Role of STA-1 and SID-3 upon Orsay virus infection in C. elegans (ChIP-Seq). Caenorhabditis elegans

Across metazoans, innate immunity is vital in defending organisms against viral infection. In mammals, antiviral innate immunity is orchestrated by interferon signaling, activating the STAT transcription factors downstream of the JAK kinases to induce expression of antiviral effector genes. In the nematode C. elegans, which lacks the interferon system, the major antiviral response so far described is RNA interference but whether additional gene expression responses are employed is not known. Here we show that, despite the absence of both interferon and JAK, the C. elegans STAT homologue STA-1 orchestrates antiviral immunity. Intriguingly, mutants lacking STA-1 show increased resistance to antiviral infection. Using gene expression analysis and chromatin immunoprecipitation we show that, in contrast to the mammalian pathway, STA-1 acts as a transcriptional repressor. Thus STA-1 might act to suppress a constitutive antiviral response in the absence of infection. Using a reverse genetic screen we identify the SID-3 as a kinase upstream of STA-1 in the response to infection. Together, our work identifies a novel STAT regulatory cascade controlling its activity in antiviral resistance, illustrating the complex evolutionary trajectory displayed by innate immune signaling pathways across metazoan organisms. Overall design: In this experiment, the DNA binding capacity of a GFP::STA-1 fusion protein was assessed by chip-sequencing with GFP immunoprecipitation followed by DNA sequencing.

在后生动物（metazoans）中，先天免疫对生物体抵御病毒感染至关重要。在哺乳动物中，抗病毒先天免疫由干扰素信号通路介导，通过激活JAK激酶下游的信号转导与转录激活因子（Signal Transducer and Activator of Transcription, STAT），诱导抗病毒效应基因的表达。在缺失干扰素系统的线虫秀丽隐杆线虫（Caenorhabditis elegans, C. elegans）中，目前已报道的主要抗病毒应答为RNA干扰（RNA interference），但尚不明确其是否还存在其他基因表达介导的抗病毒应答途径。本研究证实，尽管秀丽隐杆线虫同时缺失干扰素与JAK通路，但其STAT同源蛋白STA-1仍可介导抗病毒免疫。值得注意的是，缺失STA-1的突变体对病毒感染的抗性反而显著增强。通过基因表达分析与染色质免疫沉淀（chromatin immunoprecipitation, ChIP）实验，本研究发现与哺乳动物通路不同，STA-1发挥转录抑制因子的功能。由此推测，在未发生感染时，STA-1可能通过抑制组成型抗病毒应答来维持机体稳态。本研究通过反向遗传筛选，鉴定出SID-3为感染应答过程中STA-1的上游激酶。综上，本研究发现了一条调控STAT蛋白在抗病毒抗性中活性的新型调控级联反应，阐明了后生动物先天免疫信号通路所呈现的复杂进化轨迹。实验整体设计：本实验通过GFP免疫沉淀结合DNA测序（ChIP-sequencing），评估GFP::STA-1融合蛋白的DNA结合能力。