Binding of cationic liposomes to apoptotic cells

One of the most prominent hallmarks of apoptotic cells is the altered characteristics of their plasma membrane, with its blebbing and exposure of the anionic phospholipid, phosphatidylserine (PS), in the outer leaflet of the lipid bilayer. The latter feature provides the basis of distinguishing apoptotic cells from most normal cells due to staining with fluorescently labeled annexin V, binding specifically to PS. In this article, we report on the binding to apoptotic leukemic T cells (Jurkat cell line, treated with different apoptotic inducers) of cationic liposomes (CLs) composed of the cationic gemini surfactant SS-1 ((2S,3S)-2,3-dimethoxy-1,4-bis(N-hexadecyl-N,N-dimethylammonium)butane dibromide), the fluorescent lipid analog DOPRho (1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(lissamine rhodamine B sulfonyl)), and POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine). Control cells showed negligible and irregular binding patterns of CLs, whereas apoptotic cells revealed a strongly augmented staining of their plasma membrane. Morphological observations and comparison with standard procedures for detecting apoptotic cells further demonstrated the binding of CLs to be intense for cells undergoing apoptosis. In addition, some apoptotic cells with higher caspase-3 activity also revealed more pronounced staining by CLs. Our data suggest that the binding of CLs to apoptotic cells is mediated through an electrostatic interaction between the positively charged head group of SS-1 and the translocated anionic phospholipid PS in the plasma membrane. Because the fluorescent lipid tracer can be freely selected, this approach provides convenient and versatile means for the fluorescence detection of apoptotic cells.