bluuebunny/crossref_metadata_2025 huggingface dataset:
Channel mobility enhancement in InP metal-insulator-semiconductor field-effect transistors
Authors: Pande K. P. , Gutierrez D.
Published: July 2002

The channel mobility of the InP metal-insulator-semiconductor field-effect transistor (MISFET’s) was enhanced by incorporating a phosphorus-rich interfacial oxide between the SiO2 gate dielectric and InP substrate. This phosphorus-rich oxide was grown using an indirect, plasma-enhanced chemical vapor deposition process followed by deposition of the SiO2 gate dielectric using the same technique. The metal-insulator-semiconductor structures, formed by aluminum deposition on the SiO2, did not exhibit any hysteresis in its capacitance-voltage characteristic. An interface state density as low as 8×1010 cm−2 eV−1 was achieved for such an MIS system. A channel mobility of 3450 cm2/Vs at room temperature was achieved for 4-μm gate length InP MISFET’s. These MISFET’s did not show any significant drift of channel current during six hours of continuous operation. These excellent characteristics are postulated to result from suppression of surface defects during the deposition of the SiO2 gate dielectric.
Determination of the conduction-band discontinuities of GaAs/AlxGa1−xAs interfaces by capacitance-voltage measurements
Authors: Okumura H. , Misawa S. , Yoshida S. , Gonda S.
Published: July 2002

The relation between the conduction-band discontinuity ΔEc and the Al composition x of refined GaAs/AsxGa1−xAs (x<0.42) heterointerfaces was determined by means of capacitance-voltage measurements. The resulting relation, ΔEc=0.67ΔEg, is different from Dingle’s rule. The interface charge density σ of a series of the samples was also investigated in relation to the reliability of the determination of ΔEc. It was found that σ less than 1×1011/cm2 is required to determine ΔEc with the precision of ±10 meV.
High resolution transmission electron microscopy of proton-implanted gallium arsenide
Authors: Sadana D. K. , Zavada J. M. , Jenkinson H. A. , Sands T.
Published: July 2002

High resolution transmission electron microscopy has been performed on cross-sectional specimens from high dose (1016 cm−2) H+-implanted (100) n-GaAs (300 keV at room temperature). It was found that annealing at 500 °C created small (20–50 Å) loops on {111} planes near the projected range Rp (3.2 μm). At 550–600 °C, voids surrounded by stacking faults, microtwins, and perfect dislocations were observed near the Rp. A phenomenological model explaining the observed results is proposed.
Further evidence for free electrons as dominating the behavior of electrochromic polycrystalline WO3 films
Authors: Goldner R. B. , Norton P. , Wong K. , Foley G. , Goldner E. L. , Seward G. , Chapman R.
Published: July 2002

The results of two types of experiments on electrochromic (poly)crystalline films of rf sputtered WO3 are presented as additional new evidence that the behavior (especially scattering) of the free electrons plays a dominant role in the electrochromic properties of such films.
Negative differential resistances from Hg1−xCdxTe-CdTe single quantum barrier heterostructures
Authors: Chow D. H. , McGill T. C.
Published: July 2002

A novel negative differential resistance device is proposed and analyzed theoretically. The proposed device is a Hg1−xCdxTe-CdTe-Hg1−xCdxTe single quantum barrier heterostructure with 0.2≤x≤0.4. The anticipated negative differential resistance is a consequence of the unique properties of Hg1−xCdxTe-CdTe heterojunctions. Calculations are performed to predict the current-voltage characteristics of the proposed device. The results indicate that room-temperature negative differential resistances can be obtained over a large range of applied voltages. The high-speed capabilities associated with quantum barrier tunnel structures make the proposed device a candidate for high-frequency applications.
Direct measurements of the velocity and thickness of ‘‘explosively’’ propagating buried molten layers in amorphous silicon
Authors: Lowndes D. H. , Jellison G. E. , Pennycook S. J. , Withrow S. P. , Mashburn D. N.
Published: July 2002

Simultaneous infrared (1152 nm) and visible (633 nm) reflectivity measurements with nanosecond resolution were used to study the initial formation and subsequent motion of pulsed KrF laser-induced ‘‘explosively’’ propagating buried molten layers in ion implantation-amorphized silicon. The buried layer velocity decreases with depth below the surface, but increases with KrF laser energy density; a maximum velocity of about 14 m/s was observed, implying an undercooling-velocity relationship of ∼14 K/(m/s). Z-contrast scanning transmission electron microscopy was used to form a direct chemical image of implanted Cu ions transported by the buried layer and showed that the final buried layer thickness was <15 nm.
Induced resonance electron cyclotron quasi-optical maser in an open resonator
Authors: Sprangle P. , Tang C. M. , Serafim P.
Published: July 2002

In this letter we analyze an induced resonance electron cyclotron (IREC) quasi-optical maser configuration, which has the unique features of being highly efficient and, with the proper choice for the index of refraction, relatively insensitive to the electron beam’s energy spread. The radiation mechanism can, however, be sensitive to the beam’s pitch angle spread. A system of nonlinear coupled orbit equations, describing the dynamics of electrons in a spatially varying magnetic field, and the electromagnetic field of a steady-state open resonator configuration are derived and analyzed. Furthermore, we show that by appropriately tapering the magnetic field an induced resonance condition can be achieved which results in high interaction efficiencies. It is anticipated that an efficient, high power millimeter, submillimeter, and infrared radiation source can be realized with the IREC quasi-optical maser configuration.
Effect of Si on the reaction kinetics of Ti/AlSi bilayer structures
Authors: Nahar R. K. , Devashrayee N. M.
Published: July 2002

