Essential role of the Dishevelled DEP domain in a Wnt-dependent human-cell-based complementation assay

Semaphorin Receptors Antagonize Wnt Signaling Through Beta-Catenin Degradation

Precise control of morphogen signaling levels is essential for proper development. An outstanding question is: what mechanisms ensure proper morphogen activity and correct cellular responses? Previous work has identified Semaphorin (SEMA) receptors, Neuropilins (NRPs) and Plexins (PLXNs), as positive regulators of the Hedgehog (HH) signaling pathway. Here, we provide evidence that NRPs and PLXNs antagonize Wnt signaling in both fibroblasts and epithelial cells. Further, Nrp1/2 deletion in fibroblasts results in elevated baseline Wnt pathway activity and increased maximal responses to Wnt stimulation. Notably, and in contrast to HH signaling, SEMA receptor-mediated Wnt antagonism is independent of primary cilia. Mechanistically, PLXNs and NRPs act downstream of Dishevelled (DVL) to destabilize β-catenin (CTNNB1) in a proteosome-dependent manner. Further, NRPs, but not PLXNs, act in a GSK3β/CK1-dependent fashion to antagonize Wnt signaling, suggesting distinct repressive mechanisms for these SEMA receptors. Overall, this study identifies SEMA receptors as novel Wnt pathway antagonists that may also play larger roles integrating signals from multiple inputs.

Pathway selectivity in Frizzleds is achieved by conserved micro-switches defining pathway-determining, active conformations

The class Frizzled of G protein-coupled receptors (GPCRs), consisting of ten Frizzled (FZD1-10) paralogs and Smoothened, remains one of the most enigmatic GPCR families. This class mediates signaling predominantly through Disheveled (DVL) or heterotrimeric G proteins. However, the mechanisms underlying pathway selection are elusive. Here we employ a structure-driven mutagenesis approach in combination with an extensive panel of functional signaling readouts to investigate the importance of conserved state-stabilizing residues in FZD5 for signal specification. Similar data were obtained for FZD4 and FZD10 suggesting that our findings can be extrapolated to other members of the FZD family. Comparative molecular dynamics simulations of wild type and selected FZD5 mutants further support the concept that distinct conformational changes in FZDs specify the signal outcome. In conclusion, we find that FZD5 and FZDs in general prefer coupling to DVL rather than heterotrimeric G proteins and that distinct active state micro-switches in the receptor are essential for pathway selection arguing for conformational changes in the receptor protein defining transducer selectivity.

WNT stimulation induces dynamic conformational changes in the Frizzled-Dishevelled interaction

Frizzleds (FZDs) are G protein-coupled receptors (GPCRs) that bind to WNT family ligands. FZDs signal through multiple effector proteins, including Dishevelled (DVL), which acts as a hub for several downstream signaling pathways. To understand how WNT binding to FZD stimulates intracellular signaling and influences downstream pathway selectivity, we investigated the dynamic changes in the FZD₅-DVL2 interaction elicited by WNT-3A and WNT-5A. Ligand-induced changes in bioluminescence resonance energy transfer (BRET) between FZD₅ and DVL2 or the isolated FZD-binding DEP domain of DVL2 revealed a composite response consisting of both DVL2 recruitment and conformational dynamics in the FZD₅-DVL2 complex. The combination of different BRET paradigms enabled us to identify ligand-dependent conformational dynamics in the FZD₅-DVL2 complex and distinguish them from ligand-induced recruitment of DVL2 or DEP to FZD₅. The observed agonist-induced conformational changes at the receptor-transducer interface suggest that extracellular agonist and intracellular transducers cooperate through transmembrane allosteric interaction with FZDs in a ternary complex reminiscent of that of classical GPCRs.

T-2 Toxin Induces Kidney Fibrosis via the mtROS-NLRP3-Wnt/β-Catenin Axis

T-2 toxin causes kidney fibrosis. Wnt/β-catenin signaling promotes kidney fibrosis when sustained and activated. However, whether T-2-induced kidney fibrosis involves Wnt/β-catenin signaling activation has not been explored yet. T-2 toxin causes renal mitochondrial damage, leading to mitochondrial reactive oxygen species (mtROS) overproduction and NLRP3-inflammasome activation. The activated NLRP3-inflammasome can mediate fibrosis. However, whether the NLRP3-inflammasome can be mediated by mtROS and further regulate T-2-induced kidney fibrosis through Wnt/β-catenin signaling is unclear. In this study, first, we confirmed that T-2 toxin caused Wnt/β-catenin signaling activation in mice kidneys and HK-2 cells. Second, we confirmed that mtROS activated the NLRP3-inflammasome in T-2-exposed mice kidneys and HK-2 cells. Third, we confirmed that the NLRP3-inflammasome regulated the Wnt/β-catenin signaling in T-2 toxin-exposed mice kidneys and HK-2 cells. Finally, we confirmed that Wnt/β-catenin signaling regulated fibrosis in T-2 toxin-exposed mice kidneys and HK-2 cells. The above results confirm that T-2 toxin induces kidney fibrosis via the mtROS-NLRP3-Wnt/β-catenin axis.

Binding of DEP domain to phospholipid membranes: More than just electrostatics

Over the past decades an extensive effort has been made to provide a more comprehensive understanding of Wnt signaling, yet many regulatory and structural aspects remain elusive. Among these, the ability of Dishevelled (DVL) protein to relocalize at the plasma membrane is a crucial step in the activation of all Wnt pathways. The membrane binding of DVL was suggested to be mediated by the preferential interaction of its C-terminal DEP domain with phosphatidic acid (PA). However, due to the scarcity and fast turnover of PA, we investigated the role on the membrane association of other more abundant phospholipids. The combined results from computational simulations and experimental measurements with various model phospholipid membranes, demonstrate that the membrane binding of DEP/DVL constructs is governed by the concerted action of generic electrostatics and finely-tuned intermolecular interactions with individual lipid species. In particular, while we confirmed the strong preference for PA lipid, we also observed a weak but non-negligible affinity for phosphatidylserine, the most abundant anionic phospholipid in the plasma membrane, and phosphatidylinositol 4,5-bisphosphate. The obtained molecular insight into DEP-membrane interaction helps to elucidate the relation between changes in the local membrane composition and the spatiotemporal localization of DVL and, possibly, other DEP-containing proteins.

PI(4,5)P2-stimulated positive feedback drives the recruitment of Dishevelled to Frizzled in Wnt-β-catenin signaling

In the Wnt-β-catenin pathway, Wnt binding to Frizzled (Fzd) and LRP5 or LRP6 (LRP5/6) co-receptors inhibits the degradation of the transcriptional coactivator β-catenin by recruiting the cytosolic effector Dishevelled (Dvl). Polymerization of Dvl at the plasma membrane recruits the β-catenin destruction complex, enabling the phosphorylation of LRP5/6, a key step in inhibiting β-catenin degradation. Using purified Fzd proteins reconstituted in lipid nanodiscs, we investigated the factors that promote the recruitment of Dvl to the plasma membrane. We found that the affinity of Fzd for Dvl was not affected by Wnt ligands, in contrast to other members of the GPCR superfamily for which the binding of extracellular ligands affects the affinity for downstream transducers. Instead, Fzd-Dvl binding was enhanced by increased concentration of the lipid PI(4,5)P₂, which is generated by Dvl-associated lipid kinases in response to Wnt and which is required for LRP5/6 phosphorylation. Moreover, binding to Fzd did not promote Dvl DEP domain dimerization, which has been proposed to be required for signaling downstream of Fzd. Our findings suggest a positive feedback loop in which Wnt-stimulated local PI(4,5)P₂ production enhances Dvl recruitment and further PI(4,5)P₂ production to support Dvl polymerization, LRP5/6 phosphorylation, and β-catenin stabilization.

