Abnormal response of osteoblasts from Hyp mice to 1,25-dihydroxyvitamin D3

Insights into the Molecular and Hormonal Regulation of Complications of X-Linked Hypophosphatemia

X-linked hypophosphatemia (XLH) is characterized by mutations in the PHEX gene, leading to elevated serum levels of FGF23, decreased production of 1,25 dihydroxyvitamin D3 (1,25D), and hypophosphatemia. Those affected with XLH manifest impaired growth and skeletal and dentoalveolar mineralization as well as increased mineralization of the tendon–bone attachment site (enthesopathy), all of which lead to decreased quality of life. Many molecular and murine studies have detailed the role of mineral ions and hormones in regulating complications of XLH, including how they modulate growth and growth plate maturation, bone mineralization and structure, osteocyte-mediated mineral matrix resorption and canalicular organization, and enthesopathy development. While these studies have provided insight into the molecular underpinnings of these skeletal processes, current therapies available for XLH do not fully prevent or treat these complications. Therefore, further investigations are needed to determine the molecular pathophysiology underlying the complications of XLH.

Phosphate homeostasis disorders

Our understanding of the regulation of phosphate balance has benefited tremendously from the molecular identification and characterization of genetic defects leading to a number of rare inherited or acquired disorders affecting phosphate homeostasis. The identification of the key phosphate-regulating hormone, fibroblast growth factor 23 (FGF23), as well as other molecules that control its production, such as the glycosyltransferase GALNT3, the endopeptidase PHEX, and the matrix protein DMP1, and molecules that function as downstream effectors of FGF23 such as the longevity factor Klotho and the phosphate transporters NPT2a and NPT2c, has permitted us to understand the complex interplay that exists between the kidneys, bone, parathyroid, and gut. Such insights from genetic disorders have allowed not only the design of potent targeted treatment of FGF23-dependent hypophosphatemic conditions, but also provide clinically relevant observations related to the dysregulation of mineral ion homeostasis in health and disease.

A novel Phex mutation in a new mouse model of hypophosphatemic rickets

X-linked hypophosphatemic rickets (XLH) is a dominantly inherited disease characterized by renal phosphate wasting, aberrant vitamin D metabolism, and defective bone mineralization. It is known that XLH in humans and in certain mouse models is caused by inactivating mutations in PHEX/Phex (phosphate-regulating gene with homologies to endopeptidases on the X chromosome). By a genome-wide N-ethyl-N-nitrosourea (ENU)-induced mutagenesis screen in mice, we identified a dominant mouse mutation that exhibits the classic clinical manifestations of XLH, including growth retardation, skeletal abnormalities (rickets/osteomalacia), hypophosphatemia, and increased serum alkaline phosphatase (ALP) levels. Mapping and sequencing revealed that these mice carry a point mutation in exon 14 of the Phex gene that introduces a stop codon at amino acid 496 of the coding sequence (Phex(Jrt) also published as Phex(K496X) [Ichikawa et al., 2012]). Fgf23 mRNA expression as well as that of osteocalcin, bone sialoprotein, and matrix extracellular phosphoglycoprotein was upregulated in male mutant long bone, but that of sclerostin was unaffected. Although Phex mRNA is expressed in bone from mutant hemizygous male mice (Phex(Jrt)/Y mice), no Phex protein was detected in immunoblots of femoral bone protein. Stromal cultures from mutant bone marrow were indistinguishable from those of wild-type mice with respect to differentiation and mineralization. The ability of Phex(Jrt)/Y osteoblasts to mineralize and the altered expression levels of matrix proteins compared with the well-studied Hyp mice makes it a unique model with which to further explore the clinical manifestations of XLH and its link to FGF23 as well as to evaluate potential new therapeutic strategies.

