Regulation of glycogen synthase kinase 3beta and downstream Wnt signaling by axin

Regulation of the interaction between glycogen synthase kinase 3 and the Kaposi's sarcoma-associated herpesvirus latency-associated nuclear antigen

The Kaposi's sarcoma-associated herpesvirus (KSHV)-encoded latency-associated nuclear antigen (LANA) protein stabilizes beta-catenin by the novel mechanism of binding to the negative regulator, glycogen synthase kinase 3 (GSK-3), and depleting cytoplasmic GSK-3 levels. The two domains of LANA required for interaction with GSK-3 were further characterized. Evidence for similarity between the C-terminal LANA interaction domain and the axin GSK-3 interaction domain was obtained using GSK-3 and LANA mutants. GSK-3(F291L), which does not interact with axin, also failed to bind to LANA, and a mutation in the axin homology domain of LANA, L1132P, destroyed binding to GSK-3. The N-terminal LANA interaction domain was found to mediate interaction by acting as a substrate for GSK-3. GSK-3(R96A), a priming pocket mutant, did not bind to LANA, suggesting that LANA was a primed GSK-3 substrate. Phosphorylation of endogenous LANA precipitated from primary effusion lymphoma cells was inhibited by the GSK-3 inhibitor LiCl. GST-LANA(1-340) was phosphorylated by GSK-3, and mitogen-activated protein kinase (MAPK) and casein kinase I functioned as priming kinases in vitro. Mutation of consensus GSK-3 sites revealed that sites between LANA amino acids 219 and 268 were important for GSK-3 phosphorylation. Immunoprecipitation assays revealed that loss of GSK-3 phosphorylation of this N-terminal domain correlated with loss of GSK-3 interaction. Although LANA-associated GSK-3 actively phosphorylated LANA, GSK-3 coprecipitated with LANA was unable to phosphorylate an exogenous peptide substrate. LANA sequestration of GSK-3 may explain the ability of KSHV-infected cells to tolerate increased levels of nuclear GSK-3.

The role of Axin2 in calvarial morphogenesis and craniosynostosis

Axin1 and its homolog Axin2/conductin/Axil are negative regulators of the canonical Wnt pathway that suppress signal transduction by promoting degradation of beta-catenin. Mice with deletion of Axin1 exhibit defects in axis determination and brain patterning during early embryonic development. We show that Axin2 is expressed in the osteogenic fronts and periosteum of developing sutures during skull morphogenesis. Targeted disruption of Axin2 in mice induces malformations of skull structures, a phenotype resembling craniosynostosis in humans. In the mutants, premature fusion of cranial sutures occurs at early postnatal stages. To elucidate the mechanism of craniosynostosis, we studied intramembranous ossification in Axin2-null mice. The calvarial osteoblast development is significantly affected by the Axin2 mutation. The Axin2 mutant displays enhanced expansion of osteoprogenitors, accelerated ossification, stimulated expression of osteogenic markers and increases in mineralization. Inactivation of Axin2 promotes osteoblast proliferation and differentiation in vivo and in vitro. Furthermore, as the mammalian skull is formed from cranial skeletogenic mesenchyme, which is derived from mesoderm and neural crest, our data argue for a region-specific effect of Axin2 on neural crest dependent skeletogenesis. The craniofacial anomalies caused by the Axin2 mutation are mediated through activation of beta-catenin signaling, suggesting a novel role for the Wnt pathway in skull morphogenesis.

The links between axin and carcinogenesis

The products of the two mammalian Axin genes (Axin1 and its homologue Axin2) are essential for the degradation of beta catenin, a component of Wnt signalling that is frequently dysregulated in cancer cells. Axin is a multidomain scaffold protein that has many functions in biological signalling pathways. Overexpression of mutant [corrected] axin results in axis duplication in mouse embryos. Wnt signalling activity determines dorsal-ventral axis formation in vertebrates, implicating axin as a negative regulator of this signalling pathway. In addition, Wnts modulate pattern formation and the morphogenesis of most organs by influencing and controlling cell proliferation, motility, and fate. Defects in different components of the Wnt signalling pathway promote tumorigenesis and tumour progression. Recent biochemical studies of axins indicate that these molecules are the primary limiting components of this pathway. This review explores the intriguing connections between defects in axin function and human diseases.

Glycogen synthase kinase-3beta haploinsufficiency mimics the behavioral and molecular effects of lithium

Lithium is widely used to treat bipolar disorder, but its mechanism of action in this disorder is unknown. Several molecular targets of lithium have been identified, but these putative targets have not been shown to be responsible for the behavioral effects of lithium in vivo. A robust model for the effects of chronic lithium on behavior in mice would greatly facilitate the characterization of lithium action. We describe behaviors in mice that are robustly affected by chronic lithium. Remarkably, these lithium-sensitive behaviors are also observed in mice lacking one copy of the gene encoding glycogen synthase kinase-3beta (Gsk-3beta), a well established direct target of lithium. In addition, chronic lithium induces molecular changes consistent with inhibition of GSK-3 within regions of the brain that are paralleled in Gsk-3beta+/- heterozygous mice. We also show that lithium therapy activates Wnt signaling in vivo, as measured by increased Wnt-dependent gene expression in the amygdala, hippocampus, and hypothalamus. These observations support a central role for GSK-3beta in mediating behavioral responses to lithium.

Both the establishment and the maintenance of neuronal polarity require active mechanisms: critical roles of GSK-3beta and its upstream regulators

Axon-dendrite polarity is a cardinal feature of neuronal morphology essential for information flow. Here we report a differential distribution of GSK-3beta activity in the axon versus the dendrites. A constitutively active GSK-3beta mutant inhibited axon formation, whereas multiple axons formed from a single neuron when GSK-3beta activity was reduced by pharmacological inhibitors, a peptide inhibitor, or siRNAs. An active mechanism for maintaining neuronal polarity was revealed by the conversion of preexisting dendrites into axons upon GSK-3 inhibition. Biochemical and functional data show that the Akt kinase and the PTEN phosphatase are upstream of GSK-3beta in determining neuronal polarity. Our results demonstrate that there are active mechanisms for maintaining as well as establishing neuronal polarity, indicate that GSK-3beta relays signaling from Akt and PTEN to play critical roles in neuronal polarity, and suggest that application of GSK-3beta inhibitors can be a novel approach to promote generation of new axons after neural injuries.

