Glycosylation and palmitoylation of Wnt-3a are coupled to produce an active form of Wnt-3a

Porcn-dependent Wnt signaling is not required prior to mouse gastrulation

In mice and humans the X-chromosomal porcupine homolog (Porcn) gene is required for the acylation and secretion of all 19 Wnt ligands and thus represents a bottleneck for all Wnt signaling. We have generated a mouse line carrying a floxed allele for Porcn and used zygotic, oocyte-specific and visceral endoderm-specific deletions to investigate embryonic and extra-embryonic requirements for Wnt ligand secretion. We show that there is no requirement for Porcn-dependent secretion of Wnt ligands during preimplantation development of the mouse embryo. Porcn-dependent Wnts are first required for the initiation of gastrulation, where Porcn function is required in the epiblast but not the visceral endoderm. Heterozygous female embryos, which are mutant in both trophoblast and visceral endoderm due to imprinted X chromosome inactivation, complete gastrulation but display chorio-allantoic fusion defects similar to Wnt7b mutants. Our studies highlight the importance of Wnt3 and Wnt7b for embryonic and placental development but suggest that endogenous Porcn-dependent Wnt secretion does not play an essential role in either implantation or blastocyst lineage specification.

The way Wnt works: components and mechanism

The canonical Wnt/β-catenin pathway is an ancient and evolutionarily conserved signaling pathway that is required for the proper development of all metazoans, from the basal demosponge Amphimedon queenslandica to humans. Misregulation of Wnt signaling is implicated in many human diseases, making this pathway an intense area of research in industry as well as academia. In this review, we explore our current understanding of the molecular steps involved in the transduction of a Wnt signal. We will focus on how the critical Wnt pathway component, β-catenin, is in a "futile cycle" of constant synthesis and degradation and how this cycle is disrupted upon pathway activation. We describe the role of the Wnt pathway in major human cancers and in the control of stem cell self-renewal in the developing organism and in adults. Finally, we describe well-accepted criteria that have been proposed as evidence for the involvement of a molecule in regulating the canonical Wnt pathway.

Apicobasal secretion of Wnt11 and Wnt3a in polarized epithelial cells is regulated by distinct mechanisms

Wnts are glycan- and lipid-modified morphogens that are important for cellular responses, but how Wnt is secreted in polarized epithelial cells remains unclear. Although Wntless (Wls) has been shown to interact with Wnts and support their secretion, the role of Wls in the sorting of Wnts to the final destination in polarized epithelial cells have not been clarified. Glycosylation was shown to be important for the sorting of some transmembrane and secreted proteins, but glycan profiles and their roles in the polarized secretion of Wnts are not known. Here we show the apicobasal secretion of Wnts is regulated by different mechanisms. Wnt11 and Wnt3a were secreted apically and basolaterally, respectively, in polarized epithelial cells. Wls was localized to the basolateral membrane. Mass-spectrometric analyses revealed that Wnt11 is modified with complex/hybrid-(Asn40), high-mannose-(Asn90), and high-mannose/hybrid-(Asn300) type glycans and that Wnt3a is modified with two high-mannose-type glycans (Asn87 and Asn298). Glycosylation processing at Asn40 and galectin-3 were required for the apical secretion of Wnt11, while clathrin and adaptor protein-1 were required for the basolateral secretion of Wnt3a. By the fusion of the Asn40 glycosylation site of Wnt11, Wnt3a was secreted apically. The recycling of Wls by AP-2 was necessary for the basolateral secretion of Wnt3a but not for the apical secretion of Wnt11. These results suggest that Wls has different roles on the polarized secretion of Wnt11 and Wnt3a and that glycosylation processing of Wnts decides their secretory routes.

Role of the Wnt-Frizzled system in cardiac pathophysiology: a rapidly developing, poorly understood area with enormous potential

Abstract  The Wnt-Frizzled (Fzd) G-protein-coupled receptor system, involving 19 distinct Wnt ligands and 10 Fzd receptors, plays key roles in the development and functioning of many organ systems. There is increasing evidence that Wnt-Fzd signalling is important in regulating cardiac function. Wnt-Fzd signalling primarily involves a canonical pathway, with dishevelled-1-dependent nuclear translocation of β-catenin that derepresses Wnt-sensitive gene transcription, but can also include non-canonical pathways via phospholipase-C/Ca(2+) mobilization and dishevelled-protein activation of small GTPases. Wnt-Fzd effects vary with specific ligand/receptor interactions and associated downstream pathways. This paper reviews the biochemistry and physiology of the Wnt-Fzd complex, and presents current knowledge of Wnt signalling in cardiac remodelling processes such as hypertrophy and fibrosis, as well as disease states such as myocardial infarction (MI), heart failure and arrhythmias. Wnt signalling is activated during hypertrophy; inhibiting Wnt signalling by activating glycogen synthase kinase attenuates the hypertrophic response. Wnt signalling has complex and time-dependent actions post-MI, so that either beneficial or harmful effects might result from Wnt-directed interventions. Stem cell biology, a promising area for therapeutic intervention, is highly regulated by Wnt signalling. The Wnt system regulates fibroblast function, and is prominently altered in arrhythmogenic ventricular cardiomyopathy, a familial disease involving excess deposition of fibroadipose tissue. Wnt signalling controls connexin43 expression, thereby contributing to the regulation of cardiac electrical stability and arrhythmia generation. Although much has been learned about Wnt-Fzd signalling in hypertrophy and infarction, its role is poorly understood for a broad range of other heart disorders. Much more needs to be learned for its contributions to be fully appreciated, and to permit more effective exploitation of its enormous potential in therapeutic development.

