Human bone sialoprotein I and II enhance fibroblast attachment in vitro

Osteoclasts secrete osteopontin into resorption lacunae during bone resorption

Osteopontin (OPN) is a non-collagenous extracellular sialylated glycoprotein located in bone. It is believed to be one of the key components in osteoclast attachment to bone during resorption. In this study, we characterized OPN and other glycoproteins found in the resorption lacunae to confirm the role of osteoclasts in OPN secretion using electron microscopy and mass spectrometry. Additionally, we examined the glycan epitopes of resorption pits and the effects of different glycan epitopes on the differentiation and function of osteoclasts. Osteoarthritic femoral heads were examined by immunohistochemistry to reveal the presence of OPN in areas of increased bone metabolism in vivo. Our results demonstrate that human osteoclasts secrete OPN into resorption lacunae on native human bone and on carbonated hydroxyapatite devoid of natural OPN. OPN is associated with an elevated bone turnover in osteoarthritic bone under experimental conditions. Our data further confirm that osteoclasts secrete OPN into the resorption pit where it may function as a chemokine for subsequent bone formation. We show that α2,3- and α2,6-linked sialic acids have a role in the process of osteoclast differentiation. OPN is one of the proteins that has both of the above sialic residues, hence we propose that de-sialylation can effect osteoclast differentiation in bone.

Effect of bone sialoprotein coated three-dimensional printed calcium phosphate scaffolds on primary human osteoblasts

The combination of the two techniques of rapid prototyping 3D-plotting and bioactive surface functionalization is presented, with emphasis on the in vitro effect of Bone Sialoprotein (BSP) on primary human osteoblasts (hOBs). Our primary objective was to demonstrate the BSP influence on the expression of distinctive osteoblast markers in hOBs. Secondary objectives included examinations of the scaffolds' surface and the stability of BSP-coating as well as investigations of cell viability and proliferation. 3D-plotted calcium phosphate cement (CPC) scaffolds were coated with BSP via physisorption. hOBs were seeded on the coated scaffolds, followed by cell viability measurements, gene expression analysis and visualization. Physisorption is an effective method for BSP-coating. Coating with higher BSP concentrations leads to enhanced BSP release. Two BSP concentrations (50 and 200 μg/mL) were examined in this study. The lower BSP concentration (50 µg/mL) decreased ALP and SPARC expression, whereas the higher BSP concentration (200 μg/mL) did not change gene marker expression. Enhanced cell viability was observed on BSP-coated scaffolds on day 3. hOBs developed a polygonal shape and connected in an intercellular network under BSP influence. Quantitative cell morphology analyses demonstrated for BSP-coated CPCs an enhanced cell area and reduced circularity. The strength of the above-mentioned effects of BSP-coated scaffolds in vivo is unknown, and future work is focusing on bone ingrowth and vascularization in vivo. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 2565-2575, 2018.

Reengineered graft copolymers as a potential alternative for the bone tissue engineering application by inducing osteogenic markers expression and biocompatibility

Composite scaffolds of nano-hydroxyapatite with demineralized bone matrix were prepared and they were graft copolymerized for better bone regeneration and drug delivery applications. The graft copolymers were characterized for their physiochemical properties using conventional methods like FTIR, TGA, XRD and SEM. The scaffolds were seeded with 3T3 and MG63 cells for studying their biocompatibility and their temporal expression of ALP activity, the rate of calcium deposition and their gene expression of collagen type I (Coll-1), osteopontin (OP), osteonectin (ON), and osteocalcin (OC) were studied. In vivo studies were conducted using sub-cutaneous implantation models in male Wister rats for 6 months. Periodic radiography and post-autopsy histopathology was analysed at 15days, 1, 2, 3, 4, 5, and 6 months. The obtained in vitro results clearly confirm that the bone scaffolds prepared in this study are biocompatible, superior osteoinductivity, capable of supporting growth, maturation of MG 63 osteoblast like cells; the gene expression profile revealed that the material is capable of supporting the in vitro growth and maturation of osteoblast-like cells and maturation. The in vivo results stand a testimony to the in vitro results in proving the biocompatibility and osteoinductivity of the materials.

Wasf2: a novel target of intermittent parathyroid hormone administration

Systemic intermittent administration of parathyroid hormone (PTH) stimulates bone formation in animals and humans, and recombinant human PTH1-34 (teriparatide) is used clinically for the treatment of osteoporosis. In this study, we investigated the regulation of gene expression by intermittent PTH administration in MC3T3-E1 osteoblastic cells. We found that intermittent PTH1-34 administration downregulated Wiskott-Aldrich syndrome protein family member (Wasf) 2 mRNA expression. Wnt inhibitor, IWP-2, and protein kinase C inhibitor, Go6976, inhibited this downregulation. However, continuous PTH did not affect Wasf2 expression. Transfection of Wasf2 siRNA reduced bone sialoprotein (BSP) mRNA expression in a similar manner following intermittent PTH administration in MC3T3-E1 cells. These results identify Wasf2 as a novel target of intermittent PTH administration via the Wnt and phosphoinositide-dependent protein kinase signaling pathways, and the resulting regulation of BSP expression may contribute to the anabolic effects of PTH.

Scleral reinforcement through host tissue integration with biomimetic enzymatically degradable semi-interpenetrating polymer network

Enzymatically degradable semi-interpenetrating polymer networks (edsIPNs) were explored for their biocompatibility and ability to promote new scleral tissue growth, as a means of reinforcing the posterior wall of the eye. The edsIPNs comprised thermoresponsive poly(N-isopropylacrylamide-co-acrylic acid), customizable peptide crosslinkers cleavable by matrix metalloproteinases, and interpenetrating linear poly(acrylic acid)-graft-peptide chains to engage with cell surface receptors. Rheological studies revealed an increase in stiffness at body temperature; the complex shear modulus |G*| was 14.13 +/- 6.13 Pa at 22 degrees C and 63.18 +/- 12.24 Pa at 37 degrees C, compatible with injection at room temperature. Primary chick scleral fibroblasts and chondrocytes cultured on edsIPN increased by 15.1- and 11.1-fold, respectively, over 11 days; both exhibited delayed onset of exponential growth compared with the cells plated on tissue culture polystyrene. The edsIPN was delivered by retrobulbar injection (100 microL) to nine 2-week-old chicks to assess biocompatibility in vivo. Ocular axial dimensions were assessed using A-scan ultrasonography over 28 days, after which eyes were processed for histological analysis. Although edsIPN injections did not affect the rate of ocular elongation, the outer fibrous sclera showed significant thickening. The demonstration that injectable biomimetic edsIPNs stimulate scleral fibrous tissue growth represents proof-of-principle for a novel approach for scleral reinforcement and a potential therapy for high myopia.

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.

Gene expression of MC3T3-E1 cells on fibronectin-immobilized titanium using tresyl chloride activation technique

Fibronectin (FN) can be immobilized directly on titanium surfaces using tresyl chloride activation technique. The key advantage of tresyl chloride activation technique lies in its simplicity. In this study, we examined the cell attachment and gene expression of MC3T3-E1 cells on FN-immobilized titanium using GeneChip. Cells attached on FN-immobilized titanium at a higher rate than untreated titanium. FN altered the gene expression profile, whereby 62 genes were found to be up-regulated, while 56 genes were found to down-regulate to over twice the level on day 14. FN not only enhanced the expression levels of IBSP and OMD, but also decreased SULF1 mRNA level. Taken together, the immobilization of FN on tresylated titanium promoted early matrix mineralization and bone formation.

Root cementum may modulate gene expression during periodontal regeneration: a preliminary study in humans

BACKGROUND

Previous data demonstrated that root cementum may affect periodontal regeneration. As such, this study aimed to explore further possible mechanisms involved in this process by investigating in humans whether root cementum modulates gene expression in the regenerating tissue formed under membrane-protected intrabony defects.

