Active gels: dynamics of patterning and self-organization

Backouche, Frederic; Haviv, Lior; Groswasser, David; Bernheim‐Groswasser, Anne

doi:10.1088/1478-3975/3/4/004

Cited by 190 publications

(216 citation statements)

References 53 publications

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“…Actin filaments are compacted into dense clouds around large myosin foci and these actomyosin condensates then contract into larger superaggregates. The resulting structures are intrinsically three-dimensional (3D), in strong contrast to the more twodimensional (2D) aggregates reported previously, such as rings formed in fascin-bundled actomyosin systems (40) and vortices formed in actomyosin networks on surfaces (42). Although planar ring structures might relate to the vortex-like states predicted by active gel theories (29,31,43), such features are intrinsically 2D patterns and are topologically not possible in 3D (44).…”

Section: Discussionmentioning

confidence: 75%

“…1B i). These rings are reminiscent of actin rings observed in the presence of skeletal myosin minifilaments and the actin bundler fascin (40). However, high-resolution images of the rings with only actin fluorescently labeled (to avoid spectral crosstalk with myosin) revealed that they were not composed of a continuous band of actin filaments, but rather of a collection of compact actin aggregates with a size below 1 μm (Fig.…”

Section: Resultsmentioning

confidence: 90%

“…We expect that bundling F-actin should bring the system closer to microtubule systems. Indeed some experimental work points to this: In actomyosin systems bundled by fascin, the final steady states are networks of asters reminiscent of microtubules organized by kinesins (40). In vivo, actin asters with dense clusters of myosin at their center were observed in fibroblasts stimulated to contract with cytochalasin D (46).…”

Section: Discussionmentioning

confidence: 99%

“…Another contribution to fast pattern renewal in cells may be the transient nature of physiological actincross-linking proteins. Notably, a recent in vitro study of actin networks bundled with fascin showed that myosin could spontaneously disassemble contractile structures (40). There may be an interplay between myosin force production and force-induced dissociation of cross-link proteins that controls the lifetime of cellular contractile structures (52,53).…”

Section: Discussionmentioning

confidence: 99%

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Active multistage coarsening of actin networks driven by myosin motors

Silva

Depken

Stuhrmann

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

215

174

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In cells, many vital processes involve myosin-driven motility that actively remodels the actin cytoskeleton and changes cell shape. Here we study how the collective action of myosin motors organizes actin filaments into contractile structures in a simplified model system devoid of biochemical regulation. We show that this self-organization occurs through an active multistage coarsening process. First, motors form dense foci by moving along the actin network structure followed by coalescence. Then the foci accumulate actin filaments in a shell around them. These actomyosin condensates eventually cluster due to motor-driven coalescence. We propose that the physical origin of this multistage aggregation is the highly asymmetric load response of actin filaments: they can support large tensions but buckle easily under piconewton compressive loads. Because the motor-generated forces well exceed this threshold, buckling is induced on the connected actin network that resists motor-driven filament sliding. We show how this buckling can give rise to the accumulation of actin shells around myosin foci and subsequent coalescence of foci into superaggregates. This new physical mechanism provides an explanation for the formation and contractile dynamics of disordered condensed actomyosin states observed in vivo.active gels | molecular motors | nonequilibrium | soft condensed matter C ells undergo dramatic changes in shape and internal organization during vital processes such as migration and division. These changes involve remodeling of the cytoskeleton partly driven by collective physical interactions between molecular motors and cytoskeletal filaments. The motors use adenosine triphosphate (ATP) as fuel and hydrolyze it to actively generate forces and move along filaments (1). Multiheaded motors or complexes of motors may cross-link neighboring filaments and generate relative motion between them. Kinesin and dynein motors interact with microtubules to form the mitotic spindle, which is responsible for chromosome segregation (2). Myosin motors interact with filamentous actin (F-actin) to form complex arrays such as the contractile ring driving cell division (3, 4) and contractile networks that drive cell migration (4) and polarizing cortical flows (5, 6).To identify the biophysical processes underlying cytoskeletal organization, many in vitro model systems of purified motors and filaments that lack biochemical regulation have been recently developed. It is known that kinesins can organize microtubules into polarity sorted asters, as well as vortex or bundle states (7-10). These structures resemble physiological arrays such as the mitotic spindle (11). In contrast to microtubules, purified F-actin does not form well-defined structures when motors are added. Actin-myosin II solutions remain disordered at high levels of ATP (12-14) and generate dense condensates that appear internally unstructured if the ATP level is lowered or when the actin filaments are cross-linked (15-18). Interestingly, similar dense condensates appear in...

