bluuebunny/crossref_metadata_2025 huggingface dataset:
Transport and magnetic properties of Nd1−xPbxMnO3 single crystals
Authors: Ghosh Nilotpal , Elizabeth Suja , Bhat H. L. , Paulose P. L.
Published: September 2004

Transport and magnetic properties of flux-grown Nd1−xPbxMnO3 single crystals (x=0.15–0.5) are studied in the temperature range 300–77K and 280–2K, respectively. Magnetization measurements with a superconducting quantum interference device confirm a paramagnetic to ferromagnetic transition around 110, 121, 150, 160, and 178K for x=0.15, 0.2, 0.3, 0.4, and 0.5, respectively. Four probe resistivity measurements at low temperatures show a monotonic increase for x=0.15 which represents a ferromagnetic insulating (FMI) phase. For Nd0.8Pb0.2MnO3 there is a slope change present in the resistivity profile at 127K where metal to insulator transition (MI) sets in. For x=0.3 this MI transition is more prominent. However, both these samples have FMI phase at low temperature. When the concentration of lead increases (x>0.3) the sample displays a clear insulator to metal transition with a low temperature ferromagnetic metallic phase. On the basis of these measurements we have predicted the phase diagram of Nd1−xPbxMnO3. Magnetization measurements by a vibration sample magnetometer point out the appreciable differences between zero field cooled and field cooled profiles below the ferromagnetic to paramagnetic transition temperature for all x. These are indicative of magnetic frustration.
Electronic structure of self-assembled InAs quantum disks in an axial magnetic field and two-electron quantum-disk qubit
Authors: Dong Qing-Rui , Li Shu-Shen , Niu Zhi-Chuan , Feng Song-Lin , Zheng Hou-Zhi
Published: September 2004

We have studied the single-electron and two-electron vertically assembled quantum disks in an axial magnetic field using the effective mass approximation. The electron interaction is treated accurately by the direct diagonalization of the Hamiltonian matrix. We calculate the six energy levels of the single-electron quantum disks and the two lowest energy levels of the two-electron quantum disks in an axial magnetic field. The change of the magnetic field strongly modifies the electronic structures as an effective potential, leading to the splitting of the levels and the crossings between the levels. The effect of the vertical alignment on the electronic structures is discussed. It is demonstrated that the switching of the ground-state spin exists between S=0 and S=1. The energy difference ΔE between the lowest S=0 and S=1 states is shown as a function of the axial magnetic field. It is also found that the variation of the energy difference between the lowest S=0 and S=1 states in the strong-BS=0 state is fairly linear. Our results provide a possible realization for a qubit to be fabricated by current growth techniques.
Interfaces in AlGaSb∕GaSb multiquantum well structures
Authors: Bocchi C. , Lazzarini L. , Minelli M. , Nasi L. , Mukhamedzhanov E. Kh.
Published: September 2004

The Al0.4Ga0.6Sb∕GaSb∕Al0.4Ga0.6Sb quantum well (QW) structures were grown by a molecular beam epitaxy on the GaSb substrates at different growth temperatures (Tg) and with different Sb4∕Ga beam equivalent pressure ratios (BEPR) and were studied using high-resolution x-ray diffraction, photoluminescence (PL), and transmission electron microscopy. The x-ray diffraction analysis showed that the AlGaSb∕GaSb interfaces have smooth Al composition profiles with graded region thicknesses in the range of 0.83nm–2.17nm, depending upon the growth conditions; the normal interfaces are generally sharper than the inverse ones. The low-temperature PL spectra from QWs show relatively broad peaks and energies of the peak emission that can be related to the growth parameters. Good agreement was obtained between the PL peak energies and the transition energies calculated using a QWs model that takes into account the one-dimensionally graded confining potential for describing the experimentally determined Al composition profiles. The interface broadening can be due to Al and Ga segregation, Al diffusion, or Al and Ga intermixing. The broadening increases with the growth conditions that enhance the surface mobility of group-III species, such as high Tg and low V∕III BEPRs. The relatively large full widths at half maxima of the PL features were attributed to the Al composition fluctuations in the barriers, instead of the well thickness ones.
The impedance method in the theory of surface acoustic waves in periodic structures
Authors: Biryukov S. V. , Weihnacht M.
Published: September 2004

A method for determining surface impedances of periodic structures is proposed. The method is based on Biryukov’s equation for the surface impedance matrix of an inhomogeneous medium. This nonlinear integro-differential equation relates the impedance matrix with material parameter distributions. A solution of such equation is an alternative approach to boundary-value problems in contrast to the known finite element method. It is demonstrated that this equation in the case of periodic structures is reduced to a system of nonlinear ordinary differential equations of the first order in partial impedances. The developed impedance method is applied to a complex periodic system of elastic electrodes placed on a piezoelectric substrate. By analogy with the well-known function of surface effective permittivity, the infinite matrix of effective permittivity is introduced in an explicit form to characterize the piezoelectric substrate surface loaded by elastic strips. Due to this approach the known complex electromechanical boundary-value problem for calculation of interdigital transducers of surface acoustic waves is reduced to a pure electrical boundary-value problem. Then this last problem is solved by methods of electrostatics in the general case of arbitrary number of different electrodes per periodic cell. As a result, the electrical admittance matrix elements, relating electrode currents with voltages, are obtained. These elements are represented as ratios of dispersion equations’ left-hand side corresponding to various electrode connections. As examples, the frequency dependence of electrical admittances for some important multielectrode periodic cells are calculated.
Conducting atomic force microscopy study of phase transformation in silicon nanoindentation
Authors: Ho Shu-Te , Chang Yu-Hsien , Lin Heh-Nan
Published: September 2004

