Biological role and pharmacology of EXPERA family proteins and their effect onsterol homeostasis

Abstract EXPERA are a family of membrane proteins that include sigma-2 receptor and TM6SF2, both of which play crucial yet understudied roles in cholesterol metabolism and disease. Sigma-2, an enigmatic membrane receptor implicated in cancer, Alzheimer’s disease, and neuropathic pain, has attracted significant pharmacological interest due to its ability to bind a multitude of structurally diverse compounds with high affinity. Despite this therapeutic potential, the gene coding for sigma-2 remained unknown for decades until I cloned it from tissue and identified it as the ER membrane protein TMEM97. Subsequently, I solved the crystal structures of sigma-2 in complex with high-affinity ligands and discovered dozens of novel ligands through virtual docking. However, sigma-2's precise biological function remains unclear. It interacts with key proteins involved in sterol trafficking, such as Niemann-Pick type C1 (NPC1) and the low-density lipoprotein receptor (LDLR), but the regulatory mechanisms driving these interactions are poorly understood. Sigma-2’s dynamic ability to shift between different protein interaction networks in response to ligand binding or sterol stress offers promising therapeutic potential, though the molecular events behind these effects remain to be elucidated. TM6SF2, another member of the EXPERA family, is a regulator of lipid metabolism, particularly in the secretion of very-low-density lipoprotein (VLDL). Mutations in TM6SF2 are strongly associated with Metabolic Dysfunction-Associated Steatotic Liver Disease (MASLD), formerly known as NAFLD, which is a leading cause of chronic liver disease. Despite its potential as a therapeutic target, TM6SF2 remains largely understudied, with no known ligands and limited structural information. Understanding TM6SF2’s role in lipid homeostasis and discovering ligands that modulate its function is critical for developing therapeutic interventions. Building on my previous success in cloning and solving the structure of the sigma-2 receptor, my research aims to address these significant knowledge gaps. We will use structural biology techniques to characterize the complexes that sigma-2 forms with key cholesterol homeostasis proteins, such as NPC1. Additionally, we will investigate sigma-2’s protein interaction network and how it shifts in response to ligands and stress conditions. These studies will improve our understanding of sigma- 2’s role in cholesterol homeostasis and inform the development of targeted therapies. Simultaneously, we will conduct structural and functional studies of TM6SF2 to discover novel ligands and clarify its role in lipid metabolism. By establishing ligand-binding assays and utilizing virtual docking, we aim to uncover new druggable mechanisms for treating MASLD and related metabolic disorders. Overall, this research will advance our molecular understanding of sigma-2 and TM6SF2, two promising therapeutic targets. By elucidating their roles in cholesterol and lipid regulation, we aim to better understand these critical physiological processes, and to leverage this knowledge to develop new strategies for treating cancer, Alzheimer’s disease, MASLD, and cardiovascular diseases, addressing significant unmet clinical needs.