Helping Wingless take flight: how WNT proteins are secreted

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.

Wntless functions in mature osteoblasts to regulate bone mass

Recent genome-wide association studies of individuals of Asian and European descent have found that SNPs located within the genomic region (1p31.3) encoding the Wntless (Wls)/Gpr177 protein are associated significantly with reduced bone mineral density. Wls/Gpr177 is a newly identified chaperone protein that specifically escorts Wnt ligands for secretion. Given the strong functional association between the Wnt signaling pathways and bone development and homeostasis, we generated osteoblast-specific Wls-deficient (Ocn-Cre;Wls-flox) mice. Homozygous conditional knockout animals were born at a normal Mendelian frequency. Whole-body dual-energy X-ray absorptiometry scanning revealed that bone-mass accrual was significantly inhibited in homozygotes as early as 20 d of age. These homozygotes had spontaneous fractures and a high frequency of premature lethality at around 2 mo of age. Microcomputed tomography analysis and histomorphometric data revealed a dramatic reduction of both trabecular and cortical bone mass in homozygous mutants. Bone formation in homozygotes was severely impaired, but no obvious phenotypic change was observed in mice heterozygous for the conditional deletion. In vitro studies showed that Wls-deficient osteoblasts had a defect in differentiation and mineralization, with significant reductions in the expression of key osteoblast differentiation regulators. In summary, these results reveal a surprising and crucial role of osteoblast-secreted Wnt ligands in bone-mass accrual.

WNT6 is a novel target gene of caveolin-1 promoting chemoresistance to epirubicin in human gastric cancer cells

Resistance to chemotherapy is a major obstacle for curative treatment of human gastric cancer (GC). However, the underlying molecular mechanisms are largely unknown. Wingless-type MMTV integration site family members (WNTs) are secreted glycoproteins involved in embryogenesis and, on inappropriate expression in the adult, in cancer. Here, we show expression of WNT6 in GC patient specimens, human GC cell lines and in a mouse model of GC. In human GC cells, WNT6 expression was enhanced by caveolin-1 (Cav1), a scaffold protein of plasma membrane caveolae. WNT6 knock-down and overexpression experiments demonstrated that WNT6 increased the resistance to apoptotic cell death induced by the anthracycline chemotherapeutics epirubicin (Epi) and doxorubicin (Dox). Epi increased the activity of the human WNT6 promoter through Cav1-dependent binding of β-catenin to the proximal WNT6 promoter. Epi increased both WNT6/Wnt6 and Cav1 expression in human GC cells and within the tumor area of a murine model of GC (CEA424-SV40 TAg). In GC patients, WNT6 expression was positively associated with the tumor stage and the nodal status, and inversely correlated with the response to ECF (Epi, cisplatin, 5-fluorouracil) chemotherapy. These results showed that WNT6 and Cav1 are upregulated by chemotherapeutics and enhance the resistance of GC cells to anthracycline drugs. Understanding the molecular mechanisms driving WNT6/Cav1-induced drug resistance will provide benefits in developing new therapies for GC.

Sorting nexins provide diversity for retromer-dependent trafficking events

Sorting nexins are a large family of evolutionarily conserved phosphoinositide-binding proteins that have fundamental roles in orchestrating cargo sorting through the membranous maze that is the endosomal network. One ancient group of complexes that contain sorting nexins is the retromer. Here we discuss how retromer complexes regulate endosomal sorting, and describe how this is generating exciting new insight into the central role played by endosomal sorting in development and homeostasis of normal tissues.

Identification of an endocytosis motif in an intracellular loop of Wntless protein, essential for its recycling and the control of Wnt protein signaling

The secretion of Wnt signaling proteins is dependent upon the transmembrane sorting receptor, Wntless (Wls), which recycles between the trans-Golgi network and the cell surface. Loss of Wls results in impairment of Wnt secretion and defects in development and homeostasis in Drosophila, Caenorhabditis elegans, and the mouse. The sorting signals for the internalization and trafficking of Wls have not been defined. Here, we demonstrate that Wls internalization requires clathrin and dynamin I, components of the clathrin-mediated endocytosis pathway. Moreover, we have identified a conserved YXXϕ endocytosis motif in the third intracellular loop of the multipass membrane protein Wls. Mutation of the tyrosine-based motif YEGL to AEGL (Y425A) resulted in the accumulation of human mutant Wls on the cell surface of transfected HeLa cells. The cell surface accumulation of Wls(AEGL) was rescued by the insertion of a classical YXXϕ motif in the cytoplasmic tail. Significantly, a Drosophila Wls(AEGL) mutant displayed a wing notch phenotype, with reduced Wnt secretion and signaling. These findings demonstrate that YXXϕ endocytosis motifs can occur in the intracellular loops of multipass membrane proteins and, moreover, provide direct evidence that the trafficking of Wls is required for efficient secretion of Wnt signaling proteins.