METHODS

Thirty subjects with deep intrabony defects (> or =5 mm; 2- or 3-wall) were selected and assigned to the control or test group. The control group received scaling and root planing with the removal of granulation tissue and root cementum; the test group underwent removal of granulation tissue and soft microbial deposits by cleaning the root surface with a microbrush and saline solution, aiming at cementum preservation. Guided tissue regeneration (GTR) was applied to both groups. Twenty-one days later, the newly formed tissue under the membrane was assessed for the expression of the following genes: alkaline phosphatase (ALP), osteopontin (OPN), osteocalcin (OCN), platelet-derived growth factor-alpha (PDGFA), bone sialoprotein (BSP), and basic fibroblast growth factor (bFGF).

RESULTS

Data analysis demonstrated that mRNA levels for PDGFA, BSP, and bFGF were higher in the sites where root cementum was kept in place compared to the sites where root cementum was removed completely as part of the periodontal therapy (P <0.05); in contrast, OCN levels were lower (P <0.05). No difference for ALP or OPN was observed between the control and test groups (P >0.05).

CONCLUSION

Root cementum may modulate the expression of growth and mineral-associated factors during periodontal regeneration.

Immunohistochemical Study of Bone Sialoprotein and Osteopontin in Healthy and Diseased Root Surfaces

BACKGROUND

Periodontal disease is marked by inflammation and damage to tooth-supporting tissues. In particular, damage occurs to factors present in cementum that are thought to have the ability to influence the regeneration of surrounding tissues. Bone sialoprotein and osteopontin are major non-collagenous proteins in mineralized connective tissues associated with precementoblast chemo-attraction, adhesion to the root surface, and cell differentiation. The purpose of this investigation was to determine whether the expression and distribution of bone sialoprotein and osteopontin on root surfaces affected by periodontitis are altered compared to healthy, non-diseased root surfaces.

METHODS

Thirty healthy and 30 periodontitis-affected teeth were collected. Following fixation and demineralization, specimens were embedded in paraffin, sectioned, and exposed to antibodies against bone sialoprotein and osteopontin. Stained sections were assessed using light microscopy.

RESULTS

Bone sialoprotein was not detected in the exposed cementum (absence of overlying periodontal ligament) of diseased teeth. In most areas where the periodontal ligament was intact, bone sialoprotein was detected for healthy and diseased teeth. For teeth reactive for bone sialoprotein, the matrix of the cementum just below the periodontal ligament was moderately stained. A similar immunoreactivity pattern for osteopontin was observed.

CONCLUSIONS

The absence of bone sialoprotein and osteopontin staining along exposed cementum surfaces may be due to structural and compositional changes in matrix components associated with periodontal disease. This may influence the ability for regeneration and new connective tissue attachment onto previously diseased root surfaces.

Autologous transplantation of gingival fibroblast-like cells and a hydroxylapatite complex graft in the treatment of periodontal osseous defects: cell cultivation and long-term report of cases

Autogenous cell transplantation via hydroxylapatite (HA) vehicle has been reported to have beneficial effects on the treatment of human periodontal osseous defects. The aim of this study was to explore the possibility of using gingival fibroblast-like cells in the therapy of osseous defects caused by inflammatory periodontitis by reporting long-term results of gingival fibroblast-coated hydroxylapatite (GF-HA) grafting for healing these defects. Gingival fibroblasts were cultured from healthy gingivae of treated subjects. Growth of cells on HA particles was established in vitro, and then the GF-HA complex was transplanted into the periodontal osseous defects. Clinical parameters of gingival and plaque indices, probing depth, and periapical x-ray were monitored at baseline and at various periods from 50 months to 6 years after surgery. Grafting with only HA in the osseous defects of the same patient was used for comparison. The present study shows that GF-HA-treated sites could achieve marked pocket reduction and probing attachment gain at reentry and later recalls. Good clinical bone filling of osseous defects in GF-HA-treated sites was also demonstrated in periapical radiographs (increased bone height and reappearance of the crestal cortex) and in some reentry sites. One HA-treated site was filled with connective tissue only, and the absence of new bone formation was noted during a reentry operation. Another HA-treated site exhibited a comparable increase in radiographic density, while part of HA particles were gradually lost in longer recalls. These limited observations conclude that GF-HA grafting may provide a treatment modality leading to regeneration of periodontal tissues in periodontitis-affected osseous defects. Further studies including more cases and demonstration of the deposition of differentiated periodontal tissues are necessary before further application of this therapy.

Osteoblast-like cell adhesion to bone sialoprotein peptides

A number of studies have suggested that biomimetic peptides can be used in the design of a new generation of prosthetic implants to promote the successful biointegration of the implant materials. In the current study, the in vitro bioactivities of several peptides representing RGD (Arg-Gly-Asp)-containing sequences of bone sialoprotein (BSP) toward an osteoblast-like cell line (MC3T3-E1) were examined to provide insight into the molecular basis of BSP's interaction with bone cells. BSP residues 283-288, 281-290, 278-293 and 278-302 were coated on polystyrene surfaces in 96-well non-tissue (untreated) culture plates, and their osteoblast adhesive properties compared to intact BSP and fibronectin as positive controls. BSP peptides 278-302 and 278-293 were found to be the most potent in their adhesive activity, increasing the number of adherent cells to 350% of control levels at an added concentration of 1 microM. Since these two peptides were equivalent in potency, it is suggested that the region 294-302 beyond the RGD domain is not necessary for cell binding. In comparison, peptides 283-288 and 281-290 were only active at concentrations greater than 200 microM. 50-70% of the peptide-stimulated adhesion was inhibited by the pretreatment of cell suspensions with solution phase RGD, suggesting that a portion of the peptides' adhesive effects was specific and integrin-mediated, although other non-RGD flanking regions were probably also involved in the mechanism of adhesion. Importantly, a modified BSP peptide, in which an aspartic acid residue at position 288 of the RGD sequence was replaced by a glutamic acid residue to form RGE, was completely inactive as a cell adhesion stimulus at concentrations up to 200 microM. Thus, despite the potential role of non-RGD flanking regions, an intact RGD tripeptide was essential for all of the adhesive activity of the BSP peptides.

Enhancement of osteogenesis on hydroxyapatite surface coated with synthetic peptide (EEEEEEEPRGDT) in vitro

Some dental implants are coated with hydroxyapatite (HA), which preferentially binds to bone. Several matrix proteins have an arginine-glycine-aspartic acid (RGD) sequence where cells attach via an integrin receptor. We hypothesized that coating an HA surface with an RGD-containing peptide might enhance the attachment and differentiation of osteoblasts. The HA disks (diameter 34 mm, thickness 1 mm) were treated with a solution (50 mM Tris/HCl and 150 mM NaCl, pH 7.4) containing the peptide EEEEEEEPRGDT, in which the E repetition exerts a high affinity to HA. After washing with phosphate-buffered saline, KUSA/A1 mouse osteoblastic cells were inoculated onto the HA surface and cultured. After 30 min, the number of cells attached to the surface was counted. The DNA content and alkaline phosphatase (ALP) activity were measured after 10 days in culture. Expression of bone matrix proteins was also examined by means of reverse transcriptase-polymerase chain reaction at 7 days; the mineralized area of the culture was also evaluated by staining with Alizarin Red S after 10 days. Treatment with the peptide stimulated cell attachment and increased DNA content and ALP activity. Furthermore, matrix protein expression and mineralized nodule formation were enhanced to a greater extent on the peptide-treated surface than on the nontreated surface. Our results indicate that coating an HA surface with RGD-containing peptide enhances osteoblast attachment and differentiation. This peptide treatment of HA-coated implants may stimulate the osseointegration of the implants.

