Regulated exocytosis is a multistage process involving a merger between the vesicle and the plasma membrane, leading to the formation of a fusion pore, a channel, through which secretions are released from the vesicle to the cell exterior. A stimulus may influence the pore by either dilating it completely (full-fusion exocytosis) or mediating a reversible closure (transient exocytosis). In neurons, these transitions are short-lived and not accessible for experimentation. However, in some neuroendocrine cells and astrocytes, initial fusion pores may reopen several hundred times, indicating their stability. Frequently, these pores are too narrow to pass luminal molecules to the extracellular space (unproductive exocytosis), but their diameter can dilate upon stimulation. To explain the stability of the initial narrow fusion pores, anisotropic membrane constituents with a non-axisymmetric shape were proposed to accumulate in the fusion pore membrane. Although the nature of these is unclear, they may consist of lipids and proteins, including SNAREs, which may facilitate and regulate the pre- and post-fusional stages of exocytosis. This review highlights models and experimental studies revealing mechanisms of fusion pore stabilization in a narrow, release unproductive state. The fusion pore is a channel that forms when the vesicle and the plasma membranes merge, and mediates the release of secretions from the vesicle lumen to the cell exterior. Frequently, these pores are too narrow to pass molecules to the extracellular space. Anisotropic membrane constituents with a non-axisymmetric shape were proposed to accumulate in the fusion pore membrane. This article is part of a mini review series on Chromaffin cells (ISCCB Meeting, 2015).
Keywords: fusion pore; fusion pore stability; regulated exocytosis.
© 2016 International Society for Neurochemistry.