Hereditary hypophosphatemic rickets with hypercalciuria: a study for the phosphate transporter gene type IIc and osteoblastic function

Two cases of hereditary hypophosphatemic rickets with hypercalciuria (HHRH) were reported in Japanese female siblings. Both of them manifested short stature and bowed legs, and biochemical examination revealed hypophosphatemia, phosphaturia, and hypercalciuria. The serum concentrations of 1,25-dihydroxyvitamin D (1,25(OH)(2)D) were elevated. In the oral phosphate loading test, serum phosphate levels were markedly increased in the HHRH patients, and the elevation was much higher than that in patients affected with X-linked hypophosphatemic rickets (XLH), suggesting the increased gastrointestinal absorption of phosphate in HHRH. Bone histology studies showed increased osteoid surface and width in HHRH, which was compatible with osteomalacia. In the HHRH patients, there were no hypomineralized periosteocytic lesions, which was a hallmark of XLH in bone histology. In one of the HHRH patients, phosphate administration alone almost completely cured the osteomalacia within a year, although pharmacological doses of 1,25(OH)(2)D(3) had little effect. In osteoblasts isolated from a HHRH patient, basal alkaline phosphatase (ALP) activities and osteocalcin syntheses by a physiological concentration of 1,25(OH)(2)D(3) were not stimulated by the increased medium phosphate concentrations from 0.5 to 4 mM. In contrast, these two parameters were stimulated by the increased medium phosphate concentrations both in normal and XLH osteoblasts, although the regulatory patterns of increased osteocalcin syntheses were different from normal to XLH osteoblasts; 2 and 4 mM of phosphate concentrations at least were necessary for normal and XLH osteoblasts, respectively. The gene analysis of phosphate transporter revealed a novel heterozygous mutation (R564C) in the exon of phosphate transporter NPT type IIc. These lines of evidence suggested that the pathogenesis of osteomalacia in HHRH was different from XLH in terms of the utility of phosphate in osteoblasts. These abnormalities were speculated to be associated with the abnormal functions of phosphate transporter gene type IIc, although the exact roles of this phosphate transporter in the human osteoblast are still unknown.

Correction of the mineralization defect in hyp mice treated with protease inhibitors CA074 and pepstatin

Increased expression of several osteoblastic proteases and MEPE (a bone matrix protein) occurs in X-linked hypophosphatemic rickets (hyp). This is associated with an increased release of a protease-resistant MEPE peptide (ASARM peptide), a potent inhibitor of mineralization. Cathepsin B cleaves MEPE releasing ASARM peptide and hyp osteoblast/osteocyte cells hypersecrete cathepsin D, an activator of cathepsin B. Our aims were to determine whether cathepsin inhibitors correct the mineralization defect in vivo and whether hyp-bone ASARM peptide levels are reduced after protease treatment. Normal littermates and hyp mice (n = 6) were injected intraperitoneally once a day for 4 weeks with pepstatin, CAO74 or vehicle. Animals were then sacrificed and bones plus serum removed for comprehensive analysis. All hyp mice groups (treated and untreated) remained hypophosphatemic with serum 1,25 vitamin D3 inappropriately normal. Serum PTH was significantly elevated in all hyp mice groups relative to normal mice (P = 0.0017). Untreated hyp mice had six-fold elevated levels of serum alkaline-phosphatase and two-fold elevated levels of ASARM peptides relative to normal mice (P < 0.001). In contrast, serum alkaline phosphatase and serum ASARM peptides were significantly reduced (normalized) in hyp mice treated with CA074 or pepstatin. Serum FGF23 levels remained high in all hyp animal groups (P < 0.0001). Hyp mice treated with protease inhibitors showed dramatic reductions in unmineralized osteoid (femurs) compared to control hyp mice (Goldner staining). Also, hyp animals treated with protease inhibitors showed marked and significant improvements in growth plate width (42%), osteoid thickness (40%) and cortical area (40%) (P < 0.002). The mineralization apposition rate, bone formation rate and mineralization surface were normalized by protease-treatment. High-resolution pQCT mineral histomorphometry measurements and uCT also confirmed a marked mineralization improvement. Finally, the growth plate and cortical bone of hyp femurs contained a massive accumulation of osteoblast-derived ASARM peptide(s) that was reduced in hyp animals treated with CA074 or pepstatin. This study confirms in vivo administration of cathepsin inhibitors improves bone mineralization in hyp mice. This may be due to a protease inhibitor mediated decrease in proteolytic degradation of the extracellular matrix and a reduced release of ASARM peptides (potent mineralization inhibitors).

Overexpression of human PHEX under the human beta-actin promoter does not fully rescue the Hyp mouse phenotype

XLH in humans and the Hyp phenotype in mice are caused by inactivating Phex mutations. Overexpression of human PHEX under the human beta-actin promoter in Hyp mice rescued the bone phenotype almost completely, but did not affect phosphate homeostasis, suggesting that different, possibly independent, pathophysiological mechanisms contribute to hyperphosphaturia and bone abnormalities in XLH.

