Molecular cloning and structure of the human (GABATHG) GABA transporter gene

Age-related gene expression change of GABAergic system in visual cortex of rhesus macaque

Degradation of visual function is a common phenomenon during aging and likely mediated by change in the impaired central visual pathway. Treatment with GABA or its agonist could recover the ability of visual neurons in the primary visual cortex of senescent macaques. However, little is known about how GABAergic system change is related to the aged degradation of visual function in nonhuman primate. With the use of quantitative PCR method, we measured the expression change of 24 GABA related genes in the primary visual cortex (Brodmann's 17) of different age groups. In this study, both of mRNA and protein of glutamic acid decarboxylase (GAD65) were measured by real-time RT-PCR and Western blot, respectively. Results revealed that the level of GAD65 message was not significantly altered, but the proteins were significantly decreased in the aged monkey. As GAD65 plays an important role in GABA synthesis, the down-regulation of GAD65 protein was likely the key factor leading to the observed GABA reduction in the primary visual cortex of the aged macaques. In addition, 7 of 14 GABA receptor genes were up-regulated and one GABA receptor gene was significantly reduced during aging process even after Banjamini correction for multiple comparisons (P<0.05). These results suggested that the dysregulation of GAD65 protein might contribute to some age-related neural visual dysfunctions and most of GABA receptor genes induce a clear indication of compensatory effect for the reduced GABA release in the healthy aged monkey cortex.

A quantitative analysis of cellular and synaptic localization of GAT-1 and GAT-3 in rat neocortex

High-affinity plasma membrane GABA transporters GAT-1 and GAT-3 contribute to the modulation of GABA-mediated inhibition in adult mammalian cerebral cortex. How GATs regulate inhibition in neocortical circuits remains however poorly understood for the lack of information on key localizational features. In this study, we used quantitative pre- and post-embedding electron microscopy to define the distribution of GAT-1 and GAT-3 in elements contributing to synapses and to unveil their ultrastructural organization at adult cortical GABAergic synapses. GAT-1 and GAT-3 were found in both neuronal and astrocytic processes: GAT-1 was prevalently segregated in neuronal elements and in profiles contributing to synapses, whereas GAT-3 was mostly expressed in astrocytes and did not exhibit a preferential distribution in elements contributing to synapses. Analysis of the ultrastructural distribution of GAT-1 and GAT-3 in the plasma membrane of axon terminals and perisynaptic astrocytic processes of symmetric synapses in relation to the active zone revealed that GAT-1 was more concentrated in restricted perisynaptic and extrasynaptic regions, whereas GAT-3 was prominent in extrasynaptic areas. These studies provide a basis for understanding the role GAT-1 and GAT-3 play in the modulation of GABA-mediated phasic and tonic inhibition in cerebral cortex.

GABA transporters in the mammalian cerebral cortex: localization, development and pathological implications

The extracellular levels of gamma-aminobutyric acid (GABA), the main inhibitory neurotransmitter in the mammalian cerebral cortex, are regulated by specific high-affinity, Na+/Cl- dependent transporters. Four distinct genes encoding GABA transporters (GATs), named GAT-1, GAT-2, GAT-3, and BGT-1 have been identified using molecular cloning. Of these, GAT-1 and -3 are expressed in the cerebral cortex. Studies of the cortical distribution, cellular localization, ontogeny and relationships of GATs with GABA-releasing elements using a variety of light and electron microscopic immunocytochemical techniques have shown that: (i) a fraction of GATs is strategically placed to mediate GABA uptake at fast inhibitory synapses, terminating GABA's action and shaping inhibitory postsynaptic responses; (ii) another fraction may participate in functions such as the regulation of GABA's diffusion to neighboring synapses and of GABA levels in cerebrospinal fluid; (iii) GATs may play a role in the complex processes regulating cortical maturation; and (iv) GATs may contribute to the dysregulation of neuronal excitability that accompanies at least two major human diseases: epilepsy and ischemia.

