Acid taste, evoked mainly by protons (H(+)), is a core taste modality for many organisms. The hedonic valence of acid taste is bidirectional: animals prefer slightly but avoid highly acidic foods. However, how animals discriminate low from high acidity remains poorly understood. To explore the taste perception of acid, the fruit fly was used as a model organism. Flies employ two competing taste sensory pathways to detect low and high acidity, and the relative degree of activation of each determines either attractive or aversive responses. Moreover, one member of the fly Otopetrin family, Otopetrin-like a (OtopLa), was established as a proton channel dedicated to the gustatory detection of acid. OtopLa defines a unique subset of gustatory receptor neurons and is selectively required for attractive rather than aversive taste responses. Loss of otopla causes flies to reject normally attractive low-acid foods. Therefore, the identification of OtopLa as a low-acid sensor firmly supports a competition model of acid taste sensation. Altogether, this study has discovered a binary acid-sensing mechanism that may be evolutionarily conserved between insects and mammals (Mi, 2021).

Sour taste, like sweet, bitter, salty, and umami tastes, represents a fundamental taste modality across many species ranging from insects to mammals. Typically, humans like slightly acidic foods such as lemon juice, which potentially indicates the presence of nutrients. In contrast, humans dislike highly acidic foods, which can cause digestive tract tissue injuries. The bivalent taste response to acid is also documented in rodents. Similar to mammals, the fruit fly, Drosophila melanogaster, prefers low levels of acid, which stimulate feeding and reproduction, and avoids high acid concentrations. Therefore, although flies and humans appear drastically different, the hedonic valence of their acid-taste response is similar: it can be either attractive or aversive, depending on the acid concentration of food. It is proposed that the bidirectional characteristic of acid perception constitutes an evolutionary fitness that enables animals to choose nutritious and reject unhealthy food sources. How do animals make this seemingly challenging decision? It is hypothesized that a taste-coding mechanism underlies the opposing feeding behavior triggered by low and high levels of acids. However, the molecular and cellular nature of the acid-taste coding has remained unclear (Mi, 2021).

Several lines of research demonstrate that type III taste receptor cells (TRCs) are responsible for acid sensing in mice. Nevertheless, the type III TRC population may be heterogeneous and contain different cell subtypes. Due to the lack of molecular markers and genetic tools to manipulate different subsets of type III TRCs, the question of how type III TRCs differentially respond to different concentrations of acid appears to be difficult to address in mammals. Moreover, other than eliciting taste sensation, acid also activates trigeminal nerves in the oral cavity of mammals, leading to burning or pain sensation. This side effect further confounds the investigation of sour-taste coding and sour-taste-triggered behavior in mammals. In contrast, flies exhibit much more pronounced and distinct taste responses to varying concentrations of acid than do mammals. Therefore, the fly serves as an excellent animal model to elucidate the taste coding of acid. This study reports that the fly mainly uses two different subsets of gustatory receptor neurons (GRNs) to selectively sense low or high concentrations of acids. The taste transduction pathways orchestrated by low- and high-acid GRNs antagonize each other, and the net behavioral response to a particular concentration of acid is predominantly determined by the relative activities of low- vs. high-acid GRNs (Mi, 2021).

Animals take advantage of highly diversified taste receptors and TRCs to detect varying taste substances, including sugar, salt, acid, and bitter compounds. In mammals, in contrast to the well-characterized sweet and bitter receptors, the molecular identity of sour-taste receptor had not been determined until a recent discovery showing that the Otopetrin (Otop) protein family functions as proton channels. In mice, one of the Otop family members, Otop1, is essential for sour-taste transduction. Despite this significant finding, the exact role played by Otop1 in discriminating low- from high-acid foods remained unclear. As the Otop family is fairly conserved between mammals and insects, it was of interest to see if the Otop family is also required for taste sensation of acids in Drosophila, given that no bona fide sour-taste receptors had been established in insects. One of the fly Otop orthologues, Otopetrin-like a (OtopLa), was shown to act as a proton channel and is selectively required for attractive taste sensation of acids in Drosophila. The fly OtopLa protein is localized at the tip of the GRN dendrite, the forefront site of taste sensory cells that is responsible for directly sensing tastant stimuli. Further, OtopLa defines a novel class of GRNs, which are largely distinct from other groups of GRNs responding to sugar, salt, or bitter tastants. Furthermore, genetic analysis showed that loss of otopetrin-like a (otopla) selectively abolishes the attractive acid-taste pathway, leaving the aversive pathway intact. Notably, the otopla mutant flies became abnormally averse to low concentrations of acid. Thus, this study provides strong genetic evidence to establish not only that the attractive and aversive taste pathways responsible for acid sensation exist but also that they are genetically segregated. Finally, by establishing OtopLa as a bona fide taste receptor for acid in flies, this work overturns the long-standing view that insects and mammals use fundamentally different gustatory receptors (Mi, 2021).