Deletion of mouse Porcn blocks Wnt ligand secretion and reveals an ectodermal etiology of human focal dermal hypoplasia/Goltz syndrome

The Drosophila porcupine gene is required for secretion of wingless and other Wnt proteins, and sporadic mutations in its unique human ortholog, PORCN, cause a pleiotropic X-linked dominant disorder, focal dermal hypoplasia (FDH, also known as Goltz syndrome). We generated a conditional allele of the X-linked mouse Porcn gene and analyzed its requirement in Wnt signaling and embryonic development. We find that Porcn-deficient cells exhibit a cell-autonomous defect in Wnt ligand secretion but remain responsive to exogenous Wnts. Consistent with the female-specific inheritance pattern of FDH, Porcn hemizygous male embryos arrest during early embryogenesis and fail to generate mesoderm, a phenotype previously associated with loss of Wnt activity. Heterozygous Porcn mutant females exhibit a spectrum of limb, skin, and body patterning abnormalities resembling those observed in human patients with FDH. Many of these defects are recapitulated by ectoderm-specific deletion of Porcn, substantiating a long-standing hypothesis regarding the etiology of human FDH and extending previous studies that have focused on downstream elements of Wnt signaling, such as β-catenin. Conditional deletion of Porcn thus provides an experimental model of FDH, as well as a valuable tool to probe Wnt ligand function in vivo.

Wnt/β-catenin Signaling in Normal and Cancer Stem Cells

The ability of Wnt ligands to initiate a signaling cascade that results in cytoplasmic stabilization of, and nuclear localization of, β-catenin underlies their ability to regulate progenitor cell differentiation. In this review, we will summarize the current knowledge of the mechanisms underlying Wnt/β-catenin signaling and how the pathway regulates normal differentiation of stem cells in the intestine, mammary gland, and prostate. We will also discuss how dysregulation of the pathway is associated with putative cancer stem cells and the potential therapeutic implications of regulating Wnt signaling.

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.

Drosophila long-chain acyl-CoA synthetase acts like a gap gene in embryonic segmentation

Long-chain acyl-CoA synthetases (ACSLs) convert the long chain fatty acids to acyl-CoA esters, the activated forms participating in diverse metabolic and signaling pathways. dAcsl is the Drosophila homolog of human ACSL4 and their functions are highly conserved in the processes ranging from lipid metabolism to the establishment of visual wiring. In this study, we demonstrate that both maternal and zygotic dAcsl are required for embryonic segmentation. The abdominal segmentation defects of dAcsl mutants resemble those of gap gene knirps (kni). The central expression domain of Kni transcripts or proteins was reduced whereas the adjacent domains of another gap gene Hunchback (Hb) were correspondingly expanded in these mutants. Consequently, the striped pattern of the pair-rule gene Even-skipped (Eve) was disrupted. We propose that dAcsl plays a role in embryonic segmentation at least by shifting the anteroposterior boundaries of two gap genes.

Gpr177/mouse Wntless is essential for Wnt-mediated craniofacial and brain development

We have previously demonstrated that Gpr177, the mouse orthologue of Drosophila Wls/Evi/Srt, is required for establishment of the anterior-posterior axis. The Gpr177 null phenotype is highly reminiscent to the loss of Wnt3, the earliest abnormality among all Wnt knockouts in mice. The expression of Gpr177 in various cell types and tissues lead us to hypothesize that reciprocal regulation of Wnt and Gpr177 is essential for the Wnt-dependent developmental and pathogenic processes. Here, we create a new mouse strain permitting conditional inactivation of Gpr177. The loss of Gpr177 in the Wnt1-expressing cells causes mid/hindbrain and craniofacial defects which are far more severe than the Wnt1 knockout, but resemble the double knockout of Wnt1 and Wnt3a as well as β-catenin deletion in the Wnt1-expressing cells. Our findings demonstrate the importance of Gpr177 in Wnt1-mediated development of the mouse embryo, suggesting an overlapping function of Wnt family members in the Wnt1-expressing cells.

Expression of Gpr177, a Wnt trafficking regulator, in mouse embryogenesis

Wls/Evi/Srt encoding a multipass transmembrane protein has been identified as a regulator for proper sorting and secretion of Wnt in flies. We have previously demonstrated that Gpr177 is the mouse ortholog required for axis determination. Gpr177 is a transcriptional target of Wnt that is activated to assist its subcellular distribution in a feedback regulatory loop. We, therefore, proposed that reciprocal regulation of Wnt and Gpr177 is essential for the Wnt-dependent developmental and pathogenic processes. Here, we examine the expression pattern of Gpr177 in mouse development. Gpr177 is expressed in a variety of tissues and cell types during organogenesis. Furthermore, Gpr177 is a glycoprotein primarily accumulating in the Golgi apparatus in signal-producing cells. The glycosylation of Gpr177 is necessary for proper transportation in the secretory pathway. Our findings suggest that the Gpr177-mediated regulation of Wnt is crucial for organogenesis in health and disease.

