor α)-dependent arthritis, fibroblast-like synoviocytes constitute a major source of sclerostin and that either the lack of sclerostin or its antibody-mediated inhibition leads to an acceleration of rheumatoid arthritis (RA)-like disease in human TNFα transgenic (hTNFtg) mice with enhanced pannus formation and joint destruction. Inhibition of sclerostin also failed to improve clinical signs and joint destruction in the partially TNFα-dependent glucose-6-phosphate isomerase-induced arthritis mouse model, but ameliorated disease severity in K/BxN serum transfer-induced arthritis mouse model, which is independent of TNF receptor signaling, thus suggesting a specific role for sclerostin in TNFα signaling. Sclerostin effectively blocked TNFα- but not interleukin-1-induced activation of p38, a key step in arthritis development, pointing to a previously unrealized protective role of sclerostin in TNF-mediated chronic inflammation. The possibility of anti-sclerostin antibody treatment worsening clinical RA outcome under chronic TNFα-dependent inflammatory conditions in mice means that caution should be taken both when considering such treatment for inflammatory bone loss in RA and when using anti-sclerostin antibodies in patients with TNFα-dependent comorbidities.

Wnt signaling in mammary glands: plastic cell fates and combinatorial signaling

The mouse mammary gland is an outstanding developmental model that exemplifies the activities of many of the effector pathways known to organize mammalian morphogenesis; furthermore, there are well-characterized methods for the specific genetic manipulation of various mammary epithelial cell components. Among these signaling pathways, Wnt signaling has been shown to generate plasticity of fate determination, expanding the genetic programs available to cells in the mammary lineage. It is responsible first for the appearance of the mammary fate in embryonic ectoderm and then for maintaining bi-potential basal stem cells in adult mammary ductal trees. Recent technical developments have led to the separate analysis of various mammary epithelial cell subpopulations, spurring the investigation of Wnt-dependent interactions. Although Wnt signaling was shown to be oncogenic for mouse mammary epithelium even before being identified as the principle oncogenic driver for gut epithelium, conclusive data implicating this pathway as a tumor driver for breast cancer lag behind, and we examine potential reasons.

Notch inhibits osteoblast differentiation and causes osteopenia

Notch receptors are determinants of cell fate decisions. To define the role of Notch in the adult skeleton, we created transgenic mice overexpressing the Notch intracellular domain (NICD) under the control of the type I collagen promoter. First-generation transgenics were small and osteopenic. Bone histomorphometry revealed that NICD caused a decrease in bone volume, secondary to a reduction in trabecular number; osteoblast and osteoclast number were decreased. Low fertility of founder mice and lethality of young pups did not allow the complete establishment of transgenic lines. To characterize the effect of Notch overexpression in vitro, NICD was induced in osteoblasts and stromal cells from Rosa(notch) mice, in which a STOP cassette flanked by lox(P) sites is upstream of NICD, by transduction with an adenoviral vector expressing Cre recombinase (Cre) under the control of the cytomegalovirus (CMV) promoter (Ad-CMV-Cre). NICD impaired osteoblastogenesis and inhibited Wnt/beta-catenin signaling. To determine the effects of notch1 deletion in vivo, mice in which notch1 was flanked by lox(P) sequences (notch1(loxP/loxP)) were mated with mice expressing Cre recombinase under the control of the osteocalcin promoter. Conditional null notch1 mice had no obvious skeletal phenotype, possibly because of rescue by notch2; however, 1-month-old females exhibited a modest increase in osteoclast surface and eroded surface. Osteoblasts from notch1(loxP/loxP) mice, transduced with Ad-CMV-Cre and transfected with Notch2 small interfering RNA, displayed increased alkaline phosphatase activity. In conclusion, Notch signaling in osteoblasts causes osteopenia and impairs osteo-blastogenesis by inhibiting the Wnt/beta-catenin pathway.

Notch signaling in osteoblasts

Bone remodeling is the result of the coordinated activity of osteoblasts, which form new matrix, and osteoclasts, which resorb bone. Notch proteins are single-pass transmembrane receptors that determine cell fate. Recent gain-of-function and loss-of-function experiments reveal a suppressive effect of Notch in osteoblast and osteoclast differentiation in development and in the postnatal bone, which establishes a role for Notch signaling in bone remodeling.

β-catenin relieves I-mfa-mediated suppression of LEF-1 in mammalian cells

We have previously shown that β-catenin interacts with a transcription suppressor I-mfa and, through this interaction, canonical Wnt signaling could relieve I-mfa-mediated suppression of myogenic regulatory factors (MRFs). In this study, we found that, based on this interaction, I-mfa-mediated suppression of the Wnt transcription factor T-cell factor/lymphoid enhancing factor-1 (TCF/LEF-1) can also be relieved. Our work showed that knocking down endogenous I-mfa expression mimics canonical Wnt treatment by inducing myogenesis and increasing Wnt reporter gene activity, endogenous Wnt target gene expression and expression of MRFs in P19 cells. More importantly, these I-mfa small interfering RNA (siRNA)-induced effects could be blocked by a dominant-negative mutant of LEF-1, confirming the involvement of the TCF/LEF-1 pathway. In addition, we found that β-catenin could compete with I-mfa for binding to LEF-1 and relieve the inhibitory effects of I-mfa in overexpression systems. Furthermore, canonical Wnt was able to reduce the levels of endogenous I-mfa associated with LEF-1, while increasing that of I-mfa associated with β-catenin. All of the evidence supports a conclusion that I-mfa can suppress myogenesis by inhibiting TCF/LEF-1 and that canonical Wnt signaling may relieve the suppression through elevating β-catenin levels, which in turn relieve I-mfa-mediated suppression.

