Ex vivo gene therapy with stromal cells transduced with a retroviral vector containing the BMP4 gene completely heals critical size calvarial defect in rats

Scaffold-based bone engineering by using genetically modified cells

The first generation of clinically applied tissue engineering concepts in the area of skin, cartilage and bone marrow regeneration was based on the isolation, expansion and implantation of cells from the patient's own tissue. Although successful in selective treatments, tissue engineering needs to overcome major challenges to allow widespread clinical application with predictable outcomes. One challenge is to present the cells in a matrix to the implantation site to allow the cells to survive the wound healing contraction forces, tissue remodeling in certain tissues such as bone and biomechanical loading. Hence, several tissue engineering strategies focus on the development of load-bearing scaffold/cell constructs. From a cell source point of view, bone engineers face challenges to isolate and expand cells with the highest potential to form osseous tissue along with harvesting tissue without extensive donor site morbidity. A major hurdle to tissue engineering is de-differentiation and limited ability to control cell phenotype following in vitro expansion. Due to early successes with genetic engineering, bone tissue engineers have used different strategies to genetically alter various types of mesenchymal cells to enhance the mineralization capacity of tissue-engineered scaffold/cell constructs. Although the development of multi-component scaffold/osteogenic cell constructs requires a combination of interdisciplinary research strategies, the following review is limited to describe the general aspects of bone engineering and to present overall directions of technology platforms, which include a genetic engineering component. This paper reviews the most recent work in the field and discusses the concepts developed and executed by a collaborative effort of the multi-disciplinary teams of the two authors.

Bone regeneration in a rat cranial defect with delivery of PEI-condensed plasmid DNA encoding for bone morphogenetic protein-4 (BMP-4)

Gene therapy approaches to bone tissue engineering have been widely explored. While localized delivery of plasmid DNA encoding for osteogenic factors is attractive for promoting bone regeneration, the low transfection efficiency inherent with plasmid delivery may limit this approach. We hypothesized that this limitation could be overcome by condensing plasmid DNA with nonviral vectors such as poly(ethylenimine) (PEI), and delivering the plasmid DNA in a sustained and localized manner from poly(lactic-co-glycolic acid) (PLGA) scaffolds. To address this possibility, scaffolds delivering plasmid DNA encoding for bone morphogenetic protein-4 (BMP-4) were implanted into a cranial critical-sized defect for time periods up to 15 weeks. The control conditions included no scaffold (defect left empty), blank scaffolds (no delivered DNA), and scaffolds encapsulating plasmid DNA (non-condensed). Histological and microcomputed tomography analysis of the defect sites over time demonstrated that bone regeneration was significant at the defect edges and within the defect site when scaffolds encapsulating condensed DNA were placed in the defect. In contrast, bone formation was mainly confined to the defect edges within scaffolds encapsulating plasmid DNA, and when blank scaffolds were used to fill the defect. Histomorphometric analysis revealed a significant increase in total bone formation (at least 4.5-fold) within scaffolds incorporating condensed DNA, relative to blank scaffolds and scaffolds incorporating uncondensed DNA at each time point. In addition, there was a significant increase both in osteoid and mineralized tissue density within scaffolds incorporating condensed DNA, when compared with blank scaffolds and scaffolds incorporating uncondensed DNA, suggesting that delivery of condensed DNA led to more complete mineralized tissue regeneration within the defect area. This study demonstrated that the scaffold delivery system encapsulating PEI-condensed DNA encoding for BMP-4 was capable of enhancing bone formation and may find applications in other tissue types.

Synergy between genetic and tissue engineering: Runx2 overexpression and in vitro construct development enhance in vivo mineralization

Tissue engineering has emerged as a promising strategy to generate bone-grafting substrates. These approaches, however, are limited by an insufficient supply of committed osteoprogenitor cells and dedifferentiation of osteogenic cells during in vitro culture. To address these limitations, we engineered bone marrow stromal cells to constitutively express the osteoblastic transcription factor Runx2/Cbfa1, using retroviral gene delivery. These Runx2-modified cells were integrated into three-dimensional polymeric scaffolds to create tissue-engineered constructs. Compared with control stromal cells, Runx2 overexpression significantly upregulated osteoblastic differentiation and mineralization in vitro and in vivo in an ectopic, nonosseous subcutaneous site. More importantly, in vitro construct development to create a mineralized template before implantation dramatically enhanced subsequent in vivo mineralized tissue formation, providing a novel templating tissue-engineering strategy to improve in vivo mineralization. Finally, Runx2 overexpression and in vitro construct development synergistically enhanced in vivo mineralization compared with in vitro construct development or genetic engineering alone. This work provides a novel integrated genetic and tissue-engineering strategy to create mineralized templates for generating robust bone-grafting material.

