Osteoinduction by bone morphogenetic protein 2-expressing adenoviral vector: application of biomaterial to mask the host immune response

Biomaterial-Guided Gene Delivery for Musculoskeletal Tissue Repair

Gene therapy is a promising strategy for musculoskeletal tissue repair and regeneration where local and sustained expression of proteins and/or therapeutic nucleic acids can be achieved. However, the musculoskeletal tissues present unique engineering and biological challenges as recipients of genetic vectors. Targeting specific cell populations, regulating expression in vivo, and overcoming the harsh environment of damaged tissue accompany the general concerns of safety and efficacy common to all applications of gene therapy. In this review, we will first summarize these challenges and then discuss how biomaterial carriers for genetic vectors can address these issues. Second, we will review how limitations specific to given vectors further motivate the utility of biomaterial carriers. Finally, we will discuss how these concepts have been combined with tissue engineering strategies and approaches to improve the delivery of these vectors for musculoskeletal tissue regeneration.

Realizing the potential of gene-based molecular therapies in bone repair

A better understanding of osteogenesis at genetic and biochemical levels is yielding new molecular entities that can modulate bone regeneration and potentially act as novel therapies in a clinical setting. These new entities are motivating alternative approaches for bone repair by utilizing DNA-derived expression systems, as well as RNA-based regulatory molecules controlling the fate of cells involved in osteogenesis. These sophisticated mediators of osteogenesis, however, pose unique delivery challenges that are not obvious in deployment of conventional therapeutic agents. Viral and nonviral delivery systems are actively pursued in preclinical animal models to realize the potential of the gene-based medicines. This article will summarize promising bone-inducing molecular agents on the horizon as well as provide a critical review of delivery systems employed for their administration. Special attention was paid to synthetic (nonviral) delivery systems because they are more likely to be adopted for clinical testing because of safety considerations. We present a comparative analysis of dose-response relationships, as well as pharmacokinetic and pharmacodynamic features of various approaches, with the purpose of clearly defining the current frontier in the field. We conclude with the authors' perspective on the future of gene-based therapy of bone defects, articulating promising research avenues to advance the field of clinical bone repair.

Gelatin coating to stabilize the transfection ability of nucleic acid polyplexes

Amphiphilic polymers are effective in complexing and delivering therapeutic nucleic acids, such as plasmid DNA (pDNA) and short interfering RNA (siRNA). However, long-term stability of the complexes is not desirable, as it may have an impact on the transfection efficiency in vivo. To develop a method to preserve complex stability we first showed that pDNA complexes formed with the amphiphilic polymer linoleic acid-substituted polyethylenimine (PEI-LA) and incubated at 37°C lost ~90% of their transfection efficiency after only 24h of complex formation. Polyethyleneglycol modification of complexes to control the increase in complex size and incubation in scaffolds used for implantation did not preserve the transfection ability of the complexes. Among a variety of approaches explored, gelatin coating of complexes was found to be the best at maintaining the original transfection efficiency. Mechanistic studies suggested that improved complex uptake, not size stability, was responsible for retention of the transfection efficiency. Similarly to the results with pDNA, gelatin coating also prevented the decreases in uptake and silencing efficiency of siRNA complexes observed following incubation at 37°C. Gelatin-stabilized complexes were, furthermore, effective in vivo and led to subcutaneous transgene expression with a low pDNA dose that was otherwise ineffective. We conclude that a simple gelatin coating approach offers an efficient means to preserve the transfection efficiency of polyplexes.

Gene therapy for the regeneration of bone

Gene transfer technologies offer the prospect of enhancing bone regeneration by delivering osteogenic gene products locally to osseous defects. In most cases the gene product will be a protein, which will be synthesized endogenously within and around the lesion in a sustained fashion. It will have undergone authentic post-translational processing and lack the alterations that occur when recombinant proteins are synthesized in bioreactors and stored. Several different ex vivo and in vivo gene delivery strategies have been developed for this purpose, using viral and non-viral vectors. Proof of principle has been established in small animal models using a variety of different transgenes, including those encoding morphogens, growth factors, angiogenic factors, and transcription factors. A small number of studies demonstrate efficacy in large animal models. Developing these promising findings into clinical trials will be a long process, constrained by economic, regulatory and practical considerations. Nevertheless, the overall climate for gene therapy is improving, permitting optimism that applications in bone regeneration will eventually become available.

