Antibody-mediated osseous regeneration: a novel strategy for bioengineering bone by immobilized anti-bone morphogenetic protein-2 antibodies

Bioactive Scaffolds as a Promising Alternative for Enhancing Critical-Size Bone Defect Regeneration in the Craniomaxillofacial Region

Reconstruction of critical-size bone defects (CSDs) in the craniomaxillofacial (CMF) region remains challenging. Scaffold-based bone-engineered constructs have been proposed as an alternative to the classical treatments made with autografts and allografts. Scaffolds, a key component of engineered constructs, have been traditionally viewed as biologically passive temporary replacements of deficient bone lacking intrinsic cues to promote osteogenesis. Nowadays, scaffolds are functionalized, giving rise to bioactive scaffolds promoting bone regeneration more effectively than conventional counterparts. This review focuses on the three approaches most used to bioactivate scaffolds: (1) conferring microarchitectural designs or surface nanotopography; (2) loading bioactive molecules; and (3) seeding stem cells on scaffolds, providing relevant examples of in vivo (preclinical and clinical) studies where these methods are employed to enhance CSDs healing in the CMF region. From these, adding bioactive molecules (specifically bone morphogenetic proteins or BMPs) to scaffolds has been the most explored to bioactivate scaffolds. Nevertheless, the downsides of grafting BMP-loaded scaffolds in patients have limited its successful translation into clinics. Despite these drawbacks, scaffolds containing safer, cheaper, and more effective bioactive molecules, combined with stem cells and topographical cues, remain a promising alternative for clinical use to treat CSDs in the CMF complex replacing autografts and allografts.

Biofabrication of engineered dento-alveolar tissue

Oral health is essential for a good overall health. Dento-alveolar conditions have a high prevalence, ranging from tooth decay periodontitis to alveolar bone resorption. However, oral tissues exhibit a limited regenerative capacity, and full recovery is challenging. Therefore, regenerative therapies for dento-alveolar tissue (e.g., alveolar bone, periodontal membrane, dentin-pulp complex) have gained much attention, and novel approaches have been proposed in recent decades. This review focuses on the cells, biomaterials and the biofabrication methods used to develop therapies for tooth root bioengineering. Examples of the techniques covered are the multitude of additive manufacturing techniques and bioprinting approaches used to create scaffolds or tissue constructs. Furthermore, biomaterials and stem cells utilized during biofabrication will also be described for different target tissues. As these new therapies gradually become a reality in the lab, the translation to the clinic is still minute, with a further need to overcome multiple challenges and broaden the clinical application of these alternatives.

BMP2 immune complexes promote new bone formation by facilitating the direct contact between osteoclasts and osteoblasts

BMP2 antibody is proposed as a promising replacement for rhBMP2 in bone tissue engineering. Although studies have demonstrated its osteoinductive efficacy, the underlying osteogenic mechanism and adverse reactions of specific BMP2 antibody are not clarified yet, making it difficult to optimize the antibody for future application. By establishing BMP2 immune complexes (BMP2-ICs) ex vivo, we were able to introduce BMP2-ICs directly in vivo and found that BMP2-ICs promoted bone formation while suppressing osteoclastogenesis. However, ex vivo osteoclastogenic assays showed that BMP2-ICs promoted osteoclastogenesis by binding FcγR and activating PLCγ2 phosphorylation. Given that BMP2-ICs react with osteoblast and osteoclast lineage cells by the conjugated BMP2 domain and the Fc domain respectively, we introduced BMP2-ICs into coculture system of the two lineage cells and found that BMP2-ICs promoted osteogenesis while suppressing osteoclastogenesis by facilitating osteoblast-osteoclast contact and activating the EphrinB2-EphB4 signaling. This bidirectional function of BMP2-ICs was reproduced in the cranial bone resorption model, where osteoblast and osteoclast lineage cells co-localized. This study excluded the hidden problem of osteoclast overactivation that usually comes with rhBMP2 and clarified the first evidence of the mechanism of antibody-mediated bone regeneration, suggesting BMP2-ICs may present a promising therapy for bone diseases related with disrupted osteoclast-osteoblast interaction.

Chlorite oxidized oxyamylose differentially influences the microstructure of fibrin and self assembling peptide hydrogels as well as dental pulp stem cell behavior

Tailored hydrogels mimicking the native extracellular environment could help overcome the high variability in outcomes within regenerative endodontics. This study aimed to evaluate the effect of the chemokine-binding and antimicrobial polymer, chlorite-oxidized oxyamylose (COAM), on the microstructural properties of fibrin and self-assembling peptide (SAP) hydrogels. A further goal was to assess the influence of the microstructural differences between the hydrogels on the in vitro behavior of human dental pulp stem cells (hDPSCs). Structural and mechanical characterization of the hydrogels with and without COAM was performed by atomic force microscopy and scanning electron microscopy to characterize their microstructure (roughness and fiber length, diameter, straightness, and alignment) and by nanoindentation to measure their stiffness (elastic modulus). Then, hDPSCs were encapsulated in hydrogels with and without COAM. Cell viability and circularity were determined using confocal microscopy, and proliferation was determined using DNA quantification. Inclusion of COAM did not alter the microstructure of the fibrin hydrogels at the fiber level while affecting the SAP hydrogel microstructure (homogeneity), leading to fiber aggregation. The stiffness of the SAP hydrogels was sevenfold higher than the fibrin hydrogels. The viability and attachment of hDPSCs were significantly higher in fibrin hydrogels than in SAP hydrogels. The DNA content was significantly affected by the hydrogel type and the presence of COAM. The microstructural stability after COAM inclusion and the favorable hDPSCs' response observed in fibrin hydrogels suggest this system as a promising carrier for COAM and application in endodontic regeneration.