INTRODUCTION

Mutations in PHEX, a phosphate-regulating gene with homologies to endopeptidases on the X chromosome, are responsible for X-linked hypophosphatemia (XLH) in humans, and its mouse homologs, Hyp, Phex(Hyp-2J), Phex(Hyp-Duk), Gy, and Ska1. PHEX is thought to inactivate a phosphaturic factor, which may be fibroblast growth factor 23 (FGF)-23. Consistent with this hypothesis, FGF-23 levels were shown to be elevated in most patients with XLH and in Hyp mice. The aim of this study was, therefore, to examine whether transgenic overexpression of PHEX under the human beta-actin promoter would rescue the Hyp phenotype.

MATERIALS AND METHODS

We tested this hypothesis by generating two mouse lines expressing human PHEX under the control of a human beta-actin promoter (PHEX-tg). With the exception of brain, RT-PCR analyses showed transgene expression in all tissues examined. PHEX protein, however, was only detected in bone, muscle, lung, skin, and heart. To assess the role of the mutant PHEX, we crossed female heterozygous Hyp mice with male heterozygous PHEX-tg mice to obtain wildtype (WT), PHEX-tg, Hyp, and Hyp/PHEX-tg offspring, which were examined at 3 months of age.

RESULTS

PHEX-tg mice exhibited normal bone and mineral ion homeostasis. Hyp mice showed the known phenotype with reduced body weight, hypophosphatemia, hyperphosphaturia, and rickets. Hyp/PHEX-tg mice had almost normal body weight relative to WT controls, showed a dramatic improvement in femoral BMD, almost normal growth plate width, and, despite remaining disturbances in bone mineralization, almost normal bone architecture and pronounced improvements of osteoidosis and of halo formation compared with Hyp mice. However, Hyp and Hyp/PHEX-tg mice had comparable reductions in tubular reabsorption of phosphate and were hypophosphatemic relative to WT controls.

CONCLUSION

Our data suggest that different, possibly independent, pathophysiological mechanisms contribute to renal phosphate wasting and bone abnormalities in Hyp and XLH.

Surface plasmon resonance (SPR) confirms that MEPE binds to PHEX via the MEPE-ASARM motif: a model for impaired mineralization in X-linked rickets (HYP)

Matrix Extracellular Phospho-glycoprotEin (MEPE) and proteases are elevated and PHEX is defective in HYP. PHEX prevents proteolysis of MEPE and release of a protease-resistant MEPE-ASARM peptide, an inhibitor of mineralization (minhibin). Thus, in HYP, mutated PHEX may contribute to increased ASARM peptide release. Moreover, binding of MEPE by PHEX may regulate this process in normal subjects. The nature of the PHEX-MEPE nonproteolytic interaction(s) (direct or indirect) is/are unknown. Our aims were to determine (1) whether PHEX binds specifically to MEPE, (2) whether the binding involves the ASARM motif region, and (3) whether free ASARM peptide affects mineralization in vivo in mice. Protein interactions between MEPE and recombinant soluble PHEX (secPHEX) were measured using surface plasmon resonance (SPR). Briefly, secPHEX, MEPE, and control protein (IgG) were immobilized on a Biacore CM5 sensor chip, and SPR experiments were performed on a Biacore 3000 high-performance research system. Pure secPHEX was then injected at different concentrations, and interactions with immobilized proteins were measured. To determine MEPE sequences interacting with secPHEX, the inhibitory effects of MEPE-ASARM peptides (phosphorylated and nonphosphorylated), control peptides, and MEPE midregion RGD peptides on secPHEX binding to chip-immobilized MEPE were measured. ASARM peptide and etidronate-mediated mineralization inhibition in vivo and in vitro were determined by quenched calcein fluorescence in hind limbs and calvariae in mice and by histological Sanderson stain. A specific, dose-dependent and Zn-dependent protein interaction between secPHEX and immobilized MEPE occurs (EC50 of 553 nM). Synthetic MEPE PO4-ASARM peptide inhibits the PHEX-MEPE interaction (K(D(app)) = 15 uM and B(max/inhib) = 68%). In contrast, control and MEPE-RGD peptides had no effect. Subcutaneous administration of ASARM peptide resulted in marked quenching of fluorescence in calvariae and hind limbs relative to vehicle controls indicating impaired mineralization. Similar results were obtained with etidronate. Sanderson-stained calvariae also indicated a marked increase in unmineralized osteoid with ASARM peptide and etidronate groups. We conclude that PHEX and MEPE form a nonproteolytic protein interaction via the MEPE carboxy-terminal ASARM motif, and the ASARM peptide inhibits mineralization in vivo. The binding of MEPE and ASARM peptide by PHEX may explain why loss of functional osteoblast-expressed PHEX results in defective mineralization in HYP.

