Polyamine release and vesicular polyamine transporter expression in megakaryoblastic cells and platelets

Polyamines not only play essential roles in cell growth and function of living organisms but are also released into the extracellular space and function as regulators of chemical transduction, although the cells from which they are released and their mode of release are not well understood. The vesicular polyamine transporter (VPAT), encoded by the SLC18B1 is responsible for the vesicular storage of spermine and spermidine, followed by their vesicular release from secretory cells. Focusing on VPAT will help identify polyamine-secreting cells and new polyamine functions. In this study, we investigated the possible involvement of VPAT in vesicular release of polyamines in MEG-01 clonal megakaryoblastic cells and platelets. RT-PCR, western blotting, and immunohistochemistry revealed VPAT expression in MEG-01 cells. MEG-01 cells secreted polyamines upon A23187 stimulation in the presence of Ca2+, which is temperature-dependent and sensitive to bafilomycin A1. A23187-induced polyamine secretion from MEG-01 cells was reduced by treatment with reserpine, VPAT inhibitors, or VPAT RNA interference. Platelets also expressed VPAT, displaying a punctate distribution, and released spermidine upon A23187 and thrombin stimulation. These findings have demonstrated VPAT-mediated vesicular polyamine release from MEG-01 cells, suggesting the presence of similar vesicular polyamine release mechanisms in platelets.

Positive Regulation of S-Adenosylmethionine on Chondrocytic Differentiation via Stimulation of Polyamine Production and the Gene Expression of Chondrogenic Differentiation Factors

S-adenosylmethionine (SAM) is considered to be a useful therapeutic agent for degenerative cartilage diseases, although its mechanism is not clear. We previously found that polyamines stimulate the expression of differentiated phenotype of chondrocytes. We also found that the cellular communication network factor 2 (CCN2) played a huge role in the proliferation and differentiation of chondrocytes. Therefore, we hypothesized that polyamines and CCN2 could be involved in the chondroprotective action of SAM. In this study, we initially found that exogenous SAM enhanced proteoglycan production but not cell proliferation in human chondrocyte-like cell line-2/8 (HCS-2/8) cells. Moreover, SAM enhanced gene expression of cartilage-specific matrix (aggrecan and type II collagen), Sry-Box transcription factor 9 (SOX9), CCN2, and chondroitin sulfate biosynthetic enzymes. The blockade of the methionine adenosyltransferase 2A (MAT2A) enzyme catalyzing intracellular SAM biosynthesis restrained the effect of SAM on chondrocytes. The polyamine level in chondrocytes was higher in SAM-treated culture than control culture. Additionally, Alcian blue staining and RT-qPCR indicated that the effects of SAM on the production and gene expression of aggrecan were reduced by the inhibition of polyamine synthesis. These results suggest that the stimulation of polyamine synthesis and gene expression of chondrogenic differentiation factors, such as CCN2, account for the mechanism underlying the action of SAM on chondrocytes.

Functional characterization and tissue localization of the facilitative glucose transporter GLUT12

Facilitative glucose transporters (GLUTs) play crucial roles in glucose utilization and homeostasis. GLUT12 was initially isolated as a novel GLUT4-like transporter involved in insulin-dependent glucose transport. However, tissue distribution and biochemical properties of GLUT12 are not well understood. In this study, we investigated the basic kinetic properties and tissue distribution of GLUT12. Human GLUT12 and GLUT1 were overexpressed and purified using Ni-NTA column chromatography. Reconstituted proteoliposomes showed time-dependent d-glucose transport activity, which was inhibited by phloretin and dehydroascorbate. Dose dependence of glucose transport revealed a KM and Vmax values of 6.4 mM and 1.2 μmol/mg/min, respectively, indicating that GLUT12 is a high-affinity type GLUT. Glucose transport by GLUT12 was inhibited by ATP and glucose-1-phosphate, glucose-6-phosphate and disaccharides (properties similar to those of GLUT1). Indirect immunohistochemistry revealed the distribution of mouse GLUT12 in the apical region of distal tubules and collecting ducts in the kidney and epithelial cells of the jejunum. In addition to these cells, GLUT12 was present in chromaffin cells in the adrenal medulla, the anterior pituitary lobe, as well as the thyroid and pyloric glands. These tissue distributions suggest a unique function of GLUT12, besides that of an insulin-dependent glucose transport.

The mitochondrial inner membrane protein LETM1 modulates cristae organization through its LETM domain

LETM1 is a mitochondrial inner membrane protein that is required for maintaining the mitochondrial morphology and cristae structures, and regulates mitochondrial ion homeostasis. Here we report a role of LETM1 in the organization of cristae structures. We identified four amino acid residues of human LETM1 that are crucial for complementation of the growth deficiency caused by gene deletion of a yeast LETM1 orthologue. Substituting amino acid residues with alanine disrupts the correct assembly of a protein complex containing LETM1 and prevents changes in the mitochondrial morphology induced by exogenous LETM1 expression. Moreover, the LETM1 protein changes the shapes of the membranes of in vitro-reconstituted proteoliposomes, leading to the formation of invaginated membrane structures on artificial liposomes. LETM1 mutant proteins with alanine substitutions fail to facilitate the formation of invaginated membrane structures, suggesting that LETM1 plays a fundamental role in the organization of mitochondrial membrane morphology.

Nakamura et al find that the mitochondrial protein LETM1 can directly modulate membrane structure in vitro and identify a conserved domain involved in modulating mitochondrial membrane morphology. This study enhances our understanding of how mitochondrial cristae are organised.

