Inhibition of yeast glutathione reductase by trehalose: possible implications in yeast survival and recovery from stress

Regulation of Cat8 in energy metabolic balance and glucose tolerance in Saccharomyces cerevisiae

Cat8 is a C6 zinc cluster transcription activator in yeast. It is generally recognized that the transcription of CAT8 is inhibited and that Cat8 is inactive in the presence of high concentrations of glucose. However, our recent study found that constitutively overexpressed Cat8 played a regulatory role in Saccharomyces cerevisiae in the presence of 20 g/L glucose. To explore the regulatory network of Cat8 at high glucose concentrations, CAT8 was both overexpressed and deleted in this study. Cell growth and glucose consumption in different media were significantly accelerated by the deletion of CAT8, while the lag period was greatly shortened. RNA-seq and genetic modification showed that the deletion of CAT8 changed the type of energy metabolism in yeast cells. Many genes related to the mitochondrial respiratory chain were downregulated, resulting in a reduction in aerobic respiration and the tricarboxylic acid cycle. Meanwhile, both the energy supply of anaerobic ethanol fermentation and the Crabtree effect of S. cerevisiae were enhanced by the deletion of CAT8. CAT8 knockout cells show a higher sugar uptake rate, a higher cell growth rate, and higher tolerance to glucose than the wild-type strain YS58. This study expands the understanding of the regulatory network of Cat8 and provides guidance for modulating yeast cell growth. KEY POINTS: • The deletion of CAT8 promoted cell growth of S. cerevisiae. • Transcriptome analysis revealed the regulation network of Cat8 under 1% glucose condition. • CAT8 deletion increases the glucose tolerance of cells by enhancing the Crabtree effect.

MAL62 Overexpression Enhances Freezing Tolerance of Baker's Yeast in Lean Dough by Enhancing Tps1 Activity and Maltose Metabolism

Trehalose plays a crucial role in response to freezing stress in baker's yeast. MAL62, a gene involved in the adenosine diphosphoglucose-dependent trehalose synthesis pathway, can increase trehalose content. However, the difference between MAL62-related trehalose synthesis and traditional uridine diphosphoglucose-dependent trehalose synthesis is not well-understood. MAL62 overexpression showed less effect in enhancing intracellular trehalose compared to TPS1 overexpression. However, MAL62 overexpression elicited trehalose synthesis before fermentation with enhanced maltose metabolism and had a similar effect on cell viability after freezing. Furthermore, MAL62 and TPS1 overexpression in the NTH1 deletion background further strengthened freezing tolerance and improved leavening ability. Our results suggest that the enhancement in freezing tolerance by MAL62 overexpression may involve multiple pathways rather than simply enhancing trehalose synthesis. The results reveal valuable insights into the relationship between maltose metabolism and freezing tolerance and may help to develop better yeast strains for enhancing fermentation characteristics of frozen dough.

Lachancea yeast species: Origin, biochemical characteristics and oenological significance

The genus Lachancea, first proposed in 2003, currently comprises 12 valid species, all found to have eight chromosomes. Lachancea spp. occupy a myriad of natural and anthropic habitats, and their geographic as well as ecological origin have been identified as key drivers in the genetic variations amongst strains of several of the species. Lachancea thermotolerans is the type species of the genus and also the most widely explored, especially for its role in fermentation environments. Indeed, L. thermotolerans is desired for its ability to acidify beer and wine through the production of lactic acid, and to enhance aroma and flavor through increased production of various compounds. Similarly, L. fermentati has been characterized for its potential contribution to the chemical composition of these beverages, albeit to a lesser extent, while other species have received little attention. Overall, members of the genus Lachancea form part of the microbiomes in many fermentation ecosystems and contribute directly or indirectly to the modulation of aroma and flavor of different products. The current review provides an overview of this genus, including the latest reports on the genetic and biochemical characteristics of member species, as well as their biotechnological potential.

