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Light and circadian regulation of clock components aids flexible responses to environmental signals

Abstract (Expand)

Summary The circadian clock measures time across a 24 h period, increasing fitness by phasing biological processes to the most appropriate time of day. The interlocking feedback loop mechanism of the …he clock is conserved across species; however, the number of loops varies. Mathematical and computational analyses have suggested that loop complexity affects the overall flexibility of the oscillator, including its responses to entrainment signals. We used a discriminating experimental assay, at the transition between different photoperiods, in order to test this proposal in a minimal circadian network (in Ostreococcus tauri) and a more complex network (in Arabidopsis thaliana). Transcriptional and translational reporters in O. tauri primarily tracked dawn or dusk, whereas in A. thaliana, a wider range of responses were observed, consistent with its more flexible clock. Model analysis supported the requirement for this diversity of responses among the components of the more complex network. However, these and earlier data showed that the O. tauri network retains surprising flexibility, despite its simple circuit. We found that models constructed from experimental data can show flexibility either from multiple loops and/or from multiple light inputs. Our results suggest that O. tauri has adopted the latter strategy, possibly as a consequence of genomic reduction.

Authors: Laura E. Dixon, Sarah K. Hodge, Gerben van Ooijen, Carl Troein, Ozgur E. Akman, Andrew J. Millar

Date Published: 20th May 2014

Publication Type: Journal Article

The silicon trypanosome: a test case of iterative model extension in systems biology

Abstract (Expand)

The African trypanosome, Trypanosoma brucei, is a unicellular parasite causing African Trypanosomiasis (sleeping sickness in humans and nagana in animals). Due to some of its unique properties, it has … emerged as a popular model organism in systems biology. A predictive quantitative model of glycolysis in the bloodstream form of the parasite has been constructed and updated several times. The Silicon Trypanosome is a project that brings together modellers and experimentalists to improve and extend this core model with new pathways and additional levels of regulation. These new extensions and analyses use computational methods that explicitly take different levels of uncertainty into account. During this project, numerous tools and techniques have been developed for this purpose, which can now be used for a wide range of different studies in systems biology.

Authors: , , , , , , T. Papamarkou, , , , , , , ,

Date Published: 7th May 2014

Publication Type: Not specified

Towards a Systems Level Understanding of the Oxygen Response of Escherichia coli

Abstract (Expand)

Escherichia coli is a facultatively anaerobic bacterium. With glucose if no external electron acceptors are available, ATP is produced by substrate level phosphorylation. The intracellular redox balance … is maintained by mixed-acid fermentation, that is, the production and excretion of several organic acids. When oxygen is available, E. coli switches to aerobic respiration to achieve redox balance and optimal energy conservation by proton translocation linked to electron transfer. The switch between fermentative and aerobic respiratory growth is driven by extensive changes in gene expression and protein synthesis, resulting in global changes in metabolic fluxes and metabolite concentrations. This oxygen response is determined by the interaction of global and local genetic regulatory mechanisms, as well as by enzymatic regulation. The response is affected by basic physical constraints such as diffusion, thermodynamics and the requirement for a balance of carbon, electrons and energy (predominantly the proton motive force and the ATP pool). A comprehensive systems level understanding of the oxygen response of E. coli requires the integrated interpretation of experimental data that are pertinent to the multiple levels of organization that mediate the response. In the pan-European venture, Systems Biology of Microorganisms (SysMO) and specifically within the project Systems Understanding of Microbial Oxygen Metabolism (SUMO), regulator activities, gene expression, metabolite levels and metabolic flux datasets were obtained using a standardized and reproducible chemostat-based experimental system. These different types and qualities of data were integrated using mathematical models. The approach described here has revealed a much more detailed picture of the aerobic-anaerobic response, especially for the environmentally critical microaerobic range that is located between unlimited oxygen availability and anaerobiosis.

Authors: , , , , , , S. Kunz, , , , , ,

Date Published: 7th May 2014

Publication Type: Not specified

Agent-based modeling of oxygen-responsive transcription factors in Escherichia coli

Abstract (Expand)

In the presence of oxygen (O2) the model bacterium Escherichia coli is able to conserve energy by aerobic respiration. Two major terminal oxidases are involved in this process - Cyo has a relatively … low affinity for O2 but is able to pump protons and hence is energetically efficient; Cyd has a high affinity for O2 but does not pump protons. When E. coli encounters environments with different O2 availabilities, the expression of the genes encoding the alternative terminal oxidases, the cydAB and cyoABCDE operons, are regulated by two O2-responsive transcription factors, ArcA (an indirect O2 sensor) and FNR (a direct O2 sensor). It has been suggested that O2-consumption by the terminal oxidases located at the cytoplasmic membrane significantly affects the activities of ArcA and FNR in the bacterial nucleoid. In this study, an agent-based modeling approach has been taken to spatially simulate the uptake and consumption of O2 by E. coli and the consequent modulation of ArcA and FNR activities based on experimental data obtained from highly controlled chemostat cultures. The molecules of O2, transcription factors and terminal oxidases are treated as individual agents and their behaviors and interactions are imitated in a simulated 3-D E. coli cell. The model implies that there are two barriers that dampen the response of FNR to O2, i.e. consumption of O2 at the membrane by the terminal oxidases and reaction of O2 with cytoplasmic FNR. Analysis of FNR variants suggested that the monomer-dimer transition is the key step in FNR-mediated repression of gene expression.

