Effect of rhBMP-2 on the osteogenic potential of bone marrow stromal cells from an osteogenesis imperfecta mouse (oim)

Modeling anabolic and antiresorptive therapies for fracture healing in a mouse model of osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a genetic bone fragility disorder that features frequent fractures. Bone healing outcomes are contingent on a proper balance between bone formation and resorption, and drugs such as bone morphogenetic proteins (BMPs) and bisphosphonates (BPs) have shown to have utility in modulating fracture repair. While BPs are used for OI to increase BMD and reduce pain and fracture rates, there is little evidence for using BMPs as local agents for fracture healing (alone or with BPs). In this study, we examined wild-type and OI mice (Col1a2^+/G610C ) in a murine tibial open fracture model with (i) surgery only/no treatment, (ii) local BMP-2 (10 µg), or (iii) local BMP-2 and postoperative zoledronic acid (ZA; 0.1 mg/kg total dose). Microcomputed tomography reconstructions of healing fractures indicated BMP-2 was less effective in an OI setting, however, BMP-2 +ZA led to considerable increases in bone volume (+193% WT, p < 0.001; +154% OI, p < 0.001) and polar moment of inertia (+125% WT, p < 0.01; +248% OI, p < 0.05). Tissue histology revealed a thinning of the neocortex of the callus in BMP-2 treated OI bone, but considerable retention of woven bone in the healing callus with BMP + ZA specimens. These data suggest a cautious approach may be warranted with the sole application of BMP-2 in an OI surgical setting as a bone graft substitute. However, this may be overcome by off-label BP administration.

Modified electrospun chitosan membranes for controlled release of simvastatin

Chitosan nanofibrous membranes have immense potential in tissue engineering and drug delivery applications because of their increased surface area, high degree of biocompatibility, and their ability to mimic the extracellular matrix. However, their use is often limited due to their extreme hydrophilic nature causing them to lose their nanofibrous structure in vivo. In the present study, chitosan membranes were modified either by acylation reactions using fatty acids of different chain lengths or tert-butyloxycarbonyl (tBOC) protecting groups to increase the hydrophobicity of the membranes and protect the nanofibrous structure. The modified membranes were characterized using scanning electron microscopy, attenuated total reflectance Fourier transform infrared spectroscopy, water contact angle and elemental analysis to confirm the addition of the modification groups. These membranes were then evaluated to control the release of a hydrophobic osteogenic drug-simvastatin (SMV). The interaction between SMV and the polymer was determined using molecular modeling. Pure SMV and SMV loaded membranes were examined for their in vitro cytotoxicity and osteogenic potential using preosteoblast mouse bone marrow stromal cells. From results, it was evident that as the fatty acid chain length increased from two to six methylene groups, the hydrophobicity of the membranes increased (59.2 ± 8.2° to 94.3 ± 8.5° water contact angle). The amount of drug released from the membranes could be controlled by changing the amount of initial drug loaded and/or the type of modifications. After 4 weeks, for a 500 μg loading, the short chain fatty acid modified membranes released 17.8 ± 3.2% of the drug whereas a long chain fatty acid released only 4.8 ± 0.8%. Similarly, for a 50 μg loading, short chain modified membranes released more (73.3 ± 33.3%) of the loaded drug as compared to the long chain membranes (43.0 ± 3.5%). The long chain fatty acid membranes released SMV for extended time periods of up to 90 days. This data was further supported by molecular modeling, which revealed that SMV was more compatible with more hydrophobic membranes. Cell studies showed that pure SMV from 75 to 600 ng/ml range possessed osteogenic potential in a dose dependent manner and the amount of SMV released from the most hydrophobic FA treated membranes was not cytotoxic and supported osteogenic differentiation. Therefore, this study demonstrates our ability to control the release of a hydrophobic drug from modified chitosan membranes as per the clinical need.

Bio-Oss®acts on Stem cells derived from Peripheral Blood

OBJECTIVES

This study aims to study how Bio-Oss® can induce osteoblast differentiation in mesenchymal stem cells, the expression levels of bone related genes and mesenchymal stem cells markers using real time Reverse Transcription-Polymerase Chain Reaction.

METHODS

PB-hMSCs stem preparations were obtained for gradient centrifugation from peripheral blood of healthy anonymous volunteers, using the Acuspin System-Histopaque 1077. The samples were then cultured for 7 days for RNA processing, and the expression was quantified using real time PCR.

RESULTS

Bio-Oss® caused an induction of osteoblast transcriptional factor like RUNX2 and of bone related genes; SPP1 and FOSL1. In contrast, the expression of ENG was significantly decreased in stem cells treated with Bio-Oss® with respect to untreated cells, indicating the differentiation effect of this biomaterial on stem cells.

CONCLUSION

The results obtained can be relevant to enhance the understanding of the molecular mechanism of bone regeneration and can act as a model for comparing other materials with similar clinical effects.

Osteoplant acts on stem cells derived from peripheral blood

Objectives:

The osteoplant is an equine, flexible, heterologous, deantigenic, cortical, and spongy bone tissue, totally reabsorbable, used for implantation of bone tissue, to restore skeletal, even weight-bearing structures. However, how the osteoplant alters osteoblast activity to promote bone formation is poorly understood.

Materials and Methods:

To study how the osteoplant induces osteoblast differentiation in mesenchymal stem cells, the expression levels of bone-related genes, and mesenchymal stem cell markers are analyzed, using real time Reverse Transcription-Polymerase Chain Reaction (RT-PCR).

Results:

The osteoplant causes induction of osteoblast transcriptional factors such as osterix (RUNX2), and of bone-related genes such as osteopontin (SPP1) and osteocalcin (BGLAP). In contrast the expression of ENG (CD105) is significantly decreased in stem cells treated with osteoplant, with respect to untreated cells, indicating the differentiation effect of this biomaterial on stem cells.

Conclusion:

The obtained results can be relevant to better understand the molecular mechanism of bone regeneration and as a model for comparing other materials with similar clinical effects.

Engipore acts on human bone marrow stem cells

OBJECTIVES

Porous HA scaffolds are promising materials for tissue engineering because they offer a tridimensional support and serve as template for cell proliferation and at last tissue formation. Engipore provide a natural 3D scaffold with organic fibrous material in bone. However, how this material alters osteoblast activity to promote bone formation is poorly understood.

MATERIALS AND METHODS

To study how Engipore can induce osteoblast differentiation in mesenchymal stem cells, the expression levels of bone related genes and mesenchymal stem cells marker were analyzed.

RESULTS

Engipore causes a significant induction of osteoblast transcriptional factors like SP7 and RUNX2 and of the bone-related gene osteocalcin (BGLAP). The expression of CD105 was not significantly changed in stem cells treated with Engipore with respect to untreated cells, while SSP1 (osteopontin) was significantly down expressed thus reducing osteoclast activity.

