The developmental control of osteoblast-specific gene expression: role of specific transcription factors and the extracellular matrix environment

GPR120: A bi-potential mediator to modulate the osteogenic and adipogenic differentiation of BMMSCs

Free fatty acids display diverse effects as signalling molecules through GPCRs in addition to their involvement in cellular metabolism. GPR120, a G protein-coupled receptor for long-chain unsaturated fatty acids, has been reported to mediate adipogenesis in lipid metabolism. However, whether GPR120 also mediates osteogenesis and regulates BMMSCs remain unclear. In this study, we showed that GPR120 targeted the bi-potential differentiation of BMMSCs in a ligand dose-dependent manner. High concentrations of TUG-891 (a highly selective agonist of GPR120) promoted osteogenesis via the Ras-ERK1/2 cascade, while low concentrations elevated P38 and increased adipogenesis. The fine molecular regulation of GPR120 was implemented by up-regulating different integrin subunits (α1, α2 and β1; α5 and β3). The administration of high doses of TUG-891 rescued oestrogen-deficient bone loss in vivo, further supporting an essential role of GPR120 in bone metabolism. Our findings, for the first time, showed that GPR120-mediated cellular signalling determines the bi-potential differentiation of BMMSCs in a dose-dependent manner. Additionally, the induction of different integrin subunits was involved in the cytoplasmic regulation of a seesaw-like balance between ERK and p38 phosphorylation. These findings provide new hope for developing novel remedies to treat osteoporosis by adjusting the GPR120-mediated differentiation balance of BMMSCs.

Dynamic Expression Profiles of Marker Genes in Osteogenic Differentiation of Human Bone Marrow-derived Mesenchymal Stem Cells

OBJECTIVE

To observe the expression profiles of osteoblast-related genes in human mesenchymal stem cells (MSCs) derived from bone marrow during osteogenic differentiation.

METHODS

MSCs were induced to differentiate with MSC osteogenic differentiation medium for 7, 14, 21 and 28 days respectively. Alizarin Red staining was used to detect matrix mineralization. Expression of osteoblast-related genes, including osteocalcin, osteopontin, Runt-related transcription factor 2 (Runx2), alkaline phosphatase and collagen type 1, was assessed with quantitative reverse transcription-polymerase chain reaction.

RESULTS

On day 14 after induction of differentiation, cells were stained positively with Alizarin Red. The expression levels of these genes exhibited an upward trend as induction time was prolonged. Exposure to osteogenic differentiation medium less than 21 days did not significantly induce osteocalcin expression; osteocalcin expression levels in the differentiated cells induced for 21 and 28 days were 1.63 and 2.46 times as high as the undifferentiated cells respectively (all P<0.05). Stimulation with MSC osteogenic differentiation medium over 14 days significantly enhanced bone marrow-derived MSCs to express osteopontin and Runx2 genes (all P<0.05). Osteogenic differentiation medium could significantly induce the expressions of alkaline phosphatase and collagen type1 genes (all P<0.05). Their expressions reached the peak levels on day 21, which were increased more than 4- and 3-fold respectively.

CONCLUSION

Human bone marrow-derived MSCs could exhibit the sequential expression pattern of osteoblast marker genes during osteogenic differentiation in vitro.

Alendronate-Eluting Biphasic Calcium Phosphate (BCP) Scaffolds Stimulate Osteogenic Differentiation

Biphasic calcium phosphate (BCP) scaffolds have been widely used in orthopedic and dental fields as osteoconductive bone substitutes. However, BCP scaffolds are not satisfactory for the stimulation of osteogenic differentiation and maturation. To enhance osteogenic differentiation, we prepared alendronate- (ALN-) eluting BCP scaffolds. The coating of ALN on BCP scaffolds was confirmed by scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDS), and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR). An in vitro release study showed that release of ALN from ALN-eluting BCP scaffolds was sustained for up to 28 days. In vitro results revealed that MG-63 cells grown on ALN-eluting BCP scaffolds exhibited increased ALP activity and calcium deposition and upregulated gene expression of Runx2, ALP, OCN, and OPN compared with the BCP scaffold alone. Therefore, this study suggests that ALN-eluting BCP scaffolds have the potential to effectively stimulate osteogenic differentiation.

MicroRNA-24 Regulates Osteogenic Differentiation via Targeting T-Cell Factor-1

MicroRNAs (miRNAs) have been reported to have diverse biological roles in regulating many biological processes, including osteogenic differentiation. In the present study, we identified that miR-24 was a critical regulator during osteogenic differentiation. We found that overexpression of miR-24 significantly inhibited osteogenic differentiation, which decreased alkaline phosphatase activity, matrix mineralization and the expression of osteogenic differentiation markers. In contrast, inhibition of miR-24 exhibited an opposite effect. Furthermore, we delineated that miR-24 regulates post-transcriptionals of T-cell factor-1 (Tcf-1) via targeting the 3'-untranslated region (UTR) of Tcf-1 mRNA. MiR-24 was further found to regulate the protein expression of Tcf-1 in the murine osteoprogenitors cells and bone mesenchymal stem cells. Additionally, the positive effect of miR-24 suppression on osteoblast differentiation was apparently abrogated by Tcf-1 silencing. Taken together, our data suggest that miR-24 participates in osteogenic differentiation by targeting and regulating Tcf-1 expression in osteoblastic cells.

Increased osteogenesis in osteoporotic bone marrow stromal cells by overexpression of leptin

Osteoporosis leads to increased bone fractures and net bone loss, in part because of the dysfunction of bone marrow stromal cells (BMSCs). Leptin is an adipokine that plays important roles in many biological processes, including the regulation of the actions of mesenchymal stem cells. Our aim is to investigate the osteogenic effects of leptin in osteoporotic BMSCs in vitro and in vivo. The leptin gene was transferred into BMSCs isolated from osteoporotic rats by using recombinant adenoviruses. Once the gene and protein expression of leptin had been confirmed, MTT assays were performed; leptin overexpression was confirmed not to affect the viability of osteoporotic BMSCs. However, alkaline phosphatase (ALP) activity measurements, Alizarin red staining and analyses by quantitative real-time reverse transcription with the polymerase chain reaction revealed that leptin upregulated ALP activity, mineral deposition and the mRNA levels of runt-related transcription factor 2, ALP and collagen type І. Lastly, the effects of leptin on osteogenic differentiation were assessed in vivo. Cells transfected with leptin exhibited increased osteogenic differentiation and enhanced formation of bone-like structures. This study thus reveals, for the first time, that the overexpression of leptin in osteoporotic BMSCs (1) enhances their capacity to differentiate into osteoblasts and to form bone-like tissue and (2) might be a useful skeletal regenerative therapy in osteoporotic patients.

