Osteopontin deficiency increases mineral content and mineral crystallinity in mouse bone

Phosphate regulates osteopontin gene transcription

Extracellular inorganic phosphate (ePi) is a key regulator of cementoblast behavior, both in vivo and in vitro, and results in a marked increase in osteopontin expression in vitro. To examine the molecular mechanisms involved in ePi induction of osteopontin gene expression, we transfected a series of osteopontin promoter-luciferase constructs into OCCM-30 cementoblasts. Our results demonstrate that ePi can directly induce osteopontin gene transcription. The region responsive to ePi signaling was localized to a 53-bp region of the promoter between -1454 and -1401 that contains a glucocorticoid response element (GRE). Mutation of the GRE abolished the ePi response, suggesting that glucocorticoid receptor (GR) signaling is required for ePi-mediated transcription. In addition, treatment of cells with the GR antagonist RU-486 (Mifepristone) prevented promoter activation by ePi. The results presented support a model demonstrating that inorganic phosphate regulates OPN gene transcription in cementoblasts through a pathway that requires a functional GR.

DSPP effects on in vivo bone mineralization

Dentin sialophosphoprotein has been implicated in the mineralization process based on the defective dentin formation in Dspp null mice (Dspp-/-). Dspp is expressed at low levels in bone and Dspp-/- femurs assessed by quantitative micro-computed tomography (micro-CT) and Fourier transform infrared spectroscopic imaging (FTIRI) exhibit some mineral and matrix property differences from wildtype femurs in both developing and mature mice. Compared to wildtype, Dspp-/- mice initially (5 weeks) and at 7 months had significantly higher trabecular bone volume fractions and lower trabecular separation, while at 9 months, bone volume fraction and trabecular number were lower. Cortical bone mineral density, area, and moments of inertia in Dspp-/- were reduced at 9 months. By FTIRI, Dspp-/- animals initially (5 months) contained more stoichiometric bone apatite with higher crystallinity (crystal size/perfection) and lower carbonate substitution. This difference progressively reversed with age (significantly decreased crystallinity and increased acid phosphate content in Dspp-/- cortical bone by 9 months of age). Mineral density as determined in 3D micro-CT and mineral-to-matrix ratios as determined by 2D FTIRI in individual cortical and trabecular bones were correlated (r(2)=0.6, p<0.04). From the matrix analysis, the collagen maturity of both cortical and trabecular bones was greater in Dspp-/- than controls at 5 weeks; by 9 months this difference in cross-linking pattern did not exist. Variations in mineral and matrix properties observed at different ages are attributable, in part, to the ability of the Dspp gene products to regulate both initial mineralization and remodeling, implying an effect of Dspp on bone turnover.

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.

The effect of the microscopic and nanoscale structure on bone fragility

Bone mineral density is the gold-standard for assessing bone quantity and diagnosing osteoporosis. Although bone mineral density measurements assess the quantity of bone, the quality of the tissue is an important predictor of fragility. Understanding the macro- and nanoscale properties of bone is critical to understanding bone fragility in osteoporosis. Osteoporosis is a disease that affects more than 75 million people worldwide. The gold standard for osteoporosis prognosis, bone mineral density, primarily measures the quantity of bone in the skeleton, overlooking more subtle aspects of bone's properties. Bone quality, a measure of bone's architecture, geometry and material properties, is evaluated via mechanical, structural and chemical testing. Although decreased BMD indicates tissue fragility at the clinical level, changes in the substructure of bone can help indicate how bone quality is altered in osteoporosis. Additionally, mechanical properties which can quantify fragility, or bone's inability to resist fracture, can be changed due to alterations in bone architecture and composition. Recent studies have focused on examination of bone on the nanoscale, suggesting the importance of understanding the interactions of the mineral crystals and collagen fibrils and how they can alter bone quality. It is therefore important to understand alterations in bone that occur at the macro-, micro- and nanoscopic levels to determine what parameters contribute to decreased bone quality in diseased tissue.

Expression and processing of small integrin-binding ligand N-linked glycoproteins in mouse odontoblastic cells

OBJECTIVE

Small integrin-binding ligand N-linked glycoproteins (SIBLINGs) are expressed in dentin and believed to control dentinogenesis. Five members of SIBLING family include bone sialoprotein (BSP), osteopontin (OPN), matrix extracellular phosphoglycoprotein (MEPE), dentin matrix protein 1 (DMP1) and dentin sialophosphoprotein (DSPP). These genes are clustered on chromosome 4q in humans and share similar biological features. DSPP and DMP1 are processed into given structural/functional fragments in rat and porcine. It still remains unclear whether these evidences occur in mouse and other SIBLING members are also processed into given fragments from their parent precursors. The aim of this study was to identify expression and processing of the five proteins in two mouse odontoblastic cell lines.

DESIGN

Two mouse odontoblastic cells were used to study expression and processing of the five SIBLING proteins by immunohistochemistry and Western blot analyses.

RESULTS

Immunohistochemistry study showed that all of the five SIBLING members were expressed within the cytoplasm and cellular processes in the mouse odontoblastic cell lines. Expression levels of DMP1 and DSPP were higher in differentiated mouse odontoblasts than undifferentiated mouse odontoblasts. Immunolabelling signal of DSP and MEPE was also detected within the nucleus in the two cell lines. Western blot assay indicated that all five members were processed into at least two fragments in these cells.

CONCLUSIONS

These results suggest that different processed products and expression levels of the SIBLING proteins may play distinct biological functions in tooth development and mineralisation.

Bio-inspired Synthesis of Mineralized Collagen Fibrils

Mineralized collagen fibrils constitute a basic structural unit of collagenous mineralized tissues such as dentin and bone. Understanding of the mechanisms of collagen mineralization is vital for development of new materials for the hard tissue repair. We carried out bio-inspired mineralization of reconstituted collagen fibrils using poly-l-aspartic acid, as an analog of non-collagenous acidic proteins. Transmission electron microscopy and electron diffraction studies of the reaction products revealed stacks of ribbon-shaped apatitic crystals, deposited within the fibrils with their c-axes co-aligned with the fibril axes. Such structural organization closely resembles mineralized collagen of bone and dentin. Initial mineral deposits formed in the fibrils lacked a long range crystallographic order and transformed into crystals with time. Interestingly, the shape and organization of these amorphous deposits was similar to the crystals found in the mature mineralized fibrils. We demonstrate that the interactions between collagen and poly-l-aspartic acid are essential for the mineralized collagen fibrils formation, while collagen alone does not affect mineral formation and poly-l-aspartic acid inhibits mineralization in a concentration dependant manner. These results provide new insights into basic mechanisms of collagen mineralization and can lead to the development of novel bio-inspired nanostructured materials.

SCPP gene evolution and the dental mineralization continuum

Many genes critical to vertebrate skeletal mineralization are members of the secretory calcium-binding phosphoprotein (SCPP) gene family, which has evolved by gene duplication from a single ancestral gene. In humans, mutations in some of these SCPP genes have been associated with various diseases related to dentin or enamel hypoplasia. Recently, systematic searches for SCPP genes of various species have allowed us to investigate the history of phylogenetically variable dental tissues as a whole. One important conclusion is that not all disease-associated SCPP genes are present in tetrapods, and teleost fish probably have none, even in toothed species, having acquired their complement of SCPP genes through an independent duplication history. Here, we review comparative analyses of mineralized dental tissues, with particular emphasis on the use of SCPPs, within and between tetrapods and teleosts. Current knowledge suggests a close relationship among bone, dentin, teleost fish enameloid (enamel-like hard tissue), and tetrapod enamel. These tissues thus form a mineralized-tissue continuum. Contemporary dental tissues have evolved from an ancestral continuum through lineage-specific modifications.

