Changes In Filament Length Research Articles

Characteristics of a gill resection – Regeneration model in freshwater laboratory-reared Atlantic salmon (Salmo salar)

Impaired gill function is a critical issue in aquaculture due to factors such as poor water quality, disease, and farm management practices which can greatly compromise gill integrity and ultimately fish growth and survival. Regeneration of damaged gill tissue has been reported for some fish species, however, little is known about Atlantic salmon, which is one of the most economically important salmonid species. This study investigated the regenerative capability of gill tissue following resection in post-smolt Atlantic salmon reared in fresh water. Fish were subjected to two levels of resection severity: 30% or 50% of filament length. Changes in filament length and the cytological and functional capacity of regenerated tissue were studied at several time points post-resection (PR). On average, 38% ± 3.89 of the resected filament length regenerated in both resected groups at week-20 PR. However, absolute regeneration (mm) in 50% resected filaments was greater as compared to the 30% resected filaments. In both groups, regeneration percentage was significantly different among individuals, but was not correlated with initial body size, condition factor, or overall fish growth rate. Immunohistochemistry showed an increase in cell proliferation at the filament tip shortly after resection followed by the appearance of ionocytes, chemoreceptive neuroepithelial cells (NECs), and nerve fibres at week-4 PR. In both resected groups >50% and 90% of NECs were innervated by week-12 and -20 PR, respectively. Gill Na+/K+-ATPase activity in regenerated tissue was not significantly different from that of internal control and non-resected control fish. Our results demonstrate significant regeneration and apparent functional recovery of resected gill filaments in Atlantic salmon which has important implications for recovery from gill damage and developing best practices for maintenance of gill health of farmed salmon.

SMC protein RecN drives RecA filament translocation for in vivo homology search

While the molecular repertoire of the homologous recombination pathways is well studied, the search mechanism that enables recombination between distant homologous regions is poorly understood. Earlier work suggests that the recombinase RecA, an essential component for homology search, forms an elongated filament, nucleating at the break site. How this RecA structure carries out long-distance search remains unclear. Here, we follow the dynamics of RecA after induction of a single double-strand break on the Caulobacter chromosome. We find that the RecA-nucleoprotein filament, once formed, rapidly translocates in a directional manner in the cell, undergoing several pole-to-pole traversals, until homology search is complete. Concomitant with translocation, we observe dynamic variation in the length of the filament. Importantly invivo, the RecA filament alone is incapable of such long-distance movement; both translocation and associated length variations are contingent on action of structural maintenance of chromosome (SMC)-like protein RecN, via its ATPase cycle. In summary, we have uncovered the three key elements of homology search driven by RecN: mobility of a finite segment of RecA, changes in filament length, and ability to conduct multiple pole-to-pole traversals, which together point to an optimal search strategy.

Prediction of Solar Eruptions Using Filament Metadata

Abstract We perform a statistical analysis of erupting and non-erupting solar filaments to determine the properties related to the eruption potential. In order to perform this study, we correlate filament eruptions documented in the Heliophysics Event Knowledgebase (HEK) with HEK filaments that have been grouped together using a spatiotemporal tracking algorithm. The HEK provides metadata about each filament instance, including values for length, area, tilt, and chirality. We add additional metadata properties such as the distance from the nearest active region and the magnetic field decay index. We compare trends in the metadata from erupting and non-erupting filament tracks to discover which properties present signs of an eruption. We find that a change in filament length over time is the most important factor in discriminating between erupting and non-erupting filament tracks, with erupting tracks being more likely to have decreasing length. We attempt to find an ensemble of predictive filament metadata using a Random Forest Classifier approach, but find the probability of correctly predicting an eruption with the current metadata is only slightly better than chance.