Role of the RAM domain and ankyrin repeats on notch signaling and activity in cells of osteoblastic lineage

Notch proteins belong to a family of single pass transmembrane receptors that are activated after interactions with the membrane-bound ligands Delta and Jagged/Serrate. We determined the pathways responsible for the inhibitory effects of Notch on osteoblastogenesis and the contributions of the RAM domain and ankyrin repeats to this process in cells of the osteoblastic lineage.

INTRODUCTION

Notch receptors play a role in osteoblast differentiation. Activation of Notch results in its cleavage and the release of its intracellular domain (NICD), which interacts with the CBF1/RBP-Jkappa, Suppressor of Hairless, Lag-1 (CSL) family of transcription factors. The interaction is presumably mediated by the RBP-Jkappa-associated module (RAM) of NICD, although the role of the ankyrin repeats is uncertain.

MATERIALS AND METHODS

To determine the contributions of the RAM domain and ankyrin repeats to the inhibitory effects of Notch on osteoblastogenesis, ST-2 and MC3T3-E1 cells were transfected or transduced with vectors expressing NICD, RAM (NICD DeltaRAM), and ankyrin (NICD DeltaANK) deletion mutants.

RESULTS

Notch increased the transactivation of transiently transfected 12xCSL-Luc constructs, containing 12 repeats of an RBP-Jkappa/CSL binding site, and of the hairy and E (spl) (HES)-1 promoter. Deletion of the ankyrin repeats resulted in the loss of 12xCSL-Luc and HES-1 promoter transactivation, whereas deletion of the RAM domain caused a partial loss of 12xCSL-Luc and sustained HES-1 promoter transactivation. NICD overexpression inhibited osteocalcin mRNA levels and alkaline phosphatase activity in ST-2 cells, and deletion of the ankyrin repeats, and to a lesser extent of the RAM domain, resulted in loss of the NICD inhibitory effect. NICD inhibited Wnt signaling and deletion of ankyrin repeats or the RAM domain restored Wnt signaling activity.

CONCLUSIONS

The RAM domain and ankyrin repeats are required for Notch signaling and activity, and the CSL pathway is central to the inhibitory effect of Notch on osteoblastogenesis.

Notch 1 overexpression inhibits osteoblastogenesis by suppressing Wnt/beta-catenin but not bone morphogenetic protein signaling

Notch proteins are transmembrane receptors that control cell-fate decisions. Upon ligand binding, Notch receptors undergo proteolytic cleavage leading to the release of their intracellular domain (NICD). Overexpression of NICD impairs osteoblastogenesis, but the mechanisms are not understood. We examined consequences of the constitutive activation of Notch 1 in ST-2 cells. Notch opposed the effects of bone morphogenetic protein (BMP)-2 and Wnt 3a on alkaline phosphatase activity (APA). BMP-2 induced the phosphorylation of Smad 1/5/8 and the transactivation of a BMP/Smad-responsive construct (12xSBE-Oc-pGL3), but the effect was not modified by Notch. BMP-2 had minimal effects on the phosphorylation of the mitogen-activated protein kinases ERK, p38, and JNK, in the absence or presence of NICD. Notch overexpression decreased the transactivating effect of Wnt 3a, cytoplasmic beta-catenin levels, and Wnt-dependent gene expression. Transfection of a mutant beta-catenin expression construct, or the use of a glycogen synthase kinase 3beta inhibitor to stabilize beta-catenin, partially blocked the inhibitory effect of NICD on Wnt signaling and on APA. HES-1 or Groucho1/TLE1 RNA interference enhanced basal and induced Wnt/beta-catenin signaling opposing NICD effects, but only HES-1 silencing enhanced Wnt 3a effects on APA. In conclusion, NICD overexpression prevents BMP-2 and Wnt biological effects by suppressing Wnt but not BMP signaling. HES-1 appears to mediate effects of Notch on osteoblastogenesis.

Beta-catenin regulates myogenesis by relieving I-mfa-mediated suppression of myogenic regulatory factors in P19 cells

Wnt/beta-catenin signaling plays a critical role in embryonic myogenesis. Here we show that, in P19 embryonic carcinoma stem cells, Wnt/beta-catenin signaling initiates the myogenic process depends on beta-catenin-mediated relief of I-mfa (inhibitor of MyoD Family a) suppression of myogenic regulatory factors (MRFs). We found that beta-catenin interacted with I-mfa and that the interaction was enhanced by Wnt3a. In addition, we found that the interaction between beta-catenin and I-mfa was able to attenuate the interaction of I-mfa with MRFs, relieve I-mfa-mediated suppression of the transcriptional activity and cytosolic sequestration of MRFs, and initiate myogenesis in a P19 myogenic model system that expresses exogenous myogenin. This work reveals a mechanism for the regulation of MRFs during myogenesis by elucidating a beta-catenin-mediated, but lymphoid enhancing factor-1/T cell factor independent, mechanism in regulation of myogenic fate specification and differentiation of P19 mouse stem cells.