Authors: , , , S. Coakley, , ,

Date Published: 24th Apr 2014

Publication Type: Not specified

A mathematical model of metabolism and regulation provides a systems-level view of how Escherichia coli responds to oxygen

Abstract (Expand)

The efficient redesign of bacteria for biotechnological purposes, such as biofuel production, waste disposal or specific biocatalytic functions, requires a quantitative systems-level understanding of … energy supply, carbon, and redox metabolism. The measurement of transcript levels, metabolite concentrations and metabolic fluxes per se gives an incomplete picture. An appreciation of the interdependencies between the different measurement values is essential for systems-level understanding. Mathematical modeling has the potential to provide a coherent and quantitative description of the interplay between gene expression, metabolite concentrations, and metabolic fluxes. Escherichia coli undergoes major adaptations in central metabolism when the availability of oxygen changes. Thus, an integrated description of the oxygen response provides a benchmark of our understanding of carbon, energy, and redox metabolism. We present the first comprehensive model of the central metabolism of E. coli that describes steady-state metabolism at different levels of oxygen availability. Variables of the model are metabolite concentrations, gene expression levels, transcription factor activities, metabolic fluxes, and biomass concentration. We analyze the model with respect to the production capabilities of central metabolism of E. coli. In particular, we predict how precursor and biomass concentration are affected by product formation.

Editor:

Date Published: 27th Mar 2014

Publication Type: Not specified

Meeting report from the fourth meeting of the Computational Modeling in Biology Network (COMBINE)

Abstract (Expand)

The Computational Modeling in Biology Network (COMBINE) is an initiative to coordinate the development of community standards and formats in computational systems biology and related fields. This report … summarizes the topics and activities of the fourth edition of the annual COMBINE meeting, held in Paris during September 16-20 2013, and attended by a total of 96 people. This edition pioneered a first day devoted to modeling approaches in biology, which attracted a broad audience of scientists thanks to a panel of renowned speakers. During subsequent days, discussions were held on many subjects including the introduction of new features in the various COMBINE standards, new software tools that use the standards, and outreach efforts. Significant emphasis went into work on extensions of the SBML format, and also into community-building. This year’s edition once again demonstrated that the COMBINE community is thriving, and still manages to help coordinate activities between different standards in computational systems biology.

Authors: Dagmar Waltemath, Frank T. Bergmann, Claudine Chaouiya, Tobias Czauderna, Padraig Gleeson, , Martin Golebiewski, Michael Hucka, Nick Juty, , Nicolas Le Novère, Huaiyu Mi, Ion I. Moraru, Chris J. Myers, David Nickerson, Brett G. Olivier, Nicolas Rodriguez, Falk Schreiber, Lucian Smith, Fengkai Zhang, Eric Bonnet

Date Published: 15th Mar 2014

Publication Type: Journal Article

Analysis of Escherichia coli mutants with a linear respiratory chain

Abstract (Expand)

The respiratory chain of E. coli is branched to allow the cells' flexibility to deal with changing environmental conditions. It consists of the NADH:ubiquinone oxidoreductases NADH dehydrogenase I … and II, as well as of three terminal oxidases. They differ with respect to energetic efficiency (proton translocation) and their affinity to the different quinone/quinol species and oxygen. In order to analyze the advantages of the branched electron transport chain over a linear one and to assess how usage of the different terminal oxidases determines growth behavior at varying oxygen concentrations, a set of isogenic mutant strains was created, which lack NADH dehydrogenase I as well as two of the terminal oxidases, resulting in strains with a linear respiratory chain. These strains were analyzed in glucose-limited chemostat experiments with defined oxygen supply, adjusting aerobic, anaerobic and different microaerobic conditions. In contrast to the wild-type strain MG1655, the mutant strains produced acetate even under aerobic conditions. Strain TBE032, lacking NADH dehydrogenase I and expressing cytochrome bd-II as sole terminal oxidase, showed the highest acetate formation rate under aerobic conditions. This supports the idea that cytochrome bd-II terminal oxidase is not able to catalyze the efficient oxidation of the quinol pool at higher oxygen conditions, but is functioning mainly under limiting oxygen conditions. Phosphorylation of ArcA, the regulator of the two-component system ArcBA, besides Fnr the main transcription factor for the response towards different oxygen concentrations, was studied. Its phosphorylation pattern was changed in the mutant strains. Dephosphorylation and therefore inactivation of ArcA started at lower aerobiosis levels than in the wild-type strain. Notably, not only the micro- and aerobic metabolism was affected by the mutations, but also the anaerobic metabolism, where the respiratory chain should not be important.

