CiteULike: dchen's Mackintosh

Nonequilibrium Microtubule Fluctuations in a Model Cytoskeleton

Clifford Brangwynne — 2008-05-07T21:19:39-00:00

Physical Review Letters, Vol. 100, No. 11. (2008)

Biological activity gives rise to nonequilibrium fluctuations in the cytoplasm of cells; however, there are few methods to directly measure these fluctuations. Using a reconstituted actin cytoskeleton, we show that the bending dynamics of embedded microtubules can be used to probe local stress fluctuations. We add myosin motors that drive the network out of equilibrium, resulting in an increased amplitude and modified time dependence of microtubule bending fluctuations. We show that this behavior results from steplike forces on the order of 10 pN driven by collective motor dynamics.

Nonlinear elasticity in biological gels

Cornelis Storm — 2005-05-13T07:43:38-00:00

Nature, Vol. 435, No. 7039., pp. 191-194.

Microtubules can bear enhanced compressive loads in living cells because of lateral reinforcement

Clifford Brangwynne — 2006-11-20T22:19:20-00:00

J. Cell Biol., Vol. 173, No. 5. (5 June 2006), pp. 733-741.

Cytoskeletal microtubules have been proposed to influence cell shape and mechanics based on their ability to resist large-scale compressive forces exerted by the surrounding contractile cytoskeleton. Consistent with this, cytoplasmic microtubules are often highly curved and appear buckled because of compressive loads. However, the results of in vitro studies suggest that microtubules should buckle at much larger length scales, withstanding only exceedingly small compressive forces. This discrepancy calls into question the structural role of microtubules, and highlights our lack of quantitative knowledge of the magnitude of the forces they experience and can withstand in living cells. We show that intracellular microtubules do bear large-scale compressive loads from a variety of physiological forces, but their buckling wavelength is reduced significantly because of mechanical coupling to the surrounding elastic cytoskeleton. We quantitatively explain this behavior, and show that this coupling dramatically increases the compressive forces that microtubules can sustain, suggesting they can make a more significant structural contribution to the mechanical behavior of the cell than previously thought possible. 10.1083/jcb.200601060

Force fluctuations and polymerization dynamics of intracellular microtubules

Clifford Brangwynne — 2008-01-21T00:56:00-00:00

Proceedings of the National Academy of Sciences, Vol. 104, No. 41. (9 October 2007), pp. 16128-16133.

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. Here, we investigate the origin of these bends using a Fourier analysis approach to quantify their length and time dependence. 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 approx30 microm, approx100 times smaller than that resulting from thermal fluctuations alone. Thus, large nonthermal forces govern the growth of microtubules and can explain the highly curved shapes observed in the microtubule cytoskeleton of living cells. 10.1073/pnas.0703094104

Fluctuation-Dissipation Theorem in an Aging Colloidal Glass

Sara Farouji — 2007-11-26T20:20:36-00:00

Physical Review Letters, Vol. 98, No. 10. (2007)

We provide a direct experimental test of the fluctuation-dissipation theorem (FDT) in an aging colloidal glass. The use of combined active and passive microrheology allows us to independently measure both the correlation and response functions in this nonequilibrium situation. Contrary to previous reports, we find no deviations from the FDT over several decades in frequency (1 Hz–10 kHz) and for all aging times. In addition, we find two distinct viscoelastic contributions in the aging glass, including a nearly elastic response at low frequencies that grows during aging.