Illuminating forces that threaten living tissues across scales

Epithelial tissues are constantly exposed to forces that can lead to >50 % deformation in adult tissues, and several hundred percent during development. To function as a barrier, epithelia have to accommodate such large deformations without fracturing. Cell-cell adhesion must thus be finely tuned, or pathologies like skin blistering or cancer metastasis can occur. However, the physical principles governing tissue integrity are difficult to study, since tissue fracture is a multi-scale process spanning up to 10 orders of magnitude in both size and force. Millimetre-sized tissues can bear millinewton-forces, but tissue fracture results from the local failure of single nanometre-sized adhesion molecules that bear piconewton forces. New tools are needed to bridge these vastly different scales and understand what molecular processes lead to tissue failure. I therefore propose to combine tissue-scale mechanical testing with force sensors based on DNA-nanotechnology to directly measure how individual cell-cell adhesion molecules become loaded in tissues under stretch. I will use tissue stretchers to measure the mechanical response of suspended model epithelia, in which native adhesion proteins are replaced by artificial DNA linkers. In the chimeric cell-cell junction, a DNA hairpin will unfold when loaded above a tunable force set by the DNA design. A dye-quencher pair on either side of the hairpin will become fluorescent upon hairpin unfolding, providing a direct fluorescence-based readout of local forces. This will enable me to visualize and quantify force propagation in epithelia, measuring how different cytoskeletal networks become loaded in tissues under stretch. I will use the tool to study how different parts of the cytoskeleton dynamically share load, and how cell-cell adhesion strength impacts tissue fracture in models of healthy and diseased tissues. Together, these insights will illuminate the molecular processes that govern the integrity of living tissues.