Structural control of fibrin bioactivity by mechanical deformation

Binding of tissue plasminogen activator to endothelial cells: The effect on functional properties. Localization of a ligand in the B-chain of tPA

Tissue plasminogen activator (tPA), an arginine-specific serine protease, is an oestrogen-regulated protein in uterine and breast cancer tissue. It contains a domain which shares homology with epidermal growth factor (EGF). The aim of the present study was to determine whether specific tPA receptors or EGF receptors mediate the binding of tPA to cells and whether tPA possesses intrinsic mitogenic activity. The binding of 125I-labelled tPA to rat uterine and liver membranes was shown to be non-specific and could not be displaced by unlabelled tPA or EGF. Furthermore, acid washing of cell membranes did not unmask specific tPA-binding sites. In contrast, 125I-labelled EGF binding to both rat uterine and liver membranes was displaced in a dose-dependent manner by unlabelled EGF, and Scatchard analysis of the binding data revealed dissociation constant (Kd) values of 2.4 and 0.71 nM respectively. Unlabelled tPA (up to 20,000-fold excess) did not displace 125I-labelled EGF binding to these membranes. A study of the binding of 125I-labelled tPA and 125I-labelled EGF to endometrial carcinoma cells (Ishikawa), cervical carcinoma cells (HOG-1) and vulval carcinoma cells (A431) showed that up to a 100-fold excess of EGF or a 1000-fold excess of tPA did not displace 125I-labelled tPA binding to these cells. In contrast, 125I-labelled EGF binding was displaced by unlabelled EGF (Kd values for Ishikawa and HOG-1 cells were 2.72 and 1.92 nM respectively) but not by unlabelled tPA (1000-fold excess).(ABSTRACT TRUNCATED AT 250 WORDS)

Since the platelet surface has been shown to be a site for plasminogen conversion by tissue-type and other plasminogen activators, we examined the binding of tissue plasminogen activator (tPA) to human platelets. Resting, washed platelets were found to bind single chain, radioiodinated, recombinant tPA specifically and saturably with an apparent, estimated dissociation constant (KD) of 458 nM, binding approximately 570 molecules per platelet at saturation. Washed platelets activated with adenosine 5'-diphosphate in the presence of 0.1 mg/ml fibrinogen and 1 mM CaCl2 bound tPA with greater affinity, having an estimated apparent KD of 30.6 nM and binding approximately 29,000 molecules per platelet at saturation. Bound tPA could be completely displaced by an excess of unlabeled tPA. Interestingly, bound tPA could also be displaced from activated platelets with increasing concentrations of soluble fibrin with an estimated IC50 of 37.5 μg/ml of fibrin. In contrast, increasing concentrations of fibrinogen failed to reduce binding. These data show that tPA binds to the activated platelet surface by a mechanism that involves platelet-bound fibrinogen. In addition, these data suggest that on binding to the platelet surface, fibrinogen expresses domains that are similar to the tPA binding domains of fibrin. It is the presence of these domains within the platelet aggregate that likely supports tPA binding and facilitates plasminogen activation.

Binding of tissue plasminogen activator to human umbilical vein endothelial cells

The aim of the present study was to investigate the binding of tissue plasminogen activator (tPA) to cultured endothelial cells and to characterize binding structures present in the cultures. Studies on the binding of 125I-tPA to cultured endothelial cells from human umbilical-cord veins (HUVEC) indicated that the number of sites for specific binding of tPA is 8 x 10(5) per cell. Treatment with an excess of antibodies against plasminogen-activator inhibitor type 1 (PAI-1) caused an 80% decrease in the binding, leaving about 1.6 x 10(5) unoccupied binding sites per cell, which appeared to be different from PAI-1. About 1.9 x 10(5) binding sites/cell for tPA were found on the surface of HUVEC that had been detached from the matrix. This indicates that only minor amounts of PAI-1 occur on the surface of the cells. In addition, immunocytochemical analysis showed that PAI-1 antigen is present almost exclusively in the cytoplasm but was not observed on the surface of the cells, whereas tPA antigen is abundant on the plasma membrane of tPA-treated cells as well as intracellularly. Competition studies using unlabelled compounds showed that native tPA and tPA B-chain (the proteinase domain), as well as the inactive derivatives, B-chain inactivated with D-Phe-Pro-Arg-chloromethane and tPA-PAI-1 complex, caused a considerable quenching of the binding of 125I-tPA to HUVEC, whereas the isolated A-chain had no demonstrable effect. Two components (apparent molecular masses 38 kDa and 56 kDa) reacting with tPA but lacking PAI-1 antigen determinants were identified. Thus the data suggest that tPA binds to HUVEC by two principally different mechanisms. One is mediated by PAI-1, which binds and inactivates tPA with a functional active site. The other binding is achieved by components which react with sites on the activator molecule other than structures of the A-chain or the active site.

