Use of recombinant human bone morphogenetic protein-2 to achieve posterolateral lumbar spine fusion in humans: a prospective, randomized clinical pilot trial: 2002 Volvo Award in clinical studies

Utilisation de facteurs de croissance pour la réparation osseuse

Osteoformation is induced by numerous growth factors that play an important role in bone repair such as fracture healing. They may serve as therapeutic agent in the treatment of squeletal injuries in the orthopeadic and maxillo-facial fields. Among these proteins, Bone Morphogenetic Proteins (BMP) are the only known osteoinductive growth factors. Unfortunately, they are highly susceptible to proteolysis in vivo and require a suitable delivery system to potentiate their biological activity in a local, controlled and durable manner. In this aim, three options are under investigations: (i) osteoinductive materials made of appropriate carrier to release the protein in situ, (ii) in vivo gene therapy in which the gene is directly transfected in cells of the patient or (iii) ex vivo gene therapy in which cells are harvested from the patient, transfected with DNA in culture and then implanted in the defect. These different kinds of BMP delivery will be discussed.

Degenerative spondylolisthesis: review of current trends and controversies

STUDY DESIGN

A literature-based review.

OBJECTIVES

To review management and controversies and to present authors recommendations.

SUMMARY OF BACKGROUND DATA

There is considerable controversy regarding indication for surgery, role for decompression alone, and decompression with fusion with or without instrumentation.

METHODS

Review of English language medical literature.

RESULTS

The condition may stabilize itself with the collapse of the disc spaces and osteophytes but may continue to progress in nearly a third of the cases. It may cause predominantly back pain due to segmental instability, or radicular pain/neurogenic claudication secondary to root entrapment or spinal stenosis. When conservative treatment fails, the mainstay of surgical treatment is decompressive laminectomy and fusion, with or without instrumentation.

CONCLUSIONS

Decompression primarily relieves radicular symptoms and neurogenic claudication whereas fusion primarily relieves back pain by elimination of instability. The goals for instrumentation are to promote fusion and to correct deformity. Fusion has a better long-term outcome than decompression alone. There is evidence that instrumentation improves fusion rate but does not improve clinical outcome in a relatively short-term follow-up. However, outcome of pseudarthrosis cases deteriorates over time and solid fusion produces better long-term outcome. The benefit of instrumentation comes with a price of higher postoperative morbidity and complication rate. Bone morphogenetic proteins are being tried to increase the rate of fusion, without increasing the complication rate, but the cost is prohibitive. More recently, dynamic stabilization with instrumentation but without fusion has been introduced as an alternative treatment. The current trends of surgical treatment and controversies are discussed.

The use of recombinant human bone morphogenetic protein-2 (rhBMP-2) in orthopaedic applications

Preclinical proof-of-concept, feasibility and efficacy studies in lower animals resulted in the accumulation of data that served as a backbone for clinical trials with the recombinant, osteogenic bone morphogenetic protein-2 (BMP-2). Among the important observations was the dependence of dose and carrier on the outcome for osseous union in relation to the animal model used. Clinical outcome data for spinal applications indicate better overall results compared with traditional controls that utilised autogenous iliac crest bone graft for fusion. Parameters include less blood loss, less operating room time and costs, better fusion outcomes and increased patient satisfaction. At this juncture, the long journey from the identification of BMP-2 in demineralised bone fraction to FDA approval for use in a singular orthopaedic application has been completed. It has been demonstrated to be safe, efficacious and cost-effective, leading to increased patient satisfaction and improved clinical outcome.

