The mechanism of endogenous receptor activation functionally distinguishes prototype canonical and noncanonical Wnts

Wnt glycoproteins are developmentally essential signaling molecules, and lesions afflicting Wnt pathways play important roles in human diseases. Some Wnts signal to the canonical pathway by stabilizing beta-catenin, while others lack this activity. Frizzled serpentine receptors mediate distinct signaling pathways by both classes of Wnts. Here, we tandemly linked noncanonical Wnt5a with the C-terminal half of Dickkopf-2 (Dkk2C), a distinct ligand of the Wnt coreceptor LRP5/6. Whereas Wnt5a, Dkk2C, or both together were incapable of stimulating endogenous canonical signaling, the Wnt5a/Dkk2C chimera efficiently activated this pathway in a manner inhibitable by specific antagonists of either frizzled or LRP receptors. Thus, activation of the canonical pathway requires ligand coupling of an endogenous frizzled/LRP coreceptor complex, rather than Wnt triggering each receptor independently. Moreover, fusion of Wnt5a with Dkk2C unmasked its ability to signal to Dishevelled through multiple frizzleds, indicating that the lack of functional interaction with LRP distinguishes noncanonical Wnt5a from canonical Wnts in mammalian cells. These findings provide a novel mechanism by which the same receptor can be switched between distinct signaling pathways depending on the differential recruitment of a coreceptor by members of the same ligand family.     

A novel mechanism for Wnt activation of canonical signaling through the LRP6 receptor

LDL receptor-related protein 6 (LRP6) is a Wnt coreceptor in the canonical signaling pathway, which plays essential roles in embryonic development. We demonstrate here that wild-type LRP6 forms an inactive dimer through interactions mediated by epidermal growth factor repeat regions within the extracellular domain. A truncated LRP6 comprising its transmembrane and cytoplasmic domains is expressed as a constitutively active monomer whose signaling ability is inhibited by forced dimerization. Conversely, Wnts are shown to activate canonical signaling through LRP6 by inducing an intracellular conformational switch which relieves allosteric inhibition imposed on the intracellular domains. Thus, Wnt canonical signaling through LRP6 establishes a novel mechanism for receptor activation which is opposite to the general paradigm of ligand-induced receptor oligomerization.     

LRP6 dimerization through its LDLR domain is required for robust canonical Wnt pathway activation

Canonical Wnt/β-catenin signaling pathway plays important roles in multiple aspects of cellular responses in development and diseases. It is currently thought that Wnt receptor Frizzled (Frz) exists separately to Wnt coreceptors LRP5 and LRP6 (LRP5/6), and that Wnt–Frz–LRP5/6 triple complex formation bridged by Wnt ligand is needed for canonical pathway activation. We recently showed that Frz and LRP5/6 interact with each other in the absence of Wnt ligand binding and this interaction maintains the Frz–LRP5/6 complex in an inactive state. Here, we further show that Wnt ligand stimulation induces conformational change of the Frz–LRP6 complex and leads to hexamer formation containing the core LDLR domain-mediated LRP6 homodimer that is stabilized by two pairs of Wnt3a and Frz8, that is, Wnt3a–Frz8–LRP6–LRP6–Frz8–Wnt3a. This LDLR-mediated LRP6 dimerization is essential for robust canonical Wnt pathway activation. Our study thus suggests a previously unrecognized mode of receptor interaction in Wnt signal initiation.     

