Human skeletal muscle cells in ex vivo gene therapy to deliver bone morphogenetic protein-2

In silico evaluation of the mechanical stimulation effect on the regenerative rehabilitation for the articular cartilage local defects

Osteoarthritis is one of the most severe diseases of the human musculoskeletal system, and therefore, for many years, special attention has been paid to the search for effective methods of its treatment. However, even the most modern methods only in a limited number of cases in the early or intermediate stages of osteoarthritis lead to positive treatment results. In the later stages of development, osteoarthritis is practically incurable and most often ends with disability or the need for joint replacement for a large number of people. One of the main reasons hindering the development of osteoarthritis treatment methods is the peculiarities of articular cartilage, in which there is practically no vascular network and tissue homeostasis is carried out mainly due to the diffusion of nutrients present in the synovial fluid. In modern medicine, for the treatment of osteoarthritis, tissue engineering strategies have been developed based on the implantation of scaffolds populated with chondrogenic cells into the area of the defect. In vitro studies have established that these cells are highly mechanosensitive and, under the influence of mechanical stimuli of a certain type and intensity, their ability to proliferate and chondrogenesis increases. This property can be used to improve the efficiency of regenerative rehabilitation technologies based on the synergistic combination of cellular technologies, tissue engineering strategies, and mechanical tissue stimulation. In this work, using a regenerative rehabilitation mathematical model of local articular cartilage defects, numerical experiments were performed, the results of which indicate that the micro-and macro environment of the restored tissue, which changes during mechanical stimulation, has a significant effect on the formation of the extracellular matrix, and, consequently, cartilage tissue generally. The results obtained can be used to plan strategies for mechanical stimulation, based on the analysis of the results of cell proliferation experimental assessment after each stimulation procedure in vivo.

Graft Materials and Biologics for Spinal Interbody Fusion

Spinal fusion is the most widely performed procedure in spine surgery. It is the preferred treatment for a wide variety of pathologies including degenerative disc disease, spondylolisthesis, segmental instability, and deformity. Surgeons have the choice of fusing vertebrae by utilizing cages containing autografts, allografts, demineralized bone matrices (DBMs), or graft substitutes such as ceramic scaffolds. Autografts from the iliac spine are the most commonly used as they offer osteogenic, osteoinductive, and osteoconductive capabilities, all while avoiding immune system rejection. Allografts obtained from cadavers and living donors can also be advantageous as they lack the need for graft extraction from the patient. DBMs are acid-extracted organic allografts with osteoinductive properties. Ceramic grafts containing hydroxyapatite can be readily manufactured and are able to provide osteoinductive support while having a long shelf life. Further, bone-morphogenetic proteins (BMPs), mesenchymal stem cells (MSCs), synthetic peptides, and autologous growth factors are currently being optimized to assist in improving vertebral fusion. Genetic therapies utilizing viral transduction are also currently being devised. This review provides an overview of the advantages, disadvantages, and future directions of currently available graft materials. The current literature on growth factors, stem cells, and genetic therapy is also discussed.

Bone Tissue Engineering Using Human Cells: A Comprehensive Review on Recent Trends, Current Prospects, and Recommendations

The use of proper cells for bone tissue engineering remains a major challenge worldwide. Cells play a pivotal role in the repair and regeneration of the bone tissue in vitro and in vivo. Currently, a large number of differentiated (somatic) and undifferentiated (stem) cells have been used for bone reconstruction alone or in combination with different biomaterials and constructs (e.g., scaffolds). Although the results of the cell transplantation without any supporting or adjuvant material have been very effective with regard to bone healing. Recent advances in bone scaffolding are now becoming new players affecting the osteogenic potential of cells. In the present study, we have critically reviewed all the currently used cell sources for bone reconstruction and discussed the new horizons that are opening up in the context of cell-based bone tissue engineering strategies.

Characterization and spatiotemporal expression analysis of nine bone morphogenetic protein family genes during intermuscular bone development in blunt snout bream

Intermuscular bones (IBs) only exist in the myosepta of lower teleosts and its molecular mechanism remains to be clarified. Bone morphogenetic proteins (BMPs) have been demonstrated to be involved in various physiological processes, including bone and cartilage formation. In this study, we firstly obtained and characterized nine bmp genes for Megalobrama amblycephala, which belongs to Cyprinidae and have a certain amount of IBs. Sequence alignment and phylogenetic analysis both documented that the mature proteins of M. amblycephala bmp genes were highly conserved with other corresponding homologs, respectively, indicating that the function of each bmp gene has been conserved throughout evolution. As a step to characterize potential involvement of bmp genes in IB formation and development, spatiotemporal expressions of nine bmp genes (bmp2a, bmp2b, bmp3, bmp4, bmp5, bmp7b, bmp8a, bmp14 and bmp16) were investigated during the key development stages of IBs. During the ossification process from stage I (the IBs haven't emerged) to stage IV (all of the IBs ossified in the tail with the mature morphology), the expression profiles revealed that bmp16 was the most abundant transcript while bmp4 had the lowest abundance. The mRNA levels of bmp3, bmp4, bmp5 and bmp8a increased significantly at stage II, suggesting their roles in stimulating IB formation. The expression of bmp7b reached the highest level at stage III (the rapid period of IB development), suggesting potential involvement of bmp7b in promoting osteoblast differentiation. With the exception of bmp7b and bmp16, most bmp genes appeared a significant increase at IB maturation phase (stage IV), which means that they may play important roles in maintenance of IB morphogenesis. Spatial tissue distribution of bmp genes showed that most bmp genes were observed at the highest level in developing IBs at one year old fish. Spatiotemporal expression patterns suggest the potential key roles of these bmp genes in IBs formation and maintenance in fish, being as possible promoters or inhibitors.

* Calvarial Defects: Cell-Based Reconstructive Strategies in the Murine Model

Calvarial defects pose a continued clinical dilemma for reconstruction. Advancements within the fields of stem cell biology and tissue engineering have enabled researchers to develop reconstructive strategies using animal models. We review the utility of various animal models and focus on the mouse, which has aided investigators in understanding cranial development and calvarial bone healing. The murine model has also been used to study regenerative approaches to critical-sized calvarial defects, and we discuss the application of stem cells such as bone marrow-derived mesenchymal stromal cells, adipose-derived stromal cells, muscle-derived stem cells, and pluripotent stem cells to address deficient bone in this animal. Finally, we highlight strategies to manipulate stem cells using various growth factors and inhibitors and ultimately how these factors may prove crucial in future advancements within calvarial reconstruction using native skeletal stem cells.

The effect of BMP-7 gene activated muscle tissue implants on the repair of large segmental bone defects

BACKGROUND

This study was conducted in order to investigate the effect of Bone Morphogenetic Protein-7 (BMP-7) transduced muscle cells on bone formation and to further develop an innovative abbreviated ex vivo gene therapy for bone repair. As conventional ex vivo gene therapy methods require an elaborative and time-consuming extraction and expansion of cells we evaluated an expedited approach. Fragments of muscle tissue were directly activated by BMP-7 cDNA and implanted into bone defects.

