Nuclear receptors (NRs) are ligand-activated transcription factors that play vital roles in human physiology. 13% of all FDA-approved drugs target NRs, however they are associated with side-effects that limit their utility and safety. These side-effects are attributed to general activation of all NR transcriptional actions. Understanding the molecular mechanisms of NR transcriptional functions is essential to improve rational NR drug design. This project will focus on FXR, the NR activated by bile salts and key regulator of bile salt, lipid and glucose metabolism via transactivation. FXR also regulates inflammation status via transrepression of NF-κB. Currently, FXR is being investigated as target in the treatment of cholestasis, metabolic syndrome and inflammatory diseases. However, global activation of FXR will likely cause deleterious side-effects. Therefore, the proposed research aims to functionally dissect the different molecular mechanisms of FXR action during ligand-dependent transactivation and transrepression. Post-translational modifications and differentially recruited cofactors are crucial in differentiating between transactivation and transrepression.Key objectives:1. To investigate the upstream protein modifications in FXR both in vitro and in vivo models.2. To investigate protein-protein interactions that govern FXR signaling.Innovative unbiased assays that combine SILAC and biotinylated DNA-protein complex precipitations will be used to identify modifications and interacting proteins by mass spectrometry. In a pilot study, The Applicant identified a novel phosphorylation site (FXR-S224), important for gene transactivation but not NF-κB transrepression. These findings could advance drug design for FXR and warrant further investigation. During this fellowship The Applicant will extend her skills in molecular NR pathway analysis, important to establish herself in the NR field and prepare for setting up her own group and accomplishing her career a