Editing Vesicular monoamine transporter

{{Short description|Family of transport proteins}}
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{{redirect|VMAT|Volumetric modulated arc therapy|Radiation therapy#Volumetric modulated arc therapy (VMAT)}}
{{lead too short|date=May 2013}}

The '''vesicular monoamine transporter''' (VMAT) is a [[transport protein]] integrated into the membranes of [[synaptic vesicle]]s of [[synapse|presynaptic neurons]]. It transports [[monoamine]] neurotransmitters – such as [[dopamine]], [[serotonin]], [[norepinephrine]], [[epinephrine]], and [[histamine]] – into the [[Vesicle (biology and chemistry)|vesicles]], which release the [[neurotransmitter]]s into synapses, as chemical messages to postsynaptic neurons. VMATs utilize a [[Electrochemical gradient|proton gradient]] generated by [[V-ATPase]]s in vesicle membranes to power monoamine import.

Pharmaceutical drugs that target VMATs have possible applications for many conditions, leading to a plethora of biological research, including [[hypertension]], [[Drug-addiction|drug addiction]], psychiatric disorders, [[Parkinson's disease]], and other neurological disorders. Many drugs that target VMATs act as inhibitors and alter the kinetics of the protein. Much research regarding the effects of altered VMATs on biological systems is still ongoing.
==Monoamines==
Monoamines transported by VMATs are mainly [[noradrenaline]], [[adrenaline]], [[dopamine]], [[serotonin]], [[histamine]], and [[trace amine]]s.<ref name="E Weihe">{{cite journal |vauthors=Eiden LE, Weihe E | title = VMAT2: a dynamic regulator of brain monoaminergic neuronal function interacting with drugs of abuse | journal = Ann. N. Y. Acad. Sci. | volume = 1216 | pages = 86–98 |date=January 2011 | issue = 1 | pmid = 21272013 | doi = 10.1111/j.1749-6632.2010.05906.x | quote=VMAT2 is the CNS vesicular transporter for not only the biogenic amines DA, NE, EPI, 5-HT, and HIS, but likely also for the trace amines TYR, PEA, and thyronamine (THYR)&nbsp;...&nbsp;[Trace aminergic] neurons in mammalian CNS would be identifiable as neurons expressing VMAT2 for storage, and the biosynthetic enzyme aromatic amino acid decarboxylase (AADC). | pmc=4183197| bibcode = 2011NYASA1216...86E }}</ref> Exogenous substrates include [[guanethidine]] and [[MPP+|MPP<sup>+</sup>]].<ref name=Rang167>{{cite book |author=Rang, H. P. |title=Pharmacology |publisher=Churchill Livingstone |location=Edinburgh |year=2003 |page=167 |isbn=978-0-443-07145-4 }}</ref>

==Discovery==
VMAT research began in 1958 when [[Nils-Åke Hillarp]] discovered [[secretory vesicles]]. In the 1970s, scientists like [[Arvid Carlsson]] recognized the need to understand how transport systems and ion gradients work in different organisms in order to explore new treatment options such as [[reserpine]] (RES). Researchers discovered inhibitors that blocked the uptake of neurotransmitters into vesicles, suggesting the existence of VMATs.<ref name="Eiden, L. 2004">{{cite journal | author = Eiden L. | author2 = Schäfer M. H. | year = 2004 | title = The vesicular amine transporter family (SLC18) amine/proton antiporters required for vesicular accumulation and regulated exocytotic secretion of monoamines and acetylcholine | journal = Pflügers Archiv | volume = 447 | issue = 5| pages = 636–640 | doi=10.1007/s00424-003-1100-5 | pmid=12827358| s2cid = 20764857 |display-authors=etal}}</ref> A decade later, molecular genetic tools have improved methods for protein identification. Scientists have used these tools to analyze DNA and amino acid sequences, and discovered that transporters in bacteria and humans were very similar, which emphasized the importance and universality of transporters.<ref name="Wimalasena, K. 2011">{{cite journal | author = Wimalasena K | year = 2011 | title = Vesicular monoamine transporters: structure-function, pharmacology, and medicinal chemistry | journal = Med Res Rev | volume = 31 | issue = 4| pages = 483–519 | doi=10.1002/med.20187 | pmid=20135628 | pmc=3019297}}</ref> The transporters were first structurally identified by cloning VMATs in rats.<ref name="Eiden, L. 2004"/> The VMAT were first isolated and purified in bovine chromaffin granules, in both its native and [[Denaturation (biochemistry)|denatured]] forms.<ref name="Henry, J. P. 1994">{{cite journal |author1=Henry J. P. |author2=Botton D. | year = 1994 | title = Biochemistry and molecular biology of the vesicular monoamine transporter from chromaffin granules | journal = J Exp Biol | volume = 196 | pages = 251–62 |doi=10.1242/jeb.196.1.251 |display-authors=etal|pmid=7823026 |doi-access=free }}</ref>

==Location==
There are two types of VMATs expressed in humans: [[VMAT1]] and [[VMAT2]].<ref name="Wimalasena, K. 2011"/> VMAT1 is expressed mainly in large dense-core vesicles (LDCVs) of the peripheral nervous system. VMAT1 may be found in [[neuroendocrine cell]]s, particularly [[chromaffin cell|chromaffin]] and [[enterochromaffin cells|enterochromaffin]] granules, which are primarily found in the [[adrenal medulla|medulla]] of the [[adrenal gland]]s.

