Drosophila TRPA1 controls thermotactic behavior

Abstract

Temperature perception is an intricate process, essential for survival of many organisms. Temperatures far outside of the preferred range are usually harmful and are perceived as noxious (or painful), thus encouraging the animal to change its location or behavior. However, animals also have mechanisms to perceive more moderate, innocuous temperatures. Some animals exhibit clear directed movements in response to changes in temperature, a behavior known as thermotaxis. Thermotactic behavior has been most intensely studied in the nematode C. elegans, and a number of neurons and molecules that mediate thermotaxis in C. elegans have been identified. However, molecular mechanisms of temperature perception in general, and thermotaxis in particular, are still poorly understood. Drosophila melanogaster also exhibits strong temperature preferences and robust thermotactic behaviors, although the molecular mechanisms that mediate thermotaxis in Drosophila were unknown. We used a reverse genetic RNA interference (RNAi)-based strategy to identify Drosophila TRPA1 as a key regulator of thermotaxis. Larvae in which dTRPA1 expression has been knocked down with RNAi fail to avoid regions of moderately elevated temperature (~310C--350C).

(cont.) In heterologous cells, dTRPA1 protein is activated by warming (Viswanath, et al., 2003, Nature, 423:6942), suggesting that the role dTRPA1 plays in thermotaxis might be to sense the environmental temperature. We generated mutants in dTRPA1 using homologous recombination-mediated insertional mutagenesis. As expected, dtrpAl mutants were defective for thermotaxis at elevated temperatures, and surprisingly were also defective for thermotaxis at cold temperatures. Interestingly, dtrpAl mutants were not defective for avoidance of all cold temperatures, but the thermotactic defects of dtrpAl mutants were more pronounced at colder temperatures than at more moderate temperatures, suggesting that other mechanisms might compensate for the loss of dTRPA1 at more moderate temperatures. Temperature sensors were traditionally thought to be activated by and to mediate behavioral responses to restricted ranges of temperatures, either hot or cold temperatures, but not both. However, dTRPA1 appears to be required for thermotaxis behavior at both elevated and cold temperatures, raising an exciting possibility that dTRPA1 protein might mediate thermotaxis by serving as a sensor of both hot and cold temperatures.

(cont.) We also identified dTRPAl-expressing cells and implicated a subset of these cells in mediating thermotactic response to elevated temperatures. Surprisingly, our results suggest that the cells required for thermotaxis at elevated temperatures (dtrpAl-promoter-Gal4-expressing cells) are located in the larval central brain and not in the peripheral nervous system (PNS) where most thermal sensors are thought to function for mediating thermosensory behaviors. We further demonstrated that neither PAINLESS, a TRP channel required for proper larval responses to a high-temperature (38C--52C) nociceptive stimulus, nor the peripherally located neurons that mediatepainless behavior, were required for thermotaxis at elevated temperatures. On the other hand, dTRPA1 and the dtrpAl-promoter-Gal4-expressing central brain neurons that mediate thermotactic response to elevated temperatures appeared dispensable for larval responses to a high-temperature nociceptive stimulus. These findings suggest that different TRP channels and different neurons are used to mediate different thermosensory behaviors in Drosophila.