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.

Interaction of the mu-opioid receptor with GPR177 (Wntless) inhibits Wnt secretion: potential implications for opioid dependence

BACKGROUND

Opioid agonist drugs produce analgesia. However, long-term exposure to opioid agonists may lead to opioid dependence. The analgesic and addictive properties of opioid agonist drugs are mediated primarily via the mu-opioid receptor (MOR). Opioid agonists appear to alter neuronal morphology in key brain regions implicated in the development of opioid dependence. However, the precise role of the MOR in the development of these neuronal alterations remains elusive. We hypothesize that identifying and characterizing novel MOR interacting proteins (MORIPs) may help to elucidate the underlying mechanisms involved in the development of opioid dependence.

RESULTS

GPR177, the mammalian ortholog of Drosophila Wntless/Evi/Sprinter, was identified as a MORIP in a modified split ubiquitin yeast two-hybrid screen. GPR177 is an evolutionarily conserved protein that plays a critical role in mediating Wnt protein secretion from Wnt producing cells. The MOR/GPR177 interaction was validated in pulldown, coimmunoprecipitation, and colocalization studies using mammalian tissue culture cells. The interaction was also observed in rodent brain, where MOR and GPR177 were coexpressed in close spatial proximity within striatal neurons. At the cellular level, morphine treatment caused a shift in the distribution of GPR177 from cytosol to the cell surface, leading to enhanced MOR/GPR177 complex formation at the cell periphery and the inhibition of Wnt protein secretion.

CONCLUSIONS

It is known that chronic morphine treatment decreases dendritic arborization and hippocampal neurogenesis, and Wnt proteins are essential for these processes. We therefore propose that the morphine-mediated MOR/GPR177 interaction may result in decreased Wnt secretion in the CNS, resulting in atrophy of dendritic arbors and decreased neurogenesis. Our results demonstrate a previously unrecognized role for GPR177 in regulating cellular response to opioid drugs.

A sandwich ELISA for the detection of Wnt5a

Wnt5a is a noncanonical member of the Wnt family of signaling molecules that has been linked to various physiological and pathological processes including cell differentiation, cell migration, cell growth, vascular remodeling, cancer and chronic inflammation. To understand the role of Wnt5a in these processes, it is necessary to determine the function and expression level of Wnt5a. In this study we developed a sensitive and specific sandwich enzyme-linked immunosorbent assay (ELISA) for detecting Wnt5a. We found that a rabbit anti-human Wnt5a is a suitable capture antibody for establishing a sandwich ELISA. We used two systems to detect Wnt5a: (1) goat anti-mouse Wnt5a and horseradish peroxidase (HRP) conjugated F(ab')(2) donkey anti-goat IgG as detection and enzyme-linked antibodies respectively, or (2) biotinylated goat anti-mouse Wnt5a and HRP-streptavidin as detection antibody and enzyme-linked avidin respectively. A sandwich ELISA using either of these systems failed to detect recombinant mouse (rm)-Wnt5a diluted in Hank's balanced salt solution supplemented with Ca(2+) and Mg(2+) and 1% bovine serum albumin (HBBS+, 1% BSA). Addition of polyethylene glycol (PEG) to the HBBS+, buffer during the binding stage of rm-Wnt5a, afforded the detection of rm-Wnt5a. The use of PEG during both the binding of rm-Wnt5a and detection antibody stages of the assay yielded the maximum signal for rm-Wnt5a. The relationship between the ELISA signal and concentration of Wnt5a was linear with an R(2) of 0.9934. In summary, we have developed a specific and sensitive sandwich ELISA that detects rm-Wnt5a.

[Sorting, recycling and WNT signaling: Wntless and retromer functions]

Cell-cell signaling is essential for the development of multi-cellular organisms. Indeed, membrane traffic is required for the correct sorting and function of receptors and ligands. In the past decades, many genetic screens performed in C. elegans and Drosophila have been crucial to identify the role of intracellular traffic in signaling. In this review, we discuss recent work that led to the identification of Wntless, a sorting receptor for WNT, and of the retromer, a protein complex required for the recycling of Wntless from endosomes to the trans-Golgi network.

