High-throughput screening assay for new ligands at human melatonin receptors.

AIM: Melatonin (MT) is a neurohormone produced and secreted primarily by the pineal gland in a circadian manner, and mainly acts through 2 receptor subtypes: MT1 and MT2 in humans. The diversity in their tissue distribution is in favor of different functions for each receptor subtype. Selective modulators are therefore required to determine the physiological roles of these melatonin receptor subtypes and their implications in pathological processes. METHODS: A homogenous MT1/MT2 receptor binding assay was established for high-throughput screening of new ligands at the hMT1 and/or hMT2 receptors. The functional properties (agonists or antagonists) were assessed by a conventional guanosine-5'[gamma-(35)S] triphosphate (GTP-gammaS) assay. RESULTS: Three hMT1 receptor-selective small molecule antagonists and 1 hMT2 receptor-selective small molecule antagonist with novel structural features were identified following a high-throughput screening campaign of 48,240 synthetic and natural compounds. CONCLUSION: The findings may assist in the expansion of chemical probes to these 2 receptor subtypes.

Identification of non-peptidic neuromedin U receptor modulators by a robust homogeneous screening assay.

AIM: To develop a homogeneous binding assay for high-throughput screening (HTS) of hit compounds at human neuromedin U receptor (hNMU-R) 1 and to identify non-peptidic small molecule hNMU-R modulators through functional assessments and structure-activity relationship (SAR) analyses. METHODS: Membrane preparations of Chinese hamster ovary cells (CHO-K1) stably expressing hNMU-R1, [125I]hNMU-25, and wheat germ agglutinin-coupled microbeads were used to develop an HTS assay based on scintillation proximity assay (SPA) technology. This method was applied to a large-scale screening campaign against a diverse library of 36,000 synthetic compounds or natural products and subsequent confirmation studies. CHO-K1 cells stably expressing full-length hNMU-R1 or hNMU-R2 and a calcium-sensitive dye were employed to functionally measure intracellular calcium mobilization upon ligand stimulation. Preliminary SAR was determined based on limited structural modifications. RESULTS: The Ki value (0.7 nmol/L) of hNMU-25 (the natural ligand) at hNMU-R1 measured by the SPA method was consistent with that reported in the literature, and the Z'factor for this HTS assay was 0.81. A total of 100 hits, showing more than 30% competitive inhibition on [125I]hNMU-25 binding to hNMU-R1, were identified initially, 3 of which were confirmed thereafter to have reasonable hNMU-R1-binding affinities and similar chemical structures. Based on their common molecular skeleton, 203 analogs were synthesized and tested. Among the 16 analogs that retained variable hNMU-R1- binding abilities, 2 elicited calcium influx in both hNMU-R1 and hNMU-R2-expressing cells, but none displayed antagonist activity. CONCLUSION: The homogeneous hNMU-R1 binding assay is an efficient and robust tool for screening potential hNMU-R modulators. Two non-selective hNMU-R agonists discovered are of low molecular weight nature with novel chemical structures. The preliminary SAR investigation suggests that both the triphenyl and guanidinol groups are crucial to the bioactivities observed.