Desmosomes Are Subject To Mechanical Load

Strong cell-cell junctions, formed by adherens junctions, tight junctions, and desmosomes, are critical to the integrity of cellular tissues, including the ability to resist mechanical stress. While it is well established that tensile forces are exerted across cell-cell junctions, including actin-connected adherens junctions, it is not known if other components of the cell-cell junction, such as desmosomes, experience mechanical force. Desmosomes are intermediate filament (IF)-connected structures found in epithelial and muscle tissues that bind cells together, which consist of a transmembrane desmosomal cadherin (desmocollin or desmoglein) connected to intermediate filaments by the linker proteins plakoglobin, plakophilin, and desmoplakin. Desmosome-targeting genetic, autoimmune, and infectious diseases present clinically in the skin and heart, two organs subjected to significant mechanical force. This has led to the hypothesis that a major function of desmosomes is to resist mechanical stress at cell-cell contacts. To directly measure mechanical forces applied to desmosomes we used an existing FRET-force probe (known as TSmod) to develop a force sensor for the desmosomal cadherin desmoglein-2 (dsg-2), which is expressed in epidermal, epithelial, and cardiac muscle cells. The sensor localizes well to desmosomes (verified by electron microscopy) and retains interactions with both plakoglobin and desmoplakin. We validated the force responsiveness of the sensor using contracted and relaxed cardiomyocytes, observing that dsg-2 forces are higher in contracted cardiomyocytes. Using the dsg-2 sensor we also observed that desmosomes in epithelial cells (MDCK and A431) are subject to mechanical tension. In addition, changes in desmosome hyperadhesivity affects desmosome force. Lastly, we observed higher dsg-2 forces in MDCK cells grown as 3D acini compared to 2D monolayers. To determine the importance of desmosome forces in acini, we used dominant negative desmoplakin (DP-NTP) to prevent desmosome association with the IF cytoskeleton. DP-NTP expressing MDCK cells did not form hollow acini with a single lumen, but instead were a solid mass of cells. These results provide, to our knowledge, the first direct evidence of mechanical force across desmosomes, and by extension suggest that the intermediate filament cytoskeleton is capable of applying tensile forces at cell-cell contacts.