The mitochondrial genome and human mitochondrial diseases

To date, more than 100 point mutations and several hundreds of structural rearrangements of mitochondrial DNA (mtDNA) are known too be connected with characteristic neuromuscular and other mitochondrial syndromes varying form those causing death at the neonatal stage to diseases with late ages of onset. The immediate cause of mitochondrial disorders is a defective oxidative phosphorylation. Wide phenotypic variation and the heteroplasmy phenomenon, which some authors include in mutation load, are characteristic of human mitochondrial diseases. As the numbers of cases identified and pedigrees described increase, data on the genotype--phenotype interaction and the structure and frequency of pathogenic and conditionally pathogenic mtDNA mutations in human populations are rapidly accumulated. The data on the genetics and epidemiology of mitochondrial diseases are not only important for differential diagnosis and genetic counseling. Since both neutral and mildly pathogenic mutations of mtDNA are progressively accumulated in maternal phyletic lines, molecular analysis of these mutations permits not only reconstruction of the genealogical tree of modern humans, but also estimation of the role that these mutations play in natural selection.     

Preventing mitochondrial fission impairs mitochondrial function and leads to loss of mitochondrial DNA

Mitochondria form a highly dynamic tubular network, the morphology of which is regulated by frequent fission and fusion events. However, the role of mitochondrial fission in homeostasis of the organelle is still unknown. Here we report that preventing mitochondrial fission, by down-regulating expression of Drp1 in mammalian cells leads to a loss of mitochondrial DNA and a decrease of mitochondrial respiration coupled to an increase in the levels of cellular reactive oxygen species (ROS). At the cellular level, mitochondrial dysfunction resulting from the lack of fission leads to a drop in the levels of cellular ATP, an inhibition of cell proliferation and an increase in autophagy. In conclusion, we propose that mitochondrial fission is required for preservation of mitochondrial function and thereby for maintenance of cellular homeostasis.     

Control of mitochondrial volume by mitochondrial metabolic water

It is well-known that mitochondrial volume largely controls mitochondrial functioning. We investigate whether metabolic water produced by oxidative phosphorylation could be involved in mitochondrial volume regulation. We modulated the generation of this water in liver mitochondria and assess their volume by two independent techniques.In liver mitochondria, the mitochondrial volume was specifically decreased when no water was produced independently of energetic parameters and uncoupling activity. In all other conditions associated with water generation, there was no significant change in mitochondrial metabolic volume.Altogether these data demonstrate that mitochondrial volume is regulated, independently of energetic status, by the mitochondrial metabolic water that acts as a signal.     

Relation of mitochondrial phospholipase A activity to mitochondrial swelling

Assays of mitochondrial phospholipase A activity and mitochondrial swelling demonstrated that the phospholipase A activity is related to the swelling under the experimental conditions used. Both were stimulated by added free fatty acid and CaCl(2), not affected greatly by the addition of monoacyl phosphoglycerides, and inhibited by EDTA. The amount of fatty acid hydrolyzed from endogenous phosphatidyl ethanolamine and phosphatidyl choline during swelling was calculated to be 20-30 times less than the amount of added free fatty acid that gave comparable swelling. Under the experimental conditions about 4% of the phospholipid was hydrolyzed. Mitochondrial swelling was studied by electron microscopy and turbidity measurements. The results found were in agreement, whether oleic acid was present or not, except for those values obtained after very brief incubation (1 min) and after incubation for longer than 35 min. The lack of direct proportion between swelling and the concentration of lysosomes present indicated that the swelling is related mainly to mitochondrial phospholipase A, although swelling due to contaminating lysosomes cannot be excluded entirely. The temperature dependence of spontaneous, fatty acid-induced, or CaCl(2)-induced swelling suggested that enzymatic activities are responsible for swelling.     

Role of mitochondrial permeability transition pores in mitochondrial autophagy

During autophagy, cells rid themselves of damaged and superfluous mitochondria, as well as other organelles. This activation of mitochondrial turnover could be the result of changes in the physiological state of mitochondria. Confocal microscopy and fluorescence techniques indicate that onset of mitochondrial permeability transition is one such change. The mitochondrial permeability transition is a reversible phenomenon whereby the mitochondrial inner membrane becomes freely permeable to solutes of less than 1500 Da. At onset of the mitochondrial permeability transition, mitochondria depolarize, uncouple, and undergo large amplitude swelling due to opening of permeability transition pores, which may form by aggregation of damaged, misfolded membrane proteins. When injurious cellular stresses occur, cells may protect themselves using autophagy to remove damaged mitochondria and mutated mitochondrial DNA. Ca²⁺ overloading, reactive oxygen and nitrogen species, decreased mitochondrial membrane potential, and oxidation of pyridine nucleotides and glutathione all promote mitochondrial damage and onset of the mitochondrial permeability transition. The mitochondrial permeability transition is also associated with necrosis and apoptosis after a variety of stimuli. This review emphasizes the role of the mitochondrial permeability transition as a key event in mitochondrial autophagy.     

