Angiogenic factors in bone local environment

The "love-hate" relationship between osteoclasts and bone matrix

Osteoclasts are unique cells that destroy the mineralized matrix of the skeleton. There is a "love-hate" relationship between the osteoclasts and the bone matrix, whereby the osteoclast is stimulated by the contact with the matrix but, at the same time, it disrupts the matrix, which, in turn, counteracts this disruption by some of its components. The balance between these concerted events brings about bone resorption to be controlled and to contribute to bone tissue integrity and skeletal health. The matrix components released by osteoclasts are also involved in the local regulation of other bone cells and in the systemic control of organismal homeostasis. Disruption of this regulatory loop causes bone diseases, which may end up with either reduced or increased bone mass, often associated with poor bone quality. Expanding the knowledge on osteoclast-to-matrix interaction could help to counteract these diseases and improve the human bone health. In this article, we will present evidence of the physical, molecular and regulatory relationships between the osteoclasts and the mineralized matrix, discussing the underlying mechanisms as well as their pathologic alterations and potential targeting.

Enhanced in vitro angiogenic behaviour of human umbilical vein endothelial cells on thermally oxidized TiO2 nanofibrous surfaces

One of the major challenges in bone grafting is the lack of sufficient bone vascularization. A rapid and stable bone vascularization at an early stage of implantation is essential for optimal functioning of the bone graft. To address this, the ability of in situ TiO2 nanofibrous surfaces fabricated via thermal oxidation method to enhance the angiogenic potential of human umbilical vein endothelial cells (HUVECs) was investigated. The cellular responses of HUVECs on TiO2 nanofibrous surfaces were studied through cell adhesion, cell proliferation, capillary-like tube formation, growth factors secretion (VEGF and BFGF), and angiogenic-endogenic-associated gene (VEGF, VEGFR2, BFGF, PGF, HGF, Ang-1, VWF, PECAM-1 and ENOS) expression analysis after 2 weeks of cell seeding. Our results show that TiO2 nanofibrous surfaces significantly enhanced adhesion, proliferation, formation of capillary-like tube networks and growth factors secretion of HUVECs, as well as leading to higher expression level of all angiogenic-endogenic-associated genes, in comparison to unmodified control surfaces. These beneficial effects suggest the potential use of such surface nanostructures to be utilized as an advantageous interface for bone grafts as they can promote angiogenesis, which improves bone vascularization.

Environmental manipulation to promote stem cell survival in vivo: use of aggregation, oxygen carrier, and BMP-2 co-delivery strategies

While mesenchymal stem cell (MSC)-based strategies for critically-sized bone defect repair hold promise, poor cell survival in vivo remains a significant barrier to the translation of these therapeutics.

Smad signal pathway regulates angiogenesis via endothelial cell in an adipose-derived stromal cell/endothelial cell co-culture, 3D gel model

Co-implantation of adipose-derived stromal cells (ASCs) and endothelial cells (ECs) can markedly expedite the formation of functional microvascular beds and provides possible methods for cell-based revascularization therapies to treat various diseases. Furthermore, we investigated the role of TGFβ/Smad signaling pathway for angiogenesis in a three-dimensional (3D) collagen gel model established in vitro with co-culture between ASCs and ECs. We found that angiogenesis was attenuated in the co-culture gels after inhibition of ALK5/Smad2/3 with SB431542. Genes coding for VEGF-A, VEGF-B, VE-ca, FGF-1, PDGF, BMP-4, and BMP-7 were significantly reduced in both mono-cultured and co-cultured ECs. Furthermore, the decrease in co-cultured ECs was prominent relative to mono-cultured ECs. Taken together, these findings suggest that in the co-culture between ASCs and ECs, TGFβ/Smad signal pathway regulates angiogenesis via ECs; moreover, the findings that the co-cultured ECs were regulated more significantly than mono-cultured ECs suggest that suppression of Smad signal pathway may regulate the paracrine secretion of ASCs to further modulate angiogenesis of ECs.

Effects on Bone Repair of Osteotomy With Drills or With Erbium, Chromium: Yttrium-Scandium-Gallium-Garnet Laser: Histomorphometric and Immunohistochemical Study

BACKGROUND

The erbium, chromium:yttrium-scandium-gallium-garnet (Er,Cr:YSGG) laser has been widely used in the dental clinic; however, few studies have demonstrated the advantages of the use of this laser for bone osteotomies. The purpose of this study is to evaluate and compare the bone repair process of defects generated by the Er,Cr:YSGG laser or conventional drills.

