Lipid Modifications During Membrane Fusion in Regulated Exocytosis

Abstract

Cells have evolved specific intracellular compartments that permit local concentration of macromolecules. These macromolecules are transported from one part of the cell to another and eventually released into the extracellular space to participate in cell-to-cell communication. Neurons and neuroendocrine cells secrete neurotransmitters and hormones by exocytosis, a highly regulated process in which secretory vesicles fuse with the plasma membrane to release their contents in response to a calcium trigger. To date, many proteins that catalyze the formation, targeting and fusion of secretory vesicles have been identified. However, the lipid composition of vesicles and their target membrane is also critical and lipid modifications may be required at several stages of the exocytotic pathway. In this review, we will discuss the latest results suggesting important functions for cholesterol, phosphatidic acid (PA) and phosphatidylinositol 4,5- bisphosphate (PIP2) in the membrane merging process. We propose that exocytotic sites are determined by the local formation of lipid micro-domains, which are potentially important to allow structural and spatial organization of the exocytotic machinery. Among the lipid candidates, our results show that PA plays a decisive role in the late stages of exocytosis, most likely by changing the membrane curvature that may be required for membrane fusion to occur. The spatial and temporal coordination of the various players to form an efficient machinery for secretion now needs to be determined.