816 research outputs found
The role of Sox6 in zebrafish muscle fiber type specification
Background
The transcription factor Sox6 has been implicated in regulating muscle fiber type-specific gene expression in mammals. In zebrafish, loss of function of the transcription factor Prdm1a results in a slow to fast-twitch fiber type transformation presaged by ectopic expression of sox6 in slow-twitch progenitors. Morpholino-mediated Sox6 knockdown can suppress this transformation but causes ectopic expression of only one of three slow-twitch specific genes assayed. Here, we use gain and loss of function analysis to analyse further the role of Sox6 in zebrafish muscle fiber type specification.
Methods
The GAL4 binary misexpression system was used to express Sox6 ectopically in zebrafish embryos. Cis-regulatory elements were characterized using transgenic fish. Zinc finger nuclease mediated targeted mutagenesis was used to analyse the effects of loss of Sox6 function in embryonic, larval and adult zebrafish. Zebrafish transgenic for the GCaMP3 Calcium reporter were used to assay Ca2+ transients in wild-type and mutant muscle fibres.
Results
Ectopic Sox6 expression is sufficient to downregulate slow-twitch specific gene expression in zebrafish embryos. Cis-regulatory elements upstream of the slow myosin heavy chain 1 (smyhc1) and slow troponin c (tnnc1b) genes contain putative Sox6 binding sites required for repression of the former but not the latter. Embryos homozygous for sox6 null alleles expressed tnnc1b throughout the fast-twitch muscle whereas other slow-specific muscle genes, including smyhc1, were expressed ectopically in only a subset of fast-twitch fibers. Ca2+ transients in sox6 mutant fast-twitch fibers were intermediate in their speed and amplitude between those of wild-type slow- and fast-twitch fibers. sox6 homozygotes survived to adulthood and exhibited continued misexpression of tnnc1b as well as smaller slow-twitch fibers. They also exhibited a striking curvature of the spine.
Conclusions
The Sox6 transcription factor is a key regulator of fast-twitch muscle fiber differentiation in the zebrafish, a role similar to that ascribed to its murine ortholog
Coherent radiation reaction effects in laser-vacuum acceleration of electron bunches
The effects of coherently enhanced radiation reaction on the motion of
subwavelength electron bunches in interaction with intense laser pulses are
analyzed. The radiation reaction force behaves as a radiation pressure in the
laser beam direction, combined with a viscous force in the perpendicular
direction. Due to Coulomb expansion of the electron bunch, coherent radiation
reaction effects only occur in the initial stage of the laser-bunch interaction
while the bunch is still smaller than the wavelength. It is shown that this
initial stage can have observable effects on the trajectory of the bunch. By
scaling the system to larger bunch charges, these effects may be increased to
such an extent that they can suppress the radial instability normally found in
ponderomotive acceleration schemes, thereby enabling the full potential of
laser-vacuum electron bunch acceleration to GeV energies.Comment: 31 pages, 4 figure
Advances in the proposed electromagnetic zero-point field theory of inertia
A NASA-funded research effort has been underway at the Lockheed Martin
Advanced Technology Center in Palo Alto and at California State University in
Long Beach to develop and test a recently published theory that Newton's
equation of motion can be derived from Maxwell's equations of electrodynamics
as applied to the zero-point field (ZPF) of the quantum vacuum. In this
ZPF-inertia theory, mass is postulated to be not an intrinsic property of
matter but rather a kind of electromagnetic drag force that proves to be
acceleration dependent by virtue of the spectral characteristics of the ZPF.
The theory proposes that interactions between the ZPF and matter take place at
the level of quarks and electrons, hence would account for the mass of a
composite neutral particle such as the neutron. An effort to generalize the
exploratory study of Haisch, Rueda and Puthoff (1994) into a proper
relativistic formulation has been successful. Moreover the principle of
equivalence implies that in this view gravitation would also be electromagnetic
in origin along the lines proposed by Sakharov (1968). With regard to exotic
propulsion we can definitively rule out one speculatively hypothesized
mechanism: matter possessing negative inertial mass, a concept originated by
Bondi (1957) is shown to be logically impossible. On the other hand, the linked
ZPF-inertia and ZPF-gravity concepts open the conceptual possibility of
manipulation of inertia and gravitation, since both are postulated to be
electromagnetic phenomena. It is hoped that this will someday translate into
actual technological potential. A key question is whether the proposed
ZPF-matter interactions generating the phenomenon of mass might involve one or
more resonances. This is presently under investigation.Comment: Revised version of invited presentation at 34th AIAA/ASME/SAE/ASEE
Joint Propulsion Conference, July 13-15, 1998, Cleveland, OH, 10 pages, no
figure
Investigation of the Jahn-Teller Transition in TiF3 using Density Functional Theory
We use first principles density functional theory to calculate electronic and
magnetic properties of TiF3 using the full potential linearized augmented plane
wave method. The LDA approximation predicts a fully saturated ferromagnetic
metal and finds degenerate energy minima for high and low symmetry structures.
The experimentally observed Jahn-Teller phase transition at Tc=370K can not be
driven by the electron-phonon interaction alone, which is usually described
accurately by LDA.
