Injectable BMP-2 delivery system based on collagen-derived microspheres and alginate induced bone formation in a time- and dose-dependent manner

An Innovative Design to Enhance Osteoinductive Efficacy and Biomechanical Behavior of a Titanium Dental Implant

The present study investigated the in vivo bone-forming efficacy of an innovative titanium (Ti) dental implant combined with a collagen sponge containing recombinant human bone morphogenetic protein-2 (BMP-2) in a pig model. Two different concentrations of BMP-2 (20 and 40 µg/mL) were incorporated into collagen sponges and placed at the bottom of Ti dental implants. The investigated implants were inserted into the edentulous ridge at the canine-premolar regions of Lanyu small-ear pigs, which were then euthanized at weeks 1, 2, 4, 8, and 12 post-implantation. Specimens containing the implants and surrounding bone tissue were collected for histological evaluation of their bone-to-implant contact (BIC) ratios and calculation of maximum torques using removal torque measurement. Analytical results showed that the control and BMP-2-loaded implants presented good implant stability and bone healing for all testing durations. After 1 week of healing, the BMP-2-loaded implants with a concentration of 20 µg/mL exhibited the highest BIC ratios, ranging from 58% to 76%, among all groups (p = 0.034). Additionally, they also possessed the highest removal torque values (50.1 ± 1.3 N-cm) throughout the 8-week healing period. The BMP-2-loaded implants not only displayed excellent in vivo biocompatibility but also presented superior osteoinductive performance. Therefore, these findings demonstrate that BMP-2 delivered through a collagen sponge can potentially enhance the early-stage osseointegration of Ti dental implants.

Repair of Rat Calvarial Critical-Sized Defects Using Heparin-Conjugated Fibrin Hydrogel Containing BMP-2 and Adipose-Derived Pericytes

The repair of critical-sized calvarial defects is a challenging problem for orthopedic surgery. One of the promising strategies of bone bioengineering to enhance the efficacy of large bone defect regeneration is the combined delivery of stem cells with osteoinductive factors within polymer carriers. The purpose of the research was to study the regenerative effects of heparin-conjugated fibrin (HCF) hydrogel containing bone morphogenetic protein 2 (BMP-2) and adipose-derived pericytes (ADPs) in a rat critical-sized calvarial defect model. In vitro analysis revealed that the HCF hydrogel was able to control the BMP-2 release and induce alkaline phosphatase (ALP) activity in neonatal rat osteoblasts. In addition, it was found that eluted BMP-2 significantly induced the osteogenic differentiation of ADPs. It was characterized by the increased ALP activity, osteocalcin expression and calcium deposits in ADPs. In vivo studies have shown that both HCF hydrogel with BMP-2 and HCF hydrogel with pericytes are able to significantly increase the regeneration of critical-sized calvarial defects in comparison with the control group. Nevertheless, the greatest regenerative effect was found after the co-delivery of ADPs and BMP-2 into a critical-sized calvarial defect. Thus, our findings suggest that the combined delivery of ADPs and BMP-2 in HCF hydrogel holds promise to be applied as an alternative biopolymer for the critical-sized bone defect restoration.

The Therapeutic Potential of Pericytes in Bone Tissue Regeneration

Pericytes, as perivascular cells, are present in all vascularized organs and tissues, and they actively interact with endothelial cells in capillaries and microvessels. Their involvement includes functions like blood pressure regulation, tissue regeneration, and scarring. Studies have confirmed that pericytes play a crucial role in bone tissue regeneration through direct osteodifferentiation processes, paracrine actions, and vascularization. Recent preclinical and clinical experiments have shown that combining perivascular cells with osteogenic factors and tissue-engineered scaffolds can be therapeutically effective in restoring bone defects. This approach holds promise for addressing bone-related medical conditions. In this review, we have emphasized the characteristics of pericytes and their involvement in angiogenesis and osteogenesis. Furthermore, we have explored recent advancements in the use of pericytes in preclinical and clinical investigations, indicating their potential as a therapeutic resource in clinical applications.

Calcium phosphate ceramics combined with rhBMP6 within autologous blood coagulum promote posterolateral lumbar fusion in sheep

Posterolateral spinal fusion (PLF) is a procedure used for the treatment of degenerative spine disease. In this study we evaluated Osteogrow-C, a novel osteoinductive device comprised of recombinant human Bone morphogenetic protein 6 (rhBMP6) dispersed in autologous blood coagulum with synthetic ceramic particles, in the sheep PLF model. Osteogrow-C implants containing 74-420 or 1000-1700 µm ceramic particles (TCP/HA 80/20) were implanted between L4-L5 transverse processes in sheep (Ovis Aries, Merinolaandschaf breed). In the first experiment (n = 9 sheep; rhBMP6 dose 800 µg) the follow-up period was 27 weeks while in the second experiment (n = 12 sheep; rhBMP6 dose 500 µg) spinal fusion was assessed by in vivo CT after 9 weeks and at the end of the experiment after 14 (n = 6 sheep) and 40 (n = 6 sheep) weeks. Methods of evaluation included microCT, histological analyses and biomechanical testing. Osteogrow-C implants containing both 74-420 and 1000-1700 µm ceramic particles induced radiographic solid fusion 9 weeks following implantation. Ex-vivo microCT and histological analyses revealed complete osseointegration of newly formed bone with adjacent transverse processes. Biomechanical testing confirmed that fusion between transverse processes was complete and successful. Osteogrow-C implants induced spinal fusion in sheep PLF model and therefore represent a novel therapeutic solution for patients with degenerative disc disease.

Pilot Study on Percutaneous Delivery of Recombinant Human Bone Morphongenetic Protein-2 Augments Fusion in a Nicotine-impaired Rabbit Fusion Model

STUDY DESIGN

A nicotine-impaired spinal fusion rabbit model.

