Backbone Modification Provides a Long-Acting Inverse Agonist of Pathogenic, Constitutively Active PTH1R Variants.

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.

Altered Signaling and Desensitization Responses in PTH1R Mutants Associated with Eiken Syndrome.

The parathyroid hormone receptor type 1 (PTH1R) is a G protein-coupled receptor that plays key roles in regulating calcium homeostasis and skeletal development via binding the ligands, PTH and PTH-related protein (PTHrP), respectively. Eiken syndrome is a rare disease of delayed bone mineralization caused by homozygous PTH1R mutations. Of the three mutations identified so far, R485X, truncates the PTH1R C-terminal tail, while E35K and Y134S alter residues in the receptor's amino-terminal extracellular domain. Here, using a variety of cell-based assays, we show that R485X increases the receptor's basal rate of cAMP signaling and decreases its capacity to recruit ß-arrestin2 upon ligand stimulation. The E35K and Y134S mutations each weaken the binding of PTHrP leading to impaired ß-arrestin2 recruitment and desensitization of cAMP signaling response to PTHrP but not PTH. Our findings support a critical role for interaction with ß-arrestin in the mechanism by which the PTH1R regulates bone formation.

Expanding the role of PSMA PET in active surveillance.

INTRODUCTION: Accurate grading at the time of diagnosis is fundamental to risk stratification and treatment decision making, particularly for men being considered for Active Surveillance (AS). With the introduction of prostate-specific membrane antigen (PSMA) positron emission tomography (PET) there has been considerable improvement in sensitivity and specificity for the detection and staging of clinically significant prostate cancer. Our study aims to determine the role of PSMA PET/CT in men with newly diagnosed low or favourable intermediate risk prostate cancer to better select men for AS. METHOD: This is a retrospective single centre study performed from January 2019 and October 2022. This study includes men identified from electronic medical record system who had undergone a PSMA PET/CT following newly diagnosed low or favourable-intermediate risk prostate cancer. Primary outcome was to assess the change in management for men being considered for AS following PSMA PET/CT results on the basis of PSMA PET characteristics. RESULTS: In total, there were 11 of 30 men (36.67%) who were assigned management by AS and 19 of 30 men (63.33%) who had definitive treatment. 15 of the 19 men that needed treatment had concerning features on PSMA PET/CT results. Of the 15 men with concerning features on PSMA PET, 9 (60%) men were found to have adverse pathological features on final prostatectomy features. CONCLUSION: This retrospective study suggests that PSMA PET/CT has potential to influence the management of men with newly diagnosed prostate cancer that would otherwise be appropriate for active surveillance.

Multiparametric Magnetic Resonance Imaging of the Prostate and Prostate-specific Membrane Positron Emission Tomography Prior to Prostate Biopsy (MP4 Study).