Quantitative analyses of osteopontin mRNA expression in human proximal tubules isolated from renal biopsy tissue sections of minimal change nephrotic syndrome and IgA glomerulonephropathy patients

Osteopontin (OPN), a secreted phosphoprotein and chemotactic to monocytes/macrophages, is upregulated in renal cortical tubules in a variety of rodent models of renal injury and is believed to possibly have a role in tubulointerstitial injury. We previously reported the establishment of a system for the quantification of messenger RNA (mRNA) expression in isolated rat glomeruli using laser-manipulated microdissection and real-time polymerase chain reaction. This system was applied to human renal biopsy specimens. We quantified OPN mRNA expression in proximal tubules of 5 patients with minimal change nephrotic syndrome (MCNS) and 11 patients with mild immunoglobulin A (IgA) glomerulonephritis. We also examined the correlation between OPN mRNA expression in proximal tubules and clinical data and pathological findings in glomeruli and tubulointerstitial regions. Patients with MCNS showed a positive correlation between OPN mRNA expression in proximal tubules and urinary protein excretion (r = 0.93; P < 0.05), whereas for patients with IgA glomerulonephritis, logarithmic values of OPN mRNA expression in proximal tubules positively correlated with low urinary protein levels (r = 0.72; P < 0.05). Pathological changes, ranging from nonexistent to minor, in glomeruli and tubulointerstitium of these patients with mild IgA glomerulonephritis did not significantly correlate with OPN mRNA expression in proximal tubules. In patients with mild IgA glomerulonephritis, OPN mRNA expression in proximal tubules increased exponentially in response to a small amount of urinary protein (<1.2 g/d).

The effect of peptide surface density on mineralization of a matrix deposited by osteogenic cells

The density of Arg-Gly-Asp-containing peptides covalently grafted to solid materials has been shown to affect adhesion, spreading, and focal contact formation. The objective of this study was to examine the effect of ligand density on mineralization of the extracellular matrix deposited by osteoblasts. In particular, RGD-modified quartz surfaces with ligand densities varying over two orders (0.01-3.6 pmol/cm(2)) of magnitude were prepared to assess the long-term function of osteoblasts on peptide-derivatized surfaces. After 3 weeks in culture, surfaces modified with a 15 amino acid peptide (Ac-Cys-Gly-Gly-Asn-Gly-Glu-Pro-Arg-Gly-Asp-Thr-Tyr-Arg-Ala-Tyr-NH(2) ) at a density > or =0.62 pmol/cm(2) significantly (p<0.05) enhanced mineralization compared with a RGD surface density of 0.01 pmol/cm(2), RGE surfaces, or clean surfaces adsorbed with serum proteins. These results suggest that regulation of the surface density of adhesive ligands on biomaterial surfaces is a critical determinant in a strategy to alter the degree of extracellular matrix maturation in contact with solid surfaces (e.g., implants). Further studies are required to elucidate the intracellular signal transduction pathways that mediate long-term matrix mineralization through the initial engagement of these adhesive ligands.

Functional analysis of bone sialoprotein: identification of the hydroxyapatite-nucleating and cell-binding domains by recombinant peptide expression and site-directed mutagenesis

Mammalian bone sialoprotein (BSP) is a mineralized tissue-specific protein containing an RGD (arginine-glycine-aspartic acid) cell-attachment sequence and two distinct glutamic acid (glu)-rich regions, with each containing one contiguous glu sequence. These regions have been proposed to contribute to the attachment of bone cells to the extracellular matrix and to the nucleation of hydroxyapatite (HA), respectively. To further delineate the domains responsible for these activities, porcine BSP cDNA was used to construct expression vectors coding for two partial-length recombinant BSP peptides: P2S (residues 42-87), containing the first glutamic acid-rich domain; and P1L (residues 69-300), containing the second glutamic acid-rich region and the RGD sequence. These peptides were expressed in Escherichia coli as his-tag fusion proteins and purified by nickel affinity columns and FPLC chromatography. Digestion with trypsin released the his-tag fusion peptide, which generated P2S-TY (residues 42-87) and P1L-TY (residues 132-239). Using a steady-state agarose gel system, P2S-TY promoted HA nucleation, whereas P2S, P1L, and P1L-TY did not. This implies that the minimum requirement for nucleation of HA resides within the amino acid sequence of the first glutamic acid-rich domain, whereas the second glutamic acid-rich domain may require posttranslational modifications for activity. P1L, but not P2S, promoted RGD-mediated attachment of human gingival fibroblasts in a manner similar to that of native BSP. Deletion of the RGD domain or conversion of it to RGE (arginine-glycine-glutamic acid) abolished the cell-attachment activity of P1L. This suggests that, at least for human gingival fibroblasts, the major cell-attachment activity in the recombinant BSP peptides studied (residues 42-87 and 69-300) requires the RGD sequence located at the C-terminal domain.

Bone morphogenetic proteins secreted by breast cancer cells upregulate bone sialoprotein expression in preosteoblast cells

It is well established that bone metastases comprise bone; however, the exact factors/mechanisms involved remain unknown. We hypothesized that tumor cells secreted factors capable of altering normal bone metabolism. The aims of the present study were to (1) determine the effects of secretory products isolated from HT-39 cells, a human breast cancer cell line, on osteoprogenitor cell (MC3T3-E1 cells) behavior, and (2) identify tumor-derived factor(s) that alters osteoblast activities. Conditioned media (CM) from HT-39 cells were collected following a 24-h serum-free culture. The ability of CM to alter gene expression in MC3T3-E1 cells was determined by Northern analysis. CM effects on cell proliferation and mineralization ability were determined using a Coulter counter and von Kossa stain, respectively. MC3T3-E1 cells were treated with CM plus noggin, a factor known to block bone morphogenic proteins (BMPs), to determine whether BMPs, shown to be present in CM, were linked with CM effects on MC3T3-E1 cell activity. In addition, inhibitors of MAP kinase kinase (MEK), protein kinase C (PKC), and protein kinase A were used to identify the intracellular signaling pathway(s) by which the active factors in CM regulated osteoblast behavior. CM treatment significantly enhanced BSP mRNA (2.5-fold over control), but had no effect on cell proliferation. Mineralization assay showed that CM enhanced mineral nodule formation compared to controls. Noggin inhibited CM-induced upregulation of BSP mRNA, suggesting that BMPs were responsible for upregulating BSP gene expression in MC3T3-E1 cells. The PKC inhibitor blocked CM-mediated upregulation of BSP, suggesting involvement of the PKC pathway in regulating BSP expression. BMPs secreted by HT-39 cells may be responsible for enhancing BSP expression in MC3T3-E1 cells. Continued studies targeted at determining the role of BMPs in regulating bone metabolism are important for understanding the pathogenesis of bone diseases.

Expression of bone sialoprotein in mineralized tissues of tooth and bone and in buccal-pouch carcinomas of Syrian golden hamsters

The expression of bone sialoprotein (BSP) is normally restricted to mineralized connective tissues of bones and teeth where it has been associated with mineral crystal formation. However, recent studies have revealed ectopic expression of BSP in various lesions, including oral and extraoral carcinomas, in which it has been associated with the formation of microcrystalline deposits and the metastasis of cancer cells to bone. To develop a model to study the induction of BSP in carcinoma development, BSP expression in squamous-cell carcinomas induced by chemical carcinogen in the hamster cheek-pouch epithelium was investigated. Hamster BSP cDNA was first isolated and characterized, then used to prepare probes for Northern and in situ hybridization. The protein sequence of hamster BSP displayed 86% amino acid identity with a consensus mammalian BSP sequence and retained polyglutamate sequences, the RGD sequence and sites of phosphorylation, glycosylation and sulphation. The tissue-specific expression of hamster BSP mRNA and protein was confirmed by in situ hybridization and immunolocalization in developing tissues. Squamous-cell carcinomas induced in the buccal pouches of 5-week-old male Syrian golden hamsters treated with chemical carcinogen had BSP mRNA and BSP in the proliferating neoplastic epithelium. In contrast, neither BSP mRNA nor the protein could be detected in the stroma within which islands of the transformed tissue had formed. Thus, the hamster cheek pouch is a well-characterized model that can be used to study the induced expression of BSP in association with the development of squamous-cell carcinomas.

