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

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.

Comprehensive phenotyping of cutaneous afferents reveals early-onset alterations in nociceptor response properties, release of CGRP, and hindpaw edema following spinal cord injury

Spinal cord injury (SCI) is a complex syndrome that has profound effects on patient well-being, including the development of medically-resistant chronic pain. The mechanisms underlying SCI pain have been the subject of thorough investigation but remain poorly understood. While the majority of the research has focused on changes occurring within and surrounding the site of injury in the spinal cord, there is now a consensus that alterations within the peripheral nervous system, namely sensitization of nociceptors, contribute to the development and maintenance of chronic SCI pain. Using an ex vivo skin/nerve/DRG/spinal cord preparation to characterize afferent response properties following SCI, we found that SCI increased mechanical and thermal responding, as well as the incidence of spontaneous activity (SA) and afterdischarge (AD), in below-level C-fiber nociceptors 24 hr following injury relative to naïve controls. Interestingly, the distribution of nociceptors that exhibit SA and AD are not identical, and the development of SA was observed more frequently in nociceptors with low heat thresholds, while AD was found more frequently in nociceptors with high heat thresholds. We also found that SCI resulted in hindpaw edema and elevated cutaneous calcitonin gene-related peptide (CGRP) concentration that were not observed in naïve mice. These results suggest that SCI causes a rapidly developing nociceptor sensitization and peripheral inflammation that may contribute to the early emergence and persistence of chronic SCI pain.

PHEXL222P Mutation Increases Phex Expression in a New ENU Mouse Model for XLH Disease

PhexL222P mouse is a new ENU mouse model for XLH disease due to Leu to Pro amino acid modification at position 222. PhexL222P mouse is characterized by growth retardation, hypophosphatemia, hypocalcemia, reduced body bone length, and increased epiphyseal growth plate thickness and femur diameter despite the increase in PHEXL222P expression. Actually, PhexL222P mice show an increase in Fgf23, Dmp1, and Mepe and Slc34a1 (Na-Pi IIa cotransporter) mRNA expression similar to those observed in Hyp mice. Femoral osteocalcin and sclerostin and Slc34a1 do not show any significant variation in PhexL222P mice. Molecular dynamics simulations support the experimental data. P222 might locally break the E217-Q224 β-sheet, which in turn might disrupt inter-β-sheet interactions. We can thus expect local protein misfolding, which might be responsible for the experimentally observed PHEXL222P loss of function. This model could be a valuable addition to the existing XLH model for further comprehension of the disease occurrence and testing of new therapies.

Kidney stone formation in a novel murine model of polycystic kidney disease

Individuals with autosomal dominant polycystic kidney disease have a higher incidence of stone formation than the general population. However, there are no cystic animal models known to develop stones. Cystic mice compound heterozygous for hypomorphic Pkd1V and Pkd1RC alleles develop cystic kidneys within a few weeks of birth but live beyond 20 wk of age, allowing for the study of cystic comorbidities including stone formation. Cystic Pkd1V/RC mice were euthanized at 3, 13, or 26 wk of age, and their kidneys were analyzed by microcomputed tomography (µCT) for stone formation. Mice had occasional mineral aggregates that could be detected by µCT analysis at 3 wk of age. At 13 or 26 wk of age, numerous white masses were visible beneath the kidney surface. µCT analysis confirmed the masses to be large mineral stone deposits throughout the renal cortex, with mineral content increasing with age. Staining of histological sections with alizarin red and von Kossa suggested that the stone deposits were composed primarily of calcium and phosphate. Microdissection confirmed stones localized within cyst lumens. Analysis of individual stones by µCT and infrared spectroscopy confirmed apatite mineral composition. Urinalysis revealed elevated levels of phosphate and citrate at 3 wk of age and lower pH and elevated levels of calcium and citrate at 13 wk of age, suggesting altered phosphate and calcium homeostasis as a potential cause of mineralization and renal stone formation. This is the first animal model exhibiting overt kidney stone formation in the context of cystic kidney disease.NEW & NOTEWORTHY Compound heterozygous Pkd1V/RC mice were found to form calcium phosphate-containing stones within cysts of the renal cortex by 13 wk of age. This is the first polycystic kidney disease animal model exhibiting spontaneous stone formation. A growing body of evidence suggests a link between renal stone formation and cystic kidney disease. This mouse model may be useful for studying the interplay between stone and cyst formation and the functional role of polycystins in mineral homeostasis.

Active sites of human MEPE-ASARM regulating bone matrix mineralization

The proteolytic fragment ASARM (acidic serine- and aspartate-rich motif) of MEPE (matrix extracellular phosphoglycoprotein) (MEPE-ASARM) may act as an endogenous anti-mineralization factor involved in X-linked hypophosphatemic rickets/osteomalacia (XLH). We synthesized MEPE-ASARM peptides and relevant peptide fragments with or without phosphorylated Ser residues (pSer) to determine the active site(s) of MEPE-ASARM in a rat calvaria cell culture model. None of the synthetic peptides elicited changes in cell death, proliferation or differentiation, but the peptide (pASARM) with three pSer residues inhibited mineralization without causing changes in gene expression of osteoblast markers tested. The anti-mineralization effect was maintained in peptides in which any one of three pSer residues was deleted. Polyclonal antibodies recognizing pASARM but not ASARM abolished the pASARM effect. Deletion of six N-terminal residues but leaving the recognition sites for PHEX (phosphate regulating endopeptidase homolog, X-linked), a membrane endopeptidase responsible for XLH, intact and two C-terminal amino acid residues did not alter the anti-mineralization activity of pASARM. Our results strengthen understanding of the active sites of MEPE-pASARM and allowed us to identify a shorter more stable sequence with fewer pSer residues still exhibiting hypomineralization activity, reducing peptide synthesis cost and increasing reliability for exploring biological and potential therapeutic effects.

Modulation of calcium oxalate dihydrate growth by phosphorylated osteopontin peptides

Osteopontin (OPN) is a significant component of kidney stone matrix and a key modulator of stone formation. Here, we investigated the effects of different phosphorylated states of a synthesized peptide of OPN (the ASARM peptide; acidic, serine- and aspartate-rich motif) on calcium oxalate dihydrate (COD) crystals, a major mineral phase of kidney stones. In vitro, phosphorylated OPN-ASARM peptides strongly inhibited COD crystal growth in solution as compared to the nonphosphorylated state, with increasing inhibitory potency correlating with the degree of peptide phosphorylation. Scanning electron microscopy revealed that the inhibition from the phosphopeptides resulted in distinctive, rosette-like crystal aggregates called spherulites. The OPN-ASARM peptides preferentially bound and specifically inhibited the {1 1 0} crystallographic faces of COD, as identified by combining atomic force microscopy and computational simulation approaches. These {1 1 0} surfaces of COD have high lattice calcium occupancy (exposure), providing preferential binding sites for the highly acidic peptides; binding and inhibition by OPN-ASARM peptides at the {1 1 0} faces led to crystal aggregation and intergrowth. The crystal spherulite formations obtained in vitro when using the most phosphorylated form of OPN-ASARM peptide at a high concentration, resembled crystal morphologies observed in vivo in a rat model of urolithiasis, in which crystal deposits in the kidney contain abundant OPN as revealed by immunogold labeling. A mechanistic model for spherulite formation is proposed based on the symmetry and crystallographic structure of COD, where the phosphate groups of OPN-ASARM bind to calcium atoms at [1 1 1] step risers on the COD {1 1 0} surface, inducing the periodic emergence of new COD crystals to form spherulites.

