bluuebunny/crossref_metadata_2025 huggingface dataset:
Stimulated scattering instability of lower hybrid waves
Authors: Kuo S. P. , Lee M. C.
Published: July 2002

The excitation of a low-frequency quasimode together with two lower hybrid sidebands by a lower hybrid pump is analyzed for laboratory plasmas. This instability is excited in a narrow spectral range around the scale length of the quasimode which is about half the pump wavelength. The growth rates are of order of the ion cyclotron frequency. This instability can modify the intensity distribution of the pump wave and as a consequence affect the energy deposition of the pump wave in plasmas. The smaller parallel phase velocites of the forwardscattered lower hybrid sidebands are conducive to the acceleration of bulk electrons along the magnetic field.
Plasma formation and equilibrium in a high β tokamak
Authors: Deniz A. V. , Li X. L. , Marshall T. C.
Published: July 2002

The formation and equilibrium of a high β tokamak in the Columbia High Beta Tokamak (HBT) (Plasma Physics and Controlled Nuclear Fusion Research 1986, in press) is documented using magnetic probes as the principle diagnostic. The formation from a toriodal field stabilized Z pinch is simulated with 2-D resistive magnetohydrodynamic (MHD) equations. From the measurement of the magnetic field by high-impedence internal magnetic probes, the poloidal flux, toroidal current, and safety factor are calculated. In addition, the plasma position and cross-sectional shape are determined. The tokamak has a major radius of about 0.24 m, a minor radius of about 0.05 m, and a toroidal current density of about 106 A/m2. The high β tokamak plasma is observed to have an outward shift in major radius of both the magnetic center and the peak of the toroidal current density. The magnetic center moves inward in major radius after 20–30 μsec, presumably because the plasma maintains major radial equilibrium as its pressure decreases because of the radiation of impurity atoms.
Nonlinear flows along magnetic flux tubes: Mathematical structure and exact simple wave solutions
Authors: Ferriz-Mas A.
Published: July 2002

The mathematical structure of nonlinear, isentropic longitudinal flows of an ideal magnetohydrodynamic plasma confined to a magnetic flux tube embedded in a quiescent nonmagnetic fluid is investigated. Exact analytical solutions are derived for a special type of nonlinear flow in the absence of gravity: the simple waves, for which all the unknowns depend on single functions of space and time. These exhibit analytically the formation of shock waves. Emphasis is placed on new features introduced by the magnetic distensibility, which acts as an additional restoring force, as compared with the hydrodynamic flow of gas along a rigid tube. Introducing a series expansion in a suitable parameter, it is shown that the hydrodynamic problem can be considered as the zeroth-order approach to the magnetic flux tube problem. Finally, the motion in a magnetic flux tube under the action of a piston advancing in a prescribed manner has been briefly considered. This problem is of current interest in relation to the generation of tube-guided waves in the solar atmosphere.
Truncation effect in a Fourier series expansion on the nonlinear evolution of disturbances in free convection between vertical parallel plates
Authors: Fujimura Kaoru , Mizushima Jiro
Published: July 2002

The effect of truncation in a Fourier series expansion on the nonlinear evolution of a disturbance has been investigated numerically in free convection between vertical parallel plates with different temperatures. Qualitatively different solutions were obtained, depending on the number of terms involved in the expansion. Transition from a stationary wave to an oscillatory traveling wave associated with the breakdown of the spatial symmetry was also obtained
Lyapunov stability analysis of magnetohydrodynamic plasma equilibria with axisymmetric toroidal flow
Authors: Almaguer Juan Antonio , Hameiri Eliezer , Herrera Julio , Holm Darryl D.
Published: July 2002

Lyapunov stability conditions for ideal magnetohydrodynamic (MHD) plasmas with mass flow in axisymmetric toroidal geometry are determined in the Eulerian representation. Axisymmetric equilibrium solutions of ideal MHD are associated to critical points of a nonlinearly conserved Lyapunov functional consisting of the sum of the total energy and the following flux-weighted quantities: the circulation along field lines, the angular momentum, the toroidal flux, and the mass content within each flux tube. Conditions sufficient for Lyapunov stability of these equilibria against axisymmetric perturbations are found by taking advantage of the Hamiltonian formalism for ideal MHD. In particular [see Eq. (60)], it is sufficient for Lyapunov stability under linearized dynamics that an axisymmetric equilibrium be subsonic in the appropriate rotating frame, lie in the first elliptic regime of the Bernoulli–Grad–Shafranov (BGS) system of equations, and satisfy one additional, more complicated, condition. Effects of boundary conditions, nonlinearity, and three-dimensionality on MHD stability are also discussed.
A comparative study for an eighth harmonic cusptron tube operating in the π mode and 2π mode
Authors: Kuo S. P. , Tiong K. K. , Miller P. E. , Namkung W.
Published: July 2002

Through the use of a computer particle simulation, a study of the operational characteristics of an eighth harmonic cusptron device is presented. The functional dependence of the saturation efficiency upon (1) the beam energy and (2) the ratio of the outer and inner radii of the slotted waveguide is determined for both the π mode and 2π mode. The results show that the π mode has a better conversion efficiency than the 2π mode.
Excitation of lower hybrid waves by a gyrating ion beam in a magnetized plasma cylinder
Authors: Sharma Anamika , Tripathi V. K.
Published: July 2002

A long plasma cylinder with uniform density supports lower hybrid modes. In the presence of an energetic gyrating ion beam these modes are driven unstable via cyclotron harmonic interaction. A nonlocal theory of this process has been developed. It is seen that the modes with wavefunction maximum at the location of the beam possess a large growth rate. The instability frequency is close to the ion plasma frequency.
Cavitons are solitons: An integrable ponderomotive system
Authors: Kaup D. J.
Published: July 2002

