Suppression of complex I gene expression induces optic neuropathy

Chemotherapy for pain: reversing inflammatory and neuropathic pain with the anticancer agent mithramycin A

ABSTRACT

The persistence of inflammatory and neuropathic pain is poorly understood. We investigated a novel therapeutic paradigm by targeting gene networks that sustain or reverse persistent pain states. Our prior observations found that Sp1-like transcription factors drive the expression of TRPV1, a pain receptor, that is blocked in vitro by mithramycin A (MTM), an inhibitor of Sp1-like factors. Here, we investigate the ability of MTM to reverse in vivo models of inflammatory and chemotherapy-induced peripheral neuropathy (CIPN) pain and explore MTM's underlying mechanisms. Mithramycin reversed inflammatory heat hyperalgesia induced by complete Freund adjuvant and cisplatin-induced heat and mechanical hypersensitivity. In addition, MTM reversed both short-term and long-term (1 month) oxaliplatin-induced mechanical and cold hypersensitivity, without the rescue of intraepidermal nerve fiber loss. Mithramycin reversed oxaliplatin-induced cold hypersensitivity and oxaliplatin-induced TRPM8 overexpression in dorsal root ganglion (DRG). Evidence across multiple transcriptomic profiling approaches suggest that MTM reverses inflammatory and neuropathic pain through broad transcriptional and alternative splicing regulatory actions. Mithramycin-dependent changes in gene expression following oxaliplatin treatment were largely opposite to and rarely overlapped with changes in gene expression induced by oxaliplatin alone. Notably, RNAseq analysis revealed MTM rescue of oxaliplatin-induced dysregulation of mitochondrial electron transport chain genes that correlated with in vivo reversal of excess reactive oxygen species in DRG neurons. This finding suggests that the mechanism(s) driving persistent pain states such as CIPN are not fixed but are sustained by ongoing modifiable transcription-dependent processes.

Mitochondrial Medicine: A Promising Therapeutic Option Against Various Neurodegenerative Disorders

Abnormal mitochondrial morphology and metabolic dysfunction have been observed in many neurodegenerative disorders (NDDs). Mitochondrial dysfunction can be caused by aberrant mitochondrial DNA, mutant nuclear proteins that interact with mitochondria directly or indirectly, or for unknown reasons. Since mitochondria play a significant role in neurodegeneration, mitochondriatargeted therapies represent a prosperous direction for the development of novel drug compounds that can be used to treat NDDs. This review gives a brief description of how mitochondrial abnormalities lead to various NDDs such as Alzheimer's disease, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis. We further explore the promising therapeutic effectiveness of mitochondria- directed antioxidants, MitoQ, MitoVitE, MitoPBN, and dimebon. We have also discussed the possibility of mitochondrial gene therapy as a therapeutic option for these NDDs.

Development and validation of immune-based biomarkers and deep learning models for Alzheimer’s disease

Background: Alzheimer’s disease (AD) is the most common form of dementia in old age and poses a severe threat to the health and life of the elderly. However, traditional diagnostic methods and the ATN diagnostic framework have limitations in clinical practice. Developing novel biomarkers and diagnostic models is necessary to complement existing diagnostic procedures.

Methods: The AD expression profile dataset GSE63060 was downloaded from the NCBI GEO public database for preprocessing. AD-related differentially expressed genes were screened using a weighted co-expression network and differential expression analysis, and functional enrichment analysis was performed. Subsequently, we screened hub genes by random forest, analyzed the correlation between hub genes and immune cells using ssGSEA, and finally built an AD diagnostic model using an artificial neural network and validated it.

Results: Based on the random forest algorithm, we screened a total of seven hub genes from AD-related DEGs, based on which we confirmed that hub genes play an essential role in the immune microenvironment and successfully established a novel diagnostic model for AD using artificial neural networks, and validated its effectiveness in the publicly available datasets GSE63060 and GSE97760.

Conclusion: Our study establishes a reliable model for screening and diagnosing AD that provides a theoretical basis for adding diagnostic biomarkers for the AD gene.

Mitochondrial Genome Study Identifies Association Between Primary Open-Angle Glaucoma and Variants in MT-CYB, MT-ND4 Genes and Haplogroups

Background and purpose: Primary open-angle glaucoma (POAG) is an optic neuropathy characterized by death of retinal ganglion cells and atrophy of the optic nerve head. The susceptibility of the optic nerve to damage has been shown to be mediated by mitochondrial dysfunction. In this study, we aimed to determine a possible association between mitochondrial SNPs or haplogroups and POAG.

Methods: Mitochondrial DNA single nucleotide polymorphisms (mtSNPs) were genotyped using the Illumina Infinium Global Screening Array-24 (GSA) 700K array set. Genetic analyses were performed in a POAG case-control study involving the cohorts, Groningen Longitudinal Glaucoma Study-Lifelines Cohort Study and Amsterdam Glaucoma Study, including 721 patients and 1951 controls in total. We excluded samples not passing quality control for nuclear genotypes and samples with low call rate for mitochondrial variation. The mitochondrial variants were analyzed both as SNPs and haplogroups. These were determined with the bioinformatics software HaploGrep, and logistic regression analysis was used for the association, as well as for SNPs.

Results: Meta-analysis of the results from both cohorts revealed a significant association between POAG and the allele A of rs2853496 [odds ratio (OR) = 0.64; p = 0.006] within the MT-ND4 gene, and for the T allele of rs35788393 (OR = 0.75; p = 0.041) located in the MT-CYB gene. In the mitochondrial haplogroup analysis, the most significant p-value was reached by haplogroup K (p = 1.2 × 10⁻⁰⁵), which increases the risk of POAG with an OR of 5.8 (95% CI 2.7–13.1).

Conclusion: We identified an association between POAG and polymorphisms in the mitochondrial genes MT-ND4 (rs2853496) and MT-CYB (rs35788393), and with haplogroup K. The present study provides further evidence that mitochondrial genome variations are implicated in POAG. Further genetic and functional studies are required to substantiate the association between mitochondrial gene polymorphisms and POAG and to define the pathophysiological mechanisms of mitochondrial dysfunction in glaucoma.

Mutant three-repeat tau expression initiates retinal ganglion cell death through Caspase-2

The microtubule-associated protein tau is implicated in multiple degenerative diseases including retinal diseases such as glaucoma; however, the way tau initiates retinopathy is unclear. Previous retinal assessments in mouse models of tauopathy suggest that mutations in four-repeat (4R) tau are associated with disease-induced retinal dysfunction, while shifting tau isoform ratio to favor three-repeat (3R) tau production enhanced photoreceptor function. To further understand how alterations in tau expression impact the retina, we analyzed the retinas of transgenic mice overexpressing mutant 3R tau (m3R tau-Tg), a model known to exhibit Pick's Disease pathology in the brain. Analysis of retinal cross-sections from young (3 month) and adult (9 month) mice detected asymmetric 3R tau immunoreactivity in m3R tau-Tg retina, concentrated in the retinal ganglion and amacrine cells of the dorsal retinal periphery. Accumulation of hyperphosphorylated tau was detected specifically in the detergent insoluble fraction of the adult m3R tau-Tg retina. RNA-seq analysis highlighted biological pathways associated with tauopathy that were uniquely altered in m3R tau-Tg retina. The upregulation of transcript encoding apoptotic protease caspase-2 coincided with increased immunostaining in predominantly 3R tau positive retinal regions. In adult m3R tau-Tg, the dorsal peripheral retina of the adult m3R tau-Tg exhibited decreased cell density in the ganglion cell layer (GCL) and reduced thickness of the inner plexiform layer (IPL) compared to the ventral peripheral retina. Together, these data indicate that mutant 3R tau may mediate toxicity in retinal ganglion cells (RGC) by promoting caspase-2 expression which results in RGC degeneration. The m3R tau-Tg line has the potential to be used to assess tau-mediated RGC degeneration and test novel therapeutics for degenerative diseases such as glaucoma.