COX inhibitors and bone: A safer impact on osteoblasts by NO-releasing NSAIDs

Nonsteroidal anti-inflammatory drugs (NSAIDs) are commonly prescribed for the treatment of pain and inflammation. Although it is well known that NSAIDs can suppress bone growth, remodelling and repair, they are largely used post-operatively and post-traumatically to achieve analgesia and reduce inflammation in bone tissue.

AIMS

The impact of two NO-releasing, non-selective NSAIDs, NCX-4016 and HCT-3012 (NO-derivatives of Aspirin and Naproxen, respectively) on osteoblasts were evaluated and compared to the non-selective, parent chemicals and to the COX-2-selective inhibitor Celecoxib.

MAIN METHODS

Using MG-63 osteoblast-like cells, we considered proliferation, the early and late stage of differentiation, and the activity of proteinases thought to be involved in osteoid degradation, a preliminary fundamental event of bone remodelling.

KEY FINDINGS

Unlike Aspirin, Naproxen and Celecoxib, the two NO-NSAIDs did not alter proliferation and differentiation of osteoblasts. They also reduced the activity of plasminogen activator, metalloproteinases, and cathepsin B. Similar inhibitory effects against these proteinases were recapitulated by the NO-donor sodium nitroprusside, thereby suggesting a NO-mediated mechanism.

SIGNIFICANCE

Due to a differential effect on cell proliferation and differentiation, the two NO-NSAIDs exhibit a safer impact on osteoblast metabolism compared to Celecoxib and their parent compounds. This suggests an advantageous option for these drugs in individuals with a need of COX-inhibiting treatment, in general. In addition, their capability of modulating the proteinases involved in osteoid degradation may specifically suggest an additional safer use in comorbidity conditions of inflammation or pain with bone disorders characterized by high rate of remodelling, such as high-turnover osteoporosis in post-menopausal women.

Redox-Dependent Bone Alkaline Phosphatase Dysfunction Drives Part of the Complex Bone Phenotype in Mice Deficient for Memo1

Mediator of ErbB2-driven cell Motility 1 (MEMO1) is an intracellular redox protein that integrates growth factors signaling with the intracellular redox state. We have previously reported that mice lacking Memo1 displayed higher plasma calcium levels and other alterations of mineral metabolism, but the underlying mechanism was unresolved and the bone phenotype was not described. Here, we show that Cre/lox-mediated MEMO1 deletion in the whole body of C57Bl/6 mice (Memo cKO) leads to severely altered trabecular bone and lower mineralization, with preserved osteoblast and osteoclast number and activity, but altered osteoblast response to epidermal growth factor (EGF) and FGF2. More strikingly, Memo cKO mice display decreased alkaline phosphatase (ALP) activity in serum and in bone, while ALPL expression level is unchanged. Bone intracellular redox state is significantly altered in Memo cKO mice and we inferred that ALP dimerization was reduced in Memo cKO mice. Indeed, despite similar ALP oxidation, we found increased ALP sensitivity to detergent in Memo cKO bone leading to lower ALP dimerization capability. Thus, we report a severe bone phenotype and dysfunctional bone ALP with local alteration of the redox state in Memo cKO mice that partially mimics hypophosphatasia, independent of ALPL mutations. These findings reveal Memo as a key player in bone homeostasis and underline a role of bone redox state in controlling ALP activity.

Dentin Matrix Protein 1 (DMP1): New and Important Roles for Biomineralization and Phosphate Homeostasis

Previously, non-collagenous matrix proteins, such as DMP1, were viewed with little biological interest. The last decade of research has increased our understanding of DMP1, as it is now widely recognized that this protein is expressed in non-mineralized tissues, as well as in cancerous lesions. Protein chemistry studies have shown that the full length of DMP1, as a precursor, is cleaved into two distinct forms: the C-terminal and N-terminal fragments. Functional studies have demonstrated that DMP1 is essential in the maturation of odontoblasts and osteoblasts, as well as in mineralization via local and systemic mechanisms. The identification of DMP1 mutations in humans has led to the discovery of a novel disease: autosomal-recessive hypophosphatemic rickets. Furthermore, the regulation of phosphate homeostasis by DMP1 through FGF23, a newly identified hormone that is released from bone and targeted in the kidneys, sets a new direction for research that associates biomineralization with phosphate regulation.

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.

A unified model for bone-renal mineral and energy metabolism

The beginning of the millennium saw the discovery of a new bone-matrix protein, Matrix Extracellular PhosphoglycoprotEin (MEPE) and an associated C-terminal motif called ASARM. This motif and other distinguishing features occur in a group of proteins called SIBLINGs. These proteins include dentin matrix protein 1 (DMP1), osteopontin, dentin-sialophosphoprotein (DSPP), statherin, bone sialoprotein (BSP) and MEPE. MEPE, DMP1 and ASARM-motifs regulate expression of a phosphate regulating cytokine FGF23. Further, a trimeric interaction between phosphate regulating endopeptidase homolog X-linked (PHEX), DMP1, and α5β3-integrin that occurs on the plasma-membrane of the osteocyte mediates FGF23 regulation (FAP pathway). ASARM-peptides competitively inhibit the trimeric complex and increase FGF23. A second pathway involves specialized structures, matrix vesicles pathway (MVP). This review will discuss the FAP and MVP pathways and present a unified model for mineral and energy metabolism.

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.

The impairment of osteogenesis in bone sialoprotein (BSP) knockout calvaria cell cultures is cell density dependent

Bone sialoprotein (BSP) belongs to the "small integrin-binding ligand N-linked glycoprotein" (SIBLING) family, whose members interact with bone cells and bone mineral. BSP is strongly expressed in bone and we previously showed that BSP knockout (BSP-/-) mice have a higher bone mass than wild type (BSP+/+) littermates, with lower bone remodelling. Because baseline bone formation activity is constitutively lower in BSP-/- mice, we studied the impact of the absence of BSP on in vitro osteogenesis in mouse calvaria cell (MCC) cultures. MCC BSP-/- cultures exhibit fewer fibroblast (CFU-F), preosteoblast (CFU-ALP) and osteoblast colonies (bone nodules) than wild type, indicative of a lower number of osteoprogenitors. No mineralized colonies were observed in BSP-/- cultures, along with little/no expression of either osteogenic markers or SIBLING proteins MEPE or DMP1. Osteopontin (OPN) is the only SIBLING expressed in standard density BSP-/- culture, at higher levels than in wild type in early culture times. At higher plating density, the effects of the absence of BSP were partly rescued, with resumed expression of osteoblast markers and cognate SIBLING proteins, and mineralization of the mutant cultures. OPN expression and amount are further increased in high density BSP-/- cultures, while PHEX and CatB expression are differentiatlly regulated in a manner that may favor mineralization. Altogether, we found that BSP regulates mouse calvaria osteoblast cell clonogenicity, differentiation and activity in vitro in a cell density dependent manner, consistent with the effective skeletogenesis but the low levels of bone formation observed in vivo. The BSP knockout bone microenvironment may alter the proliferation/cell fate of early osteoprogenitors.

