Mitochondrial dysfunction as a cause of optic neuropathies

Cardiovascular Manifestations of Mitochondrial Disease

Genetic mitochondrial cardiomyopathies are uncommon causes of heart failure that may not be seen by most physicians. However, the prevalence of mitochondrial DNA mutations and somatic mutations affecting mitochondrial function are more common than previously thought. In this review, the pathogenesis of genetic mitochondrial disorders causing cardiovascular disease is reviewed. Treatment options are presently limited to mostly symptomatic support, but preclinical research is starting to reveal novel approaches that may lead to better and more targeted therapies in the future. With better understanding and clinician education, we hope to improve clinician recognition and diagnosis of these rare disorders in order to improve ongoing care of patients with these diseases and advance research towards discovering new therapeutic strategies to help treat these diseases.

miR-181a/b downregulation exerts a protective action on mitochondrial disease models

Mitochondrial diseases (MDs) are a heterogeneous group of devastating and often fatal disorders due to defective oxidative phosphorylation. Despite the recent advances in mitochondrial medicine, effective therapies are still not available for these conditions. Here, we demonstrate that the microRNAs miR-181a and miR-181b (miR-181a/b) regulate key genes involved in mitochondrial biogenesis and function and that downregulation of these miRNAs enhances mitochondrial turnover in the retina through the coordinated activation of mitochondrial biogenesis and mitophagy. We thus tested the effect of miR-181a/b inactivation in different animal models of MDs, such as microphthalmia with linear skin lesions and Leber's hereditary optic neuropathy. We found that miR-181a/b downregulation strongly protects retinal neurons from cell death and significantly ameliorates the disease phenotype in all tested models. Altogether, our results demonstrate that miR-181a/b regulate mitochondrial homeostasis and that these miRNAs may be effective gene-independent therapeutic targets for MDs characterized by neuronal degeneration.

Lactate-Mediated Protection of Retinal Ganglion Cells

Loss of retinal ganglion cells (RGCs) is a leading cause of blinding conditions. The purpose of this study was to evaluate the effect of extracellular l-lactate on RGC survival facilitated through lactate metabolism and ATP production. We identified lactate as a preferred energy substrate over glucose in murine RGCs and showed that lactate metabolism and consequently increased ATP production are crucial components in promoting RGC survival during energetic crisis. Lactate was released to the extracellular environment in the presence of glucose and detained intracellularly during glucose deprivation. Lactate uptake and metabolism was unaltered in the presence and absence of glucose. However, the ATP production declined significantly for 24 h of glucose deprivation and increased significantly in the presence of lactate. Finally, lactate exposure for 2 and 24 h resulted in increased RGC survival during glucose deprivation. In conclusion, the metabolic pathway of lactate in RGCs may be of great future interest to unravel potential pharmaceutical targets, ultimately leading to novel therapies in the prevention of blinding neurodegenerative diseases, for example, glaucoma.

Sex Hormones and Optic Nerve Disorders: A Review

Aim: This review article presents a comprehensive overview of the literature on sex hormones (estrogens, androgens, progesterone) and optic nerve disorders, with a discussion of the implications for therapy and prevention.

Methods: Epidemiological, pre-clinical and clinical studies were reviewed.

Results: Analysis of the biological basis for a relationship between eye diseases and sex hormones showed that some types of hormones can exert a protective effect either directly on the retina and optic nerve or indirectly by modulating ocular blood flow. For example, it seems that estrogen exposure has a protective effect against glaucoma, whereas its deficit may lead to early onset of the disease. If further studies confirm the data in the literature, estrogen therapy, because of its antioxidant action, may be effective in the treatment of Leber's hereditary optic neuropathy, whereas, in the light of current studies, there does not seem to be an influence of estrogen on non-arteritic anterior ischemic optic neuritis (NAION).

Conclusions: Although there is some evidence that in some optic nerve pathologies the sex hormones seem to play an important role there are still too few studies providing evidence for its wider use in clinical practice.

Optical Coherence Tomography of the Retinal Ganglion Cell Complex in Leber's Hereditary Optic Neuropathy and Dominant Optic Atrophy

Background: Mitochondrial optic neuropathies such as Leber's Hereditary Optic Neuropathy (LHON) and Dominant Optic Atrophy (DOA) have been shown to produce an optic neuropathy secondary to retinal ganglion cell loss with thinning of the retinal ganglion cell complex (RGCC). Methods: We performed a retrospective analysis assessing the thicknesses of the peripapillary retinal nerve fiber layer (pRNFL) along with the macular retinal ganglion cell-inner plexiform layer (RGC-IPL) using optical coherence tomography (OCT). We compared these changes among acute and chronic LHON, DOA, and normal healthy control patients. Results: Patients with chronic LHON exhibited statistically significant thinning of the RNFL in the superior, nasal, and inferior quadrants of the retina. In acute LHON, the RNFL was relatively thicker in all but the temporal quadrant when compared with respective quadrants in normal eyes; however, statistical significance was not achieved. In DOA, the RNFL was thinnest in the superior and inferior quadrants of the retina, measuring between acute and chronic LHON thickness values. In chronic LHON and DOA, both the pRNFL and RGC-IPL were significantly thinner in all four retinal quadrants relative to controls. Conclusions: This article represents the first comparative study of the RGCC between LHON and DOA. Our findings demonstrated significant thickness reductions in pRNFL and macular RGC-IPL in patients with LHON and DOA, with different specific patterns consistent with the general patterns of thinning classically observed. This study suggests the usefulness of the RGCC as a potential in vivo biomarker for assessing disease in patients with LHON and DOA.

[Raxone in the Leber optical neuropathy: Parisian experience]

INTRODUCTION

Leber's Hereditary Optic Neuropathy (LHON) causes a rapid and severe decrease in visual acuity. Raxone^® (Idebenone, Santhera) is the only drug to have a European Marketing Authorization for the treatment of this optic neuropathy. It can be proposed in the first months after the onset of this optic neuropathy, according to an international consensus meeting.

PATIENTS AND METHODS

Retrospective study of the efficacy of Raxone^® on the visual acuity of patients with genetically confirmed LHON who were followed in four Parisian hospitals. The primary endpoint is the best recovery of LogMar visual acuity between baseline and the end of follow-up. The secondary endpoints are the evolution of LogMar visual acuity of the best eye at baseline and change in LogMar visual acuity for each eye considered separately.

RESULTS

Seventeen patients, three women and 14 men, mean age 34.2 years, naive to treatment with Raxone^® were included in this study. The mean duration of treatment was 11.0±6.6 months. A mitochondrial DNA mutation was found in all patients. Only 2 had the 14484 mutation. A recovery of better LogMar visual acuity was found at the end of the treatment for 4 eyes (23.5 %), and a deterioration was observed for 8 (47.0 %). Only 2 eyes (11.7 %) with the best visual acuity at baseline improved. On the other hand, 17.6 % of the eyes considered separately had an improvement in their LogMar visual acuity at the end of the treatment.

CONCLUSION

The results confirm the trend of Raxone^® treatment to improve patients' visual acuity. Given the recommendations of a consensus conference, this treatment should be started early after the onset of LHON. It is therefore important to look for this diagnosis in the presence of any hereditary optic neuropathy, in order to be able to initiate this treatment.