Editor:

Date Published: 27th Jan 2014

Publication Type: Not specified

Genome-scale metabolic modelling of hepatocytes reveals serine deficiency in patients with non-alcoholic fatty liver disease.

Abstract (Expand)

Several liver disorders result from perturbations in the metabolism of hepatocytes, and their underlying mechanisms can be outlined through the use of genome-scale metabolic models (GEMs). Here we … reconstruct a consensus GEM for hepatocytes, which we call iHepatocytes2322, that extends previous models by including an extensive description of lipid metabolism. We build iHepatocytes2322 using Human Metabolic Reaction 2.0 database and proteomics data in Human Protein Atlas, which experimentally validates the incorporated reactions. The reconstruction process enables improved annotation of the proteomics data using the network centric view of iHepatocytes2322. We then use iHepatocytes2322 to analyse transcriptomics data obtained from patients with non-alcoholic fatty liver disease. We show that blood concentrations of chondroitin and heparan sulphates are suitable for diagnosing non-alcoholic steatohepatitis and for the staging of non-alcoholic fatty liver disease. Furthermore, we observe serine deficiency in patients with NASH and identify PSPH, SHMT1 and BCAT1 as potential therapeutic targets for the treatment of non-alcoholic steatohepatitis.

Authors: A. Mardinoglu, R. Agren, C. Kampf, A. Asplund, M. Uhlen, J. Nielsen

Date Published: 15th Jan 2014

Publication Type: Journal Article

Heterogeneity of glycolytic oscillatory behaviour in individual yeast cells

Abstract

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Authors: Anna-Karin Gustavsson, David D. van Niekerk, Caroline B. Adiels, Mattias Goksör, Jacky L. Snoep

Date Published: 3rd Jan 2014

Publication Type: Not specified

Systems Biology of Monovalent Cation Homeostasis in Yeast

Abstract (Expand)

Maintenance of monovalent cation homeostasis (mainly K(+) and Na(+)) is vital for cell survival, and cation toxicity is at the basis of a myriad of relevant phenomena, such as salt stress in crops and … diverse human diseases. Full understanding of the importance of monovalent cations in the biology of the cell can only be achieved from a systemic perspective. Translucent is a multinational project developed within the context of the SysMO (System Biology of Microorganisms) initiative and focussed in the study of cation homeostasis using the well-known yeast Saccharomyces cerevisiae as a model. The present review summarize how the combination of biochemical, genetic, genomic and computational approaches has boosted our knowledge in this field, providing the basis for a more comprehensive and coherent vision of the role of monovalent cations in the biology of the cell.

Authors: , Ebru Aydar, Samuel Drulhe, Daniel Ganser, , , , , Lynne Yenush, Olga Zimmermannová, G. Paul H. van Heusden, , , Chris Palmer, ,

Date Published: 2014

Publication Type: Not specified

Regulatory principles and experimental approaches to the circadian control of starch turnover

Abstract (Expand)

In many plants, starch is synthesized during the day and degraded during the night to avoid carbohydrate starvation in darkness. The circadian clock participates in a dynamic adjustment of starch … turnover to changing environmental condition through unknown mechanisms. We used mathematical modelling to explore the possible scenarios for the control of starch turnover by the molecular components of the plant circadian clock. Several classes of plausible models were capable of describing the starch dynamics observed in a range of clock mutant plants and light conditions, including discriminating circadian protocols. Three example models of these classes are studied in detail, differing in several important ways. First, the clock components directly responsible for regulating starch degradation are different in each model. Second, the intermediate species in the pathway may play either an activating or inhibiting role on starch degradation. Third, the system may include a light-dependent interaction between the clock and downstream processes. Finally, the clock may be involved in the regulation of starch synthesis. We discuss the differences among the models' predictions for diel starch profiles and the properties of the circadian regulators. These suggest additional experiments to elucidate the pathway structure, avoid confounding results and identify the molecular components involved.

Authors: D. D. Seaton, O. Ebenhoh, A. J. Millar, A. Pokhilko

Date Published: 18th Dec 2013

Publication Type: Not specified

Plerixafor induces the rapid and transient release of stromal cell-derived factor-1 alpha from human mesenchymal stromal cells and influences the migration behavior of human hematopoietic progenitor cells.

