Array Of Filaments Research Articles

The interaction of polarized microwaves with planar arrays of femtosecond laser-produced plasma filaments in air

The interaction of polarized microwaves with subwavelength arrays of parallel plasma filaments, such as those produced by the propagation of high-power femtosecond laser pulses in ambient air, was investigated by calculating the reflection and transmission coefficients as a function of the incidence angles using the finite-difference time-domain (FDTD) method. The time evolution of these coefficients was calculated and compared with experiments. It is found that the plasma filaments array becomes transparent when the polarization of the microwave radiation is perpendicular to the filaments axis, regardless the incidence angle of the microwave with respect to the filaments, except near grazing incidence. Increasing the filaments electron density or diameter, or decreasing the electron collision frequency or filaments spacing, decreases the transmission and increases the reflection. Transmission decreases when increasing the number of filament layers while reflection remains unchanged as the number of filament layers exceeds a given number (∼3 in our case). Transmission slightly increases when disorder is introduced in the filament arrays. The detailed calculation results are compared with those obtained from the simple birefringent slab model, which provides a convenient framework to calculate approximately the properties of filament arrays.

Laser Direct-Write for Low AC Loss High Temperature Superconductor Applications

The use of high temperature superconductors (HTS) in applications such as power cables, fault current limiters, and electromagnets, has seen rapid growth in recent years. While HTS exhibit zero dc resistance, appreciable ac losses are observed when the HTS are subject to an applied ac magnetic field. By dividing the HTS films into thin filament arrays and creating periodic metallic cross-overs to allow for flux penetration, the ac loss of the HTS can be significantly reduced. The HTS, YBa2Cu3O7-x (YBCO), films were prepared from the metal organic decomposition of a trifluoroacetate-based (TFA) precursor solution onto buffered substrates, manufactured by AMSC. To deposit the TFA solutions onto the buffered substrates, a new laser direct-write (LDW) technique was developed. LDW is a non-contact, non-lithographic process that relies on the mask-less printing of functional materials, such as metallic inks. The transferred TFA precursor filaments were then decomposed and subsequently characterized. In addition to the deposition of TFA precursors onto buffered substrates, LDW was used to print silver lines for metallic crossovers onto HTS tapes. These results show LDW to be a promising technique for the fabrication and modification of high temperature superconductors.

Rupture and Contraction of Crosslinked Actin Networks by Myosin Motor Activity

Self-organized contractile arrays of actin filaments and myosin motors drive cell division, migration, and tissue morphogenesis. Biophysical studies have provided detailed mechanistic insights into the mechanisms of force production by individual motor molecule. However, it is not well understood how motors and actin filaments collectively self-organize into force-generating arrays. It is for instance poorly understood how network connectivity (or crosslinking) influences active contractility. We addressed this problem by reconstituting cell-free model systems from purified actin, myosin, and actin crosslinking proteins. By studying motor-driven activity over a broad range of network connectivities, we discovered that myosin motors contract actin networks into clusters that exhibit a scale-free distribution of sizes, characteristic of a critical state. Surprisingly, this critical behavior occurs over a broad range of network connectivities. To explain this robustness, we performed simulations of contractile networks taking into account network restructuring: motors reduce connectivity by promoting crosslink unbinding. We demonstrate that this coupling between activity and connectivity drives initially well-connected networks to a critically connected state. This model provides new avenues to understand contraction and rupture phenomena occurring during cell and tissue morphogenesis.

