Keratan sulfate proteoglycan in rabbit compact bone is bone sialoprotein II

Keratan sulfate, an electrosensory neurosentient bioresponsive cell instructive glycosaminoglycan

The roles of keratan sulfate (KS) as a proton detection glycosaminoglycan in neurosensory processes in the central and peripheral nervous systems is reviewed. The functional properties of the KS-proteoglycans aggrecan, phosphacan, podocalyxcin as components of perineuronal nets in neurosensory processes in neuronal plasticity, cognitive learning and memory are also discussed. KS-glycoconjugate neurosensory gels used in electrolocation in elasmobranch fish species and KS substituted mucin like conjugates in some tissue contexts in mammals need to be considered in sensory signalling. Parallels are drawn between KS's roles in elasmobranch fish neurosensory processes and its roles in mammalian electro mechanical transduction of acoustic liquid displacement signals in the cochlea by the tectorial membrane and stereocilia of sensory inner and outer hair cells into neural signals for sound interpretation. The sophisticated structural and functional proteins which maintain the unique high precision physical properties of stereocilia in the detection, transmittance and interpretation of acoustic signals in the hearing process are important. The maintenance of the material properties of stereocilia are essential in sound transmission processes. Specific, emerging roles for low sulfation KS in sensory bioregulation are contrasted with the properties of high charge density KS isoforms. Some speculations are made on how the molecular and electrical properties of KS may be of potential application in futuristic nanoelectronic, memristor technology in advanced ultrafast computing devices with low energy requirements in nanomachines, nanobots or molecular switches which could be potentially useful in artificial synapse development. Application of KS in such innovative areas in bioregulation are eagerly awaited.

Molecular and functional analysis of PmCHST1b in nacre formation of Pinctada fucata martensii

Keratan sulfate possesses considerable amounts of negatively charged sulfonic acid groups and participates in biomineralization. In the present study, we investigated characteristics and functions of a CHST1 gene identified from the pearl oyster Pinctada fucata martensii (PmCHST1b) which participated in the synthesis of keratan sulfate. PmCHST1b amino acid sequence carried a typical sulfotransferase-3 domain (sulfotransfer-3 domain) and belonged to membrane-associated sulfotransferases. Homologous analysis of CHST1 from different species showed the conserved motif (5' PSB motif and 3' PB motif) which interacted with 3'-phosphoadenosine-5'-phosphosulfate (PAPS). Structure analysis of sulfotransferase domain indicted that PmCHST1b showed the conserved catalytic structure character and the relationships presented in the phylogenetic tree conformed to that of traditional taxonomy. Expression pattern of PmCHST1b in different tissues and development stages showed that PmCHST1b widely expressed in all the detected tissues and development stages and showed the highest expression level in the central zone of mantle (MC). PmCHST1b expressed highly in the trochophore, D-stage larvae and spat which corresponded to prodissoconch and dissoconch shell formation, respectively. RNA interference (RNAi) successfully inhibited expression level of PmCHST1b in MC (P<0.05), and sulfate polymer content in the extrapallial fluid significantly reduced (P<0.05). Crystallization of shell nacre became irregular. Results above indicated that PmCHST1b may affect nacre formation by participating in synthesis of keratan sulfate in extrapallial fluid. This study provided fundamental materials for further research on the role of sulfotransferases and keratan sulfate in nacre formation.

Keratan sulfate, a complex glycosaminoglycan with unique functional capability

From an evolutionary perspective keratan sulfate (KS) is the newest glycosaminoglycan (GAG) but the least understood. KS is a sophisticated molecule with a diverse structure, and unique functional roles continue to be uncovered for this GAG. The cornea is the richest tissue source of KS in the human body but the central and peripheral nervous systems also contain significant levels of KS and a diverse range of KS-proteoglycans with essential functional roles. KS also displays important cell regulatory properties in epithelial and mesenchymal tissues and in bone and in tumor development of diagnostic and prognostic utility. Corneal KS-I displays variable degrees of sulfation along the KS chain ranging from non-sulfated polylactosamine, mono-sulfated and disulfated disaccharide regions. Skeletal KS-II is almost completely sulfated consisting of disulfated disaccharides interrupted by occasional mono-sulfated N-acetyllactosamine residues. KS-III also contains highly sulfated KS disaccharides but differs from KS-I and KS-II through 2-O-mannose linkage to serine or threonine core protein residues on proteoglycans such as phosphacan and abakan in brain tissue. Historically, the major emphasis on the biology of KS has focused on its sulfated regions for good reason. The sulfation motifs on KS convey important molecular recognition information and direct cell behavior through a number of interactive proteins. Emerging evidence also suggest functional roles for the poly-N-acetyllactosamine regions of KS requiring further investigation. Thus further research is warranted to better understand the complexities of KS.