Coordinate regulation of N-glycosylation gene DPAGT1, canonical Wnt signaling and E-cadherin adhesion

The metabolic pathway of protein N-glycosylation influences intercellular adhesion by affecting the composition and cytoskeletal association of E-cadherin protein complexes, or adherens junctions (AJs). In sparse cells, E-cadherin is modified extensively with complex N-glycans and forms nascent AJs, while in dense cultures, hypoglycosylated E-cadherin drives the assembly of mature AJs with increased levels of γ- and α-catenins. N-glycosylation of E-cadherin is controlled by the DPAGT1 gene, a key regulator of the N-glycosylation pathway. DPAGT1 is a target of the canonical Wnt signaling pathway, with both β- and γ-catenins binding to Tcf at its promoter. We now report that DPAGT1 senses cell density through canonical Wnt signaling. In dense cells, depletion of β-catenin from the DPAGT1 promoter correlated with downregulation of its cellular abundance, while loss of nuclear γ-catenin reflected its greater recruitment to AJs. DPAGT1 itself affected canonical Wnt signaling, with forced changes in its expression resulting in corresponding changes in transcriptionally active β-catenin and canonical Wnt activity. Remarkably, a 2.4-fold increase in the DPAGT1 mRNA level resulted in increased N-glycosylation and reduced membrane localization of E-cadherin, coincident with dramatic changes in cell morphology. Lastly, we present evidence that N-glycosylation status of E-cadherin controls its antagonism of canonical Wnt signaling. Transfection of hypoglycosylated E-cadherin mutant, V13, but not fully N-glycosylated E-cadherin, into sparse cells inhibited canonical Wnt activity by depleting nuclear β- and γ-catenins. Collectively, our studies show that cells coordinate DPAGT1 expression and protein N-glycosylation with canonical Wnt signaling and E-cadherin adhesion via positive and negative feedback mechanisms.

Tiki1 is required for head formation via Wnt cleavage-oxidation and inactivation

Secreted Wnt morphogens are signaling molecules essential for embryogenesis, pathogenesis, and regeneration and require distinct modifications for secretion, gradient formation, and activity. Whether Wnt proteins can be posttranslationally inactivated during development and homeostasis is unknown. Here we identify, through functional cDNA screening, a transmembrane protein Tiki1 that is expressed specifically in the dorsal Spemann-Mangold Organizer and is required for anterior development during Xenopus embryogenesis. Tiki1 antagonizes Wnt function in embryos and human cells via a TIKI homology domain that is conserved from bacteria to mammals and acts likely as a protease to cleave eight amino-terminal residues of a Wnt protein, resulting in oxidized Wnt oligomers that exhibit normal secretion but minimized receptor-binding capability. Our findings identify a Wnt-specific protease that controls head formation, reveal a mechanism for morphogen inactivation through proteolysis-induced oxidation-oligomerization, and suggest a role of the Wnt amino terminus in evasion of oxidizing inactivation. TIKI proteins may represent potential therapeutic targets.

Wnt proteins

Wnt proteins comprise a major family of signaling molecules that orchestrate and influence a myriad of cell biological and developmental processes. Although our understanding of the role of Wnt signaling in regulating development and affecting disease, such as cancer, has been ever increasing, the study of the Wnt proteins themselves has been painstaking and slow moving. Despite advances in the biochemical characterization of Wnt proteins, many mysteries remain unsolved. In contrast to other developmental signaling molecules, such as fibroblast growth factors (FGF), transforming growth factors (TGFβ), and Sonic hedgehog (Shh), Wnt proteins have not conformed to many standard methods of protein production, such as bacterial overexpression, and analysis, such as ligand-receptor binding assays. The reasons for their recalcitrant nature are likely a consequence of the complex set of posttranslational modifications involving several highly specialized and poorly characterized processing enzymes. With the recent description of the first Wnt protein structure, the time is ripe to uncover and possibly resolve many of the remaining issues surrounding Wnt proteins and their interactions. Here we describe the process of maturation of Wnt from its initial translation to its eventual release from a cell and interactions in the extracellular environment.

Palmitoylation of Hedgehog proteins

Hedgehog (Hh) proteins are secreted signaling proteins that contain amide-linked palmitate at the N-terminus and cholesterol at the C-terminus. Palmitoylation of Hh proteins is critical for effective long- and short-range signaling. The palmitoylation reaction occurs during transit of Hh through the secretory pathway, most likely in the lumen of the ER. Attachment of palmitate to Hh proteins is independent of cholesterol modification and autoprocessing and is catalyzed by Hhat (Hedgehog acyltransferase). Hhat is a member of the membrane bound O-acyltransferase (MBOAT) family, a subgroup of multipass membrane proteins that catalyze transfer of fatty acyl groups to lipids and proteins. Several classes of secreted proteins have recently been shown to be substrates for MBOAT acyltransferases, including Hh proteins and Spitz (palmitoylated by Hhat), Wg/Wnt proteins (modified with palmitate and/or palmitoleate by Porcupine) and ghrelin (octanoylated by ghrelin O-acyltransferase). These findings highlight protein fatty acylation as a mechanism that not only influences membrane binding of intracellular proteins but also regulates the signaling range and efficacy of secreted proteins.

Tiki casts a spell on Wnt

Negative feedback is a widespread feature of signaling pathways. In an unexpected twist described in this issue, He and colleagues identify a membrane-tethered metalloprotease named Tiki that inhibits Wnt signaling by removing an essential eight-residue fragment from Wnt itself.

Wnt3a inhibits proliferation but promotes melanogenesis of melan-a cells

Melanocytes are pigment-producing cells responsible for coloration of skin and hair. Although the importance of Wnt3a in melanocyte development has been well recognized, the role of Wnt3a in mature melanocytes has not been elucidated. This study was conducted to further explore the effects of Wnt3a on melanocyte proliferation and melanogenesis, and to elucidate the possible mechanisms involved. We infected melan-a cells with AdWnt3a to serve as the production source of the Wnt3a protein. MTT assay, 5-bromodeoxyuridine incorporation assay and flow cytometric analysis showed that Wnt3a inhibited the proliferation of melan-a cells and this was associated with decrease of cells in the S phase and increase of cells in the G(1) phase. Melanin content and tyrosinase activity assay revealed that Wnt3a significantly promoted melanogenesis of melan-a cells. Furthermore, western blot analysis showed that Wnt3a upregulated the expression of microphthalmia-associated transcription factor and its downstream target genes, tyrosinase and tyrosinase-related protein 1 in melan-a cells. Collectively, our results suggest that Wnt3a plays an important role in melanocyte homeostasis.

