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The detection of water and the regulation of water intake are essential for animals to maintain proper osmotic homeostasis1. Drosophila and other insects have gustatory sensory neurons that mediate the recognition of external water sources2-4, but little is known about the underlying molecular mechanism for water taste detection. Here we identify a member of the degenerin/epithelial sodium channel family5, PPK28, as an osmosensitive ion channel that mediates the cellular and behavioural response to water. We use molecular, cellular, calcium imaging and electrophysiological approaches to show that ppk28 is expressed in water-sensing neurons, and that loss of ppk28 abolishes water sensitivity. Moreover, ectopic expression of ppk28 confers water sensitivity to bittersensing gustatory neurons in the fly and sensitivity to hypoosmotic solutions when expressed in heterologous cells. These studies link an osmosensitive ion channel to water taste detection and drinking behaviour, providing the framework for examining the molecular basis for water detection in other animals.
To uncover molecules involved in taste detection, we performed a microarray-based screen for genes expressed in taste neurons. Proboscis RNA from flies homozygous for a recessive poxn null mutation was compared to RNA from heterozygous controls. poxn mutants have a transformation of labellar gustatory chemosensory bristles into mechanosensory bristles, and therefore lack all taste neurons6,7. Whole-genome microarray comparisons showed that 256 of ,18,500 transcripts were significantly decreased in poxn mutants (.2-fold enrichment in control relative to poxn, P,0.05, moderated t-test). These included 18 gustatory receptors (representing a 21-fold enrichment in the gene set) and eight odorant-binding proteins (13-fold enrichment) (Supplementary Fig. 1 and Supplementary Table 1; Gene Expression Omnibus (GEO) accession number GSE19984).
In the mammalian gustatory system, ion channels mediate the detection of sour and salt tastes8, suggesting that ion channel genes may also participate in Drosophila taste detection. We therefore examined the expression pattern of candidate taste-enriched ion channels. The putative promoter of one gene, pickpocket 28 (ppk28), directed robust reporter expression in taste neurons on the proboscis (Fig. 1a). PPK28 belongs to the degenerin/epithelial sodium channel family (Deg/ENaC) and these channels are involved in the detection of diverse stimuli, including mechanosensory stimuli, acids and sodium ions5. In the brain, ppk28-Gal4 drives expression of green fluorescent protein (GFP) in gustatory sensory axons that project to the primary taste region,...