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Mitochondrial respiration extends beyond ATP generation, with the organelle participating in many cellular and physiological processes. Parallel changes in components of the mitochondrial electron transfer system with respiration render it an appropriate hub for coordinating cellular adaption to changes in oxygen levels. How changes in respiration under functional hypoxia (i.e., when intracellular O 2 levels limit mitochondrial respiration) are relayed by the electron transfer system to impact mitochondrial adaption and remodeling after hypoxic exposure remains poorly defined. This is largely due to challenges integrating findings under controlled and defined O 2 levels in studies connecting functions of isolated mitochondria to humans during physical exercise. Here we present experiments under conditions of hypoxia in isolated mitochondria, myotubes and exercising humans. Performing steady-state respirometry with isolated mitochondria we found that oxygen limitation of respiration reduced electron flow and oxidative phosphorylation, lowered the mitochondrial membrane potential difference, and decreased mitochondrial calcium influx. Similarly, in myotubes under functional hypoxia mitochondrial calcium uptake decreased in response to sarcoplasmic reticulum calcium release for contraction. In both myotubes and human skeletal muscle this blunted mitochondrial adaptive responses and remodeling upon contractions. Our results suggest that by regulating calcium uptake the mitochondrial electron transfer system is a hub for coordinating cellular adaption under functional hypoxia., Competing Interests: Declaration of competing interest E.G. is the founder and CEO of Oroboros Instruments., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

Mitochondrial disorders are hallmarked by the dysfunction of oxidative phosphorylation (OXPHOS) yet are highly heterogeneous at the clinical and genetic levels. Striking tissue-specific pathological manifestations are a poorly understood feature of these conditions, even if the disease-causing genes are ubiquitously expressed. To investigate the functional basis of this phenomenon, we analyzed several OXPHOS-related bioenergetic parameters, including oxygen consumption rates, electron transfer system (ETS)-related coenzyme Q (mtCoQ) redox state and production of reactive oxygen species (ROS) in mouse brain and liver mitochondria fueled by different substrates. In addition, we determined how these functional parameters are affected by ETS impairment in a tissue-specific manner using pathologically relevant mouse models lacking either Ndufs4 or Ttc19, leading to Complex I (CI) or Complex III (CIII) deficiency, respectively. Detailed OXPHOS analysis revealed striking differences between brain and liver mitochondria in the capacity of the different metabolic substrates to fuel the ETS, reduce the ETS-related mtCoQ, and to induce ROS production. In addition, ETS deficiency due to either CI or CIII dysfunction had a much greater impact on the intrinsic bioenergetic parameters of brain compared with liver mitochondria. These findings are discussed in terms of the still rather mysterious tissue-specific manifestations of mitochondrial disease., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Carlo Viscomi reports financial support was provided by Telethon Foundation. Carlo Viscomi reports financial support was provided by Associazione Luigi Comini Onlus. Erich Gnaiger reports financial support was provided by European Union. Erika Fernandez-Vizarra reports a relationship with Telethon Foundation that includes: funding grants. EG is founder and CEO of Oroboros Instruments., (Copyright © 2024. Published by Elsevier B.V.)

Donor organ biomarkers with sufficient predictive value in liver transplantation (LT) are lacking. We herein evaluate liver viability and mitochondrial bioenergetics for their predictive capacity towards the outcome in LT. We enrolled 43 consecutive patients undergoing LT. Liver biopsy samples taken upon arrival after static cold storage were assessed by histology, real-time confocal imaging analysis (RTCA), and high-resolution respirometry (HRR) for mitochondrial respiration of tissue homogenates. Early allograft dysfunction (EAD) served as primary endpoint. HRR data were analysed with a focus on the efficacy of ATP production or P - L control efficiency, calculated as 1- L / P from the capacity of oxidative phosphorylation P and non-phosphorylating respiration L . Twenty-two recipients experienced EAD. Pre-transplant histology was not predictive of EAD. The mean RTCA score was significantly lower in the EAD cohort (-0.75 ± 2.27) compared to the IF cohort (0.70 ± 2.08; p = 0.01), indicating decreased cell viability. P - L control efficiency was predictive of EAD (0.76 ± 0.06 in IF vs. 0.70 ± 0.08 in EAD-livers; p = 0.02) and correlated with the RTCA score. Both RTCA and P - L control efficiency in biopsy samples taken during cold storage have predictive capacity towards the outcome in LT. Therefore, RTCA and HRR should be considered for risk stratification, viability assessment, and bioenergetic testing in liver transplantation., Competing Interests: EG is founder and CEO of Oroboros Instruments. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2024 Meszaros, Weissenbacher, Schartner, Egelseer-Bruendl, Hermann, Unterweger, Mittelberger, Reyer, Hofmann, Zelger, Hautz, Resch, Margreiter, Maglione, Komlódi, Ulmer, Cardini, Troppmair, Öfner, Gnaiger, Schneeberger and Oberhuber.)

