A direct correlation between ischemic injury and extracellular glycine concentration in mice with genetically altered activities of the glycine cleavage multi-enzyme system

[背景与目的]缺血可引起多种氨基酸类神经递质的快速释放。谷氨酸峰激活N-甲基-D-天冬氨酸（NMDA）谷氨酸受体，触发神经元的有害过程。虽然甘氨酸是NMDA受体的促凝剂，但细胞外甘氨酸浓度对缺血性损伤的影响仍存在争议。为了解决这个问题，我们研究了小鼠的缺血性损伤与遗传改变活动的甘氨酸裂解多酶系统（GCS），这在维持细胞外甘氨酸浓度发挥了重要作用。【材料与方法】通过转基因表达GCS的关键成分甘氨酸脱羧酶（glycine decarboxylase，glycine decarboxyl通过转基因表达甘氨酸脱羧酶的显性失活突变体（低GCS小鼠），建立了另一种GCS活性降低的小鼠品系（对照组的29%）。我们研究了这些小鼠大脑中动脉短暂闭塞后神经元的损伤，用微透析法测定细胞外氨基酸浓度。[结果]高GCS组和低GCS组小鼠细胞外甘氨酸基础浓度分别显著低于对照组和高于对照组。在低GCS小鼠中，细胞外甘氨酸在缺血期间达到2-hold对照水平，并且梗死体积显著增加至对照的69%。相比之下，高GCS小鼠的梗死体积显著减小了21%。在整个实验中，细胞外谷氨酸浓度没有观察到显着差异。NMDA甘氨酸位点的拮抗剂SM-31900可缩小梗死面积，表明甘氨酸通过NMDA受体发挥作用。[结论] GCS所维持的细胞外甘氨酸浓度与脑缺血损伤有直接关系。

[Background and Purpose] Ischemia elicits rapid release of various amino acid neurotransmitters. Glutamate surge activates N-methyl-D-aspartate (NMDA) glutamate receptors, triggering deleterious process of neurons. Although glycine is a coagonist of the NMDA receptor, the effect of extracellular glycine concentration on ischemic injury remains controversial. To approach this issue we examined ischemic injury in mice with genetically altered activities of the glycine cleavage multi-enzyme system (GCS), which plays a fundamental role in maintaining extracellular glycine concentration. [Materials and Methods] A mouse line with increased GCS activity (340% of C57BL/6 control mice) was generated by transgenic expression of glycine decarboxylase, a key GCS component (high-GCS mice). Another mouse line with reduced GCS activity (29% of controls) was established by transgenic expression of a dominant negative mutant of glycine decarboxylase (low-GCS mice). We examined the neuronal injury after transient occlusion of middle cerebral artery in these mice, measuring extracellular aminoacid concentrations by microdialysis method. [Results] The high-GCS and low-GCS mice had significantly lower and higher basal concentrations of extracellular glycine than controls, respectively. In low-GCS mice, extracellular glycine reached to 2-hold control level during ischemia and the infarction volume was significantly increased by 69% of controls. In contrast, high-GCS mice had significantly smaller infarction volume by 21%. No significant difference was observed in extracellular glutamate concentrations throughout the experiments. An antagonist for the NMDA glycine site, SM-31900, attenuated infarction size, suggesting that glycine operated via NMDA receptor. [Conclusions] There is a direct correlation between ischemic injury and extracellular glycine concentration maintained by the GCS.