Background: Prostate-specific membrane antigen (PSMA) positron emission tomography/computerised tomography (PET/CT) is increasingly being utilised in the diagnostic pathway for prostate cancer (PCa). Recent publications have suggested that this might help identify those who can avoid biopsy. Objective: The primary objective of this study was to determine whether PET magnetic resonance imaging (MRI) fusion could negate the need to biopsy prior to prostatectomy in a selected population of men. Design setting and participant: Multiparametric MRI (mpMRI) for PCa is our standard of care prior to prostate biopsy. Biopsy-naïve men with one or more Prostate Imaging Reporting and Data System (PI-RADS) 4 or 5 lesions ≥10 mm on mpMRI were invited to undergo PSMA PET/CT prior to biopsy. Following ethics approval, 60 men were recruited between September 2020 and March 2021. The key exclusion criteria included a previous history of PCa and previous prostate surgery or biopsy. Outcome measurements and statistical analysis: A positive PET MRI fusion scan was defined as "consistent with" as per the Memorial Sloan Kettering Cancer Center lexicon of certainty, and concordance with biopsy results was analysed. Clinically significant PCa (csPCa) was defined as grade group (GG) ≥2 on pathology. A chi-square analysis was performed with statistical significance defined at p < 0.05. Results and limitations: A total of 71 mpMRI lesions were positive on 61 (86%) PET MRI fusion scans. Fifty-nine of 61 lesions biopsied confirmed csPCa in 54 (92%). Of five of 59 lesions for which either biopsy was negative or low-grade cancer was found, three had rebiopsy of which two were confirmed to have csPCa corroborating with PET MRI fusion and one was reconfirmed to have GG1 only. For the remaining two, both had another lesion elsewhere in the gland confirming csPCa, and hence rebiopsy was not performed. Ultimately, 56 of 59 (95%) lesions with a positive PET MRI fusion scan were confirmed to have csPCa. All GG ≥3 cancers had a positive PET MRI fusion scan. Conclusions: This prospective study of PET MRI fusion assessment of men with PI-RADS 4 or 5 lesion ≥10 mm on mpMRI confirms that the majority of men (95%) with a positive PET MRI fusion scan will have csPCa. This supports recently published retrospective data suggesting that selected men might avoid prostate biopsy prior to radical prostatectomy. Patient summary: In this research, we have confirmed that prostate-specific membrane antigen positron emission tomography/computerised tomography in combination with magnetic resonance imaging could have an important role in enabling a diagnosis of prostate cancer. Using the combination of these scans, we could confidently predict the presence of aggressive prostate cancer in some men for which treatment is warranted. This means that there are some men who could possibility proceed directly to having prostate cancer surgery without the need for a confirmatory prostate biopsy.

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.

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.

An Inverse Agonist Ligand of the PTH Receptor Partially Rescues Skeletal Defects in a Mouse Model of Jansen's Metaphyseal Chondrodysplasia.

Jansen's metaphyseal chondrodysplasia (JMC) is a rare disease of bone and mineral ion physiology that is caused by activating mutations in PTHR1. Ligand-independent signaling by the mutant receptors in cells of bone and kidney results in abnormal skeletal growth, excessive bone turnover, and chronic hypercalcemia and hyperphosphaturia. Clinical features further include short stature, limb deformities, nephrocalcinosis, and progressive losses in kidney function. There is no effective treatment option available for JMC. In previous cell-based assays, we found that certain N-terminally truncated PTH and PTHrP antagonist peptides function as inverse agonists and thus can reduce the high rates of basal cAMP signaling exhibited by the mutant PTHR1s of JMC in vitro. Here we explored whether one such inverse agonist ligand, [Leu11 ,dTrp12 ,Trp23 ,Tyr36 ]-PTHrP(7-36)NH2 (IA), can be effective in vivo and thus ameliorate the skeletal abnormalities that occur in transgenic mice expressing the PTHR1-H223R allele of JMC in osteoblastic cells via the collagen-1α1 promoter (C1HR mice). We observed that after 2 weeks of twice-daily injection and relative to vehicle controls, the IA analog resulted in significant improvements in key skeletal parameters that characterize the C1HR mice, because it reduced the excess trabecular bone mass, bone marrow fibrosis, and levels of bone turnover markers in blood and urine. The overall findings provide proof-of-concept support for the notion that inverse agonist ligands targeted to the mutant PTHR1 variants of JMC can have efficacy in vivo. Further studies of such PTHR1 ligand analogs could help open paths toward the first treatment option for this debilitating skeletal disorder. © 2019 American Society for Bone and Mineral Research.

Isolated late testicular relapse of B-cell acute lymphoblastic leukemia treated with intensive systemic chemotherapy and response-based testicular radiation: A Children's Oncology Group study.

