Confirmation of the Cardioprotective Effect of MitoGamide in the Diabetic Heart.

PURPOSE: HFpEF (heart failure with preserved ejection fraction) is a major consequence of diabetic cardiomyopathy with no effective treatments. Here, we have characterized Akita mice as a preclinical model of HFpEF and used it to confirm the therapeutic efficacy of the mitochondria-targeted dicarbonyl scavenger, MitoGamide. METHODS AND RESULTS: A longitudinal echocardiographic analysis confirmed that Akita mice develop diastolic dysfunction with reduced E peak velocity, E/A ratio and extended isovolumetric relaxation time (IVRT), while the systolic function remains comparable with wild-type mice. The myocardium of Akita mice had a decreased ATP/ADP ratio, elevated mitochondrial oxidative stress and increased organelle density, compared with that of wild-type mice. MitoGamide, a mitochondria-targeted 1,2-dicarbonyl scavenger, exhibited good stability in vivo, uptake into cells and mitochondria and reactivity with dicarbonyls. Treatment of Akita mice with MitoGamide for 12 weeks significantly improved the E/A ratio compared with the vehicle-treated group. CONCLUSION: Our work confirms that the Akita mouse model of diabetes replicates key clinical features of diabetic HFpEF, including cardiac and mitochondrial dysfunction. Furthermore, in this independent study, MitoGamide treatment improved diastolic function in Akita mice.

Identification and quantification of protein S-nitrosation by nitrite in the mouse heart during ischemia.

Nitrate (NO3-) and nitrite (NO2-) are known to be cardioprotective and to alter energy metabolism in vivo NO3- action results from its conversion to NO2- by salivary bacteria, but the mechanism(s) by which NO2- affects metabolism remains obscure. NO2- may act by S-nitrosating protein thiols, thereby altering protein activity. But how this occurs, and the functional importance of S-nitrosation sites across the mammalian proteome, remain largely uncharacterized. Here we analyzed protein thiols within mouse hearts in vivo using quantitative proteomics to determine S-nitrosation site occupancy. We extended the thiol-redox proteomic technique, isotope-coded affinity tag labeling, to quantify the extent of NO2--dependent S-nitrosation of proteins thiols in vivo Using this approach, called SNOxICAT (S-nitrosothiol redox isotope-coded affinity tag), we found that exposure to NO2- under normoxic conditions or exposure to ischemia alone results in minimal S-nitrosation of protein thiols. However, exposure to NO2- in conjunction with ischemia led to extensive S-nitrosation of protein thiols across all cellular compartments. Several mitochondrial protein thiols exposed to the mitochondrial matrix were selectively S-nitrosated under these conditions, potentially contributing to the beneficial effects of NO2- on mitochondrial metabolism. The permeability of the mitochondrial inner membrane to HNO2, but not to NO2-, combined with the lack of S-nitrosation during anoxia alone or by NO2- during normoxia places constraints on how S-nitrosation occurs in vivo and on its mechanisms of cardioprotection and modulation of energy metabolism. Quantifying S-nitrosated protein thiols now allows determination of modified cysteines across the proteome and identification of those most likely responsible for the functional consequences of NO2- exposure.

Mitochondrial hyperpolarization during chronic complex I inhibition is sustained by low activity of complex II, III, IV and V.

The mitochondrial oxidative phosphorylation (OXPHOS) system consists of four electron transport chain (ETC) complexes (CI-CIV) and the FoF1-ATP synthase (CV), which sustain ATP generation via chemiosmotic coupling. The latter requires an inward-directed proton-motive force (PMF) across the mitochondrial inner membrane (MIM) consisting of a proton (ΔpH) and electrical charge (Δψ) gradient. CI actively participates in sustaining these gradients via trans-MIM proton pumping. Enigmatically, at the cellular level genetic or inhibitor-induced CI dysfunction has been associated with Δψ depolarization or hyperpolarization. The cellular mechanism of the latter is still incompletely understood. Here we demonstrate that chronic (24h) CI inhibition in HEK293 cells induces a proton-based Δψ hyperpolarization in HEK293 cells without triggering reverse-mode action of CV or the adenine nucleotide translocase (ANT). Hyperpolarization was associated with low levels of CII-driven O2 consumption and prevented by co-inhibition of CII, CIII or CIV activity. In contrast, chronic CIII inhibition triggered CV reverse-mode action and induced Δψ depolarization. CI- and CIII-inhibition similarly reduced free matrix ATP levels and increased the cell's dependence on extracellular glucose to maintain cytosolic free ATP. Our findings support a model in which Δψ hyperpolarization in CI-inhibited cells results from low activity of CII, CIII and CIV, combined with reduced forward action of CV and ANT.

