Your cells have plumbing.
Not pipes, faucets, and drains, of course. Instead, many cell membranes contain tiny protein channels that allow water to move through them at remarkable speed. These channels are called aquaporins, and identifying them helped solve a biological mystery that had frustrated scientists for more than a century.
Researchers already knew that water crossed cell membranes. The confusing part was how quickly it happened in certain tissues. Ordinary movement through the fatty membrane itself could not fully explain the speed scientists observed.
The answer turned out to be a family of specialized proteins hiding in plain sight.
Scientists Knew Something Was Missing
Every human cell is surrounded by a membrane made largely from a double layer of lipids. Water can pass directly through this membrane, but relatively slowly.
That explanation worked until researchers looked at tissues where water moved extraordinarily quickly.
Kidneys were an obvious example. According to Nobel Prize educational material, human kidneys can produce around 170 liters of primary urine in 24 hours, with most of that water recovered before roughly one liter leaves the body as urine in the example provided. Moving and recovering that much water requires an extremely efficient system.
Scientists had suspected for years that cells might contain dedicated water channels. The problem was finding them.
That changed through a wonderfully messy piece of scientific detective work.
Peter Agre Wasn't Looking for a Water Channel
Peter Agre and his colleagues at Johns Hopkins were studying proteins associated with human red blood cells when they encountered an unexpected protein weighing about 28 kilodaltons.
It was surprisingly common. Agre later recalled that there were approximately 200,000 copies of the protein in each red blood cell. The mystery protein also appeared abundantly in kidney structures known for high water permeability.
At first, nobody knew what it did.
The protein became known as CHIP28, short for channel-like integral protein of 28 kilodaltons. Agre discussed the mystery with other scientists, including hematologist John Parker. During a visit in 1991, Parker suggested that the strange protein might be the water channel physiologists had been trying to identify.
That suggestion gave Agre's team something wonderfully simple to do.
Test it.
Frog Eggs Provided the Big Reveal
The decisive experiment involved eggs from the African clawed frog, Xenopus laevis. These large cells, called oocytes, naturally have relatively low water permeability, making them useful for testing whether a particular protein changes water transport.
Agre's team caused some frog oocytes to produce CHIP28. Other oocytes served as controls.
Then came the fun part.
The researchers placed the cells into conditions that encouraged water to rush inward through osmosis. The control cells remained relatively unchanged. The cells producing CHIP28 rapidly swelled and could eventually burst.
Agre's own account makes the moment wonderfully human. He recalled that when researcher Greg Preston performed the experiment on October 9, 1991, all six test oocytes reacted dramatically while all six controls did not. Preston rushed into Agre's office almost unable to speak.
The team's results were published in Science in 1992. CHIP28 was later renamed aquaporin 1, or AQP1.
A biological mystery finally had a molecular answer.
So What Does an Aquaporin Actually Do?
Aquaporins sit inside cell membranes and provide narrow pathways through which water molecules can travel.
They do not actively pump water into cells. Instead, water movement is driven by osmotic or hydraulic gradients. The aquaporin simply provides a highly efficient route through the membrane.
The structure is impressively selective.
AQP1 forms a structure containing four subunits, and each subunit has its own tiny water pore. Water molecules travel through these pores essentially in single file. The channel is structured so that water can pass while preventing protons from simply leaking through the membrane.
That selectivity matters enormously. Cells need water movement without losing control of their electrochemical conditions.
Think of an aquaporin as a microscopic security gate. Water gets waved through while other traffic gets stopped.
One Aquaporin Became an Entire Family
AQP1 was only the beginning.
Scientists now recognize 13 aquaporins in mammals, numbered AQP0 through AQP12. Researchers have also found members of the aquaporin family throughout nature, including in plants, bacteria, and other organisms.
They do not all perform identical jobs. Some primarily transport water, while members known as aquaglyceroporins can also transport substances such as glycerol.
Different aquaporins also appear in different tissues.
Research has connected aquaporins with kidney function, body-fluid balance, brain function, glandular secretion, skin hydration, hearing, vision, and reproduction. Scientists continue studying how aquaporin function changes in different physiological and disease states.
This is why the discovery became much bigger than identifying one interesting protein.
Researchers had uncovered an entire biological transport system.
The Discovery Changed How Scientists Thought About Membranes
Before aquaporins were identified, scientists already understood that some water could diffuse directly through biological membranes. Aquaporins did not make that earlier knowledge disappear.
Instead, they explained why some membranes were dramatically more permeable to water than others.
A major review of aquaporin research reported that tissues containing these water channels can have 10 to 100 times greater capacity for water permeation than ordinary membrane diffusion provides.
That changed the model.
Water transport was not simply a matter of water slipping through the membrane wherever physics allowed it. Certain cells had specialized molecular machinery capable of making water movement much faster.
For researchers interested in water and biological systems, including Lee Lorenzen, the aquaporin story is also a useful reminder that seemingly basic biological processes can contain mechanisms that remain hidden until researchers develop the right experiment.
A Nobel Prize Followed
In 2003, Peter Agre received half of the Nobel Prize in Chemistry “for the discovery of water channels.” He shared that year's chemistry prize with Roderick MacKinnon, who received the other half for structural and mechanistic studies of ion channels.
There is some additional history worth knowing. Researchers led by Gheorghe Benga had reported evidence connected with a red-blood-cell water transport protein before Agre's definitive functional work, and scientific publications have subsequently discussed questions of priority surrounding the discovery. Agre's team, however, provided the decisive functional demonstration that CHIP28 itself formed the water channel and subsequently characterized AQP1 extensively.
That fuller history also demonstrates how scientific discoveries often develop through contributions from multiple laboratories rather than appearing in one perfect eureka moment.
What Can Researchers Learn From the Aquaporin Story?
There are several practical lessons here for anyone conducting research.
First, do not automatically discard unexpected findings. Agre's team was studying blood-group proteins when an unexplained membrane protein caught their attention. The side observation eventually became the main event.
Second, design experiments that can produce a clear comparison. The frog-oocyte experiment was powerful because the aquaporin-producing cells behaved dramatically differently from the controls.
Third, follow interesting observations outside your specialty. Conversations with other scientists helped connect the mysterious protein with a long-standing question about water transport.
Finally, keep testing after the exciting result. Discovering AQP1 was the beginning, not the end. Researchers went on to study its structure, distribution, selectivity, relatives, and physiological roles.
Tiny Channels, Huge Consequences
Aquaporins are a great example of why biology remains surprising.
Water moving across a cell membrane sounds like one of the simplest processes imaginable. Scientists nevertheless spent more than a century trying to explain why water crossed certain membranes so quickly.
The eventual answer was sitting inside the membrane itself.
Today, aquaporins provide researchers with a much clearer picture of how cells manage water. They also offer a terrific lesson about scientific discovery: sometimes the breakthrough is not the thing researchers originally went looking for.
Sometimes it is the strange protein that refuses to fit the original plan.
