Loop Strands Research Articles

Loop Securities of Arthroscopic Sliding-Knot Techniques When the Suture Loop Is Not Evenly Tensioned

The purpose of this study was to evaluate the loop security of arthroscopic sliding knots when tension is only applied to the post strand and not the loop strand. Six different locking sliding knots (Weston, Nicky, Roeder, SMC, San Diego, and Dines) were included. Loop securities were evaluated in 2 ways: with a conventional method (equal tension applied to the suture loop) and with a worst-case scenario (WCS) method (only the post strand of the suture loop was tensioned). Differences between test methods were evaluated for significance. To help assess the applicability of each test method, loop-security testing in a cadaveric shoulder was performed with 1 type of knot (SMC). Loop securities with the conventional method versus the WCS method were as follows: 10.74 ± 4.20 N versus 6.90 ± 3.90 N for Weston, 21.25 ± 14.74 N versus 8.73 ± 3.35 N for Nicky, 26.14 ± 15.57 N versus 7.95 ± 4.23 N for Roeder, 42.67 ± 22.96 N versus 8.67 ± 4.33 N for SMC, 52.99 ± 21.36 N versus 18.25 ± 10.58 N for San Diego, and 89.27 ± 27.96 N versus 12.48 ± 3.40 N for Dines (P < .05 for each knot). All knots failed at significantly lower loads when the suture loop was not evenly tensioned. Cadaveric testing (SMC) resulted in a loop security of 5.53 ± 6.06 N, which was similar to the WCS setting. The locking mechanism of the sliding knots is maintained when the suture loop is evenly tensioned at both post and non-post strands. When tension is not applied to the non-post strand side, the knots slide more easily and fail at lower loads than previously reported. When surgeons tie locking sliding knots in single-row rotator cuff repair, they should be aware that the knots could fail at much lower loads than previously reported.

Heavy Chain-Only IgG2b Llama Antibody Effects Near-Pan HIV-1 Neutralization by Recognizing a CD4-Induced Epitope That Includes Elements of Coreceptor- and CD4-Binding Sites

The conserved HIV-1 site of coreceptor binding is protected from antibody-directed neutralization by conformational and steric restrictions. While inaccessible to most human antibodies, the coreceptor site has been shown to be accessed by antibody fragments. In this study, we used X-ray crystallography, surface plasmon resonance, and pseudovirus neutralization to characterize the gp120-envelope glycoprotein recognition and HIV-1 neutralization of a heavy chain-only llama antibody, named JM4. We describe full-length IgG2b and IgG3 versions of JM4 that target the coreceptor-binding site and potently neutralize over 95% of circulating HIV-1 isolates. Contrary to established trends that show improved access to the coreceptor-binding region by smaller antibody fragments, the single-domain (VHH) version of JM4 neutralized less well than the full-length IgG2b version of JM4. The crystal structure at 2.1-Å resolution of VHH JM4 bound to HIV-1 YU2 gp120 stabilized in the CD4-bound state by the CD4-mimetic miniprotein, M48U1, revealed a JM4 epitope that combined regions of coreceptor recognition (including the gp120 bridging sheet, V3 loop, and β19 strand) with gp120 structural elements involved in recognition of CD4 such as the CD4-binding loop. The structure of JM4 with gp120 thus defines a novel CD4-induced site of vulnerability involving elements of both coreceptor- and CD4-binding sites. The potently neutralizing JM4 IgG2b antibody that targets this newly defined site of vulnerability adds to the expanding repertoire of broadly neutralizing antibodies that effectively neutralize HIV-1 and thereby potentially provides a new template for vaccine development and target for HIV-1 therapy.

Arthroscopic Suture Anchor Tenodesis: Loop-Suture Technique

With advancements in arthroscopic surgery, arthroscopic biceps tenodesis with suture anchor recently has been reported to be a reasonable option for the treatment of biceps pathologies, especially for those that are symptomatic or accompanied by a rotator cuff tear. We introduce our technique of arthroscopic biceps tenodesis with suture anchor that we call the loop-suture technique, which is constructed with 1 loop strand and another sutured strand. This technique can help to improve biceps grip and simultaneously minimize longitudinal splitting of the tendon. In addition, it is relatively simple and can be performed with the use of conventional devices and arthroscopic portals used for rotator cuff repair, without the formation of additional portals or a separate incision for the tenodesis.

