Molecular heterogeneity of the gamma-aminobutyric acid (GABA) transport system. Cloning of two novel high affinity GABA transporters from rat brain.

cDNA clones encoding two novel gamma-aminobutyric acid (GABA) transporters (designated GAT-2 and GAT-3) have been isolated from rat brain, and their functional properties have been examined in mammalian cells. The transporters display high affinity for GABA (Km approximately 10 microM) and exhibit pharmacological properties distinct from the previously cloned neuronal GABA transporter (GAT-1). Both transporters require sodium and chloride for transport activity. The nucleotide sequences of GAT-2 and GAT-3 predict proteins of 602 and 627 amino acids, respectively, which can be modeled with 12 transmembrane domains, similar to the topology proposed for other cloned neurotransmitter transporters. Localization studies indicate that both transporters are present in brain and retina, while GAT-2 is also present in peripheral tissues. The cloning of these transporter genes from rat brain reveals previously undescribed heterogeneity in GABA transporters.     

Molecular cloning and functional characterization of a GABA transporter from the CNS of the cabbage looper, Trichoplusia ni.

A cDNA encoding a GABA transporter in the caterpillar Trichoplusia ni has been cloned and expressed in baculovirus-infected insect cells. The cDNA contains an ORF encoding a 608-residue protein, designated TrnGAT. Hydropathy analysis of the deduced amino acid sequence suggests 12 transmembrane domains, a structure similar to that of all other cloned Na+/Cl(-)-dependent GABA transporters. The deduced amino acid sequence shows high identity with a GABA transporter (MasGAT) expressed in the embryo of Manduca sexta. Expression of TrnGAT mRNA was detected only in the brain. Sf21 cells infected with recombinant baculovirus exhibited a 20- to 30-fold increase in [3H]GABA uptake compared to control-infected cells. Several blockers of GABA uptake were used to determine the pharmacological profile of TrnGAT. Although most similar to mammalian neuronal GABA transporter GAT-1 in its kinetic properties, stoichiometry of ionic dependence and pharmacological properties, TrnGAT may be distinguished from mammalian GAT-1 by the inability of cyclic GABA analogues, such as nipecotic acid and its derivatives, to inhibit GABA uptake by the insect protein. The unique pharmacology of TrnGAT suggests that the GABA transport system in the lepidopteran CNS could be a useful target in the future development of rapidly-acting neuroactive agents used to control agriculturally-important insects.     

Expression of a cloned gamma-aminobutyric acid transporter in mammalian cells.

The cDNA clone GAT-1, which encodes a Na(+)- and Cl(-)-coupled GABA transporter from rat brain, has been expressed in mammalian cells using three different systems: (1) transient expression upon transfection of mouse Ltk- cells with a eukaryotic expression vector containing GAT-1; (2) stable expression in L-cells transfected with the same vector; (3) transfection of HeLa cells infected with a recombinant vaccinia virus expressing T7 RNA polymerase. Similar results both qualitatively and quantitatively were obtained with all systems. The GABA transporter expressed in HeLa and L-cells retains all the properties described previously for GABA transport into synaptosomes and synaptic plasma membrane vesicles. It was fully inhibited by cis-3-aminocyclohexanecarboxylic acid (ACHC) and not by beta-alanine. The KM for GABA transport and the IC50 for ACHC inhibition were similar to the presynaptic transporter. Accumulated [3H]GABA was released from transfected cells by dissipating the transmembrane Na+ gradient with nigericin or by exchange with unlabeled external GABA. Accumulation was stimulated by both Na+ and Cl- in the external medium. However, in the absence of external Cl-, a small amount of GABA transport remained which was dependent on GAT-1 transfection. Functional expression of the GABA transporter was abolished by tunicamycin. An antitransporter antibody specifically immunoprecipitates a polypeptide with an apparent molecular mass of about 70 kDa from GAT-1-transfected cells. When cells were grown in the presence of tunicamycin, only a faint band of apparent mass of about 60 kDa was observed.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

