Optical tweezers microrheology of transiently crosslinked actin networks

Cells dynamically change their viscoelastic properties by restructuring networks of actin filaments, one of the most abundant proteins in the cytoskeleton. This restructuring is modulated, in part, by actin-binding proteins (ABPs) that regulate the assembly of actin filaments (F-actin) into networks and bundles, leading to the structural integrity of the cell. Myosin II, best known for its ATP-driven restructuring of actin networks, also functions as a transient cross-linker at low ATP concentrations. While the mechanics of cross-linked actin networks with common ABPs have been studied extensively, there is a lack of understanding of the mechanics and structural evolution of transiently cross-linked actin networks. Here, we use optical tweezers microrheology to impart local nonlinear strains and measure the mechanical properties of actin networks cross-linked via myosin-II. Specifically, we drag a microsphere 10 micrometers through cross-linked actin networks at a constant speed using optical tweezers and measure the network's response on the bead during and following strain. Furthermore, we simultaneously image the network via fluorescence laser scanning confocal microscopy to correlate the microscale mechanics with the structural evolution of the networks. Our measurements shed new light on how varying the transient cross-linker densities tune the mechanical properties of actin networks.