Monoamine transporters membrane proteins

The norepinephrine transporter (NET) and the vesicular monoamine transporter (VMAT) are presynaptic components of the sympathetic neurons. NET is a Na+ /Cl -dependent transport protein and responsible for the neurotransmitter uptake from the synaptic cleft into the cytoplasm of the neurons. This transport process, called uptake-1, reduces the amount and, thus, the effect of NE released into the synaptic cleft. NE is stored in the cytoplasm of the neurons in specialized vesicles by the H+-dependent transport protein VMAT. Two isoforms VMAT1 and VMAT2, are known. VMAT is localized in the vesicle membranes, and the vesicular storage protects NE from metabolism by monoamine oxidase (MAO), which is localized on the surface membrane of the mitochondria. Vice versa, nerve depolarisation causes NE release from the vesicles into the synaptic cleft by Ca+-mediated exocytose (Fig. 12) [79,132-136],... 

Molecular cloning has identified two closely related but distinct vesicular monoamine transporters, VMAT1 and VMAT2 (Liu et al., 1992 Erickson et al., 1992 Liu and Edwards, 1997). Sequence analysis predicts 12 transmembrane domains with N- and C-termini in the cytoplasm, and the proteins show no sequence similarity to plasma membrane monoamine transporters (Nguyen et al., 1983 Neal and Chater, 1987 Neyfakh et al., 1991). Rather, they show similarity to bacterial proteins involved in detoxification (Figure 2). These bacterial proteins all function as H+ exchangers, and several are even inhibited by the same drugs that inhibit VMATs (e.g., reserpine). 

Jayanthi LD, Ramamoorthy S (2005) Regulation of monoamine transporters influence of psychostimulants and therapeutic antidepressants. AAPS J 7 E728-E738 Jayanthi LD, Samuvel DJ, Ramamoorthy S (2004) Regulated intemahzation and phosphorylation of the native norepinephrine transporter in response to phorbol esters. Evidence for localization in Upid rafts and Upid raft-mediated internalization. J Biol Chem 279 19315-19326 Jess U, Betz H, Schloss P (1996) The membrane-bound rat serotonin transporter, SERTl, is an oligomeric protein. FEBS Letters 394 44 6... 

Torres GE, Gainetdinov RR, Caron MG (2003a) Plasma membrane monoamine transporters structure, regulation and function. Nat Rev Neurosci 4 13-25 Vaughan RA, Huff RA, Uhl GR, Kuhar MJ (1997) Protein kinase C-mediated phosphorylation and functional regulation of dopamine transporters in striatal synaptosomes. J Biol Chem 272 15541-15546... 

Torres GE, Yao WD, Mohn AR, Quan H, Kim KM, et al. 2001. Functional interaction between monoamine plasma membrane transporters and the synaptic PDZ domain-containing protein PICK1. Neu-ron 30 121-34... 

The monoamine transporter protein (molecular weights, 60-80 kDa) is a string of amino acids that weaves in and out of the presynaptic membrane 12 times (Fig. 21.4)—that is, 12 transmembrane domains (TMs) with a large extracellular loop between TM3 and TM4. Both the N- and C-termini of the transporters are located within the cytoplasm. There are six potential sites of phosphorylation by protein kinase A and protein kinase C, which regulate the transporters. The large extracellular loop and the cytoplasmic parts of the N- and C-termini do not appear to be the target sites for the transporter inhibitors (i.e., antidepressants). Rather, the areas important for selective monoamine... 

After release from storage sites in presynaptic monoamine transporter vesides, DA is bound by postsynaptic DA receptors, and unbound DA is broken down by enzymes, or taken back into presynaptic neurons, called reuptake into presynaptic neurons by spedfic presynaptic protein transporters. The uptake of neurotransmitters by presynaptic neurons is a major mechanism for stopping neurotransmission. The recyded neurotransmitter is repackaged in vesicles in presynaptic neurons, and then rdeased again in response to stimulation by presynaptic electrical action potentials. Tracers have been developed to assess the vesicular membranes, as well as the specific reuptake protein transporters. The selectivity with which monoaminergic neurons store DA, 5-HT, NER, or histamine in presynaptic vesides depends on the spedficity of the membrane transporters. 

