Protein s-glutathionylation in retinal pigment epithelium converts heat shock protein 70 to an active chaperone

Hsp70 in Redox Homeostasis

Cellular redox homeostasis is precisely balanced by generation and elimination of reactive oxygen species (ROS). ROS are not only capable of causing oxidation of proteins, lipids and DNA to damage cells but can also act as signaling molecules to modulate transcription factors and epigenetic pathways that determine cell survival and death. Hsp70 proteins are central hubs for proteostasis and are important factors to ameliorate damage from different kinds of stress including oxidative stress. Hsp70 members often participate in different cellular signaling pathways via their clients and cochaperones. ROS can directly cause oxidative cysteine modifications of Hsp70 members to alter their structure and chaperone activity, resulting in changes in the interactions between Hsp70 and their clients or cochaperones, which can then transfer redox signals to Hsp70-related signaling pathways. On the other hand, ROS also activate some redox-related signaling pathways to indirectly modulate Hsp70 activity and expression. Post-translational modifications including phosphorylation together with elevated Hsp70 expression can expand the capacity of Hsp70 to deal with ROS-damaged proteins and support antioxidant enzymes. Knowledge about the response and role of Hsp70 in redox homeostasis will facilitate our understanding of the cellular knock-on effects of inhibitors targeting Hsp70 and the mechanisms of redox-related diseases and aging.

A comparison of the chemical biology of hydropersulfides (RSSH) with other protective biological antioxidants and nucleophiles

The hydropersulfide (RSSH) functional group has received significant recent interest due to its unique chemical properties that set it apart from other biological species. The chemistry of RSSH predicts that one possible biological role may be as a protectant against cellular oxidative and electrophilic stress. That is, RSSH has reducing and nucleophilic properties that may combat the potentially destructive biochemistry of toxicologically relevant oxidants and electrophiles. However, there are currently numerous other molecules that have established roles in this regard. For example, ascorbate and tocopherols are potent antioxidants that quench deleterious oxidative reactions and glutathione (GSH) is a well-established and highly prevalent biological protectant against electrophile toxicity. Thus, in order to begin to understand the possible role of RSSH species as protectants against oxidative/electrophilic stress, the inherent chemical properties of RSSH versus these other protectants will be discussed and contrasted.

Thiol-based switching mechanisms of stress-sensing chaperones

Thiol-based redox switches evolved as efficient post-translational regulatory mechanisms that enable individual proteins to rapidly respond to sudden environmental changes. While some protein functions need to be switched off to save resources and avoid potentially error-prone processes, protective functions become essential and need to be switched on. In this review, we focus on thiol-based activation mechanisms of stress-sensing chaperones. Upon stress exposure, these chaperones convert into high affinity binding platforms for unfolding proteins and protect cells against the accumulation of potentially toxic protein aggregates. Their chaperone activity is independent of ATP, a feature that becomes especially important under oxidative stress conditions, where cellular ATP levels drop and canonical ATP-dependent chaperones no longer operate. Vice versa, reductive inactivation and substrate release require the restoration of ATP levels, which ensures refolding of client proteins by ATP-dependent foldases. We will give an overview over the different strategies that cells evolved to rapidly increase the pool of ATP-independent chaperones upon oxidative stress and provide mechanistic insights into how stress conditions are used to convert abundant cellular proteins into ATP-independent holding chaperones.

S-Glutathionylation of human inducible Hsp70 reveals a regulatory mechanism involving the C-terminal α-helical lid

Heat shock protein 70 (Hsp70) proteins are a family of ancient and conserved chaperones. Cysteine modifications have been widely detected among different Hsp70 family members in vivo, but their effects on Hsp70 structure and function are unclear. Here, we treated HeLa cells with diamide, which typically induces disulfide bond formation except in the presence of excess GSH, when glutathionylated cysteines predominate. We show that in these cells, HspA1A (hHsp70) undergoes reversible cysteine modifications, including glutathionylation, potentially at all five cysteine residues. In vitro experiments revealed that modification of cysteines in the nucleotide-binding domain of hHsp70 is prevented by nucleotide binding but that Cys-574 and Cys-603, located in the C-terminal α-helical lid of the substrate-binding domain, can undergo glutathionylation in both the presence and absence of nucleotide. We found that glutathionylation of these cysteine residues results in unfolding of the α-helical lid structure. The unfolded region mimics substrate by binding to and blocking the substrate-binding site, thereby promoting intrinsic ATPase activity and competing with binding of external substrates, including heat shock transcription factor 1 (Hsf1). Thus, post-translational modification can alter the structure and regulate the function of hHsp70.

Salvianolic Acid B (Sal B) Protects Retinal Pigment Epithelial Cells from Oxidative Stress-Induced Cell Death by Activating Glutaredoxin 1 (Grx1)

Protein glutathionylation, defined as the formation of protein mixed disulfides (PSSG) between cysteine residues and glutathione (GSH), can lead to cell death. Glutaredoxin 1 (Grx1) is a thiol repair enzyme which catalyzes the reduction of PSSG. Therefore, Grx1 exerts strong anti-apoptotic effects by improving the redox state, especially in times of oxidative stress. However, there is currently no compound that is identified as a Grx1 activator. In this study, we identified and characterized Salvianolic acid B (Sal B), a natural compound, as a Grx1 inducer, which potently protected retinal pigment epithelial (RPE) cells from oxidative injury. Our results showed that treatment with Sal B protected primary human RPE cells from H₂O₂-induced cell damage. Interestingly, we found Sal B pretreatment upregulated Grx1 expression in RPE cells in a time- and dose-dependent manner. Furthermore, NF-E2-related factor 2 (Nrf2), the key transcription factor that regulates the expression of Grx1, was activated in Sal B treated RPE cells. Further investigation showed that knockdown of Grx1 by small interfering RNA (siRNA) significantly reduced the protective effects of Sal B. We conclude that Sal B protects RPE cells against H₂O₂-induced cell injury through Grx1 induction by activating Nrf2 pathway, thus preventing lethal accumulation of PSSG and reversing oxidative damage.

Oxidative Stress and Huntington's Disease: The Good, The Bad, and The Ugly

Redox homeostasis is crucial for proper cellular functions, including receptor tyrosine kinase signaling, protein folding, and xenobiotic detoxification. Under basal conditions, there is a balance between oxidants and antioxidants. This balance facilitates the ability of oxidants, such as reactive oxygen species, to play critical regulatory functions through a direct modification of a small number of amino acids (e.g. cysteine) on signaling proteins. These signaling functions leverage tight spatial, amplitude, and temporal control of oxidant concentrations. However, when oxidants overwhelm the antioxidant capacity, they lead to a harmful condition of oxidative stress. Oxidative stress has long been held to be one of the key players in disease progression for Huntington's disease (HD). In this review, we will critically review this evidence, drawing some intermediate conclusions, and ultimately provide a framework for thinking about the role of oxidative stress in the pathophysiology of HD.

