Glutathione is required for growth and prespore cell differentiation in Dictyostelium

Characterization of glutathione S-transferase enzymes in Dictyostelium discoideum suggests a functional role for the GSTA2 isozyme in cell proliferation and development

In this report, we extend our previous characterization of Dictyostelium discoideum glutathione S-transferase (DdGST) enzymes that are expressed in the eukaryotic model organism. Transcript profiling of gstA1-gstA5 (alpha class) genes in vegetative, log phase cells identified gstA2 and gstA3 with highest expression (6-7.5-fold, respectively) when compared to other gstA transcripts. Marked reductions in all gstA transcripts occurred under starvation conditions, with gstA2 and gstA3 exhibiting the largest decreases (-96% and -86.6%, respectively). When compared to their pre-starvation levels, there was also a 60 percent reduction in total GST activity. Glutathione (GSH) pull-down assay and mass spectroscopy detected three isozymes (DdGSTA1, DdGSTA2 and DdGSTA3) that were predominantly expressed in vegetative cells. Biochemical and kinetic comparisons between rDdGSTA2 and rDdGSTA3 shows higher activity of rDdGSTA2 to the CDNB (1-chloro-2,4-dinitrobenzene) substrate. RNAi-mediated knockdown of endogenous DdGSTA2 caused a 60 percent reduction in proliferation, delayed development, and altered morphogenesis of fruiting bodies, whereas overexpression of rDdGSTA2 enzyme had no effect. These findings corroborate previous studies that implicate a role for phase II GST enzymes in cell proliferation, homeostasis, and development in eukaryotic cells.

Methylglyoxal-Scavenging Enzyme Activities Trigger Erythroascorbate Peroxidase and Cytochrome c Peroxidase in Glutathione-Depleted Candida albicans

γ-Glutamylcysteine synthetase (Gcs1) and glutathione reductase (Glr1) activity maintains minimal levels of cellular methylglyoxal in Candida albicans. In glutathione-depleted Δgcs1, we previously saw that NAD(H)-linked methylglyoxal oxidoreductase (Mgd1) and alcohol dehydrogenase (Adh1) are the most active methylglyoxal scavengers. With methylglyoxal accumulation, disruptants lacking MGD1 or ADH1 exhibit a poor redox state. However, there is little convincing evidence for a reciprocal relationship between methylglyoxal scavenger genes-disrupted mutants and changes in glutathione-(in)dependent redox regulation. Herein, we attempt to demonstrate a functional role for methylglyoxal scavengers, modeled on a triple disruptant (Δmgd1/Δadh1/Δgcs1), to link between antioxidative enzyme activities and their metabolites in glutathione-depleted conditions. Despite seeing elevated methylglyoxal in all of the disruptants, the result saw a decrease in pyruvate content in Δmgd1/Δadh1/Δgcs1 which was not observed in double gene-disrupted strains such as Δmgd1/Δgcs1 and Δadh1/Δgcs1. Interestingly, Δmgd1/Δadh1/Δgcs1 exhibited a significantly decrease in H₂O₂ and superoxide which was also unobserved in Δmgd1/Δgcs1 and Δadh1/Δgcs1. The activities of the antioxidative enzymes erythroascorbate peroxidase and cytochrome c peroxidase were noticeably higher in Δmgd1/Δadh1/Δgcs1 than in the other disruptants. Meanwhile, Glr1 activity severely diminished in Δmgd1/Δadh1/Δgcs1. Monitoring complementary gene transcripts between double gene-disrupted Δmgd1/Δgcs1 and Δadh1/Δgcs1 supported the concept of an unbalanced redox state independent of the Glr1 activity for Δmgd1/Δadh1/Δgcs1. Our data demonstrate the reciprocal use of Eapx1 and Ccp1 in the absence of both methylglyoxal scavengers; that being pivotal for viability in non-filamentous budding yeast.