The use of induced pluripotent stem cells for studying and treating optic neuropathies

PURPOSE OF REVIEW

The present review aims to provide an update of applications of induced pluripotent stem cells (iPSCs) for disease modeling, cell/gene therapy, and drug screening for optic neuropathies.

RECENT FINDINGS

Degeneration of retinal ganglion cells (RGCs) is a characteristic of optic neuropathies. Human iPSCs can serve as a model to investigate disease pathology and potential repair mechanisms. In recent years, significant progress has been made in generating RGCs from iPSCs. Various groups have reported the potential of iPSCs for modeling optic neuropathies, such as glaucoma. The literature also highlights the potential to use iPSC-derived cells for high-throughput drug and toxicity screening.

SUMMARY

The present review summarizes current work in the field of iPSCs in optic neuropathies. Future studies to characterize iPSC-derived RGCs in a more in-depth manner will help expand the use of iPSCs to model and treat optic neuropathic diseases. Furthermore, iPSC modeling can be used in drug development by offering a new avenue to test novel therapeutic drugs for optic neuropathies.

Leber Hereditary Optic Neuropathy: Exemplar of an mtDNA Disease

The report in 1988 that Leber Hereditary Optic Neuropathy (LHON) was the product of mitochondrial DNA (mtDNA) mutations provided the first demonstration of the clinical relevance of inherited mtDNA variation. From LHON studies, the medical importance was demonstrated for the mtDNA showing its coding for the most important energy genes, its maternal inheritance, its high mutation rate, its presence in hundreds to thousands of copies per cell, its quantitatively segregation of biallelic genotypes during both mitosis and meiosis, its preferential effect on the most energetic tissues including the eye and brain, its wide range of functional polymorphisms that predispose to common diseases, and its accumulation of mutations within somatic tissues providing the aging clock. These features of mtDNA genetics, in combination with the genetics of the 1-2000 nuclear DNA (nDNA) coded mitochondrial genes, is not only explaining the genetics of LHON but also providing a model for understanding the complexity of many common diseases. With the maturation of LHON biology and genetics, novel animal models for complex disease have been developed and new therapeutic targets and strategies envisioned, both pharmacological and genetic. Multiple somatic gene therapy approaches are being developed for LHON which are applicable to other mtDNA diseases. Moreover, the unique cytoplasmic genetics of the mtDNA has permitted the first successful human germline gene therapy via spindle nDNA transfer from mtDNA mutant oocytes to enucleated normal mtDNA oocytes. Such LHON lessons are actively being applied to common ophthalmological diseases like glaucoma and neurological diseases like Parkinsonism.

Novel Methodology for Creating Macaque Retinas with Sortable Photoreceptors and Ganglion Cells

Purpose: The ability to generate macaque retinas with sortable cell populations would be of great benefit to both basic and translational studies of the primate retina. The purpose of our study was therefore to develop methods to achieve this goal by selectively labeling, in life, photoreceptors (PRs) and retinal ganglion cells (RGCs) with separate fluorescent markers. Methods: Labeling of macaque (Macaca fascicularis) PRs and RGCs was accomplished by subretinal delivery of AAV5-hGRK1-GFP, and retrograde transport of micro-ruby™ from the lateral geniculate nucleus, respectively. Retinas were anatomically separated into different regions. Dissociation conditions were optimized, and cells from each region underwent fluorescent activated cell sorting (FACS). Expression of retinal cell type- specific genes was assessed by quantitative real-time PCR to characterize isolated cell populations. Results: We show that macaque PRs and RGCs can be simultaneously labeled in-life and enriched populations isolated by FACS. Recovery from different retinal regions indicated efficient isolation/enrichment for PRs and RGCs, with the macula being particularly amendable to this technique. Conclusions: The methods and materials presented here allow for the identification of novel reagents designed to target RGCs and/or photoreceptors in a species that is phylogenetically and anatomically similar to human. These techniques will enable screening of intravitreally-delivered AAV capsid libraries for variants with increased tropism for PRs and/or RGCs and the evaluation of vector tropism and/or cellular promoter activity of gene therapy vectors in a clinically relevant species.

Leber Hereditary Optic Neuropathy: Bringing the Lab to the Clinic

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

Structure of bacterial respiratory complex I

Complex I (NADH:ubiquinone oxidoreductase) plays a central role in cellular energy production, coupling electron transfer between NADH and quinone to proton translocation. It is the largest protein assembly of respiratory chains and one of the most elaborate redox membrane proteins known. Bacterial enzyme is about half the size of mitochondrial and thus provides its important "minimal" model. Dysfunction of mitochondrial complex I is implicated in many human neurodegenerative diseases. The L-shaped complex consists of a hydrophilic arm, where electron transfer occurs, and a membrane arm, where proton translocation takes place. We have solved the crystal structures of the hydrophilic domain of complex I from Thermus thermophilus, the membrane domain from Escherichia coli and recently of the intact, entire complex I from T. thermophilus (536 kDa, 16 subunits, 9 iron-sulphur clusters, 64 transmembrane helices). The 95Å long electron transfer pathway through the enzyme proceeds from the primary electron acceptor flavin mononucleotide through seven conserved Fe-S clusters to the unusual elongated quinone-binding site at the interface with the membrane domain. Four putative proton translocation channels are found in the membrane domain, all linked by the central flexible axis containing charged residues. The redox energy of electron transfer is coupled to proton translocation by the as yet undefined mechanism proposed to involve long-range conformational changes. This article is part of a Special Issue entitled Respiratory complex I, edited by Volker Zickermann and Ulrich Brandt.

Complex I subunit gene therapy with NDUFA6 ameliorates neurodegeneration in EAE

PURPOSE

To address the permanent disability induced by mitochondrial dysfunction in experimental autoimmune encephalomyelitis (EAE).

METHODS

Mice sensitized for EAE were rescued by intravitreal injection of adeno-associated viral vector serotype 2 with the complex I subunit gene scAAV-NDUFA6Flag. Controls were injected with a mitochondrially targeted red fluorescent protein (scAAV-COX8-cherry). Another group received scAAV-COX8-cherry, but was not sensitized for EAE. Serial pattern electroretinograms (PERGs) and optical coherent tomography (OCT) evaluated visual function and structure of the retina at 1, 3, and 6 months post injection (MPI). Treated mice were killed 6 MPI for histopathology. Immunodetection of cleaved caspase 3 gauged apoptosis. Complex I activity was assessed spectrophotometrically. Expression of NDUFA6Flag in the retina and optic nerve were evaluated between 1 week to 1 month post injection by RT-PCR, immunofluorescence and immunoblotting.

