A novel alternative splicing mechanism that enhances human 5-HT1A receptor RNA stability is altered in major depression.

The serotonin-1A (5-HT1A) receptor is a key regulator of serotonergic activity and is implicated in mood and emotion. However, its post-transcriptional regulation has never been studied in humans. In the present study, we show that the "intronless" human 5-HT1A gene (HTR1A) is alternatively spliced in its 3'-UTR, yielding two novel splice variants. These variants lack a ∼1.6-kb intron, which contains a miR135 target site. Unlike the human HTR1A, splicing was not detected in the mouse HTR1A, which lacks the splice donor/accepter sites. The two spliced mRNAs are extremely stable and are resistant to miR135-induced down-regulation, with greater translational output than the unspliced variant. Moreover, alternative HTR1A RNA splicing is oppositely regulated by the splice factors PTBP1 and nSR100, which inhibit or enhance its splicing, respectively. In post-mortem human brain tissue from both sexes, HTR1A mRNA splicing was prevalent and region-specific. Unspliced HTR1A was expressed more strongly in the hippocampus and midbrain vs. prefrontal cortex (PFC), and correlated with reduced levels of nSR100. Importantly, HTR1A RNA splicing and nSR100 levels were reduced in the PFC of individuals with major depression compared to controls. Our unexpected findings uncover a novel mechanism to regulate HTR1A gene expression through alternative splicing of miRNA sites. Altered levels of splice factors could contribute to changes in regional and depression-related gene expression through alternative splicing. SIGNIFICANCE STATEMENT Alternative splicing is prevalent in brain tissue, and increases gene diversity. The serotonin-1A receptor gene (HTR1A) is a regulator of serotonin implicated in mood and emotion. Here we show that human HTR1A RNA is alternately spliced. Splicing removes a microRNA site to generate ultra-stable RNA and increase HTR1A expression. This splicing varies in different brain regions and is reduced in major depression. We also identify specific splice factors for HTR1A RNA, showing they are also reduced in depression. Thus, we describe a novel mechanism to regulate gene expression through splicing. Altered levels of splice factors could contribute to depression by changing gene expression.