Glutathione: subcellular distribution and membrane transport 1

Glutathione (γ-l-glutamyl-l-cysteinylglycine) is a small tripeptide found at millimolar concentrations in nearly all eukaryotes as well as many prokaryotic cells. Glutathione synthesis is restricted to the cytosol in animals and fungi and to the cytosol and plastids in plants. Nonetheless, glutathione is found in virtually all subcellular compartments. This implies that transporters must exist that facilitate glutathione transport into and out of the various subcellular compartments. Glutathione may also be exported and imported across the plasma membrane in many cells. However, in most cases, the molecular identity of these transporters remains unclear. Whilst glutathione transport is essential for the supply and replenishment of subcellular glutathione pools, recent evidence supports a more active role for glutathione transport in the regulation of subcellular glutathione redox homeostasis. However, our knowledge of glutathione redox homeostasis at the level of specific subcellular compartments remains remarkably limited and the role of glutathione transport remains largely unclear. In this review, we discuss how new tools and techniques have begun to yield insights into subcellular glutathione distribution and glutathione redox homeostasis. In particular, we discuss the known and putative glutathione transporters and examine their contribution to the regulation of subcellular glutathione redox homeostasis.

Determining the Rate-Limiting Step for Light-Responsive Redox Regulation in Chloroplasts

Thiol-based redox regulation ensures light-responsive control of chloroplast functions. Light-derived signal is transferred in the form of reducing power from the photosynthetic electron transport chain to several redox-sensitive target proteins. Two types of protein, ferredoxin-thioredoxin reductase (FTR) and thioredoxin (Trx), are well recognized as the mediators of reducing power. However, it remains unclear which step in a series of redox-relay reactions is the critical bottleneck for determining the rate of target protein reduction. To address this, the redox behaviors of FTR, Trx, and target proteins were extensively characterized in vitro and in vivo. The FTR/Trx redox cascade was reconstituted in vitro using recombinant proteins from Arabidopsis. On the basis of this assay, we found that the FTR catalytic subunit and f-type Trx are rapidly reduced after the drive of reducing power transfer, irrespective of the presence or absence of their downstream target proteins. By contrast, three target proteins, fructose 1,6-bisphosphatase (FBPase), sedoheptulose 1,7-bisphosphatase (SBPase), and Rubisco activase (RCA) showed different reduction patterns; in particular, SBPase was reduced at a low rate. The in vivo study using Arabidopsis plants showed that the Trx family is commonly and rapidly reduced upon high light irradiation, whereas FBPase, SBPase, and RCA are differentially and slowly reduced. Both of these biochemical and physiological findings suggest that reducing power transfer from Trx to its target proteins is a rate-limiting step for chloroplast redox regulation, conferring distinct light-responsive redox behaviors on each of the targets.

Reactive Oxygen Species as Regulators of MDSC-Mediated Immune Suppression

Reactive oxygen species (ROS) molecules are implicated in signal transduction pathways and thereby control a range of biological activities. Immune cells are constantly confronted with ROS molecules under both physiologic and pathogenic conditions. Myeloid-derived suppressor cells (MDSCs) are immunosuppressive, immature myeloid cells and serve as major regulators of pathogenic and inflammatory immune responses. In addition to their own release of ROS, MDSCs often arise in oxidative-stress prone environments such as in tumors or during inflammation and infection. This evidently close relationship between MDSCs and ROS prompted us to summarize what is currently known about ROS signaling within MDSCs and to elucidate how MDSCs use ROS to modulate other immune cells. ROS not only activate anti-oxidative pathways but also induce transcriptional programs that regulate the fate and function of MDSCs. Furthermore, MDSCs release ROS molecules as part of a major mechanism to suppress T cell responses. Targeting redox-regulation of MDSCs thus presents a promising approach to cancer therapy and the role of redox-signaling in MDSCs in other disease states such as infection, inflammation and autoimmunity would appear to be well worth investigating.