Anoxia halts oxidative phosphorylation (OXPHOS) causing an accumulation of reduced compounds in the mitochondrial matrix which impedes dehydrogenases. By simultaneously measuring oxygen concentration, NADH autofluorescence, mitochondrial membrane potential and ubiquinone reduction extent in isolated mitochondria in real-time, we demonstrate that Complex I utilized endogenous quinones to oxidize NADH under acute anoxia. 13 C metabolic tracing or untargeted analysis of metabolites extracted during anoxia in the presence or absence of site-specific inhibitors of the electron transfer system showed that NAD +  regenerated by Complex I is reduced by the 2-oxoglutarate dehydrogenase Complex yielding succinyl-CoA supporting mitochondrial substrate-level phosphorylation (mtSLP), releasing succinate. Complex II operated amphidirectionally during the anoxic event, providing quinones to Complex I and reducing fumarate to succinate. Our results highlight the importance of quinone provision to Complex I oxidizing NADH maintaining glutamate catabolism and mtSLP in the absence of OXPHOS., (© 2024. The Author(s).)

Ubiquinol cytochrome c reductase hinge protein (UQCRH) is required for the electron transfer between cytochrome c 1 and c of the mitochondrial cytochrome bc 1 Complex (CIII). A two-exon deletion in the human UQCRH gene has recently been identified as the cause for a rare familial mitochondrial disorder. Deletion of the corresponding gene in the mouse (Uqcrh-KO) resulted in striking biochemical and clinical similarities including impairment of CIII, failure to thrive, elevated blood glucose levels, and early death. Here, we set out to test how global ablation of the murine Uqcrh affects cardiac morphology and contractility, and bioenergetics. Hearts from Uqcrh-KO mutant mice appeared macroscopically considerably smaller compared to wildtype littermate controls despite similar geometries as confirmed by transthoracic echocardiography (TTE). Relating TTE-assessed heart to body mass revealed the development of subtle cardiac enlargement, but histopathological analysis showed no excess collagen deposition. Nonetheless, Uqcrh-KO hearts developed pronounced contractile dysfunction. To assess mitochondrial functions, we used the high-resolution respirometer NextGen-O2k allowing measurement of mitochondrial respiratory capacity through the electron transfer system (ETS) simultaneously with the redox state of ETS-reactive coenzyme Q (Q), or production of reactive oxygen species (ROS). Compared to wildtype littermate controls, we found decreased mitochondrial respiratory capacity and more reduced Q in Uqcrh-KO, indicative for an impaired ETS. Yet, mitochondrial ROS production was not generally increased. Taken together, our data suggest that Uqcrh-KO leads to cardiac contractile dysfunction at 9 weeks of age, which is associated with impaired bioenergetics but not with mitochondrial ROS production. Global ablation of the Uqcrh gene results in functional impairment of CIII associated with metabolic dysfunction and postnatal developmental arrest immediately after weaning from the mother. Uqcrh-KO mice show dramatically elevated blood glucose levels and decreased ability of isolated cardiac mitochondria to consume oxygen (O 2 ). Impaired development (failure to thrive) after weaning manifests as a deficiency in the gain of body mass and growth of internal organ including the heart. The relative heart mass seemingly increases when organ mass calculated from transthoracic echocardiography (TTE) is normalized to body mass. Notably, the heart shows no signs of collagen deposition, yet does develop a contractile dysfunction reflected by a decrease in ejection fraction and fractional shortening., (© 2022. The Author(s).)

