Native rates of superoxide production from multiple sites in isolated mitochondria measured using endogenous reporters

Misprogramming of glucose metabolism impairs recovery of hippocampal slices from neuronal GLT-1 knockout mice and contributes to excitotoxic injury through mitochondrial superoxide production

We have previously reported a failure of recovery of synaptic function in the CA1 region of acute hippocampal slices from mice with a conditional neuronal knockout (KO) of GLT-1 (EAAT2, Slc1A2) driven by synapsin-Cre (synGLT-1 KO). The failure of recovery of synaptic function is due to excitotoxic injury. We hypothesized that changes in mitochondrial metabolism contribute to the heightened vulnerability to excitotoxicity in the synGLT-1 KO mice. We found impaired flux of carbon from 13C-glucose into the tricarboxylic acid cycle in synGLT-1 KO cortical and hippocampal slices compared with wild-type (WT) slices. In addition, we found downregulation of the neuronal glucose transporter GLUT3 in both genotypes. Flux of carbon from [1,2-13C]acetate, thought to be astrocyte-specific, was increased in the synGLT-KO hippocampal slices but not cortical slices. Glycogen stores, predominantly localized to astrocytes, are rapidly depleted in slices after cutting, and are replenished during ex vivo incubation. In the synGLT-1 KO, replenishment of glycogen stores during ex vivo incubation was compromised. These results suggest both neuronal and astrocytic metabolic perturbations in the synGLT-1 KO slices. Supplementing incubation medium during recovery with 20 mM D-glucose normalized glycogen replenishment but had no effect on recovery of synaptic function. In contrast, 20 mM non-metabolizable L-glucose substantially improved recovery of synaptic function, suggesting that D-glucose metabolism contributes to the excitotoxic injury in the synGLT-1 KO slices. L-lactate substitution for D-glucose did not promote recovery of synaptic function, implicating mitochondrial metabolism. Consistent with this hypothesis, phosphorylation of pyruvate dehydrogenase, which decreases enzyme activity, was increased in WT slices during the recovery period, but not in synGLT-1 KO slices. Since metabolism of glucose by the mitochondrial electron transport chain is associated with superoxide production, we tested the effect of drugs that scavenge and prevent superoxide production. The superoxide dismutase/catalase mimic EUK-134 conferred complete protection and full recovery of synaptic function. A site-specific inhibitor of complex III superoxide production, S3QEL-2, was also protective, but inhibitors of NADPH oxidase were not. In summary, we find that the failure of recovery of synaptic function in hippocampal slices from the synGLT-1 KO mouse, previously shown to be due to excitotoxic injury, is caused by production of superoxide by mitochondrial metabolism.

Transition Metal Complexes as Therapeutics: A New Frontier in Combatting Neurodegenerative Disorders through Protein Aggregation Modulation

Neurodegenerative disorders (NDDs) are a class of debilitating diseases that progressively impair the protein structure and result in neurological dysfunction in the nervous system. Among these disorders, Alzheimer's disease (AD), prion diseases such as Creutzfeldt-Jakob disease (CJD), and Parkinson's disease (PD) are caused by protein misfolding and aggregation at the cellular level. In recent years, transition metal complexes have gained significant attention for their potential applications in diagnosing, imaging, and curing these NDDs. These complexes have intriguing possibilities as therapeutics due to their diverse ligand systems and chemical properties and can interact with biological systems with minimal detrimental effects. This review focuses on the recent progress in transition metal therapeutics as a new era of hope in the battle against AD, CJD, and PD by modulating protein aggregation in vitro and in vivo. It may shed revolutionary insights into unlocking new opportunities for researchers to develop metal-based drugs to combat NDDs.

Hypoxia decreases mitochondrial ROS production in cells

We re-examined the reported increase in mitochondrial ROS production during acute hypoxia in cells. Using the Amplex Ultrared/horseradish peroxidase assay we found a decrease, not increase, in hydrogen peroxide release from HEK293 cells under acute hypoxia, at times ranging from 1 min to 3 h. The rates of superoxide/hydrogen peroxide production from each of the three major sites (site IQ in complex I and site IIIQo in complex III in mitochondria, and NADH oxidases (NOX) in the cytosol) were decreased to the same extent by acute hypoxia, with no change in the cells' ability to degrade added hydrogen peroxide. A similar decrease in ROS production under acute hypoxia was found using the diacetyldichlorofluorescein assay. Using a HIF1α reporter cell line we confirmed earlier observations that suppression of superoxide production by site IIIQo decreases HIF1α expression, and found similar effects of suppressing site IQ or NOX. We conclude that increased mitochondrial ROS do not drive the response of HIF1α to acute hypoxia, but suggest that cytosolic H2O2 derived from site IQ, site IIIQo and NOX in cells is necessary to permit HIF1α stabilization by other signals.

A mitochondrial redox switch licenses the onset of morphogenesis in animals

Embryos undergo pre-gastrulation cleavage cycles to generate a critical cell mass before transitioning to morphogenesis. The molecular underpinnings of this transition have traditionally centered on zygotic chromatin remodeling and genome activation1,2, as their repression can prevent downstream processes of differentiation and organogenesis. Despite precedents that oxygen depletion can similarly suspend development in early embryos3-6, hinting at a pivotal role for oxygen metabolism in this transition, whether there is a bona fide chemical switch that licenses the onset of morphogenesis remains unknown. Here we discover that a mitochondrial oxidant acts as a metabolic switch to license the onset of animal morphogenesis. Concomitant with the instatement of mitochondrial membrane potential, we found a burst-like accumulation of mitochondrial superoxide (O2 -) during fly blastoderm formation. In vivo chemistry experiments revealed that an electron leak from site IIIQo at ETC Complex III is responsible for O2 - production. Importantly, depleting mitochondrial O2 - fully mimics anoxic conditions and, like anoxia, induces suspended animation prior to morphogenesis, but not after. Specifically, H2O2, and not ONOO-, NO, or HO•, can single-handedly account for this mtROS-based response. We demonstrate that depleting mitochondrial O2 - similarly prevents the onset of morphogenetic events in vertebrate embryos and ichthyosporea, close relatives of animals. We postulate that such redox-based metabolic licensing of morphogenesis is an ancient trait of holozoans that couples the availability of oxygen to development, conserved from early-diverging animal relatives to vertebrates.

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 Physiology of Cellular Redox Regulations

Mitochondria (mt) represent the vital hub of the molecular physiology of the cell, being decision-makers in cell life/death and information signaling, including major redox regulations and redox signaling. Now we review recent advances in understanding mitochondrial redox homeostasis, including superoxide sources and H2O2 consumers, i.e., antioxidant mechanisms, as well as exemplar situations of physiological redox signaling, including the intramitochondrial one and mt-to-cytosol redox signals, which may be classified as acute and long-term signals. This review exemplifies the acute redox signals in hypoxic cell adaptation and upon insulin secretion in pancreatic beta-cells. We also show how metabolic changes under these circumstances are linked to mitochondrial cristae narrowing at higher intensity of ATP synthesis. Also, we will discuss major redox buffers, namely the peroxiredoxin system, which may also promote redox signaling. We will point out that pathological thresholds exist, specific for each cell type, above which the superoxide sources exceed regular antioxidant capacity and the concomitant harmful processes of oxidative stress subsequently initiate etiology of numerous diseases. The redox signaling may be impaired when sunk in such excessive pro-oxidative state.

Phytic Acid Maintains Peripheral Neuron Integrity and Enhances Survivability against Platinum-Induced Degeneration via Reducing Reactive Oxygen Species and Enhancing Mitochondrial Membrane Potential

Phytic acid (PA) has been reported to possess anti-inflammatory and antioxidant properties that are critical for neuroprotection in neuronal disorders. This raises the question of whether PA can effectively protect sensory neurons against chemotherapy-induced peripheral neuropathy (CIPN). Peripheral neuropathy is a dose-limiting side effect of chemotherapy treatment often characterized by severe and abnormal pain in hands and feet resulting from peripheral nerve degeneration. Currently, there are no effective treatments available that can prevent or cure peripheral neuropathies other than symptomatic management. Herein, we aim to demonstrate the neuroprotective effects of PA against the neurodegeneration induced by the chemotherapeutics cisplatin (CDDP) and oxaliplatin. Further aims of this study are to provide the proposed mechanism of PA-mediated neuroprotection. The neuronal protection and survivability against CDDP were characterized by axon length measurements and cell body counting of the dorsal root ganglia (DRG) neurons. A cellular phenotype study was conducted microscopically. Intracellular reactive oxygen species (ROS) was estimated by fluorogenic probe dichlorofluorescein. Likewise, mitochondrial membrane potential (MMP) was assessed by fluorescent MitoTracker Orange CMTMRos. Similarly, the mitochondria-localized superoxide anion radical in response to CDDP with and without PA was evaluated. The culture of primary DRG neurons with CDDP reduced axon length and overall neuronal survival. However, cotreatment with PA demonstrated that axons were completely protected and showed increased stability up to the 45-day test duration, which is comparable to samples treated with PA alone and control. Notably, PA treatment scavenged the mitochondria-specific superoxide radicals and overall intracellular ROS that were largely induced by CDDP and simultaneously restored MMP. These results are credited to the underlying neuroprotection of PA in a platinum-treated condition. The results also exhibited that PA had a synergistic anticancer effect with CDDP in ovarian cancer in vitro models. For the first time, PA's potency against CDDP-induced PN is demonstrated systematically. The overall findings of this study suggest the application of PA in CIPN prevention and therapeutic purposes.

