Unscheduled DNA synthesis and mitochondrial DNA synthetic rate following injury of the facial nerve.

Unscheduled DNA synthesis (UDS) of nuclear DNA and mitochondrial (mt) DNA synthetic rates were determined autoradiographically in different cell types of the rodent brain 14 days after unilateral facial nerve transection. In addition to an increased synthetic rate of mtDNA in facial motoneurons 12 h after axotomy, a significant increase of UDS, i.e., DNA repair, and mtDNA synthesis were found in the regenerating facial nucleus 4 days after axotomy. Specificity of the observed labeling was confirmed by injection of 3H2O instead of [3H]thymidine. Using electron microscopic autoradiography, it was further shown that cytoplasmic labeling of neurons was mainly due to incorporation of radioactive label into mitochondria, indicating their subsequent multiplication by division. The observation that Northern blot signals for O6-alkylguanine-DNA-alkyltransferase mRNA from homogenized facial nuclei of both the axotomized and normal side remained unchanged over 14 days after axotomy indicated that the observed DNA-repair activity was not caused by endogenously produced alkylating agents. The combined presence of transiently increased UDS, enhanced mtDNA synthesis and elevated protein synthetic rates of regenerating motoneurons (as shown in the literature) suggests that free radicals produced by mitochondria in injured nerve cells could cause unspecific DNA damage followed by immediate repair.