OBJECTIVE

To examine whether controlled delivery of morselized absorbable collagen sponge recombinant human bone morphogenetic protein-2 (rhBMP2) in a delayed manner postsurgery would allow for improved bone healing.

SUMMARY OF BACKGROUND DATA

The current delivery method of rhBMP-2 during surgery causes a burst of rhBMP-2, which is not sustained. Given that bone morphogenetic protein 2 (BMP-2) expression peaks later in the fusion process, there may be the benefit of delivery of rhBMP-2 later in the healing process.

METHODS

Sixteen male 1-year-old rabbits underwent a posterolateral spinal fusion with iliac crest bone graft at L5-L6 while being given nicotine to prevent spinal fusion as previously published. Eight were controls, whereas 8 had morselized rhBMP-2 (4.2 mg) injected at the fusion site at 4 weeks postoperatively. Histologic, radiologic, and palpation examinations were performed at 12 weeks to determine fusion status and the volume of bone formed. Hematoxylin and eosin stains were used for histology. A Student t test was used to compare the computed tomography scan measured volume of bone created between the control cohort (CC) and rhBMP-2 delayed delivery cohort (BMP-DDC).

RESULTS

Of the total, 7/8 rabbits in the BMP-DDC and 5/8 rabbits in the CC formed definitive fusion with a positive palpation examination, bridging bone between transverse processes on computed tomography scan, and an x-ray showing fusion. Histologic analysis revealed newly remodeled bone within the BMP-DDC. There was an increased average volume of bone formed within the BMP-DDC versus the CC (22.6 ± 13.1 vs 11.1 ± 3.6 cm 3 , P = 0.04).

CONCLUSION

Our study shows that injectable morselized absorbable collagen sponge/rhBMP-2 can create twice as much bone within a nicotine-impaired rabbit spine fusion model when delivered 4 weeks out from the time of surgery.

Addition of heparin binding sites strongly increases the bone forming capabilities of BMP9 in vivo

Bone Morphogenetic proteins (BMPs) like BMP2 and BMP7 have shown great potential in the treatment of severe bone defects. In recent in vitro studies, BMP9 revealed the highest osteogenic potential compared to other BMPs, possibly due to its unique signaling pathways that differs from other osteogenic BMPs. However, in vivo the bone forming capacity of BMP9-adsorbed scaffolds is not superior to BMP2 or BMP7. In silico analysis of the BMP9 protein sequence revealed that BMP9, in contrast to other osteogenic BMPs such as BMP2, completely lacks so-called heparin binding motifs that enable extracellular matrix (ECM) interactions which in general might be essential for the BMPs' osteogenic function. Therefore, we genetically engineered a new BMP9 variant by adding BMP2-derived heparin binding motifs to the N-terminal segment of BMP9's mature part. The resulting protein (BMP9 HB) showed higher heparin binding affinity than BMP2, similar osteogenic activity in vitro and comparable binding affinities to BMPR-II and ALK1 compared to BMP9. However, remarkable differences were observed when BMP9 HB was adsorbed to collagen scaffolds and implanted subcutaneously in the dorsum of rats, showing a consistent and significant increase in bone volume and density compared to BMP2 and BMP9. Even at 10-fold lower BMP9 HB doses bone tissue formation was observed. This innovative approach of significantly enhancing the osteogenic properties of BMP9 simply by addition of ECM binding motifs, could constitute a valuable replacement to the commonly used BMPs. The possibility to use lower protein doses demonstrates BMP9 HB's high translational potential.

Injectable and Cell-Laden Hydrogel in the Contained Bone Defect Animal Model: A Systematic Review

BACKGROUND

Due to its high water content and biomimetic properties simulating extracellular matrix (ECM), hydrogels have been used as preferred cell culture and delivery systems. Similarly, cell-loaded hydrogels can be easily injected into target areas in a minimally invasive manner, minimizing surgical trauma, adapting to irregular shaped defects, and benefiting patients. In this study, we systematically reviewed multiple studies on hydrogel-based bone defect research and briefly summarized the progress of injectable and cell-loaded hydrogels in bone defect repair.

METHODS

A systematic search was conducted in the PubMed and Web of Science databases using selected search terms.

RESULTS

Initially, 185 articles were retrieved from the databases. After full-text screening based on inclusion and exclusion criteria, 26 articles were included in this systematic review. Data collected from each study included culture model, seed cell type and origin, cell concentration, scaffold material, scaffold shape, experimental animal and site, bioactive agents, and binding method. This injectable and cell-loaded hydrogel shows certain feasibility in bone tissue engineering applications.

CONCLUSION

Injectable and cell-loaded hydrogels have been widely applied in bone tissue engineering research. The future direction of bone tissue engineering for bone defect treatment involves the use of new hydrogel materials and biochemical stimulation.

An engineered periosteum for efficient delivery of rhBMP-2 and mesenchymal progenitor cells during bone regeneration

During bone regeneration, the periosteum acts as a carrier for key regenerative cues, delivering osteochondroprogenitor cells and crucial growth factors to the injured bone. We developed a biocompatible, 3D polycaprolactone (PCL) melt electro-written membrane to act as a mimetic periosteum. Poly (ethyl acrylate) coating of the PCL membrane allowed functionalization, mediated by fibronectin and low dose recombinant human BMP-2 (rhBMP-2) (10-25 μg/ml), resulting in efficient, sustained osteoinduction in vitro. In vivo, rhBMP-2 functionalized mimetic periosteum demonstrated regenerative potential in the treatment of rat critical-size femoral defects with highly efficient healing and functional recovery (80%-93%). Mimetic periosteum has also proven to be efficient for cell delivery, as observed through the migration of transplanted periosteum-derived mesenchymal cells to the bone defect and their survival. Ultimately, mimetic periosteum demonstrated its ability to deliver key stem cells and morphogens to an injured site, exposing a therapeutic and translational potential in vivo when combined with unprecedentedly low rhBMP-2 doses.