Redox for Repair: Cold Physical Plasmas and Nrf2 Signaling Promoting Wound Healing

Chronic wounds and ulcers are major public health threats. Being a substantial burden for patients and health care systems alike, better understanding of wound pathophysiology and new avenues in the therapy of chronic wounds are urgently needed. Cold physical plasmas are particularly effective in promoting wound closure, irrespective of its etiology. These partially ionized gases deliver a therapeutic cocktail of reactive oxygen and nitrogen species safely at body temperature and without genotoxic side effects. This field of plasma medicine reanimates the idea of redox repair in physiological healing. This review compiles previous findings of plasma effects in wound healing. It discusses new links between plasma treatment of cells and tissues, and the perception and intracellular translation of plasma-derived reactive species via redox signaling pathways. Specifically, (i) molecular switches governing redox-mediated tissue response; (ii) the activation of the nuclear E2-related factor (Nrf2) signaling, together with antioxidative and immunomodulatory responses; and (iii) the stabilization of the scaffolding function and actin network in dermal fibroblasts are emphasized in the light of wound healing.

Mitochondrial Arabidopsis thaliana TRXo Isoforms Bind an Iron⁻Sulfur Cluster and Reduce NFU Proteins In Vitro

In plants, the mitochondrial thioredoxin (TRX) system generally comprises only one or two isoforms belonging to the TRX h or o classes, being less well developed compared to the numerous isoforms found in chloroplasts. Unlike most other plant species, Arabidopsis thaliana possesses two TRXo isoforms whose physiological functions remain unclear. Here, we performed a structure⁻function analysis to unravel the respective properties of the duplicated TRXo1 and TRXo2 isoforms. Surprisingly, when expressed in Escherichia coli, both recombinant proteins existed in an apo-monomeric form and in a homodimeric iron⁻sulfur (Fe-S) cluster-bridged form. In TRXo2, the [4Fe-4S] cluster is likely ligated in by the usual catalytic cysteines present in the conserved Trp-Cys-Gly-Pro-Cys signature. Solving the three-dimensional structure of both TRXo apo-forms pointed to marked differences in the surface charge distribution, notably in some area usually participating to protein⁻protein interactions with partners. However, we could not detect a difference in their capacity to reduce nitrogen-fixation-subunit-U (NFU)-like proteins, NFU4 or NFU5, two proteins participating in the maturation of certain mitochondrial Fe-S proteins and previously isolated as putative TRXo1 partners. Altogether, these results suggest that a novel regulation mechanism may prevail for mitochondrial TRXs o, possibly existing as a redox-inactive Fe-S cluster-bound form that could be rapidly converted in a redox-active form upon cluster degradation in specific physiological conditions.

Mechanistic basis for the evolution of chalcone synthase catalytic cysteine reactivity in land plants

Flavonoids are important polyphenolic natural products, ubiquitous in land plants, that play diverse functions in plants' survival in their ecological niches, including UV protection, pigmentation for attracting pollinators, symbiotic nitrogen fixation, and defense against herbivores. Chalcone synthase (CHS) catalyzes the first committed step in plant flavonoid biosynthesis and is highly conserved in all land plants. In several previously reported crystal structures of CHSs from flowering plants, the catalytic cysteine is oxidized to sulfinic acid, indicating enhanced nucleophilicity in this residue associated with its increased susceptibility to oxidation. In this study, we report a set of new crystal structures of CHSs representing all five major lineages of land plants (bryophytes, lycophytes, monilophytes, gymnosperms, and angiosperms), spanning 500 million years of evolution. We reveal that the structures of CHS from a lycophyte and a moss species preserve the catalytic cysteine in a reduced state, in contrast to the cysteine sulfinic acid seen in all euphyllophyte CHS structures. In vivo complementation, in vitro biochemical and mutagenesis analyses, and molecular dynamics simulations identified a set of residues that differ between basal-plant and euphyllophyte CHSs and modulate catalytic cysteine reactivity. We propose that the CHS active-site environment has evolved in euphyllophytes to further enhance the nucleophilicity of the catalytic cysteine since the divergence of euphyllophytes from other vascular plant lineages 400 million years ago. These changes in CHS could have contributed to the diversification of flavonoid biosynthesis in euphyllophytes, which in turn contributed to their dominance in terrestrial ecosystems.

