Relationships between bone protein and mineral in developing porcine long bone and calvaria

Fabrication of a cell culture scaffold that mimics the composition and structure of bone marrow extracellular matrix

Cell culture models that mimic tissue environments are useful for cell and extracellular matrix (ECM) function analysis. Decellularized tissues with tissue-specific ECM are expected to be applied as cell culture scaffolds, however, it is often difficult for seeded cells to permeate their structures. In this study, we evaluated the adhesion and proliferation of mouse fibroblasts seeded onto decellularized bone marrow scaffolds that we fabricated from adult and fetal porcine. Decellularized fetal bone marrow displays more cell attachment and faster cell proliferation than decellularized adult bone marrow. Our findings suggest that decellularized fetal bone marrow is useful as a cell culture scaffold with bone marrow ECM and structure.

Platelet-derived extracellular vesicles formulated with hyaluronic acid gels for application at the bone-implant interface: An animal study

Background/Objective

Platelet derived extracellular vesicles (pEV) are promising therapeutical tools for bone healing applications. In fact, several in vitro studies have already demonstrated the efficacy of Extracellular Vesicles (EV) in promoting bone regeneration and repair in various orthopedic models. Therefore, to evaluate the translational potential in this field, an in vivo study was performed.

Methods

Here, we used hyaluronic acid (HA) gels formulated with pEVs, as a way to directly apply pEVs and retain them at the bone defect. In this study, pEVs were isolated from Platelet Lysate (PL) through size exclusion chromatography and used to formulate 2% HA gels. Then, the gels were locally applied on the tibia cortical bone defect of New Zeland White rabbits before the surgical implantation of coin-shaped titanium implants. After eight weeks, the bone healing process was analyzed through biomechanical, micro-CT, histological and biochemical analysis.

Results

Although no biomechanical differences were observed between pEV formulated gels and non-formulated gels, biochemical markers of the wound fluid at the interface presented a decrease in Lactate dehydrogenase (LDH) activity and alkaline phosphatase (ALP) activity for pEV HA treated implants. Moreover, histological analyses showed that none of the treatments induced an irritative effect and, a decrease in the fibrotic response surrounding the implant for pEV HA treated implants was described.

Conclusion

In conclusion, pEVs improve titanium implants biocompatibility at the bone-implant interface, decreasing the necrotic effects of the surgery and diminishing the fibrotic layer associated to the implant encapsulation that can lead to implant failure.

Recombinant Proteins-Based Strategies in Bone Tissue Engineering

The increase in fracture rates and/or problems associated with missing bones due to accidents or various pathologies generates socio-health problems with a very high impact. Tissue engineering aims to offer some kind of strategy to promote the repair of damaged tissue or its restoration as close as possible to the original tissue. Among the alternatives proposed by this specialty, the development of scaffolds obtained from recombinant proteins is of special importance. Furthermore, science and technology have advanced to obtain recombinant chimera’s proteins. This review aims to offer a synthetic description of the latest and most outstanding advances made with these types of scaffolds, particularly emphasizing the main recombinant proteins that can be used to construct scaffolds in their own right, i.e., not only to impregnate them, but also to make scaffolds from their complex structure, with the purpose of being considered in bone regenerative medicine in the near future.

Oral biosciences: The annual review 2019

BACKGROUND

Journal of Oral Biosciences is devoted to the advancement and dissemination of fundamental knowledge concerning every aspect of oral biosciences.

HIGHLIGHT

This review features review articles in the fields of "Bone Cell Biology," "Microbiology," "Oral Heath," "Biocompatible Materials," "Mouth Neoplasm," and "Biological Evolution" in addition to the review articles by winners of the Lion Dental Research Award ("Role of nicotinic acetylcholine receptors for modulation of microcircuits in the agranular insular cortex" and "Phospholipase C-related catalytically inactive protein: A novel signaling molecule for modulating fat metabolism and energy expenditure") and the Rising Members Award ("Pain mechanism of oral ulcerative mucositis and the therapeutic traditional herbal medicine hangeshashinto," "Mechanisms underlying the induction of regulatory T cells by sublingual immunotherapy," and "Regulation of osteoclast function via Rho-Pkn3-c-Src pathways"), presented by the Japanese Association for Oral Biology.

CONCLUSION

These reviews in the Journal of Oral Biosciences have inspired the readers of the journal to broaden their knowledge regarding various aspects of oral biosciences. The current editorial review introduces these exciting review articles.

Degradation of extracellular matrices propagates calcification during development and healing in bones and teeth

BACKGROUND

Bone, dentin, and enamel are tissues formed through calcification, a process involving deposition of calcium phosphate minerals on extracellular organic matrices. Calcification, the underlying mechanism of which is unknown, is initiated with mineral deposition followed by advancing of the deposit and subsequent maturation of the mineral crystal.