Bioactive Materials for Bone Regeneration: Biomolecules and Delivery Systems

Novel tissue regeneration strategies are constantly being developed worldwide. Research on bone regeneration is noteworthy, as many promising new approaches have been documented with novel strategies currently under investigation. Innovative biomaterials that allow the coordinated and well-controlled repair of bone fractures and bone loss are being designed to reduce the need for autologous or allogeneic bone grafts eventually. The current engineering technologies permit the construction of synthetic, complex, biomimetic biomaterials with properties nearly as good as those of natural bone with good biocompatibility. To ensure that all these requirements meet, bioactive molecules are coupled to structural scaffolding constituents to form a final product with the desired physical, chemical, and biological properties. Bioactive molecules that have been used to promote bone regeneration include protein growth factors, peptides, amino acids, hormones, lipids, and flavonoids. Various strategies have been adapted to investigate the coupling of bioactive molecules with scaffolding materials to sustain activity and allow controlled release. The current manuscript is a thorough survey of the strategies that have been exploited for the delivery of biomolecules for bone regeneration purposes, from choosing the bioactive molecule to selecting the optimal strategy to synthesize the scaffold and assessing the advantages and disadvantages of various delivery strategies.

Improving Bone Formation by Guided Bone Regeneration Using a Collagen Membrane with rhBMP-2: A Novel Concept

We examined whether recombinant human bone morphogenetic protein-2 (rhBMP-2) when applied to collagen membranes, would reinforce them during guided bone regeneration. Four critical cranial bone defects were created and treated in 30 New Zealand white rabbits, including a control group, critical defect only; group 1, collagen membrane only; group 2, biphasic calcium phosphate (BCP) only; group 3, collagen membrane + BCP; group 4, collagen membrane with rhBMP-2 (1.0 mg/mL); group 5, collagen membrane with rhBMP-2 (0.5 mg/mL); group 6, collagen membrane with rhBMP-2 (1.0 mg/mL) + BCP; and group 7, collagen membrane with rhBMP-2 (0.5 mg/mL) + BCP. After a 2-, 4-, or 8-week healing period, the animals were sacrificed. The combination of collagen membranes with rhBMP-2 and BCP yielded significantly higher bone formation rates compared to the other groups (control group and groups 1-5 < groups 6 and 7; p < 0.05). A 2-week healing period yielded significantly lower bone formation than that at 4 and 8 weeks (2 < 4 = 8 weeks; p < 0.05). This study proposes a novel GBR concept in which rhBMP-2 is applied to collagen membranes outside instead of inside the grafted area, thereby inducing quantitatively and qualitatively enhanced bone regeneration in critical bone defects.

Effectiveness of BMP-2 and PDGF-BB Adsorption onto a Collagen/Collagen-Magnesium-Hydroxyapatite Scaffold in Weight-Bearing and Non-Weight-Bearing Osteochondral Defect Bone Repair: In Vitro, Ex Vivo and In Vivo Evaluation

Despite promising clinical results in osteochondral defect repair, a recently developed bi-layered collagen/collagen-magnesium-hydroxyapatite scaffold has demonstrated less optimal subchondral bone repair. This study aimed to improve the bone repair potential of this scaffold by adsorbing bone morphogenetic protein 2 (BMP-2) and/or platelet-derived growth factor-BB (PDGF-BB) onto said scaffold. The in vitro release kinetics of BMP-2/PDGF-BB demonstrated that PDGF-BB was burst released from the collagen-only layer, whereas BMP-2 was largely retained in both layers. Cell ingrowth was enhanced by BMP-2/PDFG-BB in a bovine osteochondral defect ex vivo model. In an in vivo semi-orthotopic athymic mouse model, adding BMP-2 or PDGF-BB increased tissue repair after four weeks. After eight weeks, most defects were filled with bone tissue. To further investigate the promising effect of BMP-2, a caprine bilateral stifle osteochondral defect model was used where defects were created in weight-bearing femoral condyle and non-weight-bearing trochlear groove locations. After six months, the adsorption of BMP-2 resulted in significantly less bone repair compared with scaffold-only in the femoral condyle defects and a trend to more bone repair in the trochlear groove. Overall, the adsorption of BMP-2 onto a Col/Col-Mg-HAp scaffold reduced bone formation in weight-bearing osteochondral defects, but not in non-weight-bearing osteochondral defects.

The Role of Microsphere Structures in Bottom-Up Bone Tissue Engineering

Bone defects have caused immense healthcare concerns and economic burdens throughout the world. Traditional autologous allogeneic bone grafts have many drawbacks, so the emergence of bone tissue engineering brings new hope. Bone tissue engineering is an interdisciplinary biomedical engineering method that involves scaffold materials, seed cells, and "growth factors". However, the traditional construction approach is not flexible and is unable to adapt to the specific shape of the defect, causing the cells inside the bone to be unable to receive adequate nourishment. Therefore, a simple but effective solution using the "bottom-up" method is proposed. Microspheres are structures with diameters ranging from 1 to 1000 µm that can be used as supports for cell growth, either in the form of a scaffold or in the form of a drug delivery system. Herein, we address a variety of strategies for the production of microspheres, the classification of raw materials, and drug loading, as well as analyze new strategies for the use of microspheres in bone tissue engineering. We also consider new perspectives and possible directions for future development.

