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

Electrospun Scaffolds of Polylactic Acid, Collagen, and Amorphous Calcium Phosphate for Bone Repair

Most electrospun scaffolds for bone tissue engineering typically use hydroxyapatite (HA) or beta tricalcium phosphate (β-TCP). However, the biological activity of these crystalline compounds can be limited due to their low solubility. Therefore, amorphous calcium phosphate (ACP) may be an alternative in bone repair scaffolds. This study analyzes the morphology, porosity, mechanical strength, and surface chemistry of electrospun scaffolds composed of polylactic acid and collagen integrated with hydroxyapatite (MHAP) or amorphous calcium phosphate (MACP). In addition, the in vitro biocompatibility, osteogenic differentiation, and growth factor production associated with bone repair using human Wharton's jelly-derived mesenchymal stem cells (hWJ-MSCs) are evaluated. The results show that the electrospun MHAP and MACP scaffolds exhibit a fibrous morphology with interconnected pores. Both scaffolds exhibit favorable biocompatibility and stimulate the proliferation and osteogenesis of hWJ-MSCs. However, cell adhesion and osteocalcin production are greater in the MACP scaffold compared to the MHAP scaffold. In addition, the MACP scaffold shows significant production of bone-repair-related growth factors such as transforming growth factor-beta 1 (TGF-β1), providing a solid basis for its use in bone tissue engineering.

Smoking Impairs Hematoma Formation and Dysregulates Angiogenesis as the First Steps of Fracture Healing

Bone fracture healing is an overly complex process in which inflammation, osteogenesis, and angiogenesis are tightly coupled, and delayed fracture repair is a very common health risk. One of the major causes of delayed healing is the formation of insufficient vasculature. Precise regulation of blood vessels in bone and their interplay with especially osteogenic processes has become an emerging topic within the last years; nevertheless, regulation of angiogenesis in (early) diseased fracture repair is still widely unknown. Here, we aim to develop an in vitro model for the analysis of early fracture healing which also enables the analysis of angiogenesis as a main influencing factor. As smoking is one of the main risk factors for bone fractures and developing a delay in healing, we model smoking and non-smoking conditions in vitro to analyze diverging reactions. Human in vitro fracture hematomas mimicking smokers’ and non-smokers’ hematomas were produced and analyzed regarding cell viability, inflammation, osteogenic and chondrogenic differentiation, and angiogenic potential. We could show that smokers’ blood hematomas were viable and comparable to non-smokers. Smokers’ hematomas showed an increase in inflammation and a decrease in osteogenic and chondrogenic differentiation potential. When analyzing angiogenesis, we could show that the smokers’ hematomas secrete factors that drastically reduced HUVEC proliferation and tube formation. With an angiogenesis array and gene expression analysis, we could identify the main influencing factors: Anpgt1/2, Tie2, and VEGFR2/3. In conclusion, our model is suitable to mimic smoking conditions in vitro showing that smoking negatively impacts early vascularization of newly formed tissue.

COMP-Ang1: Therapeutic potential of an engineered Angiopoietin-1 variant

The Angiopoietin-1/2 system is an opportune target for therapeutic intervention in a wide range of vascular pathologies, particularly through its association with endothelium. The complex multi-domain structure of native human Angiopoietin-1 has hindered its widespread applicability as a therapeutic agent, prompting the search for alternative approaches to mimicking the Ang1:Tie2 signalling axis; a system with highly complex patterns of regulation involving multiple structurally similar molecules. An engineered variant, Cartilage Oligomeric Matrix Protein - Angiopoietin-1 (COMP-Ang1), has been demonstrated to overcome the limitations of the native molecule and activate the Tie2 pathway with several fold greater potency than Ang1, both in vitro and in vivo. The therapeutic efficacy of COMP-Ang1, at both the vascular and systemic levels, is evident from multiple studies. Beneficial impacts on skeletal muscle regeneration, wound healing and angiogenesis have been reported alongside renoprotective, anti-hypertensive and anti-inflammatory effects. COMP-Ang1 has also demonstrated synergy with other compounds to heighten bone repair, has been leveraged for potential use as a co-therapeutic for enhanced targeted cancer treatment, and has received considerable attention as an anti-leakage agent for microvascular diseases like diabetic retinopathy. This review examines the vascular Angiopoietin:Tie2 signalling mechanism, evaluates the potential therapeutic merits of engineered COMP-Ang1 in both vascular and systemic contexts, and addresses the inherent translational challenges in moving this potential therapeutic from bench-to-bedside.