METHODS

Ninety-six rats were divided into two groups of 48 animals (drill group and laser group). After surgical exposure of the right tibia, the animals were subjected to a 2-mm-diameter osteotomy created by conventional drills (drill group) or by the Er,Cr:YSGG laser (laser group). The animals were sacrificed 0, 3, 7, 14, 30, and 60 days after the creation of the defect, and histologic sections were obtained and used for histomorphometric and immunohistochemical analyses for the detection of osteocalcin, osteoprotegerin, receptor activator of nuclear factor κ-B ligand, vascular endothelial growth factor, and caspase-3.

RESULTS

The osteotomy with the drill produced well-delimited and smooth walls, whereas the osteotomies in the laser group were irregular and presented an amorphous basophilic line and bone necrosis that was slowly resorbed during the repair process. Despite these characteristics, bone repair was similar between groups at various time points, and, at 60 days, the defects in both groups were completely repaired by newly formed bone.

CONCLUSION

The repair process of osteotomies created by the Er,Cr:YSGG laser, despite producing thermal damage to bone tissue, is comparable to that with conventional drills.

Bone morphogenetic protein signaling in bone homeostasis

Bone morphogenetic proteins (BMPs) are cytokines belonging to the transforming growth factor-β (TGF-β) superfamily. They play multiple functions during development and tissue homeostasis, including regulation of the bone homeostasis. The BMP signaling pathway consists in a well-orchestrated manner of ligands, membrane receptors, co-receptors and intracellular mediators, that regulate the expression of genes controlling the normal functioning of the bone tissues. Interestingly, BMP signaling perturbation is associated to a variety of low and high bone mass diseases, including osteoporosis, bone fracture disorders and heterotopic ossification. Consistent with these findings, in vitro and in vivo studies have shown that BMPs have potent effects on the activity of cells regulating bone function, suggesting that manipulation of the BMP signaling pathway may be employed as a therapeutic approach to treat bone diseases. Here we review the recent advances on BMP signaling and bone homeostasis, and how this knowledge may be used towards improved diagnosis and development of novel treatment modalities. This article is part of a Special Issue entitled "Muscle Bone Interactions".

The Angiopoietin-1 Variant COMP-Ang1 Enhances BMP2-Induced Bone Regeneration with Recruiting Pericytes in Critical Sized Calvarial Defects

Craniofacial bone defects are observed in a variety of clinical situations, and their reconstructions require coordinated coupling between angiogenesis and osteogenesis. In this study, we explored the effects of cartilage oligomeric matrix protein-angiopoietin 1 (COMP-Ang1), a synthetic and soluble variant of angiopoietin 1, on bone morphogenetic protein 2 (BMP2)-induced cranial bone regeneration, and recruitment and osteogenic differentiation of perivascular pericytes. A critical-size calvarial defect was created in the C57BL/6 mouse and COMP-Ang1 and/or BMP2 proteins were delivered into the defects with absorbable collagen sponges. After 3 weeks, bone regeneration was evaluated using micro-computed tomography and histologic examination. Pericyte recruitment into the defects was examined using immunofluorescence staining with anti-NG2 and anti-CD31 antibodies. In vitro recruitment and osteoblastic differentiation of pericyte cells were assessed with Boyden chamber assay, staining of calcified nodules, RT-PCR and Western blot analyses. Combined administration of COMP-Ang1 and BMP2 synergistically enhanced bone repair along with the increased population of CD31 (an endothelial cell marker) and NG2 (a specific marker of pericyte) positive cells. In vitro cultures of pericytes consistently showed that pericyte infiltration into the membrane pore of Boyden chamber was more enhanced by the combination treatment. In addition, the combination further increased the osteoblast-specific gene expression, including bone sialoprotein (BSP), osteocalcin (OCN) and osterix (OSX), phosphorylation of Smad/1/5/8, and mineralized nodule formation. COMP-Ang1 can enhance BMP2-induced cranial bone regeneration with increased pericyte recruitment. Combined delivery of the proteins might be a therapeutic strategy to repair cranial bone damage.

