Showing papers by "University of Bath published in 1993"

Molecular tectonics in biomineralization and biomimetic materials chemistry

Abstract: The systematic fabrication of advanced materials will require the construction of architectures over scales ranging from the molecular to the macroscopic. The basic constructional processes of biomineralization—supramolecular pre-organization, interfacial molecular recognition (templating) and cellular processing—can provide useful archetypes for molecular-scale building, or ‘molecular tectonics’, in inorganic materials chemistry.

The glucose transporter family: structure, function and tissue-specific expression.

Abstract: possess a transport system for glucose of the facilitative diffusion type; these transporters allow the movement of glucose across the plasma membrane down its chemical gradient either into or out of cells. These transporters are specific for the D-enantiomer of glucose and are not coupled to any energy-requiring components, such as ATP hydrolysis or a HI gradient [1]. The facilitative glucose transporters are distinct from the Na+dependent transporters, which actively accumulate glucose [2,3]. The importance of glucose as a cellular metabolite has led to

Crystallization at Inorganic-organic Interfaces: Biominerals and Biomimetic Synthesis

Abstract: Crystallization is an important process in a wide range of scientific disciplines including chemistry, physics, biology, geology, and materials science. Recent investigations of biomineralization indicate that specific molecular interactions at inorganic-organic interfaces can result in the controlled nucleation and growth of inorganic crystals. Synthetic systems have highlighted the importance of electrostatic binding or association, geometric matching (epitaxis), and stereochemical correspondence in these recognition processes. Similarly, organic molecules in solution can influence the morphology of inorganic crystals if there is molecular complementarity at the crystal-additive interface. A biomimetic approach based on these principles could lead to the development of new strategies in the controlled synthesis of inorganic nanophases, the crystal engineering of bulk solids, and the assembly of organized composite and ceramic materials.

Calculating Interval Forecasts

Abstract: The importance of interval forecasts is reviewed. Several general approaches to calculating such forecasts are described and compared. They include the use of theoretical formulas based on a fitted probability model (with or without a correction for parameter uncertainty), various “approximate” formulas (which should be avoided), and empirically based, simulation, and resampling procedures. The latter are useful when theoretical formulas are not available or there are doubts about some model assumptions. The distinction between a forecasting method and a forecasting model is expounded. For large groups of series, a forecasting method may be chosen in a fairly ad hoc way. With appropriate checks, it may be possible to base interval forecasts on the model for which the method is optimal. It is certainly unsound to use a model for which the method is not optimal, but, strangely, this is sometimes done. Some general comments are made as to why prediction intervals tend to be too narrow in practice to encompas...

Template mineralization of self-assembled anisotropic lipid microstructures

Abstract: THE high-fidelity replication and functional specificity of biomaterials and biopolymers is inspiring a renewal in the development of biomimetic strategies for materials synthesis1,2. Approaches involving the templating or nucleation of inorganic materials by compressed monolayers of amphiphiles3–5, the use of supramolecular lipid or protein cages in the preparation of nanoscale inorganic structures6–8 and the mineralization of bacterial fibres9 indicate the potential of controlled crystallization at inorganic–organic interfaces. Here we report the controlled formation of tubular inorganic–organic composites by using self-assembled lipid tubules as templates for the crystallization of inorganic oxides. We use microstructures formed by a sugar-based lipid galactocerebroside, doped with small amounts of an anionic sulphated derivative, to induce nucleation of magnetic and non-magnetic iron oxides. By varying the reaction conditions, we can create either tube-like or lamellar disk-like composites. Our results suggest that the variety of microstructures formed by chemically modified sugar-based lipids may provide a route to the production of mineral-containing fibres and other ceramic–organic composites.