A new set of integrable soliton equations is presented. It is demonstrated that these equations describe caviton formation in a plasma. The time evolution of the scattering data is obtained and it is found that, unlike other integrable equations where the magnitude of the reflection coefficient was always a constant of the motion, here it may grow and/or decay in time. One physical manifestation of the growth in the reflection coefficient is the pileup of density ripples in front of a microwave source. Also, only cavitons, which are the solitons, are left when the decay conditions dominate.
Extensional deformation of Newtonian liquid bridges
Authors: Gaudet S. , McKinley G. H. , Stone H. A.
Published: July 2002

A numerical investigation is presented of axisymmetric, static and elongating, viscous Newtonian liquid bridges confined between identical circular disks. The time-dependent interface shapes and applied forces on the end plates, which separate at a constant prescribed velocity, are calculated as functions of the capillary number, the viscosity ratio between the inner and outer fluids, and an initial bridge configuration characterized by the aspect ratio. The numerical simulations are in excellent agreement with available experimental data and provide useful insight into the different dynamical responses of extending liquid bridge configurations. In particular, liquid bridges surrounded by fluids of a relatively small viscosity deform in a fore-aft symmetrical manner and undergo breakup sooner than in the case of relatively viscous outer fluids, which also require a greater applied force on the end plates to maintain the desired motion. Decreasing the capillary number (increasing interfacial tension) and the initial aspect ratio result in shorter bridge lengths prior to breakup and an increase in the applied forces on the end plates.
Application of neural networks to turbulence control for drag reduction
Authors: Lee Changhoon , Kim John , Babcock David , Goodman Rodney
Published: July 2002

A new adaptive controller based on a neural network was constructed and applied to turbulent channel flow for drag reduction. A simple control network, which employs blowing and suction at the wall based only on the wall-shear stresses in the spanwise direction, was shown to reduce the skin friction by as much as 20% in direct numerical simulations of a low-Reynolds number turbulent channel flow. Also, a stable pattern was observed in the distribution of weights associated with the neural network. This allowed us to derive a simple control scheme that produced the same amount of drag reduction. This simple control scheme generates optimum wall blowing and suction proportional to a local sum of the wall-shear stress in the spanwise direction. The distribution of corresponding weights is simple and localized, thus making real implementation relatively easy. Turbulence characteristics and relevant practical issues are also discussed.
A note on the lift force on a spherical bubble or drop in a low-Reynolds-number shear flow
Authors: Legendre Dominique , Magnaudet Jacques
Published: July 2002

The analytical solution giving the lift force on a spherical drop of arbitrary viscosity moving in a low-Reynolds-number linear shear flow is derived through a matched asymptotic expansion procedure. This solution extends the result established by Saffman in the particular case of a solid sphere [J. Fluid Mech. 22, 385 (1965)]. It appears that, to leading order, the lift force is proportional to the square of the vorticity present at the surface of the drop. As a consequence the lift force decreases with the viscosity of the drop and the ratio of the forces experienced by an inviscid bubble and a solid sphere is equal to (2/3)2.
Cavity flows of elastic liquids: Purely elastic instabilities
Authors: Pakdel Peyman , McKinley Gareth H.
Published: July 2002

Experimental observations of a purely elastic flow instability occurring in the lid-driven cavity flow of two semi-dilute polymer solutions are reported and the effect of cavity aspect ratio on the dynamical structure of the unstable flow is quantitatively investigated. The spatial and temporal characteristics of the secondary flow are measured using flow visualization, laser Doppler velocimetry, and digital particle image velocimetry. At the onset conditions the disturbances appear in the form of spatially periodic flow cells which propagate along the neutral direction of the cavity. The secondary flow structure is analogous to the Taylor–Görtler vortices observed in inertially driven hydrodynamic instabilities. The critical onset conditions for two elastic test fluids and five different aspect ratios correlate with a recently proposed dimensionless stability criterion which incorporates measures of the local streamline radius of curvature and the non-Newtonian normal stresses in the flow domain.
Direct numerical simulation of turbulent channel flow up to Reτ=590
Authors: Moser Robert D. , Kim John , Mansour Nagi N.
Published: July 2002

Numerical simulations of fully developed turbulent channel flow at three Reynolds numbers up to Reτ=590 are reported. It is noted that the higher Reynolds number simulations exhibit fewer low Reynolds number effects than previous simulations at Reτ=180. A comprehensive set of statistics gathered from the simulations is available on the web at http://www.tam.uiuc.edu/Faculty/Moser/channel.
Ion acoustic solitons in finite ion temperature inhomogeneous plasmas having negative ions
Authors: Malik H. K. , Dahiya R. P.
Published: July 2002

Analytical investigations are carried out for ion acoustic soliton propagation in a spatially inhomogeneous plasma having finite temperature negative and positive ions. The soliton characteristics are described by a Korteweg–de Vries equation which has additional terms. Rarefactive solitons are possible in the negative ion containing plasma, and their existence is decided by the nonlinearity coefficient of this equation. The soliton amplitude decreases for higher-density gradients, but it increases with the increasing ratio of negative-to-positive ion density. Effect of temperature and drift velocity, of both the positive and negative ions on soliton propagation, has been analyzed.
Impossibility of plasma instabilities in isotropic quantum plasmas
Authors: Bonitz M.
Published: July 2002