α-ketoglutarate dehydrogenase complex (KGDHc), or 2-oxoglutarate dehydrogenase complex (OGDHc) is a rate-limiting enzyme in the tricarboxylic acid cycle, that has been identified in neurodegenerative diseases such as in Alzheimer's disease. The aim of the present study was to establish the role of the KGDHc and its subunits in the bioenergetics and reactive oxygen species (ROS) homeostasis of brain mitochondria. To study the bioenergetic profile of KGDHc, genetically modified mouse strains were used having a heterozygous knock out (KO) either in the dihydrolipoyl succinyltransferase (DLST +/- ) or in the dihydrolipoyl dehydrogenase (DLD +/- ) subunit. Mitochondrial oxygen consumption, hydrogen peroxide (H 2 O 2 ) production, and expression of antioxidant enzymes were measured in isolated mouse brain mitochondria. Here, we demonstrate that the ADP-stimulated respiration of mitochondria was partially arrested in the transgenic animals when utilizing α-ketoglutarate (α-KG or 2-OG) as a fuel substrate. Succinate and α-glycerophosphate (α-GP), however, did not show this effect. The H 2 O 2 production in mitochondria energized with α-KG was decreased after inhibiting the adenine nucleotide translocase and Complex I (CI) in the transgenic strains compared to the controls. Similarly, the reverse electron transfer (RET)-evoked H 2 O 2 formation supported by succinate or α-GP were inhibited in mitochondria isolated from the transgenic animals. The decrease of RET-evoked ROS production by DLST +/- or DLD +/- KO-s puts the emphasis of the KGDHc in the pathomechanism of ischemia-reperfusion evoked oxidative stress. Supporting this notion, expression of the antioxidant enzyme glutathione peroxidase was also decreased in the KGDHc transgenic animals suggesting the attenuation of ROS-producing characteristics of KGDHc. These findings confirm the contribution of the KGDHc to the mitochondrial ROS production and in the pathomechanism of ischemia-reperfusion injury.

The oxidation of proline to pyrroline-5-carboxylate (P5C) leads to the transfer of electrons to ubiquinone in mitochondria that express proline dehydrogenase (ProDH). This electron transfer supports Complexes CIII and CIV, thus generating the protonmotive force. Further catabolism of P5C forms glutamate, which fuels the citric acid cycle that yields the reducing equivalents that sustain oxidative phosphorylation. However, P5C and glutamate catabolism depend on CI activity due to NAD + requirements. NextGen-O2k (Oroboros Instruments) was used to measure proline oxidation in isolated mitochondria of various mouse tissues. Simultaneous measurements of oxygen consumption, membrane potential, NADH, and the ubiquinone redox state were correlated to ProDH activity and F 1 F O -ATPase directionality. Proline catabolism generated a sufficiently high membrane potential that was able to maintain the F 1 F O -ATPase operation in the forward mode. This was observed in CI-inhibited mouse liver and kidney mitochondria that exhibited high levels of proline oxidation and ProDH activity. This action was not observed under anoxia or when either CIII or CIV were inhibited. The duroquinone fueling of CIII and CIV partially reproduced the effects of proline. Excess glutamate, however, could not reproduce the proline effect, suggesting that processes upstream of the glutamate conversion from proline were involved. The ProDH inhibitors tetrahydro-2-furoic acid and, to a lesser extent, S-5-oxo-2-tetrahydrofurancarboxylic acid abolished all proline effects. The data show that ProDH-directed proline catabolism could generate sufficient CIII and CIV proton pumping, thus supporting ATP production by the F 1 F O -ATPase even under CI inhibition.

Iron is an essential component for metabolic processes, including oxygen transport within hemoglobin, tricarboxylic acid (TCA) cycle activity, and mitochondrial energy transformation. Iron deficiency can thus lead to metabolic dysfunction and eventually result in iron deficiency anemia (IDA), which affects approximately 1.5 billion people worldwide. Using a rat model of IDA induced by phlebotomy, we studied the effects of IDA on mitochondrial respiration in peripheral blood mononuclear cells (PBMCs) and the liver. Furthermore, we evaluated whether the mitochondrial function evaluated by high-resolution respirometry in PBMCs reflects corresponding alterations in the liver. Surprisingly, mitochondrial respiratory capacity was increased in PBMCs from rats with IDA compared to the controls. In contrast, mitochondrial respiration remained unaffected in livers from IDA rats. Of note, citrate synthase activity indicated an increased mitochondrial density in PBMCs, whereas it remained unchanged in the liver, partly explaining the different responses of mitochondrial respiration in PBMCs and the liver. Taken together, these results indicate that mitochondrial function determined in PBMCs cannot serve as a valid surrogate for respiration in the liver. Metabolic adaptions to iron deficiency resulted in different metabolic reprogramming in the blood cells and liver tissue.

