Serum levels of matrix extracellular phosphoglycoprotein (MEPE) in normal humans correlate with serum phosphorus, parathyroid hormone and bone mineral density

Correction of Vascular Calcification and Hyperphosphatemia in CKD Rats Treated with ASARM Peptide

Background

Abnormalities in calcium, phosphorus, PTH, vitamin D metabolism, bone, and vascular calcification occur in chronic kidney disease mineral bone disorder (CKD-MBD). Calciphylaxis, involving painful, ulcerative skin lesions, is also a major problem associated with CKD-MBD. There are no quality medical interventions to address these clinical issues. Bone ASARM peptides are strong inhibitors of mineralization and induce hypophosphatemia by inhibiting phosphate uptake from the gut. We hypothesize treatment of CKD-MBD rats with ASARM peptides will reverse hyperphosphatemia, reduce soft-tissue calcification, and prevent calciphylaxis.

Methods

To test our hypothesis, we assessed the effects of synthetic ASARM peptide in rats that had undergone a subtotal 5/6th nephrectomy (56NEPHREX), a rodent model of CKD-MBD. All rats were fed a high phosphate diet (2% Pi) to worsen mineral metabolism defects. Changes in serum potassium, phosphate, BUN, creatinine, PTH, FGF23, and calcium were assessed in response to 28 days of ASARM peptide infusion. Also, changes in bone quality, soft-tissue calcification, and expression of gut Npt2b (Slc34a2) were studied following ASARM peptide treatment.

Results

Rats that had undergone 56NEPHREX treated with ASARM peptide showed major improvements in hyperphosphatemia, blood urea nitrogen (BUN), and bone quality compared with vehicle controls. Also, ASARM-infused 56NEPHREX rats displayed improved renal, brain, and cardiovascular calcification. Notably, ASARM peptide infusion prevented the genesis of subdermal medial blood vessel calcification and calciphylaxis-like lesions in 56NEPHREX rats compared with vehicle controls.

Conclusions

ASARM peptide infusion corrects hyperphosphatemia and improves vascular calcification, renal calcification, brain calcification, bone quality, renal function, and skin mineralization abnormalities in 56NEPHREX rats. These findings confirm our hypothesis and support the utility of ASARM peptide treatment in patients with CKD-MBD.

Fibroblast Growth Factor-23 and Matrix Extracellular Phosphoglycoprotein Levels in Healthy Children and, Pregnant and Puerperal Women

INTRODUCTION AND OBJECTIVE

Fibroblast growth factor (FGF-23) and matrix extracellular phosphoglycoprotein (MEPE) are bone-related factors and their role in physiologic conditions and in different life stages are unknown. We aimed to evaluate age- and pregnancy-related changes in MEPE and FGF-23 levels and their correlations with calcium (Ca)-phosphate (PO4) metabolism.

METHODS

The study population included 96 healthy children (50 females) and 31 women (11 healthy, 10 pregnant, and 10 lactating). Intact FGF-23 (iFGF-23), MEPE, ferritin, parathyroid hormone (PTH), 25-OH vitamin D, alkaline phosphatase (ALP), IGF-I, IGFBP-3 and, Ca, PO4 and creatine (Cre) in serum (S) and urine (U) samples were determined. The renal phosphate threshold (TmPO4/GFR) and z-scores for the parameters that show age-related changes were calculated.

RESULTS

Serum iFGF-23 concentrations showed nonsignificant changes with age; however, MEPE decreased with age, reaching the lowest levels after 7 years. Additionally, higher serum MEPE concentrations were observed during pregnancy. Other than ALP, all other examined parameters demonstrated age-related changes. ALP, BUN, S-Cre, and U-Ca/Cre showed puerperal and pregnancy related changes together with MEPE. iFGF-23 was positively correlated with S-PO4 and TmPO4/GFR. MEPE was positively correlated with S-Ca, S-PO4 and TmPO4/GFR and negatively correlated with PTH, IGF-1, and IGFBP-3.

CONCLUSION

Not iFGF-23 but MEPE showed age-dependent changes and was affected by pregnancy. Although, MEPE and iFGF-23 did not correlate with each other, they seem to affect serum and urinary phosphate in the same direction. Additionally, we found evidence that ferritin and growth factors might have a role in serum calcium and phosphate regulation.

Effects of parathyroidectomy on the biology of bone tissue in patients with chronic kidney disease and secondary hyperparathyroidism

Secondary hyperparathyroidism is a complication of chronic kidney disease that compromises skeletal integrity. In patients with secondary hyperparathyroidism undergoing parathyroidectomy, parathyroid hormone levels dramatically decrease. The effects of parathyroidectomy on bone tissue are poorly understood, especially regarding the proteins expressed by osteocytes, such as fibroblast growth factor 23, dentin matrix protein 1, matrix extracellular phosphoglycoprotein, sclerostin, receptor activator of nuclear factor kappa B ligand (RANKL) and osteoprotegerin, which regulate bone turnover. The objective of this study was to characterize the bone expression of these proteins by immunohistochemistry and correlate these results with those of bone histomorphometry before and after parathyroidectomy. We studied bone biopsies that were obtained from 23 patients before and 12 months after parathyroidectomy. We observed an improvement in bone microarchitecture, but impaired mineralization after parathyroidectomy. We found significant increases in sclerostin and osteoprotegerin expression and a decrease in the RANKL/osteoprotegerin ratio after parathyroidectomy, suggesting that their expression is regulated by parathormone. These proteins correlated with structural and bone formation parameters. We conclude that after parathyroidectomy, significant changes occur in the bone expression of osteocyte proteins and that these proteins potentially regulate bone remodeling.

Age dependent regulation of bone-mass and renal function by the MEPE ASARM-motif

CONTEXT

Mice with null mutations in matrix extracellular phosphoglycoprotein (MEPE) have increased bone mass, increased trabecular density and abnormal cancellous bone (MN-mice). These defects worsen with age and MEPE overexpression induces opposite effects. Also, genome wide association studies show that MEPE plays a major role in bone mass. We hypothesized that the conserved C-terminal MEPE ASARM-motif is chiefly responsible for regulating bone mass and trabecular structure.

DESIGN

To test our theory we overexpressed C-terminal ASARM-peptide in MN-mice using the Col1α1 promoter (MNAt-mice). We then compared the bone and renal phenotypes of the MNAt-mouse with the MN-mouse and the X-linked hypophosphatemic rickets mouse (HYP). The HYP mouse overexpresses ASARM-peptides and is defective for the PHEX gene.