Alcohol dehydrogenase 1 and NAD(H)-linked methylglyoxal oxidoreductase reciprocally regulate glutathione-dependent enzyme activities in Candida albicans

Glutathione reductase (Glr1) activity controls cellular glutathione and reactive oxygen species (ROS). We previously demonstrated two predominant methylglyoxal scavengers-NAD(H)-linked methylglyoxal oxidoreductase (Mgd1) and alcohol dehydrogenase 1 (Adh1)-in glutathione-depleted γ-glutamyl cysteinyl synthetase-disrupted Candida albicans. However, experimental evidence for Candida pathophysiology lacking the enzyme activities of Mgd1 and Adh1 on glutathione-dependent redox regulation remains unclear. Herein, we have aimed to demonstrate that glutathione-dependent enzyme activities coupled with cellular ROS changes is regulated by methylglyoxal accumulation in Δmgd1/Δadh1 double disruptants. Δmgd1/Δadh1 showed severe growth defects and G1-phase cell cycle arrest. The observed complementary and reciprocal methylglyoxal-oxidizing and methylglyoxalreducing activities between Δmgd1 and Δadh1 were not always exhibited in Δmgd1/Δadh1. Although intracellular accumulation of methylglyoxal and pyruvate was shown in all disruptants, to a greater or lesser degree, methylglyoxal was particularly accumulated in the Δmgd1/Δadh1 double disruptant. While cellular ROS significantly increased in Δmgd1 and Δadh1 as compared to the wild-type, Δmgd1/Δadh1 underwent a decrease in ROS in contrast to Δadh1. Despite the experimental findings underlining the importance of the undergoing unbalanced redox state of Δmgd1/Δadh1, glutathione-independent antioxidative enzyme activities did not change during proliferation and filamentation. Contrary to the significantly lowered glutathione content and Glr1 enzyme activity, the activity staining-based glutathione peroxidase activities concomitantly increased in this mutant. Additionally, the enhanced GLR1 transcript supported our results in Δmgd1/Δadh1, indicating that deficiencies of both Adh1 and Mgd1 activities stimulate specific glutathione-dependent enzyme activities. This suggests that glutathione-dependent redox regulation is evidently linked to C. albicans pathogenicity under the control of methylglyoxal-scavenging activities.

Cytochrome c peroxidase regulates intracellular reactive oxygen species and methylglyoxal via enzyme activities of erythroascorbate peroxidase and glutathione-related enzymes in Candida albicans

D-erythroascorbate peroxidase (EAPX1) deficiency causes glutathione deprivation, leading to the accumulation of methylglyoxal and reactive oxygen species (ROS), and especially, induction of cytochrome c peroxidase (Ccp1) in Candida albicans. Nevertheless, reciprocal effects between changes in Ccp1 activity and the antioxidative D-erythroascorbic acid- and glutathione-dependent redox status, which reflects methylglyoxal biosynthesis altering pathophysiology are unclear in eukaryotes. To elucidate the effect of CCP1 expression on EAPX1 and glutathione reductase (Glr1) activity-mediated D-erythroascorbic acid biosynthesis and redox homeostasis, the CCP1 gene was disrupted and overexpressed. First, we demonstrated both glutathione-independent and-dependent metabolite contents and their corresponding gene transcripts and enzyme activities (i.e., Ccp1, catalase-peroxidase [KatG], superoxide dismutase [Sod], Eapx1, and Glr1) in CCP1 mutants. Second, methylglyoxal-oxidizing alcohol dehydrogenase (Adh1) and methylglyoxal-reducing oxidoreductase activity on glycolytic methylglyoxal and pyruvate production and NAD(P)H content were determined in these mutants. Contrary to our expectation, CCP1 disruption (42.19±3.22nmolO₂h^-1mgwetcell^-1) failed to affect cell respiration compared to the wild-type strain (41.62±7.11nmolO₂h^-1mgwetcell^-1) under cyanide treatment, and in contrast to hydrogen peroxide (H₂O₂) treatment (21.74±1.03nmol O₂h^-1mgwetcell^-1). Additionally, Ccp1 predominantly detoxified H₂O₂ rather than negligible scavenging activities towards methylglyoxal and other oxidants. CCP1 deficiency stimulated Sod and Adh1 activity but downregulated Glr1, Eapx1, catalase, and peroxidase activity while enhancing KatG, EAPX1, and GLR1 transcription by decreasing glutathione and D-erythroascorbic acid and increasing pyruvate. Noticeably, the ROS-accumulating CCP1-deficient mutant maintained steady-state levels of methylglyoxal, which was revealed to be regulated by methylglyoxal-oxidizing and -reducing activity with drastic changes in NAD(P)H. We confirmed and clarified our results by showing that CCP1/EAPX1 double disruptants underwent severe growth defects due to the D-erythroascorbic acid and glutathione depletion because of pyruvate overaccumulation. These observations were made in both budding and hyphal-growing CCP1 mutants. The revealed metabolic network involving Ccp1 and other redox regulators affected ROS and methylglyoxal through D-erythroascorbic acid and glutathione-dependent metabolites, thereby influencing dimorphism. This is the first report of the Ccp1-mediated D-erythroascorbic acid and glutathione biosynthesis accompanying methylglyoxal scavengers for full fungal virulence.

