Chiral symmetry in the 2-flavour lattice Schwinger model

We study the 2-flavour lattice Schwinger model: QED in D=2 with two fermion species of identical mass. In the simulation we are using Wilson fermions where chiral symmetry is explicitly broken. Since there is no known simple order parameter it is non-trivial to identify the critical line of the chiral phase transition. We therefore need to find observables which allow an identification of a possible restoration of chiral symmetry. We utilize the PCAC-relations in order to identify the critical coupling, where chiral symmetry is restored.Comment: 3 pages (LaTeX), 4 figures (EPS

Biomarkers for metabolic drug activation : towards an integrated risk assessment for drug-induced liver injury (DILI)

The term drug-induced liver injury (DILI) describes adverse effects upon therapeutic drug treatment. They are relatively rare, affecting only 1 of 10000 - 1000000 patients, and remain mostly unpredictable. Due to development of severe hepatotoxicity or death, drugs causing DILI display a high risk for patients and have been withdrawn from the market or severely restricted in use. For the pharmaceutical industry late stage attrition due to DILI represents a big burden stretching development time and effort and generating potential risk at high costs. A better characterization of the disease pattern and its contributing factors is needed. Currently experimental tools to build preclinical mitigation strategies are sparse, but urgently required to help establish an improved risk assessment. One possible mechanism of toxicity involves the formation of chemically reactive metabolites (RM) which interact with cellular macromolecules or signaling pathways. A direct link between RM formation and DILI remains speculative in most cases. Numerous studies of affected drugs demonstrate the plausible involvement of RM formation and subsequent covalent binding to proteins. Still, RMs are not detected for all DILI drugs and RMs do not lead to DILI in every case. Thus, a synergistic effect of multiple (unknown) mechanisms is supposed to result in DILI. The aim of this work was to review mechanisms leading to DILI, consisting of RM formation and other potentially contributing risk factors such as oxidative stress, cyto- or mitochondrial toxicity. Results were critically evaluated in light of the predictivity for DILI and comprise a gap analysis of current approaches. Biomarkers are proposed as complementary endpoints. Development and validation of analytical methods were conducted for in vitro experiments followed by application of tool compounds to demonstrate the correlation to in vivo studies. For the in-depth analysis of bioactivation data and its correlation to DILI, a validation set of drugs was selected. These included three groups of compounds, namely those with severe manifestation of DILI, drugs with reported DILI cases and drugs with a history of safe use. Different models were drafted to evaluate quantitative covalent binding as predictive parameter for DILI. The hypothesis was that the intrinsic property of in vitro covalent binding is not a descriptive parameter, as exposure of a toxic drug or metabolite in the body is determined by pharmacokinetic factors. E.g., low clearance drugs might result in experimental false negative results when they are not significantly activated in vitro. Thus, pharmacokinetic properties such as plasma clearance or hepatic inlet concentration were incorporated into the correlation analysis. A quantitative description of the models was established by sensitivity, specificity, precision and negative predictive value. As previously reported, a correlation between covalent binding, the daily dose and DILI was evident. This correlation further improved when adjusted for intrinsic clearance and substituting dose with the theoretical liver inlet concentration. It is further suggested to use glutathione adduct formation as surrogate for covalent binding. This approach was able to separate safe and high risk DILI drugs when evaluated in context of dose and clearance. The correlation did not hold true for medium risk drugs where a big overlap to safe drugs was noticeable. This may be due to equivocal drug classification or the fact that additional factors contribute to the development of DILI. One of the risk factors contributing to DILI is the excessive overproduction of reactive oxygen species (ROS), i.e. oxidative stress. Oxidative stress can be measured e.g. by cellular damage, biomarkers of lipid peroxidation or secondary signals like gene expression. Isoprostanes were chosen as biomarkers for further investigation. They derive from radical-catalyzed peroxidation of arachidonic acid. Selected isomers of this heterogeneous group were reported as biomarkers of ROS in the past. An online separation chromatography coupled mass spectrometry method was developed to simultaneously detect various isoprostanes and prostaglandins with a low limit of quantification. Analytical method validation allowed application of these biomarkers to a proof of concept study in primary rat and human hepatocytes. Results indicate a significant time and dose dependent cellular response for different isoprostane isomers by treatment with ferric nitrilotriacetic acid, a chemical known to cause oxidative stress. Furthermore, the value of isoprostanes as biomarkers of cellular oxidative stress was shown for DILI model compounds. The anticancer agent flutamide is known to cause hepatotoxicity, most likely by formation of reactive metabolites and impairment of mitochondrial function. Formation of imino-quinone intermediates may initiate redox cycling and cause excessive generation of ROS. In order to attenuate drug-induced ROS, hepatocyte cell culture was supplemented with pro-oxidant substrates for the in situ generation of hydrogen peroxide. Treatment of rat and human hepatocytes with flutamide induced oxidative stress as indicated by a time and dose dependent increase of isoprostane concentration. Other lipid peroxidation products, namely the hydroxynonenal (HNE) derived conjugates, HNE mercapturic acid (MA) and its reduced form dihydroxynonene MA, were found to be augmented upon treatment with flutamide as well. These were included into the biomarker panel. Under the test conditions no cytotoxicity was present, emphasizing the potential of lipid peroxidation products to early detect upcoming liver damage in in vitro systems. The described biomarkers could be translated between species from rat to human in hepatocytes. Further, results in Fischer F344 rats revealed their applicability to in vivo and enabled their classification relative to other cellular oxidative stress markers. In rats, the antioxidant response pathway was investigated via quantitative determination of mRNA for cytoprotective enzymes. In rat hepatocytes and rat liver increased RNA expression levels for glutathione-S-transferase, heme oxygenase, and NADPH:quinone oxidoreductase were detected. This suggests adaptation of cell homeostasis upon oxidative stress induced damage prior to overt cellular or organ damage. It can be assumed that pro-oxidant processes result in pathophysiological changes contributing to manifestation of DILI. Thus, the characterization of bioactivation potentials and oxidative stress conditions as contributing factor to DILI may be appropriate to characterize DILI risk. The development of new analytical tools using state of the art mass spectrometry enabled quantitative biomarker analysis and glutathione adduct screening from the same sample. In conclusion, this work describes the advances and limitations of RM characterization as risk for DILI. It highlights the value of characterizing danger signals, e.g. induced by oxidative stress. Specifically, biomarkers derived from lipid peroxidation and cell signal analysis may support preclinical risk assessment. It further stresses the importance of integrated risk mitigation strategies that are able to capture a variety of relevant drug properties and the mechanism by which they modulate toxicity. It must be also taken into account that patient related risk factors are likely to play a major role in development of DILI. Therefore, it is necessary to judge elucidated pathways on their potential to cause inter-individual differences. To minimize the general risk of adverse effects including DILI, the predominant goal in drug discovery must be the optimization of pharmacokinetic drug properties to yield low dose and selective drugs