PORCN moonlights in a Wnt-independent pathway that regulates cancer cell proliferation

Porcupine (PORCN) is a membrane-bound O-acyl transferase that is required for the palmitoylation of Wnt proteins, and that is essential in diverse Wnt pathways for Wnt-Wntless (WLS) binding, Wnt secretion, and Wnt signaling activity. We tested if PORCN was required for the proliferation of transformed cells. Knockdown of PORCN by multiple independent siRNAs results in a cell growth defect in a subset of epithelial cancer cell lines. The growth defect is transformation-dependent in human mammary epithelial (HMEC) cells. Additionally, inducible PORCN knockdown by two independent shRNAs markedly reduces the growth of established MDA-MB-231 cancers in orthotopic xenografts in immunodeficient mice. Unexpectedly, the proliferation defect resulting from loss of PORCN occurs in a Wnt-independent manner, as it is rescued by re-expression of catalytically inactive PORCN, and is not seen after RNAi-mediated knockdown of the Wnt carrier protein WLS, nor after treatment with the PORCN inhibitor IWP. Consistent with a role in a Wnt-independent pathway, knockdown of PORCN regulates a distinct set of genes that are not altered by other inhibitors of Wnt signaling. PORCN protein thus appears to moonlight in a novel signaling pathway that is rate-limiting for cancer cell growth and tumorigenesis independent of its enzymatic function in Wnt biosynthesis and secretion.

Wntless in Wnt secretion: molecular, cellular and genetic aspects

Throughout the animal kingdom, Wnt-triggered signal transduction pathways play fundamental roles in embryonic development and tissue homeostasis. Wnt proteins are modified as glycolipoproteins and are secreted into the extracellular environment as morphogens. Recent studies on the intracellular trafficking of Wnt proteins demonstrate multiple layers of regulation along its secretory pathway. These findings have propelled a great deal of interest among researchers to further investigate the molecular mechanisms that control the release of Wnts and hence the level of Wnt signaling. This review is dedicated to Wntless, a putative G-protein coupled receptor that transports Wnts intracellularly for secretion. Here, we highlight the conclusions drawn from the most recent cellular, molecular and genetic studies that affirm the role of Wntless in the secretion of Wnt proteins.

New insights into the mechanism of Wnt signaling pathway activation

Wnts compromise a large family of secreted, hydrophobic glycoproteins that control a variety of developmental and adult processes in all metazoan organisms. Recent advances in the Wnt-signal studies have revealed that distinct Wnts activate multiple intracellular cascades that regulate cellular proliferation, differentiation, migration, and polarity. Although the mechanism by which Wnts regulate different pathways selectively remains to be clarified, evidence has accumulated that in addition to the formation of ligand-receptor pairs, phosphorylation of receptors, receptor-mediated endocytosis, acidification, and the presence of cofactors, such as heparan sulfate proteoglycans, are also involved in the activation of specific Wnt pathways. Here, we review the mechanism of activation in Wnt signaling initiated on the cell-surface membrane. In addition, the mechanisms for fine-tuning by cross talk between Wnt and other signaling are also discussed.

Targeting protein lipidation in disease

Fatty acids and/or isoprenoids are covalently attached to a variety of disease-related proteins. The distinct chemical properties of each of these hydrophobic moieties allow lipid modification to serve as a mechanism to regulate protein structure, localization and function. This review highlights recent progress in identifying inhibitors of protein lipidation and their effects on human disease. Myristoylation inhibitors have shown promise in blocking the action of human pathogens. Although inhibitors that block prenylation of Ras proteins have not yet been successful for cancer treatment, they may be efficacious in the rare premature aging syndrome progeria. Agents that alter the palmitoylation status of Ras, Wnt and Hh proteins have recently been discovered, and represent the next generation of potential chemotherapeutics.

Aberrant amplification of the crosstalk between canonical Wnt signaling and N-glycosylation gene DPAGT1 promotes oral cancer

Oral cancer is one of the most aggressive epithelial malignancies, whose incidence is on the rise. Previous studies have shown that in a subset of human oral squamous cell carcinoma (OSCC) tumor specimens, overexpression of the DPAGT1 gene, encoding the dolichol-P-dependent N-acetylglucoseamine-1-phosphate transferase, a key regulator of the metabolic pathway of protein N-glycosylation, drives tumor cell discohesion by inhibiting E-cadherin adhesive function. Recently, we reported that DPAGT1 was a target of the canonical Wnt signaling pathway. Here, we link overexpression of DPAGT1 in human OSCC tumor specimens to aberrant activation of canonical Wnt signaling. We report dramatic increases in β- and γ-catenins at the DPAGT1 promoter and correlate them with reduced expression of a Wnt inhibitor, Dickkopf-1 (Dkk-1). Using human squamous carcinoma cell lines of the head and neck, we show that partial inhibition of DPAGT1 reduces canonical Wnt signaling, indicating that DPAGT1 and canonical Wnt signaling function in a positive feedback loop. We provide evidence that E-cadherin inhibits DPAGT1, canonical Wnt signaling and the OSCC cancer phenotype by depleting nuclear β- and γ-catenins, with hypoglycosylated E-cadherin being the most effective. This suggests that in human OSCC, extensive N-glycosylation of E-cadherin compromises its ability to inhibit canonical Wnt signaling and DPAGT1 expression. Our studies reveal a novel interplay between DPAGT1/N-glycosylation and canonical Wnt signaling and suggest that dysregulation of this crosstalk is a key mechanism underlying OSCC. They also suggest that partial inhibition of DPAGT1 may represent an effective way to restore normal interactions among these essential pathways in oral cancer.