CONCLUSIONS

The obtained results can be relevant to better understand the molecular mechanism of bone regeneration.

Early effects of p-15 on human bone marrow stem cells

OBJECTIVES

Peptide-15 (P-15) is an analogue of the cell binding domain of collagen. P-15 has been shown to facilitate physiological to process in a way similar to collagen, to serve as anchorage for cells, and to promote the binding, migration and differentiation of cells. However, how P-15 alters osteoblast activity to promote bone formation is poorly understood. To study the osteoinductive properties of peptide P-15, we analyzed the expression levels of bone related genes in human mesenchymal stem cells treated with this biomaterial.

MATERIAL AND METHODS

Using real time Reverse Transcription-Polymerase Chain Reaction the quantitative expression of specific genes, like transcriptional factors (RUNX2 and SP7), bone related genes (SPP1, COL1A1, COL3A1, BGLAP, ALPL, and FOSL1) and mesenchymal stem cells marker (ENG) were examined.

RESULTS

P-15 causes a considerable induction of osteoblast transcriptional factor like osterix (SP7) and of the bone related genes osteopontin (SPP1) and osteocalcin (BGLAP). In contrast the expression of endoglin (ENG) was markedly decreased in stem cells treated with P-15 respect to untreated cells, indicating the differentiation effect of this biomaterial on stem cells.

CONCLUSIONS

The present study shows the effect of P-15 on mesenchymal stem cells in the early differentiation stages: P-15 is an inducer of osteogenesis on human stem cells as indicated by the activation of bone related markers SP7, SPP1 and BGLAP.The results may allow a better understanding of the molecular mechanism of bone regeneration and as a model for comparing other materials with similar clinical effects.

A simple, lanthanide-based method to enhance the transduction efficiency of adenovirus vectors

Based upon the powerful bridging and charge-masking properties of lanthanide cations (Ln3+), we have investigated their use to improve the transduction efficiency of adenovirus vectors. Using a luciferase marker gene, it was possible to increase transgene expression by the murine mesenchymal stem cell line C3H10T(1/2) by up to four log orders when using very low multiplicities of infection in conjunction with Ln3+; La3+ was superior to Gd3+, Y3+ and Lu3+ in this regard. All Ln3+ were more effective than Ca2+. Flow cytometry, using a green fluorescent protein marker gene, confirmed that La3+ increased both the percentage of transduced cells and the level of transgene expression per cell. Transduction of primary cultures of a variety of different mesenchymal cells from human, rabbit, bovine and rat sources, as well as gene transfer to synovium and muscle in vivo, was also greatly enhanced. Our findings suggest that this lanthanide-based method holds much promise for expediting both experimental and clinical applications of gene transfer with adenoviral vectors.

Distribution of single-cell expanded marrow derived progenitors in a developing mouse model of osteogenesis imperfecta following systemic transplantation

We evaluated single-cell-expanded, marrow-derived progenitors for engraftment in a developing mouse model of osteogenesis imperfecta (OI) following systemic transplantation. The present study was initiated to evaluate the potential of mesenchymal stem cells to treat OI. Single-cell-derived progenitors were prepared from marrow stromal cells harvested from normal mice. Selected single-cell-expanded progenitors marked with green fluorescent protein were injected into the neonatal mouse model of OI, and the recipient mice were sacrificed at 2 and 4 weeks following cell transplantation. Examination of the tissues harvested from recipient mice at 2 and 4 weeks after cell transplantation demonstrated that the cells extravasated and engrafted in most of the bones as well as other tissues. Tissue sections made from the tibias and femurs of a selected recipient mouse showed that the cells were distributed in bone marrow, trabecular, and cortical bone as demonstrated by histology and confocal microscopy. The cells that engrafted in the bones of the recipient mouse synthesized and deposited type I collagen composed of alpha1(I) and alpha2(I) collagen heterotrimers. Genotyping and gene expression analysis of the cells retrieved from the bones of the recipient mouse at 2 and 4 weeks demonstrated that the cells expressed osteoblast-specific genes, suggesting that the donor cells differentiated into osteoblasts in vivo with no evidence of cell fusion. These data suggest that progenitors infused in developing mice will engraft in various tissues including bones, undergo differentiation, and deposit matrix and form bone in vivo. Disclosure of potential conflicts of interest is found at the end of this article.

Endothelial Cells Assemble into a 3-Dimensional Prevascular Network in a Bone Tissue Engineering Construct

To engineer tissues with clinically relevant dimensions, one must overcome the challenge of rapidly creating functional blood vessels to supply cells with oxygen and nutrients and to remove waste products. We tested the hypothesis that endothelial cells, cocultured with osteoprogenitor cells, can organize into a prevascular network in vitro. When cultured in a spheroid coculture model with human mesenchymal stem cells, human umbilical vein endothelial cells (HUVECs) form a 3-dimensional prevascular network within 10 days of in vitro culture. The formation of the prevascular network was promoted by seeding 2% or fewer HUVECs. Moreover, the addition of endothelial cells resulted in a 4-fold upregulation of the osteogenic marker alkaline phosphatase. The addition of mouse embryonic fibroblasts did not result in stabilization of the prevascular network. Upon implantation, the prevascular network developed further and structures including lumen could be seen regularly. However, anastomosis with the host vasculature was limited. We conclude that endothelial cells are able to form a 3-dimensional (3D) prevascular network in vitro in a bone tissue engineering setting. This finding is a strong indication that in vitro prevascularization is a promising strategy to improve implant vascularization in bone tissue engineering.

Effects of osteogenic inducers on cultures of canine mesenchymal stem cells

OBJECTIVE

To examine age-related efficacy of bone morphogenetic protein (BMP)-2, ascorbate, and dexamethasone as osteogenic inducers in canine marrow-derived stromal cells (MSCs).

SAMPLE POPULATION

Samples of femoral bone marrow obtained from 15 skeletally immature (< 1 year old) and 4 skeletally mature (> 1.5 years old) dogs.

PROCEDURE

First-passage canine MSC cultures were treated with 100 microg of ascorbate phosphate/mL, 10(-7)M dexamethasone, 100 ng of BMP-2/mL, or a combination of these osteoinducers. On day 6, cultures were harvested for quantitation of alkaline phosphatase (ALP) activity and isolation of RNA to prepare cDNA for real-time polymerase chain reaction analyses of osteoblast markers.