Metabolic consequences of NDUFS4 gene deletion in immortalized mouse embryonic fibroblasts.

Human mitochondrial complex I (CI) deficiency is associated with progressive neurological disorders. To better understand the CI pathomechanism, we here studied how deletion of the CI gene NDUFS4 affects cell metabolism. To this end we compared immortalized mouse embryonic fibroblasts (MEFs) derived from wildtype (wt) and whole-body NDUFS4 knockout (KO) mice. Mitochondria from KO cells lacked the NDUFS4 protein and mitoplasts displayed virtually no CI activity, moderately reduced CII, CIII and CIV activities and normal citrate synthase and CV (F(o)F(1)-ATPase) activity. Native electrophoresis of KO cell mitochondrial fractions revealed two distinct CI subcomplexes of ~830kDa (enzymatically inactive) and ~200kDa (active). The level of fully-assembled CII-CV was not affected by NDUFS4 gene deletion. KO cells exhibited a moderately reduced maximal and routine O(2) consumption, which was fully inhibited by acute application of the CI inhibitor rotenone. The aberrant CI assembly and reduced O(2) consumption in KO cells were fully normalized by NDUFS4 gene complementation. Cellular [NAD(+)]/[NADH] ratio, lactate production and mitochondrial tetramethyl rhodamine methyl ester (TMRM) accumulation were slightly increased in KO cells. In contrast, NDUFS4 gene deletion did not detectably alter [NADP(+)]/[NADPH] ratio, cellular glucose consumption, the protein levels of hexokinases (I and II) and phosphorylated pyruvate dehydrogenase (P-PDH), total cellular adenosine triphosphate (ATP) level, free cytosolic [ATP], cell growth rate, and reactive oxygen species (ROS) levels. We conclude that the NDUFS4 subunit is of key importance in CI stabilization and that, due to the metabolic properties of the immortalized MEFs, NDUFS4 gene deletion has only modest effects at the live cell level. This article is part of a special issue entitled: 17th European Bioenergetics Conference (EBEC 2012).

Detection and manipulation of mitochondrial reactive oxygen species in mammalian cells.

Reactive oxygen species (ROS) are formed upon incomplete reduction of molecular oxygen (O2) as an inevitable consequence of mitochondrial metabolism. Because ROS can damage biomolecules, cells contain elaborate antioxidant defense systems to prevent oxidative stress. In addition to their damaging effect, ROS can also operate as intracellular signaling molecules. Given the fact that mitochondrial ROS appear to be only generated at specific sites and that particular ROS species display a unique chemistry and have specific molecular targets, mitochondria-derived ROS might constitute local regulatory signals. The latter would allow individual mitochondria to auto-regulate their metabolism, shape and motility, enabling them to respond autonomously to the metabolic requirements of the cell. In this review we first summarize how mitochondrial ROS can be generated and removed in the living cell. Then we discuss experimental strategies for (local) detection of ROS by combining chemical or proteinaceous reporter molecules with quantitative live cell microscopy. Finally, approaches involving targeted pro- and antioxidants are presented, which allow the local manipulation of ROS levels.

NDUFA2 complex I mutation leads to Leigh disease.

Mitochondrial isolated complex I deficiency is the most frequently encountered OXPHOS defect. We report a patient with an isolated complex I deficiency expressed in skin fibroblasts as well as muscle tissue. Because the parents were consanguineous, we performed homozygosity mapping to identify homozygous regions containing candidate genes such as NDUFA2 on chromosome 5. Screening of this gene on genomic DNA revealed a mutation that interferes with correct splicing and results in the skipping of exon 2. Exon skipping was confirmed on the mRNA level. The mutation in this accessory subunit causes reduced activity and disturbed assembly of complex I. Furthermore, the mutation is associated with a mitochondrial depolarization. The expression and activity of complex I and the depolarization was (partially) rescued with a baculovirus system expressing the NDUFA2 gene.