The Role of Hydrogen Peroxide in Redox-Dependent Signaling: Homeostatic and Pathological Responses in Mammalian Cells

Hydrogen peroxide (H₂O₂) is an important metabolite involved in most of the redox metabolism reactions and processes of the cells. H₂O₂ is recognized as one of the main molecules in the sensing, modulation and signaling of redox metabolism, and it is acting as a second messenger together with hydrogen sulfide (H₂S) and nitric oxide (NO). These second messengers activate in turn a cascade of downstream proteins via specific oxidations leading to a metabolic response of the cell. This metabolic response can determine proliferation, survival or death of the cell depending on which downstream pathways (homeostatic, pathological, or protective) have been activated. The cells have several sources of H₂O₂ and cellular systems strictly control its concentration in different subcellular compartments. This review summarizes research on the role played by H₂O₂ in signaling pathways of eukaryotic cells and how this signaling leads to homeostatic or pathological responses.

Crystal structure of Arabidopsis thaliana peroxiredoxin A C119S mutant

Peroxiredoxins (Prxs), a large family of antioxidant enzymes, are abundant in all living organisms. Peroxiredoxin A (PrxA) from Arabidopsis thaliana belongs to the typical 2-Cys Prx family and is localized in the chloroplast. This article reports the crystal structure of a PrxA C119S mutant refined to 2.6 Å resolution. The protein exists as a decamer both in the crystal structure and in solution. The structure is in the reduced state suitable for the approach of peroxide, though conformational changes are needed for the resolving process.

Mitochondrial H2O2 Generation Using a Tunable Chemogenetic Tool To Perturb Redox Homeostasis in Human Cells and Induce Cell Death

Among reactive oxygen species (ROS), H₂O₂ alone acts as a signaling molecule that promotes diverse phenotypes depending on the intracellular concentration. Mitochondria have been suggested as both sources and sinks of cellular H₂O₂, and mitochondrial dysfunction has been implicated in diseases such as cancer. A genetically encoded H₂O₂ generator, d-amino acid oxidase (DAAO), was targeted to the mitochondria of human cells, and its utility in investigating cellular response to a range of H₂O₂ doses over time was assessed. Organelle-specific peroxiredoxin dimerization and protein S-glutathionylation were measured as indicators of increased H₂O₂ flux due to the activity of DAAO. Cell death was observed in a concentration- and time-dependent manner, and protein oxidation shifted in localization as the dose increased. This work presents the first systematic study of H₂O₂-specific perturbation of mitochondria in human cells, and it reveals a marked sensitivity of this organelle to increases in H₂O₂ in comparison with prior studies that targeted the cytosol.

Reduction of leucocyte cell surface disulfide bonds during immune activation is dynamic as revealed by a quantitative proteomics workflow (SH-IQ)

Communication through cell surface receptors is crucial for maintaining immune homeostasis, coordinating the immune response and pathogen clearance. This is dependent on the interaction of cell surface receptors with their ligands and requires functionally active conformational states. Thus, immune cell function can be controlled by modulating the structure of either the receptor or the ligand. Reductive cleavage of labile disulfide bonds can mediate such an allosteric change, resulting in modulation of the immune system by a hitherto little studied mechanism. Identifying proteins with labile disulfide bonds and determining the extent of reduction is crucial in elucidating the functional result of reduction. We describe a mass spectrometry-based method—thiol identification and quantitation (SH-IQ)—to identify, quantify and monitor such reduction of labile disulfide bonds in primary cells during immune activation. These results provide the first insight into the extent and dynamics of labile disulfide bond reduction in leucocyte cell surface proteins upon immune activation. We show that this process is thiol oxidoreductase-dependent and mainly affects activatory (e.g. CD132, SLAMF1) and adhesion (CD44, ICAM1) molecules, suggesting a mechanism to prevent over-activation of the immune system and excessive accumulation of leucocytes at sites of inflammation.

