The Conundrum of Hydrogen Peroxide Signaling and the Emerging Role of Peroxiredoxins as Redox Relay Hubs

Cancer metabolism and tumor microenvironment: fostering each other?

The changes associated with malignancy are not only in cancer cells but also in environment in which cancer cells live. Metabolic reprogramming supports tumor cell high demand of biogenesis for their rapid proliferation, and helps tumor cell to survive under certain genetic or environmental stresses. Emerging evidence suggests that metabolic alteration is ultimately and tightly associated with genetic changes, in particular the dysregulation of key oncogenic and tumor suppressive signaling pathways. Cancer cells activate HIF signaling even in the presence of oxygen and in the absence of growth factor stimulation. This cancer metabolic phenotype, described firstly by German physiologist Otto Warburg, insures enhanced glycolytic metabolism for the biosynthesis of macromolecules. The conception of metabolite signaling, i.e., metabolites are regulators of cell signaling, provides novel insights into how reactive oxygen species (ROS) and other metabolites deregulation may regulate redox homeostasis, epigenetics, and proliferation of cancer cells. Moreover, the unveiling of noncanonical functions of metabolic enzymes, such as the moonlighting functions of phosphoglycerate kinase 1 (PGK1), reassures the importance of metabolism in cancer development. The metabolic, microRNAs, and ncRNAs alterations in cancer cells can be sorted and delivered either to intercellular matrix or to cancer adjacent cells to shape cancer microenvironment via media such as exosome. Among them, cancer microenvironmental cells are immune cells which exert profound effects on cancer cells. Understanding of all these processes is a prerequisite for the development of a more effective strategy to contain cancers.

In vivo hydrogen peroxide diffusivity in brain tissue supports volume signaling activity

Hydrogen peroxide is a major redox signaling molecule underlying a novel paradigm of cell function and communication. A role for H₂O₂ as an intercellular signaling molecule and neuromodulator in the brain has become increasingly apparent, with evidence showing this biological oxidant to regulate neuronal polarity, connectivity, synaptic transmission and tuning of neuronal networks. This notion is supported by its ability to diffuse in the extracellular space, from source of production to target. It is, thus, crucial to understand extracellular H₂O₂ concentration dynamics in the living brain and the factors which shape its diffusion pattern and half-life. To address this issue, we have used a novel microsensor to measure H₂O₂ concentration dynamics in the brain extracellular matrix both in an ex vivo model using rodent brain slices and in vivo. We found that exogenously applied H₂O₂ is removed from the extracellular space with an average half-life of t_1/2 = 2.2 s in vivo. We determined the in vivo effective diffusion coefficient of H₂O₂ to be D* = 2.5 × 10⁻⁵ cm² s⁻¹. This allows it to diffuse over 100 μm in the extracellular space within its half-life. Considering this, we can tentatively place H₂O₂ within the class of volume neurotransmitters, connecting all cell types within the complex network of brain tissue, regardless of whether they are physically connected. These quantitative details of H₂O₂ diffusion and half-life in the brain allow us to interpret the physiology of the redox signal and lay the pavement to then address dysregulation in redox homeostasis associated with disease processes.

Bioinformatic Analyses of Peroxiredoxins and RF-Prx: A Random Forest-Based Predictor and Classifier for Prxs

Peroxiredoxins (Prxs) are a protein superfamily, present in all organisms, that play a critical role in protecting cellular macromolecules from oxidative damage but also regulate intracellular and intercellular signaling processes involving redox-regulated proteins and pathways. Bioinformatic approaches using computational tools that focus on active site-proximal sequence fragments (known as active site signatures) and iterative clustering and searching methods (referred to as TuLIP and MISST) have recently enabled the recognition of over 38,000 peroxiredoxins, as well as their classification into six functionally relevant groups. With these data providing so many examples of Prxs in each class, machine learning approaches offer an opportunity to extract additional information about features characteristic of these protein groups.In this study, we developed a novel computational method named "RF-Prx" based on a random forest (RF) approach integrated with K-space amino acid pairs (KSAAP) to identify peroxiredoxins and classify them into one of six subgroups. Our process performed in a superior manner compared to other machine learning classifiers. Thus the RF approach integrated with K-space amino acid pairs enabled the detection of class-specific conserved sequences outside the known functional centers and with potential importance. For example, drugs designed to target Prx proteins would likely suffer from cross-reactivity among distinct Prxs if targeted to conserved active sites, but this may be avoidable if remote, class-specific regions could be targeted instead.

The permeability of human red blood cell membranes to hydrogen peroxide is independent of aquaporins

Hydrogen peroxide (H₂O₂) not only is an oxidant but also is an important signaling molecule in vascular biology, mediating several physiological functions. Red blood cells (RBCs) have been proposed to be the primary sink of H₂O₂ in the vasculature because they are the main cellular component of blood with a robust antioxidant defense and a high membrane permeability. However, the exact permeability of human RBC to H₂O₂ is neither known nor is it known if the mechanism of permeation involves the lipid fraction or protein channels. To gain insight into the permeability process, we measured the partition constant of H₂O₂ between water and octanol or hexadecane using a novel double-partition method. Our results indicated that there is a large thermodynamic barrier to H₂O₂ permeation. The permeability coefficient of H₂O₂ through phospholipid membranes containing cholesterol with saturated or unsaturated acyl chains was determined to be 4 × 10⁻⁴ and 5 × 10⁻³ cm s⁻¹, respectively, at 37 °C. The permeability coefficient of human RBC membranes to H₂O₂ at 37 °C, on the other hand, was 1.6 × 10⁻³ cm s⁻¹. Different aquaporin-1 and aquaporin-3 inhibitors proved to have no effect on the permeation of H₂O₂. Moreover, human RBCs devoid of either aquaporin-1 or aquaporin-3 were equally permeable to H₂O₂ as normal human RBCs. Therefore, these results indicate that H₂O₂ does not diffuse into RBCs through aquaporins but rather through the lipid fraction or a still unidentified membrane protein.

The Peroxisome-Autophagy Redox Connection: A Double-Edged Sword?

Peroxisomes harbor numerous enzymes that can produce or degrade hydrogen peroxide (H2O2). Depending on its local concentration and environment, this oxidant can function as a redox signaling molecule or cause stochastic oxidative damage. Currently, it is well-accepted that dysfunctional peroxisomes are selectively removed by the autophagy-lysosome pathway. This process, known as "pexophagy," may serve a protective role in curbing peroxisome-derived oxidative stress. Peroxisomes also have the intrinsic ability to mediate and modulate H2O2-driven processes, including (selective) autophagy. However, the molecular mechanisms underlying these phenomena are multifaceted and have only recently begun to receive the attention they deserve. This review provides a comprehensive overview of what is known about the bidirectional relationship between peroxisomal H2O2 metabolism and (selective) autophagy. After introducing the general concepts of (selective) autophagy, we critically examine the emerging roles of H2O2 as one of the key modulators of the lysosome-dependent catabolic program. In addition, we explore possible relationships among peroxisome functioning, cellular H2O2 levels, and autophagic signaling in health and disease. Finally, we highlight the most important challenges that need to be tackled to understand how alterations in peroxisomal H2O2 metabolism contribute to autophagy-related disorders.

Shexiang Baoxin Pill for Acute Myocardial Infarction: Clinical Evidence and Molecular Mechanism of Antioxidative Stress

Acute myocardial infarction (AMI) has been a preclinical and clinical concern due to high hospitalization rate and mortality. This study was aimed at evaluating the effectiveness and safety of Shexiang Baoxin Pill (SBP) for AMI and exploring the possible mechanism of oxidative stress. Six databases were searched on March 26, 2021. Twenty-four studies were included and accessed by the RoB 2.0 or SYRCLE tool. Compared with routine treatment (RT), SBP showed the effectiveness in the clinical efficacy (RR = 1.15, 95% CI [1.06, 1.25]), left ventricular ejection fraction (LVEF) (SMD = 0.73, 95% CI [0.62, 0.95]), glutathione (GSH) (SMD = 2.07, 95% CI [1.51, 2.64]), superoxide dismutase (SOD) (SMD = 0.92, 95% CI [0.58, 1.26]), malondialdehyde (MDA) (SMD = -4.23, 95% CI [-5.80, -2.66]), creatine kinase-myocardial band (CK-MB) (SMD = -4.98, 95% CI [-5.64, -4.33]), cardiac troponin I (cTnI) (SMD = -2.17, 95% CI [-2.57, -1.76]), high-sensitivity C-reactive protein (Hs-CRP) (SMD = -1.34, 95% CI [-1.56, -1.12]), interleukin-6 (IL-6) (SMD = -0.99, 95% CI [-1.26, -0.71]), triglycerides (TG) (SMD = -0.52, 95% CI [-0.83, -0.22]), flow-mediated dilation (FMD) (SMD = 1.39, 95% CI [1.06, 1.72]), von Willebrand Factor (vWF) (SMD = -1.77, 95% CI [-2.39, -1.15]), nitric oxide (NO) (SMD = 0.89, 95% CI [0.65, 1.13]), and recurrent rate (RR = 0.30, 95% CI [0.15, 0.59]). But SBP adjunctive to RT plus PCI had no improvements in almost pooled outcomes except for the Hs-CRP (SMD = -1.19, 95% CI [-1.44, -0.94]) and TG (SMD = -0.25, 95% CI [-0.48, -0.02]). Laboratory findings showed that SBP enhanced the endothelial nitric oxide synthase (eNOS) activity and regulated laboratory indexes especially for homocysteine. In conclusion, SBP has adjunctive effects on AMI via the mechanism of antioxidative stress. The current evidence supports the use of SBP for mild and moderate AMI patients.