Properties of multistranded, impulsively heated hydrodynamic loop models

Aims. We investigate the capability of multistranded loop models subject to nanoflare heating to reproduce the properties recently observed in coronal loops at extreme ultraviolet (EUV) wavelengths. Methods. One-dimensional hydrodynamic simulations of magnetic loop strands were performed with an impulsive, footpointlocalised heating, with a moderate asymmetry between the two loop halves that was produced either by a sequence of identical nanoflares with a given cadence time tC or by a single energy pulse. The temporal evolution of the emission of a multistranded loop was modelled by simply combining the results of independent single-strand simulations, neglecting any spatial interaction among the strands, and was compared with TRACE and SDO/AIA light curves. The density excess with respect to hydrostatic equilibrium (the ψ factor) was evaluated with the filter-ratio technique. Results. Both loop models exhibit a density excess compared with hydrostatic equilibrium models, which agrees well with the observed values (1 ψ 12). However, in the single-pulse model the light curve and density excess maxima do not match. On the other hand, the models with a sequence of nanoflares predict strong emission at lower temperatures that cannot be reconciled with the available observations.

Proteolytic activation of Chlamydia trachomatis HTRA is mediated by PDZ1 domain interactions with protease domain loops L3 and LC and beta strand β5

Chlamydia trachomatis is a bacterial pathogen responsible for one of the most prevalent sexually transmitted infections worldwide. Its unique development cycle has limited our understanding of its pathogenic mechanisms. However, CtHtrA has recently been identified as a potential C. trachomatis virulence factor. CtHtrA is a tightly regulated quality control protein with a monomeric structural unit comprised of a chymotrypsin-like protease domain and two PDZ domains. Activation of proteolytic activity relies on the C-terminus of the substrate allosterically binding to the PDZ1 domain, which triggers subsequent conformational change and oligomerization of the protein into 24-mers enabling proteolysis. This activation is mediated by a cascade of precise structural arrangements, but the specific CtHtrA residues and structural elements required to facilitate activation are unknown. Using in vitro analysis guided by homology modeling, we show that the mutation of residues Arg362 and Arg224, predicted to disrupt the interaction between the CtHtrA PDZ1 domain and loop L3, and between loop L3 and loop LD, respectively, are critical for the activation of proteolytic activity. We also demonstrate that mutation to residues Arg299 and Lys160, predicted to disrupt PDZ1 domain interactions with protease loop LC and strand β5, are also able to influence proteolysis, implying their involvement in the CtHtrA mechanism of activation. This is the first investigation of protease loop LC and strand β5 with respect to their potential interactions with the PDZ1 domain. Given their high level of conservation in bacterial HtrA, these structural elements may be equally significant in the activation mechanism of DegP and other HtrA family members.

CHROMOSPHERIC EVAPORATION IN SOLAR FLARE LOOP STRANDS OBSERVED WITH THE EXTREME-ULTRAVIOLET IMAGING SPECTROMETER ON BOARDHINODE

The entire profile of the Fe XXIII line at 263.8 ?, formed at temperature 14 MK, was blueshifted by an upward velocity ?122 ? 33?km?s?1 when it was first detected by the Extreme-ultraviolet Imaging Spectrometer operating in rapid cadence (11.18?s) stare mode during a C1 solar flare. The entire profile became even more blueshifted over the next two exposures, when the upward velocity reached its maximum of ?208 ? 14?km?s?1 before decreasing to zero over the next 12 exposures. After that, a weak, secondary blueshifted component appeared for five exposures, reached a maximum upward velocity of ?206 ? 33?km?s?1, and disappeared after the maximum line intensity (stationary plus blueshifted) was achieved. Velocities were measured relative to the intense stationary profile observed near the flare's peak and early during its decline. The initial episode during which the entire profile was blueshifted lasted about 156?s, while the following episode during which a secondary blueshifted component was detected lasted about 56?s. The first episode likely corresponds to chromospheric evaporation in a single loop strand, while the second corresponds to evaporation in an additional strand, as described in multi-strand flare loop models proposed by Hori et al. and Warren & Doschek. Line emission from progressively cooler ions (Fe XVII, XVI, and XIV) brightened at successively later times, consistent with cooling of flare-heated plasma.

