Mitochondrial disorders

Mitochondrial DNA repair: a critical player in the response of cells of the CNS to genotoxic insults

Cells of the CNS are constantly exposed to agents which damage DNA. Although much attention has been paid to the effects of this damage on nuclear DNA, the nucleus is not the only organelle containing DNA. Within each cell, there are hundreds to thousands of mitochondria. Within each mitochondrion are multiple copies of the mitochondrial genome. These genomes are extremely vulnerable to insult and mutations in mitochondrial DNA (mtDNA) have been linked to several neurodegenerative diseases, as well as the normal process of aging. The principal mechanism utilized by cells to avoid DNA mutations is DNA repair. Multiple pathways of DNA repair have been elucidated for nuclear DNA. However, it appears that only base excision repair is functioning in mitochondria. This repair pathway is responsible for the removal of most endogenous damage including alkylation damage, depurination reactions and oxidative damage. Within the rat CNS, there are cell-specific differences mtDNA repair. Astrocytes exhibit efficient repair, whereas, other glial cell types and neuronal cells exhibit a reduced ability to remove lesions from mtDNA. Additionally, a correlation was observed between those cells with reduced mtDNA repair and an increase in the induction of apoptosis. To demonstrate a causative relationship, a strategy of targeting DNA repair proteins to mitochondria to enhance mtDNA repair capacity was employed. Enhancement of mtDNA repair in oligodendrocytes provided protection from reactive oxygen species- and cytokine-induced apoptosis. These experiments provide a novel strategy for protecting sensitive CNS cells from genotoxic insults and thus provide new treatment options for neurodegenerative diseases.

Energy balance in early-stage Huntington disease

BACKGROUND

Huntington disease (HD) is a genetic neurologic disorder. Weight loss is common in HD and is related to progression of the disease, but the cause of weight loss remains unclear.

OBJECTIVE

The study objective was to compare 24-h energy expenditure (EE) and energy intake in persons with early midstage HD with those of matched control subjects to determine how HD affects energy balance.

DESIGN

EE was assessed in 13 subjects with early-stage HD and in 9 control subjects via indirect calorimetry in a human respiratory chamber. Energy intake was determined by weighing all food provided and all leftovers from an ad libitum diet. Body composition was measured via air-displacement plethysmography. Stage of disease was estimated on the basis of the Unified Huntington's Disease Rating Scale and modified Mini-Mental Status examinations. Regression analysis included all 13 HD subjects; t tests were used for the comparisons between matched HD and control subjects.

RESULTS

24-h EE was 11% higher in the HD subjects than in the control subjects (NS). This difference was due to a higher (P = 0.043) waking metabolic rate, which was related to a significantly greater displacement of the center of mass by HD subjects than by control subjects (P = 0.028). On average, both groups were in positive energy balance and exceeded their energy expenditure by 2510-2929 kJ.

CONCLUSIONS

Higher 24-h EE in persons with early midstage HD is due to increased physical activity, both voluntary and involuntary. However, HD subjects are able to maintain positive energy balance when offered adequate amounts of food in a controlled setting.

Spasms in children with definite and probable mitochondrial disease*

The diagnosis of mitochondrial encephalomyopathies is complex and a system for classification of the diagnosis as definite, probable, and possible has been proposed. The objective of this study was to explore the spectrum of epileptic disorders associated with probable and definite mitochondrial disease in children using this classification system. The patient population with mitochondrial disease and epilepsy was selected from a tertiary care children's hospital. Interictal electroencephalograms and video-EEG recordings were used to characterize seizure types. Ten children fulfilled the criteria for probable or definite mitochondrial disease and had epilepsy. Four had siblings with a similar clinical phenotype. Spasms were the most common seizure type and were the initial seizure type in seven patients and two siblings. Four patients had only partial seizures, with or without generalization, and one patient had seizures that were difficult to classify. Blood lactate concentrations were elevated consistently in patients with partial seizures alone but were occasionally normal in children with spasms. Spasms were the most common presenting seizure type in children with probable and definite mitochondrial disease.

Laboratory approach to mitochondrial diseases

Dysfunction in mitochondrial processes has been related to several pathologies. In these disorders, the cell suffers oxidative imbalance that is mostly due to defects in pyruvate metabolism, mitochondrial fatty acids oxidation, the citric acid cycle or electron transport by the mitochondrial respiratory chain. These metabolic alterations produce mitochondrial diseases that have been related to inherited syndromes, such as MERRF or MELAS. The main affected organs are brain, skeletal muscle, kidney, heart and liver, because of the high energetic demand and the oxidative metabolism. Moreover, the relationship between mitochondrial dysfunction and neurodegenerative processes, such as Parkinson disease or Alzheimer disease, as well as ageing, has been shown. Because mitochondrias are the target of several xenobiotics, such as aspirin, AZT or alcohol consumption, mitochondrial impairment has also been proposed as a mechanism of toxicity. Most laboratory tests that are available in the diagnosis of mitochondrial illness are assayed in tissue biopsies and are usually difficult to interpret. Recently, it has been shown that non-invasive techniques, such as nuclear magnetic resonance or the 2-keto[1-(13)C]isocaproic acid breath test, may be useful to assess mitochondrial function. This article attempts to show the laboratory approach to mitochondrial diseases, reviewing new techniques that could be of great value in the research of mitochondrial function, such as the 2-keto[1-(13)C]isocaproic breath test.

