Delayed Endothelial Progenitor Cell Therapy Promotes Bone Defect Repair in a Clinically Relevant Rat Model

The Effect of Cryopreservation on the Bone Healing Capacity of Endothelial Progenitor Cells in a Bone Defect Model

Endothelial progenitor cells (EPCs) have proven to be a highly effective cell therapy for critical-sized bone defects. Cryopreservation can enable long-term storage of EPCs, allowing their immediate availability on demand. This study compares the therapeutic potential of EPCs before and after cryopreservation in a small animal critical-sized bone defect model. Five-millimeter segmental defects were created in the right femora of Fischer 344 rats, followed by stabilization with a miniplate and screws. The animals received 2 × 106 fresh EPCs (n = 7) or 2 × 106 cryopreserved EPCs (n = 9) delivered on a gelatin scaffold. Cryopreserved EPCs were stored for 7 days at -80°C prior to thawing and loading onto the gelatin scaffold. Biweekly radiographs were taken until the animals were euthanized 10 weeks after surgery. The operated femora were then evaluated using microscopic-computed tomography (micro-CT) and biomechanical testing. All animals treated with fresh (n = 7/7) or cryopreserved (n = 9/9) EPCs achieved radiographic union at 10 weeks. Animals treated with fresh EPCs had statistically significant higher radiographic scores at 2 weeks (p < 0.05) but showed no statistically significant differences thereafter (p > 0.05). Micro-CT analysis showed no statistically significant differences between the groups in bone volume (BV) or BV normalized to total volume (p > 0.05), with excellent bone formation in both groups. Finally, there were no differences in biomechanical outcomes between the groups (p > 0.05). These results demonstrate that cryopreserved EPCs are highly effective and equivalent to fresh EPCs for healing critical-sized bone defects in a rat model of nonunion.

Roles of lncRNA in the crosstalk between osteogenesis and angiogenesis in the bone microenvironment

Bone is a highly calcified and vascularized tissue. The vascular system plays a vital role in supporting bone growth and repair, such as the provision of nutrients, growth factors, and metabolic waste transfer. Moreover, the additional functions of the bone vasculature, such as the secretion of various factors and the regulation of bone-related signaling pathways, are essential for maintaining bone health. In the bone microenvironment, bone tissue cells play a critical role in regulating angiogenesis, including osteoblasts, bone marrow mesenchymal stem cells (BMSCs), and osteoclasts. Osteogenesis and bone angiogenesis are closely linked. The decrease in osteogenesis and bone angiogenesis caused by aging leads to osteoporosis. Long noncoding RNAs (lncRNAs) are involved in various physiological processes, including osteogenesis and angiogenesis. Recent studies have shown that lncRNAs could mediate the crosstalk between angiogenesis and osteogenesis. However, the mechanism by which lncRNAs regulate angiogenesis‒osteogenesis crosstalk remains unclear. In this review, we describe in detail the ways in which lncRNAs regulate the crosstalk between osteogenesis and angiogenesis to promote bone health, aiming to provide new directions for the study of the mechanism by which lncRNAs regulate bone metabolism.

The Effects of Antibiotic-Impregnated Spacers on Bone Healing in an Animal Model of the Induced Membrane Technique: Healing of a Critical-Size Femoral Defect in a Rat Model

Background

Surgeons performing the induced membrane technique (IMT) often incorporate antibiotics into the spacer at the first stage of the surgical procedure to prevent or treat infection. However, the effect of antibiotic use on subsequent bone healing is not clear. This study aimed to investigate if antibiotic-impregnated spacers impact subsequent bone healing in a rat model of the IMT.

Methods

Inbred male rats (Fischer 344) were randomly divided into 3 groups according to the antibiotic dose in the spacer: (1) control (no antibiotics), (2) low-dose (1.2 g tobramycin and 1.0 g vancomycin per 40 g of polymethylmethacrylate [PMMA]), and (3) high-dose (3.6 g tobramycin and 3.0 g vancomycin per 40 g of PMMA). We created a 5-mm segmental defect in the right femoral diaphysis. The bone was stabilized with a plate and screws, and the assigned spacer was inserted into the defect. Four weeks later, the spacer was removed and bone graft was placed within the defect. Radiographs made 12 weeks after grafting were scored according to union status and degree of bone healing. Micro-computed tomographic (CT) analysis and biomechanical testing were also performed at 12 weeks.

