Abstract

This thesis describes studies that have resulted in uncovering molecular and cellular mechanisms underlying the essential role of mitochondria in chronological aging of the baker’s yeast, Saccharomyces cerevisiae. The Titorenko laboratory has recently identified lithocholic acid (LCA), a bile acid and a natural compound that operates synergistically with caloric restriction (CR) to cause a substantial increase in yeast chronological lifespan under longevity-extending CR conditions. As a first step towards establishing a mechanism by which LCA extends longevity of chronologically aging yeast limited in calorie supply, I investigated the distribution of LCA within yeast cells. I found that exogenously added LCA enters yeast cells, is sorted only to mitochondria, resides mainly in the inner mitochondrial membrane and also associates with the outer membrane of mitochondria. I demonstrated that LCA-elicited changes in the membrane lipidome of mitochondria trigger age-related alterations in mitochondrial respiration, membrane potential, reactive oxygen species (ROS) quantity, oxidative macromolecular damage, respiratory complexes composition and ATP concentration. My findings provided evidence that the longevity-extending potential of LCA is due in part to its ability to alter the age-related dynamics of mitochondrially produced ROS in both chronologically ʺyoungʺ and ʺoldʺ cells, thus reducing the damaging effect of these ROS in ʺyoungʺ cells and amplifying their ʺhormeticʺ effect in ʺoldʺ cells. I demonstrated that LCA alters the age-related dynamics of changes in the levels of many mitochondrial proteins, as well as numerous proteins in cellular locations outside of mitochondria. My findings imply that LCA-driven changes in the mitochondrial lipidome alter the mitochondrial proteome and functionality, thereby enabling mitochondria to operate as signaling organelles that orchestrate the establishment of an anti-aging transcriptional program for many longevity-defining nuclear genes. Based on these findings, I proposed a model for how such LCA-driven changes early and late in the life of chronologically aging yeast cause a stepwise development of an anti-aging cellular pattern and its maintenance throughout lifespan. Moreover, I provided evidence that mitophagy, a selective autophagic degradation of aged and dysfunctional mitochondria, is a longevity assurance process that in chronologically aging yeast underlies the synergistic beneficial effects of CR and LCA on lifespan.