Filament Tension Research Articles

Dynamical behaviors of scroll rings in the presence of a dc electric field

Experimental observations of scroll rings in excitable media have shown that, a dc electric field perpendicular to the plane of the ring can change the rate at which the ring shrinks, and in some cases it may cause the scroll ring to expand even though its filament tension is positive. Here we propose a kinematical model to account for these effects by extending the theory of dynamics of the free scroll ring to include the influence of electric field. In addition, some other significant results, for example, the electric field can slow down the drift velocity of the scroll ring along its symmetry axis and in some situations may reverse its drift direction, are also predicted by the kinematical model. Direct numerical simulations of a reaction–diffusion model are performed, and the obtained results are in quantitative agreement with the numerical solutions of the kinematical model.

Mechanical Role of Nesprin-1-Mediated Nucleus-Actin Filament Binding in Cyclic Stretch-Induced Fibroblast Elongation.

The intracellular mechanical link tethering the nucleus to the cytoskeleton has been suggested to be the linker of the nucleoskeleton and cytoskeleton (LINC) complex. Previous studies have reported that knockdown of nesprin-1, a component of the LINC complex that directly binds to actin filaments, suppresses cellular morphological response to mechanical stimuli. The relation between nesprin-1 knockdown and cellular morphological changes, however, remains unclear. In this study, we examined the mechanical role of nucleus-actin filament binding in morphological changes of fibroblasts exposed to cyclic stretching. After exposure to 10% cyclic stretching for 6h, fibroblasts transfected with nesprin-1-specific small interfering RNA showed fewer elongated shapes compared with non-transfected cells. To further examine the mechanical role of the nucleus and nucleus-bound actin filaments, we applied cyclic stretching to fibroblasts treated with Trichostatin A (TSA), which decreases nuclear stiffness and thereby reduces nucleus-binding actin filament tension. TSA-treatment was found to induce more rounded cellular shapes than those of non-treated cells under both static and cyclic stretching conditions. These results suggest that the tension of nucleus-bound actin filaments plays an important role in the formation of elongated fibroblasts under cyclic stretching and that nesprin-1 knockdown causes a decrease of tension in nucleus-associated actin filaments.

Analysis of a System for Automatic Control of the Process of Rewinding Filaments

A system which ensures stabilization of filament velocity and tension has been developed to control the process of rewinding filaments. The main stage in the system’s operation is the matching of the velocities with the prescribed tension and diameter of the package. A PID controller serves as the main control element for fine adjustment of the system. To optimize the parameters of the PID controller, the dynamic characteristics of the filament rewinding process were determined by experimentally identifying recorded data on transients and then approximating them with transfer functions.

Tension-Regulated Actin Severing Revealed by Surface-Free Single-Molecule Force Spectroscopy

Actin-severing proteins, such as gelsolin, are key regulators of actin filament turnover. Studies suggest that the severing process is regulated by mechanical forces in actin filaments. However, the specific types and levels of force involved are unknown because the existing methods for applying force on actin filaments require surface attachment. These attachments introduce a combination of tension, compression, torsion and shear. We apply a surface-free force spectroscopy method that uses a high-speed cross-slot hydrodynamic trap to generate pure tension in actin filaments. Buffer containing actin filaments or severing proteins flows in from two opposite directions and exits via the two orthogonal outlets to create an elongational flow field with a stagnation point in the center. As a result, filaments near the stagnation point are stretched by the viscous drag from the flow. In addition, the pressure in one of the outlet reservoirs is electronically controlled with a high-speed feedback algorithm to stabilize the actin filament at the stagnation point. This allows us to measure the severing activity of actin filaments under pure tension. We found that the severing rate by gelsolin was independent of tension up to 10pN. In comparison, a previous magnetic tweezers experiment that applies a mixture of tension and bending forces shows a positive correlation between severing rate and pulling force for gelsolin over the range of 0.1 to 4 pN. Our result suggests that changes at actin monomer interfaces due to bending rather than tension enhances the severing rate by gelsolin. By enabling us to applying pure tension rather than a mixture of forces, our method provides a powerful tool for understand and differentiating the driving forces of structural changes in filamentous biomolecules.

Dynamics of the capillary breakup of a bridge in an elastic fluid

The tension of self-thinning filaments occurring upon the capillary breakup of liquid bridges in viscoelastic fluids is measured. The solution of high-molecular polymers with the concentration of 0.1 to 1% are studied. The stresses occurring in the filaments are determined and the effect of the hydrodynamic interaction between the filaments and the adjoining macroscopic volume (droplet) is evaluated. A theoretical analysis of the fluid inflow into the droplet shows that in the case of filament extension the rheology of the polymer liquids under consideration is different from that under the singleaxis compression in droplet flow.

