High doses of bone morphogenetic protein 2 induce structurally abnormal bone and inflammation in vivo

Local delivery of recombinant human bone morphogenetic proteins and bisphosphonate via sucrose acetate isobutyrate can prevent femoral head collapse in Legg-Calve-Perthes disease: a pilot study in pigs

PURPOSE

Legg-Calve-Perthes disease is a paediatric condition encompassing idiopathic osteonecrosis of the femoral head (ONFH). Preventing collapse and the need for subsequent joint replacement remains the major goal of clinical management. This exploratory study utilises a porcine model of surgically induced ONFH.

METHODS

rhBMP-2 with and without zoledronic acid (ZA) was delivered by intra-osseous injection in the phase-transitioning sucrose acetate isobutyrate (SAIB) in an attempt to prevent femoral head collapse. Epiphyseal quotient (EQ) at eight weeks post-surgery was the primary outcome measure. Heterotopic ossification in the joint capsule and bisphosphonate retention in the femoral head were key secondary outcomes.

RESULTS

Femoral heads with ONFH and no treatment all collapsed (3/3, EQ < 0.4, P < 0.05 compared to no ONFH). Local delivery of rhBMP-2/SAIB into the femoral head prevented collapse by EQ measurement one of four samples; however, this specimen still showed evidence of significant collapse. In contrast, the combination of local rhBMP-2 and local ZA prevented collapse in two of four samples. Confocal fluorescence microscopy showed locally dosed bisphosphonate entered and was retained in the femoral head. This group also showed strong Calcein signal, indicating new bone formation. Treatment with rhBMP-2 was associated with a limited amount of heterotrophic ossification in the joint capsules in some specimens.

CONCLUSIONS

Operators reported SAIB to be an efficient way to deliver rhBMP-2 to the femoral head. These data suggest that rhBMP-2 is ineffective for preventing femoral head collapse without the addition of bisphosphonate. Further research will be required to validate the clinical efficacy of a combined local rhBMP-2/bisphosphonate approach.

Injectable gel graft for bone defect repair

Aim: To examine the performance of an injectable gel graft made of transglutaminase (Tg)-crosslinked gelatin gel with BMP-2 (BMP-2–Tg–Gel) for bone defect repair in animal models. Materials & methods: BMP-2 mixed with gelatin gel was crosslinked using Tg. The release of tethered BMP-2 through autocrine and paracrine pathways was demonstrated by using C2C12 and NIH 3T3 cells, respectively. BMP-2–Tg–Gel was injected into the induced cranial defect site. After 14 days, the sample was removed for x-ray imaging and histological evaluation. Results: Our in vivo results demonstrated that the injectable Tg–Gel with its osteoconductivity and controllable BMP-2 activity induced bone formation in our rat models when tethered with BMP-2. Conclusion: Tg–Gel as an injectable functional bone graft may enable the use of minimally invasive surgical procedures to treat irregular-shaped bone defects. Furthermore, this novel approach is capable of incorporating and controlling the release of therapeutic agents that may advance the science of tissue regeneration.

Tunable staged release of therapeutics from layer-by-layer coatings with clay interlayer barrier

In developing new generations of coatings for medical devices and tissue engineering scaffolds, there is a need for thin coatings that provide controlled sequential release of multiple therapeutics while providing a tunable approach to time dependence and the potential for sequential or staged release. Herein, we demonstrate the ability to develop a self-assembled, polymer-based conformal coating, built by using a water-based layer-by-layer (LbL) approach, as a dual-purpose biomimetic implant surface that provides staggered and/or sustained release of an antibiotic followed by active growth factor for orthopedic implant applications. This multilayered coating consists of two parts: a base osteoinductive component containing bone morphogenetic protein-2 (rhBMP-2) beneath an antibacterial component containing gentamicin (GS). For the fabrication of truly stratified composite films with the customized release behavior, we present a new strategy-implementation of laponite clay barriers-that allows for a physical separation of the two components by controlling interlayer diffusion. The clay barriers in a single-component GS system effectively block diffusion-based release, leading to approximately 50% reduction in bolus doses and 10-fold increase in the release timescale. In a dual-therapeutic composite coating, the top GS component itself was found to be an effective physical barrier for the underlying rhBMP-2, leading to an order of magnitude increase in the release timescale compared to the single-component rhBMP-2 system. The introduction of a laponite interlayer barrier further enhanced the temporal separation between release of the two drugs, resulting in a more physiologically appropriate dosing of rhBMP-2. Both therapeutics released from the composite coating retained their efficacy over their established release timeframes. This new platform for multi-drug localized delivery can be easily fabricated, tuned, and translated to a variety of implant applications where control over spatial and temporal release profiles of multiple drugs is desired.

Osteogenic potential of BMP-2-releasing self-assembled membranes

We report here the use of novel self-assembling collagen-hyaluronic acid (HyA) membranes to deliver bone morphogenetic protein-2 (BMP-2) for orthopedic applications. Prior work has demonstrated that collagen-HyA membranes are formed initially through electrostatic interactions between the oppositely charged collagen and HyA molecules, and that membrane growth is driven by osmotic pressure imbalances between the collagen and HyA solutions. The purpose of this study was to investigate the potential of incorporating charged growth factors such as BMP-2 within the membrane for regenerative medicine applications. Membrane material properties, protein mass loss, and release kinetics of BMP-2, as well as biocompatibility and osteogenic potential in vitro and in vivo using a subcutaneous mouse model were assessed. Scanning electron microscopy and mechanical testing confirmed no loss of structural or mechanical integrity upon BMP-2 incorporation into the membranes. Slow and steady release of the growth factor was demonstrated with 17% of total loaded BMP-2 released over the course of 49 days. To test biocompatibility and osteogenic potential in vitro, human mesenchymal stem cells were cultured on collagen-HyA membranes and showed greater proliferation rates (for up to 28 days) on membranes without BMP-2, but a greater alkaline phosphatase activity and osteocalcin production on membranes releasing BMP-2. In vivo subcutaneous implantation of the membranes showed a minimal immune response with osteoblasts and mineral deposits present in the ectopic site for BMP-2-releasing membranes, further demonstrating the potential of the BMP-2-releasing membranes to induce osteogenic differentiation. This study presents a novel strategy to create self-assembled membranes using two biocompatible molecules that can deliver bioactive agents in a sustained manner to induce a local regenerative response.

