The CoQH2/CoQ Ratio Serves as a Sensor of Respiratory Chain Efficiency

Time dynamics of elevated glucose and beta-hydroxybutyrate on beta cell mitochondrial metabolism

Chronic hyperglycemia impairs mitochondrial function of beta cells. Changes in mitochondrial function preceding a negative glucose effect have not been fully characterized, nor interactions with ketones. To compare effects on beta cell mitochondrial function by short and longer exposures to elevated glucose and interactions with ketones oxygen consumption rate (OCR) was measured in intact clonal beta cells by an OROBOROS and in rat islets by a Seahorse instrument. Proteins (subunits) of mitochondrial complexes (C) were measured by immunoblotting. ATP and ROS were measured in islets. In INS-1 832/13 cells, overnight exposure to 27 vs. 11 mm glucose increased OCR and uncoupled mitochondrial respiration. These effects vanished when prolonging the exposure time of elevated glucose. C1 was decreased after two days of culture with high glucose. Interactions with racemic 5 and 20 mm beta-hydroxybutyrate (BHB) were not detected. In islets, culture overnight at 27 vs.11 mm glucose enhanced basal OCR. No decrease in glucose-induced OCR was seen after prolonging 27 mm glucose for two days. Interactions with 5 mm BHB were not detected. Prolonged exposure to 27 mm glucose enhanced basal ECAR (extracellular acidification rate) and an ECAR response to acute elevation of glucose. C1 and 3 and 4 were decreased after two days of 27 vs. 11 mm glucose. ATP levels were decreased at this time-point and extracellular ROS increased. High glucose time-dependently affects mitochondrial function in clonal beta cells and islets. C1 was uniformly decreased. Interactions with BHB were not detected.

Reductive stress and mitochondrial dysfunction: The hidden link in chronic disease

Conventional theories of oxidative stress have long focused on the deleterious consequences of excessive reactive oxygen species (ROS) formation. However, growing evidence reveals that an overload of reducing equivalents-termed reductive stress-may be equally pivotal in driving mitochondrial dysfunction and chronic disease. In this paradigm, abnormally high concentrations of NADH and NADPH create an electron "traffic jam" in the mitochondrial electron transport chain (ETC), leading to partial inhibition or reverse electron flow at upstream complexes. Paradoxically, this hyper-reduced environment promotes ROS generation by increasing electron leakage to molecular oxygen, thereby intensifying oxidative damage to lipids, proteins, and mitochondrial DNA. This review explores the intertwined nature of reductive and oxidative stress, showing how a surplus of reducing equivalents can potentiate metabolic derangements in conditions such as type 2 diabetes, nonalcoholic fatty liver disease, and neurodegenerative disorders. The review discusses common drivers of reductive overload, including chronic hyperglycemia, high-fat diets, and specific dietary patterns-particularly those enriched in polyunsaturated omega-6 fatty acids-that inundate mitochondria with electron donors. The review also highlights emerging evidence that targeted assessment of redox biomarkers (e.g., lactate:pyruvate, β-hydroxybutyrate:acetoacetate ratios) can provide clinically relevant indicators of reductive stress. Finally, the review examines how novel therapeutic strategies can address the underlying reductive imbalance, from rational nutrient modulation to pharmacologic interventions that restore NAD+ levels or optimize ETC flux. Recognizing reductive stress as a critical inflection point in mitochondrial pathophysiology underscores the need for a refined redox framework, one that moves beyond conventional oxidative paradigms to embrace the full spectrum of redox dysregulation in chronic degenerative disease.

Functional mitochondrial respiration is essential for glioblastoma tumour growth

Horizontal transfer of mitochondria from the tumour microenvironment to cancer cells to support proliferation and enhance tumour progression has been shown for various types of cancer in recent years. Glioblastoma, the most aggressive adult brain tumour, has proven to be no exception when it comes to dynamic intercellular mitochondrial movement, as shown in this study using an orthotopic tumour model of respiration-deficient glioblastoma cells. Although confirmed mitochondrial transfer was shown to facilitate tumour progression in glioblastoma, we decided to investigate whether the related electron transport chain recovery is necessary for tumour formation in the brain. Based on experiments using time-resolved analysis of tumour formation by glioblastoma cells depleted of their mitochondrial DNA, we conclude that functional mitochondrial respiration is essential for glioblastoma growth in vivo, because it is needed to support coenzyme Q redox cycling for de novo pyrimidine biosynthesis controlled by respiration-linked dihydroorotate dehydrogenase enzyme activity. We also demonstrate here that astrocytes are key mitochondrial donors in this model.

Melatonin rescues cell respiration impaired by hypoxia/reoxygenation in aortic endothelial cells and affects the mitochondrial bioenergetics targeting the F1FO-ATPase

Melatonin is evaluated as a potential molecular therapy to counteract mitochondrial dysfunction caused by hypoxia/reoxygenation (H/R) in aortic endothelial cells (pAECs). The mitochondrial permeability transition pore (mPTP) opening undergoes a desensitizing action coupled with a reduction of superoxide anion production in mitochondria treated with melatonin. The effect on mPTP has been attributed to the direct interaction of melatonin with the hydrophilic F1 domain of Ca2+-activated F1FO-ATPase. Mutual exclusion analysis highlights an overlapping binding site between melatonin and the specific F1 inhibitor NBD-Cl. The results are corroborated by melatonin inhibition of ATPase activity of the purified F1 domain in the presence of Ca2+, but not in the presence of natural cofactor Mg2+. Moreover, the impairment of bioenergetics parameters in pAECs metabolism and the increase of oxidative stress arising by H/R injury have been rescued in cells protected by melatonin treatment.

Dysregulated skeletal muscle myosin super-relaxation and energetics in male participants with type 2 diabetes mellitus

AIMS/HYPOTHESIS

Disrupted energy balance is critical for the onset and development of type 2 diabetes mellitus. Understanding of the exact underlying metabolic mechanisms remains incomplete, but skeletal muscle is thought to play an important pathogenic role. As the super-relaxed state of its most abundant protein, myosin, regulates cellular energetics, we aimed to investigate whether it is altered in individuals with type 2 diabetes.

METHODS

We used vastus lateralis biopsy specimens (obtained from patients with type 2 diabetes and control participants with similar characteristics), and ran a combination of structural and functional assays consisting of loaded 2'- (or 3')-O-(N-methylanthraniloyl)-ATP (Mant-ATP) chase experiments, x-ray diffraction and LC-MS/MS proteomics in isolated muscle fibres.

RESULTS

Our studies revealed a greater muscle myosin super-relaxation and decreased ATP demand in male participants with type 2 diabetes than in control participants. Subsequent proteomic analyses indicated that these (mal)adaptations probably originated from remodelled sarcomeric proteins and greater myosin glycation levels in patients than in control participants.

CONCLUSIONS/INTERPRETATION

Overall, our findings indicate a complex molecular dysregulation of myosin super-relaxed state and energy consumption in male participants with type 2 diabetes. Ultimately, pharmacological targeting of myosin could benefit skeletal muscle and whole-body metabolic health through enhancement of ATP consumption.

DATA AVAILABILITY

The raw MS data have been deposited to the ProteomeXchange Consortium via the PRIDE partner repository with the dataset identifier PXD053022.

MARIGOLD and MitoCIAO, two searchable compendia to visualize and functionalize protein complexes during mitochondrial remodeling

Mitochondrial proteins assemble dynamically in high molecular weight complexes essential for their functions. We generated and validated two searchable compendia of these mitochondrial complexes. Following identification by mass spectrometry of proteins in complexes separated using blue-native gel electrophoresis from unperturbed, cristae-remodeled, and outer membrane-permeabilized mitochondria, we created MARIGOLD, a mitochondrial apoptotic remodeling complexome database of 627 proteins. MARIGOLD elucidates how dynamically proteins distribute in complexes upon mitochondrial membrane remodeling. From MARIGOLD, we developed MitoCIAO, a mitochondrial complexes interactome tool that, by statistical correlation, calculates the likelihood of protein cooccurrence in complexes. MitoCIAO correctly predicted biologically validated interactions among components of the mitochondrial cristae organization system (MICOS) and optic atrophy 1 (OPA1) complexes. We used MitoCIAO to functionalize two ATPase family AAA domain-containing 3A (ATAD3A) complexes: one with OPA1 that regulates mitochondrial ultrastructure and the second containing ribosomal proteins that is essential for mitoribosome stability. These compendia reveal the dynamic nature of mitochondrial complexes and enable their functionalization.

