Strontium Enhances Osteogenic Differentiation of Mesenchymal Stem Cells and In Vivo Bone Formation by Activating Wnt/Catenin Signaling

Mesoporous strontium doped nano sized sulphate hydroxyapatite as a novel biomaterial for bone tissue applications

In this study a novel nano-structured hydroxyapatite (HA) incorporated with different fractions of Sr²⁺ and SO₄²⁻ ions has been synthesized using the wet precipitation method and characterized.

Enhanced antibacterial activity and osteoinductivity of Ag-loaded strontium hydroxyapatite/chitosan porous scaffolds for bone tissue engineering

A new design of Ag-loaded Sr-HAP/CS porous scaffolds for bone tissue engineering to promote osteogenesis and prevent infections.

New bone formation induced by surface strontium-modified ceramic bone graft substitute

OBJECTIVES

This study assessed the effect of surface strontium ion (Sr) modification on the osteogenic activity of an osteoconductive ceramic bone graft substitute with the hope of using the bone healing effect of Sr for potential application in periodontal and maxillofacial regenerative surgery.

MATERIALS AND METHODS

A simple wet chemical treatment was employed to deliver Sr to the surface of particulate porcine bone graft. The osteogenic activity of surface Sr-modified bone substitute was compared in vitro and in vivo with that of unmodified ceramic bone, other clinically available synthetic bone or osteoinductive allograft bone.

RESULTS

The resultant bone substitute showed the formation of Sr-containing microstructured surface layer along with the formation of additional nanostructures and displayed sustained Sr release. Sr modification promoted the osteogenic differentiation of bipotential ST2 stem cells. Sr-modified bone substitute increased the amount of newly formed bone at early healing period in calvarial defect of rabbits.

CONCLUSIONS

These results suggest that the surface Sr modification by wet chemical treatment is a promising approach to enhance the early bone healing capacity of osteoconductive ceramic bone substitutes.

DKK1 rescues osteogenic differentiation of mesenchymal stem cells isolated from periodontal ligaments of patients with diabetes mellitus induced periodontitis

Multiple studies have shown that diabetes mellitus is an established risk factor for periodontitis. Recently mesenchymal stem cells derived from periodontal ligament (PDLSCs) have been utilized to reconstruct tissues destroyed by chronic inflammation. However, impact of periodontitis with diabetes mellitus on PDLSCs and mechanisms mediating effects of complex microenvironments remain poorly understood. In this study, we found multiple differentiation potential of PDLSCs from chronic periodontitis with diabetes mellitus donors (D-PDLSCs) was damaged significantly. Inhibition of NF-κB signaling could rescue osteogenic potential of PDLSCs from simple chronic periodontitis patients (P-PDLSCs), whereas did not promote D-PDLSCs osteogenesis. In addition, we found expression of DKK1 in D-PDLSCs did not respond to osteogenic signal and decreased osteogenic potential of D-PDLSCs treated with DKK1 could be reversed. To further elucidate different character between P-PDLSCs and D-PDLSCs, we treated PDLSCs with TNF-α and advanced glycation end products (AGEs), and find out AGEs which enhance effect of TNF-α in PDLSCs might mediate special personality of D-PDLSCs. The adverse effect of AGEs in PDLSCs could be reversed when PDLSCs were treated with DKK1. These results suggested DKK1 mediating WNT signaling might be a therapy target to rescue potential of PDLSCs in periodontitis with diabetes mellitus.

MicroRNAs in Bone Balance and Osteoporosis

Preclinical Research Bone is a rigid and dynamic organ that undergoes continuous turnover. Bone homeostasis is maintained by osteoclast-mediated bone resorption and osteoblast-mediated bone formation. The interruption of this balance can cause various diseases, including osteoporosis a public health issue due to the rate of hip fracture, the most serious outcome of osteoporosis. The bone loss in osteoporosis results from an increase in bone resorption versus bone formation. Thus, regulation of osteoblast and osteoclast activity is a main focus in the treatment of osteoporosis. MicroRNAs (miRNAs) are a class of single stranded noncoding RNAs consisting of 18-22 nucleotides that have an important role in cell differentiation, cell fate, apoptosis, and pathogenesis in various disease states. The potential therapeutic and biomarker function of miRNAs in treating bone disorders is receiving more attention. The current review summarizes the role of miRNAs in bone function at a cellular level in the context of their therapeutic potential.

In Vitro Effects of Strontium on Proliferation and Osteoinduction of Human Preadipocytes

Development of tools to be used for in vivo bone tissue regeneration focuses on cellular models and differentiation processes. In searching for all the optimal sources, adipose tissue-derived mesenchymal stem cells (hADSCs or preadipocytes) are able to differentiate into osteoblasts with analogous characteristics to bone marrow mesenchymal stem cells, producing alkaline phosphatase (ALP), collagen, osteocalcin, and calcified nodules, mainly composed of hydroxyapatite (HA). The possibility to influence bone differentiation of stem cells encompasses local and systemic methods, including the use of drugs administered systemically. Among the latter, strontium ranelate (SR) represents an interesting compound, acting as an uncoupling factor that stimulates bone formation and inhibits bone resorption. The aim of our study was to evaluate the in vitro effects of a wide range of strontium (Sr(2+)) concentrations on proliferation, ALP activity, and mineralization of a novel finite clonal hADSCs cell line, named PA20-h5. Sr(2+) promoted PA20-h5 cell proliferation while inducing the increase of ALP activity and gene expression as well as HA production during in vitro osteoinduction. These findings indicate a role for Sr(2+) in supporting bone regeneration during the process of skeletal repair in general, and, more specifically, when cell therapies are applied.