Enhancement of BMP-2 induced bone regeneration by SDF-1α mediated stem cell recruitment

Treatment of critical size bone defects is challenging. Recent studies showed that the cytokine stromal cell-derived factor 1 alpha (SDF-1α) has potential to improve the bone regenerative effect of low bone morphogenetic protein 2 (BMP-2) concentrations. The goal of this study was to demonstrate the combined effect of SDF-1α and BMP-2 on bone regeneration and stem cell recruitment using a critical size femoral bone defect model. A total of 72 mice were randomized to six groups. External fixators were implanted onto the right femur of each mouse and 3 mm defects were created. Depending on the group affiliation, adenovirally activated fat tissue grafts expressing SDF-1α or/and BMP-2 were implanted at the defect site. One day after operation, 1×10⁶ murine mesenchymal stromal cells (MSCs), lentivirally transduced to express the gene enhanced green fluorescent protein (eGFP), firefly luciferase, and CXCR4 were injected systemically in selected groups. Migration of the injected MSCs was observed by bioluminescence imaging on days 0, 2, 4, 6, 8, 10, 12, 14, 21, 28, and 42. After 6 weeks, animals were euthanized and 80 μm CT-scans were performed. For histological investigations, hematoxylin and eosin-, tartrate-resistant acid phosphatase-, alkaline phosphatase-, and anti-eGFP-stained sections were prepared. BMP-2 and SDF-1α combined at the defect site increased bone volume (BV) (2.72 mm³; 95% CI 1.95-3.49 mm³) compared with the negative control group (1.80 mm³; 95% CI 1.56-2.04 mm³; p<0.05). In addition, histological analysis confirmed a higher degree of bone healing in the BMP-2 and SDF-1α combined group compared with the negative control group. Bioluminescence imaging demonstrated higher numbers of migrated MSCs toward the defect site in the presence of both BMP-2 and SDF-1α at the defect site. Furthermore, eGFP-labeled migrated MSCs were found in all defect areas, when cells were injected. The ratio of osteoblasts to osteoclasts, assessed by immunohistological staining, was higher and thus showed a trend toward more bone formation for the combined use of BMP-2 and SDF-1α compared with all other groups. This study demonstrated that SDF-1α enhanced BMP-2 mediated bone healing in a critical size segmental bone defect model. Notably, both proteins alone also provided a cumulative effect on MSC attraction toward the site of injury.

Sinus augmentation using BMP-2 in a bovine hydroxyapatite/collagen carrier in dogs

AIM

The objective of this study was to determine the efficacy of bone morphogenetic protein 2 (BMP-2) in a bovine hydroxyapatite/collagen (BHC) carrier to augment bone formation in a canine nasal sinus model.

METHODS

Eight mongrel dogs, approximately 12 months old and 30 kg in weight were used. Following preparation of bilateral sinus access windows, BHC alone (control) or loaded with E. coli-derived BMP-2 at 0.1 mg/ml was implanted in four animals, and BHC loaded with E. coli-derived BMP-2 at 0.5 and 1.5 mg/ml was implanted in four animals. The animals were euthanized at 20 weeks when block sections were obtained for micro-computed tomography and histometric analyses.

RESULTS

Total augmented volumes did not differ significantly between groups. Histometric analysis showed significantly enhanced bone formation for the BMP-2 groups compared with control.

CONCLUSION

BMP-2 in a BHC carrier, even at the low 0.1-mg/ml concentration, induces osteogenic activity, enhancing local bone formation in a canine sinus model.

The in vitro and in vivo treatment effects of overexpressed lentiviral vector-mediated human BMP2 gene in the femoral bone marrow stromal cells of osteoporotic rats

This study aimed to compare the treatment effects of lentiviral vector-mediated hBMP2 which was overexpressed in the femoral bone marrow stromal cells of osteoporotic rats through genetic infection in vitro and in vivo. Comparison of the two transgenic effects may be crucial to determining the lentivirus infection method to be used. Following a comparison of the rat bone marrow stromal cells (rBMSCs) in osteoporotic (MSCs OVX) and normal (MSCs CON) groups, the lentiviral vector-mediated human bone morphogenetic protein 2 (hBMP2), which overexpressed the BMSCs of osteoporotic rats in vitro (rBMSCs in OE group), was constructed. The osteogenic ability in the overexpressed (OE) group was then compared to that of the MSCs CON. The rBMSCs in the OE group (transplants of genetic infection in vitro) and the lentivirus-containing solution (injected material of genetic infection in vivo) were injected into the femurs. The treatment effect of each group was compared via bone mineral density (BMD) and bone histomorphometry. The hBMP2-modified osteoporosis rBMSCs formed by genetic infection in vitro (n=7) had an ameliorated treatment effect on the femur as compared to that of the in vivo (n=7) (BMD: 0.315 vs. 0.19 g/cm2, P<0.01; bone histomorphometry: For bone trabeculars (Tb.Ar/T.Ar): 0.301 vs. 0.114, P<0.01; for trabecular thickness (Tb.Th): 43.54 vs. 21.39 µm, P<0.01; for trabecular separation (Tb.Sp): 115.7 vs. 304.87 µm, P<0.01). The results showed that the treatment effects of osteoporotic rBMSCs on local osteoporosis performed by genetic infection were improved in vitro as compared to those in vivo.