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.

Endothelial cell modulation of bone marrow stromal cell osteogenic potential

In the context of bone development and regeneration, the intimate association of the vascular endothelium with osteogenic cells suggests that endothelial cells (ECs) may directly regulate the differentiation of osteoprogenitor cells. To investigate this question, bone marrow stromal cells (BMSCs) were cultured: in the presence of EC-conditioned medium, on EC extracellular matrix, and in EC cocultures with and without cell contact. RNA and protein were isolated from ECs and analyzed by reverse transcriptase-polymerase chain reaction and Western blotting, respectively, for expression of bone morphogenetic protein 2 (BMP-2). In animal studies, BMSCs and ECs were cotransplanted into severe combined immunodeficient mice on biodegradable polymer matrices, and histomorphometric analysis was performed to determine the extent of new bone and blood vessel formation. ECs significantly increased BMSC osteogenic differentiation in vitro only when cultured in direct contact. ECs expressed BMP-2, and experiments employing interfering RNA inhibition confirmed its production as contributing to the increased BMSC osteogenic differentiation. In vivo, cotransplantation of ECs with BMSCs resulted in greater bone formation than did transplantation of BMSCs alone. These data suggest that ECs function not only to form the microvasculature that delivers nutrients to developing bone but also to modulate the differentiation of osteoprogenitor cells in vitro and in vivo.

Combination of porous hydroxyapatite and cationic liposomes as a vector for BMP-2 gene therapy

The clinical significance of hydroxyapatite (HAP) as a bone substitute has become apparent in recent years and bone morphogenetic protein (BMP) a substance which induces bone has attracted much attention. In this study, a 1.2 cm diameter bone defects created on rabbit cranium were treated with the BMP-2 gene (cDNA plasmid) introduced with porous HAP after completion of hemostasis and the resultant bone formation was analyzed histopathologically. The amounts of bone formation was compared BMP-2 cDNA plasmids were not combined with cationic liposomes as a vector. Four groups of rabbits were compared. In the HAP group the cranial bone defect was treated with HAP containing 40 microg of liposomes and a dummy gene (PU). The BMP gene HAP group was treated with HAP soaked in liposomes and 10 microg of the BMP-2 gene. In addition, a group was treated with the gene without implanting HAP. Bone formation on the cranial defects was evaluated 3, 6 and 9 weeks after the operation, by X-ray and histopathological examinations. Three weeks after the operation there was vigorous bone formation in the cranial defect in the group which received the BMP-2 gene without HAP, and complete ossification was observed at 9 weeks. In the group which received HAP containing the BMP-2 gene, although new bone formation was evident surrounding the scaffold 3 weeks post-operation, the induced bone tissue did not fill all the pores of the scaffold even at 9 weeks post-operation. These results confirm the clinical usefulness of gene therapy for bone formation, using the BMP-2 gene combined with cationic liposomes as a vector. It is possible that the effects of administering the BMP-2 gene will be improved by specializing the microstructure of scaffold for gene therapy.

Skeletal Stem Cells in Regenerative Medicine

Postnatal stem cells have been isolated from a variety of tissues and they are highly expected to have potentiality to be utilized for cell-based clinical therapies. Bone marrow stromal stem cells (BMSSCs) derived from bone marrow stromal tissue have been identified as a population of multipotent mesenchymal stem cells that are capable of differentiating into osteoblasts, adipocytes, chondrocytes, muscle cells, and neural cells. The most significant tissue regeneration trait of BMSSCs is their in vivo bone regeneration capability, which has been widely studied for understanding molecular and cellular mechanisms of osteogenesis, and, more importantly, developing into a stem-cell-based therapy. Recent studies further demonstrated that BMSSC-mediated bone regeneration is a promising approach for regenerative medicine in clinical trials. However, there are some fundamental questions that remain to be answered prior to successful utilization of BMSSCs in clinical therapy. For instance, how to maintain stemness of BMSSCs will be a critical issue for developing methodologies to propagate multi-potential stem cells in vitro, in order to allow the development of effective clinical therapies.

