2,427 research outputs found
Stable silencing of SNAP-25 in PC12 cells by RNA interference
BACKGROUND: SNAP-25 is a synaptic protein known to be involved in exocytosis of synaptic vesicles in neurons and of large dense-core vesicles in neuroendocrine cells. Its role in exocytosis has been studied in SNAP-25 knockout mice, in lysed synaptosomes lacking functional SNAP-25 and in cells after treatment with botulinum toxins A or E that specifically cleave SNAP-25. These studies have shown that SNAP-25 appears to be required for most but not all evoked secretion. In order to further study the role of SNAP-25 in catecholamine secretion from PC12 cells we have used the recently developed technique of RNA interference to generate PC12 cell lines with virtually undetectable levels of SNAP-25. RNA interference is the sequence-specific silencing or knockdown of gene expression triggered by the introduction of double-stranded RNA into a cell. RNA interference can be elicited in mammalian cells in a number of ways, one of which is by the expression of small hairpin RNAs from a transfected plasmid. Selection of stably transfected cell lines expressing a small hairpin RNA allows one-time characterization of the degree and specificity of gene silencing and affords a continuing source of well-characterized knockdown cells for experimentation. RESULTS: A PC12 cell line stably transfected with a plasmid expressing an shRNA targeting SNAP-25 has been established. This SNAP-25 knockdown cell line has barely detectable levels of SNAP-25, but normal levels of other synaptic proteins. Catecholamine secretion elicited by depolarization of the SNAP-25 knockdown cells was reduced to 37% of control. CONCLUSION: Knockdown of SNAP-25 in PC12 cells reduces but does not eliminate evoked secretion of catecholamines. Transient expression of human SNAP-25 in the knockdown cells rescues the deficit in catecholamine secretion
Ferromagnetism in the Infinite-U Hubbard Model
We have studied the stability of the ferromagnetic state in the infinite-U
Hubbard model on a square lattice by approximate diagonalization of finite
lattices using the density matrix renormalization group technique. By studying
lattices with up to 5X20 sites, we have found the ferromagnetic state to be
stable below the hole density of 22 percent. Beyond 22 percent of hole doping,
the total spin of the ground state decreased gradually to zero with increasing
hole density.Comment: 13 pages, RevteX 3.0, seven figures appended in uuencoded form,
correcting problems with uuencoded figure
On the Geometry of Surface Stress
We present a fully general derivation of the Laplace--Young formula and
discuss the interplay between the intrinsic surface geometry and the extrinsic
one ensuing from the immersion of the surface in the ordinary euclidean
three-dimensional space. We prove that the (reversible) work done in a general
surface deformation can be expressed in terms of the surface stress tensor and
the variation of the intrinsic surface metric
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Characteristics of a Mixed Thorium-Uranium Dioxide High-Burnup Fuel
Future nuclear fuels must satisfy three sets of requirements: longer times between refueling; concerns for weapons proliferation; and development of a spent fuel form more suitable for direct geologic disposal. This project has investigated a fuel consisting of mixed thorium and uranium dioxide to satisfy these requirements. Results using the SCALE 4.3 code system indicated that the mixed Th-U fuel could be burned to 72 MWD/kg or 100 MWD/kg using 25% of 35% UO2 respectively. The uranium remained below 20% total fissile fraction throughout the cycle, making it unusable for weapons. Total plutonium production per MWD was a factor of 4.5 less in the Th-U fuel than in the conventional fuel; Pu-239 production per MWD was a factor of 6.5 less; and the plutonium produced was high in Pu-238, leading to a decay heat 5 times greater than that from plutonium derived from conventional fuel and 40 times greater than weapons grade plutonium. High decay heat would require active cooling of any crude weapon, lest the components surrounding the plutonium be melted. Spontaneous neutron production for plutonium from Th-U fuel was 2.3 times greater than that from conventional fuel and 15 times greater than that from weapons grade plutonium. High spontaneous neutron production drastically limits the probable yield of a crude weapon. Because ThO2 is the highest oxide of thorium, while UO2 can be oxidized further to U3O8, ThO2-UO2 fuel may be a superior wasteform if the spent fuel is ever to be exposed to oxygenated water. Even if the cost of fabricating the mixed Th-U fuel is $100/kg greater, the cost of the Th-U fuel is 13% to 15% less than that of the fuels using uranium only
Symmetry Breaking in Linearly Coupled Dynamical Lattices
We examine one- and two-dimensional (1D and 2D) models of linearly coupled
lattices of the discrete-nonlinear-Schr{\"{o}}dinger type. Analyzing ground
states of the systems with equal powers in the two components, we find a
symmetry-breaking phenomenon beyond a critical value of the squared -norm.