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.

Minimizing the severity of rhBMP-2-induced inflammation and heterotopic ossification with a polyelectrolyte carrier incorporating heparin on microbead templates

STUDY DESIGN

A rodent model of posterior spinal fusion.

OBJECTIVE

The aim of this study was to evaluate the efficacy of low-dose recombinant human bone morphogenetic protein-2 (rhBMP-2) delivered with a heparin based polylectrolyte complex (PEC) carrier in facilitating posterior spinal fusion while concurrently minimizing seroma and heterotopic ossification.

SUMMARY OF BACKGROUND DATA

rhBMP-2 is being used to augment spinal fusion. However, complications such as heterotopic ossification and local soft tissue swellings have been reported. These are attributed to supraphysiological amount of rhBMP-2 and the poor modulation capacity of absorbable collagen sponge.

METHODS

Forty rats were randomized into 6 groups as follows. Group I: absorbable collagen sponge without rhBMP-2 (n = 4); group II: positive control, absorbable collagen sponge + 10 μg rhBMP-2 (n = 4); group III: alginate-(poly-L-lysine)-heparin (PEC) without rhBMP-2 (n = 8); group IV: PEC + 4.5 μg rhBMP-2 (n = 8); group V: PEC + 1.5 μg rhBMP-2 (n = 8); group VI: PEC + 0.5 μg rhBMP-2 (n = 8).

RESULTS

Between postoperative days 5 and 7, seroma was observed in all rhBMP-2 implanted groups irrespective of carrier and dose. However, the rate and size of seroma differed considerably. Although all animals (100%) in positive control group showed seroma, only one animal (12.5%) in group VI developed seroma at the implant site. The size of seroma in group VI was significantly smaller than that in positive control group. Micro-computed tomography evaluation revealed comparable mean fusion scores in all rhBMP-2 implanted groups. More importantly, although new bone was well contained within the cage in group VI, heterotopic ossification beyond the cage was observed in positive control group.

CONCLUSION

A new carrier has demonstrated capacity to minimize seroma formation as well as heterotopic ossification associated with rhBMP-2 by reducing the efficacious dose needed for consistent fusion. The results of this study indicate that PEC alginate microbeads may represent a new opportunity to define an efficient rhBMP-2 carrier.

Controlled release strategies for bone, cartilage, and osteochondral engineering--Part I: recapitulation of native tissue healing and variables for the design of delivery systems

The potential of growth factors to stimulate tissue healing through the enhancement of cell proliferation, migration, and differentiation is undeniable. However, critical parameters on the design of adequate carriers, such as uncontrolled spatiotemporal presence of bioactive factors, inadequate release profiles, and supraphysiological dosages of growth factors, have impaired the translation of these systems onto clinical practice. This review describes the healing cascades for bone, cartilage, and osteochondral interface, highlighting the role of specific growth factors for triggering the reactions leading to tissue regeneration. Critical criteria on the design of carriers for controlled release of bioactive factors are also reported, focusing on the need to provide a spatiotemporal control over the delivery and presentation of these molecules.

BMP2-induced inflammation can be suppressed by the osteoinductive growth factor NELL-1

Bone-morphogenetic protein 2 (BMP2) is currently the only Food and Drug Administration-approved osteoinductive growth factor used in clinical settings for bone regeneration and repair. However, the use of BMP2 is encumbered by numerous clinical complications, including postoperative inflammation and life-threatening cervical swelling. Thus, methods to prevent BMP2-induced inflammation would have far-reaching clinical implications toward improving current BMP2-based methods for bone regeneration. For the first time, we investigate the potential role of the growth factor Nel-like molecule-1 (NELL-1) in inhibiting BMP2-induced inflammation. Adult rats underwent a femoral bone onlay procedure, treated with either BMP2 protein (4 mg/mL), NELL-1 protein (4 mg/mL), or both proteins combined. Animals were evaluated at 3, 7, and 14 days postoperatively by histology, histomorphometry, immunohistochemistry, and real-time PCR for markers of inflammation (TNFα, IL6). The relative levels of TNFα and IL6 in serum were also detected by ELISA. The mechanism for NELL-1's anti-inflammatory effect was further assessed through examining inflammatory markers and generation of reactive oxygen species (ROS) in the mouse embryonic fibroblast NIH3T3 cells. BMP2 significantly induced local inflammation, including an early and pronounced polymorphonuclear cell infiltration accompanied by increased expression of TNFα and IL6. Treatment with NELL-1 alone elicited no significant inflammatory response. However, NELL-1 significantly attenuated BMP2-induced inflammation by all markers and at all timepoints. These local findings were also confirmed using systemic serum inflammatory biomarkers (TNFα, IL6). In each case, NELL-1 fully reversed BMP2-induced systemic inflammation. Lastly, our findings were recapitulated in vitro, where NELL-1 suppressed BMP2 induced expression of inflammatory markers, as well as NF-κB transcriptional activity and generation of ROS. BMP2-induced inflammation is a serious public health concern with potentially life-threatening complications. In the present study, we observed that the growth factor, NELL-1, significantly attenuates or completely reverses BMP2-induced inflammation. The mechanisms of NELL-1's anti-inflammatory effect are only partially elucidated, and may include reduction of NF-κB transcriptional activity or ROS generation.

