Vesicular neurotransmitter transporters. Potential sites for the regulation of synaptic function

The Potential of Flavonoids and Flavonoid Metabolites in the Treatment of Neurodegenerative Pathology in Disorders of Cognitive Decline

Flavonoids are a biodiverse family of dietary compounds that have antioxidant, anti-inflammatory, antiviral, and antibacterial cell protective profiles. They have received considerable attention as potential therapeutic agents in biomedicine and have been widely used in traditional complimentary medicine for generations. Such complimentary medical herbal formulations are extremely complex mixtures of many pharmacologically active compounds that provide a therapeutic outcome through a network pharmacological effects of considerable complexity. Methods are emerging to determine the active components used in complimentary medicine and their therapeutic targets and to decipher the complexities of how network pharmacology provides such therapeutic effects. The gut microbiome has important roles to play in the generation of bioactive flavonoid metabolites retaining or exceeding the antioxidative and anti-inflammatory properties of the intact flavonoid and, in some cases, new antitumor and antineurodegenerative bioactivities. Certain food items have been identified with high prebiotic profiles suggesting that neutraceutical supplementation may be beneficially employed to preserve a healthy population of bacterial symbiont species and minimize the establishment of harmful pathogenic organisms. Gut health is an important consideration effecting the overall health and wellbeing of linked organ systems. Bioconversion of dietary flavonoid components in the gut generates therapeutic metabolites that can also be transported by the vagus nerve and systemic circulation to brain cell populations to exert a beneficial effect. This is particularly important in a number of neurological disorders (autism, bipolar disorder, AD, PD) characterized by effects on moods, resulting in depression and anxiety, impaired motor function, and long-term cognitive decline. Native flavonoids have many beneficial properties in the alleviation of inflammation in tissues, however, concerns have been raised that therapeutic levels of flavonoids may not be achieved, thus allowing them to display optimal therapeutic effects. Dietary manipulation and vagal stimulation have both yielded beneficial responses in the treatment of autism spectrum disorders, depression, and anxiety, establishing the vagal nerve as a route of communication in the gut-brain axis with established roles in disease intervention. While a number of native flavonoids are beneficial in the treatment of neurological disorders and are known to penetrate the blood–brain barrier, microbiome-generated flavonoid metabolites (e.g., protocatechuic acid, urolithins, γ-valerolactones), which retain the antioxidant and anti-inflammatory potency of the native flavonoid in addition to bioactive properties that promote mitochondrial health and cerebrovascular microcapillary function, should also be considered as potential biotherapeutic agents. Studies are warranted to experimentally examine the efficacy of flavonoid metabolites directly, as they emerge as novel therapeutic options.

Vesicular glutamate transporter modulates sex differences in dopamine neuron vulnerability to age-related neurodegeneration

Age is the greatest risk factor for Parkinson's disease (PD) which causes progressive loss of dopamine (DA) neurons, with males at greater risk than females. Intriguingly, some DA neurons are more resilient to degeneration than others. Increasing evidence suggests that vesicular glutamate transporter (VGLUT) expression in DA neurons plays a role in this selective vulnerability. We investigated the role of DA neuron VGLUT in sex- and age-related differences in DA neuron vulnerability using the genetically tractable Drosophila model. We found sex differences in age-related DA neurodegeneration and its associated locomotor behavior, where males exhibit significantly greater decreases in both DA neuron number and locomotion during aging compared with females. We discovered that dynamic changes in DA neuron VGLUT expression mediate these age- and sex-related differences, as a potential compensatory mechanism for diminished DA neurotransmission during aging. Importantly, female Drosophila possess higher levels of VGLUT expression in DA neurons compared with males, and this finding is conserved across flies, rodents, and humans. Moreover, we showed that diminishing VGLUT expression in DA neurons eliminates females' greater resilience to DA neuron loss across aging. This offers a new mechanism for sex differences in selective DA neuron vulnerability to age-related DA neurodegeneration. Finally, in mice, we showed that the ability of DA neurons to achieve optimal control over VGLUT expression is essential for DA neuron survival. These findings lay the groundwork for the manipulation of DA neuron VGLUT expression as a novel therapeutic strategy to boost DA neuron resilience to age- and PD-related neurodegeneration.

Simultaneous detection of vesicular content and exocytotic release with two electrodes in and at a single cell

We developed a technique employing two electrodes to simultaneously and dynamically monitor vesicular neurotransmitter storage and vesicular transmitter release in and at the same cell. To do this, two electrochemical techniques, single-cell amperometry (SCA) and intracellular vesicle impact electrochemical cytometry (IVIEC), were applied using two nanotip electrodes. With one electrode being placed on top of a cell measuring exocytotic release and the other electrode being inserted into the cytoplasm measuring vesicular transmitter storage, upon chemical stimulation, exocytosis is triggered and the amount of release and storage can be quantified simultaneously and compared. By using this technique, we made direct comparison between exocytotic release and vesicular storage, and investigated the dynamic changes of vesicular transmitter content before, during, and after chemical stimulation of PC12 cells, a neuroendocrine cell line. While confirming that exocytosis is partial, we suggest that chemical stimulation either induces a replenishment of the releasable pool with a subpool of vesicles having higher amount of transmitter storage, or triggers the vesicles within the same subpool to load more transiently at approximately 10–20 s. Thus, a time scale for vesicle reloading is determined. The effect of l-3,4-dihydroxyphenylalanine (l-DOPA), the precursor to dopamine, on the dynamic alteration of vesicular storage upon chemical stimulation for exocytosis was also studied. We found that l-DOPA incubation reduces the observed changes of vesicular storage in regular PC12 cells, which might be due to an increased capacity of vesicular transmitter loading caused by l-DOPA. Our data provide another mechanism for plasticity after stimulation via quantitative and dynamic changes in the exocytotic machinery.

Simultaneous measurements of IVIEC and SCA by two nanotip electrodes allows direct and dynamic comparison between vesicular transmitter content and vesicular transmitter release to shed light on stimulation-induced plasticity.

Personality traits and polymorphisms of genes coding neurotransmitter receptors or transporters: review of single gene and genome-wide association studies

BACKGROUND

The most popular tool used for measuring personality traits is the Five-Factor Model (FFM). It includes neuroticism, extraversion, openness, agreeableness and conscientiousness. Many studies indicated the association of genes encoding neurotransmitter receptors/transporters with personality traits. The relationship connecting polymorphic DNA sequences and FFM features has been described in the case of genes encoding receptors of cannabinoid and dopaminergic systems. Moreover, dopaminergic system receives inputs from other neurotransmitters, like GABAergic or serotoninergic systems.

METHODS

We searched PubMed Central (PMC), Science Direct, Scopus, Cochrane Library, Web of Science and EBSCO databases from their inception to November 19, 2020, to identify original studies, as well as peer-reviewed studies examining the FFM and its association with gene polymorphisms affecting the neurotransmitter functions in central nervous system.

RESULTS

Serotonin neurons modulate dopamine function. In gene encoding serotonin transporter protein, SLC6A4, was found polymorphism, which was correlated with openness to experience (in Sweden population), and high scores of neuroticism and low levels of agreeableness (in Caucasian population). The genome-wide association studies (GWASs) found an association of 5q34-q35, 3p24, 3q13 regions with higher scores of neuroticism, extraversion and agreeableness. However, the results for chromosome 3 regions are inconsistent, which was shown in our review paper.

CONCLUSIONS

GWASs on polymorphisms are being continued in order to determine and further understand the relationship between the changes in DNA and personality traits.

