Differences in composition of cell-attachment sialoproteins between dentin and bone

Expression of BSP-GFPtpz Transgene during Osteogenesis and Reparative Dentinogenesis

Bone sialoprotein (BSP) is a member of the SIBLING family with essential roles in skeletogenesis. In the developing teeth, although the expression and function of BSP in the formation of acellular cementum and periodontal attachment are well documented, there are uncertainties regarding the expression and function of BSP by odontoblasts and dentin. Reporter mice are valuable animal models for biological research, providing a gene expression readout that can contribute to cellular characterization within the context of a developmental process. In the present study, we examined the expression of a BSP-GFPtpz reporter mouse line during odontoblast differentiation, reparative dentinogenesis, and bone. In the developing teeth, BSP-GFPtpz was expressed at high levels in cementoblasts but not in odontoblasts or dentin. In bones, the transgene was highly expressed in osteoblasts at an early stage of differentiation. Interestingly, despite its lack of expression in odontoblasts and dental pulp during tooth development, the BSP-GFPtpz transgene was detected during in vitro mineralization of primary pulp cultures and during reparative dentinogenesis following pulp exposures. Importantly, under these experimental contexts, the expression of BSP-GFPtpz was still exclusive to DSPP-Cerulean, an odontoblast-specific reporter gene. This suggests that the combinatorial use of BSP-GFPtpz and DSPP-Cerulean can be a valuable experimental tool to distinguish osteogenic from dentinogenic cells, thereby providing an avenue to investigate mechanisms that distinctly regulate the lineage progression of progenitors into odontoblasts versus osteoblasts.

iMatrix-511 Stimulates the Proliferation and Differentiation of MDPC-23 Cells into Odontoblastlike Phenotype

INTRODUCTION

iMatrix-511 is a novel integrin-binding fragment derived from laminin-511. Previous studies showed its superiority as a culture substrate for xeno-free culture and maintenance of pluripotency in stem cells. However, its effects in the dental field remain largely unknown. The aim of the present study was to unravel the in vitro effects of iMatrix-511 in comparison with vitronectin (VN).

METHODS

Biochemical assays were performed in vitro in MDPC-23 cells. The optimal coating density for 2 proteins was determined using the cell counting kit-8. To evaluate cell proliferation to both proteins, MDPC-23 cells were directly seeded onto the iMatrix-511 or VN-modified polystyrene and analyzed by the cell counting kit-8. The phenotype of cells seeded on iMatrix-511 and VN was characterized. Phenotypic characterization included real-time reverse-transcription polymerase chain reaction and alizarin red staining.

RESULTS

The optimal coating density for iMatrix-511 and VN was determined to be 1 μg/cm² and 0.25 μg/cm², respectively. Cells cultured on iMatrix-511 showed higher cell proliferative activity than the noncoated control and VN on days 1, 2, and 4. Cell morphology observation revealed MDPC-23 cells attach preferentially to iMatrix-511 and start to spread as early as 1 hour after inoculation. MDPC-23 cells exhibited more potent odontogenic differentiation on iMatrix-511 than the control and VN as shown by the marked enhancement of dentin matrix protein 1 and dentin sialophosphoprotein messenger RNA expression. Although both proteins showed more mineralized nodule formation than the control, iMatrix-511 remained to be the one that elicited stronger calcific deposition.

CONCLUSIONS

iMatrix-511 supported the proliferation and acquisition of odontogenic cell phenotype in vitro, rendering this novel material a potential candidate for dentin regeneration.

Mineralization defects in cementum and craniofacial bone from loss of bone sialoprotein

Bone sialoprotein (BSP) is a multifunctional extracellular matrix protein found in mineralized tissues, including bone, cartilage, tooth root cementum (both acellular and cellular types), and dentin. In order to define the role BSP plays in the process of biomineralization of these tissues, we analyzed cementogenesis, dentinogenesis, and osteogenesis (intramembranous and endochondral) in craniofacial bone in Bsp null mice and wild-type (WT) controls over a developmental period (1-60 days post natal; dpn) by histology, immunohistochemistry, undecalcified histochemistry, microcomputed tomography (microCT), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and quantitative PCR (qPCR). Regions of intramembranous ossification in the alveolus, mandible, and calvaria presented delayed mineralization and osteoid accumulation, assessed by von Kossa and Goldner's trichrome stains at 1 and 14 dpn. Moreover, Bsp(-/-) mice featured increased cranial suture size at the early time point, 1 dpn. Immunostaining and PCR demonstrated that osteoblast markers, osterix, alkaline phosphatase, and osteopontin were unchanged in Bsp null mandibles compared to WT. Bsp(-/-) mouse molars featured a lack of functional acellular cementum formation by histology, SEM, and TEM, and subsequent loss of Sharpey's collagen fiber insertion into the tooth root structure. Bsp(-/-) mouse alveolar and mandibular bone featured equivalent or fewer osteoclasts at early ages (1 and 14 dpn), however, increased RANKL immunostaining and mRNA, and significantly increased number of osteoclast-like cells (2-5 fold) were found at later ages (26 and 60 dpn), corresponding to periodontal breakdown and severe alveolar bone resorption observed following molar teeth entering occlusion. Dentin formation was unperturbed in Bsp(-/-) mouse molars, with no delay in mineralization, no alteration in dentin dimensions, and no differences in odontoblast markers analyzed. No defects were identified in endochondral ossification in the cranial base, and craniofacial morphology was unaffected in Bsp(-/-) mice. These analyses confirm a critical role for BSP in processes of cementogenesis and intramembranous ossification of craniofacial bone, whereas endochondral ossification in the cranial base was minimally affected and dentinogenesis was normal in Bsp(-/-) molar teeth. Dissimilar effects of loss of BSP on mineralization of dental and craniofacial tissues suggest local differences in the role of BSP and/or yet to be defined interactions with site-specific factors.