Methylglyoxal upregulates Dictyostelium discoideum slug migration by triggering glutathione reductase and methylglyoxal reductase activity

Glutathione (GSH)-deprived Dictyostelium discoideum accumulates methylglyoxal (MG) and reactive oxygen species (ROS) during vegetative growth. However, the reciprocal effects of the production and regulation of these metabolites on differentiation and cell motility are unclear. Based on the inhibitory effects of γ-glutamylcysteine synthetase (gcsA) disruption and GSH reductase (gsr) overexpression on aggregation and culmination, respectively, we overexpressed GSH-related genes encoding superoxide dismutase (Sod2), catalase (CatA), and Gcs, in D. discoideum. Wild-type KAx3 and gcsA-overexpressing (gcsA^OE) slugs maintained GSH levels at levels of approximately 2.1-fold less than the reference GSH synthetase-overexpressing mutant; their GSH levels did not correlate with slug migration ability. Through prolonged KAx3 migration by treatment with MG and H₂O₂, we found that MG increased after the mound stage in this strain, with a 2.6-fold increase compared to early developmental stages; in contrast, ROS were maintained at high levels throughout development. While the migration-defective sod2- and catA-overexpressing mutant slugs (sod2^OE and catA^OE) decreased ROS levels by 50% and 53%, respectively, these slugs showed moderately decreased MG levels (36.2±5.8 and 40.7±1.6nmolg^-1 cells wet weight, P<0.05) compared to the parental strain (54.2±3.5nmolg^-1). Importantly, defects in the migration of gcsA^OE slugs decreased MG considerably (13.8±4.2nmolg^-1, P<0.01) along with a slight decrease in ROS. In contrast to the increase observed in migrating sod2^OE and catA^OE slugs by treatment with MG and H₂O₂, the migration of gcsA^OE slugs appeared unaffected. This behavior was caused by MG-triggered Gsr and NADPH-linked aldolase reductase activity, suggesting that GSH biosynthesis in gcsA^OE slugs is specifically used for MG-scavenging activity. This is the first report showing that MG upregulates slug migration via MG-scavenging-mediated differentiation.

Schiff bases of putrescine with methylglyoxal protect from cellular damage caused by accumulation of methylglyoxal and reactive oxygen species in Dictyostelium discoideum

Polyamines protect protein glycation in cells against the advanced glycation end product precursor methylglyoxal, which is inevitably produced during glycolysis, and the enzymes that detoxify this α-ketoaldehyde have been widely studied. Nonetheless, nonenzymatic methylglyoxal-scavenging molecules have not been sufficiently studied either in vitro or in vivo. Here, we hypothesized reciprocal regulation between polyamines and methylglyoxal modeled in Dictyostelium grown in a high-glucose medium. We based our hypothesis on the reaction between putrescine and methylglyoxal in putrescine-deficient (odc^-) or putrescine-overexpressing (odc^oe) cells. In these strains, growth and cell cycle were found to be dependent on cellular methylglyoxal and putrescine contents. The odc^- cells showed growth defects and underwent G1 phase cell cycle arrest, which was efficiently reversed by exogenous putrescine. Cellular methylglyoxal, reactive oxygen species (ROS), and glutathione levels were remarkably changed in odc^oe cells and odc̄ cells. These results revealed that putrescine may act as an intracellular scavenger of methylglyoxal and ROS. Herein, we observed interactions of putrescine and methylglyoxal via formation of a Schiff base complex, by UV-vis spectroscopy, and confirmed this adduct by liquid chromatography with mass spectrometry via electrospray ionization. Schiff bases were isolated, analyzed, and predicted to have molecular masses ranging from 124 to 130. We showed that cellular putrescine-methylglyoxal Schiff bases were downregulated in proportion to the levels of endogenous or exogenous putrescine and glutathione in the odc mutants. The putrescine-methylglyoxal Schiff base affected endogenous metabolite levels. This is the first report showing that cellular methylglyoxal functions as a signaling molecule through reciprocal interactions with polyamines by forming Schiff bases.

