Circulating Bone Marrow-Derived Osteoblast Progenitor Cells Are Recruited to the Bone-Forming Site by the CXCR4/Stromal Cell-Derived Factor-1 Pathway

Parabiosis model does not show presence of circulating osteoprogenitor cells

The goal of this study was to determine the presence of osteoprogenitor cells in the peripheral blood. Experiments were conducted with a parabiosis model in which osteoblast specific transgenic mice (Col2.3GFP or hOC-GFP) were surgically joined with a transgenic mouse where herpes virus thymidine kinase gene is under the control of the collagen alpha1 promoter (Col2.3DeltaTK). This method permits conditional ablation of osteoblasts by ganciclovir (GCV) treatment. In parabionts treated with GCV for 15 days or 1.5-2 months, GFP (hOC-GFP or Col2.3GFP) expression was not detected in histological preparations or in marrow stromal cell cultures from the Col2.3DeltaTK parabiont. Finally, Col2.3GFP/Col2.3DeltaTK pairs were treated with GCV for 15 days and allowed to recover from GCV for 3 months. Again there was a failure to detect Col2.3GFP expressing cells in the Col2.3DeltaTK parabiont. These observations, at least within the limits of this model system, allow the conclusion that osteoprogenitor cells do not readily circulate.

Hind limb unloading of mice modulates gene expression at the protein and mRNA level in mesenchymal bone cells

Background

We investigated the extent, modalities and reversibility of changes at cellular level in the expression of genes and proteins occurring upon Hind limb unloading (HU) in the tibiae of young C57BL/6J male mice. We focused on the effects of HU in chondrogenic, osteogenic, and marrow mesenchymal cells.

Methods

We analyzed for expression of genes and proteins at two time points after HU (7 and 14 days), and at 14 days after recovery from HU. Levels of mRNAs were tested by in situ hybridization. Protein levels were tested by immunohistochemistry. We studied genes involved in osteogenesis (alkaline phosphatase (AP), osteocalcin (OC), bonesialoprotein (BSP), membrane type1 matrix metalloproteinase (MT1-MMP)), in extracellular matrix (ECM) formation (procollagenases (BMP1), procollagenase enhancer proteins (PCOLCE)) and remodeling (metalloproteinase-9 (MMP9), RECK), and in bone homeostasis (Stro-1, CXCL12, CXCR4, CD146).

Results

We report the following patterns and timing of changes in gene expression induced by HU: 1) transient or stable down modulations of differentiation-associated genes (AP, OC), genes of matrix formation, maturation and remodelling, (BMP1, PCOLCEs MMP9) in osteogenic, chondrogenic and bone marrow cells; 2) up modulation of MT1-MMP in these same cells, and uncoupling of its expression from that of AP; 3) transient down modulation of the osteoblast specific expression of BSP; 4) for genes involved in bone homeostasis, up modulation in bone marrow cells at distal epiphysis for CXCR4, down modulation of CXCL12, and transient increases in osteoblasts and marrow cells for Stro1. 14 days after limb reloading expression returned to control levels for most genes and proteins in most cell types, except AP in all cells, and CXCL12, only in bone marrow.

Conclusions

HU induces the coordinated modulation of gene expression in different mesenchymal cell types and microenvironments of tibia. HU also induces specific patterns of expression for homeostasis related genes and modulation of mRNAs and proteins for ECM deposition, maturation and remodeling which may be key factors for bone maintenance.