MicroRNA-302a stimulates osteoblastic differentiation by repressing COUP-TFII expression

Chicken ovalbumin upstream promoter transcription factor II (COUP-TFII) is a potent transcription factor that represses osteoblast differentiation and bone formation. Previously, we observed that stimuli for osteoblast differentiation, such as bone morphogenetic protein 2 (BMP2), inhibits COUP-TFII expression. This study was undertaken to identify BMP2-regulated and COUP-TFII-targeting microRNAs (miRNAs), and to explore their regulatory roles in osteoblast differentiation. Based on in silico analysis, 12 miRNAs were selected and their expression in BMP2-treated MC3T3-E1 cells was examined. BMP2 induced miR-302a expression in dose- and time-dependent manners with the decrease in COUP-TFII expression. Runx2, a BMP2-downstream transcription factor, specifically regulated miR-302a expression and its promoter activity. A computer-based prediction algorithm led to the identification of two miR-302a binding sites on the 3'-untranslational region of COUP-TFII mRNA (S1: 620-626 bp, S2: 1,016-1,022 bp), and a luciferase assay showed that miR-302a directly targeted S1 and S2. Transfection of miR-302a precursor significantly enhanced expression of osteogenic marker genes with decreasing COUP-TFII mRNA and protein level, alkaline phosphatase activity and matrix mineralization. On the other hand, inhibition of miR-302a significantly attenuated BMP2-induced osteoblast specific gene expression, alkaline phosphatase activity, and matrix mineralization with increasing COUP-TFII mRNA and protein level. These results indicate that miR-302a is induced by osteogenic stimuli and promotes osteoblast differentiation by targeting COUP-TFII. MiR-302a could be a positive regulator for osteoblast differentiation.

Integrin mediated adhesion of osteoblasts to connective tissue growth factor (CTGF/CCN2) induces cytoskeleton reorganization and cell differentiation

Pre-osteoblast adhesion and interaction with extracellular matrix (ECM) proteins through integrin receptors result in activation of signaling pathways regulating osteoblast differentiation. Connective tissue growth factor (CTGF/CCN2) is a matricellular protein secreted into the ECM. Prior studies in various cell types have shown that cell adhesion to CTGF via integrin receptors results in activation of specific signaling pathways that regulate cell functions, such as differentiation and cytoskeletal reorganization. To date, there are no studies that have examined whether CTGF can serve as an adhesive substrate for osteoblasts. In this study, we used the MC3T3-E1 cell line to demonstrate that CTGF serves as an adhesive matrix for osteoblasts. Anti-integrin blocking experiments and co-immunoprecipitation assays demonstrated that the integrin α_vβ₁ plays a key role in osteoblast adhesion to a CTGF matrix. Immunofluorescence staining of osteoblasts cultured on a CTGF matrix confirmed actin cytoskeletal reorganization, enhanced spreading, formation of focal adhesions, and activation of Rac1. Alkaline phosphatase (ALP) staining and activity assays, as well as Alizarin red staining demonstrated that osteoblast attachment to CTGF matrix enhanced maturation, bone nodule formation and matrix mineralization. To investigate whether the effect of CTGF on osteoblast differentiation involves integrin-mediated activation of specific signaling pathways, we performed Western blot, chromatin immunoprecipitation (ChIP) and qPCR assays. Osteoblasts cultured on a CTGF matrix showed increased total and phosphorylated (activated) forms of focal adhesion kinase (FAK) and extracellular signal-regulated kinase (ERK). Inhibition of ERK blocked osteogenic differentiation in cells cultured on a CTGF matrix. There was an increase in runt-related transcription factor 2 (Runx2) binding to the osteocalcin gene promoter, and in the expression of osteogenic markers regulated by Runx2. Collectively, the results of this study are the first to demonstrate CTGF serves as a suitable matrix protein, enhancing osteoblast adhesion (via α_vβ₁ integrin) and promoting cell spreading via cytoskeletal reorganization and Rac1 activation. Furthermore, integrin-mediated activation of ERK signaling resulted in increased osteoblast differentiation accompanied by an increase in Runx2 binding to the osteocalcin promoter and in the expression of osteogenic markers.

Wnt/β-catenin signaling mediates osteoblast differentiation triggered by peptide-induced α5β1 integrin priming in mesenchymal skeletal cells

The α5β1 integrin is a key fibronectin (FN) receptor that binds to RGD-containing peptides to mediate cell adhesion. We previously reported that α5β1 integrin promotes osteogenic differentiation in mesenchymal skeletal cells (MSCs), but the underlying mechanisms are not fully understood. In this study, we determined the signaling mechanisms induced by α5β1 integrin interaction with its high-affinity ligand CRRETAWAC in murine and human MSCs and in vivo. We show that cyclized CRRETAWAC fully displaced MSC adhesion to FN, whereas related peptides lacking the full RRET sequence produced a partial displacement, indicating that RRET acts as an RGD-like sequence that is required to antagonize FN-mediated cell adhesion. However, all peptides increased focal adhesion kinase phosphorylation, OSE2 transcriptional activity, osteoblast gene expression, and matrix mineralization in MSCs, indicating that peptide-induced α5β1 integrin priming can promote osteogenic differentiation independently of the RRET sequence. Biochemical analyses showed that peptide-induced α5β1 integrin priming transiently increased PI3K/Akt phosphorylation and promoted Wnt/β-catenin transcriptional activity independently of RRET. Consistently, pharmacological inhibition of PI3K activity reduced osteoblast differentiation and abolished Wnt regulatory gene expression induced by α5β1 integrin priming. In vivo, systemic delivery of cyclized GACRETAWACGA linked to (DSS)6 to allow delivery to bone-forming sites for 6 weeks increased serum osteocalcin levels and improved long bone mass and microarchitecture in SAMP-6 senescent osteopenic mice. The results support a mechanism whereby α5β1 integrin priming by high-affinity ligands integrates Wnt/β-catenin signaling to promote osteoblast differentiation independently of cell adhesion, which could be used to improve bone mass and microarchitecture in the aging skeleton.

Enhancement of osteogenesis and biodegradation control by brushite coating on Mg-Nd-Zn-Zr alloy for mandibular bone repair

To diminish incongruity between bone regeneration and biodegradation of implant magnesium alloy applied for mandibular bone repair, a brushite coating was deposited on a matrix of a Mg-Nd-Zn-Zr (hereafter, denoted as JDBM) alloy to control the degradation rate of the implant and enhance osteogenesis of the mandible bone. Both in vitro and in vivo evaluations were carried out in the present work. Viability and adhesion assays of rabbit bone marrow mesenchyal stem cells (rBM-MSCs) were applied to determine the biocompatibility of a brushite-coated JDBM alloy. Osteogenic gene expression was characterized by quantitative real-time polymerase chain reaction (RT-PCR). Brushite-coated JDBM screws were implanted into mandible bones of rabbits for 1, 4, and 7 months, respectively, using 316L stainless steel screws as a control group. In vivo biodegradation rate was determined by synchrotron radiation X-ray microtomography, and osteogenesis was observed and evaluated using Van Gieson's picric acid-fuchsin. Both the naked JDBM and brushite-coated JDBM samples revealed adequate biosafety and biocompatibility as bone repair substitutes. In vitro results showed that brushite-coated JDBM considerably induced osteogenic differentiation of rBM-MSCs. And in vivo experiments indicated that brushite-coated JDBM screws presented advantages in osteoconductivity and osteogenesis of mandible bone of rabbits. Degradation rate was suppressed at a lower level at the initial stage of implantation when new bone tissue formed. Brushite, which can enhance oeteogenesis and partly control the degradation rate of an implant, is an appropriate coating for JDBM alloys used for mandibular repair. The Mg-Nd-Zn-Zr alloy with brushite coating possesses great potential for clinical applications for mandibular repair.

