Publications by authors named "Christian Weikert"

9 Publications

  • Page 1 of 1

Yeast adapts to a changing stressful environment by evolving cross-protection and anticipatory gene regulation.

Mol Biol Evol 2013 Mar 2;30(3):573-88. Epub 2012 Nov 2.

Institute of Evolutionary Biology and Environmental Studies, University of Zurich, Zurich, Switzerland.

Organisms can protect themselves against future environmental change. An example is cross-protection, where physiological adaptation against a present environmental stressor can protect an organism against a future stressor. Another is anticipation, where an organism uses information about its present environment to trigger gene expression and other physiological changes adaptive in future environments. "Predictive" abilities like this exist in organisms that have been exposed to periodic changes in environments. It is unknown how readily they can evolve. To answer this question, we carried out laboratory evolution experiments in the yeast Saccharomyces cerevisiae. Specifically, we exposed three replicate populations of yeast to environments that varied cyclically between two stressors, salt stress and oxidative stress, every 10 generations, for a total of 300 generations. We evolved six replicate control populations in only one of these stressors for the same amount of time. We analyzed fitness changes and genome-scale expression changes in all these evolved populations. Our populations evolved asymmetric cross protection, where oxidative stress protects against salt stress but not vice versa. Gene expression data also suggest the evolution of anticipation and basal gene expression changes that occur uniquely in cyclic environments. Our study shows that highly complex physiological states that are adaptive in future environments can evolve on very short evolutionary time scales.
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March 2013

Directional selection causes decanalization in a group I ribozyme.

PLoS One 2012 18;7(9):e45351. Epub 2012 Sep 18.

Department of Bioengineering, Stanford University, Stanford, California, United States of America.

A canalized genotype is robust to environmental or genetic perturbations. Canalization is expected to result from stabilizing selection on a well-adapted phenotype. Decanalization, the loss of robustness, might follow periods of directional selection toward a new optimum. The evolutionary forces causing decanalization are still unknown, in part because it is difficult to determine the fitness effects of mutations in populations of organisms with complex genotypes and phenotypes. Here, we report direct experimental measurements of robustness in a system with a simple genotype and phenotype, the catalytic activity of an RNA enzyme. We find that the robustness of a population of RNA enzymes decreases during a period of directional selection in the laboratory. The decrease in robustness is primarily caused by the selective sweep of a genotype that is decanalized relative to the wild-type, both in terms of mutational robustness and environmental robustness (thermodynamic stability). Our results experimentally demonstrate that directional selection can cause decanalization on short time scales, and demonstrate co-evolution of mutational and environmental robustness.
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February 2013

Effects of endotoxin and catecholamines on hepatic mitochondrial respiration.

Inflammation 2009 Oct;32(5):315-21

Department of Intensive Care Medicine, Inselspital, Bern University Hospital and University of Bern, Bern, Switzerland.

Catecholamines are frequently used in sepsis, but their interaction with mitochondrial function is controversial. We incubated isolated native and endotoxin-exposed swine liver mitochondria with either dopamine, dobutamine, noradrenaline or placebo for 1 h. Mitochondrial State 3 and 4 respiration and their ratio (RCR) were determined for respiratory chain complexes I, II and IV. All catecholamines impaired glutamate-dependent RCR (p = 0.046), predominantly in native mitochondria. Endotoxin incubation alone induced a decrease in glutamate-dependent RCR compared to control samples (p = 0.002). We conclude that catecholamines and endotoxin impair the efficiency of mitochondrial complex I respiration in vitro.
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October 2009

Activation of non-ischemic, hypoxia-inducible signalling pathways up-regulate cytoprotective genes in the murine liver.

J Hepatol 2007 Oct 4;47(4):538-45. Epub 2007 Jun 4.

Clinic of Visceral and Transplantation Surgery, Department of Clinical Research, University of Bern, Murtenstrase 35, 3010 Bern, Switzerland.

Background/aims: We investigated the molecular response of a non-ischemic hypoxic stress in the liver, in particular, to distinguish its hepatoprotective potential.

Methods: The livers of mice were subjected to non-ischemic hypoxia by clamping the hepatic-artery (HA) for 2h while maintaining portal circulation. Hypoxia was defined by a decrease in oxygen saturation, the activation of hypoxia-inducible factor (HIF)-1 and the mRNA up-regulation of responsive genes. To demonstrate that the molecular response to hypoxia may in part be hepatoprotective, pre-conditioned animals were injected with an antibody against Fas (Jo2) to induce acute liver failure. Hepatocyte apoptosis was monitored by caspase-3 activity, cleavage of lamin A and animal survival.