The effects of nerve growth factor on endothelial cells seeded on different titanium surfaces

Angiogenesis is critical for peri-implant bone regeneration and osseointegration. Endothelial cells (ECs) play an important role in angiogenesis during the early stage of bone formation. Nerve growth factor (NGF) is also reported to function as an angiogenic growth factor. The effects of NGF on ECs seeded on titanium surfaces are unclear. This study was done to investigate the influence of NGF on peri-implant angiogenesis in vitro and in vivo. We used two different titanium surfaces. ECs seeded on these surfaces were treated with indicated concentrations of NGF or vascular endothelial growth factor (VEGF). Proliferation, differentiation, morphological features, and amounts attached were assessed. Chicken embryo chorioallantoic membrane (CAM) was adopted to evaluate the effect of NGF in vivo. The results showed that NGF could promote EC proliferation on both titanium surfaces (F1d=2.083, P=0.156; F3d=30.857, P=0.0002; F5d=4.440, P=0.041; F7d=11.065, P=0.001). NGF and the SLA surface upregulated mRNA of NGF, TrkA, and p75 expression (FNGF=11.941, P=0.003; FTrkA=28.514, P=0.004; Fp75=7.725, P=0.01). In vivo, the supernatants of the NGF-treated group could promote neovascularization in CAM (F=17.662, P=0.009). This study demonstrated that NGF could enhance EC proliferation, gene expression on different titanium surfaces, and neovascularization in CAM. This provides novel information in relation to the promotion of early dental implant osseointegration.

Micro-CT Analysis of Radiation-Induced Osteopenia and Bone Hypovascularization in Rat

Treatment of carcinomas of the upper aerodigestive tract often requires external radiation therapy. However, radiation affects all the components of bone, with different degrees of sensitivity, and may produce severe side effects such as mandibular osteoradionecrosis (ORN). Intraosseous vascularization is thought to be decreased after irradiation, but its impact on total bone volume is still controversial. The aim of this study was to compare intraosseous vascularization, cortical bone thickness, and total bone volume in a rat model of ORN versus nonirradiated rats, using a micro-computed tomography (micro-CT) analysis after intracardiac injection of a contrast agent. The study was performed on 8-week-old Lewis 1A rats (n = 14). Eleven rats underwent external irradiation on the hind limbs by a single 80-Gy dose. Three rats did not receive irradiation and served as controls for statistical analysis. Eight weeks after the external irradiation, all the animals received a barium sulfate intracardiac injection under general anesthesia. All samples were analyzed with the micro-computed tomography system at a resolution of 5.5 μm. The images were later processed to create 3D reconstructions and study vascularization, bone volume, and cortical thickness. Data from irradiated and nonirradiated rats were compared using the Kruskal-Wallis test. No animal died after irradiation. Nineteen irradiated tibias and six nonirradiated tibias were included for micro-CT analysis. The vessel percentage was significantly lower in irradiated bones (p = 0.0001). The distance between the vessels, a marker of vascular destruction, was higher after irradiation (p = 0.001). The vessels were also more altered distally after irradiation (p = 0.028). Cortical thickness was severely decreased after irradiation, sometimes even reduced to zero. Both trabecular and cortical structures were destroyed after irradiation, with wide bone gaps. Finally, both total bone volume (p = 0.0001) and cortical thickness (p = 0.0001) were significantly decreased in irradiated tibias compared to nonirradiated tibias. These results led to multiple spontaneous fractures in the irradiated group, and the destruction of intraosseous vessels observed macroscopically with the radiographic preview. This study revealed the impact of radiation on intraosseous vasculature and cortical bone with a micro-CT analysis in a rat ORN model. Hypovascularization and osteopenia are consistent with the literature, contributing a morphological scale with high resolution. Visualization of the vasculature by micro-CT is an innovative technique to see the changes after radiation, and should help adjust bone tissue engineering in irradiated bone.

Fabrication of Biomimetic Bone Tissue Using Mesenchymal Stem Cell-Derived Three-Dimensional Constructs Incorporating Endothelial Cells

The development of technologies to promote vascularization of engineered tissue would drive major developments in tissue engineering and regenerative medicine. Recently, we succeeded in fabricating three-dimensional (3D) cell constructs composed of mesenchymal stem cells (MSCs). However, the majority of cells within the constructs underwent necrosis due to a lack of nutrients and oxygen. We hypothesized that incorporation of vascular endothelial cells would improve the cell survival rate and aid in the fabrication of biomimetic bone tissues in vitro. The purpose of this study was to assess the impact of endothelial cells combined with the MSC constructs (MSC/HUVEC constructs) during short- and long-term culture. When human umbilical vein endothelial cells (HUVECs) were incorporated into the cell constructs, cell viability and growth factor production were increased after 7 days. Furthermore, HUVECs were observed to proliferate and self-organize into reticulate porous structures by interacting with the MSCs. After long-term culture, MSC/HUVEC constructs formed abundant mineralized matrices compared with those composed of MSCs alone. Transmission electron microscopy and qualitative analysis revealed that the mineralized matrices comprised porous cancellous bone-like tissues. These results demonstrate that highly biomimetic bone tissue can be fabricated in vitro by 3D MSC constructs incorporated with HUVECs.

