The BMP2 antagonist inhibitor L51P enhances the osteogenic potential of BMP2 by simultaneous and delayed synergism

Bone morphogenetic protein 2 (BMP2) is a potent osteoinductive cytokine that plays crucial roles in bone repair. However, large amounts of BMP2 are required to induce sufficient bone formation in humans possibly due to a feedback response of BMP antagonists. The engineered BMP2 variant L51P is deficient in BMP receptor type I activation but maintains affinity for BMP antagonists and can allow for the inactivation of BMP antagonists, and eventually enhance BMP2 action. As hypothesized, simultaneous addition of L51P enhanced the BMP2-induced osteogenesis. To test the ability of L51P to competitively inactivate BMP antagonists, cell binding affinity of BMP2 ligands was investigated in the presence or absence of L51P. Because the BMP antagonists were highly expressed 3 days after exogenous BMP2 stimulation, we collected supernatants from 3-day stimulated cell cultures and used as condition culture media (CM). The results showed a significant decrease in the cell binding of BMP2 ligands when cells were incubated with exogenous BMP2 and CM, whereas L51P addition competitively rescued the suppression of BMP2-to-cell binding induced by CM incubation. In a delayed experimental model, L51P was applied 3 days after exogenous BMP2 stimulation and we could observe a striking enhancement of the BMP2-induced SMAD-1/5/8 phosphorylation and luciferase activity of the Id1 promoter compared to the simultaneous addition of the two factors. These findings provide a deeper insight into the cellular and molecular mechanisms involved in the effect of L51P in suppressing the BMP antagonists and enhancing BMP activity. Additionally, these results demonstrate that L51P is a promising down regulator of BMP-induced negative feedback, which could have a significant impact in future applications of BMP2 in research and clinical settings.