Osteoinduction by Ex Vivo Nonviral Bone Morphogenetic Protein Gene Delivery Is Independent of Cell Type

Ex vivo nonviral gene delivery of bone inductive factors has the potential to heal bone defects. Due to their inherent role in new bone formation, multipotent stromal cells (MSCs) have been studied as the primary target cell for gene delivery in a preclinical setting. The relative contribution of autocrine and paracrine mechanisms, and the need of osteogenic cells, remains unclear. This study investigates the contribution of MSCs as producer of transgenic bone morphogenetic proteins (BMPs) and to what extent the seeded MSCs participate in actual osteogenesis. Rat-derived MSCs or fibroblasts (FBs) were cotransfected with pBMP-2 and pBMP-6 or pBMP-7 via nucleofection. The bioactivity of BMP products was shown through in vitro osteogenic differentiation assays. To investigate their role in new bone formation, transfected cells were seeded on ceramic scaffolds and implanted subcutaneously in rats. Bone formation was assessed by histomorphometry after 8 weeks. As a proof of principle, we also investigated the suitability of bone marrow-derived mononuclear cells and the stromal vascular fraction isolated from adipose tissue for a one-stage gene delivery strategy. Bone formation was induced in all conditions containing cells overexpressing BMP heterodimers. Constructs seeded with FBs transfected with BMP-2/6 and MSCs transfected with BMP-2/6 showed comparable bone volumes, both significantly higher than controls. Single-stage gene delivery proved possible and resulted in some bone formation. We conclude that bone formation as a result of ex vivo BMP gene delivery can be achieved even without direct osteogenic potential of the transfected cell type, suggesting that transfected cells mainly function as a production facility for osteoinductive proteins. In addition, single-stage transfection and reimplantation of cells appeared feasible, thus facilitating future clinical translation of the method.