Current Progress in Anticalcif ication for Bioprosthetic and Polymeric Heart Valves

The use of bioprosthetic valves fabricated from fixed heterograft tissue (porcine aortic valves or bovine pericardium) in heart valve replacement surgery is limited because of calcification-related failures. The mechanism of calcification of bioprosthetic valves is quite complex and has a variety of determinants, including host factors, tissue fixation conditions, and mechanical effects. Currently, there is no effective therapy to prevent calcification in clinical settings. This article reviews a variety of anticalcification strategies that are under investigation either in advanced animal models or in clinical trials. Bisphosphonates, such as ethan hydroxybisphosphonate (EHBP), inhibit calcium phosphate crystal formation. However, because of their systemic toxicity, they are used as either tissue treatments or polymeric site-specific delivery systems. Detergent treatment, such as sodium dodecyl sulfate (SDS), extracts almost all phospholipids from bioprosthetic heart valve cuspal tissue. Procedures, such as amino oleic acid pretreatment, inhibit calcium uptake. Polyurethane trileaflet valves, investigated as alternatives to bioprosthetic or mechanical valve prostheses, undergo intrinsic and thrombus-related calcification and degradation. Calcification- and thrombus-resistant polyurethanes synthesized in our laboratory by covalent linking of EHBP or heparin (either in bulk or on surface) by unique polyepoxidation chemistry are attractive candidates for further research. Tissue-engineered heart valves may have an important place in the future.

Comparative assessment of bone among wild-type, restricted ovulator and out-of-production hens

1. The aim of this study was to assess bone characteristics in restricted ovulator (RO) hens. These hens generally are unable to ovulate due to a point mutation in the oocyte VLDL receptor gene whose protein product mediates the uptake of yolk precursors. Because these hens do not have the cyclic calcium (Ca) metabolism associated with egg formation, they could be a useful model for studying bone metabolism. 2. RO hens had greater humerus, femur and tibia ash concentrations than wild-type (WT) and out-of-production (OP) hens. Bone mineral content and density obtained with dual-energy X-ray absorptiometry (DXA) were highly correlated with the results of conventional bone assays. 3. Gross and histological examination of the femurs confirmed the presence of extremely dense medullary bone deposition in the RO hens. However, the composition of non-collagenous protein extracts of medullary bone was similar for the two genotypes. 4. Analysis of medullary bone extracts for glycosaminoglycans (GAG) confirmed the presence of large amounts of keratan sulphate (KS) in the matrix of medullary bone. 5. Plasma Ca, total GAG and KS concentrations of RO hens were markedly higher than WT and OP hens. The changes in plasma calcium and keratan sulphate are probably a reflection of elevated Ca-binding yolk precursor molecules and intensive medullary bone formation in response to increased plasma oestrogen observed by others in RO hens.

Proteoglycans: key partners in bone cell biology

The diversity of bone proteoglycan (PG) structure and localisation (pericellular, extracellular in the organic bone matrix) reflects a broad spectrum of biological functions within a unique tissue. PGs play important roles in organizing the bone extracellular matrix, taking part in the structuring of the tissue itself as active regulators of collagen fibrillogenesis. PGs also display selective patterns of reactivity with several constituents including cytokines and growth factors, such as transforming growth factor-beta or osteoprotegerin thereby modulating their bio-availability and biological activity in the bone tissue. In this review, the complex PG composition in bone will be addressed together with the specific role played by PGs (or their GAGs chains) in bone biology, as regulatory molecules for bone resorption and their involvement in bone tumor development. These roles have been determined after modulation of PG expression or mutations in their corresponding genes, which revealed specific roles for these compounds in bone pathologies (e.g. perlecan or glypican-3 mutations observed respectively in chondrodysplasia or dysmorphic syndrome). Finally, the potential therapeutic interest of PGs is discussed based on recent data, more particularly on bone tumor-associated osteolysis as these molecules are involved both in bone resorption and tumor development.

Substitution of bovine dentine sialoprotein with chondroitin sulfate glycosaminoglycan chains

Dentine sialoprotein (DSP) represents 5-8% of all non-collagenous proteins present in the tooth, but, together with dentine phosphoprotein, has been shown to be vital for correct tooth formation. Recently, the existence of a highly glycosylated form of porcine DSP has been reported and it was shown to possess glycosaminoglycan (GAG) chains. The current investigation confirms that this is also the case for bovine DSP and has further characterized these carbohydrates. Dentine sialoprotein was purified from bovine dentine extracts by anion exchange chromatography and identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), western blotting, and mass spectroscopy. An increase in molecular mass was observed, from 120 kDa to greater than 250 kDa, with a corresponding rise in anionic strength. Cellulose acetate electrophoresis and western blotting indicated the presence of chondroitin sulfate GAG chains within these dentine fractions. Further examination using sequential digestion with chondroitinase AC and N-glycosidase cleaved the samples first to 95 kDa and then to 80 kDa, respectively, confirming a high level of glycosylation. These results support the classification of bovine DSP as a proteoglycan, and that the carbohydrate substitutions may contribute to the functional properties of DSP.

Ultrastructural immunolocalization of alpha-elastin and keratan sulfate proteoglycan in normal and scoliotic lumbar disc

STUDY DESIGN

Comparative ultrastructural study of intervertebral discs from normal subjects and patients with scoliosis.

OBJECTIVE

To identify ultrastructural relations among keratan sulfate (KS) proteoglycan, alpha-elastin, collagen fibers, and elastic fibers in normal and scoliotic discs.

SUMMARY OF BACKGROUND

KS proteoglycans, elastic fibers, and collagen fibers play important mechanical roles in the intervertebral disc, but the distributions of KS proteoglycans and elastin in this tissue have received little attention.

METHODS

Tissues were fixed in 4% paraformaldehyde. Monoclonal antibody 5-D-4 (which recognizes a KS epitope on aggrecan, fibromodulin, and lumican) and polyclonal anti-alpha-elastin were visualized with a 10-nm immunogold-conjugated secondary antibody.

