VMAT
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The vesicular monoamine transporter (VMAT) is a
transport protein A transport protein (variously referred to as a transmembrane pump, transporter, escort protein, acid transport protein, cation transport protein, or anion transport protein) is a protein that serves the function of moving other materials within ...
integrated into the membranes of
synaptic vesicle In a neuron, synaptic vesicles (or neurotransmitter vesicles) store various neurotransmitters that are exocytosis, released at the chemical synapse, synapse. The release is regulated by a voltage-dependent calcium channel. Vesicle (biology), Ves ...
s of presynaptic neurons. It transports
monoamine Monoamine neurotransmitters are neurotransmitters and neuromodulators that contain one amino group connected to an aromatic ring by a two-carbon chain (such as -CH2-CH2-). Examples are dopamine, norepinephrine and serotonin. All monoamines are ...
neurotransmitters – such as
dopamine Dopamine (DA, a contraction of 3,4-dihydroxyphenethylamine) is a neuromodulatory molecule that plays several important roles in cells. It is an organic chemical of the catecholamine and phenethylamine families. It is an amine synthesized ...
,
serotonin Serotonin (), also known as 5-hydroxytryptamine (5-HT), is a monoamine neurotransmitter with a wide range of functions in both the central nervous system (CNS) and also peripheral tissues. It is involved in mood, cognition, reward, learning, ...
,
norepinephrine Norepinephrine (NE), also called noradrenaline (NA) or noradrenalin, is an organic compound, organic chemical in the catecholamine family that functions in the brain and human body, body as a hormone, neurotransmitter and neuromodulator. The ...
,
epinephrine Adrenaline, also known as epinephrine, is a hormone and medication which is involved in regulating visceral functions (e.g., respiration). It appears as a white microcrystalline granule. Adrenaline is normally produced by the adrenal glands a ...
, and
histamine Histamine is an organic nitrogenous compound involved in local immune responses communication, as well as regulating physiological functions in the gut and acting as a neurotransmitter for the brain, spinal cord, and uterus. Discovered in 19 ...
– into the
vesicles Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane * Synaptic vesicle In a neuron, synaptic vesicles (or neurotransmitter vesicles) s ...
, which release the
neurotransmitter A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a Chemical synapse, synapse. The cell receiving the signal, or target cell, may be another neuron, but could also be a gland or muscle cell. Neurotra ...
s into synapses, as chemical messages to postsynaptic neurons. VMATs utilize a
proton gradient An electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts: * The chemical gradient, or difference in solute concentration across a membrane. ...
generated by
V-ATPase Vacuolar-type ATPase (V-ATPase) is a highly conserved evolutionarily ancient enzyme with remarkably diverse functions in eukaryotic organisms. V-ATPases acidify a wide array of intracellular organelles and pump protons across the plasma ...
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 Hypertension, also known as high blood pressure, is a Chronic condition, long-term Disease, medical condition in which the blood pressure in the artery, arteries is persistently elevated. High blood pressure usually does not cause symptoms i ...
,
drug addiction Addiction is a neuropsychological disorder characterized by a persistent and intense urge to use a drug or engage in a behavior that produces natural reward, despite substantial harm and other negative consequences. Repetitive drug use can ...
, psychiatric disorders,
Parkinson's disease Parkinson's disease (PD), or simply Parkinson's, is a neurodegenerative disease primarily of the central nervous system, affecting both motor system, motor and non-motor systems. Symptoms typically develop gradually and non-motor issues become ...
, 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 Norepinephrine (NE), also called noradrenaline (NA) or noradrenalin, is an organic chemical in the catecholamine family that functions in the brain and body as a hormone, neurotransmitter and neuromodulator. The name "noradrenaline" (from ...
,
adrenaline Adrenaline, also known as epinephrine, is a hormone and medication which is involved in regulating visceral functions (e.g., respiration). It appears as a white microcrystalline granule. Adrenaline is normally produced by the adrenal glands a ...
,
dopamine Dopamine (DA, a contraction of 3,4-dihydroxyphenethylamine) is a neuromodulatory molecule that plays several important roles in cells. It is an organic chemical of the catecholamine and phenethylamine families. It is an amine synthesized ...
,
serotonin Serotonin (), also known as 5-hydroxytryptamine (5-HT), is a monoamine neurotransmitter with a wide range of functions in both the central nervous system (CNS) and also peripheral tissues. It is involved in mood, cognition, reward, learning, ...
,
histamine Histamine is an organic nitrogenous compound involved in local immune responses communication, as well as regulating physiological functions in the gut and acting as a neurotransmitter for the brain, spinal cord, and uterus. Discovered in 19 ...
, and
trace amine Trace amines are an endogenous group of trace amine-associated receptor 1 (TAAR1) agonists – and hence, monoaminergic neuromodulators – that are structurally and metabolically related to classical monoamine neurotransmitters. Compared to ...
s. Exogenous substrates include guanethidine and MPP+.