Inhibition of the Vesicular Glutamate Transporter (VGLUT) with Congo Red Analogs: New Binding Insights

The vesicular glutamate transporter (VGLUT) facilitates the uptake of glutamate (Glu) into neuronal vesicles. VGLUT has not yet been fully characterized pharmacologically but a body of work established that certain azo-dyes bearing two Glu isosteres via a linker were potent inhibitors. However, the distance between the isostere groups that convey potent inhibition has not been delineated. This report describes the synthesis and pharmacologic assessment of Congo Red analogs that contain one or two glutamate isostere or mimic groups; the latter varied in the interatomic distance and spacer properties to probe strategic binding interactions within VGLUT. The more potent inhibitors had two glutamate isosteres symmetrically linked to a central aromatic group and showed IC₅₀ values ~ 0.3-2.0 μM at VGLUT. These compounds contained phenyl, diphenyl ether (PhOPh) or 1,2-diphenylethane as the linker connecting 4-aminonaphthalene sulfonic acid groups. A homology model for VGLUT2 using D-galactonate transporter (DgoT) to dock and identify R88, H199 and F219 as key protein interactions with Trypan Blue, Congo Red and selected potent analogs prepared and tested in this report.

Molecular, Structural, Functional, and Pharmacological Sites for Vesicular Glutamate Transporter Regulation

Vesicular glutamate transporters (VGLUTs) control quantal size of glutamatergic transmission and have been the center of numerous studies over the past two decades. VGLUTs contain two independent transport modes that facilitate glutamate packaging into synaptic vesicles and phosphate (Pi) ion transport into the synaptic terminal. While a transmembrane proton electrical gradient established by a vacuolar-type ATPase powers vesicular glutamate transport, recent studies indicate that binding sites and flux properties for chloride, potassium, and protons within VGLUTs themselves regulate VGLUT activity as well. These intrinsic ionic binding and flux properties of VGLUTs can therefore be modulated by neurophysiological conditions to affect levels of glutamate available for release from synapses. Despite their extraordinary importance, specific and high-affinity pharmacological compounds that interact with these sites and regulate VGLUT function, distinguish between the various modes of transport, and the different isoforms themselves, are lacking. In this review, we provide an overview of the physiologic sites for VGLUT regulation that could modulate glutamate release in an over-active synapse or in a disease state.

Postsynaptic Mechanisms Render Syn I/II/III Mice Highly Responsive to Psychostimulants

BACKGROUND

Synapsins are encoded by SYN I, SYN II, and SYN III, and they regulate neurotransmitter release by maintaining a reserve pool of synaptic vesicles.

METHODS

Presynaptic dopamine responses to cocaine were examined by microdialysis, and postsynaptic responses were evaluated to various dopamine receptor agonists in the open field with SynI/SynII/SynIII triple knockout mice.

RESULTS

Triple knockout mice showed enhanced spontaneous locomotion in a novel environment and were hyper-responsive to indirect and direct D1 and D2 dopamine agonists. Triple knockout animals appeared sensitized to cocaine upon first open field exposure; sensitization developed across days in wild-type controls. When mutants were preexposed to a novel environment before injection, cocaine-stimulated locomotion was reduced and behavioral sensitization retarded. Baseline dopamine turnover was enhanced in mutants and novel open field exposure increased their striatal dopamine synthesis rates. As KCl-depolarization stimulated comparable dopamine release in both genotypes, their readily releasable pools appeared indistinguishable. Similarly, cocaine-induced hyperlocomotion was indifferent to blockade of newly synthesized dopamine and depletion of releasable dopamine pools. Extracellular dopamine release was similar in wild-type and triple knockout mice preexposed to the open field and given cocaine or placed immediately into the arena following injection. Since motor effects to novelty and psychostimulants depend upon frontocortical-striatal inputs, we inhibited triple knockout medial frontal cortex with GABA agonists. Locomotion was transiently increased in cocaine-injected mutants, while their supersensitive cocaine response to novelty was lost.

CONCLUSIONS

These results reveal presynaptic dopamine release is not indicative of agonist-induced triple knockout hyperlocomotion. Instead, their novelty response occurs primarily through postsynaptic mechanisms and network effects.

Positive and balancing selection on SLC18A1 gene associated with psychiatric disorders and human-unique personality traits

Maintenance of genetic variants susceptible to psychiatric disorders is one of the intriguing evolutionary enigmas. The present study detects three psychiatric disorder‐relevant genes (CLSTN2, FAT1, and SLC18A1) that have been under positive selection during the human evolution. In particular, SLC18A1 (vesicular monoamine transporter 1; VMAT1) gene has a human‐unique variant (rs1390938, Thr136Ile), which is associated with bipolar disorders and/or the anxiety‐related personality traits. 136Ile shows relatively high (20–61%) frequency in non‐African populations, and Tajima's D reports a significant peak around the Thr136Ile site, suggesting that this polymorphism has been positively maintained by balancing selection in non‐African populations. Moreover, Coalescent simulations predict that 136Ile originated around 100,000 years ago, the time being generally associated with the Out‐of‐Africa migration of modern humans. Our study sheds new light on a gene in monoamine pathway as a strong candidate contributing to human‐unique psychological traits.

Nano Secondary Ion Mass Spectrometry Imaging of Dopamine Distribution Across Nanometer Vesicles

We report an approach to spatially resolve the content across nanometer neuroendocrine vesicles in nerve-like cells by correlating super high-resolution mass spectrometry imaging, NanoSIMS, with transmission electron microscopy (TEM). Furthermore, intracellular electrochemical cytometry at nanotip electrodes is used to count the number of molecules in individual vesicles to compare to imaged amounts in vesicles. Correlation between the NanoSIMS and TEM provides nanometer resolution of the inner structure of these organelles. Moreover, correlation with electrochemical methods provides a means to quantify and relate vesicle neurotransmitter content and release, which is used to explain the slow transfer of dopamine between vesicular compartments. These nanoanalytical tools reveal that dopamine loading/unloading between vesicular compartments, dense core and halo solution, is a kinetically limited process. The combination of NanoSIMS and TEM has been used to show the distribution profile of newly synthesized dopamine across individual vesicles. Our findings suggest that the vesicle inner morphology might regulate the neurotransmitter release event during open and closed exocytosis from dense core vesicles with hours of equilibrium needed to move significant amounts of catecholamine from the protein dense core despite its nanometer size.

Discovery of novel antigiardiasis drug candidates

Giardiasis is a severe intestinal parasitic disease caused by Giardia lamblia, which inflicts many people in poor regions and is the most common parasitic infection in the United States. Current standard care drugs are associated with undesirable side effects, treatment failures, and an increasing incidence of drug resistance. As follow-up to a high-throughput screening of an approved drug library, which identified compounds lethal to G. lamblia trophozoites, we have determined the minimum lethal concentrations of 28 drugs and advanced 10 of them to in vivo studies in mice. The results were compared to treatment with the standard care drug, metronidazole, in order to identify drugs with equal or better anti-Giardia activities. Three drugs, fumagillin, carbadox, and tioxidazole, were identified. These compounds were also potent against metronidazole-resistant human G. lamblia isolates (assemblages A and B), as determined in in vitro assays. Of these three compounds, fumagillin is currently an orphan drug used within the European Union to treat microsporidiosis in immunocompromised individuals, whereas carbadox and tioxidazole are used in veterinary medicine. A dose-dependent study of fumagillin in a giardiasis mouse model revealed that the effective dose of fumagillin was ∼ 100-fold lower than the metronidazole dose. Therefore, fumagillin may be advanced to further studies as an alternative treatment for giardiasis when metronidazole fails.