The skeletal manifestations of the congenital disorders of glycosylation

The congenital disorders of glycosylation (CDG) are a rapidly expanding disease group with protean presentations. Specific end-organ involvement leads to significant morbidity and mortality, and the skeletal manifestations are often not appreciated, apart from the common association of osteopaenia with CDG-Ia. We performed a literature review of all documented skeletal manifestations in reported CDG patients, revealing a diverse range of skeletal phenotypes. We discuss the possible underlying mechanisms of these skeletal manifestations observed in CDG that are important and frequently under-recognized.

Bone sialoprotein plays a functional role in bone formation and osteoclastogenesis

Bone sialoprotein (BSP) and osteopontin (OPN) are both highly expressed in bone, but their functional specificities are unknown. OPN knockout (^−/−) mice do not lose bone in a model of hindlimb disuse (tail suspension), showing the importance of OPN in bone remodeling. We report that BSP^−/− mice are viable and breed normally, but their weight and size are lower than wild-type (WT) mice. Bone is undermineralized in fetuses and young adults, but not in older (≥12 mo) BSP^−/− mice. At 4 mo, BSP^−/− mice display thinner cortical bones than WT, but greater trabecular bone volume with very low bone formation rate, which indicates reduced resorption, as confirmed by lower osteoclast surfaces. Although the frequency of total colonies and committed osteoblast colonies is the same, fewer mineralized colonies expressing decreased levels of osteoblast markers form in BSP^−/− versus WT bone marrow stromal cultures. BSP^−/− hematopoietic progenitors form fewer osteoclasts, but their resorptive activity on dentin is normal. Tail-suspended BSP^−/− mice lose bone in hindlimbs, as expected. In conclusion, BSP deficiency impairs bone growth and mineralization, concomitant with dramatically reduced bone formation. It does not, however, prevent the bone loss resulting from loss of mechanical stimulation, a phenotype that is clearly different from OPN^−/− mice.

Altered osteoclast development and function in osteopontin deficient mice

The role of osteopontin in bone resorption was elucidated by studies of mice with knock out of the osteopontin gene generated by a different approach compared to previous models. Thus, a targeting vector with the promoter region as well as exons 1, 2, and 3 of the osteopontin gene was replaced by a loxP-flanked Neo-TK cassette, and this cassette was eliminated through transient expression of Cre recombinase. The recombined ES cells were used to create mice lacking expression of the osteopontin gene. Tissues from these mice were subjected structural and molecular analyses including morphometry and proteomics. The bone of the null mice contained no osteopontin but showed no significant alterations with regard to other bone proteins. The bone volume was normal in young null animals but in the lower metaphysis, the volume and number of osteoclasts were increased. Notably, the volume and length of the osteoclast ruffled border was several folds lower, indicating a lower resorptive capacity. The null mice did not develop the bone loss characteristic for osteoporosis demonstrated in old wild-type female animals. This quantitative study demonstrates a bone phenotype in the osteopontin null mice of all ages. The data provides further evidence for a role of osteopontin in osteoclast activity.

Osteopontin is histochemically detected by the AgNOR acid-silver staining

Silver nitrate staining of decalcified bone sections is known to reveal osteocyte canaliculi and cement lines. Nucleolar Organising Regions (NOR) are part of the nucleolus, containing argyrophilic proteins (nucleoclin/C23, nucleophosmin/B23) that can be identified by silver staining at low pH. The aim of this study was to clarify the mechanism explaining why AgNOR staining also reveals osteocyte canaliculi. Human bone and kidney sections were processed for silver staining at light and electron microscopy with a modified method used to identify AgNOR. Sections were processed in parallel for immunohistochemistry with an antibody direct against osteopontin. Protein extraction was done in the renal cortex and decalcified bone and the proteins were separated by western blotting. Purified hOPN was also used as a control. Proteins were electro-transferred on polyvinylidene difluoride membranes and stained for AgNOR proteins. In bone, Ag staining identified AgNOR in cell nuclei, as well as in osteocyte canaliculi, cement and resting lines. In the distal convoluted tubules of the kidney, silver deposits were also observed in cytoplasmic granules on the apical side of the cells. Immunolocalization of osteopontin closely matched with all these locations in bone and kidney. Ag staining of membranes at low pH revealed bands for NOR proteins and 56 KDa (kidney), 60KDa (purified hOPN) and 75 KDa (bone) bands that corresponded to osteopontin. NOR proteins and osteopontin are proteins containing aspartic acid rich regions that can bind Ag. Staining protocols using silver nitrate at low pH can identify these proteins on histological sections or membranes.

Wnt/beta-catenin inhibits dental pulp stem cell differentiation

Dental pulp stem cells (DPSCs) are a unique precursor population isolated from postnatal human dental pulp and have the ability to regenerate a reparative dentin-like complex. Canonical Wnt signaling plays a critical role in tooth development and stem cell self-renewal through beta-catenin. In this study, the regulation of odontoblast-like differentiation of DPSCs by canonical Wnt signaling was examined. DPSCs were stably transduced with canonical Wnt-1 or the active form of beta-catenin, with retrovirus-mediated infection. Northern blot analysis found that Wnt-1 strongly induced the expression of matricellular protein osteopontin, and modestly enhanced the expression of type I collagen in DPSCs. Unexpectedly, Wnt-1 inhibited alkaline phosphatase (ALP) activity and the formation of mineralized nodules in DPSCs. Moreover, over-expression of beta-catenin was also sufficient to suppress the differentiation and mineralization of DPSCs. In conclusion, our results suggest that canonical Wnt signaling negatively regulates the odontoblast-like differentiation of DPSCs.

Distribution of SIBLING proteins in the organic and inorganic phases of rat dentin and bone

The SIBLING protein family is a group of non-collagenous proteins (NCPs) that includes dentin sialophosphoprotein (DSPP), dentin matrix protein 1 (DMP1), bone sialoprotein (BSP), and osteopontin (OPN). In the present study, we compared these four proteins in different phases of rat dentin and bone. First, we extracted NCPs in the unmineralized matrices and cellular compartments using guanidium-HCl (G1). Second, we extracted NCPs closely associated with hydroxyapatite using an EDTA solution (E). Last, we extracted the remaining NCPs again with guanidium-HCl (G2). Each fraction of Q-Sepharose ion-exchange chromatography was analyzed using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), Stains-All stain, and with western immunoblotting. In dentin, the NH(2)-terminal fragment of DSPP and its proteoglycan form were primarily present in the G1 extract, whereas the COOH-terminal fragment of DSPP was present exclusively in the E extract. The processed NH(2)-terminal fragment of DMP1 was present in G1 and E extracts, whereas the COOH-terminal fragment of DMP1 existed mainly in the E extract. Bone sialoprotein was present in all three extracts of dentin and bone, whereas OPN was present only in the G1 and E extracts of bone. The difference in the distribution of the SIBLING proteins between organic and inorganic phases supports the belief that these molecular species play different roles in dentinogenesis and osteogenesis.

