Asymmetric Filament Research Articles

Cofilin drives rapid turnover and fluidization of entangled F-actin

The shape of most animal cells is controlled by the actin cortex, a thin network of dynamic actin filaments (F-actin) situated just beneath the plasma membrane. The cortex is held far from equilibrium by both active stresses and polymer turnover: Molecular motors drive deformations required for cell morphogenesis, while actin-filament disassembly dynamics relax stress and facilitate cortical remodeling. While many aspects of actin-cortex mechanics are well characterized, a mechanistic understanding of how nonequilibrium actin turnover contributes to stress relaxation is still lacking. To address this, we developed a reconstituted in vitro system of entangled F-actin, wherein the steady-state length and turnover rate of F-actin are controlled by the actin regulatory proteins cofilin, profilin, and formin, which sever, recycle, and assemble filaments, respectively. Cofilin-mediated severing accelerates the turnover and spatial reorganization of F-actin, without significant changes to filament length. We demonstrate that cofilin-mediated severing is a single-timescale mode of stress relaxation that tunes the low-frequency viscosity over two orders of magnitude. These findings serve as the foundation for understanding the mechanics of more physiological F-actin networks with turnover and inform an updated microscopic model of single-filament turnover. They also demonstrate that polymer activity, in the form of ATP hydrolysis on F-actin coupled to nucleotide-dependent cofilin binding, is sufficient to generate a form of active matter wherein asymmetric filament disassembly preserves filament number despite sustained severing.

Filament turnover tunes both force generation and dissipation to control long-range flows in a model actomyosin cortex.

Actomyosin-based cortical flow is a fundamental engine for cellular morphogenesis. Cortical flows are generated by cross-linked networks of actin filaments and myosin motors, in which active stress produced by motor activity is opposed by passive resistance to network deformation. Continuous flow requires local remodeling through crosslink unbinding and and/or filament disassembly. But how local remodeling tunes stress production and dissipation, and how this in turn shapes long range flow, remains poorly understood. Here, we study a computational model for a cross-linked network with active motors based on minimal requirements for production and dissipation of contractile stress: Asymmetric filament compliance, spatial heterogeneity of motor activity, reversible cross-links and filament turnover. We characterize how the production and dissipation of network stress depend, individually, on cross-link dynamics and filament turnover, and how these dependencies combine to determine overall rates of cortical flow. Our analysis predicts that filament turnover is required to maintain active stress against external resistance and steady state flow in response to external stress. Steady state stress increases with filament lifetime up to a characteristic time τm, then decreases with lifetime above τm. Effective viscosity increases with filament lifetime up to a characteristic time τc, and then becomes independent of filament lifetime and sharply dependent on crosslink dynamics. These individual dependencies of active stress and effective viscosity define multiple regimes of steady state flow. In particular our model predicts that when filament lifetimes are shorter than both τc and τm, the dependencies of effective viscosity and steady state stress on filament turnover cancel one another, such that flow speed is insensitive to filament turnover, and shows a simple dependence on motor activity and crosslink dynamics. These results provide a framework for understanding how animal cells tune cortical flow through local control of network remodeling.

Complementary resistive switching behaviors evolved from bipolar TiN/HfO2/Pt device

In this letter, the dynamic evolution of TiN/HfO2/Pt device from bipolar resistive switching (BRS) to complementary resistive switching (CRS) was reported. The device exhibits the uniform BRS with long retention, good endurance, and self-compliance characteristics after the asymmetric two-step electroforming. However, BRS of the device eventually transforms to CRS after the transitional processes through controlling the compliance current. Meanwhile, the effective barrier height rises up accordingly as the device evolves from BRS to CRS. These superior resistive switching performances of TiN/HfO2/Pt device here can be elucidated in views of evolution of asymmetric filament. This work confirms the intimate correlation and discrepancy between BRS and CRS, and also indicates the potential application of TiN/HfO2/Pt device for future ultra-dense resistive random access memory.

Structured illumination with particle averaging reveals novel roles for yeast centrosome components during duplication.