NELL-1 based demineralized bone graft promotes rat spine fusion as compared to commercially available BMP-2 product

BACKGROUND

Spinal fusion is among the most commonly performed orthopaedic procedures. Unfortunately, current treatments such as autologous bone grafting or recombinant proteins (BMP-2) have numerous clinical shortcomings. Here, we directly compare the efficacy of NELL-1, a novel osteoinductive growth factor, to two currently available treatments, (1) recombinant BMP-2 and (2) iliac crest bone grafting, in a spinal fusion model.

METHODS

Twenty-six skeletally mature athymic rats underwent posterolateral spine fusion of L4/L5 vertebrae. Treatment groups included NELL-1 (10 and 50 μg) in a demineralized bone matrix (DBX), as compared to BMP-2 (90 μg) in an absorbable collagen sponge (ACS) or morselized iliac crest bone. Scaffolds without recombinant protein were used as controls. Animals were sacrificed at 4 weeks post-operative and fusion was assessed by manual palpation, radiography [high-resolution X-ray, micro-computed tomography (microCT)], histology (hematoxylin and eosin, Masson's trichrome) and immunohistochemistry (osteocalcin).

RESULTS

Results showed 100 % fusion in all NELL-1- and BMP-2-treated samples. In contrast, lower rates of fusion were observed in scaffold-only and bone graft treatment groups. MicroCT scans revealed radiographic evidence of fusion among spines treated with NELL-1. Bone bridging was also observed with BMP-2 treatment, but was accompanied by inner radiolucency, suggesting cyst-like bone formation. Histologically, NELL-1-treated grafts showed increased bone formation, endochondral ossification and vascularization. Although BMP-2 treated grafts exhibited increased bone formation and angiogenesis, numerous adipocytes were also observed.

CONCLUSION

NELL-1-based bone grafts are comparable to BMP-2 + ACS in spinal fusion efficacy. Histological differences were observed however, including robust endochondral ossification with NELL-1 treatment as compared to lipid-filled bone with BMP-2 treatment. These findings suggest NELL-1 based bone grafts show promise for future efforts in skeletal tissue engineering.

Recombinant human bone morphogenetic protein-2 stimulates the osteogenic potential of the Schneiderian membrane: a histometric analysis in rabbits

This study evaluated the osteoinductive effect of the recombinant human bone morphogenetic protein-2 (rhBMP-2)-coated biphasic calcium phosphate (BCP) carrier system on the grafted sinus area, including surrounding tissues and the Schneiderian membrane. A total of 18 male rabbits were used in this study; two for in vitro and 16 for in vivo experiments. Schneiderian membranes taken from two animals were cultured with or without rhBMP-2, and quantitative reverse transcriptase-polymerase chain reaction analysis was performed. Both maxillary sinuses in each of the 16 animals were used to compare the in vivo effects of rhBMP-2-coated BCP (experimental group) and BCP alone (control group). In each animal, rhBMP-2-coated BCP was grafted into one of the maxillary sinuses, and the same amount of BCP alone was grafted into the contralateral site in random order. Radiologic and histometric analyses were performed at 2 and 8 weeks after surgery. After 2 days of culturing with or without rhBMP-2, a significant increase in the expression of early osteoblasts (RUNX2, type I collagen, alkaline phosphatase, and osteopontin) could be observed. Different histologic healing patterns were observed in experimental and control sites: newly formed bone lining the reflected sinus membrane without bone formation was observed at the central areas of experimental sites (window=0.06%; center=0%; membrane=20.86% of new bone), whereas evenly distributed new bone formation was observed at the control sites (window=7.27%; center=7.41%; membrane=15.58% of new bone).The augmented volume was well maintained at both the experimental and control sites during the experimental period, but at 2 weeks, the augmented volume was greater at the experimental sites than at the control sites (232.62 and 195.29 mm(3), respectively; p<0.001). These results suggest that good space maintenance in sinus augmentation is achieved with BCP, while the osteoinductive potential of the sinus membrane is activated at the early stage of healing with rhBMP-2.

High-concentration of BMP2 reduces cell proliferation and increases apoptosis via DKK1 and SOST in human primary periosteal cells

BMP2, a well-known osteoinductive agent approved by FDA, is currently being used for various off-label orthopedic applications. Recently, concerns about its efficacy for off-label use, concentration, and complications have emerged. Interestingly, there is an extremely large discrepancy in BMP2 concentration between clinical use (i.e. 1.5mg/ml) and in vitro studies (50-300 ng/ml). The purpose of this study was to determine the effects of a relatively high-concentration of BMP2 on cell proliferation and apoptosis using human primary periosteal cells as BMP2 is generally applied around the periosteum in orthopedic surgeries. We isolated periosteal cells from three independent patients. The cell proliferation assessed by MTT activity was significantly reduced by a high-concentration of BMP2 (~2000 ng/ml), while such a reduction was not observed by using a low-concentration of BMP2 (~200 ng/ml). The cell apoptosis assessed by caspase activity was significantly increased by high-concentration BMP2, while such an increase was not observed by low-concentration BMP2. We found that Wnt signaling activity was significantly reduced by high-concentration BMP2 along with a dramatic increase in DKK1 and SOST, key inhibitors of Wnt signaling in bone. The addition of DKK1 or SOST protein to the primary periosteal cells reduced MTT activity and significantly increased caspase activity. Silencing the DKK1 or SOST expression using the siRNA technique normalized cell proliferation and apoptosis in the periosteum-derived cells when exposed to a high-concentration BMP2. Taken together, these results suggest that a high-concentration BMP2 decreases human periosteal cell proliferation and induces apoptosis via the activation of Wnt inhibitors DKK1 and SOST. This study provides new insights to the effects of high BMP2 concentration on human periosteal cells and brings out the possibility of multiple effects of current BMP2 therapy on various skeletal tissues.

