Structures, toxicity and internalization of cell-penetrating peptides
Abstract: Cellular internalization is a highly regulated process controlled by proteins in the plasma membrane. Large and hydrophilic compounds, in particular, face difficulties conquering the plasma membrane barrier in order to gain access to intracellular environment. This puts serious constrains on the drug industry since many drugs are hydrophilic. Several methods aiming at aiding the cellular internalization of otherwise impermeable compounds have therefore been developed. One such class, so-called cell-penetrating peptides (CPPs), emerged around twenty years ago. This group constitutes hundreds of peptides that have shown a remarkable ability in translocating diverse molecules, ranging from small molecules to large proteins, over the cell membrane. The internalization mechanism of CPPs has been questioned ever since the first peptides were discovered. Initially, the consensus in the field was direct translocation but endocytosis has gradually gained ground. The confusion and the disunity within this research field through the years proceeds from divergent results between research groups that hamper comparison of the peptides.This thesis aims at characterizing several well-established CPPs with comprehensive studies on cellular toxicity, secondary structure and cellular internalization kinetics.The results demonstrate that CPPs act in general in a low or non-toxic way, but the apparent toxicity is both peptide- and cell line-dependent. Structural studies show that the CPPs have a diverse polymorphic behavior ranging from random coil to structured ?-sheet or ?-helix, depending on the environment. The ability to change secondary structure could be the key to the internalization property of the CPPs. Internalization kinetic studies of CPP conjugates reveal two sorts of internalization profiles, either fast curves that cease in few minutes or slow curves that peak in tens of minutes. Furthermore, improved synthesis of CPP conjugates is demonstrated.In conclusion, the studies in this thesis provide useful information about cytotoxicity and structural diversity of CPPs, and emphasize the importance of kinetic measurements over end-point studies in order to give better insights into the internalization mechanisms of CPPs.
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