PTH and stem cells.

At least 2 different types of cells, hematopoietic and mesenchymal, are present in the adult bone marrow, in addition to endothelial cells. Hematopoietic and mesenchymal cells are believed to originate from hematopoietic stem cells (HSC) and mesenchymal stem cells (MSC), respectively. The bone marrow stroma, a cellular microenvironment that supports HSC, is composed of non-hematopoietic cells and contains MSC. A unique expansion of the bone marrow stroma, also known as marrow fibrosis, is the hallmark of a variety of disorders including hyperparathyroidism and fibrous dysplasia. PTH is the first bone anabolic agent approved by US Food and Drug Administration for the treatment of osteoporosis. Recent studies have suggested that PTH treatment may affect the number of hematopoietic stem cells in the bone marrow and their mobilization into the bloodstream. In addition, cells with classical features of mesenchymal stem cells/progenitors have been shown to express receptors for PTH, and to increase in number and undergo redistribution in the adult bone marrow upon PTH treatment. In this review, we will summarize the up-to-date knowledge on PTH and its relation to stem cells. We will also discuss the contribution of different cell types to the development of marrow fibrosis and the involvement of PTH signaling in this pathology.

Osteoblastic cells regulate the haematopoietic stem cell niche.

Stem cell fate is influenced by specialized microenvironments that remain poorly defined in mammals. To explore the possibility that haematopoietic stem cells derive regulatory information from bone, accounting for the localization of haematopoiesis in bone marrow, we assessed mice that were genetically altered to produce osteoblast-specific, activated PTH/PTHrP receptors (PPRs). Here we show that PPR-stimulated osteoblastic cells that are increased in number produce high levels of the Notch ligand jagged 1 and support an increase in the number of haematopoietic stem cells with evidence of Notch1 activation in vivo. Furthermore, ligand-dependent activation of PPR with parathyroid hormone (PTH) increased the number of osteoblasts in stromal cultures, and augmented ex vivo primitive haematopoietic cell growth that was abrogated by gamma-secretase inhibition of Notch activation. An increase in the number of stem cells was observed in wild-type animals after PTH injection, and survival after bone marrow transplantation was markedly improved. Therefore, osteoblastic cells are a regulatory component of the haematopoietic stem cell niche in vivo that influences stem cell function through Notch activation. Niche constituent cells or signalling pathways provide pharmacological targets with therapeutic potential for stem-cell-based therapies.

A constitutively active mutant PTH-PTHrP receptor in Jansen-type metaphyseal chondrodysplasia.

A single heterozygous nucleotide exchange in exon M2 of the gene encoding the parathyroid hormone-parathyroid hormone-related peptide (PTH-PTHrP) receptor was identified in a patient with Jansen-type metaphyseal chondrodysplasia, which changes a strictly conserved histidine residue at position 223 in the receptor's first intracellular loop to arginine. Constitutive, ligand-independent adenosine 3',5'-monophosphate accumulation was observed in COS-7 cells expressing the mutant PTH-PTHrP receptor but not in cells expressing the wild-type receptor. This finding explains the severe ligand-independent hypercalcemia and hypophosphatemia, and most likely the abnormal formation of endochondral bone, in this rare form of short-limbed dwarfism.

A G protein-linked receptor for parathyroid hormone and parathyroid hormone-related peptide.

The complementary DNA encoding a 585-amino acid parathyroid hormone-parathyroid hormone-related peptide (PTH-PTHrP) receptor with seven potential membrane-spanning domains was cloned by COS-7 expression using an opossum kidney cell complementary DNA (cDNA) library. The expressed receptor binds PTH and PTHrP with equal affinity, and both ligands equivalently stimulate adenylate cyclase. Striking homology with the calcitonin receptor and lack of homology with other G protein-linked receptors indicate that receptors for these calcium-regulating hormones are related and represent a new family.

Indian hedgehog couples chondrogenesis to osteogenesis in endochondral bone development.