Mixed results with mixed disulfides

A period of research with Helmut Sies in the 1980s is recalled. Our experiments aimed at an in-depth understanding of metabolic changes due to oxidative challenges under near-physiological conditions, i.e. perfused organs. A major focus were alterations of the glutathione and the NADPH/NADP(+) system by different kinds of oxidants, in particular formation of glutathione mixed disulfides with proteins. To analyze mixed disulfides, a test was adapted which is widely used until today. The observations in perfused rat livers let us believe that glutathione-6-phosphate dehydrogenase (G6PDH), i.a. might be activated by glutathionylation. Although we did not succeed to verify this hypothesis for the special case of G6PDH, the regulation of enzyme/protein activities by glutathionylation today is an accepted posttranslational mechanism in redox biology in general. Our early experimental approaches are discussed in the context of present knowledge.

Evaluating the Oxidative Stress in Renal Diseases: What Is the Role for S-Glutathionylation?

SIGNIFICANCE

Reactive oxygen species (ROS) have long been considered as toxic derivatives of aerobic metabolism displaying a harmful effect to living cells. Deregulation of redox homeostasis and production of excessive free radicals may contribute to the pathogenesis of kidney diseases. In line, oxidative stress increases in patients with renal dysfunctions due to a general increase of ROS paralleled by impaired antioxidant ability.

RECENT ADVANCES

Emerging evidence revealed that physiologically, ROS can act as signaling molecules interplaying with several transduction pathways such as proliferation, differentiation, and apoptosis. ROS can exert signaling functions by modulating, at different layers, protein oxidation since proteins have "cysteine switches" that can be reversibly reduced or oxidized, supporting the dynamic signaling regulation function. In this scenario, S-glutathionylation is a posttranslational modification involved in oxidative cellular response.

CRITICAL ISSUES

Although it is widely accepted that renal dysfunctions are often associated with altered redox signaling, the relative role of S-glutathionylation on the pathogenesis of specific renal diseases remains unclear and needs further investigations. In this review, we discuss the impact of ROS in renal health and diseases and the role of selective S-glutathionylation proteins potentially relevant to renal physiology.

FUTURE DIRECTIONS

The paucity of studies linking the reversible protein glutathionylation with specific renal disorders remains unmet. The growing number of S-glutathionylated proteins indicates that this is a fascinating area of research. In this respect, further studies on the association of reversible glutathionylation with renal diseases, characterized by oxidative stress, may be useful to develop new pharmacological molecules targeting protein S-glutathionylation. Antioxid. Redox Signal. 25, 147-164.

Interplay between redox and protein homeostasis

The subcellular compartments of eukaryotic cells are characterized by different redox environments. Whereas the cytosol, nucleus and mitochondria are more reducing, the endoplasmic reticulum represents a more oxidizing environment. As the redox level controls the formation of intra- and inter-molecular disulfide bonds, the folding of proteins is tightly linked to its environment. The proteostasis network of each compartment needs to be adapted to the compartmental redox properties. In addition to chaperones, also members of the thioredoxin superfamily can influence the folding of proteins by regulation of cysteine reduction/oxidation. This review will focus on thioredoxin superfamily members and chaperones of C. elegans, which play an important role at the interface between redox and protein homeostasis. Additionally, this review will highlight recent methodological developments on in vivo and in vitro assessment of the redox state and their application to provide insights into the high complexity of redox and proteostasis networks of C. elegans.

Formation and Reversibility of BiP Protein Cysteine Oxidation Facilitate Cell Survival during and post Oxidative Stress

Redox fluctuations within cells can be detrimental to cell function. To gain insight into how cells normally buffer against redox changes to maintain cell function, we have focused on elucidating the signaling pathways that serve to sense and respond to oxidative redox stress within the endoplasmic reticulum (ER) using yeast as a model system. Previously, we have shown that a cysteine in the molecular chaperone BiP, a Hsp70 molecular chaperone within the ER, is susceptible to oxidation by peroxide during ER-derived oxidative stress, forming a sulfenic acid (-SOH) moiety. Here, we demonstrate that this same conserved BiP cysteine is susceptible also to glutathione modification (-SSG). Glutathionylated BiP is detected both as a consequence of enhanced levels of cellular peroxide and also as a by-product of increased levels of oxidized glutathione (GSSG). Similar to sulfenylation, we observe glutathionylation decouples BiP ATPase and peptide binding activities, turning BiP from an ATP-dependent foldase into an ATP-independent holdase. We show glutathionylation enhances cell proliferation during oxidative stress, which we suggest relates to modified BiP's increased ability to limit polypeptide aggregation. We propose the susceptibility of BiP to modification with glutathione may serve also to prevent irreversible oxidation of BiP by peroxide.

Glutaredoxin-deficiency confers bloodstream Trypanosoma brucei with improved thermotolerance

As constituents of their unusual trypanothione-based thiol metabolism, African trypanosomes express two dithiol glutaredoxins (Grxs), a cytosolic Grx1 and a mitochondrial Grx2, with so far unknown biological functions. As revealed by gel shift assays, in the mammalian bloodstream form of Trypanosoma brucei, Grx1 is in the fully reduced state. Upon diamide treatment of the cells, Grx1 forms an active site disulfide bridge that is rapidly re-reduced after stress removal; Cys76, a conserved non-active site Cys remains in the thiol state. Deletion of both grx1 alleles does not result in any proliferation defect of neither the procyclic insect form nor the bloodstream form, even not under various stress conditions. In addition, the Grx1-deficient parasites are fully infectious in the mouse model. A functional compensation by Grx2 is unlikely as identical levels of Grx2 were found in wildtype and Grx1-deficient cells. In the classical hydroxyethyl disulfide assay, Grx1-deficient bloodstream cells display 50-60% of the activity of wildtype cells indicating that the cytosolic oxidoreductase accounts for a major part of the total deglutathionylation capacity of the parasite. Intriguingly, at elevated temperature, proliferation of the Grx1-deficient bloodstream parasites is significantly less affected compared to wildtype cells. When cultured for three days at 39°C, only 51% of the cells in the wildtype population retained normal morphology with single mitochondrial and nuclear DNA (1K1N), whereas 27% of the cells displayed ≥2K2N. In comparison, 64% of the Grx1-deficient cells kept the 1K1N phenotype and only 18% had ≥2K2N. The data suggest that Grx1 plays a role in the regulation of the thermotolerance of the parasites by (in)directly interfering with the progression of the cell cycle, a process that may comprise protein (de)glutathionylation step(s).