Electrospun Gelatin/β-TCP Composite Nanofibers Enhance Osteogenic Differentiation of BMSCs and In Vivo Bone Formation by Activating Ca (2+) -Sensing Receptor Signaling

Calcium phosphate- (CaP-) based composite scaffolds have been used extensively for the bone regeneration in bone tissue engineering. Previously, we developed a biomimetic composite nanofibrous membrane of gelatin/β-tricalcium phosphate (TCP) and confirmed their biological activity in vitro and bone regeneration in vivo. However, how these composite nanofibers promote the osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) is unknown. Here, gelatin/β-TCP composite nanofibers were fabricated by incorporating 20 wt% β-TCP nanoparticles into electrospun gelatin nanofibers. Electron microscopy showed that the composite β-TCP nanofibers had a nonwoven structure with a porous network and a rough surface. Spectral analyses confirmed the presence and chemical stability of the β-TCP and gelatin components. Compared with pure gelatin nanofibers, gelatin/β-TCP composite nanofibers caused increased cell attachment, proliferation, alkaline phosphatase activity, and osteogenic gene expression in rat BMSCs. Interestingly, the expression level of the calcium-sensing receptor (CaSR) was significantly higher on the composite nanofibrous scaffolds than on pure gelatin. For rat calvarial critical sized defects, more extensive osteogenesis and neovascularization occurred in the composite scaffolds group compared with the gelatin group. Thus, gelatin/β-TCP composite scaffolds promote osteogenic differentiation of BMSCs in vitro and bone regeneration in vivo by activating Ca²⁺-sensing receptor signaling.

Recent Advances in Hydroxyapatite Scaffolds Containing Mesenchymal Stem Cells

Modern day tissue engineering and cellular therapies have gravitated toward using stem cells with scaffolds as a dynamic modality to aid in differentiation and tissue regeneration. Mesenchymal stem cells (MSCs) are one of the most studied stem cells used in combination with scaffolds. These cells differentiate along the osteogenic lineage when seeded on hydroxyapatite containing scaffolds and can be used as a therapeutic option to regenerate various tissues. In recent years, the combination of hydroxyapatite and natural or synthetic polymers has been studied extensively. Due to the interest in these scaffolds, this review will cover the wide range of hydroxyapatite containing scaffolds used with MSCs for in vitro and in vivo experiments. Further, in order to maintain a progressive scope of the field this review article will only focus on literature utilizing adult human derived MSCs (hMSCs) published in the last three years.

Osteoblastic differentiation of stem cells from human exfoliated deciduous teeth induced by thermosensitive hydrogels with strontium phosphate

Stem cells from human exfoliated deciduous teeth (SHEDs) are a novel source of multi-potential stem cells for tissue engineering because of their potential to differentiate into multiple cell lineages. Strontium exhibits an important function in bone remodeling because it can simulate bone formation and decrease bone resorption. Hydrogels can mimic the natural cellular environment. The association of hydrogels with cell viability is determined using biological tests, including rheological experiments. In this study, osteogenic differentiation was investigated through SHED encapsulation in hydrogels containing strontium phosphate. Results of 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) and proliferating cell nuclear antigen (PCNA) immunofluorescence staining indicated that the cells grew well and SHEDs proliferated in the hydrogels. Strontium-loaded chitosan-based hydrogels induced the biomineralization and high expression of alkaline phosphatase. Moreover, the expression levels of bone-related genes, including type-I collagen, Runx2, osteopontin (OP), and osteonectin (ON), were up-regulated during the osteogenic differentiation of SHEDs. This study demonstrated that strontium can be an effective inducer of osteogenesis for SHEDs. Elucidating the function of bioceramics (such as strontium) is useful in designing and developing strategies for bone tissue engineering.

The osteogenic response of mesenchymal stromal cells to strontium-substituted bioactive glasses

Bioactive glasses are known to stimulate bone healing, and the incorporation of strontium has the potential to increase their potency. In this study, calcium oxide in the 45S5 bioactive glass composition was partially (50%, Sr50) or fully (100%, Sr100) substituted with strontium oxide on a molar basis. The effects of the substitution on bioactive glass properties were studied, including density, solubility, and in vitro cytotoxicity. Stimulation of osteogenic differentiation was investigated using mesenchymal stromal cells obtained from rat bone marrow. Strontium substitution resulted in altered physical properties including increased solubility. Statistically significant reductions in cell viability were observed with the addition of bioactive glass powders to culture medium. Specifically, addition of ≥ 13.3 mg/ml of 45S5 bioactive glass or Sr50, or ≥ 6.7 mg/ml of Sr100, resulted in significant inhibition. Real‐time PCR analyses detected the upregulation of genes associated with osteoblastic differentiation in the presence of all bioactive glass compositions. Some genes, including Alpl and Bglap, were further stimulated in the presence of Sr50 and Sr100. It was concluded that strontium‐substituted bioactive glasses promoted osteogenesis in a differentiating bone cell culture model and, therefore, have considerable potential for use as improved bioactive glasses for bone tissue regeneration. © 2015 The Authors. Journal of Tissue Engineering and Regenerative Medicine Published by John Wiley & Sons Ltd.

Fabrication of gelatin-strontium substituted calcium phosphate scaffolds with unidirectional pores for bone tissue engineering

This study fabricated homogeneous gelatin-strontium substituted calcium phosphate composites via coprecipitation in a gelatin solution. Unidirectional porous scaffolds with an oriented microtubular structure were then manufactured using freeze-drying technology. The resulting structure and pore alignment were determined using scanning electron microscopy. The pore size were in the range of 200-400 μm, which is considered ideal for the engineering of bone tissue. The scaffolds were further characterized using energy dispersive spectroscopy, Fourier transform infrared spectroscopy, and X-ray diffraction. Hydroxyapatite was the main calcium phosphate compound in the scaffolds, with strontium incorporated into the crystal structure. The porosity of the scaffolds decreased with increasing concentration of calcium-phosphate. The compressive strength in the longitudinal direction was two to threefold higher than that observed in the transverse direction. Our results demonstrate that the composite scaffolds degraded by approximately 20 % after 5 weeks. Additionally, in vitro results reveal that the addition of strontium significantly increased human osteoblastic cells proliferation. Scaffolds containing strontium with a Sr-CaP/(gelatin + Sr-CaP) ratio of 50 % provided the most suitable environment for cell proliferation, particularly under dynamic culture conditions. This study demonstrates the considerable potential of composite scaffolds composed of gelatin-strontium-substituted calcium phosphate for applications in bone tissue engineering.

