An RNA-Seq Screen of the <i>Drosophila</i> Antenna Identifies a Transporter Necessary for Ammonia Detection

Many insect vectors of disease detect their hosts through olfactory cues, and thus it is of great interest to understand better how odors are encoded. However, little is known about the molecular underpinnings that support the unique function of coeloconic sensilla, an ancient and conserved class of sensilla that detect amines and acids, including components of human odor that are cues for many insect vectors. Here, we generate antennal transcriptome databases both for wild type Drosophila and for a mutant that lacks coeloconic sensilla. We use these resources to identify genes whose expression is highly enriched in coeloconic sensilla, including many genes not previously implicated in olfaction. Among them, we identify an ammonium transporter gene that is essential for ammonia responses in a class of coeloconic olfactory receptor neurons (ORNs), but is not required for responses to other odorants. Surprisingly, the transporter is not expressed in ORNs, but rather in neighboring auxiliary cells. Thus, our data reveal an unexpected non-cell autonomous role for a component that is essential to the olfactory response to ammonia. The defective response observed in a Drosophila mutant of this gene is rescued by its Anopheles ortholog, and orthologs are found in virtually all insect species examined, suggesting that its role is conserved. Taken together, our results provide a quantitative analysis of gene expression in the primary olfactory organ of Drosophila, identify molecular components of an ancient class of olfactory sensilla, and reveal that auxiliary cells, and not simply ORNs, play an essential role in the coding of an odor that is a critical host cue for many insect vectors of human disease.诸多疾病媒介昆虫通过嗅觉线索识别宿主，因此深入解析气味编码机制具有重要研究价值。然而，目前对于支撑锥形感器（coeloconic sensilla）独特功能的分子基础仍知之甚少。锥形感器是一类古老且保守的感器类群，可检测胺类与羧酸类物质，包括作为诸多疾病媒介昆虫宿主识别线索的人体气味成分。本研究构建了野生型果蝇以及缺失锥形感器的果蝇突变体的触角转录组数据库。我们利用该数据库资源，筛选出在锥形感器中高度富集表达的基因，其中包含诸多此前未被发现参与嗅觉功能的基因。其中，我们鉴定出一个铵转运蛋白基因，该基因对于一类锥形感器嗅觉受体神经元（olfactory receptor neurons，ORNs）的氨响应至关重要，但不参与其他气味物质的响应过程。令人意外的是，该转运蛋白并非在嗅觉受体神经元中表达，而是在邻近的辅助细胞中表达。因此，本研究数据揭示了一个此前未被认知的非细胞自主性功能：该组分对于氨的嗅觉响应不可或缺。该基因的果蝇突变体所表现出的响应缺陷，可通过其按蚊同源基因得以挽救；且在已检测的几乎所有昆虫物种中均存在该同源基因，表明其功能具有保守性。综上，本研究对果蝇初级嗅觉器官的基因表达进行了定量分析，鉴定出一类古老嗅觉感器的分子组分，并揭示辅助细胞而非仅嗅觉受体神经元，在编码一种气味物质时发挥了关键作用——该气味正是诸多人类疾病媒介昆虫的重要宿主识别线索。