Enhanced bone morphogenetic protein-2-induced ectopic and orthotopic bone formation by intermittent parathyroid hormone (1-34) administration

Humanized In Vivo Bone Tissue Engineering: In Vitro Preculture Conditions Control the Structural, Cellular, and Matrix Composition of Humanized Bone Organs

Bone tissue engineering (BTE) has long sought to elucidate the key factors controlling human/humanized bone formation for regenerative medicine and disease modeling applications, yet with no definitive answers due to the high number and co-dependency of parameters. This study aims to clarify the relative impacts of in vitro biomimetic 'preculture composition' and 'preculture duration' before in vivo implantation as key criteria for the optimization of BTE design. These parameters are directly related to in vitro osteogenic differentiation (OD) and mineralization and are being investigated across different osteoprogenitor-loaded biomaterials, specifically fibrous calcium phosphate-polycaprolactone (CaP-mPCL) scaffolds and gelatin methacryloyl (GelMA) hydrogels. The results show that OD and mineralization levels prior to implantation, enhanced by a mineralization medium supplement to the osteogenic medium (OM), significantly improve ectopic BTE outcomes, regardless of the biomaterial type. Specifically, preculture conditions are pivotal in achieving more faithful mimicry of human bone structure, cellular and extracellular matrix composition and organization, and provide control over bone marrow composition. This work emphasizes the potential of using biomimetic culture compositions, specifically the addition of a mineralization medium as a cost-effective and straightforward approach to enhance BTE outcomes, facilitating rapid development of bone models with superior quality and resemblance to native bone.

A composite, off-the-shelf osteoinductive material for large, vascularized bone flap prefabrication

We previously described an immortalized, genetically-engineered human Mesenchymal stromal cell line to generate BMP2-enriched Chondrogenic Matrices (MB-CM), which after devitalization and storage could efficiently induce ectopic bone tissue by endochondral ossification. In order to increase the efficiency of MB-CM utilization towards engineering scaled-up bone structures, here we hypothesized that MB-CM can retain osteoinductive properties when combined with an osteoconductive material. We first tested different volumetric ratios of MB-CM:SmartBone® (as clinically used, osteoconductive reference material) and assessed the bone formation capacity of the resulting composites following ectopic mouse implantation. After 8 weeks, as little as 25% of MB-CM was sufficient to induce bone formation and fusion across SmartBone® granules, generating large interconnected bony structures. The same composite percentage was then further assessed in a scaled-up model, namely within an axially-vascularized, confined, ectopically prefabricated flap (0.8 cm³) in rats. The material efficiently induced the formation of new bone (31% of the cross-sectional area after 8 weeks), including bone marrow and vascular elements, throughout the flap volume. Our findings outline a strategy for efficient use of MB-CM as part of a composite material, thereby reducing the amount required to fill large spaces and enabling utilization in critically sized grafts, to address challenging clinical scenarios in bone reconstruction. STATEMENT OF SIGNIFICANCE: Most bone repair strategies rely either on osteconductive properties of ceramics and devitalized bone, or osteoinductive properties of growth factors and extracellular matrices (ECM). Here we designed a composite material made of a clinically accepted osteoconductive material and an off-the-shelf tissue engineered human cartilage ECM with strong osteoinductive properties. We showed that low amount of osteoinductive ECM potentiated host cells recruitment to form large vascularized bone structures in two different animal models, one being a challenging prefabricated bone-flap model targeting challenging clinical bone repair. Overall, this study highlights the use of a promising human off-the-shelf material for accelerated healing towards clinical applications.

Chemical Bonding of Biomolecules to the Surface of Nano-Hydroxyapatite to Enhance Its Bioactivity

Hydroxyapatite (HA) is a significant constituent of bones or teeth and is widely used as an artificial bone graft. It is often used to replace the lost bones or in reconstructing alveolar bones before dental implantation. HA with biological functions finds its importance in orthopedic surgery and dentistry to increase the local concentration of calcium ions, which activate the growth and differentiation of mesenchymal stem cells (MSC). To make relevant use of HA in bone transplantation, the surfaces of orthopedic and dental implants are frequently coated with nanosized hydroxyapatite (nHA), but its low dispersibility and tendency to form aggregates, the purpose of the surface modification of bone implants is defeated. To overcome these drawbacks and to improve the histocompatibility of bone implants or to use nHA in therapeutic applications of implants in the treatment of bone diseases, various studies suggested the attachment of biomolecules (growth factors) or drugs through chemical bonding at the surface of nHA. The growth factors or drugs bonded physically at the surface of nHA are mostly unstable and burst released immediately. Therefore, reported studies suggested that the surface of nHA needs to be modified through the chemical bonding of biologically active molecules at the surface of bone implants such as proteins, peptides, or naturally occurring polysaccharides to prevent the aggregation of nHA and to get homogenous dispersion of nHA in solution. The role of irradiation in producing bioactive and antibacterial nHA through morphological variations in surfaces of nHA is also summarized by considering internal structures and the formation of reactive oxygen species on irradiation. This mini-review aims to highlight the importance of small molecules such as proteins, peptides, drugs, and photocatalysts in surface property modification of nHA to achieve stable, bioactive, and antibacterial nHA to act as artificial bone implants (scaffolds) in combination with biodegradable polymers.

