Mitochondrial disorders: an overview

Studies on effects of static electric field exposure on liver in mice

With the development of ultra-high-voltage direct-current transmission, the intensity of static electric field (SEF) under transmission lines increased, which has aroused public attention on its potential health effects. In order to examine effects of SEF exposure on liver, institute of cancer research mice were exposed to SEF with intensities of 27.5 kV/m, 34.7 kV/m and 56.3 kV/m, respectively. In each intensity of SEF exposure, a corresponding sham exposure group was used. Several indices relating to liver function (aspartate aminotransferase (AST) and alanine aminotransferase (ALT)) and oxidative stress (superoxide dismutase (SOD), glutathione peroxidase (GSH-Px) and malondialdehyde (MDA)) were tested after exposure of 7, 14, 21 and 35 days. Results showed that exposure to SEF with intensities of 27.5 kV/m and 34.7 kV/m for 35 days did not significantly influence any detected indices above. Under SEF exposure with intensity of 56.3 kV/m, the SOD activity in liver was significantly increased after exposure of 7 and 14 days. However, no significant increase was found in MDA content as well as the activities of AST and ALT between exposure group and sham exposure group during SEF exposure of 56.3 kV/m. It suggested that from three SEF intensities, only exposure to SEF with intensity of 56.3 kV/m (7 and 14 days) caused a temporary oxidative stress response in liver expressed by the increase in activity of SOD, but it did not produce oxidative damage. This biological effect may be related to the increase of mitochondrial membrane potential of hepatocytes caused by SEF exposure. When the membrane potential exceeds a threshold, Q cycle in mitochondria will be affected, which will result in an increase of superoxide anion concentration and ultimately an oxidative stress.

Mitochondrial dysfunction in a novel form of autosomal recessive ataxia

Defects in the recognition and/or repair of damage to DNA are responsible for a sub-group of autosomal recessive ataxias. Included in this group is a novel form of ataxia with oculomotor apraxia characterised by sensitivity to DNA damaging agents, a defect in p53 stabilisation, oxidative stress and resistance to apoptosis. We provide evidence here that the defect in this patient's cells is at the level of the mitochondrion. Mitochondrial membrane potential was markedly reduced in cells from the patient and ROS levels were elevated. This was accompanied by lipid peroxidation of mitochondrial proteins involved in electron transport and RNA synthesis. However, no gross changes or alteration in composition or activity of mitochondrial electron transport complexes was evident. Sequencing of mitochondrial DNA revealed a mutation, I349T, in the mitochondrial cytochrome b gene. These results describe a patient with an apparently novel form of AOA characterised by a defect at the level of the mitochondrion.

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.

Effects of new ubiquinone-imidazo[2,1-b]thiazoles on mitochondrial complex I (NADH-ubiquinone reductase) and on mitochondrial permeability transition pore

In this work we describe the synthesis of a series of imidazo[2,1-b]thiazoles and 2,3-dihydroimidazo[2,1-b]thiazoles connected by means of a methylene bridge to CoQ(0). These compounds were tested as specific inhibitors of the NADH:ubiquinone reductase activity in mitochondrial membranes. The imidazothiazole system when bound to the quinone ring in place of the isoprenoid lateral side chain, may increase the inhibitory effect (with an IC(50) for NADH-Q(1) activity ranging between 0.25 and 0.96 microM) whereas the benzoquinone moiety seems to lose the capability to accept electrons from complex I as indicated by very low maximal velocity elicited by the compounds tested. Moreover the low rotenone sensitivity for almost all of these compounds suggests that they are only partially able to interact with the physiological ubiquinone-reduction site. The compounds were investigated for the capability of increasing the permeability transition of the inner mitochondrial membrane in isolated mitochondria. Unlike CoQ(0), which is considered a mitochondrial membrane permeability transition inhibitor, the new compounds were inducers.

Mitochondria in nonalcoholic fatty liver disease

Nonalcoholic fatty liver (NAFL) is associated with fundamental issues of fat metabolism and insulin resistance. These abnormalities have been linked to impairment of ATP homeostasis, and a growing body of literature has reported mitochondrial abnormalities in various forms of hepatic steatosis. The changes are evident as structural abnormalities, including greatly increased size and the development of crystalline inclusions, and are usually regarded as pathologic, reflecting either a protective or degenerative response to injury. Although the relationships between structural changes,decreased mitochondrial function, and disease states are becoming clearer, the molecular basis for the perturbations is not well understood. Oxidative damage is the most likely causative process and may result in alterations of mitochondrial DNA (mtDNA), stimulated apoptotic pathways, and increased propensity for necrosis.Overall mitochondrial health likely depends on multiple factors including the integrity of the mtDNA, the composition of cellular lipids, lipoprotein trafficking, the balance of pro- and antioxidant factors, and the metabolic demands placed on the liver. Mitochondrial dysfunction may play a role in numerous clinical conditions associated with NAFL, such as hepatocellular carcinoma, lipodystrophy,age-related insulin resistance, gut dysmotility, cryptogenic cirrhosis, a mild form of gaze palsy, and possibly other more severe neurodegenerative diseases. The prominent role of mitochondrial dysfunction in NAFL provides a new and exciting paradigm in which to view this disorder, its complications, and potential dietary and pharmacologic intervention.