Short-term sustained hypoxia distinctly affects subpopulations of carotid body glomus cells from rats

The main O2 arterial chemoreceptors are the carotid bodies (CBs), which mediate hyperventilation in response to short-term sustained hypoxia (SH). CBs contain glomus cells expressing K+ channels, which are inhibited by hypoxia, leading to neurotransmitter release. ATP released by CBs and type II cells has been considered essential for chemosensory processing under physiological and pathophysiological conditions. Although the systemic effects of chronic activation of CBs by SH are well known, the early (first 24 h) cellular and molecular mechanisms in CBs as well as the effects of short-term SH on populations of glomus cells are still poorly understood. Here, we show that SH (10% O2 for 24 h) depolarizes the membrane potential of one population of glomus cells, mediated by increases in inward current, but does not affect the ATP release by CBs. In addition, SH promotes a reduction in their maximum outward current, mediated by voltage-gated K+ channels. SH also affected sensitivity to acute hypoxia in one glomus cell subpopulation. As for the content of mitochondrial proteins, we observed increases in the citrate synthase, Tom-20, and succinate dehydrogenase (mitochondrial complex II) per cell of CBs after SH. Our results demonstrate important cellular and molecular mechanisms of plasticity in CBs from rats after only 24 h of SH, which may contribute to the generation of cardiovascular and ventilatory adjustments observed in this experimental model.NEW & NOTEWORTHY Our study revealed two subpopulations of glomus cells of carotid bodies (CBs) with specific electrophysiological properties, which were differentially affected by short-term sustained hypoxia (SH; 10% O2 for 24 h). Our experiments showed that SH also affected the sensitivity to acute hypoxia of these glomus cell subpopulations differently. Our molecular analyses allowed us to identify important adaptations in the content of CB mitochondrial proteins that participate in the Krebs cycle and form the electron transport chain.

Reactive Oxygen Species (ROS) in Metabolic Disease-Don't Shoot the Metabolic Messenger

Reactive oxygen species (ROS) are widely considered key to pathogenesis in chronic metabolic disease. Consequently, much attention is rightly focused on minimising oxidative damage. However, for ROS production to be most effectively modulated, it is crucial to first appreciate that ROS do not solely function as pathological mediators. There are >90 gene products specifically evolved to generate, handle, and tightly buffer the cellular concentration of ROS. Therefore, it is likely that ROS plays a role as integral homeostatic signalling components and only become toxic in extremis. This review explores these commonly overlooked normal physiological functions, including how ROS are generated in response to environmental or hormonal stimuli, the mechanisms by which the signals are propagated and regulated, and how the cell effectively brings the signal to an end after an appropriate duration. In the course of this, several specific and better-characterised signalling mechanisms that rely upon ROS are explored, and the threshold at which ROS cross from beneficial signalling molecules to pathology mediators is discussed.

Exploring potential additive effects of 5-fluorouracil, thymoquinone, and coenzyme Q10 triple therapy on colon cancer cells in relation to glycolysis and redox status modulation

BACKGROUND

To investigate the anticancer effects of 5-Fluorouracil (5-FU), thymoquinone (TQ), and/or coenzyme Q10 (CQ10), alone and combined, in HT29, SW480, and SW620 human colorectal cancer (CRC) cell lines.

METHODS

Cell cycle progression and apoptosis were assessed by flow cytometry. Gene and protein expression of molecules involved in apoptosis (BLC2, survivin, BAX, Cytochrome-C, and Caspase-3), cell cycle (CCND1, CCND3, p21, and p27), the PI3K/AKT/mTOR/HIF1α oncogenic pathway, and glycolysis (LDHA, PDH, and PDHK1) were also analysed by quantitative RT-PCR and Western blot. Oxidative stress markers (ROS/RNS, MDA, and Protein carbonyl groups) and antioxidants (GSH and CAT) were quantified by ELISA.

RESULTS

All treatments resulted in anticancer effects depicted by cell cycle arrest and apoptosis, with TQ demonstrating greater efficacy than CQ10, both with and without 5-FU. However, 5-FU/TQ/CQ10 triple therapy exhibited the most potent pro-apoptotic activity in all cell lines, portrayed by the lowest levels of oncogenes (CCND1, CCND3, BCL2, and survivin) and the highest upregulation of tumour suppressors (p21, p27, BAX, Cytochrome-C, and Caspase-3). The triple therapy also showed the strongest suppression of the PI3K/AKT/mTOR/HIF1α pathway, with a concurrent increase in its endogenous inhibitors (PTEN and AMPKα) in all cell lines used. Additionally, the triple therapy favoured glucose oxidation by upregulating PDH, while decreasing LDHA and PDHK1 enzymes. The triple therapy also displayed the most significant decline in antioxidant levels and the highest increases in oxidative stress markers.

CONCLUSIONS

This study is the first to demonstrate the superior anticancer effects of TQ compared to CQ10, with and without 5-FU, in CRC treatment. Moreover, this is the first report to reveal improved anticancer effects of the 5-FU/TQ/CQ10 triple therapy, potentially through promoting oxidative phosphorylation, attenuating the PI3K/AKT/mTOR/HIF1α pathway, and increasing oxidative stress-induced apoptosis.

Formation of I2+III2 supercomplex rescues respiratory chain defects

Mitochondrial electron transport chain (ETC) complexes partition between free complexes and quaternary assemblies known as supercomplexes (SCs). However, the physiological requirement for SCs and the mechanisms regulating their formation remain controversial. Here, we show that genetic perturbations in mammalian ETC complex III (CIII) biogenesis stimulate the formation of a specialized extra-large SC (SC-XL) with a structure of I2+III2, resolved at 3.7 Å by cryoelectron microscopy (cryo-EM). SC-XL formation increases mitochondrial cristae density, reduces CIII reactive oxygen species (ROS), and sustains normal respiration despite a 70% reduction in CIII activity, effectively rescuing CIII deficiency. Consequently, inhibiting SC-XL formation in CIII mutants using the Uqcrc1DEL:E258-D260 contact site mutation leads to respiratory decompensation. Lastly, SC-XL formation promotes fatty acid oxidation (FAO) and protects against ischemic heart failure in mice. Our study uncovers an unexpected plasticity in the mammalian ETC, where structural adaptations mitigate intrinsic perturbations, and suggests that manipulating SC-XL formation is a potential therapeutic strategy for mitochondrial dysfunction.

Altered mitochondria-associated ER membrane (MAM) function shifts mitochondrial metabolism in amyotrophic lateral sclerosis (ALS)

Mitochondrial function is modulated by its interaction with the endoplasmic reticulum (ER). Recent research indicates that these contacts are disrupted in familial models of amyotrophic lateral sclerosis (ALS). We report here that this impairment in the crosstalk between mitochondria and the ER impedes the use of glucose-derived pyruvate as mitochondrial fuel, causing a shift to fatty acids to sustain energy production. Over time, this deficiency alters mitochondrial electron flow and the active/dormant status of complex I in spinal cord tissues, but not in the brain. These findings suggest mitochondria-associated ER membranes (MAM domains) play a crucial role in regulating cellular glucose metabolism and that MAM dysfunction may underlie the bioenergetic deficits observed in ALS.