BACKGROUND: The incidence of isolated testicular relapse (ITR) of acute lymphoblastic leukemia (ALL) has decreased with contemporary treatment strategies, but outcomes are suboptimal with a 58% 5-year overall survival (OS). This study aimed to improve outcome in patients with ITR of B-cell ALL (B-ALL) occurring after 18 months of first clinical remission using intensive systemic chemotherapy and to decrease long-term sequelae by limiting use of testicular radiation. PROCEDURE: Forty patients in first ITR of B-ALL were enrolled. Induction (dexamethasone, vincristine, daunorubicin, and intrathecal triple therapy) was preceded by one dose of high-dose methotrexate (MTX, 5 g/m2 ). Following induction, 25 of 26 patients who had persistent testicular enlargement underwent testicular biopsy. Eleven had biopsy-proven disease and received bilateral testicular radiation (24 Gy), whereas twenty-nine did not. RESULTS: Overall 5-year event-free survival (EFS)/OS was 65.0 ± 8.8%/73.1 ± 8.3%, with 5-year EFS 62.1 ± 11.0% vs. 72.7 ± 14.4% for patients who did not receive radiation therapy (XRT) (n = 29) compared with those who did (n = 11), respectively (P = 0.64). There were six second bone marrow relapses and six second ITRs. The proportion of second relapses was similar in the patients that received testicular radiation and those who did not. However, the 5-year OS was similar for patients who did not receive XRT (72.6 ± 10.2%) compared with those who did (72.7 ± 14.4%) (P = 0.85). CONCLUSIONS: A 5-year OS rate of 73.1 ± 8.3% was obtained in children with first ITR of B-ALL occurring after 18 months of CR1 (length of first clinical remission) using intensive chemotherapy and limiting testicular radiation.

Binding Selectivity of Abaloparatide for PTH-Type-1-Receptor Conformations and Effects on Downstream Signaling.

The PTH receptor type 1 (PTHR1) mediates the actions of two endogenous polypeptide ligands, PTH and PTHrP, and thereby plays key roles in bone biology. Based on its capacity to stimulate bone formation, the peptide fragment PTH (1-34) is currently in use as therapy for osteoporosis. Abaloparatide (ABL) is a novel synthetic analog of human PTHrP (1-34) that holds promise as a new osteoporosis therapy, as studies in animals suggest that it can stimulate bone formation with less of the accompanying bone resorption and hypercalcemic effects that can occur with PTH (1-34). Recent studies in vitro suggest that certain PTH or PTHrP ligand analogs can distinguish between two high-affinity PTHR1 conformations, R(0) and RG, and that efficient binding to R(0) results in prolonged signaling responses in cells and prolonged calcemic responses in animals, whereas selective binding to RG results in more transient responses. As intermittent PTH ligand action is known to favor the bone-formation response, whereas continuous ligand action favors the net bone-resorption/calcemic response, we hypothesized that ABL binds more selectively to the RG vs the R(0) PTHR1 conformation than does PTH (1-34), and thus induces more transient signaling responses in cells. We show that ABL indeed binds with greater selectivity to the RG conformation than does PTH (1-34), and as a result of this RG bias, ABL mediates more transient cAMP responses in PTHR1-expressing cells. The findings provide a plausible mechanism (ie, transient signaling via RG-selective binding) that can help account for the favorable anabolic effects that ABL has on bone.

A Homozygous [Cys25]PTH(1-84) Mutation That Impairs PTH/PTHrP Receptor Activation Defines a Novel Form of Hypoparathyroidism.

