11,335 research outputs found
Tyrosine kinase inhibition produces specific alterations in axon guidance in the grasshopper embryo
Tyrosine kinase signaling pathways are essential for process outgrowth and guidance during nervous system development. We have examined the roles of tyrosine kinase activity in programming growth cone guidance decisions in an intact nervous system in which neurons can be individually identified. We applied the tyrosine kinase inhibitors herbimycin A and genistein to whole 40% grasshopper embryos placed in medium, or injected the inhibitors into intact grasshopper eggs. Both inhibitors caused interneuronal axons that normally would grow along the longitudinal connectives to instead leave the central nervous system (CNS) within the segmental nerve root and grow out toward the body wall muscles. In addition, herbimycin A produced pathfinding errors in which many longitudinal axons crossed the CNS midline. To study how this drug affected guidance decisions made by individual growth cones, we dye-filled the pCC interneuron, which normally extends an axon anteriorly along the ipsilateral longitudinal connective. In the presence of herbimycin A, the pCC growth cone was redirected across the anterior commissure. These phenotypes suggest that tyrosine kinase inhibition blocks a signaling mechanism that repels the growth cones of longitudinal connective neurons and prevents them from crossing the midline
Ginzburg-Landau theory of dirty two band superconductors
In this paper we study the effect of non-magnetic impurities on two-band
superconductors by deriving the corresponding Ginzburg-Landau (GL)
equation. Depending on the strength of (impurity-induced) inter-band scattering
we find that there are two distinctive regions where the superconductors behave
very differently. In the strong impurity induced inter-band scattering regime
, where mean-life time an electron stays in one
band the two-band superconductor behaves as an effective one-band dirty
superconductor. In the other limit , the dirty two-band
superconductor is described by a network of frustrated two-band superconductor
grains connected by Josepshon tunnelling junctions. We argue that most pnictide
superconductors are in the later regime.Comment: 4 pages, 1 figur
Interactions between Type III receptor tyrosine phosphatases and growth factor receptor tyrosine kinases regulate tracheal tube formation in Drosophila
The respiratory (tracheal) system of the Drosophila melanogaster larva is an intricate branched network of air-filled tubes. Its developmental logic is similar in some ways to that of the vertebrate vascular system. We previously described a unique embryonic tracheal tubulogenesis phenotype caused by loss of both of the Type III receptor tyrosine phosphatases (RPTPs), Ptp4E and Ptp10D. In Ptp4E Ptp10D double mutants, the linear tubes in unicellular and terminal tracheal branches are converted into bubble-like cysts that incorporate apical cell surface markers. This tube geometry phenotype is modulated by changes in the activity or expression of the epidermal growth factor receptor (Egfr) tyrosine kinase (TK). Ptp10D physically interacts with Egfr. Here we demonstrate that the Ptp4E Ptp10D phenotype is the consequence of the loss of negative regulation by the RPTPs of three growth factor receptor TKs: Egfr, Breathless and Pvr. Reducing the activity of any of the three kinases by tracheal expression of dominant-negative mutants suppresses cyst formation. By competing dominant-negative and constitutively active kinase mutants against each other, we show that the three RTKs have partially interchangeable activities, so that increasing the activity of one kinase can compensate for the effects of reducing the activity of another. This implies that SH2-domain downstream effectors that are required for the phenotype are likely to be able to interact with phosphotyrosine sites on all three receptor TKs. We also show that the phenotype involves increases in signaling through the MAP kinase and Rho GTPase pathways
Stability is not open
We give an example of a symplectic manifold with a stable hypersurface such
that nearby hypersurfaces are typically unstable.Comment: 9 page
High Order Perturbation Theory for Spectral Densities of Multi-Particle Excitations: S=1/2 Two-Leg Heisenberg Ladder
We present a high order perturbation approach to quantitatively calculate
spectral densities in three distinct steps starting from the model Hamiltonian
and the observables of interest. The approach is based on the perturbative
continuous unitary transformation introduced previously. It is conceived to
work particularly well in models allowing a clear identification of the
elementary excitations above the ground state. These are then viewed as
quasi-particles above the vacuum. The article focuses on the technical aspects