Quantitative glucose and ATP sensing in mammalian cells.

The functioning and survival of mammalian cells requires an active energy metabolism. Metabolic dysfunction plays an important role in many human diseases, including diabetes, cancer, inherited mitochondrial disorders, and metabolic syndrome. The monosaccharide glucose constitutes a key source of cellular energy. Following its import across the plasma membrane, glucose is converted into pyruvate by the glycolysis pathway. Pyruvate oxidation supplies substrates for the ATP-generating mitochondrial oxidative phosphorylation (OXPHOS) system. To gain cell-biochemical knowledge about the operation and regulation of the cellular energy metabolism in the healthy and diseased state, quantitative knowledge is required about (changes in) metabolite concentrations under (non) steady-state conditions. This information can, for instance, be used to construct more realistic in silico models of cell metabolism, which facilitates understanding the consequences of metabolic dysfunction as well as on- and off-target effects of mitochondrial drugs. Here we review the current state-of-the-art live-cell quantification of two key cellular metabolites, glucose and ATP, using protein-based sensors. The latter apply the principle of FRET (fluorescence resonance energy transfer) and allow measurements in different cell compartments by fluorescence microscopy. We further summarize the properties and applications of the FRET-based sensors, their calibration, pitfalls, and future perspectives.

Extracellular acidification induces ROS- and mPTP-mediated death in HEK293 cells.

The extracellular pH (pHe) is a key determinant of the cellular (micro)environment and needs to be maintained within strict boundaries to allow normal cell function. Here we used HEK293 cells to study the effects of pHe acidification (24h), induced by mitochondrial inhibitors (rotenone, antimycin A) and/or extracellular HCl addition. Lowering pHe from 7.2 to 5.8 reduced cell viability by 70% and was paralleled by a decrease in cytosolic pH (pHc), hyperpolarization of the mitochondrial membrane potential (Δψ), increased levels of hydroethidine-oxidizing ROS and stimulation of protein carbonylation. Co-treatment with the antioxidant α-tocopherol, the mitochondrial permeability transition pore (mPTP) desensitizer cyclosporin A and Necrostatin-1, a combined inhibitor of Receptor-interacting serine/threonine-protein kinase 1 (RIPK1) and Indoleamine 2,3-dioxygenase (IDO), prevented acidification-induced cell death. In contrast, the caspase inhibitor zVAD.fmk and the ferroptosis inhibitor Ferrostatin-1 were ineffective. We conclude that extracellular acidification induces necroptotic cell death in HEK293 cells and that the latter involves intracellular acidification, mitochondrial functional impairment, increased ROS levels, mPTP opening and protein carbonylation. These findings suggest that acidosis of the extracellular environment (as observed in mitochondrial disorders, ischemia, acute inflammation and cancer) can induce cell death via a ROS- and mPTP opening-mediated pathogenic mechanism.

Monoamine oxidase-dependent endoplasmic reticulum-mitochondria dysfunction and mast cell degranulation lead to adverse cardiac remodeling in diabetes.

Monoamine oxidase (MAO) inhibitors ameliorate contractile function in diabetic animals, but the mechanisms remain unknown. Equally elusive is the interplay between the cardiomyocyte alterations induced by hyperglycemia and the accompanying inflammation. Here we show that exposure of primary cardiomyocytes to high glucose and pro-inflammatory stimuli leads to MAO-dependent increase in reactive oxygen species that causes permeability transition pore opening and mitochondrial dysfunction. These events occur upstream of endoplasmic reticulum (ER) stress and are abolished by the MAO inhibitor pargyline, highlighting the role of these flavoenzymes in the ER/mitochondria cross-talk. In vivo, streptozotocin administration to mice induced oxidative changes and ER stress in the heart, events that were abolished by pargyline. Moreover, MAO inhibition prevented both mast cell degranulation and altered collagen deposition, thereby normalizing diastolic function. Taken together, these results elucidate the mechanisms underlying MAO-induced damage in diabetic cardiomyopathy and provide novel evidence for the role of MAOs in inflammation and inter-organelle communication. MAO inhibitors may be considered as a therapeutic option for diabetic complications as well as for other disorders in which mast cell degranulation is a dominant phenomenon.