Hypocalcemia and hyperphosphatemia are encountered in idiopathic hypoparathyroidism (IHP) and pseudohypoparathyroidism type Ib (PHP1B). In contrast to PHP1B, which is caused by resistance toward parathyroid hormone (PTH), the genetic defects leading to IHP impair production of this important regulator of mineral ion homeostasis. So far, only five PTH mutations were shown to cause IHP, each of which is located in the hormone's pre-pro leader segment and thus impair hormone secretion. In three siblings affected by IHP, we now identified a homozygous arginine-to-cysteine mutation at position 25 (R25C) of the mature PTH(1-84) polypeptide; heterozygous family members are healthy. Depending on the assay used for evaluating these patients, plasma PTH levels were either low or profoundly elevated, thus leading to ambiguities regarding the underlying diagnosis, namely IHP or PHP1B. Consistent with increased PTH levels, recombinant [Cys25]PTH(1-84) and wild-type PTH(1-84) were secreted equally well by transfected COS-7 cells. However, synthetic [Cys25]PTH(1-34) was found to have a lower binding affinity for the PTH receptor type-1 (PTH1R) than PTH(1-34) and consequently a lower efficiency for stimulating cAMP formation in cells expressing this receptor. Consistent with these in vitro findings, long-term infusion of [Cys25]PTH(1-34) resulted only in minimal calcemic and phosphaturic responses, despite readily detectable levels of [Cys25]PTH(1-34) in plasma. The mineral ion abnormalities observed in the three IHP patients are thus most likely caused by the inherited homozygous missense PTH mutation, which reduces bioactivity of the secreted hormone. Based on these findings, screening for PTH(1-84) mutations should be considered when clinical and laboratory findings are consistent with PHP1B, but GNAS methylation changes have been excluded. Differentiating between IHP and PHP1B has considerable implications for genetic counseling, therapy, and long-term outcome because treatment of IHP patients with inappropriately high doses of active vitamin D and calcium can contribute to development of nephrocalcinosis and chronic kidney disease.

Actions of the small molecule ligands SW106 and AH-3960 on the type-1 parathyroid hormone receptor.

The parathyroid hormone receptor-1 (PTHR1) plays critical roles in regulating blood calcium levels and bone metabolism and is thus of interest for small-molecule ligand development. Of the few small-molecule ligands reported for the PTHR1, most are of low affinity, and none has a well-defined mechanism of action. Here, we show that SW106 and AH-3960, compounds previously identified to act as an antagonist and agonist, respectively, on the PTHR1, each bind to PTHR1-delNT, a PTHR1 construct that lacks the large amino-terminal extracellular domain used for binding endogenous PTH peptide ligands, with the same micromolar affinity with which it binds to the intact PTHR1. SW106 antagonized PTHR1-mediated cAMP signaling induced by the peptide analog, M-PTH(1-11), as well as by the native PTH(1-9) sequence, as tethered to the extracellular end of transmembrane domain (TMD) helix-1 of the receptor. SW106, however, did not function as an inverse agonist on either PTHR1-H223R or PTHR1-T410P, which have activating mutations at the cytoplasmic ends of TMD helices 2 and 6, respectively. The overall data indicate that SW106 and AH-3960 each bind to the PTHR1 TMD region and likely to within an extracellularly exposed area that is occupied by the N-terminal residues of PTH peptides. Additionally, they suggest that the inhibitory effects of SW106 are limited to the extracellular portions of the TMD region that mediate interactions with agonist ligands but do not extend to receptor-activation determinants situated more deeply in the helical bundle. The study helps to elucidate potential mechanisms of small-molecule binding at the PTHR1.

Backbone modification of a polypeptide drug alters duration of action in vivo.

Systematic modification of the backbone of bioactive polypeptides through ß-amino acid residue incorporation could provide a strategy for generating molecules with improved drug properties, but such alterations can result in lower receptor affinity and potency. Using an agonist of parathyroid hormone receptor-1 (PTHR1), a G protein-coupled receptor in the B-family, we present an approach for αâ†’ß residue replacement that enables both high activity and improved pharmacokinetic properties in vivo.

The Seattle Spine Team Approach to Adult Deformity Surgery: A Systems-Based Approach to Perioperative Care and Subsequent Reduction in Perioperative Complication Rates.