Glutathione upregulates cAMP signalling via G protein alpha 2 during the development of Dictyostelium discoideum

Despite the importance of glutathione in Dictyostelium, the role of glutathione synthetase (gshB/GSS) has not been clearly investigated. In this study, we observed that increasing glutathione content by constitutive expression of gshB leads to mound-arrest and defects in 3',5'-cyclic adenosine monophosphate (cAMP)-mediated aggregation and developmental gene expression. The overexpression of gpaB encoding G protein alpha 2 (Gα2), an essential component of the cAMP signalling pathway, results in a phenotype similar to that caused by gshB overexpression, whereas gpaB knockdown in gshB-overexpressing cells partially rescues the above-mentioned phenotypic defects. Furthermore, Gα2 is highly enriched at the plasma membrane of gshB-overexpressing cells compared to wild-type cells. Therefore, our findings suggest that glutathione upregulates cAMP signalling via Gα2 modulation during Dictyostelium development.

Candida albicans glutathione reductase downregulates Efg1-mediated cyclic AMP/protein kinase A pathway and leads to defective hyphal growth and virulence upon decreased cellular methylglyoxal content accompanied by activating alcohol dehydrogenase and glycolytic enzymes

BACKGROUND

Glutathione reductase maintains the glutathione level in a reduced state. As previously demonstrated, glutathione is required for cell growth/division and its biosynthesizing-enzyme deficiency causes methylglyoxal accumulation. However, experimental evidences for reciprocal relationships between Cph1-/Efg1-mediated signaling pathway regulation and methylglyoxal production exerted by glutathione reductase on yeast morphology remain unclear.

METHODS

Glutathione reductase (GLR1) disruption/overexpression were performed to investigate aspects of pathological/morphological alterations in Candida albicans. These assumptions were proved by observations of cellular susceptibility to oxidants and thiols, and measurements of methylglyoxal and glutathione content in hyphal-inducing conditions mainly through the activity of GLR1-overexpressing cells. Additionally, the transcriptional/translational levels of bioenergetic enzymes and dimorphism-regulating protein kinases were examined in the strain.

RESULTS

The GLR1-deficient strain was non-viable when GLR1 expression under the control of a CaMAL2 promoter was conditionally repressed, despite partial rescue of growth by exogenous thiols. During filamentation, non-growing hyphal GLR1-overexpressing cells exhibited resistance against oxidants and cellular methylglyoxal was significantly decreased, which concomitantly increased expressions of genes encoding energy-generating enzymes, including fructose-1,6-bisphosphate aldolase, glyceraldehyde-3-phosphate dehydrogenase, and alcohol dehydrogenase (ADH1), with remarkable repression of Efg1-signaling cascades.

CONCLUSIONS

This is the first report that GLR1-triggered Efg1-mediated signal transduction repression strictly reduces dimorphic switching and virulence by maintaining the basal level of methylglyoxal following the enhanced gene expressions of glycolytic enzymes and ADH1.

GENERAL SIGNIFICANCE

The Efg1 downregulatory mechanism by GLR1 expression has possibilities to involve in other complex network of signal pathways. Understanding how GLR1 overexpression affects multiple signaling pathways can help identify attractive targets for antifungal drugs.