The effects of high fat diet, bone healing, and BMP-2 treatment on endothelial cell growth and function

Angiogenesis is a vital process during the regeneration of bone tissue. The aim of this study was to investigate angiogenesis at the fracture site as well as at distal locations from obesity-induced type 2 diabetic mice that were treated with bone morphogenetic protein-2 (BMP-2, local administration at the time of surgery) to heal a femoral critical sized defect (CSD) or saline as a control. Mice were fed a high fat diet (HFD) to induce a type 2 diabetic-like phenotype while low fat diet (LFD) animals served as controls. Endothelial cells (ECs) were isolated from the lungs (LECs) and bone marrow (BMECs) 3 weeks post-surgery, and the fractured femurs were also examined. Our studies demonstrate that local administration of BMP-2 at the fracture site in a CSD model results in complete bone healing within 3 weeks for all HFD mice and 66.7% of LFD mice, whereas those treated with saline remain unhealed. At the fracture site, vessel parameters and adipocyte numbers were significantly increased in BMP-2 treated femurs, irrespective of diet. At distal sites, LEC and BMEC proliferation was not altered by diet or BMP-2 treatment. HFD increased the tube formation ability of both LECs and BMECs. Interestingly, BMP-2 treatment at the time of surgery reduced tube formation in LECs and humeri BMECs. However, migration of BMECs from HFD mice treated with BMP-2 was increased compared to BMECs from HFD mice treated with saline. BMP-2 treatment significantly increased the expression of CD31, FLT-1, and ANGPT2 in LECs and BMECs in LFD mice, but reduced the expression of these same genes in HFD mice. To date, this is the first study that depicts the systemic influence of fracture surgery and local BMP-2 treatment on the proliferation and angiogenic potential of ECs derived from the bone marrow and lungs.

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.

Assessment of the Release of Vascular Endothelial Growth Factor from 3D-Printed Poly-ε-Caprolactone/Hydroxyapatite/Calcium Sulfate Scaffold with Enhanced Osteogenic Capacity

Vascular endothelial growth factor (VEGF) is one of the most crucial growth factors and an assistant for the adjustment of bone regeneration. In this study, a 3D scaffold is fabricated using the method of fused deposition modeling. Such a fabricated method allows us to fabricate scaffolds with consistent pore sizes, which could promote cellular ingrowth into scaffolds. Therefore, we drafted a plan to accelerate bone regeneration via VEGF released from the hydroxyapatite/calcium sulfate (HACS) scaffold. Herein, HACS will gradually degrade and provide a suitable environment for cell growth and differentiation. In addition, HACS scaffolds have higher mechanical properties and drug release compared with HA scaffolds. The drug release profile of the VEGF-loaded scaffolds showed that VEGF could be loaded and released in a stable manner. Furthermore, initial results showed that VEGF-loaded scaffolds could significantly enhance the proliferation of human mesenchymal stem cells (hMSCs) and human umbilical vein endothelial cells (HUVEC). In addition, angiogenic- and osteogenic-related proteins were substantially increased in the HACS/VEGF group. Moreover, in vivo results revealed that HACS/VEGF improved the regeneration of the rabbit’s femur bone defect, and VEGF loading improved bone tissue regeneration and remineralization after implantation for 8 weeks. All these results strongly imply that the strategy of VEGF loading onto scaffolds could be a potential candidate for future bone tissue engineering.

Mesenchymal stem cells and biologic factors leading to bone formation

BACKGROUND

Physiological bone formation and bone regeneration occurring during bone repair can be considered distinct but similar processes. Mesenchymal stem cells (MSC) and associated biologic factors are crucial to both bone formation and bone regeneration.