RESULTS

In a normal disc, a regular pattern of KS labeling occurred around collagen fibers, in the cell cytoplasm, and in the rough endoplasmic reticulum; the nucleus pulposus was more densely labeled for KS than was the anulus fibrosus. In scoliotic disc anulus fibrosus, KS labeling was weak throughout the matrix and pericellularly but abundant in lysosomes and on electron-dense material in degenerate cells. Degenerate collagen fibers in scoliotic tissue bore less KS than did normal fibers. KS labeling of the microfibrillar region of elastic fibers was strong in normal disc but weak in scoliotic disc. Elastin labeling of elastic fibers was weaker in scoliotic than in normal tissue.

CONCLUSION

KS proteoglycans and elastic fibers are closely associated with the lamellar organization of the collagen fibers in a normal disc. In scoliosis, impaired regulation of collagen fibrillogenesis by lumican or fibromodulin may result in disruption of the lamellar structure.

Characterization of the non-collagenous proteins in avian cortical and medullary bone

Northern blotting, RT-PCR, and Western blotting techniques were used to characterize the matrix constituents of avian cortical and medullary bone. Extracts of bone tissue were found to contain multiple isoforms of bone sialoprotein (BSP), osteopontin (OPN), osteonectin (ON), osteocalcin (OC), and dentin matrix protein-1 (DMP-1). Only single transcripts were observed with Northern blotting; therefore it was concluded that the isoforms were due to differences in post-translational modifications. Since medullary bone is rich in keratan sulfate (KS), RT-PCR was used to investigate the expression of known keratan sulfate-containing proteoglycans (KSPGs). Although this tissue was found to express lumican and osteoglycin/mimecan, there was little evidence to suggest that these proteoglycans were a major source of the keratan sulfate glycosaminoglycans. Treatment of medullary bone extracts with keratanase resulted in the appearance of a BSP immunoactive band of approximately 59 kDa. However, it was not possible to isolate and identify the intact keratan sulfate proteoglycan.

The influence of amino acids on the biomineralization of hydroxyapatite in gelatin

The effects of pH on the calcium phosphate phase, of Tris and of amino acids, such as aspartic acid, glutamic acid, and serine on hydroxyapatite formation and morphology, were studied in double diffusion experiments. In this system, hydroxyapatite was only formed when the pH was around 7.4 or higher for the duration of the reaction. A decrease in pH resulted in the transformation of hydroxyapatite to octacalcium phosphate. Amino acids and Tris or the buffering capacity of Tris have an effect on the morphology of the synthetic hydroxyapatite. The presence of the additive results in spheres consisting of needles, blades or plates depending on the reaction system.

Posttranslational modifications to human bone sialoprotein determined by mass spectrometry

Bone sialoprotein (BSP) is an acidic 301 amino acid protein expressed by osteoblasts and at a low level by hypertrophic chondrocytes. Its expression is highest during early stages of bone formation, and it is particularly abundant in the cells lining the surface of newly formed trabeculae. BSP contains numerous substituents which are anionic in nature and apparently essential for the function of the protein. Thus, the proposed role of BSP in hydroxyapatite nucleation and growth may depend on such modifying groups. The posttranslational modifications include several acidic oligosaccharides as well as phosphate and sulfate groups. This work combines matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry with selective enzyme treatment of BSP to provide new information on the precise distribution and structure of oligosaccharides, sulfate, and phosphate groups in BSP isolated from human bone. The results provide a high level of detail in the location of these modifying groups toward the end of providing a basis for further understanding the function of BSP in bone nucleation.

Structural characterization of human recombinant and bone-derived bone sialoprotein. Functional implications for cell attachment and hydroxyapatite binding

Human bone sialoprotein (BSP) comprises 15% of the total noncollagenous proteins in bone and is thought to be involved in bone mineralization and remodeling. Recent data suggest a role for BSP in breast cancer and the development of bone metastases. We have produced full-length recombinant BSP in a human cell line and purified the protein from human bone retaining the native structure with proper folding and post-translational modifications. Mass spectrometry of bone-derived BSP revealed an average mass of 49 kDa and for recombinant BSP 57 kDa. The post-translational modifications contribute 30-40%. Carbohydrate analysis revealed 10 different complex-type N-glycans on both proteins and eight different O-glycans on recombinant BSP, four of those were found on bone-derived BSP. We could identify eight threonines modified by O-glycans, leaving the C terminus of the protein free of glycans. The recombinant protein showed similar secondary structures as bone-derived BSP. BSP was visualized in electron microscopy as a globule linked to a thread-like structure. The affinity for hydroxyapatite was higher for bone-derived BSP than for recombinant BSP. Cell adhesion assays showed that the binding of BSP to cells can be reversibly diminished by denaturation.

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.

600 MHz NMR studies of human articular cartilage keratan sulfates

The use of high-field two-dimensional 1H-correlation data is described for the detailed comparison of intact keratan sulfate polymer chains derived from human articular cartilage sources as a function of age. For fetal material the nonreducing chain termini are shown to be sparsely capped by sialyl groups which, if present, are exclusively (alpha2-3)-linked to an unsulfated galactose residue. The asialo capping segment has the structure: Gal-GlcNAc6S-Gal-GlcNAc6S-. Examination of keratan sulfate from 10-year-old cartilage shows that capping by sialyl groups is complete, with (alpha2-3)-linkages predominant; for both this and the 38-year-old cartilage the three capping structures: NeuAc(alpha2-3)-Gal-GlcNAc6S-Gal-GlcNAc6S-, NeuAc(alpha2-3)-Gal-GlcNAc6S-Gal6S-GlcNAc6S-, and NeuAc(alpha2-3)-Gal6S-GlcNAc6S-Gal6S-GlcNAc6S- are clearly recognizable. The level of (alpha2-6)-linked chain capping sialyl groups is significant for 38-year-old cartilage keratan sulfate. Structural information concerning the linkage region to protein and the distribution of galactose environments is readily obtained from the spectra. Signal complexities severely limit the usefulness of two-dimensional correlation spectroscopy at 600 MHz for the examination of N-acetylglucosamine residues within the poly(N-acetyllactosamine) repeat sequence and signals representing fucose placements remain undifferentiated. This nondestructive approach complements current degradative methods for the structural examination of keratan sulfates.

