Strontium Functionalization of Biomaterials for Bone Tissue Engineering Purposes: A Biological Point of View

Metal Ion-Containing Hydrogels: Synthesis, Properties, and Applications in Bone Tissue Engineering

Hydrogel, as a unique scaffold material, features a three-dimensional network system that provides conducive conditions for the growth of cells and tissues in bone tissue engineering (BTE). In recent years, it has been discovered that metal ion-containing hybridized hydrogels, synthesized with metal particles as the foundation, exhibit excellent physicochemical properties, osteoinductivity, and osteogenic potential. They offer a wide range of research prospects in the field of BTE. This review provides an overview of the current state and recent advancements in research concerning metal ion-containing hydrogels in the field of BTE. Within materials science, it covers topics such as the binding mechanisms of metal ions within hydrogel networks, the types and fabrication methods of various metal ion-containing hydrogels, and the influence of metal ions on the properties of hydrogels. In the context of BTE, the review delves into the osteogenic mechanisms of various metal ions, the applications of metal ion-containing hydrogels in BTE, and relevant experimental studies in vitro and in vivo. Furthermore, future improvements in bone repair can be anticipated through advancements in bone bionics, exploring interactions between metal ions and the development of a wider range of metal ions and hydrogel types.

Improvement in mechanical strength and biological function of 3D-printed trimagnesium phosphate bioceramic scaffolds by incorporating strontium orthosilicate

Trimagnesium phosphate (TMP) bioceramic scaffolds are deemed as promising bone grafts, but their mechanical and biological properties are yet to be improved. In the study, strontium orthosilicate (SrOS) was used to modify the TMP scaffolds, whose macroporous structure was constructed by the filament deposition-type 3D printing method. The new phases of SrMg2(PO4)2 and Sr2MgSi2O7, which showed nanocrystalline topography, were produced in the 3D-printed TMP/SrOS bioceramic composite scaffolds. The compressive strength (1.8-64.1 MPa) and porosity (39.7%-71.4%) of the TMP/SrOS scaffolds could be readily tailored by changing the amounts of SrOS additives and the sintering temperature. The TMP/SrOS scaffolds gradually degraded in the aqueous solution, consequently releasing ions of magnesium, strontium and silicon. In contrast with the TMP scaffolds, the TMP/SrOS bioceramic scaffolds had profoundly higher compressive strength, and enhanced cell proliferative and osteogenic activities. The TMP/SrOS scaffolds incorporated with 5 wt% SrOS had the highest mechanical strength and beneficial cellular function, which made them promising for treating different sites of bone defects.

New strontium-based coatings show activity against pathogenic bacteria in spine infection

Infections of implants and prostheses represent relevant complications associated with the implantation of biomedical devices in spine surgery. Indeed, due to the length of the surgical procedures and the need to implant invasive devices, infections have high incidence, interfere with osseointegration, and are becoming increasingly difficult to threat with common therapies due to the acquisition of antibiotic resistance genes by pathogenic bacteria. The application of metal-substituted tricalcium phosphate coatings onto the biomedical devices is a promising strategy to simultaneously prevent bacterial infections and promote osseointegration/osseoinduction. Strontium-substituted tricalcium phosphate (Sr-TCP) is known to be an encouraging formulation with osseoinductive properties, but its antimicrobial potential is still unexplored. To this end, novel Sr-TCP coatings were manufactured by Ionized Jet Deposition technology and characterized for their physiochemical and morphological properties, cytotoxicity, and bioactivity against Escherichia coli ATCC 8739 and Staphylococcus aureus ATCC 6538P human pathogenic strains. The coatings are nanostructured, as they are composed by aggregates with diameters from 90 nm up to 1 μm, and their morphology depends significantly on the deposition time. The Sr-TCP coatings did not exhibit any cytotoxic effects on human cell lines and provided an inhibitory effect on the planktonic growth of E. coli and S. aureus strains after 8 h of incubation. Furthermore, bacterial adhesion (after 4 h of exposure) and biofilm formation (after 24 h of cell growth) were significantly reduced when the strains were cultured on Sr-TCP compared to tricalcium phosphate only coatings. On Sr-TCP coatings, E. coli and S. aureus cells lost their organization in a biofilm-like structure and showed morphological alterations due to the toxic effect of the metal. These results demonstrate the stability and anti-adhesion/antibiofilm properties of IJD-manufactured Sr-TCP coatings, which represent potential candidates for future applications to prevent prostheses infections and to promote osteointegration/osteoinduction.