Vertebrate skeletogenesis requires a well-coordinated transition from chondrogenesis to osteogenesis. Hypertrophic chondrocytes in the growth plate play a pivotal role in this transition. Parathyroid hormone-related peptide (PTHrP), synthesized in the periarticular growth plate, regulates the site at which hypertrophy occurs. By comparing PTH/PTHrP receptor(-/-)/wild-type (PPR(-/-)/wild-type) chimeric mice with IHH(-/-);PPR(-/-)/wild-type chimeric and IHH(-/-)/wild-type chimeric mice, we provide in vivo evidence that Indian hedgehog (IHH), synthesized by prehypertrophic and hypertrophic chondrocytes, regulates the site of hypertrophic differentiation by signaling to the periarticular growth plate and also determines the site of bone collar formation in the adjacent perichondrium. By providing crucial local signals from prehypertrophic and hypertrophic chondrocytes to both chondrocytes and preosteoblasts, IHH couples chondrogenesis to osteogenesis in endochondral bone development.

Activated parathyroid hormone/parathyroid hormone-related protein receptor in osteoblastic cells differentially affects cortical and trabecular bone.

Parathyroid hormone (PTH), an important regulator of calcium homeostasis, targets most of its complex actions in bone to cells of the osteoblast lineage. Furthermore, PTH is known to stimulate osteoclastogenesis indirectly through activation of osteoblastic cells. To assess the role of the PTH/PTH-related protein receptor (PPR) in mediating the diverse actions of PTH on bone in vivo, we generated mice that express, in cells of the osteoblastic lineage, one of the constitutively active receptors described in Jansen's metaphyseal chondrodysplasia. In these transgenic mice, osteoblastic function was increased in the trabecular and endosteal compartments, whereas it was decreased in the periosteum. In trabecular bone of the transgenic mice, there was an increase in osteoblast precursors, as well as in mature osteoblasts. Osteoblastic expression of the constitutively active PPR induced a dramatic increase in osteoclast number in both trabecular and compact bone in transgenic animals. The net effect of these actions was a substantial increase in trabecular bone volume and a decrease in cortical bone thickness of the long bones. These findings, for the first time to our knowledge, identify the PPR as a crucial mediator of both bone-forming and bone-resorbing actions of PTH, and they underline the complexity and heterogeneity of the osteoblast population and/or their regulatory microenvironment.

Primary hyperparathyroidism caused by parathyroid-targeted overexpression of cyclin D1 in transgenic mice.

The relationship between abnormal cell proliferation and aberrant control of hormonal secretion is a fundamental and poorly understood issue in endocrine cell neoplasia. Transgenic mice with parathyroid-targeted overexpression of the cyclin D1 oncogene, modeling a gene rearrangement found in human tumors, were created to determine whether a primary defect in this cell-cycle regulator can cause an abnormal relationship between serum calcium and parathyroid hormone response, as is typical of human primary hyperparathyroidism. We also sought to develop an animal model of hyperparathyroidism and to examine directly cyclin D1's role in parathyroid tumorigenesis. Parathyroid hormone gene regulatory region--cyclin D1 (PTH--cyclin D1) mice not only developed abnormal parathyroid cell proliferation, but also developed chronic biochemical hyperparathyroidism with characteristic abnormalities in bone and, notably, a shift in the relationship between serum calcium and PTH. Thus, this animal model of human primary hyperparathyroidism provides direct experimental evidence that overexpression of the cyclin D1 oncogene can drive excessive parathyroid cell proliferation and that this proliferative defect need not occur solely as a downstream consequence of a defect in parathyroid hormone secretory control by serum calcium, as had been hypothesized. Instead, primary deregulation of cell-growth pathways can cause both the hypercellularity and abnormal control of hormonal secretion that are almost inevitably linked together in this common disorder.

Constitutively active PTH/PTHrP receptor in odontoblasts alters odontoblast and ameloblast function and maturation.