Evaluation of injectable strontium-containing borate bioactive glass cement with enhanced osteogenic capacity in a critical-sized rabbit femoral condyle defect model

The development of a new generation of injectable bone cements that are bioactive and have enhanced osteogenic capacity for rapid osseointegration is receiving considerable interest. In this study, a novel injectable cement (designated Sr-BBG) composed of strontium-doped borate bioactive glass particles and a chitosan-based bonding phase was prepared and evaluated in vitro and in vivo. The bioactive glass provided the benefits of bioactivity, conversion to hydroxyapatite, and the ability to stimulate osteogenesis, while the chitosan provided a cohesive biocompatible and biodegradable bonding phase. The Sr-BBG cement showed the ability to set in situ (initial setting time = 11.6 ± 1.2 min) and a compressive strength of 19 ± 1 MPa. The Sr-BBG cement enhanced the proliferation and osteogenic differentiation of human bone marrow-derived mesenchymal stem cells in vitro when compared to a similar cement (BBG) composed of chitosan-bonded borate bioactive glass particles without Sr. Microcomputed tomography and histology of critical-sized rabbit femoral condyle defects implanted with the cements showed the osteogenic capacity of the Sr-BBG cement. New bone was observed at different distances from the Sr-BBG implants within eight weeks. The bone-implant contact index was significantly higher for the Sr-BBG implant than it was for the BBG implant. Together, the results indicate that this Sr-BBG cement is a promising implant for healing irregularly shaped bone defects using minimally invasive surgery.

Effects on growth and osteogenic differentiation of mesenchymal stem cells by the strontium-added sol-gel hydroxyapatite gel materials

In the present study, strontium-modified hydroxyapatite gels (Sr-HA) at different concentrations were prepared using sol-gel approach and their effect on human-bone-marrow-derived mesenchymal stem cells, were evaluated. The effect of Strontium on physico-chemical and morphological properties of hydroxyapatite gel were evaluated. Morphological analyses (SEM and TEM) demonstrate that an increasing in the amount of Sr ions doped into HA made the agglomerated particles smaller. The substitution of large Sr2+ for small Ca2+ lead to denser atomic packing of the system causing retardation of crystals growth. The biological results demonstrated that hydroxyapatite gel containing from 0 to 20 mol% of Sr presented no cytotoxicity and promote the expression of osteogenesis related genes including an early marker for osteogenic differentiation ALP; a non-collagen protein OPN and a late marker for osteogenic differentiation OCN. Finally, the Sr-HA gels could have a great potential application as filler in bone repair and regeneration and used in especially in the osteoporotic disease.

Orthobiologics in the augmentation of osteoporotic fractures

Many orthobiologic adjuvants are available and widely utilized for general skeletal restoration. Their use for the specific task of osteoporotic fracture augmentation is less well recognized. Common conductive materials are reviewed for their value in this patient population including the large group of allograft adjuvants categorically known as the demineralized bone matrices (DBMs). Another large group of alloplastic materials is also examined-the calcium phosphate and sulfate ceramics. Both of these materials, when used for the proper indications, demonstrate efficacy for these patients. The inductive properties of bone morphogenic proteins (BMPs) and platelet concentrates show no clear advantages for this group of patients. Systemic agents including bisphosphonates, receptor activator of nuclear factor κβ ligand (RANKL) inhibitors, and parathyroid hormone augmentation all demonstrate positive effects with this fracture cohort. Newer modalities, such as trace ion bioceramic augmentation, are also reviewed for their positive effects on osteoporotic fracture healing.

Odontogenic differentiation of human dental pulp cells induced by silicate-based bioceramics via activation of P38/MEPE pathway

Ca₇Si₂P₂O₁₆ bioceramic significantly enhanced odontogenic protein expression (ALP activity and staining) of hDPCs.

Three-dimensional printed strontium-containing mesoporous bioactive glass scaffolds for repairing rat critical-sized calvarial defects

The development of a new generation of biomaterials with high osteogenic ability for fast osseointegration with host bone is being intensively investigated. In this study, we have fabricated three-dimensional (3-D) strontium-containing mesoporous bioactive glass (Sr-MBG) scaffolds by a 3-D printing technique. Sr-MBG scaffolds showed uniform interconnected macropores (∼400μm), high porosity (∼70%) and enhanced compressive strength (8.67±1.74MPa). Using MBG scaffolds as a control, the biological properties of Sr-MBG scaffolds were evaluated by apatite-forming ability, adhesion, proliferation, alkaline phosphatase activity and osteogenic gene expression of osteoblast-like cells MC3T3-E1. Furthermore, Sr-MBG scaffolds were used to repair critical-sized rat calvarial defects. The results showed that Sr-MBG scaffolds possessed good apatite-forming ability and stimulated MC3T3-E1 cell proliferation and differentiation. Importantly, the in vivo results revealed that Sr-MBG scaffolds had good osteogenic capability and stimulated new blood vessel formation in critical-sized rat calvarial defects within 8 weeks. Therefore, 3-D printed Sr-MBG scaffolds with favorable pore structure and high osteogenic ability have more potential applications in bone regeneration.

Strontium modified calcium phosphate cements – approaches towards targeted stimulation of bone turnover

Strontium modified calcium phosphate cements can target local bone turnover by stimulating osteoblast proliferation and differentiation (1) as well as bone mineralisation (2), reducing osteoclastogenesis (3) and resorption activity, increase osteoclast apoptosis (4) and affect osteoblast/osteoclast paracrine signalling (5).