Gene profile of soluble growth factors involved in angiogenesis, in an adipose-derived stromal cell/endothelial cell co-culture, 3D gel model

OBJECTIVES

The aim of this study was to investigate gene expressions of growth factors for angiogenesis, in a three-dimensional (3D) gel populated with adipose-derived stromal cells (ASCs) and endothelial cells (ECs) in co-culture.

MATERIALS AND METHODS

The 3D gel, mixed with green fluorescent protein (GFP)-positive ASCs and DsRed-Express-positive ECs, 1:1 ratio, was established in vitro. The phenomenon of angiogenesis was observed using confocal microscopy. To detect gene expressions for growth factor proteins in both ASCs and ECs, transwell co-culture was used, and cell lysate samples were collected at 1, 3, 5 and 7 days. Semi-quantitative polymerase chain reaction (PCR) was conducted to quantify mRNA expressions of the growth factors.

RESULTS

Angiogenesis was first observed in the gels by 7 days post-implantation. Over this time in ECs, genes coding for VEGFA/B, IGF-1, HIF-1α, FGF-1/-2 and BMP-5/-7 significantly increased. Meanwhile in ASCs, genes coding for VEGFA/B, IGF-1, HIF-1α, FGF-1/-2 and BMP-6 also were significantly enhanced. In particular, increased amounts of IGF-1 and HIF-1α in both ECs and ASCs were prominent relative to other factors.

CONCLUSIONS

Contact co-culture with ASCs and ECs at 1:1 ratio, in the 3D gel promoted angiogenesis; non-contact co-culture further confirmed gene expressions for growth factors, VEGFA/B, IGF-1, HIF-1α and FGF-1/-2 in both ASCs and ECs; BMP-5/-7 in ECs and BMP-6 in ASCs were also confirmed. This establishment of growth factor expression seemed to be responsible for enhancement of angiogenesis. This indicates that these factors could be utilized as targets for engineered angiogenesis.

Assessment of bone vascularization and its role in bone remodeling

Bone is a composite organ that fulfils several interconnected functions, which may conflict with each other in pathological conditions. Bone vascularization is at the interface between these functions. The roles of bone vascularization are better documented in bone development, growth and modeling than in bone remodeling. However, every bone remodeling unit is associated with a capillary in both cortical and trabecular envelopes. Here we summarize the most recent data on vessel involvement in bone remodeling, and we present the characteristics of bone vascularization. Finally, we describe the various techniques used for bone vessel imaging and quantitative assessment, including histology, immunohistochemistry, microtomography and intravital microscopy. Studying the role of vascularization in adult bone should provide benefits for the understanding and treatment of metabolic bone diseases.

Differential expression of vascular endothelial growth factor in human fetal skeletal site-specific tissues: Mandible versus femur

Vascular endothelial growth factor (VEGF) is a well-known mediator that signals through pathways in angiogenesis and osteogenesis. Angiogenesis and bone formation are coupled during either skeletal development or bone remodeling and repair occurring in postnatal life. In this study, we examined for the first time the expression of VEGF in human fetal mandibular and femoral bone in comparison with the respective adult tissues. Similarly to other craniofacial bones, but at variance with the axial and appendicular skeleton, during development mandible does not arise from mesoderm but neural crest cells of the neuroectoderm germ layer, and undergoes intramembranous instead of endochondral ossification. By quantitative real-time PCR technique, we could show that VEGF gene expression levels were significantly higher in fetal than in adult samples, especially in femoral tissue. Western blotting analysis confirmed higher protein expression of VEGF in the fetal femur respect to the mandible. Moreover, immunohistochemistry revealed that in both fetal tissues VEGF expression was mainly localized in pre- and osteoblasts. Differential expression of VEGF in femoral and mandibular bone tissues could be related to their different structure, function and development during organogenesis.