Discovery

VMAT research began in 1958 when Nils-Åke Hillarp discovered
secretory vesicles Secretion is the movement of material from one point to another, such as a secreted chemical substance from a cell or gland. In contrast, excretion is the removal of certain substances or waste products from a cell or organism. The classical mec ...
. In the 1970s, scientists like
Arvid Carlsson Arvid Carlsson (25 January 1923 – 29 June 2018) was a Swedish neuropharmacologist who is best known for his work with the neurotransmitter dopamine and its effects in Parkinson's disease. For his work on dopamine, Carlsson was awarded the Nob ...
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 Reserpine is a drug that is used for the treatment of hypertension, high blood pressure, usually in combination with a thiazide diuretic or vasodilator. Large clinical trials have shown that combined treatment with reserpine plus a thiazide diur ...
(RES). Researchers discovered inhibitors that blocked the uptake of neurotransmitters into vesicles, suggesting the existence of VMATs. 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. The transporters were first structurally identified by cloning VMATs in rats. The VMAT were first isolated and purified in bovine chromaffin granules, in both its native and denatured forms.


Location

There are two types of VMATs expressed in humans:
VMAT1 Vesicular monoamine transporter 1 (VMAT1), also known as chromaffin granule amine transporter (CGAT) or solute carrier family 18 member 1 (SLC18A1), is a protein that in humans is encoded by the ''SLC18A1'' gene. VMAT1 is an integral membrane pro ...
and
VMAT2 The solute carrier family 18 member 2 (SLC18A2) also known as vesicular monoamine transporter 2 (VMAT2) is a protein that in humans is encoded by the ''SLC18A2'' gene. VMAT2 is an integral membrane protein that transports monoamines—particul ...
. VMAT1 is expressed mainly in large dense-core vesicles (LDCVs) of the peripheral nervous system. VMAT1 may be found in
neuroendocrine cell Neuroendocrine cells are cells that receive neuronal input (through neurotransmitters released by nerve cells or neurosecretory cells) and, as a consequence of this input, release messenger molecules (hormones) into the blood. In this way they bri ...
s, particularly chromaffin and enterochromaffin granules, which are primarily found in the medulla of the
adrenal gland The adrenal glands (also known as suprarenal glands) are endocrine glands that produce a variety of hormones including adrenaline and the steroids aldosterone and cortisol. They are found above the kidneys. Each gland has an outer adrenal corte ...
s. VMAT2 favors expression in a variety of monoaminergic cells of the
central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain, spinal cord and retina. The CNS is so named because the brain integrates the received information and coordinates and influences the activity o ...
, such as the brain,
sympathetic nervous system The sympathetic nervous system (SNS or SANS, sympathetic autonomic nervous system, to differentiate it from the somatic nervous system) is one of the three divisions of the autonomic nervous system, the others being the parasympathetic nervous sy ...
,
mast cells A mast cell (also known as a mastocyte or a labrocyte) is a resident cell of connective tissue that contains many granules rich in histamine and heparin. Specifically, it is a type of granulocyte derived from the myeloid stem cell that is a ...
, It is prevalent in β-cells, expressed in
blood platelets Platelets or thrombocytes () are a part of blood whose function (along with the coagulation factors) is to react to bleeding from blood vessel injury by clumping to form a blood clot. Platelets have no cell nucleus; they are fragments of cytopl ...
, and co-expressed in chromaffin cells. 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.