Localization and expression of VMAT2 aross mammalian species: a translational guide for its visualization and targeting in health and disease

VMAT2 is the vesicular monoamine transporter that allows DA, NE, Epi, His, and 5-HT uptake into neurons and endocrine cells. A second isoform, VMAT1, has similar structure and function, but does not recognize histamine as a substrate. VMAT1 is absent from neurons, and its major function appears to be in endocrine cells, that is, enterochromaffin cells, which scavenge 5-HT, but not histamine, from dietary sources. This chapter provides an update on the neuroanatomical distribution of VMAT2 across various mammalian species, including human, primate, pig, rat, and mouse. When necessary, VMAT1 expression is provided as a contrast. The main purpose of this chapter is to allow clinicians, in particular endocrinologists and diagnosing neuroradiologists and neuropathologists, an acquaintanceship with the possibilities for VMAT2 as a target for in vivo imaging, and drug development, based on this updated information.

Search for the acetylcholine and vesamicol binding sites in vesicular acetylcholine transporter: the region around the lumenal end of the transport channel

Vesicular acetylcholine transporter (VAChT; TC 2.A.1.2.13) mediates storage of acetylcholine (ACh) by synaptic vesicles. A three-dimensional homology model of VAChT is available, but the binding sites for ACh and the allosteric inhibitor (-)-trans-2-(4-phenylpiperidino)cyclohexanol (vesamicol) are unknown. In previous work, mutations of invariant W331 in the lumenal beginning of transmembrane helix VIII (TM VIII) of rat VAChT led to as much as ninefold loss in equilibrium affinity for ACh and no loss in affinity for vesamicol. The current work investigates the effects of additional mutations in and around W331 and the nearby lumenal end of the substrate transport channel. Mutants of human VAChT were expressed in the PC12(A123.7) cell line and characterized using radiolabeled ligands and filtration assays for binding and transport. Properties of a new and a repeat mutation in W331 are consistent with the original observations. Of 16 additional mutations in 13 other residues (Y60 in the beginning of lumenal Loop I/II, F231 in the lumenal end of TM V, W315, M316, K317, in the lumenal end of TM VII, M320, A321, W325, A330 in lumenal Loop VII/VIII, A334 in the lumenal beginning of TM VIII, and C388, C391, F392 in the lumenal beginning of TM X), only A334F impairs binding. This mutation decreases ACh and vesamicol equilibrium binding affinities by 14- and 4-fold, respectively. The current results, combined with previous results, demonstrate existence of a spatial cluster of residues close to vesicular lumen that decreases affinity for ACh and/or vesamicol when the cluster is mutated. The cluster is composed of invariant W331, highly conserved A334, and invariant F335 in TM VIII and invariant C391 in TM X. Different models for the locations of the ACh and vesamicol binding sites relative to this cluster are discussed.

Postnatal changes in tryptophan hydroxylase and serotonin transporter immunoreactivity in multiple brainstem nuclei of the rat: implications for a sensitive period

Previously, we found that the brainstem neuronal network in normal rats undergoes abrupt neurochemical, metabolic, and physiological changes around postnatal days (P) 12-13, a critical period when the animal's response to hypoxia is also the weakest. This has special implications for sudden infant death syndrome (SIDS), insofar as seemingly normal infants succumb to SIDS when exposed to respiratory stressors (e.g., hypoxia) during a narrow postnatal window. Because an abnormal serotonergic system has recently been implicated in SIDS, we conducted a large-scale investigation of the 5-HT-synthesizing enzyme tryptophan hydroxylase (TPH) and serotonin transporter (SERT) with semiquantitative immunohistochemistry in multiple brainstem nuclei of normal rats aged P2-21. We found that 1) TPH and SERT immunoreactivity in neurons of raphé magnus, obscurus, and pallidus and SERT in the neuropil of the pre-Bötzinger complex, nucleus ambiguus, and retrotrapezoid nucleus were high at P2-11 but decreased markedly at P12 and plateaued thereafter until P21; 2) SERT labeling in neurons of the lateral paragigantocellular nucleus (LPGi) and parapyramidal region (pPy) was high at P2-9 but fell significantly at P10, followed by a gradual decline until P21; 3) TPH labeling in neurons of the ventrolateral medullary surface was stable except for a significant fall at P12; and 4) TPH and SERT immunoreactivity in a number of other nuclei was relatively stable from P2 to P21. Thus, multiple brainstem nuclei exhibited a significant decline in TPH and SERT immunoreactivity during the critical period, suggesting that such normal development can contribute to a narrow window of vulnerability in postnatal animals.

Dynamic monitoring of NET activity in mature murine sympathetic terminals using a fluorescent substrate

BACKGROUND AND PURPOSE

To validate a fluorescence approach for monitoring norepinephrine transporter (NET) transport rate in mature sympathetic terminals, and to determine how prejunctional muscarinic receptors affect NET rate.

EXPERIMENTAL APPROACH

Confocal imaging of a fluorescent NET substrate [neurotransmitter transporter uptake assay (NTUA)] as it accumulates in the mature sympathetic nerve terminals of the mouse isolated vas deferens. Fluorescence recovery after photobleaching (FRAP), enhanced green fluorescence protein (EGFP)-transgenic mice and contraction studies were also used.

KEY RESULTS

NTUA fluorescence accumulated linearly in nerve terminals, an effect that was prevented with NET inhibition with desipramine (1 microM). Such accumulation was reversed by amphetamine (10 microM), which is known to reverse the direction of transport of NET substrates. NTUA labelling was not present in cholinergic terminals (identified using EGFP fluorescence expressed in transgenic mice under a choline acetyltransferase promoter). FRAP experiments, altered nerve terminal distribution with reserpine pretreatment and co-imaging in terminals filled with a cytoplasmic marker (Alexa 594 dextran) indicated that the NTUA labelling was largely confined to vesicles within varicosities; vesicular exchange between varicosities was rare. The rate of NTUA accumulation was slower in the presence of the muscarinic agonist carbachol (10 microM) demonstrating muscarinic inhibition of NET rate.

CONCLUSIONS AND IMPLICATIONS

A straightforward protocol now exists to monitor NET transport rate at the level of the single nerve terminal varicosity, providing a useful tool to understand the physiology of NET regulation, the action of NET inhibitors on mature sympathetic terminals, dynamic vesicular tracking and to identify sympathetic terminals from mixed terminal populations in living organs.

Inhibitory effect of selective serotonin reuptake inhibitors on the vesicular monoamine transporter 2

The neuronal vesicular monoamine transporter (VMAT2) is the target molecule of action of some psychostimulants, such as methamphetamine and 3,4-methylenedioxymethamphetamine (MDMA). The present study examined the effect of antidepressants, such as selective serotonin reuptake inhibitors (SSRIs), on VMAT2 activity by measuring adenosine triphosphate-dependent [(3)H]dopamine uptake into synaptic vesicles prepared from rat striatum. SSRIs, fluoxetine, paroxetine, and fluvoxamine, inhibited vesicular [(3)H]dopamine uptake in vitro. The rank order of potency was reserpine>>fluoxetine, paroxetine>fluvoxamine, methamphetamine>MDMA. Moreover, kinetic analysis revealed that inhibition by reserpine, a typical VMAT2 inhibitor, was uncompetitive, decreasing maximum velocity and affinity for dopamine. Inhibition by fluoxetine was noncompetitive, only decreasing maximum velocity for dopamine. These results suggest that fluoxetine inhibited the activity of VMAT2 by a mechanism different from that of reserpine and did not directly interact with the active site of VMAT2.

