Lysosomal capturing of cytoplasmic injected nanoparticles by autophagy: an additional barrier to non viral gene delivery.

Autophagy or 'self-eating' is a process by which defective organelles and foreign material can be cleared from the cell's cytoplasm and delivered to the lysosomes in which degradation occurs. It remains an open question, however, whether nanoparticles that did not enter the cell through endocytosis can also be captured from the cytoplasm by autophagy. We demonstrate that nanoparticles that are introduced directly in the cytoplasm of the cells by microinjection, can trigger an autophagy response. Moreover, both polystyrene beads and plasmid DNA containing poly-ethylene-imine complexes colocalize with autophagosomes and lysosomes, as was confirmed by electron microscopy. This indicates that cytoplasmic capturing of nanoparticles can occur by an autophagy response. The capturing of nanoparticles from the cytoplasm most likely limits the time frame in which efficient nucleic acid delivery can be obtained. Hence, autophagy forms an additional barrier to non-viral gene delivery, a notion that was not often taken into account before. Furthermore, these findings urge us to reconsider the idea that a single endosomal escape event is sufficient to have the long-lasting presence of nanoparticles in the cytoplasm of the cells.