VEGF improves, whereas sFlt1 inhibits, BMP2-induced bone formation and bone healing through modulation of angiogenesis.

UNLABELLED: We studied the interaction between VEGF and BMP2 during bone formation and bone healing. Results indicate that VEGF antagonist inhibited BMP2-elicited bone formation, whereas the delivery of exogenous VEGF enhanced BMP2-induced bone formation and bone healing through modulation of angiogenesis. INTRODUCTION: Angiogenesis is closely associated with bone formation during normal bone development and is important for the bone formation elicited by BMP4. However, it remains unknown whether vascular endothelial growth factor (VEGF) also interacts with other BMPs, especially BMP2, in bone formation and bone healing. MATERIALS AND METHODS: For this study, mouse muscle-derived stem cells were transduced to express BMP2, VEGF, or VEGF antagonist (sFlt1). We studied the angiogenic process during endochondral bone formation elicited by BMP2, a prototypical osteogenic BMP. Using radiographic and histologic analyses, we also evaluated the interaction between VEGF and BMP2 during bone formation and bone healing. RESULTS: Our results indicate that BMP2-elicited bone formation comprises two phases of angiogenesis, with an early phase occurring before the appearance of hypertrophic cartilage, followed by a late phase coupled with the appearance of hypertrophic cartilage. Our finding that the administration of sFlt1, a specific antagonist of VEGF, significantly inhibited BMP2-induced bone formation and the associated angiogenesis indicates that endogenous VEGF activity is important for bone formation. Furthermore, we found that the delivery of exogenous VEGF enhanced BMP2-induced bone formation and bone healing by improving angiogenesis, which in turn led to accelerated cartilage resorption and enhanced mineralized bone formation. Our findings also indicate that the ratio between VEGF and BMP2 influences their synergistic interaction, with a higher proportion of VEGF leading to decreased synergism. Our study also revealed unique VEGF-BMP2 interactions that differ from the VEGF-BMP4 interactions that we have described previously. CONCLUSIONS: This study, along with previously published work, shows that VEGF interacts synergistically with both BMP4 and BMP2 but elicits substantially different effects with these two BMPs.

Structural and functional healing of critical-size segmental bone defects by transduced muscle-derived cells expressing BMP4.

BACKGROUND: Our previous studies have shown that muscle-derived cells, including a population of muscle stem cells, transduced with a retroviral vector expressing bone morphogenetic protein 4 (BMP4) can improve the healing of critical-size calvarial defects. However, we did not evaluate the functionality of the healed bone. The purpose of this study was to determine whether primary muscle-derived cells transduced with retroBMP4 can heal a long bone defect both structurally and functionally. METHODS: Primary muscle-derived cells were genetically engineered to express BMP4 and were implanted into 7-mm femoral defects created in syngeneic rats. Muscle-derived cells transduced with retroLacZ were used in the control group. Bone healing was monitored by radiography, histology, and biomechanical testing at designated time points. RESULTS: Most of the defects treated with muscle-derived cells expressing BMP4 formed bridging callous by 6 weeks after surgery, and exhibited radiographically evident union at 12 weeks after cell implantation. Histological analysis at 12 weeks revealed that the medullary canal of the femur was restored and the cortex was remodeled between the proximal and distal ends of each BMP4-treated defect. In contrast, the defects treated with muscle-derived cells expressing beta-galactosidase displayed nonunion at all tested time points. An evaluation of the maximum torque-to-failure in the treatment group indicated that the healed bones possessed 77 +/- 28% of the strength of the contralateral intact femora. Torsional stiffness and energy-to-failure were not significantly different between the treated and intact limbs. CONCLUSIONS: This study demonstrated that primary muscle-derived cells transduced with retroBMP4 can elicit both structural and functional healing of critical-size segmental long bone defects created in rats.

Ex vivo gene therapy-induced endochondral bone formation: comparison of muscle-derived stem cells and different subpopulations of primary muscle-derived cells.

Muscle-based gene therapy and tissue engineering hold great promise for improving bone healing. However, the relative advantage of muscle-derived stem cells (MDSCs) or primary muscle-derived cells (MDCs) remains to be defined. We compared the ability of MDSCs and different subpopulations of MDCs (PP1 and PP3) to induce bone formation via ex vivo gene therapy. We were able to efficiently transduce the MDSCs and all the other evaluated populations of MDCs (efficiency of transduction = approximately 80%) by using a retroviral vector expressing human bone morphogenetic protein 4 (BMP4). All the transduced cell populations secreted high levels of BMP4 (140-300 ng/10(6) cells/24 h), but the MDSCs differentiated toward the osteogenic lineage more effectively than did the other muscle cell populations, as indicated by the expression of alkaline phosphatase, an early osteogenic marker. von Kossa staining indicated that mineralized bone formed as early as 7 days after implantation of any of the BMP4-expressing cell populations into immunocompetent syngeneic mice; however, MDSCs expressing BMP4 produced significantly more bone than did the other MDC populations, as evidenced by both histomorphometry and biochemical analysis. Further investigation revealed that MDSCs expressing BMP4 persisted for a significantly longer period of time at the bone forming sites than did the other BMP4-expressing MDC populations. Additionally, MDSCs expressing BMP4 triggered a smaller infiltration of CD4 lymphocytes within the bone forming areas than did the other MDC populations expressing BMP4. Finally, we demonstrated that MDSCs expressing BMP4 can heal a critical-sized skull bone defect in immunocompetent mice. In summary, this study shows that MDSCs are better than primary MDCs for use as cellular vehicles in BMP4-based ex vivo gene therapy to improve bone healing. The advantage of MDSCs may be attributable, at least in part, to their lower immunogenicity and higher capacity for in vivo survival.