Abstract (Expand)

The interaction between the stromal cell-derived factor-1 alpha (SDF-1alpha, CXCL12) and its chemokine receptor CXCR4 has been reported to regulate stem cell migration, mobilization and homing. The … CXCR4 antagonist plerixafor is highly efficient in mobilizing hematopoietic progenitor cells (HPCs). However, the precise regulatory mechanisms governing the CXCR4/SDF-1alpha axis between the bone marrow niche and HPCs remain unclear. In this study, we quantify the impact of plerixafor on the interaction between human bone marrow derived mesenchymal stromal cells (MSCs) and human CD34+ HPCs. An assessment of SDF-1alpha levels in the supernatant of MSC cultures revealed that exposure to plerixafor led to a transient increase but had no long-term effect. In Transwell experiments, we observed that the addition of SDF-1alpha significantly stimulated HPC migration; this stimulation was almost completely antagonized by the addition of plerixafor, confirming the direct impact of the CXCR4/SDF-1alpha interaction on the migration capacity of HPCs. We also developed a new microstructural niche model to determine the chemotactic sensitivity of HPCs. Time-lapse microscopy demonstrated that HPCs migrated actively along an SDF-1alpha gradient within the microchannels and the quantitative assessment of the required minimum gradient initiating this chemotaxis revealed a surprisingly high sensitivity of HPCs. These data demonstrate the fine-tuned balance of the CXCR4/SDF-1alpha axis and the synergistic effects of plerixafor on HPCs and MSCs, which most likely represent the key mechanisms for the consecutive mobilization of HPCs from the bone marrow niche into the circulating blood.

Authors: P. Wuchter, C. Leinweber, R. Saffrich, M. Hanke, V. Eckstein, A. D. Ho, M. Grunze, A. Rosenhahn

Date Published: 17th Dec 2013

Publication Type: Journal Article

Handling uncertainty in dynamic models: the pentose phosphate pathway in Trypanosoma brucei

Abstract (Expand)

Dynamic models of metabolism can be useful in identifying potential drug targets, especially in unicellular organisms. A model of glycolysis in the causative agent of human African trypanosomiasis, … Trypanosoma brucei, has already shown the utility of this approach. Here we add the pentose phosphate pathway (PPP) of T. brucei to the glycolytic model. The PPP is localized to both the cytosol and the glycosome and adding it to the glycolytic model without further adjustments leads to a draining of the essential bound-phosphate moiety within the glycosome. This phosphate "leak" must be resolved for the model to be a reasonable representation of parasite physiology. Two main types of theoretical solution to the problem could be identified: (i) including additional enzymatic reactions in the glycosome, or (ii) adding a mechanism to transfer bound phosphates between cytosol and glycosome. One example of the first type of solution would be the presence of a glycosomal ribokinase to regenerate ATP from ribose 5-phosphate and ADP. Experimental characterization of ribokinase in T. brucei showed that very low enzyme levels are sufficient for parasite survival, indicating that other mechanisms are required in controlling the phosphate leak. Examples of the second type would involve the presence of an ATP:ADP exchanger or recently described permeability pores in the glycosomal membrane, although the current absence of identified genes encoding such molecules impedes experimental testing by genetic manipulation. Confronted with this uncertainty, we present a modeling strategy that identifies robust predictions in the context of incomplete system characterization. We illustrate this strategy by exploring the mechanism underlying the essential function of one of the PPP enzymes, and validate it by confirming the model predictions experimentally.

Authors: , , V. P. Alibu, R. J. Burchmore, I. H. Gilbert, M. Trybilo, N. N. Driessen, D. Gilbert, , ,

Date Published: 5th Dec 2013

Publication Type: Not specified

Model of tryptophan metabolism, readily scalable using tissue-specific gene expression data.

Abstract (Expand)

Tryptophan is utilized in various metabolic routes including protein synthesis, serotonin, and melatonin synthesis and the kynurenine pathway. Perturbations in these pathways have been associated with … neurodegenerative diseases and cancer. Here we present a comprehensive kinetic model of the complex network of human tryptophan metabolism based upon existing kinetic data for all enzymatic conversions and transporters. By integrating tissue-specific expression data, modeling tryptophan metabolism in liver and brain returned intermediate metabolite concentrations in the physiological range. Sensitivity and metabolic control analyses identified expected key enzymes to govern fluxes in the branches of the network. Combining tissue-specific models revealed a considerable impact of the kynurenine pathway in liver on the concentrations of neuroactive derivatives in the brain. Moreover, using expression data from a cancer study predicted metabolite changes that resembled the experimental observations. We conclude that the combination of the kinetic model with expression data represents a powerful diagnostic tool to predict alterations in tryptophan metabolism. The model is readily scalable to include more tissues, thereby enabling assessment of organismal tryptophan metabolism in health and disease.

Authors: A. K. Stavrum, I. Heiland, S. Schuster, P. Puntervoll, M. Ziegler

Date Published: 29th Nov 2013

Publication Type: Journal Article

Differences between healthy hematopoietic progenitors and leukemia cells with respect to CD44 mediated rolling versus adherence behavior on hyaluronic acid coated surfaces.