RESULTS

Reverse transcription-PCR and immunoblotting confirmed NDUFA6Flag overexpression with immunoprecipitation and blue native PAGE showing integration into murine complex I. Overexpression of NDUFA6Flag in the visual system of EAE mice rescued retinal complex I activity completely, axonal loss by 73%, and retinal ganglion cell (RGC) loss by 88%, RGC apoptosis by 66%, and restored the 33% loss of complex I activity in EAE to normal levels; thereby, preventing loss of vision indicated by the 43% reduction in the PERG amplitudes of EAE mice.

CONCLUSIONS

NDUFA6 gene therapy provided long-term suppression of neurodegeneration in the EAE animal model suggesting that it may also ameliorate the mitochondrial dysfunction associated with permanent disability in optic neuritis and MS patients.

Long-term evaluation of Leber's hereditary optic neuropathy-like symptoms in rotenone administered rats

Leber's hereditary optic neuropathy (LHON) is an inherited disorder affecting the retinal ganglion cells (RGCs) and their axons that lead to the loss of central vision. This study is aimed at evaluating the LHON symptoms in rats administered with rotenone microspheres into the superior colliculus (SC). Optical coherence tomography (OCT) analysis showed substantial loss of retinal nerve fiber layer (RNFL) thickness in rotenone injected rats. Optokinetic testing in rotenone treated rats showed decrease in head-tracking response. Electrophysiological mapping of the SC surface demonstrated attenuation of visually evoked responses; however, no changes were observed in the ERG data. The progressive pattern of disease manifestation in rotenone administered rats demonstrated several similarities with human disease symptoms. These rats with LHON-like symptoms can serve as a model for future investigators to design and implement reliable tests to assess the beneficial effects of therapeutic interventions for LHON disease.

Resveratrol protects cardiomyocytes from oxidative stress through SIRT1 and mitochondrial biogenesis signaling pathways

Reactive oxygen species (ROS) is generated by oxidative stress and plays an important role in various cardiac pathologies. The SIRT1 signaling pathway and mitochondrial biogenesis play essential roles in mediating the production of ROS. SIRT1 activated by resveratrol protects cardiomyocytes from oxidative stress, but the exact mechanisms by which SIRT1 prevents oxidative stress, and its relationship with mitochondrial biogenesis, remain unclear. In this study, it was observed that after stimulation with 50μMH2O2 for 6h, H9C2 cells produced excessive ROS and downregulated SIRT1. The mitochondrial protein NDUFA13 was also downregulated by ROS mediated by SIRT1. Resveratrol induced the expression of SIRT1 and mitochondrial genes NDUFA1, NDUFA2, NDUFA13 and Mn-SOD. However, the production of these genes was reversed by SIRT1 inhibitor nicotinamide. These results suggest that resveratrol inhibits ROS generation in cardiomyocytes via SIRT1 and mitochondrial biogenesis signaling pathways.

The optic nerve: a "mito-window" on mitochondrial neurodegeneration

Retinal ganglion cells (RGCs) project their long axons, composing the optic nerve, to the brain, transmitting the visual information gathered by the retina, ultimately leading to formed vision in the visual cortex. The RGC cellular system, representing the anterior part of the visual pathway, is vulnerable to mitochondrial dysfunction and optic atrophy is a very frequent feature of mitochondrial and neurodegenerative diseases. The start of the molecular era of mitochondrial medicine, the year 1988, was marked by the identification of a maternally inherited form of optic atrophy, Leber's hereditary optic neuropathy, as the first disease due to mitochondrial DNA point mutations. The field of mitochondrial medicine has expanded enormously over the last two decades and many neurodegenerative diseases are now known to have a primary mitochondrial etiology or mitochondrial dysfunction plays a relevant role in their pathogenic mechanism. Recent technical advancements in neuro-ophthalmology, such as optical coherence tomography, prompted a still ongoing systematic re-investigation of retinal and optic nerve involvement in neurodegenerative disorders. In addition to inherited optic neuropathies, such as Leber's hereditary optic neuropathy and dominant optic atrophy, and in addition to the syndromic mitochondrial encephalomyopathies or mitochondrial neurodegenerative disorders such as some spinocerebellar ataxias or familial spastic paraparesis and other disorders, we draw attention to the involvement of the optic nerve in classic age-related neurodegenerative disorders such as Parkinson and Alzheimer disease. We here provide an overview of optic nerve pathology in these different clinical settings, and we review the possible mechanisms involved in the pathogenesis of optic atrophy. This may be a model of general value for the field of neurodegeneration. This article is part of a Special Issue entitled 'Mitochondrial function and dysfunction in neurodegeneration'.

NADH-dehydrogenase type-2 suppresses irreversible visual loss and neurodegeneration in the EAE animal model of MS

To address mitochondrial dysfunction that mediates irreversible visual loss and neurodegeneration of the optic nerve in the experimental autoimmune encephalomyelitis (EAE) animal model of multiple sclerosis (MS), mice sensitized for EAE were vitreally injected with self-complementary adenoassociated virus (scAAV) containing the NADH-dehydrogenase type-2 (NDI1) complex I gene that quickly expressed in mitochondria of almost all retinal ganglion cells (RGCs). Visual function assessed by pattern electroretinograms (PERGs) reduced by half in EAE showed no significant reductions with NDI1. Serial optical coherence tomography (OCT) revealed significant inner retinal thinning with EAE that was suppressed by NDI1. Although complex I activity reduced 80% in EAE was not improved by NDI1, in vivo fluorescent probes indicated mitochondrial oxidative stress and apoptosis of the EAE retina were reduced by NDI1. Finally, the 42% loss of axons in the EAE optic nerve was ameliorated by NDI1. Targeting the dysfunctional complex I of EAE responsible for loss of respiration, mitochondrial oxidative stress and apoptosis may be a novel approach to address neuronal and axonal loss responsible for permanent disability that is unaltered by current disease modifying drugs for MS that target inflammation.

OXPHOS mutations and neurodegeneration

Mitochondrial oxidative phosphorylation (OXPHOS) sustains organelle function and plays a central role in cellular energy metabolism. The OXPHOS system consists of 5 multisubunit complexes (CI-CV) that are built up of 92 different structural proteins encoded by the nuclear (nDNA) and mitochondrial DNA (mtDNA). Biogenesis of a functional OXPHOS system further requires the assistance of nDNA-encoded OXPHOS assembly factors, of which 35 are currently identified. In humans, mutations in both structural and assembly genes and in genes involved in mtDNA maintenance, replication, transcription, and translation induce 'primary' OXPHOS disorders that are associated with neurodegenerative diseases including Leigh syndrome (LS), which is probably the most classical OXPHOS disease during early childhood. Here, we present the current insights regarding function, biogenesis, regulation, and supramolecular architecture of the OXPHOS system, as well as its genetic origin. Next, we provide an inventory of OXPHOS structural and assembly genes which, when mutated, induce human neurodegenerative disorders. Finally, we discuss the consequences of mutations in OXPHOS structural and assembly genes at the single cell level and how this information has advanced our understanding of the role of OXPHOS dysfunction in neurodegeneration.