Development of a self-inactivating tet-on retroviral vector expressing bone morphogenetic protein 4 to achieve regulated bone formation.

The aims of this study were to explore the possibility of improving the design of self-inactivating (SI) retroviral vectors and to develop an SI vector that would allow optimal tet-on-regulated therapeutic gene expression. To minimize any interference between the viral promoter and the inducible promoter, we deleted different regulatory elements in the 3'LTR and examined their effects on transgene expression in transfected or transduced cells. In transfected cells, such deletions reduced the transgene expression. The insertion of a polyadenylation sequence could not completely compensate for this effect. We observed three patterns of transgene expression in cells transduced with these tet-on retroviral vectors: (1) high levels of both basal and inducible expression, (2) low levels of both basal and inducible expression, and (3) low levels of basal and high levels of inducible expression. After using the optimal vector to transduce muscle-derived stem cells, we were able to regulate the strong in vitro expression of transgenes-including enhanced green fluorescent protein and bone morphogenetic protein 4-via the addition or withdrawal of doxycycline (Dox). Implantation of the transduced cells and subsequent Dox-dependent induction of gene expression resulted in bone formation in vivo. Thus, we have developed an optimal SI retroviral vector that maintains a high titer, efficiently transduces muscle-derived stem cells, and enables both high levels of inducible gene expression in vitro and robust regulated bone formation in vivo.

Converse relationship between in vitro osteogenic differentiation and in vivo bone healing elicited by different populations of muscle-derived cells genetically engineered to express BMP4.

UNLABELLED: In this study, we compared the use of primary muscle-derived osteoprogenitor cells (PP6 cells) for the delivery of BMP4 to improve bone healing to that of muscle-derived non-osteoprogenitor cells (PP1 cells). Surprisingly, the use of PP1 cells resulted in an improved outcome because of the lack of adverse responses to BMP4 involving cell differentiation, proliferation, and apoptosis. INTRODUCTION: Although researchers frequently opt to use osteogenic cells for osteogenic bone morphogenetic protein (BMP)-based ex vivo gene therapy to improve bone healing, it remains unclear whether the osteogenic potential of a cellular vehicle affects the outcome of bone healing applications. Here we compared the use of muscle-derived non-osteoprogenitor cells (PP1 cells) to that of primary muscle-derived osteoprogenitor cells (PP6 cells) for the delivery of BMP4 to improve the healing of bone defects. MATERIALS AND METHODS: Two distinct populations of primary rat muscle-derived cells--PP1 and PP6--were selected, transduced with retroviral vectors to express BMP4 or a marker gene (LacZ), and implanted into critical-sized calvarial defects created in syngeneic rats. The bone healing was monitored radiographically and histologically at 7 and 14 weeks after implantation. Cellular responses to BMP4 were evaluated by alkaline phosphatase histochemical staining and RT-PCR of another osteogenic marker to indicate osteogenic differentiation, a cell proliferation assay and BrdU (bromodeoxyuridine) labeling to assess cell proliferation, and the TUNEL assay to determine apoptosis. RESULTS AND CONCLUSIONS: In all animals (nine rats per group), transduced PP1 cells expressing BMP4 demonstrated significantly advanced healing compared with PP6 cells expressing BMP4 and control cells expressing LacZ. We found that constitutive BMP4 expression negatively impacted the in vitro proliferation and in vivo survival rates of PP6 cells, but not PP1 cells. BMP4 exposure also directly inhibited the proliferation and induced the apoptosis of PP6 cells, but not PP1 cells. The impairment in PP6 cell proliferation was directly associated with the osteogenic differentiation of these cells. These results indicate that PP1 cells are better suited than osteoprogenitor cells for use as cellular vehicles to deliver osteogenic BMP4 to improve bone healing and that cellular behavior in response to a particular gene can be used to predict the cells' performance as delivery vehicles in ex vivo gene therapy.

Retroviral delivery of Noggin inhibits the formation of heterotopic ossification induced by BMP-4, demineralized bone matrix, and trauma in an animal model.

