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Inhibition of GSK3 by Wnt signalling – two contrasting models

2011; The Company of Biologists; Volume: 124; Issue: 21 Linguagem: Inglês

10.1242/jcs.091991

1477-9137

Ciara Metcalfe, Mariann Bienz,

Ubiquitin and proteasome pathways

The key read-out of Wnt signalling is a change in the transcriptional profile of the cell, which is driven by β-catenin. β-catenin levels are normally kept low by a phosphorylation event that is mediated by glycogen synthase kinase 3 (GSK3, α- and β-isoforms), which targets β-catenin for ubiquitylation and proteasomal degradation. Wnt blocks this phosphorylation event, thereby allowing β-catenin to accumulate and to co-activate transcription in the nucleus. Exactly how Wnt inhibits GSK3 activity towards β-catenin is unclear and has been the focus of intensive research. Recent studies on the role of conserved PPPSPxS motifs in the cytoplasmic tail of low-density lipoprotein receptor-related protein (LRP, isoforms 5 and 6) culminated in a biochemical model: Wnt induces the phosphorylation of LRP6 PPPSPxS motifs, which consequently access the catalytic pocket of GSK3 as pseudo-substrates, thus directly blocking its activity against β-catenin. A distinct cell-biological model was proposed more recently: Wnt proteins induce the uptake of GSK3 into multivesicular bodies (MVBs), an event that sequesters the enzyme away from newly synthesised β-catenin substrate in the cytoplasm, thus blocking its phosphorylation. This new model is based on intriguing observations but also challenges a body of existing evidence, so will require further experimental consolidation. We shall consider whether the two models apply to different modes of Wnt signaling: acute versus chronic.