Abstract (Expand)

We previously demonstrated that leukemia cell lines expressing CD44 and hematopoietic progenitor cells (HPC) from umbilical cord blood (CB) showed rolling on hyaluronic acid (HA)-coated surfaces under … physiological shear stress. In the present study, we quantitatively assessed the interaction of HPC derived from CB, mobilized peripheral blood (mPB) and bone marrow (BM) from healthy donors, as well as primary leukemia blasts from PB and BM of patients with acute myeloid leukemia (AML) with HA. We have demonstrated that HPC derived from healthy donors showed relative homogeneous rolling and adhesion to HA. In contrast, highly diverse behavioral patterns were found for leukemia blasts under identical conditions. The monoclonal CD44 antibody (clone BU52) abrogated the shear stress-induced rolling of HPC and leukemia blasts, confirming the significance of CD44 in this context. On the other hand, the immobile adhesion of leukemia blasts to the HA-coated surface was, in some cases, not or incompletely inhibited by BU52. The latter property was associated with non-responsiveness to induction chemotherapy and subsequently poor clinical outcome.

Authors: M. Hanke, I. Hoffmann, C. Christophis, M. Schubert, V. T. Hoang, A. Zepeda-Moreno, N. Baran, V. Eckstein, P. Wuchter, A. Rosenhahn, A. D. Ho

Date Published: 26th Nov 2013

Publication Type: Journal Article

Functional potentials of human hematopoietic progenitor cells are maintained by mesenchymal stromal cells and not impaired by plerixafor.

Abstract (Expand)

BACKGROUND AIMS: Mesenchymal stromal cells (MSCs) resemble an essential component of the bone marrow niche for maintenance of stemness of hematopoietic progenitor cells (HPCs). Perturbation of the C-X-C … chemokine receptor type 4 (CXCR4)/stromal cell-derived factor-1alpha (SDF-1alpha) axis by plerixafor (AMD3100) mobilizes HPCs from their niche; however, little is known about how plerixafor affects interaction of HPCs and MSCs in vitro. METHODS: We monitored cell division kinetics, surface expression of CD34 and CXCR4, migration behavior and colony-forming frequency of HPCs on co-culture with MSCs either with or without exposure to plerixafor. RESULTS: Co-culture with MSCs significantly accelerated cell division kinetics of HPCs. Despite this, the proportion of CD34(+) cells was significantly increased on co-culture, whereas the expression of CXCR4 was reduced. In addition, co-culture with MSCs led to significantly higher colony-forming capacity and enhanced migration rate of HPCs compared with mono-culture conditions. The composition of MSC sub-populations-and conversely their hematopoiesis supportive functions-may be influenced by culture conditions. We compared the stromal function of MSCs isolated with three different culture media. Overall, the supporting potentials of these MSC preparations were quite similar. Perturbation by the CXCR4-antagonist plerixafor reduced the cell division kinetics of HPCs on co-culture with MSCs. However, the progenitor cell potential of the HPCs as reflected by colony-forming capacity was not affected by plerixafor. CONCLUSIONS: These results support the notion that the CXCR4/SDF-1alpha axis is critical for HPC-MSC interaction with regard to migration, adhesion and regulation of proliferation but not for maintenance of primitive progenitor cells.

Authors: A. Ludwig, R. Saffrich, V. Eckstein, T. Bruckner, W. Wagner, A. D. Ho, P. Wuchter

Date Published: 15th Oct 2013

Publication Type: Journal Article

Kinase activity of ArcB from Escherichia coli is subject to regulation by both ubiquinone and demethylmenaquinone

Abstract (Expand)

Expression of the catabolic network in Escherichia coli is predominantly regulated, via oxygen availability, by the two-component system ArcBA. It has been shown that the kinase activity of ArcB is … controlled by the redox state of two critical pairs of cysteines in dimers of the ArcB sensory kinase. Among the cellular components that control the redox state of these cysteines of ArcB are the quinones from the cytoplasmic membrane of the cell, which function in 'respiratory' electron transfer. This study is an effort to understand how the redox state of the quinone pool(s) is sensed by the cell via the ArcB kinase. We report the relationship between growth, quinone content, ubiquinone redox state, the level of ArcA phosphorylation, and the level of ArcA-dependent gene expression, in a number of mutants of E. coli with specific alterations in their set of quinones, under a range of physiological conditions. Our results provide experimental evidence for a previously formulated hypothesis that not only ubiquinone, but also demethylmenaquinone, can inactivate kinase activity of ArcB. Also, in a mutant strain that only contains demethylmenaquinone, the extent of ArcA phosphorylation can be modulated by the oxygen supply rate, which shows that demethylmenaquinone can also inactivate ArcB in its oxidized form. Furthermore, in batch cultures of a strain that contains ubiquinone as its only quinone species, we observed that the ArcA phosphorylation level closely followed the redox state of the ubiquinone/ubiquinol pool, much more strictly than it does in the wild type strain. Therefore, at low rates of oxygen supply in the wild type strain, the activity of ArcB may be inhibited by demethylmenaquinone, in spite of the fact that the ubiquinones are present in the ubiquinol form.

Authors: P. Sharma, S. Stagge, M. Bekker, K. Bettenbrock, K. J. Hellingwerf

Date Published: 7th Oct 2013

Publication Type: Not specified

[Mesenchymal stroma cells (MSCs) for the treatment of rheumatic disease].

