Adeno-associated virus-coated allografts: a novel approach for cranioplasty

Limited potential of AAV-mediated gene therapy in transducing human mesenchymal stem cells for bone repair applications

Adeno-associated viral vectors (AAV) are unique in their ability to transduce a variety of both dividing and nondividing cells, with significantly lower risk of random genomic integration and with no known pathogenicity in humans, but their role in ex vivo regional gene therapy for bone repair has not been definitively established. The goal of this study was to test the ability of AAV vectors carrying the cDNA for BMP-2 to transduce human mesenchymal stem cells (MSCs), produce BMP-2, and induce osteogenesis in vitro as compared with lentiviral gene therapy with a two-step transcriptional amplification system lentiviral vector (LV-TSTA). To this end, we created two AAV vectors (serotypes 2 and 6) expressing the target transgene; eGFP or BMP-2. Transduction of human MSCs isolated from bone marrow (BMSCs) or adipose tissue (ASCs) with AAV2-eGFP and AAV6-eGFP led to low transduction efficiency (BMSCs: 3.57% and 8.82%, respectively, ASCs: 6.17 and 20.2%, respectively) and mean fluorescence intensity as seen with FACS analysis 7 days following transduction, even at MOIs as high as 106. In contrast, strong eGFP expression was detectable in all of the cell types post transduction with LV-TSTA-eGFP. Transduction with BMP-2 producing vectors led to minimal BMP-2 production in AAV-transduced cells 2 and 7 days following transduction. In addition, transduction of ASCs and BMSCs with AAV2-BMP-2 and AAV6-BMP-2 did not enhance their osteogenic potential as seen with an alizarin red assay. In contrast, the LV-TSTA-BMP-2-transduced cells were characterized by an abundant BMP-2 production and induction of the osteogenic phenotype in vitro (p < 0.001 vs. AAV2 and 6). Our results demonstrate that the AAV2 and AAV6 vectors cannot induce a significant transgene expression in human BMSCs and ASCs, even at MOIs as high as 106. The LV-TSTA vector is significantly superior in transducing human MSCs; thus this vector would be preferable when developing an ex vivo regional gene therapy strategy for clinical use in orthopedic surgery applications.

BMP gene delivery for skeletal tissue regeneration

Musculoskeletal disorders are common and can be associated with significant morbidity and reduced quality of life. Current treatments for major bone loss or cartilage defects are insufficient. Bone morphogenetic proteins (BMPs) are key players in the recruitment and regeneration of damaged musculoskeletal tissues, and attempts have been made to introduce the protein to fracture sites with limited success. In the last 20 years we have seen a substantial progress in the development of various BMP gene delivery platforms for several conditions. In this review we cover the progress made using several techniques for BMP gene delivery for bone as well as cartilage regeneration, with focus on recent advances in the field of skeletal tissue engineering. Some methods have shown success in large animal models, and with the global trend of introducing gene therapies into the clinical setting, it seems that the day in which BMP gene therapy will be viable for clinical use is near.

BMP-2 Gene Delivery-Based Bone Regeneration in Dentistry

Bone morphogenetic protein-2 (BMP-2) is a potent growth factor affecting bone formation. While recombinant human BMP-2 (rhBMP-2) has been commercially available in cases of non-union fracture and spinal fusion in orthopaedics, it has also been applied to improve bone regeneration in challenging cases requiring dental implant treatment. However, complications related to an initially high dosage for maintaining an effective physiological concentration at the defect site have been reported, although an effective and safe rhBMP-2 dosage for bone regeneration has not yet been determined. In contrast to protein delivery, BMP-2 gene transfer into the defect site induces BMP-2 synthesis in vivo and leads to secretion for weeks to months, depending on the vector, at a concentration of nanograms per milliliter. BMP-2 gene delivery is advantageous for bone wound healing process in terms of dosage and duration. However, safety concerns related to viral vectors are one of the hurdles that need to be overcome for gene delivery to be used in clinical practice. Recently, commercially available gene therapy has been introduced in orthopedics, and clinical trials in dentistry have been ongoing. This review examines the application of BMP-2 gene therapy for bone regeneration in the oral and maxillofacial regions and discusses future perspectives of BMP-2 gene therapy in dentistry.