Iron is an essential co-factor for many cellular metabolic processes, and mitochondria are main sites of utilization. Iron accumulation promotes production of reactive oxygen species (ROS) via the catalytic activity of iron species. Herein, we investigated the consequences of dietary and genetic iron overload on mitochondrial function. C57BL/6N wildtype and Hfe -/- mice, the latter a genetic hemochromatosis model, received either normal diet (ND) or high iron diet (HI) for two weeks. Liver mitochondrial respiration was measured using high-resolution respirometry along with analysis of expression of specific proteins and ROS production. HI promoted tissue iron accumulation and slightly affected mitochondrial function in wildtype mice. Hepatic mitochondrial function was impaired in Hfe -/- mice on ND and HI. Compared to wildtype mice, Hfe -/- mice on ND showed increased mitochondrial respiratory capacity. Hfe -/- mice on HI showed very high liver iron levels, decreased mitochondrial respiratory capacity and increased ROS production associated with reduced mitochondrial aconitase activity. Although Hfe -/- resulted in increased mitochondrial iron loading, the concentration of metabolically reactive cytoplasmic iron and mitochondrial density remained unchanged. Our data show multiple effects of dietary and genetic iron loading on mitochondrial function and linked metabolic pathways, providing an explanation for fatigue in iron-overloaded hemochromatosis patients, and suggests iron reduction therapy for improvement of mitochondrial function.

Background: Triethylene glycol dimethacrylate (TEGDMA) monomers released from resin matrix are toxic to dental pulp cells, induce apoptosis, oxidative stress and decrease viability. Recently, mitochondrial complex I (CI) was identified as a potential target of TEGDMA. In isolated mitochondria supported by CI, substrates oxidation and ATP synthesis were inhibited, reactive oxygen species production was stimulated. Contrary to that, respiratory Complex II was not impaired by TEGDMA. The beneficial effects of electron carrier compound methylene blue (MB) are proven in many disease models where mitochondrial involvement has been detected. In the present study, the bioenergetic effects of MB on TEGDMA-treated isolated mitochondria and on human dental pulp stem cells (DPSC) were analyzed., Methods: Isolated mitochondria and DPSC were acutely exposed to low millimolar concentrations of TEGDMA and 2 μM concentration of MB. Mitochondrial and cellular respiration and glycolytic flux were measured by high resolution respirometry and by Seahorse XF extracellular analyzer. Mitochondrial membrane potential was measured fluorimetrically., Results: MB partially restored the mitochondrial oxidation, rescued membrane potential in isolated mitochondria and significantly increased the impaired cellular O 2 consumption in the presence of TEGDMA., Conclusion: MB is able to protect against TEGDMA-induced CI damage, and might provide protective effects in resin monomer exposed cells.

Succinate-driven reverse electron transport (RET) is one of the main sources of mitochondrial reactive oxygen species (mtROS) in ischemia-reperfusion injury. RET is dependent on mitochondrial membrane potential (Δψ m ) and transmembrane pH difference (ΔpH), components of the proton motive force (pmf); a decrease in Δψ m and/or ΔpH inhibits RET. In this study we aimed to determine which component of the pmf displays the more dominant effect on RET-provoked ROS generation in isolated guinea pig brain and heart mitochondria respiring on succinate or α-glycerophosphate (α-GP). Δψ m was detected via safranin fluorescence and a TPP + electrode, the rate of H 2 O 2 formation was measured by Amplex UltraRed, the intramitochondrial pH (pH in ) was assessed via BCECF fluorescence. Ionophores were used to dissect the effects of the two components of pmf. The K + /H + exchanger, nigericin lowered pH in and ΔpH, followed by a compensatory increase in Δψ m that led to an augmented H 2 O 2 production. Valinomycin, a K + ionophore, at low [K + ] increased ΔpH and pH in , decreased Δψ m , which resulted in a decline in H 2 O 2 formation. It was concluded that Δψ m is dominant over ∆pH in modulating the succinate- and α-GP-evoked RET. The elevation of extramitochondrial pH was accompanied by an enhanced H 2 O 2 release and a decreased ∆pH. This phenomenon reveals that from the pH component not ∆pH, but rather absolute value of pH has higher impact on the rate of mtROS formation. Minor decrease of Δψ m might be applied as a therapeutic strategy to attenuate RET-driven ROS generation in ischemia-reperfusion injury.

