The interaction of Q analogs, particularly hydroxydecyl benzoquinone (idebenone), with the respiratory complexes of heart mitochondria

Genetic variants affecting NQO1 protein levels impact the efficacy of idebenone treatment in Leber hereditary optic neuropathy

Idebenone, the only approved treatment for Leber hereditary optic neuropathy (LHON), promotes recovery of visual function in up to 50% of patients, but we can neither predict nor understand the non-responders. Idebenone is reduced by the cytosolic NAD(P)H oxidoreductase I (NQO1) and directly shuttles electrons to respiratory complex III, bypassing complex I affected in LHON. We show here that two polymorphic variants drastically reduce NQO1 protein levels when homozygous or compound heterozygous. This hampers idebenone reduction. In its oxidized form, idebenone inhibits complex I, decreasing respiratory function in cells. By retrospectively analyzing a large cohort of idebenone-treated LHON patients, classified by their response to therapy, we show that patients with homozygous or compound heterozygous NQO1 variants have the poorest therapy response, particularly if carrying the m.3460G>A/MT-ND1 LHON mutation. These results suggest consideration of patient NQO1 genotype and mitochondrial DNA mutation in the context of idebenone therapy.

p66Shc in Cardiovascular Pathology

p66Shc is a widely expressed protein that governs a variety of cardiovascular pathologies by generating, and exacerbating, pro-apoptotic ROS signals. Here, we review p66Shc’s connections to reactive oxygen species, expression, localization, and discuss p66Shc signaling and mitochondrial functions. Emphasis is placed on recent p66Shc mitochondrial function discoveries including structure/function relationships, ROS identity and regulation, mechanistic insights, and how p66Shc-cyt c interactions can influence p66Shc mitochondrial function. Based on recent findings, a new p66Shc mitochondrial function model is also put forth wherein p66Shc acts as a rheostat that can promote or antagonize apoptosis. A discussion of how the revised p66Shc model fits previous findings in p66Shc-mediated cardiovascular pathology follows.

Coenzyme Q10 effects in neurological diseases

Coenzyme Q10 (CoQ10), a lipophilic substituted benzoquinone, is present in animal and plant cells. It is endogenously synthetized in every cell and involved in a variety of cellular processes. CoQ10 is an obligatory component of the respiratory chain in inner mitochondrial membrane. In addition, the presence of CoQ10 in all cellular membranes and in blood. It is the only endogenous lipid antioxidant. Moreover, it is an essential factor for uncoupling protein and controls the permeability transition pore in mitochondria. It also participates in extramitochondrial electron transport and controls membrane physicochemical properties. CoQ10 effects on gene expression might affect the overall metabolism. Primary changes in the energetic and antioxidant functions can explain its remedial effects. CoQ10 supplementation is safe and well-tolerated, even at high doses. CoQ10 does not cause any serious adverse effects in humans or experimental animals. New preparations of CoQ10 that are less hydrophobic and structural derivatives, like idebenone and MitoQ, are being developed to increase absorption and tissue distribution. The review aims to summarize clinical and experimental effects of CoQ10 supplementations in some neurological diseases such as migraine, Parkinson´s disease, Huntington´s disease, Alzheimer´s disease, amyotrophic lateral sclerosis, Friedreich´s ataxia or multiple sclerosis. Cardiovascular hypertension was included because of its central mechanisms controlling blood pressure in the brainstem rostral ventrolateral medulla and hypothalamic paraventricular nucleus. In conclusion, it seems reasonable to recommend CoQ10 as adjunct to conventional therapy in some cases. However, sometimes CoQ10 supplementations are more efficient in animal models of diseases than in human patients (e.g. Parkinson´s disease) or rather vague (e.g. Friedreich´s ataxia or amyotrophic lateral sclerosis).

The Present and Future of Mitochondrial-Based Therapeutics for Eye Disease

Translational Relevance

Mitochondria are viable therapeutic targets for a broad spectrum of ocular diseases.

Failed, Interrupted, or Inconclusive Trials on Neuroprotective and Neuroregenerative Treatment Strategies in Multiple Sclerosis: Update 2015-2020

In the recent past, a plethora of drugs have been approved for the treatment of multiple sclerosis (MS). These therapeutics are mainly confined to immunomodulatory or immunosuppressive strategies but do not sufficiently address remyelination and neuroprotection. However, several neuroregenerative agents have shown potential in pre-clinical research and entered Phase I to III clinical trials. Although none of these compounds have yet proceeded to approval, understanding the causes of failure can broaden our knowledge about neuroprotection and neuroregeneration in MS. Moreover, most of the investigated approaches are characterised by consistent mechanisms of action and proved convincing efficacy in animal studies. Therefore, learning from their failure will help us to enforce the translation of findings acquired in pre-clinical studies into clinical application. Here, we summarise trials on MS treatment published since 2015 that have either failed or were interrupted due to a lack of efficacy, adverse events, or for other reasons. We further outline the rationale underlying these drugs and analyse the background of failure to gather new insights into MS pathophysiology and optimise future study designs. For conciseness, this review focuses on agents promoting remyelination and medications with primarily neuroprotective properties or unconventional approaches. Failed clinical trials that pursue immunomodulation are presented in a separate article.

Coenzyme Q10 Analogues: Benefits and Challenges for Therapeutics

Coenzyme Q₁₀ (CoQ₁₀ or ubiquinone) is a mobile proton and electron carrier of the mitochondrial respiratory chain with antioxidant properties widely used as an antiaging health supplement and to relieve the symptoms of many pathological conditions associated with mitochondrial dysfunction. Even though the hegemony of CoQ₁₀ in the context of antioxidant-based treatments is undeniable, the future primacy of this quinone is hindered by the promising features of its numerous analogues. Despite the unimpeachable performance of CoQ₁₀ therapies, problems associated with their administration and intraorganismal delivery has led clinicians and scientists to search for alternative derivative molecules. Over the past few years, a wide variety of CoQ₁₀ analogues with improved properties have been developed. These analogues conserve the antioxidant features of CoQ₁₀ but present upgraded characteristics such as water solubility or enhanced mitochondrial accumulation. Moreover, recent studies have proven that some of these analogues might even outperform CoQ₁₀ in the treatment of certain specific diseases. The aim of this review is to provide detailed information about these Coenzyme Q₁₀ analogues, as well as their functionality and medical applications.

Idebenone: When an antioxidant is not an antioxidant

Idebenone is a well described drug that was initially developed against dementia. The current literature widely portrays this molecule as a potent antioxidant and CoQ₁₀ analogue. While numerous papers seem to support this view, a closer look indicates that the pharmacokinetics of idebenone do not support these claims. A major discrepancy between achievable tissue levels, especially in target tissues such as the brain, and doses required to show the proposed effects, significantly questions our current understanding. This review explains how this has happened and highlights the discrepancies in the current literature. More importantly, based on some recent discoveries, a new framework is presented that can explain the mode of action of this molecule and can align formerly contradictory results. Finally, this new appreciation of the molecular activities of idebenone provides a rational approach to test idebenone in novel indications that might have not been considered previously for this drug.