According to behavioral assays, the wild-type fly displays opposing taste responses to low and high concentrations of acid: low concentrations are attractive, whereas high concentrations are aversive. Thus, the hedonic valence of acid taste is closely associated with the concentration of acid that the animals detect. The bidirectional valence of sour taste is reminiscent of salty taste, which is also dependent on salt concentrations. Given these findings, a key question arises as to how the animals discern low- from high-acid foods. Rlectrophysiology analyses of different groups of taste sensilla provide an important clue to this question. Flies were found to mainly use L- and S-type sensilla to perceive low and high concentrations of acid, respectively. The L-type sensilla mediate an attractive pathway, whereas the S-type sensilla operate an aversive pathway for the response to acids. It is postulated that acid-taste signals relayed by low- and high-acid GRNs antagonize each other in the brain, where feeding decisions are made. When the fly encounters low-acid foods, the attractive pathway mediated by low-acid GRNs dominates the aversive pathway mediated by the high-acid GRNs, driving the animal to choose the low-acid food. Conversely, when the animal encounters high-acid foods, the aversive pathway dominates the attractive pathway, leading to the avoidance of high-acid foods (Mi, 2021).

In support of this hypothesis, genetic analysis reveals that otopla is specifically required for the attractive rather than aversive acid-taste response. Further, it was found that otopla is mostly expressed in the GRNs housed within the L-type rather than the S-type sensilla. In addition, recent work shows that a member of the ionotropic receptor (IR) family, Ir7a, is selectively required for the repulsive response to high concentrations of acetic acid in Drosophila. However, flies lacking Ir7a show normal attractive feeding responses to low concentrations of acetic acid. That study, combined with the present study on otopla, provides substantial evidence to support the model that the attractive and aversive pathways for acid sensation are segregated in the peripheral taste organ (Mi, 2021).

These findings lay the foundation for a more detailed analysis of the genetic program and neural circuit involved in sour-taste perception. In mammals, type III TRCs are mainly responsible for sour-taste sensation. Nevertheless, whether distinct subgroups of type III TRCs in the taste bud selectively respond to low or high acid remains an open question. Given the conservation of acid-taste sensation between flies and mammals, the acid-taste coding mechanism identified in the fly will inform the investigation of sour-taste coding in mammals, including humans (Mi, 2021).

Using multiple lines of evidence, these studies lead to the identification of OtopLa as a long-sought taste receptor for acid in Drosophila. First, genetic analyses show that OtopLa is both necessary and sufficient to orchestrate the attractive response to foods containing low concentrations of acid. In addition, loss of otopla has no effect on sweet, bitter, or salty tastes. Second, cell biological studies reveal that OtopLa is expressed in a group of GRNs different from sweet, bitter, and salty GRNs. Moreover, OtopLa proteins selectively reside in the distal portion of the dendrite, the forefront of the GRN responsible for detecting taste substances presented from the food environment. Last but not least, patch-clamp recordings reveal that OtopLa functions as a proton channel and can be directly activated by protons. Collectively, this work establishes OtopLa as a bona fide receptor that is dedicated to the attractive taste sensation of acids in Drosophila. Recent studies in mice show that the Otop1 proton channel is both necessary and sufficient for sour-taste transduction. In light of these discoveries in flies and mice, it is concluded that the Otop family is an evolutionarily conserved proton channel dedicated to taste the sensation of acids in both insects and mammals. Over the past two decades, various types of taste receptors, including sweet, bitter, and salty taste receptors, have been identified and functionally characterized in both invertebrates and vertebrates. Although insects and mammals exhibit a striking homology in taste responses, the molecular identities of their sweet, bitter, and salty taste receptors appear to be distantly related to each other. Consequently, there is a long-held view in the chemoreception field that taste receptors for insects and mammals are evolutionarily distant from each other. In this study, the discovery in the fly acid-taste sensation has overturned this notion. The Otop family represents the first class of taste receptors that is functionally conserved between insects and mammals. From an evolutionary perspective, it is proposed that Otop is a well-conserved proton channel family involved in acid-taste sensation throughout the animal kingdom. Thus, further research is needed to explore the gustatory role of the Otop family in other animal species, including humans (Mi, 2021).

otopla is necessary for the attractive taste sensation of both the strong acid HCl and the weak acids citric acid and malic acid. Psychophysical studies in human subjects report that weak acid usually tastes more sour than strong acid at the same pH40, implying that, in addition to protons, the undissociated weak acid molecules may also elicit sourness. These studies demonstrate that the proton channel OtopLa is broadly required for the taste sensation of both strong and weak acids. Therefore, it is proposed that the taste response to acids orchestrated by OtopLa mainly results from the gustatory stimuli of protons that are dissociated from either strong or weak acids. In addition, several members of the fly IR family are involved in taste responses to carbonation and acetic acid. As there has been no evidence showing that the IRs form a proton channel, the IRs are likely to be narrowly tuned to the specific structures of weak acids rather than to protons. Collectively, the fly may use different taste transduction pathways to perceive various acid molecules present in the environment (Mi, 2021).