Beta-tricalcium phosphate particles as a controlled release carrier of osteogenic proteins for bone tissue engineering

In vivo approach of calcium deficient hydroxyapatite filler as bone induction factor

Tissue engineering combine biomaterials, cells and biologically active molecules having as a goal create functional tissues; many of the compositions are blends of a polymeric matrix with ceramic fillers, however, reduction of mechanical resistance can be a drawback on ceramic-polymer systems. In this manuscript, we investigate the potential of calcium-deficient hydroxyapatite (CDHA) whiskers, a needle shape bioceramic, to enhance mechanical and osteoconduction properties on the polymeric matrix. For this purpose, PCL scaffolds incorporating CDHA whiskers were produced by combining solvent casting and particulate leaching techniques to develop a composite scaffold that possess mechanical and biological properties which is useful for bone tissue engineering regeneration. We produced CDHA whiskers using alkaline hydrolysis of α-tricalcium phosphate and characterized by XRD, XRF and SEM. PCL/CDHA scaffolds were fabricated with a final porosity of ~70%, quantified by SEM images. Mechanical properties were evaluated by compression test. As an initial test, PCL/CDHA scaffolds were immersed in simulated body fluid to quantify apatite deposition. In vitro and in vivo studies were performed to assess cytotoxicity and bioactivity. CDHA whiskers exhibited a needle-like morphology and a Ca/P ratio equal to calcium deficient hydroxyapatite. The composite scaffolds contained interconnected pores 177 to 350 μm in size and homogeneous ceramic distribution. The addition of CDHA whiskers influences the mechanical results: higher elastic modulus and compressive strength was observed on PCL/CDHA samples. In vitro results demonstrated biocompatibility on PCL and PCL/CDHA films. In vivo data demonstrated cellular infiltration from the surrounding tissue with new bone formation that suggests bioactive potential of CDHA whiskers. Our goal was to produce a scaffold with a potential induction factor and a favorable morphology, which was proved according to this study's findings.

NELL-1 induces Sca-1+ mesenchymal progenitor cell expansion in models of bone maintenance and repair

NELL-1 is a secreted, osteogenic protein first discovered to control ossification of the cranial skeleton. Recently, NELL-1 has been implicated in bone maintenance. However, the cellular determinants of NELL-1's bone-forming effects are still unknown. Here, recombinant human NELL-1 (rhNELL-1) implantation was examined in a clinically relevant nonhuman primate lumbar spinal fusion model. Prolonged rhNELL-1 protein release was achieved using an apatite-coated β-tricalcium phosphate carrier, resulting in a local influx of stem cell antigen-1-positive (Sca-1+) mesenchymal progenitor cells (MPCs), and complete osseous fusion across all samples (100% spinal fusion rate). Murine studies revealed that Nell-1 haploinsufficiency results in marked reductions in the numbers of Sca-1+CD45-CD31- bone marrow MPCs associated with low bone mass. Conversely, rhNELL-1 systemic administration in mice showed a marked anabolic effect accompanied by increased numbers of Sca-1+CD45-CD31- bone marrow MPCs. Mechanistically, rhNELL-1 induces Sca-1 transcription among MPCs, in a process requiring intact Wnt/β-catenin signaling. In summary, NELL-1 effectively induces bone formation across small and large animal models either via local implantation or intravenous delivery. NELL-1 induces an expansion of a bone marrow subset of MPCs with Sca-1 expression. These findings provide compelling justification for the clinical translation of a NELL-1-based therapy for local or systemic bone formation.

Enhanced Mandibular Bone Repair by Combined Treatment of Bone Morphogenetic Protein 2 and Small-Molecule Phenamil

Growth factor-based therapeutics using bone morphogenetic protein 2 (BMP-2) presents a promising strategy to reconstruct craniofacial bone defects such as mandible. However, clinical applications require supraphysiological BMP doses that often increase inappropriate adipogenesis, resulting in well-documented, cyst-like bone formation. Here we reported a novel complementary strategy to enhance osteogenesis and mandibular bone repair by using small-molecule phenamil that has been shown to be a strong activator of BMP signaling. Phenamil synergistically induced osteogenic differentiation of human bone marrow mesenchymal stem cells with BMP-2 while suppressing their adipogenic differentiation induced by BMP-2 in vitro. The observed pro-osteogenic and antiadipogenic activity of phenamil was mediated by expression of tribbles homolog 3 (Trb3) that enhanced BMP-smad signaling and inhibited expression of peroxisome proliferator-activated receptor gamma (PPARγ), a master regulator of adipogenesis. The synergistic effect of BMP-2+phenamil on bone regeneration was further confirmed in a critical-sized rat mandibular bone defect by implanting polymer scaffolds designed to slowly release the therapeutic molecules. These findings indicate a new complementary osteoinductive strategy to improve clinical efficacy and safety of current BMP-based therapeutics.