Biological effects of cementum and bone extracts on human periodontal fibroblasts

BACKGROUND

Non-collagenous proteins of mineralized tissues play important roles in bone induction during mineralization and in regulating the activity of many types of mesenchymal cells. This study was conducted to determine the effects of acetic acid extracts of bone and cementum on alkaline phosphatase (ALPase) activity and in vitro mineralization of cultured human periodontal fibroblasts (hPF).

METHODS

Alveolar bone and cementum obtained from clinically healthy subjects were extracted by a solution containing 0.5 M acetic acid and enzyme inhibitors. Osteoblastic phenotypes of hPF were assayed by ALPase activity, gene expression of bone marker proteins, and the ability to produce in vitro mineralization in culture media containing 50 microg/ml ascorbic acid, 10 mM sodium beta-glycerophosphate, and 10(-7) M dexamethasone. The effects of cementum and bone extracts on the expression of osteoblastic phenotypes in hPF were also determined.

RESULTS

Many protein components, varying in molecular weight from 10 to 14 to 120 kDa, were detectable in 10% SDS-PAGE of both cementum and alveolar bone extracts. The hPF cells were found to exhibit a moderate ALPase activity when compared with rat osteosarcoma (ROS) 17/2.8 cells under the same experimental conditions. Gene expression for ALPase, osteocalcin bone sialoprotein, osteopontin, and BMP-7 at mRNA message was detected by RT-PCR in hPF and ROS 17/2.8 cells. The confluent hPF and ROS 17/2.8 cells showed evidence of calcium deposition in the extracellular milieu at 30 and 15 to 30 days' cultures, respectively, under a mineralization medium. The hPF appeared to form mineralized foci with morphological characteristics different from the mineralized nodules produced by ROS 17/2.8 cells. The addition of low concentrations (5 microg/ml) of either cementum or bone extract produced an increase in the size and number of mineralization spots, as well as greater ALPase activity in both hPF and ROS 17/2.8 cultures during the observation periods.

CONCLUSIONS

These results suggest that hPF possess certain mineralizing phenotypes, and that acetic acid extracts of bone and cementum contain components capable of stimulating osteogenic differentiation of hPF.

Bone sialoprotein mediates human endothelial cell attachment and migration and promotes angiogenesis

Bone sialoprotein (BSP) is a secreted glycoprotein primarily found in sites of biomineralization. Recently, we demonstrated that BSP is strongly upregulated in osteotropic cancers and particularly those that exhibit microcalcifications. BSP contains an Arg-Gly-Asp (RGD) motif found in other adhesive molecules that interact with cellular integrins. In bone, BSP has been shown to mediate the attachment of osteoblasts and osteoclasts via alpha(v)beta(3) integrin receptors. Ligands for alpha(v)beta(3) integrin are considered to play a central role during angiogenesis. Therefore, we used human umbilical vein endothelial cells (HUVECs) to study the potential role of BSP in angiogenesis. We found that purified eukaryotic recombinant human BSP (rhBSP) is able to promote both adhesion and chemotactic migration of HUVECs in a dose-dependent manner. These interactions involve HUVEC alpha(v)beta(3) integrin receptors and the RGD domain of BSP. Indeed, HUVECs attach to a recombinant BSP fragment containing the RGD domain, whereas this response is not observed with the same fragment in which RGD has been mutated to Lys-Ala-Glu (KAE). A cyclic RGD BSP peptide inhibits both adhesion and migration of HUVECs to rhBSP. Moreover, anti-alpha(v)beta(3) but not anti-alpha(v)beta(5) monoclonal antibodies also prevent BSP-mediated adhesion and migration of HUVECs. We observed that both rhBSP and the RGD BSP recombinant fragment stimulated ongoing angiogenesis on the chorioallantoic chick membrane assay. BSP angiogenic activity was inhibited by anti-alpha(v)beta(3) antibody, and the KAE BSP fragment was inactive. Our findings represent the first report implicating BSP in angiogenesis. BSP could play a critical role in angiogenesis associated with bone formation and with tumor growth and metastatic dissemination.

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.

Designing biomaterials to direct biological responses

We have set forth a design strategy for creating biomimetic materials that direct the formation of tissue surrounding implants or regeneration within porous scaffolds. Our studies have established that heterogeneous mimetic peptide surfaces (MPS) containing both the -RGD- (cell-binding) and-FHRRIKA- (putative heparin-binding) peptides, unique to BSP, in the ratio of 75:25 (MPS II) or 50:50 (MPS III) proved to be more biologically relevant and specific for RCO cell function. The initial response of human osteoblast-like cells to these surfaces was mediated by the collagen (alpha 2 beta 1) and vitronectin receptors (alpha v beta 3), whereas the vitronectin receptor alone dominated longer-term events (> 30 min). MPS II and III surfaces enhanced cell spreading and long-term events such as mineralization of the extracellular matrix compared to homogenous peptide surfaces and controls. Furthermore, extensive mineralization of the ECM deposited by RCOs occurred when the peptide was coupled to an interfacial interpenetrating polymer network (IPN) that resisted protein deposition (i.e., non-specific adsorption) and fouling. Work on thermo-reversible P(NIPAAm-co-AAc) hydrogels demonstrated the ability to create materials that can be delivered to the body in a minimally invasive manner and support tissue regeneration. These hydrogels can be modified to incorporate biofunctional components such as the biomimetic peptides, theoretically enhancing their ability to foster tissue regeneration. These results suggest that biomaterials can be engineered to mimic ECM components of bone (e.g., various organs) by grafting peptides in the appropriate ratios of the cell and heparin-binding domains, and ultimately modulate the expression of the osteoblast cell phenotype. Approaches similar to the one presented in this work can be used to design materials for hybrid artificial organs and other tissues.

Protein extracts of dentin affect proliferation and differentiation of osteoprogenitor cells in vitro

Proteins associated with the mineral phase of dentin are considered to have the potential to alter cell function within the local environment, during development and regeneration of tooth/periodontal tissues. Cells that may be altered include osteoblasts, ameloblasts, periodontal ligament cells, odontoblasts, and cementoblasts. However, specific factors within dentin controlling cell activity have not been elucidated. To investigate further the role of dentin proteins in regulating cell behavior, MC3T3-E1 cells, a mouse osteoprogenitor cell line, were exposed to guanidine/EDTA extracts of dentin (G/E-D) prepared from bovine teeth. Cells, with or without G/E-D (2 to 50 microg/ml), were evaluated for proliferative activity and for mRNA expression of bone-associated genes. Results indicated that G/E-D suppressed cell proliferation and caused striking morphological changes, including the conversion of cuboidal cells into fibroblastic, spindle-shaped cells. Markers of osteoblast differentiation, osteocalcin and bone sialoprotein mRNA were decreased, while osteopontin mRNA was enhanced in cells exposed to G/E-D. Since transforming growth factor beta (TGFbeta1) has been reported to influence cells in a similar fashion, G/E-D were examined for the presence of and concentration of TGFbeta using slot blot analysis and enzyme immunoassay (ELISA), respectively. These analyses demonstrated that G/E-D contained 6.6 ng/mg of TGFbeta1. Next, cells were exposed to G/E-D in conjunction with anti-TGFbeta1,2,3 antibody. When cells were exposed to antibody, G/E-D-mediated changes in morphology and gene expression were blocked. These results suggest that TGFbeta1 and perhaps other factors in dentin can regulate cell behavior and, therefore, can influence development, remodeling, and regeneration of mineralized tissues.

