Artificial exocytotic system that secretes intravesicular contents upon Ca2+ influx

Exocytosis is a crucial process of secreting various signaling molecules such as neurotransmitters, hormones, and other chemical mediators into the extracellular space. Exocytotic release is caused by membrane fusion of intracellular vesicles with the plasma membrane triggered by an increase in intracellular Ca(2+). In the present study, we developed an artificial system of exocytosis that secretes intravesicular contents upon Ca(2+) influx. We prepared artificial secretory cells using cell-sized giant unilamellar liposomal vesicles (GUVs) that contain small liposomes (SUVs) that correspond to secretory vesicles. To observe exocytosis-like secretion in an artificial system, we labeled both an intra-SUV solution and an SUV membrane with a soluble fluorescent dye and a rhodamine-labeled phospholipid, respectively. To induce membrane fusion between SUVs and a GUV as observed in exocytosis, the Ca(2+) concentration of intra-GUV was elevated by incorporating ionomycin (a Ca(2+) ionophore) into the GUV membrane. We succeeded in inducing exocytosis-like secretion by Ca(2+) elevation in a GUV together with the osmolarity difference between the intra-GUV and extra-GUV solutions.