1,522 research outputs found
Negative Electron-electron Drag Between Narrow Quantum Hall Channels
Momentum transfer due to Coulomb interaction between two parallel,
two-dimensional, narrow, and spatially separated layers, when a current
I_{drive} is driven through one layer, is studied in the presence of a
perpendicular magnetic field B. The current induced in the drag layer,
I_{drag}, is evaluated self-consistently with I_{drive} as a parameter.
I_{drag} can be positive or negative depending on the value of the filling
factor \nu of the highest occupied bulk Landau level (LL). For a fully occupied
LL, I_{drag} is negative, i.e., it flows opposite to I_{drive}, whereas it is
positive for a half-filled LL. When the circuit is opened in the drag layer, a
voltage \Delta V_{drag} develops in it; it is negative for a half-filled LL and
positive for a fully occupied LL. This positive \Delta V_{drag}, expressing a
negative Coulomb drag, results from energetically favored near-edge inter-LL
transitions that occur when the highest occupied bulk LL and the LL just above
it become degenerate.Comment: Text file in Latex/Revtex/preprint format, 7 separate PS figures,
Physical Review B, in pres
Frictional drag between non-equilibrium charged gases
The frictional drag force between separated but coupled two-dimensional
electron gases of different temperatures is studied using the non-equilibrium
Green function method based on the separation of center-of-mass and relative
dynamics of electrons. As the mechanisms of producing the frictional force we
include the direct Coulomb interaction, the interaction mediated via virtual
and real TA and LA phonons, optic phonons, plasmons, and TA and LA
phonon-electron collective modes. We found that, when the distance between the
two electron gases is large, and at intermediate temperature where plasmons and
collective modes play the most important role in the frictional drag, the
possibility of having a temperature difference between two subsystems modifies
greatly the transresistivity.Comment: 8figure
Bone morphogenetic protein 7 sensitizes O6-methylguanine methyltransferase expressing-glioblastoma stem cells to clinically relevant dose of temozolomide.
BackgroundTemozolomide (TMZ) is an oral DNA-alkylating agent used for treating patients with glioblastoma. However, therapeutic benefits of TMZ can be compromised by the expression of O6-methylguanine methyltransferase (MGMT) in tumor tissue. Here we used MGMT-expressing glioblastoma stem cells (GSC) lines as a model for investigating the molecular mechanism underlying TMZ resistance, while aiming to explore a new treatment strategy designed to possibly overcome resistance to the clinically relevant dose of TMZ (35 μM).MethodsMGMT-expressing GSC cultures are resistant to TMZ, and IC50 (half maximal inhibitory concentration) is estimated at around 500 μM. Clonogenic GSC surviving 500 μM TMZ (GSC-500 μM TMZ), were isolated. Molecular signatures were identified via comparative analysis of expression microarray against parental GSC (GSC-parental). The recombinant protein of top downregulated signature was used as a single agent or in combination with TMZ, for evaluating therapeutic effects of treatment of GSC.ResultsThe molecular signatures characterized an activation of protective stress responses in GSC-500 μM TMZ, mainly including biotransformation/detoxification of xenobiotics, blocked endoplasmic reticulum stress-mediated apoptosis, epithelial-to-mesenchymal transition (EMT), and inhibited growth/differentiation. Bone morphogenetic protein 7 (BMP7) was identified as the top down-regulated gene in GSC-500 μM TMZ. Although augmenting BMP7 signaling in GSC by exogenous BMP7 treatment did not effectively stop GSC growth, it markedly sensitized both GSC-500 μM TMZ and GSC-parental to 35 μM TMZ treatment, leading to loss of self-renewal and migration capacity. BMP7 treatment induced senescence of GSC cultures and suppressed mRNA expression of CD133, MGMT, and ATP-binding cassette drug efflux transporters (ABCB1, ABCG2), as well as reconfigured transcriptional profiles in GSC by downregulating genes associated with EMT/migration/invasion, stemness, inflammation/immune response, and cell proliferation/tumorigenesis. BMP7 treatment significantly prolonged survival time of animals intracranially inoculated with GSC when compared to those untreated or treated with TMZ alone (p = 0.0017), whereas combination of two agents further extended animal survival compared to BMP7 alone (p = 0.0489).ConclusionsThese data support the view that reduced endogenous BMP7 expression/signaling in GSC may contribute to maintained stemness, EMT, and chemoresistant phenotype, suggesting that BMP7 treatment may provide a novel strategy in combination with TMZ for an effective treatment of glioblastoma exhibiting unmethylated MGMT
Ribosomal Proteins RPS11 and RPS20, Two Stress-Response Markers of Glioblastoma Stem Cells, Are Novel Predictors of Poor Prognosis in Glioblastoma Patients.