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.

Differential stem- and progenitor-cell trafficking by prostaglandin E2

To maintain lifelong production of blood cells, hematopoietic stem cells (HSC) are tightly regulated by inherent programs and extrinsic regulatory signals received from their microenvironmental niche. Long-term repopulating HSC (LT-HSC) reside in several, perhaps overlapping, niches that produce regulatory molecules/signals necessary for homeostasis and increased output following stress/injury ^1–5. Despite significant advances in specific cellular or molecular mechanisms governing HSC/niche interactions, little is understood about regulatory function within the intact mammalian hematopoietic niche. Recently, we and others described a positive regulatory role for Prostaglandin E₂ (PGE₂) on HSC function ex vivo^6,7. While exploring the role of endogenous PGE₂ we unexpectedly observed hematopoietic egress after nonsteroidal anti-inflammatory drug (NSAID) treatment. Surprisingly, this was independent of the SDF-1/CXCR4 axis. Stem and progenitor cells were found to have differing mechanisms of egress, with HSC transit to the periphery dependent on niche attenuation and reduction in the retentive molecule osteopontin (OPN). Hematopoietic grafts mobilized with NSAIDs had superior repopulating ability and long-term engraftment. Treatment of non-human primates and healthy human volunteers confirmed NSAID-mediated egress in higher species. PGE₂ receptor knockout mice demonstrated that progenitor expansion and stem/progenitor egress resulted from reduced EP4 receptor signaling. These results not only uncover unique regulatory roles for EP4 signaling in HSC retention in the niche but also define a rapidly translatable strategy to therapeutically enhance transplantation.

Proteolytic processing of osteopontin by PHEX and accumulation of osteopontin fragments in Hyp mouse bone, the murine model of X-linked hypophosphatemia

X-linked hypophosphatemia (XLH/HYP)-with renal phosphate wasting, hypophosphatemia, osteomalacia, and tooth abscesses-is caused by mutations in the zinc-metallopeptidase PHEX gene (phosphate-regulating gene with homologies to endopeptidase on the X chromosome). PHEX is highly expressed by mineralized tissue cells. Inactivating mutations in PHEX lead to distal renal effects (implying accumulation of a secreted, circulating phosphaturic factor) and accumulation in bone and teeth of mineralization-inhibiting, acidic serine- and aspartate-rich motif (ASARM)-containing peptides, which are proteolytically derived from the mineral-binding matrix proteins of the SIBLING family (small, integrin-binding ligand N-linked glycoproteins). Although the latter observation suggests a local, direct matrix effect for PHEX, its physiologically relevant substrate protein(s) have not been identified. Here, we investigated two SIBLING proteins containing the ASARM motif-osteopontin (OPN) and bone sialoprotein (BSP)-as potential substrates for PHEX. Using cleavage assays, gel electrophoresis, and mass spectrometry, we report that OPN is a full-length protein substrate for PHEX. Degradation of OPN was essentially complete, including hydrolysis of the ASARM motif, resulting in only very small residual fragments. Western blotting of Hyp (the murine homolog of human XLH) mouse bone extracts having no PHEX activity clearly showed accumulation of an ∼35 kDa OPN fragment that was not present in wild-type mouse bone. Immunohistochemistry and immunogold labeling (electron microscopy) for OPN in Hyp bone likewise showed an accumulation of OPN and/or its fragments compared with normal wild-type bone. Incubation of Hyp mouse bone extracts with PHEX resulted in the complete degradation of these fragments. In conclusion, these results identify full-length OPN and its fragments as novel, physiologically relevant substrates for PHEX, suggesting that accumulation of mineralization-inhibiting OPN fragments may contribute to the mineralization defect seen in the osteomalacic bone characteristic of XLH/HYP.

Biological characterization of preclinical Bioluminescent Osteosarcoma Orthotopic Mouse (BOOM) model: A multi-modality approach

Osteosarcoma (OS) is a bone malignancy that affects children and adolescents. It is a highly aggressive tumor and typically metastasizes to lungs. Despite aggressive chemotherapy and surgical treatments, the current 5 year survival rate is 60–70%. Clinically relevant models are needed to understand OS pathobiology, metastatic progression from bones to lungs, and ultimately, to develop more efficacious treatment strategies and improve survival rates in OS patients with metastasis. The main goal of this study was to develop and characterize an in vivo OS model that will allow non-invasive tracking of tumor progression in real time, and aid in studying OS pathobiology, and screening of potential therapeutic agents against OS. In this study, we have used a multi-modality approach using bioluminescent imaging, electron microscopy, micro-computed tomography, and histopathology to develop and characterize a preclinical Bioluminescent Osteosarcoma Orthotopic Mouse (BOOM) model, using 143B human OS cell line. The results of this study clearly demonstrate that the BOOM model represents the clinical disease as evidenced by a spectrum of changes associated with tumor establishment, progression and metastasis, and detection of known OS biomarkers in the primary and metastatic tumor tissue. Key novel findings of this study include: (a) multimodality approach for extensive characterization of the BOOM model using 143B human OS cell line; (b) evidence of renal metastasis in OS orthotopic model using 143B cells; (c) evidence of Runx2 expression in the metastatic lung tissue; and (d) evidence of the presence of extracellular membrane vesicles and myofibroblasts in the BOOM model.

Role of cathepsin D in U18666A-induced neuronal cell death: potential implication in Niemann-Pick type C disease pathogenesis

Cathepsin D is an aspartyl protease that plays a crucial role in normal cellular functions and in a variety of neurodegenerative disorders, including Niemann-Pick type C (NPC) disease, which is characterized by intracellular accumulation of cholesterol and glycosphingolipids in many tissues, including the brain. There is evidence that the level and activity of cathepsin D increased markedly in vulnerable neurons in NPC pathology, but its involvement in neurodegeneration remains unclear. In the present study, using mouse hippocampal cultured neurons, we evaluated the significance of cathepsin D in toxicity induced by U18666A, a class II amphiphile, which triggers cell death by impairing the trafficking of cholesterol, as observed in NPC pathology. Our results showed that U18666A-mediated toxicity is accompanied by an increase in cathepsin D mRNA and enzyme activity but a decrease in the total peptide content. The cytosolic level of cathepsin D, on the other hand, was increased along with cytochrome c and activated caspase-3 in U18666A-treated neurons. The cathepsin D inhibitor, pepstatin A, partially protected neurons against toxicity by attenuating these signaling mechanisms. Additionally, down-regulation of cathepsin D level prevented, whereas overexpression of the protease increased, vulnerability of cultured N2a cells to U18666A-induced toxicity. We also showed that extracellular cathepsin D from U18666A-treated neurons or application of exogenous enzyme can induce neurotoxicity by activating the autophagic pathway. These results suggest that increased release/activation of cathepsin D can trigger neurodegeneration and possibly development of NPC pathology. Thus, targeting cathepsin D level/activity may provide a new therapeutic opportunity for the treatment of NPC pathology.