Renal Epithelial Mitochondria: Implications for Hypertensive Kidney Disease

According to the Centers for Disease Control and Prevention, 1 in 2 U.S. adults have hypertension, and more than 1 in 7 chronic kidney disease. In fact, hypertension is the second leading cause of kidney failure in the United States; it is a complex disease characterized by, leading to, and caused by renal dysfunction. It is well-established that hypertensive renal damage is accompanied by mitochondrial damage and oxidative stress, which are differentially regulated and manifested along the nephron due to the diverse structure and functions of renal cells. This article provides a summary of the relevant knowledge of mitochondrial bioenergetics and metabolism, focuses on renal mitochondrial function, and discusses the evidence that has been accumulated regarding the role of epithelial mitochondrial bioenergetics in the development of renal tissue dysfunction in hypertension. © 2024 American Physiological Society. Compr Physiol 14:5225-5242, 2024.

From amino-acid to disease: the effects of oxidation on actin-myosin interactions in muscle

Actin-myosin interactions form the basis of the force-producing contraction cycle within the sarcomere, serving as the primary mechanism for muscle contraction. Post-translational modifications, such as oxidation, have a considerable impact on the mechanics of these interactions. Considering their widespread occurrence, the explicit contributions of these modifications to muscle function remain an active field of research. In this review, we aim to provide a comprehensive overview of the basic mechanics of the actin-myosin complex and elucidate the extent to which oxidation influences the contractile cycle and various mechanical characteristics of this complex at the single-molecule, myofibrillar and whole-muscle levels. We place particular focus on amino acids shown to be vulnerable to oxidation in actin, myosin, and some of their binding partners. Additionally, we highlight the differences between in vitro environments, where oxidation is controlled and limited to actin and myosin and myofibrillar or whole muscle environments, to foster a better understanding of oxidative modification in muscle. Thus, this review seeks to encompass a broad range of studies, aiming to lay out the multi layered effects of oxidation in in vitro and in vivo environments, with brief mention of clinical muscular disorders associated with oxidative stress.

M4IDP stimulates ROS elevation through inhibition of mevalonate pathway and pentose phosphate pathway to inhibit colon cancer cells

Maintaining redox homeostasis is an essential feature of cancer cells, and disrupting this homeostasis to cause oxidative stress and induce cell death is an important strategy in cancer therapy. M4IDP, a zoledronic acid derivative, can cause the death of human colorectal cancer cells by increasing the level of intracellular reactive oxygen species (ROS). However, its potential molecular mechanism is unclear. Our in vitro studies showed that treatment with M4IDP promoted oxidative stress in HCT116 cells, as measured by the decreased ratios of GSH/GSSG and NADPH/NADP+ and increased level of MDA. M4IDP could cause the decrease of GSH content, the increase of GSSG content, the decrease of NADPH content and pentose phosphate pathway flux, the downregulation of G6PD expression, the upregulation of unprenylated Rap1A and total expression of RhoA and CDC42. The increase of ROS and cytotoxicity induced by M4IDP could be reversed by the supplementation of NADPH, the overexpression of G6PD and the supplementation of GGOH. In vivo studies showed that M4IDP inhibited tumor growth in the human colorectal cancer xenograft mouse model, which was accompanied with a decreased [18F]FDG uptake. Collectively, these results provide evidence that M4IDP can promote oxidation in colon cancer cells by inhibiting mevalonate pathway and pentose phosphate pathway and produce therapeutic effect. This study revealed for the first time a possible mechanism of bisphosphonate-induced increase of ROS in malignant tumor cells. This is helpful for the development of new molecular therapeutic targets and can provide new ideas for the combined therapy of bisphosphonates in tumors.

Exhaustive exercise alters native and site-specific H2O2 emission in red and white skeletal muscle mitochondria

Mitochondrial reactive oxygen species (ROS) homeostasis is intricately linked to energy conversion reactions and entails regulation of the mechanisms of ROS production and removal. However, there is limited understanding of how energy demand modulates ROS balance. Skeletal muscle experiences a wide range of energy requirements depending on the intensity and duration of exercise and therefore is an excellent model to probe the effect of altered energy demand on mitochondrial ROS production. Because in most fish skeletal muscle exists essentially as pure spatially distinct slow-twitch red oxidative and fast-twitch white glycolytic fibers, it provides a natural system for investigating how functional specialization affects ROS homeostasis. We tested the hypothesis that acute increase in energy demand imposed by exhaustive exercise will increase mitochondrial H2O2 emission to a greater extent in red muscle mitochondria (RMM) compared with white muscle mitochondria (WMM). We found that native H2O2 emission rates varied by up to 6-fold depending on the substrate being oxidized and muscle fiber type, with RMM emitting at higher rates with glutamate-malate and palmitoylcarnitine while WMM emitted at higher rates with succinate and glyceral-3-phosphate. Exhaustive exercise increased the native and site-specific H2O2 emission rates; however, the maximal emission rates depended on the substrate, fiber type and redox site. The H2O2 consumption capacity and activities of individual antioxidant enzymes including the glutathione- and thioredoxin-dependent peroxidases as well as catalase were higher in RMM compared with WMM indicating that the activity of antioxidant defense system does not explain the differences in H2O2 emission rates in RMM and WMM. Overall, our study suggests that substrate selection and oxidation may be the key factors determining the rates of ROS production in RMM and WMM following exhaustive exercise.

Iron-Dependent Cell Death: A New Treatment Approach against Pancreatic Ductal Adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is a devastating tumor type where a very high proportion of people diagnosed end up dying from cancer. Surgical resection is an option for only about 20% of patients, where the 5-year survival increase ranges from 10 to 25%. In addition to surgical resection, there are adjuvant chemotherapy schemes, such as FOLFIRINOX (a mix of Irinotecan, oxaliplatin, 5-Fluorouraci and leucovorin) or gemcitabine-based treatment. These last two drugs have been compared in the NAPOLI-3 clinical trial, and the NALIRIFOX arm was found to have a higher overall survival (OS) (11.1 months vs. 9.2 months). Despite these exciting improvements, PDAC still has no effective treatment. An interesting approach would be to drive ferroptosis in PDAC cells. A non-apoptotic reactive oxygen species (ROS)-dependent cell death, ferroptosis was first described by Dixon et al. in 2012. ROS are constantly produced in the tumor cell due to high cell metabolism, which is even higher when exposed to chemotherapy. Tumor cells have detoxifying mechanisms, such as Mn-SOD or the GSH-GPX system. However, when a threshold of ROS is exceeded in the tumor cell, the cell's antioxidant systems are overwhelmed, resulting in lipid peroxidation and, ultimately, ferroptosis. In this review, we point out ferroptosis as an approach to consider in PDAC and propose that altering the cellular ROS balance by combining oxidizing agents or with inhibitors of the main cellular detoxifiers triggers ferroptosis in PDAC.

Mitochondrial Cristae Morphology Reflecting Metabolism, Superoxide Formation, Redox Homeostasis, and Pathology

Significance: Mitochondrial (mt) reticulum network in the cell possesses amazing ultramorphology of parallel lamellar cristae, formed by the invaginated inner mitochondrial membrane. Its non-invaginated part, the inner boundary membrane (IBM) forms a cylindrical sandwich with the outer mitochondrial membrane (OMM). Crista membranes (CMs) meet IBM at crista junctions (CJs) of mt cristae organizing system (MICOS) complexes connected to OMM sorting and assembly machinery (SAM). Cristae dimensions, shape, and CJs have characteristic patterns for different metabolic regimes, physiological and pathological situations. Recent Advances: Cristae-shaping proteins were characterized, namely rows of ATP-synthase dimers forming the crista lamella edges, MICOS subunits, optic atrophy 1 (OPA1) isoforms and mitochondrial genome maintenance 1 (MGM1) filaments, prohibitins, and others. Detailed cristae ultramorphology changes were imaged by focused-ion beam/scanning electron microscopy. Dynamics of crista lamellae and mobile CJs were demonstrated by nanoscopy in living cells. With tBID-induced apoptosis a single entirely fused cristae reticulum was observed in a mitochondrial spheroid. Critical Issues: The mobility and composition of MICOS, OPA1, and ATP-synthase dimeric rows regulated by post-translational modifications might be exclusively responsible for cristae morphology changes, but ion fluxes across CM and resulting osmotic forces might be also involved. Inevitably, cristae ultramorphology should reflect also mitochondrial redox homeostasis, but details are unknown. Disordered cristae typically reflect higher superoxide formation. Future Directions: To link redox homeostasis to cristae ultramorphology and define markers, recent progress will help in uncovering mechanisms involved in proton-coupled electron transfer via the respiratory chain and in regulation of cristae architecture, leading to structural determination of superoxide formation sites and cristae ultramorphology changes in diseases. Antioxid. Redox Signal. 39, 635-683.