Polyelectrolyte Complex for Heparin Binding Domain Osteogenic Growth Factor Delivery

During reconstructive bone surgeries, supraphysiological amounts of growth factors are empirically loaded onto scaffolds to promote successful bone fusion. Large doses of highly potent biological agents are required due to growth factor instability as a result of rapid enzymatic degradation as well as carrier inefficiencies in localizing sufficient amounts of growth factor at implant sites. Hence, strategies that prolong the stability of growth factors such as BMP-2/NELL-1, and control their release could actually lower their efficacious dose and thus reduce the need for larger doses during future bone regeneration surgeries. This in turn will reduce side effects and growth factor costs. Self-assembled PECs have been fabricated to provide better control of BMP-2/NELL-1 delivery via heparin binding and further potentiate growth factor bioactivity by enhancing in vivo stability. Here we illustrate the simplicity of PEC fabrication which aids in the delivery of a variety of growth factors during reconstructive bone surgeries.

Vertebral Implantation of NELL-1 Enhances Bone Formation in an Osteoporotic Sheep Model

BACKGROUND

Vertebral compression fractures related to osteoporosis greatly afflict the aging population. One of the most commonly used therapy today is balloon kyphoplasty. However, this treatment is far from ideal and is associated with significant side effects. NELL-1, an osteoinductive factor that possesses both pro-osteogenic and anti-osteoclastic properties, is a promising candidate for an alternative to current treatment modalities. This study utilizes the pro-osteogenic properties of recombinant human NELL-1 (rhNELL-1) in lumbar spine vertebral defect model in osteoporotic sheep.

METHODS

Osteoporosis was induced through ovariectomy, dietary depletion of calcium and vitamin D, and steroid administration. After osteoporotic induction, lumbar vertebral body defect creation was performed. Sheep were randomly implanted with the control vehicle, comprised of hyaluronic acid (HA) with hydroxyapatite-coated β-tricalcium phosphate (β-TCP), or the treatment material of rhNELL-1 protein lyophilized onto β-TCP mixed with HA. Analysis of lumbar spine defect healing was performed by radiographic, histologic, and computer-simulated biomechanical testing.

RESULTS

rhNELL-1 treatment significantly increased lumbar spine bone formation, as determined by bone mineral density, % bone volume, and mean cortical width as assessed by micro-computed tomography. Histological analysis revealed a significant increase in bone area and osteoblast number and decrease in osteoclast number around the implant site. Computer-simulated biomechanical analysis of trabecular bone demonstrated that rhNELL-1-treatment resulted in a significantly more stress-resistant composition.

CONCLUSION

Our findings suggest rhNELL-1-based vertebral implantation successfully improved cortical and cancellous bone regeneration in the lumbar spine of osteoporotic sheep. rhNELL-1-based bone graft substitutes represent a potential new local therapy.

Enhanced Osteogenesis of Adipose-Derived Stem Cells by Regulating Bone Morphogenetic Protein Signaling Antagonists and Agonists

Although adipose-derived stem cells (ASCs) are an attractive cell source for bone tissue engineering, direct use of ASCs alone has had limited success in the treatment of large bone defects. Although bone morphogenetic proteins (BMPs) are believed to be the most potent osteoinductive factors to promote osteogenic differentiation of ASCs, their clinical applications require supraphysiological dosage, leading to high medical burden and adverse side effects. In the present study, we demonstrated an alternative approach that can effectively complement the BMP activity to maximize the osteogenesis of ASCs without exogenous application of BMPs by regulating levels of antagonists and agonists to BMP signaling. Treatment of ASCs with the amiloride derivative phenamil, a positive regulator of BMP signaling, combined with gene manipulation to suppress the BMP antagonist noggin, significantly enhanced osteogenic differentiation of ASCs through increased BMP-Smad signaling in vitro. Furthermore, the combination approach of noggin suppression and phenamil stimulation enhanced the BMP signaling and bone repair in a mouse calvarial defect model by adding noggin knockdown ASCs to apatite-coated poly(lactic-coglycolic acid) scaffolds loaded with phenamil. These results suggest novel complementary osteoinductive strategies that could maximize activity of the BMP pathway in ASC bone repair while reducing potential adverse effects of current BMP-based therapeutics.