Regulated proteolytic processing of LRP6 results in release of its intracellular domain

Low-density lipoprotein receptor-related protein 6 (LRP6) is a member of low-density lipoprotein receptor (LDLR) family which cooperates with Frizzled receptors to transduce the canonical Wnt signal. As a critical component of the canonical Wnt pathway, LRP6 is essential for appropriate brain development, however, the mechanism by which LRP6 facilitates Wnt canonical signaling has not been fully elucidated. Interestingly, LRP6 which lacks its extracellular domain can constitutively activate TCF/LEF and potentiate the Wnt signal. Further, the free cytosolic tail of LRP6 interacts directly with glycogen synthase kinase (GSK3) and inhibits GSK3's activity in the Wnt canonical pathway which results in increased TCF/LEF activation. However, whether these truncated forms of LRP6 are physiologically relevant is unclear. Recent studies have shown that other members of the LDLR family undergo gamma-secretase dependent regulated intramembrane proteolysis (RIP). Using independent experimental approaches, we show that LRP6 also undergoes RIP. The extracellular domain of LRP6 is shed and released into the surrounding milieu and the cytoplasmic tail is cleaved by gamma-secretase-like activity to release the intracellular domain. Furthermore, protein kinase C, Wnt 3a and Dickkopf-1 modulate this process. These findings suggest a novel mechanism for LRP6 in Wnt signaling: induction of ectodomain shedding of LRP6, followed by the gamma-secretase involved proteolytic releasing its intracellular domain (ICD) which then binds to GSK3 inhibiting its activity and thus activates the canonical Wnt signaling pathway.     

Both LRP5 and LRP6 receptors are required to respond to physiological Wnt ligands in mammary epithelial cells and fibroblasts

A canonical Wnt signal maintains adult mammary ductal stem cell activity, and this signal requires the Wnt signaling reception, LRP5. However, previous data from our laboratory have shown that LRP5 and LRP6 are co-expressed in mammary basal cells and that LRP6 is active, leading us to question why LRP6 is insufficient to mediate canonical signaling in the absence of LRP5. Here, we show that at endogenous levels of LRP5 and LRP6 both receptors are required to signal in response to some Wnt ligands both in vitro (in mouse embryonic fibroblasts and mammary epithelial cells) and in vivo (in mammary outgrowths). This subgroup of canonical ligands includes Wnt1, Wnt9b, and Wnt10b; the latter two are expressed in mammary gland. In contrast, the ligand commonly used experimentally, Wnt3a, prefers LRP6 and requires just one receptor regardless of cellular context. When either LRP5 or LRP6 is overexpressed, signaling remains ligand-dependent, but the requirement for both receptors is abrogated (regardless of ligand type). We have documented an LRP5-6 heteromer using immiscible filtration assisted by surface tension (IFAST) immunoprecipitation. Together, our data imply that under physiological conditions some Wnt ligands require both receptors to be present to generate a canonical signal. We have designed a model to explain our results based on the resistance of LRP5-6 heteromers to a selective inhibitor of E1/2-binding Wnt-LRP6 interaction. These data have implications for stem cell biology and for the analysis of the oncogenicity of LRP receptors that are often overexpressed in breast tumors.     

Phosphorylation of PPP(S/T)P motif of the free LRP6 intracellular domain is not required to activate the Wnt/β‐catenin pathway and attenuate GSK3β activity

The canonical Wnt/β‐catenin signaling pathway plays a critical role in numerous physiological and pathological processes. LRP6 is an essential co‐receptor for Wnt/β‐catenin signaling; as transduction of the Wnt signal is strongly dependent upon GSK3β‐mediated phosphorylation of multiple PPP(S/T)P motifs within the membrane‐anchored LRP6 intracellular domain. Previously, we showed that the free LRP6 intracellular domain (LRP6‐ICD) can activate the Wnt/β‐catenin pathway in a β‐catenin and TCF/LEF‐1 dependent manner, as well as interact with and attenuate GSK3β activity. However, it is unknown if the ability of LRP6‐ICD to attenuate GSK3β activity and modulate activation of the Wnt/β‐catenin pathway requires phosphorylation of the LRP6‐ICD PPP(S/T)P motifs, in a manner similar to the membrane‐anchored LRP6 intracellular domain. Here we provide evidence that the LRP6‐ICD does not have to be phosphorylated at its PPP(S/T)P motif by GSK3β to stabilize endogenous cytosolic β‐catenin resulting in activation of TCF/LEF‐1 and the Wnt/β‐catenin pathway. LRP6‐ICD and a mutant in which all 5 PPP(S/T)P motifs were changed to PPP(A)P motifs equivalently interacted with and attenuated GSK3β activity in vitro, and both constructs inhibited the in situ GSK3β‐mediated phosphorylation of β‐catenin and tau to the same extent. These data indicate that the LRP6‐ICD attenuates GSK3β activity similar to other GSK3β binding proteins, and is not a result of it being a GSK3β substrate. Our findings suggest the functional and regulatory mechanisms governing the free LRP6‐ICD may be distinct from membrane‐anchored LRP6, and that release of the LRP6‐ICD may provide a complimentary signaling cascade capable of modulating Wnt‐dependent gene expression. J. Cell. Biochem. 108: 886–895, 2009. © 2009 Wiley‐Liss, Inc.     