Label-free Imaging of Microtubules with Sub-nm Precision Using Interferometric Scattering Microscopy

Current in vitro optical studies of microtubule dynamics tend to rely on fluorescent labeling of tubulin, with tracking accuracy thereby limited by the quantum yield of fluorophores and by photobleaching. Here, we demonstrate label-free tracking of microtubules with nanometer precision at kilohertz frame rates using interferometric scattering microscopy (iSCAT). With microtubules tethered to a glass substrate using low-density kinesin, we readily detect sequential 8 nm steps in the microtubule center of mass, characteristic of a single kinesin molecule moving a microtubule. iSCAT also permits dynamic changes in filament length to be measured with <5 nm precision. Using the arbitrarily long observation time enabled by label-free iSCAT imaging, we demonstrate continuous monitoring of microtubule disassembly over a 30 min period. The ability of iSCAT to track microtubules with nm precision together with its potential for label-free single protein detection and simultaneous single molecule fluorescence imaging represent a unique platform for novel approaches to studying microtubule dynamics.

Transient modeling of fiber spinning with filament pull-out

Flow instabilities of wet-spun fibers in the form of draw resonance can result in radius fluctuations which impose limitations on either fiber quality or production rate. Also, at high winding velocities, if the fluid strength is sufficiently high, a filament can pull out of the capillary. Filament pull-out complicates the stability analysis of fiber spinning in the sense that the upstream boundary conditions now depend on the position in the spinneret. In addition, the filament length varies in time. Intuitively, one would expect that including filament pull-out will stabilize the spinning process since the change in filament length adjusts the spinning force to result in a process similar to spinning with a constant take-up force, which is always stable. However, from a linear stability analysis on a thin filament fiber spin model including filament pull-out, we obtained the least stable solution in a straight capillary with minimal restriction on the movement of the detachment position, see van der Walt et al. (2012) [1]. In this paper we extend the stability analysis to a fully axisymmetric transient isothermal fiber spin model including filament detachment from the capillary wall using the eXtended Pom-Pom (XPP) constitutive model. Using finite elements, we develop a numerically efficient algorithm for the dynamics of the detachment point. A comparison between the stability results of the axisymmetric and the thin filament model shows that the axisymmetric model is more stable than the thin filament pull-out model but less stable than the thin filament fixed length model for the same De (Deborah) number. We are also not able to confirm from the axisymmetric model the assumption of the force being a function of the detachment position in the thin filament model. Revisiting the integral momentum balance at the detachment point derived by Bulters and Meijer (1990) [2] we find that their force balance is not valid. There is a transient region near the detachment position from the integrated normal stress to the spinning force.

Disaggregation of Tau as a Therapeutic Approach to Tauopathies

Tau aggregation is an appealing target for therapeutic intervention. However, conformational change or aggregation needs to be targeted without inhibiting the normal biology of tau and its role in microtubule stabilization. The number of compound classes being tested at this time are very limited and include Congo red derivatives [2], anthraquinones (Pickhardt et al. 2005 [3], disputed in Crowe et al. 2007 [4]), 2,3-di(furan-2-yl)-quinoxalines , phenylthiazolyl-hydrazide (PTH) [5], polyphenols and porphyrins [6] and cyanine dyes [1, 7, 8]. Herein we have utilized a member of the cyanine dye family (C11) in an organotypic slice culture model of tangle formation. Our results demonstrate that C11 is capable of affecting tau polymerization in a biphasic, dose dependent manner. At submicromolar concentrations (0.001 microM) C11 reduced levels of aggregated tau. However, higher doses resulted in an increase in tau polymerization. These effects can also be seen at the level of individual filaments with changes in filament length and number mirroring the pattern seen via immunoblotting. In addition, this effect is achieved without altering levels of phosphorylation at disease and microtubule binding relevant epitopes.

Structural changes in the myosin filament and cross‐bridges during active force development in single intact frog muscle fibres: stiffness and X‐ray diffraction measurements