HIGHLIGHT

We have reviewed the current knowledge of how calcification proceeds during bone development, bone healing, and enamel and dentin development, based on reported studies. Previous studies reported by us and by other authors have suggested that degradation of some extracellular matrix (ECM) proteins is involved in calcification during bone and dentin development and bone healing in a manner similar to that previously reported for enamel development.

CONCLUSION

The ECM proteins may inhibit mineral deposition and calcification, similar to the role of amelogenin during enamel development. The candidates for the amelogenin equivalents in bone and dentin have not been identified. Further studies are required to elucidate the regulatory mechanisms of bone and dentin calcification in light of specific ECM proteins that prevent calcification and enzymes that degrade these ECM proteins.

Cross-talk between lung cancer and bones results in neutrophils that promote tumor progression

Lung cancer is the leading cause of cancer mortality around the world. The lack of detailed understanding of the cellular and molecular mechanisms participating in the lung tumor progression restrains the development of efficient treatments. Recently, by using state-of-the-art technologies, including in vivo sophisticated Cre/loxP technologies in combination with lung tumor models, it was revealed that osteoblasts activate neutrophils that promote tumor growth in the lung. Strikingly, genetic ablation of osteoblasts abolished lung tumor progression via interruption of SiglecF^high-expressing neutrophils supply to the tumor microenvironment. Interestingly, SiglecF^high neutrophil signature was associated with worse lung adenocarcinoma patients outcome. This study identifies novel cellular targets for lung cancer treatment. Here, we summarize and evaluate recent advances in our understanding of lung tumor microenvironment.

Site-specific differences in osteoblast phenotype, mechanical loading response and estrogen receptor-related gene expression

The osteoporosis-resistant nature of skull bones implies inherent differences exist between their cellular responses and those of other osteoporosis-susceptible skeletal sites. Phenotypic differences in calvarial and femoral osteoblastic responses to induction of osteogenesis, mechanical loading, estrogen, growth factor and cytokine stimulation were investigated. Primary rat calvarial and femoral adult male osteoblasts were cultured and osteoblastic mineralisation and maturation determined using Alizarin Red staining and expression of osteogenic marker genes assessed. Expression of known mechanically-responsive genes was compared between sites following loading of scaffold-seeded cells in a bioreactor. Cell proliferation and differentiation following growth factor and estrogen stimulation were also compared. Finally expression of estrogen receptors and associated genes during osteogenic differentiation were investigated. Calvarial osteoblasts exhibited delayed maturation (45d. vs 21d.) and produced less mineralised matrix than femoral osteoblasts when osteogenically induced. PDGF-BB and FGF2 both caused a selective increase in proliferation and decrease in osteoblastic differentiation of femoral osteoblasts. Mechanical stimulation resulted in the induction of the expression of Ccl2 and Anx2a selectively in femoral osteoblasts, but remained unchanged in calvarial cells. Estrogen receptor beta expression was selectively upregulated 2-fold in calvarial osteoblasts. Most interestingly, the estrogen responsive transcriptional repressor RERG was constitutively expressed at 1000-fold greater levels in calvarial compared with femoral osteoblasts. RERG expression in calvarial osteoblasts was down regulated during osteogenic induction whereas upregulation occurred in femoral osteoblasts. Bone cells of the skull are inherently different to those of the femur, and respond differentially to a range of stimuli. These site-specific differences may have important relevance in the development of strategies to tackle metabolic bone disorders.

EDTA soluble chemical components and the conditioned medium from mobilized dental pulp stem cells contain an inductive microenvironment, promoting cell proliferation, migration, and odontoblastic differentiation

Background

The critical challenge in tissue engineering is to establish an optimal combination of stem cells, signaling morphogenetic molecules, and extracellular matrix scaffold/microenvironment. The extracellular matrix components of teeth may be reconstituted as an inductive microenvironment in an ectopic tooth transplantation bioassay. Thus, the isolation and identification of the chemical components of the inductive microenvironment in pulp/dentin regeneration will accelerate progress towards the goal of tissue engineering of the tooth.

Methods

The teeth demineralized in 0.6 M hydrochloric acid were sequentially extracted by 4.0 M guanidine hydrochloride (GdnHCl), pH 7.4, and 0.5 M ethylenediaminetetraacetic acid (EDTA), pH 7.4. The extracted teeth were transplanted into an ectopic site in severe combined immunodeficiency (SCID) mice with mobilized dental pulp stem cells (MDPSCs). The unextracted tooth served as a positive control. Furthermore, the soluble components for the inductive microenvironment, the GdnHCl extracts, or the EDTA extracts together with or without MDPSC conditioned medium (CM) were reconstituted systematically with autoclaved teeth in which the chemical components were completely inactivated and only the physical microenvironment was preserved. Their pulp/dentin regenerative potential and angiogenic potential were compared 28 days after ectopic tooth transplantation by histomorphometry and real-time RT-PCR analysis.