Bone matrix proteins: isolation and characterization of a novel cell-binding keratan sulfate proteoglycan (osteoadherin) from bovine bone

A small cell-binding proteoglycan for which we propose the name osteoadherin was extracted from bovine bone with guanidine hydrochloride-containing EDTA. It was purified to homogeneity using a combination of ion-exchange chromatography, hydroxyapatite chromatography, and gel filtration. The Mof the proteoglycan was 85, 000 as determined by SDS-PAGE. The protein is rich in aspartic acid, glutamic acid, and leucine. Two internal octapeptides from the proteoglycan contained the sequences Glu-Ile-Asn-Leu-Ser-His-Asn-Lys and Arg-Asp-Leu-Tyr-Phe-Asn-Lys-Ile. These sequences are not previously described, and support the notion that osteoadherin belongs to the family of leucine-rich repeat proteins. A monospecific antiserum was raised in rabbits. An enzyme-linked immunosorbent assay was developed, and showed the osteoadherin content of bone extracts to be 0.4 mg/g of tissue wet weight, whereas none was found in extracts of various other bovine tissues. Metabolic labeling of primary bovine osteoblasts followed by immunoprecipitation showed the cells to synthesize and secrete the proteoglycan. Digesting the immunoprecipitated osteoadherin with N-glycosidase reduced its apparent size to 47 kD, thus showing the presence of several N-linked oligosaccharides. Digestion with keratanase indicated some of the oligosaccharides to be extended to keratan sulfate chains. In immunohistochemical studies of the bovine fetal rib growth plate, osteoadherin was exclusively identified in the primary bone spongiosa. Osteoadherin binds to hydroxyapatite. A potential function of this proteoglycan is to bind cells, since we showed it to be as efficient as fibronectin in promoting osteoblast attachment in vitro. The binding appears to be mediated by the integrin alphavbeta3, since this was the only integrin isolated by osteoadherin affinity chromatography of surface-iodinated osteoblast extracts.

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.

Phosphorylation of purified bovine bone sialoprotein and osteopontin by protein kinases

The large number of covalently bound phosphates on the extracellular phosphoproteins osteopontin (OPN) and bone sialoprotein (BSP) have been implicated in biological functions such as mineral deposition and osteoclast binding. In the present study the state of phosphorylation of BSP and OPN was evaluated by in vitro 32P labeling using a series of protein kinases and quantification. Both the purified bovine BSP and OPN were radiolabeled by [32P]ATP and factor-independent protein kinase. Quantification of 32P radioactivity incorporated on dephosphorylated BSP and OPN provided 6.6 and 8.9 mol of phosphate incorporated/mol, respectively. Native OPN incorporated 1.07 and BSP 2.46 mol of phosphate/mol by factor-independent protein kinase. These data led to calculations that OPN and BSP, respectively, contain 7.83 and 4.14 mol of phosphate/mol in their natural state. Thrombin digests of 32P-labeled BSP showed radioactivity to be associated with fragment of approximately molecular mass values 30 kDa (N-terminal half), with no observable radioactivity associated with the 40-kDa fragment (C-terminal half). Similar experiments with 32P-labeled OPN provided two radiolabeled thrombin fragments, with molecular mass 30 kDa (N-terminal half) and 20 kDa (C-terminal half), both were radioactive. The major phosphorylation was associated with the N-terminal half containing 7.0 mol of phosphate, and 1.9 mol of phosphate were associated with the C-terminal half. Additional experiments of in vitro phosphorylation of OPN and BSP by several other known protein kinases were carried out. cAMP-dependent protein kinase showed no phosphorylation of OPN or BSP, while protein kinase C and cGMP-dependent protein kinase led to minor phosphorylation, each of the latter introduced about 1 mol of phosphate/mol of OPN and BSP molecule.

Expression of rat bone sialoprotein promoter in transgenic mice

Bone sialoprotein (BSP) is a major protein of the mineralized bone extracellular matrix that has been implicated in the nucleation of hydroxyapatite crystals. Our previous studies have demonstrated that BSP mRNA is expressed by differentiated osteoblasts, odontoblasts, and cementoblasts involved in de novo mineralized tissue formation in a tissue-specific and developmentally regulated manner. To determine the basis of the selective expression of the BSP gene, we have generated four transgenic mouse lines in which 2.7 kb of the rat BSP promoter ligated to a luciferase reporter gene has been stably integrated into the mouse genome. Assays of luciferase activities in 5-day-old animals has revealed consistently high levels in bone tissues with negligible activities in various other organs including kidney, liver, stomach, intestine, and spleen. In some animals, variable expression was observed in brain and skin. Temporal analyses revealed the highest luciferase expression in neonatal bones, with expression decreasing markedly with subsequent growth and development, as observed previously for the endogenous gene in rats. Immunohistochemical analysis of luciferase activity and in situ hybridization of luciferase mRNA in bone tissues show that differentiated osteoblasts express the highest levels of luciferase, consistent with the induction of endogenous gene expression. These studies demonstrate that the regulation of the BSP gene during osteoblastic differentiation, together with its tissue-specific, developmentally regulated expression, is primarily mediated within the 2.7 kb region of the promoter.