Parathyroid hormone (PTH)-related protein (PTH-rP) is an important autocrine/paracrine attenuator of programmed cell differentiation whose expression is restricted to the epithelial layer in tooth development. The PTH/PTHrP receptor (PPR) mRNA in contrast is detected in the dental papilla, suggesting that PTHrP and the PPR may modulate epithelial-mesenchymal interactions. To explore the possible interactions, we studied the previously described transgenic mice in which a constitutively active PPR is targeted to osteoblastic cells. These transgenic mice have a vivid postnatal bone and tooth phenotype, with normal tooth eruption but abnormal, widened crowns. Transgene mRNA expression was first detected at birth in the dental papilla and, at 1 week postnatally, in odontoblasts. There was no transgene expression in ameloblasts or in other epithelial structures. Prenatally, transgenic molars and incisors revealed no remarkable change. By the age of 1 week, the dental papilla was widened, with disorganization of the odontoblastic layer and decreased dentin matrix. In addition, the number of cusps was abnormally increased, the ameloblastic layer disorganized, and enamel matrix decreased. Odontoblastic and, surprisingly, ameloblastic cytodifferentiation was impaired, as shown by in situ hybridization and electron microscopy. Interestingly, ameloblastic expression of Sonic Hedgehog, a major determinant of ameloblastic cytodifferentiation, was dramatically altered in the transgenic molars. These data suggest that odontoblastic activation of the PPR may play an important role in terminal odontoblastic and, indirectly, ameloblastic cytodifferentiation, and describe a useful model to study how this novel action of the PPR may modulate mesenchymal/epithelial interactions at later stages of tooth morphogenesis and development.

Constitutive activation of the cyclic adenosine 3',5'-monophosphate signaling pathway by parathyroid hormone (PTH)/PTH-related peptide receptors mutated at the two loci for Jansen's metaphyseal chondrodysplasia.

Two different activating PTH/PTH-related peptide (PTHrP) receptor mutations, H223R and T410P, were recently identified as the most likely cause of Jansen's metaphyseal chondrodysplasia. To assess the functional importance of either amino acid position in the human PTH/PTHrP receptor, H223 and T410 were individually replaced by all other amino acids. At position 223, only arginine and lysine led to agonist-independent cAMP accumulation; all other amino acid substitutions resulted in receptor mutants that lacked constitutive activity or were uninformative due to poor cell surface expression. In contrast, most amino acid substitutions at position 410 conferred constitutive cAMP accumulation and affected PTH/PTHrP receptor expression not at all or only mildly. Mutations corresponding to the H223R or T410P exchange in the human PTH/PTHrP receptor also led to constitutive activity when introduced into the opossum receptor homolog, but showed little or no change in basal cAMP accumulation when introduced into the rat PTH/PTHrP receptor. The PTH/PTHrP receptor residues mutated in Jansen's disease are conserved in all mammalian members of this family of G protein-coupled receptors. However, when the equivalent of either the H223R or the T410P mutation was introduced into several other related receptors, including the PTH2 receptor and the receptors for calcitonin, secretin, GH-releasing hormone, glucagon-like peptide I, and CRH, the resulting mutants failed to induce constitutive activity. These studies suggest that two residues in the human PTH/PTHrP receptor, 223 and 410, have critical roles in signal transduction, but with different sequence constrains.

Partial rescue of PTH/PTHrP receptor knockout mice by targeted expression of the Jansen transgene.