Bioactive Porous Particles as Biological and Physical Stimuli for Bone Regeneration

Even though bony defects can be recovered to their original condition with full functionality, critical-sized bone injuries continue to be a challenge in clinical fields due to deficiencies in the scaffolding matrix and growth factors at the injury region. In this study, we prepared bone morphogenetic protein-2 (BMP-2)-loaded porous particles as a bioactive bone graft for accelerated bone regeneration. The porous particles with unique leaf-stacked morphology (LSS particles) were fabricated by a simple cooling procedure of hot polycaprolactone (PCL) solution. The unique leaf-stacked structure in the LSS particles provided a large surface area and complex release path for the sufficient immobilization of BMP-2 and sustained release of BMP-2 for 26 days. The LSS was also recognized as a topographical cue for cell adhesion and differentiation. In in vitro cell culture and in vivo animal study using a canine mandible defect model, BMP-2-immobilized LSS particles provided a favorable environment for osteogenic differentiation of stem cells and bone regeneration. In vitro study suggests a dual stimulus of bone mineral-like (leaf-stacked) structure (a physical cue) and continuously supplied BMP-2 (a biological cue) to be the cause of this improved healing outcome. Thus, LSS particles containing BMP-2 can be a promising bioactive grafting material for effective new bone formation.

Bone Formation on Murine Cranial Bone by Injectable Cross-Linked Hyaluronic Acid Containing Nano-Hydroxyapatite and Bone Morphogenetic Protein

New injection-type bone-forming materials are desired in dental implantology. In this study, we added nano-hydroxyapatite (nHAp) and bone morphogenetic protein (BMP) to cross-linkable thiol-modified hyaluronic acid (tHyA) and evaluated its usefulness as an osteoinductive injectable material using an animal model. The sol (ux-tHyA) was changed to a gel (x-tHyA) by mixing with a cross-linker. We prepared two sol−gel (SG) material series, that is, x-tHyA + BMP with and without nHAp (SG I) and x-tHyA + nHAp with and without BMP (SG II). SG I materials in the sol stage were injected into the cranial subcutaneous connective tissues of mice, followed by in vivo gelation, while SG II materials gelled in Teflon rings were surgically placed directly on the cranial bones of rats. The animals were sacrificed 8 weeks after implantation, followed by X-ray analysis and histological examination. The results revealed that bone formation occurred at a high rate (>70%), mainly as ectopic bone in the SG I tests in mouse cranial connective tissues, and largely as bone augmentation in rat cranial bones in the SG II experiments when x-tHyA contained both nHAp and BMP. The prepared x-tHyA + nHAp + BMP SG material can be used as an injection-type osteoinductive bone-forming material. Sub-periosteum injection was expected.

Metformin loaded injectable silk fibroin microsphere for the treatment of spinal cord injury

The repair of spinal cord injury is a great challenge in clinical. Improving the microenvironment of the injured site is the key strategy for accelerating axon regeneration and synaptic formation. Herein, a kind of silk fibroin microspheres functionalized by metformin through dopamine was developed using water-in-oil emulsification-diffusion method and surface modification technique, and the effect on cortical neuron was evaluated. The results showed that the microspheres showed a uniform size distribution with the diameter of around 60 μm and a concave structure. Moreover, the microspheres possessed good injectability and stability. In addition, the metformin could be successfully immobilized in the silk fibroin microspheres. The cell culture results displayed that the growth and morphology of cortical neurons on the microspheres with metformin concentration of 5 mg/mL and 10 mg/mL were obviously better than that on other samples. Notably, the spread area of single cortical cell on silk fibroin microspheres was increased with the ascending metformin concentration. Therefore, the results indicated that the metformin loaded silk fibroin microsphere could obviously improve the growth and spreading behavior of cortical neuron. The study may provide an important experimental basis for the development of drug loaded injectable biomaterials scaffolds for the treatment of spinal cord injury and have great potential for spinal cord regeneration.

Peptide-Modified Nano-Bioactive Glass for Targeted Immobilization of Native VEGF

A limiting factor in large bone defect regeneration is the slow and disorganized formation of a functional vascular network in the defect area, often resulting in delayed healing or implant failure. To overcome this, strategies that induce angiogenic processes should be combined with potent bone graft substitutes in new bone regeneration approaches. To this end, we describe a unique approach to immobilize the pro-angiogenic growth factor VEGF₁₆₅ in its native state on the surface of nanosized bioactive glass particles (nBGs) via a binding peptide (PR1P). We demonstrate that covalent coupling of the peptide to amine functional groups grafted on the nBG surface allows immobilization of VEGF with high efficiency and specificity. The amount of coupled peptide could be controlled by varying amine density, which eventually allows tailoring the amount of bound VEGF within a physiologically effective range. In vitro analysis of endothelial cell tube formation in response to VEGF-carrying nBG confirmed that the biological activity of VEGF is not compromised by the immobilization. Instead, comparable angiogenic stimulation was found for lower doses of immobilized VEGF compared to exogenously added VEGF. The described system, for the first time, employs a binding peptide for growth factor immobilization on bioactive glass nanoparticles and represents a promising strategy to overcome the problem of insufficient neovascularization in large bone defect regeneration.

In situ mineralized PLGA/zwitterionic hydrogel composite scaffold enables high-efficiency rhBMP-2 release for critical-sized bone healing

Osteoconductive and osteoinductive scaffolds are highly desirable for functional restoration of large bone defects. Here, we report an in situ mineralized poly(lactic-co-glycolic acid)/poly[2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide hydrogel (PLGA/PSBMA) scaffold as a novel high-efficiency carrier for recombinant human bone morphogenetic protein-2 (rhBMP-2) for bone tissue regeneration. By virtue of the oppositely charged structure, the zwitterionic PSBMA component is able to template well-integrated dense mineralization of calcium phosphate throughout the PLGA/PSBMA scaffold. The high affinity between rhBMP-2 and the mineralized matrix, combined with the capability of the zwitterionic hydrogel to sequester and to enable sustained release of ionic proteins, endows the mineralized PLGA/PSBMA scaffolds with high-efficiency sustained release of rhBMP-2 (only 1.7% release within 35 days), thus enabling robust healing of critical-sized (5 mm) nonunion calvarial defects in rats at an ultralow dosage of rhBMP-2 (150 ng per scaffold), at which level successful healing of critical-sized bone defects has never been reported. These findings show that the mineralized PLGA/PSBMA scaffold is promising for bone defect repair.

