Soluble silica inhibits osteoclast formation and bone resorption in vitro

Intrafibrillar silicified collagen scaffold promotes in-situ bone regeneration by activating the monocyte p38 signaling pathway

Intrafibrillar silicified collagen scaffold (SCS) is a promising biomaterial for bone regeneration because it promotes cell homing and angiogenesis in bone defects via monocyte modulation. In the present study, a rat femoral defect model was used to examine the contribution of monocyte signaling pathways to SCS modulation. Activation of the monocyte p38 signaling pathway by SCS resulted in monocyte differentiation into TRAP-positive mononuclear cells. These cells demonstrated increased secretion of SDF-1α, VEGFa and PDGF-BB, which, in turn, promoted homing of bone marrow stromal cells (BMSCs) and endothelial progenitor cells (EPCs), as well as local vascularization. Monocyte differentiation and secretion were blocked after inhibition of the p38 pathway, which resulted in reduction in cell homing and angiogenesis. Taken together, these novel findings indicate that the p38 signaling pathway is crucial in SCS-modulated monocyte differentiation and secretion, which has a direct impact on SCS-induced bone regeneration.

STATEMENT OF SIGNIFICANCE

Intrafibrillar silicified collagen scaffold (SCS) is a promising biomaterial for bone regeneration. The present work demonstrates that SCS possesses favorable bone regeneration potential in a rat femoral defect model. The degrading scaffold modulates monocyte differentiation and release of certain cytokines to recruit MSCs and EPCs, as well as enhances local vascularization by activating the p38 MAPK signaling pathway. These findings indicate that SCS contributes to bone defect regeneration by stimulating host cell homing and promoting local angiogenesis and osteogenesis without the need for loading cytokines or xenogenous stem cells.

Novel Reduced Graphene Oxide/Zinc Silicate/Calcium Silicate Electroconductive Biocomposite for Stimulating Osteoporotic Bone Regeneration

In the absence of external assistance, autogenous healing of bone fracture is difficult due to impaired regeneration ability under osteoporosis pathological conditions. In this study, a reduced graphene oxide/zinc silicate/calcium silicate (RGO/ZS/CS) conductive biocomposite with an optimal surface electroconductivity of 5625 S/m was prepared by a two-step spin-coating method. The presence of lamellar apatite nanocrystals on the surfaces of the biocomposite suggests that it has good in vitro biomineralization ability. The silicon and zinc released from the biocomposite induced a significant increase in the osteogenesis of mouse bone mesenchymal stem cells (mBMSCs). Furthermore, alkaline phosphatase activities were further promoted when 3 μA direct current was applied to stimulate the mBMSCs that were cultured on the RGO/ZS/CS surface. However, electrical stimulation failed to further upregulate the osteogenesis-related gene expression. Moreover, RGO/ZS/CS extracts were found to suppress the receptor activator of nuclear factor-κB ligand-induced osteoclastic differentiation of mouse leukemic monocyte macrophages (RAW264.7 cells). Although the zinc ions in the RGO/ZS/CS extracts showed an inhibitory role in human umbilical vein endothelial cell (HUVEC) proliferation, dilutions of the RGO/ZS/CS extracts (1/16, 1/32, and 1/64) promoted HUVEC proliferation, and their angiogenesis-related gene expression was also upregulated. On the basis of the results of the in vitro angiogenesis model, more interconnected tubes formed when the above dilutions of RGO/ZS/CS extracts were added to ECMatrix. The new RGO/ZS/CS electroconductive biocomposite has potential to be used for stimulating osteoporotic bone regeneration.