METHODS

25 male, syngeneic Fischer 344 rats were used in the present study. Muscle tissue was harvested from two donor rats and either transduced with an adenovirus carrying the BMP-7 cDNA or remained unmodified. 5mm osseous defects in the right femora of 23 rats were treated with either unmodified muscle tissue (control group) or BMP-7 activated muscle tissue (treatment group). Six weeks after surgery, rat femora were evaluated by radiographs, micro-computed tomography (μCT) and histology.

RESULTS

Implantation of BMP-7 activated muscle grafts led to bony bridging in 5 out of 12 defects (41.7%) and to bone formation without bridging in 2 out of 12 defects. In 2 femoral defects of this group radiographs, μCT-imaging and histology did not reveal significant mineralization. Three animals of the BMP-7 treatment group had to be euthanized due to serious wound infection. The bone volume of the treatment group was significantly (p=0.007) higher compared to the control group.

CONCLUSION

This study shows that BMP-7 gene activated muscle fragments have the potential to regenerate critical-size segmental bone defects in rats. However, further development of this promising expedited treatment modality is required to improve the healing rate and to investigate if the high infection rate is related to treatment with BMP-7 activated muscle grafts.

A comparative study of the effects of growth and differentiation factor 5 on muscle-derived stem cells and bone marrow stromal cells in an in vitro tendon healing model

PURPOSE

To investigate the ability of muscle-derived stem cells (MDSCs) supplemented with growth and differentiation factor-5 (GDF-5) to improve tendon healing compared with bone marrow stromal cells (BMSCs) in an in vitro tendon culture model.

METHODS

Eighty canine flexor digitorum profundus tendons were assigned into 5 groups: repaired tendon (1) without gel patch interposition (no cell group), (2) with BMSC-seeded gel patch interposition (BMSC group), (3) with MDSC-seeded gel patch interposition (MDSC group), (4) with GDF-5-treated BMSC-seeded gel patch interposition (BMSC+GDF-5 group), and (5) with GDF-5-treated MDSC-seeded gel patch interposition (MDSC+GDF-5 group). After culturing for 2 or 4 weeks, the failure strength of the healing tendons was measured. The tendons were also evaluated histologically.

RESULTS

The failure strength of the repaired tendon in the MDSC+GDF-5 group was significantly higher than that of the non-cell and BMSC groups. The stiffness of the repaired tendons in the MDSC+GDF-5 group was significantly higher than that of the non-cell group. Histologically, the implanted cells became incorporated into the original tendon in all 4 cell-seeded groups.

CONCLUSIONS

Interposition of a multilayered GDF-5 and MDSC-seeded collagen gel patch at the repair site enhanced tendon healing compared with a similar patch using BMSC. However, this increase in vitro was relatively small. In the clinical setting, differences between MDSC and BMSC may not be substantially different, and it remains to be shown that such methods might enhance the results of an uncomplicated tendon repair clinically.

CLINICAL RELEVANCE

Muscle-derived stem cell implantation and administration of GDF-5 may improve the outcome of tendon repair.

A comparison of bone regeneration with human mesenchymal stem cells and muscle-derived stem cells and the critical role of BMP

Adult multipotent stem cells have been isolated from a variety of human tissues including human skeletal muscle, which represent an easily accessible source of stem cells. It has been shown that human skeletal muscle-derived stem cells (hMDSCs) are muscle-derived mesenchymal stem cells capable of multipotent differentiation. Although hMDSCs can undergo osteogenic differentiation and form bone when genetically modified to express BMP2; it is still unclear whether hMDSCs are as efficient as human bone marrow mesenchymal stem cells (hBMMSCs) for bone regeneration. The current study aimed to address this question by performing a parallel comparison between hMDSCs and hBMMSCs to evaluate their osteogenic and bone regeneration capacities. Our results demonstrated that hMDSCs and hBMMSCs had similar osteogenic-related gene expression profiles and had similar osteogenic differentiation capacities in vitro when transduced to express BMP2. Both the untransduced hMDSCs and hBMMSCs formed very negligible amounts of bone in the critical sized bone defect model when using a fibrin sealant scaffold; however, when genetically modified with lenti-BMP2, both populations successfully regenerated bone in the defect area. No significant differences were found in the newly formed bone volumes and bone defect coverage between the hMDSC and hBMMSC groups. Although both cell types formed mature bone tissue by 6 weeks post-implantation, the newly formed bone in the hMDSCs group underwent quicker remodelling than the hBMMSCs group. In conclusion, our results demonstrated that hMDSCs are as efficient as hBMMSCs in terms of their bone regeneration capacity; however, both cell types required genetic modification with BMP in order to regenerate bone in vivo.

Role of donor and host cells in muscle-derived stem cell-mediated bone repair: differentiation vs. paracrine effects

Murine muscle-derived stem cells (MDSCs) have been shown capable of regenerating bone in a critical size calvarial defect model when transduced with BMP 2 or 4; however, the contribution of the donor cells and their interactions with the host cells during the bone healing process have not been fully elucidated. To address this question, C57/BL/6J mice were divided into MDSC/BMP4/GFP, MDSC/GFP, and scaffold groups. After transplanting MDSCs into the critical-size calvarial defects created in normal mice, we found that mice transplanted with BMP4GFP-transduced MDSCs healed the bone defect in 4 wk, while the control groups (MDSC-GFP and scaffold) demonstrated no bone healing. The newly formed trabecular bone displayed similar biomechanical properties as the native bone, and the donor cells directly participated in endochondral bone formation via their differentiation into chondrocytes, osteoblasts, and osteocytes via the BMP4-pSMAD5 and COX-2-PGE2 signaling pathways. In contrast to the scaffold group, the MDSC groups attracted more inflammatory cells initially and incurred faster inflammation resolution, enhanced angiogenesis, and suppressed initial immune responses in the host mice. MDSCs were shown to attract macrophages via the secretion of monocyte chemotactic protein 1 and promote endothelial cell proliferation by secreting multiple growth factors. Our findings indicated that BMP4GFP-transduced MDSCs not only regenerated bone by direct differentiation, but also positively influenced the host cells to coordinate and promote bone tissue repair through paracrine effects.

Virus immobilization on biomaterial scaffolds through biotin-avidin interaction for improving bone regeneration

To spatially control therapeutic gene delivery for potential tissue engineering applications, a biotin-avidin interaction strategy was applied to immobilize viral vectors on biomaterial scaffolds. Both adenoviral vectors and gelatin sponges were biotinylated and avidin was applied to link them in a virus-biotin-avidin-biotin-material (VBABM) arrangement. The tethered viral particles were stably maintained within scaffolds and SEM images illustrated that viral particles were evenly distributed in three-dimensional (3D) gelatin sponges. An in vivo study demonstrated that transgene expression was restricted to the implant sites only and transduction efficiency was improved using this conjugation method. For an orthotopic bone regeneration model, adenovirus encoding BMP-2 (AdBMP2) was immobilized to gelatin sponges before implanting into critical-sized bone defects in rat calvaria. Compared to gelatin sponges with AdBMP2 loaded in a freely suspended form, the VBABM method enhanced gene transfer and bone regeneration was significantly improved. These results suggest that biotin-avidin immobilization of viral vectors to biomaterial scaffolds may be an effective strategy to facilitate tissue regeneration.