Selective function of the PDZ domain of Dishevelled in noncanonical Wnt signalling

Dishevelled is a cytoplasmic hub that transduces Wnt signals to cytoplasmic effectors, which can be broadly characterised as canonical (β-catenin dependent) and noncanonical, to specify cell fates and behaviours during development. To transduce canonical Wnt signals, Dishevelled binds to the intracellular face of Frizzled through its DEP domain and polymerises through its DIX domain to assemble dynamic signalosomes. Dishevelled also contains a PDZ domain, whose function remains controversial. Here, we use genome editing to delete the PDZ domain-encoding region from Drosophila dishevelled. Canonical Wingless signalling is entirely normal in these deletion mutants; however, they show defects in multiple contexts controlled by noncanonical Wnt signalling, such as planar polarity. We use nuclear magnetic resonance spectroscopy to identify bona fide PDZ-binding motifs at the C termini of different polarity proteins. Although deletions of these motifs proved aphenotypic in adults, we detected changes in the proximodistal distribution of the polarity protein Flamingo (also known as Starry night) in pupal wings that suggest a modulatory role of these motifs in polarity signalling. We also provide new genetic evidence that planar polarity relies on the DEP-dependent recruitment of Dishevelled to the plasma membrane by Frizzled.

Protein proximity networks and functional evaluation of the casein kinase 1 gamma family reveal unique roles for CK1γ3 in WNT signaling

Aberrant activation or suppression of WNT/β-catenin signaling contributes to cancer initiation and progression, neurodegeneration, and bone disease. However, despite great need and more than 40 years of research, targeted therapies for the WNT pathway have yet to be fully realized. Kinases are considered exceptionally druggable and occupy key nodes within the WNT signaling network, but several pathway-relevant kinases remain understudied and "dark." Here, we studied the function of the casein kinase 1γ (CSNK1γ) subfamily of human kinases and their roles in WNT signaling. miniTurbo-based proximity biotinylation and mass spectrometry analysis of CSNK1γ1, CSNK1γ2, and CSNK1γ3 revealed numerous components of the β-catenin-dependent and β-catenin-independent WNT pathways. In gain-of-function experiments, we found that CSNK1γ3 but not CSNK1γ1 or CSNK1γ2 activated β-catenin-dependent WNT signaling, with minimal effect on other signaling pathways. We also show that within the family, CSNK1γ3 expression uniquely induced low-density lipoprotein receptor-related protein 6 phosphorylation, which mediates downstream WNT signaling transduction. Conversely, siRNA-mediated silencing of CSNK1γ3 alone had no impact on WNT signaling, though cosilencing of all three family members decreased WNT pathway activity. Finally, we characterized two moderately selective and potent small-molecule inhibitors of the CSNK1γ family. We show that these inhibitors and a CSNK1γ3 kinase-dead mutant suppressed but did not eliminate WNT-driven low-density lipoprotein receptor-related protein 6 phosphorylation and β-catenin stabilization. Our data suggest that while CSNK1γ3 expression uniquely drives pathway activity, potential functional redundancy within the family necessitates loss of all three family members to suppress the WNT signaling pathway.

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.

Wnt Signaling Pathways: A Role in Pain Processing

The wingless-related integration site (Wnt) signaling pathway plays an essential role in embryonic development and nervous system regulation. It is critically involved in multiple types of neuropathic pain (NP), such as HIV-related NP, cancer pain, diabetic neuralgia, multiple sclerosis-related NP, endometriosis pain, and other painful diseases. Wnt signaling is also implicated in the pain induced by sciatic nerve compression injury and selective spinal nerve ligation. Thus, the Wnt signaling pathway may be a potential therapeutic target for NP.

Targeting the Wnt/β-Catenin Signaling Pathway as a Potential Therapeutic Strategy in Renal Tubulointerstitial Fibrosis

The Wnt/β-catenin signaling pathway plays important roles in embryonic development and tissue homeostasis. Wnt signaling is induced, and β-catenin is activated, associated with the development and progression of renal fibrosis. Wnt/β-catenin controls the expression of various downstream mediators such as snail1, twist, matrix metalloproteinase-7, plasminogen activator inhibitor-1, transient receptor potential canonical 6, and renin-angiotensin system components in epithelial cells, fibroblast, and macrophages. In addition, Wnt/β-catenin is usually intertwined with other signaling pathways to promote renal interstitial fibrosis. Actually, given the crucial of Wnt/β-catenin signaling in renal fibrogenesis, blocking this signaling may benefit renal interstitial fibrosis. There are several antagonists of Wnt signaling that negatively control Wnt activation, and these include soluble Fzd-related proteins, the family of Dickkopf 1 proteins, Klotho and Wnt inhibitory factor-1. Furthermore, numerous emerging small-molecule β-catenin inhibitors cannot be ignored to prevent and treat renal fibrosis. Moreover, we reviewed the knowledge focusing on anti-fibrotic effects of natural products commonly used in kidney disease by inhibiting the Wnt/β-catenin signaling pathway. Therefore, in this review, we summarize recent advances in the regulation, downstream targets, role, and mechanisms of Wnt/β-catenin signaling in renal fibrosis pathogenesis. We also discuss the therapeutic potential of targeting this pathway to treat renal fibrosis; this may shed new insights into effective treatment strategies to prevent and treat renal fibrosis.

CD47-Mediated Hedgehog/SMO/GLI1 Signaling Promotes Mesenchymal Stem Cell Immunomodulation in Mouse Liver Inflammation

BACKGROUND AND AIMS

The cluster of differentiation 47 (CD47)-signal regulatory protein alpha (SIRPα) signaling pathway plays important roles in immune homeostasis and tissue inflammatory response. Activation of the Hedgehog/smoothened (SMO)/GLI family zinc finger 1 (Gli1) pathway regulates cell growth, differentiation, and immune function. However, it remains unknown whether and how the CD47-SIRPα interaction may regulate Hedgehog/SMO/Gli1 signaling in mesenchymal stem cell (MSC)-mediated immune regulation during sterile inflammatory liver injury.

APPROACH AND RESULTS

In a mouse model of ischemia/reperfusion (IR)-induced sterile inflammatory liver injury, we found that adoptive transfer of MSCs increased CD47 expression and ameliorated liver IR injury. However, deletion of CD47 in MSCs exacerbated IR-induced liver damage, with increased serum ALT levels, macrophage/neutrophil infiltration, and pro-inflammatory mediators. MSC treatment augmented SIRPα, Hedgehog/SMO/Gli1, and Notch1 intracellular domain (NICD), whereas CD47-deficient MSC treatment reduced these gene expressions in IR-stressed livers. Moreover, disruption of myeloid SMO or Notch1 increased IR-triggered liver inflammation with diminished Gli1 and NICD, but enhanced NIMA related kinase 7 (NEK7) and NLR family pyrin domain containing 3 (NLRP3) activation in MSC-transferred mice. Using a MSC/macrophage co-culture system, we found that MSC CD47 and macrophage SIRPα expression were increased after LPS stimulation. The CD47-SIRPα interaction increased macrophage Gli1 and NICD nuclear translocation, whereby NICD interacted with Gli1 and regulated its target gene Dvl2 (dishevelled segment polarity protein 2), which in turn inhibited NEK7/NLRP3 activity.

CONCLUSIONS

The CD47-SIRPα signaling activates the Hedgehog/SMO/Gli1 pathway, which controls NEK7/NLRP3 activity through a direct interaction between Gli1 and NICD. NICD is a coactivator of Gli1, and the target gene Dvl2 regulated by the NICD-Gli1 complex is crucial for the modulation of NLRP3-driven inflammatory response in MSC-mediated immune regulation. Our findings provide potential therapeutic targets in MSC-mediated immunotherapy of sterile inflammatory liver injury.

Regulation of Dishevelled protein activity and stability by post-translational modifications and autophagy

As a key component of Wnt signaling, Dishevelled (Dvl/Dsh) plays essential roles in development processes and adult tissue homeostasis in multicellular organisms, and its deregulation results in human development disorders and other diseases. Dvl integrates and relays complex Wnt signals by acting as a branch-point of β-catenin-dependent canonical and β-catenin-independent noncanonical pathways. It dynamically interacts with multiple proteins to modulate Wnt signaling, while its activity and stability are tightly controlled by other proteins. This Review summarizes the current understanding of regulation of Dvl activity, localization, and stability by post-translational modifications, aggregation, and autophagy, and the impacts on Dvl function in both Wnt signaling and biological processes.