Silicon bioceramic loaded with vancomycin stimulates bone tissue regeneration

Porous ceramics doped with silicon and pure β-TCP were analyzed in terms of internal microstructure, cell behavior, and the percentage of newly formed bone. Additionally the materials were tested to determine which of the two had better properties to load and release vancomycin hydrochloride. Internal pore distribution and porosity were determined through high pressure mercury porosimetry and the specific surface area was measured by the Brunauer Emmet-Teller method. The proliferation and viability of the human osteoblast-like cell line MG-63 was studied to validate both materials. The materials were tested on eight New Zealand rabbits which created defects, 10 mm in diameter, in the calvaria bone. After 8 and 12 weeks a histological and histomorphometric analysis was performed. Si-β-TCP showed a higher porosity and specific surface area. The cytocompatibility test revealed acceptable results in terms of proliferation and viability whereas the percentage of new bone was higher in Si-β-TCP with a two-time study being statistically significant with 12 weeks of healing (p < 0.05).The vancomycin loaded within the ceramic scaffolds were burst released and the material had the ability to inhibit bacterial growth. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 2307-2315, 2018.

Exploratory Testing of Diatom Silica to Map the Role of Material Attributes on Cell Fate

Porous silica is an attractive biomaterial in many applications, including drug-delivery systems, bone-graft fillers and medical devices. The issue with porous silica biomaterials is the rate at which they resorb and the significant role played by interfacial chemistry on the host response in vivo. This paper explores the potential of diatom-biosilica as a model tool to assist in the task of mapping and quantifying the role of surface topography and chemical cues on cell fate. Diatoms are unicellular microalgae whose cell walls are composed of, amorphous nanopatterned biosilica that cannot be replicated synthetically. Their unique nanotopography has the potential to improve understanding of interface reactions between materials and cells. This study used Cyclotella meneghiniana as a test subject to assess cytotoxicity and pro-inflammatory reactions to diatom-biosilica. The results suggest that diatom-biosilica is non-cytotoxic to J774.2 macrophage cells, and supports cell proliferation and growth. The addition of amine and thiol linkers have shown a significant effect on cytotoxicity, growth and cytokine response, thus warranting further investigation into the interfacial effects of small chemical modifications to substrate surfaces. The overall findings suggest diatom-biosilica offers a unique platform for in-depth investigation of the role played by nanotopography and chemistry in biomedical applications.

Effects of MgO and SiO2 on Plasma-Sprayed Hydroxyapatite Coating: An in Vivo Study in Rat Distal Femoral Defects

Plasma-sprayed hydroxyapatite (HA)-coated titanium implants have been widely used in orthopedic applications due to their inheritance of an excellent mechanical property from titanium and great osteoconductivity from HA. However, the lack of osteoinductivity limits their further applications. In this study, 1 wt % MgO and 0.5 wt % SiO₂ were mixed with HA for making plasma-sprayed coatings on titanium implants. Plasma-sprayed HA- and MgO/SiO₂-HA-coated titanium implants showed adhesive bond strengths of 25.73 ± 1.92 and 23.44 ± 2.89 MPa, respectively. The presence of MgO and SiO₂ significantly increased the osteogenesis, osseointegration, and bone mineralization of HA-coated titanium implants by the evaluation of their histomorphology after 6, 10, and 14 weeks of implantation in rat distal femoral defects. Implant pushout tests also showed a shear modulus of 149.83 ± 3.69 MPa for MgO/SiO₂-HA-coated implants after 14 weeks of implantation, compared to 52.68 ± 10.41 MPa for uncoated implants and 83.92 ± 3.68 MPa for pure HA-coated implants; These are differences in the shear modulus of 96% and 56.4%, respectively. This study assesses for the first time the quality of the bone-implant interface of induction plasma-sprayed MgO and SiO₂ binary-doped HA coatings on load-bearing implants compared to bare titanium and pure HA coatings in a quantitative manner. Relating the osseointegration and interface shear modulus to the quality of implant fixation is critical to the advancement and implementation of HA-coated orthopedic implants.