The Wnt signaling pathway in development and disease

Tight control of cell-cell communication is essential for the generation of a normally patterned embryo. A critical mediator of key cell-cell signaling events during embryogenesis is the highly conserved Wnt family of secreted proteins. Recent biochemical and genetic analyses have greatly enriched our understanding of how Wnts signal, and the list of canonical Wnt signaling components has exploded. The data reveal that multiple extracellular, cytoplasmic, and nuclear regulators intricately modulate Wnt signaling levels. In addition, receptor-ligand specificity and feedback loops help to determine Wnt signaling outputs. Wnts are required for adult tissue maintenance, and perturbations in Wnt signaling promote both human degenerative diseases and cancer. The next few years are likely to see novel therapeutic reagents aimed at controlling Wnt signaling in order to alleviate these conditions.

Genetic evidence for involvement of maternally derived Wnt canonical signaling in dorsal determination in zebrafish

In zebrafish, the program for dorsal specification begins soon after fertilization. Dorsal determinants are localized initially to the vegetal pole, then transported to the blastoderm, where they are thought to activate the canonical Wnt pathway, which induces the expression of dorsal-specific genes. We identified a novel maternal-effect recessive mutation, tokkaebi (tkk), that affects formation of the dorsal axis. Severely ventralized phenotypes, including a lack of dorso-anterior structures, were seen in 5-100% of the embryos obtained from tkk homozygous transmitting females. tkk embryos displayed defects in the nuclear accumulation of beta-catenin on the dorsal side, and reduced or absent expression of dorsal-specific genes. Mesoderm and endoderm formation outside the dorsal axis was not significantly affected. Injection of RNAs for activated beta-catenin, dominant-negative forms of Axin1 and GSK3beta, and wild-type Dvl3, into the tkk embryos suppressed the ventralized phenotypes and/or dorsalized the embryos, and restored or induced an ectopic and expanded expression of bozozok/dharma and goosecoid. However, dorsalization by wnt RNAs was affected in the tkk embryos. Inhibition of cytoplasmic calcium release elicited an ectopic and expanded expression of chordin in the wild-type, but did not restore chordin expression efficiently in the tkk embryos. These data indicate that the tkk gene product functions upstream of or parallel to the beta-catenin-degradation machinery to control the stability of beta-catenin. The tkk locus was mapped to chromosome 16. These data provide genetic evidence that the maternally derived canonical Wnt pathway upstream of beta-catenin is involved in dorsal axis formation in zebrafish.

Combinatorial peptide library binding of mammalian spermatozoa identifies a ligand (HIPRT) in the axin protein: putative identification of a sperm surface axin binding protein and intriguing developmental implications

The identification of components in cell-cell interactions is an important research goal in reproductive and developmental biology. Such interactions are critical to gamete development, fertilization, implantation and basic development. Several proteins involved with sperm-oocyte interaction and other developmentally important phenomena have been identified. However, these are obviously only a subset of the molecular components involved in such complex cell-cell interactions. One method that has been used to identify binding partners for particular molecular targets is the use of combinatorial libraries accessible on phage surfaces. For the most part, this technique has mainly been applied to screen specific target moieties. However, in some cases whole-cell screening has been attempted. This study describes the first report of screening intact, living mammalian gametes using a proprietary whole-cell combinatorial library binding and analysis protocol. Results from the first screening protocol of mouse spermatozoa strongly identified a putative sperm-binding ligand using proprietary bioinformatic analysis. This amino acid sequence (HIPRT) precisely corresponds with a previously characterized highly conserved protein-protein interaction site in the axin protein. This sequence is found within the binding site for a known sperm surface protein, glycogen synthase kinase-3. This result not only provides proof of the utility of this technique to identify cell surface ligands in mammalian gametes, but it also suggests a potential role for spermatozoa in facilitating developmental axis formation in mammalian embryos.

A functional polymorphism in RGS6 modulates the risk of bladder cancer

RGS proteins negatively regulate heterotrimeric G protein signaling. Recent reports have shown that RGS proteins modulate neuronal, cardiovascular, and lymphocytic activity, yet their role in carcinogenesis has not been explored. In an epidemiologic study of 477 bladder cancer patients and 446 matched controls, three noncoding single-nucleotide polymorphisms (SNPs) in RGS2 and RGS6 were each associated with a statistically significant reduction in bladder cancer risk. The risk of bladder cancer was reduced by 74% in those individuals with the variant genotype at all three SNPs (odds ratio, 0.26; 95% confidence interval, 0.09-0.71). When the SNPs were analyzed separately, the RGS6-rs2074647 (C-->T) polymorphism conferred the greatest overall reduction in risk of bladder cancer (odds ratio, 0.66; 95% confidence interval, 0.46-0.95). These reductions in risk were more pronounced in ever smokers, suggesting a gene-environment interaction. In transfection assays, the RGS6-rs2074647 (C-->T) polymorphism increased the activity of a luciferase-RGS fusion protein by 2.9-fold, suggesting that this SNP is functionally significant. Finally, we demonstrate that RGS2 transcripts and several splice variants of RGS6 are expressed in bladder cancer cells. These data provide the first evidence that RGS proteins may be important modulators of cancer risk and validate RGS6 as a target for further study.

The Fbw7 tumor suppressor regulates glycogen synthase kinase 3 phosphorylation-dependent c-Myc protein degradation

Myc proteins regulate cell growth and division and are implicated in a wide range of human cancers. We show here that Fbw7, a component of the SCF(Fbw7) ubiquitin ligase and a tumor suppressor, promotes proteasome-dependent c-Myc turnover in vivo and c-Myc ubiquitination in vitro. Phosphorylation of c-Myc on threonine-58 (T58) by glycogen synthase kinase 3 regulates the binding of Fbw7 to c-Myc as well as Fbw7-mediated c-Myc degradation and ubiquitination. T58 is the most frequent site of c-myc mutations in lymphoma cells, and our findings suggest that c-Myc activation is one of the key oncogenic consequences of Fbw7 loss in cancer. Because Fbw7 mediates the degradation of cyclin E, Notch, and c-Jun, as well as c-Myc, the loss of Fbw7 is likely to elicit profound effects on cell proliferation during tumorigenesis.

