Sodium-phosphate cotransporters, nephrolithiasis and bone demineralization

PURPOSE OF REVIEW

We discuss how recent findings obtained in disorders of phosphate metabolism in humans and in animal models have provided insights into the pathogenesis of renal stone formation and bone demineralization.

RECENT FINDINGS

Mice that are null for the sodium-phosphate cotransporter (NPT)2a gene (NPT2a(-/-) mice) exhibit hypophosphataemia, increased urinary phosphate excretion, hypercalciuria and nephrolithiasis, but no bone demineralization. Mice null for the sodium-hydrogen exchanger regulatory factor (NHERF)1 (NHERF1(-/-) mice) also exhibit hypophosphataemia and increased renal phosphate excretion with decreased renal NPT2a expression, but they present with a severe sex-dependent bone demineralization. Heterozygous loss-of-function mutations in the NPT2a gene in humans induce hypophosphataemia, increased urinary phosphate excretion, hypercalciuria, nephrolithiasis in males (to date) and bone demineralization of variable severity in both sexes. Patients and experimental animals with increased circulating levels of fibroblast growth factor 23 present with hypophosphataemia, increased urinary phosphate excretion, inappropriate calcitriol synthesis and rickets/osteomalacia, but no nephrolithiasis except when treated. Low-phosphate diet in spontaneously hypercalciuric rats and disruption of the 1-alpha-hydroxylase gene in NPT2a mice prevent renal stone formation.

SUMMARY

Increased urinary phosphate excretion is a risk factor for renal calcium stone formation when it is associated with hypercalciuria. As yet undefined interplay between NPT2a, NHERF1 and possibly other cotransporters or associated proteins in bone cells may account for the diversity of bone phenotypes observed in disorders of phosphate metabolism with impaired renal phosphate reabsorption. The pathogenesis of both renal stone and bone demineralization appear to be affected by species, sex and mutation type, among other factors.

Expression of fibroblast growth factors 18 and 23 during human embryonic and fetal development

Fibroblast Growth Factor (FGF) 18 and 23 are two recently identified members of the FGF family, a family of structurally related polypeptides with diverse roles in physiological and pathological processes. Studies mostly performed in rodents and chicken have demonstrated that FGF18 is a pleiotropic growth factor involved in the development of various organs, while there are no data supporting a direct role of FGF23 in cell proliferation or differentiation either in physiology or pathology in any species. However, it is now established that FGF23 can be a humoral messenger and an important regulator of phosphate homeostasis and vitamin D metabolism. As a first step towards elucidating the roles of these FGF in human development, we examined FGF18 and FGF23 mRNA expression by in situ hybridization in whole human embryos at 30 days and 8 weeks of gestation (GW) and in specific fetal tissues at different ages. We report a highly restricted expression pattern for both FGF genes in human embryonic development.

Functional importance of Myc-associated zinc finger protein for the human parathyroid hormone (PTH)/PTH-related peptide receptor-1 P2 promoter constitutive activity

The aim of the present study was to analyze the functional importance for the parathyroid hormone (PTH)/PTH-related peptide (PTHrP) receptor (PTHR1) gene P2 promoter activity of the putative proximal Myc-associated zinc finger protein (MAZ) site localized at position bp -45 to -39 bp, taking advantage of a G/A mutation identified at position -40 in the human sequence. Wild-type 'full-length' (1285P2) and truncated (760P2) promoter sequences were inserted upstream to the luciferase basic (pLucB) and enhancer (pLucE) reporter gene expression vectors. Transient transfections in osteoblast-like SaOS-2 cells and renal cells (RC.SV3A2) showed that the -40 G/A mutation significantly impaired transcriptional activity of wild-type 1285P2-pLucB and 760P2-pLucE promoter constructs. Further truncation of the P2 sequence demonstrated that the sequence -109/-37 bp was essential for promoter activity. Co-transfection with a MAZ expression vector did not modify the wild-type 1285P2-pLucB construct reporter activity but significantly increased 2-fold the mutated construction activity (P<0.05). Electrophoretic mobility shift assays using SaOS-2 nuclear extracts and a double-stranded DNA fragment encompassing the -45 to -39 putative MAZ site (ds-MAZ-oligo) disclosed two specific DNA-protein complexes. Complex II (fast moving) had a lower affinity for the mutated MAZ motif than for the wild-type MAZ motif while complex I (slow moving) had the same affinity for both wild-type or mutated MAZ sequences. Competition studies with Sp1 consensus oligonucleotide (ds-Sp1-oligo) markedly reduced complex I intensity, with a concomitant increase in that of complex II. Finally, ribonuclease protection assays showed that P2-specific PTHR1 mRNA transcript expression was significantly decreased in SaOS-2 cells transfected with ds-MAZ-oligo as compared with that for control (P<0.001) and ds-Sp1-oligo (P<0.05). Taken together, our studies suggest that the putative -45 to -39 MAZ-binding site regulates the constitutive activity of human PTHR1 P2 promoter.