Localization of glypican-4 in different membrane microdomains is involved in the regulation of Wnt signaling

Glypicans are members of the heparan sulfate proteoglycans (HSPGs) and are involved in various growth factor signaling mechanisms. Although HSPGs affect the β-catenin-dependent and -independent pathways of Wnt signaling, how they regulate distinct Wnt pathways is not clear. It has been suggested that the β-catenin-dependent pathway is initiated through receptor endocytosis in lipid raft microdomains and the independent pathway is activated through receptor endocytosis in non-lipid raft microdomains. Here, evidence is presented that glypican-4 (GPC4) is localized to both membrane microdomains and that the localization affects its ability to regulate distinct Wnt pathways. GPC4 bound to Wnt3a and Wnt5a, which activate the β-catenin-dependent and -independent pathways, respectively, and colocalized with Wnts on the cell surface. LRP6, one of Wnt3a coreceptors, was present in lipid raft microdomains, whereas Ror2, one of Wnt5a coreceptors, was localized to non-lipid raft microdomains. Expression of GPC4 enhanced the Wnt3a-dependent β-catenin pathway and the Wnt5a-dependent β-catenin-independent pathway, and knockdown of GPC4 suppressed both pathways. A GPC4 mutant that was localized to only non-lipid raft microdomains inhibited the β-catenin-dependent pathway but enhanced the β-catenin-independent pathway. These results suggest that GPC4 concentrates Wnt3a and Wnt5a to the vicinity of their specific receptors in different membrane microdomains, thereby regulating distinct Wnt signaling.

Wnt/β-catenin signaling pathway and thioredoxin-interacting protein (TXNIP) mediate the "glucose sensor" mechanism in metastatic breast cancer-derived cells MDA-MB-231

In this study we investigated the effect of glucose on GSK3β and β-catenin expression and the involvement of the N-linked glycosylation and hexosamine pathways in the Wnt canonical pathway in response to in vitro conditions resembling normoglycemia (5  mmol) and hyperglycemia (20  mmol) in the metastatic breast cancer-derived cell line MDA-MB-231. We also investigated the relationship between this circuitry and the thioredoxin-interacting protein (TXNIP) regulation that seems to be related. MDA-MB-231 cells were grown either in 5 or 20  mM glucose chronically prior to plating. For glucose shift (5/20), cells were plated in 5  mM glucose and shifted to 20  mM at time 0. Both protein and mRNA levels for GSK3β but only the protein expression for β-catenin, were increased in response to high glucose. Furthermore, we assessed the response of GSK3β, β-catenin, and TXNIP to inhibition of the N-linked glycosylation, hexosamine, and Wnt pathways. Wnt signaling pathway activation was validated by specific reporter assay. We show that high levels of glucose regulate mRNA and protein expression of GSK3β, and consequently higher levels of activated β-catenin protein, which locates to the nucleus and is associated with increased levels of cyclin D1 expression. This event coincides with increased level of N-terminal Ser 9 phosphorylation of GSK3β protein. The inhibition of both the hexosamine pathway and N-linked glycosylation along with Wnt signaling pathway by sFRP1 and DKK1 is associated with significant decrease of the protein levels of GSK3β, β-catenin, and TXNIP RNA. Our work illuminates a novel and never described before function of this signaling pathway that relates glucose metabolism with redox regulation mechanism.

Roles of N-glycosylation and lipidation in Wg secretion and signaling

Wnt members act as morphogens essential for embryonic patterning and adult homeostasis. Currently, it is still unclear how Wnt secretion and its gradient formation are regulated. In this study, we examined the roles of N-glycosylation and lipidation/acylation in regulating the activities of Wingless (Wg), the main Drosophila Wnt member. We show that Wg mutant devoid of all the N-glycosylations exhibits no major defects in either secretion or signaling, indicating that N-glycosylation is dispensable for Wg activities. We demonstrate that lipid modification at Serine 239 (S239) rather than that at Cysteine 93 (C93) plays a more important role in regulating Wg signaling in multiple developmental contexts. Wg S239 mutant exhibits a reduced ability to bind its receptor, Drosophila Frizzled 2 (dFz2), suggesting that S239 is involved in the formation of a Wg/receptor complex. Importantly, while single Wg C93 or Wg S239 mutants can be secreted, removal of both acyl groups at C93 and S239 renders Wg incapable of reaching the plasma membrane for secretion. These data argue that lipid modifications at C93 and S239 play major roles in Wg secretion. Further experiments demonstrate that two acyl attachment sites in the Wg protein are required for the interaction of Wg with Wntless (Wls, also known as Evi or Srt), the key cargo receptor involved in Wg secretion. Together, our data demonstrate the in vivo roles of N-glycosylation and lipid modification in Wg secretion and signaling.

Dissecting the Wnt secretion pathway: key questions on the modification and intracellular trafficking of Wnt proteins

The Wnt family of signalling proteins has essential functions in development and adult tissue homoeostasis throughout the animal kingdom. Although signalling cascades triggered by Wnt proteins have been extensively studied, much remains to be learned about how Wnts are produced and secreted. Over the past few years, it has become clear that the secretion of Wnt proteins requires a specialized trafficking pathway. As this pathway has been discussed in two recent reviews (Lorenowicz & Korswagen 2009, Port & Basler 2010), we will focus our discussion on the key questions that need to be addressed to gain a more complete understanding of the mechanism and regulation of this essential secretion pathway.

Wnt5a signaling controls cytokinesis by positioning ESCRT-III to the proper site at the midbody

Wnts activate at least two signaling pathways, the β-catenin-dependent and -independent pathways. Although the β-catenin-dependent pathway is known to contribute to G1/S transition, involvement of the β-catenin-independent pathway in cell cycle regulation remains unclear. Here, we show that Wnt5a signaling, which activates the β-catenin-independent pathway, is required for cytokinesis. Dishevelled 2 (Dvl2), a mediator of Wnt signaling pathways, was localized to the midbody during cytokinesis. Beside the localization of Dvl2, Fz2, a Wnt receptor, was detected in the midbody with an endosomal sorting complex required for transport III (ESCRT-III) subunit, CHMP4B. Depletion of Wnt5a, its receptors, and Dvl increased multinucleated cells. The phenotype observed in Wnt5a-depleted cells was rescued by the addition of purified Wnt5a but not that of Wnt3a, which is a ligand for the β-catenin-dependent pathway. Moreover, depletion of Wnt5a signaling caused loss of stabilized microtubules and mislocalization of CHMP4B in the midbody, which affected abscission. Inhibition of the stabilization of microtubules at the midbody lead to the mislocalization of CHMP4B, while depletion of CHMP4B did not affect the stabilization of microtubules, suggesting that the correct localization of CHMP4B depends on microtubules. Fz2 was localized to the midbody in a Rab11-dependent manner probably along stabilized microtubules. Fz2 formed a complex with CHMP4B upon Wnt5a stimulation and was required for proper localization of CHMP4B at the midbody, while CHMP4B was not necessary for the localization of Fz2. These results suggest that Wnt5a signaling positions ESCRT-III in the midbody properly for abscission by stabilizing midbody microtubules.