Sequential antagonism of early and late Wnt-signaling by zebrafish colgate promotes dorsal and anterior fates

The establishment of the vertebrate body plan involves patterning of the ectoderm, mesoderm, and endoderm along the dorsoventral and antero-posterior axes. Interactions among numerous signaling molecules from several multigene families, including Wnts, have been implicated in regulating these processes. Here we provide evidence that the zebrafish colgate(b382) (col) mutation results in increased Wnt signaling that leads to defects in dorsal and anterior development. col mutants display early defects in dorsoventral patterning manifested by a decrease in the expression of dorsal shield-specific markers and ectopic expression of ventrolaterally expressed genes during gastrulation. In addition to these early patterning defects, col mutants display a striking regional posteriorization within the neuroectoderm, resulting in a reduction in anterior fates and an expansion of posterior fates within the forebrain and midbrain-hindbrain regions. We are able to correlate these phenotypes to the overactivation of Wnt signaling in col mutants. The early dorsal and anterior patterning phenotypes of the col mutant embryos are selectively rescued by inactivation of Wnt8 function by morpholino translational interference. In contrast, the regionalized neuroectoderm posterioriorization phenotype is selectively rescued by morpholino-mediated inactivation of Wnt8b. These results suggest that col-mediated antagonism of early and late Wnt-signaling activity during gastrulation is normally required sequentially for both early dorsoventral patterning and the specification and patterning of regional fates within the anterior neuroectoderm.

Molecular targets of lithium action

Lithium is an effective drug for both the treatment and prophylaxis of bipolar disorder. However, the precise mechanism of lithium action is not yet well understood. Extensive research aiming to elucidate the molecular mechanisms underlying the therapeutic effects of lithium has revealed several possible targets. The behavioral and physiological manifestations of the illness are complex and are mediated by a network of interconnected neurotransmitter pathways. Thus, lithium's ability to modulate the release of serotonin at presynaptic sites and modulate receptor-mediated supersensitivity in the brain remains a relevant line of investigation. However, it is at the molecular level that some of the most exciting advances in the understanding of the long-term therapeutic action of lithium will continue in the coming years. The lithium cation possesses the selective ability, at clinically relevant concentrations, to alter the PI second-messenger system, potentially altering the activity and dynamic regulation of receptors that are coupled to this intracellular response. Subtypes of muscarinic receptors in the limbic system may represent particularly sensitive targets in this regard. Likewise, preclinical data have shown that lithium regulates arachidonic acid and the protein kinase C signaling cascades. It also indirectly regulates a number of factors involved in cell survival pathways, including cAMP response element binding protein, brain-derived neurotrophic factor, bcl-2 and mitogen-activated protein kinases, and may thus bring about delayed long-term beneficial effects via under-appreciated neurotrophic effects. Identification of the molecular targets for lithium in the brain could lead to the elucidation of the pathophysiology of bipolar disorder and the discovery of a new generation of mood stabilizers, which in turn may lead to improvements in the long-term outcome of this devastating illness (1).

Glycogen synthase kinase-3 and Axin function in a β-catenin-independent pathway that regulates neurite outgrowth in neuroblastoma cells

We have sought to determine the roles of beta-catenin and the Wnt signaling pathway in neurite outgrowth using a model cell system, the Neuro-2a neuroblastoma cell line. Activation of the Wnt signaling pathway disrupts a multiprotein complex that includes beta-catenin, Axin, and glycogen synthase kinase-3 (GSK-3), which would otherwise promote the phosphorylation and degradation of beta-catenin. Stabilized beta-catenin accumulates in the cytosol and in the nucleus; in the nucleus it binds to TCF family transcription factors, forming a bipartite transcriptional activator of Wnt target genes. These events can be mimicked by lithium (Li(+)), which inhibits GSK-3 activity. Both Li(+) and the GSK-3 inhibitor SB415286 induced neurite outgrowth of Neuro-2a cells. Li(+)-induced neurite outgrowth did not require beta-catenin-/TCF-dependent transcription, and increasing levels of beta-catenin either by transfection or using Wnt-3A was not sufficient to induce neurite outgrowth. Interestingly, Axin, which is also a substrate for GSK-3, was destabilized by Li(+) and ectopic expression of Axin inhibited Li(+)-induced neurite outgrowth. Deletion analysis of Axin indicated that this inhibition required the GSK-3 binding site, but not the beta-catenin binding site. Our results suggest that a signaling pathway involving Axin and GSK-3, but not beta-catenin, regulates Li(+)-induced neurite outgrowth in Neuro-2a cells.

PP2A:B56epsilon is required for Wnt/beta-catenin signaling during embryonic development

The Wnt/beta-catenin pathway plays important roles during embryonic development and growth control. The B56 regulatory subunit of protein phosphatase 2A (PP2A) has been implicated as a regulator of this pathway. However, this has not been investigated by loss-of-function analyses. Here we report loss-of-function analysis of PP2A:B56epsilon during early Xenopus embryogenesis. We provide direct evidence that PP2A:B56epsilon is required for Wnt/beta-catenin signaling upstream of Dishevelled and downstream of the Wnt ligand. We show that maternal PP2A:B56epsilon function is required for dorsal development, and PP2A:B56epsilon function is required later for the expression of the Wnt target gene engrailed, for subsequent midbrain-hindbrain boundary formation, and for closure of the neural tube. These data demonstrate a positive role for PP2A:B56epsilon in the Wnt pathway.

Inhibitory phosphorylation of glycogen synthase kinase-3 (GSK-3) in response to lithium. Evidence for autoregulation of GSK-3

Glycogen synthase kinase-3 (GSK-3) is a critical, negative regulator of diverse signaling pathways. Lithium is a direct inhibitor of GSK-3 and has been widely used to test the putative role of GSK-3 in multiple settings. However, lithium also inhibits other targets, including inositol monophosphatase and structurally related phosphomonoesterases, and thus additional approaches are needed to attribute a given biological effect of lithium to a specific target. For example, lithium is known to increase the inhibitory N-terminal phosphorylation of GSK-3, but the target of lithium responsible for this indirect regulation has not been identified. We have characterized a short peptide derived from the GSK-3 interaction domain of Axin that potently inhibits GSK-3 activity in vitro and in mammalian cells and robustly activates Wnt-dependent transcription, mimicking lithium action. We show here, using the GSK-3 interaction domain peptide, as well as small molecule inhibitors of GSK-3, that lithium induces GSK-3 N-terminal phosphorylation through direct inhibition of GSK-3 itself. Reduction of GSK-3 protein levels, either by RNA interference or by disruption of the mouse GSK-3beta gene, causes increased N-terminal phosphorylation of GSK-3, confirming that GSK-3 regulates its own phosphorylation status. Finally, evidence is presented that N-terminal phosphorylation of GSK-3 can be regulated by the GSK-3-dependent protein phosphatase-1.inhibitor-2 complex.

