Fourier Transformed Spectral Bio-imaging for Studying the Intracellular Fate of Liposomes

Background: To improve the efficiency of liposomal drug targeting systems, it is necessary to understand the mechanism of liposome uptake by the cell and to follow the intracellular fate of internalized liposomes and their contents. Methods: We applied multiple-color fluorescence imaging spectroscopy, using a combination of five fluorescent dyes with a significant spectral overlap. pH-sensitive liposomes were labeled with the hydrophilic dye fluorescein isothiocyanate-dextran (FITC-dextran) or the lipophilic membrane marker rhodamine-B-phosphoethanolamine (Rh-PE) and incubated with COS-7 cells. Further, the cells were stained with specific markers: the cell membrane was fluorescently labeled with Vybrant DiO, lysosomes were stained with LysoTracker Red, and 4′,6 diamidino-2-phenylindole dihydrochloride was used for counterstaining the nucleus. Results. All five dyes were used simultaneously and were spectrally distinguished by the system. FITC-dextran-labeled liposomes showed a distribution pattern different from identically composed liposomes labeled with Rh-PE: the highly lipophilic Rh-PE was colocalized with the lysosomotropic dye LysoTracker Red, whereas liposomal FITC-dextran was not accompanied by LysoTracker Red in all cases. Conclusions: (a) Spectral (bio-) imaging is a powerful method for studying the intracellular fate of liposomal compounds. (b) We assume that the liposome membrane marker Rh-PE influences the uptake of particles due to its surface-modifying properties. We propose that this head-group-labeled phospholipid acts as a ligand for cellular receptors and triggers receptor-mediated (clathrin-dependent) endocytosis.