Constitutively activated receptors for parathyroid hormone and parathyroid hormone-related peptide in Jansen's metaphyseal chondrodysplasia

BACKGROUND

An activating mutation of the receptor for parathyroid hormone (PTH) and parathyroid hormone-related peptide (PTHrP) was recently found in a patient with Jansens's metaphyseal chondrodysplasia, a rare form of short-limbed dwarfism associated with hypercalcemia and normal or low serum concentrations of the two hormones. To investigate this and other activating mutations and to refine the classification of this unusual disorder, we analyzed genomic DNA from six additional patients with Jansen's disease.

METHODS

Exons encoding the PTH-PTHrP receptor were amplified by the polymerase chain reaction (PCR), and the products were analyzed by gel electrophoresis or direct nucleotide-sequence analysis. Nucleotide changes were confirmed by restriction-enzyme digestion of genomic DNA or the PCR products.

RESULTS

The previously reported mutation, which changes a histidine at position 223 to arginine (H223R), was found in genomic DNA from three of the six patients but not in DNA from their healthy relatives or 45 unrelated normal subjects. A novel missense mutation that changes a threonine in the receptor's sixth membrane-spanning region to proline (T410P) was identified in another patient but not in 62 normal subjects. In two patients with radiologic evidence of Jansen's metaphyseal chondrodysplasia but less severe hypercalcemia, no receptor mutations were detected. In COS-7 cels expressing PTH-PTHrP receptors with the T410P or H223R mutation, basal cyclic AMP accumulation was four to six times higher than in cells expressing wild-type receptors.

CONCLUSIONS

The expression of constitutively active PTH-PTHrp receptors in kidney, bone, and growth-plate chondrocytes provides a plausible genetic explanation for mineral-ion abnormalities and metaphyseal changes in patients with Jansen's disease.

Down-regulation of the receptor for parathyroid hormone (PTH) and PTH-related peptide by PTH in primary fetal rat osteoblasts

We studied the effects of parathyroid hormone (PTH) on PTH parathyroid hormone related peptide (PTHrP) receptor mRNA level, PTHrP binding and PTH-stimulated cyclic adenosine monophosphate (cAMP) accumulation in osteoblasts, derived from fetal rat calvariae (ROB). Cells isolated during 10-70 minutes of collagenase treatment were seeded at a density of 25,000 cells/cm2 and cultured for 4 days. These cells show a fast increase in cAMP production after stimulation for 5 minutes with 20 nM bovine parathyroid hormone(1-34) (bPTH(1-34)). When ROB are incubated with bPTH(1-34) (0.04-40nM) for 24 h, a dose-dependent decrease of the PTH/PTHrP receptor mRNA level, PTHrP binding, and PTH-stimulated cAMP accumulation can be observed. Pretreatment of ROB with a high concentration of bPTH(1-34) (40 nM) leads within 15 minutes to a decrease in PTH-stimulated cAMP accumulation. However, it takes > or = 3 h before a significant decrease in PTH/PTHrP receptor mRNA level can be observed. Also a significant decrease in PTHrP binding is observed after only 4 h of incubation with bPTH(1-34). Compared with bPTH(1-34), pretreatment of ROB with bPTH(3-34) (40 and 100 nM) for 24 h causes smaller decreases in PTH-stimulated cAMP accumulation, PTHrP binding, and in the PTH/PTHrP receptor mRNA level. We investigated the possible involvement of the protein kinase A signaling pathway in the regulation of the PTH/PTHrP receptor mRNA expression. Both forskolin and (Bu)2cAMP decreased PTHrP binding and PTH/PTHrP mRNA levels. These observations suggest that chronic activation of the PKA signaling pathway may down-regulate PTH/PTHrP receptor expression and thus hormone responsiveness in "normal" osteoblasts. In short, we found that the decrease of the PTH-stimulated cAMP accumulation after long-term pretreatment with bPTH(1-34) is correlated with both PTH/PTHrP receptor mRNA level and PTHrP binding. These data also suggest that the initial desensitization (< 30 minutes) of PTH-stimulated cAMP responsiveness by pretreatment with a high concentration of bPTH(1-34) (40 nM) is not dependent on the number of available PTH/PTHrP receptors. The protein kinase A signaling pathway is involved in the regulation of the PTH/PTHrP receptor, but, regarding the effect of bPTH(3-34), other signaling systems are also involved.