The wrickkened pathways of FGF23, MEPE and PHEX

The last 350 years since the publication of the first medical monograph on rickets (old English term wrickken) (Glisson et al., 1651) have seen spectacular advances in our understanding of mineral-homeostasis. Seminal and exciting discoveries have revealed the roles of PTH, vitamin D, and calcitonin in regulating calcium and phosphate, and maintaining healthy teeth and skeleton. However, it is clear that the PTH/Vitamin D axis does not account for the entire picture, and a new bone-renal metabolic milieu has emerged, implicating a novel set of matrix proteins, hormones, and Zn-metallopeptidases. The primary defects in X-linked hypophosphatemic rickets (HYP) and autosomal-dominant hypophosphatemic rickets (ADHR) are now identified as inactivating mutations in a Zn-metalloendopeptidase (PHEX) and activating mutations in fibroblast-growth-factor-23 (FGF23), respectively. In oncogenic hypophosphatemic osteomalacia (OHO), several tumor-expressed proteins (MEPE, FGF23, and FRP-4) have emerged as candidate mediators of the bone-renal pathophysiology. This has stimulated the proposal of a global model that takes into account the remarkable similarities between the inherited diseases (HYP and ADHR) and the tumor-acquired disease OHO. In HYP, loss of PHEX function is proposed to result in an increase in uncleaved full-length FGF23 and/or inappropriate processing of MEPE. In ADHR, a mutation in FGF23 results in resistance to proteolysis by PHEX or other proteases and an increase in half-life of full-length phosphaturic FGF23. In OHO, over-expression of FGF23 and/or MEPE is proposed to result in abnormal renal-phosphate handling and mineralization. Although this model is attractive, many questions remain unanswered, suggesting a more complex picture. The following review will present a global hypothesis that attempts to explain the experimental and clinical observations in HYP, ADHR, and OHO, plus diverse mouse models that include the MEPE null mutant, HYP-PHEX transgenic mouse, and MEPE-PHEX double-null-mutant.

1,25-dihydroxyvitamin D3 down-regulation of PHEX gene expression is mediated by apparent repression of a 110 kDa transfactor that binds to a polyadenine element in the promoter

The PHEX gene encodes an endopeptidase expressed in osteoblasts that inactivates an uncharacterized peptide hormone, phosphatonin, which suppresses bone mineralization as well as renal phosphate reabsorption and vitamin D bioactivation. We demonstrate that 1alpha-25-dihydroxyvitamin D (1,25(OH)2D3), the, active renal vitamin D metabolite, decreases PHEX mRNA in the rat osteoblastic cell line, UMR-106, as well as in mouse calvaria. Promoter/reporter construct analysis of the murine PHEX gene in transfected UMR-106 cells localized the repressive effect of 1,25(OH)2D3 to the -133 to -74 bp region, and gel mobility shift experiments revealed that 1,25(OH)2D3 treatment of the cells diminished the binding of a nuclear protein(s) to a stretch of 17 adenines from bp -116 to -100 in the proximal PHEX promoter. Either overexpression of a dominant-negative vitamin D receptor (VDR) or deletion of this sequence of 17 A-T base pairs abolished the repressive effect of 1,25(OH)2D3 by attenuating basal promoter activity, indicating that this region mediates the 1,25(OH)2D3 response and is involved in basal transcription. South-western blot analysis and DNA affinity purification show that an unidentified 110 kDa nuclear protein binds to the poly(A) element. Because 1,25(OH)2D3-liganded VDR neither binds to the polyadenine region of the PHEX promoter nor directly influences the association of the 110 kDa transfactor, we conclude that 1,25(OH)2D3 indirectly decreases PHEX expression via VDR-mediated repression (or modification) of this novel transactivator. Thus, we have identified a cis-element required for PHEX gene transcription that participates in negative feedback control of PHEX expression and thereby modulates the actions of phosphatonin.