Attachment, morphology, and protein expression of rat marrow stromal cells cultured on charged substrate surfaces

Although surface charge has been shown to affect the adhesion and morphology of a variety of cell types, the interactions of bone marrow stromal cells with charged surfaces still remain unclear. A novel electrical stimulation system was used to investigate the interactions between rat bone marrow stromal cells and charged substrates in this study. A conductive and transparent indium tin oxide (ITO) coating was used as an electret substrate. Rat marrow stromal cells were cultured on positive, negative, and uncharged ITO surfaces. Cell attachment, morphology, alkaline phosphatase activity, and expression of osteopontin and collagen type III were assessed using histochemical staining, immunolabeling, and fluorescence microscopy. Voltages of 0.7, 0.8, 0.9, and 1.0 V applied to the substrates created surface potentials but were insufficient to decompose the media. On positively charged ITO, cell attachment was enhanced in serum-supplemented and serum-free media. Furthermore, decreases in cell spreading, alkaline phosphatase activity, and osteopontin were observed in cells grown on the positively charged ITO. These data indicate that positively charged surfaces enhance cell attachment but suppress cell spreading and differentiation of rat marrow stromal cells.

Effect of bone proteins on human prostate cancer cell lines in vitro

BACKGROUND

Despite the high incidence and serious consequences of skeletal metastasis in prostate cancer patients, the mechanisms involved in establishing secondary lesions in bone are not well-understood. In this study, the role of the mineralized bone matrix in the process of skeletal metastasis was evaluated.

METHODS

Attachment, migration, and proliferation responses of human prostate cancer cells to a crude bone protein extract (CBE) were studied. LNCaP and DU145 cells were utilized in 24-hr attachment assays. Boyden chamber chemotactic assays and cell proliferation assays utilized DU145 cells.

RESULTS

CBE and fibronectin (FN) promoted attachment of DU145 cells, whereas only FN facilitated attachment of LNCaP cells. CBE-mediated adhesion of DU145 cells was reduced by 94% with cycloheximide, by 98% with RGD peptides, and by 94% with an antibody to alphavbeta3. Although DU145 cells migrated toward FN, CBE did not promote migration of DU145 cells. DU145 cells grown in the presence of CBE-containing media demonstrated a significant reduction in cell number by day 4. The antiproliferative effect of CBE was not due to cell toxicity.

CONCLUSIONS

In conclusion, results from this study indicate that mineralized bone proteins promote the attachment of DU145 cells in vitro and suggest that bone proteins may play a key role in vivo during the development of metastatic prostate lesions in bone.

Dentin sialoprotein (DSP) transcripts: developmentally-sustained expression in odontoblasts and transient expression in pre-ameloblasts

Dentin sialoprotein (DSP), a 53 kDa glycoprotein, is believed to be present exclusively in dentin. Using rat and mouse digoxigenin labeled (DIG)-DSP and 35S-DSP riboprobes, and in situ hybridization techniques, we have studied the presence of DSP mRNA at specific developmental stages of dentinogenesis. In mouse and rat molars and incisors, DSP transcripts were localized in young odontoblasts associated with early stages of predentin formation, as well as in mature odontoblasts, cells with cytoplasmic extensions embedded in the forming dentin. No DSP transcripts were detected in dental pulp, enamel organ, ameloblasts, epithelial root sheath, Meckel's cartilage, alveolar bone or tibia. Furthermore, no DSP mRNA was observed in other soft tissues including heart, lung, kidney, intestine, eye, and muscle. In addition to the intense and prolonged expression by odontoblasts, DSP mRNA was transiently expressed by pre-ameloblasts in both developing molars and incisors. These observations are consistent with the results of previous immunohistochemical studies (1). The transient expression of DSP in pre-ameloblasts across from young odontoblasts suggests an involvement of DSP in epithelial-mesenchymal interactions that are crucial to later stages of tooth development.

The detachment strength and morphology of bone cells contacting materials modified with a peptide sequence found within bone sialoprotein

Adhesion, spreading, and focal contact formation of primary bone-derived cells on quartz surfaces grafted with a 15 amino acid peptide that contained a -RGD-(-Arg-Gly-Asp-) sequence unique to bone sialoprotein was investigated. The peptide surfaces were fabricated by using a heterbifunctional crosslinker, sulfosuccinimidyal 4-(N-maleimidomethyl)cyclohexane-1-carboxylate, to link the peptide to amine functionalized quartz surfaces. Contact angle measurements, spectroscopic ellipsometry, and X-ray photoelectron spectroscopy were used to confirm the chemistry and thickness of the overlayers. A radial flow apparatus was used to characterize cell detachment from peptide-grafted surfaces. After 20 min of cell incubation, the strength of cell adhesion was significantly (p < 0.05) higher on the -RGD- compared to -RGE- (control) surfaces. Furthermore, the mean area of cells contacting the -RGD- was significantly (p < 0.05) higher than -RGE- surfaces. Vinculin staining showed formation of small focal contact patches on the periphery of bone cells incubated for 2 h on the -RGD- surfaces; however, few or no focal contacts were formed by cells seeded on the -RGE-grafted surfaces. The methods of peptide immobilization utilized in this study can be applied to implants, biosensors, and diagnostic devices that require specificity in cell adhesion.

Characterization of native and recombinant bone sialoprotein: delineation of the mineral-binding and cell adhesion domains and structural analysis of the RGD domain

Bone sialoprotein is a small, sulfated, and phosphorylated integrin-binding glycoprotein apparently found only in tissues that eventually mineralize. Nondenatured bone sialoprotein (BSP) purified from rat osteosarcoma cell line (UMR 106-01 BSP) culture media is shown to have a hydroxyapatite Kd approximately 2.6 x 10(-9) M, perhaps the strongest affinity for this mineral of any of the matrix proteins. Both native BSP and a 47 kD fragment of UMR-BSP (Fragment 1 approximately 133A- approximately 265Y) are more potent inhibitors of seeded hydroxyapatite crystal growth than recombinant human BSP fragments lacking post-translational modifications. The recombinant proteins, however, do show reproducible inhibitory activity, suggesting that at least some of the strong mineral-binding properties are encoded directly within the protein sequence itself. BSP facilitates the adhesion of several cell types through its integrin binding (RGD) tripeptide sequence. Nuclear magnetic resonance (NMR) analysis of a 15N-enriched 59 amino acid recombinant domain containing the RGD tripeptide shows that the structure of this isolated domain is highly flexible with or without 5 mM calcium. Previous work has also shown that an endogenous fragment of UMR-BSP (Fragment 1) supports cell adhesion in the absence of the RGD sequence. In this report, non-RGD cell adhesion sites are localized within conserved amino- and carboxy-terminal tyrosine-rich domains of recombinant human BSP. Given the proximity of the latter non-RGD cell adhesion site to the RGD tripeptide, a model of BSP-receptor interactions is presented.