A new bone-inducing biodegradable porous beta-tricalcium phosphate

A new type of degradable biomaterial with bone-inducing capacity was made by combining porous beta-tricalcium phosphate (beta-TCP) with a delivery system for recombinant human bone morphogenetic protein-2 (rhBMP-2). The BMP delivery system consisted of a block copolymer composed of poly-D,L-lactic acid with random insertion of p-dioxanone and polyethylene glycol (PLA-DX-PEG), a known biocompatible and biodegradable material. The efficacy of this biomaterial in terms of its bone-inducing capacity was examined by ectopic bone formation in the dorsal muscles of the mouse. In the beta-TCP implants coated with the PLA-DX-PEG polymer containing more than 0.0025% (w/w) of rhBMP-2, new ectopic bone tissues with marrow were consistently found on the surface of implants. The radiographic density of beta-TCP was diminished in a time-dependent manner. On histological examination, numerous multinucleated osteoclasts with positive tartrate-resistant acid-phosphatase (TRAP) staining were noted on the surface of the beta-TCP. These experimental results indicate that beta-TCP implants coated with synthetic rhBMP-2 delivery system might provide effective artificial bone-graft substitutes with osteoinductive capacity and biodegradable properties. In addition, this type of biomaterial may require less rhBMP-2 to induce significant new bone mass.

Remodeling of cortical bone allografts mediated by adherent rAAV-RANKL and VEGF gene therapy

Structural allograft healing is limited because of a lack of vascularization and remodeling. To study this we developed a mouse model that recapitulates the clinical aspects of live autograft and processed allograft healing. Gene expression analyses showed that there is a substantial decrease in the genes encoding RANKL and VEGF during allograft healing. Loss-of-function studies showed that both factors are required for autograft healing. To determine whether addition of these signals could stimulate allograft vascularization and remodeling, we developed a new approach in which rAAV can be freeze-dried onto the cortical surface without losing infectivity. We show that combination rAAV-RANKL- and rAAV-VEGF-coated allografts show marked remodeling and vascularization, which leads to a new bone collar around the graft. In conclusion, we find that RANKL and VEGF are necessary and sufficient for efficient autograft remodeling and can be transferred using rAAV to revitalize structural allografts.

Evolving concepts in bone tissue engineering

The field of tissue engineering integrates the latest advances in molecular biology, biochemistry, engineering, material science, and medical transplantation. Researchers in the developing field of regenerative medicine have identified bone tissue engineering as an attractive translational target. Clinical problems requiring bone regeneration are diverse, and no single regeneration approach will likely resolve all defects. Recent advances in the field of tissue engineering have included the use of sophisticated biocompatible scaffolds, new postnatal multipotent cell populations, and the appropriate cellular stimulation. In particular, synthetic polymer scaffolds allow for fast and reproducible construction, while still retaining biocompatible characteristics. These criteria relate to the immediate goal of determining the ideal implant. The search is becoming a reality with widespread availability of biocompatible scaffolds; however, the desired parameters have not been clearly defined. Currently, most research focuses on the use of bone morphogenetic proteins (BMPs), specifically BMP-2 and BMP-7. These proteins induce osteogenic differentiation in vitro, as well as bone defect healing in vivo. Protein-scaffold interactions that enhance BMP binding are of the utmost importance, since prolonged BMP release creates the most osteogenic microenvironment. Transition into clinical studies has had only mild success and relies on large doses of BMPs for bone formation. Advances within the field of bone tissue engineering will likely overcome these challenges and lead to more clinically relevant therapies.

Biomaterial challenges and approaches to stem cell use in bone reconstructive surgery

As life expectancy increases, so does the need to treat large bone defects. New biomaterials combined with osteogenic cells are now being developed as an alternative to autogenous bone grafts. The goal is to make the stem cells adhere to the scaffold, and then grow to differentiate into functional osteogenic cells and organize into healthy bone as the scaffold degrades. Decisive improvements have been made in the fields of stem cell biology, 3-D scaffold fabrication and tissue engineering, but the ideal bone substitute that fulfils all functional and safety requirements has yet to be developed.