RESULTS

The MN-mouse developed increased bone mass, bone strength and trabecular abnormalities that worsened markedly with age. Defects in bone formation were chiefly responsible with suppressed sclerostin and increased active β-catenin. Increased uric acid levels also suggested that abnormalities in purine-metabolism and a reduced fractional excretion of uric acid signaled additional renal transport changes. The MN mouse developed a worsening hyperphosphatemia and reduced FGF23 with age. An increase in the fractional excretion of phosphate (FEP) despite the hyperphosphatemia confirms an imbalance in kidney-intestinal phosphate regulation. Also, the MN mice showed an increased creatinine clearance suggesting hyperfiltration. A reversal of the MN bone-renal phenotype changes occurred with the MNAt mice including the apparent hyperfiltration. The MNAt mice also developed localized hypomineralization, hypophosphatemia and increased FGF23.

CONCLUSIONS

The C-terminal ASARM-motif plays a major role in regulating bone-mass and cancellous structure as mice age. In healthy mice, the processing and release of free ASARM-peptide are chiefly responsible for preserving normal bone and renal function. Free ASARM-peptide also affects renal mineral phosphate handling by influencing FGF23 expression. These findings have implications for understanding age-dependent osteoporosis, unraveling drug-targets and developing treatments.

SPR4-peptide alters bone metabolism of normal and HYP mice

CONTEXT

ASARM-peptides are substrates and ligands for PHEX, the gene responsible for X-linked hypophosphatemic rickets (HYP). PHEX binds to the DMP1-ASARM-motif to form a trimeric-complex with α5β3-integrin on the osteocyte surface and this suppresses FGF23 expression. ASARM-peptide disruption of this complex increases FGF23 expression. We used a 4.2kDa peptide (SPR4) that binds to ASARM-peptide and ASARM-motif to study DMP1-PHEX interactions and to assess SPR4 for treating inherited hypophosphatemic rickets.

DESIGN

Subcutaneously transplanted osmotic pumps were used to infuse SPR4-peptide or vehicle into wild-type mice (WT) and HYP-mice for 4 weeks.

RESULTS

Asymmetrically distributed mineralization defects occurred with WT-SPR4 femurs. Specifically, SPR4 induced negative effects on trabecular bone and increased bone volume and mineralization in cortical-bone. Markedly increased sclerostin and reduced active β-catenin occurred with HYP mice. SPR4-infusion suppressed sclerostin and increased active β-catenin in WT and HYP mice and improved HYP-mice trabecular mineralization defects but not cortical mineralization defects.

CONCLUSIONS

SPR4-peptide has bimodal activity and acts by: (1) preventing DMP1 binding to PHEX and (2) sequestering an inhibitor of DMP1-PHEX binding, ASARM-peptide. In PHEX defective HYP-mice the second pathway predominates. Although SPR4-peptide improved trabecular calcification defects, decreased sclerostin and increased active β-catenin it did not correct HYP-mice cortical mineralization defects on a normal phosphate diet. Thus, for inherited hypophosphatemic rickets patients on a normal phosphate diet, SPR4-peptide is not a useful therapeutic.

PHEX mimetic (SPR4-peptide) corrects and improves HYP and wild type mice energy-metabolism

CONTEXT

PHEX or DMP1 mutations cause hypophosphatemic-rickets and altered energy metabolism. PHEX binds to DMP1-ASARM-motif to form a complex with α5β3 integrin that suppresses FGF23 expression. ASARM-peptides increase FGF23 by disrupting the PHEX-DMP1-Integrin complex. We used a 4.2 kDa peptide (SPR4) that binds to ASARM-peptide/motif to study the DMP1-PHEX interaction and to assess SPR4 for the treatment of energy metabolism defects in HYP and potentially other bone-mineral disorders.

DESIGN

Subcutaneously transplanted osmotic pumps were used to infuse SPR4-peptide or vehicle (VE) into wild-type mice (WT) and HYP-mice (PHEX mutation) for 4 weeks.

RESULTS

SPR4 partially corrected HYP mice hypophosphatemia and increased serum 1.25(OH)2D3. Serum FGF23 remained high and PTH was unaffected. WT-SPR4 mice developed hypophosphatemia and hypercalcemia with increased PTH, FGF23 and 1.25(OH)2D3. SPR4 increased GAPDH HYP-bone expression 60× and corrected HYP-mice hyperglycemia and hypoinsulinemia. HYP-VE serum uric-acid (UA) levels were reduced and SPR4 infusion suppressed UA levels in WT-mice but not HYP-mice. SPR4 altered leptin, adiponectin, and sympathetic-tone and increased the fat mass/weight ratio for HYP and WT mice. Expression of perlipin-2 a gene involved in obesity was reduced in HYP-VE and WT-SPR4 mice but increased in HYP-SPR4 mice. Also, increased expression of two genes that inhibit insulin-signaling, ENPP1 and ESP, occurred with HYP-VE mice. In contrast, SPR4 reduced expression of both ENPP1 and ESP in WT mice and suppressed ENPP1 in HYP mice. Increased expression of FAM20C and sclerostin occurred with HYP-VE mice. SPR4 suppressed expression of FAM20C and sclerostin in HYP and WT mice.

CONCLUSIONS

ASARM peptides and motifs are physiological substrates for PHEX and modulate osteocyte PHEX-DMP1-α5β3-integrin interactions and thereby FGF23 expression. These interactions also provide a nexus that regulates bone and energy metabolism. SPR4 suppression of sclerostin and/or sequestration of ASARM-peptides improves energy metabolism and may have utility for treating familial rickets, osteoporosis, obesity and diabetes.

The osteocyte: an endocrine cell ... and more

Few investigators think of bone as an endocrine gland, even after the discovery that osteocytes produce circulating fibroblast growth factor 23 that targets the kidney and potentially other organs. In fact, until the last few years, osteocytes were perceived by many as passive, metabolically inactive cells. However, exciting recent discoveries have shown that osteocytes encased within mineralized bone matrix are actually multifunctional cells with many key regulatory roles in bone and mineral homeostasis. In addition to serving as endocrine cells and regulators of phosphate homeostasis, these cells control bone remodeling through regulation of both osteoclasts and osteoblasts, are mechanosensory cells that coordinate adaptive responses of the skeleton to mechanical loading, and also serve as a manager of the bone's reservoir of calcium. Osteocytes must survive for decades within the bone matrix, making them one of the longest lived cells in the body. Viability and survival are therefore extremely important to ensure optimal function of the osteocyte network. As we continue to search for new therapeutics, in addition to the osteoclast and the osteoblast, the osteocyte should be considered in new strategies to prevent and treat bone disease.