Secreted Wingless-interacting molecule (Swim) promotes long-range signaling by maintaining Wingless solubility

Lipid-modified Wnt/Wingless (Wg) proteins can signal to their target cells in a short- or long-range manner. How these hydrophobic proteins travel through the extracellular environment remains an outstanding question. Here, we report on a Wg binding protein, Secreted Wg-interacting molecule (Swim), that facilitates Wg diffusion through the extracellular matrix. Swim, a putative member of the Lipocalin family of extracellular transport proteins, binds to Wg with nanomolar affinity in a lipid-dependent manner. In quantitative signaling assays, Swim is sufficient to maintain the solubility and activity of purified Wg. In Drosophila, swim RNAi phenotypes resemble wg loss-of-function phenotypes in long-range signaling. We propose that Swim is a cofactor that promotes long-range Wg signaling in vivo by maintaining the solubility of Wg.

The role of Lrp5/6 in cardiac valve disease: LDL-density-pressure theory

Atherosclerosis and osteoporosis are the leading causes of mortality and morbidity in the World. Recent epidemiologic studies have demonstrated that these disease processes develop in parallel. Evidence indicates that hyperlipidemia plays a paradoxical role in both disease processes. However, the mechanism is not understood. This prospectus hypothesizes the role of lipids activate atherosclerosis within the bone and the heart to initiate the development of diseases in both of these tissues. The Prospectus on the Lrp 5/6 receptors provides a foundation for the mechanisms involved in the Lrp5/6 mediated disease biology. The LDL-Density-Pressure theory: the Role of Lrp5/6 provides a biological and a hemodynamic approach towards understanding the development of valvular heart disease and the implications in the field of bone molecular biology. This prospectus will review the current literature, provide a basis for the development of valve disease and indicate future therapeutic pathways for this disease process in the future.

Waif1/5T4 inhibits Wnt/β-catenin signaling and activates noncanonical Wnt pathways by modifying LRP6 subcellular localization

Wnt proteins can activate distinct signaling pathways, but little is known about the mechanisms regulating pathway selection. Here we show that the metastasis-associated transmembrane protein Wnt-activated inhibitory factor 1 (Waif1/5T4) interferes with Wnt/β-catenin signaling and concomitantly activates noncanonical Wnt pathways. Waif1 inhibits β-catenin signaling in zebrafish and Xenopus embryos as well as in mammalian cells, and zebrafish waif1a acts as a direct feedback inhibitor of wnt8-mediated mesoderm and neuroectoderm patterning during zebrafish gastrulation. Waif1a binds to the Wnt coreceptor LRP6 and inhibits Wnt-induced LRP6 internalization into endocytic vesicles, a process that is required for pathway activation. Thus, Waif1a modifies Wnt/β-catenin signaling by regulating LRP6 subcellular localization. In addition, Waif1a enhances β-catenin-independent Wnt signaling in zebrafish embryos and Xenopus explants by promoting a noncanonical function of Dickkopf1. These results suggest that Waif1 modulates pathway selection in Wnt-receiving cells.

Structural and functional studies of LRP6 ectodomain reveal a platform for Wnt signaling

LDL-receptor-related protein 6 (LRP6), alongside Frizzled receptors, transduces Wnt signaling across the plasma membrane. The LRP6 ectodomain comprises four tandem β-propeller-EGF-like domain (PE) pairs that harbor binding sites for Wnt morphogens and their antagonists including Dickkopf 1 (Dkk1). To understand how these multiple interactions are integrated, we combined crystallographic analysis of the third and fourth PE pairs with electron microscopy (EM) to determine the complete ectodomain structure. An extensive inter-pair interface, conserved for the first-to-second and third-to-fourth PE interactions, contributes to a compact platform-like architecture, which is disrupted by mutations implicated in developmental diseases. EM reconstruction of the LRP6 platform bound to chaperone Mesd exemplifies a binding mode spanning PE pairs. Cellular and binding assays identify overlapping Wnt3a- and Dkk1-binding surfaces on the third PE pair, consistent with steric competition, but also suggest a model in which the platform structure supports an interplay of ligands through multiple interaction sites.

Differential palmit(e)oylation of Wnt1 on C93 and S224 residues has overlapping and distinct consequences

Though the mechanisms by which cytosolic/intracellular proteins are regulated by the post-translational addition of palmitate adducts is well understood, little is known about how this lipid modification affects secreted ligands, such as Wnts. Here we use mutational analysis to show that differential modification of the two known palmit(e)oylated residues of Wnt1, C93 and S224, has both overlapping and distinct consequences. Though the relative roles of each residue are similar with respect to stability and secretion, two distinct biological assays in L cells show that modification of C93 primarily modulates signaling via a ß-catenin independent pathway while S224 is crucial for ß-catenin dependent signaling. In addition, pharmacological inhibition of Porcupine (Porcn), an upstream regulator of Wnt, by IWP1, specifically inhibited ß-catenin dependent signaling. Consistent with these observations, mapping of amino acids in peptide domains containing C93 and S224 demonstrate that acylation of C93 is likely to be Porcn-independent while that of S224 is Porcn-dependent. Cumulatively, our data strongly suggest that C93 and S224 are modified by distinct enzymes and that the differential modification of these sites has the potential to influence Wnt signaling pathway choice.