BACKGROUND: The heterotopic ossification of muscles, tendons, and ligaments is a common problem faced by orthopaedic surgeons. We investigated the ability of Noggin (a BMP [bone morphogenetic protein] antagonist) to inhibit heterotopic ossification. METHODS: Part 1: A retroviral vector carrying the gene encoding human Noggin was developed and used to transduce muscle-derived stem cells. Part 2: Cells transduced with BMP-4 were implanted into both hind limbs of mice along with either an equal number, twice the number, or three times the number of Noggin-expressing muscle-derived stem cells (treated limb) or with nontransduced muscle-derived stem cells (control limb). At four weeks, the mice were killed and radiographs were made to look for evidence of heterotopic ossification. Part 3: Eighty milligrams of human demineralized bone matrix was implanted into the hind limbs of SCID (severe combined immunodeficiency strain) mice along with 100,000, 500,000, or 1,000,000 Noggin-expressing muscle-derived stem cells (treated limbs) or nontransduced muscle-derived stem cells (control limbs). At eight weeks, the mice were killed and radiographs were made. Part 4: Immunocompetent mice underwent bilateral Achilles tenotomy along with the implantation of 1,000,000 Noggin-expressing muscle-derived stem cells (treated limbs) or nontransduced muscle-derived stem cells (control limbs). At ten weeks, the mice were killed and radiographs were made. RESULTS: Part 1: An in vitro BMP inhibition assay demonstrated that Noggin was expressed by muscle-derived stem cells at a level of 280 ng per million cells per twenty-four hours. Part 2: Three varying doses of Noggin-expressing muscle-derived stem cells inhibited the heterotopic ossification elicited by BMP-4-expressing muscle-derived stem cells. Heterotopic ossification was reduced in a dose-dependent manner by 53%, 74%, and 99%, respectively (p < 0.05). Part 3: Each of three varying doses of Noggin-expressing muscle-derived stem cells significantly inhibited the heterotopic ossification elicited by demineralized bone matrix. Heterotopic ossification was reduced by 91%, 99%, and 99%, respectively (p < 0.05). Part 4: All eleven animals that underwent Achilles tenotomy developed heterotopic ossification at the site of the injury in the control limbs. In contrast, the limbs treated with the Noggin-expressing muscle-derived stem cells had a reduction in the formation of heterotopic ossification of 83% and eight of the eleven animals had no radiographic evidence of heterotopic ossification (p < 0.05). CONCLUSIONS: The delivery of Noggin mediated by muscle-derived stem cells can inhibit heterotopic ossification caused by BMP-4, demineralized bone matrix, and trauma in an animal model. CLINICAL RELEVANCE: Gene therapy to deliver Noggin may become a powerful method to inhibit heterotopic ossification in targeted areas of the body.

Synergistic enhancement of bone formation and healing by stem cell-expressed VEGF and bone morphogenetic protein-4.

We investigated the interaction between angiogenic and osteogenic factors in bone formation and bone healing with ex vivo gene therapy using muscle-derived stem cells genetically engineered to express human bone morphogenetic protein-4 (BMP4), VEGF, or VEGF-specific antagonist (soluble Flt1). Our results show that although VEGF alone did not improve bone regeneration, it acted synergistically with BMP4 to increase recruitment of mesenchymal stem cells, to enhance cell survival, and to augment cartilage formation in the early stages of endochondral bone formation. These early effects, coupled with accelerated cartilage resorption, eventually led to a significant enhancement of bone formation and bone healing. The beneficial effect of VEGF on bone healing elicited by BMP4 depends critically on the ratio of VEGF to BMP4, with an improper ratio leading to detrimental effects on bone healing. Finally, we show that soluble Flt1 inhibits bone formation elicited by BMP4. Thus, VEGF plays an important role in bone formation elicited by BMP4, and it can significantly enhance BMP4-elicited bone formation and regeneration through multiple mechanisms. This study has important implications for the formulation of new strategies to improve bone healing through increasing mesenchymal stem cell recruitment and survival, in combination with muscle-derived stem cell-based gene therapy.

BMP4-expressing muscle-derived stem cells differentiate into osteogenic lineage and improve bone healing in immunocompetent mice.

Recent advances in molecular biology have led the way for novel approaches to improve bone healing. The ideal growth factor, vector, and delivery systems for producing bone in an immune competent animal model, however, have yet to be identified. Using a retrovirus encoding BMP4 and recently isolated muscle-derived stem cells (MDSCs), we demonstrated the following: MDSCs undergo osteogenic differentiation in response to BMP4 in a dose-dependent manner; retrovirus encoding BMP4 can efficiently transduce MDSCs, both enhancing osteogenic differentiation and inhibiting myogenic differentiation; transduced MDSCs can produce high levels of functional BMP4 as they differentiate toward an osteogenic lineage; allogeneic transduced MDSCs can induce robust de novo bone formation in immunocompetent mice despite the presence of an immune reaction, demonstrating the ability of this retroviral-BMP4-muscle construct to provide sufficient stimuli for osteoinduction in vivo; MDSCs appear to deliver BMP4, respond to the human BMP4 in an autocrine manner, and actively participate in bone formation, thus serving both osteoinductive and osteoproductive roles; and the BMP4-expressing MDSCs can induce bone formation and improve bone healing in a critical-sized skull defect in immunocompetent mice. Therefore, we believe that technology based on the MDSCs and vector system has great potential for promoting bone healing in a variety of musculoskeletal conditions.