Monocytes induce STAT3 activation in human mesenchymal stem cells to promote osteoblast formation

A major therapeutic challenge is how to replace bone once it is lost. Bone loss is a characteristic of chronic inflammatory and degenerative diseases such as rheumatoid arthritis and osteoporosis. Cells and cytokines of the immune system are known to regulate bone turnover by controlling the differentiation and activity of osteoclasts, the bone resorbing cells. However, less is known about the regulation of osteoblasts (OB), the bone forming cells. This study aimed to investigate whether immune cells also regulate OB differentiation. Using in vitro cell cultures of human bone marrow-derived mesenchymal stem cells (MSC), it was shown that monocytes/macrophages potently induced MSC differentiation into OBs. This was evident by increased alkaline phosphatase (ALP) after 7 days and the formation of mineralised bone nodules at 21 days. This monocyte-induced osteogenic effect was mediated by cell contact with MSCs leading to the production of soluble factor(s) by the monocytes. As a consequence of these interactions we observed a rapid activation of STAT3 in the MSCs. Gene profiling of STAT3 constitutively active (STAT3C) infected MSCs using Illumina whole human genome arrays showed that Runx2 and ALP were up-regulated whilst DKK1 was down-regulated in response to STAT3 signalling. STAT3C also led to the up-regulation of the oncostatin M (OSM) and LIF receptors. In the co-cultures, OSM that was produced by monocytes activated STAT3 in MSCs, and neutralising antibodies to OSM reduced ALP by 50%. These data indicate that OSM, in conjunction with other mediators, can drive MSC differentiation into OB. This study establishes a role for monocyte/macrophages as critical regulators of osteogenic differentiation via OSM production and the induction of STAT3 signalling in MSCs. Inducing the local activation of STAT3 in bone cells may be a valuable tool to increase bone formation in osteoporosis and arthritis, and in localised bone remodelling during fracture repair.     

Hey1 basic helix-loop-helix protein plays an important role in mediating BMP9-induced osteogenic differentiation of mesenchymal progenitor cells

Pluripotent mesenchymal stem cells (MSCs) are bone marrow stromal progenitor cells that can differentiate into osteogenic, chondrogenic, adipogenic, and myogenic lineages. We previously demonstrated that bone morphogenetic protein (BMP) 9 is one of the most potent and yet least characterized BMPs that are able to induce osteogenic differentiation of MSCs both in vitro and in vivo. Here, we conducted gene expression-profiling analysis and identified that Hey1 of the hairy/Enhancer of split-related repressor protein basic helix-loop-helix family was among the most significantly up-regulated early targets in BMP9-stimulated MSCs. We demonstrated that Hey1 expression was up-regulated at the immediate early stage of BMP9-induced osteogenic differentiation. Chromatin immunoprecipitation analysis indicated that Hey1 may be a direct target of the BMP9-induced Smad signaling pathway. Silencing Hey1 expression diminished BMP9-induced osteogenic differentiation both in vitro and in vivo and led to chondrogenic differentiation. Likewise, constitutive Hey1 expression augmented BMP9-mediated bone matrix mineralization. Hey1 and Runx2 were shown to act synergistically in BMP9-induced osteogenic differentiation, and Runx2 expression significantly decreased in the absence of Hey1, suggesting that Runx2 may function downstream of Hey1. Accordingly, the defective osteogenic differentiation caused by Hey1 knockdown was rescued by exogenous Runx2 expression. Thus, our findings suggest that Hey1, through its interplay with Runx2, may play an important role in regulating BMP9-induced osteoblast lineage differentiation of MSCs.     

Deletion of RBPJK in Mesenchymal Stem Cells Enhances Osteogenic Activity by Up-Regulation of BMP Signaling

Recently we have demonstrated the importance of RBPjk-dependent Notch signaling in the regulation of mesenchymal stem cell (MSC) differentiation during skeletogenesis both in vivo and in vitro. Here we further performed RBPJK loss-of-function experiments to demonstrate for the first time that RBPJK deficient MSC shows enhanced differentiation and osteogenesis acts via up-regulation of the BMP signaling. In the present study, we first compared the spontaneous and osteogenic differentiation in normal and recombination signal binding protein for immunoglobulin kappa J region (RBPJK) deficient human bone marrow-derived mesenchymal stem cells (MSCs). It was found that RBPJK highly expressed in fresh isolated MSCs and its expression was progressing down-regulated during spontaneous differentiation and even greater in osteogenic media inducted differentiation. Deletion of RBPJK in MSCs not only enhances cell spontaneous differentiation, but also significantly accelerates condition media inducted osteogenic differentiation by showing enhanced alkaline phosphatase (ALP) activity, Alizarin red staining, gene expression of Runx2, Osteopontin (OPN), Type I collagen (COL1a1) in culture. Additionally, BMP signaling responsive reporter activity and phosphor-smad1/5/8 expression were also significantly increased upon removal of RBPJK in MSCs. These data proved that inhibition of Notch signaling in MSCs promotes cell osteogenic differentiation by up-regulation of BMP signaling, and RBPJK deficient MSC maybe a better cell population for cell-based bone tissue engineering.     