Redox regulation of type-I inositol trisphosphate receptors in intact mammalian cells

A sensitization of inositol 1,4,5-trisphosphate receptor (IP₃R)-mediated Ca²⁺ release is associated with oxidative stress in multiple cell types. These effects are thought to be mediated by alterations in the redox state of critical thiols in the IP₃R, but this has not been directly demonstrated in intact cells. Here, we utilized a combination of gel-shift assays with MPEG-maleimides and LC-MS/MS to monitor the redox state of recombinant IP₃R1 expressed in HEK293 cells. We found that under basal conditions, ∼5 of the 60 cysteines are oxidized in IP₃R1. Cell treatment with 50 μm thimerosal altered gel shifts, indicating oxidation of ∼20 cysteines. By contrast, the shifts induced by 0.5 mm H₂O₂ or other oxidants were much smaller. Monitoring of biotin-maleimide attachment to IP₃R1 by LC-MS/MS with 71% coverage of the receptor sequence revealed modification of two cytosolic (Cys-292 and Cys-1415) and two intraluminal cysteines (Cys-2496 and Cys-2533) under basal conditions. The thimerosal treatment modified an additional eleven cysteines, but only three (Cys-206, Cys-767, and Cys-1459) were consistently oxidized in multiple experiments. H₂O₂ also oxidized Cys-206 and additionally oxidized two residues not modified by thimerosal (Cys-214 and Cys-1397). Potentiation of IP₃R channel function by oxidants was measured with cysteine variants transfected into a HEK293 IP₃R triple-knockout cell line, indicating that the functionally relevant redox-sensitive cysteines are predominantly clustered within the N-terminal suppressor domain of IP₃R. To our knowledge, this study is the first that has used proteomic methods to assess the redox state of individual thiols in IP₃R in intact cells.

H2O2 oxidation of cysteine residues in c-Jun N-terminal kinase 2 (JNK2) contributes to redox regulation in human articular chondrocytes

Reactive oxygen species (ROS), in particular H₂O₂, regulate intracellular signaling through reversible oxidation of reactive protein thiols present in a number of kinases and phosphatases. H₂O₂ has been shown to regulate mitogen-activated protein kinase (MAPK) signaling depending on the cellular context. We report here that in human articular chondrocytes, the MAPK family member c-Jun N-terminal kinase 2 (JNK2) is activated by fibronectin fragments and low physiological levels of H₂O₂ and inhibited by oxidation due to elevated levels of H₂O₂ The kinase activity of affinity-purified, phosphorylated JNK2 from cultured chondrocytes was reversibly inhibited by 5-20 μm H₂O₂ Using dimedone-based chemical probes that react specifically with sulfenylated cysteines (RSOH), we identified Cys-222 in JNK2, a residue not conserved in JNK1 or JNK3, as a redox-reactive site. MS analysis of human recombinant JNK2 also detected further oxidation at Cys-222 and other cysteines to sulfinic (RSO₂H) or sulfonic (RSO₃H) acid. H₂O₂ treatment of JNK2 resulted in detectable levels of peptides containing intramolecular disulfides between Cys-222 and either Cys-213 or Cys-177, without evidence of dimer formation. Substitution of Cys-222 to alanine rendered JNK2 insensitive to H₂O₂ inhibition, unlike C177A and C213A variants. Two other JNK2 variants, C116A and C163A, were also resistant to oxidative inhibition. Cumulatively, these findings indicate differential regulation of JNK2 signaling dependent on H₂O₂ levels and point to key cysteine residues regulating JNK2 activity. As levels of intracellular H₂O₂ rise, a switch occurs from activation to inhibition of JNK2 activity, linking JNK2 regulation to the redox status of the cell.

Identification of a co-target for enhancing efficacy of sorafenib in HCC through a quantitative modeling approach

Sorafenib (SFB), a multi-kinase inhibitor, is the only approved drug for treating hepatocellular carcinoma (HCC). However, SFB shows low efficacy in many cases. HCC related mortality therefore remains to be high worldwide. SFB, a multi-kinase inhibitor is also known to modulate the redox homeostasis in cancer cells. To understand the effect of SFB on the redox status, a quantitative understanding of the system is necessary. Kinetic modeling of the relevant pathways is a useful approach for obtaining a quantitative understanding of the pathway dynamics and to rank the individual factors based on the extent of influence they wield on the pathway. Here, we report a comprehensive model of the glutathione reaction network (GSH_net ), consisting of four modules and includes SFB-induced redox stress. We compared GSH_net simulations for HCC of six different etiologies with healthy liver, and correctly identified the expected variations in cancer. Next, we studied alterations induced in the system upon SFB treatment and observed differential H₂ O₂ dynamics in all the conditions. Using metabolic control analysis, we identified glutathione S-transferase (GST) as the enzyme with the highest selective control coefficient, making it an attractive co-target for potentiating the action of SFB across all six etiologies. As a proof-of-concept, we selected ethacrynic acid (EA), a known inhibitor of GST, and verified ex vivo that EA synergistically potentiates the cytotoxic effect of SFB. Being an FDA approved drug, EA is a promising candidate for repurposing as a combination therapy with SFB for HCC treatment.