Bone-chip system to monitor osteogenic differentiation using optical imaging

Human organoids and organ-on-chip systems to predict human responses to new therapies and for the understanding of disease mechanisms are being more commonly used in translational research. We have developed a bone-chip system to study osteogenic differentiation in vitro, coupled with optical imaging approach which provides the opportunity of monitoring cell survival, proliferation and differentiation in vitro without the need to terminate the culture. We used the mesenchymal stem cell (MSC) line over-expressing bone morphogenetic protein-2 (BMP-2), under Tet-Off system, and luciferase reporter gene under constitutive promoter. Cells were seeded on chips and supplemented with osteogenic medium. Flow of media was started 24 h later, while static cultures were performed using media reservoirs. Cells grown on the bone-chips under constant flow of media showed enhanced survival/proliferation, comparing to the cells grown in static conditions; luciferase reporter gene expression and activity, reflecting the cell survival and proliferation, was quantified using bioluminescence imaging and a significant advantage to the flow system was observed. In addition, the flow had positive effect on osteogenic differentiation, when compared with static cultures. Quantitative fluorescent imaging, performed using the osteogenic extra-cellular matrix-targeted probes, showed higher osteogenic differentiation of the cells under the flow conditions. Gene expression analysis of osteogenic markers confirmed the osteogenic differentiation of the MSC-BMP2 cells. Immunofluorescent staining performed against the Osteocalcin, Col1, and BSP markers illustrated robust osteogenic differentiation in the flow culture and lessened differentiation in the static culture. To sum, the bone-chip allows monitoring cell survival, proliferation, and osteogenic differentiation using optical imaging.

Recent Advances and Future of Gene Therapy for Bone Regeneration

PURPOSE OF REVIEW

The purpose of this review is to discuss the recent advances in gene therapy as a treatment for bone regeneration. While most fractures heal spontaneously, patients who present with fracture nonunion suffer from prolonged pain, disability, and often require additional operations to regain musculoskeletal function.

RECENT FINDINGS

In the last few years, BMP gene delivery by means of electroporation and sonoporation resulted in repair of nonunion bone defects in mice, rats, and minipigs. Ex vivo transfection of porcine mesenchymal stem cells (MSCs) resulted in bone regeneration following implantation in vertebral defects of minipigs. Sustained release of VEGF gene from a collagen-hydroxyapatite scaffold to the mandible of a human patient was shown to be safe and osteoinductive. In conclusion, gene therapy methods for bone regeneration are systematically becoming more efficient and show proof-of-concept in clinically relevant animal models. Yet, on the pathway to clinical use, more investigation is needed to determine the safety aspects of the various techniques in terms of biodistribution, toxicity, and tumorigenicity.

Recent advances in gene-enhanced bone tissue engineering

The loss of bone tissue represents a critical clinical condition that is frequently faced by surgeons. Substantial progress has been made in the area of bone research, providing insight into the biology of bone under physiological and pathological conditions, as well as tools for the stimulation of bone regeneration. The present review discusses recent advances in the field of gene-enhanced bone tissue engineering. Gene transfer strategies have emerged as highly effective tissue engineering approaches for supporting the repair of the musculoskeletal system. By contrast to treatment with recombinant proteins, genetically engineered cells can release growth factors at the site of injury over extended periods of time. Of particular interest are the expedited technologies that can be applied during a single surgical procedure in a cost-effective manner, allowing translation from bench to bedside. Several promising methods based on the intra-operative genetic manipulation of autologous cells or tissue fragments have been developed in preclinical studies. Moreover, gene therapy for bone regeneration has entered the clinical stage with clinical trials for the repair of alveolar bone. Current trends in gene-enhanced bone engineering are also discussed with respect to the movement of the field towards expedited, translational approaches. It is possible that gene-enhanced bone tissue engineering will become a clinical reality within the next few years.