Results

Full radiographic union was achieved in 10 (83%) of 12 control animals, 13 (100%) of 13 low-dose animals, and 8 (62%) of 13 high-dose animals (high-dose compared with low-dose: risk ratio, 11.0; p = 0.039). The control group demonstrated higher bone volume compared with the high-dose group (mean difference, 9.0 mm3; p = 0.039), and there was a trend toward higher bone volume in the low-dose group compared with the high-dose group (mean difference, 8.1 mm3; p = 0.06). The biomechanical results demonstrated that maximum stiffness was significantly higher in the low-dose group compared with the high-dose group (mean difference, 14.1 N*mm/degree; p = 0.009).

Conclusions

Our results demonstrated that low doses of antibiotics in PMMA spacers used for the IMT did not impair bone healing. However, high doses of antibiotics demonstrated inferior bone healing.

Clinical Relevance

The addition of high-dose antibiotics to the PMMA spacers used for the IMT may result in impaired bone healing and should be used with caution.

Optimal delivery of endothelial progenitor cells in a rat model of critical-size bone defects

Nonunion and segmental bone defects are complex issues in orthopedic trauma. The use of endothelial progenitor cells (EPCs), as part of a cell-based therapy for bone healing is a promising approach. In preclinical studies, culture medium (CM) is commonly used to deliver EPCs to the defect site, which has the potential for immunogenicity in humans. The goal of this study was to find an effective and clinically translatable delivery medium for EPCs. Accordingly, this study compared EPCs delivered in CM, phosphate-buffered saline (PBS), platelet-poor plasma (PPP), and platelet-rich plasma (PRP) in a rat model of femoral critical-size defects. Fischer 344 rats (n = 35) were divided into six groups: EPC+CM, EPC+PBS, EPC+PPP, EPC+PRP, PPP alone, and PRP alone. A 5 mm mid-diaphyseal defect was created in the right femur and stabilized with a miniplate. The defect was filled with a gelatin scaffold impregnated with the corresponding treatment. Radiographic, microcomputed tomography and biomechanical analyses were performed. Overall, regardless of the delivery medium, groups that received EPCs had higher radiographic scores and union rates, higher bone volume, and superior biomechanical properties compared to groups treated with PPP or PRP alone. There were no significant differences in any outcomes between EPC subgroups or between PPP and PRP alone. These results suggest that EPCs are effective in treating segmental defects in a rat model of critical-size defects regardless of the delivery medium used. Consequently, PBS could be the optimal medium for delivering EPCs, given its low cost, ease of preparation, accessibility, noninvasiveness, and nonimmunogenic properties.

Endothelial Progenitor Cell Therapy for Fracture Healing: A Dose-Response Study in a Rat Femoral Defect Model

Endothelial progenitor cell (EPC) therapy has been successfully used in orthopaedic preclinical models to heal bone defects. However, no previous studies have investigated the dose-response relationship between EPC therapy and bone healing. This study aimed to assess the effect of different EPC doses on bone healing in a rat model to define an optimal dose. Five-millimeter segmental defects were created in the right femora of Fischer 344 rats, followed by stabilization with a miniplate and screws. Rats were assigned to one of six groups (control, 0.1 M, 0.5 M, 1.0 M, 2.0 M, and 4.0 M; n = 6), receiving 0, 1 × 105, 5 × 105, 1 × 106, 2 × 106, and 4 × 106 EPCs, respectively, delivered into the defect on a gelatin scaffold. Radiographs were taken every two weeks until the animals were euthanized 10 weeks after surgery. The operated femora were then evaluated using micro-computed tomography and biomechanical testing. Overall, the groups that received higher doses of EPCs (0.5 M, 1.0 M, 2.0 M, and 4.0 M) reached better outcomes. At 10 weeks, full radiographic union was observed in 67% of animals in the 0.5 M group, 83% of animals in the 1.0 M group, and 100% of the animals in the 2.0 M and 4.0 M groups, but none in the control and 0.1 M groups. The 2.0 M group also displayed the strongest biomechanical properties, which significantly improved relative to the control and 0.1 M groups. In summary, this study defined a dose-response relationship between EPC therapy and bone healing, with 2 × 106 EPCs being the optimal dose in this model. Our findings emphasize the importance of dosing considerations in the application of cell therapies aimed at tissue regeneration and will help guide future investigations and clinical translation of EPC therapy.