Prolonged L-NAME exposure changes the vasodilator factor from NO to H2O2 in human arterioles in response to A23187

The Ca2+ ionophore A23187 induces endothelium-dependent and non-receptor-mediated vasodilation in human adipose arterioles (HAAs). The purpose of this study was to determine the mechanism of A23187-induced dilation in HAAs from patients with and without coronary artery disease (CAD). HAAs were freshly isolated from adipose tissues obtained from non-CAD (n = 25) and CAD (n = 14) patients, and vascular reactivity was studied by videomicroscopy. No difference in baseline dose response to A23187 was observed between non-CAD and CAD subjects. However, acute (30 min) incubation with N(omega)-nitro-l-arginine methyl ester (L-NAME), NO synthase inhibitor strongly reduced A23187-induced dilation in non-CAD arterioles, while catalase, an H2O2 scavenger, largely abolished dilation in CAD. Surprising, prolonged (90 min) incubation with L-NAME restored A23187 response in non-CAD subjects, which was subsequently inhibited by catalase. The action of prolonged L-NAME exposure was not reversible after washing with Krebs while the effect of acute L-NAME exposure was largely reversible. To further determine the role of mitochondria-derived ROS in A23187-induced dilation, arterioles were treated with rotenone, an inhibitor of complex I of the electron transport chain. Rotenone abolished A23187 response in CAD patients and in non-CAD arterioles after prolonged L-NAME, but not in non-CAD controls. These data indicate that NO contributes to A23187-induced dilation in HAAs from non-CAD patients and H2O2 contributes to the dilation in CAD patients. Prolonged L-NAME exposure induces a NO-H2O2 switch in the mechanism of dilation in non-CAD subjects. Moreover, the effect of prolonged L-NAME exposure is not readily reversible, while the action of acute L-NAME exposure is reversible.

A transmitochondrial sodium gradient controls membrane potential in mammalian mitochondria

Eukaryotic cell function and survival rely on the use of a mitochondrial H+ electrochemical gradient (Δp), which is composed of an inner mitochondrial membrane (IMM) potential (ΔΨmt) and a pH gradient (ΔpH). So far, ΔΨmt has been assumed to be composed exclusively of H+. Here, using a rainbow of mitochondrial and nuclear genetic models, we have discovered that a Na+ gradient equates with the H+ gradient and controls half of ΔΨmt in coupled-respiring mammalian mitochondria. This parallelism is controlled by the activity of the long-sought Na+-specific Na+/H+ exchanger (mNHE), which we have identified as the P-module of complex I (CI). Deregulation of this mNHE function, without affecting the canonical enzymatic activity or the assembly of CI, occurs in Leber's hereditary optic neuropathy (LHON), which has profound consequences in ΔΨmt and mitochondrial Ca2+ homeostasis and explains the previously unknown molecular pathogenesis of this neurodegenerative disease.

Targeting NQO1 induces ferroptosis and triggers anti-tumor immunity in immunotherapy-resistant KEAP1-deficient cancers

Immunotherapy has revolutionized cancer treatment, yet the efficacy of immunotherapeutic approaches remains limited. Resistance to ferroptosis is one of the reasons for the poor therapeutic outcomes in tumors with Kelch-like ECH-associated protein 1 (KEAP1) mutations. However, the specific mechanisms by which KEAP1-mutant tumors resist immunotherapy are not fully understood. In this study, we showed that the loss of function in KEAP1 results in resistance to ferroptosis. We identified NAD(P)H Quinone Dehydrogenase 1 (NQO1) as a transcriptional target of nuclear factor erythroid 2-related factor 2 (NRF2) and revealed that inducing NQO1-mediated ferroptosis in KEAP1-deficient tumors triggers an antitumor immune cascade. Additionally, it was found that NQO1 protein levels could serve as a candidate biomarker for predicting sensitivity to immunotherapy in clinical tumor patients. We validated these findings in several preclinical tumor models. Overall, KEAP1 mutations define a unique disease phenotype, and targeting its key downstream molecule NQO1 offers new hope for patients with resistance to immunotherapy.

Reducing the mitochondrial oxidative burden alleviates lipid-induced muscle insulin resistance in humans

Preclinical models suggest mitochondria-derived oxidative stress as an underlying cause of insulin resistance. However, it remains unknown whether this pathophysiological mechanism is conserved in humans. Here, we used an invasive in vivo mechanistic approach to interrogate muscle insulin action while selectively manipulating the mitochondrial redox state in humans. To this end, we conducted insulin clamp studies combining intravenous infusion of a lipid overload with intake of a mitochondria-targeted antioxidant (mitoquinone). Under lipid overload, selective modulation of mitochondrial redox state by mitoquinone enhanced insulin-stimulated glucose uptake in skeletal muscle. Mechanistically, mitoquinone did not affect canonical insulin signaling but augmented insulin-stimulated glucose transporter type 4 (GLUT4) translocation while reducing the mitochondrial oxidative burden under lipid oversupply. Complementary ex vivo studies in human muscle fibers exposed to high intracellular lipid levels revealed that mitoquinone improves features of mitochondrial bioenergetics, including diminished mitochondrial H2O2 emission. These findings provide translational and mechanistic evidence implicating mitochondrial oxidants in the development of lipid-induced muscle insulin resistance in humans.

Alterations in coenzyme Q10 status in a cybrid line harboring the 3243A>G mutation of mitochondrial DNA is associated with abnormal mitochondrial bioenergetics and dysregulated mitochondrial biogenesis

Mitochondrial DNA (mtDNA) mutations, including the m.3243A>G mutation that causes mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS), are associated with secondary coenzyme Q10 (CoQ10) deficiency. We previously demonstrated that PPARGC1A knockdown repressed the expression of PDSS2 and several COQ genes. In the present study, we compared the mitochondrial function, CoQ10 status, and levels of PDSS and COQ proteins and genes between mutant cybrids harboring the m.3243A>G mutation and wild-type cybrids. Decreased mitochondrial energy production, defective respiratory function, and reduced CoQ10 levels were observed in the mutant cybrids. The ubiquinol-10:ubiquinone-10 ratio was lower in the mutant cybrids, indicating blockage of the electron transfer upstream of CoQ, as evident from the reduced ratio upon rotenone treatment and increased ratio upon antimycin A treatment in 143B cells. The mutant cybrids exhibited downregulation of PDSS2 and several COQ genes and upregulation of COQ8A. In these cybrids, the levels of PDSS2, COQ3-a isoform, COQ4, and COQ9 were reduced, whereas those of COQ3-b and COQ8A were elevated. The mutant cybrids had repressed PPARGC1A expression, elevated ATP5A levels, and reduced levels of mtDNA-encoded proteins, nuclear DNA-encoded subunits of respiratory enzyme complexes, MNRR1, cytochrome c, and DHODH, but no change in TFAM, TOM20, and VDAC1 levels. Alterations in the CoQ10 level in MELAS may be associated with mitochondrial energy deficiency and abnormal gene regulation. The finding of a reduction in the ubiquinol-10:ubiquinone-10 ratio in the MELAS mutant cybrids differs from our previous discovery that cybrids harboring the m.8344A>G mutation exhibit a high ubiquinol-10:ubiquinone-10 ratio.

Facilitating Intracellular Electron Bifurcation by Mediating Flavin-Based Extracellular and Transmembrane Electron Transfer: A Novel Role of Pyrogenic Carbon in Dark Fermentation for Hydrogen Production

Pyrogenic carbon is considered an enhancer to H2-yielding dark fermentation (DF), but little is known about how it regulates extracellular electron transfer (EET) and influences transmembrane respiratory chains and intracellular metabolisms. This study addressed these knowledge gaps and demonstrated that wood waste pyrogenic carbon (biochar) could significantly improve the DF performance; e.g., addition of pyrogenic carbon produced by pyrolysis at 800 °C (PC800) increased H2 yield by 369.7%. Biochemical quantification, electrochemical analysis, and electron respiratory chain inhibition tests revealed that PC800 promoted the extracellular flavin-based electron transfer process and further activated the acceleration of the transmembrane electron transfer. Comparative metagenome/metatranscriptome analyses indicated that the flavin-containing Rnf complex was the potential transmembrane respiratory enzyme associated with PC800-mediated EET. Based on NADH/NAD+ circulation, the promoted Rnf complex could stimulate the functions of the electron bifurcating Etf/Bcd complex and startup of glycolysis. The promoted Etf/Bcd could further contribute to balance the NADH/NAD+ level for glycolytic reactions and meanwhile provide reduced ferredoxin for group A1 [FeFe]-hydrogenases. This proton-energy-linked mechanism could achieve coupling production of ATP and H2. This study verified the important roles of pyrogenic carbon in mediating EET and transmembrane/intracellular pathways and revealed the crucial roles of electron bifurcation in DF for hydrogen production.