The effect of Si on the reaction between Al and Al-2% Si films with the transition metal Ti in bilayer structures is studied by the resistivity measurements on samples isothermally annealed in the temperature range of 400–450 °C. The change in the resistivity of the structures is correlated to the rate of formation of an intermetallic compound TiAl3 due to the interaction between Ti and Al. It is found that the reaction rate of the intermetallic compound formation is reduced by a factor of 3 for Ti/AlSi compared with the Ti/Al bilayer structures. The growth mechanism in both the cases is indicated to be diffusion limited. The activation energy for the Ti/AlSi increases to 2.2 eV from 1.7 eV for the Ti/Al structure.
Stimulated emission from ultrathin InAs/GaAs quantum well heterostructures grown by atomic layer epitaxy
Authors: Tischler M. A. , Anderson N. G. , Kolbas R. M. , Bedair S. M.
Published: July 2002

We report the first observation of stimulated emission (16 K) in material grown by atomic layer epitaxy (ALE). The active region of our laser structure consists of six strained InAs quantum wells (6.6 Å thick, 7.4% strain) separated by 500 Å of GaAs, which makes these the thinnest and most highly strained quantum wells ever reported to support stimulated emission. These results demonstrate that the ALE process can be used to grow laser quality material with highly reproducible layer thickness.
Role of Ostwald ripening in islanding processes
Authors: Zinke-Allmang M. , Feldman L. C. , Nakahara S.
Published: July 2002

We report measurements of the rate of growth of Ga and Sn clusters on clean Si surfaces. The volume rate of growth is linearly dependent on time, consistent with an Ostwald ripening mechanism for island growth. Activation energies for clustering, extracted from the temperature dependence, are shown to be sensitive to the underlying surface structure. This thermodynamical approach to the description of cluster formation is generally useful in determining the operational parameters and ultimate limits to heterostructure growth.
Femtosecond dynamics of highly excited carriers in AlxGa1−xAs
Authors: Lin W. Z. , Fujimoto J. G. , Ippen E. P. , Logan R. A.
Published: July 2002

Femtosecond transient absorption saturation measurements are used to investigate the scattering of optically excited carriers in AlGaAs. With pulses as short as 35 fs at 1.98 eV, scattering times ranging from 13 to 330 fs are observed in samples of AlxGa1−xAs with x=0, 0.1, 0.2, 0.3, and 0.4. A dramatic decrease in the rate of carrier scattering out of the initial optically excited states is observed with increasing Al concentration.
Model for bulk effects on Si interstitial diffusivity in silicon
Authors: Griffin P. B. , Ahn S. T. , Tiller W. A. , Plummer J. D.
Published: July 2002

Literature values for the diffusivity of the silicon interstitial or interstitialcy, I, range over several orders of magnitude and have activation energies between 1 and 4 eV. We propose a model for bulk trapping effects on the I diffusivity which provides a consistent explanation for the observed discrepancies. It reconciles the effects of different materials (float-zone, Czochralski, and epitaxial silicon) and processes (diffusion and gettering) on the apparent value of the I diffusivity. New experimental results which directly indicate substantial bulk effects in different types of silicon support the validity of the model.
I
n s
i
t
u preparation of Y-Ba-Cu-O superconducting thin films by magnetron sputtering
Authors: Li H. C. , Linker G. , Ratzel F. , Smithey R. , Geerk J.
Published: July 2002

Thin superconducting films of YBa2 Cu3 O7 have been prepared by magnetron sputtering from targets of sintered material in an oxygen-argon atmosphere. The compositional and structural properties were studied by Rutherford backscattering and x-ray diffraction. The films were deposited at substrate temperatures between 580 and 800 °C. It was found that the material grows in the oxygen deficient tetragonal phase. In situ heat treatment at 430 °C in pure O2 atmosphere generates the orthorhombic structure and the films on sapphire and SrTiO3 coated sapphire substrates show the full superconducting transition at 83 K.
Control of pinholes in epitaxial CoSi2 layers on Si(111)
Authors: Tung R. T. , Batstone J. L.
Published: July 2002

The growth of ultrathin (<50 Å thick) uniform CoSi2 layers at low temperatures (<450 °C) has been reported recently. Pinholes are formed in these silicide layers when the temperature is raised to above ∼550 °C. An important driving force for the generation of pinholes has been identified as a change of the surface structure from CoSi2-C, stable at low temperature, to the high-temperature stable CoSi2-S. Treatment of the surface of CoSi2 facilitates this transition and prevents the formation of pinholes. A few important parameters in the silicide reaction are shown to govern the morphology of the reacted CoSi2 layers.
Inelastic scattering of electrons traversing semiconductor heterojunctions
Authors: Müller J. F. , Schmitt-Rink S. , Levi A. F. J.
Published: July 2002

We calculate the contribution of polar optic phonons to the inelastic scattering rate for an electron traversing semiconductor heterojunctions. In typical geometries, a dramatic reduction in scattering rate compared to the bulk value is found for a limited range of electron energies. This effect is related to spatial separation of initial and final electron wave functions either side of the heterojunction caused by quantum mechanical reflection at the interface. The influence of this phenomenon on the performance of devices, such as unipolar hot-electron transistors, is discussed.
Inorganic polyphosphate kinase and adenylate kinase participate in the polyphosphate:AMP phosphotransferase activity of
Escherichia coli
Authors: Ishige Kazuya , Noguchi Toshitada
Published: July 2002