A Biofabrication Strategy for a Custom-Shaped, Non-Synthetic Bone Graft Precursor with a Prevascularized Tissue Shell

Critical-sized defects of irregular bones requiring bone grafting, such as in craniofacial reconstruction, are particularly challenging to repair. With bone-grafting procedures growing in number annually, there is a reciprocal growing interest in bone graft substitutes to meet the demand. Autogenous osteo(myo)cutaneous grafts harvested from a secondary surgical site are the gold standard for reconstruction but are associated with donor-site morbidity and are in limited supply. We developed a bone graft strategy for irregular bone-involved reconstruction that is customizable to defect geometry and patient anatomy, is free of synthetic materials, is cellularized, and has an outer pre-vascularized tissue layer to enhance engraftment and promote osteogenesis. The graft, comprised of bioprinted human-derived demineralized bone matrix blended with native matrix proteins containing human mesenchymal stromal cells and encased in a simple tissue shell containing isolated, human adipose microvessels, ossifies when implanted in rats. Ossification follows robust vascularization within and around the graft, including the formation of a vascular leash, and develops mechanical strength. These results demonstrate an early feasibility animal study of a biofabrication strategy to manufacture a 3D printed patient-matched, osteoconductive, tissue-banked, bone graft without synthetic materials for use in craniofacial reconstruction. The bone fabrication workflow is designed to be performed within the hospital near the Point of Care.

Growth factor delivery using extracellular matrix-mimicking substrates for musculoskeletal tissue engineering and repair

Therapeutic approaches for musculoskeletal tissue regeneration commonly employ growth factors (GFs) to influence neighboring cells and promote migration, proliferation, or differentiation. Despite promising results in preclinical models, the use of inductive biomacromolecules has achieved limited success in translation to the clinic. The field has yet to sufficiently overcome substantial hurdles such as poor spatiotemporal control and supraphysiological dosages, which commonly result in detrimental side effects. Physiological presentation and retention of biomacromolecules is regulated by the extracellular matrix (ECM), which acts as a reservoir for GFs via electrostatic interactions. Advances in the manipulation of extracellular proteins, decellularized tissues, and synthetic ECM-mimetic applications across a range of biomaterials have increased the ability to direct the presentation of GFs. Successful application of biomaterial technologies utilizing ECM mimetics increases tissue regeneration without the reliance on supraphysiological doses of inductive biomacromolecules. This review describes recent strategies to manage GF presentation using ECM-mimetic substrates for the regeneration of bone, cartilage, and muscle.

3D Bioprinting of Human Tissues: Biofabrication, Bioinks, and Bioreactors

The field of tissue engineering has progressed tremendously over the past few decades in its ability to fabricate functional tissue substitutes for regenerative medicine and pharmaceutical research. Conventional scaffold-based approaches are limited in their capacity to produce constructs with the functionality and complexity of native tissue. Three-dimensional (3D) bioprinting offers exciting prospects for scaffolds fabrication, as it allows precise placement of cells, biochemical factors, and biomaterials in a layer-by-layer process. Compared with traditional scaffold fabrication approaches, 3D bioprinting is better to mimic the complex microstructures of biological tissues and accurately control the distribution of cells. Here, we describe recent technological advances in bio-fabrication focusing on 3D bioprinting processes for tissue engineering from data processing to bioprinting, mainly inkjet, laser, and extrusion-based technique. We then review the associated bioink formulation for 3D bioprinting of human tissues, including biomaterials, cells, and growth factors selection. The key bioink properties for successful bioprinting of human tissue were summarized. After bioprinting, the cells are generally devoid of any exposure to fluid mechanical cues, such as fluid shear stress, tension, and compression, which are crucial for tissue development and function in health and disease. The bioreactor can serve as a simulator to aid in the development of engineering human tissues from in vitro maturation of 3D cell-laden scaffolds. We then describe some of the most common bioreactors found in the engineering of several functional tissues, such as bone, cartilage, and cardiovascular applications. In the end, we conclude with a brief insight into present limitations and future developments on the application of 3D bioprinting and bioreactor systems for engineering human tissue.

Synergistic use of biomaterials and licensed therapeutics to manipulate bone remodelling and promote non-union fracture repair

Disrupted bone metabolism can lead to delayed fracture healing or non-union, often requiring intervention to correct. Although the current clinical gold standard bone graft implants and commercial bone graft substitutes are effective, they possess inherent drawbacks and are limited in their therapeutic capacity for delayed union and non-union repair. Research into advanced biomaterials and therapeutic biomolecules has shown great potential for driving bone regeneration, although few have achieved commercial success or clinical translation. There are a number of therapeutics, which influence bone remodelling, currently licensed for clinical use. Providing an alternative local delivery context for these therapies, can enhance their efficacy and is an emerging trend in bone regenerative therapeutic strategies. This review aims to provide an overview of how biomaterial design has advanced from currently available commercial bone graft substitutes to accommodate previously licensed therapeutics that target local bone restoration and healing in a synergistic manner, and the challenges faced in progressing this research towards clinical reality.

