Mobilization of endothelial progenitor cells in fracture healing and distraction osteogenesis

SDF-1/CXCR4 axis in Tie2-lineage cells including endothelial progenitor cells contributes to bone fracture healing

CXC chemokine receptor 4 (CXCR4) is a specific receptor for stromal-derived-factor 1 (SDF-1). SDF-1/CXCR4 interaction is reported to play an important role in vascular development. On the other hand, the therapeutic potential of endothelial progenitor cells (EPCs) in fracture healing has been demonstrated with mechanistic insight of vasculogenesis/angiogenesis and osteogenesis enhancement at sites of fracture. The purpose of this study was to investigate the influence of the SDF-1/CXCR4 pathway in Tie2-lineage cells (including EPCs) in bone formation. We created CXCR4 gene conditional knockout mice using the Cre/loxP system and set two groups of mice: Tie2-Cre(ER) CXCR4 knockout mice (CXCR4(-/-) ) and wild-type mice (WT). We report here that in vitro, EPCs derived from of CXCR4(-/-) mouse bone marrow demonstrated severe reduction of migration activity and EPC colony-forming activity when compared with those derived from WT mouse bone marrow. In vivo, radiological and morphological examinations showed fracture healing delayed in the CXCR4(-/-) group and the relative callus area at weeks 2 and 3 was significantly smaller in CXCR4(-/-) group mice. Quantitative analysis of capillary density at perifracture sites also showed a significant decrease in the CXCR4(-/-) group. Especially, CXCR4(-/-) group mice demonstrated significant early reduction of blood flow recovery at fracture sites compared with the WT group in laser Doppler perfusion imaging analysis. Real-time RT-PCR analysis showed that the gene expressions of angiogenic markers (CD31, VE-cadherin, vascular endothelial growth factor [VEGF]) and osteogenic markers (osteocalcin, collagen 1A1, bone morphogenetic protein 2 [BMP2]) were lower in the CXCR4(-/-) group. In the gain-of-function study, the fracture in the SDF-1 intraperitoneally injected WT group healed significantly faster with enough callus formation compared with the SDF-1 injected CXCR4(-/-) group. We demonstrated that an EPC SDF-1/CXCR4 axis plays an important role in bone fracture healing using Tie2-Cre(ER) CXCR4 conditional knockout mice.

Vascular analysis as a proxy for mechanostransduction response in an isogenic, irradiated murine model of mandibular distraction osteogenesis

INTRODUCTION

Head and neck cancer is a debilitating and disfiguring disease. Although numerous treatment options exist, an array of debilitating side effects accompany them, causing physiological and social problems. Distraction osteogenesis (DO) can avoid many of the pathologies of current reconstructive strategies; however, due to the deleterious effects of radiation on bone vascularity, DO is generally ineffective. This makes investigating the effects of radiation on neovasculature during DO and creating quantifiable metrics to gauge the success of future therapies vital. The purpose of this study was to develop a novel isogenic rat model of impaired vasculogenesis of the regenerate mandible in order to determine quantifiable metrics of vascular injury and associated damage.

METHODS

Male Lewis rats were divided into two groups: DO only (n=5) AND Radiation Therapy (XRT)+DO (n=7). Afterwards, a distraction device was surgically implanted into the mandible. Finally, they were distracted a total of 5.1mm. Animals were perfused with a radiopaque casting agent concomitant with euthanasia, and subsequently demineralization, microcomputed tomography, and vascular analysis were performed.

RESULTS

Vessel volume fraction, vessel thickness, vessel number, and degree of anisotropy were diminished by radiation. Vessel separation was increased by radiation.

CONCLUSION

The DO group experienced vigorous vessel formation during distraction and neovascularization with a clear, directional progression, while the XRT/DO group saw weak vessel formation during distraction and neovascularization. Further studies are warranted to more deeply examine the impairments in osteogenic mechanotransductive pathways following radiation in the murine mandible. This isogenic model provides quantifiable metrics for future studies requiring a controlled approach to immunogenicity.

Role of angiogenesis in bone repair

Bone vasculature plays a vital role in bone development, remodeling and homeostasis. New blood vessel formation is crucial during both primary bone development as well as fracture repair in adults. Both bone repair and bone remodeling involve the activation and complex interaction between angiogenic and osteogenic pathways. Interestingly studies have demonstrated that angiogenesis precedes the onset of osteogenesis. Indeed reduced or inadequate blood flow has been linked to impaired fracture healing and old age related low bone mass disorders such as osteoporosis. Similarly the slow penetration of host blood vessels in large engineered bone tissue grafts has been cited as one of the major hurdle still impeding current bone construction engineering strategies. This article reviews the current knowledge elaborating the importance of vascularization during bone healing and remodeling, and the current therapeutic strategies being adapted to promote and improve angiogenesis.

Stem cell-conditioned medium accelerates distraction osteogenesis through multiple regenerative mechanisms

Distraction osteogenesis (DO) successfully induces large-scale skeletal tissue regeneration, but it involves an undesirably long treatment period. A high-speed DO mouse model (H-DO) with a distraction speed twice that of a control DO model failed to generate new bone callus in the distraction gap. Here we demonstrate that the local administration of serum-free conditioned medium from human mesenchymal stem cells (MSC-CM) accelerated callus formation in the mouse H-DO model. Secretomic analysis identified factors contained in MSC-CM that recruit murine bone marrow stromal cells (mBMSCs) and endothelial cells/endothelial progenitor cells (EC/EPCs), inhibit inflammation and apoptosis, and promote osteoblast differentiation, angiogenesis, and cell proliferation. Functional assays identified MCP-1/-3 and IL-3/-6 as essential factors in recruiting mBMSCs and EC/EPCs. IL-3/-6 also enhanced the osteogenic differentiation of mBMSCs. MSC-CM that had been depleted of MCP-1/-3 failed to recruit mBMSCs, and consequently failed to promote callus formation. Taken together, our data suggest that MSCs produce a broad repertoire of trophic factors with tissue-regenerative activities that accelerate healing in the DO process.

Clinical impact of circulating CD34-positive cells on bone regeneration and healing

Failures in fracture healing after conventional autologous and allogenic bone grafting are mainly due to poor vascularization. To meet the clinical demand, recent attentions in the regeneration and repair of bone have been focused on the use of stem cells such as bone marrow mesenchymal stem cells and circulating skeletal stem cells. Circulating stem cells are currently paid a lot of attention due to their ease of clinical setting and high potential for osteogenesis and angiogenesis. In this report, we focus on the first proof-of-principle experiments demonstrating the collaborative characteristics of circulating CD34(+) cells, known as endothelial and hematopoietic progenitor cell-rich population, which are capable to differentiate into both endothelial cells and osteoblasts. Transplantation of circulating CD34(+) cells provides a favorable environment for fracture healing via angiogenesis/vasculogenesis and osteogenesis, finally leading to functional recovery from fracture. Based on a series of basic studies, we performed a phase 1/2 clinical trial of autologous CD34(+) cell transplantation in patients with tibial or femoral nonunions and reported the safety and efficacy of this novel therapy. In this review, the current concepts and strategies in circulating CD34(+) cell-based therapy and its potential applications for bone repair will be highlighted.