Influence of Various Strontium Formulations (Ranelate, Citrate, and Chloride) on Bone Mineral Density, Morphology, and Microarchitecture: A Comparative Study in an Ovariectomized Female Mouse Model of Osteoporosis

Osteoporosis stands out as a prevalent skeletal ailment, prompting exploration into potential treatments, including dietary strontium ion supplements. This study assessed the efficacy of supplementation of three strontium forms-strontium citrate (SrC), strontium ranelate (SrR), and strontium chloride (SrCl)-for enhancing bone structure in 50 female SWISS mice, aged seven weeks. In total, 40 mice underwent ovariectomy, while 10 underwent sham ovariectomy. Ovariectomized (OVX) mice were randomly assigned to the following groups: OVX (no supplementation), OVX + SrR, OVX + SrC, and OVX + SrCl, at concentrations equivalent to the molar amount of strontium. After 16 weeks, micro-CT examined trabeculae and cortical bones, and whole-bone strontium content was determined. Results confirm strontium administration increased bone tissue mineral density (TMD) and Sr content, with SrC exhibiting the weakest effect. Femur morphometry showed limited Sr impact, especially in the OVX + SrC group. This research highlights strontium's potential in bone health, emphasizing variations in efficacy among its forms.

Janus Membrane with Intrafibrillarly Strontium-Apatite-Mineralized Collagen for Guided Bone Regeneration

Commercial collagen membranes face difficulty in guided bone regeneration (GBR) due to the absence of hierarchical structural design, effective interface management, and diverse bioactivity. Herein, a Janus membrane called SrJM is developed that consists of a porous collagen face to enhance osteogenic function and a dense face to maintain barrier function. Specifically, biomimetic intrafibrillar mineralization of collagen with strontium apatite is realized by liquid precursors of amorphous strontium phosphate. Polycaprolactone methacryloyl is further integrated on one side of the collagen as a dense face, which endows SrJM with mechanical support and a prolonged lifespan. In vitro experiments demonstrate that the dense face of SrJM acts as a strong barrier against fibroblasts, while the porous face significantly promotes cell adhesion and osteogenic differentiation through activation of calcium-sensitive receptor/integrin/Wnt signaling pathways. Meanwhile, SrJM effectively enhances osteogenesis and angiogenesis by recruiting stem cells and modulating osteoimmune response, thus creating an ideal microenvironment for bone regeneration. In vivo studies verify that the bone defect region guided by SrJM is completely repaired by newly formed vascularized bone. Overall, the outstanding performance of SrJM supports its ongoing development as a multifunctional GBR membrane, and this study provides a versatile strategy of fabricating collagen-based biomaterials for hard tissue regeneration.

Effectiveness of strontium/silver-based titanium surface coatings in improving antibacterial and osteogenic implant characteristics: a systematic review of in-vitro studies