Can We Pharmacologically Target Dishevelled: The Key Signal Transducer in the Wnt Pathways?

Dishevelled (DVL) is the central signal transducer in both Wnt/β-catenin-dependent and independent signalling pathways. DVL is required to connect receptor complexes and downstream effectors. Since proximal Wnt pathway components and DVL itself are upregulated in many types of cancer, DVL represents an attractive therapeutic target in the Wnt-addicted cancers and other disorders caused by aberrant Wnt signalling. Here, we discuss progress in several approaches for the modulation of DVL function and hence inhibition of the Wnt signalling. Namely, we sum up the potential of modulation of enzymes that control post-translational modification of DVL - such as inhibition of DVL kinases or promotion of DVL ubiquitination and degradation. In addition, we discuss research directions that can take advantage of direct interaction with the protein domains essential for DVL function: the inhibition of DIX- and DEP-domain mediated polymerization and interaction of DVL PDZ domain with its ligands.

Regulation of Dishevelled DEP domain swapping by conserved phosphorylation sites

Wnt signals bind to Frizzled receptors to trigger canonical and noncanonical signaling responses that control cell fates during animal development and tissue homeostasis. All Wnt signals are relayed by the hub protein Dishevelled. During canonical (β-catenin-dependent) signaling, Dishevelled assembles signalosomes via dynamic head-to-tail polymerization of its Dishevelled and Axin (DIX) domain, which are cross-linked by its Dishevelled, Egl-10, and Pleckstrin (DEP) domain through a conformational switch from monomer to domain-swapped dimer. The domain-swapped conformation of DEP masks the site through which Dishevelled binds to Frizzled, implying that DEP domain swapping results in the detachment of Dishevelled from Frizzled. This would be incompatible with noncanonical Wnt signaling, which relies on long-term association between Dishevelled and Frizzled. It is therefore likely that DEP domain swapping is differentially regulated during canonical and noncanonical Wnt signaling. Here, we use NMR spectroscopy and cell-based assays to uncover intermolecular contacts in the DEP dimer that are essential for its stability and for Dishevelled function in relaying canonical Wnt signals. These contacts are mediated by an intrinsically structured sequence spanning a conserved phosphorylation site upstream of the DEP domain that serves to clamp down the swapped N-terminal α-helix onto the structural core of a reciprocal DEP molecule in the domain-swapped configuration. Mutations of this phosphorylation site and its cognate surface on the reciprocal DEP core attenuate DEP-dependent dimerization of Dishevelled and its canonical signaling activity in cells without impeding its binding to Frizzled. We propose that phosphorylation of this crucial residue could be employed to switch off canonical Wnt signaling.

The structural biology of canonical Wnt signalling

The Wnt signalling pathways are of great importance in embryonic development and oncogenesis. Canonical and non-canonical Wnt signalling pathways are known, with the canonical (or β-catenin dependent) pathway being perhaps the best studied of these. While structural knowledge of proteins and interactions involved in canonical Wnt signalling has accumulated over the past 20 years, the pace of discovery has increased in recent years, with the structures of several key proteins and assemblies in the pathway being released. In this review, we provide a brief overview of canonical Wnt signalling, followed by a comprehensive overview of currently available X-ray, NMR and cryoEM data elaborating the structures of proteins and interactions involved in canonical Wnt signalling. While the volume of structures available is considerable, numerous gaps in knowledge remain, particularly a comprehensive understanding of the assembly of large multiprotein complexes mediating key aspects of pathway, as well as understanding the structure and activation of membrane receptors in the pathway. Nonetheless, the presently available data affords considerable opportunities for structure-based drug design efforts targeting canonical Wnt signalling.

The nanoscale organization of the Wnt signaling integrator Dishevelled in the vegetal cortex domain of an egg and early embryo

Canonical Wnt/β-catenin (cWnt) signaling is a crucial regulator of development and Dishevelled (Dsh/Dvl) functions as an integral part of this pathway by linking Wnt binding to the Frizzled:LRP5/6 receptor complex with β-catenin-stimulated gene expression. In many cell types Dsh has been localized to ill-defined cytoplasmic puncta, however in sea urchin eggs and embryos confocal fluorescence microscopy has shown that Dsh is localized to puncta present in a novel and development-essential vegetal cortex domain (VCD). In the present study, we used super-resolution light microscopy and platinum replica transmission electron microscopy (TEM) to provide the first views of the ultrastructural organization of Dsh within the sea urchin VCD. 3D structured illumination microscopy (SIM) imaging of isolated egg cortices demonstrated the graded distribution of Dsh in the VCD, whereas higher resolution stimulated emission depletion (STED) imaging revealed that some individual Dsh puncta consisted of more than one fluorescent source. Platinum replica immuno-TEM localization showed that Dsh puncta on the cytoplasmic face of the plasma membrane consisted of aggregates of pedestal-like structures each individually labeled with the C-terminus specific Dsh antibody. These aggregates were resistant to detergent extraction and treatment with drugs that disrupt actin filaments or inhibit myosin II contraction, and coexisted with the first cleavage actomyosin contractile ring. These results confirm and extend previous studies and reveal, for the first time in any cell type, the nanoscale organization of plasma membrane tethered Dsh. Our current working hypothesis is that these Dsh pedestals represent a prepositioned scaffold organization that is important for the localized activation of the cWnt pathway at the sea urchin vegetal pole. These observations in sea urchins may also be relevant to the submembranous Dsh puncta present in other eggs and embryos.

Wnt/β-catenin signaling: Structure, assembly and endocytosis of the signalosome

Wnt/β‐catenin signaling is an ancient pathway that regulates key aspects of embryonic development, cell differentiation, proliferation, and adult stem cell homeostasis. Work from different laboratories has shed light on the molecular mechanisms underlying the Wnt pathway, including structural details of ligand–receptor interactions. One key aspect that has emerged from multiple studies is that endocytosis of the receptor complex plays a crucial role in fine‐tuning Wnt/β‐catenin signaling. Endocytosis is a key process involved in both activation as well as attenuation of Wnt signaling, but how this is regulated is still poorly understood. Importantly, recent findings show that Wnt also regulates central metabolic pathways such as the acquisition of nutrients through actin‐driven endocytic mechanisms. In this review, we propose that the Wnt pathway displays diverse characteristics that go beyond the regulation of gene expression, through a connection with the endocytic machinery.

Zooming in on the WNT/CTNNB1 Destruction Complex: Functional Mechanistic Details with Implications for Therapeutic Targeting

WNT/CTNNB1 signaling is crucial for balancing cell proliferation and differentiation in all multicellular animals. CTNNB1 accumulation is the hallmark of WNT/CTNNB1 pathway activation and the key downstream event in both a physiological and an oncogenic context. In the absence of WNT stimulation, the cytoplasmic and nuclear levels of CTNNB1 are kept low because of its sequestration and phosphorylation by the so-called destruction complex, which targets CTNNB1 for proteasomal degradation. In the presence of WNT proteins, or as a result of oncogenic mutations, this process is impaired and CTNNB1 levels become elevated.Here we discuss recent advances in our understanding of destruction complex activity and inactivation, focusing on the individual components and interactions that ultimately control CTNNB1 turnover (in the "WNT off" situation) and stabilization (in the "WNT on" situation). We especially highlight the insights gleaned from recent quantitative, image-based studies, which paint an unprecedentedly detailed picture of the dynamic events that control destruction protein complex composition and function. We argue that these mechanistic details may reveal new opportunities for therapeutic intervention and could result in the destruction complex re-emerging as a target for therapy in cancer.