RESULTS

Early markers of osteogenesis were induced in canine MSCs by BMP-2 but not dexamethasone. In young dogs, the combination of BMP-2 and ascorbate yielded the highest ALP mRNA concentrations and activity. This combination also induced significant increases in mRNA for osteopontin and runt-domain transcription factor 2. In comparison to MSCs from immature dogs, those from mature dogs had diminished ALP activity in response to BMP and ascorbate. Results for cultures treated with 3,4-dehydroproline suggested that ascorbate-induced production of extracellular matrix was important for maximal BMP-2 response in canine MSCs.

CONCLUSIONS AND CLINICAL RELEVANCE

BMP-2 was capable of inducing markers of osteogenesis in short-term cultures of canine MSCs. In MSCs obtained from skeletally immature dogs, ascorbate was required for maximal effects of BMP These results define optimal conditions for stem cell osteogenesis in dogs and will facilitate development of stem cell-based treatments for dogs with fractures.

Nucleofection-BasedEx VivoNonviral Gene Delivery to Human Stem Cells as a Platform for Tissue Regeneration

There are several gene therapy approaches to tissue regeneration. Although usually efficient, virusbased approaches may elicit an immune response against the viral proteins. An alternative approach, nonviral transfer, is safer, and can be controlled and reproduced. We hypothesized that in vivo bone formation could be achieved using human mesenchymal stem cells (hMSCs) nonvirally transfected with the human bone morphogenetic protein-2 (hBMP-2) or -9 (hBMP-9) gene. Human MSCs were transfected using nucleofection, a unique electropermeabilization-based technique. Postnucleofection, cell viability was 53.6 +/- 2.5% and gene delivery efficiency was 51% to 88% (mean 68.2 +/- 4.1%), as demonstrated by flow cytometry in enhanced green fluorescent protein (EGFP)-nucleofected hMSCs. Transgene expression lasted longer than 14 days and was very low 21 days postnucleofection. Both hBMP-2- and hBMP-9-nucleofected hMSCs in culture demonstrated a significant increase in calcium deposition compared with EGFP-nucleofected hMSCs. Human BMP-2- and hBMP-9-nucleofected hMSCs transplanted in ectopic sites in NOD/SCID mice induced bone formation 4 weeks postinjection. We conclude that in vivo bone formation can be achieved by using nonvirally nucleofected hMSCs. This could lead to a breakthrough in the field of regenerative medicine, in which safer, nonviral therapeutic strategies present a very attractive alternative.

Growth hormone injections improve bone quality in a mouse model of osteogenesis imperfecta

Systemic growth hormone injections increased spine and femur length in a mouse model of OI. Femur BMC, cross-sectional area, and BMD were increased. Smaller gains were produced in vertebral BMC and cross-sectional area. Biomechanical testing showed improvements to structural and material properties in the femur midshaft, supporting expanded testing of growth hormone therapy in children with OI.

INTRODUCTION

Osteoblasts in heterozygous Cola2oim mutant mice produce one-half the normal amounts of the alpha2 strand of type I procollagen. The mice experience a mild osteogenesis imperfecta (OI) phenotype, with femurs and vertebrae that require less force than normal to break in a biomechanical test.

MATERIALS AND METHODS

Subcutaneous injections of recombinant human growth hormone (rhGH) or saline were given 6 days per week to oim/+ mice between 3 and 12 weeks of age, in a protocol designed to simulate a trial on OI children.

RESULTS

rhGH injections promoted significant weight gain and skeletal growth compared with saline-treated control animals. Femur and spine lengths were increased significantly. Significant increases at the femur midshaft in cortical BMD (2.2%), BMC (15.5%), and cross-sectional area (13%) were produced by rhGH treatment. Increases in the same cortical bone parameters were measured in the metaphyseal region of the femur and in tail vertebrae, but lumbar vertebrae showed significant increases in BMC (9.6%) and cross-sectional area (10.1%) of trabecular bone. Three-point bending testing documented functional improvements to the femur mid-shafts. GH treatment produced significant increases in bone stiffness (23.7%), maximum load (30.8%), the energy absorbed by the femurs to the point of maximum load (44.5%), and the energy to actual fracture (40.4%). The ultimate stress endured by the bone material was increased by 14.1%.

CONCLUSIONS

Gains in bone length, cross-sectional area, BMD, BMC, structural biomechanical properties, and strength were achieved without directly addressing the genetic collagen defect in the mice. Results support expanded clinical testing of GH injections in children with OI.

Cell lines and primary cell cultures in the study of bone cell biology

Bone is a metabolically active and highly organized tissue consisting of a mineral phase of hydroxyapatite and amorphous calcium phosphate crystals deposited in an organic matrix. Bone has two main functions. It forms a rigid skeleton and has a central role in calcium and phosphate homeostasis. The major cell types of bone are osteoblasts, osteoclasts and chondrocytes. In the laboratory, primary cultures or cell lines established from each of these different cell types provide valuable information about the processes of skeletal development, bone formation and bone resorption, leading ultimately, to the formulation of new forms of treatment for common bone diseases such as osteoporosis.

The 2003 Nicolas Andry Award. Orthopaedic gene therapy

We review progress in the field of orthopaedic gene therapy since the concept of using gene transfer to address orthopaedic problems was initiated approximately 15 years ago. The original target, arthritis, has been the subject of two successful Phase I clinical trials, and additional human studies are pending in rheumatoid arthritis and osteoarthritis. The repair of damaged musculoskeletal tissues also has proved to be a fruitful area of research, and impressive enhancement of bone healing has been achieved in preclinical models. Rapid progress also is being made in the use of gene transfer to improve cartilage repair, ligament healing, and restoration of various additional tissues, including tendon and meniscus. Other applications include intervertebral disc degeneration, aseptic loosening, osteoporosis, genetic diseases, and orthopaedic tumors. Of these various orthopaedic targets of gene therapy, tissue repair is likely to make the earliest clinical impact because it can be achieved with existing technology. Tissue repair may become one of the earliest clinical successes for gene therapy as a whole. Orthopaedics promises to be a leading discipline for the use of human gene therapy.