Results

Expression of an odontoblastic marker, enamelysin, and a pulp marker, thyrotropin-releasing hormone degrading enzyme (TRH-DE), was lower, and expression of a periodontal cell marker, anti-asporin/periodontal ligament-associated protein 1 (PLAP-1), was higher in the transplant of the EDTA-extracted teeth compared with the GdnHCl-extracted teeth. The autoclaved teeth reconstituted with the GdnHCl extracts or the EDTA extracts have weak regenerative potential and minimal angiogenic potential, and the CM significantly increased this potential. Combinatorial effects of the EDTA extracts and the CM on pulp/dentin regeneration were demonstrated in vivo, consistent with their in-vitro effects on enhanced proliferation, migration, and odontoblastic differentiation.

Conclusions

The EDTA-extracted teeth demonstrated significantly lower pulp/dentin regenerative potential compared with the GdnHCl-extracted teeth. The EDTA soluble chemical components when reconstituted with the physical structure of autoclaved teeth serve as an inductive microenvironment for pulp/dentin regeneration, promoting cell proliferation, migration, and odontoblastic differentiation.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-016-0334-z) contains supplementary material, which is available to authorized users.

Osteoblasts of calvaria induce higher numbers of osteoclasts than osteoblasts from long bone

Several studies have demonstrated the existence of functional differences between osteoclasts harbored in different bones. The mechanisms involved in the occurrence of such a heterogeneity are not yet understood. Since cells of the osteoblast lineage play a critical role in osteoclastogenesis, osteoclast heterogeneity may be due to osteoblasts that differ at the different bone sites. In the present study we evaluated possible differences in the capacity of calvaria and long bone osteoblasts to induce osteoclastogenesis. Osteoblasts were isolated from calvaria and long bone of mice and co-cultured with osteoclast precursors obtained from bone marrow of both types of bone, spleen and peripheral blood. Irrespective of the source of the precursors, a significantly higher number of TRACP-positive multinucleated cells were formed with calvaria osteoblasts. The expression of osteoclastogenesis related genes was analyzed by qPCR. OPG was significantly higher expressed by long bone osteoblasts. The RANKL/OPG ratio and TNF-α gene expression were significantly higher in calvaria osteoblast cultures. OPG added to the culture system inhibited osteoclastogenesis in both groups. Blocking TNF-α had no effect on osteoclastogenesis. Calvaria and long bone osteoblasts were pre-stimulated with VitD3 for 5days. Subsequently, osteoclast precursors were added to these cultures. After a co-culture of 6days, it was shown that VitD3 pre-stimulation of long bone osteoblasts strongly improved their capacity to induce osteoclast formation. This coincided with an increased ratio of RANKL/OPG. Taken together, the data demonstrated differences in the capacity of calvaria and long bone osteoblasts to induce osteoclastogenesis. This appeared to be due to differences in the expression of RANKL and OPG. VitD3 pre-stimulation improved the ability of long bone osteoblasts to induce osteoclast formation. Our findings demonstrate bone-site specific differences in osteoblast-mediated formation of osteoclasts. The data may suggest that the heterogeneity of osteoclasts is partially due to the way the osteoblasts induce their formation.

Bone matrix calcification during embryonic and postembryonic rat calvarial development assessed by SEM-EDX spectroscopy, XRD, and FTIR spectroscopy

Bone mineral is constituted of biological hydroxyapatite crystals. In developing bone, the mineral crystal matures and the Ca/P ratio increases. However, how an increase in the Ca/P ratio is involved in maturation of the crystal is not known. The relationships among organic components and mineral changes are also unclear. The study was designed to investigate the process of calcification during rat calvarial bone development. Calcification was evaluated by analyzing the atomic distribution and concentration of Ca, P, and C with scanning electron microscopy (SEM)-energy-dispersive X-ray (EDX) spectroscopy and changes in the crystal structure with X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. Histological analysis showed that rat calvarial bone formation started around embryonic day 16. The areas of Ca and P expanded, matching the region of the developing bone matrix, whereas the area of C became localized around bone. X-ray diffraction and FTIR analysis showed that the amorphous-like structure of the minerals at embryonic day 16 gradually transformed into poorly crystalline hydroxyapatite, whereas the proportion of mineral to protein increased until postnatal week 6. FTIR analysis also showed that crystallization of hydroxyapatite started around embryonic day 20, by which time SEM-EDX spectroscopy showed that the Ca/P ratio had increased and the C/Ca and C/P ratios had decreased significantly. The study suggests that the Ca/P molar ratio increases and the proportion of organic components such as proteins of the bone matrix decreases during the early stage of calcification, whereas crystal maturation continues throughout embryonic and postembryonic bone development.

Osteoblast differentiation is enhanced by a nano-to-micro hybrid titanium surface created by Yb:YAG laser irradiation

OBJECTIVES

The aim of this study was to analyze the capacity of a new modified laser surface to stimulate calvarial osteoblasts isolated from neonatal mouse bones to differentiate and form mineralized nodules.