Mitochondrial optic neuropathies - disease mechanisms and therapeutic strategies

Leber hereditary optic neuropathy (LHON) and autosomal-dominant optic atrophy (DOA) are the two most common inherited optic neuropathies in the general population. Both disorders share striking pathological similarities, marked by the selective loss of retinal ganglion cells (RGCs) and the early involvement of the papillomacular bundle. Three mitochondrial DNA (mtDNA) point mutations; m.3460G>A, m.11778G>A, and m.14484T>C account for over 90% of LHON cases, and in DOA, the majority of affected families harbour mutations in the OPA1 gene, which codes for a mitochondrial inner membrane protein. Optic nerve degeneration in LHON and DOA is therefore due to disturbed mitochondrial function and a predominantly complex I respiratory chain defect has been identified using both in vitro and in vivo biochemical assays. However, the trigger for RGC loss is much more complex than a simple bioenergetic crisis and other important disease mechanisms have emerged relating to mitochondrial network dynamics, mtDNA maintenance, axonal transport, and the involvement of the cytoskeleton in maintaining a differential mitochondrial gradient at sites such as the lamina cribosa. The downstream consequences of these mitochondrial disturbances are likely to be influenced by the local cellular milieu. The vulnerability of RGCs in LHON and DOA could derive not only from tissue-specific, genetically-determined biological factors, but also from an increased susceptibility to exogenous influences such as light exposure, smoking, and pharmacological agents with putative mitochondrial toxic effects. Our concept of inherited mitochondrial optic neuropathies has evolved over the past decade, with the observation that patients with LHON and DOA can manifest a much broader phenotypic spectrum than pure optic nerve involvement. Interestingly, these phenotypes are sometimes clinically indistinguishable from other neurodegenerative disorders such as Charcot-Marie-Tooth disease, hereditary spastic paraplegia, and multiple sclerosis, where mitochondrial dysfunction is also thought to be an important pathophysiological player. A number of vertebrate and invertebrate disease models has recently been established to circumvent the lack of human tissues, and these have already provided considerable insight by allowing direct RGC experimentation. The ultimate goal is to translate these research advances into clinical practice and new treatment strategies are currently being investigated to improve the visual prognosis for patients with mitochondrial optic neuropathies.

Animal models of retinal disease

Diseases of the retina are the leading causes of blindness in the industrialized world. The recognition that animals develop retinal diseases with similar traits to humans has led to not only a dramatic improvement in our understanding of the pathogenesis of retinal disease but also provided a means for testing possible treatment regimes and successful gene therapy trials. With the advent of genetic and molecular biological tools, the association between specific gene mutations and retinal signs has been made. Animals carrying natural mutations usually in one gene now provide well-established models for a host of inherited retinal diseases, including retinitis pigmentosa, Leber congenital amaurosis, inherited macular degeneration, and optic nerve diseases. In addition, the development of transgenic technologies has provided a means by which to study the effects of these and novel induced mutations on retinal structure and function. Despite these advances, there is a paucity of suitable animal models for complex diseases, including age-related macular degeneration (AMD) and diabetic retinopathy, largely because these diseases are not caused by single gene defects, but involve complex genetics and/or exacerbation through environmental factors, epigenetic, or other modes of genetic influence. In this review, we outline in detail the available animal models for inherited retinal diseases and how this information has furthered our understanding of retinal diseases. We also examine how transgenic technologies have helped to develop our understanding of the role of isolated genes or pathways in complex diseases like AMD, diabetes, and glaucoma.

Leber's Hereditary Optic Neuropathy-Gene Therapy: From Benchtop to Bedside

Leber's hereditary optic neuropathy (LHON) is a maternally transmitted disorder caused by point mutations in mitochondrial DNA (mtDNA). Most cases are due to mutations in genes encoding subunits of the NADH-ubiquinone oxidoreductase that is Complex I of the electron transport chain (ETC). These mutations are located at nucleotide positions 3460, 11778, or 14484 in the mitochondrial genome. The disease is characterized by apoplectic, bilateral, and severe visual loss. While the mutated mtDNA impairs generation of ATP by all mitochondria, there is only a selective loss of retinal ganglion cells and degeneration of optic nerve axons. Thus, blindness is typically permanent. Half of the men and 10% of females who harbor the pathogenic mtDNA mutation actually develop the phenotype. This incomplete penetrance and gender bias is not fully understood. Additional mitochondrial and/or nuclear genetic factors may modulate the phenotypic expression of LHON. In a population-based study, the mtDNA background of haplogroup J was associated with an inverse relationship of low-ATP generation and increased production of reactive oxygen species (ROS). Effective therapy for LHON has been elusive. In this paper, we describe the findings of pertinent published studies and discuss the controversies of potential strategies to ameliorate the disease.

Successful amelioration of mitochondrial optic neuropathy using the yeast NDI1 gene in a rat animal model

Background

Leber's hereditary optic neuropathy (LHON) is a maternally inherited disorder with point mutations in mitochondrial DNA which result in loss of vision in young adults. The majority of mutations reported to date are within the genes encoding the subunits of the mitochondrial NADH-quinone oxidoreductase, complex I. Establishment of animal models of LHON should help elucidate mechanism of the disease and could be utilized for possible development of therapeutic strategies.

Methodology/Principal Findings

We established a rat model which involves injection of rotenone-loaded microspheres into the optic layer of the rat superior colliculus. The animals exhibited the most common features of LHON. Visual loss was observed within 2 weeks of rotenone administration with no apparent effect on retinal ganglion cells. Death of retinal ganglion cells occurred at a later stage. Using our rat model, we investigated the effect of the yeast alternative NADH dehydrogenase, Ndi1. We were able to achieve efficient expression of the Ndi1 protein in the mitochondria of all regions of retinal ganglion cells and axons by delivering the NDI1 gene into the optical layer of the superior colliculus. Remarkably, even after the vision of the rats was severely impaired, treatment of the animals with the NDI1 gene led to a complete restoration of the vision to the normal level. Control groups that received either empty vector or the GFP gene had no effects.

Conclusions/Significance

The present study reports successful manifestation of LHON-like symptoms in rats and demonstrates the potential of the NDI1 gene therapy on mitochondrial optic neuropathies. Our results indicate a window of opportunity for the gene therapy to be applied successfully after the onset of the disease symptoms.

Mitochondrial energetics and therapeutics

Mitochondrial dysfunction has been linked to a wide range of degenerative and metabolic diseases, cancer, and aging. All these clinical manifestations arise from the central role of bioenergetics in cell biology. Although genetic therapies are maturing as the rules of bioenergetic genetics are clarified, metabolic therapies have been ineffectual. This failure results from our limited appreciation of the role of bioenergetics as the interface between the environment and the cell. A systems approach, which, ironically, was first successfully applied over 80 years ago with the introduction of the ketogenic diet, is required. Analysis of the many ways that a shift from carbohydrate glycolytic metabolism to fatty acid and ketone oxidative metabolism may modulate metabolism, signal transduction pathways, and the epigenome gives us an appreciation of the ketogenic diet and the potential for bioenergetic therapeutics.

Mitochondrial optic neuropathy: In vivo model of neurodegeneration and neuroprotective strategies

This review summarizes the characteristics of a rodent toxicologic model of optic neuropathy induced by the mitochondrial complex I inhibitor rotenone. This model has been developed to fulfill the demand for a drug-screening tool providing a sound mechanistic context to address the role of mitochondrial dysfunction in the pathogenesis of neurodegenerative disorders. It features biochemical, structural, and functional retinal deficits that resemble those of patients with Leber's hereditary optic neuropathy, a mitochondrial disease characterized by selective degeneration of retinal ganglion cells, and for which an environmental component is believed to play a major triggering role. The available data support the efficiency, sensitivity, and versatility of the model for providing insights into the mechanisms of neurodegeneration, including mitochondrial dysfunction, oxidative stress and excitotoxicity. Screening work with this model has provided proof-of-principle that interventions targeting the electron transport chain, such as USP methylene blue and near-infrared light therapy, are effective at preventing neurodegeneration induced by mitochondrial dysfunction in vivo. Prospective developments of this model include the use of neuronal reporter genes for in vivo non-invasive assessment of retinal degeneration at different time points, and its combination with genetic approaches to elucidate the synergism of environmental and genetic factors in neurodegeneration.