Relevance of the bioavailable fraction of DDT and its metabolites in freshwater sediment toxicity: New insight into the mode of action of these chemicals on Dictyostelium discoideum

In this work, the toxicity of lake sediments contaminated with DDT and its metabolites DDD and DDE (collectively, DDX) was evaluated with widely used toxicity tests (i.e., Vibrio fischeri, Daphnia magna, Pseudokirchneriella subcapitata, and Lumbriculus variegatus) and with the social amoeba Dictyostelium discoideum, a model organism that is also suitable for studying pollutant-induced alterations at the molecular and cellular levels. Although the DDX concentration in the sediments was high (732.5 ppb), the results suggested a minimal environmental risk; in fact, no evidence of harmful effects was found using the different bioassays or when we considered the results of more sensitive sublethal biomarkers in D. discoideum amoebae. In line with the biological results, the chemical data showed that the concentration of DDX in the pore water (in general a highly bioavailable phase) showed a minimal value (0.0071ppb). To confirm the importance of the bioavailability of the toxic chemicals in determining their biological effects and to investigate the mechanisms of DDX toxicity, we exposed D. discoideum amoebae to 732.5ppb DDX in water solution. DDX had no effect on cell viability; however, a strong reduction in amoebae replication rate was observed, which depended mainly on a reduction in endocytosis rate and on lysosomal and mitochondrial alterations. In the presence of a moderate and transient increase in reactive oxygen species, the glutathione level in DDX-exposed amoebae drastically decreased. These results highlight that studies of the bioavailability of pollutants in environmental matrices and their biological effects are essential for site-specific ecological risk assessment. Moreover, glutathione depletion in DDX-exposed organisms is a new finding that could open the possibility of developing new pesticide mixtures that are more effective against DDT-resistant malaria vectors.

Glutathione S-transferase 4 is a putative DIF-binding protein that regulates the size of fruiting bodies in Dictyostelium discoideum

In the development of the cellular slime mold Dictyostelium discoideum, two chlorinated compounds, the differentiation-inducing factors DIF-1 and DIF-2, play important roles in the regulation of both cell differentiation and chemotactic cell movement. However, the receptors of DIFs and the components of DIF signaling systems have not previously been elucidated. To identify the receptors for DIF-1 and DIF-2, we here performed DIF-conjugated affinity gel chromatography and liquid chromatography-tandem mass spectrometry and identified the glutathione S-transferase GST4 as a major DIF-binding protein. Knockout and overexpression mutants of gst4 (gst4^- and gst4^OE, respectively) formed fruiting bodies, but the fruiting bodies of gst4^- cells were smaller than those of wild-type Ax2 cells, and those of gst4^OE cells were larger than those of Ax2 cells. Both chemotaxis regulation and in vitro stalk cell formation by DIFs in the gst4 mutants were similar to those of Ax2 cells. These results suggest that GST4 is a DIF-binding protein that regulates the sizes of cell aggregates and fruiting bodies in D. discoideum.

Polyamines regulate cell growth and cellular methylglyoxal in high-glucose medium independently of intracellular glutathione

Polyamines can presumably inhibit protein glycation, when associated with the methylglyoxal inevitably produced during glycolysis. Herein, we hypothesized a nonenzymatic interaction between putrescine and methylglyoxal in putrescine-deficient or -overexpressing Dictyostelium cells in high-glucose medium, which can control methylglyoxal production. Putrescine was essentially required for growth rescue accompanying methylglyoxal detoxification when cells underwent growth defect and cell cycle G1-arrest when supplemented with high glucose. Furthermore, methylglyoxal regulation by putrescine seemed to be a parallel pathway independent of the changes in cellular glutathione content in high-glucose medium. Consequently, we suggest that Dictyostelium cells need polyamines for normal growth and cellular methylglyoxal regulation.