Antioxidant Amelioration of Riboflavin Transporter Deficiency in Motoneurons Derived from Patient-Specific Induced Pluripotent Stem Cells

Mitochondrial dysfunction is a key element in the pathogenesis of neurodegenerative disorders, such as riboflavin transporter deficiency (RTD). This is a rare, childhood-onset disease characterized by motoneuron degeneration and caused by mutations in SLC52A2 and SLC52A3, encoding riboflavin (RF) transporters (RFVT2 and RFVT3, respectively), resulting in muscle weakness, ponto-bulbar paralysis and sensorineural deafness. Based on previous findings, which document the contribution of oxidative stress in RTD pathogenesis, we tested possible beneficial effects of several antioxidants (Vitamin C, Idebenone, Coenzyme Q10 and EPI-743, either alone or in combination with RF) on the morphology and function of neurons derived from induced pluripotent stem cells (iPSCs) from two RTD patients. To identify possible improvement of the neuronal morphotype, neurite length was measured by confocal microscopy after β-III tubulin immunofluorescent staining. Neuronal function was evaluated by determining superoxide anion generation by MitoSOX assay and intracellular calcium (Ca2+) levels, using the Fluo-4 probe. Among the antioxidants tested, EPI-743 restored the redox status, improved neurite length and ameliorated intracellular calcium influx into RTD motoneurons. In conclusion, we suggest that antioxidant supplementation may have a role in RTD treatment.

The ying and yang of idebenone: Not too little, not too much - cell death in NQO1 deficient cells and the mouse retina

Idebenone has recently been investigated as a drug therapy for Leber's hereditary optic neuropathy (LHON), a rare genetic mitochondrial disease that causes rapid and progressive bilateral vision loss. Although several studies have shown that idebenone can promote vision recovery in patients with LHON, the evidence for the efficacy of idebenone is still limited. Idebenone failed to demonstrate superiority over placebo in the primary end-points of the only published randomised, double-blind, placebo-controlled trial. There appears to be a patient-specific response to idebenone with high variability in therapeutic outcomes. A recent study suggested that the cytosolic enzyme NAD(P)H: quinone acceptor oxidoreductase (NQO1) is the major enzyme involved in the activation of idebenone, and the beneficial effects of idebenone are dependent on the expression of NQO1. Here, we confirm the NQO1-dependent activity of idebenone, but we also show, for the first time, that the cytotoxicity of idebenone is linked to cellular expression of NQO1. Upon idebenone administration, cells deficient in NQO1 show a marked decrease in viability in comparison to NQO1 expressing cells, with idebenone causing ROS production and deleterious effects on ATP levels and cell viability. In addition, our data highlights that only cells expressing NQO1 can significantly activate idebenone, indicating that other proposed metabolic activation pathways, such as complex II and glycerol-3-phosphate dehydrogenase, do not play a significant role in idebenone activation. Furthermore, we provide evidence of idebenone-induced toxicity in the retina ex-vivo, which can be explained by the variation of NQO1 expression between different cell types in the mouse retina. Idebenone mediated cell rescue in the rotenone ex vivo model also indicated that this drug has a narrow therapeutic window. These findings will help to guide the development of future therapies and drug delivery strategies including intra-ocular administration. The specific dependence of idebenone activity on NQO1 may also explain the variation in patient outcomes in clinical trials.

Idebenone Has Distinct Effects on Mitochondrial Respiration in Cortical Astrocytes Compared to Cortical Neurons Due to Differential NQO1 Activity

Idebenone is a synthetic quinone that on reduction in cells can bypass mitochondrial Complex I defects by donating electrons to Complex III. The drug is used clinically to treat the Complex I disease Leber's hereditary optic neuropathy (LHON), but has been less successful in clinical trials for other neurodegenerative diseases. NAD(P)H:quinone oxidoreductase 1 (NQO1) appears to be the main intracellular enzyme catalyzing idebenone reduction. However, NQO1 is not universally expressed by cells of the brain. Using primary rat cortical cells pooled from both sexes, we tested the hypotheses that the level of endogenous NQO1 activity limits the ability of neurons, but not astrocytes, to use idebenone as an electron donor to support mitochondrial respiration. We then tested the prediction that NQO1 induction by pharmacological activation of the transcription factor nuclear erythroid 2-related factor 2 (Nrf2) enables idebenone to bypass Complex I in cells with poor NQO1 expression. We found that idebenone stimulated respiration by astrocytes but reduced the respiratory capacity of neurons. Importantly, idebenone supported mitochondrial oxygen consumption in the presence of a Complex I inhibitor in astrocytes but not neurons, and this ability was reversed by inhibiting NQO1. Conversely, recombinant NQO1 delivery to neurons prevented respiratory impairment and conferred Complex I bypass activity. Nrf2 activators failed to increase NQO1 in neurons, but carnosic acid induced NQO1 in COS-7 cells that expressed little endogenous enzyme. Carnosic acid-idebenone combination treatment promoted NQO1-dependent Complex I bypass activity in these cells. Thus, combination drug strategies targeting NQO1 may promote the repurposing of idebenone for additional disorders.SIGNIFICANCE STATEMENT Idebenone is used clinically to treat loss of visual acuity in Leber's hereditary optic neuropathy. Clinical trials for several additional diseases have failed. This study demonstrates a fundamental difference in the way idebenone affects mitochondrial respiration in cortical neurons compared with cortical astrocytes. Cortical neurons are unable to use idebenone as a direct mitochondrial electron donor due to NQO1 deficiency. Our results suggest that idebenone behaves as an NQO1-dependent prodrug, raising the possibility that lack of neuronal NQO1 activity has contributed to the limited efficacy of idebenone in neurodegenerative disease treatment. Combination therapy with drugs able to safely induce NQO1 in neurons, as well as other brain cell types, may be able to unlock the neuroprotective therapeutic potential of idebenone or related quinones.

The NRF2 Signaling Network Defines Clinical Biomarkers and Therapeutic Opportunity in Friedreich's Ataxia

Friedreich’s ataxia (FA) is a trinucleotide repeats expansion neurodegenerative disorder, for which no cure or approved therapies are present. In most cases, GAA trinucleotide repetitions in the first intron of the FXN gene are the genetic trigger of FA, determining a strong reduction of frataxin, a mitochondrial protein involved in iron homeostasis. Frataxin depletion impairs iron–sulfur cluster biosynthesis and determines iron accumulation in the mitochondria. Mounting evidence suggests that these defects increase oxidative stress susceptibility and reactive oxygen species production in FA, where the pathologic picture is worsened by a defective regulation of the expression and signaling pathway modulation of the transcription factor NF-E2 p45-related factor 2 (NRF2), one of the fundamental mediators of the cellular antioxidant response. NRF2 protein downregulation and impairment of its nuclear translocation can compromise the adequate cellular response to the frataxin depletion-dependent redox imbalance. As NRF2 stability, expression, and activation can be modulated by diverse natural and synthetic compounds, efforts have been made in recent years to understand if regulating NRF2 signaling might ameliorate the pathologic defects in FA. Here we provide an analysis of the pharmaceutical interventions aimed at restoring the NRF2 signaling network in FA, elucidating specific biomarkers useful for monitoring therapeutic effectiveness, and developing new therapeutic tools.

