bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental study of magnetic insulation
Authors: Kraft R. , McGeoch M. W.
Published: July 2002

An experimental study is made of magnetically insulated electron flow in a cylindrical thermionic diode. Emphasis is laid on measurements of plasma waves and leakage current in the insulated state. It is found that over a wide range of conditions the leakage current can be represented by I=[(0.84×10−6ξAcV32)/deff] exp(4015x* /ξ) Amps, where Ac is the cathode area, V the anode–cathode voltage, deff is an effective interelectrode gap (m), and x* is the thickness of the Brillouin layer in contact with the cathode (m). Here ξ is a geometrical factor equal to unity in the present nearly planar geometry, increasing by 3 as the cathode radius becomes much less than the anode radius, as is the case in prior magnetron data also fitted by this formula. Unstable electrostatic, transverse magnetic (TM) waves are observed at frequencies predominantly below the electron-cyclotron frequency with wavelengths equal to the πd/n, where d=diode diameter, n=1,2,3... . The waves are believed to be driven unstable by a resistive wall instability. The correlation between the leakage current and the wave amplitude is discussed.
Nonlinear evolution of the internal kink mode in toroidal geometry for shaped tokamak plasmas
Authors: Holmes J. A. , Carreras B. A. , Charlton L. A. , Lynch V. E. , Hastie R. J. , Hender T. C.
Published: July 2002

The nonlinear evolution of the internal kink mode is studied in toroidal geometry for noncircular cross-section tokamak plasmas. The study is focused on very low-shear and hollow q profiles, with q(ρ)≥1 for which the internal kink is unstable in the latter case, even at β=0. The nonlinear evolution is dominated by ideal magnetohydrodynamics (MHD), and the instability saturates, giving a quasihelical shift to the magnetic axis. The nonlinear saturation is caused by increased field line bending. Time scales of 103 τHp and axis shifts of 20% are reached when changes in q on the order of 3×10−3 from the marginal profile are produced.
The scaling of Ne-like x-ray laser schemes to short wavelength
Authors: Rosen M. D. , London R. A. , Hagelstein P. L.
Published: July 2002

The driver and target parameters required to extend the successful results from an exploding foil Ne-like-Se soft x-ray laser at 206 Å towards wavelengths of 40 Å are investigated. The power requirements are found to scale as λ−4, which is quite costly. The principal constraint is the refraction of the x-ray laser beam in these high density, single pass, exploding foil targets. Correcting mirrors or nonrefracting target designs could reduce this costly scaling.
Spectral energy content of parametrically excited second harmonic emissions from laser plasmas
Authors: Sinha B. K. , Kumbhare S. R.
Published: July 2002

Experiments were conducted using a 1.0641 μ laser to measure the spectral profiles of second harmonic emissions from laser plasmas produced on planar, slab targets of carbon, aluminum, and copper at laser intensities varying from 2×1012 to 9×1013 W/cm2. It is observed and confirmed that the contribution to spectral energy due to parametric decay is significant and can exceed that due to resonance absorption under certain experimental conditions.
Particle simulation of radio frequency stabilization of the flute mode in a tandem mirror. II. Perpendicular antenna
Authors: Abe Hirotada , Kadoya Yutaka
Published: July 2002

A two-and-a-half-dimensional electromagnetic particle code PS2M [J. Phys. Soc. Jpn. 56, 3899 (1987)] is used to study how an electric field applied perpendicularly to the magnetic field affects the radio frequency stabilization of flute modes in a tandem mirror plasma. The electric field perpendicular to the magnetic field stabilizes or destabilizes the flute mode through the mechanism of the ponderomotive force acting on electrons and ions and through the mechanism of sideband coupling. In the simulations two typical examples have been shown: (i) when the sideband coupling effects (in which the electron terms are dominant) stabilize the flute modes and (ii) when the perpendicular ponderomotive force acting on the electrons destabilizes the flute modes.
Axisymmetric magnetohydrodynamic equilibrium equations with particle, momentum, and energy sources
Authors: Hsu J. Y. , Chu M. S.
Published: July 2002

The axisymmetric magnetohydrodynamic equilibrium equations with a flow induced by particle, momentum, and energy sources are derived.
Reynolds stresses and one-dimensional spectra for a vortex model of homogeneous anisotropic turbulence
Authors: Pullin D. I. , Saffman P. G.
Published: July 2002

Homogeneous anisotropic turbulence consisting of a collection of straight vortex structures is considered, each with a cylindrically unidirectional, but otherwise arbitrary, internal vorticity field. The orientations of the structures are given by a distribution P of appropriate Euler angles describing the transformation from laboratory to structure-fixed axes. One-dimensional spectra of the velocity components are calculated in terms of P, and the shell-summed energy spectrum. An exact kinematic relation is found in which volume-averaged Reynolds stresses are proportional to the turbulent kinetic energy of the vortex collection times a tensor moment of P. A class of large-eddy simulation models for nonhomogeneous turbulence is proposed based on application of the present results to the calculation of subgrid Reynolds stresses. These are illustrated by the development of a simplified model using a rapid-distortion-like approximation.
Anisotropy of the Lundgren–Townsend model of fine-scale turbulence
Authors: Saffman P. G. , Pullin D. I.
Published: July 2002

