Elisa Cabiscol

Redox control and oxidative stress in yeast cells

Protein structure and function can be altered by reactive oxygen species produced either by cell metabolism or by external oxidants. Although catalases, superoxide dismutases and peroxidases contribute to maintaining non-toxic levels of reactive oxygen species, modification of amino acid side chains occurs. In particular, oxidative modification of sulphydryl groups in proteins can be a two-faceted process: it could lead to impairment of protein function or, depending on the redox state of cysteine residues, may activate specific pathways involved in regulating key cell functions. In yeast cells, the thioredoxin and glutaredoxin systems participate in such redox regulation in different cell compartments, and interplay exists between both systems. In this context, glutaredoxins with monothiol activity initially characterised in Saccharomyces cerevisiae may display specific regulatory functions at the mitochondria and nuclei. Furthermore, their structural conservation in other organisms point to a conserved important role in metal homeostasis also in higher eukaryotes. Control of gene expression in response to oxidative stress is mediated by several transcription factors, among which Yap1 has a predominant role in S. cerevisiae (Pap1 in Schizosaccharomyces pombe and Cap1 in Candida albicans). In combination with Gpx3 peroxidase and Ybp1 protein, the activity of Yap1 is itself controlled depending on the redox state of some of its cysteine residues, which determines the nucleocytoplasmic location of the Yap1 molecules.

Grx5 glutaredoxin plays a central role in protection against protein oxidative damage in Saccharomyces cerevisiae

ABSTRACT Glutaredoxins are members of a superfamily of thiol disulfide oxidoreductases involved in maintaining the redox state of target proteins. In Saccharomyces cerevisiae, two glutaredoxins (Grx1 and Grx2) containing a cysteine pair at the active site had been characterized as protecting yeast cells against oxidative damage. In this work, another subfamily of yeast glutaredoxins (Grx3, Grx4, and Grx5) that differs from the first in containing a single cysteine residue at the putative active site is described. This trait is also characteristic for a number of glutaredoxins from bacteria to humans, with which the Grx3/4/5 group has extensive homology over two regions. Mutants lacking Grx5 are partially deficient in growth in rich and minimal media and also highly sensitive to oxidative damage caused by menadione and hydrogen peroxide. A significant increase in total protein carbonyl content is constitutively observed in grx5cells, and a number of specific proteins, including transketolase, appear to be highly oxidized in this mutant. The synthetic lethality of the grx5 and grx2 mutations on one hand and ofgrx5 with the grx3 grx4 combination on the other points to a complex functional relationship among yeast glutaredoxins, with Grx5 playing a specially important role in protection against oxidative stress both during ordinary growth conditions and after externally induced damage. Grx5-deficient mutants are also sensitive to osmotic stress, which indicates a relationship between the two types of stress in yeast cells.

Proteomic and oxidative stress analysis in human brain samples of Huntington disease

Huntington disease (HD) is a neurodegenerative disorder caused by expansion of CAG repeats in exon 1 of the huntingtin gene, affecting initially the striatum and progressively the cortex. This work reports a proteomic analysis of human brain postmortem samples obtained from striatum and cortex of patients with HD compared to samples of age- and sex-matched controls. Antioxidant defense proteins that were strongly induced in striatum, but also detectable in cortex, were identified as peroxiredoxins 1, 2, and 6, as well as glutathione peroxidases 1 and 6. The activities of other antioxidant enzymes such as mitochondrial superoxide dismutase and catalase were also increased in HD. Aconitase, a protein involved in energy metabolism, showed decreased activities in striatum of HD patients. Protein carbonyls, used as markers of oxidative stress, were increased in HD, and glial fibrillary acidic protein, aconitase, gamma-enolase, and creatine kinase B were identified as the main targets. Taken together, these results indicate that oxidative stress and damage to specific macromolecules would participate in the disease progression. Also, these data support the rationale for therapeutic strategies that either potentiate antioxidant defenses or avoid oxidative stress generation to delay disease progression.