Introduction: Due to the high incidence of implant failures, dual functionalization of titanium surfaces with antibacterial and osteogenic agents, like silver (Ag) and strontium (Sr), has gained significant attention in recent years. However, so far, the combined antibacterial and osteoinductive effectiveness of Ag/Sr-based titanium surface coatings has only been analyzed in individual studies. Methods: This systematic review aims to evaluate the existing scientific literature regarding the PICOS question "Does dual incorporation of strontium/silver enhances the osteogenic and anti-bacterial characteristics of Ti surfaces in vitro?". As a result of a web-based search adhering to the PRISMA Guidelines using three electronic databases (PubMed, Scopus, and Web of Science) until March 31, 2023, a total of 69 publications were identified as potentially relevant and 17 of which were considered appropriate for inclusion into this review. Results and Discussion: In all included publications, the use of Sr/Ag combination showed enhanced osteogenic and antibacterial effects, either alone or in combination with other agents. Moreover, the combination of Sr and Ag shows potential to synergistically enhance these effects. Nevertheless, further studies need to validate these findings under clinically more relevant conditions and evaluate the mechanism of antimicrobial and osteogenic activity of Sr/Ag combination.

Assessing the osseointegration potential of a strontium releasing nanostructured titanium oxide surface: A biomechanical study in the rabbit tibia plateau model

OBJECTIVES

To investigate the impact of a Ti-Sr-O technology, applied to either a turned surface or an SLA surface, on the mechanical robustness of osseointegration, benchmarked against the SLActive surface.

MATERIAL AND METHODS

Ti discs (6.25-mm-diameter and 2-mm-thick) with three different surfaces were inserted on the proximal-anterior part of the tibial plateau of adult Swedish loop rabbits: (I) turned surface modified with Ti-Sr-O (turned + Ti-Sr-O), (II) SLA surface modified with Ti-Sr-O (SLA + Ti-Sr-O), and (III) SLActive surface (SLActive). Following a healing period of 2 weeks and 4 weeks, the pull-out (PO) force needed to detach the discs from the bone was assessed, as a surrogate of osseointegration.

RESULTS

The SLActive surface exhibited statistically significant higher median PO forces, compared with the SLA + Ti-Sr-O surfaces at both 2- and 4 weeks post-op (p > .05). In this study, no single turned + Ti-Sr-O surface disk was integrated.

CONCLUSIONS

The tested Ti-Sr-O technology failed to enhance osseointegration; however, this finding may be related to the inappropriateness of the rabbit tibia plateau model for assessing third-generation implant surface technologies, due to the limited diffusion and clearance at the disk-bone interface.

Evaluating the effect of strontium ranelate and photobiomodulation on cementogenic and osteogenic differentiation of buccal fat pad-derived stem cells: An in vitro study

This study aimed to analyze the impact of strontium ranelate (Str), photobiomodulation (PBM), or their combination of the proliferation, osteogenic differentiation, and cementogenic differentiation of buccal fat pad-derived stem cells. BFPdSCs were exposed to one of the following interventions: (1) PBM (660 nm), (2) PBM (660 nm) + Str, (3) PBM (880 nm), (4) PBM (880 nm) + Str, (5) Str. All study groups had significantly higher osteogenic differentiation than the control group (p < 0.05), and no significant difference existed between the 660 and 808 nm groups (p = 0.97). Compared to the Str group, 660 nm and 880 nm group samples had significantly lower osteogenic differentiation (p < 0.0001), while other groups did not show a significant difference. Regarding cementogenic differentiation, the 660 nm group showed higher values than the 808 nm group (p < 0.01). Compared with the Str group, 660 nm, 660 nm + Str, and 808 nm + Str groups showed significantly higher gene expression (p < 0.05). In the case of osteogenic differentiation, although photobiomodulation alone had a lower inducing effect than strontium ranelate, combining 808 nm diode lasers and strontium ranelate may provide the best results. Moreover, using a 660 nm diode laser and exposing stem cells to strontium ranelate can be the most effective approach to induce cementogenic differentiation.