Combined bone morphogenetic protein-2 and -7 gene transfer enhances osteoblastic differentiation and spine fusion in a rodent model

To enhance the osteogenic activity of BMP, combination BMP2 and BMP7 gene transfer was performed. This approach led to a significant increase in osteoblastic differentiation of mesenchymal precursors compared with single BMP gene transfer in vitro. When tested in 78 rats, combination gene transfer enhanced mechanically stable spine fusion and bone formation rate versus single BMP gene transfer.

INTRODUCTION

Although clinical bone morphogenetic protein (BMP) therapy is effective, required doses are very high. Previous studies have suggested that the co-expression of two different BMP genes can result in the production of heterodimeric BMPs that may be more potent than homodimers. In this study, combined BMP2 and BMP7 gene transfer was performed to test whether this approach improves osteoblastic differentiation and bone formation compared with single BMP gene transfer.

MATERIALS AND METHODS

A producer cell (A549) was co-transfected with adenovirus vectors encoding BMP2 (AdBMP2) and BMP7 (AdBMP7) or, as controls, each vector alone, AdNull (with no transgene) or no virus. Supernatants were compared for their ability to stimulate osteoblastic differentiation of C2C12 myoblasts and MC3T3-E1 pre-osteoblasts. In a rat posterolateral spine fusion model, co-administration of AdBMP2 and AdBMP7 was compared with treatment with each vector alone, AdNull or no virus in 78 rats. The spines were assessed 8 weeks after surgery for radiographic and mechanical fusion, bone formation, and mineralization.

RESULTS

BMP2 and BMP7 were co-precipitated from supernatants of cells co-transfected with AdBMP2 and AdBMP7, indicating the presence of BMP2/7 heterodimer. Supernatants of co-transfected cells containing relatively low doses (7-140 ng/ml) of BMPs induced osteocalcin expression and alkaline phosphatase activity in both C2C12 and MC3T3-E1 cells, that were up to 6- and 40-fold higher, respectively, than levels induced by maximal doses (200-1000 ng/ml) of either BMP2 or BMP7 alone. In the spine fusion model, co-administration of AdBMP2 and AdBMP7 resulted in a significantly greater number of mechanically stable fusions and also 2-fold higher mineralization rate and bone volume in the fusion mass versus single BMP gene transfer (p < 0.02, all comparisons).

CONCLUSION

Combined BMP2 and BMP7 gene transfer is significantly more effective in inducing osteoblastic differentiation and spine fusion than individual BMP gene transfer.

Endoskopische Knochentransplantation an der Wirbels�ule

The application of autogenous bone grafts represents the golden standard for reconstruction of the load-bearing anterior column in the thoracolumbar spine. However, the osseous integration of the implanted grafts is demanding and delayed union or pseudarthrosis may occur. There are no standardized data available yet indicating the further course in such cases. The aim of this study was to evaluate the incorporation of endoscopically applied grafts and to develop therapeutic strategies for delayed or non-fusions. Twenty patients suffering from unstable injuries of the thoracolumbar spine were studied in a prospective clinical trial. After primary dorsal stabilization, the anterior column was thoracoscopically reconstructed with an autogenous iliac crest graft and a fixed-angle implant (MACS). The osseous integration of the bone grafts was detected by MSCT 1 year postoperatively. Complete integration of the transplanted bone grafts was observed in only 65% of the cases. In 25% partial integration was detected and in two cases a fracture of the transplanted iliac crest graft occurred. Despite the incomplete integration of the bone grafts, the further course without surgical intervention revealed no clinical or radiological evidence of a concomitant implant loosening or a relevant secondary loss of correction. Similar to the open technique, endoscopic reconstruction of the anterior column with autogenous bone grafts may lead to disadvantageous results concerning the integration and healing of the applied bone grafts. Decision making in such cases depends on the individual clinical and radiological findings (i.e., evidence of implant loosening and concomitant loss of correction).