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.

Sonic hedgehog gene-enhanced tissue engineering for bone regeneration

Improved methods of bone regeneration are needed in the craniofacial rehabilitation of patients with significant bone deficits secondary to tumor resection, congenital deformities, and prior to prosthetic dental reconstruction. In this study, a gene-enhanced tissue-engineering approach was used to assess bone regenerative capacity of Sonic hedgehog (Shh)-transduced gingival fibroblasts, mesenchymal stem cells, and fat-derived cells delivered to rabbit cranial bone defects in an alginate/collagen matrix. Human Shh cDNA isolated from fetal lung tissue was cloned into the replication-incompetent retroviral expression vector LNCX, in which the murine leukemia virus retroviral LTR drives expression of the neomycin-resistance gene. The rat beta-actin enhancer/promoter complex was engineered to drive expression of Shh. Reverse transcriptase-polymerase chain reaction analysis demonstrated that the transduced primary rabbit cell populations expressed Shh RNA. Shh protein secretion was confirmed by enzyme-linked immunosorbent assay (ELISA). Alginate/ type I collagen constructs containing 2 x 10(6) Shh-transduced cells were introduced into male New Zealand White rabbit calvarial defects (8 mm). A total of eight groups (N=6) were examined: unrestored empty defects, matrix alone, matrix plus the three cell populations transduced with both control and Shh expression vectors. The bone regenerative capacity of Shh gene enhanced cells was assessed grossly, radiographically and histologically at 6 and 12 weeks postimplantation. After 6 weeks, new full thickness bone was seen emanating directly from the alginate/collagen matrix in the Shh-transduced groups. Quantitative two-dimensional digital analysis of histological sections confirmed statistically significant (P<0.05) amounts of bone regeneration in all three Shh-enhanced groups compared to controls. Necropsy failed to demonstrate any evidence of treatment-related side effects. This is the first study to demonstrate that Shh delivery to bone defects, in this case through a novel gene-enhanced tissue-engineering approach, results in significant bone regeneration. This encourages further development of the Shh gene-enhanced tissue-engineering approach for bone regeneration.

Isolation and characterization of a mesenchymal cell line that differentiates into osteoblasts in response to BMP-2 from calvariae of GFP transgenic mice

We established the clonal mesenchymal cell line, GFP-C3 (C3), which differentiates into osteoblasts in response to BMP-2 from calvariae of newborn green fluorescence protein (GFP) transgenic mice. This cell line cultured with control medium expressed low levels of alkaline phosphatase (ALP) activity and osterix mRNA and undetectable ALP and osteocalcin mRNA. Incubation of these cells with rhBMP-2 increased ALP activity dose-dependently and induced substantial levels of ALP, osteocalcin and osterix mRNA expression. C3 cells infected with adenovirus vector encoding BMP-2 (AdBMP-2) or Runx2 (AdRunx2) showed greatly increased ALP mRNA expression in a time-dependent fashion. Transduction with AdRunx2-induced expression of ALP and osteocalcin mRNA, but not osterix mRNA by day 3. Transduction with AdBMP-2 induced apparent expression of ALP and osterix mRNA by day 1 after transduction, but induced only weak expression of osteocalcin mRNA day 3 after transduction. Transplantation of C3 cells transduced with AdBMP-2 into back subfascia in wild-type mice with a complex of poly-d,l-lactic-co-glycolic acid/gelatin sponge (PGS) generated ectopic bone formation involving GFP-positive osteoblasts and osteocytes 2 weeks after transplantation. C3 cells transduced with AdRunx2 or AdLacZ failed to induce ectopic bone formation. Transplantation of C3 cells transduced with AdBMP-2 into craniotomy defects in wild-type mice using PGS as a carrier induced bone formation 2 weeks after transplantation, and replaced defects 4 weeks after transplantation. C3 cells transduced with AdRunx2 failed to induce bone repair after transplantation into craniotomy defects. These results indicate that C3 cells retain differentiation potential into osteoblasts in response to BMP-2. They are useful tools for analyzing the process of osteoblast differentiation in vivo after transplantation.

