Cell membranes open "doors" for cationic nanoparticles/biomolecules: insights into uptake kinetics.

Cationic nanoparticles (NPs) and cell-penetrating peptides (CPPs) can enter cells in an energy-independent fashion, escaping the traditional endocytosis route, which is known as direct translocation. This unconventional entry, usually complementary to endocytosis, features rapid uptake and thus makes both cationic NPs and CPPs fascinating intracellular delivery agents. However, the mechanisms of the direct translocation of both cationic NPs and CPPs across cell membranes into the cytosol are not understood. Moreover, the relationship between direct translocation and endocytosis is also unclear. Here, using coarse-grained molecular dynamics simulations we show that a model cell membrane generates a nanoscale hole to assist the spontaneous translocation of cationic gold nanoparticles (AuNPs) as well as HIV-1 Tat peptides to the cytoplasm side under a transmembrane (TM) potential. After translocation, the AuNPs/Tat peptides move freely in the "cytoplasm" region and the membrane reseals itself within a microsecond, while the TM potential is strongly diminished. Furthermore, we show that the shape of the cationic object is crucial in determining if it can translocate or not across. The results provide insights into the uptake kinetics of cationic NPs/CPPs, which features the relationship between direction translocation and endocytosis. The mechanism put forward here establishes fundamental principles of the intracellular delivery of cationic nanocarriers.