On the presence of 3H-GABA uptake mechanism in bovine spermatozoa

In the present study, existence of (3)H-GABA uptake mechanism in bovine spermatozoa and the modulation of (3)H-GABA transport by GABA itself were evaluated. The hypothesis was tyrosine phosphorylation affects transporter (GAT) function. (3)H-GABA uptake assays were performed on bovine spermatozoa and it resulted to be temperature- and time-dependent and K(m) was 1.48muM. Uptake was inhibited by the metabolic inhibitor ouabain and different blockers of GAT-1 (beta-alanine, l-DABA, nipecotic acid, tiagabine). Extracellular GABA up-regulated GABA transport, while the addition of SKF89976A, a high affinity inhibitor of the rat brain GABA transporter, reduced GABA uptake. Tyrosine phosphorylation affects transporter function since genistein, a broad-spectrum tyrosine kinase inhibitor, decreased (3)H-GABA uptake. Reduction in uptake did not occur in the presence of daidzein, an inactive genistein analogue. Furthermore, the genistein-mediated reduction in transport could be prevented by the tyrosine phosphatase inhibitor pervanadate. The action of these drugs on GABA transport is likely mediated through the GABA transporter GAT-1 since SKF89976A blocked a majority of GABA uptake. Wash-out experiments indicated that the genistein effect was reversible. When the experiments were conducted using "in vitro" capacitated spermatozoa there was no detectable uptake. Present results demonstrate that the carrier-mediated GABA uptake system in bovine spermatozoa modulates its function in response to extracellular GABA, that changes in lipid distribution and membrane composition which occur during capacitation eliminates GABA uptake and suggest the involvement of tyrosine phosphorylation in GABA transport.     

Isolation of mouse and human cDNA clones encoding a protein expressed specifically in osteoblasts and brain tissues.

Using the differential hybridization screening method between osteoblastic and fibroblastic cells, a cDNA clone coding for an osteoblast specific protein, named OSF-1, consisting of 168 amino acid residues including a possible 32 amino acid long leader sequence, was isolated from murine osteoblastic cell line MC3T3-E1. The OSF-1 gene was shown by Northern blotting analysis to be expressed in mouse calvarial osteoblast-enriched cells and in mouse brain tissues, but not in thymus, spleen, kidney, liver, lung, testis or heart. The human counterpart was also found in cDNA libraries from human osteosarcoma cell line MG63 and normal brain tissues. DNA sequence analysis revealed four amino acid sequence differences between the mouse and human, of which only one is located in the mature protein. This extremely high sequence conservation suggests that OSF-1 plays a fundamental role in bone and brain functions.     

Evolutionary History of the GABA Transporter (GAT) Group Revealed by Marine Invertebrate GAT-1

The GABA transporter (GAT) group is one of the major subgroups in the solute career 6 (SLC6) family of transmembrane proteins. The GAT group, which has been well studied in mammals, has 6 known members, i.e., a taurine transporter (TAUT), four GABA transporters (GAT-1, -2, -3, - 4), and a creatine transporter (CT1), which have important roles in maintaining physiological homeostasis. However, the GAT group has not been extensively investigated in invertebrates; only TAUT has been reported in marine invertebrates such as bivalves and krills, and GAT-1 has been reported in several insect species and nematodes. Thus, it is unknown how transporters in the GAT group arose during the course of animal evolution. In this study, we cloned GAT-1 cDNAs from the deep-sea mussel, Bathymodiolus septemdierum, and the Antarctic krill, Euphausia superba, whose TAUT cDNA has already been cloned. To understand the evolutionary history of the GAT group, we conducted phylogenetic and synteny analyses on the GAT group transporters of vertebrates and invertebrates. Our findings suggest that transporters of the GAT group evolved through the following processes. First, GAT-1 and CT1 arose by tandem duplication of an ancestral transporter gene before the divergence of Deuterostomia and Protostomia; next, the TAUT gene arose and GAT-3 was formed by the tandem duplication of the TAUT gene; and finally, GAT-2 and GAT-4 evolved from a GAT-3 gene by chromosomal duplication in the ancestral vertebrates. Based on synteny and phylogenetic evidence, the present naming of the GAT group members does not accurately reflect the evolutionary relationships.     

