Exocitosis

Exocitosis, also called exocytosis, is the cellular process by which vesicles derived from the Golgi apparatus or endosomal compartments fuse with the plasma membrane to release their contents into the extracellular space. This process also transfers membrane lipids and proteins to the cell surface, contributing to membrane turnover.

In regulated exocytosis, vesicle fusion is triggered by an intracellular signal, typically a rise in cytosolic calcium ions. In neurons, calcium influx through voltage-gated channels triggers neurotransmitter release. In many secretory cells, hormones and enzymes are released in response to stimuli such as other signaling molecules or nutrients. In constitutive exocytosis, vesicles fuse with the plasma membrane in a continuous, unregulated manner, supplying membrane components and secreted proteins.

Mechanistically, vesicles are transported to the plasma membrane and dock at specific sites through interactions between v-SNAREs on the vesicle and t-SNAREs on the target membrane. Priming reactions prepare the fusion machinery; upon Ca2+ binding to sensor proteins, the SNARE complex drives membrane fusion. Fusion can be complete, with vesicle contents released and the vesicle membrane incorporated, or transient in kiss-and-run fusion, allowing rapid reuse of vesicle components.

Roles include neurotransmission, hormone secretion, enzyme release, and certain immune responses, as well as membrane remodeling and surface receptor turnover. Exocytosis is essential for cellular communication and maintenance, and its dysfunction can affect multiple tissues.

Clinical relevance includes links to secretory disorders such as diabetes mellitus (impaired insulin release) and some neurodegenerative diseases. Toxins that disrupt exocytosis, such as botulinum and tetanus toxins, cleave SNARE proteins and inhibit vesicle fusion, illustrating the pathway’s importance for normal physiology.