Structural and mechanical changes occurring in the myosin filament and myosin head domains during the development of the isometric tetanus have been investigated in intact frog muscle fibres at 4 degrees C and 2.15 microm sarcomere length, using sarcomere level mechanics and X-ray diffraction at beamline ID2 of the European Synchrotron Radiation Facility (Grenoble, France). The time courses of changes in both the M3 and M6 myosin-based reflections were recorded with 5 ms frames using the gas-filled RAPID detector (MicroGap Technology). Following the end of the latent period (11 ms after the start of stimulation), force increases to the tetanus plateau value (T(0)) with a half-time of 40 ms, and the spacings of the M3 and M6 reflections (S(M3) and S(M6)) increase by 1.5% from their resting values, with time courses that lead that of force by approximately 10 and approximately 20 ms, respectively. These temporal relations are maintained when the increase of force is delayed by approximately 10 ms by imposing, from 5 ms after the first stimulus, 50 nm (half-sarcomere)(-1) shortening at the velocity (V(0)) that maintains zero force. Shortening at V(0) transiently reduces S(M3) following the latent period and delays the subsequent increase in S(M3), but only delays the S(M6) increase without a transient decrease. Shortening at V(0) imposed at the tetanus plateau causes an abrupt reduction of the intensity of the M3 reflection (I(M3)), whereas the intensity of the M6 reflection (I(M6)) is only slightly reduced. The changes in half-sarcomere stiffness indicate that the isometric force at each time point is proportional to the number of myosin heads bound to actin. The different sensitivities of the intensity and spacing of the M3 and M6 reflections to the mechanical responses support the view that the M3 reflection in active muscle originates mainly from the myosin heads attached to the actin filament and the M6 reflection originates mainly from a fixed structure in the myosin filament signalling myosin filament length changes during the tetanus rise.

Myosin and actin filament lengths in diaphragms from emphysematous hamsters

In vitro studies of the diaphragm from emphysematous animals have, in some instances, shown an alteration in its sarcomere length-tension relationship and a decreased maximal specific tension. To our knowledge, it has never been determined whether such functional changes may be indicative of ultrastructural adaptations, e.g., changes in filament lengths and thus cross-bridge number. To address this, we compared filament lengths in diaphragms from hamsters in which emphysema was induced by endotracheal instillation of elastase (E) 5 mo before the hamsters were killed with those from control hamsters (C; saline instillation). Diaphragms were then fixed by vascular perfusion with buffered glutaraldehyde in situ at airway pressures set to approximate the physiological range of lung volumes from residual volume (RV) to total lung capacity (TLC). Ultrathin sections (50-70 nm) were taken parallel to the muscle fiber axis and examined by electron microscopy (x33,000). Sarcomere and filament length measurements were calibrated using an actin periodicity of 39 nm and an M-band width of 86 nm to correct for dimensional changes during preparation. Emphysema increased the change in lung volume from -20 to +25 cmH2O airway pressure (from RV to TLC) by approximately 88%, and the displacement volume of excised lung at 0 cmH2O airway pressure was increased by approximately 138% on average. Neither myosin (C = 1.592 +/- 0.027; E = 1.572 +/- 0.035 micron; P = 0.72) nor actin (C = 1.210 +/- 0.035; E = 1.221 +/- 0.014 micron; P = 0.76) filament lengths were affected by emphysema. Thus, filament length changes do not underlie the diaphragm functional adaptations observed previously in emphysema.

Structure and mobility of actin filaments as measured by quasielastic light scattering, viscometry, and electron microscopy.

Actin filaments of different lengths were prepared by polymerizing actin in the presence of various concentrations of gelsolin, a protein which accelerates actin polymerization by stabilizing nuclei from which filaments grow and which binds to their fast growing ends. The lengths of the actin filaments following polymerization were measured by electron microscopy and showed that the number-average filament length agreed with the predicted length if each gelsolin molecule acted as a seed for the growth of an actin filament. The distribution of lengths was independent of the actin:gelsolin ratio and was similar to that of actin filaments polymerized in the absence of gelsolin (Lw/Ln = 1.8). The mobility of these filaments in solution was studied by quasielastic light scattering and by viscometry. The translational diffusion constant determined by quasielastic light scattering was in agreement with the infinite dilution values calculated from the dimensions and the distribution of lengths determined by electron microscopy for relatively short filament lengths. Under conditions where overlap of the rotational domains of the filaments would be expected to occur, the measured diffusion rates deviated from their predicted dilute solution values and the solution viscosity increased abruptly. The dependence of the diffusion constant and the solution viscosity on the length of the actin filaments can be explained in terms of a theory that describes the restraints on diffusion of independent rigid rods in semi-dilute solution. The results suggest that the rheology of actin filaments can be accounted for by steric restraints. The length of cytoplasmic actin filaments in some cell types is such that these steric constraints are significant and could produce large changes in physical properties with small changes in filament length.

