Deciphering the combined effect of bone morphogenetic protein 6 and calcium phosphate on bone formation capacity of periosteum derived cells-based tissue engineering constructs

PI3K/AKT/mTOR signaling regulates BCP ceramic-induced osteogenesis

An increasing number of studies demonstrate that biphasic calcium phosphate (BCP) ceramics can induce bone regeneration. However, the underlying molecular mechanisms involved are still poorly understood. This work was proposed to investigate how PI3K/AKT/mTOR signaling influenced the osteogenesis mediated by BCP ceramics. The results showed that incubation with BCP ceramics promoted the proliferation of murine bone marrow-derived mesenchymal stem cells (BMSCs) in a time-dependent manner. The resulting cell proliferation was then suppressed by the selective inhibition of either PI3K, AKT, or mTOR signaling activation. Next, we confirmed that BCP ceramics up-regulated the phosphorylation levels of AKT and mTOR in BMSCs, suggesting the ability of BCP ceramics to drive the activation of PI3K/AKT/mTOR signaling in BMSCs. Furthermore, the blockade of PI3K/AKT/mTOR signaling prevented BCP ceramics-induced osteogenic differentiation and pro-angiogenesis of BMSCs by down-regulating the expression of genes encoding OPN, RUNX2 or VEGF. Moreover, the PI3K/AKT/mTOR signaling blockade suppressed stem cell infiltration and new bone formation in the implants following intra-muscular implantation of BCP ceramics in mice. Therefore, our results suggested that PI3K/AKT/mTOR signaling played a critical regulatory role in BCP ceramic-induced osteogenesis.

An Empirical Model Linking Physico-Chemical Biomaterial Characteristics to Intra-Oral Bone Formation

Facial trauma, bone resection due to cancer, periodontal diseases, and bone atrophy following tooth extraction often leads to alveolar bone defects that require bone regeneration in order to restore dental function. Guided bone regeneration using synthetic biomaterials has been suggested as an alternative approach to autologous bone grafts. The efficiency of bone substitute materials seems to be influenced by their physico-chemical characteristics; however, the debate is still ongoing on what constitutes optimal biomaterial characteristics. The purpose of this study was to develop an empirical model allowing the assessment of the bone regeneration potential of new biomaterials on the basis of their physico-chemical characteristics, potentially giving directions for the design of a new generation of dental biomaterials. A quantitative data set was built composed of physico-chemical characteristics of seven commercially available intra-oral bone biomaterials and their in vivo response. This empirical model allowed the identification of the construct parameters driving optimized bone formation. The presented model provides a better understanding of the influence of driving biomaterial properties in the bone healing process and can be used as a tool to design bone biomaterials with a more controlled and custom-made composition and structure, thereby facilitating and improving the clinical translation.

A New Osteogenic Membrane to Enhance Bone Healing: At the Crossroads between the Periosteum, the Induced Membrane, and the Diamond Concept

The lack of viability of massive bone allografts for critical-size bone defect treatment remains a challenge in orthopedic surgery. The literature has reviewed the advantages of a multi-combined treatment with the synergy of an osteoconductive extracellular matrix (ECM), osteogenic stem cells, and growth factors (GFs). Questions are still open about the need for ECM components, the influence of the decellularization process on the latter, the related potential loss of function, and the necessity of using pre-differentiated cells. In order to fill in this gap, a bone allograft surrounded by an osteogenic membrane made of a decellularized collagen matrix from human fascia lata and seeded with periosteal mesenchymal stem cells (PMSCs) was analyzed in terms of de-/recellularization, osteogenic properties, PMSC self-differentiation, and angiogenic potential. While the decellularization processes altered the ECM content differently, the main GF content was decreased in soft tissues but relatively increased in hard bone tissues. The spontaneous osteogenic differentiation was necessarily obtained through contact with a mineralized bone matrix. Trying to deepen the knowledge on the complex matrix-cell interplay could further propel these tissue engineering concepts and lead us to provide the biological elements that allow bone integration in vivo.

Periosteum and fascia lata: Are they so different?