Asymmetric states, with unequal powers in their components, emerge through a
subcritical pitchfork bifurcation, which, for very weakly coupled lattices,
changes into a supercritical one. We identify the stability of various solution
branches. Dynamical manifestations of the symmetry breaking are studied by
simulating the evolution of the unstable branches. The results present the
first example of spontaneous symmetry breaking in 2D lattice solitons. This
feature has no counterpart in the continuum limit, because of the collapse
instability in the latter case.Comment: 9 pages, 9 figures, submitted to Phys. Rev. E, Apr, 200
Theory of transverse spin dynamics in a polarized Fermi liquid and an itinerant ferromagnet
The linear equations for transverse spin dynamics in a weakly polarized
degenerate Fermi liquid with arbitrary relationship between temperature and
polarization are derived from Landau-Silin phenomenological kinetic equation
with general form of two-particle collision integral. Unlike the previous
treatment where Fermi velocity and density of states have been taken as
constants independent of polarization here we made derivation free from this
assumption. The obtained equations are applicable for description of spin
dynamics in paramagnetic Fermi liquid with finite polarization as well in an
itinerant ferromagnet. In both cases transverse spin wave frequency is found to
be proportional to the square of the wave vector with complex constant of
proportionality (diffusion coefficient) such that the damping has a finite
value at T=0. The polarization dependence of the diffusion coefficient is found
to be different for a polarized Fermi liquid and for an itinerant ferromagnet.
These conclusions are confirmed by derivation of transverse spin wave
dispersion law in frame of field theoretical methods from the integral equation
for the vortex function. It is shown that similar derivation taking into
consideration the divergency of static transverse susceptibility also leads to
the same attenuating spin wave spectrum.Comment: 7 pages, no figure
Self Consistent Expansion for the Molecular Beam Epitaxy Equation
Motivated by a controversy over the correct results derived from the dynamic
renormalization group (DRG) analysis of the non linear molecular beam epitaxy
(MBE) equation, a self-consistent expansion (SCE) for the non linear MBE theory
is considered. The scaling exponents are obtained for spatially correlated
noise of the general form . I find a lower critical dimension , above, which the linear MBE solution appears. Below the
lower critical dimension a r-dependent strong-coupling solution is found. These
results help to resolve the controversy over the correct exponents that
describe non linear MBE, using a reliable method that proved itself in the past
by predicting reasonable results for the Kardar-Parisi-Zhang (KPZ) system,
where DRG failed to do so.Comment: 16 page
Comparison of CDMS [100] and [111] oriented germanium detectors
The Cryogenic Dark Matter Search (CDMS) utilizes large mass, 3" diameter
1" thick target masses as particle detectors. The target is
instrumented with both phonon and ionization sensors and comparison of energy
in each channel provides event-by-event classification of electron and nuclear
recoils. Fiducial volume is determined by the ability to obtain good phonon and
ionization signal at a particular location. Due to electronic band structure in
germanium, electron mass is described by an anisotropic tensor with heavy mass
aligned along the symmetry axis defined by the [111] Miller index (L valley),
resulting in large lateral component to the transport. The spatial distribution
of electrons varies significantly for detectors which have their longitudinal
axis orientations described by either the [100] or [111] Miller indices.
Electric fields with large fringing component at high detector radius also
affect the spatial distribution of electrons and holes. Both effects are
studied in a 3 dimensional Monte Carlo and the impact on fiducial volume is
discussed.Comment: Low Temperature Detector 14 conference proceedings to be published in
the Journal of Low Temperature Physic
Macroscopic Quantum Tunneling of a Domain Wall in a Ferromagnetic Metal
The macroscopic quantum tunneling of a planar domain wall in a ferromagnetic
metal is studied based on the Hubbard model. It is found that the ohmic
dissipation is present even at zero temperature due to the gapless Stoner
excitation, which is the crucial difference from the case of the insulating
magnet. The dissipative effect is calculated as a function of width of the wall
and is shown to be effective in a thin wall and in a weak ferromagnet. The
results are discussed in the light of recent experiments on ferromagnets with
strong anisotropy. PACS numbers:75.60.Ch, 03.65.Sq, 75.10.LpComment: 13page
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