Live-cell assessment of mitochondrial reactive oxygen species using dihydroethidine.

Reactive oxygen species (ROS) play an important role in both physiology and pathology. Mitochondria are an important source of the primary ROS superoxide. However, accurate detection of mitochondrial superoxide especially in living cells remains a difficult task. Here, we describe a method and the pitfalls to detect superoxide in both mitochondria and the entire cell using dihydroethidium (HEt) and live-cell microscopy.

Complex I and complex III inhibition specifically increase cytosolic hydrogen peroxide levels without inducing oxidative stress in HEK293 cells.

Inhibitor studies with isolated mitochondria demonstrated that complex I (CI) and III (CIII) of the electron transport chain (ETC) can act as relevant sources of mitochondrial reactive oxygen species (ROS). Here we studied ROS generation and oxidative stress induction during chronic (24h) inhibition of CI and CIII using rotenone (ROT) and antimycin A (AA), respectively, in intact HEK293 cells. Both inhibitors stimulated oxidation of the ROS sensor hydroethidine (HEt) and increased mitochondrial NAD(P)H levels without major effects on cell viability. Integrated analysis of cells stably expressing cytosolic- or mitochondria-targeted variants of the reporter molecules HyPer (H2O2-sensitive and pH-sensitive) and SypHer (H2O2-insensitive and pH-sensitive), revealed that CI- and CIII inhibition increased cytosolic but not mitochondrial H2O2 levels. Total and mitochondria-specific lipid peroxidation was not increased in the inhibited cells as reported by the C11-BODIPY(581/591) and MitoPerOx biosensors. Also expression of the superoxide-detoxifying enzymes CuZnSOD (cytosolic) and MnSOD (mitochondrial) was not affected. Oxyblot analysis revealed that protein carbonylation was not stimulated by CI and CIII inhibition. Our findings suggest that chronic inhibition of CI and CIII: (i) increases the levels of HEt-oxidizing ROS and (ii) specifically elevates cytosolic but not mitochondrial H2O2 levels, (iii) does not induce oxidative stress or substantial cell death. We conclude that the increased ROS levels are below the stress-inducing level and might play a role in redox signaling.

BOLA1 is an aerobic protein that prevents mitochondrial morphology changes induced by glutathione depletion.

AIMS: The BolA protein family is widespread among eukaryotes and bacteria. In Escherichia coli, BolA causes a spherical cell shape and is overexpressed during oxidative stress. Here we aim to elucidate the possible role of its human homolog BOLA1 in mitochondrial morphology and thiol redox potential regulation. RESULTS: We show that BOLA1 is a mitochondrial protein that counterbalances the effect of L-buthionine-(S,R)-sulfoximine (BSO)-induced glutathione (GSH) depletion on the mitochondrial thiol redox potential. Furthermore, overexpression of BOLA1 nullifies the effect of BSO and S-nitrosocysteine on mitochondrial morphology. Conversely, knockdown of the BOLA1 gene increases the oxidation of mitochondrial thiol groups. Supporting a role of BOLA1 in controlling the mitochondrial thiol redox potential is that BOLA1 orthologs only occur in aerobic eukaryotes. A measured interaction of BOLA1 with the mitochondrial monothiol glutaredoxin GLRX5 provides hints for potential mechanisms behind BOLA1's effect on mitochondrial redox potential. Nevertheless, we have no direct evidence for a role of GLRX5 in BOLA1's function. INNOVATION: We implicate a new protein, BOLA1, in the regulation of the mitochondrial thiol redox potential. CONCLUSION: BOLA1 is an aerobic, mitochondrial protein that prevents mitochondrial morphology aberrations induced by GSH depletion and reduces the associated oxidative shift of the mitochondrial thiol redox potential.

