The role of hepatic Endoplasmic Reticulum architecture in metabolic homeostasis and disease - ABSTRACT: Metabolic dysfunction Associated Fatty Liver Disease (MAFLD) is the most common chronic liver disease, yet treatment options are limited. Within hepatocytes, the endoplasmic reticulum (ER) serves as a crucial hub for protein and lipid metabolism. Substantial evidence shows that dysfunction of hepatic ER, characterized by loss of its adaptive capacity, is a key mechanism involved in metabolic deterioration in MAFLD. However, the exact mechanisms underlying ER dysfunction in this condition are unknown. Recently, work from our lab has revealed a new angle: obesity leads to a marked loss of hepatic ER architectural organization, which significantly impacts its function. Moreover, recovering ER structure is sufficient to improve ER function and metabolic health in obese mice. Our long-term goal is to understand how alterations in ER architecture affect its function and the functionality of ER-interacting organelles; and how dysregulation of ER architecture leads to metabolic dysfunction in the liver. Our preliminary data show that fasting induces remodeling of hepatic rough ER sheets, which forms a curved membrane around the mitochondria, a structure regulated by the protein Ribosome Receptor Binding Protein (RRBP1). Obesity leads to loss of this response mainly due to downregulation of RRBP1. Moreover, RRBP1 gain of function reduces hepatic steatosis in obese mice. Based on these data, our central hypothesis is that RRBP1- driven rough ER sheet-mitochondria interaction in fasting allows the mitochondria to regulate key adaptive processes such as fatty acid oxidation. Lack of this response in obesity leads to metabolic dysfunction. We will test our hypothesis by: 1) Delineating the requirement of RRBP1-driven rough ER sheet-mitochondria interaction for hepatocyte adaptation to nutritional challenges; 2) Unraveling the mechanisms through which rough ER sheet-mitochondria interactions regulate organelle function and 3) Determining the importance of ER architectural remodeling for the development of diet-induced fatty liver disease. Under the first aim, we will thoroughly characterize the impact of liver specific RRBP1 loss and gain of function on liver metabolism in mice. In the second aim, we will test the hypothesis that rough ER-mitochondria proximity allows mitochondria to adapt to fasting. In the third aim, we will use FIB-SEM imaging and machine learning-based organelle segmentation to determine the impact of diet-induced fatty liver disease on ER structural remodeling in mice and in humans. We will also test if targeting ER structure by overexpression of RRBP1 can improve metabolic stress in MAFLD. This work opens a novel dimension in the field by unraveling how organelles reprogram their metabolic output and optimize their function by reorganizing their subcellular architecture in response to nutritional challenges. It also has the potential to unravel new therapeutic targets to treat fatty liver disease.