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.

Converting parathyroid hormone-related peptide (PTHrP) into a potent PTH-2 receptor agonist

Most of the bone and kidney-related functions of parathyroid hormone (PTH) and parathyroid hormone-related peptide (PTHrP) are thought to be mediated by the PTH/PTHrP receptor. Recently, a homologous receptor, the PTH-2 receptor, was obtained from rat and human brain cDNA libraries. This receptor displayed the remarkable property of responding potently to PTH, but not to PTHrP. To begin to define residues involved in the ligand specificity of the PTH-2 receptor, we studied the interaction of several PTH/PTHrP hybrid ligands and other related peptide analogs with the human PTH-2 receptor. The results showed that two sites in PTH and PTHrP fully account for the different potencies that the two ligands exhibited with PTH-2 receptors; residue 5 (His in PTHrP and Ile in PTH) determined signaling capability, while residue 23 (Phe in PTHrP and Trp in PTH) determined binding affinity. By changing these two residues of PTHrP to the corresponding residues of PTH, we were able to convert PTHrP into a ligand that avidly bound to the PTH-2 receptor and fully and potently stimulated cAMP formation. Changing residue 23 alone yielded [Trp23]hPTHrP-(1-36), which was an antagonist for the PTH-2 receptor, but a full agonist for the PTH/PTHrP receptor. Residues 5 and 23 in PTH and PTHrP thus play key roles in signaling and binding interactions, respectively, with the PTH-2 receptor. Receptor-selective agonists and antagonists derived from these studies could help to identify the biological role of the PTH-2 receptor and to map specific sites of ligand-receptor interaction.

PTH/PTHrP receptor in early development and Indian hedgehog-regulated bone growth

The PTH/PTHrP receptor binds to two ligands with distinct functions: the calcium-regulating hormone, parathyroid hormone (PTH), and the paracrine factor, PTH-related protein (PTHrP). Each ligand, in turn, is likely to activate more than one receptor. The functions of the PTH/PTHrP receptor were investigated by deletion of the murine gene by homologous recombination. Most PTH/PTHrP receptor (-/-) mutant mice died in mid-gestation, a phenotype not observed in PTHrP (-/-) mice, perhaps because of the effects of maternal PTHrP. Mice that survived exhibited accelerated differentiation of chondrocytes in bone, and their bones, grown in explant culture, were resistant to the effects of PTHrP and Sonic hedgehog. These results suggest that the PTH/PTHrP receptor mediates the effects of Indian Hedgehog and PTHrP on chondrocyte differentiation.

Cell-specific signal transduction of parathyroid hormone (PTH)-related protein through stably expressed recombinant PTH/PTHrP receptors in vascular smooth muscle cells

PTH-related protein activates a G protein-coupled PTH/PTHrP receptor in many cell types and produces diverse biological actions. To study the signal transduction events associated with biological activity of the PTH/PTHrP receptor in vascular smooth muscle, a principal PTHrP-responsive tissue, rat aortic smooth muscle cells (A10) were stably transfected with a plasmid encoding a PTH/PTHrP receptor and tested for ligand binding, PTHrP-(1-34)-induced cAMP levels, inositol phosphate production, and cytosolic calcium transients. Of nineteen G418-resistant lines recovered, all exhibited high affinity binding [approximately dissociation constant (Kd) > 10(-10)) of iodinated [Tyr36]hPTHrP(1-36)NH2 and ligand-induced cAMP accumulation (2- to 100-fold), which was directly proportional to PTH/PTHrP receptor number (range 4 x 10(3) to 7 x 10(7) sites/cell]. PTHrP had no effect on intracellular calcium or inositol phosphate formation in any cell line regardless of receptor number despite the presence of detectable G alpha q). Transient overexpression of individual G alpha q proteins (G alpha q, G alpha 11 or G alpha 14) into PTH/PTHrP receptor-expressing A10 cells conferred the ability of PTHrP to increase intracellular calcium and inositol phosphate formation. Ligand activation of the recombinant PTH/PTHrP receptor elicited appropriate downstream biological effects in A10 cells including inhibition of DNA synthesis and osteopontin messenger RNA (mRNA) expression. Thus, a single PTH/PTHrP receptor, though capable of coupling to different G proteins, signals exclusively through a cAMP-dependent pathway in vascular smooth muscle.