Role of adhesion in arthropod immune recognition

The recognition and inactivation of toxins and pathogens are mediated by a combination of cell-free and cellular mechanisms. A number of soluble and membrane-bound pattern recognition molecules interact with elicitors to become involved in both cell-free inactivation as well as cellular uptake reactions. Here we describe the possible recognition and effector function of key arthropod immune proteins, such as peroxinectin, hemolin, and hemomucin, as an outcome of changes in adhesiveness, which drive self-assembly reactions leading to cell-free coagulation and cellular uptake reactions. The fact that some of these proteins are essential for immune and developmental functions in some species, but are not found in closely related species, may point to the existence of multiprotein assemblies, which are conserved at the mechanistic level and can function with more than one combination of protein constituents.

Reciprocal regulation of Wnt and Gpr177/mouse Wntless is required for embryonic axis formation

Members of the Wnt family are secreted glycoproteins that trigger cellular signals essential for proper development of organisms. Cellular signaling induced by Wnt proteins is involved in diverse developmental processes and human diseases. Previous studies have generated an enormous wealth of knowledge on the events in signal-receiving cells. However, relatively little is known about the making of Wnt in signal-producing cells. Here, we describe that Gpr177, the mouse orthologue of Drosophila Wls, is expressed during formation of embryonic axes. Embryos with deficient Gpr177 exhibit defects in establishment of the body axis, a phenotype highly reminiscent to the loss of Wnt3. Although many different mammalian Wnt proteins are required for a wide range of developmental processes, the Wnt3 ablation exhibits the earliest developmental abnormality. This suggests that the Gpr177-mediated Wnt production cannot be substituted. As a direct target of Wnt, Gpr177 is activated by beta-catenin and LEF/TCF-dependent transcription. This activation alters the cellular distributions of Gpr177 which binds to Wnt proteins and assists their sorting and secretion in a feedback regulatory mechanism. Our findings demonstrate that the loss of Gpr177 affects Wnt production in the signal-producing cells, leading to alterations of Wnt signaling in the signal-receiving cells. A reciprocal regulation of Wnt and Gpr177 is essential for the patterning of the anterior-posterior axis during mammalian development.

Ferrying wingless across the synaptic cleft

Secreted Wnt morphogens mediate cell-cell communication, but the mechanism of Wnt transfer between cells is unknown. Korkut et al. (2009) report that the transmembrane protein Evi is a versatile carrier that guides Wingless to presynaptic terminals of motor neurons and then escorts it across the synaptic cleft. In postsynaptic muscles, Evi promotes Frizzled-2 trafficking.

Secretion of Hedgehog-related peptides and WNT during Caenorhabditis elegans development

There is growing awareness that endocytic trafficking plays a critical role in cell-cell communication during animal development. We are beginning to understand how endocytosis can initiate, modulate or terminate signaling. In contrast, our knowledge of the mechanisms involved in secreting signaling peptides remains more limited, particularly when it comes to secretion at the apical surface in epithelial cells. In this study, we review the mechanisms that control secretion in Caenorhabditis elegans, focusing on the role of Patched family members and the V0 complex of the vacuolar-adenosine triphosphatase (V-ATPase) in secreting Hedgehog-related peptides and of MIG-14/Wls and the retromer complex in secreting EGL-20/WNT.

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.

Coordinate Gα13 and Wnt6-β-catenin signaling in F9 embryonal carcinoma cells is required for primitive endoderm differentiation

The mouse F9 embryonal carcinoma cell line is ideally suited to study the epithelial-to-mesenchymal transition accompanying the differentiation of primitive to parietal extraembryonic endoderm. In F9 cells, the application of exogenous agents including retinoic acid or activation of signal transduction cascades downstream of G-proteins triggers widespread changes in gene expression and leads to the formation of primitive endoderm. The epithelial-to-mesenchymal transition is completed and parietal endoderm develops as of result of increasing PKA activity in primitive endoderm cells. Expression of a constitutively active form of Gα13(Q226L) is sufficient to induce F9 cells into parietal endoderm and a model is emerging that a signaling axis linking G-protein signaling to RhoA and the ERM protein moesin is required for differentiation. In this study, we found that expression of either p115RhoGEF or a constitutively active, GTPase-deficient form of RhoA(L63) promoted primitive, but not parietal, endoderm formation. The overexpression of Gα13(Q226L) or p115RhoGEF, but not Rho(L63), caused β-catenin to translocate to the nucleus. Surprisingly, the stimulation of the Wnt-β-catenin pathway was accompanied by nuclear β-catenin and primitive endoderm formation, even when a dominant negative was used to block the signaling axis at the level of p115RhoGEF or when ROCK activity was inhibited using the pharmacological agent Y-27632. Together, results indicate that the coordinate signaling by two independent pathways, one involving canonical Wnt-β-catenin activation of target genes and the other with Gα13 signaling to ERM proteins to modulate cytoarchitectural changes, is required during the retinoic acid induced differentiation of F9 cells to primitive endoderm.

