[Visual recovery as the target for glaucoma]. / Visuelle Regeneration als Ziel für das Glaukom.

BACKGROUND: Visual recovery is an established but poorly studied phenomenon in glaucoma. OBJECTIVE: To provide insights into functional recovery of retinal ganglion cells (RGCs) with a view to providing information on the development of forms of treatment that improve RGC function after injury. METHOD: A model of recoverable RGC function in the mouse eye, induced by short-term elevation of intraocular pressure (IOP). RESULTS: The RGCs manifest near complete functional recovery after a prolonged period of dysfunction following acute IOP elevation. Increasing age and a high fat diet were subsequently found to impair recovery, whereas exercise substantially improved recovery such that older mice recovered in a similar way to young mice. CONCLUSION: Injured RGCs have the capacity to restore function after periods of functional impairment. Therapies that specifically target injured RGCs and enhance their capacity to recover function may provide a new approach for treating glaucoma.

Targeting retinal ganglion cell recovery.

Accumulating evidence from experimental and clinical studies suggest that retinal ganglion cells at least in the earlier stages of glaucoma have the capacity to recover function following periods of functional loss. The capacity for recovery may be negatively impacted by advancing age but can be boosted by interventions such as diet restriction and exercise.

Emerging Mitochondrial Therapeutic Targets in Optic Neuropathies.

Optic neuropathies are an important cause of blindness worldwide. The study of the most common inherited mitochondrial optic neuropathies, Leber hereditary optic neuropathy (LHON) and autosomal dominant optic atrophy (ADOA) has highlighted a fundamental role for mitochondrial function in the survival of the affected neuron-the retinal ganglion cell. A picture is now emerging that links mitochondrial dysfunction to optic nerve disease and other neurodegenerative processes. Insights gained from the peculiar susceptibility of retinal ganglion cells to mitochondrial dysfunction are likely to inform therapeutic development for glaucoma and other common neurodegenerative diseases of aging. Despite it being a fast-evolving field of research, a lack of access to human ocular tissues and limited animal models of mitochondrial disease have prevented direct retinal ganglion cell experimentation and delayed the development of efficient therapeutic strategies to prevent vision loss. Currently, there are no approved treatments for mitochondrial disease, including optic neuropathies caused by primary or secondary mitochondrial dysfunction. Recent advances in eye research have provided important insights into the molecular mechanisms that mediate pathogenesis, and new therapeutic strategies including gene correction approaches are currently being investigated. Here, we review the general principles of mitochondrial biology relevant to retinal ganglion cell function and provide an overview of the major optic neuropathies with mitochondrial involvement, LHON and ADOA, whilst highlighting the emerging link between mitochondrial dysfunction and glaucoma. The pharmacological strategies currently being trialed to improve mitochondrial dysfunction in these optic neuropathies are discussed in addition to emerging therapeutic approaches to preserve retinal ganglion cell function.

Xenomitochondrial mice: investigation into mitochondrial compensatory mechanisms.

Xenomitochondrial mice, harboring evolutionarily divergent Mus terricolor mitochondrial DNA (mtDNA) on a Mus musculus domesticus nuclear background (B6NTac(129S6)-mt(M. terricolor)/Capt; line D7), were subjected to molecular and phenotypic analyses. No overt in vivo phenotype was identified in contrast to in vitro xenomitochondrial cybrid studies. Microarray analyses revealed differentially expressed genes in xenomitochondrial mice, though none were directly involved in mitochondrial function. qRT-PCR revealed upregulation of mt-Co2 in xenomitochondrial mice. These results illustrate that cellular compensatory mechanisms for mild mitochondrial dysfunction alter mtDNA gene expression at a proteomic and/or translational level. Understanding these mechanisms will facilitate the development of therapeutics for mitochondrial disorders.

Mitochondria in aging and Alzheimer's disease.

Two significant risk factors are inextricably linked with Alzheimer's disease: advancing age, and accumulation of the amyloid-beta peptide. Over the age of 65 the risk of developing Alzheimer's disease increases almost exponentially with age, and the amyloid-beta rich neuritic plaques of the Alzheimer's disease brain are a histopathological hallmark of the disease. Since its identification as a major constituent of neuritic plaques amyloid-beta has attracted intense research focus as the primary causative agent in the development of Alzheimer's disease. As a result, numerous reports now exist to propose potential neurotoxic mechanisms mediated by amyloid-beta. Despite these research efforts, there is still a scarcity of information on the biologic link between aging and amyloid-beta in Alzheimer's disease, and although increasing evidence indicates that intracellular amyloid-beta is acutely toxic, there is also a paucity of information on the mechanisms of neurotoxicity mediated by intracellular amyloid-beta. Functional decline of mitochondria with aging is well established, and growing evidence attributes this decline to loss of mitochondrial DNA integrity in postmitotic cells including neurons. Oxidative stress due to mitochondrial failure may drive increased amyloidogenic processing of the amyloid-beta precursor protein, contributing to a loss of amyloid-beta precursor protein functionality and increased amyloid-beta production. Importantly, recent data show that amyloid-beta accumulates within mitochondria of the Alzheimer's disease brain. We speculate that age-related somatic mutation of mitochondrial DNA may be an important factor underlying sporadic Alzheimer's disease.