Increased Activity of the Diastrophic Dysplasia Sulfate Transporter in Otosclerosis and Its Inhibition by Sodium Fluoride

HYPOTHESIS

This study investigates the function of the diastrophic dysplasia sulfate transporter (DTDST) in otosclerotic bone and the effect on it of sodium fluoride (NaF).

BACKGROUND

Otosclerosis is a localized bone dystrophy with increased bone turnover. DTDST is implicated in the regulation of the bone turnover.

MATERIALS AND METHODS

Primary cultures of cells were obtained from the stapes and external auditory canal (EAC) of 26 patients with otosclerosis and from nine control patients. Sulfate uptake was quantified under basal conditions and with NaF. The NaF signaling pathways were investigated using forskolin and verapamil.

RESULTS

The relative initial rates of sulfate uptake and the apparent Vmax values were: otosclerotic stapes > EAC > control stapes = control EAC. The sulfate uptake by the otosclerotic stapes was correlated with the loss of sensorineural hearing. The amounts of DTDST mRNA (RNase protection assay) in the four subgroups did not differ. NaF (10(-6)M, 1 hr) inhibited sulfate uptake by the otosclerotic stapes and EAC cells but not by control samples.

CONCLUSION

The authors believe that whether the increased DTDST activity is a cause or an effect of otosclerosis, it appears to be a specific target for NaF treatment.

Expression patterns of parathyroid hormone-related peptide (PTHrP) and parathyroid hormone receptor type 1 (PTHR1) during human development are suggestive of roles specific for each gene that are not mediated through the PTHrP/PTHR1 paracrine signaling pathway

Close temporal and spatial relationships between Pthrp and Pthr1 gene expression are reported during fetal life in rats, suggestive of a developmental role via a paracrine signaling pathway. In agreement, disruption of this signaling pathway is associated with developmental defects, as documented in mice and humans. Surprisingly, however, only few organs are affected. We report here PTHrP and PTHR1 gene expression during human embryo-fetal development. We describe three patterns: in some organs, both genes are active ('hand-in-glove' fashion), while in others either PTHrP or PTHR1 transcripts are detected. In addition, in some tissues, expression of PTHrP or PTHR1 genes is transient. Abnormal development has been documented only for organs expressing both genes. The patterns that we observe are compatible with specific roles for each gene not mediated through the PTHrP/PTHR1 signaling pathway.

Parathyroid hormone receptor type 1/Indian hedgehog expression is preserved in the growth plate of human fetuses affected with fibroblast growth factor receptor type 3 activating mutations

The fibroblast growth factor receptor type 3 (FGFR3) and Indian hedgehog (IHH)/parathyroid hormone (PTH)/PTH-related peptide receptor type 1 (PTHR1) systems are both essential regulators of endochondral ossification. Based on mouse models, activation of the FGFR3 system is suggested to regulate the IHH/PTHR1 pathway. To challenge this possible interaction in humans, we analyzed the femoral growth plates from fetuses carrying activating FGFR3 mutations (9 achondroplasia, 21 and 8 thanatophoric dysplasia types 1 and 2, respectively) and 14 age-matched controls by histological techniques and in situ hybridization using riboprobes for human IHH, PTHR1, type 10 and type 1 collagen transcripts. We show that bone-perichondrial ring enlargement and growth plate increased vascularization in FGFR3-mutated fetuses correlate with the phenotypic severity of the disease. PTHR1 and IHH expression in growth plates, bone-perichondrial rings and vascular canals is not affected by FGFR3 mutations, irrespective of the mutant genotype and age, and is in keeping with cell phenotypes. These results indicate that in humans, FGFR3 signaling does not down-regulate the main players of the IHH/PTHR1 pathway. Furthermore, we show that cells within the bone-perichondrial ring in controls and patients express IHH, PTHR1, and type 10 and type 1 collagen transcripts, suggesting that bone-perichondrial ring formation involves cells of both chondrocytic and osteoblastic phenotypes.