Polyphosphate kinase (PPK), responsible for the processive
synthesis of inorganic polyphosphate (polyP) from ATP in
Escherichia coli
, can transfer in reverse the terminal
phosphate residue of polyP to ADP to yield ATP. PolyP also serves as a
donor in a polyP:AMP phosphotransferase (PAP) activity observed in
extracts of
Acinetobacter johnsonii
and
Myxococcus xanthus
. We have found that overexpression of
the gene encoding PPK results in a large enhancement of PAP activity in
E. coli
. The PAP activity requires both PPK and
adenylate kinase in equimolar amounts. PPK and adenylate kinase form a
complex in the presence of polyphosphate. We discuss a phosphotransfer
mechanism that involves both enzymes and enables polyP to be a
phospho-donor to AMP.
Association of human aging with a functional variant of klotho
Authors: Arking Dan E. , Krebsova Alice , Macek Milan , Macek Milan , Arking Albert , Mian I. Saira , Fried Linda , Hamosh Ada , Dey Srabani , McIntosh Iain , Dietz Harry C.
Published: July 2002

Mice deficient inKlothogene expression exhibit a syndrome resembling premature human aging. To determine whether variation in the humanKLOTHOlocus contributes to survival, we applied two newly characterized polymorphic microsatellite markers flanking the gene in a population-based association study. In a cohort chosen for its homogeneity, Bohemian Czechs, we demonstrated significant differences in selected marker allele frequencies between newborn and elderly individuals (P< 0.05). These results precipitated a search for functional variants of klotho. We identified an allele, termed KL-VS, containing six sequence variants in complete linkage disequilibrium, two of which result in amino acid substitutions F352V and C370S. Homozygous elderly individuals were underrepresented in three distinct populations: Bohemian Czechs, Baltimore Caucasians, and Baltimore African-Americans [combined odds ratio (OR) = 2.59,P< 0.0023]. In a transient transfection assay, secreted levels of klotho harboring V352 are reduced 6-fold, whereas extracellular levels of the S370 form are increased 2.9-fold. The V352/S370 double mutant exhibits an intermediate phenotype (1.6-fold increase), providing a rare example of intragenic complementation incisby human single nucleotide polymorphisms. The remarkable conservation of F352 among homologous proteins suggests that it is functionally important. The corresponding substitution, F289V, in the closest human klotho paralog with a known substrate, cBGL1, completely eliminates its ability to cleave p-nitrophenyl-β-d-glucoside. These results suggest that the KL-VS allele influences the trafficking and catalytic activity of klotho, and that variation in klotho function contributes to heterogeneity in the onset and severity of human age-related phenotypes.
Spatial frequency and orientation tuning dynamics in area V1
Authors: Mazer James A. , Vinje William E. , McDermott Josh , Schiller Peter H. , Gallant Jack L.
Published: July 2002

Spatial frequency (SF) and orientation tuning are intrinsic properties of neurons in primary visual cortex (area V1). To investigate the neural mechanisms mediating selectivity in the awake animal, we measured the temporal dynamics of SF and orientation tuning. We adapted a high-speed reverse-correlation method previously used to characterize orientation tuning dynamics in anesthetized animals to estimate efficiently the complete spatiotemporal receptive fields in area V1 of behaving macaques. We found that SF and orientation tuning are largely separable over time in single neurons. However, spatiotemporal receptive fields also contain a small nonseparable component that reflects a significant difference in response latency for low and high SF stimuli. The observed relationship between stimulus SF and latency represents a dynamic shift in SF tuning, and suggests that single V1 neurons might receive convergent input from the magno- and parvocellular processing streams. Although previous studies with anesthetized animals suggested that orientation tuning could change dramatically over time, we find no substantial evidence of dynamic changes in orientation tuning.
Spatial order within but not between types of retinal neurons
Authors: Rockhill Rebecca L. , Euler Thomas , Masland Richard H.
Published: July 2002

We studied the mosaics of six types of retinal neurons, asking how the position of a cell relates to the positions of other cells of that same type and also to cells of different types. Every neuron studied was found to be nonrandomly positioned: Cells of a particular type were evenly spaced. However, all cells were positioned randomly with respect to members of the other cell classes. This was true even when the cells were known to be synaptically connected. It is consistent with a concept of developmental pattern formation in which (
i
) the number of cells of a particular type and their laminar distribution are specified, and (
ii
) the final spatial position of each cell is controlled exclusively by a rule that prevents cells of the same type from being positioned close to each other. This sequence would imply that a cell's final position is independent of the cell's position at the time of its specification, and we suggest a reason why, in laminar structures containing many cell types, it might be desirable for this to be so.
Endothelial cells derived from human embryonic stem cells
Authors: Levenberg Shulamit , Golub Justin S. , Amit Michal , Itskovitz-Eldor Joseph , Langer Robert
Published: July 2002

Human embryonic stem cells have the potential to differentiate into various cell types and, thus, may be useful as a source of cells for transplantation or tissue engineering. We describe here the differentiation steps of human embryonic stem cells into endothelial cells forming vascular-like structures. The human embryonic-derived endothelial cells were isolated by using platelet endothelial cell-adhesion molecule-1 (PECAM1) antibodies, their behavior was characterized
in vitro
and
in vivo
, and their potential in tissue engineering was examined. We show that the isolated embryonic PECAM1+ cells, grown in culture, display characteristics similar to vessel endothelium. The cells express endothelial cell markers in a pattern similar to human umbilical vein endothelial cells, their junctions are correctly organized, and they have high metabolism of acetylated low-density lipoprotein. In addition, the cells are able to differentiate and form tube-like structures when cultured on matrigel.
In vivo
, when transplanted into SCID mice, the cells appeared to form microvessels containing mouse blood cells. With further studies, these cells could provide a source of human endothelial cells that could be beneficial for potential applications such as engineering new blood vessels, endothelial cell transplantation into the heart for myocardial regeneration, and induction of angiogenesis for treatment of regional ischemia.
Type I collagen is thermally unstable at body temperature
Authors: Leikina E. , Mertts M. V. , Kuznetsova N. , Leikin S.
Published: July 2002