Nature and distribution of chondroitin sulphate and dermatan sulphate proteoglycans in rabbit alveolar bone

The type and distribution of mineral binding and collagenous matrix-associated chondroitin sulphate and dermatan sulphate proteoglycans in rabbit alveolar bone were studied biochemically and immunocytochemically, using three monoclonal antibodies (mAb 2B6, 3B3, and 1B5). The antibodies specifically recognize oligosaccharide stubs that remain attached to the core protein after enzymatic digestion of proteoglycans and identify epitopes in chondroitin 4-sulphate and dermatan sulphate; chondroitin 6-sulphate and unsulphated chondroitin; and unsulphated chondroitin, respectively. In addition, mAb 2B6 detects chondroitin 4-sulphate with chondroitinase ACII pre-treatment, and dermatan sulphate with chondroitinase B pre-treatment. Bone proteins were extracted from fresh specimens with a three-step extraction procedure: 4 M guanidine HCl (G-1 extract), 0.4 M EDTA (E-extract), followed by guanidine HCl (G-2 extract), to characterize mineral binding and collagenous matrix associated proteoglycans in E- and G2-extracts, respectively. Biochemical results using Western blot analysis of SDS-polyacrylamide gel electrophoresis of E- and G2-extracts demonstrated that mineral binding proteoglycans contain chondroitin 4-sulphate, chondroitin 6-sulphate, and dermatan sulphate, whereas collagenous matrix associated proteoglycans showed a predominance of dermatan sulphate with a trace of chondroitin 4-sulphate and no detectable chondroitin 6-sulphate or unsulphated chondroitin. Immunocytochemistry showed that staining associated with the mineral phase was limited to the walls of osteocytic lacunae and bone canaliculi, whereas staining associated with the matrix phase was seen on and between collagen fibrils in the remainder of the bone matrix. These results indicate that mineral binding proteoglycans having chondroitin 4-sulphate, dermatan sulphate, and chondroitin 6-sulphate were localized preferentially in the walls of the lacunocanalicular system, whereas collagenous associated dermatan sulphate proteoglycans were distributed over the remainder of the bone matrix.

Expression of bone sialoprotein mRNA during bone formation and resorption induced by colchicine in rat tibial bone marrow cavity

In the rat tibial bone marrow cavity, following colchicine injection, there is a phase of osteogenesis in which bone trabeculae replace the necrotic bone marrow tissues and fill the marrow cavity. The newly formed bone is subsequently resorbed by osteoclasts and normal bone marrow is restored. In this study, we correlated these morphologic events with the pattern of gene expression of bone sialoprotein (BSP), an extracellular matrix protein in mineralized tissues, to elucidate the possible functions of BSP in bone formation and resorption in vivo. The expressions of osteopontin (OPN) and type I collagen were also examined. Northern hybridization of the tibia demonstrated that OPN mRNA was gradually increased and expressed at a maximal level 10 days after colchicine injection (during the bone resorption process), while BSP mRNA expression already reached a maximal level at day 6 (during the initial process of bone formation). Its expression was, thus, quite temporary at the beginning of bone formation and different from that of type I collagen, which was continually elevated from days 6 to 10. In situ hybridization of the newly formed bone induced in the tibia revealed that BSP mRNA was evenly expressed in most osteoblasts and osteocytes, moreover in interconnecting colonies of spindle-shaped cells, possibly preosteoblasts, at day 6. At day 10, however, its expression became restricted to some cells on the bone surfaces, some osteoblasts, and most osteoclasts. These observations suggest that BSP may play an important role mainly in the initiation of bone formation and is also associated with the functions of osteoclast in vivo.

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.

Characterization of porcine bone sialoprotein: primary structure and cellular expression

Bone sialoprotein (BSP) is a highly glycosylated and sulphated phosphoprotein that is a major non-collagenous protein of bone. To further characterize the porcine protein and to study its expression during bone formation BSP cDNA clones were isolated from a porcine bone cDNA library. The primary sequence of the protein was derived from the nucleotide sequence of the largest cDNA insert and from the amino-terminal amino acid sequence determined by the automated Edman degradation procedure. When compared with sequences obtained from the human and rat BSPs 74% and 64% of the amino acids, respectively, were identical and a further 11% and 17%, respectively, were conservative replacements. Moreover, 60% of the amino acids in a concensus sequence derived from the primary sequences of mammalian BSPs were conserved with 16% conservative replacements. The two stretches of polyglutamic acid, through which the protein is capable of binding to hydroxyapatite, and an RGD motif that mediates cell attachment are retained in conserved sequences as are a number of potential sites of serine, threonine and tyrosine phosphorylation, glycosylation and tyrosine sulphation. Secondary structure prediction and hydrophilicity analysis indicate that the nascent BSP has an open flexible structure with the potential to form significant amounts of alpha-helix and some beta-sheet. In situ hybridization of fetal porcine bone with cRNA probes to porcine BSP mRNA shows that BSP is specifically expressed in differentiated osteoblasts on the surface of newly-forming bone trabeculae with especially high levels of hybridization at sites of de novo bone formation. The highly conserved features of BSP and its restricted distribution indicate an important role for this sialoprotein in the formation of bone.