It is well known that Vlasov plasmas with spherical momentum symmetry cannot exhibit longitudinal plasma instabilities, regardless of the monotonic behavior of the distribution function. It is shown that this property holds for quantum plasmas also, at least within the random phase approximation.
Computer modeling of plasma: Past, present, and future
Authors: Dawson John M.
Published: July 2002

Computer modeling has become a powerful tool for exploring the physics of plasmas. Early computers could handle only relatively simple models but nevertheless showed that these devices could shed a lot of light on the complex physics of plasmas. This capability has proved not only valuable to research but also is becoming an important teaching tool; modeling allows students to experience in concrete ways plasma phenomena which are otherwise presented only abstractly. Present-day plasma models combined with parallel computing provide sufficient power that numerical modeling of laboratory experiments on complex devices has become possible. Two examples of simulations are discussed in some detail: The ‘‘Beat Wave Accelerator’’ and the ‘‘Numerical Tokamak.’’
Dust acoustic waves in a direct current glow discharge
Authors: Thompson C. , Barkan A. , D’Angelo N. , Merlino R. L.
Published: July 2002

An experimental investigation of dust acoustic (DA) waves in a dc glow discharge plasma is described. The glow discharge is formed between a 3 cm anode disk and the grounded walls of a 60 cm diameter vacuum chamber which is filled with nitrogen gas at a pressure of about 100 mTorr. Dust located on a tray in the chamber is attracted into the plasma where it is trapped electrostatically. The dust acoustic waves were produced by applying a modulation signal (5–40 Hz) to the anode. The wavelength of the DA waves was measured from single frame video images of scattered light from the dust grains. The measured dispersion relation is compared with theoretical predictions.
Field controlled light scattering from nematic microdroplets
Authors: Doane J. W. , Vaz N. A. , Wu B.-G. , Žumer S.
Published: July 2002

The light scattering and electro-optic response of new material with display potential are investigated. The materials consist of microdroplets of nematic liquid crystals which are spontaneously formed in a solid polymer at the time of its polymerization. Droplet size, spacing, and distribution are readily controlled in these materials to allow optimization of displays based upon electrically controlled light scattering from the liquid crystal droplets. Preliminary experimental and theoretical studies of the light scattering properties show these materials to offer new features suitable for many display applications.
Anomalous first-phase formation in rapidly thermal annealed, thin-layered Si/Ni/Si films
Authors: Natan Menachem
Published: July 2002

The initial stages of silicide formation in very thin-layered Si/Ni/Si films reacted by rapid (pulsed) annealings were investigated using the rapid thermal annealing/transmission electron microscopy technique. At least four phases, NiSi, δNi2Si, θNi2Si, and Ni31Si12, are shown to form first after 1 s annealings in the 175-300 °C temperature regime; the actual phase and its nucleation kinetics depend on the Si:Ni ratio and on substrate deposition temperature. An amorphous (Ni+Si) mixture is shown to exist as a precursor to θNi2Si and NiSi. The multiplicity of ‘‘first’’ phases and the dependence on the Si:Ni ratio contradict various ‘‘first-phase’’ rules and steady-state annealing data obtained on thicker films and in metal-Si wafer reactions. A simple model that accounts for the stoichiometry and substrate-temperature dependences is suggested.
Bright white-light electroluminescence in SrS:Pr,K thin films
Authors: Tanaka Shosaku , Ohshio Shozo , Nishiura Junichi , Kawakami Hiroyuki , Yoshiyama Hideki , Kobayashi Hiroshi
Published: July 2002

A bright white-light electroluminescence (EL) is obtained in a SrS:Pr,K thin-film EL device. The device shows a luminance level of 500 cd/m2 with 1 kHz drive (1500 cd/m2 at 5 kHz), which is five times as high as that obtained for a ZnS:Pr,F thin-film EL device. The dominant EL excitation mechanism in the SrS:Pr,K device responsible for a higher EL efficiency is ascribed to a process wherein the ionization of the luminescent center occurs first. Subsequent recombination with an electron follows in which the Pr3+ center reaches a (4 f )2 excited state through a (4 f )(5d) excited state. Finally, luminescence due to the relaxation of the (4 f )2 electron configuration of the Pr3+ ion arises.
A rapamycin-sensitive signaling pathway contributes to long-term synaptic plasticity in the hippocampus
Authors: Tang Shao Jun , Reis Gerald , Kang Hyejin , Gingras Anne-Claude , Sonenberg Nahum , Schuman Erin M.
Published: July 2002

Many forms of long-lasting behavioral and synaptic
plasticity require the synthesis of new proteins. For example,
long-term potentiation (LTP) that endures for more than an hour
requires both transcription and translation. The signal-transduction
mechanisms that couple synaptic events to protein translational
machinery during long-lasting synaptic plasticity, however, are not
well understood. One signaling pathway that is stimulated by growth
factors and results in the translation of specific mRNAs includes the
rapamycin-sensitive kinase mammalian target of rapamycin (mTOR, also
known as FRAP and RAFT-1). Several components of this translational
signaling pathway, including mTOR, eukaryotic initiation
factor-4E-binding proteins 1 and 2, and eukaryotic initiation
factor-4E, are present in the rat hippocampus as shown by Western blot
analysis, and these proteins are detected in the cell bodies and
dendrites in the hippocampal slices by immunostaining studies. In
cultured hippocampal neurons, these proteins are present in dendrites
and are often found near the presynaptic protein, synapsin I. At
synaptic sites, their distribution completely overlaps with a
postsynaptic protein, PSD-95. These observations suggest the
postsynaptic localization of these proteins. Disruption of mTOR
signaling by rapamycin results in a reduction of late-phase LTP
expression induced by high-frequency stimulation; the early phase of
LTP is unaffected. Rapamycin also blocks the synaptic potentiation
induced by brain-derived neurotrophic factor in hippocampal slices.
These results demonstrate an essential role for rapamycin-sensitive
signaling in the expression of two forms of synaptic plasticity that
require new protein synthesis. The localization of this translational
signaling pathway at postsynaptic sites may provide a mechanism that
controls local protein synthesis at potentiated synapses.
Efficacy and age-related effects of nitric oxide-releasing aspirin on experimental restenosis
Authors: Napoli Claudio , Aldini Giancarlo , Wallace John L. , de Nigris Filomena , Maffei Roberto , Abete Pasquale , Bonaduce Domenico , Condorelli Gianluigi , Rengo Franco , Sica Vincenzo , D'Armiento Francesco P. , Mignogna Chiara , de Rosa Gaetano , Condorelli Mario , Lerman Lilach O. , Ignarro Louis J.
Published: July 2002

