A molecular glimpse of vesicular monoamine transporters

Selective cytotoxicity of topoisomerase-directed protoberberines against glioblastoma cells

Protoberberines are a new class of organic cations that are dual poisons of topoisomerases I and II. Certain protoberberines exhibit greater in vitro cytotoxicity against cell lines derived from solid tumors than from leukemias. Using a group of seventeen different protoberberine analogs, the structural basis for selective cytotoxicity toward sensitive SF-268 glioblastoma cells as compared with resistant RPMI 8402 lymphoblast cells was explored. The selective cytotoxicity is associated with the presence of an imminium ion and other structural features of protoberberines, and is not shared by drugs such as camptothecin, doxorubicin, vinblastine, and etoposide, which are either equally or more cytotoxic against RPMI 8402 cells than SF-268 cells. The selective cytotoxicity of protoberberines against SF-268 over RPMI 8402 cells is not due to differences in topoisomerase levels or known drug efflux systems such as multidrug resistance (MDR1) and multidrug-resistance protein (MRP). Comparative in vitro studies of the accumulation of coralyne, a fluorescent protoberberine, into sensitive and resistant cells demonstrated a correlation between drug accumulation and selective cytotoxicity. Inhibitors of coralyne uptake included several protoberberine-related compounds. Of these, palmatine, a minimally cytotoxic protoberberine, both inhibited coralyne accumulation and reduced cytotoxicity against SF-268 cells, but not against RPMI 8402 cells. Despite the structural resemblance of protoberberines to catecholamines, our experiments using inhibitors and cells expressing biogenic amine uptake systems have ruled out the involvement of biogenic amine uptake1, uptake2, and vesicular monoamine transport systems. Uptake systems remaining as candidates, supported by preliminary data, include transport via vesicles derived from specialized membrane invaginations and selected carrier-mediated organic amine transport systems.

Ontogeny of vesicular monoamine transporter mRNAs VMAT1 and VMAT2. II. Expression in neural crest derivatives and their target sites in the rat

We used in situ hybridization histochemistry to study the expression of the two vesicular monoamine transporters (VMAT1 and VMAT2) during embryonic development in the rat. In the adult rat VMAT2 is present exclusively in neuronal tissues and VMAT1 is present in the adrenal medulla and in certain intestinal endocrine cells. We found that both transporter molecules are more widely expressed during development. We demonstrate a complete overlap of the two VMAT mRNAs in the sympathetic nervous system between E13 and E21 days. In addition, VMAT2 (and to some extent VMAT1) mRNA is expressed in ganglionic cells of the parasympathetic nervous system and in cranial ganglia (trigeminal, vestibular and spiral ganglia) between E12 and E21. The sensory neurons of the dorsal root ganglia, which are also neural crest derivatives, express VMAT2 mRNA (E11-E21), exclusively. Both VMAT mRNAs are found in the developing GI system, but in different cells. VMAT1 mRNA was detected in organs of the endocrine system (pituitary gland, adrenal gland, testis, seminal vesicle), some connective tissue cells, and the thymus. We observed expression of both VMAT mRNAs in two separate cell groups in the placenta (E8-E10). Based on their distribution during development we suggest that monoamines, released in a controlled fashion, might affect migration and differentiation of neural crest derivatives.

Transient expression of the vesicular monoamine transporter during development in the rat thalamus and cortex

The postnatal developmental pattern of the central vesicular monoamine transporter-2 (VMAT2) was analyzed in the rat brain by means of quantitative autoradiography with a specific and high affinity ligand [3H]dihydrotetrabenazine ([3H]TBZOH). We show a dense expression of VMAT2 in the cortex (especially area 17) and thalamus (particularly the dorsal lateral geniculate nucleus) at postnatal days 1 and 8. This pattern of VMAT2 distribution was transient since it was no longer observed at day 20 or in the adult rat brain where VMAT2 density was weak and uniform in these regions. These data suggest that monoamine vesicular storage participates in the early postnatal maturation of thalamus and cortex.

Neurotransmitter transporters as molecular targets for addictive drugs

The neurotransmitter dopamine lies at or near the center of current theories of drug abuse and dependence. Multiple lines of evidence indicate that dopaminergic cells play key roles in a variety of motivated behaviors. Accordingly, it is not surprising that cocaine and amphetamines--some of the most widely used illicit drugs--elevate extraneuronal dopamine concentrations through their actions on the plasma membrane dopamine transporter. From the point of view of developing novel pharmacological interventions for the treatment or prevention of psychostimulant abuse, practical benefits may arise from an improved understanding of how neurotransmitter transporters operate and how drugs interact with them.

Dopamine levels of two classes of vesicles are differentially depleted by amphetamine

Differential depletion of neurotransmitter by amphetamine in two classes of vesicles, termed large vesicles and small vesicles, has been studied with amperometry. Carbon fiber microelectrodes have been used to monitor and quantify exocytotic events. Current transients, corresponding to individual exocytotic events, have been obtained from the cell body of the dopamine-containing neuron of Planorbis corneus. The dopamine released from individual vesicles of these cells has been compared for cells treated with D-amphetamine vs. control cells. Our results show that amphetamine has differential effects on the release of dopamine from the two classes of vesicles. Thus, it is concluded that at low concentrations, amphetamine preferentially depletes the large vesicles with a minimal effect on the small vesicles. At high concentrations, amphetamine depletes small vesicles more strongly than large vesicles although amphetamine continues to deplete the large vesicles in a dose-dependent manner. Our data appear to indicate that the two classes of vesicles observed in the Planorbis dopamine neuron might have different mechanisms associated with transmitter depletion.