Structure and function

VMAT1 and VMAT2 are acidic
glycoprotein Glycoproteins are proteins which contain oligosaccharide (sugar) chains covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known a ...
s with a molecular weight of approximately 70
kDa The dalton or unified atomic mass unit (symbols: Da or u, respectively) is a unit of mass defined as of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest. It is a non-SI unit accepted f ...
. Both isoforms are
transmembrane protein A transmembrane protein is a type of integral membrane protein that spans the entirety of the cell membrane. Many transmembrane proteins function as gateways to permit the transport of specific substances across the membrane. They frequently un ...
s with 12
transmembrane domain A transmembrane domain (TMD, TM domain) is a membrane-spanning protein domain. TMDs may consist of one or several alpha-helices or a transmembrane beta barrel. Because the interior of the lipid bilayer is hydrophobic, the amino acid residues in ...
s (TMDs). VMATs function by loading monoamines—dopamine, serotonin, histamine, norepinephrine, and epinephrine—into transport vesicles. VMATs use the same transport mechanism for all types of monoamines, and transport them from the
cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells ( intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...
into high-concentration storage vesicles. Transport vesicles are released into the space between neurons, called the
synaptic cleft Chemical synapses are biological junctions through which neurons' signals can be sent to each other and to non-neuronal cells such as those in neuromuscular junction, muscles or glands. Chemical synapses allow neurons to form biological neural ...
, 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 Autoxidation (sometimes auto-oxidation) refers to oxidations brought about by reactions with oxygen at normal temperatures, without the intervention of flame or electric spark. The term is usually used to describe the gradual degradation of organi ...
. The transporters are also known to continue biochemical modification after loading certain neurotransmitters. 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 vesicular H+-ATPase. The current model of VMAT function proposes that the efflux of two protons (H+) against the H+ gradient is coupled with influx of one monoamine. The first H+ efflux generates a transporter conformation associated with a high-affinity amine-binding site in the cytosolic phase, and the second H+ efflux is coupled with a second large
conformational change In biochemistry, a conformational change is a change in the shape of a macromolecule, often induced by environmental factors. A macromolecule is usually flexible and dynamic. Its shape can change in response to changes in its environment or othe ...
that leads to amine transport from the cytosolic side into the vesicle, reducing amine-binding affinity. Studies indicate that the amino acid 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. It has been proposed that RES inhibits VMAT by interacting with this conformation. VMAT gene sequence analysis demonstrates that four aspartic acid residues in the middle region of TMDs I, VI, X, and XI and one
lysine Lysine (symbol Lys or K) is an α-amino acid that is a precursor to many proteins. Lysine contains an α-amino group (which is in the protonated form when the lysine is dissolved in water at physiological pH), an α-carboxylic acid group ( ...
residue in TMD II have highly conserved gene sequences, suggesting these residues play a critical role in transporter structure and function. Specifically, the residues Lys139 and Asp427 are thought to compose an ion pair that promotes high-affinity interaction with VMAT substrates and inhibitors. 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.