Effects of ROS pathway inhibitors and NADH and FADH2 linked substrates on mitochondrial bioenergetics and ROS emission in the heart and kidney cortex and outer medulla

Mitochondria are major sources of reactive oxygen species (ROS), which play important roles in both physiological and pathological processes. However, the specific contributions of different ROS production and scavenging components in the mitochondria of metabolically active tissues such as heart and kidney cortex and outer medulla (OM) are not well understood. Therefore, the goal of this study was to determine contributions of different ROS production and scavenging components and provide detailed comparisons of mitochondrial respiration, bioenergetics, ROS emission between the heart and kidney cortex and OM using tissues obtained from the same Sprague-Dawley rat under identical conditions and perturbations. Specifically, data were obtained using both NADH-linked substrate pyruvate + malate and FADH2-linked substrate succinate followed by additions of inhibitors of different components of the electron transport chain (ETC) and oxidative phosphorylation (OxPhos) and other ROS production and scavenging systems. Currently, there is limited data available for the mitochondria of kidney cortex and OM, the two major energy-consuming tissues in the body only next to the heart, and scarce quantitative information on the interplay between mitochondrial ROS production and scavenging systems in the three tissues. The findings from this study demonstrate significant differences in mitochondrial respiratory and bioenergetic functions and ROS emission among the three tissues. The results quantify the rates of ROS production from different complexes of the ETC, identify the complexes responsible for variations in mitochondrial membrane depolarization and regulations of ROS production, and quantify the contributions of ROS scavenging enzymes towards overall mitochondrial ROS emission. These findings advance our fundamental knowledge of tissue-specific and substrate-dependent mitochondrial respiratory and bioenergetic functions and ROS emission. This is important given the critical role that excess ROS production, oxidative stress, and mitochondrial dysfunction in the heart and kidney cortex and OM play in the pathogenesis of cardiovascular and renal diseases, including salt-sensitive hypertension.

How the Disruption of Mitochondrial Redox Signalling Contributes to Ageing

In the past, mitochondrial reactive oxygen species (mtROS) were considered a byproduct of cellular metabolism. Due to the capacity of mtROS to cause oxidative damage, they were proposed as the main drivers of ageing and age-related diseases. Today, we know that mtROS are cellular messengers instrumental in maintaining cellular homeostasis. As cellular messengers, they are produced in specific places at specific times, and the intensity and duration of the ROS signal determine the downstream effects of mitochondrial redox signalling. We do not know yet all the processes for which mtROS are important, but we have learnt that they are essential in decisions that affect cellular differentiation, proliferation and survival. On top of causing damage due to their capacity to oxidize cellular components, mtROS contribute to the onset of degenerative diseases when redox signalling becomes dysregulated. Here, we review the best-characterized signalling pathways in which mtROS participate and those pathological processes in which they are involved. We focus on how mtROS signalling is altered during ageing and discuss whether the accumulation of damaged mitochondria without signalling capacity is a cause or a consequence of ageing.

Alteration of Mesenchymal Stem Cells Isolated from Glioblastoma Multiforme under the Influence of Photodynamic Treatment

The central hypothesis for the development of glioblastoma multiforme (GBM) postulates that the tumor begins its development by transforming neural stem cells into cancer stem cells (CSC). Recently, it has become clear that another kind of stem cell, the mesenchymal stem cell (MSC), plays a role in the tumor stroma. Mesenchymal stem cells, along with their typical markers, can express neural markers and are capable of neural transdifferentiation. From this perspective, it is hypothesized that MSCs can give rise to CSC. In addition, MSCs suppress the immune cells through direct contact and secretory factors. Photodynamic therapy aims to selectively accumulate a photosensitizer in neoplastic cells, forming reactive oxygen species (ROS) upon irradiation, initiating death pathways. In our experiments, MSCs from 15 glioblastomas (GB-MSC) were isolated and cultured. The cells were treated with 5-ALA and irradiated. Flow cytometry and ELISA were used to detect the marker expression and soluble-factor secretion. The MSCs' neural markers, Nestin, Sox2, and glial fibrillary acid protein (GFAP), were down-regulated, but the expression levels of the mesenchymal markers CD73, CD90, and CD105 were retained. The GB-MSCs also reduced their expression of PD-L1 and increased their secretion of PGE2. Our results give us grounds to speculate that the photodynamic impact on GB-MSCs reduces their capacity for neural transdifferentiation.

High Expression of COA6 Is Related to Unfavorable Prognosis and Enhanced Oxidative Phosphorylation in Lung Adenocarcinoma

Mitochondrial metabolism plays an important role in the occurrence and development of cancers. Cytochrome C oxidase assembly factor six (COA6) is essential in mitochondrial metabolism. However, the role of COA6 in lung adenocarcinoma (LUAD) remains unknown. Here we report that the expression of COA6 mRNA and protein were upregulated in LUAD tissues compared with lung normal tissues. We found that COA6 had high sensitivity and specificity to distinguish LUAD tissues from normal lung tissues shown by a receiver operating characteristic (ROC) curve. In addition, our univariate and multivariate Cox regression analysis indicated that COA6 was an independent unfavorable prognostic factor for LUAD patients. Furthermore, our survival analysis and nomogram showed that a high expression of COA6 mRNA was related to the short overall survival (OS) of LUAD patients. Notably, our weighted correlation network analysis (WGCNA) and functional enrichment analysis revealed that COA6 may participate in the development of LUAD by affecting mitochondrial oxidative phosphorylation (OXPHOS). Importantly, we demonstrated that depletion of COA6 could decrease the mitochondrial membrane potential (MMP), nicotinamide adenine dinucleotide (NAD) + hydrogen (H) (NADH), and adenosine triphosphate (ATP) production in LUAD cells (A549 and H1975), hence inhibiting the proliferation of these cells in vitro. Together, our study strongly suggests that COA6 is significantly associated with the prognosis and OXPHOS in LUAD. Hence, COA6 is highly likely a novel prognostic biomarker and therapeutic target of LUAD.

Site IQ in mitochondrial complex I generates S1QEL-sensitive superoxide/hydrogen peroxide in both the reverse and forward reactions

Superoxide/hydrogen peroxide production by site IQ in complex I of the electron transport chain is conventionally assayed during reverse electron transport (RET) from ubiquinol to NAD. However, S1QELs (specific suppressors of superoxide/hydrogen peroxide production by site IQ) have potent effects in cells and in vivo during presumed forward electron transport (FET). Therefore, we tested whether site IQ generates S1QEL-sensitive superoxide/hydrogen peroxide during FET (site IQf), or alternatively, whether RET and associated S1QEL-sensitive superoxide/hydrogen peroxide production (site IQr) occurs in cells under normal conditions. We introduce an assay to determine if electron flow through complex I is thermodynamically forward or reverse: on blocking electron flow through complex I, the endogenous matrix NAD pool will become more reduced if flow before the challenge was forward, but more oxidised if flow was reverse. Using this assay we show in the model system of isolated rat skeletal muscle mitochondria that superoxide/hydrogen peroxide production by site IQ can be equally great whether RET or FET is running. We show that sites IQr and IQf are equally sensitive to S1QELs, and to rotenone and piericidin A, inhibitors that block the Q-site of complex I. We exclude the possibility that some sub-fraction of the mitochondrial population running site IQr during FET is responsible for S1QEL-sensitive superoxide/hydrogen peroxide production by site IQ. Finally, we show that superoxide/hydrogen peroxide production by site IQ in cells occurs during FET, and is S1QEL-sensitive.

Unraveling the Peculiar Features of Mitochondrial Metabolism and Dynamics in Prostate Cancer

Prostate cancer (PCa) is the second leading cause of cancer deaths among men in Western countries. Mitochondria, the "powerhouse" of cells, undergo distinctive metabolic and structural dynamics in different types of cancer. PCa cells experience peculiar metabolic changes during their progression from normal epithelial cells to early-stage and, progressively, to late-stage cancer cells. Specifically, healthy cells display a truncated tricarboxylic acid (TCA) cycle and inefficient oxidative phosphorylation (OXPHOS) due to the high accumulation of zinc that impairs the activity of m-aconitase, the enzyme of the TCA cycle responsible for the oxidation of citrate. During the early phase of cancer development, intracellular zinc levels decrease leading to the reactivation of m-aconitase, TCA cycle and OXPHOS. PCa cells change their metabolic features again when progressing to the late stage of cancer. In particular, the Warburg effect was consistently shown to be the main metabolic feature of late-stage PCa cells. However, accumulating evidence sustains that both the TCA cycle and the OXPHOS pathway are still present and active in these cells. The androgen receptor axis as well as mutations in mitochondrial genes involved in metabolic rewiring were shown to play a key role in PCa cell metabolic reprogramming. Mitochondrial structural dynamics, such as biogenesis, fusion/fission and mitophagy, were also observed in PCa cells. In this review, we focus on the mitochondrial metabolic and structural dynamics occurring in PCa during tumor development and progression; their role as effective molecular targets for novel therapeutic strategies in PCa patients is also discussed.