MEPE is a novel regulator of growth plate cartilage mineralization

Matrix extracellular phosphoglycoprotein (MEPE) belongs to the SIBLING protein family which play key roles in biomineralization. Although the growth plates of MEPE-overexpressing mice display severe morphological disruption, the expression and function of MEPE in growth plate matrix mineralization remains largely undefined. Here we show MEPE and its cleavage product, the acidic serine aspartate-rich MEPE-associated motif (ASARM) peptide, to be localised to the hypertrophic zone of the growth plate. We also demonstrate that the phosphorylated (p)ASARM peptide inhibits ATDC5 chondrocyte matrix mineralization. Stable MEPE-overexpressing ATDC5 cells also had significantly reduced matrix mineralization in comparison to the control cells. Interestingly, we show that the addition of the non-phosphorylated (np)ASARM peptide promoted mineralization in the ATDC5 cells. The peptides and the overexpression of MEPE did not affect the differentiation of the ATDC5 cells. For a more physiologically relevant model, we utilized the metatarsal organ culture model. We show the pASARM peptide to inhibit mineralization at two stages of development, as shown by histological and μCT analysis. Like in the ATDC5 cells, the peptides did not affect the differentiation of the metatarsals indicating that the effects seen on mineralization are direct, as is additionally confirmed by no change in alkaline phosphatase activity or mRNA expression. In the metatarsal organ cultures, the pASARM peptide also reduced endothelial cell markers and vascular endothelial growth factor mRNA expression. Taken together these results show MEPE to be an important regulator of growth plate chondrocyte matrix mineralization through its cleavage to an ASARM peptide.

Overexpression of the DMP1 C-terminal fragment stimulates FGF23 and exacerbates the hypophosphatemic rickets phenotype in Hyp mice

Dentin matrix protein-1 (DMP1) or phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX) inactivation results in elevation of the phosphaturic hormone fibroblast growth factor (FGF)-23, leading to hypophosphatemia, aberrant vitamin D metabolism, and rickets/osteomalacia. Compound mutant Phex-deficient Hyp and Dmp1(ko) mice exhibit nonadditive phenotypes, suggesting that DMP1 and PHEX may have interdependent effects to regulate FGF23 and bone mineralization. To determine the relative importance of DMP1 and PHEX in regulating FGF23 and mineralization, we tested whether the transgenic expression of full-length [Dmp1(Tg(full-length))] or C-terminal Dmp1 [Dmp1(Tg(57kDa))] could rescue the phenotype of Hyp mice. We found that Dmp1(ko) and Hyp mice have similar phenotypes characterized by decreased cortical bone mineral density (-35% vs. wild type, P < 0.05) and increased serum FGF23 levels (~12-fold vs. wild type, P < 0.05). This was significantly corrected by the overexpression of either the full-length or the C-terminal transgene in Dmp1(ko) mice. However, neither of the transgenes rescued the Hyp mice phenotype. Hyp/Dmp1(Tg(full-length)) and Hyp mice were similar, but Hyp/Dmp1(Tg(57 kDa)) mice exhibited worsening of osteomalacia (-20% cortical bone mineral density) in association with increased serum FGF23 levels (+2-fold) compared with Hyp mice. Bone FGF23 mRNA expression was decreased and a 2-fold increase in the ratio of the full-length/degraded circulating FGF23 was observed, indicating that degradation of FGF23 was impaired in Hyp/Dmp1(Tg(57 kDa)) mice. The paradoxical effects of the C-terminal Dmp1 transgene were observed in Hyp/Dmp1(Tg(57 kDa)) but not in Dmp1(Tg(57 kDa)) mice expressing a functional PHEX. These findings indicate a functional interaction between PHEX and DMP1 to regulate bone mineralization and circulating FGF23 levels and for the first time demonstrate effects of the C-terminal DMP1 to regulate FGF23 degradation.

The chicken or the egg: PHEX, FGF23 and SIBLINGs unscrambled

The eggshell is an ancient innovation that helped the vertebrates' transition from the oceans and gain dominion over the land. Coincident with this conquest, several new eggshell and noncollagenous bone-matrix proteins (NCPs) emerged. The protein ovocleidin-116 is one of these proteins with an ancestry stretching back to the Triassic. Ovocleidin-116 is an avian homolog of Matrix Extracellular Phosphoglycoprotein (MEPE) and belongs to a group of proteins called Small Integrin-Binding Ligand Interacting Glycoproteins (SIBLINGs). The genes for these NCPs are all clustered on chromosome 5q in mice and chromosome 4q in humans. A unifying feature of the SIBLING proteins is an Acidic Serine Aspartate-Rich MEPE (ASARM)-associated motif. The ASARM motif and the released ASARM peptide play roles in mineralization, bone turnover, mechanotransduction, phosphate regulation and energy metabolism. ASARM peptides and motifs are physiological substrates for phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX), a Zn metalloendopeptidase. Defects in PHEX are responsible for X-linked hypophosphatemic rickets. PHEX interacts with another ASARM motif containing SIBLING protein, Dentin Matrix Protein-1 (DMP1). DMP1 mutations cause bone-renal defects that are identical with the defects caused by loss of PHEX function. This results in autosomal recessive hypophosphatemic rickets (ARHR). In both X-linked hypophosphatemic rickets and ARHR, increased fibroblast growth factor 23 (FGF23) expression occurs, and activating mutations in FGF23 cause autosomal dominant hypophosphatemic rickets (ADHR). ASARM peptide administration in vitro and in vivo also induces increased FGF23 expression. This review will discuss the evidence for a new integrative pathway involved in bone formation, bone-renal mineralization, renal phosphate homeostasis and energy metabolism in disease and health.