Cell behavior of human mesenchymal stromal cells in response to silica/collagen based xerogels and calcium deficient culture conditions

Herein, we aim to elucidate osteogenic effects of two silica-based xerogels with different degrees of bioactivity on human bone-derived mesenchymal stromal cells by means of scanning electron microscopy, quantitative PCR enhanced osteogenic effects and the formation of an extracellular matrix which could be ascribed to the sample with lower bioactivity. Given the high levels of bioactivity, the cells revealed remarkable sensitivity to extremely low calcium levels of the media. Therefore, additional experiments were performed to elucidate cell behavior under calcium deficient conditions. The results refer to capacity of the bone-derived stromal cells to overcome calcium deficiency even though proliferation, migration and osteogenic differentiation capabilities were diminished. One reason for the differences of the cellular response (on tissue culture plates versus xerogels) to calcium deficiency seems to be the positive effect of silica. The silica could be detected intracellularly as shown by time of flight-secondary ion mass spectrometry after cultivation of primary cells for 21 days on the surfaces of the xerogels. Thus, the present findings refer to different osteogenic differentiation potentials of the xerogels according to the different degrees of bioactivity, and to the role of silica as a stimulator of osteogenesis. Finally, the observed pattern of connexin-based hemichannel gating supports the assumption that connexin 43 is a key factor for calcium-mediated osteogenesis in bone-derived mesenchymal stromal cells.

Anti-osteoclastogenesis of Mineral Trioxide Aggregate through Inhibition of the Autophagic Pathway

INTRODUCTION

Mineral trioxide aggregate (MTA) regulates bone remodeling, particularly osteoclast differentiation. However, intracellular mechanisms underlying the anti-osteoclastogenesis of MTA remain unclear. This study aimed to evaluate the potential alterations of autophagic pathway during anti-osteoclastogenic effects by MTA in vitro and investigate their underlying mechanisms.

METHODS

Osteoclast precursors were treated with MTA extracts containing the receptor activator of nuclear factor-kappa B ligand (RANKL). Rapamycin was used to activate autophagy. RANKL-induced osteoclast differentiation was stained with tartrate-resistant acid phosphatase. Several specific autophagy features in osteoclast precursors were measured by using immunofluorescence, monodansylcadaverine, and transmission electron microscope. Autophagy-related proteins were investigated via Western blot analysis. The mRNA expression involved in autophagic and osteoclastic activities was detected with quantitative real-time polymerase chain reaction.

RESULTS

MTA extracts inhibited osteoclast differentiation via preventing the fusion of osteoclast precursors without cytotoxicity. MTA extracts interrupted RANKL-induced acidic vesicular organelle formation and autophagic vacuole appearance in osteoclast precursors. Moreover, autophagic genes and proteins stimulated with RANKL diminished with MTA extracts. Notably, autophagy activation through rapamycin promoted multinucleated osteoclasts formation and increased osteoclastic genes expression, which almost reversed MTA-mediated anti-osteoclastogenic effects.

CONCLUSIONS

MTA inhibited osteoclastogenesis for bone repair through attenuating the autophagic pathway.

Orthosilicic acid, Si(OH)4, stimulates osteoblast differentiation in vitro by upregulating miR-146a to antagonize NF-κB activation

Accumulating evidence over the last 40years suggests that silicate from dietary as well as silicate-containing biomaterials is beneficial to bone formation. However, the exact biological role(s) of silicate on bone cells are still unclear and controversial. Here, we report that orthosilicic acid (Si(OH)4) stimulated human mesenchymal stem cells (hMSCs) osteoblastic differentiation in vitro. To elucidate the possible molecular mechanisms, differential microRNA microarray analysis was used to show that Si(OH)4 significantly up-regulated microRNA-146a (miR-146a) expression during hMSC osteogenic differentiation. Si(OH)4 induced miR-146a expression profiling was further validated by quantitative RT-PCR (qRT-PCR), which indicated miR-146a was up-regulated during the late stages of hMSC osteogenic differentiation. Inhibition of miR-146a function by anti-miR-146a suppressed osteogenic differentiation of MC3T3 pre-osteoblasts, whereas Si(OH)4 treatment promoted osteoblast-specific genes transcription, alkaline phosphatase (ALP) production, and mineralization. Furthermore, luciferase reporter assay, Western blotting, enzyme-linked immunosorbent assay (ELISA), and immunofluorescence showed that Si(OH)4 decreased TNFα-induced activation of NF-κB, a signal transduction pathway that inhibits osteoblastic bone formation, through the known miR-146a negative feedback loop. Our studies established a mechanism for Si(OH)4 to promote osteogenesis by antagonizing NF-κB activation via miR-146a, which might be interesting to guide the design of osteo-inductive biomaterials for treatments of bone defects in humans.