Adenomatous polyposis coli/beta-catenin interaction and downstream targets: altered gene expression in gastrointestinal tumors

Gastrointestinal cancer affects 250,000 Americans a year with nearly half of those cases being colorectal cancer. The Wnt pathway is activated in most spontaneous and familial colorectal cancers and has been implicated in tumor formation at other sites in the gastrointestinal tract. In human tumors, the Wnt pathway is most often altered by mutations affecting certain components of this signal transduction cascade-the adenomatous polyposis coli (APC) tumor suppressor gene or the ss-catenin gene. Perturbations in the function of either protein lead to altered gene regulation through the interaction of ss-catenin with T-cell factor (Tcf)/lymphoid enhancer binding protein (Lef) transcription factors. This review will discuss the Wnt pathway, examine the mutations of its components that are found in human cancer, and discuss the known downstream gene targets.

SANE, a novel LEM domain protein, regulates bone morphogenetic protein signaling through interaction with Smad1

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor-beta (TGF-beta) superfamily that play important roles in bone formation, embryonic patterning, and epidermal-neural cell fate decisions. BMPs signal through pathway specific mediators such as Smads1 and 5, but the upstream regulation of BMP-specific Smads has not been fully characterized. Here we report the identification of SANE (Smad1 Antagonistic Effector), a novel protein with significant sequence similarity to nuclear envelop proteins such as MAN1. SANE binds to Smad1/5 and to BMP type I receptors and regulates BMP signaling. SANE specifically blocks BMP-dependent signaling in Xenopus embryos and in a mammalian model of bone formation but does not inhibit the TGF-beta/Smad2 pathway. Inhibition of BMP signaling by SANE requires interaction between SANE and Smad1, because a SANE mutant that does not bind Smad1 does not inhibit BMP signaling. Furthermore, inhibition appears to be mediated by inhibition of BMP-induced Smad1 phosphorylation, blocking ligand-dependent nuclear translocation of Smad1. These studies define a new mode of regulation for intracellular BMP/Smad1 signaling.

Axin negatively affects tau phosphorylation by glycogen synthase kinase 3beta

Glycogen synthase kinase 3beta (GSK3beta) is an essential protein kinase that regulates numerous functions within the cell. One critically important substrate of GSK3beta is the microtubule-associated protein tau. Phosphorylation of tau by GSK3beta decreases tau-microtubule interactions. In addition to phosphorylating tau, GSK3beta is a downstream regulator of the wnt signaling pathway, which maintains the levels of beta-catenin. Axin plays a central role in regulating beta-catenin levels by bringing together GSK3beta and beta-catenin and facilitating the phosphorylation of beta-catenin, targeting it for ubiquitination and degradation by the proteasome. Although axin clearly facilitates the phosphorylation of beta-catenin, its effects on the phosphorylation of other GSK3beta substrates are unclear. Therefore in this study the effects of axin on GSK3beta-mediated tau phosphorylation were examined. The results clearly demonstrate that axin is a negative regulator of tau phosphorylation by GSK3beta. This negative regulation of GSK3beta-mediated tau phosphorylation is due to the fact that axin efficiently binds GSK3beta but not tau and thus sequesters GSK3beta away from tau, as an axin mutant that does not bind GSK3beta did not inhibit tau phosphorylation by GSK3beta. This is the first demonstration that axin negatively affects the phosphorylation of a GSK3beta substrate, and provides a novel mechanism by which tau phosphorylation and function can be regulated within the cell.

Frizzled-9 is activated by Wnt-2 and functions in Wnt/beta -catenin signaling

Frizzled has been known to function as a Wnt receptor. Although there have been a number of mammalian Frizzled members identified, their binding specificities with Wnt and functions in mammalian cells have been poorly understood. Here, we demonstrate that rat Frizzled-9 (Rfz9) functions in Wnt/beta-catenin signaling in 293T cells. Rfz9 overexpression induces the hyperphosphorylation and relocalization of mouse Dishevelled-1 (Dvl-1) from the cytoplasm to the cell membrane and the accumulation of cytosolic beta-catenin. Transfections of Rfz9 with each of several Wnt members show that only Wnt-2 activates Rfz9 in T cell factor (TCF)-dependent transcription. Deletion mutant analysis determines that there is a difference in Rfz9 C-terminal residues required for the modifications of Dvl-1 and those required for the inductions of beta-catenin stabilization and TCF transactivation. Deletion of the Wnt-binding domain does not abolish Rfz9 activity completely, although it causes the inactivation of Wnt-2-dependent TCF transcription. Rfz9 also relocalizes Axin from the cytoplasm to the plasma membrane in the presence of Dvl-1, suggesting that one of the consequences of Dvl-1 relocalization by Rfz9 is to bring Axin to the cell membrane.

The Wnt signaling pathway in bipolar disorder

The Wnt signaling pathway is a highly conserved pathway critical for proper embryonic development. However, recent evidence suggests that this pathway and one of its key enzymes, glycogen synthase kinase 3beta, may play important roles in regulating synaptic plasticity, cell survival, and circadian rhythms in the mature CNS-all of which have been implicated in the pathophysiology and treatment of bipolar disorder. Furthermore, two structurally highly dissimilar medications used to treat bipolar disorder, lithium and valproic acid, exert effects on components of the Wnt signaling pathway. Together, these data suggest that the Wnt signaling pathway may play an important role in the treatment of bipolar disorder. Here, the authors review the modulation of the Wnt/GSK-3beta signaling pathway by mood-stabilizing agents, focusing on two therapeutically relevant aspects: neuroprotection and modulation of circadian rhythms. The future development of selective GSK-3beta inhibitors may have considerable utility not only for the treatment of bipolar disorder but also for a variety of classical neurodegenerative disorders.