Nephrolithiasis and osteoporosis associated with hypophosphatemia caused by mutations in the type 2a sodium-phosphate cotransporter

BACKGROUND

Epidemiologic studies suggest that genetic factors confer a predisposition to the formation of renal calcium stones or bone demineralization. Low serum phosphate concentrations due to a decrease in renal phosphate reabsorption have been reported in some patients with these conditions, suggesting that genetic factors leading to a decrease in renal phosphate reabsorption may contribute to them. We hypothesized that mutations in the gene coding for the main renal sodium-phosphate cotransporter (NPT2a) may be present in patients with these disorders.

METHODS

We studied 20 patients with urolithiasis or bone demineralization and persistent idiopathic hypophosphatemia associated with a decrease in maximal renal phosphate reabsorption. The coding region of the gene for NPT2a was sequenced in all patients. The functional consequences of the mutations identified were analyzed by expressing the mutated RNA in Xenopus laevis oocytes.

RESULTS

Two patients, one with recurrent urolithiasis and one with bone demineralization, were heterozygous for two distinct mutations. One mutation resulted in the substitution of phenylalanine for alanine at position 48, and the other in a substitution of methionine for valine at position 147. Phosphate-induced current and sodium-dependent phosphate uptake were impaired in oocytes expressing the mutant NPT2a. Coinjection of oocytes with wild-type and mutant RNA indicated that the mutant protein had altered function.

CONCLUSIONS

Heterozygous mutations in the NPT2a gene may be responsible for hypophosphatemia and urinary phosphate loss in persons with urolithiasis or bone demineralization.

Parathyroid hormone and parathyroid hormone-related protein exert both pro- and anti-apoptotic effects in mesenchymal cells

During bone formation, multipotential mesenchymal cells proliferate and differentiate into osteoblasts, and subsequently many die because of apoptosis. Evidence suggests that the receptor for parathyroid hormone (PTH) and parathyroid hormone-related protein (PTHrP), the PTH-1 receptor (PTH-1R), plays an important role in this process. Multipotential mesenchymal cells (C3H10T1/2) transfected with normal or mutant PTH-1Rs and MC3T3-E1 osteoblastic cells were used to explore the roles of PTH, PTHrP, and the PTH-1R in cell viability relative to osteoblastic differentiation. Overexpression of wild-type PTH-1R increased cell numbers and promoted osteocalcin gene expression versus inactivated mutant receptors. Furthermore, the effects of PTH and PTHrP on apoptosis were dramatically dependent on cell status. In preconfluent C3H10T1/2 and MC3T3-E1 cells, PTH and PTHrP protected against dexamethasone-induced reduction in cell viability, which was dependent on cAMP activation. Conversely, PTH and PTHrP resulted in reduced cell viability in postconfluent cells, which was also dependent on cAMP activation. Further, the proapoptotic-like effects were associated with an inhibition of Akt phosphorylation. These data suggest that parathyroid hormones accelerate turnover of osteoblasts by promoting cell viability early and promoting cell departure from the differentiation program later in their developmental scheme. Both of these actions occur at least in part via the protein kinase A pathway.

Parathyroid hormone-related protein in epithelial lining fluid in humans negatively correlates with the severity of lung injury

OBJECTIVE

To determine the concentration of parathyroid hormone-related protein (PTHrP; an autocrine/paracrine regulator of type-2 alveolar epithelial cells proliferation and apoptosis) in the epithelial lining fluid (ELF) from patients without pulmonary disease and from patients with acute lung injury (ALI), and to evaluate whether PTHrP concentrations correlated with the intensity of lung injury.

DESIGN

Prospective study.

SETTING

An adult trauma/surgical ICU in an urban teaching hospital.

PATIENTS

A total of 20 patients with ALI receiving mechanical ventilation (patients), and 10 patients without pulmonary disease not receiving mechanical ventilation (control subjects).