Morphological differences in BMP-2-induced ectopic bone between solid and crushed hyaluronan hydrogel templates

The possibility to affect bone formation by using crushed versus solid hydrogels as carriers for bone morphogenetic protein 2 (BMP-2) was studied. Hydrogels, based on chemical crosslinking between hyaluronic acid and poly(vinyl alcohol) derivatives, were loaded with BMP-2 and hydroxyapatite. Crushed and solid forms of the gels were analyzed both in vitro via a release study using ¹²⁵I radioactive labeling of BMP-2, and in vivo in a subcutaneous ectopic bone model in rats. Dramatically different morphologies were observed for the ectopic bone formed in vivo in the two types of gels, even though virtually identical release profiles were observed in vitro. Solid hydrogels induced formation of a dense bone shell around non-degraded hydrogel, while crushed hydrogels demonstrated a uniform bone formation throughout the entire sample. These results suggest that by crushing the hydrogel, the construct's three-dimensional network becomes disrupted. This could expose unreacted functional groups, making the fragment's surfaces reactive and enable limited chemical fusion between the crushed hydrogel fragments, leading to similar in vitro release profiles. However, in vivo these interactions could be broken by enzymatic activity, creating a macroporous structure that allows easier cell infiltration, thus, facilitating bone formation.

Affinity-selected heparan sulfate for bone repair

Bone morphogenetic protein (BMP)-2 is a potent bone healing compound produced at sites of bone trauma. Here we present a therapeutic strategy to harness the activity of endogenously produced BMP-2 by delivery of an affinity-matched heparan sulfate (HS) glycos aminoglycan biomaterial that increases the bioavailability, bioactivity and half-life of this growth factor. We have developed a robust, cost effective, peptide-based affinity platform to isolate a unique BMP-2 binding HS variant from commercially available preparations of HS, so removing the manufacturing bottleneck for their translation into the clinic. This affinity-matched HS enhanced BMP-2-induced osteogenesis through improved BMP-2 kinetics and receptor modulation, prolonged pSMAD signaling and reduced interactions with its antagonist noggin. When co-delivered with a collagen implant, the HS was as potent as exogenous BMP-2 for the healing of critical-sized bone defects in rabbits. This affinity platform can be readily tuned to isolate HS variants targeted ata range of clinically-relevant growth and adhesive factors.

Localized delivery of growth factors for angiogenesis and bone formation in tissue engineering

Angiogenesis is a key component of bone formation. Delivery of growth factors for both angiogenesis and osteogenesis is about to gain important potential as a future therapeutic tool. This review focuses on these growth factors that have dual functions in angiogenesis and osteogenesis, and their localized application. A major hurdle in the clinical development of growth factor therapy so far is how to assure safe and efficacious therapeutic use of such factors and avoid unwanted side effects and toxicity. It is now firmly established from the available information that the type, dose, combinations and delivery kinetics of growth factors all play a decisive role for the success of growth factor therapy. All of these parameters have to be adapted and optimized for each animal model or clinical case. In this review we discuss some important parameters associated with growth factor therapy and present an overview of selected preclinical studies, followed by a conceptual description of both established and proposed delivery strategies meeting therapeutic needs.

Injection and adhesion palatoplasty: a preliminary study in a canine model

BACKGROUND

Raising mucoperiosteal flaps in traditional palatoplasty impairs mid-facial growth. Hyaluronic acid-based hydrogels have been successfully tested for minimally invasive craniofacial bone generation in vivo as carriers of bone morphogenetic protein-2 (BMP-2). We aimed to develop a novel flapless technique for cleft palate repair by injecting a BMP-2 containing hydrogel.

MATERIAL AND METHODS

Dog pups with congenital cleft palate were either non-treated (n=4) or treated with two-flap palatoplasty (n=6) or with the proposed injection/adhesion technique (n=5). The experimental approach was to inject a hyaluronic acid-based hydrogel containing hydroxyapatite and BMP-2 subperiosteally at the cleft palate margins of pups aged six weeks. At week ten, a thin strip of the medial edge mucosa was removed and the margins were closed directly. Occlusal photographs and computed tomography (CT) scans were obtained up to week 20.

RESULTS

Four weeks after the gel injection the cleft palate margins had reached the midline and engineered bone had enlarged the palatal bones. Removal of the medial edge mucosa and suturing allowed complete closure of the cleft. Compared to traditional palatoplasty, the injection/adhesion technique was easier, and the post-surgical recovery was faster. CT on week 20 revealed some overlapping or "bending" of palatal shelves in the two-flap repair group, which was not observed in the experimental nor control groups.

CONCLUSION

A minimally invasive technique for cleft palate repair upon injectable scaffolds in a dog model of congenital cleft palate is feasible. Results suggest better growth of palatal bones. This represents an attractive clinical alternative to traditional palatoplasty for cleft palate patients.

