Mechanisms of Protection in the Brain by Physical Exercise in Polg Mutator Mice

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DESCRIPTION (provided by applicant): Physical exercise is inversely related to the risk of Parkinson's disease (PD), and in rodents can protect against mitochondrial dysfunction and neuronal loss induced by MPTP. Recently, exercise also has been shown to have a dramatic protective effect in Polg "mutator" mice expressing a proofreading deficient form of the mitochondrial DNA (mtDNA) polymerase ¿ (Polg). In these mice, there is an accelerated accumulation of somatic mtDNA mutations, leading to a premature aging phenotype. Exercise in these mice normalizes muscle mitochondrial function and significantly extends lifespan. Perhaps more surprisingly, physical exercise also improves brain mitochondrial function and completely prevents brain atrophy. The mechanisms of these protective effects in the brain are unknown. In skeletal muscle, exercise increases levels of mRNA of PGC-1¿, a transcriptional coactivator that upregulates mitochondrial biogenesis and antioxidant defenses. Exercise also may increase PGC-1¿ activity through posttranslational mechanisms. In muscle, exercise reduces levels of RIP140, a suppressor of PGC-1¿ activity, and induces SIRT1 dependent deacetylation of PGC-1¿, thereby promoting its activation. We hypothesize that the protective effects of exercise on brain mitochondrial function exercise may result from similar mechanisms that account for this effect in muscle. If correct, then these mechanisms may account for the association of exercise with a reduced risk of PD. This potential link between exercise and increased PGC-1¿ activity is particularly exciting in light of recent data implicating reduced brai PGC-1¿ activity in the pathogenesis of PD. Thus, increasing PGC-1¿ in brain is a promising potential neuroprotective strategy. The Polg mutator mice represent a valuable model for studying the protective effects of exercise on the brain. In addition to brain atrophy and impaired mitochondrial function, we have preliminary data indicating that the Polg mutator mice have significant behavioral (motor) deficits as well as loss of striatal tyrosine hydroxylase (TH) immunostaining intensity and reduced dopamine (DA) and dopamine metabolites, indicating that mitochondrial dysfunction caused by somatic mtDNA mutation accumulation can cause nigral-striatal pathology. These data raise the possibility that the high levels of somatic mtDNA mutations that we and others have identified in SN neurons in PD may contribute to nigral-striatal dysfunction in PD. Thus, if our hypothesis proves to be correct, then the proposed studies on the impact of exercise on somatic mtDNA mutations and PGC-1¿ activity in the brain may be of relevance to PD. The main goal of this project is to investigate potential mechanisms of the protective effect of physical exercise in the brain of Polg mutator mice, including the impact on somatic mtDNA mutation levels and on regulation of PGC-1¿ levels and activity in the brain. This targeted approach will be complemented by an unbiased metabolomics approach that may reveal a role for novel pathways linking exercise to protective effects in the brain. PUBLIC HEALTH RELEVANCE: Physical exercise improves mitochondrial function and has robust protective effects in the brain, but the mechanisms of these effects are unknown. Understanding these mechanisms provide insights into novel neuroprotective strategies of relevance to aging and age-related neurodegenerative diseases such as Parkinson's disease. We now propose to investigate the mechanisms of protection in the brain by exercise through studies of a mouse model of premature aging.