DiFrancesco Jacopo C, Cooper J Mark, Lam Amanda, Hart Paul E, Tremolizzo Lucio, Ferrarese Carlo, Schapira Antony H
Department of Neuroscience and Biomedical Technologies, University of Milano-Bicocca, Monza, Italy.
Exp Neurol. 2008 Jul;212(1):152-6. doi: 10.1016/j.expneurol.2008.03.015. Epub 2008 Mar 26.
MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes) is commonly associated with the A3243G mitochondrial DNA (mtDNA) mutation encoding the transfer RNA of leucine (UUR) (tRNA (Leu(UUR))). The pathogenetic mechanisms of this mutation are not completely understood. Neuronal functions are particularly vulnerable to alterations in oxidative phosphorylation, which may affect the function of the neurotransmitter glutamate, leading to excitotoxicity. In order to investigate the possible effects of A3243G upon glutamate homeostasis, we assessed glutamate uptake in osteosarcoma-derived cytoplasmic hybrids (cybrids) expressing high levels of this mutation. High-affinity Na(+)-dependent glutamate uptake was assessed as radioactive [(3)H]-glutamate influx mediated by specific excitatory amino acid transporters (EAATs). The maximal rate (V(max)) of Na(+)-dependent glutamate uptake was significantly reduced in all the mutant clones. Although the defect did not relate to either the mutant load or magnitude of oxidative phosphorylation defect, we found an inverse relationship between A3243G mutation load and mitochondrial ATP synthesis, without any evidence of increased cellular or mitochondrial free radical production in these A3243G clones. These data suggest that a defect of glutamate transport in MELAS neurons may be due to decreased energy production and might be involved in mediating the pathogenic effects of the A3243G mtDNA mutation.
线粒体肌病、脑病、乳酸酸中毒和卒中样发作（MELAS）通常与编码亮氨酸（UUR）转运RNA（tRNA（Leu（UUR）））的A3243G线粒体DNA（mtDNA）突变相关。这种突变的致病机制尚未完全明确。神经元功能对氧化磷酸化的改变尤为敏感，这可能会影响神经递质谷氨酸的功能，导致兴奋性毒性。为了研究A3243G对谷氨酸稳态的可能影响，我们评估了表达该高水平突变的骨肉瘤来源的细胞质杂种细胞（cybrids）对谷氨酸的摄取。高亲和力的Na⁺依赖性谷氨酸摄取通过由特定兴奋性氨基酸转运体（EAATs）介导的放射性[³H] - 谷氨酸内流来评估。在所有突变克隆中，Na⁺依赖性谷氨酸摄取的最大速率（Vmax）均显著降低。尽管这种缺陷与突变负荷或氧化磷酸化缺陷的程度均无关，但我们发现A3243G突变负荷与线粒体ATP合成之间呈负相关，在这些A3243G克隆中没有任何细胞或线粒体自由基产生增加的证据。这些数据表明，MELAS神经元中谷氨酸转运缺陷可能是由于能量产生减少所致，并且可能参与介导A3243G mtDNA突变的致病作用。
