[The biological activity of E. coli-expressed BMP-4]

BMP-4 is physiologically present in low concentrations in human bone matrix. So far the protein has only been produced in small quantities by expression in mammalian cell cultures. In this study we investigated the biological activity of E. coli-expressed BMP-4. In vitro neonatal rat muscle tissue was incubated together with BMP-4 during 4 h, followed by an incubation period of 14 days on cellulose acetate membranes in BMP-free medium. The addition of 0.4 μg BMP-4 induced cartilage formation in 1/8 samples while 4 μg BMP-4 showed chondroneogenesis in 2/10 samples. When the BMP-4 concentration was increased to 40 μg, new cartilage formation was seen in 5/7 samples. In vivo BMP-4 was implanted intramuscularly for 3 weeks in ICR mice. Amounts of 10 μg rhBMP-4 and more (up to 100 μg) constantly induced heterotopic ossicle formation. BMP-4 was also combined with a collagen carrier and implanted for 2 and 4 weeks in the abdominal muscle of SD rats. While 0.4 μg BMP-4 showed no bone or cartilage formation, the amount of 40 μg BMP-4 showed new heterotopic cartilage formation, followed by endochondral ossification in almost all samples. The results prove that E. coli-expressed BMP-4 possesses the same inductive properties as mammalian-cell-expressed BMP-4.