exocytosis

Exocytosis involves the transport of intracellular vesicles (often derived from the Golgi apparatus or secretory granules) to the plasma membrane, where they dock and fuse to release their contents outside the cell. This process is mediated by SNARE proteins (v-SNAREs on vesicles and t-SNAREs on the target membrane), which mediate membrane fusion, and is commonly triggered by an increase in intracellular calcium concentration (e.g., via voltage-gated calcium channels at synaptic terminals). Clinically, exocytosis is critical in neurotransmitter release at synapses (e.g., acetylcholine at the neuromuscular junction), insulin secretion from pancreatic beta cells, and hormone release from endocrine cells. Botulinum toxin and tetanus toxin act by cleaving SNARE proteins, blocking exocytosis of neurotransmitters (inhibiting acetylcholine release in botulism, and inhibiting release of inhibitory neurotransmitters GABA/glycine in tetanus), which is high-yield for boards. Exocytosis can be constitutive (continuous, unregulated, as in secretion of extracellular matrix components) or regulated (triggered by specific signals, as in hormone or neurotransmitter release). Distinguishing exocytosis from endocytosis (internalization of extracellular material) is important, as they represent opposite directions of vesicular trafficking that maintain membrane homeostasis and cellular communication.