The exocytotic release of neurotransmitters from synaptic vesicles underlies most information processing by the brain. Since classical neurotransmitters including monoamines, acetylcholine, GABA, and glutamate are synthesized in the cytoplasm, a mechanism is required for their accumulation in synaptic vesicles. Vesicular transporters are multitransmembrane domain proteins that mediate this process by coupling the movement of neurotransmitters to the proton electrochemical gradient across the vesicle membrane. 

Many neurotransmitters are inactivated by a combination of enzymic and non-enzymic methods. The monoamines - dopamine, noradrenaline and serotonin (5-HT) - are actively transported back from the synaptic cleft into the cytoplasm of the presynaptic neuron. This process utilises specialised proteins called transporters, or carriers. The monoamine binds to the transporter and is then carried across the plasma membrane it is thus transported back into the cellular cytoplasm. A number of psychotropic drugs selectively or non-selectively inhibit this reuptake process. They compete with the monoamines for the available binding sites on the transporter, so slowing the removal of the neurotransmitter from the synaptic cleft. The overall result is prolonged stimulation of the receptor. The tricyclic antidepressant imipramine inhibits the transport of both noradrenaline and 5-HT. While the selective noradrenaline reuptake inhibitor reboxetine and the selective serotonin reuptake inhibitor fluoxetine block the noradrenaline transporter (NAT) and serotonin transporter (SERT), respectively. Cocaine non-selectively blocks both the NAT and dopamine transporter (DAT) whereas the smoking cessation facilitator and antidepressant bupropion is a more selective DAT inhibitor. 

FIGURE 23.7 Dopamine (DA) is synthesized within neuronal terminals from the precursor tyrosine by the sequential actions of the enzymes tyrosine hydroxylase, producing the intermediary L-dihydroxyphenylalanine (Dopa), and aromatic L-amino acid decarboxylase. In the terminal, dopamine is transported into storage vesicles by a transporter protein (T) associated with the vesicular membrane. Release, triggered by depolarization and entry of Ca2+, allows dopamine to act on postsynaptic dopamine receptors (DAR). Several distinct types of dopamine receptors are present in the brain, and the differential actions of dopamine on postsynaptic targets bearing different types of dopamine receptors have important implications for the function of neural circuits. The actions of dopamine are terminated by the sequential actions of the enzymes catechol-O-methyl-transferase (COMT) and monoamine oxidase (MAO), or by reuptake of dopamine into the terminal. 

Neurons in the central nervous system communicate by chemical transmission. Of relevance to the present discussion are monoamine neurons that release dopamine, norepinephrine, or serotonin as one of their transmitters in response to an action potential. Reuptake transporter proteins embedded in the neuronal plasma membrane then clear the synapse of monoamines, typically taking up 70-80%) of the released transmitter. This reuptake is thought to be the major termination mechanism for the monoamine chemical signaling process. 

G protein—coupled receptors and nicotinic ligand-gated ion-channel receptors. Unlike monoamines that undergo reuptake by membrane-bound transporters, ACh is not returned to the presynaptic terminal but is rather readily metabolized by acetylcholinesterase (AChE) into choline and acetate. Once hydrolyzed from ACh, choline can be recycled back into the nerve terminal by choline uptake systems for further ACh bios)mthesis. 

Psychostimulant pharmacology involves an augmented release of noradrenaline and dopamine into the synaptic cleft. The presence of monoamine neurotransmitters in the synaptic cleft is regulated in a variety of ways in response to an action potential. Around 70-80% of all transmitter molecules return into the presynaptic neuron by uptake using an energy-dependent reuptake transport protein that is embedded in the neuronal membrane. A smaller fraction... 

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