Understanding the Involvement of microRNAs in Mitochondrial Dysfunction and Their Role as Potential Biomarkers and Therapeutic Targets in Parkinson's Disease

Parkinson's disease (PD) is the second most common neurodegenerative disease, affecting the elderly worldwide and causing significant movement impairments. The goal of PD treatment is to restore dopamine levels in the striatum and regulate movement symptoms. The lack of specific biomarkers for early diagnosis, as well as medication aimed at addressing the pathogenic mechanisms to decelerate the progression of dopaminergic neurodegeneration, are key roadblocks in the management of PD. Various pathogenic processes have been identified to be involved in the progression of PD, with mitochondrial dysfunction being a major contributor to the disease's pathogenesis. The regulation of mitochondrial functions is influenced by a variety of factors, including epigenetics. microRNAs (miRNAs) are epigenetic modulators involved in the regulation of gene expression and regulate a variety of proteins that essential for proper mitochondrial functioning. They are found to be dysregulated in PD, as evidenced by biological samples from PD patients and in vitro and in vivo research. In this article, we attempt to provide an overview of several miRNAs linked to mitochondrial dysfunction and their potential as diagnostic biomarkers and therapeutic targets in PD.

Diallyl trisulfide alleviates chemotherapy sensitivity of ovarian cancer via the AMPK/SIRT1/PGC1α pathway

Platinum-based chemotherapy promotes drug resistance in ovarian cancer. We investigated the antichemoresistance characteristics of diallyl trisulfide (DATS) in cisplatin-resistant ovarian cancer cells, in vitro and in vivo. Previous preclinical studies have revealed that DATS regulates distinct hallmark cancer-signaling pathways. The cell cycle pathway is the most investigated signaling pathway in DATS. Additionally, post-DATS treatment has been found to promote proapoptotic capacity through the regulation of intrinsic and extrinsic apoptotic pathway components. In the present study, we found that treating cisplatin-sensitive and cisplatin-resistant ovarian cell lines with DATS inhibited their proliferation and reduced their IC_50. It induced cell apoptosis and promoted oxidative phosphorylation through the regulation of the AMPK/SIRT1/PGC1α pathway, OXPHOS, and enhanced chemotherapy sensitivity. DATS treatment alleviated glutamine consumption in cisplatin-resistant cells. Our findings highlight the role of DATS in overcoming drug resistance in ovarian cancer in vitro and in vivo. In addition, we elucidated the role of the AMPK/SIRT1/PGC1α signaling pathway as a potential target for the treatment of drug-resistant ovarian cancer.

The role of vitamin D on redox regulation and cellular senescence

Vitamin D is considered an essential micronutrient for human health that is metabolized into a multifunctional secosteroid hormone. We can synthesize it in the skin through ultraviolet B (UVB) rays or acquire it from the diet. Its deficiency is a major global health problem that affects all ages and ethnic groups. Furthermore, dysregulation of vitamin D homeostasis has been associated with premature aging, driven by various cellular processes, including oxidative stress and cellular senescence. Various studies have shown that vitamin D can attenuate oxidative stress and delay cellular senescence, mainly by inducing the expression of nuclear factor erythroid 2-related factor 2 (Nrf2) and Klotho and improving mitochondrial homeostasis, proposing this vitamin as an excellent candidate for delaying aging. However, the mechanisms around these processes are not yet fully explored. Therefore, in this review, the effects of vitamin D on redox regulation and cellular senescence are discussed to propose new lines of research and clinical applications of vitamin D in the context of age-related diseases.

TIMP-1 Dependent Modulation of Metabolic Profiles Impacts Chemoresistance in NSCLC

The development of chemoresistance remains a significant barrier to treating NSCLC. Alteration of cancer cell metabolism is an important mechanism for chemoresistance. This study explored the role of aberrant metabolism in TIMP-1-mediated chemoresistance. Bioinformatics analysis identified an association of high TIMP-1 with altered energy metabolism. We have defined the role of depolarized mitochondria through a reduction in lactate secretion, higher ROS levels in TIMP-1 KD cells and reduced GSH levels. TIMP-1 modulates the metabolic profile via acetylation of mitochondrial STAT3 and its interaction with CD44. Intriguingly, monomers of acetylated STAT3 were critical for altered metabolism, whereas STAT3 dimers abrogated this function. Further, the mitochondrial metabolic profile was also altered in a cisplatin-resistant clone of A549 cells. We also correlated the immunoexpression of CD44, STAT3 and TIMP-1 in patient samples. This study provided evidence that TIMP-1 alters the metabolic profile by modulating mitochondrial metabolism via the CD44-STAT3 axis through its effects on STAT3 acetylation. It also lent further support to the critical role of TIMP-1 in chemoresistance. Interrogation of the TCGA-LUAD dataset revealed perturbations in the critical modulator that can alter metabolic states in cancer cells. Higher expression of a five-gene signature, including TIMP-1, correlated with immunosuppressive cells and was found to be associated with overall survival. This study identified several metabolic mechanisms that could influence therapeutic options and prognosis in NSCLC patients.

Oxidative Stress and Mitochondrial Dysfunction Associated with Peripheral Neuropathy in Type 1 Diabetes

Significance: This review highlights the many intracellular processes generating reactive oxygen species (ROS) in the peripheral nervous system in the context of type 1 diabetes. The major sources of superoxide and hydrogen peroxide (H₂O₂) are described, and scavenging systems are explained. Important roles of ROS in regulating normal redox signaling and in a disease setting, such as diabetes, contributing to oxidative stress and cellular damage are outlined. The primary focus is the role of hyperglycemia in driving elevated ROS production and oxidative stress contributing to neurodegeneration in diabetic neuropathy (within the dorsal root ganglia [DRG] and peripheral nerve). Recent Advances: Contributors to ROS production under high intracellular glucose concentration such as mitochondria and the polyol pathway are discussed. The primarily damaging impact of ROS on multiple pathways including mitochondrial function, endoplasmic reticulum (ER) stress, autophagy, and epigenetic signaling is covered. Critical Issues: There is a strong focus on mechanisms of diabetes-induced mitochondrial dysfunction and how this may drive ROS production (in particular superoxide). The mitochondrial sites of superoxide/H₂O₂ production via mitochondrial metabolism and aerobic respiration are reviewed. Future Directions: Areas for future development are highlighted, including the need to clarify diabetes-induced changes in autophagy and ER function in neurons and Schwann cells. In addition, more clarity is needed regarding the sources of ROS production at mitochondrial sites under high glucose concentration (and lack of insulin signaling). New areas of study should be introduced to investigate the role of ROS, nuclear lamina function, and epigenetic signaling under diabetic conditions in peripheral nerve.

Effects of sugars, fatty acids and amino acids on cytosolic and mitochondrial hydrogen peroxide release from liver cells

The rates of formation of superoxide and hydrogen peroxide at different electron-donating sites in isolated mitochondria are critically dependent on the substrates that are added, through their effects on the reduction level of each site and the components of the protonmotive force. However, in intact cells the acute effects of added substrates on different sites of cytosolic and mitochondrial hydrogen peroxide production are unclear. Here we tested the effects of substrate addition on cytosolic and mitochondrial hydrogen peroxide release from intact AML12 liver cells. In 30-min starved cells replete with endogenous substrates, addition of glucose, fructose, palmitate, alanine, leucine or glutamine had no effect on the rate or origin of cellular hydrogen peroxide release. However, following 150-min starvation of the cells to deplete endogenous glycogen (and other substrates), cellular hydrogen peroxide production, particularly from NADPH oxidases (NOXs), was decreased, GSH/GSSH ratio increased, and antioxidant gene expression was unchanged. Addition of glucose or glutamine (but not the other substrates) increased hydrogen peroxide release. There were similar relative increases from each of the three major sites of production: mitochondrial sites I_Q and III_Qo, and cytosolic NOXs. Glucose supplementation also restored ATP production and mitochondrial NAD reduction level, suggesting that the increased rates of hydrogen peroxide release from the mitochondrial sites were driven by increases in the protonmotive force and the degree of reduction of the electron transport chain. Long-term (24 h) glucose or glutamine deprivation also diminished hydrogen peroxide release rate, ATP production rate and (for glucose deprivation) NAD reduction level. We conclude that the rates of superoxide and hydrogen peroxide production from mitochondrial sites in liver cells are insensitive to extra added substrates when endogenous substrates are not depleted, but these rates are decreased when endogenous substrates are lowered by 150 min of starvation, and can be enhanced by restoring glucose or glutamine supply through improvements in mitochondrial energetic state.