Integrative Model of Oxidative Stress Adaptation in the Fungal Pathogen Candida albicans

The major fungal pathogen of humans, Candida albicans, mounts robust responses to oxidative stress that are critical for its virulence. These responses counteract the reactive oxygen species (ROS) that are generated by host immune cells in an attempt to kill the invading fungus. Knowledge of the dynamical processes that instigate C. albicans oxidative stress responses is required for a proper understanding of fungus-host interactions. Therefore, we have adopted an interdisciplinary approach to explore the dynamical responses of C. albicans to hydrogen peroxide (H₂O₂). Our deterministic mathematical model integrates two major oxidative stress signalling pathways (Cap1 and Hog1 pathways) with the three major antioxidant systems (catalase, glutathione and thioredoxin systems) and the pentose phosphate pathway, which provides reducing equivalents required for oxidative stress adaptation. The model encapsulates existing knowledge of these systems with new genomic, proteomic, transcriptomic, molecular and cellular datasets. Our integrative approach predicts the existence of alternative states for the key regulators Cap1 and Hog1, thereby suggesting novel regulatory behaviours during oxidative stress. The model reproduces both existing and new experimental observations under a variety of scenarios. Time- and dose-dependent predictions of the oxidative stress responses for both wild type and mutant cells have highlighted the different temporal contributions of the various antioxidant systems during oxidative stress adaptation, indicating that catalase plays a critical role immediately following stress imposition. This is the first model to encapsulate the dynamics of the transcriptional response alongside the redox kinetics of the major antioxidant systems during H₂O₂ stress in C. albicans.

Glutathione initiates the development of Dictyostelium discoideum through the regulation of YakA

Reduced glutathione (GSH) is an essential metabolite that performs multiple indispensable roles during the development of Dictyostelium. We show here that disruption of the gene (gcsA-) encoding y-glutamylcysteine synthetase, an essential enzyme in GSH biosynthesis, inhibited aggregation, and that this developmental defect was rescued by exogenous GSH, but not by other thiols or antioxidants. In GSH-depleted gcsA- cells, the expression ofa growth-stage-specific gene (cprD) was not inhibited, and we did not detect the expression of genes that encode proteins required for early development (cAMP receptor, carA/cAR1; adenylyl cyclase, acaA/ACA; and the catalytic subunit of protein kinase A, pkaC/PKA-C). The defects in gcsA cells were not restored by cAMP stimulation or by cAR1 expression. Further, the expression of yakA, which initiates development and induces the expression of PKA-C, ACA, and cAR1, was regulated by the intracellular concentration of GSH. Constitutive expression of YakA in gcsA- cells (YakA(OE)/gcsA-) rescued the defects in developmental initiation and the expression of early developmental genes in the absence of GSH. Taken together, these findings suggest that GSH plays an essential role in the transition from growth to development by modulating the expression of the genes encoding YakA as well as components thatact downstream in the YakA signaling pathway.

Robust G2 pausing of adult stem cells in Hydra

Hydra is a freshwater hydrozoan polyp that constantly renews its two tissue layers thanks to three distinct stem cell populations that cannot replace each other, epithelial ectodermal, epithelial endodermal, and multipotent interstitial. These adult stem cells, located in the central body column, exhibit different cycling paces, slow for the epithelial, fast for the interstitial. To monitor the changes in cell cycling in Hydra, we established a fast and efficient flow cytometry procedure, which we validated by confirming previous findings, as the Nocodazole-induced reversible arrest of cell cycling in G2/M, and the mitogenic signal provided by feeding. Then to dissect the cycling and differentiation behaviors of the interstitial stem cells, we used the AEP_cnnos1 and AEP_Icy1 transgenic lines that constitutively express GFP in this lineage. For the epithelial lineages we used the sf-1 strain that rapidly eliminates the fast cycling cells upon heat-shock and progressively becomes epithelial. This study evidences similar cycling patterns for the interstitial and epithelial stem cells, which all alternate between the G2 and S-phases traversing a minimal G1-phase. We also found interstitial progenitors with a shorter G2 that pause in G1/G0. At the animal extremities, most cells no longer cycle, the epithelial cells terminally differentiate in G2 and the interstitial progenitors in G1/G0. At the apical pole ~80% cells are post-mitotic differentiated cells, reflecting the higher density of neurons and nematocytes in this region. We discuss how the robust G2 pausing of stem cells, maintained over weeks of starvation, may contribute to regeneration.