Analyses of mental dysfunction-related ACSl4 in Drosophila reveal its requirement for Dpp/BMP production and visual wiring in the brain

Long-chain acyl-CoA synthetases (ACSLs) convert long-chain fatty acids to acyl-CoAs, the activated substrates essential in various metabolic and signaling pathways. Mutations in ACSL4 are associated with non-syndromic X-linked mental retardation (MRX). However, the developmental functions of ACSL4 and how it is involved in the pathogenesis of MRX remain largely unknown. The Drosophila ACSL-like protein is highly homologous to human ACSL3 and ACSL4, and we designate it as dAcsl. In this study, we demonstrate that dAcsl and ACSL4 are highly conserved in terms of ACSL4's ability to substitute the functions of dAcsl in organismal viability, lipid storage and the neural wiring in visual center. In neurodevelopment, decapentaplegic (Dpp, a BMP-like molecule) production diminished specifically in the larval brain of dAcsl mutants. Consistent with the Dpp reduction, the number of glial cells and neurons dramatically decreased and the retinal axons mis-targeted in the visual cortex. All these defects in Drosophila brain were rescued by the wild-type ACSL4 but not by the mutant products found in MRX patients. Interestingly, expression of an MRX-associated ACSL4 mutant form in a wild-type background led to the lesions in visual center, suggesting a dominant negative effect. These findings validate Drosophila as a model system to reveal the connection between ACSL4 and BMP pathway in neurodevelopment, and to infer the pathogenesis of ACSL4-related MRX.

Wnt signaling is regulated by endoplasmic reticulum retention

Precise regulation of Wnt signaling is important in many contexts, as in development of the vertebrate forebrain, where excessive or ectopic Wnt signaling leads to severe brain defects. Mutation of the widely expressed oto gene causes loss of the anterior forebrain during mouse embryogenesis. Here we report that oto is the mouse ortholog of the gpi deacylase gene pgap1, and that the endoplasmic reticulum (ER)-resident Oto protein has a novel and deacylase-independent function during Wnt maturation. Oto increases the hydrophobicities of Wnt3a and Wnt1 by promoting the addition of glycophosphatidylinositol (gpi)-like anchors to these Wnts, which results in their retention in the ER. We also report that oto-deficient embryos exhibit prematurely robust Wnt activity in the Wnt1 domain of the early neural plate. We examine the effect of low oto expression on Wnt1 in vitro by knocking down endogenous oto expression in 293 and M14 melanoma cells using shRNA. Knockdown of oto results in increased Wnt1 secretion which is correlated with greatly enhanced canonical Wnt activity. These data indicate that oto deficiency increases Wnt signaling in vivo and in vitro. Finally, we address the mechanism of Oto-mediated Wnt retention under oto-abundant conditions, by cotransfecting Wnt1 with gpi-specific phospholipase D (GPI-PLD). The presence of GPI-PLD in the secretory pathway results in increased secretion of soluble Wnt1, suggesting that the gpi-like anchor lipids on Wnt1 mediate its retention in the ER. These data now provide a mechanistic framework for understanding the forebrain defects in oto mice, and support a role for Oto-mediated Wnt regulation during early brain development. Our work highlights a critical role for ER retention in regulating Wnt signaling in the mouse embryo, and gives insight into the notoriously inefficient secretion of Wnts.

Motor neurons with axial muscle projections specified by Wnt4/5 signaling

Axial muscles are innervated by motor neurons of the median motor column (MMC). In contrast to the segmentally restricted motor columns that innervate limb, body wall, and neuronal targets, MMC neurons are generated along the entire length of the spinal cord. We show that the specification of MMC fate involves a dorsoventral signaling program mediated by three Wnt proteins (Wnt4, Wnt5a, and Wnt5b) expressed in and around the floor plate. These Wnts appear to establish a ventral^high to dorsal^low signaling gradient and promote MMC identity and connectivity by maintaining expression of the LIM homeodomain proteins Lhx3/4 in spinal motor neurons. Elevation of Wnt4/5 activity generates additional MMC neurons at the expense of other motor neuron columnar subtypes, whereas depletion of Wnt4/5 activity inhibits the production of MMC neurons. Thus, two dorsoventral signaling pathways, mediated by Shh and Wnt4/5, are required to establish an early binary divergence in motor neuron columnar identity.

Linking Abeta and tau in late-onset Alzheimer's disease: a dual pathway hypothesis

Alzheimer's disease is characterized by abnormal elevation of Abeta peptide and abnormal hyperphosphorylation of the tau protein. The "amyloid hypothesis," which is based on molecular defects observed in autosomal-dominant early-onset Alzheimer's disease (EOAD), suggests a serial model of causality, whereby elevation of Abeta drives other disease features including tau hyperphosphorylation. Here, we review recent evidence from drug trials, genetic studies, and experimental work in animal models that suggests that an alternative model might exist in late-onset AD (LOAD), the complex and more common form of the disease. Specifically, we hypothesize a "dual pathway" model of causality, whereby Abeta and tau can be linked by separate mechanisms driven by a common upstream driver. This model may account for the results of recent drug trials and, if confirmed, may guide future drug development.

