Vesicles uptake

Fig. 7.4. Vesicle uptake and release mechanisms V, vesicle, V, vesicle coalesced with membrane.

The neurotransmitter phenotype, (i.e., what type of neurotransmitter is stored and ultimately will be released from the synaptic bouton) is determined by the identity of the neurotransmitter transporter that resides on the synaptic vesicle membrane. Although some exceptions to the rule may exist all synaptic vesicles of a given neuron normally will express only one transporter type and thus will have a dehned neurotransmitter phenotype (this concept is enveloped in what is known as Dale s principle see also Reference 19). To date, four major vesicular transporter systems have been characterized that support synaptic vesicle uptake of glutamate (VGLUT 1-3), GABA and glycine (VGAT), acetylcholine (VAChT), and monoamines such as dopamine, norepinephrine, and serotonin (VMAT 1 and 2). Vesicles that store and release neuropeptides do not have specific transporters to load and concentrate the peptides but, instead, are formed with the peptides already contained within. 

Whereas acetylcholine is degraded by a membrane-anchored acetylcholine esterase (ACE) in the synaptic cleft (choline is afterwards taken up presynaptically), the biogenic amines adrenaline, noradrenaline, serotonin, and dopamine are taken up by the presynaptic membrane by transporters (Fig. 3) or by extraneuronal cells in which they are degraded by a catecholamine O-methyltransferase (COMT). These transporter have similar structure and contain 12 transmembrane regions. Once in the presynapse, the neurotransmitters are either degraded by monoamine oxidase (MAO) or taken up by synaptic vesicles. A proton pumping ATPase of the vesicle membrane (V-type ATPase as in plant vacuoles) causes an increase of hydrogen ion concentrations in the vesicles. Uptake of the neurotransmitter serotonin, adrenaline and noradrenaline could be partly achieved either via a diffusion of the free base into the vesicles where they become protonated and concentrated by an "ion trap" mechanism and via specific proton-coupled antiporters. The excitatory amino acids, acetylcholine and ATP cannot diffuse and enter the vesicles via specific transporters. 

Some inhibitors of the vesicle uptake process are listed in Table IS. The most potent inhibitors are reserpine and related substances. Kinetic studies indicate that reserpine has an affinity for the vesicle uptake sites some 10,000 times higher than that of the catecholamines. Another compound with very high affinity is prenylamine (segontin), which is about 10 times less potent than reserpine. Imi-pramine and chlorpromazine are both active as inhibitors, but are very much less potent on the vesicle system than on the neuronal membrane uptake. Other inhibitors include substances, such as N-ethylmaleimide, which block free sulphydryl groups. The action of such compounds appears to be related to an inhibition of the storage vesicle ATP-ase (Para. 5.2.4). 

Uptake of radioactive norepinephrine in hypothalamic nerve endings and accumulation in dense-core vesicles Uptake of serotonin of the ventral part of the subarachnoid space ... 

Fass S. Hammerman, M. R., and Sacktor, B., 1977, Transport of amino acids in renal brush border membrane vesicles Uptake of the neutral amino acid L-alanine, /. Biol. Chem. 252 583. 

The neurotransmitter must be present in presynaptic nerve terminals and the precursors and enzymes necessary for its synthesis must be present in the neuron. For example, ACh is stored in vesicles specifically in cholinergic nerve terminals. It is synthesized from choline and acetyl-coenzyme A (acetyl-CoA) by the enzyme, choline acetyltransferase. Choline is taken up by a high affinity transporter specific to cholinergic nerve terminals. Choline uptake appears to be the rate-limiting step in ACh synthesis, and is regulated to keep pace with demands for the neurotransmitter. Dopamine [51 -61-6] (2) is synthesized from tyrosine by tyrosine hydroxylase, which converts tyrosine to L-dopa (3,4-dihydroxy-L-phenylalanine) (3), and dopa decarboxylase, which converts L-dopa to dopamine. 

The dopamine is then concentrated in storage vesicles via an ATP-dependent process. Here the rate-limiting step appears not to be precursor uptake, under normal conditions, but tyrosine hydroxylase activity. This is regulated by protein phosphorylation and by de novo enzyme synthesis. The enzyme requites oxygen, ferrous iron, and tetrahydrobiopterin (BH. The enzymatic conversion of the precursor to the active agent and its subsequent storage in a vesicle are energy-dependent processes. 

