Exocytosis mechanism

It is believed that HA is the earliest evolutional form of glycosaminoglycans. Contrary to HA, other glycosaminoglycans are synthesized in the Golgi apparatus. They covalently bond with proteins to form the proteoglycans, which further transfer into the extracellular matrix by the exocytosis mechanism [41]. Three forms of hyaluronan synthase (HASj, HAS, HAS3) are found in human and vertebrate bodies. The enzyme HAS performs slow synthesis of high molecular weight HA, the enzyme... 

It is interesting that the enzymes responsible for the synthesis of the main extracellular polysaccharides (i.e. hyaluronan and cellulose) are localized in the cytoplasmic ceU membrane. During the synthesis, a growing polymer chain diffuses through the manbrane directly into extracellular space. Such a synthesis route is apparently more ancient and differs from the synthesis of other extracellular polysaccharides that are synthesized inside the cell and transported into extracellular media by the exocytosis mechanism. 

The LC then targets and cleaves one or more of three soluble NSF attachment protein receptor (SNARE) proteins with exquisite specificity synaptosomal-associated protein 25 (SNAP-25 BoNT/A, /C, /E) vesicle associated membrane protein 1-3 (VAMPl-3 BoNT-/B, /D, /F, /G) or syntaxin (BoNT/C). The SNARE proteins are essential components of the synaptic exocytosis mechanism, and their cleavage prevents functional assembly of the ternary complex, thereby blocking neurotransmitter release. The combination of efficient neuronal targeting and presynaptic activation renders BoNTs the most potent substances known, with estimated human lethal doses as low as 0.1-1 ng/kg. 

A second mechanism that impinges on the localization of transporters is through the association with proteins, the most prominent example being syntaxin. Syntaxin is a t-SNARE protein necessary for the fusion of vesicles with the plasma membrane (see the chapter on exocytosis). On the cell surface syntaxin consistently stabilizes the localization of GABA, noradrenaline, glycine, and 5HT transporters the PKCa isoform can sever the interaction with syntaxin suggesting a general mechanism for transporter internalization. 

Protein trafficking is the transport of proteins to their correct subcellular compartments or to the extracellular space ( secretory pathway ). Endo- and exocytosis describe vesicle budding and fusion at the plasma membrane and are by most authors not included in the term protein trafficking. Protein quality control comprize all cellular mechanisms, monitoring protein folding and detecting aberrant forms. 

Silver RB, Mackins CJ, Smith NCE, Koritchneva IL, Lefkowitz K, Lovenberg TW> Levi R Coupling of histamine H3 receptors to neuronal Na+/H+ exchange a novel protective mechanism in myocardial ischemia, Proc Natl Acad Sci USA 2001 98 2855. Silver RB, Poonwasi KS, Seyedi N, Wilson SJ, Lovenberg TW, Levi R Decreased intracellular calcium mediates the histamine H3-receptor-induced attenuation of norepinephrine exocytosis from cardiac sympathetic nerve endings. Proc Natl Acad Sci USA 2002 99 501. 

Most cells release macromolecules to the exterior by exocytosis. This process is also involved in membrane remodeling, when the components synthesized in the Colgi apparatus are carried in vesicles to the plasma membrane. The signal for exocytosis is often a hormone which, when it binds to a cell-surface receptor, induces a local and transient change in Ca concentration. Ca triggers exocytosis. Figure 41—16 provides a comparison of the mechanisms of exocytosis and endocytosis. 

Figure 41-16. A comparison of the mechanisms of endocytosis and exocytosis. Exocytosis invoives the contact of two inside surface (cytopiasmic side) mono-iayers, whereas endocytosis resuits from the contact of two outer surface mono-iayers.

Large molecules can enter or leave cells through mechanisms such as endocytosis or exocytosis. These processes often require binding of the molecule to a receptor, which affords specificity to the process. 

Nonmuscle cells perform mechanical work, including self-propulsion, morphogenesis, cleavage, endocytosis, exocytosis, intracellular transport, and changing cell shape. These cellular functions are carried out by an extensive intracellular network of filamentous structures constimting the cytoskeleton. The cell cytoplasm is not a sac of fluid, as once thought. Essentially all eukaryotic cells contain three types of filamentous struc-mres actin filaments (7-9.5 nm in diameter also known as microfilaments), microtubules (25 nm), and intermediate filaments (10-12 nm). Each type of filament can be distinguished biochemically and by the electron microscope. 

Mechanisms of Gliotransmitters Release Evidence on CXCR4-Mediated Glutamate Exocytosis from Astrocytes... 

Apart from actually demonstrating release it is important to consider how NTs are released and whether they all need to be released in the same way, especially if they do different things. The variable time-courses of NT action referred to previously may require NTs to be released at different rates and in different ways, only some of which are achievable by, or require, vesicular mechanisms and exocytosis (see Chapter 4). 

There are several ways in which activation of auto- or heteroceptors on nerve terminals could modify the amount of transmitter released by exocytosis. The fact that this will depend on the influence of second messengers is beyond doubt. What remains to be resolved is whether one mechanism is more important than the others, or whether this varies from tissue to tissue. 

Taking ai-adrenoceptors as an example, several possible mechanisms have been suggested (see Starke 1987). The first rests on evidence that these autoreceptors are coupled to a Gi (like) protein so that binding of an a2-adrenoceptor agonist to the receptor inhibits the activity of adenylyl cyclase. This leads to a fall in the synthesis of the second messenger, cAMP, which is known to be a vital factor in many processes involved in exocytosis. In this way, activation of presynaptic a2-adrenoceptors could well affect processes ranging from the docking of vesicles at the active zone to the actual release process itself... 

