Coupling Of Filaments Research Articles

Intense laser filament-solid interactions from near-ultraviolet to mid-infrared.

Studies of high-power ultrashort laser pulse interaction with matter are not only of fundamental scientific interest, but are also highly relevant to applications in the domain of remote sensing. Here, we investigate the effect of laser wavelength on coupling of femtosecond laser filaments to solid targets. Three central wavelengths have been used to produce filaments: 0.4, 0.8, and 2.0 µm. We find that, unlike the case of conventional tight focusing, use of shorter wavelengths does not necessarily produce more efficient ablation. This is explained by increased multi-photon absorption arising in near-UV filamentation. Investigations of filament-induced plasma dynamics and its thermodynamic parameters provide the foundation for unveiling the interplay between wavelength-dependent filament ablation mechanisms. In this way, strategies to increase the sensitivity of material detection via this technique may be better understood, thereby improving the analytical performance in this class of applications.

The design of MACs (minimal actin cortices)

The actin cell cortex in eukaryotic cells is a key player in controlling and maintaining the shape of cells, and in driving major shape changes such as in cytokinesis. It is thereby constantly being remodeled. Cell shape changes require forces acting on membranes that are generated by the interplay of membrane coupled actin filaments and assemblies of myosin motors. Little is known about how their interaction regulates actin cell cortex remodeling and cell shape changes. Because of the vital importance of actin, myosin motors and the cell membrane, selective in vivo experiments and manipulations are often difficult to perform or not feasible. Thus, the intelligent design of minimal in vitro systems for actin-myosin-membrane interactions could pave a way for investigating actin cell cortex mechanics in a detailed and quantitative manner. Here, we present and discuss the design of several bottom-up in vitro systems accomplishing the coupling of actin filaments to artificial membranes, where key parameters such as actin densities and membrane properties can be varied in a controlled manner. Insights gained from these in vitro systems may help to uncover fundamental principles of how exactly actin-myosin-membrane interactions govern actin cortex remodeling and membrane properties for cell shape changes. © 2013 Wiley Periodicals, Inc.

Spatiotemporal Dynamics of Actomyosin Networks

Rhodamine–phalloidin-labeled actin filaments were visualized gliding over a skeletal heavy meromyosin (HMM)-coated surface. Experiments at low filament densities showed that when two filaments collided, their paths were affected in a manner that depended on collision angle. Some collisions resulted in complete alignment of the filament paths; in others, the filaments crossed over one another. Filament crossover or alignment was equally probable at ∼40° contact angle. Filaments often underwent significant bending during collision and analysis of filament shape indicated an energy requirement of ∼13 kBT. Experiments were performed over a wide range of HMM surface density and actin filament bulk concentration. Actin filament gliding speed and path persistence plateaued above a critical HMM surface density, and at high (micromolar) actin filament concentrations, filament motion became dramatically aligned in a common direction. Spatiotemporal features of alignment behavior were determined by correlation analysis, supported by simulations. The thermal drift of individual filament tracks was suppressed as the population became more oriented. Spatial correlation analysis revealed that long-range alignment was due to incremental recruitment rather than fusion of locally ordered seed domains. The global alignment of filament movement, described by an “order parameter,” peaked at optimal actin concentrations and myosin surface densities, in contrast to previous predictions of a critical phase transition. Either hydrodynamic coupling or exchange of filaments between the surface bound and adjacent bulk phase layers might degrade order at high actin filament concentration, and high HMM surface densities might decrease alignment probability during collisions. Our results are compatible with generation of long-range order from mechanical interaction between individual actin filaments. Furthermore, we show that randomly oriented myosin motors align relatively short, submicrometer actin filaments into motile surface domains that extend over many tens of micrometers and these patterns persist for several minutes.

Finite Element Simulations of Twisted NbTi Conductors

Low-temperature superconducting wires, NbTi and Nb3Sn, designed for ac applications, such as CERN and ITER magnets, are composed of twisted multifilament structures. Under time-varying applied magnetic field, twisting decreases the induced electromotive forces between the filaments and is therefore an effective method to reduce interfilamentary coupling. In order to study coupling losses computationally with high precision, 3-D numerical models are needed. In this work, we use 3-D finite-element method simulations to study hysteresis and coupling losses in NbTi superconductors. We investigate the effect of twist pitch on ac losses. In practice, NbTi wires cannot be studied at the filament level due to the extremely complex geometries. However, the manufacturing of these wires is done by using filament bundles. Therefore, we consider wires that consist of homogenized filament bundles embedded in normal conducting matrix. In particular, we consider the effect of barriers around filament bundles and how filaments should be arranged in bundles to minimize the losses. The simulations show that the qualitative behavior of the model is consistent with the analytical results and it can be used, e.g., in optimization processes, where a comparison of wire geometries is needed. Additionally, when considering the coupling of filaments, the barrier plays a very important role in minimizing the losses.