Mesenchymal stem cells systemically injected into femoral marrow of dogs home to mandibular defects to enhance new bone formation

Musculoskeletal diseases cost the U.S. $849 billion annually. To date, there has been no proof that remote long bone mesenchymal stem cells (BMSC) can home to craniofacial defects for bone regeneration. There has been no report that systemic BMSC injection can increase new bone formation in large animals. The objectives of this study were to use a sex-mismatched canine model for systemic BMSC injection and homing to mandibular defects and to investigate appendicular BMSC migration to craniofacial defects to increase new bone formation. Male beagle dog BMSC were injected into the femoral marrow cavity of female dogs upon which mandibular defects were created. The dogs were sacrificed at 6 weeks. Cells with Y chromosome markers were detected in defects of female dogs with systemic male BMSC injection, indicating the homing of the transplanted BMSC from femoral marrow to the mandibular defect. New bone formation in dogs with systemic BMSC injection was 20-40% higher than control without BMSC injection (p<0.05). Mineralized new bone percentage was increased by 20-40% due to systemic BMSC injection (p<0.05). In conclusion, this study proved that (1) allogeneic BMSC injected into long bone marrow are capable of homing to both appendicular and craniofacial bone in large animals and (2) systemically injected BMSC can significantly increase new bone formation in dog's mandibular defects. These results may help advance the understanding of stem cell homing and present a therapy to enhance bone repair, which may have a wide applicability to the regenerative medicine field.

Negative-pressure wound therapy induces endothelial progenitor cell mobilization in diabetic patients with foot infection or skin defects

Non healing chronic wounds are difficult to treat in patients with diabetes and can result in severe medical problems for these patients and for society. Negative-pressure wound therapy (NPWT) has been adopted to treat intractable chronic wounds and has been reported to be effective. However, the mechanisms underlying the effects of this treatment have not been elucidated. To assess the vasculogenic effect of NPWT, we evaluated the systemic mobilization of endothelial progenitor cells (EPCs) during NPWT. Twenty-two of 29 consecutive patients who presented at the clinic of Seoul National Universty Hospital between December 2009 and November 2010 who underwent NPWT for diabetic foot infections or skin ulcers were included in this study. Peripheral blood samples were taken before NPWT (pre-NPWT) and 7–14 days after the initiation of NPWT (during-NPWT). Fluorescence-activated cell sorting (FACS) analysis showed that the number of cells in EPC-enriched fractions increased after NPWT, and the numbers of EPC colony forming units (CFUs) significantly increased during NPWT. We believe that NPWT is useful for treating patients with diabetic foot infections and skin ulcers, especially when these conditions are accompanied by peripheral arterial insufficiency. The systemic mobilization of EPCs during NPWT may be a mechanism for healing intractable wounds in diabetic patients with foot infections or skin defects via the formation of increased granulation tissue with numerous small blood vessels.

Higher numbers of circulating endothelial progenitor cells in stroke patients with intracranial arterial stenosis

Background

Bone marrow-derived endothelial stem cells participate in vascular repairs. Numbers of circulating endothelial progenitor cells (cEPCs) are associated with atherosclerosis. Fibrinogen plays a key role in atherosclerosis. Objective was to assess if cEPC counts were associated with atherosclerotic intracranial artery stenosis (IAS).

Methods

Three hundred subjects (108 patients with stroke and IAS (IAS), 120 control patients with stroke without IAS (CP), and 72 healthy controls (HC)) were retrospectively analyzed. cEPCs were identified and counted by flow cytometry using CD34, CD133 and KDR. Plasma fibrinogen was measured by immunoturbidimetry. cEPC counts were compared between the three groups.

Results

cEPC numbers were significantly higher in IAS (0.059 ± 0.031%) than in CP (0.026 ± 0.012%) (P < 0.001) and HC (0.021 ± 0.011%) (P < 0.001), but without difference between CP and HC (P = 0.401). Multiple logistic regression analysis showed that cEPC levels (OR 3.31, 95%CI 1.26-8.87, P = 0.025; IAS vs. CP) were independent markers of IAS after adjustment for hypertension, diabetes and smoking. No significant correlation between cEPC counts and plasma fibrinogen levels was observed (P > 0.05).

Conclusion

cEPC numbers were associated with degrees of IAS. This measurement may be useful for non-invasive evaluation of atherosclerotic IAS.

Calvarial defect healing by recruitment of autogenous osteogenic stem cells using locally applied simvastatin

Local statins implant has been shown to promote bone healing, the underlying mechanisms are unclear. The purpose of this study was to test the effect of local simvastatin implant on bone defect healing; to evaluate the mobilization, migration, and homing of bone marrow-derived mesenchymal stem cells (BMSCs) and endothelial progenitor cells (EPCs) induced by simvastatin. We found that local simvastatin implant increased bone formation by 51.8% (week 6) and 64.8% (week 12) compared with polyglycolic acid controls (P < 0.01), as verified by X-ray, CT, and histology. Simvastatin increased migration capacity of BMSCs and EPCs in vitro (P < 0.05). Local simvastatin implant increased mobilization of EPCs to the peripheral blood by 127% revealed by FACS analysis (P < 0.01), and increased osteogenic BMSCs to the peripheral blood dramatically revealed by Alizarin Red-S staining for mineralized nodules formation. Pre-transplanted GFP-transfected BMSCs as a tracing cell and bioluminescence imaging revealed that local simvastatin implant recruited GFP-labeled BMSC. Also, local simvastatin implant induced the HIF-1α and BMP-2 expression. In conclusion, local simvastatin implantation promotes bone defect healing, where the underlying mechanism appears to involve the higher expression of HIF-1α and BMP-2, thus recruit autogenous osteogenic and angiogenetic stem cells to the bone defect area implanted with simvastatin.

Mobilization of endogenous stem cell populations enhances fracture healing in a murine femoral fracture model

BACKGROUND AIMS

Delivery of bone marrow-derived stem and progenitor cells to the site of injury is an effective strategy to enhance bone healing. An alternate approach is to mobilize endogenous, heterogeneous stem cells that will home to the site of injury. AMD3100 is an antagonist of the chemokine receptor 4 (CXCR4) that rapidly mobilizes stem cell populations into peripheral blood. Our hypothesis was that increasing circulating numbers of stem and progenitor cells using AMD3100 will improve bone fracture healing.

METHODS

A transverse femoral fracture was induced in C57BL/6 mice, after which they were subcutaneously injected for 3 d with AMD3100 or saline control. Mesenchymal stromal cells, hematopoietic stem and progenitor cells and endothelial progenitor cells in the peripheral blood and bone marrow were evaluated by means of flow cytometry, automated hematology analysis and cell culture 24 h after injection and/or fracture. Healing was assessed up to 84 d after fracture by histomorphometry and micro-computed tomography.

RESULTS

AMD3100 injection resulted in higher numbers of circulating mesenchymal stromal cells, hematopoietic stem cells and endothelial progenitor cells. Micro-computed tomography data demonstrated that the fracture callus was significantly larger compared with the saline controls at day 21 and significantly smaller (remodeled) at day 84. AMD3100-treated mice have a significantly higher bone mineral density than do saline-treated counterparts at day 84.

CONCLUSIONS

Our data demonstrate that early cell mobilization had significant positive effects on healing throughout the regenerative process. Rapid mobilization of endogenous stem cells could provide an effective alternative strategy to cell transplantation for enhancing tissue regeneration.

