Spatial control of parathyroid hormone receptor-1 signaling dictates ligand-specific biological outcomes.

Parathyroid hormone receptor-1 (PTH1R) is a class B GPCR that regulates mineral ion homeostasis via parathyroid hormone (PTH) and skeletal development via parathyroid hormone-related peptide (PTHrP). While PTHrP signaling is thought to occur predominantly at the plasma membrane, PTH is considered to also signal from endosomes; however, the mechanisms that encode subcellular PTH1R actions, as well as the biological relevance of plasma membrane vs internalized PTH1R activation, remain unclear. Here, by examining the Eiken syndrome mutant R485X-PTH1R, in which C-tail truncation removes key βarrestin docking sites, we identified βarrestins as critical modulators of compartmentalized PTH1R signaling in response to endogenous ligands. Patients with Eiken syndrome display delayed bone mineralization, consistent with enhanced PTHrP actions, as well as hypocalcemia along with elevated PTH, which paradoxically indicates impaired PTH function. Mechanistic studies in cell-based assays and humanized R485X-PTH1R knock-in mice revealed that βarrestins dampen PTHrP-driven Gαs signaling at the plasma membrane but promote PTH-induced endosomal PTH1R signaling. βarrestins thus appear to impose bimodal, compartment-specific modulation of PTH1R signaling, thereby resolving the paradoxical Eiken phenotype. The findings support the notion that differences in subcellular PTH1R localization determine ligand-specific signaling outcomes in skeletal physiology and mineral ion homeostasis.