VMAT2 favors [[Gene expression|expression]] in a variety of [[Monoaminergic|monoaminergic cells]] of the [[central nervous system]], such as the brain, [[sympathetic nervous system]], [[mast cells]], {{citation needed span|and cells containing [[histamine]] in the gut.|date=October 2014}} It is prevalent in [[Beta cell|β-cells]],<ref name= "Fei">{{cite journal |author1=Fei H. |author2=Grygoruk A. | year = 2008 | title = Trafficking of vesicular neurotransmitter transporters | journal = Traffic | volume = 9| issue = 9| pages = 1425–36| pmc=2897747 | pmid=18507811 | doi=10.1111/j.1600-0854.2008.00771.x|display-authors=etal}}</ref> expressed in [[platelet|blood platelets]],<ref name="Brunk, I. 2006">{{cite book |author1=Brunk I. |author2=Höltje B. | year = 2006 | title = Regulation of vesicular monoamine and glutamate transporters by vesicle-associated trimeric G-proteins: new jobs for long-known signal transduction molecules | series = Handbook of Experimental Pharmacology | volume = 175 | issue = 175| pages = 305–25 |display-authors=etal|doi=10.1007/3-540-29784-7_15 |pmid=16722242 |isbn=978-3-540-29783-3 }}</ref><ref name="Höltje, M. 2003">{{ cite journal |author1=Höltje M. |author2=Winter S. | title = The vesicular monoamine content regulates VMAT2 activity through Galphaq in mouse platelets. Evidence for autoregulation of vesicular transmitter uptake | journal = Journal of Biological Chemistry | volume = 278 | issue = 18| date= May 2003| pages = 15850–15858 | doi = 10.1074/jbc.M212816200| pmid = 12604601 |display-authors=etal| doi-access = free }}</ref> and co-expressed in chromaffin cells.<ref name="Fei" /> Expression of the two transporters in internal organs seems to differ between species: only VMAT1 is expressed in rat adrenal medulla cells, whereas VMAT2 is the major transporter in bovine adrenal medulla cells.<ref name="Wimalasena, K. 2010">{{cite journal | author = Wimalasena K | year = 2010 | title = Vesicular monoamine transporters: structure-function, pharmacology and medicinal chemistry | journal = Medicinal Research Reviews | volume = 31 | issue = 4| pages = 483–19 | doi=10.1002/med.20187 | pmid=20135628 | pmc=3019297}}</ref>

==Structure and function==
[[File:VMAT transport 01.jpg|thumb|right|A Hydrogen atom from the inside of the vesicle binds, inducing a conformational change in the transporter]]
[[File:VMAT transport two.jpg|thumb|right|The conformational change induced by the hydrogen atom binding enables the monoamine binding to the active transport site]]
[[File:VMAT transport three.jpg|thumb|right|A second hydrogen atom binds from inside the vesicle to the transporter inducing another change]]
[[File:VMAT transport four.jpg|thumb|right|The monoamine is released inside the vesicle and the two hydrogen atoms are released into the cytosol and the transport process starts over again.]]

VMAT1 and VMAT2 are acidic [[glycoprotein]]s with a molecular weight of approximately 70 [[kDa]].<ref name="Wimalasena, K. 2011"/><ref>{{cite journal |vauthors=Liu Y, Peter D, Rogahani A, Schuldiner S, Prive GG, Eisenberg D, Brecha N, Edwards RH | year = 1992 | title = A cDNA that suppresses MPP1 toxicity encodes a vesicular amine transporter | journal = Cell | volume = 70 | issue = 4| pages = 539–551 | doi=10.1016/0092-8674(92)90425-c| pmid = 1505023 | s2cid = 6225156 }}</ref> Both isoforms are [[transmembrane protein]]s with 12 [[transmembrane domain]]s (TMDs).<ref name="Wimalasena, K. 2011"/>

VMATs function by loading monoamines—dopamine, serotonin, histamine, norepinephrine, and epinephrine—into transport vesicles.<ref>Purves, Dale, et al. Neuroscience. Sinauer Associates. 087893646</ref> VMATs use the same transport mechanism for all types of monoamines,<ref name="Henry, J. P. 1994"/> and transport them from the [[cytosol]] into high-concentration storage vesicles.<ref name="Wimalasena, K. 2011"/> Transport vesicles are released into the space between neurons, called the [[synaptic cleft]], where they convey a chemical message to the next neuron. VMATs also function in sorting, storing, and releasing neurotransmitters, and are believed to participate in protecting these neurotransmitters from [[autoxidation]].<ref name="Wimalasena, K. 2011"/> The transporters are also known to continue biochemical modification after loading certain neurotransmitters.<ref name="Wimalasena, K. 2011"/>

Vesicle packing requires a large energy source to store large quantities of neurotransmitters into a small vesicular space at high concentrations. VMAT transport relies on the pH and electrochemical gradient generated by a [[V-ATPase|vesicular H<sup>+</sup>-ATPase]].<ref name="Wimalasena, K. 2011"/><ref name="Chaudhry FA 2007">{{cite journal |vauthors=Chaudhry FA, Edwards RH, Fonnum F | year = 2007 | title = Vesicular neurotransmitter transporters as targets for endogenous and exogenous toxic substances | url = https://zenodo.org/record/897959| journal = Annu. Rev. Pharmacol. Toxicol. | volume = 48 | pages = 277–301 | doi = 10.1146/annurev.pharmtox.46.120604.141146 | pmid = 17883368 }}</ref> The current model of VMAT function proposes that the efflux of two protons (H<sup>+</sup>) against the H<sup>+</sup> gradient is coupled with influx of one monoamine.<ref name="Wimalasena, K. 2011"/><ref name="Chaudhry FA 2007"/> The first H<sup>+</sup> efflux generates a transporter [[conformational isomerism|conformation]] associated with a high-affinity amine-binding site in the cytosolic phase, and the second H<sup>+</sup> efflux is coupled with a second large [[conformational change]] that leads to amine transport from the cytosolic side into the vesicle, reducing amine-binding affinity.<ref name="Wimalasena, K. 2011"/>

