Uptake of Liposomes by Cells: Experimental Procedures and Modeling

Abstract

The uptake of liposomes by cells is viewed as a sequence of two major steps—binding to sites on the plasma membrane, followed by entry into the cell. The method of analysis that is described in the chapter is applicable both for plain lipid vesicles and for targeted liposomes. The membranes of targeted liposomes include ligands that can bind with high affinity to particular receptors on the cell surface. The mathematical model can be applied to a mode of liposome endocytosis via coated pits or via caveolae. The chapter outlines experimental procedures for discriminating between these two modes of entry, which can coexist. The total uptake or association of liposomes with cells, as a consequence of surface binding and subsequent endocytosis via a coated pit/coated vesicle system, is evaluated by several means. All the procedures involve the incubation of cells with labeled liposomes under defined experimental conditions. An alternative route of internalization into cells that is likely to contribute to total liposome uptake is the caveolar system. Caveolae are omega-shaped invaginations of the plasma membrane with a diameter of 50–100 nm. These invaginations can detach from the plasma membrane to form noncoated plasmalemmal vesicles within the cell cytoplasm. Caveolae found in a variety of cells are implicated in a number of important cellular functions, including signal transduction, potocytosis, and the endocytotic and transcytotic movements of macromolecules.