Mitochondrial DNA copy number in affected and unaffected LHON mutation carriers

OBJECTIVES

Leber's hereditary optic neuropathy (LHON) is a mitochondrial genetic disease characterized by a variable and reduced penetrance. Individuals carrying a primary LHON-causing mitochondrial DNA (mtDNA) mutation may either remain asymptomatic lifelong, as unaffected carriers, or develop sudden central visual loss that rapidly aggravates over some weeks. Over the years several genetic/environmental triggers able to modulate the risk of developing LHON have been proposed. We provided data supporting a possible correlation between LHON penetrance and the mtDNA copy number, a raw index of mitochondrial mass, whose increase could represent a compensatory response that cells implement to alleviate the pathogenic effect of the primary LHON-causing mtDNA mutations.

DATA DESCRIPTION

We collected Italian and Spanish subjects harboring one of the three common LHON primary mutations, either in heteroplasmic or homoplasmic status. For each population we were able to discriminate between affected subjects presenting typical clinical tracts of LHON and LHON-causing mutation carriers showing no symptoms correlated with vision loss. Each subject has been characterized for the presence of a LHON primary mutation, for its status of homoplasmy or heteroplasmy, and for the mtDNA content per cell, expressed as relative mtDNA/nDNA ratio respect to controls. Additional clinical information is present for all the Italian subjects.

Melanopsin Retinal Ganglion Cells and Pupil: Clinical Implications for Neuro-Ophthalmology

Melanopsin retinal ganglion cells (mRGCs) are intrinsically photosensitive RGCs that mediate many relevant non-image forming functions of the eye, including the pupillary light reflex, through the projections to the olivary pretectal nucleus. In particular, the post-illumination pupil response (PIPR), as evaluated by chromatic pupillometry, can be used as a reliable marker of mRGC function in vivo. In the last years, pupillometry has become a promising tool to assess mRGC dysfunction in various neurological and neuro-ophthalmological conditions. In this review we will present the most relevant findings of pupillometric studies in glaucoma, hereditary optic neuropathies, ischemic optic neuropathies, idiopathic intracranial hypertension, multiple sclerosis, Parkinson's disease, and mood disorders. The use of PIPR as a marker for mRGC function is also proposed for other neurodegenerative disorders in which circadian dysfunction is documented.

Leber's hereditary optic neuropathy misdiagnosed as optic neuritis and Lyme disease in a patient with multiple sclerosis

A 28-year-old Caucasian man developed sudden painless vision loss in the right eye. He was diagnosed with optic neuritis. MRI showed white matter lesions consistent with multiple sclerosis (MS), but no optic nerve enhancement. Eight months later, the left eye was affected in the same manner. Examination showed right optic atrophy and apparent left optic disc swelling. Workup revealed positive Lyme IgG. Differential diagnosis included optic neuritis and Lyme optic neuropathy, and he was treated with intravenous steroids, intravenous immunoglobulin, plasmapheresis and intravenous ceftriaxone without improvement. Neuro-ophthalmology consultation led to identification of pseudo-optic disc oedema, and Leber's hereditary optic neuropathy (LHON) was suspected and confirmed by genetic testing. LHON may occur in association with MS, and should be considered in patients with MS with vision loss atypical for optic neuritis. This is especially important as new treatments for LHON (including gene therapy) are currently undergoing clinical trials.

Central scotoma in tobacco-alcohol toxic optic neuropathy measured with semi-automated kinetic perimetry

PURPOSE

To measure the area of central scotoma obtained with semi-automated kinetic perimetry (SKP) in patients suffering from tobacco-alcohol toxic neuropathy (TATN).

METHODS

Twelve eyes of six patients with TATN were examined with SKP. Area of central scotoma was measured in square degrees (deg2). Additionally, static automated perimetry (SAP) within 60° was performed in each patient.

RESULTS

Area of central scotoma was 41.8 deg2 for III4e isopter, 22.9 deg2 for I4e isopter and 16.1 deg2 for I2e isopter in TATN patients. SAP revealed central scotoma in all patients. There was 100% of accordance between two methods.

CONCLUSION

SKP is comparable with SAP in assessing central scotoma. SKP offers advantage of measuring central scotoma and assessing remaining peripheral visual field in TATN, even with low incidence and prevalence of this clinical entity.

Dexamethasone protects retinal ganglion cells but not Müller glia against hyperglycemia in vitro

Diabetic retinopathy (DR) is a common complication of diabetes, for which hyperglycemia is a major etiological factor. It is known that retinal glia (Müller cells) and retinal ganglion cells (RGCs) are affected by diabetes, and there is evidence that DR is associated with neural degeneration. Dexamethasone is a glucocorticoid used to treat many inflammatory and autoimmune conditions, including several eye diseases like DR. Thus, our goal was to study the effect of dexamethasone on the survival of RGCs and Müller glial cells isolated from rat retinas and maintained in vitro under hyperglycemic conditions. The behavior of primary RGC cell cultures, and of mixed RGC and Müller cell co-cultures, was studied in hyperglycemic conditions (30 mM glucose), both in the presence and absence of Dexamethasone (1 μM). RGC and Müller cell survival was evaluated, and the conditioned media of these cultures was collected to quantify the inflammatory cytokines secreted by these cells using a multiplex assay. The role of IL-1β, IL-6 and TNFα in RGC death was also evaluated by adding these cytokines to the co-cultures. RGC survival decreased significantly when these cells were grown in high glucose conditions, reaching 54% survival when they were grown alone and only 33% when co-cultured with Müller glia. The analysis of the cytokines in the conditioned media revealed an increase in IL-1β, IL-6 and TNFα under hyperglycemic conditions, which reverted to the basal concentration in co-cultures maintained in the presence of dexamethasone. Finally, when these cytokines were added to co-cultures they appeared to have a direct effect on RGC survival. Hence, these cytokines could be implicated in the death of RGCs when glucose concentrations increase and dexamethasone might protect RGCs from the cell death induced in these conditions.

Clinical and genetic determinants of chronic visual pathway changes after methanol - induced optic neuropathy: four-year follow-up study

CONTEXT

Methanol poisoning induces acute optic neuropathy with possible long-term visual damage.

OBJECTIVE

To study the dynamics and key determinants of visual pathway functional changes during 4 years after acute methanol poisoning.

METHODS

A total of 42 patients with confirmed methanol poisoning (mean age 45.7 ± 4.4 years) were examined 4.9 ± 0.6, 25.0 ± 0.6, and 49.9 ± 0.5 months after discharge. The following tests were performed: visual evoked potential (VEP), retinal nerve fiber layer (RNFL) measurement, brain magnetic resonance imaging (MRI), complete ocular examination, biochemical tests, and apolipoprotein E (ApoE) genotyping.

