Re Diane B, Nafia Imane, Melon Christophe, Shimamoto Keiko, Kerkerian-Le Goff Lydia, Had-Aissouni Laurence
Interactions Cellulaires Neurodégénérescence et Neuroplasticité, IC2N, CNRS UMR 6186, Marseille, France.
Glia. 2006 Jul;54(1):47-57. doi: 10.1002/glia.20353.
Astrocytes have essential roles for neuron survival and function, so that their demise in neurodegenerative insults, such as ischemia, deserves attention. A major event of the cell death cascade in ischemia is the reversed operation of excitatory amino acid transporters (EAAT), releasing glutamate. Cytotoxicity is conventionally attributed to extracellular glutamate accumulation. We previously reported that mimicking such dysfunction by EAAT substrate inhibitors, whose uptake induces glutamate release by heteroexchange, triggers glutathione (GSH) depletion and oxidative death of differentiated astrocytes in culture. Here we demonstrate that astrocyte death, although correlated with glutamate release, is not resulting from high extracellular glutamate-mediated toxicity. L-glutamate per se was gliotoxic only at concentrations much higher than the maximum reached with the potent EAAT substrate inhibitor L-trans-pyrrolidine-2,4-dicarboxylate (PDC), and toxicity was lower. Moreover, high glutamate concentrations offered protection against PDC. Protection was also provided by L-aspartate, which is both transported by EAAT and metabolized into glutamate, and by inhibiting glutamine synthetase, which uses transported glutamate to synthesize glutamine. Neither D-aspartate, a metabolically inert EAAT substrate, nor compounds that can provide glutamate intracellularly but are not EAAT substrates offered protection. Interestingly, only the compounds providing protection prevented PDC-induced GSH depletion. These data strongly suggest that reversed uptake-mediated astrocyte death results from the leakage of glutamate from a compartmentalized intracellular metabolic pool specifically fuelled by EAAT, crucial for preserving GSH contents. In addition, we provide evidence for a minor contribution of the cystine-glutamate antiporter x(c) (-) but a major role of the 5-lipoxygenase pathway in this death mechanism.
星形胶质细胞对神经元的存活和功能起着至关重要的作用，因此它们在诸如缺血等神经退行性损伤中的死亡值得关注。缺血时细胞死亡级联反应的一个主要事件是兴奋性氨基酸转运体（EAAT）的反向运作，释放谷氨酸。细胞毒性传统上归因于细胞外谷氨酸的积累。我们之前报道过，通过EAAT底物抑制剂模拟这种功能障碍，其摄取通过异源交换诱导谷氨酸释放，会引发培养的分化星形胶质细胞的谷胱甘肽（GSH）耗竭和氧化死亡。在这里我们证明，星形胶质细胞死亡虽然与谷氨酸释放相关，但并非由高细胞外谷氨酸介导的毒性所致。L-谷氨酸本身仅在浓度远高于强效EAAT底物抑制剂L-反式-吡咯烷-2,4-二羧酸（PDC）所能达到的最高浓度时才具有神经毒性，且毒性较低。此外，高浓度的谷氨酸可提供对PDC的保护作用。L-天冬氨酸也能提供保护，它既能被EAAT转运又能代谢为谷氨酸，通过抑制谷氨酰胺合成酶（该酶利用转运来的谷氨酸合成谷氨酰胺）也能提供保护。代谢惰性的EAAT底物D-天冬氨酸以及能在细胞内提供谷氨酸但不是EAAT底物的化合物均不能提供保护。有趣的是，只有提供保护的化合物能防止PDC诱导的GSH耗竭。这些数据有力地表明，反向摄取介导的星形胶质细胞死亡是由谷氨酸从一个由EAAT专门提供燃料的区室化细胞内代谢池中泄漏所致，这对于维持GSH含量至关重要。此外，我们提供证据表明胱氨酸-谷氨酸反向转运体x(c)(-)的作用较小，但5-脂氧合酶途径在这种死亡机制中起主要作用。