INTERVENTIONS

None.

MEASUREMENTS AND RESULTS

PTHrP was detected in all BAL fluids, and ELF PTHrP concentrations (median; 25% to 75% percentiles) tended to be higher in patients (52.2 nmol/mL; 20.8 to 65.6 nmol/mL) than in control subjects (25.4 nmol/mL; 20.5 to 35.4 nmol/mL; p = 0.18). In patients, ELF PTHrP concentration correlated positively with the PaO(2)/fraction of inspired oxygen ratio (r = 0.53; p = 0.005), and negatively with lung injury score (r = - 0.44; p = 0.02), radiologic score (r = - 0.40; p = 0.04), and BAL albumin concentration (r = - 0.42; p = 0.02).

CONCLUSION

PTHrP is present in biologically significant concentrations in the alveolar milieu in humans. In patients with ALI, the PTHrP concentration correlates negatively with the degree of lung injury.

Comparison of pencil-, fan-, and cone-beam dual X-ray absorptiometers for evaluation of bone mineral content in excised rat bone

The aim of the present study was to assess the reproducibility and accuracy of measurements done on excised rat bone with three different generations of densitometers: Hologic QDR2000 pencil beam, Hologic QDR4500 fan beam, and Lunar PIXImus cone beam. The coefficients of variation for repeated measurements of bone mineral content (BMC) were 0.62 and 0.85% for pencil beam, 1.73 and 3.59% for fan beam, and 0.70 and 1.52% for cone beam for femur and tibia, respectively. BMC and ash weight were linearly correlated: 0.998 for pencil, 0.984 for fan, and 0.995 for cone beam. However, the three densitometers overestimated BMC by 10.9, 12.6, and 3.1%, respectively, and the overestimation was found to be dependent on the net BMC. The highest coefficient of correlation was found between BMC measurements from pencil and cone beam (r = 0.995). Data from cone-beam DXA were, respectively, 8.8 and 9.2% lower than those from penciland fan-beam DXA. We conclude that the three DXA instruments precisely and accurately measure BMC in excised rat bone; however, DXA overestimates BMC with a dependence on the bone ash weight. This dependence was less pronounced with the cone-beam technology.

Parathyroid hormone-related protein and its receptors: nuclear functions and roles in the renal and cardiovascular systems, the placental trophoblasts and the pancreatic islets

The cloning of the so-called 'parathyroid hormone-related protein' (PTHrP) in 1987 was the result of a long quest for the factor which, by mimicking the actions of PTH in bone and kidney, is responsible for the hypercalcemic paraneoplastic syndrome, humoral calcemia of malignancy. PTHrP is distinct from PTH in a number of ways. First, PTHrP is the product of a separate gene. Second, with the exception of a short N-terminal region, the structure of PTHrP is not closely related to that of PTH. Third, in contrast to PTH, PTHrP is a paracrine factor expressed throughout the body. Finally, most of the functions of PTHrP have nothing in common with those of PTH. PTHrP is a poly-hormone which comprises a family of distinct peptide hormones arising from post-translational endoproteolytic cleavage of the initial PTHrP translation products. Mature N-terminal, mid-region and C-terminal secretory forms of PTHrP are thus generated, each of them having their own physiologic functions and probably their own receptors. The type 1 PTHrP receptor, binding both PTH(1-34) and PTHrP(1-36), is the only cloned receptor so far. PTHrP is a PTH-like calciotropic hormone, a myorelaxant, a growth factor and a developmental regulatory molecule. The present review reports recent aspects of PTHrP pharmacology and physiology, including: (a) the identification of new peptides and receptors of the PTH/PTHrP system; (b) the recently discovered nuclear functions of PTHrP and the role of PTHrP as an intracrine regulator of cell growth and cell death; (c) the physiological and developmental actions of PTHrP in the cardiovascular and the renal glomerulo-vascular systems; (d) the role of PTHrP as a regulator of pancreatic beta cell growth and functions, and, (e) the interactions of PTHrP and calcium-sensing receptors for the control of the growth of placental trophoblasts. These new advances have contributed to a better understanding of the pathophysiological role of PTHrP, and will help to identify its therapeutic potential in a number of diseases.