LDL receptor-related proteins 5 and 6 in Wnt/beta-catenin signaling: arrows point the way

Wnt signaling through the canonical beta-catenin pathway plays essential roles in development and disease. Low-density-lipoprotein receptor-related proteins 5 and 6 (Lrp5 and Lrp6) in vertebrates, and their Drosophila ortholog Arrow, are single-span transmembrane proteins that are indispensable for Wnt/beta-catenin signaling, and are likely to act as Wnt co-receptors. This review highlights recent progress and unresolved issues in understanding the function and regulation of Arrow/Lrp5/Lrp6 in Wnt signaling. We discuss Arrow/Lrp5/Lrp6 interactions with Wnt and the Frizzled family of Wnt receptors, and with the intracellular beta-catenin degradation apparatus. We also discuss the regulation of Lrp5/Lrp6 by other extracellular ligands, and LRP5 mutations associated with familial osteoporosis and other disorders.     

Dkk1 Stabilizes Wnt Co-Receptor LRP6: Implication for Wnt Ligand-Induced LRP6 Down-Regulation

Background

The low density lipoprotein receptor-related protein-6 (LRP6) is an essential co-receptor for canonical Wnt signaling. Dickkopf 1 (Dkk1), a major secreted Wnt signaling antagonist, binds to LRP6 with high affinity and prevents the Frizzled-Wnt-LRP6 complex formation in response to Wnts. Previous studies have demonstrated that Dkk1 promotes LRP6 internalization and degradation when it forms a ternary complex with the cell surface receptor Kremen.

Methodology/Principal Findings

In the present study, we found that transfected Dkk1 induces LRP6 accumulation while inhibiting Wnt/LRP6 signaling. Treatment with Dkk1-conditioned medium or recombinant Dkk1 protein stabilized LRP6 with a prolonged half-life and induces LRP6 accumulation both at the cell surface and in endosomes. We also demonstrated that Kremen2 co-expression abrogated the effect of Dkk1 on LRP6 accumulation, indicating that the effect of Kremen2 is dominant over Dkk1 regulation of LRP6. Furthermore, we found that Wnt3A treatment induces LRP6 down-regulation, an effect paralleled with a Wnt/LRP6 signaling decay, and that Dkk1 treatment blocked Wnt3A-induced LRP6 down-regulation. Finally, we found that LRP6 turnover was blocked by an inhibitor of caveolae-mediated endocytosis.

Conclusions/Significance

Our results reveal a novel role for Dkk1 in preventing Wnt ligand-induced LRP6 down-regulation and contribute significantly to our understanding of Dkk1 function in Wnt/LRP6 signaling.     