Circulating osteogenic cells: characterization and relationship to rates of bone loss in postmenopausal women

There is increasing evidence that osteogenic cells are present not only in bone marrow (BM) but also in peripheral blood (PB). Since staining for alkaline phosphatase (AP) identifies osteoprogenitor cells in BM, we sought to further characterize BM versus PB hematopoietic lineage negative (lin-)/AP+ cells and to compare gene expression in PB lin-/AP+ cells from postmenopausal women undergoing rapid versus slow bone loss. PB lin-/AP+ cells were smaller than their BM counterparts, and both were negative for the pan-hematopoietic marker, CD45. BM and PB lin-/AP+ cells were capable of mineralization in vitro. Using whole genome linear amplification followed by quantitative polymerase chain reaction (QPCR) analysis, we found that relative to the BM cells, PB lin-/AP+ cells expressed similar levels of a number of key osteoblast marker genes (runx2, osterix, osteopontin, OPG, periostin), consistent with the PB cells being in the osteoblastic lineage. Importantly, however, compared to the BM cells, PB lin-/AP+ cells expressed lower levels of mRNAs for AP, type I collagen, and for a panel of proliferation markers, but higher levels of osteocalcin, osteonectin, and PTHR1 mRNAs, as well as those for RANKL and ICAM-1, both of which are important in supporting osteoclastogenesis. Using microarray followed by QPCR analysis, we further demonstrated that, compared to postmenopausal women undergoing slow bone loss, PB lin-/AP+ cells from women undergoing rapid bone loss expressed lower levels of mRNAs for hydroxyprostaglandin dehydrogenase, interferon regulator factor 3, Wnt1-induced secreted protein 1, and TGFbeta2, but higher levels of the Smad3 interacting protein, zinc finger DHHC-type containing 4 and col1alpha2. These data thus demonstrate that while PB lin-/AP+ cells express a number of osteoblastic genes and are capable of mineralization, they are a relatively quiescent cell population, both in terms of cell proliferation and matrix synthesis. However, their higher expression of RANKL and ICAM-1 mRNAs as compared to BM lin-/AP+ cells suggests a role for the PB lin-/AP+ cells in regulating osteoclastogenesis that warrants further investigation. Our study also provides "proof-of-concept" for the use of PB lin-/AP+ cells in clinical-investigative studies, and identifies several pathways that could potentially regulate rates of bone loss in postmenopausal women.

Recombinant human bone morphogenetic protein-7 enhances fracture healing in an ischemic environment

Ischemia predisposes orthopedic trauma patients to delayed fracture healing or nonunion. The goal of this study was to test the ability of bone morphogenetic protein 7 (BMP7) to stimulate fracture repair in an ischemic environment. Ischemic fractures were generated in male adult mice by resecting the femoral artery prior to the creation of a nonstabilized tibia fracture. Recombinant human BMP7 (rhBMP7, 50 microg) was injected into the fracture site immediately after surgery. At 7 days after injury, more tissue vascularization was observed in rhBMP7 treated fractures. Histomorphometric analyses revealed that rhBMP7 induced more cartilage at day 7, more callus and bone at days 14 and 28, and more adipose tissue and fibrous tissue at days 7, 14, and 28 compared to controls (n=5/group/time). At day 28, all fractures treated with rhBMP7 (50 microg, n=5) healed, whereas only three of five control fractures exhibited slight bony bridging. In addition, we found that rhBMP7 (both 10 and 50 microg) significantly increased the amount of cartilage compared to controls in stabilized fractures, confirming its chondrogenic effect. Lastly, using bone marrow transplantation, we determined that no donor-derived osteocytes or chondrocytes were present in rhBMP7-treated fractures, suggesting rhBMP7 did not recruit mesenchymal stem cells from the bone marrow to the fracture site. In conclusion, our results indicate that rhBMP7 is a promising treatment for fractures with severely disrupted blood supply.

Porous beta tricalcium phosphate scaffolds used as a BMP-2 delivery system for bone tissue engineering