Bioactive chitosan biguanidine-based injectable hydrogels as a novel BMP-2 and VEGF carrier for osteogenesis of dental pulp stem cells

Nowadays, vascularization and mineralization of bone defects is the main bottleneck in the bone regeneration field that is needed to be overcome and developed. Here, we prepared novel in-situ formed injectable hydrogels based on chitosan biguanidine and carboxymethylcellulose loaded with vascular endothelial growth factor (VEGF) and recombinant Bone morphogenetic protein 2 (BMP-2) and studied its influence on osteoblastic differentiation of dental pulp stem cells (DPSCs). The sequential release behavior of the VEGF and BMP-2 from hydrogels adjusted with the pattern of normal human bone growth. MTT assay exhibited that these hydrogels were non-toxic and significantly increased DPSCs proliferation. The Real-time PCR and Western blot analysis on CG11/BMP2-VEGF showed significantly higher gene and protein expression of ALP, COL1α1, and OCN. These results were confirmed by mineralization assay by Alizarin Red staining and Alkaline phosphatase enzyme activity. Based on these evaluations, these hydrogel holds potential as an injectable bone tissue engineering platform.

Overexpression Effects of miR-424 and BMP2 on the Osteogenesis of Wharton's Jelly-Derived Stem Cells

Recently, the translational application of noncoding RNAs is accelerated dramatically. In this regard, discovering therapeutic roles of microRNAs by developing synthetic RNA and vector-based RNA is attracting attention. Here, we studied the effect of BMP2 and miR-424 on the osteogenesis of Wharton's jelly-derived stem cells (WJSCs). For this purpose, human BMP2 and miR-424 DNA codes were cloned in the third generation of lentiviral vectors and then used for HEK-293T cell transfection. Lentiviral plasmids contained miR424, BMP-2, miR424-BMP2, green fluorescent protein (GFP) genes, and helper vectors. The recombinant lentiviral particles transduced the WJSCs, and the osteogenesis was evaluated by real-time PCR, Western blot, Alizarin Red staining, and alkaline phosphatase enzyme activity. According to the results, there was a significant increase in the expression of the BMP2 gene and secretion of Osteocalcin protein in the group of miR424-BMP2. Moreover, the amount of dye deposition in Alizarin Red staining and alkaline phosphatase activity was significantly higher in the mentioned group (p < 0.05). Thus, the current study results clarify the efficacy of gene therapy by miR424-BMP2 vectors for bone tissue engineering. These data could help guide the development of gene therapy-based protocols for bone tissue engineering.

Bisphosphonate-Conjugated Photo-Crosslinking Polyanionic Hyaluronic Acid Microbeads for Controlled BMP2 Delivery and Enhanced Bone Formation Efficacy

In this study, we designed bisphosphonate-conjugated polyanionic hyaluronic acid (HA) microbeads (MBs) for the controlled delivery of bone morphogenetic protein 2 (BMP2). MBs were prepared via the photo-crosslinking of bisphosphonate (alendronate)-conjugated methacrylated HA (Alen-MHA). The polyanionic Alen-MHA MBs actively absorbed cationic BMP2 up to 91.0% of the loading efficacy and displayed a sustained release of BMP2 for 10 days. BMP2/Alen-MHA MBs induced osteogenic-related genes in cellular experiments and showed the highly increased bone formation efficacy in thigh muscle injection and rat spinal fusion animal models. Thus, BMP2/Alen-MHA MBs provide a promising opportunity to improve the delivery efficiency of BMP2.

Onlay-graft of 3D printed Kagome-structure PCL scaffold incorporated with rhBMP-2 based on hyaluronic acid hydrogel

The onlay-graft, one of the most difficult graft conditions, is used for diverse clinical conditions, including plastic and dental surgery. The graft should withstand continuous pressure from overlying tissues and have excellent bone formation capability in a limited bone contact situation. We recently developed a 3D printed Kagome-structured polycaprolactone (PCL) scaffold that has a stronger mechanical property. This study evaluated the clinical feasibility of this scaffold for onlay-graft use. The value of the scaffold containing recombinant human bone morphogenetic protein-2 in a hyaluronate-based hydrogel (rhBMP-2/HA) to enhance bone regeneration was also assessed. 3D-printed Kagome-PCL scaffolds alone (n= 12, group I) or loaded with rhBMP-2/HA (n= 12, group II) were grafted using a rat calvarial onlay-graft model. Following sacrifice at 2, 4, and 8 weeks, all 3D-printed Kagome-PCL scaffolds were accurately positioned and firmly integrated to the recipient bone. Micro-computed tomography and histology analyses revealed a constant height of the scaffolds over time in all animals. New bone grew into the scaffolds in both groups, but with greater volume in group II. These results suggest the promising clinical feasibility of the 3D-printed Kagome-PCL scaffold for onlay-graft use and it could substitute the conventional onlay-graft in the plastic and dental reconstructive surgery in the near future.