Receptors for parathyroid hormone and parathyroid hormone-related peptide: from molecular cloning to definition of diseases

The parathyroid hormone/parathyroid hormone-related peptide receptor belongs to a distinct family of G protein-coupled receptors, the members of which usually signal through at least two second messenger systems, adenylate cyclase and phospholipase C. The parathyroid hormone/ parathyroid hormone-related peptide receptor is most abundantly expressed in bone, kidney and growth-plate chondrocytes, and, at lower levels, in a variety of fetal and adult tissues. To search for human diseases that are caused by parathyroid hormone/parathyroid hormone-related peptide receptor defects, genomic DNA of patients with pseudohypoparathyroidism type Ib and of patients with Jansen's metaphyseal chondrodysplasia was screened for mutations in all coding exons of the receptor gene. Inactivating parathyroid hormone/parathyroid hormone-related peptide receptor mutations were excluded in patients with pseudohypoparathyroidism type Ib. However, a receptor mutation that causes agonist-independent, constitutive cAMP accumulation was identified in a patient with Jansen's metaphyseal chondrodysplasia, a rare form of short-limbed dwarfism associated with hypercalcemia despite normal or low concentrations of parathyroid hormone and parathyroid hormone-related peptide. These findings allow the conclusion to be drawn that parathyroid hormone/parathyroid hormone-related peptide receptors mediate the endocrine actions of parathyroid hormone, which are required for the control of calcium homeostasis and the autocrine-paracrine actions of parathyroid hormone-related peptide, which are required for normal growth-plate development.

Jansen's metaphyseal chondrodysplasia: a disorder due to a PTH/PTHrP receptor gene mutation

Jansen's metaphyseal chondrodysplasia (JMC) is a rare genetic disorder that is characterized by short-limbed dwarfism and severe, agonist-independent hypercalcemia. An activating PTH/PTHrP receptor mutation that results in constitutive cAMP accumulation was recently identified in the genomic DNA of a patient with this disorder. These findings provide a plausible explanation for the abnormal regulation of growth-plate chondrocytes and mineral ion homeostasis in JMC, and may have significant implications for understanding the broader biological role of PTHrP and its receptor.

Characterization of a novel parathyroid hormone (PTH) receptor with specificity for the carboxyl-terminal region of PTH-(1-84)

Carboxyl-terminal fragments of PTH (C-PTH) appear to have biological properties different from those mediated by the amino-terminal portions of PTH and PTH-related peptide (PTHrP). To characterize a C-PTH receptor that may be involved in mediating these functions, we performed RRAs and affinity cross-linking studies with several clonal cell lines. Radiolabeled recombinant [Leu8,18,Tyr34]human PTH-(1-84)[mutPTH-(1-84) and [Tyr34] human PTH-(19-84)[mutPTH-(19-84) showed little or no specific binding to stably expressed recombinant PTH/PTHrP receptors. However, high affinity binding was observed using osteoblast-like and rat parathyroid (PT-r3) cells. The apparent Kd values were 20-30 nM for PTH-(1-84), mutPTH-(1-84), and mutPTH-(19-84), respectively; 400-800 nM for PTH-(39-84); and more than 5000 nM for PTH-(53-84). [Nle8,18,Tyr34]bovine PTH-(1-34)amide [PTH-(1-34)], PTH-(44-68), PTHrP-(37-74), and PTHrP-(109-141) showed no displacement of either radioligand. C-PTH receptor number was increased up to 2-fold by pretreating ROS 17/2.8 cells with increasing doses of PTH-(1-34), PTH-(1-84), or 8-bromo-cAMP, whereas no change was observed in response to dexamethasone or PTH-(39-84). Cross-linking studies using radiolabeled mutPTH-(1-84) or mutPTH-(19-84) revealed specific labeling of two proteins in ROS 17/2.8 cells that were approximately 40 and 90 kilodaltons in size (including the radioligand of approximately 10 kilodaltons). The intensity of affinity labeling of both proteins was dose dependently inhibited by increasing concentrations of unlabeled PTH-(1-84) and several carboxyl-terminal PTH-(1-84) fragments, but not by PTH-(1-34). Similar studies with PT-r3 cells revealed only a single protein band of about 90 kilodaltons. These data indicate that the carboxyl-terminal portion of PTH-(1-84) binds specifically to a unique receptor/binding protein distinct from the previously isolated PTH/PTHrP receptor.

