Sperm Finding May Lead to Unisex Contraceptive

By Bijal P. Trivedi
for National Geographic Today

October 15, 2001

Researchers have discovered a protein that helps sperm to swim. The
finding has stimulated immediate interest in developing a drug that
could block this protein, stopping sperm in their tracks, thereby
providing a new form of contraception that could be used by men or
women.

The protein, called CatSper, enables a single sperm cell
to vigorously whip its tail back and forth, giving it the momentum it
needs to penetrate the tough protective outer coating of a female's egg.
Without this "whiplash" action, the sperm cannot fertilize the egg.

CatSper is an "ion channel"—basically a tunnel-shaped protein that allows calcium to enter the sperm cell. If the channel is missing or blocked, calcium cannot enter the sperm cell and the tail moves only slightly.

A drug that could "block the tunnel" would cripple the sperm and prevent fertilization.

The researchers, led by David Clapham of Harvard Medical School in Boston, Massachusetts, observed the results in genetically engineered male mice that lacked the CatSper protein.

The CatSper protein is found exclusively on the tail of a sperm cell. As far as the researchers could tell, it influences only male fertility. Loss of the CatSper protein did not appear to affect the male libido, the quantity of sperm produced, or sperm development. The research is published in the October 11 issue of the journal Nature.

Because the CatSper protein is only found only on the tails of sperm, it is a good candidate for a contraceptive drug because it would be less likely to cause side effects.

Hormone-based contraceptives, like "the Pill," that contain estrogen and progestin often have significant side effects, including an increased risk of cancer and blood clots in the legs, lungs, heart, or brain, said Clapham.

A channel blocker designed specifically for CatSper would probably work only on sperm, said Clapham. "It shouldn't have effects in other parts of the body," he said.

Another advantage of a CatSper blocker is that it could be designed for men or women and be taken just before intercourse, removing the burden of taking the Pill every day.

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