Dynein Forces and Microtubule Shapes

Most essential mammalian cell functions, including migration and cell division, involve force generation by semi-flexible biopolymers called microtubules. Most experimental force measurements with microtubules have been performed in vitro and are not directly relevant to what happens in vivo. As a result, the microtubule force balance remains unclear. In conjunction with experimental and modeling groups led by Profs. Tanmay Lele and Richard Dickinson we are carrying out numerical simulations of microtubules under the action of dynein motor forces to help interpret optical microscopy experiments tracking the motion of microtubules in living cells. Numerical simulations of centrosome centering and an interpretation of the experiments can be found here.

Despite their rigidity, microtubules in living cells bend significantly during polymerization resulting in greater curvature than can be explained by thermal forces alone. However, the source of the non-thermal forces that bend growing microtubules remains obscure. We have analyzed the motion of microtubule tips in NIH-3T3 fibroblasts expressing EGFP-EB1, a fluorescent +TIP protein that specifically binds to the growing ends of microtubules. We found that dynein inhibition significantly reduced the deviation of the growing tip from its initial trajectory. Inhibiting myosin modestly reduced tip fluctuations, while simultaneous myosin and dynein inhibition caused no further decrease in fluctuations compared to dynein inhibition alone. Our results can be interpreted with a model in which dynein linkages play a key role in generating and transmitting fluctuating forces that bend growing microtubules.

Numerical simulations of microtubule bending by dynein and myosin. The model for dynein and myosin force generation is illustrated in (a). A dynein-cytomatrix linkage is formed in a force-free state at time t = 0; the light mauve circles indicate the points of attachment to the microtubule and cytomatrix. As the motor walks along the microtubule, shown by the dark mauve circle, the linkage is extended and a force is exerted in the direction shown by the black arrow. In addition the anchor point in the cytomatrix may also be moved by myosin activity. The total force in the linkage is a combination of dynein and myosin activity. A simulation of filament growth is illustrated in (b), which shows the configuration of a microtubule polymerized to a contour length of . The red circles are the positions of the dynein motors and the red lines indicate the dynein-cytomatrix linkage. (c) and (d) show the trajectories of 100 different samples (in each case) under the action of dynein forces (stationary anchor points) and dynein plus myosin forces (moving anchors) respectively.

Growth of a single microtubule with stationary anchor points. Click here to download.

Growth of a single microtubule with moving anchor points. Click here to download.

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Last updated January 2015