Emerging evidence from various studies indicates that plasmid DNA (pDNA) is internalized by cells through an endocytosis-like process when it is used for electrotransfection. To provide morphological evidence of the process, we investigated ultrastructures in cells that were associated with the electrotransfected pDNA, using immunoelectron microscopy. The results demonstrate that four endocytic pathways are involved in the uptake of the pDNA, including caveolae- and clathrin-mediated endocytosis, macropinocytosis, and the clathrin-independent carrier/glycosylphosphatidylinositol-anchored protein-enriched early endosomal compartment (CLIC/GEEC) pathway. Among them, macropinocytosis is the most common pathway utilized by cells having various pDNA uptake capacities, and the CLIC/GEEC pathway is observed primarily in human umbilical vein endothelial cells. Quantitatively, the endocytic pathways are more active in easy-to-transfect cells than in hard-to-transfect ones. Taken together, our data provide ultrastructural evidence showing that endocytosis plays an important role in cellular uptake and intracellular transport of electrotransfected pDNA.

The trafficking and processing steps that occur in cells that are infected with influenza virus play a crucial role in the outcome of infection. These steps are targets for new and future anti-viral drugs, and can affect the relative virulence of the virus and its ability to cause disease. The virus first binds to its host cell via specific sialic acid residues, which can control the species tropism of the virus. The internalisation of the virus, into the nucleus of the cell, is dependent on a low pH, and this process is therapeutically targeted by the drug amantadine. Following replication, the newly formed viral genomes leave the nucleus and assemble into infectious particles at the plasma membrane. The targeting and processing of the various viral components at this late stage of the infectious cycle can have a major effect on the ability of the virus to spread and cause disease in its host. Finally, the release of viruses is dependent on the enzyme neuraminidase (NA), and this function has recently been targeted by the NA inhibitors, a new generation of drugs against influenza virus.