Decoding Dishevelled-Mediated Wnt Signaling in Vertebrate Early Development

Dishevelled proteins are key players of Wnt signaling pathways. They transduce Wnt signals and perform cellular functions through distinct conserved domains. Due to the presence of multiple paralogs, the abundant accumulation of maternal transcripts, and the activation of distinct Wnt pathways, their regulatory roles during vertebrate early development and the mechanism by which they dictate the pathway specificity have been enigmatic and attracted much attention in the past decades. Extensive studies in different animal models have provided significant insights into the structure-function relationship of conserved Dishevelled domains in Wnt signaling and the implications of Dishevelled isoforms in early developmental processes. Notably, intra- and inter-molecular interactions and Dishevelled dosage may be important in modulating the specificity of Wnt signaling. There are also distinct and redundant functions among Dishevelled isoforms in development and disease, which may result from differential spatiotemporal expression patterns and biochemical properties and post-translational modifications. This review presents the advances and perspectives in understanding Dishevelled-mediated Wnt signaling during gastrulation and neurulation in vertebrate early embryos.

Volumetric Compression Induces Intracellular Crowding to Control Intestinal Organoid Growth via Wnt/β-Catenin Signaling

Enormous amounts of essential intracellular events are crowdedly packed inside picoliter-sized cellular space. However, the significance of the physical properties of cells remains underappreciated because of a lack of evidence of how they affect cellular functionalities. Here, we show that volumetric compression regulates the growth of intestinal organoids by modifying intracellular crowding and elevating Wnt/β-catenin signaling. Intracellular crowding varies upon stimulation by different types of extracellular physical/mechanical cues and leads to significant enhancement of Wnt/β-catenin signaling by stabilizing the LRP6 signalosome. By enhancing intracellular crowding using osmotic and mechanical compression, we show that expansion of intestinal organoids was facilitated through elevated Wnt/β-catenin signaling and greater intestinal stem cell (ISC) self-renewal. Our results provide an entry point for understanding how intracellular crowdedness functions as a physical regulator linking extracellular physical cues with intracellular signaling and potentially facilitate the design of engineering approaches for expansion of stem cells and organoids.

Feedback control of Wnt signaling based on ultrastable histidine cluster co-aggregation between Naked/NKD and Axin

Feedback control is a universal feature of cell signaling pathways. Naked/NKD is a widely conserved feedback regulator of Wnt signaling which controls animal development and tissue homeostasis. Naked/NKD destabilizes Dishevelled, which assembles Wnt signalosomes to inhibit the β-catenin destruction complex via recruitment of Axin. Here, we discover that the molecular mechanism underlying Naked/NKD function relies on its assembly into ultra-stable decameric core aggregates via its conserved C-terminal histidine cluster (HisC). HisC aggregation is facilitated by Dishevelled and depends on accumulation of Naked/NKD during prolonged Wnt stimulation. Naked/NKD HisC cores co-aggregate with a conserved histidine cluster within Axin, to destabilize it along with Dishevelled, possibly via the autophagy receptor p62, which binds to HisC aggregates. Consistent with this, attenuated Wnt responses are observed in CRISPR-engineered flies and human epithelial cells whose Naked/NKD HisC has been deleted. Thus, HisC aggregation by Naked/NKD provides context-dependent feedback control of prolonged Wnt responses.

Intersection of two key signal integrators in the cell: activator of G-protein signaling 3 and dishevelled-2

Activator of G-protein signaling 3 (AGS3, encoded by GPSM1) was discovered as a one of several receptor-independent activators of G-protein signaling, which are postulated to provide a platform for divergence between canonical and noncanonical G-protein signaling pathways. Similarly, Dishevelled (DVL) proteins serve as a point of divergence for β-catenin-dependent and -independent signaling pathways involving the family of Frizzled (FZD) ligands and cell-surface WNT receptors. We recently discovered the apparent regulated localization of dishevelled-2 (DVL2) and AGS3 to distinct cellular puncta, suggesting that the two proteins interact as part of various cell signaling systems. To address this hypothesis, we asked the following questions: (1) do AGS3 signaling pathways influence the activation of β-catenin (CTNNB1)-regulated transcription through the WNT-Frizzled-Dishevelled axis, and (2) is the AGS3 and DVL2 interaction regulated? The interaction of AGS3 and DVL2 was regulated by protein phosphorylation, subcellular distribution, and a cell-surface G-protein-coupled receptor. These data, and the commonality of functional system impacts observed for AGS3 and DVL2, suggest that the AGS3-DVL2 complex presents an unexpected path for functional integration within the cell.This article has an associated First Person interview with the first author of the paper.

Head-to-Tail Polymerization in the Assembly of Biomolecular Condensates

Clustering of macromolecules is a fundamental cellular device underlying diverse biological processes that require high-avidity binding to effectors and substrates. Often, this involves a transition between diffuse and locally concentrated molecules akin to biophysical phase separation observable in vitro. One simple mechanistic paradigm underlying physiologically relevant phase transitions in cells is the reversible head-to-tail polymerization of hub proteins into filaments that are cross-linked by dimerization into dynamic three-dimensional molecular condensates. While many diverse folds and motifs can mediate dimerization, only two structurally distinct domains have been discovered so far to undergo head-to-tail polymerization, though these are widespread among all living kingdoms.

ABL1-dependent OTULIN phosphorylation promotes genotoxic Wnt/β-catenin activation to enhance drug resistance in breast cancers

Dysregulated Wnt/β-catenin activation plays a critical role in cancer progression, metastasis, and drug resistance. Genotoxic agents such as radiation and chemotherapeutics have been shown to activate the Wnt/β-catenin signaling although the underlying mechanism remains incompletely understood. Here, we show that genotoxic agent-activated Wnt/β-catenin signaling is independent of the FZD/LRP heterodimeric receptors and Wnt ligands. OTULIN, a linear linkage-specific deubiquitinase, is essential for the DNA damage-induced β-catenin activation. OTULIN inhibits linear ubiquitination of β-catenin, which attenuates its Lys48-linked ubiquitination and proteasomal degradation upon DNA damage. The association with β-catenin is enhanced by OTULIN Tyr56 phosphorylation, which depends on genotoxic stress-activated ABL1/c-Abl. Inhibiting OTULIN or Wnt/β-catenin sensitizes triple-negative breast cancer xenograft tumors to chemotherapeutics and reduces metastasis. Increased OTULIN levels are associated with aggressive molecular subtypes and poor survival in breast cancer patients. Thus, OTULIN-mediated Wnt/β-catenin activation upon genotoxic treatments promotes drug resistance and metastasis in breast cancers.

Genotoxic agents have been shown to activate the Wnt/β-catenin signaling but the underlying mechanism remains unclear. Here, the authors show that upon DNA damage, the deubiquitinase OTULIN activates Wnt/β-catenin signaling by inhibiting linear ubiquitination, K48-linked polyubiquitination, and proteasomal degradation of β-catenin.

Wnt signaling pathway in aging-related tissue fibrosis and therapies

Fibrosis is the final hallmark of pathological remodeling, which is a major contributor to the pathogenesis of various chronic diseases and aging-related organ failure to fully control chronic wound-healing and restoring tissue function. The process of fibrosis is involved in the pathogenesis of the kidney, lung, liver, heart and other tissue disorders. Wnt is a highly conserved signaling in the aberrant wound repair and fibrogenesis, and sustained Wnt activation is correlated with the pathogenesis of fibrosis. In particular, mounting evidence has revealed that Wnt signaling played important roles in cell fate determination, proliferation and cell polarity establishment. The expression and distribution of Wnt signaling in different tissues vary with age, and these changes have key effects on maintaining tissue homeostasis. In this review, we first describe the major constituents of the Wnt signaling and their regulation functions. Subsequently, we summarize the dysregulation of Wnt signaling in aging-related fibrotic tissues such as kidney, liver, lung and cardiac fibrosis, followed by a detailed discussion of its involvement in organ fibrosis. In addition, the crosstalk between Wnt signaling and other pathways has the potential to profoundly add to the complexity of organ fibrosis. Increasing studies have demonstrated that a number of Wnt inhibitors had the potential role against tissue fibrosis, specifically in kidney fibrosis and the implications of Wnt signaling in aging-related diseases. Therefore, targeting Wnt signaling might be a novel and promising therapeutic strategy against aging-related tissue fibrosis.

