Monoamine transporters proteins

Fig. 2. In vitro efficacy of compounds rac-Sa, rac-Sh, rac-6, and rac-1 regarding serotonin, noradrenaline, and dopamine reuptake inhibition. pICso denotes the negative decadic logarithm of the half-maximum effect concentration [M]. The monoamine reuptake inhibition profiles of rac-Sa, rac-5b, rac-6, and rac-1 were genoated via radioligand transporter assays using recombinant human monoamine transporter proteins. The data represent the mean of duplicate analyses.

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... 

Interestingly, we have recently identified a mutation of a tyrosine in the third intracellular loop of the hDAT that causes a major alteration in the conformational equilibrium of the transport cycle, and thus as such is comparable to mutants on G protein-coupled receptors causing constitutive isomerization of the receptor to the active state (66). Most importantly, this conclusion is based on the observation that mutation of the tyrosine completely reverts the effect of Zn2+ at the endogenous Zn2+ binding site in the hDAT (50,51) from potent inhibition of transport to potent stimulation of transport (Fig. 6). In the absence of Zn2+, transport capacity is reduced to less than 1% of that observed for the wild-type, however, the presence of Zn2+ in only micromolar concentrations causes a close to 30-fold increase in uptake (66). Moreover, it is found that the apparent affinities for cocaine and several other inhibitors are substantially decreased, whereas the apparent affinities for substrates are markedly increased (66). Notably, the decrease in apparent cocaine affinity was around 150-fold and thus to date the most dramatic alteration in cocaine affinity reported upon mutation of a single residue in the monoamine transporters (66). 

Molecular Neuropharmacology Strategies and Methods is intended to bridge the gap between molecular biology and advanced chemistry. In addition, it attempts to include information about x-ray crystallographic analyses whenever available. This book discusses interdisciplinary interactions for monoamine transporters, amino acid transporters, ionotropic receptors, metabotropic glutamate receptors, GABA receptors, and other G protein-coupled receptors. 

Gelernter J, Cubells JF, Kidd JR, Pakstis AJ, Kidd KK (1999) Population studies of polymorphisms of the serotonin transporter protein gene. Am J Med Genet 88 61-66 Hahn MK, Blakely RD (2002) Monoamine transporter gene structure and polymorphisms in relation to psychiatric and other complex disorders. Pharmacogenomics J 2 217-235 HeUs A, Teufel A, Petri S, et al (1996) Allelic variation of human serotonin transporter gene expression. J Neurochem 66 2621-2624... 

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],... 

MDR1, multidrug resistance protein-1 MRP1, multidrug resistance-associated protein 1 NET, norepinephrine transporter SERT, serotonin reuptake transporter VMAT, vesicular monoamine transporter. 

HTxR, serotonin receptor CB1R, cannabinoid-1 DAT, dopamine transporter GABA, y-aminobutyric acid Kir3 channels, G protein-coupled inwardly rectifying potassium channels LSD, lysergic acid diethylamide i -OR, H-opioid receptor nAChR, nicotinic acetylcholine receptor NET, norepinephrine transporter NMDAR, N -methyl-D-aspartate receptor SERT, serotonin transporter VMAT, vesicular monoamine transporter indicates data not available. 

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. 

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). 

MA-TR, monoamine transporter MCP-1, monocyte chemoattractant protein 1 MG-R, a-melanocyte-stimulating hormone (a-MSH) receptor MD, methylenedioxy... 

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... 

Rcuptakc inhibition by these agents is at the level of the respective monoamine transporter via competitive inhibition of binding of the monoamine to the substrate-hinding ettm-partment. Probably the same. site on the protein is involved fur inhibitor and monoamine, but this has not yet been proved. The mechanism of rcuptakc hy monoamine transporters has been reviewed. ... 

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. 

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