A Mechanosensitive G1 Growth Rate Drives Size Reduction and Contact Inhibition in Epithelial Tissues

Epithelia are robust, barrier-forming, multicellular tissues which must sustain and recover from a variety of physical perturbations. As such, epithelial cells in mature tissues often maintain a "contact-inhibited" state which is proliferatively capable but transiently arrested by tissue-scale mechanical cues. Our previous work demonstrated that epithelial cell volumes can vary dramatically depending on physical context, and that cells are on average over two-fold smaller in mature epithelial monolayers than in subconfluent colonies, suggesting that contact inhibition of cell cycle progression is preceded by a decoupling of cell division and growth rate. Here, we employ 3D segmentation of confocal z-stacks and quantitative phase imaging of Madin-Darby Canine Kidney (MDCK) cells over time to directly measure cell cycle duration and growth rate. We show that cell volume and mass reduction is the result of G1 growth rate suppression outpacing cell cycle arrest, resulting in volume-reducing divisions. We revisit the canonical contact inhibition pathways Hippo and mTOR, which have been shown to be mechanosensitively suppressed via maturation of adherens junctions and/or cytoskeletal tension, with the goal of clarifying which causal mechanisms are downregulating cell growth as opposed to cell cycle progression. Using these tools, we offer a cohesive picture of the mechanosensitive feedbacks driving the proliferative/contact-inhibited transition in multicellular tissues.