Dissertation
How nutrients affect endoplasmic reticulum homeostasis: an interorganellar redox journey
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Spring 2022
DOI: 10.25820/etd.006470
Abstract
The endoplasmic reticulum (ER) is the entry point into the secretory pathway, and as such is sensitive to any stimulus that affects its protein synthesis burden or ability to process proteins for secretion. Because the ER must respond to stimuli from various inputs, it interacts with virtually every membrane-bound organelle in the cell; perhaps most well characterized are the interactions between the ER and mitochondria. Interactions between the ER and mitochondria are both physical and functional, facilitating exchange of lipids, ATP, calcium, and other metabolites between the ER and mitochondria. The exchange of calcium is essential for mitochondrial activity, whereas the exchange of ATP is essential for ER protein processing. Despite knowing that both mitochondrial metabolic changes and ER stress—the response to any perturbation to ER function—are implicated in metabolic diseases, including liver disease, little is known about how metabolic flux affects ER function. Prior evidence from our lab demonstrated that inhibiting mitochondrial fatty-acid oxidation attenuated ER stress and activation of the unfolded protein response (UPR)—the pathway that attempts to regain ER homeostasis by regulating transcription, translation, and metabolism—in the liver. My initial thesis work expanded upon this finding by showing that inhibiting fatty-acid oxidation attenuated UPR activation in several metabolically active cell types, including hepatocytes, myocytes, and brown adipocytes, along with demonstrating that inhibiting mitochondrial pyruvate entry attenuated UPR activity. These findings indicated that the effects of metabolic inhibition could be generalized to the tricarboxylic acid (TCA) cycle. Next, we determined that production of NADPH associated with the TCA cycle was required for metabolic activity to influence ER function. NADPH is a major redox regulator, but is unable to move between organelles, and therefore its influence on ER function is indirect. We discovered that glutathione, which exists in oxidized (GSSG) and reduced (GSH) forms and helps maintain the redox environment in the ER lumen, affects ER function in response to changes in NADPH. More specifically, NADPH promotes glutathione reduction, and therefore renders the ER lumen hypooxidizing and potentially unable to form disulfides efficiently. We then asked whether NADPH could serve as a mediator between metabolism and the ER in response to nutrient intake. We found that animals who were refed after an overnight fast exhibited increased hepatic NADPH and GSH, and had an active UPR. These findings were reproduced in primary hepatocytes, and we determined that nutrient excess increased mitochondrial and ER GSH, along with cytosolic NADPH. These subcellular changes were reversed by genetically ablating NADPH production, as was nutrient-induced UPR activation, further confirming the role of NADPH as a critical mediator between metabolism and ER function. Overall, my research establishes NADPH and glutathione as a novel mechanism linking nutrition and metabolism to ER function. These findings have the potential to influence further work in the context of overnutrition, obesity, and liver disease.
Details
- Title: Subtitle
- How nutrients affect endoplasmic reticulum homeostasis: an interorganellar redox journey
- Creators
- Erica R Gansemer
- Contributors
- D Thomas Rutkowski (Advisor)Eric B Taylor (Committee Member)Samuel B Stephens (Committee Member)Ling Yang (Committee Member)Charles Yeaman (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Anatomy and Cell Biology
- Date degree season
- Spring 2022
- Publisher
- University of Iowa
- DOI
- 10.25820/etd.006470
- Number of pages
- xv, 161 pages
- Copyright
- Copyright 2022 Erica R Gansemer
- Language
- English
- Description illustrations
- color illustrations
- Description bibliographic
- Includes bibliographical references (pages 144-161).
- Public Abstract (ETD)
When we eat, the cells in our bodies must process the nutrients in our food. These nutrients are broken down through a process broadly known as metabolism to make energy or be packaged for storage as carbohydrates and fat. Every cell in our body requires energy to function. This indicates an important balance between nutrient intake and energy usage. Too few nutrients cause our energy stores to be depleted, whereas overnutrition leads to increased storage. Both situations can contribute to disease.
An organ commonly affected by overnutrition is the liver, which plays a central role in whole-body metabolism. In addition to metabolism, the liver produces a large volume of proteins that are essential for normal physiology. When protein processing is disrupted, our cells activate a stress response to re-process the incorrectly modified proteins, shuttle them for degradation, or shut down the cell when the stress cannot be overcome.
My thesis work focuses on delineating the pathways linking metabolism to protein processing in order to understand how these processes interact under normal and diseased states. We observed that simply eating a meal causes protein misfolding stress in the liver. This was further established in isolated liver cells to rely on regulation of small molecules called redox metabolites, which assist in catalyzing protein folding and are produced during metabolic activity. Two metabolites—NADPH and glutathione—were identified as being required for metabolic activity to affect protein processing, with evidence that glutathione traffics between subcellular compartments in response to nutrient intake. Moreover, NADPH and glutathione levels increase within a few hours after eating, suggesting that they are also important mediators downstream of nutrient intake. These findings contribute to our understanding of how metabolic flux is communicated to the protein processing machinery, and have potential applications in preventing the development and progression of obesity and liver disease.
- Academic Unit
- Anatomy and Cell Biology
- Record Identifier
- 9984271355602771
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