Idebenone Protects against Acute Murine Colitis via Antioxidant and Anti-Inflammatory Mechanisms

Oxidative stress is a key player of the inflammatory cascade responsible for the initiation of ulcerative colitis (UC). Although the short chain quinone idebenone is considered a potent antioxidant and a mitochondrial electron donor, emerging evidence suggests that idebenone also displays anti-inflammatory activity. This study evaluated the impact of idebenone in the widely used dextran sodium sulphate (DSS)-induced mouse model of acute colitis. Acute colitis was induced in C57BL/6J mice via continuous exposure to 2.5% DSS over 7 days. Idebenone was co-administered orally at a dose of 200 mg/kg body weight. Idebenone significantly prevented body weight loss and improved the disease activity index (DAI), colon length, and histopathological score. Consistent with its reported antioxidant function, idebenone significantly reduced the colonic levels of malondialdehyde (MDA) and nitric oxide (NO), and increased the expression of the redox factor NAD(P)H (nicotinamide adenine dinucleotide phosphate) dehydrogenase quinone-1 (NQO-1) in DSS-exposed mice. Immunohistochemistry revealed a significantly increased expression of tight junction proteins, which protect and maintain paracellular intestinal permeability. In support of an anti-inflammatory activity, idebenone significantly attenuated the elevated levels of pro-inflammatory cytokines in colon tissue. These results suggest that idebenone could represent a promising therapeutic strategy to interfere with disease pathology in UC by simultaneously inducing antioxidative and anti-inflammatory pathways.

The Role of Mitochondria in Reactive Oxygen Species Generation and Its Implications for Neurodegenerative Diseases

Mitochondria are dynamic cellular organelles that consistently migrate, fuse, and divide to modulate their number, size, and shape. In addition, they produce ATP, reactive oxygen species, and also have a biological role in antioxidant activities and Ca²⁺ buffering. Mitochondria are thought to play a crucial biological role in most neurodegenerative disorders. Neurons, being high-energy-demanding cells, are closely related to the maintenance, dynamics, and functions of mitochondria. Thus, impairment of mitochondrial activities is associated with neurodegenerative diseases, pointing to the significance of mitochondrial functions in normal cell physiology. In recent years, considerable progress has been made in our knowledge of mitochondrial functions, which has raised interest in defining the involvement of mitochondrial dysfunction in neurodegenerative diseases. Here, we summarize the existing knowledge of the mitochondrial function in reactive oxygen species generation and its involvement in the development of neurodegenerative diseases.

CoQ10 supplementation rescues nephrotic syndrome through normalization of H2S oxidation pathway

Nephrotic syndrome (NS), a frequent chronic kidney disease in children and young adults, is the most common phenotype associated with primary coenzyme Q₁₀ (CoQ₁₀) deficiency and is very responsive to CoQ₁₀ supplementation, although the pathomechanism is not clear. Here, using a mouse model of CoQ deficiency-associated NS, we show that long-term oral CoQ₁₀ supplementation prevents kidney failure by rescuing defects of sulfides oxidation and ameliorating oxidative stress, despite only incomplete normalization of kidney CoQ levels and lack of rescue of CoQ-dependent respiratory enzymes activities. Liver and kidney lipidomics, and urine metabolomics analyses, did not show CoQ metabolites. To further demonstrate that sulfides metabolism defects cause oxidative stress in CoQ deficiency, we show that silencing of sulfide quinone oxido-reductase (SQOR) in wild-type HeLa cells leads to similar increases of reactive oxygen species (ROS) observed in HeLa cells depleted of the CoQ biosynthesis regulatory protein COQ8A. While CoQ₁₀ supplementation of COQ8A depleted cells decreases ROS and increases SQOR protein levels, knock-down of SQOR prevents CoQ₁₀ antioxidant effects. We conclude that kidney failure in CoQ deficiency-associated NS is caused by oxidative stress mediated by impaired sulfides oxidation and propose that CoQ supplementation does not significantly increase the kidney pool of CoQ bound to the respiratory supercomplexes, but rather enhances the free pool of CoQ, which stabilizes SQOR protein levels rescuing oxidative stress.

Reduction of 2-methoxy-1,4-naphtoquinone by mitochondrially-localized Nqo1 yielding NAD+ supports substrate-level phosphorylation during respiratory inhibition

Provision of NAD⁺ for oxidative decarboxylation of alpha-ketoglutarate to succinyl-CoA by the ketoglutarate dehydrogenase complex (KGDHC) is critical for maintained operation of succinyl-CoA ligase yielding high-energy phosphates, a process known as mitochondrial substrate-level phosphorylation (mSLP). We have shown previously that when NADH oxidation by complex I is inhibited by rotenone or anoxia, mitochondrial diaphorases yield NAD⁺, provided that suitable quinones are present (Kiss G et al., FASEB J 2014, 28:1682). This allows for KGDHC reaction to proceed and as an extension of this, mSLP. NAD(P)H quinone oxidoreductase 1 (NQO1) is an enzyme exhibiting diaphorase activity. Here, by using Nqo1^-/- and WT littermate mice we show that in rotenone-treated, isolated liver mitochondria 2-methoxy-1,4-naphtoquinone (MNQ) is preferentially reduced by matrix Nqo1 yielding NAD⁺ to KGDHC, supporting mSLP. This process was sensitive to inhibition by specific diaphorase inhibitors. Reduction of idebenone and its analogues MRQ-20 and MRQ-56, menadione, mitoquinone and duroquinone were unaffected by genetic disruption of the Nqo1 gene. The results allow for the conclusions that i) MNQ is a Nqo1-preferred substrate, and ii) in the presence of suitable quinones, mitochondrially-localized diaphorases other than Nqo1 support NADH oxidation when complex I is inhibited. Our work confirms that complex I bypass can occur by quinones reduced by intramitochondrial diaphorases oxidizing NADH, ultimately supporting mSLP. Finally, it may help to elucidate structure-activity relationships of redox-active quinones with diaphorase enzymes.

The idebenone metabolite QS10 restores electron transfer in complex I and coenzyme Q defects

Idebenone is a hydrophilic short-chain coenzyme (Co) Q analogue, which has been used as a potential bypass of defective complex I in both Leber Hereditary Optic Neuropathy and OPA1-dependent Dominant Optic Atrophy. Based on its potential antioxidant effects, it has also been tested in degenerative disorders such as Friedreich's ataxia, Huntington's and Alzheimer's diseases. Idebenone is rapidly modified but the biological effects of its metabolites have been characterized only partially. Here we have studied the effects of quinones generated during in vivo metabolism of idebenone with specific emphasis on 6-(9-carboxynonyl)-2,3-dimethoxy-5-methyl-1,4-benzoquinone (QS10). QS10 partially restored respiration in cells deficient of complex I or of CoQ without inducing the mitochondrial permeability transition, a detrimental effect of idebenone that may offset its potential benefits [Giorgio et al. (2012) Biochim. Biophys. Acta 1817: 363-369]. Remarkably, respiration was largely rotenone-insensitive in complex I deficient cells and rotenone-sensitive in CoQ deficient cells. These findings indicate that, like idebenone, QS10 can provide a bypass to defective complex I; and that, unlike idebenone, QS10 can partially replace endogenous CoQ. In zebrafish (Danio rerio) treated with rotenone, QS10 was more effective than idebenone in allowing partial recovery of respiration (to 40% and 20% of the basal respiration of untreated embryos, respectively) and allowing zebrafish survival (80% surviving embryos at 60 h post-fertilization, a time point at which all rotenone-treated embryos otherwise died). We conclude that QS10 is potentially more active than idebenone in the treatment of diseases caused by complex I defects, and that it could also be used in CoQ deficiencies of genetic and acquired origin.