Single-molecule dynamics of Dishevelled at the plasma membrane and Wnt pathway activation

Canonical Wnt signaling is one of the most important and widely distributed pathways in metazoan development. Dishevelled is thought to serve as an essential bridge between the membrane receptors and downstream signaling components, which has the tendency to aggregate in vitro and to form large aggregates of dubious significance in vivo, when overexpressed. To obtain a molecular understanding of the role of Dvl in Wnt signaling, while circumventing these aggregation problems, we have expressed a fluorescent-tagged Dishevelled in cells at its physiological concentration and quantified the size distribution of Dishevelled before and after Wnt treatment. We found that limited oligomerization in response to the Wnt ligand is very dynamic and provides a key step in signal transduction.

Dvl (Dishevelled) is one of several essential nonenzymatic components of the Wnt signaling pathway. In most current models, Dvl forms complexes with Wnt ligand receptors, Fzd and LRP5/6 at the plasma membrane, which then recruits the destruction complex, eventually leading to inactivation of β-catenin degradation. Although this model is widespread, direct evidence for the individual steps is lacking. In this study, we tagged mEGFP to C terminus of dishevelled2 gene using CRISPR/Cas9-induced homologous recombination and observed its dynamics directly at the single-molecule level with total internal reflection fluorescence (TIRF) microscopy. We focused on two questions: 1) What is the native size and what are the dynamic features of membrane-bound Dvl complexes during Wnt pathway activation? 2) What controls the behavior of these complexes? We found that membrane-bound Dvl2 is predominantly monomer in the absence of Wnt (observed mean size 1.1). Wnt3a stimulation leads to an increase in the total concentration of membrane-bound Dvl2 from 0.12/μm² to 0.54/μm². Wnt3a also leads to increased oligomerization which raises the weighted mean size of Dvl2 complexes to 1.5, with 56.1% of Dvl still as monomers. The driving force for Dvl2 oligomerization is the increased concentration of membrane Dvl2 caused by increased affinity of Dvl2 for Fzd, which is independent of LRP5/6. The oligomerized Dvl2 complexes have increased dwell time, 2 ∼ 3 min, compared to less than 1 s for monomeric Dvl2. These properties make Dvl a unique scaffold, dynamically changing its state of assembly and stability at the membrane in response to Wnt ligands.

Limited dishevelled/Axin oligomerization determines efficiency of Wnt/β-catenin signal transduction

In Wnt/β-catenin signaling, the transcriptional coactivator β-catenin is regulated by its phosphorylation in a complex that includes the scaffold protein Axin and associated kinases. Wnt binding to its coreceptors activates the cytosolic effector Dishevelled (Dvl), leading to the recruitment of Axin and the inhibition of β-catenin phosphorylation. This process requires interaction of homologous DIX domains present in Dvl and Axin, but is mechanistically undefined. We show that Dvl DIX forms antiparallel, double-stranded oligomers in vitro, and that Dvl in cells forms oligomers typically <10 molecules at endogenous expression levels. Axin DIX (DAX) forms small single-stranded oligomers, but its self-association is stronger than that of DIX. DAX caps the ends of DIX oligomers, such that a DIX oligomer has at most four DAX binding sites. The relative affinities and stoichiometry of the DIX-DAX interaction provide a mechanism for efficient inhibition of β-catenin phosphorylation upon Axin recruitment to the Wnt receptor complex.

Wnt regulation: exploring Axin-Disheveled interactions and defining mechanisms by which the SCF E3 ubiquitin ligase is recruited to the destruction complex

Wnt signaling plays key roles in embryonic development and adult stem cell homeostasis and is altered in human cancer. Signaling is turned on and off by regulating stability of the effector β-catenin (β-cat). The multiprotein destruction complex binds and phosphorylates β-cat and transfers it to the SCF-TrCP E3-ubiquitin ligase for ubiquitination and destruction. Wnt signals act though Dishevelled to turn down the destruction complex, stabilizing β-cat. Recent work clarified underlying mechanisms, but important questions remain. We explore β-cat transfer from the destruction complex to the E3 ligase, and test models suggesting Dishevelled and APC2 compete for association with Axin. We find that Slimb/TrCP is a dynamic component of the destruction complex biomolecular condensate, while other E3 proteins are not. Recruitment requires Axin and not APC, and Axin’s RGS domain plays an important role. We find that elevating Dishevelled levels in Drosophila embryos has paradoxical effects, promoting the ability of limiting levels of Axin to turn off Wnt signaling. When we elevate Dishevelled levels, it forms its own cytoplasmic puncta, but these do not recruit Axin. Superresolution imaging in mammalian cells raises the possibility that this may result by promoting Dishevelled:Dishevelled interactions at the expense of Dishevelled: Axin interactions when Dishevelled levels are high.

DIX Domain Polymerization Drives Assembly of Plant Cell Polarity Complexes

Cell polarity is fundamental for tissue morphogenesis in multicellular organisms. Plants and animals evolved multicellularity independently, and it is unknown whether their polarity systems are derived from a single-celled ancestor. Planar polarity in animals is conferred by Wnt signaling, an ancient signaling pathway transduced by Dishevelled, which assembles signalosomes by dynamic head-to-tail DIX domain polymerization. In contrast, polarity-determining pathways in plants are elusive. We recently discovered Arabidopsis SOSEKI proteins, which exhibit polar localization throughout development. Here, we identify SOSEKI as ancient polar proteins across land plants. Concentration-dependent polymerization via a bona fide DIX domain allows these to recruit ANGUSTIFOLIA to polar sites, similar to the polymerization-dependent recruitment of signaling effectors by Dishevelled. Cross-kingdom domain swaps reveal functional equivalence of animal and plant DIX domains. We trace DIX domains to unicellular eukaryotes and thus show that DIX-dependent polymerization is an ancient mechanism conserved between kingdoms and central to polarity proteins.

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SOSEKI proteins are deeply conserved polar proteins in land plants

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A DIX domain mediates polymerization and polarization of SOSEKI proteins

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SOSEKI polymerization allows polar recruitment of an effector protein

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DIX-dependent polymerization is shared between animal and plant polarity proteins

Comparative phosphorylation map of Dishevelled 3 links phospho-signatures to biological outputs

Background

Dishevelled (DVL) is an essential component of the Wnt signaling cascades. Function of DVL is controlled by phosphorylation but the molecular details are missing. DVL3 contains 131 serines and threonines whose phosphorylation generates complex barcodes underlying diverse DVL3 functions. In order to dissect the role of DVL phosphorylation we analyzed the phosphorylation of human DVL3 induced by previously reported (CK1ε, NEK2, PLK1, CK2α, RIPK4, PKCδ) and newly identified (TTBK2, Aurora A) DVL kinases.

Methods

Shotgun proteomics including TiO₂ enrichment of phosphorylated peptides followed by liquid chromatography tandem mass spectrometry on immunoprecipitates from HEK293T cells was used to identify and quantify phosphorylation of DVL3 protein induced by 8 kinases. Functional characterization was performed by in-cell analysis of phospho-mimicking/non-phosphorylatable DVL3 mutants and supported by FRET assays and NMR spectroscopy.

Results

We used quantitative mass spectrometry and calculated site occupancies and quantified phosphorylation of > 80 residues. Functional validation demonstrated the importance of CK1ε-induced phosphorylation of S268 and S311 for Wnt-3a-induced β-catenin activation. S630–643 cluster phosphorylation by CK1, NEK2 or TTBK2 is essential for even subcellular distribution of DVL3 when induced by CK1 and TTBK2 but not by NEK2. Further investigation showed that NEK2 utilizes a different mechanism to promote even localization of DVL3. NEK2 triggered phosphorylation of PDZ domain at S263 and S280 prevents binding of DVL C-terminus to PDZ and promotes an open conformation of DVL3 that is more prone to even subcellular localization.

Conclusions

We identify unique phosphorylation barcodes associated with DVL function. Our data provide an example of functional synergy between phosphorylation in structured domains and unstructured IDRs that together dictate the biological outcome.

