Structural Basis of WLS/Evi-Mediated Wnt Transport and Secretion

Engineering of TurboID-Wingless for the identification of Wingless interactors through in vivo proximity labelling

Wnt signalling coordinates growth and cell fate decisions during development and mis-regulation of Wnt signalling in adults is associated with a range of conditions, including cancer and neurodegenerative diseases. Therefore, means of modulating Wnt proteins and/or cofactors could have significant therapeutic potential. As a first step towards enumerating the Wnt interactome, we devised an in vivo proximity labelling strategy to identify proteins that interact with Wingless (Wg), the main Drosophila Wnt. We engineered the wingless locus to express a functional TurboID-Wg fusion at endogenous levels and identified in vivo interactors by streptavidin pull-down from embryos, followed by mass spectrometry. Further analysis may in future extend the screen coverage and deliver functional validation of the newly identified interactors.

The evolving roles of Wnt signaling in stem cell proliferation and differentiation, the development of human diseases, and therapeutic opportunities

The evolutionarily conserved Wnt signaling pathway plays a central role in development and adult tissue homeostasis across species. Wnt proteins are secreted, lipid-modified signaling molecules that activate the canonical (β-catenin dependent) and non-canonical (β-catenin independent) Wnt signaling pathways. Cellular behaviors such as proliferation, differentiation, maturation, and proper body-axis specification are carried out by the canonical pathway, which is the best characterized of the known Wnt signaling paths. Wnt signaling has emerged as an important factor in stem cell biology and is known to affect the self-renewal of stem cells in various tissues. This includes but is not limited to embryonic, hematopoietic, mesenchymal, gut, neural, and epidermal stem cells. Wnt signaling has also been implicated in tumor cells that exhibit stem cell-like properties. Wnt signaling is crucial for bone formation and presents a potential target for the development of therapeutics for bone disorders. Not surprisingly, aberrant Wnt signaling is also associated with a wide variety of diseases, including cancer. Mutations of Wnt pathway members in cancer can lead to unchecked cell proliferation, epithelial-mesenchymal transition, and metastasis. Altogether, advances in the understanding of dysregulated Wnt signaling in disease have paved the way for the development of novel therapeutics that target components of the Wnt pathway. Beginning with a brief overview of the mechanisms of canonical and non-canonical Wnt, this review aims to summarize the current knowledge of Wnt signaling in stem cells, aberrations to the Wnt pathway associated with diseases, and novel therapeutics targeting the Wnt pathway in preclinical and clinical studies.

Myeloid-derived Wnts play an indispensible role in macrophage and fibroblast activation and kidney fibrosis

Wnt/β-catenin signaling plays a pivotal role in the pathogenesis of chronic kidney diseases (CKD), which is associated with macrophage activation and polarization. However, the relative contribution of macrophage-derived Wnts in the evolution of CKD is poorly understood. Here we demonstrate a critical role of Wnts secreted by macrophages in regulating renal inflammation and fibrosis after various injuries. In mouse model of kidney fibrosis induced by unilateral ureteral obstruction (UUO), macrophages were activated and polarized to M1 and M2 subtypes, which coincided with the activation of Wnt/β-catenin signaling. In vitro, multiple Wnts were induced in primary cultured bone marrow-derived macrophages (BMDMs) after polarization. Conversely, Wnt proteins also stimulated the activation and polarization of BMDMs to M1 and M2 subtype. Blockade of Wnt secretion from macrophages in mice with myeloid-specific ablation of Wntless (Wls), a cargo receptor that is obligatory for Wnt trafficking and secretion, blunted macrophage infiltration and activation and inhibited the expression of inflammatory cytokines. Inhibition of Wnt secretion by macrophages also abolished β-catenin activation in tubular epithelium, repressed myofibroblast activation and reduced kidney fibrosis after either obstructive or ischemic injury. Furthermore, conditioned medium from Wls-deficient BMDMs exhibited less potency to stimulate fibroblast proliferation and activation, compared to the controls. These results underscore an indispensable role of macrophage-derived Wnts in promoting renal inflammation, fibroblasts activation and kidney fibrosis.

Ptip is essential for tooth development via regulating Wnt pathway

OBJECTIVE

Epigenetic regulation plays important role in stem cell maintenance. Ptip was identified as epigenetic regulator, but the role in dental progenitor cells remains unclear.

SUBJECTS AND METHODS

Dental mesenchymal progenitor cells were targeted by Sp7-icre and visualized in mTmG; Sp7-icre mice. The Ptipf/f; Sp7-icre mice were generated and the phenotype of incisors and molars were shown by micro-computerized tomography, scanning electron microscope, hematoxylin & eosin staining, and immunofluorescence. Dental mesenchymal progenitor cells were sorted by fluorescence-activated cell sorting from lower incisors and RNA sequencing was performed.

RESULTS

The Sp7-icre targets dental mesenchymal progenitor cells in incisors and molars. The Ptipf/f; Sp7-icre mice showed spontaneous fractures in the cusp of upper incisors and lower incisors at 3 weeks (w), compensative overgrowth of lower incisors at 1 month (M), and overgrowth extended to the outside at 2 M. The molars showed shortened roots. The functions of odontoblasts and dental mesenchymal progenitor cells were impaired. Mechanically, loss of Ptip activates the Wnt pathway and upregulates the expression of Wls in dental mesenchymal progenitor cells. Also, the regenerative ability of lower incisors was significantly impaired.

CONCLUSION

We first demonstrated that Ptip was crucial for tooth development via regulating Wnt signaling.