Glutathionylation of the Bacterial Hsp70 Chaperone DnaK Provides a Link between Oxidative Stress and the Heat Shock Response

DnaK is the major bacterial Hsp70, participating in DNA replication, protein folding, and the stress response. DnaK cooperates with the Hsp40 co-chaperone DnaJ and the nucleotide exchange factor GrpE. Under non-stress conditions, DnaK binds to the heat shock transcription factor σ(32)and facilitates its degradation. Oxidative stress results in temporary inactivation of DnaK due to depletion of cellular ATP and thiol modifications such as glutathionylation until normal cellular ATP levels and a reducing environment are restored. However, the biological significance of DnaK glutathionylation remains unknown, and the mechanisms by which glutathionylation may regulate the activity of DnaK are also unclear. We investigated the conditions under which Escherichia coli DnaK undergoesS-glutathionylation. We observed glutathionylation of DnaK in lysates of E. coli cells that had been subjected to oxidative stress. We also obtained homogeneously glutathionylated DnaK using purified DnaK in the apo state. We found that glutathionylation of DnaK reversibly changes the secondary structure and tertiary conformation, leading to reduced nucleotide and peptide binding ability. The chaperone activity of DnaK was reversibly down-regulated by glutathionylation, accompanying the structural changes. We found that interaction of DnaK with DnaJ, GrpE, or σ(32)becomes weaker when DnaK is glutathionylated, and the interaction is restored upon deglutathionylation. This study confirms that glutathionylation down-regulates the functions of DnaK under oxidizing conditions, and this down-regulation may facilitate release of σ(32)from its interaction with DnaK, thus triggering the heat shock response. Such a mechanism provides a link between oxidative stress and the heat shock response in bacteria.

Development of 'Redox Arrays' for identifying novel glutathionylated proteins in the secretome

Proteomics techniques for analysing the redox status of individual proteins in complex mixtures tend to identify the same proteins due to their high abundance. We describe here an array-based technique to identify proteins undergoing glutathionylation and apply it to the secretome and the proteome of human monocytic cells. The method is based on incorporation of biotinylated glutathione (GSH) into proteins, which can then be identified following binding to a 1000-protein antibody array. We thus identify 38 secreted and 55 intracellular glutathionylated proteins, most of which are novel candidates for glutathionylation. Two of the proteins identified in these experiments, IL-1 sRII and Lyn, were then confirmed to be susceptible to glutathionylation. Comparison of the redox array with conventional proteomic methods confirmed that the redox array is much more sensitive, and can be performed using more than 100-fold less protein than is required for methods based on mass spectrometry. The identification of novel targets of glutathionylation, particularly in the secretome where the protein concentration is much lower, shows that redox arrays can overcome some of the limitations of established redox proteomics techniques.

ATP-dependent molecular chaperones in plastids--More complex than expected

Plastids are a class of essential plant cell organelles comprising photosynthetic chloroplasts of green tissues, starch-storing amyloplasts of roots and tubers or the colorful pigment-storing chromoplasts of petals and fruits. They express a few genes encoded on their organellar genome, called plastome, but import most of their proteins from the cytosol. The import into plastids, the folding of freshly-translated or imported proteins, the degradation or renaturation of denatured and entangled proteins, and the quality-control of newly folded proteins all require the action of molecular chaperones. Members of all four major families of ATP-dependent molecular chaperones (chaperonin/Cpn60, Hsp70, Hsp90 and Hsp100 families) have been identified in plastids from unicellular algae to higher plants. This review aims not only at giving an overview of the most current insights into the general and conserved functions of these plastid chaperones, but also into their specific plastid functions. Given that chloroplasts harbor an extreme environment that cycles between reduced and oxidized states, that has to deal with reactive oxygen species and is highly reactive to environmental and developmental signals, it can be presumed that plastid chaperones have evolved a plethora of specific functions some of which are just about to be discovered. Here, the most urgent questions that remain unsolved are discussed, and guidance for future research on plastid chaperones is given. This article is part of a Special Issue entitled: Chloroplast Biogenesis.

Analysis of protein thiol changes occurring during rat sperm epididymal maturation

The maturation of spermatozoa throughout the epididymal environment occurs in the complete absence of nuclear protein biosynthesis. As such, these cells rely heavily on posttranslational modifications of existing proteins in order to obtain the potential for fertilization. We have used an OxiCat approach to label both free and oxidized cysteine residues in rat sperm proteins and compared the ratio of reduced:oxidized peptides as these cells undergo epididymal transit. In all, 20 peptides, corresponding to 15 proteins, underwent a change in their redox status. Included in this list were A-kinase anchoring protein 4 and fatty acid-binding protein 9. Both of these proteins undergo intradisulfide bonding, leading to reduced solubility and, in the case of the latter, is likely to cause a loss of protein function. Interestingly, two glycolytic enzymes, hexokinase-1 and lactate dehydrogenase, also display increased cysteine oxidation during epididymal transit, which may be involved in the regulation of the enzyme activities.

Hypochlorite-induced structural modifications enhance the chaperone activity of human α2-macroglobulin

Hypochlorite, an oxidant generated in vivo by the innate immune system, kills invading pathogens largely by inducing the misfolding of microbial proteins. Concomitantly, the nonspecific activity of hypochlorite also damages host proteins, and the accumulation of damaged (misfolded) proteins is implicated in the pathology of a variety of debilitating human disorders (e.g., Alzheimer's disease, atherosclerosis, and arthritis). It is well-known that cells respond to oxidative stress by up-regulating proteostasis machinery, but the direct activation of mammalian chaperones by hypochlorite has not, to our knowledge, been previously reported. In this study, we show that hypochlorite-induced modifications of human α2-macroglobulin (α2M) markedly increase its chaperone activity by generating species, particularly dimers formed by dissociation of the native tetramer, which have enhanced surface hydrophobicity. Moreover, dimeric α2M is generated in whole-blood plasma in the presence of physiologically relevant amounts of hypochlorite. The chaperone activity of hypochlorite-modified α2M involves the formation of stable soluble complexes with misfolded client proteins, including heat-denatured enzymes, oxidized fibrinogen, oxidized LDL, and native or oxidized amyloid β-peptide (Aβ1-42). Here, we show that hypochlorite-modified α2M delivers its misfolded cargo to lipoprotein receptors on macrophages and reduces Aβ1-42 neurotoxicity. Our results support the conclusion that α2M is a specialized chaperone that prevents the extracellular accumulation of misfolded and potentially pathogenic proteins, particularly during innate immune system activity.

Protein glutathionylation in cardiovascular diseases

The perturbation of thiol-disulfide homeostasis is an important consequence of many diseases, with redox signals implicated in several physio-pathological processes. A prevalent form of cysteine modification is the reversible formation of protein mixed disulfides with glutathione (S-glutathionylation). The abundance of glutathione in cells and the ready conversion of sulfenic acids to S-glutathione mixed disulfides supports the reversible protein S-glutathionylation as a common feature of redox signal transduction, able to regulate the activities of several redox sensitive proteins. In particular, protein S-glutathionylation is emerging as a critical signaling mechanism in cardiovascular diseases, because it regulates numerous physiological processes involved in cardiovascular homeostasis, including myocyte contraction, oxidative phosphorylation, protein synthesis, vasodilation, glycolytic metabolism and response to insulin. Thus, perturbations in protein glutathionylation status may contribute to the etiology of many cardiovascular diseases, such as myocardial infarction, cardiac hypertrophy and atherosclerosis. Various reports show the importance of oxidative cysteine modifications in modulating cardiovascular function. In this review, we illustrate tools and strategies to monitor protein S-glutathionylation and describe the proteins so far identified as glutathionylated in myocardial contraction, hypertrophy and inflammation.

