NanoBRET-based detection of ligand-receptor interactions at the neuropeptide FF receptor 1

The neuropeptide FF receptor 1 (NPFFR1) belongs to the RF-amide G protein-coupled receptor family. Even though it is a promising therapeutic target for the treatment of chronic pain, this receptor still has not been used as a drug target. A detailed understanding of its ligand binding and activation mechanisms is essential for the rational design of novel modulators. In this study, we developed a non-radioactive, nanoBRET-based ligand binding assay to investigate ligand interactions of neuropeptide FF (NPFF) and neuropeptide VF (NPVF) with the NPFFR1. Fluorescently labeled NPFF and NPVF analogs were synthesized by conjugating a 6-carboxytetramethylrhodamine fluorophore at distinct positions, while a nanoluciferase was fused to the N-terminus of the NPFFR1 to serve as the BRET donor. This approach enables quantitative measurement of ligand binding and provides insights into the relative orientation of the ligand and receptor. Distinct BRET signal profiles for NPFF and NPVF, with a smaller window for NPVF compared to NPFF when directly labeled with the fluorophore, indicate differences in the binding orientation. Furthermore, deletion of the N-terminal residues of the receptor revealed that this region is dispensable for ligand recognition and binding in the NPFFR1. The assay was confirmed for small molecule NPFFR1 ligands, such as hederagenin, offers new opportunities to explore subtype selectivity and will guide drug discovery targeting the RF-amide receptor family.