Epigenetic dysregulation in mesenchymal stem cell aging and spontaneous differentiation

Background

Mesenchymal stem cells (MSCs) hold great promise for the treatment of difficult diseases. As MSCs represent a rare cell population, ex vivo expansion of MSCs is indispensable to obtain sufficient amounts of cells for therapies and tissue engineering. However, spontaneous differentiation and aging of MSCs occur during expansion and the molecular mechanisms involved have been poorly understood.

Methodology/Principal Findings

Human MSCs in early and late passages were examined for their expression of genes involved in osteogenesis to determine their spontaneous differentiation towards osteoblasts in vitro, and of genes involved in self-renewal and proliferation for multipotent differentiation potential. In parallel, promoter DNA methylation and hostone H3 acetylation levels were determined. We found that MSCs underwent aging and spontaneous osteogenic differentiation upon regular culture expansion, with progressive downregulation of TERT and upregulation of osteogenic genes such as Runx2 and ALP. Meanwhile, the expression of genes associated with stem cell self-renewal such as Oct4 and Sox2 declined markedly. Notably, the altered expression of these genes were closely associated with epigenetic dysregulation of histone H3 acetylation in K9 and K14, but not with methylation of CpG islands in the promoter regions of most of these genes. bFGF promoted MSC proliferation and suppressed its spontaneous osteogenic differentiation, with corresponding changes in histone H3 acetylation in TERT, Oct4, Sox2, Runx2 and ALP genes.

Conclusions/Significance

Our results indicate that histone H3 acetylation, which can be modulated by extrinsic signals, plays a key role in regulating MSC aging and differentiation.     

Stromal derived factor-1 regulates bone morphogenetic protein 2-induced osteogenic differentiation of primary mesenchymal stem cells

Stromal derived factor-1 (SDF-1) is a chemokine signaling molecule that binds to its transmembrane receptor CXC chemokine receptor-4 (CXCR4). While we previously detected that SDF-1 was co-required with bone morphogenetic protein 2 (BMP2) for differentiating mesenchymal C2C12 cells into osteoblastic cells, it is unknown whether SDF-1 is similarly involved in the osteogenic differentiation of mesenchymal stem cells (MSCs). Therefore, here we examined the role of SDF-1 signaling during BMP2-induced osteogenic differentiation of primary MSCs that were derived from human and mouse bone marrow. Our data showed that blocking of the SDF-1/CXCR4 signal axis or adding SDF-1 protein to MSCs significantly affected BMP2-induced alkaline phosphatase (ALP) activity and osteocalcin (OCN) synthesis, markers of preosteoblasts and mature osteoblasts, respectively. Moreover, disrupting the SDF-1 signaling impaired bone nodule mineralization during terminal differentiation of MSCs. Furthermore, we detected that blocking of the SDF-1 signaling inhibited the BMP2-induced early expression of Runt-related factor-2 (Runx2) and osterix (Osx), two “master” regulators of osteogenesis, and the SDF-1 effect was mediated via intracellular Smad and Erk activation. In conclusion, our results demonstrated a regulatory role of SDF-1 in BMP2-induced osteogenic differentiation of MSCs, as perturbing the SDF-1 signaling affected the differentiation of MSCs towards osteoblastic cells in response to BMP2 stimulation. These data provide novel insights into molecular mechanisms underlying MSC osteogenesis, and will contribute to the development of MSC therapies for enhancing bone formation and regeneration in broad orthopaedic situations.     

Change in hepatocyte growth factor concentration promote mesenchymal stem cell-mediated osteogenic regeneration

Mesenchymal stem cells (MSCs) play a crucial role in tissue repair by secretion of tissue nutrient factors such as hepatocyte growth factor (HGF). However, studies examining the effects of HGF on the proliferation and differentiation of MSCs used different concentrations of HGF and reported conflicting conclusions. This study aimed to determine the mechanisms by which different concentrations of HGF regulate MSC proliferation and osteogenic differentiation, and validate the mechanism in an animal model of early stage avascular necrosis of femoral head (ANFH). Our results demonstrate that a low concentration of HGF (20 ng/ml) preferentially promotes MSC osteogenic differentiation through increased c-Met expression and phosphorylation, Akt pathway activation, and increased expression of p27, Runx2 and Osterix. In contrast, a high concentration of HGF (100 ng/ml) strongly induced proliferation by inducing strong activation of the ERK1/2 signalling pathway. As validated by animal experiments, high localized expression of HGF achieved by transplantation of HGF transgenic MSCs into ANFH rabbits increased the number of MSCs. Subsequently, 2 weeks after transplantation, HGF levels decreased and MSCs differentiated into osteoblasts and resulted in efficient tissue repair. Our results demonstrate that sequential concentration changes in HGF control the proliferation and osteogenic differentiation of MSCs in vivo. This phenomenon can be exploited therapeutically to induce bone regeneration and, in turn, improve the efficacy of pharmacological intervention for ANFH treatment.     

