Wnt/ß-catenin Signaling Dynamics in Human Cells
Massey, Joseph Kyle
Doctor of Philosophy
Many biological processes are coordinated by intercellular signaling, and all intercellular signaling pathways have been implicated in multiple roles throughout an organism’s lifetime. While the core molecular components of each signaling pathway are known, little is known about the dynamics of signal transduction, or how these dynamics might vary in different biological contexts. The Wnt/β-catenin signal pathway is crucial to all stages of life. It controls early morphogenetic events in embryos, maintains stem cell niches in adults, and is dysregulated in many types of cancer. In this work, the dynamics of signaling are probed by monitoring nuclear accumulation of β-catenin, the primary transducer of canonical Wnt signals, using quantitative live cell imaging. It is shown that β-catenin signaling responds adaptively to constant Wnt signaling in pluripotent stem cells, and that these dynamics become sustained upon differentiation. Varying dynamics were also observed in the response to Wnt in commonly used mammalian cell lines. Signal attenuation in pluripotent cells is observed even at saturating doses, where ligand stability does not affect the dynamics. TGFβ superfamily ligands Activin and BMP, which coordinate with Wnt signaling to pattern the embryo, increase the β-catenin response in a manner independent of their ability to induce new Wnt ligand production. Additionally, the effect of a variety of other variables on signaling dynamics, including rate and duration of Wnt addition, feedback through downstream Wnt and TGFß ligands, and cell density was examined. These results reveal how variables external to the pathway, including differentiation status and cross-talk with other pathways, dramatically alter Wnt/β-catenin dynamics.