Balboa Binds to Pickpocket In Vivo and Is Required for Mechanical Nociception in Drosophila Larvae

The Drosophila gene pickpocket (ppk) encodes an ion channel subunit of the degenerin/epithelial sodium channel (DEG/ENaC) family [1]. PPK is specifically expressed in nociceptive, class IV multidendritic (md) neurons and is functionally required for mechanical nociception responses [2, 3]. In this study, in a genome-wide genetic screen for other ion channel subunits required for mechanical nociception, we identify a gene that we name balboa (also known as CG8546, ppk26) [4]. Interestingly, the balboa locus encodes a DEG/ENaC ion channel subunit highly similar in amino acid sequence to PPK [5]. Moreover, laser-capture isolation of RNA from larval neurons and microarray analyses reveal that balboa is also highly enriched in nociceptive neurons. The requirement for Balboa and PPK in mechanical nociception behaviors and their specific expression in larval nociceptors led us to hypothesize that these DEG/ENaC subunits form an ion channel complex in vivo. In nociceptive neurons, Balboa::GFP proteins distribute uniformly throughout dendrites but remarkably localize to discrete foci when ectopically expressed in other neuron subtypes (where PPK is not expressed). Indeed, ectopically coexpressing ppk transforms this punctate Balboa:: GFP expression pattern to the uniform distribution observed in its native cell type. Furthermore, ppk-RNAi in class IV neurons alters the broad Balboa:: GFP pattern to a punctate distribution. Interestingly, this interaction is mutually codependent as balboa-RNAi eliminates Venus:: PPK from the sensory dendrites of nociceptors. Finally, using a GFP-reconstitution approach in transgenic larvae, we directly detect in vivo physical interactions among PPK and Balboa subunits. Combined, our results indicate a critical mechanical nociception function for heteromeric PPK and Balboa channels in vivo.