PTH1R

PTH1R is the class B GPCR that mediates both PTH's systemic calcium/phosphate homeostasis actions and PTHrP's local paracrine control of bone formation, cartilage development, and epithelial differentiation - making it simultaneously the regulator of the calcium set-point and the anabolic driver of osteoblast activity. Teriparatide (PTH 1-34) and abaloparatide (PTHrP-based) are FDA-approved anabolic osteoporosis drugs. PTH1R also generates sustained endosomal cAMP signaling - a unique class B GPCR property with major implications for pharmacological design. Every scaffold for osteoporosis, hypoparathyroidism, or bone-forming peptide research forks from this card.

PTH1R (chromosome 3p21.31, 593 aa) adopts canonical class B GPCR architecture: a 165-aa N-terminal ECD that captures the C-terminal helix of PTH/PTHrP, a seven-TM bundle that is activated by the peptide N-terminus, and a ~100-aa C-terminal tail with GRK phosphorylation sites for β-arrestin. Two-domain binding: PTH(15-34) C-terminal helix docks into the ECD hydrophobic groove (Ile38, Leu41 contacts Trp23, Leu24 of PTH; Asp137 salt-bridges PTH Arg20); this positions PTH(1-14) N-terminus to engage TM5/TM6 (Val2, Ile5 against Phe288, Leu292), driving receptor activation. The ECD is stabilized by three disulfide bonds (Cys48–Cys117, Cys108–Cys148, Cys131–Cys170). Signaling: Gαs → adenylyl cyclase → cAMP → PKA → CREB → bone anabolic genes and renal tubular phosphate excretion/calcium reabsorption (primary); Gαq/11 → PLC → Ca²⁺/PKC; Gα12/13; β-arrestin → ERK and endosomal cAMP prolongation. Sustained endosomal PTH1R-cAMP signaling after internalization is distinct from plasma membrane signaling and is prolonged by β-arrestin scaffolding - this endosomal component drives the anabolic bone effect uniquely associated with once-daily intermittent PTH dosing. Activating PTH1R mutations (H223R, T410P) cause Jansen metaphyseal chondrodysplasia; inactivating mutations cause Blomstrand lethal chondrodysplasia.

Approved: teriparatide (recombinant PTH 1-34, SC daily) for severe osteoporosis - increases bone mineral density and reduces fractures by anabolic osteoblast activation; approved in US and EU. Abaloparatide (PTHrP-based analog with an Aib²,Leu¹¹,D-Ala¹²,Aib¹⁷ sequence for enhanced G protein bias) is FDA-approved for osteoporosis, showing anabolic potency with less hypercalcemia than teriparatide. Both are approved for ≤24 months lifetime use due to theoretical osteosarcoma risk from prolonged continuous receptor activation. Cinacalcet (allosteric CaSR modulator, not PTH1R direct) reduces PTH secretion in secondary hyperparathyroidism. SEP-786 (oral small molecule PTH1R agonist) is in Phase 2 for hypoparathyroidism. For peptide research, the tractable recipes are: PTH(1-34) Aib-scanning at positions 3, 7, 10 for DPP-4/protease resistance while preserving the G protein bias toward Gs over β-arrestin; PTHrP(1-36) truncations to identify the minimum N-terminal sequence for full anabolic signaling; Gs-biased PTH analogs that maximize endosomal cAMP duration (Ala¹-modified scaffolds) for once-weekly osteoporosis dosing; and LA-PTH (long-acting PTH analogs with high-affinity ECD binding through C-terminal extensions for sustained receptor occupancy) as templates for hypoparathyroidism replacement therapy.