Homolog-scanning mutagenesis of the parathyroid hormone (PTH) receptor reveals PTH-(1-34) binding determinants in the third extracellular loop

To identify determinants in the rat PTH receptor critical for binding the agonist peptide, PTH-(1-34), we systematically replaced 12 segments (5-33 residues) of the receptor's extracellular surface with the corresponding segments of the homologous rat secretin receptor and screened the resulting mutants in COS-7 cells for altered PTH-(1-34) binding properties. Surface expression of mutant receptors was assessed by the binding of monoclonal antibody 12CA5 to the epitope (HA)-tagged receptors. Of the nine well expressed and therefore informative receptor mutants, four bound radiolabeled PTH-(1-34) at levels that were proportional to the corresponding levels of surface expression, whereas five mutants bound [125I]PTH-(1-34) to levels that were lower than predicted from the cell surface expression levels. These five mutations occurred at the extracellular (EC) end of transmembrane domain 1, the carboxy-terminal portion of the first EC loop, the second EC loop, and the third EC loop. We selected for further fine structure analysis the third EC loop; two specific residues, Trp-437 and Gln-440, were identified at which mutations caused 9- to 16-fold reductions in PTH-(1-34)-binding affinity. The same mutations had little or no effect on the binding affinity of PTH-(3-34). This study provides new information on the location of PTH receptor regions important for high affinity agonist binding and identifies two residues in the third extracellular loop which may contribute to interactions involving the hormone's critical amino terminus.

Expression of parathyroid hormone-related peptide and its receptor messenger ribonucleic acid in human amnion and chorion-decidua: implications for secretion and function

OBJECTIVE

Our purpose was to define the location and packaging of parathyroid hormone-related peptide in amnion-chorion and the potential target tissues for its action in fetal membranes.

STUDY DESIGN

We studied fetal membranes by use of light microscopic immunocytochemistry with three monoclonal antibodies against distinct regions of the parathyroid hormone-related peptide molecule. For electron microscopy immunogold analysis with a monoclonal antibody specific to the 109-141 fragment was used to observe parathyroid hormone-related peptide intracellularly in amnion membrane and in the chorion layers. Multiplex reverse transcriptase-polymerase chain reaction with Southern blotting was used to identify parathyroid hormone/parathyroid hormone-related peptide receptor and control messenger ribonucleic acids in amnion and chorion-decidua.

RESULTS

All monoclonal antibodies revealed immunoreactive parathyroid hormone-related peptide in the amniotic epithelial cells and in some fibroblast-like cells embedded in the extracellular matrix of the amnion. Parathyroid hormone-related peptide was also found in the chorion in fibroblast and trophoblast layers and in decidua. Ultrastructurally immunogold particles were evenly distributed throughout the amniotic epithelial cells and were present in apical microvilli and near the basal membranes. Electron microscopy studies of the chorion cytotrophoblast also showed freely dispersed immunogold particles of parathyroid hormone-related peptide with no packaging in secretory granules. Low to undetectable levels of parathyroid hormone/parathyroid hormone-related peptide receptor messenger ribonucleic acid were found in amnion tissue, whereas abundant receptor messenger ribonucleic acid was found in chorion-decidua.

CONCLUSIONS

These results suggest the presence of a parathyroid hormone-related peptide paracrine system within the human fetal membranes.