[Molecular aspects of phosphate homeostasis in mammals]

Renal phosphate reabsorption, the major determinant of phosphate homeostasis, is primarily dependent on dietary phosphate content and multiple hormonal factors. Over the last few years, the identification of sodium-dependent phosphate transporters in kidney, intestine and bone, as well as new insights into the molecular mechanisms involved in several hereditary hypophosphatemias, allow to set up novel phosphate reabsorption regulatory pathways. This review describes molecular players involved in these mechanisms, summarizes phosphate transport data in kidney, intestine and bone, and describes recent findings concerning the three most common hereditary hypophosphatemias.

P-glycoprotein inhibitors stimulate renal phosphate reabsorption in rats

BACKGROUND

Dipyridamole (Dip) was previously shown to increase renal phosphate (Pi) reabsorption in humans. However, the mechanism(s) underlying this renal tubular effect is not fully elucidated. It is known that Dip inhibits the activity of the P-glycoprotein (Pgp) multidrug resistance protein 1 (MDR1) expressed on the apical membrane of renal proximal tubular cells where the Na-Pi cotransporter (NPT2) is also expressed. We hypothesized that Dip could increase renal Pi reabsorption by inhibiting Pgp activity.

METHODS

To test this hypothesis, the effects of Dip, verapamil (Ver), and cyclosporine A (CsA), three unrelated Pgp inhibitors, were studied on the renal Pi reabsorption in rats.

RESULTS

All three drugs decreased the fractional excretion of Pi (FE(Pi)) in a dose-dependent manner within one hour after beginning the drug infusion, without altering the glomerular filtration rate or serum parathyroid hormone concentration. Sodium-dependent Pi uptake but not Na-glucose transport was increased in brush-border membrane vesicles (BBMVs) when comparing treated with untreated rats. Western blot analysis showed that NPT2 protein was increased in BBMVs from treated rats. Dip and Ver had no effect when applied directly to BBMVs prepared from untreated rats. Pretreatment of rats with colchicine prevented the effects of Dip on the FE(Pi) and NPT2 expression in brush-border membranes.

CONCLUSIONS

Our results suggest that inhibition of Pgp in the proximal tubule increases Pi uptake and NPT2 translocation to the apical membrane.

Absence of functional type 1 parathyroid hormone (PTH)/PTH-related protein receptors in humans is associated with abnormal breast development and tooth impaction

Recent studies in transgenic mice have demonstrated that PTH-related protein (PTHrP), signaling through the type 1 PTH/PTHrP receptor (PTHR1), regulates endochondral bone development and epithelial-mesenchymal interactions during the formation of the mammary glands and teeth. Recently, it has been shown that loss-of-function mutations in the PTHR1 gene result in a rare, lethal form of dwarfism known as Blomstrand chondrodysplasia. These patients suffer from severe defects in endochondral bone formation, but abnormalities in breast and tooth development have not been reported. To ascertain whether PTHrP signaling was important to human breast and tooth development, we studied two fetuses with Blomstrand chondrodysplasia. These fetuses lack nipples and breasts. Developing teeth were present, but they were severely impacted within the surrounding alveolar bone, leading to distortions in their architecture and orientation. Compatible with the involvement of PTHR1 and PTHrP in human breast and tooth morphogenesis, both were expressed within the developing breasts and teeth of normal human fetuses. Therefore, impairment of the PTHrP/PTHR1 signaling pathway in humans is associated with severe abnormalities in tooth and breast development. In addition to regulating human bone formation, this signaling pathway is also necessary for the normal development of the human breast and tooth.