Compartmentalised expression of Delta-like 1 in epithelial somites is required for the formation of intervertebral joints

<p>Abstract</p> <p>Background</p> <p>Expression of the mouse <it>Delta-like 1 </it>(<it>Dll1</it>) gene in the presomitic mesoderm and in the caudal halves of somites of the developing embryo is required for the formation of epithelial somites and for the maintenance of caudal somite identity, respectively. The rostro-caudal polarity of somites is initiated early on within the presomitic mesoderm in nascent somites. Here we have investigated the requirement of restricted <it>Dll1 </it>expression in caudal somite compartments for the maintenance of rostro-caudal somite polarity and the morphogenesis of the axial skeleton. We did this by overexpressing a functional copy of the <it>Dll1 </it>gene throughout the paraxial mesoderm, in particular in anterior somite compartments, during somitogenesis in transgenic mice.</p> <p>Results</p> <p>Epithelial somites were generated normally and appeared histologically normal in embryos of two independent <it>Dll1 </it>over-expressing transgenic lines. Gene expression analyses of rostro-caudal marker genes suggested that over-expression of <it>Dll1 </it>without restriction to caudal compartments was not sufficient to confer caudal identity to rostral somite halves in transgenic embryos. Nevertheless, <it>Dll1 </it>over-expression caused dysmorphologies of the axial skeleton, in particular, in morphological structures that derive from the articular joint forming compartment of vertebrae. Accordingly, transgenic animals exhibited missing or reduced intervertebral discs, rostral and caudal articular processes as well as costal heads of ribs. In addition, the midline of the vertebral column did not develop normally. Transgenic mice had open neural arches and split vertebral bodies with ectopic pseudo-growth plates. Endochondral bone formation and ossification in the developing vertebrae were delayed.</p> <p>Conclusion</p> <p>The mice overexpressing <it>Dll1 </it>exhibit skeletal dysmorphologies that are also evident in several mutant mice with defects in somite compartmentalisation. The <it>Dll1 </it>transgenic mice demonstrate that vertebral dysmorphologies such as bony fusions of vertebrae and midline vertebral defects can occur without apparent changes in somitic rostro-caudal marker gene expression. Also, we demonstrate that the over-expression of the <it>Dll1 </it>gene in rostral epithelial somites is not sufficient to confer caudal identity to rostral compartments. Our data suggest that the restricted <it>Dll1 </it>expression in caudal epithelial somites may be particularly required for the proper development of the intervertebral joint forming compartment.</p