Bone tissue engineering: recent advances and challenges

The worldwide incidence of bone disorders and conditions has trended steeply upward and is expected to double by 2020, especially in populations where aging is coupled with increased obesity and poor physical activity. Engineered bone tissue has been viewed as a potential alternative to the conventional use of bone grafts, due to their limitless supply and no disease transmission. However, bone tissue engineering practices have not proceeded to clinical practice due to several limitations or challenges. Bone tissue engineering aims to induce new functional bone regeneration via the synergistic combination of biomaterials, cells, and factor therapy. In this review, we discuss the fundamentals of bone tissue engineering, highlighting the current state of this field. Further, we review the recent advances of biomaterial and cell-based research, as well as approaches used to enhance bone regeneration. Specifically, we discuss widely investigated biomaterial scaffolds, micro- and nano-structural properties of these scaffolds, and the incorporation of biomimetic properties and/or growth factors. In addition, we examine various cellular approaches, including the use of mesenchymal stem cells (MSCs), embryonic stem cells (ESCs), adult stem cells, induced pluripotent stem cells (iPSCs), and platelet-rich plasma (PRP), and their clinical application strengths and limitations. We conclude by overviewing the challenges that face the bone tissue engineering field, such as the lack of sufficient vascularization at the defect site, and the research aimed at functional bone tissue engineering. These challenges will drive future research in the field.

Decreased pool of mesenchymal stem cells is associated with altered chemokines serum levels in atrophic nonunion fractures

Nonunion fractures can cause severe dysfunction and are often difficult to treat mainly due to a poor understanding of their physiopathology. Although many aspects of impaired fracture healing have been extensively studied, little is known about the cellular and molecular mechanisms leading to atrophic nonunion. Therefore, the aim of the present study was to assess the pools and biological functions of bone marrow-derived mesenchymal stem cells (hMSCs) and circulating endothelial progenitor cells (EPCs) in atrophic nonunion patients compared to healthy subjects, and the systemic levels of growth factors involved in the recruitment, proliferation and differentiation of these cells. In nonunions, the pool of hMSCs was decreased and their proliferation delayed. However, once committed, hMSCs from nonunions were able to proliferate, differentiate into osteoblastic cells and mineralize in vitro as efficiently as hMSCs from healthy subjects. In parallel, we found altered serum levels of chemokines and growth factors involved in the chemotaxis and proliferation of hMSCs such as leptin, interleukin-6 (IL-6) and its soluble receptor, platelet-derived growth factor-BB (PDGF-BB), stem cell factor (SCF) and insulin-like growth factor-1 (IGF-1). Moreover, we showed that the number of EPCs and their regulating growth factors were not affected in nonunion patients. If nonunion is generally attributed to a vascular defect, our results also support a role for a systemic mesenchymal and osteogenic cell pool defect that might be related to alterations in systemic levels of factors implicated in their chemotaxis and proliferation.

CXCL12/CXCR4 signaling and other recruitment and homing pathways in fracture repair

Cell recruitment, migration and homing to the fracture site are essential for the inflammatory process, neovascularization, chondrogenesis, osteogenesis and ultimately bone remodeling. Mesenchymal stem cells (MSCs) are required to navigate from local sources such as the periosteum and local bone marrow, and may also be recruited from the circulation and distant bone marrow. While the local recruitment process may involve matrix binding and degradation, systemic recruitment may utilize extravasation, a process used by leukocytes to exit the vasculature. CXCL12 (stromal cell-derived factor-1 (SDF-1)), a member of the CXC family of chemokines, is thought to have an important role in cell migration at the fracture site. However, there are many molecules upregulated in the hematoma and callus that have chemotactic potential not only for inflammatory cells but also for endothelial cells and MSCs. Surprisingly, there is little direct data to support their role in cell homing during bone healing. Current therapeutics for bone regeneration utilize local or systemic stem cell transplantation. More recently, a novel strategy that involves mobilization of large numbers of endogenous stem and progenitor cells from bone marrow into the circulation has been shown to have positive effects on bone healing. A more complete understanding of the molecular mechanisms underlying cell recruitment and homing subsequent to fracture will facilitate the fine-tuning of such strategies for bone.

Local Transplantation of Ex Vivo Expanded Bone Marrow-Derived CD34-Positive Cells Accelerates Fracture Healing

Transplantation of bone marrow (BM) CD34+ cells, an endothelial/hematopoietic progenitor-enriched cell population, has shown therapeutic efficiency in the treatment of ischemic diseases enhancing neovascularization. However, the number of CD34+ cells obtained from bone marrow is not sufficient for routine clinical application. To overcome this issue, we developed a more efficient and clinically applicable CD34+ cell expansion method. Seven-day ex vivo expansion culture of BM CD34+ cells with a cocktail of five growth factors containing VEGF, SCF, IL-6, Flt-3 ligand, and TPO resulted in reproducible more than 20-fold increase in cell number. The favorable effect of the local transplantation of culture expanded (cEx)-BM CD34+ cells on rat unhealing fractures was equivalent or higher than that of nonexpanded (fresh) BM CD34+ cells exhibiting sufficient therapeutic outcome with frequent vasculogenic/osteogenic differentiation of transplanted cEx-BM CD34+ cells and fresh BM CD34+ cells as well as intrinsic enhancement of angiogenesis/osteogenesis at the treated fracture sites. Specifically, cEx-BM CD34+ cell treatment demonstrated the best blood flow recovery at fracture sites compared with the nonexpanded BM CD34+ cells. In vitro, cEx-BM CD34+ cells showed higher colony/tube-forming capacity than nonexpanded BM CD34+ cells. Both cells demonstrated differentiation potential into osteoblasts. Since fresh BM CD34+ cells can be easily collected from fracture sites at the time of primary operation and stored for future use, autologous cEx-BM CD34+ cell transplantation would be not only a simple but also a promising therapeutic strategy for unhealing fractures in the field of orthopedic trauma surgery.

Expression of VEGF gene isoforms in a rat segmental bone defect model treated with EPCs

OBJECTIVES

Angiogenesis and osteogenesis are essential for bone growth, fracture repair, and bone remodeling. Vascular endothelial growth factor (VEGF) has an important role in bone repair by promoting angiogenesis and osteogenesis. In our previous study, endothelial progenitor cells (EPCs) promoted bone healing in a rat segmental bone defect as confirmed by radiological, histological, biomechanical, and micro-CT evaluations. Although EPCs have demonstrated effectiveness in animal models of fracture healing, the mechanism by which EPCs enhance fracture healing remains unclear. We hypothesized a possible paracrine mechanism of action, where the secretion of growth factors critical to the processes of fracture healing (such as VEGF), is responsible for the positive effects of EPC therapy. The purpose of this study was to evaluate VEGF gene expression after local EPC therapy for a rat segmental bone defect.

METHODS

Rat bone marrow-derived EPCs were isolated by the Ficoll-paque gradient centrifuge technique. The EPCs were cultured for 7-10 days in endothelial cell growth medium with supplements and collected for treatment of the rat segmental bone defect. EPCs were identified by immunocytochemistry staining with primary antibodies for CD34, CD133, fetal liver kinase-1, and Von Willebrand factor. A total of 56 rats were studied. A 5-mL segmental bone defect was created in the middle one-third of each femur followed by miniplate fixation. The treatment group received 1 × 10 EPCs locally at the bone defect on a gelfoam scaffold and control animals received the gelfoam scaffold only. Seven control and 7 EPC-treated rats were included in each group at 1, 2, 3, and 10 weeks. The animals were sacrificed at the end of the treatment period, and specimens from the fracture gap area were collected and immediately frozen. Rat VEGF mRNA was measured by reverse-transcriptase-polymerase chain reaction and quantified by VisionWorksLS. All measurements were performed in triplicate.

RESULTS

Cultured EPCs at 1 week showed positive staining for CD34, CD133, fetal liver kinase-1, and Von Willebrand factor markers. The EPC group had a greater VEGF expression than the control group at weeks 1, 2, and 3, but not at week 10. Three VEGF isoforms were detected in this rat model: VEGF120, VEGF164, and VEGF188. VEGF120 and VEGF164 levels peaked at 2 weeks, whereas VEGF188 levels peaked at 3 weeks. All 3 VEGF isoform levels were low at 10 weeks.