Development and initial characterization of xenomitochondrial mice.

Xenomitochondrial mice harboring trans-species mitochondria on a Mus musculus domesticus (MD) nuclear background were produced. We created xenomitochondrial ES cell cybrids by fusing Mus spretus (MS), Mus caroli (MC), Mus dunni (Mdu), or Mus pahari (MP) mitochondrial donor cytoplasts and rhodamine 6-G treated CC9.3.1 or PC4 ES cells. The selected donor backgrounds reflected increasing evolutionary divergence from MD mice and the resultant mitochondrial-nuclear mismatch targeted a graded respiratory chain defect. Homoplasmic (MS, MC, Mdu, and MP) and heteroplasmic (MC) cell lines were injected into MD ova, and liveborn chimeric mice were obtained (MS/MD 18 of 87, MC/MD 6 of 46, Mdu/MD 31 of 140, and MP/MD l of 9 founder chimeras, respectively). Seven MS/MD, 1 MC/MD, and 11 Mdu/MD chimeric founder females were mated with wild-type MD males, and 18 of 19 (95%) were fertile. Of fertile females, only one chimeric MS/MD (1% coat color chimerism) and four chimeric Mdu/MD females (80-90% coat color chimerism) produced homoplasmic offspring with low efficiency (7 of 135; 5%). Four male and three female offspring were homoplasmic for the introduced mitochondrial backgrounds. Three male and one female offspring proved viable. Generation of mouse lines using additional female ES cell lineages is underway. We hypothesize that these mice, when crossbred with neurodegenerative-disease mouse models, will show accelerated age-related neuronal loss, because of their suboptimal capacity for oxidative phosphorylation and putatively increased oxidative stress.

Genetic control of oxidative phosphorylation and experimental models of defects.

Energy in the form of ATP is continually produced by all cells for normal growth and function. Anaerobic glycolysis can provide enough ATP for some cells, but energetic cells such as cardiomyocytes and neurons require a more efficient ATP supply, which can only be provided by mitochondrial oxidative phosphorylation. Invented by bacteria that became symbiotically associated with other bacteria to form eukaryotic cells billions of years ago, oxidative phosphorylation carries with it a genetic legacy that is unique. The mitochondrial oxidative phosphorylation complexes are assembled from protein subunits encoded by both the mitochondrial genome (mtDNA) and the nuclear genome (nDNA, located in the chromosomes). The mtDNA is a remnant genome of the bacterial progenitor of mitochondria, and (unlike the biparental diploidy that characterizes the nuclear genome) is present in thousands of copies per cell, is replicated through life, and is inherited (cytoplasmically) only from the female parent. Oxidative phosphorylation comprises five multimeric enzyme complexes that act as a redox pathway, passing electrons from oxidizable intermediates produced by the metabolism of food to molecular oxygen in the mitochondrial matrix, while producing an electrochemical gradient by pumping protons into the intermembranal space. The proton (hydrogen ion) gradient across the inner mitochondrial membrane is used by the H+-transporting ATP synthase to produce ATP from ADP and inorganic phosphate, with the protons released into the mitochondrial matrix then combining with electronated oxygen to form water. Many of the details regarding the control of the synthesis of oxidative phosphorylation enzyme complexes remain to be elucidated. Transmitochondrial cell culture systems have been developed so that defective oxidative phosphorylation can be studied in a controlled nuclear background. Such systems may soon enable the development of mtDNA 'knockout' mice in order to better model mtDNA transmission and mitochondrial disease.

Functional analysis of lymphoblast and cybrid mitochondria containing the 3460, 11778, or 14484 Leber's hereditary optic neuropathy mitochondrial DNA mutation.