We report the study of phase transformation in the nanoindentation of Si by conducting atomic force microscopy. Distinctively high current features with a smallest size of around 20nm have been observed and correspond directly to the generated conductive Si-III and/or Si-XII phases under pressure release. Local current-voltage relationships on the high current sites have also been obtained and found to follow the Fowler-Nordheim tunneling equation.
Design of a confocal microfluidic particle sorter using fluorescent photon burst detection
Authors: Kunst Beno H. , Schots Arjen , Visser Antonie J. W. G.
Published: September 2004

An instrumental system is described for detecting and sorting single fluorescent particles such as microspheres, bacteria, viruses, or even smaller macromolecules in a flowing liquid. The system consists of microfluidic chips (biochips), computer controlled high voltage power supplies, and a fluorescence microscope with confocal optics. The confocal observation volume and detection electro-optics allow measurements of single flowing fluorescent particles. The output of the avalanche photodiode (single photon detector) is coupled to a real-time photon-burst detection device, which output can address the control of high voltage power supplies for sorting purposes. Liquid propulsion systems like electro-osmotic flow and plain electric fields to direct the particles through the observation volume have been tested and evaluated. The detection and real-time sorting of fluorescent microspheres are demonstrated. Applications of these biochips for screening of bacteriophages-type biolibraries are briefly discussed.
Reduction of spurious resonance peaks in microcantilever tuning through substrate contact surface reduction
Authors: Adams J. D. , York D. , Whisman N.
Published: September 2004

The tuning capabilities of otherwise similar tapping mode atomic force microscope (AFM) cantilever probes, for use in air, can be improved by patterning the backside of the respective probe substrate. Three designs were tested and rated against a standard unpatterned substrate. It was shown that lowering the probe substrate contact surface area decreased the occurrence of spurious resonance peaks and resulted in cleaner tuning curves as compared to probes with higher contact surface areas.
Enhanced resolution electric force microscopy with single-wall carbon nanotube tips
Authors: Wilson Neil R. , Macpherson Julie V.
Published: September 2004

Electric force microscopy (EFM) is widely used for studying small-scale electrical structures. Its applicability is limited by its spatial resolution and the difficulty of deconvoluting tip effects from the image. We demonstrate that the use of single-wall carbon nanotube (SWNT) tips increases EFM spatial resolution, allowing similar features to be distinguished with separations as small as 15nm. In addition, we show that the EFM response of the SWNT tips is consistent with a uniform-line-charge model.
Structural study of Fe doped and Ni substituted thermoelectric skutterudites by combined synchrotron and neutron powder diffraction and ab initio theory
Authors: Christensen M. , Iversen B. B. , Bertini L. , Gatti C. , Toprak M. , Muhammed M. , Nishibori E.
Published: September 2004

We present neutron and synchrotron powder-diffraction investigations as well as ab initio calculations to elucidate delicate structural features in doped skutterudites. Samples with assumed Fe doping were investigated (FeyCo4Sb12, y=0.4, 0.8, 1.0, and 1.6), as well as samples with formal Ni substitution (Co4−xNixSb12, x=0, 0.4, 0.8, and 1.2). The present study serves as a case story for the determination of fine structural details of thermoelectric skutterudites by diffraction methods in combination with ab initio calculations. We illustrate the problem of fluorescence in the conventional x-ray powder diffraction on the Fe-doped samples by a comparison with the neutron powder-diffraction data. On the series of the Ni-substituted samples, the neutron powder-diffraction data were collected to investigate the exact sitting of the Ni. The sample with the highest Ni substitution (Co2.8Ni1.2Sb12) was also used for high resolution, high-energy synchrotron powder diffraction measurements. These revealed that the sample consists of two skutterudite phases. A complete description of the Ni-substituted samples was obtained in tandem with ab initio calculations, which show that the system contains a Ni-rich (Co0.38Ni3.62Sb12) and a Ni-poor (Co3.76Ni0.24Sb12)) skutterudite phases.
Trapped charge induced gate oxide breakdown
Authors: Neugroschel Arnost , Wang Lingquan , Bersuker Gennadi
Published: September 2004

We investigate the physical mechanisms of breakdown in 21Å thick silicon dioxide gate oxides in n-channel and p-channel metal-oxide-semiconductor field-effect transistors. The oxide breakdown investigation is based on the direct measurement of the stress-induced charge trapped in the oxide using the direct-current current-voltage technique. The measurements show several parallel breakdown pathways with different oxide field dependence. Direct correlation was found between the steplike increase in the gate current and the oxide-trapped charge. The results point to the multistep character of the oxide breakdown that includes generation of precursor defects by the injection and trapping of either electrons or holes. The weakened defect (breakdown precursor) bonds may be broken by the applied electric field and temperature leading to generation of a conductive percolation path. Time-to-breakdown model is developed taking into the account the oxide field dependence of the different breakdown mechanisms.
Lattice diffusion and surface segregation of B during growth of SiGe heterostructures by molecular beam epitaxy: Effect of Ge concentration and biaxial stress
Authors: Portavoce A. , Gas P. , Berbezier I. , Ronda A. , Christensen J. S. , Svensson B.
Published: September 2004