Postnatal development of high-affinity plasma membrane GABA transporters GAT-2 and GAT-3 in the rat cerebral cortex

We investigated the developmental profile of plasma membrane gamma-aminobutyric acid (GABA) transporters (GATs) GAT-2 and GAT-3 expression by immunocytochemistry with affinity-purified polyclonal antibodies in the rat neocortex. At all developmental ages investigated, GAT-2 ir was prominent in the arachnoid and in the trabeculae of the subarachnoid space, whereas it was weak within the cortical parenchyma; the adult pattern was reached during the third week of postnatal life. GAT-3 ir was present at birth and increased rapidly in the first week, when numerous positive cells were present throughout the cortical layers; at P10, GAT-3-positive cells became less numerous and GAT-3 ir switched to the adult pattern, which was expressed at P20. Confocal and electron microscopic investigations showed that GAT-3 positive cells were both neurons and astrocytes. The present evidence indicates that early in development GAT-3 is abundantly expressed in the cerebral cortex, where its expression appears to correlate with developmental variations in GABA levels, and suggests that it accounts for the largest fraction of GABA transport observed in the neonatal cerebral cortex.

Molecular cloning and functional expression of a proline transporter from Manduca sexta

We report the molecular cloning of a L-proline transporter, MasPROT cDNA and its splice variants MasPROT.16 and MasPROT.2 from the central nervous system of Manduca sexta. Sequence analysis revealed that MasPROT belongs to a family of high affinity Na+/Cl- dependent neurotransmitter transporters. The deduced amino acid (aa) sequence of 556 aa having an estimated molecular mass of 58.9 kDa is predicted to have 12 putative transmembrane domains (TMD) and a characteristic large extracellular loop between TMD3 and TMD4. Sequence comparison to other members of the family indicates that it falls into the glycine-proline transporter subfamily. Transiently expressed MasPROT cDNA in Xenopus oocytes exclusively transported proline. Northern analysis shows that it is expressed predominantly in central nervous system, however, low levels are present in midgut, hindgut and Malpighian tubules. Two mRNA transcripts of sizes 3.6 and 8 Kb were found in all tissues except hindgut, where only a smaller transcript exists. RT-PCR and Southern blot analysis revealed the presence of MasPROT transcripts in flight muscles but not in leg muscles. Our preliminary data suggests that this transporter is an insect homologue of mammalian proline transporters. MasPROT.16 is a short splice variant encoding for 174 amino acids and shares 138 amino acids from the N terminus of MasPROT. MasPROT.2 is a long splice variant that contains six introns that coincide precisely with the previously mapped exon/intron boundaries of the members of this superfamily.

Orphanin FQ: an endogenous antagonist of rat brain dopamine transporter

Orphanin FQ, also known as nociceptin (NC),is a well-known ligand for opioid receptor-like ORLI receptor. This heptadecapeptide was identified as potently inhibiting the uptake of rat dopamine transporter (rDAT) which is stably expressed in CHO cells (designated D8 cells). Further kinetic analysis proved that this occurs through competitive inhibition with an IC50 of about 1.9 microM. Orphanin FQ also inhibits [3H]dopamine uptake by rat striatal synaptosomes, which confirmed the effect of orphanin FQ on D8 cells. Orphanin FQ was also found to inhibit GABA transporter type I (GATI) but not the serotonin transporter. These results suggest that orphanin FQ is an endogenous antagonist of dopamine transport and that it affects locomotion and other activities at least partly by inhibiting dopamine transporter and directly affecting dopamine transmission or by inhibiting GABA transporter to indirectly change dopaimne transmission.