Duplication of the yeast centrosome (called the spindle pole body, SPB) is thought to occur through a series of discrete steps that culminate in insertion of the new SPB into the nuclear envelope (NE). To better understand this process, we developed a novel two-color structured illumination microscopy with single-particle averaging (SPA-SIM) approach to study the localization of all 18 SPB components during duplication using endogenously expressed fluorescent protein derivatives. The increased resolution and quantitative intensity information obtained using this method allowed us to demonstrate that SPB duplication begins by formation of an asymmetric Sfi1 filament at mitotic exit followed by Mps1-dependent assembly of a Spc29- and Spc42-dependent complex at its tip. Our observation that proteins involved in membrane insertion, such as Mps2, Bbp1, and Ndc1, also accumulate at the new SPB early in duplication suggests that SPB assembly and NE insertion are coupled events during SPB formation in wild-type cells.

A coronal wave and an asymmetric eruptive filament in SUMER, CDS, EIT, and TRACE co-observations

The objectives of the present study is to provide a better physical understanding of the complex inter-relation and evolution of several solar coronal features comprising a double-peak flare, a coronal dimming caused by a CME, a CME-driven compression, and a fast-mode wave. For the first time, the evolution of an asymmetric eruptive filament is analysed in simultaneous SUMER spectroscopic and TRACE and EIT imaging data. We use imaging observations from EIT and TRACE in the 195A channel and spectroscopic observations from the CDS in a rastering and SUMER in a sit-and-stare observing mode. The SUMER spectra cover spectral lines with formation temperatures from logT(K) ~ 4.0 to 6.1. Although the event was already analysed in two previous studies, our analysis brings a wealth of new information on the dynamics and physical properties of the observed phenomena. We found that the dynamic event is related to a complex flare with two distinct impulsive peaks, one according to the GOES classification as C1.1 and the second - C1.9. The first energy release triggers a fast-mode wave and a CME with a clear CME driven compression ahead of it. This activity is related to, or possibly caused, by an asymmetric filament eruption. The filament is observed to rise with its leading edge moving at a speed of ~300 km/s detected both in the SUMER and CDS data. The rest of the filament body moves at only ~150 km/s while untwisting. No signature is found of the fast-mode wave in the SUMER data, suggesting that the plasma disturbed by the wave had temperatures above 600 000 K. The erupting filament material is found to emit only in spectral lines at transition region temperatures. Earlier identification of a coronal response detected in the Mg X 609.79 A line is found to be caused by a blend from the O IV 609.83 A line.

(Invited) Stress-Induced Asymmetric Switching and Filament Instability in Electrochemical Memories

Resistive switching memory based on electrochemical reaction are receiving a great interest as a future technology for nonvolatile storage class memories with fast programming and low power operation. To achieve scalable and reliable electrochemical memories, the switching and stability mechanisms must be carefully understood. This work shows that the switching characteristics of electrochemical memory devices display a strong asymmetric behavior, which is attributed to the mechanical stress around the conductive filament (CF) in the set state. CF instability after pulsed set is also evidenced and explained in terms of the compressive stress. Stress relaxation is shown to significantly stabilize ECM, thus providing new guidelines to improve memory stability at low set/reset power.

Stress-Induced Asymmetric Switching and Filament Instability in Electrochemical Memories