Function of essential chloride and arginine residue in nucleotide binding to vesicular nucleotide transporter

Vesicular nucleotide transporter (VNUT) plays a key role in purinergic signalling through its ability to transport nucleotides. VNUT belongs to the SLC17 family, which includes vesicular glutamate transporters (VGLUTs) and Type I Na+/phosphate cotransporters. All of these transporters exhibit membrane potential and Cl--dependent organic anion transport activity and have essential arginine in the transmembrane region. Previously, we reported that ketoacids inhibit these transporters through modulation of Cl- activation. Although this regulation is important to control signal transmission, the mechanisms underlying Cl--dependent regulation are unclear. Here, we examined the functional roles of Cl- and essential arginine residue on ATP binding to VNUT using the fluorescent ATP analogue trinitrophenyl-ATP (TNP-ATP). The fluorescence of TNP-ATP was enhanced by VNUT, whereas no enhancement was observed by VGLUT. Concentration-dependence curves showed that TNP-ATP was a high-affinity fluorescent probe for VNUT, with a Kd of 4.8 μM. TNP-ATP binding was competitive to ATP and showed similar specificity to transport activity. Addition of Cl- and ketoacids did not affect the apparent affinity for TNP-ATP. The Arg119 to Ala mutant retained TNP-ATP binding ability with slightly reduced affinity. Overall, these results indicated that Cl- and essential arginine were not important for ATP binding.

Outward open conformation of a Major Facilitator Superfamily multidrug/H+ antiporter provides insights into switching mechanism

Multidrug resistance (MDR) poses a major challenge to medicine. A principle cause of MDR is through active efflux by MDR transporters situated in the bacterial membrane. Here we present the crystal structure of the major facilitator superfamily (MFS) drug/H⁺ antiporter MdfA from Escherichia coli in an outward open conformation. Comparison with the inward facing (drug binding) state shows that, in addition to the expected change in relative orientations of the N- and C-terminal lobes of the antiporter, the conformation of TM5 is kinked and twisted. In vitro reconstitution experiments demonstrate the importance of selected residues for transport and molecular dynamics simulations are used to gain insights into antiporter switching. With the availability of structures of alternative conformational states, we anticipate that MdfA will serve as a model system for understanding drug efflux in MFS MDR antiporters.

The multidrug resistance transporter mediated efflux of antibiotics from the bacterial cytoplasm represents a major challenge to medicine. Here authors solve the X-ray crystallographic structure of the drug/H+ antiporter MdfA from Escherichia coli and shed light on the conformational switching mechanism.

Purification and reconstitution of polyspecific H+/organic cation antiporter human MATE1

Human MATE1 (multidrug and toxin extrusion 1, hMATE1) is a H⁺/organic cation (OC) exchanger responsible for the final step of toxic organic cation excretion in the kidney and liver. To investigate the mechanism of transport, we have established an in vitro assay procedure that includes its expression in insect cells, solubilization with octyl glucoside, purification, and reconstitution into liposomes. The resultant proteoliposomes containing hMATE1 as the sole protein component took up radiolabeled tetraethylammonium (TEA) in a ∆pH-dependent and electroneutral fashion. Furthermore, lipid-detergent micelle containing hMATE1 showed ∆pH-dependent TEA binding similar to transport. Mutated hMATE1 with replacement E273Q completely lacked these TEA binding and transport. In the case of divalent substrates, transport was electrogenic. These observations indicate that the stoichiometry of OC/H⁺ exchange is independent of substrate charge. Purification and reconstitution of hMATE1 is considered to be suitable for understanding the detailed molecular mechanisms of hMATE1. The results suggest that Glu273 of hMATE1 plays essential roles in substrate binding and transport.

Efficient Mass Spectral Analysis of Active Transporters Overexpressed in Escherichia coli

Structural analysis of purified active membrane proteins can be performed by mass spectrometry (MS). However, no large-scale expression systems for active eukaryotic membrane proteins are available. Moreover, because membrane proteins cannot easily be digested by trypsin and ionized, they are difficult to analyze by MS. We developed a method for mass spectral analysis of eukaryotic membrane proteins combined with an overexpression system in Escherichia coli. Vesicular glutamate transporter 2 (VGLUT2/SLC17A6) with a soluble α-helical protein and histidine tag on the N- and C-terminus, respectively, was overexpressed in E. coli, solubilized with detergent, and purified by Ni-NTA affinity chromatography. Proteoliposomes containing VGLUT2 retained glutamate transport activity. For MS analysis, the detergent was removed from purified VGLUT2 by trichloroacetic acid precipitation, and VGLUT2 was then subjected to reductive alkylation and tryptic digestion. The resulting peptides were detected with 88% coverage by matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) MS with or without liquid chromatography. Vesicular excitatory amino acid transporter and vesicular acetylcholine transporter were also detected with similar coverage by the same method. Thus this methodology could be used to analyze purified eukaryotic active transporters. Structural analysis with chemical modifiers by MS could have applications in functional binding analysis for drug discovery.

Vesicular nucleotide transporter mediates ATP release and migration in neutrophils

Neutrophils migrate to sites infected by pathogenic microorganisms. This migration is regulated by neutrophil-secreted ATP, which stimulates neutrophils in an autocrine manner through purinergic receptors on the plasma membrane. Although previous studies have shown that ATP is released through channels at the plasma membrane of the neutrophil, it remains unknown whether it is also released through alternate secretory systems involving vesicular mechanisms. In this study, we investigated the possible involvement of vesicular nucleotide transporter (VNUT), a key molecule for vesicular storage and nucleotide release, in ATP secretion from neutrophils. RT-PCR and Western blotting analysis indicated that VNUT is expressed in mouse neutrophils. Immunohistochemical analysis indicated that VNUT mainly colocalized with matrix metalloproteinase-9 (MMP-9), a marker of tertiary granules, which are secretory organelles. In mouse neutrophils, ATP release was inhibited by clodronate, which is a potent VNUT inhibitor. Furthermore, neutrophils from VNUT^-/- mice did not release ATP and exhibited significantly reduced migration in vitro and in vivo These findings suggest that tertiary granule-localized VNUT is responsible for vesicular ATP release and subsequent neutrophil migration. Thus, these findings suggest an additional mechanism through which ATP is released by neutrophils.