Platelet-rich plasma promotes the proliferation of human muscle derived progenitor cells and maintains their stemness

Human muscle-derived progenitor cells (hMDPCs) offer great promise for muscle cell-based regenerative medicine; however, prolonged ex-vivo expansion using animal sera is necessary to acquire sufficient cells for transplantation. Due to the risks associated with the use of animal sera, the development of a strategy for the ex vivo expansion of hMDPCs is required. The purpose of this study was to investigate the efficacy of using platelet-rich plasma (PRP) for the ex-vivo expansion of hMDPCs. Pre-plated MDPCs, myoendothelial cells, and pericytes are three populations of hMDPCs that we isolated by the modified pre-plate technique and Fluorescence Activated Cell Sorting (FACS), respectively. Pooled allogeneic human PRP was obtained from a local blood bank, and the effect that thrombin-activated PRP-releasate supplemented media had on the ex-vivo expansion of the hMDPCs was tested against FBS supplemented media, both in vitro and in vivo. PRP significantly enhanced short and long-term cell proliferation, with or without FBS supplementation. Antibody-neutralization of PDGF significantly blocked the mitogenic/proliferative effects that PRP had on the hMDPCs. A more stable and sustained expression of markers associated with stemness, and a decreased expression of lineage specific markers was observed in the PRP-expanded cells when compared with the FBS-expanded cells. The in vitro osteogenic, chondrogenic, and myogenic differentiation capacities of the hMDPCs were not altered when expanded in media supplemented with PRP. All populations of hMDPCs that were expanded in PRP supplemented media retained their ability to regenerate myofibers in vivo. Our data demonstrated that PRP promoted the proliferation and maintained the multi-differentiation capacities of the hMDPCs during ex-vivo expansion by maintaining the cells in an undifferentiated state. Moreover, PDGF appears to be a key contributing factor to the beneficial effect that PRP has on the proliferation of hMDPCs.

Intrinsic ability of adult stem cell in skeletal muscle: an effective and replenishable resource to the establishment of pluripotent stem cells

Adult stem cells play an essential role in mammalian organ maintenance and repair throughout adulthood since they ensure that organs retain their ability to regenerate. The choice of cell fate by adult stem cells for cellular proliferation, self-renewal, and differentiation into multiple lineages is critically important for the homeostasis and biological function of individual organs. Responses of stem cells to stress, injury, or environmental change are precisely regulated by intercellular and intracellular signaling networks, and these molecular events cooperatively define the ability of stem cell throughout life. Skeletal muscle tissue represents an abundant, accessible, and replenishable source of adult stem cells. Skeletal muscle contains myogenic satellite cells and muscle-derived stem cells that retain multipotent differentiation abilities. These stem cell populations have the capacity for long-term proliferation and high self-renewal. The molecular mechanisms associated with deficits in skeletal muscle and stem cell function have been extensively studied. Muscle-derived stem cells are an obvious, readily available cell resource that offers promise for cell-based therapy and various applications in the field of tissue engineering. This review describes the strategies commonly used to identify and functionally characterize adult stem cells, focusing especially on satellite cells, and discusses their potential applications.

Advanced Strategies for Articular Cartilage Defect Repair

Articular cartilage is a unique tissue owing to its ability to withstand repetitive compressive stress throughout an individual's lifetime. However, its major limitation is the inability to heal even the most minor injuries. There still remains an inherent lack of strategies that stimulate hyaline-like articular cartilage growth with appropriate functional properties. Recent scientific advances in tissue engineering have made significant steps towards development of constructs for articular cartilage repair. In particular, research has shown the potential of biomaterial physico-chemical properties significantly influencing the proliferation, differentiation and matrix deposition by progenitor cells. Accordingly, this highlights the potential of using such properties to direct the lineage towards which such cells follow. Moreover, the use of soluble growth factors to enhance the bioactivity and regenerative capacity of biomaterials has recently been adopted by researchers in the field of tissue engineering. In addition, gene therapy is a growing area that has found noteworthy use in tissue engineering partly due to the potential to overcome some drawbacks associated with current growth factor delivery systems. In this context, such advanced strategies in biomaterial science, cell-based and growth factor-based therapies that have been employed in the restoration and repair of damaged articular cartilage will be the focus of this review article.

Isolation of muscle-derived stem/progenitor cells based on adhesion characteristics to collagen-coated surfaces

Our lab developed and optimized a method, known as the modified pre-plate technique, to isolate stem/progenitor cells from skeletal muscle. This method separates different populations of myogenic cells based on their propensity to adhere to a collagen I-coated surface. Based on their surface markers and stem-like properties, including self-renewal, multi-lineage differentiation, and ability to promote tissue regeneration, the last cell fraction or slowest to adhere to the collagen-coated surface (pre-plate 6; pp6) appears to be early, quiescent progenitor cells termed muscle-derived stem/progenitor cells (MDSPCs). The cell fractions preceding pp6 (pp1-5) are likely populations of more committed (differentiated) cells, including fibroblast- and myoblast-like cells. This technique may be used to isolate MDSPCs from skeletal muscle of humans or mice regardless of age, sex or disease state, although the yield of MDSPCs varies with age and health. MDSPCs can be used for regeneration of a variety of tissues including bone, articular cartilage, skeletal and cardiac muscle, and nerve. MDSPCs are currently being tested in clinical trials for treatment of urinary incontinence and myocardial infarction. MDSPCs from young mice have also been demonstrated to extend life span and healthspan in mouse models of accelerated aging through an apparent paracrine/endocrine mechanism. Here we detail methods for isolation and characterization of MDSPCs.

Stem cells, angiogenesis and muscle healing: a potential role in massage therapies?

Skeletal muscle injuries are among the most common and frequently disabling injuries sustained by athletes. Repair of injured skeletal muscle is an area that continues to present a challenge for sports medicine clinicians and researchers due, in part, to complete muscle recovery being compromised by development of fibrosis leading to loss of function and susceptibility to re-injury. Injured skeletal muscle goes through a series of coordinated and interrelated phases of healing including degeneration, inflammation, regeneration and fibrosis. Muscle regeneration initiated shortly after injury can be limited by fibrosis which affects the degree of recovery and predisposes the muscle to reinjury. It has been demonstrated in animal studies that antifibrotic agents that inactivate transforming growth factor (TGF)-β1 have been effective at decreasing scar tissue formation. Several studies have also shown that vascular endothelial growth factor (VEGF) can increase the efficiency of skeletal muscle repair by increasing angiogenesis and, at the same time, reducing the accumulation of fibrosis. We have isolated and thoroughly characterised a population of skeletal muscle-derived stem cells (MDSCs) that enhance repair of damaged skeletal muscle fibres by directly differentiating into myofibres and secreting paracrine factors that promote tissue repair. Indeed, we have found that MDSCs transplanted into skeletal and cardiac muscles have been successful at repair probably because of their ability to secrete VEGF that works in a paracrine fashion. The application of these techniques to the study of sport-related muscle injuries awaits investigation. Other useful strategies to enhance skeletal muscle repair through increased vascularisation may include gene therapy, exercise, neuromuscular electrical stimulation and, potentially, massage therapy. Based on recent studies showing an accelerated recovery of muscle function from intense eccentric exercise through massage-based therapies, we believe that this treatment modality offers a practical and non-invasive form of therapy for skeletal muscle injuries. However, the biological mechanism(s) behind the beneficial effect of massage are still unclear and require further investigation using animal models and potentially randomised, human clinical studies.