Biological Production, Detection, and Fate of Hydrogen Peroxide

Hydrogen peroxide (H₂O₂) is generated in numerous biological processes. It transmits cellular signals, contributes to oxidative folding of exported proteins, and, in excess, can be damaging to cells and tissues. Although a strong oxidant, high activation energy barriers make it unreactive with most biological molecules. Its main reactions are with transition metal centers, selenoproteins and selected thiol proteins, with glutathione peroxidases (GPxs) and peroxiredoxins (Prxs) being major targets. It reacts slowly with most thiol proteins, and how they become oxidized during redox signal transmission is not well understood. Recent Advances: Kinetic analysis indicates that Prxs and GPxs are overwhelmingly favored as targets for H₂O₂ in cells. Studies with localized probes indicate that H₂O₂ can be produced in cellular microdomains and be consumed by highly reactive targets before it can diffuse to other parts of the cell. Inactivation of these targets alone will not confine it to its site of production. Kinetic data indicate that oxidation of regulatory thiol proteins by H₂O₂ requires a facilitated mechanism such as directed transfer from source to target or a relay mediated through a highly reactive sensor. Critical Issues and Future Directions: Absolute rates of H₂O₂ production and steady-state concentrations in cells still need to be characterized. More information on cellular sites of production and action is required, and specific mechanisms of oxidation of regulatory proteins during redox signaling require further characterization. Antioxid. Redox Signal. 29, 541-551.

Thioredoxin-like2/2-Cys peroxiredoxin redox cascade supports oxidative thiol modulation in chloroplasts

Thiol-based redox regulation is central to adjusting chloroplast functions under varying light conditions. A redox cascade via the ferredoxin-thioredoxin reductase (FTR)/thioredoxin (Trx) pathway has been well recognized to mediate the light-responsive reductive control of target proteins; however, the molecular basis for reoxidizing its targets in the dark remains unidentified. Here, we report a mechanism of oxidative thiol modulation in chloroplasts. We biochemically characterized a chloroplast stroma-localized atypical Trx from Arabidopsis, designated as Trx-like2 (TrxL2). TrxL2 had redox-active properties with an unusually less negative redox potential. By an affinity chromatography-based method, TrxL2 was shown to interact with a range of chloroplast redox-regulated proteins. The direct discrimination of thiol status indicated that TrxL2 can efficiently oxidize, but not reduce, these proteins. A notable exception was found in 2-Cys peroxiredoxin (2CP); TrxL2 was able to reduce 2CP with high efficiency. We achieved a complete in vitro reconstitution of the TrxL2/2CP redox cascade for oxidizing redox-regulated proteins and draining reducing power to hydrogen peroxide (H₂O₂). We further addressed the physiological relevance of this system by analyzing protein-oxidation dynamics. In Arabidopsis plants, a decreased level of 2CP led to the impairment of the reoxidation of redox-regulated proteins during light-dark transitions. A delayed response of protein reoxidation was concomitant with the prolonged accumulation of reducing power in TrxL2. These results suggest an in vivo function of the TrxL2/2CP redox cascade for driving oxidative thiol modulation in chloroplasts.

Redox Signaling in Neurotransmission and Cognition During Aging

SIGNIFICANCE

Oxidative stress increases in the brain with aging and neurodegenerative diseases. Previous work emphasized irreversible oxidative damage in relation to cognitive impairment. This research has evolved to consider a continuum of alterations, from redox signaling to oxidative damage, which provides a basis for understanding the onset and progression of cognitive impairment. This review provides an update on research linking redox signaling to altered function of neural circuits involved in information processing and memory. Recent Advances: Starting in middle age, redox signaling triggers changes in nervous system physiology described as senescent physiology. Hippocampal senescent physiology involves decreased cell excitability, altered synaptic plasticity, and decreased synaptic transmission. Recent studies indicate N-methyl-d-aspartate and ryanodine receptors and Ca²⁺ signaling molecules as molecular substrates of redox-mediated senescent physiology.

