Cell-penetrating peptides: Small from inception to application

Despite continuing advances in the development of macromolecules, including peptides, proteins, and oligonucleotides, for therapeutic purposes, the successful application of these hydrophilic molecules has so far been hampered by their inability to efficiently traverse the cellular plasma membrane. The discovery of a class of peptides (cell-penetrating peptides, CPPs) with the ability to mediate the non-invasive and efficient import of a whole host of cargoes, both in vitro and in vivo, has provided a new means by which the problem associated with cellular delivery can be circumvented. A complete understanding of the translocation mechanism(s) of CPPs has so far proven elusive. Initial studies indicated an ATP-independent, non-endocytotic mechanism, dependent on direct peptide-membrane interactions, making it an enticing challenge from a biophysical point of view. However, recent evidence cast doubt on many of the earlier results, and led to a re-evaluation of the translocation mechanism of CPPs. In this review a brief history of the field will be given, followed by an introduction to some of the better known and more widely used CPPs, including some of their current applications, and finally a discussion of the translocation mechanism(s) and the controversies surrounding it.