daf-16/FOXO promotes the activity of ligand-bound DAF-12/NHR to coordinate dauer recovery and post-dauer seam cell fate

Abstract

Animal development occurs in the context of varying environmental conditions. Many animal species can withstand adverse conditions by entering a stress-resistant and developmentally arrested diapause stage. If conditions improve, animals can recover and complete development. In Caenorhabditis elegans larvae that encounter adverse environments, dauer diapause can interrupt developmental progression after the second larval molt. During continuous (non-dauer) development, a heterochronic molecular timer comprised primarily of microRNAs and their targets controls the progression of stage-specific cell fates in lateral hypodermal seam cells. In unfavorable conditions, the DAF-16/FOXO transcription factor promotes dauer formation in part through opposing production of the ligand for the DAF-12 nuclear hormone receptor. Ligand-free DAF-12 promotes dauer formation and represses expression of heterochronic microRNAs in the let-7 family. Here, we show that daf-16 acts before dauer formation in anticipation of growth after dauer by promoting both dauer exit and developmental progression. We found that daf-16(0) post-dauer adults showed delayed dauer exit, reiterative heterochronic defects, and reduced expression of let-7-family microRNAs. These phenotypes were suppressed by addition of the DAF-12 ligand, dafachronic acid. Dafachronic acid is synthesized from cholesterol, and dauer larvae sequester cholesterol in the intestinal lumen until dauer exit. We found that a fluorescent cholesterol analog was not retained in daf-16 mutant larvae during dauer recovery. Timed auxin-mediated depletion of DAF-16 indicated that daf-16 is required before dauer formation to prevent reiterative seam cell fates in post-dauer animals. We propose a model whereby daf-16 acts prior to dauer formation to enable dafachronic acid synthesis by retaining cholesterol during dauer recovery. Ligand-bound DAF-12 then promotes dauer exit and expression of let-7-family microRNAs, thereby promoting developmental progression. Thus, daf-16 coordinates dauer entry with anticipation of recovery and post-dauer development.