The induced membrane technique: Optimization of bone grafting in a rat model of segmental bone defect

INTRODUCTION

The induced membrane technique (IMT) is a two-stage surgical procedure used to treat fracture nonunion and bone defects. Although there is an increasing number of animal studies investigating the IMT, few have examined the outcomes of bone healing after a second stage grafting procedure. This study aimed at comparing two bone grafting procedures, as part of the IMT, in order to establish a rat model providing consistent healing outcomes.

METHODS

In male Fischer 344 rats, we created a 5 mm defect in the right femur, stabilized the bone with a plate and screws, and inserted a polymethylmethacrylate spacer into the defect. Four weeks later, the spacer was removed. Bone graft was harvested from a donor rat and placed into the defect, followed by membrane and wound closure. Experiments were conducted in two groups. In group 1 (n = 11), the bone graft contained a variable amount of cortical and cancellous bone, the time from donor euthanasia to grafting was up to 240 min, and one donor rat provided graft for 5-6 recipients. In group 2 (n = 12), we reduced the contribution of cortical bone to the graft, included bone marrow, and kept donor euthanasia to grafting time under 150 min. One donor was used per 3-4 recipients. The volume of graft per recipient and all other elements of the protocol were the same across groups. Bone healing at 12 weeks post grafting was compared radiographically by two orthopaedic surgeons in a blinded fashion, based on union status and a modified Lane & Sandhu score.

RESULTS

Healing rates improved from 36.4% in Group 1 to 91.6% in Group 2. There was a significant relationship between the methods and resulting union status (p = 0.004). The odds of achieving full union were significantly higher in group 2 compared to group 1 (odds ratio=19.25, 95% confidence interval [1.77-209.55]; p = 0.009). The average radiographic score was also significantly higher in group 2 (p = 0.005).

CONCLUSION

The revised bone grafting method significantly improved the healing outcomes and contributed to establishing a consistent rat model of the IMT. This model can benefit preclinical investigations by allowing for reliable and clinically-relevant comparisons.

Characterization and assessment of lung and bone marrow derived endothelial cells and their bone regenerative potential

Angiogenesis is important for successful fracture repair. Aging negatively affects the number and activity of endothelial cells (ECs) and subsequently leads to impaired bone healing. We previously showed that implantation of lung-derived endothelial cells (LECs) improved fracture healing in rats. In this study, we characterized and compared neonatal lung and bone marrow-derived endothelial cells (neonatal LECs and neonatal BMECs) and further asses3sed if implantation of neonatal BMECs could enhance bone healing in both young and aged mice. We assessed neonatal EC tube formation, proliferation, and wound migration ability in vitro in ECs isolated from the bone marrow and lungs of neonatal mice. The in vitro studies demonstrated that both neonatal LECs and neonatal BMECs exhibited EC traits. To test the function of neonatal ECs in vivo, we created a femoral fracture in young and aged mice and implanted a collagen sponge to deliver neonatal BMECs at the fracture site. In the mouse fracture model, endochondral ossification was delayed in aged control mice compared to young controls. Neonatal BMECs significantly improved endochondral bone formation only in aged mice. These data suggest BMECs have potential to enhance aged bone healing. Compared to LECs, BMECs are more feasible for translational cell therapy and clinical applications in bone repair. Future studies are needed to examine the fate and function of BMECs implanted into the fracture sites.

Latest Advances in Endothelial Progenitor Cell-Derived Extracellular Vesicles Translation to the Clinic

Almost all nucleated cells secrete extracellular vesicles (EVs) that are heterogeneous spheroid patterned or round shape particles ranging from 30 to 200 nm in size. Recent preclinical and clinical studies have shown that endothelial progenitor cell-derived EVs (EPC-EVs) have a beneficial therapeutic effect in various diseases, including cardiovascular diseases and kidney, and lung disorders. Moreover, some animal studies have shown that EPC-EVs selectively accumulate at the injury site with a specific mechanism of binding along with angiogenic and restorative effects that are superior to those of their ancestors. This review article highlights current advances in the biogenesis, delivery route, and long-term storage methods of EPC-EVs and their favorable effects such as anti-inflammatory, angiogenic, and tissue protection in various diseases. Finally, we review the possibility of therapeutic application of EPC-EVs in the clinic.