OBSERVATION OF HIGH-SPEED OUTFLOWS IN CORONAL LOOPS ASSOCIATED WITH PHOTOSPHERIC MAGNETIC FIELD EVOLUTION

Using SDO/AIA instruments, we provide an EUV observation of two adjacent loop strands (Loops 1 and 2) with one side of their footpoints rooted in the boundaries of active region (AR) NOAA 11158 and the other side in the quiet-Sun regions. The AR footpoints of Loop 1 were located in monopolar magnetic areas and those of Loop 2 in mixed polar areas (SDO/HMI magnetograms). There were no apparent outflows found in Loop 1 in 10 hr of observations, whereas in Loop 2, the outflows were detected throughout the whole observation with an average speed of 120-150 km s–1. We find clear evidence of magnetic reconnections occurring in the AR footpoints of Loop 2 (the opposite magnetic polarities came close and then a part of them disappeared) and magnetic flux dispersal in the quiet-Sun footpoints (a patch of positive polarities decayed with time). Furthermore, with Hinode/SOT observations, there were no significant Ca II H brightenings detected at the loop footpoints of Loop 2 at the chromospheric heights in response to those of the AIA 171 A and 304 A channels when four strong outflow events took place in the loops, which seem to differ from the conclusions of previous studies. In other studies, the rapid coronal outflows along the coronal loops were found to originate from the chromosphere through transient events (e.g., type II spicules).

First observation of a transverse vertical oscillation during the formation of a hot post-flare loop

Aims. We report and analyse the first observation of a transverse oscillation in a hot coronal loop with the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO), following a linked coronal-flare mass-ejection event on the 3 November 2010. The oscillating coronal loop is observed off the east solar limb and exclusively in the 131 A and 94 A bandpasses, indicating a loop plasma of temperature in the range of 9-11 MK. Furthermore, the loop is not observed to cool into the other AIA channels, but just disappears from all bandpasses at the end of the oscillation. Methods. A time series analysis of the loop oscillation is conducted by taking several cuts at different positions along the loop, estimating the transverse displacements over time for two strands in the loop and fitting those with a damped cosine curve. Intensity time variations, both along the loop and for a series of cut cross-sections, are investigated. Using a three-dimensional loop geometry obtained from a comparison of STEREO-B/EUVI and AIA images, we model different modes of transverse oscillations in the uniformly filled loop. Results. Our time series analysis reveals a period of 302 ± 14 s (291 ± 9 s) and a damping time of 306 ± 43 s (487 ± 125 s) for the first (second) loop strand. A spatial phase shift along the loop of approximately 180° suggests that we observe a higher order harmonic. Intensity oscillations are consistent with an interpretation in terms of a vertically polarised mode. Our forward modelling suggests that the loop oscillates as either a second or third order harmonic of this mode. Conclusions. This is the first observation of a transverse loop oscillation observed exclusively in the hot coronal lines. The loop oscillation is vertically polarised and is dominated by a higher order harmonic mode. We conclude that the excitation mechanism of this 5 min period oscillation is directly connected with the reconnection processes that form the post flare loop, which differs from the blast wave excitation mechanism often proposed as the cause of cooler transverse loop oscillations. © 2012 ESO.