VMAT2 quantitation by PET as a biomarker for beta-cell mass in health and disease

The common pathology underlying both type 1 and type 2 diabetes (T1DM and T2DM) is insufficient beta-cell mass (BCM) to meet metabolic demands. An important impediment to the more rapid evaluation of interventions for both T1DM and T2DM lack of biomarkers of pancreatic BCM. A reliable means of monitoring the mass and/or function of beta-cells would enable evaluation of the progression of diabetes as well as the monitoring of pharmacologic and other interventions. Recently, we identified a biomarker of BCM that is quantifiable by positron emission tomography (PET). PET is an imaging technique which allows for non-invasive measurements of radioligand uptake and clearance, is sensitive in the pico- to nanomolar range and of which the results can be deconvoluted into measurements of receptor concentration. For BCM estimates, we have identified VMAT2 (vesicular monoamine transporter type 2) as a biomarker and [(11)C] DTBZ (dihydrotetrabenazine) as the transporter's ligand. VMAT2 is highly expressed in beta-cells of the human pancreas relative to other cells of the endocrine and exocrine pancreas. Thus measurements of [(11)C] DTBZ in the pancreas provide an indirect measurement of BCM. Here we summarize our ongoing efforts to validate the clinical utility of this non-invasive approach to real-time BCM measurements.

Role of vesicular monoamine transporter type 2 in rodent insulin secretion and glucose metabolism revealed by its specific antagonist tetrabenazine

Despite different embryological origins, islet beta-cells and neurons share the expression of many genes and display multiple functional similarities. One shared gene product, vesicular monoamine transporter type 2 (VMAT2, also known as SLC18A2), is highly expressed in human beta-cells relative to other cells in the endocrine and exocrine pancreas. Recent reports suggest that the monoamine dopamine is an important paracrine and/or autocrine regulator of insulin release by beta-cells. Given the important role of VMAT2 in the economy of monoamines such as dopamine, we investigated the possible role of VMAT2 in insulin secretion and glucose metabolism. Using a VMAT2-specific antagonist, tetrabenazine (TBZ), we studied glucose homeostasis, insulin secretion both in vivo and ex vivo in cultures of purified rodent islets. During intraperitoneal glucose tolerance tests, control rats showed increased serum insulin concentrations and smaller glucose excursions relative to controls after a single intravenous dose of TBZ. One hour following TBZ administration we observed a significant depletion of total pancreas dopamine. Correspondingly, exogenous L-3,4-dihydroxyphenylalanine reversed the effects of TBZ on glucose clearance in vivo. In in vitro studies of rat islets, a significantly enhanced glucose-dependent insulin secretion was observed in the presence of dihydrotetrabenazine, the active metabolite of TBZ. Together, these data suggest that VMAT2 regulates in vivo glucose homeostasis and insulin production, most likely via its role in vesicular transport and storage of monoamines in beta-cells.

Expression of the serotonin transporter (SERT) in the choroid plexuses from buffalo brain

Choroid plexuses (CPs) play pivotal roles in a wide range of processes that establish, survey, and maintain the biochemical and cellular status of the central nervous system. Mammalian CPs contain a very high density of serotonin receptors, and serotonin has been shown to affect CP functions. The serotonin transporter (SERT) regulates the entire serotonergic system, including serotonin receptors by means of modulation of serotonin concentration in the extracellular fluid. In this study, the expression of SERT in the CPs from the brain of a mammalian species, Bubalis bubalis, was established. By immunogold labeling in scanning electron microscopy, SERT immunoreactivity was found to be localized on the apical surface of the choroid epithelium. In particular, SERT positivity was detected on the apical portion of villi, and both on the membrane and in the cytoplasm of grouped cells on the surface of the choroid epithelium. Significantly, no SERT was detected in blood vessels irrigating the CPs. The expression of SERT mRNA transcripts of 440 bp in the CPs was detected by reverse-transcription polymerase chain reaction, and Western blotting analysis revealed the presence of three isoforms of the protein with molecular masses of approximately 70, 80, and 140 kDa, respectively, probably corresponding to differently glycosylated SERT. Our findings provide the first report of SERT detection in the CPs of buffalo brain and indicate that this protein is locally synthesized from the choroid epithelial cells. We suggest that SERT might have an important role in mammalian CPs, possibly regulating the serotonin flow between brain and rest of the body.

Homocysteine is toxic for dopaminergic neurons in primary mesencephalic culture

Hyperhomocysteinemia associated with L-3,4-dihydroxyphenylalanine (L-dopa) treatment has been observed in patients with Parkinson's disease. We investigated the toxicity of homocysteine (Hcy) on E14-rat-primary mesencephalic culture. Exposure to 0-5 mM Hcy decreased number of tyrosine hydroxylase (TH)-positive dopaminergic neurons and microtubule associated protein 2 (MAP2)-positive neurons in a dose-dependent manner. TH-positive neurons had vulnerability to the insult of Hcy compared with the other MAP2-positive neurons. In dopaminergic neurons, 5 microM reserpine enhanced the Hcy toxicity, whereas 50 microM alpha-methyltyrosine attenuated the toxic effect, showing that the intracellular dopamine increased the cytotoxicity of Hcy. Hcy enhanced the toxicity of 1-methyl-4-phenylpyridinium (MPP+) for dopaminergic neurons. It was suggested that the Hcy toxicity was associated with the oxidative stress. Hcy is toxic for dopaminergic neurons, and hyperhomocysteinemia may modify the clinical course of Parkinson's disease.

Identification of endophilins 1 and 3 as selective binding partners for VGLUT1 and their co-localization in neocortical glutamatergic synapses: implications for vesicular glutamate transporter trafficking and excitatory vesicle formation

1. Selective protein-protein interactions between neurotransmitter transporters and their synaptic targets play important roles in regulating chemical neurotransmission. We screened a yeast two-hybrid library with bait containing the C-terminal amino acids of VGLUT1 and obtained clones that encode endophilin 1 and endophilin 3, proteins considered to play an integral role in glutamatergic vesicle formation. 2. Using a modified yeast plasmid vector to enable more cost-effective screens, we analyzed the selectivity and specificity of this interaction. Endophilins 1 and 3 selectively recognize only VGLUT1 as the C-terminus of VGLUT2 and VGLUT3 do not interact with either endophilin isoform. We mutagenized four conserved stretches of primary sequence in VGLUT1 that includes two polyproline motifs (Pro1, PPAPPP, and Pro2, PPRPPPP), found only in VGLUT1, and two conserved stretches (SEEK, SYGAT), found also in VGLUT2 and VGLUT3. The absence of the VGLUT conserved regions does not affect VGLUT1-endophilin association. Of the two polyproline stretches, only one (Pro2) is required for binding specificity to both endophilin 1 and endophilin 3. 3. We also show that endophilin 1 and endophilin 3 co-localize with VGLUT1 in synaptic terminals of differentiated rat neocortical neurons in primary culture. These results indicate that VGLUT1 and both endophilins are enriched in a class of excitatory synaptic terminals in cortical neurons and there, may interact to play an important role affecting the vesicular sequestration and synaptic release of glutamate.

Activity-dependent regulation of vesicular glutamate and GABA transporters: a means to scale quantal size

The functional balance of glutamatergic and GABAergic signaling in neuronal cortical circuits is under homeostatic control. That is, prolonged alterations of global network activity leads to opposite changes in quantal amplitude at glutamatergic and GABAergic synapses. Such scaling of excitatory and inhibitory transmission within cortical circuits serves to restore and maintain a constant spontaneous firing rate of pyramidal neurons. Our recent work shows that this includes alterations in the levels of expression of vesicular glutamate (VGLUT1 and VGLUT2) and GABA (VIAAT) transporters. Other vesicle markers, such as synaptophysin or synapsin, are not regulated in this way. Endogenous regulation at the level of mRNA and synaptic protein controls the number of transporters per vesicle and hence, the level of vesicle filling with transmitter. Bidirectional and opposite activity-dependent regulation of VGLUT1 and VIAAT expression would serve to adjust the balance of glutamate and GABA release and therefore the level of postsynaptic receptor saturation. In some excitatory neurons and synapses, co-expression of VGLUT1 and VGLUT2 occurs. Bidirectional and opposite changes in the levels of two excitatory vesicular transporters would enable individual neocortical neurons to scale up or scale down the level of vesicular glutamate storage, and thus, the amount available for release at individual synapses. Regulated vesicular transmitter storage and release via selective changes in the level of expression of vesicular glutamate and GABA transporters indicates that homeostatic plasticity of synaptic strength at cortical synapses includes presynaptic elements.