Protein oxidation in Huntington disease affects energy production and vitamin B6 metabolism

Huntington disease (HD) is an inherited neurodegenerative disorder that initially affects the striatum and progressively the cortex. Oxidative stress in HD has been described as important to disease progression. In this study, protein carbonylation, used as a marker of protein oxidation, was analyzed in human brain striatum. A comparison of HD samples to matched controls identified 13 carbonylated proteins, including enzymes involved in the glycolytic pathway and mitochondrial proteins related to ATP production. Oxidation of the mitochondrial enzymes resulted in decreased catalytic activity, in good agreement with the energy deficiency observed in HD. We also found carbonylation of pyridoxal kinase and antiquitin 1, both involved in the metabolism of pyridoxal 5-phosphate, the active form of vitamin B6. The Tet/HD94 conditional mouse model allowed us to demonstrate that increased carbonylation in striatum is dependent on mutant huntingtin expression. As in humans, pyridoxal kinase showed decreased levels and was highly carbonylated in the gene-on mice; these modifications were reverted in the gene-off mice. We hypothesize that both pyridoxal kinase and antiquitin 1 oxidation could result in decreased pyridoxal 5-phosphate availability necessary as a cofactor in transaminations, synthesis of glutathione, and synthesis of GABA and dopamine, two neurotransmitters that play a key role in HD pathology.

Manganese is the link between frataxin and iron-sulfur deficiency in the yeast model of Friedreich ataxia.

Friedreich ataxia is a human neurodegenerative and myocardial disease caused by decreased expression of the mitochondrial protein frataxin. Proteomic analysis of the mutant yeast model of Friedreich ataxia presented in this paper reveals that these cells display increased amounts of proteins involved in antioxidant defenses, including manganese-superoxide dismutase. This enzyme shows, however, lower activity than that found in wild type cells. Our results indicate that this lack of activity is a consequence of cellular manganese deficiency, because in manganese-supplemented cultures, cell manganese content, and manganese-superoxide dismutase activity were restored. One of the hallmarks of Friedreich ataxia is the decreased activity of iron/sulfur-containing enzymes. The activities of four enzymes of this group (aconitase, glutamate synthase, succinate dehydrogenase, and isopropylmalate dehydratase) have been analyzed for the effects of manganese supplementation. Enzyme activities were recovered by manganese treatment, except for aconitase, for which, a specific interaction with frataxin has been demonstrated previously. Similar results were obtained when cells were grown in iron-limited media suggesting that manganese-superoxide dismutase deficiency is a consequence of iron overload. In conclusion, these data indicate that generalized deficiency of iron-sulfur protein activity could be a consequence of manganese-superoxide dismutase deficiency, and consequently, it opens new strategies for Friedreich ataxia treatment.

DnaK dependence of mutant ethanol oxidoreductases evolved for aerobic function and protective role of the chaperone against protein oxidative damage in Escherichia coli

The adhE gene of Escherichia coli encodes a multifunctional ethanol oxidoreductase (AdhE) that catalyzes successive reductions of acetyl-CoA to acetaldehyde and then to ethanol reversibly at the expense of NADH. Mutant JE52, serially selected for acquired and improved ability to grow aerobically on ethanol, synthesized an AdhEA267T/E568K with two amino acid substitutions that sequentially conferred improved catalytic properties and stability. Here we show that the aerobic growth ability on ethanol depends also on protection of the mutant AdhE against metal-catalyzed oxidation by the chaperone DnaK (a member of the Hsp70 family). No DnaK protection of the enzyme is evident during anaerobic growth on glucose. Synthesis of DnaK also protected E. coli from H2O2 killing under conditions when functional AdhE is not required. Our results therefore suggest that, in addition to the known role of protecting cells against heat stress, DnaK also protects numerous kinds of proteins from oxidative damage.