Vesicular amine transporter expression and isoform selection in developing brain, peripheral nervous system and gut

The vesicular monoamine transporters VMAT1 and VMAT2 are essential components of monoaminergic neurons and endocrine cells whose expression in development may provide insight into lineage pathways for chemical coding in the diffuse neuroendocrine system. Thus, the brain is a compartment in which only monoaminergic neurons are generated, the gut epithelium generates only endocrine monoamine-containing cells, and the neural crest produces both autonomic monoaminergic neurons and endocrine/paracrine monoaminergic cells. Selection of either the VMAT1 or VMAT2 isoform was examined in these three compartments during development. In the central nervous system VMAT2, but not VMAT1, was expressed in neuroepithelial cells by embryonic day 12 (E12), and all major monoaminergic cell groups by E14. Thalamocortical and hypothalamic neurons that do not express VMAT2 in adulthood were transiently VMAT2-positive from E16 to postnatal day 6 (P6). EC cells of the gut expressed exclusively VMAT1 from E19 on, while histamine-containing enterochromaffin-like (ECL) cells of the stomach expressed only VMAT2 by E19 and throughout postnatal development. VMAT2 and the vesicular acetylcholine transporter VAChT were co-expressed in early development of the primary sympathetic chain as well as in the cranial parasympathetic ganglia. VAChT was progressively restricted to a small population of VMAT2-negative post-ganglionic neurons in the adult sympathetic chain, while VMAT2 expression persisted in sympathetic principal ganglion and SIF cells but was eventually extinguished in cranial parasympathetic ganglia. VMAT1 was co-expressed with VAChT and VMAT2 mRNA in the primary sympathetic chain on E12, but progressively restricted to small intensely fluorescent (SIF) and chromaffin cells thereafter. Thus, expression of the vesicular amine transporters appropriate for chemical coding of brain neurons and gut endocrine cells are pre-determined developmentally. In contrast, the neural crest-derived sympathoadrenal and neural crest-derived parasympathetic cell groups examined here initially co-express two or more vesicular amine transporters, followed by extinction of the inappropriate transporter(s) later in development. Some neural crest-derived neuroendocrine cell populations continue to express both isoforms of VMAT even in adulthood. Lineage distinctions in ontogeny of vesicular amine transporter expression in brain, gut and autonomic nervous system make it likely that the same genes are regulated differently in the autonomic nervous system compared to brain and gut.

High-efficiency expression and characterization of the synaptic-vesicle monoamine transporter from baculovirus-infected insect cells

The full-length cDNA for the rat synaptic-vesicle monoamine transporter (VMAT2) containing a C-terminal polyhistidine epitope has been engineered into baculovirus DNA for expression in Spodoptera frugiperda (Sf9) insect cells. Using this recombinant baculovirus and cultured Sf9 cells, rVMAT2 has been expressed at levels of 7.8x10(6) transporters per cell, as assessed by [3H]dihydrotetrabenazine binding. A 1l culture of infected cells produced approx. 15 nmol (900 microg) of transporter. rVMAT2 expressed in the Sf9 cells bound [3H]dihydrotetrabenazine with a KD of 31.2 nM and a Bmax of 19.9 pmol/mg. Two polypeptides of 55 and 63 kDa were identified using the photolabel, 7-azido-8-[125I]iodoketanserin ([125I]AZIK). Photoaffinity labelling of rVMAT2 by 1 nM [125I]AZIK was protectable by 10 microM tetrabenazine and 10 microM 7-aminoketanserin. Digitonin-solubilized VMAT2 was purified to greater than 95% homogeneity using immobilized Ni2+-affinity chromatography, followed by lectin (Concanavalin A) chromatography. The purified transporter migrates as a single broad band with a molecular mass of approx. 63kDa, as analyzed by SDS/PAGE. The purified transporter retained the ability to bind ligands ([125I]AZIK and [3H]dihydrotetrabenazine). The purified VMAT2 bound [3H]dihydrotetrabenazine with a KD of 86.2 nM. As is the case with the monoamine transporter from bovine chromaffin granule membranes, purified VMAT2 is covalently modified by dicyclohexylcarbodi-imide (DCCD) and is specifically labelled by [14C]DCCD. This labelling is inhibited by tetrabenazine and ketanserin. These data indicate that VMAT2 can be overexpressed using the baculovirus expression system and purified.

Distribution of the vesicular transporter for acetylcholine in the rat central nervous system

In order to develop another selective marker for cholinergic cell bodies and fibres, we have raised a highly specific polyclonal antibody against a peptide derived from the C-terminus of a recently cloned putative vesicular acetylcholine transporter. This antibody recognizes the vesicular acetylcholine transporter protein on western blots of membranes from transfected monkey fibroblast COS cells as well as from various rat brain regions but not from untransfected COS cells or rat liver. In separate mapping studies, the antibody was found to stain cell bodies and fibres in all of the regions of the nervous system known to be cholinergic, including (i) the various nuclei of the basal nuclear complex and their projections to the hippocampus, amygdala, and cerebral cortex, (ii) the caudate-putamen nucleus, accumbens nucleus, olfactory tubercle, and islands of Calleja complex, (iii) the medial habenula, (iv) the mesopontine cholinergic complex and its projections to the thalamus, extrapyramidal motor nuclei, basal forebrain, cingulate cortex, raphe and reticular nuclei, and some cranial nerve nuclei, and (v) the somatic motor and autonomic nuclei of the cranial and spinal nerves. In many of these cholinergic neurons, it is possible to detect immunoreactivity for the vesicular acetylcholine transporter in proximal portions of processes and their branches, as well as in numerous puncta in close association with them. Some of these puncta are large and surround cell bodies and processes of neurons in several regions, including the somatic motor neurons of cranial nerve nuclei in the brainstem and in the ventral horn of the spinal cord. Double immunofluorescence studies indicated that neurons positive for the vesicular acetylcholine transporter also stained for the biosynthetic enzyme of acetylcholine, choline acetyltransferase. We conclude that antibody against the C-terminus of the putative vesicular acetylcholine transporter provides another marker for cholinergic neurons that, unlike in situ hybridization procedures, labels terminals as well as cell bodies. Therefore this antibody has the potential to reveal changes in number and morphology of cholinergic cell bodies and their terminal varicosities that occur in both physiologic and pathologic conditions.