Bone Morphogenic Protein-2 Gene Therapy for Mandibular Distraction Osteogenesis

Distraction osteogenesis (DO) requires a long consolidation period and has a low but real failure rate. Bone morphogenic proteins (BMPs) accelerate bone deposition in fractures and critical-sized bone defects, but their effects on mandibular DO are unknown. We investigated the effect of local delivery of adenovirus containing the gene for BMP-2 (Adbmp-2) on mandibular DO in a rat model. Rats (n = 54) were distracted to 3 mm over 6 days. At the start of consolidation (POD 10), Adbmp-2 or adenovirus containing the lacZgene (AdlacZ) was injected directly into the distraction zone. After 1, 2, and 4 weeks of consolidation, mandibles were evaluated for amount of bone deposition. Adbmp-2-treated specimens demonstrated an increased amount of new bone formation by radiographic, histologic, and histomorphometric analysis. This study demonstrates that local, adenovirally-mediated delivery of BMP-2 can increase bone deposition during DO, potentially shortening consolidation and enhancing DO in poorly healing mandibles, such as occurs postirradiation.

Light-activated gene transduction of recombinant adeno-associated virus in human mesenchymal stem cells

Deficiencies in skeletal tissue repair and regeneration lead to conditions like osteoarthritis, osteoporosis and degenerative disc disease. While no cure for these conditions is available, the use of human bone marrow derived-mesenchymal stem cells (HuMSCs) has been shown to have potential for cell-based therapy. Furthermore, recombinant adeno-associated viruses (rAAV) could be used together with HuMSCs for in vivo or ex vivo gene therapy. Unfortunately, the poor transduction efficiency of these cells remains a significant obstacle. Here, we describe the properties of ultraviolet (UV) light-activated gene transduction (LAGT) with rAAV in HuMSCs, an advance toward overcoming this limitation. Using direct fluorescent image analysis and real-time quantitative PCR to evaluate enhanced green fluorescent protein (eGFP) gene expression, we found that the optimal effects of LAGT with limited cytotoxicity occurred at a UV dose of 200 J/m(2). Furthermore, this UV irradiation had no effect on either the chondrogenic or osteogenic potential of HuMSCs. Significant effects of LAGT in HuMSCs could be detected as early as 12 h after exposure and persisted over 21 days, in a time and energy-dependent manner. This LAGT effect was maintained for more than 8 h after irradiation and required only a 10-min exposure to rAAV after UV irradiation. Finally, we show that the production of secreted TGFbeta1 protein from rAAV-TGFbeta1-IRES-eGFP infected to HuMSCs is highly inducible by UV irradiation. These results demonstrate that LAGT combined with rAAV is a promising procedure to facilitate gene induction in HuMSCs for human gene therapy.

Early osteoblastic differentiation induced by dexamethasone enhances adenoviral gene delivery to marrow stromal cells

We investigated the implications of induced osteogenic differentiation on gene delivery in multipotent rat marrow stromal cells (MSCs). Prior to genetic manipulation cells were cultured with or without osteogenic supplements (5x10(-8) M dexamethasone, 160 microM l-ascorbic acid 2-phosphate, and 10 mM beta-glycerophosphate). Comparison of liposome, retroviral, and adenoviral vectors demonstrated that all three vectors could mediate gene delivery to primary rat MSCs. When these vectors were applied in the absence or presence of osteogenic supplements, we found that MSCs differentiated prior to transduction with adenovirus type 5 vectors produced a 300% increase in transgene expression compared to MSCs that were not exposed to osteogenic supplements. This differentiation effect appeared specific to adenoviral mediated gene delivery, since there was minimal increase in retroviral gene delivery and no increase in liposome gene delivery when MSCs were treated with osteogenic supplements. In addition, we also determined this increase in transgene production to occur at a higher concentration of dexamethasone (5x10(-8) M) in the culture medium of MSCs prior to adenoviral transduction. We found that this increased transgene production could be extended to the osteogenic protein, human bone morphogenetic protein 2 (hBMP-2). When delivered by an adenoviral vector, hBMP-2 transgene production could be increased from 1.4 ng/10(5) cells/3 days to 4.3 ng/10(5) cells/3 days by culture of MSCs with osteogenic supplements prior to transduction. These results indicate that the utility of MSCs as a therapeutic protein delivery mechanism through genetic manipulation can be enhanced by pre-culture of these cells with dexamethasone.

