A contraceptive pill that could be taken by men as well as women could be on the horizon, after a team of scientists has discovered the secret behind the sperm's power kick.
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University of California, Berkeley, biologists discovered the switch that triggers the power kick sperm use to penetrate and fertilize a human egg, uncovering a possible source of male infertility but also a potential target for contraceptives that work in both men and women.
The switch is a protein receptor that responds to the female sex hormone progesterone, which is released by the egg or oocyte, the ultimate goal toward which sperm swim.
Thousands of these receptors sit on the surface of a sperm's tail and when the sperm gets close to the egg, the hormone activates the receptor and triggers a cascade of changes that make the tail snap like a whip, powering the sperm into and hopefully through the cells protecting the egg.
"If the receptor protein doesn't recognize progesterone, you would be infertile," said first author Melissa Miller, adding "this gives us an understanding of another pathway that is involved in human sperm activity."
A drug that inactivates this newly discovered receptor, however, might make a good "unisex" contraceptive, one that could be used by either sexual partner.
"What's really cool is that we have an actual target for unisex contraceptive development," Miller said. "If you can stop progesterone from inducing a power stroke, sperm are not going to be able to reach or penetrate the oocyte."
While there are other possible targets for a contraceptive that would prevent the initiation of the power stroke, called hyperactivation, or the simultaneous release of enzymes that cut through the protective layer around the egg, "this is one of the better options we have for a unisex contraceptive," she said.
Senior author Polina Lishko added, "Now that we know the players, the next step is to look in other tissues that express these proteins to see whether progesterone acts on them in a similar manner to affect pain threshold adjustment in pain sensing neurons, surfactant production in the lungs or the excessive smooth muscle contractions found in asthma. This may be a universal pathway in all cells. ■