Doped tricalcium phosphate scaffolds by thermal decomposition of naphthalene: Mechanical properties and in vivo osteogenesis in a rabbit femur model

Tricalcium phosphate (TCP) is a bioceramic that is widely used in orthopedic and dental applications. TCP structures show excellent biocompatibility as well as biodegradability. In this study, porous β-TCP scaffolds were prepared by thermal decomposition of naphthalene. Scaffolds with 57.64% ± 3.54% density and a maximum pore size around 100 μm were fabricated via removing 30% naphthalene at 1150°C. The compressive strength for these scaffolds was 32.85 ± 1.41 MPa. Furthermore, by mixing 1 wt % SrO and 0.5 wt % SiO2 , pore interconnectivity improved, but the compressive strength decreased to 22.40 ± 2.70 MPa. However, after addition of polycaprolactone coating layers, the compressive strength of doped scaffolds increased to 29.57 ± 3.77 MPa. Porous scaffolds were implanted in rabbit femur defects to evaluate their biological property. The addition of dopants triggered osteoinduction by enhancing osteoid formation, osteocalcin expression, and bone regeneration, especially at the interface of the scaffold and host bone. This study showed processing flexibility to make interconnected porous scaffolds with different pore size and volume fraction porosity, while maintaining high compressive mechanical strength and excellent bioactivity. Results show that SrO/SiO2 -doped porous TCP scaffolds have excellent potential to be used in bone tissue engineering applications.

A comprehensive review on the role of various materials in the osteogenic differentiation of mesenchymal stem cells with a special focus on the association of heat shock proteins and nanoparticles

Mesenchymal stem cells (MSCs) have important roles in the area of regenerative medicine and clinical applications due to their pluripotent nature. Osteogenic differentiation of MSCs has been studied extensively using various stimulants to develop models of bone repair. There are several factors that enhance the differentiation of MSCs into bone tissues. This review focuses on the effects of various inducers on the osteoblast differentiation of MSCs at different stages of cellular development. We discuss the various growth factors, hormones, vitamins, cytokines, chemical stimulants, and mechanical forces applied in bioreactors that play an essential role in the proliferation, differentiation, and matrix mineralization of stem cells during osteogenesis. Various nanoparticles have also been used recently for the same purpose and the results are promising. Moreover, we review the role of various stresses, including thermal stress, and the subsequent involvement of heat shock proteins as inducers of the proliferation and differentiation of osteoblasts. We also report how various proteasome inhibitors have been shown to induce proliferation and osteogenic differentiation of MSCs in a number of cases. In this communication, the role of peptide-based scaffolds in osteoblast proliferation and differentiation is also reviewed. Based on the reviewed information, this article proposes novel possibilities for the enhancement of proliferation, differentiation, and migration of osteoblasts from MSCs. © 2014 S. Karger AG, Basel.

Strontium potently inhibits mineralisation in bone-forming primary rat osteoblast cultures and reduces numbers of osteoclasts in mouse marrow cultures

SUMMARY

The basic mechanisms by which strontium ranelate acts on bone are still unclear. We show that an important action of strontium salts is to block calcification in cultures of osteoblasts, the bone-forming cells. These results suggest that strontium treatment could have previously overlooked effects on bone.

INTRODUCTION

The basic mechanisms of action of strontium ranelate (SrR) on bone have remained unclear. We studied the direct actions of Sr(2+) salts in functional cultures of osteoblasts and osteoclasts.

METHODS

Cultures of primary osteoblasts from rat calvariae and osteoclast-forming mouse marrow cells were treated continuously with either SrR or strontium chloride (SrCl2).

RESULTS

Abundant, discretely mineralised 'trabecular' bone structures formed in control osteoblast cultures after 14 days. SrR at 0.01, 0.1 and 1 mM inhibited mineralisation to 59, 98 and 100 % (all p < 0.001) of control values, respectively. SrCl2 at the same concentrations caused similar inhibitions. Osteoblast cell numbers and alkaline phosphatase activity were unaltered. SrR dose-dependently reduced the formation of multinucleated osteoclasts from marrow mononuclear cells cultured on dentine for 8 days in the presence of macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor kappa B ligand (RANKL), with a 50 % inhibition occurring at 1 mM; SrCl2 was slightly less effective, eliciting a maximal 30 % inhibition. Corresponding decreases in total resorption pit formation were observed, suggesting Sr(2+) salts affect osteoclast formation rather than resorptive activity.

CONCLUSION

Our findings are consistent with the documented physicochemical inhibitory action of Sr(2+) on mineralisation but contrast with reports that Sr(2+) increases osteoblast activity and number in vitro. Our results suggest that rather than acting as an agent that 'uncouples' bone formation and resorption, Sr(2+) acts as a global inhibitor of bone cell function, with particularly marked effects on mineralisation. The potential effects of long-term Sr(2+) on secondary mineralisation in bone deserve investigation.

Enhanced osseointegration of endosseous implants by predictable sustained release properties of strontium

Studies have shown that strontium (Sr) incorporated into surfaces may enhance osseointegration. Thus, we suggested that a sustained Sr release from implant surfaces could improve bone healing. This study verifies and further investigates the effect of a novel Ti-Sr-O functionalized implant surface prepared from a magnetron co-sputtering platform with a continuous release of Sr.

MATERIALS AND METHODS

Four experimental Ti-Sr-O groups, which differed from each other in Sr contents and pre-wash procedures, were tested. Implants were prepared with a Ti-Sr-O coating by means of magnetron co-sputtering and compared to Grade 4 titanium. Composition, morphology and mechanical stability were analyzed; Sr-release data were gained from in vitro washout experiments. In vivo investigations were carried out in a rat model and analyzed histologically regarding bone-to-implant contact and new bone formation 30 days after implantation.

RESULTS

Structural differences were detected between the two basis Ti-Sr-O coatings with 6.7 at.% and 8.9 at.% Sr, respectively. Different release profiles were observed with 8.9 at.% Sr coating exhibiting the highest long-term release of Sr. Median values of new bone formation and bone-to-implant contact was found to be 60.1% and 91.6%, respectively, for the best group compared to 16.6% and 70.6% for the Grade 4 titanium reference. The increase in new bone formation was found to correlate with the amount of Sr released in vitro.

CONCLUSION

The results show that sputtered Ti-Sr-O coatings with sustained release of Sr may improve osseointegration, and could thus have impact on practical applications for medical implants.