STUDY DESIGN: Retrospective consecutive case review pre- and postintervention. OBJECTIVES: Characterize the effects of the intervention. SUMMARY OF BACKGROUND DATA: Complication rates in adult spinal deformity surgery are unacceptable. System approaches are necessary to increase patient safety. This group reported on the dual-attending surgeon approach, a live multidisciplinary preoperative screening conference, and the intraoperative protocol for the management of coagulopathy. The outcomes were demonstrated by complication rates before and after the institution of this protocol. METHODS: Forty consecutive patients in Group A were managed without the 3-pronged approach. A total of 124 consecutive patients in Group B had a dual-attending surgeon approach, were presented and cleared by a live multidisciplinary preoperative conference, and were managed according to the intraoperative protocol. RESULTS: Group A had an average age of 62 years (range, 39-84 years). Group B had an average age of 64 years (range, 18-84 years). Most patients in both groups had fusions from 9 to 15 levels. Complication rates in Group B were significantly lower (16% vs. 52%) (p < .001). Group B showed significantly lower return rates to the operating room during the perioperative 90-day period (0.8% vs. 12.5%) (p < .001). Group B also had lower rates of wound infection requiring debridement (1.6% vs. 7.5%), lower rates of deep vein thrombosis/pulmonary embolism (3.2% vs. 10%), and lower rates of postoperative neurological complications (0.5% vs. 2.5%) (not significant). Group B had significantly lower rates of urinary tract infection requiring antibiotics (9.7% vs. 32.5%) (p < .001). CONCLUSIONS: These data suggests that a team approach consisting of a dual-attending surgeon approach in the operating room, a live preoperative screening conference, and an intraoperative protocol for managing coagulopathy will significantly reduce perioperative complication rates and enhance patient safety in patients undergoing complex spinal reconstructions for adult spinal deformity.

Critical role of parathyroid hormone (PTH) receptor-1 phosphorylation in regulating acute responses to PTH.

Agonist-induced phosphorylation of the parathyroid hormone (PTH) receptor 1 (PTHR1) regulates receptor signaling in vitro, but the role of this phosphorylation in vivo is uncertain. We investigated this role by injecting "knock-in" mice expressing a phosphorylation-deficient (PD) PTHR1 with PTH ligands and assessing acute biologic responses. Following injection with PTH (1-34), or with a unique, long-acting PTH analog, PD mice, compared with WT mice, exhibited enhanced increases in cAMP levels in the blood, as well as enhanced cAMP production and gene expression responses in bone and kidney tissue. Surprisingly, however, the hallmark hypercalcemic and hypophosphatemic responses were markedly absent in the PD mice, such that paradoxical hypocalcemic and hyperphosphatemic responses were observed, quite strikingly with the long-acting PTH analog. Spot urine analyses revealed a marked defect in the capacity of the PD mice to excrete phosphate, as well as cAMP, into the urine in response to PTH injection. This defect in renal excretion was associated with a severe, PTH-induced impairment in glomerular filtration, as assessed by the rate of FITC-inulin clearance from the blood, which, in turn, was explainable by an overly exuberant systemic hypotensive response. The overall findings demonstrate the importance in vivo of PTH-induced phosphorylation of the PTHR1 in regulating acute ligand responses, and they serve to focus attention on mechanisms that underlie the acute calcemic response to PTH and factors, such as blood phosphate levels, that influence it.

Engineering of therapeutic polypeptides through chemical synthesis: early lessons from human parathyroid hormone and analogues.

Application of chemical synthesis to gain access to high purity hPTH as well as more stable analogues was accomplished through a menu of extended NCL followed by metal free dethiylation.

Massive per rectal bleeding following blunt abdominal trauma: First presentation of Crohn's disease.

