Editors' ChoiceWnt signaling

Confined to Quarters?

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Science Signaling  04 Jan 2011:
Vol. 4, Issue 154, pp. ec1
DOI: 10.1126/scisignal.4154ec1

Signaling through the canonical Wnt/β-catenin pathway, which is crucial for normal development, depends on inhibition of β-catenin phosphorylation by glycogen synthase kinase 3 (GSK3); this enables β-catenin to escape proteasomal destruction and activate the transcription of target genes. GSK3 inhibition follows a series of events initiated by Wnt binding to the co-receptors Frizzled and low-density lipoprotein receptor–related protein 6 (LRP6) and formation of a multiprotein signalosome (see Niehrs and Acebron). Noting that endocytosis of the Wnt-bound receptor complex is crucial for Wnt signaling, Taelman et al. hypothesized that GSK3 inhibition could depend on its sequestration in multivesicular endosomal bodies (MVBs, organelles containing internal vesicles formed by invagination of the endosomal membrane). Fluorescence microscopy of mouse 3T3 cells expressing GSK3-RFP (GSK3 with red fluorescent protein) cocultured with human 293T cells expressing xWnt8-Venus revealed a decrease in cytosolic GSK3-RFP and its colocalization in vesicles with xWnt8-Venus. Furthermore, expression of a constitutively active form of LRP6 or other mediators of Wnt signaling promoted translocation of endogenous GSK3 to vesicles with acidic pH (as is characteristic of MVBs and lysosomes), where it colocalized with late endosomal markers. Assays of L cells with permeabilized plasma membranes revealed that Wnt signaling elicited a reduction in cytosolic GSK3 activity, which was recovered after solubilization of internal membranes, and protected GSK3 from proteinase K digestion. Indeed, cryo–immunoelectron microscopy confirmed that Wnt signaling promoted translocation of GSK3 into MVBs. Experiments using small interfering RNA directed against mammalian hepatocyte receptor tyrosine kinase substrate (HRS, also known as Vps27) or a dominant-negative form of Vps4 revealed that these two components of the ESCORT (endosomal sorting complexes required for transport) machinery, which are necessary for formation of endosomal intralumenal vesicles, were also required for Wnt signaling. Bioinformatic analysis revealed that 20% of the human proteome contained three or more potential GSK3 phosphorylation sites, and, intriguingly, analyses of cells containing radioactively labeled proteins indicated that Wnt signaling (or GSK inhibition) prolonged the half-life of many cellular proteins. The authors thus propose that canonical Wnt signaling involves sequestration of GSK3 within MVBs and leads to stabilization of numerous substrates in addition to β-catenin.

V. F. Taelman, R. Dobrowolski, J.-L. Plouhinec, L. C. Fuentealba, P. P. Vorwald, I. Gumper, D. D. Sabatini, E. M. De Robertis, Wnt signaling requires sequestration of glycogen synthase kinase 3 inside multivesicular endosomes. Cell 143, 1136–1148 (2010). [PubMed]

C. Niehrs, S. P. Acebron, Wnt signaling: Multivesicular bodies hold GSK3 captive. Cell 143, 1044–1046 (2010). [PubMed]

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