Inhibition of osteoclastogenesis by mechanically loaded osteocytes: involvement of MEPE

In regions of high bone loading, the mechanoresponsive osteocytes inhibit osteoclastic bone resorption by producing signaling molecules. One possible candidate is matrix extracellular phosphoglycoprotein (MEPE) because acidic serine- and aspartate-rich MEPE-associated motif peptides upregulate osteoprotegerin (OPG) gene expression, a negative regulator of osteoclastogenesis. These peptides are cleaved from MEPE when relatively more MEPE than PHEX (phosphate-regulating gene with homology to endopeptidases on the X chromosome) is present. We investigated whether mechanical loading of osteocytes affects osteocyte-stimulated osteoclastogenesis by involvement of MEPE. MLO-Y4 osteocytes were mechanically loaded by 1-h pulsating fluid flow (PFF; 0.7 ± 0.3 Pa, 5 Hz) or kept under static control conditions. Recombinant MEPE (0.05, 0.5, or 5 μg/ml) was added to some static cultures. Mouse bone marrow cells were seeded on top of the osteocytes to determine osteoclastogenesis. Gene expression of MEPE, PHEX, receptor activator of nuclear factor kappa-B ligand (RANKL), and OPG by osteocytes was determined after PFF. Osteocytes supported osteoclast formation under static control conditions. Both PFF and recombinant MEPE inhibited osteocyte-stimulated osteoclastogenesis. PFF upregulated MEPE gene expression by 2.5-fold, but not PHEX expression. PFF decreased the RANKL/OPG ratio at 1-h PFF treatment. Our data suggest that mechanical loading induces changes in gene expression by osteocytes, which likely contributes to the inhibition of osteoclastogenesis after mechanical loading of bone. Because mechanical loading upregulated gene expression of MEPE but not PHEX, possibly resulting in the upregulation of OPG gene expression, we speculate that MEPE is a soluble factor involved in the inhibition of osteoclastogenesis by osteocytes.

Regulation of phosphate transport by fibroblast growth factor 23 (FGF23): implications for disorders of phosphate metabolism

There are a number of hypophosphatemic disorders due to renal phosphate wasting that cannot be explained by elevated levels of parathyroid hormone. The circulating factors responsible for the phosphaturia have been designated as phosphatonins. Studies of patients with tumor-induced osteomalacia and other genetic diseases of phosphate metabolism have resulted in the identification of a number of hormones that regulate phosphate homeostasis, including matrix extracellular phosphoglycoprotein (MEPE), secreted frizzled-related protein 4 (sFRP-4), dentin matrix protein 1 (DMP1), fibroblast growth factor 7 (FGF7), fibroblast growth factor 23 (FGF23), and Klotho. Our understanding of the actions of these hypophosphatemic peptides has been enhanced by studies in mice either overexpressing or not expressing these hormones. This review focuses on FGF23 since its regulation is disordered in diseases that affect children, such as X-linked hypophosphatemia, autosomal dominant and recessive hypophosphatemic rickets as well as chronic kidney disease. Recent studies have shown that FGF23 is unique among the FGFs in its requirement for Klotho for receptor activation. Here, we also discuss new potentially clinically important data pointing to the receptor(s) that mediate the binding and action of FGF23 and Klotho.

Eight genes are highly associated with BMD variation in postmenopausal Caucasian women

Low bone mineral density (BMD) is an important risk factor for skeletal fractures which occur in about 40% of women >/=50 years in the western world. We describe the transcriptional changes in 84 trans-iliacal bone biopsies associated with BMD variations in postmenopausal females (50 to 86 years), aiming to identify genetic determinants of bone structure. The women were healthy or having a primary osteopenic or osteoporotic status with or without low energy fractures. The total cohort of 91 unrelated women representing a wide range of BMDs, were consecutively registered and submitted to global gene Affymetrix microarray expression analysis or histomorphometry. Among almost 23,000 expressed transcripts, a set represented by ACSL3 (acyl-CoA synthetase long-chain family member 3), NIPSNAP3B (nipsnap homolog 3B), DLEU2 (Deleted in lymphocytic leukemia, 2), C1ORF61 (Chromosome 1 open reading frame 61), DKK1 (Dickkopf homolog 1), SOST (Sclerostin), ABCA8, (ATP-binding cassette, sub-family A, member 8), and uncharacterized (AFFX-M27830-M-at), was significantly correlated to total hip BMD (5% false discovery rate) explaining 62% of the BMD variation expressed as T-score, 53% when adjusting for the influence of age (Z-score) and 44% when further adjusting for body mass index (BMI). Only SOST was previously associated to BMD, and the majority of the genes have previously not been associated with a bone phenotype. In molecular network analyses, SOST shows a strong, positive correlation with DKK1, both being members of the Wnt signaling pathway. The results provide novel insight in the underlying biology of bone metabolism and osteoporosis which is the ultimate consequence of low BMD.

The journey from vitamin D-resistant rickets to the regulation of renal phosphate transport

In 1937, Fuller Albright first described two rare genetic disorders: Vitamin D resistant rickets and polyostotic fibrous dysplasia, now respectively known as X-linked hypophosphatemic rickets (XLH) and the McCune-Albright syndrome. Albright carefully characterized and meticulously analyzed one patient, W.M., with vitamin D-resistant rickets. Albright subsequently reported additional carefully performed balance studies on W.M. In this review, which evaluates the journey from the initial description of vitamin D-resistant rickets (XLH) to the regulation of renal phosphate transport, we (1) trace the timeline of important discoveries in unraveling the pathophysiology of XLH, (2) cite the recognized abnormalities in mineral metabolism in XLH, (3) evaluate factors that may affect parathyroid hormone values in XLH, (4) assess the potential interactions between the phosphate-regulating gene with homology to endopeptidase on the X chromosome and fibroblast growth factor 23 (FGF23) and their resultant effects on renal phosphate transport and vitamin D metabolism, (5) analyze the complex interplay between FGF23 and the factors that regulate FGF23, and (6) discuss the genetic and acquired disorders of hypophosphatemia and hyperphosphatemia in which FGF23 plays a role. Although Albright could not measure parathyroid hormone, he concluded on the basis of his studies that showed calcemic resistance to parathyroid extract in W.M. that hyperparathyroidism was present. Using a conceptual approach, we suggest that a defect in the skeletal response to parathyroid hormone contributes to hyperparathyroidism in XLH. Finally, at the end of the review, abnormalities in renal phosphate transport that are sometimes found in patients with polyostotic fibrous dysplasia are discussed.

