Effects of continuous and pulsatile PTH treatments on rat bone marrow stromal cells

Bone marrow stromal cells (MSCs) differentiation and proliferation are controlled by numerous growth factors and hormones. Continuous parathyroid hormone (PTH) treatment has been shown to decrease osteoblast differentiation, whereas pulsatile PTH increases osteoblast differentiation. However, the effects of PTH treatments on MSCs have not been investigated. This study showed continuous PTH treatment in the presence of dexamethasone (DEX) promoted osteogenic differentiation of rat MSCs in vitro, as demonstrated by increased alkaline phosphatase (ALP) activity, number of ALP expressing cells, and up-regulation of PTH receptor-1, ALP, and osteocalcin mRNA expressions. In contrast, pulsatile PTH treatment was found to suppress osteogenesis of rat MSCs, possibly by promoting the maintenance of undifferentiated cells. Additionally, the observed effects of PTH were strongly dependent on the presence of DEX. MSC proliferation however was not influenced by PTH independent of treatment regimen and presence or absence of DEX. Furthermore, our work raised the possibility that PTH treatment may modulate stem/progenitor cell activity within MSC cultures.     

Osteoblastic cells: differentiation and trans-differentiation

The osteoblast is the bone forming cell and is derived from mesenchymal stem cells (MSC) present among the bone marrow stroma. MSC are capable of multi-lineage differentiation into mesoderm-type cells such as osteoblasts and adipocytes. Understanding the mechanisms underlying osteoblast differentiation from MSC is a central topic in bone biology that can provide insight into mechanisms of bone maintenance and also novel pharmacological targets to increase osteoblast differentiation and consequently bone formation.     

sFRP2 Suppression of Bone Morphogenic Protein (BMP) and Wnt Signaling Mediates Mesenchymal Stem Cell (MSC) Self-renewal Promoting Engraftment and Myocardial Repair

Transplantation of mesenchymal stem cells (MSCs) is a promising therapy for ischemic injury; however, inadequate survival of implanted cells in host tissue is a substantial impediment in the progress of cellular therapy. Secreted Frizzled-related protein 2 (sFRP2) has recently been highlighted as a key mediator of MSC-driven myocardial and wound repair. Notably, sFRP2 mediates significant enhancement of MSC engraftment in vivo. We hypothesized that sFRP2 improves MSC engraftment by modulating self-renewal through increasing stem cell survival and by inhibiting differentiation. In previous studies we demonstrated that sFRP2-expressing MSCs exhibited an increased proliferation rate. In the current study, we show that sFRP2 also decreased MSC apoptosis and inhibited both osteogenic and chondrogenic lineage commitment. sFRP2 activity occurred through the inhibition of both Wnt and bone morphogenic protein (BMP) signaling pathways. sFRP2-mediated inhibition of BMP signaling, as assessed by levels of pSMAD 1/5/8, was independent of its effects on the Wnt pathway. We further hypothesized that sFRP2 inhibition of MSC lineage commitment may reduce heterotopic osteogenic differentiation within the injured myocardium, a reported adverse side effect. Indeed, we found that sFRP2-MSC-treated hearts and wound tissue had less ectopic calcification. This work provides important new insight into the mechanisms by which sFRP2 increases MSC self-renewal leading to superior tissue engraftment and enhanced wound healing.     

Regulation of hematopoiesis and the hematopoietic stem cell niche by Wnt signaling pathways

Hematopoietic stem cells （HSCs） are a rare population of cells that are responsible for life-long generation of blood cells of all lineages. In order to maintain their numbers, HSCs must establish a balance between the opposing cell fates of self-renewal （in which the ability to function as HSCs is retained） and initiation of hematopoietic differentiation. Multiple signaling pathways have been implicated in the regulation of HSC cell fate. One such set of pathways are those activated by the Wnt family of ligands. Wnt signaling pathways play a crucial role during embryogenesis and deregulation of these pathways has been implicated in the formation of solid tumors. Wnt signaling also plays a role in the regulation of stem cells from multiple tissues, such as embryonic, epidermal, and intestinal stem cells. However, the function of Wnt signaling in HSC biology is still controversial. In this review, we will discuss the basic characteristics of the adult HSC and its regulatory microenvironment, the ＂niche＂, focusing on the regulation of the HSC and its niche by the Wnt signaling pathways.     

