IGF-like growth factors that couple food sensing to developmental decisions employ different release mechanisms in a single pair of C. elegans chemosensory neurons

Sensory neurons modulate organismal physiology and behavior in part by releasing neuropeptides and neurotrophins or growth factors, via the dense core vesicle (DCV) pathway. The precise matching of the sensory input to the identity of released vesicle cargo, and the timing and location of its release, is necessary to ensure appropriate responses. In some neurons, several neuropeptides or growth factors can be co-produced simultaneously, but found in distinct populations of vesicles. While this may permit their release independently of each other, in response to different stimuli, the responsible molecular pathways are not well understood. In C. elegans , the chemosensory ASI neuron pair expresses multiple neuropeptides and growth factors, including the structurally homologous C. elegans IGF-like growth factors DAF-28, INS-6, and INS-4, whose release in young larvae couples food sensing to the commitment to reproductive development. We find that these growth factors require separate molecular pathways for their release. While the axonal release of INS-6 protein required the Calcium-dependent Activator Protein for Secretion (CAPS/UNC-31), the release of DAF-28 was CAPS-independent. Consequently, the function of endogenous daf-28 , but not that of ins-6 and ins-4 , is independent of unc-31 . This difference is unexpected, as CAPS/UNC-31 is thought to control the regulated pre-synaptic/axonal release of DCV cargoes in C. elegans neurons, and demonstrates a divergence in vesicle release mechanisms. In addition, we find that mechanisms for delivering vesicles to the axons are also divergent: while neuropeptide NLP-21 was dependent on the clathrin adaptor AP-3 for its selective localization to axons, four insulin/IGF-like growth factors tested - DAF-28, INS-6, INS-4, and INS-22, were AP-3-independent for either axonal localization or function. Our data uncover molecular divergence in the pathways controlling axonal localization and release of neuropeptides and growth factors, including in a single neuron. These divergent vesicle pathways provide new means for the immediate and tunable changes in neuronal outputs, in addition to the known, less immediate mechanism of differential transcriptional regulation of DCV cargoes. Future delineation of the molecular composition of these pathways is necessary to understand how neurons match organismal responses to specific sensory inputs. Graphical Abstract Molecular divergence in the pathways controlling axonal localization and release of neuropeptides and growth factors