Matrix extracellular phosphoglycoprotein (MEPE) is a new bone renal hormone and vascularization modulator

Increased matrix extracellular phosphoglycoprotein (MEPE) expression occurs in several phosphate and bone-mineral metabolic disorders. To resolve whether MEPE plays a role, we created a murine model overexpressing MEPE protein (MEPE tgn) in bone. MEPE tgn mice displayed a growth and mineralization defect with altered bone-renal vascularization that persisted to adulthood. The growth mineralization defect was due to a decrease in bone remodeling, and MEPE tgn mice were resistant to diet-induced renal calcification. MEPE protein-derived urinary ASARM peptides and reduced urinary Ca X PO4 product mediated the suppressed renal calcification. Osteoblastic cells displayed reduced activity but normal differentiation. Osteoclastic precursors were unable to differentiate in the presence of osteoblasts. In the kidney, NPT2a up-regulation induced an increase in phosphate renal reabsorption, leading to hyperphosphatemia. We conclude MEPE and MEPE-phosphate-regulating gene with homologies to endopeptidases on the X chromosome (MEPE-PHEX) interactions are components to an age-diet-dependent pathway that regulates bone turnover and mineralization and suppresses renal calcification. This novel pathway also modulates bone-renal vascularization and bone turnover.

Small integrin-binding proteins as serum markers for prostate cancer detection

PURPOSE

The small integrin-binding ligand N-linked glycoprotein (SIBLING) gene family includes bone sialoprotein (BSP), dentin matrix protein 1 (DMP1), dentin sialophosphoprotein (DSPP), matrix extracellular phosphoglycoprotein (MEPE), and osteopontin (OPN). Previous studies have separately reported elevated expression of BSP, OPN, or DSPP in prostate tumor paraffin sections. We hypothesized that SIBLINGs may be informative serum markers for subjects with prostate cancer.

METHODS

Expression levels of SIBLINGs in biopsies of normal tissue and tumors from prostate were determined by cDNA array and by immunohistochemical staining with monoclonal antibodies. Competitive ELISAs for measuring total BSP, DSPP, MEPE, and OPN were applied to a test group of 102 subjects with prostate cancer and 110 normal subjects and a validation group of 90 subjects.

RESULTS

BSP, DMP1, DSPP, and OPN exhibited elevated mRNA expression and protein levels in biopsies. BSP, DSPP, and OPN were elevated in serum from prostate cancer subjects, with serum DSPP exhibiting the greatest difference, yielding an area under the receiver operator characteristic curve value of 0.98. Serum BSP and OPN levels were significantly elevated only in late stages, whereas DSPP was significantly elevated at all stages. Optimal serum value cutoff points derived for BSP, OPN, and DSPP were applied as a validation test to a new group of 90 subjects and DSPP yielded a sensitivity of 90% and a specificity of 100%.

CONCLUSION

Of the SIBLING gene family members, DSPP appears to be a strong candidate for use in serum assays for prostate cancer detection.

Novel mechanisms in the regulation of phosphorus homeostasis

Phosphorus plays a critical role in diverse biological processes, and, therefore, the regulation of phosphorus balance and homeostasis are critical to the well being of the organism. Changes in environmental, dietary, and serum concentrations of inorganic phosphorus are detected by sensors that elicit changes in cellular function and alter the efficiency by which phosphorus is conserved. Short-term, post-cibal responses that occur independently of hormones previously thought to be important in phosphorus homeostasis may play a larger role than previously appreciated in the regulation of phosphorus homeostasis. Several hormones and regulatory factors such as the vitamin D endocrine system, parathyroid hormone, and the phosphatonins (FGF-23, sFRP-4, MEPE) among others, may play a role only in the long-term regulation of phosphorus homeostasis. In this review, we discuss how organisms sense changes in phosphate concentrations and how changes in hormonal factors result in the conservation or excretion of phosphorus.

Quantitative trait loci, genes, and polymorphisms that regulate bone mineral density in mouse

This is an in silico analysis of data available from genome-wide scans. Through analysis of QTL, genes and polymorphisms that regulate BMD, we identified 82 BMD QTL, 191 BMD-associated (BMDA) genes, and 83 genes containing known BMD-associated polymorphisms (BMDAP). The catalogue of all BMDA/BMDAP genes and relevant literatures are provided. In total, there are substantially more BMDA/BMDAP genes in regions of the genome where QTL have been identified than in non-QTL regions. Among 191 BMDA genes and 83 BMDAP genes, 133 and 58 are localized in QTL regions, respectively. The difference was still noticeable for the chromosome distribution of these genes between QTL and non-QTL regions. These results have allowed us to generate an integrative profile of QTL, genes, polymorphisms that determine BMD. These data could facilitate more rapid and comprehensive identification of causal genes underlying the determination of BMD in mouse and provide new insights into how BMD is regulated in humans.

Matrix extracellular phosphoglycoprotein inhibits phosphate transport

The role of putative humoral factors, known as phosphatonins, in phosphate homeostasis and the relationship between phosphate handling by the kidney and gastrointestinal tract are incompletely understood. Matrix extracellular phosphoglycoprotein (MEPE), one of several candidate phosphatonins, promotes phosphaturia, but whether it also affects intestinal phosphate absorption is unknown. Here, using the in situ intestinal loop technique, we demonstrated that short-term infusion of MEPE inhibits phosphate absorption in the jejunum but not the duodenum. Simultaneous measurement of urinary phosphate excretion suggests that the phosphaturic action of MEPE correlates with a significant reduction in the protein levels of the renal sodium-phosphate co-transporter NaPi-IIa in the proximal convoluted tubules of the outer renal cortex, assessed by Western blotting and immunohistochemistry. This short-term inhibitory effect of MEPE on renal and intestinal phosphate handling occurred without any changes in circulating levels of parathyroid hormone, 1,25-dihydroxyvitamin D(3), or fibroblast growth factor 23. Taken together, these findings suggest that MEPE is a candidate phosphatonin involved in phosphate homeostasis, acting in both the kidney and the gastrointestinal tract.