Measured by ultra-slow scanning calorimetry and isothermal circular dichroism, human lung collagen monomers denature at 37°C within a couple of days. Their unfolding rate decreases exponentially at lower temperature, but complete unfolding is observed even below 36°C. Refolding of full-length, native collagen triple helices does occur, but only below 30°C. Thus, contrary to the widely held belief, the energetically preferred conformation of the main protein of bone and skin in physiological solution is a random coil rather than a triple helix. These observations suggest that once secreted from cells collagen helices would begin to unfold. We argue that initial microunfolding of their least stable domains would trigger self-assembly of fibers where the helices are protected from complete unfolding. Our data support an earlier hypothesis that in fibers collagen helices may melt and refold locally when needed, giving fibers their strength and elasticity. Apparently, Nature adjusts collagen hydroxyproline content to ensure that the melting temperature of triple helical monomers is several degrees below rather than above body temperature.
Genomewide identification of
Pseudomonas syringae
pv.
tomato
DC3000 promoters controlled by the HrpL alternative sigma factor
Authors: Fouts Derrick E. , Abramovitch Robert B. , Alfano James R. , Baldo Angela M. , Buell C. Robin , Cartinhour Samuel , Chatterjee Arun K. , D'Ascenzo Mark , Gwinn Michelle L. , Lazarowitz Sondra G. , Lin Nai-Chun , Martin Gregory B. , Rehm Amos H. , Schneider David J. , van Dijk Karin , Tang Xiaoyan , Collmer Alan
Published: July 2002

The ability of
Pseudomonas syringae
pv.
tomato
DC3000 to parasitize tomato and
Arabidopsis thaliana
depends on genes activated by the HrpL alternative sigma factor. To support various functional genomic analyses of DC3000, and specifically, to identify genes involved in pathogenesis, we developed a draft sequence of DC3000 and used an iterative process involving computational and gene expression techniques to identify virulence-implicated genes downstream of HrpL-responsive promoters. Hypersensitive response and pathogenicity (Hrp) promoters are known to control genes encoding the Hrp (type III protein secretion) machinery and a few type III effector proteins in DC3000. This process involved (
i
) identification of 9 new virulence-implicated genes in the Hrp regulon by miniTn
5gus
mutagenesis, (
ii
) development of a hidden Markov model (HMM) trained with known and transposon-identified Hrp promoter sequences, (
iii
) HMM identification of promoters upstream of 12 additional virulence-implicated genes, and (
iv
) microarray and RNA blot analyses of the HrpL-dependent expression of a representative subset of these DC3000 genes. We found that the Hrp regulon encodes candidates for 4 additional type III secretion machinery accessory factors, homologs of the effector proteins HopPsyA, AvrPpiB1 (2 copies), AvrPpiC2, AvrPphD (2 copies), AvrPphE, AvrPphF, and AvrXv3, and genes associated with the production or metabolism of virulence factors unrelated to the Hrp type III secretion system, including syringomycin synthetase (SyrE),
N
ɛ
-(indole-3-acetyl)-
l
-lysine synthetase (IaaL), and a subsidiary regulon controlling coronatine production. Additional candidate effector genes,
hopPtoA2
,
hopPtoB2
, and an
avrRps4
homolog, were preceded by Hrp promoter-like sequences, but these had HMM expectation values of relatively low significance and were not detectably activated by HrpL.
Engineering gene expression and protein synthesis by modulation of nuclear shape
Authors: Thomas Carson H. , Collier Joel H. , Sfeir Charles S. , Healy Kevin E.
Published: July 2002

The current understanding of the relationships between cell shape, intracellular forces and signaling, nuclear shape and organization, and gene expression is in its infancy. Here we introduce a method for investigating gene-specific responses in individual cells with controlled nuclear shape and projected area. The shape of the nuclei of primary osteogenic cells were controlled on microfabricated substrata with regiospecific chemistry by confining attachment and spreading of isolated cells on adhesive islands. Gene expression and protein synthesis were altered by changing nuclear shape. Collagen I synthesis correlated directly with cell shape and nuclear shape index (NSI), where intermediate values of nuclear distension (6 < NSI < 8) promoted maximum synthesis. Osteocalcin mRNA, a bone-specific differentiation marker, was observed intracellularly by using reverse transcription
in situ
PCR at 4 days in cells constrained by the pattern and not detected in unconstrained cells of similar projected area, but different NSI. Our data supports the concept of gene expression and protein synthesis based on optimal distortion of the nucleus, possibly altering transcription factor affinity for DNA, transport to the nucleus, or nuclear matrix organization. The combination of microfabricated surfaces, reverse transcription
in situ
PCR, and NSI measurement is an excellent system to study how transcription factors, the nuclear matrix, and the cytoskeleton interact to control gene expression and may be useful for studying a wide variety of other cell shape/gene expression relationships.
Erythropoietin prevents neuronal apoptosis after cerebral ischemia and metabolic stress
Authors: Sirén Anna-Leena , Fratelli Maddalena , Brines Michael , Goemans Christoph , Casagrande Simona , Lewczuk Piotr , Keenan Sonja , Gleiter Christoph , Pasquali Claudio , Capobianco Annalisa , Mennini Tiziana , Heumann Rolf , Cerami Anthony , Ehrenreich Hannelore , Ghezzi Pietro
Published: July 2002