Understanding coenzyme Q

Coenzyme Q (CoQ), also known as ubiquinone, comprises a benzoquinone head group and a long isoprenoid side chain. It is thus extremely hydrophobic and resides in membranes. It is best known for its complex function as an electron transporter in the mitochondrial electron transport chain (ETC) but is also required for several other crucial cellular processes. In fact, CoQ appears to be central to the entire redox balance of the cell. Remarkably, its structure and therefore its properties have not changed from bacteria to vertebrates. In metazoans, it is synthesized in all cells and is found in most, and maybe all, biological membranes. CoQ is also known as a nutritional supplement, mostly because of its involvement with antioxidant defenses. However, whether there is any health benefit from oral consumption of CoQ is not well established. Here we review the function of CoQ as a redox-active molecule in the ETC and other enzymatic systems, its role as a prooxidant in reactive oxygen species generation, and its separate involvement in antioxidant mechanisms. We also review CoQ biosynthesis, which is particularly complex because of its extreme hydrophobicity, as well as the biological consequences of primary and secondary CoQ deficiency, including in human patients. Primary CoQ deficiency is a rare inborn condition due to mutation in CoQ biosynthetic genes. Secondary CoQ deficiency is much more common, as it accompanies a variety of pathological conditions, including mitochondrial disorders as well as aging. In this context, we discuss the importance, but also the great difficulty, of alleviating CoQ deficiency by CoQ supplementation.

Mitochondrial puzzle in muscle: Linking the electron transport system to overweight

Human skeletal muscle mitochondria regulate energy expenditure. Research has shown that the functionality of muscle mitochondria is altered in subjects with overweight, as well as in response to nutrient excess and calorie restriction. Two metabolic features of obesity and overweight are (1) incomplete muscular fatty acid oxidation and (2) increased circulating lactate levels. In this study, I propose that these metabolic disturbances may originate from a common source within the muscle mitochondrial electron transport system. Specifically, a reorganization of the supramolecular structure of the electron transport chain could facilitate the maintenance of readily accessible coenzyme Q pools, which are essential for metabolizing lipid substrates. This approach is expected to maintain effective electron transfer, provided that there is sufficient complex III to support the Q-cycle. Such an adaptation could enhance fatty acid oxidation and prevent mitochondrial overload, thereby reducing lactate production. These insights advance our understanding of the molecular mechanisms underpinning metabolic dysregulation in overweight states. This provides a basis for targeted interventions in the quest for metabolic health.

The association of circulating bioenergetic metabolites with healthy human aging

Aging is an inevitable and gradual decline in several biological functions. Mitochondrial dysfunction is one of the most important hallmarks of aging. In this context, alterations in metabolites associated with mitochondrial dysfunction may serve as a significant biomarker. This study aimed to investigate the existence of a relationship between the key metabolites involved in bioenergetics metabolism and aging. 53 volunteers ranged 20-85 years participated in the study. We tested the association between different tricarboxylic acid (TCA) cycle metabolites, fatty acid metabolism, and amino acid metabolism with age, sex, body composition, and proxy markers of aging such as walking speed, grip strength and chair test. We found that lactic acid negatively correlated with age while several fatty acid metabolites, such as azelaic, sebacic, and linoleic acids, showed positive correlations with age (p < 0.05). Sex-specific trends, such as glycerol, and dodecanoic acid, were also observed for certain metabolites. Furthermore, citric acid levels were found to have a significant association with physical function and body composition measures. Participants with higher citric acid levels displayed improved performance in physical tests and favorable body composition indices. Additionally, fumaric acid and adipic acid showed positive correlations with fat-free body mass, while sebacic acid was negatively associated with measures of fat mass. These findings underscore the importance of understanding the role of circulating bioenergetics metabolites with age, sex variations, and their potential implications in body composition and physical performance.

RNAi screens identify HES4 as a regulator of redox balance supporting pyrimidine synthesis and tumor growth

NADH/NAD+ redox balance is pivotal for cellular metabolism. Systematic identification of NAD(H) redox regulators, although currently lacking, would help uncover unknown effectors critically implicated in the coordination of growth metabolism. In this study, we performed a genome-scale RNA interference (RNAi) screen to globally survey the genes involved in redox modulation and identified the HES family bHLH transcription factor HES4 as a negative regulator of NADH/NAD+ ratio. Functionally, HES4 is shown to be crucial for maintaining mitochondrial electron transport chain (ETC) activity and pyrimidine synthesis. More specifically, HES4 directly represses transcription of SLC44A2 and SDS, thereby inhibiting mitochondrial choline oxidation and cytosolic serine deamination, respectively, which, in turn, ensures coenzyme Q reduction capacity for DHODH-mediated UMP synthesis and serine-derived dTMP production. Accordingly, inhibition of choline oxidation preserves mitochondrial serine catabolism and ETC-coupled redox balance. Furthermore, HES4 protein stability is enhanced under EGFR activation, and increased HES4 levels facilitate EGFR-driven tumor growth and predict poor prognosis of lung adenocarcinoma. These findings illustrate an unidentified mechanism, underlying pyrimidine biosynthesis in the intersection between serine and choline catabolism, and underscore the physiological importance of HES4 in tumor metabolism.

Mitochondrial translation is the primary determinant of secondary mitochondrial complex I deficiencies

Individual complexes of the mitochondrial oxidative phosphorylation system (OXPHOS) are not linked solely by their function; they also share dependencies at the maintenance/assembly level, where one complex depends on the presence of a different individual complex. Despite the relevance of this "interdependence" behavior for mitochondrial diseases, its true nature remains elusive. To understand the mechanism that can explain this phenomenon, we examined the consequences of the aberration of different OXPHOS complexes in human cells. We demonstrate here that the complete disruption of each of the OXPHOS complexes resulted in a decrease in the complex I (cI) level and that the major reason for this is linked to the downregulation of mitochondrial ribosomal proteins. We conclude that the secondary cI defect is due to mitochondrial protein synthesis attenuation, while the responsible signaling pathways could differ based on the origin of the OXPHOS defect.

Effects of Mitoquinone (MitoQ) Supplementation on Aerobic Exercise Performance and Oxidative Damage: A Systematic Review and Meta-analysis

BACKGROUND

Contracting skeletal muscle produces reactive oxygen species (ROS) originating from both mitochondrial and cytosolic sources. The use of non-specific antioxidants, such as vitamins C and E, during exercise has produced inconsistent results in terms of exercise performance. Consequently, the effects of the mitochondrial-targeted coenzyme Q10, named Mitoquinone (MitoQ) on exercise responses are currently under investigation.

METHODS

In this study, we conducted a meta-analysis to quantitatively synthesize research assessing the impact of MitoQ on aerobic endurance performance and exercise-induced oxidative damage. PubMed, Web of Science, and SCOPUS databases were used to select articles from inception to January 16th of 2024. Inclusion criteria were MitoQ supplementation must be compared with a placebo group, showing acute exercise responses in both; for crossover designs, at least 14 d of washout was needed, and exercise training can be concomitant to MitoQ or placebo supplementation if the study meets the other inclusion criteria points. The risk of bias was evaluated through the Critical Appraisal Checklist (JBI).

RESULTS

We identified eight studies encompassing a total sample size of 188 subjects. Our findings indicate that MitoQ supplementation effectively reduces exercise-induced oxidative damage (SMD: -1.33; 95% CI: -2.24 to -0.43). Furthermore, our findings indicate that acute and/or chronic MitoQ supplementation does not improve endurance exercise performance (SMD: -0.50; 95% CI: -1.39 to 0.40) despite reducing exercise-induced oxidative stress. Notably, our sensitivity analysis reveals that MitoQ may benefit subjects with peripheral artery disease (PAD) in improving exercise tolerance.

CONCLUSION

While MitoQ effectively reduces exercise-induced oxidative damage, no evidence suggests that aerobic exercise performance is enhanced by either acute or chronic MitoQ supplementation. However, acute MitoQ supplementation may improve exercise tolerance in subjects with PAD. Future research should investigate whether MitoQ supplementation concurrent with exercise training (e.g., 4-16 weeks) alters adaptations induced by exercise alone and using different doses.