MAL62 overexpression and NTH1 deletion enhance the freezing tolerance and fermentation capacity of the baker's yeast in lean dough

BACKGROUND

Trehalose is related to several types of stress responses, especially freezing response in baker's yeast (Saccharomyces cerevisiae). It is desirable to manipulate trehalose-related genes to create yeast strains that better tolerate freezing-thaw stress with improved fermentation capacity, which are in high demand in the baking industry.

RESULTS

The strain overexpressing MAL62 gene showed increased trehalose content and cell viability after prefermention-freezing and long-term frozen. Deletion of NTH1 in combination of MAL62 overexpression further strengthens freezing tolerance and improves the leavening ability after freezing-thaw stress.

CONCLUSIONS

The mutants of the industrial baker's yeast with enhanced freezing tolerance and leavening ability in lean dough were developed by genetic engineering. These strains had excellent potential industrial applications.

Enhanced freeze tolerance of baker’s yeast by overexpressed trehalose-6-phosphate synthase gene (TPS1) and deleted trehalase genes in frozen dough

Several recombinant strains with overexpressed trehalose-6-phosphate synthase gene (TPS1) and/or deleted trehalase genes were obtained to elucidate the relationships between TPS1, trehalase genes, content of intracellular trehalose and freeze tolerance of baker’s yeast, as well as improve the fermentation properties of lean dough after freezing. In this study, strain TL301TPS1 overexpressing TPS1 showed 62.92 % higher trehalose-6-phosphate synthase (Tps1) activity and enhanced the content of intracellular trehalose than the parental strain. Deleting ATH1 exerted a significant effect on trehalase activities and the degradation amount of intracellular trehalose during the first 30 min of prefermentation. This finding indicates that acid trehalase (Ath1) plays a role in intracellular trehalose degradation. NTH2 encodes a functional neutral trehalase (Nth2) that was significantly involved in intracellular trehalose degradation in the absence of the NTH1 and/or ATH1 gene. The survival ratio, freeze-tolerance ratio and relative fermentation ability of strain TL301TPS1 were approximately twice as high as those of the parental strain (BY6-9α). The increase in freeze tolerance of strain TL301TPS1 was accompanied by relatively low trehalase activity, high Tps1 activity and high residual content of intracellular trehalose. Our results suggest that overexpressing TPS1 and deleting trehalase genes are sufficient to improve the freeze tolerance of baker’s yeast in frozen dough. The present study provides guidance for the commercial baking industry as well as the research on the intracellular trehalose mobilization and freeze tolerance of baker’s yeast.

Tight control of trehalose content is required for efficient heat-induced cell elongation in Candida albicans

The ability to form hyphae in the human pathogenic fungus Candida albicans is a prerequisite for virulence. It contributes to tissue infection, biofilm formation, as well as escape from phagocytes. Cell elongation triggered by human body temperature involves the essential heat shock protein Hsp90, which negatively governs a filamentation program dependent upon the Ras-protein kinase A (PKA) pathway. Tight regulation of Hsp90 function is required to ensure fast appropriate response and maintenance of a wide range of regulatory and signaling proteins. Client protein activation by Hsp90 relies on a conformational change of the chaperone, whose ATPase activity is competitively inhibited by geldanamycin. We demonstrate a novel regulatory mechanism of heat- and Hsp90-dependent induced morphogenesis, whereby the nonreducing disaccharide trehalose acts as a negative regulator of Hsp90 release. By means of a mutant strain deleted for Gpr1, the G protein-coupled receptor upstream of PKA, we demonstrate that elevated trehalose content in that strain, resulting from misregulation of enzymatic activities involved in trehalose metabolism, disrupts the filamentation program in response to heat. Addition of geldanamycin does not result in hyphal extensions at 30 °C in the gpr1Δ/gpr1Δ mutant as it does in wild type cells. In addition, validamycin, a specific inhibitor of trehalase, the trehalose-degrading enzyme, inhibits cell elongation in response to heat and geldanamycin. These results place Gpr1 as a regulator of trehalose metabolism in C. albicans and illustrate that trehalose modulates Hsp90-dependent activation of client proteins and signaling pathways leading to filamentation in the human fungal pathogen.