Gene therapy for bone healing

Clinical problems in bone healing include large segmental defects, spinal fusions, and the nonunion and delayed union of fractures. Gene-transfer technologies have the potential to aid healing by permitting the local delivery and sustained expression of osteogenic gene products within osseous lesions. Key questions for such an approach include the choice of transgene, vector and gene-transfer strategy. Most experimental data have been obtained using cDNAs encoding osteogenic growth factors such as bone morphogenetic protein-2 (BMP-2), BMP-4 and BMP-7, in conjunction with both nonviral and viral vectors using in vivo and ex vivo delivery strategies. Proof of principle has been convincingly demonstrated in small-animal models. Relatively few studies have used large animals, but the results so far are encouraging. Once a reliable method has been developed, it will be necessary to perform detailed pharmacological and toxicological studies, as well as satisfy other demands of the regulatory bodies, before human clinical trials can be initiated. Such studies are very expensive and often protracted. Thus, progress in developing a clinically useful gene therapy for bone healing is determined not only by scientific considerations, but also by financial constraints and the ambient regulatory environment.

Direct gene therapy for bone regeneration: gene delivery, animal models, and outcome measures

While various problems with bone healing remain, the greatest clinical change is the absence of an effective approach to manage large segmental defects in limbs and craniofacial bones caused by trauma or cancer. Thus, nontraditional forms of medicine, such as gene therapy, have been investigated as a potential solution. The use of osteogenic genes has shown great potential in bone regeneration and fracture healing. Several methods for gene delivery to the fracture site have been described. The majority of them include a cellular component as the carrying vector, an approach known as cell-mediated gene therapy. Yet, the complexity involved with cell isolation and culture emphasizes the advantages of direct gene delivery as an alternative strategy. Here we review the various approaches of direct gene delivery for bone repair, the choice of animal models, and the various outcome measures required to evaluate the efficiency and safety of each technique. Special emphasis is given to noninvasive, quantitative, in vivo monitoring of gene expression and biodistribution in live animals. Research efforts should aim at inducing a transient, localized osteogenic gene expression within a fracture site to generate an effective therapeutic approach that would eventually lead to clinical use.

COMP-Ang1, a chimeric form of Angiopoietin 1, enhances BMP2-induced osteoblast differentiation and bone formation

INTRODUCTION

Angiogenesis is closely associated with bone formation, especially endochondral ossification. Angiopoietin 1 (Ang1) is a specific growth factor functioning to generate a stable and matured vasculature through the Tie2 receptor/PI3K/AKT pathway. Recently cartilage oligomeric matrix protein (COMP)-Ang1, an Ang1 variant which is more potent than native Ang1 in phosphorylating Tie2 receptor and AKT, was developed. This study was designed to examine the effects of angiogenic COMP-Ang1 on BMP2-induced osteoblast differentiation and bone formation.

METHODS

Expression of endogenous Ang-1 and its binding receptor Tie 2 mRNA was examined in osteoblast-like cells and primary mouse calvarial cells by RT-PCR analysis, and was also monitored during osteoblast differentiation induced by BMP-2 and/or ascorbic acid and beta-glycerophosphate. Effects of COMP-Ang-1 on osteoblast differentiation and mineralization were evaluated by alkaline phosphatase (ALP) activity and osteocalcin (OC) production, and Alizarin red stain. For a molecular mechanism, Western blot and OG2 and 6xOSE promoter assays were done. For in vivo evaluation, adenoviral (Ad) vectors containing COMP-Ang-1 or BMP-2 gene were administered into thigh muscle of mice, and after 2 weeks bone formation was analyzed by micro-computed tomography and histology. Angiogenic event of COMP-Ang1 was confirmed by immunofluorescence analysis with anti-CD31 antibody.