Protection against loss of utility grid supply for a dispersed storage and generation unit

Abstract: The requirements of an islanding, or 'loss of grid', protection for a dispersed storage and generation unit are examined, and the principal methods used for this type of relaying are outlined. A new protection algorithm is introduced which is based on the rate of change of power as measured at the generator's terminals. The responses of the different measurands are examined for a selection of power system operating conditions in order to demonstrate the operation of this type of protection. The new protection algorithm is shown to trip for loss of grid and for load fluctuations while the dispersed storage and generation unit is operating independently of the utility supply following a loss of grid, and for an out-of-synchronism reconnection of the utility supply to the dispersed storage and generation unit. It is also shown to remain stable for major load fluctuations while the utility supply remains connected to the dispersed generator system. >

Crystal structure of a bacterial sialidase (from Salmonella typhimurium LT2) shows the same fold as an influenza virus neuraminidase

Abstract: Sialidases (EC 3.2.1.18 or neuraminidases) remove sialic acid from sialoglycoconjugates, are widely distributed in nature, and have been implicated in the pathogenesis of many diseases. The three-dimensional structure of influenza virus sialidase is known, and we now report the three-dimensional structure of a bacterial sialidase, from Salmonella typhimurium LT2, at 2.0-A resolution and the structure of its complex with the inhibitor 2-deoxy-2,3-dehydro-N-acetylneuraminic acid at 2.2-A resolution. The viral enzyme is a tetramer; the bacterial enzyme, a monomer. Although the monomers are of similar size (approximately 380 residues), the sequence similarity is low (approximately 15%). The viral enzyme contains at least eight disulfide bridges, conserved in all strains, and binds Ca2+, which enhances activity; the bacterial enzyme contains one disulfide and does not bind Ca2+. Comparison of the two structures shows a remarkable similarity both in the general fold and in the spatial arrangement of the catalytic residues. However, an rms fit of 3.1 A between 264 C alpha atoms of the S. typhimurium enzyme and those from an influenza A virus reflects some major differences in the fold. In common with the viral enzyme, the bacterial enzyme active site consists of an arginine triad, a hydrophobic pocket, and a key tyrosine and glutamic acid, but differences in the interactions with the O4 and glycerol groups of the inhibitor reflect differing kinetics and substrate preferences of the two enzymes. The repeating "Asp-box" motifs observed among the nonviral sialidase sequences occur at topologically equivalent positions on the outside of the structure. Implications of the structure for the catalytic mechanism, evolution, and secretion of the enzyme are discussed.

Influence of low-molecular-weight and macromolecular organic additives on the morphology of calcium carbonate

Abstract: The influence of a range of soluble biological and related molecules on the crystallization of CaCO3 from aqueous supersaturated solution has been studied by optical and scanning electron microscopy and X-ray diffraction The efficacy of monofunctional additives to induce morphological changes increased with overall anionic charge For anions of the same charge, the effect was reduced with decreasing partial charge density on the oxygen atoms of the ligand Additional factors, such as the distance between ligands and conformation were important for multifunctional molecules Orthophosphate, sulfate, various phosphonates, a polysaccharide associated with coccoliths of Emiliania huxleyi and alginate interacted specifically or pseudo-specifically with crystal faces approximately parallel to the c axis indicative of a bidentate binding motif Phenyl phosphonate stabilized {01text-decoration:overline12} faces, inferring a tridentate interaction due to steric constraints The bone protein, osteocalcin was found to be a non-specific inhibitor whereas a bone proteoglycan monomer and polygalacturonate had minimal morphological effect A carboxylated hyperbranched polymer gave oriented nucleation owing to partial segregation of the macromolecule at the air/water interface Fish anti-freeze glycopeptides and polyvinyl alcohol induced the precipitation of vaterite possibly by affecting the kinetics of cation dehydration Multifunctional additives such as diphosphonates, the coccolith polysaccharide and alginate were also effective at promoting crystal aggregation

A model mechanism for the chemotactic response of endothelial cells to tumour angiogenesis factor

Abstract: In order to accomplish the transition from avascular to vascular growth, solid tumours secrete a diffusible substance known as tumour angiogenesis factor (TAF) into the surrounding tissue. Endothelial cells which form the lining of neighbouring blood vessels respond to this chemotactic stimulus in a well-ordered sequence of events consisting, at minimum, of a degradation of their basement membrane, migration, and proliferation. A model mechanism is presented which includes the diffusion of the TAF into the surrounding host tissue and the response of the endothelial cells to the chemotactic stimulus. The model accounts for the main observed events associated with the endothelial cells during the process of angiogenesis (i.e. cell migration and proliferation); the numerical results compare very well with experimental observations. The situation where the tumour (i.e. the source of TAF) is removed and the vessels recede is also considered.

