452 research outputs found
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
Contractile units in disordered actomyosin bundles arise from F-actin buckling
Bundles of filaments and motors are central to contractility in cells. The
classic example is striated muscle, where actomyosin contractility is mediated
by highly organized sarcomeres which act as fundamental contractile units.
However, many contractile bundles in vivo and in vitro lack sarcomeric
organization. Here we propose a model for how contractility can arise in
actomyosin bundles without sarcomeric organization and validate its predictions
with experiments on a reconstituted system. In the model, internal stresses in
frustrated arrangements of motors with diverse velocities cause filaments to
buckle, leading to overall shortening. We describe the onset of buckling in the
presence of stochastic actin-myosin detachment and predict that
buckling-induced contraction occurs in an intermediate range of motor
densities. We then calculate the size of the "contractile units" associated
with this process. Consistent with these results, our reconstituted actomyosin
bundles contract at relatively high motor density, and we observe buckling at
the predicted length scale.Comment: 5 pages, 4 figures, Supporting text and movies attache
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
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
Use of perfusion bioreactors and large animal models for long bone tissue engineering
Tissue engineering and regenerative medicine (TERM) strategies for generation of new bone tissue includes the
combined use of autologous or heterologous mesenchymal stem cells (MSC) and three-dimensional (3D) scaffold
materials serving as structural support for the cells, that develop into tissue-like substitutes under appropriate
in vitro culture conditions. This approach is very important due to the limitations and risks associated with
autologous, as well as allogenic bone grafiting procedures currently used. However, the cultivation of osteoprogenitor
cells in 3D scaffolds presents several challenges, such as the efficient transport of nutrient and oxygen
and removal of waste products from the cells in the interior of the scaffold. In this context, perfusion bioreactor
systems are key components for bone TERM, as many recent studies have shown that such systems can provide
dynamic environments with enhanced diffusion of nutrients and therefore, perfusion can be used to generate
grafts of clinically relevant sizes and shapes. Nevertheless, to determine whether a developed tissue-like substitute
conforms to the requirements of biocompatibility, mechanical stability and safety, it must undergo
rigorous testing both in vitro and in vivo. Results from in vitro studies can be difficult to extrapolate to the in vivo
situation, and for this reason, the use of animal models is often an essential step in the testing of orthopedic
implants before clinical use in humans. This review provides an overview of the concepts, advantages, and
challenges associated with different types of perfusion bioreactor systems, particularly focusing on systems that
may enable the generation of critical size tissue engineered constructs. Furthermore, this review discusses some
of the most frequently used animal models, such as sheep and goats, to study the in vivo functionality of bone
implant materials, in critical size defects.Leandro Gardel acknowledges the Portuguese Foundation for Science and Technology (FCT) for the PhD scholarship (ref SFRH/BD/66714/2009)
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