Extracellular carriers control lipid-dependent secretion, delivery, and activity of WNT morphogens

WNT morphogens trigger signaling pathways fundamental for embryogenesis, regeneration, and cancer. WNTs are modified with palmitoleate, which is critical for binding Frizzled (FZD) receptors and activating signaling. However, it is unknown how WNTs are released and spread from cells, given their strong lipid-dependent membrane attachment. We demonstrate that secreted FZD-related proteins and WNT inhibitory factor 1 are WNT carriers, potently releasing lipidated WNTs and forming active soluble complexes. WNT release occurs by direct handoff from the membrane protein WNTLESS to the carriers. In turn, carriers donate WNTs to glypicans and FZDs involved in WNT reception and to the NOTUM hydrolase, which antagonizes WNTs by lipid moiety removal. WNT transfer from carriers to FZDs is greatly facilitated by glypicans that serve as essential co-receptors in Wnt signaling. Thus, an extracellular network of carriers dynamically controls secretion, posttranslational regulation, and delivery of WNT morphogens, with important practical implications for regenerative medicine.

Transport and gradient formation of Wnt and Fgf in the early zebrafish gastrula

Within embryonic development, the occurrence of gastrulation is critical in the formation of multiple germ layers with many differentiative abilities. These cells are instructed through exposure to signalling molecules called morphogens. The secretion of morphogens from a source tissue creates a concentration gradient that allows distinct pattern formation in the receiving tissue. This review focuses on the morphogens Wnt and Fgf in zebrafish development. Wnt has been shown to have critical roles throughout gastrulation, including in anteroposterior patterning and neural posterisation. Fgf is also a vital signal, contributing to involution and mesodermal specification. Both morphogens have also been found to work in finely balanced synergy for processes such as neural induction. Thus, the signalling range of Wnts and Fgfs must be strictly controlled to target the correct target cells. Fgf and Wnts signal to local cells as well as to cells in the distance in a highly regulated way, requiring specific dissemination mechanisms that allow efficient and precise signalling over short and long distances. Multiple transportation mechanisms have been discovered to aid in producing a stable morphogen gradient, including short-range diffusion, filopodia-like extensions called cytonemes and extracellular vesicles, mainly exosomes. These mechanisms are specific to the morphogen that they transport and the intended signalling range. This review article discusses how spreading mechanisms in these two morphogenetic systems differ and the consequences on paracrine signalling, hence tissue patterning.

Molecular basis of Wnt biogenesis, secretion, and Wnt7-specific signaling

Wnt proteins are enzymatically lipidated by Porcupine (PORCN) in the ER and bind to Wntless (WLS) for intracellular transport and secretion. Mechanisms governing the transfer of these low-solubility Wnts from the ER to the extracellular space remain unclear. Through structural and functional analyses of Wnt7a, a crucial Wnt involved in central nervous system angiogenesis and blood-brain barrier maintenance, we have elucidated the principles of Wnt biogenesis and Wnt7-specific signaling. The Wnt7a-WLS complex binds to calreticulin (CALR), revealing that CALR functions as a chaperone to facilitate Wnt transfer from PORCN to WLS during Wnt biogenesis. Our structures, functional analyses, and molecular dynamics simulations demonstrate that a phospholipid in the core of Wnt-bound WLS regulates the association and dissociation between Wnt and WLS, suggesting a lipid-mediated Wnt secretion mechanism. Finally, the structure of Wnt7a bound to RECK, a cell-surface Wnt7 co-receptor, reveals how RECKCC4 engages the N-terminal domain of Wnt7a to activate Wnt7-specific signaling.

Expanding the phenotypic spectrum and clinical severity associated with WLS gene

WLS (Wnt ligand secretion mediator or Wntless) orchestrates the secretion of all Wnt proteins, a family of evolutionary conserved proteins, involved in Wnt signaling pathway that has many essential biological functions including the regulation of development, cell proliferation, migration and apoptosis. Biallelic variants in WLS have recently been described in 10 patients with pleiotropic multiple congenital anomalies (MCA) known as Zaki syndrome. We identified a likely disease-causing variant in WLS (c.1579G>A, p.Gly527Arg) in a boy presented with a broad range of MCA including microcephaly, facial dysmorphism, alopecia, ophthalmologic anomalies, and complete soft tissue syndactyly. These features were reminiscent of Zaki syndrome although variable clinical severity was observed. In a detailed clinical assessment, our patient also displayed microphthalmia, dental anomalies, skeletal dysplasia with spontaneous fractures and Dandy-Walker malformation. As such, we extend the phenotype linked to Zaki syndrome. This study further highlights the importance of a thorough clinical evaluation to delineate the phenotypic spectrum associated with WLS variants and suggests that genotype-phenotype correlations due to variant localization seems likely. However, future work on additional patients and more functional studies may give further insights into genotype-phenotype correlations and the complex function of WLS.

Wnt/β-catenin signaling pathway inhibitors, glycyrrhizic acid, solanine, polyphyllin I, crocin, hypericin, tubeimoside-1, diosmin, and rutin in medicinal plants have better binding affinities and anticancer properties: Molecular docking and ADMET study

Wnt/β-catenin signaling pathway plays a role in cancer development, organogenesis, and embryogenesis. The abnormal activation promotes cancer stem cell renewal, proliferation, and differentiation. In the present study, molecular docking simulation and ADMET studies were carried out on selected bioactive compounds in search of β-catenin protein inhibitors for drug discovery against cancer. Blind docking simulation was performed using PyRx software on Autodock Vina. β-catenin protein (PDB ID: 1jdh) and 313 bioactive compounds (from PubChem database) with selected standard anticancer drugs were used for molecular docking. The ADMET properties of the best-performing compounds were calculated using SwissADME and pkCMS web servers. The results obtained from the molecular docking study showed that glycyrrhizic acid, solanine, polyphyllin I, crocin, hypericin, tubeimoside-1, diosmin, and rutin had the best binding interactions with β-catenin protein based on their binding affinities. Glycyrrhizic acid and solanine had the same and lowest binding energy of -8.5 kcal/mol. This was followed by polyphyllin I with -8.4 kcal/mol, and crocin, hypericin, and tubeimoside-1 which all had a binding energy of 8.1 kcal/mol. Other top-performing compounds include diosmin and rutin with binding energy of -8.0 kcal/mol. The ADMET study revealed that the following compounds glycyrrhizic acid, solanine, polyphyllin I, crocin, hypericin, tubeimoside-1, diosmin, rutin, and baicalin all violated Lipinski's rule of 5 which implies poor oral bioavailability. However, based on the binding energy score, it was suggested that these pharmacologically active compounds are potential molecules to be tested against cancer.