RESULTS

Abnormal VEP P1 latency was registered in 18/42 right eyes (OD) and 21/42 left eyes (OS), abnormal N1P1 amplitude in 10/42 OD and OS. Mean P1 latency shortening during the follow-up was 15.0 ± 2.0 ms for 36/42 (86%) OD and 14.9 ± 2.4 ms for 35/42 (83%) OS, with maximum shortening up to 35.0 ms. No significant change of mean N1P1 amplitude was registered during follow-up. A further decrease in N1P1 amplitude ≥1.0 mcV in at least one eye was observed in 17 of 36 patients (47%) with measurable amplitude (mean decrease -1.11 ± 0.83 (OD)/-2.37 ± 0.66 (OS) mcV versus -0.06 ± 0.56 (OD)/-0.83 ± 0.64 (OS) mcV in the study population; both p < .001). ApoE4 allele carriers had lower global and temporal RNFL thickness and longer initial P1 latency compared to the non-carriers (all p < .05). The odds ratio for abnormal visual function was 8.92 (3.00-36.50; 95%CI) for ApoE4 allele carriers (p < .001). The presence of ApoE4 allele was further associated with brain necrotic lesions (r = 0.384; p = .013) and brain hemorrhages (r = 0.395; p = .011).

CONCLUSIONS

Improvement of optic nerve conductivity occurred in more than 80% of patients, but evoked potential amplitude tended to decrease during the 4 years of observation. ApoE4 allele carriers demonstrated lower RNFL thickness, longer P1 latency, and more frequent methanol-induced brain damage compared to non-carriers.

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.

Bioactivity and gene expression profiles of hiPSC-generated retinal ganglion cells in MT-ND4 mutated Leber's hereditary optic neuropathy

Leber's hereditary optic neuropathy (LHON) is the maternally inherited mitochondrial disease caused by homoplasmic mutations in mitochondrial electron transport chain Complex I subunit genes. The mechanism of its incomplete penetrance is still largely unclear. In this study, we created the patient-specific human induced pluripotent stem cells (hiPSCs) from MT-ND4 mutated LHON-affected patient, asymptomatic mutation carrier and healthy control, and differentiated them into retinal ganglion cells (RGCs). We found the defective neurite outgrowth in affected RGCs, but not in the carrier RGCs which had significant expression of SNCG gene. We observed enhanced mitochondrial biogenesis in affected and carrier derived RGCs. Surprisingly, we observed increased NADH dehydrogenase enzymatic activity of Complex I in hiPSC-derived RGCs of asymptomatic carrier, but not of the affected patient. LHON mutation substantially decreased basal respiration in both affected and unaffected carrier hiPSCs, and had the same effect on spare respiratory capacity, which ensures normal function of mitochondria in conditions of increased energy demand or environmental stress. The expression of antioxidant enzyme catalase was decreased in affected and carrier patient hiPSC-derived RGCs as compared to the healthy control, which might indicate to higher oxidative stress-enriched environment in the LHON-specific RGCs. Microarray profiling demonstrated enhanced expression of cell cycle machinery and downregulation of neuronal specific genes.

Hormone replacement therapy in Leber's hereditary optic neuropathy: Accelerated visual recovery in vivo

Purpose

To report an accelerated course of visual recovery in a case of Leber's hereditary optic neuropathy (LHON) following treatment with idebenone and hormone replacement therapy (HRT). We hereby demonstrate the clinical utility of estrogen's protective role in LHON in vivo.

Methods

We present a case of LHON in a menopausal woman carrying the 10197 mitochondrial DNA (mtDNA) mutation, who experienced loss of vision shortly after discontinuing her estrogen replacement regimen. Functional visual outcomes are reported following treatment with idebenone and HRT.

Results

The patient exhibited an accelerated course of visual recovery, experiencing improvement in vision as early as one month and complete reversal of vision loss by eight months post-therapy.

Conclusion

Idebenone treatment combined with HRT may have a synergistic effect in enhancing cellular bioenergetics and may explain the patient's accelerated visual improvement.

Citicoline and Retinal Ganglion Cells: Effects on Morphology and Function

BACKGROUND

Retinal ganglion cells (RGCs) are the nervous retinal elements which connect the visual receptors to the brain forming the nervous visual system. Functional and/or morphological involvement of RGCs occurs in several ocular and neurological disorders and therefore these cells are targeted in neuroprotective strategies. Cytidine 5-diphosphocholine or Citicoline is an endogenous compound that acts in the biosynthesis of phospholipids of cell membranes and increases neurotransmitters' levels in the Central Nervous System. Experimental studies suggested the neuromodulator effect and the protective role of Citicoline on RGCs. This review aims to present evidence of the effects of Citicoline in experimental models of RGCs degeneration and in human neurodegenerative disorders involving RGCs.

METHODS

All published papers containing experimental or clinical studies about the effects of Citicoline on RGCs morphology and function were reviewed.

RESULTS

In rodent retinal cultures and animal models, Citicoline induces antiapoptotic effects, increases the dopamine retinal level, and counteracts retinal nerve fibers layer thinning. Human studies in neurodegenerative visual pathologies such as glaucoma or non-arteritic ischemic neuropathy showed a reduction of the RGCs impairment after Citicoline administration. By reducing the RGCs' dysfunction, a better neural conduction along the post-retinal visual pathways with an improvement of the visual field defects was observed.

CONCLUSION

Citicoline, with a solid history of experimental and clinical studies, could be considered a very promising molecule for neuroprotective strategies in those pathologies (i.e. Glaucoma) in which morpho-functional changes of RGCc occurs.

Metallosis mimicking a metabolic disorder: a case report

Metalic prosthesis or occupational exposure are potential sources of systemic cobalt and chromium ion toxicity. The resultant multisystemic clinical presentation can lead to unnecessary investigations before a final etiologic diagnosis is made; with an average delay of a year or more commonly noted.

A 58-year old man presented with cardiomyopathy, pericardial effusion, polycytaemia, polyneuropathy, visual impairment, sudden hearing loss and hypothyroidism over a 2-year period post a metal-on-polyethylene hip replacement surgery.

Biochemistry test results showed serum lactate of 3.8 mmol/L (0.5–2.2 mmol/L). Urine organic acid screen showed mild increases in excretion of tricarboxylic acid cycle intermediates and 2-ethylhydracryllate; suggestive of primary or secondary mitochondrial dysfunction. There were also slight increases in excretion of 4-hydroxyphenyllactate and 4-hydroxyphenylpyruvate suggestive of liver dysfunction. Acylcarnitine profile showed slight increase in hydroxybutyrylcarnitine and tetradeceneoylcarnitine that may reflect ketosis.

In view of his clinical presentation and abnormal metabolic investigations, the initial working diagnosis was mitochondrial disease.

Subsequently, patient presented with hip pain, and radiologic and imaging studies revealed high density collections lateral to the right proximal part of the femur, and medial to the right ilium with signal changes suggestive of metallic content. This prompted toxicology screen which revealed elevated plasma cobalt concentration (903.32 μg/L; reference range: 0.1–0.4) and chromium (71.32 μg/L; <0.5). Six months post right hip prosthesis removal the concentrations have declined and was 61.72 μg/L and chromium 23.97 μg/L. Patient felt some improvement symptomatically, without evident deterioration in his vision or hearing.