Control of osteopontin signaling and function by post-translational phosphorylation and protein folding

Osteopontin (OPN) plays roles in a variety of cellular processes from bone resorption and extracellular matrix (ECM) remodeling to immune cell activation and inhibition of apoptosis. Because it binds receptors (integrins, CD44 variants) typically engaged by ECM molecules, OPN acts as a "soluble" ECM molecule. A persistent theme throughout the characterization of how OPN functions has been the importance of phosphorylation. The source of the OPN used in specific experiments and the location of modified sites is an increasingly important consideration for OPN research. We review briefly some of the ways OPN impacts on the biology of mammalian systems with an emphasis on the importance of serine phosphorylation in modulating its signaling ability. We describe experiments that support the hypothesis that differences in the post-translational phosphorylation of OPN expressed by different cell types regulate how it impacts on target cells. Analyses of OPN's potential secondary structure reveal a possible beta-sheet conformation that offers an interpretation of certain experimental observations, specifically the effect of thrombin cleavage; it is consistent with an interaction between the C-terminal region of the protein and the central integrin-binding RGD sequence.

Matrix vesicles are carriers of bone morphogenetic proteins (BMPs), vascular endothelial growth factor (VEGF), and noncollagenous matrix proteins

Matrix vesicles (MVs) are well positioned in the growth plate to serve as a carrier of morphogenetic information to nearby chondrocytes and osteoblasts. Bone morphogenetic proteins (BMPs) carried in MVs could promote differentiation of these skeletal cells. Vascular endothelial growth factor (VEGF) in MVs could stimulate angiogenesis. Therefore, a study was undertaken to confirm the presence of bone morphogenetic protein (BMP)-1 through-7, VEGF, and the noncollagenous matrix proteins, bone sialoprotein (BSP), osteopontin (OPN), osteocalcin (OC), and osteonectin (ON) in isolated rat growth plate MVs. MVs were isolated from collagenase-digested rachitic rat tibial and femoral growth plates. The presence of BMP-1 through BMP-7, VEGF, BSP, ON, OPN, and OC was evaluated by Western blot, plus ELISA analyses for BMP-2 and-4 content. The alkaline phosphatase-raising ability of MV extracts on cultured rat growth plate chondrocytes was measured as a reflection of MV ability to promote chondroosseous differentiation. BMP-1 through-7, VEGF, BSP, ON, OPN, and OC were all detected by Western blot analyses. Chondrocytes treated with MV extracts showed a two-to threefold increase in alkaline phosphatase activity over control, indicating increased differentiation. Significant amounts of BMP-2 and BMP-4 were detected in MVs by ELISA. Combined, these data suggest that MVs could play an important morphogenetic role in growth plate and endochondral bone formation.

Osteopontin deficiency and aging on nanomechanics of mouse bone

Osteoporosis is a bone disease characterized by low bone mass and deterioration of the tissue leading to increased fragility. Osteopontin (OPN), a noncollageneous bone matrix protein, has been shown to play an important role in osteoporosis, bone resorption, and mineralization. However, OPN's role in bone mechanical properties on the submicron scale has not been studied in any detail. In this study, nanoindentation techniques were utilized to investigate how OPN and aging affect bone mechanical properties. Hardness and elastic modulus were calculated and compared between the OPN-deficient mice (OPN(-/-)) and their age and sex-matched wild-type (OPN(+/+)) controls. The results show that the mechanical properties of the young OPN(-/-) bones (age < 12 weeks) are significantly lower than that of the youngest OPN(+/+) bones. This finding was confirmed by additional microindentation testing. Biochemical analysis using micro-Raman spectroscopy indicated more mineral content in young OPN(+/+) bones. Older (age > 12 weeks) bones did not show any significant differences in mechanical properties with genotype. In addition, OPN(+/+) bones show a decrease in mechanical properties between young and older age groups. By contrast, OPN(-/-) bones showed no significant change in mechanical properties with aging.

Osteopontin

Osteopontin (OPN) is a multifunctional molecule highly expressed in chronic inflammatory and autoimmune diseases, and it is specifically localized in and around inflammatory cells. OPN is a secreted adhesive molecule, and it is thought to aid in the recruitment of monocytes-macrophages and to regulate cytokine production in macrophages, dendritic cells, and T-cells. OPN has been classified as T-helper 1 cytokine and thus believed to exacerbate inflammation in several chronic inflammatory diseases, including atherosclerosis. Besides proinflammatory functions, physiologically OPN is a potent inhibitor of mineralization, it prevents ectopic calcium deposits and is a potent inducible inhibitor of vascular calcification. Clinically, OPN plasma levels have been found associated with various inflammatory diseases, including cardiovascular burden. It is thus imperative to dissect the OPN proinflammatory and anticalcific functions. OPN recruitment functions of inflammatory cells are thought to be mediated through its adhesive domains, especially the arginine-glycine-aspartate (RGD) sequence that interacts with several integrin heterodimers. However, the integrin receptors and intracellular pathways mediating OPN effects on immune cells are not well established. Furthermore, several studies show that OPN is cleaved by at least 2 classes of proteases: thrombin and matrix-metalloproteases (MMPs). Most importantly, at least in vitro , fragments generated by cleavage not only maintain OPN adhesive functions but also expose new active domains that may impart new activities. The role for OPN proteolytic fragments in vivo is almost completely unexplored. We believe that further knowledge of the effects of OPN fragments on cell responses might help in designing therapeutics targeting inflammatory and cardiovascular diseases.

Osteopontin is an argentophilic protein in the bone matrix and in cells of kidney convoluted tubules

Nucleolar organising regions (NOR) are part of the nucleolus, containing argyrophilic proteins (nucleoclin/C23, nucleophosmin/B23). They are identified by silver staining at low pH. The method also reveals osteocyte canaliculi and cement lines and granules in the cytoplasm of kidney cells in locations that mimic osteopontin distribution. Human bone and kidney sections, benign and lymphomatous pleural effusions were processed for silver staining to identify AgNOR. Sections were processed in parallel for immunohistochemistry with an antibody direct against osteopontin. In pleural effusions, AgNORs were found increased in the nuclei of lymphoma cells. In bone, Ag staining identified AgNOR in cell nuclei, as well as in osteocyte canaliculi, cement and resting lines. In the distal convoluted tubules of the kidney, silver deposits were also observed in cytoplasmic granules on the apical side of the cells. Immunolocalization of osteopontin closely matched with all these locations in bone and kidney. NOR proteins and osteopontin are proteins containing aspartic acid rich repeats that can bind Ag. Staining protocols using silver nitrate at low pH can identify these proteins on histological sections. AgNOR is a useful histochemical method to identify osteopontin in bone sections.

