Microscale Mechanics and Structural Organization of Cross-linked Actin Networks

Actin, a globular protein, is a major component of the cytoskeleton. In the presence of ATP and magnesium, G-actin polymerizes into filamentous actin (F-actin), which plays crucial structural and mechanical roles in cell stability, motion, replication, and muscle contraction. Most of these mechanically driven structural changes in cells stem from the complex viscoelastic nature of cross-linked actin filaments. How actin networks respond to local nonlinear stresses is not well understood. Here, we use optical tweezers to impart local nonlinear strains and measure the resulting stresses during and following the strain of cross-linked actin networks. We actively drive a microsphere 10 micrometers through cross-linked actin networks at a constant speed and measure the resistive force that the network exerts on the bead during and following strain. We determine the viscoelastic response of the phalloidin stabilized actin network by varying the concentration of a cross-linker. We simultaneously image the network via fluorescence LSCM to characterize the networks' structural change and heterogeneity as the density of cross-linking protein increases. Our results shed light on how cells undergo morphological changes through varying crosslinker densities.