Application of an SPR-based receptor assay for the determination of biologically active recombinant bone morphogenetic protein-2

Human bone morphogenetic protein-2 (hBMP-2) is a member of the human transforming growth factor- superfamily. Biologically active bone morphogenetic protein-2 (BMP-2) is a dimeric protein that binds in the first step of the signal transduction cascade to specific receptors on the cell-surface. This specific interaction of the dimeric protein with the extracellular ligand-binding domain (ECD) of the receptor was used to develop a receptor-based assay based on an optical biosensor system (Biacore 2000, Biacore AB, Uppsala, Sweden). The ECD of the BMP-receptor type IA, tagged with the Fc part of IgG (BMPR-IA-Fc), was immobilised on the surface of a dextran-protein A-coated sensor chip. Calibration curves were obtained with purified and biologically active recombinant hBMP-2 (rhBMP-2) that showed a linear range from approximately 5 to 250 nM rhBMP-2. Moreover, this assay was used to quantitatively follow the generation of biologically active protein during the renaturation from unfolded and reduced monomers to biologically active dimers. A refolding mixture containing renatured dimeric rhBMP-2 and not correctly folded monomers, was used as the sample solution without any further pre-treatment. It was proven that only the biologically active dimers were recognised by the immobilised receptor, so the generation of biologically active rhBMP-2 during the renaturation process could be monitored directly and rapidly. Furthermore, the results from the optical sensor obtained during the renaturation process showed a good correlation with the data obtained by non-reducing SDS-PAGE analysis carried out at the end of the renaturation process. These data show that the disulphide-bonded dimer corresponds to the biologically active protein capable of binding the BMP-receptor type IA.