Studies indicate that the amino acid [[Residue (chemistry)#Characteristic units within a molecule|residue]] His419, located on the domain between TMDs X and XI of rat VMAT1, plays a role in energy coupling to the amine transport by assisting the first proton-dependent conformational change.<ref name="Wimalasena, K. 2011"/><ref>Shirvan A, Laskar O, Steiner-Mordoch S, Schuldiner S. (1994). "Histidine-419 plays a role in energy coupling in the vesicular monoamine transporter from rat." ''FEBS Lett',' 356:145–150.</ref> It has been proposed that RES inhibits VMAT by interacting with this conformation.{{Citation needed|date=November 2021}}

VMAT gene sequence analysis demonstrates that four [[Aspartic acid|aspartic acid residues]] in the middle region of TMDs I, VI, X, and XI and one [[lysine]] residue in TMD II have highly conserved gene sequences, suggesting these residues play a critical role in transporter structure and function.<ref name="Wimalasena, K. 2011"/><ref name = "merickel">{{cite journal |vauthors=Merickel A, Kaback HR, Edwards RH | year = 1997 | title = Charged residues in transmembrane domains II and XI of a vesicular monoamine transporter form a charge pair that promotes high affinity substrate recognition | journal = J. Biol. Chem. | volume = 272 | issue = 9| pages = 5403–5408 | doi=10.1074/jbc.272.9.5403| pmid = 9038139 | doi-access = free }}</ref> Specifically, the residues Lys139 and Asp427 are thought to compose an ion pair that promotes high-affinity interaction with VMAT substrates and inhibitors.<ref name="Wimalasena, K. 2011"/><ref name = "merickel"/> The Asp431 residue located in TMD XI is believed to be critical for amine transport, but does not interact with RES binding; it is thought to complete the substrate transport cycle.<ref name="Wimalasena, K. 2011"/><ref>{{cite journal |vauthors=Steiner-Mordoch S, Shirvan A, Schuldiner S | year = 1996 | title = Modification of the pH profile and tetrabenazine sensitivity of rat VMAT1 by replacement of aspartate 404 with glutamate | journal = J. Biol. Chem. | volume = 271 | issue = 22| pages = 13048–13054 | pmid = 8662678 | doi = 10.1074/jbc.271.22.13048 | doi-access = free }}</ref>

==Kinetics==
VMATs have a relatively low [[Michaelis–Menten kinetics|V<sub>max</sub>]], with an estimated rate of 5–20/sec depending on the substrate.<ref>{{cite journal | author = Peter D | year = 1994 | title = The chromaffin granule and synaptic vesicle amine transporters differ in substrate recognition and sensitivity to inhibitors | journal = J Biol Chem | volume = 269 | issue = 10 | pages = 7231–7237 | doi = 10.1016/S0021-9258(17)37272-1 | pmid = 8125935 |display-authors=etal| doi-access = free }}</ref> Vesicle filling may limit monoamine release from neurons with high rates of firing.

Specific amine-binding affinity varies by VMAT isoform; studies indicate that [[catecholamine]]s dopamine, norepinephrine, and epinephrine have a threefold higher affinity for VMAT2 than VMAT1 binding and uptake.<ref name="Wimalasena, K. 2011"/><ref name="Chaudhry FA 2007"/><ref name="Erickson JD 1996">{{cite journal |vauthors=Erickson JD, Schafer MK, Bonner TI, Eiden LE, Weihe E | year = 1996 | title = Distinct pharmacological properties and distribution in neurons and endocrine cells of two isoforms of the human vesicular monoamine transporter | journal = Proc. Natl. Acad. Sci. USA | volume = 93 | issue = 10| pages = 5166–5171 | doi=10.1073/pnas.93.10.5166 | pmid=8643547 | pmc=39426| bibcode = 1996PNAS...93.5166E | doi-access = free }}</ref> The imidazoleamine histamine has a thirtyfold higher affinity for VMAT2 compared to VMAT1,<ref name="Wimalasena, K. 2011"/> and is thought to bind to a different site than other monoamines.<ref name="Chaudhry FA 2007"/> Unlike catecholamines and histamine, the [[indoleamine]] serotonin binds to VMAT1 and VMAT2 with a similar affinity for both transporter isoforms.<ref name="Wimalasena, K. 2011"/><ref name="Erickson JD 1996"/>

VMAT1 has a lower turnover number and a lower affinity for most monoamine substrates than VMAT2, which may be because of VMAT2's location in the central nervous system, which demands fast recovery from neurotransmitter release in order to prepare for subsequent releases. The uptake efficiencies of each VMAT substrate can be ranked in order of efficiency as: serotonin, dopamine, epinephrine, and norepinephrine.<ref name="Wimalasena, K. 2011"/>

[[Methamphetamine|Methamphetamines]] decrease V<sub>max</sub>, while [[cocaine]] increases V<sub>max</sub> reversibly in rat brain.<ref name="Wimalasena, K. 2011"/>

==Inhibition==
The effects of VMAT inhibition have been studied in-depth in animal models. Mutant [[homozygous]] VMAT(-/-) mice move little, feed poorly, and die within a few days of birth.

More specifically, inhibition of VMAT2 may cause an increase in cytosolic catecholamine levels, which can result in an increase in efflux of catecholamines through the [[cell membrane]], depleting catecholamine concentrations and causing increased [[oxidative stress]] and oxidative damage to the neuron.