This case emphasises careful consideration of past medical history, in patients presenting with multisystemic disease suggestive of mitochondrial dysfunction, and potential causality related to exposure to toxic agents. In retrospect, the absence of a family history could be viewed as a pertinent negative finding. Not uncommonly, specialist focus on their favored system and may not search for a unifying diagnosis. It is likely further delays in diagnosis would have occurred had the patient not developed hip pains, and ultimately referred to the orthopedic surgeons more familiar with similar cases.

Toxic medications in Leber's hereditary optic neuropathy

Leber's hereditary optic neuropathy (LHON) is a maternally inherited mitochondrial disorder characterized by acute bilateral vision loss. The pathophysiology involves reactive oxygen species (ROS), which can be affected by medications. This article reviews the evidence for medications with demonstrated and theoretical effects on mitochondrial function, specifically in relation to increased ROS production. The data reviewed provides guidance when selecting medications for individuals with LHON mutations (carriers) and are susceptible to conversion to affected. However, as with all medications, the proven benefits of these therapies must be weighed against, in some cases, purely theoretical risks for this unique patient population.

Rapidly purified ganglion cells from neonatal mouse retinas allow studies of mitochondrial morphology and autophagy

Retinal explants and mixed primary cultures are currently used to investigate retinal ganglion cells (RGCs) pathophysiology and pharmacology, but information on yield, quality and quantity of contaminant cells for the available RGCs enrichment techniques is lacking. Here we compare two methods of mouse primary RGCs purification and show that mitochondrial and autophagy parameters can be measured in rapidly purified RGCs. RGCs were purified from P0 mouse eyes using two methods based on the surface antigen Thy1. In a two-step immunopanning purification, a subtraction plate bound macrophage antiserum removed contaminant macrophages and endothelial cells; unbound RGCs were then affinity selected using a plate-bound antiThy1 antibody. In an immunopanning-magnetic separation, macrophage-antiserum bound cells were first subtracted and then RGCs were positively selected using an antiThy1 antibody bound to a magnetic column. The two-steps immunopanning yielded low amounts of 90% pure RGCs, whereas RGCs represented 30% of the 6-fold more cells collected by immunopanning-magnetic separation. RGCs purified with both methods could be microelectroporated to image expressed mitochondria and autophagosomes fluorescent probes and to show that expression of pathogenic Optic atrophy 1 mutants causes mitochondrial fragmentation. Thus, these two methods purify primary mouse RGCs amenable to studies of cell morphology, mitochondrial biology and autophagy.

Neuroimaging of Mitochondrial Cytopathies

Mitochondrial diseases are a complex and heterogeneous group of genetic disorders that occur as a result of either nuclear DNA or mitochondrial DNA pathogenic variants, leading to a decrease in oxidative phosphorylation and cellular energy (ATP) production. Increasing knowledge about molecular, biochemical, and genetic abnormalities related to mitochondrial dysfunction has expanded the neuroimaging phenotypes of mitochondrial disorders. As a consequence of this growing field, the imaging recognition patterns of mitochondrial cytopathies are continually evolving. In this review, we describe the main neuroimaging characteristics of pediatric mitochondrial diseases, ranging from classical to more recent and challenging features. Due to the increased knowledge about the imaging findings of mitochondrial cytopathies, the pediatric neuroradiologist plays a crucial role in the diagnosis and evaluation of these patients.

Topographic Macular Microvascular Changes and Correlation With Visual Loss in Chronic Leber Hereditary Optic Neuropathy

PURPOSE

To study the macular microvascular networks in patients affected by chronic Leber hereditary optic neuropathy (LHON) using optical coherence tomography angiography (OCTA), and to quantify these changes in different macular sectors.

DESIGN

Prospective cross-sectional study.

METHODS

Patients with a clinical and molecularly confirmed diagnosis of LHON (affected patients in the chronic stage) were enrolled from the neuro-ophthalmology clinic at the Doheny-UCLA. Patients and controls underwent a complete ophthalmologic evaluation, including imaging with OCTA.

RESULTS

Twenty-nine eyes from 15 LHON patients (14 male) and 20 eyes from 20 healthy subjects (13 male) were included in the analysis. Mean age was 32.0 ± 14.2 years (range 16-49 years) in the LHON group and 34.2 ± 10.1 years (range 23-48 years) in the control group (P = .552). In the parafoveal region, the vessel length density was lower in LHON patients, at both the SCP (9.1% ± 0.5% and 9.3% ± 0.4%, P = .041) and DCP (9.4% ± 0.5% and 9.8% ± 0.3%, P = .008) levels. In the sectorial analysis, vascular changes remained significant only in the parafoveal nasal and inferior regions. Univariate linear regression analysis demonstrated that the strongest associations with visual acuity were with parafoveal SCP perfusion density (R² = .276, P = .045) and parafoveal SCP vessel length density (R² = .277, P = .044).

CONCLUSIONS

LHON eyes have SCP and DCP changes that are mainly confined to the nasal and inferior parafoveal sectors that correspond to the papillomacular bundle. Furthermore, visual loss is associated with the SCP flow impairment, but not with the OCT-detectable structural damage.

Clinical syndromes associated with mtDNA mutations: where we stand after 30 years

The landmark year 1988 can be considered as the birthdate of mitochondrial medicine, when the first pathogenic mutations affecting mtDNA were associated with human diseases. Three decades later, the field still expands and we are not ‘scraping the bottom of the barrel’ yet. Despite the tremendous progress in terms of molecular characterization and genotype/phenotype correlations, for the vast majority of cases we still lack a deep understanding of the pathogenesis, good models to study, and effective therapeutic options. However, recent technological advances including somatic cell reprogramming to induced pluripotent stem cells (iPSCs), organoid technology, and tailored endonucleases provide unprecedented opportunities to fill these gaps, casting hope to soon cure the major primary mitochondrial phenotypes reviewed here. This group of rare diseases represents a key model for tackling the pathogenic mechanisms involving mitochondrial biology relevant to much more common disorders that affect our currently ageing population, such as diabetes and metabolic syndrome, neurodegenerative and inflammatory disorders, and cancer.

Progressive Chronic Retinal Axonal Loss Following Acute Methanol-induced Optic Neuropathy: Four-Year Prospective Cohort Study

PURPOSE

To study the dynamics and clinical determinants of chronic retinal nerve fiber layer thickness (RNFL) loss after methanol-induced optic neuropathy.

DESIGN

Prospective cohort study.

METHODS

All patients underwent complete ophthalmic evaluation including spectral-domain optical coherence tomography 3 times during 4 years of observation: 4.9 (±0.6), 25.0 (±0.6), and 49.9 (±0.5) months after discharge.

PARTICIPANTS

Eighty-four eyes of 42 survivors of methanol poisoning, mean age (standard deviation) of 45.7 (±4.4) years; and 82 eyes of 41 controls, mean age 44.0 (±4.2) years.

MAIN OUTCOME MEASURES

Global and temporal RNFL loss.