CRITICAL ISSUES

We review redox homeostasis mechanisms and consider the chemical character of reactive oxygen and nitrogen species and their role in regulating different transmitter systems. In this regard, senescent physiology may represent the co-opting of pathways normally responsible for feedback regulation of synaptic transmission. Furthermore, differences across transmitter systems may underlie differential vulnerability of brain regions and neuronal circuits to aging and disease.

FUTURE DIRECTIONS

It will be important to identify the intrinsic mechanisms for the shift in oxidative/reductive processes. Intrinsic mechanism will depend on the transmitter system, oxidative stressors, and expression/activity of antioxidant enzymes. In addition, it will be important to identify how intrinsic processes interact with other aging factors, including changes in inflammatory or hormonal signals. Antioxid. Redox Signal. 28, 1724-1745.

Endoplasmic reticulum (ER) stress-induced reactive oxygen species (ROS) are detrimental for the fitness of a thioredoxin reductase mutant

The unfolded protein response (UPR) is constitutively active in yeast thioredoxin reductase mutants, suggesting a link between cytoplasmic thiol redox control and endoplasmic reticulum (ER) oxidative protein folding. The unique oxidative environment of the ER lumen requires tight regulatory control, and we show that the active UPR depends on the presence of oxidized thioredoxins rather than arising because of a loss of thioredoxin function. Preventing activation of the UPR by deletion of HAC1, encoding the UPR transcription factor, rescues a number of thioredoxin reductase mutant phenotypes, including slow growth, shortened longevity, and oxidation of the cytoplasmic GSH pool. This is because the constitutive UPR in a thioredoxin reductase mutant results in the generation of hydrogen peroxide. The oxidation of thioredoxins in a thioredoxin reductase mutant requires aerobic metabolism and the presence of the Tsa1 and Tsa2 peroxiredoxins, indicating that a complete cytoplasmic thioredoxin system is crucial for maintaining ER redox homeostasis.

Cardiovascular Adaptive Homeostasis in Exercise

Adaptive Homeostasis has been defined as, "The transient expansion or contraction of the homeostatic range in response to exposure to sub-toxic, non-damaging, signaling molecules or events, or the removal or cessation of such molecules or events." (Davies, 2016). I propose that one of the most significant examples of adaptive homeostasis may be the adaptation of the cardiovascular system to exercise training. In particular, endurance type training involves the generation of increased levels of free radicals such as ubisemiquinone, superoxide, nitric oxide, and other (non-radical) reactive oxygen species such as hydrogen peroxide (H2O2), in a repetitive manner, typically several times per week. As long as the training intensity and duration are sub-maximal and not exhaustive these reactive species do not cause damage, but rather activate signal transduction pathways to induce mitochondrial biogenesis-the foundation of increased exercise endurance. Particularly important are the NFκB and Nrf2 signal transduction pathways which respond to reactive oxygen and nitrogen species generated during exercise. As with other examples of adaptive homeostasis the effects are transient, lasting only as long as the training is maintained. Unfortunately, the ability to adapt to exercise training declines with age, perhaps as a result of impaired Nrf2 and NFκB signaling, as does adaptive homeostasis capacity in general. Since this is an Hypothesis/Theory Paper and not a review, I have not tried to provide a comprehensive discussion of all the literature relating to exercise adaptation and the cardiovascular system. Rather, I have attempted to develop the Hypothesis or Theory that adaptive homeostasis is the foundation for adaptation of the cardiovascular system to exercise training, largely based on work from my own laboratory, that of close collaborators, and that of key contributors over a period of almost 40 years.

