Stable long-term intracellular labelling with fluorescently tagged cationic magnetoliposomes

Iron oxide nanocrystals that are dextran coated are widely exploited biomedically for magnetic resonance imaging (MRI), hyperthermia cancer treatment and drug or gene delivery. In this study, the use of an alternative coating consisting of a phospholipid bilayer directly attached to the magnetite core is described. The flexible nature of the magnetoliposome (ML) coat, together with the simple production procedure, allows rapid and easy modification of the coating, offering many exciting possibilities for the use of these particles in biomedical applications. Upon incubation of neutral MLs with an equimolar amount of cationic 1,2-distearoyl-3-trimethylammoniumpropane (DSTAP)-bearing vesicles, approximately one third of the cationic lipids are incorporated into the ML coat. This is in line with a theoretical model predicting transferability of only the outer leaflet phospholipids of bilayer structures. Most interestingly, the use of MLs containing 3.33 % DSTAP with a positive zeta-potential of (31.3+/-7.3) mV (mean +/-SD) at neutral pH, results in very heavy labelling of a variety of biological cells (up to (70.39+/-4.52) pg of Fe per cell, depending on the cell type) without cytotoxic effects. The results suggest the general applicability of these bionanocolloids for cell labelling. Mechanistically, the nanoparticles are primarily taken up by clathrin-mediated endocytosis and follow the endosomal pathway. The fate of the ML coat after internalisation has been studied with different fluorescent lipid conjugates, which because of the unique features of the ML coat can be differentially incorporated in either the inner or the outer layer of the ML bilayer. It is shown that, ultimately, iron oxide cores surrounded by an intact lipid bilayer appear in endosomal structures. Once internalised, MLs are not actively exocytosed and remain within the cell. The lack of exocytosis and the very high initial loading of the cells by MLs result in a highly persistent label, which can be detected, even in highly proliferative 3T3 fibroblasts, for up to at least one month (equivalent to approximately 30 cell doublings), which by far exceeds any values reported in the literature.