The Body's Cellular and Molecular Response to Protein-Coated Medical Device Implants: A Review Focused on Fibronectin and BMP Proteins

Recent years have seen a marked rise in implantation into the body of a great variety of devices: hip, knee, and shoulder replacements, pacemakers, meshes, glucose sensors, and many others. Cochlear and retinal implants are being developed to restore hearing and sight. After surgery to implant a device, adjacent cells interact with the implant and release molecular signals that result in attraction, infiltration of the tissue, and attachment to the implant of various cell types including monocytes, macrophages, and platelets. These cells release additional signaling molecules (chemokines and cytokines) that recruit tissue repair cells to the device site. Some implants fail and require additional revision surgery that is traumatic for the patient and expensive for the payer. This review examines the literature for evidence to support the possibility that fibronectins and BMPs could be coated on the implants as part of the manufacturing process so that the proteins could be released into the tissue surrounding the implant and improve the rate of successful implantation.

Emerging local delivery strategies to enhance bone regeneration

In orthopedics and dentistry there is an increasing need for novel biomaterials and clinical strategies to achieve predictable bone regeneration. These novel molecular strategies have the potential to eliminate the limitations of currently available approaches. Specifically, they have the potential to reduce or eliminate the need to harvest autogenous bone, and the overall complexity of the clinical procedures. In this review, emerging tissue engineering strategies that have been, or are currently being, developed based on the current understanding of bone biology, development and wound healing will be discussed. In particular, protein/peptide based approaches, DNA/RNA therapeutics, cell therapy, and the use of exosomes will be briefly covered. The review ends with a summary of the current status of these approaches, their clinical translational potentials and their challenges.

Functionalized Scaffold and Barrier Membrane with Anti-BMP-2 Monoclonal Antibodies for Alveolar Ridge Preservation in a Canine Model

Introduction

The aim of this study was to investigate the ability of anti-bone morphogenetic protein 2 monoclonal antibody (anti-BMP-2 mAb) to functionalize scaffolds to mediate bone regeneration in a canine model.

Materials and Methods

The mandibular right premolar 4 (PM4) was extracted in eight beagle dogs and grafted with anti-BMP-2 mAb+anorganic bovine bone mineral with 10% collagen (ABBM-C) and porcine bilayer native collagen membrane (CM). The ABBM-C and CM were functionalized with either anti-BMP-2 mAb (test group) or an isotype matched control mAb (control group). Animals were euthanized at 12 weeks for radiographic, histologic, and histomorphometric analyses. Outcomes were compared between groups.

Results

3D imaging using cone beam computed tomography (CBCT) revealed that sites treated with ABBM-C and CM functionalized with anti-BMP-2 mAb exhibited significantly more remaining bone width near the alveolar crest, as well as buccal bone height, compared with control groups. Histologic and histomorphometric analyses demonstrated that in anti-BMP-2 mAb-treated sites, total tissue volume was significantly higher in the coronal part of the alveolar bone crest compared with control sites. In anti-BMP-2 mAb-treated sites, bone formation was observed under the barrier membrane.

Conclusion

Functionalization of the ABBM-C scaffold and CM appeared to have led to bone formation within healing alveolar bone sockets.

The sialylation profile of IgG determines the efficiency of antibody directed osteogenic differentiation of iMSCs by modulating local immune responses and osteoclastogenesis

Antibody-mediated osseous regeneration (AMOR) has been proved as a promising strategy for osteogenic differentiation of induced pluripotent stem cells derived MSCs (iMSCs). The key characteristic of antibody that determines the AMOR potential is largely unknown. The glycosylation profile of immunoglobulin G (IgG) represents a key checkpoint that determines its effector functions. Herein, we modified the sialylation profile of BMP2 antibodies to investigate the effects of glycosylation on antibody-mediated osteogenic differentiation of iMSCs. We found that over-sialylated BMP2 antibodies stimulated the highest amount of new bone while those non- or low-sialylated led to bone porosity and collapse. The immune response aroused by BMP2 immune complexes (BMP2-ICs) was intensified by desialylation, which contributed to an environment that favored osteoclastogenesis while inhibited osteoblastogenesis. In vitro study further demonstrated that the osteogenic potential of BMP2-ICs was not significantly affected by the degree of sialylation. On the other hand, BMP2-ICs could stimulate osteoclastogenesis by binding FcγRs on preosteoclasts directly, which was significantly intensified by desialylation and attenuated by over-sialylation. Bone defects implanted with alginate microbeads loaded with iMSCs and over-sialylated antibodies showed more bone formation than those sites with non- or low sialylated antibodies. Taken together, our study demonstrated that sialylation profile is one of the traits that decide the AMOR potential of BMP2 antibodies. Enhancement of sialylation may be a promising strategy to optimize antibody for iMSCs application in bone tissue engineering. STATEMENT OF SIGNIFICANCE: Antibody-mediated osseous regeneration (AMOR) is a promising strategy for bone tissue engineering that takes advantage of the specific reactivity of antibodies to sequester endogenous BMP2 and present it to osteoprogenitor cells. We previously demonstrated that BMP2 immune complex can drive iPSCs derived MSCs to osteogenic lineage. In this study, we analyze the effects of glycosylation profile on antibody directed osteogenic differentiation of iMSCs because glycosylation profile represents a key checkpoint that determines the effector functions of antibodies, and it is susceptible to variations in different clones. The results showed that sialylation profile is one of the traits that decides the AMOR potential of BMP2 antibody, and the enhancement of sialylation maybe a promising strategy to optimize antibodies for AMOR.