Acetylcholine release and the cholinergic genomic locus

Choline acetyltransferase and vesicular acetylcholine-transporter genes are adjacent and coregulated. They define a cholinergic locus that can be turned on under the control of several factors, including the neurotrophins and the cytokines. Hirschprung's disease, or congenital megacolon, is characterized by agenesis of intramural cholinergic ganglia in the colorectal region. It results from mutations of the RET (GDNF-activated) and the endothelin-receptor genes, causing a disregulation in the cholinergic locus. Using cultured cells, it was shown that the cholinergic locus and the proteins involved in acetylcholine (ACh) release can be expressed separately ACh release could be demonstrated by means of biochemical and electrophysiological assays even in noncholinergic cells following preloading with the transmitter. Some noncholinergic or even nonneuronal cell types were found to be capable of releasing ACh quanta. In contrast, other cells were incompetent for ACh release. Among them, neuroblastoma N18TG-2 cells were rendered release-competent by transfection with the mediatophore gene. Mediatophore is an ACh-translocating protein that has been purified from plasma membranes of Torpedo nerve terminal; it confers a specificity for ACh to the release process. The mediatophores are activated by Ca2+; but with a slower time course, they can be desensitized by Ca2+. A strictly regulated calcium microdomain controls the synchronized release of ACh quanta at the active zone. In addition to ACh and ATP, synaptic vesicles have an ATP-dependent Ca2+ uptake system; they transiently accumulate Ca2+ after a brief period of stimulation. Those vesicles that are docked close to Ca2+ channels are therefore in the best position to control the profile and dynamics of the Ca2+ microdomains. Thus, vesicles and their whole set of associated proteins (SNAREs and others) are essential for the regulation of the release mechanism in which the mediatophore seems to play a key role.

Peptide mapping of the [125I]Iodoazidoketanserin and [125I]2-N-[(3'-iodo-4'-azidophenyl)propionyl]tetrabenazine binding sites for the synaptic vesicle monoamine transporter

The full-length cDNA for the rat recombinant synaptic vesicle monoamine transporter (rVMAT2) containing a COOH-terminal polyhistidine epitope was engineered into baculovirus DNA for expression in Spodoptera frugiperda (Sf9) cells. Using this recombinant baculovirus and cultured Sf9 cells, rVMAT2 has been expressed to high levels and purified to >95% homogeneity using immobilized Ni2+-affinity chromatography followed by lectin (concanavalin A) chromatography. Purified transporter was photolabeled using [125I]-7-azido-8-iodoketanserin ([125I]AZIK) and [125I]2-N-[(3'-iodo-4'-azidophenyl)propionyl]tetrabenazine ([125I]TBZ-AIPP). Both [125I]AZIK and [125I]TBZ-AIPP photoaffinity labeling of purified rVMAT2 were protectable by 10 microM tetrabenazine (TBZ), 10 microM 7-aminoketanserin, and 1 mM concentrations of the transporter substrates dopamine, norepinephrine, and serotonin. Radiolabeled peptides were generated using enzymatic and chemical methods, purified using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and NH2-terminal microsequenced. Radiosequencing of [125I]AZIK-labeled rVMAT2 indicated derivatization of Lys-20 in the NH2 terminus, just prior to putative transmembrane domain 1 (TMD1). [125I]TBZ-AIPP derivatized a segment of rVMAT2 between Gly-408 and Cys-431 in TMD10 and 11. These data implicate juxtaposition of TMD1 and 10/11.

Expression of dopamine transporter and vesicular monoamine transporter 2 mRNAs in rat midbrain after repeated amphetamine administration

The dopamine transporter (DAT) in pre-synaptic membranes and the vesicular monoamine transporter 2 (VMAT2) in membranes of synaptic vesicles are involved in mediating the acute effects of amphetamine on dopamine transmission. Therefore, using a quantitative method of in situ hybridization and computerized image analysis, the expression of DAT and VMAT2 mRNAs was examined in rats treated for 5 days with amphetamine and killed 3 or 14 days after the last injection. We examined ventral tegmental area (VTA), substantia nigra (SN) and the transitional zone between VTA and SN. Each of these regions was further subdivided into rostral, intermediate and caudal portions. In control rats, autoradiographs revealed a gradient of both DAT and VMAT2 mRNA levels, decreasing gradually from rostral to caudal rat midbrain. After 3 days of withdrawal, a significant increase in DAT mRNA levels was found in rostral portions of VTA (117.9 + 5.8% of control group), SN (116.5 + 4.5%) and the transitional zone (119.6 + 5.6%) and in the intermediate portion of SN (113.5 + 4.3%). VMAT2 mRNA was significantly increased only in rostral and intermediate portions of the transitional zone (120.9 + 4.8 and 113.6 + 4.1%). After 14 days of withdrawal, there was a trend towards increased DAT mRNA levels in intermediate-caudal portions of midbrain, but a statistically significant increase was observed only in the intermediate portion of VTA (120.2 + 7.9%). No changes in VMAT2 mRNA levels were found. Thus, repeated amphetamine administration exerts modest and regionally selective effects on DAT and VMAT2 mRNA expression in subpopulations of midbrain dopamine neurons.

Characterization of a diamine exporter in Chinese hamster ovary cells and identification of specific polyamine substrates

Export of the diamine putrescine was studied using inside-out plasma membrane vesicles prepared from Chinese hamster cells. Putrescine uptake into vesicles was a saturable and an ATP- and antizyme-independent process. Excess amounts of a series of diamines or monoacetyl spermidine, but not monoacetyl putrescine, spermidine, or spermine, inhibited putrescine transport. Putrescine uptake into vesicles prepared at pH 7.4 was suppressed at pH 5, compared with pH 7.4; was stimulated approximately 2.5-fold at pH 7.4 in vesicles prepared at pH 6.25, compared with vesicles prepared at pH 7.4; and was not inhibited by valinomycin in the presence of potassium ions. Reserpine and verapamil blocked [3H]putrescine uptake into inverted vesicles. Verapamil treatment caused an increase in intracellular contents of putrescine, cadaverine, and N8-acetylspermidine, in unstressed proliferating cells, or of N1-acetylspermidine, in cells subjected to heat shock to induce acetylation of spermidine at N1. These data indicate that putrescine export in Chinese hamster cells is mediated by a non-electrogenic antiporter capable of using protons as the counter ion. Physiological substrates for this exporter include putrescine, cadaverine, and monoacetyl spermidine and have the general structure NH3+-(CH2)n-NH2 + R at acidic or neutral pH.

Vesicular monoamine transporter-2: immunogold localization in striatal axons and terminals

The vesicular monoamine transporter-2 (VMAT2) mediates the reserpine-sensitive neuronal uptake of monoamines into vesicles and other intracellular organelles. Accordingly, this transporter is expressed at high levels in regions that contain a dense monoamine innervation, such as the rat dorsolateral striatum. We used ultrastructural immunocytochemistry in this region to show that immunogold labeling for VMAT2 is present in varicose axonal processes, many of which also contain the catecholamine-synthesizing enzyme tyrosine-hydroxylase. Within these mainly dopaminergic processes, VMAT2 was associated with small synaptic vesicles (SSVs) and more rarely with large dense-core vesicles or tubulovesicles. These findings suggest that SSVs are the major organelles involved in the storage and release of dopamine in the dorsolateral striatum.