Cloning and analysis of cDNA clones for rat kidney alpha-spectrin

We have isolated a 3922-base pair (bp) cDNA clone for rat nonerythroid alpha-spectrin from a rat kidney lambda gt11 cDNA library. Sequence analysis revealed that this cDNA contains an open reading frame of 3090 bp encoding for the C-terminal 1030 amino acid sequence of rat kidney alpha-spectrin. The 3'-untranslated region (including a 38-bp poly(A+) tail) contains an 832-bp sequence. A single mRNA of about 8 kilobase pairs was detected in rat liver, kidney, brain, heart, intestine, lung, testis, stomach, spleen, and muscle with varying abundances, which is consistent with and further confirms the presence of spectrins in nonerythroid tissues as demonstrated previously by immunoblot analysis. Southern blot analysis suggested that there is a single gene for nonerythroid alpha-spectrin. The derived amino acid sequence contains sequence from the spectrin 106-residue internal repeat 12 to the C terminus of rat kidney alpha-spectrin. Sequence comparison with human and chicken nonerythroid alpha-spectrin showed that nonerythroid alpha-spectrin is well conserved during evolution. The rat kidney alpha-spectrin sequence, when compared to rat brain alpha-spectrin, contains an extra 76-amino-acid sequence at the C terminus. Sequence comparison of all the internal repeats available revealed that the internal repeat 3, 4, 5, 6, 7, and 8 has highest sequence similarity with internal repeat 12, 13, 14, 15, 16, and 17, respectively. Therefore, internal repeats 3-8 and 12-17 are most likely derived from an ancestral gene through gene duplication, suggesting that the spectrin gene is derived from a half-spectrin gene by gene duplication and divergence during evolution.     

Cloning and Expression of a Betaine/GABA Transporter from Human Brain

Abstract: A cDNA clone encoding a human γ-aminobutyric acid (GABA) transporter has been isolated from a brain cDNA library, and its functional properties have been examined in mammalian cells. The nucleotide sequence predicts a transporter with 614 amino acids and 12 putative transmembrane domains. The highest degree of amino acid identity is with a betaine/GABA transporter originally cloned from the dog termed BGT-1 (91%) and a related transporter from mouse brain (87%). These identities are similar to those for species homologues of other neurotransmitter transporters and suggest that the new clone represents the human homologue of BGT-1. The transporter displays high affinity for GABA (IC₅₀ of 30 µM) and is also sensitive to phloretin, l -2,4-diaminobutyric acid, and hypotaurine (IC₅₀ values of ～150–400 µM). The osmolyte betaine is ～25-fold weaker than GABA, displaying an IC₅₀ of ～1 mM. The relative potencies of these inhibitors at human BGT-1 differ from those of mouse and dog BGT-1. Northern blot analysis reveals that BGT-1 mRNA is widely distributed throughout the human brain. The cloning of the human homologue of BGT-1 will further our understanding of the roles of GABA and betaine in neural function.     