Kinetics

VMATs have a relatively low Vmax, with an estimated rate of 5–20/sec depending on the substrate. 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 A catecholamine (; abbreviated CA), most typically a 3,4-dihydroxyphenethylamine, is a monoamine neurotransmitter, an organic compound that has a catechol (benzene with two hydroxyl side groups next to each other) and a side-chain amine. Cate ...
s dopamine, norepinephrine, and epinephrine have a threefold higher affinity for VMAT2 than VMAT1 binding and uptake. The imidazoleamine histamine has a thirtyfold higher affinity for VMAT2 compared to VMAT1, and is thought to bind to a different site than other monoamines. Unlike catecholamines and histamine, the
indoleamine Indolamines are a family of neurotransmitters that share a common molecular structure. Indolamines are a classification of monoamine neurotransmitter, along with catecholamines and ethylamine derivatives. A common example of an indolamine is the ...
serotonin binds to VMAT1 and VMAT2 with a similar affinity for both transporter isoforms. 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.
Methamphetamines Methamphetamine (contracted from ) is a potent central nervous system (CNS) stimulant that is mainly used as a recreational or performance-enhancing drug and less commonly as a second-line treatment for attention deficit hyperactivity d ...
decrease Vmax, while
cocaine Cocaine is a tropane alkaloid and central nervous system stimulant, derived primarily from the leaves of two South American coca plants, ''Erythroxylum coca'' and ''Erythroxylum novogranatense, E. novogranatense'', which are cultivated a ...
increases Vmax reversibly in rat brain.


Inhibition

The effects of VMAT inhibition have been studied in-depth in animal models. Mutant
homozygous Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mos ...
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 The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...
, depleting catecholamine concentrations and causing increased
oxidative stress Oxidative stress reflects an imbalance between the systemic manifestation of reactive oxygen species and a biological system's ability to readily detoxify the reactive intermediates or to repair the resulting damage. Disturbances in the normal ...
and oxidative damage to the neuron.
Heterozygous Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mos ...
VMAT mutants display hypersensitivity to
amphetamine Amphetamine (contracted from Alpha and beta carbon, alpha-methylphenethylamine, methylphenethylamine) is a central nervous system (CNS) stimulant that is used in the treatment of attention deficit hyperactivity disorder (ADHD), narcolepsy, an ...
, cocaine, and
MPTP MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) is an organic compound. It is classified as a tetrahydropyridine. It is of interest as a precursor to the monoaminergic neurotoxin MPP+, which causes permanent symptoms of Parkinson's dise ...
(1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), the latter being a substance causally linked to
Parkinson's disease Parkinson's disease (PD), or simply Parkinson's, is a neurodegenerative disease primarily of the central nervous system, affecting both motor system, motor and non-motor systems. Symptoms typically develop gradually and non-motor issues become ...
(PD) in rodents. This suggests a protective role of VMATs against oxidative stress through removal of such substances from the cytosol. VMAT inhibitors include: *Reserpine (RES), bietaserpine, and
ketanserin Ketanserin, sold under the brand name Sufrexal, is an antihypertensive agent which is used to treat arterial hypertension and vasospasm, vasospastic disorders. It is also used in scientific research as an antiserotonergic medication, agent in th ...
(KET) (potent inhibitors of VMAT2 mediated serotonin transport) *
Tetrabenazine Tetrabenazine is a drug for the symptomatic treatment of hyperkinetic movement disorders. It is sold under the brand names Nitoman and Xenazine among others. On August 15, 2008, the US Food and Drug Administration (FDA) approved the use of tet ...
(TBZ) (specific to VMAT2) *
Phenylethylamine Phenethylamine (PEA) is an organic compound, natural monoamine alkaloid, and trace amine, which acts as a central nervous system stimulant in humans. In the brain, phenethylamine regulates monoamine neurotransmission by binding to trace amine ...
*
Amphetamine Amphetamine (contracted from Alpha and beta carbon, alpha-methylphenethylamine, methylphenethylamine) is a central nervous system (CNS) stimulant that is used in the treatment of attention deficit hyperactivity disorder (ADHD), narcolepsy, an ...
*
MDMA 3,4-Methylenedioxymethamphetamine (MDMA), commonly known as ecstasy (tablet form), and molly (crystal form), is an empathogen–entactogenic drug with stimulant and minor Psychedelic drug, psychedelic properties. In studies, it has been used ...
*''N''-Methyl-4-phenylpyridinium (MPP+) (very potent inhibitors of VMAT2 mediated serotonin transport) *
Fenfluramine Fenfluramine, sold under the brand name Fintepla, is a serotonergic medication used for the treatment of seizures associated with Dravet syndrome and Lennox–Gastaut syndrome. It was formerly used as an appetite suppressant in the treat ...
(specific to VMAT1 ) * 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. 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. RES, methoxytetrabenazine (MTBZ), and the drug
amiodarone Amiodarone is an antiarrhythmic medication used to treat and prevent a number of types of cardiac dysrhythmias. This includes ventricular tachycardia, ventricular fibrillation, and wide complex tachycardia, atrial fibrillation, and paroxys ...
bind to the RES binding site. TBZ (also called Nitoman and Xenazine), dihydrotetrabenazine (DTBZOH), ketanserin (KET), and the drug
lobeline Lobeline is a piperidine alkaloid found in a variety of plants, particularly those in the genus '' Lobelia'', including Indian tobacco ('' Lobelia inflata''), Devil's tobacco ('' Lobelia tupa''), great lobelia ('' Lobelia siphilitica''), '' Lobe ...
bind to the TBZ binding site. Amphetamine, methamphetamine and GZ-7931 are also known to interact with VMAT2. Vesicular monoamine transporter 2#Binding sites and ligands 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. The residues asp33 and ser180, 181, and 182 are believed to be involved in substrate recognition, and interact with the
protonated In chemistry, protonation (or hydronation) is the adding of a proton (or hydron, or hydrogen cation), usually denoted by H+, to an atom, molecule, or ion, forming a conjugate acid. (The complementary process, when a proton is removed from a Br ...
amino group and
hydroxyl group In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...
on the
catechol Catechol ( or ), also known as pyrocatechol or 1,2-dihydroxybenzene, is an organic compound with the molecular formula . It is the ''ortho'' isomer of the three isomeric benzenediols. This colorless compound occurs naturally in trace amounts. It ...
or
indole Indole is an organic compound with the formula . Indole is classified as an aromatic heterocycle. It has a bicyclic structure, consisting of a six-membered benzene ring fused to a five-membered pyrrole ring. Indoles are derivatives of indole ...
rings. Cocaine and
methylphenidate Methylphenidate, sold under the brand names Ritalin ( ) and Concerta ( ) among others, is a central nervous system (CNS) stimulant used in the treatment of attention deficit hyperactivity disorder (ADHD) and narcolepsy. It may be taken Oral adm ...
(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."