and includes a discussion of series extrapolation schemes. The strength of the
method is demonstrated for S=1/2 two-leg Heisenberg ladders, for which results
are presented.Comment: 21 pages, 14 figures included; to appear in Eur. Phys. J. B All
technical details for the computation of spectral densities by perturbative
CUTs Minor misprints removed, references update
Approaches and tools to manipulate the carbonate chemistry
Although the chemistry of ocean acidifi cation is very well understood (see chapter 1), its impact on marine organisms and ecosystems remains poorly known. The biological response to ocean acidifi cation is a recent field
of research, the fi rst purposeful experiments have only been carried out as late as the 1980s (Agegian, 1985)
and most were not performed until the late 1990s. The potentially dire consequences of ocean acidifi cation
have attracted the interest of scientists and students with a limited knowledge of the carbonate chemistry and
its experimental manipulation. Perturbation experiments are one of the key approaches used to investigate
the biological response to elevated p(CO2). Such experiments are based on measurements of physiological or
metabolic processes in organisms and communities exposed to seawater with normal and altered carbonate chemistry. The basics of the carbonate chemistry must be understood to perform meaningful CO2 perturbation experiments (see chapter 1). Briefl y, the marine carbonate system considers
€ CO2 ∗(aq) [the sum of CO2 and H2CO3], € HCO3 −, € CO3 2−,
H+, € OH− , and several weak acid-base systems of which borate-boric acid (€ B(OH)4 − , B(OH)3) is the most
important. As discussed by Dickson (chapter 1), if two components of the carbonate chemistry are known, all
the other components can be calculated for seawater with typical nutrient concentrations at given temperature,
salinity, and pressure. One of the possible pairs is of particular interest because both components can be
measured with precision, accuracy, and are conservative in the sense that their concentrations do not change
with temperature or pressure. Dissolved inorganic carbon (DIC) is the sum of all dissolved inorganic carbon
species while total alkalinity (AT) equals € [HCO3 − ] + 2
€ [CO3 2− ] + € [B(OH)4 − ] + € [OH− ] - [H+] + minor components, and refl ects the excess of proton acceptors over proton donors with respect to a zero level of protons (see chapter 1 for a detailed defi nition). AT is determined by the titration of seawater with a strong acid and thus can also be regarded as a measure of the buffering capacity. Any changes in any single component of the carbonate system will lead to changes in several, if not all, other components. In other words, it is not possible to vary a single component of the carbonate system while keeping all other components constant. This interdependency
in the carbonate system is important to consider when performing CO2 perturbation experiments.
To adjust seawater to different p(CO2) levels, the carbonate system can be manipulated in various ways that
usually involve changes in AT or DIC. The goal of this chapter is (1) to examine the benefi ts and drawbacks of
various manipulation methods used to date and (2) to provide a simple software package to assist the design
of perturbation experiments
Exact ground states of a spin-1/2 Ising-Heisenberg model on the Shastry-Sutherland lattice in a magnetic field
Exact ground states of a spin-1/2 Ising-Heisenberg model on the
Shastry-Sutherland lattice with Heisenberg intra-dimer and Ising inter-dimer
couplings are found by two independent rigorous procedures. The first method
uses a unitary transformation to establish a mapping correspondence with an
effective classical spin model, while the second method relies on the
derivation of an effective hard-core boson model by continuous unitary
transformations. Both methods lead to equivalent effective Hamiltonians
providing a convincing proof that the spin-1/2 Ising-Heisenberg model on the
Shastry-Sutherland lattice exhibits a zero-temperature magnetization curve with
just two intermediate plateaus at one-third and one-half of the saturation
magnetization, which correspond to stripe and checkerboard orderings of
singlets and polarized triplets, respectively. The nature of the remarkable
stripe order relevant to the one-third plateau is thoroughly investigated with
the help of the corresponding exact eigenvector. The rigorous results for the
spin-1/2 Ising-Heisenberg model on the Shastry-Sutherland lattice are compared
with the analogous results for the purely classical Ising and fully quantum
Heisenberg models. Finally, we discuss to what extent the critical fields of
SrCu2(BO3)2 and (CuCl)Ca2Nb3O10 can be described within the suggested
Ising-Heisenberg model.Comment: 15 pages, 6 figures, minor correction
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