Core-Shell NiS@SrTiO3 Nanorods on Titanium for Enhanced Osseointegration via Programmed Regulation of Bacterial Infection, Angiogenesis, and Osteogenesis

Developing bone implants with dynamic self-adjustment of antibacterial, angiogenic, and osteogenic functions in line with a bone regenerative cascade is highly required in orthopedics. Herein, a unique core-shell nanorods array featuring a thin layer of NiS coated on each SrTiO3 nanorod (NiS@SrTiO3) was in situ constructed on titanium (Ti) through a two-step hydrothermal treatment. Under near-infrared (NIR) irradiation, the photoresponsive effect of NiS layer in synergy with the physical perforation of SrTiO3 nanorods initially enabled in vitro antibacterial rates of 96.5% to Escherichia coli and 93.1% to Staphylococcus aureus. With the degradation of the NiS layer, trace amounts of Ni ions were released, which accelerated angiogenesis by upregulating the expression of vascular regeneration-related factors, while the gradual exposure of SrTiO3 nanorods could simultaneously enhance the surface hydrophilicity in favor of cell adhesion and slowly release Sr ions to promote the proliferation and differentiation of MC3T3-E1 cells. The in vivo assessment verified not only the satisfactory antibacterial effect but also the superior osteogenic ability of the NiS@SrTiO3/Ti group with the aid of NIR irradiation, finally promoting the osseointegration of the Ti implant. The modification method endowing Ti implant with antibacterial, angiogenic, and osteogenic functions provides a new strategy to improve the long-term reliability of Ti-based devices.

Dietary Effects of Nanopowder Eggshells on Mineral Contents, Bone Turnover Biomarkers, and Regulators of Bone Resorption in Healthy Rats and Ovariectomy-Induced Osteoporosis Rat Model

Postmenopausal osteoporosis is a critical issue for female health worldwide. This current study was designed to evaluate the role of nanopowder eggshell (NPES) in healthy and ovariectomy-induced osteoporosis rats. Fifty-six female rats were divided into healthy rats (35) and ovariectomized rats (21). The healthy rats were subdivided into five groups (G1-G5) and received one of the following treatments: saline, 20 or 40 mg/kg of calcium carbonate, and 20 or 40 mg/kg of NPES. The 21 ovariectomized rats were divided into three groups (G6-G8) and received either saline, 40 mg/kg of calcium carbonate, or 40 mg/kg of NPES. Biochemical and histopathological assessments of bone formation and resorption were performed. Biomarkers of bone formation (calcium and osteocalcin (OCN)) and calcium content in left femur ashes were significantly higher in healthy rats given 40-mg/kg NPES than in healthy control rats and healthy rats given 40-mg/kg calcium carbonate. The ovariectomized groups had significantly lower levels of vitamin D3, OCN, and osteoprotegerin (OPG) than the healthy control. Alanine transaminase (ALT), alkaline phosphatase (ALP), and receptor activator of nuclear factor-κB ligand (RANKL) were significantly increased in the ovariectomized group than in the healthy control group. Treatment with NPES and calcium carbonate reduced liver enzymes in ovariectomized rats. NPES treatment significantly increased Vit D3, OCN, OPG, and bone ash mineral content (calcium, magnesium, zinc, and phosphorus) in ovariectomized rats. NPES also increased femur cortical thickness, osteoblast number, and collagen fiber. The current study suggests that NPES can modulate bone turnover biomarkers and increase bone trace elements. Moreover, NPES alleviates bone resorption in ovariectomy-induced osteoporosis.