Influence of substituent heteroatoms on the cytoprotective properties of pyrimidinol antioxidants

Recently, we described the optimization of novel pyrimidinol-based antioxidants as potential therapeutic molecules for targeting mitochondrial diseases. That study focused on improving the potency and metabolic stability of pyrimidinol antioxidants. This led us to consider the possibility of altering the positions of the exocyclic alkoxy and alkylamino substituents on the pyrimidinol scaffold. Twelve new analogues were prepared and their biological activities were investigated. The metabolic stability of the prepared regioisomers was also assessed in vitro using bovine liver microsomes. Unexpectedly, the 2-alkoxy-4-alkylamino substituted pyrimidinol antioxidants were found to have properties in protecting mitochondrial function superior to the isomeric 4-alkoxy-2-alkylamino substituted pyrimidinols evaluated in all earlier studies. This observation suggests a possible mode of action involving the intermediacy of an ortho-iminoquinone, a species not previously associated with mitochondrial respiratory chain function.

Leber Hereditary Optic Neuropathy: Bringing the Lab to the Clinic

Leber hereditary optic neuropathy (LHON) was the first clinically characterized mitochondrial disorder. Since its first description in 1871, much has been discovered regarding the genetics and pathophysiology of the disease. This has enabled the development of in vitro cell and animal models that can be used to try to determine not only the effects of the genetic mutation upon the clinical phenotype but to also test potential novel therapies. Treatments for LHON have ranged from vitamins and minerals to immunosuppressants and, more recently, targeted gene therapy. This article reviews the pathophysiology and clinical features of LHON with a focus on translational research.

Natural Product Screening Reveals Naphthoquinone Complex I Bypass Factors

Deficiency of mitochondrial complex I is encountered in both rare and common diseases, but we have limited therapeutic options to treat this lesion to the oxidative phosphorylation system (OXPHOS). Idebenone and menadione are redox-active molecules capable of rescuing OXPHOS activity by engaging complex I-independent pathways of entry, often referred to as “complex I bypass.” In the present study, we created a cellular model of complex I deficiency by using CRISPR genome editing to knock out Ndufa9 in mouse myoblasts, and utilized this cell line to develop a high-throughput screening platform for novel complex I bypass factors. We screened a library of ~40,000 natural product extracts and performed bioassay-guided fractionation on a subset of the top scoring hits. We isolated four plant-derived 1,4-naphthoquinone complex I bypass factors with structural similarity to menadione: chimaphilin and 3-chloro-chimaphilin from Chimaphila umbellata and dehydro-α-lapachone and dehydroiso-α-lapachone from Stereospermum euphoroides. We also tested a small number of structurally related naphthoquinones from commercial sources and identified two additional compounds with complex I bypass activity: 2-methoxy-1,4-naphthoquinone and 2-methoxy-3-methyl-1,4,-naphthoquinone. The six novel complex I bypass factors reported here expand this class of molecules and will be useful as tool compounds for investigating complex I disease biology.

Both idebenone and idebenol are localized near the lipid-water interface of the membrane and increase its fluidity

Idebenone is a synthetic analog of coenzyme Q; both share a quinone moiety but idebenone has a shorter lipophilic tail ending with a hydroxyl group. Differential scanning calorimetry experiments showed that both idebenone and idebenol widened and shifted the phase transition of 1,2-dipalmitoylphosphatidylcholine (DPPC) to a lower temperature and a phase separation with different concentrations of these molecules was observed. Also small angle X-ray diffraction and wide angle X-ray diffraction revealed that both, idebenone and idebenol, induced laterally separated phases in fluid membranes when included in DPPC membranes. Electronic profiles showed that both forms, idebenone and idebenol, reduced the thickness of the fluid membrane. (2)H NMR measurements showed that the order of the membrane decreased at all temperatures in the presence of idebenone or idebenol, the greatest disorder being observed in the segments of the acyl chains close to the lipid-water interface. (1)H NOESY MAS NMR spectra were obtained using 1-palmitoyl-2-oleoyl-phosphatidylcholine membranes and results pointed to a similar location in the membrane for both forms, with the benzoquinone or benzoquinol rings and their terminal hydroxyl group of the hydrophobic chain located near the lipid/water interface of the phospholipid bilayer and the terminal hydroxyl group of the hydrophobic chain of both compounds located at the lipid/water interface. Taken together, all these different locations might explain the different physiological behavior shown by the idebenone/idebenol compared with the ubiquinone-10/ubiquinol-10 pair in which both compounds are differently localized in the membrane.

A randomized, placebo-controlled trial of the benzoquinone idebenone in a mouse model of OPA1-related dominant optic atrophy reveals a limited therapeutic effect on retinal ganglion cell dendropathy and visual function

Dominant optic atrophy (DOA) arises from mutations in the OPA1 gene that promotes fusion of the inner mitochondrial membrane and plays a role in maintaining ATP levels. Patients display optic disc pallor, retinal ganglion cell (RGC) loss and bilaterally reduced vision. We report a randomized, placebo-controlled trial of idebenone at 2000 mg/kg/day in 56 Opa1 mutant mice (B6;C3-Opa1(Q285STOP)), with RGC dendropathy and visual loss, and 63 wildtype mice. We assessed cellular responses in the retina, brain and liver and RGC morphology, by diolistic labeling, Sholl analysis and quantification of dendritic morphometric features. Vision was assessed by optokinetic responses. ATP levels were raised by 0.57 nmol/mg (97.73%, p=0.035) in brain from idebenone-treated Opa1 mutant mice, but in the liver there was an 80.35% (p=0.011) increase in oxidative damage. NQO1 expression in Opa1 mutant mice was reduced in the brain (to 30.5%, p=0.002) but not in retina, and neither expression level was induced by idebenone. ON-center RGCs failed to show major recovery, other than improvements in secondary dendritic length (by 53.89%, p=0.052) and dendritic territory (by 2.22 × 10(4) μm(2) or 90.24%, p=0.074). An improvement in optokinetic response was observed (by 12.2 ± 3.2s, p=0.003), but this effect was not sustained over time. OFF-center RGCs from idebenone-treated wildtype mice showed shrinkage in total dendritic length by 2.40 mm (48.05%, p=0.025) and a 47.37% diminished Sholl profile (p=0.029). Visual function in wildtype idebenone-treated mice was impaired (2.9 fewer head turns than placebo, p=0.007). Idebenone appears largely ineffective in protecting Opa1 heterozygous RGCs from dendropathy. The detrimental effect of idebenone in wildtype mice has not been previously observed and raises some concerns.

Reduction of Synthetic Ubiquinone QT Catalyzed by Bovine Mitochondrial Complex I Is Decoupled from Proton Translocation

We previously succeeded in site-specific chemical modifications of the inner part of the quinone binding pocket of bovine mitochondrial complex I through ligand-directed tosylate (LDT) chemistry using specific inhibitors as high-affinity ligands for the enzyme [Masuya, T., et al. (2014) Biochemistry 53, 2304-2317, 7816-7823]. To investigate whether a short-chain ubiquinone, in place of these specific inhibitors, serves as a ligand for LDT chemistry, we herein synthesized a LDT reagent QT possessing ubiquinone scaffold and performed LDT chemistry with bovine heart submitochondrial particles (SMP). Detailed proteomic analyses revealed that QT properly guides the tosylate group into the quinone binding pocket and transfers a terminal alkyne to nucleophilic amino acids His150 and Asp160 in the 49 kDa subunit. This result clearly indicates that QT occupies the inner part of the quinone binding pocket. Nevertheless, we noted that QT is a unique electron acceptor from complex I distinct from typical short-chain ubiquinones such as ubiquinone-1 (Q1) for several reasons; for example, QT reduction in NADH-QT oxidoreduction was almost completely insensitive to quinone-site inhibitors (such as bullatacin and piericidin A), and this reaction did not produce a membrane potential. On the basis of detailed comparisons of the electron transfer features between QT and typical short-chain quinones, we conclude that QT may accept electrons from an N2 cluster at a position different from that of typical short-chain quinones because of its unique side-chain structure; accordingly, QT reduction is unable to induce putative structural changes inside the quinone binding pocket, which are critical for driving proton translocation. Thus, QT is the first ubiquinone analogue, to the best of our knowledge, the catalytic reduction of which is decoupled from proton translocation through the membrane domain. Implications for mechanistic studies on QT are also discussed.