Wnt binding to Coatomer proteins directs secretion on exosomes independently of palmitoylation

Wnt proteins are secreted hydrophobic glycoproteins that act over long distances through poorly understood mechanisms. We discovered that Wnt7a is secreted on extracellular vesicles (EVs) following muscle injury. Structural analysis identified the motif responsible for Wnt7a secretion on EVs that we term the Exosome Binding Peptide (EBP). Addition of the EBP to an unrelated protein directed secretion on EVs. Disruption of palmitoylation, knockdown of WLS, or deletion of the N-terminal signal peptide did not affect Wnt7a secretion on purified EVs. Bio-ID analysis identified Coatomer proteins as candidates responsible for loading Wnt7a onto EVs. The crystal structure of EBP bound to the COPB2 coatomer subunit, the binding thermodynamics, and mutagenesis experiments, together demonstrate that a dilysine motif in the EBP mediates binding to COPB2. Other Wnts contain functionally analogous structural motifs. Mutation of the EBP results in a significant impairment in the ability of Wnt7a to stimulate regeneration, indicating that secretion of Wnt7a on exosomes is critical for normal regeneration in vivo . Our studies have defined the structural mechanism that mediates binding of Wnt7a to exosomes and elucidated the singularity of long-range Wnt signalling.

Compound heterozygous variants in WLS gene causes Zaki syndrome

Biallelic Wnt ligand secretion mediator (WLS gene) variants are associated with Zaki syndrome (OMIM: #619648). Here, we report the first case with Zaki syndrome in the Chinese population. Whole-exome gene sequencing (WES) identified compound heterozygous variants in the WLS gene (c.1427A > G; p.Tyr476Cys and c.415C > T, p.Arg139Cys; NM_001002292) in a 16-year-old boy presenting with facial dysmorphism, astigmatism, renal agenesis, and cryptorchidism. In vitro functional characterization showed that the two variants led to decreased WLS production and secretion of WNT3A, eventually affecting the WNT signal. We also found that the decreased mutant WLS expression can be rescued by 4-Phenylbutyric acid (4-PBA).

Efficient screening of protein-ligand complexes in lipid bilayers using LoCoMock score

Membrane proteins are attractive targets for drug discovery due to their crucial roles in various biological processes. Studying the binding poses of amphipathic molecules to membrane proteins is essential for understanding the functions of membrane proteins and docking simulations can facilitate the screening of protein-ligand complexes at low computational costs. However, identifying docking poses for a ligand in non-aqueous environments such as lipid bilayers can be challenging. To address this issue, we propose a new docking score called logP-corrected membrane docking (LoCoMock) score. To screen putative protein-ligand complexes embedded in a membrane, the LoCoMock score considers the affinity between a target ligand and the membrane. It combines the docking score of the protein-ligand complex with the logP of the target ligand. In demonstrations using several model ligands, the LoCoMock score screened more putative complexes than the conventional docking score. As extended docking, the LoCoMock score makes it possible to screen membrane proteins more effectively as drug targets than the conventional docking.

The Roles of Secreted Wnt Ligands in Cancer

Wnt ligands are secreted signaling proteins that display a wide range of biological effects. They play key roles in stimulating Wnt signaling pathways to facilitate processes such as tissue homeostasis and regeneration. Dysregulation of Wnt signaling is a hallmark of many cancers and genetic alterations in various Wnt signaling components, which result in ligand-independent or ligand-dependent hyperactivation of the pathway that have been identified. Recently, research is focusing on the impact of Wnt signaling on the interaction between tumor cells and their micro-environment. This Wnt-mediated crosstalk can act either in a tumor promoting or suppressing fashion. In this review, we comprehensively outline the function of Wnt ligands in different tumor entities and their impact on key phenotypes, including cancer stemness, drug resistance, metastasis, and immune evasion. Lastly, we elaborate approaches to target Wnt ligands in cancer therapy.

The role of Evi/Wntless in exporting Wnt proteins

Intercellular communication by Wnt proteins governs many essential processes during development, tissue homeostasis and disease in all metazoans. Many context-dependent effects are initiated in the Wnt-producing cells and depend on the export of lipidated Wnt proteins. Although much focus has been on understanding intracellular Wnt signal transduction, the cellular machinery responsible for Wnt secretion became better understood only recently. After lipid modification by the acyl-transferase Porcupine, Wnt proteins bind their dedicated cargo protein Evi/Wntless for transport and secretion. Evi/Wntless and Porcupine are conserved transmembrane proteins, and their 3D structures were recently determined. In this Review, we summarise studies and structural data highlighting how Wnts are transported from the ER to the plasma membrane, and the role of SNX3-retromer during the recycling of its cargo receptor Evi/Wntless. We also describe the regulation of Wnt export through a post-translational mechanism and review the importance of Wnt secretion for organ development and cancer, and as a future biomarker.