Emerging nano-scale delivery systems for the treatment of osteoporosis

Osteoporosis is a pathological condition characterized by an accelerated bone resorption rate, resulting in decreased bone density and increased susceptibility to fractures, particularly among the elderly population. While conventional treatments for osteoporosis have shown efficacy, they are associated with certain limitations, including limited drug bioavailability, non-specific administration, and the occurrence of adverse effects. In recent years, nanoparticle-based drug delivery systems have emerged as a promising approach for managing osteoporosis. Nanoparticles possess unique physicochemical properties, such as a small size, large surface area-to-volume ratio, and tunable surface characteristics, which enable them to overcome the limitations of conventional therapies. These nanoparticles offer several advantages, including enhanced drug stability, controlled release kinetics, targeted bone tissue delivery, and improved drug bioavailability. This comprehensive review aims to provide insights into the recent advancements in nanoparticle-based therapy for osteoporosis. It elucidates the various types of nanoparticles employed in this context, including silica, polymeric, solid lipid, and metallic nanoparticles, along with their specific processing techniques and inherent properties that render them suitable as potential drug carriers for osteoporosis treatment. Furthermore, this review discusses the challenges and future suggestions associated with the development and translation of nanoparticle drug delivery systems for clinical use. These challenges encompass issues such as scalability, safety assessment, and regulatory considerations. However, despite these challenges, the utilization of nanoparticle-based drug delivery systems holds immense promise in revolutionizing the field of osteoporosis management by enabling more effective and targeted therapies, ultimately leading to improved patient outcomes.

Incorporation/Enrichment of 3D Bioprinted Constructs by Biomimetic Nanoparticles: Tuning Printability and Cell Behavior in Bone Models

Reproducing in vitro a model of the bone microenvironment is a current need. Preclinical in vitro screening, drug discovery, as well as pathophysiology studies may benefit from in vitro three-dimensional (3D) bone models, which permit high-throughput screening, low costs, and high reproducibility, overcoming the limitations of the conventional two-dimensional cell cultures. In order to obtain these models, 3D bioprinting offers new perspectives by allowing a combination of advanced techniques and inks. In this context, we propose the use of hydroxyapatite nanoparticles, assimilated to the mineral component of bone, as a route to tune the printability and the characteristics of the scaffold and to guide cell behavior. To this aim, both stoichiometric and Sr-substituted hydroxyapatite nanocrystals are used, so as to obtain different particle shapes and solubility. Our findings show that the nanoparticles have the desired shape and composition and that they can be embedded in the inks without loss of cell viability. Both Sr-containing and stoichiometric hydroxyapatite crystals permit enhancing the printing fidelity of the scaffolds in a particle-dependent fashion and control the swelling behavior and ion release of the scaffolds. Once Saos-2 cells are encapsulated in the scaffolds, high cell viability is detected until late time points, with a good cellular distribution throughout the material. We also show that even minor modifications in the hydroxyapatite particle characteristics result in a significantly different behavior of the scaffolds. This indicates that the use of calcium phosphate nanocrystals and structural ion-substitution is a promising approach to tune the behavior of 3D bioprinted constructs.

Evaluation of Biocompatibility of New Osteoplastic Xenomaterials Containing Zoledronic Acid and Strontium Ranelate

Background. The problem of improving the functional characteristics of implanted devices and materials used in traumatology and orthopedics is a topical issue. Aim of the study to study biocompatibility of bovine bone matrix xenomaterials modified by zoledronic acid and strontium ranelate when implanted into the bone defect cavity. Methods. The study was performed on 24 male rabbits of the Soviet Chinchilla breed. Test blocks of bone matrix were implanted into the cavity of bone defects of the femur. Group 1 animals (n = 8, control group) were implanted with bone xenogenic material (Bio-Ost osteoplastic matrix). Group 2 animals (n = 8) were implanted with bone xenogenic material impregnated with zoledronic acid. Group 3 animals (n = 8) were implanted with bone xenogeneic material impregnated with strontium ranelate. Supercritical fluid extraction technology was used to purify the material and impregnate it with zoledronic acid and strontium ranelate. Radiological, pathomorphological, histological and laboratory (hematology and blood biochemistry) diagnostic methods were used to assess biocompatibility. Follow-up period was 182 days after implantation. Results. It was found out that on the 182nd day after implantation the median area of the newly-formed bone tissue in the defect modeling area in Group 1 was 79%, in Group 2 0%, in Group 3 67%. In Group 2 the maximum area by this period was filled with connective tissue 77%. Median relative area of implanted material fragments in Group 1 was 4%, in Group 2 23%, in Group 3 15%. No infection or material rejection was observed in animals of all groups. There were no signs of intoxication or prolonged systemic inflammatory reaction. Laboratory parameters did not change significantly over time. One animal in each group experienced one-time increase in C-reactive protein level against the background of leukocytosis. Two animals in Group 1 had a slight migration of implanted material under the skin, one animal developed arthritis of the knee joint. Conclusion. Osteoplastic materials based on bovine bone xenomatrix and filled with zoledronic acid and strontium ranelate have acceptable values of biocompatibility including their safety profile.