Role of phospholemman and the 70 kDa inhibitor protein in regulating Na+/K+ ATPase activity in pulmonary artery smooth muscle cells under U46619 stimulation

Treatment of bovine pulmonary smooth muscle cells with U46619 inhibited the Na(+)/K(+) ATPase activity in two parallel pathways: one of which is mediated via glutathionylation of the pump and the other by augmenting the inhibitory activity of the 70kDa inhibitor protein of Na(+)/K(+) ATPase. Although phospholemman deglutathionylates the pump leading to its activation, the inhibitor is responsible for irreversible inhibition of Na(+)/K(+) ATPase in an isoform specific manner during treatment of the cells with U46619.

Glutaredoxins in thiol/disulfide exchange

SIGNIFICANCE

Glutaredoxins (Grxs) are small oxidoreductases of the thioredoxin family of proteins regulating the thiol redox state of several proteins. Thereby, Grxs are key elements in redox signaling.

RECENT ADVANCES

Redox signaling via protein thiols depends on reversible oxidative modifications induced mainly by reactive oxygen/nitrogen species and glutathione (GSH) in form of its oxidized disulfide or S-nitroso-glutathione. Grxs contribute to redox signaling by the catalysis of glutathionylation, de-glutathionylation, as well as reduction of disulfide bridges via two distinct enzymatic mechanisms. The dithiol mechanism utilizes both active site cysteines to reduce disulfides, whereas the monothiol mechanism utilizes only the N-terminal active site cysteine for the reduction of GSH mixed disulfides. The sphere of action of Grxs continues to grow with the recent identification of novel targets.

CRITICAL ISSUES

Because of limited methodological tools, the identification of new substrates for oxidoreductases in general is one of the biggest challenges in this research area.

FUTURE DIRECTIONS

With this review, we provide a condensed summary of the current knowledge of thiol/disulfide exchange reactions catalyzed by Grxs regarding the mechanistic, structural, and functional aspects. The latter will be of high importance for future research directions, gaining novel insights into redox signaling in general, and the role of Grxs in particular.

Nitric oxide-mediated protein modification in cardiovascular physiology and pathology

Nitric oxide (NO) is a key regulator of cardiovascular functions including the control of vascular tone, anti-inflammatory properties of the endothelium, cardiac contractility, and thrombocyte activation and aggregation. Numerous experimental data support the view that NO not only acts via cyclic guanosine monophosphate (cGMP)-dependent mechanisms but also modulates protein function by nitrosation, nitrosylation, glutathiolation, and nitration, respectively. To understand how NO regulates all of these diverse biological processes on the molecular level a comprehensive assessment of NO-mediated cGMP-dependent and independent targets is required. Novel proteomic approaches allow the simultaneous identification of large quantities of proteins modified in an NO-dependent manner and thereby will considerably deepen our understanding of the role NO plays in cardiovascular physiology and pathophysiology.

Influences of temperature, oxidative stress, and phosphorylation on binding of heat shock proteins in skeletal muscle fibers

Heat shock proteins (HSPs) help maintain cellular function in stressful situations, but the processes controlling their interactions with target proteins are not well defined. This study examined the binding of HSP72, HSP25, and αB-crystallin in skeletal muscle fibers following various stresses. Rat soleus (SOL) and extensor digitorum longus (EDL) muscles were subjected in vitro to heat stress or strongly fatiguing stimulation. Superficial fibers were “skinned” by microdissection and HSP diffusibility assessed from the extent of washout following 10- to 30 min exposure to a physiological intracellular solution. In fibers from nonstressed (control) SOL muscle, >80% of each HSP is readily diffusible. However, after heating a muscle to 40°C for 30 min ∼95% of HSP25 and αB-crystallin becomes tightly bound at nonmembranous myofibrillar sites, whereas HSP72 bound at membranous sites only after heat treatment to ≥44°C. The ratio of reduced to oxidized cytoplasmic glutathione (GSH:GSSG) decreased approximately two- and fourfold after heating muscles to 40° and 45°C, respectively. The reducing agent dithiothreitol reversed HSP72 binding in heated muscles but had no effect on the other HSPs. Intense in vitro stimulation of SOL muscles, sufficient to elicit substantial oxidation-related loss of maximum force and approximately fourfold decrease in the GSH:GSSG ratio, had no effect on diffusibility of any of the HSPs. When skinned fibers from heat-treated muscles were bathed with additional exogenous HSP72, total binding increased approximately two- and 10-fold, respectively, in SOL and EDL fibers, possibly reflective of the relative sarco(endo)plasmic reticulum Ca2+-ATPase pump densities in the two fiber types. Phosphorylation at Ser59 on αB-crystallin and Ser85 on HSP25 increased with heat treatment but did not appear to determine HSP binding. The findings highlight major differences in the processes controlling binding of HSP72 and the two small HSPs. Binding was not directly related to cytoplasmic oxidative status, but oxidation of cysteine residues influenced HSP72 binding.

Differential redox proteomics allows identification of proteins reversibly oxidized at cysteine residues in endothelial cells in response to acute hypoxia

Adaptation to decreased oxygen availability (hypoxia) is crucial for proper cell function and survival. In metazoans, this is partly achieved through gene transcriptional responses mediated by hypoxia-inducible factors (HIFs). There is abundant evidence that production of reactive oxygen species (ROS) increases during hypoxia, which contributes to the activation of the HIF pathway. In addition to altering the cellular redox balance, leading to oxidative stress, ROS can transduce signals by reversibly modifying the redox state of cysteine residues in certain proteins. Using the "redox fluorescence switch" (RFS), a thiol redox proteomic technique that fluorescently labels reversibly oxidized cysteines, we analyzed endothelial cells subjected to acute hypoxia and subsequent reoxygenation. We observed a general increase in cysteine oxidation during hypoxia, which was reversed by reoxygenation, and two-dimensional electrophoresis revealed the differential oxidation of specific proteins. Using complementary derivatization techniques, we confirmed the modification of individual target proteins and identified specific cysteine residues that were oxidized in hypoxic conditions, thereby overcoming several limitations associated with fluorescence derivatization. These findings provide an important basis for future studies of the role of these modifications in HIF activation and in other acute adaptive responses to hypoxia.

S-Glutathionylation signaling in cell biology: progress and prospects

S-Glutathionylation is a mechanism of signal transduction by which cells respond effectively and reversibly to redox inputs. The glutathionylation regulates most cellular pathways. It is involved in oxidative cellular response to insult by modulating the transcription factor Nrf2 and inducing the expression of antioxidant genes (ARE); it contributes to cell survival through nuclear translocation of NFkB and activation of survival genes, and to cell death by modulating the activity of caspase 3. It is involved in mitotic spindle formation during cell division by binding cytoskeletal proteins thus contributing to cell proliferation and differentiation. Glutathionylation also interfaces with the mechanism of phosphorylation by modulating several kinases (PKA, CK) and phosphatases (PP2A, PTEN), thus allowing a cross talk between the two processes of signal transduction. Also, skeletal RyR1 channels responsible of muscle excitation-contraction coupling appear to be sensitive to glutathionylation. Members of the ryanodine receptor super family, responsible for Ca(2) release from endoplasmic reticulum stores, contain sulfhydryl groups that function as a redox "switch", which either induces or inhibits Ca(2) release. Finally, but very importantly, glutathionylation of proteins may also act on cell metabolism by modulating enzymes involved in glycosylation, in the Krebs cycle and in mitochondrial oxidative phosphorylation. In this review, we propose a greater role for glutathionylation in cell biology: not only a cellular response to oxidative stress, but an elegant and sensitive mechanism able to respond even to subtle changes in redox balance in the different cellular compartments. Given the wide spectrum of redox-sensitive proteins, we discuss the possibility that different pathways light up by glutathionylation under various pathological conditions. The feature of reversibility of this process also makes it prone to develop targeted drug therapies and monitor the pharmacological effectiveness once identified the sensor proteins involved.