A complex orthorhombic carbon allotrope in $Pbam$ symmetry with 32 atoms in its unit cell, thus termed $Pbam$-32 carbon, was recently predicted [C. Y. He et al., Phys. Rev. Lett. 121, 175701 (2018)]. Its crystal structure comprises alternating fivefold, sixfold, and sevenfold carbon rings and exhibits reduced bonding anisotropy compared to diamond, raising the prospects of finding a superstrong material with distinct and favorable mechanical properties. Here we report findings from first-principles calculations that reveal peculiar stress-strain relations in $Pbam$-32 carbon. The obtained stress responses under various tensile and shear strains display outstanding characteristics contrasting those of traditional superhard materials like diamond and cubic boron nitride ($c$-BN). The $Pbam$-32 carbon undergoes structural deformations that produce highly isotropic stress responses under a wide variety of large tensile and shear strains, showcasing unprecedented nearly degenerate stress-strain curves along multiple deformation paths extended over ultralarge, including full-range, strains up to the bond-breaking points. These deformation modes impede or even suppress the graphitization process commonly seen in highly strained diamond and $c$-BN crystals while still sustaining large peak stresses comparable to those in diamond. Most notably, we find conspicuous bond-weakening and -breaking mechanisms stemming from bonding symmetry reduction in $Pbam$-32 carbon. At large tensile strains, a sequential bond elongation process occurs, generating a more ductile deformation past the peak stress; at large shear strains, the crystal structure goes through a similar sequential bond elongation process and, interestingly, transforms into a distinct three-dimensional network containing mixed $s{p}^{2}$ and $s{p}^{3}$ bonding states, suppressing the usual graphitization process. These more gradual bonding-state changes in the severely strained $Pbam$-32 carbon improve ductility and toughness in this superstrong carbon crystal. These insights elucidate mechanisms for toughening superstrong covalent crystals via microstructural arrangements, which shed light on rational design and development of a distinct class of superstrong materials that exhibit more isotropic mechanical responses with improved toughness under diverse loading conditions.

We present a method to introduce a large biaxial tensile strain in an ultra-thin germanium-on-insulator (GOI) using selective oxidation of SiGe epilayer on silicon-on-insulator (SOI) substrate. A circular patterned Si0.81Ge0.19 mesa on SOI substrate with the sidewall protected by Si3N4 or SiO2 is selectively oxidized to generate local 12 nm GOI with high crystal quality, which shows enhanced photoluminescence due to large tensile strain. Direct band photoluminescence peak significantly shifts to longer wavelength as compared to that from bulk Ge due to a combination of strain-induced band gap reduction and quantum confinement effect.

Ge nano-belts with large tensile strain are considered as one of the promising materials for high carrier mobility metal—oxide—semiconductor transistors and efficient photonic devices. In this paper, we design the Ge nano-belts on an insulator surrounded by Si3N4 or SiO2 for improving their tensile strain and simulate the strain profiles by using the finite difference time domain (FDTD) method. The width and thickness parameters of Ge nano-belts on an insulator, which have great effects on the strain profile, are optimized. A large uniaxial tensile strain of 1.16% in 50-nm width and 12-nm thickness Ge nano-belts with the sidewalls protected by Si3N4 is achieved after thermal treatments, which would significantly tailor the band gap structures of Ge-nanobelts to realize the high performance devices.

