321 research outputs found
Stress relaxation in F-actin solutions by severing
Networks of filamentous actin (F-actin) are important for the mechanics of
most animal cells. These cytoskeletal networks are highly dynamic, with a
variety of actin-associated proteins that control cross-linking, polymerization
and force generation in the cytoskeleton. Inspired by recent rheological
experiments on reconstituted solutions of dynamic actin filaments, we report a
theoretical model that describes stress relaxation behavior of these solutions
in the presence of severing proteins. We show that depending on the kinetic
rates of assembly, disassembly, and severing, one can observe both
length-dependent and length-independent relaxation behavior
Requirements for contractility in disordered cytoskeletal bundles
Actomyosin contractility is essential for biological force generation, and is
well understood in highly organized structures such as striated muscle.
Additionally, actomyosin bundles devoid of this organization are known to
contract both in vivo and in vitro, which cannot be described by standard
muscle models. To narrow down the search for possible contraction mechanisms in
these systems, we investigate their microscopic symmetries. We show that
contractile behavior requires non-identical motors that generate large enough
forces to probe the nonlinear elastic behavior of F-actin. This suggests a role
for filament buckling in the contraction of these bundles, consistent with
recent experimental results on reconstituted actomyosin bundles.Comment: 10 pages, 6 figures; text shortene
A cycling state that can lead to glassy dynamics in intracellular transport
Power-law dwell times have been observed for molecular motors in living
cells, but the origins of these trapped states are not known. We introduce a
minimal model of motors moving on a two-dimensional network of filaments, and
simulations of its dynamics exhibit statistics comparable to those observed
experimentally. Analysis of the model trajectories, as well as experimental
particle tracking data, reveals a state in which motors cycle unproductively at
junctions of three or more filaments. We formulate a master equation for these
junction dynamics and show that the time required to escape from this
vortex-like state can account for the power-law dwell times. We identify trends
in the dynamics with the motor valency for further experimental validation. We
demonstrate that these trends exist in individual trajectories of myosin II on
an actin network. We discuss how cells could regulate intracellular transport
and, in turn, biological function, by controlling their cytoskeletal network
structures locally
Monte Carlo study of multiply crosslinked semiflexible polymer networks
We present a method to generate realistic, three-dimensional networks of
crosslinked semiflexible polymers. The free energy of these networks is
obtained from the force-extension characteristics of the individual polymers
and their persistent directionality through the crosslinks. A Monte Carlo
scheme is employed to obtain isotropic, homogeneous networks that minimize the
free energy, and for which all of the relevant parameters can be varied: the
persistence length, the contour length as well as the crosslinking length may
be chosen at will. We also provide an initial survey of the mechanical
properties of our networks subjected to shear strains, showing them to display
the expected non-linear stiffening behavior. Also, a key role for non-affinity
and its relation to order in the network is uncovered.Comment: 11 pages, revised figures, added extra information about the network
Microrheology probes length scale dependent rheology
We exploit the power of microrheology to measure the viscoelasticity of entangled F-actin solutions at different length scales from 1 to 100 mu m over a wide frequency range. We compare the behavior of single probe-particle motion to that of the correlated motion of two particles. By varying the average length of the filaments, we identify fluctuations that dissipate diffusively over the filament length. These provide an important relaxation mechanism of the elasticity between 0.1 and 30 rad/sec
Stress-Dependent Elasticity of Composite Actin Networks as a Model for Cell Behavior
Networks of filamentous actin cross-linked with the actin-binding protein filamin A exhibit remarkable strain stiffening leading to an increase in differential elastic modulus by several orders of magnitude over the linear value. The variation of the frequency dependence of the differential elastic and loss moduli as a function of prestress is consistent with that observed in living cells, suggesting that cell elasticity is always measured in the nonlinear regime, and that prestress is an essential control parameter
A Zyxin-Mediated Mechanism for Actin Stress Fiber Maintenance and Repair
