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PubMed1999

Dynamical real space renormalization group applied to sandpile models.

A general framework for the renormalization group analysis of self-organized critical sandpile models is formulated. The usual real space renormalization scheme for lattice models when applied to nonequilibrium dynamical models must be supplemented by feedback relations coming from the stationarity conditions. On the basis of these ideas the dynamically driven renormalization group is applied to describe the boundary and bulk critical behavior of sandpile models. A detailed description of the branching nature of sandpile avalanches is given in terms of the generating functions of the underlying branching process.

Journal Article↗

PubMed2000

Head-tail instability caused by electron clouds in positron storage rings

In positron or proton storage rings with many closely spaced bunches, an electron cloud can build up in the vacuum chamber due to photoemission or secondary emission. We discuss the possibility of a single-bunch two-stream instability driven by this electron cloud. Depending on the strength of the beam-electron interaction, the chromaticity and the synchrotron oscillation frequency, this instability either resembles a linac beam breakup or a head-tail instability. We present computer simulations of the instabilities, and compare the simulation results with analytical estimates.

Journal Article↗

PubMed2002

Intraocular adenoviral vector-mediated gene transfer in proliferative retinopathies.

PURPOSE: The purpose of this study was to compare levels and patterns of expression of reporter genes achieved with an E1-deleted and partially E3-deleted type 5 adenoviral (Ad) vector after intravitreous or subretinal injections, or after intravitreous injections in mouse eyes with proliferative retinopathies. METHODS: Ad vectors containing reporter gene constructs were injected into the vitreous cavity or subretinal space of wild-type mice or mice with proliferative retinopathies, and quantitative comparisons were made of expression of transgenes. RESULTS: In normal eyes, peak Ad-mediated expression of luciferase, driven by a cytomegalovirus (CMV) promoter, occurred after injection of 10(7) to 10(8) viral particles and was 10 times greater after subretinal injections than after intravitreous injections. Intravitreous injections of Ad containing beta-galactosidase (LacZ) expression constructs (AdLacZ.10) resulted in strong expression of LacZ in epithelial cells of the iris and ciliary body and focal expression in the retina. Subretinal injections of AdLacZ.10 resulted in strong expression in RPE cells. Expression of LacZ after intravitreous injection of AdLacZ.10 was significantly greater in mice with two types of proliferative retinopathy (ischemic retinopathy or transgenic mice with retina-specific expression of platelet-derived growth factor (PDGF)-BB or PDGF-AB) than littermate control animals. Cells within epiretinal membranes and activated Müller cells were preferentially transduced in eyes with proliferative retinopathy. CONCLUSIONS: These data suggest that although higher intraocular expression levels can be achieved after subretinal injection of adenoviral vectors, intravitreous injections provide good transduction of cells lining the vitreous cavity. Compared with normal eyes, eyes with proliferative retinopathy showed increased transduction, which occurred preferentially in cells participating in the disease process. Intravitreous injection of adenoviral vectors containing appropriate expression constructs may provide a good strategy for acute treatment of proliferative retinopathies, such as diabetic retinopathy and proliferative vitreoretinopathy.

Adenoviridae↗

PubMed1990

The lumazine synthase-riboflavin synthase complex of Bacillus subtilis. Crystallization of reconstituted icosahedral beta-subunit capsids.

The lumazine synthase-riboflavin synthase complex (heavy riboflavin synthase) of Bacillus subtilis consists of an icosahedral capsid of 60 beta-subunits containing a core of three alpha-subunits. The enzyme has been purified from the derepressed mutant H 94 of B. subtilis by a novel efficient procedure using column chromatography and preparative crystallization. Beta-Subunits were isolated after dissociation of the enzyme at pH 8.0. Ligand-driven renaturation of beta-subunits yields hollow icosahedral beta 60 capsids which could be crystallized in 1.55 M phosphate, pH 8.7, in three different modifications. A monoclinic modification belongs to space group C2 with unit cell dimensions of a = 235.5, b = 191.2, and c = 165.4 A and alpha = gamma = 90 degrees and beta = 134.4 degrees. The crystals contain two hollow beta 60 particles/unit cell and diffract to approximately 2.8-A resolution. A hexagonal modification has the space group P6(3)22 with unit cell dimensions of a = b = 157.2 and c = 300.8 A and alpha = beta = 90 degrees and gamma = 120 degrees. These cell parameters are similar to the dimensions of hexagonal crystals of native heavy riboflavin synthase (alpha 3 beta 60). A second hexagonal modification shows unit cell parameters of a = b = 156.3 and c = 622.6 A and alpha = beta = 90 degrees and gamma = 120 degrees. The space group of this modification could not be determined unambiguously.