Transduction of bone-marrow-derived mesenchymal stem cells by using lentivirus vectors pseudotyped with modified RD114 envelope glycoproteins

Bone-marrow-derived mesenchymal stem cells (MSCs) have attracted considerable attention as tools for the systemic delivery of therapeutic proteins in vivo, and the ability to efficiently transfer genes of interest into such cells would create a number of therapeutic opportunities. We have designed and tested a series of human immunodeficiency virus type 1 (HIV-1)-based vectors and vectors based on the oncogenic murine stem cell virus to deliver and express transgenes in human MSCs. These vectors were pseudotyped with either the vesicular stomatitis virus G (VSV-G) glycoprotein (GP) or the feline endogenous virus RD114 envelope GP. Transduction efficiencies and transgene expression levels in MSCs were analyzed by quantitative flow cytometry and quantitative real-time PCR. While transduction efficiencies with virus particles pseudotyped with the VSV-G GP were found to be high, RD114 pseudotypes revealed transduction efficiencies that were 1 to 2 orders of magnitude below those observed with VSV-G pseudotypes. However, chimeric RD114 GPs, with the transmembrane and extracellular domains fused to the cytoplasmic domain derived from the amphotropic Moloney murine leukemia virus 4070A GP, revealed about 15-fold higher titers relative to the unmodified RD114 GP. The transduction efficiencies in human MSCs of HIV-1-based vectors pseudotyped with the chimeric RD114 GP were similar to those obtained with HIV-1 vectors pseudotyped with the VSV-G GP. Our results also indicate that RD114 pseudotypes were less toxic than VSV-G pseudotypes in human MSC progenitor assays. Taken together, these results suggest that lentivirus pseudotypes bearing alternative Env GPs provide efficient tools for ex vivo modification of human MSCs.

Stem cells as vehicles for orthopedic gene therapy

Adult stem cells reside in adult tissues and serve as the source for their specialized cells. In response to specific factors and signals, adult stem cells can differentiate and give rise to functional tissue specialized cells. Adult mesenchymal stem cells (MSCs) have the potential to differentiate into various mesenchymal lineages such as muscle, bone, cartilage, fat, tendon and ligaments. Adult MSCs can be relatively easily isolated from different tissues such as bone marrow, fat and muscle. Adult MSCs are also easy to manipulate and expand in vitro. It is these properties of adult MSCs that have made them the focus of cell-mediated gene therapy for skeletal tissue regeneration. Adult MSCs engineered to express various factors not only deliver them in vivo, but also respond to these factors and differentiate into skeletal specialized cells. This allows them to actively participate in the tissue regeneration process. In this review, we examine the recent achievements and developments in stem-cell-based gene therapy approaches and their applications to bone, cartilage, tendon and ligament tissues that are the current focus of orthopedic medicine.

Efficacy and cytotoxicity of cationic-agent-mediated nonviral gene transfer into osteoblasts