Restenosis after percutaneous transluminal coronary angioplasty is caused by neointimal hyperplasia, which involves impairment of nitric oxide (NO)-dependent pathways, and may be further exacerbated by a concomitant aging process. We compared the effects of NO-releasing-aspirin (NCX-4016) and aspirin (ASA) on experimental restenosis in both adult and elderly rats. Moreover, to ascertain the efficacy of NCX-4016 during vascular aging, we fully characterized the release of bioactive NO by the drug. Sprague–Dawley rats aged 6 and 24 months were treated with NO releasing-aspirin (55 mg/kg) or ASA (30 mg/kg) for 7 days before and 21 days after standard carotid balloon injury. Histological examination and immunohistochemical double-staining were used to evaluate restenosis. Plasma nitrite and nitrate and
S
-nitrosothiols were determined by a chemiluminescence-based assay. Electron spin resonance was used for determining nitrosylhemoglobin. Treatment of aged rats with NCX-4016 was associated with increased bioactive NO, compared with ASA. NO aspirin, but not ASA, reduced experimental restenosis in old rats, an effect associated with reduced vascular smooth muscle cell proliferation. NCX-4016, but not ASA, was well tolerated and virtually devoid of gastric damage in either adult or old rats. Thus, impairment of NO-dependent mechanisms may be involved in the development of restenosis in old rats. We suggest that an NCX-4016 derivative could be an effective drug in reducing restenosis, especially in the presence of aging and/or gastrointestinal damage.
Cryptic invasion by a non-native genotype of the common reed,Phragmites australis, into North America
Authors: Saltonstall Kristin
Published: July 2002

Cryptic invasions are a largely unrecognized type of biological invasion that lead to underestimation of the total numbers and impacts of invaders because of the difficulty in detecting them. The distribution and abundance ofPhragmites australisin North America has increased dramatically over the past 150 years. This research tests the hypothesis that a non-native strain ofPhragmitesis responsible for the observed spread. Two noncoding chloroplast DNA regions were sequenced for samples collected worldwide, throughout the range ofPhragmites. Modern North American populations were compared with historical ones from herbarium collections. Results indicate that an introduction has occurred, and the introduced type has displaced native types as well as expanded to regions previously not known to havePhragmites. Native types apparently have disappeared from New England and, while still present, may be threatened in other parts of North America.
Phosphatidylinositol 3-kinase signaling mediates angiogenesis and expression of vascular endothelial growth factor in endothelial cells
Authors: Jiang Bing-Hua , Zheng Jenny Z. , Aoki Masahiro , Vogt Peter K.
Published: July 2002

Phosphatidylinositol 3-kinase (PI 3-kinase) is a signaling molecule that controls numerous cellular properties and activities. The oncogene v-
p3k
is a homolog of the gene coding for the catalytic subunit of PI 3-kinase, p110α.
P3k
induces transformation of cells in culture, formation of hemangiosarcomas in young chickens, and myogenic differentiation in myoblasts. Here, we describe a role of PI 3-kinase in angiogenesis. Overexpression of the v-P3k protein or of cellular PI 3-kinase equipped with a myristylation signal, Myr-P3k, can induce angiogenesis in the chorioallantoic membrane (CAM) of the chicken embryo. This process is characterized by extensive sprouting of new blood vessels and enlargement of preexisting vessels. Overexpression of the myristylated form of the PI 3-kinase target Akt, Myr-Akt, also induces angiogenesis. Overexpression of the tumor suppressor PTEN or of dominant-negative constructs of PI 3-kinase inhibits angiogenesis in the yolk sac of chicken embryos, suggesting that PI 3-kinase and Akt signaling is required for normal embryonal angiogenesis. The levels of mRNA for vascular endothelial growth factor (VEGF) are elevated in cells expressing activated PI 3-kinase or Myr-Akt. VEGF mRNA levels are also increased by insulin treatment through the PI 3-kinase-dependent pathway. VEGF mRNA levels are decreased in cells treated with the PI 3-kinase inhibitor LY294002 and restored by overexpression of v-P3k or Myr-Akt. Overexpression of VEGF by the RCAS vector induces angiogenesis in chicken embryos. These results suggest that PI 3-kinase plays an important role in angiogenesis and regulates VEGF expression.
Capabilities of liposomes for topological transformation
Authors: Nomura Fumimasa , Nagata Miki , Inaba Takehiko , Hiramatsu Haruka , Hotani Hirokazu , Takiguchi Kingo
Published: July 2002

