Synaptopodin couples epithelial contractility to α-actinin-4-dependent junction maturation

The epithelial junction experiences mechanical force exerted by endogenous actomyosin activities and from interactions with neighboring cells. We hypothesize that tension generated at cell-cell adhesive contacts contributes to the maturation and assembly of the junctional complex. To test our hypothesis, we used a hydraulic apparatus that can apply mechanical force to intercellular junction in a confluent monolayer of cells. We found that mechanical force induces alpha-actinin-4 and actin accumulation at the cell junction in a time- and tension-dependent manner during junction development. Intercellular tension also induces alpha-actinin-A dependent recruitment of vinculin to the cell junction. In addition, we have identified a tension-sensitive upstream regulator of alpha-actinin-4 as synaptopodin. Synaptopodin forms a complex containing alpha-actinin-4 and beta-catenin and interacts with myosin II, indicating that it can physically link adhesion molecules to the cellular contractile apparatus. Synaptopodin depletion prevents junctional accumulation of alpha-actinin-4, vinculin, and actin. Knockdown of synaptopodin and alpha-actinin-4 decreases the strength of cell-cell adhesion, reduces the monolayer permeability barrier, and compromises cellular contractility. Our findings underscore the complexity of junction development and implicate a control process via tension-induced sequential incorporation of junctional components.