Mitochondrial function and alzheimer's disease.

The brain is highly dependent on aerobic metabolism. Normal mitochondrial function is therefore likely to play a critical role in neuronal function and integrity. Defects in the mitochondrial oxidative phosphorylation pathway (OXPHOS) have been demonstrated in aging human tissue including brain. It is not clear whether underlying mitochondrial DNA mutations are responsible for the observed functional defects. The previously reported OXPHOS defects, in particular reduced cytochrome c oxidase activity, in Alzheimer's disease (AD) are not likely to be due to specific enzyme dysfunction. The falloff in cytochrome c oxidase activity in AD brains is more likely to be related to a global decline in mitochondrial activity manifested by downregulation in mitochondrial number. It is not definitely established where the observed mitochondrial changes are placed in the AD cascade. A number of factors might contribute to the observed changes in OXPHOS function including mitochondrial transport through axonal and dendritic processes, compromised regulatory feedback mechanisms responsible for individual complex-subunit synthesis, and complex assembly.

Mitochondrial respiratory chain activity in the human brain as a function of age.

Age-associated changes in mitochondrial respiratory chain activity were investigated in human brain tissue collected at autopsy. Four brain regions, the frontal cortex, superior temporal cortex, cerebellum and putamen, were studied to map any regional variation. A significant decrease in cytochrome c oxidase activity was seen in all regions studied with increasing age (P<0.05). Although a small decrease in succinate dehydrogenase-cytochrome c oxidoreductase and NADH: ubiquinone oxidoreductase activities was observed, this was not statistically significant. This study has shown that the age-related fall in cytochrome c oxidase activity affects the frontal cortex, superior temporal cortex, cerebellum and putamen. The variation in the extent of age-related oxidative phosphorylation decline was striking. We hypothesize that individuals with more severe age-related decline may be predisposed to neuronal dysfunction, whereas individuals with well preserved oxidative phosphorylation may enjoy some degree of neuronal protection.

Irregular distribution of cytochrome c oxidase protein subunits in aging and Alzheimer's disease.

This study aims to investigate the cellular distribution of human cytochrome c oxidase (COX) subunit II (CII) and COX subunit IV (CIV) in Alzheimer's disease relative to control brains. The levels of CIV and CII proteins in the cerebellar Purkinje cells were reduced in age-matched controls relative to young controls and in the Alzheimer's disease group relative to both age-matched and young controls. Results suggest that these age-associated changes are more marked in Alzheimer's disease.

Mitochondrial DNA in stroke and migraine with aura.

Patients presenting with thrombotic stroke of unexplained etiology and or migraine with aura were screened for mitochondrial (mt) DNA mutations associated with cytopathies given that both migraine and stroke-like episodes are recognised with certain mt DNA mutations. Mutations usually associated with either mitochondrial encephalopathy, lactic acidosis and stroke-like episode, myoclonic epilepsy with ragged red fibres, or those strongly linked to Leber's hereditary optic neuropathy (LHON) were not detected in patients or controls. However, increased levels of two of the secondary LHON mutations were found. The T-->C mutation at nucleotide 4216 was more common than expected in patients aged 35 years or less, as was the 13708 G-->A mutation in young stroke patients. This data lends support to the possibility that an accumulation of minor mt DNA mutations may contribute to the pathoaetiology of stroke and migraine with aura in some young patients.

The polymerase chain reaction in the study of mitochondrial genetics.

Since its development in the late 1980's, the polymerase chain reaction (PCR) has revolutionised molecular genetic studies. It has provided direct access to genetic material in quantities sufficient for meaningful analyses to be performed. Adaptations to the basic technique have resulted in a wide range of applications from basic gene amplification to the estimation of DNA species quantities within cells. The study of human mitochondrial genetics is but one of the many disciplines to benefit from the rapid ascension of PCR based technology. In this communication we outline several uses of the PCR technique in the detection, quantification and characterisation of human mitochondrial genetic defects. The data presented in this communication highlight the versatility and applicability of PCR not only to mitochondrial research but to other disciplines of medical research.

Mitochondrial DNA polymorphism in substantia nigra.

Inefficiencies in mitochondrial respiration mainly affecting complex I and IV activities, occur with increasing age and have been suggested as a possible etiological factor in age-related neurodegenerative diseases. It has been suggested that this finding may be explained by an accumulation of mtDNA mutations. We hypothesise that some polymorphic mitochondrial genomes encode less efficient respiratory protein subunits and are therefore less tolerant of acquired mutations. If this hypothesis is correct, individuals with 'less efficient' mtDNA genotypes may be predisposed both to more rapid biological aging and to neurodegenerative disease. In this study we investigate the substantia nigra mtDNA composition from 4 elderly individuals (2 non-parkinsonian and 2 with idiopathic Parkinson's disease) to determine whether there is sufficient polymorphism to account for different possible respiratory efficiencies. THe mitochondrial tRNAArg, tRNAHis, tRNAScr, tRNALeu(CUN), ND4L, ND4 and ND5 genes as well as parts of the ND3 and ND6 subunit coding regions were analysed (4221 bp), revealing the presence of multiple deletions and 48 discrete polymorphic sites. These included 23 missense, two tRNA and one nonsense polymorphism. Eight of the missense polymorphisms caused nonconservative amino acid replacements at sites of moderate to high evolutionary constraint. These findings suggest that mtDNA diversity in the ageing brain may account for a range of bioenergetic outcomes. The variation in mtDNA genotype involves both inherited (fixed familial) polymorphism and superimposed acquired mutations.