By tethering the water channel protein, Aquaporin-4 (AQP4), to the tips of brain cells, Syntrophin keeps the openings for water right next to blood vessels. Without Syntrophin, the water channels are everywhere on the brain cells except where they are supposed to be, the scientists found.
"The brain is in a very unique environment," says Peter Agre, M.D., a professor in the department of biological chemistry, part of the Johns Hopkins School of Medicine's Institute for Basic Biomedical Sciences, and an author of the report. "If you sprain your ankle, it has room to swell. If you hurt your brain, swelling can kill. The skull provides protection, but its rigidity can be a potentially lethal hazard as well."
In addition to impacting research into brain swelling, the finding also creates a new vision of the invisible barrier between the blood and the brain, one that involves a separate barrier for water, the scientists suggest.
"We think the interaction of the water channel and its tether helps regulate water flow in the brain," says lead author John Neely, Ph.D., a second-year medical student at Hopkins. "Together, AQP4 and Syntrophin appear to represent a vital control point for water in the blood-brain barrier."
Normally located in specialized brain cells called astrocytes, the water channel is clearly involved with starting down the slippery slope toward brain swelling, says Agre. Astrocytes, which make up about 50 percent of the brain's tissue, act as a bridge between blood vessels and impulse-sending neurons.
Usually pressed up to the blood vessel wall, the tips of astrocytes take on water and swell within minutes of a brain injury. AQP4 is quite prevalent in these tips, whose swelling comes before full-fledged leakiness of the blood vessels and general swelling of the brain.
"Once the astrocytes retract, in comes a lot of fluid and other matter that doesn't belong," says Neely. "It can't escape and eventually causes injury to neurons and compression of the vital structures of the brain."
The scientists say AQP4 and Syntrophin appear to be keys to understanding this process and to developing better ways to prevent or treat brain swelling. While high doses of steroids usually are used to treat brain swelling now, Neely says treatment could be improved if the steps that lead to it are better understood.
Neely's experiments showed how AQP4 is tethered to brain cells, while collaborators from the University of Washington in Seattle genetically engineered mice that lack alpha-Syntrophin. Among other differences, these animals had almost no AQP4 in their skeletal muscle, another place the water channel normally is found.
Using special imaging techniques, scientists in Norway found that distribution of AQP4 was reversed in the brain cells of mice without alpha-Syntrophin. The total amount of AQP4 in the mice's brain cells appeared normal, but the protein was on all surfaces except those next to blood vessels. The scientists are still evaluating how these changes affect the mice.
Syntrophin is one protein in a complex that fails in muscle-wasting diseases. In normal skeletal muscle, this complex helps link muscle cells to the external scaffolding that provides the muscular architecture.
"The standard answer to why muscles deteriorate in these diseases is that the muscular architecture is weakened," says Neely. "We've shown that the same protein complex involved in muscular dystrophy also holds AQP4 in its appropriate locations. If that complex is disrupted, AQP4 is in the wrong place. But we don't know what role yet, if any, this might play in muscular dystrophy."
The study was supported by the U.S. National Institutes of Health, the Muscular Dystrophy Association, the Norwegian Research Council and the Jahre Foundation. Co-authors on the report are Mahmood Amiry-Moghaddam and Ole Petter Ottersen of the University of Oslo; and Stanley Froehner and Marvin Adams of the University of Washington, Seattle.
On the Web:
http://www.hopkinsmedicine.org/press/2001/NOVEMBER/011119A.htm (PNAS, Nov. 20, 2001)
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Journal
Proceedings of the National Academy of Sciences