Structural organization of the human vesicular monoamine transporter type-2 gene and promoter analysis using the jelly fish green fluorescent protein as a reporter

The genomic structure of a human vesicle monoamine transporter, type-2 (hVMAT2) was determined from two overlapping cosmids, phVMAT2-cos1 and phVMT2-cos2, spanning more than 35 kb. The hVMAT2 open reading frame is encoded by 16 exons, with translation initiation and termination in exon 2 and exon 16, respectively. Several potential binding sites for transcriptional regulatory factors, including a cAMP response element (CRE) were identified in the 5'-upstream region of the gene. A promoter construct using the jellyfish green fluorescent protein (GFP) as reporter has been made and transfected into the human neuroblastoma cell line, SHSY-5Y. The cellular expression of the GFP was readily detected by fluorescence microscopy and cells expressing GFP could be sorted using a fluorescence-activated cell sorter (FACS), allowing the level of GFP expression in transfected SHSY-5Y cells to be quickly and reliably determined.

Charged residues in transmembrane domains II and XI of a vesicular monoamine transporter form a charge pair that promotes high affinity substrate recognition

Vesicular monoamine transporters package monoamine neurotransmitters into secretory vesicles for regulated exocytotic release. Both vesicular monoamine transporter 1 and 2 contain several charged residues predicted to reside within transmembrane domains (TMDs), and conservation of these residues in multiple species and in other members of the gene family suggest important roles in transporter structure and function. To determine the role of these residues, we have used site-directed mutagenesis. Replacement of Asp-263 in TMD6 with Asn (D263N) had no effect on transport activity. However, replacement of Lys-139 in TMD2 with Ala (K139A), Asp-400 in TMD10 with Asn (D400N), or Asp-427 in TMD11 with Asn (D427N) eliminated transport activity despite normal levels of protein expression. Remarkably, the double mutant K139A/D427N showed substantial transport activity, suggesting that Lys-139 and Asp-427 interact to form an ion pair in the native protein and hence that TMD2 occurs next to TMD11. Nonetheless, the double mutant showed reduced apparent affinity for serotonin and reduced ability of serotonin to inhibit reserpine binding, suggesting that although not required for activity, the ion pair promotes high affinity interaction with the substrate. In addition, a double mutant in which the polarity of the charged residues was reversed (K139D/D427K) showed no active transport. Remarkably, however, this mutant displayed normal reserpine binding that remained coupled to DeltaH+, but serotonin failed to inhibit reserpine binding, suggesting that the charge reversal specifically disrupts substrate recognition.

Differential distribution of the putative vesicular transporter for acetylcholine in the rat central nervous system

The organization and distribution of the mRNA for the putative vesicular transporter for acetylcholine (VAChT) was studied in the rat brain by use of digoxigenin-labeled riboprobes and in situ hybridization technology. Signal was observed in all neural regions deduced to contain cholinergic somata on the basis of previous histochemical investigations employing choline acetyltransferase riboprobes and prior immunocytochemical studies with antibodies against choline acetyltransferase. It was absent in areas believed to contain no cholinergic neurons. Anti-sense riboprobes hybridized to the mRNA for the putative VAChT: (a) the projection neurons of the various nuclei of the basal nuclear complex, (b) the local circuit cells of the dorsal and ventral striata, (c) the projection neurons of the mesopontine complex, (d) perikarya in the ventral 2/3 of the medial habenula, (e) the somatic motor and autonomic cells of cranial nerves 3-7 and 9-12, as well as perikarya in the dorsal and ventral cochlear nuclei presumably giving rise to efferent fibers of cranial nerve 8, and (f) the alpha-motor and gamma-efferent motor neurons of the spinal cord. In addition, the mRNA for the VAChT was found in a few somata, probably ectopically located cells of the basal nuclear complex, in the internal capsule, central nucleus of the amygdala, entopeduncular nucleus, and zona incerta. It was also detected in some cell bodies in the reticular part of the substantia nigra, probably the rostral extension of the mesopontine complex, in the parabigeminal nucleus, and around the central canal in the spinal cord but not in cortical, hippocampal, and cerebellar perikarya. It is concluded that, like choline acetyltransferase, the mRNA for the putative acetylcholine vesicular transporter is another specific marker for neurons utilizing acetylcholine as a neurotransmitter. Further investigations of that transporter could have important implications for various diseases involving cholinergic systems, such as Alzheimer's disease.

Glutathione is involved in the granular storage of dopamine in rat PC 12 pheochromocytoma cells: implications for the pathogenesis of Parkinson's disease

Parkinson's disease (PD) is characterized by degeneration of dopamine (DA)-containing nigro-striatal neurons. Loss of the antioxidant glutathione (GSH) has been implicated in the pathogenesis of PD. Previously, we showed that the oxidant hydrogen peroxide inhibits vesicular uptake of DA in nigro-striatal neurons. Hydrogen peroxide is scavenged by GSH and, therefore, we investigated a possible link between the process of vesicular storage of DA and GSH metabolism. For this purpose, we used rat pheochromocytoma-derived PC12 cells, a model system applied extensively for studying monoamine storage mechanisms. We show that depletion of endogenous DA stores with reserpine was accompanied in PC12 cells by a long-lasting, significant increase in GSH content the extent of which appeared to be inversely related to the rate of GSH synthesis. A similar increase in GSH content was observed after depletion of DA stores with the tyrosine hydroxylase inhibitor alpha-methyl-p-tyrosine. In the presence of alpha-methyl-p-tyrosine, refilling of the DA stores by exogenous DA reduced GSH content back to control level. Lowering of PC12 GSH content, via blockade of its synthesis with buthionine sulfoximine, however, led to a significantly decreased accumulation of exogenous [3H]DA without affecting uptake of the acetylcholine precursor [14C]choline. These data suggest that GSH is involved in the granular storage of DA in PC12 cells and that, considering the molecular characteristics of the granular transport system, it is likely that GSH is used to protect susceptible parts of this system against (possibly DA-induced) oxidative damage.

