Keratin Cortex Stabilizes Cells and Cell-Cell Contacts at High Strains

The eukaryotic cytoskeleton consists of three filament types: actin filaments, microtubules, and intermediate filaments (IFs). IF proteins are expressed in a cell-type-specific manner, and keratins are found in epithelial cells. In certain cell types, keratin forms a layer close to the membrane which may be referred to as an “IF-cortex.” It is hypothesized that this IF-cortex arranges with radial bundles in a “rim-and-spokes” structure in epithelia. Based on this hypothesis, IFs and actin filaments might add complementary mechanical properties to the cortex. It was previously shown that single IFs remain undamaged at high strains and display a nonlinear stretching behavior. We now ask whether this unique force-extension behavior of single IFs is also relevant in the context of a filament network within a cell. We show that keratin-deficient (KO) Madin-Darby canine kidney II cells readily form two-dimensional (2D) cell layers and 3D cysts and withstand high equibiaxial strains. High-resolution imaging using STED microscopy reveals altered actin cortex structures in KO cells, presumably in response to the missing keratin. We investigate the influence of the equibiaxial strain on the viscoelastic properties of wild-type (WT) and KO cells using atomic force microscopy. We find that the KO cells exhibit a higher prestress than the WT cells, likely due to the change of the cortical structure. Interestingly, both the prestress and the fluidity of the KO cells are altered already at intermediate strains, whereas the WT cells show a response only at high strain. Upon stretching, the WT cysts retain their intraluminal fluid, i.e., they possess tight cell-cell contacts, whereas a part of the KO cysts release the fluid due to deficient cell-cell contacts. The compressibility modulus is analyzed in a spatially resolved manner and we find this modulus to be increased at the cell rim, compared to the inside region, due to the geometry of the cell layer. Our results indicate that KO cells remodel their actin cytoskeleton to maintain function, but nevertheless the whole tissue is very sensitive to external strain. The intricate interplay between the actin and keratin cortices in WT cells preserves their mechanical state and the cell layer stability.