Distinct pharmacological properties and distribution in neurons and endocrine cells of two isoforms of the human vesicular monoamine transporter.

Abstract

A second isoform of the human vesicular monoamine transporter (hVMAT) has been cloned from a pheochromocytoma cDNA library. The contribution of the two transporter isoforms to monoamine storage in human neuroendocrine tissues was examined with isoform-specific polyclonal antibodies against hVMAT1 and hVMAT2. Central, peripheral, and enteric neurons express only VMAT2. VMAT1 is expressed exclusively in neuroendocrine, including chromaffin and enterochromaffin, cells. VMAT1 and VMAT2 are coexpressed in all chromaffin cells of the adrenal medulla. VMAT2 alone is expressed in histamine-storing enterochromaffin-like cells of the oxyntic mucosa of the stomach. The transport characteristics and pharmacology of each VMAT isoform have been directly compared after expression in digitonin-permeabilized fibroblastic (CV-1) cells, providing information about substrate feature recognition by each transporter and the role of vesicular monoamine storage in the mechanism of action of psychopharmacologic and neurotoxic agents in human. Serotonin has a similar affinity for both transporters. Catecholamines exhibit a 3-fold higher affinity, and histamine exhibits a 30-fold higher affinity, for VMAT2. Reserpine and ketanserin are slightly more potent inhibitors of VMAT2-mediated transport than of VMAT1-mediated transport, whereas tetrabenazine binds to and inhibits only VMAT2. N-methyl-4-phenylpyridinium, phenylethylamine, amphetamine, and methylenedioxymethamphetamine are all more potent inhibitors of VMAT2 than of VMAT1, whereas fenfluramine is a more potent inhibitor of VMAT1-mediated monamine transport than of VMAT2-mediated monoamine transport. The unique distributions of hVMAT1 and hVMAT2 provide new markers for multiple neuroendocrine lineages, and examination of their transport properties provides mechanistic insights into the pharmacology and physiology of amine storage in cardiovascular, endocrine, and central nervous system function.Storage of monoamines in secretory organelles of neurons, endocrine/paracrine cells, basophils, blood platelets, and mast cells is critical for their regulated, physiological secretion. Monoamine accumulation from the cytoplasm into storage organelles is mediated by vesicular monoamine transporters (VMATs) with an absolute dependence on a vacuolar ATPase-generated proton gradient to transport the cationic amine substrates into the storage organelle in exchange for protons (1).Recently, we cloned a VMAT (formerly named MAT) from a rat basophilic leukemia cell line (RBL-2H3) by functional expression of T7 promoter-driven cDNA sublibraries in CV-1 fibroblasts infected with T7 polymerase-expressing recombinant vaccinia virus (2). Permeabilization of the plasma membrane with digitonin provided the first demonstration that monoamine substrates could be directly accumulated by an intracellular compartment of nonneuroendocrine cells expressing this transporter in an ATP-dependent fashion that was sensitive to the specific inhibitors reserpine and tetrabenazine (TBZ). The mRNA for this transpor...

Different subcellular distributions of the vesicular monoamine transporter, VMAT2, in subclasses of sympathetic neurons

“…VMAT1 is preferentially expressed by adrenal chromaffin cells and by small intensely fluorescent cells in sympathetic ganglia (Weihe et al, 1994). By contrast, selective VMAT2 expression has been reported for noradrenergic sympathetic neurons, midbrain dopamine neurons and other monoaminergic neurons in the brain stem (Erickson et al, 1996;Weihe and Eiden, 2000;Weihe et al, 1994). Taken together, this earlier literature conveys the distinct impression that all noradrenergic sympathetic neurons are immunoreactive for VMAT2.…”

Pharmacogenomic Evaluation of the Antidepressant Citalopram in the Mouse Tail Suspension Test

“…VMAT2 is a neuronally expressed subtype of the vesicular monoamine transporter responsible for loading synaptic vesicles with the monoamine neurotransmitters serotonin, norepinephrine, dopamine, and histamine (Liu et al, 1992;Peter et al, 1994;Erickson et al, 1992Erickson et al, , 1996. Mice lacking both copies of the VMAT2 gene die during development, but heterozygotes survive and show reduced amphetamineconditioned reward, enhanced amphetamine and cocaineinduced locomotion, and enhanced MPTP toxicity (Takahashi et al, 1997;Wang et al, 1997).…”

Uptake of meta-iodobenzylguanidine in neuroendocrine tumours is mediated by vesicular monoamine transporters

“…The reason why certain carcinoid tumours cannot be visualised has not been explained, and the precise mechanism behind MIBG uptake and retention in NE tumours is unclear. It has been shown that MIBG is a substrate for VMATs, located on the membrane of secretory vesicles of NE cells (Henry et al, 1994;Erickson et al, 1996). It is therefore likely that after passing the plasma membrane, MIBG is transported into secretory granules by VMATs.…”