A direct heterotypic interaction between the DIX domains of Dishevelled and Axin mediates signaling to β-catenin

The Wnt-β-catenin signaling pathway regulates embryonic development and tissue homeostasis throughout the animal kingdom. Signaling through this pathway crucially depends on the opposing activities of two cytoplasmic multiprotein complexes: the Axin destruction complex, which destabilizes the downstream effector β-catenin, and the Dishevelled signalosome, which inactivates the Axin complex and thus enables β-catenin to accumulate and operate a transcriptional switch in the nucleus. These complexes are assembled by dynamic head-to-tail polymerization of the DIX domains of Axin or Dishevelled, respectively, which increases their avidity for signaling effectors. Axin also binds to Dishevelled through its DIX domain. Here, we report the crystal structure of the heterodimeric complex between the two DIX domains of Axin and Dishevelled. This heterotypic interface resembles the interfaces observed in the individual homopolymers, albeit exhibiting a slight rearrangement of electrostatic interactions and hydrogen bonds, consistent with the heterotypic interaction being favored over the homotypic Axin DIX interaction. Last, cell-based signaling assays showed that heterologous polymerizing domains functionally substituted for the DIX domain of Dishevelled provided that these Dishevelled chimeras retained a DIX head or tail surface capable of binding to Axin. These findings indicate that the interaction between Dishevelled and Axin through their DIX domains is crucial for signaling to β-catenin.

Acetylation of conserved DVL-1 lysines regulates its nuclear translocation and binding to gene promoters in triple-negative breast cancer

Dishevelled (DVL) proteins are central mediators of the Wnt signalling pathway and are versatile regulators of several cellular processes, yet little is known about their post-translational regulation. Acetylation is a reversible post-translational modification (PTM) which regulates the function of several non-histone proteins involved in tumorigenesis. Since we previously demonstrated that lysine deacetylase, SIRT-1, regulates DVL protein levels and its function, we reasoned that DVL could potentially be a substrate for SIRT-1 mediated deacetylation. To further examine the potential role of multiple families of lysine deacetylases in the post-translational regulation of DVL, we screened for novel acetylation sites using liquid chromatography mass-spectrometry (LC-MS/MS) analysis. Herein, we report 12 DVL-1 lysine residues that show differential acetylation in response to changes in oxygen tension and deacetylase inhibition in triple-negative breast cancer (TNBC). PTMs are well documented to influence protein activity, and cellular localization. We also identify that acetylation of two key lysine residues, K69 and K285, present on the DIX and PDZ domains respectively, promote nuclear over cytoplasmic localization of DVL-1, and influences its promoter binding and regulation of genes implicated in cancer. Collectively, these findings for the first time, uncover acetylation as a novel layer of regulation of DVL-1 proteins.

Blood vessel epicardial substance reduces LRP6 receptor and cytoplasmic β-catenin levels to modulate Wnt signaling and intestinal homeostasis

Blood vessel epicardial substance (BVES, otherwise known as POPDC1) is an integral membrane protein known to regulate tight junction formation and epithelial-mesenchymal transition. BVES is underexpressed in a number of malignancies, including colorectal cancer. BVES loss leads to activation of the Wnt pathway, suggesting that decreased BVES expression functionally contributes to tumorigenesis. However, the mechanism by which BVES modulates Wnt signaling is unknown. Here, we confirm that BVES loss increases β-catenin protein levels, leads to Wnt pathway activation in a ligand-independent fashion and coordinates with Wnt ligand to further increase Wnt signaling. We show that BVES loss increases levels and activation of the Wnt co-receptor, LRP6, in cell lines, murine adenoma tumoroids and human-derived colonoids. We also demonstrate that BVES interacts with LRP6. Finally, murine tumor modeling using a Wnt-driven genetic model and a chemically induced model of colorectal carcinogenesis demonstrate that BVES loss increases tumor multiplicity and dysplasia. Together, these results implicate BVES as an inhibitor of Wnt signaling, provide one of the first examples of a tight junction-associated protein regulating Wnt receptor levels, and expand the number of putative molecular targets for therapeutic intervention in colorectal cancer.

TMEM88 Inhibits Wnt Signaling by Promoting Wnt Signalosome Localization to Multivesicular Bodies

Wnt/β-catenin signaling is regulated in a bimodal fashion during cardiogenesis. Signaling is initially required to promote generation of precardiac mesoderm, but subsequently must be repressed for cardiac progenitor specification. TMEM88 was discovered recently as a negative regulator during the later phase of cardiac progenitor specification, but how TMEM88 functions was unknown. Based on a C-terminal PDZ-binding motif, TMEM88 was proposed to act by targeting the PDZ domain of Dishevelled, the positive Wnt signaling mediator. However, we discovered that TMEM88 acts downstream of the β-catenin destruction complex and can inhibit Wnt signaling independent of Dishevelled. TMEM88 requires the PDZ-binding motif for trafficking from Golgi to the plasma membrane and is also found in the multivesicular body (MVB) associated with the endocytosed Wnt signalosome. Expression of Tmem88 promotes association of the Wnt signalosome including β-catenin to the MVB, leading to reduced accumulation of nuclear β-catenin and repression of Wnt signaling.

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Human ESCs with a targeted TMEM88 knockout are impaired for cardiac specification

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TMEM88 does not require Dishevelled to inhibit Wnt signaling

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TMEM88 is trafficked from Golgi to plasma membrane and then to the MVB

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Expression of TMEM88 promotes association of the signalosome to the MVB

High-resolution structure of a Y27W mutant of the Dishevelled2 DIX domain

Dishevelled (Dvl) is a positive regulator of the canonical Wnt pathway that downregulates the phosphorylation of β-catenin and its subsequent degradation. Dvl contains an N-terminal DIX domain, which is involved in its homooligomerization and interactions with regulators of the Wnt pathway. The crystal structure of a Y27W mutant of the Dishevelled2 DIX domain (DIX-Y27W) has been determined at 1.64 Å resolution. DIX-Y27W has a compact ubiquitin-like fold and self-associates with neighbouring molecules through β-bridges, resulting in a head-to-tail helical molecular arrangement similar to previously reported structures of DIX domains. Glu23 of DIX-Y27W forms a hydrogen bond to the side chain of Trp27, corresponding to the Glu762...Trp766 hydrogen bond of the rat Axin DIX domain, whereas Glu23 in the Y27D mutant of the Dishevelled2 DIX domain forms a salt bridge to Lys68 of the adjacent molecule. The high-resolution DIX-Y27W structure provides details of the head-to-tail interaction, including solvent molecules, and also the plausibly wild-type-like structure of the self-association surface compared with previously published Dvl DIX-domain mutants.

WNT Activates the AAK1 Kinase to Promote Clathrin-Mediated Endocytosis of LRP6 and Establish a Negative Feedback Loop

β-Catenin-dependent WNT signal transduction governs development, tissue homeostasis, and a vast array of human diseases. Signal propagation through a WNT-Frizzled/LRP receptor complex requires proteins necessary for clathrin-mediated endocytosis (CME). Paradoxically, CME also negatively regulates WNT signaling through internalization and degradation of the receptor complex. Here, using a gain-of-function screen of the human kinome, we report that the AP2 associated kinase 1 (AAK1), a known CME enhancer, inhibits WNT signaling. Reciprocally, AAK1 genetic silencing or its pharmacological inhibition using a potent and selective inhibitor activates WNT signaling. Mechanistically, we show that AAK1 promotes clearance of LRP6 from the plasma membrane to suppress the WNT pathway. Time-course experiments support a transcription-uncoupled, WNT-driven negative feedback loop; prolonged WNT treatment drives AAK1-dependent phosphorylation of AP2M1, clathrin-coated pit maturation, and endocytosis of LRP6. We propose that, following WNT receptor activation, increased AAK1 function and CME limits WNT signaling longevity.

Functional dissection of the N-terminal extracellular domains of Frizzled 6 reveals their roles for receptor localization and Dishevelled recruitment

The Frizzled (FZD) proteins belong to class F of G protein-coupled receptors (GPCRs) and are essential for various pathways involving the secreted lipoglycoproteins of the wingless/int-1 (WNT) family. A WNT-binding cysteine-rich domain (CRD) in FZDs is N-terminally located and connected to the seven transmembrane domain-spanning receptor core by a linker domain that has a variable length in different FZD homologs. However, the function and importance of this linker domain are poorly understood. Here we used systematic mutagenesis of FZD6 to define the minimal N-terminal domain sufficient for receptor surface expression and recruitment of the intracellular scaffold protein Dishevelled (DVL). Further, we identified a triad of evolutionarily conserved cysteines in the FZD linker domain that is crucial for receptor membrane expression and recruitment of DVL. Our results are in agreement with the concept that the conserved cysteines in the linker domain of FZDs assist with the formation of a common secondary structure in this region. We propose that this structure could be involved in agonist binding and receptor activation mechanisms that are similar to the binding and activation mechanisms known for other GPCRs.