Molecular aspects of renal tubular handling and regulation of inorganic sulfate

The renal proximal tubular reabsorption of sulfate plays an important role in the maintenance of sulfate homeostasis. Two different renal sulfate transport systems have been identified and characterized at the molecular level in the past few years: NaSi-1 and Sat-1. NaSi-1 belongs to a Na(+)-coupled transporter family comprising the Na(+)-dicarboxylate transporters and the recently characterized SUT1 sulfate transporter. NaSi-1 is a Na(+)-sulfate cotransporter located exclusively in the brush border membrane of renal proximal tubular and ileal cells. Recently, NaSi-1 was shown to be regulated at the protein and mRNA level by a number of factors, such as vitamin D, dietary sulfate, glucocorticoids and thyroid hormones, which are known to modulate sulfate reabsorption in vivo. The second member of renal sulfate transporters, denoted Sat-1, belongs to a family of Na+-independent sulfate transporter family comprising the DTDST, DRA and PDS genes. Sat-1 is a sulfate/bicarbonate-oxalate exchanger located at the basolateral membrane of proximal tubular epithelial cells and canalicular surface of hepatic cells. Contrary to NaSi-1, no physiological factor has been found to date to regulate Sat-1 gene expression. Both NaSi-1 and Sat-1 transporter activities are implicated in pathophysiological states such as heavy metal intoxication and chronic renal failure. This review focuses on recent developments in the molecular characterization of NaSi-1 and Sat-1 and the mechanisms involved in their regulation.

Sulfate homeostasis, NaSi-1 cotransporter, and SAT-1 exchanger expression in chronic renal failure in rats

BACKGROUND

It is known that hypersulfatemia, like hyperphosphatemia, occurs in chronic renal failure (CRF). The aim of this study was to assess the effects of CRF on sulfate homeostasis and on sodium sulfate cotransport (NaSi-1) and sulfate/oxalate-bicarbonate exchanger (Sat-1) expression in the kidney. In addition, sulfate homeostasis was compared with phosphate homeostasis.

METHODS

Experimental studies were performed in adult male rats at three and six weeks after 80% subtotal nephrectomy (Nx) or sham-operation (S) (N = 9 per group). Transporter protein and mRNA expressions were measured by Western blot and RNase protection assay (RPA), respectively. Results were quantitated by densitometric scanning (Western) and electronic autoradiography (RPA), and were expressed in densitometric units (DUs; Western) and cpm (RPA).

RESULTS

Creatinine clearance was lower in Nx-3 compared with S-3 rats (0.23 vs. 0.51 mL/min/100 g body weight, P < 0.001) and was further impaired in Nx-6 rats (0.15 vs. 0.48, P < 0.001). Sulfatemia was significantly higher in Nx-3 rats (1.08 vs. 0.84 mmol/L, P < 0.05) and further increased in Nx-6 rats (1.42 vs. 0.90 mmol/L, P < 0.01). Fractional sulfate excretion (FESO4) was increased by twofold in Nx-3 and Nx-6 rats compared with corresponding S rats. Phosphatemia did not differ between Nx-3 rats and controls, but was increased in Nx-6 rats (P < 0.01). Total amounts of both NaSi-1 and Sat-1 proteins were significantly decreased in both Nx-3 and Nx-6 rats when compared with controls. However, NaSi-1 protein and mRNA densities did not significantly change in Nx-3 rats, but were significantly increased in Nx-6 rats when compared with controls (4.8 vs. 3.7 DU/microg protein, P < 0.05, and 7.1 vs. 2.8 cpm/microg RNA, P < 0.01, respectively, for protein and mRNA). In contrast to NaSi-1, Sat-1 protein density was significantly decreased both in Nx-3 (2.9 vs. 3.6 DU/microg protein, P < 0.05) and Nx-6 rats (2.4 vs. 3.4 DU/microg protein, P < 0.05), and Sat-1 mRNA density significantly decreased in Nx-6 rats (10.7 vs. 14.7 cpm/microg RNA, P < 0.05). Na-PO4 cotransporter (NaPi-2) protein total abundance and density were decreased at three and six weeks in Nx rats.

CONCLUSIONS

These results demonstrate that both NaSi-1 and Sat-1 total protein abundances are decreased in CRF, which may contribute to the increase in fractional sulfate excretion. Strikingly, NaSi-1 density was not decreased in CRF three weeks after Nx, and furthermore, increased six weeks after Nx, in contrast to NaPi-2 density, which was decreased at both times. The significance of this difference remains to be determined, but may explain why hypersulfatemia occurs earlier than hyperphosphatemia in CRF.