Si 1 − x Ge x ∕ Si 1 − y Ge y ∕ Si ( 100 ) heterostructures grown by molecular beam epitaxy were used in order to study B surface segregation during growth and B lattice diffusion. Ge concentration and stress effects were separated. Analysis of B segregation during growth shows that (i) for layers in epitaxy on (100)Si, B segregation decreases with increasing Ge concentration, i.e., with increased compressive stress; (ii) for unstressed layers, B segregation increases with Ge concentration; (iii) at constant Ge concentration, B segregation increases for layers in tension and decreases for layers in compression. The contrasting behaviors observed as a function of Ge concentration in compressively stressed and unstressed layers can be explained by an increase of the equilibrium segregation driving force induced by Ge additions and an increase of near-surface diffusion in compressively stressed layers. Analysis of lattice diffusion shows that (i) in unstressed layers, B lattice diffusion coefficient decreases with increasing Ge concentration; (ii) at constant Ge concentration, the diffusion coefficient of B decreases with compressive biaxial stress and increases with tensile biaxial stress; (iii) the volume of activation of B diffusion [ΔV=−kT(dlnD∕dP)] is positive for biaxial stress while it is negative in the case of hydrostatic pressure. This confirms that under a biaxial stress the activation volume is reduced to the relaxation volume.
Filled and empty states of disordered GaN studied by x-ray absorption and emission
Authors: Ruck B. J. , Koo A. , Lanke U. D. , Budde F. , Trodahl H. J. , Williams G. V. M. , Bittar A. , Metson J. B. , Nodwell E. , Tiedje T. , Zimina A. , Eisebitt S.
Published: September 2004

X-ray absorption and emission spectroscopies are used to study the effects of short-ranged ordering on the electronic states of disordered GaN. Nanocrystalline samples with crystallites as small as 3nm exhibit an electronic structure resembling a broadened version of that in crystalline GaN. The electronic structure is even more heavily broadened in amorphous GaN films containing oxygen impurities or excess gallium. The oxygen containing films show an additional peak in the density of states just above the conduction band edge, and a downward shift of the valence band edge. The signature of molecular nitrogen trapped within the films is evident in both the absorption and emission spectra.
Transmission Electron Microscopy evidence of the growth of a magnetite layer acting as a spin filter in CoFe∕Al2O3∕CoFe magnetic tunnel junctions
Authors: Snoeck E. , Serin V. , Fourmeaux R. , Zhang Z. , Freitas P. P.
Published: September 2004

Magnetic tunnel junctions (MTJs) in which an iron oxide was deposited between the Al2O3 barrier and the pinned CoFe electrode were shown to exhibit a large tunneling magnetoresistance (TMR) value of 39% after annealing at 380°C. Local transmission electron microscopy (TEM) analyses were performed on the MTJs before and after the thermal treatments. In the as-deposited state, the high-resolution transmission electron microscopy (HRTEM) and the electron-energy-loss spectroscopy (EELS) demonstrate that the pure Fe exists at the Al2O3 interface, followed by the half-metallic ferromagnetic compound Fe3O4 that is formed when oxidizing the Fe layer. Similar structural studies show that the 380°C annealing achieves the oxidation of the iron into an extra Fe3O4 layer, now in contact with the Al2O3 barrier that leads to the high TMR values. These local TEM studies indicate that further annealing at higher temperature (400°C) induces the oxidation of the pinned CoFe electrode and the diffusion of Mn in the whole junction, resulting in a large decrease of the TMR.
Design and operational characteristics of a cast steel mass spectrometer
Authors: Blantocas Gene Q. , Ramos Henry J. , Wada Motoi
Published: September 2004

A cast steel magnetic sector mass analyzer is developed for studies of hydrogen and helium ion beams generated by a gas discharge compact ion source. The optimum induced magnetic flux density of 3500 G made it possible to scan the whole spectrum of hydrogen and helium ion species. Analysis of beam characteristics shows that the mass spectrometer sensitivity, and resolving power are approximately inversely proportional. The resolution is enhanced at higher pressures and lower current discharges. In contrast, the instrument sensitivity increased at higher current discharges and decreased at higher pressures. Calculations of the ultimate resolving power with reference to analyzer dimensions yield a numerical value of 30. System anomaly in the form of spherical aberrations was also analyzed using the paraxial beam envelope equation. Beam divergence is most significant at high discharge conditions where angular spread reaches an upper limit of 8.6°.
Effects of buried oxide layer on indium diffusion in separation by implantation of oxygen
Authors: Chen Peng , Zhu Ming , Fu Ricky K. Y. , Chu Paul K. , An Zhenghua , Liu Weili , Montgomery Neil , Biswas Sukanta
Published: September 2004

The diffusion of indium in both the top silicon and the buried oxide (BOX) layers in separation by implantation of oxygen (SIMOX) is investigated. For all indium-implanted samples, there is a significant redistribution of indium atoms from the top Si-BOX interface toward the bottom BOX-Si interface, thereby affecting the indium concentrations in the two silicon-BOX interfaces. In the case of relatively high-dose and high-energy indium implantation (1×1014cm−2 at 200keV), an anomalous segregation of indium is observed in both the bulk Si and the SIMOX substrates. However, there is a notable transportation of indium atoms from the top Si layer toward the bottom BOX-Si interface in the SIMOX, thereby affecting not only the indium concentrations in the two silicon-BOX interfaces but also the indium distribution in the top silicon layer. The unique indium-diffusion behavior in the SIMOX is believed to be a composite effect of indium trapping by the two Si-BOX interfaces, indium atoms being drawn away from the top silicon layer by the buried oxide, as well as implant damages in the top silicon. The asymmetrical structure of the BOX layer with Si islands accumulating at the bottom BOX-Si interface and the abundance of oxygen-related defects in the BOX layer are also believed to be responsible for the indium-diffusion behavior in the BOX layer.
Persistent photoexcited conducting states in functionalized pentacene
Authors: Brooks J. S. , Tokumoto T. , Choi E.-S. , Graf D. , Biskup N. , Eaton D. L. , Anthony J. E. , Odom S. A.
Published: September 2004