Mitochondrial respiratory complex I: structure, function and implication in human diseases

Mitochondria are ubiquitous organelles in eukaryotic cells whose primary function is to generate energy supplies in the form of ATP through oxidative phosphorylation. As the entry point for most electrons into the respiratory chain, NADH:ubiquinone oxidoreductase, or complex I, is the largest and least understood component of the mitochondrial oxidative phosphorylation system. Substantial progress has been made in recent years in understanding its subunit composition, its assembly, the interaction among complex I and other respiratory components, and its role in oxidative stress and apoptosis. This review provides an updated overview of the structure of complex I, as well as its cellular functions, and discusses the implication of complex I dysfunction in various human diseases.

Mitochondrial T9957C Mutation in Association with NAION and Seizures but not MELAS

PURPOSE

To assess the functional significance of the mitochondrial nt-9957 mutation in a man with non-arteritic ischemic optic neuropathy (NAION). This nt-9957 mutation has been previously reported in association with mitochondrial encephalopathy, lactic acidosis, and stroke-like events (MELAS).

METHODS

The patient was examined clinically and with magnetic resonance imaging and spectroscopy. The entire coding region of the mitochondrial genome was sequenced, and mitochondrial function was assessed by flow cytometry after staining with fluorescent dihydroethidium.

RESULTS

This 76-year-old man had optic nerve disease bilaterally and seizures, but no clinical or radiological evidence of MELAS. He had no mitochondrial DNA mutation other than the 9957. Functional testing revealed a severe defect in mitochondrial complex III activity.

CONCLUSIONS

This patient had a mitochondrial functional deficit consistent with his 9957 mutation. It seems quite likely that this mutation may be responsible for optic nerve and brain injuries.

Inherited mitochondrial optic neuropathies

Leber hereditary optic neuropathy (LHON) and autosomal dominant optic atrophy (DOA) are the two most common inherited optic neuropathies and they result in significant visual morbidity among young adults. Both disorders are the result of mitochondrial dysfunction: LHON from primary mitochondrial DNA (mtDNA) mutations affecting the respiratory chain complexes; and the majority of DOA families have mutations in the OPA1 gene, which codes for an inner mitochondrial membrane protein critical for mtDNA maintenance and oxidative phosphorylation. Additional genetic and environmental factors modulate the penetrance of LHON, and the same is likely to be the case for DOA which has a markedly variable clinical phenotype. The selective vulnerability of retinal ganglion cells (RGCs) is a key pathological feature and understanding the fundamental mechanisms that underlie RGC loss in these disorders is a prerequisite for the development of effective therapeutic strategies which are currently limited.

MRI in experimental inflammatory and mitochondrial optic neuropathies

MRI is a powerful tool for evaluating structural and functional alterations in the optic nerve in experimental animal models of human disease. MRI-histopathological correlations have provided important insights into the pathogenesis of disease. Paramagnetic contrast agents have been used to serially visualize the foci and severity of disruption of the blood-optic nerve barrier and physiological neuronal alterations in living animals. Here I review the experience of our group in optic nerve imaging of experimental autoimmune encephalomyelitis and neurodegeneration induced by genetic manipulation of respiratory chain enzymes.

Medication-induced mitochondrial damage and disease

Since the first mitochondrial dysfunction was described in the 1960s, the medicine has advanced in its understanding the role mitochondria play in health and disease. Damage to mitochondria is now understood to play a role in the pathogenesis of a wide range of seemingly unrelated disorders such as schizophrenia, bipolar disease, dementia, Alzheimer's disease, epilepsy, migraine headaches, strokes, neuropathic pain, Parkinson's disease, ataxia, transient ischemic attack, cardiomyopathy, coronary artery disease, chronic fatigue syndrome, fibromyalgia, retinitis pigmentosa, diabetes, hepatitis C, and primary biliary cirrhosis. Medications have now emerged as a major cause of mitochondrial damage, which may explain many adverse effects. All classes of psychotropic drugs have been documented to damage mitochondria, as have stain medications, analgesics such as acetaminophen, and many others. While targeted nutrient therapies using antioxidants or their precursors (e. g., N-acetylcysteine) hold promise for improving mitochondrial function, there are large gaps in our knowledge. The most rational approach is to understand the mechanisms underlying mitochondrial damage for specific medications and attempt to counteract their deleterious effects with nutritional therapies. This article reviews our basic understanding of how mitochondria function and how medications damage mitochondria to create their occasionally fatal adverse effects.

Mitochondrial diseases: therapeutic approaches

Therapy of mitochondrial encephalomyopathies (defined restrictively as defects of the mitochondrial respiratory chain) is woefully inadequate, despite great progress in our understanding of the molecular bases of these disorders. In this review, we consider sequentially several different therapeutic approaches. Palliative therapy is dictated by good medical practice and includes anticonvulsant medication, control of endocrine dysfunction, and surgical procedures. Removal of noxious metabolites is centered on combating lactic acidosis, but extends to other metabolites. Attempts to bypass blocks in the respiratory chain by administration of electron acceptors have not been successful, but this may be amenable to genetic engineering. Administration of metabolites and cofactors is the mainstay of real-life therapy and is especially important in disorders due to primary deficiencies of specific compounds, such as carnitine or coenzyme Q10. There is increasing interest in the administration of reactive oxygen species scavengers both in primary mitochondrial diseases and in neurodegenerative diseases directly or indirectly related to mitochondrial dysfunction. Aerobic exercise and physical therapy prevent or correct deconditioning and improve exercise tolerance in patients with mitochondrial myopathies due to mitochondrial DNA (mtDNA) mutations. Gene therapy is a challenge because of polyplasmy and heteroplasmy, but interesting experimental approaches are being pursued and include, for example, decreasing the ratio of mutant to wild-type mitochondrial genomes (gene shifting), converting mutated mtDNA genes into normal nuclear DNA genes (allotopic expression), importing cognate genes from other species, or correcting mtDNA mutations with specific restriction endonucleases. Germline therapy raises ethical problems but is being considered for prevention of maternal transmission of mtDNA mutations. Preventive therapy through genetic counseling and prenatal diagnosis is becoming increasingly important for nuclear DNA-related disorders. Progress in each of these approaches provides some glimmer of hope for the future, although much work remains to be done.

Mitochondrial optic neuropathies: how two genomes may kill the same cell type?

Ocular involvement is a prevalent feature in mitochondrial diseases. Leber's hereditary optic neuropathy (LHON) and dominant optic atrophy (DOA) are both non-syndromic optic neuropathies with a mitochondrial etiology. LHON is associated with point mutations in the mitochondrial DNA (mtDNA), which affect subunit genes of complex I. The majority of DOA patients harbor mutations in the nuclear-encoded protein OPA1, which is targeted to mitochondria and participates to cristae organization and mitochondrial network dynamics. In both disorders the retinal ganglion cells (RGCs) are specific cellular targets of the degenerative process. We here review the clinical features and the genetic bases, and delineate the possible common pathomechanism for both these disorders.