RES binding site

Consistent with catecholamine-binding affinity, RES has a threefold higher affinity for VMAT2 than for VMAT1. The RES binding site is known to be
hydrophobic In chemistry, hydrophobicity is the chemical property of a molecule (called a hydrophobe) that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, thu ...
, which is thought to contribute to ligand binding affinity. Methamphetamine binds to the RES site on VMATs. The current working model proposes that RES and the substrate bind to a single site in a pH-gradient modulated conformational structure of the transporter. The conformation occurs after the transport of one H+ 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. Methoxytetrabenazine (MTBZ) may bind to the RES binding site, based on studies indicating that RES significantly inhibited MTBZ-binding. Amiodarone is also believed to inhibit monoamine vesicular uptake by binding to the RES binding site.


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. This site is believed to be located at the
N-terminus The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the amin ...
, based on studies done in bovine VMAT2. Tyr434 and asp461 are identified as being responsible for the high-affinity interaction of TBZ, serotonin, and histamine in VMAT2. Unlike methamphetamine, amphetamine binds to the TBZ site on hVMAT2. Unlike RES inhibition, TBZ inhibition is only affected by very high concentrations of monoamines; however, single injections of RES can inhibit TBZ binding.
ketanserin Ketanserin, sold under the brand name Sufrexal, is an antihypertensive agent which is used to treat arterial hypertension and vasospasm, vasospastic disorders. It is also used in scientific research as an antiserotonergic medication, agent in th ...
(KET) and
lobeline Lobeline is a piperidine alkaloid found in a variety of plants, particularly those in the genus '' Lobelia'', including Indian tobacco ('' Lobelia inflata''), Devil's tobacco ('' Lobelia tupa''), great lobelia ('' Lobelia siphilitica''), '' Lobe ...
also bind to the TBZ binding site conformation.


