Intracellular Microtubules Reflect Universal Non-equilibrium Dynamics of Living Cells 

Microtubules are highly dynamic biopolymer filaments involved in a wide variety of biological processes including cell division, migration, and intracellular transport. Microtubules are very rigid and form a stiff structural scaffold that resists deformation. However, despite their rigidity, inside of cells they typically exhibit significant bends on all length scales. We show that these bending fluctuations have a non-equilibrium origin, and yet these fluctuations exhibit a characteristic time dependence reminiscent of equilibrium Brownian fluctuations. This motion is consistent with our theoretical model for the collective dynamics of molecular motors in the cytoskeleton.

We show that, in cultured animal cells, bending is suppressed by the surrounding elastic cytoskeleton, and even large intracellular forces only cause significant bending fluctuations on short length scales. However, these lateral bending fluctuations also naturally cause fluctuations in the orientation of the microtubule tip. During growth, these tip fluctuations lead to microtubule bends that are frozen-in by the surrounding elastic network. This results in a persistent random walk of the microtubule, with a small apparent persistence length of 30 m, 100 times smaller than that resulting from thermal fluctuations alone. Thus, large non-thermal forces govern the growth of microtubules and can explain the highly curved shapes observed in the microtubule cytoskeleton of living cells.

Articles:

CP Brangwynne, FC MacKintosh, DA Weitz
Force fluctuations and polymerization dynamics of intracellular microtubules
PNAS, 104: 16128 (2007).

FC MacKintosh and AJ Levine
Non-equilibrium mechanics and dynamics of motor-activated gels
Physical Review Letters, 100:018104 (2008).

CP Brangwynne, GH Koenderink, FC MacKintosh, DA Weitz
Non-equilibrium microtubule fluctuations in a model cytoskeleton
Physical Review Letters, 100:118104 (2008).

CP Brangwynne, GH Koenderink, FC MacKintosh, DA Weitz
Cytoplasmic diffusion: molecular motors mix it up
J Cell Biology, 183: 583-587 (2008).