Ligand-Dependent Effects of Methionine-8 Oxidation in Parathyroid Hormone Peptide Analogues

Unraveling the Interplay of Extracellular Domain Conformational Changes and Parathyroid Hormone Type 1 Receptor Activation in Class B1 G Protein-Coupled Receptors: Integrating Enhanced Sampling Molecular Dynamics Simulations and Markov State Models

Parathyroid hormone (PTH) type 1 receptor (PTH1R), as a typical class B1 G protein-coupled receptor (GPCR), is responsible for regulating bone turnover and maintaining calcium homeostasis, and its dysregulation has been implicated in the development of several diseases. The extracellular domain (ECD) of PTH1R is crucial for the recognition and binding of ligands, and the receptor may exhibit an autoinhibited state with the closure of the ECD in the absence of ligands. However, the correlation between ECD conformations and PTH1R activation remains unclear. Thus, this study combines enhanced sampling molecular dynamics (MD) simulations and Markov state models (MSMs) to reveal the possible relevance between the ECD conformations and the activation of PTH1R. First, 22 intermediate structures are generated from the autoinhibited state to the active state and conducted for 10 independent 200 ns simulations each. Then, the MSM is constructed based on the cumulative 44 μs simulations with six identified microstates. Finally, the potential interplay between ECD conformational changes and PTH1R activation as well as cryptic allosteric pockets in the intermediate states during receptor activation is revealed. Overall, our findings reveal that the activation of PTH1R has a specific correlation with ECD conformational changes and provide essential insights for GPCR biology and developing novel allosteric modulators targeting cryptic sites.

Mutation of Methionine to Asparagine but Not Leucine in Parathyroid Hormone Mimics the Loss of Biological Function upon Oxidation

Human parathyroid hormone (PTH) is an 84-amino acid peptide that contains two methionine (Met) residues located at positions 8 and 18. It has long been recognized that Met residues in PTH are subject to oxidation to become Met sulfoxide, resulting in a decreased biological function of the peptide. However, the mechanism of the lost biological function of PTH oxidation remains elusive. To characterize whether the shift from the hydrophobic nature of the native Met residue to the hydrophilic nature of Met sulfoxide plays a role in the reduction of biological activity upon PTH oxidation, we conducted in silico and in vitro site-directed mutagenesis of Met-8 and Met-18 to the hydrophilic residue asparagine (Asn) or to the hydrophobic residue leucine (Leu) and compared the behavior of these mutated peptides with that of PTH oxidized at Met-8 and/or Met-18. Our results showed that the biological activity of the Asn-8 and Asn-8/Asn-18 mutants was significantly reduced, similar to Met-8 sulfoxide and Met-8/Met-18 sulfoxide analogues, while the functions of Asn-18, Leu-8, Leu-8/Leu-18 mutants, or Met-18 sulfoxide analogues were similar to wild-type PTH. This is rationalized from molecular modeling and immunoprecipitation assay, demonstrating disruption of hydrophobic interactions between Met-8 and Met-18 of PTH and type-1 PTH receptor (PTHR1) upon mutation or oxidation. Thus, these novel findings support the notion that the loss of biological function of PTH upon oxidation of Met-8 is due, at least in part, to the conversion from a hydrophobic to a hydrophilic residue that disrupts direct hydrophobic interaction between PTH and PTHR1.

Critical Extracellular Ca2+ Dependence of the Binding between PTH1R and a G-Protein Peptide Revealed by MD Simulations

The parathyroid hormone type 1 receptor (PTH1R), a canonical class B GPCR, is regulated by a positive allosteric modulator, extracellular Ca²⁺. Calcium ions prolong the residence time of PTH on the PTH1R, leading to increased receptor activation and duration of cAMP signaling. But the essential mechanism of the allosteric behavior of PTH1R is not fully understood. Here, extensive molecular dynamics (MD) simulations are performed for the PTH1R-G-protein combinations with and without Ca²⁺ to describe how calcium ions allosterically engage receptor-G-protein coupling. We find that the binding of Ca²⁺ stabilizes the conformation of the PTH1R-PTH-spep (the α5 helix of Gs protein) complex, especially the extracellular loop 1 (ECL1). Moreover, the MM-GBSA result indicates that Ca²⁺ allosterically promotes the interaction between PTH1R and spep, consistent with the observation of steered molecular dynamics (SMD) simulations. We further illuminate the possible allosteric signaling pathway from the stable Ca²⁺-coupling site to the intracellular G-protein binding site. These results unveil structural determinants for Ca²⁺ allosterism in the PTH1R-PTH-spep complex and give insights into pluridimensional GPCR signaling regulated by calcium ions.