Immunolocalization of sibling and RUNX2 proteins during vertical distraction osteogenesis in the human mandible

We tested the hypothesis that mechanical loading of human bone increases expression of the transcription factor RUNX2 and bone matrix proteins osteopontin (OPN), bone sialoprotein (BSP), dentin matrix protein-1 (DMP1), and matrix extracellular phosphoglycoprotein (MEPE). We examined this in tissue sections of atrophic mandibular bone taken from edentulous patients who had undergone distraction osteogenesis. In undistracted bone, weak to moderate staining for OPN and BSP was found in osteoblasts and bone matrix of immature woven bone. RUNX2 was also detectable in osteoblasts and in cells of the periosteum. In woven bone, but not in lamellar bone, a small number of osteocytes stained for all proteins tested. After distraction, staining intensity had increased in the existing old bone and staining was seen in more bone cells than before distraction. We also found a high expression of DMP1 and MEPE in many osteocytes embedded in woven bone and in some osteocytes of lamellar bone not seen before distraction. New bone trabeculae were forming in the fibrous tissue of the distraction gap containing all stages of intramembranous bone formation. Moderate to strong staining was seen for all five proteins tested in osteocytes located in woven bone of these trabeculae and for RUNX2, OPN, and BSP in osteoblasts lining the trabecular surfaces. We conclude that loading of atrophic human jawbone by distraction activates matrix synthesis of bone cells in and around existing bone. Increased staining of DMP1 and MEPE in osteocytes after loading is in line with the concept that these proteins may be involved in signaling the effector cells to adapt the bone structure to its mechanical demands.

Pyrophosphate inhibits mineralization of osteoblast cultures by binding to mineral, up-regulating osteopontin, and inhibiting alkaline phosphatase activity

Inorganic pyrophosphate (PP(i)) produced by cells inhibits mineralization by binding to crystals. Its ubiquitous presence is thought to prevent "soft" tissues from mineralizing, whereas its degradation to P(i) in bones and teeth by tissue-nonspecific alkaline phosphatase (Tnap, Tnsalp, Alpl, Akp2) may facilitate crystal growth. Whereas the crystal binding properties of PP(i) are largely understood, less is known about its effects on osteoblast activity. We have used MC3T3-E1 osteoblast cultures to investigate the effect of PP(i) on osteoblast function and matrix mineralization. Mineralization in the cultures was dose-dependently inhibited by PP(i). This inhibition could be reversed by Tnap, but not if PP(i) was bound to mineral. PP(i) also led to increased levels of osteopontin (Opn) induced via the Erk1/2 and p38 MAPK signaling pathways. Opn regulation by PP(i) was also insensitive to foscarnet (an inhibitor of phosphate uptake) and levamisole (an inhibitor of Tnap enzymatic activity), suggesting that increased Opn levels did not result from changes in phosphate. Exogenous OPN inhibited mineralization, but dephosphorylation by Tnap reversed this effect, suggesting that OPN inhibits mineralization via its negatively charged phosphate residues and that like PP(i), hydrolysis by Tnap reduces its mineral inhibiting potency. Using enzyme kinetic studies, we have shown that PP(i) inhibits Tnap-mediated P(i) release from beta-glycerophosphate (a commonly used source of organic phosphate for culture mineralization studies) through a mixed type of inhibition. In summary, PP(i) prevents mineralization in MC3T3-E1 osteoblast cultures by at least three different mechanisms that include direct binding to growing crystals, induction of Opn expression, and inhibition of Tnap activity.

Multiple roles for neurofibromin in skeletal development and growth

Neurofibromatosis type 1 (NF1) is a prevalent genetic disorder primarily characterized by the formation of neurofibromas, café-au-lait spots and freckling. Skeletal abnormalities such as short stature or bowing/pseudarthrosis of the tibia are relatively common. To investigate the role of the neurofibromin in skeletal development, we crossed Nf1flox mice with Prx1Cre mice to inactivate Nf1 in undifferentiated mesenchymal cells of the developing limbs. Similar to NF1 affected individuals, Nf1(Prx1) mice show bowing of the tibia and diminished growth. Tibial bowing is caused by decreased stability of the cortical bone due to a high degree of porosity, decreased stiffness and reduction in the mineral content as well as hyperosteoidosis. Accordingly, osteoblasts show an increase in proliferation and a decreased ability to differentiate and mineralize in vitro. The reduction in growth is due to lower proliferation rates and a differentiation defect of chondrocytes. Abnormal vascularization of skeletal tissues is likely to contribute to this pathology as it exerts a negative effect on cortical bone stability. Furthermore, Nf1 has an important role in the development of joints, as shown by fusion of the hip joints and other joint abnormalities, which are not observed in neurofibromatosis type I. Thus, neurofibromin has multiple essential roles in skeletal development and growth.

Impaired angiogenesis, early callus formation, and late stage remodeling in fracture healing of osteopontin-deficient mice

OPN is an ECM protein with diverse localization and functionality. The role of OPN during fracture healing was examined using wildtype and OPN(-/-) mice. Results showed that OPN plays an important role in regulation of angiogenesis, callus formation, and mechanical strength in early stages of healing and facilitates late stage bone remodeling and ECM organization.

INTRODUCTION

Osteopontin (OPN) is an extracellular matrix (ECM) protein with diverse localization and functionality that has been reported to play a regulatory role in both angiogenesis and osteoclastic bone remodeling, two vital processes for normal bone healing.

MATERIALS AND METHODS

Bone repair in wildtype and OPN(-/-) mice was studied using a femoral fracture model. microCT was used for quantitative angiographic measurements at 7 and 14 days and to assess callus size and mineralization at 7, 14, 28, and 56 days. Biomechanical testing was performed on intact bones and on fracture specimens at 14, 28, and 56 days. Histology and quantitative RT-PCR were used to evaluate cellular functions related to ECM formation and bone remodeling.

RESULTS

OPN deficiency was validated in the OPN(-/-) mice, which generally displayed normal levels of related ECM proteins. Intact OPN(-/-) bones displayed increased elastic modulus but decreased strength and ductility. Fracture neovascularization was reduced at 7 but not 14 days in OPN(-/-) mice. OPN(-/-) mice exhibited smaller fracture calluses at 7 and 14 days, as well as lower maximum torque and work to failure. At 28 days, OPN(-/-) mice had normal callus size but a persistent reduction in maximum torque and work to failure. Osteoclast differentiation occurred normally, but mature osteoclasts displayed reduced functionality, decreasing late stage remodeling in OPN(-/-) mice. Thus, at 56 days, OPN(-/-) fractures possessed increased callus volume, increased mechanical stiffness, and altered collagen fiber organization.

CONCLUSIONS

This study showed multiple, stage-dependent roles of OPN during fracture healing. We conclude that OPN deficiency alters the functionality of multiple cell types, resulting in delayed early vascularization, altered matrix organization and late remodeling, and reduced biomechanical properties. These findings contribute to an improved understanding of the role of OPN in vivo and provide new insight into mechanistic control of vascularization and bone regeneration during fracture repair.