Perovskite ceramic nanoparticles in polymer composites for augmenting bone tissue regeneration

There is increasing interest in the use of nanoparticles as fillers in polymer matrices to develop biomaterials which mimic the mechanical, chemical and electrical properties of bone tissue for orthopaedic applications. The objective of this study was to prepare poly(ϵ-caprolactone) (PCL) nanocomposites incorporating three different perovskite ceramic nanoparticles, namely, calcium titanate (CT), strontium titanate (ST) and barium titanate (BT). The tensile strength and modulus of the composites increased with the addition of nanoparticles. Scanning electron microscopy indicated that dispersion of the nanoparticles scaled with the density of the ceramics, which in turn played an important role in determining the enhancement in mechanical properties of the composite. Dielectric spectroscopy revealed improved permittivity and reduced losses in the composites when compared to neat PCL. Nanofibrous scaffolds were fabricated via electrospinning. Induction coupled plasma-optical emission spectroscopy indicated the release of small quantities of Ca(+2), Sr(+2), Ba(+2) ions from the scaffolds. Piezo-force microscopy revealed that BT nanoparticles imparted piezoelectric properties to the scaffolds. In vitro studies revealed that all composites support osteoblast proliferation. Expression of osteogenic genes was enhanced on the nanocomposites in the following order: PCL/CT > PCL/ST > PCL/BT > PCL. This study demonstrates that the use of perovskite nanoparticles could be a promising technique to engineer better polymeric scaffolds for bone tissue engineering.

Nrf2 is required for normal postnatal bone acquisition in mice

A large body of literature suggests that bone metabolism is susceptible to the ill effects of reactive species that accumulate in the body and cause cellular dysfunction. One of the body’s front lines in defense against such damage is the transcription factor, Nrf2. This transcription factor regulates a plethora of antioxidant and cellular defense pathways to protect cells from such damage. Despite the breadth of knowledge of both the function of Nrf2 and the effects of reactive species in bone metabolism, the direct role of Nrf2 in skeletal biology has yet to be thoroughly examined. Thus, in the current study, we have examined the role of Nrf2 in postnatal bone metabolism in mice. Mice lacking Nrf2 (Nrf2^−/−) exhibited a marked deficit in postnatal bone acquisition, which was most severe at 3 weeks of age when osteoblast numbers were 12-fold less than observed in control animals. While primary osteoblasts from Nrf2^−/− mice functioned normally in vitro, the colony forming capacity of bone marrow stromal cells (BMSCs) from these mice was significantly reduced compared to controls. This defect could be rescued through treatment with the radical scavenger N-acetyl cysteine (NAC), suggesting that increased reactive species stress might impair early osteoblastogenesis in BMSCs and lead to the failure of bone acquisition observed in Nrf2^−/− animals. Taken together, these studies suggest Nrf2 represents a key pathway in regulating bone metabolism, which may provide future therapeutic targets to treat osteoporosis.

Type III collagen regulates osteoblastogenesis and the quantity of trabecular bone

Type III collagen (Col3), a fibril-forming collagen, is a major extracellular matrix component in a variety of internal organs and skin. It is also expressed at high levels during embryonic skeletal development and is expressed by osteoblasts in mature bone. Loss of function mutations in the gene encoding Col3 (Col3a1) are associated with vascular Ehlers-Danlos syndrome (EDS). Although the most significant clinical consequences of this syndrome are associated with catastrophic failure and impaired healing of soft tissues, several studies have documented skeletal abnormalities in vascular EDS patients. However, there are no reports of the role of Col3 deficiency on the murine skeleton. We compared craniofacial and skeletal phenotypes in young (6-8 weeks) and middle-aged (>1 year) control (Col3(+/+)) and haploinsufficient (Col3(+/-)) mice, as well as young null (Col3(-/-)) mice by microcomputed tomography (μCT). Although Col3(+/-) mice did not have significant craniofacial abnormalities based upon cranial morphometrics, μCT analysis of distal femur trabecular bone demonstrated significant reductions in bone volume (BV), bone volume fraction (BV/TV), connectivity density, structure model index and trabecular thickness in young adult female Col3(+/-) mice relative to wild-type littermates. The reduction in BV/TV persisted in female mice at 1 year of age. Next, we evaluated the role of Col3 in vitro. Osteogenesis assays revealed that cultures of mesenchymal progenitors collected from Col3(-/-) embryos display decreased alkaline phosphatase activity and reduced capacity to undergo mineralization. Consistent with this data, a reduction in expression of osteogenic markers (type I collagen, osteocalcin and bone sialoprotein) correlates with reduced bone Col3 expression in Col3(+/-) mice and with age in vivo. A small but significant reduction in osteoclast numbers was found in Col3(+/-) compared to Col3(+/+) bones. Taken together, these findings indicate that Col3 plays a role in development of trabecular bone through its effects on osteoblast differentiation.

Nrf2 is a novel regulator of bone acquisition

Nuclear factor E2 p45-related factor 2 (Nrf2) is a transcription factor involved in the expression of cytoprotective genes induced by external stresses. We investigated the role of Nrf2 in osteoclast and osteoblast differentiation. Nrf2 knockdown or deletion increased osteoclastic differentiation from bone marrow-derived macrophages (BMMs) through the upregulation of NF-κB, c-Fos, and NFATc1 transcription factors. Nrf2 also inhibited osteoblast differentiation and mineralization via suppression of key regulatory proteins, such as Runx2, osteocalcin, and osterix. Micro-computed tomography and histomorphometric analyses showed an increase in bone mass of Nrf2 knockout compared to that of wild type mice. In addition, the mineral apposition rate and the number of osteoblasts in bone were higher in Nrf2 knockout mice. However, bone resorption parameters, namely DPD and CTX levels, were not affected by Nrf2 deletion. In a coculture condition where calvarial osteoblasts and BMMs from wild type and Nrf2 knockout mice were grown, deletion of Nrf2 in osteoblasts markedly reduced osteoclast formation. This effect was due to an increase in OPG expression in Nrf2 knockout osteoblasts. Taken as a whole, these results indicate that Nrf2 is intrinsically inhibitory to both osteoblast and osteoclast differentiation but its effect on osteoblasts is dominant to its effect on osteoclasts in vivo.

Collagen peptide and vitamin C additively attenuate age-related skin atrophy in Sod1-deficient mice

Age-related skin thinning is correlated with a decrease in the content of collagen in the skin. Accumulating evidence suggests that collagen peptide (CP) and vitamin C (VC) transcriptionally upregulate type I collagen in vivo. However, the additive effects of CP and VC on age-related skin changes remain unclear. We herein demonstrate that CP and a VC derivative additively corrected age-related skin thinning via reduced oxidative damage in superoxide dismutase 1 (Sod1)-deficient mice. Co-treatment with these compounds significantly normalized the altered gene expression of Col1a1, Has2, and Ci1, a proton-coupled oligopeptide transporter, in Sod1(-/-) skin. The in vitro analyses further revealed that collagen oligopeptide, a digestive product of ingested CP, significantly promoted the bioactivity of the VC derivative with respect to the migration and proliferation of Sod1(-/-) fibroblasts. These findings suggest that combined treatment with CP and VC is effective in cases of age-related skin pathology.

Osteoactivin induces transdifferentiation of C2C12 myoblasts into osteoblasts

Osteoactivin (OA) is a novel osteogenic factor important for osteoblast differentiation and function. Previous studies showed that OA stimulates matrix mineralization and transcription of osteoblast specific genes required for differentiation. OA plays a role in wound healing and its expression was shown to increase in post fracture calluses. OA expression was reported in muscle as OA is upregulated in cases of denervation and unloading stress. The regulatory mechanisms of OA in muscle and bone have not yet been determined. In this study, we examined whether OA plays a role in transdifferentiation of C2C12 myoblast into osteoblasts. Infected C2C12 with a retroviral vector overexpressing OA under the CMV promoter were able to transdifferentiate from myoblasts into osteoblasts. Immunofluorescence analysis showed that skeletal muscle marker MF-20 was severely downregulated in cells overexpressing OA and contained significantly less myotubes compared to uninfected control. C2C12 myoblasts overexpressing OA showed an increase in expression of bone specific markers such as alkaline phosphatase and alizarin red staining, and also showed an increase in Runx2 protein expression. We also detected increased levels of phosphorylated focal adhesion kinase (FAK) in C2C12 myoblasts overexpressing OA compared to control. Taken together, our results suggest that OA is able to induce transdifferentiation of myoblasts into osteoblasts through increasing levels of phosphorylated FAK.

