Misregulation of the Wnt pathway leads to diseases including osteoporosis and several cancers (Saito-Diaz et al. 2012). In the absence of the retromer complex, Wls is trapped in endosomes and subsequently degraded (Yang et al. 2008). Port et al. 2008), further confirming the role of the retromer in Wnt secretion through its regulation by Wls (Figure 1.2).
HSPGs consist of a protein core decorated with long glycosaminoglycan (GAG) chains (Häcker et al. 2005). Binding of Wnt ligands to Fz and LRP5/6 results in the formation of phosphatidylinositol (4,5)-bisphosphate (PIP2) (Pan et al. 2008). Disheveled (Dvl/Dsh) has long been known to be genetically required in the Wnt/β-catenin pathway (Klingensmith et al. 1996).
Dvl phosphorylation following Wnt signaling appears to be independent of LRP6 activation (González-Sancho et al. 2004). Finally, the deubiquitinating enzyme CYLD (encoded by the familial cylindromatosis tumor suppressor gene) has been shown to be a negative regulator of Wnt signaling (Tauriello et al. 2010). It was first identified as the gene product of the locus fused in mice (Zeng et al. 1997).
Phosphorylation of APC by GSK3 was shown to increase the binding of β-catenin by APC (Salic et al. 2000).
Dynamin
Clathrin
Caveolin
Lipid raftClathrin-dependent
Caveolin-dependent endocytosis
Caveosome
Early Endosome
Late Endosome
Lysosome
Palmitoylation of caveolin at Cys133, Cys145, and Cys156 facilitates their binding to the plasma membrane (Dietzen et al. 1995; Parton and Simons 2007). Caveolins oligomerize and interact with cholesterol-rich domains in the plasma membrane known as lipid rafts (Murata et al. 1995). Caveolar vesicles are then recycled back to the membrane for reuse (Razani et al. 2002; Pelkmans et al. 2004).
The lipid composition of the early endosomal membrane, which is rich in phosphatidylinositol-3-phosphate (PIP3), determines their specificity (Funderburk et al. 2010). Similarly, endofin interacts with the TGF receptor to induce the assembly of the SMAD2-SMAD4 complex (Chen et al. 2007). Studies show that the β2-adrenergic receptor is also recycled by this mechanism (Stenmark 2009; Puthenveedu et al. 2010).
The first evidence of the role of endocytosis in Wnt signaling was the observation that Wg (Drosophila Wnt) localizes in recipient cells in small vesicles as well as multivesicular bodies (MVBs) by electron microscopy (van den Heuvel et al. 1989). Consistent with these findings, the co-receptors Arrow and Fz mediate Wg internalization in Drosophila ( Rives et al. 2006 ). Initial studies suggested that Wg internalization led to downregulation of the Wnt pathway in Drosophila embryos ( Dubois et al. 2001 ).
Wnt3a also promotes PIP2 formation, leading to LRP6 phosphorylation and AP-2 recruitment (Pan et al. 2008). LRP6 is subsequently recycled back to the plasma membrane via Rab5- and Rab11-positive vesicles (Yamamoto et al. 2006). Wnt3a induces rapid Fz5 internalization in Rab5-dependent vesicles, and it is returned to the membrane of Rab11-positive endosomes (Yamamoto et al. 2006).
LRP5 and LRP6 are 70% identical, however LRP6 has been proposed to play a more important role in Wnt signaling transduction (He et al. 2004). The main steps are the activation of LRP6 (measured by an increase in the phosphorylation of LRP6 at Thr1572) and the accumulation of cytoplasmic -catenin (Saito-Diaz et al. 2012). To confirm that mAb7E5 inhibited Wnt signaling through direct interaction with LRP6, cells were transfected with full-length LRP6 (LRP6 FL) or a constitutively active truncated LRP6 lacking the extracellular domain (LRP6N) (Liu et al 2003).
Hinge
LRP6-1Wnt1
34;Induction of mutant dynamin specifically blocks endocytic coated vesicle formation." The Journal of Cell Biology. 34;The trimeric G protein Go induces a dual impact on axin in the Wnt/frizzled signaling pathway." Development dynamics: NA- NA. 34;Phosphoinositide-Ap-2 interactions required for targeting to plasma membrane clathrin-coated pits." The Journal of Cell Biology.
34;Cellular trafficking of the glypican Dally-like is required for full Hedgehog signaling and wingless transcytosis." 34;Casein kinase I phosphorylates and destabilizes the β-catenin degradation complex." Proceedings of the National Academy of Sciences. 34; Biochemical Characterization of Wnt-Frizzled Interactions Using a Soluble, Biologically Active Wnt Protein from Vertebrates." Proceedings of the National Academy of Sciences.
34;The Drosophila disheveled segment polarity gene encodes a novel protein required for the wingless signal response.". 34;Roles of APC and Axin emerge from experimental and theoretical analysis of the Wnt pathway." PLoS Biology 1(1): E10. 34; β-Trcp couples β-catenin phosphorylation-degradation and regulates Xenopus axis formation.” Proceedings of the National Academy of Sciences.
34;Phosphorylation of Drosophila Adherens Junction Protein Armadillo: Roles for Wingless Signal and Zeste-white 3 Kinase.". 34;Pharmacological Inhibition of the Wnt Acyltransferase PORCN Prevents WNT-Driven Breast Cancer Growth." Cancer research. 34; Spatial expression of the Drosophila armadillo segment polarity gene is posttranscriptionally regulated by the wingless cell." Cell.
34;Internalization is required for proper Wingless signaling in Drosophila melanogaster." The Journal of Cell Biology. 34;E-cadherin regulates cell growth by modulating proliferation-dependent beta-catenin transcriptional activity." The Journal of Cell Biology. 34; Isolation and characterization of a mouse homologue of the Drosophila segment polarity gene scrambled." Developmental Biology.
34;Wnt secretion is required to maintain high levels of Wnt activity in colon cancer cells." Nature Communications 4. 34;The E3 ubiquitin ligase ITCH negatively regulates canonical Wnt signaling by targeting scrambled protein." Molecular and Cellular Biology. 34;Direct binding of the PDZ domain of disheveled to a conserved internal sequence in the C-Terminal region of Frizzled.".
34;NARF, a nemo-like kinase (NLK)-associated RING finger protein regulates the ubiquitylation and degradation of T-cell factor/lymphoid enhancer factor (TCF/LEF)." The Journal of Biological Chemistry.