Trehalose overproduction affects the stress tolerance of Kluyveromyces marxianus ambiguously

A thermotolerant Kluyveromyces marxianus mutant was developed by exposing yeast cultures repeatedly to 48°C incubation temperature, and the strain was characterized with a significantly increased trehalose content. Unexpectedly, the strain was sensitive to alcohol, osmotic and oxidative stress, which correlated with the increases in the trehalose concentrations. Intracellular glutathione levels declined in both wild-type and mutant cells when exposed to elevating incubation temperatures. Finally, we reached the surprising conclusion that neither trehalose nor glutathione metabolisms should be aimed at in future strain development programs with K. marxianus.

Quantitative transcription dynamic analysis reveals candidate genes and key regulators for ethanol tolerance in Saccharomyces cerevisiae

BACKGROUND

Derived from our lignocellulosic conversion inhibitor-tolerant yeast, we generated an ethanol-tolerant strain Saccharomyces cerevisiae NRRL Y-50316 by enforced evolutionary adaptation. Using a newly developed robust mRNA reference and a master equation unifying gene expression data analyses, we investigated comparative quantitative transcription dynamics of 175 genes selected from previous studies for an ethanol-tolerant yeast and its closely related parental strain.

RESULTS

A highly fitted master equation was established and applied for quantitative gene expression analyses using pathway-based qRT-PCR array assays. The ethanol-tolerant Y-50316 displayed significantly enriched background of mRNA abundance for at least 35 genes without ethanol challenge compared with its parental strain Y-50049. Under the ethanol challenge, the tolerant Y-50316 responded in consistent expressions over time for numerous genes belonging to groups of heat shock proteins, trehalose metabolism, glycolysis, pentose phosphate pathway, fatty acid metabolism, amino acid biosynthesis, pleiotropic drug resistance gene family and transcription factors. The parental strain showed repressed expressions for many genes and was unable to withstand the ethanol stress and establish a viable culture and fermentation. The distinct expression dynamics between the two strains and their close association with cell growth, viability and ethanol fermentation profiles distinguished the tolerance-response from the stress-response in yeast under the ethanol challenge. At least 82 genes were identified as candidate and key genes for ethanol-tolerance and subsequent fermentation under the stress. Among which, 36 genes were newly recognized by the present study. Most of the ethanol-tolerance candidate genes were found to share protein binding motifs of transcription factors Msn4p/Msn2p, Yap1p, Hsf1p and Pdr1p/Pdr3p.

CONCLUSION

Enriched background of transcription abundance and enhanced expressions of ethanol-tolerance genes associated with heat shock proteins, trehalose-glycolysis-pentose phosphate pathways and PDR gene family are accountable for the tolerant yeast to withstand the ethanol stress, maintain active metabolisms, and complete ethanol fermentation under the ethanol stress. Transcription factor Msn4p appeared to be a key regulator of gene interactions for ethanol-tolerance in the tolerant yeast Y-50316.

Trehalose promotes the survival of Saccharomyces cerevisiae during lethal ethanol stress, but does not influence growth under sublethal ethanol stress

Trehalose is known to protect cells from various environmental assaults; however, its role in the ethanol tolerance of Saccharomyces cerevisiae remains controversial. Many previous studies report correlations between trehalose levels and ethanol tolerance across a variety of strains, yet variations in genetic background make it difficult to separate the impact of trehalose from other stress response factors. In the current study, investigations were conducted on the ethanol tolerance of S. cerevisiae BY4742 and BY4742 deletion strains, tsl1Delta and nth1Delta, across a range of ethanol concentrations. It was found that trehalose does play a role in ethanol tolerance at lethal ethanol concentrations, but not at sublethal ethanol concentrations; differences of 20-40% in the intracellular trehalose concentration did not provide any growth advantage for cells incubated in the presence of sublethal ethanol concentrations. It was speculated that the ethanol concentration-dependent nature of the trehalose effect supports a mechanism for trehalose in protecting cellular proteins from the damaging effects of ethanol.

