In this commentary, Anupama Dahanukar and Chun Han discuss Guntur et al. (2016) “H2O2-sensitive isoforms of Drosophila TRPA1 act in bitter-sensing gustatory neurons to promote avoidance of UV during egg laying,” which is published in this issue of GENETICS. The study reports a new function for bitter taste neurons in TRPA1-mediated sensing of bright UV and its avoidance during egg laying.
SEEING the light is of obvious importance to an animal’s life. Besides allowing visual perception of the surrounding world, animals have other light-sensing needs, for example, to regulate circadian rhythms (Yau and Hardie 2009) and to avoid harmful light. Light in the ultraviolet (UV) spectrum can cause damage to biological tissues. Therefore, it is not surprising that animals have evolved creative ways to detect and avoid bright UV light. In an article in this issue of GENETICS, Chung-Hui Yang’s group reports dTrpA1-mediated detection and avoidance of UV light for egg laying (Guntur et al. 2016). Previous studies present some information about UV sensing. For example, the nematode Caenorhabditis elegans does not have specialized light-sensing organs, but a subset of neurons located on the body can respond to UV light and elicit escape behaviors (Edwards et al. 2008; Ward et al. 2008). This ability presumably keeps the worm in the soil and prevents it from being injured by high doses of sunlight. Similar nonocular photoreceptors turn out to be important for UV sensing in insects as well. The larva of the vinegar fly Drosophila melanogaster has eye-like light-sensing structures called Bolwig organs near the head. While Bolwig organs are tuned to detecting relatively low level of light, bright UV light such as in direct sunlight is sensed by a group of peripheral somatosensory neurons located on the body wall (Xiang et al. 2010). These neurons function as nociceptors and their dendrites completely cover the body wall. Exposure of any part of the larval body to UV light will thus activate these neurons and trigger photoavoidance behaviors. Interestingly, unlike photoreceptors in dedicated light-sensing organs, the nonocular UV-sensitive neurons of both Drosophila larvae and C. elegans do not express rhodopsin proteins but rather rely on two closely related gustatory receptors (GRs) for light detection (Edwards et al. 2008; Liu et al. 2010; Xiang et al. 2010). How exactly UV light activates these neurons is still unknown.
Work conducted in Drosophila larvae also revealed the involvement of an ion channel TRPA1 in sensing UV (Xiang et al. 2010). TRPA1 belongs to the transient receptor potential (TRP) ion channel family and plays conserved roles in animal sensory functions (Julius 2013). A striking feature of TRPA1 is that it is a polymodal receptor and can respond to diverse physiological inputs. The best characterized sensory cues of TRPA1 are noxious chemicals and temperature. For example, TRPA1 is activated by environment irritants, such as acrolein and formalin (McNamara et al. 2007), and by allyl isothiocyanate, the substance in mustard that gives rise to pungent sensations (Bandell et al. 2004; Jordt et al. 2004). These compounds activate TRPA1 by covalently modifying its cysteine residues. TRPA1 in diverse animal species can also be activated by heat (Julius 2013). How does the same channel distinguish different types of stimuli? At least in Drosophila, this is achieved through alternative exon usage (Kang et al. 2012). Chemosensory and thermosensitive TRPA1 channels arise from distinct isoforms that contain different N-terminal sequences. The two isoforms are expressed in different sets of sensory neurons and mediate distinct behavioral responses. An additional type of chemical stimulus for TRPA1 is reactive oxygen species (ROS) (Andersson et al. 2008). Similar to other chemical irritants, ROS activates TRPA1 by oxidizing the thiol groups of cysteine residues. Recently, Chung-Hui Yang’s group found an interesting link between ROS action on TRPA1 and UV sensing (Guntur et al. 2015). It is well known that UV light stimulates ROS production in cells. Yang and colleagues found that Drosophila TRPA1 (dTrpA1) can indirectly detect UV light via sensing of UV-induced ROS. When ROS-sensitive dTrpA1 isoforms were expressed in motor neurons of adult flies, these neurons acquired the ability to react to UV light. But the physiological role of UV sensing by dTrpA1 remained unanswered.
