β-arrestins are known modulators of GPCR signaling, but their precise interaction with class B GPCRs is poorly understood. A new cryo-EM structure of a fully engaged parathyroid hormone receptor (PTH1R)–β-arrestin 1 complex provides a structural framework for understanding how ligands tune the receptor for arrestin versus G protein engagement.

Osteocytes are the primary mechano-sensitive cell type in bone. Mechanical loading is sensed across the dendritic projections of osteocytes leading to transient reductions in focal adhesion kinase (FAK) activity. We performed tyrosine-focused phospho-proteomic profiling in osteocyte-like Ocy454 cells to identify FAK substrates. Gsα, parathyroid hormone receptor (PTH1R), and phosphodiesterase 8A (PDE8A), three proteins associated with cAMP signaling, were found as potential FAK targets. FAK pharmacologic inhibition or gene deletion increased basal and G-protein-coupled-receptor (GPCR) ligand-stimulated cAMP levels and downstream signaling events. PDE8A inhibition mimicked FAK inhibition at the levels of increased cAMP, protein kinase A (PKA) activity, and expression of cAMP-regulated target genes. Co-immunoprecipitation assays revealed an intracellular association between FAK and PDE8A. Thus, mechanically regulated FAK can modulate intracellular cAMP levels via effects on PDE8A. These data suggest a signal transduction mechanism that mediates crosstalk between mechanical and cAMP-linked hormonal signaling in osteocytes.

Chronic hypoparathyroidism is a rare endocrine disorder characterised by hypocalcaemia due to insufficient parathyroid hormone (PTH) that is associated with a multidimensional reduced quality of life, impairing physical and cognitive functioning, and compromising emotional, psychological, and social wellbeing. This Review integrates recent findings on hypoparathyroidism diagnosis, PTH analogues, calcilytics, preclinical therapeutic agents, parathyroid tissue transplantation, and pluripotent stem cell-based therapies. Emerging diagnostic technologies, including targeted genetic testing, parathyroid autoimmunity assays, and point-of-care calcium monitoring, could enable individualised and real-time management. PTH analogues with extended action (eg, palopegteriparatide, eneboparatide, and canvuparatide) show sustained activation of PTH receptors with reduced requirements for active forms of vitamin D and calcium supplements. Calcilytics (eg, encaleret) normalise calcium-sensing receptor (CaSR) hyperactivation in patients with autosomal dominant hypocalcaemia type 1 and have the potential for broader application in other forms of hypoparathyroidism. Advances in parathyroid regenerative biology with pluripotent stem cells provide platforms for future curative therapies. Progress in parathyroid autotransplantation, allotransplantation, and graft-preservation strategies further expands the therapeutic landscape. Translational research is redefining the future of hypoparathyroidism management. Developments in receptor-targeted therapies, long-acting PTH analogues, negative allosteric modulators of CaSR, and regenerative approaches reflects a shift towards restoring mineral homoeostasis to a more physiological state.

Eneboparatide, a long-acting modified form of parathyroid hormone, increases serum calcium levels without increasing urine calcium in mice with autosomal dominant hypocalcemia type 1.

G protein-coupled receptors (GPCRs) mediate information transfer to cells from the surrounding environment. In most cases, signaling is initiated or amplified when the receptor binds to an agonist, an event that alters the conformational profile of the receptor. Signal transduction results from interaction between the agonist-receptor complex and cytosolic partners such as G proteins, GPCR kinases (GRKs), and β-arrestins. Changes in agonist structure can lead to "signal bias", i.e., changes in the relative strength of signaling involving different partners. Some GPCRs, including those activated by long peptide hormones, continue to signal after internalization. In these cases, changes in agonist structure can lead to changes in the relative extent of signaling from different sites, e.g., cell surface vs endosomes ("location bias"). Many GPCRs are targets of approved drugs or drug candidates, and tuning signal bias and/or location bias is widely considered to be important for optimizing therapeutic profiles. Here we report another mechanism of modulating outcome via agonist modification: alteration of intracellular trafficking. The synthetic peptide agonist designated SPT, which contains five β-amino acid residues, was previously shown to activate the parathyroid hormone receptor-1 (PTH1R) and cause prolonged signaling in mice by an unknown mechanism. The SPT-PTH1R complex continues to stimulate cAMP production after internalization. We now find that the SPT-PTH1R complex impairs the sorting of early endosomes into recycling endosomes relative to the receptor complexed to the drug teriparatide. These findings suggest that altering intracellular GPCR trafficking patterns represents an unappreciated strategy for achieving prolonged action in vivo.