Biocompatibility of nano-hydroxyapatite/Mg-Zn-Ca alloy composite scaffolds to human umbilical cord mesenchymal stem cells from Wharton's jelly in vitro

Seeding cells and scaffolds play pivotal roles in bone tissue engineering and regenerative medicine. Wharton's jelly-derived mesenchymal stem cells (WJCs) from human umbilical cord represent attractive and promising seeding cells in tissue regeneration and engineering for treatment applications. This study was carried out to explore the biocompatibility of scaffolds to seeding cells in vitro. Rod-like nano-hydroxyapatite (RN-HA) and flake-like micro-hydroxyapatite (FM-HA) coatings were prepared on Mg-Zn-Ca alloy substrates using micro-arc oxidation and electrochemical deposition. WJCs were utilized to investigate the cellular biocompatibility of Mg-Zn-Ca alloys after different surface modifications by observing the cell adhesion, morphology, proliferation, and osteoblastic differentiation. The in vitro results indicated that the RN-HA coating group was more suitable for cell proliferation and cell osteoblastic differentiation than the FM-HA group, demonstrating better biocompatibility. Our results suggested that the RN-HA coating on Mg-Zn-Ca alloy substrates might be of great potential in bone tissue engineering.

Hydroxyapatite and calcified elastin induce osteoblast-like differentiation in rat aortic smooth muscle cells

Vascular calcification can be categorized into two different types. Intimal calcification related to atherosclerosis and elastin-specific medial arterial calcification (MAC). Osteoblast-like differentiation of vascular smooth muscle cells (VSMCs) has been shown in both types; however, how this relates to initiation of vascular calcification is unclear. We hypothesize that the initial deposition of hydroxyapatite-like mineral in MAC occurs on degraded elastin first and that causes osteogenic transformation of VSMCs. To test this, rat aortic smooth muscle cells (RASMCs) were cultured on hydroxyapatite crystals and calcified aortic elastin. Using RT-PCR and specific protein assays, we demonstrate that RASMCs lose their smooth muscle lineage markers like alpha smooth muscle actin (SMA) and myosin heavy chain (MHC) and undergo chondrogenic/osteogenic transformation. This is indicated by an increase in the expression of typical chondrogenic proteins such as aggrecan, collagen type II alpha 1(Col2a1) and bone proteins such as runt-related transcription factor 2 (RUNX2), alkaline phosphatase (ALP) and osteocalcin (OCN). Furthermore, when calcified conditions are removed, cells return to their original phenotype. Our data supports the hypothesis that elastin degradation and calcification precedes VSMCs' osteoblast-like differentiation.

Involvement of MAPK signaling pathway in the osteogenic gene expressions of Cervi Pantotrichum Cornu in MG-63 human osteoblast-like cells

AIMS

The purposes of this study were to determine whether Cervi Pantotrichum Cornu (CPC) has osteogenic activities in human osteoblastic MG-63 cells and to investigate the underlying molecular mechanism.

MAIN METHODS

The effects of CPC on alkaline phosphatase activity, collagen synthesis, and calcium deposits were measured. The COL1A1, ALPL, BGLAP, and SPP1 expressions were measured by real-time PCR. Phosphorylated MAP kinases (ERK1/2, JNK1/2, p38, ELK1, and cJUN) were studied by western blot analysis. The involvement of MAPK pathway in osteogenic gene expressions was determined by using each selective MAPK inhibitor (PD98059, SP600125, and SB203580).

KEY FINDINGS

CPC increased alkaline phosphatase activity, collagen synthesis, and calcium deposits. CPC activated ERK1/2, JNK1/2, p38, and ELK1 phosphorylation except cJUN. CPC increased the COL1A1, ALPL, BGLAP, and SPP1 gene expressions. The elevated COL1A1 and BGLAP expressions were inhibited by PD98059, SP600125 or SB203580. The elevated ALPL expression was blocked by SB203580. The elevated SPP1 expression was inhibited by SP600125 or SB203580. CPC increased COL1A1 and BGLAP expressions via ERK1/2, JNK1/2, and p38 MAPKs pathways and SPP1 expression via JNK1/2 and p38 pathways. p38 pathway is needed for ALPL expression.

SIGNIFICANCE

These results imply that MAPK signaling pathway is an indispensable factor for bone matrix genes expression of CPC in MG-63 human osteoblast-like cells.

Potential of RAS inhibition to improve metabolic bone disorders

Metabolic bone disorder is usually caused by abnormalities of minerals and hormones metabolism. Recently, it has been proved by several studies that the renin-angiotensin system (RAS) in local bone tissue is directly involved in bone metabolism. Activation of skeletal RAS plays an important role in bone metabolic disorders. Based on in vitro, in vivo, and clinical studies, this review explains the roles of RAS in bone metabolism and also covers the potential approaches and beneficial effects of RAS inhibition on bone health. Differential strategies for inhibiting RAS can be employed to maintain bone health, which are attributed primarily to the reduced level of angiotensin II (AngII) and suppressed stimulation of the AngII signaling pathway. The use of renin inhibitors, angiotensin-converting enzyme inhibitors, and AngII receptor blockers either individually or in combination with each other could have promising results in fighting bone metabolic disorders associated with other cardiovascular diseases as well as independent bone injuries.

EB1 levels are elevated in ascorbic Acid (AA)-stimulated osteoblasts and mediate cell-cell adhesion-induced osteoblast differentiation

Osteoblasts are differentiated mesenchymal cells that function as the major bone-producing cells of the body. Differentiation cues including ascorbic acid (AA) stimulation provoke intracellular changes in osteoblasts leading to the synthesis of the organic portion of the bone, which includes collagen type I α1, proteoglycans, and matrix proteins, such as osteocalcin. During our microarray analysis of AA-stimulated osteoblasts, we observed a significant up-regulation of the microtubule (MT) plus-end binding protein, EB1, compared with undifferentiated osteoblasts. EB1 knockdown significantly impaired AA-induced osteoblast differentiation, as detected by reduced expression of osteoblast differentiation marker genes. Intracellular examination of AA-stimulated osteoblasts treated with EB1 siRNA revealed a reduction in MT stability with a concomitant loss of β-catenin distribution at the cell cortex and within the nucleus. Diminished β-catenin levels in EB1 siRNA-treated osteoblasts paralleled an increase in phospho-β-catenin and active glycogen synthase kinase 3β, a kinase known to target β-catenin to the proteasome. EB1 siRNA treatment also reduced the expression of the β-catenin gene targets, cyclin D1 and Runx2. Live immunofluorescent imaging of differentiated osteoblasts revealed a cortical association of EB1-mcherry with β-catenin-GFP. Immunoprecipitation analysis confirmed an interaction between EB1 and β-catenin. We also determined that cell-cell contacts and cortically associated EB1/β-catenin interactions are necessary for osteoblast differentiation. Finally, using functional blocking antibodies, we identified E-cadherin as a major contributor to the cell-cell contact-induced osteoblast differentiation.