Stroke and TGF-beta proteins: glial cell line-derived neurotrophic factor and bone morphogenetic protein

Recent studies have indicated that proteins in the transforming growth factor-beta superfamily alter damage induced by various neuronal injuries. Of these proteins, glial cell line-derived neurotrophic factor (GDNF) and bone morphogenetic protein-7 (BMP-7) have unique protective and regenerative effects in stroke animals. Delivery of GDNF or BMP-7 to brain tissue reduced cerebral infarction and improved motor functions in stroke animals. Pretreatment with these factors reduced caspase-3 activity and DNA fragmentation in the ischemic brain region, suggesting that antiapoptotic effects are involved. Beside the protective effects, BMP-7 given after stroke improves locomotor function. These regenerative effects of BMP-7 may involve the enhancement of dendritic growth and remodeling. In this review, we illustrate the neuroprotective and neuroregenerative properties of GDNF and BMP-7 and emphasize their therapeutic potential for stroke.

Adenoviral-mediated transfer of human BMP-6 gene accelerates healing in a rabbit ulnar osteotomy model

This study evaluated healing of rabbit bilateral ulnar osteotomies 6 and 8 weeks after surgery in response to percutaneous injection of transgenic adenoviral (Ad) bone morphogenetic protein-6 (BMP-6) vector or green fluorescent protein vector control (Ad-GFP) administered 7 days after surgery compared to untreated osteotomy controls. The amount, composition and biomechanical properties of the healing bone repair tissue were compared among groups and to historical data for intact rabbit ulnae obtained from similar studies at the same institution. Quantitative computed tomography was used to determine area, density and mineral content of the mineralized callus in the harvested ulnae. Maximum torque, torsional stiffness, and energy absorbed to failure were determined at 1.5 degrees /s. Calcified sections of excised ulnae (5 microm) were stained with Goldner's Trichrome and Von Kossa, and evaluated for callus composition, maturity, cortical continuity, and osteotomy bridging. Radiographic assessment of bone formation indicated greater mineralized callus in the ulnae injected with Ad-hBMP-6 as early as 1 week after treatment (2 weeks after surgery) compared to untreated osteotomy ulnae (p < 0.006) and Ad-GFP treated osteotomy ulnae (p < 0.002). Quantitative computed tomography confirmed greater bone area and bone mineral content at the osteotomy at 6 weeks in Ad-BMP-6 treated osteotomy as compared to untreated osteotomy ulnae (p < 0.001) and Ad-GFP treated osteotomy ulnae (p < 0.01). Ad-BMP-6 treated osteotomy ulnae were stronger (p < 0.001 and 0.003) and stiffer (p < 0.004 and 0.003) in torsion at 6 weeks than untreated osteotomy ulnae or Ad-GFP treated osteotomy ulnae, respectively. Maximum torque, torsional stiffness, and energy absorbed to failure were greater in Ad-BMP-6 treated osteotomy ulnae compared to their respective untreated contralateral osteotomy ulnae at 8 weeks [p < 0.03]. Maximum torque and torsional stiffness in the Ad-BMP-6 treated osteotomy ulnae were not different to intact ulnae values at 6 and 8 weeks. These experiments confirm that BMP-6 can be potently osteoinductive in vivo resulting in acceleration of bone repair.

A new simplified technique for producing platelet-rich plasma: a short technical note

A possible strategy to promote the wound-healing cascade in both soft and hard tissues is the preparation of an autologous platelet-rich plasma (PRP) to encourage the release of growth factors from activated platelets. In this process, PRP combines the advantage of an autologous fibrin clot that will aid in hemostasis as well as provide growth factors in high concentrations to the site of a tissue defect. The PRP preparation can be used as a biological enhancer in the healing of fractures and lumbar fusions. The local application of growth factors seems to promote initiation and early maturation of bone formation. Autologous bone or bone substitutes can be added to this mixture to increase the volume of grafting material. A simplified technique utilizing a commercially available separation system (GPS-Gravitational Platelet Separation System) is described. This system provides a less costly alternative to other previously described augmentation techniques and also presents a patient-friendly and operator-safe alternative. Further experimental studies of the actual concentrations of the growth factors in the PRP samples are necessary in order to validate the platelet concentration and growth-factor activation by laboratory evidence. In further prospective clinical trials, the safety and efficacy of PRP, in combination with autologous bone or bone graft substitutes, must be evaluated.

