Impact of the 25th chromosome on mitochondrial dysfunction in human disease

Does mitochondrial respiratory chain dysfunction have a role in common neurodegenerative disorders?

Respiratory chain dysfunction has been established as having a primary pathoaetiological role in certain relatively rare multisystem disorders (typically encephalomyopathies) and postulated as having an important role in commoner neurodegenerative disorders including Alzheimer dementia and Parkinson's disease. In the latter conditions, there is considerable evidence that non-mitochondrial factors have a key role in causation, making it unlikely that respiratory chain failure is the central event in pathoaetiology. Evidence is presented in this paper suggesting a role for mitochondrial dysfunction in upstream events predisposing to neurodegeneration, and in downstream events accelerating cell dysfunction and loss. While probably not having a primary role in pathoaetiology, respiratory chain dysfunction may still be an important risk factor for development of late life neurodegenerative disease and an important effector mechanism in neuronal dysfunction, and therefore represents a legitimate therapeutic target.

Damage to mitochondrial DNA induced by the quinolone Bay y 3118 in embryonic turkey liver

Quinolones are a class of antibiotics that induce damage to and loss of DNA from bacteria. The structural organization of bacterial DNA is more similar to eukaryotic mitochondrial DNA (mtDNA) than to eukaryotic chromosomal or nuclear DNA (nDNA). Antibiotics affecting the bacterial genome may therefore preferentially damage mtDNA rather than nDNA. We investigated the effect of a quinolone on mtDNA in avian embryonic hepatocytes in ovo. The quinolone Bay y 3118 (1-cyclopropyl-7-(2,8-diazabicyclo[4.3.0]non-8-yl) 6-fluoro-8-chloro-1,4-dihydro-4-oxo-3-quinolinecarboxylic acid hydrochloride, chemical structure see Bremm et al. [K.D. Bremm, U. Petersen, K.G. Metzger, R. Endermann, In vitro evaluation of Bay-y 3118, a new full-spectrum fluoroquinolone, Chemotherapy 38 (1992) 376-387] was injected into fertilized turkey eggs 8 days before hatching at doses of 1, 3, 10 and 30 mg per egg. The embryos were removed from the eggs after 4 days and liver samples were shock frozen. Mitochondrial DNA was purified from samples of the embryonic liver. The integrity of mtDNA was investigated by electrophoresis on agarose gels with native mtDNA and with ribonuclease-treated mtDNA. Fluorescent staining of the electrophoresis gels allows the densitometric quantification of the mtDNA of the regular band at 16 kilobases (kb) and the amount of DNA fragments of irregular size (smear). The genotoxic nitrosamine nitrosodiethylamine (NDEA) has previously been shown to reduce the content of mtDNA of the regular size of 16 kb and to induce the occurrence of smaller fragments of mtDNA [H. Enzmann, C. Kühlem, E. Löser, P. Bannasch, Damage to mitochondrial DNA induced by the hepatocarcinogen, diethylnitrosamine in ovo, Mutation Res. 329 (1995) 113-120]. After exposure to 10 and 30 mg Bay y 3118, a dose-dependent induction of damage to the mtDNA was found, whereas exposure to 3 and 1 mg showed no effect. NDEA (25 mg) was used as positive control. Testing chemical compounds in the in ovo model is a simple and rapid approach for investigations on chemically induced alterations of mtDNA.

Mitochondrial mobility in differentiating muscle heterokaryons

Ragged-red fibers, a morphological hallmark of many patients with mitochondrial encephalomyopathies who harbor mitochondrial DNA (mtDNA) mutations, usually contain varying ratios of mutated and wild-type mtDNAs. Deficient respiratory function in muscle is almost invariably segmental. To investigate whether this observation may be explained by restricted lateral movement of mitochondria within myofibers, we studied the spatial and temporal behavior of two different mitochondrial populations within multinucleate myotubes. We co-cultured normal human and mouse myoblasts, allowed them to fuse into muscle heterokaryons and investigated whether the mitochondria remained segregated, or migrated and intermixed. Human and mouse nuclei were identified by their differential staining pattern with the dye Hoechst 33 258 and mitochondria were distinguished immunologically and by in situ hybridization. Although we observed some territoriality at very early time points after myoblast fusion, there was rapid intermixing of the mitochondrial populations, as early as 48 h after myoblast fusion. We conclude that mitochondria, unlike many other muscle components, lack territorial organization in cultured, differentiating heterokaryons.

The therapy of respiratory chain encephalomyopathy: a critical review of the past and current perspective

The mitochondrial respiratory chain encephalomyopathies represent an important group of multisystem disorders. No curative treatment is currently available. A number of measures have been reported to have a theoretical potential to improve respiratory function. These treatment strategies have variable scientific support, many reports being anecdotal. We critically review the various therapeutic measures employed and suggest future treatment directions.