Resveratrol suppresses TGF-β-induced VEGF synthesis in osteoblasts: Inhibition of the p44/p42 MAPKs and SAPK/JNK pathways

Resveratrol, which is found in grape and berry skins and red wine, is generally known to be beneficial for human health due to its anti-inflammation and antioxidant effects. We have recently reported that transforming growth factor-β (TGF-β) stimulates vascular endothelial growth factor (VEGF) synthesis through Smad-independent pathways, such as the p38 mitogen-activated protein (MAP) kinase, p44/p42 MAP kinase and stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) pathways, in osteoblast-like MC3T3-E1 cells. The aim of the present study was to investigate the effect of resveratrol on the TGF-β-induced VEGF synthesis and the mechanism in osteoblast-like MC3T3-E1 cells. Resveratrol significantly suppressed the TGF-β-stimulated release of VEGF and the VEGF mRNA expression levels. SRT1720, a synthetic sirtuin 1 (SIRT1) activator, also reduced the VEGF release and the mRNA levels. With regard to the intracellular signaling in the TGF-β-stimulated VEGF synthesis, resveratrol and SRT1720 significantly attenuated the phosphorylation of p44/p42 MAP kinase and SAPK/JNK stimulated by TGF-β; however, the TGF-β-induced phosphorylation of Smad2 and p38 MAP kinase was hardly affected by resveratrol or SRT1720. These results strongly suggest that the TGF-β-stimulated VEGF synthesis is suppressed by resveratrol through the inhibition of p44/p42 MAP kinase and SAPK/JNK in osteoblasts, and that the suppressive effect is mediated, at least in part, via SIRT1 activation.

EGFL7 is expressed in bone microenvironment and promotes angiogenesis via ERK, STAT3, and integrin signaling cascades

Angiogenesis plays a pivotal role in bone formation, remodeling, and fracture healing. The regulation of angiogenesis in the bone microenvironment is highly complex and orchestrated by intercellular communication between bone cells and endothelial cells. Here, we report that EGF-like domain 7 (EGFL7), a member of the epidermal growth factor (EGF) repeat protein superfamily is expressed in both the osteoclast and osteoblast lineages, and promotes endothelial cell activities. Addition of exogenous recombinant EGFL7 potentiates SVEC (simian virus 40-transformed mouse microvascular endothelial cell line) cell migration and tube-like structure formation in vitro. Moreover, recombinant EGFL7 promotes angiogenesis featuring web-like structures in ex vivo fetal mouse metatarsal angiogenesis assay. We show that recombinant EGFL7 induces phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2), signal transducer and activator of transcription 3 (STAT3), and focal adhesion kinase (FAK) in SVEC cells. Inhibition of ERK1/2 and STAT3 signaling impairs EGFL7-induced endothelial cell migration, and angiogenesis in fetal mouse metatarsal explants. Bioinformatic analyses indicate that EGFL7 contains a conserved RGD/QGD motif and EGFL7-induced endothelial cell migration is significantly reduced in the presence of RGD peptides. Moreover, EGFL7 gene expression is significantly upregulated during growth plate injury repair. Together, these results demonstrate that EGFL7 expressed by bone cells regulates endothelial cell activities through integrin-mediated signaling. This study highlights the important role that EGFL7, like EGFL6, expressed in bone microenvironment plays in the regulation of angiogenesis in bone.

Osteocyte control of bone remodeling: is sclerostin a key molecular coordinator of the balanced bone resorption-formation cycles?

Osteocytes, entrapped within a newly mineralized bone matrix, possess a unique cellular identity due to a specialized morphology and a molecular signature. These features endow them to serve as a bone response mechanism for mechanical stress in their microenvironment. Sclerostin, a primarily osteocyte product, is widely considered as a mechanotranduction key molecule whose expression is suppressed by mechanical loading, or it is induced by unloading. This review presents a model suggesting that sclerostin is major mediator for integrating mechanical, local, and hormonal signals, sensed by the osteocytes, in controlling the remodeling apparatus. This central role is achieved through interplay between two opposing mechanisms: (1) unloading-induced high sclerostin levels, which antagonize Wnt-canonical-β-catenin signaling in osteocytes and osteoblasts, permitting simultaneously Wnt-noncanonical and/or other pathways in osteocytes and osteoclasts, directed at bone resorption; (2) mechanical loading results in low sclerostin levels, activation of Wnt-canonical signaling, and bone formation. Therefore, adaptive bone remodeling occurring at a distinct bone compartment is orchestrated by altered sclerostin levels, which regulate the expression of the other osteocyte-specific proteins, such as RANKL, OPG, and proteins encoded by "mineralization-related genes" (DMP1, PHEX, and probably FGF23). For example, under specific terms, sclerostin regulates differential RANKL and OPG production, and creates a dynamic RANKL/OPG ratio, leading either to bone formation or resorption. It also controls the expression of PHEX, DMP1, and most likely FGF23, leading to either bone matrix mineralization or its inhibition. Such opposing up- or down-regulation of remodeling phases allows osteocytes to function as an "external unit", ensuring transition from bone resorption to bone formation.Mini Abstract: The osteocyte network plays a central role in directing bone response either to mechanical loading, or to unloading, leading correspondingly to bone formation or resorption. This review shows a key role of the osteocyte-produced sclerostin as a major mediator of the molecular mechanisms involved in the process of adaptive bone remodeling.