Osteoinductivity of commercially available demineralized bone matrix. Preparations in a spine fusion model

BACKGROUND

Although autogenous bone is the most widely used graft material for spinal fusion, demineralized bone matrix preparations are available as alternatives or supplements to autograft. They are prepared by acid extraction of most of the mineralized component, with retention of the collagen and noncollagenous proteins, including growth factors. Differences in allograft processing methods among suppliers might yield products with different osteoinductive activities. The purpose of this study was to compare the efficacy of three different commercially available demineralized bone matrix products for inducing spinal fusion in an athymic rat model.

METHODS

Sixty male athymic rats underwent spinal fusion and were divided into three groups of eighteen animals each. Group I received Grafton Putty; Group II, DBX Putty; and Group III, AlloMatrix Injectable Putty. A control group of six animals (Group IV) underwent decortication alone. Six animals from each of the three experimental groups were killed at each of three intervals (two, four, and eight weeks), and the six animals from the control group were killed at eight weeks. At each of the time-points, radiographic and histologic analysis and manual testing of the explanted spines were performed.

RESULTS

The spines in Group I demonstrated higher rates of radiographically evident fusion at eight weeks than did the spines in Group III or Group IV (p < 0.05). Manual testing of the spines at four weeks revealed variable fusion rates (five of six in Group I, two of six in Group II, and none of six in Group III). At eight weeks, all six spines in Group I, three of the six in Group II, and no spine in Group III or IV had fused. Histologic analysis of the spines in Groups I, II, and III demonstrated varying amounts of residual demineralized bone matrix and new bone formation. Group-I spines demonstrated the most new bone formation.

CONCLUSIONS

This study demonstrated differences in the osteoinductive potentials of commercially available demineralized bone matrices in this animal model.

Role of Activated Growth Factors in Lumbar Spinal Fusions

BACKGROUND

The concentration of platelets into an activated growth factor (AGF) gel may stimulate graft consolidation into a fusion mass. Preoperative hemodilution and intraoperative clot activation may also reduce the overall blood loss. Consequently, the need for postoperative transfusions may also be reduced.

OBJECTIVE

The objective of this work was to report our experience with AGF platelet gels in transforaminal lumbar interbody fusion (TLIF) procedures.

METHODS

A consecutive series of patients between 1996 and 1999 undergoing one- and two-level TLIFs with AGF were compared with a consecutive series of TLIF patients who did not receive AGF. Sixty-two control subjects who did not receive AGF and 22 patients who received an AGF platelet gel were compared after 41 and 34 months of follow-up, respectively.

RESULTS

On average, the AGF group required 18 minutes of additional preincision anesthesia (P = 0.0001). No statistical differences in the operative times, estimated blood loss, postoperative drainage, percentage of patients requiring a transfusion, or length of hospitalization were appreciated between the two groups. The 19% decrease in the arthrodesis rate of the AGF group, as compared with the control group, did not reach statistical significance. Platelet counts from the AGF platelet concentrates demonstrated an average 3.5-fold increase compared with preoperative serum levels.

CONCLUSIONS

The theoretical benefits of AGF platelet gel technology were not clinically appreciated. The cost of implementing this technology may therefore outweigh its theoretical benefits.