Proton-dependent multidrug efflux systems

Multidrug efflux systems display the ability to transport a variety of structurally unrelated drugs from a cell and consequently are capable of conferring resistance to a diverse range of chemotherapeutic agents. This review examines multidrug efflux systems which use the proton motive force to drive drug transport. These proteins are likely to operate as multidrug/proton antiporters and have been identified in both prokaryotes and eukaryotes. Such proton-dependent multidrug efflux proteins belong to three distinct families or superfamilies of transport proteins: the major facilitator superfamily (MFS), the small multidrug resistance (SMR) family, and the resistance/ nodulation/cell division (RND) family. The MFS consists of symporters, antiporters, and uniporters with either 12 or 14 transmembrane-spanning segments (TMS), and we show that within the MFS, three separate families include various multidrug/proton antiport proteins. The SMR family consists of proteins with four TMS, and the multidrug efflux proteins within this family are the smallest known secondary transporters. The RND family consists of 12-TMS transport proteins and includes a number of multidrug efflux proteins with particularly broad substrate specificity. In gram-negative bacteria, some multidrug efflux systems require two auxiliary constituents, which might enable drug transport to occur across both membranes of the cell envelope. These auxiliary constituents belong to the membrane fusion protein and the outer membrane factor families, respectively. This review examines in detail each of the characterized proton-linked multidrug efflux systems. The molecular basis of the broad substrate specificity of these transporters is discussed. The surprisingly wide distribution of multidrug efflux systems and their multiplicity in single organisms, with Escherichia coli, for instance, possessing at least nine proton-dependent multidrug efflux systems with overlapping specificities, is examined. We also discuss whether the normal physiological role of the multidrug efflux systems is to protect the cell from toxic compounds or whether they fulfil primary functions unrelated to drug resistance and only efflux multiple drugs fortuitously or opportunistically.

Dopamine transporter gene expression in rat mesencephalic dopaminergic neurons is increased by direct interaction with target striatal cells in vitro

By using a semi-quantitative reverse transcriptase-PCR assay (RT-PCR) we have analyzed dopamine transporter (DAT), tyrosine hydroxylase (TH) and synaptic vesicle monoamine transporter (VMAT2) gene expression in rat mesencephalic (MES) primary cultures. Consistent with previous data obtained during rat MES ontogeny, the onset of DAT transcription in vitro is delayed in embryonic day (E)13, but not in E16, MES neurons when compared to that of TH and VMAT2. In co-culture, the addition of target striatal cells (STR) to E13 MES selectively increases DAT mRNA level in DA neurons during the first 3 days in vitro; cortical cells are ineffective. On the contrary, DAT gene does not appear up-regulated in E16 MES co-cultured with target STR cells, indicating that MES DA neurons respond to STR stimulation only at defined developmental stages. Up-regulation of DAT mRNA level by STR in E13 MES seems to require direct cell interactions since target cells do not exert their effect on DAT transcription when are separated from MES cells by a porous barrier, which only allows diffusion of soluble molecules. Thus maturation of DA neurotransmission in vitro appears to follow a developmental program which can be specifically modulated by their target STR cells.

Processing of chromogranins in chromaffin cell culture: effects of reserpine and alpha-methyl-p-tyrosine

Bovine chromaffin cell cultures were treated with either reserpine or alpha-methyl-p-tyrosine for up to 10 days. Afterwards the cells were harvested and the degree of proteolytic processing of secretogranin II, chromogranin A and chromogranin B was determined by immunoblotting and HPLC followed by RIA. There was a significant increase in the proteolysis of all three chromogranins after 4-6 days in the presence of reserpine. The small peptides formed in the presence of reserpine in vitro are also produced in vivo. A similar effect was observed with alpha-methyl-p-tyrosine, an inhibitor of tyrosine hydroxylase, but the response took up to 10 days to develop. Both drugs decreased catecholamine levels but reserpine was more effective, reaching a high degree of depletion after 4 days. In addition, experiments in vitro indicate that low millimolar amounts of either adrenaline (IC50 5.2 mM) or noradrenaline (IC50 2.4 mM) can significantly impair the proteolytic activity of recombinant murine prohormone convertase 1 when assayed with synthetic fluorogenic and/or peptidyl substrates. We conclude that a lowering of catecholamine levels in chromaffin granules leads to a concomitant increase in proteolytic processing of all secretory peptides. Apparently within chromaffin granules the endoproteases are inhibited by catecholamines and thus their removal leads to increased proteolysis.

Modification of the pH profile and tetrabenazine sensitivity of rat VMAT1 by replacement of aspartate 404 with glutamate

Vesicular monoamine transporters (VMAT) catalyze transport of serotonin, dopamine, epinephrine, and norepinephrine into subcellular storage organelles in a variety of cells. Accumulation of the neurotransmitter depends on the proton electrochemical gradient (Delta micro H+) across the organelle membrane and involves VMAT-mediated exchange of two lumenal protons with one cytoplasmic amine. Mutagenic analysis of the role of two conserved Asp residues located in transmembrane segments X and XI of rat VMAT type I reveals an important role of these two residues in catalysis. Replacement of Asp 431 with either Glu or Ser inhibits VMAT-mediated [3H]serotonin transport. The mutated proteins are unimpaired in ligand recognition as measured with the high affinity ligand [3H]reserpine or coupling to the proton electrochemical gradient as judged by its ability to accelerate [3H]reserpine binding. Therefore, the Asp residue is needed as such in this position and even a conservative replacement with Glu generates a protein that can catalyze only partial reactions but cannot complete the transport cycle. Replacement of Asp 404 with either Ser or Cys inhibits all VMAT-mediated reactions measured. However, replacement with Glu generated a protein that catalyzed [3H]serotonin transport with modified properties. Whereas the mutated protein binds [3H]reserpine to normal levels and the pH optimum of this reaction is only slightly affected, the optimum pH for transport activity shifted to the acid side and became very sharp; in addition the sensitivity to the inhibitor tetrabenazine increased significantly in this mutated protein. The results point to the need of a carboxyl moiety in position 404. A slight change in its relative location or in the environment around it has a significant effect on the pK of group(s) involved in steps after ligand recognition and coupling to the first H+.