Alternative mechanisms are equally likely. One possibility arises from evidence that activation of a2-adrenoceptors reduces Ca + influx this will have obvious effects on impulse-evoked exocytosis. In fact, the inhibition of release effected by a2-adrenoceptor agonists can be overcome by raising external Ca + concentration. Finally, an increase in K+ conductance has also been implicated this would hyperpolarise the nerve terminals and render them less likely to release transmitter on the arrival of a nerve impulse. Any, or all, of these processes could contribute to the feedback inhibition of transmitter release. Similar processes could explain the effects of activation of other types of auto-or heteroceptors. 

Figure 5.1 Mechanism of action at a chemical synapse. The arrival of an action potential at the axon terminal causes voltage-gated Ca++ channels to open. The resulting increase in concentration of Ca++ ions in the intracellular fluid facilitates exocytosis of the neurotransmitter into the synaptic cleft. Binding of the neurotransmitter to its specific receptor on the postsynaptic neuron alters the permeability of the membrane to one or more ions, thus causing a change in the membrane potential and generation of a graded potential in this neuron.

Fig. 11.4. Model for cholinergic signalling in the intestinal mucosa, providing a possible rationale for AChE secretion by parasitic nematodes. ACh released from enteric cholinergic motor neurons stimulates chloride secretion, mucus secretion and Paneth cell exocytosis through muscarinic receptors. Secretory responses may be modulated by mast cell mediators, either directly or via the induction of neural reflex programmes. The role of muscarinic receptor-positive cells in the lamina propria of rats infected with N. brasiliensis is undetermined, as are potential mechanisms of trans-epithelial transport of the enzymes. Adapted from Cooke (1984).

Figure 4.6 Likely mechanisms by which macromolecules cross cellular barriers in order to reach the bloodstream from (in this case) the lung. Transcytosis entails direct uptake of the macromolecule at one surface via endocytosis, travel of the endosome vesicle across the cell, with subsequent release on the opposite cell face via exocytosis. Paracellular transport entails the passage of the macromolecules through leaky tight junctions found between some cells...

Mechanical functions of cells require interactions between integral membrane proteins and the cyto-skeleton. These functions include organization of signaling cascades, formation of cell junctions and regulation of cell shape, motility, endo- and exocytosis. Several different families of membrane-associated proteins mediate specific interactions among integral membrane proteins, cytoskeletal proteins and contractile proteins. Many of these linker proteins consist largely of various combinations of conserved protein-association domains, which often occur in multiple variant copies. 

The constitutive pathway has not been studied as intensively as regulated secretion [54]. In particular, relatively little is known about targeting and regulatory mechanisms for these transport vesicles. Clathrin seems not to be directly involved in the constitutive secretory pathway. Antibodies that disrupt clathrin assembly in vitro inhibit endocytosis, but constitutive exocytosis is not affected [63]. 

Chemical transmission between nerve cells involves multiple steps 167 Neurotransmitter release is a highly specialized form of the secretory process that occurs in virtually all eukaryotic cells 168 A variety of methods have been developed to study exocytosis 169 The neuromuscular junction is a well defined structure that mediates the presynaptic release and postsynaptic effects of acetylcholine 170 Quantal analysis defines the mechanism of release as exocytosis 172 Ca2+ is necessary for transmission at the neuromuscular junction and other synapses and plays a special role in exocytosis 174 Presynaptic events during synaptic transmission are rapid, dynamic and interconnected 175... 

Quantal analysis defines the mechanism of release as exocytosis. Stimulation of the motor neuron causes a large depolarization of the motor end plate. In 1952, Fatt and Katz [11] observed that spontaneous potentials of approximately 1 mV occur at the motor endplate. Each individual potential change has a time course similar to the much larger evoked response of the muscle membrane that results from electrical stimulation of the motor nerve. These small spontaneous potentials were therefore called... 

The short delays between Ca2+ influx and exocytosis have important implications for the mechanism of fusion of synaptic vesicles (see Ch. 9). In this short time, a synaptic vesicle cannot move significant distances and must be already at the release site. From the diffusion constant of Ca2+ in squid axoplasm, one can calculate that Ca2+ could diffuse a distance of only 850 A, somewhat greater than the diameter of a synaptic vesicle. Therefore, in fast synapses, vesicle exocytosis sites must be close to the triggering Ca2+ channels.. Vesicles are exposed to [Ca2+] of a few hundred micromoles near the cytoplasmic mouth of the channels. 

Hormonal actions on target neurons are classified in terms of cellular mechanisms of action. Hormones act either via cell-surface or intracellular receptors. Peptide hormones and amino-acid derivatives, such as epinephrine, act on cell-surface receptors that do such things as open ion-channels, cause rapid electrical responses and facilitate exocytosis of hormones or neurotransmitters. Alternatively, they activate second-messenger systems at the cell membrane, such as those involving cAMP, Ca2+/ calmodulin or phosphoinositides (see Chs 20 and 24), which leads to phosphorylation of proteins inside various parts of the target cell (Fig. 52-2A). Steroid hormones and thyroid hormone, on the other hand, act on intracellular receptors in cell nuclei to regulate gene expression and protein synthesis (Fig. 52-2B). Steroid hormones can also affect cell-surface events via receptors at or near the cell surface. 

---