Composite electrospun nanofibers for influencing stem cell fate

The design of new bioactive materials, provided with "instructive properties" and able to regulate stem cell behavior, is the goal for several research groups involved in tissue engineering. This new function, commonly reserved for growth factors, can lead to the development of a new class of implantable scaffolds, useful for accelerating tissue regeneration in a controlled manner. In this scenario, the likely most versatile and effective tools for the realization of such scaffolds are based on nano- and microtechnology. Here, we show how exploiting the electrostatic spinning (ES) technique for producing a nanofibrillar composite structure, by mimicking topographically the extracellular matrix environment, can influence the fate of human bone marrow mesenchymal stem cells, inducing osteogenic differentiation in the absence of chemical treatments or cellular reprogramming. Basic cues on the choice of the materials and useful experimental instructions for realizing composite nanofibrous scaffolds will be given as well as fundamental tips.

Cytocompatibility and osteogenic activity of a novel calcium phosphate silicate bioceramic: Silicocarnotite

In the present study, the effect of a novel bioceramic, silicon-containing calcium phosphate ceramic (silicocarnotite, Ca5 (PO4 )2 SiO4 , CPS) on attachment, proliferation, and osteogenic differentiation of rat bone marrow-derived mesenchymal stem cells (rBMSC) has been investigated in comparison to hydroxyapatite (HA). The CPS showed a similar cell attachment behavior to HA, while the proliferation of rBMSC on CPS was significantly higher than that on HA, which indicated that CPS had a good cytocompatibility. Moreover, the expression of alkaline phosphatase activity and osteogenic-related genes, including Runx-2, osteopontin (OPN), bone sialoprotein (BSP) and osteocalcin (OC), demonstrated that CPS enhanced the osteogenic differentiation of rBMSC and accelerated the differentiation process. The results suggest that CPS ceramic exhibits a good cytocompatibility and osteogenic activity, which might be used as a potential candidate material for bone tissue engineering.

N-cadherin regulates osteogenesis and migration of bone marrow-derived mesenchymal stem cells

N-cadherin, a calcium-dependent cellular adhesive protein, plays important roles during embryonic development and bone formation. The potential of mesenchymal stem cells (MSCs) in osteoblast differentiation and homing to the sites of injury make it a promising cell resource for tissue engineering. However, the role of N-cadherin in MSCs osteoblast differentiation and migration remains still obscure. In the present study, our results showed that prolonged N-cadherin overexpression inhibited osteogenic differentiation of MSCs through negatively regulating β-catenin and ERK1/2 signaling pathways. The mRNA expression levels of osteogenesis-related genes (Osteopontin, Osteocalcin, runt-related transcription factor 2 (Runx2), alkaline phosphatase (ALP) and bone morphogenetic protein 2) were significantly inhibited by N-cadherin, as well as the ALP activity and calcium deposit as stained by Alizarin Red S. While, silencing N-cadherin using shRNA reversed this effect. Furthermore, ectopic bone formation conducted in nude mice verified that N-cadherin significantly inhibited ectopic bone formation of MSCs in vivo. In addition, we also found that the N-cadherin overexpression could promote the migration potential of MSCs. These findings reveal that N-cadherin inhibits osteogenesis but promotes migration of MSCs. The underlying mechanism of N-cadherin inhibiting osteogenesis may through suppressing β-catenin and ERK1/2 signaling pathways.

Peptide-based activation of alpha5 integrin for promoting osteogenesis

Promoting osteoblastogenesis remains a major challenge in disorders characterized by defective bone formation. We recently showed that the alpha 5 integrin subunit (ITGA5) is critically involved in human mesenchymal cell osteoblast differentiation. In this study, we determined the potential of pharmacological ITGA5 activation by a synthetic cyclic peptide (GA-CRRETAWAC-GA) on murine osteoblast differentiation and function in vitro and bone formation in vivo. Peptide-mediated activation of ITGA5 in murine C3H10T1/2 mesenchymal cells resulted in the generation of the integrin-mediated cell signals FAK and ERK1/2-MAPKs. In vitro, peptide-based activation of ITGA5 protected from cell apoptosis but did not affect cell adhesion or replication, while it enhanced the expression of the osteoblast marker genes Runx2 and type I collagen and increased extracellular matrix (ECM) mineralization as also found with bone morphogenetic protein-2 (BMP2), a standard bone anabolic factor. When injected on adult mouse cranial bone for 3 weeks, the peptide-mediated activation of ITGA5 increased bone thickness by twofold, an effect also induced by BMP2. Histomorphometric analysis showed that this anabolic effect resulted from decreased cell apoptosis and increased bone forming surfaces and bone formation rate (BFR). We conclude that pharmacological activation of ITGA5 in mesenchymal cells is effective in promoting de novo bone formation as a result of increased osteoprogenitor cell differentiation into osteoblasts and increased cell protection from apoptosis. This peptide-based approach could be used therapeutically to promote the osteogenic capacity of osteoblast progenitor cells and to induce de novo bone formation in conditions where osteoblastogenesis is compromised.

Current views on calcium phosphate osteogenicity and the translation into effective bone regeneration strategies

Calcium phosphate (CaP) has traditionally been used for the repair of bone defects because of its strong resemblance to the inorganic phase of bone matrix. Nowadays, a variety of natural or synthetic CaP-based biomaterials are produced and have been extensively used for dental and orthopaedic applications. This is justified by their biocompatibility, osteoconductivity and osteoinductivity (i.e. the intrinsic material property that initiates de novo bone formation), which are attributed to the chemical composition, surface topography, macro/microporosity and the dissolution kinetics. However, the exact molecular mechanism of action is unknown. This review paper first summarizes the most important aspects of bone biology in relation to CaP and the mechanisms of bone matrix mineralization. This is followed by the research findings on the effects of calcium (Ca²⁺) and phosphate (PO₄³⁻) ions on the migration, proliferation and differentiation of osteoblasts during in vivo bone formation and in vitro culture conditions. Further, the rationale of using CaP for bone regeneration is explained, focusing thereby specifically on the material's osteoinductive properties. Examples of different material forms and production techniques are given, with the emphasis on the state-of-the art in fine-tuning the physicochemical properties of CaP-based biomaterials for improved bone induction and the use of CaP as a delivery system for bone morphogenetic proteins. The use of computational models to simulate the CaP-driven osteogenesis is introduced as part of a bone tissue engineering strategy in order to facilitate the understanding of cell-material interactions and to gain further insight into the design and optimization of CaP-based bone reparative units. Finally, limitations and possible solutions related to current experimental and computational techniques are discussed.

Effects of mechanical vibration on proliferation and osteogenic differentiation of human periodontal ligament stem cells

OBJECTIVE

Paradental tissues (alveolar bone, periodontal ligament (PDL), and gingiva) have the capacity to adapt to their functional environment. The principal cellular elements of the PDL play an important role in normal function, regeneration of periodontal tissue and in orthodontic treatment. Recently, several studies have shown that low-magnitude, high-frequency (LMHF) mechanical vibration can positively influence bone homeostasis; however, the mechanism and optimal conditions for LMHF mechanical vibration have not been elucidated. It has been speculated that LMHF mechanical vibration stimulations have a favourable influence on osteocytes, osteoblasts and their precursors, thereby enhancing the expression of osteoblastic genes involved in bone formation and remodelling. The objective of this study was to test the effect of LMHF mechanical vibration on proliferation and osteogenic differentiation of human PDL stem cells (PDLSCs).

METHODS

Human PDLSCs were isolated from premolar teeth and randomized into vibration (magnitude: 0.3g; frequency: 10-180 Hz; 30 min/24h) and static cultures. The effect of vibration on PDLSC proliferation, differentiation and osteogenic potential was assessed at the genetic and protein level.