The effect of a statistically anisotropic distribution of stretched vortices in the Lundgren–Townsend model of the fine-scale structure of homogeneous turbulence is considered. Lundgren’s argument that anisotropy does not affect the three-dimensional energy spectrum is confirmed. Examples of velocity derivative moments and one-dimensional vorticity spectra are worked out for the case of an axisymmetric probability distribution. It is found that scaling of three-dimensional vorticity spectra may not be visible in the one-dimensional spectra.
Non-Boussinesq effect: Thermal convection with broken symmetry
Authors: Zhang Jun , Childress Stephen , Libchaber Albert
Published: July 2002

We investigate large Rayleigh number (106–109) and large Prandtl number (102–103) thermal convection in glycerol in an aspect ration one cubic cell. The kinematic viscosity of the fluid strongly depends upon the temperature. The symmetry between the top and bottom boundary layers is thus broken, the so-called non-Boussinesq regime. In a previous paper Wu and Libchaber have proposed that in such a state the two thermal boundary layers adjust their length scales so that the mean hot and cold temperature fluctuations are equal in the center of the cell. We confirm this equality. A simplified two-dimensional model for the mean center temperature based on an equation for the thermal boundary layer is presented and compared with the experimental results. The conclusion is that the central temperature adjusts itself so that heat fluxes from boundaries are equal, temperature fluctuations at the center symmetrical, at a cost of very different temperature drops and Rayleigh number for each boundary.
Linear stability and transient growth in driven contact lines
Authors: Bertozzi Andrea L. , Brenner Michael P.
Published: July 2002

Fluid flowing down an inclined plane commonly exhibits a fingering instability in which the contact line corrugates. We show that below a critical inclination angle the base state before the instability is linearly stable. Several recent experiments explore inclination angles below this critical angle, yet all clearly show the fingering instability. We explain this paradox by showing that regardless of the long time linear stability of the front, microscopic scale perturbations at the contact line grow on a transient time scale to a size comparable with the macroscopic structure of the front. This amplification is sufficient to excite nonlinearities and thus initiate finger formation. The amplification is a result of the well-known singular dependence of the macroscopic profiles on the microscopic length scale near the contact line. Implications for other types of forced contact lines are discussed.
The linear stability of flat-plate boundary-layer flow of fluid with temperature-dependent viscosity
Authors: Wall D. P. , Wilson S. K.
Published: July 2002

The linear stability of boundary-layer flow of fluid with temperature-dependent viscosity over a heated or cooled flat-plate is investigated. Decomposition of the disturbance into normal temporal modes leads to a sixth-order “modified” eigenvalue problem. Making the additional ad hoc assumption of parallel flow leads to a simpler sixth-order “parallel” eigenvalue problem which, unlike the modified problem, reduces to the classical Orr–Sommerfeld problem in the isothermal case. Two viscosity models are considered, and for both models numerically-calculated stability results for both the modified and parallel eigenvalue problems are obtained. For both viscosity models it is, perhaps surprisingly, found that for both eigenvalue problems a non-uniform decrease in viscosity across the layer stabilizes the flow while a non-uniform increase in viscosity across the layer destabilizes the flow. Results for the two eigenvalue problems are shown to be quantitatively similar with, however, the parallel problem always over-predicting the critical Reynolds number in comparison to the modified problem. Finally, we discuss the physical interpretation of our results in terms of velocity–profile shape and thin-layer effects.
Asymptotic spreading rate of initially compressible jets—experiment and analysis
Authors: Zaman K. B. M. Q.
Published: July 2002

Experimental results for the spreading and centerline velocity decay rates for round, compressible jets, from a convergent and a convergent-divergent nozzle, are presented. The spreading rate is determined from the variation of streamwise mass flux obtained from Pitot probe surveys. Results for the far asymptotic region show that both spreading and centerline velocity decay rates, when nondimensionalized by parameters at the nozzle exit, decrease with increasing “jet Mach number” MJ. Dimensional analysis with the assumption of momentum conservation, together with compressible flow calculations for the conditions at the nozzle exit, predict this Mach number dependence well. The analysis also demonstrates that an increase in the “potential core length” of the jet occurring with increasing MJ, a commonly observed trend, is largely accounted for simply by the variations in the density and static pressure at the nozzle exit. The effect of decreasing mixing efficiency with increasing compressibility is inferred to contribute only partially to the latter trend.
Computational and experimental analysis of dynamics of drop formation
Authors: Wilkes Edward D. , Phillips Scott D. , Basaran Osman A.
Published: July 2002