Stability-Indicating HPLC Assay for Determination of Idebenone in Pharmaceutical Forms

A stability-indicating method was validated for the determination in pharmaceutical forms of idebenone a coenzyme Q10-like compound. The assay was achieved by liquid chromatography analysis using a reversed-phase C18 column and a detector set at 480 nm. The optimized mobile phase consisted of isocratic flow rate at 1.0 mL/min for 3 min with methanol. The linearity of the assay was demonstrated in the range of 3.0 to 8.0 mg/mL with a correlation coefficient r (2) > 0.998. The limits of detection and quantification were 0.03 and 0.05 mg/mL, respectively. The intraday and interday precisions were less than 1.0%. Accuracy of the method ranged from 98.6 to 101.5% with RSD < 0.6%. Specificity of the assay showed no interference from tablets components and breakdown products formed by alkaline, acidic, oxidative, sunlight, and high temperature conditions. This method allows accurate and reliable determination of idebenone for drug stability assay in pharmaceutical studies.

Border between natural product and drug: comparison of the related benzoquinones idebenone and coenzyme Q10

Coenzyme Q₁₀ is a ubiquitous component of cellular membranes and belongs to the class of benzoquinones that mainly differ with regards to the length and composition of their hydrophobic tail. The characteristic quinone group can accept electrons from various biological sources and is converted by a one electron transfer to the unstable semiquinone or by a two electron transfer to the more stable hydroquinone. This feature makes CoQ₁₀ the bona fide cellular electron transfer molecule within the mitochondrial respiratory chain and also makes it a potent cellular antioxidant. These activities serve as justification for its popular use as food supplement. Another quinone with similarities to the naturally occurring CoQ₁₀ is idebenone, which shares its quinone moiety with CoQ₁₀, but at the same time differs from CoQ₁₀ by the presence of a much shorter, less lipophilic tail. However, despite its similarity to CoQ₁₀, idebenone cannot be isolated from any natural sources but instead was synthesized and selected as a pharmacologically active compound in the 1980s by Takeda Pharmaceuticals purely based on its pharmacological properties. Several recent clinical trials demonstrated some therapeutic efficacy of idebenone in different indications and as a consequence, many practitioners question if the freely available CoQ₁₀ could not be used instead. Here, we describe the molecular and pharmacological features of both molecules that arise from their structural differences to answer the question if idebenone is merely a CoQ₁₀ analogue as frequently perpetuated in the literature or a pharmaceutical drug with entirely different features.

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The benzoquinones CoQ₁₀ and idebenone have vastly different solubility.

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Both molecules need to get activated by cellular reductases.

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Due to their solubility both molecules are in different cellular compartments.

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Therefore, both quinones are activated by different enzymes.

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Thus, their solubility strongly determines their biological activities.

Idebenone and neuroprotection: antioxidant, pro-oxidant, or electron carrier?

Ubiquinone, commonly called coenzyme Q10 (CoQ), is a lipophilic electron carrier and endogenous antioxidant found in all cellular membranes. In the mitochondrial inner membrane it transfers electrons to complex III of the electron transport chain. The short chain CoQ analogue idebenone is in clinical trials for a number of diseases that exhibit a mitochondrial etiology. Nevertheless, evidence that idebenone ameliorates neurological symptoms in human disease is inconsistent. Although championed as an antioxidant, idebenone can also act as a pro-oxidant by forming an unstable semiquinone at complex I. The antioxidant function of idebenone is critically dependent on two-electron reduction to idebenol without the creation of unstable intermediates. Recently, cytoplasmic

NAD(P)H

quinone oxidoreductase 1 (NQO1) was identified as a major enzyme catalyzing idebenone reduction. While reduction allows idebenone to act as an antioxidant, evidence also suggests that NQO1 enables idebenone to shuttle reducing equivalents from cytoplasmic NAD(P)H to mitochondrial complex III, bypassing any upstream damage to the electron transport chain. In this mini-review we discuss how idebenone can influence mitochondrial function within the context of cytoprotection. Importantly, in the brain NQO1 is expressed primarily by glia rather than neurons. As NQO1 is an inducible enzyme regulated by oxidative stress and the nuclear factor erythroid 2-related factor 2 (Nrf2)/antioxidant response element (ARE) pathway, optimizing NQO1 expression in appropriate cell types within a specific disease context may be key to delivering on idebenone's therapeutic potential.

Treatment of mitochondrial disorders

OPINION STATEMENT

While numerous treatments for mitochondrial disorders have been suggested, relatively few have undergone controlled clinical trials. Treatment of these disorders is challenging, as only symptomatic therapy is available. In this review we will focus on newer drugs and treatment trials in mitochondrial diseases, with a special focus on medications to avoid in treating epilepsy and ICU patient with mitochondrial disease, which has not been included in such a review. Readers are also referred to the opinion statement in A Modern Approach to the Treatment of Mitochondrial Disease published in Current Treatment Options in Neurology 2009. Many of the supplements used for treatment were reviewed in the previous abstract, and dosing guidelines were provided. The focus of this review is on items not previously covered in depth, and our discussion includes more recently studied compounds as well as any relevant updates on older compounds . We review a variety of vitamins and xenobiotics, including dichloroacetate (DCA), arginine, coenzyme Q10, idebenone, EPI-743, and exercise training. Treatment of epilepsy, which is a common feature in many mitochondrial phenotypes, warrants special consideration due to the added toxicity of certain medications, and we provide a discussion of these unique treatment challenges. Interesting, however, with only a few exceptions, the treatment strategies for epilepsy in mitochondrial cytopathies are the same as for epilepsy without mitochondrial dysfunction. We also discuss intensive care management, building upon similar reviews, adding new dimensions, and demonstrating the complexity of overall care of these patients.

Mitochondrial enhancement for neurodegenerative movement disorders: a systematic review of trials involving creatine, coenzyme Q10, idebenone and mitoquinone

BACKGROUND

Neurodegenerative movement disorders mainly include Parkinson's disease (PD), atypical parkinsonisms, Huntington's disease (HD), and Friedreich's ataxia (FA). With mitochondrial dysfunction observed in these diseases, mitochondrial enhancement such as creatine, coenzyme Q10 (CoQ10) and its analogues (idebenone and mitoquinone) has been regarded as a potential treatment.

AIM

In this paper, we systematically analysed and summarized the efficacy of mitochondrial enhancement in improving motor and other symptoms in neurodegenerative movement disorders.

METHODS

We searched the electronic databases PubMed, EMBASE, CINAHL, Cochrane Library and China National Knowledge Infrastructure until September 2013 for eligible randomized controlled trials (RCTs), as well as unpublished and ongoing trials. We calculated the mean differences for continuous data with 95% confidence intervals and pooled the results using a fixed-effect model, if no significant statistical heterogeneity was found (I(2) < 50%).