In vivo dynamic analysis of BMP-2-induced ectopic bone formation

Bone morphogenetic protein (BMP)-2 plays a central role in bone-tissue engineering because of its potent bone-induction ability. However, the process of BMP-induced bone formation in vivo remains poorly elucidated. Here, we aimed to establish a method for intravital imaging of the entire process of BMP-2-induced ectopic bone formation. Using multicolor intravital imaging in transgenic mice, we visualized the spatiotemporal process of bone induction, including appearance and motility of osteoblasts and osteoclasts, angiogenesis, collagen-fiber formation, and bone-mineral deposition. Furthermore, we investigated how PTH1-34 affects BMP-2-induced bone formation, which revealed that PTH1-34 administration accelerated differentiation and increased the motility of osteoblasts, whereas it decreased morphological changes in osteoclasts. This is the first report on visualization of the entire process of BMP-2-induced bone formation using intravital imaging techniques, which, we believe, will contribute to our understanding of ectopic bone formation and provide new parameters for evaluating bone-forming activity.

Thiol-ene Hydrogels for Local Delivery of PTH for Bone Regeneration in Critical Size defects

Neither allograft nor commercially available bone graft substitutes provide the same quality of bone healing as autograft. Incorporation of bioactive molecules like parathyroid hormone (PTH) within bone graft substitute materials may provide similar, if not better treatment options to grafting. The goal of this work was to develop a biomaterial system for the local delivery of PTH to large bone defects for promoting bone regeneration. PTH was loaded in a thiol-ene hydrogel at several concentrations and polymerized in and around an osteoconductive poly(propylene fumarate) (PPF) scaffold. PTH was shown to be bioactive when released from the hydrogel for up to 21 days. Eighty percent of the PTH was released by day 3 with the remaining 20% released by day 14. Bone healing was quantified in rat critical size femoral defects that were treated with hydrogel/PPF and 0, 1, 3, 10, or 30 µg of PTH. Although complete osseous healing was not observed in all samples in any one treatment group, all samples in the 10 µg PTH group were bridged fully by bone or a combination of bone and cartilage containing hypertrophic chondrocytes and endochondral ossification. Outcome measures indicated improved defect bridging by a combination of bony and cartilaginous tissue in the 10 μg treatment group compared with empty bone defects and defects treated with only hydrogel/PPF (i.e., without PTH). Given the tailorability of the hydrogel, future studies will investigate the effects of prolonged gradual PTH release on bone healing. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:536-544, 2020.

Efficiency of coculture with angiogenic cells or physiological BMP-2 administration on improving osteogenic differentiation and bone formation of MSCs

Cell-based bone regeneration with mesenchymal stem cells (MSCs) represents the current challenge toward repair of bone defects and fractures. The supposed hurdles for satisfactory performance of cell-based constructs include inadequate vascularization and osteogenic signals. Considering the reported beneficial role of angiogenic cells in promoting vascularization and osteogenic differentiation and the osteogenic potential of bone morphogenetic protein 2 (BMP-2), we here evaluated the efficiency of coculture with angiogenic cells or a physiological dose of BMP-2 on improving osteogenic differentiation of MSCs and bone formation in vivo. In three dimensional (3D) collagen hydrogels in vitro, cocultured human umbilical vein endothelial cells (HUVECs) in a 1:1 ratio or with a physiological dose of BMP-2 (2 ng/μL) promoted the osteogenic potential of MSCs evidenced by enhanced alkaline phosphatase activity and gene expression of osteogenic markers. Notably, HUVECs evoked similar osteogenic stimulation as BMP-2, albeit in a delayed manner. When their bone formation capacity was further evaluated in a mouse subcutaneous implantation model, MSCs with BMP-2 demonstrated the highest efficiency with reproducible bone formation. In contrast, MSCs cocultured with HUVECs constructs displayed substantial blood vessel-like structures with fibrous tissue rather than ectopic bone as MSC monoculture controls. Our findings confirm the priority of generating cell-based bone constructs with physiological BMP-2 administration and indicate the potential of using angiogenic cells to develop vascularized constructs. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 643-653, 2019.

VEGF-mediated angiogenesis and vascularization of a fumarate-crosslinked polycaprolactone (PCLF) scaffold

PURPOSE

Revascularization of natural and synthetic scaffolds is a critical part of the scaffold's incorporation and tissue ingrowth. Our goals were to create a biocompatible polymer scaffold with 3D-printing technology, capable of sustaining vascularization and tissue ingrowth.

METHODS

We synthesized biodegradable polycaprolactone fumarate (PCLF) scaffolds to allow tissue ingrowth via large interconnected pores. The scaffolds were prepared with Poly(lactic-co-glycolic acid)(PLGA) microspheres seeded with or without different growth factors including VEGF,FGF-2, and/or BMP-2. Scaffolds were implanted into the subcutaneous tissues of rats before undergoing histologic and microCT angiographic analysis.