RESULTS

Abnormal RNFL thickness was registered in 13 of 42 (31%) survivors of methanol poisoning and chronic axonal loss in 10 of 42 (24%) patients. Significant decrease of global/temporal RNFL thickness during the observation period was found in the study population compared to the controls (P < .001). The risk estimate of chronic global RNFL loss for arterial blood pH < 7.3 at admission was 11.65 (95% confidence interval 1.91-71.12) after adjusting for age and sex. The patients with chronic axonal degeneration demonstrated progressive visual loss in 7 of 10 cases. The patients with abnormal RNFL thickness had magnetic resonance signs of brain damage in 10 of 13 vs 8 of 29 cases with normal RNFL thickness (P = .003). Signs of brain hemorrhages were present in 7 of 13 patients with abnormal RNFL thickness vs 5 of 29 cases with normal RNFL thickness (P = .015).

CONCLUSIONS

Methanol-induced optic neuropathy may lead to chronic retinal axonal loss during the following years. Arterial blood pH on admission is the strongest predictor of chronic RNFL thickness decrease. Chronic retinal neurodegeneration is associated with the progressive loss of visual functions and necrotic brain lesions.

Peripapillary vessel density changes in Leber's hereditary optic neuropathy: a new biomarker

IMPORTANCE

The contribution of the microvascular supply to the pathogenesis of Leber's hereditary optic neuropathy (LHON) is poorly understood.

BACKGROUND

We aimed at measuring the peripapillary capillary vessel density (VD) using optical coherence tomography angiography (OCT-A) at different stages of LHON.

DESIGN

Prospective, cross-sectional, multicenter, observational study.

PARTICIPANTS

Twenty-two LHON patients divided in four groups: unaffected mutation carriers (LHON-u); early sub-acute stage (LHON-e); late sub-acute stage (LHON-l); chronic stage (LHON-ch).

METHODS

OCT-A scans centred on the optic disc were obtained by spectral domain OCT system.

MAIN OUTCOME MEASURES

VD, retinal nerve fibre layer (RNFL) and ganglion cell-inner plexiform layer (GC-IPL) thickness were compared between groups.

RESULTS

Significant VD changes were detected in every sector (P < 0.0001). In LHON-e, the VD was reduced in the temporal sector compared with LHON-u and in the temporal and inferotemporal sectors compared with controls. In LHON-l, VD was reduced in whole, temporal, superotemporal and inferotemporal sectors compared with LHON-u and controls. In LHON-ch, the VD was reduced in all sectors compared to the other groups. An asynchronous pattern emerged in the temporal sector with VD changes occurring earlier than RNFL thickness changes and together with GC-IPL thinning.

CONCLUSIONS AND RELEVANCE

Significant peripapillary miscrovascular changes were detected over the different stages of LHON. Studying the vascular network separately from fibres revealed that microvascular changes in the temporal sector preceded the changes of RNFL and mirrored the GC-IPL changes. Measurements of the peripapillary vascular network may become a useful biomarker to monitor the disease process, evaluate therapeutic efficacy and elucidate pathophysiology.

Resveratrol ameliorates disorders of mitochondrial biogenesis and dynamics in a rat chronic ocular hypertension model

AIMS

To explore the roles of mitochondrial biogenesis and dynamics in both RGC-5 cells apoptosis and rat retinal damage induced by elevated pressure and their involvement in resveratrol (RSV)-induced cell protection.

MATERIALS AND METHODS

The chronic ocular hypertension (COH) model was established in rats by injecting superparamagnetic iron oxide into anterior chamber. The RGC-5 cells were incubated under ambient and elevated pressure (70 mm Hg) respectively. The intraocular pressure (IOP) was measured using a handheld Tonolab tonometer; mitochondrial dysfunction was analyzed by membrane potential (MMP) depolarization, reactive oxygen species (ROS) level and transmission electron microscope (TEM) detection. Annexin V/PI staining and the terminal deoxynucleotidy transferase dUTP nick end labeling (TUNEL) staining assay were performed for apoptosis detection. Hematoxylin-Eosin staining was performed for retinal morphology detection. The expression of mitochondrial biogenesis and dynamics relating proteins were analyzed by western blot.

KEY FINDINGS

The retinal morphology and mitochondrial function deteriorated in chronic ocular hypertension (COH) rats. The cells showed apoptosis and mitochondrial dysfunction under elevated pressure (70 mm Hg) incubation. Upregulating AMPK, NRF-1, Tfam, mfn-2, OPA1 expression with RSV-treatment could decrease the cell apoptosis, mitochondrial membrane potential depolarization, ROS generation both in in vitro and in vivo experiments, and normalized the retinal morphology in vivo.

SIGNIFICANCE

Both in vitro and in vivo experiments demonstrated that activated AMPK/PGC-1α signaling pathway and improved expression of proteins were related to mitochondrial dynamics could be the possible mechanism underlying in the RSV's mitochondrial protection.

Gene therapy for the mitochondrial genome: Purging mutations, pacifying ailments

In the recent years, the reported cases of mitochondrial disorders have reached a colossal number. These disorders spawn a sundry of pathological conditions, which lead to pernicious symptoms and even fatality. Due to the unpredictable etiologies, mitochondrial diseases are putatively referred to as "mystondria" (mysterious diseases of mitochondria). Although present-day research has greatly improved our understanding of mitochondrial disorders, effective therapeutic interventions are still at the precursory stage. The conundrum becomes further complicated because these pathologies might occur due to either mitochondrial DNA (mtDNA) mutations or due to mutations in the nuclear DNA (nDNA), or both. While correcting nDNA mutations by using gene therapy (replacement of defective genes by delivering wild-type (WT) ones into the host cell, or silencing a dominant mutant allele that is pathogenic) has emerged as a promising strategy to address some mitochondrial diseases, the complications in correcting the defects of mtDNA in order to renovate mitochondrial functions have remained a steep challenge. In this review, we focus specifically on the selective gene therapy strategies that have demonstrated prospects in targeting the pathological mutations in the mitochondrial genome, thereby treating mitochondrial ailments.

Analysis of Visual Field Defects Obtained with Semiautomated Kinetic Perimetry in Patients with Leber Hereditary Optic Neuropathy

Purpose

To analyse visual field (VF) defects obtained using semiautomated kinetic perimetry (SKP) in patients suffering from Leber hereditary optic neuropathy (LHON).

Methods

Twenty-two eyes of eleven consecutive LHON male patients with confirmed mitochondrial 11778G>A DNA mutation were prospectively examined with the V4e stimulus of SKP in both eyes. The mean time after the onset of LHON was one year. The area of obtained isopters was measured in square degrees (deg²). Additionally, static automated perimetry (SAP) within 30° was performed.

Results

Visual acuity ranged from counting fingers to 50 cm to 0.4. VFs obtained with SKP showed central scotomas in 18 eyes (82%); the periphery of the VF in these eyes remained intact. The mean area of central scotoma was 408.8 deg², and the mean area of the peripheral VF was 12291.1 deg²; SAP also revealed central scotoma in these patients. In four eyes (18%) with the worst visual acuity, only the residual central island of VF was found using SKP (mean area 898.4 deg²). SAP was difficult to obtain in these patients.

Conclusions

SKP provides additional clinical information in regard to the visual function of LHON patients. SKP enables the quantification of the area of central scotoma, preserved peripheral VF, and residual central island of vision. Using V4 stimulus is especially useful in LHON patients with poor visual acuity, when SAP is difficult to obtain.

