Molecular Basis of Compensatory Changes in Taste Preference Upon Protein Deprivation

Kai Lam, Anindya Ganguly, Craig Montell

Animals employ different regulatory mechanisms to modulate gustatory sensitivity in response to changes in both external environments and internal states. Fruit flies, for instance, increase their affinity for yeast extract and amino acids when subjected to protein deprivation. Analysis of existing RNAseq data revealed that although none of the reported amino acid taste receptors are differentially expressed upon amino acid deprivation, expression of rh2 in the proboscis is downregulated by more than four-fold. Strikingly, rh2 mutant flies fail to exhibit compensatory changes in feeding preferences upon protein deprivation. Further investigation revealed that rh2 flies have higher preference for yeast extract over sugar in binary choice assays. They also exhibited increased proboscis extension to yeast upon labellar stimulation. Since preferences for yeast extract in fruit flies is strongly dependent on its amino acid content, we compared proboscis extension upon labellar stimulation with nineteen amino acids between wild-type and rh2 flies. rh2 flies displayed elevated propensity to extend their proboscis for a broad spectrum of amino acids, with serine and tryptophan having the most significantly elevated responses. Amino acids, as reported by previous studies, could be both attractive and aversive. We hypothesize that rh2 is required for aversive amino acid taste and is downregulated in protein-deprived conditions to make amino acids more palatable. Hence, preferences for amino acids are also increased in rh2 flies. However further studies are required to understand in which subset of taste neurons rh2 function is required for gustatory aversion of amino acids.