Partitioning of the truncated insulin receptor DAF-2B between homodimers and heterodimers influences insulin signaling in C. elegans

Insulin / insulin-like growth factor signaling (IIS) in C. elegans is mediated by the DAF-2 receptor and insulin-like peptides (ILP) that can act as agonists or antagonists. DAF-2 signaling is also affected by DAF-2B, a truncated, non-signaling, secreted isoform of DAF-2 that acts as a decoy receptor by sequestering ILPs. In this study, we performed a forward genetic screen for modifiers of DAF-2B protein expression and identified a mutation in unc-31 that increased DAF-2B. UNC-31 is involved in dense core vesicle docking and is required for neuropeptide secretion, including ILPs. As a result, unc-31 mutants constitutively enter the dauer larval stage and are long-lived due to reduced IIS. We find that increased nervous system DAF-2B accumulation is associated with reduced agonist ILP availability, in both unc-31 mutants and wild type worms. Using auxin-induced degradation (AID) and fluorescence lifetime imaging microscopy-Förster resonance energy transfer (FLIM-FRET), we find that a significant fraction of nervous system DAF-2B is in the form of a heterodimeric complex with a full-length DAF-2 receptor isoform, representing a new class of DAF-2 hybrid receptor. In unc-31 mutants, DAF-2B also undergoes endocytosis in neurons in an AP2-dependent manner and genetic manipulation of daf-2b in the unc-31 mutant suggests that DAF-2B homodimers may function to reinforce a reduced insulin signaling state by clearing agonist ILPs from the extracellular space. These findings indicate that DAF-2B not only forms homodimers, but also hybrid receptors with full-length DAF-2, to regulate the activity of the large and diverse family of ILPs in C. elegans.