Homeostatic scaling of vesicular glutamate and GABA transporter expression in rat neocortical circuits

Homeostatic control of pyramidal neuron firing rate involves a functional balance of feedforward excitation and feedback inhibition in neocortical circuits. Here, we reveal a dynamic scaling in vesicular excitatory (vesicular glutamate transporters VGLUT1 and VGLUT2) and inhibitory (vesicular inhibitory amino acid transporter VIAAT) transporter mRNA and synaptic protein expression in rat neocortical neuronal cultures, using a well established in vitro protocol to induce homeostatic plasticity. During the second and third week of synaptic differentiation, the predominant vesicular transporters expressed in neocortical neurons, VGLUT1 and VIAAT, are both dramatically upregulated. In mature cultures, VGLUT1 and VIAAT exhibit bidirectional and opposite regulation by prolonged activity changes. Endogenous coregulation during development and homeostatic scaling of the expression of the transporters in functionally differentiated cultures may serve to control vesicular glutamate and GABA filling and adjust functional presynaptic excitatory/inhibitory balance. Unexpectedly, hyperexcitation in differentiated cultures triggers a striking increase in VGLUT2 mRNA and synaptic protein, whereas decreased excitation reduces levels. VGLUT2 mRNA and protein are expressed in subsets of VGLUT1-encoded neocortical neurons that we identify in primary cultures and in neocortex in situ and in vivo. After prolonged hyperexcitation, downregulation of VGLUT1/synaptophysin intensity ratios at most synapses is observed, whereas a subset of VGLUT1-containing boutons selectively increase the expression of VGLUT2. Bidirectional and opposite regulation of VGLUT1 and VGLUT2 by activity may serve as positive or negative feedback regulators for cortical synaptic transmission. Intracortical VGLUT1/VGLUT2 coexpressing neurons have the capacity to independently modulate the level of expression of either transporter at discrete synapses and therefore may serve as a plastic interface between subcortical thalamic input (VGLUT2) and cortical output (VGLUT1) neurons.

Subpopulations of rat dorsal root ganglion neurons express active vesicular acetylcholine transporter

The vesicular acetylcholine transporter (VAChT) is a transmembrane protein required, in cholinergic neurons, for selective storage of acetylcholine into synaptic vesicles. Although dorsal root ganglion (DRG) neurons utilize neuropeptides and amino acids for neurotransmission, we have previously demonstrated the presence of a cholinergic system. To investigate whether, in sensory neurons, the vesicular accumulation of acetylcholine relies on the same mechanisms active in classical cholinergic neurons, we investigated VAChT presence, subcellular distribution, and activity. RT-PCR and Western blot analysis demonstrated the presence of VAChT mRNA and protein product in DRG neurons and in the striatum and cortex, used as positive controls. Moreover, in situ hybridization and immunocytochemistry showed VAChT staining located mainly in the medium/large-sized subpopulation of the sensory neurons. A few small neurons were also faintly labeled by immunocytochemistry. In the electron microscope, immunolabeling was associated with vesicle-like elements distributed in the neuronal cytoplasm and in both myelinated and unmyelinated intraganglionic nerve fibers. Finally, [(3)H]acetylcholine active transport, evaluated either in the presence or in the absence of ATP, also demonstrated that, as previously reported, the uptake of acetylcholine by VAChT is ATP dependent. This study suggests that DRG neurons not only are able to synthesize and degrade ACh and to convey cholinergic stimuli but also are capable of accumulating and, possibly, releasing acetylcholine by the same mechanism used by the better known cholinergic neurons.

The effects of vesicular volume on secretion through the fusion pore in exocytotic release from PC12 cells

Many spikes in amperometric records of exocytosis events initially exhibit a prespike feature, or foot, which represents a steady-state flux of neurotransmitter through a stable fusion pore spanning both the vesicle and plasma membranes and connecting the vesicle lumen to the extracellular fluid. Here, we present the first evidence indicating that vesicular volume before secretion is strongly correlated with the characteristics of amperometric foot events. L-3,4-dihydroxyphenylalanine and reserpine have been used to increase and decrease, respectively, the volume of single pheochromocytoma cell vesicles. Amperometry and transmission electron microscopy have been used to determine that as vesicle size is decreased the frequency with which foot events are observed increases, the amount and duration of neurotransmitter released in the foot portion of the event decreases, and vesicles release a greater percentage of their total contents in the foot portion of the event. This previously unidentified correlation provides new insight into how vesicle volume can modulate the activity of the exocytotic fusion pore.

The vesicular acetylcholine transporter interacts with clathrin-associated adaptor complexes AP-1 and AP-2

In neuronal cells the neurotransmitter acetylcholine is transferred from the cytoplasm into synaptic vesicles by the vesicular acetylcholine transporter (VAChT). The cytoplasmic tail of VAChT has been shown to contain signals that direct its sorting and trafficking. The role of clathrin-associated protein complexes in VAChT sorting to synaptic vesicles has been examined. A fusion protein between the VAChT cytoplasmic tail and glutathione S-transferase was used to identify VAChT-clathrin-associated protein adaptor protein 1, adaptor protein 2 and adaptor protein 180 complexes from a rat brain extract. In vivo coimmunoprecipitation confirmed adaptin alpha and adaptin gamma complexes, but adaptor protein 180 complexes were not detected by this technique. Deletion and site directed mutagenesis show that the VAChT cytoplasmic tail contains multiple trafficking signals. These include a non-classical tyrosine motif that serves as the signal for adaptin alpha and a dileucine motif that serves as the signal for adaptin gamma. A classical tyrosine motif is also involved in VAChT trafficking, but does not interact with any known adaptor proteins. There appear to be two endocytosis motifs, one involving the adaptor protein 1 binding site and the other involving the adaptor protein 2 binding site. These results suggest a complex trafficking pathway for VAChT.

Direct effects of proton pump inhibitors on histamine release from rat enterochromaffin-like cells

Enterochromaffin-like (ECL) cells play a central role in the regulation of gastric acid secretion. Previous studies have shown that proton pump inhibitors accelerate histamine release from ECL cells through the effects of gastrin. However, direct effects of proton pump inhibitors on ECL cells have not been demonstrated to date because the indirect effects of gastrin are difficult to suppress. We investigated the direct effects of proton pump inhibitors medication on ECL cells using an elutriation system. ECL cells were stimulated with gastrin or rabeprazole, and histamine release from ECL cells was measured. Rabeprazole increased histamine release through a pathway that differed from that of gastrin. The histamine increase was likely due to an acceleration of vesicular monoamine transporter 2 (VMAT2). Rabeprazole increased histamine release from ECL cells directly via VMAT2, but did not affect the total amount of histamine in the cells. The results suggest that patients receiving proton pump inhibitors for extended periods must be monitored extensively because gastric tumor proliferation may be promoted by increased histamine release from ECL cells.