Senile Osteoporosis: The Involvement of Differentiation and Senescence of Bone Marrow Stromal Cells

Senile osteoporosis has become a worldwide bone disease with the aging of the world population. It increases the risk of bone fracture and seriously affects human health. Unlike postmenopausal osteoporosis which is linked to menopause in women, senile osteoporosis is due to aging, hence, affecting both men and women. It is commonly found in people with more than their 70s. Evidence has shown that with age increase, bone marrow stromal cells (BMSCs) differentiate into more adipocytes rather than osteoblasts and undergo senescence, which leads to decreased bone formation and contributes to senile osteoporosis. Therefore, it is necessary to uncover the molecular mechanisms underlying the functional changes of BMSCs. It will benefit not only for understanding the senile osteoporosis development, but also for finding new therapies to treat senile osteoporosis. Here, we review the recent advances of the functional alterations of BMSCs and the related mechanisms during senile osteoporosis development. Moreover, the treatment of senile osteoporosis by aiming at BMSCs is introduced.

A glance on the role of fibronectin in controlling cell response at biomaterial interface

The bioactivity of biomaterials is closely related to cell response in contact with them. However, shortly after their insertion, materials are soon covered with proteins that constitute the biological fluids, and which render the direct surface recognition by cells almost impossible. The control of protein adsorption at the interface is therefore desirable. Extracellular matrix proteins are of particular interest in this sense, due to their well-known ability to modulate cell behavior. Particularly, fibronectin plays a leading role, being present in both healthy and injured tissues undergoing healing and regeneration. The aim of the present work is to give an overview on fibronectin and on its involvement in the control of cell behavior providing evidence of its pivotal role in the control of cell adhesion, spreading, migration, proliferation and differentiation. A deep insight into methods to enrich biomaterials surface with fibronectin will be then discussed, as well as new cues on the possibility to design tailored platforms able to specifically retain fibronectin from the surrounding extracellular milieu.

Bacterial cellulose membrane functionalized with hydroxiapatite and anti-bone morphogenetic protein 2: A promising material for bone regeneration

Bone tissue engineering seeks to adequately restore functions related to physical and biological properties, aiming at a repair process similar to natural bone. The use of compatible biopolymers, such as bacterial cellulose (BC), as well as having interesting mechanical characteristics, presents a slow in vivo degradation rate, and the ability to be chemically modified. To promote better bioactivity towards BC, we synthesized an innovative BC membrane associated to hydroxyapatite (HA) and anti-bone morphogenetic protein antibody (anti-BMP-2) (BC-HA-anti-BMP-2). We present the physical-chemical, biological and toxicological characterization of BC-HA-anti-BMP-2. Presence of BC and HA components in the membranes was confirmed by SEM-EDS and FTIR assays. No toxic potential was found in MC3T3-E1 cells by cytotoxicity assays (XTT Assay and Clonogenic Survival), genotoxicity (Comet Assay) and mutagenicity (Cytokinesis-blocked micronucleus Test). The in vitro release kinetics of anti-BMP-2 antibodies detected gradually reducing antibody levels, reducing approximately 70% in 7 days and 90% in 14 days. BC-HA-anti-BMP-2 increased SPP1, BGLAP, VEGF, ALPL, RUNX2 and TNFRSF11B expression, genes involved in bone repair and also increased mineralization nodules and phosphatase alcalin (ALP) activity levels. In conclusion, we developed BC-HA-anti-BMP-2 as an innovative and promising biomaterial with interesting physical-chemical and biological properties which may be a good alternative to treatment with commercial BMP-2 protein.

Therapeutic antibody directed osteogenic differentiation of induced pluripotent stem cell derived MSCs

Induced pluripotent stem cells (iPSCs) are regarded as a new cell source for regenerative medicine. Recent advances in tissue engineering have brought to light the therapeutic application of induced pluripotent stem cells (iPSCs) in bone defect repair. However, a safe and efficient way to differentiate iPSCs into osteogenic lineage remains to be a major challenge. Here we describe an approach using anti-BMP2 antibodies (Abs) to mediate osteogenic differentiation of iPSC-derived mesenchymal stromal cells (iMSCs). We first proved that 3G7 (an anti-BMP2 Ab) not only bound to BMP2, but also allowed the bound BMP2 to engage the BMP2 receptors on iMSCs. Subcutaneous implantation sites loaded with iMSCs + 3G7 group showed significant bone formation and vascularization in mice while those sites with exogenous BMP2 exhibited dystrophic calcification and significantly lower vascularization. Our in vitro study demonstrated that the anti-BMP2 Ab/BMP2 immune complex were capable of dictating the acquisition of osteogenic phenotype of iMSCs and subsequent mineralization. The study provided the first evidence of antibody-mediated differentiation of iMSCs and osseous regeneration in vivo. This novel strategy takes full advantage of the endogenous bioactive molecules for osseous regeneration and its potential therapeutic application is promising.

STATEMENT OF SIGNIFICANCE

Induced pluripotent stem cells (iPSCs) and its derived cells hold significant promise for the treatment of bone defects. In present study, we carried out the concept of antibody-mediated bone regeneration into the iPSC research for the first time. We demonstrated that anti-BMP2 Ab/BMP2 immune complex was capable of promoting osteogenic differentiation of iPSC-derived MSCs (iMSCs), likely through the classical BMP2/Smad1/Runx2 pathway. Subcutaneous co-delivery of iMSCs and anti-BMP2 Abs resulted in significant bone formation and vascularization. These findings suggested antibody mediated osteogenic differentiation may be a favorable approach for iPSC-based bone tissue engineering.