Quantitative analysis of extension–torsion coupling of actin filaments

Actin filaments have a double-helix structure consisting of globular actin molecules. In many mechanical cellular activities, such as cell movement, division, and shape control, modulation of the extensional and torsional dynamics of the filament has been linked to regulatory actin-binding protein functions. Therefore, it is important to quantitatively evaluate extension–torsion coupling of filament to better understand the actin filament dynamics. In the present study, the extension–torsion coupling was investigated using molecular dynamics simulations. We constructed a model for the actin filament consisting of 14 actin subunits in an ionic solvent as a minimal functional unit, and analyzed longitudinal and twisting Brownian motions of the filament. We then derived the expected value of energy associated with extension and torsion at equilibrium, and evaluated the extension–torsion stiffness of the filament from the thermal fluctuations obtained from the MD simulations. The results demonstrated that as the analyzed sampling-window duration was increased, the extension–torsion coupling stiffness evaluated on a nanosecond scale tended to converge to a value of 7.6×10−11N. The results obtained from this study will contribute to the understanding of biomechanical events, under mechanical tension and torque, involving extension–torsion coupling of filaments.

Regulation of Hip1r by epsin controls the temporal and spatial coupling of actin filaments to clathrin-coated pits

Recently, it has become clear that the actin cytoskeleton is involved in clathrin-mediated endocytosis. During clathrin-mediated endocytosis, clathrin triskelions and adaptor proteins assemble into lattices, forming clathrin-coated pits. These coated pits invaginate and detach from the membrane, a process that requires dynamic actin polymerization. We found an unexpected role for the clathrin adaptor epsin in regulating actin dynamics during this late stage of coated vesicle formation. In Dictyostelium cells, epsin is required for both the membrane recruitment and phosphorylation of the actin- and clathrin-binding protein Hip1r. Epsin-null and Hip1r-null cells exhibit deficiencies in the timing and organization of actin filaments at clathrin-coated pits. Consequently, clathrin structures persist on the membranes of epsin and Hip1r mutants and the internalization of clathrin structures is delayed. We conclude that epsin works with Hip1r to regulate actin dynamics by controlling the spatial and temporal coupling of actin filaments to clathrin-coated pits. Specific residues in the ENTH domain of epsin that are required for the membrane recruitment and phosphorylation of Hip1r are also required for normal actin and clathrin dynamics at the plasma membrane. We propose that epsin promotes the membrane recruitment and phosphorylation of Hip1r, which in turn regulates actin polymerization at clathrin-coated pits.

A model for bidirectional traffic of cytoskeletal motors

We introduce a stochastic lattice gas model including two particle species and two parallellanes, one of which comprises exclusion interaction and directed motion while theother shows no exclusion interaction and unbiased diffusion, thus mimicking amicrotubule filament and the surrounding solution. At a high binding affinity to thefilament, jam-like situations dominate the system’s behaviour. We approximatedthe fundamental process of position exchange of two particles. In the case of amany-particle system, we were able to identify one regime in which the system is ratherhomogeneous with only small accumulations of particles and another regime in which asignificant fraction of all particles accumulate in the same cluster. Numerical dataindicate that this cluster formation will occur at all densities for large systemsizes. Coupling of several filaments leads to an increased cluster formation ascompared to the uncoupled system, suggesting that efficient bidirectional transport onone-dimensional filaments relies on long-ranged interactions and track formation.

Nonlinear assembly kinetics and mechanical properties of biopolymers

This paper discusses the role of nonlinearities in the physical description of key biomolecules that participate in crucial subcellular processes, namely actin, microtubules and ions crowding around these filaments. The assembly kinetics of actin is that of a nonlinear process that is governed by coupled nonlinear equations involving monomer concentration and filament number as the dynamical variables. The dendritic growth of actin networks in cell motility phenomena is described by the coupling of actin filaments to the protein Arp2/3. We then discuss how coupled differential equations describing the interactions between ions in solution and the filament they surround can lead to solitonic signal transmission. We also investigate the role of nonlinear dynamics in the formation of microtubules. Space-flight laboratory experiments have shown that the self-organization of microtubules is sensitive to gravitational conditions. We propose a model of self-organization of microtubules in a gravitational field based on the dominant chemical kinetics. The pattern formation of microtubule concentration is obtained in terms of a moving kink. Finally, we present a model of elastic properties of microtubules describing a microtubule as an elastic rod. We found that when the microtubule is subjected to bending forces, the tangent angle satisfies a Sine-Gordon equation whose solutions describe kink and anti-kink bending modes that may propagate at a range of velocities along the length of the microtubule.