Osteogenic Potential of Nano-Hydroxyapatite and Strontium-Substituted Nano-Hydroxyapatite

Nanohydroxyapatite (nanoHA) is the major mineral component of bone. It is highly biocompatible, osteoconductive, and forms strong bonds with native bone, making it an excellent material for bone regeneration. However, enhanced mechanical properties and biological activity for nanoHA can be achieved through enrichment with strontium ions. Here, nanoHA and nanoHA with a substitution degree of 50 and 100% of calcium with strontium ions (Sr-nanoHA_50 and Sr-nanoHA_100, respectively) were produced via wet chemical precipitation using calcium, strontium, and phosphorous salts as starting materials. The materials were evaluated for their cytotoxicity and osteogenic potential in direct contact with MC3T3-E1 pre-osteoblastic cells. All three nanoHA-based materials were cytocompatible, featured needle-shaped nanocrystals, and had enhanced osteogenic activity in vitro. The Sr-nanoHA_100 indicated a significant increase in the alkaline phosphatase activity at day 14 compared to the control. All three compositions revealed significantly higher calcium and collagen production up to 21 days in culture compared to the control. Gene expression analysis exhibited, for all three nanoHA compositions, a significant upregulation of osteonectin and osteocalcin on day 14 and of osteopontin on day 7 compared to the control. The highest osteocalcin levels were found for both Sr-substituted compounds on day 14. These results demonstrate the great osteoinductive potential of the produced compounds, which can be exploited to treat bone disease.

Biomechanical Characteristics and Analysis Approaches of Bone and Bone Substitute Materials

Bone has a special structure that is both stiff and elastic, and the composition of bone confers it with an exceptional mechanical property. However, bone substitute materials that are made of the same hydroxyapatite (HA) and collagen do not offer the same mechanical properties. It is important for bionic bone preparation to understand the structure of bone and the mineralization process and factors. In this paper, the research on the mineralization of collagen is reviewed in terms of the mechanical properties in recent years. Firstly, the structure and mechanical properties of bone are analyzed, and the differences of bone in different parts are described. Then, different scaffolds for bone repair are suggested considering bone repair sites. Mineralized collagen seems to be a better option for new composite scaffolds. Last, the paper introduces the most common method to prepare mineralized collagen and summarizes the factors influencing collagen mineralization and methods to analyze its mechanical properties. In conclusion, mineralized collagen is thought to be an ideal bone substitute material because it promotes faster development. Among the factors that promote collagen mineralization, more attention should be given to the mechanical loading factors of bone.

Strontium ion attenuates osteoarthritis through inhibiting senescence and enhancing autophagy in fibroblast-like synoviocytes

Osteoarthritis (OA) mainly occurs in the elderly population and seriously affects their quality of life (QOL). Strontium (Sr) ions have shown positive effects on bone tissue and are promising for OA treatment. However, the adequate treatment dosage and underlying mechanisms are unclear. This study investigated the effects and underlying mechanisms of different concentrations of Sr ions in a mouse model of OA induced by destabilization of the medial meniscus (DMM) surgery. DMM-induced OA mice received intra-articular injections of different concentrations of Sr ions, and a suitable concentration of Sr ions was found to improve OA. Furthermore, we investigated the mechanism by which Sr ions mediate senescence and autophagy in fibroblast-like synoviocytes (FLSs) in the synovial tissues of DMM-induced OA mice. OA mice treated with 10 µl of 5 mmol/L SrCl₂ showed the greatest improvement in pain-related behavior and cartilage damage. In addition, in vivo and in vitro experiments revealed that Sr ions inhibit senescence and improve the autophagic function of FLSs. We also found that enhancement of the autophagic function of FLSs could effectively slow down senescence. Therefore, we show that Sr ions through the AMPK/mTOR/LC3B-II signal axis improve FLSs autophagy function and delay FLSs senescence, and furthermore, improve OA. These results suggest that senescence and autophagy function of FLSs may serve as promising targets for OA treatment, and that Sr ions may inhibit OA progression through these two targets.

