(721a) Force Generation On the Nucleus by Dynein Walking On Dynamic Microtubules Is Sufficient to Explain Nuclear Rotation

Nuclear rotation and translation in fibroblasts requires dynein and microtubules, but how dynein-generated forces cause rotation remains unclear. In this study, we statistically analyzed the rotation to find the rotation angle exhibits a persistent random walk, i.e. directionally persistent on the time scale of tens of minutes, but rotationally diffusive on the longer times scale. Also, the centrosome remained stationary and did not rotate with the nucleus. These findings suggest that rotation is caused by molecular processes that are stably organized in space and time over several minutes. To quantitatively explain these observations, we formulated a model based on microtubules undergoing dynamic instability, with tensional forces between a stationary centrosome and the nuclear surface mediated by dynein. The model predicts that the persistence in rotation angle is transient configuration of microtubules exerting a net torque in one direction for a finite time period before reconfiguration by dynamic instability. A testable prediction of the model is that the rotation depends on the distance between the nucleus and the centrosome. To test this prediction, rotation was quantified in cells confined to adhesive islands on patterned substrata. In these cells, the centrosome tended to underlay the nucleus near the projected nuclear centroid. Rotation was found to be decreased in patterned cells compared to unpatterned cells. Taken together, these results suggest that force generation by retrograde motors on microtubules undergoing dynamic instability is sufficient to explain the dynamics of nuclear rotation.