METHODS

Titanium discs were subjectezd or not to laser irradiation according to specific parameters and characterized. Osteoblasts isolated from neonatal mouse calvaria were cultured over the discs, and the capacity of these cells to proliferate (MTT assay), form mineralized nodules (Alizarin red assay), and enhance alkaline phosphatase activity (ALPase activity) was analyzed. Real-time PCR was used for quantification of gene expression.

RESULTS

Laser-irradiated titanium discs (L) presented a rough nano-to-micrometric oxidized surface contrasting with the smooth pattern on polished discs (P). The Ra on the micrometric level increased from 0.32 ± 0.01 μm on P surfaces to 10.57 ± 0.39 μm on L surfaces. When compared with P, L promoted changes in osteoblast morphology, increased mineralized nodule formation in osteoblasts cultured on the surfaces for 14 days, and enhanced ALPase activity at days 7 and 14. Transcription factors triggering osteoblast differentiation (Runx2 and Sp7) and genes encoding the bone extracellular matrix proteins collagen type-1 (Col1a1), osteopontin (Spp1), and osteocalcin (Bglap) were upregulated in cells on L surfaces compared with those on P surfaces at days 1-14.

CONCLUSION

Laser treatment of titanium surfaces created a rough surface that stimulated osteoblast differentiation.

CLINICAL RELEVANCE

Laser treatment of titanium generates a reproducible and efficient surface triggering osteoblast differentiation that can be of importance for osteointegration.

Treatment with eldecalcitol positively affects mineralization, microdamage, and collagen crosslinks in primate bone

Eldecalcitol (ELD), an active form of vitamin D analog approved for the treatment of osteoporosis in Japan, increases lumbar spine bone mineral density (BMD), suppresses bone turnover markers, and reduces fracture risk in patients with osteoporosis. We have previously reported that treatment with ELD for 6 months improved the mechanical properties of the lumbar spine in ovariectomized (OVX) cynomolgus monkeys. ELD treatment increased lumbar BMD, suppressed bone turnover markers, and reduced histomorphometric parameters of both bone formation and resorption in vertebral trabecular bone. In this study, we elucidated the effects of ELD on bone quality (namely, mineralization, microarchitecture, microdamage, and bone collagen crosslinks) in OVX cynomolgus monkeys in comparison with OVX-vehicle control monkeys. Density fractionation of bone powder prepared from lumbar vertebrae revealed that ELD treatment shifted the distribution profile of bone mineralization to a higher density, and backscattered electron microscopic imaging showed improved trabecular bone connectivity in the ELD-treated groups. Higher doses of ELD more significantly reduced the amount of microdamage compared to OVX-vehicle controls. The fractionated bone powder samples were divided according to their density, and analyzed for collagen crosslinks. Enzymatic crosslinks were higher in both the high-density (≥2.0 mg/mL) and low-density (<2.0 mg/mL) fractions from the ELD-treated groups than in the corresponding fractions in the OVX-vehicle control groups. On the other hand, non-enzymatic crosslinks were lower in both the high- and low-density fractions. These observations indicated that ELD treatment stimulated the enzymatic reaction of collagen crosslinks and bone mineralization, but prevented non-enzymatic reaction of collagen crosslinks and accumulation of bone microdamage. Bone anti-resorptive agents such as bisphosphonates slow down bone remodeling so that bone mineralization, bone microdamage, and non-enzymatic collagen crosslinks all increase. Bone anabolic agents such as parathyroid hormone decrease bone mineralization and bone microdamage by stimulating bone remodeling. ELD did not fit into either category. Histological analysis indicated that the ELD treatment strongly suppressed bone resorption by reducing the number of osteoclasts, while also stimulating focal bone formation without prior bone resorption (bone minimodeling). These bidirectional activities of ELD may account for its unique effects on bone quality.

Magnesium deficiency results in an increased formation of osteoclasts

Magnesium (Mg(2+)) deficiency is a frequently occurring disorder that leads to loss of bone mass, abnormal bone growth and skeletal weakness. It is not clear whether Mg(2+) deficiency affects the formation and/or activity of osteoclasts. We evaluated the effect of Mg(2+) restriction on these parameters. Bone marrow cells from long bone and jaw of mice were seeded on plastic and on bone in medium containing different concentrations of Mg(2+) (0.8 mM which is 100% of the normal value, 0.4, 0.08 and 0 mM). The effect of Mg(2+) deficiency was evaluated on osteoclast precursors for their viability after 3 days and proliferation rate after 3 and 6 days, as was mRNA expression of osteoclastogenesis-related genes and Mg(2+)-related genes. After 6 days of incubation, the number of tartrate resistant acid phosphatase-positive (TRACP(+)) multinucleated cells was determined, and the TRACP activity of the medium was measured. Osteoclastic activity was assessed at 8 days by resorption pit analysis. Mg(2+) deficiency resulted in increased numbers of osteoclast-like cells, a phenomenon found for both types of marrow. Mg(2+) deficiency had no effect on cell viability and proliferation. Increased osteoclastogenesis due to Mg(2+) deficiency was reflected in higher expression of osteoclast-related genes. However, resorption per osteoclast and TRACP activity were lower in the absence of Mg(2+). In conclusion, Mg(2+) deficiency augmented osteoclastogenesis but appeared to inhibit the activity of these cells. Together, our in vitro data suggest that altered osteoclast numbers and activity may contribute to the skeletal phenotype as seen in Mg(2+) deficient patients.