Macroporous beta tricalcium phosphate (beta-TCP) scaffolds were evaluated as potential carriers and delivery systems for bone morphogenetic protein-2 (BMP-2). Chemical etching was performed to increase the available surface and thus the protein loading. X-ray diffraction and infrared spectrocopy analyses confirmed the preparation of pure beta-TCP scaffolds. Scanning electron microscopy revealed interconnected porosity (64%) and a microporous surface after chemical etching. Scaffolds loaded with 30 and 15 microg of BMP-2 were implanted respectively into the back muscles and into femoral defects (condyle and diaphysis) of rabbits for 4 weeks. Histological observations confirmed the activity of the BMP-2 released from the scaffolds. Intramuscularly, bone was formed within the BMP-2-loaded scaffold pores. In the bone defects, the effect of released BMP-2 was similarly noticeable, as evaluated by histomorphometry. The incorporation of BMP-2 resulted in an amount of newly formed bone that was 1.3 times higher than with unloaded scaffolds. The implant site, however, did not have an effect on bone formation as no statistical differences were measured between cortical (diaphysis) and trabecular (condyle) defects. These results indicate the suitability of chemically etched beta-TCP scaffolds as BMP-2 carriers, in the context of bone regeneration.

More insight into mesenchymal stem cells and their effects inside the body

IMPORTANCE OF THE FIELD

The pan-tissue existence and multipotency of differentiation make mesenchymal stem cells (MSCs) an attractive source of cells as tissue repair cells, seeds of engineered tissue, vehicles for gene therapy or in combination to promote tissue regeneration in wound healing and disease recovery.

AREAS COVERED IN THIS REVIEW

This review focuses on recent understanding on MSC's basic biological characteristics and the mechanisms underlying the therapeutic effects of MSCs in vivo.

WHAT THE READER WILL GAIN

The gene expression profiles for mRNA, protein, microRNA and cell surface marker of MSCs are summarized. Special attention is given to miRNA expression and its relationship with the characteristics of MSCs. The mechanisms of therapeutic effects of MSCs are attributed to their ability to migrate along chemokine gradients, differentiate into tissue-specific cells, enhance angiogenesis of wound tissue and regulate immune response. As examples, a detailed description is given on the regeneration of functional sweat glands on burned skin as well as neural cells in middle cerebral artery occlusion (MCAO) animals upon MSC transplantation.

TAKE HOME MESSAGE

Based on current data, although limited, the mesenchymal-epithelial transition is proposed to be one of the important ways for MSCs to participate tissue repair.

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.

Prospects for osteoprogenitor stem cells in fracture repair and osteoporosis

PURPOSE OF REVIEW

Bone regeneration and fracture repair are the complex processes of mesenchymal stem cell invasion, chondrogenesis, osteogenesis and angiogenesis. The coordinated actions of these principal processes result in the reconstruction of a normal bone and restoration of a structural unit. However, these normal bone regenerative mechanisms breakdown during fracture repair failure and postmenopausal osteoporosis.

RECENT FINDINGS

Recent discoveries of circulating multipotent stem cells with mixed characteristics of endothelial cell and osteogenic capacity have raised interest in new and potentially breakthrough therapies for fracture and pathologic bone loss. The cooperative actions of other mesenchymal stem cell lineage such as adipocytes and processes such as angiogenesis in bone repair could also serve as novel therapeutic targets. Recent data suggest that anabolic parathyroid hormone therapy, already approved for the treatment of osteoporosis, may recruit osteoprogenitor cells and also have a role in fracture repair.

SUMMARY

The present review will highlight recent information on stem cells and bone repair and examine potential avenues for future research.

Integrin and chemokine receptor gene expression in implant-adherent cells during early osseointegration

The mechanisms of early cellular recruitment and interaction to titanium implants are not well understood. The aim of this study was to investigate the expression of pro-inflammatory cytokines, chemokines and adhesion markers during the first 24 h of implantation. Anodically oxidized and machined titanium implants were inserted in rat tibia. After 3, 12, and 24 h the implants were unscrewed and analyzed with quantitative polymerase chain reaction. Immunohistochemistry and scanning electron microscopy revealed different cell types, morphology and adhesion at the two implant surfaces. A greater amount of cells, as indicated by higher expression of small subunit ribosomal RNA (18S), was detected on the oxidized surface. Higher expression of CXC chemokine receptor-4 (at 12 h) and integrins, alphav (at 12 h), beta1 (at 24 h) and beta2 (at 12 and 24 h) was detected at the oxidized surfaces. Significantly higher tumor necrosis factor-alpha (at 3 h) and interleukin-1beta (at 24 h) expression was demonstrated for the machined surface. It is concluded that material surface properties rapidly modulate the expression of receptors important for the recruitment and adhesion of cells which are crucial for the inflammatory and regenerative processes at implant surfaces in vivo.