Electron correlations beyond LDA are essential to lift the degeneracy of the
singly occupied Ti t2g orbital. Although the on-site Coulomb correlations are
important, the direction of the t2g-level splitting is determined by the
dipole-dipole interactions. The LDA+U functional predicts an aniferromagnetic
insulator with an orbitally ordered ground state. The input parameters U=8.1 eV
and J=0.9 eV for the Ti 3d orbital were found by varying the total charge on
the TiF ion using the molecular NRLMOL code. We estimate the
Heisenberg exchange constant for spin-1/2 on a cubic lattice to be
approximately 24 K. The symmetry lowering energy in LDA+U is about 900 K per
TiF3 formula unit.Comment: 7 pages, 9 figures, to appear in Phys. Rev.
Radiation Reaction: General approach and applications, especially to electrodynamics
Radiation reaction (but, more generally, fluctuations and dissipation) occurs
when a system interacts with a heat bath, a particular case being the
interaction of an electron with the radiation field. We have developed a
general theory for the case of a quantum particle in a general potential (but,
in more detail, an oscillator potential) coupled to an arbitrary heat bath at
arbitrary temperature, and in an external time-dependent -number field. The
results may be applied to a large variety of problems in physics but we
concentrate by showing in detail the application to the blackbody radiation
heat bath, giving an exact result for radiation reaction problem which has no
unsatisfactory features such as the runaway solutions associated with the
Abraham-Lorentz theory. In addition, we show how atomic energy and free energy
shifts due to temperature may be calculated. Finally, we give a brief review of
applications to Josephson junctions, quantum statistical mechanics, mesoscopic
physics, quantum information, noise in gravitational wave detectors, Unruh
radiation and the violation of the quantum regression theore
Theory of coherent acoustic phonons in InGaN/GaN multi-quantum wells
A microscopic theory for the generation and propagation of coherent LA
phonons in pseudomorphically strained wurzite (0001) InGaN/GaN multi-quantum
well (MQW) p-i-n diodes is presented. The generation of coherent LA phonons is
driven by photoexcitation of electron-hole pairs by an ultrafast Gaussian pump
laser and is treated theoretically using the density matrix formalism. We use
realistic wurzite bandstructures taking valence-band mixing and strain-induced
piezo- electric fields into account. In addition, the many-body Coulomb
ineraction is treated in the screened time-dependent Hartree-Fock
approximation. We find that under typical experimental conditions, our
microscopic theory can be simplified and mapped onto a loaded string problem
which can be easily solved.Comment: 20 pages, 17 figure
Human impacts in pine forests: past, present, and future
Pines (genus Pinus) form the dominant tree cover over large parts of the Northern Hemisphere. Human activities have affected the distribution, composition, and structure of pine forests for millennia. Different human-mediated factors have affected different pine species in different ways in different regions. The most important
factors affecting pine forests are altered fire regimes, altered grazing/browsing regimes, various harvesting/construction activities, land clearance and abandonment, purposeful planting and other manipulations
of natural ecosystems, alteration of biotas through species reshuffling, and pollution. These changes are occurring against a
backdrop of natural and anthropogenically driven climate change. We review past and current influence of humans in pine forests,
seeking broad generalizations. These insights are combined with perspectives from paleoecology to suggest probable trajectories
in the face of escalating human pressure. The immense scale of impacts and the complex synergies between agents of change calls
for urgent and multifaceted action.Centre of Excellence for Invasion Biolog
Finding a moral homeground: appropriately critical religious education and transmission of spiritual values
Values-inspired issues remain an important part of the British school curriculum. Avoiding moral relativism while fostering enthusiasm for spiritual values and applying them to non-curricular learning such as school ethos or children's home lives are challenges where spiritual, moral, social and cultural (SMSC) development might benefit from leadership by critical religious education (RE). Whether the school's model of spirituality is that of an individual spiritual tradition (schools of a particular religious character) or universal pluralistic religiosity (schools of plural religious character), the pedagogy of RE thought capable of leading SMSC development would be the dialogical approach with examples of successful implementation described by Gates, Ipgrave and Skeie. Marton's phenomenography, is thought to provide a valuable framework to allow the teacher to be appropriately critical in the transmission of spiritual values in schools of a particular religious character as evidenced by Hella's work in Lutheran schools
Transfer Matrices and Partition-Function Zeros for Antiferromagnetic Potts Models. V. Further Results for the Square-Lattice Chromatic Polynomial
We derive some new structural results for the transfer matrix of
square-lattice Potts models with free and cylindrical boundary conditions. In
particular, we obtain explicit closed-form expressions for the dominant (at
large |q|) diagonal entry in the transfer matrix, for arbitrary widths m, as
the solution of a special one-dimensional polymer model. We also obtain the
large-q expansion of the bulk and surface (resp. corner) free energies for the
zero-temperature antiferromagnet (= chromatic polynomial) through order q^{-47}
(resp. q^{-46}). Finally, we compute chromatic roots for strips of widths 9 <=
m <= 12 with free boundary conditions and locate roughly the limiting curves.Comment: 111 pages (LaTeX2e). Includes tex file, three sty files, and 19
Postscript figures. Also included are Mathematica files data_CYL.m and
data_FREE.m. Many changes from version 1: new material on series expansions
and their analysis, and several proofs of previously conjectured results.
Final version to be published in J. Stat. Phy
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