Exocytosis

The process by which the contents of secretory vesicles are delivered to the extracellular fluid is called exocytosis. Depending upon the type of cell, exocytosis may occur over the entire surface or be localized to a small discrete area. For example, nerve cells release their neurotransmitters only at synapses that comprise only a tiny fraction of their surface area. When an appropriate signal is received by the secretory cell, storage granules move to the cell surface generally triggered by an increased concentration of cytosolic calcium, and their surrounding membranes fuse with the plasma membrane. The area of fusion then breaks down and opens the storage vesicle to the extracellular fluid (Fig. 23). Membranes that surrounded secretory vesicles before fusion are retrieved from the plasma membrane by endocytosis. In some cells, particularly in nerve endings, these fragments of membrane are reformed into new secretory vesicles, refilled with signal molecules, and recycled many times over. In addition to providing some economy, reuptake of vesicular membranes or their equivalent prevents expansion of the cell surface area that would otherwise occur as secretory vesicles repeatedly fuse with the cell membrane.

During exocytosis, the entire contents of the secretory vesicle are released, including signal molecules, precursors, byproducts, and processing enzymes. In some instances (e.g., adrenal medullary cells, pituitary corti-cotropes), more than one of these products may have biological activity. Although one normally thinks of this process in terms of secretion of signal molecules, occasionally the processing enzyme is the critical secretory product. For example, the proteolytic enzyme renin is released by the kidney when blood volume decreases (see Chapter 29 and 40). Renin catalyzes the rate-determining step in the formation of the hormone angiotensin from its precursor, which circulates in the blood.

The cellular events that control exocytosis are still incompletely understood, but it has been recognized for many years that an increase in the concentration of free calcium in the cytosol is essential both for fusion of the vesicular membrane with the plasma membrane and for extrusion of vesicular contents. Movement of secretory granules to peripheral areas of

FIGURE 23 Exocytosis. Secretory vesicles bud off the trans-Golgi compartment (1) and move into the cytosol (2), where they await a signal for secretion. Secretion (3) is usually accompanied by increased cellular calcium which causes elements of the cytoskeleton to translocate secretory granules to the cell surface (4). The membrane surrounding the granule fuses with the plasma membrane, opening the secretory vesicle to the extracellular fluid and releasing the processed protein(s) along with enzymes and peptide fragments.

FIGURE 23 Exocytosis. Secretory vesicles bud off the trans-Golgi compartment (1) and move into the cytosol (2), where they await a signal for secretion. Secretion (3) is usually accompanied by increased cellular calcium which causes elements of the cytoskeleton to translocate secretory granules to the cell surface (4). The membrane surrounding the granule fuses with the plasma membrane, opening the secretory vesicle to the extracellular fluid and releasing the processed protein(s) along with enzymes and peptide fragments.

the cell is a necessary prelude to fusion and probably also requires calcium. Positioning of granules in the submembranous zone usually occurs before the actual secretory event. Some of the calcium-activated molecules and the proteins in the vesicular and plasma membranes that enable docking and fusion to occur have been identified, but a discussion of this complex topic is beyond the scope of this text; the interested student is encouraged to consult recent texts of cell biology.

Calcium that triggers secretion is released from intracellular organelles or gains access to the cytosol through calcium channels in the plasma membrane. In nerves and some glandular cells, the arrival of an electrical signal, the action potential, opens voltage-sensitive calcium channels. In some cells, calcium channels may open when they are phosphorylated by PKA or when they interact with an activated G protein.

These processes were considered above in the discussion of the molecular events set in motion after a signal molecule interacts with a receptor. Secretion, after all, is one of the common responses of cells to stimulation by a signal molecule.

It is obvious that synthesis must be coupled in some way with secretion, as cells rarely exhaust their supply of secretory product. It is likely that the same events that initiate secretion simultaneously initiate synthesis. It is also possible that cells have some mechanism for monitoring how much signal is stored and synthesize signal molecules when storage depots fall below some threshold level.

Clearance of Chemical Messages

For a molecule to function as a signal, it must be present in the environment of the target cell only during periods of information transfer. If the signal were not eliminated once the message was received, the target cell would remain in a state of constant activation or inhibition or would become exhausted and unresponsive to subsequent stimulation. Thus, elimination of signal molecules is as important as their secretion, and elaborate mechanisms have evolved to accomplish this task. In some cases effective signaling may be terminated simply by dilution followed by elimination at a distant site.

In general, signal molecules interact reversibly with their receptors and little is consumed or transformed in delivering their message. Target cells may take up signal molecules bound to their receptors (receptor-mediated endocytosis), separate signal from receptor in the endosomes, and then deliver the signal to the lysosomes for destruction and the receptor back to the cell membrane. Target cells may also produce hydrolytic enzymes on their surface in the vicinity of their receptors (e.g., acetylcholin esterase at the neuromuscular junction; see Chapter 6), thus efficiently destroying signal molecules that are not bound to receptors. One economical way to eliminate signal molecules is for the secreting cell to take them up again and repackage them in secretory vesicles for future use. This mechanism is utilized by terminals of sympathetic nerves and neurons in the brain. Most target cells rely on dilution of signal molecules in the large volume of extracellular fluid as they diffuse away from their receptors after the message has been delivered. Signal molecules are then destroyed by enzymes in extracellular fluid or blood or by specialized organs such as liver or kidney which alter the signals chemically and excrete the degraded products. Signals may be totally degraded or simply transformed so that they are no longer recognizable by receptors in target cells.

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