Stabilization of three-dimensional scroll waves and suppression of spatiotemporal chaos by heterogeneities.

Scroll waves in a three-dimensional medium with negative filament tension may break up and display spatiotemporal chaos. The presence of heterogeneities can influence the evolution of the medium, in particular scroll waves may pin to such heterogeneities. We show that as a result the medium may be stabilized by heterogeneities of a suitably chosen geometry. Thin rodlike heterogeneities suppress otherwise developing spatiotemporal chaos and additionally clear out already existing chaotic excitation patterns.

Spatial confinement causes lifetime enhancement and expansion of vortex rings with positive filament tension

We study the impact of spatial confinement on the dynamics of three-dimensional (3D) excitation vortices with circular filaments. In a chemically active medium we observe a decreased contraction rate of such scroll rings and even expanding ones, despite their positive filament tension. All experimentally observed regimes of spatially confined scroll ring evolution are reproduced by full 3D numerical integration of the underlying reaction–diffusion equations. Additionally, we propose a kinematical model that takes into account the interaction of the scroll ring with a no-flux boundary. Its predictions agree quantitatively with data obtained from simulations of the reaction–diffusion model.

Generalized Minimal Principle for Rotor Filaments

To a reaction-diffusion medium with an inhomogeneous anisotropic diffusion tensor D, we add a fourth spatial dimension such that the determinant of the diffusion tensor is constant in four dimensions. We propose a generalized minimal principle for rotor filaments, stating that the scroll wave filament strives to minimize its surface area in the higher-dimensional space. As a consequence, stationary scroll wave filaments in the original 3D medium are geodesic curves with respect to the metric tensor G=det(D)D(-1). The theory is confirmed by numerical simulations for positive and negative filament tension and a model with a non-stationary spiral core. We conclude that filaments in cardiac tissue with positive tension preferentially reside or anchor in regions where cardiac cells are less interconnected, such as portions of the cardiac wall with a large number of cleavage planes.

Drift of scroll waves in thin layers caused by thickness features: asymptotic theory and numerical simulations.

A scroll wave in a very thin layer of excitable medium is similar to a spiral wave, but its behavior is affected by the layer geometry. We identify the effect of sharp variations of the layer thickness, which is separate from filament tension and curvature-induced drifts described earlier. We outline a two-step asymptotic theory describing this effect, including asymptotics in the layer thickness and calculation of the drift of so-perturbed spiral waves using response functions. As specific examples, we consider drift of scrolls along thickness steps, ridges, ditches, and disk-shaped thickness variations. Asymptotic predictions agree with numerical simulations.

Mechanosensitivity of Formin-Actin Interactions

Cells are able to sense and adapt to their physical environment, translating mechanical inputs into chemical outcomes. The molecular basis for this fascinating property, referred to as “mechanotransduction”, remains largely unexplored. The actin cytoskeleton, which generates and transmits forces throughout the cell, is certainly a key player in this process. Here, I will focus on the situation where tension is applied to actin filaments that are interacting with formins at their barbed ends. In cells, formins are typically anchored to a substrate and thus undergo a mechanical force as the filament is put under tension. We have studied this situation in vitro, by manipulating individual filaments with a microfluidic flow, as they are nucleated and elongated by formin mDia1 anchored to the bottom of the microchamber. I will show how filament tension in the picoNewton range modulates the elongation rate of the actin filament, the processivity of the formin and its competition with other end-binding proteins. Quantitatively probing these mechanical properties also provides an interesting angle to test molecular models of formin-assisted filament elongation.

Drift of Scroll Wave Filaments in an Anisotropic Model of the Left Ventricle of the Human Heart.

Scroll waves are three-dimensional vortices which occur in excitable media. Their formation in the heart results in the onset of cardiac arrhythmias, and the dynamics of their filaments determine the arrhythmia type. Most studies of filament dynamics were performed in domains with simple geometries and generic description of the anisotropy of cardiac tissue. Recently, we developed an analytical model of fibre structure and anatomy of the left ventricle (LV) of the human heart. Here, we perform a systematic study of the dynamics of scroll wave filaments for the cases of positive and negative tension in this anatomical model. We study the various possible shapes of LV and different degree of anisotropy of cardiac tissue. We show that, for positive filament tension, the final position of scroll wave filament is mainly determined by the thickness of the myocardial wall but, however, anisotropy attracts the filament to the LV apex. For negative filament tension, the filament buckles, and for most cases, tends to the apex of the heart with no or slight dependency on the thickness of the LV. We discuss the mechanisms of the observed phenomena and their implications for cardiac arrhythmias.

