Evaluating the therapeutic potential of idebenone and related quinone analogues in Leber hereditary optic neuropathy

Using chanarin-dorfman syndrome patient fibroblasts to explore disease mechanisms and new treatment avenues

BACKGROUND

Chanarin-Dorfman syndrome (CDS) is a multisystemic autosomal recessive rare disorder. CDS is caused by variants in the abhydrolase domain containing 5 (ABHD5) encoding gene (CGI-58), which ultimately leads to excessive lipid storage, and therefore a high abundance of cellular lipid droplets (LDs). Although the molecular etiology of the disease was described many years ago, no treatment for CDS is currently available.

RESULTS

To further characterize the molecular basis of the disease and to uncover new treatment avenues, we used skin fibroblasts originating from a young patient diagnosed with CDS due to a homozygous nonsense mutation. We show that dysfunctional ABHD5 does not only affect LDs, but also influences other metabolic-related organelles; the mitochondria and peroxisomes. Additionally, we found that expressing functional ABHD5 in CDS patient cells reduced LD number. Finally, we developed and applied a high content-based drug repurposing screen based on a collection of ∼2500 FDA approved compounds, yielding several compounds that affected LD total area and size.

CONCLUSIONS

Our findings enhance the understanding of the dysfunction underlying CDS and propose new avenues for the treatment of CDS patients.

Idebenone improves mitochondrial respiratory activity and attenuates oxidative damage via the SIRT3-SOD2 pathway in a prion disease cell model

Prion diseases are neurodegenerative diseases that are transmitted between humans and animals, which cause spongiform brain degeneration and neuronal death. Prion diseases are difficult to treat. Mitochondrial damage and oxidative stress occurring early in disease progression. Reducing oxidative stress is a therapeutic strategy for disease. Idebenone (IDE) is an antioxidant that enhances electron transfer in the mitochondrial respiratory chain. To investigate IDE protection mechanisms in prion neuron models, we examined IDE effects on apoptosis, mitochondrial dysfunction, cellular respiratory chain damage, and oxidative stress in N2a cells treated with the prion toxic peptide PrP106-126. IDE effectively alleviated apoptosis and mitochondrial dysfunction, reduced mitochondrial reactive oxygen species (ROS), attenuated lipid peroxidation, improved glutathione percentages, increased important antioxidant enzyme (superoxide dismutase (SOD) and catalase) activities, and elevated mitochondrial DNA levels. IDE also modulated SOD2 deacetylation and oxidative damage by regulating SIRT3. Overall, IDE exerted significant antioxidant effects in our prion disease cell model and may have therapeutic applications for prion disease.

Clinical trials in Leber hereditary optic neuropathy: outcomes and opportunities

PURPOSE OF REVIEW

Leber hereditary optic neuropathy (LHON) is a mitochondrial DNA disease characterised by sequential bilateral vision loss due to loss of retinal ganglion cells. The purpose of this review is to provide an update on the results of recent clinical trials for LHON, focusing on studies of idebenone and lenadogene nolparvovec gene therapy.

RECENT FINDINGS

Evidence from three clinical studies (RHODOS, RHODOS-OFU, and LEROS) suggest that idebenone should be started early and continued for at least 24 months. Treatment effect varies according to the stage of LHON and the underlying mutation. Favourable outcomes are associated with the m.11778G>A mutation and chronic eyes with the m.14484T>C mutation. Caution should be taken in subacute/dynamic eyes with the m.3460G>A mutation, due to possible clinical worsening with idebenone. Compared to eyes from an external natural history cohort, pooled data from four clinical studies (RESCUE, REVERSE, RESTORE and REFLECT) show that a single intravitreal injection of lenadogene nolparvovec can result in sustained bilateral visual improvement in m.11778G>A LHON patients aged ≥15 years when treated within 1 year of onset. Although the treatment effect is modest, the final visual acuity of treated patients (∼1.2 logMAR) significantly differs from the published natural history of LHON and the treatment benefit is more pronounced than the effect of idebenone alone in patients with the m.11778G>A mutation.

SUMMARY

There is increasing evidence for the potential therapeutic benefit of idebenone and lenadogene nolparvovec gene therapy.

Mitochondrial diseases: from molecular mechanisms to therapeutic advances

Mitochondria are essential for cellular function and viability, serving as central hubs of metabolism and signaling. They possess various metabolic and quality control mechanisms crucial for maintaining normal cellular activities. Mitochondrial genetic disorders can arise from a wide range of mutations in either mitochondrial or nuclear DNA, which encode mitochondrial proteins or other contents. These genetic defects can lead to a breakdown of mitochondrial function and metabolism, such as the collapse of oxidative phosphorylation, one of the mitochondria's most critical functions. Mitochondrial diseases, a common group of genetic disorders, are characterized by significant phenotypic and genetic heterogeneity. Clinical symptoms can manifest in various systems and organs throughout the body, with differing degrees and forms of severity. The complexity of the relationship between mitochondria and mitochondrial diseases results in an inadequate understanding of the genotype-phenotype correlation of these diseases, historically making diagnosis and treatment challenging and often leading to unsatisfactory clinical outcomes. However, recent advancements in research and technology have significantly improved our understanding and management of these conditions. Clinical translations of mitochondria-related therapies are actively progressing. This review focuses on the physiological mechanisms of mitochondria, the pathogenesis of mitochondrial diseases, and potential diagnostic and therapeutic applications. Additionally, this review discusses future perspectives on mitochondrial genetic diseases.

Impaired mitochondrial morphological plasticity and failure of mitophagy associated with the G11778A mutation of LHON

Mitochondrial dynamics were examined in human dermal fibroblasts biopsied from a confirmed Leber's Hereditary Optic Neuropathy (LHON) patient with a homoplasmic G11778A mutation of the mitochondrial genome. Expression of the G11778A mutation did not impart any discernible difference in mitochondrial network morphology using widefield fluorescence microscopy. However, at the ultrastructural level, cells expressing this mutation exhibited an impairment of mitochondrial morphological plasticity when forced to utilize oxidative phosphorylation (OXPHOS) by transition to glucose-free, galactose-containing media. LHON fibroblasts also displayed a transient increase in mitophagy upon transition to galactose media. These results provide new insights into the consequences of the G11778A mutation of LHON and the pathological mechanisms underlying this disease.

Galactose-replacement unmasks the biochemical consequences of the G11778A mitochondrial DNA mutation of LHON in patient-derived fibroblasts

Leber's hereditary optic neuropathy (LHON) is a visual impairment associated with mutations of mitochondrial genes encoding elements of the electron transport chain. While much is known about the genetics of LHON, the cellular pathophysiology leading to retinal ganglion cell degeneration and subsequent vision loss is poorly understood. The impacts of the G11778A mutation of LHON on bioenergetics, redox balance and cell proliferation were examined in patient-derived fibroblasts. Replacement of glucose with galactose in the culture media reveals a deficit in the proliferation of G11778A fibroblasts, imparts a reduction in ATP biosynthesis, and a reduction in capacity to accommodate exogenous oxidative stress. While steady-state ROS levels were unaffected by the LHON mutation, cell survival was diminished in response to exogenous H2O2.