Strontium exerts dual effects on calcium phosphate cement: Accelerating the degradation and enhancing the osteoconductivity both in vitro and in vivo

Calcium phosphate cements (CPCs) have long been used as osteoconductive bone substitutes in the treatment of bone defects. However, the degradation rate of CPC is typically too slow to match the new bone growth rate. It is known that strontium increases the solubility of hydroxyapatite as well as exerts both anabolic and anticatabolic effects on bone. Therefore, we hypothesized that the incorporation of strontium would accelerate the degradation rate and enhance the osteoconductivity of CPC. In this study, Three groups, CPC (0% Sr-CPC), 5% Sr-CPC, and 10% Sr-CPC, were prepared, with the total molar ratio for Sr/(Sr+Ca) in the cement powder phase being 0, 5, and 10%, respectively. In the immersion test, less residual weight was observed in both 5% Sr-CPC and 10% Sr-CPC groups than CPC group. In addition, a higher osteoblastic cell proliferation rate and alkaline phosphatase activity were obtained in the strontium groups. In a rat femur bone defect model comparing CPC with 10% Sr-CPC, at 2 weeks postoperation, early endochondral ossification was found in the 10% Sr-CPC group, whereas only fibrous tissue was observed in control group; at 4-16 weeks postoperation, progressive osteoconduction toward the cement was observed in both groups. At 32 weeks, a higher peri-cement bone area and reduced cement area were noted in the 10% Sr-CPC group. In conclusion, in the 10% Sr-CPC group, strontium exerts dual effects on CPC: accelerating degradation rate and enhancing osteoconductivity, as shown here both in vitro and in vivo.

Osteogenic differentiation of amniotic epithelial cells: synergism of pulsed electromagnetic field and biochemical stimuli

BACKGROUND

Pulsed electromagnetic field (PEMF) is a non-invasive physical therapy used in the treatment of fracture nonunion or delayed healing. PEMF can facilitate the osteogenic differentiation of bone marrow mesenchymal stem cells in vitro. Amniotic epithelial cells (AECs) have been proposed as a potential source of stem cells for cell therapy. However, whether PEMF could modulate the osteogenic differentiation of AECs is unknown. In the present study, the effects of PEMF on the osteogenic differentiation of AECs were investigated.

METHODS

AECs were isolated from amniotic membrane of human placenta by trypsin digestion and were induced by PEMF and/or osteo-induction medium. After 21 days we used real time RT-PCR and immunocytochemistry to study the expression of osteoblast markers. The signal transduction of osteogenesis was further investigated.

RESULTS

The PEMF stimulation, or osteo-induction medium alone could induce osteogenic differentiation of AECs, as shown by expression of osteoblast specific genes and proteins including alkaline phosphatase and osteocalcin. Furthermore, a combination of PEMF and osteo-induction medium had synergy effects on osteogenic differentiation. In our study, the gene expression of BMP-2, Runx2, β-catenin, Nrf2, Keap1 and integrinβ1 were up-regulated in the osteogenic differentiation of AECs induced by PEMF and/or osteo-induction medium.

CONCLUSIONS

Combined application of PEMF and osteo-induction medium is synergistic for the osteogenic differentiation of AECs. It might be a novel approach in the bone regenerative medicine.

Effect of Strontium Enhanced Calcium Phosphate Coating on <i>In Vitro</i> Behavior of Human Mesenchymal Stem Cell (hMSC)

Calcium phosphate is a widely used material as coating for metallic implants. This research describes a biomimetic coating techniques based on deposition of calcium phosphate films on a Ti6Al4V plates that was used to study the effect of strontium additive on the behavior of hMSCs. In this study, strontium additive was homogenously deposited onto calcium phosphate films on a Ti6AlV plates by using a biomimetic techniques. Strontium affected composition and morphology of calcium phosphate deposited on a Ti6Al4V plates to a varying degree, according to concentration of solutions used. The effect of strontium additive on proliferation and differentiation of hMSCs depended on the solution and concentration tested. In general, all individual three coatings showed decreased hMSCs proliferation. Strontium additive demonstrated a significant increase in differentiation into osteogenic lineage when compared with the control and calcium phosphate films without strontium additive. However, no cytotoxic effect of strontium additive in the concentrations tested was detected. The Fourier transform infrared spectra showed that this new coating closely resembles bone mineral. The techniques illustrated in this study mimics bone mineral containing strontium additive, making it constructive for studying basic processes of in vitro bone formation. The results showed in this study can be used for changing bone graft substitutes by addition of strontium additive on implants in order to affect their performance in bone repair and regeneration. Also, the system can aid rapid bone formation around the implant, reducing therewith the patient’s recovery time after surgery.

Arginine enhances osteoblastogenesis and inhibits adipogenesis through the regulation of Wnt and NFATc signaling in human mesenchymal stem cells

Arginine, an α-amino acid, has been reported to exert beneficial effects that ameliorate health problems and prevent excessive fat deposition. In this study, we investigated whether the activation of cell signaling by arginine can induce osteogenic differentiation and modulate excessive adipogenic differentiation in human mesenchymal stem cells (MSCs). Arginine potently induced the expression of type Iα1 collagen, osteocalcin, and ALP in a dose-dependent manner without causing cytotoxicity. Arginine significantly increased the mRNA expression of the osteogenic transcription factors runt-related transcription factor 2 (Runx2), DIx5, and osterix. Furthermore, arginine demonstrated its antiadipogenicity by decreasing adipocyte formation and triglyceride (TG) content in MSCs and inhibiting the mRNA expression of the adipogenic transcription factors peroxisome proliferator-activated receptor γ (PPARγ), CCAAT/enhancer-binding protein α (C/EBPα), and fatty acid binding protein 4 (Fabp4). This effect was associated with increased expression of Wnt5a, and nuclear factor of activated T-cells (NFATc), and was abrogated by antagonists of Wnt and NFATc, which indicated a role of Wnt and NFATc signaling in the switch from adipogenesis to osteoblastogenesis induced by arginine. In conclusion, this is the first report of the dual action of arginine in promoting osteogenesis and inhibiting adipocyte formation through involving Wnt5a and NFATc signaling pathway.