Deciphering the function of canonical Wnt signals in development and disease: conditional loss- and gain-of-function mutations of beta-catenin in mice

Wnt signaling is one of a handful of powerful signaling pathways that play crucial roles in the animal life by controlling the genetic programs of embryonic development and adult homeostasis. When disrupted, these signaling pathways cause developmental defects, or diseases, among them cancer. The gateway of the canonical Wnt pathway, which contains >100 genes, is an essential molecule called beta-catenin (Armadillo in Drosophila). Conditional loss- and gain-of-function mutations of beta-catenin in mice provided powerful tools for the functional analysis of canonical Wnt signaling in many tissues and organs. Such studies revealed roles of Wnt signaling that were previously not accessible to genetic analysis due to the early embryonic lethality of conventional beta-catenin knockout mice, as well as the redundancy of Wnt ligands, receptors, and transcription factors. Analysis of conditional beta-catenin loss- and gain-of-function mutant mice demonstrated that canonical Wnt signals control progenitor cell expansion and lineage decisions both in the early embryo and in many organs. Canonical Wnt signaling also plays important roles in the maintenance of various embryonic or adult stem cells, and as recent findings demonstrated, in cancer stem cell types. This has opened new opportunities to model numerous human diseases, which have been associated with deregulated Wnt signaling. Our review summarizes what has been learned from genetic studies of the Wnt pathway by the analysis of conditional beta-catenin loss- and gain-of-function mice.

The long and the short of Wnt signaling in C. elegans

The simplicity of C. elegans makes it an outstanding system to study the role of Wnt signaling in development. Many asymmetric cell divisions in C. elegans require the Wnt/beta-catenin asymmetry pathway. Recent studies confirm that SYS-1 is a structurally and functionally divergent beta-catenin, and implicate lipids and retrograde trafficking in maintenance of WRM-1/beta-catenin asymmetry. Wnts also regulate short-range events such as spindle rotation and gastrulation, and a PCP-like pathway regulates asymmetric divisions. Long-range, cell non-autonomous Wnt signals regulate vulval induction. Both short-range and long-range Wnt signal s are regulated by recycling of MIG-14/Wntless via the retromer complex. These studies indicate that C. elegans continues to be useful for identifying new, conserved mechanisms underlying Wnt signaling in metazoans.

Wnt signalling and its impact on development and cancer

The Wnt signalling pathway is an ancient system that has been highly conserved during evolution. It has a crucial role in the embryonic development of all animal species, in the regeneration of tissues in adult organisms and in many other processes. Mutations or deregulated expression of components of the Wnt pathway can induce disease, most importantly cancer. The first gene to be identified that encodes a Wnt signalling component, Int1 (integration 1), was molecularly characterized from mouse tumour cells 25 years ago. In parallel, the homologous gene Wingless in Drosophila melanogaster, which produces developmental defects in embryos, was characterized. Since then, further components of the Wnt pathway have been identified and their epistatic relationships have been defined. This article is a Timeline of crucial discoveries about the components and functions of this essential pathway.

A cryptic frizzled module in cell surface collagen 18 inhibits Wnt/beta-catenin signaling

Collagens contain cryptic polypeptide modules that regulate major cell functions, such as cell proliferation or death. Collagen XVIII (C18) exists as three amino terminal end variants with specific amino terminal polypeptide modules. We investigated the function of the variant 3 of C18 (V3C18) containing a frizzled module (FZC18), which carries structural identity with the extracellular cysteine-rich domain of the frizzled receptors. We show that V3C18 is a cell surface heparan sulfate proteoglycan, its topology being mediated by the FZC18 module. V3C18 mRNA was expressed at low levels in 21 normal adult human tissues. Its expression was up-regulated in fibrogenesis and in small well-differentiated liver tumors, but decreased in advanced human liver cancers. Low FZC18 immunostaining in liver cancer nodules correlated with markers of high Wnt/β−catenin activity. V3C18 (M_r = 170 kD) was proteolytically processed into a cell surface FZC18-containing 50 kD glycoprotein precursor that bound Wnt3a in vitro through FZC18 and suppressed Wnt3a-induced stabilization of β−catenin. Ectopic expression of either FZC18 (35 kD) or its 50 kD precursor inhibited Wnt/β−catenin signaling in colorectal and liver cancer cell lines, thus downregulating major cell cycle checkpoint gatekeepers cyclin D1 and c-myc and reducing tumor cell growth. By contrast, full-length V3C18 was unable to inhibit Wnt signaling. In summary, we identified a cell-surface signaling pathway whereby FZC18 inhibits Wnt/β−catenin signaling. The signal, encrypted within cell-surface C18, is released by enzymatic processing as an active frizzledcysteine-rich domain (CRD) that reduces cancer cell growth. Thus, extracellular matrix controls Wnt signaling through a collagen-embedded CRD behaving as a cell-surface sensor of proteolysis, conveying feedback cues to control cancer cell fate.