Different effects of BMP-2 on marrow stromal cells from human and rat bone

Bone morphogenetic proteins (BMPs) promote the differentiation of osteoprogenitor cells, and also induce osteogenesis in bone marrow stromal cells (MSC) from rats and mice. However, compared to results with animal models, BMPs are relatively inefficient in inducing human MSC to undergo osteogenesis, and are much less effective in promoting bone formation in human clinical trials. Previous studies indicated that, while human MSC respond to dexamethasone with elevated levels of the osteoblast marker alkaline phosphatase, most isolates of human MSC fail to show alkaline phosphatase induction in response to BMP-2, BMP-4, or BMP-7. Several other genes known to be induced by BMPs are appropriately regulated; thus, human MSC are capable of some BMP-activated signaling. Analysis of the BMP receptors ALK-3 and ALK-6 indicated that, although ALK-6 mRNA was not expressed in human MSC, overexpressing a constitutively active ALK-6 receptor did not induce elevated alkaline phosphatase. Real-time RT-PCR was used to investigate expression of several osteoblast-related transcription factors in MSC after 6 days' exposure to BMP2 or dexamethasone. Msx-2, a transcription factor that has been reported to inhibit differentiation of osteoprogenitor cells, showed 10-fold elevation in BMP-2-treated human MSC, but not in BMP-2-treated rat MSC. Overexpression of Msx-2 in human and rat MSC, however, did not alter alkaline phosphatase levels, which suggests that absence of BMP-stimulated alkaline phosphatase was not caused by the BMP-2-induced increase in Msx-2. Although Runx2 isoforms have been implicated in control of osteoblast differentiation, levels of this transcription factor were unaffected by BMP treatment. Expression of the FKHR transcription factor, which has been reported to regulate alkaline phosphatase transcription in mouse cells, showed a modest increase in response to BMP-2, but a much greater increase in dexamethasone-treated cells. We propose that BMP regulation of the bone/liver/kidney alkaline phosphatase gene is indirect, requiring expression of new transcription factor(s) that behave differently in rodent and human MSC.

The fate of mesenchymal stem cells transplanted into immunocompetent neonatal mice: implications for skeletal gene therapy via stem cells

To explore the feasibility of skeletal gene and cell therapies, we transduced murine bone marrow-derived mesenchymal stem cells (MSCs) with a retrovirus carrying the enhanced green fluorescent protein and zeocin-resistance genes prior to transplantation into 2-day-old immunocompetent neonatal mice. Whole-body imaging of the recipient mice at 7 days post-systemic cell injection demonstrated a wide distribution of the cells in vivo. Twenty-five days posttransplantation, most of the infused cells were present in the lung as assessed by examination of the cells cultured from the lungs of the recipient mice. The cells persisted in lung and maintained a high level of gene expression and could be recovered from the recipient mice at 150 days after cell transplantation. A significant number of GFP-positive cells were also present in the bones of the recipient mice at 35 days post-cell transplantation. Recycling of the cells recovered from femurs of the recipient mice at 25 days posttransplantation by repeated injections into different neonatal mice resulted in the isolation of a clone of cells that was detected in bone and cartilage, but not in lung and liver after systemic injection. These data demonstrate that MSCs persist in immunocompetent neonatal mice, maintain a high level of gene expression, and may participate in skeletal growth and development of the recipient animals.

Bone formation following transplantation of genetically modified primary bone marrow stromal cells

Bone marrow stromal cells contain mesenchymal stem cells that can differentiate into a variety of mesenchymal tissues; in the presence of BMP-2, for example, they differentiate into osteoblasts. We constructed replication-deficient adenoviral vectors encoding human BMP-2 (BMP-2/Ad) or BMP-4 (BMP-4/Ad) and used them to transduce primary bone marrow stromal cells from the femurs of four-week-old female C3H mice, which then expressed and processed functional BMP-2 or BMP-4 protein. Enzyme assays and histochemical staining showed both groups of cells to possess alkaline phosphatase activity, a marker of differentiation into osteoblasts, though the activity was higher in cells transduced with BMP-2/Ad. When BMP-2/Ad-transduced cells were injected into the thigh muscles of immunocompetent C3H mice, ossicle development was detected on radiographs within four weeks after injection. Moreover, histological analysis indicated that newly developed ossicles contain mature osseous components, including cortical bone and bone marrow, within eight weeks. Thus, syngeneic transplantation of genetically modified primary bone marrow stromal cells induced bone formation in immunocompetent mice, perhaps indicating its potential for use in the development of therapeutic protocols aimed at enhancing bone formation.

Regulation of BMP-induced transcription in cultured human bone marrow stromal cells

BACKGROUND

Adherent bone marrow stromal cells are inducible osteoprogenitors, giving rise to cells expressing osteoblast markers including alkaline phosphatase, osteopontin, osteocalcin, and bone sialoprotein. However, the potency of inducers varies in a species-specific manner. Glucocorticoids such as dexamethasone induce alkaline phosphatase activity in both human and rat mesenchymal stem cells, while mouse bone marrow stromal cells are refractory to dexamethasone-induced alkaline phosphatase activity. In contrast, BMP induces alkaline phosphatase activity in both mouse and rat bone marrow stromal cells, while BMP effects on human bone marrow stromal cells are poorly characterized.

METHODS

Bone marrow samples were isolated from patients undergoing hip replacement. Mononuclear marrow cells were cultured and grown to confluence without or with 10 (-7) M dexamethasone. Cells from each isolate were passaged into medium containing 100 micro g/mL ascorbate phosphate and treated with dexamethasone, 100 ng/mL BMP, or no inducer. At day 6, alkaline phosphatase activity was assayed, and RNA was prepared for mRNA analyses by real-time polymerase chain reaction.

RESULTS

Bone marrow stromal cells from twenty-four of twenty-six patients showed no significant osteogenic response to BMP-2, 4, or 7 as determined by alkaline phosphatase induction. However, BMPs induced elevated levels of other genes associated with osteogenesis such as bone sialoprotein and osteopontin as well as BMP-2 and noggin. If primary cultures of human bone marrow stromal cells were pretreated with dexamethasone, BMP-2 treatment of first-passage cells induced alkaline phosphatase in approximately half of the isolates, and significantly greater induction was seen in cells from males. Dexamethasone treatment, like BMP treatment, also increased expression of the BMP-binding protein noggin.

CONCLUSIONS

Most human femur bone marrow stromal cell samples appear incapable of expressing elevated alkaline phosphatase levels in response to BMPs. Since BMP treatment induced expression of several other BMP-regulated genes, the defect in alkaline phosphatase induction is presumably not due to impaired BMP signaling. We hypothesize that the mechanism by which BMPs modulate alkaline phosphatase expression is indirect, involving a BMP-regulated transcription factor for alkaline phosphatase expression that is controlled differently in humans and rodents.

Systemically administered mesenchymal stromal cells transduced with insulin-like growth factor-I localize to a fracture site and potentiate healing

OBJECTIVE

To determine to the ability of systemically administered pluripotential mesenchymal stromal cells to localize to a fracture site and whether transduction with a therapeutic gene, insulin-like growth factor-I (D1-IGF-I), could potentiate healing.

DESIGN

Murine model, basic science study.

SETTING

Laboratory.

SPECIMENS

Closed, transverse, mid-shaft femur fractures were produced in 108 Balb/c mice after intramedullary stabilization.