The role of radiative losses in the late evolution of pulse-heated coronal loops/strands

Radiative losses from optically thin plasma are an important ingredient for modeling plasma confined in the solar corona. Spectral models are continuously updated to include the emission from more spectral lines, with significant effects on radiative losses, especially around 1 MK. We investigate the effect of changing the radiative losses temperature dependence due to upgrading of spectral codes on predictions obtained from modeling plasma confined in the solar corona. The hydrodynamic simulation of a pulse-heated loop strand is revisited comparing results using an old and a recent radiative losses function. We find significant changes in the plasma evolution during the late phases of plasma cooling: when the recent radiative loss curve is used, the plasma cooling rate increases significantly when temperatures reach 1-2 MK. Such more rapid cooling occurs when the plasma density is larger than a threshold value, and therefore in impulsive heating models that cause the loop plasma to become overdense. The fast cooling has the effect of steepening the slope of the emission measure distribution of coronal plasmas with temperature at temperatures lower than ~2 MK. The effects of changes in the radiative losses curves can be important for modeling the late phases of the evolution of pulse-heated coronal loops, and, more in general, of thermally unstable optically thin plasmas.

N for AsN – O for StrOcture? A strand–loop–strand motif for prokaryotic O‐glycosylation

So far, it has not been possible to identify a general sequence motif for O-glycosylation in bacteria. In this issue, Charbonneau et al. (2012) demonstrate why O-glycosylation is mediated by a 13-residue strand-loop-strand motif which is part of a 19-residue imperfect repeat in the passenger domain of bacterial autotransporters. This motif provides a convenient 'glycosylation cassette' and raises intriguing questions about the structural regulation of the glycosylation pathway.

EMISSION MEASURE DISTRIBUTION AND HEATING OF TWO ACTIVE REGION CORES

Using data from the Extreme-ultraviolet Imaging Spectrometer aboard Hinode, we have studied the coronal plasma in the core of two active regions. Concentrating on the area between opposite polarity moss, we found emission measure distributions having an approximate power-law form EM$\propto T^{2.4}$ from $\log\,T = 5.5$ up to a peak at $\log\,T = 6.55$. We show that the observations compare very favorably with a simple model of nanoflare-heated loop strands. They also appear to be consistent with more sophisticated nanoflare models. However, in the absence of additional constraints, steady heating is also a viable explanation.

SOLAR DYNAMICS OBSERVATORY DISCOVERS THIN HIGH TEMPERATURE STRANDS IN CORONAL ACTIVE REGIONS

One scenario proposed to explain the million degrees solar corona is a finely-stranded corona where each strand is heated by a rapid pulse. However, such fine structure has neither been resolved through direct imaging observations nor conclusively shown through indirect observations of extended superhot plasma. Recently it has been shown that the observed difference in appearance of cool and warm coronal loops (~1 MK, ~2-3 MK, respectively) -- warm loops appearing "fuzzier" than cool loops -- can be explained by models of loops composed of subarcsecond strands, which are impulsively heated up to ~10 MK. That work predicts that images of hot coronal loops (>~6 MK) should again show fine structure. Here we show that the predicted effect is indeed widely observed in an active region with the Solar Dynamics Observatory, thus supporting a scenario where impulsive heating of fine loop strands plays an important role in powering the active corona.

Use of randomly mutagenized genomic cDNA banks of potato spindle tuber viroid to screen for viable versions of the viroid genome

In an effort to study sequence space allowing the recovery of viable potato spindle tuber viroid (PSTVd) variants we have developed an in vivo selection (Selex) method to produce and bulk-inoculate by agroinfiltration large PSTVd cDNA banks in which a short stretch of the genome is mutagenized to saturation. This technique was applied to two highly conserved 6 nt-long regions of the PSTVd genome, the left terminal loop (TL bank) and part of the polypurine stretch in the upper strand of pre-melting loop 1 (PM1 bank). In each case, PSTVd accumulation was observed in a large fraction of bank-inoculated tomato plants. Characterization of the progeny molecules showed the recovery of the parental PSTVd sequence in 89 % (TL bank) and 18 % (PM1 bank) of the analysed plants. In addition, viable and genetically stable PSTVd variants with mutations outside of the known natural variability of PSTVd were recovered in both cases, although at different rates. In the case of the TL region, mutations were recovered at five of the six mutagenized positions (357, 358, 359, 1 and 3 of the genome) while for the PM1 region mutations were recovered at all six targeted positions (50-55), providing significant new insight on the plasticity of the PSTVd genome.

