Abstract

Abstract
Aging is a complex biological phenomenon that is caused by a multitude of extrinsic and intrinsic factors. At the level of the organism aging can be defined as the loss of resistance to factors promoting pathologies over time, while at the population level it is more accurately reflected by the increased mortality rate with the age of the population. Understanding the underlying intrinsic and extrinsic factors is key to understanding, and perhaps mitigating age-related functional decline. In our lab we use Saccharomyces cerevisiae as a model organism for studying the cellular and molecular mechanisms that regulate cellular aging. In the course of my project I sought to investigate the role of mitochondrial quality control, as well as lipid metabolism and storage in regulating the aging process in yeast. We found that deletion of Atg32p shortened yeast chronological lifespan (CLS) under caloric restriction growth conditions and abrogated the life-extending capabilities of lithocholic acid (LCA) – a bile acid that extends CLS even under caloric restriction conditions. Deletion of Atg32p also resulted in aberrant morphology, membrane lipid composition, and a reduction in the mitochondrial functional state. To address why deletion of Atg32p negated the beneficial effects of LCA we sought to first locate into which subcellular compartment LCA accumulates. It was found that LCA enters yeast cells and accumulates largely in the inner mitochondrial membrane. I observed that treatment with LCA alters mitochondrial lipid composition, size, and morphology. In the atg32Δ deletion background, mitochondrial lipid composition was aberrant, and these cells were highly susceptible to both H2O2 induced apoptosis and fatty acid induced liponecrotic cell death. From this I sought to understand the mechanisms through which exogenous monounsaturated fatty acids induced cell death. Using a combination of electron microscopy, biochemical assays, and mass spectrometry it was found that exogenous palmitoleic acid (POA) causes a type of cell death which is characterized by morphological and biochemical features which are unique from the three best characterized types of programmed cell death- apoptosis, necrosis, and autophagic cell death – and has henceforth been referred to as liponecrotic cell death. In addition to this, we showed that mitophagy as well as lipid storage pathways and peroxisomal beta-oxidation of free fatty acids are protective processes against POA induced programmed cell death. Together this data allowed for the formulation of a working model integrating mitochondrial quality control processes and those involved in lipid metabolism and storage in mediating the regulation of chronological lifespan in yeast.