Optic neuropathies: the tip of the neurodegeneration iceberg

The optic nerve and the cells that give origin to its 1.2 million axons, the retinal ganglion cells (RGCs), are particularly vulnerable to neurodegeneration related to mitochondrial dysfunction. Optic neuropathies may range from non-syndromic genetic entities, to rare syndromic multisystem diseases with optic atrophy such as mitochondrial encephalomyopathies, to age-related neurodegenerative diseases such as Alzheimer’s and Parkinson’s disease where optic nerve involvement has, until recently, been a relatively overlooked feature. New tools are available to thoroughly investigate optic nerve function, allowing unparalleled access to this part of the central nervous system. Understanding the molecular pathophysiology of RGC neurodegeneration and optic atrophy, is key to broadly understanding the pathogenesis of neurodegenerative disorders, for monitoring their progression in describing the natural history, and ultimately as outcome measures to evaluate therapies. In this review, the different layers, from molecular to anatomical, that may contribute to RGC neurodegeneration and optic atrophy are tackled in an integrated way, considering all relevant players. These include RGC dendrites, cell bodies and axons, the unmyelinated retinal nerve fiber layer and the myelinated post-laminar axons, as well as olygodendrocytes and astrocytes, looked for unconventional functions. Dysfunctional mitochondrial dynamics, transport, homeostatic control of mitobiogenesis and mitophagic removal, as well as specific propensity to apoptosis may target differently cell types and anatomical settings. Ultimately, we can envisage new investigative approaches and therapeutic options that will speed the early diagnosis of neurodegenerative diseases and their cure.

Haplogroup J mitogenomes are the most sensitive to the pesticide rotenone: Relevance for human diseases

There is growing evidence that the sequence variation of mitochondrial DNA (mtDNA), which clusters in population- and/or geographic-specific haplogroups, may result in functional effects that, in turn, become relevant in disease predisposition or protection, interaction with environmental factors and ultimately in modulating longevity. To unravel functional differences between mtDNA haplogroups we here employed transmitochondrial cytoplasmic hybrid cells (cybrids) grown in galactose medium, a culture condition that forces oxidative phosphorylation, and in the presence of rotenone, the classic inhibitor of respiratory Complex I. Under this experimental paradigm we assessed functional parameters such as cell viability and respiration, ATP synthesis, reactive oxygen species production and mtDNA copy number. Our analyses show that haplogroup J1, which is common in western Eurasian populations, is the most sensitive to rotenone, whereas K1 mitogenomes orchestrate the best compensation, possibly because of the haplogroup-specific missense variants impinging on Complex I function. Remarkably, haplogroups J1 and K1 fit the genetic associations previously established with Leber's hereditary optic neuropathy (LHON) for J1, as a penetrance enhancer, and with Parkinson's disease (PD) for K1, as a protective background. Our findings provide functional evidences supporting previous well-established genetic associations of specific haplogroups with two neurodegenerative pathologies, LHON and PD. Our experimental paradigm is instrumental to highlighting the subtle functional differences characterizing mtDNA haplogroups, which will be increasingly needed to dissect the role of mtDNA genetic variation in health, disease and longevity.

Peculiar combinations of individually non-pathogenic missense mitochondrial DNA variants cause low penetrance Leber's hereditary optic neuropathy

We here report on the existence of Leber’s hereditary optic neuropathy (LHON) associated with peculiar combinations of individually non-pathogenic missense mitochondrial DNA (mtDNA) variants, affecting the MT-ND4, MT-ND4L and MT-ND6 subunit genes of Complex I. The pathogenic potential of these mtDNA haplotypes is supported by multiple evidences: first, the LHON phenotype is strictly inherited along the maternal line in one very large family; second, the combinations of mtDNA variants are unique to the two maternal lineages that are characterized by recurrence of LHON; third, the Complex I-dependent respiratory and oxidative phosphorylation defect is co-transferred from the proband’s fibroblasts into the cybrid cell model. Finally, all but one of these missense mtDNA variants cluster along the same predicted fourth E-channel deputed to proton translocation within the transmembrane domain of Complex I, involving the ND1, ND4L and ND6 subunits. Hence, the definition of the pathogenic role of a specific mtDNA mutation becomes blurrier than ever and only an accurate evaluation of mitogenome sequence variation data from the general population, combined with functional analyses using the cybrid cell model, may lead to final validation. Our study conclusively shows that even in the absence of a clearly established LHON primary mutation, unprecedented combinations of missense mtDNA variants, individually known as polymorphisms, may lead to reduced OXPHOS efficiency sufficient to trigger LHON. In this context, we introduce a new diagnostic perspective that implies the complete sequence analysis of mitogenomes in LHON as mandatory gold standard diagnostic approach.

Leber’s hereditary optic neuropathy (LHON) is a common cause of maternally inherited vision loss. In the large majority of cases LHON is due to mitochondrial DNA (mtDNA) point mutations, clearly distinct from common polymorphisms normally found in the general population, affecting the mitochondrial function, thus defined as pathogenic. For the first time, we here demonstrate, on the genetic and functional ground, that unusual combinations of otherwise polymorphic and non-pathogenic mtDNA variants are sufficient for causing low-penetrance maternally inherited optic neuropathy in pedigrees fitting the LHON clinical diagnosis. Our findings bridge the blurry border between “pathogenic” and “neutral” mutations in an overall continuum that truly depends on the specific and sometime unique combination of variants characterizing each mitogenome. As a result, we conclude that, for an accurate diagnosis of LHON and possibly of other mitochondrial diseases, the only approach that can disclose all possible causative sources is complete mitogenome sequencing.

Cobalamin-Associated Superoxide Scavenging in Neuronal Cells Is a Potential Mechanism for Vitamin B12-Deprivation Optic Neuropathy

Chronic deficiency of vitamin B12 is the only nutritional deficiency definitively proved to cause optic neuropathy and loss of vision. The mechanism by which this occurs is unknown. Optic neuropathies are associated with death of retinal ganglion cells (RGCs), neurons that project their axons along the optic nerve to the brain. Injury to RGC axons causes a burst of intracellular superoxide, which then signals RGC apoptosis. Vitamin B12 (cobalamin) was recently shown to be a superoxide scavenger, with a rate constant similar to superoxide dismutase. Given that vitamin B12 deficiency causes an optic neuropathy through unknown mechanisms and that it is a potent superoxide scavenger, we tested whether cobalamin, a vitamin B12 vitamer, would be neuroprotective in vitro and in vivo. We found that cobalamin scavenged superoxide in neuronal cells in vitro treated with the reduction-oxidation cycling agent menadione. In vivo confocal scanning laser ophthalmoscopy demonstrated that optic nerve transection in Long-Evans rats increased superoxide levels in RGCs. The RGC superoxide burst was significantly reduced by intravitreal cobalamin and resulted in increased RGC survival. These data demonstrate that cobalamin may function as an endogenous neuroprotectant for RGCs through a superoxide-associated mechanism.

