Kaplan Rebecca E W, Chen Yutao, Moore Brad T, Jordan James M, Maxwell Colin S, Schindler Adam J, Baugh L Ryan
Department of Biology, Duke University, Durham, North Carolina, United States of America.
PLoS Genet. 2015 Dec 11;11(12):e1005731. doi: 10.1371/journal.pgen.1005731. eCollection 2015 Dec.
Nutrient availability has profound influence on development. In the nematode C. elegans, nutrient availability governs post-embryonic development. L1-stage larvae remain in a state of developmental arrest after hatching until they feed. This "L1 arrest" (or "L1 diapause") is associated with increased stress resistance, supporting starvation survival. Loss of the transcription factor daf-16/FOXO, an effector of insulin/IGF signaling, results in arrest-defective and starvation-sensitive phenotypes. We show that daf-16/FOXO regulates L1 arrest cell-nonautonomously, suggesting that insulin/IGF signaling regulates at least one additional signaling pathway. We used mRNA-seq to identify candidate signaling molecules affected by daf-16/FOXO during L1 arrest. dbl-1/TGF-β, a ligand for the Sma/Mab pathway, daf-12/NHR and daf-36/oxygenase, an upstream component of the daf-12 steroid hormone signaling pathway, were up-regulated during L1 arrest in a daf-16/FOXO mutant. Using genetic epistasis analysis, we show that dbl-1/TGF-β and daf-12/NHR steroid hormone signaling pathways are required for the daf-16/FOXO arrest-defective phenotype, suggesting that daf-16/FOXO represses dbl-1/TGF-β, daf-12/NHR and daf-36/oxygenase. The dbl-1/TGF-β and daf-12/NHR pathways have not previously been shown to affect L1 development, but we found that disruption of these pathways delayed L1 development in fed larvae, consistent with these pathways promoting development in starved daf-16/FOXO mutants. Though the dbl-1/TGF-β and daf-12/NHR pathways are epistatic to daf-16/FOXO for the arrest-defective phenotype, disruption of these pathways does not suppress starvation sensitivity of daf-16/FOXO mutants. This observation uncouples starvation survival from developmental arrest, indicating that DAF-16/FOXO targets distinct effectors for each phenotype and revealing that inappropriate development during starvation does not cause the early demise of daf-16/FOXO mutants. Overall, this study shows that daf-16/FOXO promotes developmental arrest cell-nonautonomously by repressing pathways that promote larval development.
营养物质的可利用性对发育有着深远影响。在线虫秀丽隐杆线虫中，营养物质的可利用性控制着胚胎后发育。L1期幼虫孵化后处于发育停滞状态，直到它们开始进食。这种“L1停滞”（或“L1滞育”）与抗逆性增强有关，有助于在饥饿状态下存活。转录因子daf-16/FOXO作为胰岛素/胰岛素样生长因子信号通路的效应器，其缺失会导致停滞缺陷和饥饿敏感的表型。我们发现daf-16/FOXO以细胞非自主的方式调节L1停滞，这表明胰岛素/胰岛素样生长因子信号通路至少调节一条额外的信号通路。我们使用mRNA测序来鉴定在L1停滞期间受daf-16/FOXO影响的候选信号分子。dbl-1/TGF-β（Sma/Mab通路的一种配体）、daf-12/NHR和daf-36/加氧酶（daf-12类固醇激素信号通路的一个上游组分）在daf-16/FOXO突变体的L1停滞期间上调。通过遗传上位性分析，我们发现dbl-1/TGF-β和daf-12/NHR类固醇激素信号通路对于daf-16/FOXO的停滞缺陷表型是必需的，这表明daf-16/FOXO抑制dbl-1/TGF-β、daf-12/NHR和daf-36/加氧酶。此前尚未发现dbl-1/TGF-β和daf-12/NHR通路会影响L1发育，但我们发现破坏这些通路会延迟进食幼虫的L1发育，这与这些通路促进饥饿状态下daf-16/FOXO突变体的发育一致。尽管对于停滞缺陷表型，dbl-1/TGF-β和daf-12/NHR通路相对于daf-16/FOXO是上位的，但破坏这些通路并不能抑制daf-16/FOXO突变体的饥饿敏感性。这一观察结果将饥饿存活与发育停滞区分开来，表明DAF-16/FOXO针对每种表型靶向不同的效应器，并揭示饥饿期间不适当的发育不会导致daf-16/FOXO突变体过早死亡。总体而言，这项研究表明daf-16/FOXO通过抑制促进幼虫发育的通路以细胞非自主的方式促进发育停滞。