Dynamics of formation of a drop of a Newtonian liquid from a capillary tube into an ambient gas phase is studied computationally and experimentally. While this problem has previously been studied computationally either (a) using a set of one-dimensional equations or (b) treating the dynamics as that of irrotational flow of an inviscid fluid or creeping flow, here the full nonlinear, transient Navier–Stokes system subject to appropriate initial and boundary conditions is solved in two dimensions to analyze the dynamics at finite Reynolds numbers. The success of the computations rests on a finite element algorithm incorporating a multiregion mesh which conforms to and evolves with the changing shape of the drop. The new algorithm is able to capture both the gross features of the phenomenon, such as the limiting length of a drop at breakup and the volume of the primary drop, and its fine features, such as a microthread that develops from a main thread or a neck in a viscous drop approaching breakup. The accuracy of the new calculations is verified by comparison of computed predictions to old and new experiments. With the new algorithm, it is shown for the first time that the interface of a viscous drop can overturn before the drop breaks. Calculations have also been carried out to determine the range of parameters over which algorithms that treat the drop liquid as inviscid and the flow inside it as irrotational can accurately predict the dynamics of formation of drops of low viscosity liquids. Limiting lengths of drops and primary drop volumes are computed over a wide range of the parameter space spanned by the relevant dimensionless groups.
Geometry and scale relationships in high Reynolds number turbulence determined from three-dimensional holographic velocimetry
Authors: Tao Bo , Katz Joseph , Meneveau Charles
Published: July 2002

Holographic particle image velocimetry measurements of a fully developed turbulent flow in a square duct (ReH=v̄cH/ν=1.2×105, η=100 μm, Reλ=310) are used for examining the relative alignment between filtered vorticity, strain-rate and subgrid-scale stress tensors. Similar to DNS and previous measurements, the filtered vorticity has a preferred alignment with the intermediate strain-rate eigendirection. Contrary to typical eddy viscosity models, the most compressive strain-rate and most extensive subgrid-scale stress eigendirections have a strongly preferred relative orientation of 34°. The orientations of the other eigendirections are less deterministic and more complex.
Magnetic reconnection in the presence of sheared plasma flow: Intermediate shock formation
Authors: La Belle-Hamer A. L. , Otto A. , Lee L. C.
Published: July 2002

Classical models of magnetic reconnection consist of a small diffusion region, which is bounded by two pairs of slow shocks. In these models, the plasma is accelerated across the shocks. It has long been postulated that violation of symmetry across the current sheet will lead to the formation of intermediate waves in the current sheet. These asymmetries are important properties of space plasma current sheets. Equally important in space plasmas is the presence of sheared flow across current sheets. In this study, the structure of steady-state reconnection is investigated in the presence of a shear flow across the current sheet with symmetric density and magnetic field strengths using two-dimensional magnetohydrodynamic (MHD) simulations. The results show that for sheared flow above the Alfvén velocity of the inflow regions no steady-state magnetic reconnection occurs. For sheared plasma flow below this critical velocity steady-state reconnection is obtained. A detailed examination of the Rankine–Hugoniot jump conditions reveals that each pair of slow shocks is replaced by a strong intermediate shock and a weak slow shock in the presence of shear flow.
Comparison of toroidal viscosity with neoclassical theory
Authors: Ida K. , Nakajima N.
Published: July 2002

Toroidal rotation profiles are measured with charge exchange spectroscopy for a plasma heated with tangential neutral beam injection (NBI) in the Compact Helical System (CHS) heliotron/torsatron device [Ida et al., Phys. Rev. Lett. 67, 58 (1991)] to estimate the toroidal viscosity. The toroidal viscosity derived from the toroidal rotation velocity shows good agreement with the neoclassical toroidal viscosity plus the perpendicular viscosity (μ⊥=2 m2/s).
Laboratory studies of waves and instabilities in dusty plasmas
Authors: Merlino R. L. , Barkan A. , Thompson C. , D’Angelo N.
Published: July 2002

Theoretical and experimental studies of low-frequency electrostatic waves in plasmas containing negatively charged dust grains are described. The presence of charged dust is shown to modify the properties of ion-acoustic waves and electrostatic ion-cyclotron waves through the quasineutrality condition even though the dust grains do not participate in the wave dynamics. If the dust dynamics is included in the analysis, new “dust modes” appear—dust acoustic and dust cyclotron modes. The results of laboratory experiments dealing with dust ion acoustic (DIA) waves and electrostatic dust ion cyclotron (EDIC) waves are shown. These modes are more easily excited in a plasma containing negatively charged dust. Finally, observations of dust acoustic (DA) waves are presented and measurements of the dispersion relation are compared with one obtained from fluid theory.
Hot-pressed neodymium-iron-boron magnets
Authors: Lee R. W.
Published: July 2002

A new method is described which produces fully dense, well-aligned Nd-Fe-B magnets from rapidly quenched ribbons. Full density is achieved by hot pressing at ∼15 kpsi in argon at 700 °C. A second hot press in a larger die cavity to allow deformation transverse to the press direction (die upsetting) introduces a preferred magnetization direction parallel to the press direction. Energy products of 40 MGOe (Br/Hci=13.5 kG/11 kOe) have been generated by this method.
Enhancement of sliding life of MoS2 films deposited by combining sputtering and high-energy ion implantation
Authors: Chevallier Jacques , Olesen Svend , So/rensen Gunnar , Gupta Balkishan
Published: July 2002

Solid lubricant films of MoS2 with improved adhesion to a stainless-steel substrate have been deposited by a combination of rf sputtering and ion implantation with inert gas ions in the 100-keV region. When the sputter film had obtained a thickness of 500 Å, it was subjected to the ionic bombardment prior to completion of the sputter deposition. Dependent on the ion dose an adhesion improvement and a sliding life enhancement of approximately a factor of 2 were obtained for the solid lubricant film.
High quality ZnSe films grown by low pressure metalorganic vapor phase epitaxy using methylalkyls
Authors: Yokogawa Toshiya , Ogura Mototsugu , Kajiwara Takao
Published: July 2002