Heparan Sulfate 6-O-endosulfatases (Sulfs) Coordinate the Wnt Signaling Pathways to Regulate Myoblast Fusion during Skeletal Muscle Regeneration

Skeletal muscle regeneration is mediated by satellite cells (SCs). Upon injury, SCs undergo self-renewal, proliferation, and differentiation into myoblasts followed by myoblast fusion to form new myofibers. We previously showed that the heparan sulfate (HS) 6-O-endosulfatases (Sulf1 and -2) repress FGF signaling to induce SC differentiation during muscle regeneration. Here, we identify a novel role of Sulfs in myoblast fusion using a skeletal muscle-specific Sulf double null (Sulf^SK-DN) mouse. Regenerating Sulf^SK-DN muscles exhibit reduced canonical Wnt signaling and elevated non-canonical Wnt signaling. In addition, we show that Sulfs are required to repress non-canonical Wnt signaling to promote myoblast fusion. Notably, skeletal muscle-relevant non-canonical Wnt ligands lack HS binding capacity, suggesting that Sulfs indirectly repress this pathway. Mechanistically, we show that Sulfs reduce the canonical Wnt-HS binding and regulate colocalization of the co-receptor LRP5 with caveolin3. Therefore, Sulfs may increase the bioavailability of canonical Wnts for Frizzled receptor and LRP5/6 interaction in lipid raft, which may in turn antagonize non-canonical Wnt signaling. Furthermore, changes in subcellular distribution of active focal adhesion kinase (FAK) are associated with the fusion defect of Sulf-deficient myoblasts and upon non-canonical Wnt treatment. Together, our findings uncover a critical role of Sulfs in myoblast fusion by promoting antagonizing canonical Wnt signaling activities against the noncanonical Wnt pathway during skeletal muscle regeneration.     

Identification of a novel inhibitor of the canonical Wnt pathway

Wnt signaling is known to regulate multiple processes including angiogenesis, inflammation, and fibrosis. Here, we identified a novel inhibitor of the Wnt pathway, pigment epithelium-derived factor (PEDF), a multifunctional serine proteinase inhibitor. Both overexpression of PEDF in transgenic mice and administration of PEDF protein attenuated Wnt signaling induced by retinal ischemia. Furthermore, PEDF knockdown by small interfering RNA (siRNA) and PEDF knockout in PEDF(-/-) mice induced activation of Wnt signaling. PEDF bound to LRP6, a Wnt coreceptor, with high affinity (K(d) [dissociation constant] of 3.7 nM) and blocked the Wnt signaling induced by Wnt ligand. The physical interaction of PEDF with LRP6 was confirmed by a coprecipitation assay, which showed that PEDF bound to LRP6 at the E1E2 domain. In addition, binding of PEDF to LRP6 blocked Wnt ligand-induced LRP6-Frizzled receptor dimerization, an essential step in Wnt signaling. These results suggest that PEDF is an endogenous antagonist of LRP6, and blocking Wnt signaling may represent a novel mechanism for its protective effects against diabetic retinopathy.     

Effects of parathyroid hormone on Wnt signaling pathway in bone

The Wnt signaling pathway has recently been demonstrated to play an important role in bone cell function. In previous studies using DNA microarray analyses, we observed a change in some of the molecular components of the canonical Wnt pathway namely, frizzled-1 (FZD-1) and axil, in response to continuous parathyroid hormone (PTH) treatment in rats. In the present study, we further explored other components of the Wnt signaling pathway in rat distal metaphyseal bone in vivo, and rat osteoblastic osteosarcoma cells (UMR 106) in culture. Several Wnt pathway components, including low-density lipoprotein-receptor-related protein 5 (LRP5), LRP6, FZD-1, Dickkopf-1 (Dkk-1), and Kremen-1 (KRM-1), were expressed in bone in vivo and in osteoblasts in vitro. Continuous exposure to PTH (1-38) both in vivo and in vitro upregulated the mRNA expression of LRP6 and FZD-1 and decreased LRP5 and Dkk-1. These effects in UMR 106 cells were associated with an increase in beta-catenin as measured by Western blots and resulted in functional activation (three to six-fold) of a downstream Wnt responsive TBE6-luciferase (TCF/LEF-binding element) reporter gene. Activation of the TBE6-luciferase reporter gene by PTH (1-38) in UMR 106 cells was inhibited by the protein kinase A (PKA) inhibitor, H89. Activation was mimicked by PTH (1-31), PTH-related protein (1-34), and forskolin, but both PTH (3-34) and (7-34) had no effect. These findings suggest that the effect of PTH on the canonical Wnt signaling pathway occurs at least in part via the cAMP-PKA pathway through the differential regulation of the receptor complex proteins (FZD-1/LRP5 or LRP6) and the antagonist (Dkk-1). Taken together, these results reveal a possible role for the Wnt signaling pathway in PTH actions in bone.     