Patterns of Retinal Ganglion Cell Damage in Neurodegenerative Disorders: Parvocellular vs Magnocellular Degeneration in Optical Coherence Tomography Studies

Many neurodegenerative disorders, such as Parkinson’s disease (PD) and Alzheimer’s disease (AD), are characterized by loss of retinal ganglion cells (RGCs) as part of the neurodegenerative process. Optical coherence tomography (OCT) studies demonstrated variable degree of optic atrophy in these diseases. However, the pattern of degenerative changes affecting the optic nerve (ON) can be different. In particular, neurodegeneration is more evident for magnocellular RGCs in AD and multiple system atrophy with a pattern resembling glaucoma. Conversely, in PD and Huntington’s disease, the parvocellular RGCs are more vulnerable. This latter pattern closely resembles that of mitochondrial optic neuropathies, possibly pointing to similar pathogenic mechanisms. In this review, the currently available evidences on OCT findings in these neurodegenerative disorders are summarized with particular emphasis on the different pattern of RGC loss. The ON degeneration could become a validated biomarker of the disease, which may turn useful to follow natural history and possibly assess therapeutic efficacy.

Leber Hereditary Optic Neuropathy and Longitudinally Extensive Transverse Myelitis

Leber Hereditary Optic Neuropathy is an inherited optic neuropathy caused by mitochondrial DNA point mutations leading to sudden, painless loss of vision. We report a case of an 8-year-old boy presenting with a radiological phenotype of longitudinally extensive transverse myelitis on a background of severe visual impairment secondary to Leber Hereditary Optic Neuropathy (LHON). He was found to have dual mitochondrial DNA mutations at 14484 (MTND6 gene) and 4160 (MTND1 gene) in a family with a severe form of LHON characterised by not only an unusually high penetrance of optic neuropathy, but also severe extra-ocular neurological complications. The m.14484T>C mutation is a common LHON mutation, but the m.4160T>C mutation is to our knowledge not reported outside this family and appears to drive the neurological manifestations. To our knowledge there have been no previous reports of spinal cord lesions in children with LHON.

International Consensus Statement on the Clinical and Therapeutic Management of Leber Hereditary Optic Neuropathy

Leber hereditary optic neuropathy (LHON) is currently estimated as the most frequent mitochondrial disease (1 in 27,000-45,000). Its molecular pathogenesis and natural history is now fairly well understood. LHON also is the first mitochondrial disease for which a treatment has been approved (idebenone-Raxone, Santhera Pharmaceuticals) by the European Medicine Agency, under exceptional circumstances because of the rarity and severity of the disease. However, what remains unclear includes the optimal target population, timing, dose, and frequency of administration of idebenone in LHON due to lack of accepted definitions, criteria, and general guidelines for the clinical management of LHON. To address these issues, a consensus conference with a panel of experts from Europe and North America was held in Milan, Italy, in 2016. The intent was to provide expert consensus statements for the clinical and therapeutic management of LHON based on the currently available evidence. We report the conclusions of this conference, providing the guidelines for clinical and therapeutic management of LHON.

Epigenetic Treatment of Neurodegenerative Ophthalmic Disorders: An Eye Toward the Future

Eye disease is one of the primary medical conditions that requires attention and therapeutic intervention in ageing populations worldwide. Further, the global burden of diabetes and obesity, along with heart disease, all lead to secondary manifestations of ophthalmic distress. Therefore, there is increased interest in developing innovative new approaches that target various mechanisms and sequelae driving conditions that result in adverse vision. The research challenge is even greater given that the terrain of eye diseases is difficult to landscape into a single therapeutic theme. This report addresses the burden of eye disease due to mitochondrial dysfunction, including antioxidant, autophagic, epigenetic, mitophagic, and other cellular processes that modulate the biomedical end result. In this light, we single out lipoic acid as a potent known natural activator of these pathways, along with alternative and potentially more effective conjugates, which together harness the necessary potency, specificity, and biodistribution parameters required for improved therapeutic outcomes.

The metabolomic signature of Leber's hereditary optic neuropathy reveals endoplasmic reticulum stress

Leber's hereditary optic neuropathy (MIM#535000), the commonest mitochondrial DNA-related disease, is caused by mutations affecting mitochondrial complex I. The clinical expression of the disorder, usually occurring in young adults, is typically characterized by subacute, usually sequential, bilateral visual loss, resulting from the degeneration of retinal ganglion cells. As the precise action of mitochondrial DNA mutations on the overall cell metabolism in Leber's hereditary optic neuropathy is unknown, we investigated the metabolomic profile of the disease. High performance liquid chromatography coupled with tandem mass spectrometry was used to quantify 188 metabolites in fibroblasts from 16 patients with Leber's hereditary optic neuropathy and eight healthy control subjects. Latent variable-based statistical methods were used to identify discriminating metabolites. One hundred and twenty-four of the metabolites were considered to be accurately quantified. A supervised orthogonal partial least squares discriminant analysis model separating patients with Leber's hereditary optic neuropathy from control subjects showed good predictive capability (Q 2cumulated = 0.57). Thirty-eight metabolites appeared to be the most significant variables, defining a Leber's hereditary optic neuropathy metabolic signature that revealed decreased concentrations of all proteinogenic amino acids, spermidine, putrescine, isovaleryl-carnitine, propionyl-carnitine and five sphingomyelin species, together with increased concentrations of 10 phosphatidylcholine species. This signature was not reproduced by the inhibition of complex I with rotenone or piericidin A in control fibroblasts. The importance of sphingomyelins and phosphatidylcholines in the Leber's hereditary optic neuropathy signature, together with the decreased amino acid pool, suggested an involvement of the endoplasmic reticulum. This was confirmed by the significantly increased phosphorylation of PERK and eIF2α, as well as the greater expression of C/EBP homologous protein and the increased XBP1 splicing, in fibroblasts from affected patients, all these changes being reversed by the endoplasmic reticulum stress inhibitor, TUDCA (tauroursodeoxycholic acid). Thus, our metabolomic analysis reveals a pharmacologically-reversible endoplasmic reticulum stress in complex I-related Leber's hereditary optic neuropathy fibroblasts, a finding that may open up new therapeutic perspectives for the treatment of Leber's hereditary optic neuropathy with endoplasmic reticulum-targeting drugs.

Optic Nerve Hypoplasia Is a Pervasive Subcortical Pathology of Visual System in Neonates

Purpose

Optic nerve hypoplasia (ONH) is the most common cause of childhood congenital blindness in developed nations, yet the fundamental pathobiology of ONH remains unknown. The objective of this study was to employ a ‘face validated' murine model to determine the timing of onset and the pathologic characteristics of ONH.

Methods

Based on the robust linkage between X-linked CASK haploinsufficiency and clinically diagnosed ONH, we hypothesized that heterozygous deletion of CASK (CASK^(+/−)) in rodents will produce an optic nerve pathology closely recapitulating ONH. We quantitatively analyzed the entire subcortical visual system in female CASK^(+/−) mice using immunohistochemistry, anterograde axonal tracing, toluidine blue staining, transmission electron microscopy, and serial block-face scanning electron microscopy.