Previously, Chung-Hui Yang’s group had found that UV avoidance emerges in Drosophila females that are in an egg-laying state (Zhu et al. 2014). In an article in this issue of GENETICS, Yang and colleagues take advantage of the same behavioral paradigm to probe the functional relevance of TRPA1-mediated UV sensing and avoidance in more detail (Guntur et al. 2016). Given that ocular UV sensors have already been implicated in behavioral responses to UV, the authors first establish that blind females retain the ability to avoid high UV that is still within the range of natural sunlight. Presented with a choice between laying eggs on a dark side or a UV-illuminated side of a chamber, control females consistently choose the dark side. The choice is not as lopsided in blind females, but they are nevertheless able to avoid UV to a significant extent. The authors then proceed to investigate the receptors and neurons that account for this avoidance. Building on their previous work, they use an arsenal of molecular genetic tools to ascertain where UV-sensitive dTrpA1 is expressed and whether or not it is required for cellular and behavioral responses to high UV. Analysis of an isoform-specific GAL4 driver coupled with RT-PCR analysis maps UV-sensitive dTrpA1 isoforms to a population of gustatory receptor neurons (GRNs) in the proboscis. These neurons, which have acquired the moniker of “bitter” taste neurons, are characterized by expression of Gr66a+ and are activated by a wide range of tastants including not only canonical bitter substances (Marella et al. 2006; Weiss et al. 2011), but also immunogenic signatures of pathogens (lipopolysaccharides) (Yanagawa et al. 2014; Soldano et al. 2016), pheromones (Lacaille et al. 2007; Miyamoto and Amrein 2008; Moon et al. 2009), and irritants sensed by dTrpA1 (Kang et al. 2010), all of which elicit rejection or avoidance behaviors in some way. The accompanying paper defines yet another capability for Gr66a+ bitter neurons as UV sensors, by showing that they are activated by UV in a fashion that depends on the presence of dTrpA1 and the accumulation of UV-induced ROS. UV sensitivity is lost in dTrpA1 mutants and in flies expressing dTrpA1-RNAi in Gr66a+ neurons. UV sensitivity is also lost in flies overexpressing catalase, an enzyme that degrades the ROS H2O2, in Gr66a+ neurons.
Next is the question of which among the large population of bitter GRNs is in fact important for egg-laying avoidance in high UV. Bitter GRNs from different taste organs have distinct representations in the subesophageal zone (SEZ), the primary taste center in the central nervous system (Thorne et al. 2004; Wang et al. 2004). This observation raises the possibility that taste input originating in different taste organs may trigger distinct behavioral outcomes. Although absolute verification of this model awaits further experimentation, evidence of diverse behavioral roles for bitter GRNs in feeding aversion, aggression, courtship inhibition, positional avoidance, and egg-laying site selection (Marella et al. 2006; Miyamoto and Amrein 2008; Koganezawa et al. 2010; Wang et al. 2011; Weiss et al. 2011; Joseph and Heberlein 2012; Charlu et al. 2013) invite the question of whether all bitter circuits can drive each of these behaviors, or whether different circuits are wired to activate different behavioral programs. Prior work has established the behavior of a gravid female fly as she is sampling and selecting a site to lay eggs as one good model for addressing just such questions (Joseph and Heberlein 2012; Yang et al. 2015). The current study reports that blind females that have their proboscis removed surgically are no longer capable of avoiding UV in the same “UV versus dark” egg-laying assays. Genetic silencing experiments with two different GAL4 drivers whose only overlap occurs in Gr66a+ neurons of the proboscis provide further support for the idea that neurons located in this organ are responsible for the observed behavior. Definitive confirmation comes from optogenetic activation of bitter neurons in the proboscis, which was achieved by labeling only the cells that express both dTrpA1-GAL4 and Gr66a-LexA with red-light-sensitive channelrhodopsin CsChrimson. As predicted, the resulting flies avoid laying eggs in red light. An obvious caveat is that the experiment relies on transgenic reporters, thus the possibility that others among the broad population of dTrpA1 cells might play overlapping or redundant roles cannot entirely be ruled out.
The overall picture that emerges from this and previous work in the Yang laboratory is that UV avoidance, which arises in egg-laying females, relies on both ocular and gustatory sensors. R7 photoreceptors, expressing Rh3 and Rh4 UV-sensitive rhodopsins, play a significant role in the eye (Zhu et al. 2014). Bitter GRNs in the proboscis, expressing UV-sensitive dTrpA1, do so in the taste system (Guntur et al. 2016). Several recent findings suggest that bitter GRNs function as polymodal sensory neurons whose activation triggers avoidance to multiple aversive stimuli (Kim et al. 2010; Weiss et al. 2011; Du et al. 2015, 2016; Soldano et al. 2016), similar to the polymodal UV-sensitive nociceptive neurons in larvae (Hwang et al. 2007; Xiang et al. 2010). How then does the gustatory sensor coordinate with the visual sensors in controlling behavioral responses to UV? First, the functional overlap in UV sensitivity between the ocular and extraocular sensors occurs in the range of high UV, creating redundant systems that may prevent even minimal exposure or egg laying in conditions that would be harmful to developing eggs and larvae. Second, ocular UV response appears to be modulated by egg-laying demand—virgin females exhibit phototactic behavior to UV rather than positional avoidance. By contrast, dTrpA1-mediated activation of bitter GRNs in response to UV is likely to trigger avoidance regardless of egg-laying state. This idea is borne out by the findings of an independent study that reported dTrpA1-dependent feeding deterrence in bright light (Du et al. 2016), and consistent with the observation that UV-sensitive dTrpA1 is also expressed in bitter GRNs in male flies. Interestingly, bitter tastants tested in similar egg-laying assays are either selected or disfavored depending on the nature of the alternative that is presented (Yang et al. 2008). With the advances reported in the current study, there is an opportunity to dissect how light is integrated with other cues to regulate positional avoidance and egg-laying behaviors in various contexts.
Footnotes
Communicating editor: M. F. Wolfner
- Received October 20, 2016.
- Accepted November 20, 2016.
- Copyright © 2017 by the Genetics Society of America
