Identification of a peptide sequence targeting mammary vasculature via RPLP0 during lactation

To find novel targeting moieties to lactating mammary gland, in vivo phage display screening was conducted with lactating rats and a peptide ligand, CLHQHNQMC (designated as MG1), which specifically homes to the mammary tissue during lactation, was identified through the consecutive in vivo biopannings. MG1 peptide ligand showed specific binding affinity to lactating mammary tissue without any preference to other organs tested in ex vivo and in vivo validation, and microscopy analysis revealed that systemically introduced MG1 could be specifically localized in the lactating mammary gland associated with mammary epithelia and alveolar vasculature. Based on the observation that binding of MG1-encoding phage to lactating mammary gland was competitively inhibited by synthetic MG1 peptide ligand, we attempted to identify a counterpart molecule corresponding to specific recognition of the MG1 and the acidic Ribosomal Protein Large P0 (RPLP0) was selected as a candidate receptor for MG1 by peptide affinity pull-down assay with protein extracts from lactating mammary tissue. We demonstrated specific expression of RPLP0 in mammary tissue, especially during lactation, by immunoblotting assays and also demonstrated that MG1 peptide ligand could be bound to, and internalized into, the cells effectively via specific interaction with RPLP0 by analysis using an in vitro endothelial cell model. The overall results suggest that the MG1 has a specific affinity with RPLP0 which are dominantly expressed on the mammary vasculature during lactation and this specific affinity enables the MG1 would be served as an effective homing ligand to deliver functional molecules to the lactating mammary gland.