The homozygous ablation of the gene encoding the PTH/PTHrP receptor (PPR(-/-)) leads to early lethality and limited developmental defects, including an acceleration of chondrocyte differentiation. In contrast to the findings in homozygous PTHrP-ablated (PTHrP(-/-)) animals, these PPR(-/-) mice show an increase in cortical bone, a decrease in trabecular bone, and a defect in bone mineralization. Opposite observations are made in Jansen's metaphyseal chondrodysplasia, a disorder caused by constitutively active PPR mutants, and in transgenic animals expressing one of these receptor mutants (HKrk-H223R) under control of the type alpha1(I) collagen promoter. Expression of the Jansen transgene under the control of the type alpha1(II) collagen promoter was, furthermore, shown to delay chondrocyte differentiation and to prevent the dramatic acceleration of chondrocyte differentiation in PTHrP(-/-) mice, thus rescuing the early lethality of these animals. In the present study we demonstrated that the type alpha1(II) collagen promoter Jansen transgene restored most of the bone abnormalities in PPR(-/-) mice, but did not prevent their perinatal lethality. These findings suggested that factors other than impaired gas exchange due to an abnormal rib cage contribute to the early death of PPR(-/-) mice.

Inverse agonism of amino-terminally truncated parathyroid hormone (PTH) and PTH-related peptide (PTHrP) analogs revealed with constitutively active mutant PTH/PTHrP receptors.

Inverse agonists, ligands that suppress spontaneous receptor signaling activity, have been described for a growing number of G protein-coupled receptors; however, none have been reported for the PTH/calcitonin/secretin receptor family. We took advantage of the constitutive signaling activity of two mutant forms of the PTH/PTH-related peptide (PTHrP) receptor, recently identified in patients with Jansen's metaphyseal chondrodysplasia, to screen for PTH and PTHrP analogs with inverse agonist activity. Two antagonist peptides, [Leu11, D-Trp12]hPTHrP(7-34)NH2 and [D-Trp12, Tyr34]bPTH-(7-34)NH2, displayed inverse agonist activity and reduced cAMP in COS-7 cells expressing either mutant receptor by 30-50% (EC50 approximately 50 nM). These data demonstrate that the concept of inverse agonism can be extended to this distinct family of G protein-coupled receptors and their cognate antagonist peptide ligands. Such ligands shall be useful probes of the multi-state conformational equilibria proposed for these receptors and could lead to new approaches for treating human diseases caused by receptor activating mutations.

Further evidence for a novel receptor for amino-terminal parathyroid hormone-related protein on keratinocytes and squamous carcinoma cell lines.

PTH and PTH-related peptides (PTHrPs) interact with a common PTH/PTHrP receptor (type I), which is expressed in many tissues, including bone and kidney. Amino-terminal PTH and PTHrPs also recognize receptors in several nonclassical PTH target tissues, and in some of these, the signaling mechanisms differ qualitatively from those of the classical type I receptor. In normal keratinocytes and squamous carcinoma cell lines, PTH and PTHrP stimulate a rise in intracellular calcium, but not cAMP, suggesting the existence of an alternate, type II PTH/PTHrP receptor. SqCC/Y1 squamous carcinoma cells stably expressing the type I receptor displayed sensitive intracellular cAMP responses to PTHrP and PTH, indicating that these cells express functional GS proteins and that the type I receptor is capable of signaling through adenylyl cyclase in this cell line. Therefore, the endogenous type II receptor in SqCC/Y1 cells differs from the cloned type I receptor. We next examined whether messenger RNA (mRNA) from keratinocytes and squamous cell lines could hybridize to a human type I PTH/PTHrP receptor complementary DNA [1.9 kilobases (kb)]. No type I receptor mRNA (2.3 kb) was detected in polyadenylated RNA from any of the squamous cell lines. However, squamous cell lines did express several mRNA transcripts that hybridized with the type I receptor probe, yet were smaller (1 and 1.5 kb) or larger (3.5-5 kb) than the cloned receptor mRNA. The predominant mRNA in two squamous carcinoma cell lines and normal keratinocytes was a 1-kb transcript. Northern analysis with five different region-specific probes that span the entire coding region of the human type I receptor was used to map homologous regions within each of the transcripts. Several of the transcripts identified in squamous lines are also present in polyadenylated RNA from SaOS-2 human bone cells, but a unique 1-kb transcript hybridizing to probe 2 (nucleotides 490-870) was observed only in squamous cells. The smaller 1- and 1.5-kb transcripts did not hybridize to probes corresponding to the extreme 5'- and 3'-coding regions of the type I receptor complementary DNA. Ribonuclease protection analysis employing riboprobes that correspond to the five region-specific DNA probes revealed strong RNA signals of the expected size in SaOS-2 cells, but no hybridization with squamous cell RNA. Several smaller, but minor, bands that were unique to squamous cells were observed with riboprobe 2 only, suggesting partial homology of this region with the type I receptor.(ABSTRACT TRUNCATED AT 400 WORDS)