The logistics of Wnt production and delivery

Wnts are secreted proteins that control stem cell maintenance, cell fate decisions, and growth during development and adult homeostasis. Wnts carry a post-translational modification not seen in any other secreted protein: during biosynthesis, they are appended with a palmitoleoyl moiety that is required for signaling but also impairs solubility and hence diffusion in the extracellular space. In some contexts, Wnts act only in a juxtacrine manner but there are also instances of long range action. Several proteins and processes ensure that active Wnts reach the appropriate target cells. Some, like Porcupine, Wntless, and Notum are dedicated to Wnt function; we describe their activities in molecular detail. We also outline how the cell infrastructure (secretory, endocytic, and retromer pathways) contribute to the progression of Wnts from production to delivery. We then address how Wnts spread in the extracellular space and form a signaling gradient despite carrying a hydrophobic moiety. We highlight particularly the role of lipid-binding Wnt interactors and heparan sulfate proteoglycans. Finally, we briefly discuss how evolution might have led to the emergence of this unusual signaling pathway.

Visualizing WNT signaling in mammalian systems

WNT/CTNNB1 signaling plays a critical role in the development of all multicellular animals. Here, we include both the embryonic stages, during which tissue morphogenesis takes place, and the postnatal stages of development, during which tissue homeostasis occurs. Thus, embryonic development concerns lineage development and cell fate specification, while postnatal development involves tissue maintenance and regeneration. Multiple tools are available to researchers who want to investigate, and ideally visualize, the dynamic and pleiotropic involvement of WNT/CTNNB1 signaling in these processes. Here, we discuss and evaluate the decisions that researchers need to make in identifying the experimental system and appropriate tools for the specific question they want to address, covering different types of WNT/CTNNB1 reporters in cells and mice. At a molecular level, advanced quantitative imaging techniques can provide spatio-temporal information that cannot be provided by traditional biochemical assays. We therefore also highlight some recent studies to show their potential in deciphering the complex and dynamic mechanisms that drive WNT/CTNNB1 signaling.

Gone with the Wnt(less): a mechanistic perspective on the journey of Wnt

Wnts are short-range signaling proteins, expressed in all metazoans from sponges to humans, critical for cell development and fate. There are 19 different Wnts in the human genome with varying expression levels and patterns, and post-translational modifications. Common to essentially all Wnts is the palmitoleation of a conserved serine by the O-acyltransferase PORCN in the endoplasmic reticulum (ER). All lipidated Wnts then bind a dedicated carrier Wntless (WLS), endowed with the task of transporting them from the ER to the plasma membrane, and ultimately facilitating their release to receptors on the Wnt-receiving cell to initiate signaling. Here, we will focus on the WLS-mediated transport step. There are currently two published structures, both obtained by single-particle cryo-electron microscopy of the Wnt/WLS complex: human Wnt8A-bound and human Wnt3A-bound WLS. We analyze the two Wnt/WLS structures - remarkably similar despite the sequence similarity between Wnt8A and Wnt3A being only ∼39% - to begin to understand the conserved nature of this binding mechanism, and ultimately how one carrier can accommodate a family of 19 different Wnts. By comparing how Wnt associates with WLS with how it binds to PORCN and FZD receptors, we can begin to speculate on mechanisms of Wnt transfer from PORCN to WLS, and from WLS to FZD, thus providing molecular-level insight into these essential steps of the Wnt signaling pathway.

Functional regulation of Wnt protein through post-translational modifications

Wnts are lipid-modified signaling glycoproteins present in all metazoans that play key roles in development and homeostasis. Post-translational modifications of Wnts regulate their function. Wnts have a unique post-translational modification, O-linked palmitoleation, that is absolutely required for their function. This Wnt-specific modification occurs during Wnt biosynthesis in the endoplasmic reticulum (ER), catalyzed by the O-acyltransferase Porcupine (PORCN). Palmitoleation is required for Wnt to bind to its transporter Wntless (WLS/Evi) as well as to its receptor Frizzled (FZD). Recent structural studies have illustrated how PORCN recognizes its substrates, and how drugs inhibit this. The abundance of WLS is tightly regulated by intracellular recycling and ubiquitylation-mediated degradation in the ER. The function of Wnt glycosylation is less well understood, and the sites and types of glycosylation are not largely conserved among different Wnts. In polarized tissues, the type of glycans can determine whether the route of trafficking is apical or basolateral. In addition, pairing of the 24 highly conserved cysteines in Wnts to form disulfide bonds is critical in maintaining proper structure and activities. Extracellularly, the amino terminus of a subset of Wnts can be cleaved by a dedicated glycosylphosphatidylinositol (GPI)-anchored metalloprotease TIKI, resulting in the inactivation of these Wnt proteins. Additionally, NOTUM is a secreted extracellular carboxylesterase that removes the palmitoleate moiety from Wnt, antagonizing its activity. In summary, Wnt signaling activity is controlled at multiple layers by post-translational modifications.

Structure of the GOLD-domain seven-transmembrane helix protein family member TMEM87A

TMEM87s are eukaryotic transmembrane proteins with two members (TMEM87A and TMEM87B) in humans. TMEM87s have proposed roles in protein transport to and from the Golgi, as mechanosensitive ion channels, and in developmental signaling. TMEM87 disruption has been implicated in cancers and developmental disorders. To better understand TMEM87 structure and function, we determined a cryo-EM structure of human TMEM87A in lipid nanodiscs. TMEM87A consists of a Golgi-dynamics (GOLD) domain atop a membrane-spanning seven-transmembrane helix domain with a large cavity open to solution and the membrane outer leaflet. Structural and functional analyses suggest TMEM87A may not function as an ion channel or G-protein coupled receptor. We find TMEM87A shares its characteristic domain arrangement with seven other proteins in humans; three that had been identified as evolutionary related (TMEM87B, GPR107, and GPR108) and four previously unrecognized homologs (GPR180, TMEM145, TMEM181, and WLS). Among these structurally related GOLD domain seven-transmembrane helix (GOST) proteins, WLS is best characterized as a membrane trafficking and secretion chaperone for lipidated Wnt signaling proteins. We find key structural determinants for WLS function are conserved in TMEM87A. We propose TMEM87A and structurally homologous GOST proteins could serve a common role in trafficking membrane-associated cargo.