Mitochondrial electron transport chain in macrophage reprogramming: Potential role in antibacterial immune response

Macrophages restrain microbial infection and reinstate tissue homeostasis. The mitochondria govern macrophage metabolism and serve as pivot in innate immunity, thus acting as immunometabolic regulon. Metabolic pathways produce electron flows that end up in mitochondrial electron transport chain (mtETC), made of super-complexes regulating multitude of molecular and biochemical processes. Cell-intrinsic and extrinsic factors influence mtETC structure and function, impacting several aspects of macrophage immunity. These factors provide the macrophages with alternate fuel sources and metabolites, critical to gain functional competence and overcoming pathogenic stress. Mitochondrial reactive oxygen species (mtROS) and oxidative phosphorylation (OXPHOS) generated through the mtETC are important innate immune attributes, which help macrophages in mounting antibacterial responses. Recent studies have demonstrated the role of mtETC in governing mitochondrial dynamics and macrophage polarization (M1/M2). M1 macrophages are important for containing bacterial pathogens and M2 macrophages promote tissue repair and wound healing. Thus, mitochondrial bioenergetics and metabolism are intimately coupled with innate immunity. In this review, we have addressed mtETC function as innate rheostats that regulate macrophage reprogramming and innate immune responses. Advancement in this field encourages further exploration and provides potential novel macrophage-based therapeutic targets to control unsolicited inflammation.

An ETFDH-driven metabolon supports OXPHOS efficiency in skeletal muscle by regulating coenzyme Q homeostasis

Coenzyme Q (Q) is a key lipid electron transporter, but several aspects of its biosynthesis and redox homeostasis remain undefined. Various flavoproteins reduce ubiquinone (oxidized form of Q) to ubiquinol (QH2); however, in eukaryotes, only oxidative phosphorylation (OXPHOS) complex III (CIII) oxidizes QH2 to Q. The mechanism of action of CIII is still debated. Herein, we show that the Q reductase electron-transfer flavoprotein dehydrogenase (ETFDH) is essential for CIII activity in skeletal muscle. We identify a complex (comprising ETFDH, CIII and the Q-biosynthesis regulator COQ2) that directs electrons from lipid substrates to the respiratory chain, thereby reducing electron leaks and reactive oxygen species production. This metabolon maintains total Q levels, minimizes QH2-reductive stress and improves OXPHOS efficiency. Muscle-specific Etfdh-/- mice develop myopathy due to CIII dysfunction, indicating that ETFDH is a required OXPHOS component and a potential therapeutic target for mitochondrial redox medicine.

Regulation of respiratory complex I assembly by FMN cofactor targeting

Respiratory complex I plays a crucial role in the mitochondrial electron transport chain and shows promise as a therapeutic target for various human diseases. While most studies focus on inhibiting complex I at the Q-site, little is known about inhibitors targeting other sites within the complex. In this study, we demonstrate that diphenyleneiodonium (DPI), a N-site inhibitor, uniquely affects the stability of complex I by reacting with its flavin cofactor FMN. Treatment with DPI blocks the final stage of complex I assembly, leading to the complete and reversible degradation of complex I in different cellular models. Growing cells in medium lacking the FMN precursor riboflavin or knocking out the mitochondrial flavin carrier gene SLC25A32 results in a similar complex I degradation. Overall, our findings establish a direct connection between mitochondrial flavin homeostasis and complex I stability and assembly, paving the way for novel pharmacological strategies to regulate respiratory complex I.

Residual Complex I activity and amphidirectional Complex II operation support glutamate catabolism through mtSLP in anoxia

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. 13C 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.

Systems-level analyses dissociate genetic regulators of reactive oxygen species and energy production

Respiratory chain dysfunction can decrease ATP and increase reactive oxygen species (ROS) levels. Despite the importance of these metabolic parameters to a wide range of cellular functions and disease, we lack an integrated understanding of how they are differentially regulated. To address this question, we adapted a CRISPRi- and FACS-based platform to compare the effects of respiratory gene knockdown on ROS to their effects on ATP. Focusing on genes whose knockdown is known to decrease mitochondria-derived ATP, we showed that knockdown of genes in specific respiratory chain complexes (I, III, and CoQ10 biosynthesis) increased ROS, whereas knockdown of other low ATP hits either had no impact (mitochondrial ribosomal proteins) or actually decreased ROS (complex IV). Moreover, although shifting metabolic conditions profoundly altered mitochondria-derived ATP levels, it had little impact on mitochondrial or cytosolic ROS. In addition, knockdown of a subset of complex I subunits-including NDUFA8, NDUFB4, and NDUFS8-decreased complex I activity, mitochondria-derived ATP, and supercomplex level, but knockdown of these genes had differential effects on ROS. Conversely, we found an essential role for ether lipids in the dynamic regulation of mitochondrial ROS levels independent of ATP. Thus, our results identify specific metabolic regulators of cellular ATP and ROS balance that may help dissect the roles of these processes in disease and identify therapeutic strategies to independently target energy failure and oxidative stress.

Redox control of antioxidants, metabolism, immunity, and development at the core of stress adaptation of the oyster Crassostrea gigas to the dynamic intertidal environment

This review uses the marine bivalve Crassostrea gigas to highlight redox reactions and control systems in species living in dynamic intertidal environments. Intertidal species face daily and seasonal environmental variability, including temperature, oxygen, salinity, and nutritional changes. Increasing anthropogenic pressure can bring pollutants and pathogens as additional stressors. Surprisingly, C. gigas demonstrates impressive adaptability to most of these challenges. We explore how ROS production, antioxidant protection, redox signaling, and metabolic adjustments can shed light on how redox biology supports oyster survival in harsh conditions. The review provides (i) a brief summary of shared redox sensing processes in metazoan; (ii) an overview of unique characteristics of the C. gigas intertidal habitat and the suitability of this species as a model organism; (iii) insights into the redox biology of C. gigas, including ROS sources, signaling pathways, ROS-scavenging systems, and thiol-containing proteins; and examples of (iv) hot topics that are underdeveloped in bivalve research linking redox biology with immunometabolism, physioxia, and development. Given its plasticity to environmental changes, C. gigas is a valuable model for studying the role of redox biology in the adaptation to harsh habitats, potentially providing novel insights for basic and applied studies in marine and comparative biochemistry and physiology.

Understanding differential aspects of microdiffusion (channeling) in the Coenzyme Q and Cytochrome c regions of the mitochondrial respiratory system

Over the past decades, models of the organization of mitochondrial respiratory system have been controversial. The goal of this perspective is to assess this "conflict of models" by focusing on specific kinetic evidence in the two distinct segments of Coenzyme Q- and Cytochrome c-mediated electron transfer. Respiratory supercomplexes provide kinetic advantage by allowing a restricted diffusion of Coenzyme Q and Cytochrome c, and short-range interaction with their partner enzymes. In particular, electron transfer from NADH is compartmentalized by channeling of Coenzyme Q within supercomplexes, whereas succinate oxidation proceeds separately using the free Coenzyme Q pool. Previous evidence favoring Coenzyme Q random diffusion in the NADH-dependent electron transfer is due to downstream flux interference and misinterpretation of results. Indeed, electron transfer by complexes III and IV via Cytochrome c is less strictly dependent on substrate channeling in mammalian mitochondria. We briefly describe these differences and their physiological implications.

Mitochondrial electron transport chain, ceramide, and coenzyme Q are linked in a pathway that drives insulin resistance in skeletal muscle

Insulin resistance (IR) is a complex metabolic disorder that underlies several human diseases, including type 2 diabetes and cardiovascular disease. Despite extensive research, the precise mechanisms underlying IR development remain poorly understood. Previously we showed that deficiency of coenzyme Q (CoQ) is necessary and sufficient for IR in adipocytes and skeletal muscle (Fazakerley et al., 2018). Here, we provide new insights into the mechanistic connections between cellular alterations associated with IR, including increased ceramides, CoQ deficiency, mitochondrial dysfunction, and oxidative stress. We demonstrate that elevated levels of ceramide in the mitochondria of skeletal muscle cells result in CoQ depletion and loss of mitochondrial respiratory chain components, leading to mitochondrial dysfunction and IR. Further, decreasing mitochondrial ceramide levels in vitro and in animal models (mice, C57BL/6J) (under chow and high-fat diet) increased CoQ levels and was protective against IR. CoQ supplementation also rescued ceramide-associated IR. Examination of the mitochondrial proteome from human muscle biopsies revealed a strong correlation between the respirasome system and mitochondrial ceramide as key determinants of insulin sensitivity. Our findings highlight the mitochondrial ceramide-CoQ-respiratory chain nexus as a potential foundation of an IR pathway that may also play a critical role in other conditions associated with ceramide accumulation and mitochondrial dysfunction, such as heart failure, cancer, and aging. These insights may have important clinical implications for the development of novel therapeutic strategies for the treatment of IR and related metabolic disorders.