Function and expression of a splicing variant of vesicular glutamate transporter 1

Vesicular glutamate transporter (VGLUT) is an active transporter responsible for vesicular storage of glutamate in synaptic vesicles and plays an essential role in glutamatergic neurotransmission. VGLUT consists of three isoforms, VGLUT1, VGLUT2, and VGLUT3. The VGLUT1 variant, VGLUT1v, with an additional 75-base pair sequence derived from a second intron between exons 2 and 3, which corresponds to 25 amino acid residues in the 1st loop of VGLUT1, is the only splicing variant among VGLUTs, although whether VGLUT1v protein is actually translated at the protein level remains unknown. In the present study, VGLUT1v was expressed in insect cells, solubilized, purified to near homogeneity, and its transport activity was examined. Proteoliposomes containing purified VGLUT1v were shown to accumulate glutamate upon imposition of an inside-positive membrane potential (Δψ). The Δψ-driven glutamate uptake activity requires Cl^- and its pharmacological profile and kinetics are comparable to those of other VGLUTs. The retinal membrane contained two VGLUT1 moieties with apparent molecular masses of 65 and 57kDa. VGLUT1v-specific antibodies against an inserted 25-amino acid residue sequence identified a 65-kDa immunoreactive polypeptide. Immunohistochemical analysis indicated that VGLUT1v immunoreactivity is present in photoreceptor cells and is associated with synaptic vesicles. VGLUT1v immunoreactivity is also present in pinealocytes, but not in other areas, including the brain. These results indicated that VGLUT1v exists in a functional state in rat photosensitive cells and is involved in glutamatergic chemical transmission.

Vesicular Polyamine Transporter Mediates Vesicular Storage and Release of Polyamine from Mast Cells

Mast cells are secretory cells that play an important role in host defense by discharging various intragranular contents, such as histamine and serotonin, upon stimulation of Fc receptors. The granules also contain spermine and spermidine, which can act as modulators of mast cell function, although the mechanism underlying vesicular storage remains unknown. Vesicular polyamine transporter (VPAT), the fourth member of the SLC18 transporter family, is an active transporter responsible for vesicular storage of spermine and spermidine in neurons. In the present study, we investigated whether VPAT functions in mast cells. RT-PCR and Western blotting indicated VPAT expression in murine bone marrow-derived mast cells (BMMCs). Immunohistochemical analysis indicated that VPAT is colocalized with VAMP3 but not with histamine, serotonin, cathepsin D, VAMP2, or VAMP7. Membrane vesicles from BMMCs accumulated spermidine upon the addition of ATP in a reserpine- and bafilomycin A₁-sensitive manner. BMMCs secreted spermine and spermidine upon the addition of either antigen or A23187 in the presence of Ca²⁺, and the antigen-mediated release, which was shown to be temperature-dependent and sensitive to bafilomycin A₁ and tetanus toxin, was significantly suppressed by VPAT gene RNA interference. Under these conditions, expression of vesicular monoamine transporter 2 was unaffected, but antigen-dependent histamine release was significantly suppressed, which was recovered by the addition of 1 mm spermine. These results strongly suggest that VPAT is expressed and is responsible for vesicular storage of spermine and spermidine in novel secretory granules that differ from histamine- and serotonin-containing granules and is involved in vesicular release of these polyamines from mast cells.

Structure, Function, and Drug Interactions of Neurotransmitter Transporters in the Postgenomic Era

Vesicular neurotransmitter transporters are responsible for the accumulation of neurotransmitters in secretory vesicles and play essential roles in chemical transmission. The SLC17 family contributes to sequestration of anionic neurotransmitters such as glutamate, aspartate, and nucleotides. Identification and subsequent cellular and molecular biological studies of SLC17 transporters unveiled the principles underlying the actions of these transporters. Recent progress in reconstitution methods in combination with postgenomic approaches has advanced studies on neurotransmitter transporters. This review summarizes the molecular properties of SLC17-type transporters and recent findings regarding the novel SLC18 transporter.

Wide expression of type I Na+-phosphate cotransporter 3 (NPT3/SLC17A2), a membrane potential-driven organic anion transporter

Membrane potential (Δψ)-driven and Cl(-)-dependent organic anion transport is a primary function of the solute carrier family 17 (SLC17) transporter family. Although the transport substrates and physiological relevance of the major members are well understood, SLC17A2 protein known to be Na(+)-phosphate cotransporter 3 (NPT3) is far less well characterized. In the present study, we investigated the transport properties and expression patterns of mouse SLC17A2 protein (mNPT3). Proteoliposomes containing the purified mNPT3 protein took up radiolabeled p-aminohippuric acid (PAH) in a Δψ- and Cl(-)-dependent manner. The mNPT3-mediated PAH uptake was inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDs) and Evans blue, common inhibitors of SLC17 family members. The PAH uptake was also inhibited by various anionic compounds, such as hydrophilic nonsteroidal anti-inflammatory drugs (NSAIDs) and urate. Consistent with these observations, the proteoliposome took up radiolabeled urate in a Δψ- and Cl(-)-dependent manner. Immunohistochemistry with specific antibodies against mNPT3 combined with RT-PCR revealed that mNPT3 is present in various tissues, including the hepatic bile duct, luminal membranes of the renal urinary tubules, maternal side of syncytiotrophoblast in the placenta, apical membrane of follicle cells in the thyroid, bronchiole epithelial cells in the lungs, and astrocytes around blood vessels in the cerebrum. These results suggested that mNPT3 is a polyspecific organic anion transporter that is involved in circulation of urate throughout the body.

AtPHT4;4 is a chloroplast-localized ascorbate transporter in Arabidopsis

Ascorbate is an antioxidant and coenzyme for various metabolic reactions in vivo. In plant chloroplasts, high ascorbate levels are required to overcome photoinhibition caused by strong light. However, ascorbate is synthesized in the mitochondria and the molecular mechanisms underlying ascorbate transport into chloroplasts are unknown. Here we show that AtPHT4;4, a member of the phosphate transporter 4 family of Arabidopsis thaliana, functions as an ascorbate transporter. In vitro analysis shows that proteoliposomes containing the purified AtPHT4;4 protein exhibit membrane potential- and Cl⁻-dependent ascorbate uptake. The AtPHT4;4 protein is abundantly expressed in the chloroplast envelope membrane. Knockout of AtPHT4;4 results in decreased levels of the reduced form of ascorbate in the leaves and the heat dissipation process of excessive energy during photosynthesis is compromised. Taken together, these observations indicate that the AtPHT4;4 protein is an ascorbate transporter at the chloroplast envelope membrane, which may be required for tolerance to strong light stress.