AIM

To perform a narrative review of the current literature regarding the role of MSC and biologic factors in bone formation with the aim of discussing the clinical relevance of in vitro and in vivo animal studies.

METHODS

The literature was searched for studies on MSC and biologic factors associated with the formation of bone in the mandible and maxilla. The search specifically targeted studies on key aspects of how stem cells and biologic factors are important in bone formation and how this might be relevant to bone regeneration. The results are summarized in a narrative review format.

RESULTS

Different types of MSC and many biologic factors are associated with bone formation in the maxilla and mandible.

CONCLUSION

Bone formation and regeneration involve very complex and highly regulated cellular and molecular processes. By studying these processes, new clinical opportunities will arise for therapeutic bone regenerative treatments.

Immunomodulatory Effects of MSCs in Bone Healing

Mesenchymal stem cells (MSCs) are capable of differentiating into multilineage cells, thus making them a significant prospect as a cell source for regenerative therapy; however, the differentiation capacity of MSCs into osteoblasts seems to not be the main mechanism responsible for the benefits associated with human mesenchymal stem cells hMSCs when used in cell therapy approaches. The process of bone fracture restoration starts with an instant inflammatory reaction, as the innate immune system responds with cytokines that enhance and activate many cell types, including MSCs, at the site of the injury. In this review, we address the influence of MSCs on the immune system in fracture repair and osteogenesis. This paradigm offers a means of distinguishing target bone diseases to be treated with MSC therapy to enhance bone repair by targeting the crosstalk between MSCs and the immune system.

Pivotal micro factors associated with endothelial cells

Objective:

Recent studies have shown the important influence of various micro factors on the general biological activity and function of endothelial cells (ECs). Vascular endothelial growth factor (VEGF) and angiogenin (ANG) are classic micro factors that promote proliferation, differentiation, and migration of ECs. The underlying pathophysiological mechanisms and related pathways of these micro factors remain the focus of current research.

Data sources:

An extensive search was undertaken in the PubMed database by using keywords including “micro factors” and “endothelial cell.” This search covered relevant research articles published between January 1, 2007 and December 31, 2018.

Study selection:

Original articles, reviews, and other articles were searched and reviewed for content on micro factors of ECs.

Results:

VEGF and ANG have critical functions in the occurrence, development, and status of the physiological pathology of ECs. Other EC-associated micro factors include interleukin 10, tumor protein P53, nuclear factor kappa B subunit, interleukin 6, and tumor necrosis factor. The results of Gene Ontology analysis revealed that variations were mainly enriched in positive regulation of transcription by the RNA polymerase II promoter, cellular response to lipopolysaccharides, negative regulation of apoptotic processes, external side of the plasma membrane, cytoplasm, extracellular regions, cytokine activity, growth factor activity, and identical protein binding. The results of the Kyoto Encyclopedia of Genes and Genomes analysis revealed that micro factors were predominantly enriched in inflammatory diseases.

Conclusions:

In summary, the main mediators, factors, or genes associated with ECs include VEGF and ANG. The effect of micro factors on ECs is complex and multifaceted. This review summarizes the correlation between ECs and several micro factors.

Tissue Constructs with Human Adipose-Derived Mesenchymal Stem Cells to Treat Bone Defects in Rats

The use of porous scaffolds created by additive manufacturing is considered a viable approach for the regeneration of critical-size bone defects. This paper investigates the xenotransplantation of polycaprolactone (PCL) tissue constructs seeded with differentiated and undifferentiated human adipose-derived mesenchymal stem cells (hADSCs) to treat calvarial critical-sized defect in Wistar rats. PCL scaffolds without cells were also considered. In vitro and in vivo biological evaluations were performed to assess the feasibility of these different approaches. In the case of cell seeded scaffolds, it was possible to observe the presence of hADSCs in the rat tissue contributing directly (osteoblasts) and indirectly (stimulation by paracrine factors) to tissue formation, organization and mineralization. The presence of bone morphogenetic protein-2 (BMP-2) in the rat tissue treated with cell-seeded PCL scaffolds suggests that the paracrine factors of undifferentiated hADSC cells could stimulate BMP-2 production by surrounding cells, leading to osteogenesis. Moreover, BMP-2 acts synergistically with growth factors to induce angiogenesis, leading to higher numbers of blood vessels in the groups containing undifferentiated and differentiated hADSCs.