Results: Clamping the HA induced a hypoxic stress in the liver in the absence of severe metabolic distress or tissue damage. The hypoxic stimulus was sufficient to activate the HIF-1 signalling pathway and up-regulate hepatoprotective genes. Pre-conditioning the liver with hypoxia was able to delay the onset of Fas-mediated apoptosis and prolong animal survival.

Conclusions: Our data reveal that hepatic cells can sense and respond to a decrease in tissue oxygenation, and furthermore, that activation of hypoxia-inducible signalling pathways function in part to promote liver cell survival.
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October 2007

Effect of endotoxin, dobutamine and dopamine on muscle mitochondrial respiration in vitro.

J Endotoxin Res 2006 ;12(6):358-66

Department of Intensive Care Medicine, University Hospital Bern, 3010 Bern, Switzerland.

Introduction: Mitochondrial respiration is impaired during endotoxemia. While catecholamines are frequently used in sepsis, their effects on mitochondrial function are controversial. We assessed effects of dobutamine and dopamine endotoxin on isolated muscle mitochondria.

Materials And Methods: Sternocleidomastoid muscle mitochondria were isolated from six anesthetized pigs. Each sample was divided into six different groups. Three groups were incubated with endotoxin, three with vehicle. After 1 h, dopamine and dobutamine at final concentrations of 100 microM were added to the vehicle and endotoxin groups. After 2 h, state 3 and 4 respiration rates were determined for all mitochondrial complexes. Oxygen consumption was determined with a Clark-type electrode.

Results: Endotoxin increased glutamate-dependent state 4 respiration from 9.3 +/- 3.6 to 31.9 +/- 9.1 (P = 0.001) without affecting state 3 respiration. This reduced the efficiency of mitochondrial respiration (RCR; state 3/state 4, 9.9 +/- 1.9 versus 3.6 +/- 0.6; P < 0.001). The other complexes were unaffected. Catecholamine partially restored the endotoxin-induced increase in complex I state 4 respiration rate (31.9 +/- 9.1 versus 17.1 +/- 6.4 and 20.1 +/- 12.2) after dopamine and dobutamine, respectively (P = 0.007), and enhanced the ADP:O ratio (P = 0.033).

Conclusions: Dopamine and dobutamine enhanced the efficiency of mitochondrial respiration after short-term endotoxin exposure.
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April 2007

Effects of prolonged endotoxemia on liver, skeletal muscle and kidney mitochondrial function.

Crit Care 2006 ;10(4):R118

Department of Intensive Care Medicine, University Hospital Bern, CH-3010 Bern, Switzerland.

Introduction: Sepsis may impair mitochondrial utilization of oxygen. Since hepatic dysfunction is a hallmark of sepsis, we hypothesized that the liver is more susceptible to mitochondrial dysfunction than the peripheral tissues, such as the skeletal muscle. We studied the effect of prolonged endotoxin infusion on liver, muscle and kidney mitochondrial respiration and on hepatosplanchnic oxygen transport and microcirculation in pigs.

Methods: 20 anesthetized pigs were randomized to receive endotoxin or saline infusion for 24 hours. Muscle, liver and kidney mitochondrial respiration was assessed. Cardiac output (thermodilution), carotid, superior mesenteric and kidney arterial, portal venous (ultrasound Doppler) and microcirculatory blood flow (laser Doppler) were measured, and systemic and regional oxygen transport and lactate exchange were calculated.

Results: Endotoxin infusion induced hyperdynamic shock and impaired the glutamate- and succinate-dependent mitochondrial respiratory control ratio (RCR) in the liver (glutamate: endotoxemia: median [range] 2.8 [2.3-3.8] vs. controls: 5.3 [3.8-7.0]; p<0.001; succinate: endotoxemia: 2.9 [1.9-4.3] vs. controls: 3.9 [2.6-6.3] p=0.003). While the ADP:O ratio was reduced with both substrates, maximal ATP production was impaired only in the succinate-dependent respiration. Hepatic oxygen consumption and extraction, and liver surface laser Doppler blood flow remained unchanged. Glutamate-dependent respiration in the muscle and kidney was unaffected.

Conclusions: Endotoxemia reduces the efficiency of hepatic but neither skeletal muscle nor kidney mitochondrial respiration, independent of regional and microcirculatory blood flow changes.
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November 2006

Reactivation of the mitosis-promoting factor in postmitotic cardiomyocytes.