NAD(+)-linked alcohol dehydrogenase 1 regulates methylglyoxal concentration in Candida albicans

We purified a fraction that showed NAD(+)-linked methylglyoxal dehydrogenase activity, directly catalyzing methylglyoxal oxidation to pyruvate, which was significantly increased in glutathione-depleted Candida albicans. It also showed NADH-linked methylglyoxal-reducing activity. The fraction was identified as a NAD(+)-linked alcohol dehydrogenase (ADH1) through mass spectrometric analyses. In ADH1-disruptants of both the wild type and glutathione-depleted cells, the intracellular methylglyoxal concentration increased significantly; defects in growth, differentiation, and virulence were observed; and G2-phase arrest was induced.

Redox regulation of the tumor suppressor PTEN by glutathione

Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) expressed in Saccharomyces cerevisiae was reversibly oxidized by hydrogen peroxide and reduced by cellular reductants. Reduction of hPTEN was delayed in each of S. cerevisiae gsh1Delta and gsh2Delta mutants. Expression of gamma-glutamylcysteine synthetase Gsh1 in the gsh1Delta mutant rescued regeneration rate of hPTEN. Oxidized hPTEN was reduced by glutathione in a concentration- and time-dependent manner. Glutathionylated PTEN was detected. Incubation of 293T cells with BSO and knockdown expression of GCLc in HeLa cells by siRNA resulted in the delay of reduction of oxidized PTEN. Also, in HeLa cells transfected with GCLc siRNA, stimulation with epidermal growth factor resulted in the increase of oxidized PTEN and phosphorylation of Akt. These results suggest that the reduction of oxidized hPTEN is mediated by glutathione.

Dictyostelium transcriptional responses to Pseudomonas aeruginosa: common and specific effects from PAO1 and PA14 strains

BACKGROUND

Pseudomonas aeruginosa is one of the most relevant human opportunistic bacterial pathogens. Two strains (PAO1 and PA14) have been mainly used as models for studying virulence of P. aeruginosa. The strain PA14 is more virulent than PAO1 in a wide range of hosts including insects, nematodes and plants. Whereas some of the differences might be attributable to concerted action of determinants encoded in pathogenicity islands present in the genome of PA14, a global analysis of the differential host responses to these P. aeruginosa strains has not been addressed. Little is known about the host response to infection with P. aeruginosa and whether or not the global host transcription is being affected as a defense mechanism or altered in the benefit of the pathogen. Since the social amoeba Dictyostelium discoideum is a suitable host to study virulence of P. aeruginosa and other pathogens, we used available genomic tools in this model system to study the transcriptional host response to P. aeruginosa infection.

RESULTS

We have compared the virulence of the P. aeruginosa PAO1 and PA14 using D. discoideum and studied the transcriptional response of the amoeba upon infection. Our results showed that PA14 is more virulent in Dictyostelium than PA01using different plating assays. For studying the differential response of the host to infection by these model strains, D. discoideum cells were exposed to either P. aeruginosa PAO1 or P. aeruginosa PA14 (mixed with an excess of the non-pathogenic bacterium Klebsiella aerogenes as food supply) and after 4 hours, cellular RNA extracted. A three-way comparison was made using whole-genome D. discoideum microarrays between RNA samples from cells treated with the two different strains and control cells exposed only to K. aerogenes. The transcriptomic analyses have shown the existence of common and specific responses to infection. The expression of 364 genes changed in a similar way upon infection with one or another strain, whereas 169 genes were differentially regulated depending on whether the infecting strain was either P. aeruginosa PAO1 or PA14. Effects on metabolism, signalling, stress response and cell cycle can be inferred from the genes affected.

CONCLUSION

Our results show that pathogenic Pseudomonas strains invoke both a common transcriptional response from Dictyostelium and a strain specific one, indicating that the infective process of bacterial pathogens can be strain-specific and is more complex than previously thought.

