Despite more than 30 years of work on the Wnt signaling pathway, the basic mechanism of how the extracellular Wnt signal increases the intracellular concentration of β-catenin is still contentious. Circumventing much of the detailed biochemistry, we used basic principles of chemical kinetics coupled with quantitative measurements to define the reactions on β-catenin directly affected by the Wnt signal. We conclude that the core signal transduction mechanism is relatively simple, with only two regulated phosphorylation steps. Their partial inhibition gives rise to the full dynamics of the response and subsequently maintains a steady state in which the concentration of β-catenin is increased.
A key step in the canonical Wnt signalling pathway is the inhibition of GSK3β, which results in the accumulation of nuclear β-catenin (also known as CTNNB1), and hence regulation of target genes. Evidence suggests that endocytosis is required for signalling, yet its role and the molecular understanding remains unclear. A recent and controversial model suggests that endocytosis contributes to Wnt signalling by causing the sequestration of the ligand–receptor complex, including LRP6 and GSK3 to multivesicular bodies (MVBs), thus preventing GSK3β from accessing β-catenin. Here, we use specific inhibitors (Dynasore and Dyngo-4a) to confirm the essential role of endocytosis in Wnt/Wingless signalling in human and Drosophila cells. However, we find no evidence that, in Drosophila cells or wing imaginal discs, LRP6/Arrow traffics to MVBs or that MVBs are required for Wnt/Wingless signalling. Moreover, we show that activation of signalling through chemical blockade of GSK3β is prevented by endocytosis inhibitors, suggesting that endocytosis impacts on Wnt/Wingless signalling downstream of the ligand–receptor complex. We propose that, through an unknown mechanism, endocytosis boosts the resting pool of β-catenin upon which GSK3β normally acts.
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