Increased mitochondrial prooxidant activity mediates up-regulation of Complex I S-glutathionylation via protein thiyl radical in the murine heart of eNOS(-/-)

Mitochondrial redox regulation and myocardial ischemia-reperfusion injury

Mitochondrial reactive oxygen species (ROS) have emerged as an important mechanism of disease and redox signaling in the cellular system. Under basal or pathological conditions, electron leakage for ROS production is primarily mediated by complexes I and III of the electron transport chain (ETC) and by the proton motive force (PMF), consisting of a membrane potential (ΔΨ) and a proton gradient (ΔpH). Several factors control redox status in mitochondria, including ROS, the PMF, oxidative posttranslational modifications (OPTM) of the ETC subunits, SOD2, and cytochrome c heme lyase (HCCS). In the mitochondrial PMF, increased ΔpH-supported backpressure due to diminishing electron transport and chemiosmosis promotes a more reductive mitochondrial physiological setting. OPTM by protein cysteine sulfonation in complex I and complex III has been shown to affect enzymatic catalysis, the proton gradient, redox status, and enzyme-mediated ROS production. Pathological conditions associated with oxidative or nitrosative stress, such as myocardial ischemia and reperfusion (I/R), increase mitochondrial ROS production and redox dysfunction via oxidative injury to complexes I and III, intensely enhancing protein cysteine sulfonation and impairing heme integrity. The physiological conditions of reductive stress induced by gains in SOD2 function normalize I/R-mediated ROS overproduction and redox dysfunction. Further insight into the cellular mechanisms by which HCCS, biogenesis of c-type cytochrome, and OPTM regulate PMF and ROS production in mitochondria will enrich our understanding of redox signal transduction and identify new therapeutic targets for cardiovascular diseases in which oxidative stress perturbs normal redox signaling.

Redox Regulation by Protein S-Glutathionylation: From Molecular Mechanisms to Implications in Health and Disease

S-glutathionylation, the post-translational modification forming mixed disulfides between protein reactive thiols and glutathione, regulates redox-based signaling events in the cell and serves as a protective mechanism against oxidative damage. S-glutathionylation alters protein function, interactions, and localization across physiological processes, and its aberrant function is implicated in various human diseases. In this review, we discuss the current understanding of the molecular mechanisms of S-glutathionylation and describe the changing levels of expression of S-glutathionylation in the context of aging, cancer, cardiovascular, and liver diseases.

Glutathione "Redox Homeostasis" and Its Relation to Cardiovascular Disease

More people die from cardiovascular diseases (CVD) than from any other cause. Cardiovascular complications are thought to arise from enhanced levels of free radicals causing impaired "redox homeostasis," which represents the interplay between oxidative stress (OS) and reductive stress (RS). In this review, we compile several experimental research findings that show sustained shifts towards OS will alter the homeostatic redox mechanism to cause cardiovascular complications, as well as findings that show a prolonged antioxidant state or RS can similarly lead to such cardiovascular complications. This experimental evidence is specifically focused on the role of glutathione, the most abundant antioxidant in the heart, in a redox homeostatic mechanism that has been shifted towards OS or RS. This may lead to impairment of cellular signaling mechanisms and elevated pools of proteotoxicity associated with cardiac dysfunction.

Protective effects of N-acetyl-cysteine in mitochondria bioenergetics, oxidative stress, dynamics and S-glutathionylation alterations in acute kidney damage induced by folic acid