Effect of 2-Cys Peroxiredoxins Inhibition on Redox Modifications of Bull Sperm Proteins

Sperm peroxiredoxins (PRDXs) are moonlighting proteins which, in addition to their antioxidant activity, also act as redox signal transducers through PRDX-induced oxidative post-translational modifications of proteins (oxPTMs). Despite extensive knowledge on the antioxidant activity of PRDXs, the mechanisms related to PRDX-mediated oxPTMs are poorly understood. The present study aimed to investigate the effect of bull sperm 2-Cys PRDX inhibition by Conoidin A on changes in oxPTM levels under control and oxidative stress conditions. The results showed that a group of sperm mitochondrial (LDHAL6B, CS, ACO2, SDHA, ACAPM) and actin cytoskeleton proteins (CAPZB, ALDOA, CCIN) is oxidized due to the action of 2-Cys PRDXs under control conditions. In turn, under oxidative stress conditions, 2-Cys PRDX activity seems to be focused on antioxidant function protecting glycolytic, TCA pathway, and respiratory chain enzymes; chaperones; and sperm axonemal tubulins from oxidative damage. Interestingly, the inhibition of PRDX resulted in oxidation of a group of rate-limiting glycolytic proteins, which is known to trigger the switching of glucose metabolism from glycolysis to pentose phosphate pathway (PPP). The obtained results are expected to broaden the knowledge of the potential role of bull sperm 2-Cys in both redox signal transmission and antioxidant activity.

Oxidized GAPDH transfers S-glutathionylation to a nuclear protein Sirtuin-1 leading to apoptosis

AIMS

S-glutathionylation is a reversible oxidative modification of protein cysteines that plays a critical role in redox signaling. Glutaredoxin-1 (Glrx), a glutathione-specific thioltransferase, removes protein S-glutathionylation. Glrx, though a cytosolic protein, can activate a nuclear protein Sirtuin-1 (SirT1) by removing its S-glutathionylation. Glrx ablation causes metabolic abnormalities and promotes controlled cell death and fibrosis in mice. Glyceraldehyde 3-phosphate dehydrogenase (GAPDH), a key enzyme of glycolysis, is sensitive to oxidative modifications and involved in apoptotic signaling via the SirT1/p53 pathway in the nucleus. We aimed to elucidate the extent to which S-glutathionylation of GAPDH and glutaredoxin-1 contribute to GAPDH/SirT1/p53 apoptosis pathway.

RESULTS

Exposure of HEK 293T cells to hydrogen peroxide (H₂O₂) caused rapid S-glutathionylation and nuclear translocation of GAPDH. Nuclear GAPDH peaked 10-15 min after the addition of H₂O₂. Overexpression of Glrx or redox dead mutant GAPDH inhibited S-glutathionylation and nuclear translocation. Nuclear GAPDH formed a protein complex with SirT1 and exchanged S-glutathionylation to SirT1 and inhibited its deacetylase activity. Inactivated SirT1 remained stably bound to acetylated-p53 and initiated apoptotic signaling resulting in cleavage of caspase-3. We observed similar effects in human primary aortic endothelial cells suggesting the GAPDH/SirT1/p53 pathway as a common apoptotic mechanism.

CONCLUSIONS

Abundant GAPDH with its highly reactive-cysteine thiolate may function as a cytoplasmic rheostat to sense oxidative stress. S-glutathionylation of GAPDH may relay the signal to the nucleus where GAPDH trans-glutathionylates nuclear proteins such as SirT1 to initiate apoptosis. Glrx reverses GAPDH S-glutathionylation and prevents its nuclear translocation and cytoplasmic-nuclear redox signaling leading to apoptosis. Our data suggest that trans-glutathionylation is a critical step in apoptotic signaling and a potential mechanism that cytosolic Glrx controls nuclear transcription factors.

Thiol-disulphide independent in-cell trapping for the identification of peroxiredoxin 2 interactors

Hydrogen peroxide (H2O2) acts as a signalling molecule by oxidising cysteine thiols in proteins. Recent evidence has established a role for cytosolic peroxiredoxins in transmitting H2O2-based oxidation to a multitude of target proteins. Moreover, it is becoming clear that peroxiredoxins fulfil their function in organised microdomains, where not all interactors are covalently bound. However, most studies aimed at identifying peroxiredoxin interactors were based on methods that only detect covalently linked partners. Here, we explore the applicability of two thiol-disulphide independent in-cell trapping methodological approaches in combination with mass spectrometry for the identification of interaction partners of peroxiredoxin 2 (Prdx2). The first is biotin-dependent proximity-labelling (BioID) with a biotin ligase A (BirA*)-fused Prdx2, which has never been applied on redox-active proteins. The second is crosslinker co-immunoprecipitation with an N-terminally His-tagged Prdx2. During the initial characterisation of the tagged Prdx2 constructs, we found that the His-tag, but not BirA*, compromises the peroxidase and signalling activities of Prdx2. Further, the Prdx2 interactors identified with each approach showed little overlap. We therefore concluded that BioID is a more reliable method than crosslinker co-immunoprecipitation. After a stringent mass spec data filtering, BioID identified 13 interactors under elevated H2O2 conditions, including subunit five of the COP9 signalosome complex (CSN5). The Prdx2:CSN5 interaction was further confirmed in a proximity ligation assay. Taken together, our results demonstrate that BioID can be used as a method for the identification of interactors of Prdxs, and that caution should be exercised when interpreting protein-protein interaction results using tagged Prdxs.

Thiol-based redox-active proteins as cardioprotective therapeutic agents in cardiovascular diseases

Thiol-based redox compounds, namely thioredoxins (Trxs), glutaredoxins (Grxs) and peroxiredoxins (Prxs), stand as a pivotal group of proteins involved in antioxidant processes and redox signaling. Glutaredoxins (Grxs) are considered as one of the major families of proteins involved in redox regulation by removal of S-glutathionylation and thereby reactivation of other enzymes with thiol-dependent activity. Grxs are also coupled to Trxs and Prxs recycling and thereby indirectly contribute to reactive oxygen species (ROS) detoxification. Peroxiredoxins (Prxs) are a ubiquitous family of peroxidases, which play an essential role in the detoxification of hydrogen peroxide, aliphatic and aromatic hydroperoxides, and peroxynitrite. The Trxs, Grxs and Prxs systems, which reversibly induce thiol modifications, regulate redox signaling involved in various biological events in the cardiovascular system. This review focuses on the current knowledge of the role of Trxs, Grxs and Prxs on cardiovascular pathologies and especially in cardiac hypertrophy, ischemia/reperfusion (I/R) injury and heart failure as well as in the presence of cardiovascular risk factors, such as hypertension, hyperlipidemia, hyperglycemia and metabolic syndrome. Further studies on the roles of thiol-dependent redox systems in the cardiovascular system will support the development of novel protective and therapeutic strategies against cardiovascular diseases.

Raising the 'Good' Oxidants for Immune Protection

Redox medicine is a new therapeutic concept targeting reactive oxygen species (ROS) and secondary reaction products for health benefit. The concomitant function of ROS as intracellular second messengers and extracellular mediators governing physiological redox signaling, and as damaging radicals instigating or perpetuating various pathophysiological conditions will require selective strategies for therapeutic intervention. In addition, the reactivity and quantity of the oxidant species generated, its source and cellular location in a defined disease context need to be considered to achieve the desired outcome. In inflammatory diseases associated with oxidative damage and tissue injury, ROS source specific inhibitors may provide more benefit than generalized removal of ROS. Contemporary approaches in immunity will also include the preservation or even elevation of certain oxygen metabolites to restore or improve ROS driven physiological functions including more effective redox signaling and cell-microenvironment communication, and to induce mucosal barrier integrity, eubiosis and repair processes. Increasing oxidants by host-directed immunomodulation or by exogenous supplementation seems especially promising for improving host defense. Here, we summarize examples of beneficial ROS in immune homeostasis, infection, and acute inflammatory disease, and address emerging therapeutic strategies for ROS augmentation to induce and strengthen protective host immunity.