Investigation on Mitochondrial tRNALeu/Lys, NDI and ATPase 6/8 in Iranian Multiple Sclerosis Patients

As with chromosomal DNA, the mitochondrial DNA (mtDNA) can contain mutations that are highly pathogenic . In fact, many diseases of the central nervous system are known to be caused by mutations in mtDNA. Dysfunction of the mitochondrial Respiratory Chain (RC) has been shown in patients with neurological disease including Alzheimer's disease (AD), Parkinson's disease (PD) and Multiple sclerosis (MS). MS is a demyelinating disease of central nervous system characterized by morphological hallmarks of inflammation, demyelination and axonal loss. Considering this importance, we decided to investigate several highly mutative parts of mtDNA for point mutations as MT-LTI (tRNA(Leucine1(UUA/G))), MT-NDI (NADH Dehydrogenase subunit 1), MT-COII (Cytochrome c oxidase subunit II), MT-TK (tRNA(Lysine)), MT-ATP8 (ATP synthase subunit F0 8) and MT-ATP6 (ATP synthase subunit F0 6) in 20 Iranian MS patients and 80 age-matched control subjects by PCR and automated DNA sequencing to evaluate any probable point mutations. Our results revealed that 15 (75%) out of 20 MS patients had point mutations. Some of point mutations were newly found in this study. This study suggested that point mutation occurred in mtDNA might be involved in pathogenesis of MS.

Mitochondrial dysfunction and molecular pathways of disease

Since the first mitochondrial dysfunction was described in the 1960s, the medicine has advanced in its understanding the role mitochondria play in health, disease, and aging. A wide range of seemingly unrelated disorders, such as schizophrenia, bipolar disease, dementia, Alzheimer's disease, epilepsy, migraine headaches, strokes, neuropathic pain, Parkinson's disease, ataxia, transient ischemic attack, cardiomyopathy, coronary artery disease, chronic fatigue syndrome, fibromyalgia, retinitis pigmentosa, diabetes, hepatitis C, and primary biliary cirrhosis, have underlying pathophysiological mechanisms in common, namely reactive oxygen species (ROS) production, the accumulation of mitochondrial DNA (mtDNA) damage, resulting in mitochondrial dysfunction. Antioxidant therapies hold promise for improving mitochondrial performance. Physicians seeking systematic treatments for their patients might consider testing urinary organic acids to determine how best to treat them. If in the next 50 years advances in mitochondrial treatments match the immense increase in knowledge about mitochondrial function that has occurred in the last 50 years, mitochondrial diseases and dysfunction will largely be a medical triumph.

Approaches to the treatment of mitochondrial diseases

Therapy for mitochondrial diseases is woefully inadequate. However, lack of a cure does not equate with lack of treatment. Palliative therapy is dictated by good medical practice and includes anticonvulsant medication, control of endocrine dysfunction, and surgical procedures. Removal of noxious metabolites is centered on combating lactic acidosis, but extends to other metabolites. Attempts to bypass blocks in the respiratory chain by administration of electron acceptors have not been successful, but this may be amenable to genetic engineering. Administration of metabolites and cofactors is the mainstay of real-life therapy and is especially important in disorders due to primary deficiencies of specific compounds, such as carnitine or coenzyme Q10 (CoQ10). There is increasing interest in the administration of reactive oxygen radicals (ROS) scavengers, both in primary mitochondrial diseases and in neurodegenerative diseases. Gene therapy is a challenge because of polyplasmy and heteroplasmy, but novel experimental approaches are being pursued. One important strategy is to decrease the ratio of mutant to wild-type mitochondrial genomes ("gene shifting") by different means: (1) converting mutated mitochondrial DNA (mtDNA) genes into normal nuclear DNA genes ("allotopic expression"); (2) importing cognate genes from other species ("xenotopic expression"); (3) correcting mtDNA mutations by importing specific restriction endonucleases; (4) selecting for respiratory function; and (5) inducing muscle regeneration. Germline therapy raises ethical problems but is being considered for prevention of maternal transmission of mtDNA mutations. Preventive therapy through genetic counseling and prenatal diagnosis is becoming increasingly important for nuclear DNA-related disorders.

Mitochondrial Protein Nitration Primes Neurodegeneration in Experimental Autoimmune Encephalomyelitis

The mechanisms of axonal and neuronal degeneration causing visual and neurologic disability in multiple sclerosis are poorly understood. Here we explored the contribution of mitochondria to neurodegeneration in the experimental autoimmune encephalomyelitis animal model of multiple sclerosis. Oxidative injury to the murine mitochondrion preceded the infiltration of inflammatory cells, classically heralded as the mediators of demyelination and axonal injury by transection. Nitration of mitochondrial proteins affected key subunits of complexes I and IV of the respiratory chain and a chaperone critical to the stabilization and translocation of proteins into the organelle. Oxidative products were associated with loss of mitochondrial membrane potential and apoptotic cell death. Reductions in the rate of synthesis of adenosine triphosphate were severe and even greater than those associated with disorders caused by mutated mitochondrial DNA. Mitochondrial vacuolization, swelling, and dissolution of cristae occurred in axons as early as 3 days after sensitization for experimental autoimmune encephalomyelitis. Our findings implicate mitochondrial dysfunction induced by protein inactivation and mediated by oxidative stress initiates a cascade of molecular events leading to apoptosis and neurodegeneration in experimental autoimmune encephalomyelitis that is not mediated by inflammatory cells.

Mitochondrial disease

Defects of mitochondrial metabolism cause a wide range of human diseases that include examples from all medical subspecialties. This review updates the topic of mitochondrial diseases by reviewing the most important recent advances in this area. The factors influencing inheritance, maintenance and replication of mtDNA are reviewed and the genotype-phenotype of mtDNA disorders has been expanded, with new insights into epidemiology, pathogenesis and its role in ageing. Recently identified nuclear gene mutations of mitochondrial proteins include mutations of frataxin causing Friedreich's ataxia, PINK1, DJ1 causing Parkinson's disease and POLG causing infantile mtDNA depletion syndrome, ophthalmoplegia, parkinsonism, male subfertility and, in a transgenic mouse model, premature senescence. Mitochondrial defects in neurodegenerative diseases include Parkinson's, Alzheimer's and Huntington's disease. Improved understanding of mtDNA inheritance and mutation penetrance patterns, and novel techniques for mtDNA modification offer significant prospects for more accurate genetic counselling and effective future therapies.

Can a Single Subunit Yeast NADH Dehydrogenase (Ndi1) Remedy Diseases Caused by Respiratory Complex I Defects?

The proton-translocating NADH-quinone oxidoreductase (complex I) is one of five enzyme complexes in the oxidative phosphorylation system in mammalian mitochondria. Complex I is composed of 46 different subunits, 7 of which are encoded by mitochondrial DNA. Defects of complex I are involved in many human mitochondrial diseases; therefore, the authors proposed to use the NDI1 gene encoding a single subunit NADH dehydrogenase of Saccharomyces cerevisiae for repair of respiratory activity. The yeast NDI1 gene was successfully introduced into 10 mammalian cell lines (two of which were complex I-deficient mutants). The expressed Ndi1 protein was correctly targeted to the matrix side of the inner mitochondrial membranes, was fully functional, and restored the NADH oxidase activity to the complex I-deficient cells. The NDI1-transduced cells were more resistant to complex I inhibitors and diminished production of reactive oxygen species. It was further shown that the Ndi1 protein can be functionally expressed in tissues such as skeletal muscles and brain of rodents. The Ndi1 expression scarcely induced an inflammatory response as assessed by hematoxylin and eosin (H&E) staining. The Ndi1 protein expressed in the substantia nigra (SN) elicited protective effects against neurodegeneration caused by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment. The Ndi1 protein has a great potential as a molecular remedy for complex I deficiencies.