Role of Cl- in electrogenic Na+-coupled cotransporters GAT1 and SGLT1

We have investigated the functional role of Cl(-) in the human Na(+)/Cl(-)/gamma-aminobutyric acid (GABA) and Na(+)/glucose cotransporters (GAT1 and SGLT1, respectively) expressed in Xenopus laevis oocytes. Substrate-evoked steady-state inward currents were examined in the presence and absence of external Cl(-). Replacement of Cl(-) by gluconate or 2-(N-morpholino)ethanesulfonic acid decreased the apparent affinity of GAT1 and SGLT1 for Na(+) and the organic substrate. In the absence of substrate, GAT1 and SGLT1 exhibited charge movements that manifested as pre-steady-state current transients. Removal of Cl(-) shifted the voltage dependence of charge movements to more negative potentials, with apparent affinity constants (K(0.5)) for Cl(-) of 21 and 115 mm for SGLT1 and GAT1, respectively. The maximum charge moved and the apparent valence were not altered. GAT1 stoichiometry was determined by measuring GABA-evoked currents and the unidirectional influx of (36)Cl(-), (22)Na(+), or [(3)H]GABA. Uptake of each GABA molecule was accompanied by inward movement of 2 positive charges, which was entirely accounted for by the influx of Na(+) in the presence or absence of Cl(-). Thus, the GAT1 stoichiometry was 2Na(+):1GABA. However, Cl(-) was transported by GAT1 because the inward movement of 2 positive charges was accompanied by the influx of one Cl(-) ion, suggesting unidirectional influx of 2Na(+):1Cl(-):1GABA per transport cycle. Activation of forward Na(+)/Cl(-)/GABA transport evoked (36)Cl(-) efflux and was blocked by the inhibitor SKF 89976A. These data suggest a Cl(-)/Cl(-) exchange mechanism during the GAT1 transport cycle. In contrast, Cl(-) was not transported by SGLT1. Thus, in both GAT1 and SGLT1, Cl(-) modulates the kinetics of cotransport by altering Na(+) affinity, but does not contribute to net charge transported per transport cycle. We conclude that Cl(-) dependence per se is not a useful criterion to classify Na(+) cotransporters.

A cognate dopamine transporter-like activity endogenously expressed in a COS-7 kidney-derived cell line

The activity of the dopamine transporter is an important mechanism for the maintenance of normal dopaminergic homeostasis by rapidly removing dopamine from the synaptic cleft. In kidney-derived COS-7, COS-1 and HEK-293 but not in other mammalian cell lines (CHO, Y1, Ltk-), we have characterized a putative functional dopamine transporter displaying a high affinity (Km approximately 250 nM) and a low capacity (approximately 0.1 pmol/10(5) cells/min) for [3H]dopamine uptake. Uptake displayed a pharmacological profile clearly indicative of the neuronal dopamine transporter. Estimated Ki values of numerous substrates and inhibitors for the COS-dopamine transporter and the cloned human neuronal transporter (human dopamine transporter) correlate well with the exception of a few notable compounds, including the endogenous neurotransmitter dopamine, the dopamine transporter inhibitor GBR 12,909 and the dopaminergic agonist apomorphine. As with native neuronal and cloned dopamine transporters, the uptake velocity was sodium-sensitive and reduced by phorbol ester pre-treatment. Two mRNA species of 3.8 and 4.0 kb in COS-7 cells were revealed by Northern blot analysis similar in size to that seen in native neuronal tissue. A reverse-transcribed PCR analysis confirmed the existence of a processed dopamine transporter. However, no immunoreactive proteins of expected dopamine transporter molecular size or [3H]WIN 35,428 binding activity were detected. A partial cDNA of 1.3 kb, isolated from a COS-1 cDNA library and encoding transmembrane domains 1-6, displayed a deduced amino acid sequence homology of approximately 96% to the human dopamine transporter. Taken together, the data suggest the existence of a non-neuronal endogenous high affinity dopamine uptake system sharing strong functional and molecular homology to that of the cloned neuronal dopamine transporter.