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Section: Discussionmentioning

confidence: 75%

Section: Resultsmentioning

confidence: 90%

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

See 2 more Smart Citations

Active multistage coarsening of actin networks driven by myosin motors

Silva

Depken

Stuhrmann

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

215

174

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show abstract

“…These macroscopic approaches based on generic symmetry considerations predict the formation of diverse patterns in actomyosin gels such as asters and ring-like structures, which have been observed in studies in vitro (15).…”

mentioning

confidence: 94%

Active contractility in actomyosin networks

Wang

Wolynes

2012

Proc. Natl. Acad. Sci. U.S.A.

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Contractile forces are essential for many developmental processes involving cell shape change and tissue deformation. Recent experiments on reconstituted actomyosin networks, the major component of the contractile machinery, have shown that active contractility occurs above a threshold motor concentration and within a window of cross-link concentration. We present a microscopic dynamic model that incorporates two essential aspects of actomyosin self-organization: the asymmetric load response of individual actin filaments and the correlated motor-driven events mimicking myosin-induced filament sliding. Using computer simulations, we examine how the concentration and susceptibility of motors contribute to their collective behavior and interplay with the network connectivity to regulate macroscopic contractility. Our model is shown to capture the formation and dynamics of contractile structures and agree with the observed dependence of active contractility on microscopic parameters, including the contractility onset. Cooperative action of load-resisting motors in a force-percolating structure integrates local contraction/buckling events into a global contractile state via an active coarsening process, in contrast to the flow transition driven by uncorrelated kicks of susceptible motors.cytoskeleton | contractile instability | anticorrelated motor kicks | nonequilibrium processes | soft active matter C ontractile forces are essential for many processes vital to development, ranging from cytokinesis and cell motility (1) to wound healing and gastrulation (2). Networks of filamentous actin (F-actin) and the molecular motor, type II myosin have been identified as the major components of the contractile machinery. The actin network provides a structural scaffold on which the myosin motors move, powered by ATP hydrolysis. Actomyosin networks generate contractile forces through the activity of myosin motors, which themselves assemble into bipolar minifilaments that generate sustained sliding of neighboring actin filaments relative to each other in order to reorganize F-actin networks and generate tension (3). When coupled to the cell substrate or using cell-cell adhesions, contractile actomyosin networks transmit forces to their environment.In addition to the microtubule-kinesin system, another important filament-motor assembly in cells that forms well-focused mitotic spindle poles driven by a polarity sorting mechanism (4, 5) to accomplish high-accuracy segregation of duplicated chromosomes, actomyosin condensates appear in diverse tissues and organisms as transient structures that coalesce into still larger arrays that exert contractile forces. Examples include the contractile rings driving cytokinesis and wound healing, and the contractile networks that deform epithelial cell layers in developing embryos and drive polarizing cortical flows (6, 7).Some recent theoretical efforts have modeled the contractile actin cortex as an active polar gel and have derived effective continuum theories within a hydrodynamic framework ...

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Cytoskeletal and Actin‐Based Polymerization Motors and Their Role in Minimal Cell Design

2019

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Life implies motion. In cells, protein‐based active molecular machines drive cell locomotion and intracellular transport, control cell shape, segregate genetic material, and split a cell in two parts. Key players among molecular machines driving these various cell functions are the cytoskeleton and motor proteins that convert chemical bound energy into mechanical work. Findings over the last decades in the field of in vitro reconstitutions of cytoskeletal and motor proteins have elucidated mechanistic details of these active protein systems. For example, a complex spatial and temporal interplay between the cytoskeleton and motor proteins is responsible for the translation of chemically bound energy into (directed) movement and force generation, which eventually governs the emergence of complex cellular functions. Understanding these mechanisms and the design principles of the cytoskeleton and motor proteins builds the basis for mimicking fundamental life processes. Here, a brief overview of actin, prokaryotic actin analogs, and motor proteins and their potential role in the design of a minimal cell from the bottom‐up is provided.

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Active gels: dynamics of patterning and self-organization

Cited by 190 publications

References 53 publications

Active multistage coarsening of actin networks driven by myosin motors

Active multistage coarsening of actin networks driven by myosin motors

Active contractility in actomyosin networks

Cytoskeletal and Actin‐Based Polymerization Motors and Their Role in Minimal Cell Design

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