Profiling patterns of glutathione reductase inhibition by the natural product illudin S and its acylfulvene analogues

Acylfulvenes (AFs) are a class of antitumor agents with favorable cytotoxic selectivity profiles compared to their natural product precursor, illudin S. Like many alkylating agents, illudin S and AFs readily react with thiol-containing small molecules such as cysteine, glutathione and cysteine-containing peptides; reduced cellular glutathione levels can affect illudin S toxicity. Glutathione reductase (GR) is a critical cellular antioxidant enzyme that regulates the intracellular ratio of reduced-oxidized glutathione. In this study, we found that acylfulvene analogues are GR inhibitors, and evaluated aspects of the drug-enzyme interactions as compared with the structurally related natural product illudin S and the known irreversible GR inhibitor, carmustine. Acylfulvene analogues exhibited concentration-dependent GR inhibitory activity with micromolar IC(50)s; however, up to 2 mM illudin S did not inhibit GR activity. The absence of NADPH attenuates GR inhibition by AFs and the presence of glutathione disulfide (GSSG), the natural GR substrate, which binds to the enzyme active site, has a minimal effect in protecting GR from AFs. Furthermore, each compound can induce GR conformation changes independent of the presence of NADPH or GSSG. These results, together with gel-filtration analysis results and mass spectrometry data, indicate AF is a reversible inhibitor and HMAF an irreversible inhibitor that can form a bis-adduct with GR by reacting with active site cysteines. Finally in a cell-based assay, illudin S and HMAF were found to inhibit GR activity, but this inhibition was not associated with the reduction of GR levels in the cell. A model accounting for differences in mechanisms of GR inhibition by the series of compounds is discussed.

The induction of trehalose and glycerol in Saccharomyces cerevisiae in response to various stresses

Trehalose and glycerol have been implicated as potential stress protectants that accumulate in yeasts during various stress conditions. We investigated the levels of glycerol and trehalose and the expression profiles of genes involved in their metabolism to determine their involvement in the response of Saccharomyces cerevisiae XQ1 to thermal, sorbitol and ethanol stresses. The results showed that the genes involved in the synthesis and degradation of trehalose and glycerol were stress induced, and that trehalose and glycerol were synthesized simultaneously during the initial stages (a sensitive response period) of diverse stress treatments. Trehalose accumulated markedly under heat treatment, but not under sorbitol or ethanol stress, whereas glycerol accumulated strikingly under sorbitol stress conditions. Interestingly, extracellular trehalose seemed to be involved in protecting cells from damage under unfavorable conditions. Moreover, our results suggest that the stress-activated futile ATP cycles of trehalose and glycerol turnover are of general importance during cellular stress adaptation.

The three trehalases Nth1p, Nth2p and Ath1p participate in the mobilization of intracellular trehalose required for recovery from saline stress in Saccharomyces cerevisiae

Trehalose accumulation is a common response to several stresses in the yeast Saccharomyces cerevisiae. This metabolite protects proteins and membrane lipids from structural damage and helps cells to maintain integrity. Based on genetic studies, degradation of trehalose has been proposed as a required mechanism for growth recovery after stress, and the neutral trehalase Nth1p as the unique degradative activity involved. Here we constructed a collection of mutants for several trehalose metabolism and transport genes and analysed their growth and trehalose mobilization profiles during experiments of saline stress recovery. The behaviour of the triple Deltanth1Deltanth2Deltaath1 and quadruple Deltanth1Deltanth2Deltaath1Deltaagt1 mutant strains in these experiments demonstrates the participation of the three known yeast trehalases Nth1p, Nth2p and Ath1p in the mobilization of intracellular trehalose during growth recovery after saline stress, rules out the participation of the Agt1p H(+)-disaccharide symporter, and allows us to propose the existence of additional new mechanisms for trehalose mobilization after saline stress.