Bacillus subtilis↗

PubMed1993

The vestibular system modulates masseter muscle activity.

The aim of this study was to investigate whether, and in what way, the vestibular input may influence the activity of the masseter muscles. The variations in the spontaneous electrical activity and the evoked responses in the masseter motor units to natural or electrical activation of the vestibular afferents were recorded in anesthetized guinea pigs. The effects of a unilateral lesion of the labyrinth on the firing rate of the masseter motor units were also studied. Results show that: 1) vestibular input elicited an excitatory tonic control on masseter muscle activity; 2) a faster labyrinthine control is driven to the contralateral than the homolateral masseter muscles; 3) vestibular macular input does exert an asymmetrical control on masseteric muscles of both sides, in relation to the head displacement in space. The latencies of responses recorded from the masseter motor units suggest that polysynaptic pathways are involved in connecting the vestibular system to the trigeminal complex. The possible anatomical substrates for this vestibulomasseteric reflex are discussed.

Animals↗

PubMed2000

Evidence for a rule governing the avoidance of superfluous escape flights.

When an imminent attack by a predator on a group of birds is signalled to non-detectors only by the departure of the detector, non-detectors may make time-wasting false-alarm flights in response to mistaken or non-predator-driven departures. The frequency of false-alarm flights might be reduced if group members assess the reason for single departures before responding. Immediate flights should only occur after multiple simultaneous departures, because these are only likely to be generated by an attack. The response delay between the detectors' departure and the next birds that respond should then be dependent on the number of detectors. On sparrowhawk attack, response delays in redshanks decreased significantly as detector number increased, controlling for raptor conspicuousness and proximity, and flock size and spacing. If response delay is modified because of risk dilution, it should increase with flock size and, consequently, the rate of alarm flights due to mistakes should decrease. However, response delay did not increase and flight frequency due to misidentification of non-raptors or non-predator-driven departures did not decrease with flock size. Significantly more feeding time was lost by birds in small flocks, suggesting that the dilution effect decreased the cost of each false-alarm flight rather than their frequency.

Animal Communication↗

PubMed1999

Stimulus-determined discrimination mechanisms for color search.

Visual attention can be goal driven, stimulus driven, or a combination of the two. Here we report evidence for an unexpectedly stimulus-driven component of visual search for a target defined by color. Observers demonstrated a surprisingly cost-free ability to incorporate multiple classifiers in search for a target of one color from among distractors of other colors. A target color was presented among distractors that could change from trial to trial (intermixed presentation) or that remained constant across all trials in a block (blocked presentation). For blocked presentation, a single search classifier (a mechanism that segregates the target from distractors in color space) could be adopted, whereas for intermixed presentation different classifiers had to be used when the distractor colors changed. The benefit of blocked presentation was very small, suggesting that the appropriate classifier was determined very quickly in trials for which the classifier changed. The results suggest that the stimulus-driven activation of an appropriate stimulus classifier can be very efficient.

Adult↗

PubMed1986

Actin filament architecture and movements in macrophage cytoplasm.

Actin filaments are the predominant structural elements in macrophage cortical cytoplasm. These fibres form a unique orthogonal network that fills all lamellae extended from the cell and which, in the cell body, bifurcates to form layers 0.2-0.5 micron thick on the cell top and bottom. Single short filaments, 0.1 micron in length, intersect in space in either T-shaped or X-shaped overlaps to form this ultrastructure. Network assembly and pseudopod extension occur when actin filaments within the network elongate. This filament growth is driven by a large storage pool of actin bound to the sequestering protein, profilin. Elongation is regulated by acumentin, gelsolin and possibly severin, proteins that bind to the end of the filaments, preventing the addition of actin monomers to the filaments. The cytosolic concentration of calcium controls whether filaments assemble or disassemble. Filaments can assemble when the filament ends are not blocked by gelsolin, a condition predicted to occur when the calcium concentration is less than 0.1 micron. Orthogonality results when actin filaments are cross-linked by molecules of actin-binding protein.

Actins↗

PubMed1994

Prediction of conformation of rat galanin in the presence and absence of water with the use of Monte Carlo methods and the ECEPP/3 force field.