Simulations of a flexible coarse-grained model are used to study silica aerogels. This model, introduced in a previous study (J. Phys. Chem. C 2007, 111, 15792), consists of spherical particles which interact through weak nonbonded forces and strong interparticle bonds that may form and break during the simulations. Small-deformation simulations are used to determine the elastic moduli of a wide range of material models, and large-deformation simulations are used to probe structural evolution and plastic deformation. Uniaxial deformation at constant transverse pressure is simulated using two methods: a hybrid Monte Carlo approach combining molecular dynamics for the motion of individual particles and stochastic moves for transverse stress equilibration, and isothermal molecular dynamics simulations at fixed Poisson ratio. Reasonable agreement on elastic moduli is obtained except at very low densities. The model aerogels exhibit Poisson ratios between 0.17 and 0.24, with higher-density gels clustered around 0.20, and Young's moduli that vary with aerogel density according to a power-law dependence with an exponent near 3.0. These results are in agreement with reported experimental values. The models are shown to satisfy the expected homogeneous isotropic linear-elastic relationship between bulk and Young's moduli at higher densities, but there are systematic deviations at the lowest densities. Simulations of large compressive and tensile strains indicate that these materials display a ductile-to-brittle transition as the density is increased, and that the tensile strength varies with density according to a power law, with an exponent in reasonable agreement with experiment. Auxetic behavior is observed at large tensile strains in some models. Finally, at maximum tensile stress very few broken bonds are found in the materials, in accord with the theory that only a small fraction of the material structure is actually load-bearing.

The effectiveness of tissue plasminogen activator (tPA) in thrombolytic therapy is dependent upon the rate at which therapeutically administered tPA reaches the clot site and the proportion of that tPA which is enzymatically active. Interactions between tPA and its main plasma inhibitor (PAI-1) and between tPA and the endothelial cells lining blood vessels are two factors which may limit efficacy. In an attempt to identify the regions of the tPA molecule involved in these interactions, we have examined a series of synthetic peptides with amino acid sequences corresponding to different regions of the tPA molecule for their ability to protect tPA from inactivation by PAI-1 and for their ability to reduce the binding of tPA to endothelial cells. Three peptides were identified which were especially effective at maintaining tPA activity in the presence of PAI-1 and three others were found which had a lesser effect. These same peptides were also found to inhibit the binding of tPA to endothelial cells. This suggests that the same regions of the tPA molecule are involved in both processes. None of the peptides inhibited the binding of tPA to fibrin. These peptides may serve as models for the development of agents for enhancing the activity of both endogenous tPA and of tPA administered in thrombolytic therapy.

Randomly oriented polycrystalline silicon (poly-Si) thin films with a typical grain size of 20×2 µm2 grown by continuous-wave laser lateral crystallization (CLC) were obtained. It was found that CLC poly-Si thin films have a large tensile strain corresponding to 0.6% of single-crystalline Si lattice in the in-plane direction. These results suggest that there is a possibility that not only grain size but also the large tensile strain in in-plane direction can affect thin film transistor (TFT) performance.

Molecular Simulation of Thermoplastic Polyurethanes under Large Compressive Deformation

Simulation of transmission electron microscopy images during tensile fracture of metal foils

Due to its simplicity in instrumentation and data analysis, uniaxial tensile tests are commonly used for determining the plastic stress-strain relation of a sheet metal. However, the tensile stress-strain data obtained using the standard procedure is valid only up to the moderate strain level just before the onset of diffuse necking in the specimen gage section. Once the diffuse necking starts, the plastic deformation of the specimen becomes increasingly non-homogenous within the necked region. Questions arise regarding the validity of extrapolating the uniaxial stress-strain data to very large strains when one analyzes the forming failure of the sheet metal. Here an experimental technique and the related application procedure for obtaining rate-dependent, anisotropic, tensile stress-strain data of sheet metals at large strains up to fracture are described. Detailed plastic strain mapping results of a tapered commercially pure titanium thin sheet specimen are presented to characterize necking of the thin sheet. A brief discussion of a hybrid experimental and numerical methodology for estimating the strain hardening, strain rate sensitivity, and plastic anisotropy of sheet metals at large tensile strains is given. Preliminary results on necking in aluminum alloys, copper alloys, and low-carbon steel thin sheets are also presented.

Deformation mechanisms of Si-doped diamond-like carbon films under uniaxial tension conditions