High quality ZnSe layers have been successfully grown by low pressure metalorganic vapor phase epitaxy (MOVPE) using methylalkyls, dimethylzinc, and dimethylselenide. Streaks and Kikuchi lines were observed by reflective high-energy electron diffraction measurements. Also the sharp and strong excitonic emission line due to free exciton and no emissions related to deep levels or residual impurities were observed by photoluminescence measurements. These results show that the ZnSe layers are of good crystalline quality. Furthermore, good selective growth of the ZnSe films on structured substrates was easily achieved by using this low pressure MOVPE technique.
Chemical warfare between microbes promotes biodiversity
Authors: Czárán Tamás L. , Hoekstra Rolf F. , Pagie Ludo
Published: July 2002

Evolutionary processes generating biodiversity and ecological mechanisms maintaining biodiversity seem to be diverse themselves. Conventional explanations of biodiversity such as niche differentiation, density-dependent predation pressure, or habitat heterogeneity seem satisfactory to explain diversity in communities of macrobial organisms such as higher plants and animals. For a long time the often high diversity among microscopic organisms in seemingly uniform environments, the famous “paradox of the plankton,” has been difficult to understand. The biodiversity in bacterial communities has been shown to be sometimes orders of magnitudes higher than the diversity of known macrobial systems. Based on a spatially explicit game theoretical model with multiply cyclic dominance structures, we suggest that antibiotic interactions within microbial communities may be very effective in maintaining diversity.
Incorporation of azides into recombinant proteins for chemoselective modification by the Staudinger ligation
Authors: Kiick Kristi L. , Saxon Eliana , Tirrell David A. , Bertozzi Carolyn R.
Published: July 2002

The introduction of chemically unique groups into proteins by
means of non-natural amino acids has numerous applications in protein
engineering and functional studies. One method to achieve this
involves the utilization of a non-natural amino acid by the cell's
native translational apparatus. Here we demonstrate that a methionine
surrogate, azidohomoalanine, is activated by the methionyl-tRNA
synthetase of
Escherichia coli
and replaces methionine
in proteins expressed in methionine-depleted bacterial cultures. We
further show that proteins containing azidohomoalanine can be
selectively modified in the presence of other cellular proteins by
means of Staudinger ligation with triarylphosphine reagents.
Incorporation of azide-functionalized amino acids into proteins
in vivo
provides opportunities for protein modification
under native conditions and selective labeling of proteins in the
intracellular environment.
Spontaneous molecular reactivation of herpes simplex virus type 1 latency in mice
Authors: Feldman Lawrence T. , Ellison Aaron R. , Voytek Cynthia C. , Yang Li , Krause Philip , Margolis Todd P.
Published: July 2002

Infection of the mouse trigeminal ganglia (TG) is the most commonly used model for the study of herpes simplex virus type 1 (HSV-1) latency. Its popularity is caused, at least in part, by the perception that latent infection can be studied in this system in the absence of spontaneous viral reactivation. However, this perception has never been rigorously tested. To carefully study this issue, the eyes of Swiss–Webster mice were inoculated with HSV-1 (KOS), and 37–47 days later the TG were dissected, serial-sectioned, and probed for HSV-1 ICP4, thymidine kinase, glycoprotein C, and latency-associated transcript RNA byin situhybridization. Serial sections of additional latently infected TG were probed with HSV-1-specific polyclonal antisera. Analysis of thousands of probed sections revealed abundant expression of viral transcripts, viral protein, and viral DNA replication in about 1 neuron per 10 TG tested. These same neurons were surrounded by a focal white cell infiltrate, indicating the presence of an antigenic stimulus. We conclude that productive cycle viral genes are abundantly expressed in rare neurons of latently infected murine TG and that these events are promptly recognized by an active local immune response. In the absence of detectable infectious virus in these ganglia, we propose the term “spontaneous molecular reactivation” to describe this ongoing process.
The movement protein NSm of tomato spotted wilt tospovirus (TSWV): RNA binding, interaction with the TSWV N protein, and identification of interacting plant proteins
Authors: Soellick T.-R. , Uhrig J. F. , Bucher G. L. , Kellmann J.-W. , Schreier P. H.
Published: July 2002

The nonstructural NSm protein of tomato spotted wilt tospovirus (TSWV) represents a putative viral movement protein involved in cell-to-cell movement of nonenveloped ribonucleocapsid structures. To study the molecular basis of NSm function, we expressed the protein in
Escherichia coli
and investigated protein–protein and protein–RNA interactions of NSm protein
in vitro
. NSm specifically interacts with TSWV N protein and binds single-stranded RNA in a sequence-nonspecific manner. Using NSm as a bait in a yeast two-hybrid screen, we identified two homologous NSm-binding proteins of the DnaJ family from
Nicotiana tabacum
and
Arabidopsis thaliana
.
Transferrin receptor 2: Continued expression in mouse liver in the face of iron overload and in hereditary hemochromatosis
Authors: Fleming Robert E. , Migas Mary C. , Holden Christopher C. , Waheed Abdul , Britton Robert S. , Tomatsu Shunji , Bacon Bruce R. , Sly William S.
Published: July 2002

