Exocytosis vesicles

Anlauf, E., and Derouiche, A. (2005). Astrocytic exocytosis vesicles and glutamate A high-resolution immunofluorescence study. Glia 49, 96-106. 

In this lecture we will be concerned by exocytosis of neurotransmitters by chromaffin cells. These cells, located above kidneys, produce the adrenaline burst which induces fast body reactions they are used in neurosciences as standard models for the study of exocytosis by catecholaminergic neurons. Prior to exocytosis, adrenaline is contained at highly concentrated solutions into a polyelectrolyte gel matrix packed into small vesicles present in the cell cytoplasm and brought by the cytoskeleton near the cell outer membrane. Stimulation of the cell by divalent ions induces the fusion of the vesicles membrane with that of the cell and hence the release of the intravesicular content into the outer-cytoplasmic region. 

During exocytosis, intracellular vesicles fuse with the plasmalemma. As a consequence, the vesicle components are incoiporated into the plasma membrane and the vesicle content is released into the extracellular space. We distinguish constitutive and regulated exocytosis. 

Constitutive exocytosis/secretion takes place in all eukaryotic cells and is essential for cell viability and growth. Trafficking vesicles destined for constitutive exocytosis originate from the trans-Golgi-network and contain secretory macromolecules derived from the... 

Neurotransmitter Transporters. Figure 3 Dopamine turnover at a presynaptic nerve terminal, (a) Dopamine is produced by tyrosine hydroxylase (TH). When secretory vesicles are filled, they join the releasable pool of vesicles at the presynaptic membrane. Upon exocytosis, the diffusion of released dopamine is limited by reuptake via DAT. (b) If DAT is inactive, dopamine spreads in the cerebrospinal fluid but cannot accumulate in secretory vesicles. This results in a compensatory increase of dopamine hydroxylase activity and a higher extracellular dopamine level mice with inactive DAT are hyperactive. 

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. 

After exocytosis, the vesicle membrane with its lipids and proteins is recycled, either by immediate re-filling with transmitter or by passing through a vesicle resting pool deeper inside the axon terminal. 

Synaptic vesicles are the organelles in axon terminals that store neurotransmitters and release them by exocytosis. There are two types, the large dense-core vesicles, diameter about 90 nm, that contain neuropeptides, and the small synaptic vesicles, diameter about 50nm, that contain non-peptide transmitters. About ten vesicles per synapse are docked to the plasma membrane and ready for release, the readily releasable pool . Many more vesicles per synapse are stored farther away from the plasma membrane, the resting pool . When needed, the latter vesicles may be recruited into the readily releasable pool. Neuronal depolarization and activation of voltage-sensitive Ca2+... 

As plant cells grow, they deposit new layers of cellulose external to the plasma membrane by exocytosis. The newest regions, which are laid down successively in three layers next to the plasma membrane, are termed the secondary cell wall. Because the latter varies in its chemical composition and structure at different locations around the cell, Golgi-derived vesicles must be guided by the cytoskeleton... 

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. 

Zenisek D, Steyer JA, Eeldman ME, Aimers W (2002) A membrane marker leaves synaptic vesicles in nulliseconds after exocytosis in retinal bipolar cells. Neuron 35 1085-1097 Zhang L, He T, Talal A, Wang G, Frankel SS, Ho DD (1998) In vivo distribution of the human immunodeficiency virus/simian immunodeficiency virus coreceptors CXCR4, CCR3, and CCR5. J Virol 72 5035-5045... 

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... 

Studies of release of noradrenaline from sympathetic neurons provided the first convincing evidence that impulse (Ca +)-dependent release of any transmitter depended on vesicular exocytosis. Landmark studies carried out in the 1960s, using the perfused cat spleen preparation, showed that stimulation of the splenic nerve not only led to the detection of noradrenaline in the effluent perfusate but the vesicular enzyme, DpH, was also present. As mentioned above, this enzyme is found only within the noradrenaline storage vesicles and so its appearance along with noradrenaline indicated that both these factors were released from the vesicles. By contrast, there was no sign in the perfusate of any lactate dehydrogenase, an enzyme that is found only in the cell cytosol. The processes by which neuronal excitation increases transmitter release were described in Chapter 4. 

---