Glycosylation sites: N- and C-linked terminals

Three to four glycosylation sites exist in the vesicular matrix on a loop between TMD I and TMD II. In biology, the vesicle
matrix Matrix (: matrices or matrixes) or MATRIX may refer to: Science and mathematics * Matrix (mathematics), a rectangular array of numbers, symbols or expressions * Matrix (logic), part of a formula in prenex normal form * Matrix (biology), the m ...
refers to the material or tissue between cells in which more specialized structures are embedded. Two of the glycosylation sites, the ''N''-linked glycosylation terminal and ''C''-linked terminal, are located in the cytosolic portion of the vesicle. 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.


C-terminus and VMAT trafficking cycle

Several motifs involved in the VMAT trafficking cycle are believed to be encoded in the C-terminus. A dileucine motif in the C-terminus is required for VMAT2
endocytosis Endocytosis is a cellular process in which Chemical substance, substances are brought into the cell. The material to be internalized is surrounded by an area of cell membrane, which then buds off inside the cell to form a Vesicle (biology and chem ...
. Studies suggest the acidic residues in the dileucine motif sort VMAT2 away from constitutive secretory vesicles and into the regulated
secretory pathway Secretion is the movement of material from one point to another, such as a secreted chemical substance from a cell (biology), cell or gland. In contrast, excretion is the removal of certain substances or waste products from a cell or organism. Th ...
. 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. 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.


Genetic expression and transporter regulation

Although both VMAT1 and VMAT2 are encoded by two different
gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...
s, the individual genetic sequences demonstrate high homology. Polymorphisms in VMAT2 that affect regulation and quantitative expression may pose genetic risk factors for PD. A specific VMAT1 gene (''SLC18A1'') has several associated polymorphisms, which have a locus 8p21.3 that has been strongly connected to
schizophrenia Schizophrenia () is a mental disorder characterized variously by hallucinations (typically, Auditory hallucination#Schizophrenia, hearing voices), delusions, thought disorder, disorganized thinking and behavior, and Reduced affect display, f ...
susceptibility. 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, post-transcriptional modifications such as
phosphorylation In biochemistry, phosphorylation is described as the "transfer of a phosphate group" from a donor to an acceptor. A common phosphorylating agent (phosphate donor) is ATP and a common family of acceptor are alcohols: : This equation can be writ ...
and
mRNA In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of Protein biosynthesis, synthesizing a protein. mRNA is ...
splicing of
exons An exon is any part of a gene that will form a part of the final mature RNA produced by that gene after introns have been removed by RNA splicing. The term ''exon'' refers to both the DNA sequence within a gene and to the corresponding sequence i ...
, 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. 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 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 Serotonin (), also known as 5-hydroxytryptamine (5-HT), is a monoamine neurotransmitter with a wide range of functions in both the central nervous system (CNS) and also peripheral tissues. It is involved in mood, cognition, reward, learning, ...
transport in blood platelets, but not in the brain, where Gαq completely inhibits VMAT2 activity. Although the exact signalling pathway for G-protein mediated regulation of VMATs is not known, it has recently been described that implicated G-proteins act directly on the VMATs.


Clinical significance

VMAT2 has been shown to contribute to many clinical neurological disorders including drug addiction, mood disorders, and stress, as well as Parkinson's disease and Alzheimer's disease.