Hereditary hemochromatosis (HH) is a common autosomal recessive disorder characterized by excess absorption of dietary iron and progressive iron deposition in several tissues, particularly liver. Liver disease resulting from iron toxicity is the major cause of death in HH. Hepatic iron loading in HH is progressive despite down-regulation of the classical transferrin receptor (TfR). Recently a human cDNA highly homologous to TfR was identified and reported to encode a protein (TfR2) that binds holotransferrin and mediates uptake of transferrin-bound iron. We independently identified a full-length murine EST encoding the mouse orthologue of the human TfR2. Although homologous to murine TfR in the coding region, the TfR2 transcript does not contain the iron-responsive elements found in the 3′ untranslated sequence of TfR mRNA. To determine the potential role for TfR2 in iron uptake by liver, we investigated TfR and TfR2 expression in normal mice and murine models of dietary iron overload (2% carbonyl iron), dietary iron deficiency (gastric parietal cell ablation), and HH (HFE −/−). Northern blot analyses demonstrated distinct tissue-specific patterns of expression for TfR and TfR2, with TfR2 expressed highly only in liver where TfR expression is low.In situhybridization demonstrated abundant TfR2 expression in hepatocytes. In contrast to TfR, TfR2 expression in liver was not increased in iron deficiency. Furthermore, hepatic expression of TfR2 was not down-regulated with dietary iron loading or in the HFE −/− model of HH. From these observations, we propose that TfR2 allows continued uptake of Tf-bound iron by hepatocytes even after TfR has been down-regulated by iron overload, and this uptake contributes to the susceptibility of liver to iron loading in HH.
Marrow stromal cells form guiding strands in the injured spinal cord and promote recovery
Authors: Hofstetter C. P. , Schwarz E. J. , Hess D. , Widenfalk J. , El Manira A. , Prockop Darwin J. , Olson L.
Published: July 2002

Marrow stromal cells (MSC) can be expanded rapidly
in vitro
and differentiated into multiple mesodermal cell types. In addition, differentiation into neuron-like cells expressing markers typical for mature neurons has been reported. To analyze whether such cells, exposed to differentiation media, could develop electrophysiological properties characteristic of neurons, we performed whole-cell recordings. Neuron-like MSC, however, lacked voltage-gated ion channels necessary for generation of action potentials. We then delivered MSC into the injured spinal cord to study the fate of transplanted MSC and possible effects on functional outcome in animals rendered paraplegic. MSC given 1 week after injury led to significantly larger numbers of surviving cells than immediate treatment and significant improvements of gait. Histology 5 weeks after spinal cord injury revealed that MSC were tightly associated with longitudinally arranged immature astrocytes and formed bundles bridging the epicenter of the injury. Robust bundles of neurofilament-positive fibers and some 5-hydroxytryptamine-positive fibers were found mainly at the interface between graft and scar tissue. MSC constitute an easily accessible, easily expandable source of cells that may prove useful in the establishment of spinal cord repair protocols.
Single point mutations affect fatty acid block of human myocardial sodium channel α subunit Na
+
channels
Authors: Xiao Yong-Fu , Ke Qingen , Wang Sho-Ya , Auktor Klaus , Yang Yinke , Wang Ging Kuo , Morgan James P. , Leaf Alexander
Published: July 2002

Suppression of cardiac voltage-gated Na
+
currents is
probably one of the important factors for the cardioprotective effects
of the n-3 polyunsaturated fatty acids (PUFAs) against lethal
arrhythmias. The α subunit of the human cardiac Na
+
channel (hH1
α
) and its mutants were expressed in human
embryonic kidney (HEK293t) cells. The effects of single amino acid
point mutations on fatty acid-induced inhibition of the
hH1
α
Na
+
current (I
Na
) were
assessed. Eicosapentaenoic acid (EPA, C20:5n-3) significantly reduced
I
Na
in HEK293t cells expressing the wild type, Y1767K, and
F1760K of hH1
α
Na
+
channels. The inhibition
was voltage and concentration-dependent with a significant
hyperpolarizing shift of the steady state of I
Na
. In
contrast, the mutant N406K was significantly less sensitive to the
inhibitory effect of EPA. The values of the shift at 1, 5, and 10 μM
EPA were significantly smaller for N406K than for the wild type.
Coexpression of the β
1
subunit and N406K further
decreased the inhibitory effects of EPA on I
Na
in HEK293t
cells. In addition, EPA produced a smaller hyperpolarizing shift of the
V
1/2
of the steady-state inactivation in HEK293t cells
coexpressing the β
1
subunit and N406K. These results
demonstrate that substitution of asparagine with lysine at the site of
406 in the domain-1-segment-6 region (D1-S6) significantly decreased
the inhibitory effect of PUFAs on I
Na
, and coexpression
with β
1
decreased this effect even more. Therefore,
asparagine at the 406 site in hH1
α
may be important for
the inhibition by the PUFAs of cardiac voltage-gated Na
+
currents, which play a significant role in the antiarrhythmic actions
of PUFAs.
Visualization of unwinding activity of duplex RNA by DbpA, a DEAD box helicase, at single-molecule resolution by atomic force microscopy
Authors: Henn Arnon , Medalia Ohad , Shi Shu-Ping , Steinberg Michal , Franceschi Francois , Sagi Irit
Published: July 2002