MicroRNA-29a-3p enhances dental implant osseointegration of hyperlipidemic rats via suppressing dishevelled 2 and frizzled 4

Background

Fine osseointegration is the basis of long-term survival of implant. In our previous study, we observed a strong correlation between hyperlipidemia and compromised osseointegration. MicroRNA-29a-3p (miR-29a-3p) has been discovered to participate in bone marrow mesenchymal stem cells (BMSCs) differentiation. However, the role and the underlying mechanisms of hyperlipidemia and miR-29a-3p in osseointegration still remain obscure.

Results

In peri-implant bone tissues of hyperlipidemia rats, bone mass, mineralization and bone trabecula formation were weakened. Alkaline phosphatase (ALP) and runt-related transcription factor 2 (Runx2), and miR-29a-3p expression were reduced. While in normal rats, implant-bone interfaces were filled with dense new bone and ALP, Runx2 and miR-29a-3p were up-regulated. Overexpressed miR-29a-3p can reverse the adverse effect of hyperlipidemia on osseointegration. Implants were tightly integrated with the surrounding dense new bone tissues, and ALP as well as Runx2 mRNAs were enhanced in miR-29a-3p overexpressed and hyperlipidemia rats, while little peri-implant bone tissue existed, ALP and Runx2 deregulated on miR-29a-3p inhibited rats. Dishevelled 2 (Dvl2) mRNA was declined in peri-implant bone tissue of high-fat (HF) group than normal group, while frizzled 4 (Fzd4) mRNA declined on day 5 and increased from day 10 to day 20 after implantation in hyperlipidemia rats than in normal rats. Next, BMSCs were cultured under HF or normal medium in vitro. In the HF group, ALP activity and mineralization, ALP and Runx2 mRNAs and proteins expression, and miR-29a-3p expression were suppressed, while adipogenesis was increased, as a result, cytoskeletons were sparse and disordered compared to control group. However, when miR-29a-3p was overexpressed in BMSCs, ALP activity, ALP, Runx2, Dvl2 and Fzd4 mRNAs and proteins expressions were up-regulated. As miR-29a-3p was inhibited in BMSCs, the reverse results were obtained. In addition, promoter assay revealed that miR-29a-3p can directly suppress Wnt/β-catenin pathway related Dvl2 and Fzd4 through binding to their 3'-UTR.

Conclusions

MiR-29a-3p facilitated implant osseointegration via targeting Wnt/β-catenin pathway-related Dvl2 and Fzd4. MiR-29a-3p/Dvl2/Fzd4 may serve as a promising therapeutic target for hyperlipidemia osseointegration.

Oligomerization of Frizzled and LRP5/6 protein initiates intracellular signaling for the canonical WNT/β-catenin pathway

Upon binding to the canonical WNT glycoproteins, Frizzled family receptors (FZDs) and low-density lipoprotein receptor-related protein 5/6 (LRP5/6) undergo a series of polymerizations on the cell surface that elicit canonical WNT/β-catenin signaling. The hyperactivation of WNT/β-catenin signaling is the major cause of tumorigenesis, but the mechanism in tumors such as hepatoma remains unclear. Here, we observed that WNT3A manifested the hyperactivity in β-catenin-dependent signaling after binding to FZD's competitive inhibitory molecule secreted Frizzled-related protein 2 (SFRP2). To understand the mechanism of FZDs in the presence of SFRP2, we explored how FZDs can bind and activate the LRP5/6 signalosome independently of WNT glycoproteins. Our findings further revealed that oligomerizations of FZDs and LRP5/6 can integrate the cytoplasmic protein Dishevelled into the LRP5/6 signalosome, resulting in a robust activation of ligand-independent β-catenin signaling. We propose that besides WNT-bridged FZD-WNT-LRP5/6 protein complexes, the homo- and hetero-oligomerizations of WNT receptors may contribute to the formation of the LRP5/6 signalosome on the cell surface. Of note, we identified four highly expressed FZDs in the hepatoma cell line HepG2, all of which significantly promoted ligand-independent LRP5/β-catenin signaling. As FZDs are ectopically expressed in numerous tumors, our findings may provide a new perspective on tumor pathologies. Furthermore, the results in our study suggest that the composition and stoichiometry of FZDs and LRP5/6 within the LRP5/6 signalosome may tune the selection of bound WNT glycoproteins and configure downstream WNT/β-catenin signaling.

Dishevelled enables casein kinase 1-mediated phosphorylation of Frizzled 6 required for cell membrane localization

Frizzleds (FZDs) are receptors for secreted lipoglycoproteins of the Wingless/Int-1 (WNT) family, initiating an important signal transduction network in multicellular organisms. FZDs are G protein-coupled receptors (GPCRs), which are well known to be regulated by phosphorylation, leading to specific downstream signaling or receptor desensitization. The role and underlying mechanisms of FZD phosphorylation remain largely unexplored. Here, we investigated the phosphorylation of human FZD₆ Using MS analysis and a phospho-state- and -site-specific antibody, we found that Ser-648, located in the FZD₆ C terminus, is efficiently phosphorylated by casein kinase 1 ϵ (CK1ϵ) and that this phosphorylation requires the scaffolding protein Dishevelled (DVL). In an overexpression system, DVL1, -2, and -3 promoted CK1ϵ-mediated FZD₆ phosphorylation on Ser-648. This DVL activity required an intact DEP domain and FZD-mediated recruitment of this domain to the cell membrane. Substitution of the CK1ϵ-targeted phosphomotif reduced FZD₆ surface expression, suggesting that Ser-648 phosphorylation controls membrane trafficking of FZD₆ Phospho-Ser-648 FZD₆ immunoreactivity in human fallopian tube epithelium was predominantly apical, associated with cilia in a subset of epithelial cells, compared with the total FZD₆ protein expression, suggesting that FZD₆ phosphorylation contributes to asymmetric localization of receptor function within the cell and to epithelial polarity. Given the key role of FZD₆ in planar cell polarity, our results raise the possibility that asymmetric phosphorylation of FZD₆ rather than asymmetric protein distribution accounts for polarized receptor signaling.

mTORC1 signaling suppresses Wnt/β-catenin signaling through DVL-dependent regulation of Wnt receptor FZD level

Wnt/β-catenin signaling plays pivotal roles in cell proliferation and tissue homeostasis by maintaining somatic stem cell functions. The mammalian target of rapamycin (mTOR) signaling functions as an integrative rheostat that orchestrates various cellular and metabolic activities that shape tissue homeostasis. Whether these two fundamental signaling pathways couple to exert physiological functions still remains mysterious. Using a genome-wide CRISPR-Cas9 screening, we discover that mTOR complex 1 (mTORC1) signaling suppresses canonical Wnt/β-catenin signaling. Deficiency in tuberous sclerosis complex 1/2 (TSC1/2), core negative regulators of mTORC1 activity, represses Wnt/β-catenin target gene expression, which can be rescued by RAD001. Mechanistically, mTORC1 signaling regulates the cell surface level of Wnt receptor Frizzled (FZD) in a Dishevelled (DVL)-dependent manner by influencing the association of DVL and clathrin AP-2 adaptor. Sustained mTORC1 activation impairs Wnt/β-catenin signaling and causes loss of stemness in intestinal organoids ex vivo and primitive intestinal progenitors in vivo. Wnt/β-catenin-dependent liver metabolic zonation gene expression program is also down-regulated by mTORC1 activation. Our study provides a paradigm that mTORC1 signaling cell autonomously regulates Wnt/β-catenin pathway to influence stem cell maintenance.