Strontium Substituted β-Tricalcium Phosphate Ceramics: Physiochemical Properties and Cytocompatibility

Sr²⁺-substituted β-tricalcium phosphate (β-TCP) powders were synthesized using the mechano-chemical activation method with subsequent pressing and sintering to obtain ceramics. The concentration of Sr²⁺ in the samples was 0 (non-substituted TCP, as a reference), 3.33 (0.1SrTCP), and 16.67 (0.5SrTCP) mol.% with the expected Ca₃(PO₄)₂, Ca_2.9Sr_0.1(PO₄)₂, and Ca_2.5Sr_0.5(PO₄)₂ formulas, respectively. The chemical compositions were confirmed by the energy-dispersive X-ray spectrometry (EDX) and the inductively coupled plasma optical emission spectroscopy (ICP-OES) methods. The study of the phase composition of the synthesized powders and ceramics by the powder X-ray diffraction (PXRD) method revealed that β-TCP is the main phase in all compounds except 0.1SrTCP, in which the apatite (Ap)-type phase was predominant. TCP and 0.5SrTCP ceramics were soaked in the standard saline solution for 21 days, and the phase analysis revealed the partial dissolution of the initial β-TCP phase with the formation of the Ap-type phase and changes in the microstructure of the ceramics. The Sr²⁺ ion release from the ceramic was measured by the ICP-OES. The human osteosarcoma MG-63 cell line was used for viability, adhesion, spreading, and cytocompatibility studies. The results show that the introduction of Sr²⁺ ions into the β-TCP improved cell adhesion, proliferation, and cytocompatibility of the prepared samples. The obtained results provide a base for the application of the Sr²⁺-substituted ceramics in model experiments in vivo.

Impact of Sr2+ and hypoxia on 3D triple cultures of primary human osteoblasts, osteocytes and osteoclasts

An in vitro bone triple culture involving human primary osteoblasts, osteocytes and osteoclasts enables the investigation of bone healing factors, drugs or biomaterials in a model system for native bone tissue. The present study analyses the impact of Sr²⁺ as well as hypoxic cultivation (5% O₂ content or chemically induced by Co²⁺) on bone cells. The three cell types were cultivated together in the presence of 100 µM Sr²⁺, hypoxic conditions or in the presence of 75 µM Co²⁺. After cultivation the cell types were separated and analysed on mRNA and protein level individually. In response to Sr²⁺ osteoblasts showed a downregulation of IBSP expression and a stimulation of ALP activity. Osteocyte gene marker expression of PDPN, MEPE, RANKL, OPG, osteocalcin and likewise the amount of secreted osteocalcin was reduced in the presence of Sr²⁺. Activity of osteoclast-specific enzymes TRAP and CAII was enhanced compared to the Sr²⁺ free control. Hypoxic conditions induced by both 5% O₂ or a Co²⁺ treatment led to decreased DNA content of all bone cells and downregulated expression of osteoblast markers ALPL and IBSP as well as osteocyte markers PDPN, RANKL and OPG. In addition, Co²⁺ induced hypoxia decreased gene and protein expression of osteocalcin in osteocytes. In response to the Co²⁺ treatment, the TRAP gene expression and activity was increased. This study is the first to analyse the effects of Sr²⁺ or hypoxia on triple cultures with primary human bone cells. The investigated in vitro bone model might be suitable to reduce animal experiments in early stages of biomaterial and drug development.