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Description
The tight junction outlines the apicolateral border of epithelial cells like a belt, sealing the paracellular space when cells form epithelial sheets. The permeability and morphology of tight junction are regulated by actomyosin contractility, which has been conventionally thought from the purse-string-like circumferential actomyosin belt along tight junction. Spatially, the tight junction is close to the apical actin network, which possibly exerts inward contractions orthogonal to the tight junction. Given the observation on the tortuous tight junctions when MDCK cells are under high tension, we propose that actomyosin networks beneath the apical surface, such as the terminal web, rather than the circumferential actomyosin belt, generate in-plane tensions across the apical surface that exert inward contractions orthogonal to the tight junction, thereby inducing its wavy morphology. To test the contribution of apical constriction in driving the curvature of tight junctions, we perturbed actomyosin activity in polarized Madin-Darby Canine Kidney (MDCK) epithelial cells using the ROCK inhibitor Y27632, which reduced tight junction tortuosity. Consistent with these findings, laser ablation of spots on the apical surface of MDCK cells severed the apical cytoskeleton network, reducing in-plane tension, increasing the apical surface area, and resulting in a less tortuous tight junction morphology. To further investigate this, we are studying genes that regulate the assembly of the terminal web, with the goal of downregulating these genes to assess their effects on tight junction tortuosity. However, visualizing the apical actin network is challenging due to the dense microvilli covering the surface when using fluorescence-based microscopy. Therefore, cryo-electron tomography (cryo-ET) will be employed to visualize the structure of the terminal web in both wild-type and gene knockout cells, enabling us to explore the relationship between the terminal web, tight junction morphology, and epithelial cell shape.