Erythropoietin (EPO) promotes neuronal survival after hypoxia and
other metabolic insults by largely unknown mechanisms.
Apoptosis and necrosis have been proposed as mechanisms of
cellular demise, and either could be the target of actions of EPO. This
study evaluates whether antiapoptotic mechanisms can account
for the neuroprotective actions of EPO. Systemic administration of EPO
(5,000 units/kg of body weight, i.p.) after middle-cerebral artery
occlusion in rats dramatically reduces the volume of infarction 24
h later, in concert with an almost complete reduction in the number of
terminal deoxynucleotidyltransferase-mediated dUTP nick-end labeling of
neurons within the ischemic penumbra. In both pure and mixed neuronal
cultures, EPO (0.1–10 units/ml) also inhibits apoptosis
induced by serum deprivation or kainic acid exposure. Protection
requires pretreatment, consistent with the induction of a gene
expression program, and is sustained for 3 days without the continued
presence of EPO. EPO (0.3 units/ml) also protects hippocampal neurons
against hypoxia-induced neuronal death through activation of
extracellular signal-regulated kinases and protein kinase
Akt-1/protein kinase B. The action of EPO is not limited to directly
promoting cell survival, as EPO is trophic but not mitogenic in
cultured neuronal cells. These data suggest that inhibition of neuronal
apoptosis underlies short latency protective effects of EPO
after cerebral ischemia and other brain injuries. The neurotrophic
actions suggest there may be longer-latency effects as well. Evaluation
of EPO, a compound established as clinically safe, as neuroprotective
therapy in acute brain injury is further supported.
Induction of behavioral associative memory by stimulation of the nucleus basalis
Authors: McLin Dewey E. , Miasnikov Alexandre A. , Weinberger Norman M.
Published: July 2002

The nucleus basalis (NB) has been implicated in memory formation indirectly, by lesions, pharmacological manipulations, and neural correlates of learning. Prior findings imply that engagement of the NB during learning promotes memory storage. We directly tested this NB-memory hypothesis by determining whether stimulation of the NB induces behavioral associative memory. Rats were trained either with paired tone (6 kHz) and NB stimulation or with the two stimuli unpaired. We later determined the specificity of cardiac and respiratory behavioral responses to the training tone and several other acoustic frequencies. Paired subjects exhibited frequency generalization gradients with a peak of 6 kHz for both cardiac and respiratory behavior. Unpaired subjects exhibited no generalization gradient. The development of such specific, associative behavioral responses indicates that tone paired with NB stimulation induced behavioral associative memory. The discovery of memory induction by direct activation of the NB supports the NB-memory hypothesis and provides a potentially powerful way to control and investigate neural mechanisms of memory.
BMP2-mediated alteration in the developmental pathway of fetal mouse brain cells from neurogenesis to astrocytogenesis
Authors: Nakashima Kinichi , Takizawa Takumi , Ochiai Wataru , Yanagisawa Makoto , Hisatsune Tatsuhiro , Nakafuku Masato , Miyazono Kohei , Kishimoto Tadamitsu , Kageyama Ryoichiro , Taga Tetsuya
Published: July 2002

We show that when telencephalic neural progenitors are briefly
exposed to bone morphogenetic protein 2 (BMP2) in culture, their
developmental fate is changed from neuronal cells to astrocytic cells.
BMP2 significantly reduced the number of cells expressing
microtubule-associated protein 2, a neuronal marker, and cells
expressing nestin, a marker for undifferentiated neural precursors, but
BMP2 increased the number of cells expressing S100-β, an astrocytic
marker. In telencephalic neuroepithelial cells, BMP2 up-regulated the
expression of negative helix–loop–helix (HLH) factors Id1, Id3, and
Hes-5 (where Hes is homologue of hairy and Enhancer of Split) that
inhibited the transcriptional activity of neurogenic HLH transcription
factors Mash1 and neurogenin. Ectopic expression of either Id1 or Id3
(where Id is inhibitor of differentiation) inhibited neurogenesis of
neuroepithelial cells, suggesting an important role for these HLH
proteins in the BMP2-mediated changes in the neurogenic fate of these
cells. Because gliogenesis in the brain and spinal cord, derived from
implanted neural stem cells or induced by injury, is responsible for
much of the failure of neuronal regeneration, this work may lead to a
therapeutic strategy to minimize this problem.
Amyotrophic lateral sclerosis: A proposed mechanism
Authors: Okado-Matsumoto Ayako , Fridovich Irwin
Published: July 2002

Missense mutations in Cu,Zn-superoxide dismutase (SOD1) account for ≈20% of familial amyotrophic lateral sclerosis (FALS) through some, as yet undefined, toxic gain of function that leads to gradual death of motor neurons. Mitochondrial swelling and vacuolization are early signs of incipient motor neuron death in FALS. We previously reported that SOD1 exists in the intermembrane space of mitochondria. Herein, we demonstrate that the entry of SOD1 into mitochondria depends on demetallation and that heat shock proteins (Hsp70, Hsp27, or Hsp25) block the uptake of the FALS-associated mutant SOD1 (G37R, G41D, or G93A), while having no effect on wild-type SOD1. The binding of mutant SOD1 to Hsps in the extract of neuroblastoma cells leads to formation of sedimentable aggregates. Many antiapoptotic effects of Hsps have been reported. We now propose that this binding of Hsps to mutant forms of a protein abundant in motor neurons, such as SOD1, makes Hsps unavailable for their antiapoptotic functions and leads ultimately to motor neuron death. It also appears that the Hsp–SOD1 complex recruits other proteins present in the neuroblastoma cell and presumably in motor neurons to form sedimentable aggregates.
Rapid and general profiling of protease specificity by using combinatorial fluorogenic substrate libraries
Authors: Harris Jennifer L. , Backes Bradley J. , Leonetti Francesco , Mahrus Sami , Ellman Jonathan A. , Craik Charles S.
Published: July 2002