Functional expression of the plasma membrane serotonin transporter but not the vesicular monoamine transporter in human placental trophoblasts and choriocarcinoma cells

We investigated the functional expression of the plasma membrane serotonin transporter and the vesicular monoamine transporter in choriocarcinoma cells and normal trophoblasts. The RBL 2H3 cells, a rat basophilic leukaemia cell line, which express both transporters were used for comparison. The choriocarcinoma cells JAr and BeWo were found to possess the plasma membrane serotonin transporter as assessed by the presence of serotonin transport activity in intact cells that was Na(+)-dependent and was sensitive to inhibition by tricyclic and non-tricyclic antidepressants. The activity of the vesicular monoamine transporter in these cells was determined by measuring serotonin transport in digitonin-permeabilized cells. The transport in permeabilized cells was very slow, was not stimulated by ATP and was insensitive to inhibition by reserpine. Under similar conditions, the vesicular monoamine transporter activity was demonstrable in RBL cells, which was stimulated by ATP and was inhibitable by reserpine, bafilomycin A1 (an inhibitor of the V-type H(+)-pump) and carbonyl cyanide p-trifluoromethoxy phenylhydrazone (a protonophore which dissipates transmembrane H+ gradients). In corroboration with these findings, mRNA transcripts hybridizable to the vesicular monoamine transporter cDNA probe were detectable in RBL cells but not in JAr choriocarcinoma cells. Similarly, there was no evidence for the expression of the vesicular monoamine transporter as assessed by Northern blot analysis in normal trophoblasts which were maintained in culture to differentiate to form multinucleated syncytial cells. It is concluded that the trophoblasts and choriocarcinoma cells express the plasma membrane serotonin transporter but not the vesicular monoamine transporter.

Reserpine affects differentially the density of the vesicular monoamine transporter and dihydrotetrabenazine binding sites

We have studied the effect of a single injection of reserpine (5 mg/kg, s.c.) on the synaptic vesicle monoamine transporter (VMAT) density in the rat striatum, using two labelling procedures: radioimmunolabelling with an antibody against VMAT, and binding of the specific ligand [(3)H]dihydrotetrabenazine ([(3)H]TBZOH). In the rostral and medial striatum, the distribution of VMAT immunoreactivity displayed the highest density in the lateral subregions. In the caudal part of the striatum, VMAT immunoreactivity showed increasing density from dorsal to ventral subregions. The VMAT immunoreactivity was not altered 2 and 30 days after the reserpine injection, whereas [(3)H]TBZOH binding site density, measured on adjacent slices, showed a dramatic decrease at day 2 and a moderate recovery at day 30, suggesting that despite a persistent blockade of [(3)H]TBZOH binding sites, VMAT protein density was unchanged.

Effect of vesamicol on the release of ATP from cortical synaptosomes

The aim of the present experiments was to test whether vesamicol alters the evoked release of ATP from nerve terminals. Continuous or cumulative release of ATP evoked by 33 mM KC1 from rat cerebrocortical synaptosomes was largely calcium-dependent. Vesamicol interfered with release of ATP from synaptosomes depolarized with KCl (33 mM) in a dose-dependent and stereoselective way. The (-)-vesamicol decreased the output of ATP in doses much lower than (+)-vesamicol. The release of the major excitatory neurotransmitter glutamate from depolarized nerve endings was not impaired by vesamicol. We suggest that vesamicol may alter the release of ATP specifically, probably by interacting with a protein similar to the vesamicol receptor found in cholinergic synaptic vesicles.

Comparison of somatodendritic and axon terminal dopamine release in the ventral tegmental area and the nucleus accumbens

Fast cyclic voltammetry at a carbon fibre microelectrode was used to measure dopamine release following electrical or chemical stimulation in rat brain slices incorporating either the ventral tegmental area or the core region of the nucleus accumbens. Electrical or chemical stimulation gave clear voltammetric signals which corresponded to dopamine; less dopamine was released in the ventral tegmental area than in the nucleus accumbens. In contrast to the nucleus accumbens, electrically stimulated dopamine release in the ventral tegmental area was not sensitive to tetrodotoxin, was not modified by the presence of dopamine uptake inhibitors, or agonist or blockers acting at dopamine D2 autoreceptors.

An overview of membrane transport proteins in Saccharomyces cerevisiae

All eukaryotic cells contain a wide variety of proteins embedded in the plasma and internal membranes, which ensure transmembrane solute transport. It is now established that a large proportion of these transport proteins can be grouped into families apparently conserved throughout organisms. This article presents the data of an in silicio analysis aimed at establishing a preliminary classification of membrane transport proteins in Saccharomyces cerevisiae. This analysis was conducted at a time when about 65% of all yeast genes were available in public databases. In addition to approximately 60 transport proteins whose function was at least partially known, approximately 100 deduced protein sequences of unknown function display significant sequence similarity to membrane transport proteins characterized in yeast and/or other organisms. While some protein families have been well characterized by classical genetic experimental approaches, others have largely if not totally escaped characterization. The proteins revealed by this in silicio analysis also include a putative K+ channel, proteins similar to aquaporins of plant and animal origin, proteins similar to Na+-solute symporters, a protein very similar to electroneural cation-chloride cotransporters, and a putative Na+-H+ antiporter. A new research area is anticipated: the functional analysis of many transport proteins whose existence was revealed by genome sequencing.

Transport of histamine by vesicular monoamine transporter-2

Histamine mediates signalling by a wide range of neural and non-neural cells including mast cells. Like other biogenic amines, histamine is released from specialized secretory vesicles and requires transport from the cytoplasm into these vesicles. Of the two vesicular monoamine transporters, histamine potently inhibits 3H-serotonin transport by one (VMAT2) but not the other (VMAT1). In addition, histamine-containing cells in both neural and non-neural cells express VMAT2. However, histamine lacks the hydroxyl groups generally considered necessary for recognition as a substrate by the vesicular monoamine transporters. Using a heterologous expression system, we now report that VMAT2 not only shows inhibition by histamine but also transports 3H-histamine. Interestingly, histamine differs from other monoamine transmitters and does not inhibit 3H-reserpine binding to VMAT2, indicating interaction at a distinct site. Surprisingly, reserpine inhibits histamine transport with much less potency than serotonin transport, suggesting a different transport mechanism. However, replacement of serines in the third transmembrane domain of VMAT2 that have been shown to be essential for recognition of other monoamines also eliminate 3H-histamine transport, suggesting that these serine residues may do more than simply recognize the hydroxyl groups on a monoamine substrate.

