Significance: For decades, researchers have been trying to unravel one of the key questions in cell biology regarding keratin intermediate filament function in protecting epithelial cells against mechanical stress. For many different reasons, however, this fundamental hypothesis was still unproven. Here we answer this pivotal question by the use of keratin KO cells lacking complete keratin gene clusters to result in total loss of keratin filaments. This lack significantly softens cells, reduces cell viscosity, and elevates plastic cell deformation on force application. Reexpression of single keratin genes facilitates biomechanical complementation of complete cluster loss. Our manuscript therefore makes a very strong case for the crucial contribution of keratins to cell mechanics, with far-reaching implications for epithelial pathophysiology.

Autophagy, a pathway primarily relevant for cell survival, and apoptosis, a process invariably leading to cell death, are the two main mechanisms of cellular self-destruction, which are essential i...

Recognition of external mechanical signals is vital for mammalian cells. Cyclic stretch, e.g. around blood vessels, is one such signal that induces cell reorientation from parallel to almost perpendicular to the direction of stretch. Here, we present quantitative analyses of both, cell and cytoskeletal reorientation of umbilical cord fibroblasts. Cyclic strain of preset amplitudes was applied at mHz frequencies. Elastomeric chambers were specifically designed and characterized to distinguish between zero strain and minimal stress directions and to allow accurate theoretical modeling. Reorientation was only induced when the applied stretch exceeded a specific amplitude, suggesting a non-linear response. However, on very soft substrates no mechanoresponse occurs even for high strain. For all stretch amplitudes, the angular distributions of reoriented cells are in very good agreement with a theory modeling stretched cells as active force dipoles. Cyclic stretch increases the number of stress fibers and the coupling to adhesions. We show that changes in cell shape follow cytoskeletal reorientation with a significant temporal delay. Our data identify the importance of environmental stiffness for cell reorientation, here in direction of zero strain. These in vitro experiments on cultured cells argue for the necessity of rather stiff environmental conditions to induce cellular reorientation in mammalian tissues.