The parathyroid hormone receptor 1 (PTH1R) transmits stimuli provided by PTH and PTH-related protein (PTHrP) and thus plays key roles in calcium and phosphate homeostasis as well as skeletal development. Variants in PTH1R have been linked to several conditions, including Jansen metaphyseal chondrodysplasia, Blomstrand chondrodysplasia, primary failure of tooth eruption, and Eiken syndrome. Here, we report a novel skeletal phenotype identified in two unrelated families associated with PTH1R variants. The clinical features include brachydactyly type E, mild short stature, and dental anomalies. A novel heterozygous PTH1R substitution (p.E469K) was identified in affected members of Family 1, while the affected individual from Family 2 had a previously described heterozygous de novo substitution (p.E465K); these two mutated sites lie within helix 8 (H8) of the PTH1R. Cell-based assays revealed reduced cell surface expression, as well as impaired basal and PTH- or PTHrP-induced cAMP signaling responses for both mutants, as compared to WT-PTH1R. Introduction of the p.E469K substitution into humanized PTH1R mice resulted in mildly increased mineralization of bones in the paws as well as shortening of long bones. Our findings demonstrate a new skeletal phenotype associated with PTH1R variants and suggest that H8 of the receptor contributes to PTH1R expression and/or signaling during bone development.

Jansen's metaphyseal chondrodysplasia (JMC) is a rare disorder caused by activating mutations in the parathyroid hormone (PTH)/PTH-related peptide (PTHrP) receptor (PTH1R). Patients exhibit short stature, dysmorphic bones, and severe growth plate abnormalities, as well as hypercalcemia, hypercalciuria, hypophosphatemia, and reduced plasma PTH levels. Humanized PTH1R (hPTH1R) mice expressing the H223R-hPTH1R JMC mutation die early without breeding. We therefore generated and characterized a stable mouse line expressing the T410R-hPTH1R allele, which confers a milder disease phenotype in patients. Mutant mice show near-normal longevity and reproductive capacity yet exhibit a profound skeletal phenotype characteristic of the disease. The long bones of T410R mice are markedly misshapen and have expanded metaphyses with disarrayed chondrocyte zones in growth plates and reduced primary spongiosa. PET/CT scanning revealed diminished uptake of [18F]-sodium fluoride in the growth plate area, consistent with reduced mineralization and vascularization. Genetic ablation of Hdac4 rescued the growth plate abnormalities in T410R mice, thereby establishing the PTH1R-Gαs-cAMP-PKA-SIK3-HDAC4/5 pathway as the main mediator of growth plate abnormalities in JMC. Serum calcium was elevated and endogenous PTH was suppressed in T410R mice, and both parameters could be normalized by acute injection of an optimized PTH inverse agonist peptide. The T410R mouse thus represents a stable animal model of JMC that recapitulates the abnormalities in skeletal development and mineral ion homeostasis which characterize this disease. The mice should help efforts to further define the cellular and molecular mechanisms underlying the JMC phenotype and to develop a potential mode of therapy.

Parathyroid hormone (PTH) analogs with improved actions in vivo could lead to optimized treatments for bone and mineral ion diseases. Rapid clearance from the circulation and short dwell times on the PTH receptor limit the efficacies of conventional PTH peptides currently in medical use. Here, we seek to enhance PTH peptide efficacy using two distinct peptide lipidation strategies. First, we append a lipid chain to the peptide's C-terminus in a fashion to promote binding to serum albumin and hence prolong the peptide's circulation half-life in vivo. Second, we append a lipid chain to a lysine side chain in a fashion designed to anchor the peptide to the cell membrane as the ligand is bound to the receptor and hence increase its dwell time on the receptor. We find that both strategies of lipidation can profoundly enhance the efficacy of PTH peptides in vitro and in mice. Our results could lead to the development of modified PTH analogs with optimized therapeutic utility.