MEPE has the properties of an osteoblastic phosphatonin and minhibin

Matrix extracellular phosphoglycoprotein (MEPE) is expressed exclusively in osteoblasts, osteocytes and odontoblasts with markedly elevated expression found in X-linked hypophosphatemic rickets (Hyp) osteoblasts and in oncogenic hypophosphatemic osteomalacia (OHO) tumors. Because these syndromes are associated with abnormalities in mineralization and renal phosphate excretion, we examined the effects of insect-expressed full-length human-MEPE (Hu-MEPE) on serum and urinary phosphate in vivo, (33)PO(4) uptake in renal proximal tubule cultures and mineralization of osteoblast cultures. Dose-dependent hypophosphatemia and hyperphosphaturia occurred in mice following intraperitoneal (IP) administration of Hu-MEPE (up to 400 microg kg(-1) 31 h(-1)), similar to mice given the phosphaturic hormone PTH (80 microg kg(-1) 31 h(-1)). Also the fractional excretion of phosphate (FEP) was stimulated by MEPE [65.0% (P < 0.001)] and PTH groups [53.3% (P < 0.001)] relative to the vehicle group [28.7% (SEM 3.97)]. In addition, Hu-MEPE significantly inhibited (33)PO(4) uptake in primary human proximal tubule renal cells (RPTEC) and a human renal cell line (Hu-CL8) in vitro (V(max) 53.4% inhibition; K(m) 27.4 ng/ml, and V(max) 9.1% inhibition; K(m) 23.8 ng/ml, respectively). Moreover, Hu-MEPE dose dependently (50-800 ng/ml) inhibited BMP2-mediated mineralization of a murine osteoblast cell line (2T3) in vitro. Inhibition of mineralization was localized to a small (2 kDa) cathepsin B released carboxy-terminal MEPE peptide (protease-resistant) containing the acidic serine-aspartate-rich motif (ASARM peptide). We conclude that MEPE promotes renal phosphate excretion and modulates mineralization.

Familial hypophosphatemic vitamin D-resistant rickets: dental findings and histologic study of teeth

A case of familial hypophosphatemic vitamin D-resistant rickets or X-linked hypophosphatemia (XLH) accompanied by specific systemic and dental findings is reported. A 15-year-old boy with XLH visited our facility complaining of a toothache in the left lower canine region. Two other family members of the patient, his younger sister and their mother, also had XLH, whereas the other 2 members, his younger brother and father, are healthy. Those with XLH show systemic signs of the disease, such as growth retardation, limb deformity, and spinal curvature disorders; however, these symptoms are more severe in the patient than in the others. The patient had multiple periodontal abscesses, but no evidence of dental caries, trauma, or periodontal disease on the corresponding teeth at the time of his oral examination. A radiographic examination showed root dysplasia and enlarged pulp chambers.A histologic examination of an extracted third molar showed marked globular dentin and an increased predentin width. The abscess was thought to be caused by pulpal infection, which came from bacterial invasion through enamel cracks and dentinal microcleavage of the teeth. The treatments provided in this case are discussed.

Hypophosphatemic rickets accompanying congenital microvillous atrophy

This report concerns an 11-year-old boy who manifested hypophosphatemic rickets associated with congenital microvillous atrophy (CMA). He had been suffering from vomiting and severe diarrhea from the first day of life and had been treated with total parenteral nutrition (TPN) since he was 67 days old. At 4 years of age, intestinal biopsy resulted in a diagnosis of CMA. He was admitted to our hospital complaining of leg pain at the age of 11. Laboratory data revealed hypophosphatemia, elevated serum 1, 25-dihydroxyvitamin D (1,25(OH)2D) levels, and hypercalciuria. A roentgenogram showed rickets in the extremities. A balance study of phosphate in urine and stool indicated that the amount of phosphate leaking into the stool was greater than that into the urine. Moreover, the total amount of phosphate leaking from both the intestine and kidney exceeded the amount of phosphate intake from TPN. The rickets was healed by increasing the phosphate concentration in TPN. This case is different from X-linked hypophosphatemic rickets but similar to hereditary hypophosphatemic rickets with hypercalciuria (HHRH) in terms of hypercalciuria and elevated serum 1,25(OH)2D levels. The effectiveness of phosphate treatments used here is also similar to that used for HHRH. However, this type of hypophosphatemic rickets is unique in that phosphate leaking into the intestine plays an important role in its pathogenesis.