Down-regulation of the receptor for parathyroid hormone (PTH) and PTH-related peptide by transforming growth factor-beta in primary fetal rat osteoblasts

We studied the effects of transforming growth factor-beta 2 (TGF beta 2) on the level of PTH/PTH-related peptide-(PTHrP) receptor messenger RNA (mRNA), PTHrP binding, and PTH-stimulated cAMP accumulation in cultured osteoblasts derived from fetal rat calvariae (ROB). When ROB were pretreated with TGF beta 2 at concentrations ranging from 1-100 pM for 24 h, dose-dependent decreases in the level of PTH/PTHrP receptor mRNA, PTHrP binding, and PTH-stimulated cAMP accumulation were observed. For the PTH/PTHrP receptor mRNA level and PTH-stimulated cAMP accumulation, the half-maximal effective concentration was approximately 4 pM. For the inhibition of PTHrP binding, the half-maximal effective concentration was much higher. A 50% decrease in both PTH/PTHrP receptor mRNA level and PTH-stimulated cAMP accumulation was obtained when ROB were treated with 100 pM TGF beta 2 for 4 h. A comparable decrease in PTHrP binding was only observed after 24 h of incubation with 100 pM TGF beta 2. Actinomycin D induced a rapid decrease in the PTH/PTHrP receptor mRNA level (70% after 4 h), indicating a half-life for the receptor mRNA of 2-3 h. Under the same conditions, PTHrP binding and PTH-stimulated cAMP accumulation did not change. When ROB were treated with cycloheximide for the same period, only a small decrease in PTHrP binding (20%) was observed, suggesting that PTH/PTHrP receptors do not have a rapid turnover. Cycloheximide also reduced PTH-stimulated cAMP production; after coincubation of cycloheximide with TGF beta 2, this inhibition was smaller than that in ROB cultures treated with TGF beta 2 exclusively. From these observations we conclude that TGF beta 2 induces a decrease in steady state levels of PTH/PTHrP receptor mRNA that results in decreased PTHrP receptor binding. The PTH-stimulated cAMP accumulation is at least to some extent independent of the PTH/PTHrP receptor availability. Furthermore, there is a high turnover of PTH/PTHrP receptor mRNA, whereas turnover of the receptor protein is much slower. Finally, protein synthesis is required for TGF beta 2-induced desensitization of cAMP responsiveness to PTH.

Functional properties of the PTH/PTHrP receptor

The PTH/PTHrP receptor belongs to a novel family of G-protein-coupled receptors which also includes an insect receptor for a diuretic hormone and the protein encoded by a genomic DNA clone from Caenorhabditis elegans. Despite significant structural conservation, rat, opossum, and human PTH/PTHrP receptor homologs display distinct functional characteristics when tested with either [Arg2, Tyr34]hPTH(1-34)amide or [Nle8.18, Tyr34]bPTH(7-34)-amide. These PTH analogs, and chimeras between rat/opossum and between rat/human PTH/PTHrP receptors, led to the identification of receptor residues that appear to be involved in ligand/receptor interaction and receptor activation, respectively. The search for mutations in the PTH/PTHrP receptor gene in genomic DNA of patients with pseudohypoparathyroidism type Ib (PHP-Ib) revealed several silent polymorphisms and a missense mutation in the receptor's tail region which did not affect receptor function. Mutations in the PTH/PTHrP receptor are therefore rarely, if at all, responsible for PHP-Ib. A mutation in the PTH/PTHrP receptor is, however, the most likely cause of Jansen-type metaphyseal chondrodysplasia, a rare form of short-limbed dwarfism which is associated with severe hypercalcemia despite normal or low levels of circulating PTH and PTHrP. A missense mutation was identified which causes constitutive, ligand-independent receptor activation, and thus explains the laboratory and the growth-plate abnormalities in affected individuals.

