The Role of Mitochondrial Deacetylase SIRT3: Delivering Benefits of Calorie Restriction and Promoting Adult Stem Cell Function

With the increase in the aging population, there has been a growing interest inunderstanding the process of age-related physical decline and increased disease risk.Research in model organisms has shown that aging, far from being a spontaneousdevelopment, is actually a controlled process, with molecular mechanisms that can alterthe pace of cellular and tissue decline. The aim of this dissertation work was to gaininsight into the molecular mechanisms that can control this aging rate.We found that SIRT3, a mitochondrial NAD+-dependent deacetylase, performs a vital roleduring calorie restriction in mice to decrease oxidative damage in tissues. These resultsled us to identify superoxide dismutase 2(SOD2) as a target protein of SIRT3, andconfirmed that SIRT3 can deacetylate SOD2 at two critical lysine residues (K53 and K89).Deacetylation of these residues on SOD2 leads to an increase in SOD2 detoxificationactivity. Furthermore, SOD2 is more deacetylated in the tissues of mice on calorierestriction, but this effect is abrogated in mice that are deficient for SIRT3. These resultsled us to develop a model whereby calorie restriction upregulates SIRT3 expression andactivity, leading to an increased deacetylation of SOD2. The increased activity level ofthe deacetylated SOD2 has the effect of decreasing oxidative damage in tissues.Concomitantly, we found that SIRT3 is required for the switch to fatty acid utilization duringcalorie restriction. SIRT3 KO mice on calorie restriction have reduced beta- oxidation, lowerlong chain-acyl CoA dehydrogenase activity (LCAD), and a preference for glucose uptakeand carbohydrate metabolism. Our findings indicate that other molecular adaptations thatoccur during calorie restriction are insufficient to compensate for a SIRT3 deficiency in thismetabolic context.We also present our findings that SIRT3 is highly expressed in hematopoietic stem cells(HSCs) as compared to differentiated hematopoietic cells, which led us to explore the roleSIRT3 was playing in this stem cell population. Our results indicate that SIRT3 is required as a stress-responsive protein that can protect stem cell function during conditions ofoxidative stress. These conditions can include serial transplant, chemical treatment, orthe increased oxidative stress associated with aging. Furthermore, we show that SIRT3expression is decreased in HSCs from aged mice, and enforced expression of SIRT3 canimprove the function of aged HSCs, suggesting the potential for rejuvenation of agedHSCs.Our studies confirm the role that SIRT3 plays in protecting cells and tissues from oxidativestress, as well as offer. Although to date, no lifespan data has been published on SIRT3KO mice, the results presented here indicate that a relatively shortened lifespan orhealthspan would not be unexpected. These findings also open avenues forunderstanding the role of SIRT3 in stem cell biology, both other stem cell types, andpotentially in human stem cell systems