Folic acid (FA)-induced acute kidney injury (AKI) is a widely used model for studies of the renal damage and its progression to chronic state. However, the molecular mechanisms by which FA induces AKI remain poorly understood. Since renal function depends on mitochondrial homeostasis, it has been suggested that mitochondrial alterations contribute to AKI development. Additionally, N-acetyl-cysteine (NAC) can be a protective agent to prevent mitochondrial and renal dysfunction in this model, given its ability to increase mitochondrial glutathione (GSH) and to control the S-glutathionylation levels, a reversible post-translational modification that has emerged as a mechanism able to link mitochondrial energy metabolism and redox homeostasis. However, this hypothesis has not been explored. The present study demonstrates for the first time that, at 24 h, FA induced mitochondrial bioenergetics, redox state, dynamics and mitophagy alterations, which are involved in the mechanisms responsible for the AKI development. On the other hand, NAC preadministration was able to prevent mitochondrial bioenergetics, redox state and dynamics alterations as well as renal damage. The protective effects of NAC on mitochondria and renal function could be related to its observed capacity to preserve the S-glutathionylation process and GSH levels in mitochondria. Taken together, our results support the idea that these mitochondrial processes can be targets for the prevention of the renal damage and its progression in FA-induced AKI model.

Mitochondrial bioenergetics, redox state, dynamics and turnover alterations in renal mass reduction models of chronic kidney diseases and their possible implications in the progression of this illness

Nowadays, chronic kidney disease (CKD) is considered a worldwide public health problem. CKD is a term used to describe a set of pathologies that structurally and functionally affect the kidney, it is mostly characterized by the progressive loss of kidney function. Current therapeutic approaches are insufficient to avoid the development of this disease, which highlights the necessity of developing new strategies to reverse or at least delay CKD progression. Kidney is highly dependent on mitochondrial homeostasis and function, consequently, the idea that mitochondrial pathologies could play a pivotal role in the genesis and development of kidney diseases has risen. Although many research groups have recently published studies of mitochondrial function in acute kidney disease models, the existing information about CKD is still limited, especially in renal mass reduction (RMR) models. This paper focuses on reviewing current experimental information about the bioenergetics, dynamics (fission and fusion processes), turnover (mitophagy and biogenesis) and redox mitochondrial alterations in RMR, to discuss and integrate the mitochondrial changes triggered by nephron loss, as well as its relationship with loss of kidney function in CKD, in these models. Understanding these mechanisms would allow us to design new therapies that target these mitochondrial alterations.

Mitochondrial Glutathione: Regulation and Functions

SIGNIFICANCE

Mitochondrial glutathione fulfills crucial roles in a number of processes, including iron-sulfur cluster biosynthesis and peroxide detoxification. Recent Advances: Genetically encoded fluorescent probes for the glutathione redox potential (E_GSH) have permitted extensive new insights into the regulation of mitochondrial glutathione redox homeostasis. These probes have revealed that the glutathione pools of the mitochondrial matrix and intermembrane space (IMS) are highly reduced, similar to the cytosolic glutathione pool. The glutathione pool of the IMS is in equilibrium with the cytosolic glutathione pool due to the presence of porins that allow free passage of reduced glutathione (GSH) and oxidized glutathione (GSSG) across the outer mitochondrial membrane. In contrast, limited transport of glutathione across the inner mitochondrial membrane ensures that the matrix glutathione pool is kinetically isolated from the cytosol and IMS.

CRITICAL ISSUES

In contrast to the situation in the cytosol, there appears to be extensive crosstalk between the mitochondrial glutathione and thioredoxin systems. Further, both systems appear to be intimately involved in the removal of reactive oxygen species, particularly hydrogen peroxide (H₂O₂), produced in mitochondria. However, a detailed understanding of these interactions remains elusive.

FUTURE DIRECTIONS

We postulate that the application of genetically encoded sensors for glutathione in combination with novel H₂O₂ probes and conventional biochemical redox state assays will lead to fundamental new insights into mitochondrial redox regulation and reinvigorate research into the physiological relevance of mitochondrial redox changes. Antioxid. Redox Signal. 27, 1162-1177.