We report a study of long-lived photoexcited states in single crystals of functionalized forms of pentacene: the 6,13-bis(triisopropylsilylethynyl)pentacene and the 6,13-bis(triethylsilylethynyl)pentacene. These materials are organic semiconductors with band gaps in the range ≈1eV. The relaxation rate of these states is thermally activated, as determined from time-and temperature-dependent measurements of the photoconductivity after illumination, and these states can be produced in a stable population at temperatures below 150K. Consequently, after the low-temperature illumination, thermally stimulated current (TSC) is observed in the dark current with an increasing temperature. Analysis of the TSC signal allows estimation of energies associated with the excited states. Possible mechanisms for the current associated with the photoexcited states are discussed, and preliminary iodine-doped studies of the material are also presented.
Chaotic itinerancy in the oscillator neural network without Lyapunov functions
Authors: Uchiyama Satoki , Fujisaka Hirokazu
Published: September 2004

Chaotic itinerancy (CI), which is defined as an incessant spontaneous switching phenomenon among attractor ruins in deterministic dynamical systems without Lyapunov functions, is numerically studied in the case of an oscillator neural network model. The model is the pseudoinverse-matrix version of the previous model [S. Uchiyama and H. Fujisaka, Phys. Rev. E 65, 061912 (2002)] that was studied theoretically with the aid of statistical neurodynamics. It is found that CI in neural nets can be understood as the intermittent dynamics of weakly destabilized chaotic retrieval solutions.
Optical trapping
Authors: Neuman Keir C. , Block Steven M.
Published: September 2004

Since their invention just over 20 years ago, optical traps have emerged as a powerful tool with broad-reaching applications in biology and physics. Capabilities have evolved from simple manipulation to the application of calibrated forces on—and the measurement of nanometer-level displacements of—optically trapped objects. We review progress in the development of optical trapping apparatus, including instrument design considerations, position detection schemes and calibration techniques, with an emphasis on recent advances. We conclude with a brief summary of innovative optical trapping configurations and applications.
Determination of thermal diffusivity in a disk shaped sample using edge heating applications to Stainless Steel
Authors: Brantschen Segolene M. , Banish R. Michael , Wessling Francis C.
Published: September 2004

We describe a methodology for determining thermal diffusivities in real time by using temperature measurements at only two locations in a cylindrical sample. The technique is based on an analytical solution of heat transfer in a circular cylinder. This methodology does not require knowing the initial temperature increase or any timing between the applied and measured response. Starting with a cylinder heated on the outer surface and unique temperature measurement locations, the analytical solution for temperature at two specific radii can be approximated, after an initial transient, by a constant plus a single term that decreases exponentially with time. There are two special radii that fulfill the required condition. The data are analyzed by taking logarithms of the differences of the temperature versus time at these two radii, resulting in lines having slopes that are proportional to the thermal diffusivity. Surprisingly, other choices of the measurement locations lead to similar results, except with longer transients. Experimental results for type 304 stainless steel from room temperature to 450 °C agree with our numerical simulations and published data. This technique is applicable to solids and to liquids if heat transport due to convection is negligible.
Proapoptotic N-truncated BCL-xL protein activates endogenous mitochondrial channels in living synaptic terminals
Authors: Jonas Elizabeth A. , Hickman John A. , Chachar Mushtaque , Polster Brian M. , Brandt Teresa A. , Fannjiang Yihru , Ivanovska Iva , Basañez Gorka , Kinnally Kathleen W. , Zimmerberg Joshua , Hardwick J. Marie , Kaczmarek Leonard K.
Published: September 2004

Neuronal death is often preceded by functional alterations at nerve terminals. Anti- and proapoptotic BCL-2 family proteins not only regulate the neuronal death pathway but also affect excitability of healthy neurons. We found that exposure of squid stellate ganglia to hypoxia, a death stimulus for neurons, causes a cysteine protease-dependent loss of full-length antiapoptotic BCL-xL, similar to previous findings in mammalian cells. Therefore, to determine the direct effect of the naturally occurring proapoptotic cleavage product of BCL-xL on mitochondria, recombinant N-truncated BCL-xL was applied to mitochondria inside the squid presynaptic terminal and to purified mitochondria isolated from yeast. N-truncated BCL-xL rapidly induced large multi-conductance channels with a maximal conductance significantly larger than those produced by full-length BCL-xL. This activity required the hydrophobic C terminus and the BH3 domain of BCL-xL. Moreover, N-truncated BCL-xL failed to produce any channel activity when applied to plasma membranes, suggesting that a component of the mitochondrial membrane is necessary for its actions. Consistent with this idea, the large channels induced by N-truncated BCL-xL are inhibited by NADH and require the presence of VDAC, a voltage-dependent anion channel present in the outer mitochondrial membrane. These observations suggest that the mitochondrial channels specific to full-length and N-truncated BCL-xL contribute to their opposite effects on synaptic transmission, and are consistent with their opposite effects on the cell death pathway.
DNA heterogeneity and phosphorylation unveiled by single-molecule electrophoresis
Authors: Wang Hui , Dunning James E. , Huang Albert P.-H. , Nyamwanda Jacqueline A. , Branton Daniel
Published: September 2004