Redox regulation of immunity and the role of small molecular weight thiols

It is thought that excessive production of reactive oxygen species (ROS) can be a causal component in many diseases, some of which have an inflammatory component. This led to an oversimplification whereby ROS are seen as inflammatory and antioxidants anti-inflammatory. This paper aims at reviewing some of the literature on thiols in host defense. The review will first summarize the mechanisms by which we survive infections by pathogens. Then we will consider how the redox field evolved from the concept of oxidative stress to that of redox regulation and how it intersects the field of innate immunity. A third section will analyze how an oversimplified oxidative stress theory of disease led to a hypothesis on the role of ROS and glutathione (GSH) in immunity, respectively as pro- and anti-inflammatory mediators. Finally, we will discuss some recent research and how to think out of the box of that oversimplification and link the role of thiols in redox regulation to the mechanisms by which we survive an infection outlined in the first section.

The Mitochondria-to-Cytosol H2O2 Gradient Is Caused by Peroxiredoxin-Dependent Cytosolic Scavenging

Fluorescent protein-based reporters used to measure intracellular H₂O₂ were developed to overcome the limitations of small permeable dyes. The two major families of genetically encoded redox reporters are the reduction-oxidation sensitive green fluorescent protein (roGFP)-based proteins fused to peroxiredoxins and HyPer and derivatives. We have used the most sensitive probes of each family, roGFP2-Tpx1.C169S and HyPer7, to monitor steady-state and fluctuating levels of peroxides in fission yeast. While both are able to monitor the nanomolar fluctuations of intracellular H₂O₂, the former is two-five times more sensitive than HyPer7, and roGFP2-Tpx1.C169S is partially oxidized in the cytosol of wild-type cells while HyPer7 is fully reduced. We have successfully expressed HyPer7 in the mitochondrial matrix, and it is ~40% oxidized, suggesting higher steady-state levels of peroxides, in the low micromolar range, than in the cytosol. Cytosolic HyPer7 can detect negligible H₂O₂ in the cytosol from mitochondrial origin unless the main H₂O₂ scavenger, the cytosolic peroxiredoxin Tpx1, is absent, while mitochondrial HyPer7 is oxidized to the same extent in wild-type and ∆tpx1 cells. We conclude that there is a bidirectional flux of H₂O₂ across the matrix and the cytosol, but Tpx1 rapidly and efficiently scavenges mitochondrial-generated peroxides and stops their steady-state cytosolic levels rising.

Central Metabolism in Mammals and Plants as a Hub for Controlling Cell Fate

Significance: The importance of oxidoreductases in energy metabolism together with the occurrence of enzymes of central metabolism in the nucleus gave rise to the active research field aiming to understand moonlighting enzymes that undergo post-translational modifications (PTMs) before carrying out new tasks. Recent Advances: Cytosolic enzymes were shown to induce gene transcription after PTM and concomitant translocation to the nucleus. Changed properties of the oxidized forms of cytosolic glyceraldehyde 3-phosphate dehydrogenase, and also malate dehydrogenases and others, are the basis for a hypothesis suggesting moonlighting functions that directly link energy metabolism to adaptive responses required for maintenance of redox-homeostasis in all eukaryotes. Critical Issues: Small molecules, such as metabolic intermediates, coenzymes, or reduced glutathione, were shown to fine-tune the redox switches, interlinking redox state, metabolism, and induction of new functions via nuclear gene expression. The cytosol with its metabolic enzymes connecting energy fluxes between the various cell compartments can be seen as a hub for redox signaling, integrating the different signals for graded and directed responses in stressful situations. Future Directions: Enzymes of central metabolism were shown to interact with p53 or the assumed plant homologue suppressor of gamma response 1 (SOG1), an NAM, ATAF, and CUC transcription factor involved in the stress response upon ultraviolet exposure. Metabolic enzymes serve as sensors for imbalances, their inhibition leading to changed energy metabolism, and the adoption of transcriptional coactivator activities. Depending on the intensity of the impact, rerouting of energy metabolism, proliferation, DNA repair, cell cycle arrest, immune responses, or cell death will be induced. Antioxid. Redox Signal. 34, 1025-1047.

Peroxiredoxins wear many hats: Factors that fashion their peroxide sensing personalities

Peroxiredoxins (Prdxs) sense and assess peroxide levels, and signal through protein interactions. Understanding the role of the multiple structural and post-translational modification (PTM) layers that tunes the peroxiredoxin specificities is still a challenge. In this review, we give a tabulated overview on what is known about human and bacterial peroxiredoxins with a focus on structure, PTMs, and protein-protein interactions. Armed with numerous cellular and atomic level experimental techniques, we look at the future and ask ourselves what is still needed to give us a clearer view on the cellular operating power of Prdxs in both stress and non-stress conditions.

Thioredoxin inhibitor PX-12 induces mitochondria-mediated apoptosis in acute lymphoblastic leukemia cells

Imbalances in redox homeostasis have been described to be involved in the development, progression and relapse of leukemia. As the thioredoxin (Trx) system, one of the major cellular antioxidant networks, has been implicated in acute lymphoblastic leukemia (ALL), we investigated the therapeutic potential of Trx inhibition in ALL. Here, we show that the Trx inhibitor PX-12 reduced cell viability and induced cell death in a dose- and time-dependent manner in different ALL cell lines. This antileukemic activity was accompanied by an increase in reactive oxygen species (ROS) levels and enhanced PRDX3 dimerization. Pre-treatment with the thiol-containing ROS scavenger N-acetylcysteine (NAC), but not with non-thiol-containing scavengers α-tocopherol (α-Toc) or Mn(III)tetrakis(4-benzoic acid) porphyrin chloride (MnTBAP), significantly rescued PX-12-induced cell death. Furthermore, PX-12 triggered activation of BAK. Importantly, knockdown of BAK reduced PX-12-stimulated ROS production and cell death. Similarly, silencing of NOXA provided significant protection from PX-12-mediated cell death. The relevance of mitochondria-mediated, caspase-dependent apoptosis was further supported by data showing that PX-12 triggered cleavage of caspase-3 and that addition of the broad-range caspase inhibitor carbobenzoxy-valyl-alanyl-aspartyl-[O-methyl]-fluoromethylketone (zVAD.fmk) potently blocked cell death upon PX-12 treatment. This study provides novel insights into the mechanisms of PX-12-induced cell death in ALL and further highlights the therapeutic potential of redox-active compounds in ALL.

Oxidative stress in phenylketonuria-evidence from human studies and animal models, and possible implications for redox signaling

Phenylketonuria (PKU) is one of the commonest inborn error of amino acid metabolism. Before mass neonatal screening was possible, and the success of introducing diet therapy right after birth, the typical clinical finds in patients ranged from intellectual disability, epilepsy, motor deficits to behavioral disturbances and other neurological and psychiatric symptoms. Since early diagnosis and treatment became widespread, usually only those patients who do not strictly follow the diet present psychiatric, less severe symptoms such as anxiety, depression, sleep pattern disturbance, and concentration and memory problems. Despite the success of low protein intake in preventing otherwise severe outcomes, PKU's underlying neuropathophysiology remains to be better elucidated. Oxidative stress has gained acceptance as a disturbance implicated in the pathogenesis of PKU. The conception of oxidative stress has evolved to comprehend how it could interfere and ultimately modulate metabolic pathways regulating cell function. We summarize the evidence of oxidative damage, as well as compromised antioxidant defenses, from patients, animal models of PKU, and in vitro experiments, discussing the possible clinical significance of these findings. There are many studies on oxidative stress and PKU, but only a few went further than showing macromolecular damage and disturbance of antioxidant defenses. In this review, we argue that these few studies may point that oxidative stress may also disturb redox signaling in PKU, an aspect few authors have explored so far. The reported effect of phenylalanine on the expression or activity of enzymes participating in metabolic pathways known to be responsive to redox signaling might be mediated through oxidative stress.