Infrared analysis of bones in magnesium-deficient rats treated with vitamin K2

In this study, we compared the effects of vitamin K(2) menatetrenone on bone mechanical properties in rats fed a low-magnesium (Mg) diet. In addition, the mechanism of bone quality was examined using Fourier transform infrared imaging (FTIRI). Thirty 4-week-old male Wistar rats were divided into three groups: intact, low-Mg-control, and low-Mg-MK-4 groups. Rats in the low-Mg groups were given a diet containing 6 mg/100 g Mg (intact, 90 mg/100 g). After an 8-week-treatment, the cortical bone mineral content (CtBMC), outer perimeter, and endo perimeter of the femoral diaphysis in the low-Mg-control group were significantly higher, while the maximum load (ML) and elastic modulus (EM) were 81% and 50% of those in the intact group, respectively (respectively, P < 0.05). In the low-Mg-MK-4 group, ML and EM were significantly higher than in the low-Mg-control group (P < 0.05), with no differences in CtBMC. The mineral/matrix ratios for the periosteal and central regions in the low-Mg-control group were 162% and 120% of those in the intact group (both, P < 0.05), respectively. MK-4 significantly inhibited these increases (P < 0.05). We found that the mineral/matrix ratios for the periosteal region of the femoral diaphysis were negatively correlated with EM, suggesting that an increase in the mineral/matrix ratio may be involved in the reduction of EM and that MK-4 may improve EM by improving the mineral/matrix ratio.

FT-IR imaging of native and tissue-engineered bone and cartilage

Fourier transform infrared (FT-IR) imaging and microspectroscopy have been extensively applied to the analyses of tissues in health and disease. Spatially resolved mid-IR data has provided insights into molecular changes that occur in diseases of connective or collagen-based tissues, including, osteoporosis, osteogenesis imperfecta, osteopetrosis and pathologic calcifications. These techniques have also been used to probe chemical changes associated with load, disuse, and micro-damage in bone, and with degradation and repair in cartilage. This review summarizes the applications of FT-IR microscopy and imaging for analyses of bone and cartilage in healthy and diseased tissues, and illustrates the application of these techniques for the characterization of tissue-engineered bone and cartilage.

Elevated skeletal osteopontin levels contribute to the hypophosphatasia phenotype in Akp2(-/-) mice

Increased levels of ePP(i) in mice deficient in TNALP (i.e., Akp2(-/-)) lead to elevated OPN concentrations. We examined the skeletal phenotype of mice lacking both OPN and TNALP and concluded that the increased OPN levels contribute to the hypophosphatasia phenotype characteristic of Akp2(-/-) mice. We also found that extracellular OPN regulates the PP(i) output by osteoblasts.

INTRODUCTION

Akp2(-/-) display mineralization deficiencies characterized by rickets/osteomalacia. This defect has been attributed to the increased levels of extracellular inorganic pyrophosphate (ePP(i)), a substrate of tissue-nonspecific alkaline phosphatase (TNALP) and a potent inhibitor of mineral deposition. Because elevated levels of ePP(i) induce Opn gene expression, the Akp2(-/-) mice also display increased levels of osteopontin (OPN), another inhibitor of mineralization.

MATERIALS AND METHODS

Akp2(-/-) mice were bred into the Opn(-/-) line. The resulting double knockout mice were analyzed for skeletal abnormalities by histology and muCT. Calvarial osteoblasts were assayed for their ability to mineralize in vitro and were probed for changes in gene expression.

RESULTS

Mice lacking both Akp2 and Opn showed partial normalization at the histological level with regard to mineral deposition and BMD. However, high ePP(i) levels remained in Akp2(-/-) mice. We found that Opn(-/-) mice have themselves elevated levels of ePP(i) attributable to an increase in Enpp1 and Ank expression and a concomitant downregulation of Akp2 expression in Opn(-/-) osteoblasts, but that Opn(-/-) mice have more mineralized osteoid than wildtype (WT) controls despite their elevated ePP(i) levels. Addition of exogenous OPN to Opn(-/-) osteoblasts results in downregulation of Enpp1 and Ank gene expression and a reduction of the PP(i) output by these cells.

CONCLUSIONS

Deletion of both Akp2 and Opn can partially rescue the hypomineralized phenotype of Akp2(-/-) mice. However, these double knockout mice do not display corrected ePP(i) levels, and we conclude that regulation of hydroxyapatite deposition requires the coordinated actions of both PP(i) and OPN and that the hypophosphatasia phenotype in Akp2(-/-) mice results from the combined inhibitory action of increased levels of both ePP(i) and OPN. Our data also suggest that the ePP(i)-mediated regulation of OPN and the OPN-mediated regulation of ePP(i) are linked counterregulatory mechanisms that control the concentrations of these two important mineralization inhibitors, OPN and ePP(i).

Variation in mineral properties in normal and mutant bones and teeth

Hydroxyapatite mineral is deposited in an organized fashion in the matrices of bones and teeth. The amount of mineral present, the composition of the mineral, and the size of the mineral crystals varies with both tissue and animal age, diet, health status, and the tissue being examined. Here, we review methods for measuring these differences in mineral properties and provide some illustrations from bones and teeth of animals in which the small leucine-rich proteoglycans (biglycan and decorin) were ablated. Differences in mineral properties between biglycan-deficient bones and teeth are related to the functions of this small proteoglycan in these tissues.

The essential role of glucocorticoids for proper human osteoblast differentiation and matrix mineralization

Glucocorticoids (GCs) exert profound effects on bone and are essential for human osteoblast differentiation. However, GCs are still interpreted as negative regulators of bone formation, mainly caused by the detrimental effects on bone after clinical use of GCs. In this paper we emphasize the importance of GCs for proper human osteoblast differentiation and matrix mineralization. We show that human osteoblast differentiation needs to be triggered by GCs in a specific time-window during the early stages of development. Exposure to GCs in the beginning of osteoblast development induces a dose dependent increase in alkaline phosphatase activity and matrix mineralization. GC-induced differentiation stimulated expression of genes involved in bone formation and suppressed genes that negatively regulate bone formation and mineralization. Furthermore we highlight the importance of local cortisol activation in osteoblasts by expression of 11beta-hydroxysteroid dehydrogenase 1 (11beta-HSD1).

Infrared imaging microscopy of bone: illustrations from a mouse model of Fabry disease

Bone is a complex tissue whose composition and properties vary with age, sex, diet, tissue type, health and disease. In this review, we demonstrate how infrared spectroscopy and infrared spectroscopic imaging can be applied to the study of these variations. A specific example of mice with Fabry disease (a lipid storage disease) is presented in which it is demonstrated that the bones of these young animals, while showing typical spatial variation in mineral content, mineral crystal size, and collagen maturity, do not differ from the bones of age- and sex-matched wild type animals.

Bisphosphonate modulates cementoblast behavior in vitro

BACKGROUND

Cementum formation is deemed to be instrumental for the successful regeneration of periodontal tissues, and thus events and modifiers of cementum formation and mineralization need to be determined. This study aimed to determine whether the bisphosphonate 1-hydroxyethylidene-1,1-bisphosphonate (HEBP) altered the behavior of immortalized cementoblasts (osteocalcin-cementoblasts [OCCM]).

METHODS

OCCM from transgenic mice were exposed to HEBP at concentrations ranging from 0.01 to 10.0 microM. The assays performed included the count of cell number for proliferation, Northern blot analysis for gene expression (up to 10 days for core binding factor alpha-1 [Cbfa1], bone sialoprotein [BSP], osteocalcin [OCN], and osteopontin [OPN], markers for cementoblast/osteoblast maturation/mineralization), von Kossa stain and alizarin red S stain for mineralization, and enzyme assay (p-nitrophenol phosphate cleavage) for alkaline phosphatase (ALP) activity.

