The Role of Mitochondria in Optic Atrophy With Autosomal Inheritance

Myelination selectively modulates BOLD signal in white matter

There is increasing recognition that blood oxygenation level dependent (BOLD) signals are detectable in white matter (WM) and reflect an important, heretofore overlooked functional activity in the brain, but their biophysical origins remain understudied and poorly understood. By integrating several disparate, multimodal data sets, we established the associations of resting state BOLD signals with key microstructural, hemodynamic and metabolic features in WM. In particular, we identified the roles of myelination and fiber type in modulating BOLD effects, and derived relationships between measurements of BOLD signal power and cerebral blood volume, flow, oxygen extraction and metabolic rate of oxygen consumption, which are predicted using a simple theory and then verified empirically. Our findings demonstrate that myelin selectively influences the fractional amplitude of low-frequency fluctuations (fALFF) in BOLD signals, and that differences in myelin content account for variations in their temporal spectra and hemodynamic response functions, but these in turn are qualitatively different in association versus projection fibers. Other determinants of BOLD in WM are further revealed by converging biological, genomic and neurochemical evidence, including measurements of neurite and mitochondrial densities. Moreover, analyses of images of the optic nerve from human subjects confirm that BOLD activations evoked by visual stimuli are preferentially localized to unmyelinated portions, with minimal responses in fully myelinated regions of the same nerve, suggesting a myelin-dependent requirement for BOLD effects in WM.

Novel in vivo models of autosomal optic atrophy reveal conserved pathological changes in neuronal mitochondrial structure and function

Autosomal optic atrophy (AOA) is a form of hereditary optic neuropathy characterized by the irreversible and progressive degermation of the retinal ganglion cells. Most cases of AOA are associated with a single dominant mutation in OPA1, which encodes a protein required for fusion of the inner mitochondrial membrane. It is unclear how loss of OPA1 leads to neuronal death, and despite ubiquitous expression appears to disproportionately affect the RGCs. This study introduces two novel in vivo models of OPA1-mediated AOA, including the first developmentally viable vertebrate Opa1 knockout (KO). These models allow for the study of Opa1 loss in neurons, specifically RGCs. Though survival is significantly reduced in Opa1 deficient zebrafish and Drosophila, both models permit the study of viable larvae. Moreover, zebrafish Opa1 KO larvae show impaired visual function but unchanged locomotor function, indicating that retinal neurons are particularly sensitive to Opa1 loss. Proteomic profiling of both models reveals marked disruption in protein expression associated with mitochondrial function, consistent with an observed decrease in mitochondrial respiratory function. Similarly, mitochondrial fragmentation and disordered cristae organization were observed in neuronal axons in both models highlighting Opa1's highly conserved role in regulating mitochondrial morphology and function in neuronal axons. Importantly, in Opa1 deficient zebrafish, mitochondrial disruption and visual impairment precede degeneration of RGCs. These novel models mimic key features of AOA and provide valuable tools for therapeutic screening. Our findings suggest that therapies enhancing mitochondrial function may offer a potential treatment strategy for AOA.

Insights on the Genetic and Phenotypic Complexities of Optic Neuropathies

Background/Objectives: Optic neuropathies are a category of illnesses that ultimately cause damage to the optic nerve, leading to vision impairment and possible blindness. Disorders such as dominant optic atrophy (DOA), Leber hereditary optic neuropathy (LHON), and glaucoma demonstrate intricate genetic foundations and varied phenotypic manifestations. This narrative review study seeks to consolidate existing knowledge on the genetic and molecular mechanisms underlying ocular neuropathies, examine genotype-phenotype correlations, and assess novel therapeutic options to improve diagnostic and treatment methodologies. Methods: A systematic literature review was performed in October 2024, utilizing PubMed, Medline, the Cochrane Library, and ClinicalTrials.gov. Search terms encompassed "optic neuropathy", "genetic variants", "LHON", "DOA", "glaucoma", and "molecular therapies". Studies were chosen according to established inclusion criteria, concentrating on the genetic and molecular dimensions of optic neuropathies and their therapeutic ramifications. Results: The results indicate that DOA and LHON are mostly associated with the mitochondrial dysfunction resulting from pathogenic variants in nuclear genes, mainly OPA1, and mitochondrial DNA (mtDNA) genes, respectively. Glaucoma, especially its intricate variants, is linked to variants in genes like MYOC, OPTN, and TBK1. Molecular mechanisms, such as oxidative stress and inflammatory modulation, are pivotal in disease progression. Innovative therapeutics, including gene therapy, RNA-based treatments, and antioxidants such as idebenone, exhibit promise for alleviating optic nerve damage and safeguarding vision. Conclusions: Genetic and molecular investigations have markedly enhanced our comprehension of ocular neuropathies. The amalgamation of genetic and phenotypic data is essential for customized medical strategies. Additional research is required to enhance therapeutic strategies and fill the gaps in our understanding of the underlying pathophysiology. This interdisciplinary approach shows potential for enhancing patient outcomes in ocular neuropathies.

