Cell-penetrating peptides (CPPs) are able to mediate the efficient cellular uptake of a wide range of cargoes. Internalization of a number of CPPs requires uptake by endocytosis, initiated by binding to anionic cell surface heparan sulfate (HS), followed by escape from endosomes. To elucidate the endosomal escape mechanism, we have modeled the process for two CPPs: penetratin (pAntp) and the N-terminal signal peptide of the unprocessed bovine prion protein (bPrPp). Large unilamellar phospholipid vesicles (LUVs) were produced encapsulating either peptide, and an ionophore, nigericin, was used to create a transmembrane pH gradient (ΔpHmem, inside acidic) similar to the one arising in endosomes in vivo. In the absence of ΔpHmem, no pAntp escape from the LUVs is observed, while a fraction of bPrPp escapes. In the presence of ΔpHmem, a significant amount of pAntp escapes and an even higher degree of bPrPp escape takes place. These results, together with the differences in kinetics of escape, indicate different escape mechanisms for the two peptides. A minimum threshold peptide concentration exists for the escape of both peptides. Coupling of the peptides to a cargo reduces the fraction escaping, while complexation with HS significantly hinders the escape. Fluorescence correlation spectroscopy results show that during the escape process the LUVs are intact. Taken together, these results suggest a model for endosomal escape of CPPs: ΔpHmem-mediated mechanism, following dissociation from HS of the peptides, above a minimum threshold peptide concentration, in a process that does not involve lysis of the vesicles.
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