RESULTS

Mineral nodule formation was inhibited at the higher doses of HEBP (1.0 and 10.0 microM) only. At early stages (1, 3, and 6 days), gene expression assays revealed only subtle changes in treated cells versus untreated cells, but by day 10, groups treated with lower doses (0.01 and 0.1 microM) were markedly different at the gene expression level. OCN was significantly downregulated (70%) at the lowest dose, with less pronounced effects at higher doses. In concurrence, the master switch gene for osteoblasts, Cbfa1, was also downregulated at the lower doses. Inversely, OPN mRNA was enhanced at the lower doses. ALP activity was not altered by HEBP.

CONCLUSION

Bisphosphonate alters cementoblast function in vitro through the regulation of gene expression and mineral formation.

Molecular basis of human dentin diseases

In recent years, substantial progress has been made regarding the molecular etiology of human structural tooth diseases that alter dentin matrix formation. These diseases have been classified into two major groups with subtypes: dentin dysplasia (DD) types I and II and dentinogenesis imperfecta (DGI) types I-III. Genetic linkage studies have identified the critical loci for DD-II, DGI-II, and DGI-II to human chromosome 4q21. Located within the common disease loci for these diseases is cluster of dentin/bone genes that includes osteopontin (OPN), bone sialoprotein (BSP), matrix extracellular phosphoglycoprotein (MEPE), dentin matrix protein 1 (DMP1), and dentin sialophosphoprotein (DSPP). To date, only mutations within dentin sialophosphoprotein have been associated with the pathogenesis of dentin diseases including DGI types-II and -III and DD-II. In this article, we overview the recent literature related to these dentin genetic diseases, their clinical features, and molecular pathogenesis.

[Osteopontin, a multi-faceted molecule]

Osteopontin (OPN) was initially isolated from bovine bone cortex, as a complex syalilated phospho-glyco-protein of around 60 kDa, with many postranslational modifications. It has been long considered a structural bone protein linking bone cells to the bone extracellular matrix (osteo : bone, pontin : bridge). It has been cloned for the first time in 1986. Since then, it was established that it is part of a protein family called SIBLINGs, which genes share common expression in bone and tooth, and encode among others a RGD motif. OPN is an intracellular as well as secreted protein, which binds to multiple organic or mineral ligands, like the integrin receptor alphaVbeta3, CD44, factor H and hydroxyapatite, depending on its final configuration (phosphorylation state). Pleiotropic functions of osteopontin have been demonstrated, and the osteopontin knock out phenotype in mice gave some new insight on the implication of the molecule in vivo. Osteopontin inhibits mineralization in bone and urine. Besides, it is a strong chemoattractive and proinflammatory molecule, implicated in tumors, like breast or prostate cancers, and in the defense against various infectious agents like tuberculosis, listeria or herpes. More recently, its key implication in TH1 mediated autoimmune diseases like multiple sclerosis and its animal model experimental autoimmune encephalomyelitis has been demonstrated. Osteopontin is a valuable therapeutic target in the animal model, and a biological tool correlating with clinical disease activity in humans. Structural, functional and pathological aspects of osteopontin are reviewed, as well as the osteopontin deficient phenotype in mouse.

Post-translationally modified residues of native human osteopontin are located in clusters: identification of 36 phosphorylation and five O-glycosylation sites and their biological implications

OPN (osteopontin) is an integrin-binding highly phosphorylated glycoprotein, recognized as a key molecule in a multitude of biological processes such as bone mineralization, cancer metastasis, cell-mediated immune response, inflammation and cell survival. A significant regulation of OPN function is mediated through PTM (post-translational modification). Using a combination of Edman degradation and MS analyses, we have characterized the complete phosphorylation and glycosylation pattern of native human OPN. A total of 36 phosphoresidues have been localized in the sequence of OPN. There are 29 phosphorylations (Ser8, Ser10, Ser11, Ser46, Ser47, Thr50, Ser60, Ser62, Ser65, Ser83, Ser86, Ser89, Ser92, Ser104, Ser110, Ser113, Thr169, Ser179, Ser208, Ser218, Ser238, Ser247, Ser254, Ser259, Ser264, Ser275, Ser287, Ser292 and Ser294) located in the target sequence of MGCK (mammary gland casein kinase) also known as the Golgi kinase (S/T-X-E/S(P)/D). Six phosphorylations (Ser101, Ser107, Ser175, Ser199, Ser212 and Ser251) are located in the target sequence of CKII (casein kinase II) [S-X-X-E/S(P)/D] and a single phosphorylation, Ser203, is not positioned in the motif of either MGCK or CKII. The 36 phosphoresidues represent the maximal degree of modification since variability at many sites was seen. Five threonine residues are O-glycosylated (Thr118, Thr122, Thr127, Thr131 and Thr136) and two potential sites for N-glycosylation (Asn63 and Asn90) are not occupied in human milk OPN. The phosphorylations are arranged in clusters of three to five phosphoresidues and the regions containing the glycosylations and the RGD (Arg-Gly-Asp) integrin-binding sequence are devoid of phosphorylations. Knowledge about the positions and nature of PTMs in OPN will allow a rational experimental design of functional studies aimed at understanding the structural and functional interdependences in diverse biological processes in which OPN is a key molecule.

Infrared spectroscopic characterization of mineralized tissues

Vibrational spectroscopy (Infrared and Raman), and in particular micro-spectroscopy and micro-spectroscopic imaging has been used to characterize developmental changes in bone and other mineralized tissues, to monitor these changes in cell cultures, and to detect disease and drug-induced modifications. Examples of the use of infrared micro-spectroscopy and micro-spectroscopic imaging are discussed in this review.

Importance of phosphorylation for osteopontin regulation of biomineralization

Previous in vitro and in vivo studies demonstrated that osteopontin (OPN) is an inhibitor of the formation and growth of hydroxyapatite (HA) and other biominerals. The present study tests the hypotheses that the interaction of OPN with HA is determined by the extent of protein phosphorylation and that this interaction regulates the mineralization process. Bone OPN as previously reported inhibited HA formation and HA-seeded growth in a gelatin-gel system. A transglutaminase-linked OPN polymer had similar effects. Recombinant, nonphosphorylated OPN and chemically dephosphorylated OPN, had no effect on HA formation or growth in this system. In contrast, highly phosphorylated milk OPN (mOPN) promoted HA formation. The mOPN stabilized the conversion of amorphous calcium phosphate (a non-crystalline constituent of milk) to HA, whereas bone OPN had a lesser effect on this conversion. Mixtures of OPN and osteocalcin known to form a complex in vitro, unexpectedly promoted HA formation. To test the hypothesis that small alterations in protein conformation caused by phosphorylation account for the differences in the observed ability of OPN to interact with HA, the conformation of bone OPN and mOPN in the presence and absence of crystalline HA was determined by attenuated total reflection (ATR) infrared (IR) spectroscopy. Both proteins exhibited a predominantly random coil structure, which was unaffected by the addition of Ca(2+). Binding to HA did not alter the secondary structure of bone OPN, but induced a small increase of beta-sheet (few percent) in mOPN. These data taken together suggest that the phosphorylation of OPN is an important factor in regulating the OPN-mediated mineralization process.

Infrared analysis of bone in health and disease

Infrared spectroscopy, microspectroscopy, and microspectroscopic imaging have been used to probe the composition and physicochemical status of mineral and matrix of bone in normal and diseased tissues using a series of validated parameters that reflect quantitative and qualitative properties. In this review, emphasis is placed on changes in bone's composition and physiochemical status during osteoporosis and the impact of currently used therapeutics on these parameters, although the impact of infrared microscopy in other pathological states is briefly discussed.