The genomic landscapes of histone H3-Lys9 modifications of gene promoter regions and expression profiles in human bone marrow mesenchymal stem cells

Mesenchymal stem cells (MSCs) of nonembryortic origins possess the proliferation and multi-lineage differentiation potentials. It has been established that epigenetic mechanisms could be critical for determining the fate of stem ceils, and MSCs derived from different origins exhibited different expression profiles individually to a certain extent. In this study, ChiP-on-chip was used to generate genome-wide historic H3-Lys9 acetylation and dimethylation profiles at gene promoters in human bone marrow MSCs. We showed that modifications of histone H3-Lys9 at gene promoters correlated well with mRNA expression in human bone marrow MSCs. Functional analysis revealed that many key cellular pathways in human bone marrow MSC self-renewal, such as the canonical signaling pathways,cell cycle pathways and cytokine related pathways may be regulated by H3-Lys9 modifications. These data suggest that gene activation and silencing affected by H3-Lys9 acetylation and dimethylation, respectively, may be essential to the maintenance of human bone marrow MSC self-renewal and multi-potency.     

Bioreactors to influence stem cell fate: Augmentation of mesenchymal stem cell signaling pathways via dynamic culture systems

Background

Mesenchymal stem cells (MSCs) are a promising cell source for bone and cartilage tissue engineering as they can be easily isolated from the body and differentiated into osteoblasts and chondrocytes. A cell based tissue engineering strategy using MSCs often involves the culture of these cells on three-dimensional scaffolds; however the size of these scaffolds and the cell population they can support can be restricted in traditional static culture. Thus dynamic culture in bioreactor systems provides a promising means to culture and differentiate MSCs in vitro.

Scope of review

This review seeks to characterize key MSC differentiation signaling pathways and provides evidence as to how dynamic culture is augmenting these pathways. Following an overview of dynamic culture systems, discussion will be provided on how these systems can effectively modify and maintain important culture parameters including oxygen content and shear stress. Literature is reviewed for both a highlight of key signaling pathways and evidence for regulation of these signaling pathways via dynamic culture systems.

Major conclusions

The ability to understand how these culture systems are affecting MSC signaling pathways could lead to a shear or oxygen regime to direct stem cell differentiation. In this way the efficacy of in vitro culture and differentiation of MSCs on three-dimensional scaffolds could be greatly increased.

General significance

Bioreactor systems have the ability to control many key differentiation stimuli including mechanical stress and oxygen content. The further integration of cell signaling investigations within dynamic culture systems will lead to a quicker realization of the promise of tissue engineering and regenerative medicine. This article is part of a Special Issue entitled Biochemistry of Stem Cells.     

TGF-β regulates β-catenin signaling and osteoblast differentiation in human mesenchymal stem cells

Human adult bone marrow-derived skeletal stem cells a.k.a mesenchymal stem cells (hMSCs) have been shown to be precursors of several different cellular lineages, including osteoblast, chondrocyte, myoblast, adipocyte, and fibroblast. Several studies have shown that cooperation between transforming growth factor β (TGF-β) and Wnt/β-catenin signaling pathways plays a role in controlling certain developmental events and diseases. Our previous data showed that agents like TGF-β, cooperation with Wnt signaling, promote chondrocyte differentiation at the expense of adipocyte differentiation in hMSCs. In this study, we tested mechanisms by which TGF-β activation of β-catenin signaling pathway and whether these pathways interact during osteoblast differentiation of hMSCs. With selective small chemical kinase inhibitors, we demonstrated that TGF-β1 requires TGF-β type I receptor ALK-5, Smad3, phosphoinositide 3-kinases (PI3K), and protein kinase A (PKA) to stabilize β-catenin, and needs ALK-5, PKA, and JNK to inhibit osteoblastogenesis in hMSCs. Knockdown of β-catenin with siRNA stimulated alkaline phosphatase activity and antagonized the inhibitory effects of TGF-β1 on bone sialoprotein (BSP) expression, suggested that TGF-β1 cooperated with β-catenin signaling in inhibitory of osteoblastogenesis in hMSCs. In summary, TGF-β1 activates β-catenin signaling pathway via ALK-5, Smad3, PKA, and PI3K pathways, and modulates osteoblastogenesis via ALK5, PKA, and JNK pathways in hMSCs; the interaction between TGF-β and β-catenin signaling supports the view that β-catenin signaling is a mediator of TGF-β's effects on osteoblast differentiation of hMSCs.     