Oxidants in Physiological Processes

A number of diseases and conditions have been associated with prolonged or persistent exposure to non-physiological levels of reactive oxygen species (ROS). Similarly, ROS underproduction due to loss-of-function mutations in superoxide or hydrogen peroxide (H₂O₂)-generating enzymes is a risk factor or causative for certain diseases. However, ROS are required for basic cell functions; in particular the diffusible second messenger H₂O₂ that serves as signaling molecule in redox processes. This activity sets H₂O₂ apart from highly reactive oxygen radicals and influences the approach to drug discovery, clinical utility, and therapeutic intervention. Here we review the chemical and biological fundamentals of ROS with emphasis on H₂O₂ as a signaling conduit and initiator of redox relays and propose an integrated view of physiological versus non-physiological reactive species. Therapeutic interventions that target persistently altered ROS levels should include both selective inhibition of a specific source of primary ROS and careful consideration of a targeted pro-oxidant approach, an avenue that is still underdeveloped. Both strategies require attention to redox dynamics in complex cellular systems, integration of the overall spatiotemporal cellular environment, and target validation to yield effective and safe therapeutics. The only professional primary ROS producers are NADPH oxidases (NOX1-5, DUOX1-2). Many other enzymes, e.g., xanthine oxidase (XO), monoamine oxidases (MAO), lysyl oxidases (LO), lipoxygenase (LOX), and cyclooxygenase (COX), produce superoxide and H₂O₂ secondary to their primary metabolic function. Superoxide is too reactive to disseminate, but H₂O₂ is diffusible, only limited by adjacent PRDXs or GPXs, and can be apically secreted and imported into cells through aquaporin (AQP) channels. H₂O₂ redox signaling includes oxidation of the active site thiol in protein tyrosine phosphatases, which will inhibit their activity and thereby increase tyrosine phosphorylation on target proteins. Essential functions include the oxidative burst by NOX2 as antimicrobial innate immune response; gastrointestinal NOX1 and DUOX2 generating low H₂O₂ concentrations sufficient to trigger antivirulence mechanisms; and thyroidal DUOX2 essential for providing H₂O₂ reduced by TPO to oxidize iodide to an iodinating form which is then attached to tyrosyls in TG. Loss-of-function (LoF) variants in TPO or DUOX2 cause congenital hypothyroidism and LoF variants in the NOX2 complex chronic granulomatous disease.

Superoxide dismutase 3 is expressed in bone tissue and required for normal bone homeostasis and mineralization

Superoxide dismutase 3 (SOD3) is an extracellular protein with the capacity to convert superoxide into hydrogen peroxide, an important secondary messenger in redox regulation. To investigate the utility of zebrafish in functional studies of SOD3 and its relevance for redox regulation, we have characterized the zebrafish orthologues; Sod3a and Sod3b. Our analyses show that both recombinant Sod3a and Sod3b express SOD activity, however, only Sod3b is able to bind heparin. Furthermore, RT-PCR analyses reveal that sod3a and sod3b are expressed in zebrafish embryos and are present primarily in separate organs in adult zebrafish, suggesting distinct functions in vivo. Surprisingly, both RT-PCR and whole mount in situ hybridization showed specific expression of sod3b in skeletal tissue. To further investigate this observation, we compared femoral bone obtained from wild-type and SOD3^-/- mice to determine whether a functional difference was apparent in healthy adult mice. Here we report, that bone from SOD3^-/- mice is less mineralized and characterized by significant reduction of cortical and trabecular thickness in addition to reduced mechanical strength. These analyses show that SOD3 plays a hitherto unappreciated role in bone development and homeostasis.

Hydrogen peroxide reactivity and specificity in thiol-based cell signalling

Reversible oxidation of thiol proteins is an important cell signalling mechanism. In many cases, this involves generation or exposure of the cells to H2O2, and oxidation of proteins that are not particularly H2O2-reactive. There is a conundrum as to how these proteins are oxidised when other highly reactive proteins such as peroxiredoxins are present. This article discusses potential mechanisms, focussing on recent evidence for oxidation being localised within the cell, redox relays involving peroxiredoxins operating in some signalling pathways, and mechanisms for facilitated or directed oxidation of specific targets. These findings help define conditions that enable redox signalling but there is still much to learn regarding mechanisms.

The Pancreatic β-Cell: The Perfect Redox System

Pancreatic β-cell insulin secretion, which responds to various secretagogues and hormonal regulations, is reviewed here, emphasizing the fundamental redox signaling by NADPH oxidase 4- (NOX4-) mediated H₂O₂ production for glucose-stimulated insulin secretion (GSIS). There is a logical summation that integrates both metabolic plus redox homeostasis because the ATP-sensitive K⁺ channel (K_ATP) can only be closed when both ATP and H₂O₂ are elevated. Otherwise ATP would block K_ATP, while H₂O₂ would activate any of the redox-sensitive nonspecific calcium channels (NSCCs), such as TRPM2. Notably, a 100%-closed K_ATP ensemble is insufficient to reach the -50 mV threshold plasma membrane depolarization required for the activation of voltage-dependent Ca²⁺ channels. Open synergic NSCCs or Cl^- channels have to act simultaneously to reach this threshold. The resulting intermittent cytosolic Ca²⁺-increases lead to the pulsatile exocytosis of insulin granule vesicles (IGVs). The incretin (e.g., GLP-1) amplification of GSIS stems from receptor signaling leading to activating the phosphorylation of TRPM channels and effects on other channels to intensify integral Ca²⁺-influx (fortified by endoplasmic reticulum Ca²⁺). ATP plus H₂O₂ are also required for branched-chain ketoacids (BCKAs); and partly for fatty acids (FAs) to secrete insulin, while BCKA or FA β-oxidation provide redox signaling from mitochondria, which proceeds by H₂O₂ diffusion or hypothetical SH relay via peroxiredoxin "redox kiss" to target proteins.

Oxidative eustress: On constant alert for redox homeostasis

In the open metabolic system, redox-related signaling requires continuous monitoring and fine-tuning of the steady-state redox set point. The ongoing oxidative metabolism is a persistent challenge, denoted as oxidative eustress, which operates within a physiological range that has been called the 'Homeodynamic Space', the 'Goldilocks Zone' or the 'Golden Mean'. Spatiotemporal control of redox signaling is achieved by compartmentalized generation and removal of oxidants. The cellular landscape of H₂O₂, the major redox signaling molecule, is characterized by orders-of-magnitude concentration differences between organelles. This concentration pattern is mirrored by the pattern of oxidatively modified proteins, exemplified by S-glutathionylated proteins. The review presents the conceptual background for short-term (non-transcriptional) and longer-term (transcriptional/translational) homeostatic mechanisms of stress and stress responses. The redox set point is a variable moving target value, modulated by circadian rhythm and by external influence, summarily denoted as exposome, which includes nutrition and lifestyle factors. Emerging fields of cell-specific and tissue-specific redox regulation in physiological settings are briefly presented, including new insight into the role of oxidative eustress in embryonal development and lifespan, skeletal muscle and exercise, sleep-wake rhythm, and the function of the nervous system with aspects leading to psychobiology.

Mechanisms and consequences of protein cysteine oxidation: the role of the initial short-lived intermediates

Thiol groups in protein cysteine (Cys) residues can undergo one- and two-electron oxidation reactions leading to the formation of thiyl radicals or sulfenic acids, respectively. In this mini-review we summarize the mechanisms and kinetics of the formation of these species by biologically relevant oxidants. Most of the latter react with the deprotonated form of the thiol. Since the pKa of the thiols in protein cysteines are usually close to physiological pH, the thermodynamics and the kinetics of their oxidation in vivo are affected by the acidity of the thiol. Moreover, the protein microenvironment has pronounced effects on cysteine residue reactivity, which in the case of the oxidation mediated by hydroperoxides, is known to confer specificity to particular protein cysteines. Despite their elusive nature, both thiyl radicals and sulfenic acids are involved in the catalytic mechanism of several enzymes and in the redox regulation of protein function and/or signaling pathways. They are usually short-lived species that undergo further reactions that converge in the formation of different stable products, resulting in several post-translational modifications of the protein. Some of these can be reversed through the action of specific cellular reduction systems. Others damage the proteins irreversibly, and can make them more prone to aggregation or degradation.

Measuring Reactive Sulfur Species and Thiol Oxidation States: Challenges and Cautions in Relation to Alkylation-Based Protocols

Significance: Redox biology is gaining ground in research related to human physiology (metabolism, signaling), pathophysiology (cancer, cardiovascular disease, neurodegeneration), and toxicology (radiation- or xenobiotic-induced damage). A major hurdle in advancing redox medicine is the current lack of understanding the mechanisms underpinning the observed detrimental or beneficial in vivo effects. To gain deeper insights into the underlying molecular pathways of redox regulation, we need to appreciate the strengths and limitations of the currently available methods. Recent Advances: Reactive sulfur species (RSS), including cysteine derivatives of peptides and proteins along with small molecules such as hydrogen sulfide or inorganic polysulfides, are major players in redox biology. RSS-mediated regulation of protein functions is a widely studied mechanism in the field, and considerable efforts have been devoted to the development of selective detection methods. Critical Issues: A large number of available methods rely on an alkylation step to freeze the dynamism of consecutive oxidation and reduction events among RSS at a particular time point inside the cell. This process uses the assumption that alkylation blocks all redox events instantaneously. We argue that unfortunately this is often not the case, which could have serious impacts on detected sulfur species speciation and confound experimental results. Future Directions: Novel technologies and prudent optimization of existing methods to accurately characterize the dynamic redox status of the thiol proteome as well as detailed understanding of regulatory and signaling capacities of protein polysulfidation are crucial to open new routes toward therapeutic interventions.