Mechanical Forces Exacerbate Periodontal Defects in Bsp-null Mice

Bone sialoprotein (BSP) is an acidic phosphoprotein with collagen-binding, cell attachment, and hydroxyapatite-nucleating properties. BSP expression in mineralized tissues is upregulated at onset of mineralization. Bsp-null (Bsp(-/-)) mice exhibit reductions in bone mineral density, bone turnover, osteoclast activation, and impaired bone healing. Furthermore, Bsp(-/-) mice have marked periodontal tissue breakdown, with a lack of acellular cementum leading to periodontal ligament detachment, extensive alveolar bone and tooth root resorption, and incisor malocclusion. We hypothesized that altered mechanical stress from mastication contributes to periodontal destruction observed in Bsp(-/-) mice. This hypothesis was tested by comparing Bsp(-/-) and wild-type mice fed with standard hard pellet diet or soft powder diet. Dentoalveolar tissues were analyzed using histology and micro-computed tomography. By 8 wk of age, Bsp(-/-) mice exhibited molar and incisor malocclusion regardless of diet. Bsp(-/-) mice with hard pellet diet exhibited high incidence (30%) of severe incisor malocclusion, 10% lower body weight, 3% reduced femur length, and 30% elevated serum alkaline phosphatase activity compared to wild type. Soft powder diet reduced severe incisor malocclusion incidence to 3% in Bsp(-/-) mice, supporting the hypothesis that occlusal loading contributed to the malocclusion phenotype. Furthermore, Bsp(-/-) mice in the soft powder diet group featured normal body weight, long bone length, and serum alkaline phosphatase activity, suggesting that tooth dysfunction and malnutrition contribute to growth and skeletal defects reported in Bsp(-/-) mice. Bsp(-/-) incisors also erupt at a slower rate, which likely leads to the observed thickened dentin and enhanced mineralization of dentin and enamel toward the apical end. We propose that the decrease in eruption rate is due to a lack of acellular cementum and associated defective periodontal attachment. These data demonstrate the importance of BSP in maintaining proper periodontal function and alveolar bone remodeling and point to dental dysfunction as causative factor of skeletal defects observed in Bsp(-/-) mice.

Effects of the bisphosphonate alendronate on molars of young rats after lateral luxation

BACKGROUND AND AIM

The bisphosphonate alendronate (ALN) was employed with the aim of investigating its effects on dental and periodontal tissues after lateral luxation of developing molars.

MATERIAL AND METHODS

Twenty-one-day-old Wistar rats had their second upper molars laterally luxated. Daily 2.5 mg kg(-1) ALN injections started at the day of the luxation; controls received sterile saline solution. The teeth were analyzed 7, 14, and 21 days after the procedure. On the days cited, the maxillae were fixed, decalcified, and embedded in paraffin or Spurr resin. The paraffin sections were stained with H&E, incubated for TRAP histochemistry or immunolabeled for osteopontin (OPN). Spurr ultrathin sections were examined in a transmission electron microscope.

RESULTS

After 21 days, the root apex of luxated molars without ALN was wide open and disorganized and also covered by an irregular layer of cellular cementum, which was not observed in ALN-treated animals. Ankylosis sites were observed in ALN rats in both luxated and non-luxated teeth. The TRAP-positive osteoclasts were more numerous in ALN group, despite their latent ultrastructural appearance without the presence of resorption apparatus compared to controls. OPN immunolabeling revealed a thick immunopositive line in the dentin that must be resultant from the moment of the luxation, while ALN-treated specimens did not present alterations in dentin.

CONCLUSION

The present findings indicate that alendronate inhibits some alterations in dentin and cementum formation induced by dental trauma.

The isolation and characterization of glycosylated phosphoproteins from herring fish bones

Past studies of bone extracellular matrix phosphoproteins such as osteopontin and bone sialoprotein have yielded important biological information regarding their role in calcification and the regulation of cellular activity. Most of these studies have been limited to proteins extracted from mammalian and avian vertebrates and nonvertebrates. The present work describes the isolation and purification of two major highly glycosylated and phosphorylated extracellular matrix proteins of 70 and 22 kDa from herring fish bones. The 70-kDa phosphoprotein has some characteristics of osteopontin with respect to amino acid composition and susceptibility to thrombin cleavage. Unlike osteopontin, however, it was found to contain high levels of sialic acid similar to bone sialoprotein. The 22-kDa protein has very different properties such as very high content of phosphoserine (∼270 Ser(P) residues/1000 amino acid residues), Ala, and Asx residues. The N-terminal amino acid sequence analysis of both the 70-kDa (NPIMA(M)ETTS(M)DSKVNPLL) and the 22-kDa (NQDMAMEASSDPEAA) fish phosphoproteins indicate that these unique amino acid sequences are unlike any published in protein databases. An enzyme-linked immunosorbent assay revealed that the 70-kDa phosphoprotein was present principally in bone and in calcified scales, whereas the 22-kDa phosphoprotein was detected only in bone. Immunohistological analysis revealed diffusely positive immunostaining for both the 70- and 22-kDa phosphoproteins throughout the matrix of the bone. Overall, this work adds additional support to the concept that the mechanism of biological calcification has common evolutionary and fundamental bases throughout vertebrate species.