Broad-spectrum analysis of DNA and RNA samples is of increasing importance in the growing field of biotechnology. We show that nanopore measurements may be used to assess the purity, phosphorylation state, and chemical integrity of nucleic acid preparations. In contrast with gel electrophoresis and mass spectrometry, an unprecedented dynamic range of DNA sizes and concentrations can be evaluated in a single data acquisition process that spans minutes. Because the molecule information is quantized and digitally recorded with single-molecule resolution, the sensitivity of the system can be adjusted in real time to detect trace amounts of a particular DNA species.
Delayed neuropathology after carbon monoxide poisoning is immune-mediated
Authors: Thom Stephen R. , Bhopale Veena M. , Fisher Donald , Zhang Jie , Gimotty Phyllis
Published: September 2004

The neuropathological sequelae of carbon monoxide (CO) poisoning cannot be explained by hypoxic stress alone. CO poisoning also causes adduct formation between myelin basic protein (MBP) and malonylaldehyde, a reactive product of lipid peroxidation, resulting in an immunological cascade. MBP loses its normal cationic characteristics, and antibody recognition of MBP is altered. Immunohistochemical evidence of degraded MBP occurs in brain over days, along with influx of macrophages and CD-4 lymphocytes. Lymphocytes from CO-poisoned rats subsequently exhibit an auto-reactive proliferative response to MBP, and there is a significant increase in the number of activated microglia in brain. Rats rendered immunologically tolerant to MBP before CO poisoning exhibit acute biochemical changes in MBP but no lymphocyte proliferative response or brain microglial activation. CO poisoning causes a decrement in learning that is not observed in immunologically tolerant rats. These results demonstrate that delayed CO-mediated neuropathology is linked to an adaptive immunological response to chemically modified MBP.
Protein KinaseCδ-Calmodulin Crosstalk Regulates Epidermal Growth Factor Receptor Exit from Early Endosomes
Authors: Lladó Anna , Tebar Francesc , Calvo Maria , Moretó Jemina , Sorkin Alexander , Enrich Carlos
Published: September 2004

We have recently shown that calmodulin antagonist W13 interferes with the trafficking of the epidermal growth factor receptor (EGFR) and regulates the mitogen-activated protein kinase (MAPK) signaling pathway. In the present study, we demonstrate that in cells in which calmodulin is inhibited, protein kinase C (PKC) inhibitors rapidly restore EGFR and transferrin trafficking through the recycling compartment, although onward transport to the degradative pathway remains arrested. Analysis of PKC isoforms reveals that inhibition of PKCδ with rottlerin or its down-modulation by using small interfering RNA is specifically responsible for the release of the W13 blockage of EGFR trafficking from early endosomes. The use of the inhibitor Gö 6976, specific for conventional PKCs (α, β, and γ), or expression of dominant-negative forms of PKCλ, ζ, or ϵ did not restore the effects of W13. Furthermore, in cells treated with W13 and rottlerin, we observed a recovery of brefeldin A tubulation, as well as transport of dextran-fluorescein isothiocyanate toward the late endocytic compartment. These results demonstrate a specific interplay between calmodulin and PKCδ in the regulation of the morphology of and trafficking from the early endocytic compartment.
The Complex of Ciliary Neurotrophic Factor-Ciliary Neurotrophic Factor Receptor α Up-Regulates Connexin43 and Intercellular Coupling in Astrocytes via the Janus Tyrosine Kinase/Signal Transducer and Activator of Transcription Pathway
Authors: Ozog Mark A. , Bernier Suzanne M. , Bates Dave C. , Chatterjee Bishwanath , Lo Cecilia W. , Naus Christian C.G.
Published: September 2004

Cytokines regulate numerous cell processes, including connexin expression and gap junctional coupling. In this study, we examined the effect of ciliary neurotrophic factor (CNTF) on connexin43 (Cx43) expression and intercellular coupling in astrocytes. Murine cortical astrocytes matured in vitro were treated with CNTF (20 ng/ml), soluble ciliary neurotrophic factor receptor α (CNTFRα) (200 ng/ml), or CNTF-CNTFRα. Although CNTF and CNTFRα alone had no effect on Cx43 expression, the heterodimer CNTF-CNTFRα significantly increased both Cx43 mRNA and protein levels. Cx43 immunostaining correlated with increased intercellular coupling as determined by dye transfer analysis. By using the pharmacological inhibitor α-cyano-(3,4-dihydroxy)-N-benzylcinnamide (AG490), the increase in Cx43 was found to be dependent on the Janus tyrosine kinase/signal transducer and activator of transcription (JAK/STAT) pathway. Immunocytochemical analysis revealed that CNTF-CNTFRα treatment produced nuclear localization of phosphorylated STAT3, whereas CNTF treatment alone did not. Transient transfection of constructs containing various sequences of the Cx43 promoter tagged to a LacZ reporter into ROS 17/2.8 cells confirmed that the promoter region between -838 to -1693 was deemed necessary for CNTF-CNTFRα to induce heightened expression. CNTF-CNTFRα did not alter Cx30 mRNA levels, suggesting selectivity of CNTF-CNTFRα for connexin signaling. Together in the presence of soluble receptor, CNTF activates the JAK/STAT pathway leading to enhanced Cx43 expression and intercellular coupling.
Sp1 Is Involved in Akt-mediated Induction of VEGF Expression through an HIF-1–independent Mechanism
Authors: Pore Nabendu , Liu Shuang , Shu Hui-Kuo , Li Bin , Haas-Kogan Daphne , Stokoe David , Milanini-Mongiat Julie , Pages Gilles , O'Rourke Donald M. , Bernhard Eric , Maity Amit
Published: September 2004