INTRODUCTION: Inflammatory bowel disease is a chronic and relatively common disorder with heterogeneous presentation. Peak incidence occurs in the second and third decades of life. We present a patient with Crohn's disease whose first presentation was profuse bleeding/rectum following blunt abdominal trauma. PRESENTATION OF CASE: A 29 year old previously healthy man presented one hour after sustaining relatively mild abdominal trauma, due to fall onto the ball during a rugby match. He complained of abdominal pain and one episode of large fresh rectal bleeding. He was pale and distressed with hypotension, tachycardia and abdominal guarding & fresh blood on digital rectal examination. With a provisional diagnosis of intestinal injury he was taken to theatre. Right hemi-colectomy was done for a thickened and inflamed segment of distal ileum, a large adjacent mesenteric haematoma & mesenteric lymph nodes and blood in distal bowel. Histology confirmed the features of Crohn's disease. DISCUSSION: Crohn's disease is unusual cause of massive lower gastrointestinal bleeding occurring in 0.9-6% of patients. Rectal bleeding associated with diarrhoea is relatively more common than massive bleeding. The presence of Crohn's disease in young patients presenting like this is unlikely to be suspected and diagnosis could only be made after laparotomy.

Acute down-regulation of sodium-dependent phosphate transporter NPT2a involves predominantly the cAMP/PKA pathway as revealed by signaling-selective parathyroid hormone analogs.

The parathyroid hormone (PTH)/PTH-related peptide (PTHrP) receptor (PTHR1) in cells of the renal proximal tubule mediates the reduction in membrane expression of the sodium-dependent P(i) co-transporters, NPT2a and NPT2c, and thus suppresses the re-uptake of P(i) from the filtrate. In most cell types, the liganded PTHR1 activates Gα(S)/adenylyl cyclase/cAMP/PKA (cAMP/PKA) and Gα(q/11)/phospholipase C/phosphatidylinositol 1,4,5-trisphosphate (IP(3))/Ca(2+)/PKC (IP(3)/PKC) signaling pathways, but the relative roles of each pathway in mediating renal regulation P(i) transport remain uncertain. We therefore explored the signaling mechanisms involved in PTH-dependent regulation of NPT2a function using potent, long-acting PTH analogs, M-PTH(1-28) (where M = Ala(1,12), Aib(3), Gln(10), Har(11), Trp(14), and Arg(19)) and its position 1-modified variant, Trp(1)-M-PTH(1-28), designed to be phospholipase C-deficient. In cell-based assays, both M-PTH(1-28) and Trp(1)-M-PTH(1-28) exhibited potent and prolonged cAMP responses, whereas only M-PTH(1-28) was effective in inducing IP(3) and intracellular calcium responses. In opossum kidney cells, a clonal cell line in which the PTHR1 and NPT2a are endogenously expressed, M-PTH(1-28) and Trp(1)-M-PTH(1-28) each induced reductions in (32)P uptake, and these responses persisted for more than 24 h after ligand wash-out, whereas that of PTH(1-34) was terminated by 4 h. When injected into wild-type mice, both M-modified PTH analogs induced prolonged reductions in blood P(i) levels and commensurate reductions in NPT2a expression in the renal brush border membrane. Our findings suggest that the acute down-regulation of NPT2a expression by PTH ligands involves mainly the cAMP/PKA signaling pathway and are thus consistent with the elevated blood P(i) levels seen in pseudohypoparathyroid patients, in whom Gα(s)-mediated signaling in renal proximal tubule cells is defective.

Altered selectivity of parathyroid hormone (PTH) and PTH-related protein (PTHrP) for distinct conformations of the PTH/PTHrP receptor.

