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180 Cell-Penetrating Peptides: Processes and Applications
FIGURE 8.8 Single-channel trace obtained with peptide 1 when incorporated into planar lipid
bilayers. (Adapted from Chaloin, L. et al., Biochim. Biophys. Acta, 1375, 52, 1998. With
permission.)
Formation of ion channels was evidenced on membranes of Xenopus oocytes and
on artificial planar lipid bilayers that allow an unambiguous assignment of the origin
of ionophore activity. For both types of membranes, the peptide induces channel
formation (Figure 8.8), thus providing an explanation for the toxic properties. 118
8.8 CONCLUSIONS AND PERSPECTIVES
Although few investigations are related to the mechanism used by cell-penetrating
peptides to penetrate into cells, the main general rule at least for shuttle peptides
that do not use the endocytosis pathway, concerns the structure. Some points, such
as the role of a linker sequence in the FP–NLS series, remain puzzling. The peptides
must show amphipathic properties, which can appear either at the primary structural
level or upon formation of an α-helix. However, for α-helical peptides, these amphipathic
properties can generate toxicity through ion channel formation according to
the peptide length. Up until now, no clear requirements have been established for
the criteria that select the internalization pathway, through endocytosis or direct
penetration into the cytoplasm.
Many aspects remain to be elucidated. They are mainly related to understanding
the CPP–lipid relationship, including the influence of the peptide conformational
state and the consequences on the lipid organization, as well as the influence of the
cargo on the conformational state of the carriers. In addition, nothing is known about
the mechanism of the release from the membrane into the cytoplasmic side of the
cells. This point is crucial and should be investigated in the near future because the
next generation of cell-penetrating peptides must bear labels that specifically recognize
the target cells through a receptor, for example. All this information will allow