Ex vivo gene transfer into osteoblastic cells is an advantageous strategy for bone tissue engineering. This study investigated the efficacy and cytotoxicity of in vitro cationic-agent-mediated nonviral gene transfer into osteoblasts. Various cationic agents, lipid, gelatin, and polyethylenimine (PEI) were tested. Each was formulated in various concentrations to form a complex with plasmid DNA encoding red fluorescent protein. The cationic agent/DNA complexes were transfected into human fetal osteoblastic cell line and rat bone-marrow-derived primary osteoblasts, as well as NIH 3T3 fibroblast controls. Rat primary osteoblasts were transfected more with cationic lipid and PEI agents than with gelatin carrier, yielding transfection efficacy up to 18.1% and 12.7 %, respectively. In contrast, human fetal osteoblastic cell line was transfected more with cationic lipid and gelatin than with PEI. There was a positive correlation between the lipid and PEI doses and cytotoxicity. When the lipid and PEI were used to transfect the rat primary osteoblasts in a dose that yielded the highest transfection efficacy, cell survival rates decreased as low as 40%. When their transfection efficacies into primary osteoblasts were compromised at two thirds of the highest value, that is, 12.6% and 8.3% for the lipid and PEI, respectively, the cell survival rate was nearly 80%. Cationic gelatin was associated with cell survival rates over 60 % in any cell type, regardless of the doses tested. These results suggest that different types of osteoblastic cells may possess different ability to the uptake and expression of cationic-agent-bound DNA. There seemed to be agent-specific threshold doses that dropped the cell survival rate. Cationic-agent-mediated nonviral gene transfer into osteoblastic cells may be successful when the agent- and dose-dependent transfection efficacy and cytotoxicity are optimized.

Gene therapy in orthopaedic surgery: the current status

The first successful gene therapy trial was reported in 1991. Since then, successful gene transfer in cultured cells and small animals has been reported by many studies, with achievement of at least transitory high levels of exogenous gene expression. Over 400 clinical protocols for gene therapy have been approved, involving over 4000 patients. However, publication of the results of these gene therapy trials has been limited, with only 80 published reports as of 2002. The majority of clinical gene therapy trials reported so far have been phase I or phase II trials, which are concerned mainly with safety issues and have focused on the treatment of malignancies and other potentially fatal conditions. The death of a patient in 1999 from systemic administration of an adenoviral vector and recent reports of leukaemia in two patients in a clinical gene therapy trial have led to a further re-evaluation of the safety of gene therapy and the role for gene therapy in clinical practice. This review outlines the current status of gene therapy as it relates to orthopaedic diseases and highlights the areas where progress is still to be made.

In Vivo Evaluation of Gene Therapy Vectors in Ex Vivo-Derived Marrow Stromal Cells for Bone Regeneration in a Rat Critical-Size Calvarial Defect Model

Cells genetically modified to produce osteoinductive factors have potential for use in enhancing bone regeneration for reconstructive applications. Genetic modification of cells can be accomplished by a variety of gene therapy vectors. In this study we evaluated the ex vivo genetic modification of rat marrow stromal cells (MSCs) by adenoviral, retroviral, and cationic lipid vectors containing the gene for human bone morphogenetic protein 2 (hBMP-2). We investigated both the in vitro and in vivo osteogeneic potential of MSCs modified by each vector. In vitro, we found that only MSCs modified with the adenoviral vector produced detectable hBMP-2 and demonstrated a statistically significant increase in endogenous alkaline phosphatase activity indicative of osteogeneic differentiation. We further investigated the ability of genetically modified MSCs seeded on a titanium mesh scaffold to facilitate bone formation in vivo. In an orthotopic critical-size defect created in the rat cranium, bone formation was observed in all conditions with MSCs modified by the adenoviral vector demonstrating a small but statistically significant increase in bone formation relative to the other vectors and control. Implants in an ectopic location demonstrated minimal bone formation relative to the orthotopic location, with MSCs modified with cationic lipids forming less bone than the other vectors and control. Our results show that MSCs genetically modified with adenovirus containing the hBMP-2 gene had enhanced osteogeneic capacity relative to unmodified MSCs or MSCs modified by the other vectors. This study was the first to compare three different gene therapy vectors for the genetic modification of cells to produce osteoinductive factors for the purpose to enhance bone regeneration.