High-Resolution Imaging of Endocytosed EGF-Quantum Dots Reveals Cellular Factors Regulating the Transitions between the Directed and Diffusive Movement of Early Endosomes

Following endocytosis, cargo in early endosomes travels tens of microns on an array of actin filaments and microtubules (MTs) to reach lysosomes in the perinuclear region for degradation. To understand how early endosomes coordinate multiple types of actin- and MT-based motors to achieve this long-distance transport, we studied their motions by live-cell imaging. Low-angle oblique, simultaneous multicolor acquisition microscopy allowed us to follow an endosomal cargo, endocytosed epidermal growth factor attached to quantum dots (EGF-Qdots), in epithelial cells relative to other cellular markers with high temporal (50ms) and spatial (20nm) resolution. We developed objective criteria to parse these complex trajectories into different types of motions. The motion of the majority of EGF-Qdots is classified as partially confined by mean-squared displacement (MSD) analysis. Treatment of cells with drugs that depolymerize (latrunculin B) or stabilize (jasplakinolide) actin revealed that the actin network induces this confinement. Net transport to the perinuclear region is achieved through bursts of rapid (up to 4μm/s) motion on MTs requiring dynein/dynactin. This directed motion is punctuated by pauses which are diffusive over 2.25s periods. However, immediately after directed motion, the diffusive motion is preferentially aligned with the MT axis, indicating the existence of a shorter time-scale transition period on the MT before free diffusion begins. We also correlated the changes in early endosome motion with the position of other organelles and the cytoskeleton. Pauses in directed movement spatially correlate with regions of dense MTs as well as other early endosomes and the endoplasmic reticulum, suggesting that early endosomes interact with these cellular features during their transport.Supported by NIH P01GM087253.

Nanoconfinement of guest materials by helical nanofilament networks of bent-core mesogens

The helical nanofilament (HNF) liquid crystal phase is a complex, hierarchical self-assembly in which bent-core molecules cooled from the isotropic melt form dense periodic arrays of nanoscale filaments of twisted smectic layers, a structure driven by an inherent tendency of the layers to be locally saddle-splayed. Here we report the results of a study of mixtures of an HNF-forming bent-core liquid crystal with a variety of organic guest molecules, showing that the high temperature, isotropic phase is completely mixed but that upon cooling the filaments form and grow, expelling the guest material from their internal volume. The nanofilaments form a network which acts as a porous nanoconfinement medium of large internal area (∼100 m2 cm−3), with the guest material confined to nanoscale interstitial volumes between the filaments. Such helical nanofilament networks represent a robust platform for potential applications in chiral separations, asymmetric synthesis, and photonic metamaterials.

CIP4 coordinates with phospholipids and actin-associated proteins to localize to the protruding edge and produce actin ribs and veils

Cdc42-interacting protein 4 (CIP4), a member of the F-BAR family of proteins, plays important roles in a variety of cellular events by regulating both membrane and actin dynamics. In many cell types, CIP4 functions in vesicle formation, endocytosis and membrane tubulation. However, recent data indicate that CIP4 is also involved in protrusion in some cell types, including cancer cells (lamellipodia and invadopodia) and neurons (ribbed lamellipodia and veils). In neurons, CIP4 localizes specifically to extending protrusions and functions to limit neurite outgrowth early in development. The mechanism by which CIP4 localizes to the protruding edge membrane and induces lamellipodial/veil protrusion and actin rib formation is not known. Here, we show that CIP4 localization to the protruding edge of neurons is dependent on both the phospholipid content of the plasma membrane and the underlying organization of actin filaments. Inhibiting phosphatidylinositol (3,4,5)-trisphosphate (PIP3) production decreases CIP4 at the membrane. CIP4 localization to the protruding edge is also dependent on Rac1/WAVE1, rather than Cdc42/N-WASP. Capping actin filaments with low concentrations of cytochalasin D or by overexpressing capping protein dramatically decreases CIP4 at the protruding edge, whereas inactivating Arp2/3 drives CIP4 to the protruding edge. We also demonstrate that CIP4 dynamically colocalizes with Ena/VASP and DAAM1, two proteins known to induce unbranched actin filament arrays and play important roles in neuronal development. Together, this is the first study to show that the localization of an F-BAR protein depends on both actin filament architecture and phospholipids at the protruding edge of developing neurons.