Changes in matrix extracellular phosphoglycoprotein expression before and during in vitro osteogenic differentiation of human dental papilla mesenchymal cells

The purpose of this study is to characterise the expression of matrix extracellular phosphoglycoprotein (MEPE) in cultured mesenchymal cells isolated from human dental papilla (PaMCs) of impacted third molars either before or during differentiation of these cells into osteo/odontoblasts. PaMCs, like mesenchymal cells deriving from human dental pulp (DPMCs), resulted positive for a number of mesenchymal markers including CD146 and STRO-1. During the first week in culture they showed a faster proliferation rate than DPMCs, coupled to an earlier down-regulation of MEPE. Also when the cells were further cultured in osteogenic medium (containing beta-glycerophosphate, ascorbic acid and dexamethasone) for 40 days, MEPE down-regulation coupled to an increased expression of osteogenic markers, such as osteocalcin and alkaline phosphatase, occurred earlier in PaMCs than in DPMCs. Thus, our data, indicating that also in PaMCs MEPE expression is higher when cells proliferate, whereas it is downregulated as cells differentiated, are in favour of a role of MEPE as an early regulator of odontogenic differentiation. We also confirm the superior proliferative potential of PaMCs in comparison with DPMCs, coupled to a more rapid induction of osteogenic differentiation. Therefore, these cells represent an optimal source to be conveniently used for dental tissue engineering and tooth regeneration.

Phosphatonins: new hormones that control phosphorus homeostasis

Phosphorus (Pi) plays an important role in nucleic acid synthesis, energy metabolism, bone mineralization and cell signaling, and is also present in sugars, phospholipids and phosphoproteins. Phosphate homeostasis is controlled by processes that regulate the intestinal absorption and renal excretion of Pi, and bone turnover. These processes are influenced by peptide and sterol hormones, such as parathyroid hormone and 1α,25-dihydroxyvitamin D (1α,25[OH]₂D₃). Recently, a new class of phosphate-regulating peptides has been discovered: phosphatonins. These factors, such as FGF-23, secreted frizzled-related protein-4, matrix extracellular phosphoglycoprotein and FGF-7, are circulating peptides with potent phosphaturic activity. These peptides inhibit Na/Pi transporters in renal epithelial cells and, therefore, increase renal Pi excretion. In addition, FGF-23 and secreted frizzled-related protein-4 inhibit 25-hydroxyvitamin D 1α-hydroxylase activity, reducing 1α,25(OH)2D3 synthesis and, thus, intestinal Pi absorption. Phosphatonins have been associated with hypophosphatemic diseases, such as tumor-induced osteomalacia, X-linked hypophosphatemic rickets, autosomal dominant hypophosphatemic rickets, autosomal recessive hypophosphatemic rickets and hyperphosphatemic disease (e.g., tumoral calcinosis). The aim of this article is to review the role of phosphatonins in Pi metabolism in normal and pathologic conditions and also to investigate the correlations among the various phosphatonins.

Degradation of MEPE, DMP1, and release of SIBLING ASARM-peptides (minhibins): ASARM-peptide(s) are directly responsible for defective mineralization in HYP

Mutations in PHEX (phosphate-regulating gene with homologies to endopeptidases on the X chromosome) and DMP1 (dentin matrix protein 1) result in X-linked hypophosphatemic rickets (HYP) and autosomal-recessive hypophosphatemic-rickets (ARHR), respectively. Specific binding of PHEX to matrix extracellular phosphoglycoprotein (MEPE) regulates the release of small protease-resistant MEPE peptides [acidic serine- and aspartate-rich MEPE-associated motif (ASARM) peptides]. ASARM peptides are potent inhibitors of mineralization (minhibins) that also occur in DMP1 [MEPE-related small integrin-binding ligand, N-linked glycoprotein (SIBLING) protein]. It is not known whether these peptides are directly responsible for the mineralization defect. We therefore used a bone marrow stromal cell (BMSC) coculture model, ASARM peptides, anti-ASARM antibodies, and a small synthetic PHEX peptide (SPR4; 4.2 kDa) to examine this. Surface plasmon resonance (SPR) and two-dimensional (1)H/(15)N nuclear magnetic resonance demonstrated specific binding of SPR4 peptide to ASARM peptide. When cultured individually for 21 d, HYP BMSCs displayed reduced mineralization compared with wild type (WT) (-87%, P < 0.05). When cocultured, both HYP and WT cells failed to mineralize. However, cocultures (HYP and WT) or monocultures of HYP BMSCs treated with SPR4 peptide or anti-ASARM neutralizing antibodies mineralized normally. WT BMSCs treated with ASARM peptide also failed to mineralize properly without SPR4 peptide or anti-ASARM neutralizing antibodies. ASARM peptide treatment decreased PHEX mRNA and protein (-80%, P < 0.05) and SPR4 peptide cotreatment reversed this by binding ASARM peptide. SPR4 peptide also reversed ASARM peptide-mediated changes in expression of key osteoclast and osteoblast differentiation genes. Western blots of HYP calvariae and BMSCs revealed massive degradation of both MEPE and DMP1 protein compared with the WT. We conclude that degradation of MEPE and DMP-1 and release of ASARM peptides are chiefly responsible for the HYP mineralization defect and changes in osteoblast-osteoclast differentiation.

Regulation of osteoclast differentiation and function by phosphate: potential role of osteoclasts in the skeletal abnormalities in hypophosphatemic conditions

Mice fed with a low Pi diet exhibited decreased osteoclast number. Hyp mice also showed decreased osteoclasts, and high Pi reversed it. Low Pi reduced osteoclast formation and bone resorption in vitro. Hypophosphatemia may suppress osteoclast differentiation/function, leading to skeletal abnormalities.

INTRODUCTION

Skeletal abnormalities seen in hypophosphatemic disorders indicate a critical role of phosphate (Pi) in skeletogenesis. However, the role of osteoclasts in the pathogenesis of the disturbed skeletogenesis is unclear.

MATERIALS AND METHODS

Mice fed with a low-Pi diet and Hyp mice that are characterized by hypophosphatemia and impaired osteogenesis were studied. Effects of Pi on osteoclast formation and bone resorption were also examined in vitro.