Correlation between molecular signals and bone bonding to titanium implants

OBJECTIVES

A better understanding of the biological processes controlling osseointegration at the bone-to-implant interface is needed. The aim of this study was to examine which are the molecular and biochemical variables that are significantly related to osseointegration, using multiple regression analysis.

MATERIALS AND METHODS

Titanium coins were placed into the tibial cortical bone of New Zealand White rabbits and evaluated using pull-out test after 4 and 8 weeks of healing. Correlations between pull-out and several markers from tissue fluid (Lactate dehydrogenase [LDH] and Alkaline phosphatase [ALP] activities and total protein content) and peri-implant bone tissue (total protein, RNA and DNA content, implant area covered with bone and gene expression of osteoblast, osteoclast and inflammation markers) were used to assess the importance of these parameters in bone healing and in relation to implant performance.

RESULTS

Our results showed a negative correlation between the content of DNA, RNA and total protein at the peri-implant bone tissue and the pull-out force, indicating that as bone matures and implant becomes more osseointegrated, the organic content of bone decreases. The negative correlation found between pull-out force and ALP activity pointed to a delayed healing in implants with lower pull-out values and primary mineralization still ongoing. LDH activity and total protein content in the tissue fluid were as well negatively correlated with the pull-out force. Finally, a positive correlation was observed between the pull-out force and the expression of the osteoblast and the bone resorption markers, being osteocalcin and collagen-I the best predictive markers for osseointegration after 4 and 8 weeks of healing respectively.

CONCLUSIONS

These results suggest that the evaluation of these markers could be relevant for the assessment of new implant surfaces for rapid bone healing and improved implant performance.

Separation of newly formed bone from older compact bone reveals clear compositional differences in bone matrix

In long bone diaphyses, woven bone forms first and then transitions into a more mineralized compact bone tissue. Prior evidence suggests that the non-collagenous protein composition of woven bone may be distinct from that of more mature bone tissue, particularly with respect to a diverse group of phosphorylated, extracellular matrix proteins. To critically test this hypothesis, we developed an in situ approach to isolate newly formed bone from more mature bone within the same long bone, and combine this anatomical approach with Western blotting to make relative comparisons of 7 phosphorylated matrix proteins important for bone physiology and biomineralization. Interestingly, 75 kDa bone sialoprotein (BSP), 63 kDa osteopontin, and the 75 kDa form of bone acidic glycoprotein-75 (BAG-75) were enriched in primary bone as opposed to more mature cortical bone, while osteonectin, fetuin A, matrix extracellular phosphoglycoprotein (MEPE) and dentin matrix protein-1 (DMP-1) appeared to be equally distributed between these two bone tissue compartments. Analyses also revealed the presence of larger sized forms of osteopontin (and to a lesser degree BSP) mostly in newly formed bone, while larger forms of BAG-75 were mostly detected in more mature cortical bone. Smaller sized forms of DMP-1 and BAG-75 were detected in both newly formed and more mature bone tissue extracts, and they are likely the result of proteolytic processing in vivo. Intact DMP-1 (97 kDa) was only detected in unmineralized matrix extracts. These findings indicate that newly formed bone exhibits a non-collagenous matrix protein composition distinct from that of more mature compact bone even within the same long bone, and suggest that the temporal fate of individual non-collagenous proteins is variable in growing bone.

Jaw and long bone marrows have a different osteoclastogenic potential

Osteoclasts, the multinucleated bone-resorbing cells, arise through fusion of precursors from the myeloid lineage. However, not all osteoclasts are alike; osteoclasts at different bone sites appear to differ in numerous respects. We investigated whether bone marrow cells obtained from jaw and long bone differed in their osteoclastogenic potential. Bone marrow cells from murine mandible and tibiae were isolated and cultured for 4 and 6 days on plastic or 6 and 10 days on dentin. Osteoclastogenesis was assessed by counting the number of TRAP(+) multinucleated cells. Bone marrow cell composition was analyzed by FACS. The expression of osteoclast- and osteoclastogenesis-related genes was studied by qPCR. TRAP activity and resorptive activity of osteoclasts were measured by absorbance and morphometric analyses, respectively. At day 4 more osteoclasts were formed in long bone cultures than in jaw cultures. At day 6 the difference in number was no longer observed. The jaw cultures, however, contained more large osteoclasts on plastic and on dentin. Long bone marrow contained more osteoclast precursors, in particular the myeloid blasts, and qPCR revealed that the RANKL:OPG ratio was higher in long bone cultures. TRAP expression was higher for the long bone cultures on dentin. Although jaw osteoclasts were larger than long bone osteoclasts, no differences were found between their resorptive activities. In conclusion, bone marrow cells from different skeletal locations (jaw and long bone) have different dynamics of osteoclastogenesis. We propose that this is primarily due to differences in the cellular composition of the bone site-specific marrow.