SIGNIFICANCE

Although stem cell-based tissue engineering strategy offers a promising alternative to repair damaged bone, direct use of stem cells alone is not adequate for challenging healing environments such as in large bone defects. This study demonstrates a novel strategy to maximize bone formation pathways in osteogenic differentiation of mesenchymal stem cells and functional bone formation by combining gene manipulation with a small molecule activator toward osteogenesis. The findings indicate promising stem cell-based therapy for treating bone defects that can effectively complement or replace current osteoinductive therapeutics.

Delivery of Phenamil Enhances BMP-2-Induced Osteogenic Differentiation of Adipose-Derived Stem Cells and Bone Formation in Calvarial Defects

Bone morphogenetic proteins (BMPs) have been widely used for bone repair in the craniofacial region. However, its high dose requirement in clinical applications revealed adverse effects and inefficient bone formation, along with high cost. Here, we report a novel osteoinductive strategy to effectively complement the osteogenic activity of BMP-2 using phenamil, a small molecule that can induce osteogenic differentiation via stimulation of BMP signaling. Treatment of adipose-derived stem cells (ASCs) with BMP-2 in combination with phenamil significantly promoted the in vitro osteogenic differentiation of ASCs. The efficacy of the combination strategy of phenamil+BMP-2 was further confirmed in a mouse calvarial defect model using scaffolds consisting of poly(lactic-co-glycolic acid) and apatite layer on their surfaces designed to slowly release phenamil and BMP-2. Six weeks after implantation, the scaffolds treated with phenamil+BMP-2 significantly promoted mouse calvarial regeneration as demonstrated by micro-computerized tomography and histology, compared with the groups treated with phenamil or BMP-2 alone. Moreover, the combination treatment reduced the BMP-2 dose without compromising calvarial healing efficacy. These results suggest promising complementary therapeutic strategies for bone repair in more efficient and cost-effective manners.

Adipose-derived stem cells and BMP-2 delivery in chitosan-based 3D constructs to enhance bone regeneration in a rat mandibular defect model

Reconstructing segmental mandiblular defects remains a challenge in the clinic. Tissue engineering strategies provide an alternative option to resolve this problem. The objective of the present study was to determine the effects of adipose-derived stem cells (ASCs) and bone morphogenetic proteins-2 (BMP-2) in three-dimensional (3D) scaffolds on mandibular repair in a small animal model. Noggin expression levels in ASCs were downregulated by a lentiviral short hairpin RNA strategy to enhance ASC osteogenesis (ASCs(Nog-)). Chitosan (CH) and chondroitin sulfate (CS), natural polysaccharides, were fabricated into 3D porous scaffolds, which were further modified with apatite coatings for enhanced cellular responses and efficient delivery of BMP-2. The efficacy of 3D apatite-coated CH/CS scaffolds supplemented with ASCs(Nog-) and BMP-2 were evaluated in a rat critical-sized mandibular defect model. After 8 weeks postimplantation, the scaffolds treated with ASCs(Nog-) and BMP-2 significantly promoted rat mandibular regeneration as demonstrated by micro-computerized tomography, histology, and immunohistochemistry, compared with the groups treated with ASCs(Nog-) or BMP-2 alone. These results suggest that our combinatorial strategy of ASCs(Nog-)+BMP-2 in 3D apatite microenvironments can significantly promote mandibular regeneration, and these may provide a potential tissue engineering approach to repair large bony defects.