Intrinsic mineralization defect in Hyp mouse osteoblasts

X-linked hypophosphatemia (XLH) is caused by inactivating mutations of PEX, an endopeptidase of uncertain function. This defect is shared by Hyp mice, the murine homologue of the human disease, in which a 3' Pex deletion has been documented. In the present study, we report that immortalized osteoblasts derived from the simian virus 40 (SV40) transgenic Hyp mouse (TMOb-Hyp) have an impaired capacity to mineralize extracellular matrix in vitro. Compared with immortalized osteoblasts from the SV40 transgenic normal mouse (TMOb-Nl), osteoblast cultures from the SV40 Hyp mouse exhibit diminished 45Ca accumulation into extracellular matrix (37 +/- 6 vs. 1,484 +/- 68 counts . min-1 . microgram protein-1) and reduced formation of mineralization nodules. Moreover, in coculture experiments, we found evidence that osteoblasts from the SV40 Hyp mouse produce a diffusible factor that blocks mineralization of extracellular matrix in normal osteoblasts. Our findings indicate that abnormal PEX in osteoblasts is associated with the accumulation of a factor(s) that inhibits mineralization of extracellular matrix in vitro.

The role of the PHEX gene (PEX) in families with X-linked hypophosphataemic rickets

For over a hundred years, the bane of rickets (a disease of bone), has been prominent in those countries that have participated in, and seeded, the industrial revolution. Industrialisation had major effects of the demography of populations, and many people moved to dark, heavily industrialised cities to find work. It soon became apparent that rickets could be cured by supplementing the diet with cod liver oil and exposure to sunlight. This in turn led to the discovery that photoactivation of 7-dehydrocholesterol was required to produce vitamin D, an indispensable regulator of bone mineral metabolism. Although inadequate exposure to light and poor dietary intake are the main causes of rickets and osteomalacia, recent research has confirmed the role of familial, and tumour forms of the disease. This review will describe the recent advances in our knowledge of the molecular defects in X-linked hypophosphataemic rickets (HYP), and oncogenic hypophosphataemic osteomalacia (OHO). Although HYP and OHO have different primary defects, both diseases have similarities that suggest a linked or overlapping pathophysiology. Also, without doubt, the recent cloning of the gene defective in HYP (the PHEX gene), has given researchers a new reagent to explore the molecular regulation of bone and its links to kidney endocrine function. The fact that the PHEX gene codes for a Zn metallopeptidase raises new and intriguing questions, and adds new momentum to the research on diseases of bone mineral metabolism.

Diagnosis of X-linked hypophosphatemic vitamin D resistant rickets

Recent advances in the pathophysiology and diagnosis of X-linked hypophosphatemic rickets (XLH) are reviewed. The recent discovery of the gene that is responsible for XLH especially enables us to understand the mechanism of hypophosphatemia in XLH. Laboratory and radiological findings are important for the diagnosis. However, dental abnormalities such as spontaneous dental abscess and a defect of dentin maturation are also notable findings.

Cloning and sequencing of human PEX from a bone cDNA library: evidence for its developmental stage-specific regulation in osteoblasts

Inactivating mutations of the neutral endopeptidase, PEX, have been identified as the cause of X-linked hypophosphatemia (XLH). Though the function of PEX is unknown, current information suggests that impaired renal phosphate conservation in XLH is due to the failure of PEX to either degrade an undefined phosphaturic factor or activate a novel phosphate-conserving hormone. The physiologically relevant target tissue for the XLH mutation has not been identified. An apparent intrinsic defect of osteoblast function in XLH implicates bone as a possible site of PEX expression. In the current investigation, we employed a polymerase chain reaction (PCR) strategy to amplify a PEX cDNA from a human bone cell cDNA library. We found that the human PEX cDNA encodes a 749 amino acid protein belonging to the type II integral membrane zinc-dependent endopeptidase family. The predicted PEX amino acid sequence shares 96.0% identify to the recently cloned mouse Pex cDNA and has 27-38% identity to other members of the metalloendopeptidase family. Using reverse transcriptase (RT)-PCR with PEX-specific primers, we detected PEX transcripts in both human osteosarcoma-derived MG-63 osteoblasts and in differentiated mouse MC3T3-E1 clonal osteoblasts but not in immature MC3T3-E1 preosteoblasts. The association of impaired mineralization of bone in XLH and the apparent developmental stage-specific expression of PEX in osteoblasts suggest that bone is a physiologically relevant site of PEX expression and that PEX may play an active role in osteoblast-mediated mineralization.