The redox state of the apoplast influences the acclimation of photosynthesis and leaf metabolism to changing irradiance

The redox state of the apoplast is largely determined by ascorbate oxidase (AO) activity. The influence of AO activity on leaf acclimation to changing irradiance was explored in wild-type (WT) and transgenic tobacco (Nicotiana tobaccum) lines containing either high [pumpkin AO (PAO)] or low [tobacco AO (TAO)] AO activity at low [low light (LL); 250 μmol m-2  s-1 ] and high [high light (HL); 1600 μmol m-2  s-1 ] irradiance and following the transition from HL to LL. AO activities changed over the photoperiod, particularly in the PAO plants. AO activity had little effect on leaf ascorbate, which was significantly higher under HL than under LL. Apoplastic ascorbate/dehydroascorbate (DHA) ratios and threonate levels were modified by AO activity. Despite decreased levels of transcripts encoding ascorbate synthesis enzymes, leaf ascorbate increased over the first photoperiod following the transition from HL to LL, to much higher levels than LL-grown plants. Photosynthesis rates were significantly higher in the TAO leaves than in WT or PAO plants grown under HL but not under LL. Sub-sets of amino acids and fatty acids were lower in TAO and WT leaves than in the PAO plants under HL, and following the transition to LL. Light acclimation processes are therefore influenced by the apoplastic as well as chloroplastic redox state.

Selective Activation of a Prodrug by Thioredoxin Reductase Providing a Strategy to Target Cancer Cells

Elevated reactive oxygen species and antioxidant defense systems have been recognized as one of the hallmarks of cancer cells. As a major regulator of the cellular redox homeostasis, the selenoprotein thioredoxin reductase (TrxR) is increasingly considered as a promising target for anticancer drug development. The current approach to inhibit TrxR predominantly relies on the modification of the selenocysteine residue in the C-terminal active site of the enzyme, in which it is hard to avoid the off-target effects. By conjugating the anticancer drug gemcitabine with a 1,2-dithiolane scaffold, an unprecedented prodrug strategy is disclosed that achieves a specific release of gemcitabine by TrxR in cells. As overexpression of TrxR is frequently found in different types of tumors, the TrxR-dependent prodrugs are promising for further development as cancer chemotherapeutic agents.

Introduction to the Thematic Minireview Series: Redox metabolism and signaling

In this sequel to the thematic collection of Minireviews on redox metabolism and signaling published last year, five articles plumb the redox metabolic pathways relevant to cell proliferation, stress response, and survival post-detachment from the extracellular matrix. The sixth article provides unexpected insights into the hepatic NAD(P)ome, revealing that more than half of these proteins reside outside the cytoplasmic and mitochondrial compartments, pointing to the paucity of knowledge on their functions. Collectively, these articles highlight the metabolic adaptability of proliferating cells to meet redox needs, the use of lipids to mediate mitochondrial stress responses, and the need for tools to interrogate compartment-specific pool sizes and oxidized/reduced ratios of antioxidants and lipids involved in redox homeostasis.

Chloroplast thioredoxin systems: prospects for improving photosynthesis

Thioredoxins (TRXs) are protein oxidoreductases that control the structure and function of cellular proteins by cleavage of a disulphide bond between the side chains of two cysteine residues. Oxidized thioredoxins are reactivated by thioredoxin reductases (TR) and a TR-dependent reduction of TRXs is called a thioredoxin system. Thiol-based redox regulation is an especially important mechanism to control chloroplast proteins involved in biogenesis, in regulation of light harvesting and distribution of light energy between photosystems, in photosynthetic carbon fixation and other biosynthetic pathways, and in stress responses of plants. Of the two plant plastid thioredoxin systems, the ferredoxin-dependent system relays reducing equivalents from photosystem I via ferredoxin and ferredoxin-thioredoxin reductase (FTR) to chloroplast proteins, while NADPH-dependent thioredoxin reductase (NTRC) forms a complete thioredoxin system including both reductase and thioredoxin domains in a single polypeptide. Chloroplast thioredoxins transmit environmental light signals to biochemical reactions, which allows fine tuning of photosynthetic processes in response to changing environmental conditions. In this paper we focus on the recent reports on specificity and networking of chloroplast thioredoxin systems and evaluate the prospect of improving photosynthetic performance by modifying the activity of thiol regulators in plants.

This article is part of the themed issue ‘Enhancing photosynthesis in crop plants: targets for improvement'.