Dynamic behaviors of liposomes caused by interactions between
liposomal membranes and surfactant were studied by direct real-time
observation by using high-intensity dark-field microscopy.
Solubilization of liposomes by surfactants is thought to be a
catastrophic event akin to the explosion of soap bubbles in the air;
however, the actual process has not been clarified. We studied this
process experimentally and found that liposomes exposed to various
surfactants exhibited unusual behavior, namely continuous shrinkage
accompanied by intermittent quakes, release of encapsulated
liposomes, opening up, and inside–out topological inversion.
A genetic model provides evidence that the receptor for atrial natriuretic peptide (guanylyl cyclase-A) inhibits cardiac ventricular myocyte hypertrophy
Authors: Kishimoto Ichiro , Rossi Kristen , Garbers David L.
Published: July 2002

Guanylyl cyclase-A (NPR-A; GC-A) is the major and possibly the only
receptor for atrial natriuretic peptide (ANP) or B-type natriuretic
peptide. Although mice deficient in GC-A display an elevated blood
pressure, the resultant cardiac hypertrophy is much greater than in
other mouse models of hypertension. Here we overproduce GC-A in the
cardiac myocytes of wild-type or GC-A null animals. Introduction of the
GC-A transgene did not alter blood pressure or heart rate as a function
of genotype. Cardiac myocyte size was larger (approximately 20%) in
GC-A null than in wild-type animals. However, introduction of the GC-A
transgene reduced cardiac myocyte size in both wild-type and null mice.
Coincident with the reduction in myocyte size, both ANP mRNA and ANP
content were significantly reduced by overexpression of GC-A, and this
reduction was independent of genotype. This genetic model, therefore,
separates a regulation of cardiac myocyte size by blood pressure from
local regulation by a GC-mediated pathway.
Aquaporin 1 regulates GTP-induced rapid gating of water in secretory vesicles
Authors: Cho Sang-Joon , Sattar A. K. M. Abdus , Jeong Eun-Hwan , Satchi Mylan , Cho Jin Ah , Dash Sudhansu , Mayes Mary Sue , Stromer Marvin H. , Jena Bhanu P.
Published: July 2002

The swelling of secretory vesicles has been implicated in exocytosis, but the underlying mechanism of vesicle swelling remains largely unknown. Zymogen granules (ZGs), the membrane-bound secretory vesicles in exocrine pancreas, swell in response to GTP mediated by a G
α
i3
protein. Evidence is presented here that the water channel aquaporin-1 (AQP1) is present in the ZG membrane and participates in rapid GTP-induced vesicular water gating and swelling. Isolated ZGs exhibit low basal water permeability. However, exposure of granules to GTP results in a marked potentiation of water entry. Treatment of ZGs with the known water channel inhibitor Hg
2+
is accompanied by a reversible loss in both the basal and GTP-stimulatable water entry and vesicle swelling. Introduction of AQP1-specific antibody raised against the carboxyl-terminal domain of AQP1 blocks GTP-stimulable swelling of vesicles. Our results demonstrate that AQP1 associated at the ZG membrane is involved in basal as well as GTP-induced rapid gating of water in ZGs of the exocrine pancreas.
Perception of biological motion without local image motion
Authors: Beintema J. A. , Lappe M.
Published: July 2002

A vivid perception of the moving form of a human figure can be obtained from a few moving light points on the joints of the body. This is known as biological motion perception. It is commonly believed that the perception of biological motion rests on image motion signals. Curiously, however, some patients with lesions to motion processing areas of the dorsal stream are severely impaired in image motion perception but can easily perceive biological motion. Here we describe a biological motion stimulus based on a limited lifetime technique that tests the perception of a moving human figure in the absence of local image motion. We find that subjects can spontaneously recognize a moving human figure in displays without local image motion. Their performance is very similar to that for classic point-light displays. We also find that tasks involving the discrimination of walking direction or the coherence of a walking figure can be performed in the absence of image motion. Thus, although image motion may generally aid processes such as segmenting figure from background, we propose that it is not the basis for the percept of biological motion. Rather, we suggest biological motion is derived from dynamic form information on body posture evolving over time.
Mechanisms of noise-resistance in genetic oscillators
Authors: Vilar José M. G. , Kueh Hao Yuan , Barkai Naama , Leibler Stanislas
Published: July 2002

A wide range of organisms use circadian clocks to keep internal sense of daily time and regulate their behavior accordingly. Most of these clocks use intracellular genetic networks based on positive and negative regulatory elements. The integration of these “circuits” at the cellular level imposes strong constraints on their functioning and design. Here, we study a recently proposed model [Barkai, N. & Leibler, S. (2000)
Nature (London)
, 403, 267–268] that incorporates just the essential elements found experimentally. We show that this type of oscillator is driven mainly by two elements: the concentration of a repressor protein and the dynamics of an activator protein forming an inactive complex with the repressor. Thus, the clock does not need to rely on mRNA dynamics to oscillate, which makes it especially resistant to fluctuations. Oscillations can be present even when the time average of the number of mRNA molecules goes below one. Under some conditions, this oscillator is not only resistant to but, paradoxically, also enhanced by the intrinsic biochemical noise.
Molecular mechanisms of translation initiation in eukaryotes
Authors: Pestova Tatyana V. , Kolupaeva Victoria G. , Lomakin Ivan B. , Pilipenko Evgeny V. , Shatsky Ivan N. , Agol Vadim I. , Hellen Christopher U. T.
Published: July 2002