Dkk2 has a role in terminal osteoblast differentiation and mineralized matrix formation

Human and mouse genetic and in vitro evidence has shown that canonical Wnt signaling promotes bone formation, but we found that mice lacking the canonical Wnt antagonist Dickkopf2 (Dkk2) were osteopenic. We reaffirmed the finding that canonical Wnt signaling stimulates osteogenesis, including the differentiation from preosteoblasts to osteoblasts, in cultured osteoblast differentiation models, but we also found that canonical Wnts upregulated the expression of Dkk2 in osteoblasts. Although exogenous overexpression of Dkk before the expression of endogenous canonical Wnt (Wnt7b) suppressed osteogenesis in cultures, its expression after peak Wnt7b expression induced a phenotype resembling terminal osteoblast differentiation leading to mineralization. In addition, osteoblasts from Dkk2-null mice were poorly mineralized upon osteogenic induction in cultures, and Dkk2 deficiency led to attenuation of the expression of osteogenic markers, which could be partially reversed by exogenous expression of Dkk2. Taken together with the finding that Dkk2-null mice have increased numbers of osteoids, these data indicate that Dkk2 has a role in late stages of osteoblast differentiation into mineralized matrices. Because expression of another Wnt antagonist, FRP3, differs from Dkk2 expression in rescuing Dkk2 deficiency and regulating osteoblast differentiation, the effects of Dkk2 on terminal osteoblast differentiation may not be entirely mediated by its Wnt signaling antagonistic activity.

Sclerostin binds to LRP5/6 and antagonizes canonical Wnt signaling

The loss of the SOST gene product sclerostin leads to sclerosteosis characterized by high bone mass. In this report, we found that sclerostin could antagonize canonical Wnt signaling in human embryonic kidney A293T cells and mouse osteoblastic MC3T3 cells. This sclerostin-mediated antagonism could be reversed by overexpression of Wnt co-receptor low density lipoprotein receptor-related protein (LRP) 5. In addition, we found that sclerostin bound to LRP5 as well as LRP6 and identified the first two YWTD-EGF repeat domains of LRP5 as being responsible for the binding. Although these two repeat domains are required for transduction of canonical Wnt signals, canonical Wnt did not appear to compete with sclerostin for binding to LRP5. Examination of the expression of sclerostin and Wnt7b, an autocrine canonical Wnt, during primary calvarial osteoblast differentiation revealed that sclerostin is expressed at late stages of osteoblast differentiation coinciding with the expression of osteogenic marker osteocalcin and trailing after the expression of Wnt7b. Given the plethora of evidence indicating that canonical Wnt signaling stimulates osteogenesis, we believe that the high bone mass phenotype associated with the loss of sclerostin may be attributed, at least in part, to an increase in canonical Wnt signaling resulting from the reduction in sclerostin-mediated Wnt antagonism.

Role of the intracellular domains of LRP5 and LRP6 in activating the Wnt canonical pathway

LDL-receptor related proteins 5 and 6 (LRP5/6) are co-receptors of Frizzled receptors that mediate Wnt-induced activation of the transcription factor family TCF/LEF-1. Even though LRP5 and LRP6 are highly homologous, LRP6, but not LRP5, is expressed primarily in the nervous system and deletion of the LRP6 gene results in significant brain abnormalities, while deletion of LRP5 results in primarily decreased bone density. Additionally, the exact function of LRP5 and LRP6 have not been clearly defined, although it is clear that they both play key roles in the Wnt canonical pathway. In this study the role of the intracellular domains of LRP5/6 in mediating Wnt signaling was examined. In the absence of exogenous Wnt 3a, full-length (FL) LRP6, but not LRP5, increased TCF/LEF-1 transcriptional activity, however both significantly potentiated Wnt 3a-induced TCF/LEF-1 activation. In contrast to the findings with the FL constructs, the intracellular domains (membrane-anchored and cytosolic) of both LRP5 and LRP6 significantly increased TCF/LEF-1 activation in the absence of Wnt 3a, and potentiated the Wnt 3a-induced decrease in beta-catenin phosphorylation, increase in free beta-catenin levels and the increase in TCF/LEF-1 activity. These findings demonstrate that: (1) LRP5 and LRP6 differentially modulate TCF/LEF-1 activation in the absence of Wnt 3a and (2) the intracellular C-terminal domains of LRP5/6 potentiate Wnt 3a-induced TCF/LEF-1 activation whether or not they are membrane-anchored. These findings provide significant new insights into the roles of LRP5/6 in modulating canonical Wnt signaling.

The LRP5 high-bone-mass G171V mutation disrupts LRP5 interaction with Mesd

The mechanism by which the high-bone-mass (HBM) mutation (G171V) of the Wnt coreceptor LRP5 regulates canonical Wnt signaling was investigated. The mutation was previously shown to reduce DKK1-mediated antagonism, suggesting that the first YWTD repeat domain where G171 is located may be responsible for DKK-mediated antagonism. However, we found that the third YWTD repeat, but not the first repeat domain, is required for DKK1-mediated antagonism. Instead, we found that the G171V mutation disrupted the interaction of LRP5 with Mesd, a chaperone protein for LRP5/6 that is required for transport of the coreceptors to cell surfaces, resulting in fewer LRP5 molecules on the cell surface. Although the reduction in the number of cell surface LRP5 molecules led to a reduction in Wnt signaling in a paracrine paradigm, the mutation did not appear to affect the activity of coexpressed Wnt in an autocrine paradigm. Together with the observation that osteoblast cells produce autocrine canonical Wnt, Wnt7b, and that osteocytes produce paracrine DKK1, we think that the G171V mutation may cause an increase in Wnt activity in osteoblasts by reducing the number of targets for paracrine DKK1 to antagonize without affecting the activity of autocrine Wnt.