Crystal Structure of a Bony Fish β2-Microglobulin: INSIGHTS INTO THE EVOLUTIONARY ORIGIN OF IMMUNOGLOBULIN SUPERFAMILY CONSTANT MOLECULES

Crystal Structure of a Bony Fish β2-Microglobulin: INSIGHTS INTO THE EVOLUTIONARY ORIGIN OF IMMUNOGLOBULIN SUPERFAMILY CONSTANT MOLECULES

SCALING LAWS AND TEMPERATURE PROFILES FOR SOLAR AND STELLAR CORONAL LOOPS WITH NON-UNIFORM HEATING

The bulk of solar coronal radiative loss consists of soft X-ray emission from quasi-static loops at the cores of Active Regions. In order to develop diagnostics for determining the heating mechanism of these loops from observations by coronal imaging instruments, I have developed analytical solutions for the temperature structure and scaling laws of loop strands for a wide range of heating functions, including footpoint heating, uniform heating, and heating concentrated at the loop apex. Key results are that the temperature profile depends only weakly on the heating distribution -- not sufficiently to be of significant diagnostic value -- and that the scaling laws survive for this wide range of heating distributions, but with the constant of proportionality in the RTV scaling law ($P_{0}L \thicksim T_{max}^3$) depending on the specific heating function. Furthermore, quasi-static analytical solutions do not exist for an excessive concentration of heating near the loop footpoints, a result in agreement with recent numerical simulations. It is demonstrated that a generalization of the solutions to the case of a strand with a variable diameter leads to only relatively small correction factors in the scaling laws and temperature profiles for constant diameter loop strands. A quintet of leading theoretical coronal heating mechanisms is shown to be captured by the formalism of this paper, and the differences in thermal structure between them may be verified through observations. Preliminary results from full numerical simulations demonstrate that, despite the simplifying assumptions, the analytical solutions from this paper are stable and accurate.

A V3 Loop-Dependent gp120 Element Disrupted by CD4 Binding Stabilizes the Human Immunodeficiency Virus Envelope Glycoprotein Trimer

Human immunodeficiency virus (HIV-1) entry into cells is mediated by a trimeric complex consisting of noncovalently associated gp120 (exterior) and gp41 (transmembrane) envelope glycoproteins. The binding of gp120 to receptors on the target cell alters the gp120-gp41 relationship and activates the membrane-fusing capacity of gp41. Interaction of gp120 with the primary receptor, CD4, results in the exposure of the gp120 third variable (V3) loop, which contributes to binding the CCR5 or CXCR4 chemokine receptors. We show here that insertions in the V3 stem or polar substitutions in a conserved hydrophobic patch near the V3 tip result in decreased gp120-gp41 association (in the unliganded state) and decreased chemokine receptor binding (in the CD4-bound state). Subunit association and syncytium-forming ability of the envelope glycoproteins from primary HIV-1 isolates were disrupted more by V3 changes than those of laboratory-adapted HIV-1 envelope glycoproteins. Changes in the gp120 beta2, beta19, beta20, and beta21 strands, which evidence suggests are proximal to the V3 loop in unliganded gp120, also resulted in decreased gp120-gp41 association. Thus, a gp120 element composed of the V3 loop and adjacent beta strands contributes to quaternary interactions that stabilize the unliganded trimer. CD4 binding dismantles this element, altering the gp120-gp41 relationship and rendering the hydrophobic patch in the V3 tip available for chemokine receptor binding.