Differential protein acetylation assists import of excess SOD2 into mitochondria and mediates SOD2 aggregation associated with cardiac hypertrophy in the murine SOD2-tg heart

SOD2 is the primary antioxidant enzyme neutralizing ^•O₂^- in mitochondria. Cardiac-specific SOD2 overexpression (SOD2-tg) induces supernormal function and cardiac hypertrophy in the mouse heart. However, the reductive stress imposed by SOD2 overexpression results in protein aggregation of SOD2 pentamers and differential hyperacetylation of SOD2 in the mitochondria and cytosol. Here, we studied SOD2 acetylation in SOD2-tg and wild-type mouse hearts. LC-MS/MS analysis indicated the presence of four acetylated lysines in matrix SOD2 and nine acetylated lysines in cytosolic SOD2 from the SOD2-tg heart. However, only one specific acetylated lysine residue was detected in the mitochondria of the wild-type heart, which was consistent with Sirt3 downregulation in the SOD2-tg heart. LC-MS/MS further detected hyperacetylated SOD2 with a signaling peptide in the mitochondrial inner membrane and matrix of the SOD2-tg heart, indicating partial arrest of the SOD2 precursor in the membrane during translocation into the mitochondria. Upregulation of HSP 70 and cytosolic HSP 60 enabled the translocation of excess SOD2 into mitochondria. In vitro acetylation of matrix SOD2 with Ac₂O deaggregated pentameric SOD2, restored the profile of cytosolic SOD2 hyperacetylation, and decreased matrix SOD2 activity. As revealed by 3D structure, acetylation of K89, K134, and K154 of cytosolic SOD2 induces unfolding of the tertiary structure and breaking of the salt bridges that are important for the quaternary structure, suggesting that hyperacetylation and HSP 70 upregulation maintain the unfolded status of SOD2 in the cytosol and mediate the import of SOD2 across the membrane. Downregulation of Sirt3, HSP 60, and presequence protease in the mitochondria of the SOD2-tg heart promoted protein misfolding that led to pentameric aggregation.

Impairment of pH gradient and membrane potential mediates redox dysfunction in the mitochondria of the post-ischemic heart

The mitochondrial electrochemical gradient (Δp), which comprises the pH gradient (ΔpH) and the membrane potential (ΔΨ), is crucial in controlling energy transduction. During myocardial ischemia and reperfusion (IR), mitochondrial dysfunction mediates superoxide (^·O₂^-) and H₂O₂ overproduction leading to oxidative injury. However, the role of ΔpH and ΔΨ in post-ischemic injury is not fully established. Here we studied mitochondria from the risk region of rat hearts subjected to 30 min of coronary ligation and 24 h of reperfusion in vivo. In the presence of glutamate, malate and ADP, normal mitochondria (mitochondria of non-ischemic region, NR) exhibited a heightened state 3 oxygen consumption rate (OCR) and reduced ^·O₂^- and H₂O₂ production when compared to state 2 conditions. Oligomycin (increases ΔpH by inhibiting ATP synthase) increased ^·O₂^- and H₂O₂ production in normal mitochondria, but not significantly in the mitochondria of the risk region (IR mitochondria or post-ischemic mitochondria), indicating that normal mitochondrial ^·O₂^- and H₂O₂ generation is dependent on ΔpH and that IR impaired the ΔpH of normal mitochondria. Conversely, nigericin (dissipates ΔpH) dramatically reduced ^·O₂^- and H₂O₂ generation by normal mitochondria under state 4 conditions, and this nigericin quenching effect was less pronounced in IR mitochondria. Nigericin also increased mitochondrial OCR, and predisposed normal mitochondria to a more oxidized redox status assessed by increased oxidation of cyclic hydroxylamine, CM-H. IR mitochondria, although more oxidized than normal mitochondria, were not responsive to nigericin-induced CM-H oxidation, which is consistent with the result that IR induced ΔpH impairment in normal mitochondria. Valinomycin, a K⁺ ionophore used to dissipate ΔΨ, drastically diminished ^·O₂^- and H₂O₂ generation by normal mitochondria, but less pronounced effect on IR mitochondria under state 4 conditions, indicating that ΔΨ also contributed to ^·O₂^- generation by normal mitochondria and that IR mediated ΔΨ impairment. However, there was no significant difference in valinomycin-induced CM-H oxidation between normal and IR mitochondria. In conclusion, under normal conditions the proton backpressure imposed by ΔpH restricts electron flow, controls a limited amount of ^·O₂^- generation, and results in a more reduced myocardium; however, IR causes ΔpH impairment and prompts a more oxidized myocardium.

