Exocytosis

Information about Exocytosis

Neuron A (transmitting) to neuron B (receiving)
1. Mitochondria
2. synaptic vesicle with neurotransmitters
3. Autoreceptor
4. Synapse with neurotransmitter released (serotonin)
5. Postsynaptic receptors activated by neurotransmitter (induction of a postsynaptic potential)
6. Calcium channel
7. Exocytosis of a vesicle
8. Recaptured neurotransmitter

Illustration of an axon releasing dopamine.

Exocytosis (ek-soh-sy-TOH-sis) is the process by which a cell directs secretory vesicles to the cell membrane. These membrane-bound vesicles contain soluble proteins to be secreted to the extracellular environment, as well as membrane proteins and lipids that are sent to become components of the cell membrane.

Types

In multicellular organisms there are two types of exocytosis: 1) Ca²⁺ triggered non-constitutive and 2) non Ca²⁺ triggered constitutive. Exocytosis in neuronal chemical synapses is Ca²⁺ triggered and serves interneuronal signalling. Constitutive exocytosis is performed by all cells and serves the release of components of the extracellular matrix, or just delivery of newly-synthesized membrane proteins that are incorporated in the plasma membrane after the fusion of the transport vesicle.

Functions

Exocytosis is needed by cells for secretion of proteins like enzymes, peptide hormones and antibodies from cells, turnover of plasma membrane, release of neurotransmitter from presynaptic neurons, placement of integral membrane proteins, acrosome reaction during fertilization, antigen presentation during the immune response and recycling of plasma membrane bound receptors.

Exocytosis is important in cellular signaling. In neuronal communication both chemical and electrical information needs to be sent throughout the cell. Exocytosis sends and converts the electrical information into chemical information. Within the neural cells, the information is electrical. In the synapse, after exocytosis has occurred, the neurotransmitters are released, and the information is chemical. The release of the neurotransmitter can either be excitatory (causing activity from the target cell) or inhibitory (preventing activity by the target cell).

Steps

Five steps are involved in exocytosis:

Vesicle trafficking

Certain vesicle-trafficking steps require the translocation of a vesicle over a significant distance. For example, vesicles that carry proteins from the Golgi apparatus to the cell surface are likely to use motor proteins and a cytoskeletal track to get close to their target before tethering would be appropriate. Both the actin- and the microtubule-based cytoskeletons are implicated in these processes, along with several motor proteins. Once the vesicles reach their targets, they come into contact with tethering factors that can restrain them.

Vesicle tethering

It is useful to distinguish between the initial, loose tethering of vesicles with their targets from the more stable, docking interactions. Tethering involves links over distances of more than about half the diameter of a vesicle from a given membrane surface (>25 nm). Tethering interactions are likely to be involved in concentrating synaptic vesicles at the synapse.

Vesicle docking

The term docking refers to the holding of two membranes within a bilayer's distance of one another (<5-10 nm). Stable docking probably represents several distinct, molecular states: the molecular interactions underlying the close and tight association of a vesicle with its target may include the molecular rearrangements needed to trigger bilayer fusion. A common feature of many proteins that function in vesicle tethering and docking is their propensity to form highly extended, coiled-coil structures. Tethering and docking of a transport vesicle at the target membrane precedes the formation of a tight core SNARE complex.

Vesicle priming

In neuronal exocytosis, the term priming has been used to include all of the molecular rearrangements and ATP-dependent protein and lipid modifications that take place after initial docking of a synaptic vesicle but before exocytosis, such that the influx of calcium ions is all that is needed to trigger nearly instantaneous neurotransmitter release. In other cell types, whose secretion is constitutive (i.e. continuous, calcium ion independent, non-triggered) there is no priming.

Vesicle fusion

The vesicle fusion is driven by SNARE proteins process of merging the vesicle membrane with the target one resulting in release of large biomolecules in the extracellular space (or in case of neurons in the synaptic cleft).

The merging of the donor and the acceptor membranes accomplishes three tasks:

*The surface of the plasma membrane increases (by the surface of the fused vesicle). This is important for the regulation of cell size, e.g., during cell growth.

*The substances within the vesicle are released into the exterior. These might be waste products or toxins, or signalling molecules like hormones or neurotransmitters during synaptic transmission.

*Proteins embedded in the vesicle membrane are now part of the plasma membrane. The side of the protein that was facing the inside of the vesicle now faces the outside of the cell. This mechanism is important for the regulation of transmembrane receptors and transporters.