Stimulation of osteopontin mRNA expression in HL-60 cells is independent of differentiation

12-O-Tetradecanoylphorbol 13-acetate (TPA) induces HL-60 cells to differentiate along the monocyte/macrophage pathway and stimulates expression of the extracellular adhesion protein osteopontin (OPN). In this study, the mechanism of TPA-mediated OPN mRNA expression and its relationship to differentiation were investigated. The induction of OPN mRNA by TPA was dose dependently inhibited by staurosporine (0.4-10.0 nM) and chelerythrine (0.1-5.0 microM), indicating that OPN expression requires PKC activation. Furthermore, the mitogen-activated protein kinase kinase (MAPKK) inhibitor, PD 098059 (1.0-10.0 microM), inhibited the effect of TPA in a dose-dependent fashion. Cycloheximide (10 microg/ml) ablated the induction of OPN mRNA by TPA. To determine if OPN mRNA expression was associated with a particular differentiational pathway, HL-60 cells were treated with RA, 9-cis-RA, calcitriol, or sodium butyrate. None of these agents stimulated OPN mRNA. Treatment with TPA subsequent to a 120-h pretreatment with retinoic acid (RA), 9-cis-RA, or calcitriol resulted in a potentiation of the induction of OPN mRNA. These results support a role for protein kinase C (PKC) in promoting OPN expression because each of these agents increased PKC levels. An hOPN promoter/reporter construct was responsive to TPA, indicating that this effect is at the level of transcription. Thus, TPA-stimulated transcription of the OPN gene apparently occurs via a PKC/MAPK-dependent mechanism that is independent of that associated with differentiation and is not dependent on the maturational state of these cells.

Electrophysiological responses of human bone cells to mechanical stimulation: evidence for specific integrin function in mechanotransduction

Bone cells respond to mechanical stimuli, but the transduction mechanisms responsible are not fully understood. Integrins, a family of heterodimeric transmembrane glycoproteins, which link components of the extracellular matrix with the actin cytoskeleton, have been implicated as mechanoreceptors. We have assessed the roles of integrins in the transduction of cyclical mechanical stimuli to human bone cells (HBCs), which results in changes in membrane potential. HBC showed membrane depolarization following 0.104 Hz mechanical stimulation and membrane hyperpolarization following stimulation at 0.33 Hz. The membrane depolarization response involved tetrodotoxin-sensitive sodium channels and could be inhibited by antibodies against alpha V, beta 1, and beta 5 integrins. In contrast, the hyperpolarization response was inhibited by gadolinium and antibodies to the integrin-associated protein (CD47), alpha 5 and beta 1 integrin. Both responses could be abrogated by ARg-Gly-Asp (RGD)-containing peptides, inhibition of tyrosine kinase activity, and disruption of the cytoskeleton. These results demonstrate differential electrophysiological responses of HBC to different frequencies of mechanical strain. Furthermore, they suggest that integrins act as HBC mechanoreceptors with distinct signaling pathways being activated by different frequencies of mechanical stimuli.

Evidence of ectokinase-mediated phosphorylation of osteopontin and bone sialoprotein by osteoblasts during bone formation in vitro

Osteopontin (OPN) and bone sialoprotein (BSP) are phosphorylated glycoproteins that, together with osteonectin/secreted protein, acidic, rich in cysteine (SPARC) and osteocalcin, comprise the major non-collagen proteins of bone. Although phosphorylation of OPN and BSP, which is known to influence the biological properties of these proteins, has been shown to occur intracellularly, recent studies have demonstrated ectokinase activity in bone cell populations [Mikuni-Takagaki, Kakai, Satoyoshi, Kawano, Suzuki, Kawase and Saito (1995) J. Bone Miner. Res. 10, 231-241]. To determine whether OPN and BSP are phosphorylated by ectokinase activity we have used [gamma-32P]ATP and [gamma-32P]GTP as cell-impenetrable phosphate donors to analyse for ectokinase activity in osteoblastic UMR106.06 cells and fetal rat calvarial cells (FRCCs). By pulse-labelling confluent cells with radiolabelled nucleotides, the phosphorylation of endogenous and exogenously added OPN and BSP was demonstrated together with the labelling of a number of cell surface proteins. These phosphorylation reactions were inhibited by a cell-impermeable ectokinase inhibitor, K252b, and cell surface phosphorylation was also inhibited by exogenously added OPN and BSP substrates, indicating competition for the ectokinase enzyme. However, phosphorylation of OPN and BSP, both of which can mediate cell attachment through Arg-Gly-Asp (RGD) motifs, was not inhibited by an RGD peptide, suggesting that binding of OPN and BSP to cell surface integrins is not required. In similar experiments, ectokinase-mediated phosphorylation of OPN and BSP was demonstrated during mineralized tissue formation by FRCCs in vitro. These studies demonstrate that OPN and BSP secreted by bone cells are phosphorylated by a casein kinase II-like ectokinase present on the surface of osteoblastic cells.

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.

Response of human osteoblasts to implant materials: integrin-mediated adhesion

The initial interaction of the human osteoblast-like cell line Saos-2 with orthopaedic implant materials was analyzed to determine the mechanism by which these cells adhere to implant surfaces. Saos-2 cells were allowed to attach to disks composed of the orthopaedic implant materials Tivanium (Ti6A14V) and Zimaloy (CoCrMo) and to control disks of glass and plastic. Serum had no effect on the number of cells that attached to Tivanium and Zimaloy at 4 or 24 hours but did increase the number of cells that attached to glass at 24 hours. Collagen synthesis was determined by [3H]proline incorporation into collagenase-digestible protein and noncollagen protein. A significant increase of 19% was found for collagen synthesized in cells cultured on Zimaloy for 24 hours compared with glass, with no differences on Tivanium and plastic. However, collagenase-digestible protein and noncollagen protein were increased the most (204 and 198%, respectively) on Tivanium compared with glass. To determine if integrins were involved in cell attachment to implant materials, the peptide GRGDSP (Gly-Arg-Gly-Asp-Ser-Pro), which blocks integrin receptors through the Arg-Gly-Asp sequence, was added to the cells in serum-free medium. This peptide inhibited cell adhesion by 28% on Tivanium and 40% on Zimaloy but had no effect on glass and plastic. The control peptide GRADSP (Gly-Arg-Ala-Asp-Ser-Pro) had no effect on adhesion. Inhibition of protein synthesis and enzymatic removal of surface proteins did not affect the ability of Arg-Gly-Asp peptides to inhibit cell attachment to the implant materials. These results suggest that integrins are able to bind directly to Tivanium and Zimaloy. Western blot analysis of integrin protein demonstrated changes in many integrin subunits, depending on the substrate to which cells attached. In particular, the beta 1 integrin subunit was increased 3.8 to 9.5-fold at 24 hours. To determine specifically which integrins may be involved in adhesion, antibodies to integrins were added. An antibody to the fibronectin receptor, alpha 5 beta 1, significantly inhibited binding of cells to Tivanium by 63% and to Zimaloy by 49% and had no effect on glass. The vitronectin receptor antibody, alpha v beta 3/beta 5, did not alter cell adhesion. In conclusion, osteoblast-like cells appear to be capable of attaching directly to implant materials through integrins. The type of substrate determines which integrins and extracellular matrix proteins are expressed by osteoblasts. These data provide information on how implant materials may affect osteoblast differentiation and bone growth.

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.

Extracellular matrix in tooth cementum and mantle dentin: localization of osteopontin and other noncollagenous proteins, plasma proteins, and glycoconjugates by electron microscopy

BACKGROUND

Noncollagenous proteins (NCPs) are considered to have multiple functions related to the formation, turnover, and repair of the collagen-based mineralized tissues. Collectively, they comprise a class of generally acidic, mineral-binding proteins showing extensive posttranslational modifications, including glycosylation, phosphorylation, and sulfation. METHODS. We have used colloidal-gold immunocytochemistry and lectin-gold cytochemistry, together with transmission electron microscopy, to examine the organic matrix composition of tooth cementum and the subjacent mantle dentin in rodent molar teeth. Molars were processed for immunocytochemistry using antibodies against osteopontin (OPN), osteocalcin (OC), bone sialoprotein (BSP), bone acidic glycoprotein-75 (BAG-75), albumin (ALB), and alpha 2HS-glycoprotein (alpha 2HS-GP), or for glycoconjugate cytochemistry using lectin-gold complexes.