Mitochondrial complex I ROS production and redox signaling in hypoxia

Mitochondria are a main source of cellular energy. Oxidative phosphorylation (OXPHOS) is the major process of aerobic respiration. Enzyme complexes of the electron transport chain (ETC) pump protons to generate a protonmotive force (Δp) that drives OXPHOS. Complex I is an electron entry point into the ETC. Complex I oxidizes nicotinamide adenine dinucleotide (NADH) and transfers electrons to ubiquinone in a reaction coupled with proton pumping. Complex I also produces reactive oxygen species (ROS) under various conditions. The enzymatic activities of complex I can be regulated by metabolic conditions and serves as a regulatory node of the ETC. Complex I ROS plays diverse roles in cell metabolism ranging from physiologic to pathologic conditions. Progress in our understanding indicates that ROS release from complex I serves important signaling functions. Increasing evidence suggests that complex I ROS is important in signaling a mismatch in energy production and demand. In this article, we review the role of ROS from complex I in sensing acute hypoxia.

Systems-level analyses dissociate genetic regulators of reactive oxygen species and energy production

Respiratory chain dysfunction can decrease ATP and increase reactive oxygen species (ROS) levels. Despite the importance of these metabolic parameters to a wide range of cellular functions and disease, we lack an integrated understanding of how they are differentially regulated. To address this question, we adapted a CRISPRi- and FACS- based platform to compare the effects of respiratory gene knockdown on ROS to their effects on ATP. Focusing on genes whose knockdown is known to decrease mitochondria-derived ATP, we showed that knockdown of genes in specific respiratory chain complexes (I, III and CoQ10 biosynthesis) increased ROS, whereas knockdown of other low ATP hits either had no impact (mitochondrial ribosomal proteins) or actually decreased ROS (complex IV). Moreover, although shifting metabolic conditions profoundly altered mitochondria-derived ATP levels, it had little impact on mitochondrial or cytosolic ROS. In addition, knockdown of a subset of complex I subunits-including NDUFA8, NDUFB4, and NDUFS8-decreased complex I activity, mitochondria-derived ATP and supercomplex level, but knockdown of these genes had differential effects on ROS. Conversely, we found an essential role for ether lipids in the dynamic regulation of mitochondrial ROS levels independent of ATP. Thus, our results identify specific metabolic regulators of cellular ATP and ROS balance that may help dissect the roles of these processes in disease and identify therapeutic strategies to independently target energy failure and oxidative stress.

Mitochondrial electron transport chain, ceramide and Coenzyme Q are linked in a pathway that drives insulin resistance in skeletal muscle

Insulin resistance (IR) is a complex metabolic disorder that underlies several human diseases, including type 2 diabetes and cardiovascular disease. Despite extensive research, the precise mechanisms underlying IR development remain poorly understood. Here, we provide new insights into the mechanistic connections between cellular alterations associated with IR, including increased ceramides, deficiency of coenzyme Q (CoQ), mitochondrial dysfunction, and oxidative stress. We demonstrate that elevated levels of ceramide in the mitochondria of skeletal muscle cells results in CoQ depletion and loss of mitochondrial respiratory chain components, leading to mitochondrial dysfunction and IR. Further, decreasing mitochondrial ceramide levels in vitro and in animal models (under chow and high fat diet) increased CoQ levels and was protective against IR. CoQ supplementation also rescued ceramide-associated IR. Examination of the mitochondrial proteome from human muscle biopsies revealed a strong correlation between the respirasome system and mitochondrial ceramide as key determinants of insulin sensitivity. Our findings highlight the mitochondrial Ceramide-CoQ-respiratory chain nexus as a potential foundation of an IR pathway that may also play a critical role in other conditions associated with ceramide accumulation and mitochondrial dysfunction, such as heart failure, cancer, and aging. These insights may have important clinical implications for the development of novel therapeutic strategies for the treatment of IR and related metabolic disorders.

Antimicrobial mitochondrial reactive oxygen species induction by lung epithelial immunometabolic modulation

Pneumonia is a worldwide threat, making discovery of novel means to combat lower respiratory tract infection an urgent need. Manipulating the lungs' intrinsic host defenses by therapeutic delivery of certain pathogen-associated molecular patterns protects mice against pneumonia in a reactive oxygen species (ROS)-dependent manner. Here we show that antimicrobial ROS are induced from lung epithelial cells by interactions of CpG oligodeoxynucleotides (ODN) with mitochondrial voltage-dependent anion channel 1 (VDAC1). The ODN-VDAC1 interaction alters cellular ATP/ADP/AMP localization, increases delivery of electrons to the electron transport chain (ETC), increases mitochondrial membrane potential (ΔΨm), differentially modulates ETC complex activities and consequently results in leak of electrons from ETC complex III and superoxide formation. The ODN-induced mitochondrial ROS yield protective antibacterial effects. Together, these studies identify a therapeutic metabolic manipulation strategy to broadly protect against pneumonia without reliance on antibiotics.

Brain mitochondrial diversity and network organization predict anxiety-like behavior in male mice

The brain and behavior are under energetic constraints, limited by mitochondrial energy transformation capacity. However, the mitochondria-behavior relationship has not been systematically studied at a brain-wide scale. Here we examined the association between multiple features of mitochondrial respiratory chain capacity and stress-related behaviors in male mice with diverse behavioral phenotypes. Miniaturized assays of mitochondrial respiratory chain enzyme activities and mitochondrial DNA (mtDNA) content were deployed on 571 samples across 17 brain areas, defining specific patterns of mito-behavior associations. By applying multi-slice network analysis to our brain-wide mitochondrial dataset, we identified three large-scale networks of brain areas with shared mitochondrial signatures. A major network composed of cortico-striatal areas exhibited the strongest mitochondria-behavior correlations, accounting for up to 50% of animal-to-animal behavioral differences, suggesting that this mito-based network is functionally significant. The mito-based brain networks also overlapped with regional gene expression and structural connectivity, and exhibited distinct molecular mitochondrial phenotype signatures. This work provides convergent multimodal evidence anchored in enzyme activities, gene expression, and animal behavior that distinct, behaviorally-relevant mitochondrial phenotypes exist across the male mouse brain.

Metabolic adaptation of human skin fibroblasts to ER stress caused by glycosylation defect in PMM2-CDG

PMM2-CDG is the most prevalent type of congenital disorders of glycosylation (CDG). It is caused by pathogenic variants in the gene encoding phosphomannomutase 2 (PMM2), which converts mannose-6-phosphate to mannose-1-phosphate and thus activates this saccharide for further glycosylation processes. Defective glycosylation can lead to an abnormal accumulation of unfolded proteins in endoplasmic reticulum (ER) and cause its stress. The ER is a key compartment for glycosylation, and its connection and communication with mitochondria has been described extensively in literature. Their crosstalk is important for cell proliferation, calcium homeostasis, apoptosis, mitochondrial fission regulation, bioenergetics, autophagy, lipid metabolism, inflammasome formation and unfolded protein response. Therefore, in the present study we posed a question, whether defective glycosylation leads to bioenergetic disruption. Our data reveal possible chronic stress in ER and activated unfolded protein response via PERK pathway in PMM2-CDG fibroblasts. Presumably, it leads to bioenergetic reorganization and increased assembly of respiratory chain complexes into supercomplexes together with suppressed glycolysis in PMM2-CDG patient cells. These changes cause alterations in Krebs cycle, which is tightly connected to electron transport system in mitochondria. In summary, we present data showing metabolic adaptation of cells to glycosylation defect caused by various pathogenic variants in PMM2.