RESULTS

Histomorphometric examination showed that mice on a low-Pi diet exhibited decreased osteoclast number. Furthermore, osteoclast number in Hyp mice was also decreased compared with wildtype (WT) mice. Of note, feeding of Hyp mice with high-Pi diet significantly reversed hypophosphatemia, improved disturbed osteogenesis, and increased osteoclast number. Osteoclast-like cell (OLC) formation and bone resorption in Hyp bone marrow cells was not different from WT bone marrow cells. On the other hand, OLC formation and bone resorption were decreased in conjunction with reduced mRNA expression of RANKL in WT bone marrow cells cultured in the medium containing low Pi (0.5 mM). Recombinant human matrix extracellular phosphoglycoprotein (MEPE), a candidate for phosphatonin, also decreased osteoclast formation, whereas fibroblast growth factor 23 (FGF23), another phosphatonin candidate, showed no effects.

CONCLUSIONS

Our results suggest that Pi controls the differentiation and function of osteoclasts. These actions of Pi on osteoclasts may be associated with the pathogenesis of the skeletal abnormalities in hypophosphatemic disorders.

Dentonin, a MEPE fragment, initiates pulp-healing response to injury

Phosphorylated extracellular matrix proteins, including matrix extracellular phosphoprotein (MEPE), are involved in the formation and mineralization of dental tissues. In this study, we evaluated the potential of Dentonin, a synthetic peptide derived from MEPE, to promote the formation of reparative dentin. Agarose beads, either soaked with Dentonin or unloaded, were implanted into the pulps of rat molars, and examined 8, 15, and 30 days after treatment. At day 8, Dentonin promoted the proliferation of pulp cells, as visualized by PCNA-labeling. RP59-positive osteoblast progenitors were located around the Dentonin-soaked beads. PCNA- and RP59-labeling were decreased at day 15, while osteopontin, weakly labeled at day 8, was increased at 15 days, but dentin sialoprotein was undetectable at any time. At 8 days, precocious reparative dentin formation occurred in pulps containing Dentonin-soaked beads, with formation slowing after 15 days. These results suggest that Dentonin affects primarily the initial cascade of events leading to pulp healing.

Emerging role of fibroblast growth factor 23 in a bone–kidney axis regulating systemic phosphate homeostasis and extracellular matrix mineralization

PURPOSE OF REVIEW

To describe emerging understanding of fibroblast growth factor 23 (FGF23) - a bone-derived hormone that inhibits phosphate reabsorption and calcitriol production by kidney and participates as the principle phosphaturic factor in a bone-kidney axis coordinating systemic phosphate homeostasis and bone mineralization.

RECENT FINDINGS

FGF23 (a circulating factor made by osteocytes in bone) inhibits phosphate reabsorption and 1,25(OH)2D production by kidney. Physiologically, FGF23 is a counter-regulatory phosphaturic hormone for vitamin D and coordinates systemic phosphate homeostasis with skeletal mineralization. Pathologically, high circulating FGF23 levels cause hypophosphatemia, decreased 1,25(OH)2D production, elevated parathyroid hormone and rickets/osteomalacia. FGF23 mutations impairing its degradation cause autosomal dominant hypophosphatemic rickets. Respective loss-of-function mutations of osteocyte gene products DMP1 and Phex cause autosomal recessive hypophosphatemic rickets and X-linked hypophosphatemic rickets, initiating increased FGF23 production. Low FGF23 levels lead to hyperphosphatemia, elevated 1,25(OH)2D, and soft-tissue calcifications. FGF23 is markedly increased in chronic renal disease, but its role remains undefined.

SUMMARY

FGF23 discovery has uncovered primary regulatory pathways and new systems biology governing bone mineralization, vitamin D metabolism, parathyroid gland function, and renal phosphate handling. FGF23 assessment will become important in diagnosing hypophosphatemic and hyperphosphatemic disorders, for which pharmacological regulation of FGF23 levels may provide novel treatments.

Phosphatonins and the Regulation of Phosphate Homeostasis

Inorganic phosphate (P(i)) is required for energy metabolism, nucleic acid synthesis, bone mineralization, and cell signaling. The activity of cell-surface sodium-phosphate (Na(+)-P(i)) cotransporters mediates the uptake of P(i) from the extracellular environment. Na(+)-P(i) cotransporters and organ-specific P(i) absorptive processes are regulated by peptide and sterol hormones, such as parathyroid hormone (PTH) and 1alpha,25-dihydroxyvitamin D (1alpha,25(OH)(2)D(3)), which interact in a coordinated fashion to regulate P(i) homeostasis. Recently, several phosphaturic peptides such as fibroblast growth factor-23 (FGF-23), secreted frizzled related protein-4 (sFRP-4), matrix extracellular phosphoglycoprotein, and fibroblast growth factor-7 have been demonstrated to play a pathogenic role in several hypophosphatemic disorders. By inhibiting Na(+)-P(i) transporters in renal epithelial cells, these proteins increase renal P(i) excretion, resulting in hypophosphatemia. FGF-23 and sFRP-4 inhibit 25-hydroxyvitamin D 1alpha-hydroxylase activity, reducing 1alpha,25(OH)(2)D(3) synthesis and thus intestinal P(i) absorption. This review examines the role of these factors in P(i) homeostasis in health and disease.

Hepatic surgery-related hypophosphatemia

This review describes pathophysiology of post-surgical hypophosphatemia (HP), which has particularly high incidence following liver transplantation. HP remains poorly understood; and there is a lack of universally accepted guidelines for its investigation and management. The pathogenesis of HP following major liver surgery has been hypothesized as being due either to excessive utilization by regenerating liver or increased urinary losses of phosphate. This review provides evidence that excessive urinary loss rather than increased Pi uptake by the liver is the most likely mechanism, and this may be mediated by recently described phosphaturic factors, known as phosphatonins. Until recently blood Pi homeostasis had been explained solely in terms of classical hormones, i.e., vitamin D and PTH. It is however increasingly recognized that phosphatonins may play a critical role in the post-operative HP, but the exact mechanism and candidate phosphaturic factor has not yet been identified. In this review, we have described likely mechanisms and suggest candidate phosphatonins that may mediate urinary Pi loss following liver transplantation. We also discuss the biochemical consequences of cellular Pi depletion, which exposes some gaps in the utilization of established knowledge and therefore in the management of HP. The main aspects of pathophysiology of HP and cellular Pi depletion are presented to provide rational for novel biochemical investigations, which are likely to improve monitoring of HP associated metabolic stress as well as extent of severity of HP, and thereby enhance management of these patients.

