260 research outputs found
First Principles Derivation of Effective Ginzburg-Landau Free Energy models for Crystalline Systems
The expression of the free energy density of a classical crystalline system
as a gradient expansion in terms of a set of order parameters is developed
using classical density functional theory. The goal here is to extend and
complete an earlier derivation by L{\"o}wen et al (Europhys. Lett.9, 791,
1989). The limitations of the resulting expressions are also discussed
including the boundary conditions needed for finite systems and the fact that
the results cannot, at present, be used to take into account elastic
relaxation.Comment: 12 pages, no figures, sumitted to Physica
Zebrafish mutants in vegfab can affect endothelial cell proliferation without altering ERK phosphorylation and are phenocopied by loss of PI3K signaling.
The formation of appropriately patterned blood vessel networks requires endothelial cell migration and proliferation. Signaling through the Vascular Endothelial Growth Factor A (VEGFA) pathway is instrumental in coordinating these processes. mRNA splicing generates short (diffusible) and long (extracellular matrix bound) Vegfa isoforms. The differences between these isoforms in controlling cellular functions are not understood. In zebrafish, vegfaa generates short and long isoforms, while vegfab only generates long isoforms. We found that mutations in vegfaa had an impact on endothelial cell (EC) migration and proliferation. Surprisingly, mutations in vegfab more strongly affected EC proliferation in distinct blood vessels, such as intersegmental blood vessels in the zebrafish trunk and central arteries in the head. Analysis of downstream signaling pathways revealed no change in MAPK (ERK) activation, while inhibiting PI3 kinase signaling phenocopied vegfab mutant phenotypes in affected blood vessels. Together, these results suggest that extracellular matrix bound Vegfa might act through PI3K signaling to control EC proliferation in a distinct set of blood vessels during angiogenesis.We would like to thank Reinhild Bussmann, Mona Finch Stephen,
Nadine Greer and Bill Vought for excellent fish care. In addition, we
would like to thank Roman Tsaryk and Zeenat Diwan for critically
reading of the manuscript and Caitlyn Parker for excellent technical
assistance. We are grateful to Federica Lunella for help with the mouse
retina dissection and immunohistochemistry. We would like to thank
William Jones and Mary Mullins for providing the pCS2þ β-galactosidase
plasmid. This work was funded by the Max-Planck-Society (A.F.S.), the
Deutsche Forschungsgemeinschaft (DFG SI-1374/4-1, DFG SI-1374/5-1
and DFG SI-1374/6-1; A.F.S.) and start-up funds from the Cardiovascular
Institute and the Department of Cell and Developmental Biology of the
University of Pennsylvania Perelman School of Medicine (A.F.S.). We
further acknowledge support from the NIH R01HL152086 (A.F.S.). Work
in R.B.’s lab was funded by the Ministerio de Economía, Industría y
Competitividad (MEIC: SAF2017-89299-P and RYC-2013-13209) and
the European Research Council (ERC-2014-StG – 638028
AngioGenesHD).S
Immunoproteomic identification and characterization of Ni2+-regulated proteins implicates Ni2+ in the induction of monocyte cell death
Nickel allergy is the most common cause of allergic reactions worldwide, with cutaneous and systemic effects potentially affecting multiple organs. Monocytes are precursors of not only macrophages but also dendritic cells, the most potent activators of nickel hypersensitivity. Monocytes are themselves important antigen-presenting cells, capable of nickel-specific T-cell activation in vivo and in vitro, in addition to being important for immediate innate immune inflammation. To elucidate early Ni2+-dependent inflammatory molecular mechanisms in human monocytes, a Ni2+-specific proteomic approach was applied. Quantitative two-dimensional (2D) differential gel electrophoresis and Delta2D software analyses coupled with matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) revealed that Ni2+ significantly regulated 56 protein species, of which 36 were analyzed by MALDI-MS. Bioinformatics analyses of all identified proteins resulted in Ni2+-associated functional annotation clusters, such as cell death, metal ion binding, and cytoskeletal remodeling. The involvement of Ni2+ in the induction of monocyte cell death, but not T-cell death, was observed at Ni2+ concentrations at or above 250 µM. Examination of caspase activity during Ni2+-mediated cell death revealed monocytic cell death independent of caspase-3 and -7 activity. However, confocal microscopy analysis demonstrated Ni2+-triggered cytoskeletal remodeling and nuclear condensation, characteristic of cellular apoptosis. Thus, Ni2+-specific peripheral blood mononuclear cell stimulation suggests monocytic cell death at Ni2+ concentrations at or above 250 µM, and monocytic effects on immune regulation at lower Ni2+ concentrations
Investigation of Living Cells in the Nanometer Regime with the Scanning Force Microscope
Membrane structures of different types of cells are imaged in the nanometer regime by scanning force microscopy (SFM). The images are compared to those obtained with a scanning electron microscope (SEM). The SFM imaging can be done on the outer cell membrane under conditions that keep the cells alive in aqueous solutions. This opens up the possibility of observing the kinematics of the structures that determine the interaction of a cell with its environment. Therefore, STM observations, together with information obtained with the electron microscope, open up new ways of studying the development of biological structures. With the currently possible resolution, the SFM gives access to processes such as antibody binding or endo- and exocytosis, including processes correlated to the infection of cells by viruses
Density-Functional Theory of Quantum Freezing: Sensitivity to Liquid-State Structure and Statistics
Density-functional theory is applied to compute the ground-state energies of
quantum hard-sphere solids. The modified weighted-density approximation is used
to map both the Bose and the Fermi solid onto a corresponding uniform Bose
liquid, assuming negligible exchange for the Fermi solid. The required
liquid-state input data are obtained from a paired phonon analysis and the
Feynman approximation, connecting the static structure factor and the linear
response function. The Fermi liquid is treated by the Wu-Feenberg cluster
expansion, which approximately accounts for the effects of antisymmetry.
