Cellular Uptake and Trafficking of Lipid Nanocarriers Using High-Resolution Electron Microscopy

Lipid based nanocarriers are a commonly used drug delivery system with cargos ranging from small molecules to complex RNA-based therapies. There are several hypotheses how such carriers can enter the cell, in which organelles they reside, and how they cross or escape the endo-lysosomal system. To provide additional insights, the cell-nanocarrier interplay was visualized exemplarily with lipid-based nanocarriers and macrophage-like cultured cells (J774A.1 cells) using high resolution electron microscopy. Nanocarrier uptake into J774A.1 cells was detectable after the first 15 min by intracellular accumulation of electron-dense material. These accumulations were identified as lysosomes and lipid droplets, indicating complete degradation and a subsequent formation of storage organelles as early as 15 min. Inhibition of lysosomal acid lipase did not block lipid droplet formation, but rather resulted in accumulation of lipid droplets within lysosomes. This suggests that other cellular lipases already degrade acylglycerols before they reach lysosomes. Chloroquine co-treatment allowed visualization of nanocarriers inside endosomal vesicles, multivesicular bodies, and lysosomes.