[[Heterozygous]] VMAT [[Mutant|mutants]] display hypersensitivity to [[amphetamine]], cocaine, and [[MPTP]] (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), the latter being a substance causally linked to [[Parkinson's disease]] (PD) in rodents.<ref name="Brunk, I. 2006"/> This suggests a protective role of VMATs against oxidative stress through removal of such substances from the cytosol.<ref name="Brunk, I. 2006"/>

VMAT inhibitors include:
*Reserpine (RES), [[bietaserpine]], and [[ketanserin]] (KET) (potent inhibitors of VMAT2 mediated serotonin transport)
*[[Tetrabenazine]] (TBZ) (specific to VMAT2)
*[[Phenylethylamine]]
*[[Amphetamine]]
*[[MDMA]]
*''N''-Methyl-4-phenylpyridinium (MPP<sup>+</sup>) (very potent inhibitors of VMAT2 mediated serotonin transport)
*[[Fenfluramine]] (specific to VMAT1 )
*[[5'-Guanylyl imidodiphosphate|Non-hydrolysable GTP-analogue guanylyllimidodiphosphate]] GMP-P(NH)P (VMAT2 only)

==Binding site structures==
===Ligand-binding affinities and structures===
Two known binding sites for VMAT inhibitors include the RES binding site and the TBZ binding site. Some evidence suggests these two sites may overlap or exist as two separate conformations of the same binding site.<ref name="Wimalasena, K. 2011"/><ref name="Chaudhry FA 2007"/> VMAT inhibitors tend to fall into two classes: those that interact with the RES binding site and those that interact with the TBZ binding site.<ref name="Chaudhry FA 2007"/>

RES, methoxytetrabenazine (MTBZ), and the drug [[amiodarone]] bind to the RES binding site. TBZ (also called Nitoman and Xenazine), [[dihydrotetrabenazine]] (DTBZOH), ketanserin (KET), and the drug [[lobeline]] bind to the TBZ binding site. Amphetamine, methamphetamine and GZ-7931 are also known to interact with VMAT2.<ref name="Wimalasena, K. 2011"/><ref name="Miller GW 1999">{{cite journal |vauthors=Miller GW, Gainetdinov RR, Levey AI, Caron MG | year = 1999 | title = Dopamine transporters and neuronal injury | journal = Trends in Pharmacological Sciences | volume = 20 | issue = 10| pages = 424–429 | doi = 10.1016/S0165-6147(99)01379-6 | pmid = 10498956 }}</ref><ref name="Fleckenstein AE 2009">{{cite journal |vauthors=Fleckenstein AE, Volz TJ, Hanson GR | year = 2009 | title = Psychostimulant-induced alterations in vesicular monoamine transporter-2 function: Neurotoxic and therapeutic implications | journal = Neuropharmacology | volume = 56 | issue = Suppl 1| pages = 133–138 | doi=10.1016/j.neuropharm.2008.07.002| pmid = 18662707 | pmc = 2634813}}</ref><ref name="en.wikipedia.org">[[Vesicular monoamine transporter 2#Binding sites and ligands]]</ref>

Inhibitor affinity varies among VMAT isoforms. RES and KET have higher inhibitory affinity for VMAT2–mediated 5HT transport than for that of VMAT1; TBZ seems to inhibit VMAT2 exclusively.<ref name="Wimalasena, K. 2011"/>

The residues asp33 and ser180, 181, and 182 are believed to be involved in substrate recognition, and interact with the [[Protonation|protonated]] amino group and [[Hydroxyl|hydroxyl group]] on the [[catechol]] or [[indole]] rings.<ref name="Chaudhry FA 2007"/>

Cocaine and [[methylphenidate]] (MPD, also known as Ritalin and Concerta) are believed to interact with VMAT2 to cause a shift in VMAT2 "from a plasmalemmal membrane-associated fraction to a vesicle-enriched, nonmembrane-associated fraction."<ref name="Fleckenstein AE 2007">{{cite journal |vauthors=Fleckenstein AE, Volz TJ, Riddle EL, Gibb JW, Hanson GR | year = 2007 | title = New insights into the mechanism of action of amphetamines | journal = Annu Rev Pharmacol Toxicol | volume = 47 | pages = 681–98 | doi = 10.1146/annurev.pharmtox.47.120505.105140 | pmid=17209801}}</ref>

====RES binding site====
Consistent with catecholamine-binding affinity, RES has a threefold higher affinity for VMAT2 than for VMAT1.<ref name="Chaudhry FA 2007"/><ref name="Erickson JD 1996"/> The RES binding site is known to be [[hydrophobic]], which is thought to contribute to ligand binding affinity.<ref name="Wimalasena, K. 2011"/> Methamphetamine binds to the RES site on VMATs.<ref name="VMAT2 amph vs meth" />

The current working model proposes that RES and the substrate bind to a single site in a pH-gradient modulated [[Conformation activity relationship|conformational]] structure of the transporter. The conformation occurs after the transport of one H<sup>+</sup> across the membrane and into the vesicle; proton transport drives the substrate recognition site from the lumen to the cytoplasmic surface of the vesicle for RES and substrate binding.<ref name="Wimalasena, K. 2011"/><ref name="Chaudhry FA 2007"/><ref name = "Darchen">{{cite journal |vauthors=Darchen F, Scherman D, Henry JP | year = 1989 | title = Reserpine binding to chromaffin granules suggests the existence of two conformations of the monoamine transporter | journal = Biochemistry | volume = 28 | issue = 4| pages = 1692–1697 | doi=10.1021/bi00430a040| pmid = 2719928 }}</ref> Methoxytetrabenazine (MTBZ) may bind to the RES binding site, based on studies indicating that RES significantly inhibited MTBZ-binding.<ref name="Wimalasena, K. 2011"/> Amiodarone is also believed to inhibit monoamine vesicular uptake by binding to the RES binding site.<ref name="Wimalasena, K. 2011"/>