Leber's hereditary optic neuropathy (LHON) is a form of blindness caused by mitochondrial DNA (mtDNA) mutations in complex I genes. We report an extensive biochemical analysis of the mitochondrial defects in lymphoblasts and transmitochondrial cybrids harboring the three most common LHON mutations: 3460A, 11778A, and 14484C. Respiration studies revealed that the 3460A mutation reduced the maximal respiration rate 20-28%, the 11778A mutation 30-36%, and the 14484C mutation 10-15%. The respiration defects of the 3460A and 11778A mutations transferred in cybrid experiments linking these defects to the mtDNA. Complex I enzymatic assays revealed that the 3460A mutation resulted in a 79% reduction in specific activity and the 11778A mutation resulted in a 20% reduction, while the 14484C mutation did not affect the complex I activity. The enzyme defect of the 3460A mutation transferred with the mtDNA in cybrids. Overall, these data support the conclusion that the 3460A and 11778A mutants result in complex I defects and that the 14484C mutation causes a much milder biochemical defect. These studies represent the first direct comparison of oxidative phosphorylation defects among all of the primary LHON mtDNA mutations, thus permitting insight into the underlying pathophysiological mechanism of the disease.

Expression of Rattus norvegicus mtDNA in Mus musculus cells results in multiple respiratory chain defects.

The production of in vitro and in vivo models of mitochondrial DNA (mtDNA) defects is currently limited by a lack of characterized mouse cell mtDNA mutants that may be expected to model human mitochondrial diseases. Here we describe the creation of transmitochondrial mouse (Mus musculus) cells repopulated with mtDNA from different murid species (xenomitochondrial cybrids). The closely related Mus spretus mtDNA is readily maintained when introduced into M. musculus mtDNA-less (rho(0)) cells, and the resulting cybrids have normal oxidative phosphorylation (OXPHOS). When the more distantly related Rattus norvegicus mtDNA is transferred to the mouse nuclear background the mtDNA is replicated, transcribed, and translated efficiently. However, function of several OXPHOS complexes that depend on the coordinated assembly of nuclear and mtDNA-encoded proteins is impaired. Complex I activity in the Rattus xenocybrid was 46% of the control mean; complex III was 37%, and complex IV was 78%. These defects combined to restrict maximal respiration to 12-31% of the control and M. spretus xenocybrids, as measured polarographically using isolated cybrid mitochondria. These defects are distinct to those previously reported for human/primate xenocybrids. It should be possible to produce other mouse xenocybrid constructs with less severe OXPHOS phenotypes, to model human mtDNA diseases.

Cloning of neuronal mtDNA variants in cultured cells by synaptosome fusion with mtDNA-less cells.

Synaptosome cybrids were used to confirm the presence of heteroplasmic mtDNA sequence variants in the human brain. Synaptosomes contain one to several mitochondria, and when fused to mtDNA-deficient (rho degrees ) mouse or human cell lines result in viable cybrid cell lines. The brain origin of mouse synaptosome cybrid mtDNAs was confirmed using sequence polymorphisms in the mtDNA COIII, ND3 and tRNA(Arg)genes. The brain origin of the human synaptosome cybrids was confirmed using a rare mtDNA Mbo I polymorphism. Fusion of synaptosomes from the brain of a 35-year-old woman resulted in 71 synaptosome cybrids. Sequencing the mtDNA control region of these cybrid clones revealed differences in the number of Cs in a poly C track between nucleotide pairs (nps) 301 and 309. Three percent of the cybrid clones had mtDNAs with 10 Cs, 76% had nine, 18% had eight and 3% had seven Cs. Comparable results were obtained by PCR amplification, cloning and sequencing of mtDNA control regions directly from the patient's brain tissue, but not when the control region was amplified and cloned from a synaptosome cybrid homoplasmic for a mtDNA with nine Cs. Thus, we have clonally recovered mtDNA control region length variants from an adult human brain without recourse to PCR, and established the variant mtDNAs within living cultured cells. This confirms that some mtDNA heteroplasmy can exist in human neurons, and provides the opportunity to study its functional significance.

A mouse model for mitochondrial myopathy and cardiomyopathy resulting from a deficiency in the heart/muscle isoform of the adenine nucleotide translocator.

In an attempt to create an animal model of tissue-specific mitochondrial disease, we generated 'knockout' mice deficient in the heart/muscle isoform of the adenine nucleotide translocator (Ant1). Histological and ultrastructural examination of skeletal muscle from Ant1 null mutants revealed ragged-red muscle fibers and a dramatic proliferation of mitochondria, while examination of the heart revealed cardiac hypertrophy with mitochondrial proliferation. Mitochondria isolated from mutant skeletal muscle exhibited a severe defect in coupled respiration. Ant1 mutant adults also had a resting serum lactate level fourfold higher than that of controls, indicative of metabolic acidosis. Significantly, mutant adults manifested severe exercise intolerance. Therefore, Ant1 mutant mice have the biochemical, histological, metabolic and physiological characteristics of mitochondrial myopathy and cardiomyopathy.