DISCUSSION AND CONCLUSIONS

EPC-based therapy for a segmental bone defect results in increased VEGF expression during the early period of fracture repair. In addition, the specific VEGF isoform may be a key regulator of the bone healing process. These findings demonstrate that EPCs may promote fracture healing by increasing VEGF levels and thus stimulating angiogenesis, a process that is essential for early callus formation and bone regeneration.

Significance of circulating endothelial progenitor cells in patients with fracture healing process

Fracture healing is a complex bone formation process, and neovascularization may contribute to new bone regeneration. The circulating endothelial progenitor cell (EPC) mobilization and homing could involve in neovascularization and vasculogenesis. In this study, we investigate the changes of circulating EPC during bone fracture healing, and the possible contribution of EPCs to increased neovascularization and fracture healing. The number of circulating EPCs was monitored in twenty-four patients with long bone traumatic fracture within the first 48 h and at 3, 5, 10, and 14 days post-fracture. The mononuclear cells which isolated from peripheral blood were analyzed by flow cytometry. Peripheral blood counts of leukocytes and platelets were measured by hematology analyzer. The amount of peripheral EPCs significantly increased in patients with fracture compared to age-matched healthy control subjects within the first 48 h after injury, and peaked at 3 days post-fracture. There was no significant difference in the change trend of early EPCs between male and female, but the number of early EPCs was significantly greater in younger patients compared to older patients. A comparison of the EPCs levels between patients with severe injury (ISS > 16) and patients with mild injury (ISS ≤ 16) revealed no statistically significant difference. The level of early EPCs was inverse correlation with the level of plate after fracture, but no correlation with the level of peripheral leucocytes. These findings suggest traumatic fracture may induce the mobilization of EPCs into the peripheral circulation. The increased EPCs may contribute to neovascularization and involve in fracture healing.

Adult stem cell mobilization enhances intramembranous bone regeneration: a pilot study

BACKGROUND

Stem cell mobilization, which is defined as the forced egress of stem cells from the bone marrow to the peripheral blood (PB) using chemokine receptor agonists, is an emerging concept for enhancing tissue regeneration. However, the effect of stem cell mobilization by a single injection of the C-X-C chemokine receptor type 4 (CXCR4) antagonist AMD3100 on intramembranous bone regeneration is unclear.

QUESTIONS/PURPOSES

We therefore asked: Does AMD3100 mobilize adult stem cells in C57BL/6 mice? Are stem cells mobilized to the PB after marrow ablation? And does AMD3100 enhance bone regeneration?

METHODS

Female C57BL/6 mice underwent femoral marrow ablation surgery alone (n = 25), systemic injection of AMD3100 alone (n = 15), or surgery plus AMD3100 (n = 57). We used colony-forming unit assays, flow cytometry, and micro-CT to investigate mobilization of mesenchymal stem cells, endothelial progenitor cells, and hematopoietic stem cells to the PB and bone regeneration.

RESULTS

AMD3100 induced mobilization of stem cells to the PB, resulting in a 40-fold increase in mesenchymal stem cells. The marrow ablation injury mobilized all three cell types to the PB over time. Administration of AMD3100 led to a 60% increase in bone regeneration at Day 21.

CONCLUSIONS

A single injection of a CXCR4 antagonist lead to stem cell mobilization and enhanced bone volume in the mouse marrow ablation model of intramembranous bone regeneration.

An emerging cell-based strategy in orthopaedics: endothelial progenitor cells

PURPOSE

The purpose of this article was to analyze the results of studies in the literature, which evaluated the use of endothelial progenitor cells (EPCs) as a cell-based tissue engineering strategy.

METHODS

EPCs have been successfully used in regenerative medicine to augment neovascularization in patients after myocardial infarction and limb ischemia. EPCs' important role as vasculogenic progenitors presents them as a potential source for cell-based therapies to promote bone healing.

RESULTS

EPCs have been shown to have prominent effects in promoting bone regeneration in several animal models. Evidence indicates that EPCs promote bone regeneration by stimulating both angiogenesis and osteogenesis through a differentiation process toward endothelial cell lineage and formation of osteoblasts. Moreover, EPCs increase vascularization and osteogenesis by increased secretion of growth factors and cytokines through paracrine mechanisms.

CONCLUSION

EPCs offer the potential to emerge as a new strategy among other cell-based therapies to promote bone regeneration. Further investigations and human trials are required to address current questions with regard to biology and mechanisms of action of EPCs in bone tissue engineering.

The characteristics of bone marrow-derived endothelial progenitor cells and their effect on glioma

Background

EPCs were isolated primarily in 1997 by Asahara et al. and recent studies indicated that bone-marrow-derived EPCs contributed little to the endothelium of tumor vessels. Tumors of the CNS system demonstrate various features of angiogenesis.

Methods

EPCs derived from rat bone marrow were isolated and cultured in M199 medium without any induced factors. EPCs were studied using immunohistochemical staining, Flow cytometry and culture under three-dimensional condition to determine EPCs’ characteristics in vitro. We also established an animal model by injecting EPCs marked with Hoechst 33342 into the back of BALB/c nude mice and performed hematoxylin-eosin (HE) and immunofluorescent staining to study EPCs’ features in vivo. To research effect of EPCs on glioma, animals bearing tumors model with C6 glioma were established. About 27 day after injection, we performed immunohistochemical staining and Immunofluorescence staining.

Results

Our results showed that EPCs derived from rat bone marrow appeared typical morphological characteristics and were positive of CD34, CD133, KDR and CD31 antigens at different time in vitro under the special M199 medium without any induced factors. The percentage of cells that expressed CD133 decreased gradually. In brief, the present study showed that EPCs derived from rat bone marrow differentiated into ECs in medium the without any induced factors and formed tubular structures in three-dimensional circumstances. Animal experiments suggested that EPCs differentiated into ECs and other else non-endothelial cells, and that EPCs contributed M199 of glioma.

Discussion

These findings provides some novel results about biological characteristics of EPCs in vivo and ex vivo, and an update on the effect of EPCs on glioma and which would be helpful for the overall understanding of EPCs and make EPCs to be implied on the clinical therapy.

Dynamic compression promotes proliferation and neovascular networks of endothelial progenitor cells in demineralized bone matrix scaffold seed

Neovascularization is required for bone formation and successful fracture healing. In the process of neovascularization, endothelial progenitor cells (EPCs) play an important role and finish vascular repair through reendothelialization to promote successful fracture healing. In this study, we found that dynamic compression can promote the proliferation and capillary-like tube formation of EPCs in the demineralized bone matrix (DBM) scaffold seed. EPCs isolated from the bone marrow of rats have been cultured in DBM scaffolds before dynamic compression and then seeded in the DBM scaffolds under dynamic conditions. The cells/scaffold constructs were subjected to cyclic compression with 5% strain and at 1 Hz for 4 h/day for 7 consecutive days. By using MTT and real-time PCR, we found that dynamic compression can significantly induce the proliferation of EPCs in three-dimensional culture with an even distribution of cells onto DBM scaffolds. Both in vitro and in vivo, the tube formation assays in the scaffolds showed that the loaded EPCs formed significant tube-like structures. These findings suggest that dynamic compression promoted the vasculogenic activities of EPCs seeded in the scaffolds, which would benefit large bone defect tissue engineering.