Reggie-1/flotillin-2 promotes secretion of the long-range signalling forms of Wingless and Hedgehog in Drosophila

The lipid-modified morphogens Wnt and Hedgehog diffuse poorly in isolation yet can spread over long distances in vivo, predicting existence of two distinct forms of these mophogens. The first is poorly mobile and activates short-range target genes. The second is specifically packed for efficient spreading to induce long-range targets. Subcellular mechanisms involved in the discriminative secretion of these two forms remain elusive. Wnt and Hedgehog can associate with membrane microdomains, but the function of this association was unknown. Here we show that a major protein component of membrane microdomains, reggie-1/flotillin-2, plays important roles in secretion and spreading of Wnt and Hedgehog in Drosophila. Reggie-1 loss-of-function results in reduced spreading of the morphogens, while its overexpression stimulates secretion of Wnt and Hedgehog and expands their diffusion. The resulting changes in the morphogen gradients differently affect the short- and long-range targets. In its action reggie-1 appears specific for Wnt and Hedgehog. These data suggest that reggie-1 is an important component of the Wnt and Hedgehog secretion pathway dedicated to formation of the mobile pool of these morphogens.

Wingless secretion requires endosome-to-Golgi retrieval of Wntless/Evi/Sprinter by the retromer complex

The glycolipoproteins of the Wnt family raise interesting trafficking issues, especially with respect to spreading within tissues. Recently, the retromer complex has been suggested to participate in packaging Wnts into long-range transport vehicles. Our analysis of a Drosophila mutant in Vps35 show that, instead, the retromer complex is required for efficient progression of Wingless (a Drosophila Wnt) through the secretory pathway. Indeed expression of senseless, a short-range target gene, is lost in Vps35-deficient imaginal discs. In contrast, Vps35 is not required for Hedgehog secretion, suggesting specificity. Overexpression of Wntless, a transmembrane protein known to be specifically required for Wingless secretion overcomes the secretion block of Vps35-mutant cells. Furthermore, biochemical evidence confirms that Wntless engages with the retromer complex. We propose that Wntless accompanies Wingless to the plasma membrane where the two proteins dissociate. Following dissociation from Wingless, Wntless is internalized and returns to the Golgi apparatus in a retromer-dependent manner. Without the retromer-dependent recycling route, Wingless secretion is impaired and, as electron microscopy suggests, Wntless is diverted to a degradative compartment.

Wingless secretion promotes and requires retromer-dependent cycling of Wntless

Wnt ligands are lipid-modified, secreted glycoproteins that control multiple steps during embryogenesis and adult-tissue homeostasis. Little is known about the mechanisms underlying Wnt secretion. Recently, Wntless (Wls/Evi/Srt) was identified as a conserved multi-pass transmembrane protein whose function seems to be dedicated to promoting the release of Wnts. Here, we describe Wls accumulation in the Golgi apparatus of Wnt/Wingless (Wg)-producing cells in Drosophila, and show that this localization is essential for Wg secretion. Moreover, Wls localization and levels critically depend on retromer, a conserved protein complex that mediates endosome-to-Golgi protein trafficking in yeast. In the absence of the retromer components Dvps35 or Dvps26, but in presence of Wg, Wls is degraded and Wg secretion impaired. Our results indicate that Wg, clathrin-mediated endocytosis and retromer sustain a Wls traffic loop from the Golgi to the plasma membrane and back to the Golgi, thereby enabling Wls to direct Wnt secretion.

High-throughput RNAi screen in Drosophila

Genetic and biochemical analyses in model systems such as the fruitfly, Drosophila melanogaster, have successfully identified several genes that play key regulatory roles in fundamental cellular and developmental processes. However, the analyses of the complete genome sequences of Drosophila, as well as of humans, now reveal that traditional methods have ascribed functions to only a fraction of the total predicted genes. Thus, the roles for many, as yet unidentified genes, in normal development and cancer remain to be discovered. The challenge presented by the various large-scale genome projects is how to derive biologically relevant information from the raw sequences. The past few years have witnessed a rapid growth in the development and implementation high-throughput screening (HTS) technologies that researchers are now using to discover "gene-function" in an unbiased, systematic, and time-efficient manner. In fact one of the most promising functional genomic approach that has emerged in the past few years is based on RNA-interference (RNAi), in which the introduction of double-stranded RNA (dsRNA) into cells or whole organisms has been shown to be an effective tool to suppress endogenous gene expression. The RNAi technology has made it feasible to query the function of every gene in the genome for their potential function in a given cell-biological process using cell-based assays. This chapter discusses the application, advantages, and limitations of this powerful technology in the identification of novel modulators of cell-signaling pathways as well as its future scope and utility in designing more efficient genome-scale screens.