TLR4 Activation Promotes Bone Marrow MSC Proliferation and Osteogenic Differentiation via Wnt3a and Wnt5a Signaling

Mesenchymal stem cells (MSCs) from adult bone marrow maintain their self-renewal ability and the ability to differentiate into osteoblast. Thus, adult bone marrow MSCs play a key role in the regeneration of bone tissue. Previous studies indicated that TLR4 is expressed in MSCs and is critical in regulating the fate decision of MSCs. However, the exact functional role and underlying mechanisms of how TLR4 regulate bone marrow MSC proliferation and differentiation are unclear. Here, we found that activated TLR4 by its ligand LPS promoted the proliferation and osteogenic differentiation of MSCs in vitro. TLR4 activation by LPS also increased cytokine IL-6 and IL-1β production in MSCs. In addition, LPS treatment has no effect on inducing cell death of MSCs. Deletion of TLR4 expression in MSCs completely eliminated the effects of LPS on MSC proliferation, osteogenic differentiation and cytokine production. We also found that the mRNA and protein expression of Wnt3a and Wnt5a, two important factors in regulating MSC fate decision, was upregulated in a TLR4-dependent manner. Silencing Wnt3a with specific siRNA remarkably inhibited TLR4-induced MSC proliferation, while Wnt5a specific siRNA treatment significantly antagonized TLR4-induced MSC osteogenic differentiation. These results together suggested that TLR4 regulates bone marrow MSC proliferation and osteogenic differentiation through Wnt3a and Wnt5a signaling. These finding provide new data to understand the role and the molecular mechanisms of TLR4 in regulating bone marrow MSC functions. These data also provide new insight in developing new therapy in bone regeneration using MSCs by modulating TLR4 and Wnt signaling activity.     

Osteogenic growth peptide enhances the proliferation of bone marrow mesenchymal stem cells from osteoprotegerin-deficient mice by CDK2/cyclin A

To promote bone formation is one of the fundamental strategies in osteoporosis treatment and fractures repair. As one of the stimulators on bone formation, osteogenic growth peptide (OGP) increases both proliferation and differentiation of the osteoblasts in vitro and in vivo, in which osteoprotegerin (OPG) has been suggested being involved. In this study, we evaluated the effects of OGP on bone marrow mesenchymal stem cells (MSCs) from OPG-deficient mice in vitro by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, alkaline phosphatase (ALP) activity assay, real-time polymerase chain reaction, and western blot analysis. Results showed that OGP stimulated MSC proliferation and increased the expression of CDK2 and cyclin A in MSCs both at mRNA and protein levels. However, no differentiative effect of OGP was shown as ALP activity and the expression levels of Runx2 and Osterix were not increased significantly by OGP. Our study suggested that OGP may increase the bone formation in OPG-deficient mice by stimulating MSC proliferation rather than differentiation, and probably by triggering CDK2/cyclin A pathway.     

骨形态发生蛋白9通过JNKs激酶途径调控间充质干细胞成骨分化

前期研究发现骨形态发生蛋白9(bone morphogenetic protein 9,BMP9)除了通过经典Smad途径外,也可通过丝裂原活化蛋白激酶(mitogen activated protein kinases,MAPKs)中的p38激酶途径调控间充质干细胞成骨分化．本研究继续探讨MAPKs的重要成员c-Jun氨基末端激酶(c-Jun N-terminal kinases,JNKs)对于BMP9诱导间充质干细胞成骨分化的调控作用．利用BMP9重组腺病毒感染间充质干细胞,通过体外细胞实验和体内动物实验,初步分析BMP9是否可通过JNKs激酶途径调控间充质干细胞成骨分化．结果表明：BMP9可通过促进JNKs激酶磷酸化而导致其活化;JNKs抑制剂SP600125可抑制由BMP9诱导的间充质干细胞的碱性磷酸酶(alkaline phosphatase,ALP)活性、骨桥蛋白(osteocpontin,OPN)和骨钙素(osteocalcin,OCN)表达以及钙盐沉积;利用抑制剂SP600125抑制JNKs激酶活性后,BMP9诱导Runx2的表达和转录活性,以及Smad经典途径的激活也相应受到抑制;RNA干扰导致JNKs基因沉默同样也可抑制BMP9诱导的间充质干细胞成骨分化以及裸鼠皮下异位成骨．因此,BMP9可通过活化JNKs激酶途径,从而调控间充质干细胞成骨分化．     

