title: How nutrients affect endoplasmic reticulum homeostasis
title: an interorganellar redox journey
titleType: Subtitle
Creators: name: Gansemer, Erica R
Publisher: name: University of Iowa
Year: 2022
Subjects: public abstract
Molecular Biology

description: 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.