Analysis of ectopic bone-inducing capabilities of BMP-2, -4, -6, and -7 in mice

Objectives: Bone morphogenetic proteins (BMPs) induce ectopic bone formation when implanted into muscle tissue. Although they are expected to be developed as novel drugs for regenerative medicine and bone fracture treatment, the precise bone formation activities of various kinds of BMPs have not been compared. In this study, we analyzed ectopic bone-inducing capabilities of BMP-2, -4, -6 and -7 in mice following implantation into back muscle tissue.
Methods: Mice (C57/BL6 strain, 6 weeks, male) were divided into the PBS (control), BMP-2, BMP-4, BMP-6, and BMP-7 groups. Collagen sponges containing each type of BMP (5 μg/mouse) or PBS were surgically implanted under dorsal muscle pouches. After 7 days, we examined for cartilage matrix as lump-like masses, in which chondrocytes express EGFP by the type II collagen promoter. On day 14, we measured the volume of ectopic bone in the lump-like masses using μCT.
Results: PBS did not induce lump-like masses or ectopic bone formation in the mice. In the BMP groups, some calcification was seen after 7 days, while large lump-like masses composed of EGFP-positive tissue (cartilage) had formed, which was later replaced with calcified bone. On day 14, the volume of calcified tissue in the lump-like masses formed by BMP-2, BMP-4, BMP-6, and BMP-7 was 5±12, 8±2, 70±14, and 81±18 mm³, respectively.
Conclusions: BMP-6 and -7 induced lump-like masses and ectopic bone volume that were significantly larger than those induced by BMP-2 and 4. Our findings suggest that BMP-6 and -7 are potential drug candidates for regenerative medicine and bone fracture treatment. Based on our observation of GFP-positive tissue (cartilage) in lump-like masses that developed during the early stage of ectopic bone formation, the final ectopic bone volume seems to depend on the amount of cartilage.