Localization of Thy-1-positive cells in the perichondrium during endochondral ossification

We elucidated the localization of Thy-1-positive cells in the perichondrium of fetal rat limb bones to clarify the distribution of osteogenic cells in the process of endochondral ossification. We also examined the formation of calcified bone-like matrices by isolated perichondrial cells in vitro. At embryonic day (E) 15.5, when the cartilage primodia were formed, immunoreactivity for Thy-1 was detected in cells of the perichondrium adjacent to the zone of hypertrophic chondrocytes. At E17.5, when the bone collar formation and the vascular invasion were initiated, fibroblast-like cells at the sites of vascular invasion, as well as in the perichondrium, showed Thy-1 labeling. Double immunostaining for Thy-1 and osterix revealed that Thy-1 was not expressed in the osterix-positive osteoblasts. Electron microscopic analysis revealed that Thy-1-positive cells in the zone of hypertrophic chondrocytes came in contact with blood vessels. Perichondrial cells isolated from limb bones showed alkaline phosphatase activity and formed calcified bone-like matrices after 4 weeks in osteogenic medium. RT-PCR demonstrated that Thy-1 expression decreased as calcified nodules formed. Conversely, the expression of osteogenic marker genes Runx2, osterix, and osteocalcin increased. These results indicate that Thy-1 is a good marker for characterizing osteoprogenitor cells.

Circulating osteogenic precursor cells in heterotopic bone formation

Cells with osteogenic potential can be found in a variety of tissues. Here we show that circulating osteogenic precursor (COP) cells, a bone marrow-derived type I collagen+/CD45+ subpopulation of mononuclear adherent cells, are present in early preosseous fibroproliferative lesions in patients with fibrodysplasia ossificans progressiva (FOP) and nucleate heterotopic ossification (HO) in a murine in vivo implantation assay. Blood samples from patients with FOP with active episodes of HO contain significantly higher numbers of clonally derived COP cell colonies than patients with stable disease or unaffected individuals. The highest level of COP cells was found in a patient just before the clinical onset of an HO exacerbation. Our studies show that even COP cells derived from an unaffected individual can contribute to HO in genetically susceptible host tissue. The possibility that circulating, hematopoietic-derived cells with osteogenic potential can seed inflammatory sites has tremendous implications and, to our knowledge, represents the first example of their involvement in clinical HO. Thus, bone formation is not limited to cells of the mesenchymal lineage, and circulating cells of hematopoietic origin can also serve as osteogenic precursors at remote sites of tissue inflammation.

Alendronate enhances osteogenic differentiation of bone marrow stromal cells: a preliminary study

Alendronate inhibits osteoclastic activity. However, some studies suggest alendronate also has effects on osteoblast activity. We hypothesized alendronate would enhance osteoblastic differentiation without causing cytotoxicity of the osteoblasts. We evaluated the effect of alendronate on the osteogenic differentiation of mouse mesenchymal stem cells. D1 cells (multipotent mouse mesenchymal stem cells) were cultured in osteogenic differentiation medium for 7 days and then treated with alendronate for 2 days before being subjected to various tests using MTT assays, Alizarin Red, enzyme-linked immunosorbent assay, energy-dispersive xray spectrophotometry, reverse transcriptase-polymerase chain reaction, confocal microscopy, and flow cytometric analysis. D1 cells differentiated into osteoblasts in the presence of osteogenic differentiation medium as confirmed by positive Alizarin Red S staining, increased alkaline phosphatase activity and osteocalcin mRNA expression, a calcium peak by energy-dispersive xray spectrophotometry, and by positive immunofluorescence staining against CD44. Osteogenic differentiation was enhanced after treatment with alendronate as confirmed by Alizarin Red S staining, elevated alkaline phosphatase activity and osteocalcin mRNA expression, a greater calcium peak by energy-dispersive xray spectrophotometry, and by immunofluorescence staining against CD44 by flow cytometric analysis. These data suggest alendronate enhances osteogenic differentiation when treated with mouse mesenchymal stem cells in osteogenic differentiation medium.