RESULTS

Expression of Tie2 receptor was significantly increased in the course of osteoblast differentiation. Treatment or overexpression of COMP-Ang1 enhanced BMP2-induced ALP activity, OC production, and mineral deposition in a dose-dependent manner. In addition, COMP-Ang1 synergistically increased OG2 and 6xOSE promoter activities of BMP2, and sustained p38, Smad and AKT phosphorylation of BMP2. Notably, in vivo intramuscular injection of COMP-Ang1 dose-dependently enhanced BMP2-induced ectopic bone formation with increases in CD31 reactivity.

CONCLUSIONS

These results suggest that COMP-Ang1 synergistically enhanced osteoblast differentiation and bone formation through potentiating BMP2 signaling pathways and angiogenesis. Combination of BMP2 and COMP-Ang1 should be clinically useful for therapeutic application to fracture and destructive bone diseases.

PDGF-B gene therapy accelerates bone engineering and oral implant osseointegration

Platelet-derived growth factor-BB (PDGF-BB) stimulates repair of healing-impaired chronic wounds such as diabetic ulcers and periodontal lesions. However, limitations in predictability of tissue regeneration occur due in part to transient growth factor bioavailability in vivo. Here, we report that gene delivery of PDGF-B stimulates repair of oral implant extraction socket defects. Alveolar ridge defects were created in rats and were treated at the time of titanium implant installation with a collagen matrix containing an adenoviral (Ad) vector encoding PDGF-B (5.5×10⁸ or 5.5×10⁹ pfu/ml), Ad encoding luciferase (Ad-Luc; 5.5×10⁹ pfu/ml; control) or recombinant human PDGF-BB protein (rhPDGF-BB, 0.3 mg/ml). Bone repair and osseointegration were measured via backscattered SEM, histomorphometry, microcomputed tomography, and biomechanical assessments. Further, a panel of local and systemic safety assessments was performed. Results demonstrated bone repair was accelerated by Ad-PDGF-B and rhPDGF-BB delivery compared to Ad-Luc, with the high dose of Ad-PDGF-B more effective than the low dose. No significant dissemination of the vector construct or alteration of systemic parameters was noted. In summary, gene delivery of Ad-PDGF-B demonstrates regenerative and safety capabilities for bone tissue engineering and osseointegration in alveolar bone defects comparable to rhPDGF-BB protein delivery in vivo.

Orthopedic gene therapy in 2008

Orthopedic disorders, although rarely fatal, are the leading cause of morbidity and impose a huge socioeconomic burden. Their prevalence will increase dramatically as populations age and gain weight. Many orthopedic conditions are difficult to treat by conventional means; however, they are good candidates for gene therapy. Clinical trials have already been initiated for arthritis and the aseptic loosening of prosthetic joints, and the development of bone-healing applications is at an advanced, preclinical stage. Other potential uses include the treatment of Mendelian diseases and orthopedic tumors, as well as the repair and regeneration of cartilage, ligaments, and tendons. Many of these goals should be achievable with existing technologies. The main barriers to clinical application are funding and regulatory issues, which in turn reflect major safety concerns and the opinion, in some quarters, that gene therapy should not be applied to nonlethal, nongenetic diseases. For some indications, advances in nongenetic treatments have also diminished enthusiasm. Nevertheless, the preclinical and early clinical data are impressive and provide considerable optimism that gene therapy will provide straightforward, effective solutions to the clinical management of several common debilitating disorders that are otherwise difficult and expensive to treat.