Glucocorticoids control beta-catenin protein expression and localization through distinct pathways that can be uncoupled by disruption of signaling events required for tight junction formation in rat mammary epithelial tumor cells

In Con8 rat mammary epithelial tumor cells, the synthetic glucocorticoid dexamethasone stimulates the remodeling of tight junctions and adherens junctions before formation of highly sealed tight junctions. In this study, the expression and localization of key components of the apical junction were examined as potential targets of glucocorticoid signaling. Western blot and RT-PCR demonstrated that dexamethasone up-regulated beta-catenin protein and transcript expression and nearly ablated beta-catenin phosphorylation under conditions that led to a significant increase in monolayer transepithelial resistance. Indirect immunofluorescence revealed that dexamethasone treatment also caused beta-catenin to localize predominantly at the cell membrane rather than the nucleus. The glucocorticoid regulation of beta-catenin expression and localization was not a consequence of dexamethasone inhibition of cell growth, because both responses were unaltered in the presence of hydroxyurea. The steroid induction of beta-catenin expression and localization can be uncoupled by altering the function of signaling pathways needed for tight junction formation. Expression of dominant-negative RasN17 abolished dexamethasone up-regulation of beta-catenin protein expression without affecting its localization at the membrane. In contrast, exogenous treatment or constitutive production of TGFalpha abolished the dexamethasone-induced alteration of beta-catenin localization without affecting the dexamethasone stimulation of beta-catenin expression. Taken together, our results demonstrate that glucocorticoids control beta-catenin at two distinct levels of cellular regulation that differ in their cell signaling requirements for the glucocorticoid regulation of mammary epithelial junctional dynamics.

Frodo interacts with Dishevelled to transduce Wnt signals

Dishevelled (Dsh) is required for the specification of cell fate and polarity by secreted Wnt proteins. Frodo, a novel conserved Dsh-binding protein, synergized with Xenopus Dsh (XDsh) in secondary axis induction in Xenopus laevis embryos. A dominant inhibitory construct and antisense oligonucleotide-mediated depletion of Frodo inhibited axial development in response to XDsh and XWnt8, and suppressed transcriptional activation of a reporter construct. At later embryonic stages, both dominant negative Frodo and antisense oligonucleotides interfered with the expression of regional neural markers and caused eye deficiencies, indicating that Frodo is required for normal eye and neural tissue development. Full-length Frodo RNA suppressed these loss-of-function phenotypes, attesting to their specificity. These findings establish a function for Frodo as an essential positive regulator of Wnt signalling.

Isolation and characterization of XKaiso, a transcriptional repressor that associates with the catenin Xp120(ctn) in Xenopus laevis

The Armadillo family of catenin proteins function in multiple capacities including cadherin-mediated cell-cell adhesion and nuclear signaling. The newest catenin, p120(ctn), differs from the classical catenins and binds to the membrane-proximal domain of cadherins. Recently, a novel transcription factor Kaiso was found to interact with p120(ctn), suggesting that p120(ctn) also possesses a nuclear function. We isolated the Xenopus homolog of Kaiso, XKaiso, from a Xenopus stage 17 cDNA library. XKaiso contains an amino-terminal BTB/POZ domain and three carboxyl-terminal zinc fingers. The XKaiso transcript was present maternally and expressed throughout early embryonic development. XKaiso's spatial expression was defined via in situ hybridization and was found localized to the brain, eye, ear, branchial arches, and spinal cord. Co-immunoprecipitation of Xenopus p120(ctn) and XKaiso demonstrated their mutual association, whereas related experiments employing differentially epitope-tagged XKaiso constructs suggest that XKaiso additionally self-associates. Finally, reporter assays employing a chimera of XKaiso fused to the GAL4 DNA binding domain indicate that XKaiso is a transcriptional repressor. These data suggest that XKaiso functions throughout development and that its repressor functions may be most apparent in the context of neural tissues. The significance of the XKaiso-p120(ctn) interaction has yet to be determined, but it may include transducing information from cadherin-mediated cell-cell contacts to transcriptional processes within the nucleus.

Differential displacement of classical cadherins by VE-cadherin

VE-cadherin is an endothelial cell-specific, type II classical cadherin that plays an important role in permeability, vasculogenesis, and vascular remodeling. Endothelial cells express equal levels of VE- and N-cadherin; VE-cadherin is present injunctions while N-cadherin is diffusely expressed over the surface of the cell. The present study was designed first to determine if the ability of VE-cadherin to displace N-cadherin from junctions was endothelial-cell specific, and second to determine if VE-cadherin could displace other classical cadherins from cell junctions. Our data suggest that VE-cadherin specifically influences the cellular localization of N-cadherin, independent of cell type, and does not effect the localization of other classical cadherins.

Second cysteine-rich domain of Dickkopf-2 activates canonical Wnt signaling pathway via LRP-6 independently of dishevelled

Recent evidence suggests that members of the Dickkopf (Dkk) family can directly bind to LDL-related protein (LRP)-6, resulting in inhibition of Wnt-activated signaling. To further characterize the interactions between Dkk and LRP proteins, conditioned media containing individually conserved cysteine-rich domains of Dkk-1 and Dkk-2 were prepared. Although full-length Dkk-1 and Dkk-2 and the second cysteine-rich domains of both Dkk molecules inhibited Wnt-3a-induced activation of lymphoid enhancing factor (LEF)-1, a downstream target of the canonical pathway, we found that the second cysteine-rich domain of Dkk-2 (Dkk-2C2) was able to stimulate the canonical pathway when LRP-6 was ectopically expressed in NIH3T3 cells. This effect of Dkk-2C2 could be blocked by a monoclonal antibody specific to the second YWTD repeat domain of LRP-5/6, suggesting that Dkk-2C2 acts via LRP-6. We also showed that while both Axin and the DIX domain of Dishevelled (Dvl) could inhibit Dkk-2C2-induced activation of LEF-1, the DEP domain of Dvl, which inhibited Wnt-induced activation of LEF-1, failed to inhibit the activation of LEF-1 by Dkk-2C2 or by an activated form of LRP-5, LRPC2. In addition, glycogen synthase kinase-3 beta, a potent inhibitor for both Dvl and Wnt, also failed to inhibit LRPC2 or Dkk-2C2. Furthermore, knocking-down the expression of Dvl molecules by short interfering RNAs specific to Dvl inhibited Wnt-induced, but not LRPC2-induced, activation of LEF-1. All the evidence indicates that Dkk-2C2 signals through LRP proteins, which does not require Dvl, while Wnt protein may employ both Dvl, presumably through Fz, and LRP to achieve more efficient signal transduction.

