InteractiveFly: GeneBrief

alkaliphile: Biological Overview | References

The sense of taste is an important sentinel governing what should or should not be ingested by an animal, with high pH sensation playing a critical role in food selection. This study explored the molecular identities of taste receptors detecting the basic pH of food using Drosophila melanogaster as a model. A chloride channel has been identified, named alkaliphile (Alka), that is both necessary and sufficient for aversive taste responses to basic food. Alka forms a high-pH-gated chloride channel and is specifically expressed in a subset of gustatory receptor neurons (GRNs). Optogenetic activation of alka-expressing GRNs is sufficient to suppress attractive feeding responses to sucrose. Conversely, inactivation of these GRNs causes severe impairments in the aversion to high pH. Altogether, this discovery of Alka as an alkaline taste receptor lays the groundwork for future research on alkaline taste sensation in other animals (Mi, 2023).

The sense of taste, which lies at the interface between the interior and the exterior of the body, ensures that food of nutritional value is consumed, whereas potentially noxious substances are rejected. Acids and bases are opposite chemical substances that are broadly present in food sources. While it is generally accepted that animals use sour taste to assess the acidity, or low pH, of food, whether animals have a taste modality to sense the basicity, or high pH, of food is a long-standing open question. Given that acid, or low pH, has a sour taste, it would be logical to hypothesize that base, or high pH, also produces a gustatory sensation (Mi, 2023).

Previous studies in humans and animal models provide initial clues to the existence of alkaline taste sensations. In the 1940s, psychophysical studies conducted on human participants reported that the tip portion of the tongue, which is enriched with taste buds, exhibits a higher sensitivity to sodium hydroxide (NaOH) than the mid-dorsal part of the tongue with few taste buds, implying that basic solutions may have taste qualities. Furthermore, electrophysiological recordings of the taste nerves in cats document that a subpopulation of the chorda tympani nerves, which relay taste input from the oral cavity to the brain, can be activated by high pH but not by other stimuli such as temperature, indicating that cats interpret the stimulation by high-pH solutions as a sense of taste rather than as an irritating noxious chemical. Moreover, insects such as beetles show robust avoidance of alkaline environments associated with unfavorable habitats and food sources. The beetle's high-pH sensitivity is mediated by pH receptor cells in the beetle's taste organ, which display increased firing activities proportional to basic pH10. Collectively, these earlier studies imply but do not resolve whether the high-pH sensation is a discrete taste modality. Since these early studies, little mechanistic research had been conducted to unravel the molecular and cellular underpinnings of alkaline taste sensation. In particular, the molecular identities of taste receptors and taste receptor cells orchestrating alkaline taste sensation had not yet been established in animals (Mi, 2023).

Like mammals, the fruit fly, Drosophila melanogaster, employs different classes of taste receptors to detect sugars, salts, acids , bitter substances and other chemicals. Given that flies have such a remarkable capability to detect a wide range of substances through taste, it was inferred that flies are also able to sense the alkalinity of food. Indeed, a vital fly gene dubbed alkaliphile (alka) was discovered that regulates gustatory responses to strong alkalinity. Molecular genetic studies revealed that alka is both necessary and sufficient to avoid highly basic food. Extensive electrophysiological assays were performed, and it was discovered that the Alka protein forms a chloride (Cl-) channel, which is specifically activated by hydroxide (OH-). Moreover, this study found that alka is expressed in a subset of gustatory receptor neurons (GRNs) in the peripheral taste organ. At the sensory cell level, alka-expressing GRNs are both necessary and sufficient for the rejection of strongly alkaline food. In summary, this work establishes Alka as the long-sought-after taste receptor responsible for sensing the basic pH of food (Mi, 2023).

As most organisms' optimal physiological activities and enzymatic reactions can occur only in a narrow pH range (around 7.4), excessively high pH can disrupt the acid-base balance and lead to alkalosis of the body, a life-threatening condition. There are many places where organisms can encounter high-pH conditions in their ecosystem such as in the food and water they may consume. Moreover, many naturally occurring toxins, including alkaloids and aqueous ammonia, are quite basic. Ethological research has documented well-defined behavioral responses to basic pH in a large variety of species, such as nematodes, insects, fish and mammals (Mi, 2023).

