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John T Potts

Publications and source records attributed to John T Potts.

10 recordsLinked to original sources
PubMed2005

Mechanisms of ligand binding to the parathyroid hormone (PTH)/PTH-related protein receptor: selectivity of a modified PTH(1-15) radioligand for GalphaS-coupled receptor conformations.

Mechanisms of ligand binding to the PTH/PTHrP receptor (PTHR) were explored using PTH fragment analogs as radioligands in binding assays. In particular, the modified amino-terminal fragment analog, (125)I-[Aib(1,3),Nle8,Gln10,homoarginine11,Ala12,Trp14,Tyr15]rPTH(1-15)NH2, (125)I-[Aib(1,3),M]PTH(1-15), was used as a radioligand that we hypothesized to bind solely to the juxtamembrane (J) portion of the PTHR containing the extracellular loops and transmembrane helices. We also employed (125)I-PTH(1-34) as a radioligand that binds to both the amino-terminal extracellular (N) and J domains of the PTHR. Binding was examined in membranes derived from cells expressing either wild-type or mutant PTHRs. We found that the binding of (125)I-[Aib(1,3),M]PTH(1-15) to the wild-type PTHR was strongly (approximately 90%) inhibited by guanosine 5'-O-(3-thio)triphosphate (GTPgammaS), whereas the binding of (125)I-PTH(1-34) was only mildly (approximately 25%) inhibited by GTPgammaS. Of these two radioligands, only (125)I-[Aib(1,3),M]PTH(1-15) bound to PTHR-delNt, which lacks most of the receptor's N domain, and again this binding was strongly inhibited by GTPgammaS. Binding of (125)I-[Aib(1,3),M]PTH(1-15) to the constitutively active receptor, PTHR-H223R, was only mildly (approximately 20%) inhibited by GTPgammaS, as was the binding of (125)I-PTH(1-34). In membranes prepared from cells lacking Galpha(S) via knockout mutation of Gnas, no binding of (125)I-[Aib(1,3),M]PTH(1-15) was observed, but binding of (125)I-[Aib(1,3),M]PTH(1-15) was recovered by virally transducing the cells to heterologously express Galpha(S). (125)I-PTH(1-34) bound to the membranes with or without Galpha(S). The overall findings confirm the hypothesis that (125)I-[Aib(1,3),M]PTH(1-15) binds solely to the J domain of the PTHR. They further show that this binding is strongly dependent on coupling of the receptor to Galpha(S)-containing heterotrimeric G proteins, whereas the binding of (125)I-PTH(1-34) can occur in the absence of such coupling. Thus, (125)I-[Aib(1,3),M]PTH(1-15) appears to function as a selective probe of Galpha(S)-coupled, active-state PTHR conformations.

Animals↗
PubMed2005

Parathyroid hormone: past and present.

Research on parathyroid hormone (PTH) has undergone four rather distinctive phases, beginning just before the turn of the 20th century. Early debates about the function of the parathyroids were resolved by 1925, when understanding the role of PTH led to comprehending the action of the glands in calcium physiology. Elucidation of the pathophysiology of hormone excess (severe bone loss) and deficiency (hypocalcemia) continued over the following decades. With the advent of advances in chemical and molecular biology, the structure of PTH and its principal receptor (PTHrP-receptor [PTHR1]) were established. Tests with purified hormonal peptide in humans led to the surprising, even paradoxical, finding that PTH can be used pharmacologically to build bone, providing a dramatic therapeutic impact on osteoporosis. These developments have stimulated the field of calcium and bone biology and posed new questions about the role of PTH as well as possible new directions in therapy.

Amino Acid Sequence↗
PubMed2004

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.

Animals↗
PubMed2002

Residue 19 of the parathyroid hormone (PTH) modulates ligand interaction with the juxtamembrane region of the PTH-1 receptor.

Recent data suggest that the binding of parathyroid hormone (PTH)-(1-34) to the PTH-1 receptor (P1R) involves a high-affinity interaction between the C-terminal (15-34) domain of the ligand and the amino-terminal extracellular (N) domain of the receptor and a low-affinity interaction between the N-terminal (1-14) portion of PTH and the juxtamembrane (J) region of the receptor, with the latter interaction giving rise to signal transduction. We investigated whether residues C-terminal of position 14 in PTH(1-34) contribute to the J component of the interaction mechanism by comparing the capacity of PTH analogues N-terminally modified to improve J domain affinity and C-terminally truncated at position 14, 20, or 34 to stimulate cAMP formation in COS-7 cells transiently transfected with P1R-delNt, a P1R construct that lacks most of the N domain. In these cells, the potency of [M]PTH(1-34) (M = Ala(1,3,12),Gln(10),Har(11),Trp(14),Arg(19)) was 120-fold greater than that of [M]PTH(1-14) (EC(50)s = 3.0 +/- 0.8 and 360 +/- 90 nM, respectively) but was equal to that of [M]PTH(1-20) (EC(50) = 2.3 +/- 0.3 nM). Reverting the Arg(19) substitution of [M]PTH(1-20) to the native Glu reduced cAMP signaling potency on P1R-delNt by 12-fold (EC(50) of [M]PTH(1-20)-Glu(19) = 27 +/- 4 nM), and it decreased the analog's capacity to inhibit the binding of the J domain-selective radioligand, (125)I-[Aib(1,3),Nle(8),M,Tyr(21)]ratPTH(1-21), to the full-length P1R stably expressed in LLC-PK1 cells by 40-fold. The Glu(19) --> Arg modification, however, did not affect the capacity of PTH(15-31) to inhibit the binding of the N domain-selective radioligand (125)I-bPTH(3-34) to the full-length receptor. The overall data suggest that residues (15-20) of PTH, and particularly residue 19, contribute to the capacity of the N-terminal portion of the ligand to interact with the juxtamembrane region of the receptor. The NMR data presented in the accompanying manuscript suggests that this role could involve intramolecular effects on secondary structure in the N-terminal portion of the ligand.

