Epithelial cells adapt to sustained stretching through a slow structural reorganization of keratin filaments and the escape of the cell nucleus from its keratin cage, according to a study published in Nature Physics.
Researchers examined Madin-Darby canine kidney cell monolayers under mechanical strain using spontaneous tissue domes and a microfluidic device designed to control luminal pressure. When subjected to a constant pressure of 200 Pa, cells exposed to large strains did not bundle their keratin filaments immediately. Instead, the structural transition began after tens of minutes with the formation of keratin-depleted zones at tricellular junctions. These depleted areas grew to an average size of 43 ± 15 square micrometers over 4.5 ± 2.3 hours before filaments shifted into thick bundles radiating from central cell nodes.
Actin interactions and bundling kinetics
The study demonstrated that interaction between actin filaments and the keratin cytoskeleton governs the speed of the bundling transition. In control cells, the average bundling time reached 424 ± 162 minutes. To test the role of actin-keratin crosslinking, the researchers overexpressed a dominant-negative plectin mutant, P1f-8–BFP, which lacks a keratin-binding domain.
Cells expressing the plectin mutant underwent the bundling transition significantly faster, recording an average bundling time of 146 ± 68 minutes. Disrupting the actin cortex directly using 1 micromolar latrunculin A 15 minutes after dome formation triggered keratin network reorganization within 60 minutes. The findings indicate that intact actin structures slow down the reorganization of keratin under sustained tension.
Computational modeling and nuclear displacement
To analyze the mechanical forces during stretching, the researchers developed two-and-a-half-dimensional and three-dimensional models using the open-source Brownian dynamics simulation suite Cytosim. In the two-and-a-half-dimensional model, keratin filaments attached to laterally mobile desmosomes and interacted with plectin linkers on the cell cortex and nesprin-3 on the nuclear envelope, establishing a rim-and-spoke architecture.
The three-dimensional model simulated cells as hexagonal prisms containing rigid spherical nuclei. Under simulated strain, keratin bundling exerted compressive stress against the nuclear surface. The model demonstrated that the physical escape of the nucleus from the surrounding keratin network relaxed this compressive stress. Experimental tracking confirmed that nuclear offset correlated directly with keratin bundling metrics across stretched cells.
The structural transformation spreads across the cell layer through a nucleation-and-growth mechanism. Keratin bundling begins in isolated individual cells before propagating to adjacent neighbors, forming interconnected networks of thick keratin bundles across multicellular clusters.
