Cell Quakes: Mechanics and Microrheology of Living Cells and Active Gels

Recent experiments on molecular motor driven in vitro F-Actin networks have found anomalously large strain fluctuations at low frequency. In addition, the shear modulus of these active networks becomes as much as one hundred times larger than that of the same system in equilibrium. We develop a two-fluid mean-field model of a semiflexible network driven by molecular motors to explore these effects and show that, relying on only simple assumptions regarding the motor activity in the system, we can quantitatively understand both the low-frequency fluctuation enhancement and the nonequilibrium stiffening of the network. We discuss the implications of the fluctuation enhancement for intracellular microrheology. In particular we show that, by observing the anti-correlated motion of tracer particles in two-particle microrheology, one can calculate the density of active motor complexes and quantitatively account for these nonequilibrium forces in the analysis of the fluctuation data. Finally, we present the results of numerical work on the motor-induced gel stiffening effect in the high motor density limit, which goes beyond the analytic mean-field model. These results have implications for the interpretation of microrheology in such active networks including the cytoskeleton of living cells. In addition, they may form the basis for theoretical studies of biomimetic nonequilibrium gels whose mechanical properties are tunable through the control of their nonequilibrium steady state.