In plants, ascorbate is synthesized in the mitochondria yet plays essential roles as an antioxidant in the chloroplast. Here, Miyaji et al. show that AtPHT4;4 is a chloroplast envelope ascorbate transporter and suggest it is required for dissipation of excess energy under light stress.

Impairment of vesicular ATP release affects glucose metabolism and increases insulin sensitivity

Neuroendocrine cells store ATP in secretory granules and release it along with hormones that may trigger a variety of cellular responses in a process called purinergic chemical transmission. Although the vesicular nucleotide transporter (VNUT) has been shown to be involved in vesicular storage and release of ATP, its physiological relevance in vivo is far less well understood. In Vnut knockout (Vnut(-/-)) mice, we found that the loss of functional VNUT in adrenal chromaffin granules and insulin granules in the islets of Langerhans led to several significant effects. Vesicular ATP accumulation and depolarization-dependent ATP release were absent in the chromaffin granules of Vnut(-/-) mice. Glucose-responsive ATP release was also absent in pancreatic β-cells in Vnut(-/-) mice, while glucose-responsive insulin secretion was enhanced to a greater extent than that in wild-type tissue. Vnut(-/-) mice exhibited improved glucose tolerance and low blood glucose upon fasting due to increased insulin sensitivity. These results demonstrated an essential role of VNUT in vesicular storage and release of ATP in neuroendocrine cells in vivo and suggest that vesicular ATP and/or its degradation products act as feedback regulators in catecholamine and insulin secretion, thereby regulating blood glucose homeostasis.

Involvement of the leaf-specific multidrug and toxic compound extrusion (MATE) transporter Nt-JAT2 in vacuolar sequestration of nicotine in Nicotiana tabacum

Alkaloids play a key role in higher plant defense against pathogens and herbivores. Following its biosynthesis in root tissues, nicotine, the major alkaloid of Nicotiana species, is translocated via xylem transport toward the accumulation sites, leaf vacuoles. Our transcriptome analysis of methyl jasmonate-treated tobacco BY-2 cells identified several multidrug and toxic compound extrusion (MATE) transporter genes. In this study, we characterized a MATE gene, Nicotiana tabacum jasmonate-inducible alkaloid transporter 2 (Nt-JAT2), which encodes a protein that has 32% amino acid identity with Nt-JAT1. Nt-JAT2 mRNA is expressed at a very low steady state level in whole plants, but is rapidly upregulated by methyl jasmonate treatment in a leaf-specific manner. To characterize the function of Nt-JAT2, yeast cells were used as the host organism in a cellular transport assay. Nt-JAT2 was localized at the plasma membrane in yeast cells. When incubated in nicotine-containing medium, the nicotine content in Nt-JAT2-expressing cells was significantly lower than in control yeast. Nt-JAT2-expressing cells also showed lower content of other alkaloids like anabasine and anatabine, but not of flavonoids, suggesting that Nt-JAT2 transports various alkaloids including nicotine. Fluorescence assays in BY-2 cells showed that Nt-JAT2-GFP was localized to the tonoplast. These findings indicate that Nt-JAT2 is involved in nicotine sequestration in leaf vacuoles following the translocation of nicotine from root tissues.

Components of foods inhibit a drug exporter, human multidrug and toxin extrusion transporter 1

Human multidrug and toxic compounds extrusion transporter 1 (hMATE1/SLC47A1) is a H(+)-coupled organic cation exporter responsible for the final step of excretion of various xenobiotics at the kidney and liver. In this study, effects of dietary constituents on hMATE1 mediated drug transport were examined to evaluate possible food-drug interactions. Bergamottin inhibited hMATE1 mediated tetraethyl ammonium transport activity, with a Ki of 98.7 µM. Coumarins, flavonols, and catechin inhibited hMATE1 activity. Among 23 compounds tested, isorhamnetin was the strongest inhibitor of hMATE1 with the Ki of 0.32 µM in a competitive manner. Since isorhamnetin is abundant in Ginkgo biloba that is widely used for herbal supplements, the findings suggest the potential hMATE1 related food-drug interactions.

Inhibitors of ATP release inhibit vesicular nucleotide transporter

Vesicular nucleotide transporter (VNUT) is responsible for vesicular ATP storage in ATP-secreting cells. In the present study, we examined the effects on VNUT-mediated transport of ATP release inhibitors such as ATP-binding cassette (ABC) proteins, hemichannels, maxi anion channels and P2X7 receptor. The ATP transport activity of proteoliposomes containing purified human VNUT was blocked by glibenclamide, carbenoxolone, 18 α-glycyrrhetinic acid, flufenamic acid, arachidonic acid and A438079 without the formation of Δψ (positive inside) as a driving force being affected. Thus, inhibitors of ATP release may inhibit VNUT and subsequent ATP release, since the previous works proved that inhibitors of ATP release blocked VNUT-mediated ATP release at the cell level.

Essential role of vesicular nucleotide transporter in vesicular storage and release of nucleotides in platelets

Nucleotides are stored in the dense granules of platelets. The release of nucleotides triggers one of the first steps in a series of cascades responsible for blood coagulation. However, the mechanism of how the nucleotides are accumulated in the granules is still far less understood. The transporter protein responsible for storage of nucleotides in the neuroendocrine cells has been identified and characterized. We hypothesized that the vesicular nucleotide transporter (VNUT) is also involved in the vesicular storage of nucleotides in platelets. In this article, we present three lines of evidence that VNUT is responsible for the vesicular storage of nucleotides in platelets and that vesicular ATP transport is crucial for platelet function, detection and characterization of VNUT activity in platelets isolated from healthy humans and MEG‐01 cells, RNA interference experiments on MEG‐01 cells, and studies on nucleotide transport and release with a selective inhibitor.

VNUT is highly expressed and associated with dense granules in platelets. VNUT plays an essential role in vesicular storage of nucleotide in platelets.