Abnormal modulation of serum osteocalcin by dietary phosphate and 1,25-dihydroxyvitamin D3 in the hypophosphatemic mouse

We evaluated in normal and hypophosphatemic (Hyp) mice whether changes in serum levels of osteocalcin in response to dietary phosphate supplementation, parathyroid hormone (PTH) and 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) administration were related to perturbations in calcium phosphate homeostasis. In normal mice, serum osteocalcin levels were not altered by phosphate supplementation. In contrast, phosphate supplementation in Hyp mice led to a 2-fold decrease in serum osteocalcin to normal levels after 3 days and to an increase in osteocalcin levels after 14 days. The decrease in osteocalcin was associated with normophosphatemia, severe hypocalcemia, and marked increases in circulating 1,25(OH)2D3 levels, whereas the increase in osteocalcin levels was associated with normophosphatemia and no change in serum calcium and 1,25(OH)2D3. Administration of PTH decreased serum osteocalcin in both genotypes. Infusion of 1,25(OH)2D3 for 3 days elicited increases in serum osteocalcin and calcium levels in normal mice, whereas in Hyp mice it produced significant decreases in osteocalcin levels and no change in serum calcium. However, with a more prolonged infusion of 1,25(OH)2D3, hypercalcemia and increases in serum osteocalcin were induced in mutant mice. Our results suggest that the abnormal osteocalcin response of Hyp mice is not directly attributable to an osteoblast dysfunction but is secondary, at least in part, to perturbations in factors that modulate the osteoblast activity, especially serum calcium and/or PTH.

The gene for X-linked hypophosphataemic rickets maps to a 200-300kb region in Xp22.1, and is located on a single YAC containing a putative vitamin D response element (VDRE)

The location of the HYP gene, which determines X-linked hypophosphataemic rickets, has been refined considerably by linkage analysis, and three new microsatellite primers isolated, Cap32 (DXS7473), Cap29 (DXS7474) and 7v2 (DXS7475). The locations of four other markers have also been determined (DXS1226, AFMa176zb1, AFMa152wc5, and AFM346azc1). Markers Cap29 and Cap32 are the closest distal markers to the gene with zetamax=11.93, thetamax= 0.018 and zetamax=12.03, thetamax = 0.015 respectively. Both Cap29 and Cap32 are proximal to DXS365 and AFMa176zb1, as deduced by screening non-chimaeric yeast artificial chromosomes (YACs) from a contig spanning the HYP gene. A single crossover places AFMa176zbl distal to the disease gene. There are no recombinations between 7v2 and HYP (zetamax=12.9, thetamax=0.0), or between 7v2 and adjacent markers Cap32, Cap29, AFMa176zb1, DXS1683 and DXS365. However screening of YAC clones encompassing the HYP gene and also P1 clones localises 7v2 distal to Cap29 and Cap32, and proximal to DXS443. Marker DXS1226 is placed outside the region containing the gene, and is located proximal to DXS274 as confirmed by a crossover for this marker and DXS41 against HYP and its presence on YAC 83B05. Genetic mapping of CEPH pedigrees, and screening of YACs places AFMa152wc5 and AFMa346zcl between DXS1683 and DXS1052. The following gene marker map presents the best order for the HYP region: Xptel-DXS43-DXS999-DXS443-(DXS365/DXS74 75/AFMa176zb1)-(DXS7474/DXS7473)-HYP- DXS1683-(AFMa152wc5/AFMa346zc1)-DXS1052-DXS 274 -(DXS41/DXS1226)-Xcen. The distance between the cluster of distal flanking markers Cap29 (DXS7474), Cap32 (DXS7473), and DXS1683 is approximately 300 kb, as deduced from physical map data from a YAC contig spanning the gene. Thus the gene for HYP is contained within a single YAC (900AO472). Of further interest, is the location of a putative vitamin D response element (VDRE) on this YAC.

