β-catenin controls Afadin cortical recruitment to regulate planar epithelial cell division

Mitotic spindle orientation is tightly controlled in epithelia to preserve polarized tissue architecture. Here, we uncover a previously unrecognized role of the adherens junction protein {beta}-catenin in regulating planar mitotic spindle positioning required for symmetric epithelial cell division. Using CRISPR/Cas9-mediated genome editing in MDCK cells, we found that {beta}-catenin--but not its close homolog {gamma}-catenin (plakoglobin)--is required for proper spindle orientation. Loss of {beta}-catenin disrupts astral microtubule anchorage and impairs the localization of LGN-NuMA spindle orientation machinery. Mechanistically, {beta}-catenin mediates spindle regulation via its N-terminal domain, independently of {beta}-catenin binding. We showed that the cortical recruitment of Afadin during mitosis depends on {beta}-catenin, but surprisingly, overexpression of either Afadin or ZO-1 rescue the spindle orientation in {beta}-catenin deficient cells by restoring cortical LGN. Afadin depletion or dominant-negative Afadin abolished ZO-1-mediated compensation, thereby establishing Afadin as the central cortical integrator, potentiated by {beta}-catenin, and operating through mass-action to recruit cortical LGN. Together, our findings define {beta}-catenin as an upstream regulator of the Afadin-based cortical-spindle linkage that coordinate the crosstalk between adherens junctions and mitotic geometry to ensure epithelial homeostasis.