Adenoviral BMP-2 gene transfer in mesenchymal stem cells: in vitro and in vivo bone formation on biodegradable polymer scaffolds

Polymeric growth factor delivery strategies for tissue engineering

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Osteogenic activity of the fourteen types of human bone morphogenetic proteins (BMPs)

BACKGROUND

Bone morphogenic proteins (BMPs) are known to promote osteogenesis, and clinical trials are currently underway to evaluate the ability of certain BMPs to promote fracture-healing and spinal fusion. The optimal BMPs to be used in different clinical applications have not been elucidated, and a comprehensive evaluation of the relative osteogenic activity of different BMPs is lacking.

METHODS

To identify the BMPs that may possess the most osteoinductive activity, we analyzed the osteogenic activity of BMPs in mesenchymal progenitor and osteoblastic cells. Recombinant adenoviruses expressing fourteen human BMPs (BMP-2 to BMP-15) were constructed to infect pluripotent mesenchymal progenitor C3H10T1/2 cells, preosteoblastic C2C12 cells, and osteoblastic TE-85 cells. Osteogenic activity was determined by measuring the induction of alkaline phosphatase, osteocalcin, and matrix mineralization upon BMP stimulation.

RESULTS

BMP-2, 6, and 9 significantly induced alkaline phosphatase activity in pluripotential C3H10T1/2 cells, while BMP-2, 4, 6, 7, and 9 significantly induced alkaline phosphatase activity in preosteoblastic C2C12 cells. In TE-85 osteoblastic cells, most BMPs (except BMP-3 and 12) were able to induce alkaline phosphatase activity. The results of alkaline phosphatase histochemical staining assays were consistent with those of alkaline phosphatase colorimetric assays. Furthermore, BMP-2, 6, and 9 (as well as BMP-4 and, to a lesser extent, BMP-7) significantly induced osteocalcin expression in C3H10T1/2 cells. In C2C12 cells, osteocalcin expression was strongly induced by BMP-2, 4, 6, 7, and 9. Mineralized nodules were readily detected in C3H10T1/2 cells infected with BMP-2, 6, and 9 (and, to a lesser extent, those infected with BMP-4 and 7).

CONCLUSIONS

A comprehensive analysis of the osteogenic activity of fourteen types of BMPs in osteoblastic progenitor cells was conducted. Our results suggest an osteogenic hierarchical model in which BMP-2, 6, and 9 may play an important role in inducing osteoblast differentiation of mesenchymal stem cells. In contrast, most BMPs are able to stimulate osteogenesis in mature osteoblasts.

Bone tissue engineering: recent advances and promising therapeutic agents

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

Effect of regional gene therapy with bone morphogenetic protein-2-producing bone marrow cells on spinal fusion in rats

BACKGROUND

Bone morphogenetic proteins (BMPs) are now being used as bone-graft substitutes to enhance spinal fusion. However, the large doses of BMP required to induce a spinal fusion in humans suggests that the delivery of these proteins should be improved. We used ex vivo adenoviral gene transfer to create BMP-2-producing bone marrow cells, and these autologous cells were found to induce a posterolateral fusion of the spine in syngeneic rats.

METHODS

Intertransverse spinal arthrodesis (L4 and L5) was attempted in ten groups of Lewis rats with 5 x 10 (6) BMP-2-producing rat bone marrow cells (Ad-BMP-2 cells), created through adenoviral gene transfer with guanidine hydrochloride-extracted demineralized bone matrix as a carrier (Group I); 5 x 10 (6) Ad-BMP-2 cells on a collagen sponge carrier (Group II); 10 micro g of recombinant BMP-2 (rhBMP-2) in a guanidine hydrochloride-extracted demineralized bone matrix carrier (Group III); 10 micro g of rhBMP-2 in a collagen sponge carrier (Group IV); autogenous iliac crest bone-grafting (Group V); 5 x 10 (6) beta-galactosidase-producing rat bone marrow cells, created through adenoviral gene transfer with guanidine hydrochloride-extracted demineralized bone matrix as a carrier (Group VI); decortication of the transverse processes alone (Group VII); 5 x 10 (6) uninfected rat bone marrow cells with a guanidine hydrochloride-extracted demineralized bone matrix carrier (Group VIII); guanidine hydrochloride-extracted demineralized bone matrix only (Group IX); or a collagen sponge alone (Group X). Each specimen underwent plain radiography, manual palpation, and histological analysis.

RESULTS

All spines in Groups I and II (BMP-2-producing bone marrow cells) and all spines in Groups III and IV were fused at four weeks postoperatively. In contrast, none of the spines in the other groups had fused at a minimum of eight weeks after implantation. Histological analysis of the specimens revealed that the spines that had received BMP-2-producing bone marrow cells (Groups I and II) were filled with coarse trabecular bone postoperatively, whereas those that had received rhBMP-2 (Groups III and IV) were filled with thin, lace-like trabecular bone. All of the other spines, including those that had been treated with autogenous iliac crest bone-grafting (Group V), produced little or no new bone.

CONCLUSION

BMP-2-producing bone marrow cells, created by adenoviral gene transfer, produce sufficient BMP to induce an intertransverse fusion in the rat spine model.

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.