SummaryTo maintain mechanical homeostasis, cells must recognize and respond to changes in cytoskeletal integrity. By imaging live cells expressing fluorescently tagged cytoskeletal proteins, we observed that actin stress fibers undergo local, acute, force-induced elongation and thinning events that compromise their stress transmission function, followed by stress fiber repair that restores this capability. The LIM protein zyxin rapidly accumulates at sites of strain-induced stress fiber damage and is essential for stress fiber repair and generation of traction force. Zyxin promotes recruitment of the actin regulatory proteins α-actinin and VASP to compromised stress fiber zones. α-Actinin plays a critical role in restoration of actin integrity at sites of local stress fiber damage, whereas both α-actinin and VASP independently contribute to limiting stress fiber elongation at strain sites, thus promoting stabilization of the stress fiber. Our findings demonstrate a mechanism for rapid repair and maintenance of the structural integrity of the actin cytoskeleton
Hydrodynamic coupling and rotational mobilities near planar elastic membranes
We study theoretically and numerically the coupling and rotational
hydrodynamic interactions between spherical particles near a planar elastic
membrane that exhibits resistance towards shear and bending. Using a
combination of the multipole expansion and Faxen's theorems, we express the
frequency-dependent hydrodynamic mobility functions as a power series of the
ratio of the particle radius to the distance from the membrane for the self
mobilities, and as a power series of the ratio of the radius to the
interparticle distance for the pair mobilities. In the quasi-steady limit of
zero frequency, we find that the shear- and bending-related contributions to
the particle mobilities may have additive or suppressive effects depending on
the membrane properties in addition to the geometric configuration of the
interacting particles relative to the confining membrane. To elucidate the
effect and role of the change of sign observed in the particle self and pair
mobilities, we consider an example involving a torque-free doublet of
counterrotating particles near an elastic membrane. We find that the induced
rotation rate of the doublet around its center of mass may differ in magnitude
and direction depending on the membrane shear and bending properties. Near a
membrane of only energetic resistance toward shear deformation, such as that of
a certain type of elastic capsules, the doublet undergoes rotation of the same
sense as observed near a no-slip wall. Near a membrane of only energetic
resistance toward bending, such as that of a fluid vesicle, we find a reversed
sense of rotation. Our analytical predictions are supplemented and compared
with fully resolved boundary integral simulations where a very good agreement
is obtained over the whole range of applied frequencies.Comment: 14 pages, 7 figures. Revised manuscript resubmitted to J. Chem. Phy
Traction stress in focal adhesions correlates biphasically with actin retrograde flow speed
How focal adhesions (FAs) convert retrograde filamentous actin (F-actin) flow into traction stress on the extracellular matrix to drive cell migration is unknown. Using combined traction force and fluorescent speckle microscopy, we observed a robust biphasic relationship between F-actin speed and traction force. F-actin speed is inversely related to traction stress near the cell edge where FAs are formed and F-actin motion is rapid. In contrast, larger FAs where the F-actin speed is low are marked by a direct relationship between F-actin speed and traction stress. We found that the biphasic switch is determined by a threshold F-actin speed of 8–10 nm/s, independent of changes in FA protein density, age, stress magnitude, assembly/disassembly status, or subcellular position induced by pleiotropic perturbations to Rho family guanosine triphosphatase signaling and myosin II activity. Thus, F-actin speed is a fundamental regulator of traction force at FAs during cell migration
Redundancy and cooperativity in the mechanics of compositely crosslinked filamentous networks
The actin cytoskeleton in living cells has many types of crosslinkers. The
mechanical interplay between these different crosslinker types is an open issue
in cytoskeletal mechanics. We develop a framework to study the cooperativity
and redundancy in the mechanics of filamentous networks with two types of
crosslinkers: crosslinkers that allow free rotations of filaments and
crosslinkers that do not. The framework consists of numerical simulations and
an effective medium theory on a percolating triangular lattice. We find that
the introduction of angle-constraining crosslinkers significantly lowers the
filament concentrations required for these networks to attain mechanical
integrity. This cooperative effect also enhances the stiffness of the network
and suppresses non-affine deformations at a fixed filament concentration. We
further find that semiflexible networks with only freely-rotating crosslinks
are mechanically very similar to compositely crosslinked flexible networks with
both networks exhibiting the same scaling behavior. We show that the network
mechanics can either be redundant or cooperative depending on the relative
energy scale of filament bending to the energy stored in the angle-constraining
crosslinkers, and the relative concentration of crosslinkers. Our results may
have implications for understanding the role of multiple crosslinkers even in a
system without bundle formation or other structural motifs.Comment: 21 pages, 5 figure
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