Effect of surface treatment on surface characteristics of carbon fibers and interfacial bonding of epoxy resin composites

This paper aims to study the effect of aqueous ammonia etching, nitric acid oxidation and aqueous ammonia/nitric acid combined treatment on surface characteristics of carbon fibers (CFs) and interfacial bonding properties of CF/epoxy resin composites. The morphologies and chemical compositions of pristine CFs and treated CFs were characterized using a Scanning Electron Microscope (SEM), Atomic Force Microscopy (AFM), and X-ray Photoelectron Spectroscopy (XPS). It was found that the surface of treated CFs has greater roughness compared with the surface of pristine CFs; moreover, the surface roughness is at maximum with the aqueous ammonia/nitric acid combined treatment. No obvious change in chemical composition takes place with aqueous ammonia etching of CFs. On the other hand, the oxygen content on the surface of CFs increases with nitric acid oxidation and aqueous ammonia/nitric acid combined treatments. The effect of surface treatment on the wettability, single filament tension strength, and interfacial shear strength (IFSS) of CF/epoxy resin composites was measured. Results show that the wettability of CFs against distilled water improves with the surface treatment. The wettability optimum with the aqueous ammonia/nitric acid combined treatment. Compared with pristine CFs, there is only a slight loss in tension strength with the treated CFs. In addition, the IFSS value of composites increased greatly. The IFSS of aqueous ammonia/nitric acid treated carbon fibers reinforced with epoxy resin composites increases by 46.36 % compared with pristine ones, This resulted from the combined action of the increase in the carbon fiber surface roughness and surface oxygen containing functional groups.

The Effects of Environmental Temperature and Additional Pressure on the Bonding Characteristics of Co-cured Carbon/Epoxy-Aluminum Single Lap Joints

The aim of this study was to investigate the bonding characteristics of a filament wound structure with an aluminum mandrel under different temperature conditions. During the filament winding process, the filament tension generates pressure in the pre-wound plies and mandrel. To simulate pressure in the filament wound structure with a single lap joint specimen, a pressure-generating device has been introduced and applied to the bonding part of the specimen. The single lap joint, composed of a carbon/epoxy prepreg and aluminum beam, was fabricated by an autoclave degassing molding process, at a pressure of 0.7 MPa (absolute including vacuum) and maximum curing temperature of 135°C. The environmental temperature conditions of −20°C, 25°C, and 50°C were used in the experimental tests, in order to consider the service condition of passenger cars. The static bonding strength and fatigue life of the specimen under different temperature conditions with and without the additional pressures were measured and compared.

Structure and Dynamics of ER: Minimal Networks and Biophysical Constraints

The endoplasmic reticulum (ER) in live cells is a highly mobile network whose structure dynamically changes on a number of timescales. The role of such drastic changes in any system is unclear, although there are correlations with ER function. A better understanding of the fundamental biophysical constraints on the system will allow biologists to determine the effects of molecular factors on ER dynamics. Previous studies have identified potential static elements that the ER may remodel around. Here, we use these structural elements to assess biophysical principles behind the network dynamics. By analyzing imaging data of tobacco leaf epidermal cells under two different conditions, i.e., native state (control) and latrunculin B (treated), we show that the geometric structure and dynamics of ER networks can be understood in terms of minimal networks. Our results show that the ER network is well modeled as a locally minimal-length network between the static elements that potentially anchor the ER to the cell cortex over longer timescales; this network is perturbed by a mixture of random and deterministic forces. The network need not have globally minimum length; we observe cases where the local topology may change dynamically between different Euclidean Steiner network topologies. The networks in the treated cells are easier to quantify, because they are less dynamic (the treatment suppresses actin dynamics), but the same general features are found in control cells. Using a Langevin approach, we model the dynamics of the nonpersistent nodes and use this to show that the images can be used to estimate both local viscoelastic behavior of the cytoplasm and filament tension in the ER network. This means we can explain several aspects of the ER geometry in terms of biophysical principles.

Analysis of anchor-size effects on pinned scroll waves and measurement of filament rigidity.

Inert, spherical heterogeneities can pin three-dimensional scroll waves in the excitable Belousov-Zhabotinsky reaction. Three pinning sites cause initially circular rotation backbones to approach equilateral triangles. The resulting stationary shapes show convex deviations that increase with decreasing anchor radii. This dependence is interpreted as a transition between filament termination at large surfaces and true, local pinning of a continuous curve. The shapes of the filament segments are described by a hyperbolic cosine function which is predicted by kinematic theory that considers filament tension and rigidity. The latter value is measured as (1.0±0.7)×10-6 cm4/s.