Abstract

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Authors: M. Schmitt, L. P. Muller, G. Keysser, H. M. Lorenz, A. D. Ho, P. Wuchter

Date Published: 6th Sep 2013

Publication Type: Journal Article

UPARSE: highly accurate OTU sequences from microbial amplicon reads

Abstract (Expand)

Amplified marker-gene sequences can be used to understand microbial community structure, but they suffer from a high level of sequencing and amplification artifacts. The UPARSE pipeline reports operational … taxonomic unit (OTU) sequences with </=1% incorrect bases in artificial microbial community tests, compared with >3% incorrect bases commonly reported by other methods. The improved accuracy results in far fewer OTUs, consistently closer to the expected number of species in a community.

Author: R. C. Edgar

Date Published: 18th Aug 2013

Publication Type: Not specified

Benchmarking various green fluorescent protein variants in Bacillus subtilis, Streptococcus pneumoniae, and Lactococcus lactis for live cell imaging

Abstract (Expand)

Green fluorescent protein (GFP) offers efficient ways of visualizing promoter activity and protein localization in vivo, and many different variants are currently available to study bacterial cell … biology. Which of these variants is best suited for a certain bacterial strain, goal, or experimental condition is not clear. Here, we have designed and constructed two "superfolder" GFPs with codon adaptation specifically for Bacillus subtilis and Streptococcus pneumoniae and have benchmarked them against five other previously available variants of GFP in B. subtilis, S. pneumoniae, and Lactococcus lactis, using promoter-gfp fusions. Surprisingly, the best-performing GFP under our experimental conditions in B. subtilis was the one codon optimized for S. pneumoniae and vice versa. The data and tools described in this study will be useful for cell biology studies in low-GC-rich Gram-positive bacteria.

Authors: W. Overkamp, K. Beilharz, R. Detert Oude Weme, A. Solopova, H. Karsens, A. Kovacs, J. Kok, ,

Date Published: 16th Aug 2013

Publication Type: Not specified

Organism-Adapted Specificity of the Allosteric Regulation of Pyruvate Kinase in Lactic Acid Bacteria

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Editor:

Date Published: 25th Jul 2013

Publication Type: Not specified

Intermediate instability at high temperature leads to low pathway efficiency for an in vitro reconstituted system of gluconeogenesis in Sulfolobus solfataricus

Abstract (Expand)

Four enzymes of the gluconeogenic pathway in Sulfolobus solfataricus were purified and kinetically characterized. The enzymes were reconstituted in vitro to quantify the contribution of temperature … instability of the pathway intermediates to carbon loss from the system. The reconstituted system, consisting of phosphoglycerate kinase, glyceraldehyde 3-phosphate dehydrogenase, triose phosphate isomerase and the fructose 1,6-bisphosphate aldolase/ phosphatase maintained a constant consumption rate of 3-phosphoglycerate and production of fructose 6-phosphate over a 1 hour period. Cofactors ATP and NADPH were regenerated via pyruvate kinase and glucose dehydrogenase. A mathematical model was constructed on the basis of the kinetics of the purified enzymes and the measured half-life times of the pathway intermediates. The model quantitatively predicted the systems uxes and metabolite concentrations. Relative enzyme concentrations were chosen such that half the carbon in the system was lost due to degradation of the thermolabile intermediates dihydroxyacetone phosphate, glyceraldehyde 3-phosphate and 1,3 bisphosphoglycerate, indicating that intermediate instability at high temperature can significantly affect pathway efficiency. This article is protected by copyright. All rights reserved.

Authors: , Dominik Esser, Julia Kort, , ,

Date Published: 20th Jul 2013

Publication Type: Not specified

A model of yeast glycolysis based on a consistent kinetic characterisation of all its enzymes.

Abstract (Expand)

We present an experimental and computational pipeline for the generation of kinetic models of metabolism, and demonstrate its application to glycolysis in Saccharomyces cerevisiae. Starting from an … approximate mathematical model, we employ a "cycle of knowledge" strategy, identifying the steps with most control over flux. Kinetic parameters of the individual isoenzymes within these steps are measured experimentally under a standardised set of conditions. Experimental strategies are applied to establish a set of in vivo concentrations for isoenzymes and metabolites. The data are integrated into a mathematical model that is used to predict a new set of metabolite concentrations and reevaluate the control properties of the system. This bottom-up modelling study reveals that control over the metabolic network most directly involved in yeast glycolysis is more widely distributed than previously thought.