Are Reactive Sulfur Species the New Reactive Oxygen Species?

Significance: Oxidative stress in moderation positively affects homeostasis through signaling, while in excess it is associated with adverse health outcomes. Both activities are generally attributed to reactive oxygen species (ROS); hydrogen peroxide as the signal, and cysteines on regulatory proteins as the target. However, using antioxidants to affect signaling or benefit health has not consistently translated into expected outcomes, or when it does, the mechanism is often unclear. Recent Advances: Reactive sulfur species (RSS) were integral in the origin of life and throughout much of evolution. Sophisticated metabolic pathways that evolved to regulate RSS were easily "tweaked" to deal with ROS due to the remarkable similarities between the two. However, unlike ROS, RSS are stored, recycled, and chemically more versatile. Despite these observations, the relevance and regulatory functions of RSS in extant organisms are generally underappreciated. Critical Issues: A number of factors bias observations in favor of ROS over RSS. Research conducted in room air is hyperoxic to cells, and promotes ROS production and RSS oxidation. Metabolic rates of rodent models greatly exceed those of humans; does this favor ROS? Analytical methods designed to detect ROS also respond to RSS. Do these disguise the contributions of RSS? Future Directions: Resolving the ROS/RSS issue is vital to understand biology in general and human health in particular. Improvements in experimental design and analytical methods are crucial. Perhaps the most important is an appreciation of all the attributes of RSS and keeping an open mind.

Reactive Oxygen Species Induced p53 Activation: DNA Damage, Redox Signaling, or Both?

Significance: The p53 tumor suppressor has been dubbed the "guardian of genome" because of its various roles in the response to DNA damage such as DNA damage repair, cell cycle arrest, senescence, and apoptosis, all of which are in place to prevent mutations from being passed on down the lineage. Recent Advances: Reactive oxygen species (ROS), for instance hydrogen peroxide derived from mitochondrial respiration, have long been regarded mainly as a major source of cellular damage to DNA and other macromolecules. Critical Issues: More recently, ROS have been shown to also play important physiological roles as second messengers in so-called redox signaling. It is, therefore, not clear whether the observed activation of p53 by ROS is mediated through the DNA damage response, redox signaling, or both. In this review, we will discuss the similarities and differences between p53 activation in response to DNA damage and redox signaling in terms of upstream signaling and downstream transcriptional program activation. Future Directions: Understanding whether and how DNA damage and redox signaling-dependent p53 activation can be dissected could be useful to develop anti-cancer therapeutic p53-reactivation strategies that do not depend on the induction of DNA damage and the resulting additional mutational load.

Genetically encoded thiol redox-sensors in the zebrafish model: lessons for embryonic development and regeneration

Important roles for reactive oxygen species (ROS) and redox signaling in embryonic development and regenerative processes are increasingly recognized. However, it is difficult to obtain information on spatiotemporal dynamics of ROS production and signaling in vivo. The zebrafish is an excellent model for in vivo bioimaging and possesses a remarkable regenerative capacity upon tissue injury. Here, we review data obtained in this model system with genetically encoded redox-sensors targeting H₂O₂ and glutathione redox potential. We describe how such observations have prompted insight into regulation and downstream effects of redox alterations during tissue differentiation, morphogenesis and regeneration. We also discuss the properties of the different sensors and their consequences for the interpretation of in vivo imaging results. Finally, we highlight open questions and additional research fields that may benefit from further application of such sensor systems in zebrafish models of development, regeneration and disease.

Intra-dimer cooperativity between the active site cysteines during the oxidation of peroxiredoxin 2

Peroxiredoxin 2 (Prdx2) and other typical 2-Cys Prdxs function as homodimers in which hydrogen peroxide oxidizes each active site cysteine to a sulfenic acid which then condenses with the resolving cysteine on the alternate chain. Previous kinetic studies have considered both sites as equally reactive. Here we have studied Prdx2 using a combination of non-reducing SDS-PAGE to separate reduced monomers and dimers with one and two disulfide bonds, and stopped flow analysis of tryptophan fluorescence, to investigate whether there is cooperativity between the sites. We have observed positive cooperativity when H₂O₂ is added as a bolus and oxidation of the second site occurs while the first site is present as a sulfenic acid. Modelling of this reaction showed that the second site reacts 2.2 ± 0.1 times faster. In contrast, when H₂O₂ was generated slowly and the first active site condensed to a disulfide before the second site reacted, no cooperativity was evident. Conversion of the sulfenic acid to the disulfide showed negative cooperativity, with modelling of the exponential rise in tryptophan fluorescence yielding a rate constant of 0.75 ± 0.08 s^-1 when the alternate active site was present as a sulfenic acid and 2.29 ± 0.08-fold lower when it was a disulfide. No difference in the rate of hyperoxidation at the two sites was detected. Our findings imply that oxidation of one active site affects the conformation of the second site and influences which intermediate forms of the protein are favored under different cellular conditions.

Mechanisms of Mitochondrial ROS Production in Assisted Reproduction: The Known, the Unknown, and the Intriguing

The consensus that assisted reproduction technologies (ART), like in vitro fertilization, to induce oxidative stress (i.e., the known) belies how oocyte/zygote mitochondria-a major presumptive oxidative stressor-produce reactive oxygen species (ROS) with ART being unknown. Unravelling how oocyte/zygote mitochondria produce ROS is important for disambiguating the molecular basis of ART-induced oxidative stress and, therefore, to rationally target it (e.g., using site-specific mitochondria-targeted antioxidants). I review the known mechanisms of ROS production in somatic mitochondria to critique how oocyte/zygote mitochondria may produce ROS (i.e., the unknown). Several plausible site- and mode-defined mitochondrial ROS production mechanisms in ART are proposed. For example, complex I catalyzed reverse electron transfer-mediated ROS production is conceivable when oocytes are initially extracted due to at least a 10% increase in molecular dioxygen exposure (i.e., the intriguing). To address the term oxidative stress being used without recourse to the underlying chemistry, I use the species-specific spectrum of biologically feasible reactions to define plausible oxidative stress mechanisms in ART. Intriguingly, mitochondrial ROS-derived redox signals could regulate embryonic development (i.e., their production could be beneficial). Their potential beneficial role raises the clinical challenge of attenuating oxidative damage while simultaneously preserving redox signaling. This discourse sets the stage to unravel how mitochondria produce ROS in ART, and their biological roles from oxidative damage to redox signaling.

Extracellular Glutathione Peroxidase GPx3 and Its Role in Cancer

Mammalian cells possess a multifaceted antioxidant enzyme system, which includes superoxide dismutases, catalase, the peroxiredoxin/thioredoxin and the glutathione peroxidase systems. The dichotomous role of reactive oxygen species and antioxidant enzymes in tumorigenesis and cancer progression complicates the use of small molecule antioxidants, pro-oxidants, and targeting of antioxidant enzymes as therapeutic approaches for cancer treatment. It also highlights the need for additional studies to investigate the role and regulation of these antioxidant enzymes in cancer. The focus of this review is on glutathione peroxidase 3 (GPx3), a selenoprotein, and the only extracellular GPx of a family of oxidoreductases that catalyze the detoxification of hydro- and soluble lipid hydroperoxides by reduced glutathione. In addition to summarizing the biochemical function, regulation, and disease associations of GPx3, we specifically discuss the role and regulation of systemic and tumor cell expressed GPx3 in cancer. From this it is evident that GPx3 has a dichotomous role in different tumor types, acting as both a tumor suppressor and pro-survival protein. Further studies are needed to examine how loss or gain of GPx3 specifically affects oxidant scavenging and redox signaling in the extracellular tumor microenvironment, and how GPx3 might be targeted for therapeutic intervention.