Bone sialoprotein in developing porcine dental tissues: cellular expression and comparison of tissue localization with osteopontin and osteonectin

Bone sialoprotein (BSP) is a highly sulphated and glycosylated phosphoprotein that is a major constituent of bone and other mineralized connective tissues. Although BSP can mediate cell attachment through an RGD sequence and binds selectively to hydroxyapatite, its precise function in mineralized tissues is unknown. To provide insights into its possible function, affinity-purified polyclonal antibodies directed against porcine BSP were used to demonstrate the histological distribution of this protein in developing porcine mandibular alveolar bone and the associated tooth tissues from 35- and 50-day fetuses. In addition, a porcine cRNA probe was used to determine the cellular expression of BSP in the same tissues by in situ hybridization. Immunoreactivity to BSP protein was restricted to the cells and matrix of the mineralized tissues of alveolar bone and dentine. In dentine, BSP was localized to the odontoblasts and their processes and to the peritubular dentine. In the alveolar bone, immunoreactivity for BSP was evident in osteoblastic cells and osteocytes and in the bone matrix; the older bone stained more strongly than newly formed bone. In addition, BSP appeared to be concentrated in the reversal lines of the rapidly remodelling bone. The distribution of BSP in these tissues revealed distinct differences when compared to osteopontin and SPARC/osteonectin, which are also prominent non-collagenous proteins of mineralized tissues. Most notable was the localization of osteopontin and especially osteonectin in non-mineralizing tissues. The immunoreactivity of osteoblasts and osteocytes for BSP in bone was consistent with the high levels of BSP mRNA revealed by in situ hybridization. However, much lower levels of hybridization were evident in the odontoblasts of developing mandibular molars. These studies demonstrate that BSP is expressed during the early formation of dentine and alveolar bone and that the protein accumulates in the peritubular dentine and bone matrix.

Development expression of bone sialoprotein mRNA in rat mineralized connective tissues

Bone sialoprotein (BSP) is a phosphorylated and sulfated glycoprotein that is a major noncollagenous protein of bone and other mineralizing connective tissues. BSP is characterized by the presence of several polyglutamic acid segments and an RGD motif that mediates cell attachment through a vitronectin-like receptor. Although the precise function of BSP is unknown, the expression of BSP in conjunction with bone formation in vitro indicates a role for this protein in the biomineralization of connective tissues. In this study we used Northern hybridization and in situ hybridization to determine the tissue-specific and developmental expression of BSP during embryogenesis and growth of rat tissues. Analysis of tissues obtained from 13, 17, and 21 day fetuses, and from 4-, 14-, and 100-day-old animals indicates that BSP mRNA expression is restricted to cells actively forming the mineralizing tissues of bone, dentin and cementum. BSP mRNA transcripts were first evident in fully differentiated osteoblasts of 17 day fetal tissues at sites of de novo intramembranous and endochondral bone formation, with maximal expression observed at 21 days of gestation. Thereafter, BSP mRNA levels decreased markedly, and in adult bone hybridization was detected only in the primary spongiosa of long bones. In comparison, mRNAs for osteopontin (OPN), alkaline phosphatase (ALP), and osteocalcin (OC) peaked at 4-14 days postpartum before declining. In the tibiae, Northern hybridization revealed a second peak of mRNA for BSP, ALP, and OPN at 14 days, reflecting an increased osteogenic activity due to the formation of the secondary centers of ossification in the epiphyseal cartilage. In situ hybridization also revealed BSP mRNA in hypertrophic chondrocytes at sites of bone formation, in odontoblasts of the incisor during dentinogenesis, and in cementoblasts during cementogenesis. In view of the restricted distribution and temporal changes in the expression of BSP mRNA that we observed together with the chemical properties of BSP, we believe that this protein has a specific role in mediating the initial stages of connective tissue mineralization.

Concepts of osteoblast growth and differentiation: basis for modulation of bone cell development and tissue formation

The combined application of molecular, biochemical, histochemical, and ultrastructural approaches has defined a temporal sequence of gene expression associated with development of the bone cell phenotype in primary osteoblast cultures. The peak levels of expressed genes reflect a developmental sequence of bone cell differentiation characterized by three principal periods: proliferation, extracellular matrix maturation and mineralization, and two restriction points to which the cells can progress but cannot pass without further signals. The regulation of cell growth and bone-specific gene expression has been examined during this developmental sequence and is discussed within the context of several unique concepts. These are (1) that oncogene expression in proliferating osteoblasts contributes to the suppression of genes expressed postproliferatively, (2) that hormone modulation of a gene is dependent upon the maturational state of the osteoblast, and (3) that chromatin structure and the presence of nucleosomes contribute to three-dimensional organization of gene promoters that support synergistic and/or antagonistic activities of physiologic mediators of bone cell growth and differentiation.

Innovative materials processing strategies: a biomimetic approach

Many organisms construct structural ceramic (biomineral) composites from seemingly mundane materials; cell-mediated processes control both the nucleation and growth of mineral and the development of composite microarchitecture. Living systems fabricate biocomposites by: (i) confining biomineralization within specific subunit compartments; (ii) producing a specific mineral with defined crystal size and orientation; and (iii) packaging many incremental units together in a moving front process to form fully densified, macroscopic structures. By adapting biological principles, materials scientists are attempting to produce novel materials. To date, neither the elegance of the biomineral assembly mechanisms nor the intricate composite microarchitectures have been duplicated by nonbiological processing. However, substantial progress has been made in the understanding of how biomineralization occurs, and the first steps are now being taken to exploit the basic principles involved.