I-mfa domain proteins interact with Axin and affect its regulation of the Wnt and c-Jun N-terminal kinase signaling pathways

I-mfa has been identified as an inhibitor of myogenic basic helix-loop-helix transcription factors, and a related human I-mfa domain-containing protein (HIC) also has been identified as a protein that regulates Tat- and Tax-mediated expression of viral promoters. HIC and I-mfa represent a family of proteins that share a highly conserved cysteine-rich domain, termed the I-mfa domain. We show here that both I-mfa domain proteins, HIC and I-mfa, interacted in vivo with the Axin complex through their C-terminal I-mfa domains. This interaction inhibited Axin-mediated downregulation of free levels of cytosolic beta-catenin. I-mfa and HIC also both directly interacted with lymphocyte enhancer factor (LEF); however, I-mfa but not HIC significantly inhibited reporter constructs regulated by beta-catenin. The overexpression of HIC but not I-mfa decreased the inhibitory effects of Axin on beta-catenin-regulated reporter constructs, while both HIC and I-mfa decreased Axin-mediated c-Jun N-terminal kinase (JNK) activation. These data reveal for the first time that I-mfa domain proteins interact with the Axin complex and affect Axin regulation of both the Wnt and the JNK activation pathways. Interestingly, HIC differs from I-mfa in that I-mfa affects both Axin function and T-cell factor- or LEF-regulated transcription in the Wnt signaling pathway while HIC affects primarily Axin function.

The Axin-like protein PRY-1 is a negative regulator of a canonical Wnt pathway in C. elegans

Axin, APC, and the kinase GSK3 beta are part of a destruction complex that regulates the stability of the Wnt pathway effector beta-catenin. In C. elegans, several Wnt-controlled developmental processes have been described, but an Axin ortholog has not been found in the genome sequence and SGG-1/GSK3 beta, and the APC-related protein APR-1 have been shown to act in a positive, rather than negative fashion in Wnt signaling. We have shown previously that the EGL-20/Wnt-dependent expression of the homeobox gene mab-5 in the Q neuroblast lineage requires BAR-1/beta-catenin and POP-1/Tcf. Here, we have investigated how BAR-1 is regulated by the EGL-20 pathway. First, we have characterized a negative regulator of the EGL-20 pathway, pry-1. We show that pry-1 encodes an RGS and DIX domain-containing protein that is distantly related to Axin/Conductin. Our results demonstrate that despite its sequence divergence, PRY-1 is a functional Axin homolog. We show that PRY-1 interacts with BAR-1, SGG-1, and APR-1 and that overexpression of PRY-1 inhibits mab-5 expression. Furthermore, pry-1 rescues the zebrafish axin1 mutation masterblind, showing that it can functionally interact with vertebrate destruction complex components. Finally, we show that SGG-1, in addition to its positive regulatory role in early embryonic Wnt signaling, may function as a negative regulator of the EGL-20 pathway. We conclude that a highly divergent destruction complex consisting of PRY-1, SGG-1, and APR-1 regulates BAR-1/beta-catenin signaling in C. elegans.

Glycogen synthase kinase-3 beta mutagenesis identifies a common binding domain for GBP and Axin

Glycogen synthase kinase-3 beta (GSK-3) is a key downstream target of Wnt signaling and is regulated by its interactions with activating and inhibitory proteins. We and others have shown that GSK-3 activity toward non-primed substrates is regulated in part through a competition between its activating (Axin) and inhibitory (GBP/FRAT) binding partners. Here we use a reverse two-hybrid screen to identify mutations in GSK-3 that alter binding to GBP and Axin. We find that these mutations overlap and propose that GBP and Axin compete for binding to the same region of GSK-3. We use these mutations to examine the ability of GSK-3 to block eye development in Xenopus embryos and suggest that GSK-3 regulates eye development through a non-Wnt pathway.

Identification of the Axin and Frat binding region of glycogen synthase kinase-3

Glycogen synthase kinase-3 (GSK-3) is a key component of several signaling pathways including those regulated by Wnt and insulin ligands. Specificity in GSK-3 signaling is thought to involve interactions with scaffold proteins that localize GSK-3 regulators and substrates. This report shows that GSK-3 forms a low affinity homodimer that is disrupted by binding to Axin and Frat. Based on the crystal structure of GSK-3, we have used surface-scanning mutagenesis to identify residues that differentially affect GSK-3 interactions. Mutations that disrupt Frat and Axin cluster at the dimer interface explaining their effect on homodimer formation. Loss of the Axin binding site blocks the ability of dominant negative GSK-3 to cause axis duplication in Xenopus embryos. The Axin binding site is conserved within all GSK-3 proteins, and its loss affects both cell motility and gene expression in the nonmetazoan, Dictyostelium. Surprisingly, we find no genetic interaction between a non-Axin-binding GSK-3 mutant and T-cell factor activity, arguing that Axin interactions alone cannot explain the regulation of T-cell factor-mediated gene expression.

The structure of phosphorylated GSK-3beta complexed with a peptide, FRATtide, that inhibits beta-catenin phosphorylation

BACKGROUND

Glycogen synthase kinase-3 (GSK-3) sequentially phosphorylates four serine residues on glycogen synthase (GS), in the sequence SxxxSxxxSxxx-SxxxS(p), by recognizing and phosphorylating the first serine in the sequence motif SxxxS(P) (where S(p) represents a phosphoserine). FRATtide (a peptide derived from a GSK-3 binding protein) binds to GSK-3 and blocks GSK-3 from interacting with Axin. This inhibits the Axin-dependent phosphorylation of beta-catenin by GSK-3.

RESULTS

Structures of uncomplexed Tyr216 phosphorylated GSK-3beta and of its complex with a peptide and a sulfate ion both show the activation loop adopting a conformation similar to that in the phosphorylated and active forms of the related kinases CDK2 and ERK2. The sulfate ion, adjacent to Val214 on the activation loop, represents the binding site for the phosphoserine residue on 'primed' substrates. The peptide FRATtide forms a helix-turn-helix motif in binding to the C-terminal lobe of the kinase domain; the FRATtide binding site is close to, but does not obstruct, the substrate binding channel of GSK-3. FRATtide (and FRAT1) does not inhibit the activity of GSK-3 toward GS.

CONCLUSIONS

The Axin binding site on GSK-3 presumably overlaps with that for FRATtide; its proximity to the active site explains how Axin may act as a scaffold protein promoting beta-catenin phosphorylation. Tyrosine 216 phosphorylation can induce an active conformation in the activation loop. Pre-phosphorylated substrate peptides can be modeled into the active site of the enzyme, with the P1 residue occupying a pocket partially formed by phosphotyrosine 216 and the P4 phosphoserine occupying the 'primed' binding site.