MULTI-STRAND CORONAL LOOP MODEL AND FILTER-RATIO ANALYSIS

We model a coronal loop as a bundle of seven separate strands or filaments. Each of the loop strands used in this model can independently be heated (near their left footpoints) by Alfv\'en/ion-cyclotron waves via wave-particle interactions. The Alfv\'en waves are assumed to penetrate the strands from their footpoints, at which we consider different wave energy inputs. As a result, the loop strands can have different heating profiles, and the differential heating can lead to a varying cross-field temperature in the total coronal loop. The simulation of TRACE observations by means of this loop model implies two uniform temperatures along the loop length, one inferred from the 171:195 filter ratio and the other from the 171:284 ratio. The reproduced flat temperature profiles are consistent with those inferred from the observed EUV coronal loops. According to our model, the flat temperature profile is a consequence of the coronal loop consisting of filaments, which have different temperatures but almost similar emission measures in the cross-field direction. Furthermore, when we assume certain errors in the simulated loop emissions (e.g., due to photometric uncertainties in the TRACE filters) and use the triple-filter analysis, our simulated loop conditions become consistent with those of an isothermal plasma. This implies that the use of TRACE/EIT triple filters for observation of a warm coronal loop may not help in determining whether the cross-field isothermal assumption is satisfied or not.

The PHCCEx domain of Tiam1/2 is a novel protein- and membrane-binding module

Tiam1 and Tiam2 (Tiam1/2) are guanine nucleotide-exchange factors that possess the PH–CC–Ex (pleckstrin homology, coiled coil and extra) region that mediates binding to plasma membranes and signalling proteins in the activation of Rac GTPases. Crystal structures of the PH–CC–Ex regions revealed a single globular domain, PHCCEx domain, comprising a conventional PH subdomain associated with an antiparallel coiled coil of CC subdomain and a novel three-helical globular Ex subdomain. The PH subdomain resembles the β-spectrin PH domain, suggesting non-canonical phosphatidylinositol binding. Mutational and binding studies indicated that CC and Ex subdomains form a positively charged surface for protein binding. We identified two unique acidic sequence motifs in Tiam1/2-interacting proteins for binding to PHCCEx domain, Motif-I in CD44 and ephrinB's and the NMDA receptor, and Motif-II in Par3 and JIP2. Our results suggest the molecular basis by which the Tiam1/2 PHCCEx domain facilitates dual binding to membranes and signalling proteins.

Analytical model of static coronal loops

By solving the energy-equilibrium equation in the stationary case, we derive analytical formulae in the form of scaling laws for non-uniformly heated and gravitationally stratified coronal loops. The heating is assumed to be localized in the chromosphere and to exponentially decrease with increasing distance along the loop strand. This exponential behavior of the heating and pressure profiles implies that we need to use the mean-value theorem, and in turn fit the mean-value parameters of the scaling laws to the results of the numerical simulations. The radiative-loss function is approximated by a power-law function of the temperature, and its effect on the resulting scaling laws for coronal loops is studied. We find that this effect is more important than the effect of varying loop geometry. We also find that the difference in lengths of the different loop strands in a loop with expanding cross-section does not produce differences in the EUV emission of these strands significant enough to explain the observed narrowness of the coronal loops.

Folding pathways of hybrid-1 and hybrid-2 G-quadruplex structures

The G-rich sequence in the human telomeric DNA can form the G-quadruplex structure. The G-quadruplex structure has become an attractive target for the anticancer drugs, because it effectively inhibits telomerase activity. Recently, the human telomere G-quadruplex in K(+) solution has been determined as a hybrid structure. This structure is called hybrid-1. More recently, the hybrid-2 G-quadruplex has been determined by NMR. Hybrid-2 G-quadruplex differs from hybrid-1 in its loop arrangement and strand orientation. Here, we propose the folding pathways of hybrid-1 and hybrid-2 G-quadruplex structures. In hybrid-1, we proposed two pathways. In one pathway, the random coil forms triplex-1; in another pathway, the random coil forms chair-1. Similarly, we proposed two pathways in hybrid-2. In one pathway, the random coil forms triplex-2; in another pathway, the random coil forms chair-2. The folding pathways of human telomeric hybrid-1 G-quadruplex and hybrid-2 G-quadruplex structures were proposed using MO calculation and molecular modelling.