Bone formation by heterodimers through non-viral gene delivery of BMP-2/6 and BMP-2/7

Non-viral gene delivery is a safe technique to release sustained physiologic dosages of bone morphogenetic protein (BMP). Co-delivery of multiple BMPs can result in the formation of more potent BMP heterodimers. In this study, non-viral co-delivery of BMP-2/6 and BMP-2/7, as a mean to produce heterodimers, was assessed. Goat MSCs were non-virally transfected with plasmid DNA encoding BMP isoforms (pBMP) known to be relevant for osteogenesis: BMP-2, -6 or -7. As a result, BMP-2, -6 and -7 were produced and detectable for up to 14 d and their combined delivery (pBMP-2 with pBMP-6 or pBMP-7) was used to create BMP-2/6 and BM-2/7 heterodimers. Formation and secretion of the heterodimer proteins was validated by sandwich enzyme-linked immunosorbent assay (ELISA). Produced BMPs and heterodimers were biologically active, as confirmed by differentiation of reporter cells and MSCs. To assess bone formation, transfected MSCs were seeded on to ceramic scaffolds and implanted subcutaneously into nude mice. Bone formation was significantly enhanced in the pBMP-2/6 condition and a trend for more bone formation was observed in the pBMP-2/7 and pBMP-6 homodimer condition. No bone was found in the pBMP-2, pBMP-7 or control condition. In conclusion, simultaneous delivery of pBMP-2 with pBMP-6 or -7 resulted in the production of heterodimers that were beneficial for bone formation as compared to BMP homodimers. Combination of BMP sequences could reduce the need for high BMP protein dosages and might enhance prolonged availability of the growth factors.

Local induction of inflammation affects bone formation

To explore the influence of inflammatory processes on bone formation, we applied a new in vivo screening model. Confined biological pockets were first created in rabbits as a response to implanted bone cement discs. These biomembrane pockets were subsequently used to study the effects of inflammatory stimuli on ectopic bone formation within biphasic calcium phosphate (BCP) constructs loaded with TNF-α, lipopolysaccharide (LPS) or lipoteichoic acid (LTA), all with or without bone morphogenetic protein (BMP)-2. Analysis of bone formation after 12 weeks demonstrated that the inflammatory mediators were not bone-inductive in combination with the BCP alone, but inhibited or enhanced BMP-induced bone formation. LPS was associated with a strong inhibition of bone formation by BMP-2, while LTA and TNF-α showed a positive interaction with BMP-2. Since the biomembrane pockets did not interfere with bone formation and prevented the leakage of pro-inflammatory compounds to the surrounding tissue, the biomembrane model can be used for in vivo approaches to study local inflammation in conjunction with new bone formation. Using this model, it was shown that the modulation of the inflammatory response could be beneficial or detrimental to the subsequent bone formation process. The co-delivery of inflammatory factors and bone-related growth factors should be further explored as a strategy to enhance the bone-forming efficacy of bone substitutes.