Deciphering the mechanism by which the yeast Phaffia rhodozyma responds adaptively to environmental, nutritional, and genetic cues

Phaffia rhodozyma is a basidiomycetous yeast that synthesizes astaxanthin (ASX), which is a powerful and highly valuable antioxidant carotenoid pigment. P. rhodozyma cells accrue ASX and gain an intense red-pink coloration when faced with stressful conditions such as nutrient limitations (e.g., nitrogen or copper), the presence of toxic substances (e.g., antimycin A), or are affected by mutations in the genes that are involved in nitrogen metabolism or respiration. Since cellular accrual of ASX occurs under a wide variety of conditions, this yeast represents a valuable model for studying the growth conditions that entail oxidative stress for yeast cells. Recently, we proposed that ASX synthesis can be largely induced by conditions that lead to reduction-oxidation (redox) imbalances, particularly the state of the NADH/NAD+ couple together with an oxidative environment. In this work, we review the multiple known conditions that elicit ASX synthesis expanding on the data that we formerly examined. When considered alongside the Mitchell's chemiosmotic hypothesis, the study served to rationalize the induction of ASX synthesis and other adaptive cellular processes under a much broader set of conditions. Our aim was to propose an underlying mechanism that explains how a broad range of divergent conditions converge to induce ASX synthesis in P. rhodozyma. The mechanism that links the induction of ASX synthesis with the occurrence of NADH/NAD+ imbalances may help in understanding how other organisms detect any of a broad array of stimuli or gene mutations, and then adaptively respond to activate numerous compensatory cellular processes.

Does high mitochondrial efficiency carry an oxidative cost? The case of the African pygmy mouse (Mus mattheyi)

Skeletal muscle mitochondria of the African pygmy mouse Mus mattheyi exhibit markedly reduced oxygen consumption and ATP synthesis rates but a higher mitochondrial efficiency than what would be expected from allometric trends. In the present study, we assessed whether such reduction of mitochondrial activity in M. mattheyi can limit the oxidative stress associated with an increased generation of mitochondrial reactive oxygen species. We conducted a comparative study of mitochondrial oxygen consumption, H₂O₂ release, and electron leak (%H₂O₂/O) in skeletal muscle mitochondria isolated from the extremely small African pygmy mouse (M. mattheyi, ~5 g) and Mus musculus, which is a larger Mus species (~25 g). Mitochondria were energized with pyruvate, malate, and succinate, after which fluxes were measured at different steady-state rates of oxidative phosphorylation. Overall, M. mattheyi exhibited lower oxidative activity and higher electron leak than M. musculus, while the H₂O₂ release did not differ significantly between these two Mus species. We further found that the high coupling efficiency of skeletal muscle mitochondria from M. mattheyi was associated with high electron leak. Nevertheless, data also show that, despite the higher electron leak, the lower mitochondrial respiratory capacity of M. mattheyi limits the cost of a net increase in H₂O₂ release, which is lower than that expected for a mammals of this size.

Mitochondrial capacity and reactive oxygen species production during hypoxia and reoxygenation in the ocean quahog, Arctica islandica

Oxygen fluctuations are common in marine waters, and hypoxia-reoxygenation (H-R) stress can negatively affect mitochondrial metabolism. The long-lived ocean quahog, Arctica islandica, is known for its hypoxia tolerance associated with metabolic rate depression, yet the mechanisms that sustain mitochondrial function during oxygen fluctuations are not well understood. We used top-down metabolic control analysis (MCA) to determine aerobic capacity and control over oxygen flux in the mitochondria of quahogs exposed to short-term hypoxia (24 h <0.01% O2) and subsequent reoxygenation (1.5 h 21% O2) compared with normoxic control animals (21% O2). We demonstrated that flux capacity of the substrate oxidation and proton leak subsystems were not affected by hypoxia, while the capacity of the phosphorylation subsystem was enhanced during hypoxia associated with a depolarization of the mitochondrial membrane. Reoxygenation decreased the oxygen flux capacity of all three mitochondrial subsystems. Control over oxidative phosphorylation (OXPHOS) respiration was mostly exerted by substrate oxidation regardless of H-R stress, whereas control by the proton leak subsystem of LEAK respiration increased during hypoxia and returned to normoxic levels during reoxygenation. During hypoxia, reactive oxygen species (ROS) efflux was elevated in the LEAK state, whereas it was suppressed in the OXPHOS state. Mitochondrial ROS efflux returned to normoxic control levels during reoxygenation. Thus, mitochondria of A. islandica appear robust to hypoxia by maintaining stable substrate oxidation and upregulating phosphorylation capacity, but remain sensitive to reoxygenation. This mitochondrial phenotype might reflect adaptation of A. islandica to environments with unpredictable oxygen fluctuations and its behavioural preference for low oxygen levels.

Cannabidiol modulation of oxidative stress and signalling

Cannabidiol (CBD), one of the primary non-euphoric components in the Cannabis sativa L. plant, has undergone clinical development over the last number of years as a therapeutic for patients with Lennox-Gastaut syndrome and Dravet syndromes. This phytocannabinoid demonstrates functional and pharmacological diversity, and research data indicate that CBD is a comparable antioxidant to common antioxidants. This review gathers the latest knowledge regarding the impact of CBD on oxidative signalling, with focus on the proclivity of CBD to regulate antioxidants and control the production of reactive oxygen species. CBD is considered an attractive therapeutic agent for neuroimmune disorders, and a body of literature indicates that CBD can regulate redox function at multiple levels, with a range of downstream effects on cells and tissues. However, pro-oxidant capacity of CBD has also been reported, and hence caution must be applied when considering CBD from a therapeutic standpoint. Such pro- and antioxidant functions of CBD may be cell- and model-dependent and may also be influenced by CBD dose, the duration of CBD treatment and the underlying pathology.

Review: Using isolated mitochondria to investigate mitochondrial hydrogen peroxide metabolism

Mitochondria are recognized as centrally important to cellular reactive oxygen species (ROS), both as a potential source and due to their substantial antioxidant capacity. While much of the initial ROS formed by mitochondria is superoxide, this is rapidly converted to hydrogen peroxide (H₂O₂) which more readily crosses membranes making H₂O₂ important in both redox signalling mechanisms and conditions of oxidative stress. Here I outline our studies on mitochondrial H₂O₂ metabolism with a focus on some of the challenges and strategies involved with developing an integrated model of mitochondria being intrinsic regulators of H₂O₂. This view of mitochondria as regulators of H₂O₂ goes beyond the simpler contention of them being net producers or consumers. Moreover, the integration of both consumption and production can then be tied to a putative mechanism linking energy sensing at the level of the mitochondrial protonmotive force. This mechanism would provide a means of mitochondria communicating their energetic status the extramitochondrial compartment via local H₂O₂ concentrations. I conclude by explaining how these concepts developed using rodent muscle as a model have high relevance and applicability to comparative studies.

Mitochondrial Dynamics, ROS, and Cell Signaling: A Blended Overview

Mitochondria are key intracellular organelles involved not only in the metabolic state of the cell, but also in several cellular functions, such as proliferation, Calcium signaling, and lipid trafficking. Indeed, these organelles are characterized by continuous events of fission and fusion which contribute to the dynamic plasticity of their network, also strongly influenced by mitochondrial contacts with other subcellular organelles. Nevertheless, mitochondria release a major amount of reactive oxygen species (ROS) inside eukaryotic cells, which are reported to mediate a plethora of both physiological and pathological cellular functions, such as growth and proliferation, regulation of autophagy, apoptosis, and metastasis. Therefore, targeting mitochondrial ROS could be a promising strategy to overcome and hinder the development of diseases such as cancer, where malignant cells, possessing a higher amount of ROS with respect to healthy ones, could be specifically targeted by therapeutic treatments. In this review, we collected the ultimate findings on the blended interplay among mitochondrial shaping, mitochondrial ROS, and several signaling pathways, in order to contribute to the dissection of intracellular molecular mechanisms involved in the pathophysiology of eukaryotic cells, possibly improving future therapeutic approaches.

Are mitochondria the main contributor of reactive oxygen species in cells?