Effects of Zinc Transporter on Differentiation of Bone Marrow Mesenchymal Stem Cells to Osteoblasts

The differentiation of bone marrow mesenchymal stem cells (MSCs) into osteoblasts is a crucial step during bone formation. However, the exact mechanisms regulating the early stages of osteogenic differentiation remain unknown. In the present study, we found that ZnT7, a member of the zinc transporter family SLC30A(ZnTs), was downregulated during dexamethasone-induced differentiation of rat MSCs into osteoblasts. Dexamethasone treatment resulted in significantly lower levels of ZnT7 compared with cocultured cells without dexamethasone. Differentiation was evaluated by measuring alkaline phosphatase (ALP) activity and staining for ALP, von Kossa, collagen type I, and osteocalcin. Overexpression of ZnT7 decreased the expression of the osteoblast alkaline phosphatase, type I collagen, as well as calcium deposition in mesenchymal cells. In contrast, knockdown of ZnT7 using siRNA promoted gene expression associated with osteoblast differentiation and matrix mineralization in vitro. Moreover, according to the ZnT7 inhibition or activation experiments, Wnt and ERK signaling pathways were found to be important signal transduction pathways in mediating the osteogenic effect of MSCs, and this effect is intensified by a decrease in the level of ZnT7 induced by dexamethasone. These findings suggest that ZnT7 is involved in the switch from the undifferentiated state of MSC to an osteogenic program, and marking the expression level of ZnT7 may be useful in the detection of early osteogenic differentiation.     

Regulation of human cranial osteoblast phenotype by FGF-2, FGFR-2 and BMP-2 signaling

The formation of cranial bone requires the differentiation of osteoblasts from undifferentiated mesenchymal cells. The balance between osteoblast recruitment, proliferation, differentiation and apoptosis in sutures between cranial bones is essential for calvarial bone formation. The mechanisms that control human osteoblasts during normal calvarial bone formation and premature suture ossification (craniosynostosis) begin to be understood. Our studies of the human calvaria osteoblast phenotype and calvarial bone formation showed that premature fusion of the sutures in non-syndromic and syndromic (Apert syndrome) craniosynostoses results from precocious osteoblast differentiation. We showed that Fibroblast Growth Factor-2 (FGF-2), FGF receptor-2 (FGFR-2) and Bone Morphogenetic Protein-2 (BMP-2), three essential factors involved in skeletal development, regulate the proliferation, differentiation and apoptosis in human calvaria osteoblasts. Mechanisms that induce the differentiated osteoblast phenotype have also been identified in human calvaria osteoblasts. We demonstrated the implication of molecules (N-cadherin, Il-1) and signaling pathways (src, PKC) by which these local factors modulate human calvaria osteoblast differentiation and apoptosis. The identification of these essential signaling molecules provides new insights into the pathways controlling the differentiated osteoblast phenotype, and leads to a more comprehensive view in the mechanisms that control normal and premature cranial ossification in humans.     

β2- and β3-, but not β1-adrenergic receptors are involved in osteogenesis of mouse mesenchymal stem cells via cAMP/PKA signaling

The osteogenic capacity of mesenchymal stem cells (MSCs) and the importance of β-adrenergic signals in bone formation and resorption have been well investigated. However, little is known about the development of β-adrenergic receptor (β-AR) systems and the role of β-adrenergic signals in osteogenic differentiation of MSCs, which is critically important in bone physiology and pharmacology. In this study, we demonstrated that both the mRNA and protein levels of β2- and β3-AR are up-regulated following osteogenesis of mouse MSCs. We also established that β-AR agonists negatively while antagonists positively affect MSC osteogenesis. Both β2- and β3-AR are involved in MSC osteogenesis, with β2-AR being dominant. The effect of β-ARs on MSC osteogenesis is partly mediated via the cAMP/PKA signaling. These findings suggest that MSC is also a target for β-adrenergic regulation and β-adrenergic signaling plays a role in MSC osteogenesis.     

调控骨骼形成的信号通路

骨骼形成后会处于不断的分解与重建中.通过骨骼形成与骨骼吸收之间的动态平衡来维持骨量.如果二者间的平衡被打破,骨吸收大于骨形成时,骨量会减少,骨骼微环境随之发生改变,脆性增加,进而引发骨质疏松、骨折等疾病.其中,骨骼形成是成骨细胞的重要功能.成骨细胞由间充质干细胞(mesenchymal stem cells,MSCs)...     