Use of transmitochondrial cybrids to assign a complex I defect to the mitochondrial DNA-encoded NADH dehydrogenase subunit 6 gene mutation at nucleotide pair 14459 that causes Leber hereditary optic neuropathy and dystonia.

A heteroplasmic G-to-A transition at nucleotide pair (np) 14459 within the mitochondrial DNA (mtDNA)-encoded NADH dehydrogenase subunit 6 (ND6) gene has been identified as the cause of Leber hereditary optic neuropathy (LHON) and/or pediatric-onset dystonia in three unrelated families. This ND6 np 14459 mutation changes a moderately conserved alanine to a valine at amino acid position 72 of the ND6 protein. Enzymologic analysis of mitochondrial NADH dehydrogenase (complex I) with submitochondrial particles isolated from Epstein-Barr virus-transformed lymphoblasts revealed a 60% reduction (P < 0.005) of complex I-specific activity in patient cell lines compared with controls, with no differences in enzymatic activity for complexes II plus III, III and IV. This biochemical defect was assigned to the ND6 np 14459 mutation by using transmitochondrial cybrids in which patient Epstein-Barr virus-transformed lymphoblast cell lines were enucleated and the cytoplasts were fused to a mtDNA-deficient (p 0) lymphoblastoid recipient cell line. Cybrids harboring the np 14459 mutation exhibited a 39% reduction (p < 0.02) in complex I-specific activity relative to wild-type cybrid lines but normal activity for the other complexes. Kinetic analysis of the np 14459 mutant complex I revealed that the Vmax of the enzyme was reduced while the Km remained the same as that of wild type. Furthermore, specific activity was inhibited by increasing concentrations of the reduced coenzyme Q analog decylubiquinol. These observations suggest that the np 14459 mutation may alter the coenzyme Q-binding site of complex I.

Production of transmitochondrial mouse cell lines by cybrid rescue of rhodamine-6G pre-treated L-cells.

Treatment of mouse LMTK- cells with the toxic mitochondrial dye rhodamine 6G (R-6G) at 2.5 micrograms/ml for 7 days prevented cell growth while maintaining viability, with less than 10(-6) cells recovering to form colonies. Pre-treatment of LMTK- cells with R-6G was followed by fusion with enucleated mouse 501-1 cells harboring a homoplasmic point mutation in the mitochondrial DNA (mtDNA) 16S rRNA gene conferring chloramphenicol resistance (CAPR). Cybrids and any surviving unfused LMTK- cells were selected in BrdU with or without CAP and their mtDNAs screened for the presence of the CAPR marker. Approximately 1 colony per 2 x 10(5) LMTK- cells appeared in the fusion plates selected both with and without CAP. Most clones investigated were confirmed to be cybrids by showing the presence of the generally homoplasmic CAPR mutation, whether or not CAP selection was used. Hence, R-6G pre-treatment permits construction of transmitochondrial cybrid cell lines carrying a variety of mtDNAs, without the need for rho 0 cell lines.

Mitochondrial DNA mutations in human degenerative diseases and aging.

A wide variety of mitochondrial DNA (mtDNA) mutations have recently been identified in degenerative diseases of the brain, heart, skeletal muscle, kidney and endocrine system. Generally, individuals inheriting these mitochondrial diseases are relatively normal in early life, develop symptoms during childhood, mid-life, or old age depending on the severity of the maternally-inherited mtDNA mutation; and then undergo a progressive decline. These novel features of mtDNA disease are proposed to be the product of the high dependence of the target organs on mitochondrial bioenergetics, and the cumulative oxidative phosphorylation (OXPHOS) defect caused by the inherited mtDNA mutation together with the age-related accumulation mtDNA mutations in post-mitotic tissues.

Cytoplasmic transfer of the mtDNA nt 8993 T-->G (ATP6) point mutation associated with Leigh syndrome into mtDNA-less cells demonstrates cosegregation with a decrease in state III respiration and ADP/O ratio.