Antibody-Mediated Osseous Regeneration for Bone Tissue Engineering in Canine Segmental Defects

Among many applications of therapeutic monoclonal antibodies (mAbs), a unique approach for regenerative medicine has entailed antibody-mediated osseous regeneration (AMOR). In an effort to identify a clinically relevant model of craniofacial defect, the present study investigated the efficacy of mAb specific for bone morphogenetic protein- (BMP-) 2 to repair canine segmental mandibular continuity defect model. Accordingly, a 15 mm unilateral segmental defect was created in mandible and fixated with a titanium plate. Anorganic bovine bone mineral with 10% collagen (ABBM-C) was functionalized with 25 μg/mL of either chimeric anti-BMP-2 mAb or isotype-matched mAb (negative control). Recombinant human (rh) BMP-2 served as positive control. Morphometric analyses were performed on computed tomography (CT) and histologic images. Bone densities within healed defect sites at 12 weeks after surgery were 1360.81 ± 10.52 Hounsfield Unit (HU), 1044.27 ± 141.16 HU, and 839.45 ± 179.41 HU, in sites with implanted anti-BMP-2 mAb, rhBMP-2, and isotype mAb groups, respectively. Osteoid bone formation in anti-BMP-2 mAb (42.99% ± 8.67) and rhBMP-2 (48.97% ± 2.96) groups was not significantly different but was higher (p < 0.05) than in sites with isotype control mAb (26.8% ± 5.35). In view of the long-term objective of translational application of AMOR in humans, the results of the present study demonstrated the feasibility of AMOR in a large clinically relevant animal model.

BBP-Functionalized Biomimetic Nanofibrous Scaffold Can Capture BMP2 and Promote Osteogenic Differentiation

Bone morphogenetic proteins (BMPs, e.g., BMP2 and 7) are potent mediators for bone repair, however, their clinical use has been limited by their safety and cost-effectiveness. Therefore, innovative strategies that can improve the efficacy of BMPs, and thereby, use a lower dose of exogenous BMPs are highly desired. Inspired by the natural interaction between extracellular matrix (ECM) and growth factors, we hypothesize that bone matrix-mimicking nanofibrous scaffold functionalized with BMP binding moieties can selectively capture and stabilize BMPs, and thereby, promote BMP-induced osteogenic differentiation. To test our hypothesis, a gelatin nanofibrous scaffold was fabricated using thermally induced phase separation together with a porogen leaching technique (TIPS&P) and functionalized by a BMP-binding peptide (BBP) through cross-linking. Our data indicated that BBP decoration largely improved the BMP2 binding and retention capacity of the nanofibrous scaffolds without compromising their macro/microstructure and mechanical properties. Importantly, the BBP-functionalized gelatin scaffolds were able to significantly promote BMP2-induced osteogenic differentiation. Moreover, BBP alone was able to significantly stimulate endogenous BMP2 expression and improve osteogenic differentiation. Compared to other affinity-based drug delivery strategies, e.g., heparin and antibody-mediated growth factor delivering techniques, we expect BBP-functionalized scaffolds will be a safer, more feasible and selective strategy for endogenous BMP stimulating and binding. Therefore, our data suggests a promising application of using the BBP-decorated gelatin nanofibrous scaffold to stimulate/capture BMPs and promote endogenous bone formation in situ in contrast to relying on the administration of high doses of exogenous BMPs and transplantation of cells.

Collagen Sponge Functionalized with Chimeric Anti-BMP-2 Monoclonal Antibody Mediates Repair of Critical-Size Mandibular Continuity Defects in a Nonhuman Primate Model

Antibody-mediated osseous regeneration (AMOR) has been introduced by our research group as a tissue engineering approach to capture of endogenous growth factors through the application of specific monoclonal antibodies (mAbs) immobilized on a scaffold. Specifically, anti-Bone Morphogenetic Protein- (BMP-) 2 mAbs have been demonstrated to be efficacious in mediating bone repair in a number of bone defects. The present study sought to investigate the application of AMOR for repair of mandibular continuity defect in nonhuman primates. Critical-sized mandibular continuity defects were created in Macaca fascicularis locally implanted with absorbable collagen sponges (ACS) functionalized with chimeric anti-BMP-2 mAb or isotype control mAb. 2D and 3D analysis of cone beam computed tomography (CBCT) imaging demonstrated increased bone density and volume observed within mandibular continuity defects implanted with collagen scaffolds functionalized with anti-BMP-2 mAb, compared with isotype-matched control mAb. Both CBCT imaging and histologic examination demonstrated de novo bone formation that was in direct apposition to the margins of the resected bone. It is hypothesized that bone injury may be necessary for AMOR. This is evidenced by de novo bone formation adjacent to resected bone margins, which may be the source of endogenous BMPs captured by anti-BMP-2 mAb, in turn mediating bone repair.

Adding Functions to Biomaterial Surfaces through Protein Incorporation

The concept of biomaterials has evolved from one of inert mechanical supports with a long-term, biologically inactive role in the body into complex matrices that exhibit selective cell binding, promote proliferation and matrix production, and may ultimately become replaced by newly generated tissues in vivo. Functionalization of material surfaces with biomolecules is critical to their ability to evade immunorecognition, interact productively with surrounding tissues and extracellular matrix, and avoid bacterial colonization. Antibody molecules and their derived fragments are commonly immobilized on materials to mediate coating with specific cell types in fields such as stent endothelialization and drug delivery. The incorporation of growth factors into biomaterials has found application in promoting and accelerating bone formation in osteogenerative and related applications. Peptides and extracellular matrix proteins can impart biomolecule- and cell-specificities to materials while antimicrobial peptides have found roles in preventing biofilm formation on devices and implants. In this progress report, we detail developments in the use of diverse proteins and peptides to modify the surfaces of hard biomaterials in vivo and in vitro. Chemical approaches to immobilizing active biomolecules are presented, as well as platform technologies for isolation or generation of natural or synthetic molecules suitable for biomaterial functionalization.