Efficient and rapid osteoinduction in an immune-competent host

Osteoinductive systems to induce targeted rapid bone formation hold clinical promise, but development of technologies for clinical use that must be tested in animal models is often a difficult challenge. We previously demonstrated that implantation of human cells transduced with Ad5F35BMP2 to express high levels of bone morphogenetic protein-2 (BMP2) resulted in rapid bone formation at targeted sites. Inclusion of human cells in this model precluded us from testing this system in an immune-competent animal model, thus limiting information about the efficacy of this approach. Here, for the first time we demonstrate the similarity between BMP2-induced endochondral bone formation in a system using human cells in an immune-incompetent mouse and a murine cell-based BMP2 gene therapy system in immune-competent animals. In both cases the delivery cells are rapidly cleared, within 5 days, and in neither case do they appear to contribute to any of the structures forming in the tissues. Endochondral bone formation progressed through a highly ordered series of stages that were both morphologically and temporally indistinguishable between the two models. Even longterm analysis of the heterotopic bone demonstrated similar bone volumes and the eventual remodeling to form similar structures. The results suggest that the ability of BMP2 to rapidly induce bone formation overrides contributions from either immune status or the nature of delivery cells.

Synergistic effects of Nell-1 and BMP-2 on the osteogenic differentiation of myoblasts

Osteogenesis is synergistically enhanced by the combined effect of complimentary factors. This study showed that Nell-1 and BMP-2 synergistically enhanced osteogenic differentiation of myoblasts and phosphorylated the JNK MAPK pathway. The findings are important because of the osteochondral specificity of Nell-1 signaling and the potential therapeutic effects of coordinated BMP-2 and Nell-1 delivery.

INTRODUCTION

BMPs play an important role in the migration and proliferation of mesenchymal cells and have a unique ability to alter the differentiation of mesenchymal cells toward chondrogenic and osteogenic lineages. Signaling upstream of Cbfa1/Runx2, BMPs effects are not limited to cells of the osteoblast lineage. Thus, additional osteoblast-specific factors that could synergize with BMP-2 would be advantageous for bone regeneration procedures. NELL-1 (NEL-like molecule-1; NEL [a protein strongly expressed in neural tissue encoding epidermal growth factor like domain]) is a novel growth factor believed to preferentially target cells committed to the osteochondral lineage.

MATERIALS AND METHODS

C2C12 myoblasts were transduced with AdLacZ, AdNell-1, AdBMP-2, or AdNell-1+AdBMP-2 overexpression viruses. Effects were studied by cell morphology, alkaline phosphatase activity, osteopontin production, and MAPK signaling. Additionally, in a nude mouse model, viruses were injected into leg muscles, and new bone formation was examined after 2 and 8 wk.

RESULTS

C2C12 myoblasts co-transduced with AdNell-1+AdBMP-2 showed a synergistic effect on osteogenic differentiation as detected by alkaline phosphatase activity and osteopontin production. Nell-1 stimulation on AdNell-1 + AdBMP-2 preconditioned C2C12 cells revealed significant activation of the non-BMP-2 associated c-Jun N-terminal kinase (JNK) MAPK signaling pathway, but not the p38 or extracellular signal-regulated kinase (ERK1/2) MAPK pathways. Importantly Nell-1 alone did not induce osteogenic differentiation of myoblasts. In a nude mouse model, injection of AdNell-1 alone stimulated no bone formation within muscle; however, injection of AdNell-1+AdBMP-2 stimulated a synergistic increase in bone formation compared with AdBMP-2 alone.

CONCLUSIONS

These findings are important because of the confirmed osteochondral specificity of Nell-1 signaling and the potential therapeutic effects of enhanced BMP-2 action with coordinated Nell-1 delivery.

Applications of gene therapy and adult stem cells in bone bioengineering

Bone tissue engineering is an emerging field, that could become a main therapeutic strategy in orthopedics in coming years. While bone has regenerative abilities that enable the self repair and regeneration of fractures, there are extreme situations in which the extent of bone loss is too large for complete regeneration to occur. In order to achieve bone regeneration, osteogenic genes (mainly from the bone morphogenetic protein family) can be delivered either directly into the target tissue, or by using adult stem cells, which are later implanted into the target site. Engineered adult stem cells combined with biodegradable polymeric scaffolds can be implanted into target sites, with or without ex vivo culture period. Several important factors influence the success of bone engineering approaches including: choice of cell and scaffold, the vector used in order to deliver the osteogenic gene, and the osteogenic gene itself. Cutting-edge imaging technologies, bioinformatics-based analysis of gene expression and exogenous regulation of transgene expression are among the tools that are being used to optimize and control bone formation in vivo. In this review we have attempted to provide an overview of the main factors that should be considered when utilizing adult stem cells and gene therapy strategies to regenerate bone defects or to promote new bone formation in vivo.