Effect of 1,25-dihydroxyvitamin D3 treatment on bone formation by transplanted cells from normal and X-linked hypophosphatemic mice

Bone cells isolated from the Hyp mouse, the murine homologue for hypophosphatemic vitamin D-resistant rickets, produce abnormal bone when transplanted to either normal or phosphate-supplemented Hyp mice. To assess whether correction of the bone formation by mutant cells transplanted into either normal or Hyp mice could be achieved in the presence of supraphysiologic serum concentrations of 1.25-dihydroxyvitamin D3 (1.25-(OH)2D3), recipient mice of both genotypes were infused continuously with 1.25-(OH)2D3 (0.2 micrograms/kg/day). Bone nodules present in transplants recovered after 14 days were characterized by measuring the osteoid thickness and volume. Administration of 1.25-(OH)2D3 to Hyp mice corrected the defective bone formation by normal cells but not by pair-transplanted Hyp cells, despite normalization of serum phosphate levels and 3-fold increases in serum 1.25-(OH)2D3. The osteoid thickness and volume in Hyp transplants into 1.25-(OH)2D3-treated Hyp mice were, however, markedly reduced down to values observed for Hyp transplants into recipient normal mice. Administration of 1.25-(OH)2D3 to normal mice improved further bone formation by mutant cells without affecting that by pair-transplanted normal cells. Administration of 24.25-(OH)2D3 (1 microgram/kg/day) combined with 1.25-(OH)2D3 to recipient mice of both genotypes prevented the sharp fall in serum 24.25-(OH)2D3 but was not more beneficial than 1.25-(OH)2D3 alone for improving bone formation by transplanted Hyp cells. These observations demonstrate an abnormal response of the mutant cells to the extracellular environment and support the concept of an intrinsic osteoblast defect in the Hyp mouse.

Phosphate transport by osteoblasts from X-linked hypophosphatemic mice

Hypophosphatemic vitamin D-resistant rickets is characterized by impaired renal reabsorption of Pi. The underlying mechanism of this abnormality remains unknown. Because the osteoblast is likely a target for the HYP mutation, we investigated the Pi transport activity in osteoblasts isolated from the murine homologue for the human disease, the Hyp mouse. Kinetic analysis of sodium-dependent Pi uptake in quiescent normal and Hyp osteoblasts indicated no significant differences in apparent maximal capacity (Vmax) and apparent affinity (Km) of the carrier for Pi. In rapidly growing cells, higher levels of Pi uptake were observed in mutants cells associated with a 1.4- to 1.7-fold increase in Vmax and no change in Km for Pi. This increase in Pi uptake seemed related to changes in the sodium electrochemical gradient inasmuch as a similar increase was observed in alanine uptake. The adaptive response of sodium-dependent Pi transport to Pi deprivation was not altered in mutant cells relative to normal cells. To test whether the expression of a Pi transport defect was dependent on a humoral factor for its expression, we evaluated the activity of the serum from Hyp mice on Pi transport in osteoblasts from both genotypes. No difference in activity was observed between sera from normal and mutant mice. In summary, cultured osteoblasts derived from Hyp mice did not express impaired sodium-dependent Pi transport when compared with cells from normal mice.

Effect of dietary phosphate deprivation and supplementation of recipient mice on bone formation by transplanted cells from normal and X-linked hypophosphatemic mice

The hypophosphatemic (Hyp) mouse is the murine homolog for human hypophosphatemic vitamin D-resistant rickets. We previously reported that bone cells isolated from normal and Hyp mice produced abnormal bone when transplanted intramuscularly into mutant mice. To assess the role of hypophosphatemia on bone formation in transplants, normal and Hyp mouse periostea were pair transplanted into control or phosphate (P)-supplemented Hyp mice and into control or P-deprived normal mice. The bone nodules formed in transplants after 2 weeks were characterized by measuring the thickness of the surrounding osteoid seams and the relative osteoid volume. P restriction in normal recipient mice impaired bone formation by transplanted normal cells and aggravated the defective bone formation by Hyp cells. The osteoid thickness and volume remained significantly higher in Hyp transplants than in normal cotransplants, however. P supplementation of Hyp recipient mice normalized bone formation by transplanted normal cells but not by Hyp cells. However, a marked decrease in osteoid thickness and volume was observed in Hyp transplants down to values observed in normal recipient mice. These results indicate that hypophosphatemia is not the only cause of abnormal bone formation in the Hyp mouse but that an osteoblast dysfunction contributes to the bone disease. These observations further support the concept that the osteoblast may be an important target for the Hyp mutation.