Beta amyloid effects on expression of multidrug efflux transporters in brain endothelial cells

ABC (ATP Binding Cassette) efflux transporters at the blood-brain barrier, P-glycoprotein (ABCB1), multidrug resistance associated protein 4 (ABCC4) and breast cancer resistance protein (ABCG2), are important for protecting the brain from circulating xenobiotics. Their expression is regulated by signals from surrounding brain tissue that may alter in CNS pathologies. Differences have been reported in transporter expression on brain vasculature of Alzheimer's subjects where raised levels of β-amyloid (Aβ) occur. The present study examines in vitro the effects of Aβ using immortalised brain endothelial cells (hCMEC/D3). Significantly lower expression of ABCB1 but not ABCC4 or ABCG2 was found following exposure to Aβ(1-42) peptide but not its scrambled equivalent. This was evident at both protein and transcript level and was reflected in lower transcriptional activity of the ABCB1 promoter as judged from the luciferase reporter gene assay and in decreases in ABCB1-mediated efflux of rhodamine 123. Aβ exposure also affected Wnt/β-catenin signalling, decreasing levels of β-catenin protein, reducing activation of TOPFLASH and increasing transcript levels of endogenous inhibitor, Dkk-1. Application of Wnt3a reversed the Aβ-induced changes to ABCB1 protein. These results suggest that Aβ may impair Wnt/β-catenin signalling at the blood-brain barrier but that activation of this pathway may restore ABCB1.

Extracellular movement of signaling molecules

Extracellular signaling molecules have crucial roles in development and homeostasis, and their incorrect deployment can lead to developmental defects and disease states. Signaling molecules are released from sending cells, travel to target cells, and act over length scales of several orders of magnitude, from morphogen-mediated patterning of small developmental fields to hormonal signaling throughout the organism. We discuss how signals are modified and assembled for transport, which routes they take to reach their targets, and how their range is affected by mobility and stability.

Drugging the cancer stem cell compartment: lessons learned from the hedgehog and Wnt signal transduction pathways

Cell-cell communication mediated by the secreted Hedgehog (Hh) and Wnt signaling molecules is essential to the coordination of cell fate decision making throughout the metazoan lifespan. From decades of genetically based interrogation, core components constituting the Hh and Wnt signal transduction pathways have been assembled, and a deep appreciation of how these signals elaborate distinct bodily tissues during development has been established. On the other hand, our incapacity to leverage similar genetic approaches to study adult organ systems has limited our understanding of how these molecules promote tissue renewal and regeneration through stem cell regulation. We discuss recent progress in the use of chemically based approaches to achieve control of these pathway activities in a broad range of biological studies and therapeutic contexts. In particular, we discuss the unique experimental opportunities that chemical modulators of these pathways afford in exploring the cancer stem cell hypothesis.

International Union of Basic and Clinical Pharmacology. LXXX. The class Frizzled receptors

The receptor class Frizzled, which has recently been categorized as a separate group of G protein-coupled receptors by the International Union of Basic and Clinical Pharmacology, consists of 10 Frizzleds (FZD(1-10)) and Smoothened (SMO). The FZDs are activated by secreted lipoglycoproteins of the Wingless/Int-1 (WNT) family, whereas SMO is indirectly activated by the Hedgehog (HH) family of proteins acting on the transmembrane protein Patched (PTCH). Recent years have seen major advances in our knowledge about these seven-transmembrane-spanning proteins, including: receptor function, molecular mechanisms of signal transduction, and the receptor's role in embryonic patterning, physiology, cancer, and other diseases. Despite intense efforts, many question marks and challenges remain in mapping receptor-ligand interaction, signaling routes, mechanisms of specificity and how these molecular details underlie disease and also the receptor's important role in physiology. This review therefore focuses on the molecular aspects of WNT/FZD and HH/SMO signaling discussing receptor structure, mechanisms of signal transduction, accessory proteins, receptor dynamics, and the possibility of targeting these signaling pathways pharmacologically.

Down-regulation of chondroitin 4-O-sulfotransferase-1 by Wnt signaling triggers diffusion of Wnt-3a

During metazoan development, Wnt molecules are secreted from Wnt-producing cells, diffuse to target cells, and determine cell fates; therefore, Wnt secretion is tightly regulated. However, the molecular mechanisms controlling Wnt diffusion are not fully elucidated. The specific chondroitin sulfate (CS) structure synthesized by chondroitin-4-O-sulfotransferase-1 (C4ST-1) binds to Wnt-3a with high affinity (Nadanaka, S., Ishida, M., Ikegami, M., and Kitagawa, H. (2008) J. Biol. Chem. 283, 27333-27343). In this study we tested whether Wnt signaling regulates sulfation patterns of cell-associated CS chains by suppressing expression of C4ST-1 to trigger release of Wnt molecules from Wnt-producing cells. C4ST-1 expression was dramatically reduced in L cells that stably expressed Wnt-3a (L-Wnt-3a cells) and had CS with low affinity for Wnt-3a. Forced expression of C4ST-1 in L-Wnt-3a cells inhibited diffusion of Wnt-3a due to structural alterations in CS chains mediated by C4ST-1. Furthermore, sustained Wnt signaling negatively regulated C4ST-1 expression in a cell-autonomous and non-cell autonomous fashion. These results demonstrated that C4ST-1 is a key downstream target of Wnt signaling that regulates Wnt diffusion from Wnt-producing cells.

Wnt signaling signaling at and above the receptor level

Wnt signaling is one of the most important developmental signaling pathways that controls cell fate decisions and tissue patterning during early embryonic and later development. It is activated by highly conserved Wnt proteins that are secreted as palmitoylated glycoproteins and act as morphogens to form a concentration gradient across a developing tissue. Wnt proteins regulate transcriptional and posttranscriptional processes depending on the distance of their origin and activate distinct intracellular cascades, commonly referred to as canonical (β-catenin-dependent) and noncanonical (β-catenin-independent) pathways. Therefore, the secretion and the diffusion of Wnt proteins needs to be tightly regulated to induce short- and long-range downstream signaling. Even though the Wnt signaling cascade has been studied intensively, key aspects and principle mechanisms, such as transport of Wnt growth factors or regulation of signaling specificity between different Wnt pathways, remain unresolved. Here, we introduce basic principles of Wnt/Wg signal transduction and highlight recent discoveries, such as the involvement of vacuolar ATPases and vesicular acidification in Wnt signaling. We also discuss recent findings regarding posttranslational modifications of Wnts, trafficking through the secretory pathway and developmental consequences of impaired Wnt secretion. Understanding the detailed mechanism and regulation of Wnt protein secretion will provide valuable insights into many human diseases based on overactivated Wnt signaling.