Length Regulation of Actin Filament Arrays

The cytoskeleton controls the movements of cellular substructures and governs dynamic changes in the cell shape. A key question in cell biology is how cytoskeletal structures are established and maintained. We use a combination of experiment and theory to develop quantitative models for describing the regulation of the actin cytoskeleton size and shape. In particular, we focus on actin cables in budding yeast cells, structures which are used for intracellular transport that is essential for polarized cell growth. In yeast, actin cables grow to span the entire length of the mother cell compartment and contour its curved surface. The regulation of cable length is essential because overgrowth of the cables causes misdirection of intracellular transport.Experiments suggest that key physical parameters in actin cable formation, such as the rates of assembly and disassembly of the actin polymers are controlled by specific actin-binding proteins. There is substantial amount of information about these individual proteins, but how their functions are integrated to produce the desired cable morphology is not well understood. Among the key proteins are formins, which catalyze both the nucleation and elongation of actin filaments. A recent experiment showed that another protein Smy1 binds to formins to decrease the actin elongation rate and that this interaction is critical for maintaining proper actin cable length and shape. Further, it was shown that the Smy1 proteins are transported by myosinV on cables to the bud neck, where the formins are anchored, suggesting a feedback mechanism that makes assembly rate length-dependent. Incorporating this mechanism, we compute the probability distribution of cable lengths as a function of parameters such as Smy1 binding strength and speed of myosin motors that deliver it to the formin. These results provide experimentally testable predictions of our proposed model of cable length control.

Topological defects, surface geometry and cohesive energy of twisted filament bundles

Cohesive assemblies of filaments are a common structural motif found in diverse contexts, ranging from biological materials such as fibrous proteins, to artificial materials such as carbon nanotube ropes and micropatterned filament arrays. In this paper, we analyze the complex dependence of cohesive energy on twist, a key structural parameter of both self-assembled and fabricated filament bundles. Based on the analysis of simulated ground states of cohesive bundles, we show that the non-linear influence of twist derives from two distinct geometric features of twisted bundles: (i) the geometrical frustration of inter-filament packing in the bundle cross-section; and (ii) the evolution of the surface geometry of bundles with twist, which dictates the cohesive cost of non-contacting filaments at the surface. Packing frustration in the bundle core gives rise to the appearance of a universal sequence of topological defects, excess 5-fold disclinations, with increasing twist, while the evolution of filament contact at the surface of the bundle generically favors twisted geometries for sufficiently long filaments. Our analysis of both continuum and discrete models of filament bundles shows that, even in the absence of external torque or intrinsic chirality, cohesive energy universally favors twisted ground states above a critical (length/radius) aspect ratio and below a critical filament stiffness threshold.

Power output of skinned skeletal muscle fibres from the cheetah (Acinonyx jubatus)

Muscle samples were taken from the gluteus, semitendinosus and longissimus muscles of a captive cheetah immediately after euthanasia. Fibres were 'skinned' to remove all membranes, leaving the contractile filament array intact and functional. Segments of skinned fibres from these cheetah muscles and from rabbit psoas muscle were activated at 20°C by a temperature-jump protocol. Step and ramp length changes were imposed after active stress had developed. The stiffness of the non-contractile ends of the fibres (series elastic component) was measured at two different stress values in each fibre; stiffness was strongly dependent on stress. Using these stiffness values, the speed of shortening of the contractile component was evaluated, and hence the power it was producing. Fibres were analysed for myosin heavy chain content using gel electrophoresis, and identified as either slow (type I) or fast (type II). The power output of cheetah type II fibre segments was 92.5±4.3 W kg(-1) (mean ± s.e., 14 fibres) during shortening at relative stress 0.15 (the stress during shortening/isometric stress). For rabbit psoas fibre segments (presumably type IIX) the corresponding value was significantly higher (P<0.001), 119.7±6.2 W kg(-1) (mean ± s.e., 7 fibres). These values are our best estimates of the maximum power output under the conditions used here. Thus, the contractile filament power from cheetah was less than that of rabbit when maximally activated at 20°C, and does not account for the superior locomotor performance of the cheetah.