Results

CASK haploinsuffiency in mice phenocopies human ONH with complete penetrance, thus satisfying the ‘face validity'. We demonstrate that the optic nerve in CASK^(+/−) mice is not only thin, but is comprised of atrophic retinal axons and displays reactive astrogliosis. Myelination of the optic nerve axons remains unchanged. Moreover, we demonstrate a significant decrease in retinal ganglion cell (RGC) numbers and perturbation in retinothalamic connectivity. Finally, we used this mouse model to define the onset and progression of ONH pathology, demonstrating for the first time that optic nerve defects arise at neonatally in CASK^(+/−)mice.

Conclusions

Optic nerve hypoplasia is a complex neuropathology of the subcortical visual system involving RGC loss, axonopathy, and synaptopathy and originates at a developmental stage in mice that corresponds to the late third trimester development in humans.

Mitochondrial disorders of the retinal ganglion cells and the optic nerve

OBJECTIVES

To summarise and discuss recent findings and future perspectives concerning mitochondrial disorders (MIDs) affecting the retinal ganglion cells and the optic nerve (mitochondrial optic neuropathy. MON).

METHOD

Literature review.

RESULTS

MON in MIDs is more frequent than usually anticipated. MON may occur in specific as well as non-specific MIDs. In specific and non-specific MIDs, MON may be a prominent or non-prominent phenotypic feature and due to mutations in genes located either in the mitochondrial DNA (mtDNA) or the nuclear DNA (nDNA). Clinically, MON manifests with painless, bilateral or unilateral, slowly or rapidly progressive visual impairment and visual field defects. In some cases, visual impairment may spontaneously recover. The most frequent MIDs with MON include LHON due to mutations in mtDNA-located genes and autosomal dominant optic atrophy (ADOA) or autosomal recessive optic atrophy (AROA) due to mutations in nuclear genes. Instrumental investigations for diagnosing MON include fundoscopy, measurement of visual acuity, visual fields, and color vision, visually-evoked potentials, optical coherence tomography, fluorescein angiography, electroretinography, and MRI of the orbita and cerebrum. In non-prominent MON, work-up of the muscle biopsy with transmission electron microscopy may indicate mitochondrial destruction. Treatment is mostly supportive but idebenone has been approved for LHON and experimental approaches are promising.

CONCLUSIONS

MON needs to be appreciated, requires extensive diagnostic work-up, and supportive treatment should be applied although loss of vision, as the most severe outcome, can often not be prevented.

A Review of Mitochondrial Optic Neuropathies: From Inherited to Acquired Forms

In recent years, the term mitochondrial optic neuropathy (MON) has increasingly been used within the literature to describe a group of optic neuropathies that exhibit mitochondrial dysfunction in retinal ganglion cells (RGCs). Interestingly, MONs include genetic aetiologies, such as Leber hereditary optic neuropathy (LHON) and dominant optic atrophy (DOA), as well as acquired aetiologies resulting from drugs, nutritional deficiencies, and mixed aetiologies. Regardless of an inherited or acquired cause, patients exhibit the same clinical manifestations with selective loss of the RGCs due to mitochondrial dysfunction. Various novel therapies are being explored to reverse or limit damage to the RGCs. Here we review the pathophysiology, clinical manifestations, differential diagnosis, current treatment, and promising therapeutic targets of MON.

A Perspective on the Müller Cell-Neuron Metabolic Partnership in the Inner Retina

The Müller cells represent the predominant macroglial cell in the retina. In recent decades, Müller cells have been acknowledged to be far more influential on neuronal homeostasis in the retina than previously assumed. With its unique localization, spanning the entire retina being interposed between the vessels and neurons, Müller cells are responsible for the functional and metabolic support of the surrounding neurons. As a consequence of major energy demands in the retina, high levels of glucose are consumed and processed by Müller cells. The present review provides a perspective on the symbiotic relationship between Müller cells and inner retinal neurons on a cellular level by emphasizing the essential role of energy metabolism within Müller cells in relation to retinal neuron survival.

Neuroprotective effect of 5ɑ-androst-3β,5,6β-triol on retinal ganglion cells in a rat chronic ocular hypertension model

Previous studies have demonstrated that 5ɑ-androst-3β,5,6β-triol (Triol), a synthesized steroid compound, showed notable neuroprotective effect in cultured cortical neurons. In the present study, we explored whether and how Triol have neuroprotective effect on retinal ganglion cells (RGCs) in a chronic ocular hypertension (COH) rat model. COH model was produced by injecting superparamagnetic iron oxide micro-beads into the anterior chamber, and Triol was administrated (4.8μg/100g, i.p., once daily for 4 weeks). Immunohistochemistry experiments showed that in whole flat-mounted COH retinas, the number of CTB-labeled survival RGCs was progressively reduced, while TUNEL-positive signals were significantly increased from 1 to 4 weeks after the micro-bead injection. Triol administration significantly attenuated the reduction in the number of CTB-labeled RGCs, and partially reduced the increased number of TUNEL-positive signals in COH retinas. Furthermore, Triol administration partially reduced the levels of malondialdehyde (MDA) and reactive oxygen species (ROS), and significantly rescued the activities of mitochondrial respiratory chain complex (MRCC) I/II/III in COH retinas. Our results suggest that Triol prevents RGCs from apoptotic death in COH retinas by reducing the lipid peroxidation and enhancing the activities of MRCCs.

Mitochondrial dynamics, transport, and quality control: A bottleneck for retinal ganglion cell viability in optic neuropathies

Retinal ganglion cells, the neurons that selectively die in glaucoma and other optic neuropathies, are endowed with an exceedingly active metabolism and display a particular vulnerability to mitochondrial dysfunction. Mitochondria are exquisitely dynamic organelles that are continually responding to endogenous and environmental cues to readily meet the energy demand of neuronal networks. The highly orchestrated regulation of mitochondrial biogenesis, fusion, fission, transport and degradation is paramount for the maintenance of energy-expensive synapses at RGC dendrites and axon terminals geared for optimal neurotransmission. The present review focuses on the progress made to date on understanding the biology of mitochondrial dynamics and quality control and how dysregulation of these processes can profoundly affect retinal ganglion cell viability and function in optic nerve diseases.

Increased Survival and Partly Preserved Cognition in a Patient With ACO2-Related Disease Secondary to a Novel Variant

ACO2 encodes aconitase 2, catalyzing the second step of the tricarboxylic acid. To date, there are only 6 reported families with 5 unique ACO2 mutations. Affected individuals can develop intellectual disability, epilepsy, brain atrophy, hypotonia, ataxia, optic atrophy, and retinal degeneration. Here, we report an 18-year-old boy with a novel ACO2 variant discovered on whole-exome sequencing. He presented with childhood-onset ataxia, impaired self-help skills comparable to severe-profound intellectual disability, intractable epilepsy, cerebellar atrophy, peripheral neuropathy, optic atrophy, and pigmentary retinopathy. His variant is the sixth unique ACO2 mutation. In addition, compared to mild cases (isolated optic atrophy) and severe cases (infantile death), our patient may be moderately affected, evident by increased survival and some preserved cognition (ability to speak full sentences and follow commands), which is a novel presentation. This case expands the disease spectrum to include increased survival with partly spared cognition.