Regulation of bone-renal mineral and energy metabolism: the PHEX, FGF23, DMP1, MEPE ASARM pathway

More than 300 million years ago, vertebrates emerged from the vast oceans to conquer gravity and the dry land. With this transition, new adaptations occurred that included ingenious changes in reproduction, waste secretion, and bone physiology. One new innovation, the egg shell, contained an ancestral protein (ovocleidin-116) that likely first appeared with the dinosaurs and was preserved through the theropod lineage in modern birds and reptiles. Ovocleidin-116 is an avian homolog of matrix extracellular phosphoglycoprotein (MEPE) and belongs to a group of proteins called short integrin-binding ligand-interacting glycoproteins (SIBLINGs). These proteins are all localized to a defined region on chromosome 5q in mice and chromosome 4q in humans. A unifying feature of SIBLING proteins is an acidic serine aspartate-rich MEPE-associated motif (ASARM). Recent research has shown that the ASARM motif and the released ASARM peptide have regulatory roles in mineralization (bone and teeth), phosphate regulation, vascularization, soft-tissue calcification, osteoclastogenesis, mechanotransduction, and fat energy metabolism. The MEPE ASARM motif and peptide are physiological substrates for PHEX, a zinc metalloendopeptidase. Defects in PHEX are responsible for X-linked hypophosphatemic rickets (HYP). There is evidence that PHEX interacts with another ASARM motif containing SIBLING protein, dentin matrix protein-1 (DMP1). DMP1 mutations cause bone and renal defects that are identical with the defects caused by a loss of PHEX function. This results in autosomal recessive hypophosphatemic rickets (ARHR). In both HYP and ARHR, increased FGF23 expression plays a major role in the disease and in autosomal dominant hypophosphatemic rickets (ADHR), FGF23 half-life is increased by activating mutations. ASARM peptide administration in vitro and in vivo also induces increased FGF23 expression. FGF23 is a member of the fibroblast growth factor (FGF) family of cytokines, which surfaced 500 million years ago with the boney fish (i.e., teleosts) that do not contain SIBLING proteins. In terrestrial vertebrates, FGF23, like SIBLING proteins, is expressed in the osteocyte. The boney fish, however, are an-osteocytic, so a physiological bone-renal link with FGF23 and the SIBLINGs was cemented when life ventured from the oceans to the land during the Triassic period, approximately 300 million years ago. This link has been revealed by recent research that indicates a competitive displacement of a PHEX-DMP1 interaction by an ASARM peptide that leads to increased FGF23 expression. This review discusses the new discoveries that reveal a novel PHEX, DMP1, MEPE, ASARM peptide, and FGF23 bone-renal pathway. This pathway impacts not only bone formation, bone-renal mineralization, and renal phosphate homeostasis but also energy metabolism. The study of this new pathway is relevant for developing therapies for several diseases: bone-teeth mineral loss disorders, renal osteodystrophy, chronic kidney disease and bone mineralization disorders (CKD-MBD), end-stage renal diseases, ectopic arterial-calcification, cardiovascular disease renal calcification, diabetes, and obesity.

Evidence for FGF23 involvement in a bone-kidney axis regulating bone mineralization and systemic phosphate and vitamin D homeostasis

Bone is involved in the maintenance of phosphate and vitamin D homeostasis via its production and secretion of FGF23 and serves as a reservoir for the storage and release of calcium and phosphate into the circulation. Alterations in mineralization of extracellular matrix and the remodeling activities of the skeleton are coupled to the kidney conservation of phosphate and production of 1,25(OH)2D via the regulation of FGF23 production by osteocytes through yet-to-be defined locally derived factors. In addition, FGF23 production is regulated by 1,25(OH)2D in a feedback loop where FGF23 stimulate Cyp24 mediated degradation of 1,25(OH)2D that serves to protect the organism from the toxic effects of vitamin D excess. In this chapter, we will review the regulation and function of FGF23.

Phosphatonins: new hormones involved in numerous inherited bone disorders

Phosphate (Pi) homeostasis is under control of several endocrine factors that play effects on bone, kidney and intestine. The control of Pi homeostasis has a significant biological importance, as it relates to numerous cellular mechanisms involved in energy metabolism, cell signaling, nucleic acid synthesis, membrane function, as well as skeletal health and integrity. Pi is essential for diverse biological processes, and negative Pi balance resulting from improperly regulated intestinal absorption, systemic utilization, and renal excretion. As results of these functions, chronic Pi deprivation causes several biological alterations, such as bone demineralization with unmineralized osateoid typical of osteomalacia in adults and rickets in developing animals and humans (1). Phosphatonins are new hormones playing an important role in the control of Pi homeostasis together with parathyroid hormone (PTH) and 1,25-dihydroxy vitamin D(3). Most insight into the underlying mechanisms was established by defining the molecular basis of different inherited disorders that are characterized by an abnormal regulation of Pi homeostasis.

Bone proteins PHEX and DMP1 regulate fibroblastic growth factor Fgf23 expression in osteocytes through a common pathway involving FGF receptor (FGFR) signaling

Fibroblastic growth factor 23 (FGF23) is a circulating phosphaturic hormone. Inactivating mutations of the endopeptidase PHEX or the SIBLING protein DMP1 result in equivalent intrinsic bone mineralization defects and increased Fgf23 expression in osteocytes. The mechanisms whereby PHEX and DMP1 regulate Fgf23 expression are unknown. We examined the possibility that PHEX and DMP1 regulate Fgf23 through a common pathway by analyzing the phenotype of compound Phex and Dmp1 mutant mice (Hyp/Dmp1(-/-)). Compared to single-mutant littermates, compound-mutant Hyp/Dmp1(-/-) mice displayed nonadditive elevations of serum FGF23 (1912 ± 183, 1715 ± 178, and 1799 ± 181 pg/ml), hypophosphatemia (P(i): 6.0 ± 0.3, 5.8 ± 0.2, and 5.4 ± 0.1 mg/dl), and severity of rickets/osteomalacia (bone mineral density: -36, -36, and -30%). Microarray analysis of long bones identified gene expression profiles implicating common activation of the FGFR pathway in all the mutant groups. Furthermore, inhibiting FGFR signaling using SU5402 in Hyp- and Dmp1(-/-)-derived bone marrow stromal cells prevented the increase in Fgf23 mRNA expression (129- and 124-fold increase in Hyp and Dmp1(-/-) vs. 1.3-fold in Hyp+SU5402 and 2.5-fold in Dmp1(-/-)+SU5402, P<0.05). For all analyses, samples collected from nonmutant wild-type littermates served as controls. These findings indicate that PHEX and DMP1 control a common pathway regulating bone mineralization and FGF23 production, the latter involving activation of the FGFR signaling in osteocytes.

Sclerostin is a locally acting regulator of late-osteoblast/preosteocyte differentiation and regulates mineralization through a MEPE-ASARM-dependent mechanism

The identity of the cell type responsive to sclerostin, a negative regulator of bone mass, is unknown. Since sclerostin is expressed in vivo by mineral-embedded osteocytes, we tested the hypothesis that sclerostin would regulate the behavior of cells actively involved in mineralization in adult bone, the preosteocyte. Differentiating cultures of human primary osteoblasts exposed to recombinant human sclerostin (rhSCL) for 35 days displayed dose- and time-dependent inhibition of in vitro mineralization, with late cultures being most responsive in terms of mineralization and gene expression. Treatment of advanced (day 35) cultures with rhSCL markedly increased the expression of the preosteocyte marker E11 and decreased the expression of mature markers DMP1 and SOST. Concomitantly, matrix extracellular phosphoglycoprotein (MEPE) expression was increased by rhSCL at both the mRNA and protein levels, whereas PHEX was decreased, implying regulation through the MEPE-ASARM axis. We confirmed that mineralization by human osteoblasts is exquisitely sensitive to the triphosphorylated ASARM-PO4 peptide. Immunostaining revealed that rhSCL increased the endogenous levels of MEPE-ASARM. Importantly, antibody-mediated neutralization of endogenous MEPE-ASARM antagonized the effect of rhSCL on mineralization, as did the PHEX synthetic peptide SPR4. Finally, we found elevated Sost mRNA expression in the long bones of HYP mice, suggesting that sclerostin may drive the increased MEPE-ASARM levels and mineralization defect in this genotype. Our results suggest that sclerostin acts through regulation of the PHEX/MEPE axis at the preosteocyte stage and serves as a master regulator of physiologic bone mineralization, consistent with its localization in vivo and its established role in the inhibition of bone formation.