Broadband and tunable mid-infrared laser source based on a transversal array of chirped filaments

A scheme for the generation of broadband and tunable mid-infrared pulses based on two-color filamentation in air is presented. The mid-infrared pulses, spanning the spectral region between 4 and 6.5 μm, result from a third-order nonlinear interaction between two short pulses, one at 800 nm and the other at 437 nm. The latter was created by frequency doubling a Raman-shifted pulse at 873 nm amplified in a chirped-pulse Raman amplifier. Tunability of this mid-infrared laser source was provided by introducing spatial chirp across the fundamental pulse profile which created a transversal array of chirped filaments.

Evidence for a cell cycle checkpoint that senses branched actin in the lamellipodium

Recent evidence indicates that branched actin might control cell progression through G1 in addition to lamellipodium protrusion.

Insights into cell division using Listeria monocytogenes infections of PtK2 renal epithelial cells

The assembly of actin into a cleavage furrow is accompanied by disassembly of the interphase actin cytoskeleton. A variation of this actin filament disassembly/assembly cycle is seen during cell division in PtK2 cells infected with the intracellular pathogen, Listeria monocytogenes, where F-actin associates with the bacteria either as a halo surrounding nonmoving bacteria, or as an array of filaments that encases the sides of moving baceteria and extends behind them like a tail. The moving Listeria are found both in the cytoplasm and in the distal ends of undulating filopodia. When infected cells enter mitosis, the distribution of moving and stationary bacteria changes. In the transition from prophase to metaphase, there is a decrease in the number of bacteria with tails of actin in the cytoplasm. The nonmoving bacteria surrounded with F-actin are excluded from the mitotic spindle and moving bacteria are seldom seen in the cytoplasm during mitosis, although small thin filopodia cluster at the edges of the cells. After completion of cytokinesis, strong tail reformation first becomes obvious in the filopodia with Listeria moving back into the cytoplasm as the daughter cells spread. In summary, the disassembly and reassembly of actin tails extending from Listeria in dividing cells is a variation of the changes in actin organization produced by stress fiber and myofibril disassembly/assembly cycles during cell division. We suggest that the same unknown factors that regulate the disassembly/assembly of stress fibers and myofibrils during mitosis and post cytokinesis also affect the movement of Listeria inside mitotic cells.

Capping Protein Modulates the Dynamic Behavior of Actin Filaments in Response to Phosphatidic Acid inArabidopsis

Remodeling of actin filament arrays in response to biotic and abiotic stimuli is thought to require precise control over the generation and availability of filament ends. Heterodimeric capping protein (CP) is an abundant filament capper, and its activity is inhibited by membrane signaling phospholipids in vitro. How exactly CP modulates the properties of filament ends in cells and whether its activity is coordinated by phospholipids in vivo is not well understood. By observing directly the dynamic behavior of individual filament ends in the cortical array of living Arabidopsis thaliana epidermal cells, we dissected the contribution of CP to actin organization and dynamics in response to the signaling phospholipid, phosphatidic acid (PA). Here, we examined three cp knockdown mutants and found that reduced CP levels resulted in more dynamic activity at filament ends, and this significantly enhanced filament-filament annealing and filament elongation from free ends. The cp mutants also exhibited more dense actin filament arrays. Treatment of wild-type cells with exogenous PA phenocopied the actin-based defects in cp mutants, with an increase in the density of filament arrays and enhanced annealing frequency. These cytoskeletal responses to exogenous PA were completely abrogated in cp mutants. Our data provide compelling genetic evidence that the end-capping activity of CP is inhibited by membrane signaling lipids in eukaryotic cells. Specifically, CP acts as a PA biosensor and key transducer of fluxes in membrane signaling phospholipids into changes in actin cytoskeleton dynamics.