Peri/Epicellular Thiol Oxidoreductases as Mediators of Extracellular Redox Signaling

Significance: Supracellular redox networks regulating cell-extracellular matrix (ECM) and organ system architecture merge with structural and functional (catalytic or allosteric) properties of disulfide bonds. This review addresses emerging evidence that exported thiol oxidoreductases (TORs), such as thioredoxin, protein disulfide isomerases (PDIs), quiescin sulfhydryl oxidases (QSOX)1, and peroxiredoxins, composing a peri/epicellular (pec)TOR pool, mediate relevant signaling. pecTOR functions depend mainly on kinetic and spatial regulation of thiol-disulfide exchange reactions governed by redox potentials, which are modulated by exported intracellular low-molecular-weight thiols, together conferring signal specificity. Recent Advances: pecTOR redox-modulates several targets including integrins, ECM proteins, surface molecules, and plasma components, although clear-cut documentation of direct effects is lacking in many cases. TOR catalytic pathways, displaying common patterns, culminate in substrate thiol reduction, oxidation, or isomerization. Peroxiredoxins act as redox/peroxide sensors, contrary to PDIs, which are likely substrate-targeted redox modulators. Emerging evidence suggests important pecTOR roles in patho(physio)logical processes, including blood coagulation, vascular remodeling, mechanosensing, endothelial function, immune responses, and inflammation. Critical Issues: Effects of pecPDIs supporting thrombosis/platelet activation have been well documented and reached the clinical arena. Roles of pecPDIA1 in vascular remodeling/mechanosensing are also emerging. Extracellular thioredoxin and pecPDIs redox-regulate immunoinflammation. Routes of TOR externalization remain elusive and appear to involve Golgi-independent routes. pecTORs are particularly accessible drug targets. Future Directions: Further understanding mechanisms of thiol redox reactions and developing assays for assessing pecTOR redox activities remain important research avenues. Also, addressing pecTORs as disease markers and achieving more efficient/specific drugs for pecTOR modulation are major perspectives for diagnostic/therapeutic improvements.

Peroxiredoxin promotes longevity and H2O2-resistance in yeast through redox-modulation of protein kinase A

Peroxiredoxins are H2O2 scavenging enzymes that also carry out H2O2 signaling and chaperone functions. In yeast, the major cytosolic peroxiredoxin, Tsa1 is required for both promoting resistance to H2O2 and extending lifespan upon caloric restriction. We show here that Tsa1 effects both these functions not by scavenging H2O2, but by repressing the nutrient signaling Ras-cAMP-PKA pathway at the level of the protein kinase A (PKA) enzyme. Tsa1 stimulates sulfenylation of cysteines in the PKA catalytic subunit by H2O2 and a significant proportion of the catalytic subunits are glutathionylated on two cysteine residues. Redox modification of the conserved Cys243 inhibits the phosphorylation of a conserved Thr241 in the kinase activation loop and enzyme activity, and preventing Thr241 phosphorylation can overcome the H2O2 sensitivity of Tsa1-deficient cells. Results support a model of aging where nutrient signaling pathways constitute hubs integrating information from multiple aging-related conduits, including a peroxiredoxin-dependent response to H2O2.

"Turn-on" ratiometric electrochemical detection of H2O2 in one drop of whole blood sample via a novel microelectrode sensor

Oxidative stress plays an important role in the pathogenesis of many diseases, while the exact mechanism that hydrogen peroxide (H₂O₂) as one of the most abundant reactive oxygen species (ROS) exerts its influence on oxidative stress remains unclear. We developed a novel turn-on ratiometric electrochemical sensor for the detection of H₂O₂ in blood samples. The electrochemical probe 5-(1,2-dithiolan-3-yl)-N-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)pent-anamide (BA) was designed and synthesized for the selective detection of H₂O₂ via a one-step amide reaction. Meanwhile, Nile Blue A (NB) was optimized as an internal reference molecule, thus enabling accurate quantification of H₂O₂ in a complex environment. BA and NB were then co-assembled onto a carbon fiber microelectrode (CFME) coated with Au cones. The oxidation peak current ratio between BA and NB demonstrated good linearity with the logarithm of the H₂O₂ concentration values ranging from 0.5 μM to 400 μM with a low detection limit of 0.02 μM. The developed sensor showed remarkable selectivity against potential interferences in whole blood samples, especially for ascorbic acid, uric acid, and dopamine. In combination with the unique characteristics of CFME, such as a small size and good biocompatibility, the microsensor was used for rapid analysis of one drop of whole blood sample. This sensor not only creates a new platform for the detection of H₂O₂ in whole blood samples, but also provides a new design strategy of other ROS analysis for early diagnosis of ROS-related diseases, drug discovery processes, and pathological mechanisms.

Redox signaling modulates Rho activity and tissue contractility in the Caenorhabditis elegans spermatheca

Actomyosin-based contractility in smooth muscle and nonmuscle cells is regulated by signaling through the small GTPase Rho and by calcium-activated pathways. We use the myoepithelial cells of the Caenorhabditis elegans spermatheca to study the mechanisms of coordinated myosin activation in vivo. Here, we show that redox signaling modulates RHO-1/Rho activity in this contractile tissue. Exogenously added as well as endogenously generated hydrogen peroxide decreases spermathecal contractility by inhibition of RHO-1, which depends on a conserved cysteine in its nucleotide binding site (C20). Further, we identify an endogenous gradient of H₂O₂ across the spermathecal tissue, which depends on the activity of cytosolic superoxide dismutase, SOD-1. Collectively, we show that SOD-1-mediated H₂O₂ production regulates the redox environment and fine tunes Rho activity across the spermatheca through oxidation of RHO-1 C20.

Real-time monitoring of peroxiredoxin oligomerization dynamics in living cells

Peroxiredoxins are most central to the cellular adaptation against oxidative stress. They act as oxidant scavengers, stress sensors, transmitters of signals, and chaperones, and they possess a unique quaternary switch that is intimately related to these functions. However, so far it has not been possible to monitor peroxiredoxin structural changes in the intact cellular environment. This study presents genetically encoded probes, based on homo-FRET (Förster resonance energy transfer between identical fluorophores) fluorescence polarization, that allow following these quaternary changes in real time, in living cells. We envisage that these probes can be used to address a broad range of questions related to the function of peroxiredoxins.

Peroxiredoxins are central to cellular redox homeostasis and signaling. They serve as peroxide scavengers, sensors, signal transducers, and chaperones, depending on conditions and context. Typical 2-Cys peroxiredoxins are known to switch between different oligomeric states, depending on redox state, pH, posttranslational modifications, and other factors. Quaternary states and their changes are closely connected to peroxiredoxin activity and function but so far have been studied, almost exclusively, outside the context of the living cell. Here we introduce the use of homo-FRET (Förster resonance energy transfer between identical fluorophores) fluorescence polarization to monitor dynamic changes in peroxiredoxin quaternary structure inside the crowded environment of living cells. Using the approach, we confirm peroxide- and thioredoxin-related quaternary transitions to take place in cellulo and observe that the relationship between dimer–decamer transitions and intersubunit disulfide bond formation is more complex than previously thought. Furthermore, we demonstrate the use of the approach to compare different peroxiredoxin isoforms and to identify mutations and small molecules affecting the oligomeric state inside cells. Mutagenesis experiments reveal that the dimer–decamer equilibrium is delicately balanced and can be shifted by single-atom structural changes. We show how to use this insight to improve the design of peroxiredoxin-based redox biosensors.

Crystal structure of sulfonic peroxiredoxin Ahp1 in complex with thioredoxin Trx2 mimics a conformational intermediate during the catalytic cycle

Peroxiredoxin (Prx) is a thiol-based peroxidase that eliminates reactive oxygen species to avoid oxidative damage. Alkyl hydroperoxide reductase Ahp1 is a novel and specific typical 2-cysteine Prx. Here, we present the crystal structure of sulfonic Ahp1 complexed with thioredoxin Trx2 at 2.12 Å resolution. This structure implies that the transient Ahp1-Trx2 complex during the catalytic cycle already have an ability to decompose the peroxides. Structural analysis reveals that the segment glutamine23-lysine32 juxtaposed to the resolving cysteine (C_R) of Ahp1 moves inward to generate a compact structure upon peroxidatic cysteine (C_P) overoxidation, resulting in the breakdown of several conserved hydrogen bonds formed by Ahp1-Trx2 complex interaction. Structural comparisons suggest that the structure of sulfonic Ahp1 represents a novel conformation of Ahp1, which can mimic a conformational intermediate between the reduced and oxidized forms. Therefore, this study may provide a new structural insight into the intermediate state in which the segment glutamine23-lysine32 juxtaposed to the cysteine31 (C_R) undergoes a conformational change upon cysteine62 (C_P) oxidation to prepare for the formation of an intermolecular C_P-C_R disulfide bond during Ahp1 catalytic cycle.

