OR21-3 Spatial Signaling Bias of a Gain-of-Function PTH1R Mutant Associated with Delayed Ossification in Eiken Syndrome

Skeletal development is orchestrated by the interaction of the parathyroid hormone related peptide (PTHrP) with the parathyroid hormone receptor type 1 (PTH1R). Knock-out or severe deficiency of either PTH1R or PTHrP by homozygous loss-of-function mutations results in premature hypertrophy of chondrocytes and accelerated bone mineralization, leading to neonatal death. In contrast, patients with a heterozygous gain-of-function PTH1R mutation (H223R, T410P/R or I458K/R) present with Jansen metaphyseal chondrodysplasia (JMC), characterized by delayed bone mineralization, hypercalcemia, hypercalciuria and hypophosphatemia. Eiken syndrome is also characterized by markedly delayed bone mineralization but lacks the biochemical abnormalities of JMC. Eiken syndrome has been described for three unrelated families, in which there are one of three homozygous PTH1R mutations: R485X, E35K or Y134S. As the functional properties of these PTH1R mutants have not been studied, we assessed their characteristics in cell-based assays. GS22A cells (HEK293/Glosensor) were transiently transfected to express the WT or a mutant PTH1R. Cell surface expression levels assessed by extracellular HA tag-directed immunofluorescence flow cytometry and normalized to PTH1R-WT (100%) were 50% for PTH1R-R485X, 90% for PTH1R-E35K and 20% for PTH1R-Y134S. Dynamic cAMP measurements via the Glosensor reporter revealed that the basal activities of PTH1R-R485X and PTH1R-E35K were 7-fold and 1.5-fold higher than that of PTH1R-WT (p<0.005), while that of PTH1R-Y134S was comparable to PTH1R-WT. Stimulation with PTH(1-34) or PTHrP(1-36) yielded similar cAMP potencies for the three mutants and PTH1R-WT (EC50s ∼ 0.5 nM, p >0.05). Intracellular calcium signaling assessed by changes in FURA2 fluorescence after PTHrP(1-36) (100 nM) addition was more robust with PTH1R-R485X than for PTH1R-WT (AUCs over 2.5 mins. = 606±9 vs 551±10, p=0.0074) and similar to PTH1R-WT with PTH1R-E35K and PTH1R-Y134S (p ≥0.2). Both basal and agonist-activated cAMP signaling by the R485X mutant were reduced by addition of a dTrp(12)-PTHrP(7-36) antagonist/inverse agonist analog. Fluorescence microscopy of HEK293/βarrestin2YFP (GBR24) cells stimulated with TMRPTH(1-34) revealed a clear association of βarrestin2YFP with PTH1R-WT, as well as with PTH1R-E35K and PTH1R-Y134S in internalized complexes, but no such association with PTH1R-R485X. Nevertheless, over-expression of βarrestin2YFP in GS22A cells markedly reduced basal and ligand-stimulated cAMP generation by PTH1R-R485X, consistent with a residual capacity of PTH1R-485X to bind βarrestin2. Treatment with the endocytosis inhibitor Dyngo4A in GS22A cells transfected with PTH1R-R485X completely blocked TMRPTH(1-34) internalization assessed by fluorescent microscopy, and enhanced ∼5-fold the cAMP response potency to PTH(1-34) vs DMSO treatment. In contrast, treatment with Dyngo4A did not increase PTH(1-34) cAMP potency in the PTH1R-WT. Collectively these results suggest that PTH1R-R485X signals via cAMP predominantly from the cell surface. The data reveal distinct functional properties for the three PTH1R mutants associated with Eiken syndrome and suggest a novel correlation between excessive PTH1R cAMP signaling due to impaired βarrestin-mediated internalization and a delay in ossification. Presentation: Monday, June 13, 2022 11:30 a.m. - 11:45 a.m.