Aqueous Fraction of Huogu Formula Promotes Osteogenic Differentiation of Bone Marrow Stromal Cells Through the BMP and Wnt Signaling Pathways

Our recent studies have shown that Huogu (HG) formula was effective both in clinic experience and in experimental osteonecrosis of the femoral head (ONFH). Given that defective of bone marrow stromal cells (MSCs) contribute to the development of osteonecrosis and MSCs show enormous potential in the treatment of ONFH, especially to aging people. How HG impacts the differentiation of MSCs and what is the underlying cellular and molecular mechanism remains largely unknown. Here, we found that an aqueous fraction of HG (HGA) significantly increased the alkaline phosphatase (ALP) activity, mineralized nodules, and migration of MSCs in a dose-dependent manner. Meanwhile, HGA could enhance the mRNA and protein expression of Runt-related transcription factor 2 (Runx2), Alp, Bmp2, osteocalcin (Ocn), and Osterix (Osx). Further investigation of the molecular mechanisms revealed that HGA treatment obviously increased expression, secretion, and activation of bone morphogenetic protein (BMP) 2 and β-catenin, two key regulators of the BMP or Wnt signaling pathway. Furthermore, osteogenic differentiation of MSCs could be blocked by using pharmacological inhibitors for these signaling pathways such as Noggin and Dkk-1. Besides, HGA could inhibit adipogenic differentiation of MSCs. Our study reveals that HGA promotes the osteogenesis of MSCs via the BMP and Wnt signaling pathways. Our findings provide mechanistic insights into the role of HG in treating ONFH.

Alginate microcapsules co-embedded with MSCs and anti-EGF mAb for the induction of hair cell-like cells in guinea pigs by taking advantage of host EGF

The strategy of co-embedding rBMSCs and anti-EGF mAb in alginate microcapsules is a promising modality for the regeneration of hair cell-like cells.

Silver nanoparticles promote osteogenesis of mesenchymal stem cells and improve bone fracture healing in osteogenesis mechanism mouse model

The potential use of osteo-conducive biomaterials in the promotion of bone fracture healing has attracted wide attention. This study investigated if silver nanoparticles (AgNps) could promote the proliferation and osteogenesis of mesenchymal stem cells (MSCs), and improve bone fracture healing. We showed that AgNps promoted MSCs' proliferation and osteogenic differentiation in vitro. Using a mouse femoral facture model, AgNps encapsulated in collagen promoted the formation of fracture callus, and induced early closure of the fracture gap. AgNps may promote the formation of the callus and the subsequent end joining of the fracture bone via multiple routes: (i) chemo-attraction of MSCs and fibroblasts to migrate to the fracture site; (ii) induction of the proliferation of MSCs; (iii) induction of osteogenic differentiation of MSCs via induction/activation of TGF-β/BMP signaling in MSCs. We concluded that AgNps might be beneficial as an adjunct treatment for bone fracture healing clinically.

FROM THE CLINICAL EDITOR

Silver nanoparticles are widely used in wound management in the clinical setting. In this article, the authors demonstrated a novel application in that these nanoparticles were efficient in promoting osteoblastic differentiation in both in-vitro and in-vivo studies. The findings may provide a new treatment direction for bone fracture in the future.

NELL-1 expression in tumors of cartilage

BACKGROUND/AIMS

NELL-1 is a novel osteochondral differentiation factor protein with increasing usage in tissue engineering. Previously, we reported the expression patterns of NELL-1 in bone-forming skeletal tumors. With increasing interest in the use of NELL-1 protein, we sought to examine the expression of NELL-1 in cartilage-forming tumors.

METHODS

Immunohistochemical expression was examined in human pathologic specimens.

RESULTS

Consistent NELL-1 overexpression across all cartilage-forming tumors was observed. Similar degrees of expression were observed in enchondroma, chondrosarcoma, and chondroblastic osteosarcoma. NELL-1 expression did not significantly vary by tumor grade.

CONCLUSION

In summary, NELL-1 demonstrates reliable and consistent expression across cartilage-forming skeletal tumors.

Biomechanical analysis of engineered bone with anti-BMP2 antibody immobilized on different scaffolds

Recently we have demonstrated the ability of monoclonal antibodies (mAb) specific for bone morphogenetic protein (BMP)-2 immobilized on different scaffolds to mediate bone formation, a process referred to as Antibody Mediated Osseous Regeneration (AMOR). One of the key properties of regenerated bone is its biomechanical strength, in particular in load-bearing areas. This study sought to test the hypothesis that the biomechanical strength of regenerated bone depends of the mode of regeneration, as well as the scaffold used. Four different scaffolds, namely titanium granules (Ti), alginate hydrogel, anorganic bovine bone mineral (ABBM), and absorbable collagen sponge (ACS) were functionalized with anti-BMP-2 or isotype control mAb and implanted into rat critical-size calvarial defects. The morphology, density and strength of the regenerated bone were evaluated after 8 weeks. Results demonstrated that scaffolds functionalized with anti-BMP-2 mAb exhibited varying degrees of bone volume and density. Ti and ABBM achieved the highest bone volume, density, and strength of bone. When anti-BMP-2 mAb was immobilized on Ti or ABBM, the strength of the regenerated bone were 80% and 77% of native bone respectively, compared with 60% of native bone in sites implanted with rh-BMP-2. Control interventions with isotype mAb did not promote considerable bone regeneration and exhibited significantly lower mechanical properties. SEM analysis showed specimens immobilized with anti-BMP-2 mAb formed new bone with organized structure bridging the crack areas. Altogether, the present data demonstrated that the morphological and mechanical properties of bone bioengineered through AMOR could approximate that of native bone, when appropriate scaffolds are used. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 104B: 1465-1473, 2016.