Introduction: The human fascia lata (HFL) is used widely in reconstructive surgery in indications other than fracture repair. The goal of this study was to compare microscopic, molecular, and mechanical properties of HFL and periosteum (HP) from a bone tissue engineering perspective.

Material and Methods: Cadaveric HP and HFL (N = 4 each) microscopic morphology was characterized using histology and immunohistochemistry (IHC), and the extracellular matrix (ECM) ultrastructure assessed by means of scanning electron microscopy (SEM). DNA, collagen, elastin, glycosaminoglycans, major histocompatibility complex Type 1, and bone morphogenetic protein (BMP) contents were quantified. HP (N = 6) and HFL (N = 11) were submitted to stretch tests.

Results: Histology and IHC highlighted similarities (Type I collagen fibers and two-layer organization) but also differences (fiber thickness and compaction and cell type) between both tissues, as confirmed using SEM. The collagen content was statistically higher in HFL than HP (735 vs. 160.2 μg/mg dry weight, respectively, p < 0.0001). On the contrary, DNA content was lower in HFL than HP (404.75 vs. 1,102.2 μg/mg dry weight, respectively, p = 0.0032), as was the immunogenic potential (p = 0.0033). BMP-2 and BMP-7 contents did not differ between both tissues (p = 0.132 and p = 0.699, respectively). HFL supported a significantly higher tension stress than HP.

Conclusion: HP and HFL display morphological differences, despite their similar molecular ECM components. The stronger stretching resistance of HFL can specifically be explained by its higher collagen content. However, HFL contains many fewer cells and is less immunogenic than HP, as latter is rich in periosteal stem cells. In conclusion, HFL is likely suitable to replace HP architecture to confer a guide for bone consolidation, with an absence of osteogenicity. This study could pave the way to a bio-engineered periosteum built from HFL.

Progress of Periosteal Osteogenesis: The Prospect of In Vivo Bioreactor

Repairing large segment bone defects is still a clinical challenge. Bone tissue prefabrication shows great translational potentials and has been gradually accepted clinically. Existing bone reconstruction strategies, including autologous periosteal graft, allogeneic periosteal transplantation, xenogeneic periosteal transplantation, and periosteal cell tissue engineering, are all clinically valuable treatments and have made significant progress in research. Herein, we reviewed the research progress of these techniques and briefly explained the relationship among in vivo microenvironment, mechanical force, and periosteum osteogenesis. Moreover, we also highlighted the importance of the critical role of periosteum in osteogenesis and explained current challenges and future perspective.

Advanced Strategies of Biomimetic Tissue-Engineered Grafts for Bone Regeneration

The failure to repair critical-sized bone defects often leads to incomplete regeneration or fracture non-union. Tissue-engineered grafts have been recognized as an alternative strategy for bone regeneration due to their potential to repair defects. To design a successful tissue-engineered graft requires the understanding of physicochemical optimization to mimic the composition and structure of native bone, as well as the biological strategies of mimicking the key biological elements during bone regeneration process. This review provides an overview of engineered graft-based strategies focusing on physicochemical properties of materials and graft structure optimization from macroscale to nanoscale to further boost bone regeneration, and it summarizes biological strategies which mainly focus on growth factors following bone regeneration pattern and stem cell-based strategies for more efficient repair. Finally, it discusses the current limitations of existing strategies upon bone repair and highlights a promising strategy for rapid bone regeneration.

Bone Morphogenetic Proteins, Carriers, and Animal Models in the Development of Novel Bone Regenerative Therapies

Bone morphogenetic proteins (BMPs) possess a unique ability to induce new bone formation. Numerous preclinical studies have been conducted to develop novel, BMP-based osteoinductive devices for the management of segmental bone defects and posterolateral spinal fusion (PLF). In these studies, BMPs were combined with a broad range of carriers (natural and synthetic polymers, inorganic materials, and their combinations) and tested in various models in mice, rats, rabbits, dogs, sheep, and non-human primates. In this review, we summarized bone regeneration strategies and animal models used for the initial, intermediate, and advanced evaluation of promising therapeutical solutions for new bone formation and repair. Moreover, in this review, we discuss basic aspects to be considered when planning animal experiments, including anatomical characteristics of the species used, appropriate BMP dosing, duration of the observation period, and sample size.