RESULTS

We included 16 studies with 1,557 randomized patients, which compared creatine, CoQ10 or its analogues with placebo in motor and other symptoms. No significant improvements were found in the motor symptoms of PD, atypical parkinsonisms or HD patients, while only the high dose of idebenone seems to be promising for motor improvement in FA. Certain benefits are found in other symptoms.

CONCLUSIONS

There is insufficient evidence to support the use of mitochondrial enhancement in patients with neurodegenerative movement disorders. More well-designed RCTs with large samples are required for further confirmation.

Nutritional recommendations for cardiovascular disease prevention

Lifestyle factors, including nutrition, play an important role in the etiology of Cardiovascular Disease (CVD). This position paper, written by collaboration between the Israel Heart Association and the Israel Dietetic Association, summarizes the current, preferably latest, literature on the association of nutrition and CVD with emphasis on the level of evidence and practical recommendations. The nutritional information is divided into three main sections: dietary patterns, individual food items, and nutritional supplements. The dietary patterns reviewed include low carbohydrate diet, low-fat diet, Mediterranean diet, and the DASH diet. Foods reviewed in the second section include: whole grains and dietary fiber, vegetables and fruits, nuts, soy, dairy products, alcoholic drinks, coffee and caffeine, tea, chocolate, garlic, and eggs. Supplements reviewed in the third section include salt and sodium, omega-3 and fish oil, phytosterols, antioxidants, vitamin D, magnesium, homocysteine-reducing agents, and coenzyme Q10.

Co-enzyme Q10 and idebenone use in Friedreich's ataxia

Friedreich's ataxia is a debilitating progressive neurodegenerative disease associated with cardiomyopathy and other features. The underlying cause is a deficiency of the mitochondrial protein frataxin which causes mitochondrial iron deposition, increased oxidative stress and impaired adenosine triphosphate production. Over the last 15 years, multiple clinical trials have assessed the efficacy of antioxidant agents in this disease. This article reviews trials of the two most important agents, namely co-enzyme Q10 and idebenone.

Synthesis of pyrido[2,1-a]isoquinolin-4-ones and oxazino[2,3-a]isoquinolin-4-ones: new inhibitors of mitochondrial respiratory chain

Benzo[a]quinolizine is an important heterocyclic framework that can be found in numerous bioactive compounds. The general scheme for the synthesis of these compounds was based on the preparation of the appropriate dihydroisoquinolines by Bischler-Napieralski cyclization with good yields, followed by the Pemberton method to form the oxazinones or pyridones derivatives via acyl-ketene imine cyclocondensation. All the synthesized compounds were assayed in vitro for their ability to inhibit mitochondrial respiratory chain. Most of the tested compounds were able to inhibit the integrated electron transfer chain, measured as NADH oxidation, which includes complexes I, III and IV, in the low micromolar range. Oxazino[2,3-a]isoquinolin-4-ones displayed greater activity than their pyrido[2,1-a]isoquinolin-4-ones analogs. Indeed, the presence of a furan ring in C₂ position of oxazino[2,3-a]isoquinolin-4-ones provided the compound (1g) with the most potent biological activity. Therefore, these compounds and especially the oxazinone derivatives are in the tendency of the new less toxic antitumor agents that target mitochondrial electron transport chain in a middle range potency.

Effects of cytoprotective antioxidants on lymphocytes from representative mitochondrial neurodegenerative diseases

Two new aza analogues of the neuroprotective agent idebenone have been synthesized and characterized. Their antioxidant activity, and ability to augment ATP levels have been evaluated in several different cell lines having suboptimal mitochondrial function. Both compounds were found to be good ROS scavengers, and to protect the cells from oxidative stress induced by glutathione depletion. The compounds were more effective than idebenone in neurodegenerative disease cells. These novel pyrimidinol derivatives were also shown to augment ATP levels in coenzyme Q(10)-deficient human lymphocytes. The more lipophilic side chains attached to the pyrimidinol redox core in these compounds resulted in less inhibition of the electron transport chain and improved antioxidant activity.

Features of idebenone and related short-chain quinones that rescue ATP levels under conditions of impaired mitochondrial complex I

Short-chain quinones have been investigated as therapeutic molecules due to their ability to modulate cellular redox reactions, mitochondrial electron transfer and oxidative stress, which are pathologically altered in many mitochondrial and neuromuscular disorders. Recently, we and others described that certain short-chain quinones are able to bypass a deficiency in complex I by shuttling electrons directly from the cytoplasm to complex III of the mitochondrial respiratory chain to produce ATP. Although this energy rescue activity is highly interesting for the therapy of disorders associated with complex I dysfunction, no structure-activity-relationship has been reported for short-chain quinones so far. Using a panel of 70 quinones, we observed that the capacity for this cellular energy rescue as well as their effect on lipid peroxidation was influenced more by the physicochemical properties (in particular logD) of the whole molecule than the quinone moiety itself. Thus, the observed correlations allow us to explain the differential biological activities and therapeutic potential of short-chain quinones for the therapy of disorders associated with mitochondrial complex I dysfunction and/or oxidative stress.

Initial experience in the treatment of inherited mitochondrial disease with EPI-743

Inherited mitochondrial respiratory chain disorders are progressive, life-threatening conditions for which there are limited supportive treatment options and no approved drugs. Because of this unmet medical need, as well as the implication of mitochondrial dysfunction as a contributor to more common age-related and neurodegenerative disorders, mitochondrial diseases represent an important therapeutic target. Thirteen children and one adult with genetically-confirmed mitochondrial disease (polymerase γ deficiency, n=4; Leigh syndrome, n=4; MELAS, n=3; mtDNA deletion syndrome, n=2; Friedreich ataxia, n=1) at risk for progressing to end-of-life care within 90 days were treated with EPI-743, a novel para-benzoquinone therapeutic, in a subject controlled, open-label study. Serial measures of safety and efficacy were obtained that included biochemical, neurological, quality-of-life, and brain redox assessments using technetium-99m-hexamethylpropyleneamine oxime (HMPAO) single photon emission computed tomography (SPECT) radionuclide imaging. Twelve patients treated with EPI-743 have survived; one polymerase γ deficiency patient died after developing pneumonia and one patient with Surf-1 deficiency died after completion of the protocol. Of the 12 survivors, 11 demonstrated clinical improvement, with 3 showing partial relapse, and 10 of the survivors also had an improvement in quality-of-life scores at the end of the 13-week emergency treatment protocol. HMPAO SPECT scans correlated with clinical response; increased regional and whole brain HMPAO uptake was noted in the clinical responders and the one subject who did not respond clinically had decreased regional and whole brain HMPAO uptake. EPI-743 has modified disease progression in >90% of patients in this open-label study as assessed by clinical, quality-of-life, and non-invasive brain imaging parameters. Data obtained herein suggest that EPI-743 may represent a new drug for the treatment of inherited mitochondrial respiratory chain disorders. Prospective controlled trials will be undertaken to substantiate these initial promising observations. Furthermore, HMPAO SPECT imaging may be a valuable tool for the detection of central nervous system redox defects and for monitoring response to treatments directed at modulating abnormal redox.