The vesicular monoamine transporter 2 is present in small synaptic vesicles and preferentially localizes to large dense core vesicles in rat solitary tract nuclei

In central neurons, monamine neurotransmitters are taken up and stored within two distinct classes of regulated secretory vesicles: small synaptic vesicles and large dense core vesicles (DCVs). Biochemical and pharmacological evidence has shown that this uptake is mediated by specific vesicular monamine transporters (VMATs). Recent molecular cloning techniques have identified the vesicular monoamine transporter (VMAT2) that is expressed in brain. This transporter determines the sites of intracellular storage of monoamines and has been implicated in both the modulation of normal monoaminergic neurotransmission and the pathogenesis of related neuropsychiatric disease. We used an antiserum against VMAT2 to examine its ultrastructural distribution in rat solitary tract nuclei, a region that contains a dense and heterogeneous population of monoaminergic neurons. We find that both immunoperoxidase and immunogold labeling for VMAT2 localize to DCVs and small synaptic vesicles in axon terminals, the trans-Golgi network of neuronal perikarya, tubulovesicles of smooth endoplasmic reticulum, and potential sites of vesicular membrane recycling. In axon terminals, immunogold labeling for VMAT2 was preferentially associated with DCVs at sites distant from typical synaptic junctions. The results provide direct evidence that a single VMAT is expressed in two morphologically distinct types of regulated secretory vesicles in central monoaminergic neurons.

Effect of reserpine treatment on enkephalin mRNA level in the rat striatum: an in situ hybridization study

We investigated the molecular mechanisms responsible for the preproenkephalin A mRNA increase following catecholamine depletion by reserpine using quantitative in situ hybridization at the cellular level. Macroscopic analysis showed that short term reserpine treatment increases the preproenkephalin A mRNA level in the rat striatum to +40.2 +/- 9%. Microautoradiography analysis demonstrated different increases in the preproenkephalin A mRNA level in different parts of the striatum: +124 +/- 22% in the dorso-median striatum, +131 +/- 19% in the dorso-lateral striatum, +119 +/- 8% in the ventro-lateral striatum and +75 +/- 6% in the ventro-median striatum. We found no difference in the number of cells expressing PPA mRNA in reserpine treated rats suggesting that these increases are only due to an increase in the number of mRNA expressed by cell.

Differential sensitivity to hydrogen peroxide of dopaminergic and noradrenergic neurotransmission in rat brain slices

Oxidative stress, induced by hydrogen peroxide, has been implicated in the pathogenesis of Parkinson's disease. Only scarce information is available if and how hydrogen peroxide, a side product of catecholamine (CA) breakdown, interferes with CAergic neurotransmission. Therefore, we investigated the effect of hydrogen peroxide on the release of [3H]dopamine (DA) and [3H]noradrenaline (NA) from rat striatal and cortical tissue slices, respectively. Hydrogen peroxide (0.01-1 mM) stimulated the spontaneous release of [3H]DA from striatal slices. Its effect on [3H]NA release from cortical slices, however, was much smaller than on DA release and occurred only in concentrations above 0.1 mM. Furthermore, only in concentrations of 1 mM or higher did a stimulation of spontaneous release of radioactivity from striatal slices incubated with [3H]choline occur. Omission of calcium significantly enhanced the effect on DA release, and an increase of calcium significantly reduced it. Blockade of vesicular storage with reserpine (0.3 microM) almost completely abolished [3H]DA release induced by hydrogen peroxide. Following incubation of striatal slices with [3H]NA in the presence of the NA (re)uptake blocker desmethylimipramine (0.3 microM), NA release was observed at a concentration (0.1 mM) at which no effect occurred in cortical slices. Moreover, under these conditions [3]NA and [3H]DA release from striatal slices reached comparable levels. Our results show that hydrogen peroxide induces a nonexocytotic release of DA and NA by interfering with the vesicular uptake and/or storage of these CAs. However, the striatal DA storage system, irrespective of the presence of either DA or NA, appeared to be substantially more sensitive to this effect than its cortical equivalent for storage of NA.

The non-specific ion channel in Torpedo ocellata fused synaptic vesicles

1. Synaptic vesicles were isolated and fused into large structures with a diameter of more than 20 microns to characterize their ionic channels. The 'cell'-attached and inside-out configurations of the patch clamp technique were used. 2. Two types of ion channels were most frequently observed: a low conductance chloride channel and a high conductance non-specific channel. 3. The non-specific channel has a main conducting state and a substate. The main conducting state has a slope conductance of 246 +/- 15 pS (+/- S.E.M., n = 15), in the presence of different combinations of KCl and potassium glutamate. 4. From the reversal potentials of the current-voltage (I-V) relation, it was concluded that this channel conducts both Cl- and K+. 5. The non-specific channel is highly voltage dependent: under steady-state voltages it has a high open probability near 0 mV and does not inactivate; when the membrane is hyperpolarized (pipette side more positive), the open probability decreases dramatically. 6. Voltage pulses showed that upon hyperpolarization (from holding potentials between -20 and + 20 mV), the channels deactivated; when the membrane was stepped back to the holding potential, the channels reactivated rapidly. 7. In a number of experiments, when the pipette side was made more negative than the bath, the open probability also decreased. 8. Frequently, a substate with a conductance of about 44 +/- 4% (+/- S.E.M., n = 3) of the main state was detected. 9. We speculate that this non-specific ion channel may have different roles at the various stages of the life cycle of the synaptic vesicle. When the synaptic vesicle is an intracellular structure, it might help its transmitter-concentrating capacity by dissipating the polarization. After fusion with the surface membrane, it might constitute an additional conductance pathway, taking part in frequency modulation of synaptic transmission.

Binding of alpha-dihydrotetrabenazine to the vesicular monoamine transporter is stereospecific

The two enantiomers of alpha-dihydrotetrabenazine were separated using chiral high performance liquid chromatography. The (+)-isomer showed high affinity in vitro (Ki = 0.97 +/- 0.48 nM) for the vesicular monoamine transporter (VMAT2) in rat brain striatum, whereas the (-)-isomer was inactive (Ki = 2.2 +/- 0.3 microM). Each isomer was then synthesized in carbon-11 labeled form, and regional brain biodistributions in mice determined after intravenous injection. Only (+)-alpha-dihydrotetrabenazine showed selective and specific accumulations in regions of dense monoaminergic innervation (e.g., striatum, hypothalamus), which could be blocked by coinjection of unlabeled tetrabenazine. Binding of alpha-dihydrotetrabenazine to the vesicular monoamine transporter is thus stereospecific.