Functional Properties of Two Distinct PTH1R Mutants Associated With Either Skeletal Defects or Pseudohypoparathyroidism

Consistent with a vital role of parathyroid hormone (PTH) receptor type 1 (PTH1R) in skeletal development, homozygous loss-of-function PTH1R mutations in humans results in neonatal lethality (Blomstrand chondrodysplasia), whereas such heterozygous mutations cause a primary failure of tooth eruption (PFE). Despite a key role of PTH1R in calcium and phosphate homeostasis, blood mineral ion levels are not altered in such cases of PFE. Recently, two nonlethal homozygous PTH1R mutations were identified in two unrelated families in which affected members exhibit either dental and skeletal abnormalities (PTH1R-V204E) or hypocalcemia and hyperphosphatemia (PTH1R-R186H). Arg186 and Val204 map to the first transmembrane helix of the PTH1R, and thus to a critical region of this class B G protein-coupled receptor. We used cell-based assays and PTH and PTH-related protein (PTHrP) ligand analogs to assess the impact of the R186H and V204E mutations on PTH1R function in vitro. In transiently transfected HEK293 cells, PTH1R-R186H mediated cyclic adenosine monophosphate (cAMP) responses to PTH(1-34) and PTHrP(1-36) that were of comparable potency to those observed on wild-type PTH1R (PTH1R-WT) (half maximal effective concentrations [EC50s] = 0.4nM to 1.2nM), whereas the response-maxima were significantly reduced for the PTH1R-V204E mutant (maximum effect [Emax] = 81%-77% of PTH1R-WT, p ≤ 0.004). Antibody binding to an extracellular hemagglutinin (HA) tag was comparable for PTH1R-R186H and PTH1R-WT, but was significantly reduced for PTH1R-V204E (maximum binding level [Bmax] = 44% ± 11% of PTH1R-WT, p = 0.002). The potency of cAMP signaling induced by a PTH(1-11) analog was reduced by ninefold and threefold, respectively, for PTH1R-R186H and PTH1R-V204E, relative to PTH1R-WT, and a PTH(1-15) radioligand analog that bound adequately to PTH1R-WT exhibited little or no specific binding to either mutant receptor. The data support a general decrease in PTH1R surface expression and/or function as a mechanism for PFE and a selective impairment in PTH ligand affinity as a potential PTH1R-mutation-based mechanism for pseudohypoparathyroidism. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Activity-based, bioorthogonal imaging of phospholipase D reveals spatiotemporal dynamics of GPCR-Gq signaling

Canonically, G-protein-coupled receptor (GPCR) signaling is transient and confined to the plasma membrane (PM). Deviating from this paradigm, the parathyroid hormone receptor (PTHR1) stimulates sustained G_s signaling at endosomes. In addition to G_s, PTHR1 activates G_q signaling; yet, in contrast to the PTHR1-G_s pathway, the spatiotemporal dynamics of the G_q branch of PTHR1 signaling and its relationship to G_s signaling remain largely ill defined. Recognizing that a downstream consequence of G_q signaling is the activation of phospholipase D (PLD) enzymes, we leverage activity-based, bioorthogonal imaging tools for PLD signaling to visualize and quantify the G_q branch of PTHR1 signaling. We establish that PTHR1-G_q signaling is short lived, exclusively at the PM, and antagonized by PTHR1 endocytosis. Our data support a model wherein G_q and G_s compete for ligand-bound receptors at the PM and more broadly highlight the utility of bioorthogonal tools for imaging PLDs as probes to visualize GPCR-G_q signaling.

Ligand-Dependent Effects of Methionine-8 Oxidation in Parathyroid Hormone Peptide Analogues

LA-PTH is a long-acting parathyroid hormone (PTH) peptide analogue in preclinical development for hypoparathyroidism (HP). Like native PTH, LA-PTH contains a methionine at position 8 (Met8) that is predicted to be critical for function. We assessed the impact of Met oxidation on the functional properties of LA-PTH and control PTH ligands. Oxidation of PTH(1-34) resulted in marked (~20-fold) reductions in binding affinity on the PTH receptor-1 (PTHR1) in cell membranes, similarly diminished potency for 3',5'-cyclic AMP signaling in osteoblastic cell lines (SaOS-2 and UMR106), and impaired efficacy for raising blood calcium in mice. Surprisingly, oxidation of LA-PTH resulted in little or no change in these functional responses. The signaling potency of oxidized-LA-PTH was, however, reduced approximately 40-fold compared to LA-PTH in cells expressing a PTHR1 construct that lacks the N-terminal extracellular domain (ECD). Molecular modeling revealed that while Met8 of both LA-PTH and PTH(1-34) is situated within the orthosteric ligand-binding pocket of the receptor's transmembrane domain bundle (TMD), the Met8 sidechain position is shifted for the 2 ligands so that on Met8 oxidation of PTH(1-34), steric clashes occur that are not seen with oxidized LA-PTH. The findings suggest that LA-PTH and PTH(1-34) engage the receptor differently in the Met8-interaction environment of the TMD bundle, and that this interaction environment can be allosterically influenced by the ECD component of the ligand-receptor complex. The findings should be useful for the future development of novel PTH-based peptide therapeutics for diseases of bone and mineral ion metabolism.