The SCPP gene repertoire in bony vertebrates and graded differences in mineralized tissues

The vertebrate tooth is covered with enamel in most sarcopterygians or enameloid in chondrichthyans and actinopterygians. The evolutionary relationship among these two tissues, the hardest tissue in the body, and other mineralized tissues has long been controversial. We have recently reported that specific combinations of secretory calcium-binding phosphoprotein (SCPP) genes are involved in the mineralization of bone, dentin, enameloid, and enamel. Thus, the early repertoire of SCPP genes would elucidate the evolutionary relationship across these tissues. However, the diversity of SCPP genes in teleosts and tetrapods and the roles of these genes in distinct tissues have remained unclear, mainly because many SCPP genes are lineage-specific. In this study, I show that the repertoire of SCPP genes in the zebrafish, frog, and humans includes many lineage-specific genes and some widely conserved genes that originated in stem osteichthyans or earlier. Expression analysis demonstrates that some frog and zebrafish SCPP genes are used primarily in bone, but also in dentin, while the reverse is true of other genes, similar to some mammalian SCPP genes. Dentin and enameloid initially use shared genes in the matrix, but enameloid is subsequently hypermineralized. Notably, enameloid and enamel use an orthologous SCPP gene in the hypermineralization process. Thus, the hypermineralization machinery ancestral to both enameloid and enamel arose before the actinopterygian-sarcopterygian divergence. However, enamel employs specialized SCPPs as structuring proteins, not used in enameloid, reflecting the divergence of enamel from enameloid. These results show graded differences in mineralized dental tissues and reinforce the hypothesis that bone-dentin-enameloid-enamel constitutes an evolutionary continuum.

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.

Involvement of the klotho protein in dentin formation and mineralization

Klotho-deficient mice exhibit multiple pathological conditions resembling human aging. Our previous study showed alterations in the distribution of osteocytes and in the bone matrix synthesis in klotho-deficient mice. Although the bone and tooth share morphological features such as mineralization processes and components of the extracellular matrix, little information is available on how klotho deletion influences tooth formation. The present study aimed to elucidate the altered histology of incisors of klotho-deficient mice-comparing the findings with those from their wild-type littermates, by using immunohistochemistry for alkaline phosphatase (ALP), osteopontin, and dentin matrix protein-1 (DMP-1), terminal deoxynucleotidyl transferase-mediated deoxyuridinetriphosphate nick end-labeling (TUNEL) detection for apoptosis, and electron probe microanalyzer (EPMA) analysis on calcium (Ca), phosphate (P), and magnesium (Mg). Klotho-deficient incisors exhibited disturbed layers of odontoblasts, predentin, and dentin, resulting in an obscure dentin-predentinal border at the labial region. Several odontoblast-like cells without ALP activity were embedded in the labial dentin matrix, and immunopositivity for DMP-1 and osteopontin was discernible in the matrix surrounding these embedded odontoblast-like cells. TUNEL detection demonstrated an apoptotic reaction in the embedded odontoblast-like cells and pulpal cells in the klotho-deficient mice. EPMA revealed lower concentrations of Ca, P, and Mg in the klotho-deficient dentin, except for the dentin around abnormal odontoblast-like cells. These findings suggest the involvement of the klotho gene in dentinogenesis and its mineralization.

Ultrastructural and immunocytochemical analyses of osteopontin in reactionary and reparative dentine formed after extrusion of upper rat incisors

Reactionary dentine and reparative dentine are two strategies used by the dentine-pulp complex to respond to injury. The reactionary dentine is secreted by original odontoblasts, while the reparative dentine is formed by odontoblast-like cells. Osteopontin (OPN) is a non-collagenous protein usually present in the repair of mineralized tissues. It is likely to be present in newly formed dentine but there are no studies attempting to detect it in reactionary and reparative dentine. The aim of the present study was to examine the ultrastructural characteristics, as well as the presence and distribution of OPN in reactionary and reparative dentine by provoking extrusion of the rat incisor. The right upper incisors of 3-month-old male rats were extruded 3 mm and then repositioned into their original sockets. At 3, 7, 10, 15, 20, 30 and 60 days after surgery, the incisors were fixed in glutaraldehyde-formaldehyde and then processed for scanning and transmission electron microscopy and for immunocytochemistry for OPN. After extrusive trauma, the dentine-pulp interface showed the presence of reactionary and reparative dentine, which varied in aspect, thickness and related cells. OPN was not detected in the physiological and reactionary dentine, while it was strongly immunoreactive in the matrix that surrounded the entrapped cells of reparative dentine. In addition, original odontoblasts subjacent to the physiological dentine contained OPN in their Golgi region. The present findings showed that reparative dentine shares some structural characteristics with primary bone, especially in relation to its OPN content. The odontoblast-like cells resemble osteoblasts rather than odontoblasts.