PDGF-BB secreted by preosteoclasts induces angiogenesis during coupling with osteogenesis

Osteogenesis during bone modeling and remodeling is coupled with angiogenesis. A recent study shows that the specific vessel subtype, strongly positive for CD31 and Endomucin (CD31^hiEmcn^hi), couples angiogenesis and osteogenesis. We found that preosteoclasts secrete platelet derived growth factor-BB (PDGF-BB), inducing CD31^hiEmcn^hi vessels during bone modeling and remodeling. Mice with depletion of PDGF-BB in tartrate-resistant acid phosphatase positive (TRAP⁺) cell lineage (Pdgfb^–/–) show significantly lower trabecular and cortical bone mass, serum and bone marrow PDGF-BB concentrations, and CD31^hiEmcn^hi vessels compared to wild-type mice. In the ovariectomized (OVX) osteoporotic mouse model, concentrations of serum and bone marrow PDGF-BB and CD31^hiEmcn^hi vessels are significantly decreased. Inhibition of cathepsin K (CTSK) increases preosteoclast numbers, resulting in higher levels of PDGF-BB to stimulate CD31^hiEmcn^hi vessels and bone formation in OVX mice. Thus, pharmacotherapies that increase PDGF-BB secretion from preosteoclasts offer a novel therapeutic target for osteoporosis to promote angiogenesis for bone formation.

Pro-angiogenic effects of Carthami Flos whole extract in human microvascular endothelial cells in vitro and in zebrafish in vivo

AIM

Carthami Flos (CF) is a Chinese herb traditionally used for cardiovascular disease and bone injury in China with pharmacological effects on improving blood circulation. The aim of this study was to investigate the angiogenic potential of CF whole extract (extracted by boiling with water, followed by ethanol) and the underlying mechanisms in human microvascular endothelial cells (HMEC-1) in vitro and in transgenic TG(fli1:EGFP)(y1)/+(AB) zebrafish with transgenic endothelial cells expressing EGFP (Enhanced Green Fluorescent Protein) in vivo.

METHODS

Effects of CF whole extract on cell proliferation, migration and tube formation in HMEC-1 cells in vitro were detected by MTT assay, wound healing assay and tube formation assay. Its angiogenic effect in zebrafish was investigated by monitoring the sprout number in the sub-intestinal vessel (SIV), and the underlying mechanisms were tested by quantitative real-time PCR.

RESULTS

CF whole extract increased cell proliferation, migration and tube formation in vitro in HMEC-1 cells. Its angiogenic effect was also confirmed in vivo in zebrafish by increasing the sprout number in the SIV. As determined by quantitative real-time PCR, CF whole extract up-regulated the expression of angiogenesis-related genes in zebrafish, including angiogenic and its associated growth factors and receptors (e.g. IGF1, CTGF, NRP2, and VEGFR3), transcription factor (e.g. HIF1A), matrix degradation and endothelial cell migration-related factors (e.g. MMP2, MMP9, TIMP2, PLG and PLAU), cell adhesion molecules (e.g. ITGAV, ITGB3, beta-catenin and PECAM1), tubule formation factors (e.g. ANGPT1, TIE-2, PDGFR-B, CDH5, S1PR1, FGF2, Shh, and TGFRB1), and blood vessel maturation/formation factor (e.g. Ephrin B2).

CONCLUSIONS

CF whole extract increased angiogenesis in HMEC-1 cells in vitro and in zebrafish in vivo with multiple mechanisms.

Conditioned media from microvascular endothelial cells cultured in simulated microgravity inhibit osteoblast activity

Background and Aims. Gravity contributes to the maintenance of bone integrity. Accordingly, weightlessness conditions during space flight accelerate bone loss and experimental models in real and simulated microgravity show decreased osteoblastic and increased osteoclastic activities. It is well known that the endothelium and bone cells cross-talk and this intercellular communication is vital to regulate bone homeostasis. Because microgravity promotes microvascular endothelial dysfunction, we anticipated that the molecular cross-talk between endothelial cells exposed to simulated microgravity and osteoblasts might be altered. Results. We cultured human microvascular endothelial cells in simulated microgravity using the rotating wall vessel device developed by NASA. Endothelial cells in microgravity show growth inhibition and release higher amounts of matrix metalloproteases type 2 and interleukin-6 than controls. Conditioned media collected from microvascular endothelial cells in simulated microgravity were used to culture human osteoblasts and were shown to retard osteoblast proliferation and inhibit their activity. Discussion. Microvascular endothelial cells in microgravity are growth retarded and release high amounts of matrix metalloproteases type 2 and interleukin-6, which might play a role in retarding the growth of osteoblasts and impairing their osteogenic activity. Conclusions. We demonstrate that since simulated microgravity modulates microvascular endothelial cell function, it indirectly impairs osteoblastic function.