Idebenone increases mitochondrial complex I activity in fibroblasts from LHON patients while producing contradictory effects on respiration

BACKGROUND

Leber's hereditary optic neuropathy (LHON) is caused by mutations in the complex I subunits of the respiratory chain. Although patients have been treated with idebenone since 1992, the efficacy of the drug is still a matter of debate.

METHODS

We evaluated the effect of idebenone in fibroblasts from LHON patients using enzymatic and polarographic measurements.

RESULTS

Complex I activity was 42% greater in treated fibroblasts compared to controls (p = 0.002). Despite this complex I activity improvement, the effects on mitochondrial respiration were contradictory, leading to impairment in some cases and stimulation in others.

CONCLUSION

These results indicate that idebenone is able to compensate the complex I deficiency in LHON patient cells with variable effects on respiration, indicating that the patients might not be equally likely to benefit from the treatment.

The effects of idebenone on mitochondrial bioenergetics

We have studied the effects of idebenone on mitochondrial function in cybrids derived from one normal donor (HQB17) and one patient harboring the G3460A/MT-ND1 mutation of Leber's Hereditary Optic Neuropathy (RJ206); and in XTC.UC1 cells bearing a premature stop codon at aminoacid 101 of MT-ND1 that hampers complex I assembly. Addition of idebenone to HQB17 cells caused mitochondrial depolarization and NADH depletion, which were inhibited by cyclosporin (Cs) A and decylubiquinone, suggesting an involvement of the permeability transition pore (PTP). On the other hand, addition of dithiothreitol together with idebenone did not cause PTP opening and allowed maintenance of the mitochondrial membrane potential even in the presence of rotenone. Addition of dithiothreitol plus idebenone, or of idebenol, to HQB17, RJ206 and XTC.UC1 cells sustained membrane potential in intact cells and ATP synthesis in permeabilized cells even in the presence of rotenone and malonate, and restored a good level of coupled respiration in complex I-deficient XTC.UC1 cells. These findings demonstrate that idebenol can feed electrons at complex III. If the quinone is maintained in the reduced state, a task that in some cell types appears to be performed by dicoumarol-sensitive NAD(P)H:quinone oxidoreductase 1 [Haefeli et al. (2011) PLoS One 6, e17963], electron transfer to complex III may allow reoxidation of NADH in complex I deficiencies.

New developments on the functions of coenzyme Q in mitochondria

The notion of a mobile pool of coenzyme Q (CoQ) in the lipid bilayer has changed with the discovery of respiratory supramolecular units, in particular the supercomplex comprising complexes I and III; in this model, the electron transfer is thought to be mediated by tunneling or microdiffusion, with a clear kinetic advantage on the transfer based on random collisions. The CoQ pool, however, has a fundamental function in establishing a dissociation equilibrium with bound quinone, besides being required for electron transfer from other dehydrogenases to complex III. The mechanism of CoQ reduction by complex I is analyzed regarding recent developments on the crystallographic structure of the enzyme, also in relation to the capacity of complex I to generate superoxide. Although the mechanism of the Q-cycle is well established for complex III, involvement of CoQ in proton translocation by complex I is still debated. Some additional roles of CoQ are also examined, such as the antioxidant effect of its reduced form and the capacity to bind the permeability transition pore and the mitochondrial uncoupling proteins. Finally, a working hypothesis is advanced on the establishment of a vicious circle of oxidative stress and supercomplex disorganization in pathological states, as in neurodegeneration and cancer.

Metabolic manipulators: a well founded strategy to combat mitochondrial dysfunction

Whilst the pathophysiology and genetics of mitochondrial disease are slowly being unraveled, currently no effective remedy for mitochondrial disorders is available. One particular strategy in mitochondrial medicine presently under study is metabolic manipulation. This approach is aimed at counteracting the deranged cell biological homeostasis caused by mitochondrial dysfunction, using dietary modifications or small molecule therapy. Cell biological alterations caused by mitochondrial dysfunction include increased reactive oxygen species production, enhanced lipid peroxidation and altered cellular calcium homeostasis. This review covers the five principles of metabolic manipulation: (1) prevention of oxidative damage by reactive oxygen species, (2) amelioration of lipid peroxidation, (3) correction of altered membrane potential, (4) restoration of calcium homeostasis, and (5) transcription regulation interference. We hypothesize that a combination of compounds targeting different metabolic pathways will abolish cellular disturbance arising as a consequences of mitochondrial dysfunction, and thereby improve or stabilize clinical features. However, only a handful of compounds have reached efficacy testing in mammals, and it remains unknown to what extent metabolic manipulation will affect the whole organism. Until a potent remedy is found, patients will remain dependent on supportive, not curative, interventions.

NQO1-dependent redox cycling of idebenone: effects on cellular redox potential and energy levels

Short-chain quinones are described as potent antioxidants and in the case of idebenone have already been under clinical investigation for the treatment of neuromuscular disorders. Due to their analogy to coenzyme Q10 (CoQ10), a long-chain quinone, they are widely regarded as a substitute for CoQ10. However, apart from their antioxidant function, this provides no clear rationale for their use in disorders with normal CoQ10 levels. Using recombinant NAD(P)H:quinone oxidoreductase (NQO) enzymes, we observed that contrary to CoQ10 short-chain quinones such as idebenone are good substrates for both NQO1 and NQO2. Furthermore, the reduction of short-chain quinones by NQOs enabled an antimycin A-sensitive transfer of electrons from cytosolic NAD(P)H to the mitochondrial respiratory chain in both human hepatoma cells (HepG2) and freshly isolated mouse hepatocytes. Consistent with the substrate selectivity of NQOs, both idebenone and CoQ1, but not CoQ10, partially restored cellular ATP levels under conditions of impaired complex I function. The observed cytosolic-mitochondrial shuttling of idebenone and CoQ1 was also associated with reduced lactate production by cybrid cells from mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes (MELAS) patients. Thus, the observed activities separate the effectiveness of short-chain quinones from the related long-chain CoQ10 and provide the rationale for the use of short-chain quinones such as idebenone for the treatment of mitochondrial disorders.

Emerging therapeutic approaches to mitochondrial diseases

Mitochondrial diseases are very heterogeneous and can affect different tissues and organs. Moreover, they can be caused by genetic defects in either nuclear or mitochondrial DNA as well as by environmental factors. All of these factors have made the development of therapies difficult. In this review article, we will discuss emerging approaches to the therapy of mitochondrial disorders, some of which are targeted to specific conditions whereas others may be applicable to a more diverse group of patients.

Coenzyme Q and mitochondrial disease

Coenzyme Q(10) (CoQ(10)) is an essential electron carrier in the mitochondrial respiratory chain and an important antioxidant. Deficiency of CoQ(10) is a clinically and molecularly heterogeneous syndrome, which, to date, has been found to be autosomal recessive in inheritance and generally responsive to CoQ(10) supplementation. CoQ(10) deficiency has been associated with five major clinical phenotypes: (1) encephalomyopathy, (2) severe infantile multisystemic disease, (3) cerebellar ataxia, (4) isolated myopathy, and (5) nephrotic syndrome. In a few patients, pathogenic mutations have been identified in genes involved in the biosynthesis of CoQ(10) (primary CoQ(10) deficiencies) or in genes not directly related to CoQ(10) biosynthesis (secondary CoQ(10) deficiencies). Respiratory chain defects, ROS production, and apoptosis contribute to the pathogenesis of primary CoQ(10) deficiencies. In vitro and in vivo studies are necessary to further understand the pathogenesis of the disease and to develop more effective therapies.

