Axial Elasticity and Mechanical Fragmentation of Desmin Intermediate Filaments

Abstract

Desmin is the intermediate filament of muscle cells. Desmin filaments are thought to play fundamental roles in cellular force transduction and the maintenance of structural integrity under mechanical exposure. Although the importance of desmin elasticity and assembly/disassembly dynamics in cellular mechanics is being increasingly recognized, the molecular basis of desmin's elasticity or its disassembly pathway are little understood. In the present work we explored the elasticity of purified desmin filaments by stretching them longitudinally with surface-tension forces.Desmin, purified from chicken gizzard, was polymerized by the addition of MgCl2. Polymerized filaments in a buffer droplet were then stretched by using a custom-built horizontal rotor assembly. Driven by centrifugal force, the droplet spread radially on freshly cleaved mica surface. Partially surface-adsorbed desmin filaments became extended by the receding meniscus and were captured on mica in the over-stretched state. The stretch force acting on the entire cross-sectional area of desmin was calculated to be 4 nN.As a result of this molecular combing, the average contour length of desmin filaments increased from 0.89 µm to 1.38 µm, which corresponds to a 1.6-fold axial stretch. Molecular combing together with EDTA-treatment caused the fragmentation of desmin filaments into short, 60- to 120-nm-long and 4-nm-wide structures displaying periodic, 34 nm-surface protrusions. Based on these calculations the observed fragments are hypothesized to be protofibril fragments composed of laterally attached desmin dimers. The orientation axis of the surface-constrained fragments deviated widely from the filament axis, suggesting that the protofibrils are not aligned in parallel within the filament during axial load. Furthermore, the emergence of protofibril fragments suggests that the head or tail domains of coiled-coil desmin dimers are the load-bearing elements during axial stretch.