STATEMENT OF SIGNIFICANCE

Accumulating evidence over 40years suggests that silicate is beneficial to bone formation. However, the biological role(s) of silicate on bone cells are still unclear and controversial. Here, we report that Si(OH)4, the simplest form of silicate, can stimulate human mesenchymal stem cells osteoblastic differentiation. We identified that miR-146a is the expression signature in bone cells treated with Si(OH)4. Further analysis of miR-146a in bone cells reveals that Si(OH)4 upregulates miR-146a to antagonize the activation of NF-κB. Si(OH)4 was also shown to deactivate the same NF-κB pathway to suppress osteoclast formation. Our findings are important to the development of third-generation cell-and gene affecting biomaterials, and suggest silicate and miR-146a can be used as pharmaceuticals for bone fracture prevention and therapy.

Nanoscale bioactive glass activates osteoclastic differentiation of RAW 264.7 cells

BACKGROUND

There is limited knowledge regarding differentiation of osteoclasts in the presence of nanoscale bioactive glass (nBG). This investigation examined increasing concentrations of 45S5 nBG and their influence on osteoclast differentiation.

MATERIALS & METHODS

Different concentrations of 45S5 nBG were cultured up to 14 days with the murine RAW264.7 cell line and human primary monocytes cultured with M-CSF and RANKL.

RESULTS

Culturing cells for 14 days with 500 μg/ml nBG showed a viability of 100%; however DNA synthesis was reduced, supporting differentiation into osteoclast-like cells. Using RAW cells, activation of nine genes, including cell fusion genes, occurred in an nBG concentration dependent manner. Low concentrations of nBG increased expression of genes involved in commitment to cell fusion, whereas high concentrations increased gene expression supporting osteoclast-like differentiation.

CONCLUSION

nBG enhances both RAW264.7 and human osteoclast differentiation. nBG controlled gene expression in a concentration dependent manner could reflect normal regulation during bone growth.

Akermanite bioceramics promote osteogenesis, angiogenesis and suppress osteoclastogenesis for osteoporotic bone regeneration

It is a big challenge for bone healing under osteoporotic pathological condition with impaired angiogenesis, osteogenesis and remodeling. In the present study, the effect of Ca, Mg, Si containing akermanite bioceramics (Ca2MgSi2O7) extract on cell proliferation, osteogenic differentiation and angiogenic factor expression of BMSCs derived from ovariectomized rats (BMSCs-OVX) as well as the expression of osteoclastogenic factors was evaluated. The results showed that akermanite could enhance cell proliferation, ALP activity, expression of Runx2, BMP-2, BSP, OPN, OCN, OPG and angiogenic factors including VEGF and ANG-1. Meanwhile, akermanite could repress expression of osteoclastogenic factors including RANKL and TNF-α. Moreover, akermanite could activate ERK, P38, AKT and STAT3 signaling pathways, while crosstalk among these signaling pathways was evident. More importantly, the effect of akermanite extract on RANKL-induced osteoclastogenesis was evaluated by TRAP staining and real-time PCR assay. The results showed that akermanite could suppress osteoclast formation and expression of TRAP, cathepsin K and NFATc1. The in vivo experiments revealed that akermanite bioceramics dramatically stimulated osteogenesis and angiogenesis in an OVX rat critical-sized calvarial defect model. All these results suggest that akermanite bioceramics with the effects of Mg and Si ions on osteogenesis, angiogenesis and osteoclastogenesis are promising biomaterials for osteoporotic bone regeneration.

Strontium-substituted bioactive glasses in vitro osteogenic and antibacterial effects

OBJECTIVES

Bioactive glass forms a bone mineral apatite interface and can be engineered to promote optimal bone regeneration. Strontium (Sr(2+)) stimulates osteoblast and inhibits osteoclast activities in vitro, and is used clinically as a treatment for osteoporosis. Dental bone defect repair requires rapid bone formation for early osseointegration but, can be subject to infection. The aim of this study was to investigate the osteogenic and antibacterial effects of strontium-substituted bioactive glasses in vitro.