TGFβ/BMP Type I Receptors ALK1 and ALK2 Are Essential for BMP9-induced Osteogenic Signaling in Mesenchymal Stem Cells

Mesenchymal stem cells (MSCs) are bone marrow stromal cells that can differentiate into multiple lineages. We previously demonstrated that BMP9 is one of the most potent BMPs to induce osteogenic differentiation of MSCs. BMP9 is one of the least studied BMPs. Whereas ALK1, ALK5, and/or endoglin have recently been reported as potential BMP9 type I receptors in endothelial cells, little is known about type I receptor involvement in BMP9-induced osteogenic differentiation in MSCs. Here, we conduct a comprehensive analysis of the functional role of seven type I receptors in BMP9-induced osteogenic signaling in MSCs. We have found that most of the seven type I receptors are expressed in MSCs. However, using dominant-negative mutants for the seven type I receptors, we demonstrate that only ALK1 and ALK2 mutants effectively inhibit BMP9-induced osteogenic differentiation in vitro and ectopic ossification in MSC implantation assays. Protein fragment complementation assays demonstrate that ALK1 and ALK2 directly interact with BMP9. Likewise, RNAi silencing of ALK1 and ALK2 expression inhibits BMP9-induced BMPR-Smad activity and osteogenic differentiation in MSCs both in vitro and in vivo. Therefore, our results strongly suggest that ALK1 and ALK2 may play an important role in mediating BMP9-induced osteogenic differentiation. These findings should further aid us in understanding the molecular mechanism through which BMP9 regulates osteogenic differentiation of MSCs.     

Comparison of gingiva‐derived and bone marrow mesenchymal stem cells for osteogenesis

Presently, bone marrow is considered as a prime source of mesenchymal stem cells; however, there are some drawbacks and limitations. Compared with other mesenchymal stem cell (MSC) sources, gingiva‐derived mesenchymal stem cells (GMSCs) are abundant and easy to obtain through minimally invasive cell isolation techniques. In this study, MSCs derived from gingiva and bone marrow were isolated and cultured from mice. GMSCs were characterized by osteogenic, adipogenic and chondrogenic differentiation, and flow cytometry. Compared with bone marrow MSCs (BMSCs), the proliferation capacity was judged by CCK‐8 proliferation assay. Osteogenic differentiation was assessed by ALP staining, ALP assay and Alizarin red staining. RT‐qPCR was performed for ALP, OCN, OSX and Runx2. The results indicated that GMSCs showed higher proliferative capacity than BMSCs. GMSCs turned more positive for ALP and formed a more number of mineralized nodules than BMSCs after osteogenic induction. RT‐qPCR revealed that the expression of ALP, OCN, OSX and Runx2 was significantly increased in the GMSCs compared with that in BMSCs. Moreover, it was found that the number of CD90‐positive cells in GMSCs elevated more than that of BMSCs during osteogenic induction. Taking these results together, it was indicated that GMSCs might be a promising source in the future bone tissue engineering.     

Thrombospondin-1 inhibits osteogenic differentiation of human mesenchymal stem cells through latent TGF-β activation

Transforming growth factor-β (TGF-β) is a critical regulator of bone development and remodeling. TGF-β must be activated from its latent form in order to signal. Thrombospondin-1 (TSP1) is a major regulator of latent TGF-β activation and TSP1 control of TGF-β activation is critical for regulation of TGF-β activity in multiple diseases. Bone marrow-derived mesenchymal stem cells (MSCs) have osteogenic potential and they participate in bone remodeling in injury and in response to tumor metastasis. Since both TSP1 and TGF-β inhibit osteoblast differentiation, we asked whether TSP1 blocks osteoblast differentiation of MSCs through its ability to stimulate TGF-β activation. TSP1 added to human bone marrow-derived MSCs under growth conditions increases active TGF-β. Cultured MSCs express TSP1 and both TSP1 expression and TGF-β activity decrease during osteoblast differentiation. TSP1 and active TGF-β block osteoblast differentiation of MSCs grown in osteogenic media as measured by decreased Runx2 and alkaline phosphatase expression. The inhibitory effect of TSP1 on osteoblast differentiation is due to its ability to activate latent TGF-β, since a peptide which blocks TSP1 TGF-β activation reduced TGF-β activity and restored osteoblast differentiation as measured by increased Runx2 and alkaline phosphatase expression. Anti-TGF-β neutralizing antibody also increased alkaline phosphatase expression in the presence of TSP1. These studies show that TSP1 regulated TGF-β activity is a critical determinant of osteoblast differentiation.