Bioengineering models of cell signaling

Strategies for rationally manipulating cell behavior in cell-based technologies and molecular therapeutics and understanding effects of environmental agents on physiological systems may be derived from a mechanistic understanding of underlying signaling mechanisms that regulate cell functions. Three crucial attributes of signal transduction necessitate modeling approaches for analyzing these systems: an ever-expanding plethora of signaling molecules and interactions, a highly interconnected biochemical scheme, and concurrent biophysical regulation. Because signal flow is tightly regulated with positive and negative feedbacks and is bidirectional with commands traveling both from outside-in and inside-out, dynamic models that couple biophysical and biochemical elements are required to consider information processing both during transient and steady-state conditions. Unique mathematical frameworks will be needed to obtain an integrated perspective on these complex systems, which operate over wide length and time scales. These may involve a two-level hierarchical approach wherein the overall signaling network is modeled in terms of effective "circuit" or "algorithm" modules, and then each module is correspondingly modeled with more detailed incorporation of its actual underlying biochemical/biophysical molecular interactions.

Foxl1 controls the Wnt/beta-catenin pathway by modulating the expression of proteoglycans in the gut

Foxl1 is a winged helix transcription factor expressed in the mesenchyme of the gastrointestinal tract. Foxl1 null mice display severe structural defects in the epithelia of the stomach, duodenum, and jejunum. Here we addressed the molecular mechanisms by which Foxl1 controls gastrointestinal differentiation. First we showed that the abnormalities found in the epithelia of the null mice are the result of an increase in the number of proliferating cells and not a change in the rate of cell migration. Next we investigated the regulatory circuits affected by Foxl1. We focused on the Wnt/beta-catenin signaling pathway as a possible target of Foxl1 as it has been shown to play a central role in gastrointestinal proliferation. We demonstrated that Foxl1 activates the Wnt/beta-catenin pathway by increasing extracellular proteoglycans, which act as co-receptors for Wnt. Thus we establish that Foxl1 is involved in the regulation of the Wnt/beta-catenin pathway, providing a novel link in mesenchymal/epithelial cross-talk in the gut. Moreover, we provide the first example implicating proteoglycans in the regulation of cellular proliferation in the gastrointestinal tract.

Constitutive activation of Wnt/beta-catenin signaling pathway in migration-active melanoma cells: role of LEF-1 in melanoma with increased metastatic potential

A constitutive complex of beta-catenin and LEF-1 has been detected in melanoma cell lines expressing either mutant beta-catenin or mutant APC (Rubinfeld et al., Science, 275, 1790-1792, 1997). However, it has been recently reported that beta-catenin mutations are rare in primary malignant melanoma, but its nuclear and/or cytoplasmic localization, a potential indicator of Wnt/beta-catenin pathway activation, is frequently observed in melanoma (Rimm et al., Am. J. Pathol., 154, 325-329, 1999). In human malignant melanoma, the appearance of the tumorigenic phase represents a capacity for metastasis and is the significant phenotypic step in disease progression. Cell motility in invasive melanoma is thought to play a crucial role in metastatic behavior. In this work, we sought to determine which transcription factor of the LEF/TCF family was preferentially involved in human melanoma from different stages of tumor progression. We show that LEF-1 mRNA expression is predominant in highly migrating cells from metastatic melanomas. These actively migrating melanoma cells showed nuclear and cytoplasmic accumulation of beta-catenin and active transcription from a reporter plasmid of the LEF/TCF binding site. These results may provide a new insight into the role of the Wnt/beta-catenin signaling pathway in the tumor progression of malignant melanoma.

WNT pathway and mammary carcinogenesis: loss of expression of candidate tumor suppressor gene SFRP1 in most invasive carcinomas except of the medullary type

Secreted Frizzled-related protein 1 (SFRP1) encodes a member of a protein family that contains a cysteine-rich domain similar to the WNT-binding site of Frizzled receptors and regulates the WNT pathway. The WNT pathway is frequently altered in human cancers. We have defined the pattern of SFRP1 mRNA expression in the progression of breast cancer. We show that SFRP1 is expressed in the epithelial component of normal breast, in the in situ component of ductal carcinomas and is lost in more than 80% of invasive breast carcinomas except the medullary type. Loss of SFRP1 expression is correlated with the presence of hormonal receptors. Conversely, the maintenance of SFRP1 in carcinomas is correlated with the presence of lymphoplasmocytic stroma. No significant association was observed between SFRP1 status and the level of apoptosis in tumoral cells.

Recruitment of (β)-catenin to cadherin-mediated intercellular adhesions is involved in myogenic induction

Cadherin-mediated cell adhesion is involved in muscle differentiation from early stages of myogenic induction to late stages of myoblast interaction and fusion. (β)-Catenin is a major constituent of cadherin-based adherens junctions and also serves as a signal transduction molecule that regulates gene expression during development. In this study, we explored the involvement of (β)-catenin in myogenic differentiation. We show here that shortly after a switch from growth to differentiation medium, (β)-catenin translocates to cell-cell junctions and its levels increase. We further show that elevation of (β)-catenin levels, induced either by inhibition of its breakdown, using LiCl, or by its overexpression, suppresses the formation of adherens junctions, resulting in a sharp decline in myogenin expression and an arrest of myogenic progression. Recruitment of (β)-catenin to adherens junctions after transfection with N-cadherin restores myogenin expression in the transfected cells. These results suggest that increased cadherin-mediated adhesion and translocation of (β)-catenin to adherens junctions are involved in activating the early steps of myogenic differentiation.