Reduction of oxidative cellular damage by overexpression of the thioredoxin TRX2 gene improves yield and quality of wine yeast dry active biomass

BackgroundWine Saccharomyces cerevisiae strains, adapted to anaerobic must fermentations, suffer oxidative stress when they are grown under aerobic conditions for biomass propagation in the industrial process of active dry yeast production. Oxidative metabolism of sugars favors high biomass yields but also causes increased oxidation damage of cell components. The overexpression of the TRX2 gene, coding for a thioredoxin, enhances oxidative stress resistance in a wine yeast strain model. The thioredoxin and also the glutathione/glutaredoxin system constitute the most important defense against oxidation. Trx2p is also involved in the regulation of Yap1p-driven transcriptional response against some reactive oxygen species.ResultsLaboratory scale simulations of the industrial active dry biomass production process demonstrate that TRX2 overexpression increases the wine yeast final biomass yield and also its fermentative capacity both after the batch and fed-batch phases. Microvinifications carried out with the modified strain show a fast start phenotype derived from its enhanced fermentative capacity and also increased content of beneficial aroma compounds. The modified strain displays an increased transcriptional response of Yap1p regulated genes and other oxidative stress related genes. Activities of antioxidant enzymes like Sod1p, Sod2p and catalase are also enhanced. Consequently, diminished oxidation of lipids and proteins is observed in the modified strain, which can explain the improved performance of the thioredoxin overexpressing strain.ConclusionsWe report several beneficial effects of overexpressing the thioredoxin gene TRX2 in a wine yeast strain. We show that this strain presents an enhanced redox defense. Increased yield of biomass production process in TRX2 overexpressing strain can be of special interest for several industrial applications.

Protein carbonylation: proteomics, specificity and relevance to aging.

Detection and quantification of protein carbonyls present in biological samples has become a popular, albeit indirect, method to determine the existence of oxidative stress. Moreover, the rise of proteomics has allowed the identification of the specific proteins targeted by protein carbonylation. This review discusses these methodologies and proteomic strategies and then focuses on the relationship between protein carbonylation and aging and the parameters that may explain the increased sensitivity of certain proteins to protein carbonylation.

Protein oxidation in Huntington disease

Huntington disease (HD) is an inherited neurodegenerative disorder caused by expansion of CAG repeats in the huntingtin gene, affecting initially the striatum and progressively the cortex. Oxidative stress, and consequent protein oxidation, has been described as important to disease progression. This review focuses on recent advances in the field, with a particular emphasis on the identified target proteins and the role that their oxidation has or might have in the pathophysiology of HD. Oxidation and the resulting inactivation and/or degradation of important proteins can explain the impairment of several metabolic pathways in HD. Oxidation of enzymes involved in ATP synthesis can account for the energy deficiency observed. Impairment of protein folding and degradation can be due to oxidation of several heat shock proteins and Valosin‐containing protein. Oxidation of two enzymes involved in the vitamin B6 metabolism could result in decreased availability of pyridoxal phosphate, which is a necessary cofactor in transaminations, the kynurenine pathway and the synthesis of glutathione, GABA, dopamine and serotonin, all of which have a key role in HD pathology. In addition, protein oxidation often contributes to oxidative stress, aggravating the molecular damage inside the cell.

Major targets of iron-induced protein oxidative damage in frataxin-deficient yeasts are magnesium-binding proteins

Iron accumulation has been associated with several pathological conditions such as Friedreich ataxia. This human disorder is caused by decreased expression of frataxin. Iron-overload triggers oxidative stress, but the main targets of such stress are not known. In yeast cells lacking the frataxin ortholog YFH1, we have identified a set of 14 carbonylated proteins, which include mitochondrial ATP synthase, phosphoglycerate kinase, pyruvate kinase, and molecular chaperones. Interestingly, most of the target proteins are magnesium- and/or nucleotide-binding proteins. This key feature leads us to postulate that when iron accumulates, chelatable iron replaces magnesium at the corresponding metal-binding site, promoting selective damage to these proteins. Consistent with this hypothesis, in vitro experiments performed with pure pyruvate kinase and phosphoglycerate kinase showed that oxidation of these proteins can be prevented by magnesium and increased by the presence of ATP. Also, chelatable iron, which forms complexes with nucleotides, showed a sevenfold increase in Deltayfh1 cells. Moreover, lowering chelatable iron in Deltayfh1 cells by desferrioxamine prevented enzyme inactivation. As a general conclusion, we propose that magnesium bound to proteins is replaced by chelatable iron when this metal accumulates. This mechanism explains selective protein oxidation and provides clues for better understanding of iron-overloading pathologies.