BMP2 is superior to BMP4 for promoting human muscle-derived stem cell-mediated bone regeneration in a critical-sized calvarial defect model

Muscle-derived cells have been successfully isolated using a variety of different methods and have been shown to possess multilineage differentiation capacities, including an ability to differentiate into articular cartilage and bone in vivo; however, the characterization of human muscle-derived stem cells (hMDSCs) and their bone regenerative capacities have not been fully investigated. Genetic modification of these cells may enhance their osteogenic capacity, which could potentially be applied to bone regenerative therapies. We found that hMDSCs, isolated by the preplate technique, consistently expressed the myogenic marker CD56, the pericyte/endothelial cell marker CD146, and the mesenchymal stem cell markers CD73, CD90, CD105, and CD44 but did not express the hematopoietic stem cell marker CD45, and they could undergo osteogenic, chondrogenic, adipogenic, and myogenic differentiation in vitro. In order to investigate the osteoinductive potential of hMDSCs, we constructed a retroviral vector expressing BMP4 and GFP and a lentiviral vector expressing BMP2. The BMP4-expressing hMDSCs were able to undergo osteogenic differentiation in vitro and exhibited enhanced mineralization compared to nontransduced cells; however, when transplanted into a calvarial defect, they failed to regenerate bone. Local administration of BMP4 protein and cell pretreatment with N-acetylcysteine (NAC), which improves cell survival, did not enhance the osteogenic capacity of the retro-BMP4-transduced cells. In contrast, lenti-BMP2-transduced hMDSCs not only exhibited enhanced in vitro osteogenic differentiation but also induced robust bone formation and nearly completely healed a critical-sized calvarial defect in CD-1 nude mice 6 weeks following transplantation. Herovici's staining of the regenerated bone demonstrated that the bone matrix contained a large amount of type I collagen. Our findings indicated that the hMDSCs are likely mesenchymal stem cells of muscle origin and that BMP2 is more efficient than BMP4 in promoting the bone regenerative capacity of the hMDSCs in vivo.

Canine oral mucosal fibroblasts differentiate into osteoblastic cells in response to BMP-2

Several lines of evidence show that transplantation of osteoblastic cells or genetically engineered nonosteogenic cells expressing osteoblast-related genes into bone defects effectively promotes bone regeneration. To extend this possibility, we investigated whether oral mucosal fibroblasts are capable of differentiating into osteoblastic cells by conducting in vitro and in vivo experiments. We investigated the effects of bone morphogenetic protein-2 (BMP-2) on osteoblast differentiation of cultured fibroblasts isolated from canine buccal mucosa. We also transplanted green fluorescence protein (GFP)-expressing fibroblasts with gelatin/BMP-2 complexes into the subfascial regions of athymic mice, and investigated the localization of GFP-positive cells in the ectopically formed bones. The cultured canine buccal mucosal fibroblasts differentiated into osteoblastic cells by increasing their alkaline phosphatase (ALP) activity and Osteocalcin, Runx2, and Osterix mRNA expression levels in response to BMP-2. Transplantation experiments of GFP-expressing oral mucosal fibroblasts with gelatin/BMP-2 complexes revealed that 17.1% of the GFP-positive fibroblasts differentiated into ALP-positive cells, and these cells accounted for 6.2% of total ALP-positive cells in the ectopically formed bone. This study suggests that oral mucosal fibroblasts can differentiate into osteogenic cells in response to BMP-2. Thus, these cells are potential candidates for cell-mediated bone regeneration therapy in dentistry.

Stem cell-based therapies for bone repair

This article provides an overview of the cellular and molecular events involved in bone repair and the current approaches to using stem cells as an adjunct to this process. The article emphasizes the key role of osteoprogenitor cells in the formation of bone and where the clinical applications of current research may lend themselves to large animal orthopaedics. The processes involved in osteogenic differentiation are presented and strategies for bone formation, including induction by osteogenic factors, bioscaffolds, and gene therapy, are reviewed.

Cell-mediated and direct gene therapy for bone regeneration

INTRODUCTION

Bone regeneration is required for the treatment of fracture non/delayed-unions and bone defects. However, most current treatment modalities have limited efficacy, and newer therapeutic strategies, such as gene therapy, have substantial benefit for bone repair and regeneration.

AREAS COVERED

This review discusses experimental and clinical applications of cell-mediated and direct gene therapy for bone regeneration. The review covers literature on this subject from 2000 to February 2012.

EXPERT OPINION

Direct gene therapy using various viral and non-viral vectors of cell-mediated genes has been demonstrated to induce bone regeneration, although use of such vectors has shown some risk in human application. Osteoinductive capability of a number of progenitor cells isolated from bone marrow, fat, muscle and skin tissues, has been demonstrated by genetic modification with osteogenic genes. Cell-mediated gene therapy using such osteogenic gene-expressing progenitor cells has shown promising results in promoting bone regeneration in extensive animal work in recent years.

Clinical potential and challenges of using genetically modified cells for articular cartilage repair

Articular cartilage defects do not regenerate. Transplantation of autologous articular chondrocytes, which is clinically being performed since several decades, laid the foundation for the transplantation of genetically modified cells, which may serve the dual role of providing a cell population capable of chondrogenesis and an additional stimulus for targeted articular cartilage repair. Experimental data generated so far have shown that genetically modified articular chondrocytes and mesenchymal stem cells (MSC) allow for sustained transgene expression when transplanted into articular cartilage defects in vivo. Overexpression of therapeutic factors enhances the structural features of the cartilaginous repair tissue. Combined overexpression of genes with complementary mechanisms of action is also feasible, holding promises for further enhancement of articular cartilage repair. Significant benefits have been also observed in preclinical animal models that are, in principle, more appropriate to the clinical situation. Finally, there is convincing proof of concept based on a phase I clinical gene therapy study in which transduced fibroblasts were injected into the metacarpophalangeal joints of patients without adverse events. To realize the full clinical potential of this approach, issues that need to be addressed include its safety, the choice of the ideal gene vector system allowing for a long-term transgene expression, the identification of the optimal therapeutic gene(s), the transplantation without or with supportive biomaterials, and the establishment of the optimal dose of modified cells. As safe techniques for generating genetically engineered articular chondrocytes and MSCs are available, they may eventually represent new avenues for improved cell-based therapies for articular cartilage repair. This, in turn, may provide an important step toward the unanswered question of articular cartilage regeneration.