Metabolic alterations and targeted therapies in prostate cancer

Cancer cells synthesize de novo large amounts of fatty acids and cholesterol, irrespective of the circulating lipid levels and benefit from this increased lipid synthesis in terms of growth advantage, self-survival and drug resistance. Key lipogenic alterations that commonly occur in prostate cancer include over-expression of the enzyme fatty acid synthase (FASN) and deregulation of the 5-AMP-activated protein kinase (AMPK). FASN is a key metabolic enzyme that catalyses the synthesis of palmitate from the condensation of malonyl-CoA and acetyl-CoA de novo and plays a central role in energy homeostasis, by converting excess carbon intake into fatty acids for storage. AMPK functions as a central metabolic switch that governs glucose and lipid metabolism. Recent interest has focused on the potential of targeting metabolic pathways that may be altered during prostate tumorigenesis and progression. Several small molecule inhibitors of FASN have now been described or in development for therapeutic use; in addition, drugs that directly or indirectly induce AMPK activation have potential benefit in prostate cancer prevention and treatment.

Wnt signaling from development to disease: insights from model systems

One of the early surprises in the study of cell adhesion was the discovery that beta-catenin plays dual roles, serving as an essential component of cadherin-based cell-cell adherens junctions and also serving as the key regulated effector of the Wnt signaling pathway. Here, we review our current model of Wnt signaling and discuss how recent work using model organisms has advanced our understanding of the roles Wnt signaling plays in both normal development and in disease. These data help flesh out the mechanisms of signaling from the membrane to the nucleus, revealing new protein players and providing novel information about known components of the pathway.

WLS-dependent secretion of WNT3A requires Ser209 acylation and vacuolar acidification

Wnt proteins are secreted post-translationally modified proteins that signal locally to regulate development and proliferation. The production of bioactive Wnts requires a number of dedicated factors in the secreting cell whose coordinated functions are not fully understood. A screen for small molecules identified inhibitors of vacuolar acidification as potent inhibitors of Wnt secretion. Inhibition of the V-ATPase or disruption of vacuolar pH gradients by diverse drugs potently inhibited Wnt/β-catenin signaling both in cultured human cells and in vivo, and impaired Wnt-regulated convergent extension movements in Xenopus embryos. WNT secretion requires its binding to the carrier protein wntless (WLS); we find that WLS is ER-resident in human cells and WNT3A binding to WLS requires PORCN-dependent lipid modification of WNT3A at serine 209. Inhibition of vacuolar acidification results in accumulation of the WNT3A-WLS complex both in cells and at the plasma membrane. Modeling predictions suggest that WLS has a lipid-binding β-barrel that is similar to the lipocalin-family fold. We propose that WLS binds Wnts in part through a lipid-binding domain, and that vacuolar acidification is required to release palmitoylated WNT3A from WLS in secretory vesicles, possibly to facilitate transfer of WNT3A to a soluble carrier protein.

N-glycosylation gene DPAGT1 is a target of the Wnt/beta-catenin signaling pathway

Protein N-glycosylation and the Wnt/β-catenin signaling pathways play critical roles in development and cancer. Although N-glycosylation has been shown to influence Wnt signaling through its effects on Wnt ligands, it is unclear whether the Wnt/β-catenin pathway impacts protein N-glycosylation. In this study, we show that promoters of the first N-glycosylation gene, DPAGT1, from Chinese hamster ovary (CHO), Madin-Darby canine kidney (MDCK), and human epidermoid carcinoma (A253) cells contain the T-cell factor/lymphoid enhancer-binding factor (TCF/LEF) consensus sequence. Treatment of cells with a Wnt activator, lithium chloride, up-regulated DPAGT1 transcript levels that correlated with an increase in the β-catenin abundance. Furthermore, exposure of cells to a Wnt receptor ligand, Wnt3a, resulted in an increase in the DPAGT1 transcript levels that was abrogated by the Wnt inhibitor, Dickkopf-1. DNA mobility shift assays revealed specific protein complexes at the DPAGT1 TCF/LEF binding region that were competed off with antibodies to either Tcf3/4 or β-catenin. Chromatin immunoprecipitation analysis confirmed the presence of β-catenin at the DPAGT1 promoter in vivo. In addition, the DPAGT1 TCF/LEF sequence drove the expression of the luciferase reporter gene. Furthermore, up-regulation of DPAGT1 transcripts by Wnt3a led to altered N-glycosylation of E-cadherin. Interestingly, the DPAGT1 TCF/LEF sequence also interacted with γ-catenin, a close homologue of β-catenin, although not in a lithium chloride-dependent manner. Our results provide the first evidence that the Wnt/β-catenin signaling pathway regulates the metabolic pathway of protein N-glycosylation by targeting DPAGT1 expression. Moreover, they suggest the existence of another regulatory mechanism involving the interaction of Tcf with γ-catenin at the DPAGT1 promoter.

Analysis of Dickkopf3 interactions with Wnt signaling receptors

Wnt signaling regulates essential biological processes ranging from embryogenesis to neurodegeneration. Recently, we demonstrated that Dickkopf3 (Dkk3) is a pro-survival glycoprotein that positively modulates Wnt signaling. An important step in understanding the mechanism of action of Dkk3 is identifying its interacting proteins in the Wnt pathway. In this study, we used a series of biochemical and functional assays to investigate the interaction between Dkk3 and the Wnt pathway receptors Kremen1 (Krm1), Kremen 2 (Krm2) and low-density lipoprotein receptor-related protein 6 (LRP6). Here, we report that, contrary to previous studies, Dkk3 interacts with Krm1 and Krm2. However, Dkk3 did not interact with, or alter expression of, LRP6. Blocking protein glycosylation did not alter the interaction between Dkk3 and Krm proteins. Additionally, Krm2 abolished Dkk3-mediated potentiation of Wnt signaling. Therefore, our data establish that Krm proteins are novel binding partners of Dkk3 and suggest a mechanism by which Dkk3 potentiates Wnt signaling.