Mitochondria in Retinal Ganglion Cells: Unraveling the Metabolic Nexus and Oxidative Stress

This review explored the role of mitochondria in retinal ganglion cells (RGCs), which are essential for visual processing. Mitochondrial dysfunction is a key factor in the pathogenesis of various vision-related disorders, including glaucoma, hereditary optic neuropathy, and age-related macular degeneration. This review highlighted the critical role of mitochondria in RGCs, which provide metabolic support, regulate cellular health, and respond to cellular stress while also producing reactive oxygen species (ROS) that can damage cellular components. Maintaining mitochondrial function is essential for meeting RGCs' high metabolic demands and ensuring redox homeostasis, which is crucial for their proper function and visual health. Oxidative stress, exacerbated by factors like elevated intraocular pressure and environmental factors, contributes to diseases such as glaucoma and age-related vision loss by triggering cellular damage pathways. Strategies targeting mitochondrial function or bolstering antioxidant defenses include mitochondrial-based therapies, gene therapies, and mitochondrial transplantation. These advances can offer potential strategies for addressing mitochondrial dysfunction in the retina, with implications that extend beyond ocular diseases.

Exploring the evidence for mitochondrial dysfunction and genetic abnormalities in the etiopathogenesis of tropical ataxic neuropathy

Tropical ataxic neuropathy (TAN) is characterised by ataxic polyneuropathy, degeneration of the posterior columns and pyramidal tracts, optic atrophy, and sensorineural hearing loss. It has been attributed to nutritional/toxic etiologies, but evidence for the same has been equivocal. TAN shares common clinical features with inherited neuropathies and mitochondrial disorders, it may be hypothesised that genetic abnormalities may underlie the pathophysiology of TAN. This study aimed to establish evidence for mitochondrial dysfunction by adopting an integrated biochemical and multipronged genetic analysis. Patients (n = 65) with chronic progressive ataxic neuropathy with involvement of visual and/or auditory pathways underwent deep phenotyping, genetic studies including mitochondrial DNA (mtDNA) deletion analysis, mtDNA and clinical exome sequencing (CES), and respiratory chain complex (RCC) assay. The phenotypic characteristics included dysfunction of visual (n = 14), auditory (n = 12) and visual + auditory pathways (n = 29). Reduced RCC activity was present in 13 patients. Mitochondrial DNA deletions were noted in five patients. Sequencing of mtDNA (n = 45) identified a homoplasmic variant (MT-ND6) and a heteroplasmic variant (MT-COI) in one patient each. CES (n = 45) revealed 55 variants in nuclear genes that are associated with neuropathy (n = 27), deafness (n = 7), ataxia (n = 4), and mitochondrial phenotypes (n = 5) in 36 patients. This study provides preliminary evidence that TAN is associated with a spectrum of genetic abnormalities, including those associated with mitochondrial dysfunction, which is in contradistinction from the prevailing hypothesis that TAN is related to dietary toxins. Analysing the functional relevance of these genetic variants may improve the understanding of the pathogenesis of TAN.

RTN4IP1-associated non-syndromic optic neuropathy and rod-cone dystrophy

BACKGROUND

Biallelic variants in RTN4IP1 are a well-established cause of syndromic and nonsyndromic early-onset autosomal recessive optic neuropathy. They have more recently been reported to cause a concomitant but later-onset rod-cone dystrophy with or without syndromic features.

METHODS

A comprehensive evaluation was performed that included assessment of visual and retinal function, clinical examination, and retinal imaging. Childhood ophthalmic records as well as the results of genetic testing were evaluated.

RESULTS

A 24-year-old female described longstanding reduced visual acuity with more recent subjective impairment of dark adaptation. Visual acuity was subnormal in both eyes. Goldmann kinetic perimetry demonstrated scotomas in a pattern consistent with the presence of both optic neuropathy and rod-cone dystrophy with fundus exam as well as retinal imaging showing corroborating findings. Full-field electroretinography further confirmed the presence of a rod-cone dystrophy. Genetic testing demonstrated biallelic variants in RTN4IP1, one of which was novel, in association with the ocular findings.

