The vesicular monoamine transporter (VMAT) is a transport protein integrated into the membranes of synaptic vesicles of presynaptic neurons. It transports monoamine neurotransmitters – such as dopamine, serotonin, norepinephrine, epinephrine, and histamine – into the vesicles, which release the neurotransmitters into synapses, as chemical messages to postsynaptic neurons. VMATs utilize a proton gradient generated by V-ATPases 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, 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 amines. Exogenous substrates include guanethidine and MPP<sup>+</sup>.

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. 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.

Location

There are two types of VMATs expressed in humans: VMAT1 and VMAT2. expressed in blood platelets, and co-expressed in chromaffin cells.

Structure and function

thumb|right|A Hydrogen atom from the inside of the vesicle binds, inducing a conformational change in the transporter

thumb|right|The conformational change induced by the hydrogen atom binding enables the monoamine binding to the active transport site

thumb|right|A second hydrogen atom binds from inside the vesicle to the transporter inducing another change

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 glycoproteins with a molecular weight of approximately 70 kDa. Both isoforms are transmembrane proteins with 12 transmembrane domains (TMDs). VMATs use the same transport mechanism for all types of monoamines, 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. 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 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.

Kinetics

VMATs have a relatively low V<sub>max</sub>, 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 catecholamines 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,

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.

RES binding site

Consistent with catecholamine-binding affinity, RES has a threefold higher affinity for VMAT2 than for VMAT1. Methoxytetrabenazine (MTBZ) may bind to the RES binding site, based on studies indicating that RES significantly inhibited MTBZ-binding. This site is believed to be located at the N-terminus, based on studies done in bovine VMAT2.

Unlike RES inhibition, TBZ inhibition is only affected by very high concentrations of monoamines; however, single injections of RES can inhibit TBZ binding.

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.

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 and mRNA 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.

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,

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 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. however, no specific diseases have been identified yet as directly resulting from a genetic mutation in an SLC18 gene, which codes for VMAT proteins. 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 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.