RESULTS

At harvest after 12 weeks, scaffolds had tissue infiltrating into their pores without signs of scar tissue formation, fibrous capsule formation, or immune responses against PCLF. Histology for M1/M2 macrophage phenotypes confirmed that there were no overt signs of immune responses. Both microCT angiography and histologic analysis demonstrated marked tissue and vessel ingrowth throughout the pores traversing the body of the scaffolds. Scaffolds seeded with microspheres containing VEGF or VEGF with either BMP-2 or FGF-2 had significantly higher vascular ingrowth and vessel penetration than controls. All VEGF-augmented scaffolds were positive for Factor-VIII and exhibited collagen tissue infiltration throughout the pores. Furthermore, scaffolds with VEGF and BMP-2 had high levels of mineral deposition throughout the scaffold that are attributable to BMP-2.

CONCLUSIONS

PCLF polymer scaffold can be utilized as a framework for vascular ingrowth and regeneration of multiple types of tissues. This novel scaffold material has promise in tissue regeneration across all types of tissues from soft tissue to bone.

The local administration of parathyroid hormone encourages the healing of bone defects in the rat calvaria: Micro-computed tomography, histological and histomorphometric evaluation

OBJECTIVE

To evaluate the effect of a single-dose local administration of PTH on bone healing in rat calvarial bone defects by means of micro-computed tomography, histological and histomorphometric analysis.

DESIGN

Critical-size cranial osteotomy defects were created in 42 male rats. The animals were randomly divided into 3 groups. In the C Group, the bone defect was only filled with a blood clot. In the S Group, it was filled with a collagen sponge and covered with bovine cortical membrane. In the PTH Group, the defect was filled with a collagen sponge soaked with PTH and covered with bovine cortical membrane. The groups were further split in two for euthanasia 15 and 60days post-surgery. Data was statistically analyzed with t-tests for independent samples or the nonparametric Mann-Whitney test when applicable. Intragroup comparisons were analyzed with paired t-tests (p<0.05).

RESULTS

Micro-CT analysis results did not demonstrate statistically significant intergroup differences. At 15days post-surgery, the histomorphometric analysis showed that the PTH Group exhibited a significantly higher percentage of bone formation compared with the S Group. At 60days post-surgery, a higher percentage of new bone was observed in the PTH group.

CONCLUSION

The results suggest that the local administration of PTH encouraged the bone healing in critical-size calvarial defects in rats.

A New Vertebral Body Replacement Strategy Using Expandable Polymeric Cages

We have developed a novel polymeric expandable cage that can be delivered via a posterior-only surgical approach for the treatment of noncontained vertebral defects. This approach is less invasive than an anterior-only or combined approach and much more cost-effective than currently used expandable metal cages. The polymeric expandable cage is composed of oligo poly(ethylene glycol) fumarate (OPF), a hydrogel that has been previously shown to have excellent nerve and bone tissue biocompatibility. OPF hydrogel cages can expand to twice their original diameter and length within a surgical time frame following hydration. Modulation of parameters such as polymeric network crosslink density or the introduction of charge to the network allowed for precise expansion kinetics. To meet specific requirements due to size variations in patient vertebral bodies, we fabricated a series of molds with varied diameters and explored the expansion kinetics of the OPF cages. Results showed a stable expansion ratio of approximately twofold to the original size within 20 min, regardless of the absolute value of the cage size. Following implantation of a dried OPF cage into a noncontained vertebral defect and its in situ expansion with normal saline, other augmentation biomaterials, such as poly(propylene fumarate) (PPF), can be injected to the lumen of the OPF cage and allowed to crosslink in situ. The OPF/PPF composite scaffold can provide the necessary rigidity and stability to the augmented spine.

A Survey of Strategies to Modulate the Bone Morphogenetic Protein Signaling Pathway: Current and Future Perspectives

Bone morphogenetic proteins (BMPs) constitute the largest subdivision of the TGF-β family of ligands and are unequivocally involved in regulating stem cell behavior. Appropriate regulation of canonical BMP signaling is critical for the development and homeostasis of numerous human organ systems, as aberrations in the BMP pathway or its regulation are increasingly associated with diverse human pathologies. In this review, we provide a wide-perspective on strategies that increase or decrease BMP signaling. We briefly outline the current FDA-approved approaches, highlight emerging next-generation technologies, and postulate prospective avenues for future investigation. We also detail how activating other pathways may indirectly modulate BMP signaling, with a particular emphasis on the relationship between the BMP and Activin/TGF-β pathways.