====TBZ binding site====
TBZ and dihydrotetrabenazine (DTBZOH) are believed to bind to a different binding site from the RES/substrate binding site, or to a different conformation of the RES/substrate binding site.<ref name="Wimalasena, K. 2011"/><ref name="Chaudhry FA 2007"/><ref>{{cite journal |vauthors=Liu Y, Edwards RH | year = 1997 | title = The role of vesicular transport proteins in synaptic transmission and neural degeneration | journal = Annu. Rev. Neurosci. | volume = 20 | pages = 125–156 | doi=10.1146/annurev.neuro.20.1.125| pmid = 9056710 }}</ref> This site is believed to be located at the [[N-terminus]], based on studies done in bovine VMAT2.<ref name="Chaudhry FA 2007"/> Tyr434 and asp461 are identified as being responsible for the high-affinity interaction of TBZ, serotonin, and histamine in VMAT2.<ref name="Chaudhry FA 2007"/> Unlike methamphetamine, amphetamine binds to the TBZ site on hVMAT2.<ref name="VMAT2 amph vs meth">{{cite journal |vauthors=Sulzer D, Sonders MS, Poulsen NW, Galli A |title=Mechanisms of neurotransmitter release by amphetamines: a review |journal=Prog. Neurobiol. |volume=75 |issue=6 |pages=406–433 |date=April 2005 |pmid=15955613 |doi=10.1016/j.pneurobio.2005.04.003 |s2cid=2359509 |quote=They also demonstrated competition for binding between METH and reserpine, suggesting they might bind to the same site on VMAT. George Uhl's laboratory similarly reported that AMPH displaced the VMAT2 blocker tetrabenazine (Gonzalez et al., 1994)....tetrabenazine and reserpine are thought to bind to different sites on VMAT (Schuldiner et al., 1993a)}}</ref>

Unlike RES inhibition, TBZ inhibition is only affected by very high concentrations of monoamines; however, single injections of RES can inhibit TBZ binding.<ref name="Chaudhry FA 2007"/> [[ketanserin]] (KET)<ref name="Wimalasena, K. 2011"/><ref name = "Darchen"/> and [[lobeline]]<ref name="Wimalasena, K. 2011"/><ref name="Chaudhry FA 2007"/> also bind to the TBZ binding site conformation.

===Glycosylation sites: N- and C-linked terminals===
Three to four [[Glycosylation|glycosylation sites]] exist in the [[Vesicle (biology and chemistry)|vesicular matrix]] on a loop between TMD I and TMD II.<ref name="Wimalasena, K. 2011"/> In biology, the vesicle [[matrix (biology)|matrix]] refers to the material or tissue between cells in which more specialized structures are embedded. Two of the glycosylation sites, the [[N-linked glycosylation|''N''-linked glycosylation terminal]] and [[C-terminus|''C''-linked terminal]], are located in the cytosolic portion of the vesicle.<ref name="Wimalasena, K. 2011"/><ref>Erickson, Eiden, & Hoffman, 1992</ref>

The highest amount of genetic variance between VMAT1 and VMAT2 exists near the N- and C- terminals in the cytosolic phase, and in the glycosylated loop between TMDs I and II.<ref name="Wimalasena, K. 2011"/>

===C-terminus and VMAT trafficking cycle===
Several motifs involved in the VMAT [[Vesicle trafficking|trafficking cycle]] are believed to be encoded in the C-terminus. A dileucine motif in the C-terminus is required for VMAT2 [[endocytosis]].<ref name= "Fei"/> Studies suggest the acidic residues in the dileucine motif sort VMAT2 away from constitutive secretory vesicles and into the regulated [[secretory pathway]].<ref name= "Fei"/> The hydrophobic residues in the dileucine motif are thought to couple with the acidic residues as a single unit to help sort VMAT2 to large dense course vesicles.<ref name= "Fei"/> Acidic glutamate residues located upstream of the dileucine motif are known to be important for localization of VMAT2 to large dense core vesicles; these residues are also conserved in VMAT1.<ref name= "Fei"/>

==Genetic expression and transporter regulation==
Although both VMAT1 and VMAT2 are encoded by two different [[gene]]s, the individual genetic sequences demonstrate high homology. [[polymorphism (biology)|Polymorphism]]s in VMAT2 that affect regulation and quantitative [[Gene Expression|expression]] may pose genetic risk factors for PD. A specific VMAT1 gene (''SLC18A1'') has several associated [[polymorphism (biology)|polymorphism]]s, which have a [[locus (genetics)|locus]] 8p21.3 that has been strongly connected to [[schizophrenia]] susceptibility.<ref>{{Cite journal|last1=Lohoff|first1=Falk W.|last2=Weller|first2=Andrew E.|last3=Bloch|first3=Paul J.|last4=Buono|first4=Russell J.|last5=Doyle|first5=Glenn A.|last6=Ferraro|first6=Thomas N.|last7=Berrettini|first7=Wade H.|date=2008|title=Association between Polymorphisms in the Vesicular Monoamine Transporter 1 Gene (VMAT1/SLC18A1) on Chromosome 8p and Schizophrenia|journal=Neuropsychobiology|language=en|volume=57|issue=1–2|pages=55–60|doi=10.1159/000129668|issn=0302-282X|pmid=18451639|s2cid=39523023}}</ref>

Over-expression of VMAT2 results in increased secretion of neurotransmitter upon cell stimulation. Data suggests that deletion of the VMAT2 genes does not affect the size of small clear-core vesicles.

VMATs may be regulated by changes in [[transcription (genetics)|transcription]], post-transcriptional modifications such as [[phosphorylation]] and [[Mrna|mRNA]] [[RNA splicing|splicing]] of [[exons]], and vesicular transport inactivation facilitated by [[heterotrimeric G-proteins]], which are thought to be possessed by chromaffin granules, and have shown to regulate small clear-core vesicles.<ref name= "Fei"/><ref name="Brunk, I. 2006"/>

Specific heterotrimeric G-protein type regulation is tissue-dependent for VMAT2; it is not known whether this is the case for VMAT1. Heterotrimeric G-protein Gαo2 decreases VMAT1 activity in pancreatic and [[Adrenal medulla|adrenal medulla cells]], and activates heterotrimeric G-proteins to inhibit VMAT2 activity in the brain, regardless of whether they are localized on small clear-core or large-dense-core vesicles. The activated heterotrimeric G-protein Gαq downregulates VMAT2-mediated [[serotonin]] transport in blood platelets, but not in the brain, where Gαq completely inhibits VMAT2 activity.<ref name="Brunk, I. 2006"/> Although the exact signalling pathway for G-protein mediated regulation of VMATs is not known,<ref name="Brunk, I. 2006"/> it has recently been described that implicated G-proteins act directly on the VMATs.<ref>Remin, R., Schuldiner, S., 2003. Vesicular neurotransmitter transporters: Pharmacology, Biochemistry and Molecular Analysis. Neurotransmitter Transporters; Structure, Function, and Regulation, pp.313-354</ref>