Endothelial progenitor cells improve directly and indirectly early vascularization of mesenchymal stem cell-driven bone regeneration in a critical bone defect in rats

Early vascularization of a composite in a critical bone defect is a prerequisite for ingrowth of osteogenic reparative cells to regenerate bone, since lack of vessels does not ensure a sufficient nutritional support of the bone graft. The innovation of this study was to investigate the direct and indirect effects of endothelial progenitor cells (EPCs) and cotransplanted mesenchymal stem cells (MSCs) on the in vivo neovascularization activity in a critical size defect at the early phase of endochondral ossification. Cultivated human EPCs and MSCs were loaded onto β-TCP in vitro. A critical-sized bone defect (5 mm) was created surgically in the femoral diaphysis of adult athymic rat and stabilized with an external fixateur. The bone defects were filled with β-TCP, MSCs seeded on β-TCP, EPCs seeded on β-TCP, and coculture of MSCs and EPCs seeded on β-TCP or autologous bone of rat. After 1 week, the rats were sacrificed. Using quantitative CD34 immunohistochemistry as well as qualitative analysis of vascularization (staining of MHC and VEGF) in decalcified serial sections were performed by means of an image analysis system. Fluorescence microscopy analyzed the direct effects and indirect effects of human implanted EPCs for vessel formation at bone regeneration site. Formation of a primitive vascular plexus was also detectable in the β-TCP, MSC, or autologous bone group, but on a significantly higher level if EPCs alone or combined with MSCs were transplanted. Moreover, highest amount of vascularization were detected when EPCs and MSCs together were implanted. Early vascularization is improved by transplanted EPCs, which formed new vessels directly. Indeed the indirect effect of EPCs to vascularization is much higher. Transplanted EPC release chemotactic factors (VEGF) to recruit EPCs of the host and stimulate vascularization in the bone defect. Transplantation of human EPCs displays a promising approach to improve early vascularization of a scaffold in a critical bone defect. Moreover, coculture of EPCs and MSCs demonstrate also a synergistic effect on new vessel formation and seems to be a potential osteogenic construct for in vivo application.

Stem cell-based therapies for bone repair

This article provides an overview of the cellular and molecular events involved in bone repair and the current approaches to using stem cells as an adjunct to this process. The article emphasizes the key role of osteoprogenitor cells in the formation of bone and where the clinical applications of current research may lend themselves to large animal orthopaedics. The processes involved in osteogenic differentiation are presented and strategies for bone formation, including induction by osteogenic factors, bioscaffolds, and gene therapy, are reviewed.

Trauma-activated polymorphonucleated leukocytes damage endothelial progenitor cells: probable role of CD11b/CD18-CD54 interaction and release of reactive oxygen species

Endothelial progenitor cells (EPCs) and polymorphonucleated leukocytes (PMNLs) migrate to and accumulate at the site of tissue injury where they express complementary sets of surface receptors (CD11b/CD18, CD54), suggesting a possible cellular interaction. Trauma-activated PMNLs release inflammatory mediators and reactive oxygen species (ROS) produced by the NADPH oxidase, which may negatively impact EPCs. To characterize the interactions between PMNLs and EPCs, we identified common surface receptors and measured the role played by NADPH oxidase and neutrophil elastase. Polymorphonucleated leukocytes were obtained from either healthy volunteers or multiple-trauma patients. After stimulation with either n-formyl-l-methionyl-l-leucyl-l-phenylalanine or phorbol 12-myristate 13-acetate, the PMNLs were incubated with DiL-prestained EPCs in a ratio of 20:1 for 3 h. Early EPCs were isolated from buffy coat. Endothelial progenitor cell killing was measured by flow cytometry, and necrotic EPCs were identified by measuring the uptake of 7-aminoactinomycin. We found that blocking CD11b, CD18, or CD54 on the EPC surface with monoclonal antibodies or blocking the intracellular production of ROS by neutralizing neutrophil's NADPH oxidase with a diphenyliodonium chloride pretreatment protected EPCs, enhancing its survival, whereas inhibiting neutrophil elastase had no effect on survival. Furthermore, we observed that native PMNLs obtained from multiple-trauma patients damaged EPCs, whereas native PMNLs from healthy volunteers did not. Our results demonstrate that EPCs and PMNLs do interact via complementary receptors and that this interaction results in PMNL-derived ROS-induced EPC damage. The effect of neutrophil-derived elastase was found to be negligible. These findings suggest that EPC damage by activated PMNLs may contribute to impaired wound healing observed after severe trauma.

Matrix metalloproteinase-9 contributes to the mobilization of bone marrow cells in the injured liver

Effective mobilization of hematopoietic stem cells (HSCs) in injured organs has not been established. Matrix metalloproteinase-9 (MMP-9) is known to release HSCs from bone marrow (BM) into the peripheral blood, but its role in the recruitment of HSCs to injured organs is unclear. In this study we tried to clarify the role of the host MMP-9 in trafficking of HSCs toward the injured liver, especially the relation of MMP-9 with the chemokine receptor 4 (CXCR4)-chemokine ligand 12 (CXCL12) axis, and to examine whether MMP-9 deficiency affects BM cell trafficking to the injured liver in mice. In vitro, we investigated the effect of MMP-9 on migration activity and CXCR4 expression on lineage-negative (Lin(-)) BM cells. In vivo, we induced acute and chronic liver injury in MMP-9 knockout (KO) and control mice by inoculation of carbon tetrachloride, followed by transplantation of Lin(-) BM cells obtained from enhanced green fluorescent protein (EGFP)-transgenic mice, and counted the BM cells mobilized in the injured liver. In a migration assay, active MMP-9, but not proMMP-9, increased the number of migrated Lin(-) BM cells, which was inhibited by tissue inhibitor of metalloproteinase-1 or a MMP inhibitor. This chemoattractant function by MMP-9 was synergistic when cotreated with CXCL12. CXCR4 expression on Lin(-) BM cells was dose- and time-dependently increased by active MMP-9. At the same time, treatment with MMP-9 enhanced CXCL12 expression, and CXCL12 reciprocally increased MMP-9 expression in BM cells. In in vivo studies, many EGFP(+) cells were seen in control recipient mice. In contrast, few EGFP(+) cells were observed in MMP-9 KO mice. BM cells tended to differentiate into desmin(+) cells. In conclusion, MMP-9 contributes to the mobilization of BM cells in the injured liver by upregulating the expression of CXCR4 on Lin(-) BM cells and attracting BM cells along its gradient of CXCL12. Therefore, host MMP-9 plays an important role in BM cell migration in the injured liver.

Progenitor cell mobilization enhances bone healing by means of improved neovascularization and osteogenesis

BACKGROUND

Although bone repair is a relatively efficient process, a significant portion of patients fail to heal their fractures. Because adequate blood supply is essential to osteogenesis, the authors hypothesize that augmenting neovascularization by increasing the number of circulating progenitor cells will improve bony healing.

METHODS

Bilateral full-thickness defects were created in the parietal bones of C57 wild-type mice. Intraperitoneal AMD3100 (n = 33) or sterile saline (n = 33) was administered daily beginning on postoperative day 3 and continuing through day 18. Circulating progenitor cell number was quantified by fluorescence-activated cell sorting. Bone regeneration was assessed with micro-computed tomography. Immunofluorescent CD31 and osteocalcin staining was performed to assess for vascularity and osteoblast density.

RESULTS

AMD3100 treatment increased circulating progenitor cell levels and significantly improved bone regeneration. Calvarial defects of AMD3100-treated mice demonstrated increased vascularity and osteoblast density.

CONCLUSIONS

Improved bone regeneration in this model was associated with elevated circulating progenitor cell number and subsequently improved neovascularization and osteogenesis. These findings highlight the importance of circulating progenitor cells in bone healing and may provide a novel therapy for bone regeneration.