Bone marrow mononuclear cells have neurovascular tropism and improve diabetic neuropathy

Bone marrow-derived mononuclear cells (BMNCs) have been shown to effectively treat ischemic cardiovascular diseases. Because diabetic neuropathy (DN) is causally associated with impaired angiogenesis and deficiency of angiogenic and neurotrophic factors in the nerves, we investigated whether DN can be ameliorated by local injection of BMNCs. Severe peripheral neuropathy, characterized by a significant decrease in the motor and sensory nerve conduction velocities (NCVs), developed 12 weeks after the induction of diabetes with streptozotocin in rats. The injection of BMNCs restored motor and sensory NCVs to normal levels and significantly improved vascular density and blood flow in diabetic nerves over 4 weeks. Fluorescent microscopic observation revealed that DiI-labeled BMNCs preferentially engrafted in sciatic nerves. Whole-mount fluorescent imaging and confocal microscopic evaluation demonstrated that many of the BMNCs localized following the course of the vasa nervorum in close proximity to blood vessels without incorporation into vasa nervorum as endothelial cells at a detectable level. Real-time reverse transcription-polymerase chain reaction analysis showed that the levels of angiogenic and neurotrophic factors were significantly increased in the nerves by BMNC injection. Local transplantation of BMNCs improved experimental DN by augmenting angiogenesis and increasing angiogenic and neurotrophic factors in peripheral nerves. These findings suggest that BMNC transplantation may represent a novel therapeutic option for treating DN.

Chemokine CXC receptor 4--mediated glioma tumor tracking by bone marrow--derived neural progenitor/stem cells

Malignant gliomas manifest frequent tumor recurrence after surgical resection and/or other treatment because of their nature of invasiveness and dissemination. The recognized brain tumor-tracking property of neural progenitor/stem cells opened the possibility of targeting malignant brain tumors using neural progenitor/stem cells. We and others have previously shown that fetal neural progenitor/stem cells can be used to deliver therapeutic molecules to brain tumors. Our recent work has further shown that gene delivery by bone marrow-derived neural progenitor/stem cells achieves therapeutic effects in a glioma model. In this study, we isolate and characterize bone marrow-derived neural progenitor/stem cells, which also express the chemokine receptor chemokine CXC receptor 4 (CXCR4). We show that CXCR4 is required for their chemotaxis and extracellular matrix invasion against a gradient of glioma soluble factors. Furthermore, beta-galactosidase-labeled bone marrow-derived neural progenitor/stem cells implanted in the contralateral side of the brain were shown to track gliomas as early as day 1 and increased through days 3 and 7. Intracranial glioma tracking by bone marrow-derived neural progenitor/stem cells is significantly inhibited by preincubation of bone marrow-derived neural progenitor/stem cells with a blocking anti-CXCR4 antibody, suggesting a CXCR4-dependent tracking mechanism. Glioma tracking bone marrow-derived neural progenitor/stem cells were found to express progenitor/stem cell markers, as well as CXCR4. Although bromodeoxyuridine incorporation assays and proliferating antigen staining indicated that tumor tracking bone marrow-derived neural progenitor/stem cells were mostly nonproliferating, these cells survive in the local tumor environment with little apoptosis. Elucidating the molecular mechanism of brain tumor tracking by adult source stem cells may provide basis for the development of future targeted therapy for malignant brain tumors.

Stromal cell-derived factor 1/CXCR4 signaling is critical for the recruitment of mesenchymal stem cells to the fracture site during skeletal repair in a mouse model

OBJECTIVE

Stromal cell-derived factor 1 (SDF-1; CXCL12/pre-B cell growth-stimulating factor) is a dominant chemokine in bone marrow and is known to be involved in inflammatory diseases, including rheumatoid arthritis. However, its role in bone repair remains unknown. The purpose of this study was to investigate the role of SDF-1 and its receptor, CXCR4, in bone healing.

