The Regulation of Mitochondrial Stress Responses in Caenorhabditis elegans

Abstract

Organismal aging has been proposed to result, at least in part, from mitochondrial dysfunction and oxidative stress. Mitochondria play an important role in energy metabolism, molecular biosynthesis, apoptosis, and cellular signaling and are therefore complexly tied to cellular and organismal health. Paradoxically, there are numerous cases in Caenorhabditis elegans, in addition to other organisms, where inhibition of mitochondrial respiration is sufficient to extend lifespan. One proposed mechanism for this pro-longevity effect is the induction of the mitochondrial unfolded protein response (UPRmt), which upregulates expression of mitochondrial-specific chaperones and proteases to re-establish protein homeostasis in the mitochondria. This thesis describes the use of C. elegans to understand the genetic regulation of the UPRmt and importantly, the role of this response in stress resistance and aging. I first describe a genome-wide RNAi screen for negative regulators of the UPRmt that takes advantage of a highly sensitive UPRmt fluorescent reporter and RNAi feeding in C. elegans. I identify 95 inducers of the UPRmt (RNAi gene knockdowns that increase reporter expression), which are enriched for mitochondrial genes that affect respiratory chain function. A subset of these positive hits differentially affect lifespan and for those that increase lifespan, do so independently of the UPRmt transcription factor ATFS-1. I also find that constitutive activation of the UPRmt is not sufficient for lifespan extension in C. elegans, and in fact, seems to harm animals. In the second part of this thesis, I follow-up on a specific gene, the cytosolic pentose phosphate pathway enzyme transaldolase, whose connection to mitochondrial proteostasis is not well understood. I find that transaldolase deficiency alters multiple parameters of mitochondrial function including respiration and mitochondrial dynamics, and promotes longevity through activation of redox-sensitive MAPK pathways and the autophagy regulator TFEB/HLH-30. I also discover that ETC RNAi extends lifespan through identical JNK MAPKs, implicating adaptive responses aside from the UPRmt in longevity control from mitochondrial stress. Finally, I describe another genome-wide RNAi screen to identify positive regulators of UPRmt signaling, to elucidate the complex network of regulatory factors that control this response. Further understanding of the mechanistic details of UPRmt regulation will provide us with insights into the evolution of mitochondrial-nuclear communication and the growing list of human diseases associated with mitochondrial dysfunction.