Glucocorticoid regulation of human BMP-6 transcription

Addition of dexamethasone (Dex) to human mesenchymal stem cells (hMSCs) resulted in a 16-fold increase in human bone morphogenetic protein-6 (hBMP-6) mRNA levels 24 h after treatment. Evaluation of luciferase expression after transfection of HeLa cells with hBMP-6 promoter/luciferase reporter constructs indicated that the hBMP-6 promoter activity was contained in a 268-bp region (-1051 to -784 where +1 is the translation start site) over 600 bases 5' to that previously published. It further showed that the promoter activity is regulated by glucocorticoid treatment. Analysis of RNA from hMSCs and HeLa cells by primer extension, RNase protection, and 5' RACE further narrowed the location of the transcription start site to an 84-bp region (-940 to -857). To determine whether this start site was regulated in hMSCs, hBMP-6 mRNA levels in control and Dex-treated cells were quantitated by RT-PCR using one primer set in the translated region of the gene and one located just 3' of the 84-bp region. Both primer sets showed hBMP-6 mRNA levels approximately 16- to 22-fold higher in the Dex-treated cells, demonstrating that hBMP-6 transcription is being regulated by glucocorticoids in the pluripotent hMSCs at the upstream transcription start site.

Engineering principles of clinical cell-based tissue engineering

Tissue engineering is a rapidly evolving discipline that seeks to repair, replace, or regenerate specific tissues or organs by translating fundamental knowledge in physics, chemistry, and biology into practical and effective materials, devices, systems, and clinical strategies. Stem cells and progenitors that are capable of forming new tissue with one or more connective tissue phenotypes are available from many adult tissues and are defined as connective tissue progenitors. There are four major cell-based tissue-engineering strategies: (1) targeting local connective tissue progenitors where new tissue is desired, (2) transplanting autogenous connective tissue progenitors, (3) transplanting culture-expanded or modified connective tissue progenitors, and (4) transplanting fully formed tissue generated in vitro or in vivo. Stem cell function is controlled by changes in stem cell activation and self-renewal or by changes in the proliferation, migration, differentiation, or survival of the progeny of stem cell activation, the downstream progenitor cells. Three-dimensional porous scaffolds promote new tissue formation by providing a surface and void volume that promotes the attachment, migration, proliferation, and desired differentiation of connective tissue progenitors throughout the region where new tissue is needed. Critical variables in scaffold design and function include the bulk material or materials from which it is made, the three-dimensional architecture, the surface chemistry, the mechanical properties, the initial environment in the area of the scaffold, and the late scaffold environment, which is often determined by degradation characteristics. Local presentation or delivery of bioactive molecules can change the function of connective tissue progenitors (activation, proliferation, migration, differentiation, or survival) in a manner that results in new or enhanced local tissue formation. All cells require access to substrate molecules (oxygen, glucose, and amino acids). A balance between consumption and local delivery of these substrates is needed if cells are to survive. Transplanted cells are particularly vulnerable. Theoretical calculations can be used to explore the relationships among cell density, diffusion distance, and cell viability within a graft and to design improved strategies for transplantation of connective tissue progenitors. Rational strategies for tissue engineering seek to optimize new tissue formation through the logical selection of conditions that modulate the performance of connective tissue progenitors in a graft site to produce a desired tissue. This increasingly involves strategies that combine cells, matrices, inductive stimuli, and techniques that enhance the survival and performance of local or transplanted connective tissue progenitors.

Biologic modification of ligamentum flavum cells by marker gene transfer and recombinant human bone morphogenetic protein-2

STUDY DESIGN

The study involves an in vitro experiment using human ligamentum flavum (LF), adenovirus lacZ construct (Ad/lacZ), and recombinant human bone morphogenetic protein-2 (BMP-2).

OBJECTIVES

To demonstrate the feasibility of marker gene transfer to human LF cells and the effect of BMP-2 on the osteogenic differentiation of human LF cells.

SUMMARY OF BACKGROUND DATA

BMP-2 is a widely known pivotal osteoinductive agent. Clinically and experimentally, BMP-2 has proven to be an effective in spinal fusion. Degenerated LF has only been implicated to be of pathophysiological significance in spinal stenosis. However, biologic modifications of LF to enhance osteogenesis have not been attempted previously.