Electron Microscopy Study of Magnetosomes in a Cultured Coccoid Magnetotactic Bacterium

Abstract: Magnetite (Fe$\_{3}$O$\_{4}$) crystals produced by two strains of cultured vibrioid magnetotactic bacteria were studied by high-resolution transmission electron microscopy (HRTEM). Both magnetotactic strains were characterized by single chains of magnetite crystals aligned along the long axes of the cells. The strains, designated as MV-2 and MV-4, produced crystals that differed markedly in size and morphology. Crystals present in MV-4 cells were generally larger than those in MV-2 cells and displayed significantly smaller aspect ratios. Crystallographic analysis of the magnetosomes of MV-2 revealed an elongated hexagonal habit based on a prism of {110} faces capped by {111} faces. The axis of elongation was parallel to the direction. This morphology closely resembles the crystal shape of magnetosomes in a previously described vibrioid species MV-1. In contrast, magnetosomes of MV-4 possessed a cubo-octahedral morphology which was modified by a small elongation along the direction. Although this morphology has not previously been observed in magnetotactic bacteria, it appears to be intermediate between the regular cubo-octahedral shape of magnetosomes in the cultured species Aquaspirillum magnetotacticum and the extensively elongated cubo-octahedral crystals of a previously studied uncultured ovoid-shaped magnetotactic bacterium. The results support the proposal that the crystal morphologies of bacterial magnetite are strain specific.

Is the steady viscous incompressible two‐dimensional flow over a backward‐facing step at Re = 800 stable?

Abstract: A detailed case study is made of one particular solution of the 2D incompressible Navier-Stokes equations. Careful mesh refinement studies were made using four different methods (and computer codes): (1) a high-order finite-element method solving the unsteady equations by time-marching; (2) a high-order finite-element method solving both the steady equations and the associated linear- stability problem; (3) a second-order finite difference method solving the unsteady equations in streamfunction form by time-marching; and (4) a spectral-element method solving the unsteady equations by time-marching. The unanimous conclusion is that the correct solution for flow over the backward-facing step at Re=800 is steady - and it is stable, to both small and large perturbations.

The families of problem drug users: a study of 50 close relatives

Abstract: The research described in this paper resulted from a collaborative multi-centre study of the relatives of problem drug users involving six practitioners and researchers, in four centres within the south-west of England. The objective of this research was simply to interview 50 close relatives of identified problem drug users, with the identification occurring through clinics and self-help groups. Quantitative and qualitative results show that large numbers of these relatives reported many negative experiences. The partners of illicit drug users reported both more and differently patterned problematic behaviours than those of prescribed tranquillizer users. The partners of illicit users also reported different problems to those of the parents. Relatives reported many negative effects in terms of how they viewed the drug user, and how the experiences had affected their health. They also described various coping mechanisms, and the extent of the support which they had received. The results are discussed in terms of coping, and similarity with research into the families of problem drinkers.

Ligation of CD28 receptor by B7 induces formation of D‐3 phosphoinositides in T lymphocytes independently of T cell receptor/CD3 activation