Low-density lipoprotein receptor-related protein-5 binds to Axin and regulates the canonical Wnt signaling pathway

To understand how the Wnt coreceptor LRP-5 is involved in transducing the canonical Wnt signals, we identified Axin as a protein that interacts with the intracellular domain of LRP-5. LRP-5, when expressed in fibroblast cells, showed no effect on the canonical Wnt signaling pathway by itself, but acted synergistically with Wnt. In contrast, LRP-5 mutants lacking the extracellular domain functioned as constitutively active forms that bind Axin and that induce LEF-1 activation by destabilizing Axin and stabilizing beta-catenin. Addition of Wnt caused the translocation of Axin to the membrane and enhanced the interaction between Axin and LRP-5. In addition, the LRP-5 sequences involved in interactions with Axin are required for LEF-1 activation. Thus, we conclude that the binding of Axin to LRP-5 is an important part of the Wnt signal transduction pathway.

The tissue polarity gene nemo carries out multiple roles in patterning during Drosophila development

Drosophila nemo was first identified as a gene required for tissue polarity during ommatidial development. We have extended the analysis of nemo and found that it participates in multiple developmental processes. It is required during wing development for wing shape and vein patterning. We observe genetic interactions between nemo and mutations in the Notch, Wingless, Frizzled and Decapentaplegic pathways. Our data support the findings from other organisms that Nemo proteins act as negative regulators of Wingless signaling. nemo mutations cause polarity defects in the adult wing and overexpression of nemo leads to abdominal polarity defects. The expression of nemo during embryogenesis is dynamic and dsRNA inhibition and ectopic expression studies indicate that nemo is essential during embryogenesis.

The cadherin superfamily: diversity in form and function

Over recent years cadherins have emerged as a growing superfamily of molecules, and a complex picture of their structure and their biological functions is becoming apparent. Variation in their extracellular region leads to the large potential for recognition properties of this superfamily. This is demonstrated strikingly by the recently discovered FYN-binding CNR-protocadherins; these exhibit alternative expression of the extracellular portion, which could lead to distinct cell recognition in different neuronal populations, whereas their cytoplasmic part, and therefore intracellular interactions, is constant. Diversity in the cytoplasmic moiety of the cadherins imparts specificity to their interactions with cytoplasmic components; for example, classical cadherins interact with catenins and the actin filament network, desmosomal cadherins interact with catenins and the intermediate filament system and CNR-cadherins interact with the SRC-family kinase FYN. Recent evidence suggests that CNR-cadherins, 7TM-cadherins and T-cadherin, which is tethered to the membrane by a GPI anchor, all localise to lipid rafts, specialised cell membrane domains rich in signalling molecules. Originally thought of as cell adhesion molecules, cadherin superfamily molecules are now known to be involved in many biological processes, such as cell recognition, cell signalling, cell communication, morphogenesis, angiogenesis and possibly even neurotransmission.

Molecular insight into medulloblastoma and central nervous system primitive neuroectodermal tumor biology from hereditary syndromes: a review

Through the study of uncommon familial syndromes, physicians and scientists have been able to illuminate the underlying mechanisms of some of the more common sporadic diseases; this is illustrated best by studies of familial retinoblastoma. A number of rare familial syndromes have been described in which affected individuals are at increased risk of developing medulloblastoma and/or supratentorial primitive neuroectodermal tumors. The descriptions of many of these syndromes are based on patients observed by clinicians in their clinical practice. Determination of the underlying genetic defects in these patients with uncommon syndromes has led to identification of a number of genes subsequently found to be mutated in sporadic medulloblastomas (tumor suppressor genes). Associated genes in the same signaling pathways have also been found to be abnormal in sporadic medulloblastoma. Identification of patients with these rare syndromes is important, as they are often at increased risk for additional neoplasms, as are family members and future children. We review the published literature describing hereditary syndromes that have been associated with an increased incidence of medulloblastoma and/or central nervous system primitive neuroectodermal tumor. Review of the underlying molecular abnormalities in comparison to changes found in sporadic neoplasms suggests pathways important for tumorigenesis.

Wnt/(beta)-catenin signaling regulates the expression of the homeobox gene Cdx1 in embryonic intestine

During mammalian development, the Cdx1 homeobox gene exhibits an early period of expression when the embryonic body axis is established, and a later period where expression is restricted to the embryonic intestinal endoderm. Cdx1 expression is maintained throughout adulthood in the proliferative cell compartment of the continuously renewed intestinal epithelium, the crypts. In this study, we provide evidence in vitro and in vivo that Cdx1 is a direct transcriptional target of the Wnt/(beta)-catenin signaling pathway. Upon Wnt stimulation, expression of Cdx1 can be induced in mouse embryonic stem (ES) cells as well as in undifferentiated rat embryonic endoderm. Tcf4-deficient mouse embryos show abrogation of Cdx1 protein in the small intestinal epithelium, making Tcf4 the likely candidate to transduce Wnt signal in this part of gut. The promoter region of the Cdx1 gene contains several Tcf-binding motifs, and these bind Tcf/Lef1/(beta)-catenin complexes and mediate (beta)-catenin-dependent transactivation. The transcriptional regulation of the homeobox gene Cdx1 in the intestinal epithelium by Wnt/(beta)-catenin signaling underlines the importance of this signaling pathway in mammalian endoderm development.

