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

肌萎缩侧索硬化症患者脊髓运动神经元Q/R位点L-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic受体亚单位GluR2的编辑效率低下是一种疾病特异性和部位选择性的分子异常。GluR2前-mRNA Q/R位点的腺苷被作用于RNA2的腺苷脱氨酶(ADAR2)转化为肌苷(A-to-I转换)，如果不编辑该位点，则含有Q/R位点未编辑的GluR2的透性AMPA受体上调。为了模拟ALS的发病机制，我们产生了转基因小鼠(命名为AR2)，其中ADAR2基因使用Cre/loxP系统有条件地靶向大约一半的运动神经元。这些AR2小鼠表现出运动功能的下降，与脊髓和颅运动神经核中ADAR2缺陷运动神经元的缓慢死亡相称。值得注意的是，在ALS中经常避免变性的动眼神经核团中的神经元数量并没有减少，尽管GluR2Q/R位点编辑显著减少。当AR2小鼠携带内源性GluR2等位基因时，所有的细胞和表型变化都被阻止了。因此，ADAR2专门编辑GluR2 Q/R位点，ADAR2活性的丧失会导致运动神经元死亡，原因是无法编辑GluR2 Q/R位点，而不是其他ADAR2介导的编辑位置。由于AR2小鼠与ALS发病机制相似，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.