Liquid-solid transitions for both systems are obtained with no adjustment of
input data. Limited quantitative agreement with simulation indicates a need for
further improvement of the liquid-state input through practical alternatives to
the Feynman approximation.Comment: IOP-TeX, 21 pages + 7 figures, to appear, J. Phys.: Condens. Matte
Polydispersity and ordered phases in solutions of rodlike macromolecules
We apply density functional theory to study the influence of polydispersity
on the stability of columnar, smectic and solid ordering in the solutions of
rodlike macromolecules. For sufficiently large length polydispersity (standard
deviation ) a direct first-order nematic-columnar transition is
found, while for smaller there is a continuous nematic-smectic and
first-order smectic-columnar transition. For increasing polydispersity the
columnar structure is stabilized with respect to solid perturbations. The
length distribution of macromolecules changes neither at the nematic-smectic
nor at the nematic-columnar transition, but it does change at the
smectic-columnar phase transition. We also study the phase behaviour of binary
mixtures, in which the nematic-smectic transition is again found to be
continuous. Demixing according to rod length in the smectic phase is always
preempted by transitions to solid or columnar ordering.Comment: 13 pages (TeX), 2 Postscript figures uuencode
Molecular dynamics study of melting of a bcc metal-vanadium II : thermodynamic melting
We present molecular dynamics simulations of the thermodynamic melting
transition of a bcc metal, vanadium using the Finnis-Sinclair potential. We
studied the structural, transport and energetic properties of slabs made of 27
atomic layers with a free surface. We investigated premelting phenomena at the
low-index surfaces of vanadium; V(111), V(001), and V(011), finding that as the
temperature increases, the V(111) surface disorders first, then the V(100)
surface, while the V(110) surface remains stable up to the melting temperature.
Also, as the temperature increases, the disorder spreads from the surface layer
into the bulk, establishing a thin quasiliquid film in the surface region. We
conclude that the hierarchy of premelting phenomena is inversely proportional
to the surface atomic density, being most pronounced for the V(111) surface
which has the lowest surface density
Probing Single-Electron Spin Decoherence in Quantum Dots using Charged Excitons
We propose to use optical detection of magnetic resonance (ODMR) to measure
the decoherence time T_{2} of a single electron spin in a semiconductor quantum
dot. The electron is in one of the spin 1/2 states and a circularly polarized
laser can only create an optical excitation for one of the electron spin states
due to Pauli blocking. An applied electron spin resonance (ESR) field leads to
Rabi spin flips and thus to a modulation of the photoluminescence or,
alternatively, of the photocurrent. This allows one to measure the ESR
linewidth and the coherent Rabi oscillations, from which the electron spin
decoherence can be determined. We study different possible schemes for such an
ODMR setup, including cw or pulsed laser excitation.Comment: 8 pages, 7 figures. Proceedings of the PASPS3 conference, Santa
Barbara, CA (USA). To appear in the Journal of Superconductivit
Crater formation by fast ions: comparison of experiment with Molecular Dynamics simulations
An incident fast ion in the electronic stopping regime produces a track of
excitations which can lead to particle ejection and cratering. Molecular
Dynamics simulations of the evolution of the deposited energy were used to
study the resulting crater morphology as a function of the excitation density
in a cylindrical track for large angle of incidence with respect to the surface
normal. Surprisingly, the overall behavior is shown to be similar to that seen
in the experimental data for crater formation in polymers. However, the
simulations give greater insight into the cratering process. The threshold for
crater formation occurs when the excitation density approaches the cohesive
energy density, and a crater rim is formed at about six times that energy
density. The crater length scales roughly as the square root of the electronic
stopping power, and the crater width and depth seem to saturate for the largest
energy densities considered here. The number of ejected particles, the
sputtering yield, is shown to be much smaller than simple estimates based on
crater size unless the full crater morphology is considered. Therefore, crater
size can not easily be used to estimate the sputtering yield.Comment: LaTeX, 7 pages, 5 EPS figures. For related figures/movies, see:
http://dirac.ms.virginia.edu/~emb3t/craters/craters.html New version uploaded
5/16/01, with minor text changes + new figure
The low temperature interface between the gas and solid phases of hard spheres with a short-ranged attraction
At low temperature, spheres with a very short-ranged attraction exist as a
close-packed solid coexisting with an infinitely dilute gas. We find that the
ratio of the interfacial tension between these two phases to the thermal energy
diverges as the range of the attraction goes to zero. The large tensions when
the interparticle attractions are short-ranged may be why globular proteins
only crystallise over a narrow range of conditions.Comment: 6 pages, no figures (v2 has change of notation to agree with that of
Stell
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