METHODS

Strontium-substituted bioactive glasses were designed and produced. Then the osteogenic potential and antibacterial effects of bioactive glass particulates were explored.

RESULTS

Alkaline phosphatase activity, cell number, Type I collagen and mineral nodule formation of MC3T3-E1 cells were significantly promoted by the 5% strontium-substituted glass (5Sr). Furthermore, after incubation with 0.001g and 0.01g glass particulates, the growth of sub-gingival bacteria, Aggregatibacter actinomycetemcomitans and Porphyromonas gingivalis was significantly inhibited; the antibacterial activity being dependent on the percentage of strontium in the glasses.

SIGNIFICANCE

These results show that strontium-substituted bioactive glasses significantly promote osteogenic responses of MC3T3-E1 osteoblast-like cells and inhibit the growth of A. actinomycetemcomitans and P. gingivalis.

Ductile silica/methacrylate hybrids for bone regeneration

Hybrids consisting of co-networks of high cross-linking density polymethacrylate and silica (class II hybrid) were synthesised as a potential new generation of scaffold materials.

Bone Anabolic Effects of Soluble Si: In Vitro Studies with Human Mesenchymal Stem Cells and CD14+ Osteoclast Precursors

Silicon (Si) is indispensable for many cellular processes including bone tissue metabolism. In this work, the effects of Si on human osteogenesis and osteoclastogenesis were characterized. Human mesenchymal stem cells (hMSC) and CD14+ stem cells, as osteoblast and osteoclast precursors, were treated with a wide range of Si concentrations, covering the physiological plasma levels. Si promoted a dose-dependent increase in hMSC proliferation, differentiation, and function, at levels similar to the normal basal plasma levels. Additionally, a decrease in the expression of the osteoclastogenic activators M-CSF and RANKL was observed. Also, Si elicited a decrease in osteoclastogenesis, which became significant at higher concentrations, as those observed after meals. Among the intracellular mechanisms studied, an upregulation of MEK and PKC signalling pathways was observed in both cell types. In conclusion, Si appears to have a direct positive effect on human osteogenesis, at basal plasma levels. On the other hand, it also seemed to be an inhibitor of osteoclastogenesis, but at higher concentrations, though yet in the physiological range. Further, an indirect effect of Si on osteoclastogenesis may also occur, through a downregulation of M-CSF and RANKL expression by osteoblasts. Thus, Si may be an important player in bone anabolic regenerative approaches.

Investigating the addition of SiO2–CaO–ZnO–Na2O–TiO2 bioactive glass to hydroxyapatite: Characterization, mechanical properties and bioactivity

Hydroxyapatite (Ca10(PO4)6(OH)2) is widely investigated as an implantable material for hard tissue restoration due to its osteoconductive properties. However, hydroxyapatite in bulk form is limited as its mechanical properties are insufficient for load-bearing orthopedic applications. Attempts have been made to improve the mechanical properties of hydroxyapatite, by incorporating ceramic fillers, but the resultant composite materials require high sintering temperatures to facilitate densification, leading to the decomposition of hydroxyapatite into tricalcium phosphate, tetra-calcium phosphate and CaO phases. One method of improving the properties of hydroxyapatite is to incorporate bioactive glass particles as a second phase. These typically have lower softening points which could possibly facilitate sintering at lower temperatures. In this work, a bioactive glass (SiO2–CaO–ZnO–Na2O–TiO2) is incorporated (10, 20 and 30 wt%) into hydroxyapatite as a reinforcing phase. X-ray diffraction confirmed that no additional phases (other than hydroxyapatite) were formed at a sintering temperature of 560 ℃ with up to 30 wt% glass addition. The addition of the glass phase increased the % crystallinity and the relative density of the composites. The biaxial flexural strength increased to 36 MPa with glass addition, and there was no significant change in hardness as a function of maturation. The pH of the incubation media increased to pH 10 or 11 through glass addition, and ion release profiles determined that Si, Na and P were released from the composites. Calcium phosphate precipitation was encouraged in simulated body fluid with the incorporation of the bioactive glass phase, and cell culture testing in MC-3T3 osteoblasts determined that the composite materials did not significantly reduce cell viability.