Rescue of odontogenesis in Dmp1-deficient mice by targeted re-expression of DMP1 reveals roles for DMP1 in early odontogenesis and dentin apposition in vivo

Dentin matrix protein 1 (DMP1) is expressed in both pulp and odontoblast cells and deletion of the Dmp1 gene leads to defects in odontogenesis and mineralization. The goals of this study were to examine how DMP1 controls dentin mineralization and odontogenesis in vivo. Fluorochrome labeling of dentin in Dmp1-null mice showed a diffuse labeling pattern with a 3-fold reduction in dentin appositional rate compared to controls. Deletion of DMP1 was also associated with abnormalities in the dentinal tubule system and delayed formation of the third molar. Unlike the mineralization defect in Vitamin D receptor-null mice, the mineralization defect in Dmp1-null mice was not rescued by a high calcium and phosphate diet, suggesting a different effect of DMP1 on mineralization. Re-expression of Dmp1 in early and late odontoblasts under control of the Col1a1 promoter rescued the defects in mineralization as well as the defects in the dentinal tubules and third molar development. In contrast, re-expression of Dmp1 in mature odontoblasts, using the Dspp promoter, produced only a partial rescue of the mineralization defects. These data suggest that DMP1 is a key regulator of odontoblast differentiation, formation of the dentin tubular system and mineralization and its expression is required in both early and late odontoblasts for normal odontogenesis to proceed.

Dentin matrix proteins and soluble factors: intrinsic regulatory signals for healing and resorption of dental and periodontal tissues?

Dentin contains numerous polypeptides and signaling molecules sequestered in a mineralized matrix. The exposure and release of these molecules occur as a consequence of injury to the pulp and periodontal ligament, which may result from luxation, orthodontic movement or infections of tooth and periodontal structures. When released at these sites, dentin constituents have the potential to act on different surrounding cells, including periodontal cells, osteoblasts, osteoclasts and inflammatory cells, and to affect the course of dental disease. Experimental studies have highlighted the interactions between dentin and cells from tooth and periodontal tissues and reveal dentin to be a cell adhesive, signaling and migratory stimulus for various mesenchymal and inflammatory cells. These results support the hypothesis that dentin molecules might function as regulatory signals for the healing and resorption of dental and periodontal tissues. Data from recent and classical investigations are summarized, many open questions are discussed, and current hypotheses concerning the mechanisms of tooth resorption and periodontal healing are outlined. Many questions regarding the importance of dentin as a source of multifunctional molecules remain unanswered and provide important directions for future studies.

A comparative study of sialic acid-rich proteins in rat bone and dentin

Four sialic acid-rich (SA-rich) proteins found in bone and dentin, osteopontin (OPN), bone sialoprotein (BSP), bone acidic glycoprotein-75 (BAG-75), and dentin matrix protein 1 (DMP1), share some common features. We used SDS-PAGE and Western immunoblots to analyze and compare SA-rich proteins in bone and dentin extracts from rats with a single chromatographic procedure. OPN was detected in dentin extracts, with a relative level less than one-seventieth of that in bone. Both bone and dentin BSP demonstrated an extremely broad distribution pattern, probably due to a high degree of heterogeneity in post-translational modifications. BAG-75 in both bone and dentin was detected as an 83 kDa band, dramatically distinct from that of DMPI. Using a polyclonal antibody raised against a purified bone 57 kDa protein (a portion of DMPI), we detected 150 kDa protein bands in bone fraction; the same bands were recognized by antirecombinant rat DMPI antibody. Bands from dentin migrating at about 150 kDa in earlier fractions and progressing to 200 kDa in later fractions showed a clear immunoreactivity to the anti-57 kDa antibody. We conclude that the majority of DMPI in rat bone is processed into fragments, whereas that in dentin remains intact.

High-resolution immunocytochemistry of noncollagenous matrix proteins in rat mandibles processed with microwave irradiation

The mineral phase in calcified tissues represents an additional factor to be considered during their preservation for ultrastructural analyses. Microwave (MW) irradiation has been shown to facilitate fixative penetration and to improve structural preservation and immunolabeling in a variety of soft tissues. The aim of the present study was to determine whether MW processing could offer similar advantages for hard tissues. Rat hemimandibles were immersed in 4% formaldehyde + 0.1% glutaraldehyde buffered with 0.1 M sodium cacodylate, pH 7.2, and exposed to MWs for three periods of 5 min at temperatures not exceeding 37C. They were then decalcified in 4.13% EDTA, pH 7.2, for 15 hr, also under MW irradiation. Osmicated and non-osmicated samples were dehydrated in graded concentrations of ethanol and embedded in LR White resin. Sections of incisor, molars, and alveolar bone were processed for postembedding colloidal gold immunolabeling using antibodies against ameloblastin, amelogenin, bone sialoprotein, or osteopontin. Ultrastructural preservation of tissues was in most cases comparable to that obtained by perfusion-fixation, and there was no difference in distribution of labeling with those previously reported for the antibodies used. However, the immunoreactivities obtained were generally more intense, particularly at early stages of tooth formation. Amelogenin was abundant between differentiating ameloblasts and labeling for osteopontin appeared over the Golgi apparatus of odontoblasts after initiation of dentine mineralization. We conclude that MW irradiation represents a simple method that can accelerate the processing of calcified tissues while yielding good structural preservation and antigen retention. (J Histochem Cytochem 49:1099-1109, 2001)

