Peptide-nanoparticle ligation mediated by cutinase fusion for the development of cancer cell-targeted nanoconjugates

The relationship between the positioning of ligands on the surface of nanoparticles and the structural features of nanoconjugates has been underestimated for a long time, albeit of primary importance to promote specific biological recognition at the nanoscale. In particular, it has been formerly observed that a proper molecular orientation can play a crucial role, first optimizing ligand immobilization onto the nanoparticles and, second, improving the targeting efficiency of the nanoconjugates. In this work, we present a novel strategy to afford peptide-oriented ligation using genetically modified cutinase fusion proteins, which combines the presence of a site-directed "capture" module based on an enzymatic unit and a "targeting" moiety consisting of the ligand terminal end of a genetically encoded polypeptide chain. As an example, the oriented presentation of U11 peptide, a sequence specific for the recognition of urokinase plasminogen activator receptor (uPAR), was achieved by enzyme-mediated conjugation with an irreversible inhibitor of cutinase, an alkylphosphonate p-nitrophenol ester linker, covalently bound to the surface of iron oxide nanoparticles. The targeting efficiency of the resulting protein-nanoparticle conjugates was assessed using uPAR-positive breast cancer cells exploiting confocal laser scanning microscopy and quantitative fluorescence analysis of confocal images. Ultrastructural analysis of transmission electron micrographs provided evidence of a receptor-mediated pathway of endocytosis. Our results showed that, despite the small average number of targeting peptides presented on the nanoparticles, our ligand-oriented nanoconjugates proved to be very effective in selectively binding to uPAR and in promoting the uptake in uPAR-positive cancer cells.