Differences in matrix composition between calvaria and long bone in mice suggest differences in biomechanical properties and resorption: Special emphasis on collagen

The mammalian skeleton consists of bones that are formed in two different ways: long bones via endochondral ossification and flat bones via intramembranous ossification. These different formation modes may result in differences in the composition of the two bone types. Using the 2D-difference in gel electrophoresis technique and mass spectrometry, we analyzed the composition of murine mineral-associated proteins of calvaria and long bone. Considerable differences in protein composition were observed. Flat bones (calvariae) contained more soluble collagen (8x), pigment epithelium derived factor (3x) and osteoglycin (4x); whereas long bones expressed more chondrocalcin (3x), thrombospondin- 1 (4x), fetuin (4x), secreted phosphoprotein 24 (3x), and thrombin (7x). Although cystatin motifs containing proteins, such as secreted phosphoprotein 24 and fetuin are highly expressed in long bone, they did not inhibit the activity of the cysteine proteinases cathepsin B and K. The solubility of collagen differed which coincided with differences in collagen crosslinking, long bone containing 3x more (hydroxylysine)-pyridinoline. The degradation of long bone collagen by MMP2 (but not by cathepsin K) was impaired. These differences in collagen crosslinking may explain the differences in the proteolytic pathways osteoclasts use to degrade bone. Our data demonstrate considerable differences in protein composition of flat and long bones and strongly suggest functional differences in formation, resorption, and mechanical properties of these bone types.

Collagen maturity, glycation induced-pentosidine, and mineralization are increased following 3-year treatment with incadronate in dogs

Collagen cross-linking is a determinant of bone quality. A three-year treatment of bisphosphonate-incadronate disodium-in beagles increased degree of mineralization, collagen maturity, and pentosidine, a compound with advanced glycation end products. The treatment had no effect on the total amount of enzymatic cross-link formation.

INTRODUCTION

Collagen cross-linking is a determinant of bone quality. Recently, we reported that long-term treatment with bisphosphonate increased microdamage accumulation. The aim of this study was to clarify the effect of a three-year treatment with bisphosphonate on degree of mineralization and immature and mature enzymatic cross-links and non-enzymatic collagen cross-link, pentosidine, in cortical bone in the same dogs.

METHODS

Twenty-nine 1-year-old beagles (15 males, 14 females) were divided into three groups that daily were given vehicle or incadronate at doses of 0.3 or 0.6 mg/kg/day orally for three years. A cortex of a rib was fractionated into low- and high-density portions. The contents of calcium, phosphorus, enzymatic immature and mature cross-links, and the non-enzymatic glycation product pentosidine were determined in each fraction.

RESULTS

Calcium, phosphorus, and pentosidine contents and the ratio of mature to immature cross-links increased significantly with incadronate in a dose-dependent manner, but the total amount of enzymatic cross-links was unchanged. The pentosidine content correlated inversely with cortical activation frequency (p < 0.01).

CONCLUSION

Long-term suppression of bone remodeling by bisphosphonate increases degree of mineralization, collagen maturity, and non-enzymatic cross-linking.

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

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

Calvarial osteoclasts express a higher level of tartrate-resistant acid phosphatase than long bone osteoclasts and activation does not depend on cathepsin K or L activity

Bone resorption by osteoclasts depends on the activity of various proteolytic enzymes, in particular those belonging to the group of cysteine proteinases. Next to these enzymes, tartrate-resistant acid phosphatase (TRAP) is considered to participate in this process. TRAP is synthesized as an inactive proenzyme, and in vitro studies have shown its activation by cysteine proteinases. In the present study, the possible involvement of the latter enzyme class in the in vivo modulation of TRAP was investigated using mice deficient for cathepsin K and/or L and in bones that express a high (long bone) or low (calvaria) level of cysteine proteinase activity. The results demonstrated, in mice lacking cathepsin K but not in those deficient for cathepsin L, significantly higher levels of TRAP activity in long bone. This higher activity was due to a higher number of osteoclasts. Next, we found considerable differences in TRAP activity between calvarial and long bones. Calvarial bones contained a 25-fold higher level of activity than long bones. This difference was seen in all mice, irrespective of genotype. Osteoclasts isolated from the two types of bone revealed that calvarial osteoclasts expressed higher enzyme activity as well as a higher level of mRNA for the enzyme. Analysis of TRAP-deficient mice revealed higher levels of nondigested bone matrix components in and around calvarial osteoclasts than in long bone osteoclasts. Finally, inhibition of cysteine proteinase activity by specific inhibitors resulted in increased TRAP activity. Our data suggest that neither cathepsin K nor L is essential in activating TRAP. The findings also point to functional differences between osteoclasts from different bone sites in terms of participation of TRAP in degradation of bone matrix. We propose that the higher level of TRAP activity in calvarial osteoclasts compared to that in long bone cells may partially compensate for the lower cysteine proteinase activity found in calvarial osteoclasts and TRAP may contribute to the degradation of noncollagenous proteins during the digestion of this type of bone.