Rat strain differences in the ectopic osteogenic potential of recombinant human BMP adenoviruses

Different animal strains have different genetic backgrounds that influence their physiological function and pathological process. The differences in genetic background may affect the efficiency of adenoviral infection and target gene expression and further cause different gene therapy results when target genes are delivered with adenoviral vectors. In this study, ectopic bone was not seen in ADCMVBMP4 injection sites, but was formed in ADCMVBMP9 injection sites in all rat strains. The mean volumes of bone induced with ADCMVBMP9 were 0.87 +/- 0.2 cm3 in Wistar, 0.26 +/- 0.1 cm3 in Long-Evans, 0.34 +/- 0.2 cm3 in Sprague-Dawley, 0.44 +/- 0.1 cm3 in ACI, 0.66 +/- 0.2 cm3 in PVG, and 0.58 +/- 0.1 cm3 in Fischer 344 rats. This indicates that ADCMVBMP9 has different bone formation potentials in different immunocompetent rat strains (P = 0.02). The basic levels of CD4+ and CD8+ T cells in blood before viral infection and titers of adenoviral neutralizing antibodies 30 days post-viral infection were significantly different among rat strains (P < 0.01). The efficiencies of target gene expression delivered with adenovirus were also significantly different in primary muscle cell cultures from different rat strains (P < 0.01). The different osteogenic potentials of ADCMVBMP9 among rat strains may be, in part, due to the differences in immune factors and target gene expression efficiency in muscle tissue.

The role of cell type in bone healing mediated by ex vivo gene therapy

BACKGROUND

The ideal cellular vehicle for use in cell-mediated gene therapy to enhance bone healing has not yet been identified. The purpose of this study was to compare the capacity of two types of cells transduced with retro-bone morphogenetic protein 4 (BMP4)-muscle-derived cells (MDCs) and unfractioned bone marrow stromal cells (BMSCs).

METHOD

Primary rat MDCs and unfractioned rat BMSCs were transduced with a retrovirus to express BMP4. A 7-mm, critical-sized femur defect was created in adult rats, and 5 x 10(6) transduced cells were implanted into the femoral defect. Bone healing was monitored radiographically and histologically at 4, 8, and 12 weeks post-implantation.

RESULTS

All specimens in the MDC-BMP4 group and BMSC-BMP4 group showed a bridging callus at 8 and 12 weeks. At 12 weeks post-implantation the calluses of the MDC-BMP4 femora displayed significantly higher bone photodensity than the BMSC-BMP4 femora (P<0.05). Histomorphometry revealed no difference between the two treatment groups. However, non-union between newly formed and original bone was observed in none of the MDC femora but in six femora from the BMSC-BMP4 group.

CONCLUSION

Both MDCs and unfractioned BMSCs can improve healing of a critical-sized bone defect following transduction of the cells with retroBMP4. However, MDCs appear to yield superior results when compared with BMSCs in terms of improved healing of segmental defects.

Osteogenic potential of five different recombinant human bone morphogenetic protein adenoviral vectors in the rat

Bone morphogenetic protein (BMP) adenoviral vectors for the induction of osteogenesis are being developed for the treatment of bone pathology. However, it is still unknown which BMP adenoviral vector has the highest potential to stimulate bone formation in vivo. In this study, the osteogenic activities of recombinant human BMP-2, BMP-4, BMP-6, BMP-7, and BMP-9 adenoviruses were compared in vitro, in athymic nude rats, and in Sprague-Dawley rats. In vitro osteogenic activity was assessed by measuring the alkaline phosphatase activity in C2C12 cells transduced by the various BMP vectors. The alkaline phosphatase activity induced by 2 x 10(5) PFU/well of BMP viral vector was 4890 x 10(-12) U/well for ADCMVBMP-9, 302 x 10(-12) U/well for ADCMVBMP-4, 220 x 10(-12) U/well for ADCMVBMP-6, 45 x 10(-12) U/well for ADCMVBMP-2, and 0.43 x 10(-12) U/well for ADCMVBMP-7. The average volume of new bone induced by 10(7) PFU of BMP vector in athymic nude rats was 0.37+/-0.03 cm(3) for ADCMVBMP-2, 0.89+/-0.07 cm(3) for ADCMVBMP-4, 1.02+/-0.07 cm(3) for ADCMVBMP-6, 0.24+/-0.05 cm(3) for ADCMVBMP-7, and 0.63+/-0.07 cm(3) for ADCMVBMP-9. In immunocompetent Sprague-Dawley rats, no bone formation was demonstrated in the ADCMVBMP-2, ADCMVBMP-4, and ADCMVBMP-7 groups. ADCMVBMP-6 at a viral dose of 10(8) PFU induced 0.10+/-0.03 cm(3) of new bone, whereas ADCMVBMP-9 at a lower viral dose of 10(7) PFU induced more bone, with an average volume of 0.29+/-0.01 cm(3).