Biochemical markers to survey bone turnover

Molecular markers of bone turnover have gained increasing relevance in the evaluation of patients with metabolic bone diseases. Their clinical applications include the assessment of future osteoporotic fracture risk, complementation of bone density measurements, diagnosis of certain metabolic osteopathies, therapeutic decision making, and monitoring of therapeutic efficacy and patient compliance. One should be aware, however, that the results from large epidemiologic or clinical trials are sometimes difficult to translate into the everyday clinical situation. The individual patient often has more than one disease that might affect either bone turnover or the handling of the parameters mentioned (or both). Analytic and biologic variability of bone markers can be significant and also needs to be considered when using these indices. In the scientific setting, conventional and new markers of bone turnover can help to elucidate formerly unknown mechanisms and pathways. Because the development of ever more specific and sensitive markers of bone metabolism is progressing rapidly, we are likely to witness new insights into the pathophysiology of bone diseases in the near future.

Serum bone sialoprotein as a marker of tumour burden and neoplastic bone involvement and as a prognostic factor in multiple myeloma

To test the potential of immunoreactive BSP, a non-collagenous bone matrix component, as a clinical guide in patients with plasma cell dyscrasias, serum BSP concentrations were measured in 62 patients with newly diagnosed multiple myeloma (MM) followed over a period of 4 years, in 46 patients with monoclonal gammopathy of undetermined significance (MGUS), in 71 patients with untreated benign vertebral osteoporosis (OPO), and in 139 healthy adults. Results were compared with clinical and laboratory data, including serum osteocalcin (OC), and urinary pyridinoline (PYD) and deoxypyridinoline (DPD) as markers of bone turnover. In MM, serum BSP, and urinary PYD and DPD were higher than in healthy controls and in MGUS or OPO (P< 0.001). BSP levels correlated with the bone marrow plasma cell content (r = 0.40, P< 0.001), and serum beta2-microglobulin (r = 0.31, P < 0.01). The differentiation of MM from healthy controls and from MGUS or OPO was highest for BSP. After chemotherapy, BSP reflected the response to treatment and correlated with the change in monoclonal protein (r = 0.55, P< 0.001). MM patients with normal baseline BSP levels survived longer than patients with initially elevated BSP values (P< 0.001, log rank test). Only serum monoclonal protein and BSP were independent predictors of survival. We conclude that in MM, BSP levels are associated with skeletal involvement and tumour cell burden. The quantification of serum BSP may be a non-invasive method for the diagnosis and follow-up, and may improve the prognostic value of conventional staging in MM.

Identification and characterization of the carboxyl-terminal region of rat dentin sialoprotein

Two acidic proteins, dentin sialoprotein (DSP) and dentin phosphoprotein (DPP), are present in the extracellular matrix of dentin but not in bone. These two proteins are expressed in odontoblasts and preameloblasts as a single cDNA transcript coding a large precursor protein termed dentin sialophosphoprotein (DSPP). DSPP is specifically cleaved into two unique proteins, DSP and DPP. However, the cleavage site(s) of DSPP and the mechanisms for regulating the cleavages are unknown. To identify the specific site(s) of DSPP that are cleaved when the initial translation product is converted to DSP and DPP, we performed a detailed analysis (Edman degradation and mass spectrometry) on selected tryptic peptides of a size originating from the COOH-terminal region of rat DSP. After cleavage with trypsin, the DSP fragments were separated by a two-dimensional method (size-exclusion chromatography followed by reversed phase high performance liquid chromatography). We characterized 13 peptides from various regions of DSP. The analyses showed that peptide Ile(409)-Tyr(421) was the major COOH-terminal fragment, ending at Tyr(421) only 9 residues from the NH(2) terminus of DPP. Peptide Gln(385)-His(406) represented a second, minor COOH-terminal peptide that terminated at His(406). Both of these residues are well beyond the COOH terminus predicted previously by two independent studies estimating that rat DSP contained 360-370 amino acids. Careful studies on two peptides showed that, among 9 potential casein kinase II phosphorylation sites, 2 serines were phosphorylated. We found that rat DSP was heterogeneous with respect to phosphorylation, because this same peptide sequence eluted in two discrete peaks, one with 2 phosphoserines and the other having 1. The finding that 3 lysines just preceding the COOH termini were modified by a 43-Da substituent (possibly a carbamoyl substituent) suggests that the lysines in this region were particularly susceptible to attachment of this substituent.