Amino- and amido-tetrabenazine derivatives: synthesis and evaluation as potential ligands for the vesicular monoamine transporter

Tetrabenazine (TBZ) and dihydrotetrabenazine are well known inhibitors of the CNS vesicular monoamine transporter (VMAT), which is responsible for the packaging of monoamine neurotransmitters in presynaptic vesicles. Amino and amido derivatives of tetrabenazine were prepared as potential ligands for the vesicular monoamine transporter. Ultimately, organotin derivatives of promising ligands were prepared for radiolabeling with 125I. The compounds were evaluated for their ability to inhibit the specific binding of a selective radioligand to the transporter in rat striatal homogenates. Of the compounds evaluated, three amine derivatives of TBZ (primary, secondary and tertiary) were found to have modest to high affinity for the transporter, while two amides exhibited low to undectable affinity. The secondary propargyl amine was found to possess the highest affinity (Ki = 7.6 nM) and was chosen for further evaluation. The organotin derivative of this compound was synthesized in order to prepare the corresponding radioiodinated ligand. However, our inability to synthesize and characterize the iodinated amine precluded its evaluation as a potential radioiodinated ligand for the transporter. Alternative approaches for decreasing the lipophilicity of TBZ analogs while maintaining high binding affinity are currently being explored.

Molecular biology of the vesicular ACh transporter

The cholinergic synapse has long been a model for biochemical studies of neurotransmission. The molecules that are responsible for synaptic transmission are being identified rapidly. The vesicular transporter for ACh, which is responsible for the concentration of ACh within synaptic vesicles, has been characterized recently, both at the molecular and functional level. Definitive identification of the cloned gene involved genetics of Caenorhabditis elegans, the specialized Torpedo electromotor system, and expression in mammalian tissue culture. Comparison of the vesicular transporter for ACh with the vesicular transporters for monoamines demonstrates a new gene family. Gene mapping has demonstrated a unique relationship between the genes for the vesicular ACh transporter and for choline acetyltransferase.

EmrE, an Escherichia coli 12-kDa multidrug transporter, exchanges toxic cations and H+ and is soluble in organic solvents

The smallest membrane protein shown to catalyze ion-coupled transport is documented in this report. A gene coding for a small 110-amino acid membrane protein (emrE or mvrC) has been previously identified and cloned and shown to render Escherichia coli cells resistant to methyl viologen and to ethidium. In this report, it is shown that the resistance is due to extrusion of the toxic compounds in a process that requires a proton electrochemical gradient rather than ATP. For this purpose, cells in which the unc gene was inactivated were used so that the interconversion between the proton gradient and ATP is not possible, and the effect of agents, which specifically affect either of them, was tested on transport of ethidium in the intact cell. In addition, EmrE has been overexpressed and metabolically labeled with [35S]methionine. Strikingly, the protein can be quantitatively extracted with a mixture of organic solvents such as chloroform:methanol and is practically pure after this extraction. Moreover, after addition of E. coli lipids to the chloroform:methanol extract, EmrE has been reconstituted in proteoliposomes loaded with ammonium chloride. Upon dilution of the proteoliposomes in ammonium-free medium, a pH gradient was formed that drove transport of ethidium and methyl viologen into the proteoliposome. Both substrates compete with each other and exchange with previously transported solute. EmrE is a multidrug transporter of a novel type, and, because of its size and its solubility properties, it provides a unique model to study structure-function aspects of transport reactions in ion-coupled processes.

Histidine-419 plays a role in energy coupling in the vesicular monoamine transporter from rat

Vesicular monoamine transporters (VMAT) catalyze transport of serotonin, dopamine, epinephrine and norepinephrine into subcellular storage organelles in a variety of cells. Accumulation of the neurotransmitter depends on the proton electrochemical gradient across the organelle membrane and involves VMAT-mediated exchange of two lumenal protons with one cytoplasmic amine. It has been suggested in the past that His residues play a role in H+ movement or in its coupling to active transport in H(+)-symporters and antiporters. Indeed VMAT-mediated transport is inhibited by reagents specific for His residues. We have identified one His residue in VMAT1 from rat which is conserved in other vesicular neurotransmitter transporters. Mutagenesis of this His (H419) to either Arg or Cys completely inhibits [3H]serotonin and [3H]dopamine accumulation. Mutagenesis also inhibits other H(+)-dependent partial reactions of VMAT such as the acceleration of binding of the high affinity ligand reserpine, but does not inhibit the [3H]reserpine binding which is not dependent on H+ translocation. It is concluded that His-419 plays a role in energy coupling in r-VMAT1.

Drug excretion mediated by a new prototype of polyspecific transporter

Cationic drugs of different types and structures (antihistaminics, antiarrhythmics, sedatives, opiates, cytostatics and antibiotics, for example) are excreted in mammals by epithelial cells of the renal proximal tubules and by hepatocytes in the liver. In the proximal tubules, two functionally disparate transport systems are involved which are localized in the basolateral and luminal plasma membrane and are different from the previously identified neuronal monoamine transporters and ATP-dependent multidrug exporting proteins. Here we report the isolation of a complementary DNA from rat kidney that encodes a 556-amino-acid membrane protein, OCT1, which has the functional characteristics of organic cation uptake over the basolateral membrane of renal proximal tubules and of organic cation uptake into hepatocytes. OCT1 is not homologous to any other known protein and is found in kidney, liver and intestine. As OCT1 translocates hydrophobic and hydrophilic organic cations of different structures, it is considered to be a new prototype of polyspecific transporters that are important for drug elimination.

Distribution of the vesicular acetylcholine transporter (VAChT) in the central and peripheral nervous systems of the rat

Expression of the acetylcholine biosynthetic enzyme choline acetyltransferase (ChAT), the vesicular acetylcholine transporter (VAChT), and the high-affinity plasma membrane choline transporter uniquely defines the cholinergic phenotype in the mammalian central (CNS) and peripheral (PNS) nervous systems. The distribution of cells expressing the messenger RNA encoding the recently cloned VAChT in the rat CNS and PNS is described here. The pattern of expression of VAChT mRNA is consistent with anatomical, pharmacological, and histochemical information on the distribution of functional cholinergic neurons in the brain and peripheral tissues of the rat. VAChT mRNA-containing cells are present in brain areas, including neocortex and hypothalamus, in which the existence of cholinergic neurons has been the subject of debate. The demonstration that VAChT is a completely adequate marker for cholinergic neurons should allow the systematic delineation of cholinergic synapses in the rat nervous system when antibodies directed to this protein are available.