Biphasic silica/apatite co-mineralized collagen scaffolds stimulate osteogenesis and inhibit RANKL-mediated osteoclastogenesis

The effects of a biphasic mineralized collagen scaffold (BCS) containing intrafibrillar silica and apatite on osteogenesis of mouse mesenchymal stem cells (mMSCs) and inhibition of receptor activator of nuclear factor κB ligand (RANKL)-mediated osteoclastogenesis were investigated in the present study. mMSCs were cultured by exposing to BCS for 7 days for cell proliferation/viability examination, and stimulated to differentiate in osteogenic medium for 7-21 days for evaluation of alkaline phosphatase activity, secretion of osteogenic deposits and expression of osteoblast lineage-specific phenotypic markers. The effect of BCS-conditioned mMSCs on osteoclastogenesis of RAW 264.7 cells was evaluated by tartrate-resistant acid phosphatase staining and resorption pit analysis. The contributions of mitogen-activated protein kinase (MAPK) and phosphatidylinositol-3 kinase (PI3K) signal transduction pathways to osteogenesis of mMSCs and their osteoprotegerin (OPG) and RANKL expressions were also evaluated. Compared with unmineralized, intrafibrillarly-silicified or intrafibrillarly-calcified collagen scaffolds, BCS enhanced osteogenic differentiation of mMSCs by activation of the extracellular signal regulated kinases (ERK)/MAPK and p38/MAPK signaling pathways. After mMSCs were exposed to BCS, they up-regulated OPG expression and down-regulated RANKL expression through activation of the p38/MAPK and PI3K/protein kinase B (Akt) pathways, resulting in inhibition of the differentiation of RAW 264.7 cells into multinucleated osteoclasts and reduction in osteoclast function. These observations collectively suggest that BCS has the potential to be used in bone tissue engineering when the demand for anabolic activities is higher than catabolic metabolism during the initial stage of wound rehabilitation.

Zebrafish: A possible tool to evaluate bioactive ions

Zebrafish is a well-established model organism with a skeletal structure that highly resembles mammalian bone. Yet its use in the research field of biomaterials has been limited. One area that could benefit from this model system is the evaluation of ionic dissolution products from different materials. As a proof of concept we have evaluated the effect of silicate ions on the zebrafish larvae and compared it to a well-known osteoblastic cell line, MC3T3-E1 subclone 14. We have shown that sodium metasilicate (125 μM and 625 μM) induces more mineralisation in a dose-dependent manner in zebrafish larvae, 9 days post fertilisation as compared to the non-treated group. Moreover the same trends were seen when adding sodium metasilicate to MC3T3-E1 cultures, with more mineralisation and higher ALP levels with higher doses of silicate (25, 125 and 625 μM). These results indicate the feasibility of zebrafish larvae for ionic dissolution studies. The zebrafish model is superior to isolated cell cultures in the aspect that it includes the whole bone remodelling system, with osteoblasts, osteoclasts and osteocytes. Zebrafish could thus provide a powerful in vivo tool and be a bridge between cell culture systems and mammalian models.

Effect of BioAggregate on Receptor Activator of Nuclear Factor-Kappa B Ligand-induced Osteoclastogenesis from Murine Macrophage Cell Line In Vitro

INTRODUCTION

This study aimed to investigate the effect of BioAggregate, a calcium silicate-based nanoparticulate bioceramic, on the regulation of receptor activator of nuclear factor-kappa B ligand (RANKL)-induced osteoclast differentiation and bone resorption in vitro, as well as to delineate the underlying molecular mechanism. The performance of BioAggregate was compared with that of ProRoot mineral trioxide aggregate (MTA).

