The cytoskeleton of animal cells is a highly dynamic network of biopolymer filaments that forms a load-bearing scaffold within cells. Cytoskeletal filaments exhibit significant bending fluctuations that result from large, non-thermal forces, such as those arising from the activity of ATP-consuming molecular motors. Microtubules are an important component of this scaffold, and are involved in a broad range of biological processes, including cell migration, intracellular transport, and mitosis. However, the precise mechanical role of microtubules in cells, and the nature of fluctuating intracellular forces in general, remain poorly understood. Here, we carefully analyze the dynamics of microtubule bending to reveal the underlying forces. We implement a Fourier analysis technique to quantify the spatial- and temporal-dependence of microtubule bending fluctuations. We first study isolated microtubules in thermal equilibrium, both in aqueous buffer solution and embedded in an entangled in vitro network of purified actin filaments. The small thermal fluctuations we observe are in quantitative agreement with the theoretically predicted behavior. In contrast, for microtubules embedded in an in vitro actin network driven by myosin motors, stochastic motor forces, of order 10 pN, give rise to large bending fluctuations. Due to the surrounding elastic network, these fluctuations are particularly apparent on short length scales, and have surprisingly diffusive-like features resulting from the step-like relaxation dynamics of the motors.
The spatial and temporal behavior of these in vitro , non-thermal microtubule bends are remarkably similar to the microtubule dynamics we observe in cells, and appear to reflect the same underlying physics. However, we also find that the instantaneous shapes of bent microtubules exhibit a surprisingly thermal-like distribution in cells, with an anomolously small persistence length of 30 μm, about 100 times smaller than in vitro . We show that this arises from non-thermal fluctuations that redirect the orientation of microtubule tips during growth, giving rise to a persistent random walk growth trajectory. The long wavelength bends that result are effectively frozen-in by the surrounding network, and the fluctuations are therefore non-ergodic . These findings suggest that the architecture of the microtubule network, as well as its mechanical response, are both intimately coupled to the fluctuating non-equilibrium activity of the composite cytoskeleton, and have important implications for the biophysical behavior of the cell.