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)

Truncation of the carboxyl-terminal region of the rat parathyroid hormone (PTH)/PTH-related peptide receptor enhances PTH stimulation of adenylyl cyclase but not phospholipase C

The functional role of the rat parathyroid hormone(PTH)/PTH-related peptide (PTHrP) receptor's carboxyl-terminal region was characterized by comparing the binding and signaling properties of receptors that have 78 and 111 amino acid deletions (R513 and R480, respectively), with those of the 591-amino acid wild-type (WT) receptor. R480 and R513 have 4- and 1.5-fold lower apparent Kd values for rat PTH-(1-34) (rPTH), compared with the WT receptor (WT, 1.81 +/- 0.19 nM; R513, 1.24 +/- 0.12 nM; R480, 0.48 +/- 0.05 nM, mean +/- S.E.). PTH (100 nM)-stimulated cAMP accumulation and polyphosphoinositide hydrolysis both correlated positively with receptor expression. However, whereas PTH-stimulated polyphosphoinositide hydrolysis was indistinguishable among WT and either truncated mutant at comparable levels of expressed receptors, maximal PTH-stimulated cAMP accumulation was 4-6- and 2-3-fold higher in cells expressing R480 and R513, respectively. Furthermore, pretreatment of COS-7 cells with 100 ng/ml of pertussis toxin (PTX) enhanced PTH-stimulated cAMP accumulation in cells expressing the WT receptor, but failed to do so in cells expressing either R480 or R513. Thus, sequences in the PTH/PTHrP receptor's carboxyl-terminal tail lower the affinity of the WT receptor for agonist; directly interact with, or indirectly facilitate the interaction of the receptor with a PTX-sensitive G protein that inhibits adenylyl cyclase; and decrease the efficacy with which the receptor interacts with Gs.

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.

Down-regulation of parathyroid (PTH)/PTH-related peptide receptor immunoreactivity and PTH binding in opossum kidney cells by PTH and dexamethasone

Recent data have shown that PTH down-regulation of its receptor on opossum kidney (OK) cells is not associated with any change in the steady state level of the PTH/PTH-related peptide (PTHrP) receptor messenger RNA. For analysis of down-regulation of the PTH/PTHrP receptor in OK cells, the present work uses a specific receptor anti-serum, SR-2, that is useful for detection and quantification of PTH/PTHrP receptor immunoreactivity on intact cells bearing the opossum PTH/PTHrP receptor. SR-2 specifically binds to COS-7 cells transiently expressing the opossum PTH/PTHrP receptor complementary DNA (OK-O), to LLCPK1 cells stably expressing the recombinant opossum PTH/PTHrP receptor (AOK cells), and to OK cells expressing endogenous PTH/PTHrP receptors, but not to mock-transfected COS-7 cells or untransfected LLCPK1 cells. SR-2 binding was also linearly correlated with PTH binding in COS-7 cells transfected with different amounts of OK-O plasmid DNA. Treatment with PTH (100 nM) for 4 and 6 h did not significantly down-regulate the PTH/PTHrP receptor immunoreactivity, although PTH binding was decreased to 51% and 49% of control, respectively, and PTH-stimulated cAMP accumulation was decreased to 27% and 28% of control, respectively. Treatment with PTH (100 nM) for 24 and 48 h significantly decreased PTH binding to 51% and 60% of control and decreased PTH/PTHrP receptor immunoreactivity to 68% and 58% of control, respectively. Incubation of OK cells with 0.1 nM to 1 microM PTH for 4 h did not down-regulate the PTH/PTHrP receptor immunoreactivity, although PTH binding was decreased dramatically. Scatchard blot analysis revealed that the binding affinity was decreased by 7-fold in OK cells treated with PTH for 4 h without change in receptor number. Conversely, treatment of OK cells with PTH for 24 h resulted in a parallel decrease in both receptor number and receptor immunoreactivity without any change in receptor binding affinity. Treatment of OK cells with dexamethasone (0.1 nM to 1 microM) had no effect on PTH binding or PTH/PTHrP receptor immunoreactivity. Incubation of OK cells with both dexamethasone (1 microM) and PTH (0.1 nM to 1 microM), however, caused a significantly greater down-regulation of both PTH binding and PTH/PTHrP receptor immunoreactivity than in cells treated with PTH alone. These data indicate that during the first 4 h of exposure of OK cells to PTH, PTH/PTHrP receptors remain on the cell surface but have lowered affinity to bind the ligand and that dexamethasone potentiates the effect of PTH on PTH/PTHrP receptor down-regulation in OK cells.

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.