Effects of porcine bone marrow-derived platelet-rich plasma on bone marrow-derived mesenchymal stem cells and endothelial progenitor cells

This study investigated the abundance of pro-regenerative growth factors in bone marrow-derived platelet-rich plasma (BM-PRP) and their effects on bone marrow-derived mesenchymal stem cells (BM-MSC) and bone marrow-derived endothelial progenitor cells (BM-EPC). Four 4-5 months-old domestic pigs were included, and each underwent bone marrow aspiration from its humerus bones and processed into bone marrow aspiration concentrate (BMAC) samples. The plasma and cellular portions of BMAC were subsequently separated and collected. The concentration of growth factors including BMP-2, PDGF-BB, TGF-β1 and VEGF in the plasma portion was measured and compared between BM-PRP and bone marrow-derived platelet-poor plasma (BM-PPP). It was found that platelet count was significantly higher in BM-PRP than in BM-PPP, but the concentration of above-mentioned growth factors was not significantly different between BM-PRP and BM-PPP. As most existing literature has indicated the regenerative potency of PRP, this study focused on assessing the effect of BM-PRP treatment on BM-MSC and BM-EPC proliferation, osteogenic differentiation and angiogenesis capacity by comparing samples with 2.5% BM-PRP treatment and samples without BM-PRP treatment (control). In response to BM-PRP treatment, the cellular doubling time increased with culturing time and was significantly shorter in the BM-PRP-treated samples than in control samples. For osteogenic differentiation, BM-PRP-treated BM-MSCs demonstrated a time-dependent increase in alkaline phosphatase (ALP) activity and expression levels of osteogenic differentiation markers. For the expression of angiogenic genes, none of the differences reached statistical significance despite a tendency of stronger expression at day 18 in BM-PRP-treated BM-EPCs. In conclusion, this in vitro study suggests that most BMP-2, PDGF-BB, TGF-β1 and VEGF-A contained in BM-PRP are not platelet-released and BM-PRP may have some stimulation (less than 1-fold) for MSC, EPC proliferation and MSC osteogenic differentiation.

Endothelial Progenitor Cell-Derived Extracellular Vesicles: Potential Therapeutic Application in Tissue Repair and Regeneration

Recently, many studies investigated the role of a specific type of stem cell named the endothelial progenitor cell (EPC) in tissue regeneration and repair. EPCs represent a heterogeneous population of mononuclear cells resident in the adult bone marrow. EPCs can migrate and differentiate in injured sites or act in a paracrine way. Among the EPCs’ secretome, extracellular vesicles (EVs) gained relevance due to their possible use for cell-free biological therapy. They are more biocompatible, less immunogenic, and present a lower oncological risk compared to cell-based options. EVs can efficiently pass the pulmonary filter and deliver to target tissues different molecules, such as micro-RNA, growth factors, cytokines, chemokines, and non-coding RNAs. Their effects are often analogous to their cellular counterparts, and EPC-derived EVs have been tested in vitro and on animal models to treat several medical conditions, including ischemic stroke, myocardial infarction, diabetes, and acute kidney injury. EPC-derived EVs have also been studied for bone, brain, and lung regeneration and as carriers for drug delivery. This review will discuss the pre-clinical evidence regarding EPC-derived EVs in the different disease models and regenerative settings. Moreover, we will discuss the translation of their use into clinical practice and the possible limitations of this process.

Vascularization Strategies in the Prevention of Nonunion Formation

Delayed healing and nonunion formation are major challenges in orthopedic surgery, which require the development of novel treatment strategies. Vascularization is considered one of the major prerequisites for successful bone healing, providing an adequate nutrient supply and allowing the infiltration of progenitor cells to the fracture site. Hence, during the last decade, a considerable number of studies have focused on the evaluation of vascularization strategies to prevent or to treat nonunion formation. These involve (1) biophysical applications, (2) systemic pharmacological interventions, and (3) tissue engineering, including sophisticated scaffold materials, local growth factor delivery systems, cell-based techniques, and surgical vascularization approaches. Accumulating evidence indicates that in nonunions, these strategies are indeed capable of improving the process of bone healing. The major challenge for the future will now be the translation of these strategies into clinical practice to make them accessible for the majority of patients. If this succeeds, these vascularization strategies may markedly reduce the incidence of nonunion formation. Impact statement Delayed healing and nonunion formation are a major clinical problem in orthopedic surgery. This review provides an overview of vascularization strategies for the prevention and treatment of nonunions. The successful translation of these strategies in clinical practice is of major importance to achieve adequate bone healing.