Glioblastoma stem cells (GSC) co-exhibiting a tumor-initiating capacity and a radio-chemoresistant phenotype, are a compelling cell model for explaining tumor recurrence. We have previously characterized patient-derived, treatment-resistant GSC clones (TRGC) that survived radiochemotherapy. Compared to glucose-dependent, treatment-sensitive GSC clones (TSGC), TRGC exhibited reduced glucose dependence that favor the fatty acid oxidation pathway as their energy source. Using comparative genome-wide transcriptome analysis, a series of defense signatures associated with TRGC survival were identified and verified by siRNA-based gene knockdown experiments that led to loss of cell integrity. In this study, we investigate the prognostic value of defense signatures in glioblastoma (GBM) patients using gene expression analysis with Probeset Analyzer (131 GBM) and The Cancer Genome Atlas (TCGA) data, and protein expression with a tissue microarray (50 GBM), yielding the first TRGC-derived prognostic biomarkers for GBM patients. Ribosomal protein S11 (RPS11), RPS20, individually and together, consistently predicted poor survival of newly diagnosed primary GBM tumors when overexpressed at the RNA or protein level [RPS11: Hazard Ratio (HR) = 11.5, p<0.001; RPS20: HR = 4.5, p = 0.03; RPS11+RPS20: HR = 17.99, p = 0.001]. The prognostic significance of RPS11 and RPS20 was further supported by whole tissue section RPS11 immunostaining (27 GBM; HR = 4.05, p = 0.01) and TCGA gene expression data (578 primary GBM; RPS11: HR = 1.19, p = 0.06; RPS20: HR = 1.25, p = 0.02; RPS11+RPS20: HR = 1.43, p = 0.01). Moreover, tumors that exhibited unmethylated O-6-methylguanine-DNA methyltransferase (MGMT) or wild-type isocitrate dehydrogenase 1 (IDH1) were associated with higher RPS11 expression levels [corr (IDH1, RPS11) = 0.64, p = 0.03); [corr (MGMT, RPS11) = 0.52, p = 0.04]. These data indicate that increased expression of RPS11 and RPS20 predicts shorter patient survival. The study also suggests that TRGC are clinically relevant cells that represent resistant tumorigenic clones from patient tumors and that their properties, at least in part, are reflected in poor-prognosis GBM. The screening of TRGC signatures may represent a novel alternative strategy for identifying new prognostic biomarkers
The relative importance of head, flux, and prior information in hydraulic tomography analysis
Using cross-correlation analysis, we demonstrate that flux measurements at observation locations during hydraulic tomography (HT) surveys carry nonredundant information about heterogeneity that are complementary to head measurements at the same locations. We then hypothesize that a joint interpretation of head and flux data, even when the same observation network as head has been used, can enhance the resolution of HT estimates. Subsequently, we use numerical experiments to test this hypothesis and investigate the impact of flux conditioning and prior information (such as correlation lengths and initial mean models (i.e., uniform mean or distributed means)) on the HT estimates of a nonstationary, layered medium. We find that the addition of flux conditioning to HT analysis improves the estimates in all of the prior models tested. While prior information on geologic structures could be useful, its influence on the estimates reduces as more nonredundant data (i.e., flux) are used in the HT analysis. Lastly, recommendations for conducting HT surveys and analysis are presented
The Quantum Hall Effect in Drag: Inter-layer Friction in Strong Magnetic Fields
We study the Coulomb drag between two spatially separated electron systems in
a strong magnetic field, one of which exhibits the quantum Hall effect. At a
fixed temperature, the drag mimics the behavior of in the quantum
Hall system, in that it is sharply peaked near the transitions between
neighboring plateaux. We assess the impact of critical fluctuations near the
transitions, and find that the low temperature behavior of the drag measures an
exponent that characterizes anomalous low frequency dissipation; the
latter is believed to be present following the work of Chalker.Comment: 13 pages, Revtex 2.0, 1 figure upon request, P-93-11-09
Frictional Coulomb drag in strong magnetic fields
A treatment of frictional Coulomb drag between two 2-dimensional electron
layers in a strong perpendicular magnetic field, within the independent
electron picture, is presented. Assuming fully resolved Landau levels, the
linear response theory expression for the transresistivity is
evaluated using diagrammatic techniques. The transresistivity is given by an
integral over energy and momentum transfer weighted by the product of the
screened interlayer interaction and the phase-space for scattering events. We
demonstrate, by a numerical analysis of the transresistivity, that for
well-resolved Landau levels the interplay between these two factors leads to
characteristic features in both the magnetic field- and the temperature
dependence of . Numerical results are compared with recent
experiments.Comment: RevTeX, 34 pages, 8 figures included in tex
Phonon mediated drag in double layer two dimensional electron systems
Experiments studying phonon mediated drag in the double layer two dimensional
electron gas system are reported. Detailed measurements of the dependence of
drag on temperature, layer spacing, density ratio, and matched density are
discussed. Comparisons are made to theoretical results [M. C. Bonsager et al.,
Phys. Rev. B 57, 7085 (1998)] which propose the existence of a new coupled
electron-phonon collective mode. The layer spacing and density dependence at
matched densities for samples with layer spacings below 2600 A do not support
the existence of this mode, showing behavior expected for independent electron
and phonon systems. The magnitude of the drag, however, suggests the alternate
limit; one in which electrons and phonons are strongly coupled. The results for
still larger layer spacing show significant discrepancies with the behavior
expected for either limit.Comment: 9 pages, 9 figures, Late
Magneto-Coulomb drag: interplay of electron--electron interactions and Landau quantization
We use the Kubo formalism to calculate the transresistivity for
carriers in coupled quantum wells in a large perpendicular magnetic field .
We find that is enhanced by approximately 50--100 times over that
of the B=0 case in the interplateau regions of the integer quantum Hall effect.
The presence of both electron--electron interactions and Landau quantization
results in (i) a twin-peaked structure of in the inter-plateau
regions at low temperatures, and, (ii) for the chemical potential at the center
of a Landau level band, a peaked temperature dependence of .Comment: 4 pages, RevTeX, 4 PS figures in text using eps
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