Translation initiation is a complex process in which initiator tRNA, 40S, and 60S ribosomal subunits are assembled by eukaryotic initiation factors (eIFs) into an 80S ribosome at the initiation codon of mRNA. The cap-binding complex eIF4F and the factors eIF4A and eIF4B are required for binding of 43S complexes (comprising a 40S subunit, eIF2/GTP/Met-tRNAi and eIF3) to the 5′ end of capped mRNA but are not sufficient to promote ribosomal scanning to the initiation codon. eIF1A enhances the ability of eIF1 to dissociate aberrantly assembled complexes from mRNA, and these factors synergistically mediate 48S complex assembly at the initiation codon. Joining of 48S complexes to 60S subunits to form 80S ribosomes requires eIF5B, which has an essential ribosome-dependent GTPase activity and hydrolysis of eIF2-bound GTP induced by eIF5. Initiation on a few mRNAs is cap-independent and occurs instead by internal ribosomal entry. Encephalomyocarditis virus (EMCV) and hepatitis C virus epitomize distinct mechanisms of internal ribosomal entry site (IRES)-mediated initiation. The eIF4A and eIF4G subunits of eIF4F bind immediately upstream of the EMCV initiation codon and promote binding of 43S complexes. EMCV initiation does not involve scanning and does not require eIF1, eIF1A, and the eIF4E subunit of eIF4F. Initiation on some EMCV-like IRESs requires additional noncanonical initiation factors, which alter IRES conformation and promote binding of eIF4A/4G. Initiation on the hepatitis C virus IRES is even simpler: 43S complexes containing only eIF2 and eIF3 bind directly to the initiation codon as a result of specific interaction of the IRES and the 40S subunit.
An important role of an inducible RNA-dependent RNA polymerase in plant antiviral defense
Authors: Xie Zhixin , Fan Baofang , Chen Chunhong , Chen Zhixiang
Published: July 2002

Plants contain RNA-dependent RNA polymerase (RdRP) activities that
synthesize short cRNAs by using cellular or viral RNAs as templates.
During studies of salicylic acid (SA)-induced resistance to viral
pathogens, we recently found that the activity of a tobacco RdRP was
increased in virus-infected or SA-treated plants. Biologically active
SA analogs capable of activating plant defense response also induced
the RdRP activity, whereas biologically inactive analogs did not. A
tobacco RdRP gene,
NtRDRP1
, was isolated and found to be
induced both by virus infection and by treatment with SA or its
biologically active analogs. Tobacco lines deficient in the inducible
RDRP activity were obtained by expressing antisense RNA for the
NtRDRP1
gene in transgenic plants. When infected by
tobacco mosaic virus, these transgenic plants accumulated significantly
higher levels of viral RNA and developed more severe disease symptoms
than wild-type plants. After infection by a strain of potato virus X
that does not spread in wild-type tobacco plants, the transgenic
NtRDRP1
antisense plants accumulated virus and developed
symptoms not only locally in inoculated leaves but also systemically in
upper uninoculated leaves. These results strongly suggest that
inducible RdRP activity plays an important role in plant antiviral
defense.
Excessive tumor-elaborated VEGF and its neutralization define a lethal paraneoplastic syndrome
Authors: Wong Alex K. , Alfert Myra , Castrillon Diego H. , Shen Qiong , Holash Jocelyn , Yancopoulos George D. , Chin Lynda
Published: July 2002

Vascular endothelial growth factor (VEGF) is a potent endothelial cell mitogen and key regulator of both physiologic and pathologic (e.g., tumor) angiogenesis. In the course of studies designed to assess the ability of constitutive VEGF to block tumor regression in an inducible RAS melanoma model, mice implanted with VEGF-expressing tumors sustained high morbidity and mortality that were out of proportion to the tumor burden. Documented elevated serum levels of VEGF were associated with a lethal hepatic syndrome characterized by massive sinusoidal dilation and endothelial cell proliferation and apoptosis. Systemic levels of VEGF correlated with the severity of liver pathology and overall clinical compromise. A striking reversal of VEGF-induced liver pathology and prolonged survival were achieved by surgical excision of VEGF-secreting tumor or by systemic administration of a potent VEGF antagonist (VEGF-TRAP
R1R2
), thus defining a paraneoplastic syndrome caused by excessive VEGF activity. Moreover, this VEGF-induced syndrome resembles peliosis hepatis, a rare human condition that is encountered in the setting of advanced malignancies, high-dose androgen therapy, and
Bartonella henselae
infection. Thus, our findings in the mouse have suggested an etiologic role for VEGF in this disease and may lead to diagnostic and therapeutic options for this debilitating condition in humans.
Inositol hexakisphosphate is a physiological signal regulating the K
+
-inward rectifying conductance in guard cells
Authors: Lemtiri-Chlieh Fouad , MacRobbie Enid A. C. , Brearley Charles A.
Published: July 2002