Electrophysiology of respiratory chain complexes and the ADP-ATP exchanger in native mitochondrial membranes

Transport of protons and solutes across mitochondrial membranes is essential for many physiological processes. However, neither the proton-pumping respiratory chain complexes nor the mitochondrial secondary active solute transport proteins have been characterized electrophysiologically in their native environment. In this study, solid-supported membrane (SSM) technology was applied for electrical measurements of respiratory chain complexes CI, CII, CIII, and CIV, the F(O)F(1)-ATPase/synthase (CV), and the adenine nucleotide translocase (ANT) in inner membranes of pig heart mitochondria. Specific substrates and inhibitors were used to validate the different assays, and the corresponding K(0.5) and IC(50) values were in good agreement with previously published results obtained with other methods. In combined measurements of CI-CV, it was possible to detect oxidative phosphorylation (OXPHOS), to measure differential effects of the uncoupler carbonyl cyanide m-chlorophenylhydrazone (CCCP) on the respective protein activities, and to determine the corresponding IC(50) values. Moreover, the measurements revealed a tight functional coupling of CI and CIII. Coenzyme Q (CoQ) analogues decylubiquinone (DBQ) and idebenone (Ide) stimulated the CII- and CIII-specific electrical currents but had inverse effects on CI-CIII activity. In summary, the results describe the electrophysiological and pharmacological properties of respiratory chain complexes, OXPHOS, and ANT in native mitochondrial membranes and demonstrate that SSM-based electrophysiology provides new insights into a complex molecular mechanism of the respiratory chain and the associated transport proteins. Besides, the SSM-based approach is suited for highly sensitive and specific testing of diverse respiratory chain modulators such as inhibitors, CoQ analogues, and uncoupling agents.

Treatment of CoQ(10) deficient fibroblasts with ubiquinone, CoQ analogs, and vitamin C: time- and compound-dependent effects

BACKGROUND

Coenzyme Q(10) (CoQ(10)) and its analogs are used therapeutically by virtue of their functions as electron carriers, antioxidant compounds, or both. However, published studies suggest that different ubiquinone analogs may produce divergent effects on oxidative phosphorylation and oxidative stress.

METHODOLOGY/PRINCIPAL FINDINGS

To test these concepts, we have evaluated the effects of CoQ(10), coenzyme Q(2) (CoQ(2)), idebenone, and vitamin C on bioenergetics and oxidative stress in human skin fibroblasts with primary CoQ(10) deficiency. A final concentration of 5 microM of each compound was chosen to approximate the plasma concentration of CoQ(10) of patients treated with oral ubiquinone. CoQ(10) supplementation for one week but not for 24 hours doubled ATP levels and ATP/ADP ratio in CoQ(10) deficient fibroblasts therein normalizing the bioenergetics status of the cells. Other compounds did not affect cellular bioenergetics. In COQ2 mutant fibroblasts, increased superoxide anion production and oxidative stress-induced cell death were normalized by all supplements.

CONCLUSIONS/SIGNIFICANCE

THESE RESULTS INDICATE THAT: 1) pharmacokinetics of CoQ(10) in reaching the mitochondrial respiratory chain is delayed; 2) short-tail ubiquinone analogs cannot replace CoQ(10) in the mitochondrial respiratory chain under conditions of CoQ(10) deficiency; and 3) oxidative stress and cell death can be counteracted by administration of lipophilic or hydrophilic antioxidants. The results of our in vitro experiments suggest that primary CoQ(10) deficiencies should be treated with CoQ(10) supplementation but not with short-tail ubiquinone analogs, such as idebenone or CoQ(2). Complementary administration of antioxidants with high bioavailability should be considered if oxidative stress is present.

Structure and organization of mitochondrial respiratory complexes: a new understanding of an old subject

The enzymatic complexes of the mitochondrial respiratory chain have been extensively investigated in their structural and functional properties. A clear distinction is possible today between three complexes in which the difference in redox potential allows proton translocation (complexes I, III, and IV) and those having the mere function to convey electrons to the respiratory chain. We also have a clearer understanding of the structure and function of most respiratory complexes, of their biogenesis and regulation, and of their capacity to generate reactive oxygen species. Past investigations led to the conclusion that the complexes are randomly dispersed and functionally connected by diffusion of smaller redox components, coenzyme Q and cytochrome c. More-recent investigations by native gel electrophoresis and single-particle image processing showed the existence of supramolecular associations. Flux-control analysis demonstrated that complexes I and III in mammals and I, III, and IV in plants kinetically behave as single units, suggesting the existence of substrate channeling. This review discusses conditions affecting the formation of supercomplexes that, besides kinetic advantage, have a role in the stability and assembly of the individual complexes and in preventing excess oxygen radical formation. Disruption of supercomplex organization may lead to functional derangements responsible for pathologic changes.

Is coenzyme Q a key factor in aging?

Coenzyme Q (Q) is a key component for bioenergetics and antioxidant protection in the cell. During the last years, research on diseases linked to Q-deficiency has highlighted the essential role of this lipid in cell physiology. Q levels are also affected during aging and neurodegenerative diseases. Therefore, therapies based on dietary supplementation with Q must be considered in cases of Q deficiency such as in aging. However, the low bioavailability of dietary Q for muscle and brain obligates to design new mechanisms to increase the uptake of this compound in these tissues. In the present review we show a complete picture of the different functions of Q in cell physiology and their relationship to age and age-related diseases. Furthermore, we describe the problems associated with dietary Q uptake and the mechanisms currently used to increase its uptake or even its biosynthesis in cells. Strategies to increase Q levels in tissues are indicated.

Reduction of hydrophilic ubiquinones by the flavin in mitochondrial NADH:ubiquinone oxidoreductase (Complex I) and production of reactive oxygen species

NADH:ubiquinone oxidoreductase (complex I) from bovine heart mitochondria is a complicated, energy-transducing, membrane-bound enzyme that contains 45 different subunits, a non-covalently bound flavin mononucleotide, and eight iron−sulfur clusters. The mechanisms of NADH oxidation and intramolecular electron transfer by complex I are gradually being defined, but the mechanism linking ubiquinone reduction to proton translocation remains unknown. Studies of ubiquinone reduction by isolated complex I are problematic because the extremely hydrophobic natural substrate, ubiquinone-10, must be substituted with a relatively hydrophilic analogue (such as ubiquinone-1). Hydrophilic ubiquinones are reduced by an additional, non-energy-transducing pathway (which is insensitive to inhibitors such as rotenone and piericidin A). Here, we show that inhibitor-insensitive ubiquinone reduction occurs by a ping-pong type mechanism, catalyzed by the flavin mononucleotide cofactor in the active site for NADH oxidation. Moreover, semiquinones produced at the flavin site initiate redox cycling reactions with molecular oxygen, producing superoxide radicals and hydrogen peroxide. The ubiquinone reactant is regenerated, so the NADH:Q reaction becomes superstoichiometric. Idebenone, an artificial ubiquinone showing promise in the treatment of Friedreich’s Ataxia, reacts at the flavin site. The factors which determine the balance of reactivity between the two sites of ubiquinone reduction (the energy-transducing site and the flavin site) and the implications for mechanistic studies of ubiquinone reduction by complex I are discussed. Finally, the possibility that the flavin site in complex I catalyzes redox cycling reactions with a wide range of compounds, some of which are important in pharmacology and toxicology, is discussed.