RESULTS

After LMHF mechanical vibration, PDLSC proliferation was decreased; however, this was accompanied by increased markers of osteogenesis in a frequency-dependent manner. Specifically, alkaline phosphatase activity gradually increased with the frequency of vibration, to a peak at 50 Hz, and the level of osteocalcin was significantly higher than control following vibration at 40 Hz, 50 Hz, 60 Hz, 90 Hz and 120 Hz. Levels of Col-I, Runx2 and Osterix were significantly increased by LMHF mechanical vibration at frequencies of 40 Hz and 50 Hz.

CONCLUSIONS

Our data demonstrates that LMHF mechanical vibration promotes PDLSC osteogenic differentiation and implies the existence of a frequency-dependent effect of vibration on determining PDLSC commitment to the osteoblast lineage.

Synergistic effect of defined artificial extracellular matrices and pulsed electric fields on osteogenic differentiation of human MSCs

In vivo, bone formation is a complex, tightly regulated process, influenced by multiple biochemical and physical factors. To develop a vital bone tissue engineering construct, all of these individual components have to be considered and integrated to gain an in vivo-like stimulation of target cells. The purpose of the present studies was to investigate the synergistic role of defined biochemical and physical microenvironments with respect to osteogenic differentiation of human mesenchymal stem cells (MSCs). Biochemical microenvironments have been designed using artificial extracellular matrices (aECMs), containing collagen I (coll) and glycosaminoglycans (GAGs) like chondroitin sulfate (CS), or a high-sulfated hyaluronan derivative (sHya), formulated as coatings on three-dimensional poly(caprolactone-co-lactide) (PCL) scaffolds. As part of the physical microenvironment, cells were exposed to pulsed electric fields via transformer-like coupling (TC). Results showed that aECM containing sHya enhanced osteogenic differentiation represented by increases in ALP activity and gene-expression (RT-qPCR) of several bone-related proteins (RUNX-2, ALP, OPN). Electric field stimulation alone did not influence cell proliferation, but osteogenic differentiation was enhanced if osteogenic supplements were provided, showing synergistic effects by the combination of sHya and electric fields. These results will improve the understanding of bone regeneration processes and support the development of effective tissue engineered bone constructs.

Nanofiber-based delivery of bioactive agents and stem cells to bone sites

Biodegradable nanofibers are important scaffolding materials for bone regeneration. In this paper, the basic concepts and recent advances of self-assembly, electrospinning and thermally induced phase separation techniques that are widely used to generate nanofibrous scaffolds are reviewed. In addition, surface functionalization and bioactive factor delivery within these nanofibrous scaffolds to enhance bone regeneration are also discussed. Moreover, recent progresses in applying these nanofiber-based scaffolds to deliver stem cells for bone regeneration are presented. Along with the significant advances, challenges and obstacles in the field as well as the future perspective are discussed.

Osteoblast-secreted collagen upregulates paracrine Sonic hedgehog signaling by prostate cancer cells and enhances osteoblast differentiation

Background

Induction of osteoblast differentiation by paracrine Sonic hedgehog (Shh) signaling may be a mechanism through which Shh-expressing prostate cancer cells initiate changes in the bone microenvironment and promote metastases. A hallmark of osteoblast differentiation is the formation of matrix whose predominant protein is type 1 collagen. We investigated the formation of a collagen matrix by osteoblasts cultured with prostate cancer cells, and its effects on interactions between prostate cancer cells and osteoblasts.

Results

In the presence of exogenous ascorbic acid (AA), a co-factor in collagen synthesis, mouse MC3T3 pre-osteoblasts in mixed cultures with human LNCaP prostate cancer cells or LNCaP cells modified to overexpress Shh (LNShh cells) formed collagen matrix with distinct fibril ultrastructural characteristics. AA increased the activity of alkaline phosphatase and the expression of the alkaline phosphatase gene Akp2, markers of osteoblast differentiation, in MC3T3 pre-osteoblasts cultured with LNCaP or LNShh cells. However, the AA-stimulated increase in Akp2 expression in MC3T3 pre-osteoblasts cultured with LNShh cells far exceeded the levels observed in MC3T3 cells cultured with either LNCaP cells with AA or LNShh cells without AA. Therefore, AA and Shh exert a synergistic effect on osteoblast differentiation. We determined whether the effect of AA on LNShh cell-induced osteoblast differentiation was mediated by Shh signaling. AA increased the expression of Gli1 and Ptc1, target genes of the Shh pathway, in MC3T3 pre-osteoblasts cultured with LNShh cells to at least twice their levels without AA. The ability of AA to upregulate Shh signaling and enhance alkaline phosphatase activity was blocked in MC3T3 cells that expressed a dominant negative form of the transcription factor GLI1. The AA-stimulated increase in Shh signaling and Shh-induced osteoblast differentiation was also inhibited by the specific collagen synthesis inhibitor 3,4-dehydro-L-proline.

Conclusions

Matrix collagen, formed by osteoblasts in the presence of AA, potentiates Shh signaling between Shh-expressing prostate cancer cells and osteoblasts. Collagen and Shh signaling exert a synergistic effect on osteoblast differentiation, a defining event in prostate carcinoma bone metastasis. Investigations into paracrine interactions among prostate cancer cells, osteoblasts, and osteoblast-synthesized matrix proteins advance our understanding of mechanisms contributing to prostate cancer bone metastasis.

[Pathophysiology of aging bone]

Deterioration of organ and systems function are the principal signs of aging. Aging is also believed to be a major factor in the loss of bone mass and quality, which in turn leads to an increase in the risk of fractures. Several factors seem to contribute to this scenario, with metabolic changes related to aging in the bone tissue itself being among them. Most of the current knowledge on the mechanisms associated with osteopenia/osteoporosis during aging has been generated from research in animal models (mainly rats and mice) and cell cultures derived from subjects of different ages. In this work, we have reviewed and summarised these studies, which have begun to establish the physiological and molecular basis of the bone alterations related to aging.

The role of connective tissue growth factor (CTGF/CCN2) in skeletogenesis

Connective tissue growth factor (CTGF) is a 38 kDa, cysteine rich, extracellular matrix protein composed of 4 domains or modules. CTGF has been shown to regulate a diverse array of cellular functions and has been implicated in more complex biological processes such as angiogenesis, chondrogenesis, and osteogenesis. A role for CTGF in the development and maintenance of skeletal tissues first came to light in studies demonstrating its expression in cartilage and bone cells, which was dramatically increased during skeletal repair or regeneration. The physiological significance of CTGF in skeletogenesis was confirmed in CTGF-null mice, which exhibited multiple skeletal dysmorphisms as a result of impaired growth plate chondrogenesis, angiogenesis, and bone formation/mineralization. Given the emerging importance of CTGF in osteogenesis and chondrogenesis, this review will focus on its expression in skeletal tissues, its effects on osteoblast and chondrocyte differentiation and function, and the skeletal implications of ablation or over-expression of CTGF in knockout or transgenic mouse models, respectively. In addition, this review will examine the role of integrin-mediated signaling and the regulation of CTGF expression as it relates to skeletogenesis. We will emphasize CTGF studies in bone or bone cells, and will identify opportunities for future investigations concerning CTGF and chondrogenesis/osteogenesis.