The impact of alkaline pH on fly physiology has been well documented across a variety of studies. It has been reported that the fly's overall body pH exhibits a dynamic change over the course of development: the body pH starts as approximately neutral at the larval stage and gradually becomes more acidic as the animal advances to the pupal and adult stages. In addition, there is remarkable variation in the luminal pH at different regions of the fly midgut, with its posterior segment more alkaline. As a result, alkaline pH sensation is strongly implicated in fly health and longevity. Flies fed moderately alkaline diets display reduced lifespan and survivability. Furthermore, chronic exposure to a highly alkaline environment impairs development, shortens lifespan and causes lethality. Consequently, female flies robustly avoid alkaline substrates when selecting a location to deposit eggs. Taken together, alkaline pH sensation serves as an essential self-protection strategy that enables flies and other animal species to effectively avoid toxic environments during food foraging and habitat selection. It is proposed that alkaline taste dramatically increases the fly's evolutionary fitness by enhancing its survival, growth and reproduction (Mi, 2023).

Alkaloids taste very bitter and are poorly soluble, meaning their strong bitterness can confound the investigation of the taste component solely contributed by high pH. In addition, ammonia is highly volatile and can interfere with the contact-dependent taste sensation by strongly activating the olfactory system. To avoid these limitations, NaOH and Na2CO3 were chosen in feeding assays because these two basic substances are simple and ecologically relevant. Molecular genetic study demonstrates that flies have the capability to avoid highly basic food mainly through their gustatory system. Based on these findings, the fly shows specific and robust taste responses to basic food, suggesting that it is a well-suited model organism to explore alkaline taste sensation. Several lines of evidence are provided supporting that Alka is a taste sensor specifically tuned to high pH. First, alka is expressed in a subset of GRNs in the fly labellum, which functions similarly to the mammalian tongue. The alka-expressing GRNs are both necessary and sufficient to detect basic food. Second, alka¹ mutant flies show remarkably decreased aversion to basic food. Third, using in vivo electrophysiological analyses, we found that the S-type sensilla of alka¹ flies exhibit considerably reduced responses to high pH but maintain normal responses to bitter compounds such as caffeine. Last, misexpressing alka in sweet GRNs attracts the flies to the otherwise aversive basic food. In summary, this work establishes that Alka is a bona fide taste receptor dedicated to sensing food basicity in Drosophila (Mi, 2023).

Over the past 20 years, the fly model has made enormous contributions to the discovery of various classes of ionotropic taste receptors. These include GRs, IRs, otopetrin (Otop) channels, TRP channels and the degenerin/epithelial Na+ channels (DEG/ENaC) or pickpocket (Ppk) channels. Notably, Alka shows remarkable differences from previously identified taste receptors because it is an anion Cl- channel, whereas the IRs, Otops, TRPs and Ppks are cation channels. Therefore, it is believed that identification of the Cl- channel Alka as an alkaline taste receptor is substantial and innovative, adding another class of receptors to the diverse taste receptor repertoire in Drosophila (Mi, 2023).

The combination of optogenetic and intersectional genetic approaches enables selective manipulation of the activity of alka-expressing GRNs at will. Acutely activating alka-expressing GRNs alone is sufficient to trigger aversive taste responses, supporting the conclusion that alka-expressing GRNs are both necessary and sufficient for alkaline taste sensation. Like bitter taste, alkaline taste can suppress sweet taste responses. Although the neuronal mechanism underlying this cross-modal inhibition is currently not clear, our assay serves as a robust behavioral paradigm allowing us to screen for the higher-order neurons mediating the integration between alkaline taste and sweet taste (Mi, 2023).