Effects of the orientation of anti-BMP2 monoclonal antibody immobilized on scaffold in antibody-mediated osseous regeneration

Recently, we have shown that anti-BMP2 monoclonal antibodies (mAbs) can trap endogenous osteogenic BMP ligands, which can in turn mediate osteodifferentiation of progenitor cells. The effectiveness of this strategy requires the availability of the anti-BMP-2 monoclonal antibodies antigen-binding sites for anti-BMP-2 monoclonal antibodies to bind to the scaffold through a domain that will leave its antigen-binding region exposed and available for binding to an osteogenic ligand. We examined whether antibodies bound to a scaffold by passive adsorption versus through Protein G as a linker will exhibit differences in mediating bone formation. In vitro anti-BMP-2 monoclonal antibodies was immobilized on absorbable collagen sponge (ACS) with Protein G as a linker to bind the antibody through its Fc region and implanted into rat calvarial defects. The biomechanical strength of bone regenerated by absorbable collagen sponge/Protein G/anti-BMP-2 monoclonal antibodies immune complex was compared to ACS/anti-BMP-2 monoclonal antibodies or ACS/Protein G/isotype mAb control group. Results demonstrated higher binding of anti-BMP-2 monoclonal antibodies/BMPs to C2C12 cells, when the mAb was initially attached to recombinant Protein G or Protein G-coupled microbeads. After eight weeks, micro-CT and histomorphometric analyses revealed increased bone formation within defects implanted with absorbable collagen sponge/Protein G/anti-BMP-2 monoclonal antibodies compared with defects implanted with absorbable collagen sponge/anti-BMP-2 monoclonal antibodies (p < 0.05). Confocal laser scanning microscopy (CLSM) confirmed increased BMP-2, -4, and -7 detection in sites implanted with absorbable collagen sponge/Protein G/anti-BMP-2 monoclonal antibodies in vivo. Biomechanical analysis revealed the regenerated bone in sites with Protein G/anti-BMP-2 monoclonal antibodies had higher mechanical strength in comparison to anti-BMP-2 monoclonal antibodies. The negative control group, Protein G/isotype mAb, did not promote bone regeneration and exhibited significantly lower mechanical properties (p < 0.05). Altogether, our results demonstrated that application of Protein G as a linker to adsorb anti-BMP-2 monoclonal antibodies onto the scaffold was accompanied by increased in vitro binding of the anti-BMP-2 mAb/BMP immune complex to BMP-receptor positive cell, as well as increased volume and strength of de novo bone formation in vivo.

NELL-1 expression in benign and malignant bone tumors

NELL-1 (NEL-like Protein 1) is an osteoinductive protein with increasing usage as a bone graft substitute in preclinical animal models. NELL-1 was first identified to have bone-forming properties by its overexpression in fusing cranial sutures. Since this time, addition of recombinant NELL-1 has been used to successfully induce bone formation in the calvarial, axial and appendicular skeleton. With increasing interest in the use of NELL-1 as a bone-graft substitute, we sought to examine the expression of NELL-1 in a wide spectrum of benign and malignant bone-forming skeletal tumors. Immunohistochemical expression was examined in human pathologic specimens. Quantitative RT-PCR evaluated NELL-1 expression among OS cell lines in vitro. Results showed NELL-1 expression in all bone tumors. Likewise, all OS cell lines demonstrated increased NELL-1 expression in comparison to non-lesional human bone marrow stromal cells. Among, benign bone tumors (osteoid osteoma and osteoblastoma), strong and diffuse staining was observed, which spatially correlated with markers of osteogenic differentiation. In contrast, a relative reduction in NELL-1 staining was observed in osteosarcoma, accompanied by increased variation between tumors. Among osteosarcoma specimens, NELL-1 expression did not correlate well with markers of osteogenic differentiation. Surprisingly, among osteosarcoma subtypes, fibroblastic osteosarcoma demonstrated the highest expression of NELL-1. In summary, NELL-1 demonstrates diffuse and reliable expression in benign but not malignant bone-forming skeletal tumors. Future studies will further define the basic biologic, diagnostic and prognostic importance of NELL-1 in bone neoplasms.

Application of AMOR in craniofacial rabbit bone bioengineering

Endogenous molecular and cellular mediators modulate tissue repair and regeneration. We have recently described antibody mediated osseous regeneration (AMOR) as a novel strategy for bioengineering bone in rat calvarial defect. This entails application of anti-BMP-2 antibodies capable of in vivo capturing of endogenous osteogenic BMPs (BMP-2, BMP-4, and BMP-7). The present study sought to investigate the feasibility of AMOR in other animal models. To that end, we examined the efficacy of a panel of anti-BMP-2 monoclonal antibodies (mAbs) and a polyclonal Ab immobilized on absorbable collagen sponge (ACS) to mediate bone regeneration within rabbit calvarial critical size defects. After 6 weeks, de novo bone formation was demonstrated by micro-CT imaging, histology, and histomorphometric analysis. Only certain anti-BMP-2 mAb clones mediated significant in vivo bone regeneration, suggesting that the epitopes with which anti-BMP-2 mAbs react are critical to AMOR. Increased localization of BMP-2 protein and expression of osteocalcin were observed within defects, suggesting accumulation of endogenous BMP-2 and/or increased de novo expression of BMP-2 protein within sites undergoing bone repair by AMOR. Considering the ultimate objective of translation of this therapeutic strategy in humans, preclinical studies will be necessary to demonstrate the feasibility of AMOR in progressively larger animal models.

