Effect of Moderate Decrease in Contractility on Epithelial Cell-Cell Contacts Under External Stretch

Epithelial tissues are characterized by extensive cell–cell contacts and often subject to stretch during normal function. Cell-cell contact adhesion strength maintains mechanical continuity from cell to cell. Mechanical forces exerted by cells via actomyosin contractility are known to strengthen cell-cell adhesions to a certain extent. While it is known that a large decrease in contractility causes cell-cell contact rupture due to weakened adhesions, it is unclear how a moderate reduction in contractility affects cell-cell junction stability. Here, we studied how a moderate inhibition of non-muscle myosin II (NMII) affects the integrity of epithelial cell-cell contacts, especially when subject to the challenge of mechanical stretch. We first modulated the contractility of epithelial cell islands strongly, or moderately, using the NMII inhibitor blebbistatin. Strong inhibition severely disrupted the actin cytoskeletal structure and induced widespread cellcell contact rupture while moderate inhibition did not. We then used traction force microscopy to show that while strong inhibition of NMII nearly abrogated the contractility, moderate inhibition led to only a moderate reduction in cell-exerted forces. Then, we subjected cell islands with moderate contractility reduction to external stretch of over 30% to ascertain how cell-cell contact integrity was affected. Surprisingly, we found that moderate contractility reduction decreased the level of cell-cell contact ruptures due to stretch. Nanoindentation experiments revealed a significant reduction in cell Young’s modulus following moderate contractility inhibition. Thus, under external stretch, stretch-induced forces are also expected to be lesser when contractility is moderately reduced. Our results can be understood with the following framework: Under normal conditions, epithelial cell-cell contact adhesion strength exceeds the forces generated by cellular contractility. External mechanical forces (e.g., due to biaxial stretch) additionally stress junctions and lead to contact rupture. When contractility is moderately reduced, both internal tension and stretch-induced forces are lowered, with this reduction exceeding any reduction in adhesion strength. Thus, overall junctional integrity is enhanced under a moderate decrease in contractility. We propose that our findings have important implications to understand processes such as morphogenesis, wound healing, and regeneration, where epithelial cells spatiotemporarily modulate their contractility.