The parathyroid hormone receptor type 1 (PTH1R) is a G protein-coupled receptor that mediates the actions of parathyroid hormone (PTH) in the regulation of blood calcium levels, as well as PTH-related protein (PTHrP) in the regulation of skeletal development. Severe loss-of-function homozygous mutations in PTH1R are incompatible with life as in Blomstrand’s lethal chondrodysplasia, characterized by accelerated growth plate ossification. More recently, homozygous mutations located in the transmembrane helices, extracellular domains and C-tail of the PTH1R were identified in patients with milder conditions characterized by variable degrees of skeletal and mineral abnormalities. These include delayed ossification in Eiken syndrome, hypocalcemia in a pseudohypoparathyroidism-like disorder, and non-syndromic primary failure of tooth eruption; which is usually caused by heterozygous PTH1R mutations. Recent detailed pharmacologic characterization of these PTH1R mutants has revealed new insights into how even subtle perturbations in PTH1R function can result in disease.

Osteoporosis, characterized by reduced bone density and strength, increases fracture risk, pain, and limits mobility. Established therapies of parathyroid hormone (PTH) analogs effectively promote bone formation and reduce fractures in severe osteoporosis, but their use is limited by potential adverse effects. In the pursuit of safer osteoporosis treatments, we investigated R25C PTH, a PTH variant wherein the native arginine at position 25 is substituted by cysteine. These studies were prompted by our finding of high bone mineral density in a hypoparathyroidism patient with the R25C homozygous mutation, and we explored its effects on PTH type-1 receptor (PTH1R) signaling in cells and bone metabolism in mice. Our findings indicate that R25C PTH(1–84) forms dimers both intracellularly and extracellularly, and the synthetic dimeric peptide, R25C PTH(1–34), exhibits altered activity in PTH1R-mediated cyclic AMP (cAMP) response. Upon a single injection in mice, dimeric R25C PTH(1–34) induced acute calcemic and phosphaturic responses comparable to PTH(1–34). Furthermore, repeated daily injections increased calvarial bone thickness in intact mice and improved trabecular and cortical bone parameters in ovariectomized (OVX) mice, akin to PTH(1–34). The overall results reveal a capacity of a dimeric PTH peptide ligand to activate the PTH1R in vitro and in vivo as PTH, suggesting a potential path of therapeutic PTH analog development.

Abstract Disclosure: J. Höppner: None. H. Noda: None. A. Khatri: None. H. Jüppner: None. T.J. Gardella: None. PTH analogs with improved actions in vivo are of interest as potential treatment options for bone and mineral ion diseases such as hypoparathyroidism and osteoporosis. The efficacies in vivo of conventional PTH peptides are generally limited by rapid rates of clearance from the circulation (t1/2 = ∼ 5-30 minutes) and relatively short-dwell times on the receptor (PTH1R). This is especially true for small fragment analogs, such as M (modified)-PTH(1-14), and M-PTH(1-11), which have only been shown to be active in vitro. We explored whether PTH fragment peptide efficacy in vivo could be enhanced by addition of a lipid chain (e.g., a C15 palmitoyl chain (palm)) to either the peptide C-terminus or to the side chain of the amino acid residue at position 11 or 13. The C-tail lipid modifications were specifically designed to promote binding to serum albumin, and hence extend circulation half-life, while the lipid chains at sidechains of residue positions 11 and 13 were designed, based on structural models, to anchor the ligand to the receptor in situ -- i.e. by projection of the acyl chain between two adjacent transmembrane helices and into the surrounding plasma membrane -- to thereby extend receptor dwell time. Assessment of cAMP signaling potencies in HEK293/hPTH1R/glosensor cells revealed that both types of lipid modifications can preserve agonist potency, and that the lipid extensions at position 11 or 13 can indeed prolong receptor dwell times, as shown by extended durations of cAMP signaling after initial binding of Palm 11 or 13-modified M-PTH(1-14) and M-PTH(1-11) peptide fragment. Single subcutaneous injection of either Palm-13-M-PTH(1-14) or Palm-11-M-PTH(1-11) into mice resulted in prolonged increases in blood serum calcium levels, while 14 days of daily injection resulted in profound increases in bone mass, as measured by uCT, and marked increases in blood levels of the bone turnover markers CTX1 and P1NP, whereas non-lipidated M-PTH(1-14) control peptides were inactive in vivo. Peptide lipidation thus holds promise as a strategy to employ in the design of new PTH analogs with improved efficacies in vivo. Presentation: 6/1/2024