A ratiometric fluorescent probe for assessing mitochondrial phospholipid peroxidation within living cells.

Mitochondrial oxidative damage contributes to a wide range of pathologies, and lipid peroxidation of the mitochondrial inner membrane is a major component of this disruption. However, despite its importance, there are no methods to assess mitochondrial lipid peroxidation within cells specifically. To address this unmet need we have developed a ratiometric, fluorescent, mitochondria-targeted lipid peroxidation probe, MitoPerOx. This compound is derived from the C11-BODIPY(581/591) probe, which contains a boron dipyromethane difluoride (BODIPY) fluorophore conjugated via a dienyl link to a phenyl group. In response to lipid peroxidation the fluorescence emission maximum shifts from ∼590 to ∼520nm. To target this probe to the matrix-facing surface of the mitochondrial inner membrane we attached a triphenylphosphonium lipophilic cation, which leads to its selective uptake into mitochondria in cells, driven by the mitochondrial membrane potential. Here we report on the development and characterization of MitoPerOx. We found that MitoPerOx was taken up very rapidly into mitochondria within cells, where it responded to changes in mitochondrial lipid peroxidation that could be measured by fluorimetry, confocal microscopy, and epifluorescence live cell imaging. Importantly, the peroxidation-sensitive change in fluorescence at 520nm relative to that at 590nm enabled the use of the probe as a ratiometric fluorescent probe, greatly facilitating assessment of mitochondrial lipid peroxidation in cells.

Trolox-sensitive reactive oxygen species regulate mitochondrial morphology, oxidative phosphorylation and cytosolic calcium handling in healthy cells.

AIMS: Cell regulation by signaling reactive oxygen species (sROS) is often incorrectly studied through extracellular oxidant addition. Here, we used the membrane-permeable antioxidant Trolox to examine the role of sROS in mitochondrial morphology, oxidative phosphorylation (OXPHOS), and cytosolic calcium (Ca(2+)) handling in healthy human skin fibroblasts. RESULTS AND INNOVATION: Trolox treatment reduced the levels of 5-(and-6)-chloromethyl-2',7'-dichlorodihydro-fluorescein (CM-H(2)DCF) oxidizing ROS, lowered cellular lipid peroxidation, and induced a less oxidized mitochondrial thiol redox state. This was paralleled by increased glutathione- and mitofusin-dependent mitochondrial filamentation, increased expression of fully assembled mitochondrial complex I, elevated activity of citrate synthase and OXPHOS enzymes, and a higher cellular O(2) consumption. In contrast, Trolox did not alter hydroethidium oxidation, cytosolic thiol redox state, mitochondrial NAD(P)H levels, or mitochondrial membrane potential. Whole genome expression profiling revealed that Trolox did not trigger significant changes in gene expression, suggesting that Trolox acts downstream of this process. Cytosolic Ca(2+) transients, induced by the hormone bradykinin, were of a higher amplitude and decayed faster in Trolox-treated cells. These effects were dose-dependently antagonized by hydrogen peroxide. CONCLUSIONS: Our findings suggest that Trolox-sensitive sROS are upstream regulators of mitochondrial mitofusin levels, morphology, and function in healthy human skin fibroblasts. This information not only facilitates the interpretation of antioxidant effects in cell models (of oxidative-stress), but also contributes to a better understanding of ROS-related human pathologies, including mitochondrial disorders.

Restoration of complex V deficiency caused by a novel deletion in the human TMEM70 gene normalizes mitochondrial morphology.

We report a fragmented mitochondrial network and swollen and irregularly shaped mitochondria with partial to complete loss of the cristae in fibroblasts of a patient with a novel TMEM70 gene deletion, which could be completely restored by complementation of the TMEM70 genetic defect. Comparative genomics analysis predicted the topology of TMEM70 in the inner mitochondrial membrane, which could be confirmed by immunogold labeling experiments, and showed that the TMEM70 gene is not restricted to higher multi-cellular eukaryotes. This study demonstrates that the role of complex V in mitochondrial cristae morphology applies to human mitochondrial disease pathology.