Probing fluctuations and active dynamics of the cellular actomyosin cortex with magnetic micropost arrays

Actomyosin networks inside living cells are an archetypical example of an active matter system. Here we use poly(dimethylsiloxane) (PDMS) micropost arrays with embedded nickel nanowires to measure the dynamical fluctuations and the local rheology of cellular actomyosin networks simultaneously. By characterizing their fluctuations using mean square displacements (MSDs), we found that the cellular cortex and stress fibers showed different dynamics. We also found large dispersion in the MSD exponent as well as the processivity time of the fluctuations in the cortex, indicating the presence of significant stochasticity in the myosin motors’ contractile dynamics. From correlations between individual posts’ deflections, we have identified specific large contractile and relaxational events in the cortex. The structure of these events is consistent with the presence of giant fluctuations in the active actomyosin system, and not with the response expected from models of step-like forces from single myosin force generators. These results thus suggest the presence of cooperativity in cortical myosin thick filament binding and unbinding.