Comparative study on the cellular activities of osteoblast-like cells and new bone formation of anorganic bone mineral coated with tetra-cell adhesion molecules and synthetic cell binding peptide

Purpose

We have previously reported that tetra-cell adhesion molecule (T-CAM) markedly enhanced the differentiation of osteoblast-like cells grown on anorganic bone mineral (ABM). T-CAM comprises recombinant peptides containing the Arg-Gly-Asp (RGD) sequence in the tenth type III domain, Pro-His-Ser-Arg-Asn (PHSRN) sequence in the ninth type III domain of fibronectin (FN), and the Glu-Pro-Asp-Ilu-Met (EPDIM) and Tyr-His (YH) sequence in the fourth fas-1 domain of βig-h3. Therefore, the purpose of this study was to evaluate the cellular activity of osteoblast-like cells and the new bone formation on ABM coated with T-CAM, while comparing the results with those of synthetic cell binding peptide (PepGen P-15).

Methods

To analyze the cell viability, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay was performed, andto analyze gene expression, northernblot was performed. Mineral nodule formations were evaluated using alizarin red stain. The new bone formations of each group were evaluated using histologic observation and histomorphometrc analysis.

Results

Expression of alkaline phosphatase mRNA was similar in all groups on days 10 and 20. The highest expression of osteopontin mRNA was observed in the group cultured with ABM/P-15, followed by those with ABM/T-CAM and ABM on days 20 and 30. Little difference was seen in the level of expression of collagen type I mRNA on the ABM, ABM/T-CAM, and ABM/P-15 cultured on day 20. There were similar growth and proliferation patterns for the ABM/T-CAM and ABM/P-15. The halo of red stain consistent with Ca²⁺ deposition was wider and denser around ABM/T-CAM and ABM/P-15 particles than around the ABM particles. The ABM/T-CAM group seemed to have bone forming bioactivity similar to that of ABM/P-15. A complete bony bridge was seen in two thirds of the defects in the ABM/T-CAM and ABM/P-15 groups.

Conclusions

ABM/T-CAM, which seemed to have bone forming bioactivity similar to ABM/P-15, was considered to serve as effective tissue-engineered bone graft material.

Precipitation of nanohydroxyapatite on PLLA/PBLG/Collagen nanofibrous structures for the differentiation of adipose derived stem cells to osteogenic lineage

Tissue engineering and nanotechnology have enabled engineering of nanostructured materials to meet the current challenges in bone treatment owing to rising occurrence of bone diseases, accidental damages and defects. Poly(L-lactic acid)/Poly-benzyl-L-glutamate/Collagen (PLLA/PBLG/Col) scaffolds were fabricated by electrospinning and nanohydroxyapatite (n-HA) was deposited by calcium-phosphate dipping method for bone tissue engineering (BTE). The abundance and accessibility of adipose derived stem cells (ADSC) may prove to be novel cell therapeutics for bone repair and regeneration. ADSCs were cultured on these scaffolds and were induced to undergo osteogenic differentiation in the presence of PBLG/n-HA for BTE. The cell-biomaterial interactions were analyzed using cell proliferation, SEM and CMFDA dye extraction techniques. Osteogenic differentiation of ADSC was confirmed using alkaline phosphatase activity (ALP), mineralization (ARS) and dual immunofluorescent staining using both ADSC marker protein and Osteocalcin, which is a bone specific protein. The utmost significance of this study is the bioactive PBLG/n-HA biomolecule introduced on the polymeric nanofibers to regulate and improve specific biological functions like adhesion, proliferation and differentiation of ADSC into osteogenic lineage. This was evident from the immunostaining and CMFDA images of ADSCs showing cuboidal morphology, characteristic of osteogenic lineage. The observed results proved that the PLLA/PBLG/Col/n-HA scaffolds promoted greater osteogenic differentiation of ADSC as evident from the enzyme activity and mineralization profiles for bone tissue engineering.

The evaluation of ectopic bone formation induced by delivery systems for bone morphogenetic protein-9 or its derived peptide

We have earlier shown that a peptide derived from the bone morphogenetic protein-9 (pBMP-9) stimulates mouse preosteoblasts MC3T3-E1 differentiation in vitro. Here, we evaluated the effects of two delivery systems (DSs) for pBMP-9, one based on collagen and the other on chitosan. The release kinetics of BMP-9 (used as control) and pBMP-9 from these DSs were first determined in vitro by using enzyme-linked immunosorbent assay and high performance liquid chromatography assays, respectively. Micro-computerized tomography and histological analysis were then performed to study in vivo the ectopic ossification induced by both DSs containing these molecules in C57BL/6 mouse quadriceps. We found that collagen DS released in vitro about 35% of its BMP-9 within 1 h, whereas chitosan DS released 80%. The pBMP-9 was released from both DSs more slowly for up to 10 days. These release kinetics seemed to fit the Korsmeyer-Peppas model. Only chitosan DS containing BMP-9 induced strong bone formation in all mice quadriceps within 24 days. All mice quadriceps treated by pBMP-9 trapped in this DS also favored bone structures that started to mineralize. However, pBMP-9 in collagen DS failed to promote ectopic ossification within 24 days in vivo. This study highlights the importance to optimize carrier, thus improving the efficiency of pBMP-9 in vivo.

Concise review: induced pluripotent stem cells and lineage reprogramming: prospects for bone regeneration

Bone tissue for transplantation therapies is in high demand in clinics. Osteodegenerative diseases, in particular, osteoporosis and osteoarthritis, represent serious public health issues affecting a respectable proportion of the elderly population. Furthermore, congenital indispositions from the spectrum of craniofacial malformations such as cleft palates and systemic disorders including osteogenesis imperfecta are further increasing the need for bone tissue. Additionally, the reconstruction of fractured bone elements after accidents and the consumption of bone parts during surgical tumor excisions represent frequent clinical situations with deficient availability of healthy bone tissue for therapeutic transplantations. Epigenetic reprogramming represents a powerful technology for the generation of healthy patient-specific cells to replace or repair diseased or damaged tissue. The recent generation of induced pluripotent stem cells (iPSCs) is probably the most promising among these approaches dominating the literature of current stem cell research. It allows the generation of pluripotent stem cells from adult human skin cells from which potentially all cell types of the human body could be obtained. Another technique to produce clinically interesting cell types is direct lineage reprogramming (LR) with the additional advantage that it can be applied directly in vivo to reconstitute a damaged organ. Here, we want to present the two technologies of iPSCs and LR, to outline the current states of research, and to discuss possible strategies for their implementation in bone regeneration.

Osteoinduction of human mesenchymal stem cells by bioactive composite scaffolds without supplemental osteogenic growth factors

The development of a new family of implantable bioinspired materials is a focal point of bone tissue engineering. Implant surfaces that better mimic the natural bone extracellular matrix, a naturally nano-composite tissue, can stimulate stem cell differentiation towards osteogenic lineages in the absence of specific chemical treatments. Herein we describe a bioactive composite nanofibrous scaffold, composed of poly-caprolactone (PCL) and nano-sized hydroxyapatite (HA) or beta-tricalcium phosphate (TCP), which was able to support the growth of human bone marrow mesenchymal stem cells (hMSCs) and guide their osteogenic differentiation at the same time. Morphological and physical/chemical investigations were carried out by scanning, transmission electron microscopy, Fourier-transform infrared (FTIR) spectroscopy, mechanical and wettability analysis. Upon culturing hMSCs on composite nanofibers, we found that the incorporation of either HA or TCP into the PCL nanofibers did not affect cell viability, meanwhile the presence of the mineral phase increases the activity of alkaline phosphatase (ALP), an early marker of bone formation, and mRNA expression levels of osteoblast-related genes, such as the Runt-related transcription factor 2 (Runx-2) and bone sialoprotein (BSP), in total absence of osteogenic supplements. These results suggest that both the nanofibrous structure and the chemical composition of the scaffolds play a role in regulating the osteogenic differentiation of hMSCs.