Bacterial Periplasmic Oxidoreductases Control the Activity of Oxidized Human Antimicrobial β-Defensin 1

The antimicrobial peptide human β-defensin 1 (hBD1) is continuously produced by epithelial cells in many tissues. Compared to other defensins, hBD1 has only minor antibiotic activity in its native state. After reduction of its disulfide bridges, however, it becomes a potent antimicrobial agent against bacteria, while the oxidized native form (hBD1ox) shows specific activity against Gram-negative bacteria. We show that the killing mechanism of hBD1ox depends on aerobic growth conditions and bacterial enzymes. We analyzed the different activities of hBD1 using mutants of Escherichia coli lacking one or more specific proteins of their outer membrane, cytosol, or redox systems. We discovered that DsbA and DsbB are essential for the antimicrobial activity of hBD1ox but not for that of reduced hBD1 (hBD1red). Furthermore, our results strongly suggest that hBD1ox uses outer membrane protein FepA to penetrate the bacterial periplasm space. In contrast, other bacterial proteins in the outer membrane and cytosol did not modify the antimicrobial activity. Using immunogold labeling, we identified the localization of hBD1ox in the periplasmic space and partly in the outer membrane of E. coli However, in resistant mutants lacking DsbA and DsbB, hBD1ox was detected mainly in the bacterial cytosol. In summary, we discovered that hBD1ox could use FepA to enter the periplasmic space, where its activity depends on presence of DsbA and DsbB. HBD1ox concentrates in the periplasm in Gram-negative bacteria, which finally leads to bleb formation and death of the bacteria. Thus, the bacterial redox system plays an essential role in mechanisms of resistance against host-derived peptides such as hBD1.

Protein S-Nitrosylation Controls Glycogen Synthase Kinase 3β Function Independent of Its Phosphorylation State

RATIONALE

GSK-3β (glycogen synthase kinase 3β) is a multifunctional and constitutively active kinase known to regulate a myriad of cellular processes. The primary mechanism to regulate its function is through phosphorylation-dependent inhibition at serine-9 residue. Emerging evidence indicates that there may be alternative mechanisms that control GSK-3β for certain functions.

OBJECTIVES

Here, we sought to understand the role of protein S-nitrosylation (SNO) on the function of GSK-3β. SNO-dependent modulation of the localization of GSK-3β and its ability to phosphorylate downstream targets was investigated in vitro, and the network of proteins differentially impacted by phospho- or SNO-dependent GSK-3β regulation and in vivo SNO modification of key signaling kinases during the development of heart failure was also studied.

METHODS AND RESULTS

We found that GSK-3β undergoes site-specific SNO both in vitro, in HEK293 cells, H9C2 myoblasts, and primary neonatal rat ventricular myocytes, as well as in vivo, in hearts from an animal model of heart failure and sudden cardiac death. S-nitrosylation of GSK-3β significantly inhibits its kinase activity independent of the canonical phospho-inhibition pathway. S-nitrosylation of GSK-3β promotes its nuclear translocation and access to novel downstream phosphosubstrates which are enriched for a novel amino acid consensus sequence motif. Quantitative phosphoproteomics pathway analysis reveals that nuclear GSK-3β plays a central role in cell cycle control, RNA splicing, and DNA damage response.

CONCLUSIONS

The results indicate that SNO has a differential effect on the location and activity of GSK-3β in the cytoplasm versus the nucleus. SNO modification of GSK-3β occurs in vivo and could contribute to the pathobiology of heart failure and sudden cardiac death.

PHB Biosynthesis Counteracts Redox Stress in Herbaspirillum seropedicae

The ability of bacteria to produce polyhydroxyalkanoates such as poly(3-hydroxybutyrate) (PHB) enables provision of a carbon storage molecule that can be mobilized under demanding physiological conditions. However, the precise function of PHB in cellular metabolism has not been clearly defined. In order to determine the impact of PHB production on global physiology, we have characterized the properties of a ΔphaC1 mutant strain of the diazotrophic bacterium Herbaspirillum seropedicae. The absence of PHB in the mutant strain not only perturbs redox balance and increases oxidative stress, but also influences the activity of the redox-sensing Fnr transcription regulators, resulting in significant changes in expression of the cytochrome c-branch of the electron transport chain. The synthesis of PHB is itself dependent on the Fnr1 and Fnr3 proteins resulting in a cyclic dependency that couples synthesis of PHB with redox regulation. Transcriptional profiling of the ΔphaC1 mutant reveals that the loss of PHB synthesis affects the expression of many genes, including approximately 30% of the Fnr regulon.