Uptake Mechanism and Direct Translocation of a New CPP for siRNA Delivery

Since their development, cell-penetrating peptides (CPPs) have been used as delivery vehicles for various genetic or therapeutic agents; however, the uptake mechanisms of CPPs and the delivery details are still unclear. Understanding the mechanisms of cellular internalization of CPPs facilitate their development of CPPs as gene delivery vectors. In the present study, we evaluated the internalization process of a previously designed CPP, STR-KV, complexed with small interference RNA (siRNA) targeting at the glyceraldehyde-3-phosphate dehydrogenase (GAPDH) gene. Using heparin treatment and chemical endocytic inhibitors, we elucidated that the electrostatic interaction of STR-KV/siRNA complex with heparin sulfate proteoglycans at the cell membrane surface triggered the energy-independent uptake of the majority of the complexes, which most likely through a direct translocation pathway. The intracellular trafficking and internalization kinetics observed by confocal microscopy also confirmed that the complex was uptaken through a nonendocytic pathway.