Endocytotic uptake of iron oxide nanoparticles by cultured brain microglial cells

Microglia are the phagocytotic cells of the brain that respond rapidly to alterations in brain homeostasis. Since iron oxide nanoparticles (IONPs) are used for diagnostic and therapeutic applications in the brain, the consequences of an exposure of microglial cells to IONPs are of particular interest. To address this topic we have synthesized and characterized fluorescent BODIPY®-labelled IONPs (BP-IONPs). The average hydrodynamic diameter and the ζ-potential of BP-IONPs in water were ∼65 nm and −49 mV, respectively. Both values increased after dispersion of the particles in serum containing incubation medium to ∼130 nm and −8 mV. Exposure of cultured rat microglial cells with BP-IONPs caused a time-, concentration- and temperature-dependent uptake of the particles, as demonstrated by strong increases in cellular iron contents and cellular fluorescence. Incubation for 3 h with 150 and 450 μM iron as BP-IONPs increased the cellular iron content from a low basal level of ∼50 nmol iron mg−1 to 219 ± 52 and 481 ± 28 nmol iron (mg protein)−1, respectively. These conditions did not affect cell viability, but exposure to higher concentrations of BP-IONPs or for longer incubation periods severely compromised cell viability. The BP-IONP fluorescence in viable microglial cells was co-localized with lysosomes. In addition, BP-IONP accumulation was lowered by 60% in the presence of the endocytosis inhibitors 5-(N-ethyl-N-isopropyl)amiloride, tyrphostin 23 and chlorpromazin. These results suggest that the rapid accumulation of BP-IONPs by microglial cells is predominantly mediated by macropinocytosis and clathrin-mediated endocytosis, which direct the accumulated particles into the lysosomal compartment.