(536e) Osteoclastic Resorption of Mineralized Fillers in the Presence of Bone Morphogenetic Protein-2

Moldable composite bone grafts (polyurethane/mineralized fillers) with the potential of maintaining weight-bearing properties during healing undergo balanced remodeling when injected into femoral plug defects in rabbits. The grafts were augmented with recombinant human Bone Morphogenetic Protein-2 (rhBMP-2) to reduce gaps between new bone deposition and matrix resorption. Delivery of rhBMP-2 increased new bone formation at 6 and 12 weeks. However, it also increased polymer degradation and matrix resorption at 6 weeks which suggests osteoclast activation. Thus, the aim of the present study was to investigate the effects of BMP-2 delivery and matrix composition on the differentiation and function of osteoclast-like cells. Mouse bone marrow cells positive for CD11b were seeded on various mineralized materials: Bioactive glass, Tricalcium phosphate, and Dentin. The cells were then treated with nuclear factor kB ligand (RANKL), macrophage colony stimulating factor (M-CSF), and BMP-2 for up to 28 days. The effect of BMP-2 and substrate composition on osteoclast differentiation was monitored through the gene expression of specific markers (TRAP, Cathepsin K, Calcitonin receptor, Atp6v0d2 proton pump). Resorptive activity of the cells was quantified by measuring secreted TRAP into the culture medium, as well as number and area of resorption pits generated on the materials. After 7 days of culture without BMP-2, dentin promoted significantly higher gene expression of osteoclastic markers than all the other groups. When BMP-2 was added to the culture, the expression of osteoclastic gene markers changed most significantly in the dentin group. These results suggest a combined effect between substrate composition and BMP-2 delivery on the differentiation and activity of osteoclasts, and support the observations of other groups related to osteoclasts expressing BMP-2 receptors (Jensen et. al, 2010) and high local doses of BMP-2 generating increased osteoclastic activity around bone (Toth et. al., 2009). Changes in the differentiation process of the cells due to their interaction with synthetic vs. natural fillers should be taken into account during the design of functional scaffolds for bone tissue engineering.