The decay of mitochondrial function is considered one among the major contributors of the age-related degenerative processes. Caloric restriction (CR) diet is widely known as the only treatment able to delay aging, increasing both the mean and maximum lifespan in a wide phylogenic range of animals including rodents and non human primates. Several evidences show that CR diet is able to prevent or reduce the age-dependent accumulation of oxidative damages to mitochondrial DNA (mtDNA) in different rodent tissues, mainly reducing the OH8dG content and the number of mtDNA deleted species. As the mitochondrial function in a cell relies not only on the integrity of mtDNA molecules but also on the copy number of mitochondrial genomes we decided to investigate the effect of CR diet on age-dependent changes of mtDNA content in different rat tissues.
We evaluated by quantitative Real Time PCR the mtDNA/nuclear DNA ratio in liver, brain and soleus muscle from adults fed ad libitum (AL), old fed ad libitum (OL) and old lifelong 40% calories-restricted (OR) Fischer 344 animals. The measurement was carried out using specific primers and TaqMan probes for mitochondrial and nuclear targets represented, respectively, by the D-loop region and β-actin gene.
The data here reported show tissue-specific age-related changes of mitochondrial DNA content. In particular, we found, in liver and soleus muscle, an age-related decrease of mtDNA content (-50% and 40%, respectively) whereas in brain there is no significant change of mtDNA content during aging. CR is able to reverse the age-related loss of mtDNA in liver and soleus, while in brain it results in a significant increase of mtDNA/nDNA ratio above the OL value. This suggests that there might exist some general age-related process leading to the loss of mtDNA both in mitotic (liver) and post-mitotic (skeletal muscle) tissues, probably through prevention or reduction of mtDNA replication. This should not occur in another post-mitotic tissue (brain) where mtDNA can replicate at a normal level during aging, increasing its total amount. Therefore, this further supports the tissue-specific effect of CR, likely due to the different dependence of tissues on external nutrients uptake.
This work has been supported by funds from Ministero dell'Istruzione, dell'Universit e della Ricerca-Programmi di Ricerca Scientifica di Rilevante Interesse Nazionale 2002 (to M.N.G.) and from the University of Bari-Progetti di Ateneo 2003 and 2004 (to A.M.S.L.). P.Cassano was supported by ESF (PON 2000-2006)