The electrochemical memory (ECM) is one of the most promising technologies for storage class memories, thanks to low-power operation, fast switching and large on/off ratio. To develop scalable ECM, the switching mechanism and filament stability must be carefully understood. In this work, we show that the switching characteristics of Ag-GeS2are strongly asymmetric, which is interpreted by the mechanical stress around the conductive filament (CF) in the set state. CF instability after pulsed set is also evidenced and explained in terms of the compressive stress. Stress relaxation is shown to significantly stabilize ECM, thus providing new guidelines to improve stability at low set/reset power.Fig. 1a shows the measured I-V curves for ECM devices with GeS2 electrolyte and Ag active top electrode (TE). Set transition at positive voltage is due to the migration of ionized Ag from the TE, resulting in the formation of a conductive filament (CF) connecting TE and bottom electrode. The reset transition at negative voltage is due to the retraction of the CF back to the TE. By changing the compliance current IC during set, the CF size (hence resistance R) can be controlled. The switching curves in Fig. 1a are asymmetric in that the reset current Ireset is markedly lower than IC. Fig. 1b, c and d summarize R, Ireset and the ratio η = Ireset/IC as a function of IC, highlighting asymmetric switching with η < 1.Set/reset transitions are modeled by CF growth driven by ion migration according to the formula:dΦ/dt = Aexp(EA/kT), (1)where Φ is the CF effective diameter controlling R, A is a pre-exponential constant, k is the Boltzmann constant and T is the local effective temperature at the CF. The activation energy is given by EA = EA0 – αqV, where EA0is the energy barrier for migration, V is the voltage across the CF and a accounts for barrier lowering. Calculation results are shown in Fig. 1, in close agreement with data.To reproduce the asymmetric switching in Fig. 1, different barriers EA0,set = 0.95 eV and EA0,reset = 0.5 eV were assumed for set and reset, respectively. The different barriers can be understood by the gradient of the mechanical stress surrounding the CF in the set state, as described in Fig. 2. At set transition (V = Vset, see Fig. 2a), ionized defects migrate from the TE, causing CF growth and mechanical-stress buildup. The calculated stress in the set state (red = compressive, blue = tensile) is shown in Fig. 2. The stress contributes an elastic energy term U in Fig. 2b which opposes to further migration. On the other hand, U enhances defect migration back to the TE during reset, since this reduces the overall elastic energy (Fig. 2c). The stress contribution to EAthus reads:EA = EA0– αqV ± U, (2)where positive and negative signs of U apply to set (increases EA) and reset (reduced EA), respectively. The opposite contributions of U during set and reset cause asymmetric switching, namely Ireset < ICin Fig. 1.The model in Fig. 2 suggests that the set state is less stable than the reset state, due to the compressive stress favoring the spontaneous retraction of the CF. This is confirmed by fast set/reset I-V curves in Fig. 3, where the set/reset sweep time was tset = treset = 1 ms, while the delay between set and reset was increased from tdelay= 1 ms (a) to 10 s (b). The latter case indicate high R during the reset sweep due to spontaneous dissolution of the CF.CF instability is instead not observed after DC set, which can be understood by the stress relaxation due to prolonged Joule heating during the set process. This is confirmed by the measured Vreset as a function of the set pulse-width tset in Fig. 3c. The increase of Vreset at increasing tset indicates that the elastic energy U, which acts as an additional driving force of defect migration back to the TE during reset in Eq. (2), decreases with tset. On the one hand, Joule heating assists migration and CF growth, resulting in a decrease of R (Fig. 3d), while on the other hand leads to suppression of stress, hence of U, evidenced by the increase of Vreset. Calculations with Eqs. (1)-(2) and a time-dependent stress-relaxation model are shown in Fig. 3. These results support careful pulse optimization and stack engineering for suppressing stress effects and stabilizing the CF in ECM devices.

Switching mechanism of double forming process phenomenon in ZrOx/HfOy bilayer resistive switching memory structure with large endurance

In this Letter, the mechanism of double forming process phenomenon revealing in ZrO2/HfO2 bilayer resistive random access memory structure is investigated. This phenomenon caused by the formation of TiON interfacial layer can be well explained by using the energy band diagram. The TiON interfacial layer will be a tunneling barrier during the first forming process when a negative voltage applied on the device, while it will breakdown when applying a positive voltage. Besides, due to the double forming process, an asymmetric conductive filament with narrower size at ZrO2/HfO2 interface is formed in the device. The point for formation and rupture of the conductive filament can be confined at the ZrO2/HfO2 interface, and it will suppress the consumption of oxygen ions during endurance test. Therefore, high speed (40 ns) and large endurance (107 cycles) characteristics are achieved in this device structure.