Role of the extracellular regions of the parathyroid hormone (PTH)/PTH-related peptide receptor in hormone binding

The PTH/PTH-related peptide receptor is a member of a newly discovered family of G-protein-coupled receptors. Strikingly conserved features among these receptors include the positioning of eight extracellular cysteines and several other residues that are located predominantly within the membrane-embedded region. Deletion mutants or receptors with point mutations of the highly conserved cysteine residues were transiently expressed in COS-7 cells to evaluate PTH binding and PTH-stimulated cAMP production. Deletion of residues 61-105, which are encoded by exon E2 in the PTH/PTH-related peptide receptor gene, did not affect receptor function. An epitope derived from Haemophilus influenza hemagglutinin was, therefore, introduced into this portion of most receptors to allow the independent assessment of cell surface expression. PTH binding capacity was not reduced by the deletion of residues 258-278 in the first extracellular loop. Receptors with deletion of either residues 31-47 in the amino-terminal extension or residues 431-440 in the third extracellular loop failed to bind PTH, although expression of the receptor on the cell surface was only marginally reduced. Most other receptor mutants, including those in which each of the six cysteines in the amino-terminus was replaced by serines, failed to be processed and/or expressed appropriately, whereas the substitution of cysteine-281 or -351 had a less severe effect. The combined replacement of both cysteines concomitantly increased PTH binding and cell surface expression, suggesting the formation of a disulfide bond between these two residues. Our data indicate that residues near the amino-terminus and within the third extracellular loop are necessary for ligand binding, whereas more than 25% of the receptor's extracellular region appears not to be involved.

Determinants of [Arg2]PTH-(1-34) binding and signaling in the transmembrane region of the parathyroid hormone receptor

Previously, we reported that [Arg2]PTH-(1-34) bound to the rat osteosarcoma cell line, ROS 17/2.8, with 2-fold higher apparent affinity than it did to the opossum kidney cell line, OK, yet the analog was only a weak partial agonist for cAMP stimulation with ROS 17/2.8 cells, whereas it was a full cAMP agonist with OK cells. These results suggested that the rat and opossum PTH receptors differ in a region recognized by the hormone's amino-terminus. In this report we show that the cloned PTH receptors derived from ROS 17/2.8 and OK cells, expressed in COS-7 cells, also displayed altered responses to [Arg2]PTH-(1-34). Thus, [Arg2]PTH-(1-34) bound to the cloned rat PTH receptor with 7-fold higher affinity than it did to the cloned opossum PTH receptor, and in cAMP stimulation assays, it was a much weaker agonist with the rat receptor than it was with the opossum receptor. Studies with rat/opossum PTH receptor chimeras suggested that the membrane-spanning region of the receptor contributed to the different binding and signaling responses to [Arg2]PTH-(1-34). Point mutation analysis identified three sites in or near the extracellular ends of transmembrane domains V and VI, which specifically affected [Arg2]PTH-(1-34) binding and signaling.

Rapid desensitization of parathyroid hormone dependent adenylate cyclase in perifused human osteosarcoma cells (SaOS-2)

The pulsatile but not the continuous application of parathyroid hormone (PTH) increase bone mass in vivo. To study the effects of intermittent hormonal administration on bone-derived cells in vitro, we established a perifusion system using the human osteosarcoma cell line SaOS-2. Cells were grown in suspension culture attached to collagen beads and were then loaded into a 3 ml syringe for perifusion experiments. The application of PTH(1-34) resulted in a dose-dependent increase of cAMP release by SaOS-2 cells into the effluent medium. Cyclic AMP accumulation was rapidly desensitized by approx. 80% after 30 min of continuous exposure to PTH(1-34) (10(-7) M), while cells remained responsive to forskolin. The recovery of PTH responsiveness required at least 2 h of hormone-free perifusion. Desensitization in the experimental setting was dose-dependent (EC50 = 1 x 10(-10) M PTH(1-34)). Neither 8Br-cAMP (2 x 10(-4) M) nor PMA(1 x 10(-7) M) had an effect on the PTH(1-34)-induced desensitization of the adenylate cyclase. Radioreceptor assays showed that [125I]-[Tyr36]hPTHrP(1-36)amide binding to SaOS-2 cells was decreased by 60-70% by PTH(1-34) (1 x 10(-6) M), bPTH(1-84) (1.8 x 10(-6) M) and bPTH(3-34) (2 x 10(-6) M), whereas 8Br-cAMP (2 x 10(-4) M) had no effect on radioligand binding. PMA (1 x 10(-7) M) appeared to slightly increase [125I]PTHrP binding. This observation is consistent with a small (3-fold) increase in PTH-induced cAMP release as a result of PMA pre-treatment. Receptor internalization was dose-dependent EC50 = 3 x 10(-7) M PTH(1-34)). The maximal effect occurred after 10-30 min and was largely reversible within 2 h. Monensin (3 x 10(-5) M) inhibited the recovery from receptor internalization. We conclude that a perifusion system using SaOS-2 cells is a suitable model to study the effect of discontinuous application of PTH on cAMP release. A rapid, homologous desensitization of PTH(1-34) stimulated cAMP accumulation has been observed that does not appear to involve protein kinase A or C.