SrO- and MgO-doped microwave sintered 3D printed tricalcium phosphate scaffolds: mechanical properties and in vivo osteogenesis in a rabbit model

The presence of interconnected macro pores allows guided tissue regeneration in tissue engineering scaffolds. However, highly porous scaffolds suffer from having poor mechanical strength. Previously, we showed that microwave sintering could successfully be used to improve mechanical strength of macro porous tricalcium phosphate (TCP) scaffolds. This study reports the presence of SrO and MgO as dopants in TCP scaffolds improves mechanical and in vivo biological performance. We have used direct three dimensional printing (3DP) technology for scaffold fabrication. These 3DP scaffolds possessed multiscale porosity, that is, 3D interconnected designed macro pores along with intrinsic micro pores. A significant increase in mechanical strength, between 37 and 41%, was achieved due to SrO and MgO doping in TCP as compared with pure TCP. Maximum compressive strengths of 9.38 ± 1.86 MPa and 12.01 ± 1.56 MPa were achieved by conventional and microwave sintering, respectively, for SrO-MgO-doped 3DP scaffolds with 500 μm designed pores. Histomorphological and histomorphometric analysis revealed a significantly higher osteoid, bone and haversian canal formation induced by the presence of SrO and MgO dopants in 3DP TCP as compared with pure TCP scaffolds when tested in rabbit femoral condyle defect model. Increased osteon and thus enhanced network of blood vessel formation, and osteocalcin expression were observed in the doped TCP scaffolds. Our results show that these 3DP SrO-MgO-doped TCP scaffolds have the potential for early wound healing through accelerated osteogenesis and vasculogenesis.

Bioactive coatings for orthopaedic implants-recent trends in development of implant coatings

Joint replacement is a major orthopaedic procedure used to treat joint osteoarthritis. Aseptic loosening and infection are the two most significant causes of prosthetic implant failure. The ideal implant should be able to promote osteointegration, deter bacterial adhesion and minimize prosthetic infection. Recent developments in material science and cell biology have seen the development of new orthopaedic implant coatings to address these issues. Coatings consisting of bioceramics, extracellular matrix proteins, biological peptides or growth factors impart bioactivity and biocompatibility to the metallic surface of conventional orthopaedic prosthesis that promote bone ingrowth and differentiation of stem cells into osteoblasts leading to enhanced osteointegration of the implant. Furthermore, coatings such as silver, nitric oxide, antibiotics, antiseptics and antimicrobial peptides with anti-microbial properties have also been developed, which show promise in reducing bacterial adhesion and prosthetic infections. This review summarizes some of the recent developments in coatings for orthopaedic implants.

Fabrication of barium- and strontium-doped silica/titania hollow nanoparticles and their synergetic effects on promoting neuronal differentiation by activating ERK and p38 pathways

Pristine, barium-doped, and strontium-doped hollow nanoparticles (p-HNPs, Ba-HNP, and Sr-HNP; HNPs) are prepared by sonication-mediated etching and redeposition (SMER) method and alkali-earth-metal hydroxide solution treatment. The HNPs are investigated to facilitate synergetic neuronal differentiation through alkali-earth-metal doping and in conjunction with nerve growth factor (NGF). PC12 cells are used as model cells for neuronal differentiation. The differentiation efficiency is improved in the presence of the HNPs+NGF, and the neurite length is in the order of Sr-HNP+NGF > Ba-HNP+NGF > p-HNP+NGF > NGF. Silica/titania have increasing effect on both differentiation efficiency and neurite length, and doped barium/strontium influences additional elongation of the average neurite length. Take advantage of hollow structure, NGF is encapsulated into HNPs, and they are further applied for directly inducing differentiation. The maximum differentiation efficiency is 67% in presence of the NGF-encapsulated Sr-HNP, which was 1.3 times higher than previous research. Furthermore, the neurite length is also 2.7 times higher than MnO2 decorated poly(3,4-ethylenedioxythiophene) nanoellipsoids. Ba- and Sr-HNP may offer a possibility for novel application of metal-hybrid nanomaterials for cell differentiation, and can be expanded to other cellular applications.

Materials as stem cell regulators

The stem cell/material interface is a complex, dynamic microenvironment in which the cell and the material cooperatively dictate one another's fate: the cell by remodelling its surroundings, and the material through its inherent properties (such as adhesivity, stiffness, nanostructure or degradability). Stem cells in contact with materials are able to sense their properties, integrate cues via signal propagation and ultimately translate parallel signalling information into cell fate decisions. However, discovering the mechanisms by which stem cells respond to inherent material characteristics is challenging because of the highly complex, multicomponent signalling milieu present in the stem cell environment. In this Review, we discuss recent evidence that shows that inherent material properties may be engineered to dictate stem cell fate decisions, and overview a subset of the operative signal transduction mechanisms that have begun to emerge. Further developments in stem cell engineering and mechanotransduction are poised to have substantial implications for stem cell biology and regenerative medicine.

Hydroxyapatite coating affects the Wnt signaling pathway during peri-implant healing in vivo

Owing to its bio- and osteoconductivity, hydroxyapatite (HA) is a widely used implant material, but its osteogenic properties are only partly evaluated in vitro and in vivo. The present study focused on bone healing adjacent to HA-coated titanium (Ti) implants, with or without incorporated lithium ions (Li(+)). Special attention was given to the Wnt signaling pathway. The implants were inserted into rat tibia for 7 or 28 days and analyzed ex vivo, mainly by histomorphometry and quantitative real-time polymerase chain reaction (qPCR). HA-coated implants showed, irrespective of Li(+) content, bone-implant contact (BIC) and removal torque values significantly higher than those of reference Ti. Further, the expression of OCN, CTSK, COL1A1, LRP5/6 and WISP1 was significantly higher in implant-adherent cells of HA-coated implants, with or without Li(+). Significantly higher β-catenin expression and significantly lower COL2A1 expression were observed in peri-implant bone cells from HA with 14 ng cm(-2) released Li(+). Interestingly, Ti implants showed a significantly larger bone area (BA) in the threads than HA with 39 ng cm(-2) released Li(+), but had a lower BIC than any HA-coated implant. This study shows that HA, with or without Li(+), is a strong activator of the Wnt signaling pathway, and may to some degree explain its high bone induction capacity.