EPC-derived exosomes promote osteoclastogenesis through LncRNA-MALAT1

Bone repair involves bone resorption through osteoclastogenesis and the stimulation of neovascularization and osteogenesis by endothelial progenitor cells (EPCs). However, the role of EPCs in osteoclastogenesis is unclear. In this study, we assess the effects of EPC-derived exosomes on the migration and osteoclastic differentiation of primary mouse bone marrow-derived macrophages (BMMs) in vitro using immunofluorescence, western blotting, RT-PCR and Transwell assays. We also evaluated the effects of EPC-derived exosomes on the homing and osteoclastic differentiation of transplanted BMMs in a mouse bone fracture model in vivo. We found that EPCs cultured with BMMs secreted exosomes into the medium and, compared with EPCs, exosomes had a higher expression level of LncRNA-MALAT1. We confirmed that LncRNA-MALAT1 directly binds to miR-124 to negatively control miR-124 activity. Moreover, overexpression of miR-124 could reverse the migration and osteoclastic differentiation of BMMs induced by EPC-derived exosomes. A dual-luciferase reporter assay indicated that the integrin ITGB1 is the target of miR-124. Mice treated with EPC-derived exosome-BMM co-transplantations exhibited increased neovascularization at the fracture site and enhanced fracture healing compared with those treated with BMMs alone. Overall, our results suggest that EPC-derived exosomes can promote bone repair by enhancing recruitment and differentiation of osteoclast precursors through LncRNA-MALAT1.

Bone Regeneration in rat using a gelatin/bioactive glass nanocomposite scaffold along with endothelial cells (HUVECs)

In our previous study, a three‐dimensional gelatin/bioactive glass nanocomposite scaffold with a total porosity of about 85% and pore sizes ranging from 200 to 500 μm was prepared through layer solvent casting combined with lamination technique. The aim of this study was to evaluate in vitro biocompatibility and in vivo bone regeneration potential of these scaffolds with and without endothelial cells when implanted into a critical‐sized rat calvarial defect. MTT assay, SEM observation, and DAPI staining were used to evaluate cell viability and adhesion in macroporous scaffolds and results demonstrated that the scaffolds were biocompatible enough to support cell attachment and proliferation. To investigate the in vivo osteogenesis of the scaffold, blank scaffolds and endothelial/scaffold constructs were implanted in critical‐sized defects, whereas in control group defects were left untreated. Bone regeneration and vascularization were evaluated at 1, 4, and 12 weeks postsurgery by histological, immunohistochemical, and histomorphometric analysis. It was shown that both groups facilitated bone growth into the defect area but improved bone regeneration was seen with the incorporation of endothelial cells. The data showed that the porous Gel/BaG nanocomposite scaffolds could well support new bone formation, indicating that the proposed strategy is a promising alternative for tissue‐engineered bone defects.

Five Days Granulocyte Colony-Stimulating Factor Treatment Increases Bone Formation and Reduces Gap Size of a Rat Segmental Bone Defect: A Pilot Study

Bone is an organ with high natural regenerative capacity and most fractures heal spontaneously when appropriate fracture fixation is provided. However, additional treatment is required for patients with large segmental defects exceeding the endogenous healing potential and for patients suffering from fracture non-unions. These cases are often associated with insufficient vascularization. Transplantation of CD34+ endothelial progenitor cells (EPCs) has been successfully applied to promote neovascularization of bone defects, however including extensive ex vivo manipulation of cells. Here, we hypothesized, that treatment with granulocyte colony-stimulating factor (G-CSF) may improve bone healing by mobilization of CD34+ progenitor cells into the circulation, which in turn may facilitate vascularization at the defect site. In this pilot study, we aimed to characterize the different cell populations mobilized by G-CSF and investigate the influence of cell mobilization on the healing of a critical size femoral defect in rats. Cell mobilization was investigated by flow cytometry at different time points after five consecutive daily G-CSF injections. In a pilot study, bone healing of a 4.5-mm critical femoral defect in F344 rats was compared between a saline-treated control group and a G-CSF treatment group. In vivo microcomputed tomography and histology were applied to compare bone formation in both treatment groups. Our data revealed that leukocyte counts show a peak increase at the first day after the last G-CSF injection. In addition, we found that CD34+ progenitor cells, including EPCs, were significantly enriched at day 1, and further increased at day 5 and day 11. Upregulation of monocytes, granulocytes and macrophages peaked at day 1. G-CSF treatment significantly increased bone volume and bone density in the defect, which was confirmed by histology. Our data show that different cell populations are mobilized by G-CSF treatment in cell specific patterns. Although in this pilot study no bridging of the critical defect was observed, significantly improved bone formation by G-CSF treatment was clearly shown.