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 an microRNA-135 (miR135) target site. Unlike the humanHTR1A, the mouseHTR1Alacks the splice donor/accepter sites. Thus, in the mouseHTR1A, splicing was not detected. The two spliced mRNAs are extremely stable, are resistant to miR135-induced downregulation, and have greater translational output than the unspliced variant. Moreover, alternativeHTR1ARNA splicing is oppositely regulated by the splice factors PTBP1 and nSR100, which inhibit or enhance its splicing, respectively. In postmortem human brain tissue from both sexes,HTR1AmRNA splicing was prevalent and region-specific. UnsplicedHTR1Awas expressed more strongly in the hippocampus and midbrain versus the prefrontal cortex (PFC), and correlated with reduced levels of nSR100. Importantly,HTR1ARNA splicing and nSR100 levels were reduced in the PFC of individuals with major depression compared with controls. Our unexpected findings uncover a novel mechanism to regulateHTR1Agene expression through alternative splicing of microRNA sites. Altered levels of splice factors could contribute to changes in regional and depression-related gene expression through alternative splicing. SIGNIFICANCE STATEMENTAlternative splicing, which is prevalent in brain tissue, increases gene diversity. The serotonin-1A receptor gene (HTR1A) is a regulator of serotonin, which is implicated in mood and emotion. Here we show that humanHTR1ARNA is alternately spliced. Splicing removes a microRNA site to generate ultrastable RNA and increaseHTR1Aexpression. This splicing varies in different brain regions and is reduced in major depression. We also identify specific splice factors forHTR1ARNA, 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.