Potential application of hydrolyzed fish collagen for inducing the multidirectional differentiation of rat bone marrow mesenchymal stem cells

Hydrolyzed fish collagen (HFC) has recently attracted considerable attention because of its outstanding bioactivity. However, few studies have been performed to determine the biological effects of HFC on bone marrow mesenchymal stem cells (BMSCs), which are often used in regenerative medicine. In this study, the molecular weight, amino acid composition, and contact angle of HFC were measured. The influence of HFC on cell viability and the multidirectional differentiation of BMSCs into osteogenic, endothelial, adipogenic, chondrogenic, and neural lineages were also assessed. Furthermore, the mechanism by which HFC promotes osteogenesis was investigated at the protein level. The molecular weight of HFC ranged from 700 to 1300 Da, the contact angle of HFC was approximately 26°, and HFC was found to be composed of various amino acids, including glycine, proline, and hydroxyproline. At a concentration of 0.2 mg/mL, HFC promoted cell viability, and significantly up-regulated the expression of osteogenic markers (RUNX2, ALP, OPN, and OCN), as well as endothelial markers (CD31, VE-cadherin, and VEGFR2). Western blot results indicated that treatment of BMSCs with 0.2 mg/mL HFC could activate the MAPK/ERK signaling pathway and then increase the protein level of RUNX2, while treatment with PD98059, a specific inhibitor of ERK1/2, could significantly inhibit the expression of P-ERK and RUNX2. Interestingly, real-time PCR demonstrated that HFC inhibited the expression of adipogenic markers (LPL and ADFP) and chondrogenic markers (aggrecan and COLII), whereas it had no effect on neural differentiation markers (MAP2 and β3-tubulin). In summary, this study suggests that without the use of any additional inducing reagent, HFC has the potential to actively promote osteogenic and endothelial differentiation because of its high hydrophilicity and the optimal extracellular microenvironment supplied by its amino acids. This research also revealed that HFC inhibited adipogenic and chondrogenic differentiation, but it had no influence on the neural differentiation of rat bone marrow mesenchymal stem cells (rBMSCs).

A signal-amplification circuit between miR-218 and Wnt/β-catenin signal promotes human adipose tissue-derived stem cells osteogenic differentiation

Human adipose-derived stem cells (hASCs) have become a highly attractive source of seed cells in bone regenerative. It has become a key issue how to effectively improve osteogenic differentiation of hASCs in the bone tissue engineering. Numerous regulatory pathways dominate osteogenic differentiation of hASCs involve transcriptional factors and signaling molecules. However, how these factors combine with each other to regulate hASCs osteogenic differentiation still remain to be illustrated. The identification of microRNAs will illuminate this and might permit finely tuning the osteogenic differentiation process. Here, we present evidence that miR-218 acts as a positive regulator of hASCs osteogenesis. Real-time PCR shows that miR-218 was up-regulated during osteogenic differentiation. Overexpression of exogenous miR-218 enhanced osteogenic differentiation in vitro, whereas inhibition of miR-218 would suppress osteogenic differentiation. Furthermore, miR-218 directly targeted SFRP2 and DKK2, which is a WNT signaling pathway antagonist, and enhanced Wnt/β-catenin signaling activity. Finally, we found that mimicking Wnt/β-catenin signal strengthened the expression level of miR-218, while blocking the signal attenuated the expression level of miR-218. This feed-forward regulatory circuit provides additional insight into how miRNAs acting as a signal amplifier interact with signal molecules during hASCs osteogenic differentiation. Taken together, we have established a regulatory network with a central role for the miR-218 in hASCs osteogenic differentiation.

A novel strontium(II)-modified calcium phosphate bone cement stimulates human-bone-marrow-derived mesenchymal stem cell proliferation and osteogenic differentiation in vitro

In the present study, the in vitro effects of novel strontium-modified calcium phosphate bone cements (SrCPCs), prepared using two different approaches on human-bone-marrow-derived mesenchymal stem cells (hMSCs), were evaluated. Strontium ions, known to stimulate bone formation and therefore already used in systemic osteoporosis therapy, were incorporated into a hydroxyapatite-forming calcium phosphate bone cement via two simple approaches: incorporation of strontium carbonate crystals and substitution of Ca(2+) by Sr(2+) ions during cement setting. All modified cements released 0.03-0.07 mM Sr(2+) under in vitro conditions, concentrations that were shown not to impair the proliferation or osteogenic differentiation of hMSCs. Furthermore, strontium modification led to a reduced medium acidification and Ca(2+) depletion in comparison to the standard calcium phosphate cement. In indirect and direct cell culture experiments with the novel SrCPCs significantly enhanced cell proliferation and differentiation were observed. In conclusion, the SrCPCs described here could be beneficial for the local treatment of defects, especially in the osteoporotic bone.

In vitro evaluation of bioactive strontium-based ceramic with rabbit adipose-derived stem cells for bone tissue regeneration

The development of bone replacement materials is an important objective in the field of orthopaedic surgery. Due to the drawbacks of treating bone defects with autografts, synthetic bone graft materials have become optional. So in this work, a bone tissue engineering approach with radiopaque bioactive strontium incorporated calcium phosphate was proposed for the preliminary cytocompatibility studies for bone substitutes. Accumulating evidence indicates that strontium containing biomaterials promote enhanced bone repair and radiopacity for easy imaging. Hence, strontium calcium phosphate (SrCaPO4) and hydroxyapatite scaffolds have been investigated for its ability to support and sustain the growth of rabbit adipose-derived mesenchymal stem cells (RADMSCs) in vitro. They were characterized via Micro-CT for pore size distribution. Cells used were isolated from New Zealand White rabbit adipose tissue, characterized by FACS and via differentiation into the osteogenic lineage by alkaline phosphatase, Masson's trichome, Alizarin Red and von Kossa staining on day 28. Material-cell interaction was observed by SEM imaging of cell morphology on contact with material. Live-Dead analysis was done by confocal laser scanning microscopy and cell cluster analysis via μCT. The in vitro biodegradation, elution and nucleation of apatite formation of the material was evaluated using simulated body fluid and phosphate buffered saline in static regime up to 28 days at 37 °C. These results demonstrated that SrCaPO4 is a good candidate for bone tissue engineering applications and with osteogenically-induced RADMSCs, they may serve as potential implants for the repair of critical-sized bone defects.