Increased expression of vascular endothelial growth factor (VEGF) contributes to the growth of many tumors by increasing angiogenesis. Although hypoxia is a potent inducer of VEGF, we previously showed that epidermal growth factor receptor amplification and loss of PTEN, both of which can increase phosphatidylinositol-3-kinase (PI3K) activity, increase VEGF expression. Using both adenoviral vectors and a cell line permanently expressing constitutively active myristoylated Akt (myrAkt), we show that activation of Akt, which is downstream of PI3K, increases VEGF expression in vitro and increases angiogenesis in a Matrigel plug assay. Transient transfection experiments using reporter constructs containing the VEGF promoter showed that up-regulation of VEGF by Akt is mediated through Sp1 binding sites located in the proximal promoter. Small interfering RNA directed against Sp1 prevented the induction of VEGF mRNA in response to myrAkt but not to hypoxia. Expression of myrAkt is associated with increased phosphorylation of Sp1 and its increased binding to a probe corresponding to the -88/-66 promoter region. In conclusion, our results indicate that Sp1 is required for transactivation of the VEGF by Akt. Others have proposed that the PI3K/Akt pathway can increase VEGF expression via the hypoxia-inducible factor 1 (HIF-1); however, our results suggest an alternative mechanism can also operate.
The Interaction of Neurofilaments with the Microtubule Motor Cytoplasmic Dynein
Authors: Wagner Oliver I. , Ascaño Jennifer , Tokito Mariko , Leterrier Jean-Francois , Janmey Paul A. , Holzbaur Erika L. F.
Published: September 2004

Neurofilaments are synthesized in the cell body of neurons and transported outward along the axon via slow axonal transport. Direct observation of neurofilaments trafficking in live cells suggests that the slow outward rate of transport is due to the net effects of anterograde and retrograde microtubule motors pulling in opposition. Previous studies have suggested that cytoplasmic dynein is required for efficient neurofilament transport. In this study, we examine the interaction of neurofilaments with cytoplasmic dynein. We used fluid tapping mode atomic force microscopy to visualize single neurofilaments, microtubules, dynein/dynactin, and physical interactions between these neuronal components. AFM images suggest that neurofilaments act as cargo for dynein, associating with the base of the motor complex. Yeast two-hybrid and affinity chromatography assays confirm this hypothesis, indicating that neurofilament subunit M binds directly to dynein IC. This interaction is blocked by monoclonal antibodies directed either to NF-M or to dynein. Together these data suggest that a specific interaction between neurofilament subunit M and cytoplasmic dynein is involved in the saltatory bidirectional motility of neurofilaments undergoing axonal transport in the neuron.
Distinct Roles for the Hsp40 and Hsp90 Molecular Chaperones during Cystic Fibrosis Transmembrane Conductance Regulator Degradation in Yeast
Authors: Youker Robert T. , Walsh Peter , Beilharz Traude , Lithgow Trevor , Brodsky Jeffrey L.
Published: September 2004

Aberrant secreted proteins can be destroyed by ER-associated protein degradation (ERAD), and a prominent, medically relevant ERAD substrate is the cystic fibrosis transmembrane conductance regulator (CFTR). To better define the chaperone requirements during CFTR maturation, the protein was expressed in yeast. Because Hsp70 function impacts CFTR biogenesis in yeast and mammals, we first sought ER-associated Hsp40 cochaperones involved in CFTR maturation. Ydj1p and Hlj1p enhanced Hsp70 ATP hydrolysis but CFTR degradation was slowed only in yeast mutated for both YDJ1 and HLJ1, suggesting functional redundancy. In contrast, CFTR degradation was accelerated in an Hsp90 mutant strain, suggesting that Hsp90 preserves CFTR in a folded state, and consistent with this hypothesis, Hsp90 maintained the solubility of an aggregation-prone domain (NBD1) in CFTR. Soluble ERAD substrate degradation was unaffected in the Hsp90 or the Ydj1p/Hlj1p mutants, and surprisingly CFTR degradation was unaffected in yeast mutated for Hsp90 cochaperones. These results indicate that Hsp90, but not the Hsp90 complex, maintains CFTR structural integrity, whereas Ydj1p/Hlj1p catalyze CFTR degradation.
Attenuation of Transient Focal Cerebral Ischemic Injury in Transgenic Mice Expressing a Mutant ICE Inhibitory Protein
Authors: Hara Hideaki , Fink Klaus , Endres Matthias , Friedlander Robert M. , Gagliardini Valeria , Yuan Junying , Moskowitz Michael A.
Published: September 2004

We used transgenic mice expressing a dominant negative mutation of interleukin-1β converting enzyme (ICE) (C285G) in a model of transient focal ischemia in order to investigate the role of ICE in ischemic brain damage. Transgenic mutant ICE mice (n = 11) and wild-type littermates (n = 9) were subjected to 3 h of middle cerebral artery occlusion followed by 24 h of reperfusion. Cerebral infarcts and brain swelling were reduced by 44% and 46%, respectively. Neurological deficits were also significantly reduced. Regional CBF, blood pressure, core temperature, and heart rate did not differ between groups when measured for up to 1 h after reperfusion. Increases in immunoreactive IL-1β levels, observed in ischemic wild-type brain at 30 min after reperfusion, were 77% lower in the mutant strain, indicating that proIL-1β cleavage is inhibited in the mutants. DNA fragmentation was reduced in the mutants 6 and 24 h after reperfusion. Hence, endogenous expression of an ICE inhibitor confers resistance to cerebral ischemia and brain swelling. Our results indicate that down-regulation of ICE expression might provide a useful therapeutic target in cerebral ischemia.
Detection of Higher-Order 50- and 10-kbp DNA Fragments before Apoptotic Internucleosomal Cleavage after Transient Cerebral Ischemia
Authors: MacManus John P. , Rasquinha Ingrid , Tuor Ursala , Preston Edward
Published: September 2004