Neuroprotective effects of coenzyme Q10 on neurological diseases: a review article

Neurological disorders affect the nervous system. Biochemical, structural, or electrical abnormalities in the spinal cord, brain, or other nerves lead to different symptoms, including muscle weakness, paralysis, poor coordination, seizures, loss of sensation, and pain. There are many recognized neurological diseases, like epilepsy, Alzheimer's disease (AD), Parkinson's disease (PD), multiple sclerosis (MS), stroke, autosomal recessive cerebellar ataxia 2 (ARCA2), Leber's hereditary optic neuropathy (LHON), and spinocerebellar ataxia autosomal recessive 9 (SCAR9). Different agents, such as coenzyme Q10 (CoQ10), exert neuroprotective effects against neuronal damage. Online databases, such as Scopus, Google Scholar, Web of Science, and PubMed/MEDLINE were systematically searched until December 2020 using keywords, including review, neurological disorders, and CoQ10. CoQ10 is endogenously produced in the body and also can be found in supplements or foods. CoQ10 has antioxidant and anti-inflammatory effects and plays a role in energy production and mitochondria stabilization, which are mechanisms, by which CoQ10 exerts its neuroprotective effects. Thus, in this review, we discussed the association between CoQ10 and neurological diseases, including AD, depression, MS, epilepsy, PD, LHON, ARCA2, SCAR9, and stroke. In addition, new therapeutic targets were introduced for the next drug discoveries.

Brightness and shadows of mitochondrial ROS in the brain

Mitochondrial reactive oxygen species (mROS) have been generally considered harmful byproducts wanted to clear when elevated to avoid brain damage. However, the abundance of mROS in astrocytes is very high -about one order of magnitude above that in neurons-, despite they are essential to preserve cell metabolism and animal behavior. Here, we have focused on this apparent ambiguity by discussing (i) the intrinsic mechanisms accounting for the higher production of mROS by the mitochondrial respiratory chain in astrocytes than in neurons, (ii) the specific molecular targets of astrocytic beneficial mROS, and (iii) how decreased astrocytic mROS causes excess neuronal mROS leading to cellular and organismal damage. We hope that this mini-review serves to clarifying the apparent controversy on the beneficial versus deleterious faces of ROS in the brain from molecular to higher-order organismal levels.

Exogenous aralar/slc25a12 can replace citrin/slc25a13 as malate aspartate shuttle component in liver

The deficiency of CITRIN, the liver mitochondrial aspartate-glutamate carrier (AGC), is the cause of four human clinical phenotypes, neonatal intrahepatic cholestasis caused by CITRIN deficiency (NICCD), silent period, failure to thrive and dyslipidemia caused by CITRIN deficiency (FTTDCD), and citrullinemia type II (CTLN2). Clinical symptoms can be traced back to disruption of the malate-aspartate shuttle due to the lack of citrin. A potential therapy for this condition is the expression of aralar, the AGC present in brain, to replace citrin. To explore this possibility we have first verified that the NADH/NAD+ ratio increases in hepatocytes from citrin(-/-) mice, and then found that exogenous aralar expression reversed the increase in NADH/NAD+ observed in these cells. Liver mitochondria from citrin (-/-) mice expressing liver specific transgenic aralar had a small (~ 4-6 nmoles x mg prot-1 x min-1) but consistent increase in malate aspartate shuttle (MAS) activity over that of citrin(-/-) mice. These results support the functional replacement between AGCs in the liver. To explore the significance of AGC replacement in human therapy we studied the relative levels of citrin and aralar in mouse and human liver through absolute quantification proteomics. We report that mouse liver has relatively high aralar levels (citrin/aralar molar ratio of 7.8), whereas human liver is virtually devoid of aralar (CITRIN/ARALAR ratio of 397). This large difference in endogenous aralar levels partly explains the high residual MAS activity in liver of citrin(-/-) mice and why they fail to recapitulate the human disease, but supports the benefit of increasing aralar expression to improve the redox balance capacity of human liver, as an effective therapy for CITRIN deficiency.

PGC-1α Is a Master Regulator of Mitochondrial Lifecycle and ROS Stress Response

Mitochondria play a major role in ROS production and defense during their life cycle. The transcriptional activator PGC-1α is a key player in the homeostasis of energy metabolism and is therefore closely linked to mitochondrial function. PGC-1α responds to environmental and intracellular conditions and is regulated by SIRT1/3, TFAM, and AMPK, which are also important regulators of mitochondrial biogenesis and function. In this review, we highlight the functions and regulatory mechanisms of PGC-1α within this framework, with a focus on its involvement in the mitochondrial lifecycle and ROS metabolism. As an example, we show the role of PGC-1α in ROS scavenging under inflammatory conditions. Interestingly, PGC-1α and the stress response factor NF-κB, which regulates the immune response, are reciprocally regulated. During inflammation, NF-κB reduces PGC-1α expression and activity. Low PGC-1α activity leads to the downregulation of antioxidant target genes resulting in oxidative stress. Additionally, low PGC-1α levels and concomitant oxidative stress promote NF-κB activity, which exacerbates the inflammatory response.

Xenotopic expression of alternative oxidase (AOX) to study mechanisms of mitochondrial disease

The mitochondrial respiratory chain or electron transport chain (ETC) facilitates redox reactions which ultimately lead to the reduction of oxygen to water (respiration). Energy released by this process is used to establish a proton electrochemical gradient which drives ATP formation (oxidative phosphorylation, OXPHOS). It also plays an important role in vital processes beyond ATP formation and cellular metabolism, such as heat production, redox and ion homeostasis. Dysfunction of the ETC can thus impair cellular and organismal viability and is thought to be the underlying cause of a heterogeneous group of so-called mitochondrial diseases. Plants, yeasts, and many lower organisms, but not insects and vertebrates, possess an enzymatic mechanism that confers resistance to respiratory stress conditions, i.e., the alternative oxidase (AOX). Even in cells that naturally lack AOX, it is autonomously imported into the mitochondrial compartment upon xenotopic expression, where it refolds and becomes catalytically engaged when the cytochrome segment of the ETC is blocked. AOX was therefore proposed as a tool to study disease etiologies. To this end, AOX has been xenotopically expressed in mammalian cells and disease models of the fruit fly and mouse. Surprisingly, AOX showed remarkable rescue effects in some cases, whilst in others it had no effect or even exacerbated a condition. Here we summarize what has been learnt from the use of AOX in various disease models and discuss issues which still need to be addressed in order to understand the role of the ETC in health and disease.

Loss of respiratory complex I subunit NDUFB10 affects complex I assembly and supercomplex formation

The orchestrated activity of the mitochondrial respiratory or electron transport chain (ETC) and ATP synthase convert reduction power (NADH, FADH₂) into ATP, the cell's energy currency in a process named oxidative phosphorylation (OXPHOS). Three out of the four ETC complexes are found in supramolecular assemblies: complex I, III, and IV form the respiratory supercomplexes (SC). The plasticity model suggests that SC formation is a form of adaptation to changing conditions such as energy supply, redox state, and stress. Complex I, the NADH-dehydrogenase, is part of the largest supercomplex (CI + CIII₂ + CIV_n). Here, we demonstrate the role of NDUFB10, a subunit of the membrane arm of complex I, in complex I and supercomplex assembly on the one hand and bioenergetics function on the other. NDUFB10 knockout was correlated with a decrease of SCAF1, a supercomplex assembly factor, and a reduction of respiration and mitochondrial membrane potential. This likely is due to loss of proton pumping since the CI P _P -module is downregulated and the P _D -module is completely abolished in NDUFB10 knock outs.

Inhibition of Pyruvate Dehydrogenase in the Heart as an Initiating Event in the Development of Diabetic Cardiomyopathy

Obesity affects a growing fraction of the population and is a risk factor for type 2 diabetes and cardiovascular disease. Even in the absence of hypertension and coronary artery disease, type 2 diabetes can result in a heart disease termed diabetic cardiomyopathy. Diminished glucose oxidation, increased reliance on fatty acid oxidation for energy production, and oxidative stress are believed to play causal roles. However, the progression of metabolic changes and mechanisms by which these changes impact the heart have not been established. Cardiac pyruvate dehydrogenase (PDH), the central regulatory site for glucose oxidation, is rapidly inhibited in mice fed high dietary fat, a model of obesity and diabetes. Increased reliance on fatty acid oxidation for energy production, in turn, enhances mitochondrial pro-oxidant production. Inhibition of PDH may therefore initiate metabolic inflexibility and oxidative stress and precipitate diabetic cardiomyopathy. We discuss evidence from the literature that supports a role for PDH inhibition in loss in energy homeostasis and diastolic function in obese and diabetic humans and in rodent models. Finally, seemingly contradictory findings highlight the complexity of the disease and the need to delineate progressive changes in cardiac metabolism, the impact on myocardial structure and function, and the ability to intercede.

