Development of Artificial System of Exocytotic Cells

Regulated exocytosis is a very important process to secrete bioactive substances from cells, such as neurotransimitters, hormones, and inflammatory mediators from immune cells. Secretory cells have intracellular small vesicles called secretory vesicles or secretory granules and they contain bioactive substances. These contents are released by membrane fusion between secretory vesicles and the plasma membrane (exocytosis). Mast cells are typical non-neuronal secretory cells and secrete inflammatory mediators such as histamine. It has been shown that SNARE (soluble N-ethyl maleimide-sensitive factor attachment protein receptor) proteins play an essential role in exocytotic release in mast cells. In this study, we tried to develop an artificial system that mimics exocytotic release of mast cells, using liposomes in which purified SNARE proteins are reconstituted. To develop a such system, we try to prepare a cell-size giant liposome that contains smaller liposomes. A giant liposome and smaller liposomes correspond to the cell membrane and secretory vesicles, respectively. Preparing giant liposomes, LUVs that contain smaller liposomes inside like mast cells were formed. Diameter of inner liposomes ranged from 1 to 3 mum and the number of inner liposomes ranged from 1 to 20. We also succeeded preparing giant liposomes that contain calcein-labeled SUV by adding SUV at the process of rehydration. These system might be useful to construct an artificial exocytotic cells.