Enhancement of actin-activated myosin ATPase by an 84K Mr actin-binding protein in vertebrate smooth muscle.

A Ca2+-dependent actin-severing 84K Mr protein prepared from bovine aorta caused four-fold activation of smooth muscle actin-activated myosin ATPase at a 1/10(2) molar ratio to actin in the presence of tropomyosin and light chain kinase-calmodulin in a Ca2+-dependent manner, while it inhibited it markedly at a 1/5 molar ratio. Purified actin-tropomyosin filaments under the experimental ATPase conditions were distributed in a range of more than 10 micron in length and the addition of the 84K Mr protein changed the filament length to around 1 micron at a 1/10(2) molar ratio to actin or less than 50 nm at a 1/5 molar ratio in the presence of Ca2+. However, the apparent length of actin filaments alone does not appear to be responsible for the activation of ATPase activity, since in the absence of tropomyosin, the ATPase activation was much less in spite of actin filament length changes. These results indicate the possibility that the 84K Mr protein plays an important role with tropomyosin in at least in vitro smooth muscle actin-myosin interaction.

Toward a theory of muscle contraction.

The general chemistry of muscle contraction was elucidated early on from test-tube reconstitutions of function:1, 2 Myosin and actin form an ATP-dissociable complex; myosin is an actin-modifiable ATPase, and the free energy of ATP hydrolysis “pays for” any work performed in contraction. The central structural feature of contraction was discovered later:3, 4 In the vertebrate sarcomere, thick (myosin) and thin (actin + control proteins) filaments interdigitate to produce shortening without change in filament length. Finally came the macroscopic “law of force”:5, 6 The “tetanic” shortening force, and/or the rate of chemical energy dissipation, is proportional to the number of interactions between (myosin) cross-bridges and actin. Even together these great discoveries are not an explanation of how muscle works because it remains unknown how myosin, actin, and ATP, located as they are, produce a force at the microscopic level, or how this force is directed and timed to produce the behavior expressed by the macroscopic law. On the other hand, it is nowadays pointless to theorize about these unknown processes unless the theory springs clearly and naturally from these firmly-established central facts.

Ultrastructure of the resting and contracted striated muscle fiber at different degrees of stretch.

Passive stretch, isometric contraction, and shortening were studied in electron micrographs of striated, non-glycerinated frog muscle fibers. The artifacts due to the different steps of preparation were evaluated by comparing sarcomere length and fiber diameter before, during, and after fixation and after sectioning. Tension and length were recorded in the resting and contracted fiber before and during fixation. The I filaments could be traced to enter the A band between the A filaments on both sides of the I band, creating a zone of overlap which decreased linearly with stretch and increased with shortening. This is consistent with a sliding filament model. The decrease in the length of the A and I filaments during isometric contraction and the finding that fibers stretched to a sarcomere length of 3.7 micro still developed 30 per cent of the maximum tetanic tension could not be explained in terms of the sliding filament model. Shortening of the sarcomeres near the myotendinous junctions which still have overlap could account for only one-sixth of this tension, indicating that even those sarcomeres stretched to such a degree that there is a gap between A and I filaments are activated during isometric contraction (increase in stiffness). Shortening, too, was associated with changes in filament length. The diameter of A filaments remained unaltered with stretch and with isometric contraction. Shortening of 50 per cent was associated with a 13 per cent increase in A filament diameter. The area occupied by the fibrils and by the interfibrillar space increased with shortening, indicating a 20 per cent reduction in the volume of the fibrils when shortening amounted to 40 per cent.

Thermal Effects in Tire Cord Materials

Consideration is given to the effect of heat on the strength and dimensional char acteristics of typical tire textile materials. The dimensional change in a hot-stretched nylon cord is divided into two components: the change in filament length and the lateral packing of the filaments. Dimensional changes are also related to the elastic behavior, to the shrinkage forces, and to the creep characteristics of the cord. The generality of the thermal effects is demonstrated by data obtained using a range of textile fibers.