Tuning the Conditions for the Deposition of Nanocrystalline Diamond by Hot Filament Chemical Vapour Deposition

Although large focus has been placed into the deposition of nanocrystalline and ultra-nanocrystalline diamond films, most of this research uses microwave plasma assisted CVD systems. However, the growth conditions used in microwave systems cannot be directly used in hot-filament CVD systems. This paper, aims to enlarge the knowledge of the diamond film depositing process. H2/CH4/Ar gas mixtures have been used to deposit micro, nano and ultra-nanocrystalline diamond films by hot-filament CVD systems. Additionally, the distance between the filaments array and the substrate was varied, in order to observe its effect and consequently the effect of a lower substrate temperature in the nucleation density and deposition. All the samples were characterized for microstructure and quality, using scanning electron microscopy and Raman spectroscopy.

Nanoscale Strain-Hardening of Keratin Fibres

Mammalian appendages such as hair, quill and wool have a unique structure composed of a cuticle, a cortex and a medulla. The cortex, responsible for the mechanical properties of the fibers, is an assemblage of spindle-shaped keratinized cells bound together by a lipid/protein sandwich called the cell membrane complex. Each cell is itself an assembly of macrofibrils around 300 nm in diameter that are paracrystalline arrays of keratin intermediate filaments embedded in a sulfur-rich protein matrix. Each macrofibril is also attached to its neighbors by a cell membrane complex. In this study, we combined atomic force microscopy based nano-indentation with peak-force imaging to study the nanomechanical properties of macrofibrils perpendicular to their axis. For indentation depths in the 200 to 500 nm range we observed a decrease of the dynamic elastic modulus at 1 Hz with increasing depth. This yielded an estimate of 1.6GPa for the lateral modulus at 1 Hz of porcupine quill’s macrofibrils. Using the same data we also estimated the dynamic elastic modulus at 1 Hz of the cell membrane complex surrounding each macrofibril, i.e., 13GPa. A similar estimate was obtained independently through elastic maps of the macrofibrils surface obtained in peak-force mode at 1 kHz. Furthermore, the macrofibrillar texture of the cortical cells was clearly identified on the elasticity maps, with the boundaries between macrofibrils being 40–50% stiffer than the macrofibrils themselves. Elasticity maps after indentation also revealed a local increase in dynamic elastic modulus over time indicative of a relaxation induced strain hardening that could be explained in term of a α-helix to β-sheet transition within the macrofibrils.

A Rho GTPase Signal Treadmill Backs a Contractile Array

Contractile arrays of actin filaments (F-actin) and myosin-2 power diverse biological processes. Contractile array formation is stimulated by the Rho GTPases Rho and Cdc42; after assembly, array movement is thought to result from contraction itself. Contractile array movement and GTPase activity were analyzed during cellular wound repair, in which arrays close in association with zones of Rho and Cdc42 activity. Remarkably, contraction suppression prevents translocation of F-actin and myosin-2 without preventing array or zone closure. Closure is driven by an underlying "signal treadmill" in which the GTPases are preferentially activated at the leading edges and preferentially lost from the trailing edges of their zones. Treadmill organization requires myosin-2-powered contraction and F-actin turnover. Thus, directional gradients in Rho GTPase turnover impart directional information to contractile arrays, and proper functioning of these gradients is dependent on both contraction and F-actin turnover.