Stromal cell-derived factor-1 enhances distraction osteogenesis-mediated skeletal tissue regeneration through the recruitment of endothelial precursors

Distraction osteogenesis (DO) is a unique therapy that induces skeletal tissue regeneration without stem/progenitor cell transplantation. Although the self-regeneration property of DO provides many clinical benefits, the long treatment period required is a major drawback. A high-speed DO mouse model (H-DO), in which the distraction was done two times faster than in control DO (C-DO) mice, failed to generate new bone callus in the DO gap. We found that this was caused by the unsuccessful recruitment of bone marrow endothelial cells (BM-ECs)/endothelial progenitor cells (EPCs) into the gap. We then tested the ability of a local application of stromal cell-derived factor-1 (SDF-1), a major chemo-attractant for BM-ECs/EPCs, to accelerate the bone regeneration in H-DO. Our data showed that, in H-DO, SDF-1 induced callus formation in the gap through the recruitment of BM-ECs/EPCs, the maturation of neo-blood vessels, and increased blood flow. These results indicate that the active recruitment of endogenous BM-ECs/EPCs may provide a substantial clinical benefit for shortening the treatment period of DO.

The dynamic changes of circulating OCN+ cells versus insulinlike growth factor-I during primary healing of orthognathic surgeries

OBJECTIVE

The objective of this study was to determine the dynamic changes of circulating osteocalcin(+) (OCN(+)) cells and insulinlike growth factor-I (IGF-I) in peripheral blood during early primary repair of jaw bones in patients with orthognathic surgery.

STUDY DESIGN

The expression of bone-related genes was detected by RT-PCR in circulating OCN(+) cells. The numbers of OCN(+) cells and serum level of IGF-I were determined by flow cytometry, immunocytochemical staining, and ELISA.

RESULTS

OCN(+) cells significantly increased in peripheral blood, and reached the peak at 1 to 2 weeks after surgery (P < .05). IGF-I in patients significantly decreased 1 week after surgery (P < .05), and then returned gradually to the normal level. There was no significant correlation between the number of circulating OCN(+) cells and the level of IGF-I (P > .05).

CONCLUSIONS

These findings suggested that circulating OCN(+) cells, at least in part, could be mobilized in response to bone injury, and contribute to bone repair in patients with orthognathic surgery.

Endothelial progenitor cells for fracture healing: a microcomputed tomography and biomechanical analysis

PURPOSE

Local treatment of segmental bone defects with ex vivo expanded endothelial progenitor cells (EPCs) has been shown to increase osteogenesis and callus formation in rat femur diaphyseal defects. The purpose of this study was to evaluate the effects of local EPC therapy on the microarchitecture and biomechanical properties of a segmental bone defect in a rat model.

METHODS

Five-millimeter segmental defects were created in the femora of 14 Fisher 344 rats and stabilized with miniplates. The treatment group (n = 7) received 1 × 10(6) EPCs, seeded on a Gelfoam scaffold, locally at the site of the bone defect, and control animals (n = 7) received Gelfoam and saline only. Animals were euthanized 10 weeks after the procedure and new bone formation was assessed with radiographs, microcomputed tomography and biomechanical testing.

RESULTS

Radiographically, all the animals in the EPC-treated group healed with bridging callus formation, whereas animals in the control group developed nonunion of the defect. Microcomputed tomography assessment demonstrated significantly superior bone formation in the EPC-treated group versus the control group for all parameters tested (P = 0.013-0.000). Biomechanical testing revealed that the EPC-treated group had significantly higher torsional strength (P = 0.000) and stiffness (P = 0.000) when compared with the control group.

CONCLUSION

The results of this study suggest that local EPC therapy significantly enhances fracture healing in a segmental defect model in a rat femur. EPC therapy results in superior radiographic bone formation and healing when compared with appropriate controls.

Angiogenesis in bone regeneration

Angiogenesis is a key component of bone repair. New blood vessels bring oxygen and nutrients to the highly metabolically active regenerating callus and serve as a route for inflammatory cells and cartilage and bone precursor cells to reach the injury site. Angiogenesis is regulated by a variety of growth factors, notably vascular endothelial growth factor (VEGF), which are produced by inflammatory cells and stromal cells to induce blood vessel in-growth. A variety of studies with transgenic and gene-targeted mice have demonstrated the importance of angiogenesis in fracture healing, and have provided insights into regulatory processes governing fracture angiogenesis. Indeed, in animal models enhancing angiogenesis promotes bone regeneration, suggesting that modifying fracture vascularization could be a viable therapeutic approach for accelerated/improved bone regeneration clinically.

Vascularization is the key challenge in tissue engineering

The main limitation in engineering in vitro tissues is the lack of a sufficient blood vessel system - the vascularization. In vivo almost all tissues are supplied by these endothelial cell coated tubular networks. Current strategies to create vascularized tissues are discussed in this review. The first strategy is based on the endothelial cells and their ability to form new vessels known as neoangiogenesis. Herein prevascularization techniques are compared to approaches in which biomolecules, such as growth factors, cytokines, peptides and proteins as well as cells are applied to generate new vessels. The second strategy is focused on scaffold-based techniques. Naturally-derived scaffolds, which contain vessels, are distinguished from synthetically manufactured matrices. Advantages and pitfalls of the approaches to create vascularized tissues in vitro are outlined and feasible future strategies are discussed.

Osteogenic Growth Peptide Accelerates Bone Healing during Distraction Osteogenesis in Rabbit Tibia

Distraction osteogenesis is a valuable treatment method that allows limb lengthening or reconstruction of large bone defects. However, its major disadvantage is the long period required for the consolidation of a distraction callus. Osteogenic growth peptide (OGP) stimulates endochondral bone formation in fracture callus, but its capacity to promote regenerate ossification during distraction osteogenesis has not been evaluated. This study investigated whether intravenously administered OGP accelerated bone healing during distraction osteogenesis in 36 male New Zealand White rabbits, randomized into two groups. The treatment group received OGP (200 ng/kg body weight) in phosphate-buffered saline (PBS), intravenously, each day; the control group received PBS alone. A 15-mm lengthening of the right lower leg was performed using the method of Ilizarov. Evidence from biomechanical, histological and radiographic evaluations demonstrated that systemic OGP treatment promoted optimal new bone formation during distraction osteogenesis in this rabbit model.

Low dose erythropoietin stimulates bone healing in mice

Beyond its classical role in regulation of erythropoiesis, erythropoietin (EPO) has been shown to exert protective and regenerative actions in a variety of non-hematopoietic tissues. However, little is known about potential actions in bone regeneration. To analyze fracture healing in mice, a femoral 0.25 mm osteotomy gap was stabilized with a pin-clip technique. Animals were treated with 500 U EPO/kg bw per day or with vehicle only. After 2 and 5 weeks, fracture healing was analyzed biomechanically, radiologically and histologically. Expression of PCNA and NFκB was examined by Western blot analysis. Vascularization was analyzed by immunohistochemical staining of PECAM-1. Circulating endothelial progenitor cells were measured by flow-cytometry. Herein, we demonstrate that EPO-treatment significantly accelerates bone healing in mice. This is indicated by a significantly greater biomechanical stiffness and a higher radiological density of the periosteal callus at 2 and 5 weeks after fracture and stabilization. Histological analysis demonstrated significantly more bone and less cartilage and fibrous tissue in the periosteal callus. Endosteal vascularization was significantly increased in EPO-treated animals when compared to controls. The number of circulating endothelial progenitor cells was significantly greater in EPO-treated animals. The herein shown acceleration of healing by EPO may represent a promising novel treatment strategy for fractures with delayed healing and non-union formation.