Type 1 sodium-dependent phosphate transporter acts as a membrane potential-driven urate exporter

SLC17A1 protein (NPT1) was the first identified member of the SLC17 phosphate transporter family, and is known to mediate Na(+)/inorganic phosphate (Pi) co-transport when expressed in Xenopus oocytes. Although this protein was suggested to be a renal polyspecific anion exporter, its transport properties were not well characterized. The clean biochemical approach revealed that proteoliposomes comprising purified NPT1 as the only protein source transport various organic anions such as urate, p-aminohippuric acid (PAH), and acetylsalicylic acid (aspirin) in a membrane potential (Δψ)-driven and Cl(-) -dependent manner. Human NPT1 carrying an SNP mutation, Thr269Ile, known to increase the risk of gout, exhibited 32% lower urate transport activity compared to the wild type protein, leading to the conclusion that NPT1 is the long searched for transporter responsible for renal urate excretion. In the present article, we summarized the history of identification of the urate exporter and its possible involvement in the dynamism of urate under physiological and pathological conditions.

Vesicular neurotransmitter transporters: an approach for studying transporters with purified proteins

Vesicular storage and subsequent release of neurotransmitters are the key processes of chemical signal transmission. In this process, vesicular neurotransmitter transporters are responsible for loading the signaling molecules. The use of a "clean biochemical" approach with purified, recombinant transporters has helped in the identification of novel vesicular neurotransmitter transporters and in the analysis of the control of signal transmission.

Vesicular GABA transporter (VGAT) transports β-alanine

Vesicular GABA transporter (VGAT) is expressed in GABAergic and glycinergic neurons, and is responsible for vesicular storage and subsequent exocytosis of these inhibitory amino acids. In this study, we show that VGAT recognizes β-alanine as a substrate. Proteoliposomes containing purified VGAT transport β-alanine using Δψ but not ΔpH as a driving force. The Δψ-driven β-alanine uptake requires Cl(-). VGAT also facilitates Cl(-) uptake in the presence of β-alanine. A previously described VGAT mutant (Glu213Ala) that disrupts GABA and glycine transport similarly abrogates β-alanine uptake. These findings indicated that VGAT transports β-alanine through a mechanism similar to those for GABA and glycine, and functions as a vesicular β-alanine transporter. Vesicular GABA transporter (VGAT) is expressed in GABAergic and glycinergic neurons, and is responsible for vesicular storage and subsequent exocytosis of these inhibitory amino acids. In the present study, we showed that proteoliposomes containing purified VGAT transport β-alanine using Δψ as a driving force. VGAT also facilitates Cl(-) uptake. Our findings indicated that VGAT functions as a vesicular β-alanine transporter.

A Na+-phosphate cotransporter homologue (SLC17A4 protein) is an intestinal organic anion exporter

The SLC17 anion transporter family comprises nine members that transport various organic anions in membrane potential (Δψ)- and Cl−-dependent manners. Although the transport substrates and physiological relevance of the majority of the members have already been determined, little is known about SLC17A4 proteins known to be Na+-phosphate cotransporter homologue (NPT homologue). In the present study, we investigated the expression and transport properties of human SLC17A4 protein. Using specific antibodies, we found that a human NPT homologue is specifically expressed and present in the intestinal brush border membrane. Proteoliposomes containing the purified protein took up radiolabeled p-aminohippuric acid (PAH) in a Cl−-dependent manner at the expense of an electrochemical gradient of protons, especially Δψ, across the membrane. The Δψ- and Cl−-dependent PAH uptake was inhibited by diisothiocyanostilbene-2,2′-disulfonic acid and Evans blue, common inhibitors of SLC17 family members. cis-Inhibition studies revealed that various anionic compounds, such as hydrophilic nonsteroidal anti-inflammatory drugs, pravastatin, and urate inhibited the PAH uptake. Proteoliposomes took up radiolabeled urate, with the uptake having properties similar to those of PAH uptake. These results strongly suggested that the human NPT homologue acts as a polyspecific organic anion exporter in the intestines. Since SLC17A1 protein (NPT1) and SLC17A3 protein (NPT4) are responsible for renal urate extrusion, our results reveal the possible involvement of a NPT homologue in urate extrusion from the intestinal duct.

Divalent cation transport by vesicular nucleotide transporter

The vesicular nucleotide transporter (VNUT) is a secretory vesicle protein that is responsible for the vesicular storage and subsequent exocytosis of ATP (Sawada, K., Echigo, N., Juge, N., Miyaji, T., Otsuka, M., Omote, H., and Moriyama, Y. (2008) Proc. Natl. Acad. Sci. U.S.A. 105, 5683-5686). Because VNUT actively transports ATP in a membrane potential (Δψ)-dependent manner irrespective of divalent cations such as Mg(2+) and Ca(2+), VNUT recognizes free ATP as a transport substrate. However, whether or not VNUT transports chelating complexes with divalent cations remains unknown. Here, we show that proteoliposomes containing purified VNUT actively took up Mg(2+) when ATP was present, as detected by atomic absorption spectroscopy. The VNUT-containing proteoliposomes also took up radioactive Ca(2+) upon imposing Δψ (positive-inside) but not ΔpH. The Δψ-driven Ca(2+) uptake required ATP and a millimolar concentration of Cl(-), which was inhibited by Evans blue, a specific inhibitor of SLC17-type transporters. VNUT in which Arg-119 was specifically mutated to alanine, the counterpart of the essential amino acid residue of the SLC17 family, lost the ability to take up both ATP and Ca(2+). Ca(2+) uptake was also inhibited in the presence of various divalent cations such as Mg(2+). Kinetic analysis indicated that Ca(2+) or Mg(2+) did not affect the apparent affinity for ATP. RNAi of the VNUT gene in PC12 cells decreased the vesicular Mg(2+) concentration to 67.7%. These results indicate that VNUT transports both nucleotides and divalent cations probably as chelating complexes and suggest that VNUT functions as a divalent cation importer in secretory vesicles under physiological conditions.