Role of protein kinase-A in homologous down-regulation of parathyroid hormone (PTH)/PTH-related peptide receptor messenger ribonucleic acid in human osteoblast-like SaOS-2 cells.

Homologous down-regulation of PTH/PTH-related peptide (PTHrP) receptor expression occurs in several PTH-responsive osteoblastic cell lines, but the mechanisms responsible are not well understood. We have used wild-type SaOS-2 human osteoblastic cells, in which homologous PTH/PTHrP receptor down-regulation occurs within 4 h, and a mutant cAMP-resistant subclone (Ca4A strain), to investigate the mechanisms by which PTH/PTHrP receptor mRNA is regulated. SaOS-2 cells expressed a single 2.2- to 2.5-kilobase transcript of PTH/PTHrP receptor mRNA, as assessed by Northern blot analysis of total RNA with a cDNA probe encoding the human PTH/PTHrP receptor. Homologous down-regulation of this PTH/PTHrP receptor mRNA first became significant when SaOS-2 cells had been treated with human (h) PTH-(1-34) (10(-7) M) for 8-12 h. By 24 h, steady state levels of PTH/PTHrP receptor mRNA were reduced by about 50%. This effect was mimicked by both (Bu)2cAMP (DBcAMP; 0.5 mM) and forskolin (Fsk; 10(-5) M). In contrast, down-regulation of PTH/PTHrP receptor mRNA by hPTH-(1-34), DBcAMP or Fsk was almost completely blocked in cAMP-resistant Ca4A cells. Short term (4-6 h) treatment with hPTH-(1-34), DBcAMP, or Fsk did not reduce steady state levels of PTH/PTHrP receptor mRNA in either SaOS-2 or Ca4A cells, although down-regulation was induced by 4-6 h of treatment with active phorbol esters such as 12-O-tetradecanoyl phorbol-13-acetate (200 nM) or phorbol-12,13-didecanoate (200 nM). Neither thapsigargin (1 microM) nor ionomycin (200 nM), both of which stimulate calcium transients in these cells, altered PTH/PTHrP receptor mRNA expression. Treatment with hPTH-(39-84) and hPTH-(53-84), which do not activate either cAMP-dependent protein kinase or protein kinase-C, but do stimulate 45Ca2+ uptake in these cells, did not alter PTH/PTHrP receptor mRNA expression. In the presence of actinomycin-D (1 microgram/ml), down-regulation of PTH/PTHrP receptor mRNA by hPTH-(1-34) was not observed. Cycloheximide (10 micrograms/ml) did not block down-regulation of PTH/PTHrP receptor mRNA induced by hPTH-(1-34). We conclude that homologous down-regulation of PTH/PTHrP receptor mRNA in SaOS-2 cells occurs later than the decline in functional surface receptors via a mechanism that does not involve enhanced mRNA degradation or new protein synthesis, but is dependent upon cAMP/cAMP-dependent protein kinase.

Selective and nonselective inverse agonists for constitutively active type-1 parathyroid hormone receptors: evidence for altered receptor conformations.