Long Noncoding RNA H19 Promotes Osteoblast Differentiation Via TGF-β1/Smad3/HDAC Signaling Pathway by Deriving miR-675

Long noncoding RNAs (lncRNAs) are emerging as important regulatory molecules at the transcriptional and post-transcriptional levels and may play essential roles in the differentiation of human bone marrow mesenchymal stem cell (hMSC). However, their roles and functions remain unclear. Here, we showed that lncRNA H19 was significantly upregulated after the induction of osteoblast differentiation. Overexpression of H19 promoted osteogenic differentiation of hMSCs in vitro and enhanced heterotopic bone formation in vivo, whereas knockdown of H19 inhibited these effects. Subsequently, we found that miR-675, encoded by exon1 of H19, promoted osteoblast differentiation of hMSCs and was partially responsible for the pro-osteogenic effect of H19. Investigating the underlying mechanism, we demonstrated that H19/miR-675 inhibited mRNA and protein expression of transforming growth factor-β1 (TGF-β1). The downregulation of TGF-β1 subsequently inhibited phosphorylation of Smad3. Meanwhile, H19/miR-675 downregulated the mRNA and protein levels of histone deacetylase (HDAC) 4/5, and thus increased osteoblast marker gene expression. Taken together, our results demonstrated that the novel pathway H19/miR-675/TGF-β1/Smad3/HDAC regulates osteogenic differentiation of hMSCs and may serve as a potential target for enhancing bone formation in vivo.

Combination therapy with PTH and DBM cannot heal a critical sized murine femoral defect

Orthopaedic surgeons continue to search for cost-effective bone graft substitutes to enhance bone repair. Teriparatide (PTH 1-34) and demineralized bone matrix (DBM) have been used in patients to promote bone healing. We evaluated the efficacy of PTH and DBM in healing a critical sized femoral defect in three lineage-specific transgenic mice expressing Col3.6GFPtopaz (pre-osteoblastic marker), Col2.3GFPemerald (osteoblastic marker) and α-SMA-Cherry (pericyte/myofibroblast marker). Mid-diaphyseal defects measuring 2 mm in length were created in the central 1/3 of mice femora using a circular saw and stabilized with an alveolar distractor device and cerclage wires. Three groups were evaluated: Group I, PTH 30 μg/kg injection daily, Group II, PTH 30 μg/kg injection daily + DBM, and Group III, DBM + 30μL saline injection. PTH was given for 28 days or until the time of sacrifice. Animals were sacrificed at 7, 14, 28, and 56 days. Radiographs at the time of sacrifice were evaluated using a 5-point scaled scoring system. Radiographs showed a lack of healing across all treatment groups at all time points: Group I, 1.57 +/- 0.68; Group II, 3.00 +/- 1.29; and Group III, 2.90 +/- 1.03. Bone formation in the defect as measured by radiographic healing score was significantly better at 56 days in Groups II (p = 0.01) and III (p < 0.01) compared to Group I. Across all treatment groups and time points the defects were largely absent of osteoprogenitor cells based on gross observation of frozen histology and quantitation of cellular based histomorphometric parameters. Quantitation of frozen histologic slides showed a limited osteoprogenitor response to PTH and DBM. Our results suggest that the anabolic agent teriparatide is unable to induce healing in a critical sized mouse femoral defect when given alone or in combination with the DBM preparation we used as a local bone graft substitute.

The rational use of animal models in the evaluation of novel bone regenerative therapies

Bone has a high potential for endogenous self-repair. However, due to population aging, human diseases with impaired bone regeneration are on the rise. Current strategies to facilitate bone healing include various biomolecules, cellular therapies, biomaterials and different combinations of these. Animal models for testing novel regenerative therapies remain the gold standard in pre-clinical phases of drug discovery and development. Despite improvements in animal experimentation, excessive poorly designed animal studies with inappropriate endpoints and inaccurate conclusions are being conducted. In this review, we discuss animal models, procedures, methods and technologies used in bone repair studies with the aim to assist investigators in planning and performing scientifically sound experiments that respect the wellbeing of animals. In the process of designing an animal study for bone repair investigators should consider: skeletal characteristics of the selected animal species; a suitable animal model that mimics the intended clinical indication; an appropriate assessment plan with validated methods, markers, timing, endpoints and scoring systems; relevant dosing and statistically pre-justified sample sizes and evaluation methods; synchronization of the study with regulatory requirements and additional evaluations specific to cell-based approaches. This article is part of a Special Issue entitled "Stem Cells and Bone".

A tissue-engineered humanized xenograft model of human breast cancer metastasis to bone

The skeleton is a preferred homing site for breast cancer metastasis. To date, treatment options for patients with bone metastases are mostly palliative and the disease is still incurable. Indeed, key mechanisms involved in breast cancer osteotropism are still only partially understood due to the lack of suitable animal models to mimic metastasis of human tumor cells to a human bone microenvironment. In the presented study, we investigate the use of a human tissue-engineered bone construct to develop a humanized xenograft model of breast cancer-induced bone metastasis in a murine host. Primary human osteoblastic cell-seeded melt electrospun scaffolds in combination with recombinant human bone morphogenetic protein 7 were implanted subcutaneously in non-obese diabetic/severe combined immunodeficient mice. The tissue-engineered constructs led to the formation of a morphologically intact ‘organ’ bone incorporating a high amount of mineralized tissue, live osteocytes and bone marrow spaces. The newly formed bone was largely humanized, as indicated by the incorporation of human bone cells and human-derived matrix proteins. After intracardiac injection, the dissemination of luciferase-expressing human breast cancer cell lines to the humanized bone ossicles was detected by bioluminescent imaging. Histological analysis revealed the presence of metastases with clear osteolysis in the newly formed bone. Thus, human tissue-engineered bone constructs can be applied efficiently as a target tissue for human breast cancer cells injected into the blood circulation and replicate the osteolytic phenotype associated with breast cancer-induced bone lesions. In conclusion, we have developed an appropriate model for investigation of species-specific mechanisms of human breast cancer-related bone metastasis in vivo.

