Prof.Alex Levine

UCLA Chemistry Department

Cell quakes: Mechanics and Microrheology in active gels and living cells

Abstract:

Experiments by Mizuno et al. [Science 315 (5810) pp. 370-373 (2007).] on molecular motor driven semiflexible networks, similar to the cytoskeleton of living cells, have found anomalously large strain fluctuations at low frequency. In addition, the shear modulus of these motor-driven �active�� networks is as much as one hundred times larger than that of the same system in thermal equilibrium. In this talk I discuss the experiment and develop a theory of both these phenomena based on a model of a low-density semiflexible network driven by molecular motors. Relying on only simple assumptions regarding the motor activity in the system, one can quantitatively understand both the low-frequency fluctuation enhancement and the nonequilibrium stiffening of the network.

There are two principal implications of this work. First, noninvasive studies of the mechanics of living cells rely on microrheology -- the use of observed intracellular strain fluctuations to determine the cell's mechanics. The appearance of large nonequilibrium fluctuations requires a reevaluation of the interpretation of cellular microrheology. Secondly, active gels, such as the cytoskeleton, represent a new class of nonequilibrium materials whose linear response properties are tunable via the material's nonequilibrium steady-state. A deeper understanding of these systems that combine inherent elastic nonlinearities and motor-induced states of stress may form the basis for the development of new biomimetic nonequilibrium gels with reversibly tunable mechanics.