The actin crosslinker Fimbrin is required for force sensing at tricellular junctions

Epithelial cells sense and respond to mechanical forces by reinforcing connections between adherens junctions and the actin cytoskeleton. Tricellular junctions are key sites of mechanotransduction in epithelial cells, but how force responses are coordinated at these structures during epithelial remodeling is poorly understood. We show that the actin crosslinking protein Fimbrin is recruited by forces to tricellular junctions in Drosophila and promotes actin reorganization and cell adhesion during epithelial remodeling. In the absence of Fimbrin, cells fail to reorganize actin or recruit junction-stabilizing proteins to tricellular junctions under tension, disrupting cell adhesion. Conversely, increasing Fimbrin activity constitutively activates multiple force-response pathways, aberrantly stabilizing adhesion. We show that Fimbrin directs force responses by amplifying actomyosin contractility, and that force-regulated actin remodeling and Plastin3 and Afadin localization occur at tricellular junctions in the mouse embryo. These results suggest that conserved mechanisms underlie force responses at tricellular junctions in fly and mouse epithelia.