Oxygen toxicity causes cyclic damage by destabilizing specific Fe-S cluster-containing protein complexes

Oxygen is toxic across all three domains of life. Yet, the underlying molecular mechanisms remain largely unknown. Here, we systematically investigate the major cellular pathways affected by excess molecular oxygen. We find that hyperoxia destabilizes a specific subset of Fe-S cluster (ISC)-containing proteins, resulting in impaired diphthamide synthesis, purine metabolism, nucleotide excision repair, and electron transport chain (ETC) function. Our findings translate to primary human lung cells and a mouse model of pulmonary oxygen toxicity. We demonstrate that the ETC is the most vulnerable to damage, resulting in decreased mitochondrial oxygen consumption. This leads to further tissue hyperoxia and cyclic damage of the additional ISC-containing pathways. In support of this model, primary ETC dysfunction in the Ndufs4 KO mouse model causes lung tissue hyperoxia and dramatically increases sensitivity to hyperoxia-mediated ISC damage. This work has important implications for hyperoxia pathologies, including bronchopulmonary dysplasia, ischemia-reperfusion injury, aging, and mitochondrial disorders.

Ubiquinone deficiency drives reverse electron transport to disrupt hepatic metabolic homeostasis in obesity

Mitochondrial reactive oxygen species (mROS) are central to physiology. While excess mROS production has been associated with several disease states, its precise sources, regulation, and mechanism of generation in vivo remain unknown, limiting translational efforts. Here we show that in obesity, hepatic ubiquinone (Q) synthesis is impaired, which raises the QH 2 /Q ratio, driving excessive mROS production via reverse electron transport (RET) from site I Q in complex I. Using multiple complementary genetic and pharmacological models in vivo we demonstrated that RET is critical for metabolic health. In patients with steatosis, the hepatic Q biosynthetic program is also suppressed, and the QH 2 /Q ratio positively correlates with disease severity. Our data identify a highly selective mechanism for pathological mROS production in obesity, which can be targeted to protect metabolic homeostasis.

Mitochondria on the move: Horizontal mitochondrial transfer in disease and health

Mammalian genes were long thought to be constrained within somatic cells in most cell types. This concept was challenged recently when cellular organelles including mitochondria were shown to move between mammalian cells in culture via cytoplasmic bridges. Recent research in animals indicates transfer of mitochondria in cancer and during lung injury in vivo, with considerable functional consequences. Since these pioneering discoveries, many studies have confirmed horizontal mitochondrial transfer (HMT) in vivo, and its functional characteristics and consequences have been described. Additional support for this phenomenon has come from phylogenetic studies. Apparently, mitochondrial trafficking between cells occurs more frequently than previously thought and contributes to diverse processes including bioenergetic crosstalk and homeostasis, disease treatment and recovery, and development of resistance to cancer therapy. Here we highlight current knowledge of HMT between cells, focusing primarily on in vivo systems, and contend that this process is not only (patho)physiologically relevant, but also can be exploited for the design of novel therapeutic approaches.

PMo12 as a redox mediator for bio-reduction of Cr(VI): Promotor or inhibitor?

Slow reduction rate and low reduction ability were the main limitations of bio-reduction of Cr(VI). As an efficient redox mediator, how phosphomolybdic acid (PMo₁₂) affected bio-reduction of Cr(VI) was worthy of exploration. In this study, short-term and long-term effects of PMo₁₂ on Cr(VI) reduction were investigated to reveal the relevant mechanism. After evaluating the short-term effect of PMo₁₂ concentration from 0.05 to 1.00 mM on Cr(VI) bio-reduction, 0.50 mM was found to be optimum by improving Cr(VI) reduction rate by 16.3 % and microbial electron transport system activity (ETSA) by 43.0 % with Cr(VI) reduction efficiency of 100 % in short-term (22 h) batch experiments. By contrast, in long-term (28 days) continuous flow experiments, 0.50 mM PMo₁₂ exhibited serious inhibition on Cr(VI) bio-reduction. The cumulative toxicity of Mo, strong oxidative stress (reactive oxygen species increased by 16.5 %), the inhibition of extracellular polymeric substances production and the reduction of microbial activity were proved to be the main inhibition mechanism. In terms of microbial electron transport system, the main electron carriers including flavin mononucleotide (FMN), nitrate reductase (NAR), nitrite reductase (NIR) were seriously inhibited. BugBase analysis confirmed that the relative abundance of biofilm forming bacteria decreased after PMo₁₂ addition, and the relative abundance of oxidative stress tolerance bacteria continued to increase.

A FRET-based respirasome assembly screen identifies spleen tyrosine kinase as a target to improve muscle mitochondrial respiration and exercise performance in mice

Aerobic muscle activities predominantly depend on fuel energy supply by mitochondrial respiration, thus, mitochondrial activity enhancement may become a therapeutic intervention for muscle disturbances. The assembly of mitochondrial respiratory complexes into higher-order "supercomplex" structures has been proposed to be an efficient biological process for energy synthesis, although there is controversy in its physiological relevance. We here established Förster resonance energy transfer (FRET) phenomenon-based live imaging of mitochondrial respiratory complexes I and IV interactions using murine myoblastic cells, whose signals represent in vivo supercomplex assembly of complexes I, III, and IV, or respirasomes. The live FRET signals were well correlated with supercomplex assembly observed by blue native polyacrylamide gel electrophoresis (BN-PAGE) and oxygen consumption rates. FRET-based live cell screen defined that the inhibition of spleen tyrosine kinase (SYK), a non-receptor protein tyrosine kinase that belongs to the SYK/ zeta-chain-associated protein kinase 70 (ZAP-70) family, leads to an increase in supercomplex assembly in murine myoblastic cells. In parallel, SYK inhibition enhanced mitochondrial respiration in the cells. Notably, SYK inhibitor administration enhances exercise performance in mice. Overall, this study proves the feasibility of FRET-based respirasome assembly assay, which recapitulates in vivo mitochondrial respiration activities.

Antimicrobial mitochondrial reactive oxygen species induction by lung epithelial metabolic reprogramming

Pneumonia is a worldwide threat, making discovery of novel means to combat lower respiratory tract infections an urgent need. We have previously shown that manipulating the lungs' intrinsic host defenses by therapeutic delivery of a unique dyad of pathogen-associated molecular patterns protects mice against pneumonia in a reactive oxygen species (ROS)-dependent manner. Here we show that antimicrobial ROS are induced from lung epithelial cells by interactions of CpG oligodeoxynucleotides (ODNs) with mitochondrial voltage-dependent anion channel 1 (VDAC1) without dependence on Toll-like receptor 9 (TLR9). The ODN-VDAC1 interaction alters cellular ATP/ADP/AMP localization, increases delivery of electrons to the electron transport chain (ETC), enhances mitochondrial membrane potential (Δ Ψm ), and differentially modulates ETC complex activities. These combined effects promote leak of electrons from ETC complex III, resulting in superoxide formation. The ODN-induced mitochondrial ROS yield protective antibacterial effects. Together, these studies identify a therapeutic metabolic manipulation strategy that has the potential to broadly protect patients against pneumonia during periods of peak vulnerability without reliance on currently available antibiotics.

Author Summary

Pneumonia is a major cause of death worldwide. Increasing antibiotic resistance and expanding immunocompromised populations continue to enhance the clinical urgency to find new strategies to prevent and treat pneumonia. We have identified a novel inhaled therapeutic that stimulates lung epithelial defenses to protect mice against pneumonia in a manner that depends on production of reactive oxygen species (ROS). Here, we report that the induction of protective ROS from lung epithelial mitochondria occurs following the interaction of one component of the treatment, an oligodeoxynucleotide, with the mitochondrial voltage-dependent anion channel 1. This interaction alters energy transfer between the mitochondria and the cytosol, resulting in metabolic reprogramming that drives more electrons into the electron transport chain, then causes electrons to leak from the electron transport chain to form protective ROS. While antioxidant therapies are endorsed in many other disease states, we present here an example of therapeutic induction of ROS that is associated with broad protection against pneumonia without reliance on administration of antibiotics.