INTERVENTIONS

A cloned, pluripotential, mesenchymal cell line, termed D1, was stably transfected with either the gene beta-galactosidase (D1-BAG) as a histologic marker or with the gene IGF-I (D1-IGF-I) growth factor. Mice received systemic injections of either D1-BAG cells for in vivo localization or D1-IGF-I for therapeutic intervention. A third group received lactated Ringer's solution and served as control.

MAIN OUTCOME MEASUREMENTS

Sections obtained from the fracture site and contralateral femurs were examined histologically and by deoxyribonucleic acid-polymerase chain reaction (DNA-PCR) to detect the presence of transplanted cells at 2, 4, and 6 weeks after fracture. Matrix mineralization and callus maturation were evaluated by histology.

RESULTS

At all time points, using histologic staining with X-gal and deoxyribonucleic acid-polymerase chain reaction for marker genes, there was a statistically greater number of transplanted cells ( p< 0.001) and significantly higher DNA-PCR for marker genes ( p< 0.001) in the fractured femurs than in the nonfractured femurs. Mice receiving D1-IGF-I cells demonstrated a greater percent of mineralized callus than controls at two weeks (p < 0.05). At 4 and 6 weeks, treated mice demonstrated on average greater mineralized matrix and accelerated progression to an osseous callus as compared with the control group.

CONCLUSIONS

Cell-based gene therapy has the potential to deliver higher therapeutic levels of growth factors specifically at the site of cell localization while minimizing wider systemic side effects. This study demonstrates that systemically injected IGF-I transduced, mesenchymal cells are able to return to and repopulate the bone marrow. More importantly, these cells localize preferentially to a fracture site and accelerated fracture healing.

Osteoblastic response to the defective matrix in the osteogenesis imperfecta murine (oim) mouse

This work examines the cellular pathophysiology associated with the weakened bone matrix found in a murine model of osteogenesis imperfecta murine (oim). Histomorphometric analysis of oim/oim bone showed significantly diminished bone mass, and the osteoblast and osteoclast histomorphometric parameters were increased in the oim/oim mice, compared with wild-type (+/+) mice. To assess osteoblast activity, a rat Col1a1 promoter linked to the chloramphenicol acetyltransferase reporter transgene was bred into the oim model. At 8 d and 1 month of age, no difference in transgene activity between oim and control mice was observed. However, at 3 months of age, chloramphenicol acetyl transferase activity was elevated in oim/oim;Tg/Tg, compared with +/+;Tg/Tg and oim/+;Tg/Tg. High levels of urinary pyridinoline crosslinks in the oim/oim;Tg/Tg mice were present at all ages, reflecting continuing high bone resorption. Our data portray a state of ineffective osteogenesis in which the mutant mouse never accumulates a normal quantity of bone matrix. However, it is only after the completion of the rapid growth phase that the high activity of the oim/oim osteoblast can compensate for the high rate of bone resorption. This relationship between bone formation and resorption may explain why the severity of osteogenesis imperfecta decreases after puberty is completed. The ability to quantify high bone turnover and advantages of using a transgene that reflects osteoblast lineage activity make this a useful model for studying interventions designed to improve the bone strength in osteogenesis imperfecta.

Gene therapy for osteoporosis: evaluation in a murine ovariectomy model

Various cytokines and cytokine antagonists hold promise as new therapeutic agents for osteoporosis, but their application is hindered by delivery problems. Gene transfer offers an attractive technology with which to obviate these restrictions. Its utility was evaluated in an animal model of osteoporosis. Disease was induced by surgical ovariectomy and monitored by measuring bone weight after 12 days, and by histomorphometry after 5 weeks. Genes were transferred to the mice by intramedullary injection of adenoviral vectors. LacZ and luciferase marker genes were used to identify the bone marrow cells transduced by this procedure, and to track the possible spread of transgenes to other organs. The effect on bone loss of transferring a cDNA encoding the human interleukin-1 receptor antagonist (IL-1Ra) was then evaluated. The intramedullary injection of adenoviral vectors transduced lining osteoblasts, osteocytes and cells within the bone marrow. Luciferase activity persisted within the injected femora and adjacent musculature for at least 3 weeks, and in the draining lymph nodes for 2 weeks. Transient, low level expression was present in the liver, but no luciferase was detected at any time in the lung or spleen. Intramedullary introduction of the IL-1Ra gene resulted in circulation of the corresponding protein at concentrations that peaked on day 3, and returned to baseline by day 12. Transfer of the IL-1Ra gene strongly reduced the early loss of bone mass occurring in response to ovariectomy. Furthermore, it completely inhibited the loss of matrix detected by histomorphometry at 5 weeks. The protective effect of this gene was not restricted to bones receiving intramedullary injection of the vector, but occurred in all bones that were evaluated. This proof of concept encourages further development of gene therapy approaches to the treatment of osteoporosis.

Comparison of lumbar spine fusion using mixed and cloned marrow cells

STUDY DESIGN

Prospective study on lumbar spine fusion using cloned and mixed marrow cells.

OBJECTIVE

To analyze the effectiveness of cloned osteoprogenitor cells in spine fusion and their differentiation in vivo using a traceable gene.

SUMMARY OF BACKGROUND DATA

Although autografts are currently the standard for stable spine fusion, supply is limited. Alternative graft materials need to be developed and evaluated.

METHODS

An osteoprogenitor cell, D1-BAG, cloned from mouse bone marrow and transduced with LacZ and neomycin resistance genes, and mixed marrow stromal cells from marrow blowouts were used in athymic rats to establish posterior spinal fusion; 2 x 10(6) cells in 100 microL Matrigel were implanted into the lumbar fusion bed in 36 animals, whereas Matrigel without cells was used in 16 animals as control. Rats were killed at 2, 3, 6, and 9 weeks, and the spines were evaluated by manual palpation, radiographs, and histology.

RESULTS

Two weeks after surgery radiopaque tissue was seen at transplantation sites with D1-BAG cells but not at sites with mixed marrow stromal cells. Successful spine fusion at 6 and 9 weeks was observed in 8 of 8 (100%) animals receiving DI-BAG cells, 4 of 8 (50%) in mixed marrow stromal cells, and 0 of 8 (0%) in control animals.

CONCLUSIONS

Compared with mixed marrow stromal cells, cloned osteoprogenitor cells can produce a larger amount of mature osseous tissue at an earlier time point during spine fusion.

