The endoplasmic reticulum (ER) is a critical organelle for protein synthesis, protein trafficking, and lipid synthesis. As such, cells have evolved a quality control system known as the ER Unfolded Protein Response (UPRER). Capable of monitoring protein folding and membrane lipid composition to preserve ER homeostasis, the UPRER is crucial for responding to cellular stress brought on by increased metabolic demand, environmental factors, and aging. In this study, we have identified let-767 as an essential gene for ER homeostasis. Through an RNAi screen of lipid droplet associated genes, we found that knockdown of let-767 resulted in reduced lipid droplets, aberrant ER morphology, and compromised UPRER induction, which impacted growth and lifespan. We found that these deficiencies in ER quality were independent of let-767’s previously characterized function in mono-methyl branched chain fatty acid synthesis (mmBCFAs), as supplementation of mmBCFAs did not ameliorate the detrimental phenotypes. However, supplementation of whole animal lysate was able to rescue the lipid droplet depletion, ER morphology, and animal growth, but not the UPRER function. The UPRER induction was instead rescued by reducing the let-767 pathway through knockdown of the upstream transcription factor sbp-1, suggesting accumulation of a potential toxic metabolite within the let-767 pathway. Our results indicate that let-767 may play a more significant role in lipid metabolism than has been previously described and highlight the importance of lipid homeostasis to protein quality control.

The connection between aging and metabolism seems obvious, but the particulars of this relationship remain obscure. Theories linking the two abound, centered on the fact that mitochondria are the location of much of the cell's free radical production and on the general correlation between lifespan and metabolic rate. The connection between longevity and mitochondrial function was strengthened by RNAi-based screens in the worm C. elegans, where RNAi knock-down of mitochondrial Electron Transport Chain (ETC) subunits of Complex I (nuo-2), Complex III (cyc-1), Complex IV (cco-1) and Complex V (atp-3) extended lifespan (Dillin et al., 2002b; Lee et al., 2002). The effects of ETC knockdown on life span appear to depend on a mechanism more complex than direct effects on mitochondrial metabolic rates. Instead, they point towards the existence of signaling programs emanating from the mitochondria and capable of regulating the life span of the entire organism. The identity of components and timing requirements of the mitochondrial ETC that influence longevity have led us to search for a mechanism imposed during early development that sets the rate of aging for the remainder of the animal's life cycle as well as the critical tissues required for the mitochondrial ETC to set the rate of aging. I have found that disruption of cco-1, in the neurons or intestine of the worm is sufficient to confer a longevity phenotype. This tissue-specific knockdown was able to uncouple some of the detrimental phenotypes of organism-wide knockdown, such as reduced brood size and slow movement. Furthermore, the long lifespan that results from ETC perturbance is dependent on the mitochondrial Unfolded Protein Response (UPRmt) gene, ubl-5. Knowing that there are tissues in which ETC function is critical as well as the necessity for a functional UPRmt might allow us to better understand the basis of mitochondria-mediated longevity.

The central nervous system coordinates peripheral cellular-stress responses, including the unfolded protein response of the mitochondria (UPRMT); however, the contexts for which this regulatory capability evolved is unknown. The UPRMT is upregulated upon pathogenic infection and in metabolic flux, and the olfactory system has been shown to regulate pathogen resistance and peripheral metabolic activity. Therefore, we asked whether the olfactory nervous system in C. elegans controls the UPRMT cell nonautonomously. In Chapter 2, we found that silencing a single inhibitory olfactory neuron pair, AWC, led to robust induction of the UPRMT and reduction of oxidative phosphorylation dependent on serotonin signaling and parkin-mediated mitophagy. Further, AWC ablation confers resistance to the pathogenic bacteria Pseudomonas aeruginosa partially dependent on the UPRMT transcription factor atfs-1, and fully dependent on mitophagy machinery. In Chapter 3, we found that in addition to UPRMT induction, olfaction regulates a distinct pathway that results in a peripheral skn-1-dependent oxidative stress response. Finally, in Chapter 4, we explore a skn-1-dependent starvation resistance phenotype regulated by olfaction. These data illustrate a role for the olfactory nervous system in regulating whole-organism mitochondrial dynamics, cellular-stress responses, and energy homeostasis, perhaps in preparation for postprandial metabolic stress, starvation, or pathogenic infection.