Co-culture with endothelial progenitor cells promotes survival, migration, and differentiation of osteoclast precursors

Engineering approaches to manipulate osteoclast behavior for bone regeneration

Extensive research has delved into the multifaceted roles of osteoclasts beyond their traditional function in bone resorption in recent years, uncovering their significant influence on bone formation. This shift in understanding has spurred investigations into engineering strategies aimed at leveraging osteoclasts to not only inhibit bone resorption but also facilitate bone regeneration. This review seeks to comprehensively examine the mechanisms by which osteoclasts impact bone metabolism. Additionally, it explores various engineering methodologies, including the modification of bioactive material properties, localized drug delivery, and the introduction of exogenous cells, assessing their potential and mechanisms in aiding bone repair by targeting osteoclasts. Finally, the review proposes current limitations and future routes for manipulating osteoclasts through biological and material cues to facilitate bone repair.

Promoting Angiogenesis Using Immune Cells for Tissue-Engineered Vascular Grafts

Implantable tissue-engineered vascular grafts (TEVGs) usually trigger the host reaction which is inextricably linked with the immune system, including blood-material interaction, protein absorption, inflammation, foreign body reaction, and so on. With remarkable progress, the immune response is no longer considered to be entirely harmful to TEVGs, but its therapeutic and impaired effects on angiogenesis and tissue regeneration are parallel. Although the implicated immune mechanisms remain elusive, it is certainly worthwhile to gain detailed knowledge about the function of the individual immune components during angiogenesis and vascular remodeling. This review provides a general overview of immune cells with an emphasis on macrophages in light of the current literature. To the extent possible, we summarize state-of-the-art approaches to immune cell regulation of the vasculature and suggest that future studies are needed to better define the timing of the activity of each cell subpopulation and to further reveal key regulatory switches.

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.

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.

Endothelial progenitor cells promote osteogenic differentiation in co-cultured with mesenchymal stem cells via the MAPK-dependent pathway

BACKGROUND

The role of bone tissue engineering is to regenerate tissue using biomaterials and stem cell-based approaches. Combination of two or more cell types is one of the strategies to promote bone formation. Endothelial progenitor cells (EPCs) may enhance the osteogenic properties of mesenchymal stem cells (MSCs) and promote bone healing; this study aimed to investigate the possible mechanisms of EPCs on promoting osteogenic differentiation of MSCs.

METHODS

MSCs and EPCs were isolated and co-cultured in Transwell chambers, the effects of EPCs on the regulation of MSC biological properties were investigated. Real-time PCR array, and western blotting were performed to explore possible signaling pathways involved in osteogenesis. The expression of osteogenesis markers and calcium nodule formation was quantified by qRT-PCR, western blotting, and Alizarin Red staining.

RESULTS

Results showed that MSCs exhibited greater alkaline phosphatase (ALP) activity and increased calcium mineral deposition significantly when co-cultured with EPCs. The mitogen-activated protein kinase (MAPK) signaling pathway was involved in this process. p38 gene expression and p38 protein phosphorylation levels showed significant upregulation in co-cultured MSCs. Silencing expression of p38 in co-cultured MSCs reduced osteogenic gene expression, protein synthesis, ALP activity, and calcium nodule formation.

CONCLUSIONS

These data suggest paracrine signaling from EPCs influences the biological function and promotes MSCs osteogenic differentiation. Activation of the p38MAPK pathway may be the key to enhancing MSCs osteogenic differentiation via indirect interactions with EPCs.

Irisin prevents microgravity-induced impairment of osteoblast differentiation in vitro during the space flight CRS-14 mission

Understanding molecular mechanisms responsible for bone cells unbalance in microgravity would allow the development of better countermeasures for astronauts, and eventually advancing terrestrial osteoporosis treatments. We conduct a unique investigation by using a controlled 3D in vitro cell model to mimic the bone microenvironment in microgravity aboard the SpaceX Dragon cargo ferry to the ISS. Osteoblasts (OBs), osteoclasts (OCs), and endothelial cells (ECs), seeded on Skelite discs, were cultured w/ or w/o rec-Irisin and exposed to 14 days of microgravity in the eOSTEO hardware. Gene expression analysis was assessed, and results were compared to ground controls treated within identical payloads. Our results show that the microgravity-induced downregulation of mRNA levels of genes encoding for OB key transcription factors (Atf4 -75%, P < .01; RunX2 -87%, P < .001, Osterix -95%, P < .05 vs ground) and proteins (Collagen I -84%, P < .05; Osteoprotegerin -94%, P < .05) were prevented by irisin. Despite it was not effective in preventing Trap and Cathepsin K mRNA increase, irisin induced a 2.8-fold increase of Osteoprotegerin (P < .05) that might act for reducing osteoclastogenesis in microgravity. Our results provide evidence that irisin supports OB differentiation and activity in microgravity and it might represent a countermeasure to prevent bone loss in astronauts.