The
Escherichia coli
protein DbpA is unique in
its subclass of DEAD box RNA helicases, because it possesses
ATPase-specific activity toward the peptidyl transferase center in 23S
rRNA. Although its remarkable ATPase activity had been well defined
toward various substrates, its RNA helicase activity remained to be
characterized. Herein, we show by using biochemical assays and atomic
force microscopy that DbpA exhibits ATP-stimulated unwinding activity
of RNA duplex regardless of its primary sequence. This work presents an
attempt to investigate the action of DEAD box proteins by a
single-molecule visualization methodology. Our atomic force microscopy
images enabled us to observe directly the unwinding reaction of a DEAD
box helicase on long stretches of double-stranded RNA. Specifically, we
could differentiate between the binding of DbpA to RNA in the absence
of ATP and the formation of a Y-shaped intermediate after its
progression through double-stranded RNA in the presence of ATP. Recent
studies have questioned the designation of DbpA, in particular, and
DEAD box proteins in general as RNA helicases. However, accumulated
evidence and the results reported herein suggest that these proteins
are indeed helicases that resemble in many aspects the DNA helicases.
Role of tumor–host interactions in interstitial diffusion of macromolecules: Cranial vs. subcutaneous tumors
Authors: Pluen Alain , Boucher Yves , Ramanujan Saroja , McKee Trevor D. , Gohongi Takeshi , di Tomaso Emmanuelle , Brown Edward B. , Izumi Yotaro , Campbell Robert B. , Berk David A. , Jain Rakesh K.
Published: July 2002

The large size of many novel therapeutics impairs their transport
through the tumor extracellular matrix and thus limits their
therapeutic effectiveness. We propose that extracellular matrix
composition, structure, and distribution determine the transport
properties in tumors. Furthermore, because the characteristics of the
extracellular matrix largely depend on the tumor–host interactions, we
postulate that diffusion of macromolecules will vary with tumor type as
well as anatomical location. Diffusion coefficients of macromolecules
and liposomes in tumors growing in cranial windows (CWs) and dorsal
chambers (DCs) were measured by fluorescence recovery after
photobleaching. For the same tumor types, diffusion of large molecules
was significantly faster in CW than in DC tumors. The greater
diffusional hindrance in DC tumors was correlated with higher levels of
collagen type I and its organization into fibrils. For molecules with
diameters comparable to the interfibrillar space the diffusion was 5-
to 10-fold slower in DC than in CW tumors. The slower diffusion in DC
tumors was associated with a higher density of host stromal cells that
synthesize and organize collagen type I. Our results point to the
necessity of developing site-specific drug carriers to improve the
delivery of molecular medicine to solid tumors.
The evolutionary impact of invasive species
Authors: Mooney H. A. , Cleland E. E.
Published: July 2002

Since the Age of Exploration began, there has been a drastic
breaching of biogeographic barriers that previously had isolated the
continental biotas for millions of years. We explore the nature of
these recent biotic exchanges and their consequences on evolutionary
processes. The direct evidence of evolutionary consequences of the
biotic rearrangements is of variable quality, but the results of
trajectories are becoming clear as the number of studies increases.
There are examples of invasive species altering the evolutionary
pathway of native species by competitive exclusion, niche displacement,
hybridization, introgression, predation, and ultimately extinction.
Invaders themselves evolve in response to their interactions with
natives, as well as in response to the new abiotic environment.
Flexibility in behavior, and mutualistic interactions, can aid in the
success of invaders in their new environment.
Reduced genetic variation and the success of an invasive species
Authors: Tsutsui Neil D. , Suarez Andrew V. , Holway David A. , Case Ted J.
Published: July 2002

Despite the severe ecological and economic damage caused by introduced species, factors that allow invaders to become successful often remain elusive. Of invasive taxa, ants are among the most widespread and harmful. Highly invasive ants are often unicolonial, forming supercolonies in which workers and queens mix freely among physically separate nests. By reducing costs associated with territoriality, unicolonial species can attain high worker densities, allowing them to achieve interspecific dominance. Here we examine the behavior and population genetics of the invasive Argentine ant (Linepithema humile) in its native and introduced ranges, and we provide a mechanism to explain its success as an invader. Using microsatellite markers, we show that a population bottleneck has reduced the genetic diversity of introduced populations. This loss is associated with reduced intraspecific aggression among spatially separate nests, and leads to the formation of interspecifically dominant supercolonies. In contrast, native populations are more genetically variable and exhibit pronounced intraspecific aggression. Although reductions in genetic diversity are generally considered detrimental, these findings provide an example of how a genetic bottleneck can lead to widespread ecological success. In addition, these results provide insights into the origin and evolution of unicoloniality, which is often considered a challenge to kin selection theory.
Nerve growth factor displays stimulatory effects on human skin and lung fibroblasts, demonstrating a direct role for this factor in tissue repair
Authors: Micera Alessandra , Vigneti Eliana , Pickholtz Dalia , Reich Reuven , Pappo Orit , Bonini Sergio , Maquart François Xavier , Aloe Luigi , Levi-Schaffer Francesca
Published: July 2002