Parathyroid hormone-parathyroid hormone-related peptide receptor expression and function in otosclerosis

The aim of this study was to investigate the possibility that an abnormality related to parathyroid hormone (PTH) action is involved in the increased bone turnover observed in otosclerosis. To do so, expression and function of the PTH-PTH-related peptide (PTHrP) receptor were studied in the involved tissue (stapes) and compared with that in control bone sample obtained from the external auditory canal (EAC) in the same patient in 10 cases of otosclerosis and in 1 case of osteogenesis imperfecta. PTH-PTHrP receptor expression was studied by RT-PCR of RNA prepared from cultured cells in three patients and RNA directly extracted from bone samples in four patients. PTH-PTHrP receptor function was assessed by measuring the stimulation of cAMP production by 0.8, 8, and 80 nM PTH in bone cell cultures in seven cases. Results showed that PTH-PTHrP receptor mRNA expression in the otosclerotic stapes was lower than that in EAC samples (P < 0.05), whereas it was higher in stapes than that in EAC in the case of osteogenesis imperfecta. cAMP production after PTH stimulation was lower in bone cells cultured from otosclerotic stapes compared with that in cells cultured from EAC (range of increase in stimulation: 0.8-4.5 and 1.5-7 in stapes and EAC bone cells, respectively, P < 0.05). In contrast, the stimulation of cAMP production by forskolin was not significantly different in otosclerotic stapes and EAC bone cells (range of increase in stimulation: 20.7-83.1 and 4.9-99.8 in stapes and EAC, respectively, P > 0.05). These results show a lower stimulation of cAMP production in response to PTH associated with a lower PTH-PTHrP receptor mRNA expression in pathological stapes from patients with otosclerosis compared with that in control EAC samples. This difference supports the hypothesis that an abnormal cellular response to PTH contributes to the abnormal bone turnover in otosclerosis.

NaPO(4) cotransport type III (PiT1) expression in human embryonic kidney cells and regulation by PTH

The aim of the present study was to characterize the type(s) of NaPO(4) cotransporter expressed in the human renal cell line HEK-293 and its regulation by parathyroid hormone (PTH) in wild-type cells and in cells transfected by the PTH/PTH-related protein (PTHrP) receptor. The results showed that human embryonic kidney HEK-293 cells expressed NaPO(4) cotransporter type III (PiT1) mRNA and protein. In contrast, type I (NPT1) or II (NPT2) cotransporter mRNA were not expressed. Na(+)-dependent phosphate uptake followed a Michaelis-Menten model (apparent maximal transport rate and affinity constant: 23.32 +/- 0.69 nmol PO(4). mg protein(-1). 10 min(-1) and 0.147 +/- 0.014 mM KH(2)PO(4), respectively), was stimulated by phosphate deprivation (maximal increase 24.5 +/- 0.8%, P < 0.001, after 15 h of phosphate deprivation), and was inhibited by increasing pH (3.6 +/- 0.2-fold decrease at pH 8.5, P < 0.0001). It was inhibited in a time- and concentration-dependent fashion by PTH in HEK-293 cells stably transfected by PTH/PTHrP receptors but not in parental HEK-293 cells. Maximal inhibition of Na(+)-dependent phosphate transport was observed at 30 min after the addition of 72 nM PTH-(1-34) (31.5 +/- 2.4% inhibition, P < 0.01). PTH inhibition of phosphate transport was maintained in phosphate-deprived cells and reversed by both GF109203X (10(-6) M) or staurosporine (5.5 nM), two protein kinase C inhibitors. Na(+)-dependent phosphate uptake was also significantly inhibited by phorbol 12-myristate 13-acetate (20.9 +/- 3.9% inhibition, P < 0.001) but not by dibutyril-cAMP (10(-4) M) or forskolin (50 microM). The physiological role played by type III NaPO(4) cotransport expression in the overall renal regulation of phosphate homeostasis remains to be established.

A novel parathyroid hormone (PTH)/PTH-related peptide receptor mutation in Jansen's metaphyseal chondrodysplasia