Retromer: multipurpose sorting and specialization in polarized transport

Retromer is an evolutionary conserved protein complex required for endosome-to-Golgi retrieval of lysosomal hydrolases' receptors. A dimer of two sorting nexins-typically, SNX1 and/or SNX2-deforms the membrane and thus cooperates with retromer to ensure cargo sorting. Research in various model organisms indicates that retromer participates in sorting of additional molecules whose proper transport has important repercussions in development and disease. The role of retromer as well as SNXs in endosomal protein (re)cycling and protein targeting to specialized plasma membrane domains in polarized cells adds further complexity and has implications in growth control, the establishment of developmental patterns, cell adhesion, and migration. This chapter will discuss the functions of retromer described in various model systems and will focus on relevant aspects in polarized transport.

C. elegans AP-2 and retromer control Wnt signaling by regulating mig-14/Wntless

While endocytosis can regulate morphogen distribution, its precise role in shaping these gradients is unclear. Even more enigmatic is the role of retromer, a complex that shuttles proteins between endosomes and the Golgi apparatus, in Wnt gradient formation. Here we report that DPY-23, the C. elegans mu subunit of the clathrin adaptor AP-2 that mediates the endocytosis of membrane proteins, regulates Wnt function. dpy-23 mutants display Wnt phenotypes, including defects in neuronal migration, neuronal polarity, and asymmetric cell division. DPY-23 acts in Wnt-expressing cells to promote these processes. MIG-14, the C. elegans homolog of the Wnt-secretion factor Wntless, also acts in these cells to control Wnt function. In dpy-23 mutants, MIG-14 accumulates at or near the plasma membrane. By contrast, MIG-14 accumulates in intracellular compartments in retromer mutants. Based on our observations, we propose that intracellular trafficking of MIG-14 by AP-2 and retromer plays an important role in Wnt secretion.

WNTers in La Jolla

A 'traditional' Wnt meeting, the first of which occurred over two decades ago as a meeting of the laboratories of Harold Varmus and Roel Nusse, was held at the University of California, San Diego, in June 2007. Organized by Karl Willert, Anthony Wynshaw-Boris and Katherine Jones, the meeting was attended by nearly 400 scientists interested in ;all things Wnt', including Wnt signal transduction mechanisms, and Wnt signaling in evolutionary and developmental biology, stem cell biology, regeneration and disease. Themes that dominated the meeting included the need for precise control over each step of the signal transduction mechanism and developing therapeutics for diseases caused by altered Wnt-signaling.

Porcupine-mediated lipid-modification regulates the activity and distribution of Wnt proteins in the chick neural tube

A long-term goal of developmental biology is to understand how morphogens establish gradients that promote proper tissue patterning. A number of reports describe the formation of the Wg (Wnt1) gradient in Drosophila and have shown that Porcupine, a predicted membrane-bound O-acyl transferase, is required for the correct distribution of Wg protein. The discovery that Wnts are palmitoylated on a conserved cysteine residue suggests that porcupine activity and Wnt palmitoylation are important for the generation of Wnt gradients. To establish the role of porcupine in Wnt gradient formation in vertebrates, we tested the role of porcupine/Wnt palmitoylation in human embryonic kidney 293T cells and in the chick neural tube. Our results lead us to conclude that: (1) vertebrate Wnt1 and Wnt3a possess at least one additional site for porcupine-mediated lipid-modification; (2)porcupine-mediated lipid-modification of Wnt proteins promotes their activity in 293T cells and in the chick neural tube; and (3) porcupine-mediated lipid-modification reduces the range of activity of Wnt1 and Wnt3a in the chick neural tube. These findings highlight the importance of porcupine-mediated lipid modifications in the formation of vertebrate Wnt activity gradients.

Wnt signaling and neurite outgrowth: insights and questions

Wnt signaling consists of a highly conserved set of biochemical pathways that have a multitude of functions during embryonic development and in the adult. The Wnt proteins are extracellular agents that often act as gradient morphogens, indicating that their distribution in tissues is tightly controlled. This attribute is also characteristic of factors that regulate neurite outgrowth and guide axons precisely to their specific destinations. Several studies in various species now have established that Wnts and their receptors have an important role in axonal guidance. Different ligand/receptor combinations have been identified that mediate this activity in many of the experimental models. Clues about downstream effector molecules have come from in vitro systems. In this article, the authors review the results from many of these models, evaluate what is known about the associated signaling pathways and speculate about the direction of future research.