Nerve growth factor (NGF) is a polypeptide which, in addition to its effect on nerve cells, is believed to play a role in inflammatory responses and in tissue repair. Because fibroblasts represent the main target and effector cells in these processes, to investigate whether NGF is involved in lung and skin tissue repair, we studied the effect of NGF on fibroblast migration, proliferation, collagen metabolism, modulation into myofibroblasts, and contraction of collagen gel. Both skin and lung fibroblasts were found to produce NGF and to express tyrosine kinase receptor (trkA) under basal conditions, whereas the low-affinity p75 receptor was expressed only after prolonged NGF exposure. NGF significantly induced skin and lung fibroblast migration in anin vitromodel of wounded fibroblast and skin migration in Boyden chambers. Nevertheless NGF did not influence either skin or lung fibroblast proliferation, collagen production, or metalloproteinase production or activation. In contrast, culture of both lung and skin fibroblasts with NGF modulated their phenotype into myofibroblasts. Moreover, addition of NGF to both fibroblast types embedded in collagen gel increased their contraction. Fibrotic human lung or skin tissues displayed immunoreactivity for NGF, trkA, and p75. These data show a direct pro-fibrogenic effect of NGF on skin and lung fibroblasts and therefore indicate a role for NGF in tissue repair and fibrosis.
Homologous DNA recombination in vertebrate cells
Authors: Sonoda Eiichiro , Takata Minoru , Yamashita Yukiko M. , Morrison Ciaran , Takeda Shunichi
Published: July 2002

TheRAD52epistasis group genes are involved in homologous DNA recombination, and their primary structures are conserved from yeast to humans. Although biochemical studies have suggested that the fundamental mechanism of homologous DNA recombination is conserved from yeast to mammals, recent studies of vertebrate cells deficient in genes of theRAD52epistasis group reveal that the role of each protein is not necessarily the same as that of the corresponding yeast gene product. This review addresses the roles and mechanisms of homologous recombination-mediated repair with a special emphasis on differences between yeast and vertebrate cells.
A methodological bias toward overestimation of molecular evolutionary time scales
Authors: Rodríguez-Trelles Francisco , Tarrío Rosa , Ayala Francisco J.
Published: July 2002

There is presently a conflict between fossil- and molecular-based evolutionary time scales. Molecular approaches for dating the branches of the tree of life frequently lead to substantially deeper times of divergence than those inferred by paleontologists. The discrepancy between molecular and fossil estimates persists despite the booming growth of sequence data sets, which increasingly feeds the interpretation that molecular estimates are older than stratigraphic dates because of deficiencies in the fossil record. Here we show that molecular time estimates suffer from a methodological handicap, namely that they are asymmetrically bounded random variables, constrained by a nonelastic boundary at the lower end, but not at the higher end of the distribution. This introduces a bias toward an overestimation of time since divergence, which becomes greater as the length of the molecular sequence and the rate of evolution decrease.
Identification and characterization of a melanin-concentrating hormone receptor
Authors: An Songzhu , Cutler Gene , Zhao Jack Jiagang , Huang Shu-Gui , Tian Hui , Li Wanbo , Liang Lingming , Rich Miki , Bakleh Amy , Du Juan , Chen Jin-Long , Dai Kang
Published: July 2002

Melanin-concentrating hormone (MCH), a neuropeptide expressed in central and peripheral nervous systems, plays an important role in the control of feeding behaviors and energy metabolism. An orphan G protein-coupled receptor (SLC-1/GPR24) has recently been identified as a receptor for MCH (MCHR1). We report here the identification and characterization of a G protein-coupled receptor as the MCH receptor subtype 2 (MCHR2). MCHR2 has higher protein sequence homology to MCHR1 than any other G protein-coupled receptor. The expression of MCHR2 has been detected in many regions of the brain. In contrast to MCHR1, which is intronless in the coding region and is located at the chromosomal locus 22q13.3, the MCHR2 gene has multiple exons and is mapped to locus 6q21. MCHR2 is specifically activated by nanomolar concentrations of MCH, binds to MCH with high affinity, and signals through Gq protein. This discovery is important for a full understanding of MCH biology and the development of potential therapeutics for diseases involving MCH, including obesity.
Life in the end-Permian dead zone
Authors: Looy Cindy V. , Twitchett Richard J. , Dilcher David L. , Van Konijnenburg-Van Cittert Johanna H. A. , Visscher Henk
Published: July 2002

The fossil record of land plants is an obvious source of information on the dynamics of mass extinctions in the geological past. In conjunction with the end-Permian ecological crisis, ≈250 million years ago, palynological data from East Greenland reveal some unanticipated patterns. We document the significant time lag between terrestrial ecosystem collapse and selective extinction among characteristic Late Permian plants. Furthermore, ecological crisis resulted in an initial increase in plant diversity, instead of a decrease. Paradoxically, these floral patterns correspond to a “dead zone” in the end-Permian faunal record, characterized by a paucity of marine invertebrate megafossils. The time-delayed, end-Permian plant extinctions resemble modeled “extinction debt” responses of multispecies metapopulations to progressive habitat destruction.
Identification of a subpopulation of rapidly self-renewing and multipotential adult stem cells in colonies of human marrow stromal cells
Authors: Colter David C. , Sekiya Ichiro , Prockop Darwin J.
Published: July 2002