Map7/7D1 and Dvl form a feedback loop that facilitates microtubule remodeling and Wnt5a signaling

The Wnt signaling pathway can be grouped into two classes, the β-catenin-dependent and β-catenin-independent pathways. Wnt5a signaling through a β-catenin-independent pathway promotes microtubule (MT) remodeling during cell-substrate adhesion, cell migration, and planar cell polarity formation. Although Wnt5a signaling and MT remodeling are known to form an interdependent regulatory loop, the underlying mechanism remains unknown. Here we show that in HeLa cells, the paralogous MT-associated proteins Map7 and Map7D1 (Map7/7D1) form an interdependent regulatory loop with Disheveled, the critical signal transducer in Wnt signaling. Map7/7D1 bind to Disheveled, direct its cortical localization, and facilitate the cortical targeting of MT plus-ends in response to Wnt5a signaling. Wnt5a signaling also promotes Map7/7D1 movement toward MT plus-ends, and depletion of the Kinesin-1 member Kif5b abolishes the Map7/7D1 dynamics and Disheveled localization. Furthermore, Disheveled stabilizes Map7/7D1. Intriguingly, Map7/7D1 and its Drosophila ortholog, Ensconsin show planar-polarized distribution in both mouse and fly epithelia, and Ensconsin influences proper localization of Drosophila Disheveled in pupal wing cells. These results suggest that the role of Map7/7D1/Ensconsin in Disheveled localization is evolutionarily conserved.

Supramolecular assembly of the beta-catenin destruction complex and the effect of Wnt signaling on its localization, molecular size, and activity in vivo

Wnt signaling provides a paradigm for cell-cell signals that regulate embryonic development and stem cell homeostasis and are inappropriately activated in cancers. The tumor suppressors APC and Axin form the core of the multiprotein destruction complex, which targets the Wnt-effector beta-catenin for phosphorylation, ubiquitination and destruction. Based on earlier work, we hypothesize that the destruction complex is a supramolecular entity that self-assembles by Axin and APC polymerization, and that regulating assembly and stability of the destruction complex underlie its function. We tested this hypothesis in Drosophila embryos, a premier model of Wnt signaling. Combining biochemistry, genetic tools to manipulate Axin and APC2 levels, advanced imaging and molecule counting, we defined destruction complex assembly, stoichiometry, and localization in vivo, and its downregulation in response to Wnt signaling. Our findings challenge and revise current models of destruction complex function. Endogenous Axin and APC2 proteins and their antagonist Dishevelled accumulate at roughly similar levels, suggesting competition for binding may be critical. By expressing Axin:GFP at near endogenous levels we found that in the absence of Wnt signals, Axin and APC2 co-assemble into large cytoplasmic complexes containing tens to hundreds of Axin proteins. Wnt signals trigger recruitment of these to the membrane, while cytoplasmic Axin levels increase, suggesting altered assembly/disassembly. Glycogen synthase kinase3 regulates destruction complex recruitment to the membrane and release of Armadillo/beta-catenin from the destruction complex. Manipulating Axin or APC2 levels had no effect on destruction complex activity when Wnt signals were absent, but, surprisingly, had opposite effects on the destruction complex when Wnt signals were present. Elevating Axin made the complex more resistant to inactivation, while elevating APC2 levels enhanced inactivation. Our data suggest both absolute levels and the ratio of these two core components affect destruction complex function, supporting models in which competition among Axin partners determines destruction complex activity.

TMEM59 potentiates Wnt signaling by promoting signalosome formation

Wnt/β-catenin signaling is crucial for adult homeostasis and stem cell maintenance, and its dysregulation is strongly associated to cancer. Upon Wnt binding, Wnt receptors assemble into large complexes called signalosomes that provide a platform for interactions with downstream effector proteins. The assembly and regulation of these signalosomes remains largely elusive. Here, we use internally tagged Wnt ligands as a tool to isolate and analyze the composition and regulation of endogenous Wnt receptor complexes. We identify a positive regulator of Wnt signaling that facilitates signalosome formation by promoting intramembrane receptor interactions. Our results reveal that the assembly of multiprotein Wnt signalosomes proceeds along well-ordered steps and involves regulated intramembrane interactions.

Wnt/β-catenin signaling controls development and adult tissue homeostasis by regulating cell proliferation and cell fate decisions. Wnt binding to its receptors Frizzled (FZD) and low-density lipoprotein-related 6 (LRP6) at the cell surface initiates a signaling cascade that leads to the transcription of Wnt target genes. Upon Wnt binding, the receptors assemble into large complexes called signalosomes that provide a platform for interactions with downstream effector proteins. The molecular basis of signalosome formation and regulation remains elusive, largely due to the lack of tools to analyze its endogenous components. Here, we use internally tagged Wnt3a proteins to isolate and characterize activated, endogenous Wnt receptor complexes by mass spectrometry-based proteomics. We identify the single-span membrane protein TMEM59 as an interactor of FZD and LRP6 and a positive regulator of Wnt signaling. Mechanistically, TMEM59 promotes the formation of multimeric Wnt–FZD assemblies via intramembrane interactions. Subsequently, these Wnt–FZD–TMEM59 clusters merge with LRP6 to form mature Wnt signalosomes. We conclude that the assembly of multiprotein Wnt signalosomes proceeds along well-ordered steps that involve regulated intramembrane interactions.

Multiprotein complexes governing Wnt signal transduction

Three multiprotein complexes have key roles in transducing Wnt signals from the plasma membrane to the cell nucleus - the β-catenin destruction complex, or Axin degradasome, which targets the Wnt effector β-catenin for proteasomal degradation in the absence of Wnt; the Wnt signalosome, assembled by polymerization of Dishevelled upon Wnt engaging its receptors, to inactivate the Axin degradasome, which allows β-catenin to accumulate; and the Wnt enhanceosome which enables β-catenin to gain access to target genes, to relieve their transcriptional repression by Groucho/TLE. This review focuses on recent advances that have highlighted mechanistic principles governing the assembly and function of these complexes.

APC Inhibits Ligand-Independent Wnt Signaling by the Clathrin Endocytic Pathway

Adenomatous polyposis coli (APC) mutations cause Wnt pathway activation in human cancers. Current models for APC action emphasize its role in promoting β-catenin degradation downstream of Wnt receptors. Unexpectedly, we find that blocking Wnt receptor activity in APC-deficient cells inhibits Wnt signaling independently of Wnt ligand. We also show that inducible loss of APC is rapidly followed by Wnt receptor activation and increased β-catenin levels. In contrast, APC2 loss does not promote receptor activation. We show that APC exists in a complex with clathrin and that Wnt pathway activation in APC-deficient cells requires clathrin-mediated endocytosis. Finally, we demonstrate conservation of this mechanism in Drosophila intestinal stem cells. We propose a model in which APC and APC2 function to promote β-catenin degradation, and APC also acts as a molecular "gatekeeper" to block receptor activation via the clathrin pathway.

Substrate clustering potently regulates the activity of WW-HECT domain-containing ubiquitin ligases

The Nedd4 family of HECT domain–containing E3 ligases ubiquitinate many transcription factors and signaling proteins, and their activity is tightly regulated. Normally, intramolecular interactions curb the catalytic activity of the HECT domain, but these can be broken by the binding of PY motifs, found on substrate molecules and adaptors, to the WW domains characteristic of this E3 ligase family. This raises the prospect of substrates automatically activating the ligases, frustrating the purpose of ligase regulation. Here we show that soluble protein substrates and adaptors such as α arrestins, even with multiple PY elements, cannot activate ligase activity efficiently. However, we found that polymerization or membrane tethering of these substrates dramatically increases the ligase activity both in vivo and in vitro. Aggregation of luciferase-containing substrates upon heat shock had a similar effect and could also expose cryptic PY elements in the substrates. We inferred that ligase activation critically requires a substantial array of clustered PY motifs and that the formation of such arrays on membranes or in polymeric aggregates may be an essential step in this mode of ligase regulation. We conclude that recruitment of α arrestins to membrane receptors and aggregation of unstable proteins after heat shock may be physiologically relevant mechanisms for triggering ubiquitination by Nedd4 family HECT domain–containing E3 ligases.