Molecular mechanism underlying death of motor neurons in model mice of amyotrophic lateral sclerosis

GluR 2是L-α-氨基-3-羟基-5-甲基-4-异恶唑丙酸（AMPA）受体的一个亚基，其在Q/R位点的RNA编辑效率低下是ALS患者脊髓运动神经元的一种疾病特异性和位点选择性分子异常。GluR 2前mRNA的Q/R位点的腺苷通过作用于RNA 2（ADAR 2）的称为腺苷脱氨酶的酶转化为肌苷（A至I转化），并且未能编辑该位点上调含有Q/R位点未编辑的GluR 2的Ca 2+可渗透的AMPA受体。为了模拟ALS发病机制，我们产生了遗传修饰的小鼠（指定为AR 2），其中ADAR 2基因使用Cre/loxP系统在约一半的运动神经元中有条件地靶向。这些AR 2小鼠表现出与脊髓和颅运动神经核中ADAR 2缺陷运动神经元的缓慢死亡相当的运动功能下降。值得注意的是，尽管GluR 2 Q/R位点编辑显著减少，但在ALS中经常逃避变性的眼神经核中的神经元数量并未减少。当小鼠携带内源性GluR 2等位基因时，AR 2小鼠中的所有细胞和表型变化都被阻止，这些等位基因被工程化以表达编辑的GluR 2而没有ADAR 2活性。因此，ADAR 2特异性编辑GluR 2 Q/R位点，并且ADAR 2活性的丧失通过不能编辑GluR 2 Q/R位点而不是其他ADAR 2介导的编辑位置而导致运动神经元死亡。由于与ALS发病机制的相似性，AR 2小鼠为ALS的研究和治疗提供了工具。

Inefficient RNA editing of GluR2, a subunit of the L-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor, at the Q/R site is a disease-specific and site-selective molecular abnormality in spinal motor neurons of ALS patients. Adenosine for the Q/R site of GluR2 pre-mRNA is converted to inosine (A-to-I conversion) by the enzyme called adenosine deaminase acting on RNA 2 (ADAR2) and failure to edit this site upregulates the Ca2+-permeable AMPA receptors containing Q/R site-unedited GluR2. To mimic the ALS pathogenesis, we generated genetically modified mice (designated as AR2) in which the ADAR2 gene was conditionally targeted in about a half of motor neurons using the Cre/loxP system. These AR2 mice showed a decline in motor function commensurate with the slow death of ADAR2-deficient motor neurons in the spinal cord and cranial motor nerve nuclei. Notably, neurons in nuclei of oculomotor nerves, which often escape degeneration in ALS, were not decreased in number despite of a significant decrease in GluR2 Q/R site-editing. All cellular and phenotypic changes in AR2 mice were prevented when the mice carried endogenous GluR2 alleles engineered to express edited GluR2 without ADAR2 activity. Thus, ADAR2 specifically edits the GluR2 Q/R site and loss of ADAR2 activity causes death of motor neurons by failure to edit the GluR2 Q/R site but not other ADAR2-mediated editing positions. Because of the similarity to the ALS pathogenesis, AR2 mice provides a tool for ALS research and therapy.