METHODS

Cells of a murine macrophage cell line RAW 264.7 were treated with various concentrations of BioAggregate and MTA extracts. Cytotoxicity of material extracts was evaluated with Cell Counting Kit-8 assay. RANKL-induced osteoclast differentiation and function were assessed with tartrate-resistant acid phosphatase staining, F-actin staining, and lacunar resorption pits assay. The mRNA expression associated with osteoclast function was detected with quantitative real-time polymerase chain reaction. Related molecular signaling pathways were investigated with Western blot and immunofluorescence.

RESULTS

BioAggregate extracts dose-dependently inhibited RANKL-induced osteoclast formation and resorption capacity without evident cytotoxicity. RAW 264.7 cells exposed to BioAggregate extracts also presented a decrease in RANKL-stimulated mRNA expression of osteoclast-related genes and transcription factors. Moreover, cells treated with BioAggregate extracts exhibited attenuated TRAF6 expression, suppressed mitogen-activated protein kinase signaling, and decreased nuclear translocation of NFATc1 and c-Fos in the presence of RANKL. Comparable effects were induced by MTA.

CONCLUSIONS

BioAggregate and MTA exhibit comparable inhibitory effect on osteoclast differentiation and function in vitro, and our findings provide valuable insights into the mechanism of bioceramic-mediated anti-osteoclastogenic activity.

Silicon: the evolution of its use in biomaterials

In the 1970s, several studies revealed the requirement for silicon in bone development, while bioactive silicate glasses simultaneously pioneered the current era of bioactive materials. Considerable research has subsequently focused on the chemistry and biological function of silicon in bone, demonstrating that the element has at least two separate effects in the extracellular matrix: (i) interacting with glycosaminoglycans and proteoglycans during their synthesis, and (ii) forming ionic substitutions in the crystal lattice structure of hydroxyapatite. In addition, the dissolution products of bioactive glass (predominantly silicic acids) have significant effects on the molecular biology of osteoblasts in vitro, regulating the expression of several genes including key osteoblastic markers, cell cycle regulators and extracellular matrix proteins. Researchers have sought to capitalize on these effects and have generated a diverse array of biomaterials, which include bioactive glasses, silicon-substituted hydroxyapatites and pure, porosified silicon, but all these materials share similarities in the mechanisms that result in their bioactivity. This review discusses the current data obtained from original research in biochemistry and biomaterials science supporting the role of silicon in bone, comparing both the biological function of the element and analysing the evolution of silicon-containing biomaterials.

Effect of BioAggregate on osteoclast differentiation and inflammatory bone resorption in vivo

AIM

To investigate the effect of BioAggregate (Innovative Bioceramix, Vancouver, BC, Canada) on lipopolysaccharide (LPS)-induced bone destruction in vivo and to compare its performance with that of mineral trioxide aggregate (MTA; Dentsply Tulsa Dental, Tulsa, OK, USA).

METHODOLOGY

Mouse bone marrow macrophages (BMMs) were primary cultured and treated with several concentrations of BioAggregate and MTA extracts. Cell viability was measured with a Cell Counting Kit-8 assay, whilst in vitro osteoclast differentiation was evaluated with tartrate-resistant acid phosphatase (TRAP) staining. LPS-induced mouse calvarial bone destruction model was established to assess the effect of BioAggregate and MTA extracts in vivo. Mice were killed on day 7, and calvarial bones were prepared for microcomputed tomography scanning, histologic analysis and double-immunofluorescence staining. Statistical tests used were one-way anova followed by Student-Newman-Keuls test.

RESULTS

BioAggregate extracts displayed no obvious cytotoxicity to BMMs and significantly inhibited (P < 0.01) the differentiation of RANKL-stimulated BMMs. Comparable effects were induced by MTA. BioAggregate and MTA extracts markedly reduced (P < 0.01) osteoclast numbers and attenuated (P < 0.05) bone resorption in LPS-challenged mouse calvaria. The expression levels of osteoclastogenic cathepsin K and its upstream regulator nuclear factor of activated T-cell cytoplasmic 1 and c-Fos were also decreased by BioAggregate and MTA extracts.