Endochondral Bone Formation from Hydrogel Carriers Loaded with BMP2-transduced Cells

The success of ex vivo viral gene therapy systems for promoting bone formation could be improved through the development of systems to spatially localize gene expression. Towards this goal, we have encapsulated adenovirus-transduced human diploid fetal lung fibroblasts (MRC-5) expressing bone morphogenetic protein-type 2 (BMP-2) within non-degradable poly(ethylene glycol)-diacrylate (PEG-DA) hydrogels and implanted these intramuscularly to promote endochondral bone formation. To optimize BMP-2 secretion, the molecular weight of the polymers and cell densities were varied. Polymers with molecular weights of 6, 10, and 20 kDa were used to prepare hydrogels containing 1, 5, or 10 million transduced cells. The results showed that 10 million transduced fibroblasts that was the maximum number of cells feasible for encapsulation within PEG-DA 10 and 20 kDa hydrogels produced the highest amount of secreted BMP-2 protein. Encapsulation of MRC-5 and transduced fibroblasts resulted in 71 and 58% cell viability, respectively. The bioactivity of secreted BMP-2 protein from the hydrogels was confirmed with an alkaline phosphatase assay. Micro-CT of the lower limb muscles of NOD/SCID mice following implantation with hydrogels showed 39.5 +/- 25.0 mm3 mineralized tissue and 31.8 +/- 7.8 mm3 for the cell-injected mice, and the bone was localized to the hydrogel surfaces. Histology revealed bone as well as cartilage for both hydrogel implanted and cell-injected animals.

Growth factor delivery for bone tissue repair: an update

Growth factors (GFs) are endogenous proteins capable of acting on cell-surface receptors and directing cellular activities involved in the regeneration of new bone tissue. The specific actions and long-term effects of GFs on bone-forming cells have resulted in exploration of their potential for clinical bone repair. The concerted efforts have led to the recent approval of two GFs, bone morphogenetic protein-2 and osteogenic protein-1, for clinical bone repair, and human parathryroid hormone (1-34) for augmentation of systemic bone mass. This review provides a selective summary of recent (2001-2004) attempts for GF delivery in bone tissue regeneration. First, a summary of non-human primate studies involving local regeneration and repair is provided, with special emphasis on the range of biomaterials used for GF delivery. Next, efforts to administer GFs for systemic augmentation of bone tissue are summarised. Finally, an alternative means of GF delivery, namely the delivery of genes coding for osteogenic proteins, rather than the delivery of the proteins, is summarised from rodent models. To conclude, future avenues of research considered promising to enhance the clinical application of GFs are discussed.

The potential of gene therapy for fracture healing in osteoporosis

Osteoporosis-associated fractures impair a patient's function and quality of life and represent one of the major public health burdens. Demographic changes predict a dramatic increase in osteoporotic fractures. Experimental data have shown that osteoporosis impairs fracture healing. Clinical observations demonstrate high failure rates of implant fixation in osteoporosis. The reduced healing capacity, including impaired bone formation, in osteoporotic humans might be due to defects in mesenchymal stem cells that lead to reduced proliferation and osteoblastic differentiation. Growth factors show remarkable promise as agents that can improve the healing of bone or increase the proliferation and differentiation capacities of mesenchymal stem cells. Their clinical utility is limited by delivery problems. The attraction of gene-transfer approaches is the unique ability to deliver authentically processed gene products to precise anatomical locations at therapeutic levels for sustained periods of time. Unlike the treatment of chronic diseases, it is neither necessary nor desirable for transgene expression to persist beyond the few weeks or months needed to achieve healing. This review presents different approaches of gene therapy to enhance fracture healing and summarizes the promising results of preclinical studies. It focuses on applications of this new technique to fracture healing in osteoporosis. In our opinion, these applications represent some of the few examples in which gene therapy has a good chance of early clinical success.