Synergistic effect of sustained release of growth factors and dynamic culture on osteoblastic differentiation of mesenchymal stem cells

Microparticles have been utilized as delivery vehicles of soluble factors to modify cellular behavior and therefore enhance tissue engineering regeneration. When incorporated into three-dimensional systems, microparticles can provide geometrical and temporal controlled release of bioactive agents, such as growth factors (GFs) to surrounding cells. This study investigates the effect of GFs release from biopolymer microparticles on osteoblastic differentiation of human mesenchymal stem cells (hMSCs) encapsulated in calcium (Ca)-alginate scaffolds while cultured in a tubular perfusion system bioreactor system. Empirical and deterministic models were used to demonstrate that poly(D,L-lactic-co-glycolic acid)-encapsulated GFs would result in a delayed release profile compared to GFs encapsulated into scaffolds directly. We hypothesized that the dual delivery of human bone-morphogenetic protein 2 (hBMP2) and human vascular endothelial growth factor to cells in dynamic culture would provide molecular and physical cues to promote differentiation. Results indicated that the exposures of hBMP2 and dynamic flow are sufficient in enhancing the osteoblastic differentiation pathway compared to no GF addition and static culture. The GF delivery system in a dynamic flow environment resulted in a synergistic effect on osteoblastic differentiation of hMSCs.

Effect of Intermittent Administration of Teriparatide (Parathyroid Hormone 1-34) on Bone Morphogenetic Protein-Induced Bone Formation in a Rat Model of Spinal Fusion

BACKGROUND

Although clinical bone morphogenetic protein (BMP) therapy is effective at enhancing bone formation in patients managed with spinal arthrodesis, the required doses are very high. Teriparatide (parathyroid hormone 1-34) is approved by the U.S. Food and Drug Administration to treat osteoporosis and is a potent anabolic agent. In this study, intermittent administration of parathyroid hormone 1-34 combined with transplantation of BMP was performed to elucidate the effect of parathyroid hormone 1-34 on the fusion rate and quality of newly formed bone in a rat model.

METHODS

A total of forty-eight male Sprague-Dawley rats underwent posterolateral lumbar spinal arthrodesis with one of three different treatments with recombinant human (rh) BMP-2: (1) 0 μg (control), (2) 2 μg (low dose), or (3) 50 μg (high dose). Each of the rhBMP-2 treatments was studied in combination with intermittent injections of either parathyroid hormone 1-34 (180 μg/kg/wk) or saline solution starting two weeks before the operation and continuing until six weeks after the operation. Osseous fusion was assessed with use of radiographs and a manual palpation test. Microstructural indices of the newly formed bone were evaluated with use of micro-computed tomography. The serum markers of bone metabolism were also quantified.

RESULTS

The fusion rate in the group treated with 2 μg of rhBMP-2 significantly increased (from 57% to 100%) with the administration of parathyroid hormone 1-34 (p < 0.05). The fusion rates in the other groups did not change significantly with the administration of parathyroid hormone 1-34. The bone volume density of the newly formed bone significantly increased in both the 2-μg and 50-μg rhBMP-2 treatment groups with the administration of parathyroid hormone 1-34 (p < 0.01). Micro-computed tomography scans of the newly formed bone clearly demonstrated an abundance of trabecular bone formation in the group treated with parathyroid hormone 1-34. In addition, serum levels of osteocalcin were significantly increased in the parathyroid hormone 1-34 treatment group.

CONCLUSIONS

Intermittent administration of parathyroid hormone 1-34 significantly increased fusion rates in the group treated with low-dose rhBMP-2, and it improved the quality of the newly formed bone in both the high and low-dose groups in a rat model of rhBMP-2-induced spinal fusion.

CLINICAL RELEVANCE

Our results suggest that the combined administration of rhBMP-2 and parathyroid hormone 1-34 may lead to efficient bone regeneration.

Annulus fibrosus cell characteristics are a potential source of intervertebral disc pathogenesis

In the end stage of intervertebral disc degeneration, cartilage, bone, endothelial cells, and neurons appear in association with the worsening condition. The origin of the abnormal cells is not clear. This study investigated the properties of progenitor cells in the annulus fibrosus (AF) using one in vitro and two in vivo models. Cultivation of rabbit AF cells with chondrogenic media significantly increased expressions of collagen and aggrecan. Upon exposure to osteogenic conditions, the cultures showed increased mineralization and expression of osteopontin, runx2, and bmp2 genes. Two models were used in the in vivo subcutaneous implantation experiments: 1) rabbit AF tissue in a demineralized bone matrix (DBM) cylinder (DBM/AF), and, 2) rat intact and needle punctured lumbar discs. Bone formation in the AF tissue was detected and hypertrophic chondrocytes and osteoblasts were present 1 month after implantation of the DBM/AF to nude mice. In addition to collagen I and II, immunostaining shows collagen X and osteocalcin expression in DBM/AF specimens 4 months after implantation. Similar changes were detected in the injured discs. Almost the entire needle punctured disc had ossified at 6 months. The results suggest that AF cells have characteristics of progenitor cells and, under appropriate stimuli, are capable of differentiating into chondrocytes and osteoblasts in vitro as well as in vivo. Importantly, these cells may be a target for biological treatment of disc degeneration.