3D printed tricalcium phosphate scaffolds: Effect of SrO and MgO doping on in vivo osteogenesis in a rat distal femoral defect model

The presence of interconnected macro pores is important in tissue engineering scaffolds for guided tissue regeneration. This study reports in vivo biological performance of interconnected macro porous tricalcium phosphate (TCP) scaffolds due to the addition of SrO and MgO as dopants in TCP. We have used direct three dimensional printing (3DP) technology for scaffold fabrication followed by microwave sintering. Mechanical strength was evaluated by scaffolds with 500 µm, 750 µm, and 1000 µm interconnected designed pore sizes. Maximum compressive strength of 12.01 ± 1.56 MPa was achieved for 500 µm interconnected designed pore size Sr-Mg doped scaffold. In vivo biological performance of the microwave sintered pure TCP and Sr-Mg doped TCP scaffolds was assessed by implanting 350 µm designed interconnected macro porous scaffolds in rat distal femoral defect. Sintered pore size of these 3D printed scaffolds were 311 ± 5.9 µm and 245 ± 7.5 µm for pure and SrO-MgO doped TCP scaffolds, respectively. These 3D printed scaffolds possessed multiscale porosity, i.e., 3D interconnected designed macro pores along with intrinsic micro pores. Histomorphology and histomorphometric analysis revealed a significant increase in osteoid like new bone formation, and accelerated mineralization inside SrO and MgO doped 3D printed TCP scaffolds as compared to pure TCP scaffolds. An increase in osteocalcin and type I collagen level was also observed in rat blood serum with SrO and MgO doped TCP scaffolds compared to pure TCP scaffolds. Our results show that these 3D printed SrO and MgO doped TCP scaffolds with multiscale porosity contributed to early healing through accelerated osteogenesis.

Enhanced osteoporotic bone regeneration by strontium-substituted calcium silicate bioactive ceramics

The regeneration capacity of the osteoporotic bones is generally lower than that of the normal bones. Current methods of bone defect treatment for osteoporosis are not always satisfactory. Recent studies have shown that the silicate based biomaterials can stimulate osteogenesis and angiogenesis due to the silicon (Si) ions released from the materials, and enhance bone regeneration in vivo. Other studies showed that strontium (Sr) plays a distinct role on inhibiting bone resorption. Based on the hypothesis that the combination of Si and Sr may have synergetic effects on osteoporotic bone regeneration, the porous Sr-substituted calcium silicate (SrCS) ceramic scaffolds combining the functions of Sr and Si elements were developed with the goals to promote osteoporotic bone defect repair. The effects of the ionic extract from SrCS on osteogenic differentiation of bone marrow mesenchymal stem cells derived from ovariectomized rats (rBMSCs-OVX), angiogenic differentiation of human umbilical vein endothelial cells (HUVECs) were investigated. The in vitro results showed that Sr and Si ions released from SrCS enhanced cell viability, alkaline phosphatase (ALP) activity, and mRNA expression levels of osteoblast-related genes of rBMSCs-OVX and expression of vascular endothelial growth factor (VEGF) without addition of extra osteogenic and angiogenic reagents. The activation in extracellular signal-related kinases (ERK) and p38 signaling pathways were observed in rBMSCs-OVX cultured in the extract of SrCS, and these effects could be blocked by ERK inhibitor PD98059, and P38 inhibitor SB203580, respectively. Furthermore, the ionic extract of SrCS stimulated HUVECs proliferation, differentiation and angiogenesis process. The in vivo experiments revealed that SrCS dramatically stimulated bone regeneration and angiogenesis in a critical sized OVX calvarial defect model, and the enhanced bone regeneration might be attributed to the modulation of osteogenic differentiation of endogenous mesenchymal stem cells (MSCs) and the inhibition of osteoclastogenesis, accompanying with the promotion of the angiogenic activity of endothelial cells (ECs).

The stimulation of osteogenic differentiation of mesenchymal stem cells and vascular endothelial growth factor secretion of endothelial cells by β-CaSiO3/β-Ca3(PO4)2 scaffolds

Porous β-CaSiO3/β-Ca3(PO4)2 (β-CS/β-TCP) composite scaffolds have been previously shown to promote bone formation in vivo. However, the mechanisms underlying such beneficial effects remain unclear. In this study, we recreated an extracellular environment using the extracts of β-CS/β-TCP composites developed in our previous in vivo study, and investigated the effects of the extracts on osteogenic differentiation of rat bone marrow-derived mesenchymal stem cells (rBMSCs) and its related mechanisms. The angiogenic potential of the extracts was also evaluated using human umbilical vein endothelial cells (HUVECs). In the absence of osteogenic supplements, the osteogenic differentiation of rBMSCs was detected by alkaline phosphatase (ALP) activity assay and the messenger RNA expression of a panel of osteoblast markers. The results showed that the soluble ions of porous β-CS/β-TCP composites were capable of promoting cell viability, directly inducing cell differentiation. The increase in phosphorylation of AMP-activated protein kinase (AMPK) and ERK1/2 were observed in rBMSCs cultured in β-CS/β-TCP composite extracts. The ALP expression, calcium deposition, and ERK1/2 phosphorylation of rBMSCs, which was promoted by ions released from β-CS/β-TCP composites, were blocked by an AMPK inhibitor, Compound C. These results indicate that bioactive ions extracted from β-CS/β-TCP composites could stimulate the osteogenic differentiation of rBMSCs via the AMPK-Erk1/2 pathway. Interestingly, the secretion of vascular endothelial growth factor and the viability of HUVECs were shown to be enhanced in the presence of extracts from the β-CS/β-TCP composite scaffolds. Our findings suggest that 50 or 80% wt. CS could promote bone regeneration by stimulating osteogenesis and angiogenesis.