Cells Tissues Organs 2003 ;175(2):61-71

Institute of Cell Biology, Swiss Federal Institute of Technology, Zürich, Switzerland.

Cardiomyocytes cease to divide shortly after birth and an irreversible cell cycle arrest is evident accompanied by the downregulation of cyclin-dependent kinase activities. To get a better understanding of the cardiac cell cycle and its regulation, the effect of functional recovery of the mitosis-promoting factor (MPF) consisting of cyclin B1 and the cyclin-dependent kinase Cdc2 was assessed in primary cultures of postmitotic ventricular adult rat cardiomyocytes (ARC). Gene transfer into ARC was achieved using the adenovirus-enhanced transferrinfection system that was characterized by the absence of cytotoxic events. Simultaneous ectopic expression of wild-type versions of cyclin B1 and Cdc2 was sufficient to induce MPF activity. Reestablished MPF resulted in a mitotic phenotype, marked by an abnormal condensation of the nuclei, histone H3 phosphorylation and variable degree of decay of the contractile apparatus. Although a complete cell division was not observed, the results provided conclusive evidence that cell cycle-related events in postmitotic cardiomyocytes could be triggered by genetic intervention downstream of the G1/S checkpoint. This will be of importance to design novel strategies to overcome the proliferation arrest in adult cardiomyocytes.
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July 2004

Cellular engineering of ventricular adult rat cardiomyocytes.

Cardiovasc Res 2003 Oct;59(4):874-82

Institute of Cell Biology, ETH-Hoenggerberg Swiss Federal Institute of Technology, CH-8093 Zurich, Switzerland.

Objective: Preparation of viable cultured adult cardiomyocytes (vARCs) is a prerequisite for cell-based transplantation and tissue engineering. Ectopic gene expression is important in this context. Here, we present an in vitro cell replating strategy using Accutase for cultured vARCs, allowing ectopic gene expression.

Methods: Cultured vARCs from 6- to 8-week-old rats were used. Transfections with EGFP (enhanced green fluorescent protein) constructs, Mlc-3f-EGFP or alpha-actinin-EGFP were performed using adenovirus-enhanced transferrin-mediated infection (AVET). Accutase (PAA Laboratories, Linz, Austria) was used for the detachment of cultured cells. Immunohistochemical analysis, together with confocal laser microscopy was used for structural analysis of the cells.

Results: Cultured vARCs could be detached with a high yield (40 to 60%) from primary cultures using Accutase. The cultivation period plays an important role in the yield of viable cells. Resultant replated vARCs (rep-vARCs) rapidly (1-2 h) acquired a rounded up shape without degradation of their contractile apparatus, which is in contrast to the rod-shaped freshly isolated vARCs (fi-vARCs). The detached cells survived passage through a narrow syringe needle. After seeding, detached cells rapidly attached to various substrates, increased their content of the contractile apparatus, and formed cell-cell contacts within 3 days after reseeding. The detached cells survived passage through a narrow syringe needle. The high recovery of cells after replating enabled the use of the AVET system for gene delivery. AVET is free of infectious particles and does not lead to expression of viral proteins. Transfection of vARCs prior to detachment had a small effect on cell recovery and ectopically synthesized proteins were properly localized after replating.

Conclusions: Detachment of cultured vARCs using Accutase is well compatible with ectopic gene expression and yields a viable transgenic population of vARCs that eventually may be suitable as transgenic cardiomyocyte grafts.
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October 2003

Use of a glycerol-limited, long-term chemostat for isolation of Escherichia coli mutants with improved physiological properties.

Microbiology (Reading) 1997 May;143 ( Pt 5):1567-1574

Institute of Biotechnology, ETH Zürich, CH-8093 Zürich, Switzerland.

The evolution of Escherichia coli MG1655 mutants was followed over 126 d in a glycerol-limited chemostat at a dilution rate of 0.05 h-1. This corresponds to a total of 217 generations at a doubling time of 13.9 h. After this time, nearly 90% of the chemostat population consisted of evolved mutant strains as determined by their altered colony morphologies on plates. Two mutants were isolated that exhibited generally improved growth phenotypes in batch cultivations on glycerol, glucose or the gluconeogenic substrate acetate. Higher specific growth rates and increased biomass yields were found for both mutants. For one mutant, this behaviour was combined with significantly reduced secretion of overflow metabolites when either glycerol or glucose was the carbon source. Additionally, during all growth phases of a batch cultivation, this mutant exhibited increased resistance to a variety of adverse conditions including heat shock, osmotic stress and nutrient deprivation. It also displayed significantly shorter lag phases.
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May 1997