Cell-penetrating Peptides

TLDR

Cell‑penetrating peptides such as Tat and penetratin were previously thought to enter cells via non‑endocytic pathways. This study reexamines how Tat 48‑60 and nona‑arginine peptides enter cells. The authors demonstrate that mild fixation creates artifactual uptake, that flow cytometry requires trypsin digestion to be valid, and that live‑cell imaging and kinetic analyses reveal endosomal localization and temperature/ATP‑dependent uptake consistent with endocytosis for both peptides and Tat‑conjugated peptide nucleic acids.

Abstract

Cellular uptake of a family of cationic cell-penetrating peptides (examples include Tat peptides and penetratin) have been ascribed in the literature to a mechanism that does not involve endocytosis. In this work we reevaluate the mechanisms of cellular uptake of Tat 48-60 and (Arg)(9). We demonstrate here that cell fixation, even in mild conditions, leads to the artifactual uptake of these peptides. Moreover, we show that flow cytometry analysis cannot be used validly to evaluate cellular uptake unless a step of trypsin digestion of the cell membrane-adsorbed peptide is included in the protocol. Fluorescence microscopy on live unfixed cells shows characteristic endosomal distribution of peptides. Flow cytometry analysis indicates that the kinetics of uptake are similar to the kinetics of endocytosis. Peptide uptake is inhibited by incubation at low temperature and cellular ATP pool depletion. Similar data were obtained for Tat-conjugated peptide nucleic acids. These data are consistent with the involvement of endocytosis in the cellular internalization of cell-penetrating peptides and their conjugates to peptide nucleic acids.