Phosphorylated acidic serine-aspartate-rich MEPE-associated motif peptide from matrix extracellular phosphoglycoprotein inhibits phosphate regulating gene with homologies to endopeptidases on the X-chromosome enzyme activity

Inactivating PHEX (phosphate regulating gene with homologies to endopeptidases on the X chromosome) mutations cause X-linked hypophosphatemia in humans and mice (Hyp) through overproduction of fibroblast growth factor 23 (FGF23) a phosphaturic factor, by osteocytes. Matrix extracellular phosphoglycoprotein (MEPE) is also elevated in Hyp and other hypophosphatemic disorders. In addition, the administration of an ASARM (acidic serine-aspartate rich MEPE-associated motif) peptide derived from MEPE causes phosphaturia and inhibits bone mineralization in mice, suggesting that MEPE also plays a role in phosphate homeostasis. Since recent studies found that MEPE binds specifically to PHEX in vitro, we tested the effect of recombinant-MEPE and its ASARM peptide on PHEX enzyme activity in vitro and FGF23 expression in bone marrow stromal cell cultures ex vivo. We found that both recombinant MEPE and synthetic phosphorylated ASARM peptide (ASARM-PO(4)) inhibit PHEX enzyme activities in an in vitro fluorescent-quenched PHEX enzyme activity assay. The ASARM-PO(4) peptide inhibits PHEX enzyme activity in a dose-dependent manner with a K(i) of 128 nM and V(max-i) of 100%. Recombinant MEPE also inhibits PHEX activity (K(i) = 2 nM and V(max-i) = 26%). Long-term bone marrow stromal cell cultures supplemented with 10 microM ASARM-PO(4) peptide resulted in significant elevation of FGF23 transcripts and inhibition of mineralization. These findings suggest that MEPE inhibits mineralization and PHEX activity and leads to increased FGF23 production. The resulting coordination of mineralization and release of a phosphaturic factor by MEPE may serve a physiological role in regulating systemic phosphate homeostasis to meet the needs for bone mineralization.

The role of mutant UDP-N-acetyl-alpha-D-galactosamine-polypeptide N-acetylgalactosaminyltransferase 3 in regulating serum intact fibroblast growth factor 23 and matrix extracellular phosphoglycoprotein in heritable tumoral calcinosis

CONTEXT

Familial tumoral calcinosis (TC) results from disruptions in phosphate metabolism and is characterized by high serum phosphate with normal or elevated 1,25 dihydroxyvitamin vitamin D concentrations and ectopic and vascular calcifications. Recessive loss-of-function mutations in UDP-N-acetyl-alpha-D-galactosamine-polypeptide N-acetylgalactosaminyltransferase 3 (GALNT3) and fibroblast growth factor-23 (FGF23) result in TC.

OBJECTIVE

The objective of the study was to determine the relationship between GALNT3 and FGF23 in familial TC.

DESIGN, SETTING, AND PATIENTS

We assessed the major biochemical defects and potential genes involved in patients with TC.

INTERVENTION

Combination therapy consisted of the phosphate binder Sevelamer and the carbonic anhydrase inhibitor acetazolamide.

RESULTS

We report a patient homozygous for a GALNT3 exon 1 deletion, which is predicted to truncate the encoded protein. This patient had high serum FGF23 concentrations when assessed with a C-terminal FGF23 ELISA but low-normal FGF23 levels when tested with an ELISA for intact FGF23 concentrations. Matrix extracellular phosphoglycoprotein has been identified as a possible regulator of phosphate homeostasis. Serum matrix extracellular phosphoglycoprotein levels, however, were normal in the family with GALNT3-TC and a kindred with TC carrying the FGF23 S71G mutation. The tumoral masses of the patient with GALNT3-TC completely resolved after combination therapy.

CONCLUSIONS

Our findings demonstrate that GALNT3 inactivation in patients with TC leads to inadequate production of biologically active FGF23 as the most likely cause of the hyperphosphatemic phenotype. Furthermore, combination therapy may be effective for reducing the tumoral burden associated with familial TC.

Emerging role of a phosphatonin in mineral homeostasis and its derangements

BACKGROUND

The study of a distinct group of renal phosphate wasting disorders with bone disease which comprise X-linked hypophosphatemic rickets (XLH), autosomal dominant hypophosphatemic rickets (ADHR) and tumour-induced osteomalacia (TIO) gave rise to the identification of different hormone-like peptides, also known as phosphatonins. These factors are responsible for the major disease features that characterize XLH, ADHR and TIO. Recent reports on one of these phosphatonins, fibroblast growth factor-23 (FGF-23), point to a general role of this factor in mineral ion metabolism.

OBJECTIVES

The main focus regards recent evidence implicating FGF-23 in normal and disordered mineral homeostasis with special emphasis on chronic kidney disease. The interactions of FGF-23 with phosphate, parathyroid hormone and vitamin D are discussed in detail.

SUMMARY

The FGF-23 has been shown to increase urinary phosphate excretion, inhibit bone mineralization and suppress 1,25-dihydroxy vitamin D(3)[1,25(OH)(2)D(3)], the main characteristics that XLH, ADHR and TIO have in common. Apart from its role in these phosphate wasting disorders serum FGF-23 is elevated in hypoparathyroidism and humoral hypercalcaemia of malignancy and responds to altered dietary phosphate and calcium supply in healthy subjects. The FGF-23 is also variably elevated in chronic kidney disease and associated secondary hyperparathyroidism where it correlates positively with serum phosphate and parathyroid hormone and negatively with 1,25(OH)(2)D(3). Such relationships, along with data from experimental studies, raise the question of whether FGF-23 contributes to the pathophysiology of chronic kidney disease.

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).

The roles of specific genes implicated as circulating factors involved in normal and disordered phosphate homeostasis: frizzled related protein-4, matrix extracellular phosphoglycoprotein, and fibroblast growth factor 23

Normal serum phosphate (Pi) concentrations are relatively tightly controlled by endocrine mediators of Pi balance. Recent data involving several disorders of Pi homeostasis have shed new light on the regulation of serum Pi balance. It has been hypothesized that circulating phosphaturic factors, or phosphatonins, exist that, when present at high serum concentrations, directly act on the kidney to induce renal Pi wasting. This review will focus upon recently discovered factors that are overexpressed in tumors associated with tumor-induced osteomalacia and have reported activity consistent with effecting Pi balance in vivo. Currently, the best-characterized group of phosphatonin-like polypeptides includes secreted frizzled related protein-4, matrix extracellular phosphoglycoprotein, and fibroblast growth factor-23. Our understanding of these factors will, in the short term, aid us in understanding normal Pi balance and, in the future, help to design novel therapeutic strategies for disorders of Pi handling.