NELL-1 injection maintains long-bone quantity and quality in an ovariectomy-induced osteoporotic senile rat model

Over 10 million Americans have osteoporosis, and is the predominant cause of fractures in the elderly. Treatment of fractures in the setting of osteoporosis is complicated by a suboptimal bone regenerative response due to a decline in the number of osteoblasts, their function, and survival. Consequently, an osteogenic therapeutic to prevent and treat fractures in patients with osteoporosis is needed. Nel-like molecule-1 (NELL-1), a novel osteoinductive growth factor, has been shown to promote bone regeneration. In this study, we aim to demonstrate the capacity of recombinant NELL-1 to prevent ovariectomy (OVX)-induced osteoporosis in a senile rat model. Ten-month-old female Sprague-Dawley rats underwent either sham surgery or OVX. Subsequently, 50 μL of 600 μg/mL NELL-1 lyophilized onto a 0-50-μm tricalcium phosphate (TCP) carrier was injected into the femoral bone marrow cavity while phosphate-buffered saline (PBS) control was injected into the contralateral femur. Our microcomputed tomography results showed that OVX+PBS/TCP control femurs showed a continuous decrease in the bone volume (BV) and bone mineral density (BMD) from 2 to 8 weeks post-OVX. In contrast, OVX+NELL-1/TCP femurs showed resistance to OVX-induced bone resorption showing BV and BMD levels similar to that of SHAM femurs at 8 weeks post-OVX. Histology showed increased endosteal-woven bone, as well as decreased adipocytes in the bone marrow of NELL-1-treated femurs compared to control. NELL-1-treated femurs also showed increased immunostaining for bone differentiation markers osteopontin and osteocalcin. These findings were validated in vitro, in which addition of NELL-1 in OVX bone marrow stem cells resulted in increased osteogenic differentiation. Thus, NELL-1 effectively enhances in situ osteogenesis in the bone marrow, making it potentially useful in the prevention and treatment of osteoporotic fractures.

Stemming the Degeneration: IVD Stem Cells and Stem Cell Regenerative Therapy for Degenerative Disc Disease

The intervertebral disc (IVD) is immensely important for the integrity of vertebral column function. The highly specialized IVD functions to confer flexibility and tensile strength to the spine and endures various types of biomechanical force. Degenerative disc disease (DDD) is a prevalent musculoskeletal disorder and is the major cause of low back pain and includes the more severe degenerative lumbar scoliosis, disc herniation and spinal stenosis. DDD is a multifactorial disorder whereby an imbalance of anabolic and catabolic factors, or alterations to cellular composition, or biophysical stimuli and genetic background can all play a role in its genesis. However, our comprehension of IVD formation and theetiology of disc degeneration (DD) are far from being complete, hampering efforts to formulate appropriate therapies to tackle DD. Knowledge of the stem cells and various techniques to manipulate and direct them to particular fates have been promising in adopting a stem-cell based regenerative approach to DD. Moreover, new evidence on the residence of stem/progenitor cells within particular IVD niches has emerged holding promise for future therapeutic applications. Existing issues pertaining to current therapeutic approaches are also covered in this review.

Combined delivery of BMP-2 and bFGF from nanostructured colloidal gelatin gels and its effect on bone regeneration in vivo

During the process of bone regeneration, a multitude of morphogenetic signaling factors regulate cellular behavior and ultimately tissue response. These factors are presented to cells under strong spatial and temporal control, which stresses the relevance of controlled delivery of multiple growth factors for bone tissue regeneration. This demand for biomimetic delivery has prompted the development of a novel generation of biomaterials that is capable of delivering multiple growth factors in a controlled manner. Therefore, the current study has exploited the strong capacity of colloidal gels solely made of oppositely charged gelatin nanospheres to obtain controlled release of angiogenic and osteogenic growth factors. The release kinetics of dual delivery of osteogenic bone morphogenetic protein-2 (BMP-2) and angiogenic basic fibroblast growth factor (bFGF) were investigated in vitro by radiolabeling the respective growth factors and monitoring their release in vitro. Furthermore, the effect of single or dual delivery of BMP-2 and bFGF on bone regeneration was evaluated in vivo using a rat femoral condyle defect model. The in vitro results confirmed that the delivery kinetics of BMP-2 and/or bFGF are more dependent on the degree of crosslinking than on the type of gelatin. Sequential release characterized by rapid release of angiogenic bFGF and more sustained release of BMP-2 was obtained by loading bFGF onto cationic nanospheres of low crosslinking density and BMP-2 onto anionic nanospheres of high crosslinking density. The in vivo study demonstrated the biocompatibility and biodegradability of bare colloidal gelatin gels, and did not show any adverse effects on the process of bone healing after 4 week of implantation since the volumes of new bone formation were comparable to empty control defects. An obvious stimulatory effect on bone regeneration was observed for the colloidal gels loaded with BMP-2, whereas bFGF-loaded colloidal gelatin gels did not influence the rate of bone regeneration. In contrast, the combined delivery of BMP-2 and bFGF resulted into an inhibitory effect on osteogenesis under the current experimental conditions. Summarizing, the current study proved that nanostructured colloidal gelatin gels are suitable carriers for programmed and sustained release of multiple therapeutic proteins for tissue regeneration.