Recent research on the growth plate: Mechanisms for growth plate injury repair and potential cell-based therapies for regeneration

Injuries to the growth plate cartilage often lead to bony repair, resulting in bone growth defects such as limb length discrepancy and angulation deformity in children. Currently utilised corrective surgeries are highly invasive and limited in their effectiveness, and there are no known biological therapies to induce cartilage regeneration and prevent the undesirable bony repair. In the last 2 decades, studies have investigated the cellular and molecular events that lead to bony repair at the injured growth plate including the identification of the four phases of injury repair responses (inflammatory, fibrogenic, osteogenic and remodelling), the important role of inflammatory cytokine tumour necrosis factor alpha in regulating downstream repair responses, the role of chemotactic and mitogenic platelet-derived growth factor in the fibrogenic response, the involvement and roles of bone morphogenic protein and Wnt/B-catenin signalling pathways, as well as vascular endothelial growth factor-based angiogenesis during the osteogenic response. These new findings could potentially lead to identification of new targets for developing a future biological therapy. In addition, recent advances in cartilage tissue engineering highlight the promising potential for utilising multipotent mesenchymal stem cells (MSCs) for inducing regeneration of injured growth plate cartilage. This review aims to summarise current understanding of the mechanisms for growth plate injury repair and discuss some progress, potential and challenges of MSC-based therapies to induce growth plate cartilage regeneration in combination with chemotactic and chondrogenic growth factors and supporting scaffolds.

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.

Parathyroid hormone 1-84 targets bone vascular structure and perfusion in mice: impacts of its administration regimen and of ovariectomy

Bone vessel functions during bone remodeling are poorly understood. They depend on both vessel network structure and vasomotor regulation. Parathyroid hormone (PTH) is a systemic vasodilator that may modulate microvascularization. Moreover, although intermittent PTH is anti-osteoporotic, continuous PTH administration can be catabolic for bone. Finally, ovariectomy (OVX) reduces bone perfusion and vessel density in mice. We reasoned that the effects of PTH on bone vascularization might depend on its administration regimen and be impacted by ovariectomy. A 100-µg/kg PTH 1-84 daily dose was administered for 15 days to 4-month-old female C57BL/6 mice, either as daily sc injection (iPTH) or continuously (cPTH; ALZET minipump). Blood pressure (BP) and tibia bone perfusion were measured in vivo with a laser Doppler device. Histomorphometry of bone and barium-contrasted vascular network were performed on the same tibia. Compared with untreated controls, both iPTH and cPTH increased bone formation but had opposite effects on resorption. Both iPTH and cPTH were slightly angiogenic. Intermittent PTH increased microvessel size (+48%, p < 0.001), whereas cPTH decreased it (-29%, p = 0.009). iPTH increased bone perfusion (27%, p < 0.001) with no change in BP, whereas cPTH did not. The vascular effects of a 15-day iPTH treatment were analyzed in OVX mice and compared with sham-operated and OVX untreated controls. Two other anti-osteoporotic drugs, zoledronate (one injection, 70 µg/kg) and propranolol, (5 mg/kg/d) were tested in OVX mice. Although no change in bone mass was observed, iPTH stimulated bone formation and prevented the OVX-induced reduction in bone perfusion and vessel density. Both zoledronate and propranolol strongly lowered bone turnover, but surprisingly, zoledronate prevented OVX-induced reduction in bone perfusion but propranolol did not. Our integrative approach thus demonstrates that the effects of PTH on bone vessel structure and function depend on its mode of administration as well as on the HPG-axis hormonal status, and that OVX-induced vascular changes are prevented by iPTH.

Netrin-4 derived from murine vascular endothelial cells inhibits osteoclast differentiation in vitro and prevents bone loss in vivo

Bone is a highly vascularized organ, thus angiogenesis is a vital process during bone remodeling. However, the role of vascular systems in bone remodeling is not well recognized. Here we show that netrin-4 inhibits osteoclast differentiation in vitro and in vivo. Co-cultures of bone marrow macrophages with vascular endothelial cells markedly inhibited osteoclast differentiation. Adding a neutralizing antibody, or RNA interference against netrin-4, restored in vitro osteoclast differentiation. Administration of netrin-4 prevented bone loss in an osteoporosis mouse model by decreasing the osteoclast number. We propose that vascular endothelial cells interact with bone in suppressing bone through netrin-4.

