Synaptopodin is required for stress fiber and contractomere assembly at the epithelial junction

Abstract

The apical junction of epithelial cells can generate force to control cell geometry and perform contractile processes while maintaining barrier function and cell-cell adhesion. Yet, the structural basis of force generation at the apical junction is not completely understood. Here, we describe 2 actomyosin structures at the apical junction containing synaptopodin, myosin IIB, and alpha-actinin-4. We showed that synaptopodin is required for the assembly of E-cadherin-associated apical stress fibers and a novel macromolecular structure, which we named contractomere. Knockdown of synaptopodin abolished both apical stress fiber and contractomere formation. Moreover, depletion of synaptopodin abolished basal stress fibers, converting myosin IIA sarcomere-like arrangement into a meshwork-type actomyosin organization. We propose a new model of junction dynamics that is dependent on contractomere movement to control epithelial cell boundary and geometry. Our findings reveal 2 actomyosin structures at the epithelial junction and underscore synaptopodin in the assembly of stress fibers and contractomeres.Summary StatementSynaptopodin assembles 2 actomyosin structures at the epithelial junction: apical stress fiber and contractomere. Synaptopodin selectively regulates myosin IIB without altering the level of myosin IIA and is responsible for converting evolutionary-conserved actomyosin meshwork into vertebrate-specific stress fibers.Graphic Abstract

Multi-level force-dependent allosteric enhancement of αE-catenin binding to F-actin by vinculin

“…At each step, polarization of F-actin is anticipated to increase the efficiency of force generation by nonmuscle myosin II, leading to a positive feedback loop between myosin contractility, catch bond formation, and F-actin polarization. This feedback loop would result in the ordered sarcomeric assemblies observed in epithelia [13, 15, 58] and cardiomyocytes [57] (Fig. 6C).…”