RNAi Screen of DAF-16/FOXO Target Genes in <em>C. elegans</em> Links Pathogenesis and Dauer Formation

The DAF-16/FOXO transcription factor is the major downstream output of the insulin/IGF1R signaling pathway controlling C. elegans dauer larva development and aging. To identify novel downstream genes affecting dauer formation, we used RNAi to screen candidate genes previously identified to be regulated by DAF-16. We used a sensitized genetic background [eri-1(mg366); sdf-9(m708)], which enhances both RNAi efficiency and constitutive dauer formation (Daf-c). Among 513 RNAi clones screened, 21 displayed a synthetic Daf-c (SynDaf) phenotype with sdf-9. One of these genes, srh-100, was previously identified to be SynDaf, but twenty have not previously been associated with dauer formation. Two of the latter genes, lys-1 and cpr-1, are known to participate in innate immunity and six more are predicted to do so, suggesting that the immune response may contribute to the dauer decision. Indeed, we show that two of these genes, lys-1 and clc-1, are required for normal resistance to Staphylococcus aureus. clc-1 is predicted to function in epithelial cohesion. Dauer formation exhibited by daf-8(m85), sdf-9(m708), and the wild-type N2 (at 27°C) were all enhanced by exposure to pathogenic bacteria, while not enhanced in a daf-22(m130) background. We conclude that knockdown of the genes required for proper pathogen resistance increases pathogenic infection, leading to increased dauer formation in our screen. We propose that dauer larva formation is a behavioral response to pathogens mediated by increased dauer pheromone production.DAF-16/FOXO转录因子（DAF-16/FOXO transcription factor）是调控秀丽隐杆线虫（C. elegans）持久态幼虫发育与衰老的胰岛素/IGF1R信号通路（insulin/IGF1R signaling pathway）的主要下游效应因子。为鉴定影响持久态幼虫形成的新型下游基因，我们采用RNA干扰（RNA interference，RNAi）技术，对此前已被DAF-16调控的候选基因进行筛选。我们使用了增敏型遗传背景[eri-1(mg366); sdf-9(m708)]，该背景可同时增强RNA干扰效率与组成型持久态幼虫形成能力（Daf-c）。在筛选的513个RNAi克隆中，有21个与sdf-9共同表现出合成型Daf-c（SynDaf）表型。其中srh-100这一基因此前已被证实与SynDaf相关，但其余20个基因此前均未被发现与持久态幼虫形成有关联。在这20个基因中，lys-1与cpr-1已知可参与先天免疫，另有6个基因被预测具有该功能，这表明免疫应答可能参与调控持久态幼虫的命运决定。进一步实验证实，lys-1与clc-1这两个基因是秀丽隐杆线虫对金黄色葡萄球菌（Staphylococcus aureus）正常抗性所必需的。clc-1被预测可参与上皮细胞黏合过程。daf-8(m85)、sdf-9(m708)突变体以及野生型N2（在27℃条件下）的持久态幼虫形成率，均在暴露于致病菌后得到增强；而在daf-22(m130)遗传背景中，该现象并未出现增强。我们据此得出结论：敲低正常病原菌抗性所需的基因会加重病原菌感染，进而导致本筛选中持久态幼虫形成率上升。我们提出，持久态幼虫的形成是一种由持久态信息素（dauer pheromone）分泌增加所介导的、针对病原菌的行为应答反应。