Bone Tissue Engineering Strategies in Co-Delivery of Bone Morphogenetic Protein-2 and Biochemical Signaling Factors

Administration of bone morphogenetic protein-2 (BMP-2), which is commercially approved by the food and drug administration to damaged bone sites has been investigated for the purpose of bone tissue regeneration. BMP-2 can promote osteoblastic differentiation of mesenchymal stem cells as well as regeneration of bone formation in early phase. This review highlights various factors such as vitamin D, dexamethasone, platelet-derived growth factor, placental growth factor, BMP-7, and NEL-like protein-1 that enhance and stimulate angiogenesis, cell differentiation, and bone regeneration. These biochemical signals and growth factors (GFs) accelerate bone repair and remodeling either synergistically or individually. Delivery systems and scaffolds are used for sustained release of these cargo molecules and support at damaged bone sites. Compared with direct administration of BMP-2, current studies have demonstrated that a combination of multiple GFs and/or therapeutic chemical factors with delivery platforms synergistically facilitates bone regeneration. Therefore, in the future, multiple combinations of various GFs, chemicals, and materials could provide patients and surgeons with non-invasive treatment options without secondary surgery and pain. To the end, this review summarizes the biological functions and synergistic effects of dual administration modalities involving BMP-2 as well as recent developments in bone tissue engineering applications.

NELL-1, HMGB1, and CCN2 Enhance Migration and Vasculogenesis, But Not Osteogenic Differentiation Compared to BMP2

Currently, autografts still represent the gold standard treatment for the repair of large bone defects. However, these are associated with donor-site morbidity and increased pain, cost, and recovery time. The ideal therapy would use biomaterials combined with bone growth factors to induce and instruct bone defect repair without the need to harvest patient tissue. In this line, bone morphogenetic proteins (BMPs) have been the most extensively used agents for clinical bone repair, but at supraphysiological doses that are not without risk. Because of the need to eliminate the risks of BMP2 use in vivo, we assessed the ability of three putative osteogenic factors, nel-like molecule type 1 (NELL-1), high mobility group box 1 (HMGB1), and CCN2, to enhance the essential processes for bone defect repair in vitro and compared them to BMP2. Although it has been reported that NELL-1, HMGB1, and CCN2 play a role in bone formation, less is known about the contribution of these proteins to the different events involved, such as cell migration, osteogenesis, and vasculogenesis. In this study, we investigated the effects of different doses of NELL-1, HMGB, CCN2, and BMP2 on these three processes as a model for the recruitment and differentiation of resident cells in the in vivo bone defect repair situation, using cells of human origin. Our data demonstrated that NELL-1, HMGB1, and CCN2 significantly induced mesenchymal stem cell migration (from 1.58-fold increase compared to control), but BMP2 did not. Interestingly, only BMP2 increased osteogenesis in marrow stromal cells, whereas it inhibited osteogenesis in preosteoblasts. Moreover, the four proteins studied promoted significantly endothelial cell migration, reaching a maximum of 2.4-fold increase compared to control, and induced formation of tube-like structures. NELL-1, HMGB1, and CCN2 had these effects at relatively low doses compared to BMP2. This work indicates that NELL-1, HMGB1, and CCN2 might enhance bone defect healing via the recruitment of endogenous cells and induction of vascularization and act via different processes than BMP2.

Differential effects of amnion and chorion membrane extracts on osteoblast-like cells due to the different growth factor composition of the extracts