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.

Inherited Optic Neuropathies: Real-World Experience in the Paediatric Neuro-Ophthalmology Clinic

Inherited optic neuropathies affect around 1 in 10,000 people in England; in these conditions, vision is lost as retinal ganglion cells lose function or die (usually due to pathological variants in genes concerned with mitochondrial function). Emerging gene therapies for these conditions have emphasised the importance of early and expedient molecular diagnoses, particularly in the paediatric population. Here, we report our real-world clinical experience of such a population, exploring which children presented with the condition, how they were investigated and the time taken for a molecular diagnosis to be reached. A retrospective case-note review of paediatric inherited optic neuropathy patients (0-16 years) in the tertiary neuro-ophthalmology service at Moorfields Eye Hospital between 2016 and 2020 identified 19 patients. Their mean age was 9.3 ± 4.6 (mean ± SD) years at presentation; 68% were male, and 32% were female; and 26% had comorbidities, with diversity of ethnicity. Most patients had undergone genetic testing (95% (n = 18)), of whom 43% (n = 8) received a molecular diagnosis. On average, this took 54.8 ± 19.5 weeks from presentation. A cerebral MRI was performed in 70% (n = 14) and blood testing in 75% (n = 15) of patients as part of their workup. Continual improvement in the investigative pathways for inherited optic neuropathies will be paramount as novel therapeutics become available.

A challenging differential diagnosis - Leber's Hereditary Optic Neuropathy

Leber's hereditary optic neuropathy (LHON) is the most common maternally inherited disease linked to mitochondrial DNA (mtDNA). The patients present with subacute asymmetric bilateral vision loss. Approximately 95% of the LHON cases are caused by m.3460G>A (MTND1), m.11778G>A (MTND4), and m.14484T>C (MTND6) mutations. The hallmark of hereditary optic neuropathies determined by mitochondrial dysfunction is the vulnerability and degeneration of retinal ganglion cells (RGC). We present the case of a 28-year-old man who came to our clinic complaining of a subacute decrease in visual acuity of his left eye. From his medical history, we found out that one month before he had the same symptoms in the right eye. From the family history, we noted that an uncle has had vision problems since childhood. We carried out complete blood tests, including specific antibodies for autoimmune and infectious diseases. Laboratory tests and MRI were within normal limits. A blood test of the mtDNA showed the presence of 11778 G>A mutation on the mtND6 gene. The medical history, the fundus appearance, the OCT, and the paraclinical investigations, made us diagnose our patient with Leber's hereditary optic neuropathy. As soon as possible, we began the treatment with systemic idebenone, 900 mg/day. We examined the patient 2, 6, and 10 weeks after initiating the treatment. Abbreviations: LHON = Leber's Hereditary Optic Neuropathy, mtDNA = mitochondrial DNA, VA = visual acuity, RE = right eye, LE = left eye, OCT = Optical coherence tomography, pRNFL = peripapillary retinal nerve fiber layer, GCL = retinal ganglion cells layer, MRI = magnetic resonance imaging, VEP = visual evoked potentials, VEP IT = VEP implicit time, VEP A = VEP amplitude.

Leber's Hereditary Optic Neuropathy (LHON): Clinical Experience and Outcomes after Long-Term Idebenone Treatment

BACKGROUND

Leber's hereditary optic neuropathy (LHON) is a rare disease. Large studies are difficult to conduct; therefore, case reports provide valuable data. Since 2015, patients have been treated with Idebenone. The aim of this paper is to share our experience with diagnosing and managing patients in different stages of LHON.

METHODS

We designed a case series study, including four patients undergoing genetic testing and ophthalmologic examination. Criteria for Idebenone administration and follow-up were presented.

RESULTS

All patients had mutation 11778G>A in MT-ND4. The first patient, an 82-year-old man, with long history of vision loss, had no indication for Idebenone. Two additional cases emerged within the same family: a 40-year-old brother and a 31-year-old sister. Both received Idebenone, with good outcomes only for the female. After a one-year regimen, they were lost to follow-up. The fourth patient, a 46-year-old man, was diagnosed in the subacute stage. Idebenone administration was deferred, allowing progression of visual field defects. After 17 months of treatment, visual improvement appeared. The treatment was continued for 36 months, with short interruptions, resulting in good outcomes.

CONCLUSIONS

Our study demonstrated positive results with long-term Idebenone use. Contrary to medical literature, our female patient had a favorable evolution, despite the delayed diagnosis.

Evaluation of Mitochondrial Dysfunction and Idebenone Responsiveness in Fibroblasts from Leber's Hereditary Optic Neuropathy (LHON) Subjects

Leber's hereditary optic neuropathy (LHON) is a disease that affects the optical nerve, causing visual loss. The diagnosis of LHON is mostly defined by the identification of three pathogenic variants in the mitochondrial DNA. Idebenone is widely used to treat LHON patients, but only some of them are responders to treatment. In our study, we assessed the maximal respiration rate (MRR) and other respiratory parameters in eight fibroblast lines from subjects carrying LHON pathogenic variants. We measured also the effects of idebenone treatment on cell growth and mtDNA amounts. Results showed that LHON fibroblasts had significantly reduced respiratory parameters in untreated conditions, but no significant gain in MRR after idebenone supplementation. No major toxicity toward mitochondrial function and no relevant compensatory effect in terms of mtDNA quantity were found for the treatment at the tested conditions. Our findings confirmed that fibroblasts from subjects harboring LHON pathogenic variants displayed impaired respiration, regardless of the disease penetrance and severity. Testing responsiveness to idebenone treatment in cultured cells did not fully recapitulate in vivo data. The in-depth evaluation of cellular respiration in fibroblasts is a good approach to evaluating novel mtDNA variants associated with LHON but needs further evaluation as a potential biomarker for disease prognosis and treatment responsiveness.

Current and Future Landscape in Genetic Therapies for Leber Hereditary Optic Neuropathy

Leber hereditary optic neuropathy (LHON) is the most common primary mitochondrial genetic disease that causes blindness in young adults. Over 50 inherited mitochondrial DNA (mtDNA) variations are associated with LHON; however, more than 95% of cases are caused by one of three missense variations (m.11778 G > A, m.3460 G > A, and m.14484 T > C) encoding for subunits ND4, ND1, and ND6 of the respiration complex I, respectively. These variants remain silent until further and currently poorly understood genetic and environmental factors precipitate the visual loss. The clinical course that ensues is variable, and a convincing treatment for LHON has yet to emerge. In 2015, an antioxidant idebenone (Raxone) received European marketing authorisation to treat visual impairment in patients with LHON, and since then it was introduced into clinical practice in several European countries. Alternative therapeutic strategies, including gene therapy and gene editing, antioxidant and neurotrophic agents, mitochondrial biogenesis, mitochondrial replacement, and stem cell therapies are being investigated in how effective they might be in altering the course of the disease. Allotopic gene therapies are in the most advanced stage of development (phase III clinical trials) whilst most other agents are in phase I or II trials or at pre-clinical stages. This manuscript discusses the phenotype and genotype of the LHON disease with complexities and peculiarities such as incomplete penetrance and gender bias, which have challenged the therapies in development emphasising the most recent use of gene therapy. Furthermore, we review the latest results of the three clinical trials based on adeno-associated viral (AAV) vector-mediated delivery of NADH dehydrogenase subunit 4 (ND4) with mitochondrial targeting sequence, highlighting the differences in the vector design and the rationale behind their use in the allotopic transfer.