CONCLUSIONS

BioAggregate and MTA showed comparable inhibitory effect on osteoclast differentiation and inflammatory bone resorption in vivo.

Regulation of the biological functions of osteoblasts and bone formation by Zn-incorporated coating on microrough titanium

To improve the biological performance of titanium implant, a series of Zn-incorporated coatings were fabricated on the microrough titanium (Micro-Ti) via sol-gel method by spin-coating technique. The successful fabrication of the coating was verified by combined techniques of scanning electron microscopy, surface profiler, X-ray diffraction, X-ray photoelectron spectroscopy, and water contact angle measurements. The incorporated zinc existed as ZnO, which released Zn ions in a sustained manner. The Zn-incorporated samples (Ti-Zn0.08, Ti-Zn0.16, and Ti-Zn0.24) efficiently inhibited the adhesion of both Gram-positive (Staphylococcus aureus) and Gram-negative (Pseudomonas aeruginosa) bacteria. The in vitro evaluations including cell activity, alkaline phosphatase (ALP), mineralization, osteogenic genes expressions (Runx2, ALP, OPG, Col I, OPN, and OC), and tartrate-resistant acid phosphatase, confirmed that Ti-Zn0.16 sample was the optimal one to regulate the proliferation or differentiation for both osteoblasts and osteoclasts. More importantly, in vivo evaluations including Micro-CT analysis, push-out test, and histological observations verified that Ti-Zn0.16 implants could efficiently promote new bone formation after implantation for 4 and 12 weeks, respectively. The resulting material thus has potential application in orthopedic field.

Nanocrystalline silicon substituted hydroxyapatite effects on osteoclast differentiation and resorptive activity

In the present study, the effects of nanocrystalline hydroxyapatite (nano-HA) and nanocrystalline Si-substituted hydroxyapatite (nano-SiHA) on osteoclast differentiation and resorptive activity have been evaluated in vitro using osteoclast-like cells. The action of these materials on proinflammatory and reparative macrophage populations was also studied. Nano-SiHA disks delayed the osteoclast differentiation and decreased the resorptive activity of these cells on their surface, as compared to nano-HA samples, without affecting cell viability. Powdered nano-SiHA also induced an increase of the reparative macrophage population. These results along with the beneficial effects on osteoblasts previously observed with powdered nano-SiHA suggest the potential of this biomaterial for modulating the fundamental processes of bone formation and turnover, preventing bone resorption and enhancing bone formation at implantation sites in treatment of osteoporotic bone and in bone repair and regeneration.

The role of osteoclasts in bone tissue engineering

The success of scaffold-based bone regeneration approaches strongly depends on the performance of the biomaterial utilized. Within the efforts of regenerative medicine towards a restitutio ad integrum (i.e. complete reconstruction of a diseased tissue), scaffolds should be completely degraded within an adequate period of time. The degradation of synthetic bone substitute materials involves both chemical dissolution (physicochemical degradation) and resorption (cellular degradation by osteoclasts). Responsible for bone resorption are osteoclasts, cells of haematopoietic origin. Osteoclasts play also a crucial role in bone remodelling, which is essential for the regeneration of bone defects. There is, however, surprisingly limited knowledge about the detailed effects of osteoclasts on biomaterials degradation behaviour. This review covers the relevant fundamental knowledge and progress made in the field of osteoclast activity related to biomaterials used for bone regeneration. In vitro studies with osteoclastic precursor cells on synthetic bone substitute materials show that there are specific parameters that inhibit or enhance resorption. Moreover, analyses of the bone-material interface reveal that biomaterials composition has a significant influence on their degradation in contact with osteoclasts. Crystallinity, grain size, surface bioactivity and density of the surface seem to have a less significant effect on osteoclastic activity. In addition, the topography of the scaffold surface can be tailored to affect the development and spreading of osteoclast cells. The present review also highlights possible areas on which future research is needed and which are relevant to enhance our understanding of the complex role of osteoclasts in bone tissue engineering.