Bone marrow mesenchymal stem cells and TGF-β signaling in bone remodeling

During bone resorption, abundant factors previously buried in the bone matrix are released into the bone marrow microenvironment, which results in recruitment and differentiation of bone marrow mesenchymal stem cells (MSCs) for subsequent bone formation, temporally and spatially coupling bone remodeling. Parathyroid hormone (PTH) orchestrates the signaling of many pathways that direct MSC fate. The spatiotemporal release and activation of matrix TGF-β during osteoclast bone resorption recruits MSCs to bone-resorptive sites. Dysregulation of TGF-β alters MSC fate, uncoupling bone remodeling and causing skeletal disorders. Modulation of TGF-β or PTH signaling may reestablish coupled bone remodeling and be a potential therapy.

The Key Role of the Blood Supply to Bone

The importance of the vascular supply for bone is well-known to orthopaedists but is still rather overlooked within the wider field of skeletal research. Blood supplies oxygen, nutrients and regulatory factors to tissues, as well as removing metabolic waste products such as carbon dioxide and acid. Bone receives up to about 10% of cardiac output, and this blood supply permits a much higher degree of cellularity, remodelling and repair than is possible in cartilage, which is avascular. The blood supply to bone is delivered to the endosteal cavity by nutrient arteries, then flows through marrow sinusoids before exiting via numerous small vessels that ramify through the cortex. The marrow cavity affords a range of vascular niches that are thought to regulate the growth and differentiation of hematopoietic and stromal cells, in part via gradients of oxygen tension. The quality of vascular supply to bone tends to decline with age and may be compromised in common pathological settings, including diabetes, anaemias, chronic airway diseases and immobility, as well as by tumours. Reductions in vascular supply are associated with bone loss. This may be due in part to the direct effects of hypoxia, which blocks osteoblast function and bone formation but causes reciprocal increases in osteoclastogenesis and bone resorption. Common regulatory factors such as parathyroid hormone or nitrates, both of which are potent vasodilators, might exert their osteogenic effects on bone via the vasculature. These observations suggest that the bone vasculature will be a fruitful area for future research.

Dynamics of bone healing after osteotomy with piezosurgery or conventional drilling - histomorphometrical, immunohistochemical, and molecular analysis

BACKGROUND

Piezosurgery is an osteotomy system used in medical and dental surgery. Many studies have proven clinical advantages of piezosurgery in terms of quality of cut, maneuverability, ease of use, and safety. However, few investigations have tested its superiority over the traditional osteotomy systems in terms of dynamics of bone healing. Therefore, the aim of this study was to evaluate the dynamics of bone healing after osteotomies with piezosurgery and to compare them with those associated to traditional bone drilling.

METHODS

One hundred and ten rats were divided into two groups with 55 animals each. The animals were anesthetized and the tibiae were surgically exposed to create defects 2 mm in diameter by using piezosurgery (Piezo group) and conventional drilling (Drill group). Animals were sacrificed at 3, 7, 14, 30 and 60 days post-surgery. Bone samples were collected and processed for histological, histomorphometrical, immunohistochemical, and molecular analysis. The histological analysis was performed at all time points (n = 8) whereas the histomorphometrical analysis was performed at 7, 14, 30 and 60 days post-surgery (n = 8). The immunolabeling was performed to detect Vascular Endothelial Growth Factor (VEGF), Caspase-3 (CAS-3), Osteoprotegerin (OPG), Receptor Activator of Nuclear Factor kappa-B Ligand (RANKL), and Osteocalcin (OC) at 3, 7, and 14 days (n = 3). For the molecular analysis, animals were sacrificed at 3, 7 and 14 days, total RNA was collected, and quantification of the expression of 21 genes related to BMP signaling, Wnt signaling, inflammation, osteogenenic and apoptotic pathways was performed by qRT-PCR (n = 5).

RESULTS

Histologically and histomorphometrically, bone healing was similar in both groups with the exception of a slightly higher amount of newly formed bone observed at 30 days after piezosurgery (p < 0.05). Immunohistochemical and qRT-PCR analyses didn't detect significant differences in expression of all the proteins and most of the genes tested.

CONCLUSIONS

Based on the results of our study we conclude that in a rat tibial bone defect model the bone healing dynamics after piezosurgery are comparable to those observed with conventional drilling.