Mutations in the WLS are associated with dental anomalies, torus palatinus, and torus mandibularis

Background

Canonical and non-canonical WNT signaling are important for odontogenesis. WNT ligand secretion mediator (WLS; MIM611514) is required to transport lipid-modified WNT proteins from the Golgi to the cell membrane, where canonical and non-canonical WNT proteins are released into the extracellular milieu. Biallelic pathogenic variants in WLS are implicated in autosomal recessive Zaki syndrome (ZKS; MIM 619648), the only genetic condition known to be caused by pathogenic variants in WLS.

Objective

To investigate molecular etiology of dental anomalies in 250 patients with or without oral exostoses.

Patients and methods

Clinical and radiographic examination, and whole exome sequencing, were performed in the case of 250 patients with dental anomalies with or without oral exostoses.

Results

Four extremely rare heterozygous missense variants (p.Ile20Thr, p.Met46Leu, p.Ser453Ile and p.Leu516Phe) in WLS were identified in 11 patients with dental anomalies. In five of these patients, a torus palatinus or a torus mandibularis was observed.

Conclusion

We report for the first time the heterozygous WLS variants in patients with dental anomalies. Root maldevelopments in patients with WLS variants supports the role of canonical and non-canonical WNT signaling in root development. We also show that variants in WLS were implicated in torus palatinus and torus mandibularis. In addition, this is the first time that heterozygous carriers of WLS variants were found to manifest phenotypes. WLS variants were likely to have adverse effects on the concentration of WNT ligands delivered to the cell membrane, resulting in aberrant canonical and non-canonical WNT signaling, and subsequent phenotypes.

Limitations of the study

Patient’s positioning during the acquisition of panoramic radiography might have affected the appearance of the tooth structures. If we had all family members of each patient to study co-segregation between genotype and phenotype, it would have strengthened the association of WLS variants and the phenotypes.

Wntless expression promotes lineage plasticity and is associated with neuroendocrine prostate cancer

Resistance to androgen receptor (AR) targeted therapies remains as the main reason for most prostate cancer related deaths. Lineage plasticity resulting in altered, treatment insensitive prostate tumor cell phenotypes such neuroendocrine differentiated prostate cancer is a common manifestation within resistant tumors upon AR-targeted therapies. The mechanisms responsible for lineage plasticity in prostate cancer remain incompletely understood. Here we demonstrate that the enzalutamide resistant MDVR cell line possesses lineage plastic characteristics associated with overexpression of the Wnt transporter Wntless (WLS). Furthermore, we present evidence that overexpression of WLS is common in varying cell line models of lineage plastic prostate cancer, is higher in neuroendocrine patient samples, and positively correlates with the neuroendocrine marker SYP in clinical data. Targeting WLS in lineage plastic cellular models reduces viability and represses lineage plasticity associated gene expression. Our study provides insight into the importance of WLS to the development of lethal resistant prostate cancer and provides a potential target for the treatment of advanced disease.

Mechanisms and inhibition of Porcupine-mediated Wnt acylation

Wnt signalling is essential for regulation of embryonic development and adult tissue homeostasis^1-3, and aberrant Wnt signalling is frequently associated with cancers⁴. Wnt signalling requires palmitoleoylation on a hairpin 2 motif by the endoplasmic reticulum-resident membrane-bound O-acyltransferase Porcupine^5-7 (PORCN). This modification is indispensable for Wnt binding to its receptor Frizzled, which triggers signalling^8,9. Here we report four cryo-electron microscopy structures of human PORCN: the complex with the palmitoleoyl-coenzyme A (palmitoleoyl-CoA) substrate; the complex with the PORCN inhibitor LGK974, an anti-cancer drug currently in clinical trials¹⁰; the complex with LGK974 and WNT3A hairpin 2 (WNT3Ap); and the complex with a synthetic palmitoleoylated WNT3Ap analogue. The structures reveal that hairpin 2 of WNT3A, which is well conserved in all Wnt ligands, inserts into PORCN from the lumenal side, and the palmitoleoyl-CoA accesses the enzyme from the cytosolic side. The catalytic histidine triggers the transfer of the unsaturated palmitoleoyl group to the target serine on the Wnt hairpin 2, facilitated by the proximity of the two substrates. The inhibitor-bound structure shows that LGK974 occupies the palmitoleoyl-CoA binding site to prevent the reaction. Thus, this work provides a mechanism for Wnt acylation and advances the development of PORCN inhibitors for cancer treatment.

Novel Dental Anomaly-associated Mutations in WNT10A Protein Binding Sites

OBJECTIVE

WNT/β-catenin signaling is initiated by binding of a WNT protein to a Frizzled (FZD) receptor and a co-receptor, low-density lipoprotein (LDL) receptor-related protein 5 or 6 (LRP5/6). The objective of this study was to find the genetic variants responsible for dental anomalies found in 4 families.

METHODS

Clinical and radiographic examination and whole exome sequencing were performed on 5 patients affected with dental anomalies and the mutant proteins modeled.

RESULTS

Five patients were heterozygous for the WNT10A variants, including c.877C>T; p.Arg293Cys, c.874A>G; p.Ser292Gly, c.1042C>T; p.Arg348Cys, and c.1039G>T; p.347GluX. The p.Arg293Cys and p.Ser292Gly mutations are located in the WNT10A N-terminal domain region with binding sites for FZD receptor, porcupine, WNTLESS, and extracellular binding proteins, so they are likely to have adverse effects on binding these proteins. The p.Arg348Cys mutation, which is located in the binding site of LRP5/6 co-receptors, is postulated to result in impaired binding to these co-receptors. The nonsense mutation p.347GluX is predicted to result in the truncation of most of the C-terminal domain, which is likely to disrupt the binding of WNT10A to WNTLESS, the membrane protein that binds lipid-acylated WNT proteins to carry them from the endoplasmic reticulum to the cell surface and FZD.