Each to their own: skeletal muscles of different function use different biochemical strategies during aestivation at high temperature

Preservation of muscle morphology depends on a continuing regulatory balance between molecules that protect, and molecules that damage, muscle structural integrity. Excessive disruption of the biochemical balance that favours reactive oxygen species (ROS) in disused muscles may lead to oxidative stress; which in turn is associated with increased atrophic or apoptotic signalling and/or oxidative damage to the muscle and thus muscle disuse atrophy. Increases in rate of oxygen consumption likely increase the overall generation of ROS in vivo. Temperature-induced increases in muscle oxygen consumption rate occur in some muscles of ectotherms undergoing prolonged muscular disuse during aestivation. In the green-striped burrowing frog, Cyclorana alboguttata, both large jumping muscles and small non-jumping muscles undergo atrophy seemingly commensurate with their rate of oxygen consumption during aestivation. However, since the extent of atrophy in these muscles is not enhanced at higher temperatures despite a temperature sensitive rate of oxygen consumption in the jumping muscle, we proposed that muscles are protected by biochemical means that when mobilised at higher temperatures inhibit atrophy. We proposed the biochemical response to temperature would be muscle-specific. We examined the effect of temperature on the antioxidant and heat shock protein systems and evidence of oxidative damage to lipids and proteins in two functionally different skeletal muscles, gastrocnemius (jumping muscle) and iliofibularis (non-jumping muscle), by aestivating frogs at 24 and 30oC for six months. We assayed small molecule antioxidant capacity, mitochondrial and cytosolic SOD and Hsp70 to show that protective mechanisms in disused muscles are differentially regulated both with respect to temperature and aestivation. High aestivation temperature results in an antioxidant response in the metabolically temperature-sensitive jumping muscle. We assayed lipid peroxidation and protein oxidation to show that oxidative damage is apparent during aestivation and its pattern is muscle-specific, but unaffected by temperature. Consideration is given to how the complex responses of muscle biochemistry inform of the different strategies muscles may use in regulating their oxidative environment during extended disuse and disuse at high temperature.

Impact of diabetes on alpha-crystallins and other heat shock proteins in the eye

Diabetes and its related complications represent a major growing health concern and economic burden worldwide. Ocular manifestations of diabetes include cataractogenesis and retinopathy, the latter being the leading cause of blindness in the working-age population. Despite numerous studies and recent progress, the exact pathophysiology of the disease remains to be fully elucidated and development of new and improved therapeutic strategies for this chronic condition are greatly needed. Heat shock proteins (Hsps) are highly conserved families of proteins, which are generally regarded as protective molecules that play a wide variety of roles and can be expressed in response to different types of cellular stresses. In recent years, numerous studies have reported their implication in various ocular diseases including diabetic retinopathy. The present review focuses on the potential implication of Hsps in ocular diabetic complications and discusses their specific mechanisms of regulation with respect to their expression, functions and alteration during diabetes. The review will conclude by examining the potential of Hsps as therapeutic agents or targets for the treatment of diabetic retinopathy.

Apoptosis-inducing factor: structure, function, and redox regulation

Apoptosis-inducing factor (AIF) is a flavin adenine dinucleotide-containing, NADH-dependent oxidoreductase residing in the mitochondrial intermembrane space whose specific enzymatic activity remains unknown. Upon an apoptotic insult, AIF undergoes proteolysis and translocates to the nucleus, where it triggers chromatin condensation and large-scale DNA degradation in a caspase-independent manner. Besides playing a key role in execution of caspase-independent cell death, AIF has emerged as a protein critical for cell survival. Analysis of in vivo phenotypes associated with AIF deficiency and defects, and identification of its mitochondrial, cytoplasmic, and nuclear partners revealed the complexity and multilevel regulation of AIF-mediated signal transduction and suggested an important role of AIF in the maintenance of mitochondrial morphology and energy metabolism. The redox activity of AIF is essential for optimal oxidative phosphorylation. Additionally, the protein is proposed to regulate the respiratory chain indirectly, through assembly and/or stabilization of complexes I and III. This review discusses accumulated data with respect to the AIF structure and outlines evidence that supports the prevalent mechanistic view on the apoptogenic actions of the flavoprotein, as well as the emerging concept of AIF as a redox sensor capable of linking NAD(H)-dependent metabolic pathways to apoptosis.

Effects of oxidative stress on behavior, physiology, and the redox thiol proteome of Caenorhabditis elegans

Accumulation of reactive oxygen species has been implicated in various diseases and aging. However, the precise physiological effects of accumulating oxidants are still largely undefined. Here, we applied a short-term peroxide stress treatment to young Caenorhabditis elegans and measured behavioral, physiological, and cellular consequences. We discovered that exposure to peroxide stress causes a number of immediate changes, including loss in mobility, decreased growth rate, and decreased cellular adenosine triphosphate levels. Many of these alterations, which are highly reminiscent of changes in aging animals, are reversible, suggesting the presence of effective antioxidant systems in young C. elegans. One of these antioxidant systems involves the highly abundant protein peroxiredoxin 2 (PRDX-2), whose gene deletion causes phenotypes symptomatic of chronic peroxide stress and shortens lifespan. Applying the quantitative redox proteomic technique OxICAT to oxidatively stressed wild-type and prdx-2 deletion worms, we identified oxidation-sensitive cysteines in 40 different proteins, including proteins involved in mobility and feeding (e.g., MYO-2 and LET-75), protein translation and homeostasis (e.g., elongation factor 1 [EFT-1] and heat shock protein 1), and adenosine triphosphate regeneration (e.g., nucleoside diphosphate kinase). The oxidative modification of some of these redox-sensitive cysteines may contribute to the physiological and behavioral changes observed in oxidatively stressed animals.