Optimisation of AAV-NDI1 Significantly Enhances Its Therapeutic Value for Correcting Retinal Mitochondrial Dysfunction

AAV gene therapy for ocular disease has become a reality with the market authorisation of Luxturna^TM for RPE65-linked inherited retinal degenerations and many AAV gene therapies currently undergoing phase III clinical trials. Many ocular disorders have a mitochondrial involvement from primary mitochondrial disorders such as Leber hereditary optic neuropathy (LHON), predominantly due to mutations in genes encoding subunits of complex I, to Mendelian and multifactorial ocular conditions such as dominant optic atrophy, glaucoma and age-related macular degeneration. In this study, we have optimised the nuclear yeast gene, NADH-quinone oxidoreductase (NDI1), which encodes a single subunit complex I equivalent, creating a candidate gene therapy to improve mitochondrial function, independent of the genetic mutation driving disease. Optimisation of NDI1 (ophNdi1) substantially increased expression in vivo, protected RGCs and increased visual function, as assessed by optokinetic and photonegative response, in a rotenone-induced murine model. In addition, ophNdi1 increased cellular oxidative phosphorylation and ATP production and protected cells from rotenone insult to a significantly greater extent than wild type NDI1. Significantly, ophNdi1 treatment of complex I deficient patient-derived fibroblasts increased oxygen consumption and ATP production rates, demonstrating the potential of ophNdi1 as a candidate therapy for ocular disorders where mitochondrial deficits comprise an important feature.

Leber's hereditary optic neuropathy: Update on current diagnosis and treatment

Leber's hereditary optic neuropathy (LHON) is a fairly prevalent mitochondrial disorder (1:50,000) arising from the dysfunction of the mitochondrial respiratory chain, which eventually leads to apoptosis of retinal ganglion cells. The usual presentation is that of a young male with a sequential reduction in visual acuity. OCT has been used to study the pattern of optic nerve involvement in LHON, showing early thickening of the inferior and superior retinal nerve fibre layer and ganglion cell layer thinning corresponding with the onset of symptoms. Of the three primary mutations for LHON, the m.14484T>C mutation has the best visual prognosis. Recent emerging therapeutic options for LHON include idebenone and the introduction of genetic vector therapy, which is currently in phase III clinical trials. Screening of family members and adequate advice to avoid environmental triggers, such as smoking and alcohol consumption, are also cornerstones in the management of LHON.

Case Report: Abnormalities of sperm motility and morphology in a patient with Leber hereditary optic neuropathy: Improvement after idebenone therapy

Case

We report the sperm characteristics of a male patient who developed, when he was 18 years old, a Leber hereditary optic neuropathy, a hereditary optic neuropathy due to mtDNA mutation as well as variants in the nuclear DNA. At the age of 30 years-old, he complained of infertility lasting for 2 years. Semen analyses showed low motility spermatozoa and a high percentage of morphological or ultrastructural abnormalities. Levels of epididymal markers were strongly atypical. Idebenone was prescribed as treatment of his Leber hereditary optic neuropathy in order to improve his visual acuity. After 5 months of this treatment, motility of spermatozoa increased, and their vitality improved. A natural conception occurred.

Outcome

This case is the first description of an anomaly of spermatozoas and of the epididymis epithelium in a patient with Leber hereditary optic neuropathy. It draws attention to sperm pathologies in patients with mitochondrial disorders. The role of the mtDNA mutations must be suspected since it plays an important role in the development and motility of spermatozoa. In addition, idebenone can by-pass the complex I and transfer electrons to complex III. It has been suspected to have a favorable effect on spermatogenesis.

Conclusion

This case confirms the possibility of sperm dysfunction in Leber hereditary optic neuropathy and the interest of idebenone as a treatment for infertility due to mtDNA mutations in human.

Developments in the Treatment of Leber Hereditary Optic Neuropathy

PURPOSEOF REVIEW

To outline the current landscape of treatments for Leber hereditary optic neuropathy (LHON) along the therapeutic delivery pipeline, exploring the mechanisms of action and evidence for these therapeutic approaches.

RECENT FINDINGS

Treatments for LHON can be broadly classified as either mutation-specific or mutation-independent. Mutation-specific therapies aim to correct the underlying mutation through the use of a gene-editing platform or replace the faulty mitochondrial DNA-encoded protein by delivering the wild-type gene using a suitable vector. Recent gene therapy clinical trials assessing the efficacy of allotopically expressed MT-ND4 for the treatment of LHON due to the m.11778G > A mutation in MT-ND4 have shown positive results when treated within 12 months of symptom onset. Mutation-independent therapies can have various downstream targets that aim to improve mitochondrial respiration, reduce mitochondrial stress, inhibit or delay retinal ganglion cell apoptosis, and/or promote retinal ganglion cell survival. Idebenone, a synthetic hydrosoluble analogue of co-enzyme Q10 (ubiquinone), is the only approved treatment for LHON. Mutation-independent approaches to gene therapy under pre-clinical investigation for other neurodegenerative disorders may have the potential to benefit patients with LHON. Although approved treatments are presently limited, innovations in gene therapy and editing are driving the expansion of the therapeutic delivery pipeline for LHON.

Solutions to a Radical Problem: Overview of Current and Future Treatment Strategies in Leber's Hereditary Opic Neuropathy

Leber's Hereditary Optic Neuropathy (LHON) is the most common primary mitochondrial DNA disorder. It is characterized by bilateral severe central subacute vision loss due to specific loss of Retinal Ganglion Cells and their axons. Historically, treatment options have been quite limited, but ongoing clinical trials show promise, with significant advances being made in the testing of free radical scavengers and gene therapy. In this review, we summarize management strategies and rational of treatment based on current insights from molecular research. This includes preventative recommendations for unaffected genetic carriers, current medical and supportive treatments for those affected, and emerging evidence for future potential therapeutics.