Objectives: Earlier studies demonstrated that dental resin monomers lower cellular viability and provoke oxidative stress. Reactive oxygen species (ROS) formation has a key role in triethylene glycol dimethacrylate (TEGDMA) induced adverse reactions. In the present study the effects of TEGDMA on mitochondrial functions were investigated to identify a direct molecular target for cytotoxicity., Methods: Mitochondria were isolated from guinea pig brain. The most important bioenergetic parameters, oxygen consumption, membrane potential (ΔΨ m ), and ATP production were assessed. Mitochondrial H 2 O 2 production and elimination and the NAD(P)H level reported on redox balance., Results: Mitochondria were supported with respiratory substrates to be oxidized by either Complex I (CI) or Complex II (CII). ΔΨ m was depolarized, respiration and ATP production was greatly diminished when applying CI substrates in the presence of TEGDMA. The same parameters remained essentially unaffected when CII substrate plus TEGDMA were applied. H 2 O 2 production by mitochondria was significantly stimulated by TEGDMA in the presence of CI substrates. In the presence of TEGDMA mitochondrial elimination of exogenous H 2 O 2 was impaired. When CII substrate supported the mitochondria in the absence of ADP the H 2 O 2 generation was decreased. NADH autofluorescence results also demonstrated the inhibitory effect of TEGDMA on CI activity., Significance: TEGDMA inhibits CI in the respiratory chain, which explains effects induced by TEGDMA on redox homeostasis, apoptotic and necrotic cell deaths described in previous studies. Identification of the molecular target of TEGDMA may influence the development of relevant biomaterials and may induce new therapeutic strategies to control the adverse effects of resin monomers., (Copyright © 2018. Published by Elsevier Inc.)

High-Resolution FluoRespirometry is a well-established and versatile approach to study mitochondrial oxygen uptake amperometrically in combination with measurement of fluorescence signals. One of the most frequently applied fluorescent dyes is Amplex UltraRed for monitoring rates of hydrogen peroxide production. Selection of an appropriate mitochondrial respiration medium is of crucial importance, the primary role of which is to support and preserve optimum mitochondrial function. For harmonization of results in a common database, we compared respiration and H 2 O 2 production of permeabilized HEK 293T cells measured in MiR05 (sucrose and K-lactobionate), Buffer Z (K-MES and KCl), MiR07 (combination of MiR05 and Buffer Z), and MiRK03 (KCl). Respiration in a simple substrate-uncoupler-inhibitor titration protocol was identical in MiR05, Buffer Z, and MiR07, whereas oxygen fluxes detected with MiRK03 were consistently lower in all coupling and electron transfer-pathway states. H 2 O 2 production rates were comparable in all four media, while assay sensitivity was comparatively low with MiR05 and MiR07 and higher but declining over time in the other two media. Stability of assay sensitivity over experimental time was highest in MiR05 but slightly less in MiR07. Taken together, MiR05 and Buffer Z yield comparable results on respiration and H 2 O 2 production. Despite the lower sensitivity, MiR05 was selected as the medium of choice for FluoRespirometry due to the highest stability of the sensitivity or calibration constant observed in experiments over periods of up to 2 h.

Although incidence and prevalence of prediabetes are increasing, little is known about its cardiac effects. Therefore, our aim was to investigate the effect of prediabetes on cardiac function and to characterize parameters and pathways associated with deteriorated cardiac performance. Long-Evans rats were fed with either control or high-fat chow for 21 wk and treated with a single low dose (20 mg/kg) of streptozotocin at week 4 High-fat and streptozotocin treatment induced prediabetes as characterized by slightly elevated fasting blood glucose, impaired glucose and insulin tolerance, increased visceral adipose tissue and plasma leptin levels, as well as sensory neuropathy. In prediabetic animals, a mild diastolic dysfunction was observed, the number of myocardial lipid droplets increased, and left ventricular mass and wall thickness were elevated; however, no molecular sign of fibrosis or cardiac hypertrophy was shown. In prediabetes, production of reactive oxygen species was elevated in subsarcolemmal mitochondria. Expression of mitofusin-2 was increased, while the phosphorylation of phospholamban and expression of Bcl-2/adenovirus E1B 19-kDa protein-interacting protein 3 (BNIP3, a marker of mitophagy) decreased. However, expression of other markers of cardiac auto- and mitophagy, mitochondrial dynamics, inflammation, heat shock proteins, Ca 2+ /calmodulin-dependent protein kinase II, mammalian target of rapamycin, or apoptotic pathways were unchanged in prediabetes. This is the first comprehensive analysis of cardiac effects of prediabetes indicating that mild diastolic dysfunction and cardiac hypertrophy are multifactorial phenomena that are associated with early changes in mitophagy, cardiac lipid accumulation, and elevated oxidative stress and that prediabetes-induced oxidative stress originates from the subsarcolemmal mitochondria., (Copyright © 2016 the American Physiological Society.)