Role of endocytosis in sonoporation-mediated membrane permeabilization and uptake of small molecules: a electron microscopy study

Sonoporation is a physical method that has been successfully used to deliver drugs into living cells both in vitro and in vivo for experimental and therapeutic purposes. Despite numerous studies on this topic, often reporting successful outcomes, very little is known about the mechanisms involved in the hypothesized membrane permeabilization processes. In this study, electron microscopy was used to investigate the ultra-structural modifications of cell membranes, induced by sonoporation. Here, we demonstrate that sonoporation in the presence of microbubbles induces the formation of a significant number of transient and permeant structures at the membrane level. These structures were transient with a half-life of 10 min and had a heterogeneous size distribution ranging from a few nanometers to 150 nm. We demonstrated that the number and the size of these structures were positively correlated with the enhanced intracellular uptake of small molecules. In addition, we showed that these structures were associated with caveolae-dependent endocytosis for two thirds of the recorded events, with the remaining one third related to non-specific routes such as membrane disruptions as well as caveolae-independent endocytosis. In conclusion, our observations provide direct evidences of the involvement of caveolae-endocytosis in cell membrane permeabilization to small molecules after sonoporation.