Kallistatin antagonizes Wnt/β-catenin signaling and cancer cell motility via binding to low-density lipoprotein receptor-related protein 6

Kallistatin, a plasma protein, exerts pleiotropic effects in inhibiting angiogenesis, inflammation and tumor growth. Canonical Wnt signaling is the primary pathway for oncogenesis in the mammary gland. In this study, we demonstrate that kallistatin bound to the Wnt coreceptor low-density lipoprotein receptor-related protein 6 (LRP6), thus, blocking Wnt/β-catenin signaling and Wnt-mediated growth and migration in MDA-MB-231 breast cancer cells. Kallistatin inhibited Wnt3a-induced proliferation, migration, and invasion of cultured breast cancer cells. Moreover, kallistatin was bound to LRP6 in breast cancer cells, as identified by immunoprecipitation followed by western blot. Kallistatin suppressed Wnt3a-mediated phosphorylation of LRP6 and glycogen synthase kinase-3β, and the elevation of cytosolic β-catenin levels. Furthermore, kallistatin antagonized Wnt3a-induced expression of c-Myc, cyclin D1, and vascular endothelial growth factor. These findings indicate a novel role of kallistatin in preventing breast tumor growth and mobility by direct interaction with LRP6, leading to blockade of the canonical Wnt signaling pathway.     

Caveolin is necessary for Wnt-3a-dependent internalization of LRP6 and accumulation of beta-catenin

beta-catenin-mediated Wnt signaling is critical in animal development and tumor progression. The single-span transmembrane Wnt receptor, low-density lipoprotein receptor-related protein 6 (LRP6), interacts with Axin to promote the Wnt-dependent accumulation of beta-catenin. However, the molecular mechanism of receptor internalization and its impact on signaling are unclear. Here, we present evidence that LRP6 is internalized with caveolin and that the components of this endocytic pathway are required not only for Wnt-3a-induced internalization of LRP6 but also for accumulation of beta-catenin. Overall, our data suggest that Wnt-3a triggers the interaction of LRP6 with caveolin and promotes recruitment of Axin to LRP6 phosphorylated by glycogen synthase kinase-3beta and that caveolin thereby inhibits the binding of beta-catenin to Axin. Thus, caveolin plays critical roles in inducing the internalization of LRP6 and activating the Wnt/beta-catenin pathway. We also discuss the idea that distinct endocytic pathways correlate with the specificity of Wnt signaling events.     

G Protein-coupled Receptor Kinases Phosphorylate LRP6 in the Wnt Pathway

Wnt ligands conduct their functions in canonical Wnt signaling by binding to two receptors, the single transmembrane low density lipoprotein receptor-related proteins 5 and 6 (LRP5/6) and seven transmembrane (7TM) Frizzled receptors. Subsequently, phosphorylation of serine/threonine residues within five repeating signature PPPSP motifs on LRP6 is responsible for LRP6 activation. GSK3β, a cytosolic kinase for phosphorylation of a downstream effector β-catenin, was proposed to participate in such LRP6 phosphorylation. Here, we report a new class of membrane-associated kinases for LRP6 phosphorylation. We found that G protein-coupled receptor kinases 5 and 6 (GRK5/6), traditionally known to phosphorylate and desensitize 7TM G protein-coupled receptors, directly phosphorylate the PPPSP motifs on single transmembrane LRP6 and regulate Wnt/LRP6 signaling. GRK5/6-induced LRP6 activation is inhibited by the LRP6 antagonist Dickkopf. Depletion of GRK5 markedly reduces Wnt3A-stimulated LRP6 phosphorylation in cells. In zebrafish, functional knock-down of GRK5 results in reduced Wnt signaling, analogous to LRP6 knock-down, as assessed by decreased abundance of β-catenin and lowered expression of the Wnt target genes cdx4, vent, and axin2. Expression of GRK5 rescues the diminished β-catenin and axin2 response caused by GRK5 depletion. Thus, our findings identify GRK5/6 as novel kinases for the single transmembrane receptor LRP6 during Wnt signaling.     