CONCLUSIONS

RTN4IP1-associated early-onset bilateral optic neuropathy with rod-cone dystrophy is a recently described clinical entity with limited reports available to-date. The present case provides additional support for this dual phenotype and identifies a novel causative variant.

Identifying therapeutic compounds for autosomal dominant optic atrophy (ADOA) through screening in the nematode C. elegans

Autosomal Dominant Optic Atrophy (ADOA) is a rare neurodegenerative condition, characterized by the bilateral loss of vision due to the degeneration of retinal ganglion cells. Its primary cause is linked to mutations in OPA1 gene, which ultimately affect mitochondrial structure and function. The current lack of successful treatments for ADOA emphasizes the need to investigate the mechanisms driving disease pathogenesis and exploit the potential of animal models for preclinical trials. Among such models, Caenorhabditis elegans stands out as a powerful tool, due its simplicity, its genetic tractability, and its relevance to human biology. Despite the lack of a visual system, the presence of mutated OPA1 in the nematode recapitulates ADOA pathology, by stimulating key pathogenic features of the human condition that can be studied in a fast and relatively non-laborious manner. Here, we provide a detailed guide on how to assess the therapeutic efficacy of chemical compounds, in either small or large scale, by evaluating three crucial phenotypes of humanized ADOA model nematodes, that express pathogenic human OPA1 in their GABAergic motor neurons: axonal mitochondria number, neuronal cell death and defecation cycle time. The described methods can deepen our understanding of ADOA pathogenesis and offer a practical framework for developing novel treatment schemes, providing hope for improved therapeutic outcomes and a better quality of life for individuals affected by this currently incurable condition.

Mitophagy in Astrocytes Is Required for the Health of Optic Nerve

Mitochondrial dysfunction in astrocytes has been implicated in the development of various neurological disorders. Mitophagy, mitochondrial autophagy, is required for proper mitochondrial function by preventing the accumulation of damaged mitochondria. The importance of mitophagy, specifically in the astrocytes of the optic nerve (ON), has been little studied. We introduce an animal model in which two separate mutations act synergistically to produce severe ON degeneration. The first mutation is in Cryba1, which encodes βA3/A1-crystallin, a lens protein also expressed in astrocytes, where it regulates lysosomal pH. The second mutation is in Bckdk, which encodes branched-chain ketoacid dehydrogenase kinase, which is ubiquitously expressed in the mitochondrial matrix and involved in the catabolism of the branched-chain amino acids. BCKDK is essential for mitochondrial function and the amelioration of oxidative stress. Neither of the mutations in isolation has a significant effect on the ON, but animals homozygous for both mutations (DM) exhibit very serious ON degeneration. ON astrocytes from these double-mutant (DM) animals have lysosomal defects, including impaired mitophagy, and dysfunctional mitochondria. Urolithin A can rescue the mitophagy impairment in DM astrocytes and reduce ON degeneration. These data demonstrate that efficient mitophagy in astrocytes is required for ON health and functional integrity.

Pyrroloquinoline quinone drives ATP synthesis in vitro and in vivo and provides retinal ganglion cell neuroprotection

Retinal ganglion cells are highly metabolically active requiring strictly regulated metabolism and functional mitochondria to keep ATP levels in physiological range. Imbalances in metabolism and mitochondrial mechanisms can be sufficient to induce a depletion of ATP, thus altering retinal ganglion cell viability and increasing cell susceptibility to death under stress. Altered metabolism and mitochondrial abnormalities have been demonstrated early in many optic neuropathies, including glaucoma, autosomal dominant optic atrophy, and Leber hereditary optic neuropathy. Pyrroloquinoline quinone (PQQ) is a quinone cofactor and is reported to have numerous effects on cellular and mitochondrial metabolism. However, the reported effects are highly context-dependent, indicating the need to study the mechanism of PQQ in specific systems. We investigated whether PQQ had a neuroprotective effect under different retinal ganglion cell stresses and assessed the effect of PQQ on metabolic and mitochondrial processes in cortical neuron and retinal ganglion cell specific contexts. We demonstrated that PQQ is neuroprotective in two models of retinal ganglion cell degeneration. We identified an increased ATP content in healthy retinal ganglion cell-related contexts both in in vitro and in vivo models. Although PQQ administration resulted in a moderate effect on mitochondrial biogenesis and content, a metabolic variation in non-diseased retinal ganglion cell-related tissues was identified after PQQ treatment. These results suggest the potential of PQQ as a novel neuroprotectant against retinal ganglion cell death.

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.