Variable effects of omaveloxolone (RTA408) on primary fibroblasts with mitochondrial defects

Omaveloxolone (RTA408) is a second-generation oleanane triterpenoid Nrf2 inducer with antioxidant and anti-inflammatory properties and was reported to improve mitochondrial bioenergetics. It is currently being tested in medical trials for Friedrich ataxia, a genetic, multi-organ disease involving mitochondrial dysfunction. Thus, omaveloxolone could potentially be beneficial for additional disorders involving mitochondrial dysfunction. To this end, we investigated its effect on primary fibroblasts derived from patients with mitochondrial complex I deficiency, mitochondrial cytochrome oxidase deficiency, and two recessive forms of Parkinson’s disease. Patients and control cells were incubated in the presence or absence of 50 nM omaveloxolone for 72 h prior to measurements. Generally, growth on galactose medium and ATP production were unaltered. Mitochondrial membrane potential was slightly but significantly decreased, while reactive oxygen species (ROS) production was variably decreased. Mitochondrial mass and mitochondrial DNA (mtDNA) contents were significantly increased in the patient’s cells. These results were partially confirmed by the results of oxygen consumption studies which disclosed increased maximal oxygen consumption rates in most cells and increased energy status in all treated cells. Further investigation is required to explore the precise effect of omaveloxolone on mitochondrial function in disease.

Pathological mitophagy disrupts mitochondrial homeostasis in Leber's hereditary optic neuropathy

Leber’s hereditary optic neuropathy (LHON), a disease associated with a mitochondrial DNA mutation, is characterized by blindness due to degeneration of retinal ganglion cells (RGCs) and their axons, which form the optic nerve. We show that a sustained pathological autophagy and compartment-specific mitophagy activity affects LHON patient-derived cells and cybrids, as well as induced pluripotent-stem-cell-derived neurons. This is variably counterbalanced by compensatory mitobiogenesis. The aberrant quality control disrupts mitochondrial homeostasis as reflected by defective bioenergetics and excessive reactive oxygen species production, a stress phenotype that ultimately challenges cell viability by increasing the rate of apoptosis. We counteract this pathological mechanism by using autophagy regulators (clozapine and chloroquine) and redox modulators (idebenone), as well as genetically activating mitochondrial biogenesis (PGC1-α overexpression). This study substantially advances our understanding of LHON pathophysiology, providing an integrated paradigm for pathogenesis of mitochondrial diseases and druggable targets for therapy.

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Autophagy and mitophagy are abnormally activated in samples carrying LHON mutations

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Autophagy and mitophagy affect LHON cells’ viability

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Therapeutic approaches targeting autophagy reverts LHON cells’ apoptotic death

Idebenone Regulates Aβ and LPS-Induced Neurogliosis and Cognitive Function Through Inhibition of NLRP3 Inflammasome/IL-1β Axis Activation

Idebenone is an analogue of coenzyme Q10, an electron donor in the mitochondrial electron transport chain, and thus may function as an antioxidant to facilitate mitochondrial function. However, whether idebenone modulates LPS- and Aβ-mediated neuroinflammatory responses and cognitive function in vivo is unknown. The present study explored the effects of idebenone on LPS- or Aβ-mediated neuroinflammation, learning and memory and the underlying molecular mechanisms in wild-type (WT) mice and 5xFAD mice, a mouse model of Alzheimer’s disease (AD). In male and female WT mice, idebenone upregulated neuroprotective NRF2 expression, rescued LPS-induced spatial and recognition memory impairments, and reduced NLRP3 priming and subsequent neuroinflammation. Moreover, idebenone downregulated LPS-mediated neurogliosis, reactive oxygen species (ROS) levels, and mitochondrial function in BV2 microglial cells and primary astrocytes by inhibiting NLRP3 inflammasome activation. In 5xFAD mice, idebenone increased neuroprotective NRF2 expression and improved amyloid beta (Aβ)-induced cognitive dysfunction. Idebenone downregulated Aβ-mediated gliosis and proinflammatory cytokine levels in 5xFAD mice by modulating the vicious NLRP3/caspase-1/IL-1β neuroinflammation cycle. Taken together, our results suggest that idebenone targets neuroglial NLRP3 inflammasome activation and therefore may have neuroprotective effects and inhibit the pathological progression of neuroinflammation-related diseases.

SARM1 Ablation Is Protective and Preserves Spatial Vision in an In Vivo Mouse Model of Retinal Ganglion Cell Degeneration

The challenge of developing gene therapies for genetic forms of blindness is heightened by the heterogeneity of these conditions. However, mechanistic commonalities indicate key pathways that may be targeted in a gene-independent approach. Mitochondrial dysfunction and axon degeneration are common features of many neurodegenerative conditions including retinal degenerations. Here we explore the neuroprotective effect afforded by the absence of sterile alpha and Toll/interleukin-1 receptor motif-containing 1 (SARM1), a prodegenerative NADase, in a rotenone-induced mouse model of retinal ganglion cell loss and visual dysfunction. Sarm1 knockout mice retain visual function after rotenone insult, displaying preservation of photopic negative response following rotenone treatment in addition to significantly higher optokinetic response measurements than wild type mice following rotenone. Protection of spatial vision is sustained over time in both sexes and is accompanied by increased RGC survival and additionally preservation of axonal density in optic nerves of Sarm1^−/− mice insulted with rotenone. Primary fibroblasts extracted from Sarm1^−/− mice demonstrate an increased oxygen consumption rate relative to those from wild type mice, with significantly higher basal, maximal and spare respiratory capacity. Collectively, our data indicate that Sarm1 ablation increases mitochondrial bioenergetics and confers histological and functional protection in vivo in the mouse retina against mitochondrial dysfunction, a hallmark of many neurodegenerative conditions including a variety of ocular disorders.

Retinal morphological and functional response to Idebenone therapy in Leber hereditary optic neuropathy

Leber hereditary optic neuropathy (LHON) is a mitochondrial disease leading to optic atrophy due to degeneration of the retinal ganglion cell. A curative treatment is not available at the moment, but a new antioxidant drug, Idebenone, is expected to reduce the progression of the disorder. Two male patients, genetically confirmed with LHON, were clinically, morphologically, and electrophysiologically evaluated, before and three, six, nine and 12 months after starting the treatment. The patient with 3460G>A mutation in mitochondrially-encoded nicotinamide adenine dinucleotide, reduced form (NADH):ubiquinone oxidoreductase core subunit (mtND)1 gene showed an improvement in visual acuity, visual field, and visual evoked potentials with no effect on morphological examinations, while the patient with 11778G>A mutation in mtND4 gene showed no functional, nor morphological recovery after one year of treatment. This study demonstrates that Idebenone, depending on the genetic profile of the disease, may be effective in functional improvement in patients with LHON.

