Defining the Impact of Hepatic ER Stress on the Integrity of the Secreted Proteome

About This Grant

ABSTRACT. Hepatic endoplasmic reticulum (ER) stress is implicated in the onset and pathogenesis of etiologically-diverse protein aggregation diseases, most notably the systemic amyloidoses. In these diseases, aggregation-prone proteins secreted from the liver aggregate into toxic oligomers and amyloid fibrils that deposit on peripheral target tissues such as the heart, gut, and peripheral nerves. Despite this link between hepatic ER stress and amyloidogenic protein aggregation, our understanding of how ER stress promotes their toxic extracellular aggregation and distal toxicity is limited. The ER maintains extracellular proteostasis through a quality control system that ensures proteins fold into their native, functional conformations before further trafficking and eventual secretion. Those proteins that cannot be properly folded are targeted for degradation to prevent proteins in non-native, aggregation-prone conformations from entering the extracellular environment. However, recent work from the Wiseman Laboratory has challenged this traditional view of ER quality control and showed that ER stress promotes the secretion of the disease-associated, amyloidogenic protein transthyretin in non-native conformations that aggregate into toxic oligomers. Despite these findings, our understanding of the molecular basis for ER stress-dependent secretion of transthyretin in non-native conformations, and the broader impact of ER stress on the conformation of the global secretory proteome, is currently unknown. I hypothesize that hepatic ER stress leads to a fundamental breakdown in key quality control pathways that broadly increase the secretion of proteins in non-native conformations that contribute to the pathogenesis of etiologically-diverse diseases linked to ER stress in the liver. Here, I will implement both established and novel proteomics approaches such covalent protein painting to define the impact of hepatic ER stress on the conformational integrity and extracellular activity of the secretory proteome. Through these efforts, I will establish a mechanistic basis for how hepatic ER stress leads to a fundamental breakdown in ER quality control pathways that increase secretion of structurally-diverse amyloidogenic proteins in non-native conformations that accelerate their extracellular aggregation into toxic conformations. This will reveal hepatic ER stress as a common contributor to the onset and pathogenesis of multiple different amyloid diseases and reveal the ER in the liver as a therapeutic target to intervene in these disorders. Further, I will expand this study to broadly probe how acute and chronic hepatic ER stress associated with metabolic diseases such as Type 2 diabetes and MAFLD/MASH broadly impacts the conformation of secreted proteins. These efforts will identify specific liver-secreted proteins whose extracellular conformation, and thus activity, are compromised by hepatic ER stress, revealing new mechanisms by which ER stress in the liver contributes to the pathogenesis of other types of diseases, most notably metabolic disorders.