Connective tissue growth factor (CTGF) is regulated by Wnt and bone morphogenetic proteins signaling in osteoblast differentiation of mesenchymal stem cells

Osteoblast lineage-specific differentiation of mesenchymal stem cells is a well regulated but poorly understood process. Both bone morphogenetic proteins (BMPs) and Wnt signaling are implicated in regulating osteoblast differentiation and bone formation. Here we analyzed the expression profiles of mesenchymal stem cells stimulated with Wnt3A and osteogenic BMPs, and we identified connective tissue growth factor (CTGF) as a potential target of Wnt and BMP signaling. We confirmed the microarray results, and we demonstrated that CTGF was up-regulated at the early stage of BMP-9 and Wnt3A stimulations and that Wnt3A-regulated CTGF expression was beta-catenin-dependent. RNA interference-mediated knockdown of CTGF expression significantly diminished BMP-9-induced, but not Wnt3A-induced, osteogenic differentiation, suggesting that Wnt3A may also regulate osteoblast differentiation in a CTGF-independent fashion. However, constitutive expression of CTGF was shown to inhibit both BMP-9- and Wnt3A-induced osteogenic differentiation. Exogenous expression of CTGF was shown to promote cell migration and recruitment of mesenchymal stem cells. Our findings demonstrate that CTGF is up-regulated by Wnt3A and BMP-9 at the early stage of osteogenic differentiation, which may regulate the proliferation and recruitment of osteoprogenitor cells; however, CTGF is down-regulated as the differentiation potential of committed pre-osteoblasts increases, strongly suggesting that tight regulation of CTGF expression may be essential for normal osteoblast differentiation of mesenchymal stem cells.     

Activation of non-canonical Wnt/JNK pathway by Wnt3a is associated with differentiation fate determination of human bone marrow stromal (mesenchymal) stem cells

The canonical Wnt signaling pathway can determine human bone marrow stromal (mesenchymal) stem cell (hMSC) differentiation fate into osteoblast or adipocyte lineages. However, its downstream targets in MSC are not well characterized. Thus, using DNA microarrays, we compared global gene expression patterns induced by Wnt3a treatment in two hMSC lines: hMSC-LRP5^T253 and hMSC-LRP5^T244 cells carrying known mutations of Wnt co-receptor LRP5 (T253I or T244M) that either enhances or represses canonical Wnt signaling, respectively. Wnt3a treatment of hMSC activated not only canonical Wnt signaling, but also the non-canonical Wnt/JNK pathway through upregulation of several non-canonical Wnt components e.g. naked cuticle 1 homolog (NKD1) and WNT11. Activation of the non-canonical Wnt/JNK pathway by anisomycin enhanced osteoblast differentiation whereas its inhibition by SP600125 enhanced adipocyte differentiation of hMSC. In conclusion, canonical and non-canonical Wnt signaling cooperate in determining MSC differentiation fate.     

Dynamic expression of the Robo ligand Slit2 in bone marrow cell populations

The bone marrow (BM) niche is essential for lifelong hematopoietic stem cell (HSC) maintenance, proliferation and differentiation. Several BM cell types, including osteoblast lineage cells (OBC), mesenchymal stem cells (MSC) and endothelial cells (EC) have been implicated in supporting HSC location and function, but the relative importance of these cell types and their secreted ligands remain controversial. We recently found that the cell surface receptors Robo4 and CXCR4 cooperate to localize HSC to BM niches. We hypothesized that Slit2, a putative ligand for Robo4, cooperates with the CXCR4 ligand SDF1 to direct HSC to specific BM niche sites. Here, we have isolated OBC, MSC and EC by flow cytometry and determined their frequency within the bone marrow and the relative mRNA levels of Slit2, SDF1 and Robo4. We found that expression of Slit2 and SDF1 were dynamically regulated in MSC and OBC-like populations following radiation, while Robo4 expression was restricted to EC. Radiation also significantly affected the cellularity and frequency of both the non-adherent and adherent cells within the BM stroma. These data support a physiological role for Slit2 in regulating the dynamic function of Robo-expressing cells within BM niches at steady state and following radiation.