Measurement of Reactive Oxygen Species, Reactive Nitrogen Species, and Redox-Dependent Signaling in the Cardiovascular System: A Scientific Statement From the American Heart Association

Reactive oxygen species and reactive nitrogen species are biological molecules that play important roles in cardiovascular physiology and contribute to disease initiation, progression, and severity. Because of their ephemeral nature and rapid reactivity, these species are difficult to measure directly with high accuracy and precision. In this statement, we review current methods for measuring these species and the secondary products they generate and suggest approaches for measuring redox status, oxidative stress, and the production of individual reactive oxygen and nitrogen species. We discuss the strengths and limitations of different methods and the relative specificity and suitability of these methods for measuring the concentrations of reactive oxygen and reactive nitrogen species in cells, tissues, and biological fluids. We provide specific guidelines, through expert opinion, for choosing reliable and reproducible assays for different experimental and clinical situations. These guidelines are intended to help investigators and clinical researchers avoid experimental error and ensure high-quality measurements of these important biological species.

Induction of mitochondrial reactive oxygen species production by GSH mediated S-glutathionylation of 2-oxoglutarate dehydrogenase

2-Oxoglutarate dehydrogenase (Ogdh) is an important mitochondria redox sensor that can undergo S-glutathionylation following an increase in H₂O₂ levels. Although S-glutathionylation is required to protect Ogdh from irreversible oxidation while simultaneously modulating its activity it remains unknown if glutathione can also modulate reactive oxygen species (ROS) production by the complex. We report that reduced (GSH) and oxidized (GSSG) glutathione control O2∙-/H₂O₂ formation by Ogdh through protein S-glutathionylation reactions. GSSG (1 mM) induced a modest decrease in Ogdh activity which was associated with a significant decrease in O2∙-/H₂O₂ formation. GSH had the opposite effect, amplifying O2∙-/H₂O₂ formation by Ogdh. Incubation of purified Ogdh in 2.5 mM GSH led to significant increase in O2∙-/H₂O₂ formation which also lowered NADH production. Inclusion of enzymatically active glutaredoxin-2 (Grx2) in reaction mixtures reversed the GSH-mediated amplification of O2∙-/H₂O₂ formation. Similarly pre-incubation of permeabilized liver mitochondria from mouse depleted of GSH showed an approximately ~3.5-fold increase in Ogdh-mediated O2∙-/H₂O₂ production that was matched by a significant decrease in NADH formation which could be reversed by Grx2. Taken together, our results demonstrate GSH and GSSG modulate ROS production by Ogdh through S-glutathionylation of different subunits. This is also the first demonstration that GSH can work in the opposite direction in mitochondria-amplifying ROS formation instead of quenching it. We propose that this regulatory mechanism is required to modulate ROS emission from Ogdh in response to variations in glutathione redox buffering capacity.

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ROS formation by Ogdh is controlled by glutathione.

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GSH amplifies ROS production by Ogdh.

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Ogdh is S-glutathionylated by GSH.

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Grx2 deglutathionylates Ogdh.

