The Endomembrane System of Neurons

Exocytosis in Neurons

Exocytosis in neurons takes on a critical role because most neurons communicate with each other using chemical extracellular signals called neurotransmitters. The mechanism of chemical neurotransmitter release is covered elsewhere. This section will discuss exocytosis of proteins.

Proteins are made in a macromolecular machine called a ribosome. Ribosomes stitch together amino acids, the monomers of proteins, using the messenger RNA code as a template. The details of this process are explained elsewhere.

Proteins for export or proteins that will be expressed on the cell surface (such as pumps, channels, or neurotransmitter receptors) are made in the rough endoplasmic reticulum (rough ER), an organelle consisting of ribosomes attached to the endomembrane system.  Protein modification then begins.

First, proteins travel into the ER-Golgi intermediate complex (ERGIC). Vesicles containing proteins bud off of the rough endoplasmic reticulum and take on an outer layer (coat) of coat protein complex II (COPII). These vesicles then fuse to form the cis face of the Golgi.

(The Golgi apparatus, also called the Golgi complex, forms at the cis face and breaks apart at the trans face. These faces are named for their relationship to the ER, with the cis face closer to the ER and the trans face further away.)

When there is a need to move proteins in the opposite direction, from the cis face of the Golgi to the rough ER, they are coated with coat protein complex Ia.

• Carbohydrate Synthesis Stage: A transition occurs when a cis cisterna loses the ability to receive COPII vesicles while acquiring the ability to receive glycosylation enzymes and sugar nucleotide transporters in COPIb vesicles. These processes convert the cis cisterna into a medial cisterna. Medial cisternae mature into trans cisternae. Both medial and trans cisternae are involved in carbohydrate synthesis, with early-acting enzymes concentrated in medial cisternae and late-acting enzymes in trans cisternae. Cisternae at the carbohydrate synthesis stage exchange material with one another via COPIb vesicles.
• Carrier Formation Stage: A transition occurs when a trans cisterna loses the ability to receive COPIb vesicles, and subsequently loses the ability to produce COPIb vesicles while acquiring the ability to produce clathrin-coated vesicles. The timing of secretory vesicle formation and the ultimate fate of carrier formation cisternae are still poorly understood. ER membranes attached to trans and/or TGN cisternae probably function in direct lipid transfer between the organelles (Hanada et al. 2009). This diagram depicts the mammalian TGN as the last cisterna in the stack.

Regulated and Constitutive Secretion in Neurons

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Endocytosis in Neurons

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