Coordination of Pickpocket ion channel delivery and dendrite growth in Drosophila sensory neurons

Sensory neurons enable an organism to perceive external stimuli, which is essential for survival. The sensory capacity of a neuron depends on the elaboration of its dendritic arbor and the delivery of sensory ion channels to the dendritic membrane. However, it is not well understood how ion channels are trafficked to sensory dendrites and whether their delivery is coordinated with dendrite growth. We investigated the trafficking of the DEG/ENaC/ASIC ion channel Pickpocket (Ppk) in peripheral sensory neurons in fruit fly larvae. We used CRISPR-Cas9 genome engineering to tag endogenous Ppk1 and visualize it live, including monitoring Ppk1 membrane localization via a novel secreted split-GFP approach. Strikingly, Ppk1 is present throughout the membrane of actively growing dendrites, and Ppk1 density scales in proportion to the dendritic membrane, even when dynein-mediated transport to dendrites is disrupted. Our data suggest that Ppk1 is integral to the membrane of growing dendrites and implicate the recycling endosome GTPase Rab11 in the forward trafficking of Ppk1 to dendrites. Together, our results suggest that Ppk channel transport is coordinated with dendrite morphogenesis, thus ensuring proper ion channel levels and distribution in sensory dendrites. Author Summary Peripheral sensory neurons are essential for an organism to interact with its environment. Neurons are composed of signal-receiving dendrites and a signal-sending axon. Ion channels distributed throughout sensory dendrites transduce external stimuli into chemical signals, however the mechanisms that localize ion channels to sensory dendrites are not well understood. Both the composition of ion channels in the dendrites and the structure of a sensory neuron’s dendritic arbor are important for how it functions to sense external stimuli. Using live imaging and genomic engineering, we have discovered that the localization of a sensory ion channel, Pickpocket, in fruit fly sensory neurons is coordinated with growth of the dendritic arbor and that Pickpocket levels scale in proportion to dendrite length, even when transport to dendrites is disrupted. We also developed a novel genetically encoded approach to visualize the membrane expression of proteins in a living organism utilizing the split-GFP system. We found that both the secretory and endosomal networks mediate the forward trafficking of Pickpocket during neuronal morphogenesis, thus coordinating membrane growth with ion channel delivery. Our findings reveal that actively growing sensory dendrites are equipped with the ion channels needed for sensing external stimuli.