MATERIALS AND METHODS

Human LF and cancellous bone from the ilium were harvested from patients with lumbar spinal stenosis. LF cells and osteoblasts were isolated and cultured, and adenovirus lacZ construct (Ad/ lacZ), luciferase construct (Ad/luciferase), and BMP-2 were designed and produced. LF cell cultures were then exposed to various concentrations of Ad/lacZ (25, 50, 75, 100, 150 multiplicity of infection) and BMP-2 (50, 100, 500, 1,000, and 1,500 ng/mL). Osteoblast cultures were used as a positive control for LF culture. LF cell cultures with Ad/luciferase served as viral controls for culture with Ad/ lacZ. The transgene expression of lacZ was assessed by X-gal stain and beta-galactosidase assay. Alkaline phosphatase, Von Kossa, and Alizarin red-S stains were used to confirm osteogenic differentiation and bone nodule formation. Immunocytochemical staining was also performed to detect osteocalcin expression.

RESULTS

LF cell cultures transduced with Ad/lacZ showed extensive X-gal expression and increased beta-galactosidase activity compared to viral (Ad/luciferase) and saline controls. In LF cultures treated with BMP-2, robust alkaline phosphatase expression, and bone nodule formations were observed as evidenced by positive Von Kossa and Alizarin red-S staining, and the strong expression of osteocalcin. The osteogenic response of LF cells to BMP-2 was dose dependent.

CONCLUSIONS

Human LF cells were found to be susceptible to adenovirus-mediated marker gene transfer, which offers the possibility of a new range of possible genetic modifications. In human LF cells, BMP-2 was found to markedly up-regulate the expression of osteogenic phenotypes and to induce bone nodule formation. The results of this study support the notion that biologically modified LF cells, i.e., LF cells treated with BMP-2, or with adenovirus-mediated BMP-2 cDNA gene transfer, may facilitate spinal fusion.

Regional gene therapy to enhance bone repair

Gene therapy presents a novel approach to the treatment of challenging bone loss problems. Recombinant, osteogenic growth factors are now available to enhance bone repair, particularly in those applications related to the treatment of fracture nonunions and the enhancement of fusion of the spine. However, there is concern that a single dose of an exogenous protein will not induce an adequate osteogenic signal in many patients, particularly in those cases where there is compromise of host bone and the surrounding soft tissue. Transfer of genes encoding osteogenic proteins has the potential to overcome the delivery problems associated with the use of the proteins themselves. Bone healing is an attractive application for gene therapy, because long-term protein production is not necessary for many bone repair problems. Therefore, the development of gene therapy strategies to treat bone repair problems promises to be easier than the application of gene therapy to treat chronic diseases. The purpose of this review is to highlight the advantages, disadvantages and clinical potential of various gene therapy strategies to enhance bone repair.

Bone tissue engineering and spinal fusion: the potential of hybrid constructs by combining osteoprogenitor cells and scaffolds

In this paper, we discuss the current knowledge and achievements on bone tissue engineering with regard to spinal fusion and highlight the technique that employs hybrid constructs of porous scaffolds with bone marrow stromal cells. These hybrid constructs potentially function in a way comparable to the present golden standard, the autologous bone graft, which comprises besides many other factors, a construct of an optimal biological scaffold with osteoprogenitor cells. However, little is known about the role of the cells in autologous grafts, and especially survival of these cells is questionable. Therefore, more research will be needed to establish a level of functioning of hybrid constructs to equal the autologous bone graft. Spinal fusion models are relevant because of the increasing demand for graft material related to this procedure. Furthermore, they offer a very challenging environment to further investigate the technique. Anterior and posterolateral animal models of spinal fusion are discussed together with recommendations on design and assessment of outcome parameters.