External links

MeSH Exocytosis

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Proteins are large organic compounds made of amino acids arranged in a linear chain and joined together by peptide bonds between the carboxyl and amino groups of adjacent amino acid residues.
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A membrane protein is a protein molecule that is attached to, or associated with the membrane of a cell or an organelle. More than half of all proteins interact with membranes.
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Lipids can be broadly defined as any fat-soluble (hydrophobic), naturally-occurring molecules. The term is more-specifically used to refer to fatty-acids and their derivatives (including tri-, di-, and monoglycerides and phospholipids) as well as other fat-soluble sterol-containing
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synapse. Synapses allow nerve cells to communicate with one another through axons and dendrites, converting electrical impulses into chemical signals.]]

Chemical synapses
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extracellular matrix (ECM) is the extracellular part of animal tissue that usually provides structural support to the cells in addition to performing various other important functions. The extracellular matrix is the defining feature of connective tissue in animals.
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vesicle is a relatively small and enclosed compartment, separated from the cytosol by at least one lipid bilayer. If there is only one lipid bilayer, they are called unilamellar vesicles; otherwise they are called multilamellar.
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Secretion is the process of segregating, elaborating, and releasing chemicals from a cell, or a secreted chemical substance or amount of substance.

Eukaryotic cells have a highly evolved process of secretion.
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Enzymes are proteins that catalyze (i.e. accelerate) chemical reactions.^[1] In enzymatic reactions, the molecules at the beginning of the process are called substrates, and the enzyme converts them into different molecules, the products.
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Peptide hormones are a class of peptides that are secreted into the blood stream and have endocrine functions in living animals.

Like other proteins, peptide hormones are synthesized from amino acids according to an mRNA template, which is itself synthesized from a DNA
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Antibodies (also known as immunoglobulins) are proteins that are found in blood or other bodily fluids of vertebrates, and are used by the immune system to identify and neutralize foreign objects, such as bacteria and viruses.
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Neurotransmitters are chemicals that are used to relay, amplify and modulate signals between a neuron and another cell. According to the prevailing beliefs of the 1960s, a chemical can be classified as a neurotransmitter if it meets the following conditions:

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An antigen or immunogen is a molecule that stimulates an immune response. The word originated from the notion that they can stimulate antibody generation. We now know that the immune system does not only consist of antibodies.
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Molecular motors are biological molecular machines that are the essential agents of movement in living organisms. Generally speaking, a motor may be defined as a device that consumes energy in one form and converts it into motion or mechanical work; for example, many protein-based
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Snare may mean:

A kind of trap used in trapping.
A kind of drum: see snare drum.
SNAREs, a family of proteins involved in vesicle fusion.
The Snares, a group of islands located approximately 200 kilometres south of New Zealand's South Island.

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Snare may mean:

A kind of trap used in trapping.
A kind of drum: see snare drum.
SNAREs, a family of proteins involved in vesicle fusion.
The Snares, a group of islands located approximately 200 kilometres south of New Zealand's South Island.

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toxin (Greek: τοξικόν, toxikon, lit. (poison) for use on arrows) is a poisonous substance produced by living cells or organisms.
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hormone (from Greek όρμή - "to set in motion") is a chemical messenger that carries a signal from one cell (or group of cells) to another. All multicellular organisms produce hormones (including plants - see phytohormone).
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For other uses, see Receptor.

In biochemistry, a receptor is a protein on the cell membrane or within the cytoplasm or cell nucleus that binds to a specific molecule (a ligand), such as a neurotransmitter, hormone, or other substance, and initiates
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Medical Subject Headings (MeSH) is a huge controlled vocabulary (or metadata system) for the purpose of indexing journal articles and books in the life sciences. Created and updated by the United States National Library of Medicine (NLM), it is used by the MEDLINE/PubMed
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The secretory pathway is a series of steps a cell uses to move proteins out of the cell; a process known as secretion. The path of a protein destined for secretion has its origins in the rough endoplasmic reticulum, a membrane bound compartment in the cell.
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Herod_Archelaus

• • [ e] Membrane transport
Passive transport	Diffusion - Facilitated diffusion - Osmosis
Active transport	Primary active transport - Secondary active transport
-cytosis	Exocytosis - Endocytosis (Phagocytosis, Pinocytosis)

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Exocytosis

Information about Exocytosis

Types

Functions

Steps

Vesicle trafficking

Vesicle tethering

Vesicle docking

Vesicle priming

Vesicle fusion

External links

See also