A Definition and Explanation of the Steps in Exocytosis

Exocytotic Vesicles

Exocytotic vesicles containing protein products are typically derived from an organelle called the Golgi apparatus, or Golgi complex. Proteins and lipids synthesized in the endoplasmic reticulum are sent to Golgi complexes for modification and sorting. Once processed, the products are contained within secretory vesicles, which bud from the trans face of the Golgi apparatus.

Other vesicles that fuse with the cell membrane do not come directly from the Golgi apparatus. Some vesicles are formed from early endosomes, which are membrane sacs found in the cytoplasm. Early endosomes fuse with vesicles internalized by endocytosis of the cell membrane. These endosomes sort the internalized material (proteins, lipids, microbes, etc.) and direct the substances to their proper destinations. Transport vesicles bud off from early endosomes sending waste material on to lysosomes for degradation, while returning proteins and lipids to the cell membrane. Vesicles located at synaptic terminals in neurons are also examples of vesicles that are not derived from Golgi complexes.

Types of Exocytosis

There are three common pathways of exocytosis. One pathway, constitutive exocytosis, involves the regular secretion of molecules. This action is performed by all cells. Constitutive exocytosis functions to deliver membrane proteins and lipids to the cell's surface and to expel substances to the cell's exterior.

Regulated exocytosis relies on the presence of extracellular signals for the expulsion of materials within vesicles. Regulated exocytosis occurs commonly in secretory cells and not in all cell types. Secretory cells store products such as hormones, neurotransmitters, and digestive enzymes that are released only when triggered by extracellular signals. Secretory vesicles are not incorporated into the cell membrane but fuse only long enough to release their contents. Once the delivery has been made, the vesicles reform and return to the cytoplasm.

A third pathway for exocytosis in cells involves the fusion of vesicles with lysosomes. These organelles contain acid hydrolase enzymes that break down waste materials, microbes, and cellular debris. Lysosomes carry their digested material to the cell membrane where they fuse with the membrane and release their contents into the extracellular matrix.

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A Definition and Explanation of the Steps in Endocytosis

By Regina Bailey

Steps of Exocytosis

Exocytosis occurs in four steps in constitutive exocytosis and in five steps in regulated exocytosis. These steps include vesicle trafficking, tethering, docking, priming, and fusing.

• Trafficking: Vesicles are transported to the cell membrane along microtubules of the cytoskeleton. Movement of the vesicles is powered by the motor proteins kinesins, dyneins, and myosins.
• Tethering: Upon reaching the cell membrane, the vesicle becomes linked to and pulled into contact with the cell membrane.
• Docking: Docking involves the attachment of the vesicle membrane with the cell membrane. The phospholipid bilayers of the vesicle membrane and cell membrane begin to merge.
• Priming: Priming occurs in regulated exocytosis and not in constitutive exocytosis. This step involves specific modifications that must happen in certain cell membrane molecules for exocytosis to occur. These modifications are required for signaling processes that trigger exocytosis to take place.
• Fusion: There are two types of fusion that can take place in exocytosis. In complete fusion, the vesicle membrane fully fuses with the cell membrane. The energy required to separate and fuse the lipid membranes comes from ATP. The fusion of the membranes creates a fusion pore, which allows the contents of the vesicle to be expelled as the vesicle becomes part of the cell membrane. In kiss-and-run fusion, the vesicle temporarily fuses with the cell membrane long enough to create a fusion pore and release its contents to the exterior of the cell. The vesicle then pulls away from the cell membrane and reforms before returning to the interior of the cell.

Exocytosis in the Pancreas

Exocytosis is used by a number of cells in the body as a means of transporting proteins and for cell to cell communication. In the pancreas, small clusters of cells called islets of Langerhans produce the hormones insulin and glucagon. These hormones are stored in secretory granules and released by exocytosis when signals are received.

When glucose concentration in the blood is too high, insulin is released from islet beta cells causing cells and tissues to take up glucose from the blood. When glucose concentrations are low, glucagon is secreted from islet alpha cells. This causes the liver to convert stored glycogen to glucose. Glucose is then released into the blood causing blood-glucose levels to rise. In addition to hormones, the pancreas also secretes digestive enzymes (proteases, lipases, amylases) by exocytosis.

Exocytosis in Neurons

Synaptic vesicle exocytosis occurs in neurons of the nervous system. Nerve cells communicate by electrical or chemical (neurotransmitters) signals that are passed from one neuron to the next. Neurotransmitters are transmitted by exocytosis. They are chemical messages that are transported from nerve to nerve by synaptic vesicles. Synaptic vesicles are membranous sacs formed by endocytosis of the plasma membrane at pre-synaptic nerve terminals.

Once formed, these vesicles are filled with neurotransmitters and sent toward an area of the plasma membrane called the active zone. The synaptic vesicle awaits a signal, an influx of calcium ions brought on by an action potential, which allows the vesicle to dock at the pre-synaptic membrane. Actual fusion of the vesicle with the pre-synaptic membrane does not occur until a second influx of calcium ions occurs.

After receiving the second signal, the synaptic vesicle fuses with the pre-synaptic membrane creating a fusion pore. This pore expands as the two membranes become one and the neurotransmitters are released into the synaptic cleft (gap between the pre-synaptic and post-synaptic neurons). The neurotransmitters bind to receptors on the post-synaptic neuron. The post-synaptic neuron may either be excited or inhibited by the binding of the neurotransmitters.

Exocytosis versus Endocytosis

While exocytosis is a form of active transport that moves substances and materials from a cell's interior to the exterior of the cell, endocytosis, is the mirror opposite. In endocytosis, substances and materials that are outside of a cell are transported into the interior of the cell. Like exocytosis, endocytosis requires energy so is also a form of active transport.

Like exocytosis, endocytosis has several different types. The different types are similar in that the basic underlying process involves the plasma membrane forming a pocket or invagination and surrounding the underlying substance that needs to be transported into the cell. There are three major types of endocytosis: phagocytosis, pinocytosis, as well as receptor mediated endocytosis.

Sources

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• _{“Exocytosis.” New World Encyclopedia, Paragon House Publishers, www.newworldencyclopedia.org/entry/Exocytosis.}
• Reece, Jane B., and Neil A. Campbell. Campbell Biology. Benjamin Cummings, 2011.
• _{Südhof, Thomas C., and Josep Rizo. “Synaptic Vesicle Exocytosis.” Cold Spring Harbor Perspectives in Biology, U.S. National Library of Medicine, 1 Dec. 2011, www.ncbi.nlm.nih.gov/pmc/articles/PMC3225952/.}