Microtubules Can Bend By Dynein-Mediated Transport Towards Transiently Pinned Locations

In this talk I will outline progress we have made in combining optical microscopy with computer simulations to elucidate the mechanisms by which microtubules are bent in living cells. Microtubules have a persistence length of the order of millimeters in vitro, but inside cells they bend over length scales of microns. How these bends are formed is unclear, especially when the microtubule tips are far from the cell membrane. Experiments with LLC-PK1 epithelial cells, using microtubules marked by photo-bleaching show that loops develop primarily by plus-end directed transport of microtubule segments towards stationary pinning points along the length of the microtubule. The pinning points are transient in nature, and their eventual release allows the bends to relax. Experiments with cells where dynein and myosin were separately inhibited suggests that dynein is the primary force generator, able to bend microtubules against stationary pinning points. Simulations of microtubule dynamics accounting for filament mechanics and dynein forces predict the development of loops of similar size and shape to those observed in cells. Furthermore, simulations show that dynein-generated loops near a microtubule tip cause persistent rotation of the tip, leading to a change the direction of microtubule growth. Collectively, these results suggest a simple physical mechanism for the bending of growing microtubules by dynein forces accumulating at pinning points.