Wnt/beta-catenin signaling: components, mechanisms, and diseases

Signaling by the Wnt family of secreted glycolipoproteins via the transcriptional coactivator beta-catenin controls embryonic development and adult homeostasis. Here we review recent progress in this so-called canonical Wnt signaling pathway. We discuss Wnt ligands, agonists, and antagonists, and their interactions with Wnt receptors. We also dissect critical events that regulate beta-catenin stability, from Wnt receptors to the cytoplasmic beta-catenin destruction complex, and nuclear machinery that mediates beta-catenin-dependent transcription. Finally, we highlight some key aspects of Wnt/beta-catenin signaling in human diseases including congenital malformations, cancer, and osteoporosis, and discuss potential therapeutic implications.

Wnt/beta-catenin signaling: components, mechanisms, and diseases

Signaling by the Wnt family of secreted glycolipoproteins via the transcriptional coactivator beta-catenin controls embryonic development and adult homeostasis. Here we review recent progress in this so-called canonical Wnt signaling pathway. We discuss Wnt ligands, agonists, and antagonists, and their interactions with Wnt receptors. We also dissect critical events that regulate beta-catenin stability, from Wnt receptors to the cytoplasmic beta-catenin destruction complex, and nuclear machinery that mediates beta-catenin-dependent transcription. Finally, we highlight some key aspects of Wnt/beta-catenin signaling in human diseases including congenital malformations, cancer, and osteoporosis, and discuss potential therapeutic implications.

Wnt/beta-catenin signaling: components, mechanisms, and diseases

Signaling by the Wnt family of secreted glycolipoproteins via the transcriptional coactivator beta-catenin controls embryonic development and adult homeostasis. Here we review recent progress in this so-called canonical Wnt signaling pathway. We discuss Wnt ligands, agonists, and antagonists, and their interactions with Wnt receptors. We also dissect critical events that regulate beta-catenin stability, from Wnt receptors to the cytoplasmic beta-catenin destruction complex, and nuclear machinery that mediates beta-catenin-dependent transcription. Finally, we highlight some key aspects of Wnt/beta-catenin signaling in human diseases including congenital malformations, cancer, and osteoporosis, and discuss potential therapeutic implications.

Laminin gamma2 mediates Wnt5a-induced invasion of gastric cancer cells

BACKGROUND & AIMS

Wnt5a expression stimulates in vitro migration and invasion of cultured gastric cancer cells by an unknown mechanism and is also correlated with aggressiveness of gastric tumors. The aim of this study was to show that Wnt5a is involved in metastasis of gastric cancer cells in vivo and to explore the molecular mechanism by which Wnt5a regulates migration and invasion.

METHODS

In an experimental liver metastasis assay, Wnt5a-knockdown gastric cancer cells were injected into the spleens of nude mice. Microarray analyses were used to compare expression patterns between mouse fibroblast L cells that stably express wild-type and a mutant form of Wnt5a to investigate Wnt5a-dependent gene expression. The expression of genes found to be regulated by Wnt5a was investigated in cultured gastric cancer cells. Immunohistochemical analyses were performed to measure levels of Wnt-regulated gene products in 153 gastric cancer samples.

RESULTS

Knockdown of Wnt5a in gastric cancer cells reduced the number of liver metastases that formed in nude mice. Microarray analyses indicated that Wnt5a activity induced expression of the gene encoding laminin gamma2, a subunit of the epithelial basement membrane protein laminin-5. Wnt5a induced the expression of laminin gamma2 through the activation of protein kinase C and c-Jun-N-terminal kinase. The invasive activity of gastric cancer cells depended on laminin gamma2; Wnt5a expression levels correlated with those of laminin gamma2 in diffuse-scattered type gastric tumor samples from patients.

CONCLUSIONS

Wnt5a contributes to gastric cancer progression by increasing metastatic potential. Wnt5a up-regulates laminin gamma2 to mediate gastric cancer cell aggressiveness.

Sailing with the Wnt: charting the Wnt processing and secretion route

Wnt proteins are members of a highly conserved family of signalling molecules that play a central role in development and disease. During the past years, the different signalling pathways that are triggered by Wnt proteins have been studied in detail, but it is still largely unknown how a functional Wnt protein is produced and secreted. The recent finding that Wnt proteins are post-translationally modified and the discovery of the Wnt binding protein Wntless and its trafficking by the retromer complex show that Wnt secretion is a complex and highly regulated process. In this review, we will give an overview of the Wnt maturation and secretion pathway and discuss how this process may influence the spreading and signalling activity of Wnt.

Lipid-modified morphogens: functions of fats

Despite their location in the aqueous extracellular environment, a number of secreted proteins carry hydrophobic lipid modifications. These modifications include glycosylphosphatidylinositol, cholesterol, and both saturated and unsaturated fatty acids, and they are attached in the secretory pathway by different classes of enzymes. Lipid attachments make crucial contributions to protein function in vivo through a diverse array of mechanisms. They can promote protein maturation and secretion, membrane tethering, targeting to specific membrane subdomains, or receptor binding and activation. Additionally, secretion of lipid-modified morphogens of the Wnt and Hh families requires dedicated accessory proteins and may involve their packaging into lipoprotein particles for long-range transport.

Mammalian Wnt3a is released on lipoprotein particles

Little is known about the release and intercellular transport of Wnt proteins from mammalian cells. Lipoproteins may act as carriers for the intercellular movement and gradient formation of the lipid-linked morphogens Wingless and Hedgehog in Drosophila. To investigate whether such a mechanism can occur in mammals, we have studied Wnt release in cultured mammalian cells. Wnt3a associated with lipoproteins in the culture medium and not with extracellular vesicles or exosomes. Although Wnt3a was associated with both high-density lipoproteins (HDL) and low-density lipoproteins, only HDL allowed Wnt3a release from mouse fibroblasts. Remarkably, Wnt3a lacking its palmitate moiety was released in a lipoprotein-independent manner, demonstrating the dual role of palmitoylation in membrane and lipoprotein binding. We additionally found that Wnt3a can be released from enterocyte cell lines on endogenously expressed lipoproteins. We further discuss the physiological implications of our findings.