ABSTRACT
Acknowledgments .............................................................................................. 104
References .......................................................................................................... 104
A reductionist and systematic study of endogenous and exogenous proteins has
demonstrated the presence of protein transduction domains (PTDs) implicated in
protein uptake. The synthesis and study of the peptides corresponding to these
domains further allowed their identification as members of the cell-penetrating
peptide (CPP) family. This is exemplified by the first studies of molecular
dissections of TAT and Antennapedia proteins that led to the definition of small
proteins able to enter cells: 60 residues for Antennapedia homeodomain
(AntpHD)1 and 72 residues for TAT.2 Further dissections of these small proteins
revealed that shorter and contiguous domains were sufficient to promote
internalization: 16 residues for the helix III of AntpHD (pAntp(43-58), named
penetratin)3 and 36 residues for TAT (TAT(37-72)).4 From these original data,
many other shorter peptides were synthesized, containing either nine or 10
residues for TAT5 and seven residues for pAntp(52-58).6 Since these pioneer
works, numerous short peptides derived from various proteins have been
identified as CPPs with the use of fluorescent or biotinyl tags and detection by
confocal microscopy or flow cytometry.6 Some of the CPPs still translocate
when coupled to hydrophobic or acidic cargos (e.g., phosphopeptides,
oligonucleotides, peptide nucleic acids, drugs, etc.). Therefore, these peptides
can be considered as Trojan peptides for their ability to deliver bioactive
molecules inside cells.7,8
Although the concept of internalization is almost consensual and accepted,9
numerous contradictory data have been reported for the uptake efficiencies and the
intracellular locations of CPPs. In order to simplify the comparison of the literature
data, we have defined a translocation coefficient, KT, that corresponds to the CPP
concentration ratio between the intra-and the extracellular media after 1 h of
incubation: KTZ[CPP]intra/[CPP]extra. An intracellular volume of 1.5 pL (measured for CHO-K1 cells) was taken whatever the cell type. A KT value greater than 1
reflects an intracellular accumulation of the CPP. The analysis of these KT values
calculated from the literature6 revealed a wide variation from one study to another.