Glucocorticoid dose determines osteocyte cell fate

In response to cellular insult, several pathways can be activated, including necrosis, apoptosis, and autophagy. Because glucocorticoids (GCs) have been shown to induce both osteocyte apoptosis and autophagy, we sought to determine whether osteocyte cell fate in the presence of GCs was dose dependent by performing in vivo and in vitro studies. Male Swiss-Webster mice were treated with slow-release prednisolone pellets at 1.4, 2.8, and 5.6 mg/kg/d for 28 d. An osteocyte cell line, MLO-Y4 cells, was treated with various doses of dexamethasone. We found that GC treatments dose dependently decreased activation of antioxidant-, autophagy-, and antiapoptosis-focused RT-PCR gene pathways in mouse cortical bone. The activation of antioxidant genes was correlated with autophagy gene expression after the GC treatments. The presence of osteocyte autophagy, as detected by immunostaining for LC3, increased ∼50% at the distal femur cortical bone region but not at trabecular bone region at the 1.4 and 2.8 mg/kg/d GC dose levels. The number of apoptotic osteocytes was increased at the cortical bone region by ∼40% initially observed at the 2.8 mg/kg/d dose level. In addition, the presence of the osteocyte autophagy was associated with an increased protein level of cathepsin K in vitro after the GC treatments. In summary, we found that GC treatment dose-dependently decreased antioxidant gene expression, with lower GC doses activating autophagy, whereas a higher dose increased apoptosis. These data suggest that autophagy may provide a mechanism for osteocytes to survive the stress after GC exposure and provide further insight into how GCs alter bone cell fate.

Biomineralization of bone: a fresh view of the roles of non-collagenous proteins

The impact of genetics has dramatically affected our understanding of the functions of non-collagenous proteins. Specifically, mutations and knockouts have defined their cellular spectrum of actions. However, the biochemical mechanisms mediated by non-collagenous proteins in biomineralization remain elusive. It is likely that this understanding will require more focused functional testing at the protein, cell, and tissue level. Although initially viewed as rather redundant and static acidic calcium binding proteins, it is now clear that non-collagenous proteins in mineralizing tissues represent diverse entities capable of forming multiple protein-protein interactions which act in positive and negative ways to regulate the process of bone mineralization. Several new examples from the author's laboratory are provided which illustrate this theme including an apparent activating effect of hydroxyapatite crystals on metalloproteinases. This review emphasizes the view that secreted non-collagenous proteins in mineralizing bone actively participate in the mineralization process and ultimately control where and how much mineral crystal is deposited, as well as determining the quality and biomechanical properties of the mineralized matrix produced.

ASARM peptides: PHEX-dependent and -independent regulation of serum phosphate

Increased acidic serine aspartate-rich MEPE-associated motif (ASARM) peptides cause mineralization defects in X-linked hypophosphatemic rickets mice (HYP) and "directly" inhibit renal phosphate uptake in vitro. However, ASARM peptides also bind to phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX) and are a physiological substrate for this bone-expressed, phosphate-regulating enzyme. We therefore tested the hypothesis that circulating ASARM peptides also "indirectly" contribute to a bone-renal PHEX-dependent hypophosphatemia in normal mice. Male mice (n = 5; 12 wk) were fed for 8 wk with a normal phosphorus and vitamin D(3) diet (1% P(i) diet) or a reduced phosphorus and vitamin D(3) diet (0.1% P(i) diet). For the final 4 wk, transplantation of mini-osmotic pumps supplied a continuous infusion of either ASARM peptide (5 mg·day(-1)·kg(-1)) or vehicle. HYP, autosomal recessive hypophosphatemic rickets (ARHR), and normal mice (no pumps or ASARM infusion; 0.4% P(i) diet) were used in a separate experiment designed to measure and compare circulating ASARM peptides in disease and health. ASARM treatment decreased serum phosphate concentration and renal phosphate cotransporter (NPT2A) mRNA with the 1% P(i) diet. This was accompanied by a twofold increase in serum ASARM and 1,25-dihydroxy vitamin D(3) [1,25 (OH)(2)D(3)] levels without changes in parathyroid hormone. For both diets, ASARM-treated mice showed significant increases in serum fibroblast growth factor 23 (FGF23; +50%) and reduced serum osteocalcin (-30%) and osteopontin (-25%). Circulating ASARM peptides showed a significant inverse correlation with serum P(i) and a significant positive correlation with fractional excretion of phosphate. We conclude that constitutive overexpression of ASARM peptides plays a "component" PHEX-independent part in the HYP and ARHR hypophosphatemia. In contrast, with wild-type mice, ASARM peptides likely play a bone PHEX-dependent role in renal phosphate regulation and FGF23 expression. They may also coordinate FGF23 expression by competitively modulating PHEX/DMP1 interactions and thus bone-renal mineral regulation.

Molecular determinants of extracellular matrix mineralization in bone and blood vessels

PURPOSE OF REVIEW

Mineralization imparts important biomechanical and other functional properties to bones and teeth. Ectopic pathologic mineralization, however, occurring in soft tissues such as blood vessels, kidneys, articular cartilage and also in body fluids, including urine and synovial fluid, is generally debilitating, often painful and typically is destructive of compromised tissue. Here we review new findings on direct molecular determinants of mineralization operating locally at the level of the extracellular matrix, with a focus on bone and blood vessels.

RECENT FINDINGS

Accumulating evidence indicates important key roles for secreted noncollagenous proteins in regulating mineralization, wherein they also contribute structurally to the scaffolding properties of the extracellular matrix. Mineral-binding proteins contain conserved acidic peptide domains (often highly phosphorylated), which bind strongly to calcium within the apatitic mineral phase of bone and calcified blood vessels to regulate crystal growth. Other recent work has underscored the importance of the small-molecule mineralization inhibitor pyrophosphate in inhibiting tissue mineralization - an inhibition released through its enzymatic cleavage by tissue-nonspecific alkaline phosphatase. Recent findings on mechanisms involved in matrix vesicle-mediated mineralization are also discussed.

SUMMARY

Mechanistic details are emerging that describe a scenario wherein the combined actions of mineral-binding noncollagenous matrix peptides/proteins within a scaffolding of collagen (and also elastin in blood vessels), phosphatases and matrix vesicles all contribute importantly to promoting or limiting mineralization.