Physiologists often assume that mitochondria are the main producers of reactive oxygen species (ROS) in cells. Consequently, in biomedicine, mitochondria are considered as important targets for therapeutic treatments, and in evolutionary biology, they are considered as mediators of life-history tradeoffs. Surprisingly, data supporting such an assumption are lacking, at least partially due to the technical difficulties in accurately measuring the level of ROS produced by different subcellular compartments in intact cells. In this Commentary, we first review three potential reasons underlying the misassumption of mitochondrial dominance in the production of cellular ROS. We then introduce some other major sites/enzymes responsible for cellular ROS production. With the use of a recently developed cell-based assay, we further discuss the contribution of mitochondria to the total rate of ROS release in cell lines and primary cells of different species. In these cells, the contribution of mitochondria varies between cell types but mitochondria are never the main source of cellular ROS. This indicates that although mitochondria are one of the significant sources of cellular ROS, they are not necessarily the main contributor under normal conditions. Intriguingly, similar findings were also observed in cells under a variety of stressors, life-history strategies and pathological stages, in which the rates of cellular ROS production were significantly enhanced. Finally, we make recommendations for designing future studies. We hope this paper will encourage investigators to carefully consider non-mitochondrial sources of cellular ROS in their study systems or models.

Superoxide produced by mitochondrial site IQ inactivates cardiac succinate dehydrogenase and induces hepatic steatosis in Sod2 knockout mice

Superoxide produced by mitochondria has been implicated in numerous physiologies and pathologies. Eleven different mitochondrial sites that can produce superoxide and/or hydrogen peroxide (O₂^.-/H₂O₂) have been identified in vitro, but little is known about their contributions in vivo. We introduce novel variants of S1QELs and S3QELs (small molecules that suppress O₂^.-/H₂O₂ production specifically from mitochondrial sites I_Q and III_Qo, respectively, without compromising bioenergetics), that are suitable for use in vivo. When administered by intraperitoneal injection, they achieve total tissue concentrations exceeding those that are effective in vitro. We use them to study the engagement of sites I_Q and III_Qo in mice lacking functional manganese-superoxide dismutase (SOD2). Lack of SOD2 is expected to elevate superoxide levels in the mitochondrial matrix, and leads to severe pathologies and death about 8 days after birth. Compared to littermate wild-type mice, 6-day-old Sod2^-/- mice had significantly lower body weight, lower heart succinate dehydrogenase activity, and greater hepatic lipid accumulation. These pathologies were ameliorated by treatment with a SOD/catalase mimetic, EUK189, confirming previous observations. A 3-day treatment with S1QEL352 decreased the inactivation of cardiac succinate dehydrogenase and hepatic steatosis in Sod2^-/- mice. S1QEL712, which has a distinct chemical structure, also decreased hepatic steatosis, confirming that O₂^.- derived specifically from mitochondrial site I_Q is a significant driver of hepatic steatosis in Sod2^-/- mice. These findings also demonstrate the ability of these new S1QELs to suppress O₂^.- production in the mitochondrial matrix in vivo. In contrast, suppressing site III_Qo using S3QEL941 did not protect, suggesting that site III_Qo does not contribute significantly to mitochondrial O₂^.- production in the hearts or livers of Sod2^-/- mice. We conclude that the novel S1QELs are effective in vivo, and that site I_Q runs in vivo and is a significant driver of pathology in Sod2^-/- mice.

Coenzyme Q redox signalling and longevity

Mitochondria are the powerhouses of the cell. They produce a significant amount of the energy we need to grow, survive and reproduce. The same system that generates energy in the form of ATP also produces Reactive Oxygen Species (ROS). Mitochondrial Reactive Oxygen Species (mtROS) were considered for many years toxic by-products of metabolism, responsible for ageing and many degenerative diseases. Today, we know that mtROS are essential redox messengers required to determine cell fate and maintain cellular homeostasis. Most mtROS are produced by respiratory complex I (CI) and complex III (CIII). How and when CI and CIII produce ROS is determined by the redox state of the Coenzyme Q (CoQ) pool and the proton motive force (pmf) generated during respiration. During ageing, there is an accumulation of defective mitochondria that generate high levels of mtROS. This causes oxidative stress and disrupts redox signalling. Here, we review how mtROS are generated in young and old mitochondria and how CI and CIII derived ROS control physiological and pathological processes. Finally, we discuss why damaged mitochondria amass during ageing as well as methods to preserve mitochondrial redox signalling with age.

Methylene Blue Bridges the Inhibition and Produces Unusual Respiratory Changes in Complex III-Inhibited Mitochondria. Studies on Rats, Mice and Guinea Pigs

Methylene blue (MB) is used in human therapy in various pathological conditions. Its effects in neurodegenerative disease models are promising. MB acts on multiple cellular targets and mechanisms, but many of its potential beneficial effects are ascribed to be mitochondrial. According to the “alternative electron transport” hypothesis, MB is capable of donating electrons to cytochrome c bypassing complex I and III. As a consequence of this, the deleterious effects of the inhibitors of complex I and III can be ameliorated by MB. Recently, the beneficial effects of MB exerted on complex III-inhibited mitochondria were debated. In the present contribution, several pieces of evidence are provided towards that MB is able to reduce cytochrome c and improve bioenergetic parameters, like respiration and membrane potential, in mitochondria treated with complex III inhibitors, either antimycin or myxothiazol. These conclusions were drawn from measurements for mitochondrial oxygen consumption, membrane potential, NAD(P)H steady state, MB uptake and MB-cytochrome c oxidoreduction. In the presence of MB and complex III inhibitors, unusual respiratory reactions, like decreased oxygen consumption as a response to ADP addition as well as stimulation of respiration upon administration of inhibitors of ATP synthase or ANT, were observed. Qualitatively identical results were obtained in three rodent species. The actual metabolic status of mitochondria is well reflected in the distribution of MB amongst various compartments of this organelle.

Superoxide produced by mitochondrial complex III plays a pivotal role in the execution of ferroptosis induced by cysteine starvation

Ferroptosis is a type of iron-dependent, non-apoptotic cell death, which is typically induced by cysteine starvation or by the inhibition of glutathione peroxidase 4 (GPX4) activity with the accompanying elevation in lipid peroxidation product levels. Despite the central role of mitochondria in oxidative metabolism and hence, as main sources of superoxide, the issue of whether mitochondrial superoxide participates in the execution of ferroptosis remains unclear. To gain additional insights into this issue, we employed suppressors of the site I_Q electron leak (S1QEL) and suppressors of the site III_Qo electron leak (S3QEL), small molecules that suppress mitochondrial superoxide production from complex I and III, respectively. The findings indicate that S3QEL, but not S1QEL, significantly protected mouse hepatoma Hepa 1-6 cells from lipid peroxidation and the subsequent ferroptosis induced by cysteine (Cys) starvation (cystine deprivation from culture media or xCT inhibition by erastin). The intracellular levels of Cys and GSH remained low irrespective of life or death. Moreover, S3QEL also suppressed ferroptosis in xCT-knockout mouse-derived embryonic fibroblasts, which usually die under conventional cultivating conditions due to the absence of intracellular Cys and GSH. Although it has been reported that erastin induces the hyperpolarization of the mitochondrial membrane potential, no correlation was observed between hyperpolarization and cell death in xCT-knockout cells. Collectively, these results indicate that superoxide production from complex III plays a pivotal role in the ferroptosis that is induced by Cys starvation, suggesting that protecting mitochondria is a promising therapeutic strategy for the treatment of multiple diseases featuring ferroptosis.

Cardiolipin deficiency in Barth syndrome is not associated with increased superoxide/H2 O2 production in heart and skeletal muscle mitochondria

Barth syndrome (BTHS) is a rare X-linked genetic disorder caused by mutations in the gene encoding the transacylase tafazzin and characterized by loss of cardiolipin and severe cardiomyopathy. Mitochondrial oxidants have been implicated in the cardiomyopathy in BTHS. Eleven mitochondrial sites produce superoxide/hydrogen peroxide (H₂ O₂ ) at significant rates. Which of these sites generate oxidants at excessive rates in BTHS is unknown. Here, we measured the maximum capacity of superoxide/H₂ O₂ production from each site and the ex vivo rate of superoxide/H₂ O₂ production in the heart and skeletal muscle mitochondria of the tafazzin knockdown mice (tazkd) from 3 to 12 months of age. Despite reduced oxidative capacity, superoxide/H₂ O₂ production was indistinguishable between tazkd mice and wild-type littermates. These observations raise questions about the involvement of mitochondrial oxidants in BTHS pathology.

Riding the tiger - physiological and pathological effects of superoxide and hydrogen peroxide generated in the mitochondrial matrix

Elevated mitochondrial matrix superoxide and/or hydrogen peroxide concentrations drive a wide range of physiological responses and pathologies. Concentrations of superoxide and hydrogen peroxide in the mitochondrial matrix are set mainly by rates of production, the activities of superoxide dismutase-2 (SOD2) and peroxiredoxin-3 (PRDX3), and by diffusion of hydrogen peroxide to the cytosol. These considerations can be used to generate criteria for assessing whether changes in matrix superoxide or hydrogen peroxide are both necessary and sufficient to drive redox signaling and pathology: is a phenotype affected by suppressing superoxide and hydrogen peroxide production; by manipulating the levels of SOD2, PRDX3 or mitochondria-targeted catalase; and by adding mitochondria-targeted SOD/catalase mimetics or mitochondria-targeted antioxidants? Is the pathology associated with variants in SOD2 and PRDX3 genes? Filtering the large literature on mitochondrial redox signaling using these criteria highlights considerable evidence that mitochondrial superoxide and hydrogen peroxide drive physiological responses involved in cellular stress management, including apoptosis, autophagy, propagation of endoplasmic reticulum stress, cellular senescence, HIF1α signaling, and immune responses. They also affect cell proliferation, migration, differentiation, and the cell cycle. Filtering the huge literature on pathologies highlights strong experimental evidence that 30-40 pathologies may be driven by mitochondrial matrix superoxide or hydrogen peroxide. These can be grouped into overlapping and interacting categories: metabolic, cardiovascular, inflammatory, and neurological diseases; cancer; ischemia/reperfusion injury; aging and its diseases; external insults, and genetic diseases. Understanding the involvement of mitochondrial matrix superoxide and hydrogen peroxide concentrations in these diseases can facilitate the rational development of appropriate therapies.