Two heterozygous PTH/PTH-related peptide (PTHrP) receptor missense mutations were previously identified in patients with Jansen's metaphyseal chondrodysplasia (JMC), a rare form of short limb dwarfism associated with hypercalcemia and normal or undetectable levels of PTH and PTHrP. Both mutations, H223R and T410P, resulted in constitutive activation of the cAMP signaling pathway and provided a plausible explanation for the abnormalities in skeletal development and mineral ion homeostasis. In the present study we analyzed genomic DNA from four additional sporadic cases with JMC to search for novel activating mutations in the PTH/PTHrP receptor, to determine the frequency of the two previously identified missense mutations, H223R and T410P, and to determine whether different mutations present with different severity of the disease. The H223R mutation was identified in three novel JMC patients and is, therefore, to date the most frequent cause of JMC. In the fourth patient, a novel heterozygous missense mutation was found that changes isoleucine 458 in the receptor's seventh membrane-spanning region to arginine (I458R). In COS-7 cells expressing the human PTH/PTHrP receptor with the I458R mutation, basal cAMP accumulation was approximately 8 times higher than that in cells expressing the wild-type receptor despite impaired surface expression of the mutant receptor. Furthermore, the I458R mutant showed higher responsiveness to PTH than the wild-type receptor in its ability to activate both downstream effectors, adenylyl cyclase and phospholipase C. Like the H223R and the T410P mutants, the I458R mutant had no detectable effect on basal inositol phosphate accumulation. Overall, the patient with the I458R mutation exhibited clinical and biochemical abnormalities similar to those in patients with the previously identified H223R and T410P mutations.

A homozygous inactivating mutation in the parathyroid hormone/parathyroid hormone-related peptide receptor causing Blomstrand chondrodysplasia

We describe a patient with Blomstrand chondrodysplasia, a lethal genetic disorder characterized by extremely advanced endochondral bone maturation, in whom a homozygous missense mutation is present in the gene coding for the PTH/PTHrP receptor that leads to the substitution of a proline for a leucine in the N-terminal portion of the receptor (P132L). PTH-induced cAMP accumulation was severely reduced in COS-7 cells expressing P132L receptors compared to that of cells expressing wild-type receptors, and PTH-induced inositol phosphate accumulation was not detectable in cells expressing the mutant receptor. Similar results were obtained using PTHrP as an agonist. Maximal specific binding of radioiodinated [Tyr36]PTHrp(1-36) by cells transfected with the P132L receptor was < 10% of that observed for cells transfected with the wild-type receptor. Despite the reduction in radioligand binding to P132L receptors, the intensity and distribution of the fluorescent signal resulting from the expression of receptors fused to GFP were similar for cells transfected with the wild-type and mutant P132L receptors, suggesting a similar degree of cell surface expression. These results firmly establish the role of abnormalities in the PTH/PTHrP receptor in the pathogenesis of Blomstrand chondrodysplasia, and thereby confirm the importance of signaling through the PTH/PTHrP receptor in human fetal skeletal development. Because the amino-acid mutated in the patient described here is otherwise conserved in all mammalian class II G protein-coupled receptors, this abnormality may provide insights into structural features needed for the normal function of this family of receptors.

Absence of functional receptors for parathyroid hormone and parathyroid hormone-related peptide in Blomstrand chondrodysplasia

We report the absence of functional parathyroid hormone (PTH)/PTH-related peptide (PTHrP) receptors (PTH/PTHrP receptor) in Blomstrand chondrodysplasia, a genetic disorder characterized by advanced endochondral bone maturation. Analysis of PTH/PTHrP receptor genomic DNA from a patient with Blomstrand chondrodysplasia demonstrated that the patient was heterozygous for a point mutation (G--> A substitution at nucleotide 1176) inherited from the mother. Analysis of PTH/PTHrP receptor cDNA demonstrated that: (a) this point mutation caused the deletion of the first 11 amino acids of exon M5 (encoding the fifth transmembrane domain of the receptor), resulting from the use of a novel splice site created by the base substitution; (b) the mutant receptor was well expressed in COS-7 cells, but did not bind PTH or PTHrP, and failed to induce detectable stimulation of either cAMP or inositol phosphate production in response to these ligands; and (c) the paternal allele was not expressed. Thus, only the abnormal and nonfunctional PTH/PTHrP receptors encoded by the maternal allele were expressed by chondrocytes from this patient. In view of the known role played by the PTH/PTHrP receptor in bone and cartilage development, these results strongly support the conclusion that the absence of functional PTH/ PTHrP receptors is responsible for the skeletal abnormalities seen in Blomstrand chondrodysplasia, abnormalities that are the mirror image of those observed in Jansen's chondrodysplasia. These findings emphasize the importance of signaling through this receptor in human fetal skeletal development.