The relevance of mitochondrial membrane topology to mitochondrial function

This review summarizes recent findings from electron tomography about the three-dimensional shape of mitochondrial membranes and its possible influence on a range of mitochondrial functions. The inner membrane invaginations called cristae are pleomorphic, typically connected by narrow tubular junctions of variable length to the inner boundary membrane. This design may restrict intra-mitochondrial diffusion of metabolites such as ADP, and of soluble proteins such as cytochrome c. Tomographic images of a variety of mitochondria suggest that inner membrane topology reflects a balance between membrane fusion and fission. Proteins that can affect cristae morphology include tBid, which triggers cytochrome c release in apoptosis, and the dynamin-like protein Mgm1, involved in inter-mitochondrial membrane fusion. In frozen-hydrated rat-liver mitochondria, the space between the inner and outer membranes contains 10-15 nm particles that may represent macromolecular complexes involved in activities that span the two membranes.     

A mitochondrial plasmid that promotes mitochondrial fusion inPhysarum polycephalum

Summary We have identified a novel mitochondrial plasmid of about 16 kbp inPhysarum polycephalum. This plasmid was apparently responsible for promoting mitochondrial fusion. Only in strains carrying the plasmid, small spherical mitochondria fused with one another to form large knotted multinucleate mitochondria which subsequently nderwent fusion between the areas (mt-nuclear) that contained the mitochondrial DNA (mtDNA) derived from individual mitochondria. Several successive mitochondrial divisions followed, accompanied by mt-nuclear divisions. The resulting mitochondria contained recombinant mtDNAs, but the plasmid was transmitted to all mitochondria without any structural change.     

线粒体疾病与核基因－线粒体基因的表达调控

线粒体与疾病是当前生物医学领域最前沿之一。本文简单介绍线粒体生物医学的基础知识、线粒体疾病的遗传模式,综述了近年来在线粒体DNA（mtDNA）突变和疾病、核基因突变和疾病等领域的研究进展,着重阐明核基因（特别是核修饰基因）调控mtDNA突变致病表达的分子机制。     

The mitochondrial brain: From mitochondrial genome to neurodegeneration

Mitochondrial DNA mutations are an important cause of neurological disease. The clinical presentation is very varied in terms of age of onset and different neurological signs and symptoms. The clinical course varies considerably but in many patients there is a progressive decline, and in some evidence of marked neurodegeneration. Our understanding of the mechanisms involved is limited due in part to limited availability of animal models of disease. However, studies on human post-mortem brains, combined with clinical and radiological studies, are giving important insights into specific neuronal involvement.     

TZDs reduce mitochondrial ROS production and enhance mitochondrial biogenesis

Although it has been reported that thiazolidinediones (TZDs) may reduce cardiovascular events in type 2 diabetic patients, its precise mechanism is unclear. We previously demonstrated that hyperglycemia-induced production of reactive oxygen species from mitochondria (mtROS) contributed to the development of diabetic complications, and metformin normalized mt ROS production by induction of MnSOD and promotion of mitochondrial biogenesis by activating the PGC-1α pathway. In this study, we examined whether TZDs could inhibit hyperglycemia-induced mtROS production by activating the PGC-1α pathway. We revealed that pioglitazone and ciglitazone attenuated hyperglycemia-induced ROS production in human umbilical vein endothelial cells (HUVECs). Both TZDs increased the expression of NRF-1, TFAM and MnSOD mRNA. Moreover, pioglitazone increased mtDNA and mitochondrial density. These results suggest that TZDs normalize hyperglycemia-induced mtROS production by induction of MnSOD and promotion of mitochondrial biogenesis by activating PGC-1α. This phenomenon could contribute to the prevention of diabetic vascular complications.     

A mutation in a mitochondrial ABC transporter results in mitochondrial dysfunction through oxidative damage of mitochondrial DNA

We have isolated a Saccharomyces cerevisiae mutant that shows an increased tendency to form cytoplasmic petites (respiration-deficient ρ⁻ or ρ⁰ mutants) in response to treatment of cells growing on a solid medium with the DNA-damaging agent methyl methanesulfonate or ultraviolet light. The mutation in this strain, atm1-1, was found to cause a single amino acid substitution in ATM1, a nuclear gene that encodes the mitochondrial ATP-binding cassette (ABC) transporter. When the mutant cells were grown in liquid glucose medium, they accumulated free iron within the mitochondria and at the same time gave rise to spontaneous cytoplasmic petite mutants, as seen previously in cells carrying a mutation in a gene homologous to the human gene responsible for Friedreich's ataxia. Analysis of the effects of free iron and malonic acid (an inhibitor of oxidative respiration in mitochondria) on the incidence of petites among the mutant cells indicated that spontaneous induction of petites was a consequence of oxidative stress rather than a direct effect of either a defect in the ATM1 gene or the accumulation of free iron. We observed an increase in the incidence of strand breaks in the mitochondrial DNA of the atm1-1 mutant cells. Furthermore, we found that rates of induction of petites and accumulation of strand breaks in mitochondrial DNA were enhanced in the atm1-1 mutant by the introduction of another mutation, mhr1-1, which results in a deficiency in mitochondrial DNA repair. These observations indicate that spontaneous induction of petites in the atm1-1 mutant is a consequence of oxidative damage to mitochondrial DNA mediated by enhanced accumulation of mitochondrial iron. Received: 26 March 1999 / Accepted: 29 June 1999