The spontaneous signaling activity of some G protein-coupled receptors and the capacity of certain ligands (inverse agonists) to inhibit such constitutive activity are poorly understood phenomena. We investigated these processes for several analogs of PTH-related peptide (PTHrP) and the constitutively active human PTH/PTHrP receptors (hP1Rcs) hP1Rc-H223R and hP1Rc-T410P. The N-terminally truncated antagonist PTHrP(5-36) functioned as a weak partial/neutral agonist with both mutant receptors but was converted to an inverse agonist for both receptors by the combined substitution of Leu(11) and D-Trp(12). The N-terminally intact analog [Bpa(2)]PTHrP(1-36)-a partial agonist with the wild-type hP1Rc-was a selective inverse agonist, in that it depressed basal cAMP signaling by hP1Rc-H223R but enhanced signaling by hP1Rc-T410P. The ability of [Bpa(2)]PTHrP(1-36) to discriminate between the two receptor mutants suggested that H223R and T410P confer constitutive receptor activity by inducing distinct conformational changes. This hypothesis was confirmed by the observations that: 1) the double mutant receptor hP1Rc-H223R/T410P exhibited basal cAMP levels that were 2-fold higher than those of either single mutant; and 2) hP1Rc-H223R and hP1Rc-T410P internalized (125)I-PTHrP(5-36) to markedly different extents. The overall results thus reveal that two different types of inverse agonists are possible for PTHrP ligands (nonselective and selective) and that constitutively active PTH-1 receptors can access different conformational states.

Identical complementary deoxyribonucleic acids encode a human renal and bone parathyroid hormone (PTH)/PTH-related peptide receptor.

Identical complementary DNAs (cDNAs) that encode a 593-amino acid human PTH (PTH)/PTH-related peptide (PTHrP) receptor were isolated by hybridization techniques from two cDNA libraries which had been constructed from human kidney and human osteoblast-like osteosarcoma cells (SaOS-2). Northern blot analysis of total RNA from human bone- and kidney-derived tissue revealed one single major messenger RNA species of about 2.5 kilobases in both tissues. The human PTH/PTHrP receptor has 91% and 81% identity, respectively, with the previously cloned rat and opossum receptors, indicating a high degree of conservation among mammals. Despite this striking degree of amino-acid conservation, the human PTH/PTHrP receptor has several unique biological properties when transiently expressed in COS-7 cells. The apparent dissociation constants for [Nle8,18,Tyr34] bovine PTH(1-34) amide [bPTH(1-34)] are similar for the human and the rat receptor (approximately 8 vs. approximately 15 nM) whereas [Tyr36]PTHrP(1-36) amide has a slightly lower affinity for the human (15-40 nM) than for the rat receptor (approximately 15 nM). Both ligands stimulate efficiently and with similar efficacy the accumulation of intracellular cAMP. The affinities for the antagonists [Nle8,18,Tyr34] bPTH(3.34) amide [bPTH(3-34)] and in particular for [Nle8,18,Tyr34] bPTH(7-34) amide [bPTH(7-34)] are considerably higher for the human receptor, e.g. approximately 8 nM vs. 30 nM for bPTH(3-34) and approximately 100 nM vs. 5000 nM for bPTH(7-34), respectively. Similar biological findings were previously attributed to differences in species- and/or organ-specific PTH/PTHrP receptors. The expression of the recombinant, highly homologous rat and human receptors in a uniform environment indicate that the moderate differences in the primary receptor structure have profound consequences for the receptor binding affinity of amino-terminally truncated PTH analogs. Furthermore, the molecular cloning of identical cDNAs encoding a human PTH/PTHrP receptor from the two major target organs for PTH, bone and kidney, provides strong evidence for one single PTH/PTHrP receptor in both organs, although additional and/or alternatively spliced receptors cannot be excluded.

Collagenase cleavage of type I collagen is essential for both basal and parathyroid hormone (PTH)/PTH-related peptide receptor-induced osteoclast activation and has differential effects on discrete bone compartments.