The Reducible Disulfide Proteome of Synaptosomes Supports a Role for Reversible Oxidations of Protein Thiols in the Maintenance of Neuronal Redox Homeostasis

The mechanisms by which neurons maintain redox homeostasis, disruption of which is linked to disease, are not well known. Hydrogen peroxide, a major cellular oxidant and neuromodulator, can promote reversible oxidations of protein thiols but the scope, targets, and significance of such oxidations occurring in neurons, especially in vivo, are uncertain. Using redox phenylarsine oxide (PAO)-affinity chromatography, which exploits the high-affinity of trivalent arsenicals for protein dithiols, this study investigated the occurrence of reducible and, therefore, potentially regulatory, protein disulfide bonds in Triton X-100-soluble protein fractions from isolated nerve-endings (synaptosomes) prepared from rat brains. Postmortem oxidations of protein thiols were limited by rapidly freezing the brains following euthanasia and, later, homogenizing them in the presence of the N-ethylmaleimide to trap reduced thiols. The reducible disulfide proteome comprised 5.4% of the total synaptosomal protein applied to the immobilized PAO columns and was overrepresented by pathways underlying ATP synaptic supply and demand including synaptic vesicle trafficking. The alpha subunits of plasma membrane Na⁺, K⁺-ATPase and the mitochondrial ATP synthase were particularly abundant proteins of the disulfide proteome and were enriched in this fraction by 3.5- and 6.7-fold, respectively. An adaptation of the commonly used "biotin-switch" method provided additional support for selective oxidation of thiols on the alpha subunit of the ATP synthase. We propose that reversible oxidations of protein thiols may underlie a coordinated metabolic response to hydrogen peroxide, serving to both control redox signaling and protect neurons from oxidant stress.

Non-redox cycling mechanisms of oxidative stress induced by PM metals

Metallic compounds contribute to the oxidative stress of ambient particulate matter (PM) exposure. The toxicity of redox inert ions of cadmium, mercury, lead and zinc, as well as redox-active ions of vanadium and chromium is underlain by dysregulation of mitochondrial function and loss of signaling quiescence. Central to the initiation of these effects is the interaction of metal ions with cysteinyl thiols on glutathione and key regulatory proteins, which leads to impaired mitochondrial electron transport and persistent pan-activation of signal transduction pathways. The mitochondrial and signaling effects are linked by the production of H₂O₂, generated from mitochondrial superoxide anion or through the activation of NADPH oxidase, which extends the range and amplifies the magnitude of the oxidative effects of the metals. This oxidative burden can be further potentiated by inhibitory effects of the metals on the enzymes of the glutathione and thioredoxin systems. Along with the better-known Fenton-based mechanisms, the non-redox cycling mechanisms of oxidative stress induced by metals constitute significant pathways for cellular injury induced by PM inhalation.

The mitochondrial peroxiredoxin displays distinct roles in different developmental stages of African trypanosomes

Hydroperoxide reduction in African trypanosomes relies on 2-Cys-peroxiredoxins (Prxs) and glutathione peroxidase-type enzymes (Pxs) which both obtain their reducing equivalents from the trypanothione/tryparedoxin couple and thus act as tryparedoxin peroxidases. While the cytosolic forms of the peroxidases are essential, the mitochondrial mPrx and Px III appear dispensable in bloodstream Trypanosoma brucei. This led to the suggestion that in this developmental stage which is characterized by a mitochondrion that lacks an active respiratory chain, only one of the two peroxidases might be required. Here we show that bloodstream cells in which the Px III gene is deleted and mPrx is down-regulated by RNA interference, proliferate as the parental cells indicating that both mitochondrial peroxidases are dispensable. However, when we raised the culture temperature to 39 °C, mPrx-depleted cells died indicating that under conditions mimicking a fever situation in the mammalian host, the protein becomes essential. In contrast, depletion of mPrx in insect stage procyclic T. brucei causes a proliferation defect under standard conditions at 27 °C, in the absence of any stress. In the absence of mPrx, a tryparedoxin-coupled roGFP2 biosensor expressed in the mitochondrial matrix is unable to respond to antimycin A treatment. Thus mPrx reduces mitochondrial H₂O₂ with the generation of trypanothione disulfide and acts as peroxidase. However, mPrx-depleted procyclic cells neither display any alteration in the cytosolic or mitochondrial trypanothione redox state nor increased sensitivity towards exogenous oxidative stressors suggesting that the peroxidase activity is not the crucial physiological function. After prolonged mPrx-depletion, the cells almost stop proliferation and display a highly elongated shape and diminished MitoTracker Red staining. In contrast to the situation in the mammalian bloodstream T. brucei and Leishmania, mPrx appears to play a constitutive role for the morphology, mitochondrial function and proliferation of the insect stage of African trypanosomes.

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In bloodstream T. brucei, both mitochondrial tryparedoxin peroxidases are dispensable.

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Heat-stressed bloodstream cells require the mitochondrial peroxiredoxin (mPrx).

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In procyclic (PC) T. brucei, mPrx plays a constitutive role for proliferation.

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Lack of mPrx affects the structure and mitochondrial membrane potential of PC cells.

Redox signalling and ageing: insights from Drosophila

Ageing and age-related diseases are major challenges for the social, economic and healthcare systems of our society. Amongst many theories, reactive oxygen species (ROS) have been implicated as a driver of the ageing process. As by-products of aerobic metabolism, ROS are able to randomly oxidise macromolecules, causing intracellular damage that accumulates over time and ultimately leads to dysfunction and cell death. However, the genetic overexpression of enzymes involved in the detoxification of ROS or treatment with antioxidants did not generally extend lifespan, prompting a re-evaluation of the causal role for ROS in ageing. More recently, ROS have emerged as key players in normal cellular signalling by oxidising redox-sensitive cysteine residues within proteins. Therefore, while high levels of ROS may be harmful and induce oxidative stress, low levels of ROS may actually be beneficial as mediators of redox signalling. In this context, enhancing ROS production in model organisms can extend lifespan, with biological effects dependent on the site, levels, and specific species of ROS. In this review, we examine the role of ROS in ageing, with a particular focus on the importance of the fruit fly Drosophila as a powerful model system to study redox processes in vivo.

Immunological Techniques to Assess Protein Thiol Redox State: Opportunities, Challenges and Solutions

To understand oxidative stress, antioxidant defense, and redox signaling in health and disease it is essential to assess protein thiol redox state. Protein thiol redox state is seldom assessed immunologically because of the inability to distinguish reduced and reversibly oxidized thiols by Western blotting. An underappreciated opportunity exists to use Click PEGylation to realize the transformative power of simple, time and cost-efficient immunological techniques. Click PEGylation harnesses selective, bio-orthogonal Click chemistry to separate reduced and reversibly oxidized thiols by selectively ligating a low molecular weight polyethylene glycol moiety to the redox state of interest. The resultant ability to disambiguate reduced and reversibly oxidized species by Western blotting enables Click PEGylation to assess protein thiol redox state. In the present review, to enable investigators to effectively harness immunological techniques to assess protein thiol redox state we critique the chemistry, promise and challenges of Click PEGylation.

Activity-Based Sensing for Site-Specific Proteomic Analysis of Cysteine Oxidation

Oxidative post-translational modifications (OxiPTMs) of cysteine residues are the molecular foundation of thiol-based redox regulation that modulates physiological events such as cell proliferation, differentiation, and migration and, when dysregulated, can lead to biomolecule damage and cell death. Common OxiPTMs of cysteine thiols (-SH) include reversible modifications such as S-sulfenylation (-SOH), S-glutathionylation (-SSG), disulfide formation (-SSR), S-nitrosylation (-SNO), and S-sulfhydration (-SSH) as well as more biologically stable modifications like S-sulfinylation (-SO₂H) and S-sulfonylation (-SO₃H). In the past decade, our laboratory has developed first-in-class chemistry-based tools and proteomic methods to advance the field of thiol-based redox biology and oxidative stress. In this Account, we take the reader through the historical aspects of probe development and application in our laboratory, highlighting key advances in our understanding of sulfur chemistry, in the test tube and in living systems. Offering superior resolution, throughput, accuracy, and reproducibility, mass spectrometry (MS)-based proteomics coupled to chemoselective "activity-based" small-molecule probes is the most rigorous technique for global mapping of cysteine OxiPTMs. Herein, we describe the evolution of this field from indirect detection to state-of-the-art site-centric quantitative chemoproteomic approaches that enable mapping of physiological and pathological changes in cysteine oxidation. These methods enable protein and site-level identification, mechanistic studies, mapping fold-changes, and modification stoichiometry. In particular, this Account focuses on activity-based methods for profiling S-sulfenylation, S-sulfinylation, and S-sulfhydration with an eye toward new reactions and methodologies developed in our group as well as their applications that have shed new light on fundamental processes of redox biology. Among several classes of sulfenic acid probes, dimedone-based C-nucleophiles possess superior chemical selectivity and compatibility with tandem MS. Cell-permeable dimedone derivatives with a bioconjugation handle are capable of detecting of S-sulfenylation in living cells. In-depth screening of a C-nucleophile library has yielded several entities with significantly enhanced reactivity over dimedone while maintaining selectivity, and reversible linear C-nucleophiles that enable controlled target release. C-Nucleophiles have also been implemented in tag-switch methods to detect S-sulfhydration. Most recently, activity-based detection of protein S-sulfinylation with electrophilic nitrogen species (ENS), such as C-nitroso compounds and electron deficient diazines, offers significant advantages in simplicity-of-use and target specificity compared to label-free methods. When feasible, the rich information provided by site-centric quantitative proteomics should not be tainted by oxidation artifacts from cell lysis. Therefore, chemoselective probes that function in a native environment with low cytotoxicity, good cell-permeability, and competitive kinetics are desired in modern redox chemoproteomics approaches. As our understanding of sulfur chemistry and redox signaling evolves, newly discovered cysteine OxiPTMs in microorganisms, plants, cells, tissues, and disease models should innovatively promote mechanistic and therapeutic research.