Human amniotic membrane extracts contain numerous growth factors and bioactive substances. However, osteogenic effects of amnion and chorion membrane extracts (AME and CME, respectively) on osteoblasts are unclear. In this study, we explored the ability of AME and CME to promote the osteogenic differentiation of osteoblast-like MG-63 cells. MG-63 cells were cultured in osteogenic induction medium (OIM) with or without exogenous AME and CME. CME enhanced the osteogenic differentiation of MG-63 cells compared with AME, as indicated by increased mineralization; alkaline phosphatase activity; and mRNA expression of osteogenic marker genes encoding integrin-binding sialoprotein (IBSP), RUNX2, OSTERIX, and osteocalcin (OCN). Interestingly, AME and CME contained different combinations of osteogenesis-related growth factors, including basic fibroblast growth factor (bFGF), transforming growth factor beta-1 (TGFβ-1), and epidermal growth factor (EGF), which differentially regulated the osteogenic differentiation of MG-63 cells. bFGF and TGFβ-1 present in CME positively regulated the osteogenic differentiation of MG-63 cells, whereas EGF present in AME negatively regulated the differentiation of MG-63 cells. Moreover, exogenous treatment of EGF antagonized CME-induced mineralization of extracellular matrix on MG-63 cells. We compared the osteogenic efficacy of CME with that of BMP2, bFGF, and TGFβ-1 alone or their combinations. We observed that CME greatly enhanced osteogenesis by providing a conductive environment for the differentiation of MG-63 cells. Together, our results indicated that human AME and CME exerted differential effects on osteogenesis because of the presence of different compositions of growth factors. In addition, our results highlighted a new possible strategy of using CME as a biocompatible therapeutic material for bone regeneration.

Therapeutic targeting of the angiopoietin-TIE pathway

The endothelial angiopoietin (ANG)-TIE growth factor receptor pathway regulates vascular permeability and pathological vascular remodelling during inflammation, tumour angiogenesis and metastasis. Drugs that target the ANG-TIE pathway are in clinical development for oncological and ophthalmological applications. The aim is to complement current vascular endothelial growth factor (VEGF)-based anti-angiogenic therapies in cancer, wet age-related macular degeneration and macular oedema. The unique function of the ANG-TIE pathway in vascular stabilization also renders this pathway an attractive target in sepsis, organ transplantation, atherosclerosis and vascular complications of diabetes. This Review covers key aspects of the function of the ANG-TIE pathway in vascular disease and describes the recent development of novel therapeutics that target this pathway.

NAMPT Is an Essential Regulator of RA-Mediated Periodontal Inflammation

Recent studies have indicated a potential correlation between rheumatoid arthritis (RA) and periodontal inflammation. We undertook this study to verify whether RA mediates periodontitis-like phenotypes in experimental mouse models of RA and to explore the role of nicotinamide phosphoribosyltransferase (NAMPT) in periodontal inflammation during RA pathogenesis. Periodontal inflammation and alveolar bone loss have been reported in mice with collagen-induced arthritis (CIA) and in genetically modified tumor necrosis factor–α (TNF-α) transgenic (TG) mouse models. Among the adipokines examined in our study, NAMPT expression was markedly upregulated in the periodontal ligament (PDL) tissues in RA mouse models and in human PDL cells stimulated by the proinflammatory cytokines, interleukin (IL) 1β and TNF-α. When NAMPT was overexpressed with the Nampt-synthesizing adenovirus vector (Ad- Nampt), the PDL cells exhibited an increased expression of cytokines (IL6), chemokines (IL8 and chemokine [C-C motif] ligand 5 [CCL5]), inflammatory mediators (cyclooxygenase 2 [COX-2]), and matrix-degrading enzymes (matrix metalloproteinase [MMP] 1 and MMP3). Inhibition of NAMPT by the intracellular NAMPT (iNAMPT) inhibitor, FK866, or by the sirtuin inhibitor, nicotinamide, in PDL cells led to inhibition of the IL1β or Ad- Nampt–induced upregulation of catabolic factors, whereas treatment with recombinant NAMPT protein or blockade of extracellular NAMPT (eNAMPT) with blocking antibody did not. Moreover, NAMPT inhibition by the intraperitoneal or intragingival injection of FK866 in CIA mice inhibited periodontal tissue damage, under conditions of RA. Thus, our results verified the co-occurrence of RA and periodontal inflammation using experimental mouse models of RA, suggesting that iNAMPT in PDL cells plays a pivotal role in the pathogenesis of RA-mediated periodontal inflammation by regulating the expression levels of catabolic genes, such as IL6, IL8, CCL5, COX-2, MMP1, and MMP3.