Gene Therapy for Cartilage Repair

The concept of using gene transfer strategies for cartilage repair originates from the idea of transferring genes encoding therapeutic factors into the repair tissue, resulting in a temporarily and spatially defined delivery of therapeutic molecules to sites of cartilage damage. This review focuses on the potential benefits of using gene therapy approaches for the repair of articular cartilage and meniscal fibrocartilage, including articular cartilage defects resulting from acute trauma, osteochondritis dissecans, osteonecrosis, and osteoarthritis. Possible applications for meniscal repair comprise meniscal lesions, meniscal sutures, and meniscal transplantation. Recent studies in both small and large animal models have demonstrated the applicability of gene-based approaches for cartilage repair. Chondrogenic pathways were stimulated in the repair tissue and in osteoarthritic cartilage using genes for polypeptide growth factors and transcription factors. Although encouraging data have been generated, a successful translation of gene therapy for cartilage repair will require an ongoing combined effort of orthopedic surgeons and of basic scientists.

Experimental construction of BMP2 and VEGF gene modified tissue engineering bone in vitro

The purpose of this study was to investigate the feasibility and advantages of constructing a novel tissue engineering bone, using β-tricalcium phosphate (β-TCP) and rat bone marrow mesenchymal stem cells (MSCs), modified with human bone morphogenetic protein 2 gene (hBMP2) and human vascular endothelial growth factor 165 gene (hVEGF165), through lentiviral transfection. Both genes were successfully co-expressed in the co-transfection group for up to eight weeks confirmed by enzyme-linked immunosorbent assay (ELISA). After seeding MSCs onto the scaffolds, scanning electron microscopy (SEM) observation showed that MSCs grew and proliferated well in co-transfection group at 7 and 14 days. There was no significant difference among all the groups in hoechst DNA assay for cell proliferation for 14 days after cell seeding (P > 0.05), but the highest alkaline phosphatase (ALP) activity was observed in the co-transfection group at 14 days after cell seeding (p < 0.01). These results demonstrated that it was advantageous to construct tissue engineering bone using β-TCP combined with MSCs lentivirally co-transfected with BMP2 and VEGF165, providing an innovative way for treating bone defects.

Direct comparison of progenitor cells derived from adipose, muscle, and bone marrow from wild-type or craniosynostotic rabbits

BACKGROUND

Reports have identified cells capable of osteogenic differentiation in bone marrow, muscle, and adipose tissues, but there are few direct comparisons of these different cell types. Also, few have investigated the potential connection between a tissue-specific abnormality and cells derived from seemingly unrelated tissues. In this article, the authors compare cells isolated from wild-type rabbits or rabbits with nonsyndromic craniosynostosis, defined as the premature fusion of one or more of the cranial sutures.

METHODS

Cells were derived from bone marrow, adipose, and muscle of 10-day-old wild-type rabbits (n = 17) or from age-matched rabbits with familial nonsyndromic craniosynostosis (n = 18). Cells were stimulated with bone morphogenetic protein-4 (BMP4), and alkaline phosphatase expression and cell proliferation were assessed.

RESULTS

In wild-type rabbits, cells derived from muscle had more alkaline phosphatase activity than cells derived from either adipose or bone marrow. The cells derived from craniosynostotic rabbit bone marrow and muscle were significantly more osteogenic than those derived from wild-type rabbits. Adipose-derived cells demonstrated no significant differences. Although muscle-derived cells were most osteogenic in wild-type rabbits, bone marrow-derived cells were most osteogenic in craniosynostotic rabbits.

CONCLUSIONS

These results suggest that cells from different tissues have different potentials for differentiation. Furthermore, cells derived from rabbits with craniosynostosis were different from cells from wild-type rabbits. Interestingly, cells derived from the craniosynostotic rabbits were not uniformly more responsive compared with wild-type cells, suggesting that specific tissue-derived cells may react differently in individuals with craniosynostosis.

Potential therapeutic applications of muscle-derived mesenchymal stem and progenitor cells

IMPORTANCE OF THE FIELD

Mesenchymal adult stem cells have properties that make them attractive for use in tissue engineering and regenerative medicine. They are inherently plastic, enabling them to differentiate along different lineages, and promote wound healing and regeneration of surrounding tissues by modulating immune and inflammatory responses, promoting angiogenesis and secreting other trophic factors. Unlike embryonic stem cells, clinical uses of mesenchymal stem cells are not encumbered by ethical considerations or legal restrictions.

AREAS COVERED IN THIS REVIEW

We discuss skeletal muscle as a source of mesenchymal stem and progenitor cells by reviewing their biology and current applications in tissue engineering and regenerative medicine. This paper covers literature from the last 5 - 10 years.

WHAT THE READER WILL GAIN

Skeletal muscle is a plentiful source of mesenchymal stem and progenitor cells. This tissue may be obtained via routine biopsy or collection after surgical debridement. We describe the biology of these cells and provide an overview of therapeutic applications currently being developed to take advantage of their regenerative properties.

TAKE HOME MESSAGE

There is potential for stem and progenitor cells derived from skeletal muscle to be incorporated in clinical interventions, either as a cellular therapy to modify the natural history of disease or as a component of engineered tissue constructs that can replace diseased or damaged tissues.

Serum after traumatic brain injury increases proliferation and supports expression of osteoblast markers in muscle cells

BACKGROUND

Traumatic brain injury is associated with an increased rate of heterotopic ossification within skeletal muscle, possibly as a result of humoral factors. In this study, we investigated whether cells from skeletal muscle adopt an osteoblastic phenotype in response to serum from patients with traumatic brain injury.

METHODS

Serum was collected from thirteen patients with severe traumatic brain injury, fourteen patients with a long-bone fracture, and ten control subjects. Primary cultures of skeletal muscle cells isolated from patients undergoing orthopaedic surgery were performed and characterized with use of immunofluorescence microscopy, reverse transcription-polymerase chain reaction, and Western blot analysis. Proliferation and osteoblastic differentiation were assessed with use of commercial cell assays, Western blot analysis (for Osterix protein), and the Villanueva bone stain.

RESULTS

All serum-treated cell populations expressed the osteoblast marker Osterix after one week in culture. Cells treated with serum from all study groups in mineralization medium had increased alkaline phosphatase activity and mineralized nodules within the mesenchymal cell subpopulation after three weeks in culture. Serum from patients with traumatic brain injury induced a significant increase (p = 0.02) in the rate of proliferation of primary skeletal muscle cells (1.87 [95% confidence interval, 1.66 to 2.09]) compared with the rate induced by serum from patients with a fracture (1.42 [95% confidence interval, 1.21 to 1.58]) or by serum from controls (1.35 [95% confidence interval, 1.15 to 1.54]).

CONCLUSIONS

Human serum supports the osteoblastic differentiation of cells derived from human skeletal muscle, and serum from patients with severe traumatic brain injury accelerates proliferation of these cells. These findings suggest the early presence of humoral factors following traumatic brain injury that stimulate the expansion of mesenchymal cells and osteoprogenitors within skeletal muscle.

Cell therapy for bone regeneration--bench to bedside

The concept of bone tissue engineering, which began in the early 1980s, has seen tremendous growth in the numbers of research studies. One of the key areas of research has been in the field of mesenchymal stem cells, where the challenge is to produce the perfect tissue-engineered bone construct. This practical review summarizes basic and applied state-of-the-art research in the area of mesenchymal stem cells, and highlights the important translational research that has already been initiated. The topics that will be covered include the sources of stem cells in use, scaffolds, gene therapy, clinical applications in nonunions, tumors, osteonecrosis, revision arthroplasties, and spine fusion. Although significant challenges remain, there exists an exceptional opportunity to translate basic research in mesenchymal stem cell technologies into viable clinical treatments for bone regeneration.