"Phosphatonins" and the regulation of phosphorus homeostasis

Phosphate ions are critical for normal bone mineralization, and phosphate plays a vital role in a number of other biological processes such as signal transduction, nucleotide metabolism, and enzyme regulation. The study of rare disorders associated with renal phosphate wasting has resulted in the discovery of a number of proteins [fibroblast growth factor 23 (FGF-23), secreted frizzled related protein 4 (sFRP-4), matrix extracellular phosphoglycoprotein, and FGF 7 (FGF-7)] that decrease renal sodium-dependent phosphate transport in vivo and in vitro. The "phosphatonins," FGF-23 and sFRP-4, also inhibit the synthesis of 1alpha,25-dihydroxyvitamin D, leading to decreased intestinal phosphate absorption and further reduction in phosphate retention by the organism. In this review, we discuss the biological properties of these proteins, alterations in their concentrations in various clinical disorders, and their possible physiological role.

Altered cathepsin D metabolism in PHEX antisense human osteoblast cells

X-linked hypophosphatemia (XLH), the most common form of hereditary rickets, is caused by loss-of-function mutations of PHEX gene in osteoblast cells, leading to rachitic bone disease and hypophosphatemia. Available evidence today indicates that the bone defect in XLH is caused not only by hypophosphatemia and altered vitamin D metabolism, but also by locally released osteoblastic mineralization inhibitory factor(s), referred to as minhibin. In our present study, we found that suppression of PHEX expression by PHEX antisense in human osteoblast cells caused an increase in cathepsin D expression at protein, but not mRNA, levels. This was associated with a decrease in cathepsin D degradation and an increased cathepsin D release into culture media. Our results also showed that lowering cathepsin D activity in antisense cell conditioned media abolished their inhibitory effect on osteoblast cell calcification, suggesting the involvement of cathepsin D in mediating the minhibin activity of the antisense cell conditioned media.

Increased cathepsin D release by Hyp mouse osteoblast cells

The X-linked hypophosphatemia (XLH), the most common form of hereditary rickets, is caused by loss-of-function mutations of PHEX (phosphate-regulating gene with homology to endopeptidases on the X chromosome) leading to rachitic bone disease and hypophosphatemia. Available evidence today indicates that the bone defect in XLH is caused not only by hypophosphatemia and altered vitamin D metabolism but also by factor(s) locally released by osteoblast cells (ObCs). The identity of these ObC-derived pathogenic factors remains unclear. In our present study, we report our finding of a prominent protein in the culture media derived from ObC of the hypophosphatemic (Hyp) mice, a murine homolog of human XLH, which was identified as the murine procathepsin D (Cat D). By metabolic labeling studies, we further confirmed that Hyp mouse ObCs released greater amount of Cat D into culture media. This increased Cat D release by Hyp mouse ObCs was unlikely to be due to nonspecific cell damage or heterogeneous cell population and was found to be associated with an increased Cat D expression at the protein level, possibly due to a reduced Cat D degradation. However, we were not able to detect a direct effect of PHEX protein on Cat D cleavage. In support of the involvement of Cat D in mediating the inhibitory effect of Hyp mouse ObC-conditioned media on ObC calcification, we found that exposure to Cat D inhibited ObC (45)Ca incorporation and that inhibition of Cat D abolished the inhibitory effect of Hyp mouse-conditioned media on ObC calcification. In conclusion, results from our present study showed that Hyp mouse ObCs release a greater amount of Cat D, which may contribute to the inhibitory effect of Hyp mouse ObC-conditioned media on ObC mineralization.

Case report of a rare syndrome associating amelogenesis imperfecta and nephrocalcinosis in a consanguineous family

A rare syndrome associating amelogenesis imperfecta (AI) with nephrocalcinosis has been reported. The purpose of this study is to characterise the phenotype of a consanguineous family presenting amelogenesis imperfecta, delayed permanent teeth eruption and nephrocalcinosis. Six family members were examined. Ground sections of the case index deciduous teeth and biopsies of enlarged dental follicles were analysed. The patients's parents were first cousins. The case index had yellow discoloration and altered teeth shapes, retention of deciduous teeth, and delayed eruption. Panoramic radiographs revealed multiple enlarged pericoronal follicles in unerupted teeth and generalised intrapulpal calcifications. Renal ultrasound showed the presence of nephrocalcinosis. No other family members presented enamel defects or nephrocalcinosis. Histologically, the enamel appeared hypoplastic, and dental follicles indicated pericoronal hamartoma. The consanguineous marriage suggests an autosomal recessive mode of inheritance. Further studies are necessary to clarify the genetic defect behind this syndrome that associates AI, nephrocalcinosis and impaired tooth eruption.

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.

Serum MEPE-ASARM-peptides are elevated in X-linked rickets (HYP): implications for phosphaturia and rickets

MEPE (Matrix Extracellular PhosphoglycoprotEin) expression is markedly elevated in X-linked-hypophosphatemic-rickets (HYP) and tumor-induced osteomalacia (TIO). In normal individuals, circulating serum-levels of MEPE are tightly correlated with serum-phosphorus, parathyroid hormone (PTH) and bone mineral density (BMD). Also, MEPE derived, C-terminal ASARM-peptides are candidate minhibins and/or phosphatonins. Our aims were to determine: 1. whether MEPE-ASARM-peptide(s) are abnormally elevated in HYP/hyp serum, and, 2. whether the ASARM-peptide(s) accumulate in hyp mice kidney renal-tubules. Using a specific competitive ELISA we measured a five fold increase (P=0.007) of serum ASARM-peptide(s) in human HYP patients (normal subjects 3.25 microM n=9; S.E.M.=0.51 and HYP-patients 15.74 microM, n=9; S.E.M.=3.32). A 6.23 fold increase (P=0.008) was measured in hyp male mice compared with their normal male siblings (normal-siblings, 3.73 muM, S.E.M.=0.57, n=3; and hyp-mice 23.4 microM, n=3, S.E.M.=4.01). Renal immuno-histological screening also revealed a dramatic increase of ASARM-peptides in regions anatomically consistent with the proximal convoluted tubules. This study demonstrates for the first time that markedly elevated serum levels of protease-resistant ASARM-peptide(s) occur in HYP/hyp and they accumulate in murine hyp kidneys. These peptides are thus likely responsible for the phosphaturia and defective mineralization in HYP/hyp and TIO.

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.