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.

Lipoteichoic acid of Enterococcus faecalis inhibits osteoclastogenesis via transcription factor RBP-J

Lipoteichoic acid (LTA) of Enterococcus faecalis is a potent stimulator of inflammatory responses, but the effects of E. faecalis LTA on osteoclastogenesis remains far from well understood. This study showed that E. faecalis LTA significantly inhibited osteoclastogenesis of wild type murine bone marrow-derived macrophages (BMMs) in the presence of a high dose of RANKL, while the inhibition of osteoclastogenesis by E. faecalis LTA was significantly removed in BMMs with deficient expression of the transcription factor RBP-J. In addition, a few small osteoclasts were generated in BMMs with only E. faecalis LTA stimulation, presumably due to the production of TNF-α and IL-6. Furthermore, both p38 and ERK1/2 MAPK signaling pathways were activated after 24 h of E. faecalis LTA treatment, but these signaling pathways were not activated after 6 d of treatment with RANKL in mature osteoclasts. In conclusion, E. faecalis LTA, which induces inflammatory response, could inhibit RANKL-induced osteoclastogenesis via RBP-J in BMMs.

Suppressing angiogenesis regulates the irradiation-induced stimulation on osteoclastogenesis in vitro

Ionizing radiation-induced bone loss is a potential health concern in radiotherapy, occupational exposure, and astronauts. Although impaired bone vasculature and reduced proliferation of bone-forming osteoblasts has been implicated in this process, it has not been clearly characterized that whether radiation affects the growth of bone-resorbing osteoclasts. The molecular crosstalk between different cell populations in the skeletal system has not yet been elucidated in detail, especially between the increased bone resorption at early stage of post-irradiation and bone marrow-derived endothelial progenitor cells (BM-EPCs). In order to further understand the mechanisms involved in radiation-induced bone loss at the cellular level, we assessed the effects of irradiation on angiogenesis of BM-EPCs and osteoclastogenesis of receptor activator for nuclear factor-κB ligand (RANKL)-stimulated RAW 264.7 cells and crosstalk between these cell populations. We herein found significantly dysfunction of BM-EPCs in response to irradiation at a dose of 2 Gy, including inhibited proliferation, migration, tube-forming abilities, and downregulated expression of pro-angiogenesis vascular endothelial growth factors A (VEGF A). Meanwhile, we observed that irradiation promoted osteoclastogenesis of RANKL-stimulated RAW 264.7 cells directly or indirectly. These results provide quantitative evidences of irradiation induced osteoclastogenesis at a cellular level, and strongly suggest the involvement of osteoclastogenesis, angiogenesis and crosstalk between bone marrow cells in the radiation-induced bone loss. This study may provide new insights for the early diagnosis and intervention of bone loss post-irradiation.

CXCR4+ CD45- Cells are Niche Forming for Osteoclastogenesis via the SDF-1, CXCL7, and CX3CL1 Signaling Pathways in Bone Marrow

Bone homeostasis comprises the balance between bone-forming osteoblasts and bone-resorbing osteoclasts (OCs), with an acceleration of osteoclastic bone resorption leading to osteoporosis. OCs can be generated from bone marrow cells (BMCs) under the tightly regulated local bone environment. However, it remained difficult to identify the critical cells responsible for providing an osteoclastogenesis niche. In this study, we used a fluorescence-activated cell sorting technique to determine the cell populations important for forming an appropriate microenvironment for osteoclastogenesis and to verify the associated interactions between osteoclast precursor cells and non-OCs. We isolated and removed a small cell population specific for osteoclastogenesis (CXCR4⁺ CD45^- ) from mouse BMCs and cultured the remaining cells with receptor activator of nuclear factor-kappa B ligand (RANKL) and macrophage-colony stimulating factor. The resulting cultures showed significantly less large osteoclast formation. Quantitative RT-PCR analysis revealed that these CXCR4⁺ CD45^- cells expressed low levels of RANK and RANKL, but high levels of critical chemokines including stromal cell derived factor 1 (SDF-1), chemokine (C-X-C motif) ligand 7 (CXCL7), and chemokine (C-X3-C motif) ligand 1 (CX3CL1). Furthermore, an SDF-1-specific antibody strongly suppressed OC formation in RAW264.7 cells and antibodies against SDF-1, CXCL7, and CX3CL1 suppressed OC formation in BMCs. These results suggest that isolated CXCR4⁺ CD45^- cells support an appropriate microenvironment for osteoclastogenesis with a direct effect on the cells expressing SDF-1, CXCL7, and CX3CL1 receptors. The regulation of CXCR4⁺ CD45^- cell function might therefore inform therapeutic strategies for diseases involving loss of bone homeostasis. Stem Cells 2016;34:2733-2743.