Growth factors in bone repair

The role of growth factors (GF) in bone repair is widely recognised, particularly for bone morphogenetic proteins (BMPs), fibroblast growth factor (FGF), insulin-like growth factors (IGFs), platelet-derived growth factor (PDGF), transforming growth factor-beta (TGF-beta) and vascular endothelial growth factor (VEGF). GF are usually stored in the extracellular matrix (ECM), but after injury are actively released by ECM, cells and platelets. In this paper, the use of different recombinant GF for bone repair stimulation is summarised in experimental research and clinical applications. Drug delivery systems, including carriers, cell or gene therapy, are needed to ensure a sustained local release of the factors, but efficacy and potential side effects of such systems require additional research prior to clinical applications. Current sources for delivery of a GF mixture into the site of bone repair are platelet gel and demineralised bone matrix. Nevertheless, the levels of GF in such preparations are affected by variability among donors and differences in preparation. Autogenous GF, produced by the patient himself during the bone repair process, potentially interfere with prosthetic devices or even have a role in implant loosening due to the periprosthetic tissue reaction. In conclusion, GF are key components of functional bone regeneration: screening of basic research results and controlled clinical trials are accelerating the development of GF in orthopaedic surgery.

Stem cells from human fat as cellular delivery vehicles in an athymic rat posterolateral spine fusion model

BACKGROUND

Mesenchymal stem cells derived from human liposuction aspirates, termed processed lipoaspirate cells, have been utilized as cellular delivery vehicles for the induction of bone formation in tissue engineering and gene therapy strategies. In this study, we sought to evaluate the efficacy of bone morphogenetic protein (BMP)-2-producing adipose-derived stem cells in inducing a posterolateral spine fusion in an athymic rat model.

METHODS

Single-level (L4-L5) intertransverse spinal arthrodesis was attempted with use of a type-I collagen matrix in five groups of athymic rats, with eight animals in each group. Group I was treated with 5 x 10(6) adipose-derived stem cells transduced with an adenoviral vector containing the BMP-2 gene; group II, with 5 x 10(6) adipose-derived stem cells treated with osteogenic media and 1 microg/mL of recombinant BMP-2 (rhBMP-2); group III, with 10 microg of rhBMP-2; group IV, with 1 microg of rhBMP-2; and group V, with 5 x 10(6) adipose-derived stem cells alone. The animals that showed radiographic evidence of healing were killed four weeks after cell implantation and were examined with plain radiographs, manual palpation, microcomputed tomography scanning, and histological analysis.

RESULTS

All eight animals in group I demonstrated successful spinal fusion, with a large fusion mass, four weeks postoperatively. Furthermore, group-I specimens consistently revealed spinal fusion at the cephalad level (L3 and L4), where no fusion bed had been prepared surgically. In contrast, despite substantial BMP-2 production measured in vitro, group-II animals demonstrated minimal bone formation even eight weeks after implantation. Of the groups treated with the application of rhBMP-2 alone, the one that received a relatively high dose (group III) had a higher rate of fusion (seen in all eight specimens) than the one that received the low dose (group IV, in which fusion was seen in four of the eight specimens). None of the group-V animals (treated with adipose-derived stem cells alone) demonstrated successful spine fusion eight weeks after the surgery.

CONCLUSIONS

Adipose-derived stem cells show promise as gene transduction targets for inducing bone formation to enhance spinal fusion in biologically stringent environments.

Muscle-derived stem cells for tissue engineering and regenerative therapy

Skeletal muscle has been recognized as an essential source of progenitor or satellite cells, which are primarily responsible for muscle regeneration. Recently, muscle has also been identified as a valuable source of postnatal stem cells that appear to be distinct from satellite cells and possess the ability to differentiate into other cell lineages. These cells, named muscle-derived stem cells, possess a high myogenic capacity and effectively regenerate both skeletal and cardiac muscle. Remarkably, when genetically modified ex vivo to express growth factors, these cells can differentiate into osteogenic and chondrogenic lineages and have been shown to promote the repair of bone and cartilage. Muscle stem cell-based regenerative therapy and tissue engineering using ex vivo gene therapy, are promising approaches for the treatment of various musculoskeletal, cardiovascular, and urological disorders.

Human muscle-derived cell injection in a rat model of stress urinary incontinence

We investigated the use of human muscle-derived cells (hMDCs) for the treatment of stress urinary incontinence (SUI) in a nude rat model. hMDCs were isolated from adult skeletal muscle. Three groups of six animals consisting of controls, animals undergoing sciatic nerve transection (SNT) with periurethral sham-injection, and SNT with hMDCs (1 x 10(6) cells/20 microl saline) were utilized. Leak point pressure (LPP) was measured 4 weeks following injection. Bilateral SNT resulted in a significantly lower LPP that was significantly higher following hMDCs than sham injection. The results demonstrate the efficacy of human muscle cell therapy alone in improving physiologic outcomes in an animal model of SUI.

Review: gene- and stem cell-based therapeutics for bone regeneration and repair

Many clinical conditions require regeneration or implantation of bone. This is one focus shared by neurosurgery and orthopedics. Current therapeutic options (bone grafting and protein-based therapy) do not provide satisfying solutions to the problem of massive bone defects. In the past few years, gene- and stem cell-based therapy has been extensively studied to achieve a viable alternative to current solutions offered by modern medicine for bone-loss repair. The use of adult stem cells for bone regeneration has gained much focus. This unique population of multipotential cells has been isolated from various sources, including bone marrow, adipose, and muscle tissues. Genetic engineering of adult stem cells with potent osteogenic genes has led to fracture repair and rapid bone formation in vivo. It is hypothesized that these genetically modified cells exert both an autocrine and a paracrine effects on host stem cells, leading to an enhanced osteogenic effect. The use of direct gene delivery has also shown much promise for in vivo bone repair. Several viral and nonviral methods have been used to achieve substantial bone tissue formation in various sites in animal models. To advance these platforms to the clinical setting, it will be mandatory to overcome specific hurdles, such as control over transgene expression, viral vector toxicity, and prolonged culture periods of therapeutic stem cells. This review covers a prospect of cell and gene therapy for bone repair as well as some very recent advancements in stem cell isolation, genetic engineering, and exogenous control of transgene expression.

Mesenchymal stem cells: molecular targets for tissue engineering

Mesenchymal stem cells (MSCs) represent an adherent, fibroblast-like population present not only in the bone marrow, but in a number of tissues, including blood, adipose tissue, muscle, and dermis. Their extensive proliferation and transdifferentiation potential makes them best suited for tissue engineering applications. Identification of growth factors and signaling pathways involved in self-renewal and differentiation is important for designing strategies to overcome replicative senescence and attain directed differentiation. Wnt, bone morphogenetic protein (BMP), and Notch pathways have been implicated to play key roles in self-renewal and differentiation of hematopoietic, intestinal, and epidermal stem cells. They are also involved in regulating MSC differentiation. However, MSC self-renewal has not received much attention, with Nucleostemin being the only recently identified proliferation molecule. Although immortalization using viral oncogenes and telomerase has been achieved, transformation in long-term cultures is a potential risk. Understanding of the mechanisms governing osteogenic differentiation of MSCs is expanding with the recent identification of two major transcription factors, Osterix and Runx2. Enhanced expansion as well as osteogenic differentiation of MSCs can be attained using a combinatorial approach involving co-expression of proliferation and differentiation genes. However, a thorough understanding of the molecular mechanism is necessary for enhancing the self-renewal ability and osteogenic potential in vitro.