CONCLUSIONS

Four novel mutations in WNT10A were identified in patients with isolated tooth agenesis. The mutations in the N-terminal domain and the interface between the N- and C-terminal domains of WNT10A in our patients are likely to disrupt its binding with FZD, LRP5/6, and various other proteins involved in WNT10A processing and transport, impair WNT and SHH signaling, and subsequently result in tooth agenesis, microdontia, and root maldevelopment.

The Wnt Pathway: From Signaling Mechanisms to Synthetic Modulators

The Wnt pathway is central to a host of developmental and disease-related processes. The remarkable conservation of this intercellular signaling cascade throughout metazoan lineages indicates that it coevolved with multicellularity to regulate the generation and spatial arrangement of distinct cell types. By regulating cell fate specification, mitotic activity, and cell polarity, Wnt signaling orchestrates development and tissue homeostasis, and its dysregulation is implicated in developmental defects, cancer, and degenerative disorders. We review advances in our understanding of this key pathway, from Wnt protein production and secretion to relay of the signal in the cytoplasm of the receiving cell. We discuss the evolutionary history of this pathway as well as endogenous and synthetic modulators of its activity. Finally, we highlight remaining gaps in our knowledge of Wnt signal transduction and avenues for future research. Expected final online publication date for the Annual Review of Biochemistry, Volume 91 is June 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Shifting the Focus of Signaling Abnormalities in Colon Cancer

Simple Summary

The major signaling pathways in colon cancer are WNT, RAS, and TGF-β. Components of these pathways are mutated in the majority of colon cancers, resulting in aberrantly high or low activity of the pathway. The functional consequences of the mutations reflect the behavior of these signaling pathways in intestinal stem cells. To better understand the roles of each pathway, we cover the basic function as well as points of intersection between the different pathways, to describe how they function individually, as well as together, to regulate cell proliferation.

Abstract

Colon cancer tumorigenesis occurs incrementally. The process involves the acquisition of mutations which typically follow an established pattern: activation of WNT signaling, activation of RAS signaling, and inhibition of TGF-β signaling. This arrangement recapitulates, to some degree, the stem cell niche of the intestinal epithelium, which maintains WNT and EGF activity while suppressing TGF-β. The resemblance between the intestinal stem cell environment and colon cancer suggests that the concerted activity of these pathways generates and maintains a potent growth-inducing stimulus. However, each pathway has a myriad of downstream targets, making it difficult to identify which aspects of these pathways are drivers. To address this, we utilize the cell cycle, the ultimate regulator of cell proliferation, as a foundation for cross-pathway integration. We attempt to generate an overview of colon cancer signaling patterns by integrating the major colon cancer signaling pathways in the context of cell replication, specifically, the entrance from G₁ into S-phase.

Structural model of human PORCN illuminates disease-associated variants and drug-binding sites

Wnt signaling is essential for normal development and is a therapeutic target in cancer. The enzyme PORCN, or porcupine, is a membrane-bound O-acyltransferase (MBOAT) that is required for the post-translational modification of all Wnts, adding an essential mono-unsaturated palmitoleic acid to a serine on the tip of Wnt hairpin 2. Inherited mutations in PORCN cause focal dermal hypoplasia, and therapeutic inhibition of PORCN slows the growth of Wnt-dependent cancers. Based on homology to mammalian MBOAT proteins, we developed and validated a structural model of human PORCN. The model accommodates palmitoleoyl-CoA and Wnt hairpin 2 in two tunnels in the conserved catalytic core, shedding light on the catalytic mechanism. The model predicts how previously uncharacterized human variants of uncertain significance can alter PORCN function. Drugs including ETC-159, IWP-L6 and LGK-974 dock in the PORCN catalytic site, providing insights into PORCN pharmacologic inhibition. This structural model enhances our mechanistic understanding of PORCN substrate recognition and catalysis, as well as the inhibition of its enzymatic activity, and can facilitate the development of improved inhibitors and the understanding of disease-relevant PORCN mutants. This article has an associated First Person interview with the joint first authors of the paper.

Extracellular WNTs: Trafficking, Exosomes, and Ligand-Receptor Interaction

WNT signaling is a key developmental pathway in tissue organization. A recent focus of research is the secretion of WNT proteins from source cells. Research over the past decade on how WNTs are produced and released into the extracellular space has unravelled very specific control mechanisms in the early secretory pathway, specialized trafficking routes, and redundant forms of packaging for delivery to target cells. In this review I discuss the findings that WNT proteins have been found on extracellular vesicles (EVs) such as exosomes and possible functional implications. There is an ongoing debate in the WNT signaling field whether EV are relevant in vivo and can fulfill specific functions, also fueled by the general preconception of EV secretion as cellular garbage disposal. As part of the EV research community, I want to give an overview of what we know and don't know about WNT secretion on EVs and offer a more unifying model that can explain current discrepancies in observations regarding WNT secretion.

Vertebrate Wnt5a - At the crossroads of cellular signalling

Wnt signalling is an essential pathway in embryogenesis, differentiation, cell motility, development, and adult tissue homeostasis in vertebrates. The Wnt signalling network can activate several downstream pathways such as the β-catenin-dependent TCF/LEF transcription, the Wnt/planar cell polarity (PCP) pathway, and the Wnt/Calcium pathway. Wnt5a is a vertebrate Wnt ligand that is most often associated with the Wnt/PCP signalling pathway. Wnt5a/PCP signalling has a well-described role in embryogenesis via binding to a receptor complex of Frizzled and its co-receptors to initiate downstream activation of the c-Jun N-terminal kinase (JNK) signalling cascade and the Rho and Rac GTPases, Rho-Kinase (ROCK). This activation results in the cytoskeletal remodelling required for cell polarity, migration, and subsequently, tissue re-arrangement and organ formation. This review will focus on more recent work that has revealed new roles for Wnt5a ligands and consequently, an emerging broader function. This is partly due to our growing understanding of the crosstalk between the Wnt/PCP pathway with both the Wnt/β-catenin pathway and other signalling pathways, and in part due to the identification of novel atypical receptors for Wnt5a that demonstrate a far broader role for this ligand.