Proresolving nanomedicines activate bone regeneration in periodontitis

Therapies to reverse tissue damage from osteolytic inflammatory diseases are limited by the inability of current tissue-engineering procedures to restore lost hard and soft tissues. There is a critical need for new therapeutics in regeneration. In addition to scaffolds, cells, and soluble mediators necessary for tissue engineering, control of endogenous inflammation is an absolute requirement for success. Although significant progress has been made in understanding natural resolution of inflammation pathways to limit uncontrolled inflammation in disease, harnessing the biomimetic properties of proresolving lipid mediators has not been demonstrated. Here, we report the use of nano-proresolving medicines (NPRM) containing a novel lipoxin analog (benzo-lipoxin A4, bLXA4) to promote regeneration of hard and soft tissues irreversibly lost to periodontitis in the Hanford miniature pig. In this proof-of-principle experiment, NPRM-bLXA4 dramatically reduced inflammatory cell infiltrate into chronic periodontal disease sites treated surgically and dramatically increased new bone formation and regeneration of the periodontal organ. These findings indicate that NPRM-bLXA4 is a mimetic of endogenous resolving mechanisms with potent bioactions that offers a new therapeutic tissue-engineering approach for the treatment of chronic osteolytic inflammatory diseases.

Roles of bone morphogenetic protein signaling in osteosarcoma

PURPOSE

Since the original extraction of bone morphogenetic proteins (BMPs) from bovine bone, research interest and clinical use has increased exponentially. With this, a concomitant analysis of BMP expression in bone tumours has been performed. BMP ligands, receptors, and signaling activity have been observed in diverse benign and malignant bone tumours. However, the reported expression, function, and importance of BMPs in bone tumours, and specifically osteosarcomas, have been far from uniform. This review highlights recent advances in understanding the role of BMP signaling in osteosarcoma biology, focusing on the sometimes divergent findings by various researchers and the challenges inherent in the study of osteosarcoma.

METHODS

We performed a literature review of all studies examining BMP signaling in osteosarcoma.

RESULTS

Overall, multiple BMP ligands and receptors are expressed in most osteosarcoma cell lines and subtypes, although BMP signaling may be reduced in comparison with benign bone-forming tumours. Studies suggest that osteosarcomas with different lineages of differentiation may have differential expression of BMP ligands. Although significant disagreement in the literature exists, the presence of BMP signaling in osteosarcoma may impart a worse prognosis. On the cellular level, BMP signaling appears to mediate promigratory effects in osteosarcoma and chondrosarcoma cell types, possibly via interaction and activation of Integrin β1.

CONCLUSIONS

BMP signaling has clear biologic importance in osteosarcoma, although it is not yet fully understood. Future questions for study include assessing the utility of BMP signaling in prognostication of osteosarcoma and the potential modulation of BMP signaling for inhibition of osteosarcomagenesis, growth and invasion.

Immobilization of murine anti-BMP-2 monoclonal antibody on various biomaterials for bone tissue engineering

Biomaterials are widely used as scaffolds for tissue engineering. We have developed a strategy for bone tissue engineering that entails application of immobilized anti-BMP-2 monoclonal antibodies (mAbs) to capture endogenous BMPs in vivo and promote antibody-mediated osseous regeneration (AMOR). The purpose of the current study was to compare the efficacy of immobilization of a specific murine anti-BMP-2 mAb on three different types of biomaterials and to evaluate their suitability as scaffolds for AMOR. Anti-BMP-2 mAb or isotype control mAb was immobilized on titanium (Ti) microbeads, alginate hydrogel, and ACS. The treated biomaterials were surgically implanted in rat critical-sized calvarial defects. After 8 weeks, de novo bone formation was assessed using micro-CT and histomorphometric analyses. Results showed de novo bone regeneration with all three scaffolds with immobilized anti-BMP-2 mAb, but not isotype control mAb. Ti microbeads showed the highest volume of bone regeneration, followed by ACS. Alginate showed the lowest volume of bone. Localization of BMP-2, -4, and -7 antigens was detected on all 3 scaffolds with immobilized anti-BMP-2 mAb implanted in calvarial defects. Altogether, these data suggested a potential mechanism for bone regeneration through entrapment of endogenous BMP-2, -4, and -7 proteins leading to bone formation using different types of scaffolds via AMOR.

Functionalization of scaffolds with chimeric anti-BMP-2 monoclonal antibodies for osseous regeneration

Recent studies have demonstrated the ability of murine anti-BMP-2 monoclonal antibodies (mAb) immobilized on an absorbable collagen sponge (ACS) to mediate de novo bone formation, a process termed antibody-mediated osseous regeneration (AMOR). The objectives of this study were to assess the efficacy of a newly generated chimeric anti-BMP-2 mAb in mediating AMOR, as well as to evaluate the suitability of different biomaterials as scaffolds to participate in AMOR. Chimeric anti-BMP-2 mAb was immobilized on 4 biomaterials, namely, titanium microbeads (Ti), alginate hydrogel, macroporous biphasic calcium phosphate (MBCP) and ACS, followed by surgical implantation into rat critical-size calvarial defects. Animals were sacrificed after 8 weeks and the degree of bone fill was assessed using micro-CT and histomorphometry. Results demonstrated local persistence of chimeric anti-BMP-2 mAb up to 8 weeks, as well as significant de novo bone regeneration in sites implanted with chimeric anti-BMP-2 antibody immobilized on each of the 4 scaffolds. Ti and MBCP showed the highest volume of bone regeneration, presumably due to their resistance to compression. Alginate and ACS also mediated de novo bone formation, though significant volumetric shrinkage was noted. In vitro assays demonstrated cross-reactivity of chimeric anti-BMP-2 mAb with BMP-4 and BMP-7. Immune complex of anti-BMP-2 mAb with BMP-2 induced osteogenic differentiation of C2C12 cells in vitro, involving expression of RUNX2 and phosphorylation of Smad1. The present data demonstrated the ability of chimeric anti-BMP-2 mAb to functionalize different biomaterial with varying characteristics to mediate osteogenesis.

