Tissue tension permits β-catenin phosphorylation to drive mesoderm specification in human embryonic stem cells.

The role of morphogenetic forces in cell fate specification is an area of intense interest. Using an ectopically expressed nonphosphorylatable mutant of β-catenin (Y654F) in human embryonic stem cell colonies, we provide evidence that impeding tension-dependent Src-mediated β-catenin phosphorylation compromises BMP4-driven Brachyury (T) expression. This impediment also disrupts the epithelial-to-mesenchymal transition essential for mesoderm specification. Mechanistically, the Y654F mutation prevents the translocation of the junctional and cytoskeletal pool of β-catenin to the nucleus. However, saturation of Wnt signaling with exogenous Wnt3a or the inhibition of GSK3β rescues mesoderm expression. These findings suggest that BMP4 initiates a force-dependent junctional β-catenin translocation upstream of both Wnt secretion and cytosolic Wnt/β-catenin stabilization to drive mesoderm specification. Ultimately, our work highlights the importance of force-dependent Wnt/β-catenin signaling in the self-organization of tissues during developmental processes, such as gastrulation, and emphasizes a role for fine-tuned molecular regulation of the Wnt signaling pathway.