Characterization of the Tritium-Labeled Analog of L-<i>threo</i>-β-Benzyloxyaspartate Binding to Glutamate Transporters
作者:Keiko Shimamoto、Yasuto Otsubo、Yasushi Shigeri、Yoshimi Yasuda-Kamatani、Masamichi Satoh、Shuji Kaneko、Takayuki Nakagawa
DOI:10.1124/mol.106.027250
日期:2007.1
l-Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system. Termination of glutamate receptor activation and maintenance of low extracellular glutamate concentrations are primarily achieved by glutamate transporters (excitatory amino acid transporters 1-5, EAATs1-5) located on both the nerve endings and the surrounding glial cells. To identify the physiological roles of each subtype, subtype-selective EAAT ligands are required. In this study, we developed a binding assay system to characterize EAAT ligands for all EAAT subtypes. We recently synthesized novel analogs of threo -β-benzyloxyaspartate (TBOA) and reported that they blocked glutamate uptake by EAATs 1-5 much more potently than TBOA. The strong inhibitory activity of the TBOA analogs suggested that they would be suitable to use as radioisotope-labeled ligands, and we therefore synthesized a tritiated derivative of (2 S ,3 S )-3-3-[4-ethylbenzoylamino]benzyloxy}aspartate ([3H]ETB-TBOA). [3H]ETB-TBOA showed significant high-affinity specific binding to EAAT-transfected COS-1 cell membranes with each EAAT subtype. The Hill coefficient for the Na+-dependence of [3H]ETB-TBOA binding revealed a single class of noncooperative binding sites for Na+, suggesting that Na+ binding in the ligand binding step is different from Na+ binding in the substrate uptake process. The binding was displaced by known substrates and blockers. The rank order of inhibition by these compounds was consistent with glutamate uptake assay results reported previously. Thus, the [3H]ETB-TBOA binding assay will be useful to screen novel EAAT ligands for all EAAT subtypes.
谷氨酸是哺乳动物中枢神经系统中的主要兴奋性神经递质。谷氨酸受体活化的终止和细胞外谷氨酸浓度的维持主要由位于神经末梢和周围胶质细胞上的谷氨酸转运体(兴奋性氨基酸转运体1-5,EAATs1-5)实现。为了确定每个亚型的生理作用,需要亚型特异的EAAT配体。在这项研究中,我们开发了一种结合测定系统,用于表征所有EAAT亚型的EAAT配体。我们最近合成了新的苏式-β-苄氧基天冬氨酸(TBOA)类似物,并报告它们比TBOA更有效地阻断EAATs 1-5对谷氨酸的摄取。TBOA类似物的强抑制活性表明它们适合用作放射性同位素标记的配体,因此我们合成了(2 S ,3 S )-3-3-[4-乙基苯甲酰氨基]苄氧基}天冬氨酸([3H]ETB-TBOA)的氚化衍生物。[3H]ETB-TBOA对每个EAAT亚型在EAAT转染的COS-1细胞膜上表现出显著的高亲和力特异性结合。[3H]ETB-TBOA结合的Na+依赖性希尔系数揭示了Na+的非合作结合位点,表明配