Protein S-glutathionlyation links energy metabolism to redox signaling in mitochondria

At its core mitochondrial function relies on redox reactions. Electrons stripped from nutrients are used to form NADH and NADPH, electron carriers that are similar in structure but support different functions. NADH supports ATP production but also generates reactive oxygen species (ROS), superoxide (O2·-) and hydrogen peroxide (H₂O₂). NADH-driven ROS production is counterbalanced by NADPH which maintains antioxidants in an active state. Mitochondria rely on a redox buffering network composed of reduced glutathione (GSH) and peroxiredoxins (Prx) to quench ROS generated by nutrient metabolism. As H₂O₂ is quenched, NADPH is expended to reactivate antioxidant networks and reset the redox environment. Thus, the mitochondrial redox environment is in a constant state of flux reflecting changes in nutrient and ROS metabolism. Changes in redox environment can modulate protein function through oxidation of protein cysteine thiols. Typically cysteine oxidation is considered to be mediated by H₂O₂ which oxidizes protein thiols (SH) forming sulfenic acid (SOH). However, problems begin to emerge when one critically evaluates the regulatory function of SOH. Indeed SOH formation is slow, non-specific, and once formed SOH reacts rapidly with a variety of molecules. By contrast, protein S-glutathionylation (PGlu) reactions involve the conjugation and removal of glutathione moieties from modifiable cysteine residues. PGlu reactions are driven by fluctuations in the availability of GSH and oxidized glutathione (GSSG) and thus should be exquisitely sensitive to changes ROS flux due to shifts in the glutathione pool in response to varying H₂O₂ availability. Here, we propose that energy metabolism-linked redox signals originating from mitochondria are mediated indirectly by H₂O₂ through the GSH redox buffering network in and outside mitochondria. This proposal is based on several observations that have shown that unlike other redox modifications PGlu reactions fulfill the requisite criteria to serve as an effective posttranslational modification that controls protein function.

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Mitochondria employ redox buffering networks to regulate bioenergetics.

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PGlu reactions link energy/nutrient metabolism to redox signaling.

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Other redox modifications lack regulatory features.

Overexpressing superoxide dismutase 2 induces a supernormal cardiac function by enhancing redox-dependent mitochondrial function and metabolic dilation

During heightened cardiac work, O2 consumption by the heart benefits energy production via mitochondria. However, some electrons leak from the respiratory chain and yield superoxide, which is rapidly metabolized into H2O2 by SOD2. To understand the systemic effects of the metabolic dilator, H2O2, we studied mice with cardiac-specific SOD2 overexpression (SOD2-tg), which increases the H2O2 produced by cardiac mitochondria. Contrast echocardiography was employed to evaluate cardiac function, indicating that SOD2-tg had a significantly greater ejection fraction and a lower mean arterial pressure (MAP) that was partially normalized by intravenous injection of catalase. Norepinephrine-mediated myocardial blood flow (MBF) was significantly enhanced in SOD2-tg mice. Coupling of MBF to the double product (Heart Rate×MAP) was increased in SOD2-tg mice, indicating that the metabolic dilator, "spilled" over, inducing systemic vasodilation. The hypothesis that SOD2 overexpression effectively enhances mitochondrial function was further evaluated. Mitochondria of SOD2-tg mice had a decreased state 3 oxygen consumption rate, but maintained the same ATP production flux under the basal and L-NAME treatment conditions, indicating a higher bioenergetic efficiency. SOD2-tg mitochondria produced less superoxide, and had lower redox activity in converting cyclic hydroxylamine to stable nitroxide, and a lower GSSG concentration. EPR analysis of the isolated mitochondria showed a significant decrease in semiquinones at the SOD2-tg Qi site. These results support a more reductive physiological setting in the SOD2-tg murine heart. Cardiac mitochondria exhibited no significant differences in the respiratory control index between WT and SOD2-tg. We conclude that SOD2 overexpression in myocytes enhances mitochondrial function and metabolic vasodilation, leading to a phenotype of supernormal cardiac function.

Thiyl radicals and induction of protein degradation

Thiyl radicals are important intermediates in the redox biology and chemistry of thiols. These radicals can react via hydrogen transfer with various C-H bonds in peptides and proteins, leading to the generation of carbon-centered radicals, and, potentially, to irreversible protein damage. This review summarizes quantitative information on reaction kinetics and product formation, and discusses the significance of these reactions for protein degradation induced by thiyl radical formation.