Homocysteine attenuates the expression of osteocalcin but enhances osteopontin in MC3T3-E1 preosteoblastic cells

It has been pointed out that very high plasma levels of homocysteine are characteristic of homocystinuria, a rare autosomal recessive disease accompanied by the early onset of generalized osteoporosis. However, it is unclear by which mechanism hyperhomocysteine induces osteoporosis, although it is known to interfere with the formation of cross-links in collagen, an essential process in bone formation. Therefore, we investigated the effect of homcysteine on expression of osteocalcin and osteopontin in MC3T3-E1 preosteoblastic cells. Confluent cells were grown in RPMI 1640 containing 10% fetal calf serum with or without homocysteine in an atmosphere of 95% humidified air, 5% CO2 at 37 degrees C. The secretion of osteocalcin from the cells increased time-dependently until the end of culture (day 34), but 500 microM homocysteine led to an approximately 61% decrease for osteocalcin after 19 days of culture as compared with the control. On the other hand, osteopontin was not inhibited by 500 microM homocysteine but rather activated, and ranged from 134%-209% of the control level in the period from 10 days until the end of culture. From analysis of RT-PCR for mRNA of osteocalcin and osteopontin at the end of the culture, homocysteine levels of 100 and 500 microM significantly increased the expression of osteopontin mRNA with the control (p < 0.05). In contrast, the expression of osteopontin mRNA was suppressed in a dose-dependent manner, showing a mirror image of the effect on osteopontin mRNA. These findings suggest that hyperhomocystenemia appears to be an independent risk factor for osteoporosis by disturbing osteoblast function.

Upregulation of osteopontin by osteocytes deprived of mechanical loading or oxygen

The pathway(s) by which disuse is transduced into locally mediated osteoclastic resorption remain unknown. We found that both acute disuse (in vivo) and direct hypoxia (in vitro) induced rapid upregulation of OPN expression by osteocytes. Within the context of OPN's role in osteoclast migration and attachment, hypoxia-induced osteocyte OPN expression may serve to mediate disuse-induced bone resorption.

INTRODUCTION

We have recently reported that disuse induces osteocyte hypoxia. Because hypoxia upregulates osteopontin (OPN) in nonconnective tissue cells, we hypothesized that both disuse and hypoxia would rapidly elevate expression of OPN by osteocytes.

MATERIALS AND METHODS

The response of osteocytes to 24 h of disuse was explored by isolating the left ulna diaphysis of adult male turkeys from loading (n = 5). Cortical osteocytes staining positive for OPN were determined using immunohistochemistry and confocal microscopy. In vitro experiments were performed to determine if OPN expression was altered in MLO-Y4 osteocytes by direct hypoxia (3, 6, 24, and 48 h) or hypoxia (3 and 24 h) followed by 24 h of reoxygenation. A final in vitro experiment explored the potential of protein kinase C (PKC) to regulate hypoxia-induced osteocyte OPN mRNA alterations.

RESULTS

We found that 24 h of disuse significantly elevated osteocyte OPN expression in vivo (145% versus intact bones; p = 0.02). We confirmed this finding in vitro, by observing rapid and significant upregulation of OPN protein expression after 24 and 48 h of hypoxia. Whereas 24 h of reoxygenation after 3 h of hypoxia restored normal osteocyte OPN expression levels, 24 h of reoxygenation after 24 h of hypoxia did not mitigate elevated osteocyte OPN expression. Finally, preliminary inhibitor studies suggested that PKC serves as a potent upstream regulator of hypoxia-induced osteocyte OPN expression.

CONCLUSIONS

Given the documented roles of OPN as a mediator of environmental stress (e.g., hypoxia), an osteoclast chemotaxant, and a modulator of osteoclastic attachment to bone, we speculate that hypoxia-induced osteocyte OPN expression may serve to mediate disuse-induced osteoclastic resorption. Furthermore, it seems that a brief window of time exists in which reoxygenation (as might be achieved by reloading bone) can serve to inhibit this pathway.

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.

Role of matrix Gla protein in parathyroid hormone inhibition of osteoblast mineralization

Parathyroid hormone (PTH) exerts biphasic effects on bone, dependent on the frequency and dose of administration. The catabolic actions of PTH on bone have been associated with continuous treatment, an increase in osteoblast-mediated resorption of bone via osteoclast activation, and inhibition of osteoblast activity and mineralization. Downregulation of differentiation markers and inhibition of mineralization by PTH have been reported in primary calvarial explants and osteoblast cell lines. Using MC3T3-E1 osteoblast-like cells, we have shown that matrix Gla protein (MGP) can be induced by PTH, and that this induction may explain the PTH-mediated inhibition of osteoblast biomineralization. MGP is a known inhibitor of mineralization, and mice deficient in Mgp show severe vascular calcification and premature bone mineralization. This review discusses the role of MGP in mineralization, comparing bone and vascular mineralization. In addition to MGP, the regulation and possible role of osteopontin, another known regulator of osteoblast mineralization, in PTH-mediated regulation of bone and vascular mineralization is discussed.

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.

Effect of sustained gene delivery of platelet-derived growth factor or its antagonist (PDGF-1308) on tissue-engineered cementum

BACKGROUND

Cementum, a mineralized tissue lining the tooth root surface, is destroyed during the inflammatory process of periodontitis. Restoration of functional cementum is considered a criterion for successful regeneration of periodontal tissues, including formation of periodontal ligament, cementum, and alveolar bone. Short-term administration of platelet-derived growth factor (PDGF) has been shown to partially regenerate periodontal structures. Nonetheless, the role of PDGF in cementogenesis is not well understood. The aim of the present study was to determine the effect of sustained PDGF gene transfer on cementum formation in an ex vivo ectopic biomineralization model.

METHODS

Osteocalcin (OC) promoter-driven SV40 transgenic mice were used to obtain immortalized cementoblasts (OCCM). The OCCM cells were transduced with adenoviruses (Ad) encoding either PDGF-A, an antagonist of PDGF signaling (PDGF-1308), a control virus (green fluorescent protein, GFP), or no treatment (NT). The transduced cells were incorporated into polymer scaffolds and implanted subcutaneously into severe combined immunodeficient (SCID) mice. The implants were harvested at 3 and 6 weeks for histomorphometric analysis of the newly formed mineralized tissues. Northern blot analysis was performed to determine the expression levels of mineral-associated genes including bone sialoprotein (BSP), OC, and osteopontin (OPN) in the cell-implant specimens at 3 and 6 weeks.

