Monell Chemical Senses Center researcher Yali Zhang and colleagues identified a previously unknown chloride ion channel, which they named alkaliphile (Alka), as a taste receptor for alkaline pH.
Alkaliphile is a new taste receptor dedicated to sensing the alkaline pH of food. Image credit: Yali Zhang, Monell Chemical Senses Center.
pH, the scale of how acidic or basic a substance is, plays an essential role for living organisms because many biological processes, such as breaking down food and enzymatic reactions, need the level of pH to be just right.
While we are familiar with sour taste, which is associated with acids and allows us to sense the acidic end of the pH scale, little is known about how animals perceive bases on the opposite end of the pH spectrum.
Detecting both acids and bases, which are commonly present in food sources, is important because they can significantly impact the nutritional properties of what animals consume.
Dr. Zhang and co-authors found that Alka is expressed in the gustatory receptor neurons (GRNs) of the fruit fly (Drosophila melanogaster) — the counterpart of taste receptor cells of mammals.
When facing neutral food versus alkaline food, wild-type flies normally choose neutral foods because of the toxicity of high pH.
In contrast, flies lacking Alka lose the ability to discriminate against alkaline food when presented with it.
If the pH of a food is too high, in humans it can be harmful and cause health concerns such as muscle spasms, nausea, and numbness.
Likewise, after fruit flies eat food with high pH, their lifespan can be shortened.
The authors demonstrated that Alka is critical for flies to stay away from harmful alkaline environments.
“Detecting the alkaline pH of food is an advantageous adaptation that helps animals avoid consuming toxic substances,” Dr. Zhang said.
To understand how Alka senses high pH, the team performed electrophysiological analyses and found that Alka forms a chloride ion (Cl–) channel that is directly activated by hydroxide ions (OH–).
Like olfactory sensory neurons in mammals, the concentration of Cl– inside the fly’s GRN is typically higher than outside this nerve cell.
The researchers propose that when exposed to high-pH stimuli, the Alka channel opens, leading to negatively charged Cl– flowing from inside to outside the fly’s GRN.
This flow of Cl– activates the GRN, ultimately signaling to the fly brain that the food is alkaline and should be avoided.
“Our work shows that Cl– and Cl– channels, which have been overlooked for a long time, have crucial functions in taste signaling to the brain,” Dr. Zhang said.
The scientists also studied how flies detect the taste of alkaline substances using light-based optogenetic tools.
They found that when they turned off alkaline GRNs, the flies were no longer bothered by the taste of alkaline food.
Conversely, they activated these alkaline GRNs by shining red light on them.
Interestingly, when these flies were given sweet food and exposed to red light at the same time, the flies did not want to eat the sweet food anymore.
“Alkaline taste can make a big impact on what flies choose to eat,” Dr. Zhang said.
The study appears today in the journal Nature Metabolism.
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Yali Zhang et al. Alka: new taste receptor dedicated to sensing the alkaline pH of food. Nature Metabolism, in press