Tissue engineering and developmental biology: going biomimetic

This article contains the collective views expressed at the first session of the workshop "Tissue Engineering--The Next Generation," which was devoted to the interactions between developmental biology and tissue engineering. Donald Ingber discussed the chasms between developmental biology and tissue engineering from the perspective of a cell biologist who has had interest in tissue engineering since its early days. Van C. Mow shared a historical perspective on the development of tissue engineering as one of the first engineers involved in the field. David Butler offered an assessment of functional tissue engineering, a new area he helped establish and promote. Laura Niklason discussed how to be more effective in developing cellular therapies for large numbers of patients. Johnny Huard described his approach to tissue engineering, based on the use of muscle-derived cells. Jeremy Mao focused on cell homing and cell density in the context of native development and relevance to tissue engineering. Ioannis Yannas proposed a set of "rules" in organ regeneration. Collectively, the faculty expressed a remarkable level of enthusiasm for bridging the gaps between developmental biology and tissue engineering and offered new ideas on how to facilitate the interaction between the two fields.

Applications of gene therapy and adult stem cells in bone bioengineering

Bone tissue engineering is an emerging field, that could become a main therapeutic strategy in orthopedics in coming years. While bone has regenerative abilities that enable the self repair and regeneration of fractures, there are extreme situations in which the extent of bone loss is too large for complete regeneration to occur. In order to achieve bone regeneration, osteogenic genes (mainly from the bone morphogenetic protein family) can be delivered either directly into the target tissue, or by using adult stem cells, which are later implanted into the target site. Engineered adult stem cells combined with biodegradable polymeric scaffolds can be implanted into target sites, with or without ex vivo culture period. Several important factors influence the success of bone engineering approaches including: choice of cell and scaffold, the vector used in order to deliver the osteogenic gene, and the osteogenic gene itself. Cutting-edge imaging technologies, bioinformatics-based analysis of gene expression and exogenous regulation of transgene expression are among the tools that are being used to optimize and control bone formation in vivo. In this review we have attempted to provide an overview of the main factors that should be considered when utilizing adult stem cells and gene therapy strategies to regenerate bone defects or to promote new bone formation in vivo.

Delayed administration of adenoviral BMP-2 vector improves the formation of bone in osseous defects

The direct, local, administration of adenovirus carrying human BMP-2 cDNA (Ad.BMP-2) heals critical-sized femoral bone defects in rabbit and rat models. However, the outcome is suboptimal and the technology needs to provide a more reliable and uniform outcome. To this end, we investigated whether the timing of Ad.BMP-2 administration influenced the formation of mineralized tissue within the defect. Critical-sized defects were created in the femora of 28 Sprague-Dawley rats. Animals were injected intralesionally with a single, percutaneous injection of Ad.BMP-2 (4 x 10(8) plaque-forming units) either intraoperatively (day 0) or 24 h (day 1), 5 days or 10 days after surgery. The femora were evaluated 8 weeks after surgery by X-ray, microcomputed tomography, dual-energy X-ray absorptiometry and biomechanical testing. The incidence of radiological union was markedly increased when administration of Ad.BMP-2 was delayed until days 5 and 10, at which point 86% of the defects healed. These time points also provided greater bone mineral content within the defect site and improved the average mechanical strength of the healed bone. Thus, delaying the injection of Ad.BMP-2 until 5 or 10 days after surgery enables a greater percentage of critical-sized, segmental defects to achieve radiological union, producing a repair tissue with enhanced mineralization and greater mechanical strength.

Effects of Runx2 genetic engineering and in vitro maturation of tissue-engineered constructs on the repair of critical size bone defects

Genetic and tissue engineering strategies are being pursued to address the clinical limitations of current bone grafting materials. Based on our previous work demonstrating that overexpression of the Runx2 osteoblastic transcription factor and in vitro construct maturation synergistically enhanced in vivo mineralization in an ectopic site (Byers et al., Tissue Eng 2004;10:1757-1766), we examined the effects of these two parameters on the repair of critical size bone defects. Primary rat bone marrow stromal cells transduced with Runx2 or control (no Runx2 insert) retroviral vector were seeded onto 3D fused deposition-modeled polycaprolactone scaffolds. Runx2-modified cells produced biologically-equivalent mineralized matrices at nearly 2-fold higher rates than control cells. Constructs cultured in vitro for 1 day (immature) or 21 days (mineralized) were subsequently implanted into critical size calvaria defects in syngeneic rats, and bone healing was analyzed by micro-CT and histomorphometry at 28 days. Runx2-modified and control constructs precultured for 1 day healed to a greater extent than defects receiving no implant. Cell-free scaffolds yielded equivalent levels of bone formation as constructs precultured for 1 day. Interestingly, defects treated with control cell-seeded constructs precultured for 21 days exhibited low bone formation compared to other construct treatments, and repair was comparable to empty defects. In contrast, Runx2-modified constructs precultured for 21 days contained twice as much bone as control constructs precultured for 21 days and equivalent levels of new bone as cell-free and 1 day precultured constructs. These results demonstrate interplay between Runx2 genetically-modified cells and in vitro construct maturation in bone healing responses.

Local immunomodulation with CD4 and CD8 antibodies, but not cyclosporine A, improves osteogenesis induced by ADhBMP9 gene therapy

This study was designed to see if immunosuppression achieved using local application of cyclosporine A (Cs. A) or CD4 and CD8 antibodies would improve bone formation following intramuscular injections of human BMP-4 and BMP-9 adenoviral vectors (ADhBMP4 and ADhBMP9) in Sprague-Dawley rats. Cs. A was injected into the thigh muscle. After 2 days, ADhBMP4, ADhBMP9, and the antibodies were separately injected into the left and right rear legs. At this time, the number of CD4+/CD3+ cells was significantly lower and the number of CD8+/CD3+ cells higher in the Cs. A group than in the control group (P < 0.01). The total number of white blood cells 3 days following injection of CD4 and CD8 antibodies was significantly lower than that before the injection (P < 0.01). At 4 weeks after the viral and antibody injections, mean bone volumes at the ADhBMP9 treatment sites were 0.29 +/- 0.01 cm3 in the viral control group, 0.17 +/- 0.03 cm3 in the Cs. A-ADhBMPs group, and 0.59 +/- 0.07 cm3 in the antibodies-ADhBMPs group. ADhBMP4 did not induce new bone formation in any group. This study demonstrates that local immunomodulation may improve the osteogenic potential of bone morphogenetic protein gene therapy in the clinical setting.