Use of dimedone-based chemical probes for sulfenic acid detection methods to visualize and identify labeled proteins

Reversible thiol modification is a major component of the modulation of cell-signaling pathways by reactive oxygen species. Hydrogen peroxide, peroxynitrite, or lipid hydroperoxides are all able to oxidize cysteines to form cysteine sulfenic acids; this reactive intermediate can be directly reduced to thiol by cellular reductants such as thioredoxin or further participate in disulfide bond formation with glutathione or cysteine residues in the same or another protein. To identify the direct protein targets of cysteine modification and the conditions under which they are oxidized, a series of dimedone-based reagents linked to affinity or fluorescent tags have been developed that specifically alkylate and trap cysteine sulfenic acids. In this chapter, we provide detailed methods using one of our biotin-tagged reagents, DCP-Bio1, to identify and monitor proteins that are oxidized in vitro and in vivo. Using streptavidin-linked agarose beads, this biotin-linked reagent can be used to affinity capture labeled proteins. Stringent washing of the beads prior to elution minimizes the contamination of the enriched material with unlabeled proteins through coimmunoprecipitation or nonspecific binding. In particular, we suggest including DTT in one of the washes to remove proteins covalently linked to biotinylated proteins through a disulfide bond, except in cases where these linked proteins are of interest. We also provide methods for targeted approaches monitoring cysteine oxidation in individual proteins, global approaches to follow total cysteine oxidation in the cell, and guidelines for proteomic analyses to identify novel proteins with redox sensitive cysteines.

Methods for analysis of protein glutathionylation and their application to photosynthetic organisms

Protein S-glutathionylation, the reversible formation of a mixed-disulfide between glutathione and protein thiols, is involved in protection of protein cysteines from irreversible oxidation, but also in protein redox regulation. Recent studies have implicated S-glutathionylation as a cellular response to oxidative/nitrosative stress, likely playing an important role in signaling. Considering the potential importance of glutathionylation, a number of methods have been developed for identifying proteins undergoing glutathionylation. These methods, ranging from analysis of purified proteins in vitro to large-scale proteomic analyses in vivo, allowed identification of nearly 200 targets in mammals. By contrast, the number of known glutathionylated proteins is more limited in photosynthetic organisms, although they are severely exposed to oxidative stress. The aim of this review is to detail the methods available for identification and analysis of glutathionylated proteins in vivo and in vitro. The advantages and drawbacks of each technique will be discussed as well as their application to photosynthetic organisms. Furthermore, an overview of known glutathionylated proteins in photosynthetic organisms is provided and the physiological importance of this post-translational modification is discussed.

Redox-based regulation of signal transduction: principles, pitfalls, and promises

Oxidants are produced as a by-product of aerobic metabolism, and organisms ranging from prokaryotes to mammals have evolved with an elaborate and redundant complement of antioxidant defenses to confer protection against oxidative insults. Compelling data now exist demonstrating that oxidants are used in physiological settings as signaling molecules with important regulatory functions controlling cell division, migration, contraction, and mediator production. These physiological functions are carried out in an exquisitely regulated and compartmentalized manner by mild oxidants, through subtle oxidative events that involve targeted amino acids in proteins. The precise understanding of the physiological relevance of redox signal transduction has been hampered by the lack of specificity of reagents and the need for chemical derivatization to visualize reversible oxidations. In addition, it is difficult to measure these subtle oxidation events in vivo. This article reviews some of the recent findings that illuminate the significance of redox signaling and exciting future perspectives. We also attempt to highlight some of the current pitfalls and the approaches needed to advance this important area of biochemical and biomedical research.

Hyperthermic stress-induced increase in the expression of glutamate-cysteine ligase and glutathione levels in the symbiotic sea anemone Aiptasia pallida

Hyperthermic stress is known to trigger the loss of unicellular algae from a number of symbiotic cnidarians, a phenomenon commonly referred to as bleaching. Oxidative and nitrosative stress have been suggested to play a major role during the process of bleaching, however the underlying molecular mechanisms are still poorly understood. In animals, the intracellular tripeptide glutathione (GSH) is involved in antioxidant defense, redox homeostasis and intracellular redox signaling. Therefore, we tested the hypothesis that hyperthermal stress-induced bleaching in Aiptasia pallida, a model for symbiotic cnidarians, results in increased levels of GSH synthesis. We report the cDNA sequence and functional analysis of the catalytic subunit of glutamate-cysteine ligase (GCLC), which catalyzes the rate-limiting step in GSH biosynthesis. In a time-series experiment, both GCLC gene expression and total GSH levels increased 4- and 1.5-fold, respectively, in response to hyperthermal stress. These results suggest that hyperthermal stress triggers adaptive increases in intracellular GSH biosynthesis in cnidarians as a protective response to oxidative/nitrosative stress. Our results show the conserved function of GCLC and GSH across animals while placing a new perspective on the role of GSH in redox signaling during cnidarian bleaching.

Nitrosative stress leads to protein glutathiolation, increased s-nitrosation, and up-regulation of peroxiredoxins in the heart

Nitric oxide (NO) is produced by different isoforms of nitric oxide synthases (NOSs) and operates as a mediator of important cell signaling pathways, such as the cGMP signaling cascade. Another mechanism by which NO exerts biological effects is mediated through S-nitrosation of target proteins. To explore thiol-based protein modifications in a situation of defined nitrosative stress, we used a transgenic mouse model with cardiac specific overexpression of inducible nitric oxide synthase (iNOS) and concomitant myoglobin deficiency (iNOS(+)/myo(-/-)). In comparison with the wild type hearts, protein glutathiolation detected by immunoblotting was significantly enhanced in iNOS(+)/myo(-/-) hearts, whereas protein S-nitrosation as measured by the biotin switch assay and two-dimensional PAGE revealed that nearly all of the detected proteins ( approximately 60) remained unchanged with the exception of three proteins. Tandem mass spectrometry revealed these proteins to be peroxiredoxins (Prxs), which are known to possess peroxidase activity, whereby hydrogen peroxide, peroxynitrite, and a wide range of organic hydroperoxides are reduced and detoxified. Immunoblotting with specific antibodies revealed up-regulation of Prx VI in the iNOS(+)/myo(-/-) hearts, whereas expression of Prx II and Prx III remained unchanged. Furthermore, the analysis of the cardiac S-nitrososubproteome identified several new proteins possibly being involved in NO-signaling pathways. Our data indicate that S-nitrosation and glutathiolation of cardiac proteins may contribute to the phenotype of NO-induced heart failure. The up-regulation of antioxidant proteins like Prx VI appears to be an additional mechanism to antagonize an excess of reactive oxygen/nitrogen species. Furthermore, S-nitrosation of Prxs may serve a new function in the signaling cascade of nitrosative stress.

Molecular mechanisms and potential clinical significance of S-glutathionylation

Protein S-glutathionylation, the reversible binding of glutathione to protein thiols (PSH), is involved in protein redox regulation, storage of glutathione, and protection of PSH from irreversible oxidation. S-Glutathionylated protein (PSSG) can result from thiol/disulfide exchange between PSH and GSSG or PSSG; direct interaction between partially oxidized PSH and GSH; reactions between PSH and S-nitrosothiols, oxidized forms of GSH, or glutathione thiyl radical. Indeed, thiol/disulfide exchange is an unlikely intracellular mechanism for S-glutathionylation, because of the redox potential of most Cys residues and the GSSG export by most cells as a protective mechanism against oxidative stress. S-Glutathionylation can be reversed, following restoration of a reducing GSH/GSSG ratio, in an enzyme-dependent or -independent manner. Currently, definite evidence of protein S-glutathionylation has been clearly demonstrated in few human diseases. In aging human lenses, protein S-glutathionylation increases; during cataractogenesis, some of lens proteins, including alpha- and beta-crystallins, form both mixed disulfides and disulfide-cross-linked aggregates, which increase with cataract severity. The correlation of lens nuclear color and opalescence intensity with protein S-glutathionylation indicates that protein-thiol mixed disulfides may play an important role in cataractogenesis and development of brunescence in human lenses. Recently, specific PSSG have been identified in the inferior parietal lobule in Alzheimer's disease. However, much investigation is needed to clarify the actual involvement of protein S-glutathionylation in many human diseases.