Bone sialoprotein

The search for a protein nucleator of hydroxyapatite crystal formation has been a focus for the isolation and characterization of the major non-collagenous proteins in bone. Of the proteins characterized to date, bone sialoprotein (BSP) has emerged as the only bona fide candidate for nucleation. BSP is a highly glycosylated and sulphated phosphoprotein that is found almost exclusively in mineralized connective tissues. Characteristically, polyglutamic acid and arginine-glycine-aspartate (RGD) motifs with the ability to bind hydroxyapatite and cell-surface integrins, respectively, have been conserved in the protein sequence. Expression of the BSP gene, which is induced in newly formed osteoblasts, is up-regulated by hormones and cytokines that promote bone formation and down-regulated by factors that suppress bone formation. Thus, BSP has the biophysical and chemical properties of a nucleator, and its temporo-spatial expression coincides with de novo mineralization in bone and cementum. Moreover, BSP has been associated with mineral crystal formation in several pathologies, including breast carcinomas. However, the ability of BSP to mediate cell attachment and to signal through the RGD motif points to alternate functions for BSP which need further investigation. In combination, the hydroxyapatite-binding polyglutamic acid sequences and the RGD provide bi-functional entities through which BSP may mediate the targeting and attachment of normal and metastasizing cells to the bone surface.

Basic principles and clinical applications of biochemical markers of bone metabolism: biochemical and technical aspects

The interest in and the need for effective measures to be used in the screening, diagnosis, and follow-up of disorders of connective tissue, bone, and mineral metabolism has markedly grown. Next to clinical and imaging techniques, indices of bone turnover have come to play an important role in the assessment of metabolic bone disease. In osteoporosis, recent research has shown that bone markers may also be used to predict future bone loss and hip fractures (in larger cohorts of older patients), identify individuals at risk for osteoporosis, select therapy, and predict and monitor the therapeutic response in individual patients. The development of new markers of bone metabolism has greatly enriched the spectrum of serum and urine analytes used in the assessment of skeletal pathologies. Besides total alkaline phosphatase, other markers such as bone-specific alkaline phosphatase, osteocalcin, or the collagen propeptides are being used to measure bone formation. Bone resorption, previously assessed only by the measurement of urinary calcium and hydroxyproline, may now be detected more precisely by a number of new serum and urine markers. Among these, the pyridinium crosslinks and the telopeptides of collagen type I are presently considered the most specific markers of bone resorption. More recently, bone sialoprotein has also been suggested as a marker of bone resorption in serum. Tartrate-resistant acid phosphatase is now measurable by immunoassay. This article surveys the biochemistry and relevant technical aspects of the currently available markers of bone metabolism.

Osteoclastic resorption of bone-like apatite formed on a plastic disk as an in vitro assay system

We have investigated the applicability of a simple and inexpensive osteoclastic assay system using bone-like apatite-coated polyethyleneterephthalate (PET) disks. A 1 microm thick apatite layer, uniform and homogeneous bone-mineral-like with no organic components, was made on PET disks using a biomimetic process. As substrates for an osteoclastic assay, these coated disks were compared with dentine as well as with bone-like or heat-treated apatite of various thicknesses on apatite- and wollastonite-containing glass ceramic (A-W GC) disks. The unfractionated bone cells, including osteoclasts, of a neonatal rabbit were seeded onto these substrates. By scanning electron microscopic examination, the resorption lacunae of the thick bone-like apatite clearly showed track-like shapes at various depths, similar to those of dentine although the border between the A-W GC and the apatite was unclear. In contrast, those of heat-treated apatite showed small and shallow shapes with irregular margins, quite different from those of dentine. By reducing the thickness of bone-like apatite to 1 microm as well as using PET as its substrate, the margins of the resorption lacunae became quite clear, and with the use of phase-contrast microscopy during culture, osteoclasts and resorption pits could be precisely observed. The resorbed area, easily measured with the aid of bright-field microscopy and an image analyzer, was found to have increased in a time-dependent manner and at the end of 4 days of culture was not statistically different from that of dentine.

Attachment of osteoblastic cells to hydroxyapatite crystals by a synthetic peptide (Glu7-Pro-Arg-Gly-Asp-Thr) containing two functional sequences of bone sialoprotein

We investigated activity of bone sialoprotein (BSP) to mediate attachment of cells to hydroxyapatite using a model peptide, Glu7-Pro-Arg-Gly-Asp-Thr, which contains a putative hydroxyapatite-binding site (poly-Glu) and a cell-attachment site. The peptide has affinity to hydroxyapatite with a dissociation constant of 13.5 microM. The peptide affected in vitro mineralization in a gel system, indicating interaction between this peptide and calcium phosphate. The osteoblastic cell line MC3T3-E1 was incubated with hydroxyapatite powder coated with the peptide or proteins. Attachment of the cells was observed on the powder coated with BSP, but not on the powder coated with serum albumin. The cells were attached to the powder coated with the peptide. The cells were flattened on the powder, and pseudopods developed. The attachment of the cells was inhibited by an excessive amount of Gly-Arg-Gly-Asp-Ser peptide. In conclusion, BSP mediated attachment of osteoblastic cells to hydroxyapatite, and this activity could be accomplished only by the poly-Glu sequence and the Arg-Gly-Asp sequence.