MPP(+) downregulates mitochondrially encoded gene transcripts and their activities in dopaminergic neuronal cells: protective role of Bcl-2

The effects of neurotoxins on levels of mitochondrially encoded gene transcripts in a dopaminergic neuronal cell line, MN9D, were examined following treatment with 200 microM N-methyl-4-phenylpyridinium (MPP(+)) or 6-hydroxydopamine (6-OHDA). As confirmed by a Northern blot analysis, levels of cytochrome c oxidase subunit 3 (COX III) and ATPase subunit 6 (ATPase 6) transcript were decreased in a time-dependent manner following treatment with MPP(+) but not with 6-OHDA. Accordingly, enzymatic activity of cytochrome c oxidase (COX) and the intracellular ATP content were also decreased in MPP(+)-treated cells while these remained unaltered in 6-OHDA-treated cells. In the cell death paradigm induced by MPP(+), overexpression of Bcl-2 in MN9D cells (MN9D/Bcl-2) significantly blocked MPP(+)-induced downregulation of COX III and ATPase 6 transcripts. In MN9D/Bcl-2 cells, MPP(+)-induced downregulation of COX activity and the intracellular level of ATP was also blocked. Treatment with a pan-caspase inhibitor, however, neither prevented MPP(+)-induced downregulation of COX activity nor affected intracellular level of ATP in MN9D cells. Taken together, our present data suggest that Bcl-2 may play a regulatory role in energy metabolism by preventing downregulation of mitochondrially encoded gene(s) at a point distinct from its known anticaspase activity in MPP(+)-induced dopaminergic neuronal death.

Two distinct mechanisms are involved in 6-hydroxydopamine- and MPP+-induced dopaminergic neuronal cell death: role of caspases, ROS, and JNK

In this study, we examined the possibility that MPTP and 6-hydroxydopamine (6-OHDA) act on distinct cell death pathways in a murine dopaminergic neuronal cell line, MN9D. First, we found that cells treated with 6-OHDA accompanied ultrastructural changes typical of apoptosis, whereas MPP+ treatment induced necrotic manifestations. Proteolytic cleavage of poly-(ADP-ribose)polymerase by caspase was induced by 6-OHDA, whereas it remained uncleaved up to 32 h after MPP+ treatment and subsequently disappeared. Accordingly, 6-OHDA- but not MPP(+)-induced cell death was significantly attenuated in the presence of a broad-spectrum caspase inhibitor, N-benzyloxy-carbonyl-Val-Ala-Asp-fluomethylketone (Z-VAD-fmk). As measured by fluorometric probes, the level of reactive oxygen species (ROS) significantly increased after 6-OHDA treatment. In contrast, the level of dihydroethidium-sensitive ROS following MPP+ treatment remained unchanged while a slight increase in dichlorofluorescin-sentive ROS was temporarily observed. As demonstrated by immunoblot analysis, the level of superoxide dismutase was down-regulated following 6-OHDA treatment, whereas it remained unchanged after MPP+ treatment. Cotreatment of cells with antioxidants such as N-acetylcysteine or Mn(III)tetrakis(4-benzoic acid)porphyrin chloride (MnTBAP, cell-permeable superoxide dismutase mimetic) rescued 6-OHDA- but not MPP(+)-induced cell death, whereas inclusion of catalase or N(G)-nitro-L-arginine had no effect in both cases. In addition, 6-OHDA induced ROS-mediated c-Jun N-terminal kinase (JNK) activation that was attenuated in the presence of N-acetylcysteine or MnTBAP but not catalase or Z-VAD-fmk. In contrast, MPP+ has little effect on JNK activity, indicating that ROS and/or ROS-induced cell death signaling pathway seems to play an essential role in 6-OHDA-mediated apoptosis but not in MPP(+)-induced necrosis in a mesencephalon-derived, dopaminergic neuronal cell line.

Pathogenesis and preclinical course of Parkinson's disease

Idiopathic parkinsonism (IP) is defined by its classic symptomology, its responsiveness to therapies which elevate dopamine levels, and by the failure to identify a specific etiological factor. The progressive and irreversible degeneration of dopaminergic neurons projecting from the substantia nigra pars compacta (SNc) to the striatum and the presence of SNc Lewy bodies are regarded as the essential pathological bases of IP, but neither the initiator(s) nor the nature of the degeneration have been determined, nor its relationship with degenerative changes in other parts of the IP brain. This paper discusses the various hypotheses that have been proposed to explain these phenomena, arguing that IP be regarded as a multisystem disorder, both at the level of individual neurons and at the whole brain level. It is probable that IP is the result of a multifactorial process, and that a cascade of interacting and overlapping biochemical mechanisms determine the course of the disease.

Decreased physical performance of congenic mice with mismatch between the nuclear and the mitochondrial genome

Maternal transmission of mitochondrial DNA (mtDNA) allows us to generate mtDNA congenic strain by repeating backcrosses of female mice to male mice of an inbred strain, which carries different mtDNA haplotype from that of the female progenitor. Since genetic backgrounds of inbred strains commonly used (e.g., C57BL/6J [B6] and BALB/c) are mainly derived from an European subspecies of Mus musculus domesticus, congenic strains, in which mtDNA originated from an Asian subspecies M. musculus musculus or an European species M. spretus, give in vivo condition that mismatch occurs between the mitochondrial and the nuclear genome. So far, little has been known how the mismatch condition affects the physiological phenotype of the mice. To address this question, we established two mtDNA congenic strains, C57BL/6J(B6)-mtSPR and BALB/c-mtSHH, which carry M. spretus- and M. m. musculus-derived mtDNAs, representing the conditions of interspecific and intersubspecific mitochondrial-nuclear genome mismatch, respectively. Using these congenic strains, we examined their physical performance by measuring their running time on a treadmill belt until exhaustion. The result clearly showed that the mtDNA congenic strains manifested a significant decrease in the level of physical performance, when compared with their progenitor strains. It also appeared that the congenic mice manifested growth rate. Thus, all results indicated that mismatch between the mitochondrial and the nuclear genome causes phenotypic changes in individuals of mice.