Parkinson's disease

Studies indicate VMAT2 mRNA is present in all cell groups damaged by Parkinson's disease (PD); 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, which may be due to a ratio between the
dopamine transporter The dopamine transporter (DAT, also sodium-dependent dopamine transporter) is a membrane-spanning protein coded for in humans by the ''SLC6A3'' gene (also known as ''DAT1''), that pumps the neurotransmitter dopamine out of the synaptic cleft ba ...
and VMAT2. Based on the understanding the increased cytosolic dopamine levels lead to dopaminergic cell death in PD, it has been proposed that regulatory polymorphisms in VMAT2 affect VMAT2 quantitative expression, and may serve as a genetic risk factor for PD. Specifically, the SLC18A2
promoter region In genetics, a promoter is a sequence of DNA to which proteins bind to initiate transcription of a single RNA transcript from the DNA downstream of the promoter. The RNA transcript may encode a protein (mRNA), or can have a function in and of it ...
for the VMAT2 gene has been identified as an area where several polymorphisms form discrete
haplotype A haplotype (haploid genotype) is a group of alleles in an organism that are inherited together from a single parent. Many organisms contain genetic material (DNA) which is inherited from two parents. Normally these organisms have their DNA orga ...
s.


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. Reduced VMAT2 levels were identified in specific subregions of the
striatum The striatum (: striata) or corpus striatum is a cluster of interconnected nuclei that make up the largest structure of the subcortical basal ganglia. The striatum is a critical component of the motor and reward systems; receives glutamat ...
involved in clinical depression, including the
nucleus accumbens The nucleus accumbens (NAc or NAcc; also known as the accumbens nucleus, or formerly as the ''nucleus accumbens septi'', Latin for ' nucleus adjacent to the septum') is a region in the basal forebrain rostral to the preoptic area of the hypo ...
shell but not the core, the
ventral tegmental area The ventral tegmental area (VTA) (tegmentum is Latin for ''covering''), also known as the ventral tegmental area of Tsai, or simply ventral tegmentum, is a group of neurons located close to the midline on the floor of the midbrain. The VTA is th ...
, and the
substantia nigra The substantia nigra (SN) is a basal ganglia structure located in the midbrain that plays an important role in reward and movement. ''Substantia nigra'' is Latin for "black substance", reflecting the fact that parts of the substantia nigra a ...
's
pars compacta The pars compacta (SNpc, SNc) is one of two subdivisions of the ''substantia nigra'' of the midbrain (the other being the pars reticulata); it is situated medial to the ''pars reticulata''. It is formed by dopaminergic neurons. It projects to th ...
. 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.


Drug addiction

Many
psychostimulant Stimulants (also known as central nervous system stimulants, or psychostimulants, or colloquially as uppers) are a class of drugs that increase alertness. They are used for various purposes, such as enhancing attention, motivation, cognition ...
drugs are known to interact with VMAT, including amphetamine analogs such as methamphetamine, cocaine, and ecstasy (MDMA).


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. RES, methoxytetrabenazine, and amiodarone bind to the RES binding site. TBZ, DTBZOH, ketanserin, and lobeline bind to the TBZ binding site. Many
psychostimulants Stimulants (also known as central nervous system stimulants, or psychostimulants, or colloquially as uppers) are a class of drugs that increase alertness. They are used for various purposes, such as enhancing attention, motivation, cognition ...
, including
substituted amphetamines Substituted amphetamines, or simply amphetamines, are a class of compounds based upon the amphetamine structure; it includes all derivative compounds which are formed by replacing, or substituting, one or more hydrogen atoms in the amphetam ...
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.


Methamphetamine

VMAT is a main target of methamphetamine. Studies indicate that substituted amphetamines including methamphetamine interact with VMAT2 at the TBZ/DTBZOH binding site. By acting as a
negative allosteric modulator In pharmacology and biochemistry, allosteric modulators are a group of substances that bind to a receptor to change that receptor's response to stimuli. Some of them, like benzodiazepines or alcohol, function as psychoactive drugs. The site that a ...
, 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
fraction A fraction (from , "broken") represents a part of a whole or, more generally, any number of equal parts. When spoken in everyday English, a fraction describes how many parts of a certain size there are, for example, one-half, eight-fifths, thre ...
to a location that is not continuous with synaptosomal preparations. Repeated amphetamine exposure may increase VMAT2 mRNA in certain brain regions with little or no decline upon withdrawal from the drug. A study performed by Sonsalla ''et al.'' demonstrated that methamphetamine treatment decreases DHTBZ binding and vesicular dopamine uptake. Another study demonstrated that multiple high doses of methamphetamine removed DTBZ binding sites from the vesicles. 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. 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. 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.