A Human Pleiotropic Multiorgan Condition Caused by Deficient Wnt Secretion

BACKGROUND

Structural birth defects occur in approximately 3% of live births; most such defects lack defined genetic or environmental causes. Despite advances in surgical approaches, pharmacologic prevention remains largely out of reach.

METHODS

We queried worldwide databases of 20,248 families that included children with neurodevelopmental disorders and that were enriched for parental consanguinity. Approximately one third of affected children in these families presented with structural birth defects or microcephaly. We performed exome or genome sequencing of samples obtained from the children, their parents, or both to identify genes with biallelic pathogenic or likely pathogenic mutations present in more than one family. After identifying disease-causing variants, we generated two mouse models, each with a pathogenic variant "knocked in," to study mechanisms and test candidate treatments. We administered a small-molecule Wnt agonist to pregnant animals and assessed their offspring.

RESULTS

We identified homozygous mutations in WLS, which encodes the Wnt ligand secretion mediator (also known as Wntless or WLS) in 10 affected persons from 5 unrelated families. (The Wnt ligand secretion mediator is essential for the secretion of all Wnt proteins.) Patients had multiorgan defects, including microcephaly and facial dysmorphism as well as foot syndactyly, renal agenesis, alopecia, iris coloboma, and heart defects. The mutations affected WLS protein stability and Wnt signaling. Knock-in mice showed tissue and cell vulnerability consistent with Wnt-signaling intensity and individual and collective functions of Wnts in embryogenesis. Administration of a pharmacologic Wnt agonist partially restored embryonic development.

CONCLUSIONS

Genetic variations affecting a central Wnt regulator caused syndromic structural birth defects. Results from mouse models suggest that what we have named Zaki syndrome is a potentially preventable disorder. (Funded by the National Institutes of Health and others.).

The Emerging Mechanisms of Wnt Secretion and Signaling in Development

Wnts are highly-conserved lipid-modified secreted proteins that activate multiple signaling pathways. These pathways regulate crucial processes during various stages of development and maintain tissue homeostasis in adults. One of the most fascinating aspects of Wnt protein is that despite being hydrophobic, they are known to travel several cell distances in the extracellular space. Research on Wnts in the past four decades has identified several factors and uncovered mechanisms regulating their expression, secretion, and mode of extracellular travel. More recently, analyses on the importance of Wnt protein gradients in the growth and patterning of developing tissues have recognized the complex interplay of signaling mechanisms that help in maintaining tissue homeostasis. This review aims to present an overview of the evidence for the various modes of Wnt protein secretion and signaling and discuss mechanisms providing precision and robustness to the developing tissues.

Cryo-EM structure of human Wntless in complex with Wnt3a

Wntless (WLS), an evolutionarily conserved multi-pass transmembrane protein, is essential for secretion of Wnt proteins. Wnt-triggered signaling pathways control many crucial life events, whereas aberrant Wnt signaling is tightly associated with many human diseases including cancers. Here, we report the cryo-EM structure of human WLS in complex with Wnt3a, the most widely studied Wnt, at 2.2 Å resolution. The transmembrane domain of WLS bears a GPCR fold, with a conserved core cavity and a lateral opening. Wnt3a interacts with WLS at multiple interfaces, with the lipid moiety on Wnt3a traversing a hydrophobic tunnel of WLS transmembrane domain and inserting into membrane. A β-hairpin of Wnt3a containing the conserved palmitoleoylation site interacts with WLS extensively, which is crucial for WLS-mediated Wnt secretion. The flexibility of the Wnt3a loop/hairpin regions involved in the multiple binding sites indicates induced fit might happen when Wnts are bound to different binding partners. Our findings provide important insights into the molecular mechanism of Wnt palmitoleoylation, secretion and signaling.

EVI/WLS function is regulated by ubiquitination and linked to ER-associated degradation by ERLIN2

WNT signalling is important for development in all metazoans and is associated with various human diseases. The ubiquitin–proteasome system (UPS) and regulatory endoplasmic reticulum-associated degradation (ERAD) have been implicated in the production of WNT proteins. Here, we investigated how the WNT secretory factor EVI (also known as WLS) is ubiquitylated, recognised by ERAD components and subsequently removed from the secretory pathway. We performed a focused immunoblot-based RNAi screen for factors that influence EVI/WLS protein stability. We identified the VCP-binding proteins FAF2 and UBXN4 as novel interaction partners of EVI/WLS and showed that ERLIN2 links EVI/WLS to the ubiquitylation machinery. Interestingly, we also found that EVI/WLS is ubiquitylated and degraded in cells irrespective of their level of WNT production. This K11, K48 and K63-linked ubiquitylation is mediated by the E2 ubiquitin-conjugating enzymes UBE2J2, UBE2K and UBE2N, but is independent of the E3 ubiquitin ligases HRD1 (also known as SYVN1) and GP78 (also known as AMFR). Taken together, our study identifies factors that link the UPS to the WNT secretory pathway and provides mechanistic details of the fate of an endogenous substrate of regulatory ERAD in mammalian cells.

This article has an associated First Person interview with the first author of the paper.

Summary: The WNT secretory factor EVI/WLS is ubiquitylated and linked to ER-associated degradation by multiple proteins, providing insight into the link between WNT signalling and the ubiquitin–proteasome system.