PTH and PTHrP use the same G protein-coupled receptor, the PTH/PTHrP receptor (PTHR), to mediate their distinct biological actions. The extent to which the mechanisms by which the two ligands bind to the PTHR differ is unclear. We examined this question using several pharmacological and biophysical approaches. Kinetic dissociation and equilibrium binding assays revealed that the binding of [(125)I]PTHrP(1-36) to the PTHR was more sensitive to GTPgammaS (added to functionally uncouple PTHR-G protein complexes) than was the binding of [(125)I]PTH(1-34) ( approximately 75% maximal inhibition vs. approximately 20%). Fluorescence resonance energy transfer-based kinetic analyses revealed that PTHrP(1-36) bound to the PTHR more slowly and dissociated from it more rapidly than did PTH(1-34). The cAMP signaling response capacity of PTHrP(1-36) in cells decayed more rapidly than did that of PTH(1-34) (t(1/2) = approximately 1 vs. approximately 2 h). Divergent residue 5 in the ligand, Ile in PTH and His in PTHrP, was identified as a key determinant of the altered receptor-interaction responses exhibited by the two peptides. We conclude that whereas PTH and PTHrP bind similarly to the G protein-coupled PTHR conformation (RG), PTH has a greater capacity to bind to the G protein-uncoupled conformation (R(0)) and, hence, can produce cumulatively greater signaling responses (via R(0)-->RG isomerization) than can PTHrP. Such conformational selectivity may relate to the distinct modes by which PTH and PTHrP act biologically, endocrine vs. paracrine, and may help explain reported differences in the effects that the ligands have on calcium and bone metabolism when administered to humans.

Novel parathyroid hormone (PTH) antagonists that bind to the juxtamembrane portion of the PTH/PTH-related protein receptor.

Current antagonists for the parathyroid hormone (PTH)/PTH-related protein (PTHrP) receptor (PTHR) are N-terminally truncated or N-terminally modified analogs of PTH(1-34) or PTHrP(1-34) and are thought to bind predominantly to the N-terminal extracellular (N) domain of the receptor. We hypothesized that ligands that bind only to PTHR region comprised of the extracellular loops and seven transmembrane helices (the juxtamembrane or J domain) could also antagonize the PTHR. To test this, we started with the J domain-selective agonists [Gln(10),Ala(12),Har(11),Trp(14),Arg(19) (M)]PTH(1-21), [M]PTH(1-15), and [M]PTH(1-14), and introduced substitutions at positions 1-3 that were predicted to dissociate PTHR binding and cAMP signaling activities. Strong dissociation was observed with the tri-residue sequence diethylglycine (Deg)(1)-para-benzoyl-l-phenylalanine (Bpa)(2)-Deg(3). In HKRK-B7 cells, which express the cloned human PTHR, [Deg(1,3),Bpa(2),M]PTH(1-21), [Deg(1,3),Bpa(2),M]PTH(1-15), and [Deg(1,3),Bpa(2),M]PTH(1-14) fully inhibited (IC(50)s = 100-700 nm) the binding of (125)I-[alpha-aminoisobutyric acid(1,3),M]PTH(1-15) and were severely defective for stimulating cAMP accumulation. In ROS 17/2.8 cells, which express the native rat PTHR, [Deg(1,3),Bpa(2),M]PTH(1-21) and [Deg(1,3),Bpa(2),M]PTH(1-15) antagonized the cAMP-agonist action of PTH(1-34), as did PTHrP(5-36) (IC(50)s = 0.7 microm, 2.6 microm, and 36 nm, respectively). In COS-7 cells expressing PTHR-delNt, which lacks the N domain of the receptor, [Deg(1,3),Bpa(2), M]PTH(1-21) and [Deg(1,3),Bpa(2),M]PTH(1-15) inhibited the agonist actions of [alpha-aminoisobutyric acid(1,3)]PTH(1-34) and [M]PTH(1-14) (IC(50)s approximately 1 microm), whereas PTHrP(5-36) failed to inhibit. [Deg(1,3),Bpa(2),M]PTH(1-14) inhibited the constitutive cAMP-signaling activity of PTHR-tether-PTH(1-9), in which the PTH(1-9) sequence is covalently linked to the PTHR J domain, as well as that of PTHR(cam)H223R. Thus, the J-domain-selective N-terminal PTH fragment analogs can function as antagonists as well as inverse agonists for the PTHR. The new ligands described should be useful for further studies of the ligand binding and activation mechanisms that operate in the critical PTHR J domain.