Experimental Study on the Effect of miR-200b Regulation of Bone Morphogenetic Protein (BMP)-2 Expression on the Proliferation and Invasion of Nasopharyngeal Carcinoma Cells

BMP is an important member of the skeletal junction proteins associated with cell invasion, metastasis, and migration. MiR-200b is associated with the progression of several tumors. This study investigated whether miR-200b plays a role in regulating BMP-2 expression and affecting the proliferation and invasion of nasopharyngeal carcinoma cells. The nasopharyngeal carcinoma tissues with different TNM stages were collected and the proliferative nasal tissues alone were used as controls to detect the expression of miR-200b and BMP by RT-PCR. The nasopharyngeal carcinoma cell line CNE1 cells were divided into miR-NC group, miR-200b mimic group, siRNA-NC group, and siRNA-BMP-2 group, to detect BMP-2 level, cell invasion and proliferation ability by transwell. The BMP-2 mRNA expressed in nasopharyngeal carcinoma tissues was significantly elevated compared to controls and correlated with TNM stage. BMP-2 was higher in tumor tissues than in controls, however, the expression profile of miR-200b was opposite to BMP-2. Transfection with miR-200b mimic or siRNA-BMP-2 significantly down-regulated BMP-2 in CNE1 cells and attenuated cell invasive and proliferative capacity. Reduced expression of miR-200b is associated with elevated BMP-2 expression and increased invasive capacity of nasopharyngeal cancer cells. Overexpression of miR-200b reduces the invasive and proliferative capacity of nasopharyngeal cancer cells by targeting and inhibiting BMP-2 expression.

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.

Polysaccharide-Based Materials Created by Physical Processes: From Preparation to Biomedical Applications

Polysaccharide-based materials created by physical processes have received considerable attention for biomedical applications. These structures are often made by associating charged polyelectrolytes in aqueous solutions, avoiding toxic chemistries (crosslinking agents). We review the principal polysaccharides (glycosaminoglycans, marine polysaccharides, and derivatives) containing ionizable groups in their structures and cellulose (neutral polysaccharide). Physical materials with high stability in aqueous media can be developed depending on the selected strategy. We review strategies, including coacervation, ionotropic gelation, electrospinning, layer-by-layer coating, gelation of polymer blends, solvent evaporation, and freezing-thawing methods, that create polysaccharide-based assemblies via in situ (one-step) methods for biomedical applications. We focus on materials used for growth factor (GFs) delivery, scaffolds, antimicrobial coatings, and wound dressings.

Boron Supplementation Promotes Osteogenesis of Tibia by Regulating the Bone Morphogenetic Protein-2 Expression in African Ostrich Chicks

The present study aimed to explore the effects of supplemental boron on osteogenesis of tibia and to investigate the possible relationship between additional boron and the expression of bone morphogenetic protein-2 (BMP-2) in tibia of ostrich chicks. Therefore, forty-eight African ostrich chicks (15 days old) were supplemented with 0 mg/L, 40 mg/L, 80 mg/L, 160 mg/L, 320 mg/L, and 640 mg/L of boron in drinking water for 75 days. The paraffin sections of tibia used to measure histomorphometric parameters by hematoxylin and eosin (HE) staining, Masson's staining, and immunohistochemistry (IHC). Enzyme-linked immunosorbent assay was performed to assess the level of BMP-2, osteocalcin (BGP), glucocorticoids (GCs), osteoprotegerin (OPG), and receptor activator of nuclear factor kappa-B ligand (RANKL) in serum. TUNEL (terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling) technique was performed to detect the cell apoptosis. The results indicated that low dose of supplemental boron (40 mg/L-160 mg/L) in drinking water promotes bone development by increasing the mature ossein. The expression of BMP2 on 45 days was higher than 90 days. Serum level of BMP-2, BGP, and GCs changed significantly in groups with low dosage of boron, and OPG/RANKL ratio was upregulated from 0 to 160 mg/L. Cell apoptosis was least in 40 mg/L and 160 mg/L groups. Taken together, low dose of boron supplemented in drinking water could promote osteogenesis and growth and development of tibia by regulating the expression and secretion of BMP-2 and providing a dynamically balanced environment for tibia growth, development, and reconstruction by regulating the concentrations of BGP, GCs, and OPG/RANKL ratio in serum.

A tough act to follow: collagen hydrogel modifications to improve mechanical and growth factor loading capabilities

Collagen hydrogels are among ​the most well-studied platforms for drug delivery and in situ tissue engineering, thanks to their low cost, low immunogenicity, versatility, biocompatibility, and similarity to the natural extracellular matrix (ECM). Despite collagen being largely responsible for the tensile properties of native connective tissues, collagen hydrogels have relatively low mechanical properties in the absence of covalent cross-linking. This is particularly problematic when attempting to regenerate stiffer and stronger native tissues such as bone. Furthermore, in contrast to hydrogels based on ECM proteins such as fibronectin, collagen hydrogels do not have any growth factor (GF)-specific binding sites and often cannot sequester physiological (small) amounts of the protein. GF binding and in situ presentation are properties that can aid significantly in the tissue regeneration process by dictating cell fate without causing adverse effects such as malignant tumorigenic tissue growth. To alleviate these issues, researchers have developed several strategies to increase the mechanical properties of collagen hydrogels using physical or chemical modifications. This can expand the applicability of collagen hydrogels to tissues subject to a continuous load. GF delivery has also been explored, mathematically and experimentally, through the development of direct loading, chemical cross-linking, electrostatic interaction, and other carrier systems. This comprehensive article explores the ways in which these parameters, mechanical properties and GF delivery, have been optimized in collagen hydrogel systems ​and examines their in vitro or in vivo biological effect. This article can, therefore, be a useful tool to streamline future studies in the field, by pointing researchers into the appropriate direction according to their collagen hydrogel design requirements.