GSK-3 inhibition by adenoviral FRAT1 overexpression is neuroprotective and induces Tau dephosphorylation and beta-catenin stabilisation without elevation of glycogen synthase activity

Glycogen synthase kinase 3 (GSK-3) has previously been shown to play an important role in the regulation of apoptosis. However, the nature of GSK-3 effector pathways that are relevant to neuroprotection remains poorly defined. Here, we have compared neuroprotection resulting from modulation of GSK-3 activity in PC12 cells using either selective small molecule ATP-competitive GSK-3 inhibitors (SB-216763 and SB-415286), or adenovirus overexpressing frequently rearranged in advanced T-cell lymphomas 1 (FRAT1), a protein proposed as a negative regulator of GSK-3 activity towards Axin and beta-catenin. Our data demonstrate that cellular overexpression of FRAT1 is sufficient to confer neuroprotection and correlates with inhibition of GSK-3 activity towards Tau and beta-catenin, but not modulation of glycogen synthase (GS) activity. By comparison, treatment with SB-216763 and SB-415286 proved more potent in terms of neuroprotection, and correlated with inhibition of GSK-3 activity towards GS in addition to Tau and beta-catenin.

The multifaceted roles of glycogen synthase kinase 3beta in cellular signaling

Glycogen synthase kinase-3beta (GSK3beta) is a fascinating enzyme with an astoundingly diverse number of actions in intracellular signaling systems. GSK3beta activity is regulated by serine (inhibitory) and tyrosine (stimulatory) phosphorylation, by protein complex formation, and by its intracellular localization. GSK3beta phosphorylates and thereby regulates the functions of many metabolic, signaling, and structural proteins. Notable among the signaling proteins regulated by GSK3beta are the many transcription factors, including activator protein-1, cyclic AMP response element binding protein, heat shock factor-1, nuclear factor of activated T cells, Myc, beta-catenin, CCAAT/enhancer binding protein, and NFkappaB. Lithium, the primary therapeutic agent for bipolar mood disorder, is a selective inhibitor of GSK3beta. This raises the possibility that dysregulation of GSK3beta and its inhibition by lithium may contribute to the disorder and its treatment, respectively. GSK3beta has been linked to all of the primary abnormalities associated with Alzheimer's disease. These include interactions between GSK3beta and components of the plaque-producing amyloid system, the participation of GSK3beta in phosphorylating the microtubule-binding protein tau that may contribute to the formation of neurofibrillary tangles, and interactions of GSK3beta with presenilin and other Alzheimer's disease-associated proteins. GSK3beta also regulates cell survival, as it facilitates a variety of apoptotic mechanisms, and lithium provides protection from many insults. Thus, GSK3beta has a central role regulating neuronal plasticity, gene expression, and cell survival, and may be a key component of certain psychiatric and neurodegenerative diseases.

Ectopic Wnt signal determines the eyeless phenotype of zebrafishmasterblindmutant

masterblind (mbl) is a zebrafish mutation characterised by the absence or reduction in size of the telencephalon, optic vesicles and olfactory placodes. We show that inhibition of Gsk3β in zebrafish embryos either by overexpression of dominant negative dn gsk3β mRNA or by lithium treatment after the midblastula transition phenocopies mbl. The loss of anterior neural tissue in mbl and lithium-treated embryos is preceded by posteriorization of presumptive anterior neuroectoderm during gastrulation, which is evident from the anterior shift of marker genes Otx2 and Wnt1. Heterozygous mbl embryos showed increased sensitivity to inhibition of GSK3β by lithium or dn Xgsk3β that led to the loss of eyes. Overexpression of gsk3β mRNA rescued eyes and the wild-type fgf8 expression of homozygous mbl embryos. emx1 that delineates the telencephalon is expanded and shifted ventroanteriorly in mbl embryos. In contrast to fgf8, the emx1 expression domain was not restored upon overexpression of gsk3β mRNA. These experiments place mbl as an antagonist of the Wnt pathway in parallel or upstream of the complex consisting of Axin, APC and Gsk3β that binds and phosphorylates β-catenin, thereby destabilising it. mbl maps on LG 3 close to a candidate gene axin1. In mbl we detected a point mutation in the conserved minimal Gsk3β-binding domain of axin1 leading to a leucine to glutamine substitution at position 399. Overexpression of wild-type axin1 mRNA rescued mbl completely, demonstrating that mutant axin1 is responsible for the mutant phenotype. Overexpression of mutant L399Q axin1 in wild-type embryos resulted in a dose-dependent dominant negative activity as demonstrated by the loss of telencephalon and eyes. We suggest that the function of Axin1/Mbl protein is to antagonise the Wnt signal and in doing so to establish and maintain the most anterior CNS. Our findings provide new insights into the mechanisms by which the Wnt pathway generates anteroposterior polarity of the neural plate.

The role of maternal axin in patterning the Xenopus embryo

Regulation of the stability of beta catenin protein is a critical role of Wnt signaling cascades. In early Xenopus development, dorsal axis specification depends on regulation of beta catenin by both cytoplasmic and nuclear mechanisms. While the cytoplasmic protein axin is known as a key component of the cytoplasmic beta catenin degradation complex, loss-of-function studies are needed to establish whether it is required for dorso-ventral patterning in the embryo, and to test where in the embryo it carries out its function. Here, we show that embryos lacking maternal axin protein have increased levels of soluble beta catenin protein and increased nuclear localization of beta catenin in ventral nuclei at the blastula stage. These embryos gastrulate abnormally and develop with excessive notochord and head structures, and reduced tail and ventral components. They show increased expression of dorsal markers, including siamois, Xnr3, chordin, gsc, Xhex, and Otx2, decreased expression of Xwnt 8 and Xbra, and little alteration of BMP4 and Xvent1 and -2 mRNA levels. The ventral halves of axin-depleted embryos at the gastrula stage have dramatically increased levels of chordin expression, and severely decreased levels of Xwnt 8 mRNA expression, while BMP4 transcript levels are only slightly reduced. This dorso-anterior phenotype is rescued by axin mRNA injected into the vegetal pole of axin-depleted oocytes before fertilization. Interestingly, the phenotype was rescued by ventral but not dorsal injection of axin mRNA, at the 4-cell stage, although dorsal injection into wild-type embryos does cause ventralization. These results show directly that the localized ventral activity of maternal axin is critical for the correct patterning of the early Xenopus embryo.