Authors: K. Smallbone, H. L. Messiha, K. M. Carroll, C. L. Winder, N. Malys, W. B. Dunn, E. Murabito, N. Swainston, J. O. Dada, F. Khan, P. Pir, E. Simeonidis, I. Spasic, J. Wishart, D. Weichart, N. W. Hayes, D. Jameson, D. S. Broomhead, S. G. Oliver, S. J. Gaskell, J. E. McCarthy, N. W. Paton, H. V. Westerhoff, D. B. Kell, P. Mendes

Date Published: 9th Jul 2013

Publication Type: Not specified

Dissecting the Catalytic Mechanism of Trypanosoma brucei Trypanothione Synthetase by Kinetic Analysis and Computational Modelling

Abstract (Expand)

In pathogenic trypanosomes, trypanothione synthetase (TryS) catalyzes the synthesis of both glutathionylspermidine (Gsp) and trypanothione [bis(glutathionyl)spermidine, T(SH)2]. Here we present a … thorough kinetic analysis of Trypanosoma brucei TryS in a newly developed phosphate buffer system at pH 7.0 and 37 °C, mimicking the physiological environment of the enzyme in the cytosol of bloodstream parasites. Under these conditions, TryS displays Km-values for GSH, ATP, spermidine and Gsp of 34, 18, 687, and 32 μM, respectively, as well as Ki-values for GSH and T(SH)2 of 1 mM and 360 μM, respectively. As Gsp hydrolysis has a Km-value of 5.6 mM, the in vivo amidase activity is probably negligible. To obtain a deeper insight in the molecular mechanism of TryS, we have formulated alternative kinetic models, with elementary reaction steps represented by linear kinetic equations. The model parameters were fitted to the extensive matrix of steady-state data obtained for different substrate/product combinations under the in vivo-like conditions. The best model describes the full kinetic profile and is able to predict time course data that were not used for fitting. This systems biology approach to enzyme kinetics led us to conclude that (i) TryS follows a ter-reactant mechanism, (ii) the intermediate Gsp dissociates from the enzyme between the two catalytic steps and (iii) T(SH)2 inhibits the enzyme by remaining bound at its product site and, as does the inhibitory GSH, by binding to the activated enzyme complex. The newly detected concerted substrate and product inhibition suggests that TryS activity is tightly regulated.

Editor:

Date Published: 3rd Jul 2013

Publication Type: Not specified

Kinetic characterization of arginine deiminase and carbamate kinase from Streptococcus pyogenes M49

Abstract (Expand)

Streptococcus pyogenes (group A Streptococcus, GAS) is an important human pathogen causing mild superficial infections of skin and mucous membranes, but also life-threatening systemic diseases. S. … pyogenes and other prokaryotic organisms use the arginine deiminase system (ADS) for survival in acidic environments. In this study, the arginine deiminase (AD), and carbamate kinase (CK) from S. pyogenes M49 strain 591 were heterologously expressed in E. coli DH5α, purified, and kinetically characterized. AD and CK from S. pyogenes M49 share high amino acid sequence similarity with the respective enzymes from Lactococcus lactis subsp. lactis IL1403 (45.6% and 53.5% identical amino acids) and Enterococcus faecalis V583 (66.8% and 66.8% identical amino acids). We found that the arginine deiminase of S. pyogenes is not allosterically regulated by the intermediates and products of the arginine degradation (E. g, ATP, citrulline, carbamoyl phosphate). The Km and Vmax values for arginine were 1.13±0.12 mM (mean ± SD) and 1.51±0.07 μmol/min/mg protein. The carbamate kinase is inhibited by ATP but unaffected by arginine and citrulline. The Km and Vmax values for ADP were 0.72±0.08 mM and 1.10±0.10 μmol/min/mg protein and the Km for carbamoyl phosphate was 0.65±0.07 mM. The optimum pH and temperature for both enzymes were 6.5 and 37°C, respectively.

Editor:

Date Published: 1st Jul 2013

Publication Type: Not specified

Regulation of the Activity of Lactate Dehydrogenases from Four Lactic Acid Bacteria

Abstract

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Authors: , , H. Messiha, , , , , , ,

Date Published: 17th May 2013

Publication Type: Not specified

A shift in the dominant phenotype governs the pH-induced metabolic switch of Clostridium acetobutylicumin phosphate-limited continuous cultures

Abstract (Expand)

In response to changing extracellular pH levels, phosphate-limited continuous cultures of Clostridium acetobutylicum reversibly switches its metabolism from the dominant formation of acids to the … prevalent production of solvents. Previous experimental and theoretical studies have revealed that this pH-induced metabolic switch involves a rearrangement of the intracellular transcriptomic, proteomic and metabolomic composition of the clostridial cells. However, the influence of the population dynamics on the observations reported has so far been neglected. Here, we present a method for linking the pH shift, clostridial growth and the acetone-butanol-ethanol fermentation metabolic network systematically into a model which combines the dynamics of the external pH and optical density with a metabolic model. Furthermore, the recently found antagonistic expression pattern of the aldehyde/alcohol dehydrogenases AdhE1/2 and pH-dependent enzyme activities have been included into this combined model. Our model predictions reveal that the pH-induced metabolic shift under these experimental conditions is governed by a phenotypic switch of predominantly acidogenic subpopulation towards a predominantly solventogenic subpopulation. This model-driven explanation of the pH-induced shift from acidogenesis to solventogenesis by population dynamics casts an entirely new light on the clostridial response to changing pH levels. Moreover, the results presented here underline that pH-dependent growth and pH-dependent specific enzymatic activity play a crucial role in this adaptation. In particular, the behaviour of AdhE1 and AdhE2 seems to be the key factor for the product formation of the two phenotypes, their pH-dependent growth, and thus, the pH-induced metabolic switch in C. acetobutylicum.