RESULTS

The results indicated mineralization was significantly reduced in both the Ad/PDGF-A and Ad/PDGF-1308 treated specimens when compared to the NT or Ad/GFP groups at 3 and 6 weeks (P<0.01). In addition, the size of the implants treated with Ad/PDGF-A and Ad/PDGF-1308 was significantly reduced compared to implants from Ad/GFP and NT groups at 3 weeks (P<0.05). At 6 weeks, the size of implants and mineral formation increased in NT, Ad/GFP, and Ad/PDGF-A groups, while the Ad/PDGF-1308 treated implants continued to decrease in size and mineral formation (P<0.01). Northern blot analysis revealed that in the Ad/PDGF-A treated implants OPN was increased, whereas OC gene expression was downregulated at 3 weeks. In the Ad/PDGF-1308 treated implants, BSP, OC, and OPN were all downregulated at 3 weeks. At 3 weeks, the Ad/PDGF-A treated implants contained significantly higher multinucleated giant cell (MNGC) density compared to NT, Ad/GFP, and Ad/PDGF-1308 specimens. The MNGC density in NT, Ad/GFP, and Ad/PDGF-A treated groups reduced over time, while the Ad/PDGF-1308 transduced implants continued to exhibit significantly higher MNGC density compared with the other treatment groups at 6 weeks.

CONCLUSIONS

The results showed that continuous exposure to PDGF-A had an inhibitory effect on cementogenesis, possibly via the upregulation of OPN and subsequent enhancement of MNGCs at 3 weeks. On the other hand, Ad/PDGF-1308 inhibited mineralization of tissue-engineered cementum possibly due to the observed downregulation of BSP and OC and a persistence of stimulation of MNGCs. These findings suggest that continuous exogenous delivery of PDGF-A may delay mineral formation induced by cementoblasts, while PDGF is clearly required for mineral neogenesis.

Concerted regulation of inorganic pyrophosphate and osteopontin by akp2, enpp1, and ank: an integrated model of the pathogenesis of mineralization disorders

Tissue-nonspecific alkaline phosphatase (TNAP) hydrolyzes the mineralization inhibitor inorganic pyrophosphate (PP(i)). Deletion of the TNAP gene (Akp2) in mice results in hypophosphatasia characterized by elevated levels of PP(i) and poorly mineralized bones, which are rescued by deletion of nucleotide pyrophosphatase phosphodiesterase 1 (NPP1) that generates PP(i). Mice deficient in NPP1 (Enpp1(-/-)), or defective in the PP(i) channeling function of ANK (ank/ank), have decreased levels of extracellular PP(i) and are hypermineralized. Given the similarity in function between ANK and NPP1 we crossbred Akp2(-/-) mice to ank/ank mice and found a partial normalization of the mineralization phenotypes and PP(i) levels. Examination of Enpp1(-/-) and ank/ank mice revealed that Enpp1(-/-) mice have a more severe hypermineralized phenotype than ank/ank mice and that NPP1 but not ANK localizes to matrix vesicles, suggesting that failure of ANK deficiency to correct hypomineralization in Akp2(-/-) mice reflects the lack of ANK activity in the matrix vesicle compartment. We also found that the mineralization inhibitor osteopontin (OPN) was increased in Akp2(-/-), and decreased in ank/ank mice. PP(i) and OPN levels were normalized in [Akp2(-/-); Enpp1(-/-)] and [Akp2(-/-); ank/ank] mice, at both the mRNA level and in serum. Wild-type osteoblasts treated with PP(i) showed an increase in OPN, and a decrease in Enpp1 and Ank expression. Thus TNAP, NPP1, and ANK coordinately regulate PP(i) and OPN levels. The hypomineralization observed in Akp2(-/-) mice arises from the combined inhibitory effects of PP(i) and OPN. In contrast, NPP1 or ANK deficiencies cause a decrease in the PP(i) and OPN pools that leads to hypermineralization.

Concerted regulation of inorganic pyrophosphate and osteopontin by akp2, enpp1, and ank: an integrated model of the pathogenesis of mineralization disorders

Tissue-nonspecific alkaline phosphatase (TNAP) hydrolyzes the mineralization inhibitor inorganic pyrophosphate (PP(i)). Deletion of the TNAP gene (Akp2) in mice results in hypophosphatasia characterized by elevated levels of PP(i) and poorly mineralized bones, which are rescued by deletion of nucleotide pyrophosphatase phosphodiesterase 1 (NPP1) that generates PP(i). Mice deficient in NPP1 (Enpp1(-/-)), or defective in the PP(i) channeling function of ANK (ank/ank), have decreased levels of extracellular PP(i) and are hypermineralized. Given the similarity in function between ANK and NPP1 we crossbred Akp2(-/-) mice to ank/ank mice and found a partial normalization of the mineralization phenotypes and PP(i) levels. Examination of Enpp1(-/-) and ank/ank mice revealed that Enpp1(-/-) mice have a more severe hypermineralized phenotype than ank/ank mice and that NPP1 but not ANK localizes to matrix vesicles, suggesting that failure of ANK deficiency to correct hypomineralization in Akp2(-/-) mice reflects the lack of ANK activity in the matrix vesicle compartment. We also found that the mineralization inhibitor osteopontin (OPN) was increased in Akp2(-/-), and decreased in ank/ank mice. PP(i) and OPN levels were normalized in [Akp2(-/-); Enpp1(-/-)] and [Akp2(-/-); ank/ank] mice, at both the mRNA level and in serum. Wild-type osteoblasts treated with PP(i) showed an increase in OPN, and a decrease in Enpp1 and Ank expression. Thus TNAP, NPP1, and ANK coordinately regulate PP(i) and OPN levels. The hypomineralization observed in Akp2(-/-) mice arises from the combined inhibitory effects of PP(i) and OPN. In contrast, NPP1 or ANK deficiencies cause a decrease in the PP(i) and OPN pools that leads to hypermineralization.

MEPE has the properties of an osteoblastic phosphatonin and minhibin

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

Induction of osteoblast differentiation indices by statins in MC3T3-E1 cells

Statins inhibit 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase, which catalyzes conversion of HMG-CoA to mevalonate, a rate-limiting step in cholesterol synthesis. The present study was undertaken to understand the events of osteoblast differentiation induced by statins. Simvastatin at 10(-7) M markedly increased mRNA expression for bone morphogenetic protein-2 (BMP-2), vascular endothelial growth factor (VEGF), alkaline phosphatase, type I collagen, bone sialoprotein, and osteocalcin (OCN) in nontransformed osteoblastic cells (MC3T3-E1), while suppressing gene expression for collagenase-1, and collagenase-3. Extracellular accumulation of proteins such as VEGF, OCN, collagenase-digestive proteins, and noncollagenous proteins was increased in the cells treated with 10(-7) M simvastatin, or 10(-8) M cerivastatin. In the culture of MC3T3-E1 cells, statins stimulated mineralization; pretreating MC3T3-E1 cells with mevalonate, or geranylgeranyl pyrophosphate (a mevalonate metabolite) abolished statin-induced mineralization. Statins stimulate osteoblast differentiation in vitro, and may hold promise drugs for the treatment of osteoporosis in the future.

Fourier transform infrared microscopic imaging: effects of estrogen and estrogen deficiency on fracture healing in rat femurs

Infrared spectroscopic imaging with 6-10 microm spatial resolution was used to characterize the changes in fracture callus mineral content, carbonate content, mineral crystallinity, and collagen maturity in femurs of 3-month-old ovariectomized rats treated with estrogen (estrogen sufficiency) or vehicle (estrogen deficiency). Comparisons were also made in these animals to cortical bone at a distance from the callus. Analyses at 4, 8, and 12 weeks post fracture demonstrated that healing was accelerated in the estrogen-sufficient animals as demonstrated by increasing mineral content and collagen maturity and decreasing carbonate incorporation.