Differential molecular assemblies underlie the dual function of Axin in modulating the WNT and JNK pathways

Axin is a multidomain scaffold protein that exerts a dual function in the Wnt signaling and MEKK1/JNK pathways. This raises a critical question as to whether Axin-based differential molecular assemblies exist and how these may act to coordinate the two separate pathways. Here we show that both wild-type glycogen synthase kinase-3 beta (GSK-3 beta) and kinase-dead GSK-3 beta-Y216F (capable of binding to Axin), but not GSK-3 beta-K85M (incapable of binding to Axin in mammalian cells), prevented MEKK1 binding to the Axin complex, thereby inhibiting JNK activation. We further show that casein kinase I epsilon also inhibited Axin-mediated JNK activation by competing against MEKK1 binding. In contrast, beta-catenin and adenomatous polyposis coli binding did not affect MEKK1 binding to the same Axin complex. This suggests that even when Axin is "switched" to activate the JNK pathway, it is still capable of sequestering free beta-catenin, which is a critical aspect for cellular homeostasis. Our results clearly demonstrate that differential molecular assemblies underlie the duality of Axin functions in the negative regulation of Wnt signaling and activation of the JNK MAPK pathway.

Eukaryotic initiation factor 2B: identification of multiple phosphorylation sites in the epsilon-subunit and their functions in vivo

Eukaryotic initiation factor (eIF) 2B is a heteromeric guanine nucleotide exchange factor that plays an important role in regulating mRNA translation. Here we identify multiple phosphorylation sites in the largest, catalytic, subunit (epsilon) of mammalian eIF2B. These sites are phosphorylated by four different protein kinases. Two conserved sites (Ser712/713) are phosphorylated by casein kinase 2. They lie at the extreme C-terminus and are required for the interaction of eIF2Bepsilon with its substrate, eIF2, in vivo and for eIF2B activity in vitro. Glycogen synthase kinase 3 (GSK3) is responsible for phosphorylating Ser535. This regulatory phosphorylation event requires both the fourth site (Ser539) and a distal region, which acts to recruit GSK3 to eIF2Bepsilon in vivo. The fifth site, which lies outside the catalytic domain of eIF2Bepsilon, can be phosphorylated by casein kinase 1. All five sites are phosphorylated in the eIF2B complex in vivo.

Molecular targets of lithium action

Lithium is highly effective in the treatment of bipolar disorder and also has multiple effects on embryonic development, glycogen synthesis, hematopoiesis, and other processes. However, the mechanism of lithium action is still unclear. A number of enzymes have been proposed as potential targets of lithium action, including inositol monophosphatase, a family of structurally related phosphomonoesterases, and the protein kinase glycogen synthase kinase-3. These potential targets are widely expressed, require metal ions for catalysis, and are generally inhibited by lithium in an uncompetitive manner, most likely by displacing a divalent cation. Thus, the challenge is to determine which target, if any, is responsible for a given response to lithium in cells. Comparison of lithium effects with genetic disruption of putative target molecules has helped to validate these targets, and the use of alternative inhibitors of a given target can also lend strong support for or against a proposed mechanism of lithium action. In this review, lithium sensitive enzymes are discussed, and a number of criteria are proposed to evaluate which of these enzymes are involved in the response to lithium in a given setting.

Wnt signaling and mammary tumorigenesis

Wnt expression patterns during mammary development support a role for Wnts in breast development and in mammary epithelial responses to systemic hormones. The deregulation of Wnt signaling also plays a role in breast cancer. Activation of the Wnt signaling pathway is a major feature of several human neoplasias and appears to lead to the cytosolic stabilization of a transcriptional co-factor, beta-catenin. This co-activator can then regulate transcription from a number of target genes including the cellular oncogenes cyclin D1 and c-myc. This review will summarize the current state of knowledge of Wnt signal transduction in a range of model systems and will then address the role of Wnts and Wnt signaling in mammary development and cancer.

Calmodulin differentially modulates Smad1 and Smad2 signaling

The members of the Smad protein family are intracellular mediators of transforming growth factor beta (TGF-beta) signaling. Smad1 transduces bone morphogenetic protein signals, inducing formation of ventral mesoderm in Xenopus embryos, whereas Smad2 transduces activin/TGF-beta signals, generating dorsal mesoderm. Calmodulin directly binds to many Smads and was shown to down-regulate Smad2 activity in a cell culture system (Zimmerman, C. M., Kariapper, M. S. T., and Mathews, L. S. (1997) J. Biol. Chem. 273, 677-680). Here, we extend those data and demonstrate that calmodulin alters Smad signaling in living embryos, increasing Smad1 activity while inhibiting Smad2 function. To characterize this regulation, we undertook a structure-function analysis and found that calmodulin binds to two distinct and conserved regions in both Smad1 and Smad2. Receptor tyrosine kinase signaling also modifies Smad activity (Kretzschmar, M., Doody, J., and Massagué, J. (1997) Nature 389, 618-622; Kretzschmar, M., Doody, J., Timokhina, I., and Massagué, J. (1999) Genes Dev. 13, 804-816; de Caestecker, M. P., Parks, W. T., Frank, C. J., Castagnino, P., Bottaro, D. P., Roberts, A. B., and Lechleider, R. J. (1998) Genes Dev. 12, 1587-1592). We show that calmodulin binding to Smads inhibits subsequent Erk2-dependent phosphorylation of Smads and vice versa. These observations suggest the presence of a cross-talk between three major signaling cascades as follows: Ca(2+)/calmodulin, receptor tyrosine kinase, and TGF-beta pathways.

RGS proteins: lessons from the RGS9 subfamily

RGS proteins enhance the time resolution of G protein signaling cascades by accelerating GTP hydrolysis of G alpha subunits of heterotrimeric G proteins. RGS9-1, a photoreceptor-specific RGS protein, is the first vertebrate member of this sizeable family whose physiological function in a well-defined G protein pathway has been identified. It is essential for normal subsecond recovery kinetics of the light responses in retinal photoreceptors. Understanding this role allows RGS9-1 to serve as a useful model for understanding how specificity and regulation of RGS function are achieved. In addition to the catalytic RGS domain, shared among all members of this family, RGS9-1 contains several other domains, which are also found in a closely related subset of RGS proteins, the RGS9 subfamily. One of these domains, the G gamma-like (GGL) domain, has been identified as the attachment site for G beta 5 proteins, which act as obligate subunits for this subfamily. Results from RGS9-1 and other subfamily members suggest that specificity is achieved by cell type-specific transcription, RNA processing, and G beta 5-dependent protein stabilization. In addition, membrane localization via specific targeting domains likely plays an important role.