Leber hereditary optic neuropathy-new insights and old challenges

Leber hereditary optic neuropathy (LHON) is the most common primary mitochondrial DNA (mtDNA) disorder with the majority of patients harboring one of three primary mtDNA point mutations, namely, m.3460G>A (MTND1), m.11778G>A (MTND4), and m.14484T>C (MTND6). LHON is characterized by bilateral subacute loss of vision due to the preferential loss of retinal ganglion cells (RGCs) within the inner retina, resulting in optic nerve degeneration. This review describes the clinical features associated with mtDNA LHON mutations and recent insights gained into the disease mechanisms contributing to RGC loss in this mitochondrial disorder. Although treatment options remain limited, LHON research has now entered an active translational phase with ongoing clinical trials, including gene therapy to correct the underlying pathogenic mtDNA mutation.

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.

Changes in Visual Function and Correlations with Inner Retinal Structure in Acute and Chronic Leber's Hereditary Optic Neuropathy Patients after Treatment with Idebenone

Progressive impairment and degeneration of retinal ganglion cells (RGC) and nerve fibers in Leber’s hereditary optic neuropathy (LHON) usually cause permanent visual loss. Idebenone is currently the only approved treatment. However, its therapeutic potential in different stages of LHON has not been definitely clarified. We aimed to investigate the changes in visual function and correlations with retinal structure in acute and in chronic LHON patients after treatment with idebenone. Twenty-three genetically confirmed LHON patients were followed during treatment using logMAR charts, automated perimetry and optical coherence tomography (OCT). Mean visual acuity improved significantly in acute patients treated within 1 year from onset (−0.52 ± 0.46 logMAR from nadir), in early chronic patients who started after 1–5 years (−0.39 ± 0.27 logMAR from baseline), and in late chronic patients with treatment initiation after >5 years (−0.33 ± 0.28 logMAR from baseline, p < 0.001 all groups). In acute and in chronic patients, strong correlations between OCT and visual function parameters were present only after treatment. This and the sustained visual recovery after treatment may indicate a reactivated signal transduction in dysfunctional RGC that survive the acute phase. Our results support previous evidence that idebenone has therapeutic potential in promoting visual recovery in LHON.

Wolfram syndrome: new pathophysiological insights and therapeutic strategies

Wolfram Syndrome (WS) is an ultra-rare, progressive neurodegenerative disease characterized by early-onset diabetes mellitus and irreversible loss of vision, secondary to optic nerve degeneration. Visual loss in WS is an important cause of registrable blindness in children and young adults and the pathological hallmark is the preferential loss of retinal ganglion cells within the inner retina. In addition to optic atrophy, affected individuals frequently develop variable combinations of neurological, endocrinological, and psychiatric complications. The majority of patients carry recessive mutations in the WFS1 (4p16.1) gene that encodes for a multimeric transmembrane protein, wolframin, embedded within the endoplasmic reticulum (ER). An increasingly recognised subgroup of patients harbor dominant WFS1 mutations that usually cause a milder phenotype, which can be limited to optic atrophy. Wolframin is a ubiquitous protein with high levels of expression in retinal, neuronal, and muscle tissues. It is a multifunctional protein that regulates a host of cellular functions, in particular the dynamic interaction with mitochondria at mitochondria-associated membranes. Wolframin has been implicated in several crucial cellular signaling pathways, including insulin signaling, calcium homeostasis, and the regulation of apoptosis and the ER stress response. There is currently no cure for WS; management remains largely supportive. This review will cover the clinical, genetic, and pathophysiological features of WS, with a specific focus on disease models and the molecular pathways that could serve as potential therapeutic targets. The current landscape of therapeutic options will also be discussed in the context of the latest evidence, including the pipeline for repurposed drugs and gene therapy.

Plain language summary

Wolfram syndrome - disease mechanisms and treatment options Wolfram syndrome (WS) is an ultra-rare genetic disease that causes diabetes mellitus and progressive loss of vision from early childhood. Vision is affected in WS because of damage to a specialized type of cells in the retina, known as retinal ganglion cells (RGCs), which converge at the back of the eye to form the optic nerve. The optic nerve is the fast-conducting cable that transmits visual information from the eye to the vision processing centers within the brain. As RGCs are lost, the optic nerve degenerates and it becomes pale in appearance (optic atrophy). Although diabetes mellitus and optic atrophy are the main features of WS, some patients can develop more severe problems because the brain and other organs, such as the kidneys and the bladder, are also affected. The majority of patients with WS carry spelling mistakes (mutations) in the WFS1 gene, which is located on the short arm of chromosome 4 (4p16.1). This gene is highly expressed in the eye and in the brain, and it encodes for a protein located within a compartment of the cell known as the endoplasmic reticulum. For reasons that still remain unclear, WFS1 mutations preferentially affect RGCs, accounting for the prominent visual loss in this genetic disorder. There is currently no effective treatment to halt or slow disease progression and management remains supportive, including the provision of visual aids and occupational rehabilitation. Research into WS has been limited by its relative rarity and the inability to get access to eye and brain tissues from affected patients. However, major advances in our understanding of this disease have been made recently by making use of more accessible cells from patients, such as skin cells (fibroblasts), or animal models, such as mice and zebrafish. This review summarizes the mechanisms by which WFS1 mutations affect cells, impairing their function and eventually leading to their premature loss. The possible treatment strategies to block these pathways are also discussed, with a particular focus on drug repurposing (i.e., using drugs that are already approved for other diseases) and gene therapy (i.e., replacing or repairing the defective WFS1 gene).

Therapeutic Options in Hereditary Optic Neuropathies

Options for the effective treatment of hereditary optic neuropathies have been a long time coming. The successful launch of the antioxidant idebenone for Leber's Hereditary Optic Neuropathy (LHON), followed by its introduction into clinical practice across Europe, was an important step forward. Nevertheless, other options, especially for a variety of mitochondrial optic neuropathies such as dominant optic atrophy (DOA), are needed, and a number of pharmaceutical agents, acting on different molecular pathways, are currently under development. These include gene therapy, which has reached Phase III development for LHON, but is expected to be  developed also for DOA, whilst most of the other agents (other antioxidants, anti-apoptotic drugs, activators of mitobiogenesis, etc.) are almost all at Phase II or at preclinical stage of research. Here, we review proposed target mechanisms, preclinical evidence, available clinical trials with primary endpoints and results, of a wide range of tested molecules, to give an overview of the field, also providing the landscape of future scenarios, including gene therapy, gene editing, and reproductive options to prevent transmission of mitochondrial DNA mutations.

Diabetic Retinopathy: The Role of Mitochondria in the Neural Retina and Microvascular Disease

Diabetic retinopathy (DR), a common chronic complication of diabetes mellitus and the leading cause of vision loss in the working-age population, is clinically defined as a microvascular disease that involves damage of the retinal capillaries with secondary visual impairment. While its clinical diagnosis is based on vascular pathology, DR is associated with early abnormalities in the electroretinogram, indicating alterations of the neural retina and impaired visual signaling. The pathogenesis of DR is complex and likely involves the simultaneous dysregulation of multiple metabolic and signaling pathways through the retinal neurovascular unit. There is evidence that microvascular disease in DR is caused in part by altered energetic metabolism in the neural retina and specifically from signals originating in the photoreceptors. In this review, we discuss the main pathogenic mechanisms that link alterations in neural retina bioenergetics with vascular regression in DR. We focus specifically on the recent developments related to alterations in mitochondrial metabolism including energetic substrate selection, mitochondrial function, oxidation-reduction (redox) imbalance, and oxidative stress, and critically discuss the mechanisms of these changes and their consequences on retinal function. We also acknowledge implications for emerging therapeutic approaches and future research directions to find novel mitochondria-targeted therapeutic strategies to correct bioenergetics in diabetes. We conclude that retinal bioenergetics is affected in the early stages of diabetes with consequences beyond changes in ATP content, and that maintaining mitochondrial integrity may alleviate retinal disease.