Mutations in the Sensitive Giant Titin Result in a Broken Heart

### Truncations of Titin Causing Dilated Cardiomyopathy Herman et al N Engl J Med. 2012;366:619–628. The specialized cytoskeleton of striated muscle cells consists of highly ordered structures, the single contractile units termed sarcomeres. Although once believed to be simply a scaffold for the assembly of structural proteins to generate force and motion, it is now widely recognized that the sarcomeric cytoskeleton is dynamic and is intimately involved in numerous cellular signaling processes that respond to a wide range of extracellular cues. Sarcomeres in vertebrates are built of an intricate array of myosin-containing thick filaments, actin-containing thin filaments, and titin filaments (Figure, A), along with a plethora of other structural and regulatory proteins. Titin (also known as connectin) is the largest protein discovered in humans to date.1,2 The human titin gene is enormous; at 363 exons, it encompasses the largest number of exons in any single gene and is predicted to encode up to 38 138 residues, or a 4.2-MDa protein.3 Remarkably, a single molecule of titin spans half the length of a sarcomere with its NH2 terminus embedded in the Z-disc and its COOH terminus anchored at the center of the sarcomere, the M-line (Figure, A). This corresponds to a molecular length of approximately 0.9 to >1.5 μm, depending on sarcomere stretch! Because titin filaments from opposite half-sarcomeres fully overlap in the Z-discs and at the M-line, titin filaments form a continuous system within myofibrils. Figure. Striated muscle sarcomere (Panel A) with locations of titin mutations linked to human dilated cardiomyopathy (Panel B). Mutations are markedly missing from the Z disc and M line; they are highly enriched in the thick filament associated A band region. While some patients presented with identical mutations, numbers reflect unique mutations. Although many unique functions for titin have been described, it is most known for its function as a molecular spring; it limits …

Actin depolymerizing factors ADF7 and ADF10 play distinct roles during pollen development and pollen tube growth

An important player in actin remodeling is the actin depolymerizing factor (ADF) which increases actin filament treadmilling rates. Previously, we had prepared fluorescent protein fusions of two Arabidopsis pollen specific ADFs, ADF7 and ADF10. These had enabled us to determine the temporal expression patterns and subcellular localization of these proteins during male gametophyte development. Here we generated stable transformants containing both chimeric genes allowing for simultaneous imaging and direct comparison. One of the striking differences between the two proteins was the localization profile in the growing pollen tube apex. Whereas ADF10 was associated with the filamentous actin array forming the subapical actin fringe, ADF7 was present in the same cytoplasmic region, but in diffuse form. This suggests that ADF7 is involved in the high actin turnover that is likely to occur in the fringe by continuously and efficiently depolymerizing filamentous actin and supplying monomeric actin to the advancing end of the fringe. The possibility to visualize both of these pollen-specific ADFs simultaneously opens avenues for future research into the regulatory function of actin binding proteins in pollen.

A Phage Tubulin Assembles Dynamic Filaments by an Atypical Mechanism to Center Viral DNA within the Host Cell

Tubulins are essential for the reproduction of many eukaryotic viruses, but historically, bacteriophage were assumed not to require a cytoskeleton. Here, we identify a tubulin-like protein, PhuZ, from bacteriophage 201φ2-1 and show that it forms filaments in vivo and in vitro. The PhuZ structure has a conserved tubulin fold, with an unusual, extended C terminus that we demonstrate to be critical for polymerization in vitro and in vivo. Longitudinal packing in the crystal lattice mimics packing observed by EM of in-vitro-formed filaments, indicating how interactions between the C terminus and the following monomer drive polymerization. PhuZ forms a filamentous array that is required for positioning phage DNA within the bacterial cell. Correct positioning to the cell center and optimal phage reproduction only occur when the PhuZ filament is dynamic. Thus, we show that PhuZ assembles a spindle-like array that functions analogously to the microtubule-based spindles of eukaryotes.

Characterization of Nanoporous WO3 Films Grown via Ballistic Deposition

We report on the preparation and characterization of high surface area, supported nanoporous tungsten oxide films prepared under different conditions on polished polycrystalline Ta and Pt(111) substrates via direct sublimation of monodispersed gas phase clusters of cyclic (WO3)3. Scanning electron microscopy and transmission electron microscopy were used to investigate the film morphology on a nanometer scale. The films consist of arrays of separated filaments that are amorphous. The chemical composition and the thermal stability of the films were investigated by means of X-ray photoelectron spectroscopy. The surface area and the distribution of binding sites on the films were measured as functions of growth temperature, deposition angle, and annealing conditions using temperature-programmed desorption of Kr. Films deposited at 20 K and at an incident angle of 65° from the substrate normal display the greatest specific surface area of ∼560 m2/g.