Impaired transitioning of the FXR ligand binding domain to an active state underlies a PFIC5 phenotype

Abstract

Nuclear receptor farnesoid X receptor (FXR) acts as a key regulator of bile acid pool homeostasis and metabolism. Within the enterohepatic circulation, reabsorbed bile acids act as agonists on FXR, which transcriptionally controls the synthesis and transport of bile acids. Binding occurs in the ligand binding domain (LBD), favoring a conformational change to the active state in which helix 12 interacts with the LBD to form an interaction surface for nuclear co-activators. The homozygous missense variant T296I, identified in a PFIC5 patient, is located close to the critical helix 12 interaction. Here, we identified reduced transcriptional activity of the variant protein on the downstream targets BSEP and SHP in vitro and within the patient's liver. Analysis of the structural dynamics of the conformational change from an inactive to an active state of the FXR LBD with molecular dynamics simulations revealed that while the wildtype protein frequently transitions into the active state, this movement and the necessary perfect placement of helix 12 was significantly impeded in the T296I mutated protein. To our knowledge, this is the first study to describe the conformational change from an inactive to an active state of the FXR LBD. This might be useful for new therapeutic approaches targeting the activation of FXR. Overall, combining in vivo data with in vitro and in silico experiments, we suggest a molecular mechanism underlying the PFIC phenotype of a patient with an FXR missense variant.