In-situ stable injectable collagen-based hydrogels for cell and growth factor delivery

Here we report development of in-situ stable injectable hydrogels for delivery of cells and growth factors based on two precursors, alginate, and collagen/calcium sulfate (CaSO4). The alg/col hydrogels were shear-thinning, injectable through commercially available needles and stable right after injection. Rheological measurements revealed that pre-crosslinked alg/col hydrogels fully crosslinked at 37°C and that the storage modulus of alg/col hydrogels increased with increasing the collagen content or the concentration of CaSO4. The viscoelastic characteristics and injectability of the alg/col hydrogels were not significantly impacted by the storage of precursor solutions for 28 days. An osteoinductive bone morphogenic protein-2 (BMP-2) loaded into alg/col hydrogels was released in 14 days. Human mesenchymal stem cells (hMSCs) encapsulated in alg/col hydrogels had over 90% viability over 7 days after injection. The DNA content of hMSC-laden alg/col hydrogels increased by 6-37 folds for 28 days, depending on the initial cell density. In addition, hMSCs encapsulated in alg/col hydrogels and incubated in osteogenic medium were osteogenically differentiated and formed a mineralized matrix. Finally, a BMP-2 loaded alg/col hydrogel was used to heal a critical size calvarial bone defect in rats after 8 weeks of injection. The alg/col hydrogel holds great promise in tissue engineering and bioprinting applications.

Bioactivity of a Novel Polycaprolactone-Hydroxyapatite Scaffold Used as a Carrier of Low Dose BMP-2: An In Vitro Study

Scaffolds of polycaprolactone-30% hydroxyapatite (PCL-30% HA) were fabricated using melt stretching and multilayer deposition (MSMD), and the in vitro response of osteoblasts to the scaffolds was assessed. In group A, the scaffolds were immersed in 10 µg/mL bone morphogenetic protein-2 (BMP-2) solution prior to being seeded with osteoblasts, and they were cultured in the medium without BMP-2. In group B, the cell-scaffold constructs without BMP-2 were cultured in medium containing 10 µg/mL BMP-2. The results showed greater cell proliferation in group A. The upregulation of runt-related transcription factor 2 and osteocalcin genes correlated with the release of BMP-2 from the scaffolds. The PCL-30% HA MSMD scaffolds appear to be suitable for use as osteoconductive frameworks and BMP-2 carriers.

Overcoming barriers confronting application of protein therapeutics in bone fracture healing

Bone fracture is a major contributor to debilitation and death among patients with bone diseases. Thus, osteogenic protein therapeutics and their delivery to bone have been extensively researched as strategies to accelerate fracture healing. To prevent morbidity and mortality of fractures, which occur frequently in the aging population, there is a critical need for development of first-line therapeutics. Bone morphogenic protein-2 (BMP-2) has been at the forefront of bone regeneration research for its potent osteoinduction, despite safety concerns and biophysiological obstacles of delivery to bone. However, continued pursuit of osteoinductive proteins as a therapeutic option is largely aided by drug delivery systems, playing an imperative role in enhancing safety and efficacy. In this work, we highlighted several types of drug delivery platforms and their biomaterials, to evaluate the suitability in overcoming challenges of therapeutic protein delivery for bone regeneration. To showcase the clinical considerations for each type of platform, we have assessed the most common route of administration strategies for bone regeneration, classifying the platforms as implantable or injectable. Additionally, we have analyzed the commonly utilized models and methodology for safety and efficacy evaluation of these osteogenic protein-loaded systems, to present clinical opinions for future directions of research in this field. It is hoped that this review will promote research and development of clinically translatable osteogenic protein therapeutics, while targeting first-line treatment status for achieving desired outcomes of fracture healing. Graphical abstract.

Bisphosphonate nanoclay edge-site interactions facilitate hydrogel self-assembly and sustained growth factor localization

Nanoclays have generated interest in biomaterial design for their ability to enhance the mechanics of polymeric materials and impart biological function. As well as their utility as physical cross-linkers, clays have been explored for sustained localization of biomolecules to promote in vivo tissue regeneration. To date, both biomolecule-clay and polymer-clay nanocomposite strategies have utilised the negatively charged clay particle surface. As such, biomolecule-clay and polymer-clay interactions are set in competition, potentially limiting the functional enhancements achieved. Here, we apply specific bisphosphonate interactions with the positively charged clay particle edge to develop self-assembling hydrogels and functionalized clay nanoparticles with preserved surface exchange capacity. Low concentrations of nanoclay are applied to cross-link hyaluronic acid polymers derivatised with a pendant bisphosphonate to generate hydrogels with enhanced mechanical properties and preserved protein binding able to sustain, for over six weeks in vivo, the localized activity of the clinically licensed growth factor BMP-2.

Nanoclays have been used in composites and for drug delivery but have suffered from a trade-off in properties when used for both. Here the authors report on the use of bisphosphonate interactions with nanoclay edges to made drug loaded composites without compromising materials properties or drug loading.

Interaction of material stiffness and negative pressure to enhance differentiation of bone marrow-derived stem cells and osteoblast proliferation

Negative pressure wound therapy (NPWT) results in improved wound repair and the combined use of NPWT with elastomeric materials may further stimulate and accelerate tissue repair. No firmly established treatment modalities using both NPWT and biomaterials exist for orthopedic application. The goal of this study was to investigate the response of osteoblasts and bone marrow-derived mesenchymal stem cells to negative pressure and to determine whether a newly developed elastic osteomimetic bone repair material (BRM), a blend of type I collagen, chondroitin 6-sulfate, and poly (octanediol citrate) could enhance the osteoblastic phenotype. The results indicate that proliferation and alkaline phosphatase activity of hFOB1.19 osteoblasts were significantly increased with exposure to 12 hr of negative pressure (-125 mmHg). Follow-on studies with rat and human mesenchymal stem cells confirmed that negative pressure enhanced osteoblastic maturation. In addition, a significant interaction of negative pressure and electrospun BRM resulted in increased mRNA expression of alkaline phosphatase, osteopontin, collagen1α2, and HIF1α, whereas little or no effect on these genes was observed on electrospun collagen or tissue culture plastic. Together, these results suggest that the use of this novel biomaterial, BRM, with NPWT may ultimately translate into a safe and cost-effective clinical application to accelerate bone repair.

