Influence of age on mitochondrial enzyme levels in Drosophila

Mitochondria and ageing in Drosophila

Studies in different organisms have revealed that ageing is a complex process involving a tight regulation of gene expression. Among other features, ageing organisms generally display an increased oxidative stress and a decreased mitochondrial function. The increase in oxidative stress can be attributable to reactive oxygen species, which are mainly produced by mitochondria as a by-product of energy metabolism. Consistent with these data, mitochondria have been suggested to play a significant role in lifespan determination. The fruitfly Drosophila melanogaster is a well-suited organism to study ageing as it is relatively short-lived, mainly composed of post-mitotic cells, has sequenced nuclear and mitochondrial genomes, and multiple genetic tools are available. It has been used in genome-wide studies to unveil the molecular signature of ageing, in different feeding and dietary restriction protocols and in overexpression and down-regulation studies to examine the effect of specific compounds or genes/proteins on lifespan. Here we review the various features linking mitochondria and ageing in Drosophila melanogaster.

Changes in the respiratory chain complexes activities and in the mitochondrial DNA content during ageing in D. subobscura

The time course (age 0-8 weeks) of the enzyme activities of respiratory chain complexes I, III and IV and of citrate synthase, and the cell mitochondrial/nuclear DNA content ratio were studied in Drosophila subobscura. The activities of the three respiratory complexes decreased with age, but with different kinetics. The activities of complexes I and III remained nearly stable between weeks 0 and 3 (falling by 6% and 15%, respectively), and then gradually decreased; after 8 weeks residual activities were about 50% of the initial value for complexes I and III. The activity of complex IV fell in the first week, decreasing continually to week 8, where residual activity was 30% of the initial value. No significant age-related change in citrate synthase activity was observed. Mitochondrial DNA (measured by mitDNA/nucDNA) increased linearly up to week 5 (2.6-fold) and then dropped by 40% in week 6 though it remained higher than initial values.

Cellular and subcellular localization of hexokinase, glutamate dehydrogenase, and alanine aminotransferase in the honeybee drone retina

Subcellular localization of hexokinase in the honeybee drone retina was examined following fractionation of cell homogenate using differential centrifugation. Nearly all hexokinase activity was found in the cytosolic fraction, following a similar distribution as the cytosolic enzymatic marker, phosphoglycerate kinase. The distribution of enzymatic markers of mitochondria (succinate dehydrogenase, rotenone-insensitive cytochrome c reductase, and adenylate kinase) indicated that the outer mitochondrial membrane was partly damaged, but their distributions were different from that of hexokinase. The activity of hexokinase in purified suspensions of cells was fivefold higher in glial cells than in photoreceptors. This result is consistent with the hypothesis based on quantitative 2-deoxy[3H]glucose autoradiography that only glial cells phosphorylate significant amounts of glucose to glucose-6-phosphate. The activities of alanine aminotransferase and to a lesser extent of glutamate dehydrogenase were higher in the cytosolic than in the mitochondrial fraction. This important cytosolic activity of glutamate dehydrogenase was consistent with the higher activity found in mitochondria-poor glial cells. In conclusion, this distribution of enzymes is consistent with the model of metabolic interactions between glial and photoreceptor cells in the intact bee retina.