==Clinical significance==
VMAT2 has been shown to contribute to many clinical neurological disorders including drug addiction, mood disorders, and stress,<ref>{{cite journal |vauthors=Tillinger A, Sollas A, Serova LI, Kvetnansky R, Sabban EL | year = 2010 | title = Vesicular monoamine transporters (VMATs) in adrenal chromaffin cells: stress-triggered induction of VMAT2 and expression in epinephrine synthesizing cells | journal = Cell Mol Neurobiol | volume = 30 | issue = 8| pages = 1459–1465 | doi =  10.1007/s10571-010-9575-z| pmid = 21046458 | s2cid = 21852075 | pmc = 11498772 }}</ref> as well as Parkinson's disease<ref>{{cite journal |vauthors=Okamura N, Villemagne VL, Drago J, Pejoska S, Dhamija RK, Mulligan RS, Ellis JR, Ackermann U, O'Keefe G, Jones G, Kung HF, Pontecorvo MJ, Skovronsky D, Rowe CC | year = 2010 | title = In vivo measurement of vesicular monoamine transporter type 2 density in Parkinson disease with (18)F-AV-133 | journal = J. Nucl. Med. | volume = 51 | issue = 2| pages = 223–228 | doi=10.2967/jnumed.109.070094 | pmid=20080893| doi-access = free }}</ref> and Alzheimer's disease.<ref>{{cite journal |vauthors=Villemagne VL, Okamura N, Pejoska S, Drago J, Mulligan RS, Chetelat G, Ackermann U, O'Keefe G, Jones G, Gong S, Tochon-Danguy H, Kung HF, Masters CL, Skovronsky DM, Rowe CC | year = 2011 | title = In vivo assessment of vesicular monoamine transporter type 2 in dementia with lewy bodies and Alzheimer disease | journal = Arch. Neurol. | volume = 68 | issue = 7| pages = 905–912 | doi=10.1001/archneurol.2011.142| pmid = 21747030 | doi-access = free }}</ref><ref>{{cite journal |vauthors=Salin A, Savli M, Lanzenberger R | year = 2011 | title = Serotonin and molecular neuroimaging in humans using PET | journal = Amino Acids | volume = 42 | issue = 6| pages = 2039–57 | doi = 10.1007/s00726-011-1078-9 | pmid = 21947614 | s2cid = 14118396 }}</ref>

===Parkinson's disease===
Studies indicate VMAT2 mRNA is present in all cell groups damaged by Parkinson's disease (PD);<ref name="Miller GW 1999 PII">{{cite journal |vauthors=Miller GW, Gainetdinov RR, Levey AI, Caron MG | year = 1999 | title = Dopamine transporters and neuronal injury | journal = TiPS | volume = 20 | issue = 10 | page = 425 | doi = 10.1016/s0165-6147(99)01379-6 | pmid = 10498956 }}</ref> these findings have identified VMAT2 as a target for preventing Parkinson's. VMAT2 presence does not independently protect neurons from PD, but a decrease in VMAT2 expression has been shown to correlate with susceptibility to the disease,<ref name="Miller GW 1999 PII"/> which may be due to a ratio between the [[dopamine transporter]] and VMAT2.<ref name="Miller GW 1999 PII"/>

Based on the understanding the increased cytosolic dopamine levels lead to dopaminergic cell death in PD, it has been proposed that regulatory [[Polymorphism (biology)|polymorphisms]] in VMAT2 affect VMAT2 quantitative expression, and may serve as a genetic risk factor for PD. Specifically, the SLC18A2 [[Promoter (genetics)|promoter region]] for the VMAT2 gene has been identified as an area where several polymorphisms form discrete [[haplotype]]s.<ref name="Wimalasena, K. 2011"/><ref name="Glatt CE, Wahner AD, White DJ, Ruiz-Linares A, Ritz B 2006 299–305">{{cite journal |vauthors=Glatt CE, Wahner AD, White DJ, Ruiz-Linares A, Ritz B | year = 2006 | title = Gain-of-function haplotypes in the vesicular monoamine transporter promoter are protective for Parkinson disease in women | journal = Hum. Mol. Genet. | volume = 15 | issue = 2| pages = 299–305 | doi=10.1093/hmg/ddi445| pmid = 16339215 | pmc = 3643966 }}</ref>

===Mood disorders===
Studies using a genetic rodent model to understand clinical depression in humans suggest that VMAT2 genetic or functional alterations may be involved in depression.<ref>{{cite journal |vauthors=Schwartz K, Yadid G, Weizman A, Rehavi M | year = 2003 | title = Decreased limbic vesicular monoamine transporter 2 in a genetic rat model of depression | journal = Brain Res. | volume = 965 | issue = 1–2| pages = 174–179 | doi=10.1016/s0006-8993(02)04167-7| pmid = 12591135 | s2cid = 26761996 }}</ref> Reduced VMAT2 levels were identified in specific subregions of the [[striatum]] involved in clinical depression, including the [[nucleus accumbens]] shell but not the core, the [[ventral tegmental area]], and the [[substantia nigra]]'s [[pars compacta]]. The reduced VMAT2 protein levels were not accompanied by similar levels of VMAT2 mRNA alterations. Based on these findings, it has been proposed that VMAT2 activity is not altered at the level of genetic expression, but may be altered at the functional level in ways that may correlate with clinical depression.<ref name="Wimalasena, K. 2011"/>

===Drug addiction===
Many [[psychostimulant]] drugs are known to interact with VMAT, including amphetamine analogs such as methamphetamine, cocaine, and ecstasy (MDMA).{{cn|date=November 2023}}

==Pharmacology==
VMAT inhibitors tend to fall into two classes; those that interact with the RES binding site and those that interact with the TBZ binding site.<ref name="Chaudhry FA 2007"/>

RES, methoxytetrabenazine, and amiodarone bind to the RES binding site.