Transfer of proalpha2(I) cDNA into cells of a murine model of human Osteogenesis Imperfecta restores synthesis of type I collagen comprised of alpha1(I) and alpha2(I) heterotrimers in vitro and in vivo

The oim mouse is a model of human Osteogenesis Imperfecta (OI) that has deficient synthesis of proalpha2(I) chains. Cells isolated from oim mice synthesize alpha1(I) collagen homotrimers that accumulate in tissues. To explore the feasibility of gene therapy for OI, a murine proalpha2(I) cDNA was inserted into an adenovirus vector and transferred into bone marrow stromal cells isolated from oim mice femurs. The murine cDNA under the control of the cytomegalovirus early promoter was expressed by the transduced cells. Analysis of the collagens synthesized by the transduced cells demonstrated that the cells synthesized stable type I collagen comprised of alpha1(I) and alpha2(I) heterotrimers in the correct ratio of 2:1. The collagen was efficiently secreted and also the cells retained the osteogenic potential as indicated by the expression of alkaline phosphatase activity when the transduced cells were treated with recombinant human bone morphogenetic protein 2. Injection of the virus carrying the murine proalpha2(I) cDNA into oim skin demonstrated synthesis of type I collagen comprised of alpha1 and alpha2 chains at the injection site. These preliminary data demonstrate that collagen genes can be transferred into bone marrow stromal cells as well as fibroblasts in vivo and that the genes are efficiently expressed. These data encourage further studies in gene replacement for some forms of OI and use of bone marrow stromal cells as vehicles to deliver therapeutic genes to bone.

Mesenchymal stem cells: building blocks for molecular medicine in the 21st century

Mesenchymal stem sells (MSCs) are present in a variety of tissues during human development, and in adults they are prevalent in bone marrow. From that readily available source, MSCs can be isolated, expanded in culture, and stimulated to differentiate into bone, cartilage, muscle, marrow stroma, tendon, fat and a variety of other connective tissues. Because large numbers of MSCs can be generated in culture, tissue-engineered constructs principally composed of these cells could be re-introduced into the in vivo setting. This approach is now being explored to regenerate tissues that the body cannot naturally repair or regenerate when challenged. Moreover, MSCs can be transduced with retroviral and other vectors and are, thus, potential candidates to deliver somatic gene therapies for local or systemic pathologies. Untapped applications include both diagnostic and prognostic uses of MSCs and their descendents in healthcare management. Finally, by understanding the complex, multistep and multifactorial differentiation pathway from MSC to functional tissues, it might be possible to manipulate MSCs directly in vivo to cue the formation of elaborate, composite tissues in situ.

An optimized method for the chemiluminescent detection of alkaline phosphatase levels during osteodifferentiation by bone morphogenetic protein 2

Differentiation of osteoprogenitor cells into osteoblasts is a pivotal step during the normal development and repair of bone. Upregulation of endogenous cellular alkaline phosphatase activity (AP) is a commonly used intracellular marker for the assessment of osteoprogenitor cell differentiation into the osteoblastic phenotype. Current methods for assaying AP involve colorimetric detection of the enzyme's activity using the synthetic substrate p-nitrophenol phosphate. In this paper, we explored an alternative method of detecting AP using the chemiluminescent substrate disodium 3-(4-methoxyspiro[1,2-dioxetane-3,2'-(5'-chloro)tricyclo[3.3.1.1(3,7)]decan]-4-yl) phenyl phosphate (CSPD) for enhanced AP sensitivity and a more simplified assay. Using calf intestinal alkaline phosphatase as a standardizing enzyme, we determined that the chemiluminescent detection system was four orders of magnitude more sensitive than the standard colorimetric method of detection. Moreover, the chemiluminescent assay was faster and markedly simpler to perform. To maximize the utility of this assay system, two osteoprogenitor cell lines were compared for their ability to generate alkaline phosphatases in vitro when exposed to recombinant human bone morphogenetic protein (rhBMP-2). The W20-17 cell line was substantially more sensitive to rhBMP-2 than the C3H10T1/2 cell line, where each cell line produced detectable increases in AP after exposure to rhBMP-2 levels of 5 and 25 ng/ml, respectively. The experimental design for AP responsiveness to rhBMP-2 was further optimized for chemiluminescent detection with the W20-17 cell line by comparing the effects of reporter cell seeding density and the day of assay. In summary, the data presented in this paper demonstrate a faster, simpler, and more sensitive chemiluminescent method to monitor changes in AP levels during osteodifferentiation.

Osteoprogenitor cells within skeletal muscle

The formation of ectopic bone within skeletal muscle is a widely observed phenomenon. However, the source of the osteoprogenitor cells responsible for ectopic bone formation remains unknown. This study was designed to test for osteogenic differentiation among cells isolated from skeletal muscle tissue. Different subpopulations of cells derived from an adult mouse skeletal muscle were tested for induction of alkaline phosphatase activity after exposure to bone morphogenetic protein-2 in vitro. A responsive subpopulation was identified, transduced with a retrovirus encoding for beta-galactosidase (Rv-lacZ) and an adenoviral construct encoding for one bone morphogenetic protein-2, and injected into the hindlimb of immune compromised (severe combined immunodeficient, or SCID) mice. The injected cells appeared to actively participate in the ectopic bone formation. The existence of lacZ-positive muscle-derived cells colocalized with osteocalcin-producing cells within lacunae of newly formed bone matrix suggests osteoblast and osteocyte differentiation. Although a specific cell was not isolated, these data support the contentions that osteoprogenitor cells reside within skeletal muscle and that muscle may represent a source other than bone marrow for the harvest of these cells.

Ex vivo gene therapy to produce bone using different cell types

Gene therapy and tissue engineering promise to revolutionize orthopaedic surgery. This study comprehensively compares five different cell types in ex vivo gene therapy to produce bone. The cell types include a bone marrow stromal cell line, primary muscle derived cells, primary bone marrow stromal cells, primary articular chondrocytes, and primary fibroblasts. After transduction by an adenovirus encoding for bone morphogenetic protein-2, all of the cell types were capable of secreting bone morphogenetic protein-2. However, the bone marrow stromal cell line and muscle derived cells showed more responsiveness to recombinant human bone morphogenetic protein-2 than did the other cell types. In vivo injection of each of the cell populations transduced to secrete bone morphogenetic protein-2 resulted in bone formation. Radiographic and histologic analyses corroborated the in vitro data regarding bone morphogenetic protein-2 secretion and cellular osteocompetence. This study showed the feasibility of using primary bone marrow stromal cells, primary muscle derived cells, primary articular chondrocytes, primary fibroblasts, and an osteogenesis imperfecta stromal cell line in ex vivo gene therapy to produce bone. The study also showed the advantages and disadvantages inherent in using each cell type.

