Have you ever wondered how animals decide what to eat? The sense of taste plays a crucial role in this decision-making process. It allows animals to sample small amounts of food before consuming larger quantities, acting as a checkpoint to ensure that they consume nourishing food and avoid dangerous options. Many animals, ranging from insects to humans, possess taste receptors that discriminate between different taste attributes.
pH is a measure of the acidity or alkalinity of a substance and plays a crucial role in the functioning of living organisms, as many important biological processes only occur at particular pH levels. Acids and bases are widely present in food sources in the natural ecosystem, and extreme pH can be detrimental to the health of an organism. For example, when fruit flies are fed diets with high pH levels, both their growth rate and lifespan decrease. Since the perception of sour taste is commonly associated with acids, it raises the question as to whether bases, or substances with a high pH value, also stimulate distinctive taste responses. Previous work in insects, cats, and humans suggests the existence of alkaline taste. Nevertheless, the molecular identities of taste receptors and taste receptor cells responsible for detecting alkaline pH were unknown. We hypothesize that animals avoid alkaline food using particular taste receptors that are responsible for sensing high pH.
To identify the potential taste receptor responsible for high pH detection, we used the fruit fly, Drosophila melanogaster, as a model organism. We chose the fruit fly because of its extensively documented taste sensations and feeding behaviors, relatively simple genetic structure, easily accessible genetic tools, and fast reproduction rate. Wild-type flies normally prefer neutral food while avoiding alkaline food. To search for the high-pH sensors, we screened a wide array of potential taste receptors in flies using two-way feeding assays to test flies’ preferences for neutral versus alkaline foods. Of great interest, we identified a mutant, alka, which exhibits an unusual preference for alkaline food.
Using gene editing, we generated a mutant lacking alka and found that these mutant flies displayed profound defects in alkaline taste avoidance. The alka gene is expressed in a subset of gustatory receptor neurons (GRNs) in the fly labellum, which is analogous to the mammalian tongue (Fig. 1a). Electrophysiological analyses demonstrate that loss of alka leads to a significant defect in the neuronal responses of GRNs evoked by alkaline pH. Our patch-clamp recordings of Alka in cell lines reveal that Alka is a high-pH-activated chloride (Cl-) channel. We propose that the Alka channel enables the flow of negatively charged Cl- ions from inside to outside to activate the GRNs in response to high- stimuli.
This is an unusual phenomenon because in mature neurons of the brain, the concentration of Cl- outside the cells is typically higher than in the cytosol. However, like mammalian olfactory sensory neurons, fly GRNs have an inverse distribution of their Cl- gradient – the Cl- levels inside the GRNs are higher than those outside. As a result of the activation of Alka by hydroxide (OH-), Cl- ions flow through the Alka channel from the interior to the exterior, causing depolarization and activation of the GRNs (Fig. 1b). This mechanism enables flies to sense the taste of alkaline substances.
We also investigated the function of alka-expressing GRNs in the fly tongue by manipulating their neuronal activities using neurogenetic and optogenetic tools. When their alkaline GRNs are inactivated, flies lose sensitivity to the noxious stimuli of alkaline pH. On the other hand, we employed an optogenetic approach to activate these GRNs using red lights. To do this, we created transgenic flies expressing a red-light activated channelrhodopsin in alkaline GRNs. We exposed them to red lights, thereby allowing us to mimic the alkaline-pH stimulation. When no red light is present, the transgenic fly stretches out its tongue to consume a drop of sucrose (Fig. 2a). In contrast, when the same fly is exposed to red lights and sucrose simultaneously, the animal’s high-pH aversion overrides its preference for sweet food, leading to significantly diminished attraction to sucrose (Fig. 2b). These results demonstrate that activation of alka-expressing GRNs can cause the flies to reject food that they would typically consume.
To summarize, our work establishes that Alka is a taste receptor for alkaline food, which resolves a long-standing issue regarding how animals perceive alkaline pH in food. As detecting alkaline pH is essential for maintaining pH balance, our findings in flies could provide valuable insights into the mechanism of alkaline taste sensation in other animals.