A method is presented for the preparation and use of fluorogenic peptide substrates that allows for the configuration of general substrate libraries to rapidly identify the primary and extended specificity of proteases. The substrates contain the fluorogenic leaving group 7-amino-4-carbamoylmethylcoumarin (ACC). Substrates incorporating the ACC leaving group show kinetic profiles comparable to those with the traditionally used 7-amino-4-methylcoumarin (AMC) leaving group. The bifunctional nature of ACC allows for the efficient production of single substrates and substrate libraries by using 9-fluorenylmethoxycarbonyl (Fmoc)-based solid-phase synthesis techniques. The approximately 3-fold-increased quantum yield of ACC over AMC permits reduction in enzyme and substrate concentrations. As a consequence, a greater number of substrates can be tolerated in a single assay, thus enabling an increase in the diversity space of the library. Soluble positional protease substrate libraries of 137,180 and 6,859 members, possessing amino acid diversity at the P4-P3-P2-P1 and P4-P3-P2 positions, respectively, were constructed. Employing this screening method, we profiled the substrate specificities of a diverse array of proteases, including the serine proteases thrombin, plasmin, factor Xa, urokinase-type plasminogen activator, tissue plasminogen activator, granzyme B, trypsin, chymotrypsin, human neutrophil elastase, and the cysteine proteases papain and cruzain. The resulting profiles create a pharmacophoric portrayal of the proteases to aid in the design of selective substrates and potent inhibitors.
Human mitochondrial topoisomerase I
Authors: Zhang Hongliang , Barceló Juana M. , Lee Benson , Kohlhagen Glenda , Zimonjic Drazen B. , Popescu Nicholas C. , Pommier Yves
Published: July 2002

Tension generated in the circular mitochondrial genome during replication and transcription points to the need for mtDNA topoisomerase activity. Here we report a 601-aa polypeptide highly homologous to nuclear topoisomerase I. The N-terminal domain of this novel topoisomerase contains a mitochondrial localization sequence and lacks a nuclear localization signal. Therefore, we refer to this polypeptide as top1mt. The pattern of top1mt expression matches the requirement for high mitochondrial activity in specific tissues. top1mt is a type IB topoisomerase that requires divalent metal (Ca
2+
or Mg
2+
) and alkaline pH for optimum activity. The
TOP1mt
gene is highly homologous to the nuclear
TOP1
gene and consists of 14 exons. It is localized on human chromosome 8q24.3.
Regulation of cyclin-dependent kinase 5 and casein kinase 1 by metabotropic glutamate receptors
Authors: Liu Feng , Ma Xiao-Hong , Ule Jernej , Bibb James A. , Nishi Akinori , DeMaggio Anthony J. , Yan Zhen , Nairn Angus C. , Greengard Paul
Published: July 2002

Cyclin-dependent kinase 5 (Cdk5) is a multifunctional neuronal
protein kinase that is required for neurite outgrowth and cortical
lamination and that plays an important role in dopaminergic signaling
in the neostriatum through phosphorylation of Thr-75 of DARPP-32
(dopamine and cAMP-regulated phosphoprotein, molecular mass 32 kDa).
Casein kinase 1 (CK1) has been implicated in a variety of cellular
functions such as DNA repair, circadian rhythm, and intracellular
trafficking. In the neostriatum, CK1 has been found to phosphorylate
Ser-137 of DARPP-32. However, first messengers for the regulation of
Cdk5 or CK1 have remained unknown. Here we report that both Cdk5 and
CK1 are regulated by metabotropic glutamate receptors (mGluRs) in
neostriatal neurons. (S)-3,5-dihydroxyphenylglycine (DHPG), an agonist
for group I mGluRs, increased Cdk5 and CK1 activities in neostriatal
slices, leading to the enhanced phosphorylation of Thr-75 and Ser-137
of DARPP-32, respectively. The effect of DHPG on Thr-75, but not on
Ser-137, was blocked by a Cdk5-specific inhibitor, butyrolactone. In
contrast, the effects of DHPG on both Thr-75 and Ser-137 were blocked
by CK1–7 and IC261, specific inhibitors of CK1, suggesting that
activation of Cdk5 by mGluRs requires CK1 activity. In support of this
possibility, the DHPG-induced increase in Cdk5 activity, measured in
extracts of neostriatal slices, was abolished by CK1–7 and IC261.
Treatment of acutely dissociated neurons with DHPG enhanced
voltage-dependent Ca
2+
currents. This enhancement was
eliminated by either butyrolactone or CK1–7 and was absent in DARPP-32
knockout mice. Together these results indicate that a CK1-Cdk5-DARPP-32
cascade may be involved in the regulation by mGluR agonists of
Ca
2+
channels.
A conditional tissue-specific transgene expression system using inducible GAL4
Authors: Osterwalder Thomas , Yoon Kenneth S. , White Benjamin H. , Keshishian Haig
Published: July 2002