Amino Acid Substitution↗
PubMed2002

Asymptomatic primary hyperparathyroidism: new issues and new questions--bridging the past with the future.

The earliest clinical descriptions of PHPT pointed to an inexorably progressive disorder. Now, in many asymptomatic individuals who do not meet any surgical guidelines. PHPT in general, does not seem to be progressive. Most asymptomatic patients seem to remain asymptomatic over many years of observation. In those with major clinical manifestations, nephrolithiasis is by far the most common. In these patients, surgery is clearly indicated. Bone densitometry is an indispensable component of the evaluation as well as in monitoring because there is evidence of bone involvement in most patients. This contrasts with the skeletal X-ray, which is invariably negative. Nevertheless, patients with PHPT can show a bone mass measurement either at the cortical or cancellous skeleton that is more than 2 SDs below age- and sex-matched control subjects. Bone density, serum calcium concentration, and/or urinary calcium excretion per se can show evidence for progression in as many as 25% of patients with asymptomatic PHPT. There are newer pharmacologic approaches to PHPT that are the subject of intense investigation. The bisphosphonates and the calcimimetics show particular promise in this regard. Unanswered are such questions as whether the lower bone density in the milder patients will increase substantially as in those who undergo successful parathyroidectomy and whether fracture risk would therefore be reduced. Can medical therapy reduce serum calcium. parathyroid hormone levels, and other biochemical indices of PHPT? Can medical therapy improve bone density without the need for surgery. The issues outlined in this presentation and further amplified in other presentations given in this workshop lead to a series of questions that, in turn, may lead to modified guidelines for the management of these patients. The questions are as follows. (1) Should there be any changes in diagnostic criteria for PHPT? (2) Should the guidelines for surgery in PHPT be changed in light of new data over the past decade? What should those guidelines be? (3) At present, is there sufficient evidence of clinical benefit with specific medical therapies to recommend their use? In which patients? (4) Can some patients be followed without surgery? If so, how should they be monitored? (5) When surgery is the preferred option, what are the relative merits of minimally invasive procedures compared with more conventional surgery? (6) What is the role of localization techniques in identifying abnormal parathyroid tissue preoperatively and intraoperatively? Are they sufficiently helpful and cost effective to warrant their use in all patients who undergo parathyroid surgery? If not, in what patients should they be recommended? (7) What items should be placed on the research agenda for PHPT over the next decade?

Bone Density↗
PubMed2002

Immunoassays for the detection of parathyroid hormone.

Biologically active parathyroid hormone (PTH) in humans with normal renal function circulates predominantly as an 84 amino acid peptide. PTH fragments of varying length arise either from metabolism of the intact hormone within the parathyroid glands or in peripheral tissues, such as liver, and the resulting carboxyl-terminal peptides are eliminated mainly by glomerular filtration and subsequent tubular degradation. Most of the initially raised anti-PTH antisera were directed against epitopes within the mid- or carboxyl-terminal regions of the hormone. These antibodies were used for the development of conventional, displacement-type radioimmunoassays, but provided only an index of the biologically active PTH(1-84) in the circulation. Subsequently developed immunometric assays use two distinct antibodies, a capture antibody usually directed against a carboxyl-terminal portion of PTH(1-84) and a radio- or enzyme-labeled detection antibody usually directed against the amino-terminal portion of the hormone. Such assays were thought to detect largely, if not exclusively, intact PTH, thus providing the concentration of biologically active hormone in blood, which is especially important for establishing the diagnosis of hyperparathyroidism. However, serum samples from normal subjects and patients with primary or secondary hyperparathyroidism have demonstrated that most immunometric two-site sandwich assays detect, besides PTH(1-84), one or more recently discovered large carboxyl-terminal PTH fragments that lack a portion of the amino-terminal end of the molecule. Some of these amino-terminally truncated PTH molecules [ntPTH(1-84)] exhibit an elution profile on high performance liquid chromatography (HPLC) that is indistinguishable from that of synthetic PTH(784). Such peptides were previously thought to be of minimal if any biological activity, but recent studies have shown that synthetic PTH(7-84) has hypocalcemic properties in vivo and that it inhibits osteoclastic bone resorption and the formation of mature osteoclasts in vitro. It is currently unclear whether important differences in disease states can be revealed by comparing results obtained with older immunometric assays that measure the full-length hormone and ntPTH(1-84) versus newer assays that measure only PTH(1-84). Therefore, whereas most immunometric PTH assay systems are appropriate for the diagnosis of primary hyperparathyroidism, it is possible that immunometric assays designed to detect only PTH(1-84) will be more useful in certain diagnostic studies, for intraoperative PTH monitoring and for assessing the pulsatility of PTH secretion. In addition, the ability to distinguish between the relative concentrations of ntPTH(1-84) versus PTH(1-84) may reveal previously unsuspected roles for the ntPTH(1-84) fragments in the pathophysiology of patients with end-stage renal disease and/or other disorders involving parathyroid hormone.

Chromatography, High Pressure Liquid↗
PubMed1998

The Institute of Medicine's report on non-heart-beating organ transplantation.

In December 1997, the Institute of Medicine (IOM) released a report on medical and ethical issues in the procurement of non-heart-beating organ donors. This report had been requested in May 1997 by the Department of Health and Human Services (DHHS). We will here describe the genesis of the IOM report, the medical and moral concerns that led the DHHS to sponsor it, the process of producing it, and its conclusions. The analyses, findings, and recommendations of the report are also reviewed, in particular the central issues that led to suggestions for policy changes.

Cadaver↗