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. Methylphenidate is believed to interact with VMAT2 in a similar fashion. In addition to mobilizing VMAT2-expressing vesicles, cocaine has been shown to increase the Vmax of VMAT2 for dopamine and increase the number of DTBZ binding sites. It has also mobilized a
synapsin The synapsins are a family of proteins that have long been implicated in the regulation of neurotransmitter release at synapses. Specifically, they are thought to be involved in regulating the number of synaptic vesicles available for release via ...
-dependent reserve pool of dopamine-containing synaptic vesicles, which interacts with the vesicular trafficking cycle to increase dopamine release. Short-term exposure to
cocaine Cocaine is a tropane alkaloid and central nervous system stimulant, derived primarily from the leaves of two South American coca plants, ''Erythroxylum coca'' and ''Erythroxylum novogranatense, E. novogranatense'', which are cultivated a ...
increases VMAT2 density in the
prefrontal cortex In mammalian brain anatomy, the prefrontal cortex (PFC) covers the front part of the frontal lobe of the cerebral cortex. It is the association cortex in the frontal lobe. The PFC contains the Brodmann areas BA8, BA9, BA10, BA11, BA12, ...
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. 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.


MDMA

MDMA is known to affect serotonergic neurons, but has been shown to inhibit synaptosomal and vesicular uptake of serotonin and dopamine to roughly the same extent ''in vitro''. ''In vivo'' studies indicate short-term MDMA exposure causes short-term reduction in VMAT2 activity, which is reversed after 24 hours.


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; however, no specific diseases have been identified yet as directly resulting from a genetic mutation in an ''SLC18'' gene, which codes for VMAT proteins. 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. 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 Schizophrenia () is a mental disorder characterized variously by hallucinations (typically, Auditory hallucination#Schizophrenia, hearing voices), delusions, thought disorder, disorganized thinking and behavior, and Reduced affect display, f ...
,
bipolar disorder Bipolar disorder (BD), previously known as manic depression, is a mental disorder characterized by periods of Depression (mood), depression and periods of abnormally elevated Mood (psychology), mood that each last from days to weeks, and in ...
, and various anxiety disorders. 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 In genetics and bioinformatics, a single-nucleotide polymorphism (SNP ; plural SNPs ) is a germline substitution of a single nucleotide at a specific position in the genome. Although certain definitions require the substitution to be present in a ...
s (SNPs) have been identified in the
coding region The coding region of a gene, also known as the coding DNA sequence (CDS), is the portion of a gene's DNA or RNA that codes for a protein. Studying the length, composition, regulation, splicing, structures, and functions of coding regions compared ...
of VMATs. The effects of some of these SNPs have been alteration of VMAT function, structure and regulation.Sager, J.J. & Torres, G.E., 2011. Proteins interacting with monoamine transporters: Current state and future challenges. Biochemistry, nlineAvailable at: ccessed 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 In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, ...
origins. α-synuclein, a cytosolic protein found mainly in 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. 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 An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...
s responsible for the synthesis of dopamine,
tyrosine hydroxylase Tyrosine hydroxylase or tyrosine 3-monooxygenase is the enzyme responsible for catalyzing the conversion of the amino acid L-tyrosine to L-3,4-dihydroxyphenylalanine (L-DOPA). It does so using molecular oxygen (O2), as well as iron (Fe2+) and ...
and amino acid aromatic decarboxylase are physically and functionally coupled with VMAT2. 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. 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. Mice are also used in drug studies, particularity studies involving the effect cocaine and methamphetamine have on VMATs. 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. Enhancing the function of VMATs may also have therapeutic value.


References


External links

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Further reading

* * * * {{Monoamine releasing agents Neurochemistry Signal transduction Receptors Neurotransmitter transporters Biogenic amines Integral membrane proteins VMAT inhibitors Amphetamine