Expression of bone sialoprotein mRNA by cells lining the mouse tooth root during cementogenesis

Adhesion molecules are considered to have an active role in controlling cell differentiation, although the mechanisms involved have yet to be determined. The developing tooth provides an excellent model to use for determining the factors/processes regulating cell differentiation. The studies presented here focused specifically on the timed and spatial expression of a bone-associated adhesion molecule, bone sialoprotein, during tooth root development. Mandibular tissues in the first molar region of CD-1 mice, at sequential stages of development, were analysed by in situ hybridization. The results demonstrate distinct expression of bone sialoprotein in surrounding bone at early stages of tooth development. At stages of active cementogenesis, bone sialoprotein transcripts were specific to cells lining the root surface, with limited expression in the surrounding connective tissue (periodontal ligament) region. The strong expression of bone sialoprotein, a mineral-specific protein having the capacity to act as a nucleator of hydroxyapatite in vitro, by cells lining the root surface at early stages of cementogenesis suggests that this molecule is operative in the cell/matrix events that accompany cementum formation.

Nucleation and inhibition of hydroxyapatite formation by mineralized tissue proteins

Many proteins found in mineralized tissues have been proposed to function as regulators of the mineralization process, either as nucleators or inhibitors of hydroxyapatite (HA) formation. We have studied the HA-nucleating and HA-inhibiting properties of proteins from bone [osteocalcin (OC), osteopontin (OPN), osteonectin (ON) and bone sialoprotein (BSP)], dentine [phosphophoryn (DPP)] and calcified cartilage [chondrocalcin (CC)] over a wide range of concentrations. Nucleation of HA was studied with a steady-state agarose gel system at sub-threshold [Ca] x [PO4] product. BSP and DPP exhibited nucleation activity at minimum concentrations of 0.3 microgram/ml (9 nM) and 10 micrograms/ml (67 nM) respectively. OC, OPN, ON and CC all lacked nucleation activity at concentrations up to 100 micrograms/ml. Inhibition of HA formation de novo was studied with calcium phosphate solutions buffered by autotitration. OPN was found to be a potent inhibitor of HA formation [IC50 = 0.32 microgram/ml (0.01 microM)] whereas OC was of lower potency [IC50 = 6.1 micrograms/ml (1.1 microM)]; BSP, ON and CC all lacked inhibitory activity at concentrations up to 10 micrograms/ml. The effect of OPN on HA formation de novo is mainly to inhibit crystal growth, whereas OC delays nucleation. These findings are consistent with the view that BSP and DPP may play roles in the initiation of mineralization in bone and dentine respectively. OPN seems to be the mineralized tissue protein most likely to function in the inhibition of HA formation, possibly by preventing phase separation in tissue fluids of high supersaturation.

Osteopontin at mineralized tissue interfaces in bone, teeth, and osseointegrated implants: ultrastructural distribution and implications for mineralized tissue formation, turnover, and repair

Currently available data describing the gene expression and regulation, secretion, distribution, and protein chemistry of osteopontin (OPN) all are consistent with the notions of this protein functioning as an inhibitor of mineralization and/or as a mediator of cell-matrix and matrix-matrix/mineral adhesion (cohesion) during the formation, turnover, and repair of normal and pathological mineralized tissues. The properties and overall integrity of mineralized tissues are in part dictated by the nature of their interfaces--sites where organic and inorganic components of the extracellular matrix interact to provide biomechanical strength, regulate mineral ion homeostasis, and influence cellular events involved in mineralized tissue modeling, remodeling, and repair. High-resolution, colloidal-gold immunocytochemistry has been used to characterize the proteinaceous composition of these interfaces and to establish that the phosphorylated sialoprotein, OPN, is a major component found at these sites where it accumulates as a dense, planar "coating" of organic material termed either a cement line or a lamina limitans. Structural/functional features of OPN predict an ability of this protein to regulate calcification in the matrix proper of mineralized tissues and to participate, more specifically, in cell-matrix and matrix-matrix/mineral adhesion in laminae limitantes and cement lines, respectively. From the ultrastructural immunocytochemical data presented herein for OPN illustrating the cellular expression and extracellular matrix distribution of this protein, it is demonstrated that the production of OPN is one of the earliest, and latest, secretory activities of the osteoblast lineage and that this activity manifests itself morphologically as a cement line or a lamina limitans, respectively, at bone matrix interfaces. In laminae limitantes at bone surfaces, OPN appears to be involved in osteoclast adhesion and possibly haptotaxis. An OPN-containing cement line is also present at hard tissue interfaces in rat tooth, against osseointegrated titanium and hydroxyapatite implants and at the margins of surgically created bone defects--and there may influence biological adhesion in a manner similar to that proposed for normal bone. It is suggested, therefore, that in addition to its potential for influencing cell adhesion/dynamics in bones and teeth, OPN in cement lines may act as an interfacial adhesion promoter between apposing substrates, therein maintaining the overall integrity of bone during the bone remodeling sequence and "bonding" dissimilar tissues (or biocompatible materials) together in biological composites such as teeth and osseointegrated implants.