A 3D vascularized bone remodeling model combining osteoblasts and osteoclasts in a CaP nanoparticle-enriched matrix

AIM

We aimed to establish a 3D vascularized in vitro bone remodeling model.

MATERIALS & METHODS

Human umbilical endothelial cells (HUVECs), bone marrow mesenchymal stem cells (BMSCs), and osteoblast (OBs) and osteoclast (OCs) precursors were embedded in collagen/fibrin hydrogels enriched with calcium phosphate nanoparticles (CaPn). We assessed vasculogenesis in HUVEC-BMSC coculture, osteogenesis with OBs, osteoclastogenesis with OCs, and, ultimately, cell interplay in tetraculture.

RESULTS

HUVECs developed a robust microvascular network and BMSCs differentiated into mural cells. Noteworthy, OB and OC differentiation was increased by their reciprocal coculture and by CaPn, and even more by the combination of the tetraculture and CaPn.

CONCLUSION

We successfully developed a vascularized 3D bone remodeling model, whereby cells interacted and exerted their specific function.

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.

C/EBPβ mediates osteoclast recruitment by regulating endothelial progenitor cell expression of SDF-1α

Integration of tissue-engineered bone grafts with the host bone is vital for the healing of critical-size bone defects. An important aspect of this process is bone resorption, which must be carried out by osteoclasts derived from the host. However, the mechanism underlying recruitment of host osteoclast precursors to graft sites remains unclear. Endothelial progenitor cells (EPCs) mobilize from the bone marrow into the circulation and home to sites of angiogenesis such as tissue remodeling. Since EPCs express SDF-1, and C/EBPβ is known to regulate SDF-1α expression, we hypothesized that EPCs may recruit CXCR4-expressing host osteoclast precursors to the repair area and that this recruitment may be mediated through C/EBPβ signaling. Using an inflammatory EPC model we showed that EPCs upregulate protein levels of both SDF-1α and C/EBPβ. A luciferase assay confirmed that C/EBPβ acts on the SDF-1α promoter in these cells, and that binding is increased under conditions of inflammation, while silencing of C/EBPβ reduces expression of SDF-1 α and C/EBPβ. Using RAW264.7 cells as a model of osteoclastic monocyte precursors, we investigated their responses to migratory factors in EPC conditioned medium. We showed that RAW264.7 cells migrate towards conditioned medium from EPCs treated with IL-1β, an effect which could be abolished by silencing C/EBPβ in EPCs, and was almost completely blocked by silencing CXCR4 in RAW264.7 cells. These findings show that EPCs respond to inflammatory stimuli by signaling to osteoclast precursors via SDF-1, and that C/EBPβ mediates this response.

Lack of ADAM10 in endothelial cells affects osteoclasts at the chondro-osseus junction

Mice lacking ADAM10 in endothelial cells (Adam10ΔEC mice) have shorter femurs, tibiae, and humeri than controls, raising questions about how endothelial cells could control long bone growth. We performed a histopathological evaluation of the femur and tibia growth plates at different postnatal stages, and assessed the distribution of TRAP-positive osteoclasts and endothelial cells at the growth plate. The growth plates in Adam10ΔEC mice appeared normal at P7 and P14, but a thickened zone of hypertrophic chondrocytes and increased trabecular bone density were apparent by P21 and later. The number of TRAP+ cells at the COJ was normal at P7 and P14, but was strongly reduced at P21 and later. Moreover, the density of endomucin-stained endothelial cells at the COJ was increased starting at P7. The defects in long bone growth in Adam10ΔEC mice could be caused by a lack of osteoclastogenesis at the COJ. Moreover, ADAM10 appears to regulate endothelial cell organization in the developing bone vasculature, perhaps in a similar manner as in the developing retinal vascular tree, where ADAM10 is thought to control Notch-dependent endothelial cell fate decisions. This study provides evidence for the regulation of osteoclast function by endothelial cells in vivo.