Identification of Commercially Available Antibodies that Block Ligand Binding by BMPR2

Osteoporosis, a disease of low bone mineral density, affects 10 million Americans and triggers significant health problems and considerable socioeconomic burdens. Current treatments for osteoporosis have significant limitations, necessitating identifying new treatment strategies via building a better understanding of the endogenous mechanisms regulating bone mass. A recent study demonstrated that removal of the BMP type 2 receptor (BMPR2) in skeletal progenitor cells of Bmpr2-cKO mice during embryonic development leads to reduced age-related bone loss by sustained elevation in bone formation rate. This present study sought to advance the translational potential of the genetic model by identifying antibodies that neutralize the ligand-binding function of the BMPR2 extracellular domain (BMPR2-ECD). This study first established a modified, cell-free immunoprecipitation assay wherein the ligand BMP2 was pulled-down by BMPR2-ECD conjugated to Protein G beads; the unbound BMP2 (found in the supernatant) was subsequently quantified by ELISA. This yielded a standard assay wherein approximately 2 ug BMPR2-ECD leads to a 70% reduction in BMP2 signal. Next, the neutralizing ability of 3F6, a mouse monoclonal antibody raised against the ligand-binding region of BMPR2, was examined and was found to cause a dose-dependent inhibition of BMPR2-ECD ligand-binding. Given the ascites preparation of 3F6, specificity of this assay was confirmed by demonstrating that ligand-binding activity of BMPR2-ECD is unchanged in the presence of non-specific, negative-control ascites. Using these results as a guide, 1F12, another mouse monoclonal antibody raised against the ligand-binding region of BMPR2, was evaluated and was also found to neutralize the ligand-binding function of BMPR2-ECD. In contrast, no effect on ligand-binding function of BMPR2-ECD was observed with 9A10 even though this mouse monoclonal antibody is also raised against the ligand-binding region of BMPR2. These results provide proof-of-concept data for future studies evaluating inhibition of BMPR2 function in vivo as a means to reduce age-related bone loss