A uni-cortical femoral defect model in the rat: evaluation using injectable hyaluronan hydrogel as a carrier for bone morphogenetic protein-2

The development of biomaterial for bone regeneration requires animal models that are reliable and designed to mimic clinically relevant situations. We have previously investigated hydrogels comprised of modified hyaluronic acid and polyvinyl alcohol in models of ectopic bone formation. This hydrogel induces bone regeneration when loaded with bone morphogenetic proteins (BMPs). To allow further optimization of hydrogels, we developed a new, femoral, non-critical-sized cortical defect model. In the rat femur, we drilled standardized, elongated unilateral cortical defects that did not require stabilization and that could be created bilaterally to allow paired comparisons of biomaterials. After optimizing the defect size, subsequent stress fractures occurred in only 8% and the defect healed partially over the 40 day study period. In a time-course experiment, we treated bone defects with the previously studied hyaluronan hydrogel loaded with 10 µg hydroxyapatite and 6 µg BMP-2. The shape of the defect allowed controlled containment of the material within the defect. The defect in the right leg was left untreated, while the left defect was filled with 40 µl of the BMP hydrogel. As determined by pQCT analysis, the treated defects had a higher bone mineral content, bone area and bone density than control defects. The relative difference was greatest between the groups at 10 and 20 days and diminished as the defect healed in the untreated legs. We conclude that this animal model allows facile and rapid screening of biomaterials for bone regeneration in cortical femoral defects without requiring external fixation.

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.

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.

Increased BMP expression in arthrofibrosis after TKA

PURPOSE

Because of the multiple possible aetiologies of painful total knee arthroplasty (TKA), the diagnosis and treatment of such patients are challenging. In a considerable number of patients, an intraarticular pathology is present, although not verifiable with clinical and diagnostic imaging techniques as in cases of primary arthrofibrosis. In these patients, the differentiation between intra- and extraarticular causes of pain remains difficult. Until now, little attention has been paid to changes of the synovial fluid and tissue in these knees. The objective of this study was to analyse the changes of the synovial environment in patients suffering from arthrofibrosis after TKA in comparison with knees with referred pain suffering from hip arthritis. The changes of the synovial environment probably provide additional diagnostic information to verify an intraarticular pathology.

METHODS

The synovial fluid of 10 consecutive knees in 10 patients presenting with a primary arthrofibrosis after TKA without signs of infection, instability, malalignment, or loosening was analysed and compared to the synovial fluid of 10 knees with referred pain serving as controls. The BMP-2 concentration was measured in the synovial fluid, and the presence of cytokines leading to an overexpression of BMP-2 was detected by measuring the change of BMP-2 expression in a synoviocyte cell line following exposing to the synovial fluid of the patients.

RESULTS

The concentration of BMP-2 in the synovial fluid was significantly higher in arthrofibrotic TKA knees (24.3 ± 6.9 pg/mL), compared with the control group 5.9 ± 4.8 pg/mL (P < 0.001). Corresponding to this finding, BMP-2 expression in synoviocytes was upregulated 11.5-fold (P < 0.05) by synovial fluid of patients suffering from arthrofibrosis after TKA, compared with the control group with referred pain.

CONCLUSION

BMP-2 is overexpressed and its concentrations are consequently higher in patients suffering from arthrofibrosis after TKA. The synovial BMP-2 concentration may be a potential marker for differentiating between intra- and extraarticular causes of pain.

LEVEL OF EVIDENCE

II.

Evaluation of BMP-2 tethered polyelectrolyte coatings on hydroxyapatite scaffolds in vivo

The goal of this in vivo study was to evaluate the osteoinductive and angio-inductive properties of a porous hydroxyapatite (HAp) scaffold with immobilized recombinant bone morphogenetic protein-2 (rhBMP-2) on the surface. It was hypothesized in this study that the use of a rhBMP-2 incorporated polyelectrolyte coating on the HAp scaffold would allow for controlled exposure of rhBMP-2 into the tissue and would provide a sound platform for tissue growth. The scaffolds were characterized for porosity and interconnectivity using pycnometry, scanning electron microscopy and micro-ct. These scaffolds were then divided into the following four groups: (a) HAp scaffold (n-HAp group), (b) rhBMP-2 physically adsorbed on HAp scaffold (HAp-BMP-2 Group), (c) polyelectrolyte coating on HAp scaffold without rhBMP-2 (HAp-PEI Scaffold Group), and (d) polyelectrolyte coating tethered with rhBMP-2 on HAp scaffold (HAp-PEI-BMP-2 Scaffold Group). Using 18 skeletally matured New Zealand white rabbits, these scaffolds were evaluated in a nonload bearing femoral condyle plug model. The negative controls for this study have defects that were left untreated and the positive controls have defects that were filled with autologous bone graft harvested from epsilateral iliac crest. Bone induction, vessel growth, and scaffold-bone contact were analyzed after 8-week implantation using micro-CT and histomorphometry. It was concluded from this study that the use of scaffold with an attached rhBMP-2 increased the vascularization around the implant when compared with the uncoated n-HAp scaffold, a necessary step of bone regeneration. The open-pore HAp scaffold was also concluded to provide a platform for tissue growth, drug loading, and tissue interaction.

Dose- and time-dependent effects of recombinant human bone morphogenetic protein-2 on the osteogenic and adipogenic potentials of alveolar bone-derived stromal cells

BACKGROUND AND OBJECTIVE

Recombinant human bone morphogenetic protein-2 (rhBMP-2) is a well-known growth factor that can induce robust bone formation, and recent studies have shown that rhBMP-2-induced osteogenesis is closely related to adipogenesis. The aim of the present study was to determine the dose- and time-dependent effects of rhBMP-2 on the osteogenic and adipogenic differentiation of human alveolar bone-derived stromal cells (hABCs) in vivo and in vitro.