Role of mitochondria in multiple sclerosis

This review presents inherited and acquired forms of mitochondrial dysfunction associated with oligodendrocytopathy and neurodegeneration in order to better understand the degenerative features of inflammatory demyelination. The recognition that various mitochondrial mechanisms are involved in the pathogenesis of multiple sclerosis leads to therapeutic considerations, re-emphasizing the importance of early neuroprotection in combination with the approved means of immune modulation.

Complex I deficiency in Persian multiple sclerosis patients

Multiple sclerosis (MS) is a demyelinating disease of the central nervous system characterized by the morphological hallmarks of inflammation, demyelination and axonal loss. Until now, little attention has been paid to the contribution of mitochondrial respiratory chain enzyme activities to MS. In this study, kinetic analysis of mitochondrial respiratory chain complex I enzyme (measured as NADH-ferricyanide reductase) was performed on intact mitochondria isolated from fresh skeletal muscle in MS patients (n = 10) and control subjects (n = 11). Mitochondrial DNA common deletion and deletions were also tested in MS patients. Our findings showed that complex I activities were significantly reduced (P = 0.007) in patients compared with control. However, we could not find deletion in mtDNA of patients with MS. The presupposition of relationship between MS and mitochondrial disorders is due to predominant maternal transmission of MS in affected parent-child pairs, pathoaetiological role of respiratory chain dysfunction in multisystem disorders and important role of it in neurodegenerative disorders, a number of patients such as LHON or other mtDNA abnormality with developed neurological symptoms indistinguishable from MS and similarity of clinical symptoms in mitochondrial disorders to those of MS. This study suggested that a biochemical defect in complex I activity may be involved in pathogenesis of MS.

The unique characteristics of Thai Leber hereditary optic neuropathy: analysis of 30 G11778A pedigrees

Leber hereditary optic neuropathy (LHON) is characterized by acute or subacute bilateral visual loss, and affects mostly young males. The most common mitochondrial DNA mutation responsible for LHON worldwide is G11778A. Despite different genetic backgrounds, which are believed to influence the disease expression, most features of LHON are quite common in different populations. However, there seem to be a few ethnic-specific differences. Analyses of our 30 G11778A LHON pedigrees in Thailand showed some characteristics different from those of Caucasians and Japanese. In particular, our pedigrees showed a lower male to female ratio of affected persons (2.6:1) and much higher prevalence of G11778A blood heteroplasmy (37% of the pedigrees contained at least one heteroplasmic G11778A individual). Heteroplasmicity seemed to influence disease manifestation in our patients but did not appear to alter the onset of the disease. The estimated overall penetrance of our G11778A LHON population was 37% for males and 13% for females. When each of our large pedigrees were considered separately, disease penetration varied from 9 to 45% between the pedigrees, and also varied between different branches of the same large pedigree. Survival analysis showed that the secondary LHON mutations G3316A and C3497T had a synergistic deleterious effect with the G11778A mutation, accelerating the onset of the disease in our patients.

Caspase-independent death of Leber's hereditary optic neuropathy cybrids is driven by energetic failure and mediated by AIF and Endonuclease G

Leber's hereditary optic neuropathy (LHON) is associated with mitochondrial DNA point mutations affecting different subunits of complex I. By replacing glucose with galactose in the medium, cybrids harboring each of the three LHON pathogenic mutations (11778/ND4, 3460/ND1, 14484/ND6) suffered a profound ATP depletion over a few hours and underwent apoptotic cell death, which was caspase-independent. Control cybrids were unaffected. In addition to cytochrome c, apoptosis inducing factor (AIF) and endonuclease G (EndoG) were also released from the mitochondria into the cytosol in LHON cybrids, but not in control cells. Exposure of isolated nuclei to cytosolic fractions from LHON cybrids maintained in galactose medium caused nuclear fragmentation, which was strongly reduced by immuno-depletion with anti-AIF and anti-EndoG antibodies. In conclusion, the caspase-independent death of LHON cybrids incubated in galactose medium is triggered by rapid ATP depletion and mediated by AIF and EndoG.

Complex I deficiency primes Bax-dependent neuronal apoptosis through mitochondrial oxidative damage

Dysfunction of mitochondrial complex I is a feature of human neurodegenerative diseases such as Leber hereditary optic neuropathy and Parkinson's disease. This mitochondrial defect is associated with a recruitment of the mitochondrial-dependent apoptotic pathway in vivo. However, in isolated brain mitochondria, complex I dysfunction caused by either pharmacological or genetic means fails to directly activate this cell death pathway. Instead, deficits of complex I stimulate intramitochondrial oxidative stress, which, in turn, increase the releasable soluble pool of cytochrome c within the mitochondrial intermembrane space. Upon mitochondrial permeabilization by the cell death agonist Bax, more cytochrome c is released to the cytosol from brain mitochondria with impaired complex I activity. Given these results, we propose a model in which defects of complex I lower the threshold for activation of mitochondrial-dependent apoptosis by Bax, thereby rendering compromised neurons more prone to degenerate. This molecular scenario may have far-reaching implications for the development of effective neuroprotective therapies for these incurable illnesses.

The mitochondrial genome in human adaptive radiation and disease: on the road to therapeutics and performance enhancement

The human mitochondrial genome consists of approximately 1500 genes, 37 encoded by the maternally inherited mitochondrial DNA (mtDNA) and the remainder encoded in the nuclear DNA (nDNA). The mtDNA is present in thousands of copies per cell and encodes proteins that are essential components of the mitochondrial energy generation pathway, oxidative phosphorylation (OXPHOS). OXPHOS generates heat to maintain our body temperature and ATP to do work. The mitochondria also produce much of the cellular reactive oxygen species (ROS) and can initiate apoptosis through activation of the mitochondrial permeability transition pore (mtPTP) in response to energy deficiency and oxidative damage. Mitochondrial ROS mutates the mtDNA and mtDNA mutations have been associated with a wide range of age-related diseases including neurodegenerative diseases, cardiomyopathy, diabetes and various cancers. The cellular accumulation of mtDNA mutations may also be the aging clock. Ancient mtDNA variants have also been adaptive and may influence individual health today. Mutations in nDNA-encoded mitochondrial genes can also disrupt OXPHOS, alter mtDNA replication, and affect mitochondrial division. In an effort to treat mitochondrial disease, both metabolic and genetic interventions have been attempted. Metabolic interventions have been directed at increasing energy output, reducing ROS production and stabilizing the mtPTP. Genetic therapies have attempted introduction of nucleic acids into the mitochondrion, nDNA-mitochondrial genes into the nucleus, and mtDNA-encoded genes into the nucleus. These therapeutic approaches might also be used to enhance performance, but we must be careful that catering to short term individual interests might undermine our capacity to adapt and survive.

Hereditary optic neuropathies: from the mitochondria to the optic nerve

PURPOSE

To review our current knowledge of inherited optic neuropathies.

DESIGN

Perspective.

METHODS

Literature review.

