Vital find was made by chance

Water channel protein was plentiful contaminant in blood cell experiment

October 09, 2003|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

In the beginning there were frogs' eggs. And then they blew up.

That explosive moment during a 1991 experiment was all Dr. Peter Agre and colleagues at the Johns Hopkins medical school needed to prove that a blood cell protein they had come across was the long-sought key to the movement of water in and out of all human cells.

The experiment took barely five minutes. The first time Agre's staff activated that protein in the frogs' eggs, the eggs immediately began to swell.

In minutes, they burst.

"We knew we had it," Agre said yesterday. Still absorbing the news that his pioneering work had earned him a Nobel Prize in chemistry, he said, "I feel kind of like a guy who was lucky enough to be up at bat when the fastball was pitched down the middle."

Agre's discovery was published in the journal Science in April 1992. Subsequent research in his lab and others around the world has revealed a growing family of similar proteins, which he named "aquaporins," or "water channels."

Eleven have been identified in human cells alone. They're at work in organs and tissues such as the blood, kidneys, brain and eyes, regulating the balance of water inside cells to keep them healthy.

Water is lost as the body flushes away waste, cools itself and moistens exposed tissues, and it has to be replaced.

Aquaporins in kidney cells, for example, play a critical role. They work to concentrate urine, recovering water so it can be reused during heavy exercise and to dilute it so water can be expelled from the body when too much has accumulated.

"We're 80 percent water," said Mark A. Knepper, chief of the kidney and electrolyte lab at the National Institutes of Health and a longtime colleague of Agre. "It is important that that water is distributed throughout the body in an appropriate manner, and that we have just the right amount of water in our bodies."

New understanding of how and where these hourglass-shaped water channels function is opening up opportunities for the treatment of disease and targets for new pharmaceuticals.

For example, some people with a certain genetic mutation, or those receiving lithium to control manic depression, develop a rare form of diabetes, called diabetes insipidus.

The disorder causes them to lose large volumes of water through their kidneys - as much as 10 to 15 liters a day. (One liter of urine output per day is normal.) They must drink enormous amounts of water to survive.

Work by Agre and his colleagues has revealed that the lithium is suppressing the work of a key water channel in the kidneys, called AQP2.

"We expect to be able to help these patients get the benefit of the lithium and not deal with the consequences," Knepper said.

Dr. Landon S. King, associate professor of medicine at Hopkins, is exploring the role of aquaporins in maintaining the viability of hair-like cilia that clear germs and toxins from airways in the lungs. His group wants to know whether reduced water channel function in some people allows those cilia to dry out, contributing to emphysema and chronic bronchitis.

"Peter has made a fundamental observation that has changed the way we view basic aspects of cell function," King said. "It's not just an incremental advance; this is a sea change."

Scientists in the 19th century knew that there had to be openings in cell walls to allow the transfer of water and salts. Some believed the water just seeped through.

By the 1950s, however, many had concluded that some sort of gateway must be operating to allow the rapid transfer of water through cell walls, while restricting other substances. But no one knew how it worked.

It was not until the work by Agre's lab in 1991 that the mechanism was identified.

Dr. William B. Guggino, a professor of physiology and pediatrics at Hopkins, and co-author of the 1992 Science paper, said Agre wasn't looking for water channels.

He was using a novel purification technique to look for new proteins on the surface of red blood cells, substances - Rh factors - that help to define blood types.

"He happened to purify a protein that was one of the major proteins in the cell membrane," Guggino said.

The stuff was a contaminant in the experiment. But it caught the researchers' attention. It was one of the most abundant proteins in the cell membrane. It turned up also in kidney cells and in plants. And yet no one had ever identified it.

"Since it was very abundant," Guggino said, "he [Agre] thought it had to do something very important."

Consulting with a former colleague, Dr. John Parker at the University of North Carolina at Chapel Hill, Agre and Guggino discussed whether it might be the long-sought cellular water channel.

That led to the experiment with the frog eggs. Because these eggs are laid in water, they must be mostly impermeable to the water around them to remain intact.

Agre and his team cloned the mysterious protein they had discovered and injected its genes into six eggs, leaving another six untouched. The eggs were then placed in fresh water.

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