Targeting cells with MR imaging probes: Cellular interaction and intracellular magnetic iron oxide nanoparticles uptake in brain capillary endothelial and choroidal plexus epithelial cells

Magnetic iron oxide nanoparticles (NPs) are considered for various diagnostic and therapeutic applications in brain including their use as contrast agent for magnetic resonance imaging. In delivery application, the critical step is the transport across cell layers and the internalization of NPs into specific cells, a process often limited by poor targeting specificity and low internalization efficiency. The development of the models of brain endothelial cells and choroidal plexus epithelial cells in culture has allowed us to investigate into these mechanisms. Our strategy is aimed at exploring different routes to the entrapment of iron oxide NPs in these brain related cells. Here we demonstrated that not only cells endowed with a good phagocytic activity like activated macrophages but also endothelial brain capillary and choroidal plexus epithelial cells do internalize iron oxide NPs. Our study of the intracellular trafficking of NPs by TEM, and confocal microscopy revealed that NPs are mainly internalized by the endocytic pathway. Iron oxide NPs were dispersed in water and coated with 3,4-dihydroxyl-L-phenylalanine (L-DOPA) using standard procedures. Magnetic lipid NPs were prepared by NANOVECTOR: water in oil in water (W/O/W) microemulsion process has been applied to directly coat different iron based NPs by lipid layer or to encapsulate them into Solid Lipid Nanoparticles (SLNs). By these coating/loading the colloidal stability was improved without strong alteration of the particle size distribution. Magnetic lipid NPs could be reconstituted after freeze drying without appreciable changes in stability. L-DOPA coated NPs are stable in PBS and in MEM (Modified Eagle Medium) medium. The magnetic properties of these NPs were not altered by the coating processes. We investigated the cellular uptake, cytotoxicity, and interaction of these NPs with rat brain capillary endothelial (REB4) and choroidal plexus epithelial (Z310) cells. By means of widefield, confocal microscopy and flow cytometry we studied the cell uptake of magnetic SLNs derivatized with a fluorescent reporter molecule and of L-DOPA-TRITC coated NPs. Inhibition of the caveolae-mediated pathway by preincubation with filipin and nystatin did not modify the cellular uptake of these NPs in both cell lines. Furthermore a mild decrease of the NPs cell uptake was obtained after chlorpromazine and NaN 3 pretreatment, which interferes with clathrin and energy-dependent endocytosis, and cytochalasin and amiloride pretreatment which interfere with macropinocytosis. NPs particle size as such can strongly affect the efficiency of cellular uptake and the mode of endocytosis. Considering that our L-DOPA and magnetic SLNs display a medium hydrodynamic size of 120 nm with a polydispersity index of 0.3, we can assume that the cell uptake process of these NPs may develop, depending the particle size, both via clathrin mediated endocytosis and macropinocytosis and only to less extent via the pathway of caveolae-mediated endocytosis. Taken together these results let us to conclude that SLNs iron loaded and iron based L-DOPA coated NPs are internalized into brain endothelial and choroidal plexus epithelial cells and this might provide the first step of an intracellular trafficking to transport these NPs between blood and brain.