The conformation of the 29-residue rat galanin neuropeptide was studied using the Monte Carlo with energy minimization (MCM) and electrostatically driven Monte Carlo (EDMC) methods. According to a previously elaborated procedure, the polypeptide chain was first treated in a united-residue approximation, in order to enable extensive exploration of the conformational space to be carried out (with the use of MCM). Then the low-energy united-residue conformations were converted to the all-atom representations, and EDMC simulations were carried out for the all-atom polypeptide chains, using the ECEPP/3 force field with hydration included. In order to estimate the effect of environment on galanin conformation, the low-energy conformations obtained as a result of these simulations were taken as starting structures for further EDMC runs that did not include hydration. The lowest-energy conformation obtained in aqueous solution calculations had a nonhelical N-terminal part packed against the nonpolar face of a residual helix that extended from Pro13 toward the C-terminus. One next lowest-energy structure was a nearly-all-helical conformation, but with a markedly higher energy. In contrast, all of the low-energy conformations in the absence of water were all-helical differing only by the extent to which the helix was kinked around Pro13. These results are in qualitative agreement with the available NMR and CD data of galanin in aqueous and nonaqueous solvents.

Amino Acid Sequence↗

PubMed1985

Patterns of ionic current through Drosophila follicles and eggs.

Large steady electrical currents traverse Drosophila follicles in vitro as well as permeabilized eggs. During the period of main follicle growth (stages 9-11), these currents enter the anterior or nurse cell end of the follicles. This inward current acts like a sodium ion influx with some calcium involvement. During the period of chorion formation (stages 12-14), foci of inward current also appear at the posterior, posterodorsal, and anterodorsal regions of follicles in vitro. In stage 14, the posterior in current acts like a chloride ion efflux. In preblastoderm eggs substantial currents continue to enter their anterior end; while weaker and less frequent ones enter their posterior end. We present models in which the currents during follicle growth are driven by the plasma membrane of the oocyte nurse cell syncitium; the external currents during choriogenesis are driven by the follicular epithelium; while the currents through the preblastoderm egg are driven by its plasma membrane. Measurements of pole-to-pole resistances and voltages across preblastoderm eggs indicate that the transcellular currents normally maintain a steady extracellular voltage gradient along the perivitelline space, with the anterior pole kept negative by perhaps 4 or 5 mV. The developmental significance of these currents is discussed.

Animals↗

PubMed2002

Nonequilibrium coupled Brownian phase oscillators.

A model of globally coupled phase oscillators under equilibrium (driven by Gaussian white noise) and nonequilibrium (driven by symmetric dichotomic fluctuations) is studied. For the equilibrium system, the mean-field state equation takes a simple form and the stability of its solution is examined in the full space of order parameters. For the nonequilbrium system, various asymptotic regimes are obtained in a closed analytical form. In a general case, the corresponding master equations are solved numerically. Moreover, the Monte Carlo simulations of the coupled set of Langevin equations of motion is performed. The phase diagram of the nonequilibrium system is presented. For the long time limit, we have found five regimes. Three of them can be obtained from the mean-field theory. One of them, the oscillating regime, cannot be predicted by the mean-field method and has been detected in the Monte Carlo numerical experiments.

Journal Article↗

PubMed2006

Domain formation and orbital ordering transition in a doped Jahn-Teller insulator.

The ground state of a double-exchange model for orbitally degenerate e(g) electrons with Jahn-Teller lattice coupling and weak disorder is found to be spatially inhomogeneous near half filling. Using a real-space Monte Carlo method we show that doping the half-filled orbitally ordered insulator leads to the appearance of hole-rich disordered regions in an orbitally ordered environment. The doping driven orbital order to disorder transition is accompanied by the emergence of metallic behavior. We present results on transport and optical properties along with spatial patterns for lattice distortions and charge densities, providing a basis for an overall understanding of the low-doping phase diagram of La1 - xCaxMnO3.

Journal Article↗

PubMedDate not supplied

Theory and design of "shortened" multiantenna microwave applicators with controllable SAR patterns.

A "shortened" multiantenna hyperthermia applicator has been designed and tested at the Mallinckrodt Institute of Radiology at Washington University School of Medicine. By shortening the distance from antenna to aperture, an applicator is obtained that produces an SAR pattern that is essentially the same as produced by a monopole antenna. By placing several properly spaced probe antennas into the same "shortened" applicator, an applicator is obtained that produces a SAR distribution that is essentially a composite of small overlapping SAR patterns produced by weakly interacting incoherently driven antennas. Such a design significantly improves the applicator's lateral heating efficiency and allows the independent control of temperatures in certain tumor areas by changing the input power to the respective antennas.