In
Drosophila
, the most widely used system for generating spatially restricted transgene expression is based on the yeast GAL4 protein and its target upstream activating sequence (UAS). To permit temporal as well as spatial control over UAS-transgene expression, we have explored the use of a conditional RU486-dependent GAL4 protein (GeneSwitch) in
Drosophila
. By using cloned promoter fragments of the embryonic lethal abnormal vision gene or the myosin heavy chain gene, we have expressed GeneSwitch specifically in neurons or muscles and show that its transcriptional activity within the target tissues depends on the presence of the activator RU486 (mifepristone). We used available UAS-reporter lines to demonstrate RU486-dependent tissue-specific transgene expression in larvae. Reporter protein expression could be detected 5 h after systemic application of RU486 by either feeding or “larval bathing.” Transgene expression levels were dose-dependent on RU486 concentration in larval food, with low background expression in the absence of RU486. By using genetically altered ion channels as reporters, we were able to change the physiological properties of larval bodywall muscles in an RU486-dependent fashion. We demonstrate here the applicability of GeneSwitch for conditional tissue-specific expression in
Drosophila
, and we provide tools to control pre- and postsynaptic expression of transgenes at the larval neuromuscular junction during postembryonic life.
Geophysical, archaeological, and historical evidence support a solar-output model for climate change
Authors: Perry Charles A. , Hsu Kenneth J.
Published: July 2002

Although the processes of climate change are not completely
understood, an important causal candidate is variation in total solar
output. Reported cycles in various climate-proxy data show a tendency
to emulate a fundamental harmonic sequence of a basic solar-cycle
length (11 years) multiplied by 2
N
(where N equals a
positive or negative integer). A simple additive model for total
solar-output variations was developed by superimposing a progression of
fundamental harmonic cycles with slightly increasing amplitudes. The
timeline of the model was calibrated to the Pleistocene/Holocene
boundary at 9,000 years before present. The calibrated model was
compared with geophysical, archaeological, and historical evidence of
warm or cold climates during the Holocene. The evidence of periods of
several centuries of cooler climates worldwide called “little ice
ages,” similar to the period
anno Domini
(A.D.)
1280–1860 and reoccurring approximately every 1,300 years, corresponds
well with fluctuations in modeled solar output. A more detailed
examination of the climate sensitive history of the last 1,000 years
further supports the model. Extrapolation of the model into the future
suggests a gradual cooling during the next few centuries with
intermittent minor warmups and a return to near little-ice-age
conditions within the next 500 years. This cool period then may be
followed approximately 1,500 years from now by a return to altithermal
conditions similar to the previous Holocene Maximum.
Adeno-associated virus vector carrying human minidystrophin genes effectively ameliorates muscular dystrophy in
mdx
mouse model
Authors: Wang Bing , Li Juan , Xiao Xiao
Published: July 2002

Duchenne muscular dystrophy (DMD) is the most common and lethal
genetic muscle disorder, caused by recessive mutations in the
dystrophin gene. One of every 3,500 males suffers from DMD, yet no
treatment is currently available. Genetic therapeutic approaches, using
primarily myoblast transplantation and adenovirus-mediated gene
transfer, have met with limited success. Adeno-associated virus (AAV)
vectors, although proven superior for muscle gene transfer, are too
small (5 kb) to package the 14-kb dystrophin cDNA. Here we have created
a series of minidystrophin genes (<4.2 kb) under the control of a
muscle-specific promoter that readily package into AAV vectors. When
injected into the muscle of
mdx
mice (a DMD model), two
of the minigenes resulted in efficient and stable expression in a
majority of the myofibers, restoring the missing dystrophin and
dystrophin-associated protein complexes onto the plasma membrane. More
importantly, this AAV treatment ameliorated dystrophic pathology in
mdx
muscle and led to normal myofiber morphology,
histology, and cell membrane integrity. Thus, we have defined minimal
functional dystrophin units and demonstrated the effectiveness of using
AAV to deliver the minigenes
in vivo
, offering a
promising avenue for DMD gene therapy.
Syntrophin-dependent expression and localization of Aquaporin-4 water channel protein
Authors: Neely John D. , Amiry-Moghaddam Mahmood , Ottersen Ole Petter , Froehner Stanley C. , Agre Peter , Adams Marvin E.
Published: July 2002

The Aquaporin-4 (AQP4) water channel contributes to brain water homeostasis in perivascular astrocyte endfeet where it is concentrated. We postulated that AQP4 is tethered at this site by binding of the AQP4 C terminus to the
P
SD95-
D
iscs large-
Z
O1 (PDZ) domain of syntrophin, a component of the dystrophin protein complex. Chemical cross-linking and coimmunoprecipitations from brain demonstrated AQP4 in association with the complex, including dystrophin, β-dystroglycan, and syntrophin. AQP4 expression was studied in brain and skeletal muscle of mice lacking α-syntrophin (α-
Syn
−/−
). The total level of AQP4 expression appears normal in brains of α-
Syn
−/−
mice, but the polarized subcellular localization is reversed. High-resolution immunogold analyses revealed that AQP4 expression is markedly reduced in astrocyte endfeet membranes adjacent to blood vessels in cerebellum and cerebral cortex of α-
Syn
−/−
mice, but is present at higher than normal levels in membranes facing neuropil. In contrast, AQP4 is virtually absent from skeletal muscle in α-
Syn
−/−
mice. Deletion of the PDZ-binding consensus (Ser-Ser-Val) at the AQP4 C terminus similarly reduced expression in transfected cell lines, and pulse–chase labeling demonstrated an increased degradation rate. These results demonstrate that perivascular localization of AQP4 in brain requires α-Syn, and stability of AQP4 in the membrane is increased by the C-terminal PDZ-binding motif.
Radiocarbon evidence for maritime pioneer colonization at the origins of farming in west Mediterranean Europe
Authors: Zilhão João
Published: July 2002