Visualization and force measurement of branching by Arp2/3 complex and N-WASP in actin filament

To determine whether the Arp2/3 complex activated by N-WASP (VCA) branches actin filaments at the side (side branching), or at the barbed (B-)end (end branching) of the mother filaments, we have directly observed the branching process of actin filaments and examined single-molecule unbinding under optical microscope. We found that side branching was predominant, though not exclusive. At the initial stage of polymerization, the branching at the B-end occurred and subsequently the side branching started to occur. In either type of branching, the mother and daughter filaments elongated at nearly the same rate (growing type). Independently of the stage of polymerization, branching due to the direct coupling of filaments with an acute angle to the mother filaments (a coupling type) occurred. Phalloidin suppressed the growing type of branching but not the coupling type, implying that actin monomers are required for the former but not the latter. We found, by single molecule measurements using optical tweezers, that the Arp2/3 complex attaches to the side of actin filaments and the N-WASP appears to detach from the actin–Arp2/3 complex at 6–7 pN.

Rapid turnover of actin in dendritic spines and its regulation by activity.

Dendritic spines are motile structures that contain high concentrations of filamentous actin. Using hippocampal neurons expressing fluorescent actin and the method of fluorescence recovery after photobleaching, we found that 85 +/- 2% of actin in the spine was dynamic, with a turnover time of 44.2 +/- 4.0 s. The rapid turnover is not compatible with current models invoking a large population of stable filaments and static coupling of filaments to postsynaptic components. Low-frequency stimulation known to induce long-term depression in these neurons stabilized nearly half the dynamic actin in the spine. This effect depended on the activation of N-methyl-D-aspartate (NMDA) receptors and the influx of calcium. In neurons from mice lacking gelsolin, a calcium-dependent actin-binding protein, activity-dependent stabilization of actin was impaired. Our studies provide new information on the kinetics of actin turnover in spines, its regulation by neural activity and the mechanisms involved in this regulation.

Membrane blebbing during apoptosis results from caspase-mediated activation of ROCK I.

The execution phase of apoptosis is characterized by marked changes in cell morphology that include contraction and membrane blebbing. The actin-myosin system has been proposed to be the source of contractile force that drives bleb formation, although the biochemical pathway that promotes actin-myosin contractility during apoptosis has not been identified. Here we show that the Rho effector protein ROCK I, which contributes to phosphorylation of myosin light-chains, myosin ATPase activity and coupling of actin-myosin filaments to the plasma membrane, is cleaved during apoptosis to generate a truncated active form. The activity of ROCK proteins is both necessary and sufficient for formation of membrane blebs and for re-localization of fragmented DNA into blebs and apoptotic bodies.

D.c. magnetization study of fine-filamentary NbTi superconductors

Measurements of d.c. magnetization M( H) and magnetic susceptibility χ( T) in perpendicular and parallel magnetic fields were performed by a SQUID magnetometer on four multifilamentary NbTi wires with a Cu matrix and a superconducting filament diameter ranging from 11.8 μm up to 0.54 μm (these samples belong to the VAMAS reference samples). Anomalies in the χ( T) and initial M( H) curves were observed in samples having lower filament diameters in the perpendicular magnetic field. The initial linear M(H) slope of these samples corresponds to perfect diamagnetism of their strands, indicating proximity effect coupling of the NbTi filaments. At higher fields, a second linear slope of M( H) was observed, corresponding to diamagnetism of individual NbTi filaments. From this slope, using the M( H) relation for the Meissner state of a cylinder in a perpendicular magnetic field, the London penetration depth λ was determined. Hysteretic effects were observed for samples having lower filament diameters in fields corresponding to the second linear M( H) slope, indicating a complex proximity effect pinning structure within the strands. Deviation from linearity of the two linear M( H) parts allows us to evaluate H c1 of the strands and NbTi filaments. Estimated H c1 values from the Ginzburg-Landau theory were lower than the measured values. Measurements in the parallel magnetic field did not indicate any proximity effect coupling within strands, with the exception of the sample with a filament diameter of 0.54 μm, where H c1 values were quite close to those calculated by the Ginzburg-Landau theory.

Coupling of (Ti–Nb) Filaments in a Superconducting Multifilament Composite Conductor

Magnetization measurements were performed on variously prepared samples of multifilament (Ti–Nb) and Cu composites. All measurements were performed at 4.2°K, utilizing peak magnetic fields of 50 kG. The magnetization data were obtained in a manner similar to that described in earlier papers. The impetus for these particular measurements was the hypothesis of P. F. Smith in regard to the coupling of the small filaments of superconductors when they are embedded in a low-resistivity matrix such as Cu. This coupling, of course, is a function of ∂Ha/∂t, but for very long lengths (e.g., magnets), or even short lengths in the case of well annealed Cu, the time constant for the decoupling is extremely long. The coupling, it was suggested by Smith, could be reduced by increasing the resistance of the matrix material, and then twisting the composite, the equivalent of ``transposition'' of the individual filaments in a two-dimensional case. Various resistance layers are formed around each strand by the proper heat treatment of these multifilament composites. The filaments of the sample composites were found to be decoupled to varying degrees depending on applied magnetic field, sample length, and matrix resistivity.