(
RS
)-2-
cis
, 4-
trans
-abscisic acid (ABA), a naturally occurring plant stress hormone, elicited rapid agonist-specific changes in
myo
-inositol hexakisphosphate (InsP
6
) measured in intact guard cells of
Solanum tuberosum
(
n
= 5); these changes were not reproduced by (
RS
)-2-
trans
, 4-
trans
-abscisic acid, an inactive stereoisomer of ABA (
n
= 4). The electrophysiological effects of InsP
6
were assessed on both
S. tuberosum
(
n
= 14) and
Vicia faba
(
n
= 6) guard cell protoplasts. In both species, submicromolar concentrations of InsP
6
, delivered through the patch electrode, mimicked the inhibitory effects of ABA and internal calcium (Ca
i
2+
) on the inward rectifying K
+
current,
I
K,in
, in a dose-dependent manner. Steady state block of
I
K,in
by InsP
6
was reached much more quickly in
Vicia
(3 min at ≈1 μM) than
Solanum
(20–30 min). The effects of InsP
6
on
I
K,in
were specific to the
myo
-inositol isomer and were not elicited by other conformers of InsP
6
(e.g.,
scyllo
- or
neo
-). Chelation of Ca
2+
by inclusion of 1,2-bis(2-aminophenoxy)ethane-
N,N,N
′
,N
′-tetraacetic acid or EGTA in the patch pipette together with InsP
6
prevented the inhibition of
I
K,in
, suggesting that the effect is Ca
2+
dependent. InsP
6
was ≈100-fold more potent than Ins(1,4,5)P
3
in modulating
I
K,in
. Thus ABA increases InsP
6
in guard cells, and InsP
6
is a potent Ca
2+
-dependent inhibitor of
I
K,in
. Taken together, these results suggest that InsP
6
may play a major role in the physiological response of guard cells to ABA.
Protacs: Chimeric molecules that target proteins to the Skp1–Cullin–F box complex for ubiquitination and degradation
Authors: Sakamoto Kathleen M. , Kim Kyung B. , Kumagai Akiko , Mercurio Frank , Crews Craig M. , Deshaies Raymond J.
Published: July 2002

The intracellular levels of many proteins are regulated by ubiquitin-dependent proteolysis. One of the best-characterized enzymes that catalyzes the attachment of ubiquitin to proteins is a ubiquitin ligase complex, Skp1-Cullin-F box complex containing Hrt1 (SCF). We sought to artificially target a protein to the SCF complex for ubiquitination and degradation. To this end, we tested methionine aminopeptidase-2 (MetAP-2), which covalently binds the angiogenesis inhibitor ovalicin. A chimeric compound, protein-targeting chimeric molecule 1 (Protac-1), was synthesized to recruit MetAP-2 to SCF. One domain of Protac-1 contains the IκBα phosphopeptide that is recognized by the F-box protein β-TRCP, whereas the other domain is composed of ovalicin. We show that MetAP-2 can be tethered to SCF
β
-TRCP
, ubiquitinated, and degraded in a Protac-1-dependent manner. In the future, this approach may be useful for conditional inactivation of proteins, and for targeting disease-causing proteins for destruction.
The human salivary peptide histatin 5 exerts its antifungal activity through the formation of reactive oxygen species
Authors: Helmerhorst Eva J. , Troxler Robert F. , Oppenheim Frank G.
Published: July 2002

Previous studies have shown that the human salivary antifungal peptide histatin 5 is taken up byCandida albicanscells and associates intracellularly with mitochondria. The purpose of the present study was to investigate the biological consequence of this specific subcellular targeting. Histatin 5 inhibited respiration of isolatedC. albicansmitochondria as well as the respiration of intact blastoconidia in a dose and time-dependent manner. A nearly perfect correlation was observed between histatin-induced inhibition of respiration and cell killing with either logarithmic- or stationary-phase cells, but stationary-phase cells were less sensitive. Because nonrespiring yeast cells are insensitive to histatin 5, the potential mechanistic relationship between histatin 5 interference with the respiratory apparatus and cell killing was explored by using an oxygen radical sensitive probe (dihydroethidium). Fluorimetric measurements showed that histatin 5 induced the formation of reactive oxygen species (ROS) inC. albicanscells as well as in isolated mitochondria and that ROS levels were highly correlated with cell death. In the presence of an oxygen scavenger (l-cysteine), cell killing and ROS formation were prevented. In addition, the membrane-permeant superoxide dismutase mimetic 2,2,6,6-tetramethylpiperidine-N-oxyl, abolished histatin-induced ROS formation in isolated mitochondria. In contrast to histatin 5, the conventional inhibitors of the respiratory chain, sodium cyanide or sodium azide, neither induced ROS nor killed yeast cells. These data provide strong evidence for a comprehensive mechanistic model of histatin-5-provoked yeast cell death in which oxygen radical formation is the ultimate and essential step.
In vivo
formation of complex microvessels lined by human endothelial cells in an immunodeficient mouse
Authors: Schechner Jeffrey S. , Nath Anjali K. , Zheng Lian , Kluger Martin S. , Hughes Christopher C. W. , Sierra-Honigmann M. Rocio , Lorber Marc I. , Tellides George , Kashgarian Michael , Bothwell Alfred L. M. , Pober Jordan S.
Published: July 2002

We have identified conditions for forming cultured human umbilical vein endothelial cells (HUVEC) into tubes within a three-dimensional gel that on implantation into immunoincompetent mice undergo remodeling into complex microvessels lined by human endothelium. HUVEC suspended in mixed collagen/fibronectin gels organize into cords with early lumena by 24 h and then apoptose. Twenty-hour constructs, s.c. implanted in immunodeficient mice, display HUVEC-lined thin-walled microvessels within the gel 31 days after implantation. Retroviral-mediated overexpression of a caspase-resistant Bcl-2 protein delays HUVEC apoptosis
in vitro
for over 7 days. Bcl-2-transduced HUVEC produce an increased density of HUVEC-lined perfused microvessels
in vivo
compared with untransduced or control-transduced HUVEC. Remarkably, Bcl-2- but not control-transduced HUVEC recruit an ingrowth of perivascular smooth-muscle α-actin-expressing mouse cells at 31 days, which organize by 60 days into HUVEC-lined multilayered structures resembling true microvessels. This system provides an
in vivo
model for dissecting mechanisms of microvascular remodeling by using genetically modified endothelium. Incorporation of such human endothelial-lined microvessels into engineered synthetic skin may improve graft viability, especially in recipients with impaired angiogenesis.
Ongoing immunoglobulin somatic mutation in germinal center B cell-like but not in activated B cell-like diffuse large cell lymphomas
Authors: Lossos Izidore S. , Alizadeh Ash A. , Eisen Michael B. , Chan Wing C. , Brown Patrick O. , Botstein David , Staudt Louis M. , Levy Ronald
Published: July 2002