Somite formation and patterning

As a consequence of their segmented arrangement and the diversity of their tissue derivatives, somites are key elements in the establishment of the metameric body plan in vertebrates. This article aims to largely review what is known about somite development, from the initial stages of somite formation through the process of somite regionalization along the three major body axes. The role of both cell intrinsic mechanisms and environmental cues are evaluated. The periodic and bilaterally synchronous nature of somite formation is proposed to rely on the existence of a developmental clock. Molecular mechanisms underlying these events are reported. The importance of an antero-posterior somitic polarity with respect to somite formation on one hand and body segmentation on the other hand is discussed. Finally, the mechanisms leading to the regionalization of somites along the dorso-ventral and medio-lateral axes are reviewed. This somitic compartmentalization is believed to underlie the segregation of dermis, skeleton, and dorsal and appendicular musculature.

Plakoglobin suppresses epithelial proliferation and hair growth in vivo

Plakoglobin regulates cell adhesion by providing a modulatable connection between both classical and desmosomal cadherins and their respective cytoskeletal linker proteins. Both plakoglobin and the related protein beta-catenin are posttranscriptionally upregulated in response to Wnt-1 in cultured cells. Upregulation of beta-catenin has been implicated in potentiating hyperproliferation and tumor formation. To investigate the role of plakoglobin in these functions we expressed a full-length (PG) and an NH(2)-terminally truncated form of plakoglobin (DeltaN80PG) in mouse epidermis and hair follicles, tissues which undergo continuous and easily observed postnatal renewal and remodeling. Expression of these constructs results in stunted hair growth, a phenotype that has also been observed in transgenic mice expressing Wnt3 and Dvl2 (Millar et al. 1999). Hair follicles from PG and DeltaN80PG mice show premature termination of the growth phase (anagen) of the hair cycle, an event that is regulated in part by FGF5 (Hebert et al. 1994). The proliferative rate of the epidermal cells was reduced and apoptotic changes, which are associated with entry into the regressive phase of the hair follicle cycle (catagen), occurred earlier than usual.

Pattern formation: Wingless on the move

Wingless is a key morphogen in Drosophila. Although it is evident that Wingless acts at a distance from its site of synthesis, there is considerable debate about how the protein travels across a field of cells. Recent studies have provided important new insights into this process, though the issue is still far from being resolved.

Brachyury is a target gene of the Wnt/beta-catenin signaling pathway

To identify target genes of the Wnt/beta-catenin signaling pathway in early mouse embryonic development we have established a co-culture system consisting of NIH3T3 fibroblasts expressing different Wnts as feeder layer cells and embryonic stem (ES) cells expressing a green fluorescent protein (GFP) reporter gene transcriptionally regulated by the TCF/beta-catenin complex. ES cells specifically respond to Wnt signal as monitored by GFP expression. In GFP-positive ES cells we observe expression of Brachyury. Two TCF binding sites located in a 500 bp Brachyury promoter fragment bind the LEF-1/beta-catenin complex and respond specifically to beta-catenin-dependent transactivation. From these results we conclude that Brachyury is a target gene for Wnt/beta-catenin signaling.

A tripartite nuclear localization signal in the PDZ-domain protein L-periaxin

The murine Periaxin gene encodes two PDZ-domain proteins in myelin-forming Schwann cells of the vertebrate peripheral nervous system (Dytrych, L., Sherman, D. L., Gillespie, C. S., and Brophy, P. J. (1998) J. Biol. Chem. 273, 5794-5800). Here we show that L-periaxin is targeted to the nucleus of embryonic Schwann cells. Subsequently, the protein redistributes to the plasma membrane processes of the myelinating Schwann cell where it is believed to function in a signaling complex. In contrast, L-periaxin remains in the nucleus when expressed ectopically in oligodendrocytes, the myelin-forming glia of the central nervous system. The nuclear localization signal (NLS) is basic and tripartite and comprises three signals that act synergistically. Nuclear targeting of L-periaxin is energy-dependent and is inhibited by cell-cell contact. These data show that L-periaxin is a member of a growing family of proteins that can shuttle between the nucleus and cortical signaling/adherence complexes.

Wnt signalling in Caenorhabditis elegans: regulating repressors and polarizing the cytoskeleton

Wnt proteins are secreted, cysteine-rich glycoprotein ligands with numerous roles during animal development. Recent studies of endoderm induction during embryogenesis in the nematode Caenorhabditis elegans challenge the prevailing view that Wnt signalling specifies cell fate by converting transcriptional repressors into activators. Instead, a mitogen-activated protein kinase (MAPK)-related pathway converges with Wnt signalling in C. elegans to relieve transcriptional repression. Furthermore, Wnt signalling induces endoderm in part by aligning the mitotic spindle in a responding cell along the anterior-posterior body axis. To orient mitotic spindles, Wnt signalling might directly target the cytoskeleton, prior to any regulation of gene transcription in responding cells.

Spatially regulated translation in embryos: asymmetric expression of maternal Wnt-11 along the dorsal-ventral axis in Xenopus

Transition from symmetry to asymmetry is a central theme in cell and developmental biology. In Xenopus embryos, dorsal-ventral asymmetry is initiated by a microtubule-dependent cytoplasmic rotation during the first cell cycle after fertilization. Here we show that the cytoplasmic rotation initiates differential cytoplasmic polyadenylation of maternal Xwnt-11 RNA, encoding a member of the Wnt family of cell-cell signaling factors. Translational regulation of Xwnt-11 mRNA along the dorsal-ventral axis results in asymmetric accumulation of Xwnt-11 protein. These results demonstrate spatially regulated translation of a maternal cell-signaling factor along the vertebrate dorsal-ventral axis and represent a novel mechanism for Wnt gene regulation. Spatial regulation of maternal RNA translation, which has been established in invertebrates, appears to be an evolutionarily conserved mechanism in the generation of intracellular asymmetry and the consequential formation of the multicellular body pattern.