Maternally inherited mitochondrial respiratory disorders: from pathogenetic principles to therapeutic implications

Maternally inherited mitochondrial respiratory disorders are rare, progressive, and multi-systemic diseases that remain intractable, with no effective therapeutic interventions. Patients share a defective oxidative phosphorylation pathway responsible for mitochondrial ATP synthesis, in most cases due to pathogenic mitochondrial variants transmitted from mother to child or to a rare de novo mutation or large-scale deletion of the mitochondrial genome. The clinical diagnosis of these mitochondrial diseases is difficult due to exceptionally high clinical variability, while their genetic diagnosis has improved with the advent of next-generation sequencing. The mechanisms regulating the penetrance of the mitochondrial variants remain unresolved with the patient's nuclear background, epigenomic regulation, heteroplasmy, mitochondrial haplogroups, and environmental factors thought to act as rheostats. The lack of animal models mimicking the phenotypic manifestations of these disorders has hampered efforts toward curative therapies. Patient-derived cellular paradigms provide alternative models for elucidating the pathogenic mechanisms and screening pharmacological small molecules to enhance mitochondrial function. Recent progress has been made in designing promising approaches to curtail the negative impact of dysfunctional mitochondria and alleviate clinical symptoms: 1) boosting mitochondrial biogenesis; 2) shifting heteroplasmy; 3) reprogramming metabolism; and 4) administering hypoxia-based treatment. Here, we discuss their varying efficacies and limitations and provide an outlook on their therapeutic potential and clinical application.

Treatment of Leber's hereditary optic neuropathy: An overview of recent developments

Leber's hereditary optic neuropathy (LHON) is a rare, maternally-inherited optic neuropathy caused by mitochondrial DNA point mutations and which can cause blindness. Currently, Raxone (idebenone) is the only available medicinal product authorised to treat LHON within the European Union and LHON remains an unmet medical need. The aim of this article was to summarise interventional clinical trials published over the past 5 years (between 2014 and 2019) with the primary purpose of treating LHON. Therapeutic approaches discussed include modulating agents of the mitochondrial electron transport chain such as Raxone, cysteamine bitartrate and KH176, inhibitors of apoptosis such as elamipretide, gene therapy medicinal products such as GS010 and scAAV2P1ND4 and retinal tissue regeneration medicinal products such as bone marrow-derived stem cells.

Decylubiquinone suppresses breast cancer growth and metastasis by inhibiting angiogenesis via the ROS/p53/ BAI1 signaling pathway

Breast cancer is one of the most common cancers worldwide with a rising incidence, and is the leading cause of cancer-related death among females. Angiogenesis plays an important role in breast cancer growth and metastasis. In this study, we identify decylubiquinone (DUb), a coenzyme Q₁₀ analog, as a promising anti-breast cancer agent through suppressing tumor-induced angiogenesis. We screened a library comprising FDA-approved drugs and found that DUb significantly inhibits blood vessel formation using in vivo chick embryo chorioallantoic membrane (CAM) and yolk sac membrane (YSM) models. DUb was further identified to inhibit angiogenesis in the rat aortic ring and Matrigel plug assay. Moreover, DUb was found to suppress breast cancer growth and metastasis in the MMTV-PyMT transgenic mouse and human xenograft tumor models. To explore whether the anticancer efficacy of DUb was directly corrected with tumor-induced angiogenesis, the MDA-MB-231 breast cancer assay on the CAM was performed. Interestingly, DUb significantly inhibits the angiogenesis of breast cancer on the CAM. Brain angiogenesis inhibitor 1 (BAI1), a member of the G protein-coupled receptor (GPCR) adhesion subfamily, has an important effect on the inhibition of angiogenesis. Further studies demonstrate that DUb suppresses the formation of tubular structures by regulating the reactive oxygen species (ROS)/p53/BAI1 signaling pathway. These results uncover a novel finding that DUb has the potential to be an effective agent for the treatment of breast cancer by inhibiting tumor-induced angiogenesis.

Bezafibrate Improves Mitochondrial Fission and Function in DNM1L-Deficient Patient Cells

Mitochondria are involved in many cellular processes and their main role is cellular energy production. They constantly undergo fission and fusion, and these counteracting processes are under strict balance. The cytosolic dynamin-related protein 1, Drp1, or dynamin-1-like protein (DNM1L) mediates mitochondrial and peroxisomal division. Defects in the DNM1L gene result in a complex neurodevelopmental disorder with heterogeneous symptoms affecting multiple organ systems. Currently there is no curative treatment available for this condition. We have previously described a patient with a de novo heterozygous c.1084G>A (p.G362S) DNM1L mutation and studied the effects of a small molecule, bezafibrate, on mitochondrial functions in this patient’s fibroblasts compared to controls. Bezafibrate normalized growth on glucose-free medium, as well as ATP production and oxygen consumption. It improved mitochondrial morphology in the patient’s fibroblasts, although causing a mild increase in ROS production at the same time. A human foreskin fibroblast cell line overexpressing the p.G362S mutation showed aberrant mitochondrial morphology, which normalized in the presence of bezafibrate. Further studies would be needed to show the consistency of the response to bezafibrate, possibly using fibroblasts from patients with different mutations in DNM1L, and this treatment should be confirmed in clinical trials. However, taking into account the favorable effects in our study, we suggest that bezafibrate could be offered as a treatment option for patients with certain DNM1L mutations.

Visual function in chronic Leber's hereditary optic neuropathy during idebenone treatment initiated 5 to 50 years after onset

PURPOSE

Leber's hereditary optic neuropathy (LHON) is a mitochondrial disease characterized by a subacute and progressive impairment and subsequent degeneration of retinal ganglion cells (RGCs). In most cases, it results in optic nerve atrophy and permanently reduced visual acuity (VA). Idebenone has recently been approved in Europe for treating LHON. However, published clinical data has only focused on efficacy in patients within the first years after disease onset. The present study is the first to evaluate possible effects of idebenone treatment in patients with LHON when initiated after more than 5 years from disease onset.

METHODS

Oral treatment with idebenone 300 mg tid was started in seven patients 5 to 51 years after LHON onset. All patients had genetically confirmed primary LHON mutations (m11778G>A, m14484T>C, and m13051G>A). Visual function of all fourteen eyes was tested every 3 months using logarithmic reading charts and automated static threshold perimetry. The obtained clinical data were analyzed retrospectively using a multivariate analysis for VA and the Wilcoxon signed-rank test for visual field data.