人肾高亲和力钠离子依赖性二羧酸转运蛋白基因的克隆和生物信息学分析

为探讨人高亲和力钠离子依赖性二羧酸转运蛋白 (humanhigh affinitysodium dependentdicar boxylatetransporter,hSDCT2orhNaDC3 )基因在人体内的生理功能及其与疾病的关系 ,借助生物信息学成功地从人肾中克隆了hSDCT2基因 (GenBank接收号 :AY0 72 810 ) .首先将大鼠SDCT2cDNA与人EST数据库进行同源性比较 ,获得具有高度同源性EST片段并用DNAstar软件将它们拼接成EST重叠群 .在重叠群上设计PCR引物从人肾总RNA中用RT PCR扩增出hSDCT2基因并测序 ,然后用软件对其结构特性、组织分布及基因定位进行分析 .序列测定结果显示 ,hSDCT2开放阅读框为180 9bp ,共编码 6 0 2个氨基酸 .蛋白同源性分析表明 ,其氨基酸序列与大鼠及小鼠SDCT2分别有85 %和 87%相同 .二级结构分析显示 ,该蛋白有 12个跨膜螺旋区 .Northern分析显示 ,该基因可在肾、肝、脑、胎盘等多种组织中表达 ,并定位于 2 0号染色体的q12～q13 1     

人SDCT2基因的两种不同转录产物选择性转录机理分析

为了克隆人高亲和力钠离子依赖性二羧酸转运蛋白 (highaffinitysodium dependentdicarboxylatetransporter,SDCT2 ,或NaDC3)基因并研究其生理功能 ,用大鼠SDCT2基因序列作为电子杂交探针对人EST数据库进行电子筛选 ,得到了一系列与大鼠SDCT2序列具有高度同源性的人EST序列 ,将它们拼接成 2个基因重叠群 ,设计特异性PCR引物通过RT PCR扩增得到 2条杂交探针用于筛选人肾cDNA文库 .从肾组织中同时克隆出了人SDCT2基因 2种mRNA变异体的全长cDNA(SDCT2α和SDCT2 β) ,两者 5′端前 3435bp序列完全一致 ,但 3′端长度不同 ,SDCT2 β在第 3435bp以后比SDCT2α多出了 5 85bp的序列 .Northern杂交和RT PCR显示 ,SDCT2α在人肾中的表达丰度最高 ,在肝、脾、胎盘、脑及结肠中也有低水平的表达 .而SDCT2 β主要在肾脏中表达 ,在脾也有低水平的表达 .基因组结构分析表明 ,虽然两种mRNAs均由 13个外显子组成 ,但是SDCT2α的第 13外显子含有 1个poly(A)加尾信号AATAAA ,而SDCT2 β的第 13外显子含有 2个poly(A)加尾信号 .这表明在肾脏和脾脏组织中 ,人SDCT2基因可能通过选择性使用位于第 13外显子不同位置的 2个poly(A)信号而转录出 2种不同长度的mRNA变异体 .     

High-affinity GABA uptake and GABA-metabolizing enzymes in pig forebrain white matter: a quantitative study

GABA receptor activation in central nervous white matter may be protective during white matter hypoxia in the adult, and it may modify axonal conduction, especially in the developing brain. GABA uptake is important for the shaping of the GABA signal, but quantitative data are lacking for GABA uptake and GABA-metabolizing enzymes in central nervous white matter. We report that high-affinity uptake of GABA in adult pig corpus callosum, fimbria, subcortical pyramidal tracts, and occipital white matter is approximately 20% of that in temporal cortex gray matter. Tiagabine (0.1 microM), an antiepileptic drug that specifically inhibits the GAT-1 GABA transporter inhibited GABA uptake 50% in temporal cortex and 60-68% in white structures. This finding indicates that GAT-1 is an important GABA transporter in white matter and suggests that white matter GABA uptake is inhibited during tiagabine therapy. GABA transaminase activity in white structures was approximately 20% of neocortical values. Glutamate decarboxylase (GAD) activity in white structures was only 4% of that in neocortex (7-12 pmol/mg tissue x min(-1) versus approximately 200 pmol/mg tissue x min(-1)). Since white matter activity of citrate synthase of the tricarboxylic acid cycle was approximately 25% of neocortical values ( approximately 0.4 nmol/mg tissue x min(-1) versus approximately 1.5 nmol/mg tissue x min(-1)), the low GAD activity suggests a slower metabolic turnover of GABA in white than in gray matter.     