Molecular cloning and functional identification of mouse vesicular glutamate transporter 3 and its expression in subsets of novel excitatory neurons

We have cloned and functionally characterized a third isoform of a vesicular glutamate transporter (VGLUT3) expressed on synaptic vesicles that identifies a distinct glutamatergic system in the brain that is partly and selectively promiscuous with cholinergic and serotoninergic transmission. Transport activity was specific for glutamate, was H(+)-dependent, was stimulated by Cl(-) ion, and was inhibited by Rose Bengal and trypan blue. Northern analysis revealed higher mRNA levels in early postnatal development than in adult brain. Restricted patterns of mRNA expression were observed in presumed interneurons in cortex and hippocampus, and projection systems were observed in the lateral and ventrolateral hypothalamic nuclei, limbic system, and brainstem. Double in situ hybridization histochemistry for vesicular acetylcholine transporter identified VGLUT3 neurons in the striatum as cholinergic interneurons, whereas VGLUT3 mRNA and protein were absent from all other cholinergic cell groups. In the brainstem VGLUT3 mRNA was concentrated in mesopontine raphé nuclei. VGLUT3 immunoreactivity was present throughout the brain in a diffuse system of thick and thin beaded varicose fibers much less abundant than, and strictly separated from, VGLUT1 or VGLUT2 synapses. Co-existence of VGLUT3 in VMAT2-positive and tyrosine hydroxylase -negative varicosities only in the cerebral cortex and hippocampus and in subsets of tryptophan hydroxylase-positive cell bodies and processes in differentiating primary raphé neurons in vitro indicates selective and target-specific expression of the glutamatergic/serotoninergic synaptic phenotype.

Regulation of vesicular acetylcholine transporter by the activation of excitatory amino acid receptors in the avian retina

1. Previous studies have shown that phorbol esters induce protein kinase C (PKC) mediated phosphorylation of the vesicular acetylcholine transporter (VAChT) and change its interaction with vesamicol. However, it is not clear whether physiological activation of receptors coupled to PKC activation can alter VAChT behavior. 2. Here we tested whether activation of kaianate (KA) receptors alters VAChT. Several studies suggest that the cholinergic amacrine cells display KA/AMPA receptors that mediate excitatory input to these neurons. In addition, KA in the chicken retina can generate intracellular messengers with the potential to regulate cellular functions. 3. In cultured chicken retina (E8C11) KA reduced vesamicol binding to VAChT by 53%. This effect was potentiated by okadaic acid, a protein phosphatase inhibitor, and was totally prevented by BIM, a PKC inhibitor. 4. Phorbol myristate acetate (PMA), but not alpha-PMA, reduced in more than 85% the number of L-[3H]-vesamicol-specific binding sites in chicken retina, confirming that activation of PKC can influence vesamicol binding to chicken VAChT. 5. The data show that activation of glutamatergic receptors reduces [3H]-vesamicol binding sites (VAChT) likely by activating PKC and increasing the phosphorylation of the ACh carrier.

Microscopic kinetics and structure-function analysis in the vesicular acetylcholine transporter

The vesicular acetylcholine transporter (VAChT) resides in synaptic vesicles of cholinergic nerve terminals. It carries out vesicular storage of ACh. The amount of ACh stored determines, along with other factors, the amount of ACh released. Knowledge of the structure-function relationship in VAChT might enable pharmacological regulation of ACh storage and release at the level of VAChT. To this end, a quantitative model for the individual steps in the overall transport cycle of VAChT has been developed. Because of the particular values of the microscopic rate constants in the model, structure-function analysis of VAChT can be misleading. Attempts to devise a pro-storage strategy to increase ACh release from cholinergic nerve terminals should take into account the microscopic kinetics of this transporter.

Identification of the differentiation-associated Na+/PI transporter as a novel vesicular glutamate transporter expressed in a distinct set of glutamatergic synapses

Glutamate transport into synaptic vesicles is a prerequisite for its regulated neurosecretion. Here we functionally identify a second isoform of the vesicular glutamate transporter (VGLUT2) that was previously identified as a plasma membrane Na+-dependent inorganic phosphate transporter (differentiation-associated Na+/P(I) transporter). Studies using intracellular vesicles from transiently transfected PC12 cells indicate that uptake by VGLUT2 is highly selective for glutamate, is H+ dependent, and requires Cl- ion. Both the vesicular membrane potential (Deltapsi) and the proton gradient (DeltapH) are important driving forces for vesicular glutamate accumulation under physiological Cl- concentrations. Using an antibody specific for VGLUT2, we also find that this protein is enriched on synaptic vesicles and selective for a distinct class of glutamatergic nerve terminals. The pathway-specific, complementary expression of two different vesicular glutamate transporters suggests functional diversity in the regulation of vesicular release at excitatory synapses. Together, the two isoforms may account for the uptake of glutamate by synaptic vesicles from all central glutamatergic neurons.

Analysis of point mutants in the Caenorhabditis elegans vesicular acetylcholine transporter reveals domains involved in substrate translocation

Cholinergic neurotransmission depends upon the regulated release of acetylcholine. This requires the loading of acetylcholine into synaptic vesicles by the vesicular acetylcholine transporter (VAChT). Here, we identify point mutants in Caenorhabditis elegans that map to highly conserved regions of the VAChT gene of Caenorhabditis elegans (CeVAChT) (unc-17) and exhibit behavioral phenotypes consistent with a reduction in vesicular transport activity and neurosecretion. Several of these mutants express normal amounts of VAChT protein and exhibit appropriate targeting of VAChT to synaptic vesicles. By site-directed mutagenesis, we have replaced the conserved amino acid residues found in human VAChT with the mutated residue in CeVAChT and stably expressed these cDNAs in PC-12 cells. These mutants display selective defects in initial acetylcholine transport velocity (K(m)), with values ranging from 2- to 8-fold lower than that of the wild-type. One of these mutants has lost its specific interaction with vesamicol, a selective inhibitor of VAChT, and displays vesamicol-insensitive uptake of acetylcholine. The relative order of behavioral severity of the CeVAChT point mutants is identical to the order of reduced affinity of VAChT for acetylcholine in vitro. This indicates that specific structural changes in VAChT translate into specific alterations in the intrinsic parameters of transport and in the storage and synaptic release of acetylcholine in vivo.

Ca2+ sensitivity of synaptic vesicle dopamine, gamma-aminobutyric acid, and glutamate transport systems

The effect of Ca2+ on the uptake of neurotransmitters by synaptic vesicles was investigated in a synaptic vesicle enriched fraction isolated from sheep brain cortex. We observed that dopamine uptake, which is driven at expenses of the proton concentration gradient generated across the membrane by the H+-ATPase activity, is strongly inhibited (70%) by 500 microM Ca2+. Conversely, glutamate uptake, which essentially requires the electrical potential in the presence of low Cl- concentrations, is not affected by Ca2+, even when the proton concentration gradient greatly contributes for the proton electrochemical gradient. These observations were checked by adding Ca2+ to dopamine or glutamate loaded vesicles, which promoted dopamine release, whereas glutamate remained inside the vesicles. Furthermore, similar effects were obtained by adding 150 microM Zn2+ that, like Ca2+, dissipates the proton concentration gradient by exchanging with H+. With respect to gamma-aminobutyric acid transport, which utilizes either the proton concentration gradient or the electrical potential as energy sources, we observed that Ca2+ or Zn2+ do not induce great alterations in the gamma-aminobutyric acid accumulation by synaptic vesicles. These results clarify the nature of the energy source for accumulation of main neurotransmitters and suggest that stressing concentrations of Ca2+ or Zn2+ inhibit the proton concentration gradient-dependent neurotransmitter accumulation by inducing H+ pump uncoupling rather than by interacting with the neurotransmitter transporter molecules.