Degree of mineralization-related collagen crosslinking in the femoral neck cancellous bone in cases of hip fracture and controls

Based on the present definition of osteoporosis, both bone density and quality are important factors in the determination of bone strength. Collagen crosslinking is a determinant of bone quality. Cross-links can form enzymatically by the action of lysyl oxidase or non-enzymatically, resulting in advanced glycation end products. Collagen crosslinking is affected by tissue maturation as well as the degree of mineralization. Homocysteine and vitamin B6 (pyridoxal) are also regulatory factors of collagen crosslinking. We elucidate the relationship between the degree of mineralization and collagen cross-links in cancellous bone from hip fracture cases. We also determined plasma levels of homocysteine and pyridoxal. Twenty-five female intracapsular hip fracture cases (78 +/- 6 years) and 25 age-matched postmortem controls (77 +/- 6 years) were included in this study. Collagen crosslinking was analyzed after each bone specimen was fractionated into low (1.7-2.0 g/ml) and high (>2.0 g/ml) density fractions. The content of enzymatic (immature reducible and mature nonreducible cross-links) and nonenzymatic cross-link (pentosidine) were determined. In the controls, there was no difference in total enzymatic cross-links between low and high density bone, while pentosidine content was significantly higher in high density bone. In the fracture cases, not only reduced enzymatic cross-links in high density bone and increased pentosidine in both low and high density bone, but also higher plasma homocysteine and lower pyridoxal levels were evident compared with the controls. These results indicate that detrimental crosslinking in both low and high mineralized bone result in impaired bone quality in osteoporotic patients.

Adsorption-induced fibronectin aggregation and fibrillogenesis

Fibronectin (Fn), a high molecular weight glycoprotein, is a central element of extracellular matrix architecture that is involved in several fundamental cell processes. In the context of bone biology, little is known about the influence of the mineral surface on fibronectin supramolecular assembly. We investigate fibronectin morphological properties induced by its adsorption onto a model mineral matrix of hydroxyapatite (HA). Fibronectin adsorption onto HA spontaneously induces its aggregation and fibrillation. In some cases, fibronectin fibrils are even found connected into a dense network that is close to the matrix synthesized by cultured cells. Fibronectin adsorption-induced self-assembly is a time-dependant process that is sensitive to bulk concentration. The N-terminal domain of the protein, known to be implicated in its self-association, does not significantly inhibit the protein self-assembly while increasing ionic strength in the bulk alters both aggregation and fibrillation. The addition of a non-ionic surfactant during adsorption tends to promote aggregation with respect to fibrillation. Ultimately, fibronectin fibrils appear to be partially structured like amyloid fibrils as shown by thioflavine T staining. Taken together, our results suggest that there might be more than one single organization route involved in fibronectin self-assembly onto hydroxyapatite. The underlying mechanisms are discussed with respect to Fn conformation, Fn/surface and Fn/Fn interactions, and a model of fibronectin fibrillogenesis onto hydroxyapatite is proposed.

Reductions in degree of mineralization and enzymatic collagen cross-links and increases in glycation-induced pentosidine in the femoral neck cortex in cases of femoral neck fracture

INTRODUCTION

Enzymatic and glycation-induced nonenzymatic cross-links play important roles in the expression of bone strength. The cross-link pattern is affected by tissue maturation and senescence. The aim of our study was to understand the distinctive posttranslational modifications of collagen in areas with different degrees of mineralization with and without hip fracture.

METHODS

Sixteen female cases of intracapsular hip fracture (78+/-6 years) and 16 age- and gender-matched postmortem controls (76+/-6 years) were included in this study. A sample of each femoral neck cortex was fractionated into low (1.7 to 2.0 g/ml) and high (>2.0 g/ml) density portions. The contents of enzymatic cross-links (dihydroxylysinonorleucine, hydroxylysinonorleucine, lysinonorleucine, pyridinoline, and deoxypyridinoline) and nonenzymatic cross-links (pentosidine) and the extent of lysine (Lys) hydroxylation were determined in each fraction.

RESULTS

In the controls, there was no significant difference in the contents of enzymatic cross-links between low- and high-mineralized bone fractions whereas pentosidine content was significantly higher in high-mineralized bone compared with low-mineralized bone (p=0.0014). When comparing enzymatic cross-link contents between controls and fracture cases, a trend toward lower (p=0.0961) cross-link content in low-mineralized bone and a significant reduction (p<0.0001) in high-mineralized bone were observed. Pentosidine content of low-mineralized bone was significantly higher in fracture cases than in controls (p<0.0001). The extent of Lys hydroxylation was significantly higher in fracture cases than in controls (p<0.001). The higher hydroxylation of Lys in collagen from fracture cases relative to controls was associated with significantly higher values of hydroxylysine-derived cross-link such that the enzymatic cross-link patterns correlated with the extent of Lys hydroxylation in the collagen molecules.