METHODS

The expression of SDF-1 during the repair of a murine structural femoral bone graft was examined by real-time polymerase chain reaction and immunohistochemical analysis. The bone graft model was treated with anti-SDF-1 neutralizing antibody or TF14016, an antagonist for CXCR4, and evaluated by histomorphometry. The functional effect of SDF-1 on primary mesenchymal stem cells was determined by in vitro and in vivo migration assays. New bone formation in an exchanging-graft model was compared with that in the autograft models, using mice partially lacking SDF-1 (SDF-1(+/-)) or CXCR4 (CXCR4(+/-)).

RESULTS

The expression of SDF1 messenger RNA was increased during the healing of live bone grafts but was not increased in dead grafts. High expression of SDF-1 protein was observed in the periosteum of the live graft. New bone formation was inhibited by the administration of anti-SDF-1 antibody or TF14016. SDF-1 increased mesenchymal stem cell chemotaxis in vitro in a dose-dependent manner. The in vivo migration study demonstrated that mesenchymal stem cells recruited by SDF-1 participate in endochondral bone repair. Bone formation was decreased in SDF-1(+/-) and CXCR4(+/-) mice and was restored by the graft bones from CXCR4(+/-) mice transplanted into the SDF-1(+/-) femur, but not vice versa.

CONCLUSION

SDF-1 is induced in the periosteum of injured bone and promotes endochondral bone repair by recruiting mesenchymal stem cells to the site of injury.

Skeletal cell fate decisions within periosteum and bone marrow during bone regeneration

Bone repair requires the mobilization of adult skeletal stem cells/progenitors to allow deposition of cartilage and bone at the injury site. These stem cells/progenitors are believed to come from multiple sources including the bone marrow and the periosteum. The goal of this study was to establish the cellular contributions of bone marrow and periosteum to bone healing in vivo and to assess the effect of the tissue environment on cell differentiation within bone marrow and periosteum. Results show that periosteal injuries heal by endochondral ossification, whereas bone marrow injuries heal by intramembranous ossification, indicating that distinct cellular responses occur within these tissues during repair. [corrected] Next, lineage analyses were used to track the fate of cells derived from periosteum, bone marrow, and endosteum, a subcompartment of the bone marrow. Skeletal progenitor cells were found to be recruited locally and concurrently from periosteum and/or bone marrow/endosteum during bone repair. Periosteum and bone marrow/endosteum both gave rise to osteoblasts, whereas the periosteum was the major source of chondrocytes. Finally, results show that intrinsic and environmental signals modulate cell fate decisions within these tissues. In conclusion, this study sheds light into the origins of skeletal stem cells/progenitors during bone regeneration and indicates that periosteum, endosteum, and bone marrow contain pools of stem cells/progenitors with distinct osteogenic and chondrogenic potentials that vary with the tissue environment.

Circulating endothelial/skeletal progenitor cells for bone regeneration and healing

An emerging strategy in the regeneration and repair of bone is to use stem cells, including bone marrow mesenchymal stem cells, which are the most investigated and reliable source for tissue engineering, as well as circulating skeletal stem/progenitor cells, which are receiving abundant attention in regenerative medicine due to their ease of isolation and high osteogenic potential. Because failures in fracture healing are largely due to poor vascularization among many environmental factors, we highlight the first proof-of-principle experiments that elucidated the collaborative multi-lineage differentiation of circulating CD34 positive cells - a cell-enriched population of endothelial/hematopoietic progenitor cells - into not only endothelial cells but also osteoblasts. These cells develop a favorable environment for fracture healing via vasculogenesis/angiogenesis and osteogenesis, ultimately leading to functional recovery from fracture. This review will also highlight current concepts of circulating stem/progenitor cell-based therapy and their potential application for bone repair.