Characterization of the family of Super conserved Receptors Expressed in Brain (SREB)

G protein-coupled receptors (GPCRs) are seven transmembrane domains proteins that constitute the largest family of membrane receptors. These receptors respond to a wide variety of extracellular stimuli, proteins, peptides, lipids, ions, sugars, and light, which trigger a conformational change in the receptor and initiate downstream signaling cascades. GPCRs are involved in most physiological and pathophysiological processes, making them crucial targets for therapeutic drugs. However, many of them remain orphan without yet identified endogenous ligands. Super conserved receptors expressed in brain (SREB) constitute a subfamily of three orphan GPCRs characterized by their high evolutionary conservation in vertebrates and their high expression in the brain. They include GPR27 (SREB1), GPR85 (SREB2), and GPR173 (SREB3). In this study we aimed to characterize this subfamily by studying their pharmacological and signaling properties. We attempted to deorphanize these receptors using an aequorin-based calcium mobilization assay, screening for their endogenous ligands in fish tissue extracts. We also tested whether they displayed constitutive activity in cell-based assays by overexpressing them in heterologous systems. Then, given their particularly strong conservation of amino acid sequences in intracellular loops, an unusual feature in GPCR subgroups, we hypothesized that SREBs might interact with unusual intracellular partners. We therefore focused our efforts on the identification of such partners, using a BioID2 proximity-labeling approach, in order to shed light on original interaction networks. Although our deorphanization and constitutive activity approaches were not successful, we identified several intracellular interactors of SREBs, the amino acid transporter subunit SLC3A2, the AKAP protein LRBA, and the cytoskeletal protein 4.1G (EPB41L2) and confirmed these physical interactions. We further characterized the interaction between EPB41L2 and SREB1 by showing the colocalization of the two proteins at the plasma membrane and the enrichment of SREB1 at cell-cell junctions. EPB41L2 knockdown by siRNAs demonstrated that this protein promotes SREB1 recruitment to the plasma membrane. Interestingly, we also observed that SREB1 is more readily solubilized by detergents when EPB41L2 is co-expressed in the cells. Taken together, our work provides a step forward in the understanding of the intracellular environment of SREBs and identified a regulatory role of EPB41L2 on the stability and subcellular localization of SREB1.Doctorat en Sciences biomédicales et pharmaceutiques (Médecine)