Different Osteogenic Potentials of Recombinant Human BMP-6 Adeno-Associated Virus and Adenovirus in Two Rat Strains

The osteogenic potential of AAV5hBMP6 was compared with that of ADhBMP6 in immunodeficient and immunocompetent rats. AAV5hBMP6 (2.3 x 10(12) particles) and ADhBMP6 (5 x 10(7) PFU) elicited viral antibody production in immunocompetent rats. Among rats that received AAV5hBMP6, the earliest time points at which the bone was visible under CT scanner were 30 days in 2-month-old Sprague-Dawley (SD) rats and 60 days in 18-month-old SD rats. The mean volumes of ectopic bone 90 days after viral injection were 0.31 +/- 0.14 cm(3) in athymic nude rats, 0.64 +/- 0.12 cm(3) in 2-month-old SD rats, and 0.21 +/- 0.10 cm(3) in 18-month-old SD rats. In contrast, among rats that received ADhBMP6, the earliest time points to observe the bone formation by CT scan were 15 days in 2-month-old rats and no bone formation in 18-month-old SD rats. The mean volumes of ectopic bone were 4.17 +/- 0.05 cm(3) in athymic nude rats and 0.06 +/- 0.03 cm(3) in 2-month-old SD rats. Although both types of viruses induced an immune response in immunocompetent animals, this response played different roles in the process of bone formation induced by the BMP6 vectors.

Current concepts of molecular aspects of bone healing

Fracture healing is a complex physiological process. It involves the coordinated participation of haematopoietic and immune cells within the bone marrow in conjunction with vascular and skeletal cell precursors, including mesenchymal stem cells (MSCs) that are recruited from the surrounding tissues and the circulation. Multiple factors regulate this cascade of molecular events by affecting different sites in the osteoblast and chondroblast lineage through various processes such as migration, proliferation, chemotaxis, differentiation, inhibition, and extracellular protein synthesis. An understanding of the fracture healing cellular and molecular pathways is not only critical for the future advancement of fracture treatment, but it may also be informative to our further understanding of the mechanisms of skeletal growth and repair as well as the mechanisms of aging.

Tissue engineering with muscle-derived stem cells

Tissue engineering and cell therapy approaches aim to take advantage of the repopulating ability and plasticity of multipotent stem cells to regenerate lost or diseased tissue. Researchers continue to investigate stem cells in mature tissues and demonstrate the potential ability of organ-specific cells to differentiate into multiple lineages. One stem cell that displays such promise is the muscle-derived stem cell (MDSC). Data supporting the existence of MDSCs have emerged as part of investigations to improve myoblast cell transplantation for the treatment of muscular dystrophies. As these efforts continue, the potential for MDSC-based therapy for other musculoskeletal injuries, as well as for cardiac and smooth muscle injuries, is currently being explored.

Application of recombinant BMP-7 on persistent upper and lower limb non-unions

The purpose of this study was to evaluate the efficacy and safety of recombinant bone morphogenetic protein 7 (rhBMP-7 or OP-1) as a bone-stimulating agent in the treatment of persistent fracture non-unions. Twenty-five consecutive patients [19 males, mean age 39.4 years (range: 18-79)] with 26 fracture non-unions were treated with rhBMP-7. There were 10 tibial non-unions, eight femoral, three humeral, three ulnar, one patellar, and one clavicular non-union. The mean follow-up was 15.3 months. The mean number of operations performed prior to rhBMP-7 application was 3.2, with autologous bone graft and bone marrow injection being used in 10 cases (38.5%). Both clinical and radiological union occurred in 24 (92.3%) cases, within a mean time of 4.2 months and 5.6 months, respectively. Of the remaining two cases, one patient ultimately underwent a below knee amputation, secondary to recurrence of deep sepsis. The other patient with recalcitrant ulnar non-union although the radiological union was incomplete, declined further intervention, as he was asymptomatic. No complications or adverse effects from the use of rhBMP-7 were encountered. This study supports the view that the application of rhBMP-7 as a bone-stimulating agent is safe and a power adjunct to be considered in the surgeon's armamentarium for the treatment of these challenging clinical conditions.

Healing of critically sized femoral defects, using genetically modified mesenchymal stem cells from human adipose tissue

The FDA has approved the clinical use of recombinant bone morphogenetic proteins (BMPs). However, the use of recombinant BMPs in humans has required large doses of the proteins to be effective, which suggests that the delivery method of bone morphogenetic proteins needs to be optimized. Gene therapy is an alternative method to deliver such recombinant proteins, and gene transfer techniques have been tested on a variety of cell types including bone marrow cells, skin fibroblasts, peripheral blood monocytes, and muscle-derived cells. In this study, we sought to determine the ability of BMP-2-producing human adipose-derived mesenchymal stem cells to heal a critically sized femoral defect in a nude rat model. After approval by the human subjects protection committee, human adipose tissue was obtained from healthy donors. The lipoaspirate was processed as previously described (De Ugarte, D.A., et al. Cells Tissues Organs 174, 101, 2003). Cells were grown in culture and infected with a BMP-2-carrying adenovirus. Five million cells were applied to a collagen- ceramic carrier and implanted into femoral defects as previously described (Zuk, P.A., et al. Mol. Biol. 13, 4279, 2002). All animals were killed at 8 weeks. Femora were dissected out and underwent radiographic, histologic, and biomechanical analysis. Eleven of the 12 femora in the group treated with human processed lipoaspirate (HPLA) cells genetically modified to overexpress BMP-2 had healed at 8 weeks. This was assessed by radiographs, by mechanical testing, and by histology. The one femur that did not heal had a subacute infection. All eight of the femora treated with the rhBMP-2-impregnated collagen-ceramic carrier healed. No statistically significant difference was detected between these two groups. Evaluation of the control groups: group II (collagen- ceramic carrier with HPLA cells) and group III (collagen-ceramic carrier alone) showed that none of the femora had healed by 8 weeks. Our results indicate that HPLA cells genetically modified by adenoviral gene transfer to overexpress BMP-2 can induce bone formation in vivo and heal a critically sized femoral defect in an athymic rat. The HPLA cells alone did not induce significant bone formation. However, when combined with an osteoinductive factor these cells may be an effective method for enhancing bone healing and the tissue engineering of bone.

Scaffold-based bone engineering by using genetically modified cells

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

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

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

Muscle injuries: biology and treatment

Muscle injuries are one of the most common traumas occurring in sports. Despite their clinical importance, few clinical studies exist on the treatment of these traumas. Thus, the current treatment principles of muscle injuries have either been derived from experimental studies or been tested only empirically. Although nonoperative treatment results in good functional outcomes in the majority of athletes with muscle injuries, the consequences of failed treatment can be very dramatic, possibly postponing an athlete's return to sports for weeks or even months. Moreover, the recognition of some basic principles of skeletal muscle regeneration and healing processes can considerably help in both avoiding the imminent dangers and accelerating the return to competition. Accordingly, in this review, the authors have summarized the prevailing understanding on the biology of muscle regeneration. Furthermore, they have reviewed the existing data on the different treatment modalities (such as medication, therapeutic ultrasound, physical therapy) thought to influence the healing of injured skeletal muscle. In the end, they extend these findings to clinical practice in an attempt to propose an evidence-based approach for the diagnosis and optimal treatment of skeletal muscle injuries.