Improved binding of acidic bone matrix proteins to cationized filters during solid phase assays

A number of commercially available matrix filter supports have been designed for the immobilization of proteins following either electrotransfer from sodium dodecyl sulfate (SDS) polyacrylamide gels or direct application during dot blotting assays. These matrices differ with respect to chemical composition, charge, pore size, and degree of hydrophobicity. It follows that the properties of the protein(s) of interest will greatly influence the degree to which they interact with and ultimately bind to various filters. Acidic bone proteins contain diverse post-translational modifications that influence their interactions with solid phase matrices such as those used in immunoblotting (Western or dot blotting) or ion binding (overlay) procedures. This communication describes the results of a study comparing binding of various mixtures of non-collagenous acidic bone matrix phosphoproteins as well as purified osteopontin and osteocalcin to various filters including nitrocellulose and cationized paper or nylon. Based on our findings, we recommend the use of cationized filters for solid phase assays requiring the binding of these acidic macromolecules to background supports.

Monoclonal antibody BRL 12 reacts with bone keratan sulphate proteoglycan

Our previous studies suggest that a monoclonal antibody, BRL 12, reacts with a specific product of differentiated cells of the osteoblastic lineage. In the present study, the bone constituent recognized by this antibody has been characterized by gel filtration, ion exchange chromatography, protein blotting and immunolocalization. Our findings show that the antibody reacts with an epitope associated with the core protein of rabbit keratan sulfate proteoglycan (KSPG), a molecule which shares considerable homology with the sialoprotein present in the bone tissue of other mammalian species.

Expression of bone sialoprotein (BSP) in developing human tissues

Bone sialoprotein (BSP) and its messenger RNA were localized in developing human skeletal and nonskeletal tissues by means of immunohistochemistry and in situ hybridization. Both protein and mRNA were found in mature, bone-forming cells but not in their immature precursors. In addition, osteoclasts displayed positive immunostaining and high densities of autoradiographic grains by in situ hybridization experiments. BSP was expressed in fetal epiphyseal cartilage cells, particularly in hypertrophic chondrocytes of growth plates. Though neither the protein nor the mRNA were identified in a variety of other connective and nonconnective tissues, an unexpected finding was the expression of BSP in the trophoblast cells of placenta. These findings show that BSP is primarily an osteoblast-derived component of the bone matrix expressed at late stages of differentiation. We have also found that osteoclasts produce BSP, possibly as a mediator of cell attachment to bone.

Structural characterization of human alveolar bone proteoglycans

Proteoglycans were extracted from EDTA-demineralized human alveolar bone under dissociative conditions using 4 M guanidinium chloride in the presence of protease inhibitors. The extract was further purified by anion-exchange chromatography on DEAE-Sephacel, using a step-wise salt gradient. The proteoglycan-rich fraction was analysed for carbohydrate, protein and amino acid composition and molecular size by SDS-PAGE. Glycosaminoglycan content was determined by cellulose acetate electrophoresis after proteolysis. The sulphate isomers of the glycosaminoglycans were confirmed by Fourier-transformed infra-red spectroscopy. Two chondroitin sulphate-proteoglycan species were identified with molecular weights of 79 and 55-65 kDa, respectively. The core proteins had molecular weights of 49 kDa for both proteoglycans, with the amino acid content rich in glycine, leucine, glutamate and aspartate. The chondroitin sulphate chains were mainly as the 4-sulphate isomer forms although low but detectable amounts of 6-sulphate isomer were also present.

The collagens and glycosaminoglycans of the extracellular matrices secreted by bone marrow stromal cells cultured in vivo in diffusion chambers

When rabbit bone marrow cells are cultured in a diffusion chamber implanted into the peritoneal cavity of an athymic mouse, the stromal cells proliferate, differentiate, and produce tissues that have the morphological features of loose fibrous tissue, woven or chondroid bone, and cartilage. The collagens synthesized during the development of the tissues from 7 to 28 days after implantation were identified using specific antibodies to rabbit types I, II, III, and V and rat type IX collagens, while the glycosaminoglycans were characterized histochemically using the dye, Alcian blue. Fibrous tissue forms in the first week and it contains types I, III, and V collagens and hyaluronan. Bone and cartilage develop within the fibrous tissue from about 12 days onwards. The bone matrix contains types I and V collagens, and chondroitin and keratan sulphates. The cartilaginous matrix contains types II and IX collagens, and chondroitin and keratan sulphates. Small amounts of type III collagen are found in the bone, and types I, III, and V collagens in the cartilage. These are thought to be the remnants of the fibrous matrix and decrease as the matrices mature. It is concluded that the tissue in diffusion chambers, formed by a small number of early precursor cells present in the soft tissues of the endosteum and marrow of young rabbits, contains extracellular matrix macromolecules similar to those found in bone and cartilage.

