MITOCHONDRIAL-DNA SYNTHESIS STUDIED AUTORADIOGRAPHICALLY IN VARIOUS CELL-TYPES IN-VIVO

It is generally accepted that mitochondria are able to proliferate eve n in postmitotic cells due to their natural turnover and also to satis fy increased cell energy requirements. However, no detailed studies ar e available, particularly with respect to specific cell types. Since [ H-3]-thymidine is incorporated not only into nuclear (n) DNA but also into the DNA of cytoplasmic mitochondria, an autoradiographic approach was developed at the light microscopy level in order to study basic q uestions of mitochondrial (mt) proliferation in organs of rodents in s itu via the cytoplasmic incorporation of [H-3]-thymidine injected into the animals 1 h before sacrifice. Experiments carried out on mice aft er X-irradiation showed that cytoplasmic labeling was not due to a pro cess such as unscheduled nuclear DNA synthesis (nUDS). Furthermore, ha lf-lives of mitochondria between 8-23 days were deduced specifically i n relation to cell types. The phase of mtDNA synthesis was about 75 mi n. Finally, mt proliferation was measured in brain cells of mice as a function of age. While all neurons showed a decreasing extent of mtDNA synthesis during old age, nUDS decreased only in distinct cell types of the cortex and hippocampus. We conclude that the leading theories e xplaining the phenomenon of aging are closely related, i.e., aging is due to a decreasing capacity of nDNA repair, which leads to unrepaired nDNA damage, or to an accumulation of mitochondria with damaged mtDNA , which leads to a deficit of cellular energy production.