Expression of a constitutively active PTH/PTHrP receptor in cells of osteoblast lineage in vivo (CL2+) causes increases in trabecular bone volume and trabecular bone formation and, conversely, a decrease in the periosteal mineral apposition rate. Collagenase-3 (matrix metalloprotease-13) is a downstream target of PTH action. To investigate the relevance of collagenase cleavage of type I collagen for the CL2+ bone phenotype, we bred CL2+ animals with mice carrying a mutated col1 alpha 1 gene that encodes a protein resistant to digestion by collagenase-3 and other collagenases (rr). Adult tibias and parietal bones from 4-wk-old double-mutant animals (CL2+/rr) and from control littermates were analyzed. Trabecular bone volume was higher in CL2+/rr than in CL2+ mice. This increase occurred despite a modest reduction in bone formation rate, which was, however, still significantly higher that in wild-type littermates, and therefore must reflect decreased bone resorption in rr mice. Osteoclast number was increased in CL2+/rr animals compared with either wild-type or CL2+ mice, suggesting that collagenase-dependent collagen cleavage affected osteoclast function rather than osteoclast number and/or differentiation. Interestingly, the periosteal mineral apposition rate was similar in CL2+/rr and CL2+ animals and was significantly lower than that in wild-type animals. Our study provides evidence that collagenase activity is important for both basal and PTH/PTHrP receptor-dependent osteoclast activation. Furthermore, it indicates that a mild impairment of osteoclast activity is still compatible with increased osteoblast function. Lastly, it supports the hypothesis that collagenases can be a downstream effector of PTH/PTHrP receptor action in trabecular bone, but not in periosteum.

The extracellular amino-terminal region of the parathyroid hormone (PTH)/PTH-related peptide receptor determines the binding affinity for carboxyl-terminal fragments of PTH-(1-34).

The recombinant human PTH/PTH-related peptide (PTHrP) receptor, when transiently expressed in COS-7 cells, binds [Nle8,18,Tyr34] bovine PTH-(7-34)amide [PTH-(7-34)], human PTH-(10-34)amide [PTH-(10-34)], and bovine PTH-(15-34)amide [PTH-(15-34)] with at least 50-fold higher affinity than does the rat receptor homolog. In contrast, PTH-(1-34) binding affinities are similar for both receptor homologs. To map those areas of the PTH/PTHrP receptors that determine the binding specificity for carboxyl-terminal fragments of PTH-(1-34), we constructed chimeric rat/human PTH/PTHrP receptors. These bound PTH-(1-34) with normal affinity and, therefore, must have an overall conformation that resembles that of native receptors. Chimeras with the amino-terminal extracellular domain of the human PTH/PTHrP receptor have a considerably higher binding affinity for PTH-(7-34), PTH-(10-34), and PTH-(15-34) than do the reciprocal receptor constructs in which the amino-terminal region is from the rat PTH/PTHrP receptor. The opossum PTH/PTHrP receptor homolog also binds PTH-(7-34) with higher affinity than the rat receptor, and studies of rat/opossum chimeras confirm the importance of the amino-terminal extracellular domain in determining the PTH-(7-34) binding specificity. Mutant rat and human PTH/PTHrP receptors in which either residues 61-105 of the extracellular region or most of the intracellular tail were deleted have PTH-(7-34) binding characteristics indistinguishable from those of either wild-type receptor. These findings indicate that the amino-terminal extracellular region of the PTH/PTHrP receptor contains a domain(s) that largely determines the binding affinity of amino-terminally truncated PTH analogs. This region, therefore, is likely to constitute a site for ligand-receptor interaction.

Hypercalcemia due to constitutive activity of the parathyroid hormone (PTH)/PTH-related peptide receptor: comparison with primary hyperparathyroidism.