Yeast responses to stresses associated with industrial brewery handling: Figure 1

During brewery handling, production strains of yeast must respond to fluctuations in dissolved oxygen concentration, pH, osmolarity, ethanol concentration, nutrient supply and temperature. Fermentation performance of brewing yeast strains is dependent on their ability to adapt to these changes, particularly during batch brewery fermentation which involves the recycling (repitching) of a single yeast culture (slurry) over a number of fermentations (generations). Modern practices, such as the use of high-gravity worts and preparation of dried yeast for use as an inoculum, have increased the magnitude of the stresses to which the cell is subjected. The ability of yeast to respond effectively to these conditions is essential not only for beer production but also for maintaining the fermentation fitness of yeast for use in subsequent fermentations. During brewery handling, cells inhabit a complex environment and our understanding of stress responses under such conditions is limited. The advent of techniques capable of determining genomic and proteomic changes within the cell is likely vastly to improve our knowledge of yeast stress responses during industrial brewery handling.

Brewing yeast genomes and genome-wide expression and proteome profiling during fermentation

The genome structure, ancestry and instability of the brewing yeast strains have received considerable attention. The hybrid nature of brewing lager yeast strains provides adaptive potential but yields genome instability which can adversely affect fermentation performance. The requirement to differentiate between production strains and assess master cultures for genomic instability has led to significant adoption of specialized molecular tool kits by the industry. Furthermore, the development of genome-wide transcriptional and protein expression technologies has generated significant interest from brewers. The opportunity presented to explore, and the concurrent requirement to understand both, the constraints and potential of their strains to generate existing and new products during fermentation is discussed.

The natural osmolyte trehalose is a positive regulator of the heat-induced activity of yeast heat shock transcription factor

In Saccharomyces cerevisiae, the intracellular concentration of trehalose increases rapidly in response to many environmental stresses, including heat shock. These high trehalose levels have been correlated with tolerance to adverse conditions and led to the model that trehalose functions as a chemical cochaperone. Here, we show that the transcriptional activity of Hsf1 during the heat shock response depends on trehalose. Strains with low levels of trehalose have a diminished transcriptional response to heat shock, while strains with high levels of trehalose have an enhanced transcriptional response to heat shock. The enhanced transcriptional response does not require the other heat-responsive transcription factors Msn2/4 but is dependent upon heat and Hsf1. In addition, the phosphorylation levels of Hsf1 correlate with both transcriptional activity and the presence of trehalose. These in vivo results support a new role for trehalose, where trehalose directly modifies the dynamic range of Hsf1 activity and therefore influences heat shock protein mRNA levels in response to stress.

Trehalose and glycerol stabilize and renature yeast inorganic pyrophosphatase inactivated by very high temperatures

A number of naturally occurring small organic molecules, primarily involved in maintaining osmotic pressure in the cell, display chaperone-like activity, stabilizing the native conformation of proteins, and protecting them from various kinds of stress. Most of them are sugars, polyols, amino acids or methylamines. Similar to molecular chaperones, most of these compounds have no substrate specificity, but some specifically stabilize certain proteins. In the present work, the capacity of trehalose and glycerol, two well-known osmolytes, to stabilize and renature inorganic pyrophosphatase is demonstrated. Both trehalose and glycerol significantly protect pyrophosphatase against thermoinactivation achieved by incubating the enzyme at temperatures up to 95 degrees C, and allow the enzyme already inactivated in the presence of these osmolytes to renature upon incubation at low temperatures. To the best of our knowledge, there are no data on the effects of these compounds on renaturation of thermoinactivated proteins. The correlation between the recovery of enzyme activity and structural changes indicated by fluorescence spectroscopy contribute to better understanding of the protein stabilization mechanism.

