A. Nerve end (synapsis) Process of Exocytosis The synaptic vesicles are directly available for release. During the firing of the nerve, neurotransmitter is released. Studies have shed some light on the inner workings by which the vesicle releases its contents. The whole process is called exocytosis. The current model of protein-mediated membrane fusion in exocytosis is as follows. When there is an activation to the nerve, a protein becomes active, which frees the vesicle from its attachment to the cytoskeleton or the microtubules. After the initial contact, the protein on the vesicle changes its shape. The protein on the vesicular membrane docks the synaptic vesicles to the cell membrane. The vesicle is then ready for release. The release sites, is close to calcium channels. The vesicle can release part or all of its contents, some of which can be recycled to form new vesicles Applying Botulinum neurotoxin selectively digests one or all of these proteins and blocks exocytosis of the vesicles, which ultimately results in muscle weakness or more profound muscle paralysis. Botulinum toxin is therapeutically used to treat spasticity or spasm in several neurologic and surgical diseases, to prevent excessive sweating, and cosmetically to correct wrinkles. B. Salivary Gland Secretion Exocytosis is the process by which the contents of a secretory granule are released across the plasma membrane. Exocytosis has been studied in different cell types including salivary gland cells. Granules are transported to the plasma membrane where they dock and undergo exocytosis of stored proteins. The extracellular discharge of cellular contents can be functionally divided into regulated and unregulated exocytosis. Regulated exocytosis occurs in response to physiological stimuli or when needed whereas unregulated exocytosis occurs continuously at low probability. Because a detailed understanding of the molecular mechanism of salivary gland exocytosis is lacking, much of the discussion in this section is drawn from other cell types. Proteins are the key driving force in exocytosis. After the energy-dependent priming step, proteins present on opposing membranes of the vesicle and the cell membrane interact loosely (termed tethering) prior to docking. The protein then initiates fusion of both membranes by exerting mechanical force on membranes to facilitate fusion and pore formation. Subsequently, the granular content is released by complete merger of the granule membrane with the plasma membrane (full fusion exocytosis).234 Following exocytosis, the protein complex dissociates into individual components233 by an energy-dependent process. This latter process is critical for preventing accumulation of byproducts. The resulting excess membrane that is inserted into the plasma membrane is retrieved by endocytosis pathways.235 Questions 1. What is the process discussed? 2. Identify the setting and the site of the material transport. 3. What are the materials moved? 4. Describe the process as best as you can. You may use images. Membrane Transport Exocytosis is a process for moving large molecules out of the cell to the cell exterior. Commonly, these macromolecules originate in storage vacuoles inside the cell and are moved to the exterior after an appropriate signal for this action. There is regulated exocytosis in which the contents of an intracellular vacuole are secreted in response to a specific signal and there is constitutive exocytosis wherein macromolecules are secreted from the cell without having to await a specific signal. In the case of constitutive exocytosis, a signal for secretion is part of the protein produced by the ribosome. Proteins thus destined for secretion transit through various compartments of the Golgi apparatus and are enclosed in a vesicle that will fuse with the cell membrane and be transported to the cell exterior through the agency of cup-shaped structures (porosomes) in the cell membrane that will dock the vesicle in the process of fusion with the cell membrane. The porosomes involve proteins. There are many regulatory factors involved in this overall process that regulate transport, the selection of specific proteins, and modification of proteins. Mouriño-Pérez, Rosa Reyna, and Meritxell Riquelme. "Recent Advances In Septum Biogenesis In Neurospora Crassa". Advances In Genetics, 2013, pp. 99-134. Elsevier, doi:10.1016/b978-0-12-407675-4.00003-1. Accessed 17 Mar 2022.
