Artificial cells: unique insights into exocytosis using liposomes and lipid nanotubes

Exocytosis is the fundamental process underlying neuronal communication. This process involves fusion of a small neurotransmitter-containing vesicle with the plasma membrane of a cell to release minute amounts of transmitter molecules. Exocytosis is thought to go through an intermediate step involving formation of a small lipid nanotube or fusion pore, followed by expansion of the pore to the final stage of exocytosis. The process of exocytosis has been studied by various methods; however, when living cells are used it is difficult to discriminate between the molecular effects of membrane proteins relative to the mechanics of lipid-membrane-driven processes and to manipulate system parameters (e.g., membrane composition, pH, ion concentration, temperature, etc.). We describe the use of liposome-lipid nanotube networks to create an artificial cell model that undergoes the later stages of exocytosis. This model shows that membrane mechanics, without protein intervention, can drive expansion of the fusion pore to the final stage of exocytosis and can affect the rate of transmitter release through the fusion pore.