Local rhBMP-2 Injection after Drill-Hole Injury in Rats: Does it Have Systemic Effects?

The aim of this study is to investigate the histopathological findings of drill hole healing and interactions of parathyroid hormone (PTH), β-catenin and transcription factor-4 (TCF7L2/Tcf-4) after local application of recombinant human bone morphogenic protein-2 (rhBMP-2). Sprague Dawley rats were used in two groups of ‘femoral cortex hole model’. In the non-treated group, a hole was opened with a 3 mm K-wire in the distal and mid third junction of the right femur. In the treated group, local rhBMP-2 protein was injected into the similar femoral hole. Sterile 18M H2O was injected into the femoral hole at contralateral femur. There was more subperiosteal membranous bone reaction in the group treated with rhBMP-2 injection compared to the non-treated group. This was also proven immunohistochemically in both ipsilateral and contralateral femur with increased anti bone morphogenic protein-2 (anti BMP-2) expression. Moreover, there was an increased subperiosteal reaction at the contralateral femur. Also, in the treated group, PTH expression was increased in cells that form callus, and nuclear beta-catenin expression was increased in chondrocytes of periosteal ossification. Future studies should try to find whether the effects of rhBMP-2 on PTH and Wnt signaling pathway changes with different fracture models, also the systemic effects of local rhBMP-2 application should be investigated.

Poor osteoinductive potential of subcutaneous bone cement-induced membranes for tissue engineered bone

BACKGROUND

Large segmental bone defects remain a challenge for reconstructive surgeons. A two-stage repair strategy may offer a potential solution. Here, we sought to evaluate the osteoinductive potential of bone cement-induced membranes in an ectopic site.

METHODS

First, bone cements were inserted into the subcutaneous tissues of 16 rabbits to induce membrane formation. After 2, 4, 6 and 8 weeks, the induced membranes were harvested to assess their vascularization and osteoinductive potential. Next, bone cements were subcutaneously inserted into 12 rabbits for 4 weeks. These bone cements were then harvested from the newly formed membranes and replaced with granular porous β-TCP, with or without bone mesenchymal stem cells. New bone formation was then evaluated after 3, 6 and 9 weeks.

RESULTS

The highest level of blood vessel formation and bone morphogenetic protein-2 expression in the membranes were found at 4 weeks (p < 0.05). In addition, vascular endothelial growth factor concentration was highest after 2 weeks (p < 0.001), persisting until 8 weeks. However, the results showed little ectopic bone formation at these time points.

CONCLUSION

While bone cement-induced membranes appear to provide a suitable environment for bone formation, they fail to drive osteoinduction in non-osseous sites for the purposes of bone tissue engineering.

Effect of bone morphogenetic protein-2, demineralized bone matrix and systemic parathyroid hormone (1-34) on local bone formation in a rat calvaria critical-size defect model

AIM

To determine the potential of recombinant human bone morphogenetic protein-2 (rhBMP-2) soak-loaded on to an absorbable collagen sponge (ACS) to induce local bone formation compared with the clinical reference demineralized bone matrix (DBM) and to investigate potential additive/synergistic effects of exogenous parathyroid hormone (PTH).

METHODS

Critical-size (8 mm), through-through calvaria osteotomy defects in 160 adult male Sprague-Dawley rats were randomized to receive one of eight interventions: rhBMP-2/ACS, DBM, ACS, or serve as controls (empty defects) combined or not with systemic PTH. Ten animals from each group were followed for 4 and 8 wks for radiographic and histometric analysis. Multivariable analysis was used to assess the effect of experimental intervention and healing time on local bone formation.

RESULTS

In the multivariable analysis, rhBMP-2/ACS exhibited significantly greater histologic bone formation than control (β ± SE: 54.76 ± 5.85, p < 0.001) and ACS (β ± SE: 9.14 ± 3.31, p = 0.007) whereas DBM showed significantly less bone formation than control (β ± SE: -32.32 ± 8.23, p < 0.001). Overall, PTH did not show a significant effect on bone formation (β ± SE: 2.72 ± 6.91, p = 0.70). No significant differences in histological defect closure were observed between 4 and 8 wks for all but the control group without PTH.

CONCLUSION

rhBMP-2/ACS significantly stimulates local bone formation whereas bone formation appears significantly limited by DBM. Systemic application of PTH provided no discernible additive/synergistic effects on local bone formation.

