Ectopic expression of Snord115 in choroid plexus interferes with editing but not splicing of 5-Ht2c receptor pre-mRNA in mice

Serotonin 5-HT2C receptor is a G-protein coupled excitatory receptor that regulates several biochemical pathways and has been implicated in obesity, mental state, sleep cycles, autism, neuropsychiatric disorders and neurodegenerative diseases. The activity of 5-HT2CR is regulated via alternative splicing and A to I editing of exon Vb of its pre-mRNA. Snord115 is a small nucleolar RNA that is expressed in mouse neurons and displays an 18-nucleotide base complementary to exon Vb of 5-HT2CR pre-mRNA. For almost two decades this putative guide element of Snord115 has wandered like a ghost through the literature in attempts to elucidate the biological significance of this complementarity. In mice, Snord115 is expressed in neurons and absent in the choroid plexus where, in contrast, 5-Ht2cr mRNA is highly abundant. Here we report the analysis of 5-Ht2cr pre-mRNA posttranscriptional processing via RNA deep sequencing in a mouse model that ectopically expresses Snord115 in the choroid plexus. In contrast to previous reports, our analysis demonstrated that Snord115 does not control alternative splicing of 5-Ht2cr pre-mRNA in vivo. We identified a modest, yet statistically significant reduction of 5-Ht2cr pre-mRNA A to I editing at the major A, B, C and D sites. We suggest that Snord115 and exon Vb of 5Ht2cr pre-mRNA form a double-stranded structure that is subject to ADAR-mediated A to I editing. To the best of our knowledge, this is the first comprehensive Snord115 gain-of-function analysis based on in vivo mouse models. Comparison of two mouse models by RNA deep sequencing5-羟色胺5-HT2C受体（serotonin 5-HT2C receptor）是一类G蛋白偶联兴奋性受体（G-protein coupled excitatory receptor），可调控多条生化通路，且与肥胖、精神状态、睡眠周期、孤独症、神经精神疾病及神经退行性疾病均存在关联。5-HT2CR的活性可通过其前体mRNA（pre-mRNA）外显子Vb的可变剪接与A-to-I编辑进行调控。Snord115是一种小核仁RNA（small nucleolar RNA），在小鼠神经元中表达，其序列与5-HT2CR前体mRNA的外显子Vb存在18个核苷酸的碱基互补区域。近二十年来，这一被认为是Snord115向导元件的互补序列，如同幽灵一般游走于相关文献之中，相关研究始终试图阐明这种互补性的生物学意义。在小鼠体内，Snord115仅在神经元中表达，而脉络丛中不表达该RNA；与之形成鲜明对比的是，脉络丛中5-Ht2cr mRNA的丰度极高。本研究通过RNA深度测序（RNA deep sequencing）技术，对在脉络丛中异位表达Snord115的小鼠模型体内的5-Ht2cr前体mRNA转录后加工过程展开分析。与既往研究结论不同，我们的分析结果显示，Snord115在体内并不会调控5-Ht2cr前体mRNA的可变剪接。我们发现，5-Ht2cr前体mRNA的主要A、B、C、D位点的A-to-I编辑水平出现了小幅但具有统计学意义的降低。我们推测，Snord115与5-Ht2cr前体mRNA的外显子Vb可形成双链结构，该结构可被RNA腺苷脱氨酶（ADAR）介导的A-to-I编辑所作用。据我们所知，这是首个基于体内小鼠模型的Snord115功能获得性全面分析研究。本研究通过RNA深度测序对两种小鼠模型进行了比较。