8,379 research outputs found
Extensile actomyosin?
Living cells move thanks to assemblies of actin filaments and myosin motors
that range from very organized striated muscle tissue to disordered
intracellular bundles. The mechanisms powering these disordered structures are
debated, and all models studied so far predict that they are contractile. We
reexamine this prediction through a theoretical treatment of the interplay of
three well-characterized internal dynamical processes in actomyosin bundles:
actin treadmilling, the attachement-detachment dynamics of myosin and that of
crosslinking proteins. We show that these processes enable an extensive control
of the bundle's active mechanics, including reversals of the filaments'
apparent velocities and the possibility of generating extension instead of
contraction. These effects offer a new perspective on well-studied in vivo
systems, as well as a robust criterion to experimentally elucidate the
underpinnings of actomyosin activity.Comment: 5 pages, 5 figure
Connecting local active forces to macroscopic stress in elastic media
In contrast with ordinary materials, living matter drives its own motion by
generating active, out-of-equilibrium internal stresses. These stresses
typically originate from localized active elements embedded in an elastic
medium, such as molecular motors inside the cell or contractile cells in a
tissue. While many large-scale phenomenological theories of such active media
have been developed, a systematic understanding of the emergence of stress from
the local force-generating elements is lacking. In this paper, we present a
rigorous theoretical framework to study this relationship. We show that the
medium's macroscopic active stress tensor is equal to the active elements'
force dipole tensor per unit volume in both continuum and discrete linear
homogeneous media of arbitrary geometries. This relationship is conserved on
average in the presence of disorder, but can be violated in nonlinear elastic
media. Such effects can lead to either a reinforcement or an attenuation of the
active stresses, giving us a glimpse of the ways in which nature might harness
microscopic forces to create active materials.Comment: 9 pages, 4 figure
Geometrical frustration yields fiber formation in self-assembly
Controlling the self-assembly of supramolecular structures is vital for
living cells, and a central challenge for engineering at the nano- and
microscales. Nevertheless, even particles without optimized shapes can robustly
form well-defined morphologies. This is the case in numerous medical conditions
where normally soluble proteins aggregate into fibers. Beyond the diversity of
molecular mechanisms involved, we propose that fibers generically arise from
the aggregation of irregular particles with short-range interactions. Using a
minimal model of ill-fitting, sticky particles, we demonstrate robust fiber
formation for a variety of particle shapes and aggregation conditions.
Geometrical frustration plays a crucial role in this process, and accounts for
the range of parameters in which fibers form as well as for their metastable
character.Comment: 6 pages, 5 figures, 1 ancillary movie; to appear in Nature Physic
A posteriori modeling error estimates in the optimization of two-scale elastic composite materials
The a posteriori analysis of the discretization error and the modeling error
is studied for a compliance cost functional in the context of the optimization
of composite elastic materials and a two-scale linearized elasticity model. A
mechanically simple, parametrized microscopic supporting structure is chosen
and the parameters describing the structure are determined minimizing the
compliance objective. An a posteriori error estimate is derived which includes
the modeling error caused by the replacement of a nested laminate
microstructure by this considerably simpler microstructure. Indeed, nested
laminates are known to realize the minimal compliance and provide a benchmark
for the quality of the microstructures. To estimate the local difference in the
compliance functional the dual weighted residual approach is used. Different
numerical experiments show that the resulting adaptive scheme leads to simple
parametrized microscopic supporting structures that can compete with the
optimal nested laminate construction. The derived a posteriori error indicators
allow to verify that the suggested simplified microstructures achieve the
optimal value of the compliance up to a few percent. Furthermore, it is shown
how discretization error and modeling error can be balanced by choosing an
optimal level of grid refinement. Our two scale results with a single scale
microstructure can provide guidance towards the design of a producible
macroscopic fine scale pattern
Fiber networks amplify active stress
Large-scale force generation is essential for biological functions such as
cell motility, embryonic development, and muscle contraction. In these
processes, forces generated at the molecular level by motor proteins are
transmitted by disordered fiber networks, resulting in large-scale active
stresses. While these fiber networks are well characterized macroscopically,
this stress generation by microscopic active units is not well understood. Here
we theoretically study force transmission in these networks, and find that
local active forces are rectified towards isotropic contraction and strongly
amplified as fibers collectively buckle in the vicinity of the active units.
This stress amplification is reinforced by the networks' disordered nature, but
saturates for high densities of active units. Our predictions are
quantitatively consistent with experiments on reconstituted tissues and
actomyosin networks, and shed light on the role of the network microstructure
in shaping active stresses in cells and tissue.Comment: 8 pages, 4 figures. Supporting information: 5 pages, 5 figure
On the microstructure of active cellular processes
Eukaryotic cells use a multitude of protein machines to regulate their own structure. In this thesis, we study how the geometrical arrangement of these interacting microscopic active elements sculpt the cell's own internal microstructure and its membrane enclosure.We first focus on the mechanisms generating actomyosin contractility, a crucial driver of cell motion and organization. We question the current position of highly organized, sarcomeric contractility as the only possible mechanism to drive contractility. We propose an alternative mechanism, and show that only it can account for the observed contractility of disordered actomyosin assemblies. It moreover yields qualitatively new effects in intracellular force transmission, including stress reversal and amplification, consistent with experimentally observations in fiber networks.We next elucidate some of the mechanisms through which the cell deforms and cuts its own membrane, thus enabling exchanges with the extracellular medium as well as between its internal compartments. We find that the function of the proteins responsible for this remodeling is strongly influenced by the mechanics of the membrane, and use these effects to elucidate the modes of operation of proteins clathrin and dynamin, as well as of protein complex ESCRT-III
Psychological pressure in competitive environments: Evidence from a randomized natural experiment: Comment
Apesteguia and Palacios-Huerta (forthcoming) report for a sample of 129 shootouts from various seasons in ten different competitions that teams kicking first in soccer penalty shootouts win significantly more often than teams kicking second. Collecting data for the entire history of six major soccer competitions we cannot replicate their result. Teams kicking first win only 53.4% of 262 shootouts in our data, which is not significantly different from random. Our findings have two implications: (1) Apesteguia and Palacios-Huerta’s results are not generally robust. (2) Using specific subsamples without a coherent criterion for data selection might lead to non-representative findings
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