Stimulation by bone sialoprotein of calcification in osteoblast-like MC3T3-E1 cells

Bone sialoprotein (BSP) containing an Arg-Gly-Asp cell-binding sequence was purified from bovine bone 4 M guanidine-HCl extract after HCl demineralization by a series of chromatographic procedures. When this protein was coated on culture dishes in the presence of type I collagen, it increased both DNA content and alkaline phosphatase (ALP) activity in osteoblast-like MC3T3-E1 cells, and stimulated calcification in the cells, whereas fibronectin, another cell-binding protein, showed a marked increase in the DNA content but had little effect on the ALP activity. These findings suggest that BSP is mitogenic for preosteoblasts and differentiating the cells into osteoblasts, thereby stimulating bone calcification.

Localization and expression of osteopontin in mineralized and nonmineralized tissues of the periodontium

To summarize results from various studies focusing on determining the expression/localization of BSP and OPN during tooth root development, there is general agreement that OPN is expressed/localized to the root surface during cementogenesis and is also seen throughout the PDL region. The expression/localization of OPN to odontoblasts and its role in dentinogenesis is less apparent. Recent studies directed at establishing odontoblast cell lines should help to resolve this conflict. Studies on BSP expression during tooth root formation indicate a very precise expression and localization of this molecule during cementogenesis indicating that this molecule may play an important role in the formation of this mineralized tissue. However, as with OPN, the expression of BSP and its role in dentin formation is not clearly defined.

Further characterization of interaction between bone sialoprotein (BSP) and collagen

Bone sialoprotein (BSP) has an affinity to collagen fibrils [25]. A role of carbohydrate chains in the affinity was examined by removing sialic acids of BSP. Neuraminidase treatment of the BSP increased the binding to collagen. Binding sites of BSP on collagen were examined by biochemical and electron-microscopic methods. Purified bovine BSP was labeled with biotin. Collagen alpha chains or CNBr peptides were separated by electrophoresis and transfered to nitrocellulose membranes. The membranes were incubated with the biotin-labeled BSP, and the bound BSP was visualized with avidin conjugated with alkaline phosphatase. The labeled BSP was preferentially bound to the alpha 2 chain, and peptides derived from alpha 2 chain. In another experiment, the labeled BSP was incubated with reconstituted native collagen fibrils. The mixture was put on a copper grid, reacted with avidin conjugated with gold particles, and observed with an electron microscope. The gold particles were seen mainly within hole zones of the fibrils. BSP bound to the alpha 2 chain within the hole zones may regulate the onset of calcification at hole zones and the cell binding to collagen fibrils.

Role of two mineral-associated adhesion molecules, osteopontin and bone sialoprotein, during cementogenesis

Adhesion molecules and their cell membrane receptors are known to play important regulatory roles in cell differentiation. Consequently, the following experiments were conducted to determine the role of two adhesion molecules, bone sialoprotein (BSP) and osteopontin (OPN) in tooth root formation. Developing murine molar tooth germs at sequential stages of development (developmental days 21-42) were analyzed using immunohistochemical and in situ hybridization techniques. While BSP was localized to alveolar bone and odontoblasts early in development, BSP was distinctly localized to the cemental root surface at latter periods coincident with the initiation of root formation and cementogenesis. Conversely, OPN was distributed in a nonspecific fashion throughout the PDL and the eruption pathway of the forming tooth. In situ hybridization confirmed that cells lining the root surface express BSP. The fact that BSP is specifically localized to the cemental surface suggests that this protein is involved in cementoblast differentiation and/or early mineralization of the cementum matrix. Localization of OPN to non-mineralized tissues further suggests that OPN functions as an inhibitor of mineralization during periodontal ligament formation. These findings collectively suggest that BSP and OPN are intimately involved in the sequence of cellular and molecular events accompanying cementogenesis.

1 alpha,25-dihydroxyvitamin D3 stimulation of osteopontin expression in rat clonal dental pulp cells

Osteopontin (OPN) is a major phosphorylated non-collagenous protein isolated from bone. Rat clonal dental-pulp cell lines RPC-C2A and RDP4-1 produce and secrete OPN as a principal phosphoprotein. 1 alpha,25-dihydroxyvitamin D3 [1,25(OH)2D3] is a potent calcitropic hormone which regulates calcified tissue metabolism including the synthesis of extracellular matrix proteins. The effects of 1,25(OH)2D3 on the expression of OPN mRNA and the synthesis of OPN protein by pulp cells in vitro were investigated. In RPC-C2A cells, 1,25(OH)2D3 markedly stimulated synthesis of both [32PO4]- and [35S]-methionine-labelled OPN. Phosphorylated OPN synthesis increased dose-dependently and showed a maximum level at 48 h after addition of 10(-11)-10(-7) M 1,25(OH)2D3. Similar stimulation was also observed in RDP4-1 cells. Northern hybridization analysis revealed that 1,25(OH)2D3 greatly increased the level of OPN mRNA in both pulp cell lines. Examination of the time course of the effects of 1,25(OH)2D3 on the level of OPN mRNA in RPC-C2A cells by dot-blot analysis showed that stimulation was detectable at 24 h and reached a maximum at 48 h after exposure to 10(-7)M 1,25(OH)2D3. These findings indicate that 1,25(OH)2D3 stimulates the production of dental-pulp OPN by a mechanism that involves de novo synthesis and transcriptional control.