Identification of anti-resorptive GPCRs by high-content imaging in human osteoclasts

Osteoporosis diagnoses are increasing in the ageing population, and although some treatments exist, these have several disadvantages, highlighting the need to identify new drug targets. G protein-coupled receptors (GPCRs) are transmembrane proteins whose surface expression and extracellular activation make them desirable drug targets. Our previous studies have identified 144 GPCR genes to be expressed in primary human osteoclasts, which could provide novel drug targets. The development of high-throughput assays to assess osteoclast activity would improve the efficiency at which we could assess the effect of GPCR activation on human bone cells and could be utilised for future compound screening. Here, we assessed the utility of a high-content imaging (HCI) assay that measured cytoplasmic-to-nuclear translocation of the nuclear factor of activated T cells-1 (NFATc1), a transcription factor that is essential for osteoclast differentiation, and resorptive activity. We first demonstrated that the HCI assay detected changes in NFATc1 nuclear translocation in human primary osteoclasts using GIPR as a positive control, and then developed an automated analysis platform to assess NFATc1 in nuclei in an efficient and unbiased manner. We assessed six GPCRs simultaneously and identified four receptors (FFAR2, FFAR4, FPR1 and GPR35) that reduced osteoclast activity. Bone resorption assays and measurements of TRAP activity verified that activation of these GPCRs reduced osteoclast activity, and that receptor-specific antagonists prevented these effects. These studies demonstrate that HCI of NFATc1 can accurately assess osteoclast activity in human cells, reducing observer bias and increasing efficiency of target detection for future osteoclast-targeted osteoporosis therapies.