Biology of bone morphogenetic protein in bone repair and regeneration: A role for autologous blood coagulum as carrier

BMPs were purified from demineralized bone matrix based on their ability to induce new bone in vivo and they represent a large member of the TGF-β superfamily of proteins. BMPs serve as morphogenic signals for mesenchymal stem cell migration, proliferation and subsequently differentiation into cartilage and bone during embryonic development. A BMP when implanted with a collagenous carrier in a rat subcutaneous site is capable of inducing new bone by mimicking the cellular events of embryonic bone formation. Based on this biological principle, BMP2 and BMP7 containing collagenous matrix as carrier have been developed as bone graft substitutes for spine fusion and long bone fractures. Here, we describe a novel autologous bone graft substitute that contains BMP6 delivered within an autologous blood coagulum as carrier and summarize the biology of osteogenic BMPs in the context of bone repair and regeneration specifically the critical role that carrier plays to support osteogenesis.

Autologous bone graft substitute containing rhBMP6 within autologous blood coagulum and synthetic ceramics of different particle size determines the quantity and structural pattern of bone formed in a rat subcutaneous assay

Bone morphogenetic proteins (BMPs) are potent osteoinductive agents for bone tissue engineering. In order to define optimal properties of a novel autologous bone graft substitute (ABGS) containing rhBMP6 within the autologous blood coagulum (ABC) and ceramic particles as a compression resistant matrix (CRM), we explored the influence of their amount, chemical composition and particle size on the quantity and quality of bone formation in the rat subcutaneous assay. Tested ceramic particles included tricalcium phosphate (TCP), hydroxyapatite (HA) and biphasic calcium phosphate ceramic (BCP), containing TCP and HA in 80/20 ratio of different particle sizes (small 74-420 μm, medium 500-1700 μm and large 1000-4000 μm). RhBMP6 was either mixed with ABC or lyophilized on CRM prior to use with ABC. The experiments were terminated on day 21 and implants were analysed by microCT, histology and histomorphometry. Addition of CRM to ABGS containing rhBMP6 in ABC significantly increased the amount of newly formed bone and the optimal CRM/ABC ratio was found to be around 100 mg/500 μL. MicroCT analyses revealed that all tested ABGS formulations induced an extensive new bone formation and there were no differences between the two methods of rhBMP6 application as determined by the bone volume. However, the particle size played a significant role in the quantity and quality of newly formed bone. ABGS containing small particles induced new bone forming a dense trabecular network, cortical bone at the rim, bone and bone marrow in apposition to and in between ceramic particles. ABGS containing medium and large particles also resulted in new bone on the surface of particles as well as inside the pores. Histomorphometric analysis revealed that the ceramics particle size correlated with the quality of trabecular pattern of newly formed bone, bone/bone marrow ratio as observed in apposition and between particles, and the ratio between the cortical and trabecular bone. By employing rat subcutaneous implant assay, we showed for the first time that the size of synthetic ceramics particles affected the osteogenesis as defined by both the quantity and quality of ectopic bone.

The Bone-Forming Properties of Periosteum-Derived Cells Differ Between Harvest Sites