RESULTS

Before treatment, VA was 0.78 ± 0.38 logMAR (range 0.24 to 1.50 logMAR). During the first year of therapy, VA improved significantly by an average of - 0.20 ± 0.10 logMAR or 10 ± 5 ETDRS letters (P = 0.002; VA range 0.06 to 1.30 logMAR). Seven of fourteen eyes showed an improvement of 2 or more lines. Visual field mean deviation increased from - 8.02 ± 6.11 to - 6.48 ± 5.26 dB after 12 months, but this change was not statistically significant (P = 0.056).

CONCLUSIONS

The increase in VA of patients who have had LHON for more than 5 years observed soon after start of treatment may not constitute a coincidental spontaneous recovery. We hypothesize that the treatment response in chronic LHON was the result of a reactivated signal transduction in surviving dysfunctional RGCs. The results of this study indicate a beneficial effect of idebenone on improvement of visual function in LHON patients with established optic atrophy.

Mitochondrial disorders and drugs: what every physician should know

Mitochondrial disorders are a group of metabolic conditions caused by impairment of the oxidative phosphorylation system. There is currently no clear evidence supporting any pharmacological interventions for most mitochondrial disorders, except for coenzyme Q10 deficiencies, Leber hereditary optic neuropathy, and mitochondrial neurogastrointestinal encephalomyopathy. Furthermore, some drugs may potentially have detrimental effects on mitochondrial dysfunction. Drugs known to be toxic for mitochondrial functions should be avoided whenever possible. Mitochondrial patients needing one of these treatments should be carefully monitored, clinically and by laboratory exams, including creatine kinase and lactate. In the era of molecular and ‘personalized’ medicine, many different physicians (not only neurologists) should be aware of the basic principles of mitochondrial medicine and its therapeutic implications. Multicenter collaboration is essential for the advancement of therapy for mitochondrial disorders. Whenever possible, randomized clinical trials are necessary to establish efficacy and safety of drugs. In this review we discuss in an accessible way the therapeutic approaches and perspectives in mitochondrial disorders. We will also provide an overview of the drugs that should be used with caution in these patients.

Primary Coenzyme Q deficiency Due to Novel ADCK3 Variants, Studies in Fibroblasts and Review of Literature

Primary deficiency of coenzyme Q10 (CoQ10 ubiquinone), is classified as a mitochondrial respiratory chain disorder with phenotypic variability. The clinical manifestation may involve one or multiple tissue with variable severity and presentation may range from infancy to late onset. ADCK3 gene mutations are responsible for the most frequent form of hereditary CoQ10 deficiency (Q10 deficiency-4 OMIM #612016) which is mainly associated with autosomal recessive spinocerebellar ataxia (ARCA2, SCAR9). Here we provide the clinical, biochemical and genetic investigation for unrelated three nuclear families presenting an autosomal form of Spino-Cerebellar Ataxia due to novel mutations in the ADCK3 gene. Using next generation sequence technology we identified a homozygous Gln343Ter mutation in one family with severe, early onset of the disease and compound heterozygous mutations of Gln343Ter and Ser608Phe in two other families with variable manifestations. Biochemical investigation in fibroblasts showed decreased activity of the CoQ dependent mitochondrial respiratory chain enzyme succinate cytochrome c reductase (complex II + III). Exogenous CoQ slightly improved enzymatic activity, ATP production and decreased oxygen free radicals in some of the patient's cells. Our results are presented in comparison to previously reported mutations and expanding the clinical, molecular and biochemical spectrum of ADCK3 related CoQ10 deficiencies.

Mitochondrial Genetic Disorders: Cell Signaling and Pharmacological Therapies

Mitochondrial fatty acid oxidation (FAO) and respiratory chain (RC) defects form a large group of inherited monogenic disorders sharing many common clinical and pathophysiological features, including disruption of mitochondrial bioenergetics, but also, for example, oxidative stress and accumulation of noxious metabolites. Interestingly, several transcription factors or co-activators exert transcriptional control on both FAO and RC genes, and can be activated by small molecules, opening to possibly common therapeutic approaches for FAO and RC deficiencies. Here, we review recent data on the potential of various drugs or small molecules targeting pivotal metabolic regulators: peroxisome proliferator activated receptors (PPARs), sirtuin 1 (SIRT1), AMP-activated protein kinase (AMPK), and protein kinase A (PKA)) or interacting with reactive oxygen species (ROS) signaling, to alleviate or to correct inborn FAO or RC deficiencies in cellular or animal models. The possible molecular mechanisms involved, in particular the contribution of mitochondrial biogenesis, are discussed. Applications of these pharmacological approaches as a function of genotype/phenotype are also addressed, which clearly orient toward personalized therapy. Finally, we propose that beyond the identification of individual candidate drugs/molecules, future pharmacological approaches should consider their combination, which could produce additive or synergistic effects that may further enhance their therapeutic potential.

Anti-Oxidant Drugs: Novelties and Clinical Implications in Cerebellar Ataxias

BACKGROUND

Hereditary cerebellar ataxias are a group of disorders characterized by heterogeneous clinical manifestations, progressive clinical course, and diverse genetic causes. No disease modifying treatments are yet available for many of these disorders. Oxidative stress has been recurrently identified in different progressive cerebellar diseases, and it represents a widely investigated target for treatment.

OBJECTIVE

To review the main aspects and new perspectives of antioxidant therapy in cerebellar ataxias ranging from bench to bedside.

METHOD

This article is a summary of the state-of-the-art on the use of antioxidant molecules in cerebellar ataxia treatments. It also briefly summarizes aspects of oxidative stress production and general characteristics of antioxidant compounds.

RESULTS

Antioxidants represent a vast category of compounds; old drugs have been extensively studied and modified in order to achieve better biological effects. Despite the vast body of literature present on the use of antioxidants in cerebellar ataxias, for the majority of these disorders conclusive results on the efficacy are still missing.

CONCLUSION

Antioxidant therapy in cerebellar ataxias is a promising field of investigations. To achieve the success in identifying the correct treatment more work needs to be done. In particular, a combined effort is needed by basic scientists in developing more efficient molecules, and by clinical researchers together with patients communities, to run clinical trials in order to identify conclusive treatments strategies.