B cell diffuse large cell lymphoma (B-DLCL) is a heterogeneous group of tumors, based on significant variations in morphology, clinical presentation, and response to treatment. Gene expression profiling has revealed two distinct tumor subtypes of B-DLCL: germinal center B cell-like DLCL and activated B cell-like DLCL. In a separate study, we determined that B-DLCL can also be subdivided into two groups based on the presence or absence of ongoing Ig gene hypermutation. Here, we evaluated the correlation between these B-DLCL subtypes established by the two different methods. Fourteen primary B-DLCL cases were studied by gene expression profiling using DNA microarrays and for the presence of ongoing mutations in their Ig heavy chain gene. All seven cases classified as germinal center B cell-like DLCL by gene expression showed the presence of ongoing mutations in the Ig genes. Five of the seven cases classified by gene expression as activated B cell-like DLCL had no ongoing somatic mutations, whereas, in the remaining two cases, a single point mutation was observed in only 2 of 15 and 21 examined molecular clones of variable heavy (V
H
) chain gene, respectively. These two cases were distantly related to the rest of the activated B cell-like DLCL tumors by gene expression. Our findings validate the concept that lymphoid malignancies are derived from cells at discrete stages of normal lymphocyte maturation and that the malignant cells retain the genetic program of those normal cells.
HIF-1α binding to VHL is regulated by stimulus-sensitive proline hydroxylation
Authors: Yu Fang , White Sarah B. , Zhao Quan , Lee Frank S.
Published: July 2002

Hypoxia-inducible factor-1α (HIF-1α)
1
is a global
transcriptional regulator of the hypoxic response. Under normoxic
conditions, HIF-1α is recognized by the von Hippel-Lindau
tumor-suppressor protein (VHL), a component of an E3 ubiquitin ligase
complex. This interaction thereby promotes the rapid degradation of
HIF-1α. Under hypoxic conditions, HIF-1α is stabilized. We have
previously shown that VHL binds in a hypoxia-sensitive manner to a
27-aa segment of HIF-1α, and that this regulation depends on a
posttranslational modification of HIF-1α. Through a combination of
in vivo
coimmunoprecipitation assays using VHL and a
panel of point mutants of HIF-1α in this region, as well as MS and
in vitro
binding assays, we now provide evidence that
this modification, which occurs under normoxic conditions, is
hydroxylation of Pro-564 of HIF-1α. The data furthermore show that
this proline hydroxylation is the primary regulator of VHL binding.
Spatiotemporal variation of metabolism in a plant circadian rhythm: The biological clock as an assembly of coupled individual oscillators
Authors: Rascher Uwe , Hütt Marc-Thorsten , Siebke Katharina , Osmond Barry , Beck Friedrich , Lüttge Ulrich
Published: July 2002

The complex dynamic properties of biological timing in organisms
remain a central enigma in biology despite the increasingly precise
genetic characterization of oscillating units and their components.
Although attempts to obtain the time constants from oscillations of
gene activity and biochemical units have led to substantial progress,
we are still far from a full molecular understanding of
endogenous rhythmicity and the physiological manifestations
of biological clocks. Applications of nonlinear dynamics have
revolutionized thinking in physics and in biomedical and life sciences
research, and spatiotemporal considerations are now advancing our
understanding of development and rhythmicity. Here we show that the
well known circadian rhythm of a metabolic cycle in a higher plant,
namely the crassulacean acid metabolism mode of photosynthesis, is
expressed as dynamic patterns of independently initiated variations in
photosynthetic efficiency (φ
PSII
) over a single leaf.
Noninvasive highly sensitive chlorophyll fluorescence imaging reveals
randomly initiated patches of varying φ
PSII
that are
propagated within minutes to hours in wave fronts, forming dynamically
expanding and contracting clusters and clearly dephased regions of
φ
PSII
. Thus, this biological clock is a spatiotemporal
product of many weakly coupled individual oscillators, defined by the
metabolic constraints of crassulacean acid metabolism. The oscillators
operate independently in space and time as a consequence of the
dynamics of metabolic pools and limitations of CO
2
diffusion between tightly packed cells.
Fruiting body formation by
Bacillus
subtilis
Authors: Branda Steven S. , González-Pastor José Eduardo , Ben-Yehuda Sigal , Losick Richard , Kolter Roberto
Published: July 2002

Spore formation by the bacterium
Bacillus subtilis
has long been studied as a model for cellular differentiation, but
predominantly as a single cell. When analyzed within the context of
highly structured, surface-associated communities (biofilms), spore
formation was discovered to have heretofore unsuspected spatial
organization. Initially, motile cells differentiated into aligned
chains of attached cells that eventually produced aerial structures, or
fruiting bodies, that served as preferential sites for sporulation.
Fruiting body formation depended on regulatory genes required early in
sporulation and on genes evidently needed for exopolysaccharide and
surfactin production. The formation of aerial structures was robust in
natural isolates but not in laboratory strains, an indication that
multicellularity has been lost during domestication of
B.
subtilis
. Other microbial differentiation processes long
thought to involve only single cells could display the spatial
organization characteristic of multicellular organisms when studied
with recent natural isolates.