The development of alternatives for autologous bone grafts is a major focus of bone tissue engineering. To produce living bone-forming implants, skeletal stem and progenitor cells (SSPCs) are envisioned as key ingredients. SSPCs can be obtained from different tissues including bone marrow, adipose tissue, dental pulp, and periosteum. Human periosteum-derived cells (hPDCs) exhibit progenitor cell characteristics and have well-documented in vivo bone formation potency. Here, we have characterized and compared hPDCs derived from tibia with craniofacial hPDCs, from maxilla and mandible, respectively, each representing a potential source for cell-based tissue engineered implants for craniofacial applications. Maxilla and mandible-derived hPDCs display similar growth curves as tibial hPDCs, with equal trilineage differentiation potential toward chondrogenic, osteogenic, and adipogenic cells. These craniofacial hPDCs are positive for SSPC-markers CD73, CD164, and Podoplanin (PDPN), and negative for CD146, hematopoietic and endothelial lineage markers. Bulk RNA-sequencing identified genes that are differentially expressed between the three sources of hPDC. In particular, differential expression was found for genes of the HOX and DLX family, for SOX9 and genes involved in skeletal system development. The in vivo bone formation, 8 weeks after ectopic implantation in nude mice, was observed in constructs seeded with tibial and mandibular hPDCs. Taken together, we provide evidence that hPDCs show different profiles and properties according to their anatomical origin, and that craniofacial hPDCs are potential sources for cell-based bone tissue engineering strategies. The mandible-derived hPDCs display - both in vitro and in vivo - chondrogenic and osteogenic differentiation potential, which supports their future testing for use in craniofacial bone regeneration applications.

Heparan sulfate co-immobilized with cRGD ligands and BMP2 on biomimetic platforms promotes BMP2-mediated osteogenic differentiation

The chemical and physical properties of the extracellular matrix (ECM) are known to be fundamental for regulating growth factor bioactivity. The role of heparan sulfate (HS), a glycosaminoglycan, and of cell adhesion proteins (containing the cyclic RGD (cRGD) ligands) on bone morphogenetic protein 2 (BMP2)-mediated osteogenic differentiation has not been fully explored. In particular, it is not known whether and how their effects can be potentiated when they are presented in controlled close proximity, as in the ECM. Here, we developed streptavidin platforms to mimic selective aspects of the in vivo presentation of cRGD, HS and BMP2, with a nanoscale-control of their surface density and orientation to study cell adhesion and osteogenic differentiation. We showed that whereas a controlled increase in cRGD surface concentration upregulated BMP2 signaling due to β₃ integrin recruitment, silencing either β₁ or β₃ integrins negatively affected BMP2-mediated phosphorylation of SMAD1/5/9 and alkaline phosphatase expression. Furthermore, the presence of adsorbed BMP2 promoted cellular adhesion at very low cRGD concentrations. Finally, we proved that HS co-immobilized with cRGD both sustained BMP2 signaling and enhanced osteogenic differentiation compared to BMP2 directly immobilized on streptavidin, even with a low cRGD surface concentration. Altogether, our results show that HS facilitated and sustained the synergy between BMP2 and integrin pathways and that the co-immobilization of HS and cRGD peptides optimised BMP2-mediated osteogenic differentiation. Statement of significance The growth factor BMP2 is used to treat large bone defects. Previous studies have shown that the presentation of BMP2 via extracellular matrix molecules, such as heparan sulfate (HS), can upregulate BMP2 signaling. The potential advantages of dose reduction and local specificity have stimulated interest in further investigations into biomimetic approaches. We designed a streptavidin model surface eligible for immobilizing tunable amounts of molecules from the extracellular space, such as HS, adhesion motifs (cyclic RGD) and BMP2. By studying cellular adhesion, BMP2 bioactivity and its osteogenic potential we reveal the combined effect of integrins, HS and BMP2, which contribute in answering fundamental questions regarding cell-matrix interaction.

A cell-based combination product for the repair of large bone defects

Regenerative cell-based implants using periosteum-derived stem cells were developed for the treatment of large 3 cm fresh and 4.5 centimeter biological compromised bone gaps in a tibial sheep model and compared with an acellular ceramic-collagen void filler. It was hypothesized that the latter is insufficient to heal large skeletal defects due to reduced endogenous biological potency. To this purpose a comparison was made between the ceramic dicalciumphosphate scaffold (CopiOs®) as such, the same ceramic coated with clinical grade Bone Morphogenetic Protein 2 and 6 (BMP) only or a BMP coated cell-seeded combination product. These implants were evaluated in 2 sheep models, a fresh 3 cm critical size tibial defect and a 4.5 cm biologically exhausted tibial defect. For the groups in which growth factors were applied, BMP-6 was chosen at a dose of 344 μg for 3 cm and 1.500 μg or 3.800 μg for 4.5 cm defects. An additional group in the 4.5 cm defect was tested using BMP-2 in a dose of 1.500 μg. For all the cell based implants autologous periosteum-derived cells were used which were cultured in monolayer during 6 weeks. For the fresh defect 408 million cells and for the biologically exhausted tibial defect 612 million cells were drop-seeded on the BMP coated scaffolds. Bone healing was studied during 16 weeks postimplantation, using standard radiographs. While fresh defects responded to all treatments, regardless the use of cells, the biologically hampered defects responded in half of the cases and only if the BMP-cell combination product was used, supporting the concept that cell-based therapies may become attractive in treating defects with a compromised biological status.