A point mutation in the mtDNA-encoded ATP6 gene (T-->G at nt 8993) associated with Leigh syndrome in two pedigrees was found to decrease ADP-stimulated (state III) respiration and the ratio of ADP molecules phosphorylated to oxygen atoms reduced (ADP/O ratio) but did not affect 2,4-dinitrophenol (DNP)-uncoupled respiration, suggesting a defective mitochondrial H(+)-translocating ATP synthase. Intact mitochondria isolated from patient and control lymphoblastoid cell lines were tested for state III, ADP-limited (state IV), and DNP-uncoupled respiration with various substrates. Mitochondria isolated from patient lymphoblasts harboring 95-100% of mtDNAs carrying the nt 8993 T-->G mutation showed state III respiration rates 26-50% lower than controls while having normal DNP-uncoupled rates. This resulted in state III/DNP ratios of 0.52-0.70 in patient mitochondria versus 0.88-0.97 in controls. The ADP/O ratio was also decreased 30-40% in patient mitochondria. Patient lymphoblasts heteroplasmic for the nt 8993 mutation were enucleated by using Percoll gradients and the cytoplasts were fused to mtDNA-deficient (rho 0) cells by electric shock. Cybrid clones homoplasmic for the wild-type nucleotide (T) at nt 8993 gave state III/DNP and ADP/O ratios similar to those of control cybrids, whereas cybrid clones homoplasmic for the mutant nucleotide (G) showed a 24-53% reduction in state III respiration, a state III/DNP ratio of 0.53-0.64, and a 30% decrease in the ADP/O ratio. Thus, the reduced state III respiration rates and ADP/O ratios are linked to the T-->G mutation at nt 8993.

A mitochondrial DNA variant, identified in Leber hereditary optic neuropathy patients, which extends the amino acid sequence of cytochrome c oxidase subunit I.

A G-to-A transition at nucleotide pair (np) 7444 in the mtDNA was found to correlate with Leber hereditary optic neuropathy (LHON). The mutation eliminates the termination codon of the cytochrome c oxidase subunit I (COI) gene, extending the COI polypeptide by three amino acids. The mutation was discovered as an XbaI restriction-endonuclease-site loss present in 2 (9.1%) of 22 LHON patients who lacked the np 11778 LHON mutation and in 6 (1.1%) of 545 unaffected controls. The mutant polypeptide has an altered mobility on SDS-PAGE, suggesting a structural alteration, and the cytochrome c oxidase enzyme activity of patient lymphocytes is reduced approximately 40% relative to that in controls. These data suggest that the np 7444 mutation results in partial respiratory deficiency and thus contributes to the onset of LHON.

Maternally transmitted diabetes and deafness associated with a 10.4 kb mitochondrial DNA deletion.

Diabetes mellitus (DM) is one of the most common chronic disorders of children and adults. Several reports have suggested an increased incidence of maternal transmission in some forms of DM. Therefore, we tested a pedigree with maternally transmitted DM and deafness for mitochondrial DNA mutations and discovered a 10.4 kilobase (kb) mtDNA deletion. This deletion is unique because it is maternally inherited, removes the light strand origin (OL) of mtDNA replication, inhibits mitochondrial protein synthesis, and is not associated with the hallmarks of mtDNA deletion syndromes. This discovery demonstrates that DM can be caused by mtDNA mutations and suggests that some of the heterogeneity of this disease results from the novel features of mtDNA genetics.

Mitochondrial theory of senescence: respiratory chain protein studies in human skeletal muscle.

In view of a previously demonstrated negative correlation between stage III respiratory activity in human mitochondria and increasing age, the relationship between human respiratory chain complex protein content and age was investigated. Quantitative immunoblot studies were carried out using holo complex I, III and IV antibody probes in human skeletal muscle mitochondrial homogenate from patients of varying ages. No significant negative correlation between increased age and respiratory complex chain protein content was seen for either total complex activity or for any of the subunits which could be reliably identified. As respiratory complex protein content is preserved with ageing, the decrease in respiratory efficiency is likely to follow aggregation of mutations in structural mitochondrial (mt) DNA genes which do not interfere with mt DNA transcription and protein translation rather than mutations in mt tRNA or ribosomal RNA genes. This is consistent with the fact that mt genes involved in protein translation only occupy a fairly small percentage of the mitochondrial genome.

Affinity chromatography isolation of human cytochrome oxidase and small-scale Western immunoblot probing of the enzyme complex in mitochondrial cytopathy patients.

Isolation of human cytochrome oxidase by a one-step affinity chromatography procedure on a Sepharose 4B-ferrocytochrome c matrix following solubilization with the nonionic detergent laurylmaltoside yields an enzyme isolate of adequate purity for producing polyclonal antisera. Such an antiserum produced a distinctive immunoreactive profile in Western immunoblot studies to that reported using the enzyme isolated with ionic detergents. A sensitive and highly reproducible Western immunoblotting method is described for probing mitochondrial fractions prepared from small frozen skeletal muscle biopsies with an antiserum against the human placenta cytochrome oxidase. Application of this method to mitochondrial cytopathy patients with partial cytochrome oxidase deficiency shows that the detected subunits are synthesized in these patients.