Intravenous delivery of bone marrow-derived endothelial progenitor cells improves survival and attenuates lipopolysaccharide-induced lung injury in rats

Acute lung injury (ALI) is a devastating disease, which is characterized by diffuse endothelium, epithelial damage, and increased pulmonary capillary permeability. Recent data have suggested that the circulating endothelial progenitor cells (EPCs) play an important role in endothelial repair after vascular injury. This study was undertaken to investigate possible endothelial-repairing effects of EPC transplantation after LPS-induced ALI in rats. Using Y-chromosome in situ hybridization and reverse transcription polymerase chain reaction assay, we detected the expression of sex-determining region y in the injured lungs of female model rats, suggesting that allogenic EPCs can migrate to the injured lung tissues. Rats that have received the EPC treatment had a reduced pulmonary edema level, inflammation, hemorrhage, and hyaline membrane formation, as well as an increased survival rate from 44% to 81%. Furthermore, anti-inflammatory cytokine IL-10 levels were dramatically increased in the EPC-treated rats compared with the phosphate buffered saline-treated rats. On the contrary, endothelin-1 and iNOS were downregulated in the EPC-treated group. These findings provide evidence that i.v. EPC treatment results in engraftment of EPCs to the injured lung tissue, which can significantly attenuate lung injury and improve survival in ALI rats. The beneficial effects of EPC engraftment is likely to come from maintaining the integrity of pulmonary alveolar-capillary barrier, reestablishing the endothelial function in vessels and ameliorating the inflammatory state.

Production of VEGF receptor 1 and 2 mRNA and protein during endochondral bone repair is differential and healing phase specific

Physiological disturbances, including temporary hypoxia, are expected to drive angiogenesis during bone repair. Evidence suggests that the angiogenic ligand vascular endothelial growth factor (VEGF)-A plays an important role in this process. We characterized the expression of two receptors that are essential for mediating VEGF signaling, VEGFR1/Flt-1 and VEGFR2/Flk-1/KDR, in a mouse rib fracture model. Their mRNA and protein levels were assessed in four healing phases, which were characterized histologically as hemorrhage formation on postfracture day (PFD) 1, inflammatory response on PFD 3, initiation of callus development on PFD 7, and the presence of a mature callus on PFD 14. Transcript was detected for VEGFR1 and VEGFR2, as well as VEGF. While mRNA expression of VEGFR1 was monophasic throughout all healing phases, VEGFR2 showed a biphasic profile with significantly increased mRNA expression during callus formation and maturation. Expression of VEGF mRNA was characterized by a more gradual increase during callus formation. The protein level for VEGFR1 was below detection sensitivity during the initial healing phase. It was then restored to a stable level, detectable through the subsequent healing phases. Hence, the VEGFR1 protein levels partially mirrored the transcript expression profile. In comparison, the protein level of VEGFR2 increased gradually during the healing phases and peaked at callus maturation. This correlated well with the transcriptional expression of VEGFR2. Intact bone from age-matched male mice had considerable protein levels of VEGFR1 and VEGF, but no detectable VEGFR2. Together, these findings uncovered expression signatures of the VEGF-VEGFR axis in endochondral bone repair.

Endothelial progenitor cells and mesenchymal stem cells seeded onto beta-TCP granules enhance early vascularization and bone healing in a critical-sized bone defect in rats

QUESTION/AIM: Lack of vessels indicates an insufficient nutritional supply of a bone graft and may limit the recruitment of bone-forming cells. Our aim was to evaluate the influence of endothelial progenitor cells (EPCs) alone or in combination with mesenchymal stem cells (MSCs) on early vascularization and bone healing in critical-sized defect (CSD) in vivo.

METHODS

MSCs from human bone marrow and EPCs from buffy coat were used. A femoral CSD in adult athymic rats was created and stabilized by an external fixateur. The remaining defects were filled with fibronectin-coated beta-tricalcium phosphate (beta-TCP) granules, EPCs seeded on beta-TCP, MSCs seeded on beta-TCP, coculture of EPCs/MSCs seeded on beta-TCP, or autologous bone. Vascularization and bone formation were determined by immunohistology, microCT analysis, and biomechanical testing after 1, 4, and 8 weeks.

RESULTS

Early vascularization was significantly improved in EPC/MSC group or EPC group, respectively. At 4 weeks bone formation increased significantly when the CSD was treated with coculture of MSCs/EPCs. Eight weeks after transplantation CSD showed significantly more bony bridgings and significantly increased ultimate load in the EPC/MSC group compared to the other groups.

DISCUSSION

This cell approach suggests that there is a synergistic effect and that the initial stage of neovascularization by EPCs is considered to be crucial for complete bone regeneration in the late phase.

Beneficial effect of autologous transplantation of endothelial progenitor cells on steroid-induced femoral head osteonecrosis in rabbits

Femoral head osteonecrosis (ON) is a serious complication of steroid administration. We examined whether implantation of autologous bone marrow-derived endothelial progenitor cells (EPCs) can augment neovascularization and bone regeneration in steroid-induced osteonecrosis of the femoral head. Forty 12-week-old male New-Zealand white rabbits were divided into group I (left untreated, n=12), group II (core decompression, n=12), and group III (core decompression + autologous EPCs implantation, n=16) after receiving an established inductive protocol for inducing steroid-associated ON. Four weeks later, these rabbits were euthanized, bilateral femora were dissected for Micro-CT-based microangiography to assess vascularization, and then the osteonecrotic changes and repair processes were examined histopathologically. Quantitative analysis showed that new vessel formation in group III was significantly greater compared with other groups at 4 weeks after treatment. The histologic and histomorphometric analyses revealed that the new bone volume was significantly higher in group III than in groups I and II 4 weeks after treatment. A combination of EPCs and core decompression enhances the neovascularization and bone regeneration in rabbit steroid-induced femoral head ON. Local implantation of EPCs may provide a novel and effective therapeutic option for early corticosteroid-induced ON.

Endothelial progenitor cells promote fracture healing in a segmental bone defect model

The objective of this study was to evaluate the effects of local endothelial progenitor cell (EPC) therapy on bone regeneration in a rat model. A segmental bone defect (5 mm) was created in the femur and fixed with a mini-plate. There were two groups: EPC-treated (N = 28) and control (N = 28). Seven animals were sacrificed from each group at 1, 2, 3, and 10 weeks postoperatively. Healing of the defect was evaluated with radiographic, histological, and quantitative micro-computed tomography (micro-CT) scans. Radiographically, mean scores of the EPC and control groups were, respectively, 1.16-0.61 (p < 0.05) at 1 week, 2.53-1.54 (p < 0.05) at 2 weeks, and 4.58-2.35 at 3 weeks (p < 0.05). At 10 weeks, all the animals in the EPC-treated group had complete union (7/7), but in the control group none achieved union (0/7). Histological evaluation revealed that specimens from EPC-treated animals had abundant new bone and vessel formation compared to that in controls. Micro-CT assessment of the samples from the animals sacrificed at 10 weeks (N = 14) showed significantly improved parameters of bone volume (36.58-10.57, p = 0.000), bone volume density (0.26-0.17, p = 0.000), model index -2.22-2.79, p = 0.000), trabecular number (1.28-0.91, p = 0.063), trabecular thickness (0.21-0.15, p = 0.001), trabecular spacing (0.63-1.07, p = 0.022), bone surface (353.75-152.08, p = 0.000), and bone surface to bone volume ratio (9.54-14.24, p = 0.004) for the EPC group compared to control, respectively. In conclusion, local EPC therapy significantly enhanced bone regeneration in a segmental defect model in rat femur diaphysis.