Thioredoxins and glutaredoxins as facilitators of protein folding

Thiol-disulfide oxidoreductase systems of bacterial cytoplasm and eukaryotic cytosol favor reducing conditions and protein thiol groups, while bacterial periplasm and eukaryotic endoplasmatic reticulum provide oxidizing conditions and a machinery for disulfide bond formation in the secretory pathway. Oxidoreductases of the thioredoxin fold superfamily catalyze steps in oxidative protein folding via protein-protein interactions and covalent catalysis to act as chaperones and isomerases of disulfides to generate a native fold. The active site dithiol/disulfide of thioredoxin fold proteins is CXXC where variations of the residues inside the disulfide ring are known to increase the redox potential like in protein disulfide isomerases. In the catalytic mechanism thioredoxin fold proteins bind to target proteins through conserved backbone-backbone hydrogen bonds and induce conformational changes of the target disulfide followed by nucleophilic attack by the N-terminally located low pK(a) Cys residue. This generates a mixed disulfide covalent bond which subsequently is resolved by attack from the C-terminally located Cys residue. This review will focus on two members of the thioredoxin superfamily of proteins known to be crucial for maintaining a reduced intracellular redox state, thioredoxin and glutaredoxin, and their potential functions as facilitators and regulators of protein folding and chaperone activity.

Targets of tyrosine nitration in diabetic rat retina

Diabetic retinopathy, a retinal vascular disease, is inhibited in animals treated with aminoguanidine, an inhibitor of inducible nitric-oxide synthase. This treatment also reduces retinal protein nitration, which is greater in diabetic rat retina than nondiabetic retina. As an approach to understanding the molecular mechanisms of diabetic retinopathy, we sought the identity of nitrotyrosine-containing proteins in retina from streptozotocin-induced diabetic rats and in a rat retinal Müller cell line grown in high glucose (25 mM). Anti-nitrotyrosine immunoprecipitation products from rat retina and Müller cells were analyzed by LC-MS/MS. Ten nitrated proteins in diabetic rat retina and three nitrated proteins in Müller cells grown in high glucose were identified; three additional nitrotyrosine-containing proteins were tentatively identified from diabetic retina. The identified nitrotyrosine-containing proteins participate in a variety of processes including glucose metabolism, signal transduction, and transcription/translation. Among the nitrated proteins were insulin-responsive glucose transporter type 4 (GLUT-4), which has been implicated previously in the pathogenesis of diabetes mellitus; exocyst complex component Exo70, which functions in insulin-stimulated glucose uptake of GLUT-4-containing vesicles; and fibroblast growth factor receptor 2, which influences retinal vascularization via fibroblast growth factor signaling. Nitration of tyrosine phosphorylation sites were identified in five proteins, including GLUT-4, exocyst complex component Exo70, protein-tyrosine phosphatase eta, sensory neuron synuclein, and inositol trisphosphate receptor 3. Quantitation of nitration and phosphorylation at common tyrosine modification sites in GLUT-4 and protein-tyrosine phosphatase eta from diabetic and nondiabetic animals suggests that nitration reduced tyrosine phosphorylation approximately 2X in these proteins from diabetic retina. The present results provide new insights regarding tyrosine nitration and its potential role in the molecular mechanisms of diabetic retinopathy.

S-glutathionylation in protein redox regulation

Protein S-glutathionylation, the reversible formation of mixed disulfides between glutathione and low-pKa cysteinyl residues, not only is a cellular response to mild oxidative/nitrosative stress, but also occurs under basal (physiological) conditions. S-glutathionylation has now emerged as a potential mechanism for dynamic, posttranslational regulation of a variety of regulatory, structural, and metabolic proteins. Moreover, substantial recent studies have implicated S-glutathionylation in the regulation of signaling and metabolic pathways in intact cellular systems. The growing list of S-glutathionylated proteins, in both animal and plant cells, attests to the occurrence of S-glutathionylation in cellular response pathways. The existence of antioxidant enzymes that specifically regulate S-glutathionylation would emphasize its importance in modulating protein function, suggesting that this protein modification too might have a role in cell signaling. The continued development of proteomic and analytical methods for disulfide analysis will help us better understand the full extent of the roles these modifications play in the regulation of cell function. In this review, we describe recent breakthroughs in our understanding of the potential role of protein S-glutathionylation in the redox regulation of signal transduction.

Redox control of neural function: background, mechanisms, and significance

The redox environment within neural cells is dependent on a series of redox couples. The glutathione disulfide/ glutathione (GSSG/GSH) redox pair forms the major redox couple in cells and as such plays a critical role in regulating redox-dependent cellular functions. Not only does GSH act as an antioxidant but it also can modulate the activity of a variety of different proteins via S-glutathionylation of cysteine sulfhydryl groups. The thioredoxin system also makes a significant contribution to the redox environment by reducing inter- and intrachain protein disulfide bonds as well as maintaining the activity of important antioxidant enzymes such as peroxiredoxins and methionine sulfoxide reductases. The redox environment affects the activity and function of a number of different protein phosphatases, protein kinases, and transcription factors. The sum of these effects will determine how changes in the redox environment alter overall cellular function, thereby playing a fundamental role in regulating neural cell fate and physiology.

Thioredoxins, glutaredoxins, and glutathionylation: new crosstalks to explore

Oxidants are widely considered as toxic molecules that cells have to scavenge and detoxify efficiently and continuously. However, emerging evidence suggests that these oxidants can play an important role in redox signaling, mainly through a set of reversible post-translational modifications of thiol residues on proteins. The most studied redox system in photosynthetic organisms is the thioredoxin (TRX) system, involved in the regulation of a growing number of target proteins via thiol/disulfide exchanges. In addition, recent studies suggest that glutaredoxins (GRX) could also play an important role in redox signaling especially by regulating protein glutathionylation, a post-translational modification whose importance begins to be recognized in mammals while much less is known in photosynthetic organisms. This review focuses on oxidants and redox signaling with particular emphasis on recent developments in the study of functions, regulation mechanisms and targets of TRX, GRX and glutathionylation. This review will also present the complex emerging interplay between these three components of redox-signaling networks.

Redox regulatory mechanisms in cellular stress responses

BACKGROUND

Reactive oxygen species are produced in a highly localized and specific pattern in biological stress responses. The present review examines the redox regulatory aspects of a number of molecular stress response mechanisms in both prokaryotes and eukaryotes.

SCOPE

The present review provides examples representing both the cytoplasmic stress response, often studied as the heat shock response, as well as the stress response of the endoplasmic reticulum, known as the unfolded protein response. The examples have been selected to illustrate the variety of ways that redox signals mediate and affect stress responses.