TBZ, DTBZOH, ketanserin, and lobeline bind to the TBZ binding site.

Many [[psychostimulants]], including [[substituted amphetamines]] and cocaine, are known to interact with VMAT2. Studies indicate that both amphetamines and cocaine act to increase non-exocytotic release of dopamine in specific regions of the brain by interacting directly with VMAT2 function.<ref name="Wimalasena, K. 2011"/><ref name="Miller GW 1999"/><ref name="Fleckenstein AE 2007"/>

===Methamphetamine===
VMAT is a main target of methamphetamine. Studies indicate that substituted amphetamines including methamphetamine interact with VMAT2 at the TBZ/DTBZOH binding site.<ref name="Wimalasena, K. 2011"/><ref name="Fleckenstein AE 2007"/> By acting as a [[competitive antagonist|negative allosteric modulator]], methamphetamine blocks the presynaptic cell's ability to use VMAT for vesicular packaging.

Methamphetamine alters the subcellular location of VMAT2, which affects the distribution of dopamine in the cell. Treatment with methamphetamine relocates VMAT2 from a vesicle-enriched [[Cell fractionation|fraction]] to a location that is not continuous with synaptosomal preparations.<ref name="Wimalasena, K. 2010"/>

Repeated amphetamine exposure may increase VMAT2 mRNA in certain brain regions with little or no decline upon withdrawal from the drug.<ref name="Wimalasena, K. 2010"/>

A study performed by Sonsalla ''et al.'' demonstrated that methamphetamine treatment decreases DHTBZ binding and vesicular dopamine uptake.<ref name="Wimalasena, K. 2011"/><ref name="Fleckenstein AE 2007"/> Another study demonstrated that multiple high doses of methamphetamine removed DTBZ binding sites from the vesicles.<ref name="Chaudhry FA 2007"/>

In addition to an interaction with the TBZ/DTBZOH binding site, some researchers propose that substituted amphetamines like methamphetamine decrease dopamine uptake because of the weak base properties of substituted amphetamines.<ref name="Fleckenstein AE 2007"/> This “Weak Base Hypothesis” proposes that amphetamine analogs enter the cell through transport and lipophilic diffusion, then diffuses through the vesicular membrane where they accumulate in synaptic vesicles and offset the proton electrochemical gradient in the vesicle that drives monoamine transport through VMAT.<ref name="Fleckenstein AE 2007"/> Amphetamine administration would prevent vesicular dopamine uptake through VMAT, and explain the finding that amphetamine administration correlates with decreased dopamine release from vesicles and a neurotoxic increase in intracellular dopamine.<ref name="Wimalasena, K. 2011"/><ref name="Fleckenstein AE 2007"/>

===Cocaine===
Unlike methamphetamine, cocaine interacts with VMAT2 by mobilizing VMAT2-expressing vesicles, causing a shift in VMAT2 proteins from a plasmalemmal (synaptosomal) membrane fraction to a vesicle-enriched fraction that is not associated with the synaptosomal membrane and not retained in synaptosomal preparations.<ref name="Wimalasena, K. 2011"/><ref name="Chaudhry FA 2007"/><ref name="Fleckenstein AE 2007"/> Methylphenidate is believed to interact with VMAT2 in a similar fashion.<ref name="Fleckenstein AE 2007"/>

In addition to mobilizing VMAT2-expressing vesicles, cocaine has been shown to increase the V<sub>max</sub> of VMAT2 for dopamine and increase the number of DTBZ binding sites.<ref name="Chaudhry FA 2007"/> It has also mobilized a [[synapsin]]-dependent reserve pool of dopamine-containing synaptic vesicles, which interacts with the vesicular trafficking cycle to increase dopamine release.<ref name="Chaudhry FA 2007"/>

Short-term exposure to [[cocaine]] increases VMAT2 density in the [[prefrontal cortex]] and striatum of mammalian brains. This is theorized to be a defensive mechanism against the depletive effects cocaine has on cytosolic dopamine through increasing monoamine storage capacity.<ref name="Wimalasena, K. 2010"/> Chronic cocaine use has been implicated with a reduction in VMAT2 immunoreactivity as well as a decrease in DTBZOH binding in humans.

Research suggests a decline in VMAT2 protein through prolonged cocaine use could play an important role in the development of cocaine-induced mood disorders.<ref name="Wimalasena, K. 2010"/>

===MDMA===
MDMA is known to affect serotonergic neurons, but has been shown to inhibit synaptosomal and vesicular uptake of serotonin and dopamine<ref name="Wimalasena, K. 2011"/> to roughly the same extent ''in vitro''.<ref name="Chaudhry FA 2007"/> ''In vivo'' studies indicate short-term MDMA exposure causes short-term reduction in VMAT2 activity, which is reversed after 24 hours.<ref name="Chaudhry FA 2007"/>

==Current research==
===Clinical research===
Genetic research models have shown that polymorphisms in ''SLC18A1'' and ''SLC18A2'', the genes that encode for VMAT1 and 2 proteins, respectively, may confer risk for some neuropsychiatric disorders;<ref name="Wimalasena, K. 2011"/><ref name="Glatt CE, Wahner AD, White DJ, Ruiz-Linares A, Ritz B 2006 299–305"/><ref name="Lawal HO 2013">{{cite journal |vauthors=Lawal HO, Krantz DE | year = 2013 | title = SLC18: Vesicular neurotransmitter transporters for monoamines and acetylcholine | journal = Molecular Aspects of Medicine | volume = 34 | issue = 2–3| pages = 360–372 | doi = 10.1016/j.mam.2012.07.005 | pmid=23506877 | pmc=3727660}}</ref> however, no specific diseases have been identified yet as directly resulting from a genetic mutation in an ''SLC18'' gene, which codes for VMAT proteins.<ref name="Lawal HO 2013"/>

Much of the current research related to VMAT explores the genetic underpinnings of neuropsychiatric disorders as they may be affected by ''SLC18A'' family mutations.