Effective dynamics of twisted and curved scroll waves using virtual filaments

Scroll waves are three-dimensional excitation patterns that rotate around a central filament curve; they occur in many physical, biological, and chemical systems. We explicitly derive the equations of motion for scroll wave filaments in reaction-diffusion systems with isotropic diffusion up to third order in the filament's twist and curvature. The net drift components define at every instance of time a virtual filament which lies close to the instantaneous filament. Importantly, virtual filaments obey simpler, time-independent laws of motion which we analytically derive here and illustrate with numerical examples. Stability analysis of scroll waves is performed using virtual filaments, showing that filament curvature and twist add as quadratic terms to the nominal filament tension. Applications to oscillating chemical reactions and cardiac tissue are discussed.

Noddy, a Mouse Harboring a Missense Mutation in Protocadherin-15, Reveals the Impact of Disrupting a Critical Interaction Site between Tip-Link Cadherins in Inner Ear Hair Cells

In hair cells of the inner ear, sound or head movement increases tension in fine filaments termed tip links, which in turn convey force to mechanosensitive ion channels to open them. Tip links are formed by a tetramer of two cadherin proteins: protocadherin 15 (PCDH15) and cadherin 23 (CDH23), which have 11 and 27 extracellular cadherin (EC) repeats, respectively. Mutations in either protein cause inner ear disorders in mice and humans. We showed recently that these two cadherins bind tip-to-tip in a "handshake" mode that involves the EC1 and EC2 repeats of both proteins. However, a paucity of appropriate animal models has slowed our understanding both of the interaction and of how mutations of residues within the predicted interface compromise tip link integrity. Here, we present noddy, a new mouse model for hereditary deafness. Identified in a forward genetic screen, noddy homozygotes lack inner ear function. Mapping and sequencing showed that noddy mutant mice harbor an isoleucine-to-asparagine (I108N) mutation in the EC1 repeat of PCDH15. Residue I108 interacts with CDH23 EC2 in the handshake and its mutation impairs the interaction in vitro. The noddy mutation allowed us to determine the consequences of blocking the handshake in vivo: tip link formation and bundle morphology are disrupted, and mechanotransduction channels fail to remain open at rest. These results offer new insights into the interaction between PCDH15 and CDH23 and help explain the etiology of human deafness linked to mutations in the tip-link interface.

Buckling of Scroll Waves

A scroll wave in a sufficiently thin layer of an excitable medium with negative filament tension can be stable nevertheless due to filament rigidity. Above a certain critical thickness of the medium, such a scroll wave will have a tendency to deform into a buckled, precessing state. Experimentally this will be seen as meandering of the spiral wave on the surface, the amplitude of which grows with the thickness of the layer, until a breakup to scroll wave turbulence happens. We present a simplified theory for this phenomenon and illustrate it with numerical examples.

Tension of freely suspended fluid filaments

Stable fluid filaments with diameters of several micrometers and slenderness ratios well above 1000 are unique objects formed by some liquid crystalline phases of bent-core mesogens. We present a technique to determine filament tensions from their deflection under defined loads. A strong temperature dependence is observed, with a minimum near the clearing temperature. Both the nonlinear relation between filament tension and diameter and the substantial increase of the tension with lower temperatures indicate contributions of volume terms, in addition to surface capillary forces. We discuss a model that relates these bulk terms to elastic forces, originating from the undulated smectic layer structure. This model can explain the origin of the filament stability.

The interrelation between mechanical characteristics of contracting muscle, cross-bridge internal structure, and the mechanism of chemomechanical energy transduction

The cross-bridge working stroke is regarded as a continuous (without jumps) change of myosin head internal state under the action of a force exerted within the nucleotide-binding site. Involvement of a concept of continuous cross-bridge conformation enables discussion of the nature of the force propelling muscle, and the Coulomb repulsion of like-charged adenosine triphosphate (ATP) fragments ADP(2-) and P (i) (2-) can quite naturally be considered as the source of this force. Two entirely different types of working stroke termination are considered. Along with the fluctuation mechanism, which controls the working stroke duration t (w) at isometric contraction, another interrupt mechanism is initially taken into account. It is triggered when the lever arm shift amounts to the maximal value S≈11nm, the back door opens, and P(i) crashes out. As a result, t (w) becomes inversely proportional to the velocity v of sliding filaments t (w)≈S/v for a wide range of values of v. Principal features of the experimentally observed dependences of force, efficiency, and rate of heat production on velocity and ATP concentration can then be reproduced by fitting a single parameter: the velocity-independent time span t (r) between the termination of the last and beginning of the next working stroke. v becomes the principal variable of the model, and the muscle force changes under external load are determined by variations in v rather than in the tension of filaments. The Boltzmann equation for an ensemble of cross-bridges is obtained, and some collective effects are discussed.