Novel approaches to fracture healing

The fractures that occur as a result of trauma frequently require multiple stage surgical procedures to achieve adequate union. Bone grafting with autogenous cancellous or cortico cancellous bone grafts is the traditional method used to repair bone defects. Most fractures will heal using this traditional procedure, however a number of fractures, up to 10% of the cases in United States alone, will result in delayed or impaired healing. Novel approaches are currently being investigated for the augmentation and acceleration of fracture healing. Some of these approaches include the use of biodegradable matrices; cell based approaches supplemented with osteogenic factors and genetic therapy. Cell based approaches for fracture healing have roused intense interest because of the great advance in the isolation and expansion of cells from the marrow that have the ability to differentiate into various types of cells including osteoblasts. In addition, the discovery and cloning of several proteins (bone morphogenetic proteins) that have the ability to induce bone formation, have contributed to the investigation of novel approaches to augment fracture healing. Use of genetic therapy for the augmentation of fracture healing has also recently gained strong interest. The attractive feature of gene therapy is that therapeutic proteins can be delivered locally to the fracture site in relatively high concentrations and in a sustained fashion. This review discusses these novel approaches and presents an assessment of their future clinical applicability.

Mechanical strain inhibits expression of osteoclast differentiation factor by murine stromal cells

Normal dynamic loading prevents bone resorption; however, the means whereby biophysical factors reduce osteoclast activity are not understood. We show here that mechanical strain (2% at 10 cycles per minute) applied to murine marrow cultures reduced 1, 25(OH)(2)D(3)-stimulated osteoclast formation by 50%. This was preceded by decreased expression of osteoclast differentiation factor (ODF/TRANCE). RT-PCR for ODF/TRANCE revealed that ODF/TRANCE mRNA in strained cultures was 59 +/- 3% of that seen in control cultures. No significant effects on total cell count, thymidine uptake, or alkaline phosphatase activity were induced by strain. To isolate the cell targeted by strain, primary stromal cells were cultured from marrow. Mechanical strain also reduced mRNA for ODF/TRANCE to 60% that of control in these cells. In contrast, mRNA for membrane-bound macrophage colony-stimulating factor was not significantly affected. Soluble ODF ( approximately 2 ng/ml) was able to reverse the effect of strain, returning osteoclast numbers to control. Because osteoclast formation is dependent upon ODF/TRANCE expression, strain-induced reductions in this factor may contribute to the accompanying reduction in osteoclastogenesis.

In vivo expression of human growth hormone by genetically modified murine bone marrow stromal cells and its effect on the cells in vitro

Human growth hormone (hGH) is frequently used clinically for growth abnormalities in children and also in adults with growth hormone deficiency. The hormone is usually administered to the individuals by frequent injections. In the present study we investigated the potential of bone marrow stromal cells as vehicles to deliver the GH in vivo by infusion of cells transduced with hGH cDNA into mice femurs. The effect of the hormone on the transduced cells in vitro was also assessed. Bone marrow stromal cells established from a mouse model of human osteogenesis imperfecta mice (oim) were transduced with a retrovirus containing hGH and neomycin resistance genes. The hGH-expressing cells were selected in a medium containing G418 and were then assessed for the hGH expression in vitro. The selected cells synthesized 15 ng/10(6) cells of hGH per 24 h in vitro and exhibited alkaline phosphatase activity when they were treated with the human recombinant bone morphogenetic protein 2 (rhBMP-2). The transduced cells also proliferated faster than the LacZ transduced cells but they did not exhibit a higher rate of matrix synthesis. When 2 x 10(6) hGH+ cells were injected into the femurs of mice, hGH was detected in the serum of the recipient mice up to 10 days after injection. The highest level of growth hormone expression, 750 pg/ml, was detected in the serum of the recipient mice I day after injection of the transduced cells. hGH was also detected in the medium conditioned by cells that were flushed from the femurs of the recipient mice at 1, 3, and 6 days after cell injection. These data indicate that bone marrow stromal cells could potentially be used therapeutically for the delivery of GH or any other therapeutic proteins targeted for bone. The data also suggest that GH may exert its effects on bone marrow stromal cells by increasing their rate of proliferation.

Genetic enhancement of fracture repair: healing of an experimental segmental defect by adenoviral transfer of the BMP-2 gene

This study evaluated the ability of gene transfer to enhance bone healing. Segmental defects were created surgically in the femora of New Zealand white rabbits. First generation adenoviruses were used as vectors to introduce into the defects genes encoding either human bone morphogenetic protein-2 (BMP-2) or, as a negative control, firefly luciferase. Representative specimens were evaluated histologically after 8 weeks. Healing of the defects was monitored radiographically for 12 weeks, after which time the repair tissue was evaluated biomechanically. By radiological criteria, animals receiving the BMP-2 gene had healed their osseous lesions after 7 weeks, whereas those receiving the luciferase gene had not. Histologic examination of representative rabbits at 8 weeks confirmed ossification across the entire defect in response to the BMP-2 gene, whereas the control defect was predominantly fibrotic and sparsely ossified. At the end of the 12-week experiment, the control femora still showed no radiological signs of stable healing. The difference in radiologically defined healing between the experimental and control groups was statistically significant (P < 0. 002). Biomechanical testing of the femora at 12 weeks demonstrated statistically significant increases in the mean bending strength (P < 0.005) and bending stiffness (P < 0.05) of the animals treated with the BMP-2 gene. Direct, local adenoviral delivery of an osteogenic gene thus led to the healing of an osseous lesion that otherwise would not do so. These promising data encourage the further development of genetic approaches to enhancing bone healing. Gene Therapy (2000) 7, 734-739.

Adenoviral transduction of human osteoblastic cell cultures: a new perspective for gene therapy of bone diseases

This article confirms the susceptibility of osteoblastic cells to adenoviral transduction. Osteoblasts were harvested from human cancellous bone. Cells were transduced, using various amounts of adenoviral vectors carrying the cDNA encoding interleukin-1 receptor antagonist (IL-1 Ra), or the marker genes beta-galactosidase and luciferase. Expression of the transgenes and the biological activity of IL-1 Ra produced by gene transfer were measured quantitatively in a time-course by ELISA. The rate of transduction was 100% after exposure to 1 x 10(7) infective particles of adeno-LacZ. No expression of IL-1Ra was seen after transduction with adeno-IL-1Ra at titers of 1 x 10(4) and less. However, after transduction at titers of 1 x 10(7), infective particles cells expressed IL-1 Ra consistently for 72 days, with levels up to 1 microg IL-1 Ra/1 x 10(6) cells/ 48 hours. None of the control samples expressed detectable levels of IL-1 Ra. The biological activity of the transgenic IL-1 Ra was demonstrated by its ability to suppress successfully IL-1-induced nitric oxide synthesis by rabbit articular chondrocytes. After transduction with 1 x 10(7) infective particles of the adenoluciferase vector, up to 81,000 Units transgenic luciferase/x 10(6) osteoblastic cells were measured 2 days after gene transfer. Our results show that adenovirus transduces osteoblastic cells at a high rate in vitro.