LDL receptor-related protein LRP6 regulates proliferation and survival through the Wnt cascade in vascular smooth muscle cells

Initial studies have established expression of low-density lipoprotein (LDL) receptor-related protein 6 (LRP6) in vascular smooth muscle cells (VSMCs). We hypothesized that LRP6 is a critical mediator governing the regulation of the canonical Wnt/beta-catenin/T cell factor 4 (Tcf-4) cascade in the vasculature. This hypothesis was based on our previous work demonstrating a role for the beta-catenin/Tcf-4 pathway in vascular remodeling as well as work in other cell systems establishing a role for LRP family members in the Wnt cascade. In line with our hypothesis, LRP6 upregulation significantly increased Wnt-1-induced Tcf activation. Moreover, a dominant interfering LRP6 mutant lacking the carboxyl intracellular domain (LRP6DeltaC) abolished Tcf activity. LRP6-induced stimulation of Tcf was blocked in VSMCs harboring constitutive expression of a dominant negative Tcf-4 transgene lacking the beta-catenin binding domain, suggesting that LRP6-induced activation of Tcf was mediated through a beta-catenin-dependent signal. Expression of the dominant interfering LRP6DeltaC transgene was sufficient to abolish the Wnt-induced survival as well as cyclin D1 activity and cell cycle progression. In conclusion, these findings provide the first evidence of a role for an LDL receptor-related protein in the regulation of VSMC proliferation and survival through the evolutionary conserved Wnt signaling cascade.     

Wnt signal amplification via activity, cooperativity, and regulation of multiple intracellular PPPSP motifs in the Wnt co-receptor LRP6

Low density lipoprotein receptor-related protein 6 (LRP6) and its homologue LRP5 serve as Wnt co-receptors that are essential for the Wnt/beta-catenin pathway. Wnt activation of LRP6 leads to recruitment of the scaffolding protein Axin and inhibition of Axin-mediated phosphorylation/destruction of beta-catenin. We showed that five conserved PPPSP motifs in the LRP6 intracellular domain are required for LRP6 function, and mutation of these motifs together abolishes LRP6 signaling activity. We further showed that Wnt induces the phosphorylation of a prototypic PPPSP motif, which provides a docking site for Axin and is sufficient to transfer signaling activity to a heterologous receptor. However, the activity, regulation, and functionality of multiple PPPSP motifs in LRP6 have not been characterized. Here we provide a comprehensive analysis of all five PPPSP motifs in LRP6. We define the core amino acid residues of a prototypic PPPSP motif via alanine scanning mutagenesis and demonstrate that each of the five PPPSP motifs exhibits signaling and Axin binding activity in isolation. We generated two novel phosphorylation-specific antibodies to additional PPPSP motifs and show that Wnt induces phosphorylation of these motifs in the endogenous LRP6 through glycogen synthase kinase 3. Finally, we uncover the critical cooperativity of PPPSP motifs in the full-length LRP6 by demonstrating that LRP6 mutants lacking a single PPPSP motif display compromised function, whereas LRP6 mutants lacking two of the five PPPSP motifs are mostly inactive. This cooperativity appears to reflect the ability of PPPSP motifs to promote the phosphorylation of one another and to interact with Axin synergistically. These results establish the critical role and a common phosphorylation/activation mechanism for the PPPSP motifs in LRP6 and suggest that the conserved multiplicity and cooperativity of the PPPSP motifs represents a built-in amplifier for Wnt signaling by the LRP6 family of receptors.