Msx2 plays an important role in BMP6-induced osteogenic differentiation of two mesenchymal cell lines: C3H10T1/2 and C2C12

Bone morphogenetic proteins (BMPs), have been shown to enhance the osteogenic differentiation of mesenchymal cells (MCs) and to promote bone formation. BMP6 is known to play an important role in the process of MCs towards osteogenic differentiation by virtue of their osteoinductive and cell type specific proliferative activity. However, the molecular mechanism relate to BMP6 osteoinductive activity is still unclear and continues to warrant further investigation. Msx2 is a member of the homeobox gene family of transcription factors and promotes calcification. Hence, we wondered if it might also play a role in BMP6-induced osteogenesis. In this study, two mouse mesenchymal cell lines were treated with BMP6, adenovirus-Msx2 (Ad-Msx2) or adenovirus-siMsx2 (Ad-siMsx2). Based on the results of mRNA and protein expression, it was indicated that BMP6 could enhance the expression of Msx2 and activate the phosphorylation of Smad 1/5/8, p38 and ERK1/2. Being transfected by Ad-Msx2, the BMP6-induced activation of phosphorylation was significantly promoted. On the contrary, two cell lines transfected by Ad-siMsx2 presented an inhibited expression of three phosphorylated proteins even after being induced by BMP6. The evaluation of ALP, OPN, OC and calcium deposits revealed the osteogenic results those were corresponding to the results of mRNA and protein. Taken together, these findings can be a novel viewpoint for the understanding of the mechanisms of BMP6-induced osteogenesis and provide therapeutic targets of bone defect.

Synthesis and Evaluation of BMMSC-seeded BMP-6/nHAG/GMS Scaffolds for Bone Regeneration

Bioactive scaffolding materials and efficient osteoinductive factors are key factors for bone tissue engineering. The present study aimed to mimic the natural bone repair process using an osteoinductive bone morphogenetic protein (BMP)-6-loaded nano-hydroxyapatite (nHA)/gelatin (Gel)/gelatin microsphere (GMS) scaffold pre-seeded with bone marrow mesenchymal stem cells (BMMSCs). BMP-6-loaded GMSs were prepared by cross-linking and BMP-6/nHAG/GMS scaffolds were fabricated by a combination of blending and freeze-drying techniques. Scanning electron microscopy, confocal laser scanning microscopy, and CCK-8 assays were carried out to determine the biocompatibility of the composite scaffolds in vitro. Alkaline phosphatase (ALP) activity was measured to evaluate the osteoinductivity of the composite scaffolds. For in vivo examination, critical-sized calvarial bone defects in Sprague-Dawley rats were randomly implanted with BMMSC/nHAG/GMS and BMMSC/BMP-6/nHAG/GMS scaffolds, and compared with a control group with untreated empty defects. The BMP-6-loaded scaffolds showed cytocompatibility by favoring BMMSC attachment, proliferation, and osteogenic differentiation. In radiological and histological analyses, the BMMSC-seeded scaffolds, especially the BMMSC-seeded BMP-6/nHAG/GMS scaffolds, significantly accelerated new bone formation. It is concluded that the BMP-6/nHAG/GMS scaffold possesses excellent biocompatibility and good osteogenic induction activity in vitro and in vivo, and could be an ideal bioactive substitute for bone tissue engineering.