Glutamate uptake

Brain tissue has a remarkable ability to accumulate glutamate. This ability is due to glutamate transporter proteins present in the plasma membranes of both glial cells and neurons. The transporter proteins represent the only (significant) mechanism for removal of glutamate from the extracellular fluid and their importance for the long-term maintenance of low and non-toxic concentrations of glutamate is now well documented. In addition to this simple, but essential glutamate removal role, the glutamate transporters appear to have more sophisticated functions in the modulation of neurotransmission. They may modify the time course of synaptic events, the extent and pattern of activation and desensitization of receptors outside the synaptic cleft and at neighboring synapses (intersynaptic cross-talk). Further, the glutamate transporters provide glutamate for synthesis of e.g. GABA, glutathione and protein, and for energy production. They also play roles in peripheral organs and tissues (e.g. bone, heart, intestine, kidneys, pancreas and placenta). Glutamate uptake appears to be modulated on virtually all possible levels, i.e. DNA transcription, mRNA splicing and degradation, protein synthesis and targeting, and actual amino acid transport activity and associated ion channel activities. A variety of soluble compounds (e.g. glutamate, cytokines and growth factors) influence glutamate transporter expression and activities. Neither the normal functioning of glutamatergic synapses nor the pathogenesis of major neurological diseases (e.g. cerebral ischemia, hypoglycemia, amyotrophic lateral sclerosis, Alzheimer's disease, traumatic brain injury, epilepsy and schizophrenia) as well as non-neurological diseases (e.g. osteoporosis) can be properly understood unless more is learned about these transporter proteins. Like glutamate itself, glutamate transporters are somehow involved in almost all aspects of normal and abnormal brain activity.

Cellular resistance to Evans blue toxicity involves an up-regulation of a phosphate transporter implicated in vesicular glutamate storage

It has recently been suggested that the brain-specific Na+-dependent phosphate inorganic co-transporter (BNPI) is able to support glutamate transport and storage in synaptic vesicles. A procedure for measuring the vesicular pool of glutamate is described and was used to select cell lines according to their glutamate storage capacity. Two cell lines were selected: C6BU-1, with a large intracellular glutamate storage capacity, and NG108-15, devoid of it. Their contents in BNPI mRNA were compared by RT-PCR. We found that both cell lines had BNPI, but in addition C6BU-1 alone expresses the other isoform, DNPI. We also carried out a clonal selection of NG108-15 cells in the presence of the dye Evans blue, a competitive inhibitor of vesicular glutamate transport, very toxic for cells in culture. It was assumed that only those that sequester and eliminate the drug by overexpressing a vesicular glutamate transporter would survive. We found that the NG108-15 clones resistant to Evans blue had an increased storage capacity for glutamate. These cells also up-regulated the BNPI isoform of the phosphate transporter as shown by RT-PCR and northern blot.

Molecular analysis of vesicular amine transporter function and targeting to secretory organelles

Vesicular transporters are responsible for the loading of neurotransmitters into specialized secretory organelles in neurons and neuroendocrine cells to make them available for regulated neurosecretion. The exocytotic release of neurotransmitter therefore depends on the functional activity of the vesicular transporters and their efficient sorting to these secretory organelles. Molecular analysis of vesicular transport proteins has revealed important information regarding structural domains responsible for their functional properties, including substrate specificity and trafficking to various classes of secretory vesicles. These studies have established the existence of an important functional relationship between transporter activity and presynaptic quantal neurosecretion.

Transport mechanisms in acetylcholine and monoamine storage

Sequence-related vesicular acetylcholine transporter (VAChT) and vesicular monoamine transporter (VMAT) transport neurotransmitter substrates into secretory vesicles. This review seeks to identify shared and differentiated aspects of the transport mechanisms. VAChT and VMAT exchange two protons per substrate molecule with very similar initial velocity kinetics and pH dependencies. However, vesicular gradients of ACh in vivo are much smaller than the driving force for uptake and vesicular gradients of monoamines, suggesting the existence of a regulatory mechanism in ACh storage not found in monoamine storage. The importance of microscopic rather than macroscopic kinetics in structure-function analysis is described. Transporter regions affecting binding or translocation of substrates, inhibitors, and protons have been found with photoaffinity labeling, chimeras, and single-site mutations. VAChT and VMAT exhibit partial structural and mechanistic homology with lactose permease, which belongs to the same sequence-defined superfamily, despite opposite directions of substrate transport. The vesicular transporters translocate the first proton using homologous aspartates in putative transmembrane domain X (ten), but they translocate the second proton using unknown residues that might not be conserved between them. Comparative analysis of the VAChT and VMAT transport mechanisms will aid understanding of regulation in neurotransmitter storage.

Constitutive phosphorylation of the vesicular inhibitory amino acid transporter in rat central nervous system

gamma-Aminobutyric acid (GABA) and glycine are stored into synaptic vesicles by a recently identified vesicular inhibitory amino acid transporter [VIAAT, also called vesicular GABA transporter (VGAT)]. Immunoblotting analysis revealed that rat brain VIAAT migrated as a doublet during sodium dodecyl sulfate-polyacrylamide gel electrophoresis, with a predominant slower band in all areas examined except olfactory bulb and retina. The slower band corresponded to a phosphorylated form of VIAAT as it was converted to the faster one by treating brain homogenates with alkaline phosphatase or with an endogenous phosphatase identified as type 2A protein-serine/threonine phosphatase using okadaic acid. In contrast, the recombinant protein expressed in COS-7 or PC12 cells co-migrated with the faster band of the brain doublet and was insensitive to alkaline phosphatase. To investigate the influence of VIAAT phosphorylation on vesicular neurotransmitter loading, purified synaptic vesicles were treated with alkaline phosphatase and assayed for amino acid uptake. However, neither GABA nor glycine uptake was affected by VIAAT phosphorylation. These results indicate that VIAAT is constitutively phosphorylated on cytosolic serine or threonine residues in most, but not all, regions of the rat brain. This phosphorylation does not regulate the vesicular loading of GABA or glycine, suggesting that it is involved at other stages of the synaptic vesicle life cycle.

VMAT-Mediated changes in quantal size and vesicular volume

It has been well established that the volume of secretory vesicles can be modulated. However, we present the first data demonstrating that the amount of transmitter in a vesicle can regulate its volume. Amperometry and transmission electron microscopy have been used to determine that l-3,4-dihydroxyphenylalanine and reserpine increase and decrease, respectively, the volume of single pheochromocytoma cell vesicles as well as their catecholamine content. Because changes in vesicular catecholamine content are tracked by changes in vesicle volume, our results indicate that when quantal size is altered via the vesicular monoamine transporter the concentration of catecholamines within the vesicles remains relatively constant. This previously unidentified cellular response provides new insight into how catecholamines can be packaged in and released from secretory vesicles.

Phosphorylation of the rat vesicular acetylcholine transporter

Metabolic labeling of a mutant PC12 cell line, A123.7, expressing recombinant rat vesicular acetylcholine transporter (VAChT) with radiolabeled inorganic phosphate was used to demonstrate phosphorylation of the transporter on a serine residue. Mutational analysis was used to demonstrate that serine 480, which is located on the COOH-terminal cytoplasmic tail, is the sole phosphorylation site. Phosphorylation of serine 480 was attributable to the action of protein kinase C. Using a permanently dephosphorylated form of rat VAChT, S480A rVAChT, it was shown that this mutant displays the same kinetics for the transport of acetylcholine and the binding of the inhibitor vesamicol as does the wild type transporter. However, sucrose gradient density centrifugation showed that, unlike wild type VAChT, the S480A mutant did not localize to synaptic vesicles. These results suggest that phosphorylation of serine 480 of VAChT is involved in the trafficking of this transporter.

A phosphorylation site regulates sorting of the vesicular acetylcholine transporter to dense core vesicles

Vesicular transport proteins package classical neurotransmitters for regulated exocytotic release, and localize to at least two distinct types of secretory vesicles. In PC12 cells, the vesicular acetylcholine transporter (VAChT) localizes preferentially to synaptic-like microvesicles (SLMVs), whereas the closely related vesicular monoamine transporters (VMATs) localize preferentially to large dense core vesicles (LDCVs). VAChT and the VMATs contain COOH-terminal, cytoplasmic dileucine motifs required for internalization from the plasma membrane. We now show that VAChT undergoes regulated phosphorylation by protein kinase C on a serine (Ser-480) five residues upstream of the dileucine motif. Replacement of Ser-480 by glutamate, to mimic the phosphorylation event, increases the localization of VAChT to LDCVs. Conversely, the VMATs contain two glutamates upstream of their dileucine-like motif, and replacement of these residues by alanine conversely reduces sorting to LDCVs. The results provide some of the first information about sequences involved in sorting to LDCVs. Since the location of the transporters determines which vesicles store classical neurotransmitters, a change in VAChT trafficking due to phosphorylation may also influence the mode of transmitter release.