Molecular cloning and characterization of a parathyroid hormone/parathyroid hormone-related peptide receptor: a member of an ancient family of G protein-coupled receptors

The parathyroid hormone (PTH)/parathyroid hormone-related peptide (PTHrP) receptor belongs to a newly discovered family of G protein-coupled receptors. Members of this family, which have been isolated from mammals, include the receptors for PTH/PTHrP, calcitonin, secretin, growth hormone-releasing hormone, vasoactive intestinal polypeptide (types 1 and 2), gastric-inhibitory polypeptide, glucagon-like peptide 1, glucagon, corticotropin-releasing factor, and the pituitary adenylate cyclase-activating peptide. Very recently, a receptor with remarkable homology to these mammalian receptors was isolated from the insect Manduca sexta, which indicates considerable conservation of these related proteins during evolution. Thus far the cognate ligands for these receptors are 27- to 46-amino-acid residues in length. Members of this novel receptor family are characterized by seven membrane-spanning domains and at least two conserved sites for N-linked glycosylation. Furthermore, 48-amino-acid residues, including eight extracellular cysteines, are identical in all receptors, and many other residues are highly conserved. The PTH/PTHrP receptor is expressed in a large variety of fetal and adult tissues, binds two ligands (PTH and PTHrP) with high affinity, and activates at least two second-messenger systems (adenylate cyclase and phospholipase C).

The rat, mouse and human genes encoding the receptor for parathyroid hormone and parathyroid hormone-related peptide are highly homologous

The organization of the PTH/PTHrP receptor gene is highly homologous in three mammalian species, rat, human and mouse. This gene extends over 22 kb and contains at least 15 exons and 14 introns. The most 5' exon we have identified (exon U) is followed by an approximately 1kb intron. The second exon (exon S) encodes the initiator methionine and the putative signal peptide and is followed by the largest intron of this gene (about 11 kb). The amino-terminal extracellular region is encoded by 4 exons (E1, E2, E3 and G); exon G contains all 4 potential glycosylation sites. Membrane-spanning domains 1-4 and portions of their connecting intracellular and extracellular loops are encoded by 4 exons (M1, M2, M3 and M4). The second extracellular loop and portions of 4th and 5th membrane-spanning domains are encoded by one exon, EL2. The 5th membrane-spanning domain and portion of the 3rd intracellular loop are encoded by one exon, M5. The 6th membrane-spanning domain, the 3rd extracellular loop and the proximal part of the 7th membrane-spanning domain are encoded by one single exon (M6/7); the remaining sequence of the 7th membrane-spanning domain is encoded by a short exon, M7. The carboxy-terminal tail of the receptor and the 3' untranslated region are encoded by one single exon, exon T. The 3' untranslated region does not contain the classical polyadenylation signal, AATAAA. Expression in COS-7 cells of a minigene constructed of a 5' rat cDNA fragment (1.3 Kb) ligated in-frame to a 3' genomic fragment at the NsiI site, which is located in exon M6/7 resulted in a transcript that was translated into a functional receptor; it bound PTH and showed PTH-stimulated accumulation of intracellular cAMP. Therefore, the PTH/PTHrP receptor gene contains alternative 3' sequences that allow cleavage and polyadenylation of its transcript.