DNA fragments of 50 and 10 kbp were found in ischemic brain in adult rats following two-vessel occlusion or in neonates following hypoxia–ischemia. These higher-order fragments were detected before any laddered oligonucleosomal DNA fragmentation characteristic of apoptosis. Both the 50- and 10-kbp fragments were also detected during necrosis produced by decapitation, but these led to smeared smaller fragments, not laddered patterns. End-group analysis showed the presence of both 3′-OH and 5′-OH ends in both the 50- and 10-kbp fragments but the predominance of 3′-OH ends in the laddered fragments. A higher proportion of 5′-OH to 3′-OH ends was found in the 10-kbp fragment compared to the larger 50-kbp fragment, suggesting a selective degradation of the 50-kbp DNA fragment to the laddered oligonucleosomal patterns. Overall, the mode of DNA fragmentation appeared different from that described in classic apoptosis of thymocytes.
Repeat Positron Emission Tomographic Studies in Transient Middle Cerebral Artery Occlusion in Cats: Residual Perfusion and Efficacy of Postischemic Reperfusion
Authors: Heiss Wolf-Dieter , Graf Rudolf , Löttgen Jan , Ohta Kouichi , Fujita Toshiaki , Wagner Rainer , Grond Martin , Weinhard Klaus
Published: September 2004

The wider clinical acceptance of thrombolytic therapy for ischemic stroke has focused more attention on experimental models of reversible focal ischemia. Such models enable the study of the effect of ischemia of various durations and of reperfusion on the development of infarctions. We used high-resolution positron emission tomography (PET) to assess cerebral blood flow (CBF), cerebral metabolic rate of oxygen (CMRO2), oxygen extraction fraction (OEF), and cerebral metabolic rate of glucose (CMRglc) before, during, and up to 24 h after middle cerebral artery occlusion (MCAO) in cats. After determination of resting values, the MCA was occluded by a transorbital device. The MCA was reopened after 30 min in five, after 60 min in 11, and after 120 min in two cats. Whereas all cats survived 30-min MCAO, six died after 60-min and one after 120-min MCAO during 6–20 h of reperfusion. In those cats surviving the first day, infarct size was determined on serial histologic sections. The arterial occlusion immediately reduced CBF in the MCA territory to <40% of control, while CMRO2 was less affected, causing an increase in OEF. Whereas in the cats surviving 24 h of reperfusion after 60- and 120-min MCAO, OEF remained elevated throughout the ischemic episode, the initial OEF increase had already disappeared during the later period of ischemia in those cats that died during the reperfusion period. After 30-min MCAO, the reperfusion period was characterized by a transient reactive hyperemia and fast normalization of CBF, CMRO2, and CMRglc, and no or only small infarcts in the deep nuclei were found in histology. After 60- and 120-min MCAO, the extent of hyperperfusion was related to the severity of ischemia, decreased CMRO2 and CMRglc persisted, and cortical/subcortical infarcts of varying sizes developed. A clear difference was found in the flow/metabolic pattern between surviving and dying cats: In cats dying during the observation period, extended postischemic hyperperfusion accompanied large defects in CMRO2 and CMRglc, large infarcts developed, and intracranial pressure increased fatally. In those surviving the day after MCAO, increased OEF persisted over the ischemic episode, postischemic hyperperfusion was less severe and shorter, and the perfusional and metabolic defects as well as the final infarcts were smaller. These results stress the importance of the severity of ischemia for the further course after reperfusion and help to explain the diverging outcome after thrombolysis, where a relation between the residual flow and the effectiveness of reperfusion was also observed.
The Effects of SB 206284A, a Novel Neuronal Calcium-Channel Antagonist, in Models of Cerebral Ischemia
Authors: Wood Nigel I. , Barone Frank C. , Benham Chris D. , Brown Tom H. , Campbell Colin A. , Cooper David G. , Evans Martyn L. , Feuerstein Giora Z. , Hamilton Tom C. , Harries Mark H. , King Penny D. , Meakin Jackie E. , Murkitt Karen L. , Patel Sara R. , Price William J. , Roberts Jenny C. , Rothaul Alan L. , Samson Nicole A. , Smith Susan J. , Hunter A. Jackie
Published: September 2004

The effects of SB 206284A, 1-[7-(4-benzyloxyphenoxy)heptyl] piperidine hydrochloride, have been investigated in vitro on calcium and sodium currents in rat-cultured dorsal root ganglion (DRG) neurones and potassium-mediated calcium influx in rat synaptosomes. Cardiovascular hemodynamic effects in both anesthetized and conscious rats, and neuroprotective activity in in vivo cerebral ischemia models were also investigated. In the rat DRG cells, SB 206284A caused almost complete block of the sustained inward Ca2+ current (IC50 = 2.4μM), suggesting that the compound is an effective blocker of slowly inactivating, high-voltage calcium current. SB 206284A reduced locomotor hyperactivity in the gerbil bilateral carotid artery occlusion model without affecting ischemia-induced damage in the hippocampal CA1 region. In the rat middle cerebral artery occlusion model, SB 206284A reduced lesion volume in the posterior forebrain, and in the rat photochemical cortical lesion model, lesion volume was reduced even when treatment was delayed until 4 hours after occlusion. At neuroprotective doses, SB 206284A had no cardiovascular effects. These findings show that SB 206284A is a novel calcium channel antagonist that shows neuroprotective properties.
Quantification of Amphetamine-Induced Changes in [11C]Raclopride Binding with Continuous Infusion
Authors: Carson Richard E. , Breier Alan , de Bartolomeis Andrea , Saunders Richard C. , Su Tom P. , Schmall Bernard , Der Margaret G. , Pickar David , Eckelman William C.
Published: September 2004