Load-induced proteoglycan orientation in bone tissue in vivo and in vitro

Previous studies of Alcian blue-induced birefringence in adult avian cortical bone showed that a short period of intermittent loading rapidly produces an increased level of orientation of proteoglycans within the bone tissue. In the absence of further loading, this persists for over 24 hours. We have proposed that this phenomenon could provide a means for "capturing" the effects of transient strains, and so provide a persistent, constantly updated strain-related influence on osteocyte populations related to the bones' averaged recent strain history, in effect, a "strain memory" in bone tissue. In our present study, we use the Alcian blue-induced birefringence technique to demonstrate that proteoglycan orientation also occurs after intermittent loading of both cortical and cancellous mammalian bone in vivo and in vitro. We also show that the change in birefringence is proportional to the magnitude of the applied strain, and that the reorientation occurs rapidly, reaching a maximal value after only 50 loading cycles. Examination of electron micrographs of bone tissue after staining with cupromeronic blue allows direct visualization and quantification of the change in proteoglycan orientation produced by loading. This shows that intermittent loading is associated with a realignment of the proteoglycan protein cores, bringing them some 5 degrees closer to the direction of collagen fibrils in the bone matrix.

A biochemical and immunohistochemical study of the proteoglycans of alveolar bone

The purpose of this investigation was to study the proteoglycans in alveolar bone of three animal species. Alveolar bone was obtained from humans, pigs, and rabbits. Portions were fixed, sectioned, and stained with monoclonal antibodies against keratan sulfate and chondroitin sulfate. In other samples, biochemical analyses were performed. After removal of the organic matrix by 4 mol/L guanidinium HCl extraction in the presence of proteinase inhibitors, proteoglycans in the mineralized matrix were extracted with 4 mol/L guanidinium HCl/0.5 mol/L EDTA/proteinase inhibitors, and characterized on the basis of their glycosaminoglycan content (cellulose acetate membrane electrophoresis), charge (DEAE-Sephacel and hydroxylapatite chromatography), size (Sepharose CL-6B chromatography and agarose/polyacrylamide gel electrophoresis), and amino acid content. The results indicated that keratan sulfate could be detected immunohistochemically and biochemically in rabbit bone only. The predominant glycosaminoglycan in pig and human alveolar bone was chondroitin sulfate, although some hyaluronate, dermatan sulfate, and heparan sulfate were also detected. The proteoglycans were found to be slightly smaller than gingival proteoglycans, but similar to those in cementum, dentin, and other bones. In addition to intact proteoglycans, some free glycosaminoglycan chains were also extracted from the mineralized matrix. Amino acid analyses showed some subtle differences between alveolar bone proteoglycan and those of the soft tissues of the periodontium.

Noncollagenous matrix proteins and their role in mineralization

Improved technologies have led to the isolation of a large number of noncollagenous matrix proteins from the mineralized connective tissues. These proteins have been postulated to have many functions, but few of their suggested roles have been verified. Many of the noncollagenous matrix proteins are thought to regulate the deposition of hydroxyapatite mineral. In this review the current information on the role of these proteins in the mineralized and mineralizing connective tissues is presented along with a synopsis of the methods currently being used to elucidate these functions.

Immunohistochemical studies using BRL 12, a monoclonal antibody reacting specifically with osteogenic tissues

A monoclonal antibody of immunoglobulin class G1 has been produced which reacts with a high molecular weight antigen apparently present exclusively in osteogenic tissues. Immunohistochemical studies have shown that the antigen is present throughout the mineralized matrix and in osteoid. None of the other tissues examined namely liver, intestine, kidney, spleen, thymus, heart, lung, skin, cartilage and skeletal muscle showed evidence of specific antibody binding. Immunohistochemical staining was also demonstrated in tissues developing from rabbit marrow cultured in vitro and in diffusion chambers in vivo. Temporal studies of antigen expression in the chambers indicated that the antigen occurs at sites of bone formation after the appearance of alkaline phosphatase but before the formation of a mineralized matrix. The results of these studies suggest that the monoclonal antibody recognises a product of differentiated osteoblasts. This antibody may therefore prove useful in studies of osteogenic differentiation.

The effects of noncollagenous matrix proteins on hydroxyapatite formation and proliferation in a collagen gel system

The effects of several noncollagenous matrix proteins on hydroxyapatite formation and growth were studied in a dynamic collagen gel system. In this system growth plate proteoglycan aggregates at concentrations of 1-10 micrograms/ml were effective inhibitors, desulfated aggregates from brachymorphic mice were less effective. Phosphophoryn at 1-100 micrograms/ml had no effect on formation; 60-120 micrograms/ml retarded mineral growth. Type X collagen at concentrations of 50-300 micrograms/ml had no effect on formation or growth.

Glycosaminoglycans of human alveolar bone

Bone proteoglycan was extracted and the glycosaminoglycan (GAG) components identified. Chondroitin-4-sulphate was the major GAG detected and represented 93.8% of the total GAG extracted. In addition, hyaluronic acid (1.3%), dermatan sulphate (3.1%) and heparan sulphate (1.8%) were identified as minor constituents.

Bone cell biology: the regulation of development, structure, and function in the skeleton

Bone cells compose a population of cells of heterogeneous origin but restricted function with respect to matrix formation, mineralization, and resorption. The local, mesenchymal origin of the cells which form the skeleton contrasts with their extraskeletal, hemopoietic relatives under which bone resorption takes place. However, the functions of these two diverse populations are remarkably related and interdependent. Bone cell regulation, presently in its infancy, is a complicated cascade involving a plethora of local and systemic factors, including some components of the skeletal matrices and other organ systems. Thus, any understanding of bone cell regulation is a key ingredient in understanding not only the development, maintenance, and repair of the skeleton but also the prevention and treatment of skeletal disorders.