The multiply-regulated gabA gene encoding the GABA permease of Aspergillus nidulans: a score of exons

We describe the cloning, sequence and expression of gabA, encoding the gamma-amino-n-butyrate (GABA) permease of the fungus Aspergillus nidulans. Sequence changes were determined for three up-promoter (gabI ) and six gabA loss-of-function mutations. The predicted protein contains 517 residues and shows 30.3% overall identity with a putative GABA permease of Arabidopsis thaliana, 29.6% identity with the yeast choline transporter and 23.4% identity with the yeast UGA4 GABA permease. Structural predictions favour 11-12 transmembrane domains. Comparison of the genomic and cDNA sequences shows the presence of 19 introns, an unusually large number of introns for, we believe, any fungal gene. In agreement with the wealth of genetic data available, transcript level analyses demonstrate that gabA is subject to carbon catabolite and nitrogen metabolite repression, omega-amino acid induction and regulation in response to ambient pH (being acid-expressed). In agreement with this, we report consensus binding sites 5' to the coding region, six each for CreA and AREA and one for PacC, the transcription factors mediating carbon catabolite and nitrogen metabolite repression and response to ambient pH respectively.

Val 70, Phe 72 and the last seven amino acid residues of C-terminal are essential to the function of norepinephrine transporter

The norepinephrine transporter(NET) is a member of the Na+/Cl- dependent neurotransmitter transporter family and constitutes the target of several clinically important antidepressants. To delineate the critical amino acid residues and the function of C-terminal in regulating transport activity of NET, here we constructed two site mutants (V70F, F72V; V70I, F72V) and one C-terminal truncated mutant (delta 611-617). The wild type and mutants of NET were expressed in Xenopus oocytes by injection of their cRNA. We found that all of these mutants lost their transport activity. These results indicate that the amino acid residues of V70 and F72, and the last seven amino acids of C-terminal are essential to the transport activity of NET.

Possible existence of quaternary structure in the high-affinity serotonin transport complex

Deletion-mutants of the cloned mouse serotonin transporter (SERT) rendered dominant negative-mutant effects upon wild-type transporter activities in heterologous expression studies; such effects were transporter-selective and did not influence the activities of co-expressed neuronal GABA transporter. Heterologous expression of linear concatenates (up to four copies) of SERT further revealed discernable uptake activities for both transporter-dimer and -tetramer, but not for the trimer. Kinetic and pharmacological analyses revealed that the monomer, dimer, and tetramer manifested comparable transport Km and potencies for known serotonin uptake inhibitors; the tetramer was distinct from the others only in manifesting notably reduced transport Vmax. Surprisingly, equivalent cocaine congener-binding activities were observed for all concatenates, including the functionally inactive trimer. These findings collectively support the existence of quaternary structure in the active 5-HT transport complex; such structure is likely to be a critical determinant of ligand transport activities, but apparently not of transporter-inhibitor interactions.

Functional sensitivity of polar surfaces on transmembrane helix 8 and cytoplasmic loop 8-9 of the Escherichia coli GABA (4-aminobutyrate) transporter encoded by gabP: mutagenic analysis of a consensus amphipathic region found in transporters from bacteria to mammals

The gab permease (GabP) catalyses transport of GABA (4-aminobutyrate) into Escherichia coli. Although GabP can recognize and transport many GABA analogues that exhibit activity at GABAergic synapses in the nervous system, the protein domains responsible for these transport and ligand recognition properties have not been studied. Here we report that an amphipathic domain extending through putative transmembrane helix 8 and into the adjoining cytoplasmic region (loop 8-9) contains a critical 20 residue zone within which mutagenesis of polar amino acids has a deleterious effect on [3H]GABA transport activity. This functionally important amphipathic domain is found to be highly conserved in the many APC family transporters that are homologous to GabP. And even though members of the GAT family of GABA transporters from the animal nervous system are not homologous to GabP, an analogous amphipathic structure is found in their loop 8-9 region. These results and observations suggest: (1) that the consensus amphipathic region (CAR) in the putative helix 8 and loop 8-9 region of GabP has functional significance, and (2) that nature has repeatedly used this CAR in transporters from bacteria to mammals.

