Structural Mapping of BMP Conformational Epitopes and Bioengineering Design of Osteogenic Peptides to Specifically...

Human bone morphogenetic proteins (BMPs) constitute a large family of cytokines related to members of the transforming growth factor-β superfamily, which fulfill biological functions by specificity binding to their cognate type I (BRI) and type II (BRII) receptors through conformational wrist and linear knuckle epitopes, respectively.

We systematically examined the intermolecular recognition and interaction between the BMP proteins and BRI receptor at structural, energetic and dynamic levels. The BRI-binding site consists of three hotspot regions on BMP surface, which totally contribute ~70% potency to the BMP-BRI binding events and represent the core sections of BMP conformational wrist epitope; the contribution increases in the order: hotspot 2 (~ 8%) < hotspot 3 (~ 20%) < hotspot 1 (~ 40%). Multiple sequence alignment and structural superposition revealed a consensus sequence pattern and a similar binding mode of the three hotspots shared by most BMP members, indicating a high conservation of wrist epitope in BMP family. The three hotspots are natively folded into wellstructured U-shaped,, loop and double-stranded conformations in BMP proteins, which, however, would become largely disordered when splitting from the protein context to derive osteogenic peptides in free state, thus largely impairing their rebinding capability to BRI receptor. In this respect, cyclization strategy was employed to constrain hotspot 1/3-derived peptides into a native-like conformation, which was conducted by adding a disulfide bond across the ending arms of linear peptides based on their native conformations. Fluorescence-based assays substantiated that the cyclization can effectively improve the binding affinities of osteogenic peptides to BRI receptor by 3-6-fold. The cyclic peptides also exhibit a good selectivity for BRI over BRII (> 5-fold), confirming that they can specifically target the wrist epitope-binding site of BRI receptor.

The rationally designed cyclic peptides can be regarded as the promising lead entities that should be further chemically modified to enhance their in vivo biological stability for further bioengineering therapeutic osteogenic peptides against chondrocyte senescence and bone disorder.