Co-encapsulation of anti-BMP2 monoclonal antibody and mesenchymal stem cells in alginate microspheres for bone tissue engineering

Recently, it has been shown that tethered anti-BMP2 monoclonal antibodies (mAbs) can trap BMP ligands and thus provide BMP inductive signals for osteo-differentiation of progenitor cells. The objectives of this study were to: (1) develop a co-delivery system based on murine anti-BMP2 mAb-loaded alginate microspheres encapsulating human bone marrow mesenchymal stem cells (hBMMSCs); and (2) investigate osteogenic differentiation of encapsulated stem cells in alginate microspheres in vitro and in vivo. Alginate microspheres of 1 ± 0.1 mm diameter were fabricated with 2 × 10(6) hBMMSCs per mL of alginate. Critical-size calvarial defects (5 mm diameter) were created in immune-compromised mice and alginate microspheres preloaded with anti-BMP mAb encapsulating hBMMSCs were transplanted into defect sites. Alginate microspheres pre-loaded with isotype-matched non-specific antibody were used as the negative control. After 8 weeks, micro CT and histologic analyses were used to analyze bone formation. In vitro analysis demonstrated that anti-BMP2 mAbs tethered BMP2 ligands that can activate the BMP receptors on hBMMSCs. The co-delivery system described herein, significantly enhanced hBMMSC-mediated osteogenesis, as confirmed by the presence of BMP signal pathway-activated osteoblast determinants Runx2 and ALP. Our results highlight the importance of engineering the microenvironment for stem cells, and particularly the value of presenting inductive signals for osteo-differentiation of hBMMSCs by tethering BMP ligands using mAbs. This strategy of engineering the microenvironment with captured BMP signals is a promising modality for repair and regeneration of craniofacial, axial and appendicular bone defects.

Antibody-mediated osseous regeneration: the early events in the healing response

Bone engineering strategies often exploit modulation of the extracellular environment, including delivery of cell and growth factors to repair and regenerate damaged tissues. During bone healing, the expression of endogenous bone morphogenetic proteins is an essential component of the healing response. However, in some situations, the inherent reparative capacity available in the local microenvironment is exceeded by the requirements of the defects. We have recently reported on a novel strategy, that exploits the specificity of antibodies to capture and make available endogenous osteogenic growth factors, referred to as "antibody-mediated osseous regeneration" (AMOR). The objective of the present study was to identify some of the cellular and molecular events involved in AMOR in an effort to begin to elucidate the mechanism of AMOR. The rat critical-sized calvarial defect model was used, where anti-bone morphogenetic protein (BMP)-2 monoclonal antibody (mAb), isotype-control mAb, or recombinant human (rh)BMP-2 were immobilized on absorbable collagen calvarial sponge (ACS) by adsorption, and then implanted into calvarial defects. The results demonstrated persistence of implanted mAbs for short term from 1 to 2 weeks after implantation. Increased cell infiltration was found in defects treated with anti-BMP-2 mAb. Examination of proteins on ACS scaffolds retrieved from defect sites demonstration increased levels of BMP-2, BMP-4, and BMP-7 proteins in sites implanted with anti-BMP-2 mAb. Moreover, BMP-2, BMP-4, and BMP-7 gene expression levels were increased in sites implanted with anti-BMP-2 mAb. Micro-computed tomography and histological analysis demonstrated that the bone within calvarial defects was fully regenerated in sites implanted with either anti-BMP-2 mAb or rhBMP-2. However, rhBMP-2-regenerated bone exhibited aberrant histomorphology with dystrophic calcification and invasion of subjacent areas. Altogether, the results revealed evidence for anti-BMP-2 mAbs to form an immune complex with BMP-2, BMP-4, and BMP-7, and bind to cells to mediate osteogenesis bone regeneration in vivo. This approach suggests a significant role for antibodies in regenerative orthopedic medicine.

Review of Signaling Pathways Governing MSC Osteogenic and Adipogenic Differentiation

Mesenchymal stem cells (MSC) are multipotent cells, functioning as precursors to a variety of cell types including adipocytes, osteoblasts, and chondrocytes. Between osteogenic and adipogenic lineage commitment and differentiation, a theoretical inverse relationship exists, such that differentiation towards an osteoblast phenotype occurs at the expense of an adipocytic phenotype. This balance is regulated by numerous, intersecting signaling pathways that converge on the regulation of two main transcription factors: peroxisome proliferator-activated receptor- γ (PPAR γ ) and Runt-related transcription factor 2 (Runx2). These two transcription factors, PPAR γ and Runx2, are generally regarded as the master regulators of adipogenesis and osteogenesis. This review will summarize signaling pathways that govern MSC fate towards osteogenic or adipocytic differentiation. A number of signaling pathways follow the inverse balance between osteogenic and adipogenic differentiation and are generally proosteogenic/antiadipogenic stimuli. These include β -catenin dependent Wnt signaling, Hedgehog signaling, and NELL-1 signaling. However, other signaling pathways exhibit more context-dependent effects on adipogenic and osteogenic differentiation. These include bone morphogenic protein (BMP) signaling and insulin growth factor (IGF) signaling, which display both proosteogenic and proadipogenic effects. In summary, understanding those factors that govern osteogenic versus adipogenic MSC differentiation has significant implications in diverse areas of human health, from obesity to osteoporosis to regenerative medicine.