Most radiocarbon dates for the earliest Neolithic cultures of west Mediterranean Europe are on samples of unidentified charcoal. If only results obtained on short lived samples (seeds, shells, and bone) of diagnostic material (domesticates, artifacts, and human remains) are considered, then the dates for the first appearance of the Neolithic package are indistinguishable statistically from central Italy to Portugal and cluster around 5400 calendar B.C. This rapidity of spread, no more than six generations, can be best explained in the framework of a maritime pioneer colonization model.
Comparison of a hair bundle's spontaneous oscillations with its response to mechanical stimulation reveals the underlying active process
Authors: Martin P. , Hudspeth A. J. , Jülicher F.
Published: July 2002

Hearing relies on active filtering to achieve exquisite sensitivity
and sharp frequency selectivity. In a quiet environment, the ears of
many vertebrates become unstable and emit one to several tones. These
spontaneous otoacoustic emissions, the most striking manifestation of
the inner ear's active process, must result from self-sustained
mechanical oscillations of aural constituents. The mechanoreceptive
hair bundles of hair cells in the bullfrog's sacculus have the ability
to amplify mechanical stimuli and oscillate spontaneously. By comparing
a hair bundle's spontaneous oscillations with its response to small
mechanical stimuli, we demonstrate a breakdown in a general principle
of equilibrium thermodynamics, the fluctuation–dissipation theorem. We
thus confirm that a hair bundle's spontaneous movements are produced
by energy-consuming elements within the hair cell. To characterize the
dynamical behavior of the active process, we introduce an effective
temperature that, for each frequency component, quantifies a hair
bundle's deviation from thermal equilibrium. The effective temperature
diverges near the bundle's frequency of spontaneous oscillation. This
behavior, which is not generic for active oscillators, can be
accommodated by a simple model that characterizes quantitatively the
fluctuations of the spontaneous movements as well as the hair bundle's
linear response function.
Oral tolerance in myelin basic protein T-cell receptor transgenic mice: suppression of autoimmune encephalomyelitis and dose-dependent induction of regulatory cells.
Authors: Chen Y , Inobe J , Kuchroo V K , Baron J L , Janeway C A , Weiner H L
Published: July 2002

Orally administered antigens induce a state of immunologic hyporesponsiveness termed oral tolerance. Different mechanisms are involved in mediating oral tolerance depending on the dose fed. Low doses of antigen generate cytokine-secreting regulatory cells, whereas high doses induce anergy or deletion. We used mice transgenic for a T-cell receptor (TCR) derived from an encephalitogenic T-cell clone specific for the acetylated N-terminal peptide of myelin basic protein (MBP) Ac-1-11 plus I-Au to test whether a regulatory T cell could be generated from the same precursor cell as that of an encephalitogenic Th1 cell and whether the induction was dose dependent. The MBP TCR transgenic mice primarily have T cells of a precursor phenotype that produce interleukin 2 (IL-2) with little interferon gamma (IFN-gamma), IL-4, or transforming growth factor beta (TGF-beta). We fed transgenic animals a low-dose (1 mg x 5) or high-dose (25 mg x 1) regimen of mouse MBP and without further immunization spleen cells were tested for cytokine production. Low-dose feeding induced prominent secretion of IL-4, IL-10, and TGF-beta, whereas minimal secretion of these cytokines was observed with high-dose feeding. Little or no change was seen in proliferation or IL-2/IFN-gamma secretion in fed animals irrespective of the dose. To demonstrate in vivo functional activity of the cytokine-secreting cells generated by oral antigen, spleen cells from low-dose-fed animals were adoptively transferred into naive (PLJ x SJL)F1 mice that were then immunized for the development of experimental autoimmune encephalomyelitis (EAE). Marked suppression of EAE was observed when T cells were transferred from MBP-fed transgenic animals but not from animals that were not fed. In contrast to oral tolerization, s.c. immunization of transgenic animals with MBP in complete Freund's adjuvant induced IFN-gamma-secreting Th1 cells in vitro and experimental encephalomyelitis in vivo. Despite the large number of cells reactive to MBP in the transgenic animals, EAE was also suppressed by low-dose feeding of MBP prior to immunization. These results demonstrate that MBP-specific T cells can differentiate in vivo into encephalitogenic or regulatory T cells depending upon the context by which they are exposed to antigen.
The hard-soft acid-base principle in enzymatic catalysis: dual reactivity of phosphoenolpyruvate.
Authors: Li Y , Evans J N
Published: July 2002

In this paper, the chemical reactivity of C3 of phosphoenolpyruvate (PEP) has been analyzed in terms of density functional theory quantified through quantum chemistry calculations. PEP is involved in a number of important enzymatic reactions, in which its C3 atom behaves like a base. In three different enzymatic reactions analyzed here, C3 sometimes behaves like a soft base and sometimes behaves like a hard base in terms of the hard-soft acid-base principle. This dual nature of C3 of PEP was found to be related to the conformational change of the molecule. This leads to a testable hypothesis: that PEP adopts particular conformations in the enzyme-substrate complexes of different PEP-using enzymes, and that the enzymes control the reactivity through controlling the dihedral angle between the carboxylate and the C==C double bond of PEP.