The Role of Translocator Protein TSPO in Hallmarks of Glioblastoma

Simple Summary

The translocator protein (TSPO) has been under extensive investigation as a specific marker in positron emission tomography (PET) to visualize brain lesions following injury or disease. In recent years, TSPO is increasingly appreciated as a potential novel therapeutic target in cancer. In Glioblastoma (GBM), the most malignant primary brain tumor, TSPO expression levels are strongly elevated and scientific evidence accumulates, hinting at a pivotal role of TSPO in tumorigenesis and glioma progression. The aim of this review is to summarize the current literature on TSPO with respect to its role both in diagnostics and especially with regard to the critical hallmarks of cancer postulated by Hanahan and Weinberg. Overall, our review contributes to a better understanding of the functional significance of TSPO in Glioblastoma and draws attention to TSPO as a potential modulator of treatment response and thus an important factor that may influence the clinical outcome of GBM.

Abstract

Glioblastoma (GBM) is the most fatal primary brain cancer in adults. Despite extensive treatment, tumors inevitably recur, leading to an average survival time shorter than 1.5 years. The 18 kDa translocator protein (TSPO) is abundantly expressed throughout the body including the central nervous system. The expression of TSPO increases in states of inflammation and brain injury due to microglia activation. Not least due to its location in the outer mitochondrial membrane, TSPO has been implicated with a broad spectrum of functions. These include the regulation of proliferation, apoptosis, migration, as well as mitochondrial functions such as mitochondrial respiration and oxidative stress regulation. TSPO is frequently overexpressed in GBM. Its expression level has been positively correlated to WHO grade, glioma cell proliferation, and poor prognosis of patients. Several lines of evidence indicate that TSPO plays a functional part in glioma hallmark features such as resistance to apoptosis, invasiveness, and proliferation. This review provides a critical overview of how TSPO could regulate several aspects of tumorigenesis in GBM, particularly in the context of the hallmarks of cancer proposed by Hanahan and Weinberg in 2011.

Mitochondrial dysfunction and endoplasmic reticulum stress in the promotion of fibrosis in obstructive nephropathy induced by unilateral ureteral obstruction

Obstructive nephropathy favors the progression to chronic kidney disease (CKD), a severe health problem worldwide. The unilateral ureteral obstruction (UUO) model is used to study the development of fibrosis. Impairment of renal mitochondria plays a crucial role in several types of CKD and has been strongly related to fibrosis onset. Nevertheless, in the UUO model, the impairment of mitochondria, their relationship with endoplasmic reticulum (ER) stress induction and the participation of both to induce the fibrotic process remain unclear. In this review, we summarize the current information about mitochondrial bioenergetics, redox dynamics, mitochondrial mass, and biogenesis alterations, as well as the relationship of these mitochondrial alterations with ER stress and their participation in fibrotic processes in UUO models. Early after obstruction, there is metabolic reprogramming related to mitochondrial fatty acid β-oxidation impairment, triggering lipid deposition, oxidative stress, (calcium) Ca²⁺ dysregulation, and a reduction in mitochondrial mass and biogenesis. Mitochondria and the ER establish a pathological feedback loop that promotes the impairment of both organelles by ER stress pathways and Ca²⁺ levels dysregulation. Preserving mitochondrial and ER function can prevent or at least delay the fibrotic process and loss of renal function. However, deeper understanding is still necessary for future clinically-useful therapies.

Mitochondrial threshold for H2O2 release in skeletal muscle of mammals

The aim of the study was to evaluate the interplay between mitochondrial respiration and H₂O₂ release during the transition from basal non-phosphorylating to maximal phosphorylating states. We conducted a large scale comparative study of mitochondrial oxygen consumption, H₂O₂ release and electron leak (% H₂O₂/O) in skeletal muscle mitochondria isolated from mammal species ranging from 7 g to 500 kg. Mitochondrial fluxes were measured at different steady state rates in presence of pyruvate, malate, and succinate as respiratory substrates. Every species exhibited a burst of H₂O₂ release from skeletal muscle mitochondria at a low rate of oxidative phosphorylation, essentially once the activity of mitochondrial oxidative phosphorylation reached 26% of the maximal respiration. This threshold for ROS generation thus appears as a general characteristic of skeletal muscle mitochondria in mammals. These findings may have implications in situations promoting succinate accumulation within mitochondria, such as ischemia or hypoxia.

Activation of transcription factor Nrf2 to counteract mitochondrial dysfunction in Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder, for which no disease-modifying therapies are available to date. Although understanding of the precise aetiology of PD is incomplete, it is clear that age, genetic predisposition and environmental stressors increase the risk. At the cellular level, oxidative stress, chronic neuroinflammation, mitochondrial dysfunction and aberrant protein aggregation have been implicated as contributing factors. These detrimental processes are counteracted by elaborate networks of cellular defence mechanisms, one of which is orchestrated by transcription factor nuclear factor-erythroid 2 p45-related factor 2 (Nrf2; gene name NFE2L2). A wealth of preclinical evidence suggests that Nrf2 activation is beneficial in cellular and animal models of PD. In this review, we summarise the current understanding of mitochondrial dysfunction in PD, the role of Nrf2 in mitochondrial function and explore the potential of Nrf2 as a therapeutic target for mitochondrial dysfunction in PD.

Low abundance of NDUFV2 and NDUFS4 subunits of the hydrophilic complex I domain and VDAC1 predicts mammalian longevity

Mitochondrial reactive oxygen species (ROS) production, specifically at complex I (Cx I), has been widely suggested to be one of the determinants of species longevity. The present study follows a comparative approach to analyse complex I in heart tissue from 8 mammalian species with a longevity ranging from 3.5 to 46 years. Gene expression and protein content of selected Cx I subunits were analysed using droplet digital PCR (ddPCR) and western blot, respectively. Our results demonstrate: 1) the existence of species-specific differences in gene expression and protein content of Cx I in relation to longevity; 2) the achievement of a longevity phenotype is associated with low protein abundance of subunits NDUFV2 and NDUFS4 from the matrix hydrophilic domain of Cx I; and 3) long-lived mammals show also lower levels of VDAC (voltage-dependent anion channel) amount. These differences could be associated with the lower mitochondrial ROS production and slower aging rate of long-lived animals and, unexpectedly, with a low content of the mitochondrial permeability transition pore in these species.

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There are species-specific differences in gene expression and protein content of Cx I.

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The achievement of a longevity phenotype is associated with low protein abundance of subunits NDUFV2 and NDUFS4 from the matrix hydrophilic domain of Cx I.

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Long-lived mammals show also lower levels of VDAC (voltage-dependent anion channel) amount.

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These differences can be causally associated with the aging rate of long-lived animals.

OXPHOS remodeling in high-grade prostate cancer involves mtDNA mutations and increased succinate oxidation

Rewiring of energy metabolism and adaptation of mitochondria are considered to impact on prostate cancer development and progression. Here, we report on mitochondrial respiration, DNA mutations and gene expression in paired benign/malignant human prostate tissue samples. Results reveal reduced respiratory capacities with NADH-pathway substrates glutamate and malate in malignant tissue and a significant metabolic shift towards higher succinate oxidation, particularly in high-grade tumors. The load of potentially deleterious mitochondrial-DNA mutations is higher in tumors and associated with unfavorable risk factors. High levels of potentially deleterious mutations in mitochondrial Complex I-encoding genes are associated with a 70% reduction in NADH-pathway capacity and compensation by increased succinate-pathway capacity. Structural analyses of these mutations reveal amino acid alterations leading to potentially deleterious effects on Complex I, supporting a causal relationship. A metagene signature extracted from the transcriptome of tumor samples exhibiting a severe mitochondrial phenotype enables identification of tumors with shorter survival times.