Ebselen prevents mitochondrial ageing due to oxidative stress: in vitro study of fish erythrocytes

Nucleated trout erythrocytes under oxidative stress suffer DNA membrane damage and inactivation of glutathione peroxidase. In addition, oxidative damage increases with the age of the cell. In the present paper, we evaluate the effects of oxidative stress and ageing on mitochondrial functionality by means of transmission electron microscopy and cytofluorimetric determination of mitochondrial membrane potential and intracellular levels of reactive oxygen species. The protective activity of the antioxidant organoselenium compound ebselen, a mimic of glutathione peroxidase, is also evaluated. Ebselen prevents the drastic structural and functional changes in mitochondria in aged RBCs induced by oxidative stress. However, the antioxidant does not prevent swelling of the mitochondria.

Identification of the full-length huntingtin- interacting protein p231HBP/HYPB as a DNA-binding factor

Neurodegeneration in Huntington's disease (HD) is associated with an elongated glutamine tract in the widely expressed huntingtin protein. Although the pathogenic mechanisms are still unknown, the distinct physical properties of mutant huntingtin in the brain suggest that other factors including huntingtin-interacting proteins might play a specific role. We have previously identified a DNA-binding motif in the proximal E1A promoter of adenovirus serotype 12 as responsible for E1A autoregulation. Here, we identified the p231HBP protein as a DNA-binding factor, the C-terminal portion of which has recently been characterized as the huntingtin-interacting protein HYPB of unknown function. We have determined the full-length cDNA sequence, identified several domains supporting its gene regulatory functions, and mapped the HBP231 gene to chromosome 3p21.2-p21.3. Our results provide an interesting molecular link between huntingtin and a DNA-binding factor, implicating that this interaction might result in the alteration of cellular gene expression involved in HD pathogenesis.

Mitochondrial abnormalities in non-alcoholic steatohepatitis

BACKGROUND/AIMS

We assessed mitochondrial morphology by electron microscopy and the prevalence of a mitochondrial gene deletion in patients with non-alcoholic steatohepatitis (NASH), alcohol-related liver disease and non-fatty liver diseases. Respiratory chain function using a cytoplasmic hybrid (cybrid) assay was further studied in NASH patients and healthy controls.

METHODS

Electron microscopy was performed in 26 specimens. Fifteen patients were studied by polymerase chain reaction to detect a 520-bp deletion product of the mitochondrial genome (dmtDNA). Cybrids were created by fusion of platelets with anaerobic neuroblastoma cells in six NASH patients and 12 controls.

RESULTS

Eight of ten NASH, one of seven alcoholics and two of nine other patients had linear crystalline inclusions in megamitochondria (p<0.05). Three of five patients with alcohol-related liver disease had dmtDNA compared to one of five NASH patients and one of five non-steatohepatitis controls. Cybrid respiratory chain function in platelets was not different from that of controls.

CONCLUSIONS

Respiratory chain dysfunction, if present in NASH, is not expressed in platelet-derived mitochondria. In contrast to alcohol-related liver disease with active drinking, NASH patients do not commonly express the 5-kb mitochondrial DNA gene deletion in liver tissue. As previously described in early alcohol-related liver disease, crystalline inclusions of unknown composition are seen in hepatic mitochondria in NASH. Their presence suggests either an adaptive process or mitochondrial injury.

Mitochondrial abnormalities in non-alcoholic steatohepatitis

BACKGROUND/AIMS

We assessed mitochondrial morphology by electron microscopy and the prevalence of a mitochondrial gene deletion in patients with non-alcoholic steatohepatitis (NASH), alcohol-related liver disease and non-fatty liver diseases. Respiratory chain function using a cytoplasmic hybrid (cybrid) assay was further studied in NASH patients and healthy controls.

METHODS

Electron microscopy was performed in 26 specimens. Fifteen patients were studied by polymerase chain reaction to detect a 520-bp deletion product of the mitochondrial genome (dmtDNA). Cybrids were created by fusion of platelets with anaerobic neuroblastoma cells in six NASH patients and 12 controls.