Adenovirus-mediated direct gene therapy with bone morphogenetic protein-2 produces bone

The need to improve bone healing permeates the discipline of orthopedic surgery. Bone morphogenetic proteins (BMPs) are capable of inducing ectopic and orthotopic bone formation. However, the ideal approach with which to deliver BMPs remains unknown. Gene therapy to deliver BMPs offers several theoretical advantages over implantation of a recombinant BMP protein, including persistent BMP delivery and eliminating the need for a foreign body carrier. A replication defective adenoviral vector was constructed to carry the rhBMP-2 gene (AdBMP-2). The direct in vivo gene therapy approach was applied in both immunodeficient and immunocompetent animals to produce intramuscular bone as early as 2 weeks following injection. Radiographic and histologic analysis revealed radiodense bone containing mature bone marrow elements. Adenovirus-mediated delivery of a marker gene (beta-galactosidase) into control animals produced no bone but indicated the cells transduced with the AdBMP-2 vector. Furthermore, comparisons between immunodeficient and immunocompetent animals illustrated the magnitude and significance of the immune response. Gene therapy to deliver BMP-2 has innumerable potential clinical applications from bone defect healing to joint replacement prosthesis stabilization. This study is the first to establish the feasibility of in vivo gene therapy to deliver active BMP-2 and produce bone.

Potential treatment of osteoarthritis by gene therapy

OA is common, debilitating, costly, incurable, and, in many cases, resistant to treatment. Novel approaches to therapy are clearly required. Progress in understanding the biology of cartilage and OA have led to our suggestion of a gene therapy approach to treatment. Genes whose products stimulate chondrogenesis or inhibit breakdown of the cartilaginous matrix are obviously candidates for therapeutic use. These genes may be transferred to the synovium or cartilage of affected joints by in vivo or ex vivo means using a variety of vectors. Transfer of such genes to chondroprogenitor cells is a particularly attractive approach.

Retrovirally transduced bone marrow stromal cells isolated from a mouse model of human osteogenesis imperfecta (oim) persist in bone and retain the ability to form cartilage and bone after extended passaging

Bone marrow stromal cells isolated from a model of osteogenesis imperfecta (oim) mice, were transduced with a retrovirus (BAG) carrying the LacZ and neor genes after passage 21. The transduced cells retained the ability to express alkaline phosphatase activity in vitro when treated with recombinant human bone morphogenetic protein two (rhBMP-2), formed cartilage in vitro in aggregate cultures and formed bone in ceramic cubes after 6 weeks of implantation in nude mice. X-gal staining of ceramic cubes seeded with the transduced cells demonstrated the presence of LacZ-positive cells on the edges of bone and also in the lacunae of the newly formed bone 6 weeks after implantation. After infusion into femurs of oim mice, the transduced cells were detected in the marrow cavity and on the edges of the trabecular bone of the injected and contralateral femurs by X-gal staining and PCR analysis at 4, 10, 20, 30 and 40 days after injection. The LacZ gene was also detected in the lung and liver of the recipient mice at 4 and 10 days after injection but not at later time-periods. The present findings suggest that long-term cultured bone marrow stromal cells from osteogenesis imperfecta (OI) animals have the potential to traffic through the circulatory system, home to bone, form bone and continue to express exogenous genes. These findings open the possibility of using these cells as vehicles to deliver normal genes to bone as an alternative approach for the treatment of some forms of OI and certain other bone acquired and genetic diseases.

Engineered pluripotent mesenchymal cells integrate and differentiate in regenerating bone: a novel cell-mediated gene therapy

BACKGROUND

Among the approximately 6.5 million fractures suffered in the United States every year, about 15% are difficult to heal. As yet, for most of these difficult cases there is no effective therapy. We have developed a mouse radial segmental defect as a model experimental system for testing the capacity of Genetically Engineered Pluripotent Mesenchymal Cells (GEPMC, C3H10T1/2 clone expressing rhBMP-2), for gene delivery, engraftment, and induction of bone growth in regenerating bone.

METHODS

Transfected GEPMC expressing rhBMP-2 were further infected with a vector carrying the lacZ gene, that encodes for beta-galactosidase (beta-gal). In vitro levels of rhBMP-2 expression and function were confirmed by immunohistochemistry, and bioassay. Differentiation was assayed using alkaline phosphatase staining. GEPMC were transplanted in vivo into a radial segmental defect. The main control groups included lacZ clones of WT-C3H10T1/2-LacZ, and CHO-rhBMP-2 cells. New bone formation was measured quantitatively via fluorescent labeling, X-ray analysis and histomorphometry. Engrafted mesenchymal cells were localized in vivo by beta-gal expression, and double immunofluorescence.

RESULTS

In vitro, GEPMC expressed rhBMP-2, beta-gal and spontaneously differentiated into osteogenic cells expressing alkaline phosphatase. Detection of transplanted cells revealed engrafted cells that had differentiated into osteoblasts and co-expressed beta-gal and rhBMP-2. Analysis of new bone formation revealed that at four to eight week post-transplantation, GEPMS significantly enhanced segmental defect repair.

CONCLUSIONS

Our study shows that cell-mediated gene transfer can be utilized for growth factor delivery to signaling receptors of transplanted cells (autocrine effect) and host mesenchymal cells (paracrine effect) suggesting the ability of GEPMC to engraft, differentiate, and stimulate bone growth. We suggest that our approach should lead to the designing of mesenchymal stem cell based gene therapy strategies for bone lesions as well as other tissues.

Potential role for gene therapy in the enhancement of fracture healing

Various proteins have the potential to initiate and accelerate fracture healing. Although osteogenic growth factors are the most prominent of these, there also may be important roles for other agents including growth factor receptors, angiogenic factors, and cytokine antagonists. Gene based delivery systems offer the potential to achieve therapeutic levels of these proteins locally within the fracture site for sustained times. Moreover, these delivery systems may deliver their products in a more biologically active form than that achieved by the exogenous application of recombinant proteins. Genes may be transferred to fractures by direct in vivo delivery or by indirect ex vivo delivery, using viral or nonviral vectors. Two examples are described in this article. With an ex vivo procedure, it was possible to transfer lac Z and neo(r) marker genes to the bones of mice, using retroviral transduction of bone marrow stromal cells. Gene expression in vivo persisted for several weeks. This procedure has the advantage of providing not only gene products but also osteoprogenitor cells to sites of bone healing. In vivo, local transfer of the lucerifase and lac Z marker genes was accomplished in a segmental defect model in the rabbit using adenoviral vectors. Under these conditions, gene expression in most tissues in and around the defect lasted between 2 and 6 weeks. These data encourage additional development of gene therapy for fracture healing. Such developments should go hand in hand with studies in the basic biology of fracture healing.