Strategies for controlled delivery of growth factors and cells for bone regeneration

The controlled delivery of growth factors and cells within biomaterial carriers can enhance and accelerate functional bone formation. The carrier system can be designed with pre-programmed release kinetics to deliver bioactive molecules in a localized, spatiotemporal manner most similar to the natural wound healing process. The carrier can also act as an extracellular matrix-mimicking substrate for promoting osteoprogenitor cellular infiltration and proliferation for integrative tissue repair. This review discusses the role of various regenerative factors involved in bone healing and their appropriate combinations with different delivery systems for augmenting bone regeneration. The general requirements of protein, cell and gene therapy are described, with elaboration on how the selection of materials, configurations and processing affects growth factor and cell delivery and regenerative efficacy in both in vitro and in vivo applications for bone tissue engineering.

Growth factor delivery: how surface interactions modulate release in vitro and in vivo

Biomaterial scaffolds have been extensively used to deliver growth factors to induce new bone formation. The pharmacokinetics of growth factor delivery has been a critical regulator of their clinical success. This review will focus on the surface interactions that control the non-covalent incorporation of growth factors into scaffolds and the mechanisms that control growth factor release from clinically relevant biomaterials. We will focus on the delivery of recombinant human bone morphogenetic protein-2 from materials currently used in the clinical practice, but also suggest how general mechanisms that control growth factor incorporation and release delineated with this growth factor could extend to other systems. A better understanding of the changing mechanisms that control growth factor release during the different stages of preclinical development could instruct the development of future scaffolds for currently untreatable injuries and diseases.

Immortalization and characterization of osteoblast cell lines generated from wild-type and Nmp4-null mouse bone marrow stromal cells using murine telomerase reverse transcriptase (mTERT)

Intermittent parathyroid hormone (PTH) adds new bone to the osteoporotic skeleton; the transcription factor Nmp4/CIZ represses PTH-induced bone formation in mice and as a consequence is a potential drug target for improving hormone clinical efficacy. To explore the impact of Nmp4/CIZ on osteoblast phenotype, we immortalized bone marrow stromal cells from wildtype (WT) and Nmp4-knockout (KO) mice using murine telomerase reverse transcriptase. Clonal lines were initially chosen based on their positive staining for alkaline phosphatase and capacity for mineralization. Disabling Nmp4/CIZ had no gross impact on osteoblast phenotype development. WT and KO clones exhibited identical sustained growth, reduced population doubling times, extended maintenance of the mature osteoblast phenotype, and competency for differentiating toward the osteoblast and adipocyte lineages. Additional screening of the immortalized cells for PTH-responsiveness permitted further studies with single WT and KO clones. We recently demonstrated that PTH-induced c-fos femoral mRNA expression is enhanced in Nmp4-KO mice and in the present study we observed that hormone stimulated either an equivalent or modestly enhanced increase in c-fos mRNA expression in both primary null and KO clone cells depending on PTH concentration. The null primary osteoblasts and KO clone cells exhibited a transiently enhanced response to bone morphogenetic protein 2 (BMP2). The clones exhibited lower and higher expressions of the PTH receptor (Pthr1) and the BMP2 receptor (Bmpr1a, Alk3), respectively, as compared to primary cells. These immortalized cell lines will provide a valuable tool for disentangling the complex functional roles underlying Nmp4/CIZ regulation of bone anabolism.

Brief review of models of ectopic bone formation

Ectopic bone formation is a unique biologic entity--distinct from other areas of skeletal biology. Animal research models of ectopic bone formation most often employ rodent models and have unique advantages over orthotopic (bone) environments, including a relative lack of bone cytokine stimulation and cell-to-cell interaction with endogenous (host) bone-forming cells. This allows for relatively controlled in vivo experimental bone formation. A wide variety of ectopic locations have been used for experimentation, including subcutaneous, intramuscular, and kidney capsule transplantation. The method, benefits and detractions of each method are summarized in the following review. Briefly, subcutaneous implantation is the simplest method. However, the most pertinent concern is the relative paucity of bone formation in comparison to other models. Intramuscular implantation is also widely used and relatively simple, however intramuscular implants are exposed to skeletal muscle satellite progenitor cells. Thus, distinguishing host from donor osteogenesis becomes challenging without cell-tracking studies. The kidney capsule (perirenal or renal capsule) method is less widely used and more technically challenging. It allows for supraphysiologic blood and nutrient resource, promoting robust bone growth. In summary, ectopic bone models are extremely useful in the evaluation of bone-forming stem cells, new osteoinductive biomaterials, and growth factors; an appropriate choice of model, however, will greatly increase experimental success.

Harnessing the parathyroid hormone, Wnt, and bone morphogenetic protein signaling cascades for successful bone tissue engineering

Tissue engineering holds great promise as a way of enhancing the normal regenerative potential of bone. By deconstructing the skeleton into its components and examining how each component influences the reparative response, it is clear that cells resident in bone, bioactive molecules produced by these cells and those brought into bone via the circulation and the unique extracellular matrix that makes up the bone itself are involved in a continuous and ever-changing set of reciprocal interactions during regeneration. Reviewed here is current information regarding the efficacy of 3 prominent signaling cascades that orchestrate bone formation, parathyroid hormone, Wnt and bone morphogenetic proteins, in enhancing bone repair. I suggest how we might successfully generate new bone in increasingly complex clinical situations by modulating the availability of these signals to cells already present within bone tissue.