Endogenous TGF-beta signaling suppresses maturation of osteoblastic mesenchymal cells

Transforming growth factor-beta (TGF-beta), one of the most abundant cytokines in bone matrix, has positive and negative effects on bone formation, although the molecular mechanisms of these effects are not fully understood. Bone morphogenetic proteins (BMPs), members of the TGF-beta superfamily, induce bone formation in vitro and in vivo. Here, we show that osteoblastic differentiation of mouse C2C12 cells was greatly enhanced by the TGF-beta type I receptor kinase inhibitor SB431542. Endogenous TGF-beta was found to be highly active, and induced expression of inhibitory Smads during the maturation phase of osteoblastic differentiation induced by BMP-4. SB431542 suppressed endogenous TGF-beta signaling and repressed the expression of inhibitory Smads during this period, possibly leading to acceleration of BMP signaling. SB431542 also induced the production of alkaline phosphatase and bone sialoprotein, and matrix mineralization of human mesenchymal stem cells. Thus, signaling cross-talk between BMP and TGF-beta pathways plays a crucial role in the regulation of osteoblastic differentiation, and TGF-beta inhibitors may be invaluable for the treatment of various bone diseases by accelerating BMP-induced osteogenesis.

Polymeric growth factor delivery strategies for tissue engineering

PURPOSE

Tissue engineering seeks to replace and regrow damaged or diseased tissues and organs from either cells resident in the surrounding tissue or cells transplanted to the tissue site. The purpose of this review is to present the application of polymeric delivery systems for growth factor delivery in tissue engineering.

METHODS

Growth factors direct the phenotype of both differentiated and stem cells, and methods used to deliver these molecules include the development of systems to deliver the protein itself, genes encoding the factor, or cells secreting the factor.

RESULTS

Results in animal models and clinical trials indicate that these approaches may be successfully used to promote the regeneration of numerous tissue types.

CONCLUSIONS

Controlling the dose, location, and duration of these factors through polymeric delivery strategies will dictate their utility in tissue regeneration.

Bone morphogenetic proteins and spinal surgery

STUDY DESIGN

A review of the literature concerning the use of recombinant human bone morphogenetic proteins 2 (rhBMP-2) and 7 (rhBMP-7) in spinal fusion.

PURPOSE

To summarize the pertinent preclinical experiments that enabled regulated human clinical trials of recombinant bone morphogenetic proteins for spinal fusion and to update clinicians on the results of those trials.

BACKGROUND

More than three decades of research involving thousands of scientists and academicians throughout the world have led to the clinical use of recombinant bone morphogenetic proteins for the treatment of spinal disease.

METHODS

The published and presented scientific literature and the author's personal experience were examined.

RESULTS AND CONCLUSIONS

Recent clinical data support the assertion that recombinant bone morphogenetic proteins can be used as complete bone graft substitutes in spinal fusion surgery. In some circumstances, the efficacy of these factors for inducing successful fusion is superior to that of autogenous bone graft. RhBMP-2 is shown to be efficacious in several fusion applications, including intervertebral and lumbar posterolateral. Similar efficacy for rhBMP-7 has not yet been demonstrated, although relevant clinical studies are currently under way. The availability of these biologic agents will improve our ability to predictably treat spinal disease and may facilitate the further development of less invasive surgical innovations.

Gene therapy: a primer for neurosurgeons

Gene therapy involves the transfer of genes into cells with therapeutic intent. Although several methods can accomplish this, vectors based on viruses still provide the most efficient approach. For neurosurgical purposes, preclinical and clinical applications in the areas of glioma therapy, spinal neurosurgery, and neuroprotection for treatment of Parkinson's disease and cerebral ischemia are reviewed. In general, therapies applied in the neurosurgical realm have proven relatively safe, despite occasional, well-publicized cases of morbidity and death in non-neurosurgical trials. However, continued clinical and preclinical research in this area is critical, to fully elucidate potential toxicities and to generate truly effective treatments that can be applied in neurological diseases.

Bone tissue engineering: recent advances and promising therapeutic agents

Bone regeneration can be accomplished with growth factors, cells and delivery systems. This review is a summary of these components that may be used for tissue regeneration. Support for the potential therapeutic applications of transcription factors in bone tissue engineering will also be discussed.