Induction of human osteoprogenitor chemotaxis, proliferation, differentiation, and bone formation by osteoblast stimulating factor-1/pleiotrophin: osteoconductive biomimetic scaffolds for tissue engineering

The process of bone growth, regeneration, and remodeling is mediated, in part, by the immediate cell-matrix environment. Osteoblast stimulating factor-1 (OSF-1), more commonly known as pleiotrophin (PTN), is an extracellular matrix-associated protein, present in matrices, which act as targets for the deposition of new bone. However, the actions of PTN on human bone progenitor cells remain unknown. We examined the effects of PTN on primary human bone marrow stromal cells chemotaxis, differentiation, and colony formation (colony forming unit-fibroblastic) in vitro, and in particular, growth and differentiation on three-dimensional biodegradable porous scaffolds adsorbed with PTN in vivo. Primary human bone marrow cells were cultured on tissue culture plastic or poly(DL-lactic acid-co-glycolic acid) (PLGA; 75:25) porous scaffolds with or without addition of recombinant human PTN (1 pg-50 ng/ml) in basal and osteogenic conditions. Negligible cellular growth was observed on PLGA scaffold alone, generated using a super-critical fluid mixing method. PTN (50 microg/ml) was chemotactic to human osteoprogenitors and stimulated total colony formation, alkaline phosphatase-positive colony formation, and alkaline phosphatase-specific activity at concentrations as low as 10 pg/ml compared with control cultures. The effects were time-dependent. On three-dimensional scaffolds adsorbed with PTN, alkaline phosphatase activity, type I collagen formation, and synthesis of cbfa-1, osteocalcin, and PTN were observed by immunocytochemistry and PTN expression by in situ hybridization. PTN-adsorbed constructs showed morphologic evidence of new bone matrix and cartilage formation after subcutaneous implantation as well as within diffusion chambers implanted into athymic mice. In summary, PTN has the ability to promote adhesion, migration, expansion, and differentiation of human osteoprogenitor cells, and these results indicate the potential to develop protocols for de novo bone formation for skeletal repair that exploit cell-matrix interactions.

Estrogens activate bone morphogenetic protein-2 gene transcription in mouse mesenchymal stem cells

Estrogens exert their physiological effects on target tissues by interacting with the estrogen receptors, ERalpha and ERbeta. Estrogen replacement is one the most common and effective strategies used to prevent osteoporosis in postmenopausal women. Whereas it was thought that estrogens work exclusively by inhibiting bone resorption, our previous results show that 17beta-estradiol (E2) increases mouse bone morphogenetic protein (BMP)-2 mRNA, suggesting that estrogens may also enhance bone formation. In this study, we used quantitative real-time RT-PCR analysis to demonstrate that estrogens increase BMP-2 mRNA in mouse mesenchymal stem cells. The selective ER modulators, tamoxifen, raloxifene, and ICI-182,780 (ICI), failed to enhance BMP-2 mRNA, whereas ICI inhibited E2 stimulation of expression. To investigate if estrogens increase BMP-2 expression by transcriptional mechanisms and if the response is mediated by ERalpha and/or ERbeta, we studied the effects of estrogens on BMP-2 promoter activity in transient transfected C3H10T1/2 cells. E2 produced a dose-dependent induction of the mouse -2712 BMP-2 promoter activity in cells cotransfected with ERalpha and ERbeta. At a dose of 10 nM E2, ERalpha induced mouse BMP-2 promoter activity 9-fold, whereas a 3-fold increase was observed in cells cotransfected with ERbeta. Tamoxifen and raloxifene were weak activators of the mouse BMP-2 promoter via ERalpha, but not via ERbeta. ICI blocked the activation of BMP-2 promoter activity by E2 acting via both ERalpha and ERbeta, indicating that mouse BMP-2 promoter activation is ER dependent. In contrast to E2 and selective ER modulators, the phytoestrogen, genistein was more effective at activating the mouse BMP-2 promoter with ERbeta, compared with ERalpha. Using a deletion series of the BMP-2 promoter, we determined that AP-1 or Sp1 sites are not required for E2 activation. A mutation in a sequence at -415 to -402 (5'-GGGCCActcTGACCC-3') that resembles the classical estrogen-responsive element abolished the activation of the BMP-2 promoter in response to E2. Our studies demonstrate that E2 activation of mouse BMP-2 gene transcription requires ERalpha or ERbeta acting via a variant estrogen-responsive element binding site in the promoter, with ERalpha being the more efficacious regulator. Estrogenic compounds may enhance bone formation by increasing the transcription of the BMP-2 gene.

Immunoselection and adenoviral genetic modulation of human osteoprogenitors: in vivo bone formation on PLA scaffold

The aim of this study was to examine the potential of immunoselected genetically modified human osteoprogenitors to form bone in vivo on porous PLA scaffolds. Human osteoprogenitors from bone marrow were selected using the antibody STRO-1 utilising a magnetically activated cell separation system. The STRO-1(+) fraction isolated 7% of nucleated marrow cells and increased fibroblastic colony formation by 300% and alkaline phosphatase activity by 190% over unselected marrow cell cultures. To engineer bone tissue, STRO-1(+) culture-expanded cells were transduced with AxCAOBMP-2, an adenovirus carrying the human BMP-2 gene, injected into diffusion chambers containing porous PLA scaffolds, and implanted in vivo. After 11 weeks the presence of bone mineral was observed by X-ray analysis and confirmed for mineral by von Kossa, as well as bone matrix composition by Sirius red staining, birefringence, and type I collagen immunohistochemistry. Bone formation in vivo indicates the potential of using immunoselected progenitor cells and ex vivo gene transfer with biodegradable scaffolds, for the development of protocols for the treatment of a wide variety of musculo-skeletal disorders.