Positron emission tomography and single-photon emission computer tomography receptor-binding ligands can be used to measure changes in neurotransmitter levels. In particular, amphetamine-induced dopamine release has been assessed with [11C]raclopride by paired bolus injections and with [123I]iodobenzamide by using a single bolus plus infusion (B/I) study. Here, we measured the change in [11C]raclopride-specifìc binding in rhesus monkeys after i.v. administration of 0.4 mg/kg amphetamine by using both the bolus and B/I paradigms. Paired bolus studies (control and postamphetamine) were analyzed using compartment modeling and graphical analysis with a new plasma metabolite model to measure the total distribution volume (VT). Specific binding, calculated with three measures linearly proportional to the binding potential, demonstrated a 22–42% reduction in the postamphetamine study. VT values from B/I studies were determined by the tissue-to-plasma ratio at equilibrium, in addition to the bolus methods. There was good agreement between the control VT values between bolus and B/I studies. The amphetamine-induced change in specific binding in B/I studies was 19 ± 16%, measured directly from tissue radioactivity levels. This study demonstrates that stimulus-induced changes in specific binding can be measured with a single [11C]raclopride study using the B/I method.
Endothelin-1 as a Mediator of Endothelial Cell–Pericyte Interactions in Bovine Brain Capillaries
Authors: Dehouck Marie-Pierre , Vigne Paul , Torpier Gérard , Breittmayer Jean Philippe , Cecchelli Roméo , Frelin Christian
Published: September 2004

Endothelial cells and pericytes are closely associated in brain capillaries. Together with astrocytic foot processes, they form the blood–brain barrier. Capillaries were isolated from bovine brain cortex. Pure populations of endothelial cells and pericytes were isolated and cultured in vitro. Polarized monolayers of endothelial cells preferentially secreted immunoreactive endothelin-1 (Et-1) at their abluminal (brain-facing) membrane. They did not express receptors for Et-1. Pericytes expressed BQ-123-sensitive ETA receptors for endothelins as evidenced by 125I-Et-1 binding experiments. These receptors were coupled to phospholipase C as demonstrated by intracellular calcium measurements using indo-1-loaded cells. Addition of Et-1 to pericytes induced marked changes in the cell morphology that were associated with a reorganization of F-actin and intermediate filaments. It is concluded that Et-1 is a paracrine mediator at the bovine blood–brain barrier and that capillary pericytes are target cells for endothelium-derived Et-1.
Nitric Oxide and Effects of Cationic Polypeptides in Canine Cerebral Arteries
Authors: Kinoshita Hiroyuki , Katusic Zvonimir S.
Published: September 2004

Cationic polypeptides are released by activated leukocytes and may play an important role in the regulation of vascular tone. Effects of cationic polypeptides on cerebral vascular tone have not been studied. The present experiments were designed to determine if synthetic cationic polypeptides, poly-L-arginine and poly-L-lysine, affect the function of cerebral arteries. Rings of canine basilar arteries with and without endothelium were suspended for isometric force recording. Poly-L-arginine (10–8–10–7 M) and poly-L-lysine (10–8–10–7 M) caused endothelium-dependent relaxations. A nitric oxide synthase inhibitor, NG-nitro-L-arginine methyl ester (10–4 M), and a nitric oxide scavenger, oxyhemoglobin (3 × 10–6 M), inhibited relaxations in response to cationic polypeptides. Negatively charged molecules, heparin (1 U/ml) and dextran sulfate (10 mg/ml), also inhibited relaxations to poly-L-arginine or poly-L-lysine. Higher concentrations of poly-L-arginine (10–6–10–5 M) and poly-L-lysine (10–6–10–5 M) induced endothelium-independent contractions. A protein kinase C inhibitor, staurosporine (10–8 M), abolished these contractions. Heparin (10 U/ml) and dextran sulfate (100 mg/ml) inhibited the contractile effect of cationic polypeptides but did not affect contractions to phorbol 12,13-dibutyrate. Poly-L-arginine (10–6 M) and poly-L-lysine (10–6 M) abolished endothelium-dependent relaxations in response to bradykinin (10–10–10–6 M) or calcium ionophore A23187 (10–9–10–6 M). Heparin (50 U/ml) and dextran sulfate (200 mg/ml) restored endothelium-dependent relaxations to bradykinin (10–10–10–6 M) in arteries exposed to poly-L-arginine (10–6 M) or poly-L-lysine (10–6 M). These studies demonstrate that in the lower concentration range (10–8–10–7 M), poly-L-arginine and poly-L-lysine induce endothelium-dependent relaxations by production of nitric oxide via charge-dependent activation of endothelial nitric oxide synthase. In the higher concentration range (10–6–10–5 M), cationic polypeptides cause endothelium-independent contractions as well as impairment of endothelium-dependent relaxations in response to bradykinin and A23187. These contractions and inhibition of endothelium-dependent relaxations are also mediated by a charge-dependent mechanism and may involve activation of protein kinase C.