RESULTS

The hereditary optic neuropathies consist of a group of disorders in which optic nerve dysfunction figures solely or prominently and direct inheritance is clinically or genetically proven. The most common of these disorders are autosomal dominant optic atrophy (Kjers' disease) and maternally-inherited Leber's hereditary optic neuropathy. Other inherited neurologic and systemic syndromic diseases will frequently manifest optic neuropathy. A selective vulnerability of the optic nerve to perturbations in mitochondrial function may underlie a final common pathway among these disorders.

CONCLUSIONS

The ophthalmologist should be familiar with the clinical characteristics and diagnosis of the hereditary optic neuropathies. Recent advances in our understanding of the underlying pathophysiology of the inherited optic neuropathies may provide insight into their treatment and the treatment of acquired optic nerve disorders.

Hereditary optic neuropathies

AIMS

To provide a clinical update on the hereditary optic neuropathies.

METHODS

Review of the literature.

RESULTS

The hereditary optic neuropathies comprise a group of disorders in which the cause of optic nerve dysfunction appears to be hereditable, based on familial expression or genetic analysis. In some hereditary optic neuropathies, optic nerve dysfunction is typically the only manifestation of the disease. In others, various neurologic and systemic abnormalities are regularly observed.

CONCLUSION

The most common hereditary optic neuropathies are autosomal dominant optic atrophy (Kjer's disease) and maternally inherited Leber's hereditary optic neuropathy. We review the clinical phenotypes of these and other inherited disorders with optic nerve involvement.

Genetic variants of Complex I in multiple sclerosis

HYPOTHESIS

A mitochondrial mechanism contributes to neurodegeneration in multiple sclerosis (MS). Genetic variants of Complex I genes may influence the nature of tissue response to inflammation in the central nervous system (CNS).

BACKGROUND

Complex I is encoded by seven mitochondrial and 38 nuclear genes. Many of the nuclear genes colocalize with regions where full genome scans detected linkage in MS. Previous studies revealed an association between variants of mitochondrial (mt)DNA encoded subunits of Complex I and MS. Biochemical studies suggested a functional involvement of Complex I in the degenerative processes downstream to inflammatory injury in the CNS.

METHODS

Patients with all MS phenotypes were included. DNA specimens of affected sib pair, trio and multiplex families were studied. Single nucleotide polymorphisms (SNP) were determined by using the Taqman assay. The association of MS with nuclear DNA encoded alleles and haplotypes of Complex I was tested by the pedigree disequilibrium test (PDT) and by the transmit program in the families. Haplotypes were further investigated by using ldmax (GOLD). The association of mtDNA encoded variants with MS was tested by the Fisher's Exact Test.

RESULTS

The previously identified MS-associated mtDNA variants and haplotypes were not increased in mothers as compared to fathers in these families. However, an association of all clinical phenotypes with haplotypes within NDUFS5 (1p34.2-p33), NDUFS7 (19p13) and NDUFA7 (19p13) was detected. The inclusion of families with primary progressive (PP)-MS phenotype did not modify the outcome and, as a subgroup alone, did not have a sufficient size to draw conclusion regarding phenotype specific associations.

CONCLUSIONS

SNP haplotypes within Complex I genes are associated with MS. Further studies are needed to refine the identification of disease relevant variants nearby or within these haplotypes. Molecular and functional properties of Complex I subunits may offer novel explanations to better understand the relationship between inflammation and neurodegeneration.

Familial multiple sclerosis and other inherited disorders of the white matter

BACKGROUND

An objective demonstration of lesions disseminated in time and space remains the core of the last revision of diagnostic criteria for multiple sclerosis (MS), but this update is now empowered by a weighted use of magnetic resonance imaging (MRI), which results in an earlier and more unambiguous diagnosis ("MS," "not MS," or "possible MS"). Nevertheless, the exclusion of other entities still remains an integral element of the diagnostic process.

REVIEW SUMMARY

Exclusion of genetic disorders can be challenging in some cases with familial recurrence of MS, particularly when the transmission is mimicking a mendelian or a maternal pattern of inheritance. Vice versa, many forms of mendelian leukodystrophies and leukoencephalopathies present with juvenile or adult onset, progressive or relapsing-remitting courses, intrafamilial phenotypic heterogeneity and MRI signs of multifocal white matter (WM) pathology, features potentially leading to a temporary confusion with MS. With the recent availability of disease modifying medications in MS, the development of specific molecular therapies in inherited WM disorders, and the general recognition of the effectiveness of early treatments, the accuracy of initial diagnostic assessment has become critical.

CONCLUSION

Considering the importance of disease specific treatments, here we review the major characteristics of familial MS and some of the inheritable diseases of the WM. Although no direct genetic link between MS and these WM abnormalities is known, molecular data from the field of rare genetic disorders may also provide some experimental paradigms to a further exploration of MS.

Mitochondrial dysfunction as a cause of optic neuropathies

Mitochondria are increasingly recognized as central players in the life and death of cells and especially of neurons. The energy-dependence of retinal ganglion cells (RGC) and their axons, which form the optic nerve, is singularly skewed. In fact, while mitochondria are very abundant in the initial, unmyelinated part of the axons anterior to the lamina cribrosa, their number suddenly decreases as the myelin sheath begins more posteriorly. The vascular system also presents different blood-brain barrier properties anterior and posterior to the lamina, possibly reflecting the different metabolic needs of the optic nerve head (unmyelinated) and of the retrobulbar optic nerve (myelinated). Mitochondrial biogenesis occurs within the cellular somata of RGC in the retina. It needs the coordinated interaction of nuclear and mitochondrial genomes. Mitochondria are then transported down the axons and distributed where they are needed. These locations are along the unmyelinated portion of the nerve, under the nodes of Ranvier in the retrobulbar nerve, and at the synaptic terminals. Efficient transportation of mitochondria depends on multiple factors, including their own energy production, the integrity of the cytoskeleton and its protein components (tubulin, etc.), and adequate myelination of the axons. Any dysfunction of these systems may be of pathological relevance for optic neuropathies with primary or secondary involvement of mitochondria. Leber's hereditary optic neuropathy (LHON) is the paradigm of mitochondrial optic neuropathies where a primary role for mitochondrial dysfunction is certified by maternal inheritance and association with specific mutations in the mitochondrial DNA (mtDNA). Clinical phenocopies of this pathology are represented by the wide array of optic neuropathies associated with vitamin depletion, toxic exposures, alcohol and tobacco abuse, and use of certain drugs. Moreover, the recent identification of mutations in the nuclear gene OPA1 as the causative factor in dominant optic atrophy (DOA, Kjer's type) brought the unexpected finding that this gene encodes for a mitochondrial protein, suggesting that DOA and LHON may be linked by similar pathogenesis. Polymorphisms in this very same gene may be associated with normal tension glaucoma (NTG), which might be considered a genetically determined optic neuropathy that again shows similarities with both LHON and DOA. Exciting new developments come from first examples of mitochondrial optic neuropathies in animal models that are genetically determined or are the result of ingenious engineering of mitochondrial gene expression, or from biochemical manipulations of the respiratory complexes. Even more exciting is the first successful attempt to correct the LHON-related complex I dysfunction by the allotopic nuclear expression of the recoded mitochondrial gene. There is hope that the genetic complexities, biochemical dysfunctions, and integrated anatomical-physiological cellular relationships will soon be precisely delineated and that promising therapeutic and prophylactic strategies will be proposed.