In Jansen's disease (JD), the hypercalcemia found in about half the cases is the result of a mutant, constitutively overactive, form of the PTH/PTHrP receptor, which in these cases also causes the skeletal dysplasia. The subject of the present report was first seen in 1956 and is still under treatment at the same medical center. We report the clinical course and a detailed study of calcium and bone metabolism carried out in 1976 and compare the results with those of six typical patients with mild primary hyperparathyroidism in whom exactly the same studies were carried out. In the patient with JD, the hypercalcemia was of early onset; chronic and nonprogressive; refractory to the administration of phosphate, glucocorticoid, and calcitonin; and accompanied by suppressed PTH levels as determined by two different immunoassays, an undetectable PTHrP level, increased excretion of nephrogenous cAMP (an in vivo bioassay of endogenous PTH production), decreased tubular reabsorption of phosphate, increased tubular reabsorption of calcium, increased biochemical indexes of bone turnover, and increased histological indexes of bone turnover on iliac bone histomorphometry after double tetracycline labeling. There was exaggerated loss of cortical bone and preservation of cancellous bone. All the results in JD relating to renal or skeletal effects of PTH excess were within or close to the ranges found in the hyperparathyroid patients, except that tubular reabsorption of phosphate was more depressed. Because PTH secretion was suppressed, any effects mediated by putative alternative receptors would have been diminished. We conclude that 1) the hypercalcemia due to constitutive overactivity of the PTH/PTHrP receptor is indistinguishable from that of mild primary hyperparathyroidism in clinical characteristics and renal tubular and skeletal features; and 2) the classic laboratory manifestations of primary hyperparathyroidism, with the possible exception of osteitis fibrosa cystica, can all be accounted for by overactivity of a single receptor.

Chromosomal localization of the parathyroid hormone/parathyroid hormone-related protein receptor gene to human chromosome 3p21.1-p24.2.

The human PTH/PTH-related peptide (PTH/PTHrP) receptor could be involved in hereditary disorders of PTH or PTHrP action. Knowledge of the gene's chromosomal location would allow studies linking it to specific disease traits. Therefore, we mapped the human PTH/PTHrP receptor gene by polymerase chain reaction of human/rodent somatic cell hybrid panels using oligonucleotide primers designed to amplify a portion of the gene from genomic DNA. The PTH/PTHrP gene was unambiguously assigned to the short arm of human chromosome 3, in the region designated 3p21.1-p24.2. Analysis of a second chromosome 3-specific mapping panel suggests that the gene is located near the 3p21.2-p21.3 boundary. The availability of highly polymorphic markers located in this region will permit exploration of the PTH/PTHrP receptor locus in genetic linkage searches for the causes of bone, calcium, and other potential disorders.

Pseudohypoparathyroidism type Ib is not caused by mutations in the coding exons of the human parathyroid hormone (PTH)/PTH-related peptide receptor gene.

Pseudohypoparathyroidism type Ib (PHP-Ib) is thought to be caused by a PTH/PTH-related peptide (PTHrP) receptor defect. To search for receptor mutations in genomic DNA from 17 PHP-Ib patients, three recently isolated human genomic DNA clones were further characterized by restriction enzyme mapping and nucleotide sequencing across intron/exon borders. Regions including all 14 coding exons and their splice junctions were amplified by polymerase chain reaction, and the products were analyzed by either temperature gradient gel electrophoresis or direct nucleotide sequencing. Silent polymorphisms were identified in exons G (1 of 17), M4 (1 of 17), and M7 (15 of 17). Two base changes were found in introns, 1 at the splice-donor site of the intron between exons E2 and E3 (1 of 17) and the other between exons G and M1 (2 of 17). Total ribonucleic acid from COS-7 cells expressing minigenes with or without the base change between exons E2 and E3 showed no difference by either Northern blot analysis or reverse transcriptase-polymerase chain reaction. Radioligand binding was indistinguishable for both transiently expressed constructs. A missense mutation (E546 to K546) in the receptor's cytoplasmic tail (3 of 17) was also found in 1 of 60 healthy individuals, and PTH/PTHrP receptors with this mutation were functionally indistinguishable from wild-type receptors. PHP-Ib thus appears to be rarely, if ever, caused by mutations in the coding exons of the PTH/PTHrP receptor gene.