Wnt Signaling: From Mesenchymal Cell Fate to Lipogenesis and Other Mature Adipocyte Functions

Wnt signaling is an ancient and evolutionarily conserved pathway with fundamental roles in the development of adipose tissues. Roles of this pathway in mesenchymal stem cell fate determination and differentiation have been extensively studied. Indeed, canonical Wnt signaling is a significant endogenous inhibitor of adipogenesis and promoter of other cell fates, including osteogenesis, chondrogenesis, and myogenesis. However, emerging genetic evidence in both humans and mice suggests central roles for Wnt signaling in body fat distribution, obesity, and metabolic dysfunction. Herein, we highlight recent studies that have begun to unravel the contributions of various Wnt pathway members to critical adipocyte functions, including carbohydrate and lipid metabolism. We further explore compelling evidence of complex and coordinated interactions between adipocytes and other cell types within adipose tissues, including stromal, immune, and endothelial cells. Given the evolutionary conservation and ubiquitous cellular distribution of this pathway, uncovering the contributions of Wnt signaling to cell metabolism has exciting implications for therapeutic intervention in widespread pathologic states, including obesity, diabetes, and cancers.

Small-molecule inhibitors of carboxylesterase Notum

Notum has recently been identified as a negative regulator of Wnt signaling through the removal of an essential palmitoleate group from Wnt proteins. There are emerging reports that Notum plays a role in human disease, with published data suggesting that targeting Notum could represent a new therapeutic approach for treating cancer, osteoporosis and neurodegenerative disorders. Complementary hit-finding strategies have been applied with successful approaches that include high-throughput screening, activity-based protein profiling, screening of fragment libraries and virtual screening campaigns. Structural studies are accelerating the discovery of new inhibitors of Notum. Three fit-for-purpose examples are LP-922056, ABC99 and ARUK3001185. The application of these small-molecule inhibitors is helping to further advance an understanding of the role Notum plays in human disease.

Targeting Wnt Signaling in Endometrial Cancer

Simple Summary

Wnt has diverse regulatory roles at multiple cellular levels and numerous targeting points, and aberrant Wnt signaling has crucial roles in carcinogenesis, metastasis, cancer recurrence, and chemotherapy resistance; based on these facts, Wnt represents an appealing therapeutic target for cancer treatment. Although preclinical data supports a role for the Wnt signaling pathway in uterine carcinogenesis, this area remains understudied. In this review, we identify the functions of several oncogenes of the Wnt/β-catenin signaling pathway in tumorigenesis and address the translation approach with potent Wnt inhibitors that have already been established or are being investigated to target key components of the pathway. Further research is likely to expand the potential for both biomarker and cancer drug development. There is a scarcity of treatment choices for advanced and recurrent endometrial cancer; investigating the sophisticated connections of Wnt signaling networks in endometrial cancer could address the unmet need for new therapeutic targets.

Abstract

This review presents new findings on Wnt signaling in endometrial carcinoma and implications for possible future treatments. The Wnt proteins are essential mediators in cell signaling during vertebrate embryo development. Recent biochemical and genetic studies have provided significant insight into Wnt signaling, in particular in cell cycle regulation, inflammation, and cancer. The role of Wnt signaling is well established in gastrointestinal and breast cancers, but its function in gynecologic cancers, especially in endometrial cancers, has not been well elucidated. Development of a subset of endometrial carcinomas has been attributed to activation of the APC/β-catenin signaling pathway (due to β-catenin mutations) and downregulation of Wnt antagonists by epigenetic silencing. The Wnt pathway also appears to be linked to estrogen and progesterone, and new findings implicate it in mTOR and Hedgehog signaling. Therapeutic interference of Wnt signaling remains a significant challenge. Herein, we discuss the Wnt-activating mechanisms in endometrial cancer and review the current advances and challenges in drug discovery.

Quantitative Profiling of WNT-3A Binding to All Human Frizzled Paralogues in HEK293 Cells by NanoBiT/BRET Assessments

The WNT signaling system governs critical processes during embryonic development and tissue homeostasis, and its dysfunction can lead to cancer. Details concerning selectivity and differences in relative binding affinities of 19 mammalian WNTs to the cysteine-rich domain (CRD) of their receptors—the ten mammalian Frizzleds (FZDs)—remain unclear. Here, we used eGFP-tagged mouse WNT-3A for a systematic analysis of WNT interaction with every human FZD paralogue in HEK293A cells. Employing HiBiT-tagged full-length FZDs, we studied eGFP-WNT-3A binding kinetics, saturation binding, and competition binding with commercially available WNTs in live HEK293A cells using a NanoBiT/BRET-based assay. Further, we generated receptor chimeras to dissect the contribution of the transmembrane core to WNT-CRD binding. Our data pinpoint distinct WNT-FZD selectivity and shed light on the complex WNT-FZD binding mechanism. The methodological development described herein reveals yet unappreciated details of the complexity of WNT signaling and WNT-FZD interactions, providing further details with respect to WNT-FZD selectivity.

Controlling Wnt Signaling Specificity and Implications for Targeting WNTs Pharmacologically

Wnt signaling is critical for proper development of the embryo and for tissue homeostasis in the adult. Activation of this signaling cascade is initiated by binding of the secreted Wnts to their receptors. With the mammalian genome encoding multiple Wnts and Wnt receptors, a longstanding question in the field has been how Wnt-receptor specificities are achieved. Emerging from these studies is a picture of exquisite control over Wnt protein production, secretion, distribution, and receptor interactions, culminating in activation of downstream signaling cascades that control a myriad of biological processes. Here we discuss mechanisms by which Wnt protein activities are tuned and illustrate how the multiple layers of regulation can be leveraged for therapeutic interventions in disease.