RESULTS

Ultrastructurally, at the advancing root edge in developing molars, OPN and BSP initially were associated with small calcification foci in the mantle dentin. With progressing mineralization, OC and alpha 2HS-GP appeared diffusely distributed throughout the calcified mantle dentin, and diminished as a gradient toward the circumpulpal dentin. Immediately following disruption of Hertwig's epithelial root sheath, cementum deposition commenced at the root surface occasionally with the appearance of a cement line rich in OPN. Cementum matrix proper contained abundant OPN, BSP, OC, and alpha 2HS-GP, but no or little BAG-75 or ALB. Protein immunolabeling, as well as lectin labeling for beta-D-galactose and N-acetyl-neuraminic acid and/or N-glycolyl-neuraminic acid, both being prominent sugars of certain NCPs, was primarily concentrated between, and at the surface of, collagen fibrils in acellular extrinsic fiber cementum. OPN, BSP, OC, and alpha 2HS-GP were also prominent components of cellular cementum and of Sharpey's fibers. In cellular cementum, laminae limitantes sometimes present delimiting cementocyte lacunae and cell process-containing canaliculi were also rich in OPN. Along the root surface, occasional cementoblasts exhibited intracellular labeling for OPN over the Golgi apparatus and secretory granules.

CONCLUSIONS

We have identified OPN, BSP, OC, and alpha 2HS-GP as being prominent organic constituents of both mantle dentin and acellular and cellular cementum, and, have elucidated the details of their distribution at the ultrastructural level. The temporal appearance and spatial distribution of these organic moieties in the teeth root are similar to those seen during bone formation and are consistent with proposals that certain NCPs may be involved in regulating calcification and/or participating in cell-matrix and matrix-matrix/mineral adhesion events.

Characterization and partial purification of microsomal casein kinase II from osteoblast-like cells: an enzyme that phosphorylates osteopontin and phosphophoryn

Microsomal casein kinase II (mCKII) is a membrane-bound enzyme present in the microsomal fractions of ROS 17/2.8 osteoblast-like cells. It phosphorylates acidic matrix phosphoproteins such as phosphophoryn and osteopontin. Addition of 1.0% Nonidet P-40 facilitates extraction of the optimum amount of detergent-solubilized and -activated enzyme from microsomal fractions. mCKII was partially purified over 3000-fold by sequential chromatography over DEAE-cellulose and heparin-agarose. SDS-polyacrylamide gels, showed that mCKII contained 43 kDa and 31 kDa polypeptides, corresponding to the alpha- and beta-subunits of the enzyme, respectively. The alpha subunit was identified by anti-CKII antiserum and the beta subunit, by its ability to undergo autophosphorylation. The enzyme was inhibited by 50% with 0.4 micrograms/ml heparin and stimulated by 100% with 1.0 mM spermine when casein was used as a substrate. The phosphorylation of phosphophoryn was reduced to 50% by 0.8 micrograms/ml heparin, but was increased to 2-2.5 fold by 5 to 15 mM spermine, which may be due to substrate-directed effects. Kinetic analysis showed that the apparent Km values for phosphophoryn (0.39 microM) and for osteopontin (2.1 microM) were lower than that for casein (21.3 microM). Vmax values of phosphophoryn and osteopontin were 2.2-fold and 4.6-fold higher than that of casein. Using the ratio Vmax/Km as a measure of kinetic specificity, osteopontin and phosphophoryn appear to be the more specific substrates than casein for mCKII. Thus, both proteins can be considered as physiological substrates for mCKII.

Bone acidic glycoprotein-75 self-associates to form large macromolecular complexes

Bone acidic glycoprotein-75 (BAG-75) displays a strong propensity to self-associate to form large fibrillar complexes above concentrations of 7 x 10(-8) M; acidic phosphoproteins osteopontin and bone sialoprotein do not form similar complexes. Although the majority of the data supporting this conclusion is derived from in vitro studies, the fact that similar sized complexes are observed in crude extracts of bone and calcified cartilage suggests that macromolecular BAG-75 complexes are also a component of mineralized matrices in vivo. An awareness of the existence of complexes in extracts from bone necessitates that these forms are accounted for in terms of the relative amounts of individual acidic phosphoproteins in bone matrix. We now estimate that the amount of BAG-75 in rat calvarial bone is equivalent to that of osteopontin. While BAG-75 is capable of binding up to 139 atoms of calcium/mole with an average affinity constant of 0.5-1.0 x 10(-3) M, millimolar concentrations of calcium are not required for self-association. Assuming macromolecular diffusion within osteoid is restricted, osteoblastic cells could control the extent of self-association through the rate at which BAG-75 is synthesized and secreted into the osteoid matrix. Based on these findings, we hypothesize that BAG-75 self-associates to form fibrillar complexes in vivo which function in a supportive mechanical role and/or as an electronegative ionic barrier. Electronegative BAG-75 barrier structures could play a role in concentrating phosphate ions within bone matrix, thus facilitating mineralization.

The anatomy of bone sialoprotein immunoreactive sites in bone as revealed by combined ultrastructural histochemistry and immunohistochemistry

Bone sialoprotein was immunolocalized at the EM level in thin Lowicryl K4M sections of rat bone. Because of the unconventional EM morphology of the bone matrix seen in thin demineralized acrylate sections, the pattern of immunolabeling was compared with detailed structural images of demineralized bone obtained using an en bloc treatment of tissue samples with the cationic electron 'dye,' Malachite Green (MG), which provides stabilization and retention of anionic material throughout specimen processing. A system of structures corresponding to the sites of bone sialoprotein (BSP) immunoreactivity, as seen in Lowicryl K4M this sections, could be readily identified in the MG-treated, epoxy thing sections. This system includes the cement lines, and aggregates of similar material within mineralized bone and mineralizing osteoid. The virtual identity of BSP distribution with the arrangement of the MG-visualized material indicates that a BSP-enriched, noncollagenous phase can be demonstrated using different, unrelated tissue preparation and imaging protocols for EM. Besides improving our understanding of the distribution of bone sialoprotein in bone, these data assign a previously unrecognized structural dimension to noncollagenous material in the bone matrix.

Specific cementum attachment protein enhances selectively the attachment and migration of periodontal cells to root surfaces

A specific cementum attachment protein (CAP) was identified in human cementum and found to bind with high affinity to non-demineralized root surfaces, hydroxyapatite and fibronectin. Attempting to elucidate the biological function of this protein and its possible role in cementogenesis the capacity of CAP to promote selective cell migration towards and attachment of various periodontal derived cell populations to root surfaces in vitro was assessed. Human gingival fibroblasts (HGF), periodontal ligament cells (HPC), and alveolar bone cells (HABC) were labeled with [3H]Thymidine during their exponential growth phase. Root slices, 300 microns thick, were incubated with increasing concentrations of CAP. Untreated and fibronectin (FN) treated root slices served as negative and positive controls, respectively. Migration was assessed by placing root slices on confluent layers of labeled cells maintained in serum free medium and determining the number of cells migrated onto the root surface 3 days thereafter. Attachment was assessed by incubating root slices with labeled cell suspensions for 2 h and determining the number of attached cells. CAP promoted both cell migration and attachment dose dependently. HABC responded better than HPC and HGF to CAP treated root slices, and HPC response was higher than that of HGF. Cell attachment was dose dependently inhibited by synthetic RGD peptides. FN did not affect the migration of HGF, barely enhanced that of HABC, and was less potent than CAP at enhancing the migration of HPC. FN was more effective than CAP in promoting the attachment of HGF to root slices, but it was as potent as CAP in supporting the attachment of HPC and HABC.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunohistochemical expression of extracellular matrix components of normal and healing periodontal tissues in the beagle dog