Effects of rat Axin domains on axis formation in Xenopus embryos

Wnt signaling plays an important role in axis formation in early vertebrate development. Axin is one Wnt signaling regulator that inhibits this pathway. The effects of the injection of mRNA of several rat Axin (rAxin) mutants on axis formation in Xenopus embryos were examined. It was found that rAxin mutants containing only a regulation of G-protein signaling (RGS) domain fragment or with deletion of the RGS domain induced axis formation. Because the RGS domain is a major adenomatous polyposis coli gene product (APC)-binding domain, APC association with glycogen synthase kinase 3beta (GSK3beta) on the Axin molecule may be important in inhibition of axis formation. The ventralizing activities of wild-type rAxin and a mutant in which the Dishevelled and Axin (DIX) domain was deleted (deltaDIX mutant) were examined. Histological examination and gene expression revealed that the ventralizing activity of the deltaDIX mutant was weaker than that of wild-type rAxin. This finding suggests that the C-terminus of rAxin contributes to the inhibition of Wnt signaling in Xenopus embryos. Furthermore, an rAxin mutant that contained both the RGS and GSK3beta-binding domains affected both the dorsal and ventral sides of blastomeres, mediated ectodermal fate and induced expansion of notochord and/or endoderm, but did not induce axis formation.

Maternally controlled (beta)-catenin-mediated signaling is required for organizer formation in the zebrafish

We have identified and characterized a zebrafish recessive maternal effect mutant, ichabod, that results in severe anterior and dorsal defects during early development. The ichabod mutation is almost completely penetrant, but exhibits variable expressivity. All mutant embryos fail to form a normal embryonic shield; most fail to form a head and notochord and have excessive development of ventral tail fin tissue and blood. Abnormal dorsal patterning can first be observed at 3.5 hpf by the lack of nuclear accumulation of (beta)-catenin in the dorsal yolk syncytial layer, which also fails to express bozozok/dharma/nieuwkoid and znr2/ndr1/squint. At the onset of gastrulation, deficiencies in expression of dorsal markers and expansion of expression of markers of ventral tissues indicate a dramatic alteration of dorsoventral identity. Injection of (beta)-catenin RNA markedly dorsalized ichabod embryos and often completely rescued the phenotype, but no measurable dorsalization was obtained with RNAs encoding upstream Wnt pathway components. In contrast, dorsalization was obtained when RNAs encoding either Bozozok/Dharma/Nieuwkoid or Znr2/Ndr1/Squint were injected. Moreover, injection of (beta)-catenin RNA into ichabod embryos resulted in activation of expression of these two genes, which could also activate each other. RNA injection experiments strongly suggest that the component affected by the ichabod mutation acts on a step affecting (beta)-catenin nuclear localization that is independent of regulation of (beta)-catenin stability. This work demonstrates that a maternal gene controlling localization of (beta)-catenin in dorsal nuclei is necessary for dorsal yolk syncytial layer gene activity and formation of the organizer in the zebrafish.

Dimerization choices control the ability of axin and dishevelled to activate c-Jun N-terminal kinase/stress-activated protein kinase

Axin and Dishevelled are two downstream components of the Wnt signaling pathway. Dishevelled is a positive regulator and is placed genetically between Frizzled and glycogen synthase kinase-3beta, whereas Axin is a negative regulator that acts downstream of glycogen synthase kinase-3beta. It is intriguing that they each can activate the c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) when expressed in the cell. We set out to address if Axin and Dishevelled are functionally cooperative, antagonistic, or entirely independent, in terms of the JNK activation event. We found that in contrast to Axin, Dvl2 activation of JNK does not require MEKK1, and complex formation between Dvl2 and Axin is independent of Axin-MEKK1 binding. Furthermore, Dvl2-DIX and Dvl2-DeltaDEP proteins deficient for JNK activation can attenuate Axin-activated JNK activity by disrupting Axin dimerization. However, Axin-DeltaMID, Axin-DeltaC, and Axin-CT proteins deficient for JNK activation cannot interfere with Dvl2-activated JNK activity. These results indicate that unlike the strict requirement of homodimerization for Axin function, Dvl2 can activate JNK either as a monomer or homodimer/heterodimer. We suggest that there may be a switch mechanism based on dimerization combinations, that commands cells to activate Wnt signaling or JNK activation, and to turn on specific activators of JNK in response to various environmental cues.

Biochemical characterization of the Drosophila axin protein

In the Wnt/Wingless pathway, accumulation of beta-catenin/Armadillo protein is a key regulatory step. Vertebrate Axin is a negative regulator of Wnt signaling, promoting glycogen synthase kinase-3beta-mediated phosphorylation of beta-catenin and thereby destabilizing beta-catenin. Using Drosophila cell culture systems, we demonstrated that a Drosophila homolog of Axin (Daxin) inhibits Wingless-induced Armadillo accumulation and Drosophila T-cell factor-dependent transcription induced by Wingless, Dishevelled, and Armadillo. The carboxy-terminal portion of Daxin is not essential for the inhibitory activity, but a mutant only consisting of this portion behaves as a dominant-negative protein. Moreover, interactions between Daxin and Zeste-white 3, Armadillo, Dishevelled, protein phosphatase 2A and Daxin itself were shown, providing direct evidence that Daxin is a scaffold protein in the Wingless pathway.

Interaction of dishevelled and Xenopus axin-related protein is required for wnt signal transduction

Signaling by the Wnt family of secreted proteins plays an important role in animal development and is often misregulated in carcinogenesis. Wnt signal transduction is controlled by the rate of degradation of beta-catenin by a complex of proteins including glycogen synthase kinase 3 (GSK3), adenomatous polyposis coli, and Axin. Dishevelled is required for Wnt signal transduction, and its activation results in stabilization of beta-catenin. However, the biochemical events underlying this process remain largely unclear. Here we show that Xenopus Dishevelled (Xdsh) interacts with a Xenopus Axin-related protein (XARP). This interaction depends on the presence of the Dishevelled-Axin (DIX) domains in both XARP and Xdsh. Moreover, the same domains are essential for signal transduction through Xdsh. Finally, our data point to a possible mechanism for signal transduction, in which Xdsh prevents beta-catenin degradation by displacing GSK3 from its complex with XARP.