Drosophila Gr5a encodes a taste receptor tuned to trehalose

Abstract

Recent studies have suggested that Drosophila taste receptors are encoded by a family of G protein-coupled receptor genes comprising at least 56 members. One of these genes, Gr5a, has been shown by genetic analysis to be required by the fly for behavioral and sensory responses to a sugar, trehalose. Here, we show that Gr5a is expressed in neurons of taste sensilla located on the labellum and legs. Expression is observed in most if not all labellar sensilla and suggests that many taste neurons express more than one receptor. We demonstrate by heterologous expression in a Drosophila S2 cell line that Gr5a encodes a receptor tuned to trehalose. This is the first functional expression of an invertebrate taste receptor.

Fig. 1.

Expression of Gr5a in taste neurons in the labellum and distal segments in the leg. Shown here are whole mount samples of labella or tarsi. Genotypes examined were as follows: Gr5a-GAL4/+; UAS-lacZ/+ (a and c) and Gr5a-GAL4/UAS-mCD8:GFP; Gr5a-GAL4/UAS-mCD8:GFP (b and d). Shown in b is a composite of a series of confocal images. Reporter gene expression was observed in ≈30 cells in each half of the labellum and 2 cells in each of the two to three distal-most segments of the leg.

Fig. 2.

Time course of trehalose response in S2-Gr5a cells. (Upper) A series of images of a single fura 2-loaded S2-Gr5a cell, taken at 5-s intervals. The first image is taken 5 s before the application of 100 mM trehalose. (Lower) A quantitative representation of the response of the same cell. Bar indicates stimulus period.

Fig. 3.

Dose-dependence of trehalose response in S2-Gr5a cells. (Upper) Divided panels of S2-Gr5a cells (Left and Center) or negative controls, transfected with GFP vector alone (Right), before and after application of either 250 mM trehalose (Left and Right) or 250 mM maltose (Center). (Lower) Images of fields of S2-Gr5a cells taken on application of different concentrations of trehalose (indicated below).

Fig. 4.

(a) Dose-response curves for selected disaccharides. Responses are normalized to response at 250 mM trehalose. 4 ≤ n ≤ 6; error bars = SEM. (b) Structures of selected disaccharides related to trehalose. With the exception of sucrose, all are composed of 2 units of glucose. The α-configuration is highlighted in yellow, and the β-configuration of the C1 carbon is highlighted in red. Isotrehalose and neotrehalose do not occur naturally. (c) Specificity of trehalose response in S2-Gr5a cells. Shown here are the responses of Gr5a-S2 cells to the various disaccharides illustrated in b. Compounds were tested at a concentration of 250 mM. 5 ≤ n ≤ 9. ”No Gr5a” cells were stimulated with 250 mM trehalose.