Protein synthesis in slowed aging : insights into shared characteristics of long-lived mouse models

Abstract

The following dissertation describes a series of experiments with the overall aim to understand the role that changes in protein synthesis have in slowed aging. The specific aims of the three sets of experiments were 1) to determine if chronic administration of the mTORC1 inhibitor rapamycin to mice increases proteostatic mechanisms in skeletal muscle, heart, and liver; 2) to determine if an underdeveloped anterior pituitary, caused by deletion of the Pit-1 gene in mice, increases proteostatic mechanisms in skeletal muscle, heart, and liver of long-lived Snell dwarf mice; 3) and to determine if transient nutrient restriction during the suckling period in mice (i.e. crowded litter), increases proteostatic mechanisms in skeletal muscle, heart, and liver later in life. In Experiment #1 we found that mitochondrial proteins were preferentially synthesized in skeletal muscle and that global protein synthesis in the heart was maintained despite reduced cellular proliferation and mTORC1 activity in mice fed rapamycin compared to normal diet controls. Originally we determined that these data were indicative of an improved somatic maintenance of skeletal muscle mitochondria and the heart proteome. Since we could not account for changes to other energetic processes (e.g. metabolism), we reasoned that our data was more consistent with proteostasis, a component of somatic maintenance. In Experiment #2 we developed a novel method for assessing proteostasis and determined that Snell dwarf mice had an increase in proteostatic mechanisms across sub-cellular fractions within skeletal muscle and heart compared control mice, despite differential rates of protein synthesis in the face of decreased mTORC1. Together with our previous investigations into rapamycin fed and caloric restriction models of long-life we concluded that increased proteostatic mechanisms may be a shared characteristic of models of slowed aging. In Experiment #3 we demonstrate that the crowded litter mouse transitions from growth to maintenance as it ages. Furthermore, in the crowded litter mouse, we demonstrate that proteostasis is not dependent upon decreased mTORC1. Our results indicate that decreased mTORC1 does not necessarily correlate to decreases in protein synthesis across all sub-cellular fractions. Discerning which proteins and the mechanism(s) of how specific proteins can be preferentially synthesized despite decreases in protein synthesis in other fractions and decreased mTORC1, may give further insight into characteristics of slowed aging. Further, we demonstrate that increases in proteostasic mechanisms are a shared characteristic of multiple unique models of slowed aging and therefore, provides a basis for future work aimed at slowing the aging process.