Electrogenic uptake of gamma-aminobutyric acid by a cloned transporter expressed in Xenopus oocytes.

GAT-1, a gamma-aminobutyric acid (GABA) transporter cloned from rat brain, was expressed in Xenopus oocytes. Voltage-clamp measurements showed concentration-dependent, inward currents in response to GABA (K0.5 4.7 microM). The transport current required extracellular sodium and chloride ions; the Hill coefficient for chloride was 0.7, and that for sodium was 1.7. Correlation of current and [3H]GABA uptake measurements indicate that flux of one positive charge occurs per molecule of GABA transported. Membrane hyperpolarization from -40 to -100 mV increased the transport current approximately 3-fold. The results indicate that the transport of one molecule of GABA involves the co-transport of two sodium ions and one chloride ion.     

Differential expression of the GABA transporters GAT-1 and GAT-3 in brains of rats,cats, monkeys and humans

The homeostasis of GABA is critical to normal brain function. Extracellular levels of GABA are regulated mainly by plasmalemmal gamma-aminobutyric acid (GABA) transporters. Whereas the expression of GABA transporters has been extensively studied in rodents, validation of this data in other species, including humans, has been limited. As this information is crucial for our understanding of therapeutic options in human diseases such as epilepsy, we have compared, by immunocytochemistry, the distributions of the GABA transporters GAT-1 and GAT-3 in rats, cats, monkeys and humans. We demonstrate subtle differences between the results reported in the literature and our results, such as the predominance of GAT-1 labelling in neurons rather than astrocytes in the rat cortex. We note that the optimal localisation of GAT-1 in cats, monkeys and humans requires the use of an antibody against the human sequence carboxyl terminal region of GAT-1 rather than against the slightly different rat sequence. We demonstrate that GAT-3 is localised mainly to astrocytes in hindbrain and midbrain regions of rat brains. However, in species such as cats, monkeys and humans, additional strong immunolabelling of oligodendrocytes has also been observed. We suggest that differences in GAT distribution, especially the expression of GAT-3 by oligodendrocytes in humans, must be accommodated in extrapolating rodent models of GABA homeostasis to humans.Grant support was provided by the National Health and Medical Research Council (Australia) grant nos. 210127 and 102448, and a Senior Research Fellowship to David Pow.     

SNAP-25/syntaxin 1A complex functionally modulates neurotransmitter gamma-aminobutyric acid reuptake

Neurotransmitter gamma-aminobutyric acid (GABA) release to the synaptic clefts is mediated by the formation of a soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex, which includes two target SNAREs syntaxin 1A and SNAP-25 and one vesicle SNARE VAMP-2. The target SNAREs syntaxin 1A and SNAP-25 form a heterodimer, the putative intermediate of the SNARE complex. Neurotransmitter GABA clearance from synaptic clefts is carried out by the reuptake function of its transporters to terminate the postsynaptic signaling. Syntaxin 1A directly binds to the neuronal GABA transporter GAT-1 and inhibits its reuptake function. However, whether other SNARE proteins or SNARE complex regulates GABA reuptake remains unknown. Here we demonstrate that SNAP-25 efficiently inhibits GAT-1 reuptake function in the presence of syntaxin 1A. This inhibition depends on SNAP-25/syntaxin 1A complex formation. The H3 domain of syntaxin 1A is identified as the binding sites for both SNAP-25 and GAT-1. SNAP-25 binding to syntaxin 1A greatly potentiates the physical interaction of syntaxin 1A with GAT-1 and significantly enhances the syntaxin 1A-mediated inhibition of GAT-1 reuptake function. Furthermore, nitric oxide, which promotes SNAP-25 binding to syntaxin 1A to form the SNARE complex, also potentiates the interaction of syntaxin 1A with GAT-1 and suppresses GABA reuptake by GAT-1. Thus our findings delineate a further molecular mechanism for the regulation of GABA reuptake by a target SNARE complex and suggest a direct coordination between GABA release and reuptake.