A STUDY OF A FAILED CORONAL MASS EJECTION CORE ASSOCIATED WITH AN ASYMMETRIC FILAMENT ERUPTION

We present the multi-wavelength observations of asymmetric filament eruption, associated CME and coronal downflows on 2012 June 17-18 during 20:00-05:00 UT. We use SDO/AIA, STEREO-B/SECCHI observations to understand the filament eruption scenario and its kinematics. While LASCO C2 observations have been analyzed to study the kinematics of the CME and associated downflows. SDO/AIA limb observations show that the filament exhibits whipping like asymmetric eruption. STEREO/EUVI disk observations reveal a two ribbon flare underneath the south-eastern part of the filament that is most probably occurred due to reconnection process in the coronal magnetic field in the wake of the filament eruption. The whipping like filament eruption later gives a slow CME in which the leading edge and the core propagate respectively with the average speed of $\approx$ 540 km s$^{-1}$ and $\approx$ 126 km s$^{-1}$ as observed in the LASCO C2 coronagraph. The CME core formed by the eruptive flux-rope shows the outer coronal downflows with the average speed of $\approx$ 56 km s$^{-1}$ after reaching up to $\approx$4.33 $R_{\sun}$. Initially, the core decelerates with $\approx$ 48 m s$^{-2}$. The plasma first decelerates gradually up to the height of $\approx$4.33 $ R_{\sun}$ and then starts accelerating downward. We suggest a self-consistent model of a magnetic flux rope representing the magnetic structure of the CME core formed by eruptive filament that lost its previous stable equilibrium when reach at a critical height. With some reasonable parameters, and inherent physical conditions the model describes the non-radial ascending motion of the flux rope in the corona, its stopping at some height, and thereafter the downward motion, which are in good agreement with the observations.

MOTIONS OF HARD X-RAY SOURCES DURING AN ASYMMETRIC ERUPTION

Filament eruptions and hard X-ray (HXR) source motions are commonly observed in solar flares, which provides critical information on the coronal magnetic reconnection. This Letter reports an event on 2005 January 15, in which we found an asymmetric filament eruption and a subsequent coronal mass ejection together with complicated motions of HXR sources during the GOES-class X2.6 flare. The HXR sources initially converge to the magnetic polarity inversion line (PIL), and then move in directions either parallel or perpendicular to the PIL depending on the local field configuration. We distinguish the evolution of the HXR source motion in four phases and associate each of them with distinct regions of coronal magnetic fields as reconstructed using a non-linear force-free field extrapolation. It is found that the magnetic reconnection proceeds along the PIL toward the regions where the overlying field decreases with height more rapidly. It is also found that not only the perpendicular but the parallel motion of the HXR sources correlates well with the HXR lightcurve. These results are discussed in favor of the torus instability as an important factor in the eruptive process.

THREE-DIMENSIONAL SHAPE AND EVOLUTION OF TWO ERUPTIVE FILAMENTS

On 2009 September 26, a dramatic and large filament (LF) eruption and a small filament (SF) eruption were observed in the He II 304 A line by the two EUVI telescopes aboard the STEREO A and B spacecraft. The LF heads out into space and becomes the bright core of a gradual coronal mass ejection (CME), while the eruption of the SF is characterized by motions of the filament materials. Using stereoscopic analysis of EUVI data, we reconstruct the three-dimensional shape and evolution of two eruptive filaments. For the first time, we investigate the true velocities and accelerations of 12 points along the axis of the LF, and find that the velocity and acceleration vary with the measured location. The highest points among the 12 points are the fastest in the first half hour, and then the points at the low-latitude leg of the LF become the fastest. For the SF, it is an asymmetric whip-like filament eruption, and the downward motions of the material lead to the disappearance of the former high-latitude endpoint and the formation of a new low-latitude endpoint. Based on the temporal evolution of the two filaments, we infer that the two filaments lie in the same filament channel. By combining the EUVI, COR1, and COR2 data of STEREO A together, we find that there is no impulsive or fast acceleration in this event. It displays a weak and persistent acceleration for more than 17 hr. The average velocity and acceleration of the LF are 101.8 km s–1 and 2.9 m s–2, respectively. The filament eruptions are associated with a slow CME with an average velocity of 177.4 km s–1. The velocity of the CME is nearly 1.6 times as large as that of the filament material. This event is one example of a gradual filament eruption associated with a gradual CME. In addition, the moving direction of the LF changes from a non-radial to a nearly radial direction with a variation of inclination angle of nearly 382.

Three-dimensional reconstruction of the muscle thin filament. Single particle analysis of an asymmetric filament

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008