Physicochemical Niche Conditions and Mechanosensing by Osteocytes and Myocytes

PURPOSE OF REVIEW

Bone and muscle mass increase in response to mechanical loading and biochemical cues. Bone-forming osteoblasts differentiate into early osteocytes which ultimately mature into late osteocytes encapsulated in stiff calcified matrix. Increased muscle mass originates from muscle stem cells (MuSCs) enclosed between their plasma membrane and basal lamina. Stem cell fate and function are strongly determined by physical and chemical properties of their microenvironment, i.e., the cell niche.

RECENT FINDINGS

The cellular niche is a three-dimensional structure consisting of extracellular matrix components, signaling molecules, and/or other cells. Via mechanical interaction with their niche, osteocytes and MuSCs are subjected to mechanical loads causing deformations of membrane, cytoskeleton, and/or nucleus, which elicit biochemical responses and secretion of signaling molecules into the niche. The latter may modulate metabolism, morphology, and mechanosensitivity of the secreting cells, or signal to neighboring cells and cells at a distance. Little is known about how mechanical loading of bone and muscle tissue affects osteocytes and MuSCs within their niches. This review provides an overview of physicochemical niche conditions of (early) osteocytes and MuSCs and how these are sensed and determine cell fate and function. Moreover, we discuss how state-of-the-art imaging techniques may enhance our understanding of these conditions and mechanisms.

Supramolecular Modification of a Sequence-Controlled Collagen-Mimicking Polymer

Structurally and functionally well-defined recombinant proteins are an interesting class of sequence-controlled macromolecules to which different crosslinking chemistries can be applied to tune their biological properties. Herein, we take advantage of a 571-residue recombinant peptide based on human collagen type I (RCPhC1), which we functionalized with supramolecular 4-fold hydrogen bonding ureido-pyrimidinone (UPy) moieties. By grafting supramolecular UPy moieties onto the backbone of RCPhC1 (UPy-RCPhC1), increased control over the polymer structure, assembly, gelation, and mechanical properties was achieved. In addition, by increasing the degree of UPy functionalization on RCPhC1, cardiomyocyte progenitor cells were cultured on "soft" (∼26 kPa) versus "stiff" (∼68-190 kPa) UPy-RCPhC1 hydrogels. Interestingly, increased stress fiber formation, focal adhesions, and proliferation were observed on stiffer compared to softer substrates, owing to the formation of stronger cell-material interactions. In conclusion, a bioinspired hydrogel material was designed by a combination of two well-known natural components, i.e., a protein as sequence-controlled polymer and UPy units inspired on nucleobases.

Follistatin Effects in Migration, Vascularization, and Osteogenesis in vitro and Bone Repair in vivo

The use of biomaterials and signaling molecules to induce bone formation is a promising approach in the field of bone tissue engineering. Follistatin (FST) is a glycoprotein able to bind irreversibly to activin A, a protein that has been reported to inhibit bone formation. We investigated the effect of FST in critical processes for bone repair, such as cell recruitment, osteogenesis and vascularization, and ultimately its use for bone tissue engineering. In vitro, FST promoted mesenchymal stem cell (MSC) and endothelial cell (EC) migration as well as essential steps in the formation and expansion of the vasculature such as EC tube-formation and sprouting. FST did not enhance osteogenic differentiation of MSCs, but increased committed osteoblast mineralization. In vivo, FST was loaded in an in situ gelling formulation made by alginate and recombinant collagen-based peptide microspheres and implanted in a rat calvarial defect model. Two FST variants (FST288 and FST315) with major differences in their affinity to cell-surface proteoglycans, which may influence their effect upon in vivo bone repair, were tested. In vitro, most of the loaded FST315 was released over 4 weeks, contrary to FST288, which was mostly retained in the biomaterial. However, none of the FST variants improved in vivo bone healing compared to control. These results demonstrate that FST enhances crucial processes needed for bone repair. Further studies need to investigate the optimal FST carrier for bone regeneration.

Biopolymeric In Situ Hydrogels for Tissue Engineering and Bioimaging Applications

BACKGROUND

Biopolymeric in situ hydrogels play a crucial role in the regenerative repair and replacement of infected or injured tissue. They possess excellent biodegradability and biocompatibility in the biological system, however only a few biopolymeric in situ hydrogels have been approved clinically. Researchers have been investigating new advancements and designs to restore tissue functions and structure, and these studies involve a composite of biometrics, cells and a combination of factors that can repair or regenerate damaged tissue.

METHODS

Injectable hydrogels, cross-linking mechanisms, bioactive materials for injectable hydrogels, clinically applied injectable biopolymeric hydrogels and the bioimaging applications of hydrogels were reviewed.

RESULTS

This article reviews the different types of biopolymeric injectable hydrogels, their gelation mechanisms, tissue engineering, clinical applications and their various in situ imaging techniques.

CONCLUSION

The applications of bioactive injectable hydrogels and their bioimaging are a promising area in tissue engineering and regenerative medicine. There is a high demand for injectable hydrogels for in situ imaging.

Sustained Release of BMP-2 from Porous Particles with Leaf-Stacked Structure for Bone Regeneration

Sustained release of bioactive molecules from delivery systems is a common strategy for ensuring their prolonged bioactivity and for minimizing safety issues. However, residual toxic reagents, the use of harsh organic solvents, and complex fabrication procedures in conventional delivery systems are considered enormous impediments toward clinical use. Herein, we describe bone morphogenetic protein-2 (BMP-2)-immobilized porous polycaprolactone particles with unique leaf-stacked structures (LSS particles) prepared using clinically feasible materials and procedures. The BMP-2 immobilized in these LSS particles is continuously released up to 36 days to provide an appropriate environment for osteogenic differentiation of human periosteum-derived cells and new bone formation. Thus, the leaf-stacked structures of these LSS particles provide a simple but clinically applicable platform for effectively delivering a variety of bioactive molecules, such as growth factors, hormones, cytokines, peptides, etc.