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Date Published: 3rd May 2013

Publication Type: Not specified

Large-scale metabolome analysis and quantitative integration with genomics and proteomics data in Mycoplasma pneumoniae.

Abstract (Expand)

Systems metabolomics, the identification and quantification of cellular metabolites and their integration with genomics and proteomics data, promises valuable functional insights into cellular biology. … However, technical constraints, sample complexity issues and the lack of suitable complementary quantitative data sets prevented accomplishing such studies in the past. Here, we present an integrative metabolomics study of the genome-reduced bacterium Mycoplasma pneumoniae. We experimentally analysed its metabolome using a cross-platform approach. We explain intracellular metabolite homeostasis by quantitatively integrating our results with the cellular inventory of proteins, DNA and other macromolecules, as well as with available building blocks from the growth medium. We calculated in vivo catalytic parameters of glycolytic enzymes, making use of measured reaction velocities, as well as enzyme and metabolite pool sizes. A quantitative, inter-species comparison of absolute and relative metabolite abundances indicated that metabolic pathways are regulated as functional units, thereby simplifying adaptive responses. Our analysis demonstrates the potential for new scientific insight by integrating different types of large-scale experimental data from a single biological source.

Authors: T. Maier, J. Marcos, J. A. Wodke, B. Paetzold, M. Liebeke, R. Gutierrez-Gallego, L. Serrano

Date Published: 20th Apr 2013

Publication Type: Not specified

Dissecting the energy metabolism in Mycoplasma pneumoniae through genome-scale metabolic modeling.

Abstract (Expand)

Mycoplasma pneumoniae, a threatening pathogen with a minimal genome, is a model organism for bacterial systems biology for which substantial experimental information is available. With the goal of … understanding the complex interactions underlying its metabolism, we analyzed and characterized the metabolic network of M. pneumoniae in great detail, integrating data from different omics analyses under a range of conditions into a constraint-based model backbone. Iterating model predictions, hypothesis generation, experimental testing, and model refinement, we accurately curated the network and quantitatively explored the energy metabolism. In contrast to other bacteria, M. pneumoniae uses most of its energy for maintenance tasks instead of growth. We show that in highly linear networks the prediction of flux distributions for different growth times allows analysis of time-dependent changes, albeit using a static model. By performing an in silico knock-out study as well as analyzing flux distributions in single and double mutant phenotypes, we demonstrated that the model accurately represents the metabolism of M. pneumoniae. The experimentally validated model provides a solid basis for understanding its metabolic regulatory mechanisms.

Authors: J. A. Wodke, J. Puchalka, M. Lluch-Senar, J. Marcos, E. Yus, M. Godinho, R. Gutierrez-Gallego, V. A. dos Santos, L. Serrano, E. Klipp, T. Maier

Date Published: 4th Apr 2013

Publication Type: Not specified

Excemplify: a flexible template based solution, parsing and managing data in spreadsheets for experimentalists.

Abstract (Expand)

In systems biology, quantitative experimental data is the basis of building mathematical models. In most of the cases, they are stored in Excel files and hosted locally. To have a public database for … collecting, retrieving and citing experimental raw data as well as experimental conditions is important for both experimentalists and modelers. However, the great effort needed in the data handling procedure and in the data submission procedure becomes the crucial limitation for experimentalists to contribute to a database, thereby impeding the database to deliver its benefit. Moreover, manual copy and paste operations which are commonly used in those procedures increase the chance of making mistakes. Excemplify, a web-based application, proposes a flexible and adaptable template-based solution to solve these problems. Comparing to the normal template based uploading approach, which is supported by some public databases, rather than predefining a format that is potentiall impractical, Excemplify allows users to create their own experiment-specific content templates in different experiment stages and to build corresponding knowledge bases for parsing. Utilizing the embedded knowledge of used templates, Excemplify is able to parse experimental data from the initial setup stage and generate following stages spreadsheets automatically. The proposed solution standardizes the flows of data traveling according to the standard procedures of applying the experiment, cuts down the amount of manual effort and reduces the chance of mistakes caused by manual data handling. In addition, it maintains the context of meta-data from the initial preparation manuscript and improves the data consistency. It interoperates and complements RightField and SEEK as well.

Authors: L. Shi, L. Jong, U. Wittig, P. Lucarelli, M. Stepath, S. Mueller, L. A. D'Alessandro, U. Klingmuller, W. Muller

Date Published: 3rd Apr 2013

Publication Type: Journal Article

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