The Influence of Light on Reactive Oxygen Species and NF-кB in Disease Progression

Reactive oxygen species (ROS) are important secondary metabolites that play major roles in signaling pathways, with their levels often used as analytical tools to investigate various cellular scenarios. They potentially damage genetic material and facilitate tumorigenesis by inhibiting certain tumor suppressors. In diabetic conditions, substantial levels of ROS stimulate oxidative stress through specialized precursors and enzymatic activity, while minimum levels are required for proper wound healing. Photobiomodulation (PBM) uses light to stimulate cellular mechanisms and facilitate the removal of oxidative stress. Photodynamic therapy (PDT) generates ROS to induce selective tumor destruction. The regulatory roles of PBM via crosstalk between ROS and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-кB) are substantial for the appropriate management of various conditions.

Mitochondrial Diagnostics: A Multiplexed Assay Platform for Comprehensive Assessment of Mitochondrial Energy Fluxes

Chronic metabolic diseases have been linked to molecular signatures of mitochondrial dysfunction. Nonetheless, molecular remodeling of the transcriptome, proteome, and/or metabolome does not necessarily translate to functional consequences that confer physiologic phenotypes. The work here aims to bridge the gap between molecular and functional phenomics by developing and validating a multiplexed assay platform for comprehensive assessment of mitochondrial energy transduction. The diagnostic power of the platform stems from a modified version of the creatine kinase energetic clamp technique, performed in parallel with multiplexed analyses of dehydrogenase activities and ATP synthesis rates. Together, these assays provide diagnostic coverage of the mitochondrial network at a level approaching that gained by molecular "-omics" technologies. Application of the platform to a comparison of skeletal muscle versus heart mitochondria reveals mechanistic insights into tissue-specific distinctions in energy transfer efficiency. This platform opens exciting opportunities to unravel the connection between mitochondrial bioenergetics and human disease.

S1QELs suppress mitochondrial superoxide/hydrogen peroxide production from site IQ without inhibiting reverse electron flow through Complex I

Mitochondria are important sources of superoxide and hydrogen peroxide in cell signaling and disease. In particular, superoxide/hydrogen peroxide production during reverse electron transport from ubiquinol to NAD⁺ though Complex I is implicated in several physiological and pathological processes. S1QELs are small molecules that suppress superoxide/hydrogen peroxide production at Complex I without affecting forward electron transport. Their mechanism of action is disputed. To test different mechanistic models, we compared the effects of two inhibitors of Complex I electron transport (piericidin A and rotenone) and two S1QELs from different chemical families on superoxide/hydrogen peroxide production and electron transport by Complex I in isolated mitochondria. Piericidin A and rotenone (and S1QEL1.1 at higher concentrations) prevented superoxide/hydrogen peroxide production from sites I_Q and I_F in Complex I by inhibiting reverse electron transport into the complex. S1QELs decreased the potency of electron transport inhibition by piericidin A and rotenone, suggesting that S1QELs bind directly to Complex I. S1QEL2.1 (and S1QEL1.1 at lower concentrations) suppressed site I_Q without affecting reverse electron transport or site I_F, showing that sites I_Q and I_F are distinct, and that S1QELs do not work simply by decreasing reverse electron transport to site I_F (or site I_Q). S1QELs did not affect the reduction of NAD⁺ or the rate of site I_F driven by reverse electron transport, therefore they do not alter the driving forces for reverse electron transport and that is not how they suppress site I_Q. We conclude that S1QELs bind to Complex I to influence the conformation of the piericidin A and rotenone binding sites and directly suppress superoxide/hydrogen peroxide production at site I_Q, which is a separate site from site I_F.

Attenuation of Equine Lentivirus Alters Mitochondrial Protein Expression Profile from Inflammation to Apoptosis

Equine infectious anemia virus (EIAV) is an equine lentivirus similar to HIV-1, targets host immune cells, and causes a life-long infection in horses. The Chinese live EIAV vaccine is attenuated from long-term passaging of a highly virulent strain in vitro The parent pathogenic strain (EIAV_DLV34) induces a host inflammatory storm to cause severe pathological injury of animals. However, the vaccine strain (EIAV_DLV121) induces a high level of apoptosis to eliminate infected cells. To investigate how these processes are regulated, we performed a comparative proteomics analysis and functional study in equine monocyte-derived macrophages (eMDMs) and found that the divergent mitochondrial protein expression profiles caused by EIAV strains with different virulence led to disparate mitochondrial function, morphology, and metabolism. This in turn promoted the distinct transformation of macrophage inflammatory polarization and intrinsic apoptosis. In EIAV_DLV34-infected cells, a high level of glycolysis and increased mitochondrial fragmentation were induced, resulting in the M1-polarized proinflammatory-type transformation of macrophages and the subsequent production of a strong inflammatory response. Following infection with EIAV_DLV121, the infected cells were transformed into M2-polarized anti-inflammatory macrophages by inhibition of glycolysis. In this case, a decrease in the mitochondrial membrane potential and impairment of the electron transport chain led to increased levels of apoptosis and reactive oxygen species. These results correlated with viral pathogenicity loss and may help provide an understanding of the key mechanism of lentiviral attenuation.IMPORTANCE Following viral infection, the working pattern and function of the cell can be transformed through the impact on mitochondria. It still unknown how the mitochondrial response changes in cells infected with viruses in the process of virulence attenuation. EIAV_DLV121 is the only effective lentiviral vaccine for large-scale use in the world. EIAV_DLV34 is the parent pathogenic strain. Unlike EIAV_DLV34-induced inflammation storms, EIAV_DLV121 can induce high levels of apoptosis. For the first time, we found that, after the mitochondrial protein expression profile is altered, EIAV_DLV34-infected cells are transformed into M1-polarized-type macrophages and cause inflammatory injury and that the intrinsic apoptosis pathway is activated in EIAV_DLV121-infected cells. These studies shed light on how the mitochondrial protein expression profile changes between cells infected by pathogenic lentivirus strains and cells infected by attenuated lentivirus strains to drive different cellular responses, especially from inflammation to apoptosis.

The use of site-specific suppressors to measure the relative contributions of different mitochondrial sites to skeletal muscle superoxide and hydrogen peroxide production

Reactive oxygen species are important signaling molecules crucial for muscle differentiation and adaptation to exercise. However, their uncontrolled generation is associated with an array of pathological conditions. To identify and quantify the sources of superoxide and hydrogen peroxide in skeletal muscle we used site-specific suppressors (S1QELs, S3QELs and NADPH oxidase inhibitors). We measured the rates of hydrogen peroxide release from isolated rat muscle mitochondria incubated in media mimicking the cytosol of intact muscle. By measuring the extent of inhibition caused by the addition of different site-specific suppressors of mitochondrial superoxide/hydrogen peroxide production (S1QELs for site I_Q and S3QELs for site III_Qo), we determined the contributions of these sites to the total signal. In media mimicking resting muscle, their contributions were each 12–18%, consistent with a previous method. In C2C12 myoblasts, site I_Q contributed 12% of cellular hydrogen peroxide production and site III_Qo contributed about 30%. When C2C12 myoblasts were differentiated to myotubes, hydrogen peroxide release increased five-fold, and the proportional contribution of site I_Q doubled. The use of S1QELs and S3QELs is a powerful new way to measure the relative contributions of different mitochondrial sites to muscle hydrogen peroxide production under different conditions. Our results show that mitochondrial sites I_Q and III_Qo make a substantial contribution to superoxide/hydrogen peroxide production in muscle mitochondria and C2C12 myoblasts. The total hydrogen peroxide release rate and the relative contribution of site I_Q both increase substantially upon differentiation to myotubes.

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S1QELs, S3QELs and NOX inhibitors report sites of superoxide/H₂O₂ generation.

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Mitochondria and NOXs are the major sources of H₂O₂ in C2C12 cells.

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H₂O₂ release increases 5-fold during differentiation of C2C12 myoblasts to myotubes.

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The relative contribution of site I_Q doubles during differentiation.

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The relative contributions of site III_Qo and NOXs remain the same.

Radical Stress Is More Cytotoxic in the Nucleus than in Other Organelles

Cells are exposed to reactive oxygen species (ROS) as a by-product of mitochondrial metabolism, especially under hypoxia. ROS are also enzymatically generated at the plasma membrane during inflammation. Radicals cause cellular damage leading to cell death, as they react indiscriminately with surrounding lipids, proteins, and nucleotides. However, ROS are also important for many physiological processes, including signaling, pathogen killing and chemotaxis. The sensitivity of cells to ROS therefore likely depends on the subcellular location of ROS production, but how this affects cell viability is poorly understood. As ROS generation consumes oxygen, and hypoxia-mediated signaling upregulates expression of antioxidant transcription factor Nrf2, it is difficult to discern hypoxic from radical stress. In this study, we developed an optogenetic toolbox for organelle-specific generation of ROS using the photosensitizer protein SuperNova which produces superoxide anion upon excitation with 590 nm light. We fused SuperNova to organelle specific localization signals to induce ROS with high precision. Selective ROS production did not affect cell viability in most organelles except for the nucleus. SuperNova is a promising tool to induce locally targeted ROS production, opening up new possibilities to investigate processes and organelles that are affected by localized ROS production.