MYASTHENIA GRAVIS

Myasthenia gravis is a disease that causes excessive fatigue and muscle weakness, in some cases leading to death. The history of how researchers have come to understand the disease, which afflicts about 150,000 people in the United States, illustrates how a number of seemingly unrelated strands of biological knowledge can merge to form a significant advance.

An important part of the story begins with curare, a poison derived from plants, insects, and snake toxins that the Indians of Central and South America used on the tips of their arrows to immobilize and kill prey and enemies, in the nineteenth century, French researchers showed in frogs and other animals that curare blocks the transmission of signals from the nervous system to muscles. However, the transmission process itself was not well understood until the 1930s, when English researchers demonstrated in animals that nerves communicate with muscles by releasing a chemical, acetylcholine, that activates receptor molecules on the muscles. Curare somehow blocked the action of acetylcholine, paralyzing the muscle.

Next, two chemists from Taiwan isolated a powerful toxin from snake venom that paralyzed animals by blocking the receptors for acetylcholine. Other investigators used this toxin to obtain large quantities of the receptor from electric eels, which have many receptors in their electricity-generating organs. When researchers injected this receptor into rabbits, the rabbits developed a syndrome virtually identical to myasthenia gravis. The rabbits were making antibodies to the injected receptors, and these antibodies were attacking the rabbits' own receptors, causing the muscle weakness characteristic of the disease.

In this way, scientists came to realize that myasthenia gravis was an autoimmune disease, in which a person's own immune system attacks acetylcholine receptors on muscles. Treatments have been available for some time to lessen the effects of the disease—by improving the transmission of signals, for instance, or by suppressing the effects of the immune system. Further research, again being conducted in animals, is seeking a permanent cure by focusing on what causes the immune system to attack the body's own acetylcholine receptors.



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