The membrane potential of a living cell is established and maintained by the flows of different cation and anion species, such as Na+, K+ and Cl-, across the plasma membrane. While extensive studies have focused on the roles played by cations and cation channels in the regulation of membrane excitability, the functional importance of the anion Cl- and Cl- channels had been overlooked. In recent years, Cl- has emerged as an essential player in electrical signal transduction and great progress has been made toward the molecular identification of various types of Cl-channels such as the acid-sensitive Cl- channel PAC/TMEM206. The curren work demonstrates that Alka functions as a distinct Cl- channel because it is strongly activated by highly basic pH (11.9). In contrast to other LGCCs such as pHCl44, Alka is insensitive to slightly basic or acidic pH. Thus, Alka is well suited to act as an external sensor to detect high pH in the natural ecosystem, which can become noxiously basic due to excessive carbonation or nitration (Mi, 2023).

Protein sequence analyses suggest that Alka is distantly related to glycine-gated Cl- channels in vertebrates. Nevertheless, this study found that Alka functions as a taste receptor rather than acting as a glycine or GABA receptor, as Alka is not activated by physiological concentrations of glycine or GABA. In support of this notion, Alka is selectively expressed in the chemosensory organs. The functional divergence of Alka from canonical GlyRs is reminiscent of the evolution of the ionotropic (IR) family in Drosophila, which adopted chemosensory functions as olfactory or gustatory receptors in the periphery rather than as glutamate neurotransmitter receptors in the brain. In summary, by discovering that Alka is a high-pH-activated Cl- channel in taste receptor cells, this work provides important insights into the highly diversified nature of Cl- channels in terms of gating and function (Mi, 2023).

Upon further interrogation of the channel properties of Alka using patch-clamp experiments, it is concluded that Alka mainly conducts Cl-. This creates a logical quandary, as mature neurons in the brain usually experience an intracellular-facing Cl- gradient and the Cl- influx in mature neurons is hyperpolarizing instead of depolarizing; however, the answer lies in the unusual distribution of the Cl- gradient across the fly GRN. From the perspective of comparative physiology, the extracellular receptor lymph of the fly gustatory receptor neuron (GRN) is analogous to the saliva bathing taste receptor cells in humans, which contains lower levels of Cl- than that of blood plasma. Furthermore, like mammalian olfactory sensory neurons and airway epithelial cells, the Cl- concentration of the extracellular receptor lymph seems to be even lower than in the cytosol of the taste receptor neuron in flies. Thus, Alka perfectly fits the unusual ionic milieu of fly GRNs. Based on this model, in response to OH- stimuli, Alka is induced to adopt an open conformation, which leads to the flow of intracellular Cl- out of the GRN. The Cl- efflux depolarizes the GRN and results in the production of action potentials, thereby enabling the animals to sense alkaline food. In summary, this work highlights the important roles played by Cl- and Cl- channels in regulating taste transduction (Mi, 2023).

In conclusion, it is proposed that Alka, which forms a previously unknown hydroxide-gated Cl- channel, is a taste receptor responsible for detecting alkaline food. As far as is known, Alka represents a notable Cl- channel identified in the animal kingdom, which is dedicated to sensing highly basic pH. Furthermore, by showing that basic pH has a discrete taste in Drosophila, this work resolves a long-standing debate as to whether alkaline taste really exists. Moreover, this study has demonstrated that Alka functions as a Cl- channel that is potently activated by highly alkaline pH. Therefore, this molecular discovery of Alka as a taste receptor of alkaline pH advances understanding of the roles and modes of activation of Cl- channels. Finally, given that detecting basic pH is crucial for food selection across many different species, this work provides the conceptual basis for investigating the neuronal mechanisms underlying alkaline taste sensations in other animals (Mi, 2023).

Search PubMed for articles about Drosophila Alkaliphile

Mi, T., Mack, J. O., Koolmees, W., Lyon, Q., Yochimowitz, L., Teng, Z. Q., Jiang, P., Montell, C. and Zhang, Y. V. (2023). Alkaline taste sensation through the alkaliphile chloride channel in Drosophila. Nat Metab 5(3): 466-480. PubMed ID: 36941450

date revised: 11 August 2023

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