Abstract: The co-stimulatory role of B7/CD28 interactions is important in promoting T cell activation. Very little is known about the intracellular events that follow CD28 engagement although recent evidence has implicated coupling of CD28 to a protein tyrosine kinase signal transduction pathway. In this study we have investigated the putative role of D-3 phosphoinositides as mediators of CD28 receptor signaling, since phosphoinositide (PI) 3-kinase, the enzyme responsible for D-3 phosphoinositide formation, is a known substrate for protein tyrosine kinases associated with certain T cell surface receptors such as CD4 and interleukin-2 receptor. The lipid products of PI 3-kinase activity have been suggested to play a role in mitogenic signaling and growth regulation in other cells. Chinese hamster ovary cells (CHO) previously transfected with B7 cDNA, induced time-dependent elevation above basal levels of phosphatidylinositol(3,4)-bisphosphate (PtdIns(3,4)P2) and PtdIns(3,4,5)P3, while parental CHO cells that did not express B7 had no effect on these lipids. Moreover, the elevation of these same lipids by CD3 ligation was potentiated in an additive manner by CHO-B7+ but not by CHO-B7- cells. CHO-B7+ and CHO-B7- cells did not activate phospholipase C as evidenced by their inability to modulate basal or CD3-induced changes in the levels of phosphatidic acid or D-4 and D-5 phosphoinositides. These data imply that PI 3-kinase but not phospholipase C, may be an important signal transduction molecule with respect to CD28-mediated co-stimulation and T cell activation following ligation by B7.

(+)‐Anatoxin‐a Is a Potent Agonist at Neuronal Nicotinic Acetylcholine Receptors

Abstract: The effects of the nicotinic agonist (+)-anatoxin-a have been examined in four different preparations, representing at least two classes of neuronal nicotinic receptors. (+)-Anatoxin-a was most potent (EC50 = 48 nM) in stimulating 86Rb+ influx into M10 cells, which express the nicotinic receptor subtype comprising alpha 4 and beta 2 subunits. A presynaptic nicotinic receptor mediating acetylcholine release from hippocampal synaptosomes was similarly sensitive to (+)-anatoxin-a (EC50 = 140 nM). alpha-Bungarotoxin-sensitive neuronal nicotinic receptors, studied using patch-clamp recording techniques, required slightly higher concentrations of this alkaloid for activation: Nicotinic currents in hippocampal neurons were activated by (+)-anatoxin-a with an EC50 of 3.9 microM, whereas alpha 7 homooligomers reconstituted in Xenopus oocytes yielded an EC50 value of 0.58 microM for (+)-anatoxin-a. In these diverse preparations, (+)-anatoxin-a was between three and 50 times more potent than (-)-nicotine and approximately 20 times more potent than acetylcholine, making it the most efficacious nicotinic agonist thus far described.

Group sequential tests for bivariate response: interim analyses of clinical trials with both efficacy and safety endpoints

Abstract: We describe group sequential tests for a bivariate response. The tests are defined in terms of the two response components jointly, rather than through a single summary statistic. Such methods are appropriate when the two responses concern different aspects of a treatment; for example, one might wish to show that a new treatment is both as effective and as safe as the current standard. We present a formulation of the bivariate testing problem, introduce group sequential tests that satisfy Type I error conditions, and show how to find the sample size guaranteeing a specified power. We describe how properties of group sequential tests for bivariate normal observations can be computed by numerical integration.

B7/BB1, the ligand for CD28, is expressed on repeatedly activated human T cells in vitro.

Abstract: The activation of T cells is now thought to require at least two distinct signals. One signal is delivered through the interaction of the antigen-specific T cell receptor with major histocompatibility complex (MHC) molecules and peptide, while the other is received from interactions with less precisely defined accessory or costimulatory molecules. In the absence of this second costimulatory signal, some T cells subsequently become unresponsive to antigenic stimulation. One of the major candidates for providing such a second signal to T cells is the molecule B7 interacting with the T cell glycoprotein CD28. In the present study we have investigated whether B7 is expressed on human T cell lines and clones, since these cells have the capacity to present antigen to each other by expressing MHC class II molecules. Our results demonstrate that B7 can be detected on T cell clones and on repeatedly activated but not freshly isolated peripheral blood T cells. The expression of B7 is dependent on the state of activation of the cells, being maximally expressed shortly after restimulation and becoming undetectable as the cells quiesce. Together, these results suggest that B7 expression may be of importance to T cells, perhaps in the avoidance of anergy.