MATERIAL AND METHODS

hABCs were isolated and cultured, and then transplanted using a carrier treated either with or without rhBMP-2 (100 μg/mL) into an ectopic subcutaneous mouse model. Comprehensive histologic and histometric analyses were performed after an 8-wk healing period. To further understand the dose-dependent (0, 10, 50, 200, 500 and 1000 ng/mL) and time-dependent (0, 3, 5, 7 and 14 d) effects of rhBMP-2 on osteogenic and adipogenic differentiation, in vitro osteogenic and adipogenic differentiation of hABCs were evaluated, and the expression of related mRNAs, including those for alkaline phosphatase, osteocalcin, bone sialoprotein, peroxisome-proliferator-activated receptor gamma-2 and lipoprotein lipase, were assessed using quantitative RT-PCR.

RESULTS

rhBMP-2 significantly promoted the osteogenic and adipogenic differentiation of hABCs in vivo, and gradually increased both the osteogenic and adipogenic potential in a dose- and time-dependent manner with minimal deviation in vitro. The expression of osteogenesis- and adipogenesis-associated mRNAs were concomitantly up-regulated by rhBMP-2.

CONCLUSION

The findings of the present study showed that rhBMP-2 significantly enhanced the adipogenic as well as the osteogenic potential of hABCs in dose- and time-dependent manner. The control of adipogenic differentiation of hABCs should be considered when regenerating the alveolar bone using rhBMP-2.

Osteophilic multilayer coatings for accelerated bone tissue growth

Osteophilic modular nanostructured multilayers containing hydroxyapatite nanoparticles complexed with a natural polymer chitosan create an osteoconductive surface for mesenchymal stem cells (MSCs). Coupled with the sustained release of physiological amounts of osteoinductive bone morphogenetic protein over several days from degradable poly(β-amino ester) based multilayers, this single coating results in a synergistic accelerated and upregulated differentiation of MSCs into osteoblasts laying down new bone tissue on orthopedic implants.

The signaling and functions of heterodimeric bone morphogenetic proteins

Heterodimeric bone morphogenetic proteins (BMPs) consist of disulfide-linked dimeric monomers derived from different BMP members. Owing to this specific constitution pattern, they bear high affinity to both type I and type II BMP receptors simultaneously. Meanwhile, the antagonism efficiency of extracellular antagonists to heterodimeric BMPs is also significantly lower than that to homodimeric ones. All these specific properties confer heterodimeric BMPs with distinct signaling and bio-functions that are characterized by more speediness, lower concentration/dose threshold and higher efficiency than homodimeric BMPs. Consequently, heterodimeric BMPs bear promising application potential in inducing osteogenesis. In addition, they may play indispensible roles in organogenesis. In this review, we summarize the current knowledge of heterodimeric BMPs in their signaling pathways and bio-functions.

Current methods of adipogenic differentiation of mesenchymal stem cells

There has been a recent increase in our understanding in the isolation, culture, and differentiation of mesenchymal stem cells (MSCs). Concomitantly, the availability of MSCs has increased, with cells now commercially available, including human MSCs from adipose tissue and bone marrow. Despite an increased understanding of MSC biology and an increase in their availability, standardization of techniques for adipogenic differentiation of MSCs is lacking. The following review will explore the variability in adipogenic differentiation in vitro, specifically in 3T3-L1 and primary MSCs derived from both adipose tissue and bone marrow. A review of alternative methods of adipogenic induction is also presented, including the use of specific peroxisome proliferator-activated receptor-gamma agonists as well as bone morphogenetic proteins. Finally, we define a standard, commonly used adipogenic differentiation medium in the hopes that this will be adopted for the future standardization of laboratory techniques--however, we also highlight the essentially arbitrary nature of this decision. With the current, rapid pace of electronic publications, it becomes imperative for standardization of such basic techniques so that interlaboratory results may be easily compared and interpreted.

Tunable dual growth factor delivery from polyelectrolyte multilayer films

A promising strategy to accelerate joint implant integration and reduce recovery time and failure rates is to deliver a combination of certain growth factors to the integration site. There is a need to control the quantity of growth factors delivered at different times during the healing process to maximize efficacy. Polyelectrolyte multilayer (PEM) films, built using the layer-by-layer (LbL) technique, are attractive for releasing controlled amounts of potent growth factors over a sustained period. Here, we present PEM films that sequester physiological amounts of osteogenic rhBMP-2 (recombinant human bone morphogenetic protein-2) and angiogenic rhVEGF₁₆₅ (recombinant human vascular endothelial growth factor) in different ratios in a degradable [poly(β-amino ester)/polyanion/growth factor/polyanion] LbL tetralayer repeat architecture where the biologic load scaled linearly with the number of tetralayers. No burst release of either growth factor was observed as the films degraded. The release of rhBMP-2 was sustained over a period of 2 weeks, while rhVEGF₁₆₅ eluted from the film over the first 8 days. Both growth factors retained their efficacy, as quantified with relevant in vitro assays. rhBMP-2 initiated a dose dependent differentiation cascade in MC3T3-E1S4 pre-osteoblasts while rhVEGF₁₆₅ upregulated HUVEC proliferation, and accelerated closure of a scratch in HUVEC cell cultures in a dose dependent manner. In vivo, the mineral density of ectopic bone formed de novo by rhBMP-2/rhVEGF₁₆₅ PEM films was approximately 33% higher than when only rhBMP-2 was introduced, with a higher trabecular thickness, which would indicate a decrease in the risk of osteoporotic fracture. Bone formed throughout the scaffold when both growth factors were released, which suggests more complete remodeling due to an increased local vascular network. This study demonstrates a promising approach to delivering precise doses of multiple growth factors for a variety of implant applications where control over spatial and temporal release profile of the biologic is desired.