Customized biomimetic scaffolds created by indirect three-dimensional printing for tissue engineering

Three-dimensional printing (3DP) is a rapid prototyping technique that can create complex 3D structures by inkjet printing of a liquid binder onto powder biomaterials for tissue engineering scaffolds. Direct fabrication of scaffolds from 3DP, however, imposes a limitation on material choices by manufacturing processes. In this study, we report an indirect 3DP approach wherein a positive replica of desired shapes was printed using gelatin particles, and the final scaffold was directly produced from the printed mold. To create patient-specific scaffolds that match precisely to a patient's external contours, we integrated our indirect 3DP technique with imaging technologies and successfully created custom scaffolds mimicking human mandibular condyle using polycaprolactone and chitosan for potential osteochondral tissue engineering. To test the ability of the technique to precisely control the internal morphology of the scaffolds, we created orthogonal interconnected channels within the scaffolds using computer-aided-design models. Because very few biomaterials are truly osteoinductive, we modified inert 3D printed materials with bioactive apatite coating. The feasibility of these scaffolds to support cell growth was investigated using bone marrow stromal cells (BMSC). The BMSCs showed good viability in the scaffolds, and the apatite coating further enhanced cellular spreading and proliferation. This technique may be valuable for complex scaffold fabrication.

Enhanced osteogenesis of adipose derived stem cells with Noggin suppression and delivery of BMP-2

Bone morphogenetic proteins (BMPs) are believed to be the most potent osteoinductive factors. However, BMPs are highly pleiotropic molecules and their supra-physiological high dose requirement leads to adverse side effects and inefficient bone formation. Thus, there is a need to develop alternative osteoinductive growth factor strategies that can effectively complement BMP activity. In this study, we intrinsically stimulated BMP signaling in adipose derived stem cells (ASCs) by downregulating noggin, a potent BMP antagonist, using an RNAi strategy. ASCs transduced with noggin shRNA significantly enhanced osteogenic differentiation of cells. The potency of endogenous BMPs was subsequently enhanced by stimulating ASCs with exogenous BMPs at a significantly reduced dose. The level of mineralization in noggin shRNA treated ASCs when treated with BMP-2 was comparable to that of control shRNA treated cell treated with 10-fold more BMP-2. The complementary strategy of noggin suppression + BMP-2 to enhance osteogenesis was further confirmed in 3D in vitro environments using scaffolds consisting of chitosan (CH), chondroitin sulfate (CS), and apatite layer on their surfaces designed to slowly release BMP-2. This finding supports the novel therapeutic potential of this complementary strategy in bone regeneration.

Anionic carbohydrate-containing chitosan scaffolds for bone regeneration

Scaffolds derived from naturally occurring polysaccharides have attracted significant interest in bone tissue engineering due to their excellent biocompatibility and hydrophilic nature favorable for cell attachment. In this study, we developed composite chitosan (CH) scaffolds containing anionic carbohydrate, such as chondroitin 4-sulfate (CS) or alginate (AG), with biomimetic apatite layer on their surfaces, and investigate their capacity to deliver progenitor cells (bone marrow stromal cells, BMSC) and model proteins with net-positive (histone) and net-negative charge (bovine serum albumin, BSA). The incorporation of CS or AG in CH scaffolds increased compressive modulus of the scaffolds and enhanced apatite formation. Initial burst release of histone was significantly higher than that of BSA from CH scaffold, while the addition of CS or AG in the scaffolds significantly reduced the initial burst release of histone, indicating strong electrostatic interaction between histone and negatively charged CS or AG. The apatite layer created on scaffold surfaces significantly reduced the initial burst release of both BSA and histone. Furthermore, apatite-coated scaffolds enhanced spreading, proliferation, and osteogenic differentiation of BMSC seeded on the scaffolds compared to non-coated scaffolds as assessed by live/dead and alamarBlue assays, scanning electron microscopy (SEM), alkaline phosphatase (ALP) activity, and Picrosirius red staining. This study suggests that apatite-coated CH/CS composite scaffolds have the potential as a promising osteogenic system for bone tissue engineering applications.

High yield isolation of BMP-2 from bone and in vivo activity of a combination of BMP-2/TGF-β1

A high-yield purification procedure for protein fractions derived from porcine bone matrix extracts is described, which has a high abundance of bone morphogenetic protein-2 (BMP-2). Naturally derived pBMP-2, ~5 μg per kilogram of porcine bone matrix, was isolated by using a 300 kDa membrane before chromatographic processing on heparin affinity media. The elution of pBMP-2 and transforming growth factor-β(1) (TGF-β(1)) revealed morphogen peaks that were unresolved on Prosep(®) medium, but resolved on hydroxyapatite medium. Antagonism was observed in animal studies when the two proteins were combined in specific doses. The TGF-β(1) fraction alone was not active in the rodent heterotopic in vivo bioassay, confirming previously obtained results.