Murine vesicular monoamine transporter 2: molecular cloning and genomic structure

The principal brain vesicular monoamine transporter (VMAT2) pumps monoamines including dopamine, norepinephrine, serotonin and histamine from neuronal cytoplasm into synaptic vesicles and is implicated in actions of certain psychostimulants and selective neurotoxins. To improve understanding of this gene and its regulation, and to facilitate study of the roles played by this important molecule in mouse genetic models, we have cloned murine VMAT2 cDNA and genomic sequences. A 4.2-kb mouse VMAT2 cDNA hybridized to a 4.3-kb mRNA expressed chiefly in brainstem. Murine cDNA and genomic DNA analyses reveal an open reading frame of 1551 bp encoding 517 amino acids that display 92, 96 and 60% amino-acid identity with human and rat VMAT2, and rat vesicular acetylcholine transporter sequences, respectively. This open reading frame is distributed over 15 of 16 identified exons, and spans > 35 kb of genomic DNA. A major transcriptional initiation site is identified 107 bp 5' to the translational initiation ATG codon using primer extension/5' rapid amplification of cDNA ends. Sequences immediately 5' of this putative transcription start site lack 'TATA' or 'CATT' boxes, but contain consensus sequences that may bind cAMP response element, Sp1, AP2 and other transcription factors. Identification of these genomic sequences facilitates construction of homologous recombinant mice, provides a template for gene structures in the vesicular transporter family, and identifies sequences elements that could contribute to the specific patterns of regulated VMAT2 expression in monoaminergic neurons.

Structure and organization of the gene encoding human dopamine transporter

The human dopamine transporter (hDAT) is a member of the subfamily of monoamine transporters which show a common topological structure and possess significant amino acid sequence homology. We isolated and characterized the hDAT gene including about 1 kb of 5'-flanking region. The hDAT gene spans over 64 kb, consisting of 15 exons separated by 14 introns. The intron-exon structure of the hDAT gene is most similar to that of the human noradrenaline transporter (hNAT) gene. Promoter sequence analysis demonstrated a 'TATA'-less, 'CAT'-less and G+C-rich structure. Two E box and several Sp-1-binding sites exist in the promoter region. These structural features are similar to that of the human D1A dopamine receptor gene and the human monoamine oxidase A gene. The transcription start site was determined by both 5'-RACE and RNase protection assay. We determined the 5' end of the mRNA by identifying the 5'-terminal cap-G residue in 5'-RACE and RNase protection assay.

Characterization of 5'-proximal sequence of mouse GABA transporter gene (GAT-1)

The cDNA molecule encoding the mouse GABA transporter gene (GAT-1) was used as probe for selecting GAT-1 gene from mouse genomic library. A positive clone, harboring the whole open reading frame of the GAT-1 protein and designated as MGABAT-G, was fished out from the library, the 5' proximal region and intron 1 were sequenced and analysed, and low homology was found in the above region between GAT-1 genes from mouse and human except some short conserved sequences. The DNA-protein interactions between DNA fragments containing the conserved sequences in the 5' proximal region and nuclear proteins from different tissues of mouse were studied by means of gel-shift assay, and Southern-Western blot. The results indicate a possible positive-negative regulation mode controlling the expression of the mouse GAT-1 gene.

Human and mouse dopamine transporter genes: conservation of 5'-flanking sequence elements and gene structures

Synaptic reaccumulation of the neurotransmitter dopamine is mediated by the dopamine transporter (DAT), a member of the family of twelve transmembrane domain, sodium- and chloride-dependent neurotransmitter transporters. Several DAT features, including its exclusive expression in dopaminergic neurons, implication in cocaine action, and prominent role in the mechanisms of Parkinsonism-inducing neurotoxins, make understanding of the DAT gene of interest. Isolation and characterization of the human and mouse DAT genes has allowed elucidation of similarities between each and other members of this transporter gene family. Sequences 5' to transcriptional start sites contain G-C rich, TATA-less, CAAT-less regions with striking conservation between human and mouse gene flanking regions. These studies suggest sequence elements that are candidates to contribute to the dopamine transporter's dopaminergic cell-specific expression.