Recent tissue engineering-based advances for effective rAAV-mediated gene transfer in the musculoskeletal system

Musculoskeletal tissues are diverse and significantly different in their ability to repair upon injury. Current treatments often fail to reproduce the natural functions of the native tissue, leading to an imperfect healing. Gene therapy might improve the repair of tissues by providing a temporarily and spatially defined expression of the therapeutic gene(s) at the site of the injury. Several gene transfer vehicles have been developed to modify various human cells and tissues from musculoskeletal system among which the non-pathogenic, effective, and relatively safe recombinant adeno-associated viral (rAAV) vectors that have emerged as the preferred gene delivery system to treat human disorders. Adapting tissue engineering platforms to gene transfer approaches mediated by rAAV vectors is an attractive tool to circumvent both the limitations of the current therapeutic options to promote an effective healing of the tissue and the natural obstacles from these clinically adapted vectors to achieve an efficient and durable gene expression of the therapeutic sequences within the lesions.

Controlling Arteriogenesis and Mast Cells Are Central to Bioengineering Solutions for Critical Bone Defect Repair Using Allografts

Although most fractures heal, critical defects in bone fail due to aberrant differentiation of mesenchymal stem cells towards fibrosis rather than osteogenesis. While conventional bioengineering solutions to this problem have focused on enhancing angiogenesis, which is required for bone formation, recent studies have shown that fibrotic non-unions are associated with arteriogenesis in the center of the defect and accumulation of mast cells around large blood vessels. Recently, recombinant parathyroid hormone (rPTH; teriparatide; Forteo) therapy have shown to have anti-fibrotic effects on non-unions and critical bone defects due to inhibition of arteriogenesis and mast cell numbers within the healing bone. As this new direction holds great promise towards a solution for significant clinical hurdles in craniofacial reconstruction and limb salvage procedures, this work reviews the current state of the field, and provides insights as to how teriparatide therapy could be used as an adjuvant for healing critical defects in bone. Finally, as teriparatide therapy is contraindicated in the setting of cancer, which constitutes a large subset of these patients, we describe early findings of adjuvant therapies that may present future promise by directly inhibiting arteriogenesis and mast cell accumulation at the defect site.

Computed Tomography and Optical Imaging of Osteogenesis-angiogenesis Coupling to Assess Integration of Cranial Bone Autografts and Allografts

A major parameter determining the success of a bone-grafting procedure is vascularization of the area surrounding the graft. We hypothesized that implantation of a bone autograft would induce greater bone regeneration by abundant blood vessel formation. To investigate the effect of the graft on neovascularization at the defect site, we developed a micro-computed tomography (µCT) approach to characterize newly forming blood vessels, which involves systemic perfusion of the animal with a polymerizing contrast agent. This method enables detailed vascular analysis of an organ in its entirety. Additionally, blood perfusion was assessed using fluorescence imaging (FLI) of a blood-borne fluorescent agent. Bone formation was quantified by FLI using a hydroxyapatite-targeted probe and µCT analysis. Stem cell recruitment was monitored by bioluminescence imaging (BLI) of transgenic mice that express luciferase under the control of the osteocalcin promoter. Here we describe and demonstrate preparation of the allograft, calvarial defect surgery, µCT scanning protocols for the neovascularization study and bone formation analysis (including the in vivo perfusion of contrast agent), and the protocol for data analysis. The 3D high-resolution analysis of vasculature demonstrated significantly greater angiogenesis in animals with implanted autografts, especially with respect to arteriole formation. Accordingly, blood perfusion was significantly higher in the autograft group by the 7(th) day after surgery. We observed superior bone mineralization and measured greater bone formation in animals that received autografts. Autograft implantation induced resident stem cell recruitment to the graft-host bone suture, where the cells differentiated into bone-forming cells between the 7(th) and 10(th) postoperative day. This finding means that enhanced bone formation may be attributed to the augmented vascular feeding that characterizes autograft implantation. The methods depicted may serve as an optimal tool to study bone regeneration in terms of tightly bounded bone formation and neovascularization.

