Deactivating Sperm’s Tail Could Be Key to Unisex Contraceptive, New Research Says

Taking the wiggle out of a sperm’s tail may be key to creating a new contraceptive that doesn’t involve hormones and can be taken by either a man or a woman, say scientists in Boston.

A protein that makes it possible for the sperm to swim to the egg can be switched off — at least in mice, which makes them infertile, say the Harvard Medical School researchers.

Along with the allure of a unisex contraceptive, another plus is that the method uses no hormones, which have been linked to side effects in oral contraceptive pills, implants and injections for women.

But it’s a tossup as to whether men would embrace such a contraceptive, says one reproductive biology expert.

In the Oct. 11 issue of Nature, researchers describe a protein that controls the whip-like movement of sperm tails, which normally allows the male reproductive cell to travel into the female reproductive tract in search of a fertile egg.

When that protein is switched off, the sperm tails lose their lash, meaning they can’t get to the ovum for fertilization to take place.

The discovery was made by a team of neurobiologists and cardiovascular scientists from Harvard Medical School studying ion channels, which are involved in the electrical activities of cells. While searching for ion channels that allow calcium into cells, they found an unusual channel, which they named CatSper.

“The most surprising part was that it was only in the sperm tails,” says David Clapham, a professor of cardiovascular medicine and neurobiology who led the research. When they deleted the gene for the CatSper protein channel in a group of mice, the mice were infertile. “The [sperm] can’t swim as well, and they can’t penetrate the coating of the egg,” he says.

In every other way that the scientists could detect, the mice were normal, although Clapham notes that the team could not test the animals’ psychiatric health. “But since the protein is only in the sperm, then you wouldn’t predict that you’d have any other effects.”

David Garbers, the director of the Cecil H. and Ida Green Center for Reproductive Biology Sciences at the University of Texas Southwestern Medical Center in Dallas, says that because the protein is located only on sperm tails, it’s an ideal target for drug therapies.

Clapham says that the goal will be to develop a drug that can block the protein channel temporarily. Alternatively, he says, it may be possible to increase the activity of the protein channel to increase sperm motility, which Garbers says could potentially help some men with fertility problems related to sperm mobility.

Moreover, a therapy that blocks the channel could be taken by either a man or a woman, says Clapham. “Sperm are deposited in the vagina, and any drug that was circulating in a female could also get to the sperm through the tissues.”

Garbers says that a female version could work as a local therapy, such as an intrauterine device that releases a compound that blocks the channel.

Or ideally, Clapham adds, it could be something taken as a pill. “If you had a pill that worked, then you’d have an easy contraceptive that didn’t have all the side effects that current hormonal contraceptives have,” says Clapham. “Hormonal therapy — progestin and estrogen — affect all tissues and have to be taken long term, every day. This could be taken just around the time of sex, and then it would not have to be taken for very long.”

But whether men would enthusiastically use this theoretical contraceptive is another issue, Garbers adds. “Up until a year or a few years ago, I think it’s safe to say that no woman in her right mind would trust a male who said he was on the pill,” he notes.

But he points out that studies have suggested that roughly 60 percent of men would like a male contraceptive, and that recent paternity cases involving DNA analysis are making some men more aware of their contraceptive options.

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