Acid- and pressure-induced (un)folding of yeast glutathione reductase: competition between protein oligomerization and aggregation

Glutathione reductase (GR) is a homodimeric flavoenzyme involved in cellular defense against oxidative stress. In the present study, we have used a combination of acidic pH and hydrostatic pressure to investigate the (un)folding transition of yeast GR. Our results indicate that at pH 2 a distinct partially folded state is stabilized, as judged by intrinsic fluorescence, bis ANS binding and circular dichroism (CD) analysis. Further characterization of this partially folded state by size exclusion chromatography revealed that it corresponds to expanded GR monomers. CD analysis at pH 2 showed a significant loss of secondary structure. The partially folded GR monomers stabilized at pH 2 were fully and reversibly unfolded using hydrostatic pressure (up to 3.5kbar) as a thermodynamic perturbant. By contrast, return to physiological pH after exposure to acidic pH led to a competing reaction between refolding dimerization and aggregation of GR. These results support the notion that a partially folded intermediate state is not only critical for folding of GR but also appears to be a seed for protein aggregation.

Trehalose and 6-aminohexanoic acid stabilize and renature glucose-6-phosphate dehydrogenase inactivated by glycation and by guanidinium hydrochloride

A number of naturally occurring small organic molecules, primarily involved in maintaining osmotic pressure in the cell, display chaperone-like activity, stabilizing the native conformation of proteins and protecting them from various kinds of stress. Most of them are sugars, polyols, amino acids or methylamines. In addition to their intrinsic protein-stabilizing activity, these small organic stress molecules regulate the activity of some molecular chaperones, and may stabilize the folded state of proteins involved in unfolding or in misfolding diseases, such as Alzheimer's and Parkinson's diseases, or alpha1-antitrypsin deficiency and cystic fibrosis, respectively. Similar to molecular chaperones, most of these compounds have no substrate specificity, but some specifically stabilize certain proteins, e.g., 6-aminohexanoic acid (AHA) stabilizes apolipoprotein A. In the present work, the capacity of 6-aminohexanoic acid to stabilize non-specifically other proteins is demonstrated. Both trehalose and AHA significantly protect glucose-6-phosphate dehydrogenase (G6PD) against glycation-induced inactivation, and renatured enzyme already inactivated by glycation and by guanidinium hydrochloride (GuHCl). To the best of our knowledge, there are no data on the effect of these compounds on protein glycation. The correlation between the recovery of enzyme activity and structural changes indicated by fluorescence spectroscopy and Western blotting contribute to better understanding of the protein stabilization mechanism.

Identification of new genes regulated by the Crt1 transcription factor, an effector of the DNA damage checkpoint pathway in Saccharomyces cerevisiae

The Crt1 (RFX1) protein in Saccharomyces cerevisiae is an effector of the DNA damage checkpoint pathway. It recognizes a 13-bp cis-regulatory element in the 5'-untranslated region (5'-UTR) of the ribonucleotide reductase genes RNR2, RNR3, and RNR4; the HUG1 gene; and itself. We calculated the weight matrix representing the Crt1p binding site motif according to analysis of the 5'-UTR sequences of the genes that are under its regulation. We subsequently searched the 5'-UTR sequences of all the genes in the yeast genome for the occurrence of this motif. The motif was found in regulatory regions of 30 genes. A statistical analysis showed that it is unlikely that a random gene cluster contains the motif conserved as well as the Crt1p binding site. Analysis of microarray data provided supporting evidence for five putative Crt1p targets: FSH3, YLR345W, UBC5, NDE2, and NTH2. We used reverse transcription-PCR to compare the expression levels of these genes in wild-type and crt1Delta strains. Our results indicated that FSH3, YLR345W, and NTH2 are indeed under the regulation of Crt1p. Sequence analysis of the FSH3p indicated that this protein may be involved in folate metabolism either by carrying serine hydrolase activity required for the novel metabolic pathway involving dihydrofolate reductase (DHFR) or by directly interacting with the DHFR enzyme. We postulate that Crt1p may influence deoxyribonucleotide synthesis not only by regulating expression of the RNR genes but also by modulating DHFR activity. FSH3p shares significant sequence similarity with the product of the human tumor suppressor gene OVCA2. YLR345Wp and NTH2p are enzymes involved in the central metabolism under stress conditions.