Drosophila APC2 is a cytoskeletally-associated protein that regulates wingless signaling in the embryonic epidermis

The tumor suppressor adenomatous polyposis coli (APC) negatively regulates Wingless (Wg)/Wnt signal transduction by helping target the Wnt effector beta-catenin or its Drosophila homologue Armadillo (Arm) for destruction. In cultured mammalian cells, APC localizes to the cell cortex near the ends of microtubules. Drosophila APC (dAPC) negatively regulates Arm signaling, but only in a limited set of tissues. We describe a second fly APC, dAPC2, which binds Arm and is expressed in a broad spectrum of tissues. dAPC2's subcellular localization revealed colocalization with actin in many but not all cellular contexts, and also suggested a possible interaction with astral microtubules. For example, dAPC2 has a striking asymmetric distribution in neuroblasts, and dAPC2 colocalizes with assembling actin filaments at the base of developing larval denticles. We identified a dAPC2 mutation, revealing that dAPC2 is a negative regulator of Wg signaling in the embryonic epidermis. This allele acts genetically downstream of wg, and upstream of arm, dTCF, and, surprisingly, dishevelled. We discuss the implications of our results for Wg signaling, and suggest a role for dAPC2 as a mediator of Wg effects on the cytoskeleton. We also speculate on more general roles that APCs may play in cytoskeletal dynamics.

beta-catenin signaling can initiate feather bud development

Intercellular signaling by a subset of Wnts is mediated by stabilization of cytoplasmic beta-catenin and its translocation to the nucleus. Immunolocalization of beta-catenin in developing chick skin reveals that this signaling pathway is active in a dynamic pattern from the earliest stages of feather bud development. Forced activation of this pathway by expression of a stabilized beta-catenin in the ectoderm results in the ectopic formation of feather buds. This construct is sufficient to induce bud formation since it does so both within presumptive feather tracts and in normally featherless regions where tract-specific signals are absent. It is also insensitive to the lateral inhibition that mediates the normal spacing of buds and can induce ectopic buds in interfollicular skin. However, additional patterning signals cooperate with this pathway to regulate gene expression within domains of stabilized beta-catenin expression. Localized activation of this pathway within the bud as it develops is required for normal morphogenesis and ectopic activation of the pathway leads to abnormally oriented buds and growths on the feather filaments. These results suggest that activation of the beta-catenin pathway initiates follicle development in embryonic skin and plays important roles in the subsequent morphogenesis of the bud.

Nuclear localization of the p120(ctn) Armadillo-like catenin is counteracted by a nuclear export signal and by E-cadherin expression

The Armadillo protein p120(ctn) associates with the cytoplasmic domain of cadherins and accumulates at cell-cell junctions. Particular Armadillo proteins such as beta-catenin and plakophilins show a partly nuclear location, suggesting gene-regulatory activities. For different human E-cadherin-negative carcinoma cancer cell lines we found expression of endogenous p120(ctn) in the nucleus. Expression of E-cadherin directed p120(ctn) out of the nucleus. Previously, we reported that the human p120(ctn) gene might encode up to 32 protein isoforms as products of alternative splicing. Overexpression of p120(ctn) isoforms B in various cell lines resulted in cytoplasmic immunopositivity but never in nuclear staining. In contrast, upon expression of p120(ctn) cDNAs lacking exon B, the isoforms were detectable within both nuclei and cytoplasm. A putative nuclear export signal (NES) with a characteristic leucine-rich motif is encoded by exon B. This sequence element was shown to be required for nuclear export and to function autonomously when fused to a carrier protein and microinjected into cell nuclei. Moreover, the NES function of endogenously or exogenously expressed p120(ctn) isoforms B was sensitive to the nuclear export inhibitor leptomycin B. Expression of exogenous E-cadherin down-regulated nuclear p120(ctn) whereas activation of protein kinase C increased the level of nuclear p120(ctn). These results reveal molecular mechanisms controlling the subcellular distribution of p120(ctn).

Membrane-tethered Drosophila Armadillo cannot transduce Wingless signal on its own

Drosophila Armadillo and its vertebrate homolog beta-catenin are key effectors of Wingless/Wnt signaling. In the current model, Wingless/Wnt signal stabilizes Armadillo/beta-catenin, which then accumulates in nuclei and binds TCF/LEF family proteins, forming bipartite transcription factors which activate transcription of Wingless/Wnt responsive genes. This model was recently challenged. Overexpression in Xenopus of membrane-tethered beta-catenin or its paralog plakoglobin activates Wnt signaling, suggesting that nuclear localization of Armadillo/beta-catenin is not essential for signaling. Tethered plakoglobin or beta-catenin might signal on their own or might act indirectly by elevating levels of endogenous beta-catenin. We tested these hypotheses in Drosophila by removing endogenous Armadillo. We generated a series of mutant Armadillo proteins with altered intracellular localizations, and expressed these in wild-type and armadillo mutant backgrounds. We found that membrane-tethered Armadillo cannot signal on its own; however it can function in adherens junctions. We also created mutant forms of Armadillo carrying heterologous nuclear localization or nuclear export signals. Although these signals alter the subcellular localization of Arm when overexpressed in Xenopus, in Drosophila they have little effect on localization and only subtle effects on signaling. This supports a model in which Armadillo's nuclear localization is key for signaling, but in which Armadillo intracellular localization is controlled by the availability and affinity of its binding partners.