How some animals experience the grainy texture of their food – sciencedaily

UC Santa Barbara professor Craig Montell and postdoctoral fellow Qiaoran Li published a study in Current biology providing the first description of how some animals experience the texture of their food based on graininess versus sweetness. They discovered that, in fruit flies, a mechanosensory channel relays this information about the texture of a food.

The channel, called TMEM63, is one of a class of receptors that appear in organisms, from plants to humans. As a result, the new findings could help shed light on some of the nuances of our own sense of taste.

“We all appreciate that the texture of food has an impact on the attractiveness of food,” said Montell, Duggan and professor emeritus in the Department of Molecular, Cellular and Developmental Biology. “But it’s something we don’t quite understand.”

Li and Montell focused on fruit flies to study the molecular and cellular mechanisms behind the effect of graininess on the palatability of foods. “We have found that they, like us, have food preferences that are influenced by this characteristic of texture,” said Montell. They devised a taste test in which they added small particles of varying sizes to sugary foods, finding that flies preferred particles of a specific size.

In previous work, Montell and his group elucidated the nuances involved in the sense of taste. In 2016, they found a channel that allows flies to feel the hardness and viscosity of their food by the movement of the tiny hairs on their tongue, or lip. More recently, they have discovered the mechanisms by which cool temperatures reduce palatability.

Now, researchers have sought to identify a receptor needed to detect graininess. They figured it would be a mechanically activated channel triggered when particles bend the hairs slightly to the taste of a fly. However, inactivation of known receptors had no impact on food preference based on sweetness and granulation.

The authors then considered the TMEM63 protein. “Fly TMEM63 is one of a class of mechanical sensors conserved from plants to humans,” Montell said, “but its roles in animals were unknown.”

With only the suspicion that he might relay information on graininess, Li and Montell knocked out the gene that codes for TMEM63 and compared the behavior of mutant flies with wild-type animals.

After withholding the animals’ food for a few hours, the researchers measured their interest in various sugar solutions mixed with particles of different sizes. They used the amount that the fly extended its trunk to measure the animal’s interest in the food presented to it. Li and Montell found that without TMEM63, flies could not distinguish between a solution of pure sugar water and a solution containing small spheres of silica about 9 microns in diameter, which is the preferred level of granulation. flies.

When they added chemicals to make the sugar solution less pleasant – a mild acid, caffeine, or moderate amounts of salt – the microspheres reversed the aversion of flies. But not in flies lacking TMEM63. During the restoration of the gene which codes for this channel in the lip of mutant flies, the animals regained their ability to detect granulation.

“It was not known before this study that flies could even discriminate foods on the basis of graininess,” Montell noted. “Now that we have discovered that the mechanosensitive channel is TMEM63, we have discovered a role for this protein in an animal.”

The TMEM63 channel functions in a single multi-dendritic neuron (md-L) in each of the two labellas at the end of the fly’s proboscis. The neuron detects the movements of most of the taste hairs on the labellum. When the hairs move slightly during the interaction with the food particle, it activates the TMEM63 channel, which stimulates the neuron that transmits sensation to the brain. Because a neuron connects to many hairs, it probably cannot transmit position data of individual particles, only a gestaltic sense of granulation.

By applying a light force to these hairs – mimicking the action of small particles in a granular solution – Montell and Li could activate the md-L neuron. However, the same procedure showed no effect in flies with knocked out TMEM63. Interestingly, both groups of animals could detect stronger forces on their hair, like those that could be caused by hard or slimy foods.

Montell’s team had previously shown that another channel called TMC, which is also expressed in md-L neurons, is important for detecting these larger forces. Both TMEM63 and TMC relay textural information about food and even activate the same neuron. However, Li and Montell’s results revealed that the two channels have distinct roles.

Texture can provide a lot of information about food. It can indicate freshness or deterioration. For example, fruits often turn mealy when they start to spoil. “Animals use all the sensory information they can to assess the palatability of food,” Montell said. “This includes not only its chemical makeup, but also its color, odor, temperature and a variety of textural characteristics.

“The 9 micron particle size that flies like the most matches in size some of their favorite foods, like budding yeast and particles from their favorite fruit,” he explained.

Montell suggested that TMEM63 almost certainly has many other roles in animals. “This protein is conserved in humans,” Montell said. “We don’t know if this has a role in texture sensation in humans, but some kind of mechanically sensitive channel probably does.”