Cytokines contribute to airway dysfunction in antigen-challenged guinea pigs : inhibition of airway hyperreactivity, pulmonary eosinophil accumulation, and tumor necrosis factor generation by pretreatment with an interleukin-1 receptor antagonist

Abstract: The role of pro-inflammatory cytokines in an animal model of allergic lung disease was examined by use of an interleukin-1 receptor antagonist (IL-1ra) and a specific bioassay for tumor necrosis factor (TNF). Ovalbumin-sensitized guinea pigs exhibit a marked bronchial hyperreactivity (assessed by airway responsiveness to intravenous histamine) and pulmonary eosinophil accumulation (assessed by bronchoalveolar lavage) 24 h after challenge with aerosolized antigen. Exposure of animals to an aerosol of IL-1ra (50 µg over 30 min) immediately before antigen challenge resulted in a marked protection against bronchial hyperreactivity and pulmonary eosinophil accumulation compared with IL-1ra vehicle-pretreated animals. Additionally, we report for the first time generation of TNF bioactivity in the bronchoalveolar lavage of antigen-challenged animals, which was significantly reduced in animals exposed to aerosolized IL-1ra before challenge. These studies point to a key role for the cytokines IL-1 and possibly TNF...

A practical approach to accurate fault location on extra high voltage teed feeders

Abstract: The basis of an alternative approach for accurately locating faults on teed feeders is described. The technique developed uses fault voltages and currents at all three ends. The method is virtually independent of fault resistance and largely insensitive to variations in source impedance, teed and line configurations, including line untransposition. The basic theory of the technique is presented. It is extensively tested using simulated primary system voltage and current waveforms, which include the transducer/hardware errors encountered in practice. The performance clearly shows a high degree of accuracy. >

Glucocorticoid-and non-glucocorticoid induction of lipocortins (annexins) 1 and 2 in rat peritoneal leucocytes in vivo.

Abstract: 1. We have studied the occurrence, distribution and disposition of lipocortins (annexins) 1, 2 and 5 in mixed peritoneal leucocytes obtained from rats in which glucocorticoid levels were altered by adrenalectomy, administration of the glucocorticoid antagonist, RU486, or by injection of dexamethasone or hydrocortisone, as well as from rats in which the peritoneal cells were elicited by inflammatory stimuli. 2. In cells obtained from untreated rats with an intact adrenal cortex, lipocortins 1, 2 and 5 were readily detectable: the majority of each of the proteins was apparently located intracellularly with much smaller amounts in the membrane. Lipocortin 1 and to a lesser extent lipocortin 5 were also seen in a Ca(2+)-dependent association with the external plasma membrane. Following administration of RU486 (2 x 20 mg kg-1) the amounts of lipocortin 1 and 2 in cells were greatly reduced. Conversely, injection of hydrocortisone (1 mg kg-1) or dexamethasone (0.08 mg kg-1) caused an increase in the amount of lipocortin 1 and 2 in peritoneal cells within 30 min. Lipocortin 5 was unchanged by any manipulation of glucocorticoid levels. 3. Lipocortins 1 and 2 were elevated in both intracellular and membrane-associated fractions of macrophages elicited by intraperitoneal injection in inflammogens. This phenomenon also occurred in adrenalectomized animals. 4. Our data indicate that glucocorticoids control the synthesis of some members of the lipocortin family in rat mixed peritoneal cells but also suggest the existence of a separate system for controlling the generation of this protein. The significance of these observations is considered in relation to the mechanism of glucocorticoid hormone action on eicosanoid production.

The role of endogenous auxin in root initiation

Abstract: This paper is the second part of a review which considers evidence for the involvement of auxin in root initiation. Part II examines the research being carried out with transformed plant tissues. Agrobacterium rhizogenes causes abundant root initiation at the site of inoculation. Ri plasmid T-DNA contains several genes which encode enzymes involved in the biosynthesis and metabolism of indole-3-acetic acid. Transfer of various fragments of the Ri plasmid has also been reported to confer increased sensitivity to auxin upon plant cells. Controlled expression of these genes in the plant genome potentially offer an insight for developmental plant physiologists into the role of plant growth substances in the process of root initiation. The importance of absolute levels of IAA in the stimulation of root initiation is discussed.