Engraftment and bone mass are enhanced by PTHrP 1-34 in ectopically transplanted vertebrae (vossicle model) and can be non-invasively monitored with bioluminescence and fluorescence imaging

Evidence exists that parathyroid hormone-related protein (PTHrP) 1-34 may be more anabolic in bone than parathyroid hormone 1-34. While optical imaging is growing in popularity, scant information exists on the relationships between traditional bone imaging and histology and bioluminescence (BLI) and fluorescence (FLI) imaging. We aimed to evaluate the effects of PTHrP 1-34 on bone mass and determine if relationships existed between radiographic and histologic findings in bone and BLI and FLI indices. Vertebrae (vossicles) from mice coexpressing luciferase and green fluorescent protein were implanted subcutaneously into allogenic nude mice. Transplant recipients were treated daily with saline or PTHrP 1-34 for 4 weeks. BLI, FLI, radiography, histology, and µCT of the vossicles were performed over time. PTHrP 1-34 increased bioluminescence the most after 2 weeks, fluorescence at all time points, and decreased the time to peak bioluminescence at 4 weeks (P ≤ 0.027), the latter of which suggesting enhanced engraftment. PTHrP 1-34 maximized vertebral body volume at 4 weeks (P < 0.0001). The total amount of bone observed histologically increased in both groups at 2 and 4 weeks (P ≤ 0.002); however, PTHrP 1-34 exceeded time-matched controls (P ≤ 0.044). A positive linear relationship existed between the percentage of trabecular bone and (1) total bioluminescence (r = 0.595; P = 0.019); (2) total fluorescence (r = 0.474; P = 0.074); and (3) max fluorescence (r = 0.587; P = 0.021). In conclusion, PTHrP 1-34 enhances engraftment and bone mass, which can be monitored non-invasively by BLI and FLI.

BMP-functionalised coatings to promote osteogenesis for orthopaedic implants

The loss of bone integrity can significantly compromise the aesthetics and mobility of patients and can be treated using orthopaedic implants. Over the past decades; various orthopaedic implants; such as allografts; xenografts and synthetic materials; have been developed and widely used in clinical practice. However; most of these materials lack intrinsic osteoinductivity and thus cannot induce bone formation. Consequently; osteoinductive functionalisation of orthopaedic implants is needed to promote local osteogenesis and implant osteointegration. For this purpose; bone morphogenetic protein (BMP)-functionalised coatings have proven to be a simple and effective strategy. In this review; we summarise the current knowledge and recent advances regardingBMP-functionalised coatings for orthopaedic implants.

PTH promotes allograft integration in a calvarial bone defect

Allografts may be useful in craniofacial bone repair, although they often fail to integrate with the host bone. We hypothesized that intermittent administration of parathyroid hormone (PTH) would enhance mesenchymal stem cell recruitment and differentiation, resulting in allograft osseointegration in cranial membranous bones. Calvarial bone defects were created in transgenic mice, in which luciferase is expressed under the control of the osteocalcin promoter. The mice were given implants of allografts with or without daily PTH treatment. Bioluminescence imaging (BLI) was performed to monitor host osteprogenitor differentiation at the implantation site. Bone formation was evaluated with the aid of fluorescence imaging (FLI) and microcomputed tomography (μCT) as well as histological analyses. Reverse transcription polymerase chain reaction (RT-PCR) was performed to evaluate the expression of key osteogenic and angiogenic genes. Osteoprogenitor differentiation, as detected by BLI, in mice treated with an allograft implant and PTH was over 2-fold higher than those in mice treated with an allograft implant without PTH. FLI also demonstrated that the bone mineralization process in PTH-treated allografts was significantly higher than that in untreated allografts. The μCT scans revealed a significant increase in bone formation in allograft + PTH treated mice comparing to allograft + PBS treated mice. The osteogenic genes osteocalcin (Oc/Bglap) and integrin binding sialoprotein (Ibsp) were upregulated in the allograft + PTH treated animals. In summary, PTH treatment enhances osteoprogenitor differentiation and augments bone formation around structural allografts. The precise mechanism is not clear, but we show that infiltration pattern of mast cells, associated with the formation of fibrotic tissue, in the defect site is significantly affected by the PTH treatment.