Nutraceutical augmentation of circulating endothelial progenitor cells and hematopoietic stem cells in human subjects

The medical significance of circulating endothelial or hematopoietic progenitors is becoming increasing recognized. While therapeutic augmentation of circulating progenitor cells using G-CSF has resulted in promising preclinical and early clinical data for several degenerative conditions, this approach is limited by cost and inability to perform chronic administration. Stem-Kine is a food supplement that was previously reported to augment circulating EPC in a pilot study. Here we report a trial in 18 healthy volunteers administered Stem-Kine twice daily for a 2 week period. Significant increases in circulating CD133 and CD34 cells were observed at days 1, 2, 7, and 14 subsequent to initiation of administration, which correlated with increased hematopoietic progenitors as detected by the HALO assay. Augmentation of EPC numbers in circulation was detected by KDR-1/CD34 staining and colony forming assays. These data suggest Stem-Kine supplementation may be useful as a stimulator of reparative processes associated with mobilization of hematopoietic and endothelial progenitors.

Bone lengthening osteogenesis, a combination of intramembranous and endochondral ossification: an experimental study in sheep

We evaluated the morphological features of the newly formed tissue in an experimental model of tibial callotasis lengthening on 24 lambs, aged from 2 to 3 months at the time of operation. A unilateral external fixator prototype Monotube Triax^® (Stryker Howmedica Osteonics, New Jersey) was applied to the left tibia. A percutaneous osteotomy was performed in a minimally traumatic manner using a chisel. Lengthening was started 7 days after surgery and was continued to 30 mm. The 24 animals were randomly divided into three groups of 8 animals each: in Group 1, lengthening took place at a rate of 1 mm/day for 30 days; in Group 2, at a rate of 2 mm/day for 15 days; in Group 3, at a rate of 3 mm/day for 10 days. In each group, 4 animals were killed 2 weeks after end of lengthening, and the other 4 animals at 4 weeks after end of lengthening. To assess bony formation in the distraction area, radiographs were taken every 2 weeks from the day of surgery. To study the process of vascularization, we used Spalteholz’s technique. After killing, the tibia of each animal was harvested, and sections were stained with hematoxylin and eosin, Masson’s trichrome, and Safranin-O. Immunohistochemistry was performed, using specific antibodies to detect collagens I and II, S100 protein, and fibronectin. A combination of intramembranous and endochondral ossification occurred together at the site of distraction. Our study provides a detailed structural characterization of the newly formed tissue in an experimental model of tibial lengthening in sheep and may be useful for further investigations on callotasis.

Distraction osteogenesis induces endothelial progenitor cell mobilization without inflammatory response in man

INTRODUCTION

Distraction osteogenesis (DO) is a unique postnatal bone formation process, which is characterized by a profuse increase in vascularization. Recently endothelial progenitor cells (EPCs) have been reported to circulate in substantial numbers under physiologic conditions and to contribute to bone regeneration. The authors investigated EPC mobilization in patients undergoing limb lengthening.

MATERIALS AND METHODS

Thirteen of 24 consecutive patients who underwent limb-lengthening procedures on weight-bearing long bones (femur and tibia) were included in this study. Peripheral blood samples were taken at four different time points from each patient, that is, before operation (pre-Op), 2 or 3 days after osteotomy (early-PO), before the start of distraction (pre-Dist), and at 7 to 14 days after the start of distraction (during-Dist). Numbers of leukocytes and levels of plasma C-reactive protein (CRP) were determined. After isolating mononuclear cells (MNCs) by centrifugation, we performed FACS analysis on freshly isolated MNCs using antibodies to the cell surface markers; CD34, CD133, vascular endothelial growth factor receptor 2 (VEGFR2), and alkaline phosphatase. MNCs were also cultured in endothelial cell growth medium and numbers of EPC colony-forming units were counted. Plasma levels of EPC-mobilizing cytokines, such as, VEGF, SDF-1, and MCP-1, were determined by ELISA.

RESULTS

Numbers of leukocytes and CRP plasma levels increased significantly during the early-PO period (p<0.01) but were maintained within normal range in the during-Dist period. FACS analysis of freshly isolated MNCs showed that EPC-enriched cell fractions increased after distraction, but that alkaline phosphatase-positive cell numbers were unchanged. Numbers of EPC colony-forming units significantly increased in the during-Dist period (p<0.01). Plasma levels of VEGF and SDF-1 significantly increased in the during-Dist period (p<0.05). In femoral lengthening patients whose healing index was less than 30 days/cm, the number of CFUs was 46.8 in the during-Dist period, whereas it was 12.7 in patients whose healing index was more than 30 days/cm (p=0.088).

CONCLUSIONS

This study demonstrates a mobilization of EPC population during distraction osteogenesis in human limb-lengthening patients. Distraction strain provoked increases in the plasma levels of EPC-mobilizing cytokines, such as, VEGF and SDF-1. These findings suggest a possibility that therapeutical approaches which modulate EPC mobilization may speed bone healing by angiogenesis-osteogenesis coupling during distraction osteogenesis.

Bone regeneration: the stem/progenitor cells point of view

After bone injuries, several molecular mechanisms establish bone repair from stem/progenitor cells. Inflammation factors attract regenerative cells which expand and differentiate in order to build up a bone highly similar to that before injury. Bone marrow (BM) mesenchymal stem cells (MSCs) as skeletal stem cells and endothelial progenitors (EPCs) are at the origin of such reparation mechanisms. However, discrepancies exist about their identities. Although cultured MSCs are extensively described, their in vivo native forms are poorly known. In addition, recent experiments show that several types of EPC exist. We therefore review up-to-date data on the characterization of such stem/progenitor cells and propose a new point of view of their function in bone regeneration.

Decreased in the number and function of circulation endothelial progenitor cells in patients with avascular necrosis of the femoral head

INTRODUCTION

Once non-traumatic avascular necrosis of the femoral head (ANFH) happened, vascular impairment and feeble collateral circulation are followed by poor outcomes. Circulating endothelial progenitor cells (EPCs) may substantially contribute to vascular homeostasis such as vascular repair and new blood vessel growth. We investigated whether abnormalities in EPCs levels and functions are present in ANFH patients.

METHODS

54 ANFH patients were enrolled, including steroid-induced (n=21), alcohol-induced (n=15) and idiopathic ANFH (n=18), and 30 healthy subjects as control (HC). The numbers of circulation EPCs were determined by fluorescence-activated cell-sorting (FACS) analysis. EPCs cultured from peripheral blood mononuclear cells on fibronectin to induce the expression of receptors for acetylated low-density lipoprotein and ulex-lectin. EPCs colony-forming units (CFUs) were observed from 54 patients and 30 healthy controls. Migratory capacity to chemo-attractants (vascular endothelial growth factor) cellular senescence levels and in vitro angiogenesis ability were assessed in age-matched subjects (n=10 per groups).

RESULTS

Mean numbers of circulating EPC were 1460+/-265 cells/ml in HC, 545+/-177 in ANFH, (P<0.001). Mean numbers of CFUs were 26.2+/-6.2 in HC, 19.6+/-7.7 in ANFH,(P<0.001). Although there were not significant differences in circulating EPC and CFUs among the steroid-induced, alcohol-induced or idiopathic three groups, all these risk factors contributed to the decreased circulating EPCs numbers and CFUs. In addition, EPCs from ANFH patients showed reduced migratory capacity and increased cellular senescence compared with EPCs from normal subjects, furthermore the ability of angiogenesis in vitro was also impaired.

CONCLUSION

Circulating endothelial progenitor cells (EPCs) numbers and functions are reduced in ANFH patients, suggesting that risk factors of ANFH may alter EPCs biology in angiogenesis and vascular repair.