Functional characterization of vesicular excitatory amino acid transport by human sialin

Sialin, the protein coded by SLC17A5, is responsible for membrane potential (Δψ)-driven aspartate and glutamate transport into synaptic vesicles in addition to H+/sialic acid co-transport in lysosomes. Rodent sialin mutants harboring the mutations associated with Salla disease in humans did not transport aspartate and glutamate whereas H+/sialic acid co-transport activity was about one-third of the wild-type protein. In this study, we investigate the effects of various mutations on the transport activities of human sialin. Proteoliposomes containing purified heterologously expressed human sialin exhibited both Δψ-driven aspartate and glutamate transport activity and H+/sialic acid co-transport activity. Aspartate and glutamate transport was not detected in the R39C and K136E mutant forms of SLC17A5 protein associated with Salla disease, whereas H+/sialic acid co-transport activity corresponded to 30-50% of the recombinant wild-type protein. In contrast, SLC17A5 protein harboring the mutations associated with infantile sialic acid storage disease, H183R and Δ268SSLRN272 still showed normal levels of Δψ-driven aspartate and glutamate transport even though H+/sialic acid co-transport activity was absent. Human sialin carrying the G328E mutation that causes both phenotypes, and P334R and G378V mutations that cause infantile sialic acid storage disease showed no transport activity. These results support the idea that people suffering from Salla disease have been defective in aspartergic and glutamatergic neurotransmissions.

A vesicular transporter that mediates aspartate and glutamate neurotransmission

Aspartate, an excitatory amino acid, is known to be stored in synaptic vesicles and exocytosed from some neurons to perform aspartergic neurotransmission. Through in vitro reconstitution, we found that sialin, a lysosomal sialic acid exporter, is responsible for the vesicular storage of aspartate in hippocampal neurons and pinealocytes. Mutations found in Salla disease cause decreased aspartate transport activity without affecting sialic acid transport. Thus, sialin is a multifunctional transporter. It is possible that people with Salla disease lose the ability of aspartergic neurotransmission, and this could explain why Salla disease involves severe neurological defects.

Characterization of the human MATE2 proton-coupled polyspecific organic cation exporter

Human multidrug and toxic compound extrusion 2 (hMATE2) is a kidney-specific isoform of hMATE1, an exporter of toxic organic cations (OCs) of exogenous and endogenous origins at the final excretion step in the kidneys and liver (Otsuka et al., 2005), and contains a splicing variant, MATE2K, that has an exon of hMATE2 deleted (Masuda et al., 2006). In the present study, we characterized the degree of expression and the transport properties of hMATE2. Quantitative PCR analysis with probes specific for hMATE2 indicated the presence of hMATE2 mRNA in the kidneys, which corresponded to 39% of total mRNA encoding both hMATE2 and hMATE2K. hMATE2-specific antibodies immunostained the renal urinary tubules. Upon expression in HEK293 cells, hMATE2 was localized in intracellular vesicular structures, and thus transport activity of tetraethylammonium (TEA), a typical substrate for MATE transporters, by the cells was not detected. The hMATE2 protein was purified and reconstituted into liposomes. An artificially imposed pH gradient (ΔpH) across the proteoliposomal membrane drove the uptake of TEA. Dissipation of ΔpH by ammonium sulfate effectively inhibited the TEA uptake, while that of the membrane potential by valinomycin had little effect. The profiles of cis-inhibition of TEA transport by hMATE2 and hMATE2K are similar to each other. Thus, both hMATE2 and hMATE2K equally operate in the human kidneys to extrude OCs into the urine.

Metabolic control of vesicular glutamate transport and release

Fasting has been used to control epilepsy since antiquity, but the mechanism of coupling between metabolic state and excitatory neurotransmission remains unknown. Previous work has shown that the vesicular glutamate transporters (VGLUTs) required for exocytotic release of glutamate undergo an unusual form of regulation by Cl(-). Using functional reconstitution of the purified VGLUTs into proteoliposomes, we now show that Cl(-) acts as an allosteric activator, and the ketone bodies that increase with fasting inhibit glutamate release by competing with Cl(-) at the site of allosteric regulation. Consistent with these observations, acetoacetate reduced quantal size at hippocampal synapses and suppresses glutamate release and seizures evoked with 4-aminopyridine in the brain. The results indicate an unsuspected link between metabolic state and excitatory neurotransmission through anion-dependent regulation of VGLUT activity.

Type 1 sodium-dependent phosphate transporter (SLC17A1 Protein) is a Cl(-)-dependent urate exporter

SLC17A1 protein (NPT1) is the first identified member of the SLC17 phosphate transporter family and mediates the transmembrane cotransport of Na(+)/P(i) in oocytes. Although this protein is believed to be a renal polyspecific anion exporter, its transport properties are not well characterized. Here, we show that proteoliposomes containing purified SLC17A1 transport various organic anions such as p-aminohippuric acid and acetylsalicylic acid (aspirin) in an inside positive membrane potential (Deltapsi)-dependent manner. We found that NPT1 also transported urate. The uptake characteristics were similar to that of SLC17 members in its Cl(-) dependence and inhibitor sensitivity. When arginine 138, an essential amino acid residue for members of the SLC17 family such as the vesicular glutamate transporter, was specifically mutated to alanine, the resulting mutant protein was inactive in Deltapsi-dependent anion transport. Heterologously expressed and purified human NPT1 carrying the single nucleotide polymorphism mutation that is associated with increased risk of gout in humans exhibited 32% lower urate transport activity compared with the wild type protein. These results strongly suggested that NPT1 is a Cl(-)-dependent polyspecific anion exporter involved in urate excretion under physiological conditions.

Tricks of the trade used to accelerate high-resolution structure determination of membrane proteins

The rate at which X-ray structures of membrane proteins are solved is on a par with that of soluble proteins in the late 1970s. There are still many obstacles facing the membrane protein structural community. Recently, there have been several technical achievements in the field that have started to dramatically accelerate structural studies. Here, we summarize these so-called 'tricks-of-the-trade' and include case studies of several mammalian transporters.