CONCLUSIONS

Redox regulatory mechanisms are intricately embedded in both the cytoplasmic and endoplasmic reticulum stress responses at multiple levels. Many different stimuli, both internal and external, activate endogenous production of reactive oxygen species as a necessary part of the intracellular communication system that activates stress responses.

The effects of glutaredoxin and copper activation pathways on the disulfide and stability of Cu,Zn superoxide dismutase

Mutations in Cu,Zn superoxide dismutase (SOD1) can cause amyotrophic lateral sclerosis (ALS) through mechanisms proposed to involve SOD1 misfolding, but the intracellular factors that modulate folding and stability of SOD1 are largely unknown. By using yeast and mammalian expression systems, we demonstrate here that SOD1 stability is governed by post-translational modification factors that target the SOD1 disulfide. Oxidation of the human SOD1 disulfide in vivo was found to involve both the copper chaperone for SOD1 (CCS) and the CCS-independent pathway for copper activation. When both copper pathways were blocked, wild type SOD1 stably accumulated in yeast cells with a reduced disulfide, whereas ALS SOD1 mutants A4V, G93A, and G37R were degraded. We describe here an unprecedented role for the thiol oxidoreductase glutaredoxin in reducing the SOD1 disulfide and destabilizing ALS mutants. Specifically, the major cytosolic glutaredoxin of yeast was seen to reduce the intramolecular disulfide of ALS SOD1 mutant A4V SOD1 in vivo and in vitro. By comparison, glutaredoxin was less reactive toward the disulfide of wild type SOD1. The apo-form of A4V SOD1 was highly reactive with glutaredoxin but not SOD1 containing both copper and zinc. Glutaredoxin therefore preferentially targets the immature form of ALS mutant SOD1 lacking metal co-factors. Overall, these studies implicate a critical balance between cellular reductants such as glutaredoxin and copper activation pathways in controlling the disulfide and stability of SOD1 in vivo.

Protein glutathiolation by nitric oxide: an intracellular mechanism regulating redox protein modification

This study was designed to examine whether NO regulates protein glutathiolation. Exposure to NO donors increased protein glutathiolation in COS-7 or rat aortic smooth muscle cells as detected by anti-protein glutathione (GSH) antibodies. This process was reversible and saturable. Stimulation with acetylcholine (ACh) increased protein glutathiolation in isolated rat aortic rings. This was prevented by inhibiting endothelial NO synthase (eNOS). In ACh-treated rings, proteins showing positive immunoreactivity with the anti-PSSG antibody (Ab) were identified by matrix assisted laser desorption-time-of-flight mass spectrometry to be actin, vimentin, and heat shock protein 70. Purified actin was more readily glutathiolated by S-nitrosoglutathione than by oxidized GSH as determined by electrospray-ionization mass spectrometry, and nitrosylated actin was glutathiolated by reduced GSH. Relative to wild-type (WT) mice, increased protein glutathiolation was observed in hearts of mice with cardiac-specific expression of inducible NO synthase (iNOS). Proteins immunoprecipitated from transgenic hearts revealed GSH-adducted peptides corresponding to adenine nucleotide translocator and the alpha-subunit of F1F0ATPase. These data suggest that exogenous NO or NO generated by eNOS or iNOS regulates protein adduction with GSH. This could be due to a direct reaction of proteins with S-nitrosoglutathione or denitrosylation of S-nitrosylated proteins by reduced GSH. Glutathiolation of cytoskeletal and mitochondrial proteins may be a significant feature of NO bioreactivity.

Redox proteomics: identification and functional role of glutathionylated proteins

Although radical oxygen and nitrogen species are harmful molecules that destroy cell functions, many operate as mediators of important cell signaling pathways when not in excess. Oxidants can modify protein function through the covalent, reversible addition of glutathione to cysteine. This review addresses different proteomic methods of identifying glutathionylation targets and emphasizes ways of defining their pattern of modification in response to oxidative stimuli in cells. Finally, the literature on nonproteomic studies that investigate the functional changes induced by glutathionylation are reviewed and future studies are commented on.

Retinal proteins modified by 4-hydroxynonenal: identification of molecular targets

The reactive aldehyde, 4-hydroxynonenal (HNE), is a product of lipid peroxidation that can covalently modify and inactivate proteins. Previously, we reported increased HNE modification of select retinal proteins resolved by one-dimensional gel electrophoresis in aged Fisher 344 x Brown Norway rats (Louie, J.L., Kapphahn, R.J., Ferrington, D.A., 2002. Proteasome function and protein oxidation in the aged retina. Exp. Eye Res. 75, 271-284). In the current study, quantitative assessment of HNE molar content using slot blot immunoassays showed HNE content is increased 30% in aged rat retina. In contrast, there was no age-related difference in HNE content in individual spots resolved by 2D gel electrophoresis suggesting the increased modification is likely on membrane proteins that are missing on 2D gels. The HNE-immunoreactive proteins resolved by 2D gel electrophoresis were identified by MALDI-TOF mass spectrometry. These proteins are involved in metabolism, chaperone function, and fatty acid transport. Proteins that were frequently modified and had the highest molar content of HNE included triosephosphate isomerase, alpha enolase, heat shock cognate 70 and betaB2 crystallin. Immunochemical detection of HNE adducts on retinal sections showed greater immune reaction in ganglion cells, photoreceptor inner segment, and the inner plexiform layer. Identification of HNE modified proteins in two alternative model systems, human retinal pigment epithelial cells in culture (ARPE19) and human donor eyes, indicated that triosephosphate isomerase and alpha enolase are generally modified. These results identify a common subset of proteins that contain HNE adducts and suggest that select retinal proteins are molecular targets for HNE modification.

Regulation of protein function by glutathionylation

The main function of reduced glutathione (GSH) is to protect from oxidative stress as a reactive oxygen scavenger. However, in the context of redox regulation, the ratio between GSH and its oxidized form (GSSG) determines the redox state of redox-sensitive cysteines in some proteins and, thus, acts as a signaling system. While GSH/GSSG can catalyze oxido-reduction of intra- and inter-chain disulfides by thiol-disulfide exchange, this review focuses on the formation of mixed disulfides between glutathione and proteins, also known as glutathionylation. The review discusses the regulatory role of this post-translational modification and the role of protein disulfide oxidoreductases (thioredoxin/thioredoxin reductase, glutaredoxin, protein disulfide isomerase) in the reversibility of this process.

S-glutathionylation: from redox regulation of protein functions to human diseases

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) play an integral role in the modulation of several physiological functions but can also be potentially destructive if produced in excessive amounts. Protein cysteinyl thiols appear especially sensitive to ROS/RNS attack. Experimental evidence started to accumulate recently, documenting that S-glutathionylation occurs in a number of physiologically relevant situations, where it can produce discrete modulatory effects on protein function. The increasing evidence of functional changes resulting from this modification, and the growing number of proteins shown to be S-glutathionylated both in vitro and in vivo support this contention, and confirm this as an attractive area of research. S-glutathionylated proteins are now actively investigated with reference to problems of biological interest and as possible biomarkers of human diseases associated with oxidative/nitrosative stress.