Marrow stromal cells are adult stem cells from bone marrow that can differentiate into multiple nonhematopoietic cell lineages. Previous reports demonstrated that single-cell-derived colonies of marrow stromal cells contained two morphologically distinct cell types: spindle-shaped cells and large flat cells. Here we found that early colonies also contain a third kind of cell: very small round cells that rapidly self-renew. Samples enriched for the small cells had a greater potential for multipotential differentiation than samples enriched for the large cells. Also, the small cells expressed a series of surface epitopes and other proteins that potentially can be used to distinguish the small cells from the large cells. The results suggested it will be important to distinguish the major subpopulations of marrow stromal cells in defining their biology and their potential for cell and gene therapy.
Intrinsic noise in gene regulatory networks
Authors: Thattai Mukund , van Oudenaarden Alexander
Published: July 2002

Cells are intrinsically noisy biochemical reactors: low reactant numbers can lead to significant statistical fluctuations in molecule numbers and reaction rates. Here we use an analytic model to investigate the emergent noise properties of genetic systems. We find for a single gene that noise is essentially determined at the translational level, and that the mean and variance of protein concentration can be independently controlled. The noise strength immediately following single gene induction is almost twice the final steady-state value. We find that fluctuations in the concentrations of a regulatory protein can propagate through a genetic cascade; translational noise control could explain the inefficient translation rates observed for genes encoding such regulatory proteins. For an autoregulatory protein, we demonstrate that negative feedback efficiently decreases system noise. The model can be used to predict the noise characteristics of networks of arbitrary connectivity. The general procedure is further illustrated for an autocatalytic protein and a bistable genetic switch. The analysis of intrinsic noise reveals biological roles of gene network structures and can lead to a deeper understanding of their evolutionary origin.
The human
VASA
gene is specifically expressed in the germ cell lineage
Authors: Castrillon Diego H. , Quade Bradley J. , Wang T. Y. , Quigley Catherine , Crum Christopher P.
Published: July 2002

To understand the origins and function of the human germ cell lineage and to identify germ cell-specific markers we have isolated a human ortholog of the
Drosophila
gene
vasa
. The gene was mapped to human chromosome 5q (near the centromere) by radiation hybrid mapping. We show by Northern analysis of fetal and adult tissues that expression of the human
VASA
gene is restricted to the ovary and testis and is undetectable in somatic tissues. We generated polyclonal antibodies that bind to the VASA protein in formalin-fixed, paraffin-embedded tissue and characterized VASA protein expression in human germ cells at various stages of development. The VASA protein is cytoplasmic and expressed in migratory primordial germ cells in the region of the gonadal ridge. VASA protein is present in fetal and adult gonadal germ cells in both males and females and is most abundant in spermatocytes and mature oocytes. The gene we have isolated is thus a highly specific marker of germ cells and should be useful for studies of human germ cell determination and function.
Regulation of plant growth by cytokinin
Authors: Werner Tomáš , Motyka Václav , Strnad Miroslav , Schmülling Thomas
Published: July 2002

Cytokinins are a class of plant-specific hormones that play a central role during the cell cycle and influence numerous developmental programs. Because of the lack of biosynthetic and signaling mutants, the regulatory roles of cytokinins are not well understood. We genetically engineered cytokinin oxidase expression in transgenic tobacco plants to reduce their endogenous cytokinin content. Cytokinin-deficient plants developed stunted shoots with smaller apical meristems. The plastochrone was prolonged, and leaf cell production was only 3–4% that of wild type, indicating an absolute requirement of cytokinins for leaf growth. In contrast, root meristems of transgenic plants were enlarged and gave rise to faster growing and more branched roots. These results suggest that cytokinins are an important regulatory factor of plant meristem activity and morphogenesis, with opposing roles in shoots and roots.
Activation of HIF1α ubiquitination by a reconstituted von Hippel-Lindau (VHL) tumor suppressor complex
Authors: Kamura Takumi , Sato Shigeo , Iwai Kazuhiro , Czyzyk-Krzeska Maria , Conaway Ronald C. , Conaway Joan Weliky
Published: July 2002

Mutations in the
VHL
tumor suppressor gene result in constitutive expression of many hypoxia-inducible genes, at least in part because of increases in the cellular level of hypoxia-inducible transcription factor HIF1α, which in normal cells is rapidly ubiquitinated and degraded by the proteasome under normoxic conditions. The recent observation that the VHL protein is a subunit of an Skp1-Cul1/Cdc53-F-box (SCF)-like E3 ubiquitin ligase raised the possibility that VHL may be directly responsible for regulating cellular levels of HIF1α by targeting it for ubiquitination and proteolysis. In this report, we test this hypothesis directly. We report development of methods for production of the purified recombinant VHL complex and present direct biochemical evidence that it can function with an E1 ubiquitin-activating enzyme and E2 ubiquitin-conjugating enzyme to activate HIF1α ubiquitination
in vitro
. Our findings provide new insight into the function of the VHL tumor suppressor protein, and they provide a foundation for future investigations of the mechanisms underlying VHL regulation of oxygen-dependent gene expression.