Vesicular inhibitory amino acid transporter is a Cl-/gamma-aminobutyrate Co-transporter

The vesicular inhibitory amino acid transporter (VIAAT) is a synaptic vesicle protein responsible for the vesicular storage of gamma-aminobutyrate (GABA) and glycine which plays an essential role in GABAergic and glycinergic neurotransmission. The transport mechanism of VIAAT remains largely unknown. Here, we show that proteoliposomes containing purified VIAAT actively took up GABA upon formation of membrane potential (Deltapsi) (positive inside) but not DeltapH. VIAAT-mediated GABA uptake had an absolute requirement for Cl(-) and actually accompanied Cl(-) movement. Kinetic analysis indicated that one GABA molecule and two Cl(-) equivalents were transported during one transport cycle. VIAAT in which Glu(213) was specifically mutated to alanine completely lost the ability to take up both GABA and Cl(-). Essentially the same results were obtained with glycine, another substrate of VIAAT. These results demonstrated that VIAAT is a vesicular Cl(-) transporter that co-transports Cl(-) with GABA or glycine in a Deltapsi dependent manner. It is concluded that Cl(-) plays an essential role in vesicular storage of GABA and glycine.

Role of glutamate residues in substrate recognition by human MATE1 polyspecific H+/organic cation exporter

Human multidrug and toxic compound extrusion 1 (hMATE1) is an electroneutral H(+)/organic cation exchanger responsible for the final excretion step of structurally unrelated toxic organic cations in kidney and liver. To elucidate the molecular basis of the substrate recognition by hMATE1, we substituted the glutamate residues Glu273, Glu278, Glu300, and Glu389, which are conserved in the transmembrane regions, for alanine or aspartate and examined the transport activities of the resulting mutant proteins using tetraethylammonium (TEA) and cimetidine as substrates after expression in human embryonic kidney 293 (HEK-293) cells. All of these mutants except Glu273Ala were fully expressed and present in the plasma membrane of the HEK-293 cells. TEA transport activity in the mutant Glu278Ala was completely absent. Both Glu300Ala and Glu389Ala and all aspartate mutants exhibited significantly decreased activity. Glu273Asp showed higher affinity for cimetidine, whereas it has reduced affinity to TEA. Glu278Asp showed decreased affinity to cimetidine. Both Glu300Asp and Glu389Asp had lowered affinity to TEA, whereas the affinity of Glu389Asp to cimetidine was fourfold higher than that of the wild-type transporter with about a fourfold decrease in V(max) value. Both Glu273Asp and Glu300Asp had altered pH dependence for TEA uptake. These results suggest that all of these glutamate residues are involved in binding and/or transport of TEA and cimetidine but that their individual roles are different.

Functional characterization of testis-specific rodent multidrug and toxic compound extrusion 2, a class III MATE-type polyspecific H+/organic cation exporter

Mammalian multidrug and toxic compound extrusion (MATE) proteins are classified into three subfamilies: classes I, II, and III. We previously showed that two of these families act as polyspecific H(+)-coupled transporters of organic cations (OCs) at final excretion steps in liver and kidney (Otsuka et al. Proc Natl Acad Sci USA 102: 17923-17928, 2005; Omote et al. Trends Pharmacol Sci 27: 587-593, 2006). Rodent MATE2 proteins are class III MATE transporters, the molecular nature, as well as transport properties, of which remain to be characterized. In the present study, we investigated the transport properties and localization of mouse MATE2 (mMATE2). On expression in human embryonic kidney (HEK)-293 cells, mMATE2 localized to the intracellular organelles and plasma membrane. mMATE2 mediated pH-dependent TEA transport with substrate specificity similar to, but distinct from, that of mMATE1, which prefers N-methylnicotinamide and guanidine as substrates. mMATE2 expressed in insect cells was solubilized and reconstituted with bacterial H(+)-ATPase into liposomes. The resultant proteoliposomes exhibited ATP-dependent uptake of TEA that was sensitive to carbonyl cyanide 3-chlorophenylhydrazone but unaffected by valinomycin in the presence of K(+). Immunologic techniques using specific antibodies revealed that mMATE2 was specifically expressed in testicular Leydig cells. Thus mMATE2 appears to act as a polyspecific H(+)/OC exporter in Leydig cells. It is concluded that all classes of mammalian MATE proteins act as polyspecific and electroneutral transporters of organic cations.

The MATE proteins as fundamental transporters of metabolic and xenobiotic organic cations

Multidrug and toxic compound extrusion (MATE) proteins, comprising the most recently designated family of multidrug transporter proteins, are widely distributed in all kingdoms of living organisms, although their function is far from understood. The bacterial MATE-type transporters that have been characterized function as exporters of cationic drugs, such as norfloxacin and ethidium, through H(+) or Na(+) exchange. Plant MATE-type transporters are involved in the detoxification of secondary metabolites, including alkaloids. Mammalian MATE-type transporters are responsible for the final step in the excretion of metabolic waste and xenobiotic organic cations in the kidney and liver through electroneutral exchange of H(+). Thus, we propose that members of the MATE family are organic cation exporters that excrete metabolic or xenobiotic organic cations from the body.

Vesicular glutamate transporter contains two independent transport machineries

Vesicular glutamate transporters (VGLUTs) are responsible for the vesicular storage of l-glutamate and play an essential role in glutamatergic signal transmission in the central nervous system. The molecular mechanism of the transport remains unknown. Here, we established a novel in vitro assay procedure, which includes purification of wild and mutant VGLUT2 and their reconstitution with purified bacterial F(o)F(1)-ATPase (F-ATPase) into liposomes. Upon the addition of ATP, the proteoliposomes facilitated l-glutamate uptake in a membrane potential (DeltaPsi)-dependent fashion. The ATP-dependent l-glutamate uptake exhibited an absolute requirement for approximately 4 mm Cl(-), was sensitive to Evans blue, but was insensitive to d,l-aspartate. VGLUT2s with mutations in the transmembrane-located residues Arg(184), His(128), and Glu(191) showed a dramatic loss in l-glutamate transport activity, whereas Na(+)-dependent inorganic phosphate (P(i)) uptake remained comparable to that of the wild type. Furthermore, P(i) transport did not require Cl(-) and was not inhibited by Evans blue. Thus, VGLUT2 appears to possess two intrinsic transport machineries that are independent of each other: a DeltaPsi-dependent l-glutamate uptake and a Na(+)-dependent P(i) uptake.