N/A

N/A

In vivo bone formation in fracture repair induced by direct retroviral-based gene therapy with bone morphogenetic protein-4

This study sought to develop an in vivo gene therapy to accelerate the repair of bone fractures. In vivo administration of an engineered viral vector to promote fracture healing represents a potential high-efficacy, low-risk procedure. We selected a murine leukemia virus (MLV)-based retroviral vector, because this vector would be expected to target transgene expression to the proliferating periosteal cells arising shortly after bone fracture. This vector transduced a hybrid gene that consisted of a bone morphogenetic protein (BMP)-4 transgene with the BMP-2 secretory signal to enhance the secretion of mature BMP-4. The MLV vector expressing this BMP-2/4 hybrid gene or beta-galactosidase control gene was administered at the lateral side of the fracture periosteum at 1 day after fracture in the rat femoral fracture model. X-ray examination by radiograph and peripheral quantitative computed tomography at 7, 14, and 28 days after fracture revealed a highly significant enhancement of fracture tissue size in the MLV-BMP-2/4-treated fractures compared to the control fractures. The tissue was extensively ossified at 14 and 28 days, and the newly formed bone exhibited normal bone histology. This tissue also exhibited strong immunohistochemical staining of BMP-4. Additional control and MLV-BMP-2/4-treated animals each were monitored for 70 days to determine the fate of the markedly enhanced fracture callus. Radiographs showed that the hard callus had been remodeled and substantial healing at the fracture site had occurred, suggesting that the union of the bone at the fracture site was at least as high in the BMP-4-treated bone as in the control bone. There was no evidence of viral vector infection of extraskeletal tissues, suggesting that this in vivo gene therapy for fracture repair is safe. In summary, we have demonstrated for the first time that a MLV-based retroviral vector is a safe and effective means of introducing a transgene to a fracture site and that this procedure caused an enormous augmentation of fracture bone formation.

Transplantation of skin fibroblasts expressing BMP-2 promotes bone repair more effectively than those expressing Runx2

We investigated the osteogenic potential of skin fibroblasts that overexpressed BMP-2 or Runx2 by using adenoviral vectors. In in vitro experiments, skin fibroblasts infected with adenovirus vector encoding BMP-2 (AdBMP-2) released substantial levels of BMP-2 proteins into culture media, and those infected with adenovirus vector encoding Runx2 (AdRunx2) produced its protein. Transduction of BMP-2 or Runx2, respectively, increased alkaline phosphatase (ALP) activity and induced expression of mRNAs of ALP, osteocalcin, and osterix in skin fibroblasts. In in vivo experiments, we investigated the bone induction activity by transplantation of a complex composed of carrier [poly-D,L-lactic-co-glycolic acid/gelatin sponge (PGS)] and skin fibroblasts (PGS/SF complex). Transplantation of PGS/SF complexes composed of skin fibroblasts transduced with AdBMP-2-induced ectopic bone formation when transplanted into the subfascia of back muscle, unlike those infected with AdRunx2. Transplantation of PGS/SF complexes composed of skin fibroblasts transduced with AdBMP-2 into craniotomy defects induced bone formation from 2 weeks after transplantation, and almost all PGS was replaced by newly synthesized bone at 6 weeks. To investigate the fate of the transplanted cells, we transplanted skin fibroblasts isolated from green fluorescence protein transgenic mice into craniotomy defects. Transplantation of these skin fibroblasts transfected with AdBMP-2 generated green fluorescence protein-positive osteoblasts and osteocytes, indicating that the transplanted skin fibroblasts differentiated into osteoblastic lineage cells during bone repair. In contrast, transplantation of PGS/SF complexes composed of skin fibroblasts transduced with AdRunx2 induced a few ALP-positive cells at 1 week after transplantation, but their number decreased depending on time after transplantation. In addition, transplantation of these complexes was insufficient to induce bone repair. Taken together, our results suggest that skin fibroblasts expressing BMP-2 are more suitable for cell-mediated therapy of bone repair than those expressing Runx2.