CONCLUSIONS

These results suggest that reductions in the degree of mineralization and enzymatic cross-links and excessive formation of pentosidine may play an important role in explaining poor bone quality in osteoporosis.

FGF upregulates osteopontin in epiphyseal growth plate chondrocytes: Implications for endochondral ossification

Fibroblast growth factor receptor 3 (FGFR3) signaling pathways are essential for normal longitudinal bone growth. Mutations in this receptor lead to various human growth disorders, including Achondroplasia, disproportionately short-limbed dwarfism, characterized by narrowing of the hypertrophic region of the epiphyseal growth plates. Here we find that FGF9, a preferred ligand for FGFR3 rapidly induces the upregulation and secretion of the matrix resident phosphoprotein, osteopontin (OPN) in cultured chicken chondrocytes. This effect was observed as early as two hours post stimulation and at FGF9 concentrations as low as 1.25 ng/ml at both mRNA and protein levels. OPN expression is known to be associated with chondrocyte and osteoblast differentiation and osteoclast activation. Unexpectedly, FGF9 induced OPN was accompanied by inhibition of differentiation and increased proliferation of the treated chondrocytes. Moreover, FGF9 stimulated OPN expression irrespective of the differentiation stage of the cells or culture conditions. In situ hybridization analysis of epiphyseal growth plates from chicken or mice homozygous for the Achondroplasia, G369C/mFGFR3 mutation demonstrated co-localization of OPN expression and osteoclast activity, as evidenced by tartarate resistant acid phosphatase positive cells in the osteochondral junction. We propose that FGF signaling directly activates OPN expression independent of chondrocytes differentiation. This may enhance the recruitment and activation of osteoclasts, and increase in cartilage resorption and remodeling in the chondro-osseus border.

Osteocrin, a novel bone-specific secreted protein that modulates the osteoblast phenotype

Although a number of secreted factors have been demonstrated to be bone regulators, none of these are unique to bone. Using a viral-based signal-trap strategy we have identified a novel gene we have termed "osteocrin." A 1280-bp mRNA encodes osteocrin producing a mature protein of 103 amino acids with a molecular mass of 11.4 kDa. Osteocrin shows no homology with any known gene except for two conserved sequence motifs reminiscent of dibasic cleavage sites found in peptide hormone precursors. Immunofluorescence and Western blot analysis confirmed the secretory nature of osteocrin. Two protein species were identified in the medium of cells overexpressing osteocrin, a full-length 11.4 kDa species and a processed approximately 5 kDa species. Mutation of the 76KKKR79 dibasic cleavage site abolished the appearance of this smaller osteocrin fragment. By in situ hybridization in mouse embryos, osteocrin was expressed specifically in Cbfa-1-positive, osteocalcin-negative osteoblasts. Immunohistochemistry on adult mouse bone showed osteocrin localization in osteoblasts and young osteocytes. By Northern blot analysis, osteocrin expression was only detected in bone, expression peaking just after birth and decreasing markedly with age. In primary osteoblastic cell cultures osteocrin expression coincided with matrix formation then decreased in very mature cultures. Treatment of cultures with 1,25-dihydroxyvitamin D3 resulted in a rapid dose-dependent down-regulation of osteocrin expression, suggesting direct regulation. Chronic treatment of primary cultures with osteocrin-conditioned media inhibited mineralization and reduced osteocalcin and alkaline phosphatase expression. These results suggest that osteocrin represents a novel, unique vitamin D-regulated bone-specific protein that appears to act as a soluble osteoblast regulator.

Basic fibroblast growth factor enhances in vitro mineralization of rat bone marrow stromal cells grown on non-woven hyaluronic acid based polymer scaffold

A biodegradable non-woven hyaluronic acid polymer scaffold (Hyaff 11) was analysed in vitro as a carrier vehicle for differentiation and mineralization of rat bone marrow stromal cells (BMSC). BMSC were grown on Hyaff 11 in a mineralizing medium in the presence/absence of basic fibroblast growth factor (bFGF). Osteoblastic differentiation was investigated by light and electron microscopy analysing the expression of osteogenic markers: calcium, alkaline phosphatase (AP), osteopontin (OP), bone sialoprotein (BSP) and collagen type 1. We also measured proliferation, AP activity and mRNA expression of AP and osteocalcin (OC). Electron microscopy and Toluidine-blue staining demonstrated that bFGF accelerated (day 20 vs. day 40) and increased mineralization. With bFGF, calcium, OP and BSP were strongly enhanced at day 40, whereas AP decreased. Our in vitro results demonstrate that Hyaff 11 is a useful vehicle for growth, differentiation and mineralization of rat BMSC, and that it permits bone development.