The insulin/insulin-like growth factor signaling (IIS) is a conserved pathway that regulates key events of animal physiology across taxa, ranging from invertebrates to vertebrates. Aside from its well-characterized role in metabolism and development, it has been implicated in behavioral plasticity. However, the mechanism underlying the function of IIS in learning remains largely unknown. Here we address this important neurobiology question using behavioral assays that quantify C. elegans olfactory learning. We find that two insulin-like peptides (ILPs), INS-6 and INS-7, antagonistically regulate the learning process.
ins-6 mutants are defective in learning, which is surprisingly rescued by mutations in
ins-7. In cell-specific rescue experiments, wild-type INS-6 rescues the learning defect of
ins-6 single mutants when it is expressed in the ASI sensory neurons, and wild-type INS-7 restores the learning defect of the
ins-6;
ins-7 double mutants when it is expressed in the URX neurons. To explore further the mechanism behind the epistatic interaction between
ins-6 and
ins-7, we have characterized the expression pattern of
ins-7 in the nervous system, using an integrated GFP transcriptional reporter for
ins-7. We have measured the expression level of
ins-7 in wild type and
ins-6 mutant animals. Intriguingly, we have observed an increase in
ins-7 expression in the
ins-6 mutant background specifically in the URX neurons, but not in other neurons that express
ins-7. In addition, we find that overexpression of INS-7 in URX produce a learning defect in wild-type animals. Together our data suggest that INS-6 from ASI promotes learning, whereas INS-7 acts in URX to inhibit it; and that INS-6 negatively regulates the transcription of
ins-7 in URX to achieve the antagonistic effects on learning regulation. This work not only characterizes the complex nature of ILP signaling in behavioral plasticity, but also reveals a novel IIS pattern that hierarchically engages multiple ILPs in a selective set of cells. This multi-step ILP-to-ILP signaling pattern employs both molecular diversity and cellular identity to achieve the sophisticated coordination of different ILP functions.