Drosophila TRP channels and animal behavior

Abstract

Multiple classes of cell surface receptors and ion channels participate in the detection of changes in environmental stimuli, and thereby influence animal behavior. Among the many classes of ion channels, Transient Receptor Potential (TRP) cation channels are notable in contributing to virtually every sensory modality, and in controlling a daunting array of behaviors. TRP channels appear to be conserved in all metazoan organisms including worms, insects and humans. Flies encode 13 TRPs, most of which are expressed and function in sensory neurons, and impact behaviors ranging from phototaxis to thermotaxis, gravitaxis, the avoidance of noxious tastants and smells and proprioception. Multiple diseases result from defects in TRPs, and flies provide an excellent animal model for dissecting the mechanisms underlying "TRPopathies." Drosophila TRPs also function in the sensation of botanically derived insect repellents, and related TRPs in insect pests are potential targets for the development of improved repellents to combat insect-borne diseases.

Figure 1

Phylogenetic tree of the Drosophila TRP channels. The tree was generated with CLC Sequence Viewer 6.4 using the amino acid sequence of the predicted transmembrane domains of the 13 Drosophila TRP channels.

Figure 2. Predicted TRP structure and the phototransduction that leads to activation of TRP and TRPL

A) Electroretinogram (ERG) recordings from trp⁺ (w¹¹¹⁸) and trp³⁴³ mutant flies (performed by Dr. Zijing Chen). B) Predicted structure of the Drosophila TRP channel. The channel contains six transmembrane domains, with the pore loop located between the 5^th and 6^th transmembrane domains. The N-terminal region contains four ankyrin repeat domains and a coiled coil domain, and the C-terminal tail contains a TRP domain, calmodulin (CaM) binding site, and an INAD binding site. C) Model of Drosophila phototransduction. Light capture by rhodopsin initiates a G^q/PLC signaling cascade that culminates in activation of the TRP and TRPL channels and influx of cations. Abbreviations: 1-MAG, monoacylglycerol; 2-MAG, 2-monoacylglycerol; DAG, diacylglycerol; FA, saturated fatty acid; IP₃, inositol 1,4,5-trisphosphate; PIP₂, phosphatidylinositol 4,5-bisphosphate; MAG, monoacylglycerol; P, pore loop indicated in TRP; PUFA, polyunsaturated fatty acid.

Figure 3. Drosophila sensory organs

A) Sensory organs in an adult fruit fly. Colored circles indicate the spatial distributions of the sensory organs. Abbreviations: ant, antennae; com. eye, compound eye; H-B e, Hofbauer-Buchner (H-B) eyelet; mp, maxillary palp; lab, labellum; oc, ocelli. The H-B eyelet is located internally in the brain between the retina and lamina, but is shown superficially here and panel B. B) Top view of a fly head showing the visual organs. C) Frontal view of a fly head. D) Larval sensory organs. The cartoon is adapted from previous drawings (Gomez-Marin and Louis, 2012). E) Arborization of the class IV multidendritic neurons that tile the body wall of larvae.

Figure 4. Simple two-way choice behavioral assays for testing temperature, olfactory and gustatory discrimination

A) Larval temperature two-way choice assay. Larvae are placed along the midline of an agarose-covered plate, which is placed on two aluminum plates kept at different temperatures by circulating water (e.g. 18°C and 24°C). The number of larvae in each temperature zone is scored after 15 minutes. The preference index is calculated using the formula: PI=(N^18°C – N^24°C)/(N^18°C + N^24°C). B) Direct airborne repellent test (DART). Two 15 ml polystyrene tubes are prepared by placing 5 µl of odorant (citronellal) or the vehicle onto small pieces of Kimwipe at the bottom of each tube. Mesh screens are positioned to prevent the flies from coming in direct contact with the odorant or the vehicle. Approximately 100 flies are tapped into the tubes and the tubes are taped together along the midline. The number of flies in test zones A and B are counted after 30 minutes and the preference index is calculated using the formula: PI=(N^{Zone B} – N^{Zone A})/(N^{Zone B} + N^{Zone A}) C) Two-way taste choice assay. Flies are starved overnight and placed into a microtiter dish containing two tastants (e.g. sugar versus sugar mixed with a bitter tastant) mixed with either red or blue food dye. Flies are allowed to feed for 2 hours before their abdomens are visually inspected for red, blue, or purple color. The preference index is calculated using the formula: PI= (N^Blue + 0.5N^Purple)/(N^Red + N^Blue + N^Purple).