The osteocyte--a novel endocrine regulator of body phosphate homeostasis

Although osteocytes are the most abundant cell type in bone, much of their biology remains enigmatic. They are known to transduce mechanical stress into signals that initiate local bone remodeling, and are targets for systemic and local endocrine signals that affect bone architecture and mineral homeostasis. However, recent data reveal that osteocytes themselves act as endocrine cells that synthesize fibroblast growth factor 23 (FGF23) and several other phosphatonins, shown to underpin the systemic regulation of phosphate homeostasis. This review will synthesize the emerging discoveries concerning the osteocytic endocrine role in phosphate homeostasis through the biology and pathophysiology of these phosphatonins. We also suggest future research paths that might resolve existing uncertainties, and look ahead at how greater understanding might improve the management of clinical disorders of phosphate homeostasis.

Abnormal presence of the matrix extracellular phosphoglycoprotein-derived acidic serine- and aspartate-rich motif peptide in human hypophosphatemic dentin

Severe dental troubles are associated with X-linked hypophosphatemic rickets and are mainly related to impaired dentin mineralization. In dentin matrix, matrix extracellular phosphoglycoprotein (MEPE) may be protected from proteolysis by a specific interaction with PHEX (phosphate regulating gene with homologies to endopeptidases on the X chromosome). The objective of our work was to determine whether PHEX impairment induces MEPE cleavage in dentin and the subsequent release of the C-terminal acidic serine- and aspartate-rich motif (ASARM) peptide, which is known to inhibit mineralization. By Western blot analysis, we explored dentin extracts from seven hypophosphatemic patients with mutations of the PHEX gene. A proteomic approach combining immunoprecipitation, surface-enhanced laser desorption/ionization-time of flight-mass spectrometry and matrix-assisted laser desorption ionization-time of flight analysis of the samples completed this exploration. This study shows a 4.1-kDa peptide containing the MEPE-derived ASARM peptide in hypophosphatemic samples. The presence of ASARM was less marked in patients treated with 1-hydroxylated vitamin D and phosphate during growth. Moreover, recombinant ASARM implanted in a rat pulp injury model disturbed the formation of the reparative dentin bridge. These results suggest that abnormal MEPE cleavage occurs when PHEX activity is deficient in humans, the ASARM peptide may be involved in the mineralization defects and the PHEX-MEPE interaction may be indirect, as ensuring a better phosphate and vitamin D environment to the mineralizing dentin prevents MEPE cleavage.

Preproenkephalin (Penk) is expressed in differentiated osteoblasts, and its deletion in Hyp mice partially rescues their bone mineralization defect

Although our understanding of the molecular mechanisms controlling osteoblast differentiation and function is steadily increasing, there are still many open questions, especially regarding the regulation of bone matrix mineralization. For instance, while there is hallmark evidence for the importance of the endopeptidase Phex, whose inactivation in Hyp mice or human patients causes X-linked hypophosphatemic rickets, it is still largely unknown how Phex controls bone mineralization since a physiological substrate for its endopeptidase activity has not been identified yet. Using a genome-wide expression analysis comparing primary calvarial osteoblasts, we have identified preproenkephalin (Penk) as a gene that is selectively expressed in mineralized cultures. Since a role of enkephalin in the regulation of bone remodeling has been suggested previously and since Leu-enkephalin is known to be cleaved by Phex, we analyzed whether Penk expression in osteoblasts is physiologically relevant. Through skeletal analysis of a Penk-deficient mouse model, we found that Penk expression is dispensable for bone development and remodeling since we could not detect any defect following nondecalcified bone histology and histomorphometry compared to wild-type littermates. When Penk was deleted in Phex-deficient Hyp mice, however, we observed a significant reduction of the osteoid enrichment at 24 weeks of age, whereas their disturbance of mineral homeostasis was not affected by the additional absence of the Penk gene. Taken together, our data provide the first in vivo analysis concerning the role of Penk in osteoblasts.

FGF-23 in bone biology

Recent studies have demonstrated that levels of fibroblast growth factor 23 (FGF-23), a key regulator of phosphorus and vitamin D metabolism, rise dramatically as renal function declines and may play a key initiating role in disordered mineral and bone metabolism in patients with chronic kidney disease (CKD). The physiologic importance of FGF-23 in mineral metabolism was first identified in human genetic and acquired rachitic diseases and further characterized in animal models. FGF-23 and its regulators, including phosphate regulating endopeptidase homolog, dentin matrix 1 (DMP1), and matrix extracellular phosphoglycoprotein, are made primarily in bone, specifically in osteocytes. Dysregulation of these proteins results in osteomalacia, implicating the osteocyte in the regulation of skeletal mineralization. Studies in pediatric patients with CKD, the majority of whom have altered skeletal mineralization in early stages of CKD, have demonstrated that skeletal expression of both FGF-23 and its regulator, DMP1, are increased in early stages of CKD and that expression of these proteins is associated with alterations in skeletal mineralization. Thus, dysregulation of osteocytic proteins occur very early in the course of CKD and appear to be central to altered bone and mineral metabolism in this patient population.

Novel regulators of Fgf23 expression and mineralization in Hyp bone

We used gene array analysis of cortical bone to identify Phex-dependent gene transcripts associated with abnormal Fgf23 production and mineralization in Hyp mice. We found evidence that elevation of Fgf23 expression in osteocytes is associated with increments in Fgf1, Fgf7, and Egr2 and decrements in Sost, an inhibitor in the Wnt-signaling pathway, were observed in Hyp bone. beta-Catenin levels were increased in Hyp cortical bone, and TOPflash luciferase reporter assay showed increased transcriptional activity in Hyp-derived osteoblasts, consistent with Wnt activation. Moreover, activation of Fgf and Wnt-signaling stimulated Fgf23 promoter activity in osteoblasts. We also observed reductions in Bmp1, a metalloproteinase that metabolizes the extracellular matrix protein Dmp1. Alterations were also found in enzymes regulating the posttranslational processing and stability of Fgf23, including decrements in the glycosyltransferase Galnt3 and the proprotein convertase Pcsk5. In addition, we found that the Pcsk5 and the glycosyltransferase Galnt3 were decreased in Hyp bone, suggesting that reduced posttranslational processing of FGF23 may also contribute to increased Fgf23 levels in Hyp mice. With regard to mineralization, we identified additional candidates to explain the intrinsic mineralization defect in Hyp osteoblasts, including increases in the mineralization inhibitors Mgp and Thbs4, as well as increases in local pH-altering factors, carbonic anhydrase 12 (Car12) and 3 (Car3) and the sodium-dependent citrate transporter (Slc13a5). These studies demonstrate the complexity of gene expression alterations in bone that accompanies inactivating Phex mutations and identify novel pathways that may coordinate Fgf23 expression and mineralization of extracellular matrix in Hyp bone.

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.

Relationship between plasma fibroblast growth factor-23 concentration and bone mineralization in children with renal failure on peritoneal dialysis

CONTEXT

Fibroblast growth factor (FGF)-23 is produced in bone, and circulating levels are markedly elevated in patients with end-stage kidney disease, but the relationship between plasma levels of FGF-23 and bone histology in dialysis patients with secondary hyperparathyroidism is unknown.