Localization of members of the gamma-glutamyl transpeptidase family identifies sites of glutathione and glutathione S-conjugate hydrolysis

gamma-Glutamyl transpeptidases (GGTs) are essential for hydrolysis of the tripeptide glutathione (gamma-glutamate-cysteine-glycine) and glutathione S-conjugates since they are the only enzymes known to cleave the amide bond linking the gamma-carboxylate of glutamate to cysteine. In Arabidopsis thaliana, four GGT genes have been identified based on homology with animal GGTs. They are designated GGT1 (At4g39640), GGT2 (At4g39650), GGT3 (At1g69820), and GGT4 (At4g29210). By analyzing the expression of each GGT in plants containing GGT:beta-glucuronidase fusions, the temporal and spatial pattern of degradation of glutathione and its metabolites was established, revealing appreciable overlap among GGTs. GGT2 exhibited narrow temporal and spatial expression primarily in immature trichomes, developing seeds, and pollen. GGT1 and GGT3 were coexpressed in most organs/tissues. Their expression was highest at sites of rapid growth including the rosette apex, floral stem apex, and seeds and might pinpoint locations where glutathione is delivered to sink tissues to supplement high demand for cysteine. In mature tissues, they were expressed only in vascular tissue. Knockout mutants of GGT2 and GGT4 showed no phenotype. The rosettes of GGT1 knockouts showed premature senescence after flowering. Knockouts of GGT3 showed reduced number of siliques and reduced seed yield. Knockouts were used to localize and assign catalytic activity to each GGT. In the standard GGT assay with gamma-glutamyl p-nitroanilide as substrate, GGT1 accounted for 80% to 99% of the activity in all tissues except seeds where GGT2 was 50% of the activity. Protoplasting experiments indicated that both GGT1 and GGT2 are localized extracellularly but have different physical or chemical associations.

Thioredoxin reductase is required for growth and regulates entry into culmination of Dictyostelium discoideum

The thioredoxin system, consisting of thioredoxin, thioredoxin reductase and NADPH, has been well established to be critical for the redox regulation of protein function and signalling. To investigate the role of thioredoxin reductase (Trr) in Dictyostelium discoideum, we generated mutant cells that underexpress or overexpress Trr. Trr-underexpressing cells exhibited severe defects in axenic growth and development. Trr-overexpressing (TrrOE) cells formed very tiny plaques on a bacterial lawn and had a lower rate of bacterial uptake. When developed in the dark, TrrOE cells exhibited a slugger phenotype, defined by a prolonged migrating slug stage. Like other slugger mutants, they were hypersensitive to ammonia, which has been known to inhibit culmination by raising the pH of intracellular acidic compartments. Interestingly, TrrOE cells showed defective acidification of intracellular compartments and decreased activity of vacuolar H+-ATPase which functions in the acidification of intracellular compartments. Moreover, biochemical studies revealed that the thioredoxin system can directly reduce the catalytic subunit of vacuolar H+-ATPase whose activity is regulated by reversible disulphide bond formation. Taken together, these results suggest that Dictyostelium Trr may be essential for growth and play a role in regulation of phagocytosis and culmination, possibly through the modulation of vacuolar H+-ATPase activity.

Reduced glutathione levels affect the culmination and cell fate decision in Dictyostelium discoideum

Glutaredoxins have been known to be glutathione-dependent oxidoreductases that participate in the redox regulation of various cellular processes. To understand the role of glutaredoxins in the development, we examined glutaredoxin 1 (Grx1) of Dictyostelium discoideum. Its mRNA was highly accumulated at the mound and the culmination stages. When Grx1-overexpressing cells were developed, their culmination was delayed, and the expression of marker genes for prespore and spore decreased. Interestingly, they had about 1.5-fold higher amount of reduced glutathione (GSH) compared with parental cells and their prolonged migration was repressed by the oxidant such as hydrogen peroxide. To confirm the effect of GSH on the culmination, glutathione reductase (Gsr) was overexpressed or underexpressed. Similar to Grx1-overexpressing cells, Gsr-overexpressing cells contained about 1.5-fold higher amount of GSH and exhibited the delayed culmination. In contrast, the knockdown mutant of Gsr had nearly 50% lower amount of GSH and showed accelerated culmination. Taken together, these data suggest that the culmination of Dictyostelium is controlled by GSH. In addition, the cells having higher GSH levels showed a prestalk tendency in the chimeric slugs with parental cells, indicating that the difference in the amount of GSH may affect the determination of cell fate.