The Molecular Basis for Water Taste in Drosophila

The sense of taste allows animals to detect and assess potentially nutritive and toxic substances prior to ingestion. Animals have evolved to detect taste substances that are present in their environment. In fliesDrosophila melanogaster, these include (but may not be limited to), sugars, salts, toxic or noxious bitter compounds, CO2, and water. How do flies detect diverse taste substances? The first part of this thesis describes the results of a microarray-based screen performed in order to identify novel taste detection components. More specifically, a screen comparing RNA from proboscises with and without gustatory neurons enriched for known taste sensillum associated transcripts (gustatory receptors and odorant binding proteins) as well as transcripts with no known gustatory ascribed function. This latter group included transcripts with homology to ion channels and transporters, cytochromes, transcription factors, and proteases. A secondary screen with transgenic flies identified genes whose putative cis-regulatory sequence directed reporter expression in specific subsets of taste neurons, including epithelial sodium channel/degenerin (ENaC/Deg) family members, ionotropic glutamate receptors (iGluRs), an orphan G-protein coupled receptor, and a carbonic anhydrase. The second part of this thesis focuses on the molecular basis for water taste. Here, I identify a member of the ENaC/Deg family, ppk28, as an osmosensitive ion channel that mediates the cellular and behavioral response to water. I use molecular, cellular, calcium imaging and electrophysiological approaches to show that ppk28 is expressed in water-sensing neurons and loss of ppk28 abolishes water sensitivity. Moreover, ectopic expression of ppk28 confers water sensitivity to bitter-sensing gustatory neurons in the fly and sensitivity to hypo-osmotic solutions when expressed in heterologous cells. These studies link an osmosensitive ion channel to water taste detection and drinking behavior, providing the framework for examining the molecular basis for water detection in other animals. The third part of this thesis describes ongoing work with two ENaC/Deg family members termed ppk23 and CG13568. These molecules are largely co-expressed in a subset of taste neurons on the proboscis. Double labeling experiments strongly suggest that these molecules label a novel class of taste neurons. Mutant analysis suggests that these molecules are not involved in salt detection. Here I describe ongoing efforts to identify ligands and chemosensory functions for these two molecules