OBJECTIVE

The aim of the study was to evaluate the correlation between plasma levels of FGF-23 and bone histology in pediatric patients with end-stage kidney disease who display biochemical evidence of secondary hyperparathyroidism.

DESIGN

We performed a cross-sectional analysis of the relationship between plasma FGF-23 levels and bone histomorphometry.

SETTING

The study was conducted in a referral center.

STUDY PARTICIPANTS

Participants consisted of forty-nine pediatric patients who were treated with maintenance peritoneal dialysis and who had serum PTH levels (1st generation Nichols assay) greater than 400 pg/ml.

INTERVENTION

There were no interventions.

MAIN OUTCOME MEASURE

Plasma FGF-23 levels and bone histomorphometry were measured.

RESULTS

No correlation existed between values of PTH and FGF-23. Bone formation rates correlated with PTH (r = 0.44; P < 0.01), but not with FGF-23. Higher FGF-23 concentrations were associated with decreased osteoid thickness (r = -0.49; P < 0.01) and shorter osteoid maturation time (r = -0.48; P < 0.01).

CONCLUSIONS

High levels of FGF-23 are associated with improved indices of skeletal mineralization in dialyzed pediatric patients with high turnover renal osteodystrophy. Together with other biomarkers, FGF-23 measurements may indicate skeletal mineralization status in this patient population.

Recent advances in the renal-skeletal-gut axis that controls phosphate homeostasis

Under physiological conditions, homeostasis of inorganic phosphate (Pi) is tightly controlled by a network of increasingly more complex interactions and direct or indirect feedback loops among classical players, such as vitamin D (1,25(OH)2D3), parathyroid hormone (PTH), intestinal and renal phosphate transporters, and the recently described phosphatonins and minhibins. A series of checks and balances offsets the effects of 1,25(OH)2D3 and PTH to enable fine-tuning of intestinal and renal Pi absorptive capacity and bone resorption and mineralization. The latter include PHEX, FGF-23, MEPE, DMP1, and secreted FRP4. Despite this large number of regulatory components with complex interactions, the system has limited redundancy and is prone to dysregulation under pathophysiological conditions. This article reviews and synthesizes recent advances to present a new model of Pi homeostasis.

Gene expression profile of the bone microenvironment in human fragility fracture bone

Osteoporosis (OP) is a common age-related systemic skeletal disease, with a strong genetic component, characterised by loss of bone mass and strength, which leads to increased bone fragility and susceptibility to fracture. Although some progress has been made in identifying genes that may contribute to OP disease, much of the genetic component of OP has yet to be accounted for. Therefore, to investigate the molecular basis for the changes in bone causally involved in OP and fragility fracture, we have used a microarray approach. We have analysed altered gene expression in human OP fracture bone by comparing mRNA in bone from individuals with fracture of the neck of the proximal femur (OP) with that from age-matched individuals with osteoarthritis (OA), and control (CTL) individuals with no known bone pathology. The OA sample set was included because an inverse association, with respect to bone density, has been reported between OA and the OP individuals. Compugen H19K oligo human microarray slides were used to compare the gene expression profiles of three sets of female samples comprising, 10 OP-CTL, 10 OP-OA, and 10 OA-CTL sample pairs. Using linear models for microarray analysis (Limma), 150 differentially expressed genes in OP bone with t scores >5 were identified. Differential expression of 32 genes in OP bone was confirmed by real time PCR analysis (p<0.01). Many of the genes identified have known or suspected roles in bone metabolism and in some cases have been implicated previously in OP pathogenesis. Three major sets of differentially expressed genes in OP bone were identified with known or suspected roles in either osteoblast maturation (PRRX1, ANXA2, ST14, CTSB, SPARC, FST, LGALS1, SPP1, ADM, and COL4A1), myelomonocytic differentiation and osteoclastogenesis (TREM2, ANXA2, IL10, CD14, CCR1, ADAM9, CCL2, CTGF, and KLF10), or adipogenesis, lipid and/or glucose metabolism (IL10, MARCO, CD14, AEBP1, FST, CCL2, CTGF, SLC14A1, ANGPTL4, ADM, TAZ, PEA15, and DOK4). Altered expression of these genes and others in these groups is consistent with previously suggested underlying molecular mechanisms for OP that include altered osteoblast and osteoclast differentiation and function, and an imbalance between osteoblastogenesis and adipogenesis.

Endocrine functions of bone in mineral metabolism regulation

Given the dramatic increase in skeletal size during growth, the need to preserve skeletal mass during adulthood, and the large capacity of bone to store calcium and phosphate, juxtaposed with the essential role of phosphate in energy metabolism and the adverse effects of hyperphosphatemia, it is not surprising that a complex systems biology has evolved that permits cross-talk between bone and other organs to adjust phosphate balance and bone mineralization in response to changing physiological requirements. This review examines the newly discovered signaling pathways involved in the endocrine functions of bone, such as those mediated by the phosphaturic and 1,25(OH)2D-regulating hormone FGF23, and the broader systemic effects associated with abnormalities of calcium and phosphate homeostasis.

MEPE-ASARM peptides control extracellular matrix mineralization by binding to hydroxyapatite: an inhibition regulated by PHEX cleavage of ASARM

Hyp mice having an inactivating mutation of the phosphate-regulating gene with homologies to endopeptidases on the X-chromosome (Phex) gene have bones with increased matrix extracellular phosphoglycoprotein (MEPE). An acidic, serine- and aspartic acid-rich motif (ASARM) is located in the C terminus of MEPE and other mineralized tissue matrix proteins. We studied the effects of ASARM peptides on mineralization and how PHEX and MEPE interactions contribute to X-linked hypophosphatemia (XLH). ASARM immunoreactivity was observed in the osteoid of wildtype bone and in the increased osteoid of Hyp mice. In wildtype bone, PHEX immunostaining was found particularly in osteoid osteocytes and their surrounding matrix. Treatment of MC3T3-E1 osteoblasts with triphosphorylated (3 phosphoserines) ASARM peptide (pASARM) caused a dose-dependent inhibition of mineralization. pASARM did not affect collagen deposition or osteoblast differentiation, suggesting that pASARM inhibits mineralization by direct binding to hydroxyapatite crystals. Binding of pASARM to mineralization foci in pASARM-treated cultures and to synthetic hydroxyapatite crystals was confirmed by colloidal-gold immunolabeling. Nonphosphorylated ASARM peptide showed little or no binding to hydroxyapatite and did not inhibit mineralization, showing the importance of ASARM phosphorylation in regulating mineralization. PHEX rescued the inhibition of osteoblast culture mineralization by pASARM, and mass spectrometry of cleaved peptides obtained after pASARM-PHEX incubations identified pASARM as a substrate for PHEX. These results, showing that pASARM inhibits mineralization by binding to hydroxyapatite and that this inhibitor can be cleaved by PHEX, provide a mechanism explaining how loss of PHEX activity can lead to extracellular matrix accumulation of ASARM resulting in the osteomalacia of XLH.

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.

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.