Secretion of L-glutamate from osteoclasts through transcytosis

Osteoclasts are involved in the catabolism of the bone matrix and eliminate the resulting degradation products through transcytosis, but the molecular mechanism and regulation of transcytosis remain poorly understood. Upon differentiation, osteoclasts express vesicular glutamate transporter 1 (VGLUT1), which is essential for vesicular storage and subsequent exocytosis of glutamate in neurons. VGLUT1 is localized in transcytotic vesicles and accumulates L-glutamate. Osteoclasts secrete L-glutamate and the bone degradation products upon stimulation with KCl or ATP in a Ca2+-dependent manner. KCl- and ATP-dependent secretion of L-glutamate was absent in osteoclasts prepared from VGLUT1-/- knockout mice. Osteoclasts express mGluR8, a class III metabotropic glutamate receptor. Its stimulation by a specific agonist inhibits secretion of L-glutamate and bone degradation products, whereas its suppression by a specific antagonist stimulates bone resorption. Finally, it was found that VGLUT1-/- mice develop osteoporosis. Thus, in bone-resorbing osteoclasts, L-glutamate and bone degradation products are secreted through transcytosis and the released L-glutamate is involved in autoregulation of transcytosis. Glutamate signaling may play an important role in the bone homeostasis.

Transmembrane topology of vesicular glutamate transporter 2

Vesicular glutamate transporter (VGLUT) plays an essential role in L-glutamate signaling in neurons and some peripheral tissues through vesicular storage of L-glutamate in secretory vesicles. To investigate the topology of VGLUT in membranes, we prepared site-directed antibodies against the amino-terminal (anti-N), 1st putative loop (anti-L), and carboxyl terminal (anti-C) regions. None of the antibodies reacted with VGLUT2 expressed in COS cells because they could not gain access to the antigen. However, both the anti-N and anti-C antibodies recognized VGLUT2 when the cells were permeabilized with digitonin, while the anti-L antibodies did not. Immunological reactivity to anti-L-antibodies appeared when the cells were permeabilized with Triton X-100. These results suggest that both the amino-terminal and carboxyl-terminal regions of VGLUT2 in membranes face the cytoplasm while the 1st loop faces the lumen.

Examination of drug resistance activity of human TAP-like (ABCB9) expressed in yeast

A half-type ABC transporter, human TAP-like (hTAPL) tagged with histidine cluster, was expressed in budding yeast protease-deficient strain BJ5457, and the effect of expression for resistance to peptide compounds including antibiotics and proteinase inhibitor was examined. Among these compounds, the yeast expressing hTAPL exhibits high sensitivity to valinomycin, a monovalent cation ionophore. A mutation in Walker A motif, which lost ATP-binding activity of hTAPL, eliminated the enhanced sensitivity to valinomycin. These findings suggest that the transport activity of hTAPL is important for conferring high valinomycin-sensitive phenotype to yeast.

Interaction of transported drugs with the lipid bilayer and P-glycoprotein through a solvation exchange mechanism

Broad substrate specificity of human P-glycoprotein (ABCB1) is an essential feature of multidrug resistance. Transport substrates of P-glycoprotein are mostly hydrophobic and many of them have net positive charge. These compounds partition into the membrane. Utilizing the energy of ATP hydrolysis, P-glycoprotein is thought to take up substrates from the cytoplasmic leaflet of the plasma membrane and to transport them to the outside of the cell. We examined this model by molecular dynamics simulation of the lipid bilayer, in the presence of transport substrates together with an atomic resolution structural model of P-glycoprotein. Taken together with previous electron paramagnetic resonance studies, the results suggest that most transported drugs are concentrated near the surface zone of the inner leaflet of the plasma membrane. Here the drugs can easily diffuse laterally into the drug-binding site of P-glycoprotein through an open cleft. It was concluded that the initial high-affinity drug-binding site was located in the interfacial surface area of P-glycoprotein in contact with the membrane interface. Based on these results and our recent kinetic studies, a "solvation exchange" drug transport mechanism of P-glycoprotein is discussed. A molecular basis for the improved colchicine transport efficiency by the much-studied colchicine-resistance G185V mutant human P-glycoprotein is also provided.

A human transporter protein that mediates the final excretion step for toxic organic cations

In mammals, toxic electrolytes of endogenous and exogenous origin are excreted through the urine and bile. Before excretion, these compounds cross numerous cellular membranes in a transporter-mediated manner. However, the protein transporters involved in the final excretion step are poorly understood. Here, we show that MATE1, a human and mouse orthologue of the multidrug and toxin extrusion family conferring multidrug resistance on bacteria, is primarily expressed in the kidney and liver, where it is localized to the luminal membranes of the urinary tubules and bile canaliculi. When expressed in HEK293 cells, MATE1 mediates H(+)-coupled electroneutral exchange of tetraethylammonium and 1-methyl-4-phenylpyridinium. Its substrate specificity is similar to those of renal and hepatic H(+)-coupled organic cations (OCs) export. Thus, MATE1 appears to be the long searched for polyspecific OC exporter that directly transports toxic OCs into urine and bile.

The remarkable transport mechanism of P-glycoprotein: a multidrug transporter

Human P-glycoprotein (ABCB1) is a primary multidrug transporter located in plasma membranes, that utilizes the energy of ATP hydrolysis to pump toxic xenobiotics out of cells. P-glycoprotein employs a most unusual molecular mechanism to perform this drug transport function. Here we review our work to elucidate the molecular mechanism of drug transport by P-glycoprotein. High level heterologous expression of human P-glycoprotein, in the yeast Saccharomyces cerevisiae, has facilitated biophysical studies in purified proteoliposome preparations. Development of novel spin-labeled transport substrates has allowed for quantitative and rigorous measurements of drug transport in real time by EPR spectroscopy. We have developed a new drug transport model of P-glycoprotein from the results of mutagenic, quantitative thermodynamic and kinetic studies. This model satisfactorily accounts for most of the unusual kinetic, coupling, and physiological features of P-glycoprotein. Additionally, an atomic detail structural model of P-glycoprotein has been devised to place our results within a proper structural context.