Redox Signaling from Mitochondria: Signal Propagation and Its Targets

Progress in mass spectroscopy of posttranslational oxidative modifications has enabled researchers to experimentally verify the concept of redox signaling. We focus here on redox signaling originating from mitochondria under physiological situations, discussing mechanisms of transient redox burst in mitochondria, as well as the possible ways to transfer such redox signals to specific extramitochondrial targets. A role of peroxiredoxins is described which enables redox relay to other targets. Examples of mitochondrial redox signaling are discussed: initiation of hypoxia-inducible factor (HIF) responses; retrograde redox signaling to PGC1α during exercise in skeletal muscle; redox signaling in innate immune cells; redox stimulation of insulin secretion, and other physiological situations.

Are the beneficial effects of 'antioxidant' lipoic acid mediated through metabolism of reactive sulfur species?

The health benefits of lipoic acid (LA) are generally attributed to mitigating the harmful effects of reactive oxygen species (ROS). ROS are chemically similar to reactive sulfur species (RSS) and signal through identical mechanisms. Here we examined the effects of LA on RSS in HEK293 cells using H₂S and polysulfide (PS) specific fluorophores, AzMC and SSP4. We show that LA concentration-dependently increased both H₂S and PS. Physioxia (5% O₂) augmented the effects of LA on H₂S production but decreased PS production. Thiosulfate, a known substrate for reduced LA, and an intermediate in the catabolism of H₂S enhanced the effects of LA on H₂S and PS production. Inhibiting peroxiredoxins with conoidin A and gluraredoxins with tiopronin augmented the effects of LA on PS and H₂S, respectively while decreasing glutathione with buthionine-sulfoximine (BSO) or diethyl maleate (DEM) decreased the stimulatory effect of LA on H₂S production but augmented LA's effect on PS. Aminooxyacetate (AOA) and propargylglycine (PPG), inhibitors of H₂S production from cysteine partially inhibited LA augmentation of H₂S production and further decreased the LA effect when applied concurrently with BSO and DEM. The selective and cell-permeable H₂S scavenger, SS20, inhibited the effects of LA on cellular H₂S. Estimates of single-cell H₂S production suggest that 0.1-0.2% of O₂ consumption is used to metabolize H₂S and these requirements may increase to 1-2% with 1 mM LA. Collectively, these results suggest that LA rescues H₂S from irreversible oxidation and that the effects of LA on RSS directly confer antioxidant, anti-inflammatory and cytoprotective responses. They also suggest that TS may be an effective supplement to increase the efficacy of LA in clinical settings.

A role for peroxiredoxins in H2O2- and MEKK-dependent activation of the p38 signaling pathway

The p38 mitogen-activated protein kinase (MAPK) signaling pathway plays an important role in the cellular response to various stresses and its deregulation accompanies pathological conditions such as cancer and chronic inflammation. Hydrogen peroxide (H2O2) is a well-established activator of the p38 MAPK signaling pathway. However, the mechanisms of H2O2-induced p38 activation are not yet fully understood. In Drosophila cells, we find that H2O2-induced activation of p38 depends on the MAPK kinase kinase (MAP3K) Mekk1. In line with the emerging role of peroxiredoxins as H2O2 sensors and signal transmitters we observe an H2O2-dependent interaction between Mekk1 and the cytosolic peroxiredoxin of Drosophila, Jafrac1. In human cells, MEKK4 (the homologue of Mekk1) and peroxiredoxin-2 (Prx2) interact in a similar manner, suggesting an evolutionarily conserved mechanism. In both organisms, H2O2 induces transient disulfide-linked conjugates between the MAP3K and a typical 2-Cys peroxiredoxin. We propose that these conjugates represent the relaying of oxidative equivalents from H2O2 to the MAP3K and that the oxidation of Mekk1/MEKK4 leads to the downstream activation of p38 MAPK. Indeed, the depletion of cytosolic 2-Cys peroxiredoxins in human cells diminished H2O2-induced activation of p38 MAPK.

Detection, identification, and quantification of oxidative protein modifications

Exposure of biological molecules to oxidants is inevitable and therefore commonplace. Oxidative stress in cells arises from both external agents and endogenous processes that generate reactive species, either purposely (e.g. during pathogen killing or enzymatic reactions) or accidentally (e.g. exposure to radiation, pollutants, drugs, or chemicals). As proteins are highly abundant and react rapidly with many oxidants, they are highly susceptible to, and major targets of, oxidative damage. This can result in changes to protein structure, function, and turnover and to loss or (occasional) gain of activity. Accumulation of oxidatively-modified proteins, due to either increased generation or decreased removal, has been associated with both aging and multiple diseases. Different oxidants generate a broad, and sometimes characteristic, spectrum of post-translational modifications. The kinetics (rates) of damage formation also vary dramatically. There is a pressing need for reliable and robust methods that can detect, identify, and quantify the products formed on amino acids, peptides, and proteins, especially in complex systems. This review summarizes several advances in our understanding of this complex chemistry and highlights methods that are available to detect oxidative modifications-at the amino acid, peptide, or protein level-and their nature, quantity, and position within a peptide sequence. Although considerable progress has been made in the development and application of new techniques, it is clear that further development is required to fully assess the relative importance of protein oxidation and to determine whether an oxidation is a cause, or merely a consequence, of injurious processes.

The Emerging Roles of Nicotinamide Adenine Dinucleotide Phosphate Oxidase 2 in Skeletal Muscle Redox Signaling and Metabolism

Significance: Skeletal muscle is a crucial tissue to whole-body locomotion and metabolic health. Reactive oxygen species (ROS) have emerged as intracellular messengers participating in both physiological and pathological adaptations in skeletal muscle. A complex interplay between ROS-producing enzymes and antioxidant networks exists in different subcellular compartments of mature skeletal muscle. Recent evidence suggests that nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (NOXs) are a major source of contraction- and insulin-stimulated oxidants production, but they may paradoxically also contribute to muscle insulin resistance and atrophy. Recent Advances: Pharmacological and molecular biological tools, including redox-sensitive probes and transgenic mouse models, have generated novel insights into compartmentalized redox signaling and suggested that NOX2 contributes to redox control of skeletal muscle metabolism. Critical Issues: Major outstanding questions in skeletal muscle include where NOX2 activation occurs under different conditions in health and disease, how NOX2 activation is regulated, how superoxide/hydrogen peroxide generated by NOX2 reaches the cytosol, what the signaling mediators are downstream of NOX2, and the role of NOX2 for different physiological and pathophysiological processes. Future Directions: Future research should utilize and expand the current redox-signaling toolbox to clarify the NOX2-dependent mechanisms in skeletal muscle and determine whether the proposed functions of NOX2 in cells and animal models are conserved into humans.

The clinical potential of thiol redox proteomics

Introduction: Protein thiols are susceptible to oxidation in health and disease. Redox proteomics methods facilitate the identification, quantification, and rationalization of oxidation processes including those involving protein thiols. These residues are crucial to understanding redox homeostasis underpinning normal cell functioning and regulation as well as novel biomarkers of pathology and promising novel drug targets.Areas covered: This article reviews redox proteomic approaches to study of protein thiols in some important human pathologies and assesses the clinical potential of individual Cys residues as novel biomarkers for disease detection and as targets for novel treatments.Expert commentary: Although protein thiols are not as routinely used as redox biomarkers as some other lesions such as carbonylation, there has been growing recent interest in their potential. Driven largely by developments in high-resolution mass spectrometry it is possible now to identify proteins that are redox modified at thiol groups or that interact with regulatory oxidoreductases. Thiols that are specifically susceptible to modification by reactive oxygen species can be routinely identified now and quantitative MS can be used to quantify the proportion of a protein that is redox modified.