The forkhead transcription factor Foxc2 promotes osteoblastogenesis via up-regulation of integrin β1 expression

The forkhead box C2 (Foxc2) protein, a member of the forkhead/winged helix transcription factor family, plays an important role in regulation of metabolism, arterial specification, and vascular sprouting. Foxc2-null mutants die prenatally or perinatally, and they exhibit hypoplasia of the vertebrae and insufficient chondrification or ossification of medial structures. However, the role of Foxc2 in osteoblastogenesis is not yet fully understood. According to the degree of differentiation of osteoblasts, we found that Foxc2 expression was gradually increased and dose-dependently up-regulated by well-known bone anabolic agents, such as hPTH(1-34) and BMP2. In ex vivo mouse calvarial organ culture, a significant reduction of the basal expression of Foxc2 induced by siFoxc2 remarkably suppressed cell proliferation and differentiation and induced cell death. Knockdown of Foxc2 expression using siFoxc2 in both MC3T3-E1 and primary mouse calvarial cells also resulted in a significant suppression of proliferation and differentiation, and induced cell death, supporting the ex vivo observations. In addition, the resistance to apoptosis induced by serum deprivation and phosphorylation of both Akt and ERK was significantly reduced after siFoxc2 treatment. Conversely, overexpression of Foxc2 increased the proliferation of MC3T3-E1 and primary mouse calvarial cells. Furthermore, we found that Foxc2 enhanced the expression of integrin β1, an important modulator of osteoblastogenesis, by direct binding to a Forkhead-binding element in its promoter. Taken together, these results indicate that Foxc2 plays an important role in osteoblastogenesis by promoting osteoblast proliferation, survival and differentiation through up-regulation of integrin β1 in response to stimuli which induce bone formation.

Myocyte enhancer factor 2c, an osteoblast transcription factor identified by dimethyl sulfoxide (DMSO)-enhanced mineralization

Rapid mineralization of cultured osteoblasts could be a useful characteristic in stem cell-mediated therapies for fracture and other orthopedic problems. Dimethyl sulfoxide (DMSO) is a small amphipathic solvent molecule capable of stimulating cell differentiation. We report that, in primary human osteoblasts, DMSO dose-dependently enhanced the expression of osteoblast differentiation markers alkaline phosphatase activity and extracellular matrix mineralization. Furthermore, similar DMSO-mediated mineralization enhancement was observed in primary osteoblast-like cells differentiated from mouse mesenchymal cells derived from fat, a promising source of starter cells for cell-based therapy. Using a convenient mouse pre-osteoblast model cell line MC3T3-E1, we further investigated this phenomenon showing that numerous osteoblast-expressed genes were elevated in response to DMSO treatment and correlated with enhanced mineralization. Myocyte enhancer factor 2c (Mef2c) was identified as the transcription factor most induced by DMSO, among the numerous DMSO-induced genes, suggesting a role for Mef2c in osteoblast gene regulation. Immunohistochemistry confirmed expression of Mef2c in osteoblast-like cells in mouse mandible, cortical, and trabecular bone. shRNAi-mediated Mef2c gene silencing resulted in defective osteoblast differentiation, decreased alkaline phosphatase activity, and matrix mineralization and knockdown of osteoblast specific gene expression, including osteocalcin and bone sialoprotein. A flow on knockdown of bone-specific transcription factors, Runx2 and osterix by shRNAi knockdown of Mef2c, suggests that Mef2c lies upstream of these two important factors in the cascade of gene expression in osteoblasts.

In vitro mineralization of human mesenchymal stem cells on three-dimensional type I collagen versus PLGA scaffolds: a comparative analysis

BACKGROUND

Development of a tissue engineered bone graft requires efficient bioactivity screening of biomaterials in clinically relevant three-dimensional systems. The authors analyzed the relative osteogenic potential of two three-dimensional biomaterials--type I collagen and poly(L-lactide-co-glycolide) (PLGA)--to support in vitro mineralization of human mesenchymal stem cells.

METHODS

Human mesenchymal stem cells were seeded onto three-dimensional PLGA or type I collagen scaffolds; incubated in osteogenic media; and harvested at 1, 4, and 7 days. Messenger RNA expression was analyzed using quantitative real-time reverse-transcriptase polymerase chain reaction for osteogenic (i.e., alkaline phosphatase, osteocalcin, bone sialoprotein, Runx2/core binding factor α-1) and angiogenic (i.e., vascular endothelial growth factor and interleukin-8) markers. Alkaline phosphatase enzyme activity was measured at 4 and 7 days. Mineralization was detected by alizarin red staining and micro-computed tomographic imaging at 8 and 12 weeks. Mineral composition was analyzed by solid-phase nuclear magnetic resonance spectroscopy.

RESULTS

Early osteogenic and angiogenic markers, and alkaline phosphatase enzyme activity, were up-regulated on PLGA versus collagen scaffolds. However, long-term mineralization endpoints favored type I collagen. By 8 weeks, human mesenchymal stem cells on collagen exhibited significantly higher mineral density by micro-computed tomographic and alizarin red staining than PLGA scaffolds. Both biomaterials deposited calcium hydroxyapatite as determined by nuclear magnetic resonance spectroscopy.

CONCLUSIONS

The authors' findings suggest that despite early PLGA induction of osteogenic gene expression, long-term mineralization occurs earlier and to a greater extent on type I collagen, highlighting collagen as a potential bone tissue engineering scaffold in the human mesenchymal stem cell niche. When screening the relative osteoinductive profiles of three-dimensional bone tissue engineering scaffolds in vitro, the authors recommend including long-term endpoints of osteogenesis.

Reciprocal roles of angiotensin II and Angiotensin II Receptors Blockade (ARB) in regulating Cbfa1/RANKL via cAMP signaling pathway: possible mechanism for hypertension-related osteoporosis and antagonistic effect of ARB on hypertension-related osteoporosis

Hypertension is a risk factor for osteoporosis. Animal and epidemiological studies demonstrate that high blood pressure is associated with increased calcium loss, elevated parathyroid hormone, and increased calcium movement from bone. However, the mechanism responsible for hypertension-related osteoporosis remains elusive. Recent epidemiological studies indicate the benefits of Angiotensin II Receptors Blockade (ARB) on decreasing fracture risks. Since receptors for angiotensin II, the targets of ARB, are expressed in both osteoblasts and osteoclasts, we postulated that angiotensin II plays an important role in hypertension-related osteoporosis. Cbfa1 and RANKL, the important factors for maintaining bone homeostasis and key mediators in controlling osteoblast and osteoclast differentiation, are both regulated by cAMP-dependent signaling. Angiotensin II along with factors such as LDL, HDL, NO and homocysteine that are commonly altered both in hypertension and osteoporosis, can down-regulate the expression of Cbfa1 but up-regulate RANKL expression via the cAMP signaling pathway. We thus hypothesized that, by altering the ratio of Cbfa1/RANKL expression via the cAMP-dependent pathway, angiotensin II differently regulates osteoblast and osteoclast differentiation leading to enhanced bone resorption and reduced bone formation. Since ARB can antagonize the adverse effect of angiotensin II on bone by lowering cAMP levels and modifying other downstream targets, including LDL, HDL, NO and Cbfa1/RANKL, we propose the hypothesis that the antagonistic effects of ARB may also be exerted via cAMP signaling pathway.