Ligand-induced activation of G protein-coupled receptors (GPCRs) can initiate signaling through multiple distinct pathways with differing biological and physiological outcomes. There is intense interest in understanding how variation in GPCR ligand structure can be used to promote pathway selective signaling ("biased agonism") with the goal of promoting desirable responses and avoiding deleterious side effects. Here we present a new approach in which a conventional peptide ligand for the type 1 parathyroid hormone receptor (PTHR1) is converted from an agonist which induces signaling through all relevant pathways to a compound that is highly selective for a single pathway. This is achieved not through variation in the core structure of the agonist, but rather by linking it to a nanobody tethering agent that binds with high affinity to a separate site on the receptor not involved in signal transduction. The resulting conjugate represents the most biased agonist of PTHR1 reported to date. This approach holds promise for facile generation of pathway selective ligands for other GPCRs.

We report on 2 patients of East African ancestry with the same novel homozygous variant in the parathyroid hormone receptor type 1 (PTH1R). Both patients shared skeletal features, including brachydactyly, extensive metacarpal pseudo-epiphyses, elongated cone-shaped epiphyses, ischiopubic hypoplasia, and deficient sacral ossification, suggestive of Eiken syndrome. Strikingly, both patients exhibited clinically manifest parathyroid hormone (PTH) resistance with hypocalcemia and elevated serum phosphate levels. These laboratory and clinical abnormalities initially suggested pseudohypoparathyroidism, which is typically associated with GNAS abnormalities. In both patients, however, a homozygous novel PTH1R variant was identified (c.710 T > A; p.IIe237Asn, p.I237N) that is located in the second transmembrane helical domain. Previously, others have reported a patient with a nearby PTH1R mutation (D241E) who presented with similar clinical features (eg, delayed bone mineralization as well as clinical PTH resistance). Functional analysis of the effects of both novel PTH1R variants (I237N- and D241E-PTH1R) in HEK293 reporter cells transfected with plasmid DNA encoding the wild-type or mutant PTH1Rs demonstrated increased basal cAMP signaling for both variants, with relative blunting of responses to both PTH and PTH-related peptide (PTHrP) ligands. The clinical presentation of PTH resistance and delayed bone mineralization combined with the functional properties of the mutant PTH1Rs suggest that this form of Eiken syndrome results from alterations in PTH1R-mediated signaling in response to both canonical ligands, PTH and PTHrP.

Parathyroid hormone 1 receptor (PTH1R) plays a key role in mediating calcium homeostasis and bone development, and aberrant PTH1R activity underlies several human diseases. Peptidic PTH1R antagonists and inverse agonists have therapeutic potential in treating these diseases, but their poor pharmacokinetics and pharmacodynamics undermine their in vivo efficacy. Herein, we report the use of a backbone-modification strategy to design a peptidic PTH1R inhibitor that displays prolonged activity as an antagonist of wild-type PTH1R and an inverse agonist of the constitutively active PTH1R-H223R mutant both in vitro and in vivo. This peptide may be of interest for the future development of therapeutic agents that ameliorate PTH1R malfunction.