Similar to C1C-5, C1C-3 is present in endosomes. It is also found in synaptic vesicles. In both instances, and similar to C1C-5, it is necessary for the efficient intravesicular acidification. The acidification of synaptic vesicles is particularly important as their uptake of neurotransmitters depends on the electrochemical proton gradient. Surprisingly, the disruption of C1C-3 in mice resulted in a drastic degeneration of the hippocampus and the retina. Much less is known about C1C-4, which, however, also appears to be present in endosomal compartments. 

GLUT4 is a glucose transporter exclusively expressed in tissues with insulin-sensitive glucose uptake (heart, muscle, fat). Under basal conditions, GLUT4 is predominantly located in intracellular vesicles, and is... 

Transport supplies nerve cells with neurotransmitter, which is used to replenish secretory vesicles, thus uptake occurs on the cell surface and on the secretory vesicle. 

Tliere are examples that cells have an intracellular reserve of functional transporter moieties EAAC1, GAT1, CHT are predominantly - while NET only in some nerve cells - localized in the cell interior where transporters are stored membrane-bound in vesicles. Regulation occuts by transporter redistribution. Increased surface density leads to increased transport capacity while transporter internalization suppresses uptake. 

Synaptic Transmission. Figure 1 Synaptic transmission. The presynaptic terminal contains voltage-dependent Na Superscript and Ca2+ channels, vesicles with a vesicular neurotransmitter transporter VNT, a plasmalemmal neurotransmitter transporter PNT, and a presynaptic G protein-coupled receptor GPCR with its G protein and its effector E the inset also shows the vesicular H+ pump. The postsynaptic cell contains two ligand-gated ion channels LGIC, one for Na+ and K+ and one for Cl-, a postsynaptic GPRC, and a PNT. In this synapse, released transmitter is inactivated by uptake into cells. 

The vesicular monoamine transporters (VMATs) were identified in a screen for genes that confer resistance to the parkinsonian neurotoxin MPP+ [2]. The resistance apparently results from sequestration of the toxin inside vesicles, away from its primary site of action in mitochondria. In addition to recognizing MPP+, the transporter s mediate the uptake of dopamine, ser otonin, epinephrine, and norepinephrine by neurons and endocrine cells. Structurally, the VMATs show no relationship to plasma membrane monoamine transporters. 

Chromaffin granules, platelet dense core vesicles, and synaptic vesicles accumulate ATP. ATP uptake has been demonstrated using chromaffin granules and synaptic vesicles and the process appears to depend on A(.lh+. It has generally been assumed that ATP is costored only with monoamines and acetylcholine, as an anion to balance to cationic charge of those transmitters. However, the extent of ATP storage and release by different neuronal populations remains unknown, and the proteins responsible for ATP uptake by secretory vesicles have not been identified. 

Mayer, L. D., Bally, M. B., Hope, M. J., and Cullis, P. R. (1985b). Uptake of antineoplastic agents into large unilamellar vesicles in response to a membrane potential, Biochim. Biophys. Acta. 816. 294-302. 

Pinocytosis is a property of all cells and leads to the cellular uptake of fluid and fluid contents. There are two types. Fluid-phase pinocytosis is a nonselective process in which the uptake of a solute by formation of small vesicles is simply proportionate to its concentration in the surrounding extracellular fluid. The formation of these vesicles is an extremely active process. Fi-... 

The reaction of choline with mitochondrial bound acetylcoenzyme A is catalysed by the cytoplasmic enzyme choline acetyltransferase (ChAT) (see Fig. 6.1). ChAT itelf is synthesised in the rough endoplasmic reticulum of the cell body and transported to the axon terminal. Although the precise location of the synthesis of ACh is uncertain most of that formed is stored in vesicles. It appears that while ChAT is not saturated with either acetyl-CoA or choline its synthesising activity is limited by the actual availability of choline, i.e. its uptake into the nerve terminal. No inhibitors of ChAT itself have been developed but the rate of synthesis of ACh can, however, be inhibited by drugs like hemicholinium or triethylcholine, which compete for choline uptake into the nerve. 

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