The Role of Endogenous Neuroprotective Mechanisms in the Prevention of Retinal Ganglion Cells Degeneration

Retinal neurons are not able to undergo spontaneous regeneration in response to damage. A variety of stressors, i.e., UV radiation, high temperature, ischemia, allergens, and others, induce reactive oxygen species production, resulting in consecutive alteration of stress-response gene expression and finally can lead to cell apoptosis. Neurons have developed their own endogenous cellular protective systems. Some of them are preventing cell death and others are allowing functional recovery after injury. The high efficiency of these mechanisms is crucial for cell survival. In this review we focus on the contribution of the most recently studied endogenous neuroprotective factors involved in retinal ganglion cell (RGC) survival, among which, neurotrophic factors and their signaling pathways, processes regulating the redox status, and different pathways regulating cell death are the most important. Additionally, we summarize currently ongoing clinical trials for therapies for RGC degeneration and optic neuropathies, including glaucoma. Knowledge of the endogenous cellular protective mechanisms may help in the development of effective therapies and potential novel therapeutic targets in order to achieve progress in the treatment of retinal and optic nerve diseases.

KL1333, a Novel NAD+ Modulator, Improves Energy Metabolism and Mitochondrial Dysfunction in MELAS Fibroblasts

Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS), one of the most common maternally inherited mitochondrial diseases, is caused by mitochondrial DNA mutations that lead to mitochondrial dysfunction. Several treatment options exist, including supplementation with CoQ10, vitamins, and nutrients, but no treatment with proven efficacy is currently available. In this study, we investigated the effects of a novel NAD⁺ modulator, KL1333, in human fibroblasts derived from a human patient with MELAS. KL1333 is an orally available, small organic molecule that reacts with NAD(P)H:quinone oxidoreductase 1 (NQO1) as a substrate, resulting in increases in intracellular NAD⁺ levels via NADH oxidation. To elucidate the mechanism of action of KL1333, we used C2C12 myoblasts, L6 myoblasts, and MELAS fibroblasts. Elevated NAD⁺ levels induced by KL1333 triggered the activation of SIRT1 and AMPK, and subsequently activated PGC-1α in these cells. In MELAS fibroblasts, KL1333 increased ATP levels and decreased lactate and ROS levels, which are often dysregulated in this disease. In addition, mitochondrial functional analyses revealed that KL1333 increased mitochondrial mass, membrane potential, and oxidative capacity. These results indicate that KL1333 improves mitochondrial biogenesis and function, and thus represents a promising therapeutic agent for the treatment of MELAS.

Leber Hereditary Optic Neuropathy-Light at the End of the Tunnel?

Leber hereditary optic neuropathy (LHON) is an important cause of mitochondrial blindness. The majority of patients harbor one of three mitochondrial DNA (mtDNA) point mutations, m.3460G>A, m.11778G>A, and m.14484T>C, which all affect complex I subunits of the mitochondrial respiratory chain. The loss of retinal ganglion cells in LHON is thought to arise from a combination of impaired mitochondrial oxidative phosphorylation resulting in decreased adenosine triphosphate (ATP) production and increased levels of reactive oxygen species. Treatment options for LHON remain limited, but major advances in mitochondrial neuroprotection, gene therapy, and the prevention of transmission of pathogenic mtDNA mutations will hopefully translate into tangible benefits for patients affected by this condition and their families.

The pathomechanism of cytochrome c oxidase deficiency includes nuclear DNA damage

Mitochondrial cytochrome c oxidase (COX, respiratory chain complex IV), contributes to ATP production via oxidative phosphorylation (OXPHOS). Clinical presentation of COX deficiency is heterogeneous ranging from mild to severe neuromuscular diseases. Anemia is among the symptoms and we have previously reported Fanconi anemia like features in COX4-1 deficiency, suggesting genomic instability and our preliminary results detected nuclear double stranded DNA breaks (DSB). We now quantified the DSB by phospho histone H2AX Ser139 staining of COX4-1 and COX6B1 deficient fibroblasts (225% and 215% of normal, respectively) and confirmed their occurrence by neutral comet assay. We further explored the mechanism of DNA damage by studying normal fibroblasts treated with micromolar concentrations of cyanide (KCN). Present results demonstrate elevated nuclear DSB in cells treated with 50 μM KCN for 24 h (170% of normal) in high-glucose medium conditions where ROS and ATP remain normal, although Glutathione content was partially decreased. In glucose-free and serum-free medium, where growth is hampered, DSB were not elevated. Additionally we demonstrate the benefit of nicotinamide riboside (NR) which ameliorated DSB in COX4-1, COX6B1 and KCN treated cells (130%, 154% and 87% of normal cells, respectively). Conversely a negative effect of a poly[ADP-ribose] polymerase (PARP) inhibitor was found. Although additional investigation is needed, our findings raise the possibility that the pathomechanism of COX deficiency and possibly also in other OXPHOS defects, include nuclear DNA damage resulting from nicotinamide adenine dinucleotide (NAD⁺) deficit combined with a replicative state, rather than oxidative stress and energy depletion.

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.

Idebenone: Novel Strategies to Improve Its Systemic and Local Efficacy

The key role of antioxidants in treating and preventing many systemic and topical diseases is well recognized. One of the most potent antioxidants available for pharmaceutical and cosmetic use is Idebenone (IDE), a synthetic analogue of Coenzyme Q10. Unfortunately, IDE's unfavorable physicochemical properties such as poor water solubility and high lipophilicity impair its bioavailability after oral and topical administration and prevent its parenteral use. In recent decades, many strategies have been proposed to improve IDE effectiveness in the treatment of neurodegenerative diseases and skin disorders. After a brief description of IDE potential therapeutic applications and its pharmacokinetic and pharmacodynamic profile, this review will focus on the different approaches investigated to overcome IDE drawbacks, such as IDE incorporation into different types of delivery systems (liposomes, cyclodextrins, microemulsions, self-micro-emulsifying drug delivery systems, lipid-based nanoparticles, polymeric nanoparticles) and IDE chemical modification. The results of these studies will be illustrated with emphasis on the most innovative strategies and their future perspectives.

Inherited eye-related disorders due to mitochondrial dysfunction

Genetic disorders due to mitochondrial dysfunction are not uncommon and the majority of these patients will have eye-related manifestations, including visual loss from the optic nerve and retinal disease, visual field loss from retrochiasmal visual pathway damage, and ptosis and ocular dysmotility from extraocular muscle involvement. Defects in both the nuclear and mitochondrial genomes cause mitochondrial dysfunction via several mechanisms, including impaired mitochondrial energy production, oxidative stress, mitochondrial DNA instability, abnormalities in the regulation of mitochondrial dynamics and mitochondrial quality control, and disturbed cellular interorganellar communication. Advances in our understanding of the molecular genetic basis of mitochondrial disease have not only improved genetic diagnosis, but they have provided important insights into the pathophysiologic basis of these disorders and potential therapeutic targets. In parallel, more sophisticated techniques for genetic manipulation are facilitating the development of animal and in vitro models that should prove powerful and versatile tools for disease modelling and therapeutic experimentation. Effective therapies for mitochondrial disorders are beginning to translate from bench to bedside along the paths of neuroprotection, gene replacement and stem cell-based regenerative paradigms. Additionally, preventing the transmission of pathogenic mtDNA mutations from mother to child is now a reality with in vitro fertilization mitochondrial replacement techniques.