Abstract Disclosure: J. Höppner: None. P. Hanna: None. M.N. Wein: None. H. Jueppner: None. T.J. Gardella: None. R. Civitelli: None. I.A. Portales-Castillo: None. Abstract The parathyroid hormone receptor-1 (PTH1R) plays a key role in bone development and acts in the growth plates in response to PTHrP ligand. Eiken syndrome is characterized by a delay in bone mineralization and is caused by homozygous PTH1R mutations. One such mutation, R485X, truncates the receptor's C-tail and removes serine phosphorylation sites involved in βarrestin binding. In HEK293 cells, R485X-hPTH1R exhibits deficient interaction with βarrestin and increased cAMP signaling both basally and in response to PTHrP (Portales-Castillo et al., Comms. Biology 2023). To understand how the R485X mutation causes delayed bone mineralization, we generated humanized R485X-hPTH1R knock-in mice. Homozygous hPTH1RR485X/R485X mice closely recapitulate the delayed bone mineralization seen in Eiken patients, as skeletal whole mount preparations from neonatal mutant mice showed reduced Alizarin red staining compared to WT controls, and metatarsal explants from the mutant mice showed a pronounced absence of mineralized bone. Tails of hPTH1RR485X/R485X mice were ∼50% shorter than those of WT littermates, and H&E-stained sections revealed only proliferative chondrocytes in the growth plates of mutant mice. This delay in growth plate chondrocyte maturation in hPTH1RR485X/R485X mice resolved with age, although older long bones and tails remained smaller in size than WT controls (Höppner et al. Presented at ASBMR 2023).Based on our in vitro findings and initial mouse characterization, we reasoned that the phenotype of Eiken mice may be explained by increased PTHrP/PTH1R signaling in growth plate chondrocyte. The class IIa histone deacetylase HDAC4 suppresses chondrocyte maturation downstream of PTH1R signaling. Therefore, we generated compound mutant mice bearing both Hdac4 deletion and the mutant R485X PTH1R allele. Indeed, Hdac4 deletion largely rescued the mineralization defect apparent in metatarsals of P1 hPTH1RR485X/R485X mice. We then assessed the contribution of endogenous PTHrP to the Eiken-like phenotype by generating hPTH1RR485X/R485X/PTHrPflox/+/Col2-Cre(tg) mice. Remarkably, these mice exhibit approximately the same tail lengths as hPTH1R-WT littermate controls, indicating that reduced PTHrP production can rescue the effects of homozygosity for R485X-PTH1R. In line, vertebral growth plates of the rescued mice exhibited normal zones of chondrocytes, including hypertrophic cells. The overall results support a disease mechanism for Eiken syndrome by which excess cAMP signaling by PTH1R-R485X, as induced in part by endogenous PTHrP, results in delayed chondrocyte differentiation and bone mineralization. Further studies employing βarr1/2-KO mice may help shed light on the specific roles of βarrestins in growth plate development by PTH1R in Eiken syndrome. Presentation: 6/3/2024

AbstractOsteoporosis, characterized by reduced bone density and strength, increases fracture risk, pain, and limits mobility. Established therapies of Parathyroid hormone (PTH) analogs effectively promote bone formation and reduce fractures in severe osteoporosis, but their use is limited by potential adverse effects. In the pursuit of safer osteoporosis treatments, we investigatedR25CPTH, a PTH variant wherein the native arginine at position 25 is substituted by cysteine. These studies were prompted by our finding of high bone mineral density in a hypoparathyroidism patient with the R25C homozygous mutation, and we explored its effects on PTH type-1 receptor (PTH1R) signaling in cells and bone metabolism in mice. Our findings indicate thatR25CPTH(1-84) forms dimers both intracellularly and extracellularly, and the synthetic dimeric peptide,R25CPTH(1-34), exhibits altered activity in PTH1R-mediated cAMP response. Upon a single injection in mice, dimericR25CPTH(1-34) induced acute calcemic and phosphaturic responses comparable to PTH(1-34). Furthermore, repeated daily injections increased calvarial bone thickness in intact mice and improved trabecular and cortical bone parameters in ovariectomized (OVX) mice, akin to PTH(1-34). The overall results reveal a capacity of a dimeric PTH peptide ligand to activate the PTH1Rin vitroandin vivoas PTH, suggesting a potential path of therapeutic PTH analog development.