Periodontal regeneration requires formation of periodontal tissues lost due to periodontal disease. To better understand the formation of new periodontal tissues during periodontal repair and regeneration, immunohistochemical expression of extracellular matrix components of normal as well as healing periodontal tissues was evaluated and compared using the avidin-biotin complex immunohistochemical technique. For this purpose, horizontal furcation defects were created around mandibular P2 and P4 of 6 dogs after extraction of P1 and P3. The root surfaces were conditioned with citric acid and expanded polytetrafluoroethylene (ePTFE) membranes were placed and retained 0.5 mm above the cemento-enamel junction. The mucoperiosteal flaps were sutured in a coronal position. Two animals were sacrificed at 2, 4, and 8 weeks, and mesio-distal tissue slices containing normal or healing periodontal tissues were demineralized, dehydrated, and embedded in paraffin. Immunohistochemical localization of type I collagen (CI), fibronectin (FN), secreted protein, acidic and rich in cysteine (SPARC), vitronectin (VN), and bone sialoprotein (BSP) was performed on 6 microns thick sections. Morphological results demonstrated that at 2 weeks after defect creation, lesions were filled primarily with granulation tissue which was gradually replaced by newly-formed fibrous connective tissue, periodontal ligament (PDL), cementum, and bone between 4 and 8 weeks. The results of immunohistochemical study revealed that at 2 weeks the granulation tissue, especially in the intercellular spaces of inflammatory cells, was intensively stained for FN and VN. At 4 and 8 weeks, staining for CI, FN, and VN was found in fibrous connective tissue, the newly-formed PDL, cementum, and osteoid. Further the attachment zone of the PDL collagen fibers to cementum showed intense staining for FN. Immunostaining for SPARC was positive in the new PDL, cementum, and bone, while staining for BSP was restricted to the new cementum and bone. Interestingly, the PDL, especially in areas adjacent to active bone formation, demonstrated intense staining for BSP. However, fibrous connective tissue and PDL proper were unstained for BSP. These results indicate that FN and VN are involved in the early stages of periodontal repair, and periodontal regeneration is achieved through formation of periodontal tissues that are composed of different matrix components specific to different types of periodontal tissues.

Commercially-prepared allograft material has biological activity in vitro

The well-established finding that implantation of demineralized bone matrix at non-skeletal sites results in formation of cartilage and bone has been attributed to bone morphogenetic proteins/factors. Commercially-available demineralized bone allograft materials are being used currently to reconstruct/regenerate bone. The studies described here focused on establishing biological activity of protein extracts prepared from commercially obtained bone graft material in vitro. Furthermore, the biological activity of these protein extracts in vitro was compared with similar extracts prepared from freshly obtained human bone. Biological activities of bone matrix proteins examined included their ability to promote proliferation, attachment, and migration of gingival fibroblasts using an in vitro system. Guanidine followed by guanidine/EDTA was used to separate bone matrix proteins into proteins associated with soft tissues of bone and proteins retained within the mineral compartment, respectively. Two preparations of each starting material were tested and the biological activity of each preparation was evaluated in triplicate at least three times. Slot blot analysis revealed that commercially-prepared material contained type I collagen; fibronectin; BSP; and BMP-2, 4, and 7. However, the freshly prepared bone extracts appeared to have higher BMP concentrations. The ability of commercial extracts to promote cell proliferation, while significant, was limited and significantly less when compared with similar extracts prepared from freshly obtained bone. All extracts promoted cell attachment significantly, while none of the extracts promoted cell migration. Thus, commercially-prepared material retained proteins having the capacity to influence cell behavior in vivo. However, some biological activity as measured in vitro was lost as a result of tissue processing.

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.

Osteopontin and the bone remodeling sequence. Colloidal-gold immunocytochemistry of an interfacial extracellular matrix protein

Relative to other noncollagenous, extracellular matrix proteins in mineralized tissues, colloidal-gold immunocytochemistry has demonstrated that the ultrastructural distribution of osteopontin (OPN) is unique in that this protein preferentially accumulates at mineralized tissue interfaces. In bone, this protein is present as a major component of cell-matrix and matrix-matrix interfacial structures called cement lines and laminae limitantes. In the present article, the implications of this distinct tissue distribution are discussed in terms of the bone remodeling sequence, and a detailed account of the secretion, accumulation and potential role of OPN is presented and related to current theory on the cellular and extracellular matrix events associated with basic multicellular unit (BMU)-based bone remodeling. In this context, a proposal is made describing the production of this protein as one of the earliest, and latest, secretory activities of the osteoblastic lineage, and that this activity manifests itself morphologically as a cement line ('plane') and a lamina limitans, respectively, at bone matrix interfaces. When integrated with other, known functional characteristics of this protein, the present morphological and compositional data indicate that OPN in cement lines and laminae limitantes may participate in initial and late extracellular matrix organization and mineralization, matrix-matrix/mineral adhesion and/or cell adhesion at bone interfaces.

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.

Osteopontin adhesion receptors on gingival fibroblasts

Osteopontin (OPN) promotes attachment and spreading of cells in an RGD dependent fashion, suggesting that OPN interacts with integrins on cell surfaces. Here in, we show that LM-609, a monoclonal antibody to the alpha v beta 3 integrin (a vitronectin receptor), inhibited OPN-mediated attachment of gingival fibroblasts. To characterize the cell surface receptors responsible for this interaction, we performed OPN-sepharose affinity chromatography using detergent extracts of 35S-methionine or 125I-surface labeled gingival fibroblasts. Proteins bound to the OPN-matrix were eluted with EDTA and subjected to SDS-PAGE under reducing conditions. EDTA eluates from both 125I-surface labeled and 35S-methionine labeled extracts demonstrated prominent bands in the 90kDa and 50kDa regions, by both autoradiography and fluorography, respectively. These studies suggest that OPN is associated with other cell surface molecules in addition to alpha v beta 3. Furthermore, these as yet to be characterized proteins, may prove to have a stronger affinity for OPN than alpha v beta 3.

Models for the study of cementogenesis

Cementum is a mineralized tissue that acts to connect the periodontal ligament to the tooth root surface. Its composition is very much like bone, being comprised mainly of type I collagen, inorganic mineral and noncollagenous proteins, however the origin of the cells and factors necessary for cementum formation have yet to be elucidated. Our laboratory has focused on the role that adhesion molecules, and their cell surface receptors, play in the formation of cementum and tooth root. In order to study this, we used a mouse molar as a model system. This system enabled us to study the formation of four distinct mineralized tissues; bone, cementum, dentin and enamel at various stages of their development. For these studies, we initiated experiments to examine potential cementoblast progenitor cells, in vitro. As a first step, we show that dental papilla and dental follicle cells, n vitro, obtained from molar tissues at day 21 of development, induce mineralized nodules, in vitro. In addition, we obtained tissues from mice where defects in root development may exist and determined bone sialoprotein (BSP) protein expression, a mineralized tissue specific adhesion molecule, in such tissues. As discussed here, we found that osteopetrotic (op/op) mice have delayed and/or defective root development and BSP does not localize in the dental tissues, at day 33 of development. In addition, dentin formation was defective and odontoblasts appeared immature, based on morphological examination. In contrast, the day 33 control molars demonstrated positive staining for BSP localized to root cementum, with normal formation of dentin.

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.