Buzz pollination: studying bee vibrations on flowers

Approximately 6% of flowering plant species possess flowers with anthers that open through small pores or slits. Extracting pollen from this type of specialised flower is achieved most efficiently by vibrating the anthers, a behaviour that has evolved repeatedly among bees. Here I provide a brief overview of studying vibrations produced by bees and their effects on pollen release. I discuss how bee morphology and behaviour affect the mechanical properties of vibrations, and how floral traits may influence the transmission of those vibrations from the bee to the anther, thus mediating pollen release, and ultimately bee and plant fitness. I suggest that understanding the evolution of buzz pollination requires studying the biomechanics of bee vibrations and their transmission on flowers

Charge Fluctuations and Counterion Condensation

We predict a condensation phenomenon in an overall neutral system, consisting of a single charged plate and its oppositely charged counterions. Based on the ``two-fluid'' model, in which the counterions are divided into a ``free'' and a ``condensed'' fraction, we argue that for high surface charge, fluctuations can lead to a phase transition in which a large fraction of counterions is condensed. Furthermore, we show that depending on the valence, the condensation is either a first-order or a smooth transition.Comment: 16 pages, 1 figure, accepted to be published in PR

Transient Brewster angle reflectometry of spiropyran monolayers

Brewster angle reflectometry has been developed as a tool for determining the absorbance and refractive index changes in molecular monolayers containing spiropyran. The method is sensitive to changes in both the real and imaginary parts of the refractive index in the monolayers. It was used to monitor the conversion of spiropyran to merocyanine and the reversal of this reaction when the molecules were immobilised on quartz using silane coupling. An analytical solution of Fresnel formula allowed the transient reflectometry data to be converted into transient absorption information. Absorbances of transients as low as ~10-6 were possible using the current apparatus with a single laser pulse transient measurement. It was found that spiropyran photoconverted to merocyanine with an efficiency of ~0.1. The photochemical reversion of converted merocyanine to spiropyran occurred with efficiencies of 0.03–0.2 and this was probably site dependent. It was found that the thermal conversion from merocyanine to spiropyran was slow and even after 10 min there was no significant thermal reversion. This measurement was possible because the real part of the refractive index of the monolayer could be monitored with time using an off-resonance probe at a wavelength where the merocyanine did not absorb light meaning that the probe did not photobleach the sample. Thus our method also provides a non-intrusive method for probing changes in molecules in thin films

Neutral and Charged Polymers at Interfaces

Chain-like macromolecules (polymers) show characteristic adsorption properties due to their flexibility and internal degrees of freedom, when attracted to surfaces and interfaces. In this review we discuss concepts and features that are relevant to the adsorption of neutral and charged polymers at equilibrium, including the type of polymer/surface interaction, the solvent quality, the characteristics of the surface, and the polymer structure. We pay special attention to the case of charged polymers (polyelectrolytes) that have a special importance due to their water solubility. We present a summary of recent progress in this rapidly evolving field. Because many experimental studies are performed with rather stiff biopolymers, we discuss in detail the case of semi-flexible polymers in addition to flexible ones. We first review the behavior of neutral and charged chains in solution. Then, the adsorption of a single polymer chain is considered. Next, the adsorption and depletion processes in the many-chain case are reviewed. Profiles, changes in the surface tension and polymer surface excess are presented. Mean-field and corrections due to fluctuations and lateral correlations are discussed. The force of interaction between two adsorbed layers, which is important in understanding colloidal stability, is characterized. The behavior of grafted polymers is also reviewed, both for neutral and charged polymer brushes.Comment: a review: 130 pages, 30 ps figures; final form, added reference