It pickpockets water

A protein makes the body realize the liquid’s worth  

By Manupriya
Published: Monday 31 May 2010

Ever wondered what it is that makes a glass of liquid as mundane as water the most satisfying of all drinks? Which part of the tongue senses this as the irreplaceable quencher on a hot and thirsty day?

Kristin Scott’s team of scientists has been working in a lab in the University of California-Berkley in usa, to understand these seemingly simple but complicated facts of life using the very ordinary-looking and almost unavoidable fruit fly as a model organism.

It has been known for long that insects taste water, which means they have specialized gustatory cells that respond, when stimulated with water. How these cells function has remained a mystery until now. In the research paper published in Nature on April 4, Scott and colleagues have found a protein localized in the boundary membranes of neurons that respond to water. Named pickpocket28 ( ppk 28), the protein is structured like a hollow channel.

It is imperative for all living organisms to recognize thirst for water, find it and then drink it. Though detection of water is “essential for all terrestrial animals, it is especially true for the insects who have a smaller body volume/surface area ratio,” said Teiichi Tanimura from the department of biology at Kyushu University in Japan. It is here that proteins like ppk 28 come into picture. ppk 28-containing neurons are located on the bristles in the mouth of flies.

The bristles were examined to see if they contributed to the urge to drink water. In mutant flies with malfunctional ppk 28 the bristles hardly responded even when there was water around. There was a 15-fold reduction in neuronal activity which means that the flies could not recognize their thirst for water. To reconfirm their observation, the scientists supplemented mutant flies with a normally functional ppk 28 and then examined their bristles. Coinciding with their expectation, the bristles pricked up considerably in response to water.

Having shown that the detection of water diminishes in absence of a fully functional ppk 28 protein, the scientists also wanted to see, if there were any behavioural changes in mutant flies. For this they monitored the duration for which flies continued drinking and, as expected, found it low. They also found that if ppk 28 was introduced in cells other than the specialized neurons, they too acquired water sensitivity.

Discovery of ppk 28 is important because it provides a framework for examining the molecular basis for water detection in other animals. Peter Cameron the graduate student involved in the work was enthused with the findings. He said he is “interested to know how many animals taste water and if there are any exceptions, why they evolved.” He also said, “It is known that ppk 28-related channels exist in humans, though they have not been investigated in relation to water taste. If water taste is important for mammals, it would be interesting to understand whether they evolved a water sensor similar to the ppk 28 proteins in the fruit fly or have an entirely different mechanism.”

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