Endocytosis and cationic cell-penetrating peptides--a merger of concepts and methods

With the identification of fixation as a major source of artefacts in the cell biological research on cell-penetrating peptides (CPPs), the past two years have witnessed a dramatic development in the CPP field. At least for some of these molecules, endocytosis is now considered to be the major if not the exclusive route of cellular import. However, endocytosis comprises a variety of different pathways with very different implications for the delivery of bioactive molecules to the cytoplasm and nucleus. The endocytosis of CPPs is governed by complex mechanisms similar to those responsible for the internalization of other molecules. Therefore the investigation of uptake and intracellular trafficking of CPPs can benefit enormously from the understanding of the endocytic machinery as well as from the tools that have already been developed for the analysis of endocytosis. This review will introduce aspects of endocytosis relevant to the analysis of CPPs. In addition to the methods, that have already been used for the analysis of CPP trafficking, we will also present other tools and approaches which can be helpful for the research of CPP uptake. Furthermore this review will analyze and summarize recent data providing new insights in endocytosis and intracellular trafficking of CPPs.