The dopaminergic neuron is known to play a central role in drug addiction and abuse and the potential role of the dopamine transporter has been well-explored as a target for amphetamine and cocaine. Current research looks toward VMAT2 as a target for such psychostimulants. A combination of imaging, neurochemical, biochemical, cell biological, genetic, and immunohistochemical evidence has been compiled to provide the most current comprehensive understanding of the role the VMAT2 plays in amphetamine and cocaine abuse and addiction through aminergic neurotransmission.<ref name="E Weihe" /><ref name="Lawal HO 2013"/>

As VMATs are membrane proteins, structural information is limited and researchers have yet to completely understand the structure of both isoforms. Further studies are needed in order to determine the structure and therefore complete function of these proteins. There is preliminary evidence that the gene for VMAT1 may be linked to susceptibility to [[schizophrenia]], [[bipolar disorder]], and various anxiety disorders.<ref name="Wimalasena, K. 2011"/> Further studies are needed in order to confirm these findings and to gain a better understanding of the role of VMATs in the central nervous system.

Multiple [[single-nucleotide polymorphism]]s (SNPs) have been identified in the [[coding region]] of VMATs. The effects of some of these SNPs have been alteration of VMAT function, structure and regulation.<ref name = "sager">Sager, J.J. & Torres, G.E., 2011. Proteins interacting with monoamine transporters: Current state and future challenges. Biochemistry, [online] Available at: <http://pubs.acs.org/doi/ipdf/10.1021/bi200405c>[Accessed 20 April 2013]</ref> Further investigation of these SNPs is required in order to distinguish whether they may be attributable to certain diseases with suspected SNP-[[mutation]] origins.

α-synuclein, a cytosolic protein found mainly in [[chemical synapse|pre-synaptic]] nerve terminals, has been found to have regulatory interactions with the trafficking of VMATs; mutations involving α-synuclein have been linked to familial PD.<ref name = "sager"/> Further research is needed to clarify the extent to which these proteins modulate the trafficking of VMATs, and whether they may be exploited in order to gather more information about the exact mechanism of how disorders such as PD occurs, and how they may potentially be treated.

Studies have shown that at the synaptic membrane, [[enzyme]]s responsible for the synthesis of dopamine, [[tyrosine hydroxylase]] and amino acid aromatic decarboxylase are physically and functionally coupled with VMAT2.<ref name="sager"/> It was initially thought that the synthesis of these substances and the subsequent packaging of them into vesicles were two entirely separate processes.

===Animal research===
Current research related to VMAT uses VMAT2 knockout mice to explore the behavioral genetics of this transporter in an animal model. VMAT2 knockouts are known to be lethal as homozygotes, but heterozygote knockouts are not lethal and are used in many studies as a durable animal model.<ref name="Wimalasena, K. 2010"/><ref name="Lawal HO 2013"/>

From knockout and knockdown mice, researchers have discovered that it is good to have over-expression or under-expression of the VMAT genes in some circumstances.<ref name="Lawal HO 2013"/> Mice are also used in drug studies, particularity studies involving the effect cocaine and methamphetamine have on VMATs.<ref name="Lawal HO 2013"/> Studies involving animals have prompted scientists to work on developing drugs that inhibit or enhance the function of VMATs. Drugs that inhibit VMATs may have use in addiction but further studies are needed.<ref name="Lawal HO 2013"/> Enhancing the function of VMATs may also have therapeutic value.<ref name="Lawal HO 2013"/>

==References==
{{Reflist}}

==External links==
* {{MeshName|Vesicular+Monoamine+Transport+Proteins}}

== Further reading ==
*{{cite journal |author=Kilbourn MR |title=''In vivo'' radiotracers for vesicular neurotransmitter transporters |journal=Nucl. Med. Biol. |volume=24 |issue=7 |pages=615–9 |year=1997 |pmid=9352531 |doi=10.1016/S0969-8051(97)00101-7}}
*{{cite journal |vauthors=Lawal HO, Krantz DE |title=SLC18: Vesicular neurotransmitter transporters for monoamines and acetylcholine |journal=Molecular Aspects of Medicine |volume=34 |issue=2–3 |pages=360–372 |year=2013 |doi=10.1016/j.mam.2012.07.005|pmid=23506877 |pmc=3727660 }}
* {{cite journal |vauthors=Weihe E, Eiden LE |title=Chemical neuroanatomy of the vesicular amine transporters |journal=FASEB J. |volume=14 |issue=15 |pages=2435–49 |year=2000 |pmid=11099461 |doi=10.1096/fj.00-0202rev|doi-access=free |citeseerx=10.1.1.334.4881 |s2cid=17798600 }}
*{{cite journal |author=Wimalasena, K. |title=Vesicular Monoamine Transporters: Structure-Function, Pharmacology, and Medicinal Chemistry |journal=Medicinal Research Reviews |volume=31 |issue=4 |pages=483–519 |year=2011 |doi=10.1002/med.20187 |pmid=20135628 |pmc=3019297}}

{{Neurotransmitter transporters}}
{{Monoamine reuptake inhibitors}}
{{Monoamine releasing agents}}

[[Category:Neurochemistry]]
[[Category:Signal transduction]]
[[Category:Receptors]]
[[Category:Neurotransmitter transporters]]
[[Category:Biogenic amines]]
[[Category:Integral membrane proteins]]
[[Category:VMAT inhibitors]]
[[Category:Amphetamine]]