Role of mediatophore in connection with proteins of the active zone in synaptic transmission

Mediatophore is a protein purified from Torpedo electric organ synaptosomes, which translocates acetylcholine (ACh) upon calcium action after reconstitution in artificial membranes. After expression in transfected cells, it endows these cells with a calcium-dependent release mechanism displaying clear quantal properties. The role of mediatophore in synaptic transmission is discussed in relation to the ultrastructural organization of the active zone and the cytosolic high calcium microdomains that transiently appear after presynaptic membrane depolarization.

SV2C is a synaptic vesicle protein with an unusually restricted localization: anatomy of a synaptic vesicle protein family

We describe here the identification and molecular characterization of a new brain protein that we named SV2C because it is homologous to the synaptic vesicle proteins SV2A and SV2B, and because it is also recognized by the monoclonal SV2 antibody that led to the initial discovery of SV2A and SV2B. SV2C is more closely related to SV2A (62% identity) than to SV2B (57% identity), and contains 12 transmembrane regions similar to these proteins. To characterize SV2C and compare its properties and localization with those of SV2A and SV2B, we raised an SV2C-specific antibody. Using this antibody, we show that SV2C is an N-glycosylated protein that is concentrated on small synaptic vesicles; in addition, it is found on microvesicles in adrenal chromaffin cells. We evaluated the relative localization of the three SV2 isoforms by staining rat brain sections with antibodies specific for SV2A, SV2B and SV2C. Analysis of the resulting staining patterns confirmed previous conclusions that SV2A is ubiquitously expressed in virtually all synapses. SV2B, although more restricted in distribution, was also found in a wide variety of synapses throughout the brain. In striking contrast to this general localization and to similarly wide distributions of other synaptic vesicle proteins, SV2C was observed only in few brain areas. High levels of SV2C were found primarily in phylogenetically old brain regions such as the pallidum, the substantia nigra, the midbrain, the brainstem and the olfactory bulb. SV2C was undetectable in the cerebral cortex and the hippocampus, and found at low levels in the cerebellar cortex. Our data suggest that closely related members of a synaptic vesicle protein family can either have very general (SV2A) or restricted distributions (SV2C), possibly in order to allow specialization in the regulation of the expression or of the function of these abundant synaptic vesicle proteins.

Differential quantal release of histamine and 5-hydroxytryptamine from mast cells of vesicular monoamine transporter 2 knockout mice

The recent availability of mice lacking the neuronal form of the vesicular monoamine transporter 2 (VMAT2) affords the opportunity to study its roles in storage and release. Carbon fiber microelectrodes were used to measure individual secretory events of histamine and 5-hydroxytryptamine (5-HT) from VMAT2-expressing mast cells as a model system for quantal release. VMAT2 is indispensable for monoamine storage because mast cells from homozygous (VMAT2(-/-)) mice, while undergoing granule-cell fusion, do not release monoamines. Cells from heterozygous animals (VMAT2(+/-)) secrete lower amounts of monoamine per granule than cells from wild-type controls. Investigation of corelease of histamine and 5-HT from granules in VMAT2(+/-) cells revealed 5-HT quantal size was reduced more than that of histamine. Thus, although vesicular transport is the limiting factor determining quantal size of 5-HT and histamine release, intragranular association with the heparin matrix also plays a significant role.

Increased methamphetamine neurotoxicity in heterozygous vesicular monoamine transporter 2 knock-out mice

Methamphetamine (METH) is a powerful psychostimulant that is increasingly abused worldwide. Although it is commonly accepted that the dopaminergic system and oxidation of dopamine (DA) play pivotal roles in the neurotoxicity produced by this phenylethylamine, the primary source of DA responsible for this effect has remained elusive. In this study, we used mice heterozygous for vesicular monoamine transporter 2 (VMAT2 +/- mice) to determine whether impaired vesicular function alters the effects of METH. METH-induced dopaminergic neurotoxicity was increased in striatum of VMAT2 +/- mice compared with wild-type mice as revealed by a more consistent DA and metabolite depletion and a greater decrease in dopamine transporter expression. Interestingly, increased METH neurotoxicity in VMAT2 +/- mice was accompanied by less pronounced increase in extracellular DA and indices of free radical formation compared with wild-type mice. These results indicate that disruption of vesicular monoamine transport potentiates METH-induced neurotoxicity in vivo and point, albeit indirectly, to a greater contribution of intraneuronal DA redistribution rather than extraneuronal overflow on mediating this effect.

The cat-1 gene of Caenorhabditis elegans encodes a vesicular monoamine transporter required for specific monoamine-dependent behaviors

We have identified the Caenorhabditis elegans homolog of the mammalian vesicular monoamine transporters (VMATs); it is 47% identical to human VMAT1 and 49% identical to human VMAT2. C. elegans VMAT is associated with synaptic vesicles in approximately 25 neurons, including all of the cells reported to contain dopamine and serotonin, plus a few others. When C. elegans VMAT is expressed in mammalian cells, it has serotonin and dopamine transport activity; norepinephrine, tyramine, octopamine, and histamine also have high affinity for the transporter. The pharmacological profile of C. elegans VMAT is closer to mammalian VMAT2 than VMAT1. The C. elegans VMAT gene is cat-1; cat-1 knock-outs are totally deficient for VMAT immunostaining and for dopamine-mediated sensory behaviors, yet they are viable and grow relatively well. The cat-1 mutant phenotypes can be rescued by C. elegans VMAT constructs and also (at least partially) by human VMAT1 or VMAT2 transgenes. It therefore appears that the function of amine neurotransmitters can be completely dependent on their loading into synaptic vesicles.

Mutational analysis of aspartate residues in the transmembrane regions and cytoplasmic loops of rat vesicular acetylcholine transporter

The vesicular acetylcholine transporter (VAChT) is responsible for the transport of the neurotransmitter acetylcholine (ACh) into synaptic vesicles using an electrochemical gradient to drive transport. Rat VAChT has a number of aspartate residues within its predicted transmembrane domains (TM) and cytoplasmic loops, which may play important structural or functional roles in acetylcholine transport. In order to identify functional charged residues, site-directed mutagenesis of rVAChT was undertaken. No effect on ACh transport was observed when any of the five aspartate residues in the cytoplasmic loop were converted to asparagine. Similarly, changing Asp-46 (D46N) in TM1 or Asp-255 (D255N) in TM6 had no effect on ACh transport or vesamicol binding. However, replacement of Asp-398 in TM10 with Asn completely eliminated both ACh transport and vesamicol binding. The conservative mutant D398E retained transport activity, but not vesamicol binding, suggesting this residue is critical for transport. Mutation of Asp-193 in TM4 did not affect ACh transport activity; however, vesamicol binding was dramatically reduced. With mutant D425N of TM11 transport activity for ACh was completely blocked, without an effect on vesamicol binding. Activity was not restored in the conservative mutant D425E, suggesting the side chain as well as the negative charge of Asp-425 is important for substrate binding. These mutants, as well as mutant D193N, clearly dissociated ACh binding and transport from vesamicol binding. These data suggest that Asp-398 in TM10 and Asp-425 in TM11 are important for ACh binding and transport, while Asp-193 and Asp-398 in TM4 and TM10, respectively, are involved in vesamicol binding.