Diathermy↗

PubMed2002

The late asthmatic response is linked with increased surface tension and reduced surfactant protein B in mice.

Pulmonary surfactant dysfunction may significantly contribute to small airway obstruction during the asthmatic response, but neither its exact role nor its regulation is clear. Surfactant function and composition was studied in an Aspergillus fumigatus (Af)-induced late-phase allergic airway response in sensitized BALB/c mice. The peak of Af-induced airway hyperresponsiveness in sensitized and challenged mice 24 h after allergen provocation coincided with a significant fall in surface activity of the pulmonary surfactant. The underlying changes included time-dependent elaboration of eotaxin and IL-5 followed by eosinophil influx into the airways. The height of airway inflammation and hyperresponsiveness was preceded by release of IL-4 and marked reductions in surfactant protein (SP)-B, a hydrophobic surfactant protein responsible for maintaining low surface tension of the lining fluid of distal air spaces. Furthermore, intratracheal administration of IL-4 significantly inhibited SP-B, indicating a regulatory role of this cytokine in the surfactant biophysical changes. Thus surfactant dysfunction induced by an IL-4-driven SP-B deficiency after allergen provocation may be an important part of the late asthmatic airway response.

Animals↗

PubMed2005

Searching sequence space: two different approaches to dihydrofolate reductase catalysis.

There are numerous examples of proteins that catalyze the same reaction while possessing different structures. This review focuses on two dihydrofolate reductases (DHFRs) that have disparate structures and discusses how the catalytic strategies of these two DHFRs are driven by their respective scaffolds. The two proteins are E. coli chromosomal DHFR (Ec DHFR) and a type II R-plasmid-encoded DHFR, typified by R67 DHFR. The former has been described as a very well evolved enzyme with an efficiency of 0.15, while the latter has been suggested to be a model for a "primitive" enzyme that has not yet been optimized by evolution. This comparison underlines what is important to catalysis in these two enzymes and concurrently highlights fundamental issues in enzyme catalysis.

Binding Sites↗

PubMed1996

A symmetry-driven search for electrostatic interaction partners in charybdotoxin and a voltage-gated K+ channel.

A structural model of charybdotoxin bound to a Shaker K+ channel has emerged from mechanistic and mutagenic analysis of toxin-channel interactions. We test this model by predicting through-space electrostatic interactions between specific pairs of channel-toxin residues. Dissociation constants of channel-toxin variants, determined by radiolabeled toxin binding to Shaker-transfected COS membrane fragments, were used to identify pairs of residues located closely enough to interact electrostatically. The results further refine the structural model of the bound complex and produce a more detailed view of the vestibule of the Shaker channel.

Animals↗

PubMed1997

Compression of visual space before saccades.

Saccadic eye movements, in which the eye moves rapidly between two resting positions, shift the position of our retinal images. If our perception of the world is to remain stable, the visual directions associated with retinal sites, and others they report to, must be updated to compensate for changes in the point of gaze. It has long been suspected that this compensation is achieved by a uniform shift of coordinates driven by an extra-retinal position signal, although some consider this to be unnecessary. Considerable effort has been devoted to a search for such a signal and to measuring its time course and accuracy. Here, by using multiple as well as single targets under normal viewing conditions, we show that changes in apparent visual direction anticipate saccades and are not of the same size, or even in the same direction, for all parts of the visual field. We also show that there is a compression of visual space sufficient to reduce the spacing and even the apparent number of pattern elements. The results are in part consistent with electrophysiological findings of anticipatory shifts in the receptive fields of neurons in parietal cortex and superior colliculi.

Humans↗

PubMed2003

Perceptual requirements for fast manual responses.

The on-line visual control of human movements can be exceptionally fast. Whether it is fast depends on the kind of visual information that is involved. In the present study we examine whether fast on-line control is specific to the magnocellular visual pathway. Fast manual responses become evident when an ongoing movement has to be adjusted, for instance because the target is displaced. We examined whether the response to such perturbations is faster for stimuli that only activate the magnocellular pathway than for equally conspicuous stimuli that only activate the parvocellular pathway. The response was indeed about 35 ms faster for stimuli that activate the magnocellular pathway. However, we argue that the slower response to stimuli that only stimulate the parvocellular pathway is due to the properties of the neurones involved and the less direct connection to the motor areas, rather than to fast reactions being driven exclusively by magnocellular input.

Hand↗