|Budget Amount *help
¥1,900,000 (Direct Cost: ¥1,900,000)
Fiscal Year 2003: ¥900,000 (Direct Cost: ¥900,000)
Fiscal Year 2002: ¥1,000,000 (Direct Cost: ¥1,000,000)
To explore the mechanism of the formation of mitochondrial DNA (mtDNA) deletion in vivo, we examined age-related 4834bp mtDNA deletion using different disease models in rats. This deletion in the brain, heart and kidney cortex started to accumulate in accordance with increase of age, however its level was grater in the brain and heart than the kidney cortex, which supports the idea that mtDNA deletion accumulate in postmitotic tissues, and proliferative tissue -could eliminate its accumulation. Based on the findings, we focused on the mtDNA deletion in the kidney cortex. Young and old rat were 5/6-nephrecomized, and maintained for 3 months. Both of the rats with 5/6 nephrectomy revealed glomerular compensatory expansion and sclerosis, and tubular expansion with epithelial cell atrophy in remaining tissue. The activity of cytochrome oxidase (COX) in tubular epithelial cells decreased drastically in nephrectomized rats, while no different was however found between the young and old rats.
MtDNA deletion was not accumulated by nephrectomy in either young or old rats, and the levels were almost equivalent to that of untreated control rats. We next used a rat model of human type 2, insulin-independent, diabetic mellitus, namely Otuska Long Evance Tokushima Fatty (OLETF) rats, which reveals obesity, hyperglycemia, and hyperlipidemia along with development of diabetic kidney diseases. During aging process, kidney disease was advanced, and 4834bp mtDNA deletion was also accumulated in the cortex. Moreover, senescent-associated beta-galactosidase (SA-β-gal), a marker of cell senescence beyond proliferative capacity, was expressed in some tubular epithelial cells. Finally, to specify the relationship between tubular senescence and mtDNA deletion, cadmium (Cd) intoxication was examined in vitro and in vivo. Cd in vitro initially perturbed mitochondrial membrane potential of tubular epithelial cells, and strongly induced apoptosis. When cells survived from Cd toxicity were cultured for more than 100 days, mtDNA deletion could not be induced. This may be due to the selective elimination mechanisms with high proliferation capacity in cultured cells. On the other hand, rats exposed for long time by low dose Cd induced kidney dysfunction and tubular epithelial damages along with decreased mitochondrial number, accumulation of oxidation products and expression of SA-B-gal. Moreover, 4834bp mtDNA deletion in kidney cortex was enormously accumulated over the occurrence of age-related mtDNA deletion. These studies indicated that when toxic stimuli are chronically loaded on tissues during senescence, mtDNA deletion even in proliferative tissue accumulates, results in functional deterioration. This phenomenon may be important in the pathogenesis, of the development of senescence-related diseases. Less