RESULTS

Eight of ten NASH, one of seven alcoholics and two of nine other patients had linear crystalline inclusions in megamitochondria (p<0.05). Three of five patients with alcohol-related liver disease had dmtDNA compared to one of five NASH patients and one of five non-steatohepatitis controls. Cybrid respiratory chain function in platelets was not different from that of controls.

CONCLUSIONS

Respiratory chain dysfunction, if present in NASH, is not expressed in platelet-derived mitochondria. In contrast to alcohol-related liver disease with active drinking, NASH patients do not commonly express the 5-kb mitochondrial DNA gene deletion in liver tissue. As previously described in early alcohol-related liver disease, crystalline inclusions of unknown composition are seen in hepatic mitochondria in NASH. Their presence suggests either an adaptive process or mitochondrial injury.

Mitochondrial physiology and pathology; concepts of programmed death of organelles, cells and organisms

The review summarizes the present state of our knowledge concerning alternative functions of mitochondria, namely energy conservation in forms of protonic potential and ATP, thermoregulatory energy dissipation as heat, production of useful substances, decomposition of harmful substances, control of cellular processes. The recent progress in understanding of some mitochondrion-linked pathologies is described. The role of reactive oxygen species in these processes is stressed. Possible mechanisms of programmed death of mitochondrion (mitoptosis), cell (apoptosis) and organism (phenoptosis) are considered. A concept is put forward assuming that mitoptosis is involved in some types of apoptosis whereas apoptosis can be a part of a phenoptotic cascade. It is hypothesized that septic shock, as well as the stress-induced brain and heart ischemic diseases and cancer, exemplify mechanisms of phenoptosis purifying population, community of organisms or kin from dangerous or useless individuals.

Role of mitochondria in oxidative stress and ageing

Mitochondria are deeply involved in the production of reactive oxygen species through one-electron carriers in the respiratory chain; mitochondrial structures are also very susceptible to oxidative stress as evidenced by massive information on lipid peroxidation, protein oxidation, and mitochondrial DNA (mtDNA) mutations. Oxidative stress can induce apoptotic death, and mitochondria have a central role in this and other types of apoptosis, since cytochrome c release in the cytoplasm and opening of the permeability transition pore are important events in the apoptotic cascade. The discovery that mtDNA mutations are at the basis of a number of human pathologies has profound implications: maternal inheritance of mtDNA is the basis of hereditary mitochondrial cytopathies; accumulation of somatic mutations of mtDNA with age has represented the basis of the mitochondrial theory of ageing, by which a vicious circle is established of mtDNA damage, altered oxidative phosphorylation and overproduction of reactive oxygen species. Experimental evidence of respiratory chain defects and of accumulation of multiple mtDNA deletions with ageing is in accordance with the mitochondrial theory, although some other experimental findings are not directly ascribable to its postulates.

Reduction of lipid peroxidation in different brain regions by a combination of alpha-tocopherol and ascorbic acid

Lipid peroxidation an outcome of free radicals activity has been hypothesized as one of the possible factor involved in the pathogenesis of neuronal damage. We investigated the effects of free radical scavengers, alpha-tocopherol (T) and ascorbic acid (A) combination (TA-combination) to attenuate tert-butyl hydroperoxide (t-BuOOH)-induced lipid peroxidation in different regions of mice brain. Examinations of effect of three regimens (100, 200, 300 mg/kg body weight) of t-BuOOH on mid brain, cerebellum, striatum, cortex and hippocampus revealed dose and time dependent increase in lipid peroxidation. We observed that prior supplementation of TA-combination reduced lipid peroxidation induced by t-BuOOH in every brain region. These findings suggest that TA-combination may play a vital role in protecting the brain tissue against free radicals.

Oxidative stress, antioxidant defences and aging

Apoptosis and aging share common mechanisms in oxidative stress and mitochondrial involvement. Treatment of cultured neuroblastoma cells with a radical initiator induced apoptosis; raise in hydrogen peroxide and release of cytochrome c from mitochondria preceded collapse of mitochondrial potential and cell death. In rat hepatocytes treated with adriamycin incubation with exogenous Coenzyme Q10 counteracted the drug-induced increase of hydrogen peroxide and the fall of the mitochondrial potential, thus demonstrating the quinone antioxidant effect. Complex I activity and its rotenone sensitivity decreased in brain cortex